Title:
HIGH STRENGTH STEEL SHEET AND METHOD FOR PRODUCING THE SAME
Kind Code:
A1


Abstract:
The present invention relates to A high strength steel sheet having a structure which is mainly composed of MD structure (Micro Duplex structure) comprising a ferrite matrix, and as a secondary phase, martensite or martensite and retained austenite, finely dispersed in said matrix, wherein the proportion that the MD structure occupies in the whole structure is 90% or more, wherein the proportion that the secondary phase present in the whole structure occupies in the whole structure is from 10 to 60%, wherein the secondary phase in the MD structure is present in ferrite grains and at grain boundary, in which the proportion of the secondary phase present in the ferrite grains is 50% or more, and wherein the average grain size of the secondary phase in the whole structure is 3 μm or less. The secondary phase is constituted of martensite, or martensite and retained austenite.



Inventors:
Saito, Kenji (Hyogo, JP)
Masuda, Tomokazu (Hyogo, JP)
Murakami, Toshio (Hyogo, JP)
Makii, Koichi (Hyogo, JP)
Kunishige, Kazutoshi (Kagawa, JP)
Ueji, Rintaro (Kagawa, JP)
Application Number:
11/873048
Publication Date:
07/31/2008
Filing Date:
10/16/2007
Assignee:
KABUSHIKI KAISHA KOBE SEIKO SHO (KOBE STEEL, LTD) (Kobe-shi, JP)
National University Corporation Kagawa University (Takamatsu-shi, JP)
Primary Class:
Other Classes:
148/320, 148/330, 148/661
International Classes:
C21D1/02; C22C38/32; C22C38/38
View Patent Images:



Primary Examiner:
LUK, VANESSA TIBAY
Attorney, Agent or Firm:
OBLON, MCCLELLAND, MAIER & NEUSTADT, L.L.P. (ALEXANDRIA, VA, US)
Claims:
What is claimed is:

1. A high strength steel sheet having a structure which is mainly composed of MD structure (Micro Duplex structure) comprising a ferrite matrix, and as a secondary phase, martensite or martensite and retained austenite, finely dispersed in said matrix, wherein the proportion that the MD structure occupies in the whole structure is 90% or more, wherein the proportion that the secondary phase present in the whole structure occupies in the whole structure is from 10 to 60%, wherein the secondary phase in the MD structure is present in ferrite grains and at grain boundary, in which the proportion of the secondary phase present in the ferrite grains is 50% or more, and wherein the average grain size of the secondary phase in the whole structure is 3 μm or less.

2. The high strength steel sheet as claimed in claim 1, wherein the secondary phase is constituted of martensite.

3. The high strength steel sheet as claimed in claim 1, wherein the secondary phase is constituted of martensite and retained austenite.

4. The high strength steel sheet as claimed in claim 2, wherein an average grain size of ferrite in the whole structure is 20 μm or less, and wherein 20 or more secondary phases on the average are present in an observation view of 50 μm×50 μm in the whole structure.

5. The high strength steel sheet as claimed in claim 3, wherein an average grain size of ferrite in the whole structure is 20 μm or less, and wherein 20 or more secondary phases on the average are present in an observation view of 50 μm×50 μm in the whole structure.

6. The high strength steel sheet as claimed in claim 5, wherein the proportion that the retained austenite present in the whole structure occupies in the whole structure is 2% or more.

7. The high strength steel sheet as claimed in claim 2, comprising, in terms of % by mass, C: 0.02 to 0.3%; Si: 0.01 to 3%; Mn: 0.5 to 3%; B: 0.0001 to 0.005%; and Al: 0.01 to 1.5%, with the remainder being Fe and inevitable impurities.

8. The high strength steel sheet as claimed in claim 3, comprising, in terms of % by mass, C: 0.02 to 0.3%; Si: 0.01 to 3%; Mn: 0.5 to 3%; B: 0.0001 to 0.005%; and Al: 0.01 to 1.5%, with the remainder being Fe and inevitable impurities.

9. The high strength steel sheet as claimed in claim 7, further comprising Mo: 0.03 to 1%.

10. The high strength steel sheet as claimed in claim 8, further comprising Mo: 0.03 to 1%.

11. The high strength steel sheet as claimed in claim 7, further comprising at least one element selected from the group consisting of Nb, Ti and V in the total amount of from 0.01 to 0.1%.

12. The high strength steel sheet as claimed in claim 8, further comprising at least one element selected from the group consisting of Nb, Ti and V in the total amount of from 0.01 to 0.1%.

13. The high strength steel sheet as claimed in claim 7, further comprising at least one of Ni: 0.5% or less, and Cu: 0.5% or less.

14. The high strength steel sheet as claimed in claim 8, further comprising at least one of Ni: 0.5% or less, and Cu: 0.5% or less.

15. The high strength steel sheet as claimed in claim 7, further comprising Cr: 1.5% or less.

16. The high strength steel sheet as claimed in claim 8, further comprising Cr: 1.5% or less.

17. The high strength steel sheet as claimed in claim 7, further comprising at least one of Ca: 0.003% or less, and REM: 0.003% or less.

18. The high strength steel sheet as claimed in claim 8, further comprising at least one of Ca: 0.003% or less, and REM: 0.003% or less.

19. A method for producing a high strength steel sheet, which comprises: heating a steel sheet material comprising, in terms of % by mass, C: 0.02 to 0.3%, Si: 0.01 to 3%, Mn: 0.5 to 3%, B: 0.0001 to 0.005%, and Al: 0.01 to 1.5%, with the remainder being Fe and inevitable impurities; and cooling the steel sheet material from a temperature of A3 point or higher to a temperature of Ms point or lower in a cooling rate of from 0.2 to 20° C./sec.

20. A method for producing a high strength steel sheet, which comprises: cooling, from a temperature of A3 point or higher, a steel sheet material comprising, in terms of % by mass, C: 0.02 to 0.3%, Si: 0.01 to 3%, Mn: 0.5 to 3%, B: 0.0001 to 0.005%, and Al: 0.01 to 1.5%, with the remainder being Fe and inevitable impurities; subjecting said material to a processing of rolling reduction of 5% or more in a temperature range of from 600 to 1,000° C.; and then cooling said material to a temperature of Ms point or lower in a cooling rate of from 0.2 to 20° C./sec.

Description:

TECHNICAL FIELD

The present invention relates to a high strength dual phase steel sheet having excellent elongation and stretch flangeability. In particular, it relates to a high strength dual phase steel sheet having excellent strength-elongation and stretch flangeability in high strength region of 590 MPa or more.

BACKGROUND OF THE INVENTION

In industrial fields of automobiles, electrical machinery and appliances, machinery and the like, steel sheets used by press forming are required to combine excellent strength and formability. In recent years, such demand characteristics are increasing more and more. Dual phase steel sheets (DP steel) having dual phase of ferrite and martensite, and TRIP steels comprising a structure containing ferrite, bainite and retained austenite are known as high strength steel sheets having excellent ductility. However, dual phase steel sheets such as DP steel and TRIP steel have the problem that stretch flangeability is poor.

To improve stretch flangeability in high strength steel sheets, a method of suppressing localization of strain by making the structure a single phase structure to homogenize processability in the structure, and a method of reducing strength difference between a soft phase and a hard phase in a dual phase structure are known. Regarding those two methods, for example, the following technologies are disclosed.

1. Single Phase Structure (See Patent Document 1)

A method of producing a martensite single phase steel sheet having tensile strength of from 880 to 1,170 MPa by bringing more balance to components and heat treatment conditions has been disclosed. For the production of a martensite single phase structure, soaking temperature necessary for austenitizing is set to 850° C. which is generally the industrially attainable temperature condition, and this made it possible to industrially achieve a martensite single phase structure. However, a steel sheet of a martensite single phase structure is that the structure is macroscopically uniform, so that stretch flangeability is excellent, but ductility (elongation) greatly deteriorates (EL<8%).

2. Reduction in Strength Difference of Dual Phase Structure (See Patent Document 2)

A steel sheet having a space factor of a low temperature transformation phase of 90% or more is heated and held in two phase regions of ferrite and austenite, thereby fine ferrite and austenite succeeding lath of a low temperature transformation phase can be formed. After the subsequent cooling, a structure in which ferrite and the low temperature transformation phase are finely dispersed in a lath state is finally formed. Such a finely dispersed low temperature transformation phase suppresses formation and growth of voids at the time of flange deformation, thereby improving stretch flangeability.

However, in this method, the structure become fine, but martensite which is a hard phase cannot be precipitated in grains. Furthermore, a grain size of martensite is 5 μm at the minimum, and the structure cannot sufficiently be homogenized. For this reason, improvement effect of stretch flangeability is not sufficient.

3. Fine Retained Austenite Steel Sheet (See Patent Document 3)

A steel sheet in which a secondary phase having an average grain size of 500 nm or less is finely dispersed in grains has been disclosed. High elongation and stretch flangeability are achieved by forming a starting point of fracture in the grains.

In this method, very expensive austenite stabilizing element such as Au, Ag or Ni must be added in order to form the secondary phase in the grains. Additionally, a concentrated region of the austenite stabilizing element must be formed in the grains in order to form the secondary phase in the grains, but to achieve this, a solution heat treatment at 1,270° C. or higher for 5 hours or more becomes necessary. Consequently, industrial problem in increase of time and cost remains in this method.

Patent Document 1: Japanese Patent No. 3729108

Patent Document 2: JP-A-2005-272954

Patent Document 3: JP-A-2005-179703

SUMMARY OF THE INVENTION

Accordingly, the present invention has an object to exhibit both elongation and stretch flangeability particularly in a high strength steel sheet of 590 MPa or more, and further to industrially realize such a high strength steel sheet.

To obtain a high strength steel sheet, it is necessary that a hard phase is a secondary phase, but when the hard phase is present at the grain boundary of a matrix, its interface becomes a starting point of fracture, and local elongation lowers. However, when a hard phase is not present, high strength cannot be obtained. When a hard phase can finely be present in the grains of a matrix, a starting point of fracture can be minimized, and as a result, local elongation of steel can be improved. It is known that local elongation of steel has a correlation with stretch flangeability thereof, and high stretch flangeability can be obtained by improving local elongation.

Thus, to improve stretch flangeability while maintaining high ductility in a high strength steel sheet of 590 MPa or more, it is important to be a dual phase steel sheet and to uniformly and finely precipitate a hard secondary phase in the grains of a matrix. Furthermore, it is important that the production of steel can industrially be realized.

In view of the above, the present inventors thought that the entire surface of a steel sheet is constituted of bainite in order to provide a steel sheet having improved stretch flangeability while maintaining high ductility even in a high strength region of 590 MPa or more, and thought to control the existence form of cementite in bainite. The reason that the matrix is bainite is that lath of bainitic ferrite constituting bainite is not grain boundary, and is therefore difficult to become a starting point of fracture. As a result of verification, cementite which is a hard phase could finely be dispersed among laths, but bainite is slightly poor in ductility, and the desired elongation was not obtained.

Subsequently, Si was added to improve elongation. By adding Si, precipitation of cementite is suppressed in the course of cooling, and austenite in which C is concentrated is formed among laths of bainitic ferrite. Furthermore, bainite transformation is completed, and cooling rate was controlled such that lath of bainitic ferrite disappears in the course of the subsequent cooling.

As a result, a structure comprising a ferrite phase and austenite finely dispersed therein was obtained in the cooling process. When austenite reached Ms point or lower, MD structure comprising a ferrite phase and martensite as a hard phase finely dispersed therein was obtained. However, in this method, a ferrite phase in which martensite is not dispersed in the ferrite phase was simultaneously formed. When such ferrite is present in the structure, strength deteriorates, and in addition, stretch flangeability deteriorats due to strength difference at the interface between the ferrite and MD.

In view of the above, the present inventors have further made researches and found that formation of ferrite which does not contain martensite can be suppressed by adding B. As a result, nearly the entire surface can be constituted of MD structure, and the present invention could be completed based on this finding.

When such unique components and heat treatment are employed, martensite having strength difference to a matrix smaller than a hard cementite can be uniformly and finely dispersed in ferrite grains. As a result, it was clarified that a dual phase steel sheet having excellent strength-stretch flangeability while maintaining high ductility can be provided even in a high strength region of 590 MPa or more.

Furthermore, DP steel, TRIP steel and the like described above are conventionally developed as the means to combine strength and ductility (elongation). DP steel comprises hard martensite and soft ferrite. The hard martensite ensures strength, and the soft ferrite ensures ductility (elongation). On the other hand, TRIP steel is that retained austenite generates deformation-induced transformation during deformation, thereby improving ductility (elongation). However, the ductility (elongation) used herein means uniform elongation, and those steel sheets are excellent to combine strength and ductility (uniform elongation), but had the disadvantage that local elongation is poor. It is known that local elongation has a high correlation with stretch flangeability, and improvement in local elongation is indispensable to obtain high stretch flangeability.

Accordingly, as a result of intensive investigations to provide a steel sheet which combines elongation and stretch flangeability even in a high strength region of 590 MPa or more, namely a steel sheet combining uniform elongation and local elongation in addition to strength, the present inventors have succeeded to develop a high strength steel sheet having excellent elongation and stretch flangeability, leading to the present invention.

Development process of the steel of the present invention and mechanism of exhibiting high properties are described below.

To combine strength and ductility, it is advantageous to be a dual phase structure in which a soft phase and a hard phase are combined, and the steel of the present invention has a dual phase structure comprising ferrite as a soft phase and martensite as a hard phase. However, even though a dual phase structure merely comprising soft ferrite and hard martensite is formed as in DP steel as described before, it is effective to combine strength and uniform elongation, but it is not sufficient to ensure excellent stretch flangeability. The reason that DP steel is poor in stretch flangeability includes size and existence position of martensite as a hard phase. Specifically, martensite in DP steel is generally large as about several ten μm, and grain boundary between martensite and ferrite is large tilt grain boundary. When deformation is added to a steel sheet, stress concentrates at grain boundary (large tilt grain boundary) as compared with the inside of grains. When interface having strength difference is present here, voids are generated in a moment, which is liable to lead to fracture. Such generation of voids and fracture have a correlation with local elongation, and a steel sheet that is liable to cause generation of voids and propagation of cracks is poor in local elongation. Grain boundary between ferrite and martensite in DP steel is just an interface having strength difference, and excellent stretch flangeability cannot be possessed for this reason. Furthermore, although it is considered that this tendency is decreased with decreasing a size of a hard phase, this effect cannot be obtained in a size of martensite in general DP steel.

In view of the above, the present inventors thought that as the means for improving stretch flangeability while maintaining excellent properties of DP steel (combining strength and uniform elongation), a dual phase structure having a soft phase (ferrite) and a hard phase (martensite) is basically formed, but martensite is finely dispersed in ferrite grains (or small tilt grain boundary). They further thought that even in the case of ensuring the same degree of strength, grain size of the individual martensite can be decreased as compared with the general DP steel, and by further making martensite present in grains (or small tilt grain boundary), it becomes possible to suppress generation of voids in the case that deformation has been generated. By those, it is possible to combine elongation and stretch flangeability, and in addition to this, it is expected that ductility (particularly, uniform elongation) is further improved by utilizing TRIP effect as a structure containing retained austenite. The steel of the present invention developed based on the above-described thoughts has a structure comprising ferrite as a matrix, and martensite and retained austenite, finely dispersed mainly in ferrite grains. Due to this structure, the steel combines elongation and stretch flangeability in addition to strength.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic view showing thermo-mechanical treatment in the Examples.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to the following (1).

(1) A high strength steel sheet having a structure which is mainly composed of MD structure (Micro Duplex structure) comprising a ferrite matrix, and as a secondary phase, martensite or martensite and retained austenite, finely dispersed in said matrix,

wherein the proportion that the MD structure occupies in the whole structure is 90% or more,

wherein the proportion that the secondary phase present in the whole structure occupies in the whole structure is from 10 to 60%,

wherein the secondary phase in the MD structure is present in ferrite grains and at grain boundary, in which the proportion of the secondary phase present in the ferrite grains is 50% or more, and

wherein the average grain size of the secondary phase in the whole structure is 3 μm or less.

In the present invention, the secondary phase is constituted of martensite, or martensite and retained austenite.

The term “proportion” used herein means a proportion of area (area fraction). Furthermore, the term “whole structure” used herein means the entire of MD structure and other structures.

It is desirable that an average grain size of ferrite in the whole structure is 20 μm or less, and 20 or more secondary phases on the average are present in an observation view of 50 μm×50 μm in the whole structure.

Furthermore, it is desirable in the high strength steel sheet that the proportion that the retained austenite present in the whole structure occupies in the whole structure is 2% or more.

The high strength steel sheet has a composition comprising, in terms of % by mass, C: 0.02 to 0.3%; Si: 0.01 to 3%; Mn: 0.5 to 3%; B: 0.0001 to 0.005%; and Al: 0.01 to 1.5%, with the remainder being Fe and inevitable impurities.

The high strength steel sheet may further contain one or two or more of:

(1) Mo: 0.03 to 1%;

(2) at least one element selected from the group consisting of Nb, Ti and V in the total amount of from 0.01 to 0.1%;

(3) at least one of Ni: 0.5% or less (not including 0%) and Cu: 0.5% or less (not including 0%);

(4) Cr: 1.5% or less (not including 0%); and

(5) at least one of Ca: 0.003% or less (not including 0%) and REM (rare earth element): 0.003% or less (not including 0%).

In addition, a method for producing a high strength steel sheet according to the present invention comprises heating a steel sheet material comprising the above-described component compositions, and cooling the steel sheet material from a temperature of A3 point or higher to a temperature of Ms point or lower in a cooling rate of from 0.2 to 20° C./sec.

The steel sheet material is produced by appropriately conducting a hot rolling step and a cold rolling step. Those steps are not particularly limited, and the conditions generally practiced can appropriately be selected and employed. For example, as the hot rolling step, conditions of holding at about 1,200° C. for 30 minutes, conducting hot rolling at A3 point or higher, cooling in an average cooling rate of about 30° C./sec, and winding up at about 500 to 600° C. can be employed. In the cold rolling step, it is recommended to conduct cold rolling in a cold rolling ratio of about 30 to 70%, but it is not particularly limited to this.

After heating the steel sheet material at a temperature of A3 point or higher and holding the same at that temperature, the steel sheet material is cooled to a temperature of Ms point or lower, generally room temperature. As a result, the structure of the present invention comprising MD structure in major portion can be obtained, and stretch flangeability can be improved while maintaining high ductility, even in a high strength region of 590 MPa or more.

When the steel sheet material is heated to a temperature of A3 point or higher and held at that temperature, the entire surface of the structure converts into austenite. Then, when the steel sheet material is cooled in a cooling rate of 0.2 to 20° C./sec, a dual phase structure of ferrite-pearlite is generally obtained. However, because Si is added in the present invention, precipitation of cementite is suppressed, and as a result, bainitic ferrite begins to precipitate from the austenite interface. With the growth of lath of bainitic ferrite, austenite decreases its space factor, and austenite is finely dispersed among laths of bainitic ferrite.

The lath of bainitic ferrite is thermally unstable, and therefore disappears in the cooling process after completion of the bainite transformation. As a result, the state that austenite is finely dispersed in the grains of ferrite having bainite block size is obtained, and when the temperature becomes Ms point or lower by further cooling, austenite transforms into martensite. As a result, MD structure comprising ferrite matrix formed by disappearance of the lath of bainitic ferrite, and a secondary phase comprising martensite finely dispersed in the matrix can be obtained. When B is further added, formation of ferrite which does not contain martensite in the grains can be suppressed. Furthermore, when Mo is added, bainite transformation can be completed in a short period of time. Mo is effective to allow the lath of bainitic ferrite to disappear.

Furthermore, the steel sheet having the above-described structure morphology can be obtained by optimizing chemical compositions and production steps (thermo-mechanical treatment conditions) in an industrially possible range. Unclear points still remain regarding appearance mechanism of the above-described structure, and all is not always clarified to the detail. However, at the present, it is considered that the following explanation is possible.

The steel of the present invention can be produced by heating a steel sheet material comprising the above-described component composition controlled in an appropriate range, cooling the material from a temperature of A3 point or higher, subjecting the material to a processing of rolling reduction of 5% or more in a temperature range of 600 to 1,000° C., and then cooling the material to a temperature of Ms point or lower in a cooling rate of 0.2 to 20° C./sec. This production step (thermo-mechanical treatment) may be conducted in a hot rolling step or may be conducted after a hot rolling step and a cold rolling step, generally conducted (in this case, this thermo-mechanical treatment is conducted by reheating).

Accordingly, MD structure comprising ferrite as a matrix, and martensite and retained austenite, which are finely dispersed mainly in ferrites grains is formed. Thus, the structure of the present invention can be produced by forming an austenite single phase structure at a temperature of A3 point or higher, cooling, adding processing at a predetermined temperature and then cooling in an appropriate range.

As described above, the structure formation mechanism of the steel of the present invention is not clarified to the detail, but one explanation that is considered at the present is described below. Therefore, the following description is not construed to limit the technical scope of the present invention.

One of various structure design guidelines devised to obtain the structure of the present invention is described below.

The present inventors thought to obtain a structure comprising ferrite as a matrix, and martensite and retained austenite finely dispersed therein, using bainite transformation generated during the cooling process. For this, bainite is formed during the cooling step after processing, but in the steel of the present invention, a mixed structure of bainitic ferrite and cementite as in general bainite is not formed. Since cementite is harder than martensite, such is harmful to stretch flangeability. Furthermore, to ensure retained austenite which is essential in the steel of the present invention, it is necessary to concentrate C in austenite, and it is therefore necessary to prevent precipitation of cementite which disturbs to concentrate C. For this reason, in the steel of the present invention, Si and Al are added to suppress precipitation of cementite in the cooling process. Accordingly, austenite in which C is concentrated among laths of bainitic ferrite is formed. This austenite is transformed into martensite in the subsequent cooling step, or remains as retained austenite without transformation.

Lath of bainitic ferrite is small tilt grain boundary, and concentration of strain during deformation as in large tilt grain boundary described before is difficult to be generated. As a result, martensite and retained austenite present among laths of bainitic ferrite are difficult to become a starting point of fracture. The term “ferrite grain boundary” used herein means large tilt grain boundary of ferrite, and lath boundary which is difficult to become a starting point of fracture is not grain boundary.

However, it is difficult to observe small tilt grain boundary at the position that martensite and retained austenite are present, in the structure obtained by the present inventors. It suggests that the steel of the present invention has further excellent structure morphology based on the following reasons.

In the steel of the present invention, processing is added at a predetermined temperature region, and as a result, bainite transformation is accelerated and bainite transformation is completed in a short period of time. Additionally, since the subsequent cooling rate is set to relatively low condition, it is still present at relatively high temperature for a certain period of time even after bainite transformation. For this reason, it is considered that even lath (small tilt grain boundary) of bainitic ferrite disappears, and as a result, it is considered that a structure (MD structure) comprising ferrite grains and martensite and retained austenite mainly dispersed therein was obtained.

In the above description, one of thoughts considered at present is shown as appearance mechanism of the structure of the present invention, but only the steel comprising the bainitic ferrite as a matrix is not the object of the present invention. In other words, ferrite which is a matrix of MD structure of the present invention is not limited to only ferrite wherein lath of bainitic ferrite becomes unclear, but includes ferrite which is precipitated at relatively high temperature, like pro-eutectoid ferrite. Thus, the present invention includes all of MD structure steel sheets having structure morphology defined in the present application.

Furthermore, the present inventors have found in the course of the above investigations that when ferrite in which martensite and retained austenite are not present is present, not only strength deteriorates, but stretch flangeability deteriorates by strength difference at the interface between ferrite and MD structure. Based on this finding, formation of such martensite-free ferrite is suppressed by adding B, thereby nearly the entire surface is constituted of MD structure.

The present invention can provide a dual phase steel sheet having excellent strength-elongation and stretch flangeability while maintaining high ductility even in high strength region of 590 MPa or more by forming a structure comprising martensite, or martensite and retained austenite which have small strength difference to a matrix as compared with hard cementite, uniformly and finely dispersed in ferrite grains.

Furthermore, according to the present invention, such a dual phase steel sheet can be produced by industrially possible means.

The structure of the present invention in the case that the secondary phase is martensite is described below.

Ferrite which is a matrix means ferrite having martensite contained in grains, formed in the cooling process from A3 point, and this ferrite includes pro-eutectoid ferrite and ferrite from which lath of bainitic ferrite has disappeared. On the other hand, this ferrite does not include ferrite which does not contain fine martensite in grains even though it is pro-eutectoid ferrite and ferrite from which lath of bainitic ferrite has disappeared. In the present invention, ferrite containing martensite in grains and ferrite which does not contain fine martensite in grains can be identified by, for example, applying the structure after completion of cooling to repeller corrosion, and showing ferrite in gray and martensite in white by image analysis. The entire surface of ferrite which does not contain fine martensite in grains is gray. On the other hand, the MD structure of the present invention is that fine martensite is contained in a ferrite phase. Therefore, fine white spot can be confirmed in ferrite grains.

The structure of the present invention is mainly composed of MD structure, and the MD structure comprises ferrite as a matrix and martensite as a secondary phase. The martensite as a secondary phase is formed such that austenite present in the ferrite matrix transforms into martensite at Ms point or lower in the cooling process. When the area fraction of the whole structure is defined as 100%, the proportion of the MD structure is required to be 90% or more. Where the proportion of the MD structure is less than 90%, influence of the remainder increases, uniformity of the structure is impaired, and stretch flangeability deteriorates. The remainder includes retained austenite, pearlite, bainite, and ferrite which is not accompanied with martensite in grains. On the other hand, the proportion that the MD structure occupies in the whole structure is better as large, and the upper limit is not particularly defined. Furthermore, when the area fraction of the whole structure is defined as 100%, the proportion of martensite as a secondary phase present in the whole structure (martensite present in MD structure and other structures) is required to be from 10 to 60%, and the desired strength can be obtained by this proportion. When the proportion is less than 10%, sufficient strength is not obtained, and when it exceeds 60%, elongation deteriorates. The proportion is preferably in a range of from 20 to 60%, more preferably in a range of from 30 to 55%, and most preferably in a range of from 40 to 50%.

Martensite in the MD structure is nearly uniformly dispersed in ferrite grains and at ferrite grain boundary. When the whole of the martensite is defined as 100%, the proportion of martensite in ferrite grains is required to be 50% or more (namely, the proportion of martensite present at ferrite grain boundary is less than 50%), and the desired stretch flangeability can be obtained by this proportion. When the proportion is less than 50%, martensite present at ferrite grain boundary acts as a starting point of fracture, and stretch flangeability deteriorates. The proportion of martensite in ferrite grains is preferably 60% or more, and more preferably 70% or more.

In the present invention, the term “grain boundary of ferrite matrix” means block boundary of bainitic ferrite formed by bainite transformation during cooling, and, for example, when orientation difference of adjacent ferrites is measured by FE/SEM-EBSP (Electron Back Scatter Diffraction Pattern), large tilt grain boundary in which the orientation difference is 15° or more is defined as block boundary, i.e., grain boundary, and a region surrounded by the block boundary is defined as grain.

Next, the structure of the present invention in the case that the secondary phase is martensite and retained austenite is described below.

The structure of the steel of the present invention is mainly composed of a structure (MD structure) comprising ferrite grains as a matrix and contained therein, fine martensite and retained austenite, as a secondary phase. Ferrite which does not contain a fine secondary phase in grains and MD structure differ in structure morphology. Furthermore, influence giving to properties differs between those, and it is therefore necessary to distinguish those.

As the identification method, for example, those can be identified by, for example, subjecting a steel sheet containing those structures to repeller corrosion, and showing ferrite in gray and martensite and retained austenite in white by image analysis. The entire surface of ferrite which does not contain the fine secondary phase in grains is gray. On the other hand, the MD structure is that fine martensite and retained austenite are contained in a ferrite phase. Therefore, fine white spot can be confirmed in ferrite grains.

When the area fraction of the whole structure is defined as 100%, the proportion of the MD structure (ferrite+martensite+retained austenite) is required to be 90% or more. When the proportion of the MD structure is less than 90%, the influence of the remainder increases, uniformity of the structure is impaired and stretch flangeability deteriorates. The remainder includes ferrite which is not accompanied with the secondary phase in grains, pearlite and bainite. On the other hand, the proportion that the MD structure occupies in the whole structure is better as large, and the upper limit thereof is not particularly defined.

When the area fraction of the whole structure is defined as 100%, the proportion of martensite and retained austenite as a secondary phase present in the whole structure (martensite and retained austenite present in MD structure and other structures) is required to be from 10 to 60%, and the desired strength can be obtained by this proportion. When the proportion is less than 10%, sufficient strength is not obtained, and when it exceeds 60%, elongation deteriorates. The proportion is preferably in a range of from 15 to 55%, more preferably in a range of from 20 to 50%, and most preferably in a range of from 20 to 35%.

The proportion of the secondary phase (martensite and retained austenite in MD structure) in ferrite matrix is that when the whole of the secondary phase in MD structure is defined as 100%, the proportion of the secondary phase in ferrite grains is required to be 50% or more (the proportion of martensite and retained austenite on grain boundary is less than 50%), and the desired stretch flangeability can be obtained by this proportion. When the proportion is less than 50%, martensite and retained austenite present on the grain boundary act as a starting point of fracture, and stretch flangeability deteriorates. The proportion is better as large, and is preferably 60% or more, and more preferably 70% or more.

In the present invention, the term “grain boundary” means large tilt grain boundary, and for example, when orientation difference of adjacent grains is measured by FE/SEM-EBSP (Electron Back Scatter Diffraction Pattern), large tilt grain boundary in which the orientation difference is 15° or more is defined as grain boundary.

When the area fraction of the whole structure is defined as 100%, the proportion that retained austenite present in the whole structure (retained austenite present in MD structure and other structures) occupies in the whole structure is desirably 2% or more. When retained austenite is present as the secondary phase, the desired ductility (mainly uniform elongation) can be obtained. When the proportion is less than 2%, the expected ductility improvement effect is diluted. The proportion is preferably 5% or more, and more preferably 8% or more. The proportion of the retained austenite is better as large, but from a practical standpoint, the upper limit is about 20%.

In the structure of the present invention, an average grain size of the secondary phase present in the whole structure (the secondary phase present in MD structure and other structures) is required to be 3 μm or less. When the average grain size of the secondary phase is larger than 3 μm, starting points of fracture are localized, and as a result, sufficient stretch flangeability is not obtained. The average grain size of the secondary phase is better as small, and is preferably 2.5 μm or less, and more preferably 2 μm or less.

An average grain size of ferrite in the whole structure (ferrite present in MD structure and other structures) is preferably 20 μm or less, and the desired stretch flangeability can be obtained by this grain size. When the grain size exceeds 20 μm, local deformability deteriorates, and stretch flangeability deteriorates, which is not preferable. The average grain size is better as small, and is preferably 15 μm or less, and more preferably 10 μm or less.

On the other hand, the secondary phase in the whole structure (martensite, or martensite and retained austenite, present in MD structure and other structures) is required to be finely dispersed, and it is preferred that 20 or more secondary phases on the average are observed in an observation view of 50 μm×50 μm. When the number of the secondary phase is less than 20, the desired strength may not be ensured, or there is the possibility that stretch flangeability deteriorates, which is not preferable. The number of the secondary phase is preferably 30 or more, and more preferably 40 or more. The number is preferable as large, such as 50 or more, 80 or more, and 100 or more.

The MD structure steel sheet having the structure morphology of the present invention has a composition comprising, in terms of % by mass, C: 0.02 to 0.3%, Si: 0.01% to 3%, Mn: 0.5 to 3%, B: 0.0001 to 0.005%, and Al: 0.01 to 1.5%, with the remainder being Fe and inevitable impurities.

The composition may further contains one or two or more of:

(1) Mo: 0.03 to 1%;

(2) at least one element selected from the group consisting of Nb, Ti and V in the total amount of from 0.01 to 0.1%;

(3) at least one of Ni: 0.5% or less (not including 0%) and Cu: 0.5% or less (not including 0%);

(4) Cr: 1.5% or less (not including 0%); and

(5) at least one of Ca: 0.003% or less (not including 0%) and REM (rare earth element): 0.003% or less (not including 0%).

The above component composition is described below. In the followings, all the percentages are defined by mass, unless otherwise indicated. Additionally, all the percentages defined by mass are the same as those defined by weight, respectively.

C: 0.02 to 0.3%

C is an effective component to obtain strength of steel. The lower limit 0.02% is the minimum necessary amount to obtain a predetermined space factor of a secondary phase, thereby obtaining the desired strength. The upper limit 0.3% is that where C is added more than this amount, strength of martensite increases, and as a result, stretch flangeability deteriorates. The content of C is in a range of preferably from 0.03 to 0.25%, and more preferably from 0.04 to 0.20%.

Si: 0.01 to 3%

Si is an element to decrease the amount of a solid solution C in ferrite, to contribute to improvement of ductility such as elongation, and to suppress precipitation of cementite during bainite transformation. Si is added in an amount of 0.01% or more. When the amount of Si added is less than 0.01%, cementite precipitates during cooling, and martensite is not formed among laths. Additionally, it is difficult to ensure the desired amount of retained austenite. The content of Si is preferably 0.3% or more, and more preferably 0.5% or more. On the other hand, when Si is added in an amount exceeding 3%, cracks may be generated, resulting in deterioration of elongation and stretch flangeability. The content of Si is preferably 2.5% or less, and more preferably 2% or less.

Mn: 0.5 to 3%

Mn enables steel to have high strength by solid-solution hardening, and at the same time, improves hardenability of steel. Mn further has a function to accelerate formation of martensite. Such a function is recognized in the steel having Mn content of 0.5% or more. Mn content is preferably 0.7% or more, and more preferably 1% or more. On the other hand, when Mn is added in an amount exceeding 3%, stretch flangeability deteriorates. Mn content is preferably 2.5% or less, and more preferably 2% or less.

B: 0.0001 to 0.005%

B is important for the present invention. When B is added, formation of pro-eutectoid ferrite can be suppressed, thereby preventing precipitation of ferrite which is not accompanied with fine secondary phase in grains, such ferrite adversely affecting stretch flangeability. This effect is exhibited when B is added in an amount of 0.0001% or more. The amount of B added is preferably 0.0002% or more, and more preferably 0.0003% or more. On the other hand, when B is added in an amount exceeding 0.005%, the degree of segregation to grains increases, resulting in deterioration of stretch flangeability. For this reason, 0.005% is defined as the upper limit of B content. The amount of B is preferably 0.004% or less, and more preferably 0.003% or less.

Al: 0.01 to 1.5%

Al is used for deoxidation of steel. When the amount of Al is less than 0.01%, silicate inclusions retain, thereby resulting in deterioration of processability of steel. Therefore, the amount of Al added must be 0.01% or more. Furthermore, similar to Si, Al has the effect to prevent precipitation of cementite, and to contribute to ensuring retained austenite. The amount of Al added is preferably 0.03% or more, and more preferably 0.05% or more. However, the amount of Al exceeding 1.5% invites increase of surface mark, and retained austenite and martensite are liable to grow, inviting deterioration of elongation and stretch flangeability. For this reason, the upper limit of Al content is set at 1.5%. The amount of Al is preferably 1% or less, and more preferably 0.5% or less.

The steel sheet material of the present invention may further contain the following components in addition to the above-described essential components.

Mo: 0.03 to 1%

Mo is an element effective to accelerate bainite transformation and to accelerate disappearance of lath of bainitic ferrite during heat treatment. Mo further has the effect to suppress ferrite transformation and to improve hardenability. To obtain such effects, Mo is necessary in an amount of 0.03% or more. The amount of Mo is preferably 0.07% or more, and more preferably 0.1% or more. On the other hand, addition of Mo in an amount exceeding 1% results in increase of strength and deterioration of stretch flangeability. Therefore, the upper limit of Mo content is set at 1%. The amount of Mo is preferably 0.8% or less, and more preferably 0.6% or less.

Total of at least one element selected from the group consisting of Nb, Ti and V: 0.01 to 0.1%

Nb, Ti and V have the function to form a carbonitride and to increase strength of steel by precipitation strengthening and the function to refine grains, and can be added according to need. Such functions are not exhibited when the total amount of at least one of Nb, Ti and V added is less than 0.01%. On the other hand, when at least one of those is added in the total amount exceeding 0.1%, precipitate increases, resulting in remarkable deterioration of stretch flangeability. For this reason, the upper limit is set at 0.1%. The total amount is preferably 0.05% or less, and more preferably 0.03% or less.

At least one of Ni: 0.5% or less (not including 0%) and Cu: 0.5% or less (not including 0%)

Ni and Cu are elements effective to realize high strength while maintaining high strength-ductility balance, and are appropriately added. However, even though those are added in excess, the effect is saturated, and additionally, productivity deteriorates, such as generation of cracks during hot rolling. Therefore, the amounts of Ni and Cu added are suppressed to 0.5% or less, respectively. The amounts of those added are preferably 0.3% or less, and more preferably 0.2% or less, respectively. To effectively exhibit the effect, it is recommended to add Ni in an amount of 0.1% or more and/or Cu in an amount of 0.1% or more.

Cr: 1.5% or less (not including 0%)

Cr is an element effective to enhance strength of steel by improving hardenability, and is appropriately added. However, even though it added in excess, the effect is saturated, and additionally, ductility deteriorates. Therefore, the amount of Cr added is preferably suppressed to 1.5% or less. The amount of Cr added is preferably 1.0% or less, and more preferably 0.8% or less. To effectively exhibit the effect, it is recommended to add Cr in an amount of 0.1% or more.

At least one of Ca: 0.003% or less (not including 0%) and REM: 0.003% or less (not including 0%)

Ca and REM (rare earth element) are elements effective to control morphology of a sulfide in steel and to improve stretch flangeability, and are appropriately added. However, even though those are added in excess, the effect is saturated, and the excessive addition is not economical. Therefore, the amounts of Ca and REM added are 0.003% or less, respectively. To effectively exhibit the effect, it is recommended to add those in an amount of 0.0003% or more, respectively. Examples of REM include Sc, Y and lanthanoid.

Inevitable Impurities

The composition of the steel sheet according to the present invention is that the remainder other than the above-described components comprises Fe and inevitable impurities. Of the inevitable impurities, P and S are admitted to contain in an amount of P: 0.07% or less (not including 0%) and S: 0.07% or less (including 0%). When processability of a steel sheet is considered, the amount of P and S added is better as small. In particular, where the content of S is high, an inclusion (MnS) increases, which remarkably adversely affects stretch flangeability of a steel sheet. However, where S is added in the above range, the addition does not give influence to properties of a steel sheet.

Next, in the steel sheet according to the present invention, production conditions for obtaining the above-described structure are described below.

The steel sheet of the present invention can be produced by conducting a hot rolling step and a cold rolling step to produce a steel sheet material, and applying a heat treatment step to the steel sheet material. Specific processes of the hot rolling step and the cold rolling step are as described before. However, the production conditions are not limited to those steps, and may appropriately be according to the necessary processes.

The purpose of the heat treatment step is to finely and uniformly disperse martensite in a ferrite phase. When the heating temperature is lower than A3 point, the whole structure does not convert into austenite during heating and holding, and ferrite which is not accompanied with martensite in the state before the heat treatment remains partially. This ferrite has low strength, and forms an interface having large strength difference to MD. As a result, stretch flangeability deteriorates. When the heating temperature is A3 point or higher, such ferrite all disappears. Therefore, the upper limit of the heating temperature is not particularly defined, but from the relationship with practical operating level, it is recommended to appropriately control the heating temperature to a proper value.

To obtain a predetermined structure of the present invention by converting a major part of the structure into MD structure, in the above-described steel composition, it is necessary to set a cooling rate to a range of from 0.2 to 20° C./sec, and cool to Ms point or lower. In general, it is cooled to room temperature. When the cooling rate is less than 0.2° C./sec, ferrite which is not accompanied with martensite as a secondary phase is formed, and the proportion of ferrite and martensite in the MD structure becomes less than 90%, resulting in deterioration of strength and stretch flangeability. Furthermore, the proportion of martensite in ferrite grains decreases, and growth of an average grain size of martensite is generated. The cooling rate is preferably 0.5° C./sec or more, and more preferably 1° C./sec or more. On the other hand, when the cooling rate exceeds 20° C./sec, time and temperature sufficient to disappear lath of bainitic ferrite cannot be ensured, and as a result, elongation and stretch flangeability deteriorate. Where the cooling rate further increases, the whole structure is converted into martensite, and as a result, elongation remarkably deteriorates. For this reason, the cooling rate is required to be 20° C./sec or less. The cooling rate is preferably 15° C./sec or less, and more preferably 10° C./sec or less.

Next, in the steel sheet according to the present invention, other production conditions for obtaining the structure are described below.

The high strength steel sheet of the present invention, that is, MD steel sheet, can be obtained by heating a steel sheet material satisfying the component system as defined above, cooling the material from a temperature of A3 point or higher (for example, 1,200° C.), subjecting the material to a processing of rolling reduction of 5% or more in a temperature range of from 600 to 1,000° C., and then cooling the material to Ms point at a cooling rate of from 0.2 to 20° C./sec.

Heating and holding the steel sheet material at a temperature of A3 point or higher, and then subjecting the material to a processing at rolling reduction of 5% or more in a temperature range of from 600 to 1,000° C.:

The steel sheet of the present invention has MD structure comprising ferrite as a matrix and a secondary phase finely dispersed therein, on nearly the entire surface thereof. When the heating temperature is lower than A3 point, the structure during heating does not converts into austenite on the entire surface, and ferrite which is not accompanied with fine secondary phase is formed. The ferrite which is not accompanied with the fine secondary phase in grains contributes to the increase of elongation, but strength difference at the interface to the MD structure increases, and as a result, stretch flangeability deteriorates. The heating temperature is sufficient to be A3 point or higher, but from the relationship with a practical operating level, it is recommended to appropriately control the heating temperature to a proper value.

The subsequent addition of the processing of rolling reduction of 5% or more in a temperature range of from 600 to 1,000° C. is, for example, to accelerate bainite transformation to thereby facilitate obtaining fine MD structure, or to decrease an average grain size of ferrite. When the processing temperature is lower than 600° C., ferrite is precipitated, and when it is higher than 1,000° C., an average grain size of ferrite increases. Furthermore, where processing ratio is lower than 5%, bainite transformation is not accelerated. The upper limit of the processing ratio is practically about 90%, but the processing ratio higher than that has no problem. The processing temperature is preferably in a range of from 620 to 980° C., and more preferably in a range of 650 to 950° C. The rolling reduction is preferably in a range of from 8 to 85%, more preferably from 10 to 80%, and further preferably from 10 to 60%.

Cooling the steel sheet material to room temperature at a cooling rate of from 0.2 to 20° C./sec:

To obtain a predetermined structure of the present invention by converting a major part of the structure into MD structure, in the above-described steel composition, it is necessary to set a cooling rate to a range of from 0.2 to 20° C./sec. In general, it is cooled to room temperature. When the cooling rate is less than 0.2° C./sec, ferrite which is not accompanied with a secondary phase in grains is formed, and the proportion of the MD structure becomes less than 90%, resulting in deterioration of strength and stretch flangeability. The cooling rate is preferably 0.5° C./sec or more, and more preferably 1° C./sec or more. On the other hand, where the cooling rate exceeds 20° C./sec, the whole structure converts into martensite, and as a result, elongation remarkably deteriorates. For this reason, the cooling rate is required to be 20° C./sec or less. The cooling rate is preferably 15° C./sec or less, and more preferably 10° C./sec or less.

EXAMPLE

Examples in the case that the secondary phase is martensite are described below.

Steels 1A to 5F having the component compositions shown in Tables 1 to 5 were melt to form slabs. The slabs were heated to 1,150° C., hot rolled to a sheet thickness of 3.0 mm at 800° C. and then taken up at 550° C. Surface scale was removed by acid pickling, followed by cold rolling to a sheet thickness of 1.2 mm. Each of the steel sheet materials thus obtained was heated and held at each temperature shown in Tables 6 to 9, and then subjected to heat treatment which cools to each stop temperature at a cooling rate shown in Tables 6 to 9.

TABLE 1
Chemical component of specimen
Chemical component (mass %)
Steel codeCSiMnPSAlBMoOtherAc3 (° C.)Ms point (° C.)Remarks
1A0.0080.691.890.0080.0050.230.00090.87950476Comparison
1B0.031.711.220.010.0050.170.00080.56969494Invention
1C0.031.552.350.0080.0040.030.00050944448Invention
1D0.081.281.310.0090.0060.130.00120.06912474Invention
1E0.070.620.670.010.0050.260.00340884506Invention
1F0.0110.090.840.010.0050.610.00110.13897531Invention
1G0.091.071.170.0070.0040.950.00190897500Invention
1H0.132.431.670.0080.0050.050.00160.45960437Invention
1I0.121.612.270.010.0050.080.00070912421Invention
1J0.180.581.340.0080.0050.130.00090.23857436Invention
1K0.192.170.660.010.0060.350.00150919466Invention
1M0.090.0081.780.0090.0051.090.00180.56867478Comparison
1N0.050.022.210.0110.0050.230.00230.87893448Invention
1O0.040.031.250.010.0070.810.00360.13875510Invention
1P0.080.0461.810.010.0050.330.00120.08857460Invention
1Q0.130.0521.190.0090.0060.760.00180839479Invention
1R0.061.321.420.0110.0050.090.00080.32929474Invention
1S0.081.341.460.010.0050.120.00090912468Invention
1T0.071.231.510.0090.0060.220.00150.28920471Invention
1U0.181.812.340.010.0050.070.00280905396Invention
1V0.072.351.020.0110.0050.340.00190.34972493Invention
1W0.122.331.910.0070.0040.670.00070944452Invention
1X0.042.681.540.010.0050.580.00350.32999490Invention
1Y0.172.761.830.0090.0050.320.00180950427Invention
1Z0.083.340.760.0110.0060.870.00090.04995517Comparison

TABLE 2
Chemical component of specimen
Chemical component (mass %)
Steel codeCSiMnPSAlBMoOtherAc3 (° C.)Ms point (° C.)Remarks
2A0.161.180.380.010.0051.070.00060.23889508Comparison
2B0.140.050.560.0080.0040.090.00140.65857477Invention
2C0.070.740.520.010.0050.320.00270889514Invention
2D0.031.421.640.0080.0050.080.00110.28947476Invention
2E0.061.30.790.0090.0060.610.00130918516Invention
2F0.042.081.270.010.0030.880.00160.18968512Invention
2G0.050.961.290.010.0040.120.00350908485Invention
2H0.041.741.850.010.0050.920.00170.24955490Invention
2I0.071.421.830.0110.0050.810.00140920478Invention
2J0.132.22.250.0090.0060.070.00330.46950415Invention
2K0.181.082.30.010.0050.540.00250872412Invention
2L0.10.962.810.0070.0071.230.00260.34899440Invention
2M0.140.742.820.010.0060.910.00440867417Invention
2N0.080.523.250.0110.0050.470.00230.65896405Comparison
2O0.031.441.360.0120.0060.0060.00270.56957484Comparison
2P0.081.371.750.0080.0050.020.00150.47929451Invention
2Q0.121.362.410.0080.0050.020.00120900413Invention
2R0.152.331.520.010.0050.980.00030.23943465Invention
2S0.040.311.980.010.0050.120.0020883462Invention
2T0.051.291.10.0080.0050.810.00110.78947509Invention
2U0.031.361.380.0090.0050.790.00150936509Invention
2V0.081.331.790.010.0031.280.00080.05914489Invention
2W0.062.310.780.010.0041.350.00040964538Invention
2X0.081.891.340.010.0051.670.00170.16942518Comparison
2Y0.090.861.590.0110.0050.080.00160.03888458Invention
2Z0.121.871.730.010.0050.450.00240.11927452Invention

TABLE 3
Chemical component of specimen
Chemical component (mass %)
Steel codeCSiMnPSAlBMoOtherAc3 (° C.)Ms point (° C.)Remarks
3A0.072.122.640.0080.0070.090.00140.34962423Invention
3B0.171.191.040.0080.0050.730.00180.81905466Invention
3C0.051.071.080.0070.0060.770.00141.19950507Comparison
3D0.040.850.660.0090.0040.3300.3917518Comparison
3E0.092.521.750.010.0050.490.00010.12966463Invention
3F0.120.122.360.010.0040.180.00010845420Invention
3G0.150.271.090.010.0050.970.00050.81869478Invention
3H0.111.522.040.0080.0050.380.00050911442Invention
3I0.032.490.550.0120.0051.230.00110.34997553Invention
3J0.081.041.670.0080.0051.030.0010899487Invention
3K0.051.821.780.0110.0060.490.00190.23953476Invention
3L0.090.262.680.010.0040.050.00220861414Invention
3M0.111.281.730.0090.0050.080.00470.28909444Invention
3N0.041.821.880.010.0050.580.00480951480Invention
3O0.191.450.890.0110.0050.690.00770.51902465Comparison
3P0.051.061.60.0070.0031.100.00120.77Ti: 0.02936499Invention
3Q0.121.620.960.010.0050.580.00090.36Ti: 0.02923485Invention
3R0.082.411.260.010.0050.220.00110.52Ti: 0.08977476Invention
3S0.041.092.170.010.0040.620.00040Ti: 0.08918470Invention
3T0.090.460.970.0080.0060.080.00120.89Ti: 0.12898478Comparison
3U0.121.661.780.0090.0050.070.00170.34Nb: 0.02925438Invention
3V0.071.241.830.0080.0070.320.00050Nb: 0.02912463Invention
3W0.170.732.140.0110.0061.040.00140.65Nb: 0.08879433Invention
3X0.050.781.480.010.0050.620.00190Nb: 0.08899493Invention
3Y0.031.831.540.120.0050.780.00210.49Nb: 0.12972500Comparison
3Z0.082.221.920.010.0040.930.00160.22V: 0.02961472Invention

TABLE 4
Chemical component of specimen
Chemical component (mass %)
Steel codeCSiMnPSAlBMoOtherAc3 (° C.)Ms point (° C.)Remarks
4A0.151.691.770.0090.0050.220.00340V: 0.02909433Invention
4B0.112.30.920.010.0030.790.00080.08V: 0.08956495Invention
4C0.080.390.590.0070.0061.170.00170V: 0.08878530Invention
4D0.071.711.180.010.0050.980.00140.04V: 0.12946504Comparison
4E0.051.960.670.010.0050.820.00180.35Ti: 0.02, Nb: 0.02963529Invention
4F0.092.072.450.010.0040.050.00230Ti: 0.02, Nb: 0.02942423Invention
4G0.061.370.720.0080.0070.090.00310.65Ti: 0.03, V: 0.03945499Invention
4H0.041.220.920.0080.0060.840.00040Ti: 0.03, V: 0.03927524Invention
4I0.111.082.150.0080.0050.480.00260.31Nb: 0.04, V: 0.04905438Invention
4J0.130.112.40.0110.0050.080.00110Nb: 0.04, V: 0.04846410Invention
4K0.081.470.860.0080.0050.330.00130.15Ti: 0.02, Nb: 0.03927495Invention
V: 0.04
4L0.051.571.390.0090.0050.380.00310Ti: 0.02, Nb: 0.03939488Invention
V: 0.04
4M0.031.241.540.0120.0050.810.00140.72Ti: 0.05, Nb: 0.05958498Comparison
V: 0.05
4N0.082.381.360.0110.0041.400.00190.37Ni: 0.3966503Invention
4O0.111.291.080.010.0050.380.00080Ni: 0.3896474Invention
4P0.10.960.780.010.0050.090.00340.79Ni: 0.6904472Comparison
4Q0.092.240.940.0110.0030.030.00450.16Cu: 0.3954478Invention
4R0.131.541.290.0080.0070.640.00290.47Cu: 0.3920467Invention
4S0.180.732.060.0070.0050.110.00130.31Cu: 0.6866400Comparison
4T0.032.081.630.0110.0050.430.00060.56Ni: 0.2, Cu: 0.2982484Invention
4U0.11.390.870.010.0050.630.00030Ni: 0.2, Cu: 0.2905497Invention

TABLE 5
Chemical component of specimen
SteelChemical component (mass %)Ac3Ms point
codeCSiMnPSAlBMoOther(° C.)(° C.)Remarks
4V0.080.541.450.010.0041.240.00060.68Cr: 0.2898494Invention
4W0.051.871.90.0080.0050.450.00180Cr: 0.2948467Invention
4X0.060.941.440.0110.0060.980.00310.22Cr: 0.5909490Invention
4Y0.091.731.510.0080.0051.190.00230Cr: 0.5926484Invention
4Z0.081.231.630.010.0050.970.00170.75Cr: 0.8931467Invention
5A0.180.570.90.010.0050.660.00220Cr: 0.8849454Invention
5B0.161.111.270.0120.0051.290.00340.36Cr: 1.6890448Comparison
5C0.072.052.880.010.0030.180.0020.09Ca: 0.002951417Invention
5D0.081.061.50.0090.0050.110.00050Ca: 0.002900466Invention
5E0.041.431.270.0080.0040.300.0040.26REM: 0.002942496Invention
5F0.130.921.660.010.0060.520.00130.05REM: 0.002880454Invention

TABLE 6
Heat treatment condition
Heat treatment condition
Cooling
HeatingCoolingstop
MaterialSteeltemperatureratetemperature
No.code(° C.)(° C./sec)(° C.)Remarks
11A1000210Comparison
21B1000210Invention
31C1000210Invention
41D800210Comparison
51D850210Comparison
61D900210Comparison
71D1000210Invention
81D1100210Invention
91D1200210Invention
101E1000210Invention
111F1000210Invention
121G1000210Invention
131H1000210Invention
141I1000210Invention
151J1000210Invention
161K1000210Invention
181M1000210Comparison
191N1000210Invention
201O1000210Invention
211P1000210Invention
221Q1000210Invention
231R10000.0510Comparison
241R10000.110Comparison
251R10000.310Invention
261R10000.510Invention
271R1000110Invention
281R1000510Invention
291R10001010Invention
301R10002010Invention
311R10003010Comparison
321R10005010Comparison
331R100010010Comparison
341R100050010Comparison
351S10000.0510Comparison

TABLE 7
Heat treatment condition
Heat treatment condition
Cooling
HeatingCoolingstop
MaterialSteeltemperatureratetemperature
No.code(° C.)(° C./sec)(° C.)Remarks
361S10000.110Comparison
371S10000.310Invention
381S10000.510Invention
391S1000110Invention
401S1000510Invention
411S10001010Invention
421S10002010Invention
431S10003010Comparison
441S10005010Comparison
451S100010010Comparison
461S100050010Comparison
471T1000210Invention
481U1000210Invention
491V1000210Invention
501W1000210Invention
511X1000210Invention
521Y1000210Invention
531Z1000210Comparison
542A1000210Comparison
552B1000210Invention
562C1000210Invention
572D1000220Invention
582D10002100Invention
592D10002300Invention
602D10002500Comparison
612E1000210Invention
622F1000210Invention
632G1000210Invention
642H1000210Invention
652I1000210Invention
662J1000210Invention
672K1000210Invention
682L1000210Invention
692M1000210Invention
702N1000210Comparison

TABLE 8
Heat treatment condition
Heat treatment condition
Cooling
HeatingCoolingstop
MaterialSteeltemperatureratetemperature
No.code(° C.)(° C./sec)(° C.)Remarks
712O1000210Comparison
722P1000210Invention
732Q1000210Invention
742R1000210Invention
752S1000210Invention
762T1000210Invention
772U1000210Invention
782V1000210Invention
792W1000210Invention
802X1000210Comparison
812Y1000210Invention
822Z1000210Invention
833A1000210Invention
843B1000210Invention
853C1000210Comparison
863D1000210Comparison
873E1000210Invention
883F1000210Invention
893G1000210Invention
903H1000210Invention
913I1000210Invention
923J1000210Invention
933K1000210Invention
943L1000210Invention
953M1000210Invention
963N1000210Invention
973O1000210Comparison
983P1000210Invention
993Q1000210Invention
1003R1000210Invention
1013S1000210Invention
1023T1000210Comparison
1033U1000210Invention
1043V1000210Invention
1053W1000210Invention

TABLE 9
Heat treatment condition
Heat treatment condition
Cooling
HeatingCoolingstop
MaterialSteeltemperatureratetemperature
No.code(° C.)(° C./sec)(° C.)Remarks
1063X1000210Invention
1073Y1000210Comparison
1083Z1000210Invention
1094A1000210Invention
1104B1000210Invention
1114C1000210Invention
1124D1000210Comparison
1134E1000210Invention
1144F1000210Invention
1154G1000210Invention
1164H1000210Invention
1174I1000210Invention
1184J1000210Invention
1194K1000210Invention
1204L1000210Invention
1214M1000210Comparison
1224N1000210Invention
1234O1000210Invention
1244P1000210Comparison
1254Q1000210Invention
1264R1000210Invention
1274S1000210Comparison
1284T1000210Invention
1294U1000210Invention
1304V1000210Invention
1314W1000210Invention
1324X1000210Invention
1334Y1000210Invention
1344Z1000210Invention
1355A1000210Invention
1365B1000210Comparison
1375C1000210Invention
1385D1000210Invention
1395E1000210Invention
1405F1000210Invention

Microstructure and dynamic properties were examined on each of steel sheets obtained above by the following procedures.

Microstructure of each steel was determined by the following method.

Each of steel sheet Nos. 1 to 140 after heat treatment was cut into a test piece for microstructure observation of 10 mm×10 mm×1.2 mm. The test piece was embedded with a cold rolled resin such that the observation position is a position of ¼ of a sheet thickness in a rolled direction. Impression as a landmark was formed with a Vickers tester so as to conduct identification on the structure observation place. The impression was corroded with repeller, and the structure was observed at five positions using an optical microscope in a magnification of 1,000. When the structure photograph after repeller corrosion was image analyzed, ferrite is observed gray, and martensite and retained austenite were observed white. After microstructure observation with an optical microscope, buff polishing and electrolysis polishing were conducted to an extent such that the Vickers impression does not disappear, and structure observation was conducted on the place at a step distance of 100 nm using FE/SEM-EBSP. A boundary having an orientation difference of 15° or more of grains was considered as grain boundary, and the grain boundary was identified.

Optical microphotograph and FE/SEM-EBSP structure photograph were combined as the standard of Vickers impression, and the structure was evaluation.

MD Structure:

The MD structure contains fine martensite (secondary phase) in ferrite grains as a matrix, as described above. The observation results of optical microscope and FE/SEM-EBSP were combined, a structure in which many martensite are present in the inside of grains surrounded by large tilt grain boundary (i.e., in the grains) was identified as MD structure, and its space factor was obtained.

There is the possibility that pearlite, bainite, retained austenite, ferrite which is not accompanied with martensite in grains, and the like are contained as other structures. Identification method of those structures is as follow.

Retained Austenite:

Retained austenite has an FCC structure, and therefore can be identified with FE/SEM-EBSP. Area fraction of austenite in the observation view was determined.

Pearlite:

Pearlite has a lamella structure of ferrite and cementite, and the cementite can be identified with FE/SEM-EBSP. In the Examples, the area fraction of pearlite in the observation view was determined.

Bainite:

Bainite is that lath is present in ferrite grains identified with FE/SEM-EBSP. Structure in which lath is present in ferrite grains was judged as bainite, and area fraction of bainite in the observation view was determined.

Ferrite which is not Accompanied with Martensite in Grains:

Structure in which martensite cannot be confirmed in ferrite grains identified with FE/SEM-EBSP is considered as ferrite which is not accompanied with martensite in grains, and is judged as a structure different from MD structure. Area fraction of ferrite which is not accompanied with martensite in grains in the observation view was determined.

Structure in which MD structure is not observed in the observation view is judged that MD is not present. When the whole structure is defined as 100%, in the case that the total of area fractions of other structures (retained austenite, pearlite, bainite, ferrite which is not accompanied with martensite in grains, and the like) exceeds 10%, the defined structure is not formed. In this case, it was judged that MD is partial.

On the other hand, in the case that the space factor of MD structure occupies 90% or more of the whole structure, the following further detailed investigation was conducted.

Space Factor and Average Grain Size of Martensite:

Space factor of martensite in the whole structure used area fraction of white portion in the whole structure by image analyzing an optical microphotograph. By combining with an image of FE/SEM-EBSP, martensite present in ferrite grains and martensite present at grain boundary were separated. Retained austenite is also observed as a white portion, but austenite can be distinguished from retained austenite by identifying retained austenite which has an FCC structure, with FE/SEM-EBSP. Retained austenite was removed by image analysis of FE/SEM-EBSP, and the space factor of martensite was determined.

On the other hand, an average grain size (particle diameter) of martensite was determined by the following method. Five places of the observation view having a size of 20 μm×20 μm were randomly extracted from a SEM observation photograph (magnification 3,000 times), an average grain size (diameter of a corresponding circle) of martensite and retained austenite in the respective observation views was determined, and its average value was used as a grain size of martensite in the whole structure.

Dynamic properties of each steel were determined by the following methods.

Tensile Properties:

JIS No. 5 test piece obtained from a vertical direction of a steel sheet rolled direction was used, and tensile strength (TS) and Elongation (EL) were measured according to JIS Z 2241. Test pieces having TS of 590 MPa or more and EL of 10% or more were considered as acceptable products. Elongation from strain corresponding to tensile strength to breaking strain was designated as local elongation.

Stretch Flangeability:

Hole expanding ratio λ was measured as stretch flangeability. The hole expanding ratio λ was measured according to JISF (The Japan Iron and Steel Federation) standard (JFST 1001-1996). Steels having λ of 80% or more were considered as acceptable products.

The results obtained are shown in Tables 10 to 15 below.

TABLE 10
Measurement results
Structure parameter
Af of α′Grain size
Af of α′in α grainof α′ inDynamic properties
PresenceAf of MDin entireof entireentireTotalLocal
MaterialSteelor absencestructurestructurestructurestructureTSelongationelongationλ
No.codeof MD(%)(%)(%)(μm)(MPa)(%)(%)(%)Remarks
11A94.518.870.91.553232.925.1169.2Comparison
21B95.927.271.91.766827.619.8136.8Invention
31C95.045.571.31.793318.810.296.5Invention
41DPartial28.168718.36.539.5Comparison
51DPartial50.171517.56.745.2Comparison
61DPartial87.476616.46.343.3Comparison
71D95.442.577.61.685417.210.5105.8Invention
81D94.143.677.71.683218.411.4108.5Invention
91D94.141.378.81.784419.412.3110.1Invention
101E90.431.967.81.972924.018.0123.4Invention
111F91.129.768.31.569625.118.9129.3Invention
121G91.139.368.32.184921.914.1107.3Invention
131H96.558.672.42.3128914.76.280.3Invention
141I94.857.471.12.3118914.76.385.7Invention
151J93.645.970.22.694919.611.695.9Invention
161K92.357.969.22.7111915.69.780.4Invention
181MNone8439.24.467.4Comparison
191N94.836.771.11.880121.913.5112.4
201O92.423.269.31.759729.322.4150.7Invention
211P93.930.170.42.071126.017.2128.3Invention
221Q91.231.668.42.372424.217.4124.3Invention
231RNone64126.210.541.5Comparison
α: Ferrite,
α′: Martensite,
Af: Area fraction,
TS: Tensile strength,
λ: Hole expanding ratio

TABLE 11
Measurement results
Structure parameter
Af of α′Grain size
Af of α′in α grainof α′ inDynamic properties
PresenceAf of MDin entireof entireentireTotalLocal
MaterialSteelor absencestructurestructurestructurestructureTSelongationelongationλ
No.codeof MD(%)(%)(%)(μm)(MPa)(%)(%)(%)Remarks
241RPartial72.973218.17.152.3Comparison
251R96.138.078.41.586719.414.8102.5Invention
261R96.739.377.41.785620.514.9112.9Invention
271R96.440.978.61.487620.215.9108.7Invention
281R96.338.580.01.487319.215.7112.4Invention
291R96.038.278.41.588420.915.9115.3Invention
301R95.438.980.31.687120.816.2105.8Invention
311RNone10219.24.774.3Comparison
321RNone11018.44.172.1Comparison
331RNone12098.13.679.4Comparison
341RNone12347.53.876.1Comparison
351SNone61125.910.538.5Comparison
361SPartial62.872217.36.945.9Comparison
371S95.137.677.61.785620.415.6100.5Invention
381S95.738.976.71.984420.914.7109.2Invention
391S95.440.577.81.686019.716.1106.5Invention
401S95.338.179.21.686118.715.5110.9Invention
411S95.037.877.61.786321.315.7112.3Invention
421S94.538.579.51.885920.516.0101.6Invention
431SNone9899.54.572.1Comparison
441SNone1088823.970.9Comparison
451SNone11737.93.476.5Comparison
461SNone11987.23.575.5Comparison
α: Ferrite,
α′: Martensite,
Af: Area fraction,
TS: Tensile strength,
λ: Hole expanding ratio

TABLE 12
Measurement results
Structure parameter
Af of α′Grain size
Af of α′in α grainof α′ inDynamic properties
PresenceAf of MDin entireof entireentireTotalLocal
MaterialSteelor absencestructurestructurestructurestructureTSelongationelongationλ
No.codeof MD(%)(%)(%)(μm)(MPa)(%)(%)(%)Remarks
471T92.139.669.02.085421.713.5106.6Invention
481U95.064.871.22.6122114.35.887.7Invention
491V93.448.470.11.998618.911.492.2Invention
501W93.663.470.22.3120114.66.784.9Invention
511X97.062.372.71.7118414.87.596.0Invention
521Y93.368.170.02.6127113.86.090.8Invention
531Z90.166.367.52.012459.13.072.3Comparison
542APartial72.566126.520.954.7Comparison
552B94.525.770.82.463527.621.7141.8Invention
562C90.924.268.21.962329.522.7146.7Invention
572D94.145.175.61.786518.713.2100.4Invention
582D94.344.578.91.887818.613.3108.1Invention
592D94.548.177.51.885517.912.8107.3Invention
602DNone9549.84.879.8Comparison
612E91.228.468.41.967625.919.7133.2Invention
622F92.946.269.71.795319.611.795.4Invention
632G91.529.068.61.869526.618.6131.4Invention
642H95.451.971.51.7103818.09.287.5Invention
652I93.349.970.01.9100918.59.890.1Invention
662J98.553.373.92.3120014.66.285.0Invention
672K94.954.071.12.6121014.56.084.4Invention
682L95.455.671.62.1110017.16.882.6Invention
692M96.658.672.42.4112915.56.389.7Invention
α: Ferrite,
α′: Martensite,
Af: Area fraction,
TS: Tensile strength,
λ: Hole expanding ratio

TABLE 13
Measurement results
Structure parameter
Af of α′Grain size
Af of α′in α grainof α′ inDynamic properties
PresenceAf of MDin entireof entireentireTotalLocal
MaterialSteelor absencestructurestructurestructurestructureTSelongationelongationλ
No.codeof MD(%)(%)(%)(μm)(MPa)(%)(%)(%)Remarks
702N98.455.473.82.0124215.24.373.1Comparison
712O96.435.172.31.77879.53.565.8Comparison
722P96.947.572.72.097219.210.693.5Invention
732Q95.256.771.42.3117414.96.386.6Invention
742R94.258.570.62.4112815.58.289.8Invention
752S93.828.870.31.769126.717.7132.1Invention
762T97.337.673.01.882422.514.9110.6Invention
772U91.835.168.81.777622.515.5116.0Invention
782V93.652.770.22.0105017.89.186.6Invention
792W92.745.669.51.993318.712.696.4Invention
802X94.866.771.13.811007.94.581.8Comparison
812Y92.737.669.62.182422.514.1110.6Invention
822Z93.956.170.42.3118615.97.386.5Invention
833A96.856.472.61.9110617.07.082.2Invention
843B97.455.673.02.6108416.19.683.0Invention
853CPartial88.780324.818.272.1Comparison
863DPartial41.958929.723.752.7Comparison
873E94.057.370.52.1112016.88.181.1Invention
883F95.144.771.32.392019.010.597.8Invention
893G97.641.573.22.487320.113.4103.1Invention
903H94.057.870.52.2111715.77.680.6Invention
913I91.943.768.91.790519.313.599.4Invention
923J92.845.169.62.093719.911.497.1Invention
α: Ferrite,
α′: Martensite,
Af: Area fraction,
TS: Tensile strength,
λ: Hole expanding ratio

TABLE 14
Measurement results
Structure parameter
Af of α′Grain size
Af of α′in α grainof α′ inDynamic properties
PresenceAf of MDin entireof entireentireTotalLocal
MaterialSteelor absencestructurestructurestructurestructureTSelongationelongationλ
No.codeof MD(%)(%)(%)(μm)(MPa)(%)(%)(%)Remarks
933K95.050.871.31.8102218.39.689.0Invention
943L96.147.672.12.196318.29.193.4Invention
953M95.352.971.52.2105317.89.286.3Invention
963N93.552.370.11.7104417.99.187.0Invention
973O94.462.770.82.7119114.78.475.6Comparison
983P98.944.674.21.892920.011.697.9Invention
993Q93.447.170.02.395618.311.994.1Invention
1003R95.754.571.82.0107717.49.584.4Invention
1013S94.444.070.81.791019.211.098.9Invention
1023T97.827.373.42.165926.620.166.6Comparison
1033U95.958.272.02.3113216.67.980.2Invention
1043V93.343.670.01.991420.411.699.5Invention
1053W96.758.272.52.6127313.75.586.7Invention
1063X92.131.769.11.873525.116.9124.2Invention
1073Y96.447.372.31.796018.210.973.8Comparison
1083Z95.455.971.62.0124815.16.382.7Invention
1094A93.156.769.82.4120514.56.984.7Invention
1104B90.953.268.22.2104916.710.385.8Invention
1114C91.227.268.42.065826.620.7136.9Invention
1124D91.544.568.61.992820.112.368.1Comparison
1134E92.337.469.31.882022.615.6111.0Invention
1144F95.454.671.52.1116515.06.383.2Invention
1154G95.030.371.21.970424.918.8127.8Invention
1164H90.326.867.71.765326.820.4137.9Invention
α: Ferrite,
α′: Martensite,
Af: Area fraction,
TS: Tensile strength,
λ: Hole expanding ratio

TABLE 15
Measurement results
Structure parameter
Af of α′Grain size
Af of α′in α grainof α′ inDynamic properties
PresenceAf of MDin entireof entireentireTotalLocal
MaterialSteelor absencestructurestructurestructurestructureTSelongationelongationλ
No.codeof MD(%)(%)(%)(μm)(MPa)(%)(%)(%)Remarks
1174I96.958.172.72.2113216.67.480.2Invention
1184J95.247.071.42.395518.39.794.2Invention
1194K91.334.268.52.077323.916.7118.0Invention
1204L91.840.068.91.886021.613.5105.9Invention
1214M98.340.673.71.785920.413.164.8Comparison
1224N94.857.171.12.0120715.67.685.2Invention
1234O90.839.468.12.285121.814.2107.0Invention
1244P96.433.772.32.176524.217.059.2Comparison
1254Q91.748.068.72.197118.011.692.7Invention
1264R95.455.871.52.3109717.19.282.8Invention
1274S96.656.972.52.6116416.27.178.0Comparison
1284T97.349.073.01.799518.810.391.4Invention
1294U90.136.967.62.180321.815.5112.1Invention
1304V97.740.273.32.086321.513.3105.5Invention
1314W93.551.670.11.8103418.19.287.9Invention
1324X93.839.570.41.985221.813.6106.8Invention
1334Y92.255.469.22.1109117.28.983.3Invention
1344Z98.951.874.12.0103718.09.687.6Invention
1355A90.241.767.62.687620.013.6102.7Invention
1365B94.458.870.82.511337.54.879.5Comparison
1375C97.558.973.21.9113315.46.188.4Invention
1385D92.236.269.12.080323.114.8113.5Invention
1395E93.636.470.21.780523.015.1113.2Invention
1405F93.149.069.92.398517.810.391.4Invention
α: Ferrite,
α′: Martensite,
Af: Area fraction,
TS: Tensile strength,
λ: Hole expanding ratio

From the results shown in Tables 10 to 15, it can be considered as follows. The following alphabets all mean steel codes in Tables 1 to 15, and the following Nos. all mean material Nos. in Tables 6 to 15. Nos. 2 to 3, 7 to 16, 19 to 22, 25 to 30, 37 to 42, 47 to 52, 55 to 59, 61 to 69, 72 to 79, 81 to 84, 87 to 96, 98 to 101, 103 to 106, 108 to 111, 113 to 120, 122 to 123, 125 to 126, 128 to 135 and 137 to 140 all are examples that high strength steel sheets having the structure defined in the present invention were produced by the production method defined in the present invention using steel species satisfying the scope of the present invention (1B to 1K, 1N to 1Y, 2B to 2M, 2P to 2W, 2Y to 3B, 3E to 3N, 3P to 3S, 3U to 3X, 3Z to 4C, 4E to 4L, 4N to 4O, 4Q to 4R, 4T to 5A and 5C to 5F in Tables 1 to 5). The high strength steel sheets shown by the above Nos. all have excellent tensile strength, elongation and stretch flangeability.

Contrary to this, the following examples that are not satisfied with any one of the requirements specified in the present invention have the following disadvantages.

No. 1 is an example using steel code 1A having a small C content. Because the proportion of martensite in the whole structure is lower than the range defined, tensile strength was decreased.

Nos. 4 to 6 each use steel code 1D having a component composition which is satisfied with the provision of the present invention. However, the heating and holding temperature of the heat treatment is not A3 point or higher, and the area fraction of α+α′ is less than 90%. Therefore, stretch flangeability was decreased.

No. 18 is an example using steel code 1M having a small Si content. Because it has a bainite structure over the entire surface and MD is not present, elongation is low.

Nos. 23 to 24 each use steel code 1R having a component composition which is satisfied with the provision of the present invention, and Nos. 35 to 36 each use steel code 1S having a component composition which is satisfied with the provision of the present invention. However, the cooling rate of the heat treatment is slower than the range defined, and as a result, ferrite which is not accompanied with martensite in grains was formed. No. 23 and No. 35 each have a ferrite-pearlite structure, and MD is not present. No. 24 and No. 36 each are that ferrite which is not accompanied with martensite in grains is formed and the area fraction of α+α′ is less than 90%. Therefore, stretch flangeability is low.

Nos. 31 to 34 each use steel code 1R having a component composition which is satisfied with the provision of the present invention, and Nos. 43 to 46 each use steel code 1S having a component composition which is satisfied with the provision of the present invention. However, the cooling rate of the heat treatment is slower than the range defined. Therefore, Nos. 31 and 32 and Nos. 43 and 44 are that lath of bainite does not completely disappear, thereby a bainitic ferrite-martensite structure is formed and MD is not present. As a result, elongation and stretch flangeability were decreased. Furthermore, No. 33 and 34 and Nos. 45 and 46 each are that a martensite single phase structure is formed, and elongation and stretch flangeability were decreased. No. 53 is an example using steel code 1Z having a large Si content. Because Si was added in a large amount, elongation and stretch flangeability were decreased.

No. 54 is an example using steel code 2A having a small Mn content.

Because pearlite was formed and α+α′ was less than 90%, stretch flangeability is low. No. 60 uses steel code 2D which is satisfied with the provision of the present invention. However, because the cooling stop temperature exceeds Ms point, a bainite structure was formed on the entire surface, and elongation and stretch flangeability were decreased.

No. 70 is an example using steel code 2N having a large Mn content, and stretch flangeability was decreased.

No. 71 is an example using steel code 2O having a small Al content.

Compounds of Si, Mn and O were unavoidably formed, and elongation and stretch flangeability were decreased.

No. 80 is an example using steel code 2X having a large Al content. Course martensite was formed in a large amount, and the area fraction of α′ in the whole structure and an average gain size of α′ exceeded the defined ranges. As a result, elongation and stretch flangeability was decreased.

No. 85 is an example using steel code 3C having a large Mo content. Because course retained austenite was formed in a large amount and α+α′ was less than 90%, stretch flangeability was decreased.

No. 86 is an example using steel code 3D having a small B content. Because ferrite which is not accompanied with martensite in grains was precipitated and α+α′ was less than 90%, stretch flangeability was decreased.

No. 97 is an example using steel code 3O having a large B content. Stretch flangeability was decreased due to segregation of B.

No. 102 is an example using steel code 3T having a large Ti content. Because precipitate was increased, stretch flangeability was decreased. No. 107 is an example using steel code 3Y having a large Nb content. Because precipitate was increased, stretch flangeability was decreased. No. 112 is an example using steel code 4D having a large V content. Because precipitate was increased, stretch flangeability was decreased. No. 121 is an example using steel code 4M having a large total addition amount of Ti, Nb and V Because precipitate was increased, stretch flangeability was decreased.

No. 124 is an example using steel code 4P having a large Ni content. Because Ni was added in a large amount, it is liable to break and stretch flangeability was decreased.

No. 127 is an example using steel code 4S having a large Cu content. Because Cu was added in a large amount, it is liable to break and stretch flangeability was decreased.

No. 136 is an example using steel code 5B having a large Cr content. Because Cr was added in a large amount, it is liable to break and stretch flangeability was decreased.

Next, examples in the case that the secondary phase is martensite and retained austenite are described below.

Steels A to W having component compositions shown in Table 16 were melted to form slabs. The slabs were heated to 1,200° C., hot rolled to a sheet thickness of 3.0 mm at 800° C. and then taken up at 550° C. The thermo-mechanical treatment (heating temperature T1 (° C.), processing temperature T2 (° C.), rolling reduction P (%), cooling rate R (° C./sec)) shown in FIG. 1 and Table 17 was conducted to cool to room temperature, and thickness reduction processing was conducted so as to be a sheet thickness of 1.2 mm.

TABLE 16
Chemical component of specimen
SteelChemical component (mass %)Ac3
codeCSiMnPSAlMoBOther(° C.)Remarks
A0.051.41.30.010.0050.050.30.0008Cr: 0.65, Ti: 0.015923Invention
B0.031.31.40.010.0050.050.30.0007927Invention
C0.081.41.40.010.0050.030.30.0007902Invention
D0.251.51.50.010.0050.0500.0009858Invention
E0.351.72.50.010.0050.050.30.0005827Comparison
F0.120.0081.40.010.0050.050.30.0005834Comparison
G0.100.031.40.010.0051.10.30.00081262Invention
H0.050.280.70.010.0050.050.30.0003893Invention
I0.073.62.50.010.0050.050.30.0008979Comparison
J0.050.40.30.010.0050.0600.0005904Comparison
K0.091.53.60.010.0050.050.30.0008845Comparison
L0.050.032.20.010.0050.0050.30.0008818Comparison
M0.051.51.50.010.0050.0500.001914Invention
N0.051.41.40.010.0050.050.70.0006934Invention
O0.071.22.20.010.0050.051.30.0008912Comparison
P0.051.20.90.010.0050.050.10921Comparison
Q0.051.51.40.010.0050.0500.0035917Invention
R0.051.51.30.010.0050.050.30.007929Comparison
S0.101.41.30.010.0050.050.30.0008Ti: 0.015, Nb: 0.012912Invention
T0.051.41.30.010.0050.050.30.0008V: 0.0012926Invention
U0.051.41.30.010.0050.050.30.0008Ni: 0.1, Cu: 0.1921Invention
V0.071.21.20.010.0050.050.30.0008Ca: 0.002, REM: 0.002910Invention
W0.010.51.20.010.0050.050.30.0008913Comparison
X0.010.31.80.010.0050.050.30.0007886Comparison

TABLE 17
Heat treatment condition
Heat treatment condition
HeatingHeatingCooling
temperaturetemperatureRollingtemperature
MaterialSteelT1T2reduction PR
No.code(° C.)(° C.)(%)(° C.)Remarks
141A12009003010Invention
142B12009003010Invention
143C12508003010Invention
144D1200800205Invention
145E12009501018Comparison
146F1200750407Comparison
147G13007005010Invention
148H12508003010Invention
149I12008003015Comparison
150J1250700400.5Comparison
151K12009002015Comparison
152L12009003010Comparison
153M12508003010Invention
154N1200650300.5Invention
155O12009001015Comparison
156P1200700300.5Comparison
157Q1200750300.5Invention
158R12009001015Comparison
159S13007003010Invention
160T13007004010Invention
161U12008003010Invention
162V12007003010Invention
163W1200700300.5Comparison
164C1200550301Comparison
165C13001050100.5Comparison
166C1200900215Comparison
167C1200900100.1Comparison
168C12009001025Comparison
169X1250900500.5Comparison

Microstructure observation was conducted on each steel sheet obtained above in the following procedures, and dynamic properties were evaluated by a tensile test and a hole expanding test.

Microstructure of each steel sheet was identified by the following method.

Each steel sheet was cut into a test piece for microstructure observation of 10 mm×10 mm×1.2 mm. The test piece was embedded in a cold rolled resin, and a position of ¼ of a sheet thickness in a rolled direction was observed. In such a case, impression as a landmark was formed with a Vickers tester so as to conduct identification on the structure observation place. The impression was corroded with repeller, and the structure was observed at five positions using an optical microscope with a magnification of 1,000. When the structure photograph after repeller corrosion was image analyzed, ferrite was observed gray, and martensite and retained austenite were observed white. After microstructure observation with an optical microscope, buff polishing and electrolysis polishing were conducted to an extent such that the Vickers impression does not disappear, and structure observation was conducted on the place at a step distance of 100 nm using FE/SEM-EBSP. A boundary having an orientation difference of 15° or more of grains was considered as grain boundary, and thus the grain boundary was identified.

Optical microphotograph and FE/SEM-EBSP structure photograph were combined as the standard of Vickers impression, and the structure was evaluated.

MD Structure:

The MD structure contains fine martensite or retained austenite (secondary phase) in ferrite grains as a matrix, as described above. The observation results of optical microscope and FE/SEM-EBSP were combined, a structure in which many martensite or retained austenite are present in the inside of grains surrounded by large tilt grain boundary (i.e., in the grains) was identified as MD structure, and its space factor was obtained.

There is the possibility that pearlite, bainite, ferrite which is not accompanied with martensite or retained austenite in grains, and the like are contained as other structures. Identification method of those structures is as follow.

Pearlite:

Pearlite has a lamella structure of ferrite and cementite, and the cementite can be identified with FE/SEM-EBSP. In the Examples, the space factor of pearlite in the observation view was determined.

Bainite:

Bainite is that lath is present in ferrite grains identified with FE/SEM-EBSP. Structure in which lath is present in ferrite grains was judged as bainite, and space factor of bainite in the observation view was determined.

Ferrite which is not Accompanied with Martensite or Retained Austenite in Grains:

Structure in which martensite or retained austenite cannot be confirmed in ferrite grains identified with FE/SEM-EBSP is considered as ferrite which is not accompanied with martensite or retained austenite in grains, and is judged as a structure different from MD structure. Space factor of ferrite which is not accompanied with martensite or retained austenite in grains in the observation view was determined.

Structure in which MD structure is not observed in the observation view is judged that MD is not present. When the whole structure is defined as 100%, in the case that the total of space factors of other structures (pearlite, bainite, ferrite which is not accompanied with martensite or retained austenite in grains, and the like) exceeds 10%, the defined structure is not formed. In this case, it was judged that MD is partial.

Proportion that secondary phase in the entire phase occupies in the whole structure:

Average Grain Size of Ferrite in the Entire Phase:

Proportion of Secondary Phase Present in Ferrite Grains in MD Structure:

First, the space factor of the secondary phase (martensite and retained austenite) in the whole structure used a space factor of white portion in the whole structure by image analyzing an optical microphotograph. Next, an average grain size (diameter of a corresponding circle) of ferrite was obtained using an image of FE/SEM-EBSP, and by combining with the optical microphotograph, the secondary phase present in the MD structure was separated into the secondary phase present in ferrite grains and the secondary phase present at grain boundary, thereby the space factor of the secondary phase present in grains of the whole secondary phase present in the MD structure was obtained.

Proportion that Retained Austenite in the Whole Structure Occupies in the Whole Structure:

On the other hand, martensite and retained austenite can be distinguished by identifying retained austenite having an FCC structure with FE/SEM-EBSP. However, retained austenite is an important structure in the present invention, and quantitative measurement of the space factor is indispensable to effectively exhibit the effect of the present invention. For this reason, a method of measuring a space factor of retained austenite by analyzing an image of FE/SEM-EBSP is considered, but in the Examples, the space factor of retained austenite was calculated by a saturated magnetization method which can obtain a measurement value of further high accuracy. It is considered that the value measured by the saturated magnetization method is a volume fraction, but the volume fraction can be regarded as being equivalent with an area fraction.

Average Grain Size and Number of Secondary Phase in the Whole Structure:

On the other hand, the average grain size (average particle size of secondary phase particles) and the number of the secondary phase (martensite and retained austenite) were obtained by the following methods. Regarding the average grain size, five places of observation view of 20 μm×20 μm are randomly extracted from SEM observation photograph (magnification; 3,000), average grain sizes (diameters of corresponding circles) of martensite and retained austenite in the respective observation views are obtained, and the average value thereof was used as an average grain size of martensite and retained austenite in the whole structure. Regarding the number, five places of observation view of 50 μm×50 μm are randomly extracted from the above optical microphotograph, and the numbers of the secondary phase particles observed thereon were averaged.

Various dynamic properties of steel were obtained by the following methods.

Tensile Properties:

Using JIS No. 5 test piece obtained from a vertical direction of a steel sheet rolled direction, tensile strength (TS), uniform elongation (u-EL), local elongation (1-EL) and total elongation (t-EL=u-EL+1-EL) were measured according to JIS Z 2241. Test pieces having TS of 590 MPa or more and t-EL of 10% or more were considered as acceptable products. Strain amount of from the maximum load point to breakage was considered as local elongation.

Stretch Flangeability:

Hole expanding ratio λ was measured as stretch flangeability. The hole expanding ratio λ was measured according to JISF (The Japan Iron and Steel Federation) standard (JFST 1001-1996). Steels having λ of 80% or more were considered as acceptable products.

The results obtained are shown in Tables 18 and 19 below.

TABLE 18
Measurement results
Structure parameter
Af of
secondAverage
structureAveragegrain
Af ofingrainsize ofNumber
secondα grainsize ofsecondaryofVf of γ
PresenceAf ofstructureinα inphasesecondaryin
orMDin entireMDentirein entirephaseentire
TestSteelabsencestructurestructurestructurestructurestructurein entirestructure
No.codeof MD(%)(%)(%)(μm)(μm)structure(%)
141A96.628.578.411.30.41530.5
142B95.826.178.711.70.61440.3
143C96.925.874.59.60.31680.8
144D95.329.474.09.21.41048.8
145ENone
146FNone
147G97.127.371.96.80.71397.3
148H96.525.474.99.50.51611.0
149INone
150JPartial68.717.373.413.50.71211.7
151KPartial83.131.772.512.33.21340.7
152LPartial86.522.770.211.60.61400.0
153M95.624.975.59.90.41660.8
154N94.226.472.97.80.41591.0
155OPartial65.862.876.211.24.1730.8
Dynamic properties
TotalUniformLocal
TestTSelongationelongationelongationλ
No.(MPa)(%)(%)(%)(%)Remarks
14187025.89.216.6101.7Invention
14261829.512.117.4103.4Invention
14378427.713.913.893.9Invention
144120522.89.912.995.1Invention
14512984.91.83.119.3Comparison
1469929.67.42.245.2Comparison
14789227.415.811.688.6Invention
14881126.912.714.296.2Invention
14913183.11.02.117.5Comparison
15043330.421.98.567.5Comparison
15111798.43.45.031.3Comparison
1526829.57.71.832.7Comparison
15384426.213.213.094.2Invention
15485224.912.812.193.4Invention
15511588.54.24.330.6Comparison
α: Ferrite,
γ: Retained austenite,
Af: Area fraction,
Vf: Volume fraction,
TS: Tensile strength,
λ: Hole expanding ratio

TABLE 19
Measurement results
Structure parameter
Af of
secondAverage
structureAveragegrain
Af ofingrainsize ofNumber
secondα grainsize ofsecondaryofVf of γ
PresenceAf ofstructureinα inphasesecondaryin
orMDin entireMDentirein entirephaseentire
TestSteelabsencestructurestructurestructurestructurestructurein entirestructure
No.codeof MD(%)(%)(%)(μm)(μm)structure(%)
156PPartial63.216.472.513.80.71311.7
157Q95.525.976.310.10.71490.9
158RPartial88.724.175.210.90.71410.9
159S95.628.370.45.40.31724.2
160T95.227.871.05.90.41694.1
161U93.825.973.99.50.51585.8
162V96.725.775.48.60.61465.1
163WPartial87.422.871.611.90.61381.5
164CPartial56.711.672.417.50.8990.8
165C94.927.578.322.41.11180.8
166CPartial71.157.372.715.83.61031.1
167CPartial60.416.570.117.20.7740.9
168CNone
169X92.49.245.88.50.6820.0
Dynamic properties
TotalUniformLocal
TestTSelongationelongationelongationλ
No.(MPa)(%)(%)(%)(%)Remarks
15651927.718.59.264.7Comparison
15799323.412.810.692.8Invention
15885018.615.03.636.2Comparison
15998728.618.410.299.3Invention
16096328.518.310.2100.1Invention
16194025.415.89.689.9Invention
16269929.318.510.897.5Invention
16351227.016.710.389.5Comparison
16471924.815.98.978.9Comparison
16576723.914.89.176.3Comparison
166108710.48.42.033.8Comparison
16763026.918.88.177.3Comparison
16813023.20.92.321.9Comparison
16948732.218.613.693.2Comparison
α: Ferrite,
γ: Retained austenite,
Af: Area fraction,
Vf: Volume fraction,
TS: Tensile strength,
λ: Hole expanding ratio

Test Nos. 141, 142, 143, 144, 147, 148, 153, 154, 157, 159, 160, 161 and 162 each were satisfied with the provision of the present invention in structure morphology, and showed good properties in all of tensile strength TS, total elongation t-EL and hole expanding ratio λ. Average grain sizes of the secondary phases are all in a range of 3 μm or less.

On the other hand, No. 145 used steel E having C content larger than the range defined in the present application. As a result, nearly the entire surface was converted into martensite structure, and total elongation and λ were deficient.

No. 146 used steel F having Si content smaller than the range defined in the present application. As a result, the structure of bainite+martensite was formed, and total elongation and λ were deficient.

No. 149 used steel I having Si content larger than the range defined in the present application. As a result, nearly the entire surface was converted into martensite structure, and total elongation and λ were deficient.

No. 150 used steel J having Mn content smaller than the range defined in the present application. As a result, hardenability was poor, and ferrite was precipitated. As a result, TS and λ were deficient.

No. 151 used steel K having Mn content larger than the range defined in the present application. As a result, the structure of MD+martensite was formed, and total elongation and λ were deficient.

No. 152 used steel L having Al content smaller than the range defined in the present application. As a result, compounds of Si, Mn and O were unavoidably formed, and total elongation and λ were deficient.

No. 155 used steel O having Mo content larger than the range defined in the present application. As a result, the structure of MD+martensite was formed, and total elongation and λ were deficient.

No. 156 used steel P having B content smaller than the range defined in the present application. As a result, ferrite was precipitated, and TS and λ were deficient.

No. 158 used steel R having B content larger than the range defined in the present application. As a result, deterioration of λ which was considered due to grain segregation was generated.

No. 163 used steel W having C content smaller than the c range defined in the present application. As a result, ferrite was precipitated, and TS was deficient.

No. 164 was that the processing temperature T2 is lower than the range defined in the present application. As a result, ferrite was precipitated, and λ was decreased.

No. 165 was that the processing temperature T2 is higher than the range defined in the present application. As a result, the average grain size of the matrix ferrite is larger than the prescribed range, and λ was decreased.

No. 166 was that rolling reduction is lower than the range defined in the present application. As a result, bainite transformation is not accelerated, thereby forming the structure of MD+martensite. As a result, λ was decreased.

No. 167 was that the cooling rate was lower than the range defined in the present application. As a result, ferrite was precipitated, and λ was decreased.

No. 168 was that the cooling rate is higher than the range defined in the present application. As a result, martensite structure was formed, and total elongation and λ were decreased.

No. 169 used steel X having C content smaller than the range defined in the present application. As a result, the proportion of the secondary phase in the whole structure was reduced, and TS was decreased.

While the present invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the scope thereof.

This application is based on Japanese Patent Application No. 2006-283517 filed Oct. 18, 2006 and Japanese Patent Application No. 2006-283579 filed Oct. 18, 2006, the entire contents thereof being hereby incorporated by reference.

Further, all references cited herein are incorporated in their entireties.