Title:
Steel for induction hardening
Kind Code:
A1


Abstract:
The first invention provides a steel product which, while minimizing an increase in hardness after forging to ensure machinability and cold workability, is improved, for example, in fatigue strength in its non-hardened part and is improved, in its hardened part, in rolling contact fatigue life, antipitting level, abrasion resistance, and fatigue strength. The highly machinable and high strength steel for induction hardening comprises, by mass, carbon (C): not less than 0.40% and less than 0.50%, silicon (Si): 0.5 to 0.9%, manganese (Mn): 0.5 to 1.0%, chromium (Cr): not more than 0.4%, sulfur (S): not more than 0.035%, and vanadium (V): 0.01 to 0.15% with the balance consisting of iron (Fe) and unavoidable impurities, said steel being forged into a component a part of which is then inductively hardened before use, the steel satisfying a carbon equivalent (Ceq) requirement represented by 0.75≦Ceq≦0.90, and a machinability index (a value) requirement represented by a value ≦0.62. The second invention provides a steel product that is excellent in cold workability after hot forging and is improved, for example, in fatigue strength in its non-hardened part and is improved, in its hardened portion, in rolling contact fatigue life, antipitting level, abrasion resistance, and fatigue strength. The steel for induction hardening, having improved cold workability, rolling fatigue life in its hardened part, and bending fatigue strength in its non-hardened part comprises, by mass, carbon (C): 0.40 to 0.60%, silicon (Si): 0.5 to 0.9%, manganese (Mn): 0.5 to 1.00%, chromium (Cr): not more than 0.4%, sulfur (S): not more than 0.035%, and vanadium (V): 0.01 to 0.15% with the balance consisting of iron (Fe) and unavoidable impurities, the percentage ferrite area after hot forging of the steel being not less than 15%, not less than 30% of the ferrite being accounted for by ferrite having a major axis/minor axis ratio of not more than 5, the steel satisfying the requirement that the equivalent of carbon represented by formula (1) is represented by formula (2):
Ceq=C %+( 1/7)Si %+(⅕)Mn %+( 1/9)Cr %−( 5/7)S %+V % (1)
0.75≦Ceq≦0.90 (2)
wherein Ceq represents the equivalent of carbon.



Inventors:
Nakamyou, Satoru (Himeji-Shi, JP)
Application Number:
11/022321
Publication Date:
09/22/2005
Filing Date:
12/22/2004
Assignee:
SANYO SPECIAL STEEL CO., LTD. (Himeji-shi, JP)
Primary Class:
Other Classes:
420/127, 420/104
International Classes:
C22C38/12; (IPC1-7): C22C38/12
View Patent Images:



Primary Examiner:
IP, SIKYIN
Attorney, Agent or Firm:
Richard L. Byrne (Pittsburgh, PA, US)
Claims:
1. A high strength steel for induction hardening, having improved machinability, said steel comprising, by mass, carbon (C): not less than 0.40% and less than 0.50%, silicon (Si): 0.5 to 0.9%, manganese (Mn): 0.5 to 1.0%, chromium (Cr): not more than 0.4%, sulfur (S): not more than 0.035%, and vanadium (V): 0.01 to 0.15% with the balance consisting of iron (Fe) and unavoidable impurities, said steel being forged into a component a part of which is then inductively hardened before use, said steel satisfying the requirement that the equivalent of carbon represented by formula (1) is represented by formula (2) and the index of machinability satisfies formula (3):
Ceq=C %+( 1/7)Si %+(⅕)Mn %+( 1/9)Cr %−( 5/7)s %+V % (1)
0.75<Ceq≦0.90 (2)
a value=C−( 1/12)Si+(⅕)Mn+( 1/9)Cr−V−( 5/7)S≦0.62 (3) wherein Ceq represents the equivalent of carbon; and a value represents the index of machinability:

2. A steel for induction hardening, having improved cold workability, rolling fatigue life in its hardened part, and bending fatigue strength in its non-hardened part, said steel comprising, by mass, carbon (C): 0.40 to 0.60%, silicon (Si): 0.5 to 0.9%, manganese (Mn): 0.5 to 1.00%, chromium (Cr): not more than 0.4%, sulfur (S): not more than 0.035%, and vanadium (V): 0.01 to 0.15% with the balance consisting of iron (Fe) and unavoidable impurities, the percentage ferrite area after hot forging of said steel being not less than 15%, not less than 30% of said ferrite being accounted for by ferrite having a major axis/minor axis ratio of not more than 5, said steel satisfying the requirement that the equivalent of carbon represented by formula (1) is represented by formula (2):
Ceq=C %+( 1/7)Si %+(⅕)Mn %+( 1/9)Cr %−( 5/7)S %+V % (1)
0.75≦Ceq<0.90 (2) wherein Ceq represents the equivalent of carbon.

Description:

RELATED APPLICATIONS

This application is a patent application claiming priority based on Japanese Patent Application Nos. 431776/2003 and 431777/2003 filed with Japanese Patent Office on Dec. 26, 2003, and the contents thereof are in conformity with these two basic applications.

TECHNICAL FIELD

The present invention relates to a steel as a steel product that is forged into a component a part of which is then inductively hardened before use, for example, as constant velocity joints or hub units.

The present invention also relates to the development of a high strength steel for induction hardening, having improved cold workability, bending fatigue strength, and rolling fatigue strength, that is inductively hardened before use as machine structural components, for example, constant velocity joint outer races and hub units.

BACKGROUND ART

Components, for example, constant velocity joints or hub units, have hitherto been produced by forming a steel product by cold forging, warm forging, or hot forging or a combination thereof and inductively hardening the forged product particularly in its portion required to have good strength. Steel products, such as JIS S 53 C, SAE 1055, and SAE 1070, are mainly used for such applications.

Due to a recent tendency toward an increase in severity of an environment under which the components are used, or a reduction in size and a reduction in thickness aimed at a reduction in weight, however, further improved rolling contact fatigue life, abrasion resistance, and fatigue strength are required of the conventional quench-hardened part. In addition, an improvement in fatigue strength in the non-hardened part, in which the fatigue strength possessed by the as-forged component has hitherto sufficed for the contemplated applications, has also become required. Further, in these components, there are many sites which should be subjected to machining after forging, and an ever-increasing demand in recent years for a reduction in working cost has led to a strong demand for improved machinability.

An increase in the content of carbon (C), silicon (Si), and chromium (Cr) or the addition of molybdenum (Mo) or the like to improve the properties required of the hardened part and, at the same time, an increase in fatigue strength of the non-hardened part by increasing the hardness of the non-hardened part are considered effective as means for meeting the above demands. Since, however, these components often undergo machining or cold working after forging, unconditionally increasing the hardness of the non-hardened part is disadvantageous from the viewpoints of machining and cold working. Further, the addition of chromium and molybdenum leads to an increase in material cost. Further, hardening of a portion, which lacks in fatigue strength, is considered effective as means for improving the fatigue strength of the non-hardened portion. This, however, disadvantageously leads to an increase in the number of steps necessary for the production of components which in turn incurs increased production cost. For this reason, simultaneously meeting a demand for an increase in fatigue strength of the non-hardened part and a demand for an improvement in properties in the hardened part while minimizing the increase in the hardness of the as-forged component to ensure machinability and cold workability of the non-hardened part is required of materials used in these components.

The applicant has developed a high strength steel for induction hardening, having excellent machinability, with a view to solving these problems of the prior art. This technique, however, is directed to an improvement in machinability. Further, this high strength steel for induction hardening is one which has been closely controlled to a carbon (C) content of 0.5 to 0.7% (see, for example, Japanese Patent Laid-Open No. 332535/2002).

On the other hand, components, for example, constant velocity joint outer races or hub units, have hitherto been produced by hot-forging a steel product and inductively hardening the forged product particularly in its part required to have good strength. Steel products of carbon steels for machine structure use, such as JIS S 48 C and JIS S 53 C, are mainly used for such applications.

Due to a recent tendency toward an increase in severity of an environment under which the components are used, or a reduction in size aimed at a reduction in weight, however, further improved rolling contact fatigue life, abrasion resistance, and fatigue strength are required of the conventional quench hardened part in the conventional components. In addition, there is an increasing demand for improved fatigue strength in the non-hardened part. Furthermore, there is a strong demand for improved cold workability because these components have sites which should be cold worked after forging.

An increase in the content of carbon (C), silicon (Si), and chromium (Cr) in the chemical composition of the steel or the addition of molybdenum or the like to the chemical composition of the steel to improve the properties required of the quench hardened part of the components and, at the same time, an increase in fatigue strength of the non-hardened part by increasing the hardness of the non-hardened part are considered effective as means for meeting the above demands. Since, however, these components have sites which are subjected to cold working after forging, unconditionally increasing the hardness of the non-hardened part disadvantageously induces cracking during cold working which makes it difficult to realize working into a desired shape.

For example, machine structural steels, which have cold forgeability and induction hardenability comparable to or better than those of S48C steel and can realize further improved rolling fatigue life, are known as machine structural steels possessing excellent cold forgeability, induction hardenability, and rolling fatigue properties (see for example, Japanese Patent Laid-Open No. 217144/1997). These steels, however, have a carbon content exceeding 0.60%.

Further, steels for induction hardening which have been improved in rolling contact fatigue life and fatigue strength without sacrificing workability by increasing the content of carbon in the chemical composition of the steel, reducing the contents of silicon and manganese in the chemical composition of the steel and further adding boron to the chemical composition of the steel to ensure the hardenability are also known (see for example, Japanese Patent Laid-Open No. 268344/1997).

Furthermore, steels for induction hardening which can be cold forged into shaft components and bearing components and can realize excellent strength properties, particularly excellent rolling contact fatigue properties, have been developed (see for example, Japanese Patent Laid-Open No. 287054/1997).

Furthermore, hardening of a part, which lacks in fatigue strength, is considered effective as means for improving the fatigue strength of the non-hardened part. This, however, disadvantageously leads to an increase in the number of steps necessary for the production of components which in turn incurs increased production cost. For this reason, simultaneously meeting a demand for an increase in fatigue strength of the non-hardened part and a demand for an improvement in properties in the hardened part while ensuring the cold workability of the non-hardened part is required of materials used in these components.

DISCLOSURE OF THE INVENTION

First Invention

Accordingly, it is an object of the first invention to solve the above problems of the prior art despite a further reduction in carbon content from the carbon content of the high strength steel for induction hardening developed by the applicant and to provide a steel product which, while minimizing an increase in as-forged hardness to ensure machinability and cold workability, is improved, for example, in fatigue strength in its non-hardened part and is improved, in its hardened part, in rolling contact fatigue life, antipitting level, abrasion resistance, and fatigue strength.

The above object can be attained by a high strength steel for induction hardening, having improved machinability, said steel comprising, by mass, carbon (C): not less than 0.40% and less than 0.50%, silicon (Si): 0.5 to 0.9%, manganese (Mn): 0.5 to 1.0%, chromium (Cr): not more than 0.4%, sulfur (S): not more than 0.035%, and vanadium (V): 0.01 to 0.15% with the balance consisting of iron (Fe) and unavoidable impurities, said steel being forged into a component a part of which is then inductively hardened before use, said steel satisfying the requirement that the equivalent of carbon represented by formula (1) is represented by formula (2) and the index of machinability satisfies formula (3):
Ceq=C %+( 1/7)Si %+(⅕)Mn %+( 1/9)Cr %−( 5/7)S %+V % (1)
0.75≦Ceq≦0.90 (2)
a value=C−( 1/12)Si+(⅕)Mn+( 1/9)Cr−V−( 5/7)S≦0.62 (3)

    • wherein Ceq represents the equivalent of carbon; and a value represents the index of machinability.
      Second Invention

In the second invention, unlike the above-described prior art method in which the contents of silicon and manganese are reduced to reduce the hardness of the steel after hot forging, the above problems of the prior art are solved by regulating the percentage area and form of ferrite after hot forging.

Thus, the above problems of the prior art can be solved by a steel for induction hardening, having improved cold workability, rolling contact fatigue life in its hardened part, and bending fatigue strength in its non-hardened part, said steel comprising, by mass, carbon (C): 0.40 to 0.60%, silicon (Si): 0.5 to 0.9%, manganese (Mn): 0.5 to 1.00%, chromium (Cr): not more than 0.4%, sulfur (S): not more than 0.035%, and vanadium (V): 0.01 to 0.15% with the balance consisting of iron (Fe) and unavoidable impurities, the percentage ferrite area after hot forging of said steel being not less than 15%, not less than 30% of said ferrite being accounted for by ferrite having a major axis/minor axis ratio of not more than 5, said steel satisfying the requirement that the equivalent of carbon represented by formula (1) is represented by formula (2):
Ceq=C %+( 1/7)Si %+(⅕)Mn %+( 1/9)Cr %−( 5/7)S %+V % (1)
0.75≦Ceq≦0.90 (2)
wherein Ceq represents the equivalent of carbon.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a graph showing the relationship between carbon equivalent and hardness (HRC).

BEST MODE FOR CARRYING OUT THE INVENTION

First Invention

The first invention is directed to a high strength steel for induction hardening, having improved machinability, said steel comprising, by mass, carbon (C): not less than 0.40% and less than 0.50%, silicon (Si): 0.5 to 0.9%, manganese (Mn): 0.5 to 1.0%, chromium (Cr): not more than 0.4%, sulfur (S): not more than 0.035%, and vanadium (V): 0.01 to 0.15% with the balance consisting of iron (Fe) and unavoidable impurities, said steel being forged into a component a part of which is then inductively hardened before use, said steel satisfying the requirement that the equivalent of carbon represented by formula (1) is represented by formula (2) and the index of machinability satisfies formula (3):
Ceq=C %+( 1/7)Si %+(⅕)Mn %+( 1/9)Cr %−( 5/7)S %+V % (1)
0.75≦Ceq≦0.90 (2)
a value=C−( 1/12)Si+(⅕)Mn+( 1/9)Cr−V−( 5/7)S≦0.62 (3)
wherein Ceq represents the equivalent of carbon; and a value represents the index of machinability.

The reason for the limitation of the chemical composition of the steel according to the first invention will be described. In the following description, “%” is by mass.

Carbon (C): not less than 0.40% and less than 0.50%, preferably 0.43 to 0.50%, more preferably 0.45 to 0.50%.

Carbon is an element for ensuring the hardenability. The lower limit of the carbon content is 0.40%. When the carbon content exceeds 0.50%, regarding the structure, the amount of proeutectoid ferrite in the non-hardened part is reduced, leading to a significant reduction in machinability. In the present invention, the machinability index, i.e., a value, is brought to not more than 0.62, and, at the same time, the carbon content is brought to not less than 0.40% and less than 0.50%.

Silicon (Si): 0.5 to 0.9%, preferably 0.6 to 0.9%, more preferably 0.6 to 0.8%.

The content of silicon plays the most important role in the present invention, and silicon functions to improve fatigue strength and machinability and further contributes to an improvement in rolling contact fatigue life, antipitting level, abrasion resistance, and fatigue strength in the hardened part while minimizing the increase in hardness of the non-hardened portion. The addition of silicon in an amount of not less than 0.5% is useful for attaining the effect of improving the rolling contact fatigue life, the antipitting level, the abrasion resistance, and the fatigue strength in the hardened portion. When the amount of silicon added exceeds 1.0%, the contemplated effect is saturated. Regarding the machinability, the addition of silicon in an amount of 0.7 to 0.9% can offer the best effect. For the above reason, the silicon content is limited to 0.5 to 0.9%.

Manganese (Mn): 0.5 to 1.0%, preferably 0.6 to 1.0%, more preferably 0.6 to 0.9%.

When the manganese content is lowered, the austenitization by heating for a short period of time in the induction hardening is unsatisfactory and, consequently, satisfactory as-quenched hardness cannot be provided. In the steel according to the present invention, the manganese content should be at least 0.5%. A manganese content of not less than 0.6% is preferred: Increasing the manganese content improves the fatigue strength of the non-hardened portion, but on the other hand, the amount of proeutectoid ferrite is reduced resulting in a significant reduction in machinability. For this reason, the upper limit of the manganese content is 1.0%.

Chromium (Cr): not more than 0.4%, preferably not more than 0.35%, more preferably not more than 0.25%.

Chromium, even when not intentionally added, is unavoidably contained in an amount of about 0.05 to 0.35%. If necessary, chromium may be added. However, it should be noted that, when the content of chromium exceeds 0.4%, chromium concentrates in the cementite and, at the time of heating before hardening, inhibits the dissolution of carbon in the matrix to form a solid solution. For this reason, the upper limit of the chromium content is 0.4% from the viewpoint of avoiding the inhibition of induction hardening.

Sulfur (S): not more than 0.035%, preferably not more than 0.030%, more preferably not more than 0.020%.

Sulfur is an element which functions to improve the machinability, and increasing the amount of sulfur added is advantageous from the viewpoint of machinability. Since, however, sulfur forms MnS as a nonmetallic inclusion, the rolling contact fatigue life is deteriorated. Therefore, the upper limit of the sulfur content is 0.035% from the viewpoint of avoiding the influence of the addition of sulfur on the rolling contact fatigue life.

Vanadium (V): 0.01 to 0.15%, preferably 0.01 to 0.08%, more preferably 0.02 to 0.06%.

Vanadium, as well as silicon, is an element that plays an important role in the present invention and contributes to an improvement in fatigue strength of the non-hardened part and an improvement in machinability. The addition of vanadium permits proeutectoid ferrite, which is the weakest portion in the structure, to be strengthened by precipitation hardening of VC, and, as a result, the fatigue strength is improved. Further, proeutectoid ferrite is stably precipitated in a spheroidal form using VN, which is produced by the addition of vanadium, as a nucleus to significantly improve the machinability. When the carbon content is not more than 0.5%, despite the addition of vanadium, proeutectoid ferrite is precipitated in a satisfactory amount although the proeutectoid ferrite is precipitated in a layer form. Therefore, in this case, the effect of improving the machinability cannot be substantially attained by the addition of vanadium. On the other hand, when the carbon content is not less than 0.7%, the proeutectoid ferrite is hardly precipitated by the addition of vanadium and, consequently, the effect of improving the machinability cannot be attained. Only in the carbon content range of 0.5 to 0.7% which causes the precipitation of a slight amount of layered proeutectoid ferrite and is unsatisfactory in the machinability, the machinability can be improved by adding vanadium to stably precipitate spheroidal proeutectoid ferrite.

Reason for the adoption of 0.75≦Ceq≦0.90 is as follows.

The as-hot-forged hardness and the as-warm-forged hardness can be predicted from the carbon equivalent Ceq. A higher as-forged 2.5 hardness is more advantageous from the viewpoint of the fatigue strength, but on the other hand, is more disadvantageous from the viewpoint of working. The relationship between the carbon equivalent and the as-hot-forged hardness was examined by experimentation on chemical compositions around the scope of the steel of the present invention. As a result, a relationship as shown in FIG. 1 was obtained. Specifically, when the carbon equivalent is 0.75≦Ceq≦0.90, an as-hot-forged hardness of 19.5 to 26.5 HRC is obtained which can realize a combination of the contemplated fatigue strength in the non-hardened part with desired workability by conventional working method. Therefore, in the chemical composition of the steel, when the carbon equivalent Ceq falls within the above defined range, a steel satisfying both workability and fatigue strength requirements can be surely produced. In particular, in the case where hot forging is included in the step of working, this Ceq range is in many cases adopted. More preferably, 0.75≦Ceq≦0.83 is satisfied.

The reason why the a value as the machinability index should satisfy the formula: a value=C−( 1/12)Si+(⅕)Mn+( 1/9)Cr−V−( 5/7)S≦0.62 is as follows.

The present inventor has found that the machinability can be predicted by the a value as the machinability index represented by the above formula. In order to realize a significant improvement in machinability, for the steel having C: not less than 0.4% and less than 0.50%, the a value as the machinability index should be closely regulated to not more than 0.62. The a value as the machinability index is further preferably not more than 0.58.

As described above, in the first invention, the carbon and manganese contents are limited to a range that does not sacrifice the workability and, at the same time, the silicon content is increased to enhance the strength, whereby excellent effects not attained by the prior art technique can be attained in a high strength steel product for induction hardening that is forged into a component a part of which is then inductively hardened before use, that is, a steel product for induction hardening can be realized which, while minimizing an increase in hardness after forging to ensure machinability and cold workability, is improved, for example, in fatigue strength in its non-hardened part and is improved, in its hardened part, in rolling contact fatigue life, antipitting level, abrasion resistance, and fatigue strength.

Second Invention

The second invention provides a steel for induction hardening, having improved cold workability, rolling contact fatigue life in its hardened part, and bending fatigue strength in its non-hardened part, said steel comprising, by mass, carbon (C): 0.40 to 0.60%, silicon (Si): 0.5 to 0.9%, manganese (Mn): 0.5 to 1.00%, chromium (Cr): not more than 0.4%, sulfur (S): not more than 0.035%, and vanadium (V): 0.01 to 0.15% with the balance consisting of iron (Fe) and unavoidable impurities, the percentage ferrite area after hot forging of said steel being not less than 15%, not less than 30% of said ferrite being accounted for by ferrite having a major axis/minor axis ratio of not more than 5, said steel satisfying the requirement that the equivalent of carbon represented by formula (1) is represented by formula (2):
Ceq=C %+( 1/7)Si %+(⅕)Mn %+( 1/9)Cr %−( 5/7)S %+V % (1)
0.75≦Ceq≦0.90 (2)
wherein Ceq represents the equivalent of carbon.

The reason for the limitation of the chemical composition of the steel according to the second invention will be described. In the following description, “%” is by mass.

Carbon (C): 0.40 to 0.60%, preferably 0.40 to 0.50%, more preferably 0.45 to 0.50%.

Carbon is an element for ensuring the hardenability. The lower limit of the carbon content is 0.40% from the viewpoint of ensuring the hardenability after induction hardening. When the carbon content is excessively high, however, the cold workability is deteriorated. For the above reason, the upper limit of the carbon content is 0.60%.

Silicon (Si): 0.5 to 0.9%, preferably 0.6 to 0.9%, more preferably 0.6 to 0.8%.

Silicon plays the most important role in the present invention, and silicon functions to improve fatigue strength, cold workability and machinability at the time of non-hardening and contributes to an improvement in rolling contact fatigue life, antipitting level, abrasion resistance, and fatigue strength at the time of hardening. The contemplated effect can be attained by the addition of silicon in an amount of not less than 0.5%. When the amount of silicon added exceeds 1.0%, the contemplated effect is saturated. Regarding the machinability, the addition of silicon in an amount of 0.7 to 0.9% can offer the best effect. For the above reason, the silicon content is limited to 0.5 to 0.9%.

Manganese (Mn): 0.5 to 1.0%, preferably 0.6 to 1.0%, more preferably 0.6 to 0.9%.

Manganese is an element that is necessary for ensuring induction hardenability. When the content of manganese is less than 0.5%, the austenitization by heating for a short period of time in the induction hardening is unsatisfactory and, consequently, satisfactory as-quenched hardness cannot be provided. In the steel according to the present invention, the manganese content should be at least 0.5%. However, a manganese content of not less than 0.6% is preferred. Increasing the manganese content improves the fatigue strength of the non-hardened portion, but on the other hand, the amount of proeutectoid ferrite is reduced resulting in a significant reduction in machinability. For this reason, the upper limit of the manganese content is 1.0%.

Chromium (Cr): not more than 0.4%, preferably not more than 0.35%, more preferably not more than 0.25%.

Chromium, even when not intentionally added, is unavoidably contained in an amount of about 0.05 to 0.35%. If necessary, chromium may be added. However, it should be noted that, when the content of chromium exceeds 0.4%, chromium concentrates in the cementite and, at the time of heating before hardening, inhibits the dissolution of carbon in the matrix to form a solid solution. For this reason, the upper limit of the chromium content is 0.4% from the viewpoint of avoiding the inhibition of induction hardening.

Sulfur (S): not more than 0.035%, preferably not more than 0.030%, more preferably not more than 0.025%.

Sulfur is an element which functions to improve the machinability, and increasing the amount of sulfur added is advantageous from the viewpoint of machinability. Since, however, sulfur forms MnS as a nonmetallic inclusion, the rolling contact fatigue life is deteriorated. Therefore, the upper limit of the sulfur content is 0.035% from the viewpoint of avoiding the influence of the addition of sulfur on the rolling contact fatigue life.

Vanadium (V): 0.01 to 0.15%, preferably 0.01 to 0.08%, more preferably 0.02 to 0.06%.

Vanadium, as well as silicon, is an element that plays an important role in the present invention and contributes to an improvement in fatigue strength of the non-hardened part and an improvement in machinability. The addition of vanadium permits proeutectoid ferrite, which is the weakest portion in the structure, to be strengthened by precipitation hardening of VC, and, as a result, the fatigue strength is improved. Further, proeutectoid ferrite is stably precipitated in a spheroidal form using VN, which is produced by the addition of vanadium, as a nucleus to significantly improve the machinability. When the vanadium content exceeds 0.15%, however, the cold workability and the machinability are deteriorated. For the above reason, the vanadium content is limited to 0.01 to 0.15%.

Percentage area of ferrite after hot forging: not less than 15%, preferably not less than 16%, more preferably not less than 19%.

When the percentage area of ferrite after hot forging is less than 15%, cracking occurs during cold working. Therefore, the percentage area of ferrite after hot forging should be closely regulated to not less than 15%.

The proportion of ferrite having a major axis/minor axis ratio of not more than 5 in the whole ferrite: not less than 30%, preferably not less than 35%, more preferably not less than 40%.

When the proportion of ferrite having a major axis/minor axis ratio of not more than 5 in the whole ferrite is less than 30%, cracking occurs during cold working. Therefore, the proportion of ferrite having a major axis/minor axis ratio of not more than 5 in the whole ferrite should be closely regulated to not less than 30%.
0.75≦Ceq≦0.90

The as-hot-forged hardness and the as-warm-forged hardness can be predicted from the carbon equivalent Ceq. A higher as-forged hardness is more advantageous from the viewpoint of the fatigue strength, but on the other hand, is more disadvantageous from the viewpoint of cold working. The relationship between the carbon equivalent and the as-hot-forged hardness was examined by experimentation on chemical compositions around the scope of the steel of the present invention. As a result, it was found that, when the carbon equivalent is 0.75≦Ceq≦0.90, an as-hot-forged hardness is obtained which can realize a combination of the contemplated fatigue strength with desired cold workability. Therefore, in the chemical composition of the steel, when the carbon equivalent Ceq falls within the above defined range, a steel satisfying both cold workability and fatigue strength requirements can be surely produced. When Ceq as the carbon equivalent exceeds 0.90, however, the hardness is excessively increased, making it difficult to deform the steel at the time of cold working. Therefore, the Ceq as the carbon equivalent is limited to 0.75≦Ceq≦0.90, more preferably 0.75≦Ceq≦0.83.

In the steel according to the second invention, the form of ferrite in the structure is regulated, so that not less than 30% of the whole ferrite is accounted for by ferrite having a major axis/minor axis ratio of not more than 5 and the percentage ferrite area is improved to not less than 15%, by increasing the content of silicon and adding vanadium, whereby a steel can be realized which can suppress cracking during cold working after hot forging and, in addition, as compared with the conventional steel, is improved in fatigue strength in its non-hardened part and is improved, in its hardened part, in rolling contact fatigue life, antipitting level, abrasion resistance, and fatigue strength.

EXAMPLES

Example A

The best mode for carrying out the first invention will be described with reference to the following Examples. Steel ingots of test steels of respective steel Nos. of the invention, conventional steels, and comparative steels specified in Table A1 are produced by a melt process in a 100-kg vacuum melting furnace. In this connection, it should be noted that oxygen (O) and nitrogen (N) in the test steels specified in the tables are elements that are unavoidably contained as impurities. The steel ingots thus obtained were hot forged into steel bars of 30φ and 20φ which were then allowed to cool.

Example A1

TABLE A1
(mass %, O and N in ppm)
No.ClassificationCSiMnSCrONVa valueRemarks
1Steel of invention0.490.590.600.0300.0581240.100.44
20.480.650.610.0250.15121140.070.48
30.450.710.900.0230.0991450.040.52
40.480.600.800.0170.1891400.050.55
50.490.730.640.0240.16111280.110.45
60.460.770.800.0140.20101110.040.53
70.490.650.710.0150.2281380.030.56
80.430.800.850.0180.1891470.040.50
90.490.700.700.0230.23101300.030.56
10Conventional steel0.53*0.20*0.830.0190.20889—*0.69*JIS S 53 C
11Comparative steel0.70*0.20*0.600.0300.15882—*0.79*
120.35*0.650.610.0250.15151140.070.35
130.59*0.710.900.0230.1291420.040.66*
140.481.10*0.800.0170.1891220.050.51
150.490.23*0.640.0240.1671010.030.49
160.460.770.800.0140.2010147—*0.57
170.430.820.710.039*0.2281380.030.46
180.52*0.600.900.0180.1891090.020.64*
190.490.520.950.0170.206960.010.63*a value: outside the
scope of claim

*Values outside the scope of claims of the present invention.

TABLE A2
Number of boresRolling service life: L10
No.Classificationa valueformed by drillingFatigue strength, MPaservice life, cycle
1Steel of invention0.44129380>5.0 × 107
20.48132360>5.0 × 107
30.52118365>5.0 × 107
40.55109375>5.0 × 107
50.45120400>5.0 × 107
60.53112380>5.0 × 107
70.56110350>5.0 × 107
80.50119360>5.0 × 107
90.56105390>5.0 × 107
10Conventional steel0.69* 60*270 3.0 × 107
11Comparative steel0.79* 50*380>5.0 × 107
120.35139 240* 1.2 × 107*
130.66* 67*430>5.0 × 107
140.51 85*420>5.0 × 107
150.49102 260* 2.9 × 107*
160.57 82*300 4.8 × 107
170.46121280 2.1 × 107*
180.64* 68*380>5.0 × 107
190.63* 79*360>5.0 × 107

*Values outside the scope of claims of the present invention and below the target value.

For nine steels of the invention, that is, Nos. 1 to 9 in Table A1, among the above test steels, the results of evaluation are shown in steel Nos. 1 to 9 of invention in Table A2. Steel Nos. 2 and 7 are Cr-added materials, and the other steels are Cr-non-added materials. For the conventional steel of No. 10 in Table A1, the carbon content and the silicon content are outside the carbon and silicon content ranges specified for the steels of the present invention, and, in addition, the a value as the machinability index specified in Table A2 is outside the a value range specified for the steels of the present invention. For the nine comparative steels of Nos. 11 to 19 in Table A1, the values marked with an asterisk are outside the ranges specified for the steels of the present invention. The specimens of the test steels were subjected to the following tests. The results are shown in Table A2.

Regarding the results of evaluation of the a value as the machinability index in the test steels, as shown in Table A2, for steel Nos. 1 to 9, the a values are 0.44 to 0.56. That is, for all steel Nos. 1 to 9, the a value is not more than 0.62.

1) Machinability (Drill Service Life Test)

A hot forged material of 30 mmφ was worked by a milling machine to prepare a rectangular material having a size of 24 mm×18 mm×300 mm which was then subjected to a drilling test, and the machinability was evaluated in terms of the number of bores which could be formed by drilling until the drill failed. Testing conditions are as follows. Diameter of drill: 5 mmφ; material of drill: SKH 51; cutting speed: 20 m/min; feed rate: 0.2 mm/rev; cutting oil: not used (dry type); bore depth: 15 mm; and evaluation method: number of bores which could be formed until the drill no longer could form a bore.

2) Fatigue Test (Rotating Bending Fatigue Test)

A hot forged material of 20 mmφ was turned to prepare a specimen, for a rotating bending fatigue test, having a test portion size of 8 mmφ, and the specimen was subjected to a rotating bending fatigue test to evaluate the fatigue strength.

3) Rolling Service Life Test (Thrust Load)

A forged material of 65 mmφ was turned to prepare a specimen having a size of 60 mmφ×7.2 mm. The specimen was inductively hardened, was tempered, was surface polished, and was then subjected to a rolling service life test. The test was carried out under conditions of Pmax=5292 MPa, load=thrust direction, and temperature=room temperature. The rolling service life test was evaluated in terms of L10 service life.

4) Rolling Service Life Test (Radial Load)

A forged material of 20 mmφ was turned to prepare a specimen having a size of 12 mmφ×22 mm. The specimen was inductively hardened, was tempered, was surface polished, and was then subjected to a rolling service life test. The test was carried out under conditions of Pmax=5880 MPa, load=radial direction, and temperature=room temperature. The rolling service life test was evaluated in terms of L10 service life.

In the above tests, the target value in the evaluation of the machinability (drill service life test) as the test 1) is not less than 90 in number of bores formed by drilling which is 50% larger than specified in JIS S 53 C. For all steel Nos. 1 to 9 of the present invention, the number of bores formed by drilling is not less than 105.

The target value in the evaluation of the fatigue test (rotating bending fatigue test) as the test 2) is not less than the value specified in JIS S 53 C, that is, not less than 270 MPa in rotating bending fatigue strength. For all the steel Nos. 1 to 9 of the present invention, the rotating bending fatigue strength is not less than 350 MPa.

The target value in the evaluation of the rolling service life test (radial load) as the test 3) is not less than the value specified in JIS S 53C, that is, not less than 3.0×107 in rolling fatigue service life. For all the steel Nos. 1 to 9 of the present invention, the rolling fatigue service life exceeds 5.0×107.

Example B

The best mode for carrying out the second invention will be described with reference to the following Examples. The chemical compositions of steels of the present invention, conventional steels, and comparative steels are shown in Table B1.

TABLE B1
(mass %, O and N in ppm)
No.ClassificationCSiMnSCrONVRemarks
1Steel of invention0.500.590.600.0300.13111540.10
20.420.650.610.0250.15151140.07
30.450.710.900.0230.099900.04
40.480.600.800.0170.3291030.05
50.490.730.640.0240.2371110.11
60.460.770.800.0140.20101300.04
70.430.650.710.0150.2281870.03
80.410.800.850.0180.1891340.04
90.540.650.800.0200.2281200.05
100.510.700.700.0230.23101300.03
110.570.610.650.0250.1581090.06
12Conventional steel0.530.20*0.830.0190.20889—*JIS S 53 C
130.480.19*0.810.0170.177100—*JIS S 48 C
14Comparative steel0.68*0.20*0.600.0300.05889—*
150.35*0.650.610.0250.15101290.07
160.58*0.710.900.0230.0991090.04
170.480.10*0.800.0170.1891220.05
180.510.23*0.640.0240.1671010.02
190.460.770.800.0140.201098—*
200.480.650.750.0190.23988—*
210.430.820.710.039*0.2281380.03
220.410.801.20*0.0180.1891100.04

*Values outside the scope of claims of the present invention.

Example B1

Steel ingots of test steels having respective chemical compositions specified in Table B1 were produced by a melt process in a 100-kg vacuum melting furnace. Steel Nos. 2 and 7 of the present invention are Cr-added materials, and the other steels are Cr-non-added materials. These steel ingots thus obtained were hot forged into steel bars of 30 mmφ and 20 mmφ which were then allowed to cool. The bars produced by hot forging were subjected to the following tests.

(1) Cold Workability (Swaging Test)

A specimen having a size of 14 mmφ×21 mm was taken off from the hot forged material of 20 mmφ and was subjected to a cold swaging test to measure the percentage final swaging necessary for causing cracking.

(2) Machinability (Drill Service Life Test)

A hot forged material of 30 mmφ was worked by a milling machine to prepare a rectangular material having a size of 24 mm×18 mm×300 mm which was then subjected to a drilling test, and the machinability was evaluated in terms of the number of bores which could be formed by drilling until the drill failed. Testing conditions are as follows. Diameter of drill: 5 mmφ; material of drill: SKH 51; cutting speed: 20 m/min; feed rate: 0.2 mm/rev; cutting oil: not used (dry type); bore depth: 15 mm; and evaluation method: number of bores which could be formed until the drill no longer could form a bore.

(3) Fatigue Strength (Rotating Bending Fatigue Test)

A specimen of 20 mmφ was turned to prepare a specimen, for a rotating bending fatigue test, having a test portion size of 8 mmφ, and the specimen was subjected to a rotating bending fatigue test to evaluate the fatigue strength.

(4) Rolling Service Life Test (Thrust Load)

A forged material of 20 mmφwas turned to prepare a specimen having a size of 20 mmφ×22 mm. The specimen was inductively hardened, was tempered, was surface polished, and was then subjected to a rolling service life test. The test was carried out under conditions of Pmax=5880 MPa, load=radial direction, and temperature=room temperature. The rolling service life test was evaluated in terms of L10 service life.

The results of tests, that is, the percentage ferrite area, the proportion of ferrite having a major axis/minor axis ratio of not more than 5, the percentage final swaging in the swaging test, the number of bores formed by drilling, the fatigue strength determined by the rotating bending fatigue test, and the rolling service life in the radial load are shown in Table 2.

TABLE B2
Proportion of ferriteNumber ofFatigueRolling service life:
Percentagehaving ferrite from ratioPercentage finalbores formedstrength,L10 service life,
No.Classificationferrite area, %of not more than 5, %swaging, %by drillingMPacycle
1Steel of invention348062121380>5.0 × 107
2256670130300>5.0 × 107
3236866 96365>5.0 × 107
4226965 92375>5.0 × 107
5258366120400>5.0 × 107
6326766 91380>5.0 × 107
7415373136320>5.0 × 107
8306771112330>5.0 × 107
9185461 93410>5.0 × 107
10236765 98380>5.0 × 107
11165768109410>5.0 × 107
12Conventional 12* 23* 56* 60*270 3.0 × 107
13steel15 22*57 80 240* 2.1 × 107*
14Comparative 5* 11* 51* 50*380>5.0 × 107
15steel337172139 240* 1.2 × 107*
16186658 67*430>5.0 × 107
17176057 89 260* 2.9 × 107*
18 14*5158100 260* 2.9 × 107*
1916 24* 55* 78*300 4.8 × 107
20 14* 22* 54* 69*300 4.7 × 107
212854 53*107280 2.1 × 107*
222055 56* 68*360>5.0 × 107

*Values below the target value and outside the scope of claims of the present invention.

The target value of the present invention in Table B2 is (1) percentage final swaging: not less than 57% (not less than the value specified in JIS S 48 C), (2) Number of bores formed by drilling: not less than 80 (not less than the value specified in JIS S 48 C), (3) rotating bending fatigue test: not less than 270 MPa (not less than the value specified in JIS S 53 C), and (4) L10 service life as rolling fatigue service life: not less than 3.0×107 (not less than the value specified in JIS S 53 C).

As shown in Table B2, the steels of the present invention satisfied all the target values for the percentage final swaging, the number of bores formed by drilling, the fatigue strength, and the L10 service life, whereas both the conventional steels and the comparative steels did not satisfy the target value for any one of the percentage final swaging, the number of bores formed by drilling, the fatigue strength, and the L10 service life.