Title:
C+N AUSTENITIC STAINLESS STEEL HAVING GOOD LOW-TEMPERATURE TOUGHNESS AND A FABRICATION METHOD THEREOF
Kind Code:
A1


Abstract:
A C+N austenitic stainless steel includes 15 to 20% by weight of chromium (Cr), 8 to 12% by weight of manganese (Mn), 3% or less by weight of nickel (Ni), and 0.5 to 1.0% by weight of a total content (C+N) of carbon (C) and nitrogen (N), remainder iron (Fe), and other inevitable impurities. A ratio (C/N) of the carbon (C) to the nitrogen (N) ranges from 0.5 to 1.5. It is possible to provide austenitic stainless steel having excellent low-temperature toughness while satisfying the requirements of strength, ductility, and pitting corrosion resistance and minimizing the nickel content



Inventors:
Hwang, Byoung Chul (Gyeongsangnam-do, KR)
Lee, Tae Ho (Gyeongsangnam-do, KR)
Application Number:
13/707699
Publication Date:
06/13/2013
Filing Date:
12/07/2012
Assignee:
Korea Institute of Machinery & Materials (Daejeon, KR)
Primary Class:
Other Classes:
164/55.1
International Classes:
C22C38/58; B22D25/06; C22C38/00
View Patent Images:



Foreign References:
JP2011190513A2011-09-29
Other References:
JP 2011-190513 machine translation
"Impurities in Steel." CODECOGS® http://www.codecogs.com/library/engineering/metallurgy/impurities-in-steel.php. Accessed September 2, 2015.
Primary Examiner:
HILL, STEPHANI A
Attorney, Agent or Firm:
HOLLAND & KNIGHT LLP (BOSTON, MA, US)
Claims:
What is claimed is:

1. A C+N austenitic stainless steel comprising: 15 to 20% by weight of chromium (Cr); 8 to 12% by weight of manganese (Mn); 3% or less by weight of nickel (Ni); and 0.5 to 1.0% by weight of a total content (C+N) of carbon (C) and nitrogen (N); remainder iron (Fe); and other inevitable impurities, wherein a ratio (C/N) of the carbon (C) to the nitrogen (N) ranges from 0.5 to 1.5.

2. The C+N austenitic stainless steel of claim 1, wherein the ratio (C/N) of the carbon (C) to the nitrogen (N) ranges from 0.5 to 1.0.

3. A fabrication method of C+N austenitic stainless steel, comprising: a master alloy charging step of charging a master alloy into a vacuum melting furnace; a vacuum maintaining step of maintaining a vacuum state of the vacuum melting furnace into which the master alloy is charged; a master alloy melting step of melting the master alloy by heating the vacuum melting furnace; a nitrogen content adjusting step of injecting a nitrogen gas into the vacuum melting furnace; a molten ally stirring step of stirring the molten master alloy; an ingot forming step of forming an ingot by tapping the molten alloy stirred in the vacuum melting furnace; a hot rolling step of hot-rolling the formed ingot; and a cooling step of cooling the hot-rolled stainless steel, wherein the C+N austenitic stainless steel includes 15 to 20% by weight of chromium (Cr); 8 to 12% by weight of manganese (Mn); 3% or less by weight of nickel (Ni); and 0.5 to 1.0% by weight of a total content (C+N) of carbon (C) and nitrogen (N); remainder iron (Fe); and other inevitable impurities, and a ratio (C/N) of the carbon (C) to the nitrogen (N) ranges from 0.5 to 1.5.

4. The fabrication method of claim 3, wherein the ratio (C/N) of the carbon (C) to the nitrogen (N) ranges from 0.5 to 1.0.

Description:

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority from Korean Patent Application No. 10-2011-0133822 filed on Dec. 13, 2011 in the Korean Intellectual Property Office, and all the benefits accruing therefrom under 35 U.S.C. 119, the contents of which in its entirety are herein incorporated by reference.

BACKGROUND

1. Technical Field

The present inventive concept relates to a C+N austenitic stainless steel having good low-temperature toughness and a fabrication method thereof.

2. Description of the Related Art

In general, carbon steel has an excellent strength and ductility through thermomechanical treatment and phase transformation using various thermomechanical treatment processes.

However, unlike the carbon steel, austenitic stainless steel is difficult to expect the improvement of characteristics by thermal treatment. Thus, the improvement of the characteristics of austenitic stainless steel mainly depends on addition of alloying elements.

The existing austenitic stainless steel reported by the study or invention contains 16˜20 wt % chromium (Cr), 6˜12 wt % nickel (Ni), 0˜2 wt % molybdenum (Mo) and 0.03˜0.15 wt % carbon (C), thereby implementing the mechanical properties including a tensile strength of 500˜600 MPa, and elongation of 40%.

In this case, nickel (Ni) among the alloying elements is an efficient austenite stabilizing element, and also has an advantage of contributing to the improvement of workability. Accordingly, 65% or more of the total amount of supply and demand of nickel (Ni) is used as an alloying element of austenitic stainless steel.

However, the price of nickel (Ni) has risen by 700% or more in six years since 2001, and particularly, has soared by more than twice for one year in 2007, and acts as a key indicator to price the stainless steel.

Further, the nickel (Ni) causes human allergy in addition to the above-mentioned economical aspect and emits harmful gases during recycling, thereby causing a problem against human health and environment affinity.

Accordingly, as new stainless steel which has been developed recently in order to solve several problems of the existing stainless steel having a high content of nickel (Ni), there is high nitrogen stainless steel whose characteristics have been improved by actively utilizing advantages of nitrogen serving as an alloying element and a Fe—Cr—Mn-based alloy known as a STS-200-based alloy.

The nitrogen is a strong austenite stabilizing element and has several advantages that a solid-solution strengthening effect is large, a reduction in ductility due to an increase in strength is small, and corrosion resistance including pitting corrosion resistance is improved. Conventionally, the development of high nitrogen steel has not been conducted actively because of difficulty in a fabrication process to stably ensure nitrogen in a steel material. However, recently, a lot of research and development have been underway by virtue of the development of a variety of fabricating process technologies such as solid-phase nitridation, powder metallurgy method, pressurized electroslag remelting (PESR) and pressurized induction melting under a nitrogen atmosphere.

However, the biggest obstacle to commoditizing the high nitrogen steel is that the equipment is expensive, and a special fabricating process such as PESR or pressurized induction melting that requires complicated fabricating steps needs to be performed.

More specifically, in the case of pressurized process, there is an advantage of minimizing a delta ferrite region in which the nitrogen solid solubility is drastically reduced during solidification while ensuring a high nitrogen content in a liquid state. Accordingly, the pressurized process is necessary in the fabrication of a large ingot of high nitrogen steel. However, several problems have been raised in the commercialization because the fabricating process equipment which has been used in the fabrication of the existing stainless steel should be modified or the introduction of new facilities is inevitable.

In order to solve such problems, recently, H. Berns group has filed International Patent PCT/EP/008960 which discloses austenitic stainless steel using a minimum amount of nickel (Ni), and containing 16˜21 wt % chromium (Cr), 16˜21 wt % manganese (Mn), 0.5˜2 wt % molybdenum (Mo), and total carbon (C) and nitrogen (N) of 0.8 wt % or more. However, since it also has a high percentage of manganese (Mn), there is a problem such that the pitting corrosion resistance is low.

Meanwhile, as the use environment of steels for shipbuilding, offshore structures, or line pipes in which stainless steel is mainly used becomes increasingly harsh, there is a growing demand for excellent low-temperature toughness in order to ensure the structural safety even at low temperatures.

However, it is necessary to develop stainless steel having excellent low-temperature toughness characteristics at the same time while satisfying such strength, ductility, and pitting corrosion resistance requirements. Therefore, currently, in the field of austenitic stainless steel, it is important to ensure excellent characteristics such as strength, ductility, corrosion resistance and low-temperature toughness. Further, it is an important technical challenge in the alloy development to offer an advantage in terms of manufacturing costs by minimizing the addition of expensive alloying elements or replacing them with other elements.

SUMMARY

In view of the above, the present invention provides stainless steel having excellent low-temperature toughness while satisfying the requirements of strength, ductility, and pitting corrosion resistance.

The present invention also provides economically superior stainless steel while ensuring excellent characteristics such as strength, ductility, corrosion resistance and low-temperature toughness.

The objects of the present invention are not limited thereto, and the other objects of the present invention will be described in or be apparent from the following description of the embodiments.

According to an aspect of the present invention, there is provided a C+N austenitic stainless steel comprising: 15 to 20% by weight of chromium (Cr); 8 to 12% by weight of manganese (Mn); 3% or less by weight of nickel (Ni); and 0.5 to 1.0% by weight of a total content (C+N) of carbon (C) and nitrogen (N); remainder iron (Fe); and other inevitable impurities, wherein a ratio (C/N) of the carbon (C) to the nitrogen (N) ranges from 0.5 to 1.5.

In the C+N austenitic stainless steel, the ratio (C/N) of the carbon (C) to the nitrogen (N) ranges from 0.5 to 1.0.

According to another aspect of the present invention, there is provided a fabrication method of C+N austenitic stainless steel, comprising: a master alloy charging step of charging a master alloy into a vacuum melting furnace; a vacuum maintaining step of maintaining a vacuum state of the vacuum melting furnace into which the master alloy is charged; a master alloy melting step of melting the master alloy by heating the vacuum melting furnace; a nitrogen content adjusting step of injecting a nitrogen gas into the vacuum melting furnace; a molten ally stirring step of stirring the molten master alloy; an ingot forming step of forming an ingot by tapping the molten alloy stirred in the vacuum melting furnace; a hot rolling step of hot-rolling the formed ingot; and a cooling step of cooling the hot-rolled stainless steel, wherein the C+N austenitic stainless steel includes 15 to 20% by weight of chromium (Cr); 8 to 12% by weight of manganese (Mn); 3% or less by weight of nickel (Ni); and 0.5 to 1.0% by weight of a total content (C+N) of carbon (C) and nitrogen (N); remainder iron (Fe); and other inevitable impurities, and a ratio (C/N) of the carbon (C) to the nitrogen (N) ranges from 0.5 to 1.5.

In the fabrication method, the ratio (C/N) of the carbon (C) to the nitrogen (N) ranges from 0.5 to 1.0.

According to the present invention, it is possible to provide austenitic stainless steel having excellent low-temperature toughness while satisfying the requirements of strength, ductility, and pitting corrosion resistance and minimizing the nickel content compared with commercial austenitic stainless steel or conventional C+N austenitic stainless steel.

In addition, according to the present invention, it is possible to provide economically superior austenitic stainless steel because it can be fabricated by a melting process at atmospheric pressure without a pressurized melting process.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects and features of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings, in which:

FIGS. 1A and 1B are flowcharts showing a fabrication method of C+N austenitic stainless steel according to the present invention;

FIG. 2 is a graph showing the impact absorbed energy according to the test temperature;

FIG. 3 illustrates SEM photographs obtained by observing the fracture surfaces of the specimens which were broken by the impact test; and

FIG. 4 shows photographs illustrating the results obtained by observing the distribution of carbon by using a nano-secondary ion mass spectrometry (Nano-SIMS).

DETAILED DESCRIPTION OF THE EMBODIMENTS

Advantages and features of the present invention and methods of accomplishing the same may be understood more readily by reference to the following detailed description of exemplary embodiments and the accompanying drawings. The present invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete and will fully convey the concept of the invention to those skilled in the art, and the present invention will only be defined by the appended claims

The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. Like numbers refer to like elements throughout. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items.

It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another element. Thus, for example, a first element, a first component or a first section discussed below could be termed a second element, a second component or a second section without departing from the teachings of the present invention.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It is noted that the use of any and all examples, or exemplary terms provided herein is intended merely to better illuminate the invention and is not a limitation on the scope of the invention unless otherwise specified. Further, unless defined otherwise, all terms defined in generally used dictionaries may not be overly interpreted.

Spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper” and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, the exemplary term “below” can encompass both an orientation of above and below. The device may be otherwise oriented and the spatially relative descriptors used herein interpreted accordingly.

Hereinafter, an embodiment of the present invention will be described in detail with reference to the accompanying drawings.

First, C+N austenitic stainless steel according to the present invention includes 15 to 20% by weight of chromium (Cr), 8 to 12% by weight of manganese (Mn), 3% or less by weight of nickel (Ni), 0.5 to 1.0% by weight of the total content (C+N) of carbon (C) and nitrogen (N), and remainder iron (Fe) and other inevitable impurities. In this case, a ratio (C/N) of the carbon (C) to the nitrogen (N) is 0.5 to 1.5. Further, more preferably, ratio (C/N) of the carbon (C) to the nitrogen (N) is 0.5 to 1.0.

Hereinafter, a configuration of the C+N austenitic stainless steel according to the present invention will be described in more detail.

First, the chromium (Cr) is an essential alloying element to ensure the corrosion resistance required for stainless steel, and 15% or more by weight of chromium (Cr) is added to most of austenitic stainless steels. However, if the chromium (Cr) is excessively added, excessive delta ferrite may remain after solidification, or production of various types of harmful second precipitate may be promoted in the thermal treatment, thereby reducing the workability and corrosion resistance of the stainless steel. Accordingly, it is preferable that the content of chromium (Cr) in the stainless steel is 15 to 20 wt %.

Next, the manganese (Mn) is an austenite stabilizing element which can replace expensive nickel (Ni), and is added to the stainless steel to increase the nitrogen solid solubility and increase the strength of the material. However, if the manganese (Mn) is excessively added, it is combined with sulfur (S) or oxygen (O) that is an impurity element to form a non-metallic inclusion such as manganese sulfide (MnS) or manganese oxide (MnO). In this case, the generated non-metallic inclusion serves as a main pitting corrosion generating source to reduce the pitting corrosion resistance of the austenitic stainless steel. Accordingly, it is preferable that the content of manganese (Mn) ranges from 8 to 12 wt %.

In other words, the content of the manganese is lower than the content (16 to 21 wt %) of manganese included in the conventional stainless steel disclosed in PCT/EP/008960 filed by H. Berns group. Thus, it is possible to improve the pitting corrosion resistance of the stainless steel according to the present invention.

Next, the nickel (Ni) has a high austenite stabilization function, but it is expensive and harmful to the environment and human body. Accordingly, it is desirable to minimize the amount of nickel added.

However, if a small amount of nickel (Ni) is added to the austenitic stainless steel, it is possible to improve hot and cold workability and suppress the formation of delta ferrite in the solidification from the liquid phase. Thus, it is preferable to add a small amount of nickel (Ni). However, as mentioned above, since it is desirable to minimize the content of nickel, and it is preferable that the content of nickel (Ni) added is limited to be equal to less than 3 wt %.

Next, the nitrogen (N) is an austenite stabilizing element together with carbon (C) and manganese (Mn), and is added to replace the nickel (Ni) having the above-mentioned problems. Further, the nitrogen (N) is an element to increase the corrosion resistance including the pitting corrosion resistance and increase the strength without a large decrease in ductility. For this effect, it is preferable that 0.3% by weight of nitrogen is used.

However, if nitrogen (N) is excessively added, there is a problem of causing the brittleness as well as reducing the ductility. Accordingly, as will be described later, it is preferable to limit the total content (C+N) of carbon (C) and nitrogen (N).

Next, the carbon (C) is added for austenite stabilization similarly to the nitrogen (N), and serves to improve the strength of the stainless steel through the solid-solution strengthening effect. However, if the carbon (C) is excessively added, the mechanical characteristics (typically toughness) are degraded, and carbides such as M23C6 and M6C are created at the grain boundaries to promote the sensitization of the austenitic stainless steel, thereby resulting in a decrease in corrosion resistance.

In this case, it is preferable that the total content (C+N) of carbon (C) and nitrogen (N) contained in the C+N austenitic stainless steel of the present invention ranges from 0.5 to 1.0 wt %.

That is, it is known that the nitrogen (N) is added as an alloying element to increase the free electron density of the austenitic base, and promote the metallic bonding, thereby increasing the short-range ordering in the austenitic base.

Because of the uniqueness of the atomic bonding occurring in the addition of nitrogen, the formation of harmful second phase due to segregation of the alloying element is suppressed, and the ductility and the corrosion resistance are improved. In other words, it can be said based on the physical grounds that the addition of nitrogen (N) improves the entire characteristics of the steel due to an increase in the concentration of free electrons.

The addition of carbon (C) in the content of the similar interstitial element does not have a significant impact on the concentration of free electrons of the steel. On the other hand, the nitrogen (N) effectively increases the concentration of free electrons in a predetermined content range.

However, if carbon (C) and nitrogen (N) are compositely added, the concentration of free electrons largely increases compared to a case of adding only nitrogen (N) due to a synergistic effect of the two elements. The concentration of free electrons has a maximum value when the total content (C+N) of carbon (C) and nitrogen (N) is 0.85 wt %, and is reduced again.

Thus, in the present invention, based on the above-described physical grounds and in order to prevent the formation of harmful second precipitate generated in the case of excessively adding carbon (C) and nitrogen (N), it is preferable that the total content (C+N) of carbon (C) and nitrogen (N) added as alloying elements is limited to 0.5 to 1.0 wt %.

Further, in the present invention, the ratio (C/N) of the carbon (C) to the nitrogen (N) is 0.5 to 1.5. More preferably, the ratio (C/N) of the carbon (C) to the nitrogen (N) is 0.5 to 1.0.

If the ratio (C/N) of the carbon (C) to the nitrogen (N) is out of the range of 0.5 to 1.5, the ductile-to-brittle transition temperature is high, and the low-temperature toughness is not excellent. Further, if the ratio (C/N) of the carbon (C) to the nitrogen (N) is 0.5 to 1.0, the ductile-to-brittle transition temperature is low, and the low-temperature toughness is excellent.

Hereinafter, a fabrication method of C+N austenitic stainless steel according to the present invention will be described in more detail.

FIGS. 1A and 1B are flowcharts showing a fabrication method of C+N austenitic stainless steel according to the present invention.

First, referring to FIG. 1A, a fabrication method of C+N austenitic stainless steel according to the present invention includes a master alloy charging step (S100) of charging a master alloy containing electrolytic iron, Fe-50% Mn, Fe-60% Cr, Fe-58.8% Cr-6.6% N, and 75.1% Mn-17.4% Fe-6.8% C into a vacuum melting furnace, a vacuum maintaining step (S200) of maintaining a vacuum state of the vacuum melting furnace into which the master alloy is charged, a master alloy melting step (S300) of melting the master alloy by heating the vacuum melting furnace, a nitrogen content adjusting step (S400) of injecting a nitrogen gas into the vacuum melting furnace, a molten ally stirring step (S500) of stirring the molten master alloy, an ingot forming step (S600) of forming an ingot by tapping the molten alloy stirred in the vacuum melting furnace, a hot rolling step (S700) of hot-rolling the formed ingot, and a cooling step (S800) of cooling the hot-rolled stainless steel to suppress the precipitation of carbide harmful to the mechanical properties and corrosion resistance.

Further, referring to FIG. 1B, the nitrogen content adjusting step may include a nitrogen injection process (S420) and a pressure adjustment process (S440).

In this case, the vacuum maintaining step may be a process of allowing the inside of the vacuum melting furnace to have a vacuum level of 10−3 torr or less. The nitrogen content adjusting step may include a nitrogen injection process of injecting a nitrogen gas into the vacuum melting furnace, and a pressure adjustment process of adjusting a partial pressure of nitrogen in the vacuum melting furnace to 1 atm.

According to the present invention configured as described above, there is an advantage that it can be variously applied to a fabrication process of an austenitic stainless steel casting, forging, or rolling material having a high strength and corrosion resistance with low fabrication and raw material costs.

Hereinafter, the present invention will be described in detail through examples and comparative examples. However, the following examples and comparative examples are merely intended to illustrate the present invention, and the embodiments of the present invention are not limited by the following examples.

Examples 1 and 2, and Comparative Examples 1 and 2

In the fabrication of the austenitic stainless steel according to Examples 1 and 2, and Comparative Examples 1 and 2, for chromium (Cr) which has a high melting point and is difficult to dissolve, Fe-60% Cr master alloy was used, and for manganese (Mn) which as a low vapor pressure and may cause fume generation and segregation during dissolution, Fe-50% Mn master alloy was used.

As shown in FIGS. 1A and 1B, according to the composition of Table 1 below, the Fe-50% Mn, Fe-60% Cr, electrolytic iron, Fe-58.8% Cr-6.6% N master alloy for controlling the nitrogen content, 75.1% Mn-17.4% Fe-6.8% C master alloy for controlling the carbon content were selectively charged into the vacuum melting furnace (S100). Then, after deaeration was performed until the inside of the vacuum melting furnace reached to a vacuum level of 10−3 torr or less, while maintaining the vacuum (S200), the vacuum melting furnace was heated such that the master alloy and electrolytic iron charged in the vacuum melting furnace were melted (S300). When the master alloy and electrolytic iron were melted, a nitrogen gas was injected into the vacuum melting furnace (S420), and the pressure was adjusted such that a partial pressure of nitrogen in the vacuum melting furnace was 1 atm (the partial pressure of nitrogen was 2.5 atm in the case of Comparative Example 1) (S440) to adjust the nitrogen content (S400). Then, in order to remove segregation of the alloying elements through electromagnetic induction stirring, molten alloy was stirred (S500), and when the temperature of molten metal formed by melting the electrolytic iron and master alloy in the molten ally stirring step (S500) was 1,450, an ingot was formed by tapping it from the vacuum melting furnace (S600). The formed ingot was fabricated into a plate, tube, rod, wire or the like by hot rolling (S700), and a cooling process was performed to suppress the precipitation of carbide harmful to the mechanical properties and corrosion resistance (S800).

TABLE 1
nitrogen
N +partial
CrMnNiNCCC/Npressure
Example18.3410.082.100.340.230.570.681
1(A)
Example18.389.982.110.340.430.771.261
2(B)
Comparative18.5710.012.050.530.020.550.042.5
Example
1(C)
Comparative18.359.982.100.290.660.952.281
Example
2(D)

The materials according to the composition of Table 1 above are four types of Fe-18Cr-10Mn-2Ni alloys having different carbon contents. After preparation of the alloys, the materials were hot rolled to be 12 mm thick. After setting an appropriate temperature based on the equilibrium state diagram obtained from the thermodynamic calculation results in order to obtain a uniform single-phase austenite structure, a solution treatment was performed at a temperature of 1100 to 1230° C. for 30 minutes. Specimen C is high-nitrogen steel (HNS) to which only nitrogen is added, and each of Specimens A, B and D is high-interstitial alloy (HIA) to which both nitrogen and carbon are added.

The following tests were conducted using Specimens A, B, C and D.

First, a tensile test was carried out at a crosshead speed of 2 mm/min and at room temperature by using a 10-ton capacity Instron testing machine after processing the specimens into sub-size and plate-shaped specimens in accordance with ASTM E8 test method.

An impact test was carried out within a temperature range from −196° C. to +100° C. after processing the specimens into Charpy V-notch (CVN) impact specimens of 10×10×55 mm in accordance with ASTM E23 test method. A ductile-to-brittle transition temperature (DBTT) was determined as a temperature having energy corresponding to an average of upper-shelf energy and lower-shelf energy through hyperbolic tangent fitting.

The experimental results are shown in Table 2 below, and the impact absorbed energy according to the test temperature is shown in FIG. 2.

TABLE 2
Tensile CharacteristicsImpact Characteristics
Yield StrengthTensile StrengthElongationUpper-shelf Energy
(MPa)(MPa)(%)(J)DBTT (° C.)
Example 1(A)44086470317−121
Example 2(B)45289367321−96
Comparative46283267285−77
Example 1(C)
Comparative43290265290−64
Example 2(D)

As the analysis results of the microstructure obtained after solution treatment, it was confirmed that all specimens have a single-phase austenite structure.

Further, referring to FIG. 2, the impact absorbed energy of all the specimens exhibits a ductile-to-brittle transition behavior to be markedly reduced in a certain temperature interval as the test temperature decreases.

Meanwhile, referring to Table 2, the yield strength ranges from 430 to 460 MPa, and the percentage of elongation ranges from 65 to 70%, which are almost the same, but the tensile strength slightly increases in proportion to the amount of interstitial elements. Further, the upper-shelf energy of HIA is higher than that of HNS, but generally decreases as the carbon content increases.

In this case, it can be seen from Example 1 and Comparative Example 1 that the DBTT of HIA is lower by about 45° C. than that of HNS at the same content of interstitial elements (Example 1: 0.57, Comparative Example 1: 0.55).

In other words, the C/N ratio of Example 1 is 0.68, and the C/N ratio of Comparative Example 1 is 0.04. Accordingly, it can be seen that the DBTT is significantly influenced by the C/N ratio as well as the content of interstitial elements.

Further, referring to Example 1 and Example 2, the C/N ratio of Example 1 is 0.68, and the C/N ratio of Example 2 is 1.26. Referring to the DBTT of Examples 1 and 2, it can be seen that the DBTT of Example 1 is −121° C., and the DBTT of Example 2 is −96° C.

In other words, it can be seen that when the C/N ratio becomes equal to or greater than a predetermined value, the DBTT characteristics are degraded. Thus, the present invention is characterized in that a ratio (C/N) of the carbon (C) to the nitrogen (N) ranges from 0.5 to 1.5. Further, more preferably, it is characterized in that the ratio (C/N) of the carbon (C) to the nitrogen (N) ranges from 0.5 to 1.0.

FIG. 3 illustrates SEM photographs obtained by observing the fracture surfaces of the specimens which were broken by the impact test.

Referring to FIG. 3, Specimens A and C have a transgranular brittle fracture surface in a facet form along with a ductile fracture region including dimples, but Specimen D has a typical intergranular fracture surface. The transgranular brittle fracture surface shown in Specimens A and C is known as an activated {111} slip plane, and slip lines intersect each other thereon.

It was found that HIA has a DBTT lower than that of HNS and absorbed energy higher than that of HNS because it has relatively many ductile fracture regions at the same contents of interstitial elements.

On the other hand, it was found that Specimen D having a high content of carbon in the HIA has absorbed energy lower than that of Specimen A due to intergranular fracture, and DBTT which increases as the carbon content increases.

FIG. 4 shows photographs illustrating the results obtained by observing the distribution of carbon by using a nano-secondary ion mass spectrometry (Nano-SIMS).

Referring to the results obtained by observing the distribution of carbon by using a nano-secondary ion mass spectrometry (Nano-SIMS, model: Nano-SIMS 50, CAMECA), carbon segregation frequently occurred at austenite grain boundaries in Specimen D having a relatively high content of carbon, while carbon segregation hardly occurred in Specimen A. This means that in the case of HIA having a relatively high carbon content, carbon segregation occurs at austenite grain boundaries, which may affect the intergranular fracture.

According to the present invention as described above, it is possible to provide austenitic stainless steel having excellent low-temperature toughness while satisfying the requirements of strength, ductility, and pitting corrosion resistance and minimizing the nickel content compared with commercial austenitic stainless steel or conventional C+N austenitic stainless steel.

In addition, it is possible to provide economically superior austenitic stainless steel because it can be fabricated by a melting process at atmospheric pressure without a pressurized melting process.

In concluding the detailed description, those skilled in the art will appreciate that many variations and modifications can be made to the preferred embodiments without substantially departing from the principles of the present invention. Therefore, the disclosed preferred embodiments of the invention are used in a generic and descriptive sense only and not for purposes of limitation.