EP4177369A1

EP4177369A1 - Austenitic stainless steel and manufacturing method thereof

Info

Publication number: EP4177369A1
Application number: EP21864595.0A
Authority: EP
Inventors: Sangseok KIM; Mi-Nam Park
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2020-09-03
Filing date: 2021-08-26
Publication date: 2023-05-10
Also published as: CN116075600A; KR20220030722A; EP4177369A4; JP7603147B2; JP2023538602A; WO2022050635A1; CN116075600B; US20230279531A1; KR102448735B1

Abstract

Disclosed are an austenitic stainless steel which may be utilized as various materials for an outer panel of a vehicle, a component for construction and the like and a manufacturing method thereof.According to an embodiment, the austenitic stainless steel includes, in percent by weight (wt%), 0.005 to 0.03% of C, 0.1 to 1% of Si, 0.1 to 2% of Mn, 6 to 9% of Ni, 16 to 19% of Cr, 0.2% or less of N, and the remainder being Fe and unavoidable impurities. Assuming that a total thickness of the steel material is t, a value of an average grain size d in a thickness range of 1/4t to 3/4t is 5 µm or less, an ASP value represented by Formula (1) below is from 10 to 25, a value represented by Formula (2) below is 435 or more, and a value represented by Formula (3) below is 6000 or more. 551−462*C+N−9.2*Si−8.1*Mn−13.7*Cr−29*Ni+Cu−18.5*Mo−68*Nb+V1600*N+700/d+4*ASP−20*Ni+100YS*EL−500*Ni+CrIn Formulae (1), (2), and (3), [C], [N], [Si], [Mn], [Cr], [Ni], [Cu], [Mo], [Nb], and [V] represent weight percentages (wt%) of respective elements, YS represents yield strength (MPa), and EL represents elongation (%).

Description

[Technical Field]

The present disclosure relates to an austenitic stainless steel which may be utilized as various materials for an outer panel of a vehicle, a component for construction, and the like and a manufacturing method thereof.

[Background Art]

Austenitic stainless steels have been applied for various uses to manufacture components for transportation and construction due to excellent formability, work hardenability, and weldability. However, 304 series stainless steels or 301 series stainless steels, which are representative general-use austenitic stainless steels, have low yield strengths of 200 to 350 MPa, and thus there are limits to apply these stainless steels to structural materials due to low strength.
Although a skin pass rolling process may additionally be performed as a method for increasing yield strength of austenitic stainless steels, this method causes problems in increasing manufacturing costs due to the additional process and significantly deteriorating elongation of materials.
Patent Document 1 discloses a method for manufacturing a 300 series stainless steel having a small curvature even after half etching by performing stress relief (SR) heat treatment twice after skin pass rolling a cold-rolled, annealed material for a laser metal mask for photoetching. However, Patent Document 1 relates to a manufacturing method to control etchability and curvature after etching. Because an austenitic stability parameter (ASP) value is from 30 to 50, strain-induced martensite transformation rapidly occurs during formation, resulting in deterioration of elongation.
Patent Document 2 relates a nuclear power component. In order to manufacture an austenitic stainless steel having an average grain size of 10 µm or less, heat treatment was performed for a long time over 48 hours at a temperature of 600 to 700°C. According to Patent Document 2, heat treatment is required for a long time, productivity decreases in the case of being implemented in a real production line, and manufacturing costs increase.

(Patent Document 1) International Patent Application Publication No. WO2016-043125A1 (March 14, 2016 )
(Patent Document 2) Japanese Patent Application Laid-Open No. JP2020-50940A (April 2, 2020 )

[Disclosure]

[Technical Problem]

To solve the foregoing problems, provided is an ultrafine-grained 300 series stainless steel having simultaneously satisfying high yield strength and excellent elongation.

[Technical Solution]

In accordance with an aspect of the present disclosure, an austenitic stainless steel includes, in percent by weight (wt%), 0.005 to 0.03% of C, 0.1 to 1% of Si, 0.1 to 2% of Mn, 6 to 9% of Ni, 16 to 19% of Cr, 0.2% or less of N, and the remainder being Fe and unavoidable impurities, wherein assuming that a total thickness of the steel material is t, a value of an average grain size d in a thickness range of 1/4t to 3/4t is 5 µm or less, an ASP value represented by Formula (1) below is from 10 to 25, a value represented by Formula (2) below is 435 or more, and a value represented by Formula (3) below is 6000 or more. $\begin{array}{l} 551 - 462 * ([C] + [N]) - 9.2 * [Si] - 8.1 * [Mn] - 13.7 * [Cr] - \\ 29 * ([Ni] + [Cu]) - 18.5 * [Mo] - 68 * ([Nb] + [V]) \end{array}$

$(1600 * [N]) + (700 / \sqrt{d}) + (4 * ASP) - (20 * [Ni]) + 100$

$YS * EL - 500 * ([Ni] + [Cr])$
In Formulae (1), (2), and (3), [C], [N], [Si], [Mn], [Cr], [Ni], [Cu], [Mo], [Nb], and [V] represent weight percentages (wt%) of respective elements, YS represents a yield strength (MPa), and EL represents an elongation (%).
In addition, in the austenitic stainless steel according to the present disclosure, a value of Formula (4) below may be 200 or more.
$Hv - ([Ni] + [Cr])$
In Formula (4), Hv represents a Vickers hardness (Hv) and [Ni] and [Cr] represent weight percentages (wt%) of respective elements.
In addition, the austenitic stainless steel according to the present disclosure may further include, in percent by weight (wt%), at least one of 0.4% or less of Cu, 0.2% or less of Mo, 0.25% or less of Nb, and 0.25% or less of V.
In addition, in the austenitic stainless steel according to the present disclosure, the t may be from 0.4 to 2.0 mm.
In addition, in the austenitic stainless steel according to the present disclosure, a pitting potential, measured by dipping in a 3.5% NaCl solution at 30°C, may be 250 mV or more.
In accordance with another aspect of the present disclosure, a method of manufacturing the austenitic stainless steel includes: hot rolling a slab comprising, in percent by weight (wt%), 0.005 to 0.03% of C, 0.1 to 1% of Si, 0.1 to 2% of Mn, 6 to 9% of Ni, 16 to 19% of Cr, 0.2% or less of N, and the remainder being Fe and unavoidable impurities, and cold rolling the hot-rolled steel material at room temperature with a reduction ratio of 40% or more; and annealing the cold-rolled steel material at a temperature of 700 to 850°C.
In addition, in the method of manufacturing the austenitic stainless steel according to the present disclosure, the slab may be hot-rolled and then cold-rolled without being annealed.
In addition, the method of manufacturing the austenitic stainless steel according to the present disclosure may further include skin pass rolling with a reduction ratio of 60% or more.

[Advantageous Effects]

The present disclosure may provide an ultrafine-grained 300 series stainless steel simultaneously satisfying high strength and high ductility.
The present disclosure provides an austenitic stainless steel simultaneously satisfying high strength and high ductility with high economic feasibility by reducing the contents of Ni and Cr which are high-priced alloying elements.
The present disclosure provides an austenitic stainless steel simultaneously satisfying high strength and high ductility with excellent corrosion resistance.

[Description of Drawings]

FIG. 1 is a graph showing ranges of values of Formulae (2) and (3) of inventive examples and comparative examples.
FIGS. 2A to 2C are views for comparison of average grain size d among the inventive examples and comparative examples.
FIGS. 3A to 3C are graphs showing Strain (%)-Stress (MPa) curves of inventive examples.
FIGS. 4A to 4C are graphs showing Strain (%)-Stress (MPa) curves of comparative examples.

[Best Mode]

An austenitic stainless steel according to an embodiment of the present disclosure includes, in percent by weight (wt%), 0.005 to 0.03% of C, 0.1 to 1% of Si, 0.1 to 2% of Mn, 6 to 9% of Ni, 16 to 19% of Cr, 0.2% or less of N, and the remainder being Fe and unavoidable impurities, wherein assuming that a total thickness of the steel material is t, a value of an average grain size d in a thickness range of 1/4t to 3/4t is 5 µm or less, an ASP value represented by Formula (1) below is from 10 to 25, a value represented by Formula (2) below is 435 or more, and a value represented by Formula (3) below is 6000 or more. $\begin{array}{l} 551 - 462 * ([C] + [N]) - 9.2 * [Si] - 8.1 * [Mn] - 13.7 * [Cr] - \\ 29 * ([Ni] + [Cu]) - 18.5 * [Mo] - 68 * ([Nb] + [V]) \end{array}$

$(1600 * [N]) + (700 / \sqrt{d}) + (4 * ASP) - (20 * [Ni]) + 100$

$YS * EL - 500 * ([Ni] + [Cr])$

wherein in Formulae (1), (2), and (3), [C], [N], [Si], [Mn], [Cr], [Ni], [Cu], [Mo], [Nb], and [V] represent weight percentages (wt%) of respective elements, YS represents yield strength (MPa), and EL represents elongation (%).

[Modes of the Invention]

Hereinafter, preferred embodiments of the present disclosure will now be described. However, the present disclosure may 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 scope of the disclosure to those skilled in the art.
The terms used herein are merely used to describe embodiments. Thus, an expression used in the singular encompasses the expression of the plural, unless it has a clearly different meaning in the context. In addition, it is to be understood that the terms such as "including" or "having" are intended to indicate the existence of features, steps, functions, components, or combinations thereof disclosed in the specification, and are not intended to preclude the possibility that one or more other features, steps, functions, components, or combinations thereof may exist or may be added.
Meanwhile, unless otherwise defined, all terms used herein have the same meaning as those commonly understood by one of ordinary skill in the art to which this disclosure belongs. Thus, these terms should not be interpreted in an idealized or overly formal sense unless expressly so defined herein. As used herein, the singular forms are intended to include the plural forms as well, unless the context clearly indicates otherwise.
In addition, the terms "about", "substantially", etc. used throughout the specification mean that when a natural manufacturing and substance allowable error are suggested, such an allowable error corresponds a value or is similar to the value, and such values are intended for the sake of clear understanding of the present disclosure or to prevent an unconscious infringer from illegally using the disclosure of the present disclosure.
Although austenitic stainless steels have been used for various uses due to excellent formability, work hardenability, and weldability, yield strength thereof is low. In the case of performing skin pass rolling to increase yield strength, a problem of deteriorating elongation may occur. The present inventors have focused on an ultrafine austenitic stainless steel as a steel material simultaneously satisfying high yield strength and excellent elongation. However, not all of ultrafine stainless steels satisfy both high yield strength and excellent elongation. Ultrafine grained austenitic stainless steels may have considerably different yield strength and elongation, because the Ni content and the Cr content vary according to steel types, the amount of martensite transformation during cold working may vary according to the stability of the austenite phase, and tensile curve characteristics may vary according to the transformation induced plasticity (TRIP) transformation behavior. The present inventors have found an ultrafine grained austenitic stainless steel capable of simultaneously satisfying high strength and high ductility in consideration the above-described effects.
An austenitic stainless steel according to an embodiment of the present disclosure includes, in percent by weight (wt%), 0.005 to 0.03% of C, 0.1 to 1% of Si, 0.1 to 2% of Mn, 6 to 9% of Ni, 16 to 19% of Cr, 0.2% or less of N, and the remainder being Fe and unavoidable impurities. In addition, the austenitic stainless steel may further include at least one of 0.4% or less of Cu, 0.2% or less of Mo, 0.25% or less of Nb, and 0.25% or less of V.
Hereinafter, reasons for numerical limitations on the contents of alloying elements in the embodiment of the present disclosure will be described.
The content of carbon (C) is from 0.005 to 0.03 wt%.
C is an austenite phase-stabilizing element. In consideration thereof, C is added in an amount of 0.005 wt% or more. However, since an excess of C causes a problem of forming a chromium carbide during low-temperature annealing to deteriorate grain boundary corrosion resistance, the C content is controlled to 0.03 wt% or less in the present disclosure.
The content of silicon (Si) is from 0.1 to 1 wt%.
Si is an element added as a deoxidizer during a steel-making process and has an effect on improving corrosion resistance of a steel by forming an Si oxide in a passivated layer in the case of performing a bright annealing process. In consideration thereof, Si is added in an amount of 0.1 wt% or more in the present disclosure. However, since an excess of Si causes a problem of deteriorating ductility, the Si content is controlled to 1.0 wt% or less in the present disclosure.
The content of manganese (Mn) is from 0.1 to 1.0 wt%.
Mn is an austenite phase-stabilizing element. In consideration thereof, Mn is added in an amount of 0.1 wt% or more in the present disclosure. However, since an excess of Mn causes a problem of deteriorating corrosion resistance, the Mn content is controlled to 1.0 wt% or less in the present disclosure.
The content of nickel (Ni) is from 6.0 to 9.0 wt%.
Ni, as an austenite phase-stabilizing element, has an effect on softening a steel material. In consideration thereof, Ni is added in an amount of 6.0 wt% or more in the present disclosure. However, since an excess of Ni causes a problem of increasing costs, the Ni content is controlled to 9.0 wt% or less in the present disclosure.
The content of chromium (Cr) is from 16.0 to 19.0 wt%.
Cr is a major element for improving corrosion resistance of a stainless steel. In consideration thereof, Cr is added in an amount of 16.0 wt% or more in the present disclosure. However, since an excess of Cr causes problems of hardening of a steel material and inhibiting strain-induced martensite transformation during cold rolling, the Cr content is controlled to 19.0 wt% or less in the present disclosure.
The content of nitrogen (N) is 0.2 wt% or less.
N is an austenite phase-stabilizing element and improves strength of a steel material. However, since an excess of N causes problems of hardening a steel material and deteriorating hot workability, the N content is controlled to 0.2 wt% or less in the present disclosure.
Hereinafter, reasons for numerical limitations on the contents of optional alloying elements Cu, Mo, Nb, and V will be described in detail.
The content of copper (Cu) may be from 0.4 wt% or less.
Cu is an austenite phase-stabilizing element. However, since an excess of Cu causes problems of deteriorating corrosion resistance of a steel material and increasing costs, the Cu content is controlled to 0.4 wt% or less in the present disclosure.
The content of molybdenum (Mo) may be 0.2 wt% or less.
Mo has an effect on improving corrosion resistance and workability. However, since an excess of Mo causes a problem of increasing costs, the Mo content is controlled to 0.2 wt% or less in the present disclosure.
The contents of niobium (Nb) or vanadium (V) may be 0.25 wt% or less.
Nb and V have an effect on suppressing the growth of grains by forming (Nb,V)(C,N) precipitates. However, when the contents of Nb and V are excessive, a problem of increasing manufacturing coast may occur, and thus the contents of Nb and V are controlled to 0.25 wt% or less, respectively, in the present disclosure.
The remaining component of the composition of the present disclosure is iron (Fe). However, the composition may include unintended impurities inevitably incorporated from raw materials or surrounding environments, and thus addition of other alloying elements is not excluded. These impurities are known to any person skilled in the art of manufacturing and details thereof are not specifically mentioned in the present disclosure.
As well as limiting the contents of the alloying elements of the stainless steel according to the present disclosure as described above, the relationships therebetween may further be limited as follows.
An Austenitic Stability Parameter (ASP) value represented by Formula (1) below may be from 10 to 25. $\begin{array}{l} 551 - 462 * ([C] + [N]) - 9.2 * [Si] - 8.1 * [Mn] - 13.7 * [Cr] - \\ 29 * ([Ni] + [Cu]) - 18.5 * [Mo] - 68 * ([Nb] + [V]) \end{array}$
In Formula (1) above, [C], [N], [Si], [Mn], [Cr], [Ni], [Cu], [Mo], [Nb], and [V] represent weight percentages (wt%) of respective elements. For an element not added, 0 wt% is substituted into Formula (1).
Formula (1) indicates a temperature at which 50% of austenite is transformed into martensite when a stainless steel is deformed with a true strain of 0.3 and is used as an index of stability of the austenite phase. A lower value of Formula (1) means higher stability of the austenite phase indicating a smaller amount of strain-induced martensite during transformation.
When the value of Formula (1) is less than 10, a fraction of martensite in a cold-rolled steel material decreases and a fraction of the retained austenite increases due to a low amount of TRIP transformation, which is transformation from an austenite phase into a martensite phase by cold rolling. As the amount of strain-induced martensite decreases, a ratio of reverted austenite by low-temperature annealing decreases, and the fraction of the retained austenite phase without being transformed into martensite increases, making it difficult to obtain ultrafine grains. When the value of Formula (1) exceeds 25, TRIP transformation by cold rolling is activated, but a problem of decreasing elongation occurs due to a too high TRIP transformation rate.
A value of Formula (2) below may be 435 or more.
$(1600 * [N]) + (700 / \sqrt{d}) + (4 * ASP) - (20 * [Ni]) + 100$
In Formula (2), [N] and [Ni] represent weight percentages (wt%) of respective elements. ASP means the ASP value of Formula (1). Assuming that a total thickness of the steel material is t, d indicates an average grain size in a thickness range of 1/4t to 3/4t.
According to an embodiment of the present disclosure, t may be from 0.4 to 2.0 mm. A material having a thickness of 0.4 to 2.0 mm has been widely applied to structural components. According to the present disclosure, an austenitic stainless steel having high strength and high ductility may be provided within the above-described thickness range.
According to an embodiment of the present disclosure, d may be 5 µm or less.
When d exceeds 5 µm, a yield strength decreases by Hall-Petch equation, resulting in a decrease in a YS^∗El value.
Formula (2) is a parameter to obtain high strength, derived in consideration of factors affecting strength, such as the N content, the Ni content, grain size, and austenitic stability parameter. When the value of Formula (2) is less than 435, sufficient strength cannot be obtained.
A value of Formula (3) below may be 6000 or more.
$YS * EL - 500 * ([Ni] + [Cr])$
In Formula (3), YS represents yield strength (MPa), EL represents elongation (%), and [Ni] and [Cr] represent weight percentages (wt%) of respective elements.
In Formula (3), the YS*EL value is considerably affected by the contents of Cr and Ni. For example, in the case where the contents of Cr and Ni are small, TRIP transformation easily occurs and the YS*EL value tends to increase. When the value of Formula (3) is less than 6000, high strength and high ductility cannot be satisfied simultaneously.
A value of Formula (4) below may be 200 or more.
$Hv - ([Ni] + [Cr])$
In Formula (4), Hv represents Vickers hardness (Hv) and [Ni] and [Cr] represent weight percentages (wt%) of respective elements.
In Formula (4), the Hv value is considerably affected by the contents of Cr and Ni. For example, in the case where the contents of Cr and Ni are small, TRIP transformation easily occurs, and thus an amount of strain-induced martensite transformation increases during cold deformation. As a result, hardness of a steel material increases. When the value of Formula (4) is less than 200, sufficient hardness cannot be obtained.
The austenitic stainless steel according to the present disclosure has not only high strength and high ductility but also excellent corrosion resistance. The austenitic stainless steel according to an embodiment may have a pitting potential of 250 mV or more when measured by dipping in an 3.5% NaCl solution at 30°C.
The austenitic stainless steel according to an embodiment of the present disclosure may have a tensile strength of 1750 MPa or more after skin pass rolling.
A method of manufacturing an austenitic stainless steel according to an embodiment of the present disclosure includes: hot rolling a slab including, in percent by weight (wt%), 0.005 to 0.03% of C, 0.1 to 1% of Si, 0.1 to 2% of Mn, 6 to 9% of Ni, 16 to 19% of Cr, 0.2% or less of N, and the remainder being Fe and unavoidable impurities, and cold rolling the hot-rolled steel material at room temperature with a reduction ratio of 40% or more; and annealing the cold-rolled steel material at a temperature of 700 to 850°C.
When the reduction ratio is less than 40% during cold rolling, the austenitic stainless steel of the present disclosure having an ASP value of 10 to 25 has a too low amount of TRIP transformation, resulting in a decrease in the fraction of martensite in the cold-rolled steel material and an increase in the fraction of the retained austenite phase. As the amount of the strain-induced martensite decreases, the ratio of the reverted austenite phase during the subsequent low-temperature annealing decreases, and the fraction of the retained austenite phase without being transformed into martensite increases, making it difficult to obtain ultrafine grains.
According to the present disclosure, the hot-rolled material is cold-rolled and annealed at a low temperature of 700 to 850°C. When the temperature of the low-temperature annealing is below 700°C, recrystallization from the strain-induced martensite phase into the reverted austenite phase does not occur. On the contrary, when the temperature of the low-temperature annealing is higher than 850°C, the grain size of the reverted austenite may increase, thereby decreasing the yield strength.
According to an embodiment of the present disclosure, the slab may be hot-rolled and then cold-rolled without conducting annealing.
In addition, the method may further include skin pass rolling with a reduction ratio of 60% or more to further increase strength.
Hereinafter, the present disclosure will be described in more detail through examples. However, it is necessary to note that the following examples are only intended to illustrate the present disclosure in more detail and are not intended to limit the scope of the present disclosure. This is because the scope of the present disclosure is determined by matters described in the claims and able to be reasonably inferred therefrom.

Examples

Slabs having the compositions of alloying elements shown in Table 1 below were hot-rolled and cold-rolled with a total reduction ratio of 40% or more at room temperature without conducting annealing. Subsequently, the resultant was annealed at a temperature of 700 to 850°C to prepare a cold-rolled, annealed material with a thickness of 0.4 to 2.0 mm.
In Table 1 below, ASP refers to a value obtained by substituting the contents of alloying elements of Table 1 into Formula (1) below indicating stability of an austenite phase. $\begin{array}{l} 551 - 462 * ([C] + [N]) - 9.2 * [Si] - 8.1 * [Mn] - 13.7 * [Cr] - \\ 29 * ([Ni] + [Cu]) - 18.5 * [Mo] - 68 * ([Nb] + [V]) \end{array}$

In Formula (1), [C], [N], [Si], [Mn], [Cr], [Ni], [Cu], [Mo], [Nb], and [V] represent weight percentages (wt%) of respective elements.

Table 1

	Composition of alloying element (wt%)										ASP
	C	Si	Mn	Cr	Ni	Cu	Mo	N	Nb	V	ASP
Inventive Example 1	0.023	0.53	1.24	17.5	6.4	0	0	0.17	0	0	21.6
Inventive Example 2	0.023	0.53	1.24	17.5	6.4	0	0	0.17	0	0	21.6
Inventive Example 3	0.019	0.3	0.46	17.3	6.3	0.25	0.1	0.15	0.21	0	23.3
Inventive Example 4	0.019	0.3	0.46	17.3	6.3	0.25	0.1	0.15	0.21	0	23.3
Inventive Example 5	0.017	0.32	1.79	16.7	6.85	0.25	0.1	0.15	0	0	19.9
Inventive Example 6	0.017	0.32	1.79	16.7	6.85	0.25	0.1	0.15	0	0	19.9
Inventive Example 7	0.02	0.5	1	18.1	8	0.25	0.1	0.04	0	0.1	14.7
Inventive Example 8	0.02	0.5	1	18.1	8	0.25	0.1	0.04	0	0.1	14.7
Comparative Example 1	0.023	0.53	1.24	17.5	6.4	0	0	0.17	0	0	21.6
Comparative Example 2	0.019	0.3	0.46	17.3	6.3	0.25	0.1	0.15	0.21	0	23.3
Comparative Example 3	0.017	0.32	1.79	16.7	6.85	0.25	0.1	0.15	0	0	19.9
Comparative Example 4	0.02	0.3	0.5	18.1	8.02	0.25	0.1	0.04	0.05	0	23.4
Comparative Example 5	0.02	0.5	1	18.1	8	0.25	0.1	0.04	0	0.1	14.7
Comparative Example 6	0.02	0.51	0.98	17.3	6.3	0	0	0.1	0	0	63.2
Comparative Example 7	0.02	0.51	0.98	17.3	6.3	0	0	0.1	0	0	63.2
Comparative Example 8	0.02	0.29	0.49	16.6	5.98	0.25	0.1	0.18	0	0	42.0
Comparative Example 9	0.02	0.29	0.49	16.6	5.98	0.25	0.1	0.18	0	0	42.0
Comparative Example 10	0.02	0.29	0.49	16.6	5.98	0.25	0.1	0.18	0	0	42.0
Comparative Example 11	0.022	0.31	0.29	18.2	8.09	0.25	0.1	0.02	0	0	33.3
Comparative Example 12	0.022	0.31	0.29	18.2	8.09	0.25	0.1	0.02	0	0	33.3
Comparative Example 13	0.022	0.31	0.29	18.2	8.09	0.25	0.1	0.02	0	0	33.3
Comparative Example 14	0.047	0.57	0.78	18.4	9.13	0.24	0.097	0.029	0	0	-21.3
Comparative Example 15	0.019	0.3	1.5	18.1	8.27	1.27	0.1	0.041	0	0	-18.1
Comparative Example 16	0.016	0.4	1.2	18.7	10.3	0.25	0.1	0.015	0	0	-40.7
Comparative Example 17	0.02	0.6	1.1	16.2	10.1	0.25	2.1	0.02	0	0	-42.8
Comparative Example 18	0.079	0.4	9	15.2	3.5	1.68	0	0.115	0	0	26.3

In Table 2, assuming that a total thickness of the cold-rolled, annealed material of Table 1 is t, d indicates an average grain size (µm) in a thickness range of 1/4t to 3/4t. Formula (2) of Table 2 means a value obtained by Formula (2) below.
$(1600 * [N]) + (700 / \sqrt{d}) + (4 * ASP) - (20 * [Ni]) + 100$
The value of Formula (2) was obtained by substituting the contents (wt%) of [N] and [Ni] and the ASP value of Table 1 and substituting the d value of Table 2 thereinto.
Formula (3) of Table 2 means a value obtained by Formula (3) below.
$YS * EL - 500 * ([Ni] + [Cr])$
The value of Formula (3) was obtained by substituting a yield strength (YS, MPa) and an elongation (EL, %) measured under the conditions described below and the contents (wt%) of [Ni] and [Cr] of Table 1 thereinto.
The yield strength (YS, MPa) and the elongation (EL, %) were measured after preparing a sample of the cold-rolled, annealed material according to the JIS 13B standards and conducting a tensile test at room temperature in a crosshead range of 10 mm/min to 20 mm/min.
Formula (4) of Table 2 represents a value obtained by Formula (4) below.
$Hv - ([Ni] + [Cr])$
The value of Formula (4) was derived by substituting a Vickers hardness (Hv) measured under the conditions described below and the contents (wt%) of [Ni] and [Cr] of Table 1 thereinto.
The Vickers hardness (Hv) is a value measured using a Vickers hardness tester with a load of 2 kgf.

Pitting potential (mV) of Table 2 is a value measured after dipping a cold-rolled, annealed material in a 3.5% NaCl solution at 30°C.

Table 2

Example	d (µm)	Formula (2)	Formula (3)	Formula (4)	Pitting potential (mV)
Inventive Example 1	1.2	969	19131	298	287
Inventive Example 2	4.2	672	16772	267	290
Inventive Example 3	0.5	1297	12463	365	274
Inventive Example 4	2.2	779	19048	329	280
Inventive Example 5	1.2	921	17547	271	254
Inventive Example 6	3.3	668	14699	239	257
Inventive Example 7	1	763	7243	304	291
Inventive Example 8	3.2	454	7835	232	297
Comparative Example 1	25.5	469	9123	193	307
Comparative Example 2	15.5	485	1600	226	360
Comparative Example 3	27.5	416	6689	173	257
Comparative Example 4	28.8	228	8173	138	303
Comparative Example 5	17.5	230	-3712	134	320
Comparative Example 6	3.7	751	5980	252	272
Comparative Example 7	24.5	528	-3514	174	317
Comparative Example 8	1.1	1104	4768	295	255
Comparative Example 9	3.5	811	5890	273	262
Comparative Example 10	26.7	572	735	189	311
Comparative Example 11	1.3	718	3270	217	342
Comparative Example 12	3.4	483	1215	189	325
Comparative Example 13	27.5	237	-4415	125	312
Comparative Example 14	6.5	153	2446	175	311
Comparative Example 15	7.8	178	3037	156	325
Comparative Example 16	8.2	0	-1278	165	320
Comparative Example 17	7.8	9	271	181	425
Comparative Example 18	2.9	730	8882	272	30

Referring to Tables 1 and 2, Inventive Examples 1 to 8 satisfied the ASP values of 10 to 25, the d value of 5 µm or less, the value of Formula (2) of 435 or more, and the value of Formula (3) of6000 or more, and thus high strength, high ductility, and excellent corrosion resistance were obtained. Also, referring to Table 2, in Inventive Examples 1 to 8, the values of Formula (4) were at least 200 and pitting potentials were at least 250 mV. In Comparative Examples 6 to 18, the ASP values were out of the range defined in the present disclosure. In Comparative Examples 6 to 13 and 18 in which the ASP values exceeded 25, low elongation was observed due to too high TRIP transformation rates during formation. In Comparative Examples 14 to 17 in which the ASP values were less than 10, ultrafine grains could not be obtained due to high fractions of the retained austenite phase.
In Comparative Examples 1 to 5, 7, 10, and 13 to 17, coarse grains were formed because the d values were out of the range suggested by the present disclosure. As a result, high strength and high ductility could not be satisfied simultaneously.
The range of Formula (2) suggested by the present disclosure was not satisfied in Comparative Examples 3 to 5 and 13 to 17. As a result, high strength and high ductility could not be satisfied simultaneously.
The range of Formula (3) suggested by the present disclosure was not satisfied in Comparative Examples 2 and 5 to 17. As a result, high strength and high ductility could not be satisfied simultaneously.
The range of Formula (4) suggested by the present disclosure was not satisfied in Comparative Examples 1, 3 to 5, 7, 10, 12, and 14 to 17. As a result, sufficient hardness could not be obtained.
Corrosion resistance deteriorated in Comparative Example 18 due to a large amount of Mn contained therein, and a pitting potential of 30 mV was obtained.
FIG. 1 is a graph showing ranges of values of Formulae (2) and (3) of inventive examples and comparative examples. Referring to FIG. 1, it may be confirmed that all inventive examples satisfied the ranges of Formulae (2) and (3) defined in the present disclosure, and thus austenitic stainless steels simultaneously satisfying the ranges of high strength and high ductility may be provided. On the contrary, in the case of the comparative examples, the values of Formula (2) and/or Formula (3) were not satisfied, and in this case it is difficult to simultaneously satisfy high strength and high ductility characteristics.
FIGS. 2A to 2C are views for comparison of average grain size d among the inventive examples and comparative examples. FIGS. 2A and 2B show samples according to the present disclosure, and FIG. 2C shows a sample according to a comparative example. Upon comparison of FIGS. 2A and 2B with 2C, it may be confirmed that the austenitic stainless steel according to the present disclosure are in the form of ultrafine grains with a d value of 5 µm or less.
FIGS. 3A to 3C are graphs showing Strain (%)-Stress (MPa) curves of the inventive examples.
FIGS. 4A to 4C are graphs showing Strain (%)-Stress (MPa) curves of the comparative examples.
While the present disclosure has been particularly described with reference to exemplary embodiments, it should be understood by those of skilled in the art that the scope of the present disclosure is not limited thereby and various changes in form and details may be made without departing from the spirit and scope of the present disclosure.

[Industrial Applicability]

The austenitic stainless steel according to an embodiment of the present disclosure may have both high strength and high ductility, be economically feasible by reducing the contents of Ni and Cr which are high-priced alloying elements, have excellent corrosion resistance. Therefore, the austenitic stainless steel may be applied to outer panels of vehicles, components for construction, and the like, as various materials.

Claims

An austenitic stainless steel comprising, in percent by weight (wt%), 0.005 to 0.03% of C, 0.1 to 1% of Si, 0.1 to 2% of Mn, 6 to 9% of Ni, 16 to 19% of Cr, 0.2% or less of N, and the remainder being Fe and unavoidable impurities,
wherein assuming that a total thickness of the steel material is t, a value of an average grain size d in a thickness range of 1/4t to 3/4t is 5 µm or less, an ASP value represented by Formula (1) below is from 10 to 25, a value represented by Formula (2) below is 435 or more, and a value represented by Formula (3) below is 6000 or more: $\begin{array}{l} 551 - 462 * ([C] + [N]) - 9.2 * [Si] - 8.1 * [Mn] - 13.7 * [Cr] - 29 * ([Ni] + [Cu]) - \\ 18.5 * [Mo] - 68 * ([Nb] + [V]) \end{array}$
$(1600 * [N]) + (700 / \sqrt{d}) + (4 * ASP) - (20 * [Ni]) + 100$
$YS * EL - 500 * ([Ni] + [Cr])$

(wherein in Formulae (1), (2), and (3), [C], [N], [Si], [Mn], [Cr], [Ni], [Cu], [Mo], [Nb], and [V] represent weight percentages (wt%) of respective elements, YS represents a yield strength (MPa), and EL represents an elongation (%)).
The austenitic stainless steel according to claim 1, wherein a value of Formula (4) below is 200 or more: $Hv - ([Ni] + [Cr])$
(wherein in Formula (4), Hv represents a Vickers hardness (Hv) and [Ni] and [Cr] represent weight percentages (wt%) of respective elements).
The austenitic stainless steel according to claim 1, further comprising, in percent by weight (wt%), at least one of 0.4% or less of Cu, 0.2% or less of Mo, 0.25% or less of Nb, and 0.25% or less of V.
The austenitic stainless steel according to claim 1, wherein the t is from 0.4 to 2.0 mm.
The austenitic stainless steel according to claim 1, wherein a pitting potential, measured by dipping in a 3.5% NaCl solution at 30°C, is 250 mV or more.
A method of manufacturing the austenitic stainless steel according to claim 1, the method comprising:
hot rolling a slab comprising, in percent by weight (wt%), 0.005 to 0.03% of C, 0.1 to 1% of Si, 0.1 to 2% of Mn, 6 to 9% of Ni, 16 to 19% of Cr, 0.2% or less of N, and the remainder being Fe and unavoidable impurities, and cold rolling the hot-rolled steel material at room temperature with a reduction ratio of 40% or more; and

annealing the cold-rolled steel material at a temperature of 700 to 850°C.
The method according to claim 6, wherein the slab is hot-rolled and then cold-rolled without being annealed.
The method according to claim 6, further comprising skin pass rolling with a reduction ratio of 60% or more.