AUSTENITIC STAINLESS STEEL AND MANUFACTURING METHOD THEREOF

Abstract

 Disclosed are a ultrafine austenitic stainless steel simultaneously satisfying a high strength, a high elongation, and a high yield ratio and a method for manufacturing the same. An austenitic stainless steel according to an embodiment of the present disclosure includes, in percent by weight (wt%), 0.005 to 0.03% of carbon (C), 0.1 to 1.0% of silicon (Si), 0.1 to 2.0% of manganese (Mn), 6.0 to 12.0% of nickel (Ni), 16.0 to 20.0% of chromium (Cr), 0.01 to 0.2% of nitrogen (N), 0.25% or less of niobium (Nb), and the balance of iron (Fe) and inevitable impurities, wherein a thickness central region has an average grain size d of 5 µm or less, and a fraction of a unrecrystallized area in a band form is 10% or less.

Description

[Technical Field]

[0001] The present disclosure relates to an austenitic stainless steel with a high yield strength and a method for manufacturing the same, and more particularly, to a ultrafine austenitic stainless steel simultaneously satisfying a high strength, a high elongation, and a high yield ratio and a method for manufacturing the same.

[Background Art]

[0002] In general, 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 have low yield strengths of 200 to 350 MPa, and there are limits to apply these stainless steels to structural materials. A skin pass rolling process is generally conducted to increase yield strength of 300 series stainless steels for common use. However, the skin pass rolling process may cause problems in increasing manufacturing costs and significantly deteriorating elongation of materials.

[0003] Patent Document 0001 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-annealed steel material. However, the method disclosed in Patent Document 0001 is a method used to control etchability and curvature after etching. When formation occurs with an austenitic stability parameter (ASP) value of 30 to 50, strain-induced martensite transformation rapidly occurs, resulting in deterioration of elongation.

[0004] Patent Document 2 discloses a method of performing heat treatment for a long time over 48 hours in a temperature range of 600 to 700°C to adjust an average grain size to 10 µm or less. According to Patent Document 2, productivity decreases in the case of being implemented in a real production line, and manufacturing costs increase.

(Patent Document 0001) International Patent Application Publication No. WO2016-043125A1 (March 14, 2016)

(Patent Document 0002) Japanese Patent Application Laid-Open No. JP2020-50940A (April 2, 2020)

[Disclosure]

[Technical Problem]

[0005] To solve the problem as described above, provided are a ultrafine austenitic stainless steel simultaneously satisfying a high strength, a high elongation, and a high yield ratio and a method for manufacturing the same.

[Technical Solution]

[0006] 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 carbon (C), 0.1 to 1.0% of silicon (Si), 0.1 to 2.0% of manganese (Mn), 6.0 to 12.0% of nickel (Ni), 16.0 to 20.0% of chromium (Cr), 0.01 to 0.2% of nitrogen (N), 0.25% or less of niobium (Nb), and the balance of iron (Fe) and inevitable impurities, wherein a thickness central region has an average grain size d of 5 µm or less, and a fraction of a unrecrystallized area in a band form is 10% or less.

[0007] In addition, the austenitic stainless steel according to an embodiment of the present disclosure may have a yield strength of at least 700 MPa but not more than 1113 MPa.

[0008] In addition, the austenitic stainless steel according to an embodiment of the present disclosure may have an elongation of at least 20% but not more than 41.2%.

[0009] In addition, the austenitic stainless steel according to an embodiment of the present disclosure may have a yield ratio of at least 0.8 but not more than 0.96.

[0010] In addition, a method for manufacturing an austenitic stainless steel includes: hot rolling a slab including 0.005 to 0.03% of C, 0.1 to 1.0% of Si, 0.1 to 2.0% of Mn, 6.0 to 12.0% of Ni, 16.0 to 20.0% of Cr, 0.01 to 0.2% of N, 0.002 to 0.25% of Nb, and the balance of Fe and inevitable impurities, wherein a thickness central region has an average grain size d of 5 µm or less, and a fraction of a unrecrystallized area in a band form is 10% or less, cold rolling the hot-rolled slab at room temperature with a reduction ratio of 40% or more, and cold annealing the resultant to satisfy a S2 value of 0.8 or more represented by Equation (1) below.

[0011] Meanwhile, in Equation (1), [C], [Si], [Mn], [Cr], [Ni], [N], and [Nb] represent weight percentages (wt%) of respective elements, Md30 is a value defined by 551-462([C]+[N])-9.2*[Si]-8.1*[Mn]-13.7*[Cr]-29([Ni]+[Cu])-18.5*[Mo]-68([Nb]+[V]), and Temp is a cold annealing temperature (°C).

[0012] In addition, according to the method for manufacturing an austenitic stainless steel according to an embodiment of the present disclosure, the cold rolling may be performed after the hot rolling without performing hot annealing.

[Advantageous Effects]

[0013] According to an embodiment of the present disclosure, provide are a ultrafine austenitic stainless steel simultaneously satisfying a high strength, a high elongation, and a high yield ratio, and a method for manufacturing the same.

[Description of Drawings]

[0014] 

FIG. 1 is a graph illustrating a stress-deformation curve of Example 1.

FIG. 2 is a graph illustrating a stress-deformation curve of Comparative Example 3.

FIG. 3 is an image of a microstructure of a thickness central region of Example 3 obtained by an electron backscatter diffraction (EBSD) pattern analyzer.

FIG. 4 is an image of a microstructure of a thickness central region of Comparative Example 2 obtained by an EBSD pattern analyzer.

[Best Mode]

[0015] An austenitic stainless steel according to an embodiment of the present disclosure includes, in percent by weight (wt%), 0.005 to 0.03% of carbon (C), 0.1 to 1.0% of silicon (Si), 0.1 to 2.0% of manganese (Mn), 6.0 to 12.0% of nickel (Ni), 16.0 to 20.0% of chromium (Cr), 0.01 to 0.2% of nitrogen (N), 0.25% or less of niobium (Nb), and the balance of iron (Fe) and inevitable impurities, wherein a thickness central region has an average grain size d of 5 µm or less, and a fraction of a unrecrystallized area in a band form is 10% or less.

[Modes of the Invention]

[0016] 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.

[0017] The terms used herein are merely used to describe particular 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, processes, functions, components, or combinations thereof disclosed in the specification, and are not intended to preclude the possibility that one or more other features, processes, functions, components, or combinations thereof may exist or may be added.

[0018] Meanwhile, unless otherwise defined, all terms used herein have the same meaning as 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.

[0019] 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 invention or to prevent an unconscious infringer from illegally using the disclosure of the present invention.

[0020] An austenitic stainless steel according to an embodiment of the present disclosure includes, in percent by weight (wt%), 0.005 to 0.03% of carbon (C), 0.1 to 1.0% of silicon (Si), 0.1 to 2.0% of manganese (Mn), 6.0 to 12.0% of nickel (Ni), 16.0 to 20.0% of chromium (Cr), 0.01 to 0.2% of nitrogen (N), 0.25% or less of niobium (Nb), and the balance of iron (Fe) and inevitable impurities.

[0021] Hereinafter, reasons for numerical limitations on the contents of the alloying elements will be described.

[0022] The content of carbon (C) may be from 0.005 to 0.03%.

[0023] C is an austenite phase-stabilizing element. In consideration thereof, C may be added in an amount of 0.005% or more. However, an excess of C may cause a problem of forming a chromium carbide during low-temperature annealing to deteriorate grain boundary corrosion resistance. In consideration thereof, an upper limit of the C content may be set to 0.03 wt%.

[0024] The content of silicon (Si) may be from 0.1 to 1.0%.

[0025] 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 may be added in an amount of 0.1 wt% or more in the present disclosure. However, an excess of Si may cause a problem of deteriorating ductility. In consideration thereof, an upper limit of the Si content may be set to 1.0 wt% or less.

[0026] The content of manganese (Mn) may be from 0.1 to 2.0%.

[0027] Mn is an austenite phase-stabilizing element. In consideration thereof, Mn may be added in an amount of 0.1% or more. However, an excess of Mn may cause a problem of deteriorating corrosion resistance. In consideration thereof, an upper limit of the Mn content may be set to 2.0%.

[0028] The content of nickel (Ni) may be from 6.0 to 12.0%.

[0029] Ni is an austenite phase-stabilizing element and has an effect on softening a steel material. In consideration thereof, Ni may be added in an amount of 6.0% or more. However, an excess of Ni may cause a problem of increasing manufacturing costs. In consideration thereof, an upper limit of Ni may be set to 12.0%.

[0030] The content of chromium (Cr) may be from 16.0 to 20.0%.

[0031] Cr is a major element for improving corrosion resistance of a stainless steel. In consideration thereof, Cr may be added in an amount of 16.0 wt% or more. However, an excess of Cr may cause problems of hardening a steel material and inhibiting strain-induced martensite transformation during cold rolling. In consideration thereof, an upper limit of the Cr content may be set to 20.0%.

[0032] The content of nitrogen (N) may be from 0.01 to 0.2%.

[0033] N is an austenite phase-stabilizing element and enhances strength of a steel material. In consideration thereof, N may be added in an amount of 0.01% or more. However, an excess of N may cause problems such as hardening of a steel material and deteriorating hot workability. In consideration thereof, an upper limit of the N content may be set to 0.2%.

[0034] The content of niobium (Nb) may be from 0.25% or less. Addition of Nb that induces formation of Nb-based Z-phase precipitates has an effect on inhibiting the growth of crystal grains. However, an excess of Nb may cause a problem of increasing manufacturing costs. In consideration thereof, an upper limit of the Nb content may be set to 0.25%.

[0035] 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 alloy components is not excluded. The impurities are not specifically mentioned in the present disclosure, as they are known to any person skilled in the art of manufacturing.

[0036] By adjusting the composition of the alloying elements in the austenitic stainless steel according to an embodiment of the present disclosure, a thickness central region may have an average grain size d of 5 µm or less, and a fraction of a unrecrystallized area in a band form may be 10% or less.

[0037] In general, in order to implement a ultrafine microstructure, TRIP transformation to transform an austenite phase to a martensite phase is used. In the austenitic stainless steel according to an embodiment of the present disclosure, an average grain size d of the thickness central region is controlled to 5 µm or less by TRIP transformation. Meanwhile, when the average grain size d of the thickness central region exceeds 5 µm, a yield strength decreases by Hall-Petch equation.

[0038] A portion remaining without being transformed into the martensite phase during cold rolling is shown as a unrecrystallized area. When there are many unrecrystallized areas, a problem of decreasing ductility may be cause. Therefore, it is preferable to adjust the fraction of the unrecrystallized area to 10% or less.

[0039] The austenitic stainless steel according to an embodiment of the present disclosure may have a yield strength of at least 700 MPa not more than 1113 MPa.

[0040] The austenitic stainless steel according to an embodiment of the present disclosure may have an elongation of at least 20% but not more than 41.2%.

[0041] The austenitic stainless steel according to an embodiment of the present disclosure may have a yield ratio of at least 0.8 but not more than 0.96. The yield ratio refers to a value obtained by dividing a yield strength by a tensile strength.

[0042] A method for 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 1.0% of Si, 0.1 to 2.0% of Mn, 6.0 to 12.0% of Ni, 16.0 to 20.0% of Cr, 0.01 to 0.2% of N, 0.002 to 0.25% of Nb, and the balance of Fe and inevitable impurities, wherein a thickness central region has an average grain size d of 5 µm or less, and a fraction of a unrecrystallized area in a band form is 10% or less, cold rolling the hot-rolled slab at room temperature with a reduction ratio of 40% or more, and cold annealing a resultant to satisfy a S2 value of 0.8 or more represented by Equation (1) below.

[0043] Meanwhile, in Equation (1), [C], [Si], [Mn], [Cr], [Ni], [N], and [Nb] represent weight percentages (wt%) of respective elements, Md30 is a value defined by 551-462([C]+[N])-9.2*[Si]-8.1*[Mn]-13.7*[Cr]-29([Ni]+[Cu])-18.5*[Mo]-68([Nb]+[V]), and Temp is a cold annealing temperature (°C).

[0044] Reasons for limitations on the composition of alloying elements are as described above, and hereinafter, processes of the manufacturing method thereof will be described in more detail.

[0045] The slab may be prepared as a hot-rolled steel material by a hot rolling process. Subsequently, the hot-rolled steel material may be cold-rolled at room temperature to prepare a cold-rolled steel material.

[0046] When the reduction ratio is less than 40% during the cold rolling, a fraction of the martensite phase of the cold-rolled steel material decreases and a fraction of the retained austenite phase increases due to a too low amount of TRIP transformation. 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.

[0047] Subsequently, the prepared cold-rolled steel material may be cold-annealed. The cold annealing may be performed in a temperature range of 700 to 850°C to satisfy the Ω value represented by Equation (1) above to be 0.8 or more.

[0048] When the temperature of the cold annealing is below 700°C, recrystallization does not sufficiently occur, resulting in a decrease in elongation. On the contrary, when the temperature of the cold annealing is above 850°C, grains coarsen making formation of ultrafine grains with a grain size of 5 µm or less difficult.

[0049] In addition, according to the method of manufacturing an austenitic stainless steel according to an embodiment of the present disclosure, the steel material may be cold-rolled after the hot rolling without performing an annealing process. In the case where a separate annealing process is not performed after the hot rolling, productivity increases and manufacturing costs may be reduced.

[0050] Hereinafter, the present disclosure will be described in more detail through examples.

Examples

[0051] The slabs including the elements listed in Table 1 below were hot-rolled and cold-rolled with a total thickness reduction ratio of 40% or more after performing an annealing process at a temperature of 1000 to 1150°C or without performing the annealing process. Then, annealing was performed in temperature ranges shown in Table 1 below to prepare cold-annealed materials.

Table 1

Category	Composition of alloying elements (wt%)										Temp (°C)
	C	Si	Mn	Cr	Ni	Cu	Mo	N	Nb	V
Example 1	0.023	0.53	1.24	17.5	6.4	0	0	0.17	0	0	750
Example 2	0.02	0.51	0.98	17.3	6.3	0	0	0.1	0	0	750
Example 3	0.019	0.3	0.46	17.3	6.3	0.25	0.1	0.15	0.21	0	750
Example 4	0.018	0.3	0.3	18.1	7.96	0.24	0.1	0.021	0.1	0	750
Example 5	0.021	0.41	1	17.3	7.19	0.24	0.1	0.15	0	0.2	750
Example 6	0.019	0.3	0.46	17.3	6.3	0.25	0.1	0.15	0.21	0	800
Example 7	0.02	0.41	0.99	17.3	7.04	0.25	0.1	0.15	0.2	0	800
Example 8	0.019	0.3	0.46	17.3	6.3	0.25	0.1	0.15	0.21	0	850
Example 9	0.02	0.41	0.99	17.3	7.04	0.25	0.1	0.15	0.2	0	850
Comparative Example 1	0.02	0.31	0.5	18.2	8.02	0.27	0.1	0.041	0.053	0	750
Comparative Example 2	0.02	0.41	0.99	17.3	7.04	0.25	0.1	0.15	0.2	0	750
Comparative Example 3	0.02	0.29	0.49	16.6	5.98	0.25	0.1	0.18	0	0	750
Comparative Example 4	0.019	0.31	0.5	18.1	8.05	0.25	0.1	0.1	0	0	750
Comparative Example 5	0.022	0.44	0.99	18.1	8.05	0.25	0.1	0.08	0	0	750
Comparative Example 6	0.023	0.53	1.24	17.5	6.4	0	0	0.17	0	0	800
Comparative Example 7	0.02	0.51	0.98	17.3	6.3	0	0	0.1	0	0	800
Comparative Example 8	0.02	0.29	0.49	16.6	5.98	0.25	0.1	0.18	0	0	800
Comparative Example 9	0.017	0.32	1.79	16.7	6.85	0.25	0.1	0.15	0	0	800
Comparative Example 10	0.022	0.31	0.29	18.2	8.09	0.25	0.1	0.02	0	0	800
Comparative Example 11	0.02	0.31	0.5	18.2	8.02	0.27	0.1	0.041	0.053	0	800
Comparative Example 12	0.019	0.31	0.5	18.1	8.05	0.25	0.1	0.1	0	0	800
Comparative Example 13	0.02	0.39	1	17.4	7.13	0.25	0.1	0.16	0	0	800
Comparative Example 14	0.021	0.41	1	17.3	7.19	0.24	0.1	0.15	0	0.2	800
Comparative Example 15	0.022	0.44	0.99	18.1	8.05	0.25	0.1	0.08	0	0	800
Comparative Example 16	0.023	0.53	1.24	17.5	6.4	0	0	0.17	0	0	850
Comparative Example 17	0.02	0.51	0.98	17.3	6.3	0	0	0.1	0	0	850
Comparative Example 18	0.02	0.29	0.49	16.6	5.98	0.25	0.1	0.18	0	0	850
Comparative Example 19	0.017	0.32	1.79	16.7	6.85	0.25	0.1	0.15	0	0	850
Comparative Example 20	0.022	0.31	0.29	18.2	8.09	0.25	0.1	0.02	0	0	850
Comparative Example 21	0.018	0.3	0.3	18.1	7.96	0.24	0.1	0.021	0.1	0	850
Comparative Example 22	0.02	0.31	0.5	18.2	8.02	0.27	0.1	0.041	0.053	0	850
Comparative Example 23	0.019	0.31	0.5	18.1	8.05	0.25	0.1	0.1	0	0	850
Comparative Example 24	0.02	0.39	1	17.4	7.13	0.25	0.1	0.16	0	0	850
Comparative Example 25	0.021	0.41	1	17.3	7.19	0.24	0.1	0.15	0	0.2	850
Comparative Example 26	0.022	0.44	0.99	18.1	8.05	0.25	0.1	0.08	0	0	850
Comparative Example 27	0.023	0.53	1.24	17.5	6.4	0	0	0.17	0	0	1050
Comparative Example 28	0.02	0.51	0.98	17.3	6.3	0	0	0.1	0	0	1050
Comparative Example 29	0.019	0.3	0.46	17.3	6.3	0.25	0.1	0.15	0.21	0	1050
Comparative Example 30	0.02	0.29	0.49	16.6	5.98	0.25	0.1	0.18	0	0	1050
Comparative Example 31	0.017	0.32	1.79	16.7	6.85	0.25	0.1	0.15	0	0	1050
Comparative Example 32	0.022	0.31	0.29	18.2	8.09	0.25	0.1	0.02	0	0	1050
Comparative Example 33	0.018	0.3	0.3	18.1	7.96	0.24	0.1	0.021	0.1	0	1050
Comparative Example 34	0.02	0.31	0.5	18.2	8.02	0.27	0.1	0.041	0.053	0	1050
Comparative Example 35	0.019	0.31	0.5	18.1	8.05	0.25	0.1	0.1	0	0	1050
Comparative Example 36	0.02	0.39	1	17.4	7.13	0.25	0.1	0.16	0	0	1050
Comparative Example 37	0.02	0.41	0.99	17.3	7.04	0.25	0.1	0.15	0.2	0	1050
Comparative Example 38	0.021	0.41	1	17.3	7.19	0.24	0.1	0.15	0	0.2	1050
Comparative Example 39	0.022	0.44	0.99	18.1	8.05	0.25	0.1	0.08	0	0	1050

[0052] The values of Equation (1) of the cold-annealed materials prepared as described above are shown in Table 2 below. The values of Equation (1) shown in Table 2 below refer to values derived from parameters defined by Equation (1): S2 = 3.35 - 14.6*[C] + 0.105*[Si] + 0.0058*[Mn] + 0.0321*[Cr] - 0.222*[Ni] - 2.02*[N] + 0.340*[Nb] - 0.00538*Md30 - 0.00124*Temp.

[0053] In Equation (1) above, [C], [Si], [Mn], [Cr], [Ni], [N], and [Nb] represent weight percentages (wt%) of respective elements, Md30 refers to values defined by 551-462([C]+[N])-9.2*[Si]-8.1*[Mn]-13.7*[Cr]-29([Ni]+[Cu])-18.5*[Mo]-68([Nb]+[V]), and Temp refers to cold annealing temperature (°C).

[0054] The prepared cold-annealed material was prepared as a sample having a thickness of 0.1 to 3.0 mm. Subsequently, average grain sizes d, fractions of the unrecrystallized area, yield strengths, tensile strengths, elongations, and yield ratios of the thickness central regions of the samples were measured and shown in Table 2 below.

[0055] The average grain size d and the fraction of the unrecrystallized area were measured by analyzing orientations of the thickness central region by using an electron backscatter diffraction (EBSD) pattern analyzer with Model No. of e-Flash FS.

[0056] The yield strength, tensile strength, and elongation were measured by using a universal test machine (UTM).

[0057] The yield ratio refers to a value obtained by dividing a yield strength by a tensile strength.

Table 2

Category	Md30	Equatio n (1) Ω	d (µm)	Fraction of the unrecrystalli zed area (%)	Yield strength (MPa)	Tensile strength (MPa)	Elongatio n (%)	Yield ratio
Example 1	21.6	0.83	1.2	3	993	1059	34.5	0.94
Example 2	63.2	0.80	1.0	0	930	1083	20.8	0.86
Example 3	23.3	0.98	0.5	0	1113	1172	21.8	0.95
Example 4	33.4	0.82	1.2	0	910	1011	22.3	0.9
Example 5	-7.8	0.86	2.5	6	887	973	31.9	0.91
Example 6	23.3	0.91	2.2	0	964	1006	32	0.96
Example 7	-3.2	0.89	3.5	0	864	938	35.8	0.92
Example 8	23.3	0.85	4.0	0	810	987	30.4	0.82
Example 9	-3.2	0.83	4.5	0	702	869	41.2	0.81
Comparative Example 1	20.7	0.79	2.1	25	955	1076	11.1	0.89
Comparative Example 2	-3.2	0.95	3.5	32	1143	1222	11.5	0.94
Comparative Example 3	42	0.78	1.2	5	868	1118	20.8	0.78
Comparative Example 4	-1.4	0.78	2.7	8	663	857	39.1	0.77
Comparative Example 5	1.3	0.78	3.1	9	546	796	37.9	0.69
Comparative Example 6	21.6	0.77	3.5	0	679	940	42.3	0.72
Comparative Example 7	63.2	0.74	2.2	0	678	960	28	0.71
Comparative Example 8	42	0.72	2.1	0	741	1076	24.6	0.69
Comparative Example 9	19.9	0.76	4.5	0	587	830	45.1	0.71
Comparative Example 10	33.3	0.64	3.4	3	435	742	36.6	0.59
Comparative Example 11	20.7	0.73	3.4	4	618	801	39.7	0.77
Comparative Example 12	-1.4	0.72	4.3	0	503	771	43.6	0.65
Comparative Example 13	1.9	0.76	4.2	0	585	833	43.3	0.7
Comparative Example 14	-7.8	0.79	4.8	0	646	865	40	0.75
Comparative Example 15	1.3	0.71	3.6	0	460	751	42.1	0.61
Comparative Example 16	21.6	0.70	4.6	0	627	911	44.1	0.69
Comparative Example 17	63.2	0.68	3.7	0	595	908	25.4	0.66
Comparative Example 18	42	0.65	3.9	0	655	1019	28	0.64
Comparative Example 19	19.9	0.70	4.3	0	538	809	45.8	0.67
Comparative Example 20	33.3	0.58	3.9	0	384	730	38.3	0.53
Comparative Example 21	33.4	0.69	2.1	0	503	746	36.3	0.67
Comparative Example 22	20.7	0.67	3.2	0	475	745	44.7	0.64
Comparative Example 23	-1.4	0.65	4.8	0	475	755	44.3	0.63
Comparative Example 24	1.9	0.69	4.9	0	541	808	43.8	0.67
Comparative Example 25	-7.8	0.74	4.4	0	602	842	42.5	0.71
Comparative Example 26	1.3	0.65	2.5	0	427	734	44.3	0.58
Comparative Example 27	21.6	0.46	22.0	0	414	835	50.9	0.5
Comparative Example 28	63.2	0.43	25.0	0	341	948	24.3	0.36
Comparative Example 29	23.3	0.60	15.0	0	482	956	27.8	0.5
Comparative Example 30	42	0.41	32.0	0	409	974	29.4	0.42
Comparative Example 31	19.9	0.45	25.0	0	373	735	49.5	0.51
Comparative Example 32	33.3	0.33	27.0	0	225	701	38.8	0.32
Comparative Example 33	33.4	0.44	21.0	0	237	687	39.4	0.34
Comparative Example 34	20.7	0.42	28.0	0	256	670	47.7	0.38
Comparative Example 35	-1.4	0.41	32.0	0	325	675	56.5	0.48
Comparative Example 36	1.9	0.45	33.0	0	385	730	53.6	0.53
Comparative Example 37	-3.2	0.58	17.0	0	508	821	44.9	0.62
Comparative Example 38	-7.8	0.49	36.0	0	391	722	54.4	0.54
Comparative Example 39	1.3	0.40	34.0	0	298	654	56	0.46

[0058] Referring to Tables 1 and 2 above, in all of Examples 1 to 9, the S2 vales of Equation (1) satisfied 0.8 or more and the average grain sizes d satisfied 5 µm or less. In addition, in all of Examples 1 to 9, the fraction of a unrecrystallized area in a band form satisfied 10% or less.

[0059] Accordingly, Examples 1 to 9 satisfied a yield strength of at least 700 MPa but not more than 1113 MPa, an elongation of at least 20% but not more than 41.2%, and a yield ratio of at least 0.8 but not more than 0.96. That is, Examples 1 to 9 simultaneously satisfied the high strength, high elongation, and high yield ratio.

[0060] On the contrary, in Comparative Examples 1 and 2, the fraction of the unrecrystallized area exceeded 10%. Accordingly, in Comparative Examples 1 and 2, the elongation was less than 20% indicating poor elongation.

[0061] Comparative Examples 3 and 8 exhibited low average grain sizes d and satisfied a yield strength of at least 700 MPa but not more than 1113 MPa. However, in Comparative Examples 3 and 8, the tensile strength was relatively high compared to the yield strength. Accordingly, Comparative Examples 3 and 8 did not satisfy the yield ratio of at least 0.8 but not more than 0.96.

[0062] The Ω value represented by Equation (1) of 0.8 or more was not satisfied in Comparative Examples 4 to 7 and 9 to 39. Accordingly, the yield strength of at least 700 MPa but not more than 1113 MPa and the yield ratio of at least 0.8 but not more than 0.96 were not satisfied in Comparative Examples 4 to 7 and 9 to 39.

[0063] Comparative Examples 27 to 39 exhibited high cold annealing temperatures. Accordingly, the average grain sizes d of 5 µm or less were not satisfied in Comparative Examples 27 to 39.

[0064] FIGS. 1 and 2 are graphs illustrating stress-deformation curves of an example and a comparative example. FIG. 1 is a graph of Example 1, and FIG. 2 is a graph of Comparative Example 3. Upon comparison between FIGS. 1 and 2, the austenitic stainless steel according to an embodiment of the present disclosure may simultaneously satisfy the high strength, the high elongation, and the high yield ratio because a stress change according to the degree of deformation is not relatively large.

[0065] FIGS. 3 and 4 are images of microstructures of thickness central regions of an example and a comparative example obtained by an electron backscatter diffraction (EBSD) pattern analyzer. FIG. 3 is an image of Example 3, and FIG. 4 is an image of Comparative Example 2. Upon comparison between FIGS. 3 and 4, a band-shaped unrecrystallization was not observed in the austenitic stainless steel according to an embodiment of the present disclosure.

[0066] 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]

[0067] According to an embodiment of the present disclosure, a ultrafine austenitic stainless steel simultaneously satisfying a high strength, s high elongation, and a high yield ratio and a method for manufacturing the same may be provided.

Claims

1. An austenitic stainless steel comprising, in percent by weight (wt%), 0.005 to 0.03% of carbon (C), 0.1 to 1.0% of silicon (Si), 0.1 to 2.0% of manganese (Mn), 6.0 to 12.0% of nickel (Ni), 16.0 to 20.0% of chromium (Cr), 0.01 to 0.2% of nitrogen (N), 0.25% or less of niobium (Nb), and the balance of iron (Fe) and inevitable impurities,
wherein a thickness central region has an average grain size d of 5 µm or less, and a fraction of a unrecrystallized area in a band form is 10% or less.

2. The austenitic stainless steel according to claim 1, wherein a yield strength is at least 700 MPa but not more than 1113 MPa.

3. The austenitic stainless steel according to claim 1, wherein an elongation is at least 20% but not more than 41.2%.

4. The austenitic stainless steel according to claim 1, wherein a yield ratio is at least 0.8 but not more than 0.96.

5. A method for manufacturing an austenitic stainless steel, the method comprising:

hot rolling a slab comprising, in percent by weight (wt%), 0.005 to 0.03% of C, 0.1 to 1.0% of Si, 0.1 to 2.0% of Mn, 6.0 to 12.0% of Ni, 16.0 to 20.0% of Cr, 0.01 to 0.2% of N, 0.002 to 0.25% of Nb, and the balance of Fe and inevitable impurities, wherein a thickness central region has an average grain size d of 5 µm or less, and a fraction of a unrecrystallized area in a band form is 10% or less;

cold rolling the hot-rolled slab at room temperature with a reduction ratio of 40% or more; and

cold annealing a resultant to satisfy a Ω value, represented by Equation (1) below of 0.8 or more:

(wherein in Equation (1), [C], [Si], [Mn], [Cr], [Ni], [N], and [Nb] represent weight percentages (wt%) of respective elements, Md30 is a value defined by 551-462([C]+[N])-9.2*[Si]-8.1*[Mn]-13.7*[Cr]-29([Ni]+[Cu])-18.5*[Mo]-68([Nb]+[V]), and Temp is a cold annealing temperature (°C)).

6. The method according to claim 5, wherein the cold rolling is performed after the hot rolling without performing hot annealing.

Drawing