EP4343013A1

EP4343013A1 - Austenitic stainless steel and manufacturing method thereof

Info

Publication number: EP4343013A1
Application number: EP22828661.3A
Authority: EP
Inventors: Sangseok KIM; Minam PARK; Kihoon Jo
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2021-06-21
Filing date: 2022-06-15
Publication date: 2024-03-27
Also published as: WO2022270814A1; CN117529571A; KR20220169655A

Abstract

Disclosed are aa austenitic stainless steel free of surface cracks and having excellent surface roughness in a bent portion and a manufacturing method thereof.The 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 12% of Ni, 16 to 20% of Cr, 0.01 to 0.2% of N, 0.25% or less of Nb, and the balance of Fe and inevitable impurities, wherein an average grain size (d) of a central portion in a thickness direction is 5 µm or less, and a martensite area fraction measured in the bent portion after a 180° bending test is 10% or less.

Description

[Technical Field]

The present disclosure relates to an austenitic stainless steel free of surface cracks and having excellent surface roughness in a bent portion and a manufacturing method thereof.

[Background Art]

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, because 304 series stainless steels or 301 series stainless steels have low yield strengths of 200 to 350 MPa, there are limits to apply these stainless steels to structural materials. Thus, 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.
Patent Document 0001 discloses a method of performing heat treatment for a long time over 48 hours in a temperature range of 600 to 700°C to obtain an average grain size of 10 µm or less. However, the method disclosed in Patent Document 0001 may cause problems of deteriorating productivity in the case of implementing the method in an actual production line and increasing manufacturing costs.
(Patent Document 0001) Japanese Patent Laid-open Publication No. 2020-050940A (Publication Date: April 2, 2020 )

[Disclosure]

[Technical Problem]

To solve the problem as described above, provided are an austenitic stainless steel free of surface cracks and having excellent surface roughness in a bent portion and a manufacturing method thereof by presenting a ultra-fine grain manufacturing technology that realizes bending formability and sound surface properties in the bent portion.

[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 12% of Ni, 16 to 20% of Cr, 0.01 to 0.2% of N, 0.25% or less of Nb, and the balance of Fe and inevitable impurities, wherein an average grain size (d) of a central portion in a thickness direction is 5 µm or less, and a martensite area fraction measured in a bent portion after a 180° bending test may be 10% or less.
In addition, the austenitic stainless steel according to an embodiment of the present disclosure may have a center line average height Ra of 0.5 µm or less and a ten point average roughness Rz of 3 µm or less in the bent portion as surface roughness.
In addition, the austenitic stainless steel according to an embodiment of the present disclosure may have a pitting potential of 250 mV or more when measured by a 3.5% NaCl solution at 30°C.
In accordance with another aspect of the present disclosure, a method of manufacturing an austenitic stainless steel 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 12% of Ni, 16 to 20% of Cr, 0.01 to 0.2% of N, 0.25% or less of Nb, and the balance of Fe and inevitable impurities, cold rolling the hot-rolled steel sheet at room temperature, and cold annealing the cold-rolled steel sheet to satisfy a Ω value represented by Equation (1) below to at least -10 but not more than 10. $\begin{array}{l} Ω = 406 - 2127 * [C] - 26.2 * [Mn] - 31.5 * [Ni] - 127 * [N] \\ - 48.2 * [Nb] - 0.108 * Temp \end{array}$
Meanwhile, in Equation (1), [C], [Mn], [Ni], [N], and [Nb] represent weight percentages (wt%) of respective elements and Temp refers to cold annealing temperature (°C).
In addition, according to the method of 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]

According to an embodiment of the present disclosure, an austenitic stainless steel free of surface cracks and having excellent surface roughness in a bent portion and a method of manufacturing the same may be provided by applying a ultra-fine grain manufacturing technology that realizes bending formability and sound surface properties in the bent portion.

[Description of Drawings]

FIG. 1 is a photograph of Inventive Example 5 showing surface properties and occurrence of surface cracks in a bent portion after a 180° bending test.
FIG. 2 is a photograph of Comparative Example 4 showing surface properties and occurrence of surface cracks in a bent portion after a 180° bending test.
FIG. 3 is a photographs of Inventive Example 5 showing surface roughness of a bent portion after a 180° bending test.
FIG. 4 is a photographs of Comparative Example 14 showing surface roughness of a bent portion after a 180° bending test.
FIG. 5 is a photograph of Inventive Example 5 showing a microstructure of a central portion in the thickness direction measured by electron backscatter diffraction (EBSD).
FIG. 6 is a photograph of Comparative Example 14 showing a microstructure of a central portion in the thickness direction measured by EBSD.

[Best Mode]

An austenitic stainless steel according to an embodiment of the present disclosure may include, 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 12% of Ni, 16 to 20% of Cr, 0.01 to 0.2% of N, 0.25% or less of Nb, and the balance of Fe and inevitable impurities, wherein an average grain size (d) of a central portion in a thickness direction may be 5 µm or less, and a martensite area fraction measured in the bent portion after a 180° bending test may be 10% or less.

[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 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.
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.
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.
An austenitic stainless steel according to an embodiment of the present disclosure may include, 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 12% of Ni, 16 to 20% of Cr, 0.01 to 0.2% of N, 0.25% or less of Nb, and the balance of Fe and inevitable impurities.
Hereinafter, reasons for numerical limitations on the contents of the alloying elements will be described.
The content of carbon (C) may be from 0.005 to 0.03%.
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 controlled to 0.03 wt%.
The content of silicon (Si) may be from 0.1 to 1.0%.
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. However, an excess of Si may cause a problem of deteriorating ductility. In consideration thereof, an upper limit of the Si content may be controlled to 1.0 wt%.
The content of manganese (Mn) may be from 0.1 to 2.0%.
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 controlled to 2.0%.
The content of nickel (Ni) may be from 6.0 to 12.0%.
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 controlled to 12.0%.
The content of chromium (Cr) may be from 16.0 to 20.0%.
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 controlled to 20.0%.
The content of nitrogen (N) may be from 0.01 to 0.2%.
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 deterioration of hot workability. In consideration thereof, an upper limit of the N content may be controlled to 0.2%.
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 controlled to 0.25%.
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.
By adjusting the composition of the alloying elements of the austenitic stainless steel according to an embodiment of the present disclosure, the average grain size (d) of a central portion in a thickness direction may be 5 µm or less, and a martensite area fraction measured in the bent portion after a 180° bending test may be 10% or less.
In general, a method of promoting TRIP transformation to transform an austenite phase to a martensite phase is used to implement a ultra-fine grain microstructure. However, in the case of using the method of promoting TRIP transformation, an amount of strain-induced martensite transformation increases during cold deformation. As a result, a hardness of a material increases, and surface properties of a processed portion may deteriorate in the case of processing the material.
Therefore, according to an embodiment of the present disclosure, by adjusting the composition of the alloying elements such as C, Mn, Ni, N, and Nb, a ultra-fine grain microstructure may be implemented and a martensite area fraction measured in a bent portion may be reduced, so that an austenitic stainless steel with excellent surface properties may be provided.
Hereinafter, physical properties of the austenitic stainless steel according to an embodiment of the present disclosure will be described in detail.
The austenitic stainless steel according to an embodiment of the present disclosure may have a center line average height Ra of 0.5 µm or less and a ten point average roughness Rz of 3 µm or less in a bent portion after a 180° bending test. The 180° bending test may be performed by setting a curvature R value of a bent portion to be identical to a thickness of a material and conducting a bending process once.
The pitting potential is a critical potential at which corrosion occurs in the form of holes in a passivated metal material, and the austenitic stainless steel according to an embodiment of the present disclosure may have a pitting potential of 250 mV or more when measured by immersing the stainless steel in a NaCl solution and applying a potential thereto. In this regard, a temperature of the NaCl may be 30°C and a concentration thereof may be 3.5%.
Hereinafter, a method of manufacturing the austenitic stainless steel according to an embodiment of the present disclosure will be described in detail.
A method of manufacturing an austenitic stainless steel according to an embodiment of the present disclosure may include 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 12% of Ni, 16 to 20% of Cr, 0.01 to 0.2% of N, 0.25% or less of Nb, and the balance of Fe and inevitable impurities, cold rolling the hot-rolled slab at room temperature, and cold annealing the cold-rolled steel sheet to satisfy a Ω value represented by Equation (1) below to at least -10 but not more than 10. $\begin{array}{l} Ω = 406 - 2127 * [C] - 26.2 * [Mn] - 31.5 * [Ni] - 127 * [N] \\ - 48.2 * [Nb] - 0.108 * Temp \end{array}$
Meanwhile, in Equation (1), [C], [Mn], [Ni], [N], and [Nb] represent weight percentages (wt%) of respective elements and Temp refers to cold annealing temperature (°C).
Reasons for numerical limitations on the contents of the alloying elements are as described above and will be described in more detail.
A slab having the composition of alloying elements may be processed to prepare a hot-rolled steel sheet by a hot rolling process. Then, the hot-rolled steel sheet may be cold-rolled at room temperature to prepare a cold-rolled steel sheet.
Then, the prepared cold-rolled steel sheet may be cold-annealed. The cold annealing may be performed in a temperature range of 700 to 850°C such that the Ω value represented by Equation (1) above satisfies at least -10 but not more than 10.
When the cold annealing temperature is below 700°C, recrystallization does not sufficiently occur to decrease elongation. On the contrary, when the cold annealing temperature is above 850°C, crystal grains coarsen making it difficult to form ultra-fine grains with a size of 5 µm or less, thereby causing problems of occurrence of surface cracks and deterioration of surface roughness in a bent portion of the austenitic stainless steel.
In addition, according to the method of manufacturing an austenitic stainless steel according to an embodiment of the present disclosure, the cold rolling process may be performed after the hot rolling process without performing a hot annealing process. In the case where a separate annealing process is not conducted after a hot rolling process, productivity may be increased and manufacturing costs may be reduced.
Hereinafter, the present disclosure will be described in more detail through examples.

Examples

Slabs having the compositions of alloying elements shown in Table 1 below were hot-rolled and then cold-rolled at room temperature with a total reduction rate of 40% or more after performing a hot annealing process at a temperature of 1000 to 1150°C or without performing the hot annealing process. Then, a cold annealing process was performed in temperature range of 700 to 850°C to prepare cold-annealed steel sheets.
Values of Equation (1) of the prepared cold-annealed steel sheets are shown in Table 1 below. In Table 1, the values of Equation (1) refer to values derived from parameters defined by Equation (1): S2 = 406 - 2127*[C] - 26.2*[Mn] - 31.5*[Ni] - 127*[N] - 48.2*[Nb] - 0.108*Temp.

In Equation (1), [C], [Mn], [Ni], [N], and [Nb] represent weight percentages (wt%) of respective elements and Temp refers to cold annealing temperature (°C).

Table 1

Category	Composition of alloying elements (wt%)									Equati on (1) Ω	Tern p (°C)
Category	C	Si	Mn	Cr	Ni	Cu	Mo	N	Nb	Equati on (1) Ω	Tern p (°C)
Inventive Example 1	0.02	0.31	0.5	18.2	8.02	0.27	0.1	0.041	0.053	9.8	742
Inventive Example 2	0.02	0.31	0.5	18.2	8.02	0.27	0.1	0.041	0.053	8.9	751
Inventive Example 3	0.02	0.31	0.5	18.2	8.02	0.27	0.1	0.041	0.053	3.5	801
Inventive Example 4	0.019	0.31	0.5	18.1	8.05	0.25	0.1	0.1	0	5.2	750
Inventive Example 5	0.019	0.31	0.5	18.1	8.05	0.25	0.1	0.1	0	0	798
Inventive Example 6	0.019	0.31	0.5	18.1	8.05	0.25	0.1	0.1	0	-5.6	850
Inventive Example 7	0.023	0.53	1.24	17.5	6.4	0	0	0.17	0	9.6	850
Inventive Example 8	0.017	0.32	1.79	16.7	6.85	0.25	0.1	0.15	0	7.1	750
Inventive Example 9	0.017	0.32	1.79	16.7	6.85	0.25	0.1	0.15	0	1.7	800
Inventive Example 10	0.017	0.32	1.79	16.7	6.85	0.25	0.1	0.15	0	-3.6	849
Inventive Example 11	0.022	0.31	0.29	18.2	8.09	0.25	0.1	0.02	0	7.8	800
Inventive Example 12	0.022	0.31	0.29	18.2	8.09	0.25	0.1	0.02	0	2.4	850
Inventive Example 13	0.018	0.3	0.3	18.1	7.96	0.24	0.1	0.021	0.1	9.6	850
Comparative Example 1	0.023	0.53	1.24	17.5	6.4	0	0	0.17	0	20.4	750
Comparative Example 2	0.023	0.53	1.24	17.5	6.4	0	0	0.17	0	15.1	799
Comparative Example 3	0.02	0.51	0.98	17.3	6.3	0	0	0.1	0	45.6	750
Comparative Example 4	0.02	0.51	0.98	17.3	6.3	0	0	0.1	0	40.2	800
Comparative Example 5	0.02	0.51	0.98	17.3	6.3	0	0	0.1	0	34.8	850
Comparative Example 6	0.019	0.3	0.46	17.3	6.3	0.25	0.1	0.15	0.21	44.9	750
Comparative Example 7	0.019	0.3	0.46	17.3	6.3	0.25	0.1	0.15	0.21	39.5	800
Comparative Example 8	0.019	0.3	0.46	17.3	6.3	0.25	0.1	0.15	0.21	34.1	850
Comparative Example 9	0.02	0.29	0.49	16.6	5.98	0.25	0.1	0.18	0	58.3	750
Comparative Example 10	0.02	0.29	0.49	16.6	5.98	0.25	0.1	0.18	0	52.9	801
Comparative Example 11	0.02	0.29	0.49	16.6	5.98	0.25	0.1	0.18	0	47.5	850
Comparative Example 12	0.022	0.31	0.29	18.2	8.09	0.25	0.1	0.02	0	13.2	750
Comparative Example 13	0.018	0.3	0.3	18.1	7.96	0.24	0.1	0.021	0.1	20.6	750
Comparative Example 14	0.018	0.3	0.3	18.1	7.96	0.24	0.1	0.021	0.1	15.8	795

The prepared cold-annealed steel sheet were cut into samples with a thickness of 0.1 to 3.0 mm. Then, for each sample, an average grain size (d) of a central portion in a thickness direction, a pitting potential, a martensite area fraction of a bent portion, cracks in the bent portion, surface properties of the bent portion, a center line average height Ra of the bent portion, and a ten point average roughness Rz of the bent portion were measured and the results are shown in Table 2 below.
The average grain size (d) was measured by electron backscatter diffraction (EBSD) (model no. e-Flash FS) by analyzing an orientation of the central portion.
The pitting potential refers to a potential value at which pits are formed after immersing the sample in a NaCl solution and applying a potential thereto. A NaCl solution maintained at 30°C and having a concentration of 3.5% was used.
The martensite area fraction of the bent portion (%) refers to an area fraction of martensite in the bent portion after the 180° bending test. The martensite area fraction (%) was measured by using a ferrite content measuring device (model no. FMP30).
The cracks in the bent portion, the surface properties of the bent portion, the center line average height Ra of the bent portion, and the ten point average roughness Rz of the bent portion were measured after the 180° bending test. The 180° bending test was performed by setting a curvature R value of a bent portion to be identical to the thickness of the cold-annealed steel sheet and conducting a bending process once.
In the cracks in the bent portion of Table 2 below, 'O' indicates a fine state of cracks in bent portion. 'X' indicates occurrence of cracks in the bent portion.

In the surface properties of the bent portion of Table 2 below, 'O' indicates fine surface properties of the bent portion. 'X' indicates poor surface properties of the bent portion.

Table 2

	d (µm )	Pitting potential (mV)	Martensite area fraction of bent portion (%)	Cracks in bent portion	Surface properti es of bent portion	Ra (µm)	Rz (µm)
Inventive Example 1	2.8	301	4.0	O	O	0.42	2.95
Inventive Example 2	3.6	309	10.0	O	O	0.39	1.95
Inventive Example 3	4.2	314	4.0	O	O	0.25	1.80
Inventive Example 4	3.8	325	1.0	O	O	0.35	2.54
Inventive Example 5	4.2	372	1.0	O	O	0.28	1.92
Inventive Example 6	4.6	392	1.0	O	O	0.22	1.82
Inventive Example 7	4.5	292	0.0	O	O	0.45	2.89
Inventive Example 8	3.9	257	2.0	O	O	0.32	2.05
Inventive Example 9	4.2	273	1.0	O	O	0.22	1.92
Inventive Example 10	4.7	293	1.0	O	O	0.21	1.82
Inventive	3.4	325	3.0	O	O	0.41	2.22
Example 11
Inventive Example 12	4.1	327	2.0	O	O	0.32	1.80
Inventive Example 13	4.1	305	5.0	O	O	0.48	2.98
Comparative Example 1	1.2	287	25.0	X	X	Occurren ce of cracks	Occurre nce of cracks
Comparative Example 2	4.2	290	15.0	X	X
Comparative Example 3	3.7	272	45.0	X	X
Comparative Example 4	4.0	281	40.0	X	X
Comparative Example 5	4.2	283	39.0	X	X
Comparative Example 6	0.5	274	44.0	X	X
Comparative Example 7	2.2	280	44.0	X	X
Comparative Example 8	3.2	292	31.0	X	X
Comparative Example 9	1.1	255	61.0	X	X
Comparative Example 10	3.5	262	52.0	X	X
Comparative Example 11	3.8	267	45.0	X	X
Comparative Example 12	1.3	342	16.0	O	X	1.16	7.05
Comparative Example 13	2.5	298	21.0	O	X	3.92	16.20
Comparative Example 14	3.2	301	19.0	O	X	1.57	8.74

Referring to Tables 1 and 2, in all of Inventive Examples 1 to 13, the S2 values of Equation (1) satisfied the range of at least -10 but not more than 10, and the average grain size (d) satisfied 5 µm or less. In addition, the martensite area fraction (%) measured in the bent portion after the 180° bending test satisfied 10% or less in Inventive Examples 1 to 13.
Accordingly, no surface cracks occurred in the bent portion in all of Inventive Examples 1 to 13, and as the surface roughness, the center line average height Ra of 0.5 µm or less and the ten point average roughness Rz of 3 µm or less were obtained, indicating excellent surface properties.
On the contrary, in Comparative Examples 1 to 11, the S2 values of Equation (1) did not satisfy the range of at least -10 but not more than 10 and the martensite area fraction (%) measured in the bent portion after the 180° bending test exceeded 10%. Accordingly, surface cracks occurred in the bent portion of Comparative Examples 1 to 11.
Because the contents of Ni, which softens steel materials, of Comparative Examples 12 to 14 were more than that of Comparative Examples 1 to 11, surface cracks did not occur in the bent portion. However, bend-shaped uncrystallized portions were formed in Comparative Examples 12 to 14 due to low cold annealing temperatures. Therefore, Comparative Examples 12 to 14 exhibited poor surface properties because the center line average heights Ra were 1.16 to 3.92 µm and the ten point average roughnesses Rz were 7.05 to 16.20 µm in the bent portion as surface roughnesses.
FIGS. 1 and 2 are photographs for comparison of occurrence of cracks in a bent portion after a 180° bending test between an inventive example and a comparative example. FIG. 1 is a photograph of Inventive Example 5, and FIG. 2 is a photograph Comparative Example 4. Upon comparison between FIGS. 1 and 2, it was confirmed that surface cracks did not occur in the austenitic stainless steel according to an embodiment of the present disclosure.
FIGS. 3 and 4 are photographs for comparison of surface properties of a bent portion after a 180° bending test between an inventive example and a comparative example. FIG. 3 is a photograph of Inventive Example 5, and FIG. 4 is a photograph of Comparative Example 14. Upon comparison between FIGS. 3 and 4, it was confirmed that the austenitic stainless steel according to an embodiment of the present disclosure had excellent surface properties.
FIGS. 5 and 6 are photographs of a central portion in the thickness direction of an inventive example and a comparative example measured by electron backscatter diffraction (EBSD). FIG. 5 is a photograph of Inventive Example 5, and FIG. 6 is a photograph of Comparative Example 14. Upon comparison between FIGS. 5 and 6, it was confirmed that the austenitic stainless steel according to an embodiment of the present disclosure had ultra-fine grains without band-shaped uncrystallized portions.
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 various changes in form and details may be made without departing from the spirit and scope of the present disclosure.

[Industrial Applicability]

According to an embodiment of the present disclosure, an austenitic stainless steel free of surface cracks and having excellent surface roughness in a bent portion and a manufacturing method may be provided by presenting a ultra-fine grain manufacturing technology that realizes bending formability and sound surface properties in the bent portion.

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 12% of Ni, 16 to 20% of Cr, 0.01 to 0.2% of N, 0.25% or less of Nb, and the balance of Fe and inevitable impurities,
wherein an average grain size (d) of a central portion in a thickness direction is 5 µm or less, and a martensite area fraction measured in a bent portion after a 180° bending test is 10% or less.
The austenitic stainless steel according to claim 1, wherein the bent portion has a center line average height Ra of 0.5 µm or less and a ten point average roughness Rz of 3 µm or less as surface roughness.
The austenitic stainless steel according to claim 1, wherein a pitting potential value measured by a 3.5% NaCl solution at 30°C is 250 mV or more.
A method of manufacturing an austenitic stainless steel, the method comprising:
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 12% of Ni, 16 to 20% of Cr, 0.01 to 0.2% of N, 0.25% or less of Nb, and the balance of Fe and inevitable impurities;

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

cold annealing the cold-rolled steel sheet to satisfy a Ω value represented by Equation (1) below to at least -10 but not more than 10: $\begin{array}{l} Ω = 406 - 2127 * [C] - 26.2 * [Mn] - 31.5 * [Ni] - 127 * [N] - \\ 48.2 * [Nb] - 0.108 * Temp \end{array}$

(wherein in Equation (1), [C], [Mn], [Ni], [N], and [Nb] represent weight percentages (wt%) of respective elements and Temp refers to cold annealing temperature (°C))
The method according to claim 4, wherein the cold rolling is performed after the hot rolling without performing hot annealing.