EP4606923A1

EP4606923A1 - High-strength austenitic stainless steel and manufacturing method thereof

Info

Publication number: EP4606923A1
Application number: EP23903674.2A
Authority: EP
Inventors: Hanseop NOH; Jaehwa Lee; Nayeon CHU
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2022-12-12
Filing date: 2023-09-04
Publication date: 2025-08-27
Also published as: EP4606923A4; JP2025540836A; WO2024128470A1; KR20240087433A; CN120322586A

Abstract

A high-strength austenitic stainless steel according to an embodiment comprises 0.05% to 0.1% of C, 0.1% to 1.0% of Si, 1.0% to 5.0% of Mn, less than 0.05% of P, less than 0.03% of S, 14.0% to 18.0% of Cr, 1.0% to 5.0% of Ni, 0.1% to 2.0% of Cu, 0.1% to 0.2% of N, and the balance being Fe and inevitable impurities, and may have a diameter deviation of 2 or smaller between crystal grains in the thickness direction.

Description

[Technical Field]

The present invention relates to a high-strength austenitic stainless steel and a method of manufacturing the same.

[Background Art]

Recently, due to rising in Ni prices and increasing price volatility, demand for low-Ni austenitic stainless steels has been growing, but such steels have a yield strength of 250 MPa or less, which is considered inferior.
In addition, efforts have been made to reduce the content of costly Ni by substituting Ni with austenite stabilizing elements, such as Mn and N, but this has led to inferior corrosion resistance due to the generation of MnS.
In order to increase the strength of austenitic stainless steels, methods such as work hardening through temper rolling or the addition of a large amount of interstitial elements, such as C and N, are utilized. However, the temperred materials have inferior elongation, limiting the applicability, the addition of a high amount of C reduces the weldability, and the addition of a high amount of N reduces the hot workability.
Patent Document 0001 discloses a fine-grained austenitic stainless steel having excellent strength and ductility. However, Patent Document 0001 does not disclose the variation in grain refinement caused by difference in austenite phase stability according to the composition, nor the change in hot workability due to grain refinement.

[Related Art Document]

(Patent Document 1) Korean Laid-Open Patent Publication NO. 10-2007-0067905 (2007.06.29)

[Disclosure]

[Technical Problem]

To resolve the above-described issues, the present invention is directed to providing a low-Ni austenitic stainless steel having high yield strength, excellent hot workability and corrosion resistance through compositional control and grain refinement, and a method of manufacturing the same.

[Technical Solution]

A high-strength austenitic stainless steel according to an embodiment includes: 0.05% to 0.1% of C, 0.1% to 1.0% of Si, 1.0% to 5.0% of Mn, less than 0.05% of P, less than 0.03% of S,14.0% to 18.0% of Cr, 1.0% to 5.0% of Ni, 0.1% to 2.0% of Cu, 0.1% to 0.2% of N, and the balance being Fe and inevitable impurities, and having a grain diameter deviation in a thickness direction of 2 or less.
In addition, the high-strength austenitic stainless steel may have a value of Expression (1) below of less than 1.0. $0.18 Si + 0.45 C + 4.4 N$
In Expression (1), Si, C, and N may represent the content (wt %) of each element.
In addition, the high-strength austenitic stainless steel may have a value of Expression (2) below of 3.6 or less. $0.02 Si + 0.21 Mn + 0.09 Cr + 0.15 Ni + 2.73 (C + N)$
In Expression (2), Si, Mn, Cr, Ni, C, and N may represent the content (wt %) of each element.
In addition, the high-strength austenitic stainless steel may have a value of Expression (3) below of 0.35 or more. $- 0.04 Si + 0.05 Mn - 0.01 Cr + 0.05 Ni + 1.18 (C + N)$
In Expression (3), Si, Mn, Cr, Ni, C, and N may represent the content (wt %) of each element.
In addition, an average grain diameter of a central portion in a thickness direction of the high-strength austenitic stainless steel may be 10 µm or less.
In addition, the high-strength austenitic stainless steel may have a yield strength of 600 MPa or more.
In addition, the high-strength austenitic stainless steel may have an elongation of 35% or more.
In addition, the high-strength austenitic stainless steel may have a corrosion potential of 200 mV or more.
In addition, the high-strength austenitic stainless steel may have a thickness of 0.5 to 2.0 mm.
A method of manufacturing a high-strength austenitic stainless steel according to an embodiment includes: preparing a slab comprising 0.05% to 0.1% of C, 0.1% to 1.0% of Si, 1.0% to 5.0% of Mn, less than 0.05% of P, less than 0.03% of S,14.0% to 18.0% of Cr, 1.0% to 5.0% of Ni, 0.1% to 2.0% of Cu, 0.1% to 0.2% of N, and the balance being Fe and inevitable impurities; hot-rolling the slab and then reheating to produce a hot-rolled steel sheet; and cold-rolling the hot-rolled steel sheet and then performing cold-rolled annealing at 850 to 900°C to produce a cold-rolled steel sheet.
The slab may have a value less than 1.0 in Expression (1) below:
$0.18 Si + 0.45 C + 4.4 N$
In Expression (1), Si, C, and N may represent the content (wt %) of each element.
In addition, the slab may have a value of 3.6 or less in Expression (2) below:
$0.02 Si + 0.21 Mn + 0.09 Cr + 0.15 Ni + 2.73 (C + N)$
In Expression (2), Si, Mn, Cr, Ni, C, and N may represent the content (wt %) of each element.
In addition, the slab may have a value of 0.35 or more in Expression (3) below:
$- 0.04 Si + 0.05 Mn - 0.01 Cr + 0.05 Ni + 1.18 (C + N),$
In Expression (3), Si, Mn, Cr, Ni, C, and N represent the content (wt %) of each element.
The reheating may be performed at 1000 to 1150°C.
The cold rolling may be performed with a thickness reduction ratio of 60% or more.

[Advantageous Effects]

According to an embodiment of the disclosed invention, a low-Ni austenitic stainless steel implementing high yield strength while having excellent hot workability and corrosion resistance by controlling phase stability, and a method of manufacturing the same, can be provided.

[Description of Drawings]

FIG. 1 is an image of the microstructure of a high-strength austenitic stainless steel according to an example of the disclosed invention, captured using electron backscatter diffraction (EBSD).

[Modes of the Invention]

Hereinafter, examples of the present invention will be described in detail with reference to the accompanying drawings. The following examples are provided to fully convey the spirit of the present invention to a person having ordinary skill in the art to which the present invention belongs. The present invention is not limited to the examples shown herein but may be embodied in other forms. In order to make the description of the present invention clear, unrelated parts are not shown and, the sizes of components are exaggerated for clarity.
Throughout the specification, when a part is referred to as "including", "comprising" and/or "having" a certain element, it is understood that, unless expressed otherwise, the description does not preclude the presence or addition of one or more elements.
The singular form of a noun corresponding to an item may include one or a plurality of the items unless clearly indicated otherwise in a related context.
Hereinafter, the reasons for numerically limiting the alloy composition in the embodiments of the present invention will be described. Unless otherwise specified, the units thereof are expressed in weight percent (wt %).
A high-strength austenitic stainless steel according to an embodiment includes: 0.05% to 0.1% of C, 0.1% to 1.0% of Si, 1.0% to 5.0% of Mn, less than 0.05% of P, less than 0.03% of S, 14.0% to 18.0% of Cr, 1.0% to 5.0% of Ni, 0.1% to 2.0% of Cu, 0.1% to 0.2% of N, and the balance being Fe and inevitable impurities.
The content of C (carbon) may be 0.05% or more and 0.1% or less.
C is a highly effective and inexpensive element for stabilizing austenite. C is an interstitial element and contributes to the improvement of strength through solid solution strengthening. Considering this, C may be added in an amount of 0.05% or more. However, when the content of C is excessive, sensitization may occur due to precipitation of carbides, such as Cr23 C6 at grain boundaries in a heat-affected zone after welding, which may degrade ductility, toughness, corrosion resistance, and the like. Considering this, the upper limit of the C content may be limited to 0.1%. Preferably, the content of C may be 0.058% or more and 0.096% or less.
The content of Si (silicon) may be 0.1% or more and 1.0% or less.
Si is an element that acts as a deoxidizer during the steelmaking process and effective in improving corrosion resistance. Considering this, Si may be added in an amount of 0.1% or more. However, when the content of Si is excessive, a delta-ferrite phase may be formed by the peritectic reaction during casting, which may degrade hot workability. In consideration of this, the upper limit of the Si content may be limited to 1.0%. Preferably, the content of Si may be 0.34% or more and 0.97% or less.
The content of Mn (manganese) may be 1.0% or more and 5.0% or less.
Mn is an inexpensive element that stabilizes the austenite phase. In addition, Mn is an effective element for increasing low-temperature impact toughness by suppressing heat-induced and mechanically-induced martensitic transformation. In consideration of this, Mn may be added in an amount of 1.0% or more. However, when the content of Mn is excessive, the amount of inclusions (MnS) may increase, which may degrade the corrosion resistance and hot workability of the steel. In consideration of this, the upper limit of the Mn content may be limited to 5.0%. Preferably, the content of Mn may be 1.02% or more and 3.79% or less.
The content of P (phosphorus) may be from 0% to less than 0.05% or from more than 0% to less than 0.05%.
P is an impurity that is inevitably contained in steel and is an element that reduces corrosion resistance and hot workability. Considering this, the content of P may be 0% or may be more than 0% but less than 0.05%. Preferably, the content of P may be 0.02% or less. In this case, the effect of reducing process costs may be further improved while minimizing the influence of the content of P on the physical properties.
The content of S (sulfur) may be 0% to less than 0.03%, or more than 0% to less than 0.03%.
S, similar to P, is an impurity that is inevitably contained in steel and is an element that reduces corrosion resistance and hot workability. Considering this, the content of S may be 0% or may be more than 0% to less than 0.03%. Preferably, the content of S may be 0.002% or less. In this case, the effect of reducing the process cost may be further improved while minimizing the influence of the content of S on the physical properties.
The content of Cr (chromium) may be 14.0% or more and 18.0% or less.
Cr is an essential element for ensuring corrosion resistance and phase stability. Considering this, Cr may be added in an amount of 14.0% or more. However, when the content of Cr is excessive, a delta-ferrite phase may be formed due to the peritectic reaction, which may degrade the hot workability. Considering this, the upper limit of the Cr content may be limited to 18.0%.
The content of Ni (nickel) may be 1.0% or more and 5.0% or less.
Ni is a strong element that stabilizes the austenite phase. In addition, Ni is effective in preventing a decrease in toughness at extremely low temperatures by suppressing heat-induced and mechanically-induced martensite transformation. In addition, when Ni is added, hot workability and cold workability may be facilitated. Considering this, Ni may be added in an amount of 1.0% or more. However, when the content of Ni is excessive, grain refinement may be hindered. In addition, excessive Ni content may lead to an increase in raw material cost. Considering this, the upper limit of the Ni content may be limited to 5.0%. Preferably, the content of Ni may be 2.61% or more and 4.45% or less.
The content of Cu (copper) may be 0.1% or more and 2.0% or less.
Cu is an element effective in stabilizing the austenite phase. In addition, Cu is an element effective in suppressing heat-induced and mechanically-induced martensitic transformation. Considering this, Cu may be added in an amount of 0.1% or more. However, when the content of Cu is excessive, hot workability may be degraded due to Cu solidification segregation. Considering this, the upper limit of the Cu content may be limited to 2.0%. Preferably, the content of Cu may be 1.19% or more and 1.88% or less.
The content of N (nitrogen) may be 0.1% or more and 0.2% or less.
N is an element highly effective in stabilizing the austenite phase and inexpensive. In addition, N is an element effective in increasing strength through solid solution strengthening and improving corrosion resistance. Considering this, N may be added in an amount of 0.1% or more. However, when the content of N is excessive, hot workability may be degraded. Considering this, the upper limit of the content of N may be limited to 0.2%. Preferably, the content of N may be 0.173% or more and 0.189% or less.
The remainder of the disclosed invention is iron (Fe). However, unintended impurities may inevitably be introduced from raw materials or the surrounding environment in a typical manufacturing process, and thus cannot be excluded. Since such impurities may be well known to those skilled in the art of conventional manufacturing processes, details thereof are not described in this specification.
In general, austenitic stainless steels may develop mechanically-induced martensite phases (mechanically induced ε-martensite and α'-martensite) during cold rolling. The development of mechanically-induced phases tends to vary depending on the phase stability of the austenite. In austenitic stainless steels with a low phase stability, ε-martensite bands develop at the beginning of deformation, and as the amount of deformation increases, α'-martensite may be generated from the intersections within the bands.
In order to reduce expensive Ni and increase cost competitiveness, the phase stability needs to be controlled by utilizing other austenite phase-stabilizing elements besides Ni. To this end, it is required to control the free energy change (ΔG_γ-α) value from the austenite phase to the martensite phase at room temperature.
Meanwhile, when a temper-rolled material is subjected to cold annealing heat treatment, a reverse transformation from the mechanically-induced martensite phase to the austenite phase may occur. The reverse transformation process may be largely divided into diffusional reversion and shear reversion. The reverse transformation process may proceed depending on the free energy change (ΔG_α-γ) from the martensite phase to the austenite phase during the annealing stage. In general, the martensite shear reversion requires more free energy change (ΔG_α-γ) than the diffusion reversion.
Therefore, the disclosed invention is provided to suppress the decrease in hot workability due to the reduction of Ni by controlling the alloy elements. In addition, the disclosed invention is provided to facilitate the transformation from austenite to martensite by controlling the room temperature free energy change (ΔG_γ-α) during cold rolling. In addition, the disclosed invention is provided to facilitate the reverse transformation recrystallization from martensite to austenite during cold-rolled annealing by controlling the free energy change (ΔG_α-γ) during cold-rolled annealing.
In addition, the disclosed invention is provided to ensure excellent strength and corrosion resistance by controlling the grain diameter and uniformity.
The high-strength austenitic stainless steel according to an embodiment may have a value of Expression (1) of less than 1.0, preferably 0.48 or more and less than 1.0. $0.18 Si + 0.45 C + 4.4 N$
In Expression (1), Si, C, and N represent the content (wt %) of each element.
Expression (1) indicates Hot Rolling Index (HRI) as a hot rolling index.
When the value of Expression (1) is controlled to be less than 1.0, hot workability may be improved, and slab edge cracks may not occur. In addition, when the value of Expression (1) is controlled to be 0.48 or more, the effect of grain refinement may be improved. Specifically, the value of Expression (1) may be 0.49 or more and less than 1.0, more specifically 0.76 or more and less than 1.0, and even more specifically 0.86 or more and less than 1.0. Within the range, the high-strength austenitic stainless steel according to an embodiment of the present invention may have an improved hot workability without the occurrence of slab edge cracks while achieving a further improved balance of compositional control and grain refinement, thereby providing more excellent hot workability and corrosion resistance.
In the disclosed invention, the phase stability of the austenite phase and the martensite phase is evaluated using a thermodynamic analysis program (Thermo-Calc. TCFE 6.0) thermodynamic database. Through this, the change in free energy of the austenite phase and the ferrite phase according to the alloying element content and temperature change is calculated to derive a phase stability index.
The high-strength austenitic stainless steel according to an embodiment may have a value of Expression (2) of 3.6 or less, preferably 2.0 or more and 3.6 or less. $0.02 Si + 0.21 Mn + 0.09 Cr + 0.15 Ni + 2.73 (C + N)$
In Expression (2), Si, Mn, Cr, Ni, C, and N represent the content (wt %) of each element.
Expression (2) indicates Austenite Stability Index (ASI) as an indicator of the phase stability of austenite.
When the value of Expression (2) is controlled to be 3.6 or less, the free energy change (ΔG_γ-α) from the austenite phase to the martensite phase may be -2.1 kJ/mol or less. Therefore, when the value of Expression (2) is controlled to be 3.6 or less, transformation into martensite may occur more easily, which may compensate for the austenite phase stability due to the lowered Ni content and promote grain refinement. In addition, when the value of Expression (2) is controlled to be 3.6 or less, the formation of austenite residual structure remaining as a band structure during cold rolling may be minimized, so that uniform grains may be implemented. When the value of Expression (2) is controlled to be 2.0 or more, improvements in hot workability and grain refinement may be enhanced. Specifically, the value of Expression (2) may be 2.6 or more and 3.6 or less, more specifically 2.9 or more and 3.6 or less, and even more specifically 3.1 or more and 3.6 or less. Within the range, the high-strength austenitic stainless steel according to an embodiment of the present invention may exhibit a free energy change (ΔG_γ-α) from the austenite phase to the martensite phase of -2.1 kJ/mol or less while achieving a further improved balance of compositional control and grain refinement, thereby achieving more excellent hot workability and corrosion resistance.
The high-strength austenitic stainless steel according to an embodiment may have a value of Expression (3) of 0.35 or more, preferably 0.35 or more and 0.71 or less. $- 0.04 Si + 0.05 Mn - 0.01 Cr + 0.05 Ni + 1.18 (C + N)$
In Expression (3), Si, Mn, Cr, Ni, C, and N represent the content (wt %) of each element.
Expression (3) represents Austenite Recrystallization Index (ARI) as an indicator of recrystallization of the austenite phase.
When the value of Expression (3) is 0.35 or more, the free energy change (ΔG_α-γ) from the martensite phase to the austenite phase may be -0.44 kJ/mol or less. Therefore, when the value of Expression (3) is 0.35 or more, recrystallization by diffusion reverse transformation during cold-rolled annealing may be facilitated. In addition, when the value of Expression (3) is 0.35 or more, the formation of the residual martensite band structure is minimized, thereby suppressing defect generation during forming, and achieving an improved surface appearance. When the value of Expression (3) is 0.71 or less, the free energy change (ΔG_α-γ) is -0.44 kJ/mol or less while the effect of grain refinement may be further improved. Specifically, the value of Expression (3) may be 0.4 or more and 0.71 or less, more specifically 0.44 or more and 0.71 or less, and even more specifically 0.44 or more and less than 0.6. Within the range, the high-strength austenitic stainless steel according to an embodiment of the present invention may have a free energy change (ΔG_α-γ) from the martensite phase to the austenite phase of -0.44 kJ/mol or less while achieving a further improved balance of compositional control and grain refinement, thereby realizing more excellent hot workability and corrosion resistance.
The high-strength austenitic stainless steel according to an embodiment may have a free energy change (ΔG_γ-α) from the austenite phase to the martensite phase of -2.1 kJ/mol or less, specifically -2.19 kJ/mol or less, more specifically -2.19 kJ/mol or less at 25°C. In addition, the lower limit of the free energy change (ΔG_γ-α) from the austenite phase to the martensite phase at 25°C may be, for example, -5.0 kJ/mol or more, -3.0 kJ/mol or more. Within the range, the high-strength austenitic stainless steel according to an embodiment may compensate for the austenite phase stability due to the lowered Ni content and promote grain refinement, thereby realizing higher yield strength and excellent hot workability and corrosion resistance.
The high-strength austenitic stainless steel according to an embodiment may have a free energy change (ΔG_α-γ) from a martensite phase to an austenite phase at 850°C of -0.44 kJ/mol or less, specifically - 0.5 kJ/mol or less, more specifically -0.55 kJ/mol or less. In addition, the lower limit of the free energy change (ΔG_α-γ) from a martensite phase to an austenite phase at 850°C may be, for example, -1.9 kJ/mol or more, -0.9 kJ/mol or more. Within the range, the high-strength austenitic stainless steel according to an embodiment is more effective in recrystallization by diffusion transformation during cold-rolled annealing, and is effective in minimizing the formation of residual martensite band structure, thereby realizing higher yield strength and excellent hot workability and corrosion resistance.
By controlling the above described alloy elements, Expressions (1), (2), and (3), and a manufacturing method described below, the high-strength austenitic stainless steel according to an embodiment may have a grain diameter deviation in the thickness direction of 2 or less, and an average grain diameter of a central portion in the thickness direction of 10 µm or less. In this case, the balance of compositional control and grain refinement may be improved, thereby providing a low-Ni austenitic stainless steel having high yield strength, excellent hot workability, and corrosion resistance.
In addition, the high-strength austenitic stainless steel according to an embodiment may have a yield strength of 600 MPa or more, an elongation of 35% or more, and a corrosion potential of 200 mV or more.
In addition, the high-strength austenitic stainless steel according to an embodiment may implement sufficient strength and corrosion resistance and thus has a thickness of 0.5 to 2.0 mm.
Next, a method of manufacturing a high-strength austenitic stainless steel according to another aspect of the disclosed invention will be described.
A method of manufacturing a high-strength austenitic stainless steel according to one embodiment may include: preparing a slab comprising 0.05% to 0.1% of C, 0.1% to 1.0% of Si, 1.0% to 5.0% of Mn, less than 0.05% of P, less than 0.03% of S,14.0% to 18.0% of Cr, 1.0% to 5.0% of Ni, 0.1% to 2.0% of Cu, 0.1% to 0.2% of N, and the balance being Fe and inevitable impurities; hot-rolling the slab and then reheating to produce a hot-rolled steel sheet; and cold-rolling the hot-rolled steel sheet and then performing cold-rolled annealing at 850 to 900°C to produce a cold-rolled steel sheet.
The slab may have a value of Expression (1) of less than 1.0, preferably 0.48 or more and less than 1.0. $0.18 Si + 0.45 C + 4.4 N$
In Expression (1), Si, C, and N represent the content (wt %) of each element.
In addition, the slab may have a value of Expression (2) of less than 3.6, preferably 2.0 or more and 3.6 or less. $0.02 Si + 0.21 Mn + 0.09 Cr + 0.15 Ni + 2.73 (C + N)$
In Expression (2), Si, Mn, Cr, Ni, C, and N represent the content (wt %) of each element.
In addition, the slab may have a value of Expression (3) of 0.35 or more, and preferably 0.35 or more and 0.71 or less. $- 0.04 Si + 0.05 Mn - 0.01 Cr + 0.05 Ni + 1.18 (C + N)$
In Expression (3), Si, Mn, Cr, Ni, C, and N represent the content (wt %) of each element.
The reasons for numerically limiting the component range of each alloy composition, Expression (1), Expression (2), and Expression (3) are as described above, and the following provides details of manufacturing operations.
After preparing a slab satisfying the above alloy composition, Expression (1), Expression (2), and Expression (3), a series of processes including hot rolling, reheating, cold rolling, and cold-rolled annealing processes may be performed.
First, the slab may be heated at 1200 to 1350°C, hot-rolled, and then reheated to 1000 to 1150°C to produce a hot-rolled steel sheet.
By the reheating at 1000 to 1150°C, coarse precipitates generated during the production of the hot-rolled steel sheet may be re-decomposed, and the internal grain sizes may be appropriately controlled.
Next, the hot-rolled steel sheet may be cold-rolled with a thickness reduction ratio of 60% or more.
By the cold-rolling with a thickness reduction ratio of 60% or more, most of the structure is transformed into martensite, thereby compensating for the austenite phase stability while implementing grain refinement.
After the cold-rolling, cold-rolled annealing may be performed at 850 to 900°C to produce a cold-rolled steel sheet.
By the cold-rolled annealing at 850 to 900°C, the reverse transformation recrystallization from martensite to austenite may be easily performed. Preferably, the cold-rolled annealing may be performed at 850 to 890°C.
Hereinafter, the present invention will be described in more detail through embodiments. However, the descriptions of the embodiments are only for illustrating the implementation of the present invention, and the present invention is not limited by the descriptions of the embodiments. This is because the scope of the rights of the present invention is determined by matters described in the scope of claims and matters reasonably inferred therefrom.

{Embodiment}

For the various alloy composition ranges shown in Table 1 below, slabs were prepared in a vacuum induction melting furnace. The prepared slabs were heated at 1250°C for 2 hours and then hot-rolled to a thickness of 3.0 mm to produce hot-rolled materials. The hot-rolled materials were reheated at 1100°C for 10 minutes and then water-cooled to produce hot-rolled steel sheets. The hot-rolled steel sheets were cold rolled with a thickness reduction ratio of 70% to produce a cold-rolled material having a thickness of 0.9 mm. The cold-rolled materials were subjected to cold-rolled annealing at 850°C to produce cold-rolled steel sheets.

[Table 1]

Classification	Alloy composition (wt %)
Classification	C	Si	Mn	P	S	Cr	Ni	Cu	N
Example 1	0.096	0.34	1.02	0.02	0.001	14.39	4.24	1.61	0.189
Example 2	0.058	0.97	3.79	0.01	0.001	16.21	4.15	1.72	0.173
Example 3	0.067	0.36	3.70	0.02	0.002	14.84	2.61	1.30	0.183
Example 4	0.064	0.68	2.35	0.02	0.001	14.48	4.45	1.19	0.186
Example 5	0.061	0.35	3.72	0.02	0.001	17.8	3.61	1.88	0.182
Comparative Example 1	0.061	0.26	5.85	0.02	0.002	17.59	2.00	2.00	0.153
Comparative Example 2	0.100	0.53	9.71	0.01	0.002	17.44	0.06	1.36	0.161
Comparative Example 3	0.060	0.72	1.55	0.02	0.001	15.19	4.84	1.08	0.224
Comparative Example 4	0.070	0.94	1.34	0.01	0.001	14.46	0.54	0.54	0.181
Comparative Example 5	0.076	0.31	6.06	0.01	0.002	16.05	1.93	0.51	0.213

Table 2 below shows the values of Expression (1), Expression (2), Expression (3), the calculated values of thermodynamic free energy change ΔG_γ-α (25°C), and the calculated values of ΔG_α-γ (850°C). The value of Expression (1) was calculated by Expression (1) below. $0.18 Si + 0.45 C + 4.4 N$
In Expression (1), Si, C, and N represent the contents (weight%) of each element.
The value of Expression (2) was calculated by Expression (2) below. $0.02 Si + 0.21 Mn + 0.09 Cr + 0.15 Ni + 2.73 (C + N)$
In Expression (2), Si, Mn, Cr, Ni, C, and N represent the contents (weight%) of each element.
The value of Expression (3) was calculated by Expression (3) below. Expression (3): -0.04 Si + 0.05 Mn - 0.01 Cr + 0.05 Ni + 1.18 (C + N)
In Expression (3), Si, Mn, Cr, Ni, C, and N represent the content (weight%) of each element.

The calculated values of the thermodynamic free energy change ΔG_γ-α (25°C) and ΔG_α-γ (850°C) were obtained by calculating the change in free energy of the austenite phase and the ferrite phase based on the alloying element contents and temperature variations, using the Thermo-Calc. TCFE 6.0 thermodynamic database.

[Table 2]

Classification	Expression (1)	Expression (2)	Expression (3)	ΔG_γ-α (25°C) (kJ/mol)	ΔG_α-γ (85°C) (kJ/mol)
Example 1	0.94	2.93	0.44	-2.61	-0.60
Example 2	0.96	3.53	0.47	-2.19	-0.55
Example 3	0.90	3.19	0.45	-2.50	-0.52
Example 4	0.97	3.16	0.46	-2.51	-0.57
Example 5	0.89	3.60	0.46	-2.19	-0.55
Comparative Example 1	0.75	3.70	0.46	-2.06	-0.45
Comparative Example 2	0.85	4.34	0.60	-1.65	-0.42
Comparative Example 3	1.14	3.21	0.47	-2.48	-0.59
Comparative Example 4	1.00	2.37	0.21	-3.17	-0.28
Comparative Example 5	1.03	3.80	0.57	-2.05	-0.54

Table 3 below shows an average grain diameter, a grain-to-grain diameter deviation, the occurrence of hot-rolled steel sheet cracks, the occurrence of cold-rolled steel sheet recrystallization, a yield strength, an elongation, and a corrosion potential. The average grain diameter was measured by photographing a thickness central portion of the cold-rolled steel sheet with a scanning electron microscope (SEM). Meanwhile, in the disclosed invention, the "average" refers to the average value of the values measured at five random points. In addition, the thickness central portion refers to a region between 1/4 t and 3/4 t when the thickness is t.
The grain-to-grain diameter deviation was expressed by photographing the thickness central portion of the cold-rolled steel sheet with a SEM and calculating the grain-to-grain diameter deviation in the thickness direction.
Meanwhile, in the disclosed invention, the deviation is calculated using the standard deviation by conventional methods.
The occurrence of hot-rolled steel sheet cracks was determined based on whether an edge crack has occurred in the hot-rolled steel sheet. The occurrence of hot-rolled steel sheet cracks was indicated as "good" when no edge cracks have occurred, and "bad" when edge cracks have occurred.
Meanwhile, in the disclosed invention, when a crack of 3 mm or more in the width direction appeared, it is considered that an edge crack has occurred.
The occurrence of cold-rolled steel sheet recrystallization was determined based on an area fraction of residual martensite band structures by photographing the thickness central portion of the cold-rolled steel sheet with a SEM. The occurrence of cold-rolled steel sheet recrystallization was indicated as "good" when the residual martensite band structure was less than 3%, and "bad" when the residual martensite band structure was 3% or more.
The yield strength and the elongation were measured by performing a test at room temperature using a Zwick Roell tensile tester, using a JIS13B tensile test specimen at a tensile speed of 20 mm per minute.

The corrosion potential was measured using a potentiostat according to the KS D 0238 standard. In this case, when stainless steel was immersed in a NaCl solution and a voltage of 20 mV/min was applied, the value of the potential at which the current reached 100 µA was measured (a pitting potential). Here, the temperature of the NaCl solution was 30°C, and the concentration of the NaCl solution was set to 3.5%. Meanwhile, a higher corrosion potential value indicates a better corrosion resistance.

[Table 3]

Classification	Average grain diameter (µm)	Grain-to-grain diameter deviation	Occurrence of hot-rolled steel sheet cracks	Occurrence of cold-rolled steel sheet recrystallizatio n	Yield strength (MPa)	Elongation (%)	Corrosion potential (mV)
Example 1	4.5	1.6	O	O	618	37	241
Example 2	7.3	1.8	O	O	605	38	237
Example 3	4.2	1.2	O	O	610	40	235
Example 4	5.1	1.7	O	O	623	40	222
Example 5	7.1	1.8	O	O	622	41	205
Comparative Example 1	11.2	3.6	O	O	590	43	212
Comparative Example 2	13.1	2.5	O	O	580	40	148
Comparative Example 3	8.3	1.7	X	O	620	36	231
Comparative Example 4	9.2	2.6	X	X	710	31	232
Comparative Example 5	11.2	3.3	X	O	585	40	173

Referring to Tables 2 and 3, Examples 1 to 5 satisfied the alloy compositions, the values of Expressions (1) to (3), and the manufacturing methods presented in the disclosed invention. Therefore, Examples 1 to 5 satisfied the grain diameter deviation in the thickness direction of 2 or less, the average grain diameter in the thickness central portion of 10 µm or less, and the occurrence of hot-rolled steel sheet cracks and the occurrence of cold-rolled steel sheet recrystallization were good. In addition, Examples 1 to 5 satisfied the yield strength of 600 MPa or more, the elongation of 35% or more, and the corrosion potential of 200 mV or more. However, Comparative Examples 1, 2, and 5 did not satisfy the value of Expression (2) being 3.6 or less, and the value of ΔG_γ-α (25°C) being -2.1 kJ/mol or less. Therefore, Comparative Examples 1, 2, and 5 did not satisfy the grain diameter deviation in the thickness direction of 2 or less and the average grain diameter in the thickness central portion of 10 µm or less, and did not satisfy the yield strength of 600 MPa or more. That is, Comparative Examples 1, 2, and 5 did not have sufficient mechanically-induced martensite transformation, resulting in austenite residual structure remaining as a coarse structure, thereby leading to inferior strength.
Comparative Examples 3 to 5 did not satisfy the value of Expression (1) being less than 1.0. Therefore, Comparative Examples 3 to 5 were poor in the hot-rolled steel sheet cracking, resulting in reduced hot workability.
Comparative Example 4 did not satisfy the value of Expression (3) being 0.35 or more. Therefore, Comparative Example 4, despite a low austenite phase stability, has a large amount of residual martensite structures, leading to poor cold-rolled steel sheet recrystallization. Therefore, Comparative Example 4 had a high yield strength, but did not achieve an elongation of 35% or more. In addition, Comparative Example 4 had a large number of unrecrystallized structures, which caused defects during forming processing and resulted in poor quality.
Comparative Examples 2 and 5 contained excessive Mn. Therefore, Comparative Examples 2 and 5 did not satisfy a corrosion potential of 200 mV or more. Therefore, Comparative Examples 2 and 5 had poor corrosion resistance.
FIG. 1 is an image of the microstructure of a high-strength austenitic stainless steel according to an example of the disclosed invention, captured using electron backscatter diffraction (EBSD).
Referring to FIG. 1, it can be seen that, according to an example of the disclosed invention, high yield strength and corrosion resistance may be achieved by implementing grain refinement.
According to an embodiment of the disclosed invention, by controlling a phase stability to achieve grain refinement, a low-cost austenitic stainless steel having high yield strength and corrosion resistance and improved manufacturability and a method of manufacturing the same may be provided.

Claims

A high-strength austenitic stainless steel comprising:
0.05% to 0.1% of C, 0.1% to 1.0% of Si, 1.0% to 5.0% of Mn, less than 0.05% of P, less than 0.03% of S,14.0% to 18.0% of Cr, 1.0% to 5.0% of Ni, 0.1% to 2.0% of Cu, 0.1% to 0.2% of N, and the balance being Fe and inevitable impurities, and

having a grain diameter deviation in a thickness direction of 2 or less.
The high-strength austenitic stainless steel of claim 1, wherein a value of Expression (1) below is less than 1.0: $0.18 Si + 0.45 C + 4.4 N$ (wherein in Expression (1), Si, C, and N represent the content (wt %) of each element).
The high-strength austenitic stainless steel of claim 1, wherein a value of Expression (2) below is 3.6 or less: $0.02 Si + 0.21 Mn + 0.09 Cr + 0.15 Ni + 2.73 (C + N)$ (wherein in Expression (2), Si, Mn, Cr, Ni, C, and N represent the content (wt %) of each element).
The high-strength austenitic stainless steel of claim 1, wherein a value of Expression (3) below is 0.35 or more: $- 0.04 Si + 0.05 Mn - 0.01 Cr + 0.05 Ni + 1.18 (C + N)$ (wherein in Expression (3), Si, Mn, Cr, Ni, C, and N represent the content (wt %) of each element).
The high-strength austenitic stainless steel of claim 1, wherein an average grain diameter of a central portion in a thickness direction is 10 µm or less.
The high-strength austenitic stainless steel of claim 1, wherein a yield strength is 600 MPa or more.
The high-strength austenitic stainless steel of claim 1, wherein an elongation is 35% or more.
The high-strength austenitic stainless steel of claim 1, wherein a corrosion potential is 200 mV or more.
The high-strength austenitic stainless steel of claim 1, wherein a thickness is from 0.5 to 2.0 mm.
A method of manufacturing a high-strength austenitic stainless steel, the method comprising:
preparing a slab comprising 0.05% to 0.1% of C, 0.1% to 1.0% of Si, 1.0% to 5.0% of Mn, less than 0.05% of P, less than 0.03% of S,14.0% to 18.0% of Cr, 1.0% to 5.0% of Ni, 0.1% to 2.0% of Cu, 0.1% to 0.2% of N, and the balance being Fe and inevitable impurities;

hot-rolling the slab and then reheating to produce a hot-rolled steel sheet; and

cold-rolling the hot-rolled steel sheet and then performing cold-rolled annealing at 850 to 900°C to produce a cold-rolled steel sheet.
The method of claim 10, wherein the slab has a value of less than 1.0 in Expression (1) below: $0.18 Si + 0.45 C + 4.4 N$ (wherein in Expression (1), Si, C, and N represent the content (wt %) of each element).
The method of claim 10, wherein the slab has a value of 3. 6 or less in Expression (2) below: $0.02 Si + 0.21 Mn + 0.09 Cr + 0.15 Ni + 2.73 (C + N)$ (wherein in Expression (2), Si, Mn, Cr, Ni, C, and N represent the content (wt %) of each element).
The method of claim 10, wherein the slab has a value of 0.35 or more in Expression (3) below: $- 0.04 Si + 0.05 Mn - 0.01 Cr + 0.05 Ni + 1.18 (C + N),$ (wherein in Expression (3), Si, Mn, Cr, Ni, C, and N represent the content (wt %) of each element).
The method of claim 10, wherein the reheating is performed at 1000 to 1150°C.
The method of claim 10, wherein the cold rolling is performed with a thickness reduction ratio of 60% or more.