Austenitic stainless steel and its manufacturing method

Abstract

The present invention provides an austenitic stainless steel that has excellent cost competitiveness while ensuring excellent yield strength and elongation, has high corrosion resistance, and can be used as an industrial material. Specifically, the present invention provides a high-strength, high-ductility austenitic stainless steel sheet that can be used for automobile exterior panels, building components, etc., through the ultrafine grain manufacturing technology, and a manufacturing method thereof. [Solution] An austenitic stainless steel characterized by containing, by weight, carbon (C): 0.005-0.060%, silicon (Si): 0.1-1.5%, manganese (Mn): 5.0-10.0%, nickel (Ni): more than 0% and not more than 3%, chromium (Cr): 14.0-18.0%, copper (Cu): more than 0% and not more than 2.0%, nitrogen (N): 0.01-0.25%, and the balance being iron (Fe) and unavoidable impurities, having an α value defined by the following formula (1) of 10,000 or more, and a β value defined by the following formula (2) of 100 or more. Formula (1): α=YS×EL-200(8[Ni]+[Cr]+3[Mn]) TIFF2025500865000016.tif11147

Description

[Technical field]

[0001] The present invention relates to an austenitic stainless steel and a manufacturing method thereof, and more particularly to an austenitic stainless steel that can simultaneously ensure high strength and ductility even when annealed at low temperatures, and has excellent cost competitiveness and corrosion resistance, and a manufacturing method thereof. [Background technology]

[0002] Stainless steel has excellent corrosion resistance and does not require additional capital investment to improve corrosion resistance, making it suitable for mass production of small varieties, which is a recent market trend, as well as transportation and construction parts. In particular, austenitic stainless steel has the advantage of being easy to make into complex shapes to meet various customer requests, as it has excellent formability and elongation, and has the advantage of having a beautiful appearance.

[0003] However, the yield strength of frequently used austenitic stainless steel is 200 to 350 MPa, which is lower than that of carbon steel, and the application of the stainless steel to structures is limited. In order to obtain a higher yield strength in such general-purpose 300 series stainless steel, an additional temper rolling process is performed after the final annealing, but this causes problems of increased costs and a sharp decrease in the elongation rate and formability of the material. In addition, general stainless steel contains expensive alloy components, which causes a problem of poor cost competitiveness. In particular, Ni contained in austenitic stainless steel not only has unstable supply and demand due to extreme fluctuations in material prices, but also makes it difficult to ensure stability in supply prices, and at the same time, the material price itself is high, which causes problems in terms of price competitiveness.

[0004] For example, Patent Document 1 relates to an austenitic stainless steel and a manufacturing method thereof, and discloses an austenitic stainless steel containing C: 0.10% or less, Si: 1.0% or less, Mn: 2.1 to 6.0%, P: 0.045% or less, S: 0.1% or less, Ni: 8.0 to 16.0%, Cr: 15.0 to 30.0%, Mo: 1.0 to 5.0%, N: 0.05 to 0.45%, Nb: 0 to 0.50%, and V: 0 to 0.50%, with the balance being Fe and impurities, having a chemical composition that satisfies a specific formula (1), has a grain size number of less than 8.0, and has a tensile strength of 690 MPa or more. However, there is a problem in that a large nickel content is required.

[0005] Meanwhile, the technology to realize ultra-fine grains in austenitic stainless steel generally involves transforming the austenite phase into martensite phase through cold rolling, and then annealing at a low temperature to realize ultra-fine grains. However, even if ultra-fine grains are realized, it is not easy to realize a material with excellent yield strength and elongation at the same time. This is because the content of Ni, Cr, Mn, etc. varies within a range where cost competitiveness can be ensured, the amount of martensite phase transformation due to cold working differs depending on the ASP (Austenitic Stability Parameter) value, and the elongation changes depending on the TRIP (Transformation Induced Plasticity) transformation behavior during tensile testing.

[0006] Therefore, there is a need to develop a material and a manufacturing method thereof that can ensure excellent yield strength, elongation, and corrosion resistance while minimizing the content of expensive alloy elements such as Ni. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Korean Patent Publication No. 10-2016-0138277 Summary of the Invention [Problem to be solved by the invention]

[0008] The present invention provides an austenitic stainless steel that has excellent cost competitiveness while ensuring excellent yield strength and elongation, has high corrosion resistance, and can be used as an industrial material.

[0009] Specifically, the present invention provides a high-strength, high-ductility austenitic stainless steel sheet that can be used for automobile exterior panels, building components, etc., through the ultrafine grain manufacturing technology, and a manufacturing method thereof.

[0010] However, the problems to be solved by the present application are not limited to those mentioned above, and other problems not mentioned will be clearly understood by those skilled in the art from the following description. [Means for solving the problem]

[0011] In order to achieve the above-mentioned object, the austenitic stainless steel of the present invention contains, by weight%, 0.005 to 0.060% carbon (C), 0.1 to 1.5% silicon (Si), 5.0 to 10.0% manganese (Mn), more than 0% and not more than 3% nickel (Ni), 14.0 to 18.0% chromium (Cr), more than 0% and not more than 2.0% copper (Cu), 0.01 to 0.25% nitrogen (N), with the balance being iron (Fe) and unavoidable impurities, and is characterized in that it has an α value defined by the following formula (1) of 10,000 or more, and a β value defined by the following formula (2) of 100 or more.

[0012] Formula (1): α=YS×EL-200(8[Ni]+[Cr]+3[Mn])

[0013] TIFF2025500865000002.tif11147

[0014] In the formulas (1) and (2), [Ni], [Cr], and [Mn] represent the content (wt%) of each element, YS represents the yield stress (MPa), EL represents the elongation rate (%), ASP (Austenitic Stability Parameter) is a value calculated by the following formula (2-1), and d represents the average crystal grain size (μm) at the thickness center,

[0015] Formula (2-1): ASP=551-462([C]+[N])-9.2[Si]-8.1[Mn]-13.7[Cr]-29([Ni]+[Cu])

[0016] In the formula (2-1), [C], [N], [Si], [Mn], [Cr], [Ni], and [Cu] mean the content (wt%) of each element.

[0017] The average crystal grain size (d) at the thickness center portion is also 5.0 μm or less.

[0018] The ASP value of the formula (2) satisfies the range of 15-70.

[0019] Moreover, the austenitic stainless steel has a pitting potential value of 100 mV or more.

[0020] The austenitic stainless steel has a yield strength of 540.0 MPa or more and an elongation of 30.0% or more.

[0021] In order to achieve the above-mentioned object, the method for producing an austenitic stainless steel of the present invention comprises, by weight percent, 0.005-0.060% carbon (C), 0.1-1.5% silicon (Si), 5.0-10.0% manganese (Mn), more than 0% and not more than 3% nickel (Ni), 14.0-18.0% chromium (Cr), more than 0% and not more than 2.0% copper (Cu), 0.01-0.25% nitrogen (N), and the balance being iron (Fe) and unavoidable impurities. The present invention provides a steel sheet having a final annealing step, the final annealing step comprising the steps of: providing a steel material; hot rolling the steel material to produce a hot-rolled steel sheet; cold rolling the hot-rolled steel sheet to produce a cold-rolled steel sheet; and final annealing the cold-rolled steel sheet, the final annealed steel sheet being controlled to have an α value defined by the following formula (1) of 10,000 or more, a β value defined by the following formula (2) of 100 or more, and a γ value defined by the following formula (3) of 0 or more.

[0022] Formula (1): α=YS×EL-200(8[Ni]+[Cr]+3[Mn])

[0023] TIFF2025500865000003.tif11147

[0024] TIFF2025500865000004.tif10147

[0025] In the formulas (1) to (3), [Ni], [Cr], [Mn], and [N] represent the content of each element (wt%), YS represents the yield stress (MPa), EL represents the elongation rate (%), ASP (Austenitic Stability Parameter) is a value calculated by the following formula (2-1), d represents the average crystal grain size (μm) at the center of the thickness, and Temp represents the final annealing temperature (°C).

[0026] Formula (2-1): ASP=551-462([C]+[N])-9.2[Si]-8.1[Mn]-13.7[Cr]-29([Ni]+[Cu])

[0027] In the formula (2-1), [C], [N], [Si], [Mn], [Cr], [Ni], and [Cu] mean the content (wt%) of each element.

[0028] The final annealing step is performed at a temperature in the range of 750 to 850°C.

[0029] The method further includes a step of performing primary annealing on the hot-rolled steel sheet at 1000 to 1150° C. before the cold rolling.

[0030] In the cold rolling step, the cold rolled steel sheet is rolled at a thickness reduction rate of 50% or more of the hot rolled steel sheet in a room temperature range.

[0031] Moreover, the austenitic stainless steel satisfies the range of 15-70 for the ASP value.

[0032] Furthermore, the final annealed steel sheet has an average grain size (d) at the center of the thickness of 5.0 μm or less. Effect of the Invention

[0033] According to the present invention, it is possible to provide an austenitic stainless steel which is excellent in yield strength and elongation, and has a low raw material cost due to a minimal Ni content.

[0034] The present invention can provide an ultrafine-grained austenitic stainless steel that is cost-competitive while also having excellent strength, ductility, and corrosion resistance, and a method for producing the same. [Brief description of the drawings]

[0035] [Figure 1] The ranges of the examples and the comparative examples are shown based on the α value calculated by formula (1) and the β value calculated by formula (2). [Diagram 2] 1 shows the average crystal grain size at the center of the thickness of the austenitic stainless steels according to the examples and the comparative examples. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0036] The austenitic stainless steel of the present invention contains, by weight, 0.005-0.060% carbon (C), 0.1-1.5% silicon (Si), 5.0-10.0% manganese (Mn), more than 0% and not more than 3% nickel (Ni), 14.0-18.0% chromium (Cr), more than 0% and not more than 2.0% copper (Cu), 0.01-0.25% nitrogen (N), with the balance being iron (Fe) and unavoidable impurities, and satisfies an α value defined by the following formula (1) of 10,000 or more, and a β value defined by the following formula (2) of 100 or more.

[0037] Formula (1): α=YS×EL-200(8[Ni]+[Cr]+3[Mn])

[0038] TIFF2025500865000005.tif11147

[0039] In the formula (1) and formula (2), [Ni], [Cr], and [Mn] mean the content of each element (wt%). YS stands for yield stress (MPa) EL means elongation rate (%), ASP (Austenitic Stability Parameter) is a value calculated by the following formula (2-1): d means the average crystal grain size (μm) at the center of the thickness.

[0040] Formula (2-1): ASP=551-462([C]+[N])-9.2[Si]-8.1[Mn]-13.7[Cr]-29([Ni]+[Cu])

[0041] In the formula (2-1), [C], [N], [Si], [Mn], [Cr], [Ni], and [Cu] mean the content (wt%) of each element.

[0042] The terms used herein are for the purpose of describing the invention and are not intended to be limiting of the invention. Additionally, as used herein, the singular forms include the plural forms unless the relevant definition clearly indicates otherwise.

[0043] Hereinafter, unless otherwise specified, the unit is % by weight. Furthermore, when a part "contains" a certain component, this does not mean that other components are excluded, but that other components may be further contained, unless otherwise specified.

[0044] Unless otherwise defined, all terms, including technical and scientific terms, used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Dictionary-defined terms are to be interpreted as having a meaning consistent with the relevant technical literature and the present disclosure.

[0045] In addition, in this specification, the terms "about," "substantially," and the like are used to mean a numerical value or a value close to that numerical value when the tolerances of manufacturing and materials inherent in the referred meaning are given, and are used to prevent unscrupulous infringers from unfairly using the disclosure content in which precise and absolute numerical values ​​are mentioned to aid in the understanding of the present invention.

[0046] Ultra-fine grain (UFG) materials have characteristics such as excellent strength-elongation balance, fatigue resistance, etching workability, etc., and the present invention provides an austenitic stainless steel having ultra-fine grains that can realize high strength-high ductility and a manufacturing method thereof.Furthermore, the present invention is characterized by providing an austenitic stainless steel sheet having a yield strength and elongation suitable for structural members such as automobile exterior panels and building parts.

[0047] In addition, the present invention uses manganese and nitrogen to maintain excellent performance while reducing the use of expensive elements such as nickel to improve the poor cost competitiveness of austenitic stainless steels.

[0048] The austenitic stainless steel of the present invention will be described in detail below.

[0049] The austenitic stainless steel of the present invention contains, by weight, carbon (C): 0.005-0.060%, silicon (Si): 0.1-1.5%, manganese (Mn): 5.0-10.0%, copper (Cu): more than 0% and not more than 2.0%, nickel (Ni): more than 0% and not more than 3%, chromium (Cr): 14.0-18.0%, nitrogen (N): 0.01-0.25%, and the remainder being iron (Fe) and unavoidable impurities.

[0050] [Component range] Carbon (C):0.005%~0.060% Carbon is an effective element for stabilizing the austenite phase, and may be added in an amount of 0.005% or more to ensure the yield strength of austenitic stainless steels. However, if the carbon content is excessive, it not only reduces cold workability due to the solid solution strengthening effect, but also induces grain boundary precipitation of Cr carbides during low-temperature annealing, which may adversely affect ductility, toughness, corrosion resistance, etc., so the upper limit may be limited to 0.060%.

[0051] Silicon (Si): 0.1% to 1.5% Silicone is an element that acts as a deoxidizer in the steelmaking process and is effective in improving corrosion resistance, and may be added in an amount of 0.01% or more. However, silicon is an element that is effective in stabilizing the ferrite phase, and when added in excess, it not only promotes the formation of delta (δ) ferrite in the casting material, reducing hot workability, but may also adversely affect the ductility and impact properties of the material, so the upper limit may be limited to 1.5%.

[0052] Manganese (Mn): 5.0% to 10.0% Manganese is an austenite phase stabilizing element added in place of nickel (Ni) in the present invention, and may be added in an amount of 5.0% or more to improve austenite stability. However, if its content is excessive, it may form excessive amounts of S-based inclusions (MnS), which may reduce the ductility, toughness, and corrosion resistance of austenitic stainless steel, and may generate Mn fumes during the steelmaking process, which may pose a manufacturing risk, so its upper limit may be limited to 10.0%.

[0053] Nickel (Ni): over 0% and up to 3.0% Nickel is a strong austenite phase stabilizing element and is essential for ensuring good hot and cold workability. However, Ni is an expensive element, and adding a large amount of Ni leads to an increase in raw material costs. Therefore, taking into consideration both the cost and efficiency of the steel, the upper limit of Ni may be limited to 3.0%.

[0054] Chromium (Cr): 14.0% to 18.0% Chromium is a ferrite stabilizing element, but is also effective in suppressing the formation of martensite phase, and is a basic element that ensures the corrosion resistance required of stainless steel, so it may be added in an amount of 14.0% or more. However, if the content is excessive, the manufacturing cost increases, and a large amount of delta (δ) ferrite is formed in the material, which reduces hot workability and adversely affects material properties, so the upper limit may be limited to 18.0%.

[0055] Copper (Cu): More than 0% but less than 2.0% Copper is an austenite phase stabilizing element, and is added in place of nickel (Ni) in the present invention. Cu may be added as an element to improve corrosion resistance in a reducing environment. However, if the content is excessive, not only will the material cost increase, but there will also be problems with liquefaction and low-temperature brittleness. Therefore, in consideration of the cost-effectiveness and material properties of the steel, the upper limit may be limited to 2.0%.

[0056] Nitrogen (N): 0.01% to 0.25% Nitrogen is a strong austenite stabilizing element and is effective in improving the corrosion resistance and yield strength of austenitic stainless steels, and may be added in an amount of 0.01% or more. However, if the content is excessive, the material may become hard and its hot workability may be deteriorated due to the solid solution strengthening effect, so the upper limit may be limited to 0.25%.

[0057] Other Ingredients The austenitic stainless steel of the present invention may further contain at least one of phosphorus (P) and sulfur (S), which are unavoidable impurities.

[0058] The phosphorus (P) content is 0.035% or less. Phosphorus (P) is an impurity inevitably contained in steel, and is an element that is the main cause of grain boundary corrosion and impairs hot workability, so it is preferable to control the content as low as possible. In the present invention, the upper limit of the P content is controlled to 0.035% or less.

[0059] The sulfur (S) content is 0.01% or less. Sulfur (S) is an impurity inevitably contained in steel, and is an element that segregates at grain boundaries and is the main cause of impairing hot workability, so it is preferable to control the content as low as possible. In the present invention, the upper limit of the S content is controlled to 0.01% or less.

[0060] The remaining component of the present invention is iron (Fe). However, in normal manufacturing processes, unintended impurities may be inevitably mixed in from raw materials or the surrounding environment, and it is not possible to exclude them. Since any technician in normal manufacturing processes would know about these impurities, the contents of all of them are not specifically mentioned in this specification.

[0061] [Parameters and properties] In addition, the austenitic stainless steel is characterized in that the α value defined by the following formula (1) is 10,000 or more. When the α value is 10,000 or more, there is an advantage in that high strength and high ductility can be realized while being cost competitive.

[0062] Formula (1): α=YS×EL-200(8[Ni]+[Cr]+3[Mn])

[0063] In the formula (1), [Ni], [Cr], and [Mn] represent the content (wt%) of each element, YS represents the yield stress (MPa), and EL represents the elongation rate (%).

[0064] In addition, the austenitic stainless steel is characterized in that the β value defined by the following formula (2) is 100 or more. When the β value is 100 or more, ultrafine grains are formed, and high strength and high ductility can be simultaneously realized.

[0065] TIFF2025500865000006.tif11147

[0066] In the above formula (2), ASP (Austenitic Stability Parameter) is a value calculated by the following formula (2-1), and d means the average crystal grain size (μm) at the center of the thickness.

[0067] Formula (2-1): ASP=551-462([C]+[N])-9.2[Si]-8.1[Mn]-13.7[Cr]-29([Ni]+[Cu])

[0068] In the formula (2-1), [C], [N], [Si], [Mn], [Cr], [Ni], and [Cu] mean the content (wt%) of each element.

[0069] The average crystal grain size (d) at the thickness center portion is preferably ultrafine grains of 5.0 μm or less, and the ASP value of the formula (2) preferably satisfies the range of 15-70.

[0070] Austenitic stainless steel has the advantage of excellent corrosion resistance, with a pitting potential value of 100mV or more when measured in a 3.5% NaCl solution at 30°C.

[0071] Austenitic stainless steel has the advantage of having excellent strength with a yield strength of 540.0 MPa or more, and excellent ductility with an elongation rate of 30.0% or more.

[0072] The coil is obtained by hot rolling the material produced by the continuous casting process of the present invention, but without annealing, and then performing final cold rolling at room temperature with a total thickness reduction rate of 50% or more, and then performing final annealing at an annealing temperature of 750 to 850°C, or the coil is obtained by hot rolling the material produced by the continuous casting process, annealing it at an annealing temperature of 1000 to 1150°C, performing final cold rolling at room temperature with a total thickness reduction rate of 50% or more, and then performing final annealing at an annealing temperature of 750 to 850°C, and the coil satisfies that the average grain size (d) value at the center of the thickness of the material is 5 μm or less, and when the pitting potential value is measured in a 3.5% NaCl solution (30°C), the pitting potential value is 100 mV or more.

[0073] [Method of manufacturing austenitic stainless steel] The method for producing the austenitic stainless steel of the present invention will be described below.

[0074] The method for producing an austenitic stainless steel according to the present invention includes the steps of providing a material, hot rolling the material to produce a hot-rolled steel sheet, cold rolling the hot-rolled steel sheet to produce a cold-rolled steel sheet, and final annealing the cold-rolled steel sheet, and may further include a step of performing primary annealing before the cold rolling.

[0075] In the material providing step, a material (ingot or slab) containing, by weight, carbon (C): 0.005-0.060%, silicon (Si): 0.1-1.5%, manganese (Mn): 5.0-10.0%, nickel (Ni): more than 0% and not more than 3.0%, chromium (Cr): 14.0-18.0%, copper (Cu): more than 0% and not more than 2.0%, nitrogen (N): 0.01-0.25%, with the remainder being iron (Fe) and unavoidable impurities may be provided.

[0076] The composition of the material preferably satisfies the range of 15 to 70 for the ASP value defined by the following formula (2-1).

[0077] Formula (2-1): ASP=551-462([C]+[N])-9.2[Si]-8.1[Mn]-13.7[Cr]-29([Ni]+[Cu])

[0078] In the above formula, [C], [N], [Si], [Mn], [Cr], [Ni], and [Cu] mean the content (wt%) of each element.

[0079] In addition, the method for producing austenitic stainless steel according to the present invention includes controlling the α value defined by the following formula (1) to be 10,000 or more. When the α value is controlled to be 10,000 or more, there is an advantage in that high strength and high ductility can be realized while maintaining cost competitiveness.

[0080] Formula (1): α=YS×EL-200(8[Ni]+[Cr]+3[Mn])

[0081] In the formula (1), Ni, Cr, and Mn represent the content (wt %) of each element, YS represents the yield stress (MPa), and EL represents the elongation rate (%).

[0082] The method for producing an austenitic stainless steel according to the present invention also includes controlling the β value defined by the following formula (2) to be 100 or more. When the β value is controlled to be 100 or more, ultrafine grains having an average grain size (d) of 5.0 μm or less are formed in the center of the thickness, thereby realizing high strength and high ductility at the same time.

[0083] TIFF2025500865000007.tif11147

[0084] In the formula (2), the austenitic stability parameter (ASP) is a value calculated by the above formula (2-1), and it is preferable that the ASP value is in the range of 15 to 70.

[0085] d means the average crystal grain size (μm) at the center of the thickness, and is preferably 5.0 μm or less.

[0086] After the hot rolling step, cold rolling may be performed without annealing, or cold rolling may be performed after the primary annealing. If the primary annealing is performed, the primary annealing may be performed at a temperature of 1000 to 1150°C.

[0087] The cold rolling step may be performed such that the hot rolled steel sheet is rolled at a thickness reduction rate of 50% or more in a room temperature range.

[0088] The final annealing step may be performed in a temperature range of 750 to 850°C, and the final annealing temperature is preferably controlled so that the γ value defined by the following formula (3) is equal to or greater than 0. When the γ value is controlled to be equal to or greater than 0, the composition balance of manganese, chromium, nitrogen, etc. is excellent, and sufficient corrosion resistance can be ensured even when low-temperature annealing is performed.

[0089] TIFF2025500865000008.tif10147

[0090] In the formula (3), [Cr], [N], and [Mn] represent the content (wt%) of each element, and Temp represents the final annealing temperature (° C.).

[0091] Specifically, when the γ value is controlled to be 0 or more, when the pitting potential is measured in a 3.5% NaCl solution at 30°C, the measured pitting potential is 100mV or more, thereby achieving excellent corrosion resistance.

[0092] [Example] The present invention will now be described in more detail with reference to the following examples.

[0093] The alloy components and composition ranges of the steel types used in the examples and comparative examples of austenitic stainless steel and the Austenite Stability Parameter (ASP) values, which are main component parameters calculated by the following formula (2-1), are summarized in Table 1 below.

[0094] Formula (2-1): ASP=551-462([C]+[N])-9.2[Si]-8.1[Mn]-13.7[Cr]-29([Ni]+[Cu])

[0095] [Table 1]

[0096] Of the steels in Table 1, some were vacuum melted in a laboratory to produce ingots, and some were subjected to an electric furnace-continuous casting process to produce slabs, and coils of Examples 1 to 10 and Comparative Examples 1 to 24 were produced at the final annealing temperatures (Temp; °C) shown in Table 2 below. The average grain size (d) was measured at the center of the thickness of the produced material, and the yield strength (YS; MPa) and elongation (EL; %) were measured by performing a tensile test on a JIS 13B tensile test piece at room temperature with a crosshead speed of 10 mm / min to 20 mm / min, and the pitting potential was measured in a 3.5% NaCl solution (30 °C), and the results are shown in Table 2 below.

[0097] Based on the compositions and measured values ​​in Table 1, the α, β, and γ values ​​defined by the following formulas (1) to (3) were calculated and summarized in Table 2 below. FIG. 1 shows the ranges of the examples and the comparative examples based on the α and β values.

[0098] Formula (1): α=YS(MPa)×EL(%)-200(8[Ni]+[Cr]+3[Mn])

[0099] TIFF2025500865000010.tif11147

[0100] TIFF2025500865000011.tif10147

[0101] [Table 2]

[0102] As shown in Tables 1 and 2, Examples 1 to 10 have an ASP value in the range of 15 to 70, and an average grain size (d) value in the thickness center of the material is 5 μm or less. On the other hand, it can be confirmed that Comparative Examples 1 to 9 and 12 to 24 have an ASP value outside the range of 10 to 70, or an average grain size (d) value in the thickness center of the material is 5.1 μm or more. Examples 1 to 10 are cost-saving austenitic stainless steels that can realize high strength and high ductility and have excellent corrosion resistance properties, and Examples 1 to 10 all satisfy an α value of 10,000 or more, a β value of 100 or more, a γ value of 0 or more, and a pitting potential value of 100 mV or more.

[0103] Comparative Examples 1 to 6 are commercially produced standard austenitic stainless steels, and are inferior in cost competitiveness because they use steel types that do not fall within the composition range of the present invention. In addition, they all have an α value of less than 10,000, which does not meet the α value requirements, and therefore have the problem of not being able to realize high strength and high ductility with cost competitiveness.

[0104] Comparative Examples 7 to 8, 12, 14, and 16 to 21 all have an α value of less than 10,000, which does not meet the α value requirements, and therefore have the problem of not being able to realize high strength and high ductility with cost competitiveness.

[0105] Comparative Examples 7 to 9, 12 to 15, and 18 to 24 not only fail to satisfy the average grain size (d) of 5 μm or less at the center of the material thickness, but also have the problem that the β value is less than 100. Fig. 2 shows the grain sizes of Example 1 and Comparative Example 9. Comparative Examples not satisfying the requirement of 5 μm or less have the problem that coarse grains are formed as shown on the right side of Fig. 2 (Comparative Example 9) and ultrafine grains as shown on the left side of Fig. 2 (Example 1) cannot be realized, resulting in the problem that high strength and high ductility cannot be realized at the same time.

[0106] Comparative Examples 10 to 11 have a problem that the γ value is less than 0. This is the result of an incorrect balance of components consisting of a large amount of Mn and low amounts of Cr, N, etc., and there is a problem that it is difficult to ensure sufficient corrosion resistance of the material by performing low-temperature annealing, and high strength and high ductility with excellent corrosion resistance cannot be realized at the same time.

[0107] Although exemplary embodiments of the present invention have been described above, the present invention is not limited thereto, and a person having ordinary skill in the art will understand that various changes and modifications are possible without departing from the concept and scope of the claims set forth below. [Industrial Applicability]

[0108] The austenitic stainless steel according to the present invention is cost-competitive and has excellent strength, ductility and corrosion resistance, and therefore is found to be industrially applicable.

Claims

1. The alloy contains, by weight, 0.005-0.060% carbon (C), 0.1-1.5% silicon (Si), 5.0-10.0% manganese (Mn), 0%-3% or less nickel (Ni), 14.0-18.0% chromium (Cr), 0%-2.0% or less copper (Cu), 0.01-0.25% nitrogen (N), and the remainder being iron (Fe) and unavoidable impurities; The α value defined by the following formula (1) is 10,000 or more, An austenitic stainless steel characterized in that the β value defined by the following formula (2) is 100 or more. Formula (1): α=YS×EL-200 (8[Ni]+[Cr]+3[Mn]) In the formula (1) and the formula (2), [Ni], [Cr], and [Mn] mean the content (wt%) of each element, YS means yield stress (MPa); EL means elongation rate (%), ASP (Austenitic Stability Parameter) is a value calculated by the following formula (2-1), where d means the average crystal grain size (μm) at the center of the thickness. Formula (2-1): ASP=551-462([C]+[N])-9.2[Si]-8.1[Mn]-13.7[Cr]-29([Ni]+[Cu]) In the formula (2-1), [C], [N], [Si], [Mn], [Cr], [Ni], and [Cu] mean the content (wt %) of each element.

2. 2. The austenitic stainless steel according to claim 1, wherein the average crystal grain size (d) at the thickness center portion is 5.0 μm or less.

3. 2. The austenitic stainless steel according to claim 1, characterized in that the ASP value of the formula (2) satisfies the range of 15 to 70.

4. The austenitic stainless steel according to claim 1, wherein the austenitic stainless steel has a pitting potential value of 100 mV or more.

5. 2. The austenitic stainless steel according to claim 1, wherein the austenitic stainless steel has a yield strength of 540.0 MPa or more and an elongation of 30.0% or more.

6. providing a material containing, by weight percent, 0.005-0.060% carbon (C), 0.1-1.5% silicon (Si), 5.0-10.0% manganese (Mn), 0%-3.0% nickel (Ni), 14.0-18.0% chromium (Cr), 0%-2.0% copper (Cu), 0.01-0.25% nitrogen (N), and the remainder being iron (Fe) and unavoidable impurities; hot rolling the raw material to produce a hot rolled steel sheet; cold rolling the hot rolled steel sheet to produce a cold rolled steel sheet; and The cold rolled steel sheet is subjected to final annealing, The final annealed steel sheet is controlled so that the α value defined by the following formula (1) is 10,000 or more and the β value defined by the following formula (2) is 100 or more, A method for producing an austenitic stainless steel, characterized in that the γ value defined by the following formula (3) is 0 or more. Formula (1): α=YS×EL-200 (8[Ni]+[Cr]+3[Mn]) In the formulas (1) to (3), [Ni], [Cr], [Mn], and [N] mean the content (wt%) of each element, YS means yield stress (MPa); EL means elongation rate (%), ASP (Austenitic Stability Parameter) is a value calculated by the following formula (2-1), where d is the average crystal grain size (μm) at the thickness center. Temp means the final annealing temperature (°C); Formula (2-1): ASP=551-462([C]+[N])-9.2[Si]-8.1[Mn]-13.7[Cr]-29([Ni]+[Cu]) In the formula (2-1), [C], [N], [Si], [Mn], [Cr], [Ni], and [Cu] mean the content (wt %) of each element.

7. The method for producing an austenitic stainless steel according to claim 6, wherein the final annealing step is performed at a temperature in the range of 750 to 850°C.

8. 7. The method of claim 6, further comprising the step of performing primary annealing of the hot-rolled steel sheet at 1000 to 1150° C. before the cold rolling.

9. 7. The method of claim 6, wherein in the cold rolling, the cold rolled steel sheet is rolled at a thickness reduction rate of 50% or more of the hot rolled steel sheet in a room temperature range.

10. The method for producing an austenitic stainless steel according to claim 6, characterized in that the austenitic stainless steel has an ASP value in the range of 15 to 70.

11. 7. The method for producing an austenitic stainless steel according to claim 6, wherein the final annealed steel sheet has an average crystal grain size (d) of 5.0 μm or less at a center portion of the thickness of the steel sheet.

Images