Austenitic stainless steel and method for manufacturing the same

Abstract

Provided are an austenitic stainless steel and a method for producing the same, which can simultaneously achieve high strength, high ductility, and high corrosion resistance by realizing ultra-fine grain characteristics while having price competitiveness. 【Solution means】The austenitic stainless steel of the present invention consists of, by weight%, C: 0.05% or more and 0.1% or less, Si: 0.2% or more and 0.7% or less, Mn: 2.0% or more and 4.0% or less, P: more than 0% and less than 0.1%, S: more than 0% and less than 0.01%, Cr: 17% or more and 19% or less, Ni: 2.0% or more and 4.0% or less, Cu: 1.0% or more and 2.5% or less, N: 0.15% or more and 0.25% or less, the balance being iron (Fe) and unavoidable impurities, and is characterized in that the average crystal grain size at the center of the thickness is 5 μm or less.

Description

Technical Field

[0001] The present invention relates to austenitic stainless steel and a method for producing the same, and more particularly, to austenitic stainless steel having ultrafine grain characteristics and simultaneously improved high strength, high ductility, and corrosion resistance, and a method for producing the same.

Background Art

[0002] The 304 steel, which is a generally used austenitic stainless steel, has a yield strength in the range of 200 to 350 MPa, so there is a limit to its application as a structural member. In the 304 steel, in order to obtain a higher yield strength, an additional temper rolling process is required, which not only increases the cost but also causes a sharp decrease in the elongation and formability. In addition, since the 304 steel contains a large amount of expensive alloy components, there is a problem of inferior cost competitiveness.

[0003] Patent Document 1 relates to an austenitic stainless steel and a method for producing the same, and discloses an austenitic stainless steel having a tensile strength of 600 MPa or more, but it has a large Ni content and inferior price competitiveness. However, when the thickness of the steel material increases, the rolling pressure applied by rolling is limited, and it becomes difficult to form fine crystal grains by rolling, especially closer to the center of the steel material. This is because the austenite crystal grains tend to grow as the temperature is higher and the heating time is longer at a temperature above Ae3. On the other hand, it is often difficult to ensure crystal grains of a sufficiently small size only by slab reheating and rolling in the process of austenite crystal grain refinement. In particular, the higher the temperature of the steel material to be rolled, the lower the deformation resistance during rolling, so rolling becomes easier. Therefore, slab reheating is carried out at a temperature much higher than the Ae3 temperature. At this time, the austenite crystal grains will grow large. When the crystal grain refinement effect by rolling is not sufficient, a further austenite crystal grain refinement effect can be expected through post-rolling heat treatment. Generally, normalizing heat treatment corresponds to this.

[0004] On the other hand, grain refinement technology is attracting attention as a technique for simultaneously improving strength and ductility. In particular, severe plastic deformation (SPD) is gaining prominence as a method for making steel materials for structural members ultra-fine. Severe plastic deformation is a method that applies strong shear stress to a material to create new grain boundaries within the existing grain boundaries, thereby achieving finer grains. However, severe plastic deformation has problems such as reduced productivity and limitations on product size.

[0005] Patent Document 2 discloses a method for achieving an average crystal grain size of 10 μm or less by performing long-term heat treatment at 600-700°C for 48 hours or more. However, the method disclosed in Patent Document 2 has the problem of reduced productivity and increased manufacturing costs. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Korean Published Patent Publication No. 10-2016-0138277 [Patent Document 2] Japanese Patent Publication No. 2020-050940 [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] The objective of the present invention, which was made to solve the above-mentioned problems, is to provide an austenitic stainless steel and a method for producing the same that can simultaneously achieve high strength, high ductility, and high corrosion resistance by realizing ultra-fine grain properties while maintaining price competitiveness. [Means for solving the problem]

[0008] The austenitic stainless steel of the present invention is characterized by comprising, by weight percent, C: 0.05% to 0.1%, Si: 0.2% to 0.7%, Mn: 2.0% to 4.0%, P: more than 0% but less than 0.1%, S: more than 0% but less than 0.01%, Cr: 17% to 19%, Ni: 2.0% to 4.0%, Cu: 1.0% to 2.5%, N: 0.15% to 0.25%, with the remainder being iron (Fe) and unavoidable impurities, and having an average grain size of 5 μm or less at the center of the thickness.

[0009] The austenitic stainless steel of the present invention preferably has an ASP (Austenite Stability Parameter) value represented by the following formula (1) between -30 and 30. Formula (1): 551-462×([C]+[N])-9.2×[Si]-8.1×[Mn]-13.7×[Cr]-29×([Ni]+[Cu]) In formula (1) above, [C], [N], [Si], [Mn], [Cr], [Ni], and [Cu] represent the content (weight %) of each element.

[0010] The austenitic stainless steel of the present invention preferably has an SSP (Strength Stability Parameter) value of 0 or greater, represented by the following formula (2). Formula (2): 58+132×[C]-7.9×[Si]+1.0×[Mn]-5.6×[Cr]+7.0×[Ni]+3.9×[Cu]+1.7×[N] In formula (2) above, [C], [Si], [Mn], [Cr], [Ni], [Cu], and [N] represent the content (weight %) of each element.

[0011] The austenitic stainless steel of the present invention can have a pitting resistance equivalet number (PREN) of 17 or higher, as represented by the following formula (3). Formula (3): [Cr]-0.5×[Mn]+16×[N] In formula (3) above, [Cr], [Mn], and [N] represent the content (weight %) of each element.

[0012] The austenitic stainless steel of the present invention preferably has a yield strength of 600 MPa or higher. Furthermore, the extension ratio can be 30% or more.

[0013] The austenitic stainless steel of the present invention preferably has a pitting potential value of 200 mV or higher. Furthermore, the thickness can be between 0.4 and 2.0 mm.

[0014] The present invention relates to a method for producing austenitic stainless steel, comprising the steps of: producing an ingot consisting of, by weight percent, C: 0.05% or more and 0.1% or less, Si: 0.2% or more and 0.7% or less, Mn: 2.0% or more and 4.0% or less, P: more than 0% but less than 0.1%, S: more than 0% but less than 0.01%, Cr: 17% or more and 19% or less, Ni: 2.0% or more and 4.0% or less, Cu: 1.0% or more and 2.5% or less, N: 0.15% or more and 0.25% or less, the remaining iron (Fe) and unavoidable impurities; hot rolling the ingot to produce a hot-rolled steel sheet; cold rolling the hot-rolled steel sheet to produce a cold-rolled steel sheet; and final annealing of the cold-rolled steel sheet.

[0015] In the manufacturing method of the present invention, the ingot preferably has an ASP (Austenite Stability Parameter) value represented by the following formula (1) between -30 and 30. Formula (1): 551-462×([C]+[N])-9.2×[Si]-8.1×[Mn]-13.7×[Cr]-29×([Ni]+[Cu]) In formula (1) above, [C], [N], [Si], [Mn], [Cr], [Ni], and [Cu] represent the content (weight %) of each element.

[0016] In the manufacturing method of the present invention, it is preferable that the ingot has an SSP (Strength Stability Parameter) value of 0 or greater, which is represented by the following formula (2). Formula (2): 58 + 132×[C] - 7.9×[Si] + 1.0×[Mn] - 5.6×[Cr] + 7.0×[Ni] + 3.9×[Cu] + 1.7×[N] In the above formula (2), [C], [Si], [Mn], [Cr], [Ni], [Cu], and [N] represent the content (% by weight) of each element.

[0017] In the manufacturing method of the present invention, the ingot can have a pitting resistance equivalent number (PREN) represented by the following formula (3) of 17 or more. Formula (3): [Cr] - 0.5×[Mn] + 16×[N] In the above formula (3), [Cr], [Mn], and [N] represent the content (% by weight) of each element.

[0018] The manufacturing method of the present invention can further include a step of intermediate annealing the hot-rolled steel sheet before the cold rolling. In the manufacturing method of the present invention, the intermediate annealing temperature is preferably carried out at 1050 - 1150 °C.

[0019] In the manufacturing method of the present invention, the final annealing temperature is preferably carried out at 800 - 850 °C. In the manufacturing method of the present invention, the cold rolling is preferably carried out at room temperature such that the thickness reduction rate of the hot-rolled steel sheet is 50% or more.

Advantages of the Invention

[0020] According to an embodiment of the present invention, it is possible to provide an austenitic stainless steel and a manufacturing method thereof that can simultaneously achieve high strength, high ductility, and high corrosion resistance by realizing ultrafine grain characteristics while having price competitiveness.

Brief Description of the Drawings

[0021] [Figure 1]This is a photograph taken with a scanning electron microscope (SEM) of the central cross-section of an austenitic stainless steel material according to one embodiment of the present invention. [Modes for carrying out the invention]

[0022] An austenitic stainless steel according to one embodiment of the present invention is characterized by comprising, by weight percent, C: 0.05% to 0.1%, Si: 0.2% to 0.7%, Mn: 2.0% to 4.0%, P: more than 0% but less than 0.1%, S: more than 0% but less than 0.01%, Cr: 17% to 19%, Ni: 2.0% to 4.0%, Cu: 1.0% to 2.5%, N: 0.15% to 0.25%, the remaining iron (Fe) and unavoidable impurities, and having an average grain size of 5 μm or less at the center of the thickness.

[0023] Embodiments of the present invention will now be described in detail with reference to the accompanying drawings. The following embodiments are provided to fully convey the spirit of the invention to those who have ordinary skill in the art to which the invention pertains. The present invention is not limited to the embodiments presented herein and may be embodied in other forms. In order to clarify the invention, the drawings may omit illustrations of parts not relevant to the description and may exaggerate the sizes of components to some extent to aid understanding. When, throughout the specification, a part of a specification is said to "include" a certain component, this means, unless otherwise stated, that it may include other components rather than excluding them. Unless otherwise clearly indicated by the context, singular expressions include plural forms.

[0024] The following explains the reasons for the numerical limitations on the alloy component content in the embodiments of the present invention. Unless otherwise specified, the unit is weight percent. An austenitic stainless steel according to one embodiment of the present invention consists of, by weight percent, C: 0.05% to 0.1%, Si: 0.2% to 0.7%, Mn: 2.0% to 4.0%, P: more than 0% but less than 0.1%, S: more than 0% but less than 0.01%, Cr: 17% to 19%, Ni: 2.0% to 4.0%, Cu: 1.0% to 2.5%, N: 0.15% to 0.25%, with the remainder being iron (Fe) and unavoidable impurities.

[0025] The carbon (C) content should ideally be between 0.05% and 0.1%. Carbon (C) is an effective element for stabilizing the austenite phase and needs to be added appropriately to ensure the yield strength of the steel. Considering this, it is preferable to add 0.05% or more C. On the other hand, excessive C content can reduce cold workability due to solid solution strengthening. Furthermore, excessive C content may adversely affect ductility and corrosion resistance during low-temperature annealing due to grain boundary precipitation of chromium carbides. Considering this, it is preferable to limit the upper limit of C content to 0.1%.

[0026] The silicon (Si) content should preferably be between 0.2% and 0.7%. Si is an element added to steel for deoxidation and is effective in improving corrosion resistance. Considering this, it is preferable to add Si at a concentration of 0.2% or more. On the other hand, if the Si content is excessive, the stabilizing effect of the ferrite phase may promote the formation of delta ferrite in the casting material. Therefore, if the Si content is excessive, hot workability may decrease, and ductility and impact properties may be adversely affected. Considering this, it is preferable to limit the upper limit of the Si content to 0.7%. Preferably, Si is added at a concentration of 0.3% to 0.4%.

[0027] The manganese (Mn) content should ideally be between 2.0% and 4.0%. In this invention, Mn is an austenite phase stabilizing element added in place of Ni. Considering this, it is preferable to add 2.0% or more of Mn. On the other hand, if the Mn content is excessive, there is a risk of excessive formation of sulfur-based inclusions (MnS), which may reduce ductility and corrosion resistance. Considering this, it is preferable to limit the upper limit of the Mn content to 4.0%. Preferably, Mn is added in an amount of 3.6% to 3.9%.

[0028] The phosphorus (P) content should ideally be between 0% and less than 0.1%. P is an impurity that is inevitably present in steel and is an element that causes intergranular corrosion, thereby hindering hot workability. Therefore, it is desirable to control the P content to be as low as possible. Taking this into consideration, it is best to control the upper limit of the P content to less than 0.1%.

[0029] The sulfur (S) content should ideally be between 0% and less than 0.01%. S, like P, is an impurity inevitably present in steel and is an element that segregates at grain boundaries, hindering hot workability. Therefore, it is desirable to control the S content to be as low as possible. Taking this into consideration, the upper limit of the S content can be controlled to less than 0.01%.

[0030] The chromium (Cr) content should ideally be between 17% and 19%. Cr is an effective element for suppressing the formation of the martensite phase and ensuring corrosion resistance. Considering this, it is preferable to add 17% or more Cr. On the other hand, if the Cr content is excessive, manufacturing costs will increase, and there is a risk of forming a large amount of delta ferrite in the material, reducing hot workability. Considering this, it is preferable to limit the upper limit of the Cr content to 19%. Preferably, Cr should be added in an amount between 17.2% and 18%.

[0031] The nickel (Ni) content should ideally be between 2.0% and 4.0%. Ni is a strong austenite phase stabilizing element and is essential for ensuring good hot and cold workability. Therefore, even with the addition of a certain amount of Mn, it is preferable to add Ni at a concentration of 2.0% or more. On the other hand, if the Ni content is excessive, the martensitic transformation onset temperature (Ms) may become too low, making it difficult to generate stress-induced martensite during cold working. Furthermore, an excessive Ni content may lead to increased raw material costs. Considering this, it is preferable to limit the upper limit of the Ni content to 4.0%. Preferably, Ni should be added at a concentration of 3.4% to 3.7%.

[0032] The copper (Cu) content should ideally be between 1.0% and 2.5%. Cu is effective in softening materials as an austenite phase stabilizing element. Considering this, it is preferable to add 1.0% or more Cu. On the other hand, if the Cu content is excessive, the martensitic transformation initiation temperature (Ms) may become too low, making it difficult to generate stress-induced martensite during cold working. Furthermore, excessive Cu content may increase material costs and induce hot brittleness. Considering this, it is preferable to limit the upper limit of the Cu content to 2.5%. Preferably, Cu should be added in an amount between 1.5% and 2.0%.

[0033] The nitrogen (N) content should ideally be between 0.15% and 0.25%. N is an effective element for stabilizing the austenite phase and improving corrosion resistance. Considering this, N can be added in amounts of 0.15% or more. On the other hand, if the N content is excessive, the solid solution strengthening effect may reduce cold workability, and the martensitic transformation onset temperature (Ms) may become too low, making it difficult to generate stress-induced martensite during cold working. Furthermore, excessive N content may cause quality defects due to porosity formation during casting. Considering this, it is preferable to limit the upper limit of the N content to 0.25%. Preferably, N should be added in an amount between 0.16% and 0.21%.

[0034] The remaining component of this invention is iron (Fe). However, in the normal manufacturing process, unintended impurities from the raw materials or the surrounding environment may inevitably be introduced and cannot be eliminated. These impurities are known to any technician in the normal manufacturing process, and not all of them are specifically mentioned herein.

[0035] An austenitic stainless steel according to one embodiment of the present invention exhibits ultra-fine grain characteristics, and preferably has an average crystal grain size of 5 μm or less at the center of its thickness. Here, the center of the thickness refers to the portion from 1 / 4t to 3 / 4t when the thickness of the stainless steel is t. In this invention, "average" refers to the average value of measurements taken at any five locations.

[0036] An austenitic stainless steel according to one embodiment of the present invention can have an ASP (Austenite Stability Parameter) value represented by the following formula (1) ranging from -30 to 30. Formula (1): 551-462×([C]+[N])-9.2×[Si]-8.1×[Mn]-13.7×[Cr]-29×([Ni]+[Cu]) In formula (1) above, [C], [N], [Si], [Mn], [Cr], [Ni], and [Cu] represent the content (weight %) of each element.

[0037] Equation (1) represents the temperature at which 50% of austenite transforms into martensite when stainless steel is deformed at a true strain of 0.3, and is used as an indicator of austenite phase stabilization. A lower value of Equation (1) indicates higher austenite phase stabilization, meaning that less work-induced martensite transforms during deformation. If the value of equation (1) is less than -30, the amount of TRIP transformation from the austenite phase to the martensite phase decreases, resulting in less processing-induced martensite. Therefore, if the value of equation (1) is less than -30, the proportion of reverted austenite phase formed by low-temperature annealing decreases, making it difficult to secure ultrafine grains. However, if the value of equation (1) exceeds 30, the yield strength and elongation may decrease due to the premature TRIP transformation.

[0038] An austenitic stainless steel according to one embodiment of the present invention may have an SSP (Strength Stability Parameter) value of 0 or greater, represented by the following formula (2). Formula (2): 58+132×[C]-7.9×[Si]+1.0×[Mn]-5.6×[Cr]+7.0×[Ni]+3.9×[Cu]+1.7×[N] In formula (2) above, [C], [Si], [Mn], [Cr], [Ni], [Cu], and [N] represent the content (weight %) of each element. If the value of equation (2) above is less than 0, it may become difficult to achieve ultrafine-grained microstructure characteristics over a wide range of final annealing temperatures. In other words, in order to achieve ultrafine-grained microstructure characteristics in the entire range of 800 to 850°C, which is the final annealing temperature range presented in this invention, it is necessary to control the value of equation (2) above to 0 or greater.

[0039] An austenitic stainless steel according to one embodiment of the present invention can have a pitting resistance equivalet number (PREN) of 17 or higher, as expressed by the following formula (3). Formula (3): [Cr]-0.5×[Mn]+16×[N] In formula (3) above, [Cr], [Mn], and [N] represent the content (weight %) of each element. If the value of equation (3) above is less than 17, the pitting potential measured in a 3.5% NaCl solution (30°C) may not satisfy the requirement of 200mV or higher. In other words, if the value of equation (3) above is less than 17, it becomes difficult to achieve the target high corrosion resistance.

[0040] The alloy composition, parameters, and manufacturing method presented in this invention make it possible to realize ultrafine grain properties. Therefore, an austenitic stainless steel according to one embodiment of the present invention can have a yield strength of 600 MPa or more and an elongation ratio of 30% or more. Furthermore, the austenitic stainless steel according to one embodiment of the present invention preferably has a pitting potential value of 200 mV or higher. An austenitic stainless steel according to one embodiment of the present invention has a thickness of 0.4~ A thickness of 2.0 mm is preferable. However, it is not limited to this, and can be manufactured in various thicknesses depending on the purpose.

[0041] Next, a method for producing austenitic stainless steel according to another aspect of the present invention will be described. A method for producing austenitic stainless steel according to one embodiment of the present invention may include the steps of: producing an ingot consisting of, by weight percent, C: 0.05% or more and 0.1% or less, Si: 0.2% or more and 0.7% or less, Mn: 2.0% or more and 4.0% or less, P: more than 0% but less than 0.1%, S: more than 0% but less than 0.01%, Cr: 17% or more and 19% or less, Ni: 2.0% or more and 4.0% or less, Cu: 1.0% or more and 2.5% or less, N: 0.15% or more and 0.25% or less, the remaining iron (Fe) and unavoidable impurities; hot rolling the ingot to produce a hot-rolled steel sheet; cold rolling the hot-rolled steel sheet to produce a cold-rolled steel sheet; and final annealing of the cold-rolled steel sheet.

[0042] The aforementioned ingot should preferably have an ASP (Austenite Stability Parameter) value, represented by the following formula (1), between -30 and 30. Formula (1): 551-462×([C]+[N])-9.2×[Si]-8.1×[Mn]-13.7×[Cr]-29×([Ni]+[Cu]) In formula (1) above, [C], [N], [Si], [Mn], [Cr], [Ni], and [Cu] represent the content (weight %) of each element.

[0043] Furthermore, in a method for manufacturing austenitic stainless steel according to one embodiment of the present invention, it is preferable that the ingot has an SSP (Strength Stability Parameter) value of 0 or greater, represented by the following formula (2). Formula (2): 58+132×[C]-7.9×[Si]+1.0×[Mn]-5.6×[Cr]+7.0×[Ni]+3.9×[Cu]+1.7×[N] In formula (2) above, [C], [Si], [Mn], [Cr], [Ni], [Cu], and [N] represent the content (weight %) of each element.

[0044] Furthermore, in a method for manufacturing austenitic stainless steel according to one embodiment of the present invention, the ingot is preferably of 17 or higher, having a pitting resistance equivalet number (PREN) represented by the following formula (3). Formula (3): [Cr]-0.5×[Mn]+16×[N] In formula (3) above, [Cr], [Mn], and [N] represent the content (weight %) of each element. On the other hand, depending on the purpose and application, it may also be manufactured as a slab instead of an ingot.

[0045] The reasons for the component ranges of each alloy composition and the numerical limitations of the values ​​in equations (1), (2), and (3) are as described above. The manufacturing stages will be explained in more detail below. First, an ingot satisfying the aforementioned alloy composition can be prepared, followed by a series of hot rolling, cold rolling, and final annealing processes. After heating the ingot to 1150-1300°C, hot-rolled steel sheets are produced by hot-rolling. If the heating temperature is too low, it may become difficult to re-decompose the coarse precipitates generated during the manufacturing of the ingot. Considering this, the heating temperature should be 1150°C or higher. However, if the heating temperature is too high, the internal crystal grains may become too coarse, leading to severe surface oxidation and potentially causing surface defects. Considering this, the upper limit of the heating temperature should be restricted to 1300°C. Next, the process may further include a step of intermediate annealing the hot-rolled steel sheet before cold rolling. The intermediate annealing step may be performed as needed or omitted. When performing the intermediate annealing described above, it is preferable to do so at a temperature of 1000 to 1150°C.

[0046] If the intermediate annealing temperature is too low, the residual martensite fraction may increase, leading to reduced workability. On the other hand, if the intermediate annealing temperature is too high, there is a risk of reduced strength due to grain coarsening. The final annealing temperature is preferably 800-850°C. Similar to the intermediate annealing temperature, if the final annealing temperature is too low, the workability may decrease. On the other hand, if the final annealing temperature is too high, the strength may decrease due to grain coarsening. The cold rolling should be carried out such that the thickness reduction rate of the hot-rolled steel sheet at room temperature is 50% or more. If the thickness reduction rate during cold rolling is less than 50%, the amount of work-induced martensite decreases, and the proportion of the ultrafine-grained reverse-transformed austenite phase decreases during low-temperature annealing, which may make it difficult to ensure sufficient strength.

[0047] The present invention will be described in more detail below through embodiments. However, these embodiments are for illustrative purposes only and do not limit the present invention. The scope of the present invention is determined by the matters described in the claims and matters that can be reasonably inferred therefrom. {Example}

[0048] Various alloy composition ranges shown in Table 1 below were cast in a vacuum induction melting furnace in the form of 150 mm thick, 35 kg ingots. The cast ingots were heated in a 1250°C heating furnace for 2 hours, then hot-rolled to a width of 200 mm and a thickness of 4 mm to produce hot-rolled steel sheets, which were then air-cooled. The air-cooled hot-rolled steel sheets were subjected to intermediate annealing at 1100°C for 1 minute, then pickled, and cold-rolled to a thickness of 1.2 mm to produce cold-rolled steel sheets. The cold-rolled steel sheets were then annealed at the final annealing temperature shown in Table 2 to produce the final product.

[0049] [Table 1]

[0050] Table 2 below shows the values ​​of equation (1), equation (2), and equation (3), the final annealing temperature, average grain size, pitting potential, yield strength, and elongation. The value of equation (1) is shown by calculating equation (1) below. Formula (1): 551-462×([C]+[N])-9.2×[Si]-8.1×[Mn]-13.7×[Cr]-29×([Ni]+[Cu]) In formula (1) above, [C], [N], [Si], [Mn], [Cr], [Ni], and [Cu] represent the content (weight %) of each element.

[0051] The value of equation (2) was obtained by calculating equation (2) below. Formula (2): 58+132×[C]-7.9×[Si]+1.0×[Mn]-5.6×[Cr]+7.0×[Ni]+3.9×[Cu]+1.7×[N] In formula (2) above, [C], [Si], [Mn], [Cr], [Ni], [Cu], and [N] represent the content (weight %) of each element.

[0052] The value of equation (3) was obtained by calculating equation (3) below. Formula (3): [Cr]-0.5×[Mn]+16×[N] In formula (3) above, [Cr], [Mn], and [N] represent the content (weight %) of each element.

[0053] The average grain size was measured by taking a cross-sectional image of the center of the steel using a scanning electron microscope (SEM) model named JSM-7001F. The pitting potential was measured using a potentiostat. The values ​​shown represent the pitting potential at which a current of 100 μA was reached when the steel was immersed in an NaCl solution and a voltage of 20 mV / min was applied. The temperature of the NaCl solution was 30°C, and the concentration was set to 3.5%. A higher pitting potential value indicates better corrosion resistance.

[0054] Yield strength and elongation were measured using a Zwick Roell tensile testing machine, testing JIS 13B tensile specimens at a tensile speed of 15 mm per minute at room temperature.

[0055] [Table 2]

[0056] Referring to Table 2, Examples 1 to 4 satisfied the alloy composition, component range, parameters, and manufacturing process presented in the present invention. Accordingly, Examples 1 to 4 satisfied the average grain size of 5 μm or less, yield strength of 600 MPa or more, elongation of 30% or more, and pitting potential of 200 mV or more. In other words, Examples 1 to 4 simultaneously satisfied high strength, high ductility, and high corrosion resistance, but in the case of Comparative Examples 1 to 3, the value of equation (1) could not be satisfied between -30 and 30, and the value of equation (2) could not be satisfied above 0. Therefore, the grain size of 5 μm or less and the yield strength of 600 MPa or more could not be satisfied. In particular, it was difficult to realize the ultrafine grain microstructure characteristics in the entire range of 800 to 850°C, which is the final annealing temperature range presented in the present invention. In Comparative Examples 2 and 4, the Mn content was excessive, so the value of equation (3) could not be satisfied as 17 or higher. As a result, in Comparative Examples 2 and 4, the pitting potential value could not be satisfied as 200 mV or higher. Comparative Examples 2 and 4 were shown to have poor corrosion resistance.

[0057] Figure 1 is a photograph taken with a scanning electron microscope (SEM) of the central cross-section of an austenitic stainless steel according to one embodiment of the present invention. Referring to Figure 1, it can be confirmed that the austenitic stainless steel according to one embodiment of the present invention satisfies the requirement that the average grain size at the center of the thickness is 5 μm or less. In other words, it can be seen that ultra-fine grain properties can be realized according to one embodiment of the present invention. [Industrial applicability]

[0058] According to one embodiment of the present invention, an austenitic stainless steel and a method for producing the same can be provided that simultaneously achieve high strength, high ductility, and high corrosion resistance by embodying ultra-fine grain characteristics while maintaining price competitiveness, and thus has industrial applicability.

Claims

1. In weight percent, it consists of C: 0.05% to 0.1%, Si: 0.2% to 0.7%, Mn: 2.0% to 4.0%, P: more than 0% but less than 0.1%, S: more than 0% but less than 0.01%, Cr: 17% to 19%, Ni: 2.0% to 4.0%, Cu: 1.0% to 2.5%, N: 0.15% to 0.25%, with the remainder being iron (Fe) and unavoidable impurities. An austenitic stainless steel sheet characterized by having an average grain size of 5 μm or less at the center of its thickness.

2. The austenitic stainless steel sheet according to claim 1, characterized in that the ASP (Austenite Stability Parameter) value represented by the following formula (1) is between -30 and 30. Formula (1): 551-462×([C]+[N])-9.2×[Si]-8.1×[Mn]-13.7×[Cr]-29×([Ni]+[Cu]) (In formula (1) above, [C], [N], [Si], [Mn], [Cr], [Ni], and [Cu] represent the content (weight %) of each element.)

3. The austenitic stainless steel sheet according to claim 1, characterized in that the SSP (Strength Stability Parameter) value represented by the following formula (2) is 0 or greater. Formula (2): 58+132×[C]-7.9×[Si]+1.0×[Mn]-5.6×[Cr]+7.0×[Ni]+3.9×[Cu]+1.7×[N] (In formula (2) above, [C], [Si], [Mn], [Cr], [Ni], [Cu], and [N] represent the content (weight %) of each element.)

4. The austenitic stainless steel sheet according to claim 1, characterized in that its Pitting Resistance Equivalet Number (PREN), represented by the following formula (3), is 17 or higher. Formula (3): [Cr]-0.5×[Mn]+16×[N] (In formula (3) above, [Cr], [Mn], and [N] represent the content (weight %) of each element.)

5. The austenitic stainless steel sheet according to claim 1, characterized in that it has a yield strength of 600 MPa or more.

6. The austenitic stainless steel sheet according to claim 1, characterized in that the elongation rate is 30% or more.

7. The austenitic stainless steel sheet according to claim 1, characterized in that the pitting potential value is 200 mV or more.

8. The austenitic stainless steel sheet according to claim 1, characterized in that its thickness is 0.4 to 2.0 mm.

9. The process of manufacturing an ingot consisting of, by weight percent, C: 0.05% to 0.1%, Si: 0.2% to 0.7%, Mn: 2.0% to 4.0%, P: more than 0% but less than 0.1%, S: more than 0% but less than 0.01%, Cr: 17% to 19%, Ni: 2.0% to 4.0%, Cu: 1.0% to 2.5%, N: 0.15% to 0.25%, with the remainder being iron (Fe) and unavoidable impurities. The step of hot-rolling the ingot to produce a hot-rolled steel sheet, The steps of manufacturing a cold-rolled steel sheet by cold-rolling the hot-rolled steel sheet, and The step includes the final annealing of the cold-rolled steel sheet, A method for manufacturing an austenitic stainless steel sheet, characterized in that the average grain size at the center of the thickness is 5 μm or less.

10. The method for manufacturing an austenitic stainless steel sheet according to claim 9, characterized in that the ingot has an ASP (Austenite Stability Parameter) value represented by the following formula (1) of -30 to 30. Formula (1): 551-462×([C]+[N])-9.2×[Si]-8.1×[Mn]-13.7×[Cr]-29×([Ni]+[Cu]) (In formula (1) above, [C], [N], [Si], [Mn], [Cr], [Ni], and [Cu] represent the content (weight %) of each element.)

11. The method for manufacturing an austenitic stainless steel sheet according to claim 9, characterized in that the ingot has an SSP (Strength Stability Parameter) value of 0 or greater, which is represented by the following formula (2). Formula (2): 58+132×[C]-7.9×[Si]+1.0×[Mn]-5.6×[Cr]+7.0×[Ni]+3.9×[Cu]+1.7×[N] (In formula (2) above, [C], [Si], [Mn], [Cr], [Ni], [Cu], and [N] represent the content (weight %) of each element.)

12. The method for manufacturing an austenitic stainless steel sheet according to claim 9, characterized in that the ingot has a Pitting Resistance Equivalet Number (PREN) represented by the following formula (3) of 17 or more. Formula (3): [Cr]-0.5×[Mn]+16×[N] (In formula (3) above, [Cr], [Mn], and [N] represent the content (weight %) of each element.)

13. The method for manufacturing an austenitic stainless steel sheet according to claim 9, further comprising the step of intermediate annealing of the hot-rolled steel sheet before the cold rolling.

14. The method for manufacturing an austenitic stainless steel sheet according to claim 13, characterized in that the intermediate annealing temperature is 1050 to 1150°C.

15. The method for manufacturing an austenitic stainless steel sheet according to claim 9, characterized in that the final annealing temperature is 800 to 850°C.

16. The method for manufacturing an austenitic stainless steel sheet according to claim 9, characterized in that the cold rolling is performed such that the thickness reduction rate of the hot-rolled steel sheet at room temperature is 50% or more.

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