Austenitic stainless steel and method for producing same

Abstract

In the present specification, an austenitic stainless steel having improved high strength, high ductility, and corrosion resistance while achieving ultra-fine grain characteristics, and a method for manufacturing the same are disclosed. The austenitic stainless steel according to one embodiment of the present invention may contain, in wt%, 0.05% to 0.1% of C, 0.2% to 0.7% of Si, 2.0% to 4.0% of Mn, more than 0% and less than 0.1% of P, more than 0% and less than 0.01% of S, 17% to 19% of Cr, 2.0% to 4.0% of Ni, 1.0% to 2.5% of Cu, 0.15% to 0.25% of N, and the balance of iron (Fe) and unavoidable impurities, the average crystal grain diameter in the thickness center of the austenitic stainless steel may be 5 [mu] m or less.

Description

Technical Field

[0001] The present invention relates to an austenitic stainless steel and a method for manufacturing the same, and more particularly to an austenitic stainless steel that realizes ultrafine grain characteristics while improving high strength, high ductility and corrosion resistance and a method for manufacturing the same. Background Art

[0002] The yield strength of 304 steel, a commonly used austenitic stainless steel, is 200-350 MPa, which limits its application in structural parts. The additional temper rolling process of 304 steel to obtain a higher yield strength not only increases the cost, but also causes a sharp decrease in elongation and formability. In addition, 304 steel contains a large amount of expensive alloy components, so it has the problem of poor cost competitiveness.

[0003] Patent Document 0001 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. However, due to the high Ni content, the price competitiveness is reduced.

[0004] In addition, grain refinement technology has attracted much attention as a technology for improving both strength and ductility. In particular, severe plastic processing (Severe Plastic Deformation (SPD)) has attracted attention as a method for ultra-fine graining of steel for structural parts. The severe plastic processing method is a method of achieving fine grains by applying strong shear stress to the material to generate new grain boundaries within the grain boundaries. However, the severe plastic processing method has problems such as reduced productivity and limited product size.

[0005] Patent Document 0002 discloses a method of performing a long-term heat treatment at 600-700° C. for more than 48 hours to achieve an average grain size of 10 μm or less. However, the method disclosed in Patent Document 0002 has problems of reduced productivity and increased manufacturing costs.

[0006] (Prior art literature)

[0007] Patent Document 0001: Patent Publication No. 10-2016-0138277 (Publication Date: December 2, 2016)

[0008] Patent Document 0002: Japanese Patent Publication No. 2020-050940 (Publication Date: April 2, 2020) Summary of the invention

[0009] 1. Technical issues to be resolved

[0010] An object of the present invention to solve the above-mentioned problems is to provide an austenitic stainless steel having price competitiveness and realizing ultrafine grain characteristics to simultaneously realize high strength, high ductility and high corrosion resistance, and a method for producing the same.

[0011] (II) Technical solution

[0012] The austenitic stainless steel according to one embodiment of the present invention may contain, in weight %, C: 0.05% to 0.1%, Si: 0.2% to 0.7%, Mn: 2.0% to 4.0%, P: more than 0% to less than 0.1%, S: more than 0% to 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 balance being iron (Fe) and inevitable impurities, and the average grain diameter of the austenitic stainless steel in the thickness center portion may be 5 μm or less.

[0013] In addition, the austenite stainless steel according to one embodiment of the present invention may have an Austenite Stability Parameter (ASP) value represented by the following formula (1) of -30 to 30.

[0014] Formula (1): 551-462×([C]+[N])-9.2×[Si]-8.1×[Mn]-13.7×[Cr]-29×([Ni]+[Cu])

[0015] In the above formula (1), [C], [N], [Si], [Mn], [Cr], [Ni], and [Cu] represent the content (weight %) of each element.

[0016] Furthermore, the austenitic stainless steel according to one embodiment of the present invention may have a Strength Stability Parameter (SSP) value represented by the following formula (2) of 0 or more.

[0017] Formula (2): 58+132×[C]-7.9×[Si]+1.0×[Mn]-5.6×[Cr]+7.0×[Ni]+3.9×[Cu]+1.7×[N]

[0018] In the above formula (2), [C], [Si], [Mn], [Cr], [Ni], [Cu], and [N] represent the content (weight %) of each element.

[0019] Furthermore, the pitting resistance equivalent number (PREN) of the austenitic stainless steel according to one embodiment of the present invention, which is represented by the following formula (3), may be 17 or more.

[0020] Formula (3): [Cr]-0.5×[Mn]+16×[N]

[0021] In the above formula (3), [Cr], [Mn], and [N] represent the content (weight %) of each element.

[0022] Furthermore, the yield strength of the austenitic stainless steel according to one embodiment of the present invention may be 600 MPa or more.

[0023] Furthermore, the elongation of the austenitic stainless steel according to one embodiment of the present invention may be 30% or more.

[0024] Furthermore, the pitting potential value of the austenitic stainless steel according to one embodiment of the present invention may be 200 mV or more.

[0025] In addition, the thickness of the austenitic stainless steel according to one embodiment of the present invention may be 0.4-2.0 mm.

[0026] In addition, a method for manufacturing austenitic stainless steel according to an embodiment of the present invention may include the following steps: manufacturing a steel ingot, the steel ingot containing, in weight %, C: 0.05% to 0.1%, Si: 0.2% to 0.7%, Mn: 2.0% to 4.0%, P: more than 0% to less than 0.1%, S: more than 0% to 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%, and the balance of iron (Fe) and inevitable impurities; hot rolling the steel ingot to manufacture a hot-rolled steel sheet; cold rolling the hot-rolled steel sheet to manufacture a cold-rolled steel sheet; and final annealing the cold-rolled steel sheet.

[0027] Furthermore, in the method for producing austenitic stainless steel according to one embodiment of the present invention, the steel ingot may have an austenite stability parameter (ASP) value represented by the following formula (1) of -30 to 30.

[0028] Formula (1): 551-462×([C]+[N])-9.2×[Si]-8.1×[Mn]-13.7×[Cr]-29×([Ni]+[Cu])

[0029] In the above formula (1), [C], [N], [Si], [Mn], [Cr], [Ni], and [Cu] represent the content (weight %) of each element.

[0030] Furthermore, in the method for producing austenitic stainless steel according to one embodiment of the present invention, the steel ingot may have a strength stability parameter (SSP) value represented by the following formula (2) of 0 or more.

[0031] Formula (2): 58+132×[C]-7.9×[Si]+1.0×[Mn]-5.6×[Cr]+7.0×[Ni]+3.9×[Cu]+1.7×[N]

[0032] In the above formula (2), [C], [Si], [Mn], [Cr], [Ni], [Cu], and [N] represent the content (weight %) of each element.

[0033] Furthermore, in the method for producing austenitic stainless steel according to one embodiment of the present invention, the pitting resistance equivalent value (PREN) of the steel ingot represented by the following formula (3) may be 17 or more.

[0034] Formula (3): [Cr]-0.5×[Mn]+16×[N]

[0035] In the above formula (3), [Cr], [Mn], and [N] represent the content (weight %) of each element.

[0036] In addition, the method for producing austenitic stainless steel according to an embodiment of the present invention may further include a step of performing intermediate annealing on the hot-rolled steel sheet before the cold rolling.

[0037] Furthermore, in the method for manufacturing austenitic stainless steel according to an embodiment of the present invention, the intermediate annealing temperature may be 1050-1150°C.

[0038] Furthermore, in the method for producing austenitic stainless steel according to one embodiment of the present invention, the final annealing temperature may be 800-850°C.

[0039] Furthermore, in the method for producing austenitic stainless steel according to one embodiment of the present invention, the cold rolling may be performed so that a thickness reduction rate of the hot-rolled steel sheet at room temperature is 50% or more.

[0040] (III) Beneficial effects

[0041] According to one embodiment of the present invention, an austenitic stainless steel having price competitiveness and realizing ultrafine grain characteristics while realizing high strength, high ductility and high corrosion resistance, and a method for manufacturing the same can be provided. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 This is a photograph of a cross section of a thickness center portion of an austenitic stainless steel according to an example of the present invention, taken using a scanning electron microscope (SEM). Best Mode for Carrying Out the Invention

[0043] The austenitic stainless steel according to one embodiment of the present invention may contain, in weight %, C: 0.05% to 0.1%, Si: 0.2% to 0.7%, Mn: 2.0% to 4.0%, P: more than 0% to less than 0.1%, S: more than 0% to 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 balance being iron (Fe) and inevitable impurities, and the average grain diameter of the austenitic stainless steel in the thickness center portion may be 5 μm or less. DETAILED DESCRIPTION

[0044] The embodiments of the present invention will be described in detail below with reference to the accompanying drawings. The following embodiments are proposed to fully convey the ideas of the present invention to those skilled in the art to which the present invention belongs. The present invention is not limited to the embodiments proposed in this specification, but may also be embodied in other forms. In order to make the present invention clear, the drawings may omit the illustration of parts that are not related to the description, and the sizes of the constituent elements may be slightly exaggerated to help understand.

[0045] Throughout the specification, when a part is described as “including” or “comprising” a certain constituent element, unless otherwise specifically stated to the contrary, it means that other constituent elements may be further included or comprised, rather than excluding other constituent elements.

[0046] Unless the context clearly indicates otherwise, an expression in the singular includes an expression in the plural.

[0047] The reasons for limiting the numerical values ​​of the alloy component contents in the embodiment of the present invention are described below. Unless otherwise specified, the units are weight %.

[0048] The austenitic stainless steel according to one embodiment of the present invention may contain, in terms of weight %, C: 0.05% to 0.1%, Si: 0.2% to 0.7%, Mn: 2.0% to 4.0%, P: more than 0% to less than 0.1%, S: more than 0% to 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%, and the balance of iron (Fe) and inevitable impurities.

[0049] The content of carbon (C) may be 0.05% or more and 0.1% or less.

[0050] C is an element effective for stabilizing the austenite phase, and needs to be appropriately added to ensure the yield strength of the steel. Considering this, more than 0.05% C can be added. However, when the C content is too much, the cold workability may be reduced due to the solid solution strengthening effect. Also, when the C content is too much, the grain boundary precipitation of Cr carbides during low temperature annealing may cause an adverse effect on ductility and corrosion resistance. Considering this, the upper limit of the C content can be limited to 0.1%.

[0051] The content of silicon (Si) may be greater than or equal to 0.2% and less than or equal to 0.7%.

[0052] Si can be added for deoxidation of steel, and Si is an effective element for improving corrosion resistance. In view of this, more than 0.2% Si can be added. However, when the content of Si is too much, the formation of delta ferrite in the casting material may be promoted due to the ferrite phase stabilization effect. Therefore, when the content of Si is too much, hot workability may be reduced, and ductility and impact properties may be adversely affected. In view of this, the upper limit of the Si content can be limited to 0.7%. Preferably, more than 0.3% and less than 0.4% Si can be added.

[0053] The content of manganese (Mn) may be 2.0% or more and 4.0% or less.

[0054] In the present invention, Mn is an austenite phase stabilizing element added in place of Ni. In view of this, 2.0% or more of Mn may be added. However, when the content of Mn is too much, excessive S-based inclusions (MnS) are formed, which may reduce ductility and corrosion resistance. In view of this, the upper limit of the Mn content may be limited to 4.0%. Preferably, 3.6% or more and 3.9% or less of Mn may be added.

[0055] The content of phosphorus (P) may be more than 0% and less than 0.1%.

[0056] P is an impurity inevitably contained in steel and is an element that causes intergranular corrosion and hinders hot workability. Therefore, it is preferred to control the P content to be as low as possible. In view of this, the upper limit of the P content can be controlled to less than 0.1%.

[0057] The content of sulfur (S) may be more than 0% and less than 0.01%.

[0058] Like P, S is an impurity inevitably contained in steel and is an element that segregates at grain boundaries and hinders hot workability. Therefore, it is preferred to control the S content to be as low as possible. In view of this, the upper limit of the S content can be controlled to less than 0.01%.

[0059] The content of chromium (Cr) may be 17% or more and 19% or less.

[0060] Cr is an effective element for suppressing the formation of martensite phase and ensuring corrosion resistance. Considering this, 17% or more of Cr can be added. However, when the Cr content is too much, the manufacturing cost increases, and a large amount of delta ferrite is formed in the material, which may reduce hot workability. Considering this, the upper limit of the Cr content can be limited to 19%. Preferably, 17.2% or more and 18% or less of Cr can be added.

[0061] The content of nickel (Ni) may be 2.0% or more and 4.0% or less.

[0062] Ni is a strong austenite phase stabilizing element, an element necessary for ensuring good hot workability and cold workability. Therefore, even if a specified amount of Mn is added, more than 2.0% Ni may be added. However, when the content of Ni is too much, the martensitic transformation starting temperature (Ms) becomes too low, and it may be difficult to generate stress-induced martensite during cold working. And, when the content of Ni is too much, it may cause an increase in raw material costs. In view of this, the upper limit of the Ni content may be limited to 4.0%. Preferably, more than 3.4% and less than 3.7% Ni may be added.

[0063] The content of copper (Cu) may be 1.0% or more and 2.5% or less.

[0064] Cu is an austenite phase stabilizing element, which is effective for softening the material. Considering this, more than 1.0% Cu can be added. However, when the content of Cu is too much, the martensitic transformation starting temperature (Ms) becomes too low, and it may be difficult to generate stress-induced martensite during cold working. And, when the content of Cu is too much, it may induce an increase in material cost and hot brittleness. Considering this, the upper limit of the Cu content can be limited to 2.5%. Preferably, more than 1.5% and less than 2.0% Cu can be added.

[0065] The content of nitrogen (N) may be 0.15% or more and 0.25% or less.

[0066] N is an element effective for improving the stabilization of the austenite phase and corrosion resistance. In view of this, more than 0.15% N can be added. However, when the content of N is too much, due to the solution effect, the cold workability may be reduced, and the martensitic phase transformation starting temperature (Ms) becomes too low, so it may be difficult to generate stress-induced martensite during cold working. In addition, when the content of N is too much, quality defects may be caused due to the formation of holes during casting. In view of this, the upper limit of the N content can be limited to 0.25%. Preferably, more than 0.16% and less than 0.21% N can be added.

[0067] The remaining component of the present invention is iron (Fe). However, in the conventional manufacturing process, undesirable impurities may inevitably be mixed from the raw materials or the surrounding environment, so these impurities cannot be eliminated. These impurities are well known to technicians in the conventional manufacturing process, so all of them are not specifically described in this specification.

[0068] The austenitic stainless steel according to one embodiment of the present invention realizes ultrafine grain characteristics, so that the average grain diameter of the thickness center portion can be 5 μm or less, wherein the thickness center portion refers to a portion of 1 / 4t to 3 / 4t when the thickness of the stainless steel is t.

[0069] In the present invention, the term “average” refers to the average value of values ​​measured at any five locations.

[0070] The austenite stability parameter (ASP) value of the austenitic stainless steel according to one embodiment of the present invention, which is represented by the following formula (1), may be -30 to 30.

[0071] Formula (1): 551-462×([C]+[N])-9.2×[Si]-8.1×[Mn]-13.7×[Cr]-29×([Ni]+[Cu])

[0072] In the above formula (1), [C], [N], [Si], [Mn], [Cr], [Ni], and [Cu] represent the content (weight %) of each element.

[0073] Formula (1) refers to the temperature at which 50% of austenite transforms into martensite when stainless steel is strained to a true strain of 0.3, and is used as an indicator of austenite phase stability. The lower the value of formula (1), the higher the austenite phase stability and the less the amount of transformation into work-induced martensite during strain.

[0074] When the value of formula (1) is less than -30, the amount of TRIP phase from the austenite phase to the martensite phase is low, so that the amount of processing-induced martensite is reduced. Therefore, when the value of formula (1) is less than -30, the ratio of the reversed austenite phase may be reduced due to low temperature annealing, and it may be difficult to ensure ultrafine grains. However, when the value of formula (1) exceeds 30, the yield strength and elongation may be reduced due to excessively rapid TRIP phase transformation.

[0075] The austenitic stainless steel according to one embodiment of the present invention may have a strength stability parameter (SSP) value represented by the following formula (2) of 0 or more.

[0076] Formula (2): 58+132×[C]-7.9×[Si]+1.0×[Mn]-5.6×[Cr]+7.0×[Ni]+3.9×[Cu]+1.7×[N]

[0077] In the above formula (2), [C], [Si], [Mn], [Cr], [Ni], [Cu], and [N] represent the content (weight %) of each element.

[0078] When the value of the formula (2) is less than 0, it may be difficult to achieve ultrafine grain microstructure characteristics in a wide range of final annealing temperatures. That is, in order to achieve ultrafine grain microstructure characteristics in all regions of 800-850°C as the final annealing temperature range proposed in the present invention, it is necessary to control the value of the formula (2) to be greater than 0.

[0079] The austenitic stainless steel according to one embodiment of the present invention may have a pitting resistance equivalent value (PREN) represented by the following formula (3) of 17 or more.

[0080] Formula (3): [Cr]-0.5×[Mn]+16×[N]

[0081] In the above formula (3), [Cr], [Mn], and [N] represent the content (weight %) of each element.

[0082] When the value of the formula (3) is less than 17, the pitting potential value measured with a 3.5% NaCl solution (30° C.) may not satisfy 200 mV or more. That is, when the value of the formula (3) is less than 17, it may be difficult to achieve the desired high corrosion resistance.

[0083] The ultrafine grain characteristics can be achieved by the alloy composition, parameters and manufacturing method proposed in the present invention. Therefore, the yield strength of the austenitic stainless steel according to one embodiment of the present invention can be 600 MPa or more, and the elongation can be 30% or more.

[0084] Furthermore, the pitting potential value of the austenitic stainless steel according to one embodiment of the present invention may be 200 mV or more.

[0085] The thickness of the austenitic stainless steel according to one embodiment of the present invention may be 0.4 to 2.0 mm, but is not limited thereto and may be various thicknesses depending on the purpose.

[0086] Next, a method for producing austenitic stainless steel according to another aspect of the present invention will be described.

[0087] A method for manufacturing austenitic stainless steel according to an embodiment of the present invention may include the following steps: manufacturing a steel ingot, the steel ingot containing, in weight %, C: 0.05% to 0.1%, Si: 0.2% to 0.7%, Mn: 2.0% to 4.0%, P: more than 0% to less than 0.1%, S: more than 0% to 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%, and the balance of iron (Fe) and inevitable impurities; hot rolling the steel ingot to manufacture a hot-rolled steel sheet; cold rolling the hot-rolled steel sheet to manufacture a cold-rolled steel sheet; and final annealing the cold-rolled steel sheet.

[0088] The steel ingot may have an austenite stability parameter (ASP) value represented by the following formula (1) of -30 to 30.

[0089] Formula (1): 551-462×([C]+[N])-9.2×[Si]-8.1×[Mn]-13.7×[Cr]-29×([Ni]+[Cu])

[0090] In the above formula (1), [C], [N], [Si], [Mn], [Cr], [Ni], and [Cu] represent the content (weight %) of each element.

[0091] Furthermore, in the method for producing austenitic stainless steel according to one embodiment of the present invention, the steel ingot may have a strength stability parameter (SSP) value represented by the following formula (2) of 0 or more.

[0092] Formula (2): 58+132×[C]-7.9×[Si]+1.0×[Mn]-5.6×[Cr]+7.0×[Ni]+3.9×[Cu]+1.7×[N]

[0093] In the above formula (2), [C], [Si], [Mn], [Cr], [Ni], [Cu], and [N] represent the content (weight %) of each element.

[0094] Furthermore, in the method for producing austenitic stainless steel according to one embodiment of the present invention, the pitting resistance equivalent value (PREN) of the steel ingot represented by the following formula (3) may be 17 or more.

[0095] Formula (3): [Cr]-0.5×[Mn]+16×[N]

[0096] In the above formula (3), [Cr], [Mn], and [N] represent the content (weight %) of each element.

[0097] In addition, slabs may be produced instead of the ingots according to the purpose and application.

[0098] The reasons for limiting the component ranges of the above-mentioned alloy compositions and the numerical values ​​of the formulas (1), (2) and (3) are as described above. Below, each production step will be described in more detail.

[0099] First, a steel ingot satisfying the alloy composition is prepared, and then may go through a series of processes of hot rolling, cold rolling and final annealing.

[0100] First, the steel ingot may be heated at 1150-1300° C. and then hot-rolled to manufacture a hot-rolled steel sheet.

[0101] When the heating temperature is low, it may be difficult to decompose the coarse precipitates generated in the process of manufacturing the steel ingot. Considering this, the heating temperature may be 1150°C or higher. However, when the heating temperature is too high, the internal grains may become too coarse, and severe surface oxidation may occur, which may cause surface defects. Considering this, the upper limit of the heating temperature may be limited to 1300°C.

[0102] Next, the method may further include performing intermediate annealing on the hot rolled steel sheet before the cold rolling. The intermediate annealing step may be performed or omitted as required.

[0103] When the intermediate annealing is performed, it may be performed at a temperature of 1000-1150°C.

[0104] When the intermediate annealing temperature is low, the fraction of retained martensite becomes high, and thus workability may be reduced. However, when the intermediate annealing temperature is too high, strength reduction due to grain coarsening may occur.

[0105] The final annealing temperature may be 800-850°C.

[0106] Like the intermediate annealing temperature, when the final annealing temperature is too low, workability may be reduced. However, when the final annealing temperature is too high, strength may be reduced due to coarsening of crystal grains.

[0107] The cold rolling may be performed so that the thickness reduction rate of the hot-rolled steel sheet at room temperature is 50% or more. When the thickness reduction rate during cold rolling is less than 50%, the amount of work-induced martensite decreases, so the proportion of the ultrafine grain reverse transformation austenite phase decreases during low temperature annealing, and it may be difficult to ensure strength.

[0108] Hereinafter, the present invention will be described in more detail by way of examples. However, the description of these examples is only used to illustrate the implementation of the present invention, and the present invention is not limited by these examples. This is because the scope of the present invention is determined by the matters described in the claims and the matters reasonably derived therefrom.

[0109] {Example}

[0110] For various alloy composition ranges shown in Table 1 below, a 35 kg ingot with a thickness of 150 mm was cast in a vacuum induction melting furnace. The cast ingot was heated in a heating furnace at 1250° C. for 2 hours, and then hot rolled to a plate width of 200 mm and a thickness of 4 mm to produce a hot-rolled steel plate and air-cooled. The air-cooled hot-rolled steel plate was intermediate annealed at 1100° C. for 1 minute, and then pickled and cold rolled to a thickness of 1.2 mm to produce a cold-rolled steel plate. The cold-rolled steel plate was annealed at the final annealing temperature of Table 2 to produce the final product.

[0111] [Table 1]

[0112]

[0113] The values ​​of formula (1), formula (2), formula (3), final annealing temperature, average grain size, pitting potential, yield strength and elongation are shown in the following Table 2. The value of formula (1) is calculated by the following formula (1) and shown.

[0114] Formula (1): 551-462×([C]+[N])-9.2×[Si]-8.1×[Mn]-13.7×[Cr]-29×([Ni]+[Cu])

[0115] In the above formula (1), [C], [N], [Si], [Mn], [Cr], [Ni], and [Cu] represent the content (weight %) of each element.

[0116] The value of formula (2) is calculated and shown by the following formula (2).

[0117] Formula (2): 58+132×[C]-7.9×[Si]+1.0×[Mn]-5.6×[Cr]+7.0×[Ni]+3.9×[Cu]+1.7×[N]

[0118] In the above formula (2), [C], [Si], [Mn], [Cr], [Ni], [Cu], and [N] represent the content (weight %) of each element.

[0119] The value of formula (3) is calculated and shown by the following formula (3).

[0120] Formula (3): [Cr]-0.5×[Mn]+16×[N]

[0121] In the above formula (3), [Cr], [Mn], and [N] represent the content (weight %) of each element.

[0122] The average grain size was measured by photographing a cross section of the center of the steel using a scanning electron microscope (SEM) of Model JSM-7001F.

[0123] The pitting potential is measured using a potentiostat. At this time, when the steel is immersed in a NaCl solution and a voltage of 20mV / minute is applied, the value of the potential (pitting potential) when the measured current reaches 100μA is shown. The temperature of the NaCl solution is set to 30°C and the concentration is set to 3.5%. In addition, the higher the pitting potential value, the better the corrosion resistance.

[0124] The yield strength and elongation were measured by testing a JIS 13B tensile test piece at a tensile speed of 15 mm per minute at room temperature using a tensile tester from Zwick Roell.

[0125] [Table 2]

[0126]

[0127] Referring to Table 2, Examples 1 to 4 satisfy the alloy composition, component range, parameters and manufacturing process proposed in the present invention. Therefore, Examples 1 to 4 satisfy the average grain diameter of less than 5 μm, the yield strength of more than 600 MPa, the elongation of more than 30%, and the pitting potential value of more than 200 mV. That is, Examples 1 to 4 simultaneously satisfy high strength, high ductility and high corrosion resistance. However, in the case of Comparative Examples 1 to 3, the value of formula (1) cannot satisfy -30 to 30, and the value of formula (2) cannot satisfy more than 0. Therefore, the grain diameter of less than 5 μm and the yield strength of more than 600 MPa cannot be satisfied. In particular, in all regions of 800-850°C, which is the final annealing temperature range proposed in the present invention, it is difficult to achieve ultrafine grain microstructure characteristics.

[0128] In the case of Comparative Examples 2 and 4, the Mn content is too high and the value of Formula (3) cannot satisfy 17 or more. Therefore, in the case of Comparative Examples 2 and 4, the pitting potential value cannot satisfy 200 mV or more. That is, Comparative Examples 2 and 4 have poor corrosion resistance.

[0129] Figure 1 This is a photograph of a cross section of a thickness center portion of an austenitic stainless steel according to an example of the present invention, taken with a scanning electron microscope (SEM).

[0130] Reference Figure 1 It can be confirmed that the average grain size of the thickness center portion of the austenitic stainless steel according to one example of the present invention satisfies 5 μm or less. That is, it can be seen that the ultrafine grain characteristics can be achieved according to one example of the present invention.

[0131] Industrial Applicability

[0132] According to one embodiment of the present invention, an austenitic stainless steel and a method for manufacturing the same are provided. The austenitic stainless steel has price competitiveness and realizes ultrafine grain characteristics to simultaneously achieve high strength, high ductility and high corrosion resistance, and thus has industrial applicability.

Claims

1. An austenitic stainless steel, wherein: The austenitic stainless steel comprises, in weight %, C: 0.05% to 0.1%, Si: 0.2% to 0.7%, Mn: 2.0% to 4.0%, P: more than 0% to less than 0.1%, S: more than 0% to 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%, and the balance is iron (Fe) and inevitable impurities, and the average grain size of the austenitic stainless steel at the center of the thickness is 5 μm or less.

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

3. The austenitic stainless steel according to claim 1, wherein: The austenitic stainless steel has a strength stability parameter (SSP) value represented by the following formula (2) of 0 or more, 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 of each element, and the unit is weight %.

4. The austenitic stainless steel according to claim 1, wherein: The pitting resistance equivalent value (PREN) of the austenitic stainless steel represented by the following formula (3) is 17 or more, Formula (3): [Cr]-0.5×[Mn]+16×[N] In the above formula (3), [Cr], [Mn], and [N] represent the content of each element, and the unit is weight %.

5. The austenitic stainless steel according to claim 1, wherein: The yield strength of the austenitic stainless steel is 600 MPa or more.

6. The austenitic stainless steel according to claim 1, wherein: The austenitic stainless steel has an elongation of 30% or more.

7. The austenitic stainless steel according to claim 1, wherein: The pitting potential value of the austenitic stainless steel is 200 mV or more.

8. The austenitic stainless steel according to claim 1, wherein: The thickness of the austenitic stainless steel is 0.4-2.0 mm.

9. A method for manufacturing austenitic stainless steel, comprising the following steps: A steel ingot is manufactured, wherein the steel ingot contains, in terms of weight percent, C: 0.05% to 0.1%, Si: 0.2% to 0.7%, Mn: 2.0% to 4.0%, P: more than 0% to less than 0.1%, S: more than 0% to 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%, and the balance is iron (Fe) and unavoidable impurities; hot rolling the steel ingot to produce a hot-rolled steel plate; cold rolling the hot-rolled steel sheet to produce a cold-rolled steel sheet; as well as The cold rolled steel sheet is subjected to final annealing.

10. The method for producing austenitic stainless steel according to claim 9, wherein: The austenite stability parameter (ASP) value of the steel ingot represented by the following formula (1) is -30 to 30, Formula (1): 551-462×([C]+[N])-9.2×[Si]-8.1×[Mn]-13.7×[Cr]-29×([Ni]+[Cu]) In the above formula (1), [C], [N], [Si], [Mn], [Cr], [Ni], and [Cu] represent the content of each element, and the unit is weight %.

11. The method for producing austenitic stainless steel according to claim 9, wherein: The steel ingot has a strength stability parameter (SSP) value represented by the following formula (2) of 0 or more, 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 of each element, and the unit is weight %.

12. The method for producing austenitic stainless steel according to claim 9, wherein: The pitting resistance equivalent value (PREN) of the steel ingot represented by the following formula (3) is 17 or more, Formula (3): [Cr]-0.5×[Mn]+16×[N] In the above formula (3), [Cr], [Mn], and [N] represent the content of each element, and the unit is weight %.

13. The method for producing austenitic stainless steel according to claim 9, wherein: The manufacturing method further includes the step of performing intermediate annealing on the hot-rolled steel sheet before the cold rolling.

14. The method for producing austenitic stainless steel according to claim 13, wherein: The intermediate annealing temperature is 1050-1150°C.

15. The method for producing austenitic stainless steel according to claim 9, wherein: The final annealing temperature is 800-850°C.

16. The method for producing austenitic stainless steel according to claim 9, wherein: The cold rolling is performed so that the thickness reduction rate of the hot-rolled steel sheet at room temperature is 50% or more.

Images