Austenitic stainless steel and method for producing 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 manufacturing 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 manufacturing the same.

Background Art

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

[0003] Patent Document 1 relates to austenitic stainless steel and a method for manufacturing the same, and discloses austenitic stainless steel having a tensile strength of 600 MPa or more, but it has a large Ni content and poor price competitiveness. However, as 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, in the process of austenite crystal grain refinement, it is often difficult to ensure crystal grains of a sufficiently small size only by slab reheating and rolling. In particular, the higher the temperature of the steel material to be rolled, the lower the deformation resistance during rolling, making rolling 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 effect of crystal grain refinement 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 one hand, the grain refinement technology has been attracting attention as a technique for simultaneously improving strength and ductility. In particular, as a method for ultrafine-graining steel materials for structural members, severe plastic deformation (SPD) has been in the spotlight. The severe plastic working method is a method of applying a strong shear stress to a material to generate new grain boundaries within the grain boundaries and realizing fine grains. However, the severe plastic working method has problems such as a decrease in productivity and limitations on product size.

[0005] Patent Document 2 discloses a method of performing a heat treatment for 48 hours or more at 600 to 700 °C in order to achieve an average crystal grain diameter of 10 μm or less. However, the method disclosed in Patent Document 2 has problems such as a decrease in productivity and an increase in manufacturing cost.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0007] An object of the present invention made to solve the above-described problems is to provide an austenitic stainless steel capable of simultaneously realizing high strength, high ductility, and high corrosion resistance by realizing ultrafine grain characteristics while having price competitiveness, and a method for manufacturing the same.

Means for Solving the Problems

[0008] The austenitic stainless steel of the present invention, by weight%, contains 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, and consists of the remaining iron (Fe) and inevitable impurities, and is characterized in that the average crystal grain size at the center of the thickness is 5 μm or less.

[0009] The austenitic stainless steel of the present invention preferably 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 the above formula (1), [C], [N], [Si], [Mn], [Cr], [Ni], [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 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], [N] represent the content (weight%) of each element.

[0011] The austenitic stainless steel of the present invention 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], [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 more. Also, the elongation can be 30% or more.

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

[0014] The method for manufacturing the austenitic stainless steel of the present invention comprises, in 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 step of manufacturing an ingot composed of the remaining iron (Fe) and inevitable impurities, the step of hot rolling the ingot to manufacture a hot-rolled steel sheet, the step of cold rolling the hot-rolled steel sheet to manufacture a cold-rolled steel sheet, and the step of finally annealing 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) of -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], [Cu] mean the content (weight %) of each element.

[0016] In the manufacturing method of the present invention, the ingot preferably has an SSP (Strength Stability Parameter) value represented by the following formula (2) of 0 or more. Equation (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 Equation (2), [C], [Si], [Mn], [Cr], [Ni], [Cu], and [N] represent the content (wt%) 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 Equation (3) of 17 or more. Equation (3): [Cr] - 0.5×[Mn] + 16×[N] In the above Equation (3), [Cr], [Mn], and [N] represent the content (wt%) 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 so 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

Embodiments for Carrying Out the Invention

[0022] An austenitic stainless steel according to an embodiment of the present invention, in weight %, consists of 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 remaining iron (Fe) and inevitable impurities, and is characterized in that the average crystal grain size at the center of the thickness is 5 μm or less.

[0023] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. The following embodiments are presented to fully convey the idea of the present invention to those with ordinary knowledge in the technical field to which the present invention pertains. The present invention is not limited only to the embodiments presented in this specification and may be embodied in other forms. The drawings may omit the illustration of parts not related to the explanation for clarity and may represent the sizes of the components somewhat exaggerated to assist understanding. Throughout the specification, when a certain part refers to a certain component as "including", this means that, unless otherwise stated to the contrary, it does not exclude other components but may further include other components. Singular expressions include plural expressions unless there is an obvious exception in the context.

[0024] Hereinafter, the reasons for the numerical limitations of the alloy component contents in the embodiments of the present invention will be described. Hereinafter, unless otherwise specified, the unit is weight %. The austenitic stainless steel according to one embodiment 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, and the balance being iron (Fe) and inevitable impurities.

[0025] The content of C (carbon) is preferably 0.05% or more and 0.1% or less. 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, C is preferably added in an amount of 0.05% or more. On the other hand, when the content of C is excessive, the cold workability will be deteriorated due to the solid solution strengthening effect. Also, when the C content is excessive, it may adversely affect the ductility and corrosion resistance due to the grain boundary precipitation of Cr carbides during low-temperature annealing. Considering this, the upper limit of the C content is preferably limited to 0.1%.

[0026] The content of Si (silicon) is preferably 0.2% or more and 0.7% or less. Si is an element added for deoxidation of the steel and is effective for improving corrosion resistance. Considering this, Si is preferably added in an amount of 0.2% or more. On the other hand, when the content of Si is excessive, there is a risk of promoting the formation of delta ferrite in the casting material due to the stabilizing effect of the ferrite phase. Therefore, when the Si content is excessive, the hot workability may be deteriorated, and there is a risk of adversely affecting the ductility and impact properties. Considering this, the upper limit of the Si content is preferably limited to 0.7%. Preferably, Si is added in an amount of 0.3% or more and 0.4% or less.

[0027] The content of Mn (manganese) is preferably 2.0% or more and 4.0% or less. In the present 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, when the content of Mn is excessive, there is a risk of excessively forming S-based inclusions (MnS), which may reduce ductility and corrosion resistance. Considering this, the upper limit of the Mn content is preferably limited to 4.0%. Preferably, Mn is added in an amount of 3.6% or more and 3.9% or less.

[0028] The content of P (phosphorus) is preferably more than 0% and less than 0.1%. P is an impurity inevitably contained in steel and is an element that causes intergranular corrosion and inhibits hot workability. Therefore, it is desirable to control the content of P as low as possible. Considering this, the upper limit of the P content is preferably controlled to less than 0.1%.

[0029] The content of S (sulfur) is preferably more than 0% and less than 0.01%. Similar to P, S is an impurity inevitably contained in steel and is an element that segregates at the grain boundaries and inhibits hot workability. Therefore, it is desirable to control the content of S as low as possible. Considering this, the upper limit of the S content can be controlled to less than 0.01%.

[0030] The content of Cr (chromium) is preferably 17% or more and 19% or less. Cr is an element effective for suppressing the formation of the martensite phase and ensuring corrosion resistance. Considering this, it is preferable to add 17% or more of Cr. On the other hand, when the content of Cr is excessive, the manufacturing cost increases, and there is a risk of forming a large amount of delta ferrite in the material, which may reduce hot workability. Considering this, the upper limit of the Cr content is preferably limited to 19%. Preferably, Cr is added in an amount of 17.2% or more and 18% or less.

[0031] The content of Ni (nickel) is preferably 2.0% or more and 4.0% or less. Ni is a strong austenite phase stabilizing element and is an essential element for ensuring good hot workability and cold workability. Therefore, even when a certain amount of Mn is added, it is advisable to add Ni in an amount of 2.0% or more. On the other hand, when the Ni content is excessive, the martensite transformation start temperature (Ms) becomes too low, and it may become difficult to generate stress-induced martensite during cold working. Also, when the Ni content is excessive, there is a risk of increasing the raw material cost. Considering this, the upper limit of the Ni content is preferably limited to 4.0%. Preferably, Ni is added in an amount of 3.4% or more and 3.7% or less.

[0032] The content of Cu (copper) is preferably 1.0% or more and 2.5% or less. As an austenite phase stabilizing element, Cu is effective in softening the material. Considering this, it is preferable to add Cu in an amount of 1.0% or more. On the other hand, when the Cu content is excessive, the martensite transformation start temperature (Ms) becomes too low, and there is a risk of making it difficult to generate stress-induced martensite during cold working. Also, when the Cu content is excessive, there is a risk of increasing the material cost and inducing hot brittleness. Considering this, the upper limit of the Cu content is preferably limited to 2.5%. Preferably, Cu is added in an amount of 1.5% or more and 2.0% or less.

[0033] The content of N (nitrogen) is preferably 0.15% or more and 0.25% or less. N is an element effective for stabilizing the austenite phase and improving corrosion resistance. Considering this, N can be added in an amount of 0.15% or more. On the other hand, when the N content is excessive, there is a risk of reducing cold workability due to the solid solution strengthening effect, and the martensite transformation start temperature (Ms) becomes too low, making it difficult to generate stress-induced martensite during cold working. Also, when the N content is excessive, it may cause quality defects due to pore formation during casting. Considering this, the upper limit of the N content is preferably limited to 0.25%. Preferably, N is added in an amount of 0.16% or more and 0.21% or less.

[0034] The remaining component of the present invention is iron (Fe). However, in normal manufacturing processes, it is inevitable that unintended impurities may be mixed in from raw materials or the surrounding environment, and thus it is impossible to eliminate them. These impurities are known to any engineer in normal manufacturing processes, and not all of their details are specifically mentioned in this specification.

[0035] The austenitic stainless steel according to an embodiment of the present invention exhibits ultrafine grain characteristics, and the average crystal grain size at the center of the thickness is preferably 5 μm or less. Here, the center of the thickness means the portion from 1 / 4t to 3 / 4t when the thickness of the stainless steel is t. In addition, the average in the present invention means the average value of the values measured at any five locations.

[0036] The austenitic stainless steel according to an embodiment of the present invention can have 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 the above formula (1), [C], [N], [Si], [Mn], [Cr], [Ni], and [Cu] represent the content (weight %) of each element.

[0037] Formula (1) means the temperature at which 50% of austenite transforms into martensite when the stainless steel is deformed with a true strain of 0.3, and is utilized as an index of the austenite phase stabilization degree. The lower the value of formula (1), the higher the austenite phase stabilization degree, and the smaller the amount of strain-induced martensite that transforms during deformation. When the value of formula (1) is less than -30, the amount of TRIP transformation from the austenite phase to the martensite phase decreases, and the amount of strain-induced martensite decreases. Therefore, when the value of formula (1) is less than -30, the proportion of reverted austenite phase formed by low-temperature annealing may be low, making it difficult to ensure ultrafine grains. However, when the value of formula (1) exceeds 30, the yield strength and elongation may decrease due to premature TRIP transformation.

[0038] The austenitic stainless steel according to one embodiment of the present invention can have an SSP (Strength Stability Parameter) 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 (weight %) of each element. When the value of the above formula (2) is less than 0, it may be difficult to realize the ultrafine grain microstructure characteristics at a wide range of final annealing temperatures. That is, in order to realize the ultrafine grain microstructure characteristics in all regions of the final annealing temperature range of 800 to 850 °C presented in the present invention, it is necessary to control the value of the above formula (2) to be 0 or more.

[0039] The austenitic stainless steel according to one embodiment of the present invention 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 (weight %) of each element. When the value of the above formula (3) is less than 17, the pitting potential value measured in a 3.5% NaCl solution (30 °C) may not satisfy 200 mV or more. That is, when the value of the above formula (3) is less than 17, it becomes difficult to achieve the target high corrosion resistance.

[0040] With the alloy components, parameters, and manufacturing method presented in the present invention, ultrafine grain characteristics can be realized. Therefore, the austenitic stainless steel according to an embodiment of the present invention can have a yield strength of 600 MPa or more and an elongation of 30% or more. In addition, the austenitic stainless steel according to an embodiment of the present invention preferably has a pitting potential value of 200 mV or more. The austenitic stainless steel according to an embodiment of the present invention has a thickness of 0.4 to It is preferably 2.0 mm. However, it is not limited thereto, and it can be manufactured in various thicknesses according to the purpose.

[0041] Next, a method for manufacturing austenitic stainless steel according to another aspect of the present invention will be described. The method for manufacturing austenitic stainless steel according to an embodiment of the present invention includes, 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, producing an ingot composed of the remaining iron (Fe) and inevitable 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 finally annealing the cold-rolled steel sheet.

[0042] The ingot preferably 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 the above formula (1), [C], [N], [Si], [Mn], [Cr], [Ni], and [Cu] represent the content (% by weight) of each element.

[0043] Further, in the method for manufacturing austenitic stainless steel according to an embodiment of the present invention, it is preferable that the ingot has an SSP (Strength Stability Parameter) 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 (% by weight) of each element.

[0044] Further, in the method for manufacturing austenitic stainless steel according to an embodiment of the present invention, it is preferable that the ingot has 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. On the other hand, depending on the purpose and application, it is also possible to manufacture using a slab instead of the ingot.

[0045] The component ranges of the above alloy compositions and the reasons for the numerical limitations of the values of formula (1), formula (2), and formula (3) are as described above. Hereinafter, each manufacturing stage will be described in more detail. First, after providing an ingot that satisfies the above alloy composition, it can undergo a series of hot rolling, cold rolling, and final annealing processes. After heating the ingot at 1150 to 1300°C, it is hot-rolled to produce a hot-rolled steel sheet. When the heating temperature is low, there is a risk that it may be difficult to redissolve the coarse precipitates generated during the production of the ingot. Considering this, the heating temperature is preferably 1150°C or higher. However, if the heating temperature is too high, there is a risk that the internal crystal grains may become too coarse, surface oxidation may become intense, and surface defects may be caused. Considering this, the upper limit of the heating temperature is limited to 1300°C. Next, before the cold rolling, the step of intermediate annealing the hot-rolled steel sheet can be further included. The step of intermediate annealing can be performed as necessary or can be omitted. When performing the intermediate annealing, it is preferably performed at a temperature of 1000 to 1150°C.

[0046] When the intermediate annealing temperature is low, the residual martensite fraction may increase, and the workability may decrease. On the other hand, if the intermediate annealing temperature is too high, there is a risk of a decrease in strength due to grain coarsening. The final annealing temperature is preferably 800 to 850°C. Similar to the intermediate annealing temperature, if the final annealing temperature is too low, there is a risk that the workability may decrease. On the other hand, if the final annealing temperature is too high, there is a risk that the strength may decrease due to grain coarsening. The cold rolling is preferably performed at room temperature so that the thickness reduction rate of the hot-rolled steel sheet is 50% or more. If the thickness reduction rate during cold rolling is less than 50%, the amount of strain-induced martensite decreases, and the ratio of the ultrafine grain reverse transformation austenite phase during low-temperature annealing becomes low, making it difficult to ensure strength.

[0047] Hereinafter, the present invention will be described in more detail through embodiments. However, the description of these embodiments is for exemplifying the embodiments of the present invention, and the present invention is not limited by such a description of the embodiments. The scope of the rights of the present invention is determined by the matters described in the claims and matters reasonably inferred therefrom. {Examples}

[0048] For various alloy component ranges shown in Table 1 below, ingots with a thickness of 150 mm and a weight of 35 kg were cast in a vacuum induction melting furnace. The cast ingots were heated in a heating furnace at 1250 °C for 2 hours, then hot-rolled into hot-rolled steel sheets with a plate width of 200 mm and a thickness of 4 mm, and 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 annealed at the final annealing temperature shown in Table 2 to produce the final products.

[0049]

Table 1

[0050] Table 2 below shows the values of formula (1), formula (2), formula (3), final annealing temperature, average crystal grain size, pitting potential, yield strength, and elongation. The value of formula (1) was calculated according to the following formula (1). 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], [Cu] represent the content (weight %) of each element.

[0051] The value of formula (2) was calculated according to 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], [N] represent the content (weight %) of each element.

[0052] The value of formula (3) was calculated according to the following formula (3). Formula (3): [Cr] - 0.5×[Mn] + 16×[N] In the above formula (3), [Cr], [Mn], [N] represent the content (weight %) of each element.

[0053] The average crystal grain size was measured by photographing the cross-section of the center part of the steel with a scanning electron microscope (SEM) of model name JSM-7001F. The pitting potential was measured using a potentiostat device. At this time, the steel was immersed in an NaCl solution, and when a voltage of 20 mV / min was applied, the potential (pitting potential) at which the current reached 100 μA was measured. Here, the temperature of the NaCl solution was 30 °C and the concentration was set to 3.5%. On the other hand, it means that the higher the pitting potential value, the better the corrosion resistance.

[0054] The yield strength and elongation were measured using a tensile testing machine from Zwick Roell. A JIS13B tensile test piece was tested at room temperature at a tensile speed of 15 mm per minute.

[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. Therefore, Examples 1 to 4 satisfied an average crystal grain size of 5 μm or less, a yield strength of 600 MPa or more, an elongation of 30% or more, and a pitting potential value of 200 mV or more. That is, Examples 1 to 4 simultaneously satisfied high strength, high ductility, and high corrosion resistance. However, in the case of Comparative Examples 1 to 3, the value of formula (1) could not satisfy -30 to 30, and the value of formula (2) could not satisfy 0 or more. Therefore, it was not possible to satisfy a crystal grain size of 5 μm or less and a yield strength of 600 MPa or more. In particular, it was difficult to realize the ultrafine grain microstructure characteristics in all regions of 800 to 850 °C, which is the final annealing temperature range presented in the present invention. In the case of Comparative Examples 2 and 4, since the Mn content was excessive, the value of formula (3) could not satisfy 17 or more. As a result, in the case of Comparative Examples 2 and 4, the pitting potential value could not satisfy 200 mV or more. 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 a cross-section of the center of the thickness of an austenitic stainless steel according to an embodiment of the present invention. Referring to Figure 1, it can be confirmed that the average crystal grain size in the center of the thickness of the austenitic stainless steel according to an embodiment of the present invention satisfies 5 μm or less. That is, according to an embodiment of the present invention, it can be seen that ultrafine grain characteristics can be realized.

Industrial Applicability

[0058] According to an embodiment of the present invention, it is possible to provide an austenitic stainless steel and a method for manufacturing the same that can simultaneously achieve high strength, high ductility, and high corrosion resistance by realizing ultrafine grain characteristics while having price competitiveness, and industrial applicability is recognized.

Claims

1. 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, consisting of the remaining iron (Fe) and inevitable impurities, An austenitic stainless steel characterized in that the average crystal grain size at the center of the thickness is 5 μm or less.

2. The austenitic stainless steel according to Claim 1, wherein the ASP (Austenite Stability Parameter) value 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], [Cu] mean the content (weight %) of each element.)

3. The austenitic stainless steel according to Claim 1, wherein the SSP (Strength Stability Parameter) value represented by the following formula (2) is 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], [N] mean the content (weight %) of each element.)

4. The austenitic stainless steel according to Claim 1, wherein the pitting resistance equivalent number (PREN) 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], [N] mean the content (weight %) of each element.)

5. The austenitic stainless steel according to Claim 1, characterized in that the yield strength is 600 MPa or more.

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

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

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

9. In terms of 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 step of manufacturing an ingot consisting of the remaining iron (Fe) and inevitable impurities, The step of hot rolling the ingot to produce a hot rolled steel sheet, The step of cold rolling the hot rolled steel sheet to produce a cold rolled steel sheet, and The step of final annealing the cold rolled steel sheet, a method for manufacturing an austenitic stainless steel, characterized by including.

10. The method for manufacturing an austenitic stainless steel 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 the above formula (1), [C], [N], [Si], [Mn], [Cr], [Ni], [Cu] mean the content (weight%) of each element.)

11. The method for manufacturing an austenitic stainless steel according to claim 9, characterized in that the ingot has an SSP (Strength Stability Parameter) 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], [N] mean the content (weight%) of each element.)

12. The method for manufacturing an austenitic stainless steel according to claim 9, characterized in that the ingot has 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 (wt%) of each element.)

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

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

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

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

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