Austenitic stainless steel sheet and method for manufacturing the same

Abstract

Provided are ultrafine-grained austenitic stainless steel that simultaneously satisfies high strength, high elongation and high yield ratio, and a method for producing the same. [Solution] The austenitic stainless steel of the present invention is characterized in that it consists, by weight, of 0.005-0.03% C (carbon), 0.1-1.0% Si (silicon), 0.1-2.0% Mn (manganese), 6.0-12.0% Ni (nickel), 16.0-20.0% Cr (chromium), 0.01-0.2% N (nitrogen), 0.25% or less Nb (niobium), with the balance being Fe (iron) and other unavoidable impurities, and has an average crystal grain size (d) value at the center in the thickness direction of 5 μm or less, and an unrecrystallized area fraction in the band form of 10% or less.

Description

[Technical Field]

[0001] The present invention relates to austenitic stainless steel. board and a manufacturing method thereof, and more particularly, an ultrafine-grained austenitic stainless steel that simultaneously satisfies high strength, high elongation, and high yield ratio. board and a manufacturing method thereof. [Background technology]

[0002] In general, austenitic stainless steels are used in a variety of applications, such as transportation and construction components, due to their excellent formability, work hardening ability, and weldability. However, the yield strength of 304 series stainless steel and 301 series stainless steel is only 200-350 MPa, limiting their application to structures. Therefore, in order to obtain higher yield strength in general-purpose 300 series stainless steel, a temper rolling process is typically used. However, this temper rolling process has the drawbacks of increasing costs and significantly reducing the elongation rate of the material.

[0003] Patent Document 1 discloses a manufacturing method for 300 series stainless steel that has small bending even after half etching by temper rolling cold-rolled annealed material and then performing two stress relief (SR) heat treatments. However, the method presented in Patent Document 1 relates to a manufacturing technology for controlling the etching property and bending after etching, and there is a risk that deformation-induced martensitic transformation will occur rapidly during forming because the austenitic stability parameter (ASP) value is 30 to 50, resulting in a decrease in elongation. Patent Document 2 proposes a method of performing heat treatment for a long period of time, such as 48 hours or more, at a temperature in the range of 600 to 700°C in order to control the average crystal grain size to 10 μm or less. However, the method proposed in Patent Document 2 has problems in that it is too low in productivity to be implemented on an actual production line, and manufacturing costs increase. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2016 / 043125 [Patent Document 2] Japanese Patent Application Publication No. 2020-50940 Summary of the Invention [Problem to be solved by the invention]

[0005] The present invention has been made to solve the above problems, and its object is to provide an ultrafine-grained austenitic stainless steel that simultaneously satisfies high strength, high elongation, and a high yield ratio. board and a method for producing the same. [Means for solving the problem]

[0006] The austenitic stainless steel according to the present invention has quality The alloy is characterized by the following content in terms of content: C (carbon): 0.005-0.03%, Si (silicon): 0.1-1.0%, Mn (manganese): 0.1-2.0%, Ni (nickel): 6.0-12.0%, Cr (chromium): 16.0-20.0%, N (nitrogen): 0.01-0.2%, Nb (niobium): 0.25% or less, and the remainder being Fe (iron) and other unavoidable impurities; the average crystal grain size (d) value at the center in the thickness direction is 5 μm or less; and the unrecrystallized area fraction in the band form is 10% or less.

[0007] The austenitic stainless steel of the present invention preferably has a yield strength of 700 MPa or more and 1113 MPa or less. The austenitic stainless steel of the present invention may have an elongation ratio of 20% or more and 41.2% or less. The austenitic stainless steel of the present invention has a yield ratio 0.80 It is preferable that the ratio is not less than 0.96.

[0008] The method for producing austenitic stainless steel of the present invention comprises the steps of: qualityThe method is characterized by comprising the steps of hot rolling a slab, which is composed of, in terms of content, C: 0.005-0.03%, Si: 0.1-1.0%, Mn: 0.1-2.0%, Ni: 6.0-12.0%, Cr: 16.0-20.0%, N: 0.01-0.2%, Nb: 0.002-0.25%, and the balance being Fe and unavoidable impurities, and which has an average crystal grain size (d) value at the center in the thickness direction of 5 μm or less and an unrecrystallized area fraction in the form of a band of 10% or less; cold rolling the slab at room temperature with a reduction rate of 40% or more; and cold rolling annealing the slab so that the Ω value represented by the following formula (1) is 0.8 or more. Formula (1): Ω=3.35-14.6*[C]+0.105*[Si]+0.0058*[Mn]+0.0321*[Cr]-0.222*[Ni]-2.02*[N]+0.340*[Nb]-0.00538*Md30-0.00124*Temp In formula (1), [C], [Si], [Mn], [Cr], [Ni], [N], and [Nb] are the values ​​of each element. quality Md30 refers to the value defined as 551-462([C]+[N])-9.2*[Si]-8.1*[Mn]-13.7*[Cr]-29([Ni]+[Cu])-18.5*[Mo]-68([Nb]+[V]), and Temp refers to the cold rolling annealing temperature (°C).

[0009] In the method for producing austenitic stainless steel according to the present invention, cold rolling can be performed without annealing after the hot rolling step. [Effects of the Invention]

[0010] According to one embodiment of the present invention, it is possible to provide an ultrafine-grained austenitic stainless steel that simultaneously satisfies high strength, high elongation, and high yield ratio, and a method for manufacturing the same. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a graph showing a stress-strain curve for Example 1. [Figure 2] 10 is a graph showing a stress-strain curve for Comparative Example 3. [Figure 3]1 is a photograph of the microstructure at the center in the thickness direction of Example 3 taken by an electron backscatter diffraction pattern analyzer (EBSD). [Figure 4] 1 is a photograph of the microstructure at the center in the thickness direction of Comparative Example 2 taken by an electron backscatter diffraction pattern analyzer (EBSD). DETAILED DESCRIPTION OF THE INVENTION

[0012] The austenitic stainless steel according to one embodiment of the present invention is quality The content is, in terms of content percentages, C (carbon): 0.005 to 0.03%, Si (silicon): 0.1 to 1.0%, Mn (manganese): 0.1 to 2.0%, Ni (nickel): 6.0 to 12.0%, Cr (chromium): 16.0 to 20.0%, N (nitrogen): 0.01 to 0.2%, Nb (niobium): 0.25% or less, with the remainder being Fe (iron) and other unavoidable impurities. The average crystal grain size (d) value at the center in the thickness direction is 5 μm or less, and the unrecrystallized area fraction in the form of bands is 10% or less.

[0013] Preferred embodiments of the present invention will be described below. However, the embodiments of the present invention may be modified in various different forms, and the technical concept of the present invention is not limited to the embodiments described below. Furthermore, the embodiments of the present invention are provided to more completely explain the present invention to those having average knowledge in the art. The terms used in the present invention are merely used to describe specific examples. Thus, for example, singular expressions include plural expressions unless the context clearly dictates that they are singular. Furthermore, it should be noted that the terms "comprise" or "include" used in the present invention are used to clearly indicate the presence of features, steps, functions, components, or combinations thereof described in the specification, and are not used to preliminarily exclude the presence of other features, steps, functions, components, or combinations thereof.

[0014] On the other hand, unless otherwise defined, all terms used herein should be considered to have the same meaning as commonly understood by a person of ordinary skill in the art to which the present invention belongs. Therefore, unless clearly defined herein, specific terms should not be interpreted in an overly ideal or formal sense. For example, in this specification, singular expressions include plural expressions unless the context clearly dictates otherwise. Furthermore, in this specification, when tolerances for manufacturing and materials inherent in the meanings mentioned are given, the terms "about," "substantially," and the like are used to mean a numerical value or a value close to that numerical value, and are used to prevent unscrupulous infringers from unfairly using the disclosure content in which precise and absolute numerical values ​​are mentioned to aid in the understanding of the present invention.

[0015] An austenitic stainless steel according to an example of the present invention is quality In terms of quantity, the components are: C (carbon): 0.005-0.03%, Si (silicon): 0.1-1.0%, Mn (manganese): 0.1-2.0%, Ni (nickel): 6.0-12.0%, Cr (chromium): 16.0-20.0%, N (nitrogen): 0.01-0.2%, Nb (niobium): 0.25% or less, and the remainder is Fe (iron) and other unavoidable impurities. The reasons for limiting the alloy composition will now be specifically explained.

[0016] The C (carbon) content is 0.005 to 0.03%. C is an austenite phase stabilizing element. Taking this into consideration, it is recommended that C be added in an amount of 0.005% or more. However, if the C content is excessive, there is a risk that chromium carbide will be formed during low-temperature annealing, which may reduce intergranular corrosion resistance. Taking this into consideration, the upper limit of the C content is limited to 0.03%.

[0017] The Si (silicon) content is 0.1 to 1.0%. Silicon is added as a deoxidizer during steelmaking. It forms silicon oxide in the passivation film during bright annealing, improving the corrosion resistance of steel. Therefore, silicon content should be 0.1% or more. However, excessive silicon content can reduce the ductility of steel. Therefore, the upper limit of silicon content is set at 1.0%.

[0018] The Mn (manganese) content is 0.1 to 2.0%. Mn is an austenite phase stabilizing element. Taking this into consideration, it is recommended that 0.1% or more of Mn be added. However, if the Mn content is excessive, there is a risk of a problem of reduced corrosion resistance. Taking this into consideration, the upper limit of the Mn content is limited to 2.0%.

[0019] The Ni (nickel) content is 6.0 to 12.0%. Ni is an austenite phase stabilizing element and has the effect of softening steel. Taking this into consideration, it is preferable to add Ni in an amount of 6.0% or more. However, if the Ni content is excessive, there is a risk of a problem of increased costs. Taking this into consideration, the upper limit of the Ni content is limited to 12.0%.

[0020] The Cr (chromium) content is 16.0 to 20.0%. Cr is a key element for improving the corrosion resistance of stainless steel. Taking this into consideration, it is recommended that Cr be added in an amount of 16.0% or more. However, excessive Cr content can cause problems, such as the steel becoming hard and suppressing the deformation-induced martensitic transformation during cold rolling. Taking this into consideration, the upper limit of the Cr content is set to 20.0%.

[0021] The N (nitrogen) content is 0.01 to 0.2%. N is an austenite phase stabilizing element and improves the strength of steel. Taking this into consideration, it is recommended that N be added in an amount of 0.01% or more. However, if the N content is excessive, the steel may become hard and the hot workability may deteriorate. Taking this into consideration, the upper limit of the N content is set to 0.2%.

[0022] The Nb (niobium) content is 0.25% or less. When added, Nb forms Nb-based z-phase precipitates, which has the effect of suppressing grain growth. However, if the Nb content is excessive, there is a risk of a problem of increased costs. Taking this into consideration, the upper limit of the Nb content is set to 0.25%.

[0023] The remaining component is iron (Fe). However, in the normal manufacturing process, unintentional impurities may be inevitably mixed in from the raw materials or the surrounding environment, and this cannot be eliminated. Since these impurities are known to anyone skilled in the normal manufacturing process, not all of the contents of these impurities will be specifically mentioned in this specification.

[0024] In an austenitic stainless steel according to one example of the present invention, by controlling the alloy component composition ratio, the average crystal grain size (d) value at the center in the thickness direction can be made 5 μm or less, and the band-shaped unrecrystallized area fraction can be made 10% or less.

[0025] Generally, to achieve an ultrafine-grained microstructure, TRIP transformation, which transforms the austenite phase to martensite, is used. In an austenitic stainless steel according to an embodiment of the present invention, the average grain size (d) at the center of the thickness direction is controlled to 5 μm or less through TRIP transformation. However, if the average grain size (d) at the center of the thickness direction exceeds 5 μm, the yield strength decreases according to the Hall-Petch equation.

[0026] The portion that does not transform into martensite during cold rolling and remains as it is appears as unrecrystallized. If a large amount of unrecrystallized material is present, the problem of reduced ductility occurs. Therefore, it is preferable that the unrecrystallized area fraction be 10% or less.

[0027] The austenitic stainless steel according to one example of the present invention preferably has a yield strength of 700 MPa or more and 1113 MPa or less. The austenitic stainless steel according to one example of the present invention may have an elongation ratio of 20% or more and 41.2% or less. The austenitic stainless steel according to one example of the present invention has a yield ratio of 0.80 The yield ratio is preferably not less than 0.96. The yield ratio is a value obtained by dividing the yield strength by the tensile strength.

[0028] A method for producing austenitic stainless steel according to one embodiment of the present invention includes the steps of: quality The method is characterized by comprising the steps of hot rolling a slab, which is composed of, in terms of content, C: 0.005-0.03%, Si: 0.1-1.0%, Mn: 0.1-2.0%, Ni: 6.0-12.0%, Cr: 16.0-20.0%, N: 0.01-0.2%, Nb: 0.002-0.25%, and the balance being Fe and unavoidable impurities, and which has an average crystal grain size (d) value at the center in the thickness direction of 5 μm or less and an unrecrystallized area fraction in the form of a band of 10% or less; cold rolling the slab at room temperature with a reduction rate of 40% or more; and cold rolling annealing the slab so that the Ω value represented by the following formula (1) is 0.8 or more. Here, equation (1) is Ω = 3.35 - 14.6 * [C] + 0.105 * [Si] + 0.0058 * [Mn] + 0.0321 * [Cr] - 0.222 * [Ni] - 2.02 * [N] + 0.340 * [Nb] - 0.00538 * Md30 - 0.00124 * Temp, In formula (1), [C], [Si], [Mn], [Cr], [Ni], [N], and [Nb] are the respective elements. qualityMd30 refers to the value defined as 551-462([C]+[N])-9.2*[Si]-8.1*[Mn]-13.7*[Cr]-29([Ni]+[Cu])-18.5*[Mo]-68([Nb]+[V]), and Temp refers to the cold rolling annealing temperature (°C).

[0029] The reasons for limiting the range of each alloying element are as described above, and will be explained in more detail in the following manufacturing steps. The slab is produced as a hot-rolled material through a hot rolling process, and then the hot-rolled material is cold-rolled at room temperature to produce a cold-rolled material. If the reduction ratio during cold rolling is less than 40%, the amount of TRIP transformation is too small, resulting in a low martensite fraction in the cold-rolled material and a high fraction of retained austenite. As the amount of strain-induced martensite decreases, the proportion of the austenite phase that reverses after low-temperature annealing decreases, and the fraction of retained austenite that does not transform to martensite increases, making it difficult to obtain ultrafine crystal grains.

[0030] Next, the produced cold-rolled material is subjected to cold-roll annealing, which is preferably carried out in the range of 700 to 850°C so that the Ω value represented by the formula (1) satisfies 0.8 or more. If the cold rolling annealing temperature is less than 700°C, recrystallization is insufficient, resulting in a low elongation ratio. On the other hand, if the cold rolling annealing temperature exceeds 850°C, the grains become coarse, making it difficult to form ultrafine grains of 5 μm or less. In addition, in the method for manufacturing austenitic stainless steel according to an embodiment of the present invention, cold rolling may be performed without annealing after hot rolling. When a separate annealing process is not performed after hot rolling, productivity can be increased and manufacturing costs can be reduced. {Example}

[0031] The present invention will be described in more detail below through preferred embodiments. Slabs having the chemical compositions shown in Table 1 below were hot rolled and then annealed at 1000 to 1150°C, or cold rolled at room temperature with a total thickness reduction of 40% or more without annealing. Then, the slabs were annealed at a cold rolling annealing temperature (Temp. in Table 1 below) to produce cold rolled and annealed materials.

[0032] TIFF0007787209000001.tif235157

[0033] The values ​​of formula (1) for the cold-rolled and annealed steel produced above are shown in Table 2. In Table 1 below, the Ω value in formula (1) refers to a value derived from parameters defined by formula (1): Ω = 3.35 - 14.6 * [C] + 0.105 * [Si] + 0.0058 * [Mn] + 0.0321 * [Cr] - 0.222 * [Ni] - 2.02 * [N] + 0.340 * [Nb] - 0.00538 * Md30 - 0.00124 * Temp. In the formula (1), [C], [Si], [Mn], [Cr], [Ni], [N], and [Nb] represent the respective elements. quality Md30 refers to the value defined as 551-462([C]+[N])-9.2*[Si]-8.1*[Mn]-13.7*[Cr]-29([Ni]+[Cu])-18.5*[Mo]-68([Nb]+[V]), and Temp refers to the cold rolling annealing temperature (°C).

[0034] Test pieces with thicknesses of 0.1 to 3.0 mm were prepared from the cold-rolled and annealed steel sheets prepared as described above. The average grain size (d) at the center of the thickness direction, the unrecrystallized area fraction, the yield strength, the tensile strength, the elongation, and the yield ratio were measured for the test pieces, and the results are shown in Table 2 below. The average grain size (d) and unrecrystallized area fraction were measured by analyzing the orientation at the center of the thickness direction using an electron backscatter diffraction (EBSD) analyzer (model name: e-Flash FS). The yield strength, tensile strength and elongation were measured using a Universal Test Machine (UTM). The yield ratio is the value obtained by dividing the yield strength by the tensile strength.

[0035] [Table 2]

[0036] Referring to Tables 1 and 2, Examples 1 to 9 all satisfied the Ω value of formula (1) of 0.8 or more, the average crystal grain size (d) value of 5 μm or less, and the band-shaped unrecrystallized area fraction of 10% or less. As a result, Examples 1 to 9 have a yield strength of 700 MPa or more and 1113 MPa or less, an elongation ratio of 20% or more and 41.2% or less, and a yield ratio 0.80 or more and 0.96 or less. That is, Examples 1 to 9 simultaneously satisfied the requirements of high strength, high elongation, and high yield ratio.

[0037] On the other hand, the unrecrystallized area fraction exceeded 10% in Comparative Examples 1 and 2. As a result, the elongation ratios in Comparative Examples 1 and 2 were less than 20%, which was extremely poor. In Comparative Examples 3 and 8, the average grain size (d) value was low, and the yield strength was 700 MPa or more and 1113 MPa or less. However, in Comparative Examples 3 and 8, the tensile strength was relatively high compared to the yield strength. Therefore, in Comparative Examples 3 and 8, the yield ratio 0.80 The value did not meet the criteria of 0.96 or less. In Comparative Examples 4 to 7 and 9 to 39, the Ω value of formula (1) did not satisfy 0.8 or more. As a result, in Comparative Examples 4 to 7 and 9 to 39, the yield strength was 700 MPa or more and 1113 MPa or less, and the yield ratio was 0.80 The value did not meet the criteria of 0.96 or less. The cold rolling annealing temperature was high in Comparative Examples 27 to 39. As a result, Comparative Examples 27 to 39 did not satisfy the requirement that the average crystal grain size (d) be 5 μm or less.

[0038] Figures 1 and 2 are graphs showing stress-strain curves of examples and comparative examples. Figure 1 is the graph of Example 1, and Figure 2 is the graph of Comparative Example 3. Comparing Figures 1 and 2, it can be seen that the austenitic stainless steel according to one embodiment of the present invention has a relatively small rate of stress change depending on the degree of deformation, and therefore can simultaneously satisfy high strength, high elongation, and a high yield ratio.

[0039] Figures 3 and 4 are photographs of the microstructures at the center of the thickness direction for the example and comparative examples taken by an electron backscatter diffraction pattern analyzer (EBSD). Figure 3 is a photograph for Example 3, and Figure 4 is a photograph for Comparative Example 2. Comparing Figures 3 and 4, it can be seen that no band-shaped unrecrystallization occurred in the austenitic stainless steel according to one example of the present invention.

[0040] Although exemplary embodiments of the present invention have been described above, the present invention is not limited thereto, and it will be understood by those skilled in the art that various changes and modifications can be made without departing from the concept and scope of the claims set forth below. [Industrial Applicability]

[0041] According to one embodiment of the present invention, it is possible to provide an ultrafine-grained austenitic stainless steel that simultaneously satisfies high strength, high elongation, and a high yield ratio, and a method for producing the same.

Claims

1. In mass%, the composition is: C (carbon): 0.005 to 0.03%, Si (silicon): 0.1 to 1.0%, Mn (manganese): 0.1 to 2.0%, Ni (nickel): 6.0 to 12.0%, Cr (chromium): 16.0 to 20.0%, N (nitrogen): 0.01 to 0.2%, Nb (niobium): 0.25% or less, with the remainder being Fe (iron) and other unavoidable impurities; The average crystal grain size (d) value at the center in the thickness direction is 5 μm or less, and the band-shaped unrecrystallized area fraction is 10% or less; An austenitic stainless steel sheet having a yield ratio of 0.80 or more and 0.96 or less.

2. 2. The austenitic stainless steel sheet according to claim 1, wherein the yield strength is 700 MPa or more and 1113 MPa or less.

3. 2. The austenitic stainless steel sheet according to claim 1, wherein the elongation is 20% or more and 41.2% or less.

4. hot rolling a slab consisting of, in mass%, C: 0.005 to 0.03%, Si: 0.1 to 1.0%, Mn: 0.1 to 2.0%, Ni: 6.0 to 12.0%, Cr: 16.0 to 20.0%, N: 0.01 to 0.2%, Nb: 0.25% or less, with the balance being Fe and unavoidable impurities; cold rolling at room temperature with a reduction of 40% or more; 2. The method for producing an austenitic stainless steel sheet according to claim 1, further comprising a step of cold rolling and annealing such that the Ω value expressed by the following formula (1) satisfies 0.8 or more. Formula (1): Ω=3.35-14.6*[C]+0.105*[Si]+0.0058*[Mn]+0.0321*[Cr]-0.222*[Ni]-2.02*[N]+0.340*[Nb]-0.00538*Md30-0.00124*Temp (In formula (1), [C], [Si], [Mn], [Cr], [Ni], [N], and [Nb] mean the mass% of each element, Md30 means a value defined as 551-462([C]+[N])-9.2*[Si]-8.1*[Mn]-13.7*[Cr]-29([Ni]+[Cu])-18.5*[Mo]-68([Nb]+[V]), and Temp means the cold rolling annealing temperature (°C).)

5. 5. The method for manufacturing an austenitic stainless steel sheet according to claim 4, wherein the hot rolling step is followed by cold rolling without annealing.

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