Ferritic stainless steel with improved formability and method for producing the same

Abstract

The present invention aims to provide a ferritic stainless steel with improved formability by controlling the average crystal grain size through alloying components and manufacturing processes, and a manufacturing method thereof. [Solution] The ferritic stainless steel with improved formability contains, by weight, C: 0.0005-0.02%, N: 0.005-0.02%, Si: 0.01-1.0%, Mn: 0.01-1.0%, P: 0.001-0.05%, Cr: 10.0-30.0%, Nb: 0.05-0.5%, Ti: 0.05-0.5%, with the remainder being Fe and unavoidable impurities, and is characterized by having an Erichsen height of 6-50 mm, an average grain size of 5-20 μm, a thickness of 200 μm or less, and a (thickness) / (average grain size) value of 5-10.

Description

[Technical Field]

[0001] The present invention relates to a ferritic stainless steel having improved formability and a method for manufacturing the same, and more particularly to a ferritic stainless steel having improved formability by controlling the average crystal grain size through alloying components and manufacturing processes, and a method for manufacturing the same. [Background technology]

[0002] In recent years, the demand for electric vehicles and hydrogen fuel cell vehicles has increased with the global trend toward carbon neutrality. In particular, hydrogen fuel cell vehicles are becoming increasingly important as eco-friendly cars of the future. Therefore, the development of hydrogen fuel cell separation membranes has become more active.

[0003] Hydrogen fuel cell separation membranes can be made of metal materials, and ferritic stainless steels in particular have been attracting attention due to their advantages in fuel cell performance and economic efficiency.

[0004] The ferritic stainless steel used as the fuel cell separation membrane is an extremely thin material with a thickness of 200 μm or less, and the formability of the separation membrane components is required when manufacturing fuel cells.

[0005] Therefore, there is a growing demand for improved formability of ultra-thin ferritic stainless steels that can be used for hydrogen fuel cell separation membranes. Summary of the Invention [Problem to be solved by the invention]

[0006] The object of the disclosed invention to solve the above-mentioned problems is to provide a ferritic stainless steel with improved formability by controlling the average crystal grain size through the alloying elements and manufacturing process, and a manufacturing method thereof. [Means for solving the problem]

[0007] The ferritic stainless steel with improved formability contains, by weight, C: 0.0005-0.02%, N: 0.005-0.02%, Si: 0.01-1.0%, Mn: 0.01-1.0%, P: 0.001-0.05%, Cr: 10.0-30.0%, Nb: 0.05-0.5%, Ti: 0.05-0.5%, with the remainder being Fe and unavoidable impurities, and is characterized by having an Erichsen height of 6-50 mm.

[0008] The ferritic stainless steel with improved formability may have an average crystal grain size of 5 to 20 μm.

[0009] The ferritic stainless steel with improved formability may have a thickness of 200 μm or less.

[0010] Ferritic stainless steel with improved formability may have a thickness / average grain size ratio of 5-10.

[0011] A method for producing a ferritic stainless steel with improved formability includes the steps of producing a slab containing, by weight, C: 0.0005-0.02%, N: 0.005-0.02%, Si: 0.01-1.0%, Mn: 0.01-1.0%, P: 0.001-0.05%, Cr: 10.0-30.0%, Nb: 0.05-0.5%, Ti: 0.05-0.5%, and the remainder being Fe and unavoidable impurities; reheating the slab to 1100-1300°C, followed by hot rolling and hot annealing to produce a hot-rolled material; and cold rolling and cold annealing the hot-rolled and annealed material two to five times to produce a steel sheet, wherein the cold annealing is performed at 850-1000°C.

[0012] The hot rolling annealing can be carried out at 900 to 1100°C.

[0013] In the step of producing the steel sheet, the final cold rolling reduction may be 50% or more. [Effects of the Invention]

[0014] According to the present invention, it is possible to provide a ferritic stainless steel having improved formability by controlling the average crystal grain size, and a method for producing the same. [Brief explanation of the drawings]

[0015] [Figure 1] 1 is an image of the microstructure of Comparative Example 7 photographed by EBSD (Electron Backscatter Diffraction). [Figure 2] 1 is an image of the microstructure of Example 3 taken by EBSD (Electron Backscatter Diffraction). [Figure 3] 1 is a graph showing the correlation between the (thickness) / (average crystal grain size) value and the Erichsen height. DETAILED DESCRIPTION OF THE INVENTION

[0016] The ferritic stainless steel with improved formability may contain, by weight, C: 0.0005 to 0.02%, N: 0.005 to 0.02%, Si: 0.01 to 1.0%, Mn: 0.01 to 1.0%, P: 0.001 to 0.05%, Cr: 10.0 to 30.0%, Nb: 0.05 to 0.5%, Ti: 0.05 to 0.5%, with the remainder being Fe and unavoidable impurities, and may have an Erichsen height of 6 to 50 mm.

[0017] Hereinafter, embodiments of the disclosed invention will be described in detail with reference to the accompanying drawings. The following embodiments are presented to fully convey the concept of the disclosed invention to those skilled in the art to which the disclosed invention pertains. The disclosed invention is not limited to the embodiments presented herein and may be embodied in other forms. In the drawings, parts not relevant to the description may be omitted to clarify the disclosed invention, and the sizes of components may be slightly exaggerated to facilitate understanding.

[0018] Throughout the specification, when a part is said to "comprise" certain elements, this means that it may further include other elements, rather than excluding other elements, unless specifically stated to the contrary.

[0019] The singular expression includes the plural expression unless the context clearly indicates otherwise.

[0020] The reasons for limiting the alloying component contents in the present invention will be explained below.

[0021] Ferritic stainless steel with improved formability contains, by weight, C: 0.0005-0.02%, N: 0.005-0.02%, Si: 0.01-1.0%, Mn: 0.01-1.0%, P: 0.001-0.05%, Cr: 10.0-30.0%, Nb: 0.05-0.5%, Ti: 0.05-0.5%, with the remainder being Fe and unavoidable impurities.

[0022] The C (carbon) content may be 0.0005 to 0.02%.

[0023] If the C content is less than 0.0005%, the refining cost for producing a high-purity product increases. Therefore, C can be added in an amount of 0.0005% or more. However, if the C content is excessive, the ductility may decrease, resulting in a decrease in formability. Furthermore, if the C content is excessive, the corrosion resistance may be deteriorated. In consideration of this, the upper limit of the C content can be limited to 0.02%. Preferably, the C content can be set to 0.0092 to 0.0122%.

[0024] The N (nitrogen) content may be 0.005 to 0.02%.

[0025] If the N content is less than 0.005%, TiN crystallization may decrease, resulting in a low equiaxed crystal fraction of the slab. In consideration of this, N can be added in an amount of 0.005% or more. However, if the N content is excessive, the strength may increase excessively, potentially reducing formability. Furthermore, if the N content is excessive, the corrosion resistance may be reduced. In consideration of this, the upper limit of the N content can be limited to 0.02%. Preferably, the N content can be set to 0.0083 to 0.0114%.

[0026] The Si (silicon) content may be 0.01 to 1.0%.

[0027] If the Si content is less than 0.01%, there is a problem of increased refining costs. In consideration of this, Si can be added in an amount of 0.01% or more. However, if the Si content is excessive, impurities may increase and formability may decrease. In consideration of this, the upper limit of the Si content can be limited to 1.0%. Preferably, the Si content can be set to 0.2 to 0.3%.

[0028] The Mn (manganese) content may be 0.01 to 1.0%.

[0029] If the Mn content is less than 0.01%, there is a problem of increased refining costs. In consideration of this, Mn can be added in an amount of 0.01% or more. However, if the Mn content is excessive, impurities may increase and formability may decrease. In consideration of this, the upper limit of the Mn content can be limited to 1.0%. Preferably, the Si content can be set to 0.4 to 0.5%.

[0030] The P (phosphorus) content may be 0.001 to 0.05%.

[0031] If the P content is less than 0.001%, there is a problem of increased refining costs. In consideration of this, P can be added in an amount of 0.001% or more. However, if the P content is excessive, impurities increase, which may reduce formability and hot workability. In consideration of this, the upper limit of the P content can be limited to 0.05%. Preferably, the Si content can be set to 0.02 to 0.03%.

[0032] The Cr (chromium) content may be 10.0 to 30.0%.

[0033] Cr is an effective element for ensuring the corrosion resistance of steel. Taking this into consideration, Cr can be added in an amount of 10% or more. However, excessive Cr content can cause the formation of a large amount of delta ferrite in the material, which can reduce hot workability and formability. Taking this into consideration, the upper limit of Cr content can be limited to 30%.

[0034] The Nb (niobium) content may be 0.05 to 0.5%.

[0035] Nb is an element that forms fine Nb precipitates, inhibits grain growth, and is effective in preventing the grain size from increasing during annealing of ultra-thin steel sheets. Taking this into consideration, Nb can be added in an amount of 0.05% or more. However, if the Nb content is excessive, the manufacturing cost increases and excessive precipitates may be formed, potentially reducing formability. Taking this into consideration, the upper limit of the Nb content can be limited to 0.5%. Preferably, the Nb content can be set to 0.12 to 0.32%.

[0036] The content of Ti (titanium) may be 0.05 to 0.5%.

[0037] Ti is an element that is effective in forming recrystallization during hot rolling. Taking this into consideration, Ti can be added in an amount of 0.05% or more. However, if the Ti content is excessive, a large number of steelmaking inclusions may be generated. Taking this into consideration, the upper limit of the Ti content can be limited to 0.5%. Preferably, the Ti content can be set to 0.10 to 0.21%.

[0038] The remaining component of the present invention is iron (Fe). However, in a normal manufacturing process, unintentional impurities may inevitably be mixed in from raw materials or the surrounding environment, and this cannot be excluded. Since these impurities are known to anyone skilled in normal manufacturing processes, not all of the contents of these impurities will be specifically mentioned in this specification.

[0039] The disclosed invention aims to improve formability by controlling the microstructure using the alloying elements described above and the manufacturing method described below. Therefore, the ferritic stainless steel with improved formability can have an Erichsen height of 6 to 50 mm.

[0040] The Erichsen height can be measured by the Erichsen test, which is a widely used test for evaluating the workability of stainless steels. The test involves applying pressure to a test piece until a crack appears, and measuring the height of deformation.

[0041] If the Erichsen height is less than 6 mm, it may be difficult to ensure sufficient formability, but if the Erichsen height exceeds 50 mm, it may be difficult to control the shape, and production efficiency may decrease.

[0042] The ferritic stainless steel of the present invention with improved formability can have an Erichsen height of 6 mm to 40 mm, more specifically, 6 mm to 30 mm. Within this range, the ferritic stainless steel of the present invention can achieve both formability and production efficiency. In this case, it can realize properties that are advantageous for application to separator parts such as fuel cells.

[0043] The ferritic stainless steel with improved formability may have an Erichsen height of 6 mm to 20 mm. Within this range, the ferritic stainless steel of the present invention can achieve both formability and production efficiency while simultaneously maintaining or improving the physical properties required for separator components in fuel cells and the like.

[0044] The ferritic stainless steel with improved formability may have an average crystal grain size of 5 to 20 μm, preferably 5 to 15 μm, and more preferably 10 to 15 μm.

[0045] If the average grain size is too large, the slip systems present in the thickness direction of the ultra-thin material will decrease, which may result in poor formability. However, if the average grain size is too small, some unrecrystallized regions may be included, which may result in poor formability. Therefore, it is necessary to optimize and control the average grain size.

[0046] On the other hand, in the present invention, the term "average" means the average value of values ​​measured at any five points.

[0047] The ferritic stainless steel with improved formability may have a thickness of 200 μm or less, preferably 5 to 200 μm, and more preferably 5 to 150 μm. Within this range, the ferritic stainless steel of the present invention may exhibit advantageous properties suitable for application to separator components in fuel cells and the like.

[0048] The ferritic stainless steel with improved formability may have a thickness of 10 μm to 100 μm. Within this range, the ferritic stainless steel of the present invention can achieve both formability and production efficiency while simultaneously maintaining or improving the physical properties required for separator components such as fuel cells.

[0049] The present invention may be applied to an extremely thin material of 200 μm or less so that it can be used as a hydrogen fuel cell separation membrane, but is not limited to this and can be applied to various fields in which the disclosed invention can be used.

[0050] The ferritic stainless steel with improved formability may have a (thickness) / (average crystal grain size) value of 5-10.

[0051] Due to the nature of the present invention being an extremely thin material, it is necessary to control the ratio between the thickness and the average grain size.

[0052] If the value of (thickness) / (average crystal grain size) is less than 5, there will be fewer than five crystal grains in the thickness direction of the stainless steel, resulting in too few slip systems. Therefore, if the value of (thickness) / (average crystal grain size) is less than 5, the formability of the stainless steel may be reduced. However, if the value of (thickness) / (average crystal grain size) is more than 20, the average crystal grain size of the stainless steel will be too fine, which may result in reduced formability due to non-recrystallization.

[0053] The ferritic stainless steel of the present invention with improved formability may have a (thickness) / (average crystal grain size) value of 5.4 to 10. Within the above range, the ferritic stainless steel of the present invention can suppress deterioration in formability.

[0054] In another example, the ferritic stainless steel of the present invention with improved formability may have a (thickness) / (average crystal grain size) value of 5.4 to 9.3. Within the above range, the ferritic stainless steel of the present invention can suppress a decrease in formability.

[0055] Next, a method for producing the ferritic stainless steel having improved formability according to the present invention will be described.

[0056] A method for producing ferritic stainless steel with improved formability includes the steps of producing a slab containing, by weight, C: 0.0005-0.02%, N: 0.005-0.02%, Si: 0.01-1.0%, Mn: 0.01-1.0%, P: 0.001-0.05%, Cr: 10.0-30.0%, Nb: 0.05-0.5%, Ti: 0.05-0.5%, and the remainder being Fe and unavoidable impurities; reheating the slab to 1100-1300°C, followed by hot rolling and hot annealing to produce a hot-rolled material; and cold rolling the hot-rolled and annealed material two to five times to produce a steel sheet.

[0057] The reasons for limiting the range of the components of each alloy composition are as described above, and each manufacturing step will be described in more detail below.

[0058] After a slab satisfying the above alloy composition is produced, it can undergo a series of steps including reheating, hot rolling, hot roll annealing, cold rolling, and cold roll annealing.

[0059] First, the slab is reheated to 1100 to 1300° C., and then hot-rolled and hot-rolled annealed to produce a hot-rolled material.

[0060] If the heating temperature is low, it may be difficult to redissolve the coarse precipitates formed during slab production. In consideration of this, the heating temperature can be 1100°C or higher. However, if the heating temperature is too high, the internal crystal grains may become too coarse. In consideration of this, the upper limit of the heating temperature can be limited to 1300°C.

[0061] The hot rolling annealing can be carried out at 900 to 1100°C.

[0062] If the hot rolling annealing temperature is too low, it is difficult to sufficiently remove the stress caused by rolling, but if the hot rolling annealing temperature is too high, the crystal grains may become too coarse.

[0063] The hot rolled and annealed hot rolled material can be cold rolled and cold roll annealed two to five times.

[0064] The initial cold rolling and cold rolling annealing cause deformation of the hot rolled material, and subsequently precipitate formation can be smoothly induced during the process.

[0065] Through the second and subsequent cold rolling and cold rolling annealing, a large number of precipitates are precipitated, which induces recrystallization and ensures fine crystal grains.

[0066] On the other hand, since the manufacturing cost may increase as the number of cold rolling and cold roll annealing steps increases, the number of cold rolling and cold roll annealing steps may be limited to five or less.

[0067] The cold rolling annealing can be carried out at a temperature of 850 to 1000°C.

[0068] If the cold rolling annealing temperature is too low, the stress formed during rolling may not be sufficiently removed, resulting in a decrease in workability, whereas if the cold rolling annealing temperature is too high, the formability may decrease due to grain coarsening.

[0069] In the step of producing the steel sheet, a final cold rolling reduction may be 50% or more.

[0070] If the final cold rolling reduction is less than 50%, the amount of deformation is insufficient, making it difficult to realize fine crystal grains.

[0071] The present invention will be described in more detail below through examples. However, the description of these examples is for illustrating the implementation of the present invention, and the present invention is not limited by the description of these examples. The scope of the present invention is determined by the matters described in the claims and matters that can be reasonably inferred from them. [Example]

[0072] Slabs were produced in a vacuum induction melting furnace for the various alloy composition ranges shown in Table 1 below. The produced slabs were reheated in a heating furnace at 1250°C, hot rolled at 1100°C, and then hot-rolled and annealed to produce hot-rolled materials. The hot-rolled materials were cold-rolled and cold-rolled and annealed to produce test specimens measuring 100 mm wide x 100 mm long. Cold rolling and cold-roll annealing were performed three times. The thicknesses of the test specimens produced in this manner are listed in Table 2, and the final rolling reduction of each test specimen was 60%.

[0073] [Table 1]

[0074] The thickness, average grain size, (thickness) / (average grain size) ratio, and Erichsen height are shown in Table 2. The average grain size was measured by photographing the entire thickness of a cross section perpendicular to the rolling direction of the test specimen using an EBSD (Electron Backscatter Diffraction) analyzer.

[0075] The Erichsen height was measured by an Erichsen test. The Erichsen test was performed as follows: A test specimen was inserted between an upper die and a lower die, and the outer periphery of the test specimen was fixed with a force of 20 kN. Then, a spherical punch with a diameter of 20 mm was used to deform the test specimen at a speed of 5 to 20 mm / min. The punch was then inserted until the test specimen broke, and the deformation height of the test specimen at the time of breakage was measured. A higher Erichsen height indicates better formability.

[0076] [Table 2]

[0077] As shown in Table 2, Examples 1 to 8 satisfied the alloy composition and manufacturing method proposed in the disclosed invention. Therefore, the Erichsen height was 6 to 50 mm, the average grain size was 5 to 20 μm, the thickness was 200 μm or less, and the (thickness) / (average grain size) value was 5 to 10. In other words, Examples 1 to 8 can be considered to have excellent formability. However, Comparative Examples 1 to 10, although satisfying the alloy composition, did not satisfy the (thickness) / (average grain size) value of 5 to 10. Therefore, the Erichsen height did not satisfy the 6 to 50 mm requirement. In other words, Comparative Examples 1 to 10 had inferior formability.

[0078] Comparative Examples 11 to 15 did not satisfy the alloy composition and manufacturing method proposed by the present invention. Therefore, the value of (thickness) / (average crystal grain size) did not satisfy 5 to 10, and the Erichsen height did not satisfy 6 to 50 mm. In other words, Comparative Examples 11 to 15 had poor formability.

[0079] FIG. 1 is an image of the microstructure of Comparative Example 7 taken by EBSD (Electron Backscatter Diffraction: Backscattered Electron Diffraction Pattern Analyzer), and FIG. 2 is an image of the microstructure of Example 3 taken by EBSD (Electron Backscatter Diffraction: Backscattered Electron Diffraction Pattern Analyzer).

[0080] As shown in FIGS. 1 and 2, it can be seen that the ferritic stainless steel according to the disclosed invention realizes excellent formability by controlling the grain size to be small.

[0081] FIG. 3 is a graph showing the correlation between the (thickness) / (average crystal grain size) value and the Erichsen height.

[0082] Referring to FIG. 3, it can be seen that the Erichsen height can be adjusted to 6 to 50 mm by controlling the value of (thickness) / (average crystal grain size) to 5 to 10.

[0083] According to the disclosed invention, as described above, it is possible to provide a ferritic stainless steel having improved formability by controlling the average crystal grain size, and a method for producing the same. [Industrial Applicability]

[0084] INDUSTRIAL APPLICABILITY The present invention provides a ferritic stainless steel having improved formability by controlling the average crystal grain size, and a method for producing the same, and is therefore found to be industrially applicable.

Claims

1. The alloy contains, by weight, C: 0.0005 to 0.02%, N: 0.005 to 0.02%, Si: 0.01 to 1.0%, Mn: 0.01 to 1.0%, P: 0.001 to 0.05%, Cr: 10.0 to 30.0%, Nb: 0.05 to 0.5%, Ti: 0.05 to 0.5%, and the remainder being Fe and unavoidable impurities; A ferritic stainless steel having improved formability, characterized in that the Erichsen height is 6 to 50 mm.

2. 2. The ferritic stainless steel with improved formability according to claim 1, wherein the average crystal grain size is 5 to 20 μm.

3. 2. The ferritic stainless steel with improved formability according to claim 1, characterized in that the thickness is 200 μm or less.

4. 2. The ferritic stainless steel with improved formability according to claim 1, wherein the value of (thickness) / (average crystal grain size) is 5 to 10.

5. producing a slab containing, in weight percent, C: 0.0005-0.02%, N: 0.005-0.02%, Si: 0.01-1.0%, Mn: 0.01-1.0%, P: 0.001-0.05%, Cr: 10.0-30.0%, Nb: 0.05-0.5%, Ti: 0.05-0.5%, with the remainder being Fe and unavoidable impurities; reheating the slab to 1100 to 1300°C, followed by hot rolling and hot rolling annealing to produce a hot rolled material; and cold rolling and cold annealing the hot-rolled material two to five times to manufacture a steel sheet; The method for producing ferritic stainless steel with improved formability is characterized in that the cold rolling annealing is carried out at 850 to 1000°C.

6. 6. The method for producing a ferritic stainless steel with improved formability according to claim 5, wherein the hot rolling annealing is carried out at a temperature of 900 to 1100°C.

7. In the step of manufacturing the steel plate, 6. The method for producing a ferritic stainless steel having improved formability according to claim 5, wherein the final cold rolling reduction is 50% or more.

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