Ferritic stainless steel having improved formability and method for producing same

Abstract

The disclosed specification relates to a ferritic stainless steel having improved formability by controlling the average crystal grain diameter through alloy components and a manufacturing process, and a method for manufacturing the ferritic stainless steel. The ferritic stainless steel having improved formability according to one embodiment may include, in wt%, 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 balance Fe and unavoidable impurities, and may have an Erichsen height of 6 to 50 mm.

Description

Technical Field

[0001] The present invention relates to a ferritic stainless steel having improved formability and a method for manufacturing the same. Background Art

[0002] In recent years, with the global trend of carbon neutrality, the demand for electric vehicles and hydrogen fuel cell vehicles is increasing. In particular, hydrogen fuel cell vehicles are becoming increasingly important as future environmentally friendly vehicles. Therefore, the development of hydrogen fuel cell diaphragms is also more actively carried out.

[0003] Metal materials can be used for hydrogen fuel cell diaphragms. In particular, ferritic stainless steels, which are advantageous in terms of fuel cell performance and economy, have received more attention.

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

[0005] Therefore, there is an increasing demand for improving the formability of ultra-thin ferritic stainless steels that can be used as hydrogen fuel cell diaphragms. Summary of the Invention

[0006] (I) Technical Problems to be Solved

[0007] The object of the disclosed invention for solving the above problems is to provide a ferritic stainless steel and a method for manufacturing the same, which control the average grain diameter through alloying components and manufacturing processes to improve formability.

[0008] (II) Technical Solutions

[0009] The ferritic stainless steel having improved formability according to an embodiment may contain, 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%, the balance being Fe and unavoidable impurities, and the Erichsen height may be 6 - 50 mm.

[0010] The average grain diameter of the ferritic stainless steel having improved formability according to an embodiment may be 5 - 20 μm.

[0011] The thickness of the ferritic stainless steel having improved formability according to an embodiment may be 200 μm or less.

[0012] The thickness / average grain diameter value of the ferritic stainless steel having improved formability according to an embodiment may be 5 to 10.

[0013] A method for manufacturing a ferritic stainless steel with improved formability according to an embodiment may include the following steps: manufacturing a slab which, by weight %, contains: 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%, the balance being Fe and unavoidable impurities; reheating the slab to 1100 - 1300 °C and then performing hot rolling and hot rolling annealing to manufacture a hot-rolled material; performing cold rolling and cold rolling annealing 2 to 5 times on the hot-rolled material subjected to the hot rolling annealing to manufacture a steel sheet, wherein the cold rolling annealing may be performed at 850 - 1000 °C.

[0014] The hot rolling annealing may be performed at 900 - 1100 °C.

[0015] In the step of manufacturing the steel sheet, the final cold rolling reduction ratio may be 50% or more.

[0016] (III) Beneficial effects

[0017] According to an embodiment of the disclosed invention, a ferritic stainless steel and a method for manufacturing the same that improve formability by controlling the average grain diameter may be provided. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 A diagram for photographing the microstructure of Comparative Example 7 using an Electron BackScatter Diffraction (EBSD) pattern analyzer.

[0019] Figure 2 A diagram for photographing the microstructure of Example 3 using an Electron BackScatter Diffraction (EBSD) pattern analyzer.

[0020] Figure 3 A diagram showing the correlation between the (thickness) / (average grain diameter) value and the Erichsen height. BEST MODE

[0021] A ferritic stainless steel with improved formability according to an embodiment may contain, 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%, the balance being Fe and unavoidable impurities, and the Erichsen height may be 6 - 50 mm. Detailed Embodiments

[0022] Hereinafter, embodiments of the disclosed invention will be described in detail with reference to the accompanying drawings. The following embodiments are presented in order to fully convey the idea 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 in this specification, but can also be embodied in other forms. To make the disclosed invention clear, illustrations of parts irrelevant to the description may be omitted in the drawings, and the dimensions of the components may be slightly exaggerated to aid understanding.

[0023] Throughout the specification, when a certain part "includes" or "comprises" a certain component, unless there is a particularly contrary description otherwise, it means that other components can be further included or comprised, rather than excluding other components.

[0024] Unless there is a clear exception in the context, a singular expression includes a plural expression.

[0025] 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 specifically stated, the unit is wt%.

[0026] A ferritic stainless steel having improved formability according to one embodiment may contain, 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%, the balance being Fe and inevitable impurities.

[0027] The content of carbon (C) may be 0.0005 - 0.02%.

[0028] When the content of C is less than 0.0005%, the refining cost for producing high-purity products increases. Therefore, C can be added in an amount of 0.0005% or more. However, when the content of C is too high, the ductility decreases and the formability may be reduced. In addition, when the content of C is too high, the corrosion resistance may deteriorate. Considering this, the upper limit of the content of C can be limited to 0.02%. Preferably, the content of C may be 0.0092 - 0.0122%.

[0029] The content of nitrogen (N) may be 0.005 - 0.02%.

[0030] When the content of N is less than 0.005%, the crystallinity of TiN decreases and the equiaxed degree of the slab decreases. Considering this, N above 0.005% can be added. However, when the content of N is too high, the strength increases excessively and the formability may decrease. In addition, when the content of N is too high, the corrosion resistance may deteriorate. Considering this, the upper limit of the content of N can be limited to 0.02%. Preferably, the content of N can be 0.0083 - 0.0114%.

[0031] The content of silicon (Si) can be 0.01 - 1.0%.

[0032] When the content of Si is less than 0.01%, there is a problem of increased refining cost. Considering this, Si above 0.01% can be added. However, when the content of Si is too high, impurities increase and thus the formability may decrease. Considering this, the upper limit of the content of Si can be limited to 1.0%. Preferably, the content of Si can be 0.2 - 0.3%.

[0033] The content of manganese (Mn) can be 0.01 - 1.0%.

[0034] When the content of Mn is less than 0.01%, there is a problem of increased refining cost. Considering this, Mn above 0.01% can be added. However, when the content of Mn is too high, impurities increase and thus the formability may decrease. Considering this, the upper limit of the content of Mn can be limited to 1.0%. Preferably, the content of Si can be 0.4 - 0.5%.

[0035] The content of phosphorus (P) can be 0.001 - 0.05%.

[0036] When the content of P is less than 0.001%, there is a problem of increased refining cost. Considering this, P above 0.001% can be added. However, when the content of P is too high, due to the increase of impurities, the formability decreases and the hot workability may deteriorate. Considering this, the upper limit of the content of P can be limited to 0.05%. Preferably, the content of Si can be 0.02 - 0.03%.

[0037] The content of chromium (Cr) can be 10.0 - 30.0%.

[0038] Cr is an effective element to ensure the corrosion resistance of steel. Considering this, Cr above 10% can be added. However, when the content of Cr is too high, a large amount of δ-ferrite is formed in the material, the hot workability decreases, and the formability may decrease. Considering this, the upper limit of the content of Cr can be limited to 30%.

[0039] The content of niobium (Nb) can be 0.05 - 0.5%.

[0040] Nb is an effective element that forms fine Nb precipitates and inhibits grain growth, thereby preventing the grain size from increasing during annealing of ultra-thin materials. Considering this, Nb can be added in an amount of more than 0.05%. However, when the content of Nb is too high, the manufacturing cost increases, excessive precipitates are formed, and the formability may decrease. Considering this, the upper limit of the content of Nb can be limited to 0.5%. Preferably, the content of Nb can be 0.12 - 0.32%.

[0041] The content of titanium (Ti) can be 0.05 - 0.5%.

[0042] Ti is an effective element that forms recrystallization during hot rolling. Considering this, Ti can be added in an amount of more than 0.05%. However, when the content of Ti is too high, a large amount of steelmaking inclusions may be generated. Considering this, the upper limit of the content of Ti can be limited to 0.5%. Preferably, the content of Ti can be 0.10 - 0.21%.

[0043] The remaining component of the present invention is iron (Fe). However, in the conventional manufacturing process, undesired impurities may inevitably be mixed in from raw materials or the surrounding environment, so these impurities cannot be excluded. These impurities are well known to those skilled in the conventional manufacturing process, so not all of their contents will be specifically described in this specification.

[0044] The object of the disclosed invention is to control the fine structure to improve formability through the above alloying components and the following manufacturing method. Therefore, the Erichsen height of the ferritic stainless steel with improved formability according to one embodiment can be 6 - 50 mm.

[0045] The Erichsen height can be measured by the Erichsen test. The Erichsen test is a test widely used to evaluate the workability of stainless steel, and it is a test that applies pressure until cracks occur in the test piece and measures the height of deformation.

[0046] When the Erichsen height is less than 6 mm, it may be difficult to fully ensure formability. However, when the Erichsen height exceeds 50 mm, it is not conducive to shape control, so the production efficiency decreases.

[0047] The Erichsen height of the ferritic stainless steel with improved formability according to one embodiment of the present invention can specifically be 6 mm or more and 40 mm or less, and more specifically 6 mm or more and 30 mm or less. Within the above range, the ferritic stainless steel according to one embodiment of the present invention can have both formability and production efficiency. In this case, characteristics favorable for application to diaphragm components such as fuel cells can be achieved.

[0048] In one example, the Erichsen value of the ferritic stainless steel with improved formability according to an embodiment of the present invention may be 6 mm or more and 20 mm or less. Within the above range, the ferritic stainless steel according to an embodiment of the present invention has both formability and production efficiency, and at the same time can achieve the effect of maintaining or improving the physical properties required for diaphragm components such as fuel cells.

[0049] The average grain diameter of the ferritic stainless steel with improved formability according to an embodiment may be 5 - 20 μm, preferably 5 - 15 μm, and more preferably 10 - 15 μm.

[0050] When the average grain diameter is too large, the slip systems existing in the thickness direction of the ultra-thin material decrease, resulting in possible reduction in formability. However, when the average grain diameter is too small, it may include a part of the non-recrystallized region, so the formability may deteriorate. Therefore, it is necessary to optimize and control the average grain diameter.

[0051] In addition, in the disclosed invention, "average" refers to the average value of the measured values at any 5 positions.

[0052] The thickness of the ferritic stainless steel with improved formability according to an embodiment may be 200 μm or less, preferably 5 - 200 μm, and more preferably 5 - 150 μm. Within the above range, the ferritic stainless steel according to an embodiment of the present invention can achieve characteristics beneficial for application to diaphragm components such as fuel cells.

[0053] In one example, the thickness of the ferritic stainless steel with improved formability according to an embodiment of the present invention may be 10 μm to 100 μm. Within the above range, the ferritic stainless steel according to an embodiment of the present invention has both formability and production efficiency, and at the same time can achieve the effect of maintaining or improving the physical properties required for diaphragm components such as fuel cells.

[0054] The disclosed invention can be an ultra-thin material of 200 μm or less to be used as a hydrogen fuel cell diaphragm. However, the present invention is not limited thereto, and the disclosed invention can be applied to various fields where it can be utilized.

[0055] The (thickness) / (average grain diameter) value of the ferritic stainless steel with improved formability according to an embodiment may be 5 to 10.

[0056] Due to the characteristics of the ultra-thin material, the disclosed invention needs to control the ratio between the thickness and the average grain diameter.

[0057] When the value of (thickness) / (average grain diameter) is less than 5, there are less than 5 grains in the thickness direction of the stainless steel, and the slip system is too few. Therefore, when the value of (thickness) / (average grain diameter) is less than 5, the formability of the stainless steel may be reduced. However, when the value of (thickness) / (average grain diameter) exceeds 20, the average grain diameter of the stainless steel is too fine, and the formability may be reduced due to non-recrystallization.

[0058] In one example, the value of (thickness) / (average grain diameter) of a ferritic stainless steel having improved formability according to an embodiment of the present invention can be 5.4 to 10. Within the above range, the ferritic stainless steel according to an embodiment of the present invention can suppress the reduction of formability.

[0059] In another example, the value of (thickness) / (average grain diameter) of a ferritic stainless steel having improved formability according to an embodiment of the present invention can be 5.4 to 9.3. Within the above range, the ferritic stainless steel according to an embodiment of the present invention can suppress the reduction of formability.

[0060] Next, a method for manufacturing a ferritic stainless steel having improved formability according to another aspect of the disclosed invention will be described.

[0061] A method for manufacturing a ferritic stainless steel having improved formability according to an embodiment may include the following steps: manufacturing a slab, which, by weight%, contains: 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%, the balance being Fe and inevitable impurities; reheating the slab to 1100 - 1300 °C and then performing hot rolling and hot rolling annealing to manufacture a hot-rolled material; performing cold rolling and cold rolling annealing 2 to 5 times on the hot-rolled material subjected to the hot rolling annealing to manufacture a steel sheet.

[0062] The reasons for limiting the numerical values of the component ranges of the respective alloy compositions are as described above, and the following is a more detailed description of each manufacturing step.

[0063] After manufacturing a slab that satisfies the alloy composition, it can undergo a series of processes of reheating, hot rolling, hot rolling annealing, cold rolling, and cold rolling annealing.

[0064] First, the slab can be reheated at 1100 - 1300 °C and then subjected to hot rolling and hot rolling annealing to manufacture a hot-rolled material.

[0065] When the heating temperature is low, it is difficult to re-dissolve the coarse precipitates generated during the production of the slab. Taking this into consideration, the heating temperature can be 1100 °C or higher. However, when the heating temperature is too high, the internal grains may become overly coarse. Taking this into consideration, the upper limit of the heating temperature can be restricted to 1300 °C.

[0066] The hot rolling and annealing can be carried out at 900 - 1100 °C.

[0067] When the hot rolling and annealing temperature is too low, it is difficult to fully remove the stress generated during rolling. However, when the hot rolling and annealing temperature is too high, the grains may become overly coarse.

[0068] The hot-rolled material after the hot rolling and annealing can be cold-rolled and cold-rolled annealed 2 to 5 times.

[0069] By means of the initial cold rolling and cold rolling annealing, the steel of the hot-rolled material is deformed, and during subsequent processes, it is possible to induce the smooth formation of precipitates.

[0070] Through cold rolling and cold rolling annealing after the second time, a large amount of precipitates are precipitated and recrystallization is induced, thereby ensuring fine grains.

[0071] In addition, as the number of cold rolling and cold rolling annealing increases, the manufacturing cost may increase. Therefore, the cold rolling and cold rolling annealing can be carried out 5 times or less.

[0072] The cold rolling annealing can be carried out at 850 - 1000 °C.

[0073] When the cold rolling annealing temperature is low, the stress formed during rolling cannot be fully removed, so the workability may be reduced. However, when the cold rolling annealing temperature is too high, the formability may be reduced due to grain coarsening.

[0074] In the step of manufacturing the steel plate, the final cold rolling reduction ratio can be 50% or more.

[0075] When the final cold rolling reduction ratio is less than 50%, the strain is insufficient, and it may be difficult to obtain fine grains.

[0076] Hereinafter, the present invention will be described in more detail by way of examples. However, the description of these examples is only for illustrating the implementation of the present invention, and the present invention is not limited by these examples. This is because the scope of rights of the present invention is determined by the matters described in the claims and the matters reasonably deduced therefrom.

[0077] {Examples}

[0078] For various alloy composition ranges shown in Table 1 below, slabs are manufactured in a vacuum induction melting furnace. The obtained slabs are reheated in a heating furnace at 1250 °C, then hot-rolled at 1100 °C, and then hot-rolled and annealed to manufacture hot-rolled materials. The hot-rolled materials are cold-rolled and cold-rolled annealed to manufacture test pieces with dimensions of 100 mm in the transverse direction × 100 mm in the longitudinal direction. At this time, cold rolling and cold-rolled annealing are performed 3 times. The thicknesses of the test pieces manufactured as described above are listed in Table 2, and the final reduction ratio of each test piece is 60%.

[0079] [Table 1]

[0080]

[0081] The thickness, average grain diameter, (thickness) / (average grain diameter) value, and Erichsen height are shown in Table 2 below. For the average grain diameter, the entire thickness of the cross-section perpendicular to the rolling direction of the test piece is photographed with an electron backscatter diffraction pattern analyzer (EBSD) and the average grain diameter is measured.

[0082] The Erichsen height is measured by the Erichsen test. The Erichsen test measurement is performed as follows. The test piece is inserted between the upper die and the lower die, and then the outer peripheral part of the test piece is fixed with a force of 20 kN. After that, a spherical punch with a diameter of 20 mm is used to apply deformation to the test piece at a speed of 5 - 20 mm / min. After that, it is performed by inserting the punch until the test piece breaks and then measuring the deformation height of the test piece at the time of fracture. The higher the Erichsen height, the better the formability.

[0083] [Table 2]

[0084]

[0085] Referring to Table 2, Examples 1 to 8 satisfy the alloy composition and manufacturing method proposed by the disclosed invention. Therefore, the Erichsen height satisfies 6 - 50 mm, the average grain diameter satisfies 5 - 20 μm, the thickness satisfies 200 μm or less, and the (thickness) / (average grain diameter) value satisfies 5 to 10. That is, Examples 1 to 8 can be considered to have excellent formability. However, although Comparative Examples 1 to 10 satisfy the alloy composition, the (thickness) / (average grain diameter) value does not satisfy 5 to 10. Therefore, the Erichsen height does not satisfy 6 - 50 mm. That is, the formability of Comparative Examples 1 to 10 is poor.

[0086] Comparative Examples 11 to 15 do not satisfy the alloy composition and manufacturing method proposed by the disclosed invention. Therefore, the (thickness) / (average grain diameter) value does not satisfy 5 to 10, and the Erichsen height does not satisfy 6 - 50 mm. That is, the formability of Comparative Examples 11 to 15 is poor.

[0087] Figure 1 The figure of the microstructure of Comparative Example 7 was taken with an electron backscatter diffraction pattern analyzer (EBSD). Figure 2 The figure of the microstructure of Example 3 was taken with an electron backscatter diffraction pattern analyzer (EBSD).

[0088] Referring to Figure 1 and Figure 2 , it can be confirmed that the ferritic stainless steel according to the disclosed invention achieves excellent formability by controlling the grain size to be small.

[0089] Figure 3 The figure shows the correlation between the (thickness) / (average grain diameter) value and the Erichsen height.

[0090] Referring to Figure 3 , it can be confirmed that the Erichsen height can be adjusted to 6 - 50 mm by controlling the (thickness) / (average grain diameter) value to be between 5 and 10.

[0091] According to one embodiment of the disclosed invention, as described above, a ferritic stainless steel and a manufacturing method thereof that improve formability by controlling the average grain diameter can be provided.

[0092] Industrial Applicability

[0093] According to the present invention, a ferritic stainless steel and a manufacturing method thereof that improve formability by controlling the average grain diameter can be provided, and thus have industrial applicability.

Claims

1. A ferritic stainless steel with improved formability, in weight %, the ferritic stainless steel comprises: 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%, the balance Fe and inevitable impurities, The Erichsen height of the ferritic stainless steel is 6 - 50 mm.

2. The ferritic stainless steel having improved formability according to claim 1, wherein, The average grain diameter is 5 - 20 μm.

3. The ferritic stainless steel having improved formability according to claim 1, wherein, The thickness is 200 μm or less.

4. The ferritic stainless steel having improved formability according to claim 1, wherein, The (thickness) / (average grain diameter) value is 5 to 10.

5. A method for manufacturing a ferritic stainless steel with improved formability, which comprises the following steps: Manufacturing a slab, in weight %, the slab comprises: 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%, the balance Fe and inevitable impurities; Reheating the slab to 1100 - 1300 °C and then performing hot rolling and hot rolling annealing to manufacture a hot rolled material; Performing cold rolling and cold rolling annealing 2 to 5 times on the hot rolled material after the hot rolling annealing to manufacture a steel sheet, wherein, the cold rolling annealing is performed at 850 - 1000 °C.

6. The method for manufacturing a ferritic stainless steel having improved formability according to claim 5, wherein, The hot rolling annealing is performed at 900 - 1100 °C.

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

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