Ferritic stainless steel with improved formability and methods for manufacturing it.

Abstract

Invention details;

Description

Ferritic stainless steel with improved formability and its manufacturing method

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

[0002] In line with the recent global trend toward carbon neutrality, demand for electric and hydrogen fuel cell vehicles is increasing. In particular, hydrogen fuel cell vehicles are gaining increasing importance as future eco-friendly vehicles. Consequently, development of hydrogen fuel cell separators is becoming increasingly active.

[0003] Hydrogen fuel cell separators can utilize metal materials. In particular, ferritic stainless steel, which offers advantages in fuel cell performance and cost-effectiveness, is attracting increasing attention.

[0004] Ferritic stainless steel used as a fuel cell separator is an ultra-thin material with a thickness of less than 200㎛, and formability of the separator parts is required when manufacturing fuel cells.

[0005] Accordingly, there is an increasing demand for improving the formability of ultra-thin ferritic stainless steel that can be used in hydrogen fuel cell separator membranes.

[0006] The purpose of the disclosed invention to solve the above-described problem is to provide a ferritic stainless steel with improved formability and a method for manufacturing the same by controlling the average grain diameter through the alloying components and manufacturing process.

[0007] A ferritic stainless steel having improved formability according to one embodiment comprises, 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%, the remainder being Fe and unavoidable impurities, and may have an Erichsen height of 6 to 50 mm.

[0008] According to one embodiment, a ferritic stainless steel having improved formability may have an average grain diameter of 5 to 20 μm.

[0009] According to one embodiment, a ferritic stainless steel with improved formability may have a thickness of 200 μm or less.

[0010] A ferritic stainless steel having improved formability according to one embodiment may have a thickness / average grain diameter value of 5 to 10.

[0011] A method for manufacturing a ferritic stainless steel having improved formability according to one embodiment comprises the steps of: manufacturing a slab including, 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%, the remainder being Fe and unavoidable impurities; reheating the slab to 1100 to 1300°C and then hot-rolling and hot-annealing to manufacture a hot-rolled material; It may include a step of manufacturing a steel sheet by cold rolling and cold annealing the hot-rolled material that has been hot-rolled and annealed two to five times, and the cold annealing may be performed at 850 to 1000°C.

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

[0013] In the step of manufacturing the above steel plate, the final cold rolling reduction ratio may be 50% or more.

[0014] According to one embodiment of the disclosed invention, a ferritic stainless steel having improved formability by controlling the average grain diameter and a method for manufacturing the same can be provided.

[0015] Figure 1 is an image of the microstructure of Comparative Example 7 taken using EBSD (Electron BackScatter Diffraction).

[0016] Figure 2 is an image of the microstructure of Example 3 taken using EBSD (Electron BackScatter Diffraction).

[0017] Figure 3 is a graph showing the correlation between the (thickness) / (average grain diameter) value and the Eriksen height.

[0018] A ferritic stainless steel having improved formability according to one embodiment comprises, 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%, the remainder being Fe and unavoidable impurities, and may have an Erichsen height of 6 to 50 mm.

[0019] Hereinafter, embodiments of the disclosed invention will be described in detail with reference to the accompanying drawings. The following embodiments are provided to sufficiently convey the spirit of the disclosed invention to those skilled in the art. The disclosed invention is not limited to the embodiments presented herein and may be embodied in other forms. To clarify the disclosed invention, the drawings may omit portions irrelevant to the description, and the sizes of components may be slightly exaggerated to aid understanding.

[0020] Throughout the specification, whenever a part is said to "include" a component, this does not mean that it excludes other components, but rather that it may include other components, unless otherwise stated.

[0021] Singular expressions include plural expressions unless the context clearly indicates otherwise.

[0022] Hereinafter, the reasons for numerical limitations on the alloy component content in the embodiments of the present invention will be described. Hereinafter, unless otherwise specified, the unit is weight percent.

[0023] A 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%, the remainder being Fe and unavoidable impurities.

[0024] The C (carbon) content can be 0.0005 to 0.02%.

[0025] When 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, when the C content is excessive, ductility may decrease, resulting in poor formability. Furthermore, when the C content is excessive, corrosion resistance may be deteriorated. Considering this, the upper limit of the C content may be limited to 0.02%. Preferably, the C content may be 0.0092 to 0.0122%.

[0026] The content of N (nitrogen) can be 0.005 to 0.02%.

[0027] When the content of N is less than 0.005%, TiN crystallization may be reduced, which may lower the equiaxed crystallinity of the slab. Considering this, N may be added in an amount of 0.005% or more. However, when the content of N is excessive, the strength may increase excessively, which may lower the formability. In addition, when the content of N is excessive, the corrosion resistance may be deteriorated. Considering this, the upper limit of the content of N may be limited to 0.02%. Preferably, the content of N may be 0.0083 to 0.0114%.

[0028] The content of Si (silicon) can be 0.01 to 1.0%.

[0029] When the Si content is less than 0.01%, there is a problem of increased refining costs. Considering this, Si can be added in amounts greater than 0.01%. However, when the Si content is excessive, impurities increase, which can reduce formability. Considering this, the upper limit of the Si content can be limited to 1.0%. Preferably, the Si content can be 0.2 to 0.3%.

[0030] The content of Mn (manganese) can be 0.01 to 1.0%.

[0031] When the manganese content is less than 0.01%, there is a problem of increased refining costs. Considering this, manganese can be added in amounts of 0.01% or more. However, if the manganese content is excessive, impurities may increase, resulting in reduced formability. Considering this, the upper limit of the manganese content may be limited to 1.0%. Preferably, it may be 0.4 to 0.5%, which is the same as the silicon content.

[0032] The content of P(phosphorus) can be 0.001 to 0.05%.

[0033] When the P content is less than 0.001%, there is a problem that the refining cost increases. Considering this, P can be added in an amount of 0.001% or more. However, when the P content is excessive, impurities increase, which may reduce formability and hot workability. Considering this, the upper limit of the P content may be limited to 0.05%. Preferably, it may be 0.02 to 0.03%, which is the content of Si.

[0034] The content of Cr (chromium) can be 10.0 to 30.0%.

[0035] Cr is an effective element for ensuring steel's corrosion resistance. Considering this, Cr can be added at levels exceeding 10%. However, excessive Cr content can lead to the formation of large amounts of delta ferrite within the material, which can deteriorate hot workability and formability. Considering this, the upper limit of Cr content may be limited to 30%.

[0036] The content of Nb (niobium) can be 0.05 to 0.5%.

[0037] Niobium (Nb) is an effective element in preventing grain size from increasing during annealing of ultra-thin steel by forming fine Nb precipitates and inhibiting grain growth. Considering this, Nb may be added in amounts of 0.05% or more. However, excessive Nb content may increase manufacturing costs and reduce formability due to excessive precipitate formation. Considering this, the upper limit of the Nb content may be limited to 0.5%. Preferably, the Nb content may be 0.12 to 0.32%.

[0038] The content of Ti (titanium) can be 0.05 to 0.5%.

[0039] Ti is an element that is effective in forming recrystallization during hot rolling. Considering this, Ti can be added in amounts of 0.05% or more. However, if the Ti content is excessive, numerous steelmaking inclusions may occur. Considering this, the upper limit of the Ti content can be limited to 0.5%. Preferably, the Ti content can be 0.10 to 0.21%.

[0040] The remaining component of the present invention is iron (Fe). However, during the typical manufacturing process, unintended impurities from raw materials or the surrounding environment may inevitably be mixed in, and thus cannot be excluded. Since these impurities are readily apparent to anyone skilled in the art of typical manufacturing, their full details are not specifically discussed in this specification.

[0041] The disclosed invention seeks to improve formability by controlling the microstructure through the aforementioned alloy composition and the manufacturing method described below. Accordingly, a ferritic stainless steel with improved formability according to one embodiment may have an Erichsen height of 6 to 50 mm.

[0042] The Erichsen height can be measured using the Erichsen test. This test is widely used to evaluate the workability of stainless steel. It involves applying pressure to a specimen until a crack appears and measuring the resulting deformation height.

[0043] When the Eriksen height is less than 6 mm, it may be difficult to ensure sufficient formability. However, when the Eriksen height exceeds 50 mm, shape control may be difficult, resulting in lower production efficiency.

[0044] The ferritic stainless steel with improved formability according to one embodiment of the present invention may specifically have an Erichsen height of 6 mm to 40 mm, more specifically, 6 mm to 30 mm. Within this range, the ferritic stainless steel according to one embodiment of the present invention can achieve both formability and production efficiency. In this case, it can realize advantageous properties for application to separator components such as fuel cells.

[0045] In one example, a ferritic stainless steel with improved formability according to one embodiment of the present invention may have an Erichsen height of 6 mm or more and 20 mm or less. Within this range, the ferritic stainless steel according to one embodiment of the present invention can achieve both formability and production efficiency, while simultaneously maintaining or improving the properties required for separator components such as fuel cells.

[0046] A ferritic stainless steel with improved formability according to one embodiment may have an average grain diameter of 5 to 20 μm, preferably 5 to 15 μm, and more preferably 10 to 15 μm.

[0047] If the average grain diameter is too large, the slip system existing in the thickness direction of the ultra-thin material may be reduced, resulting in poor formability. However, if the average grain diameter is too small, some unrecrystallized regions may be included, resulting in poor formability. Therefore, it is necessary to optimize and control the average grain diameter.

[0048] Meanwhile, in the disclosed invention, the average means the average value of values ​​measured at five arbitrary locations.

[0049] The ferritic stainless steel with improved formability according to one embodiment may have a thickness of 200 μm or less, preferably 5 to 200 μm, and more preferably 5 to 150 μm. Within the above range, the ferritic stainless steel according to one embodiment of the present invention may exhibit advantageous properties for application to separator components of fuel cells and the like.

[0050] In one example, the ferritic stainless steel with improved formability according to one embodiment of the present invention may have a thickness of 10 μm to 100 μm. Within this range, the ferritic stainless steel according to one embodiment of the present invention can achieve both formability and production efficiency, while simultaneously maintaining or improving the properties required for separator components such as fuel cells.

[0051] The disclosed invention may be an ultra-thin material having a thickness of 200 μm or less, suitable for use as a hydrogen fuel cell separator. However, the disclosed invention is not limited thereto, and can be applied to various fields in which it can be utilized.

[0052] A ferritic stainless steel having improved formability according to one embodiment may have a (thickness) / (average grain diameter) value of 5 to 10.

[0053] The disclosed invention requires control of the ratio between the thickness and the average grain diameter due to the nature of the material being ultra-thin.

[0054] When the (thickness) / (average grain diameter) value is less than 5, the number of grains existing in the thickness direction of the stainless steel is less than 5, and the slip system is too small. Therefore, when the (thickness) / (average grain diameter) value is less than 5, the formability of the stainless steel may be poor. However, when the (thickness) / (average grain diameter) value exceeds 20, the average grain diameter of the stainless steel is too fine, and a decline in formability due to non-recrystallization may occur.

[0055] In one example, the ferritic stainless steel with improved formability according to one embodiment of the present invention may have a (thickness) / (average grain diameter) value of 5.4 to 10. Within this range, the ferritic stainless steel according to one embodiment of the present invention can suppress a decrease in formability.

[0056] In another example, the ferritic stainless steel with improved formability according to one embodiment of the present invention may have a (thickness) / (average grain diameter) value of 5.4 to 9.3. Within this range, the ferritic stainless steel according to one embodiment of the present invention can suppress a decrease in formability.

[0057] Next, a method for manufacturing a ferritic stainless steel with improved formability according to another aspect of the disclosed invention is described.

[0058] A method for manufacturing a ferritic stainless steel having improved formability according to one embodiment may include the steps of: manufacturing a slab including, 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%, the remainder Fe and unavoidable impurities; reheating the slab to 1100 to 1300°C and then hot-rolling and hot-annealing to manufacture a hot-rolled material; and cold-rolling and cold-annealing the hot-rolled and annealed material 2 to 5 times to manufacture a steel sheet.

[0059] The reason for limiting the numerical value of the component range of each alloy composition is as described above, and each manufacturing step is explained in more detail below.

[0060] After manufacturing a slab that satisfies the above alloy composition, it can undergo a series of processes including reheating, hot rolling, hot annealing, cold rolling, and cold annealing.

[0061] First, the above slab can be reheated at 1100 to 1300°C, then hot-rolled and hot-annealed to produce a hot-rolled material.

[0062] If the heating temperature is low, it may be difficult to redissolve the coarse precipitates generated during slab manufacturing. Considering this, the heating temperature may be 1100°C or higher. However, if the heating temperature is excessively high, the internal grains may become too coarse. Considering this, the upper limit of the heating temperature may be limited to 1300°C.

[0063] The above hot rolling annealing can be performed at 900 to 1100°C.

[0064] If the hot rolling annealing temperature is too low, it is difficult to sufficiently remove the stress caused by rolling. However, if the hot rolling annealing temperature is too high, the grains may become excessively coarse.

[0065] The above hot-rolled and hot-annealed hot-rolled material can be cold-rolled and cold-annealed 2 to 5 times.

[0066] The initial cold rolling and cold annealing can cause deformation of the hot-rolled material and facilitate the formation of precipitates during subsequent processes.

[0067] Through secondary and subsequent cold rolling and cold annealing, a large number of precipitates can be precipitated, and recrystallization can be induced to secure fine grains.

[0068] Meanwhile, as the number of cold rolling and cold annealing cycles increases, manufacturing costs may increase, so cold rolling and cold annealing cycles can be performed no more than five times.

[0069] The above cold rolling annealing can be performed at 850 to 1000°C.

[0070] If the cold rolling annealing temperature is low, the stress formed during rolling may not be sufficiently removed, resulting in poor workability. However, if the cold rolling annealing temperature is too high, grain coarsening may result in poor formability.

[0071] In the step of manufacturing the above steel plate, the final cold rolling reduction ratio may be 50% or more.

[0072] If the final cold rolling reduction is less than 50%, it may be difficult to achieve fine grains due to insufficient deformation.

[0073] Hereinafter, the present invention will be described in more detail through examples. However, the description of these examples is merely intended to illustrate the implementation of the present invention and is not intended to limit the present invention. This is because the scope of the present invention is determined by the matters set forth in the claims and matters reasonably inferred therefrom.

[0074] {Example}

[0075] For the various alloy composition ranges shown in Table 1 below, slabs were manufactured in a vacuum induction melting furnace. The manufactured slabs were reheated in a heating furnace at 1250℃, hot-rolled at 1100℃, and then hot-annealed to manufacture hot-rolled materials. The hot-rolled materials were cold-rolled and cold-annealed to manufacture specimens measuring 100 mm in width X 100 mm in length. At this time, cold rolling and cold-annealing were performed three times. The thicknesses of the specimens manufactured in this way are shown in Table 2, and the final reduction ratio of each specimen was 60%.

[0076] Steel alloy composition (weight %)CNSiMnPCrNbTiA0.01220.01140.30.40.0225.60.320.10B0.01040.00830.20.50.0321.20.170.15C0.00920.00900.20.40.0216.20.120.21D0.00840.00990.30.40.0226.40.010.22E0.01020.00840.20.40.0217.30.620.02

[0077] Table 2 below shows the thickness, average grain diameter, (thickness) / (average grain diameter) value, and Eriksen height. The average grain diameter was measured by photographing the full thickness of the cross-section perpendicular to the rolling direction of the specimen using EBSD (Electron BackScatter Diffraction).

[0078] The Ericksen height was measured using the Ericksen test. The Ericksen test was measured as follows. After the specimen was inserted between the upper and lower dies, the outer periphery of the specimen was fixed with a force of 20 kN. Then, the specimen was deformed at a rate of 5 to 20 mm / min using a spherical punch with a diameter of 20 mm. The punch was inserted until the specimen broke, and the deformation height of the specimen at the time of breakage was measured. A higher Ericksen height indicates better formability.

[0079] Classification Steel grade Cold rolling annealing temperature (℃) Thickness (㎛) Average grain diameter (㎛) (Thickness) / (Average grain diameter) Ericsen Height (mm) Example 1 A 9 0 0 5 0 6.7 7.5 6.5 Example 2 A 9 2 0 5 0 9.15 5 6.3 Example 3 B 9 4 0 8 0 9.38 6 6.6 Example 4 B 9 6 0 8 0 13.46 0 6.7 Example 5 B 9 8 0 8 0 14.95 46.4 Example 6 C 9 4 0 10 0 10.89 36.7 Example 7 C 9 6 0 10 0 15.06 7 7.1 Example 8 C 9 8 0 10 0 18.45 46.6 Comparative Example 1 A 8 4 0 5 0 4.5 1 1.15 7 Comparative Example 2 A 9 6 0 5 0 1 1.44 45.3 Comparative Example 3 A 9 8 0 5 0 16.13 15.2 Comparative Example 4 A 1 0 0 5 0 19.3 2.64.7Comparative Example 5B880807.510.75.6Comparative Example 6B10208018.54.35.3Comparative Example 7B10408025.33.25.1Comparative Example 8C8801008.312.05.4Comparative Example 9C102010024.34.15.4Comparative Example 10C104010029.13.44.7Comparative Example 11D10208022.43.65.7Comparative Example 12D10408025.63.15.1Comparative Example 13D10608032.12.54.6Comparative Example 14E840805.813.83.9Comparative Example 15E860807.311.04.5

[0080] Referring to Table 2, Examples 1 to 8 satisfied the alloy composition and manufacturing method presented in the disclosed invention. Accordingly, the Erickson height was 6 to 50 mm, the average grain diameter was 5 to 20 ㎛, the thickness was 200 ㎛ or less, and the (thickness) / (average grain diameter) value was 5 to 10. That is, Examples 1 to 8 can be considered to have excellent formability. However, Comparative Examples 1 to 10 did not satisfy the (thickness) / (average grain diameter) value of 5 to 10 even though they satisfied the alloy composition. Therefore, the Erickson height did not satisfy 6 to 50 mm. That is, Comparative Examples 1 to 10 had inferior formability.

[0081] Comparative Examples 11 to 15 did not satisfy the alloy composition and manufacturing method suggested by the disclosed invention. Accordingly, the (thickness) / (average grain diameter) value did not satisfy 5 to 10, and the Eriksen height did not satisfy 6 to 50 mm. In other words, Comparative Examples 11 to 15 had poor formability.

[0082] Fig. 1 is an image of the microstructure of Comparative Example 7 taken using EBSD (Electron BackScatter Diffraction), and Fig. 2 is an image of the microstructure of Example 3 taken using EBSD (Electron BackScatter Diffraction).

[0083] Referring to FIGS. 1 and 2, it can be confirmed that the ferritic stainless steel according to the disclosed invention implements excellent formability by controlling the crystal grains to be small.

[0084] Figure 3 is a graph showing the correlation between the (thickness) / (average grain diameter) value and the Eriksen height.

[0085] Referring to Fig. 3, it can be confirmed that the Eriksen height can be adjusted from 6 to 50 mm by controlling the (thickness) / (average grain diameter) value to 5 to 10.

[0086] According to one embodiment of the disclosed invention, as described above, a ferritic stainless steel having improved formability by controlling the average grain diameter and a method for manufacturing the same can be provided.

[0087] According to the present invention, a ferritic stainless steel having improved formability by controlling the average grain diameter and a method for manufacturing the same can be provided, and thus, its industrial applicability is recognized.

Claims

DEPCT6908 / 09 / 25681. Ferritic stainless steel with improved formability, composed of the following elements in percentage by weight (% by weight): Carbon (C) 0.0005 to 0.02%, Nitrogen (N) 0.005 to 0.02%, Silicon (Si) 0.01 to 1.0%, Manganese (Mn) 0.01 to 1.0%, Phosphorus (P) 0.001 to 0.05%, Chromium (Cr) 10.0 to 30.0%, Niobium (Nb) 0.05 to 0.5%, Titanium (Ti) 0.05 to 0.5%, the remainder being iron (Fe) and unavoidable impurities, and having an Erichsen height of 6 to 50 mm.

2. Ferritic stainless steel according to 1. Ferritic stainless steel according to Patent 1 with an average grain diameter of 5 to 20 micrometers.

3. Ferritic stainless steel according to Patent 1 with a thickness of 200 micrometers or less.

4. Ferritic stainless steel according to Patent 1 with a (thickness) / (average grain diameter) value of 5 to 10.

5. A method for producing ferritic stainless steel with improved formability in which the assembly method involves preparing flat bars incorporating the following elements in percentage by weight (% by weight): Carbon (C) 0.0005 to 0.02%, Nitrogen (N) 0.The composition is 0.05 to 0.02%, silicon (Si) 0.01 to 1.0%, manganese (Mn) 0.01 to 1.0%, phosphorus (P) 0.001 to 0.05%, chromium (Cr) 10.0 to 30.0%, niobium (Nb) 0.05 to 0.5%, titanium (Ti) 0.05 to 0.5%, with the remainder being iron (Fe) and unavoidable impurities, and an Eriksen height of 6 to 50 mm. The flat bars are reheated to 1100 to 1300 degrees Celsius, and then hot rolling and hot annealing are performed on the reheated bars to produce hot-rolled material. Cold rolling and cold annealing on hot-rolled material which has been hot-annealed two to five times to produce steel sheets, where the cold annealing is performed at 850 to 1000 degrees Celsius; 6. Ferritic stainless steel according to claim 5, where the hot annealing is performed at 900 to 1100 degrees Celsius; 7. Ferritic stainless steel according to claim 5, where in the production of steel sheets the final cold rolling reduction ratio is 50% or more;

Images