Steel for battery shell and preparation method thereof

Abstract

The invention relates to the technical field of metal materials, and discloses steel for a battery shell and a preparation method of the steel. The steel for the battery case comprises the following chemical components in percentage by weight: 0.001%-0.004% of C, 0.01%-0.02% of Si, 0.12%-0.22% of Mn, 0.004%-0.01% of P, less than or equal to 0.004% of S, 0.2%-0.4% of Cr, 0.02%-0.05% of Al, 0.015%-0.035% of Ti, 0.001%-0.003% of B, 0.01%-0.03% of Nb and the balance of Fe and inevitable impurities. By accurately controlling the components in the steel, particularly controlling the C content, the niobium content and the chromium content in a proper range, the yield strength, the tensile strength and the ductility of the steel can be improved, and particularly the low-temperature impact toughness of the steel can be improved.

Description

Technical Field

[0001] The present invention relates to the technical field of metal materials, and particularly to a steel for battery cases and a preparation method thereof. Background Art

[0002] With the rapid development of the new energy industry, in fields such as polar scientific research equipment and electric vehicles in alpine regions, the battery case not only has to withstand the complex chemical reaction stress inside and external mechanical shocks, but also needs to maintain excellent impact toughness under low-temperature conditions after the first stamping process to ensure the safety of personnel, equipment, and property.

[0003] The current battery case steel materials on the market have significant defects in dealing with low-temperature impact toughness. Most traditional steels have a sharp drop in toughness in a low-temperature environment, resulting in cracks or even fractures easily occurring during impact, seriously affecting the production efficiency and quality of the battery case, and further threatening the safety and reliability of the battery in a low-temperature environment. Although some existing patents have improved the performance of the battery case steel, they still cannot provide an ideal solution in comprehensively balancing low-temperature toughness and impact performance. Some solutions ignore the overall synergistic optimization due to excessive focus on a certain performance, or are difficult to meet the urgent market demand for high-performance, highly reliable, and economically practical steel for battery cases due to complex production processes and high costs.

[0004] Currently, there are two component systems for battery case steel on the market, mainly as follows: Domestic patent CN1174109C, "Ultra-thin Steel Strip for Battery Case and Its Manufacturing Method", discloses an ultra-thin steel strip for battery case and its production method. The weight percentages of its chemical components are: C ≤ 0.0050%, Si ≤ 0.020%, Mn: 0.15 - 0.30%, P: 0.010% - 0.030%, S: ≤ 0.015%, N ≤ 0.0040%, Al: 0.020% - 0.07%, Ti: 0.010% - 0.030%, Nb: 0.010% - 0.025%, and the balance is Fe. The P element in this patent is relatively high, which has a greater adverse impact on the impact toughness. At the same time, there is a large difference in the thickness of the head and tail during single-stand rolling, and the consistency of the stamping shell is poor.

[0005] Domestic patent CN100560770C, "Steel for battery case with excellent planar isotropy and its manufacturing method", discloses a steel for battery case and its manufacturing method. The chemical composition of the steel for battery case is as follows by weight percentage: C: 0.01% - 0.05%, Si ≤ 0.03%, Mn: 0.10% - 0.50%, P ≤ 0.020%, S ≤ 0.015%, Als: 0.010% - 0.10%, N: 0.0020% - 0.0070%, Ti: 0.0050% - 0.020%, and the balance is Fe and inevitable impurities. Its production steps include hot metal pretreatment, converter smelting, in-furnace refining, hot rolling, pickling, cold rolling, bell annealing, skin pass rolling, and finishing into finished coils. This patent mainly focuses on the aluminum-killed steel system, with poor plastic toughness, which is not conducive to deep drawing, and poor low-temperature stamping toughness.

[0006] Domestic patent CN102286699B, "Steel for corrosion-resistant battery case with punching speed ≥ 150 per minute and its preparation method", discloses a steel for battery case with rapid stamping forming and its preparation method. The chemical composition of the steel for battery case is as follows by weight percentage: C: 0.0001% - 0.005%, Mn: 0.10% - 0.20%, Al: 0.010% - 0.050%, N: 0.00010% - 0.0040%, Nb: 0.010% - 0.030%, and control P ≤ 0.020%, S ≤ 0.0150%, Cu ≤ 0.050%, Ni ≤ 0.050%, Cr ≤ 0.080%, Mo ≤ 0.050%, Si ≤ 0.02%, and the balance is Fe and inevitable impurities. Production steps: smelt according to the clean steel process and continuously cast into slabs; heat the continuous casting slabs; rough rolling; finish rolling in the single-phase austenite region; coiling; pickling; cold rolling; degreasing; annealing in a full-hydrogen bell furnace; skin pass rolling and ready for use. This patent uses conventional bell annealing, with large fluctuations in coil properties, long holding time during annealing, large grain growth tendency, and poor low-temperature toughness.

[0007] Domestic patent application CN106148803A, "Production method of steel for deep-drawing battery case", discloses a production method of steel for deep-drawing battery case. The weight percentage of its chemical composition is: C: 0.0150% - 0.0350%, Si ≤ 0.020%, Mn: 0.15% - 0.25%, P: ≤ 0.018%, S: ≤ 0.015%, N ≤ 0.0030%, Alt: 0.030% - 0.060%, Ti: 0.008% - 0.015%, and the balance is Fe. This patent adopts the aluminum-killed steel system, with relatively poor stamping performance, difficult to meet the requirements of rapid stamping of steel for battery case, and this patent does not involve the production process of cold rolling.

[0008] It can be seen that the low-temperature impact toughness of the steel used for battery cases in the prior art is generally poor. Therefore, it is urgent to develop a new steel for battery cases to meet the market requirements for low-temperature impact toughness. Summary of the Invention

[0009] The purpose of the present invention is to overcome the problem of poor low-temperature impact toughness of the steel used for battery cases in the prior art, and to provide a steel for battery cases and a preparation method thereof. The steel for battery cases has high yield strength, tensile strength, elongation and excellent low-temperature impact toughness.

[0010] To achieve the above purpose, on the one hand, the present invention provides a steel for battery cases, which contains the following chemical components in weight percentage: C: 0.001% - 0.004%, Si: 0.01% - 0.02%, Mn: 0.12% - 0.22%, P: 0.004% - 0.01%, S: ≤0.004%, Cr: 0.2% - 0.4%, Al: 0.02% - 0.05%, Ti: 0.015% - 0.035%, B: 0.001% - 0.003%, Nb: 0.01% - 0.03%, and the balance is Fe and unavoidable impurities.

[0011] Preferably, the yield strength of the steel for battery cases is 200 MPa - 260 MPa, the tensile strength is 320 MPa - 380 MPa, and the elongation ≥35%.

[0012] On the second aspect, the present invention provides a method for preparing the steel for battery cases described above. The method includes hot metal pretreatment, converter smelting, refining, continuous casting, hot rolling, coiling, pickling, first cold rolling, continuous annealing and second cold rolling; Among them, the continuous annealing includes a soaking section and a cooling section; The soaking temperature of the soaking section is 700 °C - 720 °C, and the time is 40 s - 140 s; The cooling process of the cooling section includes: first rapidly cooling at 12 - 16 °C / s to 500 - 550 °C, and then cooling at 5 - 8 °C / s to 300 - 350 °C; During the first cold rolling process, the last stand is a bright roll rolling, and the surface roughness of the steel plate is controlled to be 0.3 μm - 0.5 μm.

[0013] Preferably, in the hot metal pretreatment process, the KR method is used for desulfurization and the double slag method is used for dephosphorization.

[0014] Preferably, the refining process includes RH vacuum degassing, VD vacuum degassing treatment and LF furnace refining.

[0015] Preferably, the refining slag composition of the LF furnace refining is of the CaO - Al2O3 - SiO2 system.

[0016] Preferably, electromagnetic stirring is carried out during the continuous casting process, and full protection casting is adopted.

[0017] Preferably, during the hot rolling process, the heating temperature of the heating furnace is 1230°C - 1250°C, and the finishing rolling temperature is 880°C - 920°C.

[0018] Preferably, the coiling temperature is 560°C - 600°C.

[0019] Preferably, during the first cold rolling process, the total reduction ratio ≥ 82%.

[0020] Preferably, during the second cold rolling process, the reduction ratio is 3% - 5%, and the surface roughness of the second cold rolling finished product ≤ 0.6μm.

[0021] Compared with the prior art, the present invention has at least the following beneficial effects: (1) By precisely controlling the composition in the steel, especially controlling the C content, niobium content, and chromium content within a suitable range, the present invention can improve the yield strength, tensile strength, and elongation of the steel, and especially can improve the low-temperature impact toughness of the steel.

[0022] (2) The method of the present invention passes through processes such as hot rolling, pickling, first cold rolling, continuous annealing, and second cold rolling. During the continuous annealing process, a specific annealing method is adopted to optimize the tissue morphology, and in the first cold rolling process, the last stand is rolled with a smooth roll to control the surface roughness of the steel plate within a suitable range. Thus, while ensuring that the steel has excellent low-temperature impact toughness, the strength and elongation of the steel are improved. The yield strength of the steel for battery cases prepared by the method of the present invention reaches 200MPa - 260MPa, the tensile strength is 320MPa - 380MPa, the elongation is not less than 35%, and excellent toughness is exhibited in a low-temperature environment (-20°C - -40°C), which can effectively avoid cracks and fractures. Detailed Embodiments

[0023] The following provides a detailed description of the specific embodiments of the present invention. It should be understood that the specific embodiments described herein are only for the purpose of illustrating and explaining the present invention, and are not used to limit the present invention.

[0024] In the ranges disclosed herein, the endpoints and any values are not limited to the exact range or value. These ranges or values should be understood to include values close to these ranges or values. For numerical ranges, between the endpoint values of each range, between the endpoint values of each range and individual point values, and between individual point values, they can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed herein.

[0025] The existing steel for battery cases is difficult to meet the application of batteries in extreme environments. Therefore, through research, the inventor provides a steel for battery cases that can exhibit excellent toughness in low-temperature environments and can effectively avoid cracks and fractures.

[0026] As described above, the steel for battery cases provided by the present invention contains the following chemical components in weight percentages: C: 0.001% - 0.004%, Si: 0.01% - 0.02%, Mn: 0.12% - 0.22%, P: 0.004% - 0.01%, S: ≤0.004%, Cr: 0.2% - 0.4%, Al: 0.02% - 0.05%, Ti: 0.015% - 0.035%, B: 0.001% - 0.003%, Nb: 0.01% - 0.03%, and the balance is Fe and unavoidable impurities.

[0027] The steel for battery cases described in the present invention contains specific components, and each component is controlled within a specific range. In particular, the C content, niobium content, and chromium content are controlled within a suitable range, which can ensure that the prepared steel has excellent toughness and strength in low-temperature environments (-20°C to -40°C).

[0028] Furthermore, the steel for battery cases has high strength and elongation. In some embodiments, the yield strength of the steel for battery cases is 200 MPa - 260 MPa, the tensile strength is 320 MPa - 380 MPa, and the elongation is ≥35%.

[0029] The following will detail the functions of each component and its content in the steel for battery cases.

[0030] Carbon (C): Maintaining an ultra-low carbon content can greatly reduce the brittleness of the steel at low temperatures, reduce the precipitation of carbide at grain boundaries, effectively improve low-temperature toughness. At the same time, trace carbon ensures the basic strength and meets the load-bearing requirements. Therefore, the present invention controls the C content to be 0.001% - 0.004%.

[0031] Silicon (Si): An appropriate amount of silicon enhances the strength of the steel, but too high a content will exacerbate low-temperature brittleness. Therefore, the present invention controls the Si content to be 0.01% - 0.02%, which can enhance the strength of the steel without damaging the low-temperature impact toughness.

[0032] Manganese (Mn): It can lower the transformation temperature from austenite to ferrite, refine the grains, and improve strength and toughness. Therefore, the present invention controls the Mn content within the range of 0.12% - 0.22%, which can optimize the comprehensive performance and ensure low-temperature stability and formability.

[0033] Phosphorus (P): A small amount of phosphorus has a strengthening effect, but it is prone to segregation. Excessive phosphorus will seriously reduce toughness and stamping performance. Therefore, in the present invention, the P content is controlled to be 0.004% - 0.01%, which can balance strength and low-temperature performance stability.

[0034] Sulfur (S): As a harmful element, sulfur forms sulfide inclusions, greatly reducing the toughness and workability of steel, especially reducing the toughness and workability of steel at low temperatures. Therefore, in the present invention, the sulfur content is controlled ≤ 0.004%, which can reduce the negative impact and improve the purity and low-temperature toughness of steel.

[0035] Aluminum (Al): Used for deoxidation and grain refinement, alumina particles are formed during the solidification of molten steel to promote grain refinement, enhancing the toughness and stamping performance of steel. In the present invention, controlling the Al content to be 0.02% - 0.05% can effectively optimize the microstructure and improve the comprehensive performance and low-temperature impact toughness of steel.

[0036] Chromium (Cr): Can improve the strength, hardness and corrosion resistance of steel, and cooperate with other elements to improve low-temperature performance. In the present invention, controlling the Cr content to be 0.2% - 0.4% can balance strengthening, corrosion resistance and workability, and contribute to the improvement of low-temperature impact toughness.

[0037] Titanium (Ti): As a strong carbide and nitride forming element, it fixes interstitial atoms, refines grains, and improves toughness and stamping performance. In the present invention, controlling the Ti content to be 0.015% - 0.035% can give full play to its role, avoid the formation of coarse precipitates in excess and affect performance, and ensure the low-temperature impact toughness of steel.

[0038] Boron (B): Trace amounts of boron can significantly improve the hardenability of steel, refine grains, and enhance the strength and toughness of steel at low temperatures. In the present invention, controlling boron within the range of 0.001% - 0.003% can effectively improve the low-temperature impact performance of steel.

[0039] Niobium (Nb): Niobium can combine with carbon and nitrogen in steel to form fine carbides and nitrides, effectively hindering grain growth, refining the grain structure, and improving the strength and toughness of steel. In the present invention, controlling the niobium content to be 0.01% - 0.03% can optimize the low-temperature impact performance of steel.

[0040] The second aspect of the present invention provides a method for preparing the steel for battery cases described above, and this method includes hot metal pretreatment, converter smelting, refining, continuous casting, hot rolling, coiling, pickling, primary cold rolling, continuous annealing and secondary cold rolling; Among them, continuous annealing includes a soaking section and a cooling section; The soaking temperature of the soaking section is 700°C - 720°C, and the time is 40s - 140s; The cooling process of the cooling section includes: first, rapidly cooling to 500 - 550°C at a rate of 12 - 16°C / s, and then cooling to 300 - 350°C at a rate of 5 - 8°C / s; During the first cold rolling process, the last stand is rolled with a smooth roll, and the surface roughness of the steel plate is controlled to be 0.3μm - 0.5μm.

[0041] The method of the present invention includes processes such as hot rolling, pickling, first cold rolling, continuous annealing, and second cold rolling. And during the continuous annealing process, the conditions of the soaking section and the cooling process of the cooling section are strictly controlled, so as to optimize the tissue morphology. And during the first cold rolling process, the last stand is rolled with a smooth roll, and the surface roughness of the steel plate is controlled within a suitable range, improving the strength and elongation of the steel while ensuring excellent low-temperature impact toughness of the steel.

[0042] In the present invention, the hot metal pretreatment process uses the KR method for desulfurization and the double slag method for dephosphorization to deeply purify the hot metal, strictly control the content of harmful elements such as sulfur and phosphorus at an extremely low level, and reduce the source of inclusion formation. Further, during the desulfurization process, the addition amount and stirring intensity of the desulfurizer can be adjusted to ensure the best desulfurization effect; during dephosphorization, the slag-making process and the lance position of the oxygen lance are precisely controlled to achieve efficient dephosphorization.

[0043] In the present invention, the converter smelting adopts the top and bottom combined blowing process to strengthen the molten bath stirring, promote the uniformity of the molten steel composition and temperature, and reduce the aggregation of inclusions caused by local composition segregation. During the blowing process, the oxygen supply intensity and the addition amount of slag-making materials are adjusted in a timely manner according to the changes in the molten steel composition and temperature to ensure the smooth progress of the smelting process.

[0044] In some embodiments, the refining process includes RH vacuum degassing, VD vacuum degassing treatment, and LF furnace refining. Specifically, adding VD vacuum degassing treatment on the basis of RH vacuum cyclic degassing treatment can further deeply remove gas impurities such as hydrogen and nitrogen in the molten steel and reduce the risk of gas inclusion formation; LF furnace refining makes a refining slag with high alkalinity and low melting point to deeply desulfurize and deoxidize the molten steel and adsorb the inclusions in the molten steel. During the LF furnace refining process, bottom blowing argon stirring is adopted to control the appropriate stirring intensity and time to promote the floating and removal of inclusions and avoid slag entrainment at the same time.

[0045] In some embodiments, the composition of the refining slag for LF furnace refining is of the CaO - Al2O3 - SiO2 system. Selecting the CaO - Al2O3 - SiO2 system for the refining slag composition can optimize the slag system composition and refining process parameters to ensure the refining effect.

[0046] In the present invention, electromagnetic stirring can be carried out during the continuous casting process, and full protection casting is adopted. Specifically, by using electromagnetic stirring technology, the solidification structure of the slab can be improved, the grains can be refined, and segregation can be reduced; by adopting full protection casting, secondary oxidation of the molten steel can be prevented, slag retaining walls, weirs and other devices are arranged in the tundish and the mold to effectively remove large inclusions in the molten steel and ensure the purity of the molten steel entering the mold. High-frequency vibration technology is adopted in the mold to improve the surface quality of the slab and reduce the surface defects and the entrainment of inclusions. The reduction of inclusions is beneficial to the qualification rate of stamping products on the one hand and to the improvement of the low-temperature toughness of the material on the other hand.

[0047] In some embodiments, during the hot rolling process, the heating temperature of the heating furnace is 1230°C to 1250°C, and the finishing rolling temperature is 880°C to 920°C. Specifically, the heating temperature is controlled at 1230°C to 1250°C. At high temperatures, the second-phase particles dissolve in the steel, which is beneficial to rolling. Fine particles precipitate again during the coiling process, which is beneficial to improving the stamping performance and corrosion resistance of the material; the finishing rolling temperature is controlled at 850°C to 920°C. This temperature can ensure that the finishing rolling temperature is controlled above the austenite temperature to avoid the mixed crystal phenomenon caused by two-phase zone rolling.

[0048] In some embodiments, the coiling temperature is 560°C to 600°C. Setting the coiling temperature to 560°C to 600°C can achieve fine and uniform grains and precipitates.

[0049] In the present invention, the pickling process can adopt an efficient process to remove mill scale and ensure the cleanliness of the plate surface. During the pickling process, the acid concentration, temperature and pickling time are controlled to ensure that the mill scale is removed thoroughly and the surface of the steel plate is not over-corroded, and to avoid introducing new inclusions or surface defects due to improper pickling.

[0050] In some embodiments, during the first cold rolling process, the total reduction ratio ≥ 82%. Specifically, multi-stand tandem cold rolling is adopted in the first cold rolling, and the total reduction ratio is controlled to be ≥ 82%, which can increase the grain distortion energy and reduce the recrystallization temperature. Among them, the last stand is light roll rolling, and the surface roughness of the steel plate is controlled to be 0.3μm to 0.5μm, which can improve the surface quality and is beneficial to improving the low-temperature impact performance. When the roughness < 0.3μm, it is very easy to cause deviation during continuous annealing production, resulting in production failures; when the roughness > 0.5μm, a low-roughness surface (roughness ≤ 0.6μm) cannot be achieved during the second cold rolling process, which will lead to stress concentration during the stamping process. In a low-temperature environment, stress concentration is likely to cause crack initiation and propagation, affecting the low-temperature impact resistance.

[0051] The continuous annealing process described in the present invention includes a soaking section, a cooling section and air cooling. Among them, the soaking temperature of the soaking section is controlled at 700℃~720℃, and the time is 40s~140s, which can achieve the effect of sufficient recrystallization, eliminate work hardening, and restore plastic toughness. The cooling process adopts variable temperature cooling, first rapidly cooling to 500~550℃ at 12~16℃ / s to inhibit unfavorable phase transformation; then cooling to 300~350℃ at 5~8℃ / s to promote further transformation of the organization and reduce internal stress; finally air cooling to room temperature. This variable temperature cooling can optimize the organizational morphology, and can improve strength and plasticity while ensuring low-temperature toughness.

[0052] In some embodiments, the reduction rate during the secondary cold rolling process is 3% to 5%. During the secondary cold rolling process, the secondary cold rolling reduction rate is controlled at 3% to 5%. The reduction rate is relatively low, and moderate processing further strengthens the steel, which can improve the strength and surface quality, ensure that the roughness of the secondary cold rolled finished product is ≤0.6μm, and retain good low-temperature impact performance, ensuring that the battery shell manufacturing precision requirements are met.

[0053] The present invention ensures that the steel has excellent toughness and strength in low temperature environment (-20℃~-40℃) by precisely adjusting the chemical composition and carefully optimizing each link of the production process. Specifically, the yield strength reaches 200MPa~260MPa, the tensile strength is in the range of 320MPa~380MPa, and the elongation is not less than 35%; and it shows excellent toughness during low temperature impact, effectively avoiding cracks and fractures, greatly improving the manufacturing yield and product quality of battery shells, laying a solid material foundation for the safe and stable operation of new energy batteries in low temperature environments, and effectively promoting the development of the new energy industry.

[0054] The present invention will be described in detail below by way of examples, but the protection scope of the present invention is not limited thereto. In the following examples, unless otherwise specified, the processes used are all conventional processes. Example 1

[0055] The battery shell steel provided in this embodiment contains the following chemical components in weight percentage: C: 0.0013%, Si: 0.010%, Mn: 0.12%, P: 0.004%, S: 0.003%, Cr: 0.20%, Al: 0.02%, Ti: 0.015%, B: 0.0011%, Nb: 0.023, and the balance is Fe and inevitable impurities.

[0056] The preparation method of the battery shell steel includes molten iron pretreatment, converter smelting, refining, continuous casting, hot rolling, coiling, pickling, primary cold rolling (acid rolling), continuous annealing and secondary cold rolling; The molten iron pretreatment process adopts KR method desulfurization and double slag method dephosphorization to deeply purify the molten iron; The converter smelting adopts the top and bottom combined blowing process to strengthen the bath stirring, promote the uniformity of molten steel composition and temperature, and reduce the inclusion aggregation caused by local composition segregation; The refining process includes RH vacuum degassing, VD vacuum degassing treatment and LF furnace refining. During the LF furnace refining process, argon gas is blown from the bottom for stirring, and the appropriate stirring intensity and time are controlled to promote the floating and removal of inclusions, while avoiding the phenomenon of slag entrainment; The refining slag composition of the LF furnace refining is of the CaO-Al2O3-SiO2 system; During the continuous casting process, electromagnetic stirring can be carried out, and full protection casting is adopted; High-frequency vibration technology is adopted in the mold to improve the surface quality of the billet, reduce surface defects and the entrainment of inclusions; During the hot rolling process, the heating temperature of the heating furnace is 1235°C, and the finishing rolling temperature is 885°C; The coiling temperature is 565°C; During the first cold rolling process, the total reduction ratio is 83%, the last stand is rolled with a bright roll, and the surface roughness of the steel plate is controlled to be 0.32μm; The continuous annealing process includes a soaking section, a cooling section and air cooling. Among them, the soaking temperature in the soaking section is 700°C, and the holding time is 40s; The cooling process in the cooling section adopts variable temperature cooling, first quickly cooling to 520°C at a rate of 14°C / s, then cooling to 330°C at a rate of 6°C / s, and finally air cooling to room temperature; During the second cold rolling process, the reduction ratio is 3%, and the surface roughness of the second cold rolling finished product is 0.46μm.

[0057] Examples 1-5 and Comparative Examples 1-7 were implemented according to the method of Example 1. The difference is that the content of the components in the steel for battery cases is different and the production process parameters are different. See Tables 1 and 2 for details.

[0058] Table 1 Chemical composition and content (wt%) Number C Si Mn P S Cr Al Ti Nb B Example 1 0.0013 0.010 0.12 0.004 0.003 0.20 0.02 0.015 0.023 0.0011 Example 2 0.0025 0.015 0.18 0.008 0.004 0.30 0.040 0.030 0.021 0.0023 Example 3 0.0033 0.020 0.20 0.009 0.004 0.35 0.045 0.032 0.015 0.0025 Example 4 0.0015 0.012 0.16 0.006 0.003 0.25 0.035 0.020 0.018 0.0015 Example 5 0.0040 0.020 0.22 0.010 0.004 0.40 0.050 0.035 0.026 0.0030 Comparative Example 1 0.0230 0.025 0.10 0.020 0.010 0.15 0.015 0.010 0.003 0.0005 Comparative Example 2 0.0018 0.008 0.30 0.003 0.002 0.45 0.040 0.025 0.022 0.0025 Comparative Example 3 0.0045 0.030 0.20 0.013 0.009 0.32 0.025 0.038 0.034 0.0041 Comparative Example 4 0.0036 0.016 0.18 0.018 0.007 0.60 0.041 0.050 0.021 0.0055 Comparative Example 5 0.0025 0.024 0.28 0.005 0.004 0.35 0.030 0.022 - 0.0022 Comparative Example 6 0.0035 0.010 0.15 0.010 0.008 0.40 0.018 0.030 0.015 0.0033 Comparative Example 7 0.0013 0.010 0.12 0.004 0.003 0.20 0.02 0.015 0.023 0.0011 Table 2 Production process parameters Number Heating Temperature / °C Final Rolling Temperature / °C Coiling Temperature / °C Reduction Ratio / % Pickling and Rolling Roughness / μm Soaking Annealing Temperature in Soaking Section / °C Soaking Time in Soaking Section / s Fast Cooling Rate / °C / s Reduction Ratio of Secondary Cold Rolling / % Final Product Roughness / μm Example 1 1235 885 565 83 0.32 700 40 14 3.0 0.46 Example 2 1230 913 572 85 0.46 718 125 16 3.5 0.55 Example 3 1233 893 585 87 0.43 710 140 12 4.1 0.54 Example 4 1240 885 595 84 0.35 705 64 13 3.6 0.46 Example 5 1242 890 570 89 0.35 712 133 14 4.2 0.43 Comparative Example 1 1236 880 620 80 0.34 730 180 14 2.6 0.45 Comparative Example 2 1244 883 570 86 0.46 726 30 10 3.6 0.55 Comparative Example 3 1295 894 550 88 0.49 715 130 12 3.3 0.59 Comparative Example 4 1275 893 575 82 0.42 705 70 13 5.5 0.42 Comparative Example 5 1234 892 580 84 0.34 725 50 14 2.4 0.45 Comparative Example 6 1190 905 595 87 0.35 693 110 15 3.2 0.48 Comparative Example 7 1235 885 565 83 0.82 700 40 14 3.0 0.96 Test Example The mechanical property values and low-temperature impact resistance detection conditions of the final products of the test examples and comparative examples are shown in Table 3.

[0059] The low-temperature impact resistance test method is as follows: Process the steel plate into round pieces, and then stamp them into sample cups in the shape of cylindrical battery cases. Cool and keep them warm in an environment of -40°C for 10 minutes respectively. Lift the hammer head of the testing machine to a height of 1000mm ± 5mm. Take out the sample cup from the cooling device, place it upright on the base directly below the hammer head, and release the hammer to let it fall freely to impact the sample cup. The process from taking out the sample cup from the cooling device to impacting the sample cup should be completed within 3 seconds. Observe whether the sample cup is cracked. If it is cracked, it indicates that the low-temperature toughness is unqualified; otherwise, the low-temperature toughness is qualified.

[0060] The test methods for yield strength Rp0.2, tensile strength Rm, and elongation A50 are carried out in accordance with the requirements of the national standard GB / T228 "Test Method for Tensile Test of Metallic Materials at Room Temperature".

[0061] Table 3 Mechanical Properties

[0062] From the results in Table 3, it can be seen that the mechanical properties of Examples 1 to 5 can meet the requirements of yield strength of 200MPa to 260MPa, tensile strength of 320MPa to 380MPa, and elongation ≥ 35%, and the low-temperature impact resistance tests are all qualified.

[0063] Comparative Example 1 uses the composition of ordinary aluminum-killed steel, with a low elongation rate of performance, and the low-temperature impact test result is unqualified.

[0064] In Comparative Example 2, the annealing fast cooling rate is too slow, resulting in coarse grains and ultimately low strength.

[0065] In Comparative Example 3, the carbon content exceeds 0.004%, and the niobium element content also exceeds 0.03%, ultimately resulting in high performance values and low elongation rate, and cracking is likely to occur during the stamping process.

[0066] In Comparative Example 4, the chromium element exceeds 0.4%, the elongation rate is unqualified, and cracking is likely to occur during the stamping process.

[0067] Comparative Example 5 does not contain niobium element, and finally the yield strength is low, and the low-temperature impact test is unqualified.

[0068] In Comparative Example 6, due to the low annealing temperature, the strength is high, the elongation rate is low, and the low-temperature impact resistance is unqualified.

[0069] In Comparative Example 7, the acid rolling roughness is high, the internal stress of the material increases after stamping, and the low-temperature impact resistance is unqualified.

[0070] It should be understood that the parts not elaborated in detail in this specification all belong to the prior art.

[0071] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited thereto. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solutions of the present invention, including any other suitable combination of each technical feature. These simple modifications and combinations should also be regarded as the content disclosed by the present invention and fall within the protection scope of the present invention.

Claims

1. A steel for battery case, characterized in that, The steel used for the battery case contains the following chemical components by weight percentage: C: 0.001% - 0.004%, Si: 0.01% - 0.02%, Mn: 0.12% - 0.22%, P: 0.004% - 0.01%, S: ≤0.004%, Cr: 0.2% - 0.4%, Al: 0.02% - 0.05%, Ti: 0.015% - 0.035%, B: 0.001% - 0.003%, Nb: 0.01% - 0.03%, and the balance is Fe and unavoidable impurities.

2. The steel for battery case according to claim 1, characterized in that, The yield strength of the steel used for the battery case is 200 MPa - 260 MPa, the tensile strength is 320 MPa - 380 MPa, and the elongation is ≥35%.

3. A method for preparing the steel for the battery case according to claim 1 or 2, characterized in that, The method includes hot metal pretreatment, converter smelting, refining, continuous casting, hot rolling, coiling, pickling, first cold rolling, continuous annealing, and second cold rolling; Among them, the continuous annealing includes a soaking section and a cooling section; The soaking temperature in the soaking section is 700°C - 720°C, and the time is 40 s - 140 s; The cooling process in the cooling section includes: first rapidly cooling at 12 - 16°C / s to 500 - 550°C, and then cooling at 5 - 8°C / s to 300 - 350°C; During the first cold rolling process, the last stand is a bright roll rolling, and the surface roughness of the steel plate is controlled to be 0.3 μm - 0.5 μm.

4. The method according to claim 3, wherein In the hot metal pretreatment, the KR method is used for desulfurization and the double slag method is used for dephosphorization.

5. The method according to claim 3, wherein The refining process includes RH vacuum degassing, VD vacuum degassing treatment, and LF furnace refining.

6. The method according to claim 5, wherein The refining slag composition of the LF furnace refining is of the CaO - Al2O3 - SiO2 system.

7. The method according to claim 3, characterized in that During the continuous casting process, electromagnetic stirring is carried out, and full protection casting is adopted.

8. The method according to claim 3, wherein During the hot rolling process, the heating temperature of the heating furnace is 1230°C - 1250°C, and the finish rolling temperature is 880°C - 920°C; And / or, the coiling temperature is 560°C - 600°C.

9. The method according to claim 3, characterized in that, During the first cold rolling process, the total reduction rate is ≥82%.

10. The method according to claim 3, characterized in that, During the second cold rolling process, the reduction rate is 3% - 5%, and the surface roughness of the second cold rolling finished product is ≤0.6 μm.

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