Steel with high surface hardness for battery shell and preparation method

Abstract

The invention discloses steel with high surface hardness for a battery shell and a preparation method. The steel comprises the following chemical components in percentage by weight: 0.004 to 0.007 percent of C, 0.01 to 0.03 percent of Si, 0.25 to 0.40 percent of Mn, 0.004 to 0.01 percent of P, less than or equal to 0.002 percent of S, 0.05 to 0.1 percent of Cr, 0.02 to 0.05 percent of Al, 0.035 to 0.055 percent of Ti, 0.001 to 0.003 percent of B, 0.02 to 0.04 percent of Nb and the balance of Fe and inevitable impurities. By optimizing the chemical components and the production process, the steel shows good deformation uniformity in the high-speed stamping process, the flat elongation is effectively improved, stamping defects are reduced, meanwhile, the surface hardness of the shell is remarkably improved, and the mechanical damage resistance of the battery shell is enhanced.

Description

Technical Field

[0001] The present invention belongs to the technical field of metal materials, and in particular relates to a steel for battery shells with high surface hardness and a preparation method thereof. Background Art

[0002] With the vigorous development of the new energy industry, the performance requirements for battery shell steel are becoming increasingly stringent. In the high-speed stamping production process, there are many problems with the existing battery shell steel. On the one hand, the elongation of some high-strength steels is low, resulting in poor deformation uniformity of the material during stamping, which is prone to local stress concentration, and then produces defects such as cracks and wrinkles, which seriously affect the molding quality and production efficiency of the battery shell. On the other hand, the surface hardness of existing steel grades is also difficult to meet the requirements, which makes the battery shell insufficient to resist external mechanical damage and internal expansion during subsequent use, reducing the safety and reliability of the battery.

[0003] Common battery casing steels currently on the market, such as aluminum-killed steel and IF steel, have certain performance advantages, but still leave much room for improvement in overall performance optimization. Some patents involve complex and costly production processes and fail to effectively address the synergistic improvement of elongation and surface hardness. For example, the following patents struggle to meet market demand for high-performance, low-cost battery casing steel.

[0004] Announcement No. CN1174109C, "Ultra-thin steel strip for battery shells and its manufacturing method", reports an ultra-thin steel strip for battery shells and its production method. The weight percentage of its chemical composition is: 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 remainder is Fe. The P element in this patent is relatively high, which makes it difficult to meet the requirements of high-speed stamping. At the same time, single-stand rolling will result in a large difference in thickness between the head and tail, and poor consistency of the stamped shell.

[0005] Publication No. CN100560770C, "Steel for Battery Cases with Excellent Planar Isotropy and Its Manufacturing Method," reports a battery case steel and its manufacturing method. The battery case steel's chemical composition by weight is as follows: 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%, with the remainder being Fe and unavoidable impurities. The production process includes hot metal pretreatment, converter smelting, furnace refining, hot rolling, pickling, cold rolling, bell annealing, leveling, and finishing into finished coils. This patent primarily addresses aluminum-killed steel systems, which have poor plasticity and toughness, making them unsuitable for high-speed stamping.

[0006] Announcement No. CN102286699B "Corrosion-resistant battery shell steel with a punching speed ≥150 per minute and its preparation method" reports a rapid stamping battery shell steel and its preparation method. The chemical composition weight percentage of the battery shell steel is as follows: 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 P≤0.020%, S≤0.0150%, Cu≤0.050%, Ni≤0.050%, Cr≤0.080%, Mo≤0.050%, Si≤0.02%, and the rest is Fe and unavoidable impurities. Production steps: smelting and continuous casting into billets using a pure steel process; heating the continuous cast billets; rough rolling; finish rolling in the single-phase austenite region; coiling; pickling; cold rolling; degreasing; annealing in a full hydrogen bell furnace; leveling and preparation for use. This patented method uses conventional bell annealing, which results in significant fluctuations in coiling performance, long holding times during annealing, rapid grain growth, and poor ductility.

[0007] Publication number CN106148803A, "A Method for Producing Deep-Drawn Battery Case Steel," discloses a method for producing deep-drawn battery case steel. The steel's chemical composition by weight is as follows: 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%, with the remainder being Fe. This patent utilizes an aluminum-killed steel system, resulting in relatively poor stamping performance and difficulty meeting the high-speed stamping requirements for battery case steel. Furthermore, the patent does not address the cold rolling process. Summary of the Invention

[0008] The purpose of the present invention is to solve the above technical problems and provide a battery shell steel with high surface hardness, excellent elongation and suitable for high-speed stamping and a production process thereof.

[0009] To achieve the above objectives, the present invention provides a steel for a battery shell with high surface hardness, the chemical composition of the steel comprising, by weight percentage, C: 0.004-0.007%, Si: 0.01-0.03%, Mn: 0.25-0.40%, P: 0.004-0.01%, S: ≤0.002%, Cr: 0.05-0.1%, Al: 0.02-0.05%, Ti: 0.035-0.055%, B: 0.001-0.003%, Nb: 0.02-0.04%, and the rest being Fe and unavoidable impurities.

[0010] The role of the alloying elements in the steel grades of the present invention is mainly based on the following principles: Carbon (C): Controlling the carbon content at a lower level can not only reduce the precipitation of carbides at grain boundaries and reduce the brittleness of steel, but also ensure a certain strength foundation to meet the load-bearing requirements of the battery shell. Control C: 0.004~0.007%.

[0011] Silicon (Si): A moderate amount of silicon can strengthen the steel matrix, but too high a content will affect the toughness and stamping performance of the steel. It should be controlled within a reasonable range to balance strength and other properties. The Si content should be controlled at 0.01~0.03%.

[0012] Manganese (Mn): reduces the phase transition temperature from austenite to ferrite, refines grains, improves the strength and toughness of steel, and enhances deformation capacity during high-speed stamping. The Mn content is controlled at 0.25-0.40%, preferably 0.30-0.36%.

[0013] Phosphorus (P): A small amount of phosphorus can play a strengthening role, but excessive phosphorus will lead to a decrease in toughness and stamping performance. Its content must be strictly controlled to ensure stable overall performance. The P content is controlled at 0.004~0.01%.

[0014] Sulfur (S): As a harmful element, it easily forms sulfide inclusions, which reduces the performance of steel. The sulfur content should be controlled at an extremely low level to improve the purity of steel and control S≤0.002%.

[0015] Chromium (Cr): Improves the strength, hardness and corrosion resistance of steel, and works with other elements to improve the performance of steel during high-speed stamping and use. Control Cr: 0.05~0.1%, preferably 0.08~0.095%.

[0016] Aluminum (Al): used for deoxidation and grain refinement, forming alumina particles when the molten steel solidifies, optimizing the organizational structure, improving toughness and stamping performance. Control Al: 0.02~0.05%, preferably 0.03~0.042%.

[0017] Titanium (Ti): Combined with carbon, nitrogen and other elements, it fixes interstitial atoms, refines grains, improves the toughness and stamping performance of steel, and avoids the formation of coarse precipitates. Control Ti: 0.035~0.055%, preferably 0.04~0.048%.

[0018] Boron (B): Trace amounts of boron can significantly refine grains and enhance the strength and toughness of steel under low-temperature and high-speed stamping conditions. The B content is controlled at 0.001~0.003%, preferably 0.002~0.0028%.

[0019] Niobium (Nb): forms fine carbides and nitrides with carbon and nitrogen, hindering grain growth, refining grain structure, improving strength and toughness of steel, and optimizing stamping performance. Control Nb: 0.02~0.04%, preferably 0.025~0.033%.

[0020] When Ti, B, and Nb coexist, this combination forms a unique "trinity" nucleation and grain boundary control mechanism. During the continuous casting process, the nanoscale TiC, TiN, NbC, and NbN precipitates formed by Ti and Nb not only serve as heterogeneous nucleation cores, but also have extremely high stability at high temperatures. These precipitates form a "lattice trap" at the crystallization front, making iron atoms more inclined to nucleate around them during crystallization, thereby greatly improving the nucleation rate. At the same time, B atoms will form a layer of "atomic barrier" at the grain boundaries, which cooperates with the precipitates of Ti and Nb to further inhibit the migration of grain boundaries. This synergistic effect improves the grain refinement effect by more than 50% compared to traditional processes.

[0021] When Mn, Cr, and Al work together, Mn refines the grains by lowering the austenite-to-ferrite phase transition temperature, increasing the number of nuclei and inhibiting grain growth, thus providing a fine-grained foundation for uniform deformation of the steel. Cr, while dissolving in the ferrite matrix, induces lattice distortion to achieve solid solution strengthening, while also promoting the formation of fine, dispersed carbides, hindering grain boundary migration and dislocation motion. Al, through deoxidation, generates particles that serve as heterogeneous nucleation cores, while the nitrides formed simultaneously pin the grain boundaries, further refining the grains and improving the purity of the molten steel. These three elements work together to form a "fine-grained matrix + dispersed strengthening phase + high purity" microstructure in the steel, optimizing the match between grain boundary strength and matrix toughness. During high-speed stamping, the fine-grained structure promotes uniform deformation, the dispersed carbides suppress local stress concentration, and the high purity reduces inclusion-induced defects, resulting in excellent deformation uniformity.

[0022] A method for preparing the steel as described above is also provided, wherein the production process is as follows: molten iron pretreatment → converter smelting → refining → continuous casting → flame cleaning → hot rolling and coiling → pickling → cold rolling → continuous annealing → leveling; Hot metal pretreatment: Utilizing a combination of KR desulfurization and dual-slag dephosphorization technologies, the desulfurizer dosage and stirring intensity are precisely controlled based on real-time hot metal composition and temperature data to ensure a stable sulfur content below 0.002%. During the dephosphorization stage, a slag-forming process optimization model is employed to precisely control the oxygen lance position and slag-forming material addition timing, achieving a dephosphorization rate exceeding 90%, effectively reducing the hot metal phosphorus content to below 0.01%, and significantly minimizing the sources of inclusion formation.

[0023] Converter smelting: Utilizing dynamic top-bottom combined blowing technology, we monitor the composition and temperature of the molten steel in the molten pool in real time, adjusting the oxygen supply and slag-forming material addition to ensure uniform molten steel composition and minimize temperature fluctuations. Furthermore, we employ pulsed bottom-blowing argon to enhance stirring, promote inclusion floating, effectively reduce inclusion accumulation, and improve molten steel purity.

[0024] Refining: An integrated refining process is adopted, organically combining RH vacuum cycle degassing, VD vacuum degassing, and LF furnace refining to achieve deep purification of molten steel. During the RH and VD degassing processes, the vacuum degree, degassing time, and cooling rate are precisely controlled to reduce the hydrogen content to below 1.5ppm and the nitrogen content to below 30ppm. During LF furnace refining, a self-developed high-activity refining slag is used. This slag is based on the CaO-Al2O3-SiO2 system and contains special active ingredients, which significantly improves the desulfurization, deoxidation, and inclusion adsorption capabilities. By optimizing the bottom-blown argon stirring parameters and adopting a variable-intensity stirring method, the inclusions are fully floated and removed, while slag curling is avoided, thereby improving the quality of the molten steel.

[0025] Continuous Casting: During the continuous casting process, a combination of electromagnetic stirring and automatic mold level control technology is employed. The electromagnetic stirring device dynamically adjusts the stirring current and frequency based on the cross-sectional dimensions of the ingot, the characteristics of the steel grade, and the casting speed, thereby refining the ingot grains, improving the solidification structure, and reducing segregation. Automatic mold level control technology utilizes high-precision sensors to monitor the liquid level in real time. By automatically adjusting the opening of the stopper rod or slide, liquid level fluctuations are controlled within ±3mm, ensuring the surface quality of the ingot and reducing the occurrence of surface defects. Furthermore, a special mold slag is used in the mold to optimize its performance, improve the lubrication between the ingot and the mold wall, and further reduce the incidence of surface defects.

[0026] Flame cleaning: The continuous casting billet is flame cleaned before entering the subsequent process. The high-temperature flame is used to rapidly heat the billet surface, causing the iron oxide scale, inclusions and minor defects on the billet surface to oxidize, melt and peel off under the action of high temperature. The flame cleaning speed is controlled at 0.5~0.8m / min, the oxygen pressure is maintained at 0.6~0.8MPa, and the propane pressure is controlled at 0.05~0.08MPa. Ensuring a uniform cleaning depth can effectively remove surface defects without causing excessive damage to the billet matrix. The surface quality of the billet after flame cleaning is significantly improved, providing a better foundation for subsequent hot rolling, cold rolling and other processes, and reducing product quality problems caused by surface defects.

[0027] During hot rolling and coiling, the heating temperature is controlled between 1230°C and 1250°C. At high temperatures, the second-phase particles dissolve in the steel, facilitating rolling. Fine particles are then reprecipitated during the coiling process, improving the material's stamping performance and corrosion resistance. The final rolling temperature is controlled between 850°C and 920°C, ensuring it remains above the austenite temperature and avoiding the formation of mixed crystals caused by rolling in the two-phase region. The coiling temperature is set between 560°C and 600°C to achieve fine and uniform grains and precipitates.

[0028] Pickling and Rolling: Pickling removes scale and ensures a clean plate surface. Cold rolling is performed using multiple stands with a total reduction of ≥85% to increase grain distortion energy and lower the recrystallization temperature. The final stand uses smooth rollers to control the roughness to 0.3-0.5 μm, improving surface quality. During the pickling process, the acid concentration, temperature, and pickling time are controlled to ensure thorough removal of scale without excessive corrosion of the steel plate surface. This prevents the introduction of new inclusions or surface defects due to improper pickling.

[0029] Continuous annealing: Heating to 700-720°C at a heating rate of 20-30°C / s, followed by cooling to 340-350°C at a cooling rate of 12-16°C / s using hydrogen, and then heating to 360-370°C at a heating rate of 5-8°C / s. After continuous annealing, leveling is performed using a small roller diameter mode with a reduction ratio of 1.0-1.5%. Precisely controlling the heating rate to 20-30°C / s reduces the time the steel spends at high temperatures and effectively inhibits grain growth. Hydrogen is introduced during the cooling process, initially rapidly cooling to 340-350°C at a rate of 12-16°C / s. Rapid cooling not only helps suppress unfavorable phase transformations and stabilizes the steel's microstructure, but also significantly improves its strength and hardness. This is because rapid cooling prevents the supersaturated solid solution in the steel from fully decomposing, retaining more solute atoms and producing a solid solution strengthening effect. Furthermore, the high density of dislocations formed during rapid cooling enhances dislocation strengthening, effectively improving strength and hardness. Then heat it to 360~370℃ at 5~8℃ / s. Heating within this temperature range increases the over-aging effect, promotes the precipitation of interstitial atoms such as carbon and nitrogen in the steel, optimizes the internal structure of the steel, and significantly improves the stamping ability of the product.

[0030] Flattening: After annealing, the steel is lightly flattened using a small roller diameter and a reduction ratio of 1.2-1.5%. This cold rolling process not only further improves the surface flatness of the steel, but also slightly increases the surface hardness through work hardening, while also making the surface hardness distribution more uniform.

[0031] During the annealing process, when the steel plate is rapidly cooled to 340-350°C at a rate of 12-16°C / s, the Ti and Nb atoms are forced to remain in the ferrite matrix in a supersaturated state due to limited diffusion rate, while the B atoms form a metastable segregation layer at the grain boundaries. This supersaturated state provides sufficient precipitation driving force for the subsequent aging process. At the same time, since the surface of the steel plate is in direct contact with the hydrogen cooling medium, the cooling rate is 10%-15% faster than that of the core, prompting the Ti and Nb atoms in the surface layer to preferentially precipitate nano-scale carbonitrides (size 5-10nm), forming a surface "precipitation strengthening shell", while the core cools slightly slower, and the precipitate phase size is slightly larger (10-15nm), forming a "tough core", achieving a gradient distribution of precipitation phases between the surface and the core. During the leveling process, the surface layer dislocation density (10 12 ~10 13 cm -2 ) is 30% higher than in the core. The high-density dislocations act as diffusion channels for Ti and Nb atoms, prompting nano-precipitates (TiC and NbC) to align along dislocation lines (with spacing of 50-100 nm), forming a "dislocation-precipitate strengthening network." This oriented distribution extends the dislocation slip path in the surface layer by 40%, significantly improving the surface hardness.

[0032] Compared with the existing technology, the present invention has the following advantages: by optimizing the chemical composition and production process, the present invention enables the steel to exhibit good deformation uniformity during high-speed stamping, effectively improves the flat elongation, reduces stamping defects, and at the same time, significantly improves the surface hardness of the shell and enhances the battery shell's resistance to mechanical damage. The steel plate has a yield strength of 230-290MPa, a tensile strength of 350-410MPa, an elongation of not less than 35%, and a surface hardness of 130≤HV 0.1 ≤160, core hardness 110≤HV5≤130, thereby greatly improving the manufacturing quality and production efficiency of the battery shell, providing reliable material guarantee for the safe and stable operation of new energy batteries, and promoting the further development of the new energy industry. DETAILED DESCRIPTION

[0033] The present invention will be further described below with reference to specific embodiments.

[0034] The chemical composition of the molten steel is shown in Table 1. The remainder is Fe and unavoidable impurity elements. Examples 1 to 5 and Comparative Examples 1 to 6 were not smelted according to the smelting scheme of the present invention.

[0035] Table 1 Chemical composition, wt% All production processes were trial-produced according to Table 2, and the finished product thickness specification was 0.30mm. The large reduction rate can achieve finer grains at the edge.

[0036] Table 2 Production process The mechanical properties and surface hardness of the final product as well as the high-speed stamping conditions are shown in Table 3. The surface hardness test was conducted using a Vickers hardness tester, and the HV values ​​were measured at different locations on the steel plate surface (5 mm apart). 0.1 The surface hardness data was obtained by taking the average value of HV5 hardness and the high-speed stamping test was completed on a steel shell stamping equipment with a speed of 150 pieces / min.

[0037] Table 3 Mechanical properties The above description only provides a specific exemplary description of the present invention. It should be noted that the specific implementation of the present invention is not limited to the above-mentioned method. As long as various non-substantial improvements are made using the technical concept and technical solution of the present invention, or the technical concept and technical solution of the present invention are directly applied to other occasions without improvement, they are all within the scope of protection of the present invention.

Claims

1. A steel for battery shell with high surface hardness, characterized by: The chemical composition of the steel includes, by weight percentage, C: 0.004-0.007%, Si: 0.01-0.03%, Mn: 0.25-0.40%, P: 0.004-0.01%, S: ≤0.002%, Cr: 0.05-0.1%, Al: 0.02-0.05%, Ti: 0.035-0.055%, B: 0.001-0.003%, Nb: 0.02-0.04%, and the rest is Fe and unavoidable impurities.

2. The steel for battery case with high surface hardness according to claim 1, characterized in that: The weight percentage of Mn is 0.30-0.36%.

3. The steel for battery case with high surface hardness according to claim 1, characterized in that: The weight percentage of Cr is 0.08-0.095%.

4. The steel for battery case with high surface hardness according to claim 1, characterized in that: The weight percentage of Al is 0.03-0.042%.

5. The steel for battery case with high surface hardness according to claim 1, characterized in that: The weight percentage of Ti is 0.04-0.048%.

6. The battery case steel with high surface hardness according to claim 1, characterized in that: The weight percentage of B is 0.002-0.0028%.

7. The steel for battery case with high surface hardness according to claim 1, characterized in that: The weight percentage of Nb is 0.025-0.033%.

8. A method for preparing the steel according to any one of claims 1 to 8, wherein the production process is as follows: molten iron pretreatment → converter smelting → refining → continuous casting → flame cleaning → hot rolling and coiling → pickling → cold rolling → continuous annealing → leveling; characterized in that: In the continuous annealing process, the steel sheet is heated to 700-720°C at a heating rate of 20-30°C / s, cooled to 340-350°C at a cooling rate of 12-16°C / s by filling with hydrogen, and then heated to 360-370°C at a heating rate of 5-8°C / s. After the continuous annealing is completed, the steel sheet is leveled using a small roller diameter mode with a reduction rate of 1.0-1.5%.

9. The method for preparing steel according to claim 8, characterized in that: In the refining process, during the RH and VD degassing processes, the hydrogen content is reduced to below 1.5 ppm and the nitrogen content is controlled below 30 ppm; in the flame cleaning process, the flame cleaning speed is 0.5-0.8 m / min, the oxygen pressure is 0.6-0.8 MPa, and the propane pressure is 0.05-0.08 MPa.

10. The method for preparing steel according to claim 8, characterized in that: In the hot rolling and coiling process, the heating temperature is 1230-1250°C, the finishing temperature is 850-920°C, and the coiling temperature is 560-600°C.

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