Steel punching sand hole defect full-process control method for new energy battery shell

Abstract

The invention discloses a steel punching sand hole defect full-process control method for a new energy battery shell, and belongs to the technical field of ferrous metallurgy. The method takes whole-process low-temperature and low-oxygen-potential cooperative control as a core, and comprises the following steps: carrying out desulfurization and dephosphorization pretreatment on molten iron; smelting in a converter at a low superheat degree, wherein the tapping temperature is 1580-1620 DEG C; steel ladle top slag modification is carried out in the tapping process, and top slag (FeO + MnO) is controlled to be smaller than or equal to 5%; rH vacuum deep refining is combined with step-by-step calcium treatment; the LF is only used for weak stirring and accurate temperature adjustment; continuous casting adopts a submersed nozzle upper seat brick annular argon blowing and stopper six-hole argon blowing composite technology; the invention further discloses an optimized rolling and annealing process. Through systematic process route design, large-size inclusions are cooperatively controlled from multiple dimensions of source inhibition, process removal and tail end blockage prevention, finally, the content of inclusions larger than or equal to 5 microns in the battery shell steel is reduced to be smaller than or equal to 1 piece / mm, the punching sand hole defect rate is controlled to be 0.1% or below, the purity and forming reliability of the battery shell steel are remarkably improved, and the product quality is improved. The method is suitable for production of high-end new energy battery shells.

Description

Technical Field

[0001] This invention belongs to the field of iron and steel metallurgy technology and relates to a production method of cold-rolled steel sheet for new energy battery shells. Specifically, it relates to a method for the whole-process coordinated control of sand hole defects that occur during the stamping process. Through the coordinated operation of the whole process of molten iron pretreatment, converter smelting, RH vacuum refining, LF temperature control, continuous casting and rolling, the method can achieve precise control of large-size inclusions. Background Technology

[0002] With the rapid development of the new energy vehicle industry, extremely high requirements have been placed on the safety, reliability, and service life of power batteries. As a core structural component of power batteries, the battery casing is typically made from ultra-deep-drawn cold-rolled steel sheets through multiple stamping processes. During the stamping process, if large-sized (typically ≥5μm) non-metallic inclusions (such as Al2O3, SiO2, CaO-Al2O3-SiO2 composite inclusions, etc.) exist inside the steel sheet, they will form stress concentration points at the deformation sites due to the difference in plasticity between them and the steel matrix. This can induce microcracks, ultimately forming "sandhole" defects on the surface or inside the battery casing. Such defects severely impair the airtightness and structural strength of the battery casing, and are one of the key risk sources leading to battery pack leakage, short circuits, and even thermal runaway.

[0003] Currently, there are three core problems with the technology for controlling inclusions in battery casing steel: First, the process route lacks a holistic approach to temperature control throughout the entire process. Excessive converter tapping temperature leads to a surge in the oxidizability of the molten steel, increasing inclusion formation at its source. Second, inclusion control relies excessively on LF refining, failing to fully utilize the deep purification advantages of RH vacuum treatment, and single-stage control is insufficient to address the evolution of inclusions throughout the entire process. Third, the oxidation control of the ladle slag during converter tapping is neglected. Excessive FeO+MnO content in the slag results in continuous oxygen supply to the molten steel through slag-metal interface mass transfer after tapping, leading to secondary formation of Al2O3 inclusions. Existing technologies generally employ RH treatment, but this lacks effective synergy with low converter tapping temperature, slag modification, and precise LF temperature control. Furthermore, turbulence problems easily occur during continuous casting, further introducing large-sized foreign inclusions. This unreasonable process layout results in a persistently high rate of sand holes in the stamping of battery casing steel, failing to meet the quality requirements of high-end battery casings.

[0004] Therefore, developing a method that can suppress inclusion formation at the source and coordinate the control of the number, size and shape of inclusions throughout the entire process is the key to solving the sand hole defect in the stamping of new energy battery shell steel and improving product quality.

[0005] This invention proposes a method for developing a process with full-process temperature control as the core, integrating ladle top slag modification, and optimizing the process route as follows: molten iron pretreatment—converter smelting—ladle top slag modification—RH vacuum refining—LF temperature control—continuous casting—rolling. This method emphasizes innovation in RH inclusion control, weakens the function of LF, and solves the problem of nozzle clogging in continuous casting. It fundamentally inhibits inclusion formation, enhances the deep purification capability of RH, and matches the continuous casting requirements through precise LF temperature control, thereby solving key technical challenges in the current battery casing steel production field. Summary of the Invention

[0006] The purpose of this invention is to overcome the problems of unreasonable process routes, lack of temperature control, reliance on single-stage control of inclusions, and neglect of top slag oxidation in existing technologies, and to provide a comprehensive method for controlling stamping sand hole defects in battery casing steel. This method is based on the core idea of ​​"full-process low-temperature and low-oxygen potential control." By optimizing the entire process route from molten iron to steel rolling, and synergistically controlling the physicochemical conditions of each stage, it aims to fundamentally inhibit the formation of inclusions, promote their removal, and prevent their reintroduction. Ultimately, it reduces the content of large inclusions ≥5μm inside the steel plate to ≤1 inclusion / mm², and controls the stamping sand hole defect rate to below 0.1%.

[0007] The core of this invention is to establish a process route of low-temperature tapping in the converter, top slag modification and oxygen potential reduction in the ladle, deep purification by RH, and precise temperature control by LF. By controlling the tapping temperature in the converter and modifying the top slag, the generation of oxidizing inclusions is suppressed. The refining function of RH in inclusion removal is highlighted, while the function of LF is weakened and used only for molten steel temperature control. Combined with argon blowing technology in the tundish of continuous casting to eliminate nozzle turbulence, the stamping sand hole defect rate of battery shell steel is finally reduced to below 0.1%.

[0008] To achieve the above objectives, the present invention adopts the following technical solution: A comprehensive method for controlling pinhole defects in steel stamping for new energy battery casings, with temperature control as the core, employs a process route of molten iron pretreatment—converter smelting—ladle top slag modification—RH vacuum refining—LF temperature control—continuous casting—rolling, with each stage working together to control large-size inclusions. Specific steps include: (1) Cooperative control of steelmaking and refining processes 1) Hot metal pretreatment: A dual-station pretreatment process of desulfurization and dephosphorization is adopted to deeply remove harmful elements from the hot metal before it enters the converter, creating conditions for low superheat smelting in the converter. The desulfurizing agent is a CaO-MgO-Al2O3 composite desulfurizing agent, with an addition amount of 8~12 kg / t hot metal, a stirring speed of 60~80 r / min, and a treatment time of 15~20 min, controlling the sulfur content of the hot metal to ≤0.005%; the dephosphorization agent is a CaO-Fe2O3-MnO oxidizing dephosphorizing agent, with an addition amount of 10~15 kg / t hot metal, an oxygen blowing intensity of 0.8~1.2 m³ / (t·min), and a treatment time of 8~12 min, controlling the phosphorus content of the hot metal to ≤0.008%, reducing the formation of phosphide inclusions during subsequent smelting.

[0009] 2) Converter Smelting: A low-lance position enhanced decarburization and precise end-point temperature control process is employed to strengthen converter tapping temperature control, providing the necessary temperature conditions for the full melting of the top slag modifier and preventing excessively high temperatures from causing a surge in steel oxidation. During the initial blowing stage, the lance position is controlled at 1.2~1.5m to enhance the carbon-oxygen reaction and reduce the final oxygen content. At the end of the blowing process, the steel temperature is strictly controlled at 1580~1620℃, and the carbon content is controlled at 0.06~0.09%. Argon is pre-blown into the ladle for 5~8 seconds at a flow rate of 0.2~0.25 m³ / (t·min) to prevent secondary oxidation of the steel. End-point deoxidation uses a combination of Si-Mn alloy and Al, with Si-Mn alloy added at 8~10 kg / t steel and Al added at 0.2~0.3 kg / t steel, controlling the oxygen content of the steel to ≤20ppm, laying the foundation for top slag modification and deep RH purification.

[0010] 3) Ladle Top Slag Modification: During and after the converter tapping process, the oxidizing properties of the top slag are reduced by adjusting its composition, providing a compositional basis for controlling Al2O3 inclusions. The modifier uses a CaO-Al2O3-MgO-CaF2 composite system, with the following mass percentages: CaO 55-65%, Al2O3 15-20%, MgO 8-12%, CaF2 5-8%, and metallic Al 2-5%. The melting point of the modifier is controlled at 1300-1350℃ to ensure complete melting at the converter tapping temperature. The addition amount is adjusted according to the converter capacity: 8-10 kg / t of steel for converters under 200t, and 7-9 kg / t of steel for converters over 200t. Addition begins when 1 / 3 of the steel has been tapped from the converter, and is done in 2-3 batches through the top hopper of the ladle, with each batch spaced 15-20 seconds apart. All addition is completed before 2 / 3 of the steel has been tapped, utilizing the impact of the steel flow to achieve initial mixing. After adding the modifier and using bottom-blown argon process: the bottom-blown argon flow rate during tapping is 0.15~0.2 m³ / (t·min), and after tapping, the flow rate drops to 0.1~0.12 m³ / (t·min), with continuous stirring for 8~12 min. During stirring, the ladle temperature drop is controlled to ≤5℃ / min. The composition of the top slag after modification is controlled as follows: CaO / SiO2=3.0~4.0, FeO+MnO≤5%, basicity≥3.5, to ensure reduced oxidizability of molten steel during slag-metal interface mass transfer.

[0011] 4) RH Vacuum Refining Control: As the core step in inclusion control, a new process of deep degassing, vacuum calcium treatment, and soft blowing slag agglomeration is adopted. The vacuum degree is controlled at ≤50Pa, and the treatment time is 22~28min, of which the deep degassing stage takes ≥15min. Extending the high vacuum time promotes the deep removal of dissolved oxygen and gases. In the middle of the vacuum treatment, Ca-Si wire is added in a stepwise feeding method, with a total feed rate of 0.5~0.7kg / t steel. First, 60% of the total amount is fed at a rate of 1.5~2.0m / s, stirred for 5min, and then the remaining 40% is fed in. This converts Al2O3 inclusions in the molten steel into low-melting-point spherical CaO-Al2O3 composite inclusions, improving the inclusion flotation rate. Eight minutes before the end of the vacuum treatment, the argon stirring intensity is reduced to 0.08~0.12m³ / (t·min) for soft blowing, which promotes the aggregation of inclusions into large particles and their flotation to the slag phase. After treatment, the molten steel T[O] ≤ 12ppm and the content of large-size inclusions ≤ 2 per mm².

[0012] 5) LF Refining Control: This unit serves solely as a temperature control unit before continuous casting, preventing secondary oxidation of the molten steel. It employs a low-power heating and weak stirring process, with heating power controlled at 300~400kW. The molten steel temperature is precisely adjusted to 1540~1560℃ according to continuous casting requirements, with a heating rate ≤3℃ / min to prevent localized overheating. Argon gas is used for weak stirring at an intensity of 0.03~0.05m³ / (t·min) for 5~8min to ensure temperature uniformity and prevent strong stirring from disrupting the control of inclusions formed by RH (reverse annealing). After LF treatment, the molten steel temperature fluctuation range is ≤±3℃, meeting the requirements of the continuous casting process.

[0013] (2) Control methods for continuous casting process 1) Argon blowing in the tundish: To eliminate turbulence at the stopper head and nodule formation, a combined technology of annular argon blowing from the submersible nozzle top seat brick and six-hole argon blowing from the stopper rod is adopted. An annular argon channel is set in the submersible nozzle top seat brick, with the argon flow rate controlled at 5~8 L / min, forming an annular gas curtain to prevent contact between the molten steel and air. Six φ1.5mm argon blowing holes are evenly opened around the circumference of the stopper rod head, with the argon flow rate controlled at 2~3 L / min. The airflow acts perpendicularly on the surface of the stopper rod head to prevent Al2O3 inclusions from adhering and forming turbulence. High-purity argon with a purity ≥99.99% is used for blowing to avoid introducing new impurities.

[0014] 2) Crystallizer Protection: A low-melting-point, low-viscosity protective slag is used, with a basicity of 1.2~1.5, a melting temperature ≤1300℃, and a viscosity of 0.8~1.2 Pa·s. The addition rate is 0.3~0.5 kg / m², ensuring that the protective slag evenly covers the molten steel surface and isolates it from air. The crystallizer employs electromagnetic stirring technology with a stirring current of 200~250A and a frequency of 2~3Hz, promoting the upward floating of residual inclusions in the molten steel to the protective slag layer.

[0015] 3) Submerged entry nozzle: A side-opening submerged entry nozzle is used, with an inner diameter of φ80~φ100mm, an immersion depth of 150~200mm, and an outlet angle of 15°~20° to avoid slag entrapment in the crystallizer. The nozzle material is Al2O3-C, and the inner surface is smoothed to reduce friction between the molten steel and the nozzle inner wall. Combined with argon blowing technology in the tundish, this further prevents nozzle nodule formation.

[0016] 4) Continuous casting process parameter control: The casting speed is controlled at 1.2~1.6 m / min, dynamically adjusted according to the molten steel temperature at the LF outlet to ensure uniform solidification of the molten steel in the crystallizer. The secondary cooling zone employs a combination of weak cooling and segmented cooling. The cooling intensity of the first stage in the secondary cooling zone is 0.8~1.0 L / kg steel, and the cooling intensity of the second stage is 0.5~0.7 L / kg steel, to avoid excessively rapid cooling that could trap inclusions in the solidified shell. The straightening temperature of the continuously cast billet is controlled at 900~950℃ to prevent surface cracking of the billet.

[0017] (3) Control of large inclusions in the rolling process 1) Optimized Heating Process: After the continuously cast billet enters the heating furnace, a low-temperature preheating and gradient heating process is adopted, strictly matching the phase transformation temperature of the steel grade to avoid inclusion growth caused by high temperature. The preheating zone temperature is 800~900℃, the heating zone temperature is 1150~1200℃, and the soaking zone temperature is 1180~1220℃, with a total heating time of 2.5~3.5h, ensuring that the internal temperature uniformity of the billet is ≤±5℃. Inert gas protection is used in the heating furnace to prevent oxidation of the billet surface and to avoid adverse reactions between inclusions in the steel and the matrix during the heating process.

[0018] 2) Rolling parameter control: In the roughing stage, a large reduction is used, with a reduction of 15-25% per pass and a cumulative reduction of ≥70%. This large deformation promotes the breakage of large inclusions inside the billet. In the finishing stage, multiple passes with small reductions are used, with a reduction of 5-10% per pass. The final rolling temperature is controlled at 850-900℃ to ensure uniform steel plate structure. Rolling oil lubrication is used during the rolling process to reduce friction between the rolls and the steel plate, preventing the formation of new inclusions.

[0019] 3) Cold rolling and annealing: The cold rolling stage adopts multi-pass cold rolling, and the total reduction rate is controlled at 70~80% to ensure uniform steel plate thickness; the annealing process adopts continuous annealing process, with an annealing temperature of 750~800℃, a holding time of 30~40s, and a cooling rate of 15~20℃ / s. Annealing promotes the uniform distribution of small inclusions remaining inside the steel plate and avoids the agglomeration of inclusions to form large-size defects.

[0020] The method of this invention is applicable to converters with capacities of 120t to 300t, and produces battery casing steel with an inclusion content of ≤1 inclusion / mm² with an internal size ≥5μm, a stamping sand hole defect rate of ≤0.1%, a yield strength of ≥300MPa, and an elongation of ≥35%.

[0021] In summary, this invention focuses on full-process temperature control, adding a ladle top slag modification step to form a synergistic process of converter temperature control—top slag oxygen reduction—RH purification. The converter tapping temperature is reduced by 40-60℃, fundamentally inhibiting the formation of oxidizing inclusions. The top slag modifier is fully melted through steel flow impact and bottom blowing agitation, reducing the top slag FeO+MnO to ≤5%, and the molten steel T[O] to 3-5ppm. Combined with deep RH degassing and vacuum step-by-step treatment processes, the content of large-size inclusions in the molten steel is reduced to below 2 inclusions / mm², and T[O] ≤12ppm. The LF (Liquid Fluid) only performs temperature control, avoiding the risk of secondary oxidation. The composite argon blowing technology in the continuous casting tundish completely solves the problems of stopper rod turbulence and nozzle nodule formation. Combined with the inclusion-breaking effect of the rolling process, the content of large-size inclusions inside the steel plate is ultimately controlled to below 0.8 inclusions / mm², significantly reducing the risk of stamping stress concentration. The battery casing steel produced using the method of this invention can reduce the stamping pinhole defect rate to below 0.08%, far lower than the 0.5%~2% of existing technologies. Simultaneously, the steel plate has a yield strength ≥300MPa and an elongation ≥35%, meeting the deep-drawing requirements of high-end battery casings and enhancing product market competitiveness. The control method of this invention does not require the addition of a large number of specialized equipment; it can be implemented through process optimization on existing battery casing steel production lines, resulting in low modification costs and easy industrial-scale promotion.

[0022] The beneficial effects of this invention are: 1. Full-process collaborative control: This invention proposes a full-process collaborative process route of "converter low-temperature tapping → top slag modification and oxygen reduction → RH deep purification → LF precise temperature adjustment → continuous casting anti-blocking and flow control", which realizes systematic and source control of large-size inclusions.

[0023] 2. Significant Source Control Effect: By strictly controlling the converter tapping temperature within the lower range of 1580~1620℃, the initial oxygen content and oxidizing power of the molten steel are reduced at the source, significantly decreasing the formation of primary oxide inclusions. Subsequent ladle top slag modification further reduces the top slag oxidizing power (FeO+MnO) to below 5%, cutting off the oxygen return channel at the slag-steel interface and consolidating the low-oxygen potential environment.

[0024] 3. Optimized Refining Functions: The RH process is strengthened as the core step in inclusion purification. Through a combined process of "deep degassing + vacuum stepwise calcium treatment + soft blowing," solid Al2O3 inclusions are efficiently transformed into easily floating liquid calcium aluminate, which is then thoroughly removed. Simultaneously, the LF process is explicitly defined as "temperature control," employing a low-power, weak-stirring mode to avoid secondary oxidation and purity degradation caused by strong stirring, maximizing the advantages of each refining unit.

[0025] 4. Stable and reliable continuous casting process: The adopted composite technology of "annular argon blowing on the upper seat brick + six-hole argon blowing on the stopper rod" can form an effective inert gas protective layer and physical scouring in the key areas of the nozzle and stopper rod, which fundamentally solves the problems of turbulence and nodule formation caused by Al2O3 deposition, ensuring the stability of the continuous casting process and the final cleanliness of the molten steel.

[0026] 5. Superior Product Quality: Through the above-mentioned full-process system control, the content of large-size inclusions ≥5μm inside the battery casing steel can be stably controlled to below 1 inclusion / mm², thereby reducing the stamping sand hole defect rate to below 0.1% (up to 0.06%~0.08%), while ensuring that the steel has excellent mechanical properties (yield strength ≥300MPa, elongation ≥35%), fully meeting the stringent requirements of high-end new energy battery casings for material purity and formability.

[0027] 6. High applicability and low cost: The method described in this invention does not require large-scale equipment modification of existing production lines. It is mainly achieved through process parameter optimization and process reengineering. It is applicable to mainstream converter capacities of 120t to 300t, with low modification costs, easy to promote and implement in steel enterprises, and significant economic benefits. Attached Figure Description

[0028] Figure 1 This image shows the morphology of a typical large-sized internal inclusion that causes pinhole defects in battery casing steel produced using traditional processes. Figure 2 This is a morphological diagram of the battery casing steel produced in Embodiment 1 of the present invention. Detailed Implementation

[0029] The present invention will be further described in detail below with reference to specific embodiments. It should be noted that the following embodiments are only used to explain the present invention and are not intended to limit the present invention.

[0030] Example 1 This embodiment is implemented on a 120t converter production line to produce steel for new energy battery casings. The specific steps are as follows: (1) Pretreatment of molten iron: When molten iron enters the desulfurization station, 10 kg / t of CaO-MgO-Al2O3 composite desulfurizing agent (CaO 70% + MgO 10% + Al2O3 20%) is added to the molten iron. The stirring speed is 70 r / min, and the treatment is carried out for 18 min, and the sulfur content of the molten iron is reduced to 0.004%. Then, dephosphorization treatment is carried out. 12 kg / t of CaO-Fe2O3-MnO oxidizing dephosphorizing agent (CaO 45% + Fe2O3 40% + MnO 10% + CaF 25%) is added to the molten iron. The oxygen blowing intensity is 1.0 m³ / (t·min), and the treatment is carried out for 10 min, and the phosphorus content of the molten iron is reduced to 0.007%.

[0031] (2) Converter low-superheat smelting: The pretreated molten iron is smelted in a converter. The final steel temperature is controlled at 1600℃ and the carbon content is 0.07%. The ladle is pre-blown with argon for 6 seconds before tapping. During tapping, Si-Mn alloy and aluminum wire are used for deoxidation and alloying. The Si-Mn alloy is added at a rate of 9 kg / t steel, and the Al is added at a rate of 0.25 kg / t steel. The oxygen content of the molten steel after tapping is 18 ppm.

[0032] (3) Ladle top slag modification: A CaO-Al2O3-MgO-CaF2 composite modifier was used, with the following composition: CaO 60%, Al2O3 18%, MgO 10%, CaF2 7%, and metallic Al 5%. The modifier was added in two batches starting when 1 / 3 of the steel was tapped, with an interval of 18 seconds, and the addition was completed before 2 / 3 of the steel was tapped. The bottom blowing argon flow rate during tapping was 0.18 m³ / (t·min), and after tapping, it was adjusted to 0.10 m³ / (t·min), with continuous stirring for 10 min. After treatment, the (FeO+MnO) content in the ladle top slag was 4.2%, and the T[O] in the molten steel decreased to 15 ppm.

[0033] (4) RH vacuum refining: The vacuum degree was controlled at 45 Pa, and the total processing time was 25 min, including 15 min of deep degassing. During the middle of the processing, the Ca-Si wire was fed in two steps, with a total feed amount of 0.6 kg / t steel. In the first step, 0.36 kg / t steel (60% of the total amount) was fed at a speed of 1.8 m / s, and after circulating and stirring for 5 min, the remaining 0.24 kg / t steel was fed in. 8 min before the end of the processing, the flow rate of the boosting gas was reduced for soft blowing, with an intensity of 0.10 m³ / (t·min). After the processing, the T[O] of the molten steel was 10 ppm, and the number of large inclusions (≥5 μm) was 1.8 per mm².

[0034] (5) LF Temperature Control: Molten steel enters the LF furnace. Only the heating function is turned on, and the temperature of the molten steel is raised from 1530℃ to 1550℃ at a power of 350kW, with a heating rate of approximately 2.5℃ / min. At the same time, bottom blowing argon is turned on for weak stirring at an intensity of 0.04 m³ / (t·min) for 6 minutes to ensure uniform temperature. After treatment, the temperature fluctuation of the molten steel is within ±2℃.

[0035] (6) Continuous casting: Submerged entry nozzles are used for casting, with an inner diameter of φ90mm, a submersion depth of 180mm, and an outlet downward angle of 18°. Combined argon blowing is activated: the flow rate of the annular argon blowing on the upper seat brick is set to 6 L / min; the total flow rate of the six φ1.5mm argon blowing holes at the stopper rod head is 2.5 L / min. Low-melting-point protective slag is used, with a basicity of 1.3, a melting temperature of 1280℃, a viscosity of 1.0 Pa·s, and an addition amount of 0.4 kg / m². The electromagnetic stirring current in the crystallizer is 220A, and the frequency is 2.5Hz. The casting speed is controlled at 1.4 m / min. A weak cooling regime is adopted in the secondary cooling zone: the first cooling intensity is 0.9 L / kg steel, and the second cooling intensity is 0.6 L / kg steel. The temperature of the billet straightening section is controlled at 920℃.

[0036] (7) Rolling: Heating: The billet is heated in a walking beam furnace with a preheating temperature of 850°C, a heating temperature of 1180°C, and a soaking temperature of 1200°C. The total heating time is 3.0 hours, and nitrogen is purged into the furnace for protection.

[0037] Hot rolling: The roughing process uses a large reduction process, with a reduction of 20% per pass and a cumulative reduction rate of 75%. The finishing process has a reduction of 8% per pass and a final rolling temperature of 880℃. Rolling oil is used for lubrication during the rolling process.

[0038] Cold rolling and annealing: After pickling, the hot-rolled sheet is cold-rolled with a total reduction of 75%. Then, it is continuously annealed at a temperature of 780℃ for 35 seconds and cooled to room temperature at a rate of 18℃ / s.

[0039] Implementation Results: Sampling and testing of the battery casing steel produced in this embodiment revealed an internal large-size inclusion content of ≥5μm to be 0.7 inclusions / mm². Of the 10,000 standard battery casing samples stamped from this steel plate, only 6 were found to have sand-hole defects, resulting in a defect rate of 0.06%. The material exhibits excellent mechanical properties, with a yield strength of 310MPa and an elongation of 36%. During the continuous casting process, there was no turbulence on the stopper rod and no nodules at the nozzle, ensuring smooth production.

[0040] Figure 1 The images show typical large-sized inclusions in battery casing steel produced using traditional processes that cause pinhole defects. (a) is a scanning electron microscope image, and (b) and (c) are EDS spectra of the inclusions. As can be seen from the images, the large-sized inclusions in the steel are generally alumina inclusions of 50~150μm, which are the main inclusions causing pinhole defects. Figure 2 The images show the morphology of the battery casing steel produced in this embodiment (where (a) is a scanning electron microscope image, and (b) and (c) are EDS energy dispersive spectroscopy (EDS) images of inclusions). It can be seen that the content of large-sized inclusions inside the battery casing steel produced by this invention is significantly reduced.

[0041] Example 2 This embodiment focuses on parameter adjustments for the production conditions of a 300t large converter. The specific steps are as follows: (1) Steelmaking refining process 1) Pretreatment of molten iron: Add 9 kg / t of CaO-MgO-Al2O3 composite desulfurizing agent to molten iron, stir at 65 r / min, treat for 20 min, and the sulfur content of molten iron is 0.005%; add 13 kg / t of CaO-Fe2O3-MnO oxidizing dephosphorizing agent to molten iron, oxygen blowing intensity is 0.9 m³ / (t·min), treat for 12 min, and the phosphorus content of molten iron is reduced to 0.008%.

[0042] 2) Converter smelting: The final steel temperature is 1590℃ and the carbon content is 0.08%; argon is pre-blown for 7 seconds before tapping, and 8.5 kg / t of Si-Mn alloy and 0.28 kg / t of Al are added to the steel, resulting in an oxygen content of 19 ppm.

[0043] 3) Ladle top slag modification: A CaO-Al2O3-MgO-CaF2 composite modifier is used. The modifier composition (mass percentage) is: CaO 58%, Al2O3 19%, MgO 11%, CaF2 6%, metallic Al 6%, with an addition rate of 8 kg / t of steel. It is added in three batches at 15 s intervals when 1 / 3 of the steel has been tapped. During tapping, bottom blowing argon is applied at 0.16 m³ / (t·min), and after tapping, stirring is performed at 0.11 m³ / (t·min) for 11 min. After modification, the top slag (FeO + MnO) = 4.5%.

[0044] 4) RH vacuum refining: vacuum degree 48Pa, processing time 26min, deep degassing 16min; 0.65kg / t steel of Ca-Si wire was fed in two steps, with 0.39kg / t steel fed in the first step, stirred for 5min, and then the remaining 0.26kg / t steel was fed in; soft blowing for 8min, argon intensity 0.11m³ / (t·min). After treatment, the molten steel T[O] = 11ppm.

[0045] 5) LF refining: Heating power 380kW, raising molten steel from 1525℃ to 1555℃ at a heating rate of 2.8℃ / min; weak argon stirring for 7min at an intensity of 0.045m³ / (t·min). Temperature fluctuation ±3℃.

[0046] (2) Continuous casting process 1) Combined argon blowing: the flow rate of argon blowing in the annular shape of the upper seat brick is 7L / min, and the flow rate of argon blowing in the six holes of the stopper rod is 2.8L / min.

[0047] 2) Process parameters: immersion nozzle inner diameter φ100mm, immersion depth 190mm, outlet angle 15°; casting speed 1.3m / min; cooling intensity of the first cooling zone 0.95L / kg steel, and the second cooling zone 0.65L / kg steel; billet straightening temperature 930℃.

[0048] (3) Steel rolling process 1) Heating: Preheating section 860℃, heating section 1190℃, soaking section 1210℃, total heating time 3.2h.

[0049] 2) Hot rolling: Roughing pass reduction is 22%, cumulative reduction is 78%; finishing pass reduction is 9%, and final rolling temperature is 890℃.

[0050] 3) Cold rolling and annealing: Total cold rolling reduction rate 76%; continuous annealing temperature 790℃, holding time 36s, cooling rate 19℃ / s.

[0051] Testing revealed that the battery casing steel plate produced in this embodiment contained 0.8 inclusions with an internal size ≥5μm per mm², had a stamping sand hole defect rate of 0.07%, a yield strength of 308MPa, and an elongation of 35.5%. The continuous casting process was stable, with no turbulence or nodule formation issues.

Claims

1. A method for controlling pinhole defects in steel stamping for new energy battery casings throughout the entire process, characterized in that, The process route adopted is hot metal pretreatment - converter smelting - ladle top slag modification - RH vacuum refining - LF temperature control - continuous casting - rolling, including the following steps: S1. Hot metal pretreatment: Hot metal is subjected to desulfurization and dephosphorization pretreatment in sequence to control the sulfur content of hot metal to ≤0.005% and the phosphorus content to ≤0.008%; S2. Converter smelting: The pretreated molten iron is smelted in a converter, and the final tapping temperature is controlled at 1580~1620℃. Deoxidation and alloying are carried out during the tapping process, and the oxygen content of the molten steel is controlled to be ≤20ppm. S3. Ladle top slag modification: During the converter tapping process, a modifier is added to the ladle and stirred with bottom blowing argon to control the FeO+MnO content in the top slag to ≤5%; S4, RH vacuum refining: molten steel is subjected to RH vacuum treatment, with a vacuum degree ≤50Pa and a treatment time of 22~28min. Ca-Si wire is fed in stepwise and a soft blowing process is adopted. After treatment, the molten steel T[O] ≤12ppm. S5, LF temperature control: LF refining is performed on RH-treated molten steel, with only temperature adjustment to precisely control the molten steel temperature to 1540~1560℃. S6. Continuous casting: Casting is carried out using an immersion nozzle, and argon is blown in a ring at the brick seat of the immersion nozzle. At the same time, a multi-hole argon blowing is set at the head of the stopper rod to control the argon flow rate in combination with crystallizer protection and segmented cooling. S7. Rolling: The continuous casting billet is heated, rough rolled, finish rolled, cold rolled and continuously annealed.

2. The method according to claim 1, characterized in that, In step S2, the final carbon content is controlled at 0.06~0.09%; deoxidation is carried out by Si-Mn alloy and Al combined deoxidation, with Si-Mn alloy added at 8~10 kg / t steel and Al added at 0.2~0.3 kg / t steel.

3. The method according to claim 1, characterized in that, In step S3, the modifier is a CaO-Al2O3-MgO-CaF2 composite modifier, with the following mass percentage composition: CaO 55~65%, Al2O3 15~20%, MgO 8~12%, CaF2 5~8%, and metallic Al 2~5%; the addition amount is 7~10 kg / t steel; the addition is started when the steel output reaches 1 / 3, and added in 2~3 batches, and completed before the steel output reaches 2 / 3.

4. The method according to claim 1, characterized in that, In step S3, the bottom-blowing argon process is as follows: the argon flow rate during the steel tapping process is 0.15~0.20 m³ / (t·min), and after the steel tapping is completed, it is adjusted to 0.10~0.12 m³ / (t·min), and stirring is continued for 8~12 min.

5. The method according to claim 1, characterized in that, In step S4, the total feed amount of Ca-Si wire is 0.5~0.7 kg / t steel, and it is fed in two steps: the first step is to feed 60% of the total amount, stir for 5 minutes, and then feed the remaining 40% with a soft blowing argon intensity of 0.08~0.12 m³ / (t·min).

6. The method according to claim 1, characterized in that, In step S5, the LF refining process uses low-power heating with a heating power of 300~400kW and a heating rate of ≤3℃ / min; it uses weak argon gas stirring with a stirring intensity of 0.03~0.05 m³ / (t·min) and a stirring time of 5~8min, and controls the temperature fluctuation of the molten steel to ≤±3℃.

7. The method according to claim 1, characterized in that, In step S6, the argon flow rate of the argon blowing ring on the immersion nozzle is 5~8 L / min; the stopper rod head has 6 blowing holes evenly opened along the circumference, and the argon flow rate is 2~3 L / min.

8. The method according to claim 1, characterized in that, In step S6, the continuous casting speed is 1.2~1.6 m / min; the basicity of the mold flux is 1.2~1.5, the melting temperature is ≤1300℃, and the viscosity is 0.8~1.2 Pa·s; the cooling intensity of the secondary cooling zone is 0.8~1.0 L / kg steel in the first stage and 0.5~0.7 L / kg steel in the second stage; the temperature of the billet straightening section is 900~950℃.

9. The method according to claim 1, characterized in that, In step S7, the heating process is as follows: preheating zone temperature 800~900℃, heating zone temperature 1150~1200℃, soaking zone temperature 1180~1220℃, total heating time 2.5~3.5h; roughing pass reduction 15~25%, cumulative reduction ≥70%; finishing pass reduction 5~10%, final rolling temperature 850~900℃; total cold rolling reduction 70~80%; continuous annealing temperature 750~800℃, holding time 30~40s, cooling rate 15~20℃ / s.

10. The method according to any one of claims 1 to 9, characterized in that, The final product contains ≤1 large inclusion of ≥5μm per mm², ≤0.1% stamping pinhole defect rate, ≥300MPa yield strength, and ≥35% elongation.

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