High-strength steel for battery pack frame, square tube and preparation method

Abstract

The invention relates to high-strength steel for a battery pack frame, a square tube and a preparation method, and belongs to the technical field of automobile part preparation. The high-strength steel comprises the following chemical components in percentage by mass: 0.04 percent to 0.12 percent of C, 1.8 percent to 2.8 percent of Mn, 0.3 percent to 1.0 percent of Si, 0.3 percent to 1.0 percent of Cr, 0.1 percent to 0.5 percent of Mo, 0.002 percent to 0.06 percent of Nb, 0.01 percent to 0.1 percent of Ti, 0.01 percent to 0.1 percent of V and the balance of Fe and inevitable impurities. The high-precision square tube manufacturing method is provided in a matched mode for solving the tube manufacturing process problem caused by the fact that the hardness of high-strength steel is improved, and the core technical bottlenecks that high-strength steel is remarkable in work hardening and low in forming qualification rate are effectively solved. By adopting the high-strength steel and the square tube preparation method provided by the invention, when the obtained square tube is used for preparing a battery pack frame, the weight redundancy problem of the battery pack frame can be effectively solved, and the light weight of an electric commercial vehicle is realized.

Description

Technical Field

[0001] This application relates to the field of automotive parts manufacturing technology, and in particular to a high-strength steel, square tube, and manufacturing method for a battery pack frame. Background Technology

[0002] In the field of electric commercial vehicles, the power battery pack plays a crucial role as the core energy supply unit. Its energy density and structural design optimization directly affect the vehicle's range and load-bearing capacity. With the rapid development of the electric vehicle market and the increasing demands of consumers for vehicle performance, increasing the number of battery packs or enlarging the size of individual battery packs has become a common method to improve energy storage. However, while this technological approach brings energy gains, it also leads to a significant increase in the weight of the battery system, posing a challenge to lightweight vehicle design. The battery pack frame, as a key component supporting the battery modules, is mainly composed of square tubing. The material selection and mechanical properties of the square tubing directly determine the load-bearing capacity and reliability of the entire battery bracket. Currently, traditional battery pack frames mostly use low-strength steel, such as QSTE700TM, to meet mechanical load requirements such as vibration and impact (following the QC / T 989-2014 standard). However, due to limitations in material strength, to meet structural safety requirements, over-design is often necessary, resulting in a single frame weighing up to approximately 500 kg, indicating a significant weight redundancy problem.

[0003] The gross vehicle weight of commercial vehicles is subject to strict regulations (such as GB 1589-2016). Therefore, achieving lightweight battery pack frames through innovative material applications and optimized frame design, while ensuring structural safety, has become a key technological challenge in balancing the range and load-bearing capacity of electric commercial vehicles. This not only improves vehicle economy but also promotes the development of green transportation, reducing energy consumption and environmental pollution. Furthermore, the work hardening of high-strength steel leads to difficulties in tube manufacturing processes, such as forming cracks and weld strength attenuation, which are also technological challenges that need to be addressed. Summary of the Invention

[0004] This application provides a high-strength steel, square tube, and manufacturing method for a battery pack frame to solve the following technical problem: how to meet the requirements of lightweight and high strength for battery pack frames.

[0005] In a first aspect, embodiments of this application provide a high-strength steel for a battery pack frame, wherein the chemical composition of the high-strength steel, by mass fraction, is: C: 0.04%–0.12%, Mn: 1.8%–2.8%, Si: 0.3%–1.0%, Cr: 0.3%–1.0%, Mo: 0.1%–0.5%, Nb: 0.002%–0.06%, Ti: 0.01%–0.1%, V: 0.01%–0.1%, with the balance being Fe and unavoidable impurities.

[0006] Optionally, the high-strength steel meets at least one of the following properties: yield strength ≥750MPa, tensile strength ≥950MPa.

[0007] Optionally, the microstructure of the high-strength steel is martensite + ferrite, wherein the volume fraction of the ferrite is 5% to 30%.

[0008] Optionally, the thickness d of the high-strength steel is 2mm to 8mm.

[0009] Secondly, this application provides a square tube for a battery pack frame, the square tube being made of the high-strength steel described in the first aspect, wherein the cross-sectional length a of the square tube is 10mm to 100mm, and the cross-sectional width b of the square tube is 20mm to 200mm.

[0010] Thirdly, this application provides a method for manufacturing a square tube for a battery pack frame as described in the second aspect, the method comprising:

[0011] The high-strength steel is roll-formed to obtain a pipe blank to be welded;

[0012] The tube blank to be welded is subjected to high-frequency welding to obtain a square tube for the finished battery pack frame.

[0013] Optionally, the step of roll forming the high-strength steel to obtain the pipe blank to be welded includes:

[0014] The high-strength steel is directly formed into a square cross section through multiple progressive bending processes to obtain a semi-finished square tube.

[0015] The semi-finished square tube is shaped to form an I-shaped butt joint, thus obtaining a tube blank to be welded.

[0016] Each right angle of the square tube is completed through 10 to 15 bends, and the edge strain value ε of each bend satisfies:

[0017] R represents the radius of the roll forming roll, and d represents the thickness of the high-strength steel.

[0018] The final bending angle D of the tube blank to be welded is d to 4d, and the height difference between the two sides of the tube blank to be welded is <0.05d, where d is the thickness of the high-strength steel in mm.

[0019] Optionally, the step of performing high-frequency welding on the tube blank to obtain the finished battery pack frame square tube includes:

[0020] Based on the thickness of the high-strength steel, a high-frequency welding machine with an appropriate frequency is selected. Where ρ is the resistivity of high-strength steel at 1400℃, and its value is 138 μΩ·cm; μ r d represents the magnetic permeability of the high-strength steel at 1400℃, with a value of 1H / cm; d represents the thickness of the high-strength steel.

[0021] Select appropriate welding power P and welding speed v based on the cross-sectional dimensions of the tube blank to be welded. Where k is a constant with a value of 0.68 to 0.85, a is the cross-sectional length of the square tube, and b is the cross-sectional width of the square tube;

[0022] The welding speed v is 5m / min to 30m / min;

[0023] The edges of the tube blank to be welded are heated using the high-frequency welding machine.

[0024] Pressure is applied to the edge to fuse the melted edge into a weld, wherein the extrusion amount S of the applied pressure is 0.8 to 3 times the thickness d of the high-strength steel;

[0025] Remove the excess weld seam material from the outer seam to ensure the weld seam is free of defects;

[0026] The weld is cooled by spraying coolant at a distance of 0.3m to 1.5m from the weld.

[0027] The tube blank to be welded after being sprayed and cooled is cut to length to obtain the finished square tube for the battery pack frame.

[0028] Optionally, the weld seam of the square tube used for the finished battery pack frame does not crack when fully flattened, and the impact energy at -20℃ reaches more than 90% of that at room temperature.

[0029] Optionally, the battery pack frame made from the battery pack frame using square tubing is used in electric commercial vehicles.

[0030] The technical solutions provided in this application have the following advantages compared with the prior art:

[0031] This application provides a high-strength steel for a battery pack frame. The chemical composition of the high-strength steel, by mass fraction, is: C: 0.04%–0.12%, Mn: 1.8%–2.8%, Si: 0.3%–1.0%, Cr: 0.3%–1.0%, Mo: 0.1%–0.5%, Nb: 0.002%–0.06%, Ti: 0.01%–0.1%, V: 0.01%–0.1%, with the balance being Fe and unavoidable impurities. On the one hand, through precise composition design, the steel maintains high strength while also possessing good plasticity and toughness, providing a foundation for subsequent processing and forming. On the other hand, addressing the challenges in tube manufacturing processes caused by the increased hardness of high-strength steel, a corresponding method for preparing square tubes was proposed. This method employs a "direct square forming" roll forming process, using a fixed-length bending method where the radius gradually decreases during forming. Each 90° right angle requires 10 to 15 passes to complete the forming. This method effectively reduces peak stress and strain at the corners, minimizing the risk of cracking. Rollers and guide rings shape the tube blank into an I-shaped butt joint, ensuring the height difference between the two sides of the weld section is less than a certain value, providing favorable conditions for subsequent welding. A welding machine with an appropriate frequency is selected based on the thickness of the tube blank. The skin effect and proximity effect are used to concentrate heating on the edges, rapidly melting them. Pressure is applied by extrusion rollers to bond the molten metal, forming a weld. The extrusion amount is controlled to ensure weld quality. Excess height from both the inner and outer seams is removed to ensure weld flatness and appearance quality. A spray cooling method is used to rapidly cool and solidify the weld, improving its strength and toughness. Square tubes prepared using this method exhibit completely flattened welds without cracking. To address the challenges in tube manufacturing caused by the increased hardness of high-strength steel, a high-precision square tube fabrication method is proposed, effectively solving the core technical bottlenecks of significant work hardening and low forming yield in high-strength steel. Using the high-strength steel and square tube fabrication method provided in this application, the resulting square tube can be used to fabricate battery pack frames, effectively solving the weight redundancy problem and achieving lightweighting of electric commercial vehicles. Attached Figure Description

[0032] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.

[0033] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0034] Figure 1 This is a schematic flowchart illustrating a method for fabricating a square tube for a battery pack frame, as provided in an embodiment of this application. Detailed Implementation

[0035] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0036] The range descriptions used herein, such as numerical ranges and proportional ranges, include all possible sub-ranges and single numerical values ​​within that range. For example, the range descriptions of "1 to 6" or "1 to 6" cover all sub-ranges (such as 1 to 3, 2 to 5, etc.) and single numbers (such as 1, 2, 3, 4, 5, 6) between 1 and 6. Unless otherwise specified, the terms "including" and "comprise" as used herein mean "including but not limited to"; relational terms such as "first" and "second" are used only to distinguish different entities or operations and do not imply an actual order or relationship; "and / or" indicates that multiple situations can exist individually or simultaneously; expressions such as "at least one," "multiple," and "at least one" refer to any combination of the corresponding objects, including combinations of single or multiple objects. The proportional relationships mentioned herein, such as mass ratios and molar ratios, should be understood as the correspondence between the first and second terms of a proportional formula, according to the order of description. The raw materials, reagents, instruments, and equipment used herein can all be obtained through commercial purchase or prepared using existing methods.

[0037] In a first aspect, embodiments of this application provide a high-strength steel for a battery pack frame, wherein the chemical composition of the high-strength steel, by mass fraction, is: C: 0.04%–0.12%, Mn: 1.8%–2.8%, Si: 0.3%–1.0%, Cr: 0.3%–1.0%, Mo: 0.1%–0.5%, Nb: 0.002%–0.06%, Ti: 0.01%–0.1%, V: 0.01%–0.1%, with the balance being Fe and unavoidable impurities;

[0038] High-strength steel (HSS) refers to steel whose yield strength or tensile strength is significantly higher than that of ordinary structural steel through alloy design, heat treatment, or controlled rolling and cooling processes. Its strength index is typically defined as a yield strength (σs) ≥ 460 MPa or a tensile strength (σb) ≥ 590 MPa (the exact values ​​may vary slightly depending on the standard). It also possesses good plasticity, toughness, and weldability. Through precise composition design, the content of various elements in the steel, such as C, Mn, Si, Cr, Mo, Nb, Ti, and V, is adjusted. These elements strengthen the matrix and improve hardness and toughness. Specific compositional ratios allow the steel to maintain high strength while also possessing good plasticity and toughness, providing a foundation for subsequent processing and forming.

[0039] In some embodiments, the high-strength steel satisfies at least one of the following properties: yield strength ≥ 750 MPa, tensile strength ≥ 950 MPa.

[0040] In this embodiment, the high-strength steel has a yield strength ≥750MPa and a tensile strength ≥950MPa, which are significantly higher than the low-strength grade steel used in traditional battery pack frames. Because the strength of the high-strength steel is significantly improved, the cross-sectional dimensions and thickness of the battery pack frame can be reduced while maintaining the same load-bearing capacity, thus achieving lightweighting.

[0041] In some embodiments, the microstructure of the high-strength steel is martensite + ferrite, wherein the volume fraction of the ferrite is 5% to 30%.

[0042] This structure allows the steel to maintain high strength while also possessing a certain degree of toughness, avoiding brittleness caused by excessive hardening.

[0043] In some embodiments, the thickness d of the high-strength steel is 2mm to 8mm.

[0044] For example, the thickness d of the high-strength steel can be 2mm, 3mm, 4mm, 5mm, 6mm, 7mm, 8mm, etc.

[0045] Secondly, this application provides a square tube for a battery pack frame, the square tube being made of the high-strength steel described in the first aspect, wherein the cross-sectional length a of the square tube is 10mm to 100mm, and the cross-sectional width b of the square tube is 20mm to 200mm.

[0046] For example, the cross-sectional length 'a' of the square tube can be 10mm, 20mm, 30mm, 40mm, 50mm, 60mm, 70mm, 80mm, 90mm, 100mm, etc.; and the cross-sectional width 'b' of the square tube can be 20mm, 40mm, 60mm, 80mm, 100mm, 120mm, 140mm, 160mm, 180mm, 200mm, etc.

[0047] Figure 1 This is a schematic flowchart illustrating a method for fabricating a square tube for a battery pack frame, as provided in an embodiment of this application.

[0048] Thirdly, this application provides a method for manufacturing a square tube for a battery pack frame as described in the second aspect, the method comprising:

[0049] The high-strength steel is roll-formed to obtain a pipe blank to be welded;

[0050] The tube blank to be welded is subjected to high-frequency welding to obtain a square tube for the finished battery pack frame.

[0051] In some embodiments, before the high-strength steel is rolled, the steel strip needs to be uncoiled. In this embodiment, the high-strength steel can be uncoiled by an uncoiler, and then the uncoiled steel strip can be processed by a leveling machine to eliminate its curling stress, ensure the flatness of the plate surface, and obtain a flat steel strip.

[0052] In some embodiments, the step of roll forming the high-strength steel to obtain a pipe blank for welding includes:

[0053] The high-strength steel is directly formed into a square cross section through multiple progressive bending processes to obtain a semi-finished square tube.

[0054] The semi-finished square tube is shaped to form an I-shaped butt joint, thus obtaining a tube blank to be welded.

[0055] Each right angle of the square tube is completed through 10 to 15 bends, and the edge strain value ε of each bend satisfies:

[0056] R represents the radius of the roll forming roll, and d represents the thickness of the high-strength steel.

[0057] In this embodiment, a flat steel strip is formed using a "direct square" roll forming process, and then bent using a fixed-length method. Through multi-pass progressive forming (10 to 15 passes per 90° right angle), the bending radius is gradually reduced, significantly dispersing corner stress and strain peaks. This process controls the edge strain value ε at each pass within ε... 极限 At and below, this effectively avoids crack defects caused by local overload in the material.

[0058] In some embodiments, the final bending angle D of the tube blank to be welded is d to 4d, and the height difference between the two sides of the tube blank to be welded is <0.05d, where d is the thickness of the high-strength steel in mm.

[0059] The pre-formed square tube is further processed by rollers and guide rings to form an I-shaped butt joint. During this process, it is necessary to ensure that the final bending angle D is within the range of d to 4d, and that the height difference between the two sides of the section to be welded is less than 0.05d, so as to obtain a weldable tube blank that meets the requirements.

[0060] "Height difference between the two sides of the section to be welded" refers to the difference in height between the two edges to be welded in an I-shaped butt joint. "<0.05d" means that this height difference should be less than 0.05 times the thickness of the high-strength steel, which aims to ensure the accuracy and quality of the weld. Controlling the height difference between the two sides of the section to be welded is crucial for ensuring the uniformity and strength of the weld. If the height difference is greater than or equal to 0.05d, it may lead to incomplete welding, poor weld quality, and other problems, thus affecting the overall performance of the finished square tube.

[0061] In some embodiments, the step of performing high-frequency welding on the tube blank to obtain the finished square tube for the battery pack frame includes:

[0062] Based on the thickness of the high-strength steel, a high-frequency welding machine with an appropriate frequency is selected. Where ρ is the resistivity of high-strength steel at 1400℃, and its value is 138 μΩ·cm; μ r d represents the magnetic permeability of the high-strength steel at 1400℃, with a value of 1H / cm; d represents the thickness of the high-strength steel.

[0063] Select appropriate welding power P and welding speed v based on the cross-sectional dimensions of the tube blank to be welded. Where k is a constant with a value of 0.68 to 0.85, a is the cross-sectional length of the square tube, and b is the cross-sectional width of the square tube;

[0064] The welding speed v is 5m / min to 30m / min;

[0065] The edges of the tube blank to be welded are heated using the high-frequency welding machine.

[0066] Pressure is applied to the edge to fuse the melted edge into a weld, wherein the extrusion amount S of the applied pressure is 0.8 to 3 times the thickness d of the high-strength steel;

[0067] Remove the excess weld seam material from the outer seam to ensure the weld seam is free of defects;

[0068] The weld is cooled by spraying coolant at a distance of 0.3m to 1.5m from the weld.

[0069] The tube blank to be welded after being sprayed and cooled is cut to length to obtain the finished square tube for the battery pack frame.

[0070] In this embodiment, the welding machine frequency is selected based on the thickness of the pipe blank to be welded (i.e., the thickness of the high-strength steel), and the skin effect and proximity effect are used to concentrate the heating of the edges. When a high-frequency current passes through a conductor, the current tends to concentrate on the surface of the conductor rather than being uniformly distributed throughout the conductor. This phenomenon is called the skin effect. The skin effect occurs because the magnetic field generated by the high-frequency current inside the conductor causes eddy currents to form on the surface of the conductor, resulting in the maximum current density at the surface. When two conductors are close together, their magnetic fields influence each other, causing a change in the current distribution. This phenomenon is called the proximity effect. In high-frequency welding, the edge portion of the pipe blank to be welded experiences an increased current density due to the proximity effect, resulting in more concentrated heating. The high-frequency current generated by the welding machine utilizes the skin effect to concentrate the current mainly on the edge portion of the steel pipe. Simultaneously, due to the proximity effect, the edge portions on both sides of the pipe blank to be welded generate a stronger current density as they approach each other, further heating this area. During the heating process, the metal at the edges melts due to the high temperature. Subsequently, pressure is applied by the extrusion rollers to fuse the molten metal together, forming a weld.

[0071] The selection of the welding machine frequency is primarily based on the thickness of the tube blank to be welded and the physical properties of the material. In this embodiment, resistivity determines the difficulty of current flow in the material, thus affecting heat generation during welding, while permeability reflects the material's response to a magnetic field, influencing the electromagnetic induction heating effect in high-frequency welding. Based on the resistivity and permeability of high-strength steel at 1400℃, the optimal frequency range of the welding machine can be calculated to ensure efficient and concentrated heating of the steel pipe's edges during welding, achieving good welding results.

[0072] The welding power P directly determines the heat input during the welding process. Excessive power may lead to overheating and welding defects; insufficient power may fail to fully melt the edges of the steel pipe, resulting in a weak weld. Therefore, in this embodiment, a suitable power is selected based on the cross-sectional dimensions of the pipe blank to be welded and the required heat input.

[0073] The welding speed has a significant impact on the welding quality. In this embodiment, a speed faster than 30 m / min may lead to insufficient welding and defects such as lack of fusion; a speed slower than 5 m / min may reduce production efficiency and increase the risk of welding deformation. For example, the welding speed v of high-frequency welding can be 5 m / min, 10 m / min, 15 m / min, 20 m / min, 25 m / min, 30 m / min, etc.

[0074] In this embodiment, the edge of the tube blank to be welded is heated to a molten state by a high-frequency current. Under the pressure of the extrusion rollers, the molten metal tightly bonds together to form a weld. The extrusion amount S is the degree to which the metal at the weld is squeezed when the extrusion rollers apply pressure, and its magnitude has a significant impact on the weld quality. The extrusion amount S should be controlled within the range of 0.8d to 3d, where d is the thickness of the tube blank to be welded (i.e., the thickness of the high-strength steel). An extrusion amount less than 0.8d may result in a loose weld bond and the presence of porosity; an extrusion amount greater than 3d may lead to excessive deformation of the weld, or even cracks. For example, the extrusion amount S applied under pressure can be 0.8 times, 1.2 times, 1.6 times, 2.0 times, 2.4 times, 2.8 times, 3 times, etc., the thickness d of the high-strength steel.

[0075] The main purpose of spray cooling is to rapidly reduce the temperature of the weld and its surrounding area after high-frequency welding, thereby solidifying the weld, eliminating thermal stress generated during welding, and preventing microstructural changes in the weld area. Simultaneously, it helps remove oxides and other impurities from the weld surface, improving weld quality. In this embodiment, the distance between the coolant and the weld should be controlled within the range of 0.3m to 1.5m. If the distance is less than 0.3m, the coolant may prematurely contact the weld, cooling it before it is fully solidified, affecting its strength and toughness. If the distance is greater than 1.5m, the coolant may not be able to effectively reduce the weld temperature in a timely manner, leading to microstructural changes in the weld area, such as coarse grains and reduced hardness, thus affecting weld quality. In practice, the distance between the coolant and the weld can be precisely controlled by adjusting the position and spray angle of the cooling nozzle. For example, during spray cooling, the distance between the coolant and the weld can be 0.3m, 0.6m, 0.9m, 1.2m, 1.5m, etc.

[0076] In some embodiments, the weld seam of the square tube used for the finished battery pack frame does not crack when fully flattened, and the impact energy at -20°C reaches more than 90% of that at room temperature.

[0077] In some embodiments, the battery pack frame, which is made of square tubing, is used in electric commercial vehicles.

[0078] This application's embodiments, on the one hand, through precise composition design, enable the steel to maintain high strength while also possessing good plasticity and toughness, providing a foundation for subsequent processing and forming. On the other hand, addressing the challenges in tube manufacturing processes caused by the increased hardness of high-strength steel, a corresponding method for preparing square tubes is proposed. This method employs a "direct square forming" roll forming process, using a fixed-length bending method, where the radius gradually decreases during forming. Each 90° right angle requires 10 to 15 passes to complete the forming process. This method effectively reduces the peak stress and strain at the corners in each pass, minimizing the risk of cracking. Rollers and guide rings shape the tube blank into an I-shaped butt joint, ensuring the height difference between the two sides of the weld section is less than a certain value, providing favorable conditions for subsequent welding. A welding machine with an appropriate frequency is selected based on the thickness of the tube blank. The skin effect and proximity effect are used to concentrate heating on the edges, rapidly melting them. Pressure is applied by extrusion rollers, causing the molten metal to bond and form a weld. Simultaneously, the extrusion amount is controlled to ensure weld quality. The excess height of the inner and outer seams is removed to ensure the flatness and appearance quality of the weld. A spray cooling method is used to rapidly cool and solidify the weld, improving its strength and toughness. The square tube obtained through this preparation method exhibits welds that are completely flattened without cracking, and its impact energy at -20℃ reaches over 90% of room temperature, ensuring the reliability of the square tube in harsh environments. This technical solution addresses the challenges in tube manufacturing processes caused by the increased hardness of high-strength steel, proposing a high-precision square tube preparation method that effectively solves the core technical bottleneck of significant work hardening and low forming qualification rate in high-strength steel. Using the high-strength steel and square tube preparation method provided in this application, the resulting square tube can effectively solve the weight redundancy problem when used to prepare battery pack frames, achieving lightweighting of electric commercial vehicles.

[0079] The present application is further illustrated below with reference to specific embodiments. Experimental methods in the following embodiments that do not specify specific conditions are generally determined according to national / industry standards; if there is no corresponding national / industry standard, they are performed according to general international standards, conventional conditions, or conditions recommended by the manufacturer.

[0080] Example 1

[0081] This example provides a high-strength steel with the following composition: C: 0.05%, Mn: 2.6%, Si: 0.5%, Cr: 0.5%, Mo: 0.1%, Nb: 0.02%, Ti: 0.05%, V: 0.02%, with the balance being Fe and unavoidable impurities. The steel plate has a yield strength of 780 MPa, a tensile strength of 1040 MPa, a microstructure of 90% martensite + 10% ferrite, and a thickness of 3 mm. The target square tube has a cross-sectional dimension of 60 mm × 50 mm and a bending angle D = 6 mm. A fixed-length bending method is used, with each 90° right angle formed in 12 passes, and the edge strain value ε < 0.006 in each pass. The tube blank to be welded is shaped into an I-shaped butt joint using rolls and guide rings, with a height difference of 0.05 mm between the two sides of the cross-section to be welded. The selected welding machine had a frequency of 250kHz, an output power of 54kW, an extrusion depth of 3mm, a welding speed of 15m / min, and a cooling water distance of 0.5m from the weld. The resulting square tubes underwent flattening tests, with the weld at 90° and 0° to the applied force direction, respectively. The square tubes remained intact even after complete flattening. The impact energy of the weld at room temperature and -20°C was 28J and 26J, respectively.

[0082] Example 2

[0083] This example provides a high-strength steel with the following composition: C: 0.06%, Mn: 2.2%, Si: 0.4%, Cr: 0.4%, Mo: 0.1%, Nb: 0.03%, Ti: 0.02%, V: 0.04%, with the balance being Fe and unavoidable impurities. The steel plate has a yield strength of 785 MPa, a tensile strength of 1067 MPa, a microstructure of 90% martensite + 8% ferrite, and a thickness of 4 mm. The target square tube has a cross-sectional dimension of 50 mm × 80 mm and a bending angle D = 8 mm. A fixed-length bending method is used, with each 90° right angle formed in 12 passes, and the edge strain value ε < 0.008 for each pass. The tube blank to be welded is shaped into an I-shaped butt joint using rolls and guide rings, with a height difference of 0.06 mm between the two sides of the cross-section to be welded. The selected welding machine had a frequency of 200kHz, an output power of 60kW, an extrusion depth of 6mm, a welding speed of 12m / min, and a cooling water distance of 0.5m from the weld. The resulting square tubes underwent flattening tests, with the weld at 90° and 0° to the applied force direction, respectively. The square tubes remained crack-free even after complete flattening. The impact energy of the weld at room temperature and -20°C was 32J and 30J, respectively.

[0084] Example 3

[0085] This example provides a high-strength steel with the following composition: C: 0.05%, Mn: 2.3%, Si: 0.4%, Cr: 0.4%, Mo: 0.1%, Nb: 0.02%, Ti: 0.02%, V: 0.02%, with the balance being Fe and unavoidable impurities. The steel plate has a yield strength of 752 MPa, a tensile strength of 974 MPa, a microstructure of 90% martensite + 15% ferrite, and a thickness of 6 mm. The target square tube has a cross-sectional dimension of 80 mm × 130 mm and a bending angle D = 12 mm. A fixed-length bending method is used, with each 90° right angle formed in 12 passes, and the edge strain value ε < 0.012 in each pass. The tube blank to be welded is shaped into an I-shaped butt joint using rolls and guide rings, with a height difference of 0.06 mm between the two sides of the cross-section to be welded. The selected welding machine had a frequency of 200kHz, an output power of 85kW, an extrusion depth of 9mm, a welding speed of 10m / min, and a cooling water distance of 0.7m from the weld. The resulting square tubes underwent flattening tests, with the weld at 90° and 0° to the applied force direction, respectively. The square tubes remained crack-free even after complete flattening. The impact energy of the weld at room temperature and -20°C was 60J and 56J, respectively.

[0086] Comparative Example 1

[0087] This example provides a high-strength steel with the following composition: C: 0.08%, Mn: 2.3%, Si: 0.4%, Cr: 0.4%, Mo: 0.1%, Nb: 0.06%, Ti: 0.02%, V: 0.02%, with the balance being Fe and unavoidable impurities. The steel plate has a yield strength of 790 MPa, a tensile strength of 1132 MPa, a microstructure of 90% martensite + 7% ferrite, and a thickness of 2 mm. The target square tube has a cross-sectional dimension of 80 mm × 130 mm, a bending angle D = 6 mm, and cracks during the rolling process.

[0088] Comparative Example 2

[0089] This example provides a high-strength steel with the following composition: C: 0.06%, Mn: 2.3%, Si: 0.4%, Cr: 0.4%, Mo: 0.1%, Nb: 0.02%, Ti: 0.02%, V: 0.03%, with the balance being Fe and unavoidable impurities. The steel plate has a yield strength of 776 MPa, a tensile strength of 989 MPa, a microstructure of 90% martensite + 12% ferrite, and a thickness of 4 mm. The target square tube has a cross-sectional dimension of 80 mm × 130 mm and a bending angle D = 16 mm. A fixed-length bending method is used, with each 90° right angle formed in 12 passes, and the edge strain value ε < 0.008 for each pass. The tube blank to be welded is shaped into an I-shaped butt joint using rolls and guide rings, with a height difference of 0.05 mm between the two sides of the cross-section to be welded. The selected welding machine had a frequency of 200kHz, an output power of 75KW, an extrusion depth of 2mm, a welding speed of 10m / min, and a cooling water distance of 0.7m from the weld. The obtained square tubes were subjected to flattening tests, with the weld at 90° and 0° to the direction of force, respectively. Both square tubes cracked when completely flattened.

[0090] Furthermore, one or more technical solutions in the embodiments of the present invention have at least the following technical effects or advantages:

[0091] Significantly improved material performance: By optimizing the chemical composition design of high-strength steel, excellent mechanical properties of yield strength ≥750MPa and tensile strength ≥950MPa were achieved, providing a solid material foundation for the lightweighting of the battery pack frame.

[0092] Achieving lightweight design: The high-strength steel and square tube manufacturing method of this application are used to prepare the square tube for the battery pack frame, which can effectively reduce the weight redundancy of the frame, meet the urgent need for lightweighting of electric commercial vehicles, and thus improve the vehicle's range and economy.

[0093] Improving the precision of tube manufacturing process: In response to the challenges in tube manufacturing caused by the increased hardness of high-strength steel, the square tube preparation method proposed in this invention effectively solves the problems of significant work hardening and low forming qualification rate of high-strength steel, and improves the dimensional accuracy and mechanical properties of square tubes.

[0094] Enhanced durability of square tubes: The weld seams of the prepared square tubes are completely flattened without cracking, and the impact energy at -20℃ reaches more than 90% of that at room temperature, indicating that the square tubes have good low-temperature toughness and weld strength, which can meet the requirements of battery pack frames under complex working conditions.

[0095] Promoting the development of green transportation: The method provided in this embodiment of the invention helps to reduce the overall weight of electric commercial vehicles, reduce energy consumption and environmental pollution, which is in line with the development trend of green transportation and is of great significance to promoting the sustainable development of the transportation industry.

[0096] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed in this application.

Claims

1. A high-strength steel for a battery pack frame, wherein the chemical composition of the high-strength steel, by mass fraction, is: C: 0.04%–0.12%, Mn: 1.8%–2.8%, Si: 0.3%–1.0%, Cr: 0.3%–1.0%, Mo: 0.1%–0.5%, Nb: 0.002%–0.06%, Ti: 0.01%–0.1%, V: 0.01%–0.1%, with the balance being Fe and unavoidable impurities.

2. The high-strength steel according to claim 1, characterized in that, The high-strength steel meets at least one of the following properties: yield strength ≥ 750 MPa, tensile strength ≥ 950 MPa.

3. The high-strength steel according to claim 1, characterized in that, The microstructure of the high-strength steel is martensite + ferrite, wherein the volume fraction of the ferrite is 5% to 30%.

4. The high-strength steel according to claim 1, characterized in that, The thickness d of the high-strength steel is 2mm to 8mm.

5. A square tube for a battery pack frame, said square tube being made of high-strength steel as described in any one of claims 1 to 4, wherein, The cross-sectional length a of the square tube is 10mm to 100mm, and the cross-sectional width b of the square tube is 20mm to 200mm.

6. A method for manufacturing a square tube for a battery pack frame as described in claim 5, the method comprising: The high-strength steel is roll-formed to obtain a pipe blank to be welded; The tube blank to be welded is subjected to high-frequency welding to obtain a square tube for the finished battery pack frame.

7. The method according to claim 6, characterized in that, The process of roll forming the high-strength steel to obtain a pipe blank for welding includes: The high-strength steel is directly formed into a square cross section through multiple progressive bending processes to obtain a semi-finished square tube. The semi-finished square tube is shaped to form an I-shaped butt joint, thus obtaining a tube blank to be welded. Each right angle of the square tube is completed through 10 to 15 bends, and the edge strain value ε of each bend satisfies: R represents the radius of the roll forming roll, and d represents the thickness of the high-strength steel; The final bending angle D of the tube blank to be welded is d to 4d, and the height difference between the two sides of the tube blank to be welded is <0.05d, where d is the thickness of the high-strength steel in mm.

8. The method according to claim 6, characterized in that, The process of performing high-frequency welding on the tube blank to obtain a finished square tube for the battery pack frame includes: Based on the thickness of the high-strength steel, a high-frequency welding machine with an appropriate frequency is selected. Where ρ is the resistivity of high-strength steel at 1400℃, and its value is 138 μΩ·cm; μ r d represents the magnetic permeability of the high-strength steel at 1400℃, with a value of 1H / cm; d represents the thickness of the high-strength steel. Select appropriate welding power P and welding speed v based on the cross-sectional dimensions of the tube blank to be welded. Where k is a constant with a value of 0.68 to 0.85, a is the cross-sectional length of the square tube, and b is the cross-sectional width of the square tube; The welding speed v is 5m / min to 30m / min; The edges of the tube blank to be welded are heated using the high-frequency welding machine. Pressure is applied to the edge to fuse the melted edge into a weld, wherein the extrusion amount S of the applied pressure is 0.8 to 3 times the thickness d of the high-strength steel; Remove the excess weld seam material from the outer seam to ensure the weld seam is free of defects; The weld is cooled by spraying coolant at a distance of 0.3m to 1.5m from the weld. The tube blank to be welded after being sprayed and cooled is cut to length to obtain the finished square tube for the battery pack frame.

9. The method according to claim 8, characterized in that, The weld seams of the square tube used for the finished battery pack frame do not crack when fully flattened, and the impact energy at -20℃ reaches more than 90% of that at room temperature.

10. The method according to claim 5, characterized in that, The battery pack frame, fabricated from the aforementioned battery pack frame using square tubing, is used in electric commercial vehicles.

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