A method for manufacturing a battery pack case pipe by roll forming a shot blasting-free, non-plated hot-formed steel

Abstract

The present application relates to the technical field of steel metallurgical production, and especially relates to a method for manufacturing battery pack box pipe fittings by rolling forming of a shot-free and non-coating hot forming steel, which adopts a heating device and a cooling device added after a rolling forming process to complete the pipe fitting manufacturing of the battery pack box, and the manufacturing process comprises unwinding-rolling forming-heating-cooling; the battery pack box pipe fitting is heated to 900-950 DEG C, and the heating time is 30-90s.The present application has the beneficial effects that: the subsequent shot blasting treatment is not needed, the production efficiency is further improved on the basis of improving the strength of the battery pack box pipe fitting, the method has simple design structure and low investment cost, and is conducive to realizing industrialization and wide application.

Description

Technical Field

[0001] The present invention relates to the technical field of hot forming metal processing, and particularly relates to a method for manufacturing battery pack box pipe members by roll forming of shot-free and coating-free hot-formed steel. Background Art

[0002] The battery pack box is a core safety component of new energy vehicles, which has extremely high requirements for its strength, rigidity, sealing performance, dimensional accuracy and collision resistance. Currently widely used DP steel is difficult to accurately predict and control the dimensional accuracy and poses a sealing risk due to its high springback and residual stress. Meanwhile, due to the high strength and work hardening characteristics of DP steel, it is difficult to perform secondary processing such as punching and flanging after forming, and cracking is prone to occur.

[0003] Although roll forming of DP steel can theoretically manufacture battery pack box pipe members with high strength and controllable cost, the huge challenges in sealing performance, dimensional accuracy and lightweight make it not the optimal solution, especially in the field of passenger vehicles with strict performance requirements.

[0004] In recent years, there has been a practice of using Mn-B system hot-formed steel to prepare pipe members by adding heating and cooling devices at the position after forming on a conventional roll forming line and before pipe cutting. However, due to the poor high-temperature oxidation resistance of Mn-B system hot-formed steel, the parts need to be shot-blasted after hot forming to remove the surface scale generated during the hot forming process. Meanwhile, due to its poor hardenability, the cooling conditions after heating are relatively strict, and a cooling rate greater than 27°C / s can only be adopted to finally form a fully martensitic structure, so as to ensure the strength requirements of the parts. For the battery pack box, a square tube is first made by roll forming. If Mn-B system hot-formed steel is used, it is difficult to remove the scale on the inner wall, and the cooling capacity requirement for the "day"-shaped square tube part in the battery pack box is higher, which adversely affects the production efficiency, production cost and equipment investment of the parts.

[0005] Patent Publication No. CN109794506B entitled "Hot-formed Steel Plate Roll Forming Apparatus and Method" discloses a hot-formed steel plate roll forming apparatus comprising an apparatus main body, on which an initial positioning roller, a forming roller, a heating device, a control box, a quenching device, a shaping positioning roller and a blanking positioning roller are arranged. The heating and quenching devices are located between the forming roller and the shaping positioning roller, the heating temperature is 800°C to 880°C, and the used material is Mn-B system hot-formed steel, which has poor hardenability, and can only adopt a cooling rate greater than 27°C / s to finally form a fully martensitic structure, so as to ensure the strength requirements of the parts, and the cooling conditions after heating are relatively strict.

[0006] Patent Publication No. CN116511310A entitled "A Continuous Hot Roll Forming Process and Part Preparation Method" discloses a continuous hot roll forming process and part preparation method, which includes: a pretreatment stage; a roll forming stage; a heating stage, where the heating stage includes a temperature rising stage and a heat preservation stage; a pre-cooling stage, as well as a shape correction quenching stage, a 3D bent pipe quenching stage and a post-treatment stage. Its heating, heat preservation and quenching stages are located between the roll forming stage and the part cutting stage, the heating and heat preservation temperature is 850℃ to 1000℃, and the material used is Mn-B system hot stamping steel. The scale on the inner wall of the steel is difficult to remove, and meanwhile, the "日"-shaped square tube part in the battery pack box has higher requirements for cooling capacity, all of which adversely affect the production efficiency, production cost and equipment investment of the parts. Contents of the Invention

[0007] To overcome the deficiencies of the prior art, the present invention provides a method for manufacturing battery pack box pipe fittings by roll forming of shot-blasting-free and coating-free hot stamping steel. After the battery pack box parts are formed and cut, a heating device and a cooling device are added to heat and cool the pipe fittings, so as to achieve more uniform heating and more complete cooling of the pipe fittings, which is beneficial to the homogenization of the structure and properties of the pipe fittings.

[0008] To achieve the above purpose, the present invention adopts the following technical solutions: A method for manufacturing battery pack box pipe fittings by roll forming of shot-blasting-free and coating-free hot stamping steel, the chemical composition of the hot stamping steel in weight percentage is as follows: C: 0.22 to 0.28%, Mn: 1.3 to 2.0%, Si: 2 to 2.5%, S: <0.01%, P: <0.015%, Al: 0.02 to 0.04%, Cr: 1.2 to 2.0%, Nb: 0.01 to 0.05%, V: 0.01 to 0.05%, Ti: 0.01 to 0.03%, Cu: 0.1 to 0.3%, Ni: 0.05 to 0.15%, with the balance being Fe and other unavoidable impurities; In the method for manufacturing battery pack box pipe fittings by roll forming of hot stamping steel, a heating device and a cooling device are added after the roll forming process to continuously heat and cool the cut pipe fittings to complete the manufacture of the battery pack box pipe fittings. The manufacturing steps include uncoiling - roll forming - heating - cooling; the specific manufacturing steps include the following contents: S1. Uncoiling - Roll Forming: The hot stamping steel coil is slitted according to the width and then loaded on the machine, and the battery pack box is prepared and cut after roll forming; S2. Heating: An induction heating method is adopted to heat the battery pack box to 900 to 950℃, the heating time is 30 to 90s, and a nitrogen atmosphere protection is adopted in the heating device; S3. Cooling: The cooling device uses a nitrogen atmosphere for protection and employs a fan cooling method to cool the pipes to room temperature before they are finished products. The cooling rate is >5℃ / s.

[0009] Furthermore, the forming roller in step S1 includes: an initial positioning roller, a forming roller, a straightening positioning roller, a welding positioning roller, a straightening roller, and a blanking positioning roller.

[0010] Furthermore, the thickness of the iron oxide scale on the surface of the battery pack casing tube after roll forming is less than 500 nm, so no subsequent shot blasting treatment is required.

[0011] Furthermore, the sheet metal used for roll forming of battery pack casing tubes has a thickness of 0.8–1.8 mm and a width of 1000–1500 mm.

[0012] Furthermore, the metallographic structure of the hot-formed steel is: 95% martensite + 5% retained austenite.

[0013] Furthermore, the yield strength of the hot-formed steel after hot forming can reach 1000-1150MPa, the tensile strength can reach 1500-1700MPa, the elongation can reach more than 7%, and the bending angle is greater than 60°.

[0014] Compared with the prior art, the beneficial effects of the present invention are: 1) Material Innovation: For the first time, Cr-Si alloy system hot-formed steel is used for the roll forming of battery pack casings and tubing. This product features high strength, high hardenability, and good resistance to high-temperature oxidation. The synergistic effect of Si and Cr provides excellent oxidation resistance, resulting in virtually no iron oxide scale formation after hot forming, eliminating the need for shot blasting. Due to the high hardenability of Cr, the critical cooling rate is only 5℃ / s, far lower than the 27℃ / s of 22MnB5 steel. Therefore, under nitrogen atmosphere protection, cooling to room temperature using a fan is sufficient. These advantages solve the application pain point of difficult iron oxide scale removal from the inside of battery pack casings and tubing. Furthermore, compared to roll forming with Mn-B hot-formed steel, it reduces the requirements for cooling conditions and eliminates the need for quenching, requiring only fan cooling to room temperature under nitrogen atmosphere protection.

[0015] 2) Process Flow Innovation: After the battery pack casing tubes are formed and cut, heating and cooling devices are added to heat and cool the tubes. The purpose of this process flow design is to achieve more uniform heating and more complete cooling of the tubes, which is conducive to the homogenization of the tube structure and performance. The heating device adopts induction heating mode, which can heat the tubes to 900-950℃ in 30-90 seconds, greatly shortening the heating time. The cooling device only uses fan cooling, eliminating the need for quenching, simplifying the structure of the cooling device, and is more conducive to improving production efficiency and reducing investment costs.

[0016] 3) Performance improvement: The yield strength can reach 1000 to 1150 MPa, the tensile strength can reach 1500 to 1700 MPa, the elongation is more than 7%, and the bending angle is >60°, which improves the strength of parts and is conducive to advancing the lightweight process of automobiles.

[0017] 4) The heating and cooling devices added to the conventional roll forming production line have a simple design structure and low investment cost, which is beneficial to wide industrial application.

[0018] 5) Production efficiency improvement: The perfect combination of roll forming and hot forming, that is, after adding heating devices and cooling devices to the existing roll forming production line, realizes the application of shot-blasting-free uncoated hot-formed steel products in battery pack housing parts, effectively solves the application pain points of uniformly improving the performance of pipe fittings, especially the "day"-shaped square pipe, and the difficulty in removing the scale on the inner wall, and no shot blasting treatment is required after hot forming. On the basis of improving the strength of the battery pack housing pipe fittings, the production efficiency is further improved. Compared with the existing related patents, the production efficiency can be increased by at least more than 30%. Brief Description of the Drawings

[0019] Figure 1 is the system flow chart of the roll forming manufacturing of the battery pack housing using the shot-blasting-free uncoated hot-formed steel according to the present invention.

[0020] Figure 2 is the metallographic structure diagram of the battery pack housing pipe fitting according to the present invention.

[0021] Figure 3 is the structure diagram of the scale thickness of the battery pack housing pipe fitting according to the present invention. Detailed Description of the Embodiments

[0022] The specific embodiments of the present invention are further described below with reference to the accompanying drawings: A method for manufacturing a battery pack housing by roll forming of shot-blasting-free uncoated hot-formed steel, the method adopts adding a heating device and a cooling device after the roll forming process to complete the manufacture of the pipe fittings of the battery pack housing, and the manufacturing process includes uncoiling - roll forming - heating - cooling; the specific manufacturing method includes the following contents: S1. Uncoiling - Roll forming: The hot-formed steel coil is slit to the required width and then loaded onto the machine. The loaded steel coil passes through the initial positioning rollers, forming rollers, shaping positioning rollers, welding positioning wheels, straightening wheels, and blanking positioning rollers to complete the preparation and cutting of the battery pack housing pipe fittings. The thickness of the roll-formed battery pack housing pipe fitting is 0.8 to 1.8 mm, and the width is 1000 to 1500 mm; S2. Heating: After the battery pack casing tubes are formed and cut, a heating device is added to heat the tubes first. The purpose of this process design is to achieve more uniform heating of the tubes, which is conducive to the uniformity of the tube structure and performance. The heating device adopts induction heating, which can heat the battery pack casing tubes to 900-950℃ in 30-90 seconds, greatly shortening the heating time. The heating device is protected by a nitrogen atmosphere.

[0023] S3, Cooling: A cooling device is added after the heating device. The cooling device is protected by a nitrogen atmosphere and is cooled to room temperature by a fan before the finished product is produced. The cooling rate is >5℃ / s, eliminating the need for quenching and simplifying the structure of the cooling device. Subsequent shot blasting is not required, which is more conducive to improving production efficiency and reducing investment costs.

[0024] After roll forming, the thickness of the iron oxide scale on the battery pack casing is less than 500 nm.

[0025] Example: The chemical composition of the steel in the embodiments of the present invention is shown in Table 1; the hot forming process parameters in the embodiments of the present invention are shown in Table 2.

[0026] The comparative example is the production of battery pack casing tubing using a conventional hot forming roll forming method. The material used is Mn-B system hot forming steel. The performance of the finished product is compared with that of the finished product in the embodiment of the present invention, as shown in Table 3.

[0027] Table 1 - Chemical composition (wt, %) of the steel in the embodiments of the present invention: Table 2 - Thermoforming process parameters of embodiments of the present invention: Table 3 Comparison of the performance of finished products from embodiments of the present invention and comparative examples: From the above embodiments, it can be concluded that: Performance improvements in battery pack casing components: After roll forming, 22MnB5 hot-formed steel achieves a yield strength of approximately 900-1000 MPa, a tensile strength of 1300-1500 MPa, an elongation of 5-7%, and a bending angle of approximately 55°. The shot-blast-free, uncoated hot-formed steel described in this invention, after roll forming, achieves a yield strength of 1000-1150 MPa, a tensile strength of 1500-1700 MPa, an elongation exceeding 7%, and a bending angle greater than 60°. Compared to 22MnB5, this represents an 11% increase in yield strength, a 15% increase in tensile strength, a 40% increase in elongation, and a 9% increase in bending angle.

[0028] While improving the strength of the pipe fittings, the iron oxide scale thickness is only 260nm to 460nm, which means that subsequent welding and painting can be carried out without shot blasting, effectively solving the problem of removing iron oxide scale from the inner wall.

[0029] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for manufacturing battery pack casing tubular components by roll forming of hot-formed steel without shot blasting or coating, characterized in that, The chemical composition of the hot-formed steel, expressed as a percentage by weight, is as follows: C: 0.22–0.28%, Mn: 1.3–2.0%, Si: 2–2.5%, S: <0.01%, P: <0.015%, Al: 0.02–0.04%, Cr: 1.2–2.0%, Nb: 0.01–0.05%, V: 0.01–0.05%, Ti: 0.01–0.03%, Cu: 0.1–0.3%, Ni: 0.05–0.15%, with the balance being Fe and other unavoidable impurities. The method for manufacturing battery pack casing tubing by hot forming of steel employs a heating and cooling device after the roll forming process to continuously heat and cool the cut tubing, thus completing the manufacturing of the battery pack casing tubing. The manufacturing steps include uncoiling, roll forming, heating, and cooling; the specific manufacturing steps include the following: S1. Uncoiling-roll forming: After being slit into strips according to width, the hot-formed steel coils are rolled and cut to complete the preparation and cutting of the battery pack casing. S2, Heating: The battery pack housing is heated to 900-950°C using induction heating for 30-90 seconds, and the heating device is protected by a nitrogen atmosphere. S3, Cooling: The cooling device is protected by a nitrogen atmosphere. The pipes are cooled to room temperature by a fan and then the finished product is produced. The cooling rate is >5℃ / s. After roll forming, the surface iron oxide scale of the battery pack casing is less than 500nm thick, so no subsequent shot blasting is required. The microstructure of hot-formed steel is 95% martensite + 5% retained austenite. After hot forming, the yield strength of hot-formed steel can reach 1000-1150MPa, the tensile strength can reach 1500-1700MPa, the elongation can reach more than 7%, and the bending angle is greater than 60°.

2. The method for manufacturing battery pack casing tubing by roll forming of hot-formed steel without shot blasting or coating, as described in claim 1, is characterized in that... The forming rollers in step S1 include: an initial positioning roller, a forming roller, a straightening positioning roller, a welding positioning roller, a straightening roller, and a blanking positioning roller.

3. The method for manufacturing battery pack housing tubular components by roll forming of hot-formed steel without shot blasting or coating, as described in claim 1, is characterized in that... The sheet metal used for roll forming of battery pack casing tubes has a thickness of 0.8–1.8 mm and a width of 1000–1500 mm.

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