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Process development for lightweight lightweight steel panels
As China's automobile industry continues to grow and develop, the production, sales, and ownership of motor vehicles have seen a significant increase. In response to stricter government regulations on vehicle emissions, energy consumption, and safety, automakers are now focusing more on enhancing ride comfort, noise reduction, corrosion resistance, and aesthetics. To meet these demands, car manufacturers are constantly improving vehicle performance and features. One effective strategy is lightweighting, which helps reduce COâ‚‚ emissions, fuel consumption, raw material usage, and improves ride comfort by reducing the overall weight of the vehicle.
In a complete vehicle, the body-in-white accounts for about 25% of the total mass, making it a critical area for weight reduction. High-strength steel plates offer an excellent solution, as they allow for a lighter body while maintaining structural strength and safety. With growing demands for improved safety, reduced fuel consumption, and lower emissions, the use of high-strength steel in vehicle manufacturing has been steadily increasing. Proper application and development of high-strength steel components play a vital role in accelerating product upgrades and shortening development cycles.
This paper takes the middle channel front elongate piece as a case study to illustrate the application of high-strength steel in automotive body construction and the tooling development process. Through mechanical property analysis and CAE forming simulations, an optimal stamping plan was determined, resulting in a final product that meets all quality requirements.
**Product Description**
High-strength steel sheets come in various grades, each with different mechanical properties. Selecting the right grade is crucial for product design. The initial design of the channel front elongate piece used B340/590DP steel with a thickness of 1.2mm. However, this component presented several challenges: large external dimensions, irregular shape, low elongation, and difficulty in forming.
B340/590DP has a yield strength of 340–500 MPa, tensile strength of ≥590 MPa, and an elongation after fracture of 18%. Compared to conventional low-carbon steel, its formability is poorer, leading to potential issues like cracking, wrinkling, and uneven thickness during stamping. Additionally, high residual stresses and springback can affect the final shape and dimensional accuracy of the part.
Due to these limitations, traditional methods used for low-carbon steel cannot be directly applied. The use of high-strength steel significantly impacts stamping design, process development, mold structure, and debugging.
**CAE Forming Analysis**
During early design stages, CAE forming analysis of the product model (see Figure 2) revealed severe cracking, indicating that the part could not be formed properly. Many plane strain points were above the critical section, leading to cracks across the sheet.
The draw forming process involves clamping the material between the die and punch, where plastic deformation occurs due to two-dimensional strain. If local strain is too high, failure can occur. The material’s ability to redistribute strain is closely related to its work hardening capacity. Higher work hardening means better formability, while lower work hardening leads to poor drawability.
After thorough analysis, it was found that the main issue was the low elongation and poor formability of B340/590DP. As a result, the material was re-selected to one with higher elongation and better formability—B220P2. This material has a yield strength of 220–320 MPa, tensile strength of ≥380 MPa, and elongation after fracture of 28%, significantly improving the forming capability.
Following multiple rounds of optimization, the final data model (Figure 3) showed good results, meeting the draw forming requirements. The simulation indicated that most plane strain points were below the critical section, with no risk of cracking (see Figure 4).
**Stamping Process Plan**
Alongside data modifications, the stamping processability of the part was carefully analyzed. The middle channel front elongate member has a complex shape and large undulations. A single-action stretching approach was chosen, with additional steps added to prevent negative angles. To reduce rebound and avoid cracking or distortion, the part was designed to fully enter the cavity during forming.
To streamline the process, punching and trimming were combined into a single operation. A stretch-forming approach using a shaped material was also adopted to ensure smooth forming.
After two rounds of die debugging, the stamping process was successfully implemented. The final product (Figure 5) showed no cracks or wrinkles, demonstrating the effectiveness of the process.
The final product (Figure 6) met all quality requirements, with no major defects detected during trial assembly.
**Conclusion**
High-strength steel offers numerous advantages for vehicle lightweighting, but its use requires careful consideration of material formability and flow characteristics. Only by selecting materials that meet the formability requirements of digital molds can the full benefits of high-strength steel be realized. At the same time, choosing the right stamping process and designing an efficient workflow are essential to avoid cracking and achieve high-quality parts. By following these principles, automakers can ensure both product quality and timely development.