Effects of Cold‐Rolling Reduction on Microstructure, Thickness, Surface Roughness of AS20/20 and on Bending Toughness of AS20/20‐Coated Press‐Hardened Steel

Abstract

Lightweightness becomes an important trend in the automobile industry, as the requirements for energy conservation and emission reduction in this industry of all countries are continually raised. [1,2] When vehicle weight is lightened by 10%, oil consumption can be saved by 6%-8%. [3,4] To make lightweight vehicles satisfy the collision safety requirement, automobile host factories start to focus on research and application of high-strength materials while optimizing body structures. Currently, the advanced high-strength steel suitable for coldstamping forming and the press-hardened steel (PHS) suitable for hot-stamping forming can both be stamped to 3D body parts with complex shapes, and thus become the main materials in the construction parts of vehicle bodies.Along with the steel strength improvement, the elongation rate and forming performance of materials will be largely reduced, which will lead to lower product rate (product cracking, wrinkling), large resilience, and severe die abrasion during cold-stamping forming. [5] The third-generation advanced high-strength steel (e.g., medium manganese steel, quenching and partitioning steel) has tensile strength up to 1000-1200 MPa and can ensure the elongation rate up to 15%-40%. Compared with two-phase steel and low-alloy transformation-induced plasticity steel, the third-generation advanced high-strength steel has not been put into large-scale industrial production owing to many restrictions (e.g., instability in smelting and production process), though the strength and elongation rates are largely improved. [6,7] During hot-stamping forming, the steel blanks are heated to the austenitization status, stamped at high temperature, and rapidly quenched in the die to the martensite phase. This process of first forming and then hardening perfectly solves the conflict between strength and forming. The PHS is stronger than the third-generation advanced high-strength steel, and has low resilience and high success rate. Thus, the applied proportion of PHS in the body in white of vehicles is increasing. [8][9][10][11][12][13] The 22MnB5 steel, the commonly used PHS in industrial applications, is formed by heating steel blanks to 920-960 °C and preserving there for 3-6 min to induce austenitization. Then, the steel blanks are rapidly removed using manipulators to presser dies, where the blanks are formed at high temperature and rapidly cooled down at the rate above 30 °C s À1 to the martensite phase. [14][15][16][17] When uncoated PHS blanks are austenitized in a heating furnace and then transferred to dies, oxidation and decarbonization will inevitably occur. Hence, during the production, the iron oxide skins falling into the dies shall be periodically cleaned, [18,19] and be discarded by shot-peening the whole surface of the formed parts, which will raise production procedures and costs. [10] Hot-dipped Al-Si coatings are widely adopted as the surface protective layer to solve the surface oxidation of steel blanks during the hot-stamping of PHS. The hot-dipped Al-Si coat...