Influence of rapid cooling pretreatment on microstructure and mechanical property of hot stamped AHSS part

Based on the analyses of how stress state influences the microstructure, thickness distribution and mechanical properties, rapid cooling pretreatment is proposed to solve the cracking issues of hot stamped AHSS (advanced high strength steel) parts with complex shapes. Rather than stamping the part right after the blank is fully austenitized at around 900-950 °C as in the traditional hot stamping process, the proposed process cools the blank rapidly to around 700 °C then followed by forming and quenching. The rapid cooling pretreatment not only enables the AHSS to have a higher hardening exponent n value so as better formability in tension dominant areas, but also increases the probability of additional nucleation of martensite in austenitic parent phase that helps obtain a dense microstructure and improved strength and toughness in compression dominant areas. Experiments show the rapid cooling prior to forming does help form AHSS parts without cracking. The process also leads to a microstructure with refined martensitic phase arrangement and uniformly distributed macro hardness. The rapid cooling method is thereby proved scientific and effective. It breaks through the bottleneck of the cracking problem in hot stamping very complex AHSS parts and establishes the theoretical foundation for the massive production intent manufacturing process.