Process development and mechanism analysis for improving the formability of complex hot forming AHSS parts

Abstract

Based on the analyses of how stress state affects the microstructure, thickness distribution and mechanical properties, rapid cooling pretreatment is proposed to solve the cracking issues of hot forming AHSS (Advanced High Strength Steel) parts with complex shapes. Rather than forming the part right after the blank is fully austenitized at around 900~950℃ as in the traditional hot forming process, the proposed process cools the blank rapidly to around 700℃ followed by forming. The rapid cooling pretreatment not only enables the material to have a higher hardening exponent n so as better formability in tension dominated areas, but also increases the probability of new nucleation of martensite in austenitic parent phase to obtain a dense microstructure and improved strength and toughness in compression dominated 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 of 460 HV or higher. The performance requirements of high strength and toughness are met. The rapid cooling method is thereby proved scientific and effective. It breaks through the bottleneck of the cracking problem in hot forming complex AHSS parts and establishes the theoretical basis of our own developed manufacturing process.