改善复杂形状的热成形先进高强钢汽车结构件成形性的工艺开发和机理分析

针对具有形状要求的先进高强钢(AHSS)汽车结构件热成形时经常发生破裂现象,从其各处分布的不同应力方式对样件微观结构、厚度分布和力学性能影响作用存在差异的角度分析,首次利用高速加热炉、急冷处理室和带有冷却水道模具的一体化实验设备,通过在冲压前设计急冷处理新方法,即由现有工艺的奥氏体化后直接冲压成形,转变成先经急冷处理,然后到700°C左右成形,其目的是:使得样件上以拉应力为主的加载区,因具有较好硬化指数n值而获得较好成形性;以压应力为主的某些抑制马氏体相变区域,因提前的急冷处理而在微观奥氏体母相中增加马氏体新相形核的几率,获得致密的组织结构,以改善强韧性.实验证明,通过冲压前的急冷处理,样件更为完整,未出现开裂现象;微观具有明显细化的马氏体排列形态;宏观硬度分布均匀,且都在460 Hv以上,能够满足高强度、高韧性的性能要求.从而,验证了该急冷方法的科学有效,并突破了国内现有热成形AHSS易开裂的瓶颈问题,为建立我国自有知识产权、复杂形状的热成形AHSS结构件生产工艺路线提供了依据.

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

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°C as in the traditional hot forming process, the proposed process cools the blank rapidly to around 700 oC 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.