先进超高强钢搅拌摩擦焊超细组织演化与力学行为研究

Advanced ultra-high strength steel (AUHSS) has received more and more attention due to the large demand in automotive industry. However, coarse structures easily formed in weld zone of traditional fusion welded AUHSS, and serious softening ocurred usually at heat affected zone, which severely restrict the wide application of AUHSS. Friction stir welding (FSW) is a solid state joining technology, and weld softening can be greatly reduced under low heat input, providing an ideal method in welding of AUHSS with sound properties. In this project, two typical AUHSS of dual phase steel (DP 1180) and quenching and partitioning steel (Q&P 1180) produced by Bao Steel Group will be FSWed with low heat input. Based on the investigation of microstructral evolution and mechanical behaviors of the ultra-fine strucutres in the weld zone of FSW AUHSS joints, the grain refining and phase transformation mechanisms under the combination of severe plastic deformation and high temperature are expected to be well explained, static and dynamic deformation behaviors of typical ultra-fine grained steels will be further understood, and the relationship between dislocations and different grain boundaries and phase boundaries during deformation will be clarified. Meanwhile, new methods of improving the mechaical properties of AUHSS via adjusting the microstructure will be discussed.The results of this project will provide essential experimental data and theory support for the optimized design on the microstructure of the AUHSS welds.

在汽车工业需求的牵引下，先进超高强钢（AUHSS）受到了越来越广泛的关注。然而采用传统熔化焊工艺对AUHSS焊接时焊缝组织极易粗化、热影响区软化严重，限制了这种新型材料的广泛应用。搅拌摩擦焊(FSW)为固态连接技术，采用低热输入的FSW可大幅降低接头的软化，为AUHSS提供了一种理想的高质量焊接工艺。本项目以宝钢生产的两种典型AUHSS—双相钢DP1180和淬火配分钢Q&P1180为研究对象，在低热输入参数下进行FSW，通过对焊核区超细组织的演变及力学行为的深入研究，揭示在FSW大塑性变形和温度共同作用下钢的晶粒细化及相转变机制，加深对钢的典型超细晶组织静态及动态力学行为的理解，阐明不同特征的晶界及相界在变形过程中与位错的交互作用机制，探讨通过微观组织优化提高AUHSS力学性能的可能性，从而为AUHSS接头结构优化设计提供实验依据和理论指导。

随着轻量化的发展，先进超高强钢（AUHSS）受到了越来越广泛的关注。然而采用传统熔化焊工艺对AUHSS焊接时焊缝组织极易粗化、热影响区软化严重，限制了这种新型材料的广泛应用。搅拌摩擦焊(FSW)为固态连接技术，采用低热输入的FSW可大幅降低接头的软化，为AUHSS提供了一种理想的高质量焊接工艺。而基于FSW发展的搅拌摩擦加工（FSP）技术可利用大塑性变形和温升的特点制备超细晶组织，提升焊缝/加工区的性能。本项目以几种强度为1 GPa左右的超高强钢为研究对象，系统研究了FSW过程中焊接参数对材料流动及缺陷形成的影响，从而得出优化的焊接工艺；对焊接过程中各区域微观组织演变及相转变机制进行了深入研究，对FSW过程中温度场进行了数值模拟，并建立相关模型。同时利用FSP技术制备超细复相组织，并对相关微观组织演变机制和力学、腐蚀性能进行了深入研究。通过本项目研究，结合数值模拟阐明了高强钢FSW接头各区域的典型微观组织演变与温度场特征，首次实现了强度1 GPa以上Q&P1180超高强钢的等强焊接；利用低热输入FSP结合后续退火工艺在低碳钢中成功制备出超细铁素体、马氏体、残余奥氏体的混合复相等轴组织，其强韧性可达到中锰钢的水平，此外通过FSP处理的超高强高氮不锈钢塑性和耐蚀性得到明显改善。本研究揭示了在FSW/FSP大塑性变形和温度共同作用下钢的晶粒细化及相转变机制，加深了对钢的典型超细晶组织力学、化学行为的理解，提供了通过微观组织优化提高AUHSS力学、化学性能的新思路，从而为AUHSS及其接头结构优化设计提供了实验依据和理论指导。