Partitioning-related microstructure evolution and mechanical behavior in a δ-quenching and partitioning steel

Abstract

The splitting of the quenching and partitioning of quenching - tempering & partitioning (Q-T&P) process permits us to investigate the as-quenched and partitioned states of the novel δ-quenching & partitioning (δ-QP) steel, separately. Both quenched and partitioned steels consisted of ferrite and martensite/austenite islands. After tempering at 350 °C, C and Mn partitioned into the austenite during partitioning. Besides, the films comprised of martensitic-based dislocation sub-structures were formed, and these were enriched with all solutes including Al. The deformation behavior of δ-QP steels was significantly adjusted by partitioning treatment. The fracture strain was improved by 38% after partitioning step. The higher tensile strength was attributed to the strengthened ferrite and more efficient transformation induced plasticity (TRIP) effect. The unpinned dislocations slipping in ferrite of partitioned steel induced an obvious recovery of work hardening rate. Meanwhile, the improved deformation uniformity and martensite ductility contributed to the higher ductility. Based on such an analysis of microstructure-property relationship, it is conducive to develop high-grade δ-QP steels and give some guidelines on the optimal design of low-density alloy system.

Keywords

• Microstructural evolution
• Deformation
• Low-density steel
• High-Al
• δ-TRIP steel