Batch annealing enhances the yield strength of a high quenching temperature Q&P steel through accelerated bainitic transformation

Abstract

Hot-dip galvanized quenching and partitioning (Q&P) steels exhibit excellent corrosion resistance and strength-ductility synergies. However, production using conventional continuous galvanizing lines (CGLs) is challenging because these lines cannot meet the required quenching and reheating capacity. The presence of ferrite and absence of sufficient primary martensite led to a substantial decrease in the yield strength of the material. To solve this problem, we propose a simple batch annealing (BA) treatment prior to continuous annealing and galvanization. The BA treatment resulted in spheroidization of cementite in the coiled samples and enrichment of Mn. During the intercritical annealing process, the higher Mn content in the cementite of the BA sample retards cementite dissolution. The retained cementite particles served as nucleation sites and significantly promoted bainite formation during quenching and partitioning. Consequently, the yield strength increased by approximately 90%, from 438 MPa to 821 MPa. The total elongation of the BA samples was not affected due to the simultaneous stabilization of the retained austenite. The effect of batch annealing time was also investigated, and the balance between Mn enrichment in cementite and the number density of spheroidized cementite before Q&P treatment was revealed.