Pre-quenching induced lath structures and enhanced TRIP effect to optimize the strength-ductility of ultrahigh-strength hot-galvanized steel

Abstract

Compared with the conventional hot-dip galvanized steel, an optimized strength-ductility combination was obtained by the combination of continuous galvanizing line (CGL) compatible quenching and austempering (QAT) treatment and pre-quenching, i.e., “pre-quenching + CGL-QAT”. The effects of the pre-quenching on the microstructure and mechanical performance were analyzed by using dilatometry, SEM, AES-EBSD, TEM and XRD technique. Compared with QAT samples, pre-quenching could eliminate the blocky M-A islands and promote the formation of lath reversed austenite and bainite ferrite in “pre-quenching + CGL-QAT” samples. Meanwhile, a large fraction (22.7–25.8%) of lath RA were obtained, such that brittle intergranular fracture can be prevented. The lath bainite/ferrite structures and sufficient TRIP effect enhanced the strength-ductility combination with an UTS reaching 1000 MPa and TEL around 30%. The fracture mainly presents ductile fracture and results in a ductile fracture surface with large dimples. The excellent strength-ductility combination with a UTS above 980 MPa, a YS of 625 MPa, a TEL of 29.1% and a higher PSE around 30 GPa·% can be obtained, which exceeds the current commercial hot-dip galvanized steel grades.