Structure-property relationship in novel low carbon hot-rolled TRIP steels via thermo-mechanical controlled processing and coiling

Abstract

A novel process involving thermo-mechanical controlled processing (TMCP) and coiling process was adopted to obtain multiple-phase mixed microstructure. The coiling temperature governed microstructural evolution, stability of retained austenite (RA) and microstructure-properties relationships. The results indicated that the optimum properties were obtained at a coiling temperature of 485 °C with combination of tensile strength of 1006 MPa, elongation of 13.3%, hole-expansion ratio of 28% and impact toughness of 14.7 J (1/4-Charpy size sample at −20 °C). A small amount of RA was present when the coiling temperature was 430 °C (close to the Ms point), resulting in low elongation. The size of blocky RA located at grain boundary was ~450 nm, whereas, the film-like RA embedded in bainite laths was ~40–110 nm. The film-like RA was more stable than blocky RA, which enabled superior formability. Both types of RA provided positive and continuous TRIP effect during deformation. But the presence of a large amount of high-carbon martensite adversely affected impact toughness and formability because it acted as a source for crack initiation as a hard phase in the matrix.