Effects of coiling temperature on the microstructures and mechanical properties of a hot-rolled medium-Mn steel processed by simulated coiling process

A hot-rolled medium-Mn steel was processed by a simulated coiling process, which significantly saves energy by replacing traditional off-line reheating process. The effects of coiling temperatures (above and below M s temperature) on the microstructures and mechanical properties of hot-rolled sheet were studied. The results showed carbon partitioning occurred during simulated coiling process whereas Mn diffusion was not noticeable. Coiling temperature affected the martensite/bainite transformation, which altered the morphology and grain size of untransformed austenite as well as carbon partitioning behavior, leading to distinct microstructures in different matrix structures. When the coiling temperature (290 °C) was below M s temperature, a martensite matrix was obtained, accompanied by carbides, film-like retained austenite and blocky retained austenite. This microstructure yielded the highest tensile strength of 1528 ± 1 MPa and the lowest total elongation of 10.4% ± 0.4%. In contrast, at coiling temperatures ranging from 400 °C to 600 °C (above the M s temperature), a bainitic ferrite matrix was formed, along with various second phases including film-like retained austenite, blocky retained austenite, martensite-austenite island, lath martensite and twin martensite. These samples exhibited a better combination of tensile strength (1231–1357 MPa) and total elongation (15.5%–17.3%) compared to the steel with a martensite matrix. This study indicates hot-rolled medium-Mn steel can be produced utilizing coiling process in the industrial hot rolling production line without additional off-line heat treatment.