Microstructure evolution and enhanced performance of a novel Nb-Mo microalloyed medium Mn alloy fabricated by low-temperature rolling and warm stamping

Abstract

A combined low-temperature rolling and warm stamping process was proposed for a Fe-5.6Mn-0.19C-1.2Al-0.05Nb-0.22Mo alloy to improve the tensile ductility and surface quality of the stamped parts. The results show that the present warm-stamped U-shaped parts exhibited the relatively uniform ultrafine multiphase microstructure and desirable mechanical properties, e.g. the average products of ultimate tensile strength and total elongation (PSE) values reached 27.5–31.3GPa·%, which are more than double or triple times higher than those of the hot-stamped boron steels with similar strength level. As the stamping temperature decreased from 760 to 700°C, both the fractions of ferrite and austenite increased, resulting in the enhanced tensile ductility (up to 24.5%) with sacrificing tensile strength from 1717±117 to 1424±53MPa. The difference in stamping temperature greatly affected the tensile deformation behavior of the stamped parts: at 700°C, the tensile curves exhibited Lüders band propagation (LBP) behavior, and both Portevin-Le Chatelier (PLC) and transformation induced plasticity (TRIP) effects happened, while at 760°C, the tensile curves exhibited continuous yielding behavior, and only TRIP effect happened over the entire plastic deformation.