Strategy of complex carbides in microalloyed 4Mn steels processed by quenching and austenite reversion

This work describes a strategy to employ complex carbides for high-yield microalloyed 4Mn steels processed by quenching and reversion (Q&R). The nano/microstructural details are investigated by using electron microscopy and atom probe. It is found that complex carbides are classified into two types and contribute to yield strength in different ways. First, coarse carbides (MC1), which are retained after austenization and quenching, provide the Zener pinning effect and contribute to strengthening by microstructure refinement and precipitation hardening. Second, nanoscale carbides (MC2), which precipitate in martensite during austenite reversion, provide precipitation hardening. Furthermore, the precipitation of MC1 accelerates austenite reversion due to the finer martensite microstructure. Hence, the strengthening effects and TRIP effect can be tuned by controlling the proportion of the two types of carbide. This concept is validated in Ti-Mo 4Mn steel, which exhibits a high yield strength of about 850 MPa, a high ultimate tensile strength of about 950 MPa, and excellent elongation of over 20%. The complex carbide strategy is able to increase yield strength without sacrificing much ductility for Q&R medium-Mn steels.