Effect Of Heating Process On The Microstructure And Mechanical Properties Of Medium-Mn Steel

Along with the development of the automotive industry, higher requirements are puton safety, energy conservation and emission reduction. The medium manganese alloyedTRIP steel with its low cost and good mechanical properties has been widely researched.The organizational control theory of M3(Multiphase, Metastable and Multiscale) makesa new development direction of manganese steel heat treatment process.In this study, the new type medium manganese steels were used as experimentalmaterials, the microstructural evolution during heating was characterized by transmissionelectronic microscopy and electron backscatter diffraction. The amount of retainedaustenite was measured by X-ray diffraction and tensile test was used to characterizemechanical properties. The results showed that heating with a high rate or with aninterruption at low temperature leads to more austenite formed during the intercriticalannealing and then more retained after quenching. An isothermal holding at lowtemperature results in extensive precipitation of transition carbide which can becompletely dissolved during the intercritical annealing and acts as nucleation sites ofaustenite grains, leading to accelerated austenitization kinetics; whilst the high heating ratecould suppress both precipitation of cementite and recovery of the matrix during heating,promoting austenitization before recrystallization. After annealed at the intercriticalregion(between610℃and720℃) for10min，the tensile strength(Rm) w increased withthe temperature increasing. The elongation (A) increased at first and lowered later. Themaximum values Rm*A (26.96GPa%) are obtained for the steel annealed at670℃for10min.Based on the analysis of the relationship between microstructure and mechanicalproperties of manganese steel, the heating with a higher rate or an interruption at lower temperature can lead to more austenite formed during intercritical annealing and moreretained afterwards. And the comprehensive mechanical properties (Rm*A) increase withretained austenite fraction, which implies that TRIP effect is the key factor to obtain highstrength and ductility.