Effect of Tempering on the Microstructure and Tensile Properties of a Martensitic Medium-Mn Lightweight Steel

In the present study a Fe-8.8Mn-5.1Al-0.31C (wt pct) medium-Mn lightweight steel with martensitic matrix was newly designed, and the effect of tempering on its microstructure and tensile properties was investigated. When the medium-Mn lightweight steel specimen was quenched from annealing temperature of 1373 K (1100 °C), it revealed a triple-phase microstructure consisting of 64.6 pct fresh martensite, 20.2 pct ferrite, and 15.2 pct retained austenite. Tempering the as-quenched specimen at temperatures less than 573 K (300 °C) induced the precipitation of thin cementite or κ-carbide platelets in tempered martensite. With the increasing tempering temperature, the type of precipitate changed from cementite to κ-carbide, and thickening of platelets with the longer interspacing was observed by means of transmission electron microscopy and atom probe tomography. While the as-quenched specimen revealed high strain-hardening rate (SHR) and poor elongation (< 2 pct), tempered specimens exhibited moderate SHR and improved elongation (> 16.8 pct) mainly due to gradual transformation-induced plasticity in retained austenite stabilized by tempering. In particular, the 373 K (100 °C)-tempered specimen with tempered martensite embedded with thin cementite platelets revealed ultrahigh strengths (yield strength of 945 MPa and ultimate tensile strength of ~ 1.56 GPa) with moderate elongation of ~ 16.8 pct.