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 1373K (1100 degrees 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 573K (300 degrees C) induced the precipitation of thin cementite or kappa-carbide platelets in tempered martensite. With the increasing tempering temperature, the type of precipitate changed from cementite to kappa-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 373K (100 degrees C)-tempered specimen with tempered martensite embedded with thin cementite platelets revealed ultrahigh strengths (yield strength of 945 MPa and ultimate tensile strength of similar to 1.56 GPa) with moderate elongation of similar to 16.8 pct. (C) The Minerals, Metals & Materials Society and ASM International 2019