The microstructures, Vickers hardness, and electromagnetic properties of the automotive die–quenched steel were investigated under various die temperatures.
The positive correlations between hardness and the coercivity of magnetic hysteresis loop and between hardness and electrical resistance were confirmed above the
hardness of HV370. EBSD and TEM observations reveal that the die–quenched steels contain both fresh martensite and auto–tempered martensite due to the high
martensite transformation temperature of the steel. Neural network analysis shows an increase of auto–tempered martensite with increasing die–temperature, which
causes the positive correlation. This study demonstrates the usefulness of nondestructive electromagnetic evaluation of the hardness in die–quenched automotive
steel components during manufacturing process.