Improved uniformity of hardness by continuous low temperature bainitic transformation in prehardened mold steel with large section

摘要

Expanding automobile industry energized the demand for large section plastic mold steel, large (465 mm × 1325 mm × length) pre-hardened mold (PHM) steel was trial-produced by quenching the forged slab at temperature between start (Ms) and finish (Mf) temperature of martensite, followed by tempering, according to the thermal expansion experiments and finite element modeling (FEM) of cooling rate from core to surface. Continuous low temperature bainitic transformation in PHM steel created high volume fractions of refined bainitic-ferrite laths, filmy retained austenite and dispersed carbides. Uniformity of microstructure and hardness, and the impact toughness of the PHM steel were remarkably superior to the one produced by conventional quenching & tempering (QT) process.