Abstract
The effects of variations in the rate of post-austenitization cooling of a 0.2%C-1.5%Si-1.5%Mn-1.0%Cr-0.2%Mo-1.5%Ni-0.05%Nb (mass%) transformation-induced plasticity (TRIP)-aided steel with a lath martensite structure matrix on the Charpy impact toughness were investigated, with the aim of improving the material properties for automotive body applications. When cooled at 1.2°C/s after austenitization, the TRIP-aided steel showed a higher upper-shelf Charpy impact absorbed value (90 J/cm2) and a lower ductile-brittle fracture appearance transition temperature (−126°C), compared with the values determined (82 J/cm2, −98°C) for steel cooled at 53.5°C/s. The lower cooling rate yielded a higher volume fraction and carbon concentration of metastable retained austenite, finer martensite-austenite constituents, and a lower carbide fraction in the wide lath martensite structure in the TRIP-aided steel. These improved microstructural characteristics resulted in superior impact toughness.
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