Substructure of M/B mixed structure in a low carbon NiCrMoV steel obtained through different cooling mode were quantitative analysis by optical microscope, SEM, EBSD and TEM to investigate the effect of the substructure on toughness. The results indicated that with the same prior austenite grain size and mixed by different ratio of M and B, toughness of steel enhanced with decrease of packet and block size and the lath width was about 0.38um. Calculation showed that packet and block boundary have the same effect on delaying crack extension, furthermore, effect of block width on impact energy was notably bigger than packet one. Therefore, block could work as a contributing microstructure controlled substructure in low carbon martensite steels, which means that block size was “the effect grain size controlling toughness”.
lath martensite/bainite mixed structure;toughness;EBSD;crack extension