Grain boundary bulging during tempering in lath-martensltic steel

Ultralow carbon martensitic steel undergoes discontinuous recrystallization through the grain boundary bulging during tempering that occurs after quenching. In this study, the crystallographic character of the recrystallized grains was examined with Crystal Orientation Maps (COM) obtained by EBSP method in a ultralow carbon martensitic steel (Fe-1.5%Mn-0.0018%B). In addition, the strain distribution in the recovered martensite and recrystallized grain was observed with Image Quality Maps (IQM). The COM and IQM analysis revealed that the recrystallized grains (area where the bulging boundary swept) completely inherit the crystallographic orientation of the original martensite, the strain (dislocation density) in the newly formed recrystallized grains is found to be quite low, It was also found that the direction of boundary migration is dependent on the difference in dislocation density between both grains adjacent to the migrating boundary; the grain boundary always migrates to the grain with higher dislocation density. Each grain of martensite tends to be recovered differently, that is, the dislocation density becomes different in each grain after tempering even though the lath martensite originally has uniform distribution of dislocation density. This inhomogeneous recovery leads to the generation of driving force of the grain boundary migration.