Conditions for Grain Boundary Bulging during Tempering of Lath Martensite in Ultra-low Carbon Steel

摘要

Lath martensite in steel contains a high density of dislo- cations around 10!°/m?. This dislocation density corre- sponds to that in 80% cold-rolled ferrite.!’ Due to the large stored energy by dislocations, as-quenched martensitic steel undergoes recrystallization during tempering even without pre-working when the carbon content is reduced to an ultra- low level.'” In addition, it was demonstrated that the re- crystallization in lath martensite occurs through bulging of the pre-existing grain boundaries (bulge nucleation’! and growth mechanism”). Considering that lath martensite con- sists of fine substructures, namely, packets, blocks and laths, a very fine-grained recrystallization structure would be formed just only by quenching and tempering if all the grain boundaries could cause bulging. However, the grain boundary bulging during tempering does not frequently occur in spite of the high density of grain boundaries in martensitic structure, resulting in a formation of coarse grained structure after the completion of recrystalliza- tion.'”) This fact suggests that there is some necessary con- dition for grain boundary to migrate and cause bulging in lath martensite and that only the limited boundaries satisfy- ing the condition could become a preferential nucleation site for recrystallized grains. In order to understand the unique recrystallization behavior of lath martensite, various unknown factors such as the grain boundary character of bulging boundaries, the substructural change which triggers grain boundary migration, the growing behavior of recrys- tallized grains efc. must be clarified. In this study, EBSD analysis was performed to examine the characteristics of bulging boundaries and matrix structure adjacent to the boundary. The conditions for grain boundary bulging was then discussed in terms of the mobility of grain boundary and the driving force for grain boundary migration.