Effects of carbon content on the formation of nano/ultrafine grained low-carbon steel treated by martensite process

Martensite process consists of simple cold rolling and annealing of martensite structure. This process can produce a nano/ultrafine grained structure in carbon steels. The key to the process is to start from martensite structure. Carbon can hardly affect the morphology of lath martensite. In this study, the effects of carbon content on required rolling reduction and formation mechanisms of nano/ultrafine ferrite grains are investigated. For this purpose, two low-carbon steels containing 0.04 and 0.08 wt% carbon are processed. Rolling reduction ranging from 40 to 80% is used. Specimens are annealed at various temperatures from 450 to 700 °C. After annealing at 500 °C, ultrafine ferrite grains are obtained in the 80% cold-rolled specimen containing 0.08 wt% C. The formation of these ultrafine ferrite grains is interpreted here in terms of the relationship between the size of the laths and the blocks of the lath martensite and required rolling reduction for dividing them into sub-micron scale sections. The required rolling reduction increases with decreasing the carbon content of steel.