Effect of rolling reduction on ultrafine grained structure and mechanical properties of low-carbon steel thermomechanically processed from martensite starting structure

The present authors have invented a novel and simple thermomechanical processing to realize the ultrafine grained microstructure in carbon steels. The key of the process is to start from martensite structure. In the previous study, it has been clarified that conventional cold-rolling to a reduction in thickness of only 50% (equivalent strain of 0.8) and subsequent annealing at warm temperature around 500 degreesC fabricates the multi-phased ultrafine grained structure composed of the ultrafine ferrite grains with mean grain size of 180 nm, uniformly precipitated nano cementite and tempered martensite. In this study, the effect of the rolling reduction ranging from 25 to 70% (equivalent strains of 0.3-1.5) on the ultrafine grained structure and the mechanical properties of the plain low-carbon steel (Fe-0.13 wt% C) processed from martensite starting structure was studied. In the as-deformed specimen, the area fraction of the region showing the lamellar structure, which is typical for severely rolled metals, increased with increasing the rolling reduction and the strength also increased. After annealing at warm temperature around 500 degreesC, the multi-phased ultrafine grained microstructures were obtained in all the examined rolling reductions. The area fraction of the region showing the ultrafine ferrite grains increased with increasing the rolling reduction. At higher temperature, conventional recrystallization took place, and the recrystallization temperature became lower with increasing the reduction. Tensile test exhibited that the specimen rolled to the intermediate reduction (50%) performed the best strength-ductility balance (870 MPa of tensile strength and 9% of uniform elongation). The reason for the good strength-ductility balance of the specimen rolled to the intermediate reduction was discussed on the basis of the observed microstructures. (C) 2003 Elsevier Ltd. All rights reserved.