SOLUTE PARTITIONING AND MICROSTRUCTURAL DEVELOPMENT IN DUAL PHASE STEELS (ELECTRON MICROSCOPY)

A dual phase steel is one of the high strength steels characterized by a microstructure consisting of dispersed martensite in a ferrite matrix. A study of solute partitioning in a dual phase structure is of considerable importance because it controls not only the hardenability of austenite but also its growth rate during intercritical annealing. A systematic study was made of solute partitioning with particular emphasis on the relation between solute partitioning and microstructure formation for low carbon steels containing 15 wt% manganese, 2 wt% silicon, and 1 wt% aluminum.; Two types of heat treatments were employed to obtain dual phase structures: (1) step cooling from the austenite region, (2) reheating into the intercritical region, followed by quenching in both cases. The volume fraction of martensite was measured as a function of intercritical annealing time. Concentration line profiles and partitioning coefficients were measured using x-ray microanalysis. These measurements enabled the effect of alloying elements and heat treatments on the growth of the transformed phase to be determined.; It was found that in the manganese steel, the growth rate of austenite in the reheating treatment was higher than that in the step cooling treatment. In the step cooling treatment, long range carbon diffusion was interrupted by the high manganese barrier, whereas in the reheating treatment, the growth of austenite was controlled by carbon diffusion in the early stage of austenite growth.; Ferrite growth in a silicon containing steel was faster than that in a manganese steel in the step cooling treatment. This fact is due to the difference of partitioning coefficients and carbon activities in austenite between silicon and manganese steels.; Finally, based on these results, optimum alloy design and heat treatments are proposed.