Grain Refinement of Dual Phase Steel via Tempering of Cold-Rolled Martensite

A microstructure consisting of ultrafine grained (UFG) ferrite with average grain size of ~ 0.7 µm and dispersed nano-sized carbides was produced by cold-rolling and tempering of the martensite starting microstructure in a low carbon steel. Subsequently, fine grained dual phase (DP) steel consisting of equiaxed ferrite grains with average size of ~ 5 µm and martensite islands with average size of ~ 3 µm was produced by intercritical annealing of this microstructure. Coarse grained DP steel with average ferrite grain size of ~ 20 µm and average martensite island size of ~ 5 µm was also produced by intercritical annealing of the as-received ferritic-pearlitic microstructure. The UFG microstructure showed high strength, low ductility, and poor work hardening response due to intense grain refinement. The fine grained DP steel had higher tensile strength and total elongation compared with the coarse grained one, which was related to the improved work-hardening behavior by microstructural refinement.