Effect of Starting Microstructure on the Grain Refinement in Cold-Rolled Low-Carbon Steel During Annealing at Two Different Heating Rates

Cold-rolled samples of different starting microstructures, namely ferrite–pearlite (F+P) and ferrite–martensite structures, with blocky martensite (F+M_b) and fibrous martensite (F+M_f) morphologies were reheated at two different heating rates (1 and 300 K/s) to sub-critical and inter-critical annealing temperatures [773 K to 1173 K (500 °C to 900 °C)] and immediately water quenched. Grain refinement after annealing of cold-rolled samples depends on the rate of recovery, recrystallization, transformation, and grain growth. Rapid recrystallization during annealing weakened the recrystallization–transformation interaction in F+P sample. Higher rate of ferrite recovery reduced the driving force for recrystallization, which weakened the recrystallization–transformation interaction during annealing of F+M_b sample. As a result, coarser grain structures were obtained after annealing of cold-rolled F+P and F+M_b starting structures. Strong recrystallization–transformation interaction and suppression of grain growth by uniformly distributed carbide particles and austenite islands (after transformation) offered finest ferrite grain size and best combination of strength and ductility in F+M_f sample.