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Annealing of deformed martensite (high-temperature tempering) in St37 steel was studied. Different reductions in thickness were considered and compared with the behavior of as-quenched martensite during tempering. tempering of the asquenched martensite was accompanied by the formation of carbide particles, incomplete disappearance of the lath martensite morphology, and continuous decrease in hardness until reaching low values. However, during tempering of the cold rolled martensite, the precipitation of carbides in the lamellar structure, development of distinct equiaxed ultrafine grains through a continuous recrystallization mechanism, and a sudden hardness drop were characterized. the importance of cold rolling reduction and its amount were also discussed.

The effect of thermo-mechanical processing routes on the microstructure and mechanical properties of st37 low carbon steel was studied. Several dual phase (DP) ferritic-martensitic steels were produced by intercritical annealing of the martensitic, cold-rolled martensitic, and bimodal-sized ferritic microstructures. The latter microstructure was produced by subcritical tempering of cold-rolled DP steel to develop an aggregate of ultrafine and large ferrite grains. The DP steel obtained by intercritical annealing of cold-rolled martensite was shown to exhibit better strength-ductility balance compared with that obtained by intercritical annealing of the as-quenched martensite due to much finer microstructure and enhancement of work-hardening behavior in the former. The bimodal-sized ferritic structure showed high yield stress, yieldpoint elongation, and less pronounced work-hardening regime. The DP steel obtained by intercritical annealing of bimodal-sized ferritic structure exhibited inferior strength-ductility tradeoff compared with that obtained by intercritical annealing of cold-rolled martensite due to the coarser microstructure in the former. Conclusively, it is possible to enhance the mechanical properties of st37 low carbon steel by simple processing routes. K e y w o r d s : dual phase steels, microstructure, mechanical properties, strain hardening rate

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