Effect of Microstructure on the Mechanical Properties of Bainitic Steel

Microalloyed medium carbon steels had been developed for automobile parts. These steels had a little inferior toughness to that of quenched-tempered medium carbon steel. So, application of these steels were restricted to the automobile components of less severe circumstance. In this report, low carbon bainitic steel, RK603 (0.2C-0.85Mn-1.7Cr-0.06Nb) is studied especially focussing to the mechanical properties and the microstructures. The following results are obtained. (1) Tensile strength of RK603 depends on hardness and grain size, 0.2% proof stress, moreover, depends on morphology of bainitic structure. (2) Toughness of RK603 are improved by lowering the isothremal transformation temperature, but, does not depend on grain size. The reason of improving toughness can be explained by refinement of unit crack path. (3) Fatigue strength of RK603 depends on isothermal transformation temperature, and are a little inferior to that of quenched-tempered SCM440.