REGULATION OF RETAINED AUSTENITE AND ITS EFFECT ON THE MECHANICAL PROPERTIES OF LOW CARBON STEEL

The development of high performance steels needs to realize the combination of high strength, high plasticity and high toughness. Multiphase microstructure which contains a specific proportion of retained austenite is conductive to enhance the toughness and plasticity of the steel. Making use of the quenching+intercritical reheating-quenching and partitioning (IQ&P) process, a multiphase microstructure which was composed of intercritical ferrite, martensite and well distributed retained austenite (primarily distributed in the prior austenitic grain boundary and the phase boundary) can be obtained in the 0.23C-1.8Mn-1.35Si steel. By means of SEM, XRD and EBSD, microstructures of the steel in different heat treatment stages were characterized. The results indicated that the obtention of the retained austenite was mainly dependent on the following two stages: the first one is the enrichment of the carbon and manganese in the reversed austenite during the intercritical reheating process; the second stage is the secondary enrichment of carbon in retained austenite during the following quenching and partitioning process. After the two stages of element enrichment treatment, more than 10% volume fraction of retained austenite was obtained, and the second stage of treatment plays an important role in the formation and stabilization of the metastable austenite. Due to the strengthening and toughening effect of the widely distrubuted retained austenite, this kind of steel obtained a continuous work hardening ability, and thus achieved a good combination of strength and plasticity. Test results indicated that steel treated by the IQ&P process shows excellent comprehensive mechanical properties: the product of strength and elongation is greater than 26 GPa.%, the yield strength and tensile stength is more than 600 and 900 MPa respectively, the uniform elongationg is above 16%, and the half thickness size impact toughness at room temperature reaches to 39 J.