Effect of Precursor Microstructure on Retained Austenite and Mechanical Property of Low Carbon Steels with IQ&P Treatments

The microstructure, retained austenite and mechanical property of low-carbon steels with different precursor microstructures were studied by means of intercritical reheating-quenching and partitioning (IQ&P) processes. The results show that the structure of IQ&P-Ⅰ multiphase steel with precursor ferrite + pearlite (F+P) is blocky ferrite, martensite (M). The blocky retained austenite(RA) of IQ&P-Ⅰ multiphase steel exists in the boundary of ferrite and martensite and film-like retained austenite distributes around the lath martensite as the shape of thin slice. The transformation induced plastic effect of IQ&P-Ⅰ multiphase steel with less content of retained austenite is unapparent. The tensile strength of IQ&P-Ⅰ multiphase steel reaches 957 MPa, but the tensile elongation is only 20%, and the production of strength and elongation is 19905.6 MPa·%. The structure of IQ&P-Ⅱ multiphase steel with precursor martensite is needle or lath grey-black ferrite and martensite. The fine needle like martensite is distributed evenly on the ferrite matrix. The film-like retained austenite is only distributed on the ferrite matrix. The content of retained austenite of IQ&P-Ⅱ multiphase steel reaches 13.2% with more obvious TRIP effect and higher stability. The production of strength and elongation of IQ&P-Ⅱ multiphase steel is 21560 MPa·% with good combination of strength and plasticity