EFFECT OF PRECURSOR MICROSTRUCTURE ON MORPHOLOGY FEATURE AND MECHANICAL PROPERTY OF C-Mn-Si STEEL

The effects of different precursor microstructure on the morphology and mechanical properties of the 0.22C-1.9Mn-1.32Si multiphase steel which was obtained by the treatment of intercritical reheating-quenching and partitioning (IQ&P) heat treatment were examined. Under the same IQ&P heat treatment parameters, multiphase microstructure which contains lath-like ferrite matrix and film or short needle-like retained austenite can be obtained by the martensite (M) precursor steel; while multiphase steel which has a bainite-ferrite (B-F) precursor can obtain a microstructure of equiaxed-like ferrite matrix and particale like retained austenite. After the IQ&P process, tensile strength of the multiphase steel which has a B-F precursor is up to 976 MPa, but elongation of this kind of steel is only 26.7%, and thus the product of strength and elongation of this kind of steel is only 26 GPa.%;, while multiphase steel which has a M precursor has realized the combined properties of high strength and excellent ductility, product of strength and elongation of this kind of steel reaches 31 GPa%. As for the work hardening behavior of the uniform elongation stage, although B-F precursor multiphase steel has a higher work hardening index n than the M precursor multiphase steel, stability of the retained austenite in this kind of steel is relatively poor, variation behavior curve of the instantaneous n value with true strain for this kind of steel shows a notched-like shape; as for the multiphase steel which has a M precursor, retained austenite in this kind of steel is relatively stable, variation behavior curve of the instantaneous n value with true strain for this kind of steel is much more steady, which shows a trend of gradual increasing. The reason for the different tensile testing and work hardening results above is related to the morphology, proportion and distribution state of the retained austenite and matrix microstructures, which is due to the effect of different morphology and microstructure characteristics of the precursor phases by the roots.