Microstructure regulation and mechanical properties of low-carbon multiphase steels

Two kinds of 6 mm hot rolled plate steels, (0.22/0.17)C-(1.91/1.85)Mn-(1.32/0.94)Si, which have different precursor microstructures were heat treated by the critical region reheating, quenching and medium temperature isothermal processes (named T1 and T2). After the processes, a multiphase microstructure, which is composed of ferrite, bainite, martensite and well-distributed retained austenite (primarily distributed in prior austenitic grain boundaries and phase boundaries), was obtained for both of the steels, The microstructures of the steels in different heat treatment stages were characterized by scanning electron microscopy, X-ray diffraction and electron backscatter diffraction. It is found that making use of different precursor microstructures can well lead to the regulation of morphology, proportion and carbon content of the reversed austenite during the process of critical region reheating, and then achieve the regulation and control of the multiphase microstructure by the subsequent quenching and medium temperature isothermal processes. After the process of T1, 0.22C steel which has a martensite precursor microstructure can obtain a multiphase mierostructure with acicular-like ferrite as the matrix, and its product of strength and elongation is greater than 30 GPa.%. After the process of T2, 0.17C steel which has a ferrite + martensite precursor microstructure can obtain a multiphase microstructure with block-like ferrite as the matrix, and its product of strength and elongation is greater than 27 GPa.%.