Strain hardening behavior and deformation characteristics of multiphase microstructure in a medium-carbon quenching and partitioning bainitic steel

A 0.3C-1.4Si-1.8Mn (wt%) steel was subjected to a modified Q&P process leading to the formation of a multiphase microstructure comprising bainite, martensite and retained austenite. The strain hardening behavior and deformation characteristics of the multiphase microstructure were investigated by tensile tests and electron backscattered diffraction (EBSD), respectively. The results show that the multiphase microstructure containing about 50% bainite exhibits a better combination of strength (1617MPa), elongation (18.6%), the product of strength and elongation (PSE, 30.1GPa%) and toughness (103J) compared to the fully bainite. The strain hardening exponent-strain curves indicate that the multiphase microstructure has a much broader strain hardening plateau and the strain hardening exponent calculated for the multiphase microstructure and fully bainite is 0.085 and 0.064, respectively. The analysis results of EBSD demonstrate that the local stress concentration of the multiphase microstructure mainly appears in martensite and the bainite/martensite interfaces, and can more effectively spread to the interior of bainite with deformation increasing. In addition, the more low Taylor Factor regions under the large deformation condition and some retained austenite grain rotation can be observed in the multiphase microstructure. These caused better plastic deformation of multiphase microstructure with subsequent improvement in ductility.