Study On Microstructure And Mechanical Properties Of Low-carbon Quenching And Partitioning Steels

To achieve the lightweight of automotive steel, energy saving and environmental protection, it is necessary to develop high strength and high plasticity automotive steel. Based on the quenching and partitioning (Q&P) treatment, which is proposed by Prof. Speer and applied to the new generation of advanced high strength steel, a new low-carbon steel was designed in the study. The chemical composition is Fe-0.176C-1.31Si-1.58Mn-0.26A1-0.3Cr (wt.%) The effect of austenitized temperature, quenching temperature and partitioning time on microstructural evolution and mechanical properties of the new Q&P steel is investigated using scanning electron microscopy (SEM), electron back scatter diffraction (EBSD), transmission electron microscopy (TEM), X-ray diffraction (XRD) and tensile tests. Meanwhile, transformation mechanism of retained austenite during the deformation and the effect of different types of retained austenite on increasing plasticity of the steel were systemically studied via in-situ tensile tests and EBSD analysis. The major conclusions are as follows:(1) The best original quenching temperature of the new designed Q&P steel was theoretically predicted by J Mat Pro software and constrained carbon theory (CCE). According to the original quenching temperature, a more reasonable Q&P heat treatment was designed. The results of tensile tests show that all the specimens show a good combination of tensile strength (1217～1280 MPa), yield strength (945～1025 MPa) and elongation (10.4～15.4%). From the microstructural observations of the steel, it can be seen that the microstructure is mainly comprised of lath martensite, retained austenite and lower bainite as well as a few of tempered martensite, twin martensite and twin austenite. Especially, the twin austenite is observed for the first time in Q&P steel. There is no substantial difference in microstructure of the specimen quenched at different temperatures. In addition, comparing the results of XRD experiment with the results of tensile tests, it can conclude that the effect of retained austenite on plasticity of the steel not only depends on volume fraction of retained austenite, but also depends on carbon content in retained austenite.(2) The effect of austenizited temperature and partitioning time on the microstructure and properties was studied by resetting the heat treatment conditions. The results show that retained austenite in specimen after partial austenitization was carbon-enriched twice, while retained austenite in specimen after full austenitization was only carbon-enriched once. Therefore, the former can get more stable retained austenite at room temperature. Meanwhile, as the increasing of austenitized temperature or partitioning time, upper bainite, tempered martensite and lower bainite successively appeared in the microstructure. Besides, comparing with the specimens after full austenitization, the specimens after partial austenitization show a lower strength and a higher elongation. Especially, the specimen after partial austenitization at 800℃ show an excellent elongation. The highest elongation of the specimen reaches 37.1%.(3) It can be seen from the analysis results of XRD and EBSD that as the partitioning time increases, the process of carbon partitioning and the homogenization of carbon in austenite could significantly influence the interface migration of martenite and austenite, then can influence volume fraction, carbon content and grain size of retained austenite. While in turn, the change of volume fraction, carbon content and grain size of retained austenite can influence the interface migration of martenite and austenite. Namely, they act on and influence each other, until the carbon content in both sides of interface of marteniste and austenite reaches the dynamic balance at last.(4) The specimens after full and partial austenitization were analyzed respectively using in-situ tensile tests and EBSD experiments. The results show that retained austenite in these two specimen under the strain has some common transformation behaviors:As true strain increased, the blocky retained austenite distributed at triple edges and twin austenite transformed quickly. While, as true strain increased, the film-like retained austenite located between martensite before transformation prone to rotate. Moreover, retained austenite rotated along a specific slip plane and slip direction. However, it has some difference in transformation behavior of retained austenite in these two specimen. It has another type of retained austenite in specimen after partial austenitization. This type of retained austenite is completely embedded in ferrite and its transformation behavior is similar to the transformation behavior of retained austenite located between martensite. That is, these retained austenite before transformation prone to rotate during true strain.(5) These retained austenite grains distributed at different locations show different transformation behaviors, which can be attributed to the different abilities of resisting the strain in these retained austenite grains and the different stresses imposed by their surrounding grains. The grain rotation is conducive to improve plasticity of the steel. Furthermore, under the strain, the ability of rotation stronger, the ability of improving plasticity of the steel stronger.