| Hot-rolled dual-phase steel has been applied to automobile manufacture more widely due to its excellent comprehensive properties. However, there are some problems for the development of hot-rolled dual-phase steel above700MPa grade in our country at present, such as complicated technology, high cost, unstable properties and so on. The research objective of this thesis is the development and manufacturing of nano Ti precipitation strengthening hot-rolled dual-phase steel characterized by low Si content and adding trace element Ti and using ultra-fast cooling technology. The main works and research results are as follows:(1) Dynamic recrystallization behavior of experimental steel was investigated through single pass reduction experiments. It was showed that dynamic recrystallization activation energy critical strain of experimental steel were gradually increased with the increase of Ti content because of the inhibitory effect of element Ti on austenite dynamic recrystallization process.(2) The curves of continuous cooling transformation of experimental steel showed that Ti could improve the stability of supercooled austenite, make the ferrite phase transformation zone moves to the right and expand the bainite transformation zone. With the increase of Ti content and cooling rate, microstructure was refined and hardness of experimental steel was improved. And it was more easily to obtain ferrite-martensite dual phase structure for the experimental steel of Ti content of0.015%by controlled rolling and cooling.(3) Effects of deformation temperature, holding temperature and Ti content on dual-phase microstructure and properties of experimental steel were researched through the thermal simulation experiment. Meanwhile, the precipitation behavior of Ti was studied. The precipitation forms of nano TiC particles in ferrite were dispersed precipitation and interphase precipitation. And the strengthening effect of interphase precipitation was greater than dispersed precipitation. Ti content and holding time affected the precipitation behavior of TiC. The size of TiC particles remained unchanged and interphase precipitations were increased with prolonging the holding time.(4) Effects of finishing delivery temperature, ultra-fast cooling export temperature, air cooling time, and deformation amount of non-recrystallization zone on microstructure and properties of experimental steels were investigated through hot rolling experiment in laboratory. And optimum TMCP parameters of controlled rolling and cooling.were given. The mechanical properties of nano Ti precipitation strengthened hot-rolled dual-phase steel were attained as follows:with respect to the experimental steel of Ti content of0.05%, tensile strength was745-815MPa, yield ratio was0.62～0.69, n value was0.13～0.17and elongation was18～23.4%. With respect to the experimental steel of Ti content of0.1%, tensile strength was745-850MPa, yield ratio was0.7～0.75,n value was0.13～0.14and elongation was18～21.4%.(5) The precipitation behavior of nano TiC of hot-rolled dual-phase steel was researched. Enhancing finishing delivery temperature increased the precipitation amount in ferrite matrix and improved the precipitation strengthening effect. Increasing Ti content was conducive to the interphase precipitate. The fine grain strengthening had the greatest contribution on the yield strength but the main reason of the yield strength difference was the precipitation strengthening, so grain strengthening and precipitation strengthening should be considered comprehensively to improve performance of hot-rolled dual-phase steel. |
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