| Application of high strength steel in auto manufacturing has increased with the development of research on light-weighting of auto body. For the excellent mechanical properties of dual phase (DP) steel, such as low YS/TS ratio, high original work hardening ratio, well combination of strength and plasticity, DP steel has been one of advanced high strength steels used most widely, and is accord to requirement of light-weighting, high performance, safety, environment protection and energy saving of auto materials.By way of research and development of high strength Nb bearing hot-rolled DP steel, tested steels with different alloy component series were designed. Then, transformation rule during continuous cooling of tested steels deformed in Gleeble-1500 thermal simulator were studied, and thermal-mechanical experiments were carried out to analyze the effect of process data on dual phase microstructure evolution of tested steels. The controlled rolling and controlled cooling experiments were carried out in lab to control microstructure and properties of tested steel, and strengthening and toughening mechanism of high strength Nb bearing hot-rolled DP steel were discussed.In the light of transformation rule and microstructure evolution during step-cooling process of tested steels of different alloying component series, effects of elements and process data on microstructure and properties of DP steels were analyzed, and the relationship of alloying design and process control stability during microstructure and properties control was described. It is shown that C-Si-Mn-Nb steel with simple alloying design of low cost is provided with low hardenability, and has strong process sensibility during microstructure evolution. DP microstructure of ferrite and martensite (F+M) of C-Si-Mn-Nb steel just can be obtained by coiling at low temperature of 250℃after ferrite transformation followed by water -cooling. The process control stability of C-Si-Mn-Cr-Nb steel is stronger than that of C-Si-Mn-Nb steel. Alloy element Cr can effectively control morphology of bainite, and is benefit to obtain ferrite and bainite (F+B) DP microstructure by coiling at medium temperature of 500℃, to obtain F+M DP microstructure by air cooling at temperature of 550℃. C-Si-Mn-Cr-Mo-Nb steel with complex alloying design of high cost is provided with excellent process control stability. Alloy element Mo can promote stabilizing of metastable austenite to obtain F+M DP microstructure of by coiling at medium temperature of 550~600℃, but, has few effect on mechanical properties of F+M DP microstructure coiled at 500℃, compare to that of C-Si-Mn-Cr-Nb steel.On the basic of adjusting transformation condition of retained metastable austenite to control strengthening phase in DP microstructure, flexible control technique of microstructure and property of high strength Nb bearing hot-rolled DP steel was put forward. The same alloying component design tested steel can obtain DP microstructure of F+B or F+M with different properties, by adjusting coiling temperature to control transformation product of metastable austenite. The same strength grade objective also can be met by different alloying design tested steel corresponding to specifically process, according with practical equipment capacity and process control condition.On the basic of research on combination mechanism of strengthening and toughening including phase transformation, grain refining precipitation of high strength Nb bearing hot rolled DP steel, the microstructure characteristic of interaction between ferrite matrix and strengthening phase and relationship of microstructure and properties were described, and effect of different strengthening phase on continuous yielding and fracture toughness of DP steel was analyzed. Phase transformation strengthening is main strengthening method, and it decreases yield strength, increases tensile strength by interaction of ferrite matrix and strengthening phase and its effect on state of dislocation in ferrite matrix, so, mechanical properties such as continuous yielding, low YS/TS ratio, high origin work hardening ratio, and well matching of strength and plasticity are obtained. F+B DP steel has high YS and low TS, but has well fracture toughness, comparing to F+M DP steel. On the basic of phase transformation strengthening, micro-alloying of Nb is important to promote strength grade of DP steel by strengthening of grain refining and precipitation.
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