| The excellent comprehensive performance of dual-phase steel makes it suitable for the development direction of energy saving,environmental protection,safety and aesthetics of the automobile industry,and thus has been widely used in structural parts such as automobile panels,wheels,girders and chassis.Among the dual-phase steels that have been comercialized,the cold-rolled and heat-treated dual-phase steels occupy a large proportion,but the hot-rolled dual-phase steels occupy a very small proportion and low maximum intensity level.There are still unstable microstructures and properties,poor controllability and other issues in actual production.In this paper,the optimal production plan of 600MPa grade hot-rolled dual-phase steel is explored to achieve controllable and stable production by adjusting TMCP.The main works and results of this paper are as follows:(1)On the basis of C-Si-Mn steel,a small amount of Mo and Nb elements were added to design the chemical composition of the experimental steel:0.07%～0.09%C,0.15%～0.25%Si,1.3%～1.5%Mn,0.1%～0.2%Mo,0.02%～0.04%Nb.(2)The high temperature deformation behavior of austenite in experimental steel was studied by thermal simulation experiments.It is shown that the recrystallization model is established by selecting the peak stress value,and the dynamic recrystallization activation energy is 445.3kJ/mol.The deformation resistance model of the experimental steel is fitted by the experimental data processing,and explored the affects of deformation condition to the deformation resistance.(3)Through the phase transformation experiment,the continuous cooling transition curve of the experimental steel with undeformation and 40%deformation is drawn.The research shows that when there is no deformation,the ferrite and bainite transformation occurs at cooling rate of 0.5～1℃/s,the bainite transformation occurs when the cooling rate is 1～10℃/s,and bainite and martensite transformation at 10～25℃/s.The cooling rate is greater than 25℃/s,only martensite transformation occurs Deformation promotes the austenite transformation to increase the phase transformation temperature,so that the CCT curve moves to the upper left,and has the effect of refining the grains.(4)Through the laboratory hot rolling experiment,the influence of process parameters on the microstructure performance of the experimental steel in the laminar cooling+air cooling+fast cooling three-stage cooling mode was studied,and the process window of F+B and F+M biphasic organization is obtained..The research shows that when the start rolling temperature is 970～980℃,the final rolling temperature is 810～830℃,the relaxation temperature is 720～750℃,the relaxation time is 15～20s,the cooling rate is more than 30℃/s,and the coiling temperature is 200～350℃,F+M dual-phase steel can be obtained,whose tensile strength is 660～680MPa,yield strength is 400～410MPa,elongation is 23%～24%,and yield ratio is less than 0.6.When the other process parameters are unchanged,through adjusting the cooling rate to 10～25℃/s and the coiling temperature to 420～450℃,the F+B+RA(small amount)dual phase structure is obtained.The tensile strength reaches 570～629MPa,the elongation reaches 23.3%～28.3%,and the yield ratio is in the range of 0.69～0.78.(5)Industrial trial production was carried out with different process parameters,and analyzed the microstructure properties of the experimental steel after rolling.The results show that the F+B dual phase structure is obtained at the medium temperature coiling,whose tensile strength reaches 583～632MPa,the elongation is 22.7%～27.8%,and the hole expansion ratio is 113%～152%.At low temperature coiling,F+M+B(small amount)structure is obtained.The tensile strength reaches 728～740MPa,the yield strength is 488～512MPa,the yield ratio is 0.66～0.7,the elongation is 22.3%～26.4%,and the hole expansion rate is 51%～93%,which meets the technical requirements of 600MPa grade F+M dual phase steel. |
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