Research And Development Of Low Cost500～700MPa Hot Rolled Dual Phase Steel

With the development of reduction technology on vehicle manufacturing, hot rolled dual phase steel has been applied to automobile manufacture more widely. Compared with foreign countries, the development of the500～700MPa hot rolled dual phase steel in our country is relatively late, there are some deficiencies in production of hot rolled dual-phase steel for automobile structure, such as high cost, complex process, unstable microstructure and properties and so on.The objects of this thesis are C-Mn-Cr and C-Mn-Nb-Ti steels, whose targets are to develop low cost500-700MPa hot rolled dual phase steel. The main works and research results of this thesis are as follows:(1) Two low cost composition system hot rolled dual phase steels were designed. C-Mn-Cr steel was used to develop500-580MPa hot rolled dual phase steel, whose process stability was improved and the cost was down. C-Mn-Nb-Ti steel was used to develop650-700MPa and above hot rolled dual phase steel, which could give full play to fine grain, transformation and precipitation strengthening combined with UFC technology.(2) The dynamic recrystallization behavior was researched through single pass reduction experiments. The results showed that the dynamic recrystallization activation energy of C-Mn-Cr steel was383.835kJ/mol and the deformation resistance was small. The dynamic recrystallization activation energy of C-Mn-Nb-Ti steel was433.981kJ/mol, so the dynamic recrystallization was more difficult to occur. C-Mn-Nb-Ti steel had a larger recrystallization rolling window and was conducive to grain refinement. When the deformation rate was0.1s-1and deformation temperature was less than800℃, there were peaks in the stress-strain curves, because the deformation of austenite induced ferrite transformation.(3) The curves of static and dynamic continuous cooling transformation were established by the application of thermal dilation method and metallographic analysis. The results showed that it was more easily to obtain ferrite-martensite dual phase structure for C-Mn-Cr steel by controlled rolling and cooling. Deformation could enhance the starting temperature of transformation, make the CCT curve move to the upper left and significantly enlarge the area of ferrite phase transformation. The combined effects of deformation with accelerated cooling could make the microstructure of deformed austenite after continuous cooling small and uniform.(4) The thermal simulation tester was used to simulate the quick cooling+holding+quenching process, and investigated the effects of various process parameters on microstructure evolution and mechanical properties. The results showed that the deformation temperature had no effect on microstructure and properties of C-Mn-Cr steel, but the ferrite volume fraction of C-Mn-Nb-Ti steel was gradually decreased with the decrease of deformation temperature. And the size of martensite was smaller and more diffuse when the deformation temperature was low. The small cooling rate could enhance the precipitation of ferrite, and reduce the size of martensite islands. The peak temperature of amount of transformation ferrite for these two test steels were about700℃and660℃. The relationship curves of the amount of ferrite transformation and holding temperature showed "S" pattern. And the "S" curves would move to upper left with the decrease of holding temperature.(5) Microstructure and mechanical properties of test steels with the UFC+air cooling+laminar cooling and one-stage continuous cooling strategies were researched through hot rolling experiment at laboratory, and obtained optimum process parameters. The results showed that the effect of process parameters on microstructure and properties of C-Mn-Nb-Ti steel were more obvious, compared with C-Mn-Cr steel. The finishing temperature, UFC temperature and air cooling time commonly affected the morphology and amount of ferrite, and then affected the subsequent transformation and microstructure and properties; the coiling temperature was the key process parameters of obtaining martensite and continuous yielding property.(6) The microstructure and properties of540-580MPa C-Mn-Cr dual phase steel was excellent, when the finishing temperature was about840℃, the UFC temperature was690-745℃, the air cooling time was4-6s, and the coiling temperature was below350℃; the steel had good properties, with the structure of ferrite+martensite+(little)bainite, while using the one-stage continuous cooling strategy. The microstructure and properties of650～710MPa C-Mn-Cr dual phase steel was excellent, when the finishing temperature was810℃, the UFC temperature was670～735℃, the air cooling time was5～7s, and the coiling temperature was below300℃; the microstructure and strength of C-Mn-Nb-Ti steel realized flexible control.