| The rapid development of automobile industry not only has brought convenient transportation to mankind, but also caused serious environmental pollution. Saving energy and crash safety has been paid more and more attention. And the requirements of lightweight vehicles, safety, environmental protection and other indicators are increasingly high. The high strength steel is the preferred material to fulfil the mentioned requirements, but also the major development trends of automobile steel in the futher. In this paper,"MeiSteel economical high strength hot rolled steel technology research and development" is the research background. This article has developed a new generation hot rolled high strength steel with ferrite precipitation strengthening by using ultra fast cooling process. The main works and results involved as follows:(1) The continuous cooling transformation behavior and effects of cooling path on phase transformation of experimental steels have been investigated by means of thermal simulation experiments. And the traditional continuous cooling transformation curves and cooling transformation curves under ultra fast cooling conditions are obtained. The results showed that increasing the foreward cooling rate and lowering the foreward final cooling temperature could significantly refine the ferrite grain size. The application of ultra fast cooling after deformation can greatly shorten the ferrite phase transformation start time (incubation period), lower ferrite transformation start temperature, and move the ferrite phase transformation zone to the lower right. It has little effect of adding V element on phase behavior of experimental steels, while adding Mo element can remarkablly reduce the ferrite transformation start temperature. According to Pham’s additivity rule, the time-temperature-transformation (TTT) curves of experimental steels are fitted by origin software, which has important significance for making new cooling path.(2) Study of isothermal temperature and isothermal time on the ferrite phase transformation behavior and precipitation behavior has been made by using isothermal quenching experiments. The nose temperatures of Ti, V-Ti and Mo-Ti microalloyed steels are700℃,700～680℃and680℃, respectively. The lower the isothermal temperature is, the smaller the ferrite grain size is, and the higher the microhardness. The volume fraction of ferrite increases with the isothermal time. For Ti microalloyed steel, interphase precipitaton occurred in the later stage of the ferrite phase transformation when isothermality at750℃. However, a large number of supersaturatd precipitation particles can only observed when isthermality at650℃. Additionally, adding Mo can lower ferrite transformation temperature, and impede the coarsening behavior of precipitates.(3) Based on thermal simulation results,1#Ti-bearing steel and2#Nb-Ti steel were studied through hot rolling experiments at laboratory. The optimum process window of these two steels is that heating temperature1250℃, final rolling temperature910～930℃, and coiling temperature640℃. The tensile strength of steel is more than730MPa, elongation is above21%, and-20℃low temperature impact energy is more than35J. Under the same hot rolling process, Ti-bearing steel added more0.04wt.%C element has reached the strengthening effect of Nb-Ti steel obtained by adding more0.025wt.%Nb element, which enormously reduce production costs. Therefore, Ti-bearing steel has great prospects in industrial production and applications. |
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