Investigation On Hot-Rolling Process Of Precipitation Strengthened "Ferrite&Martensite" Dual-Phase Steel

As an Advanced High Strength Steels (AHSS) with superior comprehensive properties, dual-phase (DP) steel has achieved wide application in automobile industry. However, dual-phase steel is susceptible to crack during forming process, such as bending and stretch flanging, due to the large difference in strength between ferrite and martensite. This has greatly limited its application. The present work is aimed at improving the formability of "ferrite&martensite" DP steel and meanwhile promoting its strength level. This is realized through the strengthening of ferrite matrix by micro-alloy carbonitride and therefore increasing the deformation homogeneity between ferrite and martensite during formation. The chief original work of this paper is as follows.(1) Static isothermal phase transformation test has been done for Fe-0.1C-0.45V to measure the austenite to ferrite transformation kinetics. On the basis of diffusional transformation theory, the kinetic model for phase transformation has been established under para-equilibrium and negligible partition local equilibrium. TEM analysis was conducted to study the precipitation morphology in ferrite. Results showed that, interphase precipitation behavior was induced by the coupling of phase transformation and precipitation kinetics(2) The decarburization equipment was set up for the investigation of interphase precipitation kinetics. Through the experimental study of Fe-0.5C-0.035Nb and Fe-0.1C-0.45V, the applicability and feasiblity of using decarburization method for the investigation of interphase precipitation in Fe-C-X alloys was explored. In the study of Fe-0.1C-0.45V, precipitates were found to line up in ferrite. Some of them were coarsened and some of them were dissolved during the long holding period in decarburization. Further TEM analysis at the austenite/ferrite interphase boundary is required.(3) Isothermal transformation tests were conducted in a low carbon, vanadium and titanium microalloyed steel. Large fraction of ferrite phase was only observed at630℃and650℃. However, deformation has promote the ferrite transformation and noticed ferrite transformation took place at the temperature range of630-710℃. Interphase precipitates were detected at every isothermally transformed ferrite phase.630-650℃was considered as the best temperature range for interphase precipitation.(4) Continuous cooling transformation experiments were carried out in the low carbon, vanadium and titanium microalloyed steel. Dynamic and static continuous cooling transformation diagram (CCT) was constructed. The micro structure of precipitate strengthened ferrite was obtained by using "UFC+slow cooling" process. Due to the homogeneous single phase structure, it has exhibited a superior formability with the hole-expansion ratio of up to200%. Using the "UFC+air cooling+quenching" process, the precipitate strengthened "ferrite&martensite" dual-phase structure was achieved. The strength difference between ferrite and martensite was reduced by the precipitate strengthening in ferrite, resulting in an average hole-expansion ratio of120%.