Study On Microstructure And Properties Of Hot-Rolled Ti Microalloyed TRIP Steels

In recent years, with the development of automotive industry in our country, the demand for TRIP steel of high strength-ductility balance is increasing remarkably. Hot-rolled TRIP steels have the advantages of short process and low cost, mainly used for buses and trucks of large capacity. Besides, the strength grade of TRIP steel can be further improved with the addition of Ti. Therefore it is necessary to study systematically on microstructure and properties of hot-rolled Ti microalloyed TRIP steels. This paper explores the reasonable production process and analyses the influence of different TMCP parameters on microstructure and properties of TRIP steels through the thermal simulation and hot rolling experiments with two different chemical composition tested steels as the object of the research, which provides theoretical basis for actual production. The main works and research results of are as follows:(1) The continuous cooling transformation behaviors of overcooling austenite were studied and the static CCT curves were acquired with thermal expansion method and microstructural observation. Results showed that ferrite transformation temperature gradually decreased and the volume fraction of banite and martensite increased with the increase of cooling rates. Proeutectoid ferrite transformation temperature increased and ferrite transformation region expanded in steel A with the increase of start cooling temperature.(2) Effects of finish rolling temperature, finish slow cooling temperature, isothermal holding time in bainite transformation region and double-pass compression deformation amount on the microstructural evolution of tested steels were investigated by process simulation experiments. The distribution of C, Si, Mn elements for different isothermal holding time in bainite region was analysed according to EPMA elemental mappings.(3) Different hot rolling processes were carried out for tested steels at laboratory with two-stage cooling processes(air cooling plus water cooling).The process of the best mechanical properties was explored and the relationship between microstructure and properties under different finish rolling temperature and finish air cooling temperature was analysed. Results showed that the strength-ductility balance of tested steels reached the maximum when finish rolling temperature800℃and finish air cooling temperature722℃were adopted. The plasticity of tested steels mainly depended on the grain size of ferrite and the strain induced transformation of retained austenite. The mian factors affecting strength were the volume fraction of bainite and TRIP effect.(4) Retained austenite content and its carbon content of tested steels were calculated by XRD experiments. The amount of retained austenite first increased then decreased in steel A and gradually increased in steel B with the decrease of finish rolling temperature. The carbon content of retained austenite reached the maximum when the amount of ferrite was greatest.