Study On Microstructure Evolution And Production Process Of Low Cost C-Si-Mn-Cr Hot-rolled Dual-phase

Hot-rolled dual-phase steel due to its excellent comprehensive properties has been widely used in many fields, such as automobiles, petroleum, shipping and construction, and so on. In recent years, particularly high-strength automotive steel in the "cold" thermal generation of products not only can save a lot of technology, reduce energy consumption, but also allows the car weight, so that both the security and energy-saving goal. Hot-rolled dual-phase steel is a wide range of applications, a very market competitive advantage, steel materials, the rapid development of automobile industry in China will play an increasingly important role.This thesis is based on the subject named development of high strength steel which advanced by RAL and Lian steel.Production of hot-rolled dual phase steel in the low Si-Cr component design principles in order to reduce costs and reduce the alloying elements added. Melting different Si and Cr contents,controling the the finishing rolling temperature and the temperature after UFC and coiling temperature,through thermal simulation and hot rolling experiments in order to research microstructure evolution and production process of low cost C-Si-Mn-Cr hot-rolled dual-phase. With ultra-fast cooling technology, providing a theoretical basis and experimental data for the actual production of dual phase steels, in order to meet the needs of the site mass production. Thesis work and research results are as follows:(1) Using the optimizing processing can be obtained corresponding to the phase transformation kinetics curves by the thermal curves.Phase change kinetic curves can be a good reaction out of a new phase to form a process and new phase formation rate, combined with phase transformation kinetics curves and the thermal expansion curve, the critical temperature can be obtained. Based on the Austin-Rickett index n can describe the first eutectoid transformation procession,especially the first eutectoid transformation process.(2) The continuous cooling transformation experiments of austenite was studied with the object.Fs is665-580℃,Bs is531-501℃. Fs is668-808℃in the dynamic thermal simulation experiment. Deformation promotes the ferrite phase transformation. As the cooling rate increases, continuous cooling of undercooled austenite phase transition temperature is reduced.(3)The addition of alloying elements Si and Cr would increase the temperature of Ac1and AC3, while the temperature of Ms would decrease, but the decrease is very small. The addition of alloying elements of Si in the low cooling rate on ferrite phase transformation temperature increased by nearly30℃, the addition of alloying elements Si help to accelerate the ferrite phase transformation. As the increase of the content of alloying elements Cr,Martensite began to appear in40℃/s.The increase of alloying elements Cr contribute to the precipitation of the martensite.Also,it make inhibition of bainite transformation.(4) Austenite grain size before the phase transition gradually decreases with the decrease of austenitizing temperature.Reducing the grain size before the phase traintion will promote precipitation and precipitation rate of edge ferrite.What is more,as the cooling rate increase,the phase change time was significantly shortened.In addition,the ferrite volume fraction on the edge sige increase when the cooling rate accelerate. The temperature of Ferrite phase transformation and the precipitation of ferrite will rise When the austenitizing temperature reduced.While in the case of not fully austenitic, there are o lot ferritic in the process of follow-up phase transitions. As the austenitizing minimize, the bainite phase transformation region is increasing,however, the range expansion of the volume fraction is reduced, the occurrence of the bainite transformation temperature is gradually reduced, which carbon content in the main, the phase transformation driving force of the impact is relatively more weakening. With the lower austenitizing temperature, the pearlite transformation and the martensitic transformation range were expanding.(5) Increasing the contents of Si and Cr will help to improve the yield and tensile strength of the experimental steel. With the final rolling temperature and ultra fast cooling outlet temperature reduction, the experimental steel yield strength and tensile strength are improved. The semi-rolling can effectively improve plate quality, reduce wear with the process resulting in roll wear, improve the finished product rate.(6) Combination of laboratory studies and Lian Steel site rolling process can obtain the best process window of the experimental steel:the finishing rolling temperature was controlled at795-814℃,the temperature after UFC was controlled at680-700℃,air-colling time control in2.6-3.3s,the coiling temperature control in200-300℃. All of the microstructure are ferrite and martensite in this process window. In addition, the yield strength was350-395MPa, the tensile strength was600-645MPa,the yield ratio was0.56-0.68,the elongation was25-31%. All of this meet the standard requirements of DP580.