Study On Deformation Mechanism Of Medium Manganese Steel Based On Q-P-T Process

With the increase of the car ownership,the automobile steel is also increasing rapidly.The future direction of the automobile steel towards safety and lightweight determines the requirements for automotive steel is more stringent.In order to make it and achieve win-win situation,the need to research both low-cost and high-strength of the third generation of advanced high-strength steel is urgent.Based on the Quenching-Partitioning-Tempering(Q-P-T)process proposed by Xu Zuyao team in 2007,we designed a new medium manganese steel,the study of this steel was studied by optical microscope(OM),scanning electron microscope(SEM),transmission electron microscope(TEM),X-ray diffraction(XRD),Gleeble thermal simulation test and mechanical property test.The main research contents and the final results of this study are as follows:(1)A new medium manganese steel containing Nb was designed.The continuous cooling transformation(CCT)curve of this experimental steel was obtained by thermal expansion test.Several key temperatures of this experimental steel were acquired: Ac3 was813 ?,Ac1 was 594 ?,martensite start temperature(sM)was 360? and martensite finish temperature(fM)was 80?.Speer put forward the theory of “Constrained Carbon Paraequilibrium"(CCE)thermodynamics model.We could use the theory to predict the residual austenite content,quenching temperature of the experimental steel and the optimum quenching temperature.Above all,we got the final process: The first step is an austenitizing at a temperature and then deformed to generate dynamic recrystallization.The second step is a rapid quenching to a specific quenching temperature betweensM andfM to fabricate martensite partially.The third step is carbon partitioning treatment to make the carbon atom transferred from martensite to austenite to form carbon-enriched retained austenite which is stable at room temperature.(2)After the heat treatment,the final microstructure of the Q-P-T experimental steel is lath martensite and retained austenite can be seen clearly by the microscope.The XRD results show the trend of retained austenite content is "rising and then descending" atdifferent deformation temperatures.The retained austenite content is gradually increased under the same deformation temperature.(3)Through the mechanical properties test,we found the maximum of product of strength and elongation can reach to 34307MPa%,which is fitting the requirements of the AHSS.Comparing the trend of strong plasticity with the retained austenite content,it can be seen the trend is basically the same.It also shows that the retained austenite content plays a key role in the performance of the AHSS.The results show the microstructures are lath martensite and retained austenite.With the increasing of the deformation,tissue size decreases,tensile strength increases and austenite content is changed.Above the recrystallization temperature,with the deformation temperature decreases,the organization decreases,the austenite content decreases but the strength increases.In the non-recrystallization temperature range,the deformation temperature has different effects on the microstructure and properties.In short,the introduction of deformation based on the Q-P-T process,the product of strength and elongation of this experimental steel meets the requirements of the third generation of AHSS.