Evolution Of Multiphase Microstructures In C-Si-Mn Q&P Steel With Ultra-fast Heat Treatment Process

Recently, continuous improvement of automotive safety are required considering environmental protection, resources reducing and energy saving. Q&P steel is a new type of martensite steels with high strength, high plasticity, high toughness and an excellent combination of strength and ductility. As an automotive structural steel, it can greatly reduce vehicle weight and enhance impactability, which results in a great improvement of operational safety and satisfaction requirement of people for automobile safety performance. Because the ultra-fast cooling process not only can reduce pollution and the heat treatment time but also improve the internal organizational structure and mechanical properties of Q&P steel, it can provide technology conditions for the development of high strength steel. Now the induction heating technology has have been applied in various fields.This study focused on commercial C-Si-Mn TRIP steel as experimental materials. Q&P heat treatment process was performed by the DIL78RITA Quenching Dilatometer. Experimental methods such as optical microscopy(OM), scanning electron microscope(SEM), transmission electron microscopy(TEM) and macro hardness tester were used to study effect of austenitizing temperature(AT), austenitic the holding time(At), quenching temperature (QT), partitioning temperature(PT) and partitioning time(Pt) on the volume of retained austenite in the steels. The main results are as follows:(1) Under condition of ultra-fast Q&P heat treatment, with the increasing of heating rate, the austenite transformation temperature rises. However, the time for austenitizing, the grain size of primeval austenite and microstructure after quenched decrease, as well as the austenitizing degree will decrease at the same AT increases. With the same heating rate, the austenitizing temperature is higher, the grain austenitizing degree is higher. In addition, both the grain size of austenite and the lath of martensite after quenched decrease.(2) Under the condition of quick Q&P heat treatment, with the increasing of cooling rate, the lath of martensite after quenched are more small and the hardness are more higher. With the QT increasing, the volume of retained austenite increases at the same cooling rate. The austenite locates in martensite laths with laminar film and thin strips.(3) Under the condition of ultra-fast Q&P heat treatment, with the more higher partitioning temperature, the laths of martensite laths is more flatness and orderness at the same partitioning time. Wth the rising of partitioning temperature and time, the microstructure of it becomes discontinuous and smallish because the supersaturation C gather in the phase boundary between martensite and austenite in the partitioning progress.(4) Under the condition of ultra-fast Q&P heat treatment, ε-carbide precipitates with different characteristics. Simultaneously, the C atoms partitions unevenly which leads to the formation of twin martensite with the condition of ultra-fast cooling.