A New Q-P-T Heat Treatment Process Of Research

In an international attention of saving energy and raw materials as well as protecting environment, the government should investigate and develop the third generation of advanced high strength steels(AHSS) for reducing the weight of the car. In order to achieving “multi-phase, metastable, multi-scale” of the third AHSS, XU Zuyao proposed the novel quenching-partitioning-tempering(Q-P-T) process. According to this heat treatment, the novel combined with hot deformation, phase transformation and carbon partitioning to study Q-P-T steels. The composition of medium carbon Q-P-T steel have been designed in this work. Scanning election microscopy(SEM), transmission electron microscopy(TEM), X-ray diffraction(XRD) and mechanical tests were employed to study the effects on microstructures and mechanical properties. The main results and achievements are expressed below.(1)In this work, a medium carbon Fe-0.41C-0.24Si-0.58Mn-0.88Cr-0.014Ni(wt.%) steel has been designed and developed the heat treatment process. The process included hot deformation, phase transformation and carbon partitioning. The test process is as follows: The first step is an austenitizing at a temperature and austenite is deformed at 870 ℃so as to generate dynamic recrystallization. The process make austenite refine the grain size. The second step is a rapid quenching to a specific quenching temperature between martensite start temperature(Ms) and martensite finish temperature(Mf) to fabricate partially martensite. The third step is a carbon partitioning treatment. The aim is to make the carbon from supersaturated martensite to untransformed austenite to form carbon-enriched retained austenite which is stable at room temperature.(2)The medium carbon steel after the Q-P-T heat treatment can obtain martensite and residual austenite. When the tempering time is longer, the residual austenite can decompose ε-carbide and cementite. The Q-P-T steel by deformation can obtained more residual austenite with XRD test. The existence of residual austenite can effectively hinger the crack propagation and improve the ductility of the steel.(3)By the Q-P-T heat treatment, the ultimate tensile strength of the steel is 1487.5MPa; the elongation is 19.9%. When the quenching temperature is 300 ℃and the tempering temperature is 400℃/30 s, the max of product of strength and elongation can reaches 26.3GPa%. These mechanical properties can conform the third generation of advanced high strength steels.(4)The Q-P-T steel has refined multiphase microstructure: lath martensite and retained austenite films. The existence of the double microstructures ensures the steel has high strength and good plasticity. In the process of deformation, residual austenite can produce the transformation induced plasticity effect and residual austenite hinders the crack propagagation effect. The two effects ensure the steel has excellent ductility.