Study On High Strength And Elongation Steels Treated By Hot Deformation & QP Process

It is an effective way for energy saving, environmental protection and automobile safety to explore high strength steels, which have low cost, better processing property and service performance. Therefore, in order to develop advanced high strength steels which possess ultra-high strength with corresponding plasticity, low cost and good quality steel, we put forward two advanced heat treatment methods: Hot Stamping-Quenching & Partitioning (HS-QP) and Deformation Induced Ferrite Transformation-Quenching & Partitioning (DIFT-QP). The new types of high strength multiphase steels with corresponding plasticity were obtained through hot deformation plus Q&P process. The performances of steels meet the requirements of excellent properties of the third generation high strength automobile steel. The new designed steels were characterized and measured by means of optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), thermal expansion device (TED) and Zwick T1-FR020TN A50 universal testing machine. The main contents and important results are showed as follows.Firstly, according to the results of the present study, we recommend the integration of hot deformation, phase transformation and carbon partitioning can be conducted to obtain refined multiphase microstructure. Grain refinement can be realized through hot deformation at austenitizing temperature. Quenching and partitioning process is used to obtain hard phase martensite and soft phase retained austenite. Also,we put forward a technologically advanced heat treatment method of combination of hot stamping and controlled quenching: Hot Stamping-Quenching & Partitioning (HS-QP). HS-QP process includes: The first step is an austenitizing at a temperature and austenite is deformed at higher temperature so as to 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 transformed supersaturated martensite and untransformed austenite. The third step is a carbon partitioning treatment from supersaturated martensite to untransformed austenite to increase the carbon content of untransformed austenite in absence of carbide formation. Finally, a refined multiphase microstructure of carbon-depleted martensite and carbon-enriched retained austenite films is fabricated at room temperature. Through proper optimize manufacturing process parameters, the new type of high strength steel (0.22C-1.58Mn-0.81Si-0.022Ti-0.0024B) with tensile strength higher than 1500 MPa, elongation higher than 14% was obtained during the HS-QP process, and the anticipant results were achieved. Compared with the sample treated only by hot stamping, the product of strength and elongation of the sample treated by HS-QP process can increase from 11000MPa% to 22000MPa%。The results showed that a mixed microstructure of HS-QP steel was made up of refined martensite lathes (about 100~200nm) and slender interlath retained austenite (about 20~40nm).Secondly, grain refining technique of Deformation Induced Ferrite Transformation (DIFT) is introduced to Q&P process. We proposed another novel heat treatment processes: Deformation Induced Ferrite Transformation-Quenching & Partitioning (DIFT-QP). Grain refinement and generation of ferrite can be realized through DIFT. Quenching and partitioning process is used to obtain hard phase martensite and soft phase retained austenite. DIFT-QP process includes: The first step is an austenitizing at a temperature with a period of time; The second step is a rapid quenching to a specific quenching temperature near the intercritical temperature and microstructure is deformed at this temperature so as to refine the grain size and produce ferrite. The third step is a rapid quenching to a specific quenching temperature between martensite start temperature (Ms) and martensite finish temperature (Mf) to fabricate partially transformed supersaturated martensite and untransformed austenite. The fourth step is a carbon partitioning treatment from supersaturated martensite to untransformed austenite to increase the carbon content of untransformed austenite in absence of carbide formation. High dislocation density can provide diffusing channel for carbon, which is very important to improve the stability of retained austenite. Finally, a refined multiphase microstructure of carbon-depleted martensite, carbon-enriched retained austenite films and deformation induced ferrite is fabricated at room temperature. Through proper optimize manufacturing process parameters, the new type of high strength steel (0.22C-1.58Mn-0.81Si-0.022Ti-0.0024B) with tensile strength higher than 1700 MPa, with the yield strength above 900 MPa and elongation more than 15% was obtained during the DIFT-QP process, and the anticipant results were achieved. The refined multiphase microstructure is made up of carbon-depleted martensite, carbon-enriched retained austenite films and deformation induced ferrite. The evolution of refined multiphase microstructure of DIFT-QP process was revealed and the mechanism of martensite formation during DIFT-QP process was analyzed.Lastly, the evolution model of refined multiphase microstructure treated by HS-QP process has been proposed: in the initial deformation stage, dislocation crystallographic slips result in the formation of multiple configuration of small angle dislocation interface, namely the sub-grain boundary, deformation zone and dislocation wall, etc. Big-angle grain boundary is easy to move in the initial deformation stage and the original grain boundary becomes curved under the condition of stress. Austenite may become unstable and elongated microstructure unit and sub-grain. With the increase of rolling reduction dislocation interface orientation convert to random orientation interface. The cumulative strain to a certain extent, these sub-grains may fracture and lead to formation of refined grain. Dislocation in austenite treated by hot deformation was inherited for the martensite during Q&P process. Nucleation and growth of martensite transformation were affected by sub-grain boundary generated by deformation. The role of refined martensite is very important to refine grain size and grain can be further segmented to small grain. Moreover, the mechanisms of microstructure refinement and elongation enhancement of the refined multiphase microstructure obtained by hot deformation, phase transformation and carbon partitioning have been analyzed. The refined multiphase microstructure can make high strength steel perform good strength and toughness and the combined heat treatment process is simple. Also, the combination of hot deformation and Q&P process can reduce cost and save resources. Therefore, the new combined heat treatment process will have wide prospect of industrial applications.