Research On Q-P-T Process Of Low-carbon Microalloy Steel

In recent years, auto industry has been promptly developed all around the world. And the world is facing with the problems including enegy, environment and safety, then the steel used in cars should posess higher strength and higher elongation. A sieries of auto steel with exellent strength and plasticity are produced to meet the demand, such as DP steel, TRIP steel and so on. In2003, a new process of heat treatment was put forward by professor Speer, and a themodynamic model was built to describe how the carbon patitioned during the process, that is’Constrained Carbon Paraequilibrium’. After the quenching and partitioning process, the mixture of martesite and austenite was obtained ultimately. The martensite endows the steel with high strength during the deformation process and high elongation is obtained because of the TRIP effect of the retained austenite, and then excellent strength and plasticity was gained.In consideration of that the CCE model didn’t refer to the carbide precipitation and the migration of boundary, academician XU Zuyao proposed Q-P-T process, which is based on Q&P process, by adding micro-alloy elements and the formation of precipitation strengthening, the strength is increased. Preliminarily proving test showed that the propertie of steel obtained through Q-P-T heat treatment is superior to the propertie treated throngh Q&P process.With the support of national natural science fund project, this paper explored the theory of Q-P-T process. Taking the steel consisting of0.19C-0.19Al-0.053Nb-0.94Ni-0.89Cr as the research object, the steel with strength above1500MPa, elongation above10%was produced through Q-P-T process. In this paper, the effects of quenching temperature, partitioning temperature, partitioning time on the mechanical properties of steel is explored. The results show that with the quenching teperatures’increasing, the strength of steel increases. this is because the stability of retained austente is weakened and transformed to martensite when quenching to room temperature after partitioning. In the begining, with the partitioning temperatures increasing, the tensile strength of steel decreases. Continuely improving the partitioning temperature, tensile strength increases. The reason for this phenomenon is that higher partioning temperature stabilizes the retained austenite when the partioning temperatures are not high enough. But at the high partitioning temperatures, transformation of retained austenite to bainite gives rise to the strength. At the beginning, prolonging the partioning time, carbon can gradually fully partitions from the martensite to the austenite. And then increase the partitionig temperatures, austenite transform to bainite. Continuely prolonging the partitioning time, martensite’s strength decreases due to the precipitation of microalloy carbides reduces the carbon content of martensite.At the same time, retained austenite where carbon content is low transformed to bainitic ferrite. As a whole, with the partitioning time prolonging, in the beginning, the strength of steel decreses, and then increses, at last decreses, while the elongation changes is converse to the strength changes.In this paper, a new heat treatment that the qenching after partitioning is changed to air cooling after partitioning is proposed. And the effects of quenching temperature, partitioning temperature, and partitioning time on the steel is also studied. It can be seen from the result that the strength of steel treated through the new process is about1400MPa, lower than the one treated through the former Q-P-T (quenching after partitioning) process, and the elongation increses to above15%. During the air cooling after partioning process, retained austenite will not transform to martensite. The effects of the quenching temperatures, partitioning temperatures, partitioning time are different from the former Q-P-T process.CoMPared the microstructures and mechanical properties of the steel treated through the process in air cooling after partitioning instead of the steel quenched after partitioning through the former Q-P-T process, the martensite laths are in disoder and the areas of ferrite expand. The lower strength, higher elongation and higer product of tensile strength and ductility can be obtained, the mechanical properties are improved.