Research On Microstrcuture And Mechanical Properties Of Q&P High-Property Steel

With the deterioration of environment and energy crisis in the world, the main trend of modern car industry is to reduce weight, conserve energy, protect environment and improve safety. Using new high-strength steels can reduce weight, conserve resource and reduce emissions of tail gas of the automobile. These trends demand that automobile steel should be lighter and with higher strength and higher ductility.In order to further improve the mechanical properties of automobile steel, Professor Speer, etc., from Colorado School of Mines, put forward a new heat-treatment process-Q&P Process (Quenching and Partitioning Process). This process mainly suppresses carbide precipitation by controlling the partitioning process after quenching and its purpose is to enrich retained austenite with carbon. It changes the microstructures of steel and increases the mechanical properties of steels.In this thesis, two kinds of steel0.19C-1.53Mn-1.52Si-0.14Al-0.048Nb and0.19C-1.48Mn-1.52Si-0.15Al have been mainly studied. By comparing the different technolo gical parameters of Q&P process treatment on the two kinds of steels under the cond itions of laboratory, the effects of quenching temperature, partitioning temperature, part itioning time and alloy element Nb on microstructures and mechanical properties of th e steels were researched. The contents of the study mainly include the following ma nners and conclusions:(1)The matrix structures of Q&P steel are lath martensite and carbon-enriched retained austenite. It has higher strength and well ductility.(2)With quenching temperature’s increasing, the size of lath martensite in the steels increases. The strength of the steel decreases gradually and the ductility is improved. But with the further temperature rising, the content of retained austenite after quenching becomes more and more. Because of carbon content in the steel is not enough to make austenite stable, retained austenite transforms again. So the strength rebounds with ductility’s reducing.(3)With the rising of partitioning temperature, the strength of the steels decreases, but the ductility of steel rises. It is found that the diffusing speed of carbon rises so that the carbon content in martensite is reduced, but the one in austenite rises. This explains the reason of reducing strength with ductility rising. When the temperature is continuously raised, the strength increases while the ductility decreases. Because carbides precipitate when the temperature is higher than a determinable value (about400℃) and this results in the decrease of diffusing carbon in the steels. It directly decreases the carbon in austenite and it’s harmful to give full scope to ductility of Q&P steel.(4)During partitioning process, the strength of the steels continuously decreases along with the extention of partitioning time. Carbon in supersaturated martensite always diffuses into retained austenite during the process, so carbon content in martensite and the strength of the steels has been reduced. But the ductility of the steels continuously rises, resulted from higher degree of carbon-enrichment.(5)The microstructure of steel with Nb is finer and more homogenous than that of steel without Nb. Under the Transmission Electron Microscope, the density of precipitation in the steel with Nb is higher than the steel without Nb. It has been discovered that the addition of Nb can bring effects of grain refining strengthening and precipitate strengthening during the Q&P process by experiments and improves the mechanical properties of the steel.