A Study On Microstructure,Mechanical Properties And Retained Austenite Mechanical Stability Of Al-Bearing TRIP Steel

Energy conservation,environmental protection and safety are the main problems of the world automotive industry.Under the case of the increasingly serious environmental pollution problem,controlling automotive emission pollution is very important for energy conservation and emission reduction as well as alleviating environmental pressure.Automobile lightweighting is an important way and method to save energy and reduce emissions.By reducing the weight of the car as much as possible,it can effectively improve the power,reduce fuel consumption and exhaust emissions.Therefore,the research and development of high strength and high plastic automobile steel is the research focus of vehicle lightweight.Transformation induced plasticity(TRIP)steel has attracted much attention due to its good combination of strength,plasticity,excellent formability and strong energy absorption capacity.It is a very promising advanced high strength steel.In order to obtain higher content and stability of retained austenite,conventional TRIP steel was added to the steel,and the high silicon content will lead to deterioration of the surface quality of the steel sheet and a decrease in coating performance.Considering that aluminum can also inhibit the formation of cementite and don’t detetiorate the surface quality of steel,so silicon was replaced by aluminum in this paper.The microstructure,properties and retained austenite mechanical stability of experimental TRIP steel were investigated.The main contents can be drawn.(1)Three kinds of experimental steels with different aluminium contents were designed to study the effect of aluminum content on the structure and mechanical properties of Al-bearing TRIP steels.The results show that the microstructures of 2%Al and 4%Al steels at three annealing temperatures are composed of ferrite,bainite and retained austenite;while the microstructure of 6%Al steel annealed at 800℃ is ferrite and κ-precipitate,and the typical three-phase structures annealed are obtained at 850℃ and 900℃.At any annealing temperature,the yield strength and tensile strength increase as a whole with the increase of aluminum content,while total elongation and product of strength and elongation decrease with increasing of aluminum content,and the maximum product of strength and elongation can be obtained at 2%Al.The maximum volume fraction of retained austenite can be obtained at 800℃ for 2%Al steel,and the maximum product of strength and elongation can be obtained at the optimum aluminum content of 2%and annealing temperature of 800 ℃.(2)The effect of deformation temperature on mechanical properties and mechanical stability of retained austenite in 2%Al TRIP steel was studied.The results show that the yield strength and tensile strength decrease as a whole with increasing deformation temperature,the elongation and the product of strength and elongation first increase and then decrease with the increase of deformation temperature.The mechanical stability of retained austenite increases with the increase of deformation temperature.(3)The effect of stress state on mechanical stability of retained austenite in 2%Al steel was studied.The results show that the volume fraction of retained austenite decreases with increasing strain under any deformation mode.The rate of phase transformation of retained austenite is different under different stress states,and the retained austenite content at the end of deformation is also different;The mechanical stability of retained austenite increases in turn from biaxial tension,uniaxial tension to single shear.