A Study On Deformation Behavior And Retained Austenite Stability Of Al-bearing TRIP Steel

With the rapid development of modern society,the transport vehicle is more and more popular.Requirements for safety,environmental protection and energy saving are needed in automobile industry.It is essential to develop automobile steel with high strength and plasticity.However,high strength and high plasticity of steel is a pair of contradiction.For automotive structural parts,increasing strength of the steel tends to reduce its plasticity,or even reduce the formability.Transformation-induced plasticity(TRIP)steel has good prospects in automobile manufacture due to its excellent combination of strength and plasticity.High content silicon added to traditional TRIP steel can inhibit the formation of cementite and stabilize the retained austenite.However,high content silicon will lead to the decrease of the coating performance of the steel plate.Considering that aluminum has the same effect as the silicon,silicon can be replaced by aluminum to improve the coating properties of steel and surface quality.Under this frame,some research works have been conducted and the main contents of this thesis are summarized as follows.(1)Four kinds of heat treatment processes were conducted for this 2wt%Al-bearing TRIP steel and the corresponding microstructures were observed and analyzed by using optical microscope(OM),scanning electron microscope(SEM),transmission electron microscope(TEM)and X-ray diffraction(XRD).The effects of morphology of retained austenite and grain size on mechanical properties and retained austenite stability were investigated for this TRIP steel.(2)After heat treatment and subsequent tensile deformation,the retained austenite in Al-bearing TRIP steel transformed to martensite,i.e.TRIP effect is generated.TRIP steel with more blocky retained austenite has higher strength,while TRIP steel has better plasticity with more film-like retained austenite.After 2 cycles of rapid heating and rapid cooling,the experimental steel has the highest tensile strength,elongation,product of tensile strength and elongation.(3)For TRIP steel containing typical blocky retained austenite,quasi-static tensile and dynamic tensile deformation tests were performed to investigate the mechanical properties under different strain rates.Yield strength,tensile strength and elongation decrease with increasing the strain rate at quasi-static tensile strain rate range.Tensile strength and elongation increase with increasing the strain rate at dynamic tensile range.Tensile strength of the Al-bearing TRIP steel decrease tensile deformed at strain rates of 100 s-1 and 1000 s-1 due to the adiabatic heat.(4)Comparison of the fracture morphologies of the Al-bearing TRIP steel deformed at different strain rates indicated that the fracture is mainly of ductile and there are many dimples at the fracture surface.The micro structures of tensile specimen were observed at strain rates of 100 s-1 and 1000 s-1.Twin martensite was observed when deformed at 100 s-1,while twin and lath martensites were both observed when deformed at 1000 s-1.