An Investigation On Microstructures And Mechanical Properties Of TRIP Steel With High Al And Low Si Contents

TRIP (Transformation-induced Plasticity, TRIP) steel is one of the most potential automobile steel at present. Owing to TRIP effect, TRIP steel has a combination of higher strength and higher ductility such that TRIP steel possesses the excellent properties. Generally, TRIP steel can reduce the weight and save the fuel consumption, while it can keep safety. However, this steel contains higher silicon content and it cannot be applied to manufacture automotive parts due to low surface quality. In order to improve the surface quality, we choose the aluminum element to substitute silicon to stabilize the retained austenite in C-Mn-Si system TRIP steel.According to a compositional designing of low silicon and high aluminum contents, a new type of C-Mn-Al-Si system TRIP steel was designed in this thesis, and then it was remelted and rolled in laboratory. By means of optical microstructure (OM), scanning electron microcopy (SEM), transmission electron microcopy (TEM), X-ray diffraction (XRD), dilatometric simulation, and tensile testing, the effects of alloying elements and heat treatment process, e.g. intereritical annealing and isothermal bainitie transformation parameters, on microstructure and mechanical properties were investigated. The optimal heat treatment process was obtained, and their influences of heat treatment processes and chemical composition on the microstructures and mechanical properties were also discussed.The experimental results show that after substitution of Si by Al this steel has higher Ac1、Ac3 temperature than that of the conventional TRIP steels. By conducting various annealling experiments, we found that the steel has similar mechanical properties when annealed from 840℃ to 960℃. The tensile strength of this steel is lower when annealed at 1000℃ due to the reduction of the carbon content in retained austenite and thus unsteady state of retained austenite, which could easily transform retained austenite to martensite. So, the TRIP effect is not so obvious and it is not beneficial to the mechanical properties. During the IBT holding, there is no much difference of the strengthes when annealed either at 840℃ or at 960℃, but the elongation to failure is slightly reduced with increasing the IBT temperature. Especially, when the IBT temperature was at 500℃, there was pearlite transformation which reduced the mechanical properties. When the annealing temperature was at 840℃, much better mechanical properties could be obtained, since a finer bainite lath and granular bainite at ferrite grain boundaries could be achieved as compared with that at high annealing temperature. For the present experimental steel, the optimal holding time is 3-5 min.