Study On The Microstructures And Mechanical Properties Of CMnAl-TRIP Steels

In order to meet the automobile industry's need for weight reduction and safety improvement, the application of advanced high strength steel sheets such as TRIP steel have been examined for suspension and structural part. TRIP steel is the most beneficial steel for increasing the strength of steel without deteriorating the strength and elongation balance. Generally, TRIP steel can reduce the weight and save the fuel without losing the safety. However, the steel containing high silicon elements can not be applied automotive parts due to low surface condition. In this study, aims to promote the surface quality substitute silicon by aluminum base on 0.15C-1.5 Mn-1.5Al-TRIP steel, the effects of alloying elements, heat treatment condition, such as intercritical annealing(IA)and isothermal bainitic transformation(IBT) parameters on microstructure and mechanical properties were investigated, by means of X-ray diffraction(XRD), dilatometric simulation, scanning electron microcopy (SEM), transmission electron microcopy (TEM), optical microstructure (OM) and tensile testing.The experimental results show that the three steels have higher Ac₁ Ac₃ temperature than conventional TRIP steels, with 750℃of Ac₁ for 0.05P steel and 757℃for the others. The Ac₃ is not found for the steels until heating up to 1100℃. Before cooling to the IBT zone, the pro-eutectoid ferrite is found and it affects the volume fraction and carbon content of the austenite which formed in the intercritical zone, the higher IA temperature is favor the formation of pro-eutectoid ferrite.Through dilatometric simulation, we find that 0.05P steel has volume fraction and carbon content of the retained austenite of 25% and 0.54% at 770℃, 35% and 0.40% at 820℃, 45% and 0.31% at 870℃, respectively. But the tendency is not monotony with the IA temperature, the volume fraction and carbon content of the retained austenite arrives the highest at 820℃, with the number of 14% and 1.36%, respectively. The bainite transformation kinetics is increase with the increase of IBT temperature, which the cementite is found at 500℃holding for 30s. The 0.05P steel had the Avrami content of 1.011 at 680℃and 0.77 at 720℃, which are lower than the ideal Avrami content around 2-4.The three steels have more than 10% volume fraction of retained austenite, with the lowest amount at 800℃and the highest at 770℃, showing the increase tendency during 800℃to 890℃. The volume fraction of ferrite decreases with the increase of IA temperature, while the bainite shows the reverse tendency. The tensile strength is similar to the tendency of retained austenite; yield strength is increase and elongation were decrease with the increase of IA temperature. The IA temperature gives ferrite and austenite fraction of 65%:35% is the best selection, in which the 0.05P and 0.1P steel have the elongation of 39.3% and 36.1% at 820℃. The product of tensile strength and elongation were greater than 22000 MPa%, with the n value greater than 0.2 at necking.The martensite is founded at shorter IBT holding time, at the same time, the volume fraction of retained austenite was little. During the IBT holding, the retained austenite is carbon enriched through the bainitic transformation and can resist the following cooling, then the martensite is diminished and retained austenite is increased. The cementite is founded at 450℃for 300s, while it appears at 30s for 500℃.The cementite appears reduced the volume fraction of retained austenite and its carbon content, which would harm the mechanical properties. All the samples show the highest of the product of tensile strength and elongation at 450℃, and 0.05P steel with the tensile strength of 646.5MPa, elongation of 39.3% and product of tensile strength and elongation of 25400 MPa % at 450℃for 60s, respectively. In industrial manufacture, the IBT parameters of 450℃for 60s can be selected for the continual galvannealing lines and the galvanizing rigs.P and Cu can increase the volume fraction of retained austenite but low its carbon content. The retained austenite fraction is increased by the increment of P content from 0.05wt% to 0.1 wt%. The volume fraction of retained austenite fraction reaches 13% for 0.05P steel and 15% for 0.1P steel after isothermal holding for 60 seconds at IBT temperature. The increase of tensile strength by the increment of P content is ascribed to the higher fraction of strain-induced transformed martensite as well as the solid-solution hardening effect of P. Despite of the increase of retained austenite fraction, the elongation of TRIP-aided steel is deteriorated by the increment of P content. It originates from the lower mechanical stability of retained austenite in 0.1P steel, which brings about rapid strain-induced martensite transformation and thus inhibits persistent work hardening during deformation.Compared with the instantaneous n value and the mechanical stability of retained austenite, there are all affected by the TRIP effect. If the mechanical stability of retained austenite is low, most transforms to martensite at early stage of deform and shows the stronger TRIP effect and higher instantaneous n value; the TRIP effect is weak at the late stage of deform and shows lower instantaneous n value, together with low uniform and total elongation. When the mechanical stability of retained austenite is high, it can continually transform to martensite with high instantaneous n value during whole deform. Not only the volume fraction of retained austenite, but also the mechanical stability is considered for TRIP steel.The carbon content of retained austenite is the mainly factor influence its mechanical stability, the heat treatment parameters and the alloying affect the mechanical stability through the carbon content of the retained austenite, the refined retained austenite grain size also increases the stability. The instinct stacking-fault energy affects the nucleation of martesite as well as the retained austenite transforms during deform, aluminum increases the instinct stacking-fault energy and promotes the stability of retained austenite.