Evolution Of Microstructure And Mechanical Properties Of A Fe-30Mn-3Si-4Al Steel Under Deformation

The automobile industry currently plays a very important role in domestic economy and the fast development of vehicles has promoted the related industries, especially the steels for automobile production. Moreover, the ongoing research of the steel industry mainly focuses on high strength steels with excellent formability. This optimal combination is particularly attractive for automotive applications such as structural reinforcements and energy absorption parts. Especially, the increase in the oil price and environmental concerns, have lead the automobile industry to address seriously energy saving issues. Among various measurements considered, the reduction of the vehicle weight is proved to be the most effective, which can be achieved by developing new types of cost-effectively steels including the twinning induced plasticity (TWIP) steel. The strong points such as the good formability and high energy absorption ability of TWIP steel are extremely attractive for the development of new generation steels. The present study investigated the evolution of microstructures and mechanical properties of a TWIP steel with a chemical composition of Fe-30Mn-3Si-4Al-0.097C. Firstly, the TWIP steel was annealed at750℃for1h, and then was deformed under a cold rolling reduction of10%,20%,30%,40%,50%,60%, and70%, respectively. Subsequently, the deformed samples were annealed for1h at500℃and deformed under tension. The X-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques were used to investigate the microstructures of TWIP steel. The main results are as follows:(1) The mechanical properties of TWIP steel with different cold-rolling reductions have been investigated. The tensile results have revealed that the TWIP steel shows high strength and significant ductility (elongation to failure>30%) as the rolling reduction less than40%. Both the yield strength and ultimate-tensile-strength increase with an increasing in cold-rolling reductions, but the ductility exhibits a reverse trend. For instance, the TWIP steel with a rolling reduction of10%shows a yield strength of610MPa and tensile strength of766MPa. The values increase as1260MPa and1378MPa for the sample with a rolling reduction of70%. However, the tensile strain decreases from31%to2%with increasing the rolling reduction from10%to70%.(2) The value of strength multiplied strain dramatically decreases from47492MPa%to9233MPa%with increasing the cold-rolling reduction from10%to70%。(3) The XRD analysis indicated that the TWIP steel had a pure austenite phase before and after cold-rolling. The calculated result revealed that the stacking fault energy of the investigated steel is36.11mJ/m2at room temperature, which is well consistent with the value for the twinning. The deformation of austenite with the SFE above20mJ/m2is manifested by mechanical twinning, suggesting the deformation process has been dominated by twinning other than phase transformation. The results from both the optical and TEM observations have shown that a number of twins in the deformed samples and the amounts of twins increase with the rolling reductions.(4) The work-hardening ability has been investigated for the present TWIP steel with a chemical composition of Fe-30Mn-3Si-4Al-0.097C. It suggested that the deformation process should be dominated by the initial dislocation slip and the subsequent twinning resulting from the increasing stress. The interaction between dislocation slip and the deformation twins impressively enhanced the work-hardening ability of the present steel. However, the tensile elongation of the TWIP steel decreases with increasing cold-rolling reduction, especially more than a reduction of30%. It is suggested that an adjustment of heat-treatment process may be preferential to the improvement of ductility for the investigated steel.(5) The texture evolution of the TWIP steel under different rolling reduction was investigated. And the evolution of α-fiber, β-fiber and τ-fiber texture and volume fraction of various texture were studied. The results show that the current steel only exhibits obvious texture as the rolling reduction more than40%. The twinning process inhibited the texture evolution at some extent.