Effect Of Torsion Deformation On Microstructure And Mechanical Properties Of Twin Induced Plasticity (TWIP) Steel In Fe-Mn-Al-Si System

Twinning induced plasticity(TWIP)steel has very high work hardening ability,high plasticity and high strength.As a whole,it shows very excellent comprehensive mechanical properties.It has a wide application prospect in the automotive industry.Since frommeyer and other scholars first proposed TWIP effect and developed TWIP Steel from 1997 to 1998,TWIP Steel with high strength and high toughness has attracted much attention.Compared with other advanced high strength steel(AHSS),TWIP Steel has very high strength plastic product,but its yield strength is very low,and its anti-collision ability is obviously poor,which seriously restricts its application in automobile industry.Taking Fe-25Mn-3Al-3Si-0.3C-0.3V TWIP Steel as the main research object,the effects of torsion deformation on microstructure and mechanical properties of TWIP Steel were studied by means of optical microscope(OM),scanning electron microscope(SEM),X-ray diffraction(XRD),transmission electron microscope(TEM)and tensile test at room temperature.The main results are as follows:Torsion deformation can effectively improve the strength of TWIP Steel,and different torsion angles can obtain different hardness.Among them,the hardness of the experimental steel is the highest at 1988° torsion.There are not a lot of deformation twins and a lot of dislocation defects in the experimental steel after 15° torsion,but a lot of deformation twins appear in the experimental steel after 15° torsion.This is because the strengthening mechanism of the experimental steel in the torsion process changes from dislocation slip at room temperature to twin deformation.The tensile strength and yield strength of the experimental steel can be improved after pre torsion.Compared with the direct tensile samples,the tensile strength of the samples with pre torsion of 30°,60° and 90° is improved,and the larger the pre torsion angle is,the more obvious the improvement is.There are deformation twins in the matrix of the three groups of experimental steels before stretching.During the stretching process,because the tensile stress promotes the occurrence of dislocation slip,the sliding deformation is the main deformation.A certain number of deformation twins are formed in the microstructure of the pre twisted experimental steel,and dislocation stacking exists in the matrix.After the introduction of tensile stress,dislocation slip is promoted and twinning deformation is restrained.When the specimen is pre twisted by 30° and subjected to tensile stress,the shear stress produced by tensile stress and slip deformation together lead to fracture of the specimen,in which shear stress is the main factor and slip deformation is the secondary factor.The results show that there is a slip system in the matrix of pre twisted 90° specimens after torsion.When tensile stress is applied,the tensile force produces a shear stress on the slip surface,and the shear stress promotes the slip deformation.Therefore,the main reason for the fracture of this group of experimental steels is the slip deformation.The strain of the experimental steel is increased from the axis to the edge,and the strain hardening effect is different,which results in the gradual increase of hardness in the radial direction.In addition,the existence of strain gradient is also the reason for the formation of nano twin after tensile.