Study On Tensile Mechanical Properties And Mechanism Of High Strength And Toughness Medium Manganese Steel

Medium Manganese steel(MMS)has a relatively high strength-plasticity product(the product of tensile strength and elongation after fracture)with less alloying elements,so it is termed as the third generation of advanced high-strength steel(AHSS).In this paper,we formed a surface gradient structure by performing sliding friction treatment(SFT)on the surface of the MMS.Optical Microscope,Scanning Electron Microscope,Transmission Electron Microscopy,and Electron Backscattered Diffraction were used to study systematically the microstructure characteristics of different regions after SFT,and the grain refinement mechanism were analysed deeply;Hardness test and Tensile test were performed to obtain the mechanical properties of the current MMS after SFT.Finally,the influence of microstructure on mechanical properties was analyzed.The main research conclusions are as follows:(1)Nanocrystalline grains are prepared on the surface layer of MMS by SFT.The microstructure of the surface layer changes gradient with the depth direction,and it can be divided into nano grain(NG)layer,sub micron grain(SMG)layer and coarse grain(CG)layer.The thickness of the nano grain layer is about 20 microns,and the grain size is about 0.5 to 1.0 ?m.Next to the nano grain layer is the sub micron layer from 20 microns to 40 microns in depth,with a grain size of about 0.5 to 2.2 ?m.The grain size of the coarse grained layer is similar to that of traditional MMS,and the size of a small number of new formed fine grains can reach 1.4 ± 0.7 ?m.(2)The grain refinement mechanism of the larger grain regions of the CG layer and the SMG layer is through discontinuous dynamic recrystallization(DDR);while in the finer grain regions of the SMG layer and the NG layer,the grains are generally formed by continuous dynamic recrystallization(CDR)mechanism,except for some special regions with accumulated defects.(3)After SFT,the microhardness of MMS increases significantly,and the hardness value decreases with increasing depth.The hardness value of 20 microns from the surface layer can reach as much as 700 HV,which is more than double the hardness value of the coarse grain region.The most important reason for the increase in surface hardness is grain refinement and work hardening.The tensile strength of traditional MMS is 950 Mpa and the elongation is 22% while after SFT,the tensile strength of MMS is 1158 Mpa and the elongation can still reach 20%.Gradient nanostructures make MMS obtain high strength and ductility.The fracture surface analysis shows that the CG layer is fracture through typical ductile style,which is the main contribution of elongation,whereas SMG layer is a transition zone and the NG layer shows a brittle fracture.