Microstructure And Properties Of 12MnNb Steel Processed By Equal Channel Angular Processing

Equal Channel Angular Processing (ECAP) is a severe plastic deformation technology, which can effectively process bulk ultra-fine grains. Therefore, ECAP has recently attracted the attention of many researchers.ECAP was successfully carried out in 12MnNb Steel to a total equivalent true strain of ～4 at room temperature. The microstructure evolution and mechanical properties were investigated by means of optical microscopy (OM) , transmission electron microscopy (TEM) and electronic tensile equipment. The main results are as following:The equiaxed submicrometer grains with high-angle boundaries were formed and the average grain size was about 0.3μm after four passes ECAP deformation in room temperature. In rout C, the first pass significantly refined the grains, and then the angle of orientation enlarged in the subsequent passes. The strength increased with the increase of deformation passes, the ultimate tensile strength was up to 895MPa with preserving better ductility, the elongation of 12.4%. In ECAP, ferrite first broke into bands structure, further deformation increased the density of dislocation in bands structure and formed dislocation cell structure, lastly these structure evolved into ultra-fine grains. In order to accommodate the severe plastic deformation in ECAP with rout C, the deformation mode of in steel include bending deformation, necking deformation and kinking deformation. Cementite lamellae partly dissolved and speroidized. On normalization experiments, Nb nano-carbonitride can effectively prevent the growth of grains and control the size of the ferrite size. Above 950℃, the ferrite grains by normalizing is coarser, which show that the second phase particle gradually lose the hindrance on the growth of grains.