Effect of cold rolling and annealing on the microstructure and mechanical properties of Mn-substituted-for-Ni alumina-forming austenitic stainless steel

The microstructural evolution of low-Ni alumina-forming austenitic (8Mn-AFA) steel during cold rolling with different reductions (from 45% to 80%) and annealing was investigated using electron backscatter diffraction (EBSD) and X-ray diffraction (XRD) techniques. Compared with conventional AFA steel (low Mn content), 8Mn-AFA steel exhibits a higher strength in the transverse direction (TD) than in the rolling direction (RD) during plastic deformation. This anisotropy increases with the cold rolling deformation. When the cold rolling defor-mation increases from 45% to 80%, the ultimate tensile strength (Rm) in the RD and TD increases from 1091 to 1306 MPa and from 1162 to 1442 MPa, respectively. The stacking fault energy (SFE) of 8Mn-AFA steel is only 46.79 mJ/m2 due to the substitution of 8Mn for Ni. The low SFE induces the formation of many deformation twins arranged along or close to the TD (the angle between them is less than 30 degrees) in the austenite grain, yielding a higher strength in the TD than in the RD. Compared with cold-rolled samples, Rp and Rm decrease in annealed samples, and the anisotropy in the TD and RD almost disappear as the number of deformation twins decreases. The high strength of the cold-rolled 8Mn-AFA steel can be attributed to the dislocation enhancement, grain boundary enhancement, and texture-induced anisotropy strengthening. The dislocation enhancement contributes the most to the strength, exceeding 70%, followed by grain boundary contribution, about 6.9 - 10.8%, while the contribution of texture-induced anisotropy is 5 - 9.2%.