Enhanced Mechanical Properties of a Gradient Nanostructured Medium Manganese Steel and Its Grain Refinement Mechanism

As the third generation of advanced high strength steel (AHSS), medium manganese steel (MMS) has been widely emphasized by scholars around the world. Presently, we applied sliding friction treatment (SFT) of severe plastic deformation (SPD) on the surface of MMS to form surface gradient nanostructures, the formation mechanism of microstructure and the corresponding mechanical behavior was studied. The results show that the deformation layer can be divided into nano-grain (NG), submicron grain (SMG) and coarse grain (CG) in terms of grain size. It has been demonstrated that in the CG layer and a part of SMG layer, new fine grains can be formed through discontinuous dynamic recrystallization (DDR) mechanism, while continuous dynamic recrystallization (CDR) is a favorable nucleation mechanism for the new formed small grains in the SMG layer and the NG layer. The SFT process increases microhardness sharply in the surface region. Compared with conventional MMS, it is apparent that the yield strength (YS) and the ultimate tensile strength (UTS) of gradient medium manganese steel specimens have been greatly improved, while the elongation does not decrease significantly. Fracture surface analysis demonstrates that the fracture morphology of different layers can be generally characterized by different fracture mechanisms, i.e., cleavage, quasi-cleavage and dimple.