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.

作为第三代高级高强度钢（AHSS），中锰钢（MMS）已被世界各地的学者广泛重视。目前，我们在MMS表面进行了严重塑性变形（SPD）的滑动摩擦处理（SFT），以形成表面梯度纳米结构，研究了微观结构的形成机理和相应的力学行为。结果表明，根据晶粒尺寸，变形层可分为纳米晶粒（NG），亚微米晶粒（SMG）和粗晶粒（CG）。已经证明，在CG层和SMG层的一部分中，可以通过不连续动态重结晶（DDR）机制形成新的细晶粒，连续动态重结晶（CDR）是SMG层和NG层中新形成的小晶粒的有利成核机制。SFT工艺在表面区域中急剧增加了显微硬度。与常规MMS相比，显然梯度中锰钢试样的屈服强度（YS）和极限抗拉强度（UTS）有了很大提高，而伸长率却没有明显降低。断裂表面分析表明，不同层的断裂形态通常可以通过不同的断裂机制来表征，例如，劈开，准劈开和陷窝。显然，梯度中锰钢试样的屈服强度（YS）和极限抗拉强度（UTS）有了很大提高，而伸长率却没有明显降低。断裂表面分析表明，不同层的断裂形态通常可以通过不同的断裂机制来表征，例如，劈开，准劈开和陷窝。显然，梯度中锰钢试样的屈服强度（YS）和极限抗拉强度（UTS）有了很大提高，而伸长率却没有明显降低。断裂表面分析表明，不同层的断裂形态通常可以通过不同的断裂机制来表征，例如，劈开，准劈开和陷窝。