马氏体组织对SFPB处理双相钢表面纳米结构及力学性能的影响

目的 通过对不同微观组织铁素体/马氏体双相钢进行表面纳米化处理，探究材料表面晶粒细化和塑性变形机理。方法 采用超音速微粒轰击（SFPB）技术对经临界区退火（IA）、中间淬火（IQ）和分级淬火（SQ）后的双相钢进行纳米化处理，采用SEM、OM和XRD研究试验钢表面SFPB前后的微观组织特征，采用显微硬度仪测试其表面硬度，采用拉伸实验测试其力学性能。结果 热处理后，IA、IQ和SQ试样马氏体组织分别呈岛状、纤维状和块状，IQ试样平均晶粒尺寸最小，但马氏体体积分数最大。SFPB工艺处理后，双相钢表面形成了一定厚度的梯度纳米晶层（GNS），该晶层内的晶粒尺寸均达到纳米级别，且随距离表面深度的增大而增大。IA-GNS、IQ-GNS和SQ-GNS试样表面硬度分别为285.9、266.7、382.1HV，抗拉强度分别为771.30、820.02、663.81 MPa，延伸率分别为8.89%、14.70%、10.04%。IQ-GNS试样断口以韧窝为主，SQ-GNS和IA-GNS试样断口韧窝较少，有明显裂纹开口。结论 表面产生强烈塑性变形时，由于位错的分割作用，表面晶粒尺寸细化至纳米级，材料强度大幅提高，同时纳米级纤维状马氏体微观组织的存在使得IQ-GNS试样保持了较高的塑韧性。

Influence of Martensite Characteristics on Surface Nanostructure and Mechanical Properties of Dual Phase Steel Processed by SFPB

The work aims to investigate the mechanisms of grain refinement and plastic deformation of the surface nano-crystallization by conducting surface nano-treatment to ferrite/martensite (F/M) steels with different microstructures. Supersonic Fine Particles Bombarding (SFPB) technology was used to conduct nano-treatment to the dual phase steel after intercritical an-nealing (IA), intermediate quenching (IQ) and step quenching (SQ). SEM, OM and XRD were applied to study the surface mi-crostructure of experimental steel before and after SFPB. The surface microhardness was measured by Microhardness Tester and the tensile experiment was used to test the mechanical property. The martensites specimens after IA, IQ and SQ appeared island, fibrous, and massive shape after the heat treatment. The specimen after IQ had the smallest average grain size and the largest martensite volume fraction. After SFPB process, the gradient nanostructured surface (GNS) layer with certain thickness was fabricated on the dual phase steel and the average minimum grain size reached nanoscale and grew with increase of the in-depth distance to the surface. The surface microhardness of IA-GNS, IQ-GNS and SQ-GNS specimens was 285.9, 266.7 and 382.1HV, the tensile strength was 771.30, 820.02 and 663.81 MPa, and the elongation rate was 8.89%, 14.70% and 10.04%, respectively. Many dimples were found from the fracture surface of IQ-GNS specimen, while fewer dimples and clear cracks existed in SQ-GNS and IA-GNS specimens. The grain size can be refined to the nanoscale due to the segmentation of dislocation as a result of the improvement of the material strength when the severe plastic deformation occurs, and the IQ-GNS specimen can still keep the good toughness owing to the existence of the fibrous martensite microstructure.