对硅钢板材分别进行异步和同步轧制, 研究了轧制参数包括速比、压下量和道次对板材表面显微组织的演变的作用. 结果表明, 异步轧制硅钢板材表面形成了晶粒尺寸为10~50 nm, 取向接近随机分布的纳米晶, 而同步轧制板材的表面只形成了位错胞, 证明异步轧制可以诱发表面纳米化. 异步轧制板材表面纳米晶的形成过程为: 在剪切力的反复作用下, 高密度位错形成、滑移、湮灭和重组形成亚微米尺度的亚微晶/位错胞. 随着压下量和轧制道次增加, 高密度位错重复以上过程使晶粒尺寸减小、取向差增大, 最终形成取向接近随机分布的纳米晶组织. 大压下量和多道次是异步轧制诱发板材表面纳米化的关键, 而速比的增加可以加快纳米化进程.

Surface nanocrystallization (SNC) can effectively enhance the surface and global properties of the metallic materials, such as microhardness, intensity, fatigue, wear and corrosion resistances, therefore provides more promising practical industrial applicability. Up to now, several SNC treatment methods were developed based either on the principles of ball impactions or friction sliding, however, difficulty still exists for the surface treatment of large-dimensional samples with high efficiency. Recently, more attentions were focused on the asymmetric rolling, of which upper and lower rolls rotate with different circumferential speeds, and then an extra shear strain was applied to metal sheet in addition to compression strain. The shear strain could refine the grains into micro- or submicro-scales. In order to investigate the possibility to realize the SNC for metal sheet in the rolling process and examine the effects of rolling parameters, silicon steel sheet was rolled by means of asymmetric rolling and conventional rolling respectively, the microstructural evolution in the top surface layer was observed for the samples rolled for different parameters including mismatch speed ratio, rolling reduction and rolling pass. Experimental results show that after the asymmetric rolling, nanocrystallines about 10~50 nm in size with nearly random orientations form in the top-surface layer of sheet. Meanwhile, dislocation cells can be observed after conventional rolling, which indicates that the asymmetric rolling can be utilized for the surface nanocrystallization of the cubic metal sheets. The surface nanocrystallization mechanism induced by asymmetric rolling was summarized as follows: (1) upon the application of repeated shear force, submicro-grains/dislocation cells form through formations, slips, annihilations and recombinations of high density of dislocations; (2) with a further increment of rolling reduction and rolling pass, high density of dislocations in the refined cells/grains developing in above route lead to reduction of grain size and increment of misorientations between the refined grains; (3) nanocrystallines with nearly random orientations form. Larger reduction and multi-passes are necessary for the surface nanocrystallization induced by asymmetric rolling, and the increment of mismatch speed ratio can accelerate the grain refinement process.