Studies On Texture Evolution And Microstructural Characteristics Of IF Steel By Special Rolling And Pure Iron By Surface Nanocrystallization

IF steel is widely used in many industry fields. Its deformation and annealing procedure under conventional cold rolling have been investigated systematically. With the rapid development of modern industry, higher performance of IF steel is required, and the conventional treatments can not meet the practical application. Recent years, some special deformation methods have been suggested and well developed. Grain can be refined and some special texture components can be obtained in IF steel by means of these special deformation methods(especially with different initial texture), which significantly influence the properties of IF steel. However, this aspect has not been thoroughly investigated. In the present study, a special rolling procedure of IF steel (i.e. side rolling in Chinese) is used to fabricate samples with special initial texture components. The studies will provide theorical references for the study of transfer of important texture component during cold rolling and annealing of IF steel. Side rolling was conducted on the IF steel after it has been subjected a cold rolling with80%reduction. Microstructures and microhardness of the samples were investigated by metallurgical observation, SEM, TEM, ODF, EBSD analysis and microhardness test. Comparing with the conventional cold rolling sample, the crystal rotation path and evolution of texture of the sample after side rolling were studied. The variation of important texture components during side rolling and annealing were analysed. Additionally, texture evolution in different plastic deformation layer of the pure iron sample after surface nanocrystalization was investigated. The main results are as follows:Different initial textures of0°,30°,60°and90°samples were prepared by side rolling. When side rolling reduction is in the range of0%-48%, initial textures were converged at {111}<110> and{001}<100>(or{001}<110>) along different rotation route. Rotation paths of the0°and90°sample are{112}<110>°{223}<110>→{111}<110>;{110}<110>→{111}<110>;{110}<111>→{110}<112>→{111}<110>。In addition to the above rotation paths occurred in the0°and90°samples, rotation paths of the30°and60°samples also include the following paths:the tube fiber with{140}<410>,{160}<610>{114}<172>components in30°samples rotate to{001}<100> and the tube fiber with {140}<410>,{127}<141>,{113}<581Components in60°samples rotate to{001}<100>. When the side rolling reduction is48%, intense texture component and cube{001}<100>(or (001}<110>) texture were formed in these four samples which takes strong point{111}<110> as center and spread about20°. However, texture of0°sample was almost single spherical {111}<110> texture, and the{001}<110> texture is weak. When the side rolling reduction increases from48%to92%, textures in all four samples evolved in almost the same way:the spherical{111}<110> texture gradually diffuses to{111}<112> along the y skeleton line and eventually forms y fiber texture; the cube{001}<100>(or{001}<110>) texture gradually diffuses to other components of the<001>//ND texture and eventually forms planar <001>//ND texture with strong point at{001}<100>(or (001}<110>). After severe side rolling, ultrafine grained and nanocrystalline structure can be formed in the samples.After annealing, the final stable texture components of0°sample were{111}<112>{111}<110> and{113}<471>. The<001>//ND deformed matrix formed during side rolling disappears. At the early stage of recrystallization of the side rolled0°sample,{111}<112> and{111}<110> recrystallized nuclei initiated in the cold rolled matrix with{111}<110> and {111}<112>orientation, while{113}<471> recrystallized nuclei mainly initiated in the cold rolled matrix with{001}<110>(belonging to<001>//ND texture) deformed matrix at the same time. The formation of{113}<471> recrystallized nuclei can be attributed to the preferred nucleation. After nucleation the{113}<471> nuclei grows by consumption of matrix with<001>//ND orientation.. The∑9、∑11and∑17b CSL boundaries appears between{113}<471> grains and other orientation grains, which accelerates the growth of {113}<471> recrystallized grains with several grains grow abnormally.When the IF sample was cold-rolled with80%reduction through conventional rolling and subsequently annealing, the annealing temperature at the stage of grain growth is the main factor influencing distribution of grain size and texture characteristics of grains with different size. The distributions of grain size and texture characteristics are distinct at different annealing temperature. At the same annealing temperature, the distributions of grain size and texture characteristics are similar irrespective of their holding time. When the holding time increases, partial texture characteristics remains unchanged in the large, medium and small grain group, whereas its intensity increases with the duration. In different groups of grain, the relationship between the texture and the average grain size is nearly linear.After SMAT, the surface layer of the pure iron sample can be divided into four layers along the depth from the top surface:nano-sized regime, submicro-sized regime, micro-sized regime and plastic deformation regime. The size of surface nanocrystalline regime in the SMATed30and60min samples are similar. The prolonged time of SMAT only leads to increase in thickness of layers mentioned above except nano-sized regime. The main texture of pure iron after SMAT is the<110>//ND fiber texture with random orientation in nano-sized regime. The texture remains unchanged along the depth direction, whereas texture intensity increases and reaches the maximum in the micro-sized regime with{110}<111> texture increases the fastest. Textures of the SMATed30and60min samples are similar. However, the variation of texture in the60min sample is significant, and typical cube texture can be formed. After annealing at500℃and550℃in vacuum, the structure in the top surface of the SMATed samples is still nanocrystalline with high hardness. The texture characteristics of the nano-sized regime and submicro-sized regime remain unchanged, whereas the texture intensity reduces.