Texture Evolution And Its Mechanism In Warm Rolled Ti-IF Steel Sheet During Cold Rolling And Annealing

IF steel is widely used to due to its excellent deep drawability. It is well known that the drawability of IF steel is closely related to the texture formed during recrystallization and that a high volume fraction of near{111} components is essential for good drawability. Ferritic rolling, namely, warm rolling has been generally accepted as an effective method to improve {111} components in IF steel. Many researches have been conducted on the warm rolling process, however researches on the subsequent cold rolling and annealing process are quite scarce. In this paper, texture evolution during warm rolling, cold rolling and annealing of the Ti-IF steel is systematically investigated by means of the micro structure observation, X-Ray diffraction and EBSD analysis. The main work and the results are as follows:1) The hot rolled texture has decisive effect on the cold rolled and annealed texture due to the heredity of texture. The texture evolution during cold rollng and annealing process has comprehensively studied by XRD analysis with different initial texture hot bands. The results show that an ideal y fiber beneficial for improving deep drawability can be formed after annealing only when a well-proportioned y fiber dominates in the cold rolled texture. This requires the formation of strong y fiber in the hot band, which can be obtained by high temperature coiling or direct annealing after warm rolling.2) The microstructure, the orientation of the nuclei, the nucleation site at the early stage of recrystallization and the growth mechanism of the nuclei have been studied by EBSD analysis in Ti-IF steel during direct annealing after warm rolling. It is concluded that the formation of the recrystallization texture is dominated by the oriented nucleation mechanism. The recrystallized nuclei with y-orientation emerged preferentially at the beginning of recrystallization and preferred to form in the deformed bands withγ-orientation and on the boundaries betweenγandα-orientation. The recrystallized grains first consumed their neighboring y-oriented matrix, and then consumed the a-oriented deformed bands at the late stage of recrystallization, leading to strong y-fiber.3) The texture evolution during cold rolling with various reductions has been investigated in warm rolled and subsequent high and low temperature coiled Ti-IF steels. The orientation rotates along the following path:{111}<112>-{111}<110>-{223}<110>-{112} <110>-{113}<110>-{114}<110>-{001}<110>. The characteristic of the recrystallization texture has been analyzed from the point of cold rolled texture and the best cold rolling reduction is 75% which is determined by analyzing the properties and the recrystallization texture.4) The texture evolution during annealing after cold rolling of warm rolled Ti-IF steel has been systematically studied in this thesis. It is found that 710-750℃is the optimal annealing temperature range. When the annealing temperature is 710℃and 750℃, a strong and well-proportioned y fiber is obtained.When the annealing temperature is 600℃, the reduced a fiber transforms to{554}<225>-{332}<113> components as well asγfiber at the late stage of recrystallization. The EBSD analysis shows that orientated-nucleation dominates during recrystallization, but the orientation of the nuclei includes{332}<113> component besides y orientation.5) The texture evolution and the properties of the austenite rolled and the warm rolled high strength Ti-IF steels have been compared. Aithough the texture types have no differences, the intensity of the texture in the warm rolled one is much higher which leads to higher mechanical properties in the warm rolled, cold rolled and annealed high strength Ti-IF steel. It is proved that warm rolling is an effective method to improve the drawability of high strength Ti-IF steel.