Study On Effects Of Metallurgical Processing On Microstructure And Properties Of Ferritic Hot-Rolled Deep-Drawing Interstitial-Free (IF) Sheet Steel

Interstitial-Free (IF) steels which represent a new generation of deep drawing steels have been widely used as raw materials of automobile sheet because of its excellent deep drawability and no-aging property. Their conventional production includes multi-step processes, such as continuous casting, hot-rolling, cold-rolling, annealing etc.. The process is complicated and costly. With the increase of market competition, in order to simplify the process and reduce costs, the idea that producing steel sheets for deep-drawing operation by hot-rolling has attracted the attentions of both steel company and steel users. In recent years, it has been founded that the dominant texture is {111} texture after IF steels are hot-rolled in ferrite region, this fact make it possible that deep-drawing steel can be produced by hot-rolling. Hot-rolled IF steel is characteristic of shorter production cycle and lower cost. It is an advanced technology product and can bring significant economic benefits. Therefore, to carry out research work of hot-rolled IF steel is not only of great theoretical significance but also has practical value.In this paper, the effects of metallurgical processing on mechanical properties, microstructure and texture of ferritic hot-rolled IF steel were investigated by means of electron back-scatter diffraction (EBSD), X-ray diffractometer (XRD), optical microscope (OM), scanning electron microscope (SEM), electro-probe microanalyzer (EPMA), transmission electron microscope (TEM) and tensile test.By investigating the effects of chemical composition on microstructure and mechanical properties of hot-rolled IF steel, it has been shown that solute C, N cause detrimental effect on deep drawability of hot-rolled IF steel. In ultra-low-carbon (ULC) steel with no Ti addition, where solute C, N atoms exist, the deep drawability cannot be obtained even if hot-rolling is performed in ferrite region. But in Ti-stabilized IF steel, where Ti can combine C, N atoms to form TiN, TiS, Ti₄C₂S₂ and TiC precipitates and thus remove them from the matrix, good deep drawability is got after ferritic hot-rolling. The significant difference of deep drawability between the two experimental steels is mainly attributed to the different content of in-grain shear bands, and the difference of shear band behavior is in turn caused by the interaction of solute C, N and dislocation movement during ferritic hot-rolling. P enhances the strength of hot-rolled IF steel significantly, but it also reduces its deep drawability. The addition of P changes the precipitation behavior of Ti-IF steel, a large number of small FeTiP new phases precipitate along grain boundaries during ferritic hot rolling and annealing. That is to say, in hot-rolled Ti+P-IF steel, the phosphorus exists in the form of solid solution P atoms and compounds FeTiP particles. Solid solution strengthening of solute P atoms is the main factors in strength enhancing, and the inhibition of FeTiP particles to the development of favorable texture during annealing is the main reason for deep drawability reducing.Effects of ferritic hot-rolling temperature and reduction on microstructure, texture and mechanical properties were studied. Results show that ferritic rolling temperature and reduction have less influence on the yield strength, tensile strength, total elongation and n|--value of hot-rolled Ti-IF steel, but have a great influence on the r|--value. With the decrease of rolling temperature and the increase of reduction, the r|--value increases significantly. With the decrease of rolling temperature, stored energy of {111}<112> and nearby {554}<225> deformed grains increase which is beneficial for them to have priority to nucleation and grow up during recrystallization, so a stronger {111}<112> and {554}<225> recrystallization texture and a weaker {001}<110>～{223}<110> recrystallization texture are formed after annealing which is the reason for the increase of the r|--value. At the same hot-rolling temperature, the intensity of beneficial texture components is enhanced with the increase of reduction in ferrite region and their intensity would be further improved after annealing, so the r|--value increases.Lubrication condition has an important effect on deep drawability of hot-rolled IF steel, r|--value reduced significantly without lubricant condition. Inhomogeneous deformation is occurred when ferritic hot-rolled without using lubricant. Large amounts of {110} equiaxed fine grains are formed in the surface layer as a result of shear deformation during hot-rolling and the number of {110} grains reduces accompanied by grain growth during annealing, finally a weak texture is formed in the surface layer. In the center layer, a small amount of {111} recrystallization grain are formed after annealing. The misorientation of both surface and center layer are close to random distribution. This kind of texture inhomogeneity is the essence reason for r|--value decrease. Uniform deformed microstructure along the thickness direction is obtained when hot-rolling is done with good lubrication condition, long fibrous {001}, {112} and {111} deformed grains are formed in both surface and center layer. After annealing, large number of {111} recrystallized grains are developed in both layers, misorientation deviates from random distribution and the content of small-angle grain boundary is higher than that in the un-lubricant condition. Therefore, to obtain excellent deep drawability, good lubricant condition is necessary. The recrystallization behavior of hot-rolled IF steel sheet during batch annealing was investigated. The results show that it takes 3 hours for hot-rolled Ti-IF steel to be fully recrystallized at the annealing temperature of 650℃and when annealed at 750℃, the time that is needed is only 1 minute. Annealing temperature has significant effect on the recrystallization of hot-rolled Ti-IF steel, and the effect of holding time on recrystallization depends on the annealing temperature. Sufficient holding time is necessary for the complete recrystallization of hot-rolled Ti-IF steel when annealed at lower temperature. The economical and reasonable batch annealing process for hot-rolled Ti-IF steel is: annealing temperature 750℃and holding time 3 hours.The texture evolution of hot-rolled Ti-IF steel during annealing was investigated by using ODF analysis and EBSD techniques. It has been founded that a strongα-<110>//RD fiber texture and a weakγ-<111>//ND fiber texture are formed in ferritic hot-rolled Ti-IF steel and the stored energy of main deformed grains gradual increase in the sequence of {001}<110>, {112}<110>, {111} <110> and {111}<112> orientations. At the early stage of recrystallization, the {111} deformed grains which have higher stored energy will give priority to form {111}-oriented nuclei. Texture changes occur among the y fiber texture, that is, {111} recrystallization texture forms from {111} deformed grains. At the latter stage of recrystallization,α-oriented deformed grains are consumed gradually, and the intensity of a fiber texture decreased significantly and the {001}<110> component is consumed finally because of its smallest stored energy, while the intensity ofγfiber recrystallization texture is increased. The texture characteristics of recrystallized grains at the early and latter stage of recrystallization are basically the same as that at complete recrystallization. The initial recrystallization nucleation characteristics determines recrystallization texture and the oriented nucleation mechanism play a dominant role in the formation of recrystallization texture in hot-rolled Ti-IF steel.