| Electrical steel is an iron-based alloy with excellent soft magnetic properties,which is mainly used in the iron core of motors or transformers,to realize the conversion between electrical energy and mechanical energy.60%-70%of the power generation in a country is consumed by motors,of which 10%-30%is caused by the iron loss of electrical steel.Low-and medium-grades non-oriented electrical steels are the main products in electrical steel market,which directly leads to a large amount of power consumption.Reducing iron loss and increasing magnetic induction can improve the conversion efficiency of the motor and save resources.Currently,improving the {100} and Goss textures through texture optimization is an important method to control magnetic properties.Non-oriented electrical steel is divided into low-grade electrical steel with phase transformation and medium-and high-grade electrical steel without phase transformation.For the medium-and high-grade electrical steel,the initial hot rolling microstructure and texture are affected by the columnar grains,start rolling temperature,final rolling temperature,coiling temperature.The subsequent normalizing annealing can effectively coarsen the microstructure of the hot rolled sheet,and then affect the magnetic properties of final products.For the low-grade electrical steel,the phase transformation generally weakens the texture,but columnar grains still exist in the continuous cast slab at room temperature,which is similar to the solidification structure at high temperature,i.e,the morphological memory and textural memory phenomena occur after ????? phase transformations.Therefore,based on the fact that columnar grains generally exist in continuous cast slab,this dissertation firstly studied the basic laws of columnar grains phase transformation,the reasons for the formation of subgrains,and the effect of different initial structure and heating and cooling rate related to columnar grains on the actual phase transformation temperature.Finally,the effect of thermal deformation temperature on {100} texture is studied by thermal simulation.Based on the characteristics of columnar grains that are not easily transformed,the actual columnar grains hot rolling is carried out to determine whether the rapid short-time and low-temperature heating method can effectively retain {100} ferrite and coarsen grain size.For medium-and high-grade electrical steels,the texture evolution during hot rolling,normalization,cold rolling and recrystallization annealing are studied.Firstly,the effects of normalization are analyzed,and secondly,the nucleation position and growth ability of grains with different orientations during recrystallization annealing are discussed.All research aims to improve the magnetic properties of industrial final products and provide new ideas for the industry.The main conclusions are summarized in details as follows:The cast slab of low-grade electrical steel is composed of approximately 80%coarse {100} columnar grains and 20%fine equiaxed grains.The morphological memory and textural memory phenomena can be due to the retention of the initial high temperature ?-ferritie.Due to the large size and stress hindrance,the high temperature ?-ferrite columnar grains are not swallowed up by the new small austenite grains during phase transformation.Many small equiaxed ferrite grains show ?3 misorientations relationship with columnar grains,which implies that the phase transformation obeys the K-S relationship and the phase transformation strain is significant.The results of ????? phase transformation show that the interior of the columnar grains is the main nucleation site rather than the low energy columnar grain boundaries.The coarse columnar grains significantly affect the phase transformation kinetics resulting in the insufficient transformation.The small {100} subgrain is a phase transformation variant.During the twice phase transformations,most {100} orientations return to the near {100}orientation,i.e.the texture memory appears,and the probability of occurrence of the {100} variants is 33.3%.The phase transformation strain induced by {100}subgrain variants is large,while that induced by small equiaxed grains which conform to ?3 relationship with columnar grain is lower.Considering the phase transformation resistance,the released strain energy is the larger when the ?3 variant pairs appear,while the releasesd strain energy of the single variant appeared is little.This shows that {100} subgrains variants are mainly single variant.The reason of the appearance of {100} subgrain can be attributed to the lower energy of low-angle grain boundary except for ?3.The appearance of the large number of ?3 relationships is to reduce the overall energy,rather than the simple strain accommodation behavior between variants.Beyond that,the actual phase transformation does not follow the strict K-S relationship.The initial structures of the columnar grains have significantly effects on the transformation temperature and the extent of phase transformation,especially during heating.Deformation can reduce the thermal hysteresis more effectively than decreasing the heating rate,and the phase transformation hysteresis of the deformed sample is the smallest during heating,followed by the columnar grained sample with a slow heating rate,whereas the columnar grained sample with a fast heating rate has the largest phase transformation hysteresis.The coarse columnar grains affect the transformation kinetics more remarkably than the fine columnar grains,both of which cause the insufficient transformation.It is proposed that the smaller dilatational amount is related to the incomplete phase transformation of the columnar grains,and also related to the suppression of the dilatational amount of the small grains by the surrounding columnar grains.In contrast,owing to the relatively sufficient phase transformation,the recrystallized sample has the largest dilatational amount.Moreover,the dilatational amount has little change with the heating rate,but changes greatly with the cooling rate.The change of the transformation temperature and the dilatational amount are mainly induced by the grain size effect and the inhomogeneity of the structures,and it should be noted that texture can affect the uniformity of grain size.The coarse structures with strong {100} texture are obtained by simulating final hot rolling in ferrite region and the annealing close to phase transformation temperature after final rolling.Hot rolling of {100} columnar grains by rapid short-time and low-temperature heating method is conducive to the retention of the {100} orientation,and the magnetic properties of its final products are better than those of final products prepared by the traditional hot rolling and normalizing method in industries.Normalization can significantly coarsen {110} oriented grains in the surface layer and {114}<481>oriented grains in the central layer,of which {114}<481>has the strongest growth ability.During cold rolling,{100}<021>-cube orientation rotates around<100>//ND axis to ?100}<021>,and then around<110>//RD axis to {114}<481>,and finally around<114>//ND axis to ? fiber.During recrystallization annealing,?-oriented grains nucleate in deformed grains with variant orientations.Moreover,the nucleation rate of ?*-oriented deformed grains is high,and the recrystallized grains are mostly cube grains.The surface layer of the normalized final products is {053}<100>texture,while the central layer is {100}<021>texture,that is,the recrystallized final sheets inherit the texture gradient of the normalized sample.The normalized final products always show higher magnetic induction and higher iron loss,while those of the hot rolled samples are lower.With the increase of final annealing temperature,the magnetic properties of the normalized final products change from high magnetic induction and high iron loss to low magnetic induction and low iron loss.It should be noted that the 940 ? normalized final products always have the best magnetic properties.Moreover,the non-{111}-fiber-oriented grains show a higher growth ability than {111}-fiber-oriented grains during the final high-temperature and long-time annealing. |
PDF Full Text Request