Study On The Formation Of The Columnar Grain And Texture In The Electrical Steels Controlled By Phase Transformation

Non-oriented electrical steel is an indispensable soft magnetic material in electrical power applications due to the special magnetic properties.At present,the medium and low grade products dominate in the non-oriented electrical steels in our country.This will cause a lot of energy loss.The {100} texture is the key factor to improve the magnetic properties of the non-oriented electrical steels.Thus,the control of texture is very important for obtaining desirable magnetic properties.At present,the commercial non-oriented electrical steels are produced by ordinary cold rolling and recrystallization annealing processes combining the controls of the purity of the alloy and chemical compositions.However,the intensity of {100} texture is weak.There is still room for improvement of the magnetic properties through texture control.In previous reports,the intensity of the detrimental {111} texture could be reduced by α→γ→α transformation in steels.And α→γ→α transformation can further improve the microstructure and texture,which is a novel preparation method for the non-oriented electrical steels.The columnar grains with sharp{100} texture can be obtained in the electrical steel containing high silicon(3%Si)by vacuum annealing with α→γ→α transformation,which exhibits excellent magnetic properties.However,the vacuum annealing time is too long.Industrial production has not yet been achieved,and the formation mechanism of the {100}texture is not clear.For the non-oriented electrical steel containing ultra-low carbon and low/no-silicon,the phase transformation method is suitable only for the limited range of chemical composition.The favorable texture is difficult to be obtained by α→γ→α transformation in the industrial non-oriented electrical steels.Therefore,based on the above problems,α→γ→α transformation is applied in the non-oriented electrical steel containing low carbon high Si and the non-oriented electrical steel containing ultra-low carbon low Si or no-Si in this work.The primary results of the thesis are described in details as follows.The formation mechanism of columnar grains in Fe-3%Si during α→γ→αnon-full phase transformation in vacuum is explored.The decarburization,removal of manganese and recrystallization occur on the surface before the formation of the y phase.Even as the sample is heated to higher temperatures,theγ phase might not appear on the surface;thus,recrystallized and recovered alpha grains are retained on the surface as the thin a layer.The contents of Mn and C in the y phases near the thin alpha layer would both be reduced as vacuum annealing proceeded,and then the ly phases adjacent to the thin columnar grain layer were unstable.The deformed and recrystallized alpha grains on the surface would consume the γ phases to grow towards the interior.The thin alpha grains on the surface gradually grew into the interior as columnar grains by the evaporation and diffusion of Mn and C.The formation mechanism of the {100}<011>texture in the surface layer during the vacuum annealing with γ→α phase transformation is analysed.The relation among deformation,recrystallization and phase transformation during the evolution of the texture is expounded.The sufficient recovery takes place in the deformed {100}<011>grains and then {100}<011>grains are retained in the heating process.The elastic strain is induced in the thin columnar grain layer during the inward growth of the surface columnar grain via γ→α transformation.The {100} oriented a grain has the lowest elastic strain energy.The retained{100}<011>grains prefer to grow by consuming the non-{100} a grains in the surface layer by minimizing elastic strain energy during γ→α transformation.Finally,the sharp {100}<011>texture develops in the surface layer.The columnar grains with nearly 100%{100}<011>texture in the surface layer are obtained within 30 min by the controls of annealing temperature,the thickness reduction,the chemical compositions stabilizing the y phase and the heating rate.The strong {100}<0vw>and {100}<011>textures are obtained during the vacuum annealing with α→γ→α transformation by different cold rolling reductions.The retaining and orientation rotation of {100}<0vw>seeds are related with the cold rolling reductions.The {100}<0uv>seeds are retained during a low cold rolling reduction.The sharp {100}<0uv>texture develops in the surface layer by minimizing elastic strain energy during γ→α transformation.The texture in Fe-0.46%Mn is improved by α→γ→α full phase transformation,and the intensity of {111} texture is reduced obviously.The orientation gradient in the interior of grains appeared on the surface of steel sheets after α→γ→α transformation in different atmospheres.The intensity of the texture is weakened,and the orientation gradient originates from the surface.The formation mechanism of orientation gradient is related with the oxidation behavior on the surface.The combined effect of phase transformation stress and thermal stress caused the plastic deformation of the thin ferrite grains on the surface at the initial stage of γ→α phase transformation on the surface.The orientations of ferrite grains on the surface gradually rotate and cause the orientation gradient in the interior of grains.The {100} oriented a grains formed by y → a phase transformation may rotate toward {110} orientation under the thermal stress and phase transformation stress.Industrial 800 grade non-oriented electrical steel is prepared by α→γ→a transformation.The effects of aluminum and phosphorus on the formation of microstruction are analysed.Aluminum and phosphorus can obviously increase the critical temperatures,and increase the difference of critical temperatures A1 and A3.The probability of nucleating in the interior of the steel sheet will increase during the cooling process with γ → α phase transformation,and then the process that preferred nucleating on the surface and growing into the center is difficult to achieve.In summary,this thesis mainly focuses on the exploration for the evolutions of microstructure and texture during α→γ→α transformation in high-Si,low-Si and no-Si non-oriented electrical steels.The principles of controlling the columnar grains and texture by non-full phase transformation in the high-Si electrical steel are investigated,and the regularities for the evolutions of the microstructure and texture during α→γ→α transformation in low/no-Si electrical steels are investigated.The system of controlling the microstructure and texture by full and non-full phase transformation in non-oriented electrical steels containing different silicon contents is preliminarily established.