Study On Microstructure,Texture And Magnetic Properties Of Strip-cast Non-oriented Silicon Steel With High Magnetic Induction

As an important soft magnetic alloy for making cores of generators and motors,non-oriented silicon steel is the key material to improve the efficiency of electrification equipment and reduce energy consumption.The conventional production process with many influencing factors for non-oriented silicon steel is complicated,and the product process is lengthy.Twin-roll strip casting technology has the characteristics of short process and environmental protection and high efficiency.Its special short process and sub-rapid solidification characteristics provide a new technical way for flexible regulation of the structure,texture and precipitate and optimization of magnetic properties.In this paper,the technology of twin-roll strip casting is introduced into the preparation process of non-oriented silicon steel.Based on the physical metallurgical characteristics of sub-rapid solidification,the effects of normalization heat treatment,rolling temperature and two-stage rolling process on the structure,texture,precipitates and magnetic properties of non-oriented silicon steel are systematically studied,and explain the whole process control mechanism of strip normalization-cold rolling-heat treatment.The main work and research results are as follows:(1)The formation law of solidification structure and texture of non-oriented silicon steel strip in twin-roll strip casting process with different superheat and silicon content was studied.The grain size,morphology and orientation of initial solidification structure of non-oriented silicon steel with different composition were obtained,which laid a foundation technology for optimizing final annealed structure and texture and improving magnetic properties.The as-cast strip is a symmetrical solidification structure on both sides of the thickness direction.The surface and center layers are uniform equiaxed grains with irregular shape,and the sub-surface layer is coarse columnar grains approximately parallel to the normal direction of the thickness of the strip,which dominated by strong {100} and {110} orientations.The columnar crystal ratio is higher and {100} texture is stronger with the increase of casting superheat.And columnar crystals are subjected to high temperature deformation,and the orientation gradient is obvious in crystal,and {110} orientation is enhanced with the decrease of superheat.The increase of silicon content decreases the thermal conductivity of solid alloys,which increases the temperature gradient,therefore the super cooling of liquid steel is reduced at the solid-liquid interface during solidification and the columnar crystal growth is more developed.2)The influence of normalized heat treatment on solidification structure,texture and precipitate of cast strip was clarified.The mechanism of the formation and coarsening of Al₂O₃-MnS composite precipitate and its effect on the structure and texture in the subsequent cold rolling and annealing process were analyzed.The cold rolled non-oriented silicon steel with excellent magnetic properties was obtained.After normalized heat treatment,the structure and texture of the cast strip are similar to those of the initial solidification,but the homogeneity of the structure is greatly improved.The precipitation behavior of MnS particles is inhibited by the sub-rapid solidification process of twin-roll strip casting.A large number of MnS particles and Al₂O₃-MnS composite precipitates are precipitated after normalized treatment.Fine precipitates hinder the slip of dislocations and crystal rotation during cold rolling process.A large number of ingrain shear bands are formed in the cold rolling structure,which improves the energy storage of deformation structure and promotes the nucleation and growth of recrystallization.Normalized heat treatment can effectively improve the strength of favorable a and ? oriented and weaken the strength of y oriented in cold rolled sheets.The precipitates coarsen further during annealing,the pinning of dislocations and the hindrance of grain boundaries are reduced,and the recrystallization structure is coarse and uniform.The average grain size is increased from?29 to?82 ?m.Moreover,the annealing texture is strongly ?oriented.The magnetic induction intensity B₅₀ increased from 1.747 to 1.782 T,while the iron loss W_15/50 decreased from 6.212 to 4.631 W/kg.(3)The effect of rolling temperature on the deformation structure and texture of Fe-3%Si cast strip during warm rolling was clarified,and the mechanism of dynamic strain aging and low temperature recovery on the crystal rotation,geometric softening and shear deformation behavior was clarified.The critical role of warm rolling in promoting shear band formation and weakening deformation texture was clarified,and the optimization of texture and magnetic properties were realized.The obvious dynamic strain aging occurs during warm rolling at 350?,which improves the shear band density in the deformed structure,increases the deformation energy storage,and weakens the strength of the plane deformation texture significantly.At the same time,strong {110}<110>oriented shear texture is formed.With the increase of rolling temperature,the average grain size of annealed plate increases,and the homogeneity of structure improves.Moreover the magnetic properties are greatly optimized with the increase of ?texture and decrease of y texture.The magnetic induction strength of annealed sheet is the highest(B₅₀=1.788 T)when rolling at 350?,and the iron loss is the lowest(P_15/50=3.042 W/kg)when rolling at 500?.With the increase of rolling temperature to 650?,the diffusion rate of solid solution atoms increases and the effect of dynamic strain aging decreases.The shear band disappears and some recrystallization occurs with the recovery effect increases,and the deformation texture strength weakens significantly.A large number of fine grains appeared in the annealed plate,the homogeneity of the structure deteriorated.The strength of ? and ? orientation decreased,and the Goss orientation increased,therefore the magnetic properties decreased.(4)The origin and evolution of Cube texture in annealed structure of twin-roll strip casting non-oriented silicon steel were analyzed,and the nucleation and growth behavior of Cube oriented in different orientation deformation matrices in each stage of recrystallization were analyzed.By optimizing the deformation structure and texture,strong ? oriented high magnetic induction non-oriented silicon steel with Cube dominant orientation was obtained.The deformation textures of strong ? and ?,weak ? oriented and the strong{110}<110>shear texture obtained by warm rolling of as-cast strip have important effects on the nucleation of Cube grain.In the early stage of recrystallization annealing,the Cube recrystallization preferentially nucleate on the shear bands of {110}<110>oriented deformed matrix with high energy storage and the grain boundaries of {411}<148>oriented deformed grains.With the increase of recrystallization level,Cube and Goss orientation competition nucleation and growth appear in the shear band of {111}<110>and {111}<112>deformed matrix.In addition,a large amount of Cube recrystallization can be obtained by the original position nucleation and recovery of the Cube oriented deformation region.In the later stage of recrystallization process,Cube grains acquiring the advantages of size and quantity,swallow the surrounding matrix and grains and grow continuously.Through directional nucleation and selective growth mechanism,Cube texture strength in annealed structure is enhanced and the magnetic properties of annealed plate are improved.(5)The effects of one-stage and two-stage cold rolling processes on the structure,texture and magnetic properties of thin non-oriented silicon steel were compared,and the control mechanism of intermediate annealing on the texture of products was studied.The relationship between preparation process,texture and properties of cold-rolled thin non-oriented silicon steel was clarified.The magnetic properties of medium and high frequency were enhanced with the improving of the structure and texture of annealed sheets.During one-stage cold rolling,the grains are severely broken and the fibre structure is formed.The deformation texture is determined by stronger y texture and weaker ? texture.The annealed sheet with fine grains,poor uniformity,strong ? texture and weak ? texture;has low magnetic properties.In the two-stage cold rolling process,intermediate annealing is beneficial to coarsening grains and improving the strength of Cube orientation.Finally,the shear band density of cold rolled sheets increases,and the texture of ? and ?increase,while the texture of ? decreases.The annealed structure is coarse and uniform with strong ? texture.The magnetic properties of 0.2 mm annealed sheets are B₅₀=1.758 T,P_15/50=2.065 W/kg,P_10/400=15.173 W/kg and P_10/1k=59.48 W/kg.Increasing the ratio of ? texture in annealed sheets is beneficial to increase the magnetic induction intensity and decrease the magnetic anisotropy.Reducing the thickness can greatly reduce eddy current loss and reduce high frequency iron loss.Optimizing texture is beneficial to reduce hysteresis loss.