| Silicon steel,which is the most common soft magnetic material,is widely used for various products in power industry,electronics and military industries,and is of strategic importance to the steel industry.Grain-oriented silicon steel is the only one product manufactured by applying the secondary recrystallization phenomenon in the steel industry.Ithas been manufactured through many complicated steps,leading to such steel products being honored as "artistic product of iron and steel".The strip casting technology provides a possibility to eliminate the continuous casting and hot rolling processes,resulting in obvious advantages on environment protection,energy saving and efficiency improving.Based on its superiority in sub-rapid solidification,near-net-shape characteristics and controlling the inhibitor precipitation,strip casting process makes it possible to manufacture grain-oriented silicon steel and provides a new method to produce thin-gauge grain-oriented silicon steel with high magnetic properties.In this paper,ultra-low carbon grain-oriented silicon steel strips with new inhibitors were produced by using a twin-roll strip caster.Exploratory studies were carried out on the evolution of microstructure,texture and precipitates in the process of casting,normalization,cold-rolling,primary and secondary recrystallization by OM,EBSD,EPMA,XRD and TEM.The main conclusions are as follows:(1)A novel ultra-low carbon route based on strip casting was proposed to fabricate high permeability grain oriented silicon steel.Owing to the characteristic of sub-rapid solidification,more inhibitor elements could be dissolved in the as-cast strip,Furthermore,some novel inhibitors could be adopted,providing a distinct method to produce thin-gauge grain-oriented silicon steel with high magnetic properties.Nb and V were adopted as supplemental inhibitor elements in the present steel.The elements were in supersaturated solution state in the as-cast strip and precipitated during normalization annealing and intermediate annealing processes.For the final sheet of 0.2mm thickness,the magnetic induction B8 is 1.83T,and the iron loss P1.7/50 is 1.82W/kg when normalized at 1050?and intermediate annealed at 950?.For 0.08mm thin-gauge sheet,the magnetic induction B8 is 1.94T,and the iron losses P1.7/50=1.0W/kg,P1.0/400=6.18W/kg,P1.5/400=13W/kg,P1.0/1000=23SW/kg.(2)The solidification process and solutionizing-precipitation behavior of inhibitors were investigated.Optimizing the superheat was a suitable way to control the initial solidification microstructure.The precipitates formed at high temperature dragged the grain boundaries motion,refining the microstructure of as-cast strip.The inhomogeneity of microstructure and texture through thickness and width of as-cast strip were related to the temperature profile and fluid flow during casting process.Meanwhile,hot rolling deformation between kiss-point and nip-point during casting process also had obvious influence on the structure of as-cast strip.In the process of sub-rapid solidification,colmmar grains with pronounced {100} fiber and weak {110} fiber were well developed at high melt superheat.And the equiaxed grains displaying nearly random texture were obtained at low melt superheat.Typical solidification microstructure can be classified into three layers.The surface layer shown fine equiaxed grains with random texture.The subsurface layer was characterized by {110} fiber and shear deformed texture.And the large equiaxed grains at the middle of as-cast strip exhibited a weak and nearly random texture.Few precipitates containing MnS and(Nb,V)N were observed in the as-cast strip,which played a key role in refining the initial microstructure.(Nb,V)N mainly precipitated on the dislocation lines in bar morphology with a size range of 25-45nm.Additionally,few(Nb,V)N precipitated with MnS,showing an size of 80-120nm.By comparison,the as-cast strip with MnS and AIN as inhibitors was characterized by MnS together with few AIN.The size range of the precipitates was 120-150nm.(3)The evolution of distinct microstructure,texture and precipitation in the grain oriented silicon steel with new inhibitors were clarified.The normalization process was beneficial to the homogeneity of strip microstructure and the precipitation of second-phase particles.In addition,two-stage cold rolling and intermediate annealing refine the primary microstructure and optimize the primary texture,providing the suitable matrix prior to the secondary recrystallization.The normalized band of grain-oriented silicon steel exhibited random texture and homogeneous microstructure.A large number of precipitates were formed while fine(Nb,V)N tended to re-dissolution during normalizing annealing.The size distribution of these precipitates was homogeneous and the area density was higher than that in the as-cast strip.After cold rolling and intermediate annealing processes,lots of finely dispersed inhibitors were obtained.The a-fiber,y-fiber together with Goss texture dominated the intermediate annealed sheet.The primary annealed sheet with(Nb,V)N as supplemental inhibitors was characterized by uniform equiaxed grains with an average size of?11 ?m.Finely dispersed(Nb,V)N with a diameter range of 30-100nm and MnS with a diameter range of 40-70nm could inhibit grain growth in primary and secondary recrystallization effectively.The silicon steel with MnS and AIN as inhibitors exhibited finely dispersed AIN after primary annealing.(4)The Goss "seeds" originate from the shear bands during the second-stage cold rolling process.The abnormal growth for Goss grains could be explained by the CSL and HE theories,while the size-advantage may be the main factor during the advanced stage of the abnormal growth.The rolling direction of second stage rolling had influences on the texture of primary annealed sheet.But the secondary recrystallization of Goss orientation could evolve completely after the high temperature annealing with different rolling direction,indicating the Goss "seeds" originate from the shear bands during the cold rolling process.Rapid heating treatment retarded both recovery and recrystallization at high temperature.The texture exhibited stronger Goss and relatively weaker y fiber.Goss grains did not exhibit size and frequency advantages over other orientations prior to the secondary recrystallization,but they were surrounded by the higher proportion of CSL and HE boundaries in the primary recrystallization matrix.These special grain boundaries had higher mobility,and the difference of density of precipitates between orientations decided the direction of abnormal Goss growth.During the later stage of secondary recry stallization,the {111}<112>,Cube and{100}<110>orientation grains was normally grown,which could be consumed by the abnormal growth Goss grains. |
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