Research On Microstructure,Texture And Magnetic Properties Of Non-oriented 6.5 Wt.%Si Steel Fabricated By Strip Casting

This paper focuses on the basic research on the microstructure,texture evolution,recrystallization behavior and magnetic property of the strip casting non-oriented 6.5 wt.%Si steel.The original work of this paper are as follows:(1)Tensile ductilities and fracture mechanisms of strip casting non-oriented 6.5 wt.%Si steel at elevated temperatures are systematically investigated,which lays a solidly theoretical and practical foundation for the demonstration and optimization of the strip casting route by which the non-oriented 6.5 wt.%Si steel is fabricated.The elevated-temperature tensile experiments demonstrates that the yield and tensile strength of the as-cast strip gradually decrease with the tensile temperature increasing,whereas the elongation continually increases and the fracture morphology gradually transforms from cleavage pattern to dimple pattern.The ductile-brittle transition of the as-cast strip occurs at 350?.The tensile ductilities of the as-cast strip rely heavily on the deforming temperature rather than the ordered degree.The reordering of the antiphase boundary and the deformation induced disordering at 350? and 400? give rise to the Portevin-Le Chatelier(PLC)effect,leading to the serrated flow behavior at the end of the engineering strain-stress curve.Cerium can capture the harmful O and S in the melt and form Ce2O3 and Ce2O2S precipitates.The dispersive Ce2O2S in the melt,the lattice misfit of which between ?-ferrite is as low as 3.7%,can enhance the heterogeneous nucleation,and thus substantially refine and homogenize the microstructure of the as-cast strip.The average grain size of the as-cast strip is refined to 127?m from the original 258?m.Benefiting from the grain boundary cleaning effect of cerium,the tensile ductility of the as-cast strip at 600? increases from 22.8%to 56.8%and the fracture morphology transforms from intergranular dimple pattern to developed dimple pattern.(2)Warm rolling at relatively low temperatures facilitates the optimization of the annealing texture and the improvement of the magnetic induction of the strip casting non-oriented 6.5 wt.%Si steel.The mechanism of the textural optimization is clarified.The result of the macro-texture investigation indicates that the texture of the annealed sheet previously warm rolled at 150-450? is dominated by relatively strong ?,{115}<5 101>and weak ?-fiber textures,whereas that of the annealed sheet prior warm rolled at 650-850 ?is deteriorated and is characterized by very strong ?,? and relatively weak ?-fiber texture.The TEM bright field image and optical microstructure show that the sheet warm rolled at 150-450? bears a higher dislocation density and the dislocation frequently tangles and piles up,which promotes the nucleation of shear band.As the rolling temperature increases to 650-850 ?,the softening effect involving dynamic recovery,static recovery and recrystallization is dramatically intensified,leading to a lower dislocation density,a uniform dislocation configuration and a gradually decreasing density up to total absence of shear bands.The EBSD result indicates that the ?,? and {115}<5 10 1>nuclei preferentially nucleate within shear bands of the deformed ? grains at the early stage of recrystallization,which results in an obvious advantage in number over that of the ? nuclei.Afterwards,the ?,?and {115}<5 10 1>texture fully develop under the operation of the oriented nucleation mechanism,whereas the development of the detrimental ?-fiber texture is sufficiently hindered.Thus,the shear band in the warm rolled sheet is inferred to be conductive to the optimization of the annealing texture and the enhancement of the magnetic induction of the final product.The highest average magnetic induction measured at 800A/m(B8)approaches 1.388T which is 0.118T higher than that of the Chemical Vapor Deposition(CVD)product with the same thickness.(3)The effect of rolling reduction on the microstructure,texture evolution and magnetic properties is investigated.The textural formation mechanism at different rolling routes is clarified.The rolling scheme according to which the high-quality strip casting non-oriented 6.5 wt.%Si steel is fabricated is further developed.As the hot rolling reduction increases,partial recrystallization occurs during hot rolling and small recrystallized grains appear near the boundary of deformed grains.When the hot rolling reduction is relatively low(0-32%),the hot rolling texture does not reveal a classical deforming texture pattern.With the hot rolling reduction further increases,? and ?-fiber textures appear and gradually develop.With the warm rolling reduction increasing,the deformed grains are further flattened and the density of shear bands increases.The warm rolling texture is dominated by ? and ?-fiber textures.The intensity of the ?-fiber texture is gradually polarized and the textural maximums are respectively concentrated on the {001 }<11 0>and {223}<1 1 0>components,whereas that of the ?-fiber texture tends to be homogeneous.With the warm rolling reduction increasing,the average annealing grain size gradually decreases and the annealing ?-fiber texture is intensified.The magnetic induction(B8,B25 and B50)of the annealed sheet initially increases and then decreases,whereas the iron loss(P10/400 and P2/1000)initially decreases and then increases.When the hot and warm rolling reductions are respectively 52%and 70.8%,the correspondingly annealed sheet acquires the best magnetic properties,the annealing texture of which is well optimized and contains a relatively strong ?-fiber texture with the strongest cube component and a moderate y-fiber texture.When the warm rolling reduction is higher than 79%,the warm rolling microstructure is mainly composed of {001 }<1 1 0>and ? deformed grains.The grain boundaries of the ?deformed grains that gain the advantage in volume provide abundant nucleation sites for the ?nuclei,which goes against the optimization of the annealing texture.When the hot and warm rolling reduction are moderate,the multiple nucleation behaviors near grain boundaries and shear bands facilitate the oriented nucleation of the ?,and parallel ?(?*)nuclei,and thus hinder the excessive development of the recrystallization ?-fiber texture,which contributes to the optimization of the annealing texture.When the hot rolling reduction is higher than 68%,the number of shear bands and the stored energy of the warm rolling microstructure decreases,which hinders the oriented nucleation of the ? and a*nuclei.On the contrary,the preferential nucleation of the ? nuclei near the grain boundary of the y deformed grains deteriorates the annealing texture.(4)The effect of warm temper rolling reduction on the microstructure,texture evolution and magnetic properties of strip casting non-oriented 6.5 wt.%Si steel is investigated.The mechanism of the microstructure and texture evolution during the final annealing is revealed,which contributes to the optimization of the magnetic properties of the final product.As the warm temper rolling reduction increases,the final annealing microstructure becomes uniform and the average grain size gradually decreases.When the warm temper rolling reduction is 2.7-7%,the final annealing texture is composed of?,?*,?-fiber and many other hard-magnetization texture components.At 12%and 14.4%warm temper rolling reduction,the final annealing texture is optimized and characterized by fewer texture components mainly involving weak ?,? and relatively strong ?*-fiber textures.The best magnetic properties is obtained at 14.4%warm temper rolling reduction.Compared with the annealed sheet free from prior warm temper rolling,the iron loss P15/50,P10/400 and P10/1000 along the rolling direction decreases by 0.473W/kg,4.26W/kg and 7.9W/kg,respectively.Strain induced boundary migration(SIBM)is responsible for the microstructure and texture evolution during final annealing.When the warm temper rolling reduction is lower than 7%,the stored energy mainly concentrates near the surface and sub-surface layers of the temper rolled sheet.At the early stage of annealing,only a minority of grains can acquire the advantage in size through SIBM.Most of the grains slowly grow up and tend to be invaded.However,the The annealing time is insufficient for the few large grains to completely invade the adjacently small grains,leading to a heterogeneous annealing microstructure.By contrast,if the warm temper rolling reduction is higher than 7%,the stored energy is accumulated thougth the thickness,which absolutely increases the number of the grains growing by SIBM.In this way,the rest of the small grains can be totally invaded,leading to a homogeneous annealing microstructure.(5)The microstructure,texture evolution and magnetic properties of the strip casting non-oriented 6.5 wt.%Si steel doped with cerium is investigated,which contributes to the further optimization of the magnetic properties of the final product.The formation mechanism of the {114}<4 8 1>texture is systematically clarified.The experimental result enriches the chemical composition design and the processing route by which the strip casting non-oriented 6.5 wt.%Si steel characterized by high quality is fabricated.At a relatively high melt superheat of 70 ?,the macro-texture of the as-cast strip through the whole thickness is dominated by ?-fiber texture.The macro-texture of the 0.3mm thick product which is treated by hot rolling at 1050?,warm rolling at 500 ?,and annealing at 1150? for 1h is dramatically optimized and dominated by strong ?,Goss and very weak y-fiber textures,leading to a superior magnetic properties.The magneic induction B8 along the rolling direction reaches up to 1.42 1T which is 0.15T higher than that of the CVD product with the same thickness,and the iron loss P10/400 is close to the latter.The formation of the relatively strong ?114}<4 8 1>component in the 0.3mm thick product results from the oriented nucleation,oriented growth during recrystallization and the grain size effect at the grain growing stage.The nucleation sites of the {114}<4 8 1>nuclei are abundant at the early stage of annealing.A considerable number of the {114}<4 8 1>grains that acquire the advantage in size after recrystallization continually invade the adjacently small grains and further grow up at the grain growing stage,which contributes to the full development of the corresponding texture.By contrast,the nucleation sites of the {111?<1 2 1>nuclei are monotonous and mainly focused on the the shear band of the {111 }<1 1 0>deformed grain,which tends to result in the occurrence of the orientation pinning effect.Thus,the further coarsening the {111}<1 2 1>nuclei is impeded.Afterwards,most of the {111}<1 2 1>grains that lose the advantage in size after recrystallization tended to be invaded by the adjacently large grains during the grain growing stage,which contributes to the substantial weakening of the annealing {111 }<1 2 1>component.Part of the researching results have been checked and accepted by the specialists in some or other steel corporation.The on-site applications have reached the original target,which facilitates the semi-industrialization of the strip casting non-oriented 6.5 wt.%Si steel.