Study Of The Microstructure,Texture,Precipitation And Magnetic Properties Of Electrical Steel Produced By Twin Roll Strip Casting

The microstructure and texture evolution and the control of magnetic properties in non-oriented silicon steel,grain-oriented Fe-3%Si and Fe-6.5%Si steels were investigated based on the twin-roll strip casting process.This study mainly focused on the effects of strip casting process on the microstructure,texture and magnetic properties of non-oriented silicon steel,the manufacture process of ultra-low carbon high magnetic inductions grain-oriented Fe-3%Si and Fe-6.5%Si steels and the distribution of special grain boundaries formed during the sub-rapid solidification process.The main innovations are as follows:(1)The metallurgical mechanisms of the advantages of strip casting electrical steel were clarified.The contradiction between conventional process and the microstructure and texture requirements in low and middle grades non-oriented silicon steels can be solved by applying the strip casting process,leading to the better magnetic properties.On the condition of the same composition system,cold rolling and annealing process,the strip casting non-oriented silicon steels have larger recrystallization grain size,stronger Cube texture and lower harmful ?-fiber texture compared to the conventional non-oriented silicon steel.Therefore,the magnetic induction B50 of strip casting non-oriented silicon steels is higher at least 0.06T than that of conventional non-oriented silicon steels,while the iron loss value is lower at least 0.04W/kg.Moreover,as for strip casting non-oriented silicon steel,the magnetic property differences between the transverse and longitudinal direction silicon steels are smaller,and the stability of magnetic properties is significantly superior to that of conventional non-oriented silicon steel during magnetic aging process.(2)The manufacture process of ultra-low carbon grain-oriented silicon steel was proposed based on strip casting process.The amount of super saturation of inhibitor elements was accurately controlled by the rapid solidification of strip casting process.The abnormal growth of Goss grain was explained by the high energy grain boundary mechanism.A large number of inhibitor elements were in solid solution state because of the sub-rapid solidification.These elements were then precipitated during two-stage cold rolling and annealing process.The fine and dispersed inhibitors significantly refined and stabilized the primary recrystallized matrix.The size of secondary Goss grains was 10?30mm.The magnetic induction B8 was 1.94T.The first cold rolling reduction significantly affected the inhibitors precipitation behaviors during intermediate annealing and the homogeneity of primary recrysatllized microstructure,resulting in the different complete degrees of secondary recrystallization.The Goss grain was nucleated at shear bands and then grew and gathered during secondary recryatallization annealing.The coalescence of Goss grain with small misorientations was the main growth mechanism of the onset of secondary recrystallization,the high energy grain boundaries around Goss grain promoted the coarsening of inhibitors and then led to the abnormal growth of Goss grain.(3)The grain-oriented Fe-6.5%Si steel was produced by the special inhibitor design and strip casting process.Due to the high temperature precipitation of MnS and the special grain boundaries,the as-cast strip was refined considerably and its plastic forming ability was significantly improved.The precipitation behavior and deformation conditions were controlled by warm and cold rolling,and a large number of inhibitors were obtained during primary annealing,which is the necessary condition for abnormal growth of Goss grain.A fine Fe-6.5%Si as-cast strip can be obtained when a low superheating was applied.On one hand,the low superheating increased the nucleation rates and the proportion of special grain boundaries.On the other hand,the precipitation of MnS during the transformation of liquid to solid has a pinning effect of grain boundaries and the average grain size of as-cast strip was?35?m.During plastic deformation,the mobility of slip system and the rotation of grains were difficult,which promoted the formation of in-grain shear bands during warm and cold rolling.In addition,a few order phases B2 with size of 100?500nm were observed in as-cast strip.After the warm rolling and annealing,a new D03 order phase was found.The average primary annealed grain size was?11.4?m,the annealing texture was dominated by strong{111}<112>component.During secondary annealing,the Goss grain grew abnormally in the temperature range of 1079?1100?.This narrow temperature range can be explained by the high energy grain boundaries theory and the existence of ?3 grain boundaries inherited from as-cast strip.The grain oriented high silicon steel has a very accurate orientation and the magnetic induction of B8 was 1.74T and the Bg/Bs was not lower than 0.961.(4)A large number of E3 grain boundaries and their special formation mechanism were found in the sub-rapid solidified single ferrite microstructure.A control route was established to obtain the excellent plastic properties by forming E3 grain boundary and stabilizing the solidified microstructure in strip casting process.?3 grain boundary has a good lattice symmetry relationship and relative low energy,which is the most important special boundary that applied to control the metal properties.The ferrite steels have relatively high stacking fault energy.In addition,the coarse solidification interface between solid and liquid phase limited the nucleation conditions.Thus,the ?3 grain boundaries were rarely observed in the solidification microstructures of ferritic steels.However,in strip casting process,the lower superheating led to a large number of nuclei in tundish and melt pool.When the nuclei contacted the solidification interface,two conditions can decrease the system interface energy.One was that the orientation of the nuclei was the same as that of the grains nucleated at cast rollers,resulting in no boundary or small angle boundary between them.The other was that two close-packed planes were parallel and a special large angle boundary was formed with the relationship of a rotation of 60° around<110>axis,referred as ?3.Both of the above two conditions can form stable metallic bond rapidly in the flowing of steel liquid,leading to the nuclei attached at the solidified shell.After solidification,this kind of special grain boundary was further developed due to its low energy.At the subsurface and center layers of Fe-6.5%Si as-cast strip,the proportion of ?3 grain boundary was more than 20%,indicating that the strip casting process has a significant influence on the distributions of solidified grain boundaries.As a result,fine solidified grains with a large proportion of special grain boundaries were obtained.These results provided an efficient way to overcome the plastic processing problems of Fe-6.5%Si steel.(5)The rotations of the grains with special orientations and the formation of shear bands during cold rolling were investigated in detail.The excellent annealing texture and magnetic properties were obtained by controlling the strip casting parameters.The Fe-1.3%Si as-cast strips produced by steel cast rollers were characterized by coarse and homogeneous solidification grains together with the {100} and {110}orientations.During cold rolling,the grains with special orientations located at the center layer of as-cast strip are inclined to form shear bands and accomplish plastic deformation by geometrical softening mechanism.The dislocation cells inside the shear bands are rotated to the ?-fiber such as cube and Goss orientation due to the effect of minimum strain energy.The shear bands in {110}<110>grains are easy to form Cube dislocation cells whereas,the shear bands in {111}<112>and{111}<110>grains are easy to form Goss texture.It should be noted that the{111}<110>grains only exhibited limited shear bands.After recrystallization annealing,a strong Goss texture was obtained by the oriented nucleation mechanism and the magnetic properties were significantly improved.On the other hand,the non-oriented Fe-2.4%Si as-cast strip was composed of complete coarse columnar grains the abnormal grain growth can be observed due to its size advantages after two stage annealing process.