Microstructure Evolution And Texture Regulation Mechanism Of Electrical Steel Fabricated By Strip Casting Process

Electrical steels are mainly used as core material of electric machinery,transformer and other power conversion equipment,which are the most widely used metallic functional materials in the national economic construction,and represent country's special steel production level."Made in China 2025”,upgrading of new generation rural power network,the long-term plan of railway network and other major national strategic plans have put forward urgent requirements for high quality electrical steel products.The characteristics of sub-rapid solidification and near-net-shape via strip casting provided unique advantages in the control of microstructure and texture in electrical steels.In this study,the research focused on the key techniques and relevant mechanisms of high-quality electrical steel,and the relationship between chemical composition,process;microstructure and properties based on strip casting process was systematically studied.The main innovative results are as follows:(1)The control of inhibitors for grain-oriented silicon steel is difficult and the types of inhibitors are limited.Based on the advantage of sub-rapid solidification on inhibitor control in strip casting process,Nb and Bi were added respectively as inhibitor element on the basis of MnS and AIN.The inhibitor force for strip casting grain-oriented silicon steel was strengthened and the magnetic properties were improved.The results indicated that majority of the inhibitor elements remained in solution in the as-cast strip during the solidification and cooling process.Few MnS mainly precipitated at grain boundaries at high temperature,while the precipitation of AIN was inhibited in the as-cast strip with conventional inhibitor.For the as-cast strip with Nb addition,fine NbN nucleated in the interior of the grain.While in the as-cast strip with Bi addition,dispersion of MnS and AlN were present.During the cold rolling and annealing process,the density of inhibitor increased and the average size decreased in grain-oriented silicon steel with new inhibitors.Meanwhile,high density of fine NbN precipitates significantly refined the microstructure and enhanced y texture intensity in grain-oriented silicon steel with Nb addition.A large amount of MnS and AlN precipitates promoted shear deformation and enhanced Goss texture intensity in grain-oriented silicon steel with Bi addition.Grain-oriented silicon steels with Nb and Bi addition both experienced complete abnormal growth during secondary recrystallization annealing,and the enhanced magnetic induction B₈ was high at 1.91 T and 1.88 T respectively,while 1.84 T for conventional grain-oriented silicon steel.(2)The effects of heat treatment and rolling temperature on microstructure and magnetic properties of strip casting grain-oriented silicon steel were investigated.The characteristic of supersaturated solid solution of inhibitor elements in as-cast strip is beneficial for precipitation control.In view of process characteristics of batch annealing and continuous annealing,the heat treatment process involving low temperature batch annealing in intermediate annealing process without normalization process was proposed to produce grain-oriented silicon steel with high magnetic induction.It was shown that during batch annealing process with slow heating rate(60 ?/h)at 750?950 ?,precipitation was adequate and fine inhibitors were obtained,which can reduce dependence of precipitation regulation on the normalization process.The corresponding magnetic induction B₈ reached 1.93 T.The optimum temperature for continuous annealing process with rapid heating(20 C/s)was 950?1050?,and the magnetic induction B₈ was 1.87 T.Considering that the coarse solidification microstructure tended to form inhomogeneous primary recrystallization microstructure after cold rolling,warm rolling was adopted instead of cold rolling.Microstructure optimization of strip-cast grain-oriented silicon steel was achieved via dynamic strain-aging effect(DSA)on rolling in the appropriate temperature range,with improved magnetic properties.The results showed that obvious DSA behavior was observed during warm rolling in the temperature range of 200?400? in strip-cast grain-oriented silicon steel.Intense shear bands and high stored energy were obtained regardless of the grain orientation in the warm-rolled specimens at the DSA temperature,accompanied by strong Goss and ? texture.Fine-grained homogeneous microstructure was obtained in the primary annealed sheet,which was responsible for perfect secondary recrystallization.The magnetic induction B₈ of 300? warm rolling sample was high at 1.88 T,which was 0.1 T higher than the cold rolling sample.(3)During high temperature annealing process,inhibitors in thin-gauge grain-oriented silicon steel were prone to premature failure,which may lead to poor secondary recrystallization.The sequential inhibition behavior of MnS,(Nb,V)N and AIN was designed to provide strong inhibitor force,which induced secondary recrystallization in thin-gauge grain-oriented silicon steel.The 0.08-0.15 mm thin-gauge grain-oriented silicon steels were successfully prepared.In the comparative experiment,commercial grain-oriented silicon steel(B₈=1.88 T)was adopted to prepare thin-gauge grain-oriented silicon steel.The primary annealed sheet exhibited strong Goss and ?*texture.During high temperature annealing,the further development of Goss textures was inhibited by normal growth grains with{210}<001>and {411}<148>.The magnetic induction B₈ of 0.08 mm sheet was 1.88 T.Different from the conventional method based on texture inheritance,the strip casting technology provided a new way to produce thin-gauge grain-oriented silicon steel based on inhibitor-induced secondary recrystallization mechanism.During the final annealing process,MnS,(Nb,V)N and AlN precipitates provided strong pinning effect on grain boundaries at 950?1050 ?.The "sequential inhibition behavior" can effectively avoid the premature failure of inhibitor caused by surface effect,which was responsible for the complete secondary recrystallization of exact Goss grains.The magnetic induction B₈ of 0.08 mm sheet was as high as 1.97 T,which was 0.09 T higher than the conventional method.(4)The origin and evolution of Goss texture in strip-cast grain-oriented silicon steel were studied,and the effect of shear deformation on recrystallization nucleation was clarified.The abnormal growth of Goss grains in consideration of high energy boundary and inhibitor behaviors was established,which provided guidance for the control of secondary recrystallization.The results indicated that severe shear deformation occurred during cold rolling of strip-cast silicon steel,and the rolling direction of crystal in shear band regions tended to be parallel to<001>,which has the minimum elastic modulus.Thus,during the early stage of recrystallization,the special orientation of shear band ensured preferentially nucleation of ? nuclei such as Goss and Cube in shear band regions.And accurate Goss nuclei that formed in the shear band in the interior of {111}<112>deformed matrix acted as nuclei for secondary recrystallization in strip-cast grain-oriented silicon steel,which is different from that in conventional process.During the secondary recrystallization process,Goss grains with high frequently of high energy grain boundaries ensured that boundaries around Goss grains were less dragged by the inhibitor,and promoted the abnormal growth of Goss grains.The stability of matrix was strongly depended on the inhibitor strength,which laid the foundation for secondary recrystallization.Under the condition of premature failure of inhibitor,non-Goss grains grew obviously,which hindered the development of Goss grain.(5)Remarkable abnormal grain growth of {100} grains in as-cast strip was observed,and high performance {100} textured non-oriented silicon steel was obtained based on the inheritance of {100} texture.Meanwhile the formation mechanism of Cube grain in strip-cast non-oriented silicon steel was elucidated.During the isothermal hydrogen annealing at the upper limit temperature of ferrite region,remarkable abnormal grain growth of {100} grains occurred in Fe-1.5%Si as-cast strip.This phenomenon is considered to be the synergistic effect involving the size advantage of coarse columnar grains and surface energy.During cold rolling,the matrix orientation rotated along ? and ?*fiber towards {115}<120>and a texture({118}<110>-{113}<110>).Some Cube texture was retained in the severe deformation region such as the shear band and y texture was not detected obviously.The main nucleation sites for Cube grains were identified:? near the grain boundary and shear band in{411}<148>deformed matrix.?near the grain boundary in {114}<110>deformed matrix.?? orientation shear band in the interior of {112}<110>?{111}<110>deformed regions.Strong Cube texture was formed in the final annealed sheet.Thus,the non-oriented silicon steel exhibited superior magnetic properties with high magnetic induction(B50 as high as 1.87 and 1.86 T,respectively in the rolling direction and transverse direction)and low iron loss(P1.5/50 as low as 4.1 and 4.2 W/kg).The magnetic induction in this study was 0.11 T higher than the existing non-oriented silicon steel product with high magnetic induction.