Microstructure,Texture And Magnetic Properties Of Grain-oriented High Silicon Steel Processed By Strip Casting

Grain-oriented high silicon steels containing 4.5%-6.5%Si exhibit low core losses and high permeability,which are the ideal soft magnetic materials in industrial.However,the development of these steels has been limited so far.This is because high silicon content deteriorates the workability and makes it difficult to control the microstructure and texture.In this study,grainoriented high silicon steels were produced by the strip casting method.The detailed microstructure and texture evolution was characterized with focuses on the Goss texture development,secondary recrystallization mechanism and core losses reduction.The main research conclusions are as follows:(1)Grain-oriented 4.5%Si steel was successfully produced based on strip casting by adopting warm rolling and a new inhibitor system containing Nb elements.The effect of the second rolling reduction ratio on the evolution of secondary recrystallization was investigated.Moreover,a new model describing the relationship between the second rolling reduction ratio,the inhibiting force and the finally magnetic properties was established.The results showed that the as-cast strip was characterized by equiaxed grains and random texture.Most of the inhibitor elements were supersaturated soluted in the as-cast strip,and only a few MnS and NbN appeared at the sub-grain boundaries.After intermediate annealing,the microstructure consisted of equiaxed microstructure,and significant Goss texture occurred for the first time.Another result was that by increasing the second rolling reduction ratio from 53.5%to 82.7%,the grain sizes in the primary annealed sheets were decreased but the y texture increased.This reduced the onset temperature of secondary recrystallization,and thus enhanced the precise Goss grains growth.The maximum magnetic induction B8 of the present grain-oriented 4.5%Si steel was 1.77 T,which was similar to 1.89 T of grain-oriented 3.0%Si steel.The corresponding core loss P10/1000 was 23.97 W/kg.(2)The origin of the Goss texture in the grain-oriented high silicon steel was investigated.Furthermore,the mechanism of secondary recrystallization was revealed and kinetic Monte Carlo simulation was used to study the boundaries in front of the secondary Goss grain.The results showed that Goss nuclei were originated in the shear bands of {111}<112>and{111}<110>deformed grains during intermediate annealing.After the nuclei were formed,they grew at a smaller rate than the whole recrystallization grains.During secondary annealing,the Goss grains were surrounded by a high fraction of HE boundaries.These boundaries migrated preferentially and led to abnormal grain growth.After the onset of abnormal grain growth,the fraction of HE boundaries in front of the large Goss grain varied within a certain ranges.(3)The evolution of microstructure and texture during isothermal secondary annealing was investigated,and the related secondary recrystallization behavior was elucidated.The results showed that the relatively complete abnormal grain growth occurred in the grain-oriented high silicon steel after isothermal secondary annealing at 1025-1050? for 15 min.Given to these high annealing temperatures,both of the Goss and {110}<227>grains grew abnormally which decreased the intensity of the final Goss texture.In addition,the grains with less deviation in orientation occurred in "ribbon" after primary annealing.This made the secondary grains grew faster along RD than TD,resulting in the retained matrix grain colonies.The magnetic induction B8 was 1.67-1.68 T and the high frequency core losses P10/400 was 9.75-10.08 W/kg,P10/1000 was 38.19-40.40 W/kg.The magnetic properties were superior to the non-orinted 6.5%Si steel.(4)The ultra-thin gauge grain-oriented high silicon steel was produced,the rolling and recrystallization behaviors of Goss and {110}<227>grains were investigated.The results showed that Goss grains rotated to {111}<112>grains during rolling.By comparison,a part of the {110}<227>grain rotated to {111}<112>orientation and others rotated to {112}<241>orientation.During the initial stage of recrystallization,the Goss and {210}<001>grain occurred in the {111}<112>grain from Goss grains.The {110}<227>grains formed in the {111}<112>region from the {110}<227>grians.In addition,the {112}<241>region from the {110}<227>grians grain was consumed by the {210}<001>grains or continuously recrystallized.During the latter stage of recrystallization,the {210}<001>grains showed growth advantage than the other grains and the final sheet was characterized by sharp {210}<001>texture.The magnetic induction B8 of the present ultra-thin gauge grain-oriented high silicon steel was 1.71 T,and the core loss P10/400 was 6.98 W/kg and P10/1000 was 23.68 W/kg.(5)The relationship between the initial structure of the strip casting and the secondary recrystallization behavior was clarified.The results showed that the microstructure characteristics of the as-cast strip will be inherited into the subsequent process.When the initial microstructure was coarse columnar crystal(grain size-240 ?m),a large number of ? and a grains occurred after final rolling.This induced incompleted microstructure after primary annealing and retarded the abnormal grain growth during secondary annealing.By comparison,when the equiaxed microstructure was used(grain size?110 ?m),strong {111}<112>texture and fine equiaxed grains were observed after primary annealing.This led to completed abnormal grain growth after secondary annealing.The magnetic induction B8 of the grain-oriented 6.5%Si steel was 1.65 T,which was 0.3 T higher than the non-oriented 6.5%Si steel.The core loss P10/400 was 7.23 W/kg and P10/1000 was 27.71 W/kg.Such core losses were 20%-30%lower than the non-oriented 6.5%Si steel.