Study On Heat Treatment Process For High-grade Non-oriented Silicon Steel Produced By Strip Casting

High-grade non-oriented silicon steel is an important core material for electrical machines such as large motors and generators.Compared with the conventional hot rolling process,twin-roll strip casting has the application potential of manufacturing non-oriented silicon steel sheets due to its short process,low energy consumption,environmental protection and advantage on controlling microstructure and texture.In this paper,the microstructures,textures and precipitates of a Fe-2.6%Si as-cast strip and a Fe-3%Si conventional hot rolled band were comparatively analyzed.These two specimens were subjected to different normalizing heat treatments,and then cold rolled and annealed.The effects of normalizing heat treatment on the microstructure,texture,precipitates and magnetic properties for strip cast and conventional hot rolled non-oriented silicon steels were investigated.In addition,the origin of recrystallized grains with Cube orientation in the annealed sheets was described by EBSD(Electron Backscatter Diffraction)measurements.The results show that:(1)The microstructures along the transverse direction(TD)of as-cast strip is not uniform,indicating the solidification end points are different.This is caused by the decrease of melt superheat during molten steel flows along TD,The microstructure of strip is mainly composed of columnar grains and its texture focuses on {100} and {110}.However,the hot rolled band is characterized by fine equiaxed grains at the surface layer and elongated grains at the center layer.And it shows orientation gradients along normal direction(ND).The texture at the surface layer are {112}<111>,{110}<001>,?110}<112>and texture at the center layer is{001}<110>.(2)As-cast strip exhibits a supersaturated solution state which is related to the rapid solidification rate.Precipitates in strip are mainly AIN and AlN-MnS complex,without any separate MnS.The corresponding number density is about 51 in 5000?m and the size range is 50-900nm.By comparison,the number density of particles in hot rolled band is 2 in 5000?m2 and size range is 0.5-8?m.(3)Strong Cube and Goss texture are developed in the annealed sheets produced by cold rolling directly from as-cast strip.The magnetic induction(B50)is about 1.725T which is 0.065T higher than that of conventional annealed sheets.Compared with the monotonically decreasing of magnetic inductions from rolling direction(RD)to TD in the conventional products,strip cast products show a multi changes.The B50 firstly decreases from 0°(RD)to 45°and then increases from 45° to 90°(TD).The difference between RD and TD is<0.02T.However,it is of importance to note that a large number of fine MnS with size of 20-70nm are formed during annealing process.These particles will inhibit grain growth,resulting in the high core loss(P15/50)of 4.07W/kg.(4)Heat treatments for as-cast strip under 1100? have little effect on microstructure,but great on precipitates.The solute atoms in strip will undergo two stages during heat treatments.Firstly,these atoms diffuse to initial precipitates in strip and promote the growth of these particles in the early stages.Secondly,with increasing in time,these atoms will gather to boundaries,sub-boundaries and dislocations,forming precipitates nuclei and then grow when driving force is big enough.This process is easier to occur at high temperatures whereases atoms are more likely to diffuse to initial precipitates at low temperatures.(5)Grain boundary pinning is effective at a mean precipitate size of 20-70 nm.Particles with 100-220nm have great effect on core loss of products.Suitable normalizing process for strip can reduce core loss greatly by increasing the size of grains and decreasing the number of precipitates with 20-220nm in the annealed sheets.The core loss is reduced to 3.26 W/kg(B50=1.732T)when experiment strip is subjected to heat treatment at 900? for 120min.(6)There are four main sources for the Cube grains in the annealed sheets produced by strip casting.The first is the shear bands in {111}<112>.{111}<110>deformed grains and boundaries of {111}<110>deformed grains.The second is the shear bands with Cube orientation in {110}<110>deformed grains.The third is the grain boundaries of {411}<148>deformed regions.The fourth is the deformation bands with Cube orientation in Cube deformed grains and the recovery of Cube deformed grains.