Study On Microstructure,Texture And Inhibitor Precipitation Behavior Of Strip-cast Grain-Oriented 6.5wt%Si Steel

Grain-oriented 6.5wt%Si steel is an ideal soft magnetic material with high permeability,high resistivity and near zero magnetostrictive coefficient.Because of the high silicon content which makes it hard and brittle,6.5wt%Si steel has long been focused on optimizing its fabrication methods.As a new manufacture technology with the characteristics of near-netshape and sub-rapid solidification,strip casting process has unique advantages in fabricating 6.5wt%Si steel,controlling inhibitor precipitation and cost reduction.Based on the strip casting process,in this paper,the microstructure,texture evolution and inhibitor precipitation behavior of 6.5wt%Si steel were studied by OM,EBSD,EPMA,XRD and TEM.(1)The effect of pouring temperature on the microstructure of as-cast strips was investigated by using the same cooling process with different pouring temperature.The study found that the grain size of the as-cast strip with higher pouring temperature is larger,and the microstructure gradient along the thickness and width direction is more obvious.Along the thickness direction,it is a typical three layer distribution of cast structure,that is,the surface equiaxed crystal,subsurface columnar crystal and the central equiaxed fine-grained zone.The columnar crystals in the sub-side along the width direction are more developed than other locations.(2)The inhibition effect of water cooling on particle precipitation was investigated by using the same pouring temperature with different cooling process.The study found that the water cooling can effectively inhibit the precipitation of particles and make particles easier to precipitate along high energy locations such as grain boundaries and dislocations,and the ratio can reach 29.9%.The thermodynamic calculation shows that the initial precipitation temperature of AlN particles is lower,and the gap is larger between the maximum nucleation temperature.The effect of inhibition of AIN particles is most obvious.(3)The as-cast strip of 1500℃ pouring and water cooling was treated at 825℃×10min、900℃ ×10min and 975℃×10min.The study found that during the process of normalizing,the grain in the subsurface grain is dominated by the polygonal process of grain boundary migration.At the same time,the grains are coarsened.The maximum grain size can reach 700μm when the temperature is 975℃.The grain size of the center layer is the homogenization process.When the temperature is 825℃,the average size is 53μm.When the temperature reaches 975℃,the average size decreases to 47μm.During the process,the columnar crystal is destroyed and the texture of the {100} texture was weakened,but the columnar crystal is still difficult to be eliminated completely when the temperature rises to 975℃.(4)The precipitation behavior of cast strips with different normalizing temperatures was studied by EPMA and TEM.The study found that the amount of precipitation in the cast strip after 825℃×10min normalization treatment is less.The surface density of precipitated particles larger than 200nm is 0.0061/μm2.And the surface density of precipitated particles less than 200nm is 0.69/μm2.In the process of 975 ℃ ×10min normalizing,the particle coarsening rate is high because of the high temperature and the surface density of precipitated particles larger than 200nm can reach 0.0337/μm2.During the treatment of 900℃×10min,it is difficult to make the particles rapidly coarsened because of the low temperature.It leads to the high density of the particle size less than 200nm which can reach 2.4/μm2.This precipitated state can be inherited to affect the pinning effect of the inhibitor in the primary annealed microstructure.(5)By studying the whole process of fabricating finished sheets with different normalizing temperatures,it is found that the finished sheet of 975℃ normalizing sample has the best magnetic properties,its magnetic induction is 1.53T and iron loss is 1.86W/kg.The main reason is that the spacing of grain boundaries in the cold rolled microstructure of 975℃ normalizing sample is larger,which provides enough space for the growth of Goss grains.After the primary annealing,the orientation density of the {111}<112>texture is the highest.(6)Through the study of the whole process of the fabricating finished sheets with different warm rolling temperatures,it is found that the 700℃ warm rolled sample have the best magnetic properties.According to the analysis of cold rolling microstructure,the angle between the shear bands and the RD is concentrated between 25°～35°,which is most conducive to the formation of Goss grains with accurate orientation.Meanwhile,the high warm rolling temperature reduces the energy storage of the cold rolled sheet,and then reduces the orientation pinning between the new Goss grains,and the Goss texture ratio can reach 0.035 after the primary annealing.From the point of view of the inhibitor,the precipitated particles in the cold rolled 700℃ warm rolled sample are relatively weak,which can inhibit the Non-goss grains growth during the primary annealing.From the texture analysis,the accurate {111}<112>texture in the 700℃ warm rolled sample can be obtained after primary annealing,and the orientation density can reach 15.(7)The microstructure,texture and coarsening behavior of the inhibitor particles during the secondary recrystallization process were studied by interrupting the recrystallization.Studies have found that the microstructure and texture change little under 1050℃,and the Goss grains failed to obtain the size and quantity advantage.The grain size increased significantly at 1100℃,and the unusual orientation which relate to the matrix grain orientation with Σ5 or Σ9 appeared abnormal growth trend.During 1100℃～1150℃,the Goss grains and nearly Goss oriented grains begin to grow rapidly.In the secondary recrystallization,the nitrogen compounds of Nb and Al have a distinct tendency to separate from MnS.The AlN particles precipitate at 1050℃,and the NbN particles precipitate at 1100℃.The particle coarsening rate is the fastest at the temperature of 1050℃～1100℃.