Evolution Of Microstructure,Texture And Effect Of Annealing Heating Rate Of High Temperature On Secondary Recrystallization Of High Magnetic Induction Grain-Oriented Silicon Steel

Grain-oriented silicon steel is an important soft magnetic material which widely used in UHV AC and DC transmission and distribution,manned space engineering,high-speed railway,new energy vehicles and other key fields,as well as household appliances,motors,transformers and other industries.With the implementation of the new national energy efficiency standards,the increasing requirements for energy saving and emission reduction,the continuous increasing of energy efficiency of transformers,and the continuous development of the emerging industries such as wind power,photovoltaic power,and energy storage,the market demand for high magnetic induction grain oriented silicon steel（Hi-B steel）for grade 1 and grade 2 energy efficiency transformers continues to increase.High temperature annealing is the final process of preparing Hi-B steel,and it is also the key segment to ensure the complete secondary recrystallization.Due to its“black box”properties,it is difficult to study the microstructure and texture of the Hi-B steel.In this study,the industrial trial production of Hi-B steel was carried out by“one-time cold rolled method+decarburization and nitriding technology”.The product with 5.47cm average grain size of Goss oriented grains,the magnetic induction intensity B₈ reaching1.902T,and the iron loss P_1.7/50 being 0.975W/Kg,reached the level of 23Q100.The whole process was sampled,and the evolution of microstructure and texture in different processes was studied by means of XRD and EBSD.The results show that:The proportion of recrystallization layer increases from 9.89%to 59.38%along the width of hot rolled plate from edge to center;The hot rolled plate is divided into equiaxed crystals with ferrite and pearlite in surface layer,mixture of recrystallization and banded fiber microstructure in 1/4 layer,and banded fiber microstructure in 1/2 layer along the thickness direction;The main texture gradient distribution of the hot rolled plate from the surface layer to the 1/2 layer is:（{112}<111>+{110}<114>）→{441}<014>→{001}～{111}<110>.The average grain sizes of equiaxed grains on the upper and lower surfaces of the normalized plate are 45.9μm and 67.1μm,respectively,and the normalized surface layer is mainly{110}<113>.The transition layer is composed of banded fiber and recrystallized microstructure,and the 1/4 layer is mainly composed of scatteredαand{441}<014>.The1/2 layer is mainly strongαtexture{001}～{111}<110>.The texture of cold rolled sheet is concentrated in{001}～{332}<110>;The initial average grain size of the decarburized nitriding sheet is 26.4μm,and the texture is strongα*-fiber and{111}<112>;For Hi-B steel,the Goss secondary nucleus originated from the surface layer and 1/4 layer of the hot rolled plate,and the initial content in the 1/4 layer of the hot rolled plate was 11.5%and reached highest.With the normalization and cold rolling,the content of the Goss texture decreased.During the decarburization annealing and high temperature annealing process,the Goss grains nucleated again,and the content reached 1.03%before the abnormal growth of the Goss grains（1030°C）.The high temperature annealing’interrupt’test was carried out on the decarburized nitriding plate,and the influence of heating rate on the secondary recrystallization process was discussed.The optimum heating rate was obtained,and the mechanism of secondary recrystallization growth was studied.The results show that:Compared with the heating rates of 5℃/h and 25℃/h,at heating rate of 15℃/h the average grain size of the matrix before the occurrence of secondary recrystallization reaches a minimum of 25.27μm,and the secondary recrystallization temperature range is 1030°C～1050°C,and the primary recrystallization grains are concentrated in{114}<481>and{111}<112>textures,which are beneficial to the abnormal growth of Goss grains.At the heating rate of 15℃/h,the content of 20°～45°high energy grain boundaries around{114}<481>and{111}<112>oriented grains before secondary recrystallization is 16.81%～21.38%and 0.27%～6.14%,respectively,and the content of 20°～45°high energy grain boundaries around{110}<001>oriented grains is60%～62.22%.The HE grain boundary theory is more suitable for the secondary recrystallization mechanism of Hi-B steel.