Basic Research On Low Temperature Hi-B Steel Craft And Industrial Application

Grain-oriented silicon steel is an indispensable functional material in the power industry,and it is mainly used to manufacture transformer.On June 1,2021,the GB20052-2020 "Minimum allowable values of energy efficiency and the energy efficiency grades for power transformers" standard was officially implemented.The implementation of the new standard will promote the use of oriented silicon steel by one or two grades on the current basis.Compared to conventional grain oriented silicon steel(CGO steel),low-temperature Hi-B steel has lower production costs and better magnetic properties.However,the production process parameters are very precise,and the production technology is strictly confidential,so a few steel mills in China have full process production capacity.Based on the industrial trial production of low-temperature Hi-B steel,theoretical analysis of Al N precipitation,primary recrystallization annealing experiment,decarburization and nitriding annealing experiment,and high-temperature annealing extraction experiment were conducted.Analysis and detection techniques such as optical microscopy,X-ray diffraction,EBSD,scanning electron microscopy,transmission electron microscopy,and glow discharge atomic emission spectroscopy were used to study the following content: the precipitation of Al N,evolution of microstructure and texture,recrystallization(primary and secondary recrystallization),formation of oxide layer on decarburized plates and its influence on nitriding.Finally,the industrial production process of low-temperature Hi-B steel was optimized through process basic research.The main conclusions are as follows:(1)Within the composition range of low-temperature Hi-B steel,the Al N precipitation amounts of 1150℃ homogenization,950℃ hot rolled final rolling,1120℃normalization,and 900℃ normalization are 0.0130%～0.0169%,0.0198%～0.0229%,0.0148%～0.0185%,and 0.0202%～0.0232%,respectively.During the hot rolling process of low-temperature Hi-B steel,Al N mainly forms dislocation nucleation,so the final rolling temperature should be controlled at around 976℃.The controlling element for Al N growth in both ferrite and austenite is Al,and Al N grows faster in ferrite.(2)After annealing at 760 ℃ for 3.5 minutes,the deformation structure of the lowtemperature Hi-B steel cold-rolled sheet completely disappears,that is,completely recrystallizes.During the primary recrystallization process,{111}<112>and{114}<481>recrystallized grains mainly nucleate within and at grain boundaries of{112}<110>grains.As recrystallization progresses,α weaken,but γ,{114}<481> and{001}<120> texture enhance.As the annealing temperature increases from 760 ℃ to880 ℃,the average grain size of the primary recrystallization increases from 15.2 μm to 26.7 μm.Taking into account the influence of annealing temperature on the average grain size,{110}<001> grains,and their grain boundary characteristics,the appropriate decarburization annealing temperature for low-temperature Hi-B steel is 820 ℃.(3)The surface layer of the oxide layer is mainly composed of spherical oxides,and the interface between the oxide layer and the substrate is mainly composed of lamellar oxides.Prolonging the annealing time or increasing the dew point of the atmosphere can increase the thickness of the oxide layer on the surface of the decarburized plate.When the dew point increases from 30 ℃ to 40 ℃,the thickness of the oxide layer increases by 1.7 μ m.and the thickness of the oxide layer increases the most.When the annealing temperature increases from 840 ℃ to 870 ℃,the oxide density in the oxide layer increases,affecting the diffusion of O and reducing the thickness of the oxide layer.The lamellar oxide in the oxide layer is more conducive to nitriding than the spherical oxide.(4)Before the secondary recrystallization occurs during the high-temperature annealing process,the texture type of the steel matrix remains unchanged.During the heating stage from 900 ℃ to 975 ℃,the area fraction of {110}<001>components show an increasing trend,but Goss grains have no size advantage.When the high temperature annealing process reaches 980 ℃ without insulation,a complete secondary recrystallization occurs and the magnetic properties are good.The proportion of highenergy grain boundaries around abnormally grown Goss grains is higher than that of other oriented grains,allowing them to grow through the advantage of grain boundary migration rate,then obtain gain size advantage,and ultimately swallow other oriented grains to complete secondary recrystallization.(5)Through basic research on the low-temperature Hi-B steel process,the industrial production process has been optimized.The target temperature of hot rolling finishing was raised to 980℃,the decarburization annealing temperature was reduced to 820℃,the 980℃ insulation process during high temperature annealing was cancelled,and other key measures were taken.After the process optimization,the inhibitor’s inhibitory ability was enhanced,while the average grain size of the decarburized and nitrided sheets decreased,and the qualified rate of magnetic properties of the finished coils increased from 79% to 92%.