Process Study And Magnetic Property Regulation Of Aluminium-free Non-oriented Electrical Steel Produced By TSCR

Thin slab continuous casting and rolling（TSCR）can realize low temperature heating,high temperature initial rolling,high temperature final rolling and high temperature coiling,which is suitable for low energy consumption and short process production of non-oriented electrical steel with excellent magnetic properties and fine strip profile.However,due to the thinness of TSCR casting slab,narrow mold and submerged entry nozzle（SEN）,the inclusion is easy to noddle at the SEN nozzle,resulting in continuous casting stoppage,and maintaining continuous industrial production is challenging.In addition,due to the compact TSCR process and limited microstructural control ability,the non-oriented electrical steel produced by TSCR always has the problems of corrugating defects and high core loss.Therefore,this paper designs and produces an aluminium-free non-oriented electrical steel which is suitable for TSCR process.The key processes of TSCR technology are simulated and researched.The influence of"replacing Al with Si"on the evolution of inclusions during smelting process was analyzed.The effects of primary recrystallization annealing,skin-passed temper rolling,and stress relief annealing（SRA）on microstructures and magnetic properties was investigated.The main conclusions are as follows:（1）The"replacing Al with Si"aluminium-free non-oriented electrical steel has been successfully prepared by the TSCR process.The core loss of P_1.5/50is 5.32W/kg,which is similar to the aluminium-bearing non-oriented electrical steel.The magnetic induction B₅₀is 1.755T,which is 0.044T（440Gs）higher than the aluminium-bearing non-oriented electrical steel,and the magnetic properties are better.Aluminium-free steel has good castability,with an average continuous casting of 11 furnaces,an increase of 323%.The incidence rate of linear defects on the surface was only 0.48%,a decrease of 96%.（2）Through industrial production sampling,the evolution law of inclusions in the whole process from RH（Ruhstahl Hausen）vacuum refining,continuous casting to casting slab is systematically analyzed.Al₂O₃-Si O₂-Mn O composite inclusions are the main ones with a melting point lower than 1500℃.Compared with aluminium-bearing steels containing Al₂O₃-Si O₂-Ca O and Al₂O₃-Mg O-Ca O composite inclusions with the melting point higher than 1700℃,the inclusions in aluminium-free non-oriented electrical steel are not easy to accumulate in SEN nozzle to form nodding,and have good castability.With the proceeding of RH smelting and continuous casting,inclusions increased by 39.39%in the unstable casting stage,but gradually decreased in other stages.The final removal rate of inclusions reached 95.74%.Among them,the removal rate of micro-inclusions with a size less than 1μm that deteriorated to the magnetic properties,reached 90.54%.It mainly occurs in the stage of decarburization,alloying and net circulation of RH smelting vacuum cycle.The removal rate in this stage reaches 79.50%.In order to improve the properties of non-oriented electrical steel in industrial productions,it is necessary to increase the vacuum cycle time of RH decarburization,deoxidation alloying and net cycle stage,so as to increase the removal rate of fine inclusions,and enhance the protective casting of molten steel in the unstable casting stage,to reduce the increase of inclusions caused by secondary oxidation.（3）The relationship between the rheology stress（deformation resistance）at high temperature and the strain rate,strain,and temperature of the experimental steel was studied by the simulation test of hot compression,and its numerical model was established.According to the simulation results,the temperature range ofα-γphase transformation was determined,namely 890～970℃(2s^-1and 5s^-1)and 870～950℃(10s^-1).According to this,the critical hot rolling process of the steel was developed,with the initial rolling temperature FT0≥980℃and the final rolling temperature FT7≤890℃,the reduction rate of the first rack F1 and the second rack F2 by45%～60%,and the reduction rate of the final rack F7 by 15～25%,avoiding corrugating defects and improving magnetic properties.（4）The effect of primary recrystallization annealing,temper rolling and SRA annealing on magnetic properties of aluminium-free non-oriented electrical steel were investigated.It is found that the higher recrystallization annealing temperature is,the lower the core loss,the higher the maximum permeabilityμ_max,and the maximum permeabilityμ_1.5/50is at 800℃.The core loss and magnetic induction decreased first and then increased with the higher temper rolling reduction rate.The core loss was the lowest when the reduction rate was 8%,while the magnetic induction was the lowest when the reduction rate was 5%.The core loss decreases with the increased SRA annealing time,and tends to be stable when the annealing time exceeds 15min.With the extension of SRA annealing time,the magnetic induction first increased and then decreased,reaching its highest value at 3 min and tending to be stable after 15min.In summary,when the annealing temperature of primary recrystallization is 800℃,the reduction rate is 8%,and the SRA anneals at780℃for more than 15min,the magnetic properties of the product are the best.（5）The strain gradient and non-uniform deformation induced by temper rolling can cause the grain to grow from the surface to the core layer under SRA annealing.The optimal grain size is about 100μm,and the lowest core loss can be obtained after SRA annealing when the reduction rate is 8%.With SRA annealing,the experimental steel transitions from aγtexture to a composite texture with a weak Goss texture,and then to a random texture.When the annealing time of SRA is less than 3min,the intensity of unfavourableγtexture is reduced,the distortion energy of the material is decreased,which leads to the increase of magnetic induction.However,when the annealing time of SRA is more than 3min,the magnetic induction is decreased due to grain coarsening.