Research On The Microstructure,Texture And Properties Of High Strength Non-oriented Electrical Steel Processed By Strip Casting Method

With the increasingly serious environmental problems and the unceasing reduction of fossil fuels reserves,the research and development of new energy vehicles has attracted extensive attention worldwide.Non-oriented electrical steels with high strength are one of the key materials in driving motor rotor core,while its immature preparation technologies have been restricting the development of new energy vehicles to some extent.Thus it is a compelling demand to develop high strength non-oriented electrical steels which exhibit both of the improved mechanical properties and magnetic property.Due to the particularities in preparing high quality electrical steels,twin-roll strip casting shows remarkable technical advantages comparing with the conventional technologies.To investigate the processing theory and technology on the thin-gauge non-oriented electrical steels with high strength but low iron loss prepared by twin-roll strip casting,is of significance for not only enriching and developing the processing theories and technologies with China’s independent intellectual property rights,but also meeting the urgent needs of the development of new energy vehicles.In this paper,the twin-roll strip casting technology was used to prepare the investigated high strength non-oriented electrical steels.The evolutions of microstructure,texture,precipitated phases during the whole process and the mechanical and magnetic properties for the corresponding samples were systematicly studied.The main aims of this paper was to tap the technical potential of twin-roll strip casting,to provide theoretical basis and technical guidance for the industrialization of high strength non-oriented electrical steels.The main works and results of this paper are as follows:(1)The effects of Ni addition on the microstructure and texture evolution were characterized,and the mechanical and magnetic properties of the investigated sheets were tested and studied as well.The results showed that the typical columnar grains were formed,and the strong λ({100}//ND)texture were obtained in the Ni-alloyed as-cast strips due to the decrease of solidification temperature.More shear bands and stronger λ texture were obtained in cold rolled sheets,thus the intensity of λ texture component which was beneficial to magnetic properties was enhanced,while that of magnetic-properties-harmful γ({111}//ND)was relatively low in the final annealed sheets.Moreover,as the annealing recrystallization process could be delayed and the recrystallization rate was reduced to some extent,the grains growth was inhibited in Ni-alloyed sheets,not only enhancing their mechanical properties,but also improving their magnetic properties.For the final annealed sheets,its yield strength enhanced 65 MPa,and its magnetic induction increased 0.03 T in comparison with the sheets without Ni additions.(2)The effects of hot rolling and normalizing on the evolution of microstructure,texture,and the precipitating behaviors in cold rolled and annealed sheets were analyzed.The experimental results showed that the precipitating behaviors of AIN and MnS particles were influenced by normalizing,affecting the microstructure,texture and magnetic properties of corresponding sheets.The precipitates grew up rapidly and then could not pin the grains boundaries during annealing in the sheets which were conducted to normalizing after hot rolling,resulting in the coarse grains annealed sheets finally.Meanwhile,at the initial stage of annealing,the strong {100}<011>cold rolled texture was favourable to the preferential nucleation of {411}<148>nucleus at the boundaries of {100}<011>grains,inducing strong{411}<148>but relatively weaker y recrystallization texture.Compared with the sheets without normalizing,the annealed sheet with normalizing presented improved magnetic induction,B50 increased from 1.69 T to 1.71 T;while both of the hysteresis loss and coercive force decreased remarkably under either low or high frequency,specifically,P10/1000 decreased from 122.31 W/kgto 104.70 W/kg.(3)The effects of reduction partition in hot and cold rolling on the evolution of microstructure,texture,and magnetic properties were analyzed for the investigated sheets.The results showed that the deformation stored energy was influenced by different rolling reduction partition,then affecting the evolution of microstructure,texture,and magnetic properties.With the decrease of deformation stored energy in cold rolled sheets,the texture components mainly distributed at α({110}//RD)and y orientation lines.Moreover,the recrystallization rate decreased gradually,while the grains size increased with the increasing hot rolled reduction and relatively decreasing cold rolled reduction.When the cold rolling reduction was high as 84%,the y recrystallization grains was at a remarkable advantage at the initial stage of annealing,and then developed as the mainly recrystallization texture at the later stage of annealing,deteriorating the magnetic properties.When the cold rolling reduction was controlled at 72%,the magnetic properties decreased as the strong a and y recrystallization texture was obtained after annealing.The best magnetic properties could be obtained when the cold rolling reduction was controlled at a medium value of 77%.The y recrystallization grains were not at an advantage in quantity at the initial stage of annealing,while the {001}<100>grains had obvious advantages in quantity and size and then forming as the dominated recrystallization texture at the later stage of annealing.(4)The effects of Nb addition on the microstructure and texture evolution were characterized,and the mechanical and magnetic properties of the investigated sheets were tested and studied as well.On one hand,the {111}<112>nucleus preferentially nucleated at the boundaries of y deformed grains at the initial stage,and then gradually grew larger by annexing those grains with other orientation.Therefore,strong {111}<112>texture was obtained at the later stage of annealing.On the other hand,NbC particles were mainly precipitated during annealing,obviously delaying the recrystallization process in the Nb addition sheets and then refining the grains and improving the mechanical properties in comparison with the sheets without Nb addition.The yield strength increased from 687 MPa to 789 MPa after annealing at the temperature of 780℃.(5)The effects of Cu additions on the evolution of microstructure,texture,and the precipitating behaviors through the whole process were analyzed in detail.The yield strength and tensile strength both increased with the increasing of aging temperature,and reached to the maximum for those sheets which were conducted to age at the temperature of 550℃.The grains size and the main texture component hardly changed during isothermal aging at 550℃.Specifically,the dominated texture component was {411}<148>,meanwhile lots of nanoscale dispersive Cu precipitates were observed in the annealed sheets.Moreover,the crystal structure evolved as bcc→9R→fcc during precipitating and coarsening.These nanoscale precipitates of Cu could not only played a significant precipitation strengthening effect,but also hardly worsen the magnetic property.(6)Non-oriented electrical steels with different strength grades were successfully prepared by twin-roll strip casting in laboratory.Compared with the 35HXT590T products fabricated by Nippon Steel&Sumitomo Metal,the investigated non-oriented electrical steels with Cu additions presented obvious improvement in both of mechanical and magnetic properties.Specifically,the yield strength was basically controlled at the same grade,the magnetic induction of B50 was increased from 1.65 T to 1.71 T,while P10/400 was decreased from 42.4 W/kg to 26.4 W/kg.