| The strength of high strength electrical steel is higher than that of ordinary electrical steel by more than 200MPa,and it is widely used in ultra-high-speed motors with a speed exceeding 10×104r/min,large hydroelectric generators and new energy electric vehicle drive motors.Internal permanent magnet synchronous motors,which are the mainstream choice for the drive motor of new energy electric vehicles in China,possess a special rotor structure.The rotor core of the internal permanent magnet synchronous motor may be broken due to its special structure design even though the speed is not very high.Therefore,high strength electrical steel is demanded.The key to the preparation of high strength electrical steel is greatly increasing the strength of electrical steel without deteriorating its magnetic properties.At present,preparing high strength electrical steel with excellent mechanical properties and magnetic properties has become one of research hot points in this field.In this thesis,the high strength electrical steel with balanced properties were successfully prepared through solid solution strengthening,dislocation strengthening and precipitation strengthening.The microstructure and properties,evolution of texture,and structure of nanoprecipitates were studied.The main conclusions and innovations are listed as follows:(1)The influence of Si element on the mechanical and magnetic properties of cold rolled electrical steel sheets was studied,and the feasibility of preparing high strength electrical steel by adding Si element was explored.Cold rolled sheets with silicon content of 3.0%,3.5%,4.0%and 4.5%were prepared by rolling process,and their mechanical properties and magnetic properties were measured after final annealing.After annealing,the yield strength of the cold rolled sheets increased with the increase of silicon content and the decrease of annealing temperature.The iron loss P1.0T/400Hz of the cold rolled sheet decreased with the increase of silicon content and annealing temperature.The performance of cold rolled electrical steel was better than commericial high strength electrical steel when the silicon content exceeds 4.2%.(2)The change of mechanical properties and magnetic properties of Fe-4.5%Si alloy cold rolled sheet during the recrystallization process was studied.The influence of annealing temperatures on the mechanical properties and magnetic properties of cold rolled sheets were systematically studied.Results showed that cold rolled sheets possessed excellent performance after annealing at 550-600℃for 2h.The yield stress of cold rolled sheets had a linear relationship with the recrystallization fraction.The three-point bending test showed that the hot rolled sheet has good plasticity,and crimping process can be completed at room temperature.The iron loss P1.0T/400Hz of the high strength electrical steel sheet prepared with Fe-4.5%Si alloy was lower than that of commercial products by 20%when the yield stress was close.(3)Effects of normalizing on the mechanical and magnetic properties of Fe-4.5%Si cold-rolled sheets and the evolution of microstructure and texture during rolling and annealing were studied.After normalizing,The microstructure of hot rolled sheets gradually transformed from a sandwich-like structure to a structure composed of coarse equiaxed grains.There was no significant change in the type of macrotexture and the X fiber texture was dominant.However.the intensity of macrotexture decreased.After cold rolling of hot rolled sheets normalized at higher temperature,the microstructure of surface layer transformed from narrow elongated grains to wide elongated grains,and the center layer were composed of wide elongated grains.The macrotextures of cold rolled sheets exhibited pronounced{112}<110>component and {100}<011>component.The macrotextures after cold rolling for the hot rolled sheets normalized at 1000℃ exhibited pronounced{111}<110>component and {100}<011>component,and its intensity was significantly lower than that for the unnormalized ones.After annealing,the new grians of cold rolled sheet mainly nucleated at grian boundaries and the microtexture of new grains are mainly {111}<112>component.The new grians after cold rolling for the hot rolled sheets normalized at 1000℃ mainly nucleated at shear bands and the microtexture of new grains are mainly {111}<112>component,however,the intensity of favorable texture {100}<001>,{114}<481>and {100}<013>was significantly increased.In the later stage of recrystallization,deformed grains with orientation of<100>//ND recrystallized through continuous recrystallization,which further increases the intensity of favorable textures.(4)The control factors of iron loss at different strength grades were systematically analyzed,and the performance of cold rolled sheets for normalized hot rolled sheets were compared with that of commercial products.For high strength grades,the main controlling factor for iron loss was the width of the elongated grains of the cold rolled sheets;for low strength grades,the main controlling factor for iron loss was the orientation of recrystallized grains.The iron loss P1.0T/400Hz decreased 13%and 25%at stress level of 570MPa and 780MPa,respectively,comparing with commercial high strength electrical steel.(5)High strength electrical steel cold rolled sheets with excellent properties was prepared by copper alloying.Coherent precipitation of copper-rich precipitates greatly improved the strength of cold rolled electrical steel sheets.The iron loss and yield strength of cold rolled sheets decreased with the increase of the solution temperature.When the solution temperature exceeded 950℃,the grains of the cold rolled sheet were throughout whole thickness,and the yield strength was greatly reduced.The under-aged and peak-aged precipitates had a body-centered cubic structure and were completely coherent with the matrix.As the aging time increases,the structure of the copper-rich precipitates changed from a body-centered cubic structure to a twin 9R structure and a twin face-centered cubic structure.Precipitates with complex structure were also observed.The size of copper-rich precipitates was only 2-10nm,which had a small hindering effect on the movement of magnetic domains,and had little effect on the deterioration of magnetic properties compared with carbonitrides.The decrease in iron loss at the beginning of the aging process was mainly caused by the decrease of hysteresis loss.The ratio of yield stress to iron loss was employed to evaluate high strength electrical steel with different strength grades.The cold rolled sheets preparesd by Fe-4.0%Si-2.0%Cu alloy had obvious advantages over commercial products and its ratio was twice of that for the commercial products. |
PDF Full Text Request