Investigation On Continuous Rolling Process And Deformation Mechanism In Fe-6.5wt.%Si Alloy

Fe-6.5wt.%Si alloy(high silicon electrical steel)is an excellent soft magnetic material with high permeability,high resistivity,low iron loss and near-zero magnetostriction,which can be used to improve the efficiency,save energy,and reduce noise in electric devices.However,the appearance of ordered structures(B2 and D03)leads to its room temperature brittleness and poor workability,which seriously restricts its application in industry.Recently,the ultra-thin Fe-6.5wt.%Si alloy sheets with a final thickness of 0.05 mm have been successfully fabricated by the advanced technique of stepwise plasticization(casting-hot forging-hot rolling-warm rolling-cold rolling)in our group.However,there are few reports on the process exploration and investigation from the lab-scale level to the pilot-scale experiment(magnification of ingot size and continuous rolling).At the same time,the systematic investigation on the deformation mechanism of Fe-6.5wt.%Si alloy is limited.Based on technique and theory of stepwise plasticization(casting-hot forging-hot rolling-warm rolling-cold rolling),in this thesis,the scientific problems during the development and optimization of continuous rolling process(including casting of large-size ingots,directly hot rolling without forging,warm rolling with tension,cold rolling with tension and heat treatment)were systematically studied,and the theory was used to guide the process and production practice.The main research contents and results are as following:(l)The casting process of large-size ingots was optimized,and large-size ingots with excellent formability were prepared.The investigation suggests that homogeneous microstructures with a high proportion of equiaxed grains were observed under the condition of furnace cooling,but the ordered structures were fully generated,and the degree of order is high.Inhomogeneous microstructures with a low proportion of equiaxed grains were formed under air-cooling condition,but the formation of ordered structures was partially inhibited,especially for D03.Under the condition of furnace cooling,the effects of uniform equiaxed grains and high degree of order on the workability of the alloy were contradictory.The formability was tested through the flow stress during the compression and the microstructure after the compression.The results show that both the hot formability and the wann workability of the furnace cooled sample are better than those of the air cooled sample.For large-scale ingots,the control of the as-cast microstructure(grain morphology)and residual stress becomes more significant than the control of degree of order.Slowly cooling during casting is beneficial to obtaining large-scale ingots with excellent formability.(2)Directly hot rolling(without forging)of the large-scale ingots has been realized.The evolution of structure and texture of large-scale ingots with equiaxed grains during directly hot rolling was studied.The mechanical properties of hot-rolled sheets with layered structure were analyzed.Serrated flow behavior was found during tensile deformation,and the reason of the serrated flow behavior was revealed.During hot rolling of the large-scale ingots with equiaxed grains,the structure of the central part of the ingots was gradually shifted to the {001}<110>texture.The transition layer was mainly composed of the {110}<001>texture.Dynamic recrystallization occurred in surface layer due to large shear deformation,and the surface layer was mainly composed of {011}<100>texture,{112}<111>texture and {011}<211>texture.The serrated flow of the hot-rolled sheet is mainly attributed to the serration in the center part.The serrated flow behavior results from interaction of solutes with mobile dislocations.(3)The work-softening mechanism during warm deformation was clarified.An appropriate deformation temperature window for improving the formability of the Fe-6.5wt.%Si alloy was determined,and continuous warm rolling with tension was developed.The influence of the ordered phases on the flow stress gradually weakens with increasing deformation temperature.The dynamic recovery and deformation-induced disorder result in the work-softening behavior.Considering the microstructure,the breakage degree of ordered phase,dislocation configurations after deformation and the flow stress during compression,the appropriate deformation temperature window for improving the formability of the Fe-6.5wt.%Si alloy is about 500-600?.The welding process was optimized,and the welding quality can be significantly improved by adopting the method of double-sided welding,pre-heating before welding,heat preservation after welding,and slow cooling.Continuous warm rolling with tension was developed.At the same time,the warm-rolled steel coil with a weight of 15 kg was successfully prepared,which laid the foundation for the pilot-scale experiment of warm rolling.(4)The work-softening range at the relatively low temperature(300? and below)was found.The cold rolling process was optimized.Continuous cold rolling with tension was developed.In the first stage of the strain(0-22%compression ratio),the flow stress increased rapidly.The ordered phases were basically destroyed.In the second stage of the strain(22-43%compression ratio),there is a work softening range.In the third stage of the strain(more than 43%compression ratio),the work hardening is further enhanced.The main reasons for the work softening are deformation-induced disorder and a softening dislocation structure.Based on the study,the edge cracks during cold rolling were reduced.Continuous cold rolling with tension was developed.The cold rolling with tension and coiling of the sheet with a thickness of 0.2 mm were realized.(5)The effects of the ordered degree(ordered structures),grain orientation,grain size,aging,strain rate and deformation temperature on deformation twinning were systematically analyzed,and the mechanism of deformation twinning was revealed.The heat treatment process of Fe-6.5wt.%Si alloy was optimized.The investigation on deformation twinning characteristics of Fe-6.5wt.%Si alloy suggests that deformation twinning can occur in Fe-6.5wt.%Si alloy with low degree of order and relatively large grain size.Large Schmid factor values of deformation twinning appear in the grains with a<001>orientation,and the<001>orientation along the tensile direction can promote deformation twinning by reducing the critical twinning stress.Meanwhile,high strain rate and low deformation temperature can further promote the formation of the twinning by dislocations pile-up and high work hardening rate.The effect of aging on deformation twinning is essentially the influence of degree of order on deformation twinning.Deformation twinning in the aged samples at 400? for 20 h and 500“C was completely inhibited.Relatively appropriate process parameters(400?.for 10 h)for stress relief annealing were obtained by optimizing the degree of order and residual stress,and the plasticity of quenched samples was improved.Based on the above results and understanding,the continuous rolling process from the lab-scale level to the pilot-scale experiment was explored and the deformation mechanism of Fe-6.5wt.%Si alloy was systematically revealed.The understandings and research work provide theoretical references and process guidance for the deformation mechanism and pilot-scale experiment of Fe-6.5wt.%Si alloy.