Basic Research On Warm-rolling And Cold-rolling Processing Of Fe-6.5%Si Electrical Steel With Columnar Grains

Fe-6.5%Si electrical steel (weight percent, referred to "high silicon electrical steel") has excellent soft magnetic properties, such as high permeability, high resistivity, near zero magnetostriction, and low magnetic loss, which make it to be an ideal magnetic material for the high efficiency, low noise, miniaturization and lightening of the equipment in the field of electric power, electric vehicle and high frequency electromagnetic. However, high silicon electrical steel strips are difficult to be fabricated by the conventional rolling process to meet the usage requirements due to its intrinsic brittleness. Previous studies of our research group indicated that the plasticity of high silicon electrical steel at room and intermediate temperatures can be improved remarkablely by the plasticization methods though the controls of orientation, heat treatment and deformation. These related results break-through the bottleneck technology of the plastic processing of high silicon electrical steel at room temperature, and provide important basic works for developing the fabrication of high silicon electrical steel strips.Based on previous works, the change rules of microstructure, ordered structure and residual stress, as well as the influence mechanism of them on the plastic deformation properties of the high silicon electrical steel samples during the heat treatment and deformation processing were investigated in this paper. The methods of controls of rolling temperature and deformation reduction, warm-rolling with gradually decreasing temperature, and stress relief annealing before cold-rolling were proposed, by which the cold-rolling workability and cold-rolling yield of the high silicon electrical steel was improved significantly. In addition, the plasticity and processability of the recrystallization annealed strip at room temperature can be significantly improved though optimizing the process of recrystallization annealing. The work of this paper has established the important technological basis for the rolling processing of high silicon electrical steel strips.The main results show that, under the condition of compression with reduction of 50%, the columnar-grained high silicon electrical steel sample has good deformation property without crack when deformation at 400 ℃ or above. After deformed at 700℃ or below, the columnar grains remain intact without occurrence of recrystallization in the sample. The dominant deformation mechanism of the columnar-grained high silicon electrical steel is the twinning at the deformation temperature of 300～500℃. When the temperature is increased to 550℃ or above, the dominant deformation mechanism is the slipping instead of the deformation twinning. The steel has the characteristics of deformation disordering and reordering when deformation at intermediate temperatures. The order degreees of the samples deformed at 400～600 ℃ are lower than that of the un-deformed sample, while, this relationship is opposite at 700～800℃ that the order degreees of the deformed samples are higher than that of the un-deformed sample. Therefore, the reasonable deformation temperature for improving the plasticity and subsequent workability of the high silicon electrical steel is in the range of 400～600 ℃.Compared with the process of warm-rolling with constant temperature of 500℃, the process of warm-rolling with gradually decreasing temperature (500℃/3 passes/total reduction of 50%+400℃/5 passes/total reduction of 60%+ 300℃/4 passes/total reduction of 64%) with the same total reduction of 93% can improve the three point bending fracture deflections by above 80% and the maximum reductions of single pass cold-rolling without edge crack from 50% to 62%. The plasticity improvement of the high silicon electrical steel can be attributed to both reductions of surface oxidation degree and order degree of the high silicon electrical steel by warm-rolling with gradually decreasing temperature.After annealed at 400 ℃ for 1 h, the room-temperature plasticity of the warm-rolled high silicon electrical steel samples was significantly improved that the average elongation to failure increased from 0.73% to 1.19%. However, the average elongation to failure decreased to 0.41% in the case of the sample annealed at 650 ℃, and the fracture morphology transformed from a mixture of cleavage fracture and dimple fracture in the sample annealed at 400 ℃ to a mixture of intergranular fracture and cleavage fracture in the sample annealed at 650℃. Considering the comprehensive effects of annealing on the recrystallization, reordering, residual stress, and mechanical properties of the warm-rolled high silicon electrical steel samples with columnar grains, the appropriate annealing condition is an annealing temperature of 300～400℃ for 1 h. After being annealed under this condition, the warm-rolled high silicon electrical steel sheets could be cold-rolled to a total reduction of more than 71.4% without obvious edge cracks.After the cold-rolled strip samples were recrystallization annealed at 800～1200℃ for 1 h followed by furnace-cooling, the plasticity decrease sharply, which makes it difficult to be deformed in subsequent. Improving the cooling temperature and cooling rate of the recrystallization annealing can improve the plasticity significantly. For instance, when recrystallization annealing at 1000℃ for 1 h following by oil-quenching from 900 ℃, the average elongation to failure are increased from 0.2% of furnace-cooled sample to 5.2% and the secondary cold-rolling can be performed without obvious edge cracks. The refinement of the ordered phase is responsible for the improvement of the plasticity at room temperature, e.g., the size of ordered phase in the sample oil-quenched below 600℃ is～5 um, while it is refined to<25 nm by oil-quenched from 900℃.Based on above results, a process way of "directional solidification the slab of high silicon electrical steelâ†'warm-rolling with gradually decreasing temperatureâ†'stress relief annealing before cold-rollingâ†'cold-rolling" was proposed for fabricating the cold-rolled high silicon electrical steel strip. By this process way, a bright-surface strip with a thickness of less than 0.1 mm could be fabricated without the formation of obvious edge cracks under laboratory condition, and the strip is flexible enough to be coiled.