Si-steel is widely used as soft magnetic material in transformers, motors and generators and its electromagnetic properties are improved with increasing Si content, such as the increase of specific resistivity and the decrease of the magnetic anisotropy. At 6.5% Si the specific resistivity is increased to 60~80 μΩcm, while it is 45 ~ 48 μΩcm at 3% Si, and the eddy current loss can be reduced greatly. Furthermore at 6.5% Si the magnetostriction is reduced to almost zero and the noise of the electric machine can be reduced. However, with increasing Si, the hardness and brittleness increase and the cold rolling of the steel to thin sheets become difficult for Si content over 4%. So in this research the characteristics of cold workability and ordered phase for high Si-steels are investigated by comparing ingot cast samples and spray formed samples with varying heat treating conditions of homogenization and suppression of ordered phase formation, Si content and rolling temperature.
The alloy was prepared by melting pure iron and metallic silicon in induction furnace under Ar atmosphere. Ingot samples were cast in two different sizes, a thicker (45×45×40 mm) and a thinner (10×90×50 mm) ingot in steel mold. The spray formed samples were sprayed with Ar gas on a rotating substrate. Ingot samples were homogenized before hot rolling, which was performed in the temperature range of 1200∼800℃ up to 75% reduction in thickness. Hot rolled specimens were annealed in the A2 single phase range of 750~1000℃ for 10 min to 24 hour to dissolve B2 ordered phase and then quenched in different media (ice brine, oil, water and air) to find out the condition to suppress the formation of ordered phases. These specimens were tensile tested and cold rolled at various temperature. Micro-Vickers hardness and nano-hardness were investigated to understand the effect of B2 ordered phase on the mechanical property of high Si steels.
The critical quenching rate to suppress B2 ordered phase increased exponentially with increase of Si content. Fe-5.7%Si could be suppressed by ice-brine quenching after annealing at 850℃, while oil quenching was not fast enough to achieve it. The removal of solidification segregation was necessary in order to reduce the amount of B2 ordered phase and lower the temperature for annealing to dissolve B2 ordered phase. The adverse effect of the ordered phase on cold rolling seems to appear after a certain critical amount and size. The tensile elongation of high Si-steels was found to depend on the Si-content and heat treating conditions and increased greatly by increasing the testing temperature from room temperature to 150℃. A long homogenization annealing at a high temperature reduced the elongation because of grain growth and oxidation. The tensile elongation of Fe-5.4% Si was increased greatly even at room temperature by reducing homogenization annealing time from 10 hours to 40 min. The Fe-6% Si specimen was successfully cold rolled to a thickness reduction of 50% during 13 passes by increasing the rolling temperature to 200℃. This result may be due to cross-slip of dislocation. The adverse effect of B2 phases of several tens nm on cold workability was disappeared by increasing the deformation temperature above 150℃. A large B2 phases around 1000 nm decreased ductility and increased flow stress in 6% Si steels. The short-range order in A2 disordered phase caused a similar hardness as B2 phase and Si content only determined the micro-hardness. The electrical resistivity and core-loss ($W_{10/50}$) of 6% Si steel sheet with 0.5 mm thickness were 78 μΩcm and 0.89 W/kg at maximum induction of 1 T.
