Steels with high Si content (up to 6.5% Si) are excellent soft magnetic materials, however, as the Si content is increased, the material becomes extremely brittle and it is very difficult to produce thin sheet by conventional rolling. An alternative production route has been developed through hot dipping in a high Si-bath followed by diffusion annealing. Experiments were carried out in a hot dip simulator using as substrate a steel with 0.35 mm thickness and 3.2% Si. Samples were dipped in an Al–Si hypereutectic bath at 800 °C for different times. After dipping, the coating consists of Fe–Al–Si phases and primary silicon crystals within a matrix of eutectic Al–Si. During a first annealing inside the hot dip simulator, diffusion of Si and Al into the substrate and phase transformation occurs in the layer producing a series of ternary intermetallics of the Fe–Si–Al system. A second annealing was performed outside the hot dip simulator under vacuum at 1250 °C with different holding times. This diffusion annealing allows the homogenisation of the composition obtaining a homogeneous concentration of 6%–7% Si and/or 2%–5% Al over the thickness in function of the previous hot-dip parameters. The material was characterized by SEM. Magnetic properties were measured and it was shown that the magnetic losses after hot dipping and diffusion annealing were reduced at 50 Hz from 3.7 W/kg (original material) to 0.64 W/kg (1 T), and at 400 Hz from 47 W/kg (original material) to 10 W/kg (1 T).
The oxidation behaviour of a series of high silicon steels was studied by means of optical microscopy and scanning electron microscopy. The samples were cast with amounts of silicon that ranged from 3 . 3 to 11 . 6 wt-%. The experimental samples were cut from the ingots, polished and cleaned before placing them in an electric furnace at temperatures that varied from 1000 to 1230uC for a total time of 50 min. Specimens were then mounted and polished for examination. The analysis of X-ray spectra taken in different regions of the oxide crust indicated the presence of different oxide species. It was found that growth of the layer depended on the temperature at which the treatment was made, and on the amount of silicon in the steel, as it was found that the diffusion of silicon to form fayalite reduced the rate of growth.CEST/2141
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