High-silicon steel produced by hot dipping and diffusion annealing

Abstract

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).