High Temperature Straining Behaviour of High FeSi Electrical Steel by Torsion Tests

Steel with an increased Si-content has better magnetic properties in electrical applications in terms of high electrical resistivity, reduced energy losses and low magnetostriction. Nevertheless, the oxygen affinity of this element at high working temperatures and the poor ductility observed at room temperature caused by order structures make the thermomechanical processing of these alloys rather difficult. Since these materials do not present a phase transformations from ferrite to austenite, a fundamental study of their workability using torsion tests will help to understand and to optimise their production process. Important critical temperatures in these materials are T-ord (the temperature above which the material is disordered), T-nr (the temperature below which static recrystallisation is not taking place any more) and other restoration temperatures appearing during processing. Fe-Si electrical steels, with silicon concentrations of 2, 3 and 4 wt.-%, were tested according to a multi-deformation torsion schedule under continuous of cooling conditions in 18 passes, with temperature ranges from 1150 to 810 degrees C, at a strain rate of 1 s(-1), the interpass time and the amount of plastic deformation were varied from 20 to 5 see and from 0.1 to 0.3, respectively. Different critical temperatures, important for the processing of these alloys, were calculated from the dependence of the mean flow stress (MFS) on inverse temperature, based on their changes of slope. The temperatures at which the restorations mechanism, the recrystallization and the recovery stops, T, were determined and can be described using the relation developed here, based on their dependence on composition, deformation parameters and cooling rate. The metallographic analysis of quenched samples is in good agreement with the critical temperatures obtained through the measurement of the MFS

A study of texture evolution during deformation and annealing in Fe 3 wt.-% Si by orientation contrast microscopy

Electrical steels, in particular Fe-Si alloys, are used as magnetic flux carrier in transformers and motors because of their excellent magnetic properties. They owe these magnetic properties in part to the presence of specific texture components such as the Goss ({110} ) or the cube components ({001} ), but also to the chemical composition which is optimum with 6.5 wt. % Si. This high silicon content provides a stable BCC lattice structure to the alloy over the entire solid state domain, but also renders the material more brittle. This embrittlement, which is induced by ordering phenomena, makes it impossible to produce the alloy in a conventional rolling process unless a specific thermomechanical route at high temperature is applied. In order to examine the working behaviour of high Si electrical steels, a series of room temperature plane strain compression tests was carried out on a Fe-3%Si alloy in hot band condition. The samples were compressed with a constant strain rate of 20 s(-1) to a reduction of 10, 35 and 70% and subsequently annealed for different times at 800 and 900 degrees C in an electrical furnace without protecting atmosphere. The hot rolled microstructure displayed an average grain size of 195 mu m and the texture showed on the cube component ({001} ) of maximum 5x random levels. After plane strain compression the samples developed the conventional a ( // RD) / gamma ( // ND) fibre texture by plastic shear which was also accommodated, in part, by mechanical twinning. With regard to the annealed material, it was observed that the recrystallisation started in grains with the higher stored energy and within the shear bands. After a reduction of 70% the samples that were annealed at 900 degrees C for 4 hours displayed an average grain size of 27 mu m and a relative maximum of 4x random on the cube component. Also other less intense components such as the rotated cube ({001} ) and the Goss ({110} ) were present in the annealing texture. The samples that were annealed at 900 C, after a reduction of 70%, were characterized by an average grain size of 36 mu m and by the appearance of the {111} gamma fibre component with an intensity of 4.7

Workability and straining behaviour of high silicon steel (3,0 - 6,3 wt.-% Si) by hot torsion and RT compression tests

High silicon steel (up to 6.5 wt.-%Si) is important for the electrical industry because of its magnetic properties. However, its production in low thickness by cold rolling is difficult due to extreme brittleness, mainly caused by ordering processes, making dislocation motion more complex. Nevertheless, these materials appear to be deformable at higher temperatures. The cooling rate after hot deformation, the temperature from which it is cooled and the time delay prior to cold deformation are important elements for the understanding of their workability. Hot torsion tests were performed on Fe-Si steel (4.2 and 5.6 wt.-%Si) under continuous cooling to study the influence of strain and interpass time on ordering and non-recrystallization temperatures. Compression tests at a constant strain rate were used to study the effect of continuous cooling to RT and the delay time between deformations for series of silicon alloys (from 3.3 to 6.3 wt.-% Si) with different thermomechanical treatments. An aging phenomenon due to an ordering reaction at RT was observed. Finally, extrapolating the hot torsion and compression tests parameters to the rolling mill a suitable schedule for hot rolling was found guaranteeing good conditions for further cold rolling

On the effect of texture in experimental grades of high-silicon electrical steel

Crystallographic texture has an important effect on the magnetic quality of electrical steel: a specific texture parameter A is defined and used to estimate the magnetic quality of texture components. It is shown that obtaining the best possible texture in non oriented electrical steel can reduce the losses with 1,5 W/kg. Two production schemes for high silicon electrical steel are described: a conventional processing through hot and cold rolling with adequate temperatures and cooling rates and an immersion-diffusion process by hot dipping in a Si- and Al-rich bath followed by diffusion annealing. The texture evolution in these experimental materials is under study and first results are reported for conventional alloys (rolling procedure) and for immersion-diffusion alloys, which are annealed after dipping in order to obtain a controlled concentration gradient with high Si and/or Al at the surface or a homogeneous Si and/or Al-content over the thickness