Tanya Ros-Yáñez

High-silicon steel produced by hot dipping and diffusion annealing

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 anne...

Characterization of TRIP-assisted multiphase steel surface topography by atomic force microscopy

Transformation-induced plasticity (TRIP)-assisted multiphase steels have a complicated microstructure consisting of different phases, mainly ferrite, retained austenite, bainite and martensite. Atomic force microscopy has been used for the phase identification and characterization of the phases in this kind of steel. A series of tests has been made on a C-Mn-Si and a C-Mn-Al TRIP-assisted steel after two different heat treatments: intercritical annealing followed by quench, and intercritical annealing followed by aging. After the aging process, the C-Mn-Al alloy was tempered in order to make metallographic observation easier, except the samples for mechanical testing, XRD or Mossbauer spectroscopy. It has been possible to identify the different phases and their topographic characteristics and to study their morphology using atomic force microscopy. The fine and complex microstructures of TRIP-assisted multiphase, steels require improvements of the existing observation techniques, like electron backscattered diffraction and atomic force microscopy. Results of these techniques are presented

Magnetic properties of high Si steel with variable ordering obtained through thermomechanical processing

Alloys with a Si content of 4.2 and 5.4 wt % Si were produced to investigate the effect of increasing the Si on the materials processing and properties and to understand the effect of the order–disorder phenomenon on its magnetic properties. Different cooling rates after hot rolling were applied: Slow cooling from 780 °C to room temperature in 26 h, air cooling and water quench, followed by cold rolling until 0.5 to 0.7 mm thickness. Magnetic properties were measured after pickling and annealing at 950 °C for 2 h. 57 Fe Mossbauer spectroscopy was used to study the effect of thermomechanical cycles on the ordering phenomena. It was noticed that the quenched samples have the highest values for the magnetic polarization, while slowly cooled samples have the lowest, for the power losses higher values are obtained for the quenched materials. The highest values for the polarization in the quenched samples were explained as a result of a higher B2 ordering.

Magnetic properties of electrical steel with Si and Al concentration gradients

Electrical steel with a Si content up to 6.5 wt% is a good soft magnetic material, because the power losses are reduced through the increased electrical resistivity of the bulk material by the presence of alloying elements like Si and Al. Nevertheless, with the increasing of the alloying elements, the concentration of Fe atoms in the bulk is reduced and a lower magnetic saturation is reached. Samples are produced by hot dipping in a Si-Al hypereutectic bath, followed by annealing treatments for diffusion. Series of different concentration profiles have been obtained and the magnetic properties measured. A short immersion is sufficient to form a Si-Al enriched layer with D0/sub 3/ ordered structure (25at% Si-Al) in the steel surface: 10 /spl mu/m of D0/sub 3/ or more can be obtained after 20 s immersion (dependent on substrate and experimental conditions). A short diffusion annealing lowers and broadens this level, e.g., to a classical level of 6.5 wt% of Si, while the center of the steel remains at the original level (e.g., 3 wt% Si). Similar gradients are obtained with the simultaneous presence of Si and Al. Magnetic measurements were performed at 50 and 400 Hz after different thermal cycles, leading to different gradients of Si and Al. The power losses are reduced more than 50% in the final material compared with the original. It appears that a short annealing after the dipping already gives low power losses, even before a homogeneous concentration profile is obtained over the sample thickness. A smooth concentration gradient is preferable, with a lower Si content in the center, in order to maintain a good magnetic saturation. This can also be shown through modeling of the magnetic behavior with the concentration gradient. The reduction of power losses is more pronounced at 400 Hz, proving the importance of the skin effect. A further advantage of this procedure is the shorter annealing time after hot dipping.

Evolution of magnetic properties and microstructure of high-silicon steel during hot dipping and diffusion annealing

Hot dipping and diffusion annealing is an alternative production process to obtain steel with high silicon contents while avoiding rolling problems. Surface alloying with Si and Al is achieved on a low-Si steel substrate (/spl ap/ 3%Si) by hot dipping in a hypereutectic Al-27%Si molten bath. To obtain a sufficient amount of Al and Si in solid solution over the thickness, a diffusion annealing treatment is performed after hot dipping. A diffusion annealing at 1250/spl deg/C allows the homogenization of the composition obtaining a concentration of 6.3% Si and 4.5% Al over the thickness. The evolution of microstructure, crystallographic texture, and magnetic properties in each step of the process is reported. After dipping and annealing, the losses decreased proportionally with the total Si-Al concentration achieved during the process. Values as low as 0.6 W/kg at 50 Hz (1T) and 10 W/kg at 400 Hz (1T) have been obtained in the experimental materials with high Si + Al content and 0.35-mm thickness.

Chemical and physical interactions of Si-rich steel substrates during hot dipping experiments in a molten Al-(25 wt%)Si alloy

The chemical and physical interaction between Fe-Si alloys in the range 0-3.8 Si wt% and a molten Al-(Si 25wt%) alloy at 800 °C has been studied for different reaction times (from 0.1 to 200s) by hot dipping tests. Several intermetallic phases have been identified, Fe2Al5, τ1-Al3Fe3Si2, τ2-Al12Fe6Si5, τ3- Al2FeSi and τ4- Al3FeSi2, which already were reported in the literature dealing with the interaction between iron and molten Al-Si alloys. In addition an ordered phase Fe3Si (D03) appears in contact with the Fe-Si substrate. Diffusion reaction and solidification phenomena appear to be involved in the developing of the coating. The growth kinetic has been studied and diffusion appears as the step controlling the intermetallic compounds growth. Special attention was paid to the effect of the microstructure of the dipped sheet on the interaction with the molten alloy. The higher deformed structures react faster; this effect can be explained by the faster diffusion through high diffusivity paths like grain boundaries and dislocations.

Plane strain compression of high silicon steel

Abstract A series of plane strain compression tests were conducted at room temperature on a high silicon steel to study the behaviour of this material owing to the similarity of this test with rolling. The tests were performed in a specially designed machine able to control the strain rate profile and the amount of deformation. The material was cast and hot rolled to a thickness of 4 mm from which the experimental samples were machined. Plane strain compression tests were carried out at three constant rates of strain (0·01, 0·1 and 1 s−1) up to four levels of equivalent strain (0·15, 0·30, 0·45 and 0·60). 57Fe Mössbauer spectroscopy analyses indicate that the degree of order of the material decreases with the amount of deformation until a strain of 0·45 is achieved; a further increase in strain augments the order of the material. It is found that the strain rate sensitivity close to yielding is negative, but converts to positive once a stain of ∼0·04 is surpassed. Microstructural analyses of deformed samples show that deformation is by twinning and shear. Electron backscattered diffraction analyses indicate that twinning occurs only at an early stage of deformation within grains with {111} planes parallel to the plane of deformation. Deformation twinning takes place in all grains at later stages of deformation.

Influence of Ordering Phenomena on the Thermomechanical Processing of High‐Si Electrical Steel Studied by Mössbauer Spectroscopy

Fe‐Si alloys with Si‐contents up to 14 at.%Si (7.6 wt.%Si ) were produced by conventional casting and hot/cold rolled with different conditions of temperatures, pass reductions and cooling rates. The obtained thickness of the final sheet was between 0.5 and 0.7 mm for Si‐steel up to 10 at.%Si (5.5 wt.%). Mossbauer spectroscopy was used to characterize the order evolution with the Si‐content and the thermomechanical processing of the material. A new fitting program was developed for this purpose due to the known complexity of the Fe‐alloys spectra. Results show that there is never complete disorder, i.e. binomial distribution of the atoms, whatever the Si‐concentration is. Moreover, a clear tendency towards D03 ordering is observed. The obtained long‐range order parameters prove a patent increase of the D03 order above 7 at.%Si (3.6 wt.%) which can be associated to the observed brittleness beyond this Si‐amount. Concerning the relationship between order and thermomechanical processing, no big effect was fo...

Effect of atomic order on the electrical and magnetic properties of Fe/sub 100-x/Si/sub x/ (6</spl times/<14) alloys

The influence of order phenomena on the electrical and magnetic properties of a series of Fe-Si alloys is analyzed. Electrical resistivity and hysteresis loop measurements are performed on alloys after different thermal treatments, including annealing and quenching from several temperatures. Mo/spl uml/ssbauer spectroscopy and differential scanning calorimetry were used to characterize both the type of order existing in the alloys and the order-disorder transition temperature. For the highest quenching temperature, the highest magnetic losses are measured due to internal stresses and disordering induced during quenching. High electrical resistivity is obtained by high-temperature quench only when the Si content was over 10.6 at.%. This occurred due to an order-disorder transition, which reduces the B2 degree of order present in the alloy. On the other hand, disordering appears to be detrimental for the excess losses due to the creation of antiphase domain boundaries.

Study of precipitates formed in a wrought aluminum alloy by means of atomic force microscopy

A study of the particles precipitated during heat treating a Type 6061 wrought aluminum alloy was carried out by means of atomic force microscopy and scanning electron microscopy (SEM). The specimens were cut from hot-extruded bars and were heat-treated to peak hardness and overaged stages to obtain precipitates of different sizes. The samples were prepared by standard metallographic techniques before being examined with either type of microscope. Observations with the atomic force microscope (AFM) were made in the topographic and lateral force modes, as each method yields different information. The former mode is issued to identify the particles and measure their size and shape, whereas the latter allows for data related to superficial characteristics. It is concluded that both types of microscopic examinations complement each other and that they can be used in conjunction to obtain a deeper understanding of precipitation behaviour and kinetics.