A review of ordering phenomena in iron-silicon steels

Silicon steel is an industrially-desired alloy of iron and silicon, characterised by soft magnetic properties, low eddy-current losses, and low magnetostriction. Silicon steels have narrow hysteresis cycles, making them particularly advantageous in applications using electromagnetic fields, such as transformers, generators, electric motor cores, and few other components in industry. Despite its incontestable industrial value, there is not much agreement on the atomic structure of silicon steel. Gaining better understanding of e.g. ordering processes in Fe-Si alloys could not only explain their magnetic properties, but also open opportunities to reduce their weaker characteristics, such as brittleness that adversely affects silicon steel workability and its associated high production costs. This review summarises the state-of-the-art knowledge about ordering in silicon steel and describes the most relevant experimental techniques used for studying its microstructure. In addition, the process of building the iron rich part of the Fe-Si phase diagram is explained. Lastly, the influence of order on the alloy's magnetic and mechanical properties is illustrated

Control of the Austenite recrystallization in Niobium Microalloyed steels

The use of heavy gauge steel sheets for structural applications very often requires a combination of high yield strength and adequate toughness. The most cost effective way to realize a high yield strength and a high ductility in a low alloyed steel is grain refinement. In industrial practice, this refinement is realized by controlled processing. This process consists of controlling the slab reheating temperature, applying a large amount of hot deformation below the nonrecrystallization temperature (T-nr) and accelerated cooling. A better knowledge of T-nr could optimize the process and the best mechanical properties could be reached against the lowest cost. T-nr can be raised by the addition of microalloying elements such as Nb. Nb can retard the static recrystallization of austenite at low temperatures either by solute drag or by precipitation pinning. In this study, the recrystallization behavior of five Nb-microalloyed model alloys with various Nb contents, was evaluated by double hit compression tests. Further, the precipitation state of the materials was investigated experimentally by Inductively Couples Mass Spectroscopy and X-ray Diffraction. The construction of recrystallization-time-temperature diagrams and precipitation-time-temperature diagrams showed that both mechanisms, i.e. recrystallization and precipitation, strongly influence each other

Freeform determination of a nonlinear diffusion coefficient by a reduced adjoint system

In this article we deal with the determination of a diffusion coefficient function in a quasi-linear parabolic system. The motivation comes from a metallurgy setting. The solution method is based on the output least-squares approach (OLS) with minimization applying the adjoint equation. As the diffusion coefficient has an a-priori unknown form, we apply freeform determination in which the coefficient is defined by a linear interpolation. To avoid higher order of interpolation of the coefficient, as well as to enable the use of an adjoint system which is of the same complexity as the original partial differential equation, a reduced adjoint equation is used, applying a mapping strategy. This method is compared with a standard Levenberg-Marquardt approach

OIM analysis of microstructure and texture of a TRIP assisted steel after static and dynamic deformation

TRIP-assisted steel with a composition of 0.2%C, 1.6%Mn, 1.5%Al was studied in the undeformed state, after the application of 10 and 30 % static tensile strain parallel to rolling the direction of the sheet and after dynamic (Hopkinson) fracture test. Detailed examination of the microstructure and microtexture by means of electron backscattered diffraction (EBSD) was carried out in order to quantify the microstructural constituents and to study the strain distribution. The microtexture evolution and the distribution of the specific texture components between the BCC and FCC phases were studied as a function of the external strain and the strain mode-static or dynamic. The strain localization and strain distribution between the structural constituents were quantified based on local misorientation maps. The full constraint Taylor model was used to predict the texture changes in the material and the results were compared to the experimental findings. Comparing the local misorientation data it was found that at low strains the ferrite accommodates approximately 10 times more deformation than the retained austenite. The strain localizes initially on the BCC-FCC phase boundaries and is then spread in the BCC constituents (ferrite and bainite) creating a deformation skeleton in the BCC phase. It was found that the observed texture changes in the measured retained austenite texture after deformation do not correspond exactly to the model prediction. The austenite texture components which were predicted by the Taylor model were not found in the measured austenite texture after deformation which means that they are first transformed to martensite, which is considered as an indication for the selective transformation of austenite under strain

Correlation between microstructure, texture, and magnetic induction in nonoriented electrical steels

Although it is well known that the magnetic induction of electrical steels at a given applied field critically depends on the microstructure and on the present crystallographic texture, there is still no quantitative model to describe this relation in the whole range of inductions. In this paper, the existing different models for the dependence of B-8, B-25, and B-50 on the texture intensities will be evaluated in detail. Finally, a more general model is proposed for the dependence of the magnetic induction at a given applied field as a function of the mean grain size, a texture related parameter and the Si content of the material

Dimensional effects on magnetic properties of Fe-Si steels due to laser and mechanical cutting

Microstructural deterioration near the cut line and presence of residual stresses both affect the magnetic properties of cut parts. In this paper, the differences between microstructural deterioration resulting from mechanical and laser cutting as well as the sample size effects observed upon hysteresis will be discussed. It will be shown that the underlying mechanism for changes in magnetic properties due to mechanical cutting is distinct from that of laser cutting