Yvan Houbaert

Comparison of the effects of silicon and aluminium on the tensile behaviour of multiphase TRIP-assisted steels

Ghent University,Laboratory for Iron and Steelmaking, Technologiepark 9, B-9052 Ghent, Belgium(Received July 12, 2000)(Accepted in revised form November 21, 2000)Keywords: TRIP-steels; Microstructure; Phase transformations; Mechanical propertiesIntroductionMultiphase TRIP-assisted steels are a new generation of low alloy high strength steels that exhibitexceptional formability [1]. The remarkable strength to ductility balance results from the occurrenceduring testing of the Transformation Induced Plasticity (TRIP) phenomenon [2], which involves thestrain-induced transformation of austenite to martensite. The presence of austenite in the initialmicrostructure appears to be critical to the achievement of the desired properties. The retention ofaustenite is usually obtained by the combined effect of an appropriate chemistry and a typicalheat-treatment. In this respect, it is known that silicon and aluminium may both retard the kinetics ofcarbide formation and thus favour the austenite stabilisation by a bainitic holding stage [3]. Despite thisqualitative knowledge, very little literature can be found that rigorously compares the effect of siliconand aluminium on the austenite retention, on the extent of the TRIP effect, and on the resulting tensilebehaviour, all other chemical constituents have been kept constant [4]. The objective of this paper is toquantitatively assess the influence of aluminium and silicon contents, in view of the development ofmultiphase TRIP-assisted steels.Materials and Experimental ProcedureThe chemical compositions of the steels studied in this work are given in Table 1. Specific care wastaken to keep the same carbon content for each alloy. The slabs were initially hot and cold-rolled tothicknesses between 0.8mm and 1.0mm, following classical processing routes.The desired multiphase microstructure was obtained as displayed in Figure 1. The cold-rolledmaterial was first annealed for 4 minutes in the (a1g) region at a temperature 25°C above its Ac1temperature. It was then rapidly cooled and held at an intermediate temperature (i.e. between 375°C and450°C), where bainite formation takes place and contributes to the stabilisation of the austenite. Theheat-treatment was eventually interrupted by quenching the samples to room temperature. After the

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.

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...

Effect of laser cutting on microstructure and on magnetic properties of grain non-oriented electrical steels

Non-oriented electrical steels have been cut with two different techniques, the laser cutting and the mechanical cutting. In order to investigate the effect of the first technique on magnetic properties, different cutting parameters have been tested. Despite this, the best magnetic properties have been obtained after mechanical cutting. The laser cutting causes a coercive field increase and a permeability drop. Due to thermal effect, internal stress seems to be the main process drawback. No correlation between the heat affected zone and the magnetic properties has been found.

The effect of the guillotine clearance on the magnetic properties of electrical steels

Measurements of the bulk magnetic properties of four non-oriented electrical steels after guillotine cutting with varying clearances are reported. A heterogeneous state of deformation is observed at the sheared edge. The deformation-affected zone extends to several millimeters away from the cut edge and possibly occupies the entire sample volume. An obvious correlation was established between the clearance of the guillotine cut and the general deterioration of the magnetic properties such as the total core losses (at 50 Hz), the coercive field, the relative permeability and the remanent induction. The existence of a transition zone was demonstrated in the range of applied clearances for which the magnetic properties were severely deteriorated. This clearance transition zone corresponds with the drastic change of one shearing parameter: the knife displacement at fracture. The observed effects are discussed in terms of the strain heterogeneities that are introduced in the sample by the guillotine cutting process.

Thermomechanical processing of high Si-steel (up to 6.3% Si)

Good properties for magnetostriction and hysteresis losses are obtained in steel with 6.5% Si, making high-Si steels interesting in electrical applications. A research program is described to evaluate the possibility to produce high-silicon steel up to 6.5% Si using an appropriate hot rolling program followed by cold rolling. A final thickness between 0.50 and 0.40 mm should be obtained. Thermomechanical processing of high silicon steel (up to 6.3% Si) by hot rolling appears to be possible whenever special conditions of temperature and rolling are maintained. The texture and microstructure of the materials are described. Magnetic properties of some annealed cold rolled samples were also measured.

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

Constitutive equations for the room temperature deformation of commercial purity aluminum

Abstract A series of compression tests were conducted using a servo-hydraulic, computer driven testing machine to deduce the constitutive equations that describe the behavior of commercial purity aluminum deformed at room temperature over a wide range of strain rates. The experimental samples were tested with and without the use of either mineral oil or polytetrafluoroethylene (PTFE) tape. All the tests were performed at constant strain rates up to an equivalent strain close to 1. The load–displacement curves were converted into stress–strain curves using different coefficients of friction. The constitutive equations deduced assuming that a steady state is achieved at large strains are considered to be good enough for using them to predict the strength of the material on a wide range of strains and strain rates. The friction coefficient found to yield the best results was 0.05 for PTFE tape, 0.10 for mineral oil and 0.20 when no lubricant was used.

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.

Effect of Hot and Cold Rolling on Grain Size and Texture in Fe-2.4wt%Si Strips

The magnetic properties of electrical steel such as magnetization behavior and specific magnetic losses are related to the microstructure and texture of the steel. The interest in the case of FeSi-alloys is to realize a low intensity on the {111} fiber, which is a different goal than that of conventional steels, where a high intensity on the {111} fiber and small grain size is desired. In this paper, we present and discuss some results of our recent studies on FeSi-alloys without phase transformation. The resulting grain structure and the relevant magnetic texture components for nonoriented electrical steels before and after cold rolling as well as after annealing were analyzed.