Manuel Bobadilla

Inverse finite element modelling and identification of constitutive parameters of UHS steel based on Gleeble tensile tests at high temperature

The rheological behaviour of an ultra high strength (UHS) steel is investigated by Gleeble tensile tests at low-deformation rates and high temperature, from 1200°C to solidus temperature. Results show that large thermal gradients exist in specimens, resulting in heterogeneous deformation, which makes the identification of constitutive parameters difficult from the directly deduced nominal stress–strain curves. The advantages of an inverse identification method – associating a direct finite element model of Gleeble tests and an optimization module – are demonstrated in such conditions. The constitutive parameters identified by this technique have been successfully applied to additional tests, more complex in nature than those used for the identification of parameters. However, such tests combining successive loading and relaxation stages have revealed some limitations of the considered constitutive model.

Study of hot tearing and macrosegregation through ingot bending test and its numerical simulation

Ingot bending tests are performed on the already formed solid shell of a 450 kg solidifying ingot. It is designed in order to be representative of two defects occurring during secondary cooling in steel continuous casting: hot tearing and macrosegregation. Ingots show the defects which result from the application of bending. In order to understand the different physical phenomena, finite element numerical modelling of the test has been developed, using two different approaches. A first approach consists of a 3D finite element thermomechanical simulation. Hot tearing criteria, based on strain, strain rate and two values for solid fraction are then evaluated. It is demonstrated that such a strain based criterion has an excellent capability to predict the formation of hot tears. A second approach consists of a 2D planar finite element simulation in the median section of the ingot. A two-phase formulation is used, in which the velocity of the liquid and solid phases are distinguished. The simulation shows how solutes are redistributed through the ingot under the effect of bending. Solute-depleted and enriched regions are well reproduced, but peak values of macrosegregation are underestimated.

FINITE ELEMENT MODELLING OF TENSILE TEST FOR MICRO-ALLOYED LOW CARBON STEEL AT HIGH TEMPERATURE

In view of the numerical inverse identification of constitutive models, a forward numer- ical modelling of Gleeble tension tests is conducted. A coupled electrical-thermal-mechanical model is proposed for the resolution of electrical, energy and momentum conservation equations by means of finite element method. In momentum equation, the mixed rheological model in multi-phase region (e.g. δ-ferrite and γ austenite (δ+γ mixture)) is developed to consider the δ/γ phase transformation phenomenon for micro-alloyed low carbon steel at high temperature. Experimental and numerical results reveal that significant thermal gradients exist in specimen along longitudinal and radial direc- tions. Such thermal gradients will lead to phase fraction gradient in specimen at high temperature, such as δ fraction gradient or liquid fraction gradient. All these gradients will contribute to the het- erogeneous deformation of specimen and severe stress non-uniform distribution, which is the major difficulty for the identification of constitutive models, especially for the simple identification method based on nominal stress-strain. The proposed model can be viewed as an important achievement for further inverse identification methods, which should be used to identify constitutive parameters for steel at high temperature in the presence of thermal gradients.