Studies of the thermo-mechanical material response of a Boron steel by inverse modelling

Abstract

In numerical forming simulations of the hot stamping process, the lack of reliable material data, especially at higher temperatures gives quite rough estimations of the stamping forces, strains, residual stresses and achieved final product shape. Traditionally, the material is characterised by several isothermal compression or tension tests at elevated temperatures and strain rates. The present work have pointed out an alternative/complementary approach to reduce the number of experiments and to obtain good results by using Gleeble compression tests at continuous cooling with different compression start temperatures. The isothermal mechanical response was established by means of inverse modeling, with start values obtained from isothermal test results. Three different functions describing the flow stress as function of temperature and plastic strain have been tested and compared in usefulness for the steel grade used. The main object function consists of three parts, one from each compression start temperature. The object function in the inverse problem is based on the deviation in compression force-displacement data and radial displacement at the mid point of the specimen. Minimisation of the objective function with respect to the material parameters were performed using an in-house optimization software which is based on the subplex method. The established material response has been evaluated by comparison with data from a separate forming test. For the boron steel in the austenitic phase, the Nemat-Nasser model was found to describe the material response with acceptable agreement within the temperature and strain interval studied in the experimental validation.