Fundamentals of materials modelling for hot stamping of UHSS panels with graded properties

Abstract

The aim of this study is to develop the fundamentals of materials modelling to enable effective process control of hot stamping for forming UHSS panels with graded properties for optimised functional performance. A selective heating and press hardening strategy is adopted to grade the microstructural distribution of a press hardened component through differential heat treatment of the blank prior to forming. Comprehensive material models, to enable prediction of austenite formation and deformation behaviours of boron steel under hot forming conditions, as well as the dynamic response of a press hardened part with tailored properties in collision situations, have been developed based on experimental investigations and mechanism studies. The research work is concerned with four aspects: feasibility of the selective heating and press hardening strategy, austenite formation in boron steel during selective heating, thermo-mechanical properties of boron steel under hot stamping, and mechanical properties of boron steel with various microstructures at room temperature. Feasibility studies for the selective heating and press hardening strategy were carried out through a designed experimental programme. A lab-scale demonstrator part was designed and relevant manufacturing and property-assessment processes were defined. A heating technique and selective-heating rigs were designed to enable certain microstructural distributions in blanks to be obtained. A hot stamping tool set was designed for forming and quenching the parts. Test pieces were formed under various heating conditions to obtain demonstrator parts having variously graded microstructures. Microstructural distributions in the as-formed parts were determined through hardness testing and microstructural observation. Ultimately, the structural performance of the parts was evaluated through bending tests. Heat treatment tests were performed to study the formation of austenite in boron steel during selective heating. Characterisation of the effects of heating rate and temperature on transformation behavior was conducted based on the test results. A unified austenite formation model, capable of predicting full or partial austenite formation under both isothermal and non-isothermal conditions, was developed, and determined from the heat treatment test results. Hot tensile tests were performed to study the thermo-mechanical properties of the austenite and initial phase (ferrite and pearlite) of boron steel. The viscoplastic deformation behaviours of the both phase states were analysed in terms of strain rate and temperature dependence based on the test results. A viscoplastic-damage constitutive model, capable of describing the thermo-mechanical response of boron steel in a state corresponding to hot stamping after selective heating, was proposed. Values of constants in the model for both the austenite and initial phase were calibrated from the hot tensile test results. Dynamic and quasi-static tensile testes combined with hardness testing and microstructural observation were carried out to study the mechanical properties of press hardened boron steel with various microstructures at room temperature. Based on the results, the strain rate sensitivity of the martensite and initial phase of boron steel was characterised; the relationships between mechanical properties (true ultimate tensile strength, 0.2% proof stress, elongation, and hardness) and phase composition (volume fraction of martensite), for boron steel with various microstructures, were rationalised. Finally, a viscoplastic-damage constitutive model, capable of predicting the mechanical response of a press hardened boron steel part with graded properties being subjected to crash situations in automobiles, were developed, and determined from the test results.