Numerical simulation of sheet metal forming for high strength steels

Sammanfattning: New demands for passenger safety, vehicle performance and fuel economy have led to an increase in the use of advanced high strength steel. An increase in strength decreases the formability of the material and increases the spring back behaviour. Recently the development of high strength steel has rapidly advanced, requiring verification of earlier material models suitable for describing the plasticity behaviour in sheet metal forming. The aim of the here conducted numerical simulations is to verify the deep drawing process and the shape of the final component of a simple hat profile geometry for studying spring-back of high strength steels. Four advanced high strength steels were selected for detailed investigation, namely the dual phase steels DP600 and DP750, the triple phase steel TRIP700 and the stainless steel HYTENS800. The plastic properties of these steels have been assessed through intrinsic and simulative tests, leading to verification and comparison at different levels. The hat profile serves as a simple test geometry for deep drawing due to elimination of the lateral dimension in first order. The corresponding simpler plasticity behaviour in space facilitates systematic analysis Experiments and simulations were carried out, leading to comparison of the resulting draw in, strain, thinning, final shape and spring-back. The verification and analysis concerns the friction coefficient, two software codes, Finite Element properties and the two material models Hill48 and Hill90. The simulation provides a good qualitative coincidence with experimental results, which enables to develop a process theory and to study the individual mechanisms involved. The friction coefficient, varied from 0 to 0.1, shows very low sensitivity on the process. The simulation underestimates the spring-back by 8-12° at the flange edge. Among the four materials studied basically the stainless steel HYTENS800 shows the largest deviations during comparison. In general the results partially reveal distinct quantitative discrepancies, in particular in the critical bending regions, demanding for improved material models and better knowledge of the boundary conditions.