Hot Stamping Of High Strength Steels Research Articles

Application of a Model for Quenching and Partitioning in Hot Stamping of High-Strength Steel

Application of quenching and partitioning process in hot stamping has proven to be an effective method to improve the plasticity of advanced high-strength steels (AHSSs). In this study, the hot stamping and partitioning process of advanced high-strength steel 30CrMnSi2Nb is investigated with a hot stamping mold. Given the specific partitioning time and temperature, the influence of quenching temperature on the volume fraction of microstructure evolution and mechanical properties of the above steel are studied in detail. In addition, a model for quenching and partitioning process is applied to predict the carbon diffusion and interface migration during partitioning, which determines the retained austenite volume fraction and final properties of the part. The predicted trends of the retained austenite volume fraction agree with the experimental results. In both cases, the volume fraction of retained austenite increases first and then decreases with the increasing quenching temperature. The optimal quenching temperature is approximately 290 °C for 30CrMnSi2Nb with the partition conditions of 425 °C and 20 seconds. It is suggested that the model can be used to help determine the process parameters to obtain retained austenite as much as possible.

A thermal forming limit prediction method considering material damage for 22MnB5 sheet

Hot stamping of high-strength steel is an innovative technology to achieve automobile lightweight and guarantee security simultaneously. However, the formability of high-strength steel is still limited at elevated temperatures by the evolution of void damage inside materials. Thus, the establishment of an efficient forming limit prediction method is urgently demanded. In the present work, the Gurson-Tvergaard-Needleman (GTN) model is extended by containing Hosford anisotropic yield criterion which can characterize void damage in normal anisotropic materials. The flow behavior of matrix material 22MnB5 at different forming temperatures is simulated by the modified Norton-Hoff hardening law. And the damage-related parameters are calibrated through FEM inverse analysis. Furthermore, the prediction method of forming limit combined with void damage is realized within the framework of the Marciniak and Kuczynski (M-K) model. Meanwhile, the high-temperature Nakazima test is conducted to obtain experimental thermal forming limit diagram (TFLD). The comparison of results by theoretical prediction and experimental test shows good consistency. Based on the proposed method, the effects of temperature, void damage, initial thickness imperfection, and anisotropy on the forming limit are analyzed. The formability improved with the increasing of deformation temperature but worse for other factors.

Cracking Prediction in Hot Stamping of High-Strength Steel by a Temperature-Dependent Forming Limit Surface Approach

Hot stamping of high-strength steel (HSS) can significantly improve ultimate tensile strength (UTS) of hot-stamped part and thus meet the increasing demands for weight reduction and safety standards in vehicles. However, the prediction of forming defect such as cracking in hot stamping using traditional forming limit curve (FLC) is still challenging. In this paper, to predict HSS BR1500HS cracking in hot stamping, a temperature-dependent forming limit surface (FLS) is developed by simulations combined with experiments of biaxial tension of the plate with a groove at different temperatures. Different from the FLC, the newly developed FLS in which temperature is included suits the hot stamping of HSS. Considering the interplay among phase transformation, stress and strain, a finite element (FE)-coupled thermo-mechanical model of the hot stamping is developed and implemented under ABAQUS/Explicit platform where the developed FLS is built-in to predict strain distributions and HSS BR1500HS cracking in the hot stamping. Finally, the developed FLS is used to evaluate hot formability of HSS BR1500HS by using a hot stamping experiment for forming a box-shaped part. Results confirm that the developed FLS can accurately predict HSS BR1500HS cracking occurrence in the hot stamping.

Testing formability in the hot stamping of HSS

This paper presents an innovative experimental procedure, based on Nakazima test, for evaluating the formability limits in the hot stamping of high strength steel (HSS) which is capable of generating formability data suitable for an FE modelling of the process. The approach is based on two complementary tests which are aimed at evaluating the actual phase transformation kinetics for the material under deformation conditions and the combinations of microstructure, temperature and straining path that lead to localized necking or fracture.