Abstract
Ultra-high-strength steels (UHSS) combined with tailor-stamping technologies are increasingly being adopted in automotive body production due to crashworthiness improvements and part weight reduction, which meet safety and energy saving demands. Recently, USIBOR®2000 (37MnB5) steel has been added to the family of UHSS. This new material allows higher performance with respect to its predecessor USIBOR®1500 (22MnB5). In this work, the two steels are compared for the manufacturing of an automotive B-Pillar by press-hardening with a tailored tool tempering approach. A Finite Element (FE) model has been developed for the numerical simulation of thermomechanical cycles of the press-hardening process. The FE-simulations have been performed with the aim of obtaining soft zones in the part, by varying the quenching time and the temperature of heated tools. The effects of these parameters on the mechanical properties of the part have been experimentally evaluated thanks to hardness and tensile tests performed on specimens subjected to the numerical thermo-mechanical cycles using the Geeble-3180 physical simulator. The results show that for both UHSS, an increase in quenching time leads to a decrease in hardness up to a threshold value, which is lower for the USIBOR®1500. Moreover, higher mechanical resistance and lower elongation at break values are derived for the USIBOR®2000 steel than for USIBOR®1500 steel.
Highlights
Automotive components design include crashworthiness and high strength to weight ratio requirements, in order to meet safety and energy-saving demands
In the post-forming characterization phase, the thermomechanical cycles obtained by Finite Element (FE) simulation varying the quenching time and hot tool temperature, have been physically reproduced on USIBOR® 2000 and USIBOR® 1500 steel specimens (Figure 2). 8These speciof 19 mens have been subjected to Vickers hardness tests using a Qness hardness tester, after grinding and polishing of the specimen surface
These results confirm a significant effect of temperature and strain rate on steel strength and deformability
Summary
Automotive components design include crashworthiness and high strength to weight ratio requirements, in order to meet safety and energy-saving demands. T. Taylor et al [3] compared the performance of the hot stamping process of the two steels showing that USIBOR® 2000 has a yield strength up to 1400 MPa and an ultimate tensile strength up to 2000 MPa. The press-hardening process of a quenchable steel is a thermo-mechanical sheet metalforming process in which the blank is heated to austenitization temperature, and forming occurs with the steel is in an austenite state and the subsequent quenching in the forming tools allows a martensite microstructure [4]. 2022, 6, 11 the aim to obtain thermo-mechanical cycles imposed in the part area where greater ductility is required, by varying process parameters of the quenching phase (in particular the quenching time and the temperature of the heated tools). The more hardenability of USIBOR® 2000 steel suggests better performance when tailored blank heating approach is adopted
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