Figure 1: Component usually made in PHS1500 (left), mechanical characteristics of different PHS grades (right, source ArcelorMittal)
Safety, CO2 emission reduction and lightweighting are some of the main drivers of the recent automotive industry history. All these aspects are directly linked to the material choice. The passive safety results of the metals applied on the body in white and chassis components, while lightweight is, by definition, the amount of the material used, which impacts in the fuel usage and the CO2 emissions.
The right material in right place motto is becoming day by day the key to success. It is necessary to investigate new materials, new solution and archetypes, which could tailor novel materials to end users.
Press Hardening Steel (PHS): process and materials in automotive field.
The Press Hardening process combines high formability with extremely high mechanical properties at the end of the process thanks to high temperature (more than 700°C) stamping.
The process steps are:
- Cutting the blanks from the coil
- Heat up the blanks at880°C to 930°C
- Transferring the heated blanks from the oven to the press
- Stamping and quenching the material during the die closing
Figure 2: Hot press forming process (Source: Benteler)
The conventional and most used PHS grades in the automotive industry are the Usibor1500®, Ductibor450® and Ductibor500®. These steels are, in general, used for crash relevant components such as A and B pillars, windshield reinforcements or side sills to limit the intrusion and improve the absorption of energy. Their use enables car makers to reduce the weight and increase the passive safety performance.
Usibor2000® and the test carried out to evaluate its performance
The last innovation in terms of PHS is the newest Usibor2000® steel of ArcelorMittal, with a tensile strength near to 2000MPa (Table 1). During the FormPlanet project, a a case study was carried out to test this new material for automotive applications. The Centro Riserche Fiat (CRF), in FCA (now Stellantis) Cassino plant stamped several A pillar reinforcement parts using the Usibor2000® instead of the conventional Usibor1500®.
First results indicated that, without make any changes to the press, die or oven, the components could be produced in the same geometry of the current production parts without finding any thinning or wrinkles.
The components were cut and samples were used to perform:
- Hydrogen embrittlement tests
- Chemical analyses
- Tensile and bending tests
- Advance Digital Image Correlation Test
Table 1. Mechanical properties of Usibor2000®
Michele Maria Tedesco
Master’s degree in Mechanical Engineering from the Politecnico of Turin. He started working on the Sheet Metal Department at Centro Ricerche Fiat (CRF) to become today the responsible of the Metals Department, including sheets, bulk, failure analysis and fatigue in its competences.
