Tests at the ESRF aim to improve advanced high-strength steels

The automobile industry has come a long way in terms of safety and its latest challenge is to provide environmentally friendly cars by reducing fuel consumption and CO₂ emission. One measure that helps to achieve this is using advanced high-strength steels (AHSS) in their manufacture.

Car makers started using AHSS in the late 1990s in the so-called body-in-white structure of the cars. The main aim was to increase the material’s strength, giving the opportunity to make the cars lighter while keeping a high energy absorption during a crash. In comparison to the conventional high-strength steels (HSS) having yield strengths lower than 550 MPa, the AHSS exhibits yield strengths between 600 and 1500 MPa.

Dual-phase steels (DP) are the main metallurgical solutions for AHSS. The microstructure consists of hard martensite islands embedded in a soft ferritic matrix. Thanks to this bi-phase microstructure, the material has a good combination of strength and ductility. In order to go further it is now necessary to limit as much as possible the damage induced by deformation by microstructural design. The fraction of martensite used and its hardness play a major part in the damage nucleation but also in increasing the steel resistance. The task of researchers at ArcelorMittal is to find the right balance to avoid damage, while maintaining a high resistance. Caroline Landron, a PhD student funded by ArcelorMittal and based at INSA, explains that, when DP steels are forming, voids could appear in the structure. “We have to understand why and what are the impacts of the microstructure.”

The first step is to be able to characterise damage appearing when tensile tests are performed. For this, the team uses X-ray microtomography to picture the DP steels at the ESRF. “This is almost exclusively the technique that we use to track the void nucleation,” explains Eric Maire, director of research at INSA. The team has used ID15 and ID19, the first one being faster at scanning and the second one providing better resolution images. The researchers applied tensile tests to various DP steels (with different proportions of ferrite and martensite) on both beamlines and managed to quantify the damage. The results allowed the team to create and validate models of the nucleation and the growth of voids.

Damage nucleation (in red) during a tensile test on advanced high-strength steels (AHSS).

In the future, it could be desirable to carry out X-ray tomography on the material while it is being tested with even more realistic conditions than tension. “Once we have found the patterns on how the damage is made and where it takes place, we will try to design new microstructures that better resist damage,” explains Olivier Bouaziz, expert at ArcelorMittal.

Reference:
C Landron et al. 2010 Scripta Materialia 63 973–976.

M Capellas

This article appeared in ESRFnews, December 2010. 

To register for a free subscription and to rapidly receive the current issue, please go to: ESRFNews Digital Edition Subscription.

Top image: Possible uses of AHSS in the body-in-white structure (the orange parts). Image credit: Arcelor Mittal.