Abstract
The application of ultra high-strength steel sheets in automotive bodies is one of the most important ways to satisfy needs for both the weight reduction and crash safety of a vehicle. Recently, as well, various kinds of high-strength steel sheets have been applied to several automobile parts, and the majority of high-strength steel sheets used for key structural areas usually have a tensile strength of up to 590 MPa. Unfortunately, there are only limited applications for ultra high-strength steel sheets with tensile strengths ranging up to 980 and 1470 MPa, such as those used for bumper reinforcements and door beams. One of the main reasons for such a limited range of application is due to the relatively poor formability and less variations of galvannealed steel sheets, even though they have good corrosion resistance. In order to expand the applications of such ultra high-strength steel sheets, it is therefore necessary to develop such sheets with both good formability and high corrosion resistance.
In this study, ultra high-strength galvannealed steel sheets with a tensile strength of 1180 MPa were developed, and their subsequent performance, such as their mechanical properties, weldability, adhesive resistance of Zn coatings, and their crashworthiness, has been evaluated. These newly developed ultra high-strength galvannealed steel sheets have comparable mechanical properties and weldability (i.e., a good combination between tensile strength/elongation and spot-weld joint strength) to conventional ultra high-strength steel sheets. With regard to adhesion of Zn coatings, the amount of powdering after V-bend tests was measured, and the newly developed steel sheets have the same adhesive resistance as conventional galvannealed mild steel sheets. Furthermore, regarding crashworthiness, absorbed energies measured using bending drop tests were also investigated and calculated. It was found that absorbed energy increased correspondingly to the tensile strength of the steel sheets tested.
In conclusion, ultra high-strength galvannealed steel sheets with a tensile strength of 1180 MPa have been developed for automotive bodies and can contribute to improvements in both the weight reduction and crashworthiness of a vehicle.
Keywords: Ultra high-strength steel, Galvannealed steel, Lightweight body, Crashworthiness