High Mn TWIP Steels for Automotive Applications

Abstract

Modern car design puts an increasing emphasis on the notion that a material used in building the body-in-white (BIW) should be selected on the basis of how well it helps achieving specific engineering targets such as low vehicle weight, high passive safety, stability, stiffness, comfort, acoustics, corrosion, and recycling. Steel is at present still the material of choice for car bodies, with 99% of the passenger cars having a steel body, and 6070% of the car weight consisting of steel or steel-based parts. The automotive industry is however continuously making excursions in the area of light materials applications. At present, most car makers are routinely testing multi-materials concepts, which are not limited to the obvious use of light materials for closures, e.g. the use of Al for the front lid or thermosetting resins for trunk lids. The steel industry has made a sustained effort to innovate and create advanced steels and original steel-based solutions and methods in close collaboration with the manufacturers by an early involvement in automotive projects, but also by involving automakers in their own developments. Carmakers have increasingly built passenger cars with body designs which emphasize passenger safety in the event of a collision, and most passenger cars currently achieve high ratings in standardized crash simulations such as the EURO NCAP or the North American NHST tests. The safety issue directly related to the BIW materials is passive safety. High impact energy absorption is required for frontal crash and rear collision, and anti-intrusion properties are required in situations when passenger injury must be avoided, i.e. during a side impact and in case of a roll over, with its associated roof crush. Increased consumer expectations have resulted in cars which have steadily gained in weight as illustrated in figure 1. This weight spiral is a direct result of improvements in vehicle safety, increased space, performance, reliability, passenger comfort and overall vehicle quality. This trend has actually resulted in an increased use of steel in car body manufacturing in absolute terms, and this increase may in certain cases be as high as 25%. The weight issue is therefore high on the agenda of BIW design, as it is directly related to environmental concerns, i.e. emissions of CO2, and the economics of the gas mileage. Reports on weight saving resulting from the use of Advanced High Strength Steels (AHSS) are difficult to evaluate as these tend to focus on the use of advanced steels and improved designs for a single part, rather than the entire car body. The use of Dual Phase (DP) and Transformation-Induced Plasticity (TRIP) steels has been