Mechanical Properties of Trip Aided Bainitic Ferrite TBF Steels in Production and Service Conditions

I n the automotive industry, one of the most common methods to reduce the weight of the body components is to downgage the sheets using higher strength steels. In the design phase, engineers typically use the material properties of the incoming material, such as the yield strength and the elongation. For forming analyses, however, more detailed characterization is required flow curves, anisotropy, forming limit curves, etc. . Once the components are formed in the press shop, the yield strength increases due to work strain hardening. The parts are then welded in the body shop, and the body-in-white goes to the paint shop where it is baked to cure the paint. Most steels’ yield strength changes during this paint bake cycle, which determines its final properties in service. Bake hardening and in some cases, bake softening is measured by Bake Hardening Index BHI as defined by EN 10325-2006. The standard dictates relatively low pre-strain 2% and baking temperature 170°C . In real production conditions however, higher strains are achieved and baking temperatures may exceed 170°C to shorten the baking time. In this study, a new generation Advanced High Strength Steel AHSS grade TBF 1050 was characterized for metal forming purposes and its bake hardening response was studied both as the standard suggests and as the real production cycle dictate