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

Abstract

In 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, suchas the yield strength and the elongation. For forming analyses, however, more detailedcharacterization is required (flow curves, anisotropy, forming limit curves, etc.). Once thecomponents 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 thepaint shop where it is baked to cure the paint. Most steels’ yield strength changes duringthis paint bake cycle, which determines its final properties in service. Bake hardening (andin some cases, bake softening) is measured by Bake Hardening Index (BHI) as defined byEN 10325-2006. The standard dictates relatively low pre-strain (2%) and baking temperature (170°C). In real production conditions however, higher strains are achieved andbaking temperatures may exceed 170°C to shorten the baking time. In this study, a newgeneration Advanced High Strength Steel (AHSS) grade TBF 1050 was characterized formetal forming purposes and its bake hardening response was studied both as the standardsuggests and as the real production cycle dictates.