Material Characterization of Uncoated Boron Steel for Automotive Body Structure Applications

Abstract

Use of Advanced High Strength Steels in automotive applications is increasing. One of these materials is boron steel, which is commercially available in coated and uncoated sheets. Automotive manufacturers are using boron steel in body structure applications to produce light weight parts and to address safety requirements. Boron steel is available in a non heat-treated condition (also referred to as “green state”) which typically has a yield strength around 350 MPa. The yield strength for a fully temperature hardened boron steel increases to above 1000 MPa, depending on heat treatment temperature and quenching methods used. In this report, the static and fatigue properties of uncoated boron steel were evaluated. One objective was to understand whether these properties varied with respect to the material rolling direction (longitudinal, transverse and diagonal). For static strength analysis three different gages (1.0 mm, 1.5 mm and 2.0 mm) were evaluated. For fatigue evaluation, 3.0 mm thickness boron steel was evaluated. Based on the mechanical test data, ultimate tensile strength was not statistically significant in all three directions (longitudinal, transverse and diagonal) among three gages chosen. However, within the same gage, ultimate tensile strength is statistically significant in all three directions. 0.2% offset Yield strength and total elongation are uniform in all gages as well as in all three directions within each gage. However, uniform elongation (at max. load condition) was significant among the gages as well as within the same gages. A comparison of the monotonic and cyclic stress strain curves indicates boron steel is a strain-softening material.

Elemental segregation during resistance spot welding of boron containing advanced high strength steels

“…It was suggested that the higher boron content steel resulted in a larger weld because it exhibited a higher contact resistance during resistance seam welding. A new generation of borontreated automotive steels are currently of interest because they can be hot stamped and subsequently quenched to produce a martensitic structure with high strength, typically in the range of 900 to 1200 MPa [17]. Studies on the effect of boron content on the weldability of high-strength steels indicate that increasing boron concentration above 100 ppm in the weld metal results in cracking [18].…”