The Effects of Boron and Stabilization Chemistry on the Spot Weld Fatigue and Secondary Cold Work Embrittlement of Hot Dip Galvanized Interstitial-Free Steels 2000-01-2700

The fatigue strength of conventionally produced spot welds on a series of interstitial-free (I-F^®) and low carbon (LC) hot dip galvanized steels was experimentally investigated. The secondary cold work embrittlement transition temperature of cups drawn from the same series of materials was also evaluated. The goal of the investigation was to determine the effects of carbon content, stabilization chemistry and boron additions on the lap shear and cross tension spot weld fatigue (SWF) and secondary cold work embrittlement (SCWE) properties of hot dip galvanized sheet steels.

Testing was conducted on titanium and titanium plus niobium I-F^® steels both with and without boron, one standard low carbon steel and one low carbon steel containing sufficient titanium and niobium to fully stabilize the carbon. The low carbon material was tested in nominal gauges of 0.8 and 1.2 mm while the base titanium-stabilized I-F^® steel was tested at nominally 0.8, 1.0 and 1.2 mm gauges.

The standard titanium-stabilized interstitial-free chemistry showed lower SCWE and SWF resistance than the low carbon steel. While the dual-stabilized (titanium plus niobium) I-F^® steel was not found to be significantly superior to the standard titanium stabilized material in spot weld fatigue, niobium additions did significantly reduce the SCWE transition temperature. The addition of boron drastically improved the spot weld fatigue performance of both the Ti- and dual-stabilized I-F^® steels, and reduced the SCWE transition temperature of the Ti-stabilized material substantially. The addition of 6 ppm boron to a 0.986-mm Ti-stabilized I-F^® steel improved the lap shear spot weld fatigue performance to equal that of a 1.212 mm Ti-stabilized I-F^® steel without boron additions.

I-F^® is a registered trademark of AK Steel Corporation Unfortunately, direct comparisons between I-F^® steels containing boron and low carbon steel were not possible due to thickness differences between the materials. However, the application of a simple thickness normalization routine shows the spot weld fatigue performance of the low carbon and the boron-containing I-F^® materials to be equivalent.