Effect of titanium on the weldability of thermomechanically rolled high-strength cold-formable steels

This research is concerned with the effect of titanium in the range 0.01 to 0.13 wt. % on the base plate and weldability properties of laboratory hot-rolled cold-formable steels with yield strengths in the range 500 - 900 MPa (S500MC - S900MC). Different strength levels were achieved by varying the contents of boron, chromium, molybdenum and manganese. For the base metal, titanium had a small strengthening effect and it also raised the impact transition temperature. In MAG welding (t8/5 = 5 s), titanium had a strengthening effect probably due to precipitation strengthening which was seen both in the strength of the welded steels and in the lower hardness difference between the base metal and the HAZ. Titanium, especially with higher concentrations, had a clear negative effect on the impact toughness of the fusion line and a somewhat smaller negative effect on the impact toughness of the fusion line + 1 mm position. This was probably due to the presence of large angular TiN inclusions promoting cleavage crack nucleation. Also unlike small TiN particles precipitated from the solid state, these inclusions are ineffective in preventing austenite grain coarsening in the coarse-grained HAZ (CGHAZ). Overall, high titanium contents have been shown to have a detrimental effect on HAZ properties: although titanium reduces the softening of the HAZ experienced in these types of steels, it has a clear negative effect on the HAZ impact toughness with low heat input.