Control of non-metallic inclusions and micro-structure of an Al-killed steel through adjusting titanium content

Effect of titanium on the precipitation of Ti-bearing inclusions, microstructure and mechanical properties of an Al-killed high-strength steel was investigated coupling thermodynamic calculations. Titanium wire was fed into the molten steel at the mold. Intragranular acicular ferrites and polygonal ferrites were observed in the Ti-bearing steel after welding thermal simulation tests, whereas the titanium-free steel consisted mostly of polygonal ferrites. The thermodynamic calculations proved the generation of Ti 3 O 5 inclusions rather than Ti 2 O 3 and TiO 2 ones in the molten steel, especially under the condition with a 1.78 ratio of the dissolved titanium [Ti]/the dissolved aluminum [Al] and a <60 ppm [Al]. The welding thermal simulation test demonstrated that Ti-bearing inclusions promoted the nucleation of intragranular ferrites in the welding heat-affected zone (HAZ). In the Ti-free steel, the formation of ferrite side-plates, which are often present in the HAZ, was prohibited by the formation of intragranular ferrites.