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

Abstract

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 Ti3O5 inclusions rather than Ti2O3 and TiO2 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.

Keywords

• Ti-bearing inclusions
• Microstructure
• Thermodynamic calculations
• Ship hull steel