Influence of oxide particles and residual elements on microstructure and toughness in the heat-affected zone of low-carbon steel deoxidized with Ti and Zr

The microstructure and toughness in Fe-0.04%C-1.85%Mn-0.03%Si-0.018%Nb steel deoxidized with Ti and Zr have been studied as functions of particle characteristics, austenite grain size and soluble Ti and Zr contents using a simulated HAZ (heat affected zone) thermal cycle (peak temperature, 1 400°C; peak holding time, 60 s; time of cooling from 800 to 500°C, 70 s) and submerged arc welding (heat input of 15 kJ/mm), respectively. Microstructures were studied in samples containing 1.0 to 1.5 μm-diameter oxide particles numbering 500 to 2 000 mm_-2 and with a soluble oxygen content of 10 to 30 ppm (measured before casting) and soluble Ti and Zr contents of 50-150 ppm. The γgrain size after HAZ thermal cycle in the range between 200 and 600 μm is controlled by pinning and solute drag. Small γgrain size below 300μm was obtained with high soluble Ti and Zr contents of 110-160 ppm, whereas large γgrain size above 300 μm was obtained with low soluble Ti and Zr contents of 60-110 ppm. Two types of microstructures that showed high Charpy absorbed energy (VE(-10°C)= 150-250 J and VE(-50°C)= 50-150 J) were observed independent of γgrain size: One is acicular ferrite and a small amount of grain boundary ferrite (GBF) and ferrite side plate (FSP) and the other is GBF, FSP and granular bainitic ferrite. It was observed that low VE(T) values are attributed to the formation of porosity, large-size particles, carbides (+nitrides) and lathe bainitic ferrite.