Improvement of Impact Toughness of the Welding Heat-Affected Zone in High-Strength Low-Alloy Steels through Ca Deoxidation

Thick high-strength low-alloy (HSLA) steel plates with excellent large heat input welding properties are increasingly demanded to build super-large container ships for the globally flourishing marine trade. This article describes the effect of Ca content on the TiN particles, microstructure and impact toughness of the simulated coarse-grained heat-affected zone (CGHAZ) in HSLA steels after large heat input welding at 400 kJ/cm. The quantitative analyses on TiN particles demonstrate that the number densities of the submicron- and nano-scale particles increase with increasing Ca content, but the particle sizes appear to be independent of the Ca content. The prior austenite grains in the CGHAZs of the HSLA steels are comparably coarse, including a small number of extremely large grains, except for 25Ca steel, whose grains are relatively fine and uniform. Moreover, only the impact toughness of the CGHAZ in 25Ca steel is satisfactorily improved with a small test variability, and this could be attributed to the improved precipitation of the submicron-scale TiN particles that effectively refine the grains. Based on the experimental and thermodynamic results, it is strongly recommended to increase the Ca content to > 0.0019 wt pct for the related HSLA steel system in the future steel optimization.