Study on surface longitudinal crack defects of medium and high alloy steel continuous casting round billet

Φ400 mm P91 grade round billets produced by vertical continuous casting machines are prone to surface longitudinal crack defects. The formation of such longitudinal cracks impairs both the quality of the billets themselves and the surface quality of subsequent finished products. As a peritectic steel, P91 heat-resistant steel exhibits high sensitivity to temperature fluctuations, which easily leads to the initiation of initial microcracks on the billet surface. These microcracks further propagate after the billets pass through the secondary cooling zone and the radiation cooling zone. In this study, the formation mechanism of surface longitudinal cracks was investigated, and the effects of key influencing factors on this mechanism were clarified. Specifically, adjusting the mold cooling system involves increasing the cooling water velocity to 8 m/s, configuring 48 cooling water channels, and setting the water channel diameter to 12 mm. These adjustments effectively reduce the incidence of surface longitudinal cracks.