Factors on Slab Surface Longitudinal Crack of Hypo-Peritectic Steel

摘要

The appearance, grain structure of longitudinal crack and non-metallic inclusions in the longitudinal crack of hypo-peritectic steel slabs were investigated with SEM and microscope method. It was found that, longitudinal cracks often occur at turbulent area around port of SEN (submerged entry nozzle).Longitudinal cracks form in mold, and the short longitudinal cracks in dendrite crystal form on the surfaces (3～5mm from surfaces) of slabs. The cracks extend along the dendritic crystals branches, there are two kinds of fracture type, one is intergranular fracture, and the other is transgranular fracture. Factors on longitudinal crack, casting speed, superheat temperature, ratio of manganese and sulphur, average heat flux and mold level fluctuation were discussed. It was found that high superheat temperature and strong secondary cooling can cause longitudinal cracks on the slab surface, High casting speed results in cracks. Inner curved surface of strand is easier to form longitudinal cracks than outer curved surface of strand under the same casting speed. The longitudinal cracks are promoted by uneven average heat flux of mold.