Analysis on the Formation of Surface Crack on Casting Bloom of Hypo-peritectic Steel Q345B

Large amounts of surface cracks were found on the 300 mm × 360 mm continuous casting blooms for hypo-peritectic steel produced by EAF + LF + VD processes after cooling.The cracks were analyzed by optical microscopy,SEM and EDS,and thermo dynamical calculation was carried out at the same time in order to find out the reasons for formation of the cracks.The results indicate that decarburized layer and ferrite film are found near the cracks and enrichment of P and S found at the cracks; inclusions containing elements of Al,Mg,Ca and Si wreck continuity of the steel base; mold flux and precipitates have significant effects on surface quality of the casting bloom.In order to protect casting bloom from crack forming,it is necessary to control the superheat of steel between 15 and 30℃,maintain the liquid powder layer between 8.5 and 9 mm,and increase the crystallization rate of mold powder.Keeping the N content of the steel at 35 × 10^-6 to 45 × 10^-6 and increasing the Ti treatment effeciency to decrease the amount of AlN in bloom also contributes to improve the quality of the casting bloom.