Surface Transversal Crack on S355ML Beam Blank

The causes of surface transverse crack on S355 ML beam blanks had been investigated by metallographic,SEM,TEM observations and nozzle flow field analysis,in combination with thermal imaging temperature of continuous casting billet. Results show that the flow field distribution of through-type nozzle is unsuitable. The surface of beam blank exist great temperature gradient,especially from top of flange to the web R Angle,thus profiled billet fall in brittle zone during straightening. Furthermore,lots of carbonitride precipitation in S355 ML shaped blank damage high temperature ductility of steel,and thus increase the slab crack sensitivity. By optimizing secondary cooling system and controlling composition at narrow range,the billet can be avoided to fall in Ⅲ brittle zone when being straighten and then slab surface quality can be improved.