Abstract: It was found that the tarnsverse corner cracks occurring on the Al-Si killed medium carbon steel cc slab produced by Baoshan Iron and Steel Company.The hot ductility of GR4151 cc slab was measured with Gleeble-1500 tensile test machine.Some of the fractures were examined by means of optical microscopy and SEM.There exist two low ductility temperature regions for the GR4151 steel within the temperature range from melting points to 700℃,i.e.Region I from the melting point to 1330℃ and Region Ⅲ from 860~740℃.In Region Ⅲ,the ductility loss is due,in part to strain-induced precipitation of the fine particles of AlN at the austenite grain boundaries and, in part to the austenite-ferrite phase transformation.Counter measures against transverse corner cracks were proposed.