Abstract: Aiming at the problem of severe edge-cracking defects of ultra-low carbon steel hot rolled strips for cold rolled sheets of automotive produced by Qiansteel, where the occurrence rate is as high as 30%. The severe burr defects would appeared after cold rolling and difficult to polish clean lead to iron chip adhesion on furnace of rolls in the continuous annealing furnace area, and it formed scale tumors and caused periodic indentation defects that seriously affect the surface quality of automotive sheets. Through microscopic analysis of the defects, a large number of oxide particles and copper element enrichment were found around the defects, tracing the root cause could be backed to before slab heating, i.e. the precipitation and enrichment of copper elements on the slab surface were the fundamental cause of the edge-cracking defects. Combining the macroscopic law that the occurrence rate of edge-cracking defects in flame-cleaned (abbreviated as mechanical cleaning) slabs is as high as 30%, while non-mechanical cleaning slabs show no such defects, inspections on the quality of mechanical cleaning slabs and mechanical cleaning equipment were conducted. It was determined that the root cause of edge-cracking defects is the local burning damage of copper foil in the narrow-face burner gap during mechanical cleaning of slabs, leading to copper element infiltration into the narrow face of the slab and inducing "copper embrittlement"-type defects. Effective solutions to the edge-cracking defects were achieved through measures such as controlling mechanical cleaning burners and optimizing the heating and scale removal processes in the hot rolling process.