Surface peeling defect occurred in stamping process.QSTE500TM high strength hot-rolled sheet manufactured by Baosteel was applied.A layer of certain substances similar to slag is found underneath the surface about 160 μm by observing its cross-section.By analyzing composition and morphology,it is found that the substances contains alumina( Al2O3) and calcium oxide( CaO) as its matrix and the Ca/Al ratio is quite close to calcium trialuminate( CaO · 3Al2O3 ),and it also contains a small amount of magnesium,sodium,potassium and fluorine correspond to typical mold slag composition.Micro zone analysis indicates a small amount of zirconia ( 2rO2) granularly distributes with a diameter of about 3.5 μm.In continuous casting process,zirconia only has a high content in slag line area of submerged nozzle.therefore cross-section of the new submerged nozzle section is analyzed firstly.The result shows its anti-oxidation coating,porous media and matrix do not contain calcium,magnesium,potassium and fluorine,and magnesium content is quite low.Zirconia granules are only present in porous media and matrix with a diameter above 100 μm.Consequently calcium trialuminate comes from reaction product between calcium in molten steel and material of nozzle.Sodium,potassium and fluorine is from tundish slag.Zircoma is not possibly from matrix of new nozzle.After submerged nozzle is used,the morphology and composition of the internal surface above slag line area indicates calcium oxide reacts with zirconia producing calcium zirconate ( CaZrO3 ) which precipitates on the internal surface dendritically,and its trunk is about 5 μm.However,due to the high temperature of molten steel,there is no zirconia found on the internal surface of lower part of nozzle.Zirconia grains found in defect substances probably come from the internal surface of upper part of submerged nozzle refers to its size and morphology.Given that the substances underneath the surface contains not only the typical composition of slag but also small zirconia grains,it is included that after a series of reactions between upper part of slag line area and molten steel,the defect substances deposits on the internal side of submerged nozzle,then it is scoured and melts into tundish slag and finally goes into rolling process.