Abstract: The enrichment mechanism of Cu, As and Sn in HSLA continuously cast slabs and medium plates were studied by scanning electron microscope and X-ray energy dispersive spectroscopy (X-EDS). It is shown that Cu, As and Sn are simultaneous enriched in the scale and the scale/steel matrix interface of continuously cast slabs. There are Cu, As and Sn enriched phases in the scale/steel matrix interface of medium plates, and the contents of Cu, As and Sn in the steel matrix are large than those in the scale. The contents of Cu, AS and Sn at grain boundaries of hot rolled plates are more than those in grains. Segregation of Cu, AS and Sn at austenite grain boundaries and selective oxidation of Fe result in enrichment of Cu, As and Sn, reheating in the reheating furnace exacerbates the enrichment, and the penetration and diffusion of Cu, As and Sn toward the steel matrix deteriorates the hot ductility of steel tremendously, which lead to lots of surface microcracks in medium plates. The effect meehanism of Cu, As and Sn enriched phases on the surface microcracks was analyzed.