Analysis on surface upwarp defects of hot rolled plate of high strength alloying steel Q345E produced by 120 t converter-RH-CC melting process has been carried out by using scanning electron microscope and optical microscope

and it is found that there are ferroferric oxides

residuum of mold shielding slag and secondary oxides in micro-regions of upwarp

and in the upwarp regions the arrangement of ferrite and pearlite is out of order

and the proportion of ferrite is higher than normal with serious decarburization. Further found the main reason for formation of surface upwarp defects of hot rolled plate is the higher argon flow rate in tundish which leads to bubble wrapping slag into liquid and forming subskin blowhole of casting slab

re-oxidizing in heating to form oxide scale then being pressed in plate during rolling. The occurrence rate of upwarp defects decreases from 3. 86% to 0. 8% by adopting the process measures including decreasing argon flow rate in tundish from 65 L/min to 50 L/min and adjusting casting speed from 1.0 m/min to 1.2 m/min.