The Mechanism And Control Of Surface Cracks For Microalloying Wide And Heavy Plate

In order to reduce the rate of surface cracks on microalloying wide and heavy plate, the formation mechanism of plate surface defects occurring in the practical production was investigated by the metallographic observer, scanning electron microscope, and energy spectrum analysis. Meanwhile, the properties of mold flux, the mechanical properties of typical steel at high temperature and cold state characteristics of secondary cooling nozzles which have a direct effect on the occurrence of slab surface crack were studied in the laboratory. Finally, with the adjustment of steel composition, optimization properties of mold flux, casting system and the secondary cooling system, the incidence of various types of surface defects was substantially reduced. The major contents and conclusions are summarized as follows:(1) The forming reasons of cracks were complex, where the high temperature mechanic properties of steel determined by chemical composition of steel was the internal factors of cracks forming, while processing parameters and conditions were external factors.(2) Marginal transversal crack, surface longitudinal crack and surface star crack were generally derived from the original cracks of slab. Marginal longitudinal crack, surface transversal crack and defects such as detonation on plate sidepiece were mainly under the influence of rolling technology and operating conditions.(3) Gleeble3800was used to simulate actual continuous casting conditions, and test high temperature mechanical properties of AH36, Q345E,45and A36-1B. The third steel high temperature brittleness zone mainly determined by the reduction rate of the cross-section was lower than900℃,650℃～875℃,lower than825℃and700℃～900℃respectively, which was very important to the development of continuous casting process and improvement of slab quality.(4) The flux characteristic, water flow density and spraying angle were obtained from the test of the cold state characteristics of nozzles in the slab caster. Meanwhile, the paper analyzed the secondary cooling nozzle layout and the existing secondary cooling system in terms of the testing results of nozzles.(5) With the adjustment of steel composition, optimization properties of mold flux, casting system and the secondary cooling system, the incidence of various types of surface defects was substantially reduced.