| Mechanical properties and dimensional accuracy controlling problems of hot-rolled plates have made great progress in China,but the surface quality problem is becoming increasingly prominent.Surface defects of hot-rolled plate include pits,residual oxide scale pressed,cracks,nodules,etc.,in which pit is one of the main factors that worsen the steel surface quality.Pits are usually caused by hot-rolled scale oxide defects.By analysis of oxidation kinetics,surface defect types and their formation mechanism and simulated hot-rolled process conditions on the morphology of the oxide scale,oxide scale controlling technology is proposed.With the help of the improved production technology,amendment rate of the target steel due to oxide scale defects is reduced 90%.Main work and results of this study are as follows:(1)Parabolic law is achieved by isothermal oxidation experiments,and oxidative activation energy of A36 and X70 are 188.864kJ/mol and 186.742kJ/mol separately.(2)By analyzing the defect samples collected in the factory the plate surface defects are classified,and mechanism of its formation is also discussed.Pits and pressed residual oxide scale are the most common defects.Pits are caused by crushed residual oxide scale during finishing stage.Pressed residual oxide scale are caused by crushed fluxes from slab surface and secondary oxide scale during roughing stage.(3)By analyzing effects of simulated hot-rolling conditions on morphology of oxide scale,it is found that with rolling temperature increasing,oxide scale thickness increases,and outermost layer of Fe2O3 thickness also increases.Finishing temperature has little effect on oxide scale thickness,but has a significant impact on its structure.(4)By reducing silicon content,adjusting heating temperature and optimizing descaling process,under the premise of guaranteed mechanical properties,amendment rate of the target steel due to oxide scale defects is reduced 90%compared to the rate before process improved. |
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