Study On The Formation And Control Of Pitting Defects In Hot-rolled Pickling Plate

In recent years,automobile,machinery,light industry and other industries develop rapidly,huge market competitiveness makes companies’ pursuit of production costs and product quality increasingly.With the characteristics of low cost,high surface quality,precision and surface smoothness,hot-rolled and pickled sheet has been recognized by an increasing number of users.However,due to the high surface quality requirement,it is more difficult to produce hot-rolled and pickled sheet than to produce common hot-rolled sheet.The high-efficient production of hot-rolled and pickled sheet has been restricted at home by the defects such as the press-in of scales on the surface of the steel plate,landscape painting and overpickling.Especially for the production of extreme-dimensioned high-strength hot-rolled and pickled sheet,the aesthetic measure has been severely affected by pitting defect on the surface of the steel plate.Up to now,the domestic iron and steel enterprises are still unable to stably control the pitting defect,particularly the old hot rolling production lines show more limited control ability due to the insufficient equipment capacity.In this paper,the extreme-dimensioned high-strength hot-rolled and pickled sheets are provided by a domestic enterprise and they are produced by a 1580 hot rolling production line of this enterprise.The effects of Si on scale formation kinetics were investigated using thermal gravimetric analyzer(TGA)and electron probe microanalyzers(EPMA).The numerical calculation program of roll temperature field of the hot rolling production line in a domestic enterprise was developed by the use of C++ programming language and the relationship between working roll temperature and cooling system of the finishing mill group in this enterprise was simulated using the developed program.Finally,the experimental and simulated results were applied in the field production and the production process parameters were optimized by tracking the production process,which significantly reduced the occurrence rate of pitting defect of the hot-rolled and pickled sheet produced by a domestic enterprise.The main works and results of this paper are as follows:(1)The effects of temperature and alloying element Si on scale formation kinetics of low Si(Si<0.1 wt.%)high-strength hot-rolled and pickled sheets were studied by carrying out a series of high temperature oxidation weight gain experiments in the temperature ranges of 950～1050℃ and the oxidation kinetics models of the experimental steels were also established.The results showed that with the increase in Si content,the oxidation rate constants corresponding to both slow and rapid oxidation stages decreased,which is due to the fact that Fe2SiO4 formed on the surface of the experimental steels can improve the resistance to oxidation of the experimental steels.With the increase in temperature,the oxidation rate constants corresponding to both slow and rapid oxidation stages increased and the binding ability of the scale to experimental steel matrix was worse.(2)A semi-3d numerical calculation model of roll temperature field was built by finite difference method and the water cooling interface was simulated according to the actual production condition of a domestic enterprise.The results indicated that the pressure,temperature and flow rate of cooling water and the stored water in water-cutting board could affect the temperature of rolls,the change of water-cooled boundary conditions to the roll temperature is also given.(3)The effects of rolling process on the formation of pitting defect were investigated by tracking the production process.And the effects of the roll grinding process,roasting roll process,production scheduling and rolling rhythm on the production of hot-rolled and pickled sheet were analyzed in detail.In addition,the rolling plans about cooling water supply,roasting roll process,production scheduling and rolling rhythm used in a domestic enterprise were improved referring to the results of oxidation kinetics experiments and the simulation calculation of roll temperature field.By tracking the production process,one thing is clear that the occurrence rate of pitting defect of extreme-dimensioned high-strength hot-rolled and pickled sheets produced from 3.0%to 0.5%,achieving the desired effect.