| Hot-rolled free-pickling process is a process technology can directly enter into the subsequent deep processing by saving the pickling process of strip surface,which makes dark oxide scale mainly composed of Fe3O4 form on the surface of steels through the control of thickness and structure of oxide scale of hot-rolled steel.At present,the free-pickling technology has been widely used in low-strength,thin-gauge hot-rolled automobile girder steels,but the application of the free-pickling technology in high strength and thick gauge steel is still lacking of relevant researches.Based on this,in this paper,high-temperature oxidation behavior of target steel under different conditions was studied by using TGA?SEM?EPMA and other equipment,and high temperature deformation behavior of oxide scales and the effects of coiling temperature and cooling rate after coiling on the structure of oxide scale of the experimental steel were analyzed,finally industrial trial was conducted according to the results of theoretical researches in the laboratory.The main research contents of this paper are as follows:(1)Through the experiment of high temperature oxidation kinetics,the oxidation weight gain curve of the target steel 700L at different temperatures was obtained.The oxidation weight gain and oxidation time of the steel samples in the air showed a parabolic relationship,and the oxidation weight gain per unit area increased with the increase of temperature.By linear fitting of the experimental data,the oxidation rate constant kw at different temperatures was obtained,and the Q value of the oxidation activation energy of the sample steel was calculated.Based on the analysis of alloy elements in the section of oxide scales,the enrichment of alloy elements at the interface between oxide layer and substrate at different temperatures was different.(2)The influence of heating furnace system on the oxide scale of the experimental steel was studied.It was found that the thickness of the oxide scale can be significantly reduced by reducing the final holding temperature in the heating furnace,which will have a positive efect on the subsequent removal of oxide scale.The growth regularity of oxide scale during short-time oxidation was explored,pre oxidation weight gain followed linear law,then it transformed into the parabolic law when the oxidation weight reached a certain value and the oxidation parabolic weight value increased with temperature increasing.The effect of steam atmosphere on high temperature oxidation behavior was studied.It was found that steam can accelerate the oxidation process.With the increase of relative humidity,the thickness of oxide scale increased and the rate constant of oxidation increased.(3)The influence of rolling process on deformation behavior of oxide scale was systematically studied.The results show that the oxide scale had certain plasticity in the range of 900 to 1000?,and the plastic was better with the increase of temperature.The deformation rate of the oxide scale at different temperatures roughly agreed with the reduction rate of the substrate.In the range of 800?880?,the deformation mechanism of the oxide scale was the mixed deformation of plastic deformation and brittle fracture.(4)Coiling temperature and cooling rate have a significant effect on the structure of the oxide scale at the final room temperature.Through the experimental study,it was concluded that the structural transformation of oxide scale was in accordance with the "C" curve under continuous cooling conditions,and the temperature of the nose was between 450?500?.The progress of eutectoid reaction was closely related to the cooling rate.The smaller cooling rate,the more eutectoid product.When the cooling rate was greater than 20?/min,the eutectoid reaction was inhibited.When the coiling temperature was below 400?,the diffusion of ions was inhibited because of the low temperature,and the eutectoid reaction was also hindered.(5)According to the research of laboratory theory,industrial trial of the target steel was conducted by controlling the soaking temperature of the heating furnace,adjusting the finish rolling inlet temperature,finish rolling outlet temperature and cooling rate to control the thickness and structure of the oxide scale in order to satisfy pickling process requirements. |
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