| The temporary protective coating is a simple and effective measure to reduce the oxidation loss of billets in the reheating process before hot rolling.Most of the existing anti-oxidation coatings are mainly designed based on the gas shielding effect of the dense structure of coating,and the effect of ions diffusion on the oxidation of billets at high temperature has not been deeply studied.However,when heated at high temperature for a long time,Fe2+/Fe3+ ions in the matrix will diffuse through the cation vacancy in the oxide scale,resulting in the oxidation reaction at the gas-solid interface,which makes the coatings hardly suitable for the anti-oxidation protection of carbon steel due to the high-temperature failure and long-time failure.In view of the above problems,based on the protection mechanism of gas shielding and the oxidation characteristics of the carbon steel at high temperature,the coupling protective mechanism of gas shielding and ions diffusion inhibition is put forward in this paper according to the ions diffusion law in different crystal structure.Under the guidance of this mechanism,the study on preparation of high temperature protective coating for carbon steel is conducted with typical carbon steel Q235B as the research object The main research contents are as follows:(1)The oxide scale structure and the oxidation characteristics of carbon steel Q235B at high temperature are investigated.At the initial oxidation stage,the oxidation reaction rate of carbon steel Q235B is controlled by the process of interfacial oxidation reaction.When the oxidation reaction lasts for a period of time and the oxide scale reaches to a certain thickness,the oxidation reaction rate of carbon steel Q235B is controlled by the process of ions diffusion in the oxide scale.Due to the presence of C element,the oxide scale of carbon steel Q235B is mainly composed of FeO and Fe3O4,and only a small amount of Fe2O3 exists at the gas-solid interface.Because the content of Si and Mn elements is low,the structure of oxide scale is not significantly affected by them.(2)The diffusion law of O2-ions and Fe2+/Fe3+ ions in different crystal structures are simulated by molecular dynamics method.The diffusion coefficients of Fe2+/Fe3+ions in FeO,Fe3O4,Fe2Al2O4 and Fe2Al4Si5O18 are much higher than those of O2-ions.Therefore,the diffusion of Fe2+/Fe3+ ions dominate in the above four crystal structures at high temperature.Meanwhile,the diffusion coefficients of Fe2+/Fe3+ ions in FeAl2O4 and Fe2Al4SisO18 are much lower than those in FeO and Fe3O4.Namely,compared with FeO and Fe3O4 in the oxide scale of carbon steel Q235B,FeAl2O4 and Fe2Al4Si5O18 can inhibit the Fe2+/Fe3+ ions diffusion at high temperature.Combined with the oxidation characteristics at high temperature and molecular dynamics simulation results of carbon steel Q235B,the coupling protection mechanism of gas shielding and ion diffusion inhibition is introduced on the basis of gas shielding protection mechanism.According to this theory,the design principle of protective coating for carbon steel is established.(3)Based on the inhibition effect of Fe2Al4Si5O18 on Fe2+/Fe3+ ions diffusion at high temperature,cordierite ceramic powders are selected as the raw materials to prepare ceramic coating with Fe2Al4Si5O18 as the interface layer.The protective effect of cordierite based ceramic coating is 18.9%at 1100? for 120 min.Furthermore,the MgO-Al2O3-SiO2 glass-ceramics are prepared with the industrial solid waste of vanadium titanium slag and coal gangue as raw materials,and MgO-Al2O3-SiO2 glass-ceramics coating is prepared with the unnucleated and uncrystallized powders of primary glass as raw materials instead of cordierite ceramic powders.Compared with cordierite based ceramic coating,the porosity of MgO-Al2O3-SiO2 glass-ceramic coating is decreased by 20.88%and anti-oxidation effect is improved by 24.3%due to the presence of amorphous phase.(4)Based on the inhibition effect of FeAl2O4 on the diffusion of Fe2+/Fe3+ ions at high temperature,the Al2O3-SiO2 amorphous ceramic powders prepared by sol-gel method is used as raw materials to prepare Al2O3-SiO2 based ceramic coating with FeAl2O4 as the interface layer.The applicable temperature of Al2O3-SiO2 based ceramic coating increased to 1200? compared with that of MgO-Al2O3-SiO2 glass-ceramic coating.The protective effect of Al2O3-SiO2 based ceramic coating is 63.4%at 1200? for 120 min.On this basis,aluminum powder modified with nano-SiO2(SiO2@Al)is introduced as thermite agent,and Al2O3 powder is used as raw materials instead of Al2O3-SiO2 amorphous ceramic powders to prepare SiO2@Al-Al2O3 composites based ceramic coating.Compared with Al2O3-SiO2 based ceramic coating,the applicable temperature of SiO2@Al-Al2O3 composites based ceramic coating is increased to 1250? due to the decrease of SiO2 content in the coating.The anti-oxidation effect of SiO2@Al composite Al2O3 ceramic coating is 80.3%at 1250? for 120 min.(5)Carbon steel J55,of which the elemental composition is similar to that of carbon steel Q235B,is selected as the research object to carry out industrial test to explain the equipment,process parameters and key issues in the actual application of the coatings.According to the reheating parameters of carbon steel J55,SiO2@Al-Al2O3 composites based ceramic coating is selected to protect it from oxidation at high temperature.According to the experimental results,the applicability of SiO2@Al-Al2O3 composites based ceramic coating to similar kinds of carbon steel is verified.Under the protection of SiO2@Al-Al2O3 composites based ceramic coating,the yield of hot rolling carbon steel J55 is increased by 0.56%,and 5.6 kg per ton steel is saved.The thickness of residual oxide scale on the surface of billet is reduced by 41.3%,and the descaling performance of billets is improved.Apart from the equipment depreciation and wages,the benefit of the coating is about 11.77 yuan per ton,which has a good application prospect and industrial value. |
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