Research On Anti-oxidation Coating Of Billet At High Temperature

In hot-working process,high-temperature oxidation is one of the main reasons for the mass loss of iron and steel materials,not only increasing energy consumption and production costs,but also destroying surface quality of billet.Three main methods reducing the high-temperature oxidation loss of billet are inert atmosphere protection,vacuum protection and high-temperature coating protection.Compared with the other two methods,high-temperature coating protection has the advantages of simple operation,wide application,low costs,becoming the hot spots in recent years.The research and application situations of anti-oxidation coating were reviewed in this paper.Aiming at the applicable temperature of the existing oxidation resistant coatings being not high,limited protection performance and single action,taking the billet before rolling as the research object,a kind of silicon nitride based anti-oxidation coating was prepared,effectively reducing the oxidation loss of billet at high temperature of 1200?.Different with most silicate glass coatings using protective mechanism of inert melt shield,this coating uses three kinds of mechanism of low-temperature mutual melting reaction,consuming reaction and high-temperature inert film shield at the same time,being suitable for high temperature and having large protection range.According to the performance requirements of anti-oxidation coating,the physicochemical stability,surface tension and thermal expansion coefficient of coating materials in high temperature were considered.The best coating formula of50.85%Si₃N₄,7.63%Si C,6.35%H₃BO₃,10.17%Mica powder,10%Al₂O₃,8%Zr O₂,4%Ti O₂,2%Ca O,1%Mg O,5%of sodium silicate,45%of silica sol and 4%of bentonite of the mass of powder was getting by means of coating composition explore-experiments and orthogonal experiments.The protective performance and mechanism of the coating in high temperature were studied by comparing oxidation weightlessness,analyzing SEM morphology,metallography and phases by X-ray diffraction of the steel samples.It shows that:(1)The silicon nitride based anti-oxidation coating can reduce the oxidation loss of steel samples by 48%after keeping warm for one hour at the temperature of1200?.The increasing extent of the loss rate of the coated steel sample was lower with holding time prolonged.The stability of the coating at high temperature was good.The average oxide thickness on the steel surface is about 45?m after coating protection,reducing about 78%.The volume of surface oxide turned smaller.The oxide edge was smooth.The matrix elements were distributed evenly,avoiding sharp fluctuations in composition,improving the surface quality of steel samples obviously.And meanwhile the coating had good effect on preventing decarburization,the total decarburization layer on the steel surface disappearing,only existing a small part of the decarburization layer.The depth of decarburization layer decreased significantly overall.(2)The B₂O₃ provided protection at low temperature by mutual melting reaction,making coating film-forming earlier by analyzing the protective mechanism of the coating by XRD.The Si C,generating Si O₂ and reducing contact between iron and oxygen,provided protection at middle temperature by consuming reaction.The Si₃N₄provided protection by inert film shield.And the Si₂N₂O and Mg Al₂O₄ were found in coating at 1200?,helpful to increase the compactness of the coating,preventing oxygen from diffusing into the coating.The performance of anti-oxidation of the coating was improved at high temperature.