Study On Formation And Shedding Control Of Oxide Layer In Hot Rolling Of 700XL Steel

Hot rolled high strength steel has the benefits of energy saving,reducing cost reduct,but it is easy to produce a large number of oxide scale in the rolling process,which causes environmental pollution and product surface defects.In order to improve the surface quality of hotrolled high strength steel and solve the problem of oxide layer falling off,this paper takes 700 XL steel as the research object,adopts simulation experiment and combined with field process system,the structural transformation and peeling off of oxide layer are studied by means of SEM,EBSD,EPMA,XRD and other detection,so as to control the oxide scale and provide guidance for on-site production.Results indicate that:The oxidation rate of 700 XL steel is very slow below 850℃,and the oxidation weight gain curve conforms to the linear growth law;The oxidation rate increases sharply above 850℃,and the curve conforms to the logarithmic growth law.The oxidation activation energy of 700 XL steel is 198.94KJ/mol.With the increase of rough rolling inlet temperature,the thickness of Fe₃O₄ layer is almost the same,the thickness of FeO layer increases gradually,and the proportion of FeO increases,the lower exit temperature of finishing mill is beneficial to the formation of Fe₃O₄.FeO phase transition is mainly affected by coiling temperature and cooling rate.The coiling temperature above 600℃ is not conducive to the eutectoid transformation of FeO;When coiling temperature drops to 550℃,FeO begins to precipitate a large number of proeutectoid Fe₃O₄ particles and a small amount of eutectoid structure;When coiling at 450⁵00℃,the grain size of oxide layer is small,which promotes the formation of eutectoid structure,slow cooling is conducive to the transformation of eutectoid structure.The peeling rate of oxide layer is mainly affected by the thickness and structure of oxide layer.The larger the thickness of the oxide layer,the less the residual FeO content and the more the Fe₃O₄ eutectoid structure,the lower the exfoliation rate of the oxide layer.The thickness of the oxide layer can be reduced to about 10μm by controlling the furnace temperature to 1200℃,rough rolling inlet temperature to 1040℃,finishing rolling outlet temperature to 830℃,coiling temperature to 550℃ and slowly cooling to room temperature,FeO is completely transformed into Fe₃O₄ and eutectoid structure,the defects of oxide layer on the surface are basically improved,and the comprehensive properties are good.