Study On Surface Quality And Rust Prevention Of Cold-rolled Bell-type Annealing Strip

Cold-rolled strip steel products are widely used in various sectors of the national economy such as automobiles,household appliances and construction due to their good surface quality and mechanical properties.With the development of the market,users have put forward higher requirements for the surface quality of cold-rolled strip products,and the surface quality has gradually become a key factor affecting the market competitiveness of cold-rolled strip products.Bell-type annealing strip is one of the most important cold-rolled products.Due to its process characteristics,it has many surface quality problems and is difficult to improve,such as rust,bonding,oxidation color,leveling yellow patches,which have been bothering major steel plants at home and abroad.Among them,surface rust is the most difficult problem for coastal steel plants.Based on the cold-rolled bell-type annealing unit of a coastal steel plant,combined with the production practice,this paper makes an in-depth and systematic analysis and study of the main surface quality defects of cold-rolled bell-type annealing strip steel products,including bonding,oxidation color,leveling yellow patches and rust,gives their occurrence regularity,generation mechanism and influencing factors,and proposes targeted solutions,which have been applied in a steel plant and finally achieved significant improvement effect,creating considerable economic benefits for the unit.The main research results are as follows:(1)Bonding defects on the surface of bell-type annealing strip are more likely to occur in the annealing,cooling and cooling stage than in the heating and heating stage.The occurrence of bonding can be reduced or avoided by controlling the plate type,coiling tension and surface roughness of the rolling mill,reducing the annealing heating speed,holding temperature,cooling speed,improving stacking mode,controlling the flat uncoiling angle,and increasing the uncoiling speed,etc.Through the application of the research results in this paper,the incidence of bonding defects in a steel plant has decreased by 58%,creating an economic benefit of 350,000 yuan per year for the enterprise.(2)There are mainly two types of oxidation color defects on the surface of bell-type annealing strip:yellow edge and white edge.Among them,the yellow oxidation color at the edge is caused by Fe,Mn and other elements at the edge of the strip,which are oxidized by atmosphere during the annealing process and mainly occurs in the cooling stage of the annealing process.The white oxidation color at the edge is caused by the internal oxidation of Mn,Co and other alloy elements at the edge of the strip during the annealing process and the dissolution and diffusion of oxygen elements into the alloy,which mainly occurs in the heating stage of the annealing process.The occurrence of oxidation color can be reduced or avoided by improving the sealing performance of furnace equipment and optimizing the purging process of protective gas.Through the application of the research results in this paper,the occurrence rate of oxidation color defects in a steel plant has decreased by 69%,creating an economic benefit of 1 million yuan per year for the enterprise.(3)The leveling yellow patches on the surface of bell-type annealing strip are caused by electrochemical rust due to the residual leveling liquid.The occurrence of leveling yellow patches can be reduced or avoided by reducing the leveling feeding temperature,improving the surface cleanliness of the strip steel before leveling,increasing the film forming speed of leveling liquid,and improving the purging effect of the leveling machine.Through the application of the research results in this paper,the occurrence rate of leveling yellow patches in a steel plant has decreased by 97%,creating an economic benefit of 1.9 million yuan per year for the enterprise.(4)On the macro level,the rust defects on the surface of bell-type annealing strip mainly show irregular and randomly distributed spot rust with a diameter of millimeter.Microscopically,the corroded parts are loose,and the main components include Al,Si,Ca and other impurity elements in addition to Fe and O.Its occurrence has the following rules:Rust mainly occurs in May to October of each year.The rust level of products of different varieties,steel grades,thickness and width specifications is the same.The rust rate on both sides of strip steel is much higher than that in the middle,especially on the upper side after annealing.The rust rate at the head and tail of strip steel is much higher than that at the middle,while the level at the head and tail is the same.(5)Through on-site production test and using MINITAB statistical tool,the significant factors affecting rust on the surface of bell-type annealing strip are analyzed as follows:cooling time of final cooling station,storage time of intermediate storage,temperature setting of intermediate storage,temperature of leveling liquid,purging pressure of leveling machine,dust in environment,and their influence law on rust is obtained.(6)Based on the analysis and summary of the influencing factors of rust on the surface of bell-type annealing strip,a solution to the rust problem was proposed and applied in a steel plant.After application,the incidence of rust defects decreased by 53%,creating an economic benefit of 1.56 million yuan per year for the enterprise.