| Cold-rolled strip steels often undergo color difference defects after annealing in a bell furnace.The occurrence of surface defects not only affects the product yield,but also increases economic losses.The surface quality of the strip is an important indicator of the quality of the cold-rolled strip.In this paper,the discoloration of the cold-rolled strip hood is taken as the research object,and a combination of theoretical analysis,experimental research and numerical simulation is used to analyze the influencing factors of the hood's discoloration.The correlation between strip surface roughness,rolling mill emulsion,steel coil exit temperature,etc.and the fading discoloration difference were analyzed.Testing equipment such as XPS,SEM,SPM,VHX-5000 were used to test the surface composition,morphology,and roughness of the light yellow color difference;the field insert test was used to monitor the heating process of the steel coil during the annealing of the bell furnace;The finite element software performs numerical simulation analysis on the temperature field and hydrogen flow field in the coil during the annealing process of the full hydrogen hood furnace.The main research results are as follows:(1)The surface layer of the cold-rolled strip cover has a layer of iron oxide film and a carbon film,and the thickness of the iron oxide film is 10-20 nm.The surface layer without color difference contains a layer of iron oxide film with a thickness of 5-10 nm.(2)The average surface roughness of the fading area of the mask was 1.06 ?m,and the average surface roughness of the strip steel at no color difference was 0.95 ?m.The surface roughness of the two ribs at the edge of the strip is large,and residues such as emulsions are easily attached.Oxygen that cracks out of the residue after annealing oxidizes with the strip to produce color difference.(3)The surface of the cold-rolled strip steel sample coated with emulsion showed yellow,blue,and purple after annealing.For 2 # emulsion,the temperature change interval with obvious color difference is 400 ?-450 ?.The temperature change range of 4 # emulsion with obvious color difference is 450 ?-500 ?.(4)The temperature at which the steel coil comes out of the oven indicates that the critical point at which the yellowish color difference appears is 160 ° C.(5)The field insert test results show that the temperature of the coil during the withdrawal process is unevenly distributed.During the heating stage,the temperature difference between the hot spot and the cold spot outside the coil is between20-200 ?;during the cooling stage,the temperature difference between the hot spot and the cold spot outside the coil is 30-120 ?.After the temperature of the steel coil is high,it will lead to different degrees of oxidation.(6)The numerical simulation results show that the flow velocity of the hydrogen gradually increases when it starts to flow down the central axis of the coil after passing the top.The flow velocity of hydrogen when passing through the position ofthe convection plate is below 12 m/s.The cold spot temperature of the steel coil increases significantly when the hydrogen velocity is 39.3 m/s.In the heat preservation stage,the cold spot and hot spot temperature of the steel coil held for 5hours was the lowest,and the cold spot and hot spot temperature of the steel coil held for 9.5 hours was the highest.During the whole annealing process,the different flow rates of hydrogen have no effect on the temperature uniformity of the coil,but the different holding time causes the temperature of the coil to be unevenly distributed.The coil is heated unevenly,and the coil is liable to produce color difference during annealing. |
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