Surface Decarburization Control Of Hot-rolled Steel Strip During Hydrogen Reduction Descaling

The hydrogen reduction is environmentally friendly.However,the surface of strip steel will generates the decarburization layer and reduction layer and grain coarsening after hydrogen reduction.These defects may be conserved in finished product after cold rolling and annealing.Therefore,the experimental study on controlling surface decarburization during hydrogen reduction is of great signification for the popularization and application of hydrogen reduction descaling process.The Q235 carbon-steel is used as raw experimental materials.The oxide scale on the sample surface is reduced by homemade furnace with different reduction atmosphere,different reduction temperature and different reduction time.The reduction atmosphere includes hydrogen mixed with carbon monoxide,hydrogen mixed with ethanol and hydrogen mixed with solid carbon.Reduction temperature is600?~900? and reduction time is 1min~15min.The cross-section microstructures of samples are observed by SEM and optional microscopy.The surface hardness of the samples is tested by HVS-1000 digital Vickers.The microstructures and properties of the samples before reduction and after reduction under different conditions are analyzed.We project the comparison experiments to explore the decarburization kinetic in the process of reduction.We study internal organization evolution of the samples reduction in different atmosphere,detailed analyze the influence of annealing temperature and annealing time on microstructure and the reasons of decarburization on strip steel surface after hydrogen reduction descaling.The experimental result shows that the cross-section microstructure of strip steel after hydrogen reduction include reduction layer,semi-decarburization layer,decarburization layer and steel matrix.There contains a small amount of carbide in the decarburization layer close to the strip steel matrix,so the carbon content there higher than near the surface of strip,but the cross-section microstructure of specimens after pickling is uniform and no decarburization phenomenon.During hydrogen reduction descaling process,oxide deoxidizing on the surface of strip steel accompanied with decarburization of matrix,the total weight reduction including oxygen reduction and carbon reduction.The thickness of the decarburization layer increases with the increasing of reduction temperature and reduction time,,the color of the decarburization layer brighter and the total weight reduction increased gradually.The acid pickled and cold-rolled samples decarburization effect is obvious in wet hydrogen,which shows that decarburization phenomenon should be attributedto the water vapor that produced by iron oxide and hydrogen during hydrogen reduction process.After high temperature annealing,the acid pickled and cold-rolled samples have taken place in recovery,recrystallization and grain growth phenomenon,the higher annealing temperature and longer annealing time,the more serious grain growth.From the steel substrate to the decarburization layer,the grain size increases gradually.In the process of hot-rolled strip steel reduction descaling,if the reduction atmosphere is hydrogen mingled with carbon monoxide,then adjust the proportion of them to 18.5:1 approximately;if the reduction atmosphere is hydrogen mingled with ethanol,then adjust ethanol flow velocity to 45ml/h ~ 51 ml/h when the flow of hydrogen is 100L/h;If the reduction atmosphere is hydrogen mingled with solid carbon,then join enough charcoal when the flow of hydrogen is 100L/h.The above methods can reduce even eliminate decarburization phenomenon on the surface of hot-rolled strip steel.Using the above three methods can completely control the decarburization defect on the surface of steel strip in the process of hydrogen reduction descaling.