Mechanical Processes for Descaling of Steel Sep08

Mechanical Processes for Descaling of Steel...

Mechanical Processes for Descaling of Steel Scale is the product of oxidation which takes place during hot rolling. The oxidation and scale formation of steel is an unavoidable phenomenon during the process of hot rolling which involve reheating of steel in a reheating furnace, multi-pass hot rolling and air-cooling in the inter-pass delay times and after rolling. Scale formed during the heating of steel to rolling temperatures in the reheating furnace is known as primary scale. This primary scale is removed before hot rolling. It is usually done for producing steel products with high surface quality and for reducing roll wear. However, secondary scale continues to form on the descaled steel surface during the inter-pass delay time in the roughing and intermediate rolling mills. The colour of primary mill scale is generally bluish black while that of the secondary scale is blue.  The secondary scale gives the steel an appearance which is similar to that of a lacquer coating finish and is often mistaken for a blue coloured primer. The primary scale is composed of three well defined layers of iron oxides. Adjacent to the steel is the thickest layer consisting of wustite having an approximate composition of FeO. The intermediate layer consists of magnetite (Fe3O4) while the outermost layer is hematite (Fe2O3). The thicknesses of these layers depend on several factors linked to the rolling of the steel and the availability of oxygen at the steel surface. The layer at the surface of the steel is richest in oxygen and constitutes 0.5 % to 2 % of scale thickness. The layer at the metal surface is richest in iron and constitutes about 85 % of the scale thickness. The intermediate layer of scale constitutes around 13 % to 14.5 % of scale thickness....