Iron oxide scale can be divided into primary iron oxide scale, secondary iron oxide scale, tertiary iron oxide scale and red iron oxide scale.
Primary iron oxide:
Steel is often heated and insulated at 1100~1300 °C before hot rolling. At this temperature, the steel surface is oxidized by contact with the high-temperature furnace gas to produce a primary scale of 1 to 3 mm thick and insufficient lateral pressure by rough rolling and incomplete descaling. This primary scale is also referred to as an iron oxide scale. There is a large cavity inside the primary scale, and the primary iron oxide scale is a gray-black scale layer, which covers the surface of the steel sheet in a sheet shape. The main component of the scale is composed of magnetite (Fe3O4).
Secondary iron oxide scale:
After the hot-rolled billet is discharged from the heating furnace, the scale is removed by high-pressure water, that is, the surface iron oxide peels off and rough rolling is performed. In the short-time rough rolling process, the surface of the slab is in contact with water and air, and a secondary scale is formed on the surface of the slab, which is also called a primary iron oxide scale. The secondary scale is affected by horizontal rolling, and the thickness is thin. The interface stress between the billet and the scale is small, so the peelability is poor. If the high-pressure water is sprayed, the secondary scale cannot be completely removed, and the scale is left on the surface of the steel sheet for finish rolling, and defects appear on the surface of the product. The secondary scale is red scale, which is obviously long and pressed, and is distributed along the rolling direction. The main components of the scale are composed of particles such as ferroalloy (FeO) and hematite (Fe2O3).
Three iron oxide scales:
During the hot rolling and finishing process, the surface iron oxide skin layer is produced when the strip enters each rolling mill. Iron oxide scales are again produced by the final descaling after rolling or between each rolling mill. Therefore, the surface condition of the strip under the action of the rolls will depend on the amount and characteristics of the scale formed before entering each of the stands. The iron oxide skin at this time is called tertiary iron oxide skin because it is after descaling. Formed before entering the finishing mill.
Three times of iron oxide defects are visible to the naked eye:
Dark brown, small boat. It is relatively dense, fine and scattered in the surface of the defective strip. It has a fine touch and a pinhole-like small pit with different depths in the surface defects of the strip after pickling. They are in normal hot rolled strip. The surface is invisible. The results of the defect strip and normal strip surface under low magnification metallurgical microscope are shown in Figures 3-4 and 3-5:
Red iron oxide scale:
The red iron oxide scale only occurs on specific steel grades such as high silicon content, mainly due to the strong meshing of the surface oxide with the base metal during the heating of the billet. No obvious depth, irregular flaky.
There are two types of red iron oxide skin:
One type is non-uniformly distributed in the width direction of the board, mainly distributed in the middle, and is biased toward the operation side. The red and blue areas have obvious watermarks, and are not uniform in the longitudinal direction of the steel sheet, and the individual parts are slightly lighter. The red iron oxide scale is thicker and can be collapsed when straightened. It can be blown off by high-pressure wind. The residual red color is easy to wipe off. The red iron oxide scale is called red rust.
Another type of red iron oxide scale is evenly distributed along the width of the plate, generally slightly heavier than 100mm in the side, and the outer part of the roll is heavier than the inside. This red iron oxide scale is thin and difficult to rub down. The thicker the steel plate, the heavier the red. This kind of red iron oxide is also present in some other steel grades and has certain universality.
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