The outer iron sheet of ND steel pipe mainly consists of four kinds: primary oxide sheet, secondary oxide sheet, tertiary oxide sheet and red oxide sheet. Each layer of iron oxide scale is divided according to its production process and specific material. Finally, four kinds of iron scale are combined to form the outer structure of ND steel pipe. For four kinds of outer iron sheets, we have a simple understanding. 一次氧化铁皮：钢在热轧前，往往要在１１００～１３００℃加热和保温。在此温度下，钢表面于高温炉气接触发生氧化反应，生成１～３ｍｍ厚的一次鳞以及由粗轧侧压不充分、除鳞不彻底所致。该一次鳞也称为一次氧化铁皮。一次鳞的内部存在有较大的空穴，nd钢管一次氧化铁皮为灰黑色鳞层，呈片状覆盖在钢板表面。
Primary scale: Before hot rolling, steel is often heated and kept warm at 1100 - 1300 C. At this temperature, oxidation reaction occurs on the steel surface in contact with high temperature furnace gas, resulting in a primary scale 1-3 mm thick and caused by inadequate side pressure and incomplete descaling during rough rolling. The primary scale is also called primary iron oxide scale. There are large holes in the primary scale. The primary oxide scale of ND steel pipe is grey-black scale, which covers the surface of steel plate in sheet form.
The scales are mainly composed of magnets (Fe3O4). Secondary oxide scale: After hot-rolled billet comes out of the heating furnace, the scale is removed by high pressure water, that is, the oxide scale on the surface falls off and rough rolling is carried out. During short roughing process, the surface of billet contacts with water and air, and the surface of billet produces secondary scale. The ND steel pipe is also called primary oxide scale.
The thickness of secondary scale affected by horizontal rolling is thin, the interface stress between billet and scale is small, and the stripping property of ND steel pipe is poor. If spraying high-pressure water can not completely remove the secondary scale, and the scale remains on the surface of the steel plate for finishing rolling, defects will appear on the surface of the product.
The secondary oxide scale is red scale. The ND steel pipe is obviously long and pressed, and distributes in strip along rolling direction. The scale is mainly composed of iron ore (FeO), hematite (Fe2O3) and other particles.
Third-order oxide scale for ND steel pipe:
In the process of hot rolling and finishing, iron oxide coating will be produced on the surface of strip when it enters each rolling mill. Iron oxide scale will be produced again after rolling through final descaling or between each rolling mill. Therefore, the surface condition of strip under roll action will depend on the quantity and characteristics of oxide scale formed before entering each stand of rolling mill. At this time, the scale of iron oxide is called tertiary scale, because it is after descaling. ND steel pipe formed before entering finishing mill.
The third iron oxide scale defect is visible to the naked eye.
Black-brown, boat-shaped. Relatively dense, fine and loose sand distribute on the surface of defective strip steel, which has a fine feel. After pickling, pinhole pits with different depths are left on the surface of defective strip steel. They are invisible on the surface of normal hot rolled strip steel.
Red oxide scale:
Red oxide scale only occurs on special steel grades with high silicon content, mainly due to the strong meshing between surface oxide and matrix metal during billet heating. No obvious depth, irregular sheet.
There are two kinds of red oxide scales:
A kind of non-uniform distribution in the direction of plate width, mainly distributed in the middle, biased to the operation side, ND steel pipe red and blue have obvious watermarking, in the direction of steel plate length is not uniform, some parts are slightly lighter. This kind of red oxide scale is thicker, can collapse when straightening, can be blown away by high-pressure wind, residual red is easy to rub off color, this red oxide scale is called red rust more appropriate.
Another kind of red oxide sheet distributes evenly along the width of the sheet, which is slightly heavier within 100 mm of the edge and heavier outside than inside of the ND steel tube coil. This kind of red oxide scale is thinner, and it is not easy to rub off the color. The thicker the steel plate, the heavier the red. ND steel pipe, a kind of red oxide scale, also exists in some other steels, which has certain universality.