nd钢管的外层铁皮主要有四大类组成，分别是一次氧化铁皮，二次氧化铁皮，三次氧化铁皮和红色氧化铁皮。nd钢管每一层的氧化铁皮由于其生产工艺和具体的材质不同而进行划分，最终四种铁皮组合后变成nd钢管的外层结构。对于四种外层铁皮，我们来进行一些简单的了解。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。

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. Primary scale: Before hot rolling, steel is often heated and kept warm at 1100 - 1300 (?) 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, as well as inadequate side pressure during rough rolling and incomplete descaling. 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. 鳞层主要成分由磁铁（Ｆｅ３Ｏ４）组成。二次氧化铁皮：热轧钢坯从加热炉出来后，经高压水除去一次鳞后，即表面氧化铁皮脱落，进行粗轧。在短时间的粗轧过程中钢坯表面与水和空气接触，钢坯表面产生了二次鳞，nd钢管也称为一次氧化铁皮。

The scales are mainly composed of magnets (Fe3O4). Secondary oxide scale: After hot-rolled billet comes out of the heating furnace, the scales are 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.

二次鳞受水平轧制的影响厚度较薄，钢坯与鳞的界面应力小，nd钢管所以剥离性差。如果喷射高压水不能完全除去二次鳞，鳞残留在钢板表面的情况下进行精轧，产品表面就会出现缺陷。

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.

二次氧化铁皮为红色鳞层，nd钢管呈明显的长条、压入状，沿轧制方向带状分布，鳞层主要成分由方铁矿（ＦｅＯ）、赤铁矿（Ｆｅ２Ｏ３）等微粒组成。

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.

nd钢管三次氧化铁皮：

Third-order oxide scale for ND steel pipe:

热轧精轧过程中，带钢进入每架轧机时都将产生表面氧化铁皮层。轧制后通过最终的除鳞或在每架轧机之间时还将再次产生氧化铁皮。因此，轧辊作用下的带钢表面条件将取决于进入各架轧机前形成的氧化铁皮的数量和特性。这时的氧化铁皮称为三次氧化铁皮，因为它是在除鳞之后。nd钢管进入精轧机之前形成的。

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 strip surface conditions under the action of rolls will depend on the number and characteristics of oxide scales 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.

三次氧化铁皮缺陷肉眼可见：

The third iron oxide defect is visible to the naked eye.

黑褐色、小舟状。相对密集、细小、散沙状地分布在缺陷带钢表面，细摸有手感，酸洗后在带钢表面缺陷处留下深浅不一的针孔状小麻坑，它们在正常热轧带钢的表面上是看不见的。

Black-brown, boatlike. Relatively dense, fine and scattered sand distributes 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, which are invisible on the surface of normal hot rolled strip steel.

红色氧化铁皮：

Red oxide scale:

红色氧化铁皮仅发生在高硅含量等特定的钢种上，主要由于在钢坯加热过程中，表面氧化物与基体金属强烈啮合所致。无明显深度，呈不规则片状。

The red oxide scale only occurs on special steel grades such as high silicon content. It is mainly due to the strong meshing between surface oxide and matrix metal during billet heating. No obvious depth, irregular sheet.

红色氧化铁皮分两种：

There are two kinds of red oxide scales:

一种在板宽方向非均匀分布，主要分布在中间，偏向操作侧，nd钢管红色与兰色处有明显水印，在钢板长度方向上也不均匀，个别部位稍轻些。这种红色氧化铁皮较厚，矫直时可崩起，可用高压风吹去，残余红色很易擦下色，此红色氧化铁皮称红锈较贴切。

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. The red oxide scale is thicker and can collapse when straightening. It can be blown away by high pressure wind. The residual red color is easy to rub off. This red oxide scale is called red rust.

另一种红色氧化铁皮沿板宽分布比较均匀，一般靠边部１００ｍｍ内稍重些，nd钢管卷外部比内部重些。这种红色氧化铁皮较薄，不易擦下色，钢板越厚红色越重。nd钢管这种红色氧化铁皮其他一些钢种也存在，具有一定的普遍性。

Another kind of red oxide sheet distributes evenly along the width of the sheet, which is slightly heavier within 100 mm on the edge and heavier outside than inside of ND steel tube coil. This kind of red iron oxide scale is thin and difficult to rub off the color. The thicker the steel plate, the heavier the red. The red oxide scale of ND steel pipe also exists in some other steels, which has certain universality.