Hot-rolled black steel pipe

Hot-rolled black steel pipe is a common steel pipe material. Its name comes from the black coating on its surface, which is mainly used to prevent corrosion of steel pipes during transportation and storage. Hot rolling is relative to cold rolling. Cold rolling is rolling performed below the recrystallization temperature, while hot rolling is rolling performed above the crystallization temperature. To put it simply, a piece of steel billet is heated and rolled several times, then trimmed and straightened into a steel plate. This is called hot rolling. Hot rolling can significantly reduce energy consumption and reduce costs. Hot-rolled black steel pipes are widely used in construction, manufacturing, and various other industries and are favored for their excellent properties.

What is the surface treatment and coating of hot-rolled black steel pipes? In the production process of black steel pipes, the steel billet needs to be heated to the appropriate temperature first, and then processed into square or circular cross-section steel pipes through hot rolling and other processes. Next, hot-rolled black steel pipes usually undergo surface treatment processes such as pickling, rust removal, and grinding to remove surface oxides, rust, and other dirt. Finally, the surface of the steel pipe will be coated. Common coatings include black paint coating and hot-dip galvanized coating. A black paint coating provides an extra layer of protection against corrosion, while a hot-dip galvanized coating protects the steel pipes from corrosion and oxidation.

Advantages and disadvantages of hot-rolled black steel pipe:

1.Advantages

Hot rolling can destroy the casting structure of the steel ingot, refine the grains of the steel, and eliminate defects in the microstructure, thereby making the steel structure dense and improving the mechanical properties. This improvement mainly occurs along the rolling direction, so that the black steel pipe is no longer isotropic to a certain extent; bubbles, cracks and looseness formed during pouring can also be welded under high temperature and pressure.

2. Disadvantages

(1) Black steel pipes appear to be layered (interlayered)

After hot rolling, the non-metallic inclusions (mainly sulfides, oxides, and silicates) inside the steel are pressed into thin sheets, resulting in delamination (sandwiching). Delamination greatly deteriorates the tensile properties of the steel along the thickness direction and may cause interlaminar tearing as the weld shrinks. The local strain induced by weld shrinkage often reaches several times the yield point strain, which is much larger than the strain caused by load;

(2)Residual stress caused by uneven cooling.

Residual stress is the internal self-balanced stress in the absence of external force. Hot-rolled steel sections of various sections have such residual stress. Generally, the larger the cross-section size of the section steel, the greater the residual stress. Although residual stress is self-balanced, it still has a certain impact on the performance of black steel pipes under the action of external forces. For example, it may have adverse effects on deformation, stability, fatigue resistance, etc.

(3)Unable to control the thickness and widening of black steel pipes

We are familiar with thermal expansion and contraction. Even if the length and thickness are up to standard after hot rolling at the beginning, there will still be a certain negative difference after cooling. The wider the side width and the thicker the thickness, the more obvious this negative difference will be. Therefore, for large-sized steel, the side width, thickness, length, angle, and edge lines of the steel cannot be too precise.

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