Advantages and Disadvantages of Hot-Rolled Seamless Steel Pipes

The production processes for seamless steel pipes can generally be divided into two types: cold-drawn and hot-rolled. As the name suggests, hot rolling involves rolling at high temperatures, which reduces deformation resistance and allows for significant deformation. For example, in steel plate rolling, the thickness of continuous casting slabs is typically around 230mm, which is reduced to 1-20mm after rough rolling and finishing rolling. Due to the smaller width-to-thickness ratio of steel plates and the relatively low precision requirements for dimensions, shape issues are less likely to occur, and controlling crown (convexity) becomes the primary focus.

Hot-Rolled Seamless Steel Pipes: Hot rolling is carried out above the recrystallization temperature, in contrast to cold rolling, which is performed below the recrystallization temperature.

Advantages:

Improved Structure and Mechanical Properties: Hot rolling can break down the casting structure of the steel ingots, refine the grains of the steel, and eliminate microstructural defects, resulting in a denser and stronger steel structure. This improvement is mainly evident along the rolling direction, making the steel less anisotropic. Moreover, defects such as bubbles, cracks, and looseness formed during casting can be welded together under high temperature and pressure.

Large Deformation Capability: The high temperature reduces the resistance to deformation, enabling large deformations. This makes hot rolling suitable for producing large-sized steel components with complex shapes.

Disadvantages:

Layering and Inclusions: After hot rolling, non-metallic inclusions (mainly sulfides, oxides, and silicates) within the steel are pressed into thin sheets, leading to a phenomenon known as layering (or lamination). Layering significantly deteriorates the steel's tensile properties in the thickness direction and can cause interlayer tearing when the weld shrinks. The local strain induced by weld shrinkage often reaches several times the yield point strain, far exceeding the strain caused by loads.

Dimensional Accuracy and Surface Quality: Hot-rolled products typically have less precision in dimensions and poorer surface quality compared to cold-rolled products. This is due to the high temperatures causing oxidation and surface scaling.

Seamless Steel Pipes are manufactured using hot rolling and other thermal processing methods, resulting in pipes without weld seams. If necessary, the pipes can undergo further cold processing to achieve the desired shape, size, and performance. Currently, seamless steel pipes (DN15-600) are the most commonly used pipes in petrochemical production facilities.

Based on the manufacturing process, seamless steel pipes are categorized into hot-rolled (extruded) seamless steel pipes and cold-drawn (rolled) seamless steel pipes. Cold-drawn (rolled) pipes are further classified into round pipes and shaped pipes.

Process Overview:

Hot-Rolled (Extruded) Seamless Steel Pipes:

Round billet → Heating → Piercing → Three-roller skew rolling, continuous rolling or extrusion → Tube removal → Sizing (or reducing) → Cooling → Tube billet → Straightening → Hydrostatic testing (or flaw detection) → Marking → Storage.

Cold-Drawn (Rolled) Seamless Steel Pipes:

Round billet → Heating → Piercing → Head forming → Annealing → Pickling → Lubrication (copper plating) → Multiple pass cold drawing (cold rolling) → Tube billet → Heat treatment → Straightening → Hydrostatic testing (flaw detection) → Marking → Storage.

This detailed process description highlights the steps involved in the production of both hot-rolled and cold-drawn seamless steel pipes, emphasizing the differences in their respective manufacturing techniques and applications.