Generally, the production processes of seamless steel tubes can be divided into two categories: cold drawing and hot rolling. Hot rolling, as the name suggests, involves rolling at high temperatures, which reduces deformation resistance and allows for significant deformation. Taking the rolling of steel plates as an example, the typical thickness of a continuous casting slab is around 230mm, which, after roughing and finishing, can be reduced to a final thickness of 1-20mm. Moreover, due to the relatively small width-to-thickness ratio of steel plates and lower dimensional accuracy requirements, plate shape issues are less likely to occur, with the main focus being on controlling convexity.
Hot-rolled seamless steel tubes refer to the process where rolling takes place above the recrystallization temperature, in contrast to cold rolling, which occurs below this temperature.
Advantages:
It can disrupt the casting structure of steel ingots, refine the grain size of steel, and eliminate defects in the microstructure, resulting in a denser steel structure and improved mechanical properties, particularly along the rolling direction. This transformation makes steel no longer isotropic to a certain extent.
Bubbles, cracks, and porosity formed during casting can be welded shut under high temperature and pressure.
Disadvantages:
After hot rolling, non-metallic inclusions (primarily sulfides, oxides, and silicates) within the steel are compressed into thin layers, leading to lamination or interlayering. This lamination significantly deteriorates the steel's tensile properties along the thickness direction and may cause interlayer tearing during weld contraction. Local strains induced by weld contraction can often reach several times the yield point strain, far exceeding strains caused by loading.
Seamless steel tubes are manufactured through hot-working methods such as piercing and hot rolling, without any welding seams. When necessary, the tubes can undergo further cold working after hot processing to achieve the desired shape, size, and properties. Currently, seamless steel tubes (DN15-600) are the most widely used pipes in petrochemical production facilities.
Based on manufacturing processes, seamless steel tubes are further classified into hot-rolled (or extruded) seamless steel tubes and cold-drawn (or cold-rolled) seamless steel tubes. Cold-drawn (or cold-rolled) tubes are further divided into round tubes and special-shaped tubes.
a. Overview of Production Processes:
Hot-rolled (or Extruded) Seamless Steel Tubes: Round billet → Heating → Piercing → Three-roll skew rolling, continuous rolling, or extrusion → Tube stripping → Sizing (or reducing) → Cooling → Tube blank → Straightening → Hydrostatic testing (or flaw detection) → Marking → Warehousing.
Cold-drawn (or Cold-rolled) Seamless Steel Tubes: Round billet → Heating → Piercing → Heading → Annealing → Pickling → Oil coating (or copper plating) → Multi-stage cold drawing (or cold rolling) → Tube blank → Heat treatment → Straightening → Hydrostatic testing (or flaw detection) → Marking → Warehousing.
