① Main production processes (main inspection processes) of hot-rolled seamless steel pipes: Billet preparation and inspection → Billet heating → Piercing → Rolling → Steel pipe reheating → Sizing (reduction) → Heat treatment → Finished pipe straightening → Finishing → Inspection (non-destructive, physical and chemical, bench inspection) → Warehousing
② Main production processes of cold-rolled (drawn) seamless steel pipes: Billet preparation → Pickling and lubrication → Cold rolling (drawing) → Heat treatment → Straightening → Finishing → Inspection The production process of general seamless steel pipes can be divided into cold drawing and hot rolling. The production process of cold-rolled seamless steel pipes is generally more complex than that of hot rolling. The billet first undergoes three-roll continuous rolling. After extrusion, a sizing test is performed. If there are no cracks on the surface, the round pipe is cut into billets approximately one meter in length. Then it enters the annealing process. Annealing requires pickling with acidic liquid. During pickling, attention should be paid to whether a large amount of bubbling is generated on the surface. If a large amount of bubbling is generated, it indicates that the quality of the steel pipe does not meet the corresponding standards. In terms of appearance, cold-rolled seamless steel pipes are shorter than hot-rolled seamless steel pipes. The wall thickness of cold-rolled seamless steel pipes is generally smaller than that of hot-rolled seamless steel pipes, but the surface appears brighter, with less roughness and fewer burrs.
Hot-rolled seamless steel pipes are generally delivered in their hot-rolled state after heat treatment. After quality inspection, hot-rolled seamless steel pipes undergo rigorous manual selection by staff. Following quality inspection, the surface is oiled, and then multiple cold-drawing tests are performed. After hot rolling, a piercing test is conducted; if the piercing diameter is too large, straightening is required. After straightening, the pipes are conveyed to a flaw detector for flaw detection. Finally, labels are affixed, specifications are arranged, and the pipes are placed in the warehouse.
Seamless steel pipes are manufactured by piercing steel ingots or solid tube blanks to form a rough tube, which is then hot-rolled, cold-rolled, or cold-drawn.
The specifications of seamless steel pipes are expressed in millimeters as outer diameter * wall thickness.
Hot-rolled seamless pipes generally have an outer diameter greater than 32mm and a wall thickness of 2.5-200mm. Cold-rolled seamless steel pipes can have an outer diameter down to 6mm and a wall thickness down to 0.25mm. Thin-walled pipes can have an outer diameter down to 5mm and a wall thickness less than 0.25mm. Cold-rolled pipes have higher dimensional accuracy than hot-rolled pipes. Seamless steel pipes are generally made from high-quality carbon structural steels such as 10, 20, 30, 35, and 45, low-alloy structural steels such as 16Mn and 5MnV, or alloy structural steels such as 40Cr, 30CrMnSi, 45Mn2, and 40MnB, through hot rolling or cold rolling. Seamless pipes made from low-carbon steels such as 10 and 20 are mainly used for fluid transportation pipelines. Seamless pipes made from medium-carbon steels such as 45 and 40Cr are used to manufacture mechanical parts, such as load-bearing parts for automobiles and tractors. Seamless steel pipes generally require guarantees of strength and flattening tests. Hot-rolled steel pipes are delivered in hot-rolled or heat-treated condition; cold-rolled pipes are delivered in heat-treated condition.
Hot rolling, as the name suggests, involves rolling at high temperatures, resulting in low deformation resistance and allowing for large deformation amounts. Taking steel plate rolling as an example, the thickness of a continuously cast slab is generally around 230mm, while after roughing and finishing rolling, the final thickness is 1-20mm. Meanwhile, due to the small width-to-thickness ratio of the steel plate, the dimensional accuracy requirements are relatively low, and plate shape problems are not easy to occur, with the main focus being on controlling convexity. For those with specific microstructure requirements, controlled rolling and cooling are generally used, i.e., controlling the initial rolling temperature and the final rolling temperature of the finishing roll. Round billet → heating → piercing → heading → annealing → pickling → oiling (copper plating) → multi-pass cold drawing (cold rolling) → billet tube → heat treatment → straightening → hydrostatic test (flaw detection) → marking → warehousing.
