There are two main manufacturing methods for seamless steel pipes: hot rolling and cold drawing.
1. Hot rolling (extruded seamless steel pipe): Round tube billet → Heating → Piercing → Three roll oblique rolling, continuous rolling or extrusion → Tube detachment → Sizing (or reducing) → Cooling → Straightening → Hydrostatic testing (or flaw detection) → Marking → Warehousing
The raw material for rolling seamless pipes is round tube blanks, which need to be cut and processed by a cutting machine to form a billet with a length of about 1 meter, and then sent to the furnace for heating through a conveyor belt. The billet is fed into the furnace for heating, with a temperature of approximately 1200 degrees Celsius. The fuel is hydrogen or acetylene. Temperature control in the furnace is a crucial issue. After the round tube billet is discharged from the furnace, it needs to be pierced through by a pressure piercing machine.
The commonly used piercing machine is a conical roller piercing machine, which has high production efficiency, good product quality, large perforation and expansion capacity, and can pierce various steel grades. After perforation, the round tube blank is sequentially rolled, continuously rolled, or extruded by three rollers. After squeezing, the pipe needs to be detached and sized. The sizing machine uses a conical drill bit to rotate at high speed into the steel billet for drilling, forming a steel pipe. The inner diameter of the steel pipe is determined by the outer diameter length of the sizing machine drill bit. After the steel pipe is sized, it enters the cooling tower and is cooled by water spray. After cooling, the steel pipe needs to be straightened.
After straightening, the steel pipe is transported by a conveyor belt to a metal inspection machine (or hydraulic test) for internal inspection. If there are cracks, bubbles, and other issues inside the steel pipe, they will be detected. After the quality inspection of steel pipes, strict manual selection is also required. After the quality inspection of the steel pipe, the number, specification, production batch number, etc. are sprayed with paint. And lifted into the warehouse by a crane.
2. Cold drawn (rolled) seamless steel pipe: round tube blank → heating → perforation → heading → annealing → acid washing → oil coating (copper plating) → multi pass cold drawing (cold rolling) → blank tube → heat treatment → straightening → hydraulic test (flaw detection) → marking → warehousing.
The rolling method of cold drawn (rolled) seamless steel pipes is more complex than that of hot rolled (extruded seamless steel pipes). The first three steps of their production process are basically the same. The difference starts from the fourth step, after the round tube blank is punched out, it needs to be headed and annealed. After annealing, special acidic liquid should be used for pickling.
After pickling, apply oil. Then, it is followed by multiple passes of cold drawing (cold rolling) followed by specialized heat treatment for the billet. After heat treatment, it needs to be straightened. After straightening, the steel pipe is transported by a conveyor belt to a metal inspection machine (or hydraulic test) for internal inspection. If there are cracks, bubbles, and other issues inside the steel pipe, they will be detected. After the quality inspection of steel pipes, strict manual selection is also required.
After the quality inspection of steel pipes, the standard classification of seamless steel pipes is quoted using painted steel pipes. The production and manufacturing methods of thick wall pipes can be divided into hot rolled pipes, cold rolled pipes, cold drawn pipes, extruded pipes, etc. according to different production methods. Hot rolled seamless pipes are generally produced on automatic rolling mills, and the solid pipe blank is inspected and removed from surface defects to cut into the required length, Centering is carried out on the end face of the piercing end of the pipe blank, and then sent to the heating furnace for heating. The piercing is carried out on the piercing machine while continuously rotating and advancing. Under the action of the rolling roller and head, a cavity is gradually formed inside the pipe blank, which is called a capillary. It is then sent to the automatic pipe rolling machine for further rolling. Finally, the entire machine is evenly adjusted for wall thickness, and the diameter is determined by the sizing machine to meet the specification requirements.
The use of continuous rolling mills to produce hot-rolled seamless steel pipes is a more advanced method. To obtain smaller and better quality seamless pipes, cold rolling and cold drawing or a combination of both methods must be used. Cold rolling is usually carried out on a two roll mill, where the steel pipe is rolled in a circular pass composed of a variable cross-section circular groove and a fixed conical head, Cold drawing is usually carried out on a single chain or double chain cold drawing machine by extrusion method. The heated tube blank is placed in a closed extrusion cylinder, and the perforated rod and extrusion rod move together to extrude the extruded part from the smaller mold hole. This method can produce steel pipes with smaller diameters.
