Seamless pipe is perforated by the whole piece of round steel, and the steel pipe with no weld on the surface is called seamless pipe.

The production method of seamless steel pipe is roughly divided into the diagonal rolling method (Mennesmann method) and the extrusion method. The diagonal rolling method (Mennesmann method) is to first perforate the tube embryo with a slanting roller and then extend it with a rolling mill. This method has a fast production speed, but it has a high requirement for the machinability of the tube billet, which is mainly suitable for the production of carbon steel and low alloy steel pipes.

The seamless process of steel pipe is mainly accomplished by stretch reducing, which is a continuous rolling process of hollow base metal without mandrel. Under the condition of ensuring the welding quality of the master pipe, the welded pipe stretch reducing process is to heat the welded pipe as a whole to more than 950 degrees Celsius, and then after the stretch reducing machine (a total of 24 passes of stretch reducing machine) into a variety of outer diameter and wall thickness of the finished pipe. The hot-rolled steel pipe produced by this process is essentially different from the ordinary high-frequency welded pipe. After heating by the reheating furnace, the metallographic structure and mechanical properties of the weld can be completely consistent with that of the parent pipe. In addition, the dimensional accuracy of the steel pipe (especially the roundness and wall thickness accuracy of the pipe) is better than that of similar seamless pipes through the multi-pass stretch reducer rolling and automatic control.

The production equipment consists of piercing machine, automatic tube rolling machine, reeling machine, sizing machine and reducing machine. The production process of seamless steel pipes is divided into the following procedures: round tube billet → heating → piercing → three roll cross rolling, continuous rolling or squeezing → off pipe → sizing (reducing) → cooling → straightening → water pressure test (crack detection) → marking → warehousing.