Seamless steel pipes are made by perforating steel ingots or solid tubes into rough tubes, and then hot rolling, cold rolling or cold drawing.
1. Preparation of tube billets
It is necessary to select suitable steel ingots or solid tube billets as raw materials. After that, according to production requirements, the tube billets are pre-treated by re-inspection, fixed cutting, centering, peeling and other pre-treatment operations to ensure that the quality and size of the tube billets meet the requirements of subsequent processing.
2. Heating
Whether the oblique rolling method or extrusion method is used, the tube billet or steel ingot must be heated first. The purpose of heating is to improve the plasticity of the material, reduce the deformation resistance, and facilitate subsequent perforation and rolling processing. The heated tube billet or steel ingot is used to produce hot-rolled tubes.
3. Perforation and extension
Oblique rolling method (Mennesmann method): first perforate the tube billet with an oblique rolling roller, and then extend it with a rolling mill. That is, through the action of the oblique rolling roller, a perforation is formed in the center of the tube billet, and then further processed by the rolling mill, so that the tube billet gradually becomes a steel pipe with a certain size.
Extrusion method: After heating the tube billet or ingot, it is extruded into a steel pipe through an extrusion device.
4. Cold processing stage (optional)
Steel pipes produced by hot processing can sometimes be cold processed as needed. Cold processing can improve the dimensional accuracy and processing finish of the steel pipe, improve the mechanical properties of the material, etc. There are two methods for cold processing:
Cold drawing method
Pulling the steel pipe through a tube drawing die to gradually thin and elongate the steel pipe, thereby achieving the required dimensional accuracy and surface quality.
Cold rolling method
The method of applying the hot rolling mill invented by the Munnesmann brothers to cold processing to achieve further processing of the steel pipe.
5. Seamless processing
The seamlessness of the steel pipe is mainly achieved through tension reduction. The tension reduction process is a continuous rolling process of the hollow base material without a mandrel. Under the condition of ensuring the welding quality of the base pipe, the tension reduction process of the welded pipe is to heat the welded pipe as a whole to above 950 degrees Celsius, and then roll it into finished pipes of various outer diameters and wall thicknesses through a tension reducer (the tension reducer has a total of 24 passes). The hot-rolled steel pipe produced by this process is essentially different from the ordinary high-frequency welded pipe. After being heated by the heating furnace, the metallographic structure and mechanical properties of the weld can be completely consistent with those of the parent pipe. In addition, through multiple passes of tension reducing mill rolling and automatic control, the dimensional accuracy of the steel pipe (especially the roundness and wall thickness accuracy of the pipe body) is better than that of similar seamless pipes.
6. Subsequent processing
After completing the above main processing steps, a series of subsequent processing is required for the seamless steel pipe, including solution heat treatment (i.e. annealing), straightening, pipe cutting, pickling or passivation, finished product inspection, labeling, packaging, etc., and finally the qualified seamless steel pipe is shipped out of the factory.
