Choosing Blanks for Extra-Large-Diameter Hot-Rolled Seamless Pipes

First, the wall thickness of the blank pipe is calculated based on the wall thickness of the finished seamless steel pipe and the degree of wall thinning caused by diameter expansion. When the pipe is pushed and expanded, the length of the blank pipe shortens. The greater the diameter expansion is, the more the length is reduced. The length of the blank tube should be calculated based on the volume invariance law, the degree of diameter expansion, and the reduction in wall thickness. To obtain a seamless steel pipe with a length of about 12m after expansion, the blank pipe needs to be longer. Therefore, the billet supply unit must have the capability to produce ultra-long steel pipes. The wall thickness deviation of the steel pipe after push expansion will be greater than that of the blank pipe. Thus, the wall thickness accuracy of the blank pipe must be higher than that required for the expanded steel pipe. During the pushing and expanding process, tensile stress occurs on the outer surface of the blank tube, leading to deformation that produces and exposes defects. Therefore, the selected billet pipe must be made of pure steel without defects, and cracks on the outer surface are strictly prohibited. When pushing and expanding the pipe, the risk of defects and the degree of wall thickness deterioration are correlated with the diameter expansion rate. A larger diameter blank pipe should be selected when possible to reduce diameter expansion and ensure the quality of the expanded steel pipe.
 

The blank used for the secondary cross-rolling pipe expansion is the capillary pipe perforated during the previous process. The capillary tube is directly sent to the front end of the punching machine (either after heating and temperature equalization) for two or three stages of extension and diameter expansion. This process requires the perforated capillary tube to be free from quality defects, with a qualified outer diameter and wall thickness, and the temperature must meet the process specifications.
 

Similar to the push-pipe expansion process, the spin-rolling pipe expansion process also requires selecting qualified steel pipes as raw materials. Since the rotary rolling pipe expansion machine improves wall thickness accuracy and conveniently controls the steel pipe’s outer diameter and wall thickness, it corrects the uneven wall thickness of the defective material pipe. If the rotary rolling pipe expansion machine can be connected to the rolling mill (or sizing machine) that supplies the billet pipe, and the billet pipe is hot-sent through temperature compensation and uniform heating, it can save energy and reduce surface oxidation on the steel pipe. This helps reduce manufacturing costs and improve product quality.
 

As mentioned above, the production processes of extra-large-diameter thick-walled hot-rolled seamless steel pipes include the extruded pipe process, pipe jacking process, two-roller cross-rolling process, and periodic pipe rolling process.
 

The extruded pipe process is suitable for producing high value-added steel pipes, such as high alloys, corrosion-resistant alloys, high-temperature alloys, and stainless steel. Generally, electroslag steel is required. Currently, the electroslag steel hollow blank used in the extrusion tube process is mainly obtained by heating and expanding the electroslag steel ingot. If the electroslag hollow steel ingot production process could directly obtain hollow electroslag billets, it would greatly simplify the hollow billet production process in the extruded tube process and reduce the cost of defective material manufacturing. Currently, there have been reports on the manufacturing technology of electroslag steel hollow ingots. For other varieties that do not require electroslag steel, centrifugally cast hollow tube blanks or molded hollow steel ingots can be used.
 

The steel pipe varieties produced by the large pipe jacking process, the two-roll cross-rolling pipe process, and the periodic pipe rolling process have a lower alloy content than those produced by the extruded pipe process. Generally, injection-molded steel ingots, electroslag steel ingots or continuous casting methods are used. The capillary tube, obtained by directly perforating the round tube blank, is used as the blank. The original pressure punching process that provided defective materials for the large pipe jacking unit was also changed to a cross-rolling and punching process.
 

(1) The development history of extra-large-diameter hot-rolled seamless steel pipe varieties and equipment in China is introduced, and the characteristics of the production process for extra-large-diameter hot-rolled seamless steel pipes are analyzed.
(2) The extruded pipe unit can produce various pipes, including carbon steel pipes, alloy steel pipes, stainless steel pipes, high-temperature alloy pipes, and corrosion-resistant alloy pipes; however, the production cost is high.
 (3) The two-roll cross-rolled pipe unit can produce carbon steel pipes and alloy steel pipes with high production efficiency and wall thickness accuracy. However, difficulties arise in producing high-alloy steel, stainless steel, high-temperature alloys, and corrosion-resistant alloy steel pipes.
(4) The periodic pipe rolling unit is suitable for a wider range of steel pipes than the cross-rolling pipe unit, and while the deformation during rolling is greater, the dimensional accuracy is lower.
(5) It provides billet supply solutions for various extra-large-diameter hot-rolled seamless steel pipe production units.