Tube-Rolling Unit

a system of rolling mills and other machinery that performs all operations in the production of rolled seamless metal pipes and tubes, from removal of the initial product from storage to quality control and shipment of the finished pipe to the consumer.

The main operations of a tube-rolling unit are as follows: (1) heating of the initial product (ingots or round billets), (2) piercing of the workpiece, usually on a piercing mill, by forming a longitudinal opening at its center and creating a shell, (3) subsequent rolling of the shell on a draw mill to increase its length and to reduce its wall thickness, (4) sizing, (5) straightening, (6) end trimming, and (7) quality control of the finished product. All machines that perform these operations are connected by handling equipment, which provides full automation and continuity of production.

The size of tube-rolling units is determined by the largest pipe diameter rolled, and the type of the unit depends mainly on the operating system of the draw mill. There are four main types of draw mills: continuous, short-mandrel, three-high, and Pilger mills.

Tube-rolling units using continuous draw mills are the most efficient; they have been widely used since the 1950’s for the production of pipe up to 110 mm in diameter. The Soviet machine-building industry played a major role in the development of modern tube-rolling units of this type. In the 1960’s it produced two unique tube-rolling units that were capable of record output (400,000–600,000 tons a year). Draw mills of this type have been successfully put into operation at the Pervoural’sk New Pipe Plant and the Nikopol’ Southern Pipe Metallurgical Works. A continuous draw mill, which is installed after the piercing mill, consists of nine roll stands. The shell, produced on the piercing mill without intermediate heating, is rolled on a long mandrel and made into a thin-walled pipe. Because of its continuous operation and high rolling speed, this unit has an hourly output of up to 400 pipes more than 25–30 m long. The following machines are installed after the continuous draw mill: a machine for extracting mandrels from the pipe, a saw for trimming the pipe ends, an induction furnace for reheating, a sizing mill, and a conveyor for pipe cooling. To attain maximum efficiency, these tube-rolling units are usually used for the production of pipe of the largest possible diameter. In order to obtain a required pipe diameter, the pipe undergoes additional rolling on a continuous reducing mill, which is installed parallel to the sizing mill. The pipe is cooled and is conveyed to the finishing machines, where it is straightened, cut, heat-treated, checked for quality, packed, and shipped to the customer.

Tube-rolling units with short-mandrel draw equipment were originally developed in the late 19th and early 20th centuries by R. Stiefel (Sweden); they are also known as Stiefel mills. They are used to produce pipes ranging from 60 to more than 450 mm in diameter. Rolled round billets are used as the initial material. A tube-rolling mill using short-mandrel draw equipment includes a piercing mill. Two piercing mills are used to roll pipe with a diameter exceeding 200–300 mm; the second piercing mill is used to decrease the pipe wall thickness and increase the diameter of the shell. The short-mandrel, or automatic, draw mill is installed after the piercing mill. It consists of a single-stand two-high rolling mill with rolls 600–1,100 mm in diameter. The pipe is rolled on a short mandrel located on the end of a long rod in the pass between the rolls. The pipe wall decreases in thickness as it deforms between the rolls and the mandrel. The pipe is then returned to the original roll stand for a repeat pass. Two rolling mills are located parallel to the draw mill. The pipe is passed through each mill in turn and is subjected to additional rotary rolling on a short mandrel to obtain a more uniform wall thickness. Beyond the reducing mills are located a multistand sizing mill, a continuous conveyor for pipe cooling, and a set of finishing machines similar to those used for tube-rolling units with continuous drawing mills.

In 1975, Soviet rolling mill designers made several improvements in these tube-rolling units. The single-stand, double-pass short-mandrel rolling mill was replaced by the tandem, consisting of two short-mandrel stands located one behind the other. In addition, backward travel of the pipe was eliminated, increasing output and improving the dimensional accuracy of the pipe.

A tube-rolling unit using a three-high draw mill is used to produce thick-walled pipe 35–200 mm in diameter, with small variation of wall thickness, intended primarily for the manufacture of collars for rolling-contact bearings. Mills of this type, designed by V. Hassel (USA), were put into use in the 1920’s. The distinguishing feature of this type of mill is the use of rotary rolling to elongate the shell. The Hassel mill has three conical rolls, 250–500 mm in diameter, which are set at an angle to the pipe axis and which all rotate in the same direction. The pipe, which is gripped between the rolls, revolves in the opposite direction and, at the same time, advances along its own axis. Such helical motion causes deformation of the wall of the pipe between the rolls and the long cylindrical mandrel, resulting in a decrease in wall thickness and an increase in length. Equipment installed farther down the line includes a machine for extracting the mandrels from the pipe, a pipe reheating furnace, a three-high sizing mill, and a set of finishing machines.

A tube-rolling unit including a Pilger mill is used primarily to manufacture seamless pipe of large diameter (400–700 mm). The initial material is round ingots, which may be solid and cast in ingot molds or hollow and prepared by continuous casting; hollow billets produced on hydraulic presses are also used. The billets are preheated and rolled on a piercing mill and then on a Pilger tube-drawing mill, which is a two-high mill with periodic control of the rolls. The pipe is rolled on a cylindrical mandrel, with stepped feed by a special mechanism at each revolution of the rolls. The pipe is reheated after rolling and is then sized, straightened, and put through the finishing process.