TPT July 2009

Manufacturing technology of medium size seamless pipe By Kenichi Sasaki, Akihito Yamane, Yuji Arai, Sumitomo Metal Industries Ltd, Japan

Introduction Sumitomo Metal Industries (SMI) constructed a 16¾" seamless mill at Wakayama Steel Works in 1997 with an annual capacity of 640,000 tons. Features of the plant are the short production lead- time and the quality of produced tubes and pipes. Direct connection of the billet casting, the pipe rolling process and heat treatment process enable SMI to shorten the production lead-time to one third. High-toe-angle, high-expansion piercing technology and a new method of measuring and controlling hot wall thickness with the mandrel mill produce high quality tubes and pipes. Due to the brisk activity in exploring for oil and gas in critical environments, the demand for high-strength pipes and stainless steel pipes is increased and dimension accuracy is increasingly important. In order to produce high-strength pipes and stainless pipes without defects, the company developed the high-toe-angle and high-expansion piercing technology. With the high toe-angle and high-expansion piercer, it is possible to decrease circumferential shear strain and reduce a piercing elongation. In order to make the pipes accurate, a new method of measuring and controlling hot wall thickness with the mandrel mill was developed. When the plant was constructed a hot wall thickness measuring system was installed to monitor hot wall thickness. A system was developed to control the wall thickness by adjusting the position of the mandrel mill’s rolls. At every rolling, the rolls’ positions are adjusted and the wall thickness measured again. Through repeated adjusting and measuring, pipe dimensions are extremely accurate.

quenched and tempered. The outline of the simple and compact medium-size mill is described below. This is a state of-the-art facility, which began operation in 1997. The entire process of manufacturing seamless pipe is directly connected and the rolling line arranged in a compact design, as shown in figure 1-b; consequently, reduction of production cost and shortening of delivery time are achieved. In order to mass-produce heat-treated seamless pipe with high efficiency, the facility of in-line heat treatment was installed. As a result, almost all heat-treated pipe, such as high grade OCTG or project line pipe that used to be heat-treated in an off-line process, is now treated in an in-line process. 2. Billet making Pig iron is processed in a blast furnace and refined in a converter. The molten steel is then treated in an RH (Rheinstahl Huttenwerke & Heraus) degassing process to reduce impurities and improve cleanliness, and then poured into the continuous caster with a round-shaped mould. The round billet casting facility and the rolling plant are connected through the intermediate storage yards; the billet is supplied smoothly to the mill. 3. Billet heating furnace A walking beam furnace is installed for the billet-heating furnace. The aim is to minimise the circumferential temperature distribution of the billet, which causes eccentricity of the wall thickness during the piercing operation. With the walking beam furnace, circumferential temperature distribution is controlled within ±5°C by applying the billet rotation in the furnace. Hollow shells of high dimensional tolerances can be obtained, suppressing eccentricity during piercing. 4. Piercer For piercing, a high-toe-angle piercer with disc guide, originally developed by SMI, was installed. Using the high-toe-angle piercer inside bore defects are drastically reduced, and it is easier to pierce hard workability materials, stainless steels for example, compared with using a conventional piercer.

Manufacturing process 1. Seamless mill line

A schematic diagram of a conventional production line for seamless pipe is shown, as reference, in figure 1-a. Molten steel was cast into the bloom by caster and the bloom heated and forged into a round billet. The billet was again heated, pierced and rolled into the seamless pipe, cooled to room temperature, heated again,  Figure 1-a : Schematic illustration of conventional production line for seamless pipe

Moreover, with the high-toe-angle piercer, high expansionpiercingbecomespossible. Highexpansion piercing is suitable to pierce hard workability materials, such as stainless steels, more than the parallel piercing. Utilising expansion piercing, it is possible to obtain a thin wall hollow shell of stainless steels. 5. Mandrel mill The 5-stand mandrel mill consists of two high mills, alternately arranged at 45 degrees to the vertical

104

J uly 2009

www.read-tpt.com

›