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The Continuous Rolling Process of the Seamless Steel Pipe

Posted: 08/24/2021 09:27:21 Hits: 344

The development history of the continuous rolling process of the steel pipe
The production of seamless steel pipes originated from the cross rolling piercing process of the MF Brothers in Germany. When they straightened round steel bars on a slanted roll straightening machine, they found that the center of the material was loose and there were cavities. They conducted a theoretical analysis of this and determined that the cross rolling process can form the center cavity of the bar, which could be used to manufacture seamless steel pipes. After long-term experiments, the two-roll Mannesmann cross rolling piercing technology was finally invented.

Taiyuan University of Science and Technology has developed the three-roll combined piercing and rolling process for a seamless steel pipe, and has done a lot of research on the structure of production equipment and new production processes. The biggest feature of this process is to realize multiple processes such as piercing and rolling on a set of rolls. The so-called combination means that the three processes of piercing, rolling, and leveling are continuously completed in one process. The German SMS Meer company developed the three-roll combined piercing and rolling process (3RCM). The process is implemented on the same three-roll combined piercing and rolling mill for piercing and elongation. The billet and tubular billet enter the rolling mill from both sides of the rolling mill.

With the development and perfection of the cross rolling piercing process, the production of seamless steel pipes through rolling methods has developed rapidly. Longitudinal rolling is the most commonly used rolling method for metal plastic processing. The characteristics of steel pipe's longitudinal rolling are that the longitudinal axes of the rollers are parallel to each other. In the rolling process, the rolled piece moves along its own axis, which is perpendicular to the longitudinal axis of the rolled pipe, and the metal extension is mainly along the axis of the rolled piece. The continuous rolling process is a typical metal longitudinal rolling process, and the continuous pipe mill is the leading model with the fastest development for longitudinal rolling production technology in the past 40 years. In this process, the tubular billet is put on the long mandrel, so that the mandrel and the rolled piece are simultaneously sent into several rack passes arranged along the rolling line for rolling. According to the working mode of the mandrel, the continuous rolling mill can be divided into full floating mandrel continuous rolling mill, fixed mandrel continuous rolling mill and semi floating mandrel continuous rolling mill. According to the number of racks, the continuous rolling mill can be classified into two types, that is, conventional (MPM, 7 to 9 racks) and few racks (MINI-MPM). The continuous rolling mill can be divided into two-roll and three-roll continuous pipe rolling mills based on the number of rollers.

The four stages of the development of the continuous rolling process
(1) Two-roll full floating mandrel continuous rolling process (From 1964 to 1983)
This process is that in the metal's rolling process, the mandrel involved in metal’s deformation is in a floating state, and the mandrel moves by friction with the inner surface of the pipe. This process will cause the outer diameter and wall thickness of the two ends of the steel pipe to fluctuate in a wide range, and the end must be cut to ensure that the product geometry is within a certain tolerance range, thereby reducing the metal's yield.

(2) Two-roll semi-floating mandrel continuous rolling processes (From 1977 to 1995)
This process is represented by DEMAG's MRK-S model. Its typical process features are that most of the rolling processes have features of the retained mandrel, and only when the rolling is carried out to the end of the tube, the mandrel limiting mechanism is released in advance to realize full floating rolling. The offline rod penetrating mechanism is used with the production capacity of about 180 pieces per hour. Four sets of continuous pipe rolling mills which were newly built and renovated during this period had a total production capacity of about 2 million tons, and the two-roll full-floating, semi-floating, and constrained mandrel processes coexisted, which is a period of transition from the two-roller full floating mandrel rolling process to two-roller retained mandrel rolling process.

(3) Two-roller mandrel continuous rolling processes (From 1978 to 2009)
The representative model of this process is the MPM tube rolling mill. The restricted mandrel effectively shortens the length of the mandrel by 50% compared with the full floating process. The working mode of the restricted mandrel promotes the metal flow in the hole pattern deformation zone, reduces the peak load of the pipe rolling mill. The biting conditions of the first rack of the continuous miller are improved. The deformation of the first rack's metal is improved; the loosening process is eliminated. In 1994, the world's first Mini-MPM continuous rolling mill with 5 racks was built in Baotou City. This process is a new technology developed from the MPM process. The Mini-MPM process has a few racks, and 3 large racks and 2 small racks are used to reduce the space between them. The racks are arranged vertically and horizontally in staggered in order to reduce the required installation height. The hydraulic pressure adjustment mechanism is adopted, and dynamic adjustment is used in the rolling process. The length of the mandrel is shorter and it can be turned, reducing consumption of tools. The total equipment weight is reduced by 1/3 compared to MPM; the thin-walled tubular billet with a wall thickness less than and equal to 16 mm adopts on-line penetrating rod to reduce the tubular billet’s temperature drop and uneven temperatures.

(4) Three-roller mandrel continuous rolling processes (From 2003 to now)
The three-roller limited mandrel continuous rolling mill is a typical representative of this stage, and it is the most advanced continuous rolling process today. The process was first officially launched by the Italian company Innse at the Hong Kong International Conference in 1993. The first three-roller limited mandrel continuous rolling mill was jointly designed and built by German SMS Meer, SMS DEMAG, Innse and Tianjin Steel Pipe Company in August 2003, and it was put into use in Tianjin Steel Pipe Company. This process is called "PQF" by SMS in Germany and "FQM" by Danieli in Italy. In terms of continuous rolling mill process control, the most advanced control technology in use is the CARTA process control technology developed by SMS Meer, including the perforating auxiliary design system CPM, the continuous rolling mill process auxiliary design system PQF, and the sizing mill process auxiliary design system SRM and online quality assurance system QAS. In China, this model can be designed and manufactured. In 2010, the TZφ180 mm continuous tube mill designed and manufactured by Taiyuan Heavy Machinery Group Co., Ltd. was put into use in Molong, Shandong Province, China.

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