China piping solution supplier: www.epowermetals.com

Production process and production technology of hot rolled seamless steel pipe

What is hot rolled seamless steel pipe?

Hot rolled seamless steel pipe: compared with cold rolling, cold rolling seamless steel pipe is seamless steel pipe rolled under recrystallization temperature, and hot-rolled seamless steel pipe is seamless steel pipe rolled above recrystallization temperature.

Advantages and disadvantages of hot rolled seamless steel pipe

Advantages of hot rolled seamless steel pipe

It can destroy the casting structure of ingot, refine the grain size of steel, and eliminate the defects of microstructure, so that the steel structure is dense and the mechanical properties are improved. This improvement is mainly reflected in the rolling direction, so that the steel is no longer isotropic to a certain extent; the bubbles, cracks and porosity formed during pouring can also be welded under the action of high temperature and pressure.

Disadvantages of hot rolled seamless steel pipe

  • 1. After hot rolling, the non-metallic inclusions (mainly sulfides and oxides, as well as silicates) inside the steel are pressed into thin sheets, resulting in delamination (interlayer). Delamination greatly deteriorates the tensile properties of the steel along the thickness direction and may cause interlaminar tearing when the weld shrinks. The local strain induced by weld shrinkage often reaches several times of the yield point strain, which is much larger than that caused by load.
  • 2. Residual stress caused by uneven cooling. The residual stress is the internal self-equilibrium stress without external force. All kinds of hot-rolled sections have this kind of residual stress. The larger the section size of general section steel, the greater the residual stress. Although the residual stress is self balanced, it still has some influence on the performance of steel members under external force. Such as deformation, stability, anti fatigue and other aspects may have adverse effects.
  • 3. It is difficult to control the thickness and side width of hot rolled steel products. We are familiar with thermal expansion and cold contraction. Since at the beginning of hot rolling, even if the length and thickness are up to the standard, there will still be a certain negative difference after cooling. The wider the negative difference is, the thicker the thickness is, the more obvious it will be. Therefore, for large steel, the width, thickness, length, angle and edge line of steel can not be required to be too accurate.

Classification of hot rolled seamless steel tubes

Hot rolled seamless steel pipe is divided into general steel pipe, low and medium pressure boiler steel pipe, high pressure boiler steel pipe, alloy steel pipe, carbon steel pipe, stainless steel pipe, petroleum cracking pipe, geological steel pipe and other steel pipes.
Cold rolled (drawn) seamless steel pipe includes carbon thin-walled steel pipe, alloy thin-walled steel pipe, alloy thin-walled steel pipe, alloy thin-walled steel pipe, stainless steel pipe, stainless steel pipe and special-shaped steel pipe. Generally, the outer diameter of hot-rolled seamless pipe is greater than 32mm, and the wall thickness is 2.5-75mm. The outer diameter and wall thickness of cold-rolled seamless steel pipe can reach 6 mm and 0.25 mm, respectively. The outer diameter of thin-walled pipe can reach 5 mm and the wall thickness is less than 0.25 mm. General seamless steel pipe: it is made of 10, 20, 30, 35, 45 and other high-quality carbon Bonded Steel 16Mn, 5mnv and other low-alloy structural steel or 40Cr, 30CrMnSi, 45Mn2, 40MnB alloy steel hot-rolled or cold-rolled. 10. The seamless pipe made of 20 grade low carbon steel is mainly used for fluid transportation pipeline. The seamless pipe made of 45, 40Cr and other medium carbon steel is used to manufacture mechanical parts, such as the stressed parts of automobile and tractor. Generally, the strength and flattening test of seamless steel pipe shall be ensured. Hot rolled steel pipes are delivered as hot rolled or heat treated; cold rolled pipes are delivered in heat treated condition.

Specification table of hot rolled seamless steel pipe

Specifications (mm)
Specifications (mm)
Specifications (mm)
12×1.5
73×4-4.5-5-6-7-8-9-10-12-15
219×30-40
14×1.5-2.5-3
76×4-4.5-5-6-7-8-9-10-12-14
245×8-10-14-17-20-22
16×1.5-2.5-4
83×4.5-5-10/ 12 14 16 18 20
245×25-28-30-34-40
18×2.3-4-5
89×4.5-8-10/12-14 16 -18 20
273×7-9-12-15-18-20-25
20×2.5-3-5
95×4.5-6-8/10-12-14-16-20
273×30-35-38-40
24×2.5-3-4-5-6
102×4.5-5-6-7-8-10-12-14-16-20
299×8-12-16-20-24-28
25×2.5-3-4-5-6
108×4.5-5-6-7-8-10/12-14-18-20-22
299×30-32-36-38-40
28×3-3.5-4-5-6
114×4.5-5-6-7-8-10-12-14-18-20
325×8-12-14-18-38
32×3.5-4-5-6-8
121×4.5-5-6-7-8-10-12-14-16-20
325×28-32-34-38
34×4-5-6-8
127×4.5-5-6-7-8-10-12-14-16-20
351×10-16-20-25-40
36×3.5-5-6-8
133×4.5-6-8-12-14-16-18-20-25-30
355×6-18-20-25-30-40
38×4-5-6-7-8-9-10
140×5-6-7-8-9-10/12-14-16-20-22
377×8-12-15-20-24-28
39×3.5-5-8-10
146×4.5-6-8-12-14-16-18-20-25-30
377×30/37/40
42×3.5-4-5-6-7-8-9-10
152×4.5-6-8-12-14-16-18-20-25-30
402×10/15-20-25-30-35
45×3.5-4-5-6-7-8-9
159×4.5-6-10-12-14
406×10/20-25-28-35-40
48×3.5-4-5-6-7-8-9-10
159×16-18-20-22-25-30
426×10/12-14-16-20-25-30-35-40
51×3.5-4-5-6-7-8-9-10
168×8-10/12/14/20/25/30/35
450×10/12-14-16-20-25-34-40
56×3.5-4-5-6-7-8-9-10
180×8-12-16-25-30
480×10/ 12 14 16 18 20 25 30 40
57×3.5-5-6-7-8-9-10-12
180×32-34-38-40
500×10/12-14-16-20-25-34-40
60×3.5-4.5-5-6-7-8-9-12-14
194×6-8-12-16-20-25-30-40
510×10/12-14-16-20-25-34-40
63×3.5-4.5-5-6-7-8-9-10-12-14
203×6-7-10-15-20-25-30
530×10/ 12 14 16 18 20 30-40
68×4-4.5-5-6-7-8-9-10-12-14-16
203×32-36-40
70×4-4.5-5-6-7-8-9-10-12-15-16
219×6-8-12-16-18-20-25

Production process of hot rolled seamless steel pipe

General process flow

The production process of hot-rolled seamless steel pipe includes several basic processes, such as preparation before rolling, billet heating, piercing, rolling, sizing and reducing, cooling and finishing of steel pipe.

20200918100502 94313 - Production process and production technology of hot rolled seamless steel pipe

At present, there are three main deformation processes in the production of hot-rolled seamless steel pipe: piercing, pipe rolling and sizing and reducing

  • Piercing: to change a solid tube blank into a hollow tube; we can understand it as shaping, that is, to fix the cross section of the rolled piece into a circular ring; its equipment is called a piercer. The requirements of piercing process are as follows: firstly, the wall thickness of the pierced capillary is uniform, the ovality is small, and the geometric dimension accuracy is high; secondly, the inner and outer surface of the capillary should be smooth, without scab, folding, crack and other defects; thirdly, there should be corresponding piercing speed and rolling cycle to adapt to the production rhythm of the whole unit, so that the final rolling temperature of the capillary can meet the requirements of the pipe mill 。
  • Pipe rolling: the thick wall of the capillary into thin-walled (close to the finished wall thickness) of the raw pipe; we can regard it as a fixed wall, that is, according to the subsequent process reducing amount and empirical formula to determine the wall thickness value of the raw pipe in this process; the equipment is called the pipe rolling mill. The requirements of pipe rolling process are as follows: first, when the thick wall tube is turned into a thin-walled tube (reduced wall extension), the first thing is to ensure that the raw pipe has a high wall thickness uniformity; second, the raw pipe has good internal and external surface quality.
  • Sizing and reducing (including stretch reducing): large circle becomes small circle, referred to as sizing; the corresponding equipment is sizing (reducing) mill, whose main function is to eliminate the difference of outer diameter (the same piece or the same batch) caused by the rolling process in the previous process, so as to improve the outer diameter accuracy and roundness of hot-rolled finished pipe. The requirements of sizing and reducing process are as follows: firstly, the sizing can be achieved under the condition of a certain total reducing rate and a small single stand reducing rate; secondly, it can realize the task of producing multi specification finished pipe by using one kind of tube blank; thirdly, it can further improve the external surface quality of steel pipe.

Table.1 process method of tube wall reduction

Deformation principle Features of equipment and tools Processing technology Elongation coefficient
External tools and equipment Mandrel
Longitudinal rolling Single rack Short (fixed) plug mill 1.5~2.1
Multi stand continuous rolling Long (floating) MM 3~4.5
Medium length (semi floating, limited motion) Neuval-R,MRK-S、MPM、MINI-MPM、PQF 3~6.5
Cross rolling Two rollers Guide plate Short (fixed, limited) Secondary piercing and stretching machine <2.5
Guide plate Long (floating) Stretcher(Diescher) 2~5.0
Medium length (limited motion) ACCU-ROLL 2~5.0
Three rollers Medium and long (floating, limiting and retreating) Three roll pipe mill(Assel,Transval) 1.3~3.5
Multi roll Medium length (fixed) Planetary pipe mill (PSW) 5~14
Forging and rolling Periodic section roller Medium length (reciprocating) Pilger 8~15
Pipe jacking method A row of die holes Long (synchronous with outlet pipe end) Pipe jacking machine 4~16.5
Extrusion method Single mode hole Medium length (fixed) An extruder 1.2~30

Several forms of continuous pipe mill

After long mandrel is inserted into the capillary, the continuous pipe mill is arranged in sequence through multiple stands, and the adjacent stand roll gap is staggered (two roll roll gap is staggered by 90 ° as shown in Fig. 1; three roll roll roll gap is staggered by 60 ° each other). It is the most widely used longitudinal rolling method. In the rolling process of continuous pipe mill, the deformation of rolled piece is actually affected by the repeated action of 4 ~ 8 groups of rolls and mandrel, from circle to ellipse The process from ellipse to circle.
The continuous pipe mill has a long history of development. As early as the end of the 19th century, it was tried to roll pipe on long mandrel. But for various reasons, there were only six continuous pipe mills in the world by 1950. After 1960, with the progress of science and technology and the development of production, especially the rapid development and application of computer technology, the continuous pipe mill has been gradually improved in the production process and equipment, and has been rapidly developed and popularized. On the basis of the floating mandrel mill, the process test of the limited mandrel mill was carried out in the mid-1960s, and gratifying results were obtained. In 1978, the first MPM mandrel mill in Italy was put into operation, and a new set of mandrel mill was developed in the world. At the end of 1990s, the technology of three roll continuous pipe mill (PQF) was introduced, which made the process equipment of continuous rolling pipe jump to a higher level.
Before the appearance of PQF, the continuous pipe mill was of two roll type, that is, the pass was composed of two rolls, The two roll stands are staggered at 45 ° to the ground, or perpendicular and horizontal to the ground; PQF is three roll, i.e. three roll pass is formed; MPM and PQF pass composition are shown in Fig. 2; during continuous pipe rolling, the metal at the top of the pass extends axially and extends horizontally to the circumference due to the external pressure of the roll and the internal pressure of the mandrel, However, the metal in the side wall of the pass does not contact with the mandrel, but it is extended axially by the additional tensile stress of the metal extending axially at the top of the pass, and at the same time, the axial tension and contraction occurs. No matter two high or three high continuous pipe mill, according to the operation mode of mandrel, it can be divided into the following three forms.
20200918102405 15513 - Production process and production technology of hot rolled seamless steel pipe

Floating mandrel mill (or full floating mandrel mill)

MM (mandrel mill) is generally equipped with 8 racks. In the rolling process, the mandrel speed is not controlled, and the mandrel is driven by the friction force of the rolled metal to follow the pipe freely through the rolling mill; During the rolling process, the running speed of the mandrel fluctuates with the biting in and steel throwing of each stand, resulting in the fluctuation of the wall thickness of the pipe; after the rolling, the mandrel is rolled out with the raw pipe to the output roller table behind the continuous rolling mill, and almost the whole length of the mandrel is in the raw tube when rolling the medium and thin-walled pipes, as shown in Fig. 3; the raw pipe with mandrel moves horizontally to the stripper line, and the mandrel is extracted from the tube by the stripper After cooling and lubricating, it can be recycled. It has the characteristics of shorter tube length (177.4 mm) and shorter mandrel length (less than 177.4 mm) of tube diameter, and it is suitable for the production of steel tube with shorter tube length (less than 177.4 mm). The representative floating mandrel mills are RK2 of millheim plant in Germany and 140 mm mill of Baosteel in China.
The working characteristic of the floating mandrel mill is that the mandrel speed is not controlled during rolling, so the mandrel speed changes many times in the whole rolling process. For example, on an 8-stand continuous pipe mill, when the metal enters the first stand, the mandrel runs at the rolling speed close to the first stand under the action of friction force; when the metal enters the second stand, the mandrel speed will change and run at a certain speed between the first and second stands; when entering the third stand, the mandrel speed has changed to the first and second rolling speed And so on, until entering the eighth stand, the mandrel speed has gone through eight changes, has run at a certain speed between 1 and 8 stands, and has entered a relatively stable rolling stage. At this stage, the rolling speed of the front stand is slower than that of the mandrel (called the slow stand), and the rolling speed of the rear stand is faster than that of the mandrel (called the fast stand). If the rolling speed of a middle stand is exactly the same as the running speed of the mandrel, it is called synchronous stand. Then, when the metal is gradually rolled out of the relevant stand, the mandrel speed changes to a certain speed between 2 and 8 stands; when the metal is rolled out from the second stand, the mandrel speed changes to a certain speed between the third and the eighth stands, and so on, until the metal is rolled out from the eighth stand.
It can be seen from the above that in the rolling process of steel pipe, the speed of mandrel must be changed at least 15 times, and the change of mandrel speed will lead to the change of metal flow conditions. The metal flow of the floating mandrel mill is changed due to the change of the mandrel speed during the rolling process, and the longitudinal wall thickness and diameter of the steel pipe are changed due to the irregular metal flow. Although many measures have been taken and some results have been achieved, when the rolling conditions change, the dimensional accuracy of the product pipe is not as good as that of the limited mandrel mill. In addition, the long mandrel increases the manufacturing cost and makes it difficult to manufacture. Moreover, the weight of the long mandrel is also very large. The steel pipe running on the roller with heavy mandrel will cause damage to the steel pipe surface. Therefore, at present, the floating mandrel mills are used in small mills.
In continuous rolling, the tube can be regarded as the continuous rolling between rolls of different diameters. The mandrel in the steel tube can be regarded as the inner roll with infinite radius of curvature. In the process of floating mandrel rolling, the mandrel has no other external force except the force transmitted by the roll through the rolled piece. When the head of the rolled piece is bitten by the first stand, the number of stands acting on the mandrel increases as the rolled pieces move towards the rear extension rack one by one, so the mandrel speed increases continuously. This stage is called “bite in” stage. When the head of the rolled piece enters the last stand, the whole rolling piece is in the rolling process of all stands of the continuous pipe mill, and the mandrel speed remains unchanged, which is called the “urgent stage of stable rolling”. When the end of the rolled piece leaves the first stand, the mandrel speed increases gradually until the rolling out extension, which is called “rolling out” stage. The working circumferential speed of roll is set under the condition of “stable rolling”. In the rolling process, the rolled piece follows the law of volume invariance. However, the increase of the rolling speed caused by the mandrel will inevitably increase the metal flowing into the back frame, that is to say, the back stand will feed more metal than the set circumferential speed of the roller by the mandrel, which will lead to the metal accumulation that increases the sectional area. Although the large section caused by the gradual inflow of additional metal was processed on the final frame, it still led to the diameter and wall thickness of some parts of the pipe, which was called “slub”. In principle, “slub” may appear on the whole steel pipe. Obviously, the phenomenon of “slub” is uneven longitudinal wall thickness, which is disadvantageous to subsequent stretch reducing mill rolling and should be prevented as far as possible.
In order to prevent or reduce the formation of “slub”, the reduction of the first few frames should be increased appropriately under the premise of keeping the total extension unchanged. In this way, the sudden increase of mandrel speed can be weakened in the rear several frames, so as to reduce the influence of mandrel speed change. Good mandrel lubrication is beneficial to extension and energy consumption reduction, and also can reduce the formation of slub. The electronic control technology can also be used to prevent the occurrence of bamboo knots. It is preset by computer, and the roll speed changes according to the requirements. When the rolled piece passes, the roll is calibrated to make the exit speed of each stand adapt to the change of mandrel speed.
The floating mandrel mill was popular in the 1970s. Due to the limitation of mandrel weight, this kind of mill can only produce steel pipe with diameter less than 177.8mm.

Semi floating mandrel mill

The German name is mrk-s (Mannesmann Bohr kontimill stripper); the French name is neuval-r. Generally, there are 7-8 stands of semi floating mandrel mill.
The process designed in Germany is: in the first half of the rolling process, the mandrel does not move freely with the rolling piece, but is controlled by the limited moving mechanism. When the mandrel moves forward at a constant speed, the velocity difference distribution between the mandrel and the rolled piece is inconsistent, and the exit speed of the first stand is less than that of the mandrel; from the second stand, the speed of the rolled piece is faster than that of the mandrel, forming a stable differential rolling state When the main deformation is completed and the pipe is separated from the penultimate stand, the limiting mechanism accelerates the release of the mandrel, and the mandrel is taken out of the mill by the steel pipe like a floating mandrel. The German style semi floating mandrel mill was built and put into operation in Japan’s Bafan plant in the early 1980s.
The process developed in France is: the mandrel is released only when the steel pipe is rolled out from the last stand, that is, the mandrel is released after the final rolling and the mandrel is released after the final rolling; the mandrel is transported to the output roller table after the continuous rolling mill along with the pipe. The French style semi floating mandrel mill was put into production in the late 1970s at Saint SOFF steel tube plant in France.
No matter the German process or French process, after the semi floating mandrel rolling mill is finished, about 1 / 3 of the pipe (tail) is wrapped around the front end of the mandrel, as shown in Fig. 4; the raw pipe with mandrel moves transversely to the stripper line, and the mandrel is drawn out from the tube by the stripper for cooling and lubrication. It is characterized by high precision and fast rhythm of raw pipe wall thickness. It can roll 3 or more tubes per minute. Although the length of mandrel is much shorter than that of floating mandrel mill, it is slightly longer than that of limited mandrel mill. It is equipped with rod stripper process and its process is longer. It is suitable for the production of seamless steel pipe with smaller specification (outer diameter less than 219mm). The representative units of German mode include the φ 194 mm unit of Bafan factory in Japan and the φ 89 mm unit of Hengyang in China; up to now, there is only one set of French model unit, that is, the φ 127 mm unit of Saint suff factory of French V & M company.
The semi floating mandrel mill also controls the mandrel speed during the rolling process, but the mandrel is released before the end of rolling. Like the floating mandrel mill, the mandrel is taken out of the mill by the steel pipe, and then the mandrel is extracted from the raw pipe by the stripper. When the mandrel speed is limited, the problem of metal flow regularity is solved to a certain extent. After releasing the mandrel, it is necessary to consider the limit of rod removal condition as the floating mandrel mill. Therefore, the diameter of steel pipe rolled by semi floating mandrel mill should not be too large.
The semi floating mandrel mill takes into account the advantages of both the limited mandrel mill and the floating mandrel mill. It not only maintains a high rolling rhythm, but also ensures the wall thickness accuracy and internal and external surface quality of the steel pipe. However, the upper limit of the rolling specification is limited due to the need to set up a stripper.

Limited mandrel mill

MPM (multi stand pipe mill) is widely used by inxi company in Italy, with 7-8 stands in general. During pipe rolling, the running speed of mandrel is limited and the speed is controllable; the speed of mandrel should be higher than the biting speed of the first stand and lower than the rolling speed of the first stand. During the whole rolling process, the mandrel speed is constant, so as to ensure the accuracy of pipe wall thickness. When rolling different pipes, the mandrel speed can be adjusted within a certain range. After the rolling, the mandrel stops, and the tube is removed from the mandrel by the pipe stripper, and then the mandrel is sent back to leave the rolling mill, and the mandrel is pulled out of the rolling line for cooling and lubrication before recycling. It is characterized by high precision of the wall thickness of the raw pipe, replacing the rod stripper with a pipe stripper, which shortens the process flow and shortens the mandrel; however, the rolling rhythm is slow, and it can roll two or more tubes per minute; it is suitable for the production of medium-sized seamless steel tubes (outer diameter less than 460mm). The representative units are the φ 356 mm unit of dalmin in Italy and the φ 250 mm unit of Tianjin steel pipe company of China.
In order to solve the problem of irregular metal flow in the rolling process of floating mandrel continuous rolling mill, shorten the length of mandrel and solve the difficulties in mandrel manufacturing, the limited mandrel rolling experiment was started abroad in the 1960s, and it was successful in the 1970s, and it was put into industrial production in dalmin plant in Italy.
The basic feature of the MPM is to control the running speed of the mandrel so that the mandrel can move forward at a constant speed lower than the first stand metal rolling speed in the whole binding process. This is a very important process improvement, which makes the limited mandrel mill have incomparable advantages over the floating mandrel mill. The practice in recent years shows that the speed of mandrel should be higher than the biting speed of the first stand and lower than the rolling speed of the first stand. In this way, the moving speed of the mandrel is lower than that of all stands in the whole rolling process, which avoids the irregular metal flow and the change of rolling conditions. Because the mandrel speed is controlled, the rolling pressure of each stand is small, the metal flow is regular, and the elongation coefficient can reach some, which can obtain very good wall thickness deviation.
The length of mandrel can be greatly shortened due to the speed limit of mandrel. The working length of mandrel is only 15m when rolling 32m steel pipe. After the steel tube is rolled out of the mandrel, it is pulled out from the front end of the mandrel by the pipe stripper, and the mandrel returns quickly. Unlike in the floating mandrel mill, it is not restricted by the stripping conditions, so medium and large seamless steel tubes can be produced.
The limited mandrel mill is developed on the basis of floating mandrel mill. Compared with the floating mandrel mill, the limited mandrel mill has the following advantages:

  • 1) The tool consumption is reduced. As the mandrel of the limited mandrel mill is shorter than that of the floating mandrel mill, the contact time between the steel pipe and the mandrel is shorter, so the service life of the mandrel is improved, and the mandrel consumption is generally reduced to about 1 kg per ton of steel pipe.
  • 2) The quality of steel pipe is improved. Due to the rolling property of limited mandrel mill (relative motion between mandrel and inner surface of steel tube), which is conducive to metal extension, and with micro tension rolling state, transverse deformation is reduced. There is no “slub” phenomenon caused by floating mandrel rolling, which greatly improves the internal and external surface and dimensional accuracy of steel pipe.
  • 3) The stripper is cancelled, the process flow is shortened and the final rolling temperature of steel pipe is increased. The reheating process before sizing can be omitted for some varieties, so as to save energy.
  • 4) Expanded product specifications. Due to the use of restrained mandrel rolling, the length of mandrel can be reduced, the weight of mandrel can be reduced, the diameter of mandrel can be increased, and the maximum outer diameter of steel pipe can be expanded from 177.8mm to 426mm or even larger. In addition, the MPM can also roll steel pipes with larger diameter to wall ratio (D / s > 40).

Limited mandrel mill represents the advanced technology of modern seamless steel pipe production, which embodies the development trend of continuous, high efficiency, mechanization and industrial automation of seamless steel pipe production. Since 1980s, it has been playing a leading role in seamless steel pipe production.
Mini-MPM is a process introduced by inxi company of Italy in 1990s. Its essence is the same as that of MPM. At that time, it was mainly designed for the technical transformation of Tosa plant in South Africa. It basically retained the advantages of MPM. Compared with MPM, it realized the rolling of long steel pipe with shorter mandrel, and the working section of mandrel was longer The total length of mandrel can be shortened by about 5 m. The mandrel can be made into a whole, and both ends are processed, and the limited moving head can be used to reduce the consumption of mandrel. With the popularization and application of cone-shaped roll piercer, it is possible to move forward the deformation of hot-rolled seamless steel pipe. The extension of continuous rolling process can be appropriately reduced. The required hot-rolling deformation can be realized without setting 7-8 stands in the continuous rolling mill. Therefore, the Mini-MPM with 5 stands is mostly used in the later limited mandrel continuous rolling mill. The representative units are 180 unit of Baotou Iron and steel company, 159 unit of Anshan Iron and steel company, 273 unit of Hengyang Steel Pipe Plant and 340 unit of Chengdu steel pipe plant.

20200918112305 89695 - Production process and production technology of hot rolled seamless steel pipe

Fig. 2 pass structure of continuous rolling pipe

PQF (high quality finishing)

PQF (high quality finishing) is also a limited mandrel mill, but each stand consists of three rolls. The pass with three roll design has better roundness than the traditional two roll design, and the pass radius difference is small, which is conducive to the uniform deformation of rolled piece and uniform wear of roll. The circumferential velocity difference between the bottom of the groove and the top of the groove is small, so that the metal deformation can be more uniform under stable conditions. Flange area (the area of the pipe not in contact with the roll or mandrel). That is to say, the wall thickness at the roll gap / the bulge area at the outer diameter is reduced, that is, the amount of metal flowing to the flange is reduced. This advantage is particularly important in rolling the end of steel pipe which is not restrained by the outer end and other stands. In fact, when the end of the steel tube is rolled on the three roll pipe mill, it is controlled because of the smaller flange area (about 30% smaller than that of the two roll mill) and the smaller circumferential velocity difference between the bottom of the groove and the top of the groove. Therefore, the tube end folding and the formation of flying wings can be avoided or greatly reduced. Because of the high circumferential compressive stress, the risk of longitudinal tensile stress at the roll gap can be greatly reduced during rolling. The stability of mandrel in pass is high. PQF unit can produce high strength (above P110) special grade oil well pipe, high pressure boiler pipe and 13Cr, 304L stainless steel pipe. The biggest advantage of PQF is that because the radius difference of three roll pass is less than that of two roll pass, the deformation of rolled piece is more uniform and stable, and the wall thickness accuracy and surface quality of products are higher than that of MPM. Due to the unique mandrel operation mode of the 168mm PQF unit, the rolling rhythm reaches 24 seconds / piece. The representative unit is the 168 mm unit of Tianjin steel pipe company.
There are two modes of mandrel operation in the MPM: one is that the mandrel stops moving at the end of rolling, and after the tube is removed from the mandrel, the mandrel returns quickly, pulls out the rolling line, cools and lubricates it for recycling. The traditional MPM adopts this operation mode; the other operation mode is: at the end of rolling, the mandrel stops moving and the waste pipe is removed from the mandrel by the pipe stripper After delivery, the mandrel does not return, but runs forward rapidly, following the tube through the pipe stripper in turn. After the mandrel passes through the stripper, it is pulled out of the rolling line and then recycled, cooled and lubricated. This method reduces the on-line waiting time of mandrel (non rolling steel). Therefore, the rolling cycle is shortened and the rolling rhythm is accelerated. PQF adopts this operation mode; the main difference between the two operation modes is that after the tube removal, the mandrel runs backward with the raw tube and pulls out the rolling line for cooling, lubrication and recycling after leaving the rolling mill; or the mandrel runs in the same direction and leaves the rolling mill before passing through the tube stripper to cool, lubricate and recycle. In the second method, because the mandrel passes through the stripper, the roll of the stripper must have the function of quick opening and closing when rolling the thin-walled tube (the diameter reduction of the stripper is greater than or equal to 2 times of the wall thickness of the raw tube), so as to prevent the mandrel from damaging the roll of the stripper.

20200918114210 46024 - Production process and production technology of hot rolled seamless steel pipe

Fig. 3 mandrel / tube after floating mandrel rolling

20200918114227 93361 - Production process and production technology of hot rolled seamless steel pipe
Fig. 4 mandrel / tube after semi floating mandrel rolling

About pipe stripper and rod stripper

In order to separate the raw pipe and mandrel from the continuous pipe mill, and to further process the finished steel pipe, two methods are generally adopted:
First, after rolling, the tube / mandrel is moved out of the rolling line together, and the tube is restrained axially, and the mandrel is drawn out from the tube by the device; we call this kind of equipment that the tube is not moved and the mandrel moves is called the rod stripper. When the raw pipe with mandrel enters the rod removal position, the rod removal clamping device on the rod removal chain will hook the tail handle of the mandrel, while the hydraulic opening and closing clamping plate will block the waste pipe, and the rod removal chain will draw out the mandrel from the waste pipe. The distance of the chain is about 1.1 times of the length of the mandrel. The maximum speed of the stripper is more than 4.5m/s. The error of reaching the final position is ± 50 mm. The released mandrel is sent to the mandrel positioning lifting baffle through the conveying roller table, and then the mandrel is sent to the mandrel cooling tank by the mandrel transfer device for recycling. The installation position of the stripper is parallel to the continuous pipe mill. The stripper has two rows of stripper chains. The two chains are connected by beams. There are two rod removal crossbeams and several load-bearing beams between the two chains. The stripper beam is used to extract the mandrel from the tube, while the loading beam is used to support the mandrel during the stripping process. The rod removal clamping device related to the size of rolled piece is fastened with bolts on the stripper beam. When the rolling mandrel is replaced, it is necessary to replace the clamping device.
The other is that after rolling, the mandrel stops moving, and the tube is removed from the mandrel by the device on line; we call this kind of device that the mandrel does not move and the tube moves is called the pipe stripper. There are two roll and three roll stripper. The setting of the pipe stripper has two important technological purposes: one is to extract the waste pipe from the mandrel to complete the purpose of pipe removal. In the rolling line, the stripping machine is omitted, the process flow is shortened and the final rolling temperature is increased; the second is to play the role of calibration (sizing), that is, in the production of each steel pipe, the machine also has the function of extension and sizing. In order to produce thin-walled tube and medium thick wall tube, the nominal diameter of pass of each stripper must be smaller than that of mandrel of tube rolling mill. The thin-walled tube is not easy to be stripped out of the tube in the production. Safety mortar is installed on each stripping machine to prevent damage to roll and related mechanical parts when mandrel enters into the stripper. In case of an accident, if the raw pipe with mandrel enters the pass of the stripper, if the radial load borne by the roll of the stripper is greater than the preset value, the roll pass will be opened correspondingly to prevent the mandrel from damaging the roll and equipment.

Source: China Steel Pipe Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

If you want to have more information about the article or you want to share your opinion with us, contact us at sales@epowermetals.com

Please notice that you might be interested in the other technical articles we’ve published:

Summary
production process and production technology of hot rolled seamless steel pipe - Production process and production technology of hot rolled seamless steel pipe
Article Name
Production process and production technology of hot rolled seamless steel pipe
Description
The production process of hot-rolled seamless steel pipe includes several basic processes, such as preparation before rolling, billet heating, piercing, rolling, sizing and reducing, cooling and finishing of steel pipe.
Author
Publisher Name
www.epowermetals.com
Publisher Logo
PREV

RELATED POSTS

Leave a Reply

*

*

Inquery now

SUBSCRIBE TO OUR NEWSLETTER

FOLLOW US

YouTube
العربية简体中文繁體中文NederlandsEnglishFrançaisDeutschItaliano日本語한국어LatinPortuguêsРусскийEspañolTürkçe
  • Email me
    Mail to us