Due to the limitations of billet conditions and the extension capacity of the piercing machine, the size and accuracy of the rough pipe after perforation cannot meet the user’s requirements. The rough pipe needs to be further processed. There are many methods for hot-processing and extending seamless steel pipes. In addition to the three types of machines introduced above, the following methods are currently commonly used.
5.4.1 Automatic pipe rolling machine
The automatic pipe rolling machine was invented by the Swiss Stephen in 1903, and the first set of units was established in 1906. Before the 1980s, it was one of the main methods for hot-rolling seamless steel pipes. Due to the limitations of its rolled pipe length, wall thickness accuracy, etc., it has been gradually replaced by continuous pipe rolling units; currently, the best automatic pipe rolling unit in my country is the 400 unit in Baotou. Except for some automatic pipe rolling units in the former Soviet Union and Eastern Europe that are still in use, most of the others have been dismantled. The automatic pipe rolling machine consists of three parts: the main machine, the front desk, and the back desk. The main machine is a two-roller irreversible longitudinal rolling mill, which is characterized by a pair of high-speed reverse-rotating return rollers installed behind the working rollers. At the same time, to meet the needs of returning steel pipes, a quick lifting mechanism for the upper working roller and the lower return roller is provided. The working roller has a round hole type. The rough pipe sent by the piercing machine and the stretching machine is rolled in the annular hole type composed of a round hole type and a head (conical head or spherical head). Usually, two passes are rolled. After each rolling pass, the upper working roller and the lower return roller are lifted to a certain height, and the rough pipe is sent back to the front stage by the return roller, and then the rolled pipe is restored to the original working position, and the steel pipe is turned 90°, and then the second pass is rolled in the same hole type. The deformation amount of each pass is adjusted by the difference in the head diameter of the two passes. After the rolled steel pipe is returned to the front stage, it is moved horizontally to the leveling machine for leveling. Its deformation process also goes through three stages: flattening, diameter reduction, and wall reduction.
The advantage of automatic tube rolling mills is that the specifications of production can be adjusted flexibly. As far as steel types are concerned, the applicable range is wide, and low and medium carbon steel, low alloy steel, stainless steel, etc. can be produced; it is suitable for small batch and multi-variety production. Its disadvantages are poor deformation ability, and the total extension of two passes is only less than 2.5; the wall thickness is uneven, and internal scratches often occur, which must be eliminated by a leveling machine; the length of the rough pipe is short, which affects the improvement of the yield rate. Low production efficiency (slow-rolling rhythm, but lightweight).
5.4.2 Accu-Roll tube mill
Accu-Roll tube mill started in Yantai, Chengdu, and other places in my country in the early 1990s. It was very popular at that time and had the momentum to replace other oblique rolling and continuous rolling units. However, after practical tests, it was found that the short length of the raw tubes it rolled limited the production of 3-times-long tubes of some specifications, and the deep spiral marks on the surface of the raw tubes when rolling thin-walled tubes affected the appearance quality of the steel tubes. So far, it has only survived in my country, especially recently some private enterprises have newly built a batch of small Accu-Roll tube mills. So far, there has been no report on the construction of this type of tube mill abroad. This type of machine is not suitable for the production of large and medium-diameter seamless steel pipes. It is a two-roller horizontal long mandrel oblique rolling mill with an active guide plate.
The mill structure has the following characteristics:
The two rollers are conical. Like the tapered roller piercing machine, it has both a feeding angle and a rolling angle, so that the roller diameter gradually increases along the rolling direction, which is conducive to reducing sliding, promoting the longitudinal extension of the metal, and reducing additional torsional deformation.
Two large-diameter active guide discs are used.
The limited mandrel operation mode is adopted.
The roller type without roller shoulder is adopted. It is reported that this overcomes the problem that the ASSEL reduces the wall reduction amount of the roller shoulder part, which reduces the roller life and the wall uniformity effect, thereby improving the wall thickness accuracy of the rough pipe.
5.4.3 Pipe jacking machine pipe jacking
The pipe jacking method for producing seamless steel pipes was proposed by Heinrich Erhard of Germany as early as 1892. The piercing process of the early pipe jacking unit is divided into the hydraulic piercing method, which uses a vertical hydraulic press to squeeze the steel ingot placed in the mold into a rough pipe with a cup bottom, and then uses a crane to take out the rough pipe, lay it down, and put the cup-shaped rough pipe on the long mandrel. The mandrel is pushed to make the cup-shaped rough pipe pass through a group of annular die holes with decreasing diameters in turn to obtain diameter reduction, wall reduction, and extension. The power of deformation is all concentrated at the tail of the jacking rod. After the jacking, the rod needs to be removed and then the cup bottom is cut off. The characteristics are low productivity, serious uneven wall thickness, and limited L/D of the steel pipe. At present, only this method is used to produce large-diameter (400-1400m seamless steel pipe). Another method is called the CPE method, which uses the oblique rolling and perforation method to produce rough pipes, and the method of shrinking one end of the rough pipe provides rough pipes for the jacking machine. It can improve production and product quality, and restore vitality for the production of small-diameter seamless steel pipes by the jacking process.
The advantages of the jacking method are:
1) Low investment, simple equipment and tools, and low production cost.
2) The extension of the jacking unit is large, up to 10-17. Therefore, the number of equipment and tools required for rolling similar products by the jacking method can be less.
3) A wide range of varieties and specifications.
The disadvantage is that the wall thickness accuracy is not high, and scratch defects are prone to occur on the inner and outer surfaces.
5.4.4 Extruded steel pipe
The so-called extrusion method refers to a method of placing a metal billet in a “closed” container composed of an extrusion cylinder, an extrusion die, and an extrusion rod, and applying pressure by the extrusion rod to force the metal to flow out of the extrusion die hole to obtain metal plastic forming. This is a method of manufacturing seamless steel pipes with a long history. According to the relative relationship between the force direction of the extrusion rod and the metal flow direction, the extrusion method can be divided into two types: positive extrusion and reverse extrusion. The force direction of positive extrusion is consistent with the metal flow direction, while reverse extrusion is the opposite. Reverse extrusion has the advantages of small extrusion force, large extrusion ratio, fast extrusion speed, lower extrusion temperature, improved extrusion conditions, easy-to-achieve isothermal/isobaric/constant speed extrusion, improved product structure performance and dimensional accuracy, reduced metal pressure surplus at the end of extrusion, and increased metal recovery rate; however, its operation is relatively inconvenient, and the cross-sectional size of the product is limited by the size of the extrusion rod.
The application of metal extrusion technology in industry has a history of more than 100 years, but the use of hot extrusion technology in the production of steel has gradually developed after “Seshi” invented glass extrusion lubricant in 1941. In particular, the development of non-oxidative heating, high-speed extrusion technology, mold materials, and tension reduction technology have made hot extrusion production of seamless steel pipes more economical and reasonable, greatly improving the output and quality, and further broadening the range of varieties, thus attracting the attention of various countries.
At present, the product range of steel pipes produced by extrusion is generally: outer diameter: 18.4~340mm, minimum wall thickness can reach 2mm, length is about 15m, and small-diameter pipes can get 60m steel pipes. The capacity of the extruder is generally 2000~4000 tons, and the maximum is 12000 tons.
Compared with other hot rolling methods, the production of extruded seamless steel pipes has the following advantages:
Fewer processing steps, which can save investment under the same output.
Because the extruded metal is in a three-dimensional compressive stress state, it can produce materials that are difficult or impossible to roll and forge, such as nickel-based alloys.
Due to the large amount of metal deformation during extrusion (large extrusion ratio), and the complete deformation is completed in a very short time, the product has a uniform structure and good performance.
There are few defects on the internal and external surfaces, and the geometric dimension accuracy is high.
The production organization is flexible and suitable for small-batch and multi-variety production.
It can produce pipes and bimetallic composite pipes with complex sections.
The disadvantages are:
1) High requirements for lubricants and heating, which increases production costs.
2) As well as low tool life, large consumption, and high prices.
3) The yield rate is low, which reduces the competitiveness of the product.
5.4.5 Cycle tube mill (Pilger tube mill) tube rolling
The cycle tube mill was put into industrial production in 1990. It is a single-frame two-roller mill. There is a variable cross-section hole on the roll. The two rolls rotate in opposite directions, and the rough pipe is fed in the opposite direction of the roll. The roll rotates one circle and pushes out the rough pipe so that the rough pipe is reduced in diameter, reduced in the wall, and finished in the hole to complete the rolling of a section of the rough pipe. Then the rough tube is fed again for rolling. A rough tube needs to be circulated back and forth in the hole many times to complete the entire rolling process, so it is called a periodic tube rolling mill, also known as a Pilger tube rolling mill. The tube is periodically processed by a variable-section roller hole, and the feeding and rotation operations of the tube material are combined to make the tube wall undergo multiple cumulative deformations to obtain a larger wall reduction and elongation.
The characteristics of this production method are:
1) It is more suitable for the production of thick-walled tubes, and its wall thickness can reach 60-120mm;
2) The range of processed steel types is relatively wide. Since its deformation method is a combination of forging and rolling, it can produce tubes of low-plasticity and difficult-to-deform metals, and the mechanical properties of the steel tubes are excellent.
3) The length of the rolled steel tube is large, up to 35m.
4) The productivity of the rolling mill is low, generally 60-80%, so the output is low; therefore, a piercing machine needs to be equipped with two periodic tube rolling mills to balance.
5) The tail cannot be processed, resulting in large cutting losses and a low yield rate.
6) Poor surface quality and serious uneven wall thickness.
7) Large tool consumption, generally 9-35kg/t.
5.4.6 Hot expansion of steel pipes
The maximum outer diameter of finished steel pipes produced by hot-rolled seamless steel pipe units is less than 530mm for automatic pipe rolling units; less than 460mm for continuous pipe rolling units; and less than 660mm for large piles. When a larger diameter steel pipe is required, in addition to the jacking method and the extrusion method, the steel pipe hot expansion method can be used. This method can currently produce a thin-walled pipe with a maximum outer diameter of 1500mm for seamless steel pipes.
There are three methods for hot expansion of steel pipes: oblique rolling, drawing, and pushing. These three methods started in the 1930s. Oblique rolling and drawing require heating the steel pipe as a whole before deformation processing can be carried out, while the pushing method does not require heating the entire steel pipe.
Oblique rolling expansion machine:
The process flow of oblique rolling expansion is: the heated pipe material is transported to the oblique rolling expansion machine for expansion. The oblique rolling expansion machine consists of two rollers of the same shape. The axes of the two rollers are at an angle of 30° to the rolling line, and the two rollers are driven by motors separately to rotate in the same direction. The plug participates in the deformation in the expansion deformation zone, and the steel pipe makes a spiral motion in the deformation zone. The pipe wall is rolled by the rollers and the plug so that the expansion diameter is enlarged and the wall thickness is thinned. The axial force of the plug is borne by the push rod, which can be arranged on the inlet side or installed on the outlet side.
Oblique rolling expansion can produce steel pipes with a wall thickness of 6 to 30 mm and a maximum outer diameter of 710 mm. Its disadvantage is that there are residual spiral marks on the inner and outer surfaces of the steel pipe, which reduces the surface quality. For this reason, a leveling machine and a sizing machine must be installed. This type of expansion machine has large equipment, high investment costs, and certain restrictions on varieties, and cannot produce thick-walled pipes.
Drawing expansion machine:
Drawing expansion is a production method with low production capacity, but it is still in use because of its simple equipment and processing and easy mechanized operation. The drawing expansion machine can be used for both cold drawing and hot drawing expansion. When the expansion amount is not large and the physical and mechanical properties and dimensional accuracy of the steel pipe need to be improved, cold drawing expansion can be used. The process flow of hot drawing expansion of steel pipes is heating of pipe material, expansion of pipe ends, expansion and drawing, straightening, cutting heads and tails, and inspection. The expansion rate of each heating is 60-70%, and the maximum diameter of steel pipes of 750mm can be produced.
The main working principle of drawing hot expansion is: through a group (generally 1-4) of plugs with gradually increasing diameters, insert and pass through the entire length of the inner hole of the steel pipe, so that the diameter of the steel pipe is expanded, the wall thickness is thinned, and the length is slightly shortened.
The main tools of the drawing expansion machine are expansion plugs, expansion plugs, and ejector rods. The advantages are simple equipment, convenient operation, and ease to master; a wide range of product varieties and specifications, and can also to produce rectangular and other special-shaped steel pipes. The disadvantages are long production cycle, low productivity, and high consumption of tools and metal.
Push-type expander: The working principle of the push-type expander is to place the raw steel pipe in the medium-frequency induction coil. After medium-frequency induction heating, the hydraulic cylinder piston or the pusher head of the winch moves to push the tail of the steel pipe so that the steel passes through the axially fixed conical core rod from the pipe head in sequence to achieve the purpose of expansion; when the tail of the steel pipe is pushed into the core rod, a new steel pipe to be processed is added behind it, and the pusher head returns to continue to push the tail of the new steel pipe. The head of the new steel pipe pushes the tail of the previous steel pipe through the core rod, thereby completing the expansion of the steel pipe. Since only the steel pipe in the deformed section is heated, the deformed steel pipe is easy to bend, and the wall thickness and length of the expanded pipe are limited. The advantages of the pusher expander are a high metal recovery rate, simple equipment, and low energy consumption. The disadvantages are that the performance consistency of the steel pipe in the length direction is slightly poor and the production efficiency is low.
Post time: Oct-31-2024