Due to the limitations of billet conditions and the extension capacity of the piercing machine, the size and accuracy of the rough pipe after piercing 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.
1. Automatic pipe rolling machine rolling
The automatic pipe rolling machine consists of three parts: the main machine, the front, and the back. 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 rolls. At the same time, to meet the needs of returning steel pipes, a quick lifting mechanism for the upper working roll and the lower return roll is provided. The working roll has a round hole type. The rough pipe sent by the piercing machine and the stretching machine is rolled in a ring 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 roll and the lower return roll are lifted to a certain height, and the rough pipe is sent back to the front stage by the return roll, 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 the automatic pipe rolling machine is that the production specifications can be adjusted flexibly. As far as the steel type is 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, the total extension of the two passes is only less than 2.5; the wall thickness is uneven, and internal scratches often occur, which must be eliminated by the 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).
2. Accu-Roll tube mill
It is a two-roller horizontal long mandrel inclined rolling mill with active guide discs. The mill structure has the following features: the two rolls are tapered. Like the tapered roller piercing machine, there are both feeding angles and rolling angles, so that the roll 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 of ASSEL’s concentrated wall reduction on the shoulder part, which reduces the roll life and wall uniformity effect, thereby improving the wall thickness accuracy of the rough pipe.
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 perforation process of the initial jacking unit is divided into the hydraulic perforation method, which uses a vertical hydraulic press to squeeze the steel ingot placed in the mold into a rough tube with a cup bottom, and then uses a crane to take out the rough tube, lay it down, and put the cup-shaped rough tube on the long core rod. The core rod is pushed to make the cup-shaped rough tube 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 push 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 is to produce rough tubes by oblique rolling and perforation method and provide rough tubes for the jacking machine by shrinking one end of the rough tube. It can improve production and product quality, and restore vitality to 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, which can reach 10 to 17. Therefore, the number of equipment and tools required for the jacking method of rolling similar products can be less.
3) The range of varieties and specifications is wide. The disadvantage is that the wall thickness accuracy is not high, and the inner and outer surfaces are prone to scratch defects.
4. Extrusion of steel pipes
The so-called extrusion method refers to placing the 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 positive extrusion and reverse extrusion. The force direction of positive extrusion is consistent with the metal flow direction, and the reverse extrusion is opposite. Reverse extrusion has the advantages of small extrusion force, large extrusion ratio, fast extrusion speed, reduced extrusion temperature, improved extrusion conditions, easy-to-achieve isothermal/isobaric/isospeed extrusion, improved product structure performance and dimensional accuracy, can reduce the metal pressure surplus at the end of extrusion, and improve metal recovery rate; but 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:
1) Fewer processing steps, which can save investment under the same output.
2) Since the extruded metal is in a triaxial compressive stress state, it can produce materials that are difficult or impossible to roll and forge, such as nickel-based alloys.
3) 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 structure is uniform and the performance is good.
4) There are few defects on the internal and external surfaces, and the geometric dimension accuracy is high.
5) The production organization is flexible and suitable for small-batch and multi-variety production.
6) It can produce pipes and bimetallic composite pipes with complex sections.
Disadvantages are:
1) High requirements for lubricants and heating, which increases production costs.
2) And the tool life is short, the consumption is large, and the price is expensive.
3) The yield rate is low, which reduces the competitiveness of the product.
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 the roll pushes out the rough pipe so that the rough pipe is reduced in diameter, reduced in 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 mill productivity 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.
6. Hot expansion of steel pipe
The maximum outer diameter of the finished steel pipe produced by the hot-rolled seamless steel pipe unit is less than 530mm for the automatic pipe rolling unit; less than 460mm for the continuous pipe rolling unit; and less than 660mm for the large Pilger. When a larger diameter steel pipe is required, in addition to the jacking method and 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 pipe. 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 pushing does not require heating the entire steel pipe.
Oblique rolling expansion machine
The oblique rolling expansion process 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. The oblique rolling expansion can produce steel pipes with a wall thickness of 6 to 30 mm and a maximum outer diameter of 710 mm. The 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 that can be produced is 750mm. 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 reduced, 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, ease of mastering; 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 mandrel from the pipe head in sequence to achieve the purpose of expansion; when the tail of the steel pipe is pushed into the mandrel, a new steel pipe to be processed is added behind it. After the pusher head returns, it continues 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 mandrel, 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. Due to its low investment, many sets have been newly built in North my country recently. The top-pushing expansion process can currently produce steel pipes with a wall thickness of 6 to 30 mm and a maximum outer diameter of 860 mm.
Post time: Sep-30-2024