A method for manufacturing a hot-rolled seamless steel pipe having an outer diameter of 38 to 50 mm

Abstract

The present application belongs to the technical field of steel pipe production, and particularly relates to a manufacturing method of hot-rolled seamless steel pipe with an outer diameter of 38-50 mm, which comprises the following steps: S1, preparing a solid round pipe blank, cutting the solid round pipe blank, and then heating and reducing the pipe blank to prepare a blank pipe; S2, longitudinally rolling and removing the blank pipe to prepare a hollow pipe; S3, heating and uniformly controlling the temperature of the hollow pipe, and then performing diameter reduction on the hollow pipe by a tension reducing machine to obtain a long finished product size steel pipe; and S4, performing online controlled cooling, pre-cutting, straightening, nondestructive flaw detection and fixed-length sawing on the long finished product size steel pipe to prepare the hot-rolled seamless steel pipe with an outer diameter of 38-50 mm. The product produced by the present application has high size precision, stable and small fluctuation of organization performance, and the present application has simple process, less equipment investment, energy saving and environmental protection, compact production, and a yield of 95%.

Description

Technical Field

[0001] This invention belongs to the field of steel pipe production technology, specifically relating to a method for manufacturing small-diameter hot-rolled seamless steel pipes with an outer diameter of 38mm to 50mm. Background Technology

[0002] With the vigorous implementation of my country's energy conservation and emission reduction policies and the continuous expansion of the circular economy, the market demand for high-precision, small-diameter seamless steel pipes used in high-temperature, high-pressure waste heat boilers, gas-fired boilers, and waste incineration boilers is increasing. However, most seamless steel pipes with an outer diameter ≤50mm are manufactured using traditional cold-forming processes, whose high investment, high cost, high pollution, and low efficiency have seriously affected market demand. The replacement of small-diameter cold-formed seamless steel pipes with high-precision, small-diameter hot-rolled seamless steel pipes will inevitably become a development trend.

[0003] Currently, the main forming methods for small-diameter seamless steel pipes are cold drawing and cold rolling.

[0004] Cold drawing forming involves using large-sized tube blanks, heating them at high temperatures, and then hot forging or hot rolling them individually to produce small-sized round bars, i.e., small-diameter tube blanks. A centering hole is machined at the center of one end face of the tube blank, followed by heating and piercing to form a rough tube. Then, through processes such as head making, softening annealing, pickling, phosphating, and lubrication, the tube is cold-drawn to achieve the final finished steel pipe size. This process typically requires multiple drawing passes. Finally, the finished steel pipe undergoes heat treatment to obtain the final product. The head making process requires making each pipe individually. Pickling, phosphating, and lubrication are to reduce the cooling temperature. While cold drawing resists deformation and ensures the surface quality of cold-drawn steel pipes, reducing tool and die wear, it increases the cost of surface treatment and waste liquid disposal. Cold drawing deformation aims to achieve the required dimensions, but the deformation size per pass is small, and the drawing speed is slow. Finished product heat treatment aims to achieve the physical, mechanical, and technological properties of the steel pipe. Due to cold deformation stress and work hardening, cold-drawn steel pipes, whether carbon or alloy steel, require appropriate finished product heat treatment, increasing energy costs, carbon emissions, and metal oxide loss. In short, high cost, high pollution, and low efficiency are inherent characteristics of the cold drawing forming method.

[0005] The basic process of cold rolling forming is the same as that of cold drawing forming. It adopts a cold rolling deformation process. Cold rolling deformation is used to achieve the required dimensions of the steel pipe. However, the deformation dimension of a single cold rolling pass is small and the rolling speed is slower. To achieve the final finished size of the steel pipe, multiple rolling passes are generally required. During the rolling process, heavy machine oil needs to be sprayed onto the inner and outer surfaces of the steel pipe for deformation lubrication. Therefore, the cost of steel pipe deformation processing is significantly increased and the production efficiency is significantly reduced. At the same time, the management cost caused by oil pollution is increased. It has the characteristics of high cost, high pollution and low efficiency.

[0006] It is evident that the existing processes for producing seamless steel pipes with an outer diameter of 38mm to 50mm all suffer from high costs, high pollution, and low efficiency, and cannot fully meet the urgent production requirements of high efficiency, high quality, low energy consumption, low pollution, and low emissions. Summary of the Invention

[0007] This invention proposes a method for manufacturing hot-rolled seamless steel pipes with an outer diameter of 38-50 mm, which solves the technical problems existing in the prior art.

[0008] To achieve the above objectives, the present invention adopts the following technical solution:

[0009] A method for manufacturing a hot-rolled seamless steel pipe with an outer diameter of 38-50 mm includes the following steps:

[0010] Step S1: A solid round tube blank with an outer diameter of 150mm is obtained by feeding and unloading a large-diameter solid continuous casting round tube blank with a diameter of 230mm. The obtained solid round tube blank is cut off, heated, and then subjected to diameter reduction and piercing to obtain a rough tube with an outer diameter of 130mm and a wall thickness of 12.2mm.

[0011] Step S2: The tube is longitudinally rolled and stripped to obtain a rough tube with a diameter of 112 mm and a wall thickness of 3.80 mm;

[0012] Step S3: After the rough pipe is heated and the temperature is controlled by medium frequency induction heating, it is sized by tension reducing machine to obtain a long finished steel pipe.

[0013] Step S4: Long finished steel pipes are subjected to online controlled cooling, pre-cutting, straightening, non-destructive testing, and fixed-length sawing to produce hot-rolled seamless steel pipes with an outer diameter of 38-50mm.

[0014] As a further improvement, in step S1, the solid round tube blank is placed in an annular heating furnace for heating, and then pierced using a piercing machine to obtain a tube with an outer diameter of 130 mm and a wall thickness of 12.2 mm.

[0015] As a further improvement, the solid round tube blank is heated in an annular heating furnace with a heating rate controlled at 780℃ / h, a preheating section of 850℃, a first heating section of 1100℃, a second heating section of 1260℃, a soaking section of 1270-1290℃, and a total heating time of 100 minutes.

[0016] As a further improvement, in step S2, a three-roll five-stand hot rolling mill is used to shorten the mandrel length to 10 meters. The mandrel is preheated using medium-frequency induction rapid preheating, and the hardness of the rolls is controlled between 400 HB and 440 HB. A combination of 130 mm to 120 mm stand pass profiles is used to roll a 130 mm diameter tube into a rough tube with a diameter of 119.43 mm. The rough tube is then separated from the mandrel by a three-roll three-stand tube stripper. The rough tube with a diameter of 119.43 mm is further rounded and reduced in diameter to obtain a rough tube with a diameter of 112 mm and a wall thickness of 3.80 mm.

[0017] As a further improvement, in step S2, during the three-roll five-stand hot rolling process, the temperature of the inlet tube is 1100℃~1130℃, the diameter of the pass set for the first stand is 123.40mm, the diameter of the pass set for the second stand is 120.80mm, the diameter of the pass set for the third stand is 119.43mm, the diameter of the pass set for the fourth stand is 118.82mm, the diameter of the pass set for the fifth stand is 118.82mm, the diameter of the mandrel is 111.5mm, and the total rolling elongation coefficient is 3.72.

[0018] During the tube removal process of the three-roller, three-stand tube remover, the diameter of the hole in the first stand is 117.84 mm, the ellipticity is 1.057, the diameter of the hole in the second stand is 113.59 mm, the ellipticity is 1.024, and the diameter of the hole in the third stand is 112.00 mm, the ellipticity is 1.005, and the elongation coefficient is 1.04.

[0019] As a further improvement, in step S3, a medium-frequency induction furnace is used for rapid induction heating, and the rough tube passes through at a speed of 1.2 m / s, raising the temperature of the rough tube from 840°C to 920°C, and the final rolling temperature of the steel pipe is 880°C.

[0020] Tension reduction and sizing are performed using a 3-roll 24-stand tension reduction machine, and the CEC control function is activated to obtain a finished steel pipe with a suitable outer diameter. In step S4, the steel pipe is subjected to online controlled cooling on a 64-meter-long online cooling bed. The initial temperature of controlled cooling is ≥860℃, and the cooling rate is controlled at 40℃ / min~60℃ / min.

[0021] As a further improvement, in step S1, the piercing machine used for diameter reduction piercing includes a frame, on which a two-roll skew-roll conical piercing mechanism, a three-roll skew-roll drum-shaped piercing mechanism, and a push rod mechanism are slidably mounted. The two-roll skew-roll conical piercing mechanism is provided with two conical rolls, two guide plates, and a conical roll drive mechanism. The three-roll skew-roll drum-shaped piercing mechanism is provided with three drum-shaped rolls and a drum-shaped roll drive mechanism. The push rod mechanism is rotatably provided with a first push rod, the end of which is provided with a first head. The first head is detachably connected to a second push rod coaxially mounted. The second push rod is provided with a second head. The diameter of the first push rod is larger than the diameter of the second push rod, and the diameter of the first head is larger than the diameter of the second head.

[0022] The first push head is provided with a threaded hole, and the end of the second push rod connected to the first push head is provided with an external thread. The second push rod is threadedly connected to the first push head. The end of the first push head facing the second push head is provided with an annular platform. The second push rod is provided with an umbrella-shaped cap. The area of ​​the bottom end of the umbrella-shaped cap is the same as the area of ​​the annular platform. The umbrella-shaped cap abuts against the annular platform.

[0023] The second push rod has an external thread section near the second push head, and the external thread section has a material groove. The material groove extends in a direction parallel to the axial direction of the second push rod. There are two or more material grooves, and the two or more material grooves are evenly distributed along the circumference of the second push rod.

[0024] As a further improvement, the two-roll skew rolling conical piercing mechanism and the three-roll skew rolling drum-shaped piercing mechanism each include a machine base. Gear mounting seats are slidably installed at both ends of the machine base. Hydraulic cylinders are provided on both sides of the two gear mounting seats. The piston rods of the hydraulic cylinders abut against the gear mounting seats. A central gear is fixedly installed in the middle of the opposite side of the two gear mounting seats. A gear ring is rotatably installed on the gear mounting seat outside the central gear. A roll mounting gear is meshed between the central gear and the gear ring. The roller shafts of the conical roll and the drum-shaped roll are respectively mounted on the roll mounting gear through spherical bearings. The outer circumferential surface of the gear ring is provided with gear teeth. A rotating adjusting rod driven by a hydraulic cylinder is slidably provided on the gear mounting seat. A rack that meshes with the gear teeth on the outer circumferential surface of the gear ring is provided on the rotating adjusting rod.

[0025] As a further improvement, the roll composition by mass percentage is: carbon 3.2%–3.6%, silicon 1.3%–1.8%, chromium 0.2%–0.5%, molybdenum 0.7%–1.0%, manganese 0.4%–0.8%, nickel 2.5%–3.5%, with the balance being iron.

[0026] As a further improvement, the mass percentage composition of the tension reducing roller of the tension reducing machine is as follows: carbon 2.9%–3.6%, silicon 1.0%–2.0%, chromium 0.2%–1.0%, molybdenum 0.2%–1.0%, manganese 0.4%–1.0%, nickel 1.2%–3.5%, with the balance being iron, and the hardness of the tension reducing roller is 60HSD–65HSD.

[0027] Due to the adoption of the above technical solution, the beneficial effects of the present invention are as follows:

[0028] This invention uses a large-diameter solid continuously cast round tube billet with a diameter of 230mm. This diameter continuously cast round tube billet can be produced by a hot rolling production line with fewer stands, shorter process and lower investment, resulting in low cost. Compared with small-diameter rolled tube billets, it can save the cost per ton of steel. Under the current market conditions, it can save at least 300 yuan / ton of steel cost.

[0029] The longitudinal rolling of this invention utilizes a three-roll mill to minimize the radius difference between the top of the roll pass and the edge of the roll, thereby mitigating the adverse effects of large rolling speed differences on metal deformation. A five-stand mill is used to shorten the mandrel length to a maximum of 10 meters, reducing the difficulty and cost of manufacturing small mandrels. To ensure uniform graphite coating on the mandrel, medium-frequency induction rapid preheating is employed. Compared to traditional heating furnaces, this method has a shorter preheating cycle, lower capital investment, smaller footprint, and higher heat utilization rate. It also reduces the number of mandrels used in cyclic operation. By controlling the chemical composition of the rolls, the roll hardness is controlled between 400 HB and 440 HB, thereby improving the rolls' wear resistance and ensuring the wall thickness accuracy and surface flatness of the continuously rolled rough tubes. Based on the above improvements, a combination of 130mm to 120mm frame hole patterns is used to roll a 130mm diameter tube into a rough tube with a diameter of 119.43mm. Then, the rough tube is separated from the mandrel by a tube stripper, and the 119.43mm diameter rough tube is further rounded and reduced in diameter to obtain a rough tube with a diameter of 112mm.

[0030] The online medium-frequency induction heating of this invention, through the combined action of intelligent rapid temperature control, six sets of thyristor-controlled medium-frequency rapid induction electric heating furnaces, and the steel pipe moving at a speed of 1.25 m / s, ensures a longitudinal temperature difference of ±10℃ and a circumferential temperature difference of ±5℃ for the steel pipe. This guarantees precise temperature control before tension sizing of the rough pipe. Utilizing the rapid speed of medium-frequency induction heating, the grains of the rough pipe are fine and uniform after heating, resulting in a significant improvement in the overall performance of the steel pipe material. The fast speed and short duration of medium-frequency induction heating reduce oxidation of the rough pipe surface in the air, ensuring surface cleanliness and significantly improving the surface quality of the steel pipe after tension sizing. Compared with traditional gas-fired heating furnaces, the medium-frequency induction heating furnace has a smaller footprint, lower capital investment, higher heat utilization rate, lower carbon emissions, and no waste gas pollution.

[0031] This invention uses a tension reducing mill for sizing. By redistributing the deformation parameters in the preceding piercing and continuous rolling processes, the inlet diameter of the tension-reduced rough tube is reduced to 112mm, and the total reduction rate of the 38 series is reduced to 66%. The 38 series small-diameter tubes are prone to bending; by adding a straightening stand, the straightness of the finished tube is ensured, guaranteeing smooth production. The use of CEC (Continuous Elongation) on the tension reducing mill shortens the thickened ends, ensuring a high yield. Specifically, the die-cutting of the tension reducing mill can be performed using an independent lathe, in conjunction with... Compared to existing processing methods, changing the processing method ensures the dimensional accuracy of the finished frame; the outer diameter tolerance of small-diameter pipes is small, which is controlled by the chemical composition of the tension reduction rolls: carbon 2.9%~3.6%, silicon 1.0%~2.0%, chromium 0.2%~1.0%, molybdenum 0.2%~1.0%, manganese 0.4%~1.0%, nickel 1.2%~3.5%, thereby controlling the roll hardness to 60HSD~65HSD, ensuring the wear resistance of the rolls, and thus making the outer diameter of the steel pipe reliable and stable.

[0032] This invention can produce high-precision hot-rolled seamless steel with a diameter of 38mm to 50mm, involving steel grades such as 10Cr9MoW2VJJNbBN, 10Cr9Mo1VNbN, X10CrWMoVNb9-2, X10CrMoVNb9-1, T92, T91, P92, P91 and below.

[0033] In this invention, during the diameter reduction piercing process, the solid round tube blank first undergoes diameter reduction piercing through a two-roll skew rolling conical piercing mechanism, and then through a three-roll skew rolling drum-shaped piercing mechanism. Combined with a unique push rod mechanism, continuous diameter reduction piercing is achieved in a single process. This combines the advantages of both two-roll skew rolling conical piercing and three-roll skew rolling drum-shaped piercing. The two-roll skew rolling conical piercing offers high efficiency, while the three-roll skew rolling drum-shaped piercing provides good quality. The diameter reduction piercing is completed in two steps, compared to existing methods that use only two-roll skew rolling conical piercing or three-roll skew rolling drum-shaped piercing. In comparison, the individual piercing and diameter reduction amounts of the two-roll skew rolling conical piercing and the three-roll skew rolling drum-shaped piercing in this technical solution are significantly reduced, and the piercing efficiency and quality are significantly improved. In specific use, the diameter of the conical roll is smaller than the diameter of the two-roll skew rolling conical piercing when using it alone for the same specifications, and the diameter reduction amount is also smaller, which makes it easier to reduce the diameter during piercing. In order to take advantage of the two-roll skew rolling conical piercing, the diameter reduction amount of the two-roll skew rolling piercing in this technical solution is greater than that of the three-roll skew rolling piercing.

[0034] The piercing mechanism includes a first piercing rod and a second piercing rod that are coaxially connected. The second piercing rod has a threaded section near the second mandrel. When piercing, the piercing rod rotates, and the external thread contacts the inner wall of the tube blank after piercing. The two generate an interaction force, which provides a pushing assistance to the solid round tube blank. It also helps to center the piercing rod, reduces the thrust of the piercing rod, and reduces the wear on the rolls. Attached Figure Description

[0035] Figure 1 is a process flow diagram of an embodiment of the present invention;

[0036] Figure 2 This is a schematic diagram of the perforation machine;

[0037] Figure 3 This is a structural diagram of the push rod;

[0038] Figure 4 This is a schematic diagram of the connection structure between the first mandrel and the second mandrel;

[0039] Figure 5 This is a schematic diagram of the structure of the two-roll skew rolling conical piercing mechanism;

[0040] Figure 6 This is a schematic diagram of the distribution of tapered rolls;

[0041] Figure 7 This is a schematic diagram of the distribution of drum-shaped rolls.

[0042] Among them, 1-two-roll skew rolling conical piercing mechanism, 11-conical roll, 2-three-roll skew rolling drum-shaped piercing mechanism, 21-drum roll, 3-top rod mechanism, 31-first top rod, 32-first top head, 321-annular platform, 33-second top rod, 331-umbrella-shaped cap, 332-external thread section, 333-material trough, 34-second top head, 4-machine base, 5-gear mounting seat, 6-center gear, 7-gear ring, 8-roll mounting gear, 9-rotation adjusting rod, 10-solid round tube blank. Detailed Implementation

[0043] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0044] like Figure 1 As shown, a method for manufacturing a hot-rolled seamless steel pipe with an outer diameter of 38-50 mm includes the following steps:

[0045] Step S1: A solid round tube blank 10 with an outer diameter of 150 mm is obtained by loading and unloading a large-diameter solid continuous casting round tube blank with a diameter of 230 mm. The obtained solid round tube blank 10 is cut off, heated, and then subjected to diameter reduction and piercing to obtain a rough tube with an outer diameter of 130 mm and a wall thickness of 12.2 mm.

[0046] Step S2: The tube is longitudinally rolled and stripped to obtain a rough tube with a diameter of 112 mm and a wall thickness of 3.80 mm;

[0047] Step S3: After the rough pipe is heated and the temperature is controlled by medium frequency induction heating, it is sized by tension reducing machine to obtain a long finished steel pipe.

[0048] Step S4: Long finished steel pipes are subjected to online controlled cooling, pre-cutting, straightening, non-destructive testing, and fixed-length sawing to produce hot-rolled seamless steel pipes with an outer diameter of 38-50mm.

[0049] In step S1 of this embodiment, a solid round tube blank is placed in an annular heating furnace for heating, and then pierced using a piercing machine to obtain a tube with an outer diameter of 130 mm and a wall thickness of 12.2 mm.

[0050] Large-diameter solid continuously cast round tube billets with a diameter of 230mm are produced using a hot rolling production line with fewer stands, shorter process, and lower investment. This results in lower costs and can save at least 300 yuan per ton of steel compared to small-diameter rolled tube billets. Under current market conditions, this can save at least 300 yuan per ton of steel.

[0051] In this embodiment, the solid round tube blank is heated in an annular heating furnace with a heating rate controlled at 780℃ / h. The preheating section is 850℃, the first heating section is 1100℃, the second heating section is 1260℃, the soaking section is 1270-1290℃, and the total heating time is 100 minutes.

[0052] In step S2 of this embodiment, a three-roll, five-stand hot rolling mill is used. The three-roll mill minimizes the radius difference between the top of the roll pass and the roll edge, thereby mitigating the adverse effects of large rolling speed differences on metal deformation. The five-stand mill shortens the mandrel length to 10 meters, reducing the difficulty and cost of manufacturing small mandrels. Medium-frequency induction rapid preheating is used for mandrel preheating to ensure uniform graphite coating. Compared with traditional heating furnaces, this method has a shorter preheating cycle, lower capital investment, smaller footprint, and higher heat utilization rate. It also reduces the number of mandrels used in cyclic operation. By controlling the chemical composition of the rolls, the roll hardness is controlled between 400 HB and 440 HB, improving the rolls' wear resistance and ensuring the wall thickness accuracy and surface flatness of the continuously rolled rough tube. A combination of 130mm to 120mm stand passes is used to roll a 130mm diameter tube into a 119.43mm diameter tube. The rough tube is then separated from the mandrel by a three-roller three-stand tube stripper. The rough tube with a diameter of 119.43 mm is further rounded and reduced in diameter to obtain a rough tube with a diameter of 112 mm and a wall thickness of 3.80 mm.

[0053] Specifically, the mass percentage composition of the roll is as follows: carbon 3.2%–3.6%, silicon 1.3%–1.8%, chromium 0.2%–0.5%, molybdenum 0.7%–1.0%, manganese 0.4%–0.8%, nickel 2.5%–3.5%, with the balance being iron. This ensures the stability of the roll hardness and allows for relatively precise control of the roll hardness within a small range, thereby guaranteeing the control of rolling precision and other indicators.

[0054] In step S2 of this embodiment, during the three-roll, five-stand hot rolling process, the temperature of the incoming roll tube is 1100℃~1130℃. The roll pass diameter is set to 123.40mm for the first stand, 120.80mm for the second stand, 119.43mm for the third stand, 118.82mm for the fourth stand, and 118.82mm for the fifth stand. The mandrel diameter is 111.5mm, and the total rolling elongation is 3.72. The three-roll system minimizes the gap between the top of the roll pass and the edge of the roll. The difference in the radius of the edge is minimized, thereby reducing the adverse effects of large rolling speed differences on metal deformation. The length of the mandrel is shortened to 10 meters using a five-stand mill, which reduces the difficulty and cost of manufacturing small mandrels. To ensure uniform graphite coating on the mandrel, medium-frequency induction rapid preheating is used. Compared with traditional heating furnaces, it has a shorter preheating cycle, lower capital investment, smaller footprint, and higher heat utilization rate. At the same time, it reduces the number of mandrels in the cycle. By controlling the chemical composition of the rolls, the hardness of the rolls is controlled between 400HB and 440HB, thereby improving the wear resistance of the rolls and ensuring the wall thickness accuracy and surface flatness of the continuously rolled rough tubes.

[0055] The tube is removed using a three-roll, three-stand tube stripper. During the stripping process, the first stand has a die diameter of 117.84 mm and an ellipticity of 1.057, the second stand has a die diameter of 113.59 mm and an ellipticity of 1.024, and the third stand has a die diameter of 112.00 mm, an ellipticity of 1.005, and an elongation coefficient of 1.04. A combination of stand die diameters of 130 mm to 120 mm is used to roll a 130 mm diameter tube into a rough tube with a diameter of 119.43 mm. The tube stripper then separates the rough tube from the mandrel and further rounds and reduces the diameter of the 119.43 mm rough tube to obtain a rough tube with a diameter of 112 mm.

[0056] In step S3 of this embodiment, rapid induction heating is performed using an online medium-frequency induction furnace. This furnace employs intelligent rapid temperature control and six sets of thyristor-controlled medium-frequency rapid induction heating furnaces. The rough tube passes through at a speed of 1.2 m / s, ensuring a longitudinal temperature difference of ±10℃ and a circumferential temperature difference of ±5℃. This guarantees precise temperature control before tension sizing of the rough tube, raising its temperature from 840℃ to 920℃. The final rolling temperature of the steel tube is 880℃. Utilizing the rapid speed of medium-frequency induction heating, the grains of the rough tube are fine and uniform after heating, significantly improving the overall performance of the steel material. The fast speed and short duration of medium-frequency induction heating reduce oxidation of the rough tube surface in air, ensuring surface cleanliness and significantly improving the surface quality of the steel tube after tension sizing. Compared to traditional gas-fired furnaces, medium-frequency induction heating furnaces have a smaller footprint, lower capital investment, higher heat utilization rate, lower carbon emissions, and no waste gas pollution.

[0057] Tension reduction sizing is performed using a 3-roll 24-stand tension reduction mill. By redistributing the deformation parameters of the preceding piercing and continuous rolling processes, the inlet diameter of the tension reduction rough tube is reduced to 112mm, and the total reduction rate of the 38 series is reduced to 66%. The 38 series small-diameter tubes are prone to bending. By adding a straightening stand, the straightness of the finished tube is ensured, and the production is guaranteed to be smooth. The CEC control function is activated to shorten the thickened ends at the head and tail, ensuring the yield and obtaining finished steel tubes with suitable outer diameters. In step S4, the steel tubes are subjected to online controlled cooling on a 64-meter-long online cooling bed. The initial temperature of the controlled cooling is ≥860℃, and the cooling rate is controlled at 40℃ / min~60℃ / min.

[0058] The tension reduction mill's die profile machining utilizes an independent lathe, and the machining method has been modified to ensure the dimensional accuracy of the finished mill frame. The outer diameter tolerance of small-diameter pipes is small. By controlling the mass percentage content of the tension reduction rolls to: carbon 2.9%–3.6%, silicon 1.0%–2.0%, chromium 0.2%–1.0%, molybdenum 0.2%–1.0%, manganese 0.4%–1.0%, nickel 1.2%–3.5%, with the balance being iron, the hardness of the tension reduction rolls is controlled to 60HSD–65HSD, ensuring the wear resistance of the rolls and thus making the outer diameter dimensions of the steel pipes reliable and stable.

[0059] like Figures 2-7 As shown, in step S1, the piercing machine used for diameter reduction piercing includes a frame. A two-roll skew-roll conical piercing mechanism 1, a three-roll skew-roll drum-shaped piercing mechanism 2, and a push rod mechanism 3 are slidably mounted on the frame. The two-roll skew-roll conical piercing mechanism 1 has two conical rolls 11, two guide plates, and a conical roll drive mechanism. The three-roll skew-roll drum-shaped piercing mechanism 2 has three drum-shaped rolls 21 and a drum-shaped roll drive mechanism. Both the conical roll drive mechanism and the drum-shaped roll drive mechanism are existing technologies and will not be described in detail here. The push rod mechanism 3 rotates... The device is equipped with a first push rod 31, and a first push head 32 is provided at the end of the first push rod 31. The first push head 32 is detachably connected to a second push rod 33 coaxially arranged. The second push rod 33 is provided with a second push head 34. The diameter of the first push rod 31 is larger than the diameter of the second push rod 33, and the diameter of the first push head 32 is larger than the diameter of the second push head 34. The first push head 32 corresponds to the position of the three-roll skew rolling drum-shaped piercing mechanism 2, and the second push head 34 corresponds to the position of the two-roll skew rolling conical piercing mechanism 1.

[0060] The first push head 32 is provided with a threaded hole. The end of the second push rod 33 connected to the first push head 32 is provided with an external thread. The second push rod 33 is threadedly connected to the first push head 32. The end of the first push head 32 facing the second push head 34 is provided with an annular platform 321. The second push rod 33 is provided with an umbrella-shaped cap 331. The area of ​​the bottom end of the umbrella-shaped cap 331 is the same as the area of ​​the annular platform 321. The umbrella-shaped cap 331 abuts against the annular platform 321. In actual use, the first push rod 31, the first push head 32, the second push rod 33 and the second push head 34 are all provided with cooling channels. The alignment of this technical solution has not been improved and is not marked in the figure.

[0061] The second push rod 33 has an external thread section 332 near the second push head. The external thread section 332 has a material groove 333, which extends parallel to the axial direction of the second push rod 33. There are two or more material grooves 333, which are evenly distributed around the circumference of the second push rod 33. During piercing, the push rod rotates, and the external thread section 332 contacts the inner wall of the solid round tube blank 10 after piercing, generating an interaction force between them. In actual use, the rotation direction of the roller and the push rod is adjusted so that the rotation direction of the solid round tube blank driven by the roller is opposite to the rotation direction of the push rod. When the push rod rotates, the external thread section screws in, so that the external thread section 332 provides a pushing assistance to the solid round tube blank, which is also beneficial for the centering of the push rod, reducing the thrust of the push rod, reducing the wear on the roller, and the material groove can accommodate steel chips generated during the piercing process.

[0062] In this embodiment, the two-roll skew rolling conical piercing mechanism 1 and the three-roll skew rolling drum-shaped piercing mechanism 2 each include a machine base 4. Gear mounting seats 5 are slidably installed at both ends of the machine base 4. Specifically, mutually cooperating slide grooves and slide rails can be provided on the machine base 4 and the gear mounting seats 5. Hydraulic cylinders are provided on both sides of the two gear mounting seats 5. The piston rods of the hydraulic cylinders abut against the gear mounting seats 5. By controlling the hydraulic cylinders, the position of the gear mounting seats 5 and the components on them can be adjusted. A central gear 6 is fixedly installed on the gear mounting seat 5, and a gear ring 7 is rotatably installed. A roll mounting gear 8 is meshed between the central gear 6 and the gear ring 7. The roller shafts of the conical roll 11 and the drum-shaped roll 21 are respectively mounted on the roll mounting gear through spherical bearings. On gear 7, the outer circumferential surface of the gear ring 7 is provided with gear teeth. A rotating adjusting rod 9, driven by a hydraulic cylinder, is slidably mounted on the gear mounting base 5. The rotating adjusting rod 9 has a rack that meshes with the gear teeth on the outer circumferential surface of the gear ring 7. By controlling the operation of the hydraulic cylinder, the rotating adjusting rod 9 can be moved. When the rotating adjusting rod 9 moves, it drives the gear ring 7 to rotate. Since the central gear 6 is fixedly mounted on the gear mounting base 5, the rotation of the gear ring 7 drives the roll mounting gear 8 to rotate around the central gear 6. Specifically, the conical roll 11 and the drum-shaped roll 21 are evenly distributed along the circumference of the central gear 6. When the gear ring 7 rotates, it drives the conical roll 11 or the drum-shaped roll 21 within the same gear ring 7 to rotate synchronously, achieving synchronous adjustment of the rolls and improving adjustment accuracy and efficiency. Specifically, the mating structure between the central gear 6, the roll mounting gear 8, and the gear ring 7 can all adopt existing technologies, as long as they can meet the mating requirements of this technical solution.

[0063] In this embodiment, the second mandrel 34 is provided with a piercing area, a rolling area, and a rounding area. During the diameter reduction piercing process, the heated solid round billet contacts the rolls and is bitten into the conical rolls in one go, spiraling forward until it reaches the end area before the mandrel, which is the pre-mandrel compression area. In this area, part of the solid round billet flows laterally and flows into the roll gap. It is stopped by the guide plate. At this time, the solid round billet is only supported by the guide plate to avoid affecting the rolls' biting of the billet. Because the guide plate spacing is greater than the roll spacing, the billet becomes elliptical, and the solid round... The billet mainly extends axially in layers, with the outer metal layer forming a layered structure. After the solid round billet's end face encounters the second mandrel, the second mandrel rotates. Due to the sufficient pulling force of the rolls on the billet, the second mandrel pushes the solid round billet through, thus piercing it. Corresponding to the piercing area of ​​the second mandrel, the gap between the second mandrel and the tapered rolls gradually thins, and the wall thickness of the rough tube gradually decreases. At this point, it is crucial to control the contact between the billet and the guide plate. If contact occurs too early, the resistance of the guide plate combined with the resistance of the mandrel can easily cause jamming, preventing secondary biting. The piercing process is affected by the fact that if the billet contacts the guide plate too late, the billet will have a large elliptical deformation, which can easily lead to central porosity and cavities. At this stage, the guide plate controls the lateral deformation of the billet, and together with the conical rolls and the second mandrel, forms an annular piercing pattern to allow the piercing process to proceed. The solid round billet continues to the rolling zone. The gap between the second mandrel and the conical rolls is uniform. In this section, the rough tube is rolled, which improves and ensures the accuracy of the wall thickness and the quality of the outer surface, playing a role in uniformity. At this stage, the guide plate... The plate and the piercing zone serve the same purpose. The rough tube continues to enter the rounding zone. The outer wall of the rough tube first separates from the guide plate, and the wall of the rough tube separates from the mandrel. It is rounded between the rolls, thus completing the piercing process. If the rough tube is still in contact with the guide plate in this zone, it is easy to be flattened. Throughout the piercing process, the distance between the guide plates is greater than the distance between the tapered rolls. In this embodiment, the parameters of the main piercing tools, the guide plate and the mandrel, are redesigned and matched. The two-roll skew rolling tapered piercing mechanism can reduce the diameter of a 150mm solid round tube blank and roll it into a 135-138mm diameter tube. Then, the three-roll skew rolling drum piercing mechanism reduces the diameter of the 135-138mm tube and pierces it into a 130mm tube.

[0064] This invention can produce high-precision hot-rolled seamless steel with a diameter of 38mm to 50mm, involving steel grades such as 10Cr9MoW2VJJNbBN, 10Cr9Mo1VNbN, X10CrWMoVNb9-2, X10CrMoVNb9-1, T92, T91, P92, P91 and below.

[0065] This embodiment takes the production of Φ38*3.5 steel pipe of grade 15CrMoG as an example.

[0066] Step S1: Load and unload a large-diameter solid continuously cast round tube billet with a diameter of 230mm to obtain a solid round tube billet with an outer diameter of 150mm and a single length of 1420mm. Place the solid round tube billet in an annular heating furnace with a diameter of 20 meters and heat it. The heating rate is controlled at 780℃ / h, the preheating section is 850℃, the first heating section is 1100℃, the second heating section is 1260℃, and the soaking section is 1270-1290℃. The total heating time is 100 minutes. Then, use a piercing machine to pierce the tube to obtain a capillary tube with an outer diameter of 130mm and a wall thickness of 12.2mm.

[0067] Step S2: Hot rolling is performed using a three-roll five-stand continuous rolling mill to obtain a rough tube with an outer diameter of 119.26 mm and a wall thickness of 3.80 mm. The temperature of the rough tube entering the rolling mill is 1100℃~1130℃. The diameter of the pass set for the first stand is 123.40 mm, the diameter of the pass set for the second stand is 120.80 mm, the diameter of the pass set for the third stand is 119.43 mm, the diameter of the pass set for the fourth stand is 118.82 mm, the diameter of the pass set for the fifth stand is 118.82 mm, the diameter of the mandrel is 111.5 mm, and the total elongation coefficient of rolling is 3.72.

[0068] The tube was removed using a three-roller, three-stand tube remover to obtain a rough tube with an outer diameter of 112 mm and a wall thickness of 3.80 mm. The first stand has a die diameter of 117.84 mm and an ellipticity of 1.057, the second stand has a die diameter of 113.59 mm and an ellipticity of 1.024, and the third stand has a die diameter of 112.00 mm, an ellipticity of 1.005, and an elongation coefficient of 1.04.

[0069] Step S3: Rapid induction heating is performed using a medium-frequency induction furnace. The rough tube passes through at a speed of 1.2 m / s, raising the temperature of the rough tube from 840°C to 920°C, ensuring the final rolling temperature of the steel tube is 880°C, and realizing controlled rolling normalizing instead of offline normalizing.

[0070] Using a 3-roll 24-stand tension reduction mill for tension sizing and with CEC function control enabled, a finished steel pipe with an outer diameter of 38.28 mm, a wall thickness of 3.54 mm, and a length of 62000 mm was obtained. The diameter of each stand's pass, the reduction rate, and the intended use are shown in Table 1. The total elongation coefficient of the tension reduction mill is 3.46, the total wall thickness change rate is 9.36%, and the total reduction rate is 65.73%.

[0071] Table 1. Frame profile diameter, reduction ratio, and application.

[0072]

[0073] Step S4: The steel pipe is subjected to online controlled cooling on a 64-meter-long online cooling bed. The initial temperature of controlled cooling is ≥860℃, and the cooling rate is controlled at 40℃ / min~60℃ / min. This results in the internal structure of the steel pipe being ferrite, pearlite, and granular bainite. Then, 62-meter-long hot-rolled finished steel pipes are pre-cut using disc spacing at multiples of 12.4 meters.

[0074] After tempering heat treatment, seamless steel pipe samples were taken and subjected to performance tests according to GB / T5310-2017. The performance results are shown in Table 2. The samples fully meet the performance indicators specified in standard GB / T5310-2017.

[0075] Table 2 Sampling test performance results

[0076]

[0077] The steel pipes were subjected to online flaw detection, with a flaw level of L2 and a pass rate of 99%. After the flaw detection blind zone was removed, the pipes were cut to length.

[0078] The wall thickness of the steel pipe was measured by ultrasonic full-length measurement. The test results showed that the wall thickness range was controlled between 3.23mm and 3.76mm, while the product standard specifies 3.05mm to 3.95mm. The outer diameter was measured at three points: the beginning, middle, and end of the finished steel pipe. The test results showed that the outer diameter was between 37.8mm and 38.2mm, while the product standard specifies 37.6mm to 38.4mm.

[0079] Finally, the products are labeled and put into storage.

[0080] This invention utilizes a combination of process equipment and parameters, including a 150mm diameter solid continuously cast round billet, a piercing mill for diameter reduction and piercing, a three-roll five-stand rolling mill, medium-frequency induction preheating with mandrels, a three-roll three-stand tube stripping mill, medium-frequency rapid induction with intelligent temperature control for temperature control and homogenization of the rough tube, a three-roll twenty-four-stand tension reduction mill for controlled rolling and sizing, and online controlled cooling, to produce seamless steel pipes with diameters ranging from 38mm to 50mm. This method not only produces products with high dimensional accuracy, stable microstructure and low fluctuations, but also features simple processes, lower equipment investment, energy saving and environmental protection, compact production, a yield rate of 95%, and a single production line capacity of 200,000 tons / year.

[0081] In conclusion, the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method of manufacturing a hot-rolled seamless steel pipe having an outer diameter of 38 to 50 mm, characterized by, Includes the following steps: Step S1: A solid round tube billet with an outer diameter of 150mm is obtained by feeding and unloading a large-diameter solid continuous casting round tube billet with a diameter of 230mm. The obtained solid round tube billet is cut off, and then the solid round tube billet is placed in an annular heating furnace for heating. Then, a piercing machine is used to reduce the diameter and pierce to obtain a capillary tube with an outer diameter of 130mm and a wall thickness of 12.2mm. During heating, the heating rate is controlled at 780℃ / h, the preheating section is 850℃, the first heating section is 1100℃, the second heating section is 1260℃, the soaking section is 1270-1290℃, and the total heating time is 100 minutes. Step S2: The tube is longitudinally rolled and stripped to obtain a rough tube with a diameter of 112 mm and a wall thickness of 3.80 mm; Step S3: After the rough pipe is heated and the temperature is controlled by medium frequency induction heating, it is sized by tension reducing machine to obtain a long finished steel pipe. Step S4: Long finished steel pipes are subjected to online controlled cooling, pre-cutting, straightening, non-destructive testing, and fixed-length sawing to produce hot-rolled seamless steel pipes with an outer diameter of 38-50mm. In step S1, the piercing machine used for diameter reduction piercing includes a frame. A two-roll skew-roll conical piercing mechanism, a three-roll skew-roll drum-shaped piercing mechanism, and a push rod mechanism are slidably mounted on the frame. The two-roll skew-roll conical piercing mechanism has two conical rolls, two guide plates, and a conical roll drive mechanism. The three-roll skew-roll drum-shaped piercing mechanism has three drum-shaped rolls and a drum-shaped roll drive mechanism. The push rod mechanism is rotatably equipped with a first push rod. The end of the first push rod is equipped with a first head. The first head is detachably connected to a second push rod coaxially mounted. The second push rod is equipped with a second head. The diameter of the first push rod is larger than the diameter of the second push rod, and the diameter of the first head is larger than the diameter of the second head.

2. The method of producing a hot-rolled seamless steel pipe having an outer diameter of 38 to 50 mm according to claim 1, characterized by, In step S2, the mandrel length is shortened to 10 meters using a three-roll five-stand hot rolling mill. The mandrel is preheated using medium-frequency induction rapid preheating, and the hardness of the rolls is controlled between 400HB and 440HB. A combination of 130mm and 120mm stand pass profiles is used to roll a 130mm diameter tube into a rough tube with a diameter of 119.43mm. The rough tube is then separated from the mandrel by a three-roll three-stand tube stripper. The rough tube with a diameter of 119.43mm is further rounded and reduced in diameter to obtain a rough tube with a diameter of 112mm and a wall thickness of 3.80mm.

3. The method of producing a hot-rolled seamless steel pipe having an outer diameter of 38 to 50 mm according to claim 2, characterized by, In step S2, during the three-roll five-stand hot rolling process, the temperature of the inlet tube is 1100℃~1130℃, the diameter of the pass on the first stand is set to 123.40mm, the diameter of the pass on the second stand is set to 120.80mm, the diameter of the pass on the third stand is set to 119.43mm, the diameter of the pass on the fourth stand is set to 118.82mm, the diameter of the pass on the fifth stand is set to 118.82mm, the diameter of the mandrel is 111.5mm, and the total elongation coefficient is 3.

72. During the tube removal process of the three-roller, three-stand tube remover, the diameter of the hole in the first stand is 117.84 mm, the ellipticity is 1.057, the diameter of the hole in the second stand is 113.59 mm, the ellipticity is 1.024, and the diameter of the hole in the third stand is 112.00 mm, the ellipticity is 1.005, and the elongation coefficient is 1.

04.

4. The method for manufacturing a hot-rolled seamless steel pipe with an outer diameter of 38-50 mm according to claim 1, characterized in that, In step S3, a medium-frequency induction furnace is used for rapid induction heating. The rough tube passes through at a speed of 1.2 m / s, raising the temperature of the rough tube from 840°C to 920°C. The final rolling temperature of the steel tube is 880°C. Tension reduction and sizing are performed using a 3-roll 24-stand tension reduction machine, and the CEC control function is activated to obtain a finished steel pipe with a suitable outer diameter. In step S4, the steel pipe is subjected to online controlled cooling on a 64-meter-long online cooling bed. The initial temperature of controlled cooling is ≥860℃, and the cooling rate is controlled at 40℃ / min~60℃ / min.

5. The method of producing a hot-rolled seamless steel pipe having an outer diameter of 38 to 50 mm according to claim 1, characterized in that, The first push head is provided with a threaded hole, and the end of the second push rod connected to the first push head is provided with an external thread. The second push rod is threadedly connected to the first push head. The end of the first push head facing the second push head is provided with an annular platform. The second push rod is provided with an umbrella-shaped cap. The area of ​​the bottom end of the umbrella-shaped cap is the same as the area of ​​the annular platform. The umbrella-shaped cap abuts against the annular platform. The second push rod has an external thread section near the second push head, and the external thread section has a material groove. The material groove extends in a direction parallel to the axial direction of the second push rod. There are two or more material grooves, and the two or more material grooves are evenly distributed along the circumference of the second push rod.

6. The method of producing a hot-rolled seamless steel pipe having an outer diameter of 38 to 50 mm according to claim 5, characterized in that, The two-roll skew rolling conical piercing mechanism and the three-roll skew rolling drum-shaped piercing mechanism each include a machine base. Gear mounting seats are slidably installed at both ends of the machine base. Hydraulic cylinders are provided on both sides of the two gear mounting seats. The piston rods of the hydraulic cylinders abut against the gear mounting seats. A central gear is fixedly installed in the middle of one of the opposite sides of the two gear mounting seats. A gear ring is rotatably installed on the gear mounting seat outside the central gear. A roll mounting gear is meshed between the central gear and the gear ring. The roller shafts of the conical roll and the drum-shaped roll are respectively mounted on the roll mounting gear through spherical bearings. The outer circumferential surface of the gear ring is provided with gear teeth. A rotating adjusting rod driven by a hydraulic cylinder is slidably provided on the gear mounting seat. A rack that meshes with the gear teeth on the outer circumferential surface of the gear ring is provided on the rotating adjusting rod.

7. The method of producing a hot-rolled seamless steel pipe having an outer diameter of 38 to 50 mm according to claim 2, characterized by The mass percentage composition of the rolls used in the three-roll five-stand hot rolling process in step S2 is as follows: carbon 3.2%–3.6%, silicon 1.3%–1.8%, chromium 0.2%–0.5%, molybdenum 0.7%–1.0%, manganese 0.4%–0.8%, nickel 2.5%–3.5%, with the balance being iron.

8. The method of producing a hot-rolled seamless steel pipe having an outer diameter of 38 to 50 mm according to claim 4, characterized by, The mass percentage composition of the tension reducing roller of the tension reducing machine is as follows: carbon 2.9%~3.6%, silicon 1.0%~2.0%, chromium 0.2%~1.0%, molybdenum 0.2%~1.0%, manganese 0.4%~1.0%, nickel 1.2%~3.5%, with the balance being iron. The hardness of the tension reducing roller is 60HSD~65HSD.

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