Low-cost preparation method of large-diameter seamless steel tube

Abstract

The invention provides a low-cost preparation method of a large-diameter seamless steel tube. The low-cost preparation method comprises the following steps: S1, tempering or annealing a vacuum consumable steel ingot; s2, a hard layer on the surface of the steel ingot is removed, the head and the tail are cut off, and a pipe blank is obtained; s3, heating and hot piercing are conducted on the pipe blank, and a tubular billet is obtained; s4, the tubular billet is subjected to finish rolling to reduce the wall thickness, and a pierced billet is obtained; s5, the outer diameter of the pierced billet is reduced through a reducing mill, and a steel pipe is obtained; s6, the steel pipe is evenly cooled through a cooling bed; s7, the steel pipe is subjected to tempering or annealing treatment; s8, the steel pipe is subjected to external grinding and head flattening treatment; s9, performing ultrasonic flaw detection on the steel pipe to obtain a qualified steel pipe; the vacuum consumable steel ingot is directly used for hot rolling perforation, the traditional forging cogging link is thoroughly omitted, material loss and energy consumption in the forging process are avoided, the utilization rate of raw materials is remarkably increased, meanwhile, the production process is simplified, the production period is shortened, and the equipment and human input cost is reduced.

Description

Technical Field

[0001] The present invention relates to the technical field of seamless steel pipe production, and in particular to a low-cost preparation method for large-diameter seamless steel pipes. Background Art

[0002] D406A is a low-alloy, ultra-high-strength steel independently developed in my country and widely used in aerospace engine casings. The manufacturing process typically involves the metallurgical plant providing the ingots, the forging plant rolling the rings, and heat-treating the steel before rough turning for delivery. The OEM is responsible for machining, spinning, welding, and other processes.

[0003] In recent years, the applicant has developed ultra-large diameter seamless steel pipes with the largest outer diameter produced 930mm, the process includes: making steel ingots through a vacuum consumable process, forging the steel ingots to prepare tube billets, and hot rolling and hot expanding the tube billets to obtain ultra-large diameter seamless steel pipes.

[0004] Since the above process requires forging steel ingots to prepare tube blanks, and the forging blank yield rate is 74%-78%, the material loss is large and the forging cycle is long, resulting in high steel pipe preparation costs and low efficiency, which is difficult to meet market demand. Summary of the Invention

[0005] The present invention provides a low-cost preparation method for large-diameter seamless steel pipes, which solves the problems of high cost and low efficiency in the preparation of ultra-large-diameter seamless steel pipes in the prior art.

[0006] The technical solution of the present invention is achieved as follows: The present invention provides a low-cost preparation method for a large-diameter seamless steel pipe, comprising the following steps: S1, tempering or annealing the selected vacuum consumable steel ingot. The main purpose of this process is to reduce the hardness of the steel ingot, eliminate the stress during the solidification process of the vacuum consumable steel ingot, and prevent cracking; S2, removing the hard layer on the surface of the steel ingot by turning (due to the characteristics of the vacuum consumable process, the inclusions in the steel are discharged to the surface of the steel ingot to form a hard layer with poor plasticity. The surface defects must be removed by turning before hot working deformation can be carried out), and cutting off the head and tail of the steel ingot (the metallurgical quality of the head and tail of the steel ingot does not meet the requirements and cannot be used) to obtain the tube blank; S3, heating the tube billet in a ring furnace (this step requires the heating temperature to reach the temperature range where the material has good plasticity and sufficient holding time to ensure that the steel ingot is burned through and the internal and external temperatures are uniform). The heated tube billet is hot-pierced by a mandrel (selecting a mandrel of appropriate size for hot piercing to obtain a reasonable inner hole size). After hot piercing is completed, the mandrel is removed to obtain a capillary tube; S4, placing a mandrel in the inner hole of the shell tube and performing finish rolling on the shell tube until the shell tube wall thickness is reduced to the target range (this step controls the inner hole size by the mandrel to remain unchanged. During the shell tube rolling process, the wall thickness decreases and gradually reaches the target requirement). After the finish rolling is completed, the mandrel is removed to obtain a rough tube; S5, reducing the outer diameter of the rough pipe by a reducing mill until the outer diameter of the rough pipe is reduced to a target range, thereby obtaining a steel pipe; S6, using the cooling bed to evenly cool the steel pipe to prevent deformation and control the ovality and straightness of the finished steel pipe; S7, tempering the steel pipe. The main purpose of this step is to eliminate the stress caused by thermal deformation of the steel pipe, prevent cracking of the steel pipe, and secondly reduce the hardness; S8, external grinding and flattening of the steel pipe; S9, conduct ultrasonic flaw detection on the steel pipe (the main purpose is to determine whether there are defects on the surface and inside of the steel pipe that affect its use) to obtain qualified steel pipes.

[0007] Specifically, in step S1, the ingot shape of the vacuum consumable steel ingot is selected as 660 mm, and the diameter of the steel ingot obtained after turning in step S2 is 590-625 mm.

[0008] Specifically, in step S2, the length of the head and tail of the steel ingot cut off is 100 to 200 mm.

[0009] Preferably, before step S3, the surface of the tube blank is inspected for flaws, and if local cracks or slag pits are found, the local cracks or slag pits are repaired by grinding.

[0010] Specifically, in step S3, the heating temperature of the annular furnace is 1150-1270° C., and the heating time is 10-20 hours. Before hot perforation, a bell mouth is opened at the end of the tube blank for positioning the ejector rod.

[0011] Specifically, in step S4, the target range of the capillary wall thickness is 25 to 50 mm; in step S5, the target range of the steel pipe outer diameter is 580 to 635 mm.

[0012] Specifically, in step S8, the external grinding treatment is to remove the oxide scale on the outer surface of the steel pipe and ensure that the roughness of the outer surface of the steel pipe is no more than 6.3μm, which meets the requirements of ultrasonic testing; the flat head treatment is to cut off 150 to 250mm of the head and tail of the steel pipe respectively (after the steel pipe is pierced and rolled, a concave and convex surface is formed at the starting and end ends. The size of this part does not meet the requirements and must be removed).

[0013] Preferably, after step S9, the method further includes: S10, cutting the steel pipe to a predetermined length to obtain multiple sections of steel pipe; S11, the steel pipe is subjected to normalizing and spheroidizing degradation treatment.

[0014] Specifically, in step S10, the allowable length tolerance range of the cut-to-length sawing is 0 to +10 mm. The thin-walled steel pipe is affected by its own weight during the long heat treatment process, and deformation occurs after the heat treatment is completed. This deformation is particularly serious when the D / S value is ≥14. By reserving sufficient machining allowance (0 to +10 mm) during the cut-to-length sawing, it is ensured that the original surface defect layer can be completely removed after final machining.

[0015] Specifically, in step S11, the normalizing heating temperature is 900-950°C, and the holding time is 1-5 hours; the spheroidizing annealing adopts a three-stage heat treatment, with the first stage annealing temperature at 750-850°C, the holding time at 2-6 hours, and the furnace cooling to the second stage annealing temperature; the second stage annealing temperature at 700-760°C, the holding time at 20-30 hours, and the furnace cooling to the third stage annealing temperature; the third stage annealing temperature at 640-720°C, and the holding time at 6-14 hours. The purpose of this process is mainly to significantly reduce the hardness and obtain spherical pearlite structure, improve the plasticity of the material, and mainly ensure the smooth progress of subsequent spinning deformation without cracking.

[0016] Compared with the prior art, the present invention has the following beneficial effects: (1) The present invention directly uses vacuum consumable steel ingots for hot rolling and piercing, completely eliminating the traditional forging process of blanking, avoiding material loss and energy consumption in the forging process, significantly improving the utilization rate of raw materials, and at the same time simplifying the production process, shortening the production cycle, and reducing equipment and manpower investment costs; (2) The present invention adopts a hot rolling process with a large compression ratio (hot piercing + finishing rolling) to ensure that the steel ingot is fully plastically deformed to refine the grains during the process of solid to hollow transformation, thereby obtaining a uniform and dense organizational structure, so that the mechanical properties of the finished steel pipe fully meet aerospace-grade standards; (3) The present invention is aimed at thin-walled tubes with a high diameter-to-thickness ratio (D / S≥14). By placing the cut-to-length sawing before the heat treatment and strictly limiting the one-way positive tolerance (0~+10mm), sufficient margin is reserved for subsequent turning, completely eliminating the risk of dimensional deviation and surface "black skin" residue caused by heat treatment deformation, and ensuring the qualified rate of spinning forming. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0018] Figure 1 The figure is a schematic flow chart of a low-cost preparation method of a large-diameter seamless steel pipe according to the present invention. DETAILED DESCRIPTION

[0019] The following will clearly and completely describe the technical solutions of the present invention in conjunction with the embodiments of the present invention. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts shall fall within the scope of protection of the present invention.

[0020] Reference Figure 1 The present invention provides a low-cost preparation method for a large-diameter seamless steel pipe, comprising the following steps: S1, tempering or annealing the selected vacuum consumable steel ingot. The main purpose of this process is to reduce the hardness of the steel ingot, eliminate the stress during the solidification process of the vacuum consumable steel ingot, and prevent cracking; S2, removing the hard layer on the surface of the steel ingot by turning (due to the characteristics of the vacuum consumable process, the inclusions in the steel are discharged to the surface of the steel ingot to form a hard layer with poor plasticity. The surface defects must be removed by turning before hot working deformation can be carried out), and cutting off the head and tail of the steel ingot (the metallurgical quality of the head and tail of the steel ingot does not meet the requirements and cannot be used) to obtain the tube blank; S3, heating the tube billet in a ring furnace (this step requires the heating temperature to reach the temperature range where the material has good plasticity and sufficient holding time to ensure that the steel ingot is burned through and the internal and external temperatures are uniform). The heated tube billet is hot-pierced by a mandrel (selecting a mandrel of appropriate size for hot piercing to obtain a reasonable inner hole size). After hot piercing is completed, the mandrel is removed to obtain a capillary tube; S4, placing a mandrel in the inner hole of the shell tube and performing finish rolling on the shell tube until the shell tube wall thickness is reduced to the target range (this step controls the inner hole size by the mandrel to remain unchanged. During the shell tube rolling process, the wall thickness decreases and gradually reaches the target requirement). After the finish rolling is completed, the mandrel is removed to obtain a rough tube; S5, reducing the outer diameter of the rough pipe by a reducing mill until the outer diameter of the rough pipe is reduced to a target range, thereby obtaining a steel pipe; S6, using the cooling bed to evenly cool the steel pipe to prevent deformation and control the ovality and straightness of the finished steel pipe; S7, tempering the steel pipe. The main purpose of this step is to eliminate the stress caused by thermal deformation of the steel pipe, prevent cracking of the steel pipe, and secondly reduce the hardness; S8, external grinding and flattening of the steel pipe; S9, conduct ultrasonic flaw detection on the steel pipe (the main purpose is to determine whether there are defects on the surface and inside of the steel pipe that affect its use) to obtain qualified steel pipes.

[0021] The beneficial effects of the present invention are described below by means of specific embodiments and comparative examples: Example 1 (steel ingot passing through pipe): Steel grade: D406A, Steel pipe size: 630×25mm (outer diameter 630mm, wall thickness 25mm), size after lower section 630 mm× 580 mm × 1370 mm (the outer diameter of the steel pipe after cutting to length is 630 mm, the inner diameter is 580 mm, and the length is 1370 mm).

[0022] The specific processing technology includes the following steps: 1) Use D406A consumable round ingot, ingot shape 660mm, the composition and metallurgical quality of the steel ingot must meet the requirements of GJB3325A-2019 standard; 2) Ingot Tempering: This process is designed to eliminate stress during the solidification of the vacuum consumable steel ingot, prevent cracking, and reduce the hardness of the material to facilitate steps 3) and 4). D406A steel ingots are tempered at a temperature of 730-750°C for 15-20 hours, followed by air cooling after removal from the furnace. 3) Ingot turning: D406A vacuum consumable round ingot. Due to the characteristics of the vacuum consumable process, the inclusions in the steel are removed to form a hard layer on the surface of the ingot, which has poor plasticity. The surface defects must be removed by turning before hot working deformation can be carried out. The target size of the ingot is 590~ 625mm; 4) Removal of the head and tail of the ingot: The removal length is 100-200mm respectively. The head and tail of the ingot do not meet the metallurgical quality requirements and cannot be used. The existing technology removes them after forging, blanking and annealing. Since the next process of the present invention is ingot finishing and acceptance, heating and steel pipe forming, according to the basic requirements of quality management, this process must provide qualified intermediate products for the next process. Therefore, the unqualified material must be removed. 5) Steel ingot finishing and acceptance: If there are surface cracks on the surface of the steel ingot, the cracks will expand during the subsequent hot working process, and the local size may not meet the requirements after finishing.

[0023] According to the quality management requirements, the surface of the billet must meet the requirements of hot working. Cracks, slag pits and other defects are not allowed on the surface of the steel ingot. If any defects exist, they must be cleaned up. Billet acceptance: Acceptance through magnetic particle inspection technology. If cracks are found, they can only be judged as qualified after rework and grinding and magnetic particle re-inspection. 6) Ring furnace heating: This process allows the billet to be burned through, the inner and outer metals to be heated evenly, and good thermoplasticity to be obtained, which is a prerequisite for the smooth progress of subsequent processes; the heating temperature is 1150-1270℃, and the total heating time is 10-20h; 7) Hot piercing of steel ingots (controlling inner diameter): A bell mouth is opened at the end of the blank to facilitate the positioning and engagement of the ejector pin. The ejector pin size is selected and a reasonable inner hole size is obtained through hot perforation. After the hot perforation is completed, the ejector pin is withdrawn and the finished product of this process is output as a capillary tube. Key parameter settings: plug diameter 570mm, roller pitch 550mm, lead distance 590mm, capillary outer diameter 635~643mm. Rotate the long guide roller for 10~40 seconds before piercing to remove iron oxide scale; the roller speed is ≤150 rpm; 8) Finish rolling (control wall thickness): In this step, a mandrel is placed in the inner hole to control the inner hole size to remain unchanged. During the rough tube rolling process, the wall thickness decreases and gradually reaches the target requirement. After the hot rolling is completed, the mandrel is withdrawn and the finished product of this process is output as a rough tube. Key parameter settings: core rod size 570mm, roller pitch 610~630mm, guide pitch 640~660mm, rolling current ≤3000A; 9) Sizing of rough pipe (controlling outer diameter): In this step, the inner hole no longer uses a core rod. When the rough pipe passes through the reducing mill, the outer diameter is reduced. This process is designed based on the reducing principle. When the rough pipe passes through the reducing mill to adjust the outer diameter, the inner diameter remains basically unchanged. This process mainly controls the outer diameter size to meet the target requirements. The output of this process is a finished steel pipe.

[0024] The outer diameter is controlled by four frames including A654, A646, A640 and C638, and the target size is achieved. 638×27mm (outer diameter 638mm, wall thickness 27mm); 10) Slow cooling of steel pipes: In this process, the finished steel pipes are cooled slowly and evenly on the cooling bed to prevent deformation of the steel pipes and control the ovality and straightness of the finished steel pipes.

[0025] 11) Steel Pipe Tempering: This process primarily eliminates heat stress and prevents cracking in finished steel pipes. It also reduces hardness to facilitate subsequent flattening and sawing. Key parameters: Heating temperature 700-760°C, holding temperature 2-8 hours, and air cooling after exiting the furnace.

[0026] 12) Steel pipe finishing: External grinding: The main purpose of external grinding of steel pipes is to remove the oxide scale on the outer surface of the steel pipe, and secondly to make the roughness of the outer surface of the steel pipe meet the requirements of ultrasonic testing, roughness Ra ≤ 6.3μm.

[0027] Flat head (including sampling): After the steel pipe is pierced and rolled, a concave and convex surface is formed at the starting and end. The size of this part does not meet the requirements and must be cut off, with 150-250mm cut at the head and tail respectively.

[0028] 13) Ultrasonic testing: Ultrasonic testing of steel pipes is a non-destructive test used to assess the internal quality of steel pipes. Steel pipes meeting GB / T5777 standard U2.5 grade are transferred to the next process after passing the flaw detection.

[0029] 14) Steel pipe cutting to length: Cut to length according to the user's application length requirements. Thin-walled steel pipes are affected by their own weight during the long heat treatment process. After the heat treatment process is completed, there is a phenomenon of deformation. Especially when the D / S value (ratio of outer diameter to wall thickness) is ≥14, this tendency is more serious. Therefore, in order to minimize the dimensional deformation of the steel pipe before and after heat treatment in process (15), and to ensure sufficient processing allowance in the turning process, cut to length according to the user's application length of 1.37m, with a length tolerance of (0~+10mm) and number the products in sequence.

[0030] 15) Normalizing + spheroidizing annealing: The primary purpose of normalizing and spheroidizing annealing is to improve the as-rolled microstructure and reduce hardness. The resulting microstructure is spherical pearlite, which offers low hardness and good uniformity. This allows the steel pipe to exhibit good plasticity during spinning and resist cracking. This process places special demands on equipment, ensuring stable material properties through temperature uniformity in the resistance furnace.

[0031] The normalizing + spheroidizing annealing process requirements are as follows: Normalizing: Heating temperature 900-950°C, holding time 1-5 hours, air cooling after exiting the furnace. Spheroidizing annealing: A three-stage heat treatment is used, with the first stage at 750-850°C, holding time 2-6 hours, furnace cooling to the second stage: temperature 700-760°C, holding time 20-30 hours; furnace cooling to the third stage: temperature 640-720°C, holding time 6-14 hours.

[0032] Example 2 (steel ingot through pipe): The difference between this embodiment and embodiment 1 is that the steel grade is 30CrMnSiNi2A, the steel pipe size is: 590×50mm (outer diameter 590mm, wall thickness 50mm), size after lower section 590 mm× 490 mm × 1000 mm (the outer diameter of the steel pipe after cutting to length is 590 mm, the inner diameter is 490 mm, and the length is 1000 mm).

[0033] The specific processing technology includes the following steps: 1) Select 30CrMnSiNi2A consumable round ingot, ingot type 660mm, the composition and metallurgical quality of the steel ingot must meet the requirements of GJB3325A-2019 standard.

[0034] 2) Ingot Annealing: This process eliminates stress during the solidification of the vacuum consumable steel ingot, preventing cracking and reducing the material's hardness to facilitate steps 3) and 4). The 30CrMnSiNi2A steel ingot is annealed at a heating temperature of 600-700°C for 15-20 hours, followed by furnace cooling.

[0035] 3) Ingot turning: Due to the characteristics of vacuum consumable process, the inclusions in the steel are removed to form a hard layer on the surface of the ingot, which has poor plasticity. The surface defects must be removed by turning before hot working deformation can be carried out. The target size of the ingot is 590~ 625mm.

[0036] 4) Removal of the head and tail of the ingot: The removal length is 100-200mm respectively. The head and tail of the ingot do not meet the metallurgical quality requirements and cannot be used. The existing technology removes these parts after forging, slab opening, and annealing. However, since the next steps in this invention are ingot finishing and acceptance, heating, and steel pipe forming, according to the basic requirements of quality management, this step must provide qualified intermediate products for the next step. Therefore, the unsatisfactory material must be removed.

[0037] 5) Steel ingot finishing and acceptance: If there are surface cracks on the surface of the steel ingot, the cracks will expand during the subsequent hot working process, and the local size may not meet the requirements after finishing.

[0038] In accordance with quality management requirements, the surface of the billet must meet hot working requirements. Cracks, slag pits, and other defects are not permitted on the surface of the steel ingot. If any are present, they must be cleaned. Billet acceptance: Acceptance is through magnetic particle inspection. If cracks are present, rework and grinding followed by magnetic particle re-inspection are required before acceptance is considered qualified.

[0039] 6) Ring furnace heating: This process allows the billet to be burned through, the inner and outer metals to be heated evenly, and good thermoplasticity to be obtained, which is a prerequisite for the smooth progress of subsequent processes; the heating temperature is 1150-1270℃, and the total heating time is 10-20h.

[0040] 7) Hot piercing of steel ingots (controlling inner diameter): A bell mouth is opened at the end of the blank to facilitate the positioning and engagement of the ejector pin. The ejector pin size is selected and a reasonable inner hole size is obtained through hot perforation. After the hot perforation is completed, the ejector pin is withdrawn and the finished product of this process is output as a capillary tube. Key parameter settings: plug diameter 480mm, capillary outer diameter 595~608mm. Rotate the long guide roller for 10~40 seconds before piercing to remove iron oxide scale; the roller speed is ≤150 rpm.

[0041] 8) Finish rolling (control wall thickness): In this step, a mandrel is placed in the inner hole to control the inner hole size to remain unchanged. During the rough tube rolling process, the wall thickness decreases and gradually reaches the target requirement. After the hot rolling is completed, the mandrel is withdrawn and the finished product of this process is output as a rough tube. Key parameter settings: core rod size 480mm, roller pitch 570~590mm, guide pitch 600~620mm, rolling current ≤3000A.

[0042] 9) Sizing of rough pipe (controlling outer diameter): In this step, the inner hole no longer uses a core rod. When the rough pipe passes through the reducing mill, the outer diameter is reduced. This process is designed based on the reducing principle. When the rough pipe passes through the reducing mill to adjust the outer diameter, the inner diameter remains basically unchanged. This process mainly controls the outer diameter size to meet the target requirements. The output of this process is a finished steel pipe.

[0043] 10) Slow cooling of steel pipes: In this process, the finished steel pipes are cooled slowly and evenly on the cooling bed to prevent deformation of the steel pipes and control the ovality and straightness of the finished steel pipes.

[0044] 11) Steel Pipe Annealing: This process primarily eliminates heat-processing stresses and prevents cracking in finished steel pipes. It also reduces hardness to facilitate subsequent flattening and sawing. Key parameters: Heating temperature 600-720°C, holding temperature 2-8 hours, and air cooling after exiting the furnace.

[0045] 12) Steel pipe finishing: External grinding: The main purpose of external grinding of steel pipes is to remove the oxide scale on the outer surface of the steel pipe, and secondly to make the roughness of the outer surface of the steel pipe meet the requirements of ultrasonic testing, roughness Ra ≤ 6.3μm.

[0046] Flat head (including sampling): After the steel pipe is pierced and rolled, a concave and convex surface is formed at the starting and end. The size of this part does not meet the requirements and must be cut off, with 150-250mm cut at the head and tail respectively.

[0047] 13) Ultrasonic testing: Ultrasonic testing of steel pipes is a non-destructive test used to assess the internal quality of steel pipes. Steel pipes meeting GB / T5777 standard U2.5 grade are transferred to the next process after passing the flaw detection.

[0048] 14) Steel pipe cutting to length: Cut to length according to the user's application length requirements. Thin-walled steel pipes are affected by their own weight during the long heat treatment process. After the heat treatment process is completed, there is a phenomenon of deformation. Especially when the D / S value (ratio of outer diameter to wall thickness) is ≥14, this tendency is more serious. Therefore, in order to minimize the dimensional deformation of the steel pipe before and after heat treatment in process (15), and to ensure sufficient processing allowance in the turning process, cut to length according to the user's application length of 1m, with a length tolerance of (0~+10mm) and the products are numbered in sequence.

[0049] 15) Normalizing + spheroidizing annealing: The primary purpose of normalizing and spheroidizing annealing is to improve the as-rolled microstructure and reduce hardness. The resulting microstructure is spherical pearlite, which offers low hardness and good uniformity. This allows the steel pipe to exhibit good plasticity during spinning and resist cracking. This process places special demands on equipment, ensuring stable material properties through temperature uniformity in the resistance furnace.

[0050] Comparative Example 1 (forging billet through pipe): Grade: D406A, Steel pipe size: 630×25mm, bottom cut size 630 mm× 580 mm×1370 mm.

[0051] The difference from Example 1 is that the production steps of the forged tube blank include (4) to (9). Among them, (1) to (3) are the selection, tempering and polishing of D406A consumable ingots, and steps (11) to (20) are the production process of steel tubes, which are equivalent to (6) to (15) in Example 1. The biggest difference between the two is that Example 1 eliminates the forging process and uses a cut-off steel ingot instead of a forged blank as the tube blank.

[0052] Comparative Example 1 The specific implementation process is as follows: (1) D406A consumable round ingot 660mm, the composition and metallurgical quality of the steel ingot must meet the requirements of GJB3325A-2019 standard.

[0053] (2) Ingot tempering: The main purpose of this process is to eliminate stress during the solidification process of the vacuum consumable steel ingot, prevent cracking, and reduce the hardness of the material to facilitate the implementation of steps (3) and (4). D406A is tempered at a heating temperature of 730-750℃, a holding time of 15-20h, and air cooling after leaving the furnace.

[0054] (3) Ingot turning: D406A vacuum consumable round ingot, due to the characteristics of the vacuum consumable process, the inclusions in the steel are removed to form a hard layer on the surface of the ingot, which has poor plasticity. It must be turned to remove the surface defects before hot working deformation can be carried out. The target size of the ingot is 590~ 625mm.

[0055] (4) Ingot heating: This process allows the billet to burn through, heat the inner and outer metals evenly, and obtain good thermoplasticity. It is a prerequisite for the smooth progress of process (5). The heating temperature is 1150-1200℃, and the total heating time is 3-8h.

[0056] (5) Rapid forging: This process obtains a round tube with good density through forging deformation. Target size: 600mm, tolerance; (-5mm, +5mm), forging by three-upsetting and three-drawing process, forging ratio ≥5, stop forging temperature ≥800℃.

[0057] (6) Annealing: This process eliminates the stress of forging thermal deformation and reduces the hardness through annealing to facilitate sawing operation. The specific process parameters are as follows: heating temperature 600-760℃, holding time 20-40h, and cooling with the furnace.

[0058] (7) Blank finishing: This process mainly involves straightening and grinding wheel peeling. The tube blank may be bent after forging, and the main purpose of straightening is to correct the straightness of the tube blank.

[0059] (8) Cutting the head and tail of the blank: According to the process regulations, the unusable parts of the head and tail of the steel ingot are removed. The purpose of this process is the same as that of step (4) in Example 1. The difference is that this process removes the head and tail in the forging state, while step (4) in Example 1 removes the head and tail in the steel ingot state.

[0060] (9) Ultrasonic testing: This process uses ultrasonic testing to detect whether there are cracks or other defects inside the tube billet that may affect the quality of subsequent products. If qualified, it also serves as an acceptance check for the internal quality of the billet.

[0061] (10) Tube finishing and acceptance: If cracks are present on the surface of the tube billet, they will expand during subsequent hot working, and local dimensions may not meet requirements after finishing. According to quality management requirements, the billet surface must meet hot working requirements. Cracks, slag pits, and other defects are not permitted, and any existing defects must be cleaned. The billet can be inspected and accepted using magnetic particle inspection. If cracks are present, the billet can only be considered qualified after rework, grinding, and magnetic particle recarburization.

[0062] (11) Ring furnace heating: This process allows the billet to be burned through, the inner and outer metals to be heated evenly, and good thermoplasticity to be achieved. It is a prerequisite for the smooth progress of processes (4), (5), and (6). The heating temperature is 1150-1270°C, and the total heating time is 10-20 hours.

[0063] (12) Hot piercing of steel ingots (controlling inner diameter): The end of the blank is flared to facilitate the positioning and engagement of the ejector pin. The size of the ejector pin and the hot piercing process can obtain a reasonable inner hole size. After the hot piercing is completed, the ejector pin is withdrawn and the finished product of this process is output as a capillary tube.

[0064] Key parameter settings: plug diameter 570mm, roller pitch 550mm, lead distance 590mm, capillary outer diameter 635~643mm. Before piercing, rotate the long guide roller for 10-40 seconds to remove iron oxide scale; the roller speed is ≤150 rpm.

[0065] (13) Finish rolling (control wall thickness): In this step, a mandrel is placed in the inner hole to control the inner hole size to remain unchanged. During the rough tube rolling process, the wall thickness decreases and gradually reaches the target requirement. After the hot rolling is completed, the mandrel is withdrawn and the finished product of this process is output as a rough tube.

[0066] Key parameter settings: core rod size 570mm, roller pitch 610~630mm, guide pitch 640~660mm, rolling current ≤3000A.

[0067] (14) Sizing of rough pipe (controlling outer diameter): In this step, the inner hole no longer uses a core rod. When the rough pipe passes through the reducing mill, the outer diameter is reduced. This process is designed based on the reducing principle. When the rough pipe passes through the reducing mill to adjust the outer diameter, the inner diameter remains basically unchanged. This process mainly controls the outer diameter size to meet the target requirements. The output of this process is a finished steel pipe.

[0068] The outer diameter is controlled by four frames including A654, A646, A640 and C638, and the target size is achieved. 638×27mm.

[0069] (15) Slow cooling of steel pipes: In this process, the finished steel pipes are cooled slowly and evenly on the cooling bed to prevent deformation of the steel pipes and control the ovality and straightness of the finished steel pipes.

[0070] (16) Steel pipe tempering: This process is mainly used to eliminate heat treatment stress and prevent cracking of finished steel pipes. Secondly, it is used to reduce the hardness and facilitate the subsequent flattening and sawing. Key parameters: heating temperature 700-760℃, holding temperature 2-8h, and air cooling after leaving the furnace.

[0071] (17) Steel pipe finishing: External grinding: The main purpose of external grinding of steel pipes is to remove the oxide scale on the outer surface of the steel pipe, and secondly to make the roughness of the outer surface of the steel pipe meet the requirements of ultrasonic testing, roughness Ra ≤ 6.3μm.

[0072] Flat head (including sampling): After the steel pipe is pierced and rolled, a concave and convex surface is formed at the starting and end. The size of this part does not meet the requirements and must be cut off, with 150-250mm cut at the head and tail respectively.

[0073] (18) Ultrasonic testing: Ultrasonic testing of steel pipes is a non-destructive test used to assess the internal quality of steel pipes. Steel pipes meeting GB / T5777 standard U2.5 grade are transferred to the next process after passing the flaw detection.

[0074] (19) Steel pipe cutting to length: Cut to length according to the user's application length requirements. Thin-walled steel pipes are affected by their own weight during the long heat treatment process. After the heat treatment process is completed, there is a phenomenon of deformation. Especially when the D / S value (ratio of outer diameter to wall thickness) is ≥14, this tendency is more serious. Therefore, in order to minimize the dimensional deformation of the steel pipe before and after heat treatment in process (15), sufficient processing allowance is guaranteed during the turning process. According to the user's application length requirement of 1.37m, cut to length, with a length tolerance (0~+10mm) and the products are numbered in sequence.

[0075] (20) Normalizing + spheroidizing annealing: The main purpose of normalizing + spheroidizing annealing is to improve the as-rolled microstructure and reduce hardness. The resulting microstructure is spheroidal pearlite, which has low hardness and good uniformity. The steel pipe has good plasticity during the spinning process and is not prone to cracking. The normalizing + spheroidizing annealing process requirements are as follows: Normalizing: Heating temperature 900-950℃, holding time 1-5 hours, air cooling after exiting the furnace. Spheroidizing annealing: A three-stage heat treatment is used. The first stage is 750-850℃, holding time 2-6 hours, furnace cooling to the second stage: temperature 700-760℃, holding time 20-30 hours; furnace cooling to the third stage: temperature 640-720℃, holding time 6-14 hours.

[0076] Comparative Example 2 (forging billet through pipe): Grade: 30CrMnSiNi2A, Steel pipe size: 590×50mm. The difference from Example 2 is that the production steps of the forged tube blank include (4) to (10), of which (1) to (3) are the selection, tempering and polishing of D406A consumable ingots, and steps (11) to (18) are the production process of the steel pipe, which are equivalent to (6) to (13) in Example 2. The specific implementation process of Comparative Example 2 is as follows: (1) Consumable ingot size 590~ 625mm, ingot type 660mm, the composition and metallurgical quality of the steel ingot must meet the standard requirements.

[0077] (2) Ingot annealing: The main purpose of this process is to eliminate the stress during the solidification process of the vacuum consumable steel ingot, prevent cracking, and reduce the hardness of the material to facilitate the implementation of steps (3) and (4). 30CrMnSiNi2A is annealed at a heating temperature of 600-700℃, a holding time of 15-20h, and furnace cooling.

[0078] (3) Ingot turning: Due to the characteristics of the vacuum consumable process, the inclusions in the steel are removed to form a hard layer on the surface of the ingot, which has poor plasticity. The surface defects must be removed by turning before hot working deformation can be carried out. The target size of the ingot is 590~ 625mm.

[0079] (4) Ingot heating: This process allows the blank to be burned through, the inner and outer metals to be heated evenly, and good thermoplasticity to be achieved, which is a prerequisite for the smooth progress of process (5). The heating temperature is 1150-1200℃, and the total heating time is 3-8h.

[0080] (5) Rapid forging: This process obtains a round tube with good density through forging deformation. Target size: 600mm, tolerance (-5mm, +5mm); forging is carried out using three-upsetting and three-drawing process, forging ratio ≥5, and stop forging temperature ≥800℃.

[0081] (6) Annealing: This process eliminates the stress of forging thermal deformation and reduces the hardness through annealing to facilitate sawing operation. The specific process parameters are as follows: heating temperature 600-720℃, holding time 20-40h, and cooling with the furnace.

[0082] (7) Blank finishing: This process mainly involves straightening and grinding wheel peeling. The tube blank may be bent after forging. The main purpose of straightening is to correct the straightness of the tube blank. (8) Cutting the head and tail of the blank: According to the process regulations, the unusable parts of the head and tail of the steel ingot are removed. The purpose of this process is the same as that of step (4) in Example 2. The difference is that this process removes the head and tail in the forging step, while step (4) in Example 2 removes the head and tail in the steel ingot state.

[0083] (9) Ultrasonic testing: This process uses ultrasonic testing to detect whether there are cracks or other defects inside the tube billet that may affect the quality of subsequent products. If qualified, it also serves as an acceptance check for the internal quality of the billet.

[0084] (10) Tube finishing and acceptance: If there are surface cracks on the surface of the tube blank, the cracks will expand during the subsequent hot working process, and the local size after finishing may not meet the requirements.

[0085] According to quality management requirements, the surface of the blank must meet the requirements of hot working. Cracks, slag pits and other defects are not allowed. If any defects are found, they must be cleaned. The blank can be inspected by magnetic particle inspection. If cracks are found, they can only be considered qualified after rework, grinding and magnetic particle recarburization.

[0086] (11) Ring furnace heating: This process allows the billet to be burned through, the inner and outer metals to be heated evenly, and good thermoplasticity to be achieved. It is a prerequisite for the smooth progress of processes (12), (13), and (14). The heating temperature is 1150-1270°C, and the total heating time is 10-20 hours.

[0087] (12) Hot piercing of steel ingots (controlling inner diameter): The end of the blank is flared to facilitate the positioning and engagement of the ejector pin. The size of the ejector pin and the hot piercing process can obtain a reasonable inner hole size. After the hot piercing is completed, the ejector pin is withdrawn and the finished product of this process is output as a capillary tube.

[0088] Key parameter settings: plug diameter 480mm, capillary outer diameter 600~620mm. Rotate the long guide roller for 10-60 seconds before piercing to remove iron oxide scale. The roller speed should be ≤150 rpm.

[0089] (13) Finish rolling (control wall thickness): In this step, a mandrel is placed in the inner hole to control the inner hole size to remain unchanged. During the rough tube rolling process, the wall thickness decreases and gradually reaches the target requirement. After the hot rolling is completed, the mandrel is withdrawn and the finished product of this process is output as a rough tube.

[0090] Key parameter settings: mandrel size 480mm, roller pitch 570~590mm, lead length 600~620mm, rolling current ≤3000A.

[0091] (14) Sizing of rough pipe (controlling outer diameter): In this step, the inner hole no longer uses a mandrel. When the rough pipe passes through the reducing mill, the outer diameter is reduced. This process is designed based on the reducing principle. When the rough pipe passes through the reducing mill to adjust the outer diameter, the inner diameter remains basically unchanged. This process mainly controls the outer diameter size to meet the target requirements. The output of this process is the finished steel pipe. Target size 598×52mm.

[0092] (15) Slow cooling of steel pipes: In this process, the finished steel pipes are cooled slowly and evenly on the cooling bed to prevent deformation of the steel pipes and control the ovality and straightness of the finished steel pipes.

[0093] (16) Steel pipe tempering: This process is mainly used to eliminate heat treatment stress and prevent cracking of finished steel pipes. Secondly, it is used to reduce the hardness and facilitate the subsequent flattening and sawing. Key parameters: heating temperature 600-720℃, holding temperature 2-8h, and air cooling after leaving the furnace.

[0094] (17) Steel pipe finishing: External grinding: The main purpose of external grinding of steel pipes is to remove the oxide scale on the outer surface of the steel pipe, and secondly to make the roughness of the outer surface of the steel pipe meet the requirements of ultrasonic testing, roughness Ra ≤ 6.3μm.

[0095] Flat head (including sampling): After the steel pipe is pierced and rolled, a concave and convex surface is formed at the starting and end. The size of this part does not meet the requirements and must be cut off, with 150 to 250 mm cut at each end.

[0096] (18) Ultrasonic testing: Ultrasonic testing of steel pipes is a non-destructive test used to assess the internal quality of steel pipes. Steel pipes meeting GB / T5777 standard U2.5 grade are transferred to the next process after passing the flaw detection.

[0097] In each embodiment or comparative example, two samples, one at the head and one at the tail, were taken for experiment in the "flat head" process, and thus two sets of experimental data were obtained for each embodiment or comparative example.

[0098] By testing the finished steel pipes prepared in Example 1 and Comparative Example 1, a comparison table of technical indicators is obtained as shown in Table 1 below: Table 1 Comparison of technical indicators of finished steel pipes prepared in Example 1 and Comparative Example 1

[0099] By testing the finished steel pipes prepared in Example 2 and Comparative Example 2, a comparison table of technical indicators is obtained as shown in Table 2 below: Table 2 Comparison of technical indicators of finished steel pipes prepared in Example 2 and Comparative Example 2

[0100] By testing the finished steel pipes prepared in Examples 1 and 2 and Comparative Examples 1 and 2, a comparison table of economic indicators is obtained as shown in Table 3 below: Table 3 Comparison of economic indicators of finished steel pipes prepared in Examples 1 and 2 and Comparative Examples 1 and 2

[0101] According to Tables 1 to 3, the technical indicators of the embodiments and comparative examples of the two materials are comparable. Compared with comparative examples 1 and 2, the economic indicators of the steel pipes prepared in Examples 1 and 2 of the present invention are that the yield rate is increased by more than 10%, and the production cycle is shortened by more than 30 days. Therefore, the steel pipe preparation method of the present invention greatly reduces the production cost of the steel pipe while ensuring the quality of the steel pipe, compared with the traditional steel pipe preparation method.

[0102] 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 in the scope of protection of the present invention.

Claims

1. A low-cost preparation method for large-diameter seamless steel pipes, characterized in that: The following steps are involved: S1, tempering or annealing the selected vacuum consumable steel ingot to eliminate stress; S2, removing the hard layer on the surface of the steel ingot by turning, and cutting off the head and tail of the steel ingot to obtain a tube blank; S3, heating the tube blank in a ring furnace, and performing hot perforation on the heated tube blank by a push rod. After the hot perforation is completed, the push rod is withdrawn to obtain a capillary tube; S4, placing the mandrel into the inner hole of the shell, and performing finish rolling on the shell until the shell wall thickness is reduced to the target range. After the finish rolling is completed, the mandrel is withdrawn to obtain a rough shell; S5, reducing the outer diameter of the rough pipe by a reducing mill until the outer diameter of the rough pipe is reduced to a target range, thereby obtaining a steel pipe; S6, using a cooling bed to uniformly cool the steel pipe; S7, tempering or annealing the steel pipe to eliminate stress; S8, external grinding and flattening of the steel pipe; S9, perform ultrasonic flaw detection on the steel pipe to obtain qualified steel pipe.

2. A low-cost method for preparing a large-diameter seamless steel pipe according to claim 1, characterized in that: In step S1, the ingot shape of the vacuum consumable steel ingot is selected as 660 mm, and the diameter of the steel ingot obtained after turning in step S2 is 590-625 mm.

3. The low-cost preparation method for a large-diameter seamless steel pipe according to claim 1, characterized in that: In step S2, the length of the head and tail of the steel ingot is cut off to 100 to 200 mm.

4. A low-cost method for preparing a large-diameter seamless steel pipe according to claim 1, characterized in that: Before step S3, the surface of the tube blank is inspected for flaws. If local cracks or slag pits are found, they are repaired by grinding.

5. The low-cost preparation method for a large-diameter seamless steel pipe according to claim 1, characterized in that: In step S3, the heating temperature of the annular furnace is 1150-1270° C., and the heating time is 10-20 hours. Before hot perforation, a bell mouth is opened at the end of the tube blank for positioning the ejector rod.

6. A low-cost method for preparing a large-diameter seamless steel pipe according to claim 1, characterized in that: In step S4, the target range of the capillary wall thickness is 25 to 50 mm; in step S5, the target range of the steel pipe outer diameter is 580 to 635 mm.

7. A low-cost method for preparing a large-diameter seamless steel pipe according to claim 1, characterized in that: In step S8, the outer grinding treatment is to remove the oxide scale on the outer surface of the steel pipe and ensure that the roughness of the outer surface of the steel pipe is not greater than 6.3 μm; the flattening treatment is to cut off 150 to 250 mm from the head and tail of the steel pipe respectively.

8. The low-cost method for preparing a large-diameter seamless steel pipe according to claim 1, characterized in that: After step S9, the method further includes: S10, cutting the steel pipe to a predetermined length to obtain multiple sections of steel pipe; S11, the steel pipe is subjected to normalizing and spheroidizing degradation treatment.

9. A low-cost method for preparing a large-diameter seamless steel pipe according to claim 8, characterized in that: In step S10, the length tolerance range allowed for the cut to length sawing is 0 to +10 mm.

10. A low-cost method for preparing a large-diameter seamless steel pipe according to claim 8, characterized in that: In step S11, the normalizing heating temperature is 900-950°C, and the holding time is 1-5 hours; the spheroidizing annealing adopts a three-stage heat treatment, the first stage annealing temperature is 750-850°C, the holding time is 2-6 hours, and the furnace is cooled to the second stage annealing temperature; the second stage annealing temperature is 700-760°C, the holding time is 20-30 hours, and the furnace is cooled to the third stage annealing temperature; the third stage annealing temperature is 640-720°C, and the holding time is 6-14 hours.

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