A production process for simplifying the manufacturing process of T91 seamless steel pipes

Abstract

The present application belongs to the technical field of seamless steel pipe production, and discloses a production process for simplifying the manufacturing process of T91 seamless steel pipe, which adopts a manufacturing process of inclined bottom heating furnace heating+inclined rolling piercing+tension reducing+annealing+cold rolling+heat treatment to produce T91 finished seamless steel pipe. The present application utilizes the residual heat of the finished steel pipe piercing to perform tension reducing on the steel pipe, to reduce the size of the capillary tube, to reduce the subsequent cold rolling times of the T91 seamless steel pipe and the intermediate annealing and pickling process, to reduce the discharge of waste acid; in addition, the roller speed distribution of the reducing mill is optimized, the roller speed of each rack of the reducing mill is reasonably distributed, the defects of uneven wall thickness are avoided, and the T91 seamless pipe yield is improved. The energy consumption of the steel pipe production is reduced, the production efficiency is improved, the T91 seamless steel pipe produced has good inner and outer surface quality, high precision, stable performance and good organization.

Description

Technical Field

[0001] This invention pertains to seamless steel pipe manufacturing processes, and specifically relates to a simplified manufacturing process for T91 seamless steel pipes. Background Technology

[0002] T91 steel possesses high strength, good plasticity, and excellent oxidation and corrosion resistance, making it suitable for manufacturing heating surface tubes such as reheaters and superheaters in high-pressure boilers with metal wall temperatures <625℃, and steam ducts with metal wall temperatures ≤600℃. Existing production processes generally include the following steps: heating, piercing, sizing, annealing, pickling, cold drawing / cold rolling, final heat treatment, and finishing. Specifically, the aforementioned production process for T91 seamless steel pipes typically begins by heating the round steel billet to a high temperature above 1150℃, followed by piercing. The billet is then sizing using a tension reducing mill to obtain a rough tube with a reduction rate of 20%–25%. After annealing, pickling, and phosphating, the rough tube undergoes cold drawing / cold rolling to obtain the finished pipe, with a cold rolling deformation of 50%–60%.

[0003] T91 round steel exhibits good piercing performance within the range of 1100℃ to 1070℃. However, current production processes involve heating the T91 tube blank at high temperatures without considering the heat generated during high-speed rotation and friction during piercing. The inner wall temperature is approximately 80℃ to 120℃ higher than the heating temperature, easily leading to overheating and burning, resulting in piercing failure. Furthermore, the low reduction rate leads to excessive deformation during subsequent cold rolling, causing cracking and rolling impediments, resulting in tube scrap and shortening the lifespan of the rolling mill. Inappropriate roll speed distribution on the tension reduction mill also affects the surface quality of the tube. Multiple cold working processes are cumbersome, resulting in long production cycles, low efficiency, and high costs. Additionally, multiple annealing, pickling, and phosphating treatments further increase production costs and raise waste acid emissions, posing a potential pollution risk.

[0004] This invention optimizes the production process of T91 seamless tubes based on the above-mentioned problems, improves tube blank heating, diameter reduction processing, and cold rolling deformation, and optimizes the synergistic effect of each process to improve the production efficiency of enterprises and significantly improve the mechanical properties of T91 seamless tubes. Summary of the Invention

[0005] The technical problem to be solved by the present invention is: based on the above-mentioned technical problem, the present invention provides a production process that simplifies the manufacturing process of T91 seamless steel pipes.

[0006] The technical solution adopted by this invention to solve its technical problem is: a simplified production process for manufacturing T91 seamless steel pipes, including the following steps: billet blanking → heating → skew rolling piercing → tension reduction → stress relief annealing → surface treatment → cold rolling → degreasing → final heat treatment → physical and chemical testing → finishing and non-destructive testing → marking and warehousing.

[0007] The specific steps are as follows:

[0008] (1) Cutting: Cut the T91 round steel pipe blank to a suitable length and then peel and grind it;

[0009] (2) Hot rolling piercing: The heating furnace is evacuated and filled with nitrogen for protection. The round steel tube billet is heated to 1080℃~1170℃ and held at that temperature for 80min~100min. During the heating process, the steel is turned over multiple times to ensure uniform heating. After heating, it is conveyed to the piercing machine for piercing.

[0010] (3) Tension Reduction: The diameter of T91 steel pipes is reduced using a tension reduction machine, with a reduction of more than 30%. A 12-stand 130 unit reduction machine is preferred for reducing the diameter of the pierced T91 tubes, with roller speeds of stands 1-12 all above 300 r / min. Further preferred roller speeds are 350-380 r / min for stands 1-4, 400-420 r / min for stands 5-8, and 440-460 r / min for stands 9-12. The roller speeds of each stand of the reduction machine are rationally distributed, with the roller speeds increasing progressively.

[0011] (4) Annealing treatment: T91 is subjected to recrystallization annealing treatment to eliminate the internal stress generated in the hot rolling piercing and diameter reduction of the tube blank, so as to make good microstructure preparation for the next process; further, the annealing treatment conditions are 860~880℃ for 90min, and the annealing furnace is evacuated and filled with nitrogen for protection.

[0012] (5) Surface treatment: After the T91 seamless steel pipe cools down, pickling, phosphating and saponification are carried out.

[0013] The pickling process involves immersing the steel pipe in a 60°C pickling solution for 70-90 minutes, followed by 10 minutes of pickling. Afterward, the pipe is lifted once to observe the removal of oxide scale from the T91 steel pipe surface, preventing over-pickling.

[0014] The phosphating treatment is as follows: the steel pipe is immersed in a phosphating solution at 75°C for 20 to 30 minutes, wherein the mass ratio of water to phosphoric acid mother liquor is 10:1.

[0015] The saponification process involves immersing the steel pipe in a lubricant at 70°C for no more than 10 minutes.

[0016] (6) The surface-treated T91 seamless steel pipe is subjected to only one large-deformation cold rolling, with the cold rolling deformation reaching more than 70%, and rolled to the required finished specifications. Further optimization is to make the cold rolling deformation 70% to 80%.

[0017] (7) Degrease the cold-rolled T91 steel pipe to remove the rolling oil adsorbed on the inner and outer surfaces during the cold rolling process, so as to avoid contaminating the surface of the steel pipe during the final heat treatment process.

[0018] (8) The surface-treated T91 steel pipe is subjected to normalizing and tempering treatment to obtain the finished pipe.

[0019] Before normalizing, preheat once: the preheating temperature is 860℃ and the holding time is 10min.

[0020] The normalizing process is as follows: temperature 1040℃, holding time calculated at 1.8 min / mm, followed by air cooling to 820℃ and then air cooling. The tempering process is as follows: temperature 780℃, holding time 90 min, and air cooling.

[0021] (9) Conduct physical and chemical tests, straighten and non-destructive testing, and put T91 steel pipes that pass the tests and testing into the warehouse.

[0022] The beneficial effects of this invention are:

[0023] (1) The present invention optimizes the heating temperature of T91 round steel tube blank and strictly controls the holding time so that it reaches the optimal plastic processing temperature during the piercing process. The surface quality after piercing can be significantly improved. In addition, reducing the heating temperature will also reduce the energy consumption of enterprises and reduce production costs.

[0024] (2) In this invention, the diameter is reduced by using the residual heat of the tube after hot rolling and piercing, and then formed by a second large deformation cold rolling. The deformation is large, the dislocation entanglement is serious, and the grains are completely broken. This can provide more nucleation points during the heat treatment of the finished tube, which is conducive to refining the grains and improving the mechanical properties.

[0025] (3) The roller speed distribution of the tension reducing mill has been optimized. The roller speed is distributed in an increasing manner to avoid defects such as rolling and bending, and wall thickness deviation. This improves the wall thickness uniformity of T91 seamless pipe and is beneficial for subsequent reprocessing.

[0026] (4) The present invention optimizes the final heat treatment. Compared with conventional heat treatment process, the present invention introduces nitrogen gas as a protective atmosphere during the heating process to reduce oxidation and decarburization. Furthermore, preheating and precooling are added before and after normalizing, which can reduce the tendency of T91 seamless tubes to deform and crack during normalizing and improve the yield of tubes. Attached Figure Description

[0027] Figure 1 This is a flowchart illustrating the overall manufacturing process of a simplified T91 seamless steel pipe manufacturing technology. Detailed Implementation

[0028] The present invention will be further described below with reference to embodiments, but is not limited thereto.

[0029] Example 1

[0030] (1) Select a T91 round steel pipe blank with a steel pipe specification of Φ88mm, cut it to a length of 1600mm, check for appearance defects, and perform peeling and grinding treatment.

[0031] (2) Heat the T91 tube blank to 1080℃ and hold for 90 minutes. Vacuum the heating furnace and fill it with nitrogen for protection. Then, transport it to the piercing machine for piercing. The T91 tube blank after piercing has a specification of Φ90mm*9.3mm.

[0032] (3) A 12-stand 130 unit reducing mill was used to reduce the diameter of the pierced T91 tube. The roll speeds for stands 1-4 were 350 r / min, for stands 5-8 were 400 r / min, and for stands 9-12 were 445 r / min. The reduction was 31%, the final rolling temperature was 870℃, and the T91 tube after reduction had a diameter of Φ65mm*9.3mm.

[0033] (4) The reduced diameter tube is subjected to stress-relieving annealing. The annealing furnace is evacuated and filled with nitrogen for protection. The annealing temperature is 860℃ and the holding time is 90min. Then pickling, phosphating and lubrication are performed.

[0034] (5) The surface-treated T91 tube is cold-rolled with a cold rolling deformation of 72%, and the rolled steel pipe has a specification of Φ35mm*4mm.

[0035] (6) After the cold-rolled T91 steel pipe is degreased, it is then subjected to a final heat treatment of normalizing and tempering. The normalizing furnace is evacuated and filled with nitrogen for protection. It is preheated once at a temperature of 860℃ and held for 10 minutes. The normalizing temperature is 1040℃ and the holding time is calculated at 1.8 mm / min. Then it is air-cooled to 820℃ and then air-cooled. After air-cooling to room temperature, it is tempered at a temperature of 780℃ and held for 90 minutes.

[0036] (7) After heat treatment, T91 steel pipes are subjected to physical and chemical tests, straightening and non-destructive testing. T91 steel pipes that pass the tests are marked and put into storage.

[0037] Example 2

[0038] (1) Select a T91 round steel pipe blank with a steel pipe specification of Φ88mm, cut it to a length of 1600mm, check for appearance defects, and perform peeling and grinding treatment.

[0039] (2) Heat the T91 tube blank to 1170℃ and hold for 80 minutes. Evacuate the heating furnace and fill it with nitrogen for protection. Then, transfer it to the piercing machine for piercing. The pierced T91 tube has a diameter of Φ90mm*9.3mm. The surface is slightly overburned. After treatment, further processing can be carried out, but this will increase the time and labor costs.

[0040] (3) A 12-stand 130 unit reducing mill was used to reduce the diameter of the pierced T91 tube. The roll speeds for stands 1-4 were 380 r / min, for stands 5-8 were 420 r / min, and for stands 9-12 were 450 r / min. The reduction was 31%, the final rolling temperature was 870℃, and the T91 tube after reduction had a diameter of Φ65mm*9.3mm.

[0041] (4) The reduced diameter tube is subjected to stress-relieving annealing. The annealing furnace is evacuated and filled with nitrogen for protection. The annealing temperature is 880℃ and the holding time is 90min. Then pickling, phosphating and lubrication are performed.

[0042] (5) The surface-treated T91 tube is cold-rolled with a cold rolling deformation of 73%, and the rolled steel pipe has a specification of Φ32mm*5mm.

[0043] (6) After the cold-rolled T91 steel pipe is degreased, it is then subjected to a final heat treatment of normalizing and tempering. The normalizing furnace is evacuated and filled with nitrogen for protection. First, it is preheated at 860℃ and held for 10 min. The normalizing temperature is 1040℃ and the holding time is calculated at 1.8 min / mm. Then, it is air-cooled to 820℃ and then air-cooled. After air-cooling to room temperature, it is tempered at 780℃ and held for 90 min.

[0044] (7) After heat treatment, T91 steel pipes are subjected to physical and chemical tests, straightening and non-destructive testing. T91 steel pipes that pass the tests are marked and put into storage.

[0045] Example 3

[0046] In step (3) of Example 1, the roller speeds of frames 1 to 12 were all adjusted to 400 r / min, and other operations were the same as in Example 1. After the diameter reduction, the capillary tube exhibited eccentricity, and roller marks appeared on the tube body.

[0047] Example 4

[0048] In step (3) of Example 1, the roller speeds of stands 1-4 are adjusted to 445 r / min, the roller speeds of stands 5-8 to 410 r / min, and the roller speeds of stands 9-12 to 375 r / min. Other operations are the same as in Example 1. After the diameter reduction, the tube shows an inner hexagonal shape, and the tube body shows rolling defects.

[0049] Example 5

[0050] The cold rolling deformation in step (5) of Example 1 is adjusted to 52%, and the outer diameter after rolling is Φ48mm*5mm.

[0051] Example 6

[0052] In Example 1, the diameter reduction in step (3) was adjusted to 25%, and the cold rolling deformation in step (5) was adjusted to 81%. The outer diameter after rolling was Φ35mm*4mm. The diameter reduction did not reach 30%. After rolling, it was found that the tube body had rolling cracks due to excessive deformation, resulting in scrap.

[0053] Example 7

[0054] (1) Select a T91 round steel pipe blank with a steel pipe specification of Φ88mm, cut it to a length of 1600mm, check for appearance defects, and perform peeling and grinding treatment.

[0055] (2) Heat the T91 tube blank to 1080℃ and keep it at that temperature for 90 minutes. Then, it is sent to the piercing machine for piercing. The T91 tube blank after piercing has a diameter of Φ90mm*9.3mm.

[0056] (3) The perforated tube is subjected to stress-relief annealing at 880℃ for 90 minutes, followed by pickling, phosphating and lubrication.

[0057] (4) The T91 tube after surface treatment is cold rolled in two passes. The first deformation is 60%, and the rolled specification is Φ48mm*7.5mm. After annealing, pickling, phosphating and lubrication, the second cold rolling is carried out with a deformation of 55%. The rolled steel pipe specification is Φ32mm*5mm.

[0058] (6) After the cold-rolled T91 steel pipe is degreased, it is then subjected to a final heat treatment of normalizing and tempering. The normalizing temperature is 1040℃ and the holding time is calculated at 1.8min / mm. After being air-cooled to room temperature, it is tempered at 780℃ and the holding time is 90min.

[0059] (7) After heat treatment, T91 steel pipes are subjected to physical and chemical tests, straightening and non-destructive testing. T91 steel pipes that pass the tests are marked and put into storage.

[0060] Example 7 describes a T91 seamless tube cold rolling process that uses two passes with annealing between adjacent passes. This reduces the strengthening effect of the cold rolling process and lowers production efficiency. The present invention not only eliminates the need for cold rolling passes and intermediate annealing, but also significantly improves the overall performance of the final T91 seamless tube.

[0061] Table 1 Surface condition of T91 capillary tube after diameter reduction

[0062]

[0063] Table 2 Mechanical properties of finished T91 seamless tubes under different processes

[0064]

[0065]

[0066] As shown in Table 1, lowering the heating temperature can prevent overheating. Reasonable roller speed distribution can effectively affect the surface morphology and dimensions of the tube. An increasing roller speed distribution results in tubes with good surface condition and precise dimensions, which is beneficial for subsequent processing. Table 2 shows that the mechanical properties of T91 seamless tubes meet the standard values ​​after simplifying the manufacturing process. Conventional cold rolling of T91 seamless tubes uses two passes with annealing between adjacent passes, which reduces the strengthening effect of cold rolling and lowers production efficiency. This invention eliminates the intermediate cold rolling and annealing processes, resulting in a significant improvement in the overall performance of the T91 seamless tube. The mechanical properties after large deformation rolling are also superior to those after small deformation rolling. This is because a single large deformation cold rolling pass increases the dislocation density, providing more nucleation points for subsequent heat treatment and refining the grains. However, excessive deformation can cause rolling cracks in the T91 seamless tube during cold rolling, leading to scrap. Normalizing further refines the microstructure, while tempering forms dispersed alloy carbides / nitrides in the T91 seamless pipe. This dispersion strengthening further improves the strength and hardness of the seamless steel pipe, without a significant decrease in plasticity. The introduction of nitrogen gas as a protective atmosphere prevents oxidation and decarburization.

[0067] Based on the above-described preferred embodiments of the present invention, and through the foregoing description, those skilled in the art can make various changes and modifications without departing from the inventive concept. The technical scope of this invention is not limited to the contents of the specification, but must be determined according to the scope of the claims.

Claims

1. A simplified manufacturing process for T91 seamless steel pipes, characterized in that, The process includes tube blank blanking → heating → hot rolling piercing → tension reduction → stress relief annealing → surface treatment → cold rolling → degreasing → heat treatment; The steps are as follows: (1) Cutting: Select T91 round steel pipe blanks, cut them, and perform peeling and grinding; (2) Heating and hot rolling piercing: The heating furnace is evacuated and filled with nitrogen for protection. The round steel tube blank is heated to 1080℃~1170℃ and held for 80min~100min. After heating, it is transported to the piercing machine for piercing. (3) Tension reduction: The T91 steel pipe is reduced in diameter using a tension reduction machine, and the reduction amount reaches more than 30%; specifically, the T91 capillary tube after piercing is reduced in diameter using a 130 unit reduction machine with 12 stands, and the roller speed of stands 1 to 12 is above 300 r / min, and the roller speed is distributed in an increasing manner; among them, the roller speed of stands 1 to 12 in step (3) is 350 to 380 r / min for stands 1 to 4, 400 to 420 r / min for stands 5 to 8, and 440 to 460 r / min for stands 9 to 12. (4) Stress-relief annealing: (5) Surface treatment: after annealing, pickling, phosphating and lubrication are performed; (6) Cold rolling and degreasing: The surface-treated T91 tube is cold rolled in one pass, and the cold rolling deformation reaches more than 70% to the required finished product specifications. The cold-rolled T91 steel tube is then degreased. (7) Heat treatment: The degreased T91 steel pipe is subjected to normalizing and tempering treatment. Before normalizing, a preheating is performed: the normalizing temperature is 1040℃, the tempering temperature is 780℃, the preheating temperature before normalizing is 860℃, and after normalizing, a precooling is performed, which is air-cooled to 820℃ and then air-cooled to obtain the finished pipe.

2. The simplified T91 seamless steel pipe manufacturing process according to claim 1, characterized in that, The annealing conditions for step (4) are 860~880℃ for 90 minutes.

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