Production process of duplex stainless steel seamless steel tube for heat exchanger

Abstract

The invention relates to a production process of a duplex stainless steel seamless steel tube for a high-precision heat exchanger, the steel grade of the duplex stainless steel seamless steel tube is 022Cr23Ni5Mo3N, and the duplex stainless steel seamless steel tube comprises the following chemical components in percentage by weight: less than or equal to 0.03% of C, less than or equal to 1.00% of Si, less than or equal to 2.00% of Mn, 22.00-23.00% of Cr, 4.50-6.50% of Ni, 3.00-3.50% of Mo, less than or equal to 0.03% of S and less than or equal to 0.030% of P. And the typical specifications of the high-frequency frequency converter are phi 25.4 * 1.651, phi 31.75 * 1.651 and phi 38.1 * 2. The manufacturing method comprises the following steps: (1) manufacturing a pipe blank; (2) hot punching; (3) cold rolling; (4) heat treatment; (5) straightening; and (6) pickling. The room-temperature tensile strength of the finished pipe is larger than or equal to 655 MPa, the room-temperature yield strength is larger than or equal to 485 MPa, the percentage elongation after fracture is larger than or equal to 25%, the hardness HRC is smaller than or equal to 28, the structure is uniform, the ferrite content is 45%-55%, the heat conductivity coefficient is larger than or equal to 17.6 W (m.K)-1 (0 DEG C), and the spot corrosion rate lt is achieved. And 0.01 g / m < 2 > / 24Hrs (25 DEG C, 6% FeCl3 solution, 72 hours) can meet the requirements of the heat exchanger.

Description

Technical Field

[0001] The invention relates to the technical field of stainless steel seamless pipe production, and in particular to a production process of a high-precision duplex stainless steel seamless pipe, in particular to a production process of a duplex stainless steel seamless pipe for manufacturing a heat exchanger. Background Art

[0002] A heat exchanger is a device that transfers part of the energy of a hot fluid to a cold fluid. It is also called a heat exchanger. Its function is to transfer heat from a hot fluid to a cold fluid. Heat exchangers are widely used in many fields, such as industrial production, heating systems, refrigeration systems, etc.

[0003] Duplex stainless steel seamless pipes are widely used in heat exchangers because of their excellent high strength and corrosion resistance, which can effectively improve the durability and reliability of equipment. Their dimensional accuracy has a great influence on the production cost and reliability of heat exchangers. If the dimensional error of seamless pipes is too large, it may lead to problems such as reduced performance of heat exchangers, poor sealing performance or premature failure. Therefore, it is necessary to propose a production process for high-precision duplex stainless steel seamless pipes for efficient heat exchangers.

[0004] The purpose of the present invention is to solve the technical problems in the production process of high-precision duplex stainless steel seamless pipes and to provide a production process of duplex stainless steel seamless pipes with high dimensional accuracy and good surface quality for efficient heat exchangers. Summary of the invention

[0005] The purpose of the present invention is to provide a production process of duplex stainless steel seamless steel pipe for heat exchanger in view of the above problems.

[0006] The object of the present invention is achieved as follows: A production process of duplex stainless steel seamless steel pipe for heat exchanger, comprising the following steps: Step 1: Raw material smelting: round steel billet is obtained by smelting in converter + AOD + LF furnace, die casting, electroslag remelting, initial rolling and blanking, and the composition requirements of the round steel billet are: C≤0.02%, Ni≤5%, 0.18%≤N≤0.2%, P≤0.025%, S≤0.001%; ​​Step 2: Pipe billet preparation: the round steel billet is peeled, sawed and centered to become a billet, and the outer diameter of the billet after peeling is 63-63.5m m, length tolerance (+5,0) mm, billet surface roughness Ra≤1.6μm; Step 3: Hot perforation: After the billet is heated to 1130-1150℃ in an inclined bottom furnace, it enters the perforator and perforates into a rough pipe with an outer diameter of 66-67mm and a wall thickness of 5-5.5mm; the temperature of the preheating section of the inclined bottom furnace is not higher than 700℃, and the preheating time is 120-150min; the temperature of the heating section is 1050-1100℃, and the heating time is 120-150min; the temperature of the soaking section is 1140-116 0℃, heating time in soaking section 60-70min; billet flips 180° every 3-5min; Step 4: Grinding: both inner and outer surfaces of the rough pipe are polished; Step 5: Cold rolling: After the rough pipe is pickled to remove the oxide scale, it is cold rolled. The first pass is cold rolled to the specification Φ(38-42)×(2.5-3.5)mm, the mill feed is 4-6mm, and the rolling speed is 50-65 times / min; the second pass is cold rolled to the specification Φ(25.4-38.1)×(1.651-2)mm, the mill feed is 2.5 -6mm, rolling speed 40-65 times / min; remove oil stains on the inner and outer surfaces after cold rolling; Step 6: Heat treatment: Both the intermediate semi-finished products and the finished products are annealed and heat treated. The heat treatment temperature of the intermediate semi-finished products is 1100±10℃, and the holding time is 12-15min; the annealing temperature of the finished product is 1060±10℃, and the holding time is 8-10min; Step 7: Straightening: After the finished steel pipe is solution heat treated, it is straightened to achieve a curvature of ≤0.8mm / m; Step 8: Pickling: The acid used for pickling is HNO 3 +HF mixed acid, HNO 3 : 150-300g / L, HF: 35-45g / L; acid temperature 50-70℃, first immersion time 2h-2h10min, second immersion time 1h30min-1h40min, if the local oxide scale is still not washed off, continue to soak for 1h-1h10min.

[0007] The pickling in step 5 is specifically as follows: the acid used for pickling is HNO 3 +HF mixed acid, HNO 3: 150-300g / L, HF: 35-45g / L; acid temperature 50-70℃, first immersion time 2h-2h10min, second immersion time 1h30min-1h40min, if the local oxide scale is still not washed off, continue to soak for 1h-1h10min.

[0008] The beneficial effects of the present invention are as follows: the performance and parameters of the duplex stainless steel pipe produced by the present invention meet the requirements of GB / T21833.1 and NB / T47019.6 standards, have the characteristics of high dimensional accuracy, good surface quality and excellent performance, and can meet the high performance and high parameter requirements of the heat exchanger.

[0009] The present invention adopts targeted component design and high-temperature solution treatment process to ensure its excellent mechanical properties and corrosion resistance, adopts oblique rolling and perforation to improve the yield rate and reduce production costs, polishes the inner and outer surfaces of the rough pipe and adopts a high-precision two-roller pilger high-speed rolling mill for two-pass rolling to improve the product size accuracy and surface quality, and ensure the flow rate and flow rate of the fluid in the pipeline. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The present invention will be further described below in conjunction with the accompanying drawings.

[0011] Figure 1 It is the design drawing of the oblique rolling piercing plug of the present invention.

[0012] Figure 2 It is the front view of the design drawing of the oblique rolling perforated guide plate of the present invention.

[0013] Figure 3 It is a top view of the design drawing of the oblique rolling perforated guide plate of the present invention.

[0014] Figure 4 It is a left view of the design drawing of the oblique rolling perforated guide plate of the present invention. DETAILED DESCRIPTION

[0015] The present invention provides a production process of duplex stainless steel seamless steel pipe for heat exchanger, which realizes batch production of duplex stainless steel seamless steel pipe with high dimensional accuracy and good surface quality by optimizing key steps such as raw material smelting, tube blank processing, hot perforation, cold rolling, heat treatment, pickling, etc. The implementation of the process includes the following steps: 1. Raw material smelting: φ65 round steel billet is obtained through converter + AOD + LF furnace smelting, die casting, electroslag remelting, and initial rolling. The composition of the tube billet meets the requirements of GB / T21833.1 and NB / T47019.6 standards, and C≤0.02%, Ni≤5%, N≥0.18%, P≤0.025%, and S≤0.001%.

[0016] 2. Preparation of tube blanks: round steel is peeled, sawn and centered to become blanks; after peeling, the outer diameter of the blank is in the range of 63-63.5mm, the allowable length deviation is (+5,0)mm, and the surface roughness of the blank is Ra≤1.6μm.

[0017] 3. Hot piercing: After the tube billet is heated by the inclined bottom furnace, it enters the piercing machine to be pierced into a rough tube with an outer diameter of 66-67mm and a wall thickness of 5-5.5mm. The piercing temperature is 1130-1150℃.

[0018] 4. Grinding: The inner and outer surfaces of the rough pipe are polished to ensure that there are no cracks or other defects on the inner and outer surfaces.

[0019] 5. Cold rolling: After the rough pipe is pickled to remove the oxide scale, it is cold rolled using a two-roll high-speed pilger mill. The first pass is cold rolled to the specification of Φ(38-42)×(2.5-3.5)mm, and the second pass is cold rolled to the specification of Φ(25.4-38.1)×(1.651-2)mm; after cold rolling, the oil stains on the inner and outer surfaces must be removed.

[0020] 6. Heat treatment: The intermediate and finished products are subjected to annealing heat treatment between the two cold rolling processes. The intermediate semi-finished product heat treatment temperature is 1100±10℃, and the finished product annealing temperature is 1060±10℃.

[0021] 7. Straightening: After solution heat treatment, the finished steel pipe is straightened to achieve a curvature of ≤0.8mm / m.

[0022] 8. Pickling: The acid used for pickling is HNO 3 +HF mixed acid, HNO 3 :150-300g / L, HF: 35-45g / L.

[0023] Example 1

[0024] Raw material smelting: The raw materials are melted by electricity in the EAF furnace, and then reduced after melting. After the composition is adjusted to the internal control requirements, the reduction slag is removed and the steel is tapped. The smelting processes such as decarburization, degassing, and removal of inclusions are carried out in the AOD furnace. The final carbon is strictly controlled according to the internal control requirements. Al and Si-Fe are added in a certain proportion during reduction. After reduction, the slag is ensured to change color, and the slag is pulled. The slag is pulled fully. After AOD reduction, S≤0.001% is generally ensured before steel can be tapped. The composition and temperature are fine-tuned in the LF furnace. The chemical composition mass percentage of steel smelted by this method is C: 0.019%, Si: 0.6%, Mn: 0.5%, P: 0.024%, Cr: 22.5%, Ni: 4.6%, Mo: 3.1%, N: 0.19%, and the balance is Fe.

[0025] Round steel forging: After converter + AOD + LF furnace smelting, die casting, electroslag remelting, and initial rolling, a 220mm diameter round steel billet is obtained, which is then rolled into a 65mm diameter, 15m long round steel through multiple passes.

[0026] Raw material processing: The round steel is peeled, sawn and centered to become a billet with a diameter of 63.5mm and a length of 500mm. The cutting slope of the pipe end is less than 1mm, and there are no obvious cracks or black skin on the outer surface. The size of the centering hole at the pipe end is 13*15mm.

[0027] Hot piercing: Place the processed billet into an inclined bottom furnace for heating. One end of the centering hole is the biting direction of the piercing machine. The billet should be placed in the center of the furnace. The heating process of the billet is as shown in the following table.

[0028] Furnace temperature time Preheating section 700℃ 120min Heating section 1090℃ 120min Heat soaking section 1140℃ 60min

[0029] The billet tapping temperature is 1142°C. The billet in the furnace is turned 180° every 3 minutes.

[0030] The die specifications for the punching machine are shown in the following table.

[0031] Head Guide mandrel 51*111 65-70 50

[0032] The punching machine parameters are adjusted as shown in the following table.

[0033] Guide plate distance Roller distance Plug extension factor Elongation coefficient 62 58 55 2.754

[0034] The perforated rough pipe has a specification of 67mm*5.5mm and is immediately put into water for solid solution after perforation. The production rhythm is 30 pieces / hour.

[0035] Grinding: After pickling to remove the oxide scale, the rough pipe is first repaired locally to eliminate the cracks, pits, scars, guide plate marks and other defects with a depth of ≥ 0.3mm on the outer surface through spot repair. After ensuring that there are no obvious defects on the outer surface of the product, it is then polished; the outer surface is polished one by one (5%); the inner surface is endoscopy + internal polishing one by one (1.5%); the semi-finished products are inspected by endoscopy one by one.

[0036] Cold rolling: A two-roll high-speed pilger mill is used for cold rolling. The first pass is cold rolled to a specification of 38×3.0mm, a feed amount of 4mm, and a rolling speed of 60 times / min; the second pass is cold rolled to a specification of 25.4×1.651mm, a feed amount of 4mm, and a rolling speed of 60 times / min. Annealing heat treatment is performed between the two cold rollings.

[0037] Intermediate annealing heat treatment process: annealing temperature 1090 ~ 1110 ℃, keep warm for 15 minutes, and water cool to room temperature after taking out of the furnace.

[0038] Finished product solution heat treatment process: solution temperature 1050 ~ 1060 ℃, keep warm for 10 minutes, and water cool to room temperature after taking out of the furnace.

[0039] After solution heat treatment, the finished steel pipe is straightened on a five-roll straightening machine to achieve a curvature of ≤0.8mm / m.

[0040] Pickling process for intermediate and finished pipes: The acid used for pickling is HNO 3 +HF mixed acid, HNO 3 : 150-300g / L, HF: 35-45g / L; acid solution heating temperature 60℃, first immersion time 2h, second immersion time 1.5h, after two immersions, the product is taken out of the tank.

[0041] The above description is only a specific embodiment of the present invention, but the structural features of the protection scope of the present invention are not limited thereto. Any changes or modifications made by any technician in the field of the present invention are all covered by the patent scope of the present invention.

Claims

1. A production process for duplex stainless steel seamless steel pipe for heat exchanger, characterized in that: The following steps are involved: Step 1: Raw material smelting: round steel billet is obtained through converter + AOD + LF furnace smelting, die casting, electroslag remelting, initial rolling and blanking. The composition requirements of round steel billet are: C≤0.02%, Ni≤5%, 0.18%≤N≤0.2%, P≤0.025%, S≤0.001%; Step 2: Preparation of tube billet: The round steel billet is peeled, sawed and centered to become a billet. After peeling, the outer diameter of the billet is 63-63.5mm, the length tolerance is (+5,0)mm, and the surface roughness of the billet is Ra≤1.6μm; Step 3: Hot perforation: After the billet is heated to 1130-1150℃ in the inclined bottom furnace, it enters the perforator to be perforated into a rough tube with an outer diameter of 66-67mm and a wall thickness of 5-5.5mm; the preheating section temperature of the inclined bottom furnace is not higher than 700℃, and the preheating section time is 120-150min; the heating section temperature is 1050-1100℃, and the heating section time is 120-150min; the soaking section temperature is 1140-1160℃, and the soaking section heating time is 60-70min; the billet is turned 180° every 3-5min; Step 4: Grinding: The inner and outer surfaces of the rough pipe are polished; Step 5: Cold rolling: After the rough pipe is pickled to remove the oxide scale, it is cold rolled. The first pass is cold rolled to the specification of Φ(38-42)×(2.5-3.5)mm, the mill feed is 4-6mm, and the rolling speed is 50-65 times / min; the second pass is cold rolled to the specification of Φ(25.4-38.1)×(1.651-2)mm, the mill feed is 2.5-6mm, and the rolling speed is 40-65 times / min; after cold rolling, remove the oil stains on the inner and outer surfaces; Step 6: Heat treatment: Both the intermediate semi-finished products and finished products are annealed. The heat treatment temperature of the intermediate semi-finished products is 1100±10℃, and the insulation time is 12-15min; the annealing temperature of the finished products is 1060±10℃, and the insulation time is 8-10min; Step 7: Straightening: After the finished steel pipe is solution heat treated, it is straightened to achieve a curvature of ≤0.8mm / m; Step 8: Pickling: The acid used for pickling is HNO3+HF mixed acid, HNO3: 150-300g / L, HF: 35-45g / L; the acid temperature is 50-70℃, the first immersion time is 2h-2h10min, the second immersion time is 1h30min-1h40min, if the local oxide scale is still not washed off, continue to soak for 1h-1h10min.

2. The production process of a duplex stainless steel seamless steel pipe for heat exchanger according to claim 1, characterized in that: The pickling in step five is specifically as follows: the acid solution used for pickling is HNO3+HF mixed acid, HNO3: 150-300g / L, HF: 35-45g / L; the acid solution temperature is 50-70℃, the first immersion time is 2h-2h10min, the second immersion time is 1h30min-1h40min, if the local oxide scale is still not washed off, continue to soak for 1h-1h10min.

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