Martensite heat-resistant steel G115 seamless steel tube for 630 DEG C ultra-supercritical power station and manufacturing method thereof

Abstract

The invention discloses a martensite heat-resistant steel G115 seamless steel pipe for a 630 DEG C ultra-supercritical power station and a manufacturing method thereof, the martensite heat-resistant steel G115 seamless steel pipe with excellent structure and performance is prepared by adopting an extrusion and cold rolling pipe manufacturing process, and the high requirements of the 630 DEG C ultra-supercritical thermal power generating unit on the structure and the performance can be met.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of heat-resistant seamless steel pipe processing, in particular to a martensitic heat-resistant steel G115 seamless steel pipe for 630℃ ultra-supercritical power station and a manufacturing method thereof. BACKGROUND

[0002] With the rapid development of economy, the demand for energy is also increasing, so the demand for high-efficiency, low-emission high-parameter thermal power generating units is becoming more and more urgent. The higher the steam temperature and pressure parameters of coal-fired power generation, the lower the coal consumption and the less the pollutant emissions, but the higher the requirements for the performance of the material. P92 used in 600℃ parameter ultra-supercritical units cannot meet the requirements of higher parameter units; to build a 630℃ parameter ultra-supercritical unit, the high-temperature resistance of the material must reach 650℃.

[0003] 9Cr-3W-3CoVNbCuBN(G115) is a new type of martensitic heat-resistant steel invented by Academician Liu Zhengdong's team of the Iron and Steel Research Institute (Chinese patent CN103045962B). The heat-resistant steel adopts the principle of composite reinforcement, contains W, Co, Cu, B, N and other strengthening elements, and its high-temperature long-time aging organizational stability, high-temperature endurance strength and oxidation resistance are all significantly higher than P92, and it is suitable for long-term use under high-pressure working conditions below 650℃. Comparative studies have found that the endurance strength and oxidation resistance of G115 at 650℃ are superior to the performance of P92 at 600℃. Currently, G115 has begun to be applied to the world's first 630℃ ultra-supercritical demonstration power station, and the product meets the requirements of CSTM standards and enterprise standards such as "CSTM 00017-2021 Martensitic Heat-Resistant Steel 08Cr9W3Co3VNbCuBN(G115) Seamless Steel Pipe for Power Station", "Q / OAPD 2253-2022 New Type of Martensitic Heat-Resistant Steel 08Cr9W3Co3VNbCuBN(G115) Seamless Steel Pipe for Power Station". However, this technology mainly involves the composition design, strengthening concept, manufacturing method, performance characteristics, etc. of G115, and does not involve the specific manufacturing method of G115 seamless pipe.

[0004] Chinese patent CN108998650A relates to a manufacturing method of G115 large-diameter thick-wall seamless steel pipe for 630℃ ultra-supercritical units, and similarly, this technology only involves the extrusion pipe process of G115 large-diameter thick-wall seamless steel pipe.

[0005] Chinese patent CN108950148A relates to a method for improving the radial organization and performance uniformity of G115 large-diameter thick-wall pipe, and similarly, this technology does not involve the specific manufacturing method of G115 seamless pipe.

[0006] In view of the above, it is urgent to develop a manufacturing technology of martensitic heat-resistant steel G115 seamless pipe, which can obtain G115 seamless pipe with excellent structure and performance to meet the high requirements of 630 DEG C ultra-supercritical thermal power generating unit on structure and performance. SUMMARY

[0007] In view of the defects in the prior art, the purpose of the present application is to provide a martensitic heat-resistant steel G115 seamless pipe for 630 DEG C ultra-supercritical power station and a manufacturing method thereof, which adopts an extrusion + cold rolling pipe manufacturing process to prepare the martensitic heat-resistant steel G115 seamless pipe with excellent structure and performance, and can meet the high requirements of 630 DEG C ultra-supercritical thermal power generating unit on structure and performance.

[0008] In order to achieve the above-mentioned purpose, the technical scheme adopted by the present application is as follows:

[0009] The first aspect of the present application provides a martensitic heat-resistant steel G115 seamless pipe for 630 DEG C ultra-supercritical power station, and the chemical composition of the martensitic heat-resistant steel G115 seamless pipe is as follows: C: 0.060-0.100, Si≤0.55, Mn: 0.27-0.73, P≤0.020, S≤0.010, Cr: 8.40-9.60, W: 2.33-3.17, Co: 2.80-3.25, Cu: 0.40-1.20, V: 0.13-0.27, Nb: 0.03-0.10, N: 0.005-0.019, B: 0.008-0.022, Ni: ≤0.13, Ti≤0.02, Al≤0.015, O≤0.0040, As≤0.015, Sb≤0.015, Bi≤0.005, Sn≤0.020, Pb≤0.015, As+Sb+Bi+Sn+Pb≤0.035, and the balance is iron and inevitable impurities.

[0010] The microstructure of the martensitic heat-resistant steel G115 seamless pipe is tempered martensite, and does not contain ferrite, and the grain size is 4-7 levels.

[0011] Preferably, the performance of the martensitic heat-resistant steel G115 seamless pipe is as follows:

[0012] The room temperature tensile strength is ≥660 MPa, the yield strength is ≥480 MPa, and the elongation is ≥20%; the high temperature yield strength at 630 DEG C is ≥271 MPa, the room temperature impact energy KV2 is ≥40 J, the hardness HBW is 195-250; the flattened seamless pipe is flattened to an inner wall spacing of 2 times the wall thickness, and there is no crack on the flattened seamless pipe; the 650 DEG C extrapolated 100,000 hour stress rupture strength is >80 MPa.

[0013] The second aspect of the present application provides a manufacturing method of a martensitic heat-resistant steel G115 seamless pipe for 630 DEG C ultra-supercritical power station, which comprises the following steps:

[0014] S1, smelting and forging, smelting according to the chemical composition ratio of the supercritical ultra-supercritical power station used martensitic heat-resistant steel G115 seamless steel pipe, and then forging into a round pipe blank;

[0015] S2, hot extrusion, after preheating the round pipe blank, heating and holding in the ring furnace, blanking on the piercer, and then heating and holding in the induction furnace, and then extruding to obtain an extruded pipe, water cooling to room temperature, and then annealing and pickling;

[0016] S3, cold working, cold rolling the extruded pipe to obtain a finished pipe;

[0017] S4, heat treatment, normalizing and tempering heat treatment of the finished pipe;

[0018] S5, surface treatment, pickling the finished pipe to obtain martensitic heat-resistant steel G115 seamless steel pipe.

[0019] Preferably, in the step S1:

[0020] The forging process is to heat the smelted ingot or electroslag ingot, deform by multiple upsetting and elongation, and then shape by radial forging machine, and obtain a round pipe blank after annealing and bright turning;

[0021] In the forging process, the forging deformation ratio is ≥6;

[0022] The surface roughness Ra of the round pipe blank is ≤1.6 μm.

[0023] Preferably, the process of step S2 is as follows:

[0024] S21, preheat the round pipe blank to 700±50℃, heat to 900-930℃ in the ring furnace, and blank in the piercer after holding;

[0025] S22, heat the blanked blank to 1140-1220℃ in the induction furnace, and extrude to obtain an extruded pipe in the horizontal extruder after holding;

[0026] S23, rapidly water cooling the extruded pipe to room temperature, and then annealing and pickling.

[0027] Preferably, in the step S21, the heating and holding time of the ring furnace is 0.5-1.5 minutes per millimeter thickness of the round pipe blank.

[0028] Preferably, in the step S22:

[0029] The heating and holding time of the induction furnace is 1-3 minutes per millimeter thickness of the blank;

[0030] During the extrusion tube making process, lubricant is applied to the inner and outer walls of the billet and to the inner wall of the extrusion cylinder of the extrusion die. Then, the lubricated billet is placed into the extrusion cylinder.

[0031] During the extrusion tube manufacturing process, the extrusion ratio is 2 to 20, and the extrusion speed is 100 to 200 mm / s.

[0032] Preferably, in step S23:

[0033] The annealing temperature during the annealing process is 760-790℃, and the holding time is 30 minutes.

[0034] The pickling process uses sulfuric acid / hydrochloric acid with a percentage concentration of 5-20% and a pickling temperature of 45-70℃.

[0035] Preferably, in step S3, during the cold rolling process, the deformation amount of each pass is 30-60%. When there are more than two cold rolling passes, high-temperature tempering and pickling treatment are performed between adjacent cold rolling passes, and the high-temperature tempering temperature is 860-900℃.

[0036] Preferably, in step S4:

[0037] The normalizing heat treatment temperature is 1060–1090°C;

[0038] The tempering heat treatment temperature is 760–790°C.

[0039] Preferably, in step S5, the pickling uses sulfuric acid / hydrochloric acid with a percentage concentration of 5-20% and the pickling temperature is 45-70°C.

[0040] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0041] 1. In this invention, the steel ingots used for producing tube blanks are first subjected to rapid forging to fully deform them, with a forging ratio ≥6, to ensure that the as-cast structure is fully broken and that carbides and Laves phases are dispersed, thereby increasing the plasticity and uniformity of the material.

[0042] 2. This invention uses an extrusion method to manufacture raw tubes, which has a large deformation range and controllable temperature rise of the inner wall and tail end, avoiding inner wall defects. By controlling process parameters such as heating temperature, extrusion speed, and lubrication, a uniform and defect-free extruded raw tube is produced.

[0043] 3. This invention uses cold rolling and matching heat treatment processes to produce finished tubes to specifications. By controlling the deformation amount of each cold rolling pass and adopting a suitable heat treatment regime, the grains of the finished tubes can be controlled to be fine and uniform, thereby obtaining G115 seamless tubes with excellent microstructure and performance, which can meet the high requirements of 630℃ ultra-supercritical thermal power units for microstructure and performance.

[0044] 4. This invention employs an extrusion + cold rolling tube manufacturing process, producing G115 seamless steel pipes with uniform microstructure, grain size of 4-7, room temperature tensile strength ≥660MPa, yield strength ≥480MPa, elongation ≥20%; high-temperature yield strength at 630℃ ≥271MPa, room temperature impact absorption energy KV2 ≥40J, hardness HBW: 195-250; the flattened inner wall spacing is twice the wall thickness, and no cracks are allowed on the flattened sample; each pipe undergoes a 20MPa hydrostatic test without leakage; the 100,000-hour creep strength at 650℃ is >80MPa, meeting the requirements for use in 630℃ ultra-supercritical power plants. Detailed Implementation

[0045] The present invention will now be described in detail with reference to specific embodiments. These embodiments will help those skilled in the art to further understand the present invention, but do not limit the invention in any way.

[0046] The martensitic heat-resistant steel G115 seamless steel pipe for 630℃ ultra-supercritical power plants provided by this invention has the following chemical composition by weight percentage: C: 0.060~0.100, Si≤0.55, Mn: 0.27~0.73, P≤0.020, S≤0.010, Cr: 8.40~9.60, W: 2.33~3.17, Co: 2.80~3.25, Cu: 0.40~1.20, V: 0.13~0.27 Nb: 0.03~0.10, N: 0.005~0.019, B: 0.008~0.022, Ni: ≤0.13, Ti≤0.02, Al≤0.015, O≤0.0040, As≤0.015, Sb≤0.015, Bi≤0.005, Sn≤0.020, Pb≤0.015, As+Sb+Bi+Sn+Pb≤0.035, balance being iron and unavoidable impurity elements;

[0047] The microstructure of the aforementioned martensitic heat-resistant steel G115 seamless steel pipe is tempered martensite, which does not contain ferrite, has a uniform structure, and its grain size is 4 to 7.

[0048] The properties of the aforementioned martensitic heat-resistant steel G115 seamless steel pipe are as follows: room temperature tensile strength ≥660MPa, yield strength ≥480MPa, elongation ≥20%; high-temperature yield strength at 630℃ ≥271MPa, room temperature impact absorption energy KV2 ≥40J, hardness HBW 195~250, and extrapolated 100,000-hour creep strength at 650℃ >80MPa. The flattened inner wall spacing is twice the wall thickness, and no cracks appear on the seamless steel pipe after flattening; each pipe undergoes a 20MPa hydrostatic test without leakage.

[0049] The manufacturing method of the aforementioned seamless martensitic heat-resistant steel G115 pipe for 630℃ ultra-supercritical power plants includes the following steps:

[0050] S1, smelting and forging, smelting according to the chemical composition ratio of G115 seamless martensitic heat-resistant steel pipe for ultra-supercritical power plants at 630℃, and then forging into round tube billets.

[0051] The steel is smelted according to the chemical composition of G115 seamless martensitic heat-resistant steel for 630℃ ultra-supercritical power plants, and then forged. The smelted steel ingot / electroslag ingot is heated and subjected to multiple upsetting and drawing deformations using a high-speed forging mill. Then, it is forged into a round tube billet using a radial forging mill. After forging, the round tube billet is obtained after annealing and machining. During the forging process, the forging deformation ratio is ≥6; the surface finish Ra of the round tube billet is ≤1.6μm.

[0052] S2, hot extrusion, the round tube billet is preheated, heated and kept at a temperature in a ring furnace, formed on a piercing mill, heated and kept at a temperature in an induction furnace, and then extruded to obtain an extruded rough tube. After being cooled to room temperature by water, it is annealed and pickled.

[0053] Using a ring furnace for heating and forming the billet, followed by extrusion in an induction furnace, ensures uniform temperature throughout the billet and improves the surface quality of the extruded tube. The specific process is as follows:

[0054] S21. After preheating the round tube billet to 700±50℃, heat it to 900~930℃ in an annular furnace, hold it at that temperature, and then form the billet on a piercing mill. The heating and holding time in the annular furnace is 0.5~1.5 minutes per millimeter of round tube billet thickness.

[0055] S22, the prepared billet is heated to 1140-1220℃ in an induction furnace, and after holding at the temperature, it is extruded on a horizontal extrusion press to obtain an extruded rough tube.

[0056] The heating and holding time in the induction furnace is 1-3 minutes per millimeter of billet thickness. During extrusion tube making, lubricant is applied to the inner and outer walls of the heated billet, and lubricant is also applied to the inner wall of the extrusion cylinder of the extrusion die. The lubricated billet is then placed into the extrusion cylinder. Glass powder lubricant can be used, with a melting point of -20 to 30°C from the heating temperature (i.e., the melting point of the lubricant is 20-30°C lower than the heating temperature of the induction furnace). During the extrusion tube making process, the extrusion ratio is 2-20 to ensure the uniformity of the billet structure and the dispersed distribution of precipitated phases during the extrusion process. The extrusion speed is 100-200 mm / s to ensure the stability of the billet temperature during the extrusion process, control the appropriate temperature rise, and ensure the consistency of the structure at the beginning and end of the extruded tube.

[0057] S23, the extruded tube is rapidly water-cooled to room temperature, and then annealed and pickled.

[0058] The extruded rough tube is rapidly water-cooled to room temperature, and then annealed at 760–790°C for 30 minutes. After annealing, the rough tube is pickled to remove oxide scale and lubricant. The pickling solution is a 5–20% sulfuric acid / hydrochloric acid solution at 45–70°C. Following pickling, the tube is rinsed with high-pressure water. The rough tube can then be manually ground to obtain a defect-free surface.

[0059] S3, cold working, cold rolling of extruded rough tubes to obtain finished tubes;

[0060] G115 is a martensitic heat-resistant steel with severe work hardening and high resistance to cold deformation, requiring multiple cold rolling processes. To achieve excellent cold forming, microstructure, and dimensions of the product, while minimizing damage to the cold rolling mill, this invention requires controlling the cold rolling deformation per pass to 30-60%. When there are more than two cold rolling passes, high-temperature tempering and pickling are performed between adjacent cold rolling passes; the high-temperature tempering temperature is 860-900℃.

[0061] During pickling, the pipe is immersed in a sulfuric acid / hydrochloric acid solution at 45℃~70℃ to remove oxide scale for 20~30 minutes. Then it is rinsed with high-pressure water, and the folds, cracks and pits on the surface of the pipe are locally ground. The sulfuric acid / hydrochloric acid concentration in the solution is 5~20%.

[0062] S4, heat treatment, the finished tube is subjected to normalizing and tempering heat treatment;

[0063] The normalizing heat treatment temperature is 1060~1090℃, and the heat treatment time can be 120 minutes;

[0064] The tempering heat treatment temperature is 760–790℃, and the heat treatment time can be 180 minutes. The tempering temperature should not exceed that of G115 (A grade). C1 This ensures that tempering does not enter the two-phase region and does not produce residual austenite, achieving uniform microstructure and properties to meet the high requirements of 630℃ ultra-supercritical thermal power units for microstructure and properties.

[0065] S5, Surface treatment, pickling the finished pipe to obtain martensitic heat-resistant steel G115 seamless steel pipe.

[0066] Pickling is performed using sulfuric acid / hydrochloric acid with a percentage concentration of 5-20%, and the pickling temperature is 45-70℃.

[0067] This invention achieves uniform microstructure and meets standard performance requirements by controlling the deformation amount during tube blank forging, the heating temperature during hot extrusion, the extrusion ratio and extrusion speed, the deformation amount per pass during cold working, and the heat treatment regime.

[0068] The following section provides a further description of the 630℃ ultra-supercritical power plant martensitic heat-resistant steel G115 seamless steel pipe and its manufacturing method, using specific examples.

[0069] Example 1

[0070] This embodiment describes the preparation of a G115 seamless tube with a specification of Φ48×5.

[0071] 1) Steel ingots are forged into round tube blanks.

[0072] G115 smelted using the EAF+LF+VD method is cast into 2.3-ton steel ingots. The steel ingots are heated and forged using a 4000-ton high-speed forging machine. After three upsetting and drawing processes, the ingots are forged to an octagonal shape with a forging deformation ratio of 6.5. They are then hot-transmitted to a heating furnace for further heating and finally forged into Φ241 black-skinned tube blanks using a 1300-ton radial forging machine. After annealing, the tube blanks are machined, and the surface finish Ra of the machined round tube blanks is ≤1.6um.

[0073] 2) Extrusion of tube blanks into rough tubes

[0074] The round tube blank obtained in step 1) was extruded into a tube using a 6000-ton bedroom extrusion press according to the following extrusion process. The size of the extruded tube was Φ114×14. Then, the mixture of oxide scale and glass powder was removed in sulfuric acid solution, and the defects on the surface of the extruded tube were manually ground.

[0075] The hot extrusion process is as follows:

[0076] a) After preheating the tube blank to 700±50℃, heat it to 900~930℃ in a ring furnace, hold it for 160 minutes, and then roll it out in a 3000-ton piercing machine.

[0077] b) Heat the prepared billet in an induction furnace to 1140-1220℃ and hold for 27 minutes. Apply glass powder lubricant evenly to the inner and outer walls and apply glass powder lubricant to the inner wall of the die extrusion cylinder. Extrude the billet on a 6000-ton bedroom extruder with an extrusion ratio of 8.5 and an extrusion speed of 150mm / s.

[0078] c) The extruded rough tubes are rapidly water-cooled to room temperature;

[0079] d) Anneal the extruded rough tube at a temperature of 760-790℃ for 30 minutes.

[0080] e) Pickle the annealed extruded tubes to remove oxide scale. Soak them in a 15% sulfuric acid solution at 60°C for 50 minutes, then rinse with high-pressure water.

[0081] 3) The extruded rough tube with no surface defects obtained in step 2) is subjected to three cold rolling passes to produce a finished tube with a diameter of Φ48×5. The deformation amount of each cold rolling pass is 33-42%. The tube after each cold rolling pass is subjected to annealing heat treatment and pickling to remove oxide scale.

[0082] 4) Heat treatment

[0083] The finished tubes undergo normalizing and tempering heat treatment. The normalizing heat treatment temperature is 1060-1090℃ and the normalizing heat treatment time is 120 minutes. The tempering heat treatment temperature is 760-790℃ and the tempering heat treatment time is 180 minutes.

[0084] 5) Surface treatment of finished pipes

[0085] The finished tubes are pickled using a 15% sulfuric acid solution at a temperature of 45℃~70℃.

[0086] 6) Finished product inspection

[0087] The microstructure of the G115 finished pipe is tempered martensite, free of ferrite, with a grain size of 5.0 grade. Its room temperature tensile strength is 708 MPa, yield strength is 612 MPa, and elongation is 28%. The high-temperature yield strength at 630℃ is 282 MPa, the room temperature impact absorption energy is 2119 J (KV), and the hardness (HBW) is 223. The flattened inner wall spacing is twice the wall thickness, and the flattened sample shows no cracks, indicating 100% compliance. Each pipe undergoes a 20 MPa hydrostatic test, with 100% passing. The 100,000-hour creep rupture strength at 650℃ is 105 MPa, meeting the requirements for use in ultra-supercritical power plants at 630℃.

[0088] Example 2

[0089] This embodiment describes the preparation of a G115 seamless tube with a specification of Φ89×15.

[0090] 1) Steel ingots are forged into round tube blanks.

[0091] G115 smelted using the EAF+LF+VD method is cast into 2.3-ton steel ingots. The steel ingots are heated and forged using a 4000-ton high-speed forging machine. After three upsetting and drawing processes, the ingots are forged to an octagonal shape with a forging deformation ratio of 6.5. They are then hot-transmitted to a heating furnace for further heating and finally forged into Φ241 black-skinned tube blanks using a 1300-ton radial forging machine. After annealing, the tube blanks are machined, and the surface finish Ra of the machined round tube blanks is ≤1.6um.

[0092] 2) Extrusion of tube blanks into rough tubes

[0093] The round tube blank obtained in step 1) was extruded into a rough extruded tube using a 6000-ton bedroom extrusion press according to the following extrusion process. The size of the rough extruded tube was Φ114×23. Then, the mixture of oxide scale and glass powder was removed in sulfuric acid solution, and the defects on the surface of the rough extruded tube were manually ground.

[0094] The hot extrusion process is as follows:

[0095] a) After preheating the round tube billet to 700±50℃, heat it to 900~930℃ in a ring furnace, hold it for 160 minutes, and then roll it out in a 3000-ton piercing machine.

[0096] b) Heat the prepared billet in an induction furnace to 1140-1220℃ and hold for 27 minutes. Apply glass powder lubricant evenly to the inner and outer walls and apply glass powder lubricant to the inner wall of the die extrusion cylinder. Extrude the billet on a 6000-ton bedroom extruder with an extrusion ratio of 8.5 and an extrusion speed of 150mm / s.

[0097] c) The extruded rough tubes are rapidly water-cooled to room temperature;

[0098] d) Anneal the extruded rough tube at a temperature of 760-790℃ for 30 minutes.

[0099] e) Pickle the annealed extruded tubes to remove oxide scale. Soak them in a 15% sulfuric acid solution at 60°C for 50 minutes, then rinse with high-pressure water.

[0100] 3) The extruded rough tube with no surface defects obtained in step 2) is subjected to two cold rolling passes to produce a finished tube with a diameter of Φ89×15. The deformation amount of each cold rolling pass is 30-40%. The tube after each cold rolling pass is subjected to annealing heat treatment and pickling to remove oxide scale.

[0101] 4) Heat treatment

[0102] The finished tubes undergo normalizing and tempering heat treatment. The normalizing heat treatment temperature is 1060-1090℃ and the normalizing heat treatment time is 120 minutes. The tempering heat treatment temperature is 760-790℃ and the tempering heat treatment time is 180 minutes.

[0103] 5) Surface treatment of finished pipes

[0104] The finished tubes are pickled using a 15% sulfuric acid solution at a temperature of 45℃~70℃.

[0105] 6) Finished product inspection

[0106] The microstructure of the G115 finished pipe is tempered martensite, free of ferrite, with a grain size of 4.5. Its room temperature tensile strength is 698 MPa, yield strength is 594 MPa, and elongation is 31%. The high-temperature yield strength at 630℃ is 295 MPa, the room temperature impact absorption energy is 2134 J (KV), and the hardness (HBW) is 221. When flattened to a distance twice the wall thickness, the flattened sample shows no cracks and is 100% qualified. Each pipe undergoes a 20 MPa hydrostatic test, and all samples pass. The 100,000-hour creep rupture strength at 650℃ is 103 MPa, meeting the requirements for use in ultra-supercritical power plants at 630℃.

[0107] Example 3

[0108] This embodiment describes the preparation of a G115 seamless tube with a specification of Φ114×18.

[0109] 1) Steel ingots are forged into round tube blanks.

[0110] G115 smelted using the EAF+LF+VD method is cast into 2.3-ton steel ingots. The steel ingots are heated and forged using a 4000-ton high-speed forging machine. After three upsetting and drawing processes, the ingots are forged to an octagonal shape with a forging deformation ratio of 6.5. They are then hot-transmitted to a heating furnace for further heating and finally forged into Φ241 black-skinned tube blanks using a 1300-ton radial forging machine. After annealing, the tube blanks are machined, and the surface finish Ra of the machined round tube blanks is ≤1.6um.

[0111] 2) Extrusion of tube blanks into rough tubes

[0112] The round tube blank obtained in step 1) was extruded into a rough extruded tube using a 6000-ton bedroom extrusion press according to the following extrusion process. The rough extruded tube had a size of Φ133×24. Then, the mixture of oxide scale and glass powder was removed in sulfuric acid solution, and the defects on the surface of the rough extruded tube were manually ground.

[0113] The hot extrusion process is as follows:

[0114] k) After preheating the round tube billet to 700±50℃, heat it to 900~930℃ in a ring furnace, hold it for 160 minutes, and then roll it out in a 3000-ton piercing machine.

[0115] l) Heat the prepared billet in an induction furnace to 1140-1220℃, hold for 27 minutes, evenly coat the inner and outer walls with glass powder lubricant, and extrude it on a 6000-ton bedroom extruder with an extrusion ratio of 8.5 and an extrusion speed of 150mm / s.

[0116] m) The extruded rough tubes are rapidly water-cooled to room temperature;

[0117] n) Anneal the extruded rough tube at a temperature of 760-790℃ for 30 minutes.

[0118] o) Pickle the annealed extruded tubes to remove oxide scale, soak them in a 15% sulfuric acid solution at 60°C for 50 minutes, and then rinse them with high-pressure water.

[0119] 3) The extruded rough tube with no surface defects obtained in step 2) is cold rolled once to produce a finished tube of Φ114×18. The deformation amount of each cold rolling is 34%. The tube after each cold rolling is annealed and pickled to remove the oxide scale.

[0120] 4) Heat treatment

[0121] The finished tubes undergo normalizing and tempering heat treatment. The normalizing heat treatment temperature is 1060-1090℃ and the normalizing heat treatment time is 120 minutes. The tempering heat treatment temperature is 760-790℃ and the tempering heat treatment time is 180 minutes.

[0122] 5) Surface treatment of finished pipes

[0123] The finished tubes are pickled using a 15% sulfuric acid solution at a temperature of 45℃~70℃.

[0124] 6) Finished product inspection

[0125] The microstructure of the G115 finished pipe is tempered martensite, free of ferrite, with a grain size of 4.0. Its room temperature tensile strength is 688 MPa, yield strength is 579 MPa, and elongation is 29%. The high-temperature yield strength at 630℃ is 279 MPa, the room temperature impact absorption energy is 2120 J (KV), and the hardness is HBW220. When flattened to a distance twice the wall thickness, the flattened sample shows no cracks and is 100% qualified. Each pipe undergoes a 20 MPa hydrostatic test, and all samples pass. The 100,000-hour creep rupture strength at 650℃ is 101 MPa, meeting the requirements for use in ultra-supercritical power plants at 630℃.

[0126] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A seamless steel pipe of martensitic heat-resistant steel G115 for 630°C ultra supercritical power stations, characterized by, The chemical composition weight percentage is: C: 0.060-0.100, Si≤0.55, Mn: 0.27-0.73, P≤0.020, S≤0.010, Cr: 8.40-9.60, W: 2.33-3.17, Co: 2.80-3.25, Cu: 0.40-1.20, V: 0.13-0.27, Nb: 0.03-0.10, N: 0.005-0.019, B: 0.008-0.022, Ni: ≤0.13, Ti≤0.02, Al≤0.015, O≤0.0040, As≤0.015, Sb≤0.015, Bi≤0.005, Sn≤0.020, Pb≤0.015, As+Sb+Bi+Sn+Pb≤0.035, the balance being iron and inevitable impurities; The microstructure of the seamless steel pipe of the martensitic heat-resistant steel G115 is tempered martensite without ferrite, and the grain size is 4-7.

2. The seamless steel pipe of martensitic heat-resistant steel G 115 for 6300C ultra supercritical power plants according to claim 1, characterized in that, The performance of the seamless steel pipe of the martensitic heat-resistant steel G115 is as follows: The tensile strength at room temperature is ≥660 MPa, the yield strength is ≥480 MPa, and the elongation is ≥20%; the high-temperature yield strength at 630℃ is ≥271 MPa, the impact energy at room temperature KV2 is ≥40 J, the hardness HBW is 195-250; the flattened seamless steel pipe is flattened to the distance between the inner walls being 2 times of the wall thickness, and no cracks are found on the flattened seamless steel pipe; the 10 million hour stress-rupture strength at 650℃ is >80 MPa.

3. A method of manufacturing a seamless steel pipe of martensitic heat-resistant steel G115 for 630°C ultra supercritical power stations, characterized by, The method comprises the following steps: S1, smelting and forging, smelting according to the chemical composition ratio of the seamless steel pipe of the martensitic heat-resistant steel G115 for 630℃ ultra-supercritical power station, and then forging into a round pipe blank; S2, hot extrusion, preheating the round pipe blank, heating and keeping in a ring furnace, blanking in a piercing mill, then heating and keeping in an induction furnace, and then extruding to obtain an extruded rough pipe, and then water cooling to room temperature, annealing and pickling; S3, cold working, cold rolling the extruded rough pipe to obtain a finished pipe; S4, heat treatment, normalizing and tempering the finished pipe; S5, surface treatment, pickling the finished pipe to obtain the seamless steel pipe of the martensitic heat-resistant steel G115.

4. The method of producing a 630°C ultra supercritical power station- use martensitic heat-resistant steel pipe G 115 according to claim 3, characterized by In the step S1, The forging process is heating the smelted ingot or electroslag ingot, adopting a fast forging machine to deform by multiple upsetting and elongating, then forging into a round pipe blank by a radial forging machine, and obtaining the round pipe blank after annealing and polishing; In the forging process, the forging deformation ratio is ≥6; The surface roughness Ra of the round pipe blank is ≤1.6 μm.

5. The method of producing a 630°C ultra supercritical power station- use martensitic heat-resistant steel pipe G 115 according to claim 3, characterized by The process of the step S2 is as follows: S21, preheating the round pipe blank to 700±50℃, heating to 900-930℃ in a ring furnace, blanking in a piercing mill after keeping, heating to 1140-1220℃ in an induction furnace, and then extruding to obtain an extruded rough pipe in a horizontal extruding machine after keeping; S22, rapidly water cooling the extruded rough pipe to room temperature, and then annealing and pickling; In the step S21, the heating and keeping time of the ring furnace is 0.5-1.5 minutes per millimeter thickness of the round pipe blank.

6. The method of producing a 630°C ultra supercritical power station- use martensitic heat-resistant steel pipe G 115 according to claim 5, characterized by ​ 7. The method of producing a 630°C ultra supercritical power station- use martensitic heat-resistant steel pipe G 115 according to claim 5, characterized by, ​ The heating and holding time of the induction furnace is 1-3 minutes per millimeter thickness of the blank; During the extrusion pipe manufacturing, lubricant is applied to the inner and outer walls of the blank, and the inner wall of the extrusion cylinder of the extrusion die is also coated with lubricant, and then the blank coated with lubricant is put into the extrusion cylinder; During the extrusion pipe manufacturing, the extrusion ratio is 2-20, and the extrusion speed is 100-200 mm / s.

8. The method of producing a 630°C ultra supercritical power station- use martensitic heat-resistant steel pipe G 115 according to claim 5, characterized by, In the step S23: The annealing temperature during the annealing treatment is 760-790 DEG C, and the holding time is 30 minutes; The pickling uses sulfuric acid / hydrochloric acid with a percentage concentration of 5-20%, and the pickling temperature is 45-70 DEG C.

9. The method of producing a 630°C ultra supercritical power station- use martensitic heat-resistant steel pipe G 115 according to claim 3, characterized by, In the step S3, during the cold rolling, the deformation of each pass is 30-60%, and when the cold rolling passes are more than two, high-temperature tempering and pickling treatment are performed between the adjacent cold rolling passes, the high-temperature tempering temperature is 860-900 DEG C.

10. The method of producing a 630°C ultra supercritical power station- use martensitic heat-resistant steel pipe G 115 according to claim 3, characterized by, In the step S4: The normalizing heat treatment temperature is 1060-1090 DEG C; The tempering heat treatment temperature is 760-790 DEG C.

11. The method of producing a 630°C ultra supercritical power station- use martensitic heat-resistant steel pipe G 115 according to claim 3, characterized by, In the step S5, the pickling uses sulfuric acid / hydrochloric acid with a percentage concentration of 5-20%, and the pickling temperature is 45-70 DEG C.