A manufacturing method of a small-bore G115 martensitic heat-resistant steel thin-walled pipe with a wall thickness of less than or equal to 5 mm

Abstract

The application discloses a manufacturing method of a small-diameter G115 martensitic heat-resistant steel thin-wall pipe with a wall thickness of less than 5 mm, which comprises the following steps: 1) smelting and forging, 2) hot rolling, 3) cold rolling, 4) welding, 5) sizing, 6) heat treatment, and 7) surface treatment; the pipe manufacturing process adopts the plate hot rolling + plate cold rolling + welding + steel pipe cold rolling sizing process, under the premise that the performance meets the requirements, the production cycle of the small-diameter G115 martensitic heat-resistant steel thin-wall pipe with a wall thickness of less than 5 mm is shortened, and the production cost is reduced; the room temperature tensile strength is greater than or equal to 660 MPa, the yield strength is greater than or equal to 480 MPa, and the elongation is greater than or equal to 20%; the high-temperature yield strength at 630 DEG C is greater than or equal to 271 MPa, the room temperature impact energy KV2 is greater than or equal to 40 J, the hardness HBW is 195-250; the small-diameter thin-wall pipe is flattened to the inner wall spacing of 2 times of the wall thickness, and no cracks are generated on the small-diameter thin-wall pipe after flattening; the 10 million hour stress-rupture strength at 650 DEG C is greater than or equal to 80 MPa.

Description

Technical Field

[0001] This invention belongs to the technical field, specifically relating to a method for manufacturing a small-diameter G115 martensitic heat-resistant steel thin-walled tube with a wall thickness ≤5mm. Background Technology

[0002] G115 is a new type of martensitic heat-resistant steel, currently the only high-performance heat-resistant material in the world that can be engineered for metal wall temperatures of 630-650℃. This steel grade is significantly superior to similar products in terms of strength, oxidation resistance, and creep resistance, enabling pipe wall thickness to be reduced by one-third and weight to be halved, providing key material support for the core components of ultra-supercritical generator sets.

[0003] The demand for small-diameter G115 thin-walled tubes has surged in critical applications such as boiler heating surfaces and steam connection pipes. These tubes must meet requirements for high temperature resistance, long service life, and high reliability. Currently, multi-pass cold rolling processes are commonly used to produce small-diameter G115 tubes. For example, to produce hot-extruded 25.4×1.65mm tubes, forging is required to produce a 240mm round billet, followed by hot extrusion to produce a 114×12mm cold-rolled billet, and then five cold-drawing passes (76×7.5→57×4→32×2.5→30×1.65→25.4×1.65) to achieve the final 25.4×1.65mm diameter. It is evident that the cold-drawing process for producing small-diameter G115 tubes involves many production passes, a long production cycle, and high production costs, severely hindering the further promotion and application of G115 products.

[0004] G115 steel, as described in Chinese patent CN103045962B, is a heat-resistant steel that employs a composite strengthening principle, containing multiple strengthening elements such as W, Co, Cu, B, and N. Its high-temperature long-term aging stability, high-temperature creep strength, and oxidation resistance are significantly higher than P92, making it suitable for long-term use under high-pressure conditions below 650℃. Comparative studies have shown that G115 exhibits superior creep strength and oxidation resistance at 650℃ compared to P92 at 600℃. Currently, G115 has begun to be used in the world's first 630℃ ultra-supercritical demonstration power plant. The product meets the requirements of standards such as "CSTM 00017-2021 Seamless Martensitic Heat-Resistant Steel 08Cr9W3Co3VNbCuBN (G115) for Power Plants" and "Q / OAPD 2253-2022 New Martensitic Heat-Resistant Steel 08Cr9W3Co3VNbCuBN (G115) for Power Plants".

[0005] Chinese patent CN103045962B is an invention patent for G115, which mainly involves the composition design, strengthening concept, manufacturing method and performance characteristics of G115, but does not involve the specific manufacturing method of G115 small diameter thin-walled tube.

[0006] Chinese patent CN108998650A relates to a manufacturing method for G115 large-diameter thick-walled seamless steel pipes in 630℃ ultra-supercritical units. This technology only involves the extrusion process for G115 large-diameter thick-walled seamless steel pipes and does not cover specific manufacturing methods for G115 small-diameter thin-walled pipes. The production processes for small-diameter thin-walled pipes differ from those for large-diameter pipes. Large-diameter pipes generally have a wall thickness of not less than 20mm, produced through hot rolling or hot extrusion, achieving a wall thickness uniformity range of ±2mm. Small-diameter pipes generally have a wall thickness ≤5mm, produced through cold working, with a wall thickness uniformity typically within ±0.3mm. Furthermore, small-diameter pipes have extremely high requirements for wall thickness uniformity. If the wall thickness is uneven, it is prone to lateral bending during cold rolling, resulting in non-compliant straightness. Therefore, it is impossible to directly produce small-diameter thin-walled pipes using the same methods as large-diameter seamless pipes.

[0007] Chinese patent CN108950148A relates to a method for improving the radial microstructure and performance uniformity of G115 large-diameter thick-walled tubes. However, this technology does not involve a specific manufacturing method for G115 small-diameter thin-walled tubes.

[0008] Chinese patent CN119702759B relates to a hot rolling production process for G115 seamless steel pipes. It mainly produces large-diameter pipes through a precision forming process of "hot piercing + three-roll continuous rolling". However, this technology does not involve a specific manufacturing method for G115 small-diameter thin-walled pipes.

[0009] In summary, current research focuses primarily on developing production processes for G115 seamless steel pipes, and these studies are mainly aimed at obtaining large-diameter, thick-walled seamless steel pipes. There is relatively little research on how to improve the production process of small-diameter G115 pipes, shorten their long production cycle, and reduce production costs. Summary of the Invention

[0010] The purpose of this invention is to provide a manufacturing method for small-diameter G115 martensitic heat-resistant steel thin-walled tubes with a wall thickness ≤5mm. Under the premise of meeting performance requirements, this method shortens the production cycle and reduces production costs. The tubes exhibit the following characteristics: 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 of 195~250; flattening to an inner wall spacing of twice the wall thickness; no cracks on the small-diameter thin-walled tube after flattening; and an extrapolated 100,000-hour creep strength at 650℃ ≥80MPa.

[0011] To achieve the above objectives, the technical solution of the present invention is as follows: A method for manufacturing a small-diameter G115 martensitic heat-resistant steel thin-walled tube with a wall thickness ≤ 5mm includes the following steps: 1) Smelting and forging The steel is smelted according to the chemical composition ratio of G115 martensitic heat-resistant steel and then forged into a flat billet. 2) Hot-rolled The flat billet is heated and held at a certain temperature before being hot rolled. After hot rolling, it is cooled to room temperature by water, followed by annealing and pickling to obtain a hot-rolled billet. The heating and holding temperature is 1240~1280℃, the heating and holding time is 1~4h, the annealing temperature is 760~790℃, the annealing and holding time is 30~90min, and the hot-rolled billet thickness is 3~8mm. 3) Cold rolling Hot-rolled billets are cold-rolled to obtain cold-rolled billets with a wall thickness of ≤5mm. After annealing and pickling, they are slit to obtain cold-rolled strips, with a deformation of ≤45% per pass. 4) Welding Cold-rolled strip is welded into small-diameter thin-walled tubes with an outer diameter ≤50mm and a wall thickness ≤5mm. 5) Sizing After welding, the small-diameter thin-walled tube blank is annealed and pickled, and then cold rolled in one pass to produce the finished product specifications. 6) Heat treatment Perform normalizing and tempering heat treatment; The normalizing heat treatment temperature is 1110~1140℃, and the normalizing heat treatment time is 4~8 minutes; the tempering heat treatment temperature is 760~790℃, and the tempering heat treatment time is 30~90 minutes. 7) Surface treatment The finished tube is obtained after pickling.

[0012] Preferably, in step 1), the forging process involves heating the smelted steel ingot or electroslag ingot, directly drawing it to the target specification using a high-speed forging machine, and obtaining a flat billet after annealing, grinding, milling, or planing.

[0013] Preferably, in step 1), the surface finish Ra of the blank is ≤6.4μm.

[0014] Preferably, in step 2), the pickling uses sulfuric acid / hydrochloric acid with a percentage concentration of 5-20% and the pickling temperature is 45-70℃.

[0015] Preferably, in step 3), the cold rolling passes are one or more. When there are ≥ 2 cold rolling passes, annealing and pickling are performed between adjacent cold rolling passes. The annealing temperature is 760~790℃ and the annealing time is 30~90 minutes.

[0016] Preferably, in step 5), the annealing temperature is 760~790℃ and the annealing time is 30~90min.

[0017] Preferably, in step 5), the pickling is performed using sulfuric acid / hydrochloric acid with a percentage concentration of 5-20%, the pickling temperature is 45-70℃, and the pickling is followed by rinsing with high-pressure water.

[0018] Preferably, in step 7), the pickling uses sulfuric acid / hydrochloric acid with a percentage concentration of 5-20% and the pickling temperature is 45-70℃.

[0019] The microstructure of the G115 martensitic heat-resistant steel thin-walled tube obtained by this invention is tempered martensite with a grain size ≥ 4.

[0020] The G115 martensitic heat-resistant steel thin-walled tube obtained by this invention has a room temperature tensile strength ≥660MPa, yield strength ≥480MPa, and elongation ≥20%; a high temperature yield strength at 630℃ ≥271MPa, a room temperature impact absorption energy KV2 ≥40J, and a hardness HBW of 195~250; when flattened to a distance of twice the wall thickness, there are no cracks on the small-diameter thin-walled tube after flattening; the extrapolated 100,000-hour creep strength at 650℃ ≥80MPa; the wall thickness tolerance of the thin-walled tube is ≤8%, and the straightness is ≤1.5mm / m.

[0021] The chemical composition (weight percentage) of the G115 martensitic heat-resistant steel described in this invention 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. 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 is Fe and unavoidable impurity elements.

[0022] In the manufacturing method of the small-diameter G115 martensitic heat-resistant steel thin-walled tube with a wall thickness ≤ 5mm described in this invention: The flat billet obtained by this invention is hot-rolled after being held at a relatively high temperature. The heating and holding temperature is controlled at 1240~1280℃ and the heating and holding time is controlled at 1~4h, while the general heating and holding temperature is 1140~1180℃. This invention raises the heating temperature of G115 steel to 1240~1280℃ through thermal simulation experiments. The material has high high-temperature deformation resistance, with an equivalent deformation of about 3.46, which can achieve large deformation in a single pass, ensure that the as-cast structure is fully broken, and that the precipitates such as carbides and Laves phase are dispersed and the grain size is fine.

[0023] After hot rolling, G115 hot-rolled coils contain deformation stress, which needs to be relieved by annealing to prevent cracking. Since G115 steel will subsequently undergo cold rolling, annealing is necessary to reduce the strength and hardness of the G115 hot-rolled coils, while improving their plasticity and toughness, thereby enhancing their cold workability.

[0024] Because G115 steel is a martensitic steel, it is prone to work hardening and stress concentration during cold working, resulting in a lower elongation than conventional austenitic stainless steel. Therefore, its cold rolling deformation should be ≤45% to ensure that cracking does not occur during cold rolling. This invention sets the annealing temperature after cold rolling of G115 to 760~790℃, and the holding time to 30~90 minutes. If the annealing temperature is lower than 760℃ or the holding time is lower than 30 minutes, the G115 hot-rolled coil will not soften sufficiently, and cracking of the steel strip is likely to occur when the subsequent cold rolling deformation approaches 45%. If the annealing temperature exceeds 790℃, the martensitic transformation temperature of G115 is approximately 800℃, and conventional heating furnaces have a temperature error of about ±10℃. Therefore, the actual heating temperature of the hot-rolled coil may exceed 800℃, causing G115 to transform into austenite. After annealing and cooling, G115 transforms from austenite to martensite, increasing its strength and hardness while decreasing its plasticity and toughness, thus deteriorating its cold workability. Holding time exceeding 90 minutes has no significant impact on the properties of G115 steel and is beneficial for improving its cold workability. However, extending the time will increase production costs and reduce the yield of G115. Therefore, a holding temperature limit of 90 minutes is recommended.

[0025] Furthermore, when there are ≥2 cold rolling passes, annealing is performed between adjacent cold rolling passes at a temperature of 760~790℃ for 30~90 minutes. This is mainly because when there is a large difference between the thickness of the cold rolled billet and the thickness of the final steel strip, it is impossible to complete the process in one cold rolling pass. Intermediate annealing is required to soften the material and prevent cold rolling cracking.

[0026] Cold-rolled strip is welded to form small-diameter thin-walled tubes with an outer diameter ≤50mm and a wall thickness ≤5mm. Due to thermal stress and martensitic transformation stress in the weld and surrounding heat-affected zone after welding, the material's plasticity and toughness are reduced, necessitating stress relief through annealing. Similarly, the annealing temperature after welding is controlled at 760~790℃, and the holding time is controlled at 30~90 minutes to eliminate welding stress and prevent cracking during subsequent sizing processes.

[0027] The obtained small-diameter thin-walled tubes are produced to the finished product specifications through a single cold rolling sizing process. Subsequently, heat treatment is combined to control the grain size of the finished tubes to be fine and uniform, thereby obtaining G115 small-diameter thin-walled tubes with excellent microstructure and performance, which can meet the high requirements of 630℃ ultra-supercritical thermal power units for microstructure and performance.

[0028] The heat treatment adopted is a quenching and tempering heat treatment consisting of normalizing and tempering.

[0029] The normalizing heat treatment process, combined with the dissolution characteristics of precipitates in G115 steel, ensures the full transformation of the G115 matrix structure into austenitic grains. This process also allows for the complete dissolution of carbides precipitated during heat treatment, while minimizing grain size growth. Based on this design concept, the normalizing heat treatment temperature and holding time were determined. The conventional normalizing temperature for G115 is 1050~1120℃. This invention utilizes a continuous heat treatment furnace, resulting in a shorter holding time. Considering the temperature difference between the furnace and the steel, the normalizing temperature was increased, controlled between 1100~1140℃. If the heating temperature is below 1100℃, the superheat for dissolution of chromium carbides is insufficient, requiring a longer holding time and resulting in poor dissolution. If the heating temperature is above 1140℃, a large amount of molybdenum carbides in the G115 steel dissolves, significantly reducing the number of carbides pinning grain boundaries, making grain aggregation and growth easier. This invention specifies the holding time. If the holding time is insufficient, the dissolution of G115 steel at 1110℃ will be incomplete. If the holding time is too long, the temperature of G115 steel at 1140℃ will be too high, causing a large amount of molybdenum carbides pinning the grain boundaries to dissolve and the grains to grow rapidly. Therefore, the normalizing holding time of this invention is 4 to 8 minutes.

[0030] Tempering heat treatment, taking into account the precipitation patterns of carbides and precipitates in G115 steel, significantly improves the mechanical properties of G115 steel through precipitation strengthening by secondary precipitation of dispersed fine carbides and precipitates from the matrix. Based on this design concept, the tempering heat treatment temperature and holding time are determined. If the tempering temperature is below 760℃, the carbide precipitation rate decreases, and incomplete precipitation of precipitates may occur. If the tempering temperature is above 790℃, a certain amount of austenite will form in G115 steel, which will transform into martensite during subsequent cooling, increasing the cracking risk of G115. Therefore, this invention sets the tempering temperature to 760~790℃. If the tempering time is too low, at 760℃, the precipitation of carbides and precipitates will be incomplete, failing to fully improve the mechanical properties of G115 steel. If the tempering time is too long, small-sized carbides and precipitates will dissolve and large-sized carbides and precipitates will grow during the heat preservation process of G115 steel at 790℃. As the carbides and precipitates continue to grow, the interface relationship between the carbides, precipitates and the matrix will gradually change from a coherent interface to a semi-coherent interface and an incoherent interface, reducing the precipitation strengthening effect of carbides and precipitates. Therefore, the present invention sets the tempering heat preservation time to 30~90 minutes.

[0031] This invention employs a pipe-making process of hot rolling of sheet metal, cold rolling of sheet metal, welding, and cold rolling and sizing of steel pipe to obtain small-diameter G115 martensitic heat-resistant steel thin-walled pipes with an outer diameter ≤50mm and a wall thickness ≤5mm.

[0032] Because welded pipes are manufactured through welding, the weld creates a heat-affected zone with a relatively coarse microstructure and poor bonding. This invention achieves seamless welded pipes through a combination of welding, cold rolling, and heat treatment, utilizing deformation and recrystallization. Simultaneously, it improves the mechanical properties of the weld and surrounding areas, giving them properties similar to cold-drawn pipes, thus meeting application requirements. The invention also produces uniformly thickened pipe blanks, enabling the production of G115 cold-rolled pipes with a wall thickness tolerance ≤8% and a straightness ≤1.5mm / m.

[0033] The microstructure of the G115 martensitic heat-resistant steel thin-walled tube is tempered martensite, free of ferrite, with a grain size ≥4, wall thickness tolerance ≤8%, and straightness ≤1.5mm / m. It exhibits a room temperature tensile strength ≥660MPa, yield strength ≥480MPa, and elongation ≥20%; a high-temperature yield strength at 630℃ ≥271MPa; room temperature impact absorption energy KV2 ≥40J; and a hardness HBW of 195~250. When flattened to a distance twice the wall thickness, no cracks appear on the small-diameter thin-walled tube. The extrapolated 100,000-hour creep strength at 650℃ ≥80MPa.

[0034] Compared with the prior art, the beneficial effects of the present invention are as follows: In the existing technology, G115 small-diameter pipes with an outer diameter ≤50mm and a wall thickness ≤5mm are produced by "multi-pass cold drawing" or "hot extrusion + multi-pass cold drawing", which involves many production passes, long production cycle and high production cost.

[0035] This invention proposes a tube-making process employing hot rolling of sheet metal, cold rolling of sheet metal, welding, and cold rolling sizing of the steel pipe. High hot rolling temperatures are used to achieve large deformation in a single pass, ensuring thorough fragmentation of the as-cast microstructure and a dispersed distribution of carbides and Laves phase precipitates with fine grain size. Annealing after hot rolling eliminates internal stress, improves the material's plasticity and toughness, and prevents cracking during subsequent cold rolling. Cold rolling to the target thickness yields steel strips with high surface finish and small thickness tolerances, laying the foundation for subsequent welding into pipes and ensuring the final finished pipe's wall thickness tolerance (≤8%) and straightness (≤1.5 mm / m). The cold rolling process introduces a large amount of dislocation and distortion energy, providing a basis for obtaining a fine tempered martensite microstructure.

[0036] The cold-rolled strip undergoes a process of welding, cold rolling of steel pipe, and heat treatment to eliminate welding stress, refine grains, and optimize precipitates, ultimately obtaining a fully tempered martensitic structure. This avoids the adverse effects of welding and enables the G115 thin-walled tube to meet the performance requirements (especially the 650℃ creep strength) of core components for ultra-supercritical generator sets above 630℃.

[0037] If conventional hot extrusion + cold drawing is used to produce G115 seamless small-diameter pipes, hot-rolled rough tubes are required first. The length of hot-rolled rough tubes is usually 6-12 meters, and is limited by the capacity of the extruder. To cold draw the large-diameter rough tubes to a small diameter, 3-6 cold drawing passes are usually required, with a production speed of about 0.5-3 meters per minute. The thicker the wall, the lower the production speed. Annealing (softening) must be performed after each cold drawing pass, resulting in low production efficiency.

[0038] The welded pipe process described in this invention can obtain cold-rolled billets through a single hot rolling process. The billet thickness can be controlled to ≤8mm, resulting in low resistance to deformation during subsequent cold rolling and high production speed. Simultaneously, multiple hot-rolled coils can be welded into a single hot-rolled coil, with a single hot-rolled billet weighing 10-20 tons, significantly improving the continuity of cold-rolled steel strip production. Cold rolling provides precise wall thickness and width, followed by welding to produce thin-walled tubes. Cold drawing is only responsible for diameter reduction and weld strengthening. This process, compared to extrusion + cold drawing, shortens the production cycle, reduces production costs, and better controls wall thickness uniformity, resulting in higher dimensional accuracy and better surface quality. It is beneficial for producing thin-walled tubes with uniform wall thickness and high straightness requirements. Detailed Implementation

[0039] The present invention will be further described below with reference to the embodiments.

[0040] Example 1 A method for manufacturing a small-diameter G115 martensitic heat-resistant steel thin-walled tube with specifications of Φ48×5mm (wall thickness) includes the following steps: 1) Smelting and forging According to the chemical composition ratio of G115 martensitic heat-resistant steel in Table 1, G115 smelted by EAF+LF+VD method is cast into 2.3-ton steel ingots. The steel ingots are heated and drawn into flat billets of 100mm×310mm by forging machine. The surface finish of the milled flat billets is Ra≤6.4um. 2) Hot-rolled After the flat billet is heated and kept at a certain temperature, it is hot rolled in one pass. After hot rolling, it is cooled to room temperature by water, followed by annealing and pickling to obtain a hot-rolled coil with a specification of 8mm×315mm. The heating and holding temperature is 1280℃, the heating and holding time is 4h, the annealing temperature is 780℃, and the annealing and holding time is 60min. After annealing, the sample is pickled with a 15% sulfuric acid solution at a temperature of 60°C for 45 minutes, followed by high-pressure water rinsing. 3) Cold rolling The hot-rolled billet is cold-rolled once to obtain a cold-rolled sheet with a wall thickness of 6mm. Then it is annealed at a temperature of 790℃ for 60 minutes. After annealing, it is pickled with a 15% sulfuric acid solution at a temperature of 60℃ for 45 minutes. After pickling, it is rinsed with high-pressure water. After rinsing, it is polished and slit to obtain a 6×150.8mm cold-rolled steel strip. 4) Welding Cold-rolled steel strips of 6×150.8mm are welded together by argon arc welding to form a cold-rolled tube blank of Φ47×6mm. 5) Sizing After welding, the small-diameter thin-walled tube blank is annealed and pickled, and then cold rolled in one pass to produce the finished product specifications. The annealing temperature is 790℃ and the annealing time is 60 minutes. Pickling is done with a 15% sulfuric acid solution at a temperature of 60℃ for 45 minutes. The oxide scale is removed by rinsing with high-pressure water. 6) Heat treatment Perform normalizing and tempering heat treatment; The normalizing heat treatment temperature is 1140℃ and the normalizing heat treatment time is 8 minutes; the tempering heat treatment temperature is 790℃ and the tempering heat treatment time is 60 minutes. 7) Surface treatment Pickling was performed using a 15% sulfuric acid solution at a temperature of 60°C for 45 minutes.

[0041] The microstructure of the G115 finished pipe obtained in Example 1 of this invention is tempered martensite, without ferrite, with a grain size of 7.0 grade, a room temperature tensile strength of 739 MPa, a yield strength of 626 MPa, and an elongation of 25%; a high-temperature yield strength of 294 MPa at 630℃, a room temperature impact absorption energy KV2 of 114 J, and a hardness HBW of 234; the flattened inner wall spacing is twice the wall thickness, and there are no cracks on the flattened sample, indicating 100% qualification; each pipe is subjected to a 20 MPa hydrostatic test, and 100% of them pass; the extrapolated 100,000-hour creep strength at 650℃ is 112 MPa, the wall thickness tolerance is ≤8%, and the straightness is ≤1.5 mm / m.

[0042] Example 2 A method for manufacturing a small-diameter G115 martensitic heat-resistant steel thin-walled tube with specifications of Φ30×4mm (wall thickness) includes the following steps: 1) Smelting and forging According to the chemical composition ratio of G115 martensitic heat-resistant steel in Table 1, G115 smelted by EAF+LF+VD method is cast into 2.3-ton steel ingots. The steel ingots are heated and drawn into flat billets of 60mm×310mm by forging machine. The surface finish of the milled flat billets is Ra≤6.4um. 2) Hot-rolled After the flat billet is heated and kept at a certain temperature, it is hot rolled in one pass. After hot rolling, it is cooled to room temperature by water, followed by annealing and pickling to obtain a hot-rolled coil with a specification of 6mm×315mm. The heating and holding temperature is 1270℃, the heating and holding time is 2h, the annealing temperature is 790℃, and the annealing and holding time is 30min. After annealing, the sample is pickled with a 5% sulfuric acid solution at a temperature of 70°C for 35 minutes, followed by high-pressure water rinsing. 3) Cold rolling The hot-rolled billet is cold-rolled once to obtain a cold-rolled sheet with a wall thickness of 5mm. Then it is annealed at a temperature of 760℃ for 90 minutes. After annealing, it is pickled with a 10% sulfuric acid solution at a temperature of 45℃ for 70 minutes. After pickling, it is rinsed with high-pressure water. After rinsing, it is polished and slit to obtain a 5×94.2mm cold-rolled steel strip. 4) Welding 5×94.2mm cold-rolled steel strips were welded by argon arc welding to form Φ39×5mm cold-rolled tube blanks; after welding, the annealing temperature was 760℃ and the annealing holding time was 40 minutes. After annealing, 10% sulfuric acid solution was used for pickling at 70℃ for 35 minutes. After pickling, the oxide scale was removed by rinsing with high pressure water. 5) Sizing After welding, the small-diameter thin-walled tube blank is annealed and pickled, and then cold rolled in one pass to produce the finished product specifications. The annealing temperature is 760℃ and the annealing time is 40 minutes. Pickling is done with a 15% sulfuric acid solution at a temperature of 70℃ for 35 minutes. The oxide scale is removed by rinsing with high-pressure water. 6) Heat treatment Perform normalizing and tempering heat treatment; The normalizing heat treatment temperature is 1140℃ and the normalizing heat treatment time is 4 minutes; the tempering heat treatment temperature is 770℃ and the tempering heat treatment time is 70 minutes. 7) Surface treatment Pickling was performed using a 10% sulfuric acid solution at a temperature of 70°C for 35 minutes.

[0043] The microstructure of the G115 finished pipe obtained in Example 2 is tempered martensite, without ferrite, with a grain size of 5.5. Its room temperature tensile strength is 702 MPa, yield strength is 611 MPa, and elongation is 27%. The high-temperature yield strength at 630℃ is 283 MPa, the room temperature impact absorption energy (KV2) is 131 J, and the hardness (HBW) is 219. The flattened inner wall spacing is twice the wall thickness, and no cracks were found on the flattened sample, indicating 100% compliance. Each pipe underwent a 20 MPa hydrostatic test, with 100% compliance. The extrapolated 100,000-hour creep rupture strength at 650℃ is 104 MPa, with a wall thickness tolerance of ≤8% and a straightness of ≤1.5 mm / m.

[0044] Example 3 A method for manufacturing a small-diameter G115 martensitic heat-resistant steel thin-walled tube with specifications of Φ12×3mm (wall thickness) includes the following steps: 1) Smelting and forging According to the chemical composition ratio of G115 martensitic heat-resistant steel in Table 1, G115 smelted by EAF+LF+VD method is cast into 2.3-ton steel ingots. The steel ingots are heated and drawn into flat billets of 40mm×310mm by forging machine. The surface finish of the milled flat billets is Ra≤6.4um. 2) Hot-rolled After the flat billet is heated and kept at a certain temperature, it is hot rolled in one pass. After hot rolling, it is cooled to room temperature by water, followed by annealing and pickling to obtain a hot-rolled coil with a specification of 7mm×315mm. The heating and holding temperature is 1240℃, the heating and holding time is 1 hour, the annealing temperature is 770℃, and the annealing and holding time is 90 minutes. After annealing, the sample is pickled with a 10% sulfuric acid solution at a temperature of 45°C for 70 minutes, followed by high-pressure water rinsing. 3) Cold rolling The hot-rolled billet is cold-rolled once to obtain a cold-rolled sheet with a wall thickness of 4mm. Then it is annealed at 780℃ for 30 minutes. After annealing, it is pickled with a 10% sulfuric acid solution at 70℃ for 35 minutes. After pickling, it is rinsed with high-pressure water. After rinsing, it is polished and slit to obtain a 4×37.7mm cold-rolled steel strip. 4) Welding Cold-rolled steel strips of 4×37.7mm were welded by argon arc welding to form a cold-rolled tube blank of Φ15×4mm; after welding, the annealing temperature was 770℃ and the annealing holding time was 80 minutes. After annealing, a 10% sulfuric acid solution was used for pickling at a temperature of 40℃ for 70 minutes. After pickling, the oxide scale was removed by rinsing with high-pressure water. 5) Sizing After welding, the small-diameter thin-walled tube blank is annealed and pickled, and then cold rolled in one pass to produce the finished product specifications. The annealing temperature is 770℃ and the annealing time is 80 minutes. Pickling is done with a 15% sulfuric acid solution at a temperature of 40℃ for 70 minutes. The oxide scale is removed by rinsing with high-pressure water. 6) Heat treatment Perform normalizing and tempering heat treatment; The normalizing heat treatment temperature is 1120℃ and the normalizing heat treatment time is 6 minutes; the tempering heat treatment temperature is 760℃ and the tempering heat treatment time is 80 minutes. 7) Surface treatment Pickling was performed using a 15% sulfuric acid solution at a temperature of 40°C for 70 minutes.

[0045] Example 3: The microstructure of the G115 finished pipe is tempered martensite, without ferrite, with a grain size of grade 5. Its room temperature tensile strength is 694 MPa, yield strength is 603 MPa, and elongation is 26%. The high-temperature yield strength at 630℃ is 271 MPa, the room temperature impact absorption energy (KV2) is 137 J, and the hardness (HBW) is 220. The flattened inner wall spacing is twice the wall thickness, and the flattened sample shows no cracks, indicating 100% pass rate. Each pipe undergoes a 20 MPa hydrostatic test, with 100% pass rate. The 100,000-hour creep rupture strength at 650℃ is 101 MPa, with a wall thickness tolerance ≤8% and straightness ≤1.5 mm / m.

[0046] 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.

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[0050]

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Claims

1. A method for manufacturing a small-diameter G115 martensitic heat-resistant steel thin-walled tube with a wall thickness ≤ 5 mm, characterized in that, Includes the following steps: 1) Smelting and forging The steel is smelted according to the chemical composition ratio of G115 martensitic heat-resistant steel and then forged into a flat billet. 2) Hot-rolled The flat billet is heated and held at a certain temperature before being hot rolled. After hot rolling, it is cooled to room temperature by water, followed by annealing and pickling to obtain a hot-rolled billet. The heating and holding temperature is 1240~1280℃, the heating and holding time is 1~4h, the annealing temperature is 760~790℃, the annealing and holding time is 30~90min, and the hot-rolled billet thickness is 3~8mm. 3) Cold rolling Hot-rolled billets are cold-rolled to obtain cold-rolled billets with a wall thickness of ≤5mm. After annealing and pickling, they are slit to obtain cold-rolled strips. The deformation amount of each cold rolling pass is ≤45%. 4) Welding Cold-rolled strip is welded into small-diameter thin-walled tubes with an outer diameter ≤50mm and a wall thickness ≤5mm. 5) Sizing After welding, the small-diameter thin-walled tube blank is annealed and pickled, and then cold rolled in one pass to produce the finished product specifications. 6) Heat treatment The normalizing heat treatment temperature is 1110~1140℃, and the normalizing heat treatment time is 4~8 minutes; The tempering heat treatment temperature is 760~790℃, and the tempering heat treatment time is 30~90 minutes; 7) Surface treatment The finished tube is obtained after pickling.

2. The manufacturing method of the small-diameter G115 martensitic heat-resistant steel thin-walled tube with a wall thickness ≤ 5mm as described in claim 1, characterized in that, In step 1), the forging process involves heating the smelted steel ingot or electroslag ingot, drawing it directly to the target size using a high-speed forging machine, and obtaining a flat billet after annealing, grinding, milling, or planing.

3. The manufacturing method of the small-diameter G115 martensitic heat-resistant steel thin-walled tube with a wall thickness ≤ 5mm as described in claim 1, characterized in that, In step 1), the surface finish Ra of the blank is ≤ 6.4 μm.

4. The manufacturing method of the small-diameter G115 martensitic heat-resistant steel thin-walled tube with a wall thickness ≤ 5mm as described in claim 1, characterized in that, In step 2), the pickling uses sulfuric acid / hydrochloric acid with a percentage concentration of 5-20% and the pickling temperature is 45-70℃.

5. The manufacturing method of the small-diameter G115 martensitic heat-resistant steel thin-walled tube with a wall thickness ≤ 5mm as described in claim 1, characterized in that, In step 3), the cold rolling passes are one or more. When there are ≥ 2 cold rolling passes, annealing and pickling are performed between adjacent cold rolling passes. The annealing temperature is 760~790℃ and the annealing time is 30~90 minutes.

6. The method for manufacturing a small-diameter G115 martensitic heat-resistant steel thin-walled tube with a wall thickness ≤ 5mm as described in claim 1, characterized in that, In step 5), the annealing temperature is 760~790℃ and the annealing time is 30~90min.

7. The method for manufacturing a small-diameter G115 martensitic heat-resistant steel thin-walled tube with a wall thickness ≤ 5mm as described in claim 1, characterized in that, In step 5), the pickling uses sulfuric acid / hydrochloric acid with a percentage concentration of 5-20% and a pickling temperature of 45-70℃. After pickling, the solution is rinsed with high-pressure water.

8. The method for manufacturing a small-diameter G115 martensitic heat-resistant steel thin-walled tube with a wall thickness ≤ 5mm as described in claim 1, characterized in that, In step 7), the pickling uses sulfuric acid / hydrochloric acid with a percentage concentration of 5-20% and the pickling temperature is 45-70℃.

9. The method for manufacturing a small-diameter G115 martensitic heat-resistant steel thin-walled tube with a wall thickness ≤ 5 mm as described in any one of claims 1 to 8, characterized in that, The obtained G115 martensitic heat-resistant steel thin-walled tube has a tempered martensite microstructure with a grain size ≥ 4.

10. The method for manufacturing a small-diameter G115 martensitic heat-resistant steel thin-walled tube with a wall thickness ≤ 5 mm as described in any one of claims 1 to 9, characterized in that, The obtained G115 martensitic heat-resistant steel thin-walled tube has a room temperature tensile strength ≥660MPa, yield strength ≥480MPa, and elongation ≥20%; a high-temperature yield strength at 630℃ ≥271MPa, a room temperature impact absorption energy KV2 ≥40J, and a hardness HBW of 195~250; the flattened inner wall spacing is twice the wall thickness, and there are no cracks on the small-diameter thin-walled tube after flattening; the extrapolated 100,000-hour creep strength at 650℃ ≥80MPa; the wall thickness tolerance of the thin-walled tube is ≤8%, and the straightness is ≤1.5mm / m.

11. The method for manufacturing a small-diameter G115 martensitic heat-resistant steel thin-walled tube with a wall thickness ≤ 5 mm as described in any one of claims 1 to 10, characterized in that, The chemical composition (by weight percentage) of the G115 martensitic heat-resistant steel 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: 0.03–0.1%. 0%, 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 is Fe and unavoidable impurity elements.

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