Hot Rolling Production Process of G115 Seamless Steel Pipe

Abstract

The present invention relates to the technical field of seamless steel pipe production, and discloses a hot rolling production process for G115 seamless steel pipe, comprising the following steps: preparing a G115 steel hollow billet with a diameter smaller than that of a finished pipe; hot rolling, piercing and rolling the hollow billet to obtain a rough pipe; sizing the rough pipe to obtain a rough pipe; heat treating the rough pipe to form a semi-finished pipe; grinding and machining the semi-finished pipe to obtain a finished pipe. By adopting a production process of hollow billets with a diameter smaller than that of finished pipes + hot rolling, the production batch is flexible, the specification applicability is strong, and it is suitable for industrialized production of factories with complex specifications, the production efficiency and yield rate are significantly improved, the product surface quality is good, the metallographic structure and the longitudinal and transverse mechanical properties are uniform, the process is controllable, the quality is stable, the yield rate is high, and the economic benefits are significant.

Description

Technical Field

[0001] The present invention relates to the technical field of seamless steel pipe production, and particularly to a hot rolling production process for G115 seamless steel pipes. Background Art

[0002] G115 steel is a 650°C martensitic heat-resistant steel developed by the Central Iron and Steel Research Institute with independent intellectual property rights. Under the guidance of the "multi-element composite strengthening" theory, this steel adopts the "selective strengthening and toughening" design concept, combines the research basis of MARBN (9W3Co3BN) steel, and further improves the strength of the steel by adding precipitation-strengthening element Cu. At the same time, by making full use of the "principles of hot-strength steel grain boundary engineering", it is developed through the matching and precise control of elements such as B, N, and Al. This steel has excellent tissue stability performance in the temperature range of 620 - 650°C. At 650°C, its creep strength is 1.5 times that of P92 steel, and its high-temperature steam oxidation resistance and weldability are comparable to those of P92 steel. It can be applied to thick-walled components such as large-diameter pipes and headers in the temperature range of 620 - 650°C, and has broad market prospects.

[0003] In the Chinese patent document with the publication number CN 108998650 A and the invention title "Manufacturing Method of G115 Large-Diameter Thick-Walled Seamless Steel Pipe for 630°C Ultra-Supercritical Unit", a manufacturing method of G115 large-diameter thick-walled seamless steel pipe for 630°C ultra-supercritical unit is disclosed, including the following steps: 1. Preparation of raw materials (forged billets / ingots); 2. Closed upsetting + backward extrusion punching. Heat the forged billets (ingots) to 1140 ± 10°C and soak. Remove the surface oxide scale, spray glass lubricant, and then perform closed upsetting on the billet press. The upsetting ratio ≥ 1.3, and backward extrusion punching. The piercing ratio ≥ 1.1diw to make a hollow billet; 3. Extrusion forming. Heat the hollow billet to 1240 ± 10°C, remove the surface oxide scale, return to the furnace for heating for 30 minutes, take out and spray glass lubricant, and then extrude the hollow billet into a tube blank on the extruder. The extrusion speed is 15 - 30 mm / s, and the extrusion ratio ≥ 6; 4. After extrusion, perform rapid cooling + annealing heat treatment; 5. Performance heat treatment.

[0004] It can be seen from the above-mentioned prior art that in the manufacture of G115 large-diameter seamless steel pipes, after preparing hollow billets larger than the finished product specifications, the hot extrusion compression molding process is used to make the tube blanks. If the heating temperature is lower than 1150°C, the plastic deformation is poor, the deformation is uneven, and the mold is easily damaged. The hot extrusion process is generally divided into a filling extrusion stage, a breakthrough extrusion stage, and a stable extrusion stage. In the filling extrusion stage, after the initial contact between the hollow billet and the extrusion die, it will cause local uneven deformation. As the extrusion continues, the deformation of the deformation zone increases, and the uneven deformation intensifies. Then, the outer gap and the inner gap are filled respectively, and the entire extrusion die can be completely filled; then the extrusion continues, and the steel pipe is squeezed out of the die, that is, the breakthrough extrusion stage is reached. At this time, the extrusion force is the largest. Due to the uneven deformation in the filling extrusion stage and the large extrusion force in the breakthrough extrusion stage, the follower core rod is very easy to be eccentric, so that the steel pipe has serious uneven wall thickness when it is extruded out of the die; if the temperature and friction are not properly controlled in the stable extrusion stage, the uneven wall thickness will be aggravated. Therefore, after obtaining a large diameter tube billet through hot extrusion, it is necessary to machine the tube billet using turning and boring processes, resulting in a material loss of about 10%, eliminating the uneven wall thickness defect, and making the tube diameter and wall thickness meet the required specifications of the finished steel pipe. This process can only use tube billets with a larger diameter than the finished tube. If the finished diameter reaches more than 900mm, it is difficult to batch the tube billets, and the load stress on the equipment is more severe. The yield rate of G115 steel pipes produced by this process method is ≤50%, the machining workload is large, and the material loss is about 20%~50%, resulting in a high waste of raw materials and low economic benefits. Summary of the invention

[0005] The purpose of the present invention is to provide a hot rolling production process for G115 seamless steel pipe, break through the existing production process limitations of G115 seamless steel pipe, improve the yield rate of G115 steel pipe, and improve its economic benefits.

[0006] In order to solve the above technical problems, the present invention provides a technical solution as follows:

[0007] A hot rolling production process for a G115 seamless steel pipe comprises the following steps: preparing a G115 steel hollow billet with a diameter smaller than that of a finished pipe; hot rolling and piercing the hollow billet to obtain a rough pipe, wherein the first hot rolling and piercing temperature is controlled at 1000° C.-1140° C.; sizing the rough pipe to obtain a rough pipe; heat treating the rough pipe to form a semi-finished pipe; and grinding and machining the semi-finished pipe to obtain a finished pipe.

[0008] Furthermore, the method for manufacturing a G115 steel hollow billet with a diameter smaller than the finished pipe includes: smelting raw materials according to the G115 steel composition to obtain a steel billet, and manufacturing the G115 steel hollow billet by any one of the following methods: hot extruding the steel billet into a hollow billet, forging the steel billet after continuous casting and then machining to drill holes to form a hollow billet, or machining to drill holes to form a hollow billet after electroslag remelting of the continuous casting electrode. The material loss during hole drilling is ≤5%.

[0009] Furthermore, obtaining a rough tube by hot rolling and piercing the hollow billet includes heating the hollow billet to a temperature of 1120°C ± 20°C and then holding the temperature. The relationship between the holding time and the thickness of the hollow billet is set to 0.9 - 1 min / mm. The hollow billet is placed on a piercing mill to pierce and roll into a rough tube, and the temperature of the final rolling deformation is not lower than 900°C.

[0010] Furthermore, placing the hollow billet on a piercing mill to pierce and roll into a rough tube includes multiple piercing and rolling processes. The heating temperature during the intermediate rolling deformation is 1100°C ± 20°C, the wall thickness deformation ratio during piercing and rolling is ≥3, and the length elongation ratio is ≥3.

[0011] Furthermore, after the hollow billet is placed on a piercing mill to pierce and roll into a rough tube, the rough tube is annealed. The annealing process conditions are set as follows: the annealing temperature is 780 ± 10°C, the furnace temperature is ≤400°C when entering the furnace, the heating rate is 150°C / h, timing starts when the workpiece reaches 770°C, the holding time is 300 minutes, and the workpiece is air-cooled after leaving the furnace.

[0012] Furthermore, sizing the rough tube to obtain a semifinished tube includes heating the rough tube, and then sizing the heated rough tube on a three-high five-stand sizing mill to obtain a semifinished tube. The heating temperature is controlled at 1100 ± 15°C, and the relationship between the holding time and the wall thickness of the above-mentioned rough tube is set to 0.9 - 1.1 min / mm.

[0013] Furthermore, cold drawing or cold rolling processes are used for sizing the rough tube to obtain a semifinished tube, and the diameter reduction is ≤50 mm.

[0014] Furthermore, heat treating the semifinished tube to form a semi-finished product tube includes heat treating the semifinished tube by normalizing + tempering. The heat treatment process conditions are as follows: the normalizing temperature is 1080 ± 10°C, the furnace temperature is ≤650°C when entering the furnace, the heating rate is set to 190°C / h below 700°C and 100°C / h from 700 to 1080°C. Timing starts when the workpiece reaches 1070°C, the holding time is 80 minutes, and the workpiece is air-cooled after leaving the furnace; the tempering temperature is 790 ± 10°C, the furnace temperature is ≤400°C when entering the furnace, the heating rate is set to 160°C / h below 660°C and 120°C / h from 660 to 780°C. Timing starts when the workpiece reaches 785°C, and the holding time is 240 minutes.

[0015] Furthermore, the heat-treated rough pipe is heat-straightened to form a semi-finished pipe, the heat-straightening temperature is 300-750°C, and the straightness of the rough pipe after straightening is ≤3.0mm / m, and the total length is ≤0.10%.

[0016] The hot rolling production process of G115 seamless steel pipe provided by the present invention is different from the process of producing G115 large-diameter seamless steel pipe by hollow billet with a diameter larger than that of finished pipe + hot extrusion + machining commonly used in the prior art. It creatively proposes a process of producing rough pipe by hollow billet with a diameter smaller than that of finished pipe + low-temperature, high-stress and large-deformation hot rolling. The rough pipe sizing adopts the production process of hot rolling, cold drawing or cold rolling, so that the production batch of G115 seamless steel pipe is flexible and the process is controllable. The production efficiency and yield rate of the produced G115 seamless steel pipe are significantly improved, the surface quality is good, the metallographic structure and longitudinal and transverse mechanical properties of the produced finished pipe are uniform, and all meet the relevant requirements of GBT5310-2023 and T / CSTM 00017-2021. It can be used for the main steam pipeline, reheat steam pipeline and corresponding high-temperature header and other components of super (super) critical thermal power units with stable quality, high yield rate and significant economic benefits. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 This is a process flow chart of hot rolling production of G115 seamless steel pipe in an embodiment of the present invention;

[0018] Figure 2 This is a metallographic structure view of the outer surface of the finished tube Φ424×53mm in the embodiment of the present invention, magnified 100 times;

[0019] Figure 3 This is a metallographic structure view of the middle of the wall thickness of the finished tube Φ424×53mm in the embodiment of the present invention, which is magnified 100 times;

[0020] Figure 4 This is a metallographic structure view of the inner surface of the finished tube Φ424×53mm in the embodiment of the present invention, magnified 100 times;

[0021] Figure 5 This is a metallographic structure view of the outer surface of the finished tube Φ978×38mm in the embodiment of the present invention, magnified 100 times;

[0022] Figure 6 This is a metallographic structure view of the middle of the wall thickness of the finished tube Φ978×38mm in the embodiment of the present invention, magnified 100 times;

[0023] Figure 7 This is a metallographic structure view of the inner surface of the finished tube Φ978×38mm in the embodiment of the present invention, magnified 100 times. DETAILED DESCRIPTION

[0024] To make the objectives, technical solutions, and advantages of the present invention clearer, the following will elaborate on various embodiments of the present invention in conjunction with the accompanying drawings. However, those of ordinary skill in the art can understand that in various embodiments of the present invention, many technical details are provided to help readers better understand the present application. However, even without these technical details and various changes and modifications based on the following embodiments, the technical solutions claimed in the various claims of the present application can still be achieved.

[0025] As Figure 1 shown, in one embodiment of the present invention, a hot rolling production process for G115 seamless steel pipes is involved, including the following steps:

[0026] Manufacture a hollow billet of G115 steel. According to the Chinese patent document CN103045962B, a steel for ultra-supercritical thermal power generation units with steam temperature and its preparation method are disclosed. The chemical composition of the steel for the hollow billet in weight percentage is as follows: carbon: 0.06 - 0.10%; silicon: 0.1 - 0.5%; manganese: 0.2 - 0.8%; phosphorus: ≤ 0.004%; sulfur: ≤ 0.002%; chromium: 8.0 - 9.5%; tungsten: 2.5 - 3.5%; cobalt: 2.5 - 3.5%; niobium: 0.03 - 0.07%; vanadium: 0.10 - 0.30%; copper: 0.80 - 1.20%; nitrogen: 0.006 - 0.010%; boron: 0.010 - 0.016%; rare earth cerium: 0.01 - 0.04%; nickel: ≤ 0.01%; aluminum: ≤ 0.005%; titanium: ≤ 0.01%; zirconium: ≤ 0.01%; the balance is iron and inevitable impurity elements. It can be smelted by the technological process of EAF + AOD + protective atmosphere ESR or VIM + protective atmosphere ESR, or can also be smelted by other suitable technological processes in the existing technologies. Among them, EAF (Electric Arc Furnace): a smelting device using arc heating, mainly used for melting scrap steel or ferroalloys, suitable for steel smelting; AOD (Argon Oxygen Decarburization): a steel smelting device, which decarburizes and refines through a mixed gas of argon and oxygen, reduces the carbon content and controls the composition; ESR (Electroslag Remelting): a refining process, which removes impurities, improves the material uniformity and density through electroslag remelting, and is used for the production of high-quality alloy steels and special alloys. VIM (Vacuum Induction Melting): a process of melting metals by induction heating in a vacuum environment, avoiding oxidation and gas pollution, suitable for the production of high-purity alloys and special materials. After obtaining the steel billet through smelting, the G115 steel hollow billet can be made by any one of the following methods: the steel billet is hot-extruded into a hollow billet, the steel billet is forged after continuous casting and then machined to drill holes to form a hollow billet, or the steel billet is electroslag remelted with a continuous casting electrode and then machined to drill holes to form a hollow billet. The allowable deviation of the diameter of the hollow billet is set to ±1%, and the allowable deviation of the wall thickness is 10%.

[0027] The hot rolling piercing of the hollow billet is carried out to obtain a rough tube. First, the hollow billet is heated. The heating quality is a key factor for the wall thickness uniformity of the rolled tube and the surface quality of the rough tube. The heating equipment can use a 35-meter ring heating furnace. The hot rolling method of the G115 steel hollow billet is the piercing rolling of a round tube billet, and the stress mode is three-dimensional internal and external surface shear stress. Since G115 contains 2.5 - 3.5% tungsten and 2.5 - 3.5% cobalt, tungsten and cobalt are heavy atom metals, and their carbides are difficult to dissolve in the matrix even at high temperatures and are distributed at the grain boundaries. The higher the temperature, the weaker the grain boundaries. When the hot deformation temperature is higher than 1150 °C, the grain boundaries will crack under the shear rolling stress, and then the tube will crack. If the heating temperature of the hollow billet is too low, the hot plasticity of the material will decrease sharply, and the reduction of the hollow billet by the piercing mill will decrease, resulting in the inability to establish the piercing pass and the failure to achieve hot rolling piercing. Therefore, the first hot rolling piercing temperature range is set at 1000 - 1140 °C, which is much lower than that of other alloy steel pipes. Considering the rolling force load of the hot rolling mill, the heating temperature is set at 1120 °C ± 20 °C. The residence time of the hollow billet in the high-temperature zone should not be too long, otherwise it is easy to cause serious overoxidation, element depletion, and grain coarsening. The relationship between the holding time and the thickness of the hollow billet is set at 0.9 - 1 min / mm. Then, the hollow billet is placed on the piercing mill to pierce and roll the tube to make a rough tube. Since the low-temperature deformation resistance of G115 steel is relatively large, the final rolling temperature should not be lower than 900 °C, the wall thickness deformation ratio of piercing rolling ≥ 3, and the length elongation ratio ≥ 3. When multiple piercing rolling deformations are required according to the piercing rolling process requirements, the heating temperature for the next rolling deformation is 1100 °C ± 20 °C. Compared with the process of making rough tubes by hot extrusion compression molding above 1200 °C in the prior art, it can complete the rolling deformation of the rough tube under low temperature and high stress, effectively reduce the energy consumption in the production process, reduce the manufacturing cost, and improve the forming quality. Before rolling, glass powder is sprinkled on the inner surfaces at both ends of the hollow billet for lubrication. After the rough tube is made by piercing rolling, the rough tube is annealed. The equipment used for annealing is a car-bottom LPG furnace, and the fuel used for this heat treatment furnace is natural gas. The annealing process conditions are set as follows: annealing temperature 780 ± 10 °C, the temperature ≤ 400 °C when entering the furnace, heating rate 150 °C / h, timing starts when the workpiece reaches 770 °C, holding for 300 minutes, and air cooling after leaving the furnace.

[0028] The sizing of the rough tube is carried out to obtain a semifinished tube. First, the rough tube is heat-treated, and the heating temperature is controlled at 1100 ± 15 °C. The relationship between the holding time and the wall thickness of the above-mentioned rough tube is set at 0.9 - 1.1 min / mm. After the holding is completed, it is taken out of the furnace for sizing. Then, the heated rough tube is sized on a three-roll five-stand sizing mill to obtain a semifinished tube. In one embodiment, according to actual needs, the sizing of the rough tube to obtain a semifinished tube can also be carried out by cold drawing or cold rolling processes to make a semifinished tube. To ensure the process quality, the diameter reduction ≤ 50 mm.

[0029] The raw pipe is heat-treated to form a semi-finished pipe. The equipment used for heat treatment is a trolley-type heat treatment furnace, and a heat treatment process combining normalizing and tempering is adopted. Among them, the normalizing process is set as follows: the normalizing temperature is 1080 ± 10 °C, the temperature for furnace entry is ≤ 650 °C, the heating rate is set as follows: below 700 °C, 190 °C / h; from 700 to 1080 °C, 100 °C / h. When the workpiece reaches 1070 °C, timing starts, and it is kept warm for 80 minutes, then taken out of the furnace and air-cooled. The tempering process is set as follows: the tempering temperature is 790 ± 10 °C, the temperature for furnace entry is ≤ 400 °C, the heating rate is set as follows: below 660 °C, 160 °C / h; from 660 to 780 °C, 120 °C / h. When the workpiece reaches 785 °C, timing starts, and it is kept warm for 240 minutes. To prevent the raw pipe from cracking, the raw pipe is hot straightening immediately after tempering out of the furnace. The hot straightening temperature is 300 - 750 °C. After the raw pipe is straightened, the straightness is ≤ 3.0 mm / m, and the overall length is ≤ 0.10%.

[0030] The semi-finished pipe is polished and machined to produce a finished pipe. Oxide layers are formed on the inner and outer surfaces of the raw pipe after heat treatment to form a semi-finished pipe. The surface oxide scale is removed by internal and external polishing to obtain a finished pipe meeting the technical requirements. For surface defects that cannot be eliminated by polishing, further machining can be carried out to eliminate the defects.

[0031] Example 1: Produce G115 seamless steel pipes according to the hot rolling production process of G115 seamless steel pipes provided by the present invention. The diameter specification of the finished pipe is Φ424mm, and the wall thickness specification is 53mm, that is, the specification of the finished pipe is Φ424×53mm. The specific process includes: producing a G115 steel hollow blank with a diameter smaller than that of the finished pipe. The steel billet is obtained by smelting raw materials according to the composition of G115 steel. The hollow blank is formed by extruding the steel billet or by machining and punching after forging the continuously cast steel billet. The specification of the hollow blank is Φ400×170mm. The allowable deviation of the diameter of the hollow blank is 1%, and the allowable deviation of the wall thickness is 10%. The nominal diameter of the hollow blank is 400mm, so the allowable diameter value is 396mm - 404mm, the nominal wall thickness is 170mm, and the allowable wall thickness value is 153mm - 187mm. The actual measured value of the diameter of the hollow blank is 398.0mm~404.0, and the actual measured value of the wall thickness of the hollow blank is 170mm - 175mm. The dimensions of the hollow blank meet the requirements; the hollow blank is subjected to hot rolling piercing to obtain a rough pipe. The first hot rolling piercing temperature is controlled at 1000℃ - 1140℃. Specifically, a ring heating furnace is used to heat the hollow blank Φ400×170mm. When the hollow blank is below 800℃, it starts to be heated into the furnace. The heating rate is 100℃ / h. When the heating temperature reaches 1120℃±20℃, it is kept warm for 170 minutes. After the heat preservation ends, it is taken out of the furnace and directly pierced and rolled into a pipe. The first piercing temperature is controlled at 1000℃ - 1140℃. The piercing and rolling into a pipe includes multiple piercing and rolling deformations. The heating temperature of the second rolling deformation is 1100℃±20℃, and the final rolling temperature is controlled at 900℃ - 1100℃. The rough pipe is obtained by piercing and rolling into a pipe. The specification of the rough pipe is Φ450×57mm, and the wall thickness deformation ratio of the piercing and rolling is ≥3, and the length extension ratio is ≥3; after rolling into a pipe, the rough pipe is annealed. The equipment used for annealing is a car-bottom LPG furnace, and the fuel used in this heat treatment furnace is natural gas. The annealing process conditions are set as follows: the annealing temperature is 780±10℃, the temperature ≤400℃ when entering the furnace, the heating rate is 150℃ / h, timing starts when the workpiece reaches 770℃, keep warm for 300 minutes, and take out of the furnace and air-cool; the rough pipe is sized to obtain a semifinished pipe. Specifically, when the rough pipe is below 800℃, it starts to be heated into the furnace. The heating temperature is controlled at 1100±15℃, and the heat preservation time is controlled at 0.9 - 1.1min / mm. Preferably, the heat preservation time is 60 minutes. After the heat preservation ends, it is taken out of the furnace and sized. The heated rough pipe is sized on a three-roll five-stand sizing mill to obtain a semifinished pipe. The specification of the obtained semifinished pipe is Φ428×57.5mm; optionally, the rough pipe Φ450×57mm is sized by cold drawing or cold rolling to obtain a semifinished pipe. The specification of the semifinished pipe is Φ428×57.5mm, and the diameter reduction amount ≤50mm.

[0032] The semifinished pipe is obtained by heat-treating the raw pipe Φ428×57.5mm. The equipment used for heat treatment is a trolley-type heat treatment furnace, and a heat treatment process combining normalizing and tempering is adopted. Among them, the normalizing process is set as follows: the normalizing temperature is 1080±10°C, the temperature enters the furnace at ≤650°C, the heating rate is set at below 700°C, 190°C / h for 700 - 1080°C, 100°C / h. When the workpiece reaches 1070°C, timing starts, and it is kept warm for 80 minutes, then taken out of the furnace and air-cooled; the tempering process is set as follows: the tempering temperature is 790±10°C, the temperature enters the furnace at ≤400°C, the heating rate is set at below 660°C, 160°C / h for 660 - 780°C, 120°C / h. When the workpiece reaches 785°C, timing starts, and it is kept warm for 240 minutes; to prevent the raw pipe from cracking, the raw pipe is hot-straightened immediately after tempering and taken out of the furnace. The hot-straightening temperature is 300 - 750°C. After the raw pipe is straightened, the straightness is ≤3.0mm / m, and the overall length is ≤0.10%; the semifinished pipe is polished and machined to produce the finished pipe. The oxide scale and surface defects on the surface of the semifinished pipe are removed to obtain the finished pipe. The specifications of the finished pipe are Φ424×53mm.

[0033] According to the standards of GB / T5310-2023 and T / CSTM00017‐2021, ultrasonic, eddy current and magnetic particle inspections are carried out on the produced finished pipe Φ424×53mm. Among them, the ultrasonic inspection is carried out according to the standard of GB / T5777-2019, the artificial defect grade is U2, and the defect size is 25×0.5×1.5 (groove length×width×depth); the eddy current inspection is carried out according to the standard of GB / T7735-2016, the artificial defect grade is E2, and the defect size is 25×0.5×1.5 (groove length×width×depth); the magnetic particle inspection is carried out according to the standard of NB / T47013.4-2015, and the results are all qualified; the outer surface quality, outer diameter, wall thickness, length, straightness, PMI, etc. of the finished pipe Φ424×53mm are detected, and all meet the standard requirements. As shown in Table 1, the size accuracy measurement record form of the finished pipe Φ424×53mm:

[0034]

[0035] According to the standards of GB / T5310-2023 and T / CSTM00017-2021, room temperature mechanical property tests are carried out on the finished pipe Φ424×53mm. The room temperature mechanical property specimens adopt transverse and longitudinal round bar specimens with a diameter of 10mm and a fixed gauge length of 50mm. The two ends of the specimen along the axis are marked as end A and end B, and the test is carried out according to the standard regulations. As shown in Table 2, the room temperature mechanical property test record form of the finished pipe Φ424×53mm:

[0036]

[0037] The test results of room temperature tensile properties, impact resistance and hardness show that the sample pipe has good properties at room temperature, and its mechanical properties, impact resistance and hardness are relatively uniform, meeting the requirements of relevant standards.

[0038] According to the standards of GBT5310-2023 and T / CSTM00017-2021, for the finished pipe Φ424×53mm, high-temperature short-time tensile tests were carried out by taking transverse and longitudinal samples at both ends of the steel pipe at test temperatures of 630°C and 650°C respectively. The diameter of the specimen is 10mm, the calibrated gauge length is 50mm, and the two ends of the specimen along the axis are marked as end A and end B. The test results are shown in Table 3 Record Table of High-temperature Tensile Properties Test of Finished Pipe Φ424×53mm:

[0039]

[0040] As can be seen from the above, the yield strength of the G115 large-diameter finished pipe provided by the present invention at test temperatures of 630°C and 650°C both meets the relevant regulations of GBT5310-2023 and T / CSTM00017-2021. At the same temperature, the high-temperature mechanical properties in the longitudinal and transverse directions are basically the same, the strength values along the wall thickness direction are relatively uniform, and there is a large margin.

[0041] Samples were taken from the near outer surface, the middle of the wall thickness and the near inner surface of the finished pipe Φ424×53mm respectively for metallographic structure and grain size inspection. As Figure 2 shown, it is a metallographic structure view of the near outer surface of the finished pipe Φ424×53mm magnified 100 times. The metallographic structure shows tempered martensite and the grain size is grade 0. Figure 3 It is a metallographic structure view of the middle of the wall thickness of the finished pipe Φ424×53mm magnified 100 times. The metallographic structure shows tempered martensite and the grain size is grade 0; Figure 4 It is a metallographic structure view of the near inner surface of the finished pipe Φ424×53mm magnified 100 times. The metallographic structure shows tempered martensite and the grain size is grade 0, meeting the requirements of GBT5310-2023 and T / CSTM00017-2021.

[0042] According to the standards of GBT5310-2023 (Seamless Steel Tubes for High-Pressure Boilers) and T / CSTM00017-2021 (Seamless Steel Tubes of Martensitic Heat-Resistant Steel 08Cr9W3Co3VNbCuBN for Power Stations), the non-metallic inclusions in the steel pipe were rated by method A in GB / T10561 (Microscopic Examination Method of Standard Rating Charts for Determination of Content of Non-Metallic Inclusions in Steel). The non-metallic inclusions in the finished pipe Φ424×53mm were rated, as shown in Table 4 Inspection Results of Non-Metallic Inclusions in Steel Pipe:

[0043]

[0044] As can be seen from the above, the non-metallic inclusion content of the finished pipe Φ424×53mm meets the requirements of GBT5310-2023 and T / CSTM00017‐2021.

[0045] In this implementation, the production of G115 large-diameter seamless steel pipes was successfully achieved by using the process of hollow billet + hot rolling + sizing (hot drawing or cold drawing or cold rolling). For the G115 large-diameter target finished pipe Φ424×53mm, the hollow billet Φ400×170mm was rolled through piercing to obtain the rough pipe Φ450×57mm, and then the rough pipe was sized to obtain the semi-finished pipe Φ428×57.5mm. The semi-finished pipe was ground and machined to successfully obtain the finished pipe Φ424×53mm. The finished pipe was inspected for dimensions, tested for room-temperature mechanical properties, inspected for high-temperature mechanical properties, examined for metallographic structure, grain size, and non-metallic inclusions. Compared with the hot extrusion production process, it meets the standards of G115 seamless steel pipes for martensitic heat-resistant steels used in power stations.

[0046] Example 2: The G115 seamless steel pipe is produced according to the hot rolling production process of G115 seamless steel pipe provided by the present invention. The diameter specification of the finished pipe is Φ978mm, and the wall thickness specification is 38mm, that is, the finished pipe specification is Φ978×38mm. The specific process includes: preparing a G115 steel hollow billet with a diameter smaller than that of the finished pipe. The method for preparing the G115 steel hollow billet includes: smelting raw materials according to the composition of G115 steel to obtain a steel billet, using continuous casting electrodes for the steel billet, and after electroslag remelting, machining and drilling to make a hollow billet. The specification of the hollow billet is Φ700×200mm. The size of the hollow billet meets the requirements; the hollow billet is subjected to piercing rolling to obtain a rough pipe. Specifically, an annular heating furnace is used to heat the hollow billet Φ700×200mm. When the temperature of the hollow billet is lower than 800°C, it starts to be heated in the furnace. The heating rate is 100°C / h. When the heating temperature reaches 1120°C±20°C, it is kept warm for 200 minutes. After the heat preservation ends, it is taken out of the furnace and directly pierced and rolled into a pipe. The specification of the first piercing and rolling is Φ710×125mm. It is heated to 1100°C for the second piercing and rolling with a specification of Φ870×60mm. The final piercing and rolling specification is Φ998×42mm. The final rolling temperature is controlled at 900°C - 1100°C. Preferably, the final rolling temperature is 1100°C. The rough pipe obtained by piercing and rolling has a specification of Φ998×42mm. The wall thickness deformation ratio of piercing and rolling is ≥3, and the length elongation ratio is ≥3; after rolling the pipe, the rough pipe is annealed. The equipment used for annealing is a car-bottom LPG furnace. The fuel used in this heat treatment furnace is natural gas. The annealing process conditions are set as follows: the annealing temperature is 780±10°C, the temperature ≤400°C when entering the furnace, the heating rate is 150°C / h, timing starts when the workpiece reaches 770°C, it is kept warm for 300 minutes, and then taken out of the furnace and air-cooled; the rough pipe is sized to obtain a semifinished pipe. Specifically, when the temperature of the rough pipe is lower than 800°C, it starts to be heated in the furnace. The heating temperature is controlled at 1100±15°C, and the heat preservation time is controlled at 0.9 - 1.1min / mm. Preferably, the heat preservation time is 40 minutes. After the heat preservation ends, it is taken out of the furnace and sized. The heated rough pipe is sized on a three-high five-stand sizing mill to obtain a semifinished pipe. The specification of the obtained semifinished pipe is Φ982×42mm; optionally, the rough pipe Φ998×42mm is sized by cold drawing or cold rolling to obtain a semifinished pipe. The specification of the semifinished pipe is Φ982×42mm, and the diameter reduction amount ≤50mm.

[0047] The semi-finished pipe is obtained by heat-treating the raw pipe. The equipment used for heat treatment is a trolley-type heat treatment furnace, and a heat treatment process combining normalizing and tempering is adopted. Among them, the normalizing process is set as follows: the normalizing temperature is 1080±10°C, the pipe is put into the furnace when the temperature is ≤650°C, the heating rate is set to be below 700°C, 190°C / h, and from 700 to 1080°C, 100°C / h. When the workpiece reaches 1070°C, timing starts, and it is kept warm for 80 minutes, then taken out of the furnace and air-cooled; the tempering process is set as follows: the tempering temperature is 790±10°C, the pipe is put into the furnace when the temperature is ≤400°C, the heating rate is set to be below 660°C, 160°C / h, and from 660 to 780°C, 120°C / h. When the workpiece reaches 785°C, timing starts, and it is kept warm for 240 minutes; to prevent the raw pipe from cracking, the raw pipe is hot-straightened immediately after tempering and taken out of the furnace. The hot-straightening temperature is 300 - 750°C. After the raw pipe is straightened, the straightness is ≤3.0mm / m, and the overall length is ≤0.10%; the semi-finished pipe is polished and machined to produce the finished pipe. The oxide scale and surface defects on the surface of the semi-finished pipe are removed to obtain the finished pipe. The specification of the finished pipe is Φ978×38mm.

[0048] According to the standards of GB / T5310-2023 and T / CSTM00017‐2021, ultrasonic, eddy current and magnetic particle inspections were carried out on the produced finished pipe Φ978×38mm. Among them, the ultrasonic inspection was carried out according to the standard of GB / T5777-2019, the artificial defect grade was U2, and the defect size was 25×0.5×1.5 (groove length×width×depth); the eddy current inspection was carried out according to the standard of GB / T7735-2016, the artificial defect grade was E2, and the defect size was 25×0.5×1.5 (groove length×width×depth); the magnetic particle inspection was carried out according to the standard of NB / T47013.4-2015, and the results were all qualified; the outer surface quality, outer diameter, wall thickness, length, straightness, PMI, etc. of the finished pipe Φ978×38mm were detected, and all met the standard requirements. As shown in Table 5, the size accuracy measurement record form of the finished pipe Φ978×38mm:

[0049]

[0050] According to the standards of GB / T5310-2023 and T / CSTM00017-2021, room temperature mechanical property tests were carried out on the finished pipe Φ978×38mm. The room temperature mechanical property specimens were transverse and longitudinal round bar specimens with a diameter of 10mm and a specified gauge length of 50mm. The two ends of the specimen along the axis were marked as end A and end B, and the test was carried out according to the standard regulations. As shown in Table 6, the room temperature mechanical property test record form of the finished pipe Φ978×38mm:

[0051]

[0052] The test results of room temperature tensile properties, impact resistance and hardness show that the sample pipe has good properties at room temperature, and its mechanical properties, impact resistance and hardness are relatively uniform, meeting the requirements of relevant standards.

[0053] According to the GBT5310-2023 and T / CSTM00017-2021 standards, for the finished pipe Φ978×38mm, high-temperature short-time tensile tests were carried out by taking transverse and longitudinal samples at both ends of the steel pipe at test temperatures of 630°C and 650°C respectively. The diameter of the specimen is 10mm and the calibrated gauge length is 50mm. The two ends of the specimen along the axis are marked as end A and end B. The test results are shown in Table 7 Record Table of High-temperature Tensile Properties Test of Finished Pipe Φ978×38mm:

[0054]

[0055] As can be seen from the above, the yield strength of the G115 large-diameter finished pipe provided by the present invention at test temperatures of 630°C and 650°C meets the relevant regulations of GBT5310-2023 and T / CSTM00017-2021. The high-temperature mechanical properties in the longitudinal and transverse directions at the same temperature are basically the same, the strength values along the wall thickness direction are relatively uniform, and there is a large margin.

[0056] Samples were taken from the near outer surface, middle of the wall thickness and near inner surface of the finished pipe Φ978×38mm respectively for metallographic structure and grain size inspection. As Figure 5 shown, it is a metallographic structure view of the near outer surface of the finished pipe Φ978×38mm magnified 100 times. The metallographic structure shows tempered martensite and the grain size is grade 4. Figure 6 It is a metallographic structure view of the middle of the wall thickness of the finished pipe Φ978×38mm magnified 100 times. The metallographic structure shows tempered martensite and the grain size is grade 4; Figure 7 It is a metallographic structure view of the near inner surface of the finished pipe Φ978×38mm magnified 100 times. The metallographic structure shows tempered martensite and the grain size is grade 4, meeting the requirements of GBT5310-2023 and T / CSTM00017-2021.

[0057] According to the GBT5310-2023 and T / CSTM00017-2021 standards, the non-metallic inclusions in the steel pipe were rated by method A in GB / T10561, and the non-metallic inclusions in the finished pipe Φ978×38mm were rated. The results are shown in Table 8 Inspection Results of Non-metallic Inclusions in Steel Pipe:

[0058]

[0059] As can be seen from the above, the content of non-metallic inclusions in the finished pipe Φ978×38mm meets the regulations of GBT5310-2023 and T / CSTM00017-2021.

[0060] In this embodiment, the process of hollow billet + hot rolling + sizing (hot rolling or cold drawing or cold rolling) is adopted to successfully realize the production of G115 large-diameter seamless steel pipe. For the G115 large-diameter target finished pipe Φ978×38mm, the hollow billet Φ978×38mm is hot rolled (piercing rolling) to obtain a rough pipe Φ998×42mm, and then the rough pipe is sizing (hot rolling or cold drawing or cold rolling) to make a rough pipe Φ982×42mm. The rough pipe is ground and machined to successfully obtain a finished pipe Φ978×38mm. The finished pipe is inspected by size, room temperature mechanical properties test, high temperature mechanical properties test, metallographic structure, grain size, non-metallic inclusions inspection and relative hot extrusion production process, and all meet the standard of martensitic heat-resistant steel G115 seamless steel pipe for power stations.

[0061] The hot rolling production process of G115 seamless steel pipe provided by the present invention is adopted to trial-produce finished pipes of Φ424×53mm and finished pipes of Φ978×38mm in small batches, and the yield rate of seamless steel pipes is tested and verified. The yield rate = the mass of finished pipe / the mass of hollow billet. The test results are shown in Table 9, the statistical table of the yield rate of G115 seamless steel pipes:

[0062]

[0063] The hot rolling production process for G115 seamless steel pipe provided by the present invention is used, and the results of batch production data verification show that the yield rate of G115 steel pipe is ≥80%. In the prior art, a process of obtaining a hollow billet and then hot extruding to make a finished pipe is adopted, and the yield rate of G115 steel pipe is about 50%. Compared with the prior art, the hot rolling production process for G115 seamless steel pipe provided by the present invention has a yield rate of ≥80%, which significantly improves the yield rate of G115 seamless steel pipe.

[0064] The hot rolling production process of G115 seamless steel pipe provided by the present invention is different from the process of producing G115 large-diameter seamless steel pipe by hollow billet with a diameter larger than that of finished pipe + hot extrusion + machining commonly used in the prior art. It creatively proposes a process of producing rough pipe by hollow billet with a diameter smaller than that of finished pipe + low-temperature, high-stress and large-deformation hot rolling. The rough pipe sizing adopts the production process of hot rolling, cold drawing or cold rolling, so that the production batch of G115 seamless steel pipe is flexible, the production process is controllable, and the specification applicability is strong. It is suitable for industrial production of factories with complex specifications. The surface quality of the produced G115 seamless steel pipe is good, and the metallographic structure and longitudinal and transverse mechanical properties of the finished pipe are uniform, which meet the relevant requirements of GBT5310-2023 and T / CSTM00017-2021. It can be used for the main steam pipeline, reheat steam pipeline and corresponding high-temperature header and other components of super (super) critical thermal power units with stable quality, high yield rate and significant economic benefits.

[0065] Those of ordinary skill in the art can understand that the above embodiments are specific examples for implementing the present invention, and in practical applications, various changes can be made to them in form and details without departing from the spirit and scope of the present invention.

Claims

1. A hot rolling production process for G115 seamless steel pipe, characterized in that: The following steps are involved: A G115 steel hollow billet with a diameter smaller than that of a finished tube is prepared; the hollow billet is subjected to multiple hot rolling, piercing and rolling to obtain a rough tube, the first hot rolling piercing temperature is controlled at 1000°C-1140°C, and the final rolling temperature is controlled at 900°C-1100°C; the rough tube is annealed, and the annealing process conditions are set as follows: annealing temperature 780±10°C, temperature ≤400°C into the furnace, heating rate 150°C / h, timing starts when the workpiece reaches 770°C, heat preservation for 300 minutes, and air cooling after the workpiece is taken out of the furnace; the rough tube is sized to obtain a rough tube; the rough tube is heat treated to form a semi-finished tube; the semi-finished tube is polished and machined to obtain a finished tube.

2. The hot rolling production process for G115 seamless steel pipe according to claim 1, characterized in that: The preparation of the G115 steel hollow billet with a diameter smaller than that of the finished pipe includes: smelting raw materials according to the composition of G115 steel to obtain a billet, hot extruding the billet into a hollow billet, forging the billet after continuous casting and then machining to form a hollow billet, remelting the billet with continuous casting electrodes and electroslag and then machining to form a hollow billet. Any one of the following methods is used to prepare the G115 steel hollow billet, and the loss of the punching material is ≤5%.

3. The hot rolling production process of G115 seamless steel pipe according to claim 1, characterized in that: The method of hot-rolling, piercing and rolling the hollow billet to obtain a rough tube includes heating the hollow billet to a temperature of 1120°C±20°C and then keeping the billet warm, wherein the relationship between the keeping time and the thickness of the hollow billet is set to 0.9-1min / mm; and placing the hollow billet on a piercing unit to perform piercing and tube rolling to obtain a rough tube.

4. The hot rolling production process for G115 seamless steel pipe according to claim 3, characterized in that: The multiple hot rolling and piercing rolling include intermediate rolling deformation, the heating temperature of the intermediate rolling deformation is 1100°C±20°C, the piercing rolling wall thickness deformation ratio is ≥3, and the length extension ratio is ≥3.

5. The hot rolling production process of G115 seamless steel pipe according to claim 1, characterized in that: The process of sizing the rough tube to produce a rough tube includes: heating the rough tube, and then sizing the heated rough tube on a three-roller five-frame sizing machine to produce a rough tube, the heating temperature is controlled at 1100±15°C, and the insulation time is set to 0.9-1.1 min / mm in relation to the wall thickness of the rough tube.

6. The hot rolling production process of G115 seamless steel pipe according to claim 1, characterized in that: The rough tube is sizing to produce a rough tube by cold drawing or cold rolling, and the diameter reduction is ≤50mm.

7. The hot rolling production process for G115 seamless steel pipe according to claim 5 or 6, characterized in that: The rough pipe is heat treated to form a semi-finished pipe, including heat treatment of the rough pipe by normalizing + tempering, and the heat treatment process conditions are: normalizing temperature 1080±10℃, temperature ≤650℃ into the furnace, heating rate is set to below 700℃, 190℃ / h, 700~1080℃, 100℃ / h, start timing when the workpiece reaches 1070℃, keep warm for 80 minutes, and air cool out of the furnace; tempering temperature 790±10℃, temperature ≤400℃ into the furnace, heating rate is set to below 660℃, 160℃ / h, 660~780℃, 120℃ / h, start timing when the workpiece reaches 785℃, and keep warm for 240 minutes.

8. The hot rolling production process for G115 seamless steel pipe according to claim 7, characterized in that: The rough pipe after tempering heat treatment is heat straightened to form a semi-finished pipe. The heat straightening temperature is 300-750°C. The straightness of the rough pipe after straightening is ≤3.0mm / m and the total length is ≤0.10%.

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