Hot rolling production process of G115 seamless steel tube

Abstract

The invention relates to the technical field of seamless steel pipe production, and discloses a G115 seamless steel pipe hot rolling production process which comprises the following steps: preparing a G115 steel hollow blank with the diameter smaller than that of a finished pipe; carrying out hot-rolling perforation rolling on the hollow blank to obtain a tubular billet; sizing the tubular billet to prepare a pierced billet; performing heat treatment on the pierced billet to form a semi-finished tube; and the semi-finished pipe is polished and machined, and a finished pipe is manufactured. By adopting the production process of the hollow blank with the diameter smaller than that of the finished pipe and hot rolling, the production grouping is flexible, the specification applicability is high, the method is suitable for industrial production of factories with complex specifications, the production efficiency and the yield are remarkably improved, the product surface quality is good, the metallographic structure and the uniformity of longitudinal and transverse mechanical properties are good, the technological process is controllable, and the production cost is low. And the quality is stable, the yield is high, and economic benefits are remarkable.

Description

Technical Field

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

[0002] G115 steel is a 650℃ martensitic heat-resistant steel with independent intellectual property rights developed by the Central Iron and Steel Research Institute. Under the guidance of the "multi-element composite strengthening" theory, the steel adopts the "selective toughening" design concept, combines the research basis of MARBN (9W3Co3BN) steel, and adds the precipitation element Cu to further improve the strength of the steel. At the same time, it makes full use of the "thermal steel grain boundary engineering principle" and is developed through the matching and precise control of elements such as B, N, and Al. The steel has excellent organizational stability in the temperature range of 620-650℃. Its endurance strength at 650℃ is 1.5 times that of P92 steel. Its high-temperature steam oxidation resistance and weldability are equivalent to those of P92 steel. It can be used in thick-walled parts such as large-diameter pipes and headers in the temperature range of 620-650℃, and has broad market prospects.

[0003] In the Chinese patent document with publication number CN 108998650 A and invention name "630℃ ultra-supercritical unit G115 large-diameter thick-walled seamless steel pipe manufacturing method", a 630℃ ultra-supercritical unit G115 large-diameter thick-walled seamless steel pipe manufacturing method is disclosed, which includes the following steps: 1. Raw material (forging billet / steel ingot) preparation; 2. Closed upsetting + reverse extrusion punching, heating the forging billet (steel ingot) to 1140±10℃ , uniform heating; remove the surface oxide scale, spray glass lubricant and then perform closed upsetting on the billet press, the upsetting ratio ≥1.3, reverse extrusion punching, the perforation ratio ≥1.1diw, to make a hollow billet; 3. Extrusion molding, heat the hollow billet to 1240±10℃, remove the surface oxide scale, return to the furnace for heating for 30min, spray glass lubricant after taking out of the furnace, and then extrude the hollow billet into a tube billet on the extruder, the extrusion speed is 15~30mm / s, and the extrusion ratio is ≥6; 4. After extrusion, it is subjected to 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: 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.

[0007] Furthermore, 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 of the following methods is used to prepare the G115 steel hollow billet, and the loss of the punching material is ≤5%.

[0008] Further, the hollow billet is hot-rolled and pierced to obtain a rough tube, which includes heating the hollow billet to a temperature of 1120°C±20°C and then keeping it warm, wherein the relationship between the insulation time and the thickness of the hollow billet is set to 0.9-1min / mm; the hollow billet is set on a piercing unit for piercing and tube rolling to form a rough tube, and the temperature of the final rolling deformation is not less than 900°C.

[0009] Further, the hollow billet is placed on a piercing unit for piercing and rolling to form a rough tube, including multiple piercing and rolling, the heating temperature of the intermediate rolling deformation is 1100°C±20°C, the piercing and rolling wall thickness deformation ratio is ≥3, and the length extension ratio is ≥3.

[0010] Furthermore, after the hollow billet is placed on the perforating unit for perforation and tube rolling to form a rough tube, the rough tube is annealed, and the annealing process conditions are set as follows: annealing temperature 780±10℃, temperature ≤400℃ into the furnace, heating rate 150℃ / h, timing starts when the workpiece reaches 770℃, heat preservation for 300 minutes, and air cooling after the workpiece is taken out of the furnace.

[0011] Furthermore, 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 0.9-1.1min / mm in relationship to the wall thickness of the rough tube.

[0012] Furthermore, the rough tube is sizing to produce a rough tube by cold drawing or cold rolling, and the diameter reduction is ≤50mm.

[0013] Further, the rough pipe is heat treated to form a semi-finished pipe, including heat treating 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.

[0014] 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%.

[0015] 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

[0016] Figure 1 This is a process flow chart of hot rolling production of G115 seamless steel pipe in an embodiment of the present invention; 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; 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; 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; 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; 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; 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

[0017] In order to make the purpose, technical scheme and advantages of the present invention clearer, the following will be described in detail with reference to the accompanying drawings. However, it will be appreciated by those skilled in the art that in the various embodiments of the present invention, many technical details are provided in order to enable the reader to better understand the present application. However, even without these technical details and various changes and modifications based on the following embodiments, the technical scheme claimed for protection in the claims of the present application can be implemented.

[0018] like Figure 1 As shown, one embodiment of the present invention relates to a hot rolling production process for a G115 seamless steel pipe, comprising the following steps: Prepare G115 steel hollow billet. According to Chinese patent document CN103045962B, which discloses steel for steam temperature ultra-supercritical thermal power unit and its preparation method, the chemical composition weight fraction of the steel for the hollow billet is: 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%;Nitrol: 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 unavoidable impurity elements. It can be smelted by EAF+AOD+ protective atmosphere ESR or VIM+ protective atmosphere ESR process flow, or by other suitable process flow in the prior art. Among them, EAF (Electric Arc Furnace): a smelting equipment using electric arc heating, mainly used to melt scrap steel or ferroalloys, suitable for steel smelting; AOD (Argon Oxygen Decarburization): equipment for steel smelting, decarburization and refining through a mixed gas of argon and oxygen to reduce carbon content and control composition; ESR (Electroslag Remelting): a refining process that removes impurities and improves material uniformity and density through electroslag remelting, used for the production of high-quality alloy steel and special alloys. VIM (Vacuum Induction Melting): a process of melting metal by induction heating in a vacuum environment to avoid oxidation and gas pollution, suitable for the production of high-purity alloys and special materials. After the steel billet is obtained by smelting, the G115 steel hollow billet is made by hot extrusion, forging and then machining to form a hollow billet, or remelting the steel billet by continuous casting electrode electroslag and then machining to form a hollow billet. The diameter tolerance of the hollow billet is set to ±1%, and the wall thickness tolerance is 10%.

[0019] The hollow billet is hot-rolled and pierced to obtain a rough tube. First, the hollow billet is heated. The heating quality is a key factor in the uniformity of the wall thickness of the rolled tube and the surface quality of the rough tube. The heating equipment can be a 35-meter annular heating furnace. The hot rolling method of the G115 steel hollow billet is round tube billet piercing rolling, and the force mode is three-dimensional internal and external surface shear stress. Since G115 contains 2.5 to 3.5% tungsten and 2.5 to 3.5% cobalt, tungsten and cobalt are heavy atomic metals, and their carbides are difficult to dissolve in the matrix at high temperatures, but are distributed at the grain boundaries. The higher the temperature, the weaker the grain boundaries. When the thermal deformation temperature is higher than 1150°C, the shear rolling stress will cause the grain boundaries to crack, and then the tube will crack. If the heating temperature of the hollow billet is too low, the material's hot plasticity will drop sharply, and the pressing deformation of the hollow billet by the piercing machine will be reduced, resulting in the inability to establish the perforation hole type and the inability to achieve hot rolling perforation. Therefore, the first hot rolling perforation temperature range is set to 1000-1140 degrees, which is much lower than the range of other alloy steel pipes. Considering the rolling force load of the hot rolling unit, the heating temperature is set to 1120℃±20℃. 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 to 0.9~1min / mm. Then, the hollow billet is set on the piercing unit for perforation and tube rolling to make a rough tube. Since G115 steel has a large low-temperature deformation resistance, the final rolling temperature should not be lower than 900℃, the perforation rolling wall thickness deformation ratio is ≥3, and the length extension ratio is ≥3. According to the perforation rolling process requirements, when multiple perforation rolling deformations are required, the heating temperature for the second rolling deformation is 1100℃±20℃. Compared with the process of hot extrusion compression molding of the rough tube at 1200℃ or above in the prior art, the rough tube rolling deformation can be completed at low temperature and high stress, which can 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 surface of both ends of the hollow billet for lubrication. After the rough tube is obtained by perforation rolling, the rough tube is annealed. The equipment used for annealing is a car-bottom LPG furnace, and the fuel used in the heat treatment furnace is natural gas. The annealing process conditions are set as follows: annealing temperature 780±10℃, temperature ≤400℃ into the furnace, heating rate 150℃ / h, start timing when the workpiece reaches 770℃, keep warm for 300 minutes, and air cool out of the furnace.

[0020] The rough tube is sized to produce a rough tube. First, the rough tube is heated, and the heating temperature is controlled at 1100±15℃. The insulation time is set to 0.9-1.1min / mm in relation to the wall thickness of the rough tube. After the insulation is completed, the rough tube is taken out of the furnace and sized. Then, the heated rough tube is sized on a three-roller five-frame sizing machine to produce a rough tube. In one embodiment, according to actual needs, the rough tube can also be sized by cold drawing or cold rolling. To ensure the process quality, the diameter reduction is ≤50mm.

[0021] The rough pipe is heat treated to form a semi-finished pipe. The equipment used for heat treatment is a trolley-type heat treatment furnace, which adopts a heat treatment process combining normalizing and tempering, wherein the normalizing process is set as follows: 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. The tempering process is set as follows: 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. In order to prevent the rough pipe from cracking, the rough pipe is heat straightened after tempering. The heat straightening temperature is 300-750℃. After straightening, the straightness of the rough pipe is ≤3.0mm / m and the total length is ≤0.10%.

[0022] The semi-finished pipe is polished and machined to obtain a finished pipe. The rough pipe is heat treated to form an oxide layer on the inner and outer surfaces of the semi-finished pipe. The surface oxide scale is removed by polishing the inner and outer surfaces to obtain a finished pipe that meets the technical requirements. Surface defects that cannot be eliminated by polishing can be further machined to eliminate the defects.

[0023] Embodiment 1: According to the hot rolling production process of G115 seamless steel pipe provided by the present invention, a G115 seamless steel pipe is produced, and the finished pipe diameter specification is Φ424mm, and the wall thickness specification is 53mm, that is, the finished pipe specification is Φ424×53mm. The specific process includes: producing a G115 steel hollow billet with a diameter smaller than the finished pipe, smelting raw materials according to the composition of G115 steel to obtain a steel billet, extruding the steel billet to form a hollow billet, or forging the steel billet after continuous casting and then machining and hollowing to form a hollow billet, and the hollow billet specification is Φ400×170mm. The allowable deviation of the diameter of the hollow billet is 1%, and the allowable deviation of the wall thickness is 10%. The nominal diameter of the hollow billet is 400mm, the allowable value of the diameter is 396mm~404mm, the nominal wall thickness is 170mm, the allowable value of the wall thickness is 153mm~187mm, the actual measured value of the diameter of the hollow billet is 398.0mm~404.0, the actual measured value of the wall thickness of the hollow billet is 170mm~175mm, and the size of the hollow billet meets the requirements; the hollow billet is hot-rolled and pierced to obtain a rough tube, and the first hot-rolling piercing temperature is controlled at 1000℃-1140℃. Specifically, a ring-shaped heating furnace is used to perform hot-rolling and piercing on the hollow billet Φ400×170mm The hollow billet is heated when the temperature is lower than 800℃, and the heating rate is 100℃ / h. When the heating temperature reaches 1120℃±20℃, the billet is kept warm for 170 minutes. After the end of the warming, the billet is taken out of the furnace and directly perforated and rolled. The first perforation temperature is controlled at 1000℃-1140℃. The perforated and rolled billet includes multiple perforation 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 billet is perforated and rolled to obtain a rough pipe. The specification of the rough pipe is Φ450×57mm, the perforation and rolling wall thickness deformation ratio is ≥3, and the length extension ratio is ≥3. After rolling, the rough pipe is annealed, and the annealing equipment used is a car bottom LPG The heat treatment furnace uses natural gas as the fuel, and the annealing process conditions are set as follows: annealing temperature 780±10℃, temperature ≤400℃ entering the furnace, heating rate 150℃ / h, start timing when the workpiece reaches 770℃, keep warm for 300 minutes, and air cool out of the furnace; the rough tube is sized to produce a rough tube, specifically, the rough tube starts to enter the furnace for heating when it is lower than 800℃, the heating temperature is controlled at 1100±15℃, the holding time is controlled at 0.9-1.1min / mm, preferably, the holding time is 60 minutes, and the rough tube is taken out of the furnace for sizing after the holding is completed, and the heated rough tube is sized on a three-roller five-frame sizing machine to produce a rough tube, and the resulting rough tube specification is Φ428×57.5mm; optionally, the rough tube Φ450×57mm is sized by cold drawing or cold rolling to produce a rough tube, and the rough tube specification is Φ428×57.5mm, and the diameter reduction is ≤50mm.

[0024] The rough pipe Φ428×57.5mm is heat treated to obtain a semi-finished pipe. The equipment used for the heat treatment is a trolley-type heat treatment furnace, and a heat treatment process combining normalizing and tempering is adopted. The normalizing process is set as follows: 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 process is set as follows: tempering temperature 790±10℃, temperature ≤40 The temperature of the raw pipe is 0℃, and the heating rate is set to be below 660℃, 160℃ / h, 660~780℃, 120℃ / h. The timing starts when the workpiece reaches 785℃ and the temperature is kept at this temperature for 240 minutes. To prevent the raw pipe from cracking, the raw pipe is heat-straightened after being tempered out of the furnace. The heat-straightening temperature is 300~750℃. The straightness of the raw pipe after straightening is ≤3.0mm / m, and the total length is ≤0.10%. The semi-finished pipe is polished and machined to obtain 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 Φ424×53mm.

[0025] According to GB / T5310-2023 and T / CSTM00017-2021 standards, the finished tube Φ424×53mm was subjected to ultrasonic, eddy current and magnetic particle inspections. The ultrasonic inspection was carried out according to GB / T5777-2019 standard, the artificial sample damage level was U2, and the sample damage size was 25×0.5×1.5 (groove length×width×depth); the eddy current inspection was carried out according to GB / T7735-2016 standard, the artificial sample damage level was E2, and the sample damage size was 25×0.5×1.5 (groove length×width×depth); the magnetic particle inspection was carried out according to NB / T47013.4-2015 standard, and the results were all qualified; the outer surface quality, outer diameter, wall thickness, length, straightness, PMI, etc. of the finished tube Φ424×53mm were tested, and all met the standard requirements. As shown in Table 1, the dimensional accuracy measurement record of the finished tube Φ424×53mm:

[0026] According to GB / T5310-2023 and T / CSTM00017-2021 standards, the room temperature mechanical properties test of the finished pipe Φ424×53mm is carried out. The room temperature mechanical properties test specimens are transverse and longitudinal round bar specimens with a diameter of 10mm and a calibration distance of 50mm. The two ends of the specimen along the axial direction are marked as end A and end B. The test is carried out according to the standard, as shown in Table 2, the room temperature mechanical properties test record of the finished pipe Φ424×53mm:

[0027] The room temperature tensile properties, impact and hardness test results show that the room temperature properties of the sample tube are good, the mechanical properties, impact and hardness properties are relatively uniform, and meet the requirements of relevant standards.

[0028] According to GBT5310-2023 and T / CSTM00017-2021 standards, the finished pipe Φ424×53mm was sampled at both ends of the steel pipe at a test temperature of 630℃ and 650℃ for high-temperature short-time tensile test. The sample diameter was 10mm and the calibration distance was 50mm. The two ends of the sample along the axial direction were marked as end A and end B. The test results are shown in Table 3. High-temperature tensile performance test record of finished pipe Φ424×53mm:

[0029] It can be seen from the above that the yield strength of the G115 large-diameter finished pipe provided by the present invention at the test temperatures of 630°C and 650°C meets the relevant provisions of GBT5310-2023 and T / CSTM00017-2021, and the high-temperature mechanical properties in the longitudinal and transverse directions at the same temperature are basically the same, and the strength value along the wall thickness direction is relatively uniform with a large margin.

[0030] Samples were taken from the outer surface, the middle of the wall thickness and the inner surface of the finished tube Φ424×53mm for metallographic structure and grain size inspection. Figure 2 The figure shows the metallographic structure of the finished tube Φ424×53mm near the outer surface magnified 100 times. The metallographic structure is tempered martensite with a grain size of 0. Figure 3 This is a 100-fold magnified metallographic structure view of the middle of the wall thickness of the finished tube Φ424×53mm. The metallographic structure shows tempered martensite with a grain size of 0; Figure 4 This is a 100-fold magnified metallographic structure view of the inner surface of the finished tube Φ424×53mm. The metallographic structure shows tempered martensite with a grain size of grade 0, which meets the requirements of GBT5310-2023 and T / CSTM 00017-2021.

[0031] According to GBT5310-2023 (seamless steel tube for high-pressure boiler) and T / CSTM00017-2021 (martensitic heat-resistant steel 08Cr9W3Co3VNbCuBN seamless steel tube for power station), the non-metallic inclusions of steel tubes were rated by method A in GB / T10561 (microscopic inspection method for standard rating chart for determination of non-metallic inclusion content in steel), and the non-metallic inclusions of finished tubes Φ424×53mm were rated, as shown in Table 4.

[0032] From the above, it can be seen that the non-metallic inclusion content of the finished pipe Φ424×53mm meets the requirements of GBT5310-2023 and T / CSTM00017-2021.

[0033] 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 Φ424×53mm, the hollow billet Φ400×170mm is perforated and rolled to obtain a rough pipe Φ450×57mm, and then the rough pipe is sizing to make a rough pipe Φ428×57.5mm. The rough pipe is ground and machined to successfully obtain a finished pipe Φ424×53mm. 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 standards of martensitic heat-resistant steel G115 seamless steel pipe for power stations.

[0034] Embodiment 2: According to the hot rolling production process of G115 seamless steel pipe provided by the present invention, a G115 seamless steel pipe is produced, and the finished pipe diameter specification is Φ978mm, and the wall thickness specification is 38mm, that is, the finished pipe specification is Φ978×38mm. The specific process includes producing a G115 steel hollow billet with a diameter smaller than the finished pipe, and the method for producing the G115 steel hollow billet includes: smelting raw materials according to the composition of G115 steel to obtain a steel billet, using a continuous casting electrode for the steel billet, and machining and punching holes after electroslag remelting to produce a hollow billet, and the hollow billet specification is Φ700×200mm. The size of the hollow billet meets the requirements; the hollow billet is pierced and rolled to produce a rough tube. Specifically, the hollow billet Φ700×200mm is heated in an annular heating furnace. When the hollow billet is lower than 800°C, the furnace is started to heat, the heating rate is 100°C / h, and the heating temperature reaches 1120°C±20°C, and the temperature is kept for 200 The pipe is directly perforated and rolled after the heat preservation is finished. The specification of the first perforated and rolled pipe is Φ710×125mm. The specification of the second perforated and rolled pipe is Φ870×60mm after heating to 1100℃. The specification of the final perforated and rolled pipe is Φ998×42mm. The final rolling temperature is controlled at 900℃-1100℃. Preferably, the final rolling temperature is 1100℃. The specification of the rough pipe obtained by perforating and rolling is Φ998×42mm. The wall thickness deformation ratio of perforating and rolling is ≥3, and the length extension ratio is ≥3. After rolling, the rough pipe is annealed. The equipment used for annealing is a car-bottom LPG furnace. The fuel used by the heat treatment furnace is natural gas. The annealing process conditions are set as follows: annealing temperature 780±10℃, temperature ≤400℃ into the furnace, heating rate 150℃ / h, timing starts when the workpiece reaches 770℃, and heat preservation 300 minutes, and then take it out of the furnace for air cooling; the rough tube is sized to produce a rough tube. Specifically, when the rough tube is lower than 800°C, it starts to be heated in the furnace, the heating temperature is controlled at 1100±15°C, and the insulation time is controlled at 0.9-1.1min / mm. Preferably, the insulation time is 40 minutes. After the insulation is completed, the rough tube is taken out of the furnace for sizing, and the heated rough tube is sized on a three-roller five-frame sizing machine to produce a rough tube. The obtained rough tube specification is Φ982×42mm; optionally, the rough tube Φ998×42mm is sized by cold drawing or cold rolling to produce a rough tube. The rough tube specification is Φ982×42mm, and the diameter reduction is ≤50mm.

[0035] The rough pipe is heat treated to obtain the semi-finished pipe. The equipment used for heat treatment is a trolley heat treatment furnace, and a heat treatment process combining normalizing and tempering is adopted. Among them, the normalizing process is set as follows: 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 process is set as follows: tempering temperature 790±10℃, temperature ≤400℃ into the furnace, heating The heat rate is set to be below 660℃, 160℃ / h, 660~780℃, 120℃ / h, start timing when the workpiece reaches 785℃, and keep warm for 240 minutes; to prevent the rough pipe from cracking, the rough pipe is heat-straightened after tempering, and the heat-straightening temperature is 300~750℃. The straightness of the rough pipe after straightening is ≤3.0mm / m, and the total length is ≤0.10%; the semi-finished pipe is polished and machined to obtain the finished pipe, and 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.

[0036] According to GB / T5310-2023 and T / CSTM00017-2021 standards, the finished tube Φ978×38mm was subjected to ultrasonic, eddy current and magnetic particle inspections. The ultrasonic inspection was carried out according to GB / T5777-2019 standard, the artificial sample damage level was U2, and the sample damage size was 25×0.5×1.5 (groove length×width×depth); the eddy current inspection was carried out according to GB / T7735-2016 standard, the artificial sample damage level was E2, and the sample damage size was 25×0.5×1.5 (groove length×width×depth); the magnetic particle inspection was carried out according to NB / T47013.4-2015 standard, and the results were all qualified; the outer surface quality, outer diameter, wall thickness, length, straightness, PMI, etc. of the finished tube Φ978×38mm were tested, and all met the standard requirements. As shown in Table 5, the dimensional accuracy measurement record of the finished tube Φ978×38mm:

[0037] According to GB / T5310-2023 and T / CSTM00017-2021 standards, the room temperature mechanical properties test of the finished pipe Φ978×38mm is carried out. The room temperature mechanical properties test specimens are transverse and longitudinal round bar specimens with a diameter of 10mm and a calibration distance of 50mm. The two ends of the specimen along the axial direction are marked as end A and end B. The test is carried out according to the standard, as shown in Table 6, the room temperature mechanical properties test record of the finished pipe Φ978×38mm:

[0038] The room temperature tensile properties, impact and hardness test results show that the room temperature properties of the sample tube are good, the mechanical properties, impact and hardness properties are relatively uniform, and meet the requirements of relevant standards.

[0039] According to GBT5310-2023 and T / CSTM00017-2021 standards, the finished pipe Φ978×38mm is sampled at both ends of the steel pipe at a test temperature of 630℃ and 650℃ for high-temperature short-time tensile test. The sample diameter is 10mm and the calibration distance is 50mm. The two ends of the sample along the axial direction are marked as A end and B end. The test results are shown in Table 7. High-temperature tensile performance test record table of finished pipe Φ978×38mm:

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

[0041] Samples were taken from the outer surface, the middle of the wall thickness and the inner surface of the finished tube Φ978×38mm for metallographic structure and grain size inspection. Figure 5 The figure shows the metallographic structure of the finished tube Φ978×38mm near the outer surface magnified 100 times. The metallographic structure is tempered martensite with a grain size of 4. Figure 6 This is a 100-fold magnified metallographic structure view of the middle of the wall thickness of the finished tube Φ978×38mm. The metallographic structure shows tempered martensite with a grain size of level 4; Figure 7 This is a 100-fold magnified metallographic structure view of the inner surface of the finished tube Φ978×38mm. The metallographic structure shows tempered martensite with a grain size of level 4, which meets the requirements of GBT5310-2023 and T / CSTM00017-2021.

[0042] According to GBT5310-2023 and T / CSTM00017-2021 standards, the non-metallic inclusions of steel pipes are rated using method A in GB / T10561, and the non-metallic inclusions of finished pipes Φ978×38mm are rated, as shown in Table 8. From the above, it can be seen that the non-metallic inclusion content of the finished pipe Φ978×38mm meets the requirements of GBT5310-2023 and T / CSTM00017-2021.

[0043] 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.

[0044] 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:

[0045] 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.

[0046] 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.

[0047] Those skilled in the art will appreciate that the above-mentioned embodiments are specific examples for implementing the present invention, and in actual applications, various changes may be made thereto in form and detail 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 hot rolling and piercing to obtain a rough tube, and the first hot rolling and piercing temperature is controlled at 1000° C.-1140° C.; 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; placing the hollow billet on a piercing unit for piercing and rolling to form a rough tube, wherein the temperature of the final rolling deformation is not less than 900°C.

4. The hot rolling production process for G115 seamless steel pipe according to claim 3, characterized in that: The hollow billet is placed on the perforating unit for perforating and rolling to form a rough tube, which includes multiple perforating and rolling. The heating temperature of the intermediate rolling deformation is 1100℃±20℃, the perforating and rolling wall thickness deformation ratio is ≥3, and the length extension ratio is ≥3.

5. The hot rolling production process for G115 seamless steel pipe according to claim 3 or 4, characterized in that: After the hollow billet is placed on the perforating unit for perforation and tube rolling to form a rough tube, the rough tube is annealed. The annealing process conditions are set as follows: annealing temperature 780±10℃, temperature ≤400℃ into the furnace, heating rate 150℃ / h, start timing when the workpiece reaches 770℃, keep warm for 300 minutes, and air cool out of the furnace.

6. 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.

7. 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.

8. The hot rolling production process for G115 seamless steel pipe according to claim 6 or 7, 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.

9. The hot rolling production process for G115 seamless steel pipe according to claim 8, 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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