Manufacturing method of martensite heat-resistant steel small-caliber thick-wall pipe

Abstract

The invention discloses a manufacturing method of a martensitic heat-resistant steel small-caliber thick-wall pipe, which specifically comprises the following steps of: (1) preparing round steel of martensitic heat-resistant steel G115, peeling, cutting off, grinding and inspecting a pipe blank, performing hot piercing, and tempering, pickling, cutting the pipe, grinding, inspecting and cleaning a pierced billet after piercing; (2) carrying out cold rolling of a deformation process on the pierced billet obtained in the step (1), and then carrying out heat treatment to obtain a finished product specification; and (3) performing visual inspection, size inspection, cleaning, mark spraying and finishing procedures on the finished pipe obtained in the step (2), and finally packaging and warehousing. The technological process provided by the invention is standardized, simple to operate and suitable for batch production, meanwhile, good balance is achieved between product performance indexes and cost control, and technical support is provided for application of the G115 steel in the small-caliber thick-wall pipe.

Description

Technical Field

[0001] The invention relates to a method for manufacturing a seamless pipe, in particular to a method for manufacturing a small-diameter thick-walled pipe made of martensitic heat-resistant steel, and belongs to the technical field of material processing. Background Art

[0002] G115 steel is a new generation of martensitic heat-resistant steel, specially developed for ultra supercritical (USC) thermal power units, aiming to meet the extreme requirements for material performance in high temperature and high pressure steam environments. With the growth of global energy demand and the intensification of environmental pressure, thermal power units have increased steam temperature and pressure, which has become a key means to improve coal-fired efficiency, reduce coal consumption and reduce carbon emissions. Against this background, G115 steel came into being.

[0003] G115 steel is based on P92 steel used in 600℃ USC units, and has achieved significant performance improvements by adjusting and optimizing the chemical composition. Its development goal is to provide higher creep endurance strength and oxidation resistance under high temperature conditions of 650℃ and above, while maintaining good economy and processing performance. Compared with nickel-based alloys and austenitic heat-resistant steels, G115 steel has low cost, small thermal expansion coefficient, and high thermal stability, and is more suitable for large boiler pipes and pressure parts of ultra-supercritical units.

[0004] G115 steel adopts a multi-element composite strengthening and selective strengthening strategy. On the basis of P92 steel, key alloying elements such as W, Co, Cu and B are introduced: tungsten (W) replaces part of molybdenum (Mo), enhances the solid solution strengthening ability, and inhibits the coarsening of M23C6 carbides, thereby improving creep strength; the addition of cobalt (Co) inhibits the formation of δ-ferrite, prevents brittle fracture, and improves the comprehensive strength of the material; copper (Cu) is to form a nano-scale Cu-rich phase, which significantly improves the dispersion strengthening effect and enhances the organizational stability; boron (B) precipitates M23C6 carbides at the grain boundaries, thereby improving the grain boundary strength and inhibiting the coarsening of carbides.

[0005] At present, the technology for manufacturing large-diameter thick-walled tubes of G115 is relatively mature. Patent CN114635023B "A method for producing martensitic heat-resistant steel billets" only studies the production of G115 billets, focusing on the heat treatment process of the billets, and the quality and mechanical properties of the heat-resistant steel need to be improved; Patent CN115074504B "Manufacturing method of G115 large-diameter thick-walled seamless steel pipes for 630℃ ultra-supercritical units" is to extrude G115 billets to obtain seamless steel pipes, and then heat treat them to obtain finished products. It does not involve cold deformation and rolling processes, and can only manufacture large-diameter thick-walled seamless steel pipes. The surface quality of the finished product is far inferior to that of seamless steel pipes after cold rolling.

[0006] G115 has high strength and hardness due to the addition of strengthening elements such as W, Co and Cu, and has great resistance during plastic deformation. Especially in the process of manufacturing small-diameter thick-walled tubes, metal deformation is more restricted, which puts higher requirements on rolling and extrusion equipment. At the same time, small-diameter thick-walled tubes have extremely high requirements for dimensional tolerance and wall thickness uniformity. Slight deviations in the manufacturing process may lead to stress concentration, thereby affecting creep strength and service life. The high-temperature performance of G115 depends on the stability of its tempered martensite and precipitate phases (such as M23C6 and Cu-rich phases), while the temperature uniformity of the inner and outer layers of thick-walled tubes is difficult to control during heat treatment. Long-term heat preservation is required to ensure the uniformity of the organization, but too long a time may cause the precipitate phase to coarsen and reduce the material properties.

[0007] Based on this, developing a production process for G115 small-diameter thick-walled pipes that can overcome the above-mentioned problems and developing G115 small-diameter thick-walled finished pipes whose chemical composition, mechanical properties, metallographic structure, corrosion resistance, etc. can meet the technical requirements have become technical problems that need to be urgently solved by technical personnel in this field. Summary of the invention

[0008] The technical problem to be solved by the present invention is to overcome the shortcomings of the prior art and provide a method for manufacturing a small-diameter thick-walled tube of martensitic heat-resistant steel. The manufacturing method solves the problems of difficulty in manufacturing small-diameter thick-walled tubes of G115, poor quality of finished products, and low yield rate by strictly limiting process optimization. The products produced by this method have high dimensional accuracy, stable organizational properties, and are conducive to mass production.

[0009] In order to solve the above technical problems, the present invention provides a method for manufacturing a small-diameter thick-walled martensitic heat-resistant steel tube, which specifically comprises the following steps:

[0010] (1) Prepare round steel of martensitic heat-resistant steel G115, peel, cut, grind and inspect the tube blank, and then perform hot perforation. After perforation, the rough tube is tempered, pickled, cut, grind, inspected and cleaned;

[0011] (ii) cold rolling the rough pipe obtained in step (i) through a deformation process, and then heat treating it to the finished product specifications, specifically:

[0012] The rough pipe is cold rolled twice on the cold rolling unit. After each cold rolling, it is degreased, heat treated, straightened, cut, pickled, and polished. Then, it is ultrasonically, hydraulically, sampled, physically and chemically tested, and finally cut to obtain the finished pipe.

[0013] After the first cold rolling, the outer diameter deviation is controlled within ±0.3mm, and the wall thickness deviation is controlled within ±0.3mm;

[0014] After the second cold rolling, the outer diameter deviation is controlled within ±0.2mm, and the wall thickness deviation is controlled within ±0.3mm;

[0015] The first heat treatment after cold rolling is tempering heat treatment, the heat treatment temperature is 760-800℃, the holding time is 0.5-2h, and the steel is air-cooled to room temperature after being taken out of the furnace;

[0016] The heat treatment after the second cold rolling is normalizing and tempering. The heat treatment temperature for normalizing is 1030-1140℃, the holding time is 0.5-1h, and the steel is air-cooled to room temperature after being taken out of the furnace. The heat treatment temperature for tempering is 760-800℃, the holding time is 0.5-2h, and the steel is air-cooled to room temperature after being taken out of the furnace.

[0017] (iii) The finished tubes obtained in step (ii) are subjected to visual inspection, dimensional inspection, cleaning, labeling and finishing processes, and finally packaged and stored.

[0018] The technical solution further defined in the present invention is:

[0019] Furthermore, in the aforementioned method for manufacturing a small-diameter thick-walled martensitic heat-resistant steel tube, the insulation temperature during hot perforation in step (i) is 1060-1150° C., and the insulation time is 2-3 hours.

[0020] In the aforementioned method for manufacturing a small-diameter thick-walled martensitic heat-resistant steel tube, the heat treatment temperature during the tempering heat treatment in step (i) is 760-800°C, the holding time is 0.5-2h, and the tube is air-cooled to room temperature after being taken out of the furnace.

[0021] In the aforementioned method for manufacturing a small-diameter thick-walled martensitic heat-resistant steel tube, the heat treatment in step (ii) is carried out in a box-type furnace, and the atmosphere in the furnace is an oxidizing atmosphere.

[0022] In the aforementioned method for manufacturing a small-diameter thick-walled martensitic heat-resistant steel tube, the room temperature mechanical properties of the small-diameter thick-walled tube are controlled to be: yield strength ≥480 MPa, tensile strength ≥660 MPa, elongation A ≥20%, impact absorption energy KV2 ≥40 J, and hardness HBW between 195-250;

[0023] The high temperature tensile properties of the small-diameter thick-walled tube are as follows: the yield strength is ≥316MPa at 600°C, the yield strength is ≥271MPa at 630°C, the yield strength is ≥244MPa at 650°C, and the yield strength is ≥182MPa at 680°C.

[0024] In the aforementioned method for manufacturing a small-diameter thick-walled martensitic heat-resistant steel tube, the small-diameter thick-walled tube manufactured by the method has an outer diameter of 10-100 mm and a wall thickness of 3-25 mm; the ratio of the outer diameter to the wall thickness is less than 10.

[0025] The beneficial effects of the present invention are:

[0026] The present invention is based on the rough pipe obtained by hot perforation, and finally obtains the G115 martensitic heat-resistant steel small-diameter thick-walled pipe after two rolling passes. Tempering heat treatment is required before each rolling pass. This is because the cold working hardening caused by the high strength of G115 during the rolling process may cause the seamless pipe to break or the rolling mill to be damaged during the rolling process. Therefore, tempering heat treatment is required before each rolling pass. The addition of pickling and grinding processes before rolling ensures the size and surface quality of the seamless pipe after rolling, and can also significantly improve the quality of the final product. Normalizing + tempering heat treatment is used for the finished product heat treatment. Normalizing heat treatment The treatment refines the grains through high-temperature austenitization, homogenizes the internal structure of the material, provides an ideal initial organizational basis for the finished product, improves the mechanical properties and dimensional stability of the material, and then regulates the organization through tempering to promote the reasonable distribution of precipitated phases (such as M23C6 carbides and Cu-rich phases), enhance high-temperature creep performance, oxidation resistance and fatigue resistance, so as to meet the long-term service requirements of small-diameter thick-walled pipes in the high temperature and high pressure environment of ultra-supercritical units. Further tempering after normalizing can balance the strength and toughness of the material, avoid the contradiction between high strength and brittleness resistance that a single heat treatment process cannot take into account, and ensure that the final product performance reaches the best.

[0027] The present invention manufactures small-diameter thick-walled tubes, and the number of rolling passes cannot be too many (an increase in the number of passes will significantly increase the cold working hardening degree of G115, increase equipment loss, reduce efficiency and increase costs). Therefore, the size of the rough tube obtained during hot perforation should be as close to the finished product size as possible, and rough tubes with too large outer diameters and wall thicknesses cannot be manufactured. Therefore, the insulation temperature and time during hot perforation must be strictly controlled to ensure the qualified rate of the rough tubes.

[0028] The present invention ensures that G115 can be cold-rolled without defects and damage to the machine through a reasonable heat treatment system, including normalizing and tempering heat treatment; at the same time, the reasonable heat treatment system ensures the organizational stability of G115. A box-type furnace is used for uniform heating under an oxidizing atmosphere. The temperature field inside the box-type furnace is uniform, which can effectively reduce the temperature difference between the inner and outer walls of the thick-walled tube, thereby avoiding the problems of organizational coarsening and inconsistent performance caused by local overheating or uneven cooling, thereby ensuring the overall organizational uniformity of the material and improving the mechanical properties and dimensional stability; for G115 steel, a material that has extremely high requirements for organizational stability, the uniform heating characteristics of the box-type furnace are particularly important. At the same time, the heat treatment of the G115 thick-walled tube requires a long insulation time to ensure the uniformity of the organization. The box-type furnace can provide stable and long-term heating conditions to meet the sufficient insulation time required for the heat treatment of the thick-walled tube, effectively release residual stress, and avoid the internal stress concentration and crack risks that may be caused by uneven cooling, rapid heating and cooling.

[0029] The present invention adopts hot perforation technology combined with two-pass cold rolling process, so that in the manufacturing process of G115 small-diameter thick-walled tubes, the problem of excessive forming difficulty is avoided, the yield rate of finished tubes is guaranteed, and the production cost is reduced.

[0030] Through process optimization, the G115 small-diameter thick-walled tubes produced have stable performance and meet the stringent requirements of ultra-supercritical units for high-temperature and high-pressure components. Its room temperature and high-temperature tensile properties, impact absorption energy and hardness all meet or even exceed industry standards, providing a guarantee for the long-term safe service of boiler pipes and key pressure-bearing components.

[0031] The process provided by the present invention is standardized and easy to operate, suitable for mass production, and achieves a good balance between product performance indicators and cost control, providing technical support for the application of G115 steel in small-diameter thick-walled pipes. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 The present invention is a manufacturing flow chart of a method for manufacturing a small-diameter, thick-walled martensitic heat-resistant steel tube according to an embodiment of the present invention;

[0033] Figure 2 This is a 100X metallographic structure diagram of a G115 finished tube manufactured by the method for manufacturing a small-diameter thick-walled martensitic heat-resistant steel tube according to an embodiment of the present invention;

[0034] Figure 3 This is a metallographic structure diagram of a G115 finished tube 500X manufactured by the method for manufacturing a small-diameter thick-walled martensitic heat-resistant steel tube according to an embodiment of the present invention. DETAILED DESCRIPTION

[0035] The present invention will be described clearly and completely below in conjunction with specific embodiments. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0036] Example 1

[0037] This embodiment provides a method for manufacturing a small-diameter thick-walled martensitic heat-resistant steel tube, the flow chart of which is as follows: Figure 1 As shown, the specific steps include:

[0038] (I) Prepare Φ110mm martensitic heat-resistant steel G115 round steel, peel, cut, grind and inspect the tube blank, and then put it into the inclined bottom heating furnace for hot perforation. After perforation, the rough tube is tempered, pickled, cut, grinded, inspected and cleaned. Specifically:

[0039] The Φ110mm round steel is peeled 1mm on one side to remove the surface oxide scale and defects. After the round steel is cut according to the finished product size requirements, the surface defects of the tube billet are thoroughly ground. The ground tube billet is placed in an inclined bottom heating furnace for heating, and the temperature is controlled at 1060-1150℃ and kept warm for 2-3 hours to ensure uniform internal temperature. Then the heated tube billet is hot-perforated to obtain a rough tube with a specification of Φ114×18mm. The rough tube is placed in a roller hearth furnace for heat treatment. The treatment temperature is set at 760-800℃ and the holding time is 0.5-2 hours. After the heat treatment is completed, it is taken out of the furnace and slowly cooled to room temperature in the air, and pickling, tube cutting, grinding, inspection and cleaning are carried out in sequence to ensure that the quality of the tube billet meets the subsequent process requirements;

[0040] (ii) cold rolling the rough pipe obtained in step (i) through a deformation process, and then heat treating it to the finished product specifications, specifically:

[0041] The Φ114×18mm G115 rough pipe obtained after tempering heat treatment is rolled in two passes on the LG60 cold rolling unit, and the rolling sizes are Φ76×16mm and Φ54×13.5mm respectively. The specific cold rolling parameters are shown in Table 1, and the dimensional data during cold rolling are shown in Table 2. After each cold rolling pass, degreasing, heat treatment, straightening, pipe cutting, pickling, grinding, and then ultrasonic, water pressure, sampling, physical and chemical inspection, and final cutting are performed to obtain the finished pipe;

[0042] Due to the high strength of G115 steel, cold working hardening is prone to occur during the cold rolling process, which will significantly increase the difficulty of rolling and may even cause damage to the rolling equipment or cracks in the pipe. Therefore, tempering heat treatment must be carried out before rolling. The internal stress of the material can be released through appropriate heating and holding time, and its tempered martensite structure is optimized to improve plasticity. The tempering heat treatment temperature is set at 760-800°C, the holding time is 0.5-2 hours, and it is air-cooled to room temperature after being taken out of the furnace; in addition, the deformation amount needs to be strictly controlled during the rolling process. The deformation amount of the two passes is basically the same and not more than 50%. Step-by-step rolling is used to avoid excessive deformation stress from having an adverse effect on material properties and product quality, thereby ensuring the dimensional accuracy and organizational stability of the final product;

[0043] Table 1 Cold rolling parameters

[0044] Billet specifications (mm) Rolling specification (mm) Elongation coefficient Deformation Φ114×18mm Φ76×16mm 1.8 44.44％ Φ76×16mm Φ54×13.5mm 1.76 43.05％

[0045] Table 2 Technical requirements dimensional accuracy and actual measured dimensions

[0046]

[0047] The heat treatment of intermediate products and finished products is completed in a box furnace, and the atmosphere in the furnace is set to an oxidizing environment. The intermediate product is subjected to tempering heat treatment, and the treatment temperature is controlled at 760-800°C, the holding time is 0.5-2 hours, and it is naturally air-cooled to room temperature after heat treatment. The finished product is subjected to normalizing + tempering heat treatment. The holding temperature of the normalizing stage is set at 1030-1140°C, the holding time is 0.5-1 hour, and it is also slowly cooled to room temperature in the air after treatment. The tempering heat treatment process is the same as the tempering heat treatment process of the intermediate product. This process ensures that G115 can be smoothly rolled and deformed, while ensuring its organizational uniformity and performance stability in the finished product stage, providing reliable guarantee for subsequent use; the physical and chemical properties of the martensitic heat-resistant steel thick-walled tube obtained in the end meet the technical requirements, as shown in Tables 3 to 6;

[0048] Table 3 Mechanical properties at room temperature

[0049]

[0050] Table 4 High temperature tensile test

[0051]

[0052] Table 5 Chemical composition test results (%)

[0053]

[0054] The chemical composition of the small-diameter thick-walled tube material is partially tested as shown in Table 5. The material needs to meet the corresponding chemical composition requirements because the chemical composition directly determines the mechanical properties, high temperature resistance, oxidation resistance, corrosion resistance and processing performance of the material. The appropriate element ratio can optimize the strength, toughness and durability of the material, ensuring its long-term stability under extreme environments such as high pressure and high temperature.

[0055] Table 6 Non-metallic inclusions

[0056]

[0057] The metallographic appearance of the finished steel pipe is as follows: Figure 2 , 3 As shown, the microstructure is tempered martensite, in which high temperature ferrite (delta ferrite) and free ferrite do not exist, and the finished steel pipe meets the grain size of not less than level 4.

[0058] Finished steel pipes need to undergo flattening tests:

[0059] First, the ductility test is carried out, and the test is pressed to a distance of H between two and a half bottles, and H is calculated according to formula (1). In the formula, H is the distance between the two plates in millimeters (mm); S is the wall thickness of the steel pipe in millimeters (mm); and D is the outer diameter of the steel pipe in millimeters (mm).

[0060]

[0061] When the test pressure reaches the distance H between the two plates, there are no cracks or splits on the specimen.

[0062] Then the two walls of the sample were brought into contact with each other. During the entire flattening test, no visible delamination, white spots or inclusions appeared on the sample.

[0063] Non-destructive testing was carried out by ultrasonic testing on the entire length of each pipe in accordance with the provisions of GB / T 31925-2015, and the results were qualified.

[0064] The water pressure test pressure is calculated according to formula (1), and the maximum test pressure is 20MPa. In the formula, P is the test pressure in MPa. When P<7MPa, it is rounded to the nearest 0.5MPa, and when P≥7MPa, it is rounded to the nearest 1MPa. S is the wall thickness of the steel pipe in millimeters (mm); D is the outer diameter of the steel pipe in millimeters (mm); R is the allowable stress, which is 80% of the specified yield strength, in megapascals (MPa). Under the calculated test pressure, the pressure was maintained for 10 seconds, and no leakage occurred in the steel pipe.

[0065] P=2SR / D (2)

[0066] (iii) The finished tubes obtained in step (ii) are subjected to visual inspection, dimensional inspection, cleaning, labeling and finishing processes, and finally packaged and stored.

[0067] The method of the present invention solves the technical difficulties existing in the process of manufacturing small-diameter thick-walled G115 pipes by regulating the heat treatment parameters and deformation process: by performing tempering heat treatment before each rolling, the cold working hardening problem caused by the high strength of the material is effectively alleviated, thereby ensuring the stability of the rolling process and the safety of the equipment. At the same time, the two-pass cold rolling process is adopted, combined with strict dimensional control, which significantly improves the dimensional accuracy and wall thickness uniformity. In addition, by performing uniform heat treatment in a box furnace, the internal organizational stability of the pipe is optimized, and the performance degradation problem caused by the coarsening of the precipitated phase is avoided. Combined with the pickling and grinding process, the present invention also significantly improves the surface quality of the pipe, solves the problem of surface defects in the traditional process, and greatly improves the yield rate. The overall process realizes the high-quality manufacturing of small-diameter thick-walled G115 pipes.

[0068] In addition to the above embodiments, the present invention may also have other implementation modes. Any technical solution formed by equivalent replacement or equivalent transformation falls within the protection scope required by the present invention.

Claims

1. A method for manufacturing a small-diameter thick-walled martensitic heat-resistant steel tube, characterized in that: The specific steps include: (I) Prepare round steel of martensitic heat-resistant steel G115, peel, cut, grind and inspect the tube blank, and then perform hot piercing. After piercing, the rough tube is tempered, pickled, cut, grinded, inspected and cleaned; (ii) cold rolling the rough pipe obtained in step (i) through a deformation process, and then heat treating it to the finished product specifications, specifically: The rough pipe is cold rolled twice on the cold rolling unit. After each cold rolling, it is degreased, heat treated, straightened, cut, pickled, and polished. Then, it is ultrasonically, hydraulically, sampled, physically and chemically tested, and finally cut to obtain the finished pipe. After the first cold rolling, the outer diameter deviation is controlled within ±0.3mm, and the wall thickness deviation is controlled within ±0.3mm; After the second cold rolling, the outer diameter deviation is controlled within ±0.2mm, and the wall thickness deviation is controlled within ±0.3mm; The first heat treatment after cold rolling is tempering heat treatment, the heat treatment temperature is 760-800℃, the holding time is 0.5-2h, and the steel is air-cooled to room temperature after being taken out of the furnace; The heat treatment after the second cold rolling is normalizing and tempering. The heat treatment temperature for normalizing is 1030-1140℃, the holding time is 0.5-1h, and the steel is air-cooled to room temperature after being taken out of the furnace. The heat treatment temperature for tempering is 760-800℃, the holding time is 0.5-2h, and the steel is air-cooled to room temperature after being taken out of the furnace. (iii) The finished tubes obtained in step (ii) are subjected to visual inspection, dimensional inspection, cleaning, labeling and finishing processes, and finally packaged and stored.

2. The method for manufacturing a small-diameter thick-walled martensitic heat-resistant steel tube according to claim 1, characterized in that: In the step (i) of heat perforation, the insulation temperature is 1060-1150° C. and the insulation time is 2-3 hours.

3. The method for manufacturing a small-diameter thick-walled martensitic heat-resistant steel tube according to claim 1, characterized in that: During the tempering heat treatment in step (i), the heat treatment temperature is 760-800°C, the heat preservation time is 0.5-2h, and the product is air-cooled to room temperature after being taken out of the furnace.

4. The method for manufacturing a small-diameter thick-walled martensitic heat-resistant steel tube according to claim 1, characterized in that: The heat treatment in step (ii) is carried out in a box-type furnace, and the atmosphere in the furnace is an oxidizing atmosphere.

5. The method for manufacturing a small-diameter thick-walled martensitic heat-resistant steel tube according to claim 1, characterized in that: The room temperature mechanical properties of the small-diameter thick-walled tube are controlled as follows: yield strength ≥480Mpa, tensile strength ≥660MPa, elongation A ≥20%, impact absorption energy KV2 ≥40J, and hardness HBW between 195-250; The high temperature tensile properties of the small-diameter thick-walled tube are as follows: the yield strength is ≥316MPa at 600°C, the yield strength is ≥271MPa at 630°C, the yield strength is ≥244MPa at 650°C, and the yield strength is ≥182MPa at 680°C.

6. The method for manufacturing a small-diameter thick-walled martensitic heat-resistant steel tube according to claim 1, characterized in that: The small-diameter thick-walled tube manufactured by the manufacturing method has an outer diameter of 10-100 mm and a wall thickness of 3-25 mm; the ratio of the outer diameter to the wall thickness is less than 10.

Images