A method for manufacturing a high-chromium heat-resistant seamless pipe

Abstract

The present application relates to the field of seamless pipe preparation, and particularly relates to a high-chromium heat-resistant steel seamless pipe preparation method, which uses a protective slag heat preservation technology and a molten steel ladle heat preservation technology to preserve the high-chromium heat-resistant steel liquid tundish, adopts a three-section combined electromagnetic stirring technology with interval distribution in the high-chromium heat-resistant steel liquid pouring process, ensures that the produced large-diameter high-chromium heat-resistant steel continuous casting billet has stable composition, small alloy element segregation, and defects and segregation are concentrated in the center, greatly improves the as-cast defects, removes the center defects of the continuous casting billet through the centering through hole, provides favorable biting conditions for subsequent cross piercing and reduces the rolling deformation stress of the continuous casting billet, greatly improves the size precision, surface quality and yield of the large-diameter high-chromium heat-resistant steel seamless pipe, realizes the direct production of the seamless pipe by replacing the die casting forging billet with the continuous casting billet, reduces the forging process of the billet, shortens the pipe preparation process, improves the preparation efficiency of the high-chromium heat-resistant steel seamless pipe, and reduces the cost.

Description

Technical Field

[0001] This invention belongs to the field of seamless steel pipe manufacturing, and particularly relates to the manufacturing of a high-chromium heat-resistant steel seamless pipe. Background Technology

[0002] Currently, coal-fired power generation accounts for nearly 80% of my country's main energy power generation. The pressure to reach carbon peak has led to the development of coal-fired power plants towards more efficient ultra-supercritical units. Ultra-supercritical coal-fired power generation technology can improve power supply efficiency, reduce coal consumption by nearly 70 grams per kilowatt-hour, and reduce carbon dioxide emissions by 14%, which is the development path of clean coal-fired power generation technology.

[0003] The demand for heat-resistant steel seamless pipes in ultra-supercritical units is further increasing. Large-diameter high-chromium heat-resistant steel seamless pipes have broad application prospects in the fields of main steam pipelines, reheat steam pipelines and high-temperature headers of ultra-supercritical thermal power units, as well as petrochemical pipelines and fittings.

[0004] Due to the high alloy content of high-chromium heat-resistant steel, it has serious defects in the as-cast structure, high resistance to high-temperature deformation, and is prone to cracking during piercing and rolling. In order to obtain qualified seamless tubes, the existing technology uses die casting and forging machining to produce small batches of seamless tubes from high-chromium heat-resistant steel billets. This process is lengthy, results in high material loss, and is costly.

[0005] Replacing die-cast forging billets with continuously cast billets to directly produce seamless pipes can reduce the forging process of cast billets, shorten the pipe manufacturing process, improve the production efficiency of heat-resistant steel seamless pipes, and reduce costs.

[0006] Existing tube billet continuous casting processes are constrained by technical bottlenecks in the continuous casting of large-diameter heat-resistant steel. The elimination of as-cast defects is poor, and the compositional segregation is severe. High-chromium heat-resistant steel continuous casting billets have three distinct crystal regions and solidification defects such as segregation and shrinkage porosity. They are also prone to forming martensitic structures, making them extremely sensitive to cracking. They are very easy to crack during heating, piercing, and rolling, making it difficult to achieve uniform plastic deformation and obtain qualified seamless steel pipes. Summary of the Invention

[0007] The purpose of this invention is to provide a method for preparing seamless tubes of high-chromium heat-resistant steel, overcoming the problems of poor removal of as-cast defects and severe compositional segregation in large-diameter continuous casting billets of high-chromium heat-resistant steel. This method allows for the direct fabrication of seamless tubes from continuously cast billets of high-chromium heat-resistant steel, replacing forged billets.

[0008] To achieve the above objectives, the specific technical solution for the preparation method of high-chromium heat-resistant steel seamless tubes of the present invention is as follows:

[0009] A method for preparing a high-chromium heat-resistant steel seamless tube includes the following steps:

[0010] High-chromium heat-resistant steel molten steel is continuously cast to form a continuous casting billet;

[0011] The continuous casting billet is machined with a full-length axial center through hole;

[0012] The continuous casting billet, after the machining of the full-length axial center through hole, is heated and skewed to form the through hole;

[0013] In the continuous casting process of high-chromium heat-resistant steel to form a continuous casting billet, protective slag is added to the molten steel in the tundish to reduce heat loss. The outer wall of the tundish is covered with nano-insulation material to further reduce heat loss. A continuous temperature probe is built into the molten steel in the tundish to monitor the temperature change and provide operators with real-time, continuous, and accurate molten steel temperature. During the pouring of high-chromium heat-resistant steel, a three-stage combined electromagnetic stirring with interval distribution is used.

[0014] Furthermore, the three-section combined electromagnetic stirring consists of an internal and external crystallizer electromagnetic stirrer, a secondary cooling zone electromagnetic stirrer, and a terminal electromagnetic stirrer located at the rear end of the pouring channel.

[0015] Furthermore, the machining of the full-length axial center through hole involves using a coaxiality adjustment device to clamp and position the continuous casting billet, and using a combination of high wear-resistant and heat-resistant cutting tools and a chip removal device to drill the continuous casting billet, forming a full-length center through hole. The central axis of the center through hole coincides with the central axis of the continuous casting billet.

[0016] Furthermore, the skew rolling piercing forming method involves heating the continuously cast billet after the axial center through hole has been processed, and then forming a closed deformation die through an internal reinforced mandrel, an external guide plate, and skew rolls. The skew rolls are asymmetrically arranged, and a pre-set pre-pressing amount is used. The frictional force of the circumferential rolls and the extrusion force of the axial mandrel generate huge shearing and rolling forces on the continuously cast billet, driving the continuously cast billet to rotate circumferentially and advance axially, performing three-dimensional double-sided rolling with internal extrusion and external pressure, so that the circumferential and axial deformation are uniform with a strain ratio of more than 3.0, and the circumferential and axial deformation are uniform.

[0017] Furthermore, the pre-set reduction amount of the skew rolling piercing is 5% to 8%, the roll rolling angle is 8° to 15°, and the skew rolling piercing deformation ratio is 3 to 6.

[0018] Furthermore, the continuous casting billet has a length of 4m-8m, a diameter of Φ300-Φ800mm, and a hardness of 250HV-300HV.

[0019] Furthermore, the diameter of the central through hole is 90mm-150mm.

[0020] The high-chromium heat-resistant steel seamless tube preparation method of the present invention has the following advantages over the prior art: By using protective slag insulation technology, tundish insulation and continuous temperature measurement technology in the tundish, and a three-stage combined electromagnetic stirring technology with interval distribution during the high-chromium heat-resistant steel molten casting process, the composition of the produced large-diameter high-chromium heat-resistant steel continuous casting billet is stable, the alloy element segregation is small, and defects and segregation are concentrated in the center, which greatly improves the casting defects. The central defects of the continuous casting billet are removed through the central through hole, and favorable bite conditions are provided for subsequent skew rolling piercing and the rolling deformation stress of the continuous casting billet is reduced. This significantly improves the dimensional accuracy, surface quality and yield of large-diameter high-chromium heat-resistant steel seamless tubes, realizes the direct production of seamless tubes by continuously casting billets instead of die-casting forging billets, reduces the forging process of the billet and subsequent machining processes, shortens the tube manufacturing process, improves the preparation efficiency of high-chromium heat-resistant steel seamless tubes, and reduces costs. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the three-stage combined electromagnetic stirring technology in the method for preparing high-chromium heat-resistant steel seamless tubes of the present invention;

[0022] Figure 2 This is a schematic diagram of segregation defects in the cross-section of a continuously cast billet in the method for preparing high-chromium heat-resistant steel seamless tubes of the present invention;

[0023] Figure 3 This is a schematic diagram of the center defect of the continuously cast billet in the method for preparing high-chromium heat-resistant steel seamless tubes of the present invention;

[0024] Figure 4 This is a schematic diagram of the skew rolling piercing process in the method for preparing high-chromium heat-resistant steel seamless tubes of the present invention;

[0025] The markings in the diagram are as follows: 1. Electromagnetic stirrer for crystallizer; 2. Electromagnetic stirrer for secondary cooling zone; 3. Terminal electromagnetic stirrer; 4. Tundish; 5. Central through hole; 6. Reinforcing mandrel; 7. External guide plate; 8. Inclined roll. Detailed Implementation

[0026] To better understand the purpose, structure, and function of this invention, the following detailed description of a method for preparing a high-chromium heat-resistant steel seamless tube is provided in conjunction with the accompanying drawings.

[0027] The method for preparing high-chromium heat-resistant steel seamless tubes of the present invention includes the following steps:

[0028] First, high-chromium heat-resistant steel is continuously cast into a billet. In this step, protective slag is added to the molten steel in the tundish to form a covering layer on the surface, providing insulation and reducing heat loss. The outer wall of the tundish is covered with nano-insulating material to further reduce heat loss. A continuous temperature probe is embedded in the tundish to monitor temperature changes, providing operators with real-time, continuous, and accurate steel temperature information. During the pouring of the high-chromium heat-resistant steel, a three-stage combined electromagnetic stirring system with intervals is employed.

[0029] like Figure 1 As shown, the protective slag is an important auxiliary material in continuous casting production. It is added to the molten steel in the tundish, covering the surface of the high-chromium heat-resistant molten steel in tundish 4 to provide insulation and reduce heat loss. Simultaneously, a nano-insulating material coating is applied to the outer wall of tundish 4 to further reduce heat exchange loss between the molten steel and the outside. A continuous temperature probe is used to monitor the temperature of the molten steel in tundish 4 in real time, providing operators with accurate and continuous temperature information so they can respond promptly to temperature changes. This ensures the temperature of the molten steel in the tundish remains stable within a small fluctuation range during casting, reducing component segregation in the continuously cast billet. In the crystallizer area, an electromagnetic stirrer 1, an electromagnetic stirrer 2 in the secondary cooling zone, and an electromagnetic stirrer 3 at the terminal are respectively installed. These three types of electromagnetic stirrers are spaced apart to control the flow direction and shape of the molten steel during the solidification process, enhancing the flow, heat transfer, and migration processes during solidification, effectively improving the internal microstructure of the continuously cast billet and enhancing surface quality. Electromagnetic stirring in internal and external crystallizers increases the equiaxed crystal ratio, reduces surface and subcutaneous porosity defects, and homogenizes the billet shell; electromagnetic stirring in the secondary cooling zone further increases the equiaxed crystal ratio, reduces internal cracks, and improves center segregation; terminal electromagnetic stirring refines the equiaxed crystals and improves center segregation and V-shaped segregation, thereby maximizing the improvement of as-cast defects.

[0030] like Figure 2 As shown, the combined effects of protective slag insulation, tundish nano-coating insulation, tundish continuous temperature measurement technology and three-stage combined electromagnetic stirring technology ensured the stable composition of the produced large-diameter high-chromium heat-resistant steel continuous casting billet, small alloy element segregation, stable quality without harmful δ-ferrite phases, and concentrated defects and segregation to the center, thus improving the overall uniformity of the billet composition and structure.

[0031] Secondly, the continuous casting billet is machined with a full-length axial through hole. The machining of the full-length axial center through hole is carried out by using a coaxiality adjustment device to clamp and position the continuous casting billet, and using a combination of high wear-resistant and heat-resistant cutting tools and a chip removal device to drill the continuous casting billet to form a full-length center through hole. The central axis of the center through hole coincides with the central axis of the continuous casting billet.

[0032] like Figure 3 As shown, defects such as central cracks, porosity, and shrinkage cavities in high-chromium heat-resistant steel continuously cast billets cannot be eliminated during the continuous casting stage. If direct piercing rolling is performed, these defects will expand, resulting in cracks, pits, and peeling on the inner surface of the steel pipe. This severely affects the forming, service life, and safety of large-diameter high-chromium heat-resistant steel seamless pipes. Therefore, axial through-hole machining is required before piercing rolling of the continuously cast billet to remove central defects. The high-chromium heat-resistant steel continuously cast billets targeted in this invention have a length of 5m-8m, a diameter of Φ300mm-Φ800mm, and a hardness of 230HV-300HV.

[0033] In one embodiment, a high-chromium heat-resistant steel continuous casting billet with a length of 8m and a diameter of Φ800mm weighs up to 40 tons and has a hardness of 280HV. This invention utilizes a large-diameter, ultra-long high-chromium heat-resistant steel billet centering through-hole technology. A coaxiality adjustment device is used to clamp and position the billet. A novel high-wear-resistant and heat-resistant combination tool and a dedicated chip removal device tailored to the billet's length are used to drill the billet, forming a central through-hole. The novel high-wear-resistant and heat-resistant combination tool can handle drilling operations on high-hardness, ultra-long heat-resistant steel continuous casting billets, reducing tool consumption, improving production efficiency, and achieving a coaxiality of ±3m for the central through-hole. Within m, with a central through-hole diameter of 90mm-150mm, the coaxiality adjustment device clamps and precisely positions the billet, enabling the machining of the central through-hole along the entire length of the continuous casting billet while effectively reducing the through-hole eccentricity. This facilitates uniform heating of the high-chromium heat-resistant steel continuous casting billet and the skew rolling piercing bite of large-diameter continuous casting billets. It also increases the axial and circumferential friction of the continuous casting billet in the skew rolling forming die, reduces the rolling deformation stress of the continuous casting billet, and makes the axial and circumferential rolling strain and stress distribution more uniform. This significantly improves the wall thickness uniformity of difficult-to-deform high-chromium heat-resistant steel pipes, and greatly improves the dimensional accuracy, surface quality, and yield of the product while fully utilizing the advantages of continuous casting billets.

[0034] Finally, the continuous casting billet is skewed and pierced after the axial through hole is processed. The pre-pressing amount of the skewed piercing is 5% to 8%, the roll rolling angle is 8° to 15°, and the skewed piercing deformation ratio is 3 to 6.

[0035] like Figure 4As shown, the continuously cast billet with a central through hole 5 after the axial center through hole is machined is skewed and rolled into a seamless tube. A closed deformation die is formed by an internal reinforcing mandrel 6, an external guide plate 7, and skew rolls 8. The skew rolls 8 are asymmetrically arranged, with a roll rolling angle of 8°–15° and a pre-set pre-compression reduction of 5%–8%. The circumferential roll friction and axial mandrel extrusion force generate enormous shear and rolling forces on the continuously cast billet, driving the billet to rotate circumferentially and advance axially, achieving a unique three-dimensional double-sided rolling process with internal extrusion and external pressure. With a large area, a skew rolling piercing deformation ratio of 3-6, and uniform circumferential and axial deformation, the casting defects of raw materials are eliminated to the greatest extent, the microstructure is uniform, and the grains are refined. Grain boundary strengthening, martensitic lath strengthening, and carbonitride precipitation strengthening are achieved. The microstructure and properties are controlled and thermoplastic deformation is achieved during forming, which improves the material properties. In one embodiment, the plastic deformation of the heat-resistant steel continuous casting billet during the tube-piercing process is stable and reliable. The large-diameter high-chromium heat-resistant steel seamless tube manufactured has a creep strength of more than 108 MPa at 600℃ / 105h under laboratory conditions.

[0036] This invention discloses a method for preparing seamless high-chromium heat-resistant steel tubes. In the continuous casting process of high-chromium heat-resistant steel molten steel, it utilizes protective slag insulation, tundish insulation, and continuous tundish temperature measurement technologies, along with a three-stage combined electromagnetic stirring technology. This achieves microstructural control of the continuously cast billet, concentrating defects in the billet center as much as possible. Specialized equipment is used to perform a central through-hole operation on the billet, removing central defects. Simultaneously, the preparation of the central through-hole significantly improves the heating and rolling uniformity of the billet. Through the use of a built-in reinforcing mandrel, external guide plates, and skew rolls to form a closed deformation die, and the asymmetrical arrangement of the skew rolls, the method enables the direct fabrication of seamless tubes from heat-resistant steel continuously cast billets. This allows for the mass production of large-diameter seamless high-chromium heat-resistant steel tubes by directly fabricating seamless tubes from continuously cast billets instead of using ingot casting or forging processes. This reduces the forging and subsequent machining steps of the billet, shortens the tube manufacturing process, improves the production efficiency of high-chromium heat-resistant steel seamless tubes, and reduces costs.

[0037] It is understood that the present invention has been described through some embodiments, and those skilled in the art will recognize that various changes or equivalent substitutions can be made to these features and embodiments without departing from the spirit and scope of the invention. Furthermore, under the teachings of the present invention, these features and embodiments can be modified to adapt to specific situations and materials without departing from the spirit and scope of the invention. Therefore, the present invention is not limited to the specific embodiments disclosed herein, and all embodiments falling within the scope of the claims of this application are within the protection scope of the present invention.

Claims

1. A method for preparing a high-chromium heat-resistant steel seamless tube, characterized in that, Includes the following steps: High-chromium heat-resistant steel molten steel is continuously cast to form a continuous casting billet; The continuous casting billet is machined with a full-length axial center through hole to remove the center defect of the continuous casting billet; The continuous casting billet, after the machining of the full-length axial center through hole, is heated and skewed to form the through hole; In the continuous casting process of high-chromium heat-resistant steel to form a continuous casting billet, protective slag is added to the molten steel in the tundish to reduce heat loss. The outer wall of the tundish is covered with nano-insulation material to further reduce heat loss. A continuous temperature probe is embedded in the tundish to monitor temperature changes and provide operators with real-time, continuous, and accurate molten steel temperature readings. During the pouring of the high-chromium heat-resistant steel, a three-stage combined electromagnetic stirring system with interval distribution is used. The skew rolling piercing forming method involves adding... After heating, a closed deformation die is formed by an internal reinforced mandrel, an external guide plate, and skew rolls. The skew rolls are asymmetrically arranged, with a pre-set pre-top reduction. The pre-set pre-top reduction for the skew rolling piercing is 5%~8%, the roll rolling angle is 8°~15°, and the skew rolling piercing deformation ratio is 3~6. The frictional force of the circumferential rolls and the extrusion force of the axial mandrel generate huge shearing and rolling forces on the continuously cast billet, driving the continuously cast billet to rotate circumferentially and advance axially, performing three-dimensional double-sided rolling with internal extrusion and external pressure, so that the circumferential and axial deformation are uniform with a strain ratio of more than 3.0, and the circumferential and axial deformation are uniform.

2. The method for preparing high-chromium heat-resistant steel seamless tubes according to claim 1, characterized in that, The three-section combined electromagnetic stirring consists of an internal and external crystallizer electromagnetic stirrer, a secondary cooling zone electromagnetic stirrer, and a terminal electromagnetic stirrer located at the rear end of the pouring channel.

3. The method for preparing high-chromium heat-resistant steel seamless tubes according to claim 1, characterized in that, The machining of the full-length axial center through hole involves using a coaxiality adjustment device to clamp and position the continuous casting billet, and using a combination of high wear-resistant and heat-resistant cutting tools and a chip removal device to drill the continuous casting billet, forming a full-length center through hole. The central axis of the center through hole coincides with the central axis of the continuous casting billet.

4. The method for preparing high-chromium heat-resistant steel seamless tubes according to claim 1, characterized in that, The continuous casting billet has a length of 4m-8m, a diameter of Φ300-Φ800mm, and a hardness of 250HV-300HV.

5. The method for preparing high-chromium heat-resistant steel seamless tubes according to claim 3, characterized in that, The diameter of the central through hole is 90mm-150mm.

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