Production method of D406A seamless steel tube

Abstract

The invention discloses a production method of a D406A seamless steel pipe, which comprises the following steps: S1, preparing a blank: forming a steel ingot by using iron as a main raw material through a double vacuum smelting process VIM + VAR; s2, forging and pressing a forging rod, forging the steel ingot into the forging rod, and performing annealing treatment; s3, hot extrusion forming is conducted, specifically, the annealed forging rod is machined into a chambering rod material, and then the chambering rod material is machined into a medium-thick-wall steel pipe through a hot extrusion forming technology; and S4, annealing heat treatment is conducted, specifically, the medium-thick-wall steel pipe is sequentially subjected to annealing treatment, normalizing treatment, quenching treatment and tempering treatment, and then a finished pipe is obtained. According to the method, the technological method combining double-vacuum steel ingot smelting, large-compression-ratio upsetting and drawing forging, hot extrusion forming and annealing heat treatment is adopted, the produced D406A seamless steel pipe material is high in compactness and refined in grain size, the defects of air holes, looseness and the like can be avoided, and the excellent mechanical property is achieved.

Description

Technical Field

[0001] This invention belongs to the field of seamless steel pipe processing technology, specifically relating to a production method of D406A seamless steel pipe. Background Technology

[0002] D406A, a low-alloy ultra-high-strength steel independently developed in my country, is closely related to the country's high-end equipment manufacturing needs, breakthroughs in materials technology, and industrialization progress. The successful development of D406A marks a historic leap for my country in the field of ultra-high-strength steel, moving from "following" to "keeping pace," breaking through foreign technological blockades and providing a reliable domestic material option for key equipment. This material significantly reduces dependence on imported ultra-high-strength steel, especially in the military equipment sector, and powerfully enhances the autonomy, controllability, and security of my country's high-end equipment supply chain.

[0003] D406A exhibits extremely high fracture toughness (KIC≥80MPa·m¹ / ²), a significantly superior metric compared to other traditional materials. In the aerospace and defense fields, it can be used to manufacture pressure shells for solid rocket engines, capable of withstanding high temperatures and pressures (>3000℃ gas erosion) and complex alternating stresses from high-speed flight, preventing catastrophic brittle fracture of the shell under high pressure and impact. D406A also possesses excellent low-temperature impact toughness (≥50J) and creep resistance. In the aerospace field, it can be used to manufacture aircraft landing gear and engine mounts, maintaining stable performance within a temperature range of -50℃ to 200℃.

[0004] Currently, the domestic manufacturing process for D406A seamless steel pipes generally involves first producing raw bar stock using an electric furnace + ladle refining + vacuum degassing process, and then forming the seamless steel pipe through hot piercing. This production process has the following problems: Firstly, the D406A raw bar stock manufactured by the electric furnace + ladle refining + vacuum degassing process has defects such as high inclusion content, high porosity, and high degree of component segregation, resulting in poor performance in all aspects of the final seamless steel pipe. Secondly, the hot piercing forming process is lengthy, requiring piercing, rolling, and sizing before the pipe can be formed. Heat loss is significant during product transfer, and the final rolling temperature fluctuates considerably between pipes, making it difficult to ensure consistency in forming process parameters for the same batch of raw materials, resulting in variations in finished product performance. Furthermore, diagonal cracks are unavoidable when forming pipes using the hot piercing forming process, leading to poorer finished product performance. Summary of the Invention

[0005] To address the aforementioned shortcomings in the existing technology, this invention aims to provide a method for producing D406A seamless steel pipes, thereby improving the overall performance of the finished pipes.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows: a method for producing D406A seamless steel pipe, comprising the following steps performed sequentially. S1. Billet preparation: Steel ingots are formed by VIM+VAR double vacuum smelting process using iron as the main raw material. S2. Forging bar: The steel ingot is forged into a shaped bar and then annealed. S3. Hot extrusion forming: The annealed forging bar is processed into a hole-expanding bar stock, and then the hole-expanding bar stock is processed into a medium-thick wall steel pipe through a hot extrusion forming process. S4. Annealing heat treatment: The medium-thick wall steel pipe is subjected to annealing, normalizing, quenching and tempering treatment in sequence to obtain the finished pipe. The composition of the finished pipe steel by mass percentage is as follows: C: 0.27~0.32, Si: 1.35~1.75, Mn: 0.65~1.05, P: ≤0.01, S: ≤0.008, Cr: 0.95~1.35, Mo: 0.38~0.57, V: 0.06~0.17, Cu: ≤0.25, with the remainder being Fe and unavoidable impurities.

[0007] As a limitation of the present invention, the outer diameter of the finished tube is 80-325 mm and the wall thickness is 15-50 mm; The finished tube has the following properties: hardness ≤ 285HBW; tensile strength ≥ 1620MPa; yield strength ≥ 1325MPa (Rp0.2); elongation ≥ 9.0%; reduction of area ≥ 40.0%; and average full-size impact energy at room temperature with longitudinal Charpy U-notch ≥ 40J.

[0008] As a further limitation of the present invention, the main raw materials in step S1 include pure iron and refined materials, wherein the refined materials are ferromolybdenum and / or ferrosilicon.

[0009] As another limitation of the present invention, step S2 uses an upsetting forging process to forge the heated steel ingot into a forging bar, with a compression ratio of 4.0 to 8.0, an initial forging temperature of 1160 to 1200°C, a final forging temperature of 850 to 950°C, a reduction of 30 to 50 mm per pass, and a feed rate of 60 to 100 mm.

[0010] As a further limitation of the present invention, the heating in step S2 is as follows: heating the steel ingot to a furnace temperature of 500-550°C and holding it at that temperature for 3.5 hours; then heating the steel ingot to a furnace temperature of 1160-1200°C at a heating rate of 100-120°C and holding it at that temperature for 3-5 hours.

[0011] As a further limitation of the present invention, the annealing described in step S2 is carried out in a chamber furnace: the annealing temperature is 810-870°C, the holding time is 6-12 hours, and then the temperature is cooled to 300-350°C in the chamber furnace and then air-cooled to room temperature after being removed from the furnace.

[0012] As a third limitation of the present invention, the process of processing the forging bar into a hole-expanding bar in step S3 is as follows: the outer circle of the forging bar is machined by 6-10 mm on one side, the oxide scale on the outer surface of the forging bar is removed, and then a hole-expanding guide hole with a diameter of 45-80 mm is machined to obtain the hole-expanding bar.

[0013] As a further limitation of the present invention, the hot extrusion molding process described in step S3 is as follows: the expanded bar material is heated to 1100-1150°C in an electromagnetic induction furnace, and then descaled with 18-23MPa high-pressure water and lubricated with glass powder, and then extruded into a medium-thick wall steel pipe using a horizontal extrusion press.

[0014] As a further limitation of the present invention, in step S3, the expanded hole bar is heated twice in an electromagnetic induction furnace; the first heating power is 350kW until the outer surface temperature of the expanded hole bar reaches 1000-1050°C; the second heating power is 700kW until the outer surface temperature of the expanded hole bar reaches 1100-1150°C, thus completing the heating process. The process of extruding medium-thick wall steel pipes using a horizontal extrusion press in step S3 is as follows: ① Expanding the hole with a reamer at a speed of 150-250 mm / s and an expansion ratio of 1.01-1.40; ② Extruding with a horizontal extrusion press at a speed of 100-200 mm / s and an extrusion ratio of 3-12.

[0015] As another limitation of the present invention, step S4 includes the following steps performed sequentially. S4.1 Annealing treatment: The medium-thick wall steel pipe is fed into a roller hearth continuous furnace and heated to 870±10℃, held for 60~90min, cooled in the furnace to below 350℃, removed from the furnace and air-cooled, and then pressure straightened. S4.2 Normalizing treatment: The medium-thick wall steel pipe is sent into a chamber furnace and heated to 920±10℃, held for 60 minutes, and then air-cooled after being taken out of the furnace. S4.3 Quenching treatment: The medium-thick wall steel pipe is sent into a chamber furnace and heated to 930±10℃, held for 60 minutes, and then immersed in oil for rapid cooling quenching. When the temperature of the medium-thick wall steel pipe reaches below 50℃, it is air-cooled and the residual oil is cleaned. S4.4 Tempering treatment: The medium-thick wall steel pipe is sent into a chamber furnace and heated to 300±10℃, held for 150 minutes, and then air-cooled to obtain the finished pipe.

[0016] By adopting the above-described technical solution, the beneficial effects achieved by this invention compared to the prior art are as follows: This invention employs a process combining double-vacuum smelting of steel ingots, high compression ratio upsetting and forging, hot extrusion forming, and annealing heat treatment. The resulting D406A seamless steel pipe material has high density and fine grain size, avoiding defects such as porosity and looseness, and possesses excellent mechanical properties.

[0017] Among these advancements, the dual-vacuum smelting process allows for precise control of the gas content in steel ingots, resulting in more accurate control over impurity elements, reduced segregation in the finished product, and refined grain size. Furthermore, the dual-vacuum smelting process effectively improves the purity of the material; these improvements at the raw material level fundamentally enhance the performance of the finished pipe in various aspects. The high compression ratio upsetting forging process can improve the density of the forging material, completely eliminating defects such as porosity, thereby reducing the scrap rate of tubes caused by porosity defects and increasing the product yield. At the same time, the high compression ratio upsetting forging process can make the material structure of the forging bar more interwoven, which is beneficial to subsequent hot deformation processing; Hot extrusion molding allows forging bars to be unaffected by tension during hot deformation, thus preventing the formation of oblique cracks. Secondly, the use of electromagnetic induction heating technology in hot extrusion molding, compared to ring furnace heating, eliminates the problem of severe surface oxidation and decarburization, helping to improve and stabilize the mechanical properties of the finished tubes. Furthermore, the hot extrusion molding process is tightly integrated, consisting of only two main steps: pore expansion and extrusion. Heat loss during product transfer is significantly less than in existing hot piercing processes, preventing large temperature fluctuations between tubes and ensuring consistency in molding process parameters for the same batch of raw materials. This provides a good foundation for subsequent annealing heat treatment and guarantees consistent performance of the finished tubes. Detailed Implementation

[0018] This embodiment discloses a D406A seamless steel pipe, which is a seamless pipe with an outer diameter of 80-325mm and a wall thickness of 15-50mm. Except for Fe and unavoidable impurities, its composition is shown in Table 1.

[0019] The properties of the D406A seamless steel pipe disclosed in this embodiment are as follows: hardness ≤285HBW; tensile strength ≥1620MPa; yield strength Rp0.2 ≥1325MPa; elongation ≥9.0%; reduction of area ≥40.0%; average full-size impact energy of longitudinal Charpy U-notch at room temperature ≥40J.

[0020] This embodiment also discloses a production process for D406A seamless steel pipe. Taking a D406A seamless steel pipe with an outer diameter of 220mm and a wall thickness of 40mm as an example, the production process includes the following steps performed in sequence: S1. Billet Preparation: Using pure iron and refined materials as the main raw materials, steel ingots with a diameter of 600mm are formed through a double vacuum smelting process (VIM+VAR). In this embodiment, the refined materials are ferromolybdenum and / or ferrosilicon.

[0021] S2. Forged Bars: Steel ingots are forged into shaped bars and then annealed. In this step, the heated steel ingot is forged into a forged bar using the uplift forging process, with a compression ratio of 4.0 to 8.0. The uplift forging process can meet the requirements of the corresponding compression ratio, making the material structure more compact and eliminating defects such as porosity. At the same time, it can make the material structure more interwoven, which is beneficial to subsequent hot deformation processing. Specifically, when heating the steel ingot, it is first heated to a furnace temperature of 500–550℃ and held for 3.5 hours; then, it is heated to a furnace temperature of 1160–1200℃ at a rate of 100–120℃ and held for 3–5 hours. When forging the heated steel ingot using the upsetting and drawing forging process, the initial forging temperature is 1160–1200℃, the final forging temperature is 850–950℃, the reduction per pass is 30–50 mm, and the feed rate is 60–100 mm. A 600 mm diameter steel ingot is forged into a 368 mm diameter forging bar. The annealing of the forged bars is carried out in a chamber furnace at a temperature of 810–870℃ for 6–12 hours. The bars are then cooled to 300–350℃ in the chamber furnace and air-cooled to room temperature to reduce their hardness.

[0022] S3. Hot Extrusion Forming: Annealed forging bars are processed into expanded bar stock, which is then processed into medium-thick wall steel pipes through hot extrusion forming. S3.1, Processing of the expanded hole bar stock: The outer diameter of the expanded hole bar stock is machined by 6-10mm on one side to remove the oxide scale on the outer surface, and then a reaming guide hole with a diameter of 45-80mm is machined to obtain the expanded hole bar stock. In this embodiment, the outer diameter of the expanded hole bar stock is 358mm, the inner diameter is 45mm, and the length is 900mm; S3.2 The hot extrusion molding process is as follows: The cold-state expanded steel bar is heated to 1100–1150℃ in an electromagnetic induction furnace, then descaled with 18–23MPa high-pressure water and lubricated with glass powder. The resulting medium-thick-walled steel pipe is then extruded using a horizontal extrusion press. The cold-state expanded steel bar is heated twice in the electromagnetic induction furnace. The first heating power is 350kW until the outer surface temperature of the expanded steel bar reaches 1000–1050℃; the second heating power is 700kW until the outer surface temperature reaches 1100–1150℃, completing the heating process. For lubrication of the expanded steel bar, medium-alkali powder is used for internal lubrication, and GW7 type glass powder from Tianlichuang Company is used for external lubrication. Medium-alkali powder is also used for the glass padding. When using a horizontal extrusion press to extrude medium-thick wall steel pipes, the process includes: ① expanding the hole with a reamer at a speed of 150-250 mm / s and an expansion ratio of 1.01-1.40; ② extruding with a horizontal extrusion press at a speed of 100-200 mm / s and an extrusion ratio of 3-12.

[0023] S4. Annealing heat treatment: The medium-thick wall steel pipe is subjected to annealing, normalizing, quenching and tempering in sequence to obtain the finished pipe. S4.1 Annealing treatment: The medium-thick wall steel pipe is fed into a roller hearth continuous furnace and heated to 870±10℃. It is held for 60 to 90 minutes and then cooled in the furnace to below 350℃. After being taken out of the furnace, it is air cooled and then pressure straightened. S4.2 Normalizing treatment: The medium-thick wall steel pipe after the annealing treatment in step S4.1 is sent into a chamber furnace and heated to 920±10℃, held for 60 minutes, and then air-cooled after being taken out of the furnace. S4.3 Quenching treatment: The medium-thick wall steel pipe after the normalizing treatment in step 4.2 is placed in a chamber furnace and heated to 930±10℃, held for 60 minutes, and then immersed in oil for rapid cooling quenching to ensure performance while preventing water-cooled cracking. Once the temperature of the medium-thick wall steel pipe reaches below 50℃, it is air-cooled and residual oil is cleaned. S4.4 Tempering treatment: The medium-thick wall steel pipe after quenching treatment in step S4.3 is sent into a chamber furnace and heated to 300±10℃, held for 150min, and then air-cooled to obtain the finished pipe.

[0024] After annealing in step S4.1, the hardness of the medium-thick wall steel pipe is tested, and the test data is shown in Table 2 below. After tempering in step S4.4, the tensile strength, Rp0.2 yield strength, elongation, reduction of area, impact energy and other properties of the finished pipe are tested, and the test data is shown in Table 2 below.

[0025]

[0026] As can be seen from the table above, the finished tube obtained in this embodiment has excellent mechanical properties.

Claims

1. A method for producing D406A seamless steel pipe, characterized in that: Including the following steps performed sequentially, S1. Billet preparation: Steel ingots are formed by VIM+VAR double vacuum smelting process using iron as the main raw material. S2. Forged bar: The steel ingot is forged into a shaped bar using the upsetting and drawing forging process, and then annealed. S3. Hot extrusion forming: The annealed forging bar is processed into a hole-expanding bar stock, and then the hole-expanding bar stock is processed into a medium-thick wall steel pipe through a hot extrusion forming process. S4. Annealing heat treatment: The medium-thick wall steel pipe is subjected to annealing, normalizing, quenching and tempering treatment in sequence to obtain the finished pipe. The composition of the finished pipe steel by mass percentage is as follows: C: 0.27~0.32, Si: 1.35~1.75, Mn: 0.65~1.05, P: ≤0.01, S: ≤0.008, Cr: 0.95~1.35, Mo: 0.38~0.57, V: 0.06~0.17, Cu: ≤0.25, with the remainder being Fe and unavoidable impurities.

2. The method for producing D406A seamless steel pipe according to claim 1, characterized in that: The outer diameter of the finished tube is 80–325 mm, and the wall thickness is 15–50 mm. The finished tube has the following properties: hardness ≤ 285HBW; tensile strength ≥ 1620MPa; yield strength ≥ 1325MPa (Rp0.2); elongation ≥ 9.0%; reduction of area ≥ 40.0%; and average full-size impact energy at room temperature with longitudinal Charpy U-notch ≥ 40J.

3. The method for producing D406A seamless steel pipe according to claim 2, characterized in that: The main raw materials in step S1 include pure iron and refined materials, wherein the refined materials are ferromolybdenum and / or ferrosilicon.

4. The method for producing D406A seamless steel pipe according to any one of claims 1-3, characterized in that: Step S2 uses an upsetting forging process to forge the heated steel ingot into a forging bar with a compression ratio of 4.0 to 8.0, an initial forging temperature of 1160 to 1200℃, a final forging temperature of 850 to 950℃, a reduction of 30 to 50 mm per pass, and a feed rate of 60 to 100 mm.

5. The method for producing D406A seamless steel pipe according to claim 4, characterized in that: The heating described in step S2 is as follows: heating the steel ingot to a furnace temperature of 500-550°C and holding it at that temperature for 3.5 hours; then heating the steel ingot to a furnace temperature of 1160-1200°C at a heating rate of 100-120°C and holding it at that temperature for 3-5 hours.

6. The method for producing D406A seamless steel pipe according to claim 5, characterized in that: The annealing described in step S2 is carried out in a chamber furnace: the annealing temperature is 810-870℃, the holding time is 6-12h, and then the temperature is cooled to 300-350℃ in the chamber furnace and then air-cooled to room temperature after being removed from the furnace.

7. The method for producing D406A seamless steel pipe according to any one of claims 1-3, 5, and 6, characterized in that: The process of processing the forged bar into a reamed bar in step S3 is as follows: the outer diameter of the forged bar is machined by 6-10 mm on one side, the oxide scale on the outer surface of the forged bar is removed, and then a reamed guide hole with a diameter of 45-80 mm is machined to obtain the reamed bar.

8. The method for producing D406A seamless steel pipe according to claim 7, characterized in that: The hot extrusion molding process described in step S3 is as follows: the expanded bar material is heated to 1100-1150°C in an electromagnetic induction furnace, and then descaled with 18-23MPa high-pressure water and lubricated with glass powder. After that, the medium-thick wall steel pipe is extruded using a horizontal extrusion press.

9. The method for producing D406A seamless steel pipe according to claim 8, characterized in that: In step S3, the expanded hole bar is heated twice in an electromagnetic induction furnace; the first heating power is 350kW until the outer surface temperature of the expanded hole bar reaches 1000-1050℃; the second heating power is 700kW until the outer surface temperature of the expanded hole bar reaches 1100-1150℃, thus completing the heating process. The process of extruding medium-thick wall steel pipes using a horizontal extrusion press in step S3 is as follows: ① Expanding the hole with a reamer at a speed of 150-250 mm / s and an expansion ratio of 1.01-1.40; ② Extruding with a horizontal extrusion press at a speed of 100-200 mm / s and an extrusion ratio of 3-12.

10. The method for producing D406A seamless steel pipe according to any one of claims 1-3, 5, 6, 8, and 9, characterized in that: Step S4 includes the following steps performed sequentially: S4.1 Annealing treatment: The medium-thick wall steel pipe is fed into a roller hearth continuous furnace and heated to 870±10℃, held for 60~90min, cooled in the furnace to below 350℃, removed from the furnace and air-cooled, and then pressure straightened. S4.2 Normalizing treatment: The medium-thick wall steel pipe is sent into a chamber furnace and heated to 920±10℃, held for 60 minutes, and then air-cooled after being taken out of the furnace. S4.3 Quenching treatment: The medium-thick wall steel pipe is sent into a chamber furnace and heated to 930±10℃, held for 60 minutes, and then immersed in oil for rapid cooling quenching. When the temperature of the medium-thick wall steel pipe reaches below 50℃, it is air-cooled and the residual oil is cleaned. S4.4 Tempering treatment: The medium-thick wall steel pipe is sent into a chamber furnace and heated to 300±10℃, held for 150 minutes, and then air-cooled to obtain the finished pipe.

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