A method for producing large-diameter, thin-walled X60N grade pipeline pipe

Abstract

A method for producing large-diameter, thin-walled X60N grade pipeline pipes is disclosed. The chemical composition of the steel, by weight percentage, is C=0.18–0.22, Si=0.33–0.43, Mn=1.35–1.65, V=0.08–0.10, Nb=0.030–0.050, Al=0.015–0.050, Ti=0.002–0.005, with the balance being Fe and unavoidable impurities. During the smelting process, vacuum degassing and electromagnetic stirring are employed to homogenize the billet composition. The heating temperature of the steel pipe during hot expansion is controlled to ensure complete entry into the austenite region. The steel pipe is heated to above the critical point (A3 or Ac3) and then cooled in air, or by blowing or spraying, to obtain suitable normalized microstructure and properties of the hot-expanded pipe. This invention can effectively reduce subsequent heat treatment processes for large-diameter, thin-walled X60N pipeline pipes, saving energy and improving efficiency.

Description

Technical Field

[0001] This invention belongs to the field of metallurgical technology and relates to a production method for large-diameter thin-walled X60N grade steel pipeline pipes. Background Technology

[0002] Large-diameter thin-walled seamless steel pipes generally refer to seamless steel pipes with a large D / S (outer diameter / wall thickness) ratio, which are difficult to directly roll out using conventional rolling methods. Their outer diameter is ≥508mm, and D / S ≥35. Existing manufacturing methods first roll out a small-diameter hot-rolled master pipe, then use a hot expander to enlarge the outer diameter of the steel pipe, and finally heat treat it in a heat treatment furnace. After finishing and inspection processes, the finished steel pipe is produced. The production process for large-diameter thin-walled X60N pipeline pipes commonly uses hot expansion + heat treatment normalizing. The disadvantages of this heat treatment process are: when large-diameter thin-walled X60N pipeline pipes are heated in the heat treatment furnace, the steel enters the austenite region, resulting in a large D / S ratio, making it prone to deformation and causing a large ellipticity in the steel pipe. This fails to meet the pipe end ellipticity requirements of the API 5L 46th standard for pipeline pipes, or the surface is easily dented or scratched, causing quality defects that are difficult to repair and lead to high costs.

[0003] Chinese patent CN201210033609.X discloses a "heat treatment method for improving the comprehensive mechanical properties of a heat-expanding tube." When the heat-expanding tube is heated by a heat-expanding unit, the temperature is increased to 850–920°C. After the tube changes diameter and exits the induction coil, an atomizing cooling device is immediately activated. The length of the heat-expanding tube in the centralized cooling section is approximately 200 mm, and the heat expansion speed is 180–220 mm / min. After the heat-expanding tube is cooled to 500–600°C by atomized water, it is air-cooled at a rate controlled at 8–12°C / s. While this patent does not specify the chemical composition, its induction heating temperature is too low, and some steel grades fail to fully enter the austenite phase, affecting material properties. Furthermore, the patent cools the heat-expanding tube to 500–600°C before air-cooling. At higher temperatures, the material has better plasticity, making it prone to dents and scratches during subsequent movement. Additionally, the ends of the steel pipe are prone to collapse under gravity, resulting in a large ellipticity in the outer diameter of the steel pipe. Furthermore, the performance of this patented material after thermal expansion is generally low, below X46 steel grade.

[0004] Chinese patent CN 107326280 B focuses on the development of a large-diameter, thin-walled X42 grade, low-temperature impact resistant hot-expanded seamless steel pipe and its production method. The chemical composition is designed as follows: C=0.07~0.11, Mn=1.25~1.35, Al=0.020~0.050. The hot expansion temperature is controlled at 735~755℃. The steel pipe does not enter the complete austenite temperature range. After cooling, the microstructure transformation is not completed, and a large amount of the original microstructure is retained, which is not conducive to improving mechanical properties.

[0005] Both of the above patents focus on developing lower-grade steel pipes, with the highest grade being X42N, and do not involve the production of high-grade, extra-large diameter thin-walled steel pipes. Summary of the Invention

[0006] This invention aims to provide a method for producing large-diameter, thin-walled X60N grade steel pipeline pipes. The produced X60N grade steel pipes have the following properties: yield strength ratio ≤0.85, grain size ≥8.0, and full-size transverse impact energy ≥80J at -40℃ (10×10×52mm). The pipe surface is free of pits, dents, and other damage. After subsequent straightening, the ellipticity of the pipe ends is within ±1.6mm, and the geometric dimensions fully meet the API 5L 46th standard.

[0007] The technical solution of this invention:

[0008] A method for producing large-diameter, thin-walled X60N grade steel pipeline pipe, wherein the chemical composition of the steel by weight percentage is C=0.18~0.22, Si=0.33~0.43, Mn=1.35~1.65, V=0.08~0.10, Nb=0.030~0.050, Al=0.015~0.050, Ti=0.002~0.005, with the balance being Fe and unavoidable impurities; the key process steps include:

[0009] (1) Steelmaking is carried out according to the chemical composition of steel by weight percentage, EAF+LF+VD argon blowing for 15 minutes, vacuum degassing + electromagnetic stirring + continuous casting to form round tube billets.

[0010] (2) The billet is pierced by a piercing mill and rolled by a continuous rolling mill to obtain a hot-rolled rough tube;

[0011] (3) Hot-rolled rough tubes are normalized online to obtain small-diameter hot-rolled steel pipes; the temperature before entering the furnace is controlled to be ≤550℃, and the reheating temperature of the walking beam furnace is 880~950℃;

[0012] (4) After sizing, strong fog cooling is performed, with the cooling rate controlled at 5-15℃ / s. After reaching 300-500℃, air cooling is performed to obtain a small-diameter hot-expanded mother tube.

[0013] (5) The heating temperature for hot expansion and normalizing is 880~930℃, the target temperature deviation is ≤±10℃, and the mandrel advance or forward speed is 150~350mm / min;

[0014] (6) Thermal expansion internal propulsion conical core;

[0015] (7) After heat expansion, blow air for rapid cooling. The length of the cooling section is controlled at 300mm to 800mm, and the cooling rate is controlled at 5 to 15℃ / S. After cooling to 300 to 400℃, air cooling is performed.

[0016] The X60N steel grade properties obtained using the above methods are: yield strength ratio ≤0.85, grain size ≥8.0, and a full-size transverse impact energy ≥80J at -40℃ for 10×10×52mm pipes. The steel pipe surface is free of pits, dents, and other damage. After subsequent straightening, the ovality of the pipe ends is within ±1.6mm, and the geometric dimensions fully meet the API 5L 46th standard.

[0017] Explanation of the inventive principle of this invention:

[0018] This invention fully considers the following factors in the chemical composition design of steel: Mn is an economical element that increases strength and toughness, and the appropriate addition of manganese improves strength. The addition of trace amounts of grain-refining elements such as Nb, V, or Al allows for dispersed distribution during normalization and recrystallization, increasing the number of nuclei and refining the grains.

[0019] Using molten iron and a certain proportion of scrap steel, steelmaking is carried out in an electric arc furnace (EAF) combined with a ladle furnace. Secondary refining employs an LF+VD argon blowing process, where VD (vacuum degassing) reduces hydrogen and nitrogen content, minimizing the impact of free hydrogen and nitrogen in the steel. The molten steel is then electromagnetically stirred in a continuous casting crystallizer to homogenize its composition and ensure uniform distribution of second-phase particles such as alumina, NbC, VN, and VC during subsequent cooling. This dispersion strengthening and austenite grain boundary pinning effect are achieved. The resulting round continuous casting billets are then rolled into tubes using a conical piercing mill, and finally rolled into small-diameter rough tubes of suitable specifications using a continuous rolling mill.

[0020] Small-diameter rough tubes are hot-rolled and then normalized online. The temperature before reheating in the walking beam furnace is controlled to be ≤550℃, and the heating temperature in the walking beam furnace is ≥Ar3, generally controlled between 880 and 950℃. After heating in the walking beam furnace, the rough tubes are sized or reduced in diameter by a sizing mill. The inner and outer surfaces of the steel tubes are then rapidly cooled by spray cooling at a rate of 3–15℃ / s to below 300–500℃, reducing the tendency for grain growth due to high-temperature conditions. Based on the conventional recrystallization process, a rapid cooling process after sizing is added, which can further refine the grains to obtain small-diameter hot-rolled steel tubes for use as hot-expansion mother tubes.

[0021] On the hot expansion machine, medium-frequency induction heating is used to heat the steel pipe to the austenitic region. The normalizing heating temperature for low-carbon steel is Ac3+100~150℃, using 880~930℃, supplemented by a certain cooling rate, either air cooling or mist cooling, with the cooling rate controlled at 3~15℃ / s. After cooling to 300~500℃, it is air-cooled to obtain the normalized microstructure of large-diameter thin-walled steel pipe. The hot expansion process is described in [link to hot expansion process description]. Figure 1 Among these, controlling the heating temperature during hot expansion is crucial, with a target temperature fluctuation range of ±10℃. If the temperature deviation is large, it can easily lead to a large fluctuation range in the mechanical properties of the steel pipe after normalizing. Therefore, it is necessary to strictly control the fluctuation range of the hot expansion temperature.

[0022] The cooling section of the hot-expanded tube is approximately 300–800 mm long, using spray or air cooling at a rate controlled at 5–15 °C / s, cooling to 300–400 °C to prevent deformation or impact damage at high temperatures, which could affect the outer diameter of the tube ends. It is then air-cooled to room temperature, transforming the material from austenite in its high-temperature state to a pearlite + a small amount of bainite microstructure, achieving X60 steel grade properties. Its transverse impact resistance at -40 °C meets a full-size average J ≥ 80 J, and its hardness ≤ 230 HV10. The steel tube's geometric dimensions meet API SPEC 5L 46. th The surface must be free of scratches or dents, meeting API Spec 5L 46 requirements. th The requirements for the surface of steel pipes are as follows: steel pipes must not have dents with a depth of ≥6.4mm, and the length of the dent in any direction must not be ≥D / 2.

[0023] The beneficial effects of this invention are as follows: By controlling the steel composition and adding trace amounts of grain-refining elements such as Nb, V, or Al, dislocations are pinned. During normalization, Nb, V, and Al second-phase particles are dispersed, increasing the number of nuclei and refining the grains. Furthermore, it prevents significant grain growth caused by hot expansion heating, which would affect low-temperature impact performance. During the smelting process, vacuum degassing and electromagnetic stirring are used to homogenize the billet composition. Secondly, the heating temperature of the steel pipe during hot expansion is controlled to ensure it fully enters the austenite region. The steel pipe is heated to above the critical point (A3 or Ac3) and then cooled in air, or by blowing or spraying, at a certain cooling rate, thereby obtaining a suitable normalized microstructure and properties for the hot-expanded pipe. Finally, the hot-expanded pipe of this invention needs to be continuously cooled to below 300-400°C. At higher temperatures, the pipe ends are prone to deformation, resulting in a large ellipticity of the pipe end outer diameter. This causes problems such as misaligned welding at the pipe ends and unusability during subsequent use. This invention requires further cooling of the hot-expanded pipe to reduce circumferential deformation or collapse of the steel pipe at high temperatures. This invention can effectively reduce the subsequent heat treatment process for large-diameter thin-walled X60N pipeline pipes, saving energy and improving efficiency. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of hot expansion and normalizing of steel pipes.

[0025] Figure 2 This is a microstructure diagram of the finished product X60N from Example 1.

[0026] Figure 3 This is a microstructure diagram of the finished product X60N from Example 2. Detailed Implementation

[0027] The following examples further illustrate this point.

[0028] Example 1:

[0029] To produce API SPEC 5L 46th standard steel requiring φ609.6×14.27mm X60N, a φ351×16mm hot-expanded mother tube is used to produce API SPEC 5L 46th standard steel requiring φ609.6×14.27mm X60N PSL2. The main chemical composition of the steel is shown in Table 1. The process steps include:

[0030] (1) Steelmaking: Steelmaking is carried out according to the composition in Table 1, EAF+LF+VD argon blowing for 15min (vacuum degassing) + electromagnetic stirring + continuous casting of round tube billets;

[0031] (2) The billet is pierced by a piercing mill and rolled by a continuous rolling mill to obtain a hot-rolled rough tube;

[0032] (3) Online normalization of hot-rolled rough tubes; process control: the temperature before entering the furnace is ≤550℃, and the reheating temperature of the walking beam furnace is 930±10℃;

[0033] (4) After sizing, strong fog cooling is performed, with the cooling rate controlled at 5-15℃ / s. After reaching 300-500℃, air cooling is performed to obtain a small-diameter hot-expanded mother tube.

[0034] (5) The heating temperature for hot expansion and normalizing is 900-920℃, and the mandrel advance or forward speed is 200-300mm / min;

[0035] (6) The sizing band of the thermally expanded inner push conical mandrel is φ578mm, and the inner diameter of the induction coil is φ720mm;

[0036] (7) After heat expansion, blow air to cool quickly. The cooling length is controlled at 400-600 mm, the cooling rate is controlled at 5-15℃ / S, and after cooling to 300-400℃, air cool.

[0037] Randomly select both ends of a steel pipe and test its mechanical properties (see Table 2). Metallographic images are shown below. Figure 2 .

[0038] According to the metallographic images, the microstructure of the product is pearlite + ferrite, with a grain size of approximately 8.0, obtained by using a hot expansion normalizing followed by air cooling process.

[0039] Table 1 Composition of φ609.6×14.27mm X60N

[0040] .

[0041] Table 2 Mechanical properties of φ609.6×14.27mm X60N

[0042] .

[0043] As shown in Table 2, the φ609.6×14.27mm X60N produced by the hot expansion and simultaneous normalizing process has superior mechanical properties, with strength reaching the performance of X60 steel grade. It has finer grains, a larger impact tolerance, and a better balance of strength and toughness. Moreover, the steel pipe surface has no obvious pits or deformation. After subsequent straightening, the ellipticity of the pipe end meets the ±1.6mm requirement, and the ellipticity of the pipe body meets the standard ±0.75%D deviation requirement, which fully meets the requirements of API SPEC 5L 46th standard.

[0044] Example 2:

[0045] To produce φ559×12.7mm X60N steel, a φ351×14mm hot-expanded mother tube is used to manufacture φ559×12.7mm X60N PSL2 steel, meeting API SPEC 5L 46th standard requirements. The chemical composition (weight percentage) of the steel is shown in Table 3. The process steps include:

[0046] (1) Steelmaking: Steelmaking is carried out according to the composition in Table 1, EAF+LF+VD argon blowing for 15min (vacuum degassing) + electromagnetic stirring for 5min + continuous casting of round tube billets;

[0047] (2) The billet is pierced by a piercing mill and rolled by a continuous rolling mill to obtain a hot-rolled rough tube;

[0048] (3) Online normalization of hot-rolled rough tubes; process control: the temperature before entering the furnace is ≤550℃, and the reheating temperature of the walking beam furnace is 920±10℃;

[0049] (4) After sizing, rapid cooling is performed, with the cooling rate controlled at 5-15℃ / min. After reaching 300-500℃, air cooling is performed to obtain a small-diameter hot-expanded mother tube.

[0050] (5) The heating temperature for hot expansion and normalizing is 890-910℃, and the mandrel advance or forward speed is 250-350mm / min;

[0051] (6) The sizing band of the thermally expanded inner push conical mandrel is φ540mm, and the inner diameter of the induction coil is φ720mm;

[0052] (7) After heat expansion, blow air to cool quickly. The cooling length is controlled at 300-500 mm, the cooling rate is controlled at 5-15℃ / S, and after cooling to 300-400℃, air cool.

[0053] Randomly select both ends of a steel pipe and test its mechanical properties (see Table 4). The metallographic structure is shown in Table 4. Figure 3 .

[0054] According to the metallographic images, the microstructure is pearlite + ferrite with a grain size of approximately 8.5, achieved by using a hot expansion normalizing followed by air cooling process.

[0055] Table 3 Composition of φ559×12.7mm X60N

[0056] .

[0057] Table 4 Mechanical properties of φ559×12.7mm X60N

[0058] .

[0059] Table 4 shows that the φ559×12.7mm X60N produced by the online normalizing + sizing followed by rapid cooling and hot expansion normalizing process has superior mechanical properties, achieving the strength of X60 steel grade, with finer grains, a larger impact tolerance, and a better strength-toughness ratio, meeting API SPEC 5L 46. th Standard. The steel pipe surface has no obvious pits or deformation. After subsequent straightening, the ellipticity of the pipe end meets ±1.6mm, and the ellipticity of the pipe body meets the standard ±0.75%D deviation requirement, which fully meets the requirements of API SPEC5L 46th standard.

Claims

1. A method of producing large diameter, thin wall, X60N grade line pipe characterized in that: The steel has a chemical composition by weight percent of C=0.18-0.22, Si=0.33-0.43, Mn=1.35-1.65, V=0.08-0.10, Nb=0.030-0.050, Al=0.015-0.050, Ti=0.002-0.005, and the balance of Fe and inevitable impurities; The key process steps include: (1) Steelmaking according to the chemical composition by weight percent, EAF+LF+VD argon blowing for 15 minutes, vacuum degassing+electromagnetic stirring+continuous casting; (2) Hot-rolled blank pipe is obtained by piercing and rolling of the cast blank by a piercing mill and a continuous rolling mill; (3) On-line normalizing of the hot-rolled blank pipe; the temperature before entering the furnace is controlled to be ≤550℃, and the reheating temperature in the step furnace is 880-950℃; the blank pipe after heating in the step furnace is sized by a sizing mill or is reduced in diameter; (4) After sizing or reducing in diameter, the blank pipe is strongly mist-cooled at a cooling speed of 5-15℃ / s, and is air-cooled after being cooled to 300-500℃, to obtain a small-diameter hot-expanded mother pipe; (5) Hot-expanding normalizing heating temperature is 880-930℃, the target temperature is allowed to deviate by ≤±10℃, and the pushing speed of the conical mandrel is 150-350mm / min; (6) The conical mandrel is pushed in the hot-expanding process; (7) After hot-expanding, the blank pipe is air-cooled after being blown and fast-cooled, the length of the cooling section is controlled to be 300-800mm, the cooling speed is controlled to be 5-15℃ / s, and the blank pipe is air-cooled after being cooled to 300-400℃.

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