Steel pipe and preparation method thereof

Abstract

The present invention provides a steel pipe and a method for preparing the same. The steel pipe comprises, by mass percentage, the following elements: 0.06-0.10% C, 0.24-0.36% Si, 1.34-1.46% Mn, 0.08-0.16% Cu, 0.04-0.11% Mo, 0.04-0.09% V, 0.034-0.051% Nb, 0.01-0.06% Al, 0.007-0.013% N, 0.2-0.26% Ni, 0.09-0.16% Cr, S ≤ 0.006%, P ≤ 0.006%, with the balance being Fe. The steel pipe has a tensile strength of 550-700 MPa and a yield strength of 450-700 MPa. At -100°C, the steel pipe has an average impact energy of 55-500 AKv / J. The above steel pipes have excellent low-temperature impact properties.

Description

Technical Field

[0001] The present invention relates to the technical field of steel pipes, and in particular to a steel pipe and a preparation method thereof. Background Art

[0002] With the widespread use of cryogenic liquefied products such as liquefied natural gas (LNG) and liquefied ethane, the demand for steel pipe materials with excellent low-temperature toughness and strength is increasing. Under extremely low temperature conditions, the toughness of steel pipes decreases significantly, making them susceptible to brittle fracture. While commonly used stainless steel and high-nickel steels offer excellent low-temperature performance, their high alloy content makes them expensive, making them particularly costly for long-distance pipeline construction.

[0003] The wall thickness of steel pipes directly affects their pressure-bearing capacity and structural safety. To ensure strength and toughness in low-temperature environments, the wall thickness is often increased, but this also increases material usage and costs. Furthermore, steel pipes often operate in environments containing corrosive media, such as hydrogen sulfide (H2S), which can cause severe corrosion, impacting service life and safety. Summary of the Invention

[0004] The main purpose of the present invention is to provide a steel pipe and a preparation method thereof to solve the problems in the prior art that steel pipe materials are difficult to balance wall thickness and strength, have poor corrosion resistance, poor impact toughness at low temperatures, and have high preparation costs.

[0005] To achieve the above object, according to one aspect of the present invention, a steel pipe is provided, which comprises, by mass percentage, the following elements: 0.06-0.10% C, 0.24-0.36% Si, 1.34-1.46% Mn, 0.08-0.16% Cu, 0.04-0.11% Mo, 0.04-0.09% V, 0.034-0.051% Nb, 0.01-0.06% Al, 0.007-0.013% N, 0.2-0.26% Ni, 0.09-0.16% Ti, 0.11-0.2% TiO, 0.11-0.2% TiO, 0.11-0.2% TiO, 0.11-0.2% TiO, 0.11-0.2% TiO, 0.11-0.2% TiO, 0.11-0.2% TiO, 0.11-0.2% TiO The total content of Cr element, S element ≤0.006%, P element ≤0.006% and unavoidable impurities ≤0.15%, and the balance is Fe element; the tensile strength of the steel pipe is 550~700MPa, and the yield strength of the steel pipe is 450~700MPa; at -100℃, the average impact energy of the steel pipe is 55~500AKv / J; after immersion in the test solution for 96h, the maximum crack length rate, the maximum crack thickness rate and the maximum crack sensitivity rate of the steel pipe are each independently 0; wherein the test solution consists of 0.5wt% CH3COOH solution and 5wt% NaCl solution.

[0006] Furthermore, the steel pipe includes the following elements, measured in percentage by mass: 0.07-0.09% of C, 0.25-0.35% of Si, 1.35-1.45% of Mn, 0.09-0.15% of Cu, 0.05-0.10% of Mo, 0.05-0.08% of V, 0.035-0.05% of Nb, 0.02-0.05% of Al, 0.008-0.012% of N, 0.20-0.25% of Ni, 0.10-0.15% of Cr, ≤0.005% of S, ≤0.005% of P, and the total content of unavoidable impurities ≤0.15%, with the balance being Fe.

[0007] Furthermore, the mass ratio of the Al element to the N element is 2-3.5:1; and / or the mass ratio of the Ni element to the Cu element is 1.2-2.0:1.

[0008] Furthermore, the wall thickness of the steel pipe is 4-50 mm, and / or the outer diameter of the steel pipe is Φ48-762 mm; and / or the average grain size of the steel pipe is 2.8-22 μm, and / or the recrystallization texture content of the steel pipe is 40-70%, and / or the dislocation density of the steel pipe is 10 14 ~10 16 m -2 .

[0009] According to another aspect of the present invention, a method for preparing the above-mentioned steel pipe is provided, which comprises: step S1, after the raw materials corresponding to the steel pipe are prepared, smelting, annealing, heat treatment, hot rolling piercing, rolling and sizing are carried out in sequence to obtain a sized steel pipe; and step S2, after the sized steel pipe is quenched pre-treated, quenched and tempered in sequence to obtain a steel pipe; wherein the temperature of the quenching pre-treatment is 845-855°C; the time of the quenching pre-treatment is controlled according to the wall thickness coefficient of the sized steel pipe, and the wall thickness coefficient of the sized steel pipe is 2.0-2.5 min / mm.

[0010] Furthermore, the above step S2 further includes: step S21, quenching and cooling the sized steel pipe to obtain a quenched steel pipe; step S22, quenching and cooling the quenched steel pipe to obtain a quenched steel pipe; step S23, tempering and cooling the quenched steel pipe to obtain a steel pipe; wherein the quenching pretreatment time is 120-150 minutes, and the average grain size of the quenched steel pipe is 2.8-18 μm; and / or the quenching temperature is 870~880℃, the quenching time is controlled according to the wall thickness coefficient of the steel pipe after quenching pretreatment, and the wall thickness coefficient of the steel pipe after quenching pretreatment is 3.0~3.5min / mm; preferably, the quenching time is 120~145min; and / or, the tempering temperature is 650~670℃, the tempering time is controlled according to the wall thickness coefficient of the steel pipe after quenching, the wall thickness coefficient of the steel pipe after quenching is 4.0~4.5min / mm, and the tempering time is 120~300min.

[0011] Furthermore, in the above-mentioned step S1, the heating treatment process includes preheating, multi-stage heating and soaking performed in sequence; wherein, the preheating time is 1~2h; the soaking temperature is 1170~1190℃, and the soaking time is 1.5~2h; the multi-stage heating includes one-stage heating, two-stage heating, three-stage heating and four-stage heating performed in sequence, the temperature of the one-stage heating is 1010~1030℃, and the time of the one-stage heating is 1~2h; and / or, the temperature of the second-stage heating is 1110~1130℃, and the time of the second-stage heating is 1.5~2h; and / or, the temperature of the third-stage heating is 1145~1165℃, and the time of the third-stage heating is 1.5~2h; and / or, the temperature of the fourth-stage heating is 1170~1190℃, and the time of the fourth-stage heating is 1.5~2h.

[0012] Furthermore, the above step S1 also includes sequentially performing a peeling treatment and an oxidation layer spraying on the annealed tube blank to obtain a tube blank coated with an anti-oxidation coating: wherein the thickness of the anti-oxidation coating is 1.5-1.8 mm.

[0013] Furthermore, in the above step S1, the temperature of hot rolling piercing is 1180-1210°C; and / or the temperature of rolling is 980-1000°C; and / or the temperature of sizing is 930-950°C.

[0014] Furthermore, in the above step S1, the annealing temperature is 950-1020° C.; and the annealing time is 9-20 hours.

[0015] By applying the technical solution of the present invention, the present application controls the content of each element in the steel pipe within the above range, thereby enabling the tensile strength and yield strength of the steel pipe to reach the above range, and having good corrosion resistance. In particular, under the harsh temperature conditions of -100°C, the steel pipe of the present application has excellent low-temperature impact performance, and its average impact energy can reach the above range, so that it can be better applied to the storage and transportation of liquid gases at -100°C. Specifically, the S element easily forms precipitates MnS with the metal element Mn, thereby reducing the low-temperature toughness of the steel pipe material; the P element easily segregates at the grain boundaries, thereby reducing the low-temperature toughness, crack growth resistance and corrosion resistance of the steel pipe material. Therefore, it is preferred that the content of the S element and the P element be within the above range, which can reduce the tendency of the steel pipe to be hot brittle at -100°C. The Mn element is an austenite stabilizing element and is also a matrix strengthening element. It can not only increase the strength through precipitation strengthening, but also improve the hardenability of the steel pipe material. If the content of Mn is too low, the strength will not meet the requirements, and if it is too high, the toughness of the material will be affected. Therefore, it is preferred that the content of Mn is within the above range, which can take into account both the strength and toughness of the steel pipe. The addition of V, Mo, Ni and Nb can improve the room temperature strength, low temperature toughness and sulfide corrosion resistance of the steel pipe material while further controlling the cost. Ni is an austenite stabilizing element, especially important for the impact stability of the material at low temperatures, and can reduce the ductile-brittle transition temperature. Si is a deoxidizing element in the steelmaking process, which can reduce the harmful element O in the material and also improve the strength of the steel pipe. However, too high Si content is not conducive to improving the low temperature impact performance of the steel pipe. Therefore, it is preferred that the content of Si is within the above range, which can take into account both the strength and the impact performance at low temperatures. C can improve the strength of the steel pipe through interstitial solid solution. Too high a content affects the low temperature impact performance of the steel pipe, and too low a content will make the strength of the steel pipe fail to meet the requirements. Therefore, it is preferred that the content of C is within the above range, which can take into account both the strength and the impact performance at low temperatures of the steel pipe. The Cu element is an austenitizing stabilizing element, and the retained austenite is beneficial to improving the low-temperature toughness of the steel pipe. An excessively high Cu content is likely to cause surface cracks during rolling. Therefore, the content of the Cu element is preferably within the above-mentioned range, which can improve the low-temperature toughness of the steel pipe while avoiding surface cracks during rolling. The Cr element can improve the hardenability of the steel pipe material, thereby increasing the strength of the steel pipe. An excessively high Cr content affects the low-temperature impact performance of the steel pipe. Therefore, the content of the Cr element is preferably within the above-mentioned range, which can take into account both the strength of the steel pipe and the impact performance at low temperatures. In addition, compared with stainless steel, the wall thickness of the steel pipe of the present application can be further thinned at the same strength, and the material cost is low, only 1 / 4 of the cost of stainless steel materials, which has a significant cost advantage. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The accompanying drawings, which constitute part of this application, are intended to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are intended to explain the present invention and do not constitute an undue limitation of the present invention. In the accompanying drawings:

[0017] Figure 1 The microstructure diagram of the steel pipe in Example 1 of the present application is shown. DETAILED DESCRIPTION

[0018] It should be noted that, in the absence of conflict, the embodiments and features of the embodiments in this application can be combined with each other. The present invention will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0019] As analyzed in the background technology of this application, there are problems in the existing technology such as it is difficult for steel pipe materials to balance wall thickness and strength, poor corrosion resistance, poor impact toughness at low temperatures, and high preparation costs. In order to solve the above problems, this application provides a steel pipe and a preparation method thereof.

[0020] In a typical embodiment of the present application, a steel pipe is provided, which comprises the following elements, in percentage by mass: 0.06-0.10% C, 0.24-0.36% Si, 1.34-1.46% Mn, 0.08-0.16% Cu, 0.04-0.11% Mo, 0.04-0.09% V, 0.034-0.051% Nb, 0.01-0.06% Al, 0.007-0.013% N, 0.2-0.26% Ni, 0.09-0.16% C. The total content of r element, S element ≤0.006%, P element ≤0.006% and unavoidable impurities ≤0.15%, and the balance is Fe element; the tensile strength of the steel pipe is 550~700MPa, and the yield strength of the steel pipe is 450~700MPa; at -100°C, the average impact energy of the steel pipe is 55~500AKv / J; after immersion in the test solution for 96h, the maximum crack length rate, the maximum crack thickness rate and the maximum crack sensitivity rate of the steel pipe are each independently 0; wherein the test solution consists of 0.5wt% CH3COOH solution and 5wt% NaCl solution.

[0021] By controlling the content of each element in the steel pipe within the above range, the present application can make the tensile strength and yield strength of the steel pipe reach the above range, and have good corrosion resistance. Especially under the harsh temperature conditions of -100°C, the steel pipe of the present application has excellent low-temperature impact performance, and its average impact energy can reach the above range, so that it can be better applied to the storage and transportation of liquid gas at -100°C. Specifically, the S element easily forms precipitates MnS with the metal element Mn, thereby reducing the low-temperature toughness of the steel pipe material; the P element easily segregates at the grain boundaries, thereby reducing the low-temperature toughness, crack propagation resistance and corrosion resistance of the steel pipe material. Therefore, it is preferred that the content of the S element and the P element be within the above range, which can reduce the tendency of the steel pipe to be hot brittle at -100°C. The Mn element is an austenite stabilizing element and is also a matrix strengthening element. It can not only improve the strength through precipitation strengthening, but also improve the hardenability of the steel pipe material. If the content of Mn is too low, the strength will not meet the requirements, and if it is too high, the toughness of the material will be affected. Therefore, it is preferred that the content of Mn is within the above range, which can take into account both the strength and toughness of the steel pipe. The addition of V, Mo, Ni and Nb can improve the room temperature strength, low temperature toughness and sulfide corrosion resistance of the steel pipe material while further controlling the cost. Ni is an austenite stabilizing element, especially important for the impact stability of the material at low temperatures, and can reduce the ductile-brittle transition temperature. Si is a deoxidizing element in the steelmaking process, which can reduce the harmful element O in the material and also improve the strength of the steel pipe. However, too high Si content is not conducive to improving the low temperature impact performance of the steel pipe. Therefore, it is preferred that the content of Si is within the above range, which can take into account both the strength and the impact performance at low temperatures. C can improve the strength of the steel pipe through interstitial solid solution. Too high a content affects the low temperature impact performance of the steel pipe, and too low a content will make the strength of the steel pipe fail to meet the requirements. Therefore, it is preferred that the content of C is within the above range, which can take into account both the strength and the impact performance at low temperatures of the steel pipe. The Cu element is an austenitizing stabilizing element, and the retained austenite is beneficial to improving the low-temperature toughness of the steel pipe. An excessively high Cu content is likely to cause surface cracks during rolling. Therefore, the content of the Cu element is preferably within the above-mentioned range, which can improve the low-temperature toughness of the steel pipe while avoiding surface cracks during rolling. The Cr element can improve the hardenability of the steel pipe material, thereby increasing the strength of the steel pipe. An excessively high Cr content affects the low-temperature impact performance of the steel pipe. Therefore, the content of the Cr element is preferably within the above-mentioned range, which can take into account both the strength of the steel pipe and the impact performance at low temperatures. In addition, compared with stainless steel, the wall thickness of the steel pipe of the present application can be further thinned at the same strength, and the material cost is low, only 1 / 4 of the cost of stainless steel materials, which has a significant cost advantage.

[0022] The test solution was prepared according to NACE TM0284 standard.

[0023] There are significant differences in the impact properties of steel pipe materials at different temperatures. This is mainly because the toughness of steel pipe materials decreases as the temperature decreases, especially when it is close to or below the ductile-brittle transition temperature of the material. At -10℃~0℃ or room temperature, steel pipe materials can exhibit good toughness because the temperature does not approach or reach the ductile-brittle transition point of the material. However, at extremely low temperatures, such as -100℃, the sensitivity of the microstructure of the steel pipe material to temperature is significantly enhanced, resulting in a significant decrease in the toughness of the material at this temperature, and even brittle fracture may occur. Therefore, by controlling the content of each element in the steel pipe within the above range, the steel pipe of the present application can not only have good impact properties at room temperature, but also have excellent low-temperature impact properties even under such harsh temperature conditions as -100℃.

[0024] In order to further improve the strength, corrosion resistance and low-temperature impact performance of the steel pipe, in one embodiment of the present application, the steel pipe includes the following elements, calculated by mass percentage: 0.07-0.09% of C, 0.25-0.35% of Si, 1.35-1.45% of Mn, 0.09-0.15% of Cu, 0.05-0.10% of Mo, 0.05-0.08% of V, 0.035-0.05% of Nb, 0.02-0.05% of Al, 0.008-0.012% of N, 0.2-0.25% of Ni, 0.10-0.15% of Cr, ≤0.005% of S, ≤0.005% of P, ≤0.005% of the total content of unavoidable impurities, ≤0.15%, and the balance being Fe.

[0025] In one embodiment of the present application, the mass ratio of the Al element to the N element is 2-3.5:1; and / or the mass ratio of the Ni element to the Cu element is 1.2-2:1.

[0026] The mass ratio of Al to N is preferably within the above range, which helps form AlN, thereby improving the low-temperature impact resistance of the steel pipe. The mass ratio of Ni to Cu is preferably within the above range, which helps form a nickel-copper-rich layer, thereby improving the low-temperature impact resistance of the steel pipe.

[0027] The preferred mass ratio of Si element to Mn element is 1-1.5:4-5.5, which is beneficial to inhibiting the segregation of the two elements, thereby further improving the low-temperature impact resistance and corrosion resistance of the steel pipe.

[0028] The preferred mass ratio of Si element to P element is 50~70:1, which helps Si element to inhibit P element from segregating at grain boundaries, thereby further improving the low-temperature impact resistance and corrosion resistance of the steel pipe.

[0029] In one embodiment of the present application, the wall thickness of the steel pipe is 4-50 mm, and / or the outer diameter of the steel pipe is Φ48-762 mm; and / or the average grain size of the steel pipe is 2.8-22 μm, and / or the recrystallization texture content of the steel pipe is 40-70%, and / or the dislocation density of the steel pipe is 10 14 ~10 16 m -2 .

[0030] The wall thickness and outer diameter of the steel pipe are preferably controlled within the above ranges to help achieve both strength and low-temperature impact performance while also considering cost. The average grain size, recrystallization texture content, and dislocation density of the steel pipe are also preferably controlled within the above ranges to help refine the grains, thereby ensuring that the steel pipe material achieves both strength and low-temperature impact performance.

[0031] In another typical embodiment of the present application, a method for preparing the above-mentioned steel pipe is provided, which comprises: step S1, after the raw materials corresponding to the steel pipe are prepared, smelting, annealing, heat treatment, hot rolling piercing, rolling and sizing are carried out in sequence to obtain a sizing steel pipe; and step S2, the sizing steel pipe is subjected to quenching pretreatment, quenching and tempering in sequence to obtain a steel pipe; wherein the temperature of the quenching pretreatment is 845~855℃; the time of the quenching pretreatment is controlled according to the wall thickness coefficient of the sizing steel pipe, and the wall thickness coefficient of the sizing steel pipe is 2.0~2.5min / mm.

[0032] The present application adopts the above-mentioned heat treatment process, namely quenching pretreatment, quenching and tempering, which can finely control the grain size of the steel pipe after quenching pretreatment, thereby greatly improving the low-temperature toughness and corrosion resistance of the steel pipe material and maintaining a high strength. At the same time, controlling the pre-quenching temperature and the time of quenching pretreatment according to the wall thickness coefficient of the steel pipe after sizing within the above-mentioned range can promote the formation of solid solution inside the steel pipe material and avoid abnormal growth of grains, thereby obtaining a steel pipe with a uniform and fine grain structure, thereby further improving the low-temperature impact performance of the steel pipe. The hardness and strength of the steel pipe material can be improved by quenching. The brittleness of the steel pipe material can be reduced by tempering, and its toughness and plasticity can be improved. In addition, the preparation cost of the present application is low and the process is simple.

[0033] Among them, the wall thickness coefficient is used as a correction coefficient to consider factors such as material properties and working conditions when calculating the wall thickness of steel pipes.

[0034] The smelting includes LF refining, VD vacuum degassing and continuous casting in sequence to obtain billets. The temperature at the beginning of continuous casting is 1600~1650℃.

[0035] In one embodiment of the present application, the above step S2 further includes: step S21, quenching and cooling the sizing steel pipe to obtain a quenched steel pipe; step S22, quenching and cooling the quenched steel pipe to obtain a quenched steel pipe; step S23, tempering and cooling the quenched steel pipe to obtain a steel pipe; wherein the quenching pretreatment time is 120-150 minutes, and the average grain size of the quenched steel pipe is 2.8-18 μm; and / or, the quenching The temperature is 870-880°C, the quenching time is controlled according to the wall thickness coefficient of the steel pipe after quenching pretreatment, and the wall thickness coefficient of the steel pipe after quenching pretreatment is 3.0-3.5 min / mm; preferably, the quenching time is 120-145 min; and / or, the tempering temperature is 650-670°C, the tempering time is controlled according to the wall thickness coefficient of the steel pipe after quenching, the wall thickness coefficient of the steel pipe after quenching is 4.0-4.5 min / mm, and the tempering time is 120-300 min.

[0036] It is preferable to control the quenching pretreatment time within the above range, which helps to refine the grains, thereby improving the strength and toughness of the steel pipe, especially in a low temperature environment, which helps to further improve the impact toughness of the steel pipe.

[0037] Controlling the quenching temperature and time within the above ranges helps improve the uniformity of the grain size of the quenched steel pipe and reduces localized hardening or softening. Air cooling after quenching helps stabilize the grain structure of the quenched steel pipe in a short period of time, forming a martensitic structure, further improving the hardness and strength of the steel pipe material.

[0038] The tempering time is calculated as the wall thickness of the steel pipe × 2. It is further preferred to control the tempering temperature and time within the above range, which helps to eliminate the internal stress generated during the quenching process, thereby stabilizing the microstructure of the steel pipe material and optimizing its mechanical properties, especially the impact toughness at low temperatures.

[0039] In one embodiment of the present application, in the above-mentioned step S1, the heating treatment process includes preheating, multi-stage heating and soaking performed in sequence; wherein, the preheating time is 1~2h; the soaking temperature is 1170~1190℃, and the soaking time is 1.5~2h; the multi-stage heating includes one-stage heating, two-stage heating, three-stage heating and four-stage heating performed in sequence, the temperature of the one-stage heating is 1010~1030℃, and the time of the one-stage heating is 1~2h; and / or, the temperature of the two-stage heating is 1110~1130℃, and the time of the two-stage heating is 1.5~2h; and / or, the temperature of the three-stage heating is 1145~1165℃, and the time of the three-stage heating is 1.5~2h; and / or, the temperature of the four-stage heating is 1170~1190℃, and the time of the four-stage heating is 1.5~2h.

[0040] Adopting the above-mentioned heating treatment and controlling the preheating time, the temperature and time of each heating stage in multi-stage heating, and the temperature and time of soaking within the above-mentioned ranges helps to make the temperature inside the steel pipe uniform, reduce the stress concentration generated during the heating treatment, and thus improve the dimensional accuracy and surface quality of the steel pipe.

[0041] In one embodiment of the present application, the above-mentioned step S1 further includes sequentially performing peeling treatment and oxide layer spraying on the annealed tube blank to obtain a tube blank coated with an anti-oxidation coating: wherein the thickness of the anti-oxidation coating is 1.5~1.8mm.

[0042] The preferred anti-oxidation coating thickness helps reduce the formation of iron oxide scale during heat treatment at temperatures above 1000°C, effectively reducing oxidation and improving the surface quality of the steel pipe. The specific process for spraying the oxide layer is as follows: the anti-oxidation coating uses the oxides Al2O3 and SiO2 as the base materials for the coating, adds carbide powder SiC, and uses an aqueous potassium silicate solution as a binder. The oxides, carbide powder, and binder are mixed into a paste and applied to the surface of the tube blank to form the anti-oxidation coating, resulting in a tube blank coated with the anti-oxidation coating.

[0043] In one embodiment of the present application, in the above step S1, the temperature of hot rolling piercing is 1180~1210℃; and / or the temperature of sizing is 930~950℃; and / or the temperature of rolling is 980~1000℃.

[0044] Hot rolling piercing is the process of passing a tube billet through a piercing mill into a hollow, capillary tube. The preferred temperature for hot rolling piercing is within the above range, which helps improve the billet's plasticity and fluidity, thereby facilitating a smoother piercing process and reducing equipment wear and energy consumption caused by material hardening.

[0045] Sizing is the process of fine-tuning the pipe's dimensions after hot rolling and piercing, using a sizing mill. Sizing temperatures within the above range are preferred to effectively reduce the pipe's resistance to deformation, allowing the pipe to reduce its outer diameter more evenly as it passes through the sizing mill, thereby improving dimensional accuracy while maintaining a good surface finish.

[0046] The rolling temperature is preferably within the above range, which helps the pipe to be effectively deformed while maintaining a certain plasticity, thereby reducing the wall thickness and refining the pipe diameter.

[0047] In one embodiment of the present application, in the above step S1, the annealing temperature is 950-1020° C.; and the annealing time is 9-12 hours.

[0048] It is preferred to control the annealing temperature and time within the above ranges, which not only helps to eliminate the stress inside the billet, thereby improving its plasticity, but also helps to optimize the microstructure of the billet and increase the solid solubility of alloy elements, thereby improving the performance stability and consistency of the steel pipe in subsequent processing.

[0049] The beneficial effects of the present application will be further illustrated below with reference to examples.

[0050] Example 1

[0051] Calculated in mass percentage, the raw materials of the steel pipe are prepared according to the following ingredients: 0.08% C element, 0.30% Si element, 1.40% Mn element, 0.12% Cu element, 0.08% Mo element, 0.07% V element, 0.04% Nb element, 0.025% Al element, 0.010% N element, 0.23% Ni element, 0.13% Cr element, 0.005% S element, 0.005% P element, the total content of unavoidable impurities ≤ 0.15%, and the balance Fe element, and then subjected to smelting treatment, namely LF refining, VD vacuum degassing and continuous casting, to obtain a billet, and the temperature at the start of continuous casting is 1625°C.

[0052] The billet is subjected to high temperature diffusion annealing at 950° C. to obtain an annealed tube billet.

[0053] After annealing, the tube blank is peeled and then sprayed with an oxidation layer. The anti-oxidation coating uses the oxides Al2O3 and SiO2 as the base materials, adds carbide powder SiC, and uses a potassium silicate aqueous solution as a binder. The oxides, carbide powder, and binder are mixed into a paste and applied to the tube blank to form an anti-oxidation coating. The resulting tube blank is coated with the anti-oxidation coating. The thickness of the coating is 1.6 mm.

[0054] The tube billet coated with the anti-oxidation coating is sequentially heated in a ring furnace, hot-rolled and pierced at 1200°C, rolled at 990°C, and sized at 940°C to obtain the sized steel pipe. The heating process includes preheating, multi-stage heating, and soaking. The preheating temperature depends on the furnace temperature and the preheating temperature is 1.5 hours. The multi-stage heating includes one, two, three, and four stages of heating, with the first stage heating temperature at 1020°C and the first heating time at 1.52 hours. The second stage heating temperature is 1120°C and the second heating time is 1.8 hours. The third stage heating temperature is 1155°C and the third heating time is 1.8 hours. The fourth stage heating temperature is 1180°C and the fourth heating time is 1.8 hours. The soaking temperature is 1180°C and the soaking time is 1.8 hours.

[0055] The sizing steel pipe was quenched and air-cooled at 850°C to obtain a quenched steel pipe. The quenching time was controlled according to the wall thickness coefficient of the sizing steel pipe, which was 2.3 min / mm. The quenching time was set to 120 min. The quenched steel pipe was quenched and air-cooled at 875°C to obtain a quenched steel pipe. The quenching time was controlled according to the wall thickness coefficient of the quenching steel pipe, which was 3.3 min / mm. The quenching time was set to 120 min. The quenched steel pipe was tempered and air-cooled at 660°C to obtain a steel pipe. The tempering time was controlled according to the wall thickness coefficient of the quenched steel pipe, which was 4.3 min / mm. The tempering time was set to 120 min.

[0056] Example 2

[0057] The difference from Example 1 is that, in terms of mass percentage, the raw material of the steel pipe comprises 0.07% of C element, 0.35% of Si element, 1.35% of Mn element, 0.15% of Cu element, 0.05% of Mo element, 0.08% of V element, 0.035% of Nb element, 0.05% of Al element, 0.008% of N element, 0.25% of Ni element, 0.10% of Cr element, 0.004% of S element, 0.004% of P element, the total content of unavoidable impurities is ≤0.15%, and the balance is Fe element, and the raw material is smelted, i.e., LF refining, VD vacuum degassing and continuous casting to obtain a billet, and finally a steel pipe.

[0058] Example 3

[0059] The difference from Example 1 is that, in terms of mass percentage, the raw material of the steel pipe comprises 0.09% of C element, 0.25% of Si element, 1.45% of Mn element, 0.09% of Cu element, 0.10% of Mo element, 0.05% of V element, 0.05% of Nb element, 0.02% of Al element, 0.012% of N element, 0.2% of Ni element, 0.15% of Cr element, 0.003% of S element, 0.003% of P element, the total content of unavoidable impurities is ≤0.15%, and the balance is Fe element, and the raw material is smelted, i.e., LF refining, VD vacuum degassing and continuous casting to obtain a billet, and finally a steel pipe.

[0060] Example 4

[0061] The difference from Example 1 is that the total mass content of Al element and N element is 0.035%, the mass ratio of Al element to N element is 2:1, and a steel pipe is finally obtained.

[0062] Example 5

[0063] The difference from Example 1 is that the total mass content of Al element and N element is 0.035%, the mass ratio of Al element to N element is 4:1, and a steel pipe is finally obtained.

[0064] Example 6

[0065] The difference from Example 1 is that the total mass content of Ni and Cu elements is 0.35%, the mass ratio of Ni and Cu elements is 2:1, and a steel pipe is finally obtained.

[0066] Example 7

[0067] The difference from Example 1 is that the total mass content of Ni element and Cu element is 0.35%, the mass ratio of Ni element to Cu element is 2.5:1, and a steel pipe is finally obtained.

[0068] Example 8

[0069] The difference from Example 1 is that the total mass content of Si and Mn elements is 1.7%, the mass ratio of Si and Mn elements is 1:4, and a steel pipe is finally obtained.

[0070] Example 9

[0071] The difference from Example 1 is that the total mass content of Si element and Mn element is 1.7%, the mass ratio of Si element to Mn element is 1:6, and a steel pipe is finally obtained.

[0072] Example 10

[0073] The difference from Example 1 is that the total mass content of Si element and P element is 0.305%, the mass ratio of Si element to P element is 55:1, and a steel pipe is finally obtained.

[0074] Example 11

[0075] The difference from Example 1 is that the total mass content of Si element and P element is 0.305%, the mass ratio of Si element to P element is 75:1, and a steel pipe is finally obtained.

[0076] Example 12

[0077] The difference from Example 1 is that the sizing steel pipe is quenched at 855°C to obtain a quenched steel pipe, wherein the quenching time is controlled according to the wall thickness coefficient of the sizing steel pipe of 2.5 min / mm, and the quenching time is set to 120 min to finally obtain a steel pipe.

[0078] Example 13

[0079] The difference from Example 1 is that the steel pipe after quenching pretreatment is quenched and cooled at 880°C to obtain a quenched steel pipe; wherein the quenching time is controlled according to the wall thickness coefficient of the steel pipe after quenching pretreatment of 3.5 min / mm, and the quenching time is set to 145 min, and finally a steel pipe is obtained.

[0080] Example 14

[0081] The difference from Example 1 is that the quenched steel pipe is tempered and cooled at 670°C to obtain a steel pipe; wherein the tempering time is controlled according to the wall thickness coefficient of the quenched steel pipe of 4.5 min / mm, and the tempering time is set to 150 min, and finally a steel pipe is obtained.

[0082] Example 15

[0083] The difference from Example 1 is that the quenched steel pipe is tempered and cooled at 630°C to obtain a steel pipe; wherein the tempering time is controlled according to the wall thickness coefficient of the quenched steel pipe of 3.5 min / mm, and the tempering time is set to 100 min, and finally a steel pipe is obtained.

[0084] Example 16

[0085] The difference from Example 1 is that the anti-oxidation coated tube blank is sequentially heated in a ring furnace, hot-rolled and pierced at 1200°C, rolled at 990°C, and sized at 940°C to obtain a sized steel pipe. The heating process includes preheating, multi-stage heating, and soaking. The multi-stage heating includes a first heating stage, a second heating stage, a third heating stage, and a fourth heating stage, with the first heating stage at 1010°C, the second heating stage at 1110°C, the third heating stage at 1145°C, and the fourth heating stage at 1170°C, to obtain a steel pipe.

[0086] Example 17

[0087] The difference from Example 1 is that the anti-oxidation coated tube blank is sequentially heated in a ring furnace, hot-rolled and pierced at 1200°C, rolled at 990°C, and sized at 940°C to produce a sized steel pipe. The heating process consists of preheating, primary heating, and soaking. The primary heating temperature is 1200°C, resulting in a steel pipe.

[0088] Comparative Example 1

[0089] The difference from Example 1 is that, in terms of mass percentage, the raw material of the steel pipe comprises 0.13% of C element, 0.45% of Si element, 1.65% of Mn element, 0.02% of Cu element, 0.10% of Mo element, 0.12% of V element, 0.02% of Nb element, 0.06% of Al element, 0.012% of N element, 0.4% of Ni element, 0.2% of Cr element, 0.015% of S element, 0.015% of P element, the total content of unavoidable impurities is ≤0.15%, and the balance is Fe element, and the raw material is smelted, i.e., LF refining, VD vacuum degassing and continuous casting to obtain a billet, and finally a steel pipe.

[0090] Comparative Example 2

[0091] The difference from Example 1 is that the sizing steel pipe is quenched at 830° C. to obtain a quenched steel pipe, wherein the quenching time is controlled according to the wall thickness coefficient of the sizing steel pipe of 1.5 min / mm, and the quenching time is set to 100 min to finally obtain a steel pipe.

[0092] Comparative Example 3

[0093] The difference from Example 1 is that the sizing steel pipe is subjected to normalizing heat treatment at 900° C. to obtain the steel pipe.

[0094] Comparative Example 4

[0095] The difference from Example 1 is that, in terms of mass percentage, the raw material of the steel pipe comprises 0.13% of C element, 0.15% of Si element, 1.65% of Mn element, 0.2% of Cu element, 0.30% of Mo element, 0.03% of V element, 0.02% of Nb element, 0.015% of Al element, 0.003% of N element, 0.1% of Ni element, 0.2% of Cr element, 0.003% of S element, 0.015% of P element, the total content of unavoidable impurities is ≤0.15%, and the balance is Fe element, and the raw material is smelted, i.e., LF refining, VD vacuum degassing and continuous casting to obtain a billet, and finally a steel pipe.

[0096] Comparative Example 5

[0097] The difference from Example 1 is that, in terms of mass percentage, the raw material of the steel pipe comprises 0.14% of C element, 0.2% of Si element, 1.25% of Mn element, 0.04% of Cu element, 0.03% of Mo element, 0.02% of V element, 0.03% of Nb element, 0.015% of Al element, 0.006% of N element, 0.05% of Ni element, 0.06% of Cr element, 0.001% of S element, 0.008% of P element, the total content of unavoidable impurities is ≤0.15%, and the balance is Fe element, and the raw material is subjected to smelting treatment, i.e., LF refining, VD vacuum degassing and continuous casting to obtain a billet, and finally a steel pipe.

[0098] Comparative Example 6

[0099] The difference from Example 1 is that, in terms of mass percentage, the raw material of the steel pipe is smelted, i.e., LF refining, VD vacuum degassing and continuous casting, to obtain a billet and finally a steel pipe.

[0100] Test method:

[0101] Outer diameter and wall thickness measurement: Use outside diameter calipers and ultrasonic thickness gauges to measure the outer diameter and wall thickness of steel pipes.

[0102] Average grain size, recrystallization texture content and dislocation density test: The average grain size is tested according to GBT6394 standard, the dislocation density is tested according to GB1554 standard, and the recrystallization texture content is tested according to GB / T 20832.

[0103] Yield strength, tensile strength and elongation test: Test in accordance with GBT228.1 "Metallic materials tensile test part 1 room temperature test method".

[0104] Impact energy test at -100°C: According to GBT229 "Charpy pendulum impact test method for metallic materials", a set of three impact energy data is obtained. The average impact energy is the average of the three impact energy data.

[0105] The steel pipes of the above examples and comparative examples were subjected to performance tests, and the test results are shown in Tables 1 and 2.

[0106]

[0107]

[0108] in, Figure 1 is the microstructure diagram of the steel pipe in Example 1. Figure 1 It can be seen from the figure that the structure of the steel pipe is tempered martensite with fine and uniform grains.

[0109] After the steel pipes in the above examples and comparative examples were immersed in the test solution for 96 hours, the maximum crack length rate, maximum crack thickness rate, and maximum crack sensitivity rate of the steel pipes in the examples were all 0. The maximum crack length rate, maximum crack thickness rate, and maximum crack sensitivity rate of Comparative Example 1 were 2%, 3.5%, and 6%, respectively. The maximum crack length rate, maximum crack thickness rate, and maximum crack sensitivity rate of Comparative Example 2 were 2.6%, 3.9%, and 6.7%, respectively. The maximum crack length rate, maximum crack thickness rate, and maximum crack sensitivity rate of Comparative Example 3 were 3%, 4.2%, and 6.9%, respectively. The test solution consisted of a 0.5 wt% CH3COOH solution and a 5 wt% NaCl solution.

[0110] The element contents in the steel pipe materials of Comparative Examples 1 and 4 to 5 are not within the scope of the present application. Therefore, their average impact energy at -100°C is low, indicating that their low-temperature impact performance at -100°C is poor.

[0111] From the above description, it can be seen that the above embodiments of the present invention achieve the following technical effects:

[0112] By controlling the content of each element in the steel pipe within the above range, the present application can make the tensile strength and yield strength of the steel pipe reach the above range, and have good corrosion resistance. Especially under the harsh temperature conditions of -100°C, the steel pipe of the present application has excellent low-temperature impact performance, and its average impact energy can reach the above range, so that it can be better applied to the storage and transportation of liquid gas at -100°C. Specifically, the S element easily forms precipitates MnS with the metal element Mn, thereby reducing the low-temperature toughness of the steel pipe material; the P element easily segregates at the grain boundaries, thereby reducing the low-temperature toughness, crack propagation resistance and corrosion resistance of the steel pipe material. Therefore, it is preferred that the content of the S element and the P element be within the above range, which can reduce the tendency of the steel pipe to be hot brittle at -100°C. The Mn element is an austenite stabilizing element and is also a matrix strengthening element. It can not only improve the strength through precipitation strengthening, but also improve the hardenability of the steel pipe material. If the content of Mn is too low, the strength will not meet the requirements, and if it is too high, the toughness of the material will be affected. Therefore, it is preferred that the content of Mn is within the above range, which can take into account both the strength and toughness of the steel pipe. The addition of V, Mo, Ni and Nb can improve the room temperature strength, low temperature toughness and sulfide corrosion resistance of the steel pipe material while further controlling the cost. Ni is an austenite stabilizing element, especially important for the impact stability of the material at low temperatures, and can reduce the ductile-brittle transition temperature. Si is a deoxidizing element in the steelmaking process, which can reduce the harmful element O in the material and also improve the strength of the steel pipe. However, too high Si content is not conducive to improving the low temperature impact performance of the steel pipe. Therefore, it is preferred that the content of Si is within the above range, which can take into account both the strength and the impact performance at low temperatures. C can improve the strength of the steel pipe through interstitial solid solution. Too high a content affects the low temperature impact performance of the steel pipe, and too low a content will make the strength of the steel pipe fail to meet the requirements. Therefore, it is preferred that the content of C is within the above range, which can take into account both the strength and the impact performance at low temperatures of the steel pipe. The Cu element is an austenitizing stabilizing element, and the retained austenite is beneficial to improving the low-temperature toughness of the steel pipe. An excessively high Cu content is likely to cause surface cracks during rolling. Therefore, the content of the Cu element is preferably within the above-mentioned range, which can improve the low-temperature toughness of the steel pipe while avoiding surface cracks during rolling. The Cr element can improve the hardenability of the steel pipe material, thereby increasing the strength of the steel pipe. An excessively high Cr content affects the low-temperature impact performance of the steel pipe. Therefore, the content of the Cr element is preferably within the above-mentioned range, which can take into account both the strength of the steel pipe and the impact performance at low temperatures. In addition, compared with stainless steel, the wall thickness of the steel pipe of the present application can be further thinned at the same strength, and the material cost is low, only 1 / 4 of the cost of stainless steel materials, which has a significant cost advantage.

[0113] The above are merely embodiments of the present invention and are not intended to limit the present invention. Those skilled in the art will readily appreciate that the present invention is susceptible to various modifications and variations. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention shall be included within the scope of protection of the present invention.

Claims

1. A steel pipe, characterized in that: The steel pipe comprises the following elements in percentage by mass: 0.06~0.10% C element, 0.24~0.36% Si element, 1.34~1.46% Mn element, 0.08~0.16% Cu element, 0.04~0.11% Mo element, 0.04~0.09% V element, 0.034~0.051% Nb element, 0.01~0.06% Al element, 0.007~0.013% N element, 0.2~0.26% Ni element, 0.09~0.16% Cr element, S element ≤0.006%, P element ≤0.006%, the total content of unavoidable impurities ≤0.15%, the balance is Fe element; The tensile strength of the steel pipe is 550-700 MPa, and the yield strength of the steel pipe is 450-700 MPa; At -100°C, the average impact energy of the steel pipe is 55-500 AKv / J; After immersion in the test solution for 96 hours, the maximum crack length rate, the maximum crack thickness rate, and the maximum crack sensitivity rate of the steel pipe are each independently 0; wherein the test solution consists of a 0.5 wt % CH 3 COOH solution and a 5 wt % NaCl solution; and / or, the average grain size of the steel pipe is 2.8 to 22 μm, and / or, the recrystallization texture content of the steel pipe is 40 to 70%, and / or, the dislocation density of the steel pipe is 10 14 ~10 16 m -2 .

2. The steel pipe according to claim 1, characterized in that The steel pipe comprises the following elements in percentage by mass: 0.07-0.09% of the C element, 0.25-0.35% of the Si element, 1.35-1.45% of the Mn element, 0.09-0.15% of the Cu element, 0.05-0.10% of the Mo element, 0.05-0.08% of the V element, 0.035-0.05% of the Nb element, 0.02-0.05% of the Al element, 0.008-0.012% of the N element, 0.20-0.25% of the Ni element, 0.10-0.15% of the Cr element, the S element ≤0.005%, the P element ≤0.005%, the total content of unavoidable impurities ≤0.15%, and the balance being the Fe element.

3. The steel pipe according to claim 1 or 2, characterized in that The mass ratio of the Al element to the N element is 2-3.5:

1.

4. The steel pipe according to claim 1 or 2, characterized in that The wall thickness of the steel pipe is 4-50 mm, and / or the outer diameter of the steel pipe is Φ48-762 mm.

5. A method for preparing the steel pipe according to any one of claims 1 to 4, characterized in that: The preparation method comprises: Step S1, after preparing the raw materials corresponding to the steel pipe, smelting, annealing, heat treatment, hot rolling piercing, rolling and sizing are carried out in sequence to obtain a sized steel pipe; and Step S2, sequentially performing quenching pretreatment, quenching, and tempering on the sized steel pipe to obtain the steel pipe; The temperature of the quenching pretreatment is 845-855° C. The time of the quenching pretreatment is controlled according to the wall thickness coefficient of the sizing steel pipe, and the wall thickness coefficient of the sizing steel pipe is 2.0-2.5 min / mm.

6. The preparation method according to claim 5, characterized in that The step S2 further includes: Step S21, performing the quenching pretreatment and cooling on the sizing steel pipe to obtain a quenched pretreated steel pipe; Step S22, quenching and cooling the steel pipe after the quenching pretreatment to obtain a quenched steel pipe; Step S23, tempering and cooling the quenched steel pipe to obtain the steel pipe; The quenching pretreatment time is 120-150 min, and the average grain size of the steel pipe after the quenching pretreatment is 2.8-18 μm; And / or, the quenching temperature is 870-880° C., the quenching time is controlled according to the wall thickness coefficient of the steel pipe after the quenching pretreatment, the wall thickness coefficient of the steel pipe after the quenching pretreatment is 3.0-3.5 min / mm; the quenching time is 120-145 min; And / or, the tempering temperature is 650-670° C., the tempering time is controlled according to the wall thickness coefficient of the quenched steel pipe, the wall thickness coefficient of the quenched steel pipe is 4.0-4.5 min / mm, and the tempering time is 120-300 min.

7. The preparation method according to claim 5 or 6, characterized in that: In step S1, the heating process includes preheating, multi-stage heating and soaking in sequence; wherein the preheating time is 1 to 2 hours; the soaking temperature is 1170 to 1190° C., and the soaking time is 1.5 to 2 hours; The multi-stage heating includes one-stage heating, two-stage heating, three-stage heating and four-stage heating performed in sequence, the temperature of the one-stage heating is 1010~1030℃, and the time of the one-stage heating is 1~2h; and / or, the temperature of the two-stage heating is 1110~1130℃, and the time of the two-stage heating is 1.5~2h; and / or, the temperature of the three-stage heating is 1145~1165℃, and the time of the three-stage heating is 1.5~2h; and / or, the temperature of the four-stage heating is 1170~1190℃, and the time of the four-stage heating is 1.5~2h.

8. The preparation method according to claim 5 or 6, characterized in that: The step S1 further includes sequentially performing a peeling treatment and an oxidation layer spraying on the annealed tube blank to obtain a tube blank coated with an anti-oxidation coating, wherein the thickness of the anti-oxidation coating is 1.5-1.8 mm.

9. The preparation method according to claim 5 or 6, characterized in that: In the step S1, the temperature of the hot rolling piercing is 1180-1210°C; and / or the temperature of the rolling is 980-1000°C; and / or the temperature of the sizing is 930-950°C.

10. The preparation method according to claim 5 or 6, characterized in that: In the step S1, the annealing temperature is 950-1020° C. and the annealing time is 9-20 hours.

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