Pipeline pipe and preparation method thereof

Abstract

The invention belongs to the field of pipes, and particularly discloses a pipeline pipe and a preparation method thereof. The pipeline pipe is prepared from the following chemical components in percentage by mass: 0.13 to 0.16 percent of C, 0.3 to 0.45 percent of Si, 1.3 to 1.5 percent of Mn, less than or equal to 0.02 percent of P, less than or equal to 0.01 percent of S, 0.02 to 0.03 percent of V, 0.015 to 0.06 percent of Nb, 0.001 to 0.02 percent of Ti, 0.015 to 0.05 percent of Al and the balance of iron and impurities. The pipeline pipe meets the requirements of X52N type pipes in GB 9711-2017, is good in toughness, large in impact allowance and high in weldability, and is particularly suitable for serving in low-temperature and extremely-low-temperature environments.

Description

Technical Field

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

[0002] Line pipes are used to connect energy extraction points and energy processing plants or end customers, and are often used to transport fluids, such as water, oil or natural gas. However, most energy extraction areas are in remote areas with few people, such as Gobi, deserts, oceans, etc. These areas have complex environments and are far away from end customers, requiring special pipelines for energy transportation. In the process of using pipelines to transport energy, the energy transportation capacity can be improved by increasing the transportation pressure, but it is not only accompanied by an increase in cost, but also with the increase in transportation pressure, the corresponding pipe grade requirements are higher. And because the line pipes are connected by welding, low-carbon equivalent micro-alloyed steel is generally required as the pipe material to ensure weldability, but this type of pipe generally has low impact toughness and small impact margin. In addition, with seasonal changes, steel pipes often need to serve in extremely low temperatures, requiring excellent low-temperature impact toughness, and ordinary line pipes cannot meet the requirements of such extreme environments.

[0003] Therefore, there is still a need to develop a line pipe with high room temperature toughness and even good low temperature impact toughness. Summary of the invention

[0004] In view of the problem of poor toughness of the line pipe involved in the above-mentioned prior art, the present invention provides a line pipe and a preparation method thereof.

[0005] To achieve the above purpose, the following technical solutions are specifically included:

[0006] On the one hand, the present invention provides a pipeline pipe, comprising the following chemical composition in percentage by mass: C 0.13-0.16%, Si 0.3-0.45%, Mn 1.3-1.5%, P≤0.02%, S≤0.01%, V 0.02-0.03%, Nb 0.015-0.06%, Ti 0.001-0.02%, Al 0.015-0.05%, and the balance iron and impurities.

[0007] C is a carbide-forming element, which can effectively improve the strength and hardenability of steel. The strength of steel increases with the increase of carbon content, but the impact toughness and elongation decrease significantly, which is not conducive to welding performance. Therefore, the C content is controlled within the above-mentioned content range of the present invention, which can maintain the strength and toughness of the steel without adversely affecting the welding performance.

[0008] Mn is an austenite forming element, which can improve the strength of the steel pipe, improve the hardenability of the steel pipe and reduce the quenching temperature. It also has the effects of deoxidation and desulfurization, which can weaken the adverse effects of sulfur. However, when the Mn content is too high, it will cause grain coarsening and increase the sensitivity of temper brittleness, which is not conducive to the toughness of the seamless steel pipe. Therefore, the Mn content is controlled within the above-mentioned content range of the present invention to ensure the strength of the material.

[0009] Si can remove oxygen from steel, thereby improving the toughness of the steel pipe. Too high Si content reduces plasticity and affects welding performance. Moreover, Si has poor heat permeability. If more Si is added, rolling defects such as inward folding and cracking are likely to occur during the rolling process of the steel billet. Therefore, the Si content is controlled within the above-mentioned content range of the present invention to achieve the required deoxidation without any other adverse effects.

[0010] V easily forms extremely stable carbides and nitrides with C and N in steel, and vanadium carbide and vanadium nitride usually exist in steel in a fine, dispersed form, so they can significantly refine the structure and grains of steel, increase the grain coarsening temperature, and reduce the overheating sensitivity of steel, thereby improving the strength and toughness of steel at the same time. However, V is expensive, so from an economical point of view, it is recommended to add as little V as possible on the basis of achieving the corresponding performance.

[0011] Nb can refine the grains. It forms carbides and nitrides with C and N in the steel, inhibiting the heating and growth of austenite in the steel. However, it is easy to be saturated. If the Nb content is too high, cracks and external folds may easily appear on the surface of the steel pipe during billet rolling.

[0012] Ti is a grain-refining element, which forms Ti(C, N) in steel, pinning the austenite grain boundaries and preventing grain growth at high temperatures. It is dispersed in the steel and can prevent the growth of austenite grains and refine the grains. Moreover, the solid solubility of Ti in steel is low, so adding a small amount can play a role in refining the grains.

[0013] In addition, Nb, V, and Ti can combine with C (carbon) and N (nitrogen) in steel to form carbides, nitrides, and carbonitrides. These compounds dissolve at high temperatures and precipitate at low temperatures, thereby inhibiting grain growth and precipitation strengthening. However, excessive addition will affect the toughness of the material. Therefore, the total mass percentage of V and Nb is controlled to be ≤0.07%, and the total mass percentage of V, Nb, and Ti is ≤0.12%.

[0014] Al is a good deoxidizer in steel. It can cooperate with Si to remove oxygen from the raw materials and generate highly fine and ultra-micro oxides, which can effectively prevent the growth of grains when the steel is heated and refine the grains. Since the deoxidation effect of Al is saturated and inclusions are prevented from appearing in the form of agglomerates, adding excessive Al to Cr-containing steel will form a more dense iron oxide scale on the surface of the steel pipe, which is not easy to remove under high-pressure water. Therefore, it is more appropriate to control the Al content within the above-mentioned content range of the present invention.

[0015] Nb, Al and Ti are all grain-refining elements, which can play a role in grain refinement. Adding one or more of them can play a corresponding role.

[0016] Preferably, the impurities include Cr, Mo, Ni, and Cu.

[0017] Preferably, the line pipe comprises the following chemical compositions in mass percentage: C 0.12-0.15%, Si 0.33-0.36%, Mn 1.39-1.44%, P 0.012-0.016%, S 0.002-0.0036%, Cr 0.07-0.1%, Mo 0.009-0.031%, Ni 0.02-0.03%, Cu 0.026-0.042%, V 0.02-0.026%, Nb 0.019-0.05%, Ti 0.002-0.003%, Al 0.024-0.033%, and the balance is iron.

[0018] Further preferably, the percentage of Nb in the line pipe is 0.020-0.028%. The inventors of the present invention have found that increasing the content of Nb in the line pipe will not significantly deteriorate the impact performance, but at the temperature of rolling deformation, the carbonitride of Nb in the hard and brittle phase is difficult to melt into the matrix, and the steel pipe is prone to inward or outward bending during the rolling process, and the yield rate of the product is slightly reduced. Therefore, it is further preferred that the percentage of Nb is 0.020-0.028%, which can maintain good impact performance and yield rate.

[0019] Preferably, the carbon equivalent of the line pipe is ≤ 0.42%.

[0020] When C ≥ 0.12%, CE IIW Measuring carbon equivalent, CE IIW =C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15, where each element represents the content of each element for calculation; when CE IIW ≤0.42%, can simultaneously obtain high strength and toughness and brittleness, and good welding performance. IIW When the carbon equivalent CE is ≥0.42%, the welding performance also deteriorates.IIW ≤0.42%, the carbon content and carbon equivalent are both low, and the pipeline has excellent weldability.

[0021] Preferably, the line pipe has a yield strength of 370-450 MPa, a tensile strength of 510-570 MPa, an elongation of 47-49%, a yield strength ratio of 0.73-0.78, an impact KV8 value of 170-250 J at 0°C, a shear ratio of 90-95% at 0°C, and an impact KV8 value of 140-210 J at -40°C.

[0022] The line pipe of the present invention meets the X52N requirements in GB 9711-2017, has good toughness, large impact margin, high weldability, and is particularly suitable for service in low temperature and ultra-low temperature environments.

[0023] Preferably, the wall thickness of the line pipe is ≤16 mm.

[0024] The present invention also provides a method for preparing the pipeline pipe, comprising the following steps:

[0025] (1) The material is prepared according to the chemical composition of the line pipe, and the pipe blank is obtained through smelting, refining, vacuum degassing, and arc continuous casting in sequence;

[0026] (2) punching and rolling the tube blank in sequence at 1000-1400° C. to obtain a rough tube;

[0027] (3) Cooling the rough pipe to 350-500° C., with the cooling rate being ≤4° C. / s;

[0028] (4) heating to 900-950° C. to reduce the diameter, cooling to 500-700° C. and air cooling to obtain a line pipe, wherein the cooling rate is greater than 4° C. / s.

[0029] Preferably, in step (1), when preparing the ingredients, the raw materials used include molten iron, scrap steel, and ferroalloy, and the mass percentage of the molten iron is ≥50%.

[0030] In step (1), when heating to 1000-1400° C., the heating rate of the heating process is not particularly limited, and a conventional heating rate may be used, such as a heating rate of 5-30° C. / min.

[0031] Preferably, in step (4), the cooling is sequentially performed by water cooling and spray cooling.

[0032] In step (4), when heating to 900-950°C, the heating rate of the heating process is not particularly limited, and a conventional heating rate can be used, such as a heating rate of 5-30°C / min.

[0033] In large-scale industrial production, in order to pursue output, a large number of cooling fans or mist cooling (cooling temperature > 4°C / s) are often turned on during the cooling process of the rough pipe, resulting in excessive cooling speed. On the one hand, because the starting transformation temperature of bainite is 562°C and the ending transformation temperature is 536.3°C, the cooling structure is ferrite, pearlite and bainite; on the other hand, when the cooling rate is further increased (reaching above 12.8°C / s), martensite structure will be produced; the overall result is that the austenite fails to be completely transformed into ferrite and pearlite due to supercooling, and is heated to the austenite zone in a non-equilibrium state. This part of the austenite will grow with the original undecomposed austenite as the core, resulting in coarse grains, followed by rapid cooling to form bainite and pearlite structures, which greatly reduces the toughness of the steel pipe and affects the effect of online normalization. In the preparation method of the present invention, firstly, a tube blank is obtained by a conventional smelting method, and the tube blank is pierced and rolled in sequence to obtain a rough tube; secondly, the rough tube is cooled to 350-500°C at a cooling rate of ≤4°C / s, which is below the recrystallization temperature, and the structure is transformed from austenite to ferrite and pearlite. The appropriate cooling rate can effectively ensure that austenite is transformed into ferrite and pearlite, thereby improving the toughness of the pipeline pipe; the rough tube is reheated to 900-950°C and re-austenitized; finally, in the diameter reduction process, the steel pipe is deformed by fixed diameter reduction rolling, and then rapidly cooled to 500-700°C at a rate of >4°C / s, which can achieve rapid cooling below the austenite zone, prevent grain growth at high temperature, and effectively refine the grains; and after cooling to below 500-700°C, air cooling is adopted to utilize the residual heat of the steel pipe to eliminate the residual stress caused by rapid cooling of the steel pipe, thereby further improving the toughness of the pipeline pipe.

[0034] Compared with the prior art, the present invention has the following beneficial effects: the line pipe of the present invention meets the X52N requirements in GB 9711-2017, has good toughness, large impact margin, high weldability, and is particularly suitable for service in low temperature and ultra-low temperature environments. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 This is a metallographic image of the line pipe of Example 1.

[0036] Figure 2 The Jmat-pro software simulated phase transformation curve diagram of the pipeline, wherein Ferrite (0.1%) represents the ferrite transformation start curve, Pearlite (0.1%) represents the pearlite transformation start curve, Bainite (0.1%) represents the bainite transformation start curve, Pearlite (99.9%) represents the pearlite transformation end curve, and Bainite (99.9%) represents the bainite transformation end curve. DETAILED DESCRIPTION

[0037] In order to better illustrate the purpose, technical scheme and advantages of the present invention, the present invention will be further described below through specific examples. The test methods used in the examples and / or comparative examples are conventional methods unless otherwise specified; the materials, reagents, etc. used can be obtained from commercial channels unless otherwise specified.

[0038] Example 1

[0039] (1) The raw materials for the batching include molten iron, pipe heads, pure materials, scrap steel, ferroalloys and aluminum, and the mass percentage of the molten iron is ≥50%; the molten steel is smelted in an electric furnace to a temperature of about 1600° C., and then the temperature is raised to 1650° C. and then refined in a refining furnace, vacuum degassed, and then arc continuous casting is performed at 1545° C. to obtain a continuously cast round pipe billet;

[0040] (2) In a ring furnace, the tube blank is heated to 1300° C., and then taken out of the furnace for perforation (temperature is about 1200° C.) to obtain a rough tube; at this time, the temperature of the rough tube is about 1100° C., and then the rough tube at this temperature is rolled by a limited mandrel continuous tube mill (MPM) or a three-roll limited mandrel continuous tube mill (PQF) to obtain a rough tube, and at this time, the temperature of the rough tube is about 1050° C.;

[0041] (3) The rough pipe is cooled to 400°C at a cooling rate of 3°C / s, then heated to 900°C in a forward furnace, and reduced in diameter on a sizing mill. The temperature of the steel pipe out of the sizing mill is ≥830°C;

[0042] (4) After sizing, the steel pipe is rapidly cooled by a continuous cooling water ring. The inner ring of the cooling water ring is equipped with a high-pressure nozzle. After cooling by the cooling water ring, the steel pipe is spray-cooled. After cooling to 600°C, it is air-cooled. After straightening, water pressure testing and flaw detection, the pipeline pipe is obtained. The size of the pipeline pipe is φ323.9×12.7mm.

[0043] Embodiment 2-6

[0044] The only difference between Examples 2-6 and Example 1 is that the ingredients of the line pipe are different, as shown in Table 1.

[0045] Comparative Examples 1-3

[0046] The difference between Comparative Examples 1-3 and Example 1 is that the ingredients of the line pipes are different, as shown in Table 1.

[0047] Comparative Example 4

[0048] The only difference between this comparative example and Example 1 is that the ingredients of the line pipe are different, as shown in Table 1, and the cooling rate in step (3) is different. The cooling rate of this comparative example is 6°C / s. The specific steps are as follows:

[0049] (1) The raw materials for the batching include molten iron, pipe heads, pure materials, scrap steel, ferroalloys and aluminum, and the mass percentage of the molten iron is ≥50%; the molten steel is smelted in an electric furnace to a temperature of 1600° C. or above, and the temperature is raised to about 1650° C. and then refined in a refining furnace, vacuum degassed, and then arc-shaped continuous casting is performed at 1545° C. to obtain a continuously cast round pipe billet;

[0050] (2) In a ring furnace, the tube blank is heated to 1300° C., and then taken out of the furnace for perforation (temperature is about 1200° C.) to obtain a rough tube; at this time, the temperature of the rough tube is about 1100° C., and then the rough tube at this temperature is rolled by a limited mandrel continuous tube mill (MPM) or a three-roll limited mandrel continuous tube mill (PQF) to obtain a rough tube, and at this time, the temperature of the rough tube is about 1050° C.;

[0051] (3) The rough pipe is cooled to 400°C at a cooling rate of 6°C / s, then heated to 900°C in a forward furnace, and reduced in diameter on a sizing mill. The temperature of the steel pipe out of the sizing mill is ≥830°C;

[0052] (4) After sizing, the steel pipe is rapidly cooled by a continuous cooling water ring. The inner ring of the cooling water ring is equipped with a high-pressure nozzle. After cooling by the cooling water ring, the steel pipe is spray-cooled. After cooling to 600°C, it is air-cooled. After straightening, water pressure and flaw detection, the line pipe is obtained. The size of the line pipe is φ323.9×12.7mm.

[0053] Table 1 (% by mass, the balance is Fe)

[0054]

[0055]

[0056] Note: Cr, Mo, Ni and Cu are all impurity elements remaining from smelting and are not added intentionally. Moreover, the above elements have been taken into consideration in the calculation of carbon equivalent.

[0057] The pipelines prepared in the above embodiments and comparative examples were subjected to performance tests, and the test methods were as follows:

[0058] (1) The tensile properties, 0°C impact and shear properties, and -40°C impact properties of the pipeline were tested according to ASTM A370-24 standard; (2) The metallographic structure of the pipeline of Example 1 was observed using a metallographic microscope.

[0059] Table 2

[0060]

[0061] It can be seen from Examples 1-6 that the line pipe of the present invention includes the following chemical components in mass percentage: C 0.12-0.15%, Si 0.33-0.36%, Mn 1.39-1.44%, P 0.012-0.016%, S 0.002-0.0036%, Cr 0.07-0.1%, Mo 0.009-0.031%, Ni 0.02-0.03%, Cu 0.026-0.042%, V 0.02-0.026%, Nb 0.019-0.05%, Ti 0.002-0.003%, Al 0.024-0.033%, and the balance is iron. The pipeline pipe of the present invention has an Rt0.5 yield strength of 370-450Mpa, a tensile strength of 510-570Mpa, an elongation of 47-49%, a yield strength ratio of 0.73-0.78, an impact KV8 value of 170-250J at 0°C, a shear ratio of 90-95% at 0°C, and an impact KV8 value of 140-210J at -40°C, which meets the requirements of X52N type pipeline pipes. The method of the present invention produces X52N pipeline pipes. After hot rolling, the pipeline pipes are directly cooled and reduced in diameter without the need for a normalizing process (offline) after hot rolling, thereby eliminating the conventional off-line normalizing process of steel pipes after hot rolling, saving costs and realizing low-cost production. In addition, the method of the present invention adopts microalloying components, has low carbon equivalent, good weldability, low yield strength ratio, significantly improved impact toughness, high impact values ​​at 0°C and -40°C, and high shear ratio at 0°C, and is particularly suitable for service in severe cold and low temperature areas.

[0062] By the attached Figure 1 and 2 It can be seen that the metallographic structure of the pipeline steel material finally obtained by the present invention is ferrite and pearlite.

[0063] It can be seen from Examples 4 and 6 that, within a certain range, increasing the Nb content in the steel pipe will not significantly deteriorate the impact. However, at the temperature of rolling deformation, the hard and brittle Nb carbonitrides are difficult to melt into the matrix, and the steel pipe is prone to inward or outward folding during the rolling process, and the product yield rate is slightly reduced.

[0064] It can be seen from Example 4 and Comparative Example 1 that as the carbon content increases, the strength of the steel pipe is significantly improved after online normalization, but its carbon equivalent also increases, the impact toughness decreases sharply, the 0°C impact shear ratio is unqualified, and especially the -40°C low-temperature impact toughness decreases.

[0065] It can be seen from Example 1 and Comparative Example 2 that no V element for grain refinement is added to the steel pipe, and only a trace amount of V is introduced by smelting residues. After online normalization, its strength fails to meet the requirements of X52N pipeline pipe.

[0066] It can be seen from Example 3 and Comparative Example 3 that increasing the Mn content in the steel pipe significantly improves the strength, but the impact resistance drops sharply, and the 0°C impact shear ratio is unqualified.

[0067] It can be seen from Example 1 and Comparative Example 4 that the cooling speed before sizing is too fast, resulting in the material having non-equilibrium structure, supercooled austenite and partial bainite. After entering the furnace for reheating, the austenite continues to grow to form coarse austenite. After cooling, the coarse pearlite structure is formed, the toughness decreases, the 0°C impact shear ratio is unqualified, and the -40°C impact margin is not much.

[0068] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the essence and scope of the technical solution of the present invention.

Claims

1. A line pipe, characterized in that: The invention comprises the following chemical compositions in percentage by mass: C 0.13-0.16%, Si 0.3-0.45%, Mn 1.3-1.5%, P≤0.02%, S≤0.01%, V 0.02-0.03%, Nb 0.015-0.06%, Ti 0.001-0.02%, Al 0.015-0.05%, and the balance is iron and impurities.

2. The line pipe according to claim 1, wherein: The invention comprises the following chemical components in percentage by mass: C 0.12-0.15%, Si 0.33-0.36%, Mn 1.39-1.44%, P 0.012-0.016%, S 0.002-0.0036%, Cr 0.07-0.1%, Mo 0.009-0.031%, Ni 0.02-0.03%, Cu 0.026-0.042%, V 0.02-0.026%, Nb 0.019-0.05%, Ti 0.002-0.003%, Al 0.024-0.033%, and the balance is iron.

3. The line pipe according to claim 1, wherein: The carbon equivalent of the line pipe is ≤ 0.42%.

4. The line pipe according to claim 1, wherein: The line pipe Rt0.5 has a yield strength of 370-450 MPa, a tensile strength of 510-570 MPa, an elongation of 47-49%, a yield strength ratio of 0.73-0.78, an impact KV8 value of 170-250 J at 0°C, a shear ratio of 90-95% at 0°C, and an impact KV8 value of 140-210 J at -40°C.

5. The line pipe according to claim 1, wherein: The wall thickness of the pipeline pipe is ≤16mm.

6. A method for preparing a line pipe according to any one of claims 1 to 5, characterized in that: The steps include: (1) The material is prepared according to the chemical composition of the line pipe, and the pipe blank is obtained through smelting, refining, vacuum degassing, and arc continuous casting in sequence; (2) punching and rolling the tube blank in sequence at 1000-1400° C. to obtain a rough tube; (3) Cooling the rough pipe to 350-500° C., with the cooling rate being ≤4° C. / s; (4) heating to 900-950° C. to reduce the diameter, cooling to 500-700° C. and air cooling to obtain a line pipe, wherein the cooling rate is greater than 4° C. / s.

7. The method for preparing a line pipe according to claim 6, characterized in that: In step (1), the raw materials used in the mixing process include molten iron, scrap steel and ferroalloy, and the mass percentage of the molten iron is ≥50%.

8. The method for preparing a line pipe according to claim 6, characterized in that: In step (4), the cooling is carried out successively by water cooling and spray cooling.

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