High frequency resistance welded steel pipe and method for manufacturing the same

Abstract

A high-frequency resistance welded steel pipe and a preparation method thereof. The present application provides a high-frequency resistance welded steel pipe, which comprises C: 0.025-0.054%, Si: 0.10-0.30%, Mn: 1.21-1.45%, Ni: 0.05-0.20%, Cr: 0.11-0.25%, Mo: 0.01-0.15%, Nb: 0.030-0.055%, Ti: 0.005-0.020%, Al: 0.020-0.050%, and Ca: 0.001-0.004%. The present application also provides a preparation method of the steel pipe, which comprises slab rolling into a coil; producing a steel strip; edge milling; producing a rough pipe; high-frequency welding and welding the rough pipe into a welded pipe under the action of an extrusion roller; heat treating the weld joint by using medium-frequency induction heating; and sizing. The high-frequency resistance welded steel pipe has good DWTT crack arrest performance and weld joint impact toughness at low temperature, and can be applied to the field of low-temperature service high-end submarine pipelines.

Description

Technical Field

[0001] This invention belongs to the field of welded pipe production, and particularly relates to a high-frequency resistance welded steel pipe with excellent low-temperature crack arrest performance and its preparation method. Background Technology

[0002] High-frequency resistance welded steel pipe (HFW welded pipe) is a steel pipe made from hot-rolled steel coils through roll forming and high-frequency resistance welding. It has high production efficiency and dimensional accuracy, and is also economical compared to other pipe types. At present, HFW welded steel pipe has been widely used in submarine pipeline projects.

[0003] In addition to meeting the pipeline steel pipe specification API Spec 5L, high-frequency resistance welded steel pipes used for submarine pipelines must also comply with the submarine pipeline standard DNVGL-ST-F101. Conventional submarine pipelines only need to meet the general requirements of the standard, while some high-pressure gas transmission submarine pipeline projects with higher requirements will also have additional crack arrest performance clauses in the supplementary requirements of DNVGL-ST-F101, which require steel pipes with an outer diameter >400mm to meet the low-temperature drop hammer tear test (DWTT) requirements, which is quite difficult for small-diameter thick-walled steel pipes.

[0004] It should be noted that HFW welded pipes used for submarine pipelines typically have a large thickness-to-diameter ratio (thickness / outer diameter). During the cold forming process of pipe manufacturing, hot-rolled coils undergo significant plastic deformation, leading to a decrease in toughness. This is particularly true for small-diameter, thick-walled specifications with an outer diameter ≤508mm and a wall thickness ≥14.3mm, where DWTT performance will significantly decline. Furthermore, the toughness will decrease further during the flattening process of the DWTT sample. Therefore, optimizing the design of pipe manufacturing raw materials and processes to ensure that HFW welded pipes possess excellent low-temperature DWTT crack arrest performance while simultaneously meeting other general technical requirements, especially weld impact toughness, is a technical challenge that needs to be addressed by those skilled in the art. Moreover, the lower the service temperature, the greater the technical difficulty.

[0005] Chinese patent document CN110904391A, entitled "A Thick-gauge ERW Submarine Pipe with Excellent Weld Quality and Its Manufacturing Method," discloses a thick-gauge ERW submarine pipeline welded pipe. The technical solution disclosed in this patent document incorporates Mo and V elements, contains no Cr, exhibits high weld impact toughness at 0℃, and the pipe body adopts a needle-like ferrite microstructure design.

[0006] Chinese patent document CN101701315A, entitled "Manufacturing Method of Submarine Pipeline Steel Pipe," discloses an ERW steel pipe for submarine pipelines. The technical solution disclosed in this patent document features a low-carbon, low-manganese, and micro-alloyed chemical composition, resulting in a product with high strength, corrosion resistance, and high-pressure resistance.

[0007] Chinese patent document CN102284780B, entitled "Welding Production Process of Low-Temperature Resistant HFW Steel Pipe for Stations," discloses a low-temperature resistant HFW steel pipe. The technical solution disclosed in this patent document uses a welding power of 350–500 kW to provide a welding production process for low-temperature resistant HFW steel pipes used in stations, ensuring the normal operation of oil and gas transmission pipelines under low-temperature conditions.

[0008] Chinese patent document CN103966505B, entitled "A Manufacturing Process for X70M Grade HFW Pipeline with Large Wall Thickness," discloses an X70M grade high-frequency resistance welded steel pipe. The technical solution disclosed in this patent document employs a low-carbon, micro-alloyed composition design to produce a high-frequency welded pipe that meets the requirements for tensile properties, impact toughness, and microstructure, and can be used for transporting coal slurry, mineral slurry, oil, gas, etc.

[0009] Chinese patent document CN103526108B, entitled "An X70MS ERW welded pipe with excellent resistance to SSCC stress corrosion and its manufacturing method," discloses an X70MS corrosion-resistant high-frequency resistance welded steel pipe. In the technical solution disclosed in this patent document, the Mn and Cr content is low, the microstructure of the produced welded pipe base material and weld seam are basically consistent, and it exhibits excellent resistance to H2S corrosion.

[0010] Japanese patent document JP2015190026A, entitled "Thick High Strength Electroseamed SteelPipe for Linepipe and Manufacturing Method," discloses a high-strength, high-frequency resistance-welded steel pipe. The technical solution disclosed in this patent document incorporates Cu, Ni, Mo, Nb, and V elements, employing a ferrite-pearlite microstructure design. The resulting welded pipe exhibits high strength, high toughness, and excellent acid resistance and resistance to hydrogen-induced cracking.

[0011] However, the main purpose of the aforementioned patents in improving welded pipes is to enhance their strength, toughness, corrosion resistance, and welding process, but they do not improve the low-temperature DWTT crack arrest performance of welded pipes. Summary of the Invention

[0012] To address the issue that existing HFW welded pipes, while meeting other general technical requirements, struggle to simultaneously satisfy low-temperature DWTT crack arrest performance, this paper proposes a high-frequency resistance welded steel pipe with excellent low-temperature crack arrest performance. At low temperatures, particularly -20°C, it exhibits good DWTT crack arrest performance and weld impact toughness, making it suitable for high-end subsea pipelines operating at low temperatures requiring crack arrest.

[0013] This invention provides a high-frequency resistance welded steel pipe, which is composed of the following chemical elements by mass percentage:

[0014] C: 0.025–0.054%, Si: 0.10–0.30%, Mn: 1.21–1.45%, Ni: 0.05–0.20%, Cr: 0.11–0.25%, Mo: 0.01–0.15%, Nb: 0.030–0.055%, Ti: 0.005–0.020%, Al: 0.020–0.050%, Ca: 0.001–0.004%; balance Fe and unavoidable impurities.

[0015] The above scheme, which uses a low-carbon, low-alloy composition design, with Mo+Cr reinforcement and the addition of Ni, can achieve good low-temperature crack arrest performance.

[0016] According to another specific embodiment of the present invention, the high-frequency resistance welded steel pipe provided by the present invention has a C+(Cr+Mo) / 5 mass percentage controlled in the range of 0.07 to 0.12%.

[0017] By adopting the above scheme, the mass percentage range of C+(Cr+Mo) / 5 is controlled so that the desired DWTT crack arrest performance can be achieved while meeting a certain strength requirement.

[0018] According to another specific embodiment of the present invention, the high-frequency resistance welded steel pipe provided by the present invention contains unavoidable impurities, wherein P, S, and B are controlled within the following mass percentages: P≤0.013%, S≤0.003%, and B≤0.0005%.

[0019] By adopting the above scheme, the content of harmful impurity elements is strictly controlled, thereby reducing the impact of harmful impurity elements on the quality of high-frequency resistance welded steel pipes.

[0020] According to another specific embodiment of the present invention, the high-frequency resistance welded steel pipe provided by the present invention has a microstructure of polygonal ferrite + granular bainite; wherein the grain size of the polygonal ferrite is ≤10μm; and the volume percentage of the polygonal ferrite is in the range of 10-40%.

[0021] This invention also provides a method for preparing high-frequency resistance welded steel pipes, comprising the following steps:

[0022] S1: Rolling the slab into a coil;

[0023] S2: Cut off the beginning and end of each coil, with the cut at an angle of 2 to 5 degrees to the plane where the coil is transverse. Weld the beginning and end of two adjacent coils together by carbon dioxide gas shielded welding to form a continuous steel strip.

[0024] S3: Mill the steel strip. The width of the milled steel strip is determined according to the outer diameter specification of the high-frequency resistance welded steel pipe.

[0025] S4: The steel strip is formed into a rough tube by using a roller forming method, which involves warp forming and precision forming.

[0026] S5: High-frequency welding is used to weld the edges of the rough tube together under the action of extrusion rollers to form a welded tube; wherein the high-frequency welding frequency is controlled to be ≥200kHz.

[0027] S6: The weld seam of the welded pipe is heat-treated by medium frequency induction heating, with a heat treatment temperature of 950~1010℃ and a holding time of ≥15s; after heat treatment, it is cooled by spray cooling water to a temperature range of 550~650℃, and then air-cooled.

[0028] S7: The heat-treated welded pipe is radially compressed using a sizing stand to achieve the specified dimensions.

[0029] Using the above scheme, the high-frequency welding process uses high frequency to enhance the skin effect, ensuring that the edge of the steel strip is in a good welding state; medium-frequency induction heating is used to heat treat the weld of the welded pipe, and excellent strength and toughness are obtained through weld heat treatment.

[0030] According to another specific embodiment of the present invention, in step S5 of the method for preparing high-frequency resistance welded steel pipe provided by the present invention, the welding power is controlled to be ≥700kW.

[0031] By adopting the above scheme, high welding power is used to ensure that the edge of the steel strip is fully melted, avoiding cold welding. At the same time, it is also conducive to the outward flow of molten metal under the action of electromagnetic force, effectively removing oxides.

[0032] According to another specific embodiment of the present invention, in the method for preparing high-frequency resistance welded steel pipe provided by the present invention, in step S5, the extrusion amount of the extrusion roller is controlled to be in the range of 10-25mm.

[0033] By adopting the above scheme, the extrusion roller uses a large extrusion amount to further extrude the oxides and other inclusions formed in the molten state, thereby obtaining a high-purity weld. By adopting a high-frequency, high-power, and large-extrusion welding process, defects such as welding oxides are effectively removed, resulting in good weld quality.

[0034] According to another specific embodiment of the present invention, in the method for preparing high-frequency resistance welded steel pipe provided by the present invention, in step S5, the opening V angle at the welding junction is controlled within the range of 3 to 6°.

[0035] By adopting the above scheme, the welding process can be made to remove inclusions by controlling the opening V angle, while ensuring that the welding temperature does not decrease.

[0036] According to another specific embodiment of the present invention, the method for preparing high-frequency resistance welded steel pipe provided by the present invention includes step S1 as follows:

[0037] S11: Heating the slab at a heating temperature of 1160~1210℃;

[0038] S12: The slab is rough rolled at a final pass temperature of ≥960℃;

[0039] S13: Finish rolling is performed with an initial rolling temperature of ≤930℃ and a final rolling temperature of 790~840℃;

[0040] S14: Laminar flow cooling is performed at a cooling rate of 25-35℃ / s; winding is performed at a temperature of 450-520℃.

[0041] By adopting the above scheme, grain refinement can be achieved and phase transformation structure can be effectively controlled by controlling the process parameters of rolling slabs into coils, so as to obtain high-frequency resistance welded steel pipes with a microstructure of fine polygonal ferrite + granular bainite.

[0042] The beneficial effects of this invention are:

[0043] The high-frequency resistance welded steel pipe provided by this invention adopts a low-carbon, low-alloy composition design, and with the optimized TMCP (Thermo-Mechanical Control Process) to achieve grain refinement and effectively control the phase transformation structure, the resulting high-frequency resistance welded steel pipe has a microstructure of fine polygonal ferrite + granular bainite, wherein the grain size of the polygonal ferrite is ≤10μm and the volume fraction is in the range of 10-40%. The resulting high-frequency resistance welded steel pipe has high strength and excellent low-temperature crack arrest performance; it can be applied to the field of high-end subsea pipelines with low-temperature service and crack arrest requirements.

[0044] The tensile properties of the high-frequency resistance welded steel pipe of this invention can achieve: pipe body yield strength ≥ 459 MPa, pipe body tensile strength ≥ 545 MPa, and weld tensile strength ≥ 549 MPa; the DWTT fracture shear area ratio of the high-frequency resistance welded steel pipe at -20℃ is above 91%, and it also meets the following requirements: pipe body Charpy impact energy at -20℃ ≥ 200 J, and weld Charpy impact energy at -20℃ ≥ 100 J. In addition to meeting other general technical requirements, the high-frequency resistance welded steel pipe of this invention also meets the low-temperature DWTT crack arrest performance requirements. Attached Figure Description

[0045] Figure 1 This is a microstructure diagram of the high-frequency resistance welded steel pipe obtained in Embodiment 1 of the present invention. Detailed Implementation

[0046] For the purposes of the detailed description below, it should be understood that, except in any operational instance or otherwise indicated, all figures representing the amounts of ingredients used, for example, in the specification and claims, should be understood to be modified in all cases by the term "about". Therefore, unless otherwise stated, the numerical parameters set forth in the following specification and appended claims are approximations varying according to the desired performance to be obtained in this application. It is not at least not an attempt to limit the application of the doctrine of equivalents to the scope of the claims, and each numerical parameter should be interpreted at least according to the number of significant figures reported and by applying ordinary rounding techniques.

[0047] The terminology used in this application is for the purpose of describing particular embodiments only and is not to be construed as limiting. As used herein, the singular forms “a” and “the” are also intended to include the plural forms unless the context clearly indicates otherwise. Expressions such as “at least one of…” modify the entire list of elements when preceding or following it, without modifying any individual elements in the list.

[0048] Furthermore, the terms "comprising" or "including" as used in this application, when used in this specification, indicate the presence of the stated features, areas, integrals, steps, operations, elements, and / or components, but do not exclude the presence or addition of one or more additional features, areas, integrals, steps, operations, elements, components, and / or combinations thereof.

[0049] As used herein, “about” or “approximately” includes the described value and means, for example, an acceptable range of deviation for a specific value, determined by a person of ordinary skill in the art, taking into account the measurement in question and errors associated with the measurement of the specific quantity (i.e., limitations of the measurement system). Unless otherwise specified, all parameter ranges disclosed include endpoint values ​​and all values ​​in between.

[0050] In the description of this invention, unless otherwise specified, the meanings of the terms are the same as those generally understood by those skilled in the art, but if there are any differences, the definitions of this invention shall prevail; unless otherwise specified, the test methods are all conventional methods; unless otherwise specified, the raw materials and test materials used in this invention are all conventionally available.

[0051] To make the objectives, technical solutions, and advantages of the present invention clearer, the embodiments of the present invention will be described in detail below.

[0052] This invention provides a high-frequency resistance welded steel pipe, which is composed of the following chemical elements by mass percentage:

[0053] C: 0.025–0.054%, Si: 0.10–0.30%, Mn: 1.21–1.45%, Ni: 0.05–0.20%, Cr: 0.11–0.25%, Mo: 0.01–0.15%, Nb: 0.030–0.055%, Ti: 0.005–0.020%, Al: 0.020–0.050%, Ca: 0.001–0.004%; balance Fe and unavoidable impurities.

[0054] The design principles of each chemical element in the high-frequency resistance welded steel pipe provided by this invention are as follows:

[0055] C (Carbon): In the high-frequency resistance welded steel pipe of this invention, C is the most effective strengthening element. However, it should be noted that when the C content is too high, it is easy to form more carbides or Mao islands, and it is also easy to form banded hard phase structure in the center of the thickness, which has an adverse effect on the crack arrest performance of the material. Based on this, in the high-frequency resistance welded steel pipe of this invention, the mass percentage content of C is controlled between 0.025% and 0.054%.

[0056] Mn (Manganese): In the high-frequency resistance welded steel pipe of this invention, Mn enhances strength through solid solution strengthening and has the effect of refining grains. However, when the Mn content is too high, lath bainite structure is easily formed during the rapid cooling and low-temperature coiling process of hot-rolled coils via TMCP, which has an adverse effect on low-temperature crack arrest performance. Based on this, in the high-frequency resistance welded steel pipe of this invention, the mass percentage content of Mn is controlled between 1.21% and 1.45%.

[0057] Si (silicon): In the high-frequency resistance welded steel pipe of the present invention, Si is a solid solution strengthening element and also a deoxidizing element in steel. However, when the Si content is too high, oxide inclusions are easily formed during high-frequency welding, which is detrimental to the weld quality. Based on this, in the high-frequency resistance welded steel pipe of the present invention, the mass percentage of Si is controlled between 0.10% and 0.30%.

[0058] Ni (Ni): In the high-frequency resistance welded steel pipe of this invention, Ni is an important element for improving low-temperature crack arrest performance. Ni can reduce dislocation slip resistance at low temperatures, alleviate low-temperature embrittlement, and also promote cross-slip of screw dislocations at low temperatures by increasing stacking fault energy, thereby increasing the energy consumed in crack propagation and improving local crack arrest capability. However, Ni is expensive and its addition should not be excessive. Therefore, in this invention, the mass percentage of Ni is controlled between 0.05% and 0.20%.

[0059] Cr (chromium): In the high-frequency resistance welded steel pipe of this invention, Cr can improve the hardenability and strength of the material, and is particularly helpful in improving the strength of the HFW weld. However, if the Cr content is too high, it will increase the hardness of bainite and reduce its toughness. Therefore, in this invention, the mass percentage of Cr is controlled between 0.11% and 0.25%.

[0060] Mo (Mo): In the high-frequency resistance welded steel pipe of this invention, Mo plays a role in controlling phase transformation and improving strength. Mo can lower the γ→α phase transformation temperature of steel, thus refining the microstructure. Furthermore, in low-carbon pipeline steel, it effectively promotes the formation of high-toughness granular bainite and inhibits pearlite. However, excessively high Mo content can easily lead to the formation of harder structures such as lath bainite, which is detrimental to low-temperature crack arrest performance. Therefore, in this invention, the mass percentage of Mo is controlled between 0.01% and 0.15%.

[0061] Niobium (Nb): In the high-frequency resistance welded steel pipe of this invention, Nb is the most important microalloying element for refining grains and can effectively improve the strength and toughness of the welded steel pipe. However, after the Nb content exceeds a certain amount, the grain refining effect no longer increases with the increase of Nb content. Therefore, in this invention, the mass percentage of Nb is controlled between 0.030% and 0.055%.

[0062] Ti (Titanium): In the high-frequency resistance welded steel pipe of this invention, adding a trace amount of Ti element can form TiN. During the slab heating process, TiN can prevent austenite grain growth and play a role in refining the original austenite grains. Therefore, in this invention, the mass percentage content of Ti element is controlled between 0.005% and 0.020%.

[0063] Al (aluminum): In the high-frequency resistance welded steel pipe of this invention, Al is a deoxidizing element, which is beneficial to improving the purity of the steel. However, excessive addition can easily form inclusions such as alumina. Therefore, in this invention, the mass percentage of Al is controlled between 0.020% and 0.050%.

[0064] Ca (calcium): In the high-frequency resistance welded steel pipe of the present invention, the use of trace amounts of Ca element treatment can avoid the formation of long strip-shaped MnS inclusions. However, excessive Ca addition can easily lead to the agglomeration of inclusions such as CaO and CaS. Therefore, in the high-frequency resistance welded steel pipe of the present invention, the mass percentage content of Ca element is controlled between 0.001% and 0.004%.

[0065] The high-frequency resistance welded steel pipe of this invention adopts a low-carbon, low-alloy composition design, which uses Mo+Cr for strengthening and adds Ni element to achieve good low-temperature crack arrest performance.

[0066] In this invention, since C, Cr, and Mo are important alloying elements for ensuring strength, a total content that is too low cannot guarantee strength, while a content that is too high is detrimental to toughness. Therefore, according to another specific embodiment of this invention, the mass percentage of C+(Cr+Mo) / 5 is controlled in the range of 0.07 to 0.12%.

[0067] According to another specific embodiment of the present invention, the unavoidable impurities P, S, and B are controlled within the following mass percentages: P ≤ 0.013 wt%, S ≤ 0.003 wt%, and B ≤ 0.0005%.

[0068] It should be noted that phosphorus (P), sulfur (S), and boron (B) are unavoidable harmful impurities in welded pipes. S easily forms elongated MnS inclusions, P reduces the low-temperature toughness of welded pipes, and B is a strong hardenability element that easily leads to a decrease in the plasticity and toughness of the material. Therefore, the high-frequency resistance welded steel pipe of this invention controls P ≤ 0.013%, S ≤ 0.003%, and B ≤ 0.0005%.

[0069] According to another specific embodiment of the present invention, the chemical composition is designed according to the present invention, and an optimized preparation process is used to obtain a fine granular bainitic microstructure to achieve the desired DWTT crack arrest performance. Specifically, the microstructure of the steel pipe is polygonal ferrite + granular bainite; wherein the grain size of the polygonal ferrite is ≤10μm; and the volume percentage of the polygonal ferrite is in the range of 10-40%.

[0070] The present invention also provides a method for preparing the high-frequency resistance welded steel pipe of the present invention, comprising the following steps:

[0071] S1: According to the composition design of the high-frequency resistance welded steel pipe of the present invention, a slab with a specific composition is obtained by smelting and continuous casting; the slab is rolled into a coil.

[0072] The slab contains the following chemical elements by mass percentage:

[0073] C: 0.025~0.054%, Si: 0.10~0.30%, Mn: 1.21~1.45%, Ni: 0.05~0.20%, Cr: 0.11~0.25%, Mo: 0.01~0.15%, Nb: 0.030~0.055%, Ti: 0.005~0.020%, Al: 0.020~0.050%, Ca: 0.001~0.004%.

[0074] Furthermore, in addition to the above-mentioned components, the balance is Fe and unavoidable impurities. Even further, among the unavoidable impurities, P, S, and B are controlled within the following mass percentages: P ≤ 0.013%, S ≤ 0.003%, B ≤ 0.0005%.

[0075] The steps of rolling a slab into a coil include slab heating, rough rolling, finish rolling, cooling, and coiling. Further, according to one specific embodiment of the present invention, the specific steps of rolling a slab into a coil include:

[0076] S11: The slab needs to be heated to a certain temperature to ensure that the alloying elements are fully dissolved, but excessively high heating temperature will lead to excessively large original austenite grain size; the present invention controls the heating temperature range of 1160~1210℃ to heat the slab.

[0077] S12: The rough rolling process refines the austenite grains through recrystallization, so it should be carried out within the recrystallization temperature range. In this invention, the temperature of the last pass of rough rolling is controlled to be ≥960℃.

[0078] S13: During the finishing rolling process, the austenite accumulates strain energy and deforms in the deformation zone, accompanied by recovery. Therefore, the rolling should be carried out in the non-recrystallization zone. To avoid the partial recrystallization zone, this invention controls the finishing rolling start temperature to ≤930℃; at the same time, it controls a relatively low finishing rolling finish temperature to reduce the recovery of strain energy, increase the phase deformation nucleation rate, and achieve the effect of refining the microstructure. Therefore, this invention controls the finishing rolling finish temperature to 790~840℃.

[0079] S14: The laminar cooling and coiling process is a transformation of deformed austenite. A higher cooling rate helps refine the grains and improve strength and toughness. Therefore, this invention controls the cooling rate at 25–35 °C / s for laminar cooling. The coiling temperature is the stopping temperature for cooling the hot-rolled coil. This invention aims to obtain a microstructure of fine polygonal ferrite + granular bainite. Therefore, it is necessary to suppress quasi-polygonal ferrite and pearlite, and the coiling temperature should not be too high. On the other hand, to avoid lath bainite structure, the coiling temperature should not be too low. Therefore, under the composition system of this invention, to obtain the target microstructure, the coiling temperature range is controlled to be 450–520 °C.

[0080] S2: Plate and coil butt welding. The head and tail of each plate and coil are cut off, and the cut is at an angle of 2 to 5 degrees to the plane where the plate and coil are transverse. The heads and tails of two adjacent plate and coil are welded together by carbon dioxide gas shielded welding to form a continuous steel strip.

[0081] S3: Milling the steel strip edge, precisely controlling the width of the steel strip after milling according to the outer diameter specifications of the designed high-frequency resistance welded steel pipe.

[0082] S4: The steel strip is formed into a rough tube by using a roller forming method, which involves warp forming and precision forming.

[0083] S5: High-frequency welding is used to weld the edges of the rough tube together under the action of extrusion rollers to form a welded tube; the high-frequency welding frequency is controlled to be ≥200kHz. Using a high frequency can enhance the skin effect and ensure that the steel strip edges are in a good welding state. The steel strip edges melt under the skin effect of high-frequency current and are welded together under the action of extrusion rollers. Then, the extruded internal and external burrs are scraped off to form a welded tube.

[0084] Furthermore, high welding power ensures that the steel strip edges are fully melted, avoiding cold welding, and also facilitates the outward flow of molten metal under the action of electromagnetic force, effectively removing oxides. Therefore, according to one specific embodiment of the present invention, the high-frequency welding power is controlled to be ≥700KW.

[0085] Furthermore, by employing a large extrusion amount on the extrusion roller, inclusions such as oxides formed in the molten state can be further extruded to obtain a high-purity weld. Therefore, according to one specific embodiment of the present invention, the extrusion amount of the extrusion roller is controlled to be between 10 and 25 mm, specifically, the extrusion amount = the circumference of the rough tube - the circumference after extrusion.

[0086] This invention employs a high-frequency, high-power, and high-extrusion welding process to effectively eliminate defects such as welding oxides and obtain good weld quality.

[0087] Furthermore, if the opening V-angle at the weld junction is too small, inclusions are difficult to remove; if the opening V-angle is too large, the welding temperature will decrease. Therefore, according to one specific embodiment of the present invention, the opening V-angle at the weld junction is controlled within the range of 3 to 6°.

[0088] S6: The weld seam of the welded pipe is heat-treated using medium-frequency induction heating. Due to the temperature difference between the inner and outer walls of the induction-heated pipe, the heat treatment temperature should not be too low to ensure complete austenitization of the weld microstructure. However, excessively high heat treatment temperatures can lead to coarsening of the weld microstructure and reduced impact toughness. Therefore, this invention controls the heat treatment temperature to 950–1010℃ and the holding time to ≥15s. After medium-frequency induction heat treatment, the weld seam is sprayed with cooling water to a temperature range of 550–650℃, followed by air cooling. Excellent strength and toughness are achieved through this weld seam heat treatment.

[0089] S7: The heat-treated welded pipe is radially compressed using a sizing stand to achieve the specified dimensions.

[0090] This invention employs a low-carbon, low-alloy composition design, combined with an optimized TMCP process to achieve grain refinement and effectively control phase transformation structure, resulting in a high-frequency resistance welded steel pipe with a microstructure of fine polygonal ferrite + granular bainite. The polygonal ferrite grain size is ≤10μm, and the volume fraction is in the range of 10-40%. The resulting high-frequency resistance welded steel pipe has high strength and excellent low-temperature crack arrest performance.

[0091] The preparation method of the steel plate for low-temperature high-strength and tough pipe fittings of the present invention will be further described below with reference to specific embodiments and accompanying drawings.

[0092] Examples 1-6

[0093] The high-frequency resistance welded steel pipes in Examples 1-6 were all prepared using the following steps:

[0094] S1: (1) Slabs are obtained by smelting and continuous casting according to the chemical composition shown in Table 1.

[0095] Table 1 Chemical composition (wt%) of Examples 1-6, balance being Fe and other unavoidable impurities besides P, S, and B.

[0096]

[0097]

[0098] (2) Rolling the slab into coils: This includes slab heating, rough rolling, finish rolling, laminar flow cooling, and coiling to obtain hot-rolled coils. Specifically, the slab heating temperature is controlled at 1160–1210℃, the final rough rolling temperature is controlled at ≥960℃, the initial finish rolling temperature is controlled at ≤930℃, and the final finish rolling temperature is controlled at 790–840℃; the laminar flow cooling rate is controlled at 25–35℃ / s, and the coiling temperature is controlled at 450–520℃. The specific process parameters for rolling the high-frequency resistance welded steel pipes of Examples 1-6 into coils are shown in Table 2-1.

[0099] Table 2-1 Specific process parameters for the rolling process in Examples 1-6

[0100]

[0101] S2: Plate and coil butt welding. The head and tail of each plate and coil are cut off, and the cut is at an angle of 2 to 5 degrees to the plane where the plate and coil are transverse. The heads and tails of two adjacent plate and coil are welded together by carbon dioxide gas shielded welding to form a continuous steel strip.

[0102] S3: Mill the steel strip. The width of the milled steel strip is determined according to the outer diameter specification of the high-frequency resistance welded steel pipe.

[0103] S4: The steel strip is formed into a rough tube by using a roller forming method, which involves warp forming and precision forming.

[0104] S5: High-frequency welding is used to weld the edges of the rough tube together under the action of the extrusion roller. Then, the extruded inner and outer burrs are scraped off to form a welded tube. The high-frequency welding frequency is controlled to be ≥200kHz, the welding power is controlled to be ≥700KW, the extrusion amount of the extrusion roller is controlled to be in the range of 10 to 25mm, and the opening V angle at the welding junction is controlled to be in the range of 3 to 6°.

[0105] S6: The weld seam of the welded pipe is heat-treated by medium frequency induction heating, with a heat treatment temperature of 950~1010℃ and a holding time of ≥15s; after heat treatment, it is cooled by spray cooling water to a temperature range of 550~650℃, and then air-cooled.

[0106] S7: The heat-treated welded pipe is radially compressed using a sizing stand to achieve the specified dimensions.

[0107] The specific process parameters for steps S5-S7 above are shown in Table 2-2.

[0108] Table 2-2 Specific process parameters for steps S5-S7 in Examples 1-6

[0109]

[0110] Performance testing:

[0111] The high-frequency resistance welded steel pipes obtained in Examples 1-6 were sampled and subjected to performance testing, including tensile testing, impact testing, and drop hammer tear test (DWTT). It should be noted that the tensile and impact tests were conducted according to ASTM A370 standard, "Methods and Definitions for Testing the Mechanical Properties of Steel Products," and the drop hammer tear test (DWTT) was conducted according to API RP 5L3 standard, "Drop Hammer Tear Test Method for Pipeline Steel."

[0112] The performance test results of Examples 1-6 are shown in Tables 3 and 4, respectively; Table 3 lists the tensile property test results of the high-frequency resistance welded steel pipes of Examples 1-6, and Table 4 lists the DWTT and impact toughness test results of the high-frequency resistance welded steel pipes of Examples 1-6.

[0113] Table 3. Tensile property test results of high-frequency resistance welded steel pipes in Examples 1-6

[0114]

[0115] Table 4. DWTT and impact toughness test results of high-frequency resistance welded steel pipes in Examples 1-6.

[0116]

[0117]

[0118] As can be seen from Tables 3 and 4, the high-frequency resistance welded steel pipes of Examples 1-6 of the present invention have excellent comprehensive performance, not only with high strength, but also with excellent DWTT crack arrest performance and impact toughness.

[0119] As can be seen from Table 3, in this invention, the high-frequency resistance welded steel pipes of Examples 1-6 have a pipe body yield strength ≥459MPa, a pipe body tensile strength ≥545MPa, and a weld tensile strength ≥549MPa.

[0120] Furthermore, as can be seen from Table 4, in this invention, the high-frequency resistance welded steel pipes of Examples 1-6 all have a DWTT fracture shear area ratio of over 91% at -20℃, and also meet the following requirements: Charpy impact energy of the pipe body at -20℃ ≥ 200J, and Charpy impact energy of the weld at -20℃ ≥ 100J.

[0121] As shown in Table 1, the C+(Cr+Mo) / 5 content in the high-frequency resistance welded steel pipe of Example 5 was controlled at 0.123 (the highest among the six examples). Further, Table 4 shows that the Charpy impact energy of the pipe body at -20℃ in Example 5 was 267J, the lowest among the six examples, proving that an excessively high C+(Cr+Mo) / 5 content is detrimental to toughness. As shown in Table 1, the C+(Cr+Mo) / 5 content in the high-frequency resistance welded steel pipe of Example 6 was controlled at 0.073 (the lowest among the six examples). Further, Table 3 shows that the yield strength of the pipe body in Example 6 was 459MPa, and the tensile strength was 545MPa, the lowest among the six examples, proving that an excessively low C+(Cr+Mo) / 5 content cannot guarantee strength.

[0122] According to Table 2-2, the high-frequency resistance welded steel pipes of Examples 1-3 have the same outer diameter, with Example 2 having the smallest thickness-to-diameter ratio and Example 3 having the largest thickness-to-diameter ratio. Furthermore, according to Table 4, among the three examples, the Charpy impact energy (272J) and DWTT shear area ratio (93%) of the pipe body at -20℃ of Example 3 are lower than those of Examples 1-2, indicating that the thickness-to-diameter ratio affects the impact toughness and DWTT performance, and the toughness decreases as the thickness-to-diameter ratio increases.

[0123] In summary, it can be seen that the high-frequency resistance welded steel pipe of the present invention, through reasonable chemical composition design and optimized process, can obtain high strength and impact toughness, and in particular, has excellent low-temperature crack arrest performance. It can be applied to the field of high-end submarine pipelines with low-temperature service and crack arrest requirements, and has a very broad application prospect.

[0124] Figure 1 This is a microstructure diagram of the high-frequency resistance welded steel pipe obtained in Example 1.

[0125] like Figure 1As shown, the microstructure of the high-frequency resistance welded steel pipe in Example 1 is fine polygonal ferrite + granular bainite, and the analysis shows that the grain size of the polygonal ferrite is ≤10μm and the volume fraction is in the range of 10-40%.

[0126] The above description, in conjunction with specific embodiments, provides a further detailed explanation of the present invention. It should not be construed that the specific implementation of the present invention is limited to these descriptions. For those skilled in the art, various simple deductions or substitutions can be made without departing from the concept of the present invention, and all such modifications and substitutions should be considered within the scope of protection of the present invention.

Claims

1. A method for preparing a high-frequency resistance welded steel pipe, characterized in that, The high-frequency resistance welded steel pipe is composed of the following chemical elements by mass percentage composition: C: 0.025~0.054%, Si: 0.10~0.30%, Mn: 1.21~1.45%, Ni: 0.05~0.20%, Cr: 0.11~0.25%, Mo: 0.01~0.15%, Nb: 0.030~0.055%, Ti: 0.005~0.020%, Al: 0.020~0.050%, Ca: 0.001~0.004%; the balance is Fe and unavoidable impurities; wherein, the mass percentage of C+(Cr+Mo) / 5 is controlled in the range of 0.07~0.12%; The microstructure of the high-frequency resistance welded steel pipe is polygonal ferrite + granular bainite; wherein the grain size of the polygonal ferrite is ≤10μm; and the volume percentage of the polygonal ferrite is in the range of 10~40%. The method includes the following steps: S1: Rolling the slab into a coil, including laminar flow cooling and coiling processes, wherein laminar flow cooling is performed at a cooling rate of 25~35℃ / s; and coiling is performed at a temperature of 450~520℃. S2: Cut off the beginning and end of each coil of steel strip, with the cut at an angle of 2 to 5° to the plane in which the coil is transverse. Weld the beginning and end of two adjacent coils together by carbon dioxide gas shielded welding to form a continuous steel strip. S3: Mill the edge of the steel strip, and the width of the steel strip after milling is determined according to the outer diameter specification of the high-frequency resistance welded steel pipe; S4: The steel strip is formed into a rough tube by using a roller forming method and wire forming and precision forming. S5: The edges of the rough tube are welded together by high-frequency welding under the action of the extrusion roller to form a welded tube; wherein the high-frequency welding frequency is controlled at ≥200kHz to enhance the skin effect and ensure that the steel strip edge is in a good welding state; and the welding power is controlled at ≥700kW, and the extrusion amount of the extrusion roller is controlled in the range of 10 to 25mm. S6: The weld of the welded pipe is heat-treated by medium-frequency induction heating, wherein the heat treatment temperature is 950~1010℃ and the holding time is ≥15s; after heat treatment, the weld is cooled by spray cooling water to the range of 550~650℃, and then air-cooled. S7: The heat-treated welded pipe is radially compressed using a sizing stand to achieve the specified dimensions.

2. The method for preparing high-frequency resistance welded steel pipe as described in claim 1, characterized in that, Of the unavoidable impurities, P, S, and B are controlled within the following mass percentages: P ≤ 0.013%, S ≤ 0.003%, and B ≤ 0.0005%.

3. The method for preparing high-frequency resistance welded steel pipe as described in claim 1, characterized in that, In step S5, the opening V-angle at the welding junction is controlled to be within the range of 3~6°.

4. The method for preparing high-frequency resistance welded steel pipe according to any one of claims 1-3, characterized in that, In step S1, prior to the laminar flow cooling and winding process, the following is included: S11: The slab is heated at a temperature of 1160~1210℃; S12: The slab is rough rolled at a final pass temperature ≥960℃; S13: Finish rolling is performed at an initial rolling temperature of ≤930℃ and a final rolling temperature of 790~840℃.

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