High-strength anti-fatigue steel for tension leg tendon connector and preparation method of high-strength anti-fatigue steel

Abstract

The invention relates to the technical field of high-performance steel for ocean engineering, in particular to high-strength anti-fatigue steel for a tension leg tendon connector and a preparation method of the high-strength anti-fatigue steel. The steel comprises the following chemical components: 0.02%-0.04% of C, 0.1%-0.35% of Si, 1.0%-1.2% of Mn, less than or equal to 0.013% of P, less than or equal to 0.004% of S, 1.13%-1.33% of Cu, 1.9%-2.2% of Ni, less than or equal to 0.005% of Ca, 0.5%-0.7% of Cr, 0.3%-0.5% of Mo, 0.02%-0.045% of Al, 0.04%-0.06% of Nb, less than or equal to 0.009% of N, less than or equal to 0.0002% of H and less than or equal to 0.0018% of O. The carbon equivalent Ceq is less than or equal to 0.8%. The high-strength, high-toughness and anti-fatigue steel for the tendon connector of the tension leg is prepared by adopting a low-carbon copper-containing component design according to the special requirements of the tendon connector on steel, forging, fully utilizing the precipitation strengthening effect of a copper-containing element and matching a reasonable heat treatment system. And high-quality raw materials are provided for ocean equipment such as tension leg tendon connectors.

Description

Technical Field

[0001] The present invention relates to the technical field of high-performance steel for ocean engineering, and particularly to a high-strength anti-fatigue steel for a tension leg tendon connector and a preparation method thereof. Background Art

[0002] A tension leg platform is a floating platform used for deep-sea oil and gas development. It is anchored to the seabed through vertical tension legs and maintains stability by combining buoyancy and the tension of the tension legs. Its working principle is to utilize the semi-compliant and semi-rigid platform to generate a buoyancy far greater than the self-weight of the structure, so as to balance the pre-tension, thereby providing a relatively stable and safe working environment. Moreover, its upright floating cylinder structure can also endow it with good motion performance. In the field of deep and far-offshore platforms, the tension leg platform has become one of the most ideal facilities for developing deep and far-sea energy due to its good vertical motion performance and compact system layout.

[0003] A high-strength, high-toughness and corrosion-resistant tendon anchoring system is the key to maintaining the stability of a tension leg platform. The tension leg platform usually uses steel pipes as the main material of the tendon. The material of the steel pipe is generally low-carbon alloy steel (such as pipeline steel with the grade of X70). And the whole tendon is generally formed by connecting many steel pipes dozens of meters long end to end. When connecting the steel pipes, a tendon connector is required as a fixing and connecting device between two steel pipes.

[0004] Since the tension leg tendon steel pipe needs to bear a large tension, therefore, the tendon connector itself also has to bear a large stress, which requires the material of the tendon connector to have a certain strength. And the tendon connector itself works in a seawater corrosion environment, requiring it to have a certain corrosion resistance; at the same time, the tension leg platform needs to ensure a certain motion coupling and stability, requiring the tendon connector to have a certain anti-fatigue ability. The above requirements determine that the tendon connector cannot be made of carbon steel that is not corrosion-resistant. Therefore, it is necessary to develop a material specifically for the tension leg tendon connector.

[0005] The Chinese patent application with the publication number of CN118391327A discloses "a tension leg tendon connector and its usage method", but it only relates to the structure and usage method of the tendon connector, does not involve the material and performance indexes of the tendon connector, and even less involves the production process of the tendon connector material.

[0006] Chinese Patent Application No. CN118547212A discloses a "high-strength steel for marine equipment with excellent low-temperature fracture toughness and its preparation method". The chemical composition of the steel by weight percentage is as follows: C 0.03% - 0.05%, Si 0.15% - 0.35%, Mn 1.2% - 1.4%, P ≤ 0.013%, S ≤ 0.004%, Ni 2.6% - 3.0%, Cr 0.5% - 0.7%, Cu 1.0% - 1.3%, Mo 0.35% - 0.5%, Al 0.02% - 0.045%, Nb 0.04% - 0.06%, N ≤ 0.009%, H ≤ 0.0002%, O ≤ 0.0018%, and the balance is Fe and inevitable impurity elements. However, the contents of main elements such as C, Mn, Cu, and Ni in the composition of this high-strength steel for marine equipment are different from those of the present invention. It adopts the steel plate rolling process, uses the primary online quenching and secondary offline quenching processes, and the aging temperature is 620 - 650°C, while the present invention adopts the forging process, and the temperatures and times of heat treatments such as quenching and aging are completely different from it. In addition, the strength and other properties of this high-strength steel for marine equipment are also different from those of the high-strength fatigue-resistant steel described in the present invention, and it does not involve the welding performance with low-carbon steel plates. The high-strength fatigue-resistant steel described in the present invention is specially designed for the tension leg tendon connector and can be welded to low-carbon steel.

[0007] In view of the working environment of the tension leg tendon connector, the present invention adopts the forging process, utilizes the precipitation strengthening technology of copper, selects a suitable heat treatment process, and develops a new type of steel suitable for the tension leg tendon connector. Summary of the Invention

[0008] The present invention provides a high-strength fatigue-resistant steel for tension leg tendon connectors and its preparation method. In view of the special requirements of the tendon connector for steel, a low-carbon copper-containing composition design is adopted. Through forging, the precipitation strengthening effect of the copper element is fully utilized, and a reasonable heat treatment system is matched to prepare a high-strength, high-toughness and fatigue-resistant steel for tension leg tendon connectors, providing high-quality raw materials for marine equipment such as tension leg tendon connectors.

[0009] To achieve the above object, the present invention is realized by the following technical solutions:

[0010] A high-strength anti-fatigue steel for a tension leg tendon connector, the chemical components in the steel are by mass percentage: C: 0.02% - 0.04%, Si: 0.1% - 0.35%, Mn: 1.0% - 1.2%, P ≤ 0.013%, S ≤ 0.004%, Cu: 1.13% - 1.33%, Ni: 1.9% - 2.2%, Ca ≤ 0.005%, Cr: 0.5% - 0.7%, Mo: 0.3% - 0.5%, Al: 0.02% - 0.045%, Nb: 0.04% - 0.06%, N ≤ 0.009%, H ≤ 0.0002%, O ≤ 0.0018%; the carbon equivalent Ceq ≤ 0.8%, and Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15; the balance is Fe and inevitable impurity elements.

[0011] The yield strength of the finished steel ≥ 520 MPa, the tensile strength ≥ 640 MPa, the surface Brinell hardness ≥ 190 HB10, the elongation ≥ 24%, and the absorbed energy KV2 of the -20°C V-notch impact test ≥ 220 J.

[0012] The fatigue limit strength of the finished steel ≥ 300 MPa.

[0013] A preparation method of a high-strength anti-fatigue steel for a tension leg tendon connector, comprising the following steps:

[0014] 1) Smelting and casting: Smelting is carried out by an electric arc furnace according to the set chemical composition, degassing treatment is carried out by LF furnace refining, further decarburization and degassing treatment is carried out by VOD vacuum furnace, and then the molten steel is cast into an ingot.

[0015] 2) Forging: The ingot is forged by the upsetting and drawing process, and the total forging ratio is not less than 4.

[0016] 3) Normalizing treatment: The forging is heated to 955 - 995°C and held, the holding time is calculated as 1 h / (20 - 30) mm thickness, and the total holding time is not less than 3 h, and then the forging is air-cooled to room temperature.

[0017] 4) The first water quenching treatment: The forging after normalizing treatment is heated to 920 ± 10°C, the holding time is calculated as 1 h / (20 - 30) mm thickness, and the total holding time is not less than 3 h; then immediately the forging is put into water and quenched to below 200°C, and the forging is cooled to room temperature after being taken out of the water.

[0018] 5) The second water quenching treatment: The forging after the first water quenching treatment is heated to 895° ± 10°C, the holding time is calculated as 1 h / (20 - 30) mm thickness, and the total holding time does not exceed 2 h; then immediately the forging is put into water and quenched to below 200°C, and the forging is cooled to room temperature after being taken out of the water.

[0019] 6) Aging treatment: Heat the forging that has undergone the second water quenching treatment to 660 ± 5 °C, calculate the holding time according to 1 h / (20 - 30) mm thickness, with the total holding time not exceeding 3 h, and then air cool the forging to room temperature.

[0020] Compared with the prior art, the beneficial effects of the present invention are:

[0021] 1) In view of the special requirements of tendon connectors for steel, a low-carbon copper-containing composition design is adopted, making full use of the precipitation strengthening effect of copper-containing steel, matching a reasonable heat treatment system, and preparing high-strength, high-toughness and fatigue-resistant steel for tension leg tendon connectors through forging; providing high-quality materials for marine equipment such as tension leg tendon connectors.

[0022] 2) The high-strength fatigue-resistant steel prepared by the present invention can be welded to low-carbon alloy steel, and is particularly suitable for tension leg platform tendon connectors. Description of the Drawings

[0023] Figure 1 It is the quenching and aging heat treatment process curve diagram of the steel described in the present invention. Specific Embodiments

[0024] For the high-strength fatigue-resistant steel for tension leg tendon connectors described in the present invention, the chemical components in the steel are by mass percentage: C: 0.02% - 0.04%, Si: 0.1% - 0.35%, Mn: 1.0% - 1.2%, P ≤ 0.013%, S ≤ 0.004%, Cu: 1.13% - 1.33%, Ni: 1.9% - 2.2%, Ca ≤ 0.005%, Cr: 0.5% - 0.7%, Mo: 0.3% - 0.5%, Al: 0.02% - 0.045%, Nb: 0.04% - 0.06%, N ≤ 0.009%, H ≤ 0.0002%, O ≤ 0.0018%; the carbon equivalent Ceq ≤ 0.8%, and Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15; the balance is Fe and unavoidable impurity elements.

[0025] The yield strength of the finished steel ≥ 520 MPa, the tensile strength ≥ 640 MPa, the surface Brinell hardness ≥ 190 HB10, the elongation ≥ 24%, and the absorbed energy KV2 of the -20 °C V-notch impact test ≥ 220 J.

[0026] The fatigue limit strength of the finished steel ≥ 300 MPa.

[0027] A preparation method of high-strength fatigue-resistant steel for tension leg tendon connectors, comprising the following steps:

[0028] 1) Smelting and casting: The steel is smelted in an electric arc furnace according to the set chemical composition, degassed by refining in an LF furnace, further decarburized and degassed in a VOD vacuum furnace, and then the molten steel is cast into ingots.

[0029] 2) Forging: The ingots are forged using the upsetting and drawing process, and the total forging ratio is not less than 4.

[0030] 3) Normalizing treatment: The forgings are heated to 955 - 995 °C and held for heat preservation. The heat preservation time is calculated as 1 h / (20 - 30) mm thickness, and the total heat preservation time is not less than 3 h. Then the forgings are air-cooled to room temperature.

[0031] 4) First water quenching treatment: The forgings after normalizing treatment are heated to 920 ± 10 °C, and the heat preservation time is calculated as 1 h / (20 - 30) mm thickness, with the total heat preservation time not less than 3 h. Then the forgings are immediately put into water and quenched to below 200 °C. After the forgings are taken out of the water, they are cooled to room temperature.

[0032] 5) Second water quenching treatment: The forgings after the first water quenching treatment are heated to 895 °± 10 °C, and the heat preservation time is calculated as 1 h / (20 - 30) mm thickness, with the total heat preservation time not exceeding 2 h. Then the forgings are immediately put into water and quenched to below 200 °C. After the forgings are taken out of the water, they are cooled to room temperature.

[0033] 6) Aging treatment: The forgings after the second water quenching treatment are heated to 660 ± 5 °C, and the heat preservation time is calculated as 1 h / (20 - 30) mm thickness, with the total heat preservation time not exceeding 3 h. Then the forgings are air-cooled to room temperature.

[0034] The quenching and aging heat treatment process curve of the steel described in the present invention is as Figure 1 shown.

[0035] The composition design basis of the high-strength anti-fatigue steel for a tension leg tendon connector described in the present invention is as follows:

[0036] C: Carbon is the main strengthening element in steel. However, the design concept of the present invention is different from that of conventional carbon steel. The present invention does not improve the strength of the material by the carbon content, that is, a super-low carbon composition design is adopted, and the content of element C is preferably controlled at 0.02% - 0.04%.

[0037] Si: Silicon is used as a deoxidizer and reducing agent in the steelmaking process. Excessive Si content will affect the toughness of the material. The content of element Si is preferably 0.1% - 0.35% in the present invention.

[0038] Mn: Manganese can improve the strength and corrosion resistance of carbon steel and is a widely used strengthening element in steel. The content of element Mn is preferably 1.0% - 1.2% in the present invention.

[0039] P: Phosphorus is a harmful element in steel, which has an adverse effect on the plasticity and toughness of the material. In the present invention, the content of P element is preferably ≤0.013%.

[0040] S: Sulfur is a harmful element in steel, which has an adverse effect on the plasticity and toughness of the material. In the present invention, the content of S element is preferably ≤0.004%.

[0041] Cu: Copper element acts as a precipitation strengthening element in the steel described in the present invention. Its strengthening mechanism is as follows: First, through austenite homogenization, the copper element is dissolved into austenite, then a supersaturated solid solution structure is obtained by quenching, and then aging treatment is used to precipitate the copper element from the supersaturated solid solution to form fine precipitation phases containing Cu element. These fine precipitation phases can hinder the movement of dislocations, thereby improving the strength and hardness of the material without reducing the toughness of the material. In the present invention, the content of Cu element is preferably 1.13% - 1.33%.

[0042] Ni: Nickel element can improve the toughness of the material and also has a good effect on improving the strength and corrosion resistance of steel; especially adding Ni element to copper-containing steel can reduce the tendency of hot cracking of the material. The mechanism is as follows: After the Cu element precipitates, the Ni element precipitates around the Cu element with the Cu element as the core, forming a wrapping effect on the Cu element to prevent the Cu element from forming harmful phases. In the present invention, the content of Ni element is preferably 1.9% - 2.2%.

[0043] Ca: Calcium element can control the shape of inclusions in steel, but the residual Ca element has an adverse effect on the material properties. In the present invention, the content of Ca element is preferably ≤0.005%.

[0044] Cr: Chromium element can improve the corrosion resistance and hardenability of the material. In the present invention, the content of Cr element is preferably 0.5% - 0.7%.

[0045] Mo: Molybdenum element can improve the hardenability of the material and also reduce temper brittleness. In the present invention, the content of Mo element is preferably 0.3% - 0.5%.

[0046] Al: Aluminum element is often used as a deoxidizer. In the present invention, the content of Al element is preferably 0.02% - 0.045%.

[0047] Nb: Niobium element can refine the grain size after quenching by hindering the growth of austenite, improving the strength and toughness of the steel. In the present invention, the content of Nb element is preferably 0.04% - 0.06%.

[0048] N: Nitrogen element is an element that strongly expands and stabilizes austenite, but too high nitrogen content will cause a decrease in the toughness of the material. In the present invention, the content of N element is preferably ≤0.009%.

[0049] O and H: Both are harmful gas elements. Oxygen is likely to form inclusions, and hydrogen is likely to cause flakes, greatly reducing the plasticity and toughness of steel and leading to defects such as delayed cracks. Therefore, it is necessary to strictly control the content of both. In the present invention, it is preferably to control the O content below 0.0018% and the H content below 0.0002%.

[0050] The preparation method of the high-strength anti-fatigue steel for a tension leg tendon connector according to the present invention has the following process design basis:

[0051] The use of the high-strength anti-fatigue steel according to the present invention is to manufacture a tendon connector for connecting tendon steel pipes (made of low-carbon low-alloy steel, such as X70 pipeline steel) in a tension leg platform. Therefore, it is necessary to match the strength and toughness of the tendon steel pipe. However, different from conventional low-carbon steel, the steel for the tendon connector itself requires relatively high toughness and corrosion resistance. This requires that the alloy system of traditional carbon steel cannot be used for the steel for the tendon connector. Although the alloy system of stainless steel can solve the problem of corrosion resistance, its weldability is poor and it cannot solve the welding problem between the tendon connector and the tendon steel pipe.

[0052] The Cu element can produce a strong precipitation strengthening effect in steel, improve the strength of the material without reducing its toughness, and also ensure the corrosion resistance and weldability of the material. Therefore, through the precipitation strengthening effect of Cu and a reasonable heat treatment process, excellent physical properties are obtained in the present invention. By forging, a material with high internal density and a specific shape can be obtained.

[0053] The heat treatment process adopted in the present invention is "normalizing treatment + first water quenching treatment + second water quenching treatment + aging treatment", and its process design basis is as follows:

[0054] Quenching of the material helps to obtain a high-strength martensite structure. However, the grain size of the structure obtained by the first quenching does not meet the requirements; in order to further refine the grains and give full play to the role of fine grain strengthening, a heat treatment process of secondary quenching is selected. The target heating temperature of the first quenching is set at 920 °C and controlled within the range of ±10 °C. At the same time, the holding time is set at one hour for every 20 - 30 mm thickness of heating, but the total heating time is not less than 3 hours to ensure that Cu and other solute elements are fully dissolved and uniformly distributed in the austenite matrix. The first water quenching temperature is set below 200 °C to ensure that the water quenching temperature is below the martensite transformation temperature to form a supersaturated solid solution.

[0055] During the first water quenching treatment, the austenitizing heating temperature is relatively high and the holding time is relatively long. As a result, the austenite grain size is relatively large, and the obtained quenched martensite grain size is also relatively large, which cannot meet the requirements of further heat treatment. Therefore, it is necessary to perform a second quenching treatment on the material obtained from the first quenching. The target heating temperature for the second quenching is set at 895 °C and controlled within the range of ±10 °C to ensure the austenite homogenization of the material. At the same time, the holding time is set at one hour of heating for every 20 - 30 mm of thickness, and the total heating time does not exceed 2 hours. The main purpose of the second quenching is to further refine the grains and improve the strength of the material, preparing for the next aging treatment.

[0056] The aging temperature and aging time during the aging treatment are key parameters for solute element precipitation strengthening. For example, if the aging temperature is too low, the strength of the material remains relatively high but the toughness will decrease; if the aging temperature is too high, the strength of the material will decrease but the toughness is relatively high. In order to obtain a high-strength, high-toughness and fatigue-resistant material, the present invention adopts the idea of high-temperature over-aging. The target heating during the aging treatment is set at 660 °C and controlled within the range of ±5 °C. The holding time is one hour of heating for every 20 - 30 mm of thickness, but the total heating time does not exceed 3 hours. The purpose of the aging treatment is to ensure the sufficient precipitation of copper elements, thereby providing sufficient strength for the material.

[0057] Since the present invention adopts the design idea of ultra-low carbon, the generation of hard carbide is reduced, making the stress distribution inside the material more uniform and reducing the probability of fatigue crack initiation. At the same time, the uniform precipitation of copper elements helps to inhibit the propagation of fatigue cracks in the material, making the material have good fatigue resistance.

[0058] The addition of elements such as copper and nickel increases the corrosion potential of the steel, making the material have good corrosion resistance.

[0059] The function of the tension leg tendon connector is to ensure the stable and rapid docking of the tendon steel pipe. One end of the tendon connector is welded to the low-carbon steel pipe, and the other end is used for docking operations with other tendon connectors. After docking, it is necessary to ensure that the overall structure has a certain degree of movement coupling and stability. Therefore, the steel used for the tendon connector is required to be able to be welded to the low-carbon steel pipe and also have a certain fatigue resistance.

[0060] To achieve welding, good weldability and a low carbon equivalent need to be ensured. The carbon equivalent Ceq can be calculated by the following formula: Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15. According to this calculation formula, the carbon equivalent of the high-strength and fatigue-resistant steel for the tension leg tendon connector described in the present invention is approximately 0.632% (mass percentage). That is, the Ceq value of the steel does not exceed 0.8%, and it can be welded to the low-carbon steel.

[0061] To more intuitively illustrate the present invention, the embodiments of the present invention will be further described in conjunction with the examples. The following examples are only preferred specific embodiments of the present invention, but the protection scope of the present invention is not limited thereto. Any technical solutions that can be obviously obtained by those skilled in the art within the technical scope disclosed by the present invention, including simple changes or equivalent replacements, shall fall within the protection scope of the present invention.

[0062]

Example

[0063] The chemical compositions of the steel in each example are shown in Table 1; the production process parameters are shown in Table 2; the finished product performance parameters are shown in Table 3; the performance of the welded joint between the finished product and low-carbon steel is shown in Table 4.

[0064] Table 1 Chemical Compositions of Steel (mass percentage %)

[0065]

[0066] Table 2 Main Production Process Parameters

[0067]

[0068] Table 3 Finished Product Performance

[0069]

[0070] Table 4 Basic Performance of the Welded Joint between the Finished Product and Low-Carbon Steel

[0071]

[0072] As mentioned above, the above are only preferred specific embodiments of the present invention, but the protection scope of the present invention is not limited thereto. Any technical personnel familiar with the technical field within the technical scope disclosed by the present invention, according to the technical solution and inventive concept of the present invention, making equivalent replacements or changes, shall be covered by the protection scope of the present invention.

Claims

1. A high-strength anti-fatigue steel for a tendon connector of a tension leg, characterized in that, The chemical components in the steel are by mass percentage: C: 0.02% - 0.04%, Si: 0.1% - 0.35%, Mn: 1.0% - 1.2%, P ≤ 0.013%, S ≤ 0.004%, Cu: 1.13% - 1.33%, Ni: 1.9% - 2.2%, Ca ≤ 0.005%, Cr: 0.5% - 0.7%, Mo: 0.3% - 0.5%, Al: 0.02% - 0.045%, Nb: 0.04% - 0.06%, N ≤ 0.009%, H ≤ 0.0002%, O ≤ 0.0018%; the carbon equivalent Ceq ≤ 0.8%, and Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15; the balance is Fe and inevitable impurity elements.

2. The high-strength anti-fatigue steel for a tendon connector of a tension leg according to claim 1, wherein The yield strength of the finished steel is ≥520 MPa, the tensile strength is ≥640 MPa, the surface Brinell hardness is ≥190 HB10, the elongation is ≥24%, and the absorbed energy KV2 of the -20°C V-notch impact test is ≥220 J.

3. The high-strength anti-fatigue steel for a tension leg tendon connector according to claim 1, wherein The fatigue limit strength of the finished steel is ≥300 MPa.

4. The preparation method of the high-strength anti-fatigue steel for a tension leg tendon connector according to claim 1 or 2 or 3, characterized in that, It includes the following steps: 1) Smelting and casting: Smelt according to the set chemical components using an electric arc furnace, carry out degassing treatment using an LF furnace refining, further carry out decarburization and degassing treatment using a VOD vacuum furnace, and then pour the molten steel into an ingot. 2) Forging: Forge the ingot using the upsetting and drawing process, and the total forging ratio is not less than 4. 3) Normalizing treatment: Heat the forging to 955 - 995°C and hold it, and the holding time is calculated as 1 h / (20 - 30) mm thickness, and the total holding time is not less than 3 h, and then air-cool the forging to room temperature. 4) First water quenching treatment: Heat the forging after normalizing treatment to 920 ± 10°C, and the holding time is calculated as 1 h / (20 - 30) mm thickness, and the total holding time is not less than 3 h; then immediately put the forging into water and quench it to below 200°C, and after taking the forging out of the water, cool it to room temperature. 5) Second water quenching treatment: Heat the forging after the first water quenching treatment to 895° ± 10°C, and the holding time is calculated as 1 h / (20 - 30) mm thickness, and the total holding time does not exceed 2 h; then immediately put the forging into water and quench it to below 200°C, and after taking the forging out of the water, cool it to room temperature. 6) Aging treatment: Heat the forging after the second water quenching treatment to 660 ± 5°C, and the holding time is calculated as 1 h / (20 - 30) mm thickness, and the total holding time does not exceed 3 h, and then air-cool the forging to room temperature.

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