Seamless square tube for offshore crane jib

Abstract

The application discloses a seamless square tube for a crane boom of offshore engineering, relates to the technical field of steel pipes, and improves the purity of square tube steel, solves the problems of insufficient corrosion fatigue resistance of the existing seamless square tube for the crane boom of offshore engineering in a harsh marine environment and poor low-temperature toughness caused by the difficulty in balancing the strength and toughness, and the seamless square tube comprises the following components: C: 0.14%-0.18%, Si: 0.25%-0.45%, Mn: 1.40%-1.60%, P<=0.015%, S<=0.010%, Cu<=0.20%, Ni<=0.25%, Cr: 0.30%-0.40%, Mo: 0.25%-0.30%, V: 0.05%-0.09%, W: 0.40%-0.60%, Nb<=0.05%, Ti<=0.05%, and Al: 0.02%-0.05%.

Description

Technical Field

[0001] This application relates to the field of marine engineering crane technology, and more specifically, to a seamless square tube for the boom of a marine engineering crane. Background Technology

[0002] The boom of a marine engineering crane is the most prominent and critical load-bearing component in heavy marine equipment. In the turbulent ocean environment, the boom must bear enormous loads, lifting hundreds or even thousands of tons of material. It must precisely deliver these loads to designated locations through luffing or telescoping, thus requiring a wide operating radius. Furthermore, the boom must withstand complex dynamic loads, enduring the combined impacts and vibrations from the inertia, centrifugal force, and wind loads caused by the ship's swaying in wind and waves. The performance of the marine engineering crane boom directly determines the crane's lifting height, working radius, load-bearing capacity, and operational efficiency, making it a vital force in offshore wind power, bridge construction, and oil drilling platform installation projects.

[0003] However, the corrosion fatigue resistance of existing seamless square tubing used in marine crane booms still needs improvement in harsh marine environments, affecting the long-term safety and lifespan of the structure. Furthermore, existing seamless square tubing for marine crane booms struggles to achieve a balance between strength and toughness. In pursuit of high strength to reduce weight, low-temperature impact toughness sometimes has to be sacrificed. When operating in cold sea areas, the tubing's resistance to brittle fracture is insufficient, leading to unstable safety. Therefore, there is an urgent need for a seamless square tubing with good resistance to marine corrosion, high strength, and good low-temperature impact toughness to meet the tubing requirements of marine crane booms. Summary of the Invention

[0004] This application provides a seamless square tube for the boom of a marine engineering crane, which solves the problems of insufficient corrosion fatigue resistance and poor low-temperature toughness caused by the difficulty in balancing strength and toughness in existing seamless square tubes for the boom of marine engineering cranes in harsh marine environments.

[0005] The specific technical solution is as follows:

[0006] This application provides a seamless square tube for the boom of a marine engineering crane, the chemical composition of which, by mass percentage, is:

[0007] C: 0.14%~0.18%, Si: 0.25%~0.45%, Mn: 1.40%~1.60%, P≤0.015%, S≤0.010%, Cu≤0.20%, Ni≤0.25%, Cr: 0.30%~0.40%, Mo: 0.25%~0.30%, V: 0.05%~0.09%, W: 0.40%~0.60%, Nb≤0.05%, Ti≤0.05%, Al: 0.02%~0.05%, with the balance being Fe and unavoidable impurities. The sum of the mass percentages of all the above components is 100%.

[0008] The seamless square tube for the boom of the marine engineering crane is prepared by the following method:

[0009] Includes the following steps:

[0010] S1, Electric arc furnace steelmaking: Steel is smelted in sequence using a roughing furnace, a refining furnace, and a VD furnace;

[0011] S2, Continuous casting: The molten steel is cast into a continuous casting billet through an arc continuous casting machine;

[0012] S3, Hot rolling: The continuously cast billet is hot rolled into a seamless round steel pipe;

[0013] S4, Quenching and tempering treatment: Quenching and high-temperature tempering treatment of seamless round steel pipes obtained by hot rolling;

[0014] S5, Performance Testing: Mechanical property testing of seamless round steel pipes after quenching and tempering;

[0015] S6, Surface treatment: Seamless round steel pipes that meet the performance test standards are sequentially pickled, phosphated and saponified.

[0016] S7, Cold drawing: Cold drawing a surface-treated seamless round steel pipe into a square steel pipe;

[0017] S8, Heat treatment: Stress-relieving annealing of cold-drawn square steel pipes;

[0018] S9, Finishing process: The heat-treated square steel pipes are straightened, inspected for flaws, sawed, weighed, marked, and packaged for storage.

[0019] In some embodiments of this application, step S3 specifically includes the following steps: heating the continuously cast billet, piercing it to form a rough tube, then using a three-roll continuous rolling mill to roll the rough tube into a blank tube, and then using a sizing machine to sizing the blank tube to obtain the seamless circular steel pipe of the preset size.

[0020] In some embodiments of this application, in step S4, the specific process of the quenching and tempering treatment is as follows: the quenching temperature is 910℃±10℃, the holding time is 40 minutes, followed by oil cooling or water cooling; the tempering temperature is 620℃±10℃, the holding time is 80 minutes, followed by air cooling.

[0021] In some embodiments of this application, in step S7, the seamless circular steel pipe is cold-drawn into the square steel pipe using a 600-ton hydraulic cold drawing machine.

[0022] In some embodiments of this application, in step S7, the cold drawing forming specifically includes: using a mold with a circular inlet and a square outlet, cold drawing a surface-treated seamless circular steel pipe into a square steel pipe in one or more cold drawing processes.

[0023] In some embodiments of this application, in step S8, the stress-relief annealing temperature is 500℃±10℃ and the holding time is 90 minutes.

[0024] In some embodiments of this application, in step S9, the flaw detection is ultrasonic flaw detection.

[0025] In some embodiments of this application, the seamless square tube for the boom of the marine engineering crane has a room temperature yield strength of 690~862MPa, a tensile strength of 770~940MPa, an elongation after fracture ≥15%, and a V-notch impact energy ≥46J at -40℃.

[0026] In some embodiments of this application, the seamless square tube used for the boom of the marine engineering crane has a wall thickness accuracy of ≤ ±10% × wall thickness, a side length accuracy of ≤ ±1% × side length, a concavity / convexity of ≤ 1% × side length, a side length perpendicularity of ≤ 90° ± 1°, a torsion of ≤ 2 + 0.5 mm / m × finished product length, and a straightness of ≤ 1.0 mm / m.

[0027] Compared with the prior art, this application has at least the following beneficial effects:

[0028] The seamless square tube for the boom of the marine engineering crane in this application improves the chemical composition of the product and effectively improves the purity of the square tube steel by controlling the range of some trace elements. It has excellent comprehensive performance, high strength, excellent strength and toughness matching, excellent impact resistance, excellent resistance to low temperature brittleness and corrosion resistance, and good weldability.

[0029] By controlling the low C content and utilizing the precipitation strengthening and grain refinement effects of carbonitrides from microalloying elements such as V, Nb, and Ti during rolling and subsequent processing, the grain size is significantly refined. This fine grain structure is key to ensuring the high strength and excellent low-temperature impact toughness of the steel, effectively preventing crack propagation and thus endowing the seamless square tube for booms with extremely high impact absorption energy, meeting the stringent requirements for impact resistance under harsh working conditions. Strict restrictions on the content of harmful elements such as P and S greatly improve the purity of the seamless square tube for booms, directly improving its toughness at low temperatures. In particular, the addition of Ni significantly lowers the ductile-brittle transition temperature of the steel, a core alloy design element that ensures the seamless square tube for booms maintains high toughness and prevents brittle fracture in low-temperature environments, enabling it to be suitable for applications in polar and deep-sea environments. By adding certain amounts of alloying elements such as Cr, Cu, and Ni, a dense and stable protective oxide film can be formed on the surface of the seamless square tube used for booms, effectively blocking the intrusion of corrosive media. Simultaneously, the combined addition of Mo and W elements further synergistically enhances the steel pipe's resistance to pitting and uniform corrosion in harsh environments. Furthermore, by controlling the carbon equivalent to a low level, excellent weldability of the seamless square tube for booms is ensured, reducing the susceptibility to cold cracking during welding, facilitating on-site welding, and guaranteeing the reliability of welded joints.

[0030] This application adopts a production process of "micro-alloy hot-rolled seamless steel pipe + quenching and tempering treatment + cold drawing into square tube + stress-relief annealing". It finds a better balance between the strength and plasticity of square tube materials, which are usually contradictory properties. This results in better overall performance of the material, making it more suitable for specific application requirements. It optimizes comprehensive performance, improves product dimensional accuracy, and reduces manufacturing costs. It solves the technical problems in the prior art of difficult and costly subsequent processing of seamless steel pipes after quenching and tempering, as well as the high metallurgical quality requirements and contradictions between various performance matching of seamless square tubes. Attached Figure Description

[0031] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0032] Figure 1 A schematic diagram of the process flow for the fabrication method of a seamless square tube for a marine engineering crane boom provided in this application;

[0033] Figure 2 Metallographic diagram of the seamless square tube for marine engineering crane boom prepared in Example 3 of this application. Detailed Implementation

[0034] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this application without creative effort are within the scope of protection of this application.

[0035] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. The terms "comprising" and "having," and any variations thereof, in the embodiments and drawings of this application are intended to cover non-exclusive inclusion. For example, a process, method, system, product, or device that includes a series of steps or units is not limited to the listed steps or units, but may optionally include steps or units not listed, or may optionally include other steps or units inherent to these processes, methods, products, or devices.

[0036] Seamless square tubing for marine crane booms operates in harsh marine environments. On one hand, corrosive media in the marine environment can corrode ordinary alloy steel; on the other hand, seamless square tubing for booms must withstand low-temperature operations down to -40°C. Therefore, seamless square tubing for marine crane booms must meet comprehensive performance requirements, including high strength, impact resistance, low-temperature resistance, and corrosion resistance.

[0037] Based on the design and manufacturing requirements of seamless square tubes for marine engineering crane booms, this application provides the following technical solutions.

[0038] This application provides a seamless square tube for the boom of a marine engineering crane. The chemical composition of the seamless square tube for the boom, by mass percentage, is as follows: C: 0.14%–0.18%, Si: 0.25%–0.45%, Mn: 1.40%–1.60%, P≤0.015%, S≤0.010%, Cu≤0.20%, Ni≤0.25%, Cr: 0.30%–0.40%, Mo: 0.25%–0.30%, V: 0.05%–0.09%, W: 0.40%–0.60%, Nb≤0.05%, Ti≤0.05%, Al: 0.02%–0.05%, with the balance being Fe and unavoidable impurities. The sum of the mass percentages of the above components is 100%.

[0039] By applying the technical solution of this application, and through improving the chemical composition of the product and rationally controlling the content range of each element, the material strength is guaranteed while also ensuring good toughness and weldability. Specifically, the adoption of a low-carbon and micro-alloying design, combined with specific element ratios, helps to refine the grains and optimize the microstructure during subsequent processing, thereby obtaining the required mechanical properties. Limiting the P and S content helps to improve the purity of the steel, ensuring the material's toughness and resistance to crack propagation. Furthermore, controlling the carbon equivalent to an appropriate level ensures that the seamless square tubing for booms possesses excellent comprehensive performance, especially weldability and corrosion resistance, meeting the requirements of high-standard pipeline applications.

[0040] The following explains the reasons for the chemical composition of the seamless square tube for the boom of the marine engineering crane in this application as specified above, wherein the "%" indicating the content (concentration) of the chemical composition means "mass %".

[0041] C: 0.14%~0.18%

[0042] Carbon (C) is a carbide-forming element that effectively improves the strength and hardenability of steel. As the carbon content increases, the strength of the steel increases accordingly; however, excessive carbon content significantly impairs impact toughness, elongation, and weldability. Therefore, controlling the C content between 0.14% and 0.18% ensures sufficient strength in the steel pipe while also maintaining a certain level of toughness and weldability.

[0043] Si: 0.25%~0.45%

[0044] Silicon (Si), as a deoxidizer, can effectively remove oxygen from steel and improve toughness. However, excessive Si content can promote temper brittleness, reduce plasticity, and affect weldability. In addition, Si has poor heat penetration, and excessive content can easily cause defects such as internal folds and cracks during rolling. Based on deoxidation requirements, the lower limit of Si content is set at 0.25%, and the upper limit is set at 0.45% to control brittleness and weldability.

[0045] Mn: 1.40%~1.60%

[0046] Manganese (Mn), as an austenite-forming element, can improve the strength and hardenability of steel and lower the quenching temperature. It also has a desulfurizing effect, mitigating the harmful effects of sulfur. However, excessive Mn content can lead to grain coarsening and increased temper brittleness, which is detrimental to low-temperature impact toughness and CTOD performance. Therefore, the upper limit of Mn content is controlled at 1.60%, and the lower limit is set at 1.40% to ensure the necessary strengthening and hardenability effects.

[0047] P≤0.015%, S≤0.010%

[0048] Phosphorus (P) and sulfur (S) are both harmful elements. P is a highly brittle element that raises the brittle transition temperature of steel; reducing its content helps improve low-temperature impact resistance. S easily forms sulfide inclusions, severely deteriorating toughness and weldability. Strictly controlling P ≤ 0.015% and S ≤ 0.010% helps improve steel purity and ensures low-temperature toughness and resistance to crack propagation. Furthermore, S forms sulfides between grains, creating low potential points and causing intergranular corrosion; setting S below 0.010% better meets corrosion resistance requirements.

[0049] Cu≤0.20%

[0050] Copper (Cu) is a harmful element that causes hot brittleness, so the lower the content, the better. However, Cu has the effect of resisting seawater corrosion, which can improve the corrosion resistance of steel. Excessive Cu content can easily lead to copper brittleness during hot working, affecting surface quality and thermoplasticity. Therefore, its upper limit is set at 0.20%.

[0051] Ni≤0.25%

[0052] Nickel (Ni) stabilizes austenite, improves strength, significantly enhances low-temperature toughness, and lowers the brittle transition temperature, making it crucial for steel pipes used in low-temperature environments. However, excessive Ni content can easily lead to saturation and negatively impacts weldability; therefore, its content is capped at 0.25%.

[0053] Cr: 0.30%~0.40%

[0054] Chromium (Cr), as a carbide-forming element and a strong hardenability element, produces more martensite during heat treatment and quenching, which can improve strength and hardenability, and form fine and stable carbides, refining the grain and improving tempering stability. However, excessive Cr content will increase the brittle transition temperature, reduce low-temperature impact toughness, and affect weldability. Therefore, its range is set at 0.30% to 0.40%.

[0055] Mo: 0.25%~0.30%

[0056] Molybdenum (Mo) can improve hardenability by forming dispersed carbides at grain boundaries, thereby increasing strength, refining grains, and suppressing temper brittleness. This increases hardenability and helps to simultaneously improve strength and impact toughness. However, excessive Mo content can impair weldability and toughness; therefore, its content is controlled between 0.25% and 0.30%.

[0057] V: 0.05%~0.09%

[0058] Low vanadium (V) forms stable carbonitrides with C and N, precipitating as fine dispersed phases, which can significantly refine grains, increase grain coarsening temperature, and improve strength and toughness. However, when the V content is too high, secondary hardening is prone to occur during tempering, and coarse carbonitrides are precipitated in the grains, resulting in a decrease in toughness. Therefore, its range is set at 0.05% to 0.09%.

[0059] W: 0.40%~0.60%

[0060] Tungsten (W) is a strong carbide-forming element that can improve the hot strength, tempering stability, and corrosion resistance of steel. In particular, it works synergistically with Mo in high-temperature or corrosive environments to enhance creep resistance and pitting corrosion resistance. However, excessive W content will increase costs and may affect processability and toughness, so its content is controlled at 0.40% to 0.60%.

[0061] Nb≤0.05%, Ti≤0.05%

[0062] Niobium (Nb) and titanium (Ti) are both strong carbonitride forming elements, which can significantly improve strength and toughness by refining grains and precipitating. However, excessive addition can easily form coarse precipitates, which can impair toughness and weldability. Therefore, the content of both is limited to 0.05%.

[0063] Al: 0.02%~0.05%

[0064] Aluminum (Al) acts as a deoxidizer and grain refiner, effectively refining grains, improving toughness, and enhancing overall performance. However, excessive Al content can easily form large inclusions, affecting the purity and surface quality of the steel. Therefore, its range is set at 0.02%–0.05%.

[0065] The seamless square tube used for the boom in this application has a chemical composition balance of Fe and unavoidable impurities. Here, impurities refer to elements that are mixed in during steel manufacturing from raw materials such as ore and waste, or from the environment during the manufacturing process. For example, in the steelmaking process, N is a residual element that cannot be completely avoided, and even without active addition, steel usually contains trace amounts of N (e.g., 0.003% to 0.008%).

[0066] In this application, low C, low P, and low S content increases the purity of the steel and reduces the carbon equivalent, which is beneficial to the overall performance of the steel, especially its weldability and corrosion resistance. Appropriate contents of elements such as Mn, Cr, Mo, Al, and W refine the grain structure, significantly improving the steel's strength, toughness, and other comprehensive properties. Low V and low Nb content further refine the grain structure and improve overall performance. Mo, in addition to increasing strength, also prevents temper brittleness and enhances the steel's low-temperature impact resistance. Furthermore, reducing the P content to ≤0.015% improves low-temperature impact resistance. Simultaneously, the composition design in this application optimizes the overall performance of seamless square tubing for booms through the synergistic effect of various elements.

[0067] First, synergistic strengthening and toughening: Microalloying elements such as V, Nb, and Ti combine with C and N to form fine carbonitrides, effectively inhibiting austenite grain growth and strengthening through precipitation. The addition of Mo and W further improves tempering stability and high-temperature strength, enabling the steel pipe to maintain excellent low-temperature toughness while achieving high strength. The introduction of Ni significantly lowers the ductile-brittle transition temperature of the material, synergistically with the grain-refining strengthening effect of microalloying elements, ensuring the safe use of the steel pipe in low-temperature environments.

[0068] Secondly, synergistic microstructure control: Elements such as Cr, Mo, and Mn collectively improve the hardenability of steel, which is beneficial for obtaining a uniform and fine bainitic structure during controlled cooling. Si and Al, as deoxidizing elements, synergistically control the oxygen content in steel, reduce non-metallic inclusions, and improve the cleanliness of the steel. Mo can also effectively suppress temper brittleness that may be caused by elements such as Mn and P, further improving the stability of the microstructure.

[0069] Third, synergistic corrosion resistance: Cr, Cu, and Ni work together to form a dense and strongly adhered protective oxide film on the steel pipe surface, significantly improving its resistance to uniform corrosion. The combined addition of Mo and W, especially the introduction of W, enhances the steel pipe's resistance to pitting corrosion in harsh environments containing chloride ions, forming a multi-layered corrosion protection system with Cr, Cu, and Ni.

[0070] Fourth, weldability and purity control: By controlling the C content and carbon equivalent, and strictly limiting the content of harmful elements such as P and S, a foundation for good weldability is laid. The deoxidizing effect of Al and Si, combined with strict control of S content, ensures the high purity of the steel, which not only benefits toughness but also reduces the sensitivity of the weld heat-affected zone to crack formation.

[0071] The seamless square tube for the boom of the marine engineering crane in this application improves the chemical composition of the product by controlling the range of some trace elements, effectively improving the purity of the square tube steel. It has excellent comprehensive performance, high strength, excellent strength and toughness matching, excellent impact resistance, excellent resistance to low temperature brittleness and corrosion resistance, and good weldability.

[0072] This application also provides a method for manufacturing seamless square tubes for marine engineering crane booms, employing a process route of electric furnace steelmaking → continuous casting → hot rolling → quenching and tempering → performance testing → surface treatment → cold drawing → heat treatment → finishing. This method optimizes the overall performance of the seamless square tubes for marine engineering crane booms, reduces the difficulty and cost of subsequent production and processing, and significantly improves the dimensional accuracy of the seamless square tubes. Specifically, as shown... Figure 1 As shown, the preparation method includes the following steps:

[0073] S1, Electric arc furnace steelmaking: Steel is smelted in sequence using a roughing furnace, a refining furnace, and a VD furnace.

[0074] S2, Continuous casting: The molten steel is cast into a continuous casting billet through an arc-shaped continuous casting machine.

[0075] S3, Hot Rolled: Hot rolling of continuously cast billets into seamless round steel pipes.

[0076] S4, Quenching and tempering treatment: Quenching and high-temperature tempering treatment of seamless round steel pipes obtained by hot rolling.

[0077] S5, Performance Testing: Mechanical property testing is performed on seamless round steel pipes after quenching and tempering.

[0078] S6, Surface treatment: Seamless round steel pipes that meet the performance test standards are sequentially pickled, phosphated and saponified.

[0079] S7, Cold drawing: Cold drawing a surface-treated seamless round steel pipe into a square steel pipe.

[0080] S8, Heat treatment: Stress-relieving annealing is performed on the cold-drawn square steel pipe.

[0081] S9, Finishing process: The heat-treated square steel pipes are straightened, inspected for flaws, sawed, weighed, marked, and packaged for storage.

[0082] In the method for manufacturing seamless square tubes for marine engineering crane booms in this application, the steelmaking process employs a roughing furnace + refining furnace + VD vacuum degassing method to precisely control chemical composition and inclusions, ensuring compliance with design requirements. Furthermore, during the continuous casting of the arc-shaped steel billet, the billet drawing speed and cooling rate are strictly controlled according to existing operating procedures to prevent loose central structure and shrinkage cracks in the billet.

[0083] In one embodiment of this application, step S3 specifically includes the following steps: heating the continuously cast billet, piercing it to form a rough tube, then using a three-roll continuous rolling mill to roll the rough tube into a blank tube, and finally using a sizing mill to sizing the blank tube to obtain a seamless circular steel pipe of a preset size. Furthermore, the three-roll continuous rolling mill in this application can preferably be a PQF type continuous rolling mill provided by SMS Group, achieving billet piercing → three-roll continuous rolling → sizing by a sizing mill, with a steel pipe wall thickness accuracy ≤ ±10% × wall thickness, ultrasonic testing meeting the GB / T5777-2019 U2 grade standard requirements, and no surface defects.

[0084] In one embodiment of this application, the specific process of the quenching and tempering treatment in step S4 is as follows: quenching temperature is 910℃±10℃, holding time is 40 minutes, followed by oil cooling or water cooling; tempering temperature is 620℃±10℃, holding time is 80 minutes, followed by air cooling. More specifically, the quenching and tempering heat treatment of the hot-rolled seamless steel pipe of this application involves quenching at 910℃ for 40 minutes, tempering at 620℃ for 80 minutes, and performance testing after quenching and tempering to verify the success of the quenching and tempering process, that is, to verify whether the quenching and tempering treatment (quenching + high-temperature tempering) achieves the purpose of giving the material "excellent strength and toughness matching", and to verify the uniformity and stability of the microstructure, so as to ensure that the performance of the steel pipe meets the design specifications.

[0085] In one embodiment of this application, in step S7, the seamless round steel pipe is cold-drawn into a square steel pipe using a 600-ton hydraulic cold drawing machine. This ensures excellent wall thickness uniformity, right-angle accuracy, and straightness of the square pipe. The high precision of the square pipe, through deformation strengthening, further improves the strength of the material on the basis of tempering, contributing to lightweighting and enhancing strength. Simultaneously, it provides a smooth, dense surface, laying a good foundation for corrosion resistance and fatigue resistance, improving the surface finish. Furthermore, the large-tonnage hydraulic equipment ensures the feasibility and stability of the process for processing large-size, high-strength products, ensuring that the dimensions fully meet design requirements. Further, the seamless round steel pipe that meets performance testing standards undergoes pickling, phosphating, and saponification treatments in a pickling tank, phosphating tank, and saponification tank, followed by a cold drawing forming process. The cold drawing forming specifically includes: using a mold with a round inlet and a square outlet, the surface-treated seamless round steel pipe is cold-drawn into a square steel pipe in one or more cold drawing processes.

[0086] In one embodiment of this application, in step S8, the stress-relief annealing temperature is 500℃±10℃, and the holding time is 90 minutes, to eliminate residual stress in the steel pipe, thereby improving the dimensional stability, reliability, and service life of the seamless square tube used for the boom. More specifically, the stress-relief annealing process includes: heating the cold-drawn square steel pipe to 500℃±10℃ at a preset heating rate, then holding it at 500℃±10℃ for 90 minutes; after holding, cooling it in the furnace to below 250℃ at a preset cooling rate, and then air-cooling it. The preset heating rate can be no greater than 100℃ / h, and the preset cooling rate can be no greater than 50℃ / h.

[0087] This application involves stress-relieving heat treatment after cold drawing and before finishing to eliminate internal stress, ensure dimensional stability and structural integrity, prevent deformation and warping, guarantee dimensional accuracy, reduce the risk of stress corrosion cracking, improve fatigue resistance, restore some toughness and plasticity, and improve mechanical properties.

[0088] In one embodiment of this application, in step S9, the flaw detection is ultrasonic flaw detection, which mainly realizes non-destructive detection of internal defects, confirms that there are no potential dangerous defects inside the seamless square tube of the boom, and ensures that it will not break from the inside under high pressure conditions, thus ensuring structural integrity.

[0089] In one embodiment of this application, the seamless square tube for the boom of a marine engineering crane has a room temperature yield strength of 690~862MPa, a tensile strength of 770~940MPa, an elongation after fracture ≥15%, and a V-notch impact energy ≥46J at -40℃. Furthermore, the seamless square tube for the boom of a marine engineering crane has the following specifications: wall thickness accuracy ≤±10%×wall thickness, side length accuracy ≤±1%×side length, unevenness ≤1%×side length, side length perpendicularity ≤90°±1°, torsion ≤2+0.5mm / m×finished length, and straightness ≤1.0mm / m. In addition, the seamless square tube for the boom of a marine engineering crane has a PREN value ≥32 for corrosion resistance.

[0090] The production process of seamless square tubes for marine engineering crane booms in this application is as follows: electric arc furnace steelmaking (roughing furnace, refining furnace, VD furnace) → arc-shaped continuous casting billet → hot-rolled seamless steel tube → quenching and tempering heat treatment → performance testing of round steel tubes → pickling, phosphating, saponification → cold drawing into square tubes → stress-relief annealing → straightening → flaw detection → sawing → weighing → marking → packaging and warehousing. This production process integrates three core technologies: micro-alloying design, quenching and tempering heat treatment, and cold drawing, systematically endowing the final product with excellent comprehensive performance. The smelting process of "electric arc furnace steelmaking → refining → VD vacuum degassing" ensures the purity and precision of the steel composition, laying the foundation for high performance. Subsequently, "quenching and tempering heat treatment" achieves the optimal match between high strength and high toughness, establishing the core of the material's excellent mechanical properties. Furthermore, "cold drawing" further enhances strength while achieving dimensional accuracy and superior surface quality far exceeding that of hot-rolled tubes. Meanwhile, key auxiliary processes such as stress-relief annealing, straightening, and ultrasonic testing throughout the entire process effectively ensure the dimensional stability, operational safety, and internal quality reliability of the product. This production process addresses the multiple stringent requirements of marine engineering equipment in harsh environments, demanding high strength, high toughness, corrosion and fatigue resistance, high precision, and high safety from structural components. It achieves a simultaneous leap in product performance and reliability, and the subsequent processing of seamless steel pipes after heat treatment is simple and cost-effective.

[0091] It is important to note that existing technologies often employ a production process of "cold drawing hot-rolled seamless steel pipes into square tubes, followed by tempering heat treatment." During tempering heat treatment, the four corners of the square tubes are prone to cracking, and the performance of the four corners differs significantly from that of the straight edges, making the heat treatment process complex. This application, however, adopts a production process of "micro-alloy hot-rolled seamless steel pipe + tempering treatment + cold drawing into square tubes + stress-relief annealing." By tempering the hot-rolled seamless steel pipes to achieve the mechanical properties of Q690E, followed by cold drawing to deform them into square tubes, and then stress-relief annealing heat treatment, cold deformation hardening is eliminated, ensuring the mechanical properties of the square tubes. Straightening is also performed to ensure that its geometric dimensions fully meet the requirements. This solves the aforementioned technical problems in existing technologies, such as the four corners being prone to cracking during tempering heat treatment, the significant performance difference between the four corners and straight edges, and the complexity of the heat treatment process.

[0092] In summary, this application discloses a seamless square tube for marine engineering crane booms. By improving the chemical composition of the product and controlling the range of trace elements, the purity and overall performance of the square tube steel are effectively enhanced. It not only boasts high strength and excellent strength-toughness matching, but also superior impact resistance, excellent resistance to low-temperature brittleness and corrosion resistance, and good weldability. The production process of "micro-alloy hot-rolled seamless steel pipe + quenching and tempering + cold drawing into square tube + stress-relief annealing" finds a better balance between the often contradictory properties of strength and plasticity in square tube materials. This results in better overall material performance, making it more suitable for specific application requirements, optimizing overall performance, improving product dimensional accuracy, and reducing manufacturing costs. It solves the technical problems of difficult and costly subsequent processing of quenched and tempered seamless steel pipes in existing technologies, as well as the high metallurgical quality requirements and contradictory performance matching of seamless square tubes.

[0093] The beneficial effects of this application are further illustrated below with reference to typical but non-limiting embodiments.

[0094] Example 1

[0095] By manufacturing This application describes the specifications for seamless square tubing.

[0096] The seamless square tube for the boom of a marine engineering crane manufactured in this embodiment contains the following components by weight percentage:

[0097] C: 0.17%, Si: 0.36%, Mn: 1.53%, P: 0.012%, S: 0.002%, Cu: 0.002%, Ni: 0.003%, Cr: 0.36%, Mo: 0.28%, V: 0.06%, W: 0.53%, Nb: 0.014%, Ti: 0.012%, Al: 0.029%, with the balance being Fe and unavoidable impurities. The sum of the mass percentages of the above components is 100%.

[0098] The above-mentioned seamless square tube is prepared according to the following method:

[0099] S1 uses high-quality scrap steel, pig iron, and / or direct reduced iron (DRI) as main raw materials. Based on a microalloying design, ferrosilicon and ferromanganese are added in the roughing furnace at the end of melting and during the oxidation phase for preliminary alloying. Then, dephosphorization and decarburization are performed through oxygen blowing, with precise control over the final carbon content and temperature. Once the steel composition and temperature meet the requirements, it is tapped into a ladle. Synthetic slag is added to the ladle for white slag formation, deep desulfurization, and deoxidation. Microalloying elements are precisely added according to the formula. The steel temperature is precisely adjusted to the superheat range required for continuous casting through electric arc heating, completing the refining furnace refining. Finally, the ladle is transferred to a VD furnace for VD furnace vacuum treatment.

[0100] S2 adopts a fully protected casting method, using an arc-shaped continuous casting machine to cast the molten steel into a continuous casting billet.

[0101] S3. The continuously cast billet is fed into a walking beam furnace and heated to 1180-1250℃, with sufficient holding time to allow the microalloying elements to fully dissolve. A piercing mill is used to pierce the billet into a rough tube, which is then rolled into a blank tube using a three-roll continuous rolling mill. Finally, a sizing mill is used to sizing the blank tube to obtain a seamless circular steel pipe with an outer diameter of 210mm and a wall thickness of 6mm.

[0102] S4 involves heating the hot-rolled seamless round steel pipe to its austenitizing temperature of 910℃, holding it at that temperature for 40 minutes, and then subjecting it to water quenching or high-intensity water mist quenching to form a martensitic structure inside. The quenched steel pipe is then reheated to 620℃, held for 80 minutes, and then air-cooled.

[0103] S5. Samples are taken from the heat-treated steel pipes for performance testing to ensure that the performance meets or exceeds the requirements of Q690E level. The testing items include, but are not limited to, room temperature tensile test (to determine yield strength, tensile strength, and elongation after fracture) and V-notch impact test at -40℃ (to determine impact absorption energy).

[0104] S6. Seamless round steel pipes that have passed performance testing are placed in an acid pickling tank, where hydrochloric acid or sulfuric acid solution is used to thoroughly remove the iron oxide scale from the surface of the steel pipes. Afterwards, they are placed in a phosphating tank for phosphating treatment, forming a uniform and dense phosphate conversion film on the surface of the steel pipes. Finally, the phosphated steel pipes are immersed in a saponification solution (such as sodium stearate) to form a lubricating layer on the phosphate film.

[0105] The S7 uses a 600-ton hydraulic cold drawing machine to draw phosphated and saponified round steel pipes into square steel pipes through a special mold.

[0106] S8 involves heating the cold-drawn square steel pipe to 500°C in a heating furnace, holding it at that temperature for 90 minutes, and then air-cooling it to achieve stress-relief annealing.

[0107] S9. A multi-roller straightener is used to straighten the steel pipe to ensure its straightness meets the standard requirements. Then, ultrasonic testing is used to perform 100% inspection of the finished square tube to ensure there are no internal defects such as white spots, cracks, or inclusions, reaching the qualified level specified in the standard (e.g., JB / T4730.3). Afterwards, according to... The required lengths are cut, weighed, and marked (with information such as coating specifications, furnace number, and applicable standards), and finally packaged and stored.

[0108] Example 2

[0109] By manufacturing This application describes the specifications for seamless square tubing.

[0110] The seamless square tube for the boom of a marine engineering crane manufactured in this embodiment contains the following components by weight percentage:

[0111] C: 0.16%, Si: 0.35%, Mn: 1.54%, P: 0.012%, S: 0.002%, Cu: 0.002%, Ni: 0.003%, Cr: 0.35%, Mo: 0.27%, V: 0.06%, W: 0.53%, Nb: 0.013%, Ti: 0.012%, Al: 0.028%, with the balance being Fe and unavoidable impurities. The sum of the mass percentages of the above components is 100%.

[0112] The above-mentioned seamless square tube is prepared according to the following method:

[0113] S1 uses high-quality scrap steel, pig iron, and / or direct reduced iron (DRI) as main raw materials. Based on a microalloying design, ferrosilicon and ferromanganese are added in the roughing furnace at the end of melting and during the oxidation phase for preliminary alloying. Then, dephosphorization and decarburization are performed through oxygen blowing, with precise control over the final carbon content and temperature. Once the steel composition and temperature meet the requirements, it is tapped into a ladle. Synthetic slag is added to the ladle for white slag formation, deep desulfurization, and deoxidation. Microalloying elements are precisely added according to the formula. The steel temperature is precisely adjusted to the superheat range required for continuous casting through electric arc heating, completing the refining furnace refining. Finally, the ladle is transferred to a VD furnace for VD furnace vacuum treatment.

[0114] S2 adopts a fully protected casting method, using an arc-shaped continuous casting machine to cast the molten steel into a continuous casting billet.

[0115] S3. The continuously cast billet is fed into a walking beam furnace and heated to 1180-1250℃, with sufficient holding time to allow the microalloying elements to fully dissolve. A piercing mill is used to pierce the billet into a rough tube, which is then rolled into a blank tube using a three-roll continuous rolling mill. Finally, a sizing mill is used to sizing the blank tube to obtain a seamless circular steel pipe with an outer diameter of 210mm and a wall thickness of 6mm.

[0116] S4 involves heating the hot-rolled seamless round steel pipe to its austenitizing temperature of 910℃, holding it at that temperature for 40 minutes, and then subjecting it to water quenching or high-intensity water mist quenching to form a martensitic structure inside. The quenched steel pipe is then reheated to 620℃, held for 80 minutes, and then air-cooled.

[0117] S5. Samples are taken from the heat-treated steel pipes for performance testing to ensure that the performance meets or exceeds the requirements of Q690E level. The testing items include, but are not limited to, room temperature tensile test (to determine yield strength, tensile strength, and elongation after fracture) and V-notch impact test at -40℃ (to determine impact absorption energy).

[0118] S6. Seamless round steel pipes that have passed performance testing are placed in an acid pickling tank, where hydrochloric acid or sulfuric acid solution is used to thoroughly remove the iron oxide scale from the surface of the steel pipes. Afterwards, they are placed in a phosphating tank for phosphating treatment, forming a uniform and dense phosphate conversion film on the surface of the steel pipes. Finally, the phosphated steel pipes are immersed in a saponification solution (such as sodium stearate) to form a lubricating layer on the phosphate film.

[0119] The S7 uses a 600-ton hydraulic cold drawing machine to draw phosphated and saponified round steel pipes into square steel pipes through a special mold.

[0120] S8 involves heating the cold-drawn square steel pipe to 500°C in a heating furnace, holding it at that temperature for 90 minutes, and then air-cooling it to achieve stress-relief annealing.

[0121] S9. A multi-roller straightener is used to straighten the steel pipe to ensure its straightness meets the standard requirements. Then, ultrasonic testing is used to perform 100% inspection of the finished square tube to ensure there are no internal defects such as white spots, cracks, or inclusions, reaching the qualified level specified in the standard (e.g., JB / T4730.3). Afterwards, according to... The required lengths are cut, weighed, and marked (with information such as coating specifications, furnace number, and applicable standards), and finally packaged and stored.

[0122] Example 3

[0123] By manufacturing This application describes the specifications for seamless square tubing.

[0124] The seamless square tube for the boom of a marine engineering crane manufactured in this embodiment contains the following components by weight percentage:

[0125] C: 0.16%, Si: 0.34%, Mn: 1.54%, P: 0.011%, S: 0.001%, Cu: 0.002%, Ni: 0.003%, Cr: 0.36%, Mo: 0.27%, V: 0.06%, W: 0.52%, Nb: 0.015%, Ti: 0.012%, Al: 0.026%, with the balance being Fe and unavoidable impurities. The sum of the mass percentages of the above components is 100%.

[0126] The above-mentioned seamless square tube is prepared according to the following method:

[0127] S1 uses high-quality scrap steel, pig iron, and / or direct reduced iron (DRI) as main raw materials. Based on a microalloying design, ferrosilicon and ferromanganese are added in the roughing furnace at the end of melting and during the oxidation phase for preliminary alloying. Then, dephosphorization and decarburization are performed through oxygen blowing, with precise control over the final carbon content and temperature. Once the steel composition and temperature meet the requirements, it is tapped into a ladle. Synthetic slag is added to the ladle for white slag formation, deep desulfurization, and deoxidation. Microalloying elements are precisely added according to the formula. The steel temperature is precisely adjusted to the superheat range required for continuous casting through electric arc heating, completing the refining furnace refining. Finally, the ladle is transferred to a VD furnace for VD furnace vacuum treatment.

[0128] S2 adopts a fully protected casting method, using an arc-shaped continuous casting machine to cast the molten steel into a continuous casting billet.

[0129] S3. The continuously cast billet is fed into a walking beam furnace and heated to 1180-1250℃, with sufficient holding time to allow the microalloying elements to fully dissolve. A piercing mill is used to pierce the billet into a rough tube, which is then rolled into a blank tube using a three-roll continuous rolling mill. Finally, a sizing mill is used to sizing the blank tube to obtain a seamless circular steel pipe with an outer diameter of 280mm and a wall thickness of 7.5mm.

[0130] S4 involves heating the hot-rolled seamless round steel pipe to its austenitizing temperature of 910℃, holding it at that temperature for 40 minutes, and then subjecting it to water quenching or high-intensity water mist quenching to form a martensitic structure inside. The quenched steel pipe is then reheated to 620℃, held for 80 minutes, and then air-cooled.

[0131] S5. Samples are taken from the heat-treated steel pipes for performance testing to ensure that the performance meets or exceeds the requirements of Q690E level. The testing items include, but are not limited to, room temperature tensile test (to determine yield strength, tensile strength, and elongation after fracture) and V-notch impact test at -40℃ (to determine impact absorption energy).

[0132] S6. Seamless round steel pipes that have passed performance testing are placed in an acid pickling tank, where hydrochloric acid or sulfuric acid solution is used to thoroughly remove the iron oxide scale from the surface of the steel pipes. Afterwards, they are placed in a phosphating tank for phosphating treatment, forming a uniform and dense phosphate conversion film on the surface of the steel pipes. Finally, the phosphated steel pipes are immersed in a saponification solution (such as sodium stearate) to form a lubricating layer on the phosphate film.

[0133] The S7 uses a 600-ton hydraulic cold drawing machine to draw phosphated and saponified round steel pipes into square steel pipes through a special mold.

[0134] S8 involves heating the cold-drawn square steel pipe to 500°C in a heating furnace, holding it at that temperature for 90 minutes, and then air-cooling it to achieve stress-relief annealing.

[0135] S9. A multi-roller straightener is used to straighten the steel pipe to ensure its straightness meets the standard requirements. Then, ultrasonic testing is used to perform 100% inspection of the finished square tube to ensure there are no internal defects such as white spots, cracks, or inclusions, reaching the qualified level specified in the standard (e.g., JB / T4730.3). Afterwards, according to... The required lengths are cut, weighed, and marked (with information such as coating specifications, furnace number, and applicable standards), and finally packaged and stored.

[0136] Example 4

[0137] By manufacturing This application describes the specifications for seamless square tubing.

[0138] The seamless square tube for the boom of a marine engineering crane manufactured in this embodiment contains the following components by weight percentage:

[0139] C: 0.15%, Si: 0.34%, Mn: 1.55%, P: 0.012%, S: 0.001%, Cu: 0.002%, Ni: 0.002%, Cr: 0.35%, Mo: 0.26%, V: 0.07%, W: 0.53%, Nb: 0.015%, Ti: 0.011%, Al: 0.027%, with the balance being Fe and unavoidable impurities. The sum of the mass percentages of the above components is 100%.

[0140] The above-mentioned seamless square tube is prepared according to the following method:

[0141] S1 uses high-quality scrap steel, pig iron, and / or direct reduced iron (DRI) as main raw materials. Based on a microalloying design, ferrosilicon and ferromanganese are added in the roughing furnace at the end of melting and during the oxidation phase for preliminary alloying. Then, dephosphorization and decarburization are performed through oxygen blowing, with precise control over the final carbon content and temperature. Once the steel composition and temperature meet the requirements, it is tapped into a ladle. Synthetic slag is added to the ladle for white slag formation, deep desulfurization, and deoxidation. Microalloying elements are precisely added according to the formula. The steel temperature is precisely adjusted to the superheat range required for continuous casting through electric arc heating, completing the refining furnace refining. Finally, the ladle is transferred to a VD furnace for VD furnace vacuum treatment.

[0142] S2 adopts a fully protected casting method, using an arc-shaped continuous casting machine to cast the molten steel into a continuous casting billet.

[0143] S3. The continuously cast billet is fed into a walking beam furnace and heated to 1180-1250℃, with sufficient holding time to allow the microalloying elements to fully dissolve. A piercing mill is used to pierce the billet into a rough tube, which is then rolled into a blank tube using a three-roll continuous rolling mill. Finally, a sizing mill is used to sizing the blank tube to obtain a seamless circular steel pipe with an outer diameter of 450mm and a wall thickness of 13.5mm.

[0144] S4 involves heating the hot-rolled seamless round steel pipe to its austenitizing temperature of 910℃, holding it at that temperature for 40 minutes, and then subjecting it to water quenching or high-intensity water mist quenching to form a martensitic structure inside. The quenched steel pipe is then reheated to 620℃, held for 80 minutes, and then air-cooled.

[0145] S5. Samples are taken from the heat-treated steel pipes for performance testing to ensure that the performance meets or exceeds the requirements of Q690E level. The testing items include, but are not limited to, room temperature tensile test (to determine yield strength, tensile strength, and elongation after fracture) and V-notch impact test at -40℃ (to determine impact absorption energy).

[0146] S6. Seamless round steel pipes that have passed performance testing are placed in an acid pickling tank, where hydrochloric acid or sulfuric acid solution is used to thoroughly remove the iron oxide scale from the surface of the steel pipes. Afterwards, they are placed in a phosphating tank for phosphating treatment, forming a uniform and dense phosphate conversion film on the surface of the steel pipes. Finally, the phosphated steel pipes are immersed in a saponification solution (such as sodium stearate) to form a lubricating layer on the phosphate film.

[0147] The S7 uses a 600-ton hydraulic cold drawing machine to draw phosphated and saponified round steel pipes into square steel pipes through a special mold.

[0148] S8 involves heating the cold-drawn square steel pipe to 500°C in a heating furnace, holding it at that temperature for 90 minutes, and then air-cooling it to achieve stress-relief annealing.

[0149] S9. A multi-roller straightener is used to straighten the steel pipe to ensure its straightness meets the standard requirements. Then, ultrasonic testing is used to perform 100% inspection of the finished square tube to ensure there are no internal defects such as white spots, cracks, or inclusions, reaching the qualified level specified in the standard (e.g., JB / T4730.3). Afterwards, according to... The required lengths are cut, weighed, and marked (with information such as coating specifications, furnace number, and applicable standards), and finally packaged and stored.

[0150] Product Inspection and Testing

[0151] Figure 2 The metallographic structure of the seamless square tube for marine engineering crane boom prepared in Example 3 is shown. The seamless square tubes prepared in Examples 1–4 were subjected to room temperature performance testing, a -40℃ tensile test, finished product dimensional accuracy testing, and corrosion resistance testing, respectively. The testing methods were performed according to conventional methods in the art, and the results are as follows:

[0152] (1) Room temperature performance test and tensile test at -40℃

[0153] ;

[0154] The data in Table 1 shows that:

[0155] The seamless square tubes prepared in Examples 1–4 exhibit room temperature yield strengths ranging from 690 to 862 MPa, significantly higher than the 690 MPa required by the Q690E standard, and even approaching the strength level of Q800. This indicates that the materials prepared in Examples 1–4 possess extremely high resistance to deformation and load-bearing capacity, making them highly advantageous for achieving lightweight designs in structures such as crane booms.

[0156] The seamless square tubes prepared in Examples 1–4 exhibit tensile strengths ranging from 770 to 940 MPa, significantly exceeding the tensile strength requirements of Q690E. Furthermore, their strength-to-yield ratio (Rm / Rt0.7) is approximately between 1.09 and 1.10, a highly desirable value. This indicates that the materials prepared in Examples 1–4 retain sufficient work hardening capacity after yielding, preventing immediate fracture and providing a safety margin.

[0157] The seamless square tubes prepared in Examples 1–4 exhibited elongation after fracture ≥17%, significantly exceeding the ≥14% requirement of the Q690E standard. This demonstrates that the materials prepared in Examples 1–4 possess excellent plastic deformation capacity before fracture. They can absorb a large amount of energy before excessive deformation (failure warning) occurs in the engineering structure, avoiding brittle fracture without warning and ensuring high safety.

[0158] The seamless square tubes prepared in Examples 1–3 exhibited V-notch impact energy between 115 and 119 J at -40°C, with Example 4 reaching an even higher 191 J. These figures far exceed the Q690E standard's requirement for impact energy at -40°C, reaching 2.5 to 4 times the standard requirement. Such high low-temperature impact energy indicates that the material retains exceptional toughness even in extremely cold environments like -40°C, effectively preventing crack initiation and propagation. This directly solves the problem of insufficient resistance to brittle fracture in existing technologies for pipes operating in cold sea areas.

[0159] (2) Inspection of finished product dimensional accuracy

[0160] The seamless square tubes prepared in Examples 1–4 were subjected to dimensional accuracy testing. The results were as follows: wall thickness accuracy ≤ ±10% × wall thickness, side length accuracy ≤ ±1% × side length, unevenness ≤ 1% × side length, side length perpendicularity ≤ 90° ± 1°, torsion ≤ 2 + 0.5 mm / m × finished product length, and straightness ≤ 1.0 mm / m. These results fully meet the design requirements.

[0161] Among these, wall thickness accuracy ≤ ±10% × wall thickness: high-precision wall thickness uniformity ensures uniform structural stress, avoids local stress concentration, and helps improve product fatigue life and load-bearing safety; side length accuracy ≤ ±1% × side length: high side length accuracy ensures stable pipe cross-sectional dimensions, providing great convenience and precision for subsequent welding and assembly, enabling "stress-free" or "low-stress" assembly; unevenness ≤ 1% × side length: indicates that the pipe surface is very smooth, without obvious bulges or depressions, which not only affects aesthetics but also ensures good fit with other components, ensuring smooth force flow. Sliding transmission; perpendicularity of side length ≤90°±1°, close to a perfect right angle, 1° tolerance ensures precise positioning when constructing the frame structure, and the force line transmission does not generate eccentric bending moment, which is the basis for structural stability; torsion ≤2+0.5mm / m×finished length, indicating that the pipe has not undergone obvious spiral twisting in the length direction. For long booms, preventing torsion is crucial to ensuring its overall movement trajectory and stability; straightness ≤1.0mm / m, excellent straightness ensures that the structure will not buckle under pressure, runs smoothly with low resistance during movement, and has a tall and beautiful appearance.

[0162] (3) Corrosion resistance test

[0163] The seamless square tubes prepared in Examples 1–4 were subjected to corrosion resistance testing, and the results showed that the PREN value for corrosion resistance was ≥32 in all cases. This result indicates that the steel used in the seamless square tubes prepared in Examples 1–4 possesses a medium to high level of resistance to pitting and crevice corrosion, making them suitable for harsh marine environments (especially seawater environments).

[0164] The product's physical and chemical properties and dimensional accuracy fully meet the design requirements.

[0165] It should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit it. This application uses the above embodiments to illustrate the detailed process equipment and process flow of this application, but this application is not limited to the above detailed process equipment and process flow, that is, it does not mean that this application must rely on the above detailed process equipment and process flow to be implemented. Those skilled in the art should understand that any improvements to this application, equivalent substitutions of the raw materials of this application's products, addition of auxiliary components, selection of specific methods, etc., all fall within the protection scope and disclosure scope of this application.

Claims

1. A seamless square tube for the boom of a marine engineering crane, characterized in that, Its chemical composition, by mass percentage, is as follows: C: 0.14%~0.18%, Si: 0.25%~0.45%, Mn: 1.40%~1.60%, P≤0.015%, S≤0.010%, Cu≤0.20%, Ni≤0.25%, Cr: 0.30%~0.40%, Mo: 0.25%~0.30%, V: 0.05%~0.09%, W: 0.52%~0.53%, Nb≤0.05%, Ti≤0.05%, Al: 0.02%~0.05%, with the balance being Fe and unavoidable impurities. The sum of the mass percentages of the above components is 100%. The seamless square tube for the boom of the marine engineering crane is prepared by the following method: Includes the following steps: S1, Electric arc furnace steelmaking: Steel is smelted in sequence using a roughing furnace, a refining furnace, and a VD furnace; S2, Continuous casting: The molten steel is cast into a continuous casting billet through an arc continuous casting machine; S3, Hot rolling: The continuously cast billet is hot rolled into a seamless round steel pipe; S4, Quenching and tempering treatment: Quenching and high-temperature tempering treatment of seamless round steel pipes obtained by hot rolling; S5, Performance Testing: Mechanical property testing of seamless round steel pipes after quenching and tempering; S6, Surface treatment: Seamless round steel pipes that meet the performance test standards are sequentially pickled, phosphated and saponified. S7, Cold drawing: Cold drawing a surface-treated seamless round steel pipe into a square steel pipe. S8, Heat treatment: The cold-drawn square steel pipe is subjected to stress-relief annealing at a temperature of 500℃±10℃ and a holding time of 90 minutes. S9, Finishing process: The heat-treated square steel pipes are straightened, inspected for flaws, sawed, weighed, marked, and packaged for storage. The seamless square tube used for the boom of the marine engineering crane has a room temperature yield strength of 690~862MPa, a tensile strength of 770~940MPa, an elongation after fracture of ≥15%, and a V-notch impact energy of ≥46J at -40℃.

2. The seamless square tube for the boom of a marine engineering crane according to claim 1, characterized in that, Step S3 specifically includes the following steps: heating the continuously cast billet, piercing it to form a rough tube, then using a three-roll continuous rolling mill to roll the rough tube into a blank tube, and then using a sizing machine to sizing the blank tube to obtain the seamless circular steel pipe of the preset size.

3. The seamless square tube for the boom of a marine engineering crane according to claim 1, characterized in that, In step S4, the specific process of the quenching and tempering treatment is as follows: the quenching temperature is 910℃±10℃, the holding time is 40 minutes, followed by oil cooling or water cooling; the tempering temperature is 620℃±10℃, the holding time is 80 minutes, followed by air cooling.

4. The seamless square tube for the boom of a marine engineering crane according to claim 1, characterized in that, In step S7, the seamless circular steel pipe is cold-drawn into the square steel pipe using a 600-ton hydraulic cold drawing machine.

5. The seamless square tube for the boom of a marine engineering crane according to claim 4, characterized in that, In step S7, the cold drawing forming specifically includes: using a mold with a circular inlet and a square outlet, the surface-treated seamless circular steel pipe is cold-drawn into a square steel pipe in one or more cold drawing processes.

6. The seamless square tube for the boom of a marine engineering crane according to claim 1, characterized in that, In step S9, the flaw detection is ultrasonic flaw detection.

7. The seamless square tube for the boom of a marine engineering crane according to claim 1, characterized in that, The seamless square tubes used for the boom of the marine engineering crane have the following specifications: wall thickness accuracy ≤ ±10% × wall thickness, side length accuracy ≤ ±1% × side length, concavity / convexity ≤ 1% × side length, side length perpendicularity ≤ 90° ± 1°, torsion ≤ 2 + 0.5 mm / m × finished product length, and straightness ≤ 1.0 mm / m.

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