A fatigue-resistant, 550 MPa grade low-temperature toughness marine steel and its manufacturing method

Abstract

This invention relates to the field of marine engineering steel technology, specifically to a low-temperature toughness marine engineering steel with a fatigue resistance of 550 MPa and its manufacturing method. It is composed of the following chemical composition by weight percentage: C: 0.06%~0.08%, Si: 0.25%~0.45%, Mn: 1.5%~2.0%, P≤0.02%, S≤0.01%, Als: 0.04%~0.06%, Ni: 0.95%~2.05%, Mo: 0.001%~0.15%, Cu: 0.22%~0.28%, Co: 0.001%~0.15%, Nb: 0.03%~0.05%, Ti: 0.008%~0.02%, N: 0.002%~0.004%, with the remainder being Fe and unavoidable impurities. The steel plate has a yield strength ≥550 MPa, tensile strength 640~820 MPa, elongation ≥19%, Charpy impact energy at -60℃ ≥120 J, and yield strength ratio ≤0.84.

Description

Technical Field

[0001] This invention relates to the field of marine engineering steel technology, specifically to a low-temperature toughness marine engineering steel with a fatigue resistance of 550 MPa and its manufacturing method. Background Art

[0002] Since the beginning of the 21st century, the shipbuilding and marine engineering sector has developed rapidly, with increasing demand for cold-water and Arctic shipping routes, placing higher demands on related technologies and equipment. Simultaneously, as research into marine engineering steel materials for extreme environments deepens, new types of marine engineering steel materials are constantly emerging to meet increasingly stringent requirements. In recent years, the development of cold-water and Arctic shipping routes has intensified. The Arctic region possesses abundant energy resources such as oil and natural gas, thus holding enormous development potential. Along with the growth of Arctic shipping, higher demands are placed on the technology and performance of related ships and marine engineering equipment, including resistance to low temperatures, corrosion, and fatigue. Furthermore, environmental protection issues associated with Arctic shipping routes, such as ship emissions and marine pollution, also require the support of relevant technologies and equipment. To adapt to these changes and challenges, the development of new marine engineering steel materials has become a key area. Currently, research on marine engineering steel materials focuses on advanced technologies such as micro-alloying, controlled rolling cooling, and heat treatment processes to improve their freeze resistance, corrosion resistance, and other properties.

[0003] Microalloying technology is also a hot research area in marine engineering steel materials. By adding appropriate trace alloying elements, the fatigue resistance and service life of steel plates can be significantly improved. Controlled rolling, controlled cooling, and optimized heat treatment processes are also important technical directions. Controlled rolling processes can effectively control the microstructure and phase transformation of steel, improving its toughness and fatigue resistance.

[0004] The fatigue resistance of steel plates is of great significance to the development of marine engineering equipment, as it can significantly improve their reliability, safety, and economic efficiency. The fatigue resistance of steel plates can be effectively improved by adding appropriate trace alloying elements and employing advanced heat treatment processes. To address the new requirements for the fatigue performance of marine engineering steel plates, fatigue crack propagation testing, fatigue life testing, and low-cycle fatigue strength testing can be used.

[0005] When selecting steel materials and processing techniques, factors such as performance, cost, and feasibility need to be comprehensively considered. Although new types of marine engineering steel materials have emerged, their prices remain relatively high. Therefore, in specific applications, a reasonable choice must be made based on different needs and economic conditions. Furthermore, in terms of improving fatigue resistance, in addition to micro-alloying technology and heat treatment processes, ultrasonic surface treatment is also available.

[0006] Currently, marine engineering steel can meet most of the market demand in the marine engineering field. However, special steels with lower ductile-brittle transition temperature and excellent comprehensive performance in terms of fatigue fracture resistance are still the development goal of countries around the world. High-strength steel plates with high service safety are difficult to research, have strict production processes, high equipment requirements, and are difficult to develop.

[0007] Chinese patent CN113366133B discloses a "High-strength Steel Plate and its Manufacturing Method," which yields high-strength steel plates with excellent fatigue strength in resistance spot welds. However, this invention uses a high-Si, high-Mn alloy system and only employs conventional cold rolling and annealing processes, limiting it to thin-gauge steel plates with poor low-temperature toughness. Chinese patent CN111534740A discloses a "550MPa Fatigue-Resistant High-Strength and High-Toughness Steel Plate and its Manufacturing Method," which uses a lower alloy composition and a rolling + quenching and tempering process, similarly limiting it to producing steel plates with thicknesses of 10-60mm. Summary of the Invention

[0008] To overcome the shortcomings of the prior art, the present invention provides a fatigue-resistant 550Mpa grade low-temperature toughness marine steel and its manufacturing method. The maximum thickness of the steel plate can reach 120mm, and it has good fatigue fracture resistance and low-temperature toughness.

[0009] To achieve the above objectives, the present invention employs the following technical solution:

[0010] A fatigue-resistant, 550 MPa grade low-temperature toughness marine steel is composed of the following chemical composition by weight percentage:

[0011] C: 0.06%~0.08%, Si: 0.25%~0.45%, Mn: 1.5%~2.0%, P≤0.02%, S≤0.01%, Als: 0.04%~0.06%, Ni: 0.95%~2.05%, Mo: 0.001%~0.15%, Cu: 0.22%~0.28%, Co: 0.001%~0.15%, Nb: 0.03%~0.05%, Ti: 0.008%~0.02%, N: 0.002%~0.004%, with the remainder being Fe and unavoidable impurities.

[0012] This invention employs a compositional design that combines C, Mn, Ni, Mo, Cu, Co with microalloying elements Nb and Ti, highlighting the role of the selected alloying elements and their respective concentrations:

[0013] (1) Carbon (C) can improve the yield and tensile strength of steel plates. Too low a C content will lead to a decrease in C solid solution content and carbide content, resulting in a lower grain refinement effect and insufficient steel plate strength. Too high a C content will produce a large amount of hardened structure, increasing the risk of cracking and fracture due to fatigue cycles during service. Low-temperature toughness will also be significantly reduced. Therefore, the C content in steel should be precisely controlled. The C content should be 0.06%~0.08%.

[0014] (2) In the steelmaking process, Si acts as a reducing agent and deoxidizer, which can reduce the O content in steel and improve the purity of billets. At the same time, Si can also improve the strength of steel plates. When the Si content is below 0.25%, the deoxidation effect is not obvious. When the Si content is above 0.45%, the brittle phase in high-alloy steel plates will increase significantly, and the fatigue resistance and low-temperature toughness and plasticity will decrease. The Si content of this invention is 0.25%~0.45%.

[0015] (3) Mn can improve the strength of steel plates, but excessive Mn can cause core segregation during the solidification of thick billets, affecting the low-temperature toughness of the core of extra-thick steel plates. Adding an appropriate amount of Mn to steel in combination with Co can effectively improve the elongation performance of thick steel plates, reduce the probability of fatigue cracking, and resist hydrogen-induced cracking. In this invention, an appropriate amount of Mn is selected, with an Mn content of 1.5% to 2.0%.

[0016] (4) P and S elements have no benefit to the mechanical properties of steel plates, especially elongation. P should be controlled to ≤0.02% and S to ≤0.01%.

[0017] (5) Al is the main deoxidizing element in steel. When the Al content is too low, the deoxidation effect is not good. Adding a certain amount of Al to steel can refine the grains, improve the low-temperature impact toughness and fatigue resistance of the steel plate. When combined with elements such as Mn in steel, it can effectively reduce hydrogen embrittlement, improve corrosion resistance, and further improve the fatigue resistance of the steel plate. However, if the Al content is too high, large inclusions will be formed. The Al content is 0.04%~0.06%.

[0018] (6) The role of Ni is to improve the strength and toughness of steel plates, reduce the yield strength ratio of steel plates, and improve fatigue resistance. The combination of Ni and Mo and Nb elements in steel can ensure that the steel plate has high strength. At the same time, adding a large amount of Ni to steel can reduce the ductile-brittle transition temperature of steel and improve the fatigue resistance and low-temperature toughness of steel. The Ni content is 0.95%~2.05%.

[0019] (7) Mo can refine the grain size in steel. The beneficial effect of grain refinement is to improve the strength and fatigue resistance of the steel plate. The addition of Mo can also improve the tempering stability of the steel plate during tempering. Mo combined with Cu and Ni can also improve the pitting corrosion resistance of the steel plate, further improving the service life of the steel plate under fatigue service conditions. The Mo content is 0.001%~0.15%.

[0020] (8) Cu can improve the strength and hardness of steel, thus improving the wear resistance and fatigue resistance of steel plates. At the same time, Cu can improve the pitting corrosion resistance of steel plates, reduce the generation of fatigue crack initiation, and increase the fatigue cycle. Excessive addition of Cu will increase the tendency for hot cracking. The Cu content is 0.22%~0.28%.

[0021] (9) Co is dissolved in the matrix of steel, which can effectively improve the hardness and strength of steel plate; thus significantly improve the fatigue resistance of steel plate. When combined with elements such as Mn and Ni, it can improve fatigue resistance while ensuring low temperature toughness. At the same time, Co can prevent the performance deterioration caused by carbide aggregation during tempering. The Co content of this invention is 0.001%~0.15%.

[0022] (10) Nb is an important additive element in this invention. Nb has a strong binding force with C and N elements in steel, forming stable CN compounds. Fine and stable Nb (CN) is dispersed on the austenite grain boundaries, which can hinder the growth of austenite grains when the steel is heated. Nb can effectively delay the recrystallization of deformed austenite, reduce the overheating sensitivity of steel, increase the austenite recrystallization temperature, refine the grains, and improve strength and toughness. Nb can also improve the steel's resistance to hydrogen and fatigue resistance. In this invention, the Nb content is 0.03%~0.05%.

[0023] (11) Ti, as a deoxidizing element in steel, can purify the steel. At the same time, adding a certain amount of Ti to steel can combine with elements such as C / N to improve strength and low-temperature toughness. Ti can also prevent intergranular corrosion to a certain extent and improve fatigue resistance. The Ti content is 0.008%~0.02%.

[0024] (12) As a solid solution strengthening element, nitrogen can work together with microalloying elements during heat treatment to improve the strength and hardness of steel plates. However, excessive addition of nitrogen will reduce the toughness of steel plates and increase crack sensitivity. The nitrogen content is 0.002%~0.004%.

[0025] The aforementioned fatigue-resistant 550MPa grade low-temperature toughness marine steel has a yield strength ≥550MPa, tensile strength 640~820MPa, elongation ≥19%, Charpy impact energy at -60℃ ≥120J, and yield strength ratio ≤0.84. It withstands cyclic stresses of 0.6 and 0.9 times the yield strength, with 10 cycles. 7 Under these cycles, the steel plate does not experience fatigue fracture. The maximum thickness of the steel plate can reach 120mm.

[0026] The manufacturing method of the aforementioned fatigue-resistant 550 MPa grade low-temperature toughness marine steel specifically includes the following steps:

[0027] 1) Steel refining further reduces the content of P, S and non-metallic inclusions.

[0028] 2) Casting of billet.

[0029] 3) Ingot casting or continuous casting.

[0030] 4) Heating the billet:

[0031] The billet is placed in a heating furnace at a temperature of 500~650℃ and held for more than 60 minutes. The purpose is to keep the billet at a consistent temperature in the thickness direction during the low-temperature stage, so as to ensure the uniformity of the microstructure of the extra-thick slab.

[0032] Heating rate: 3~5℃ / min; heating temperature: 1150~1250℃; heat soaking temperature: 1150~1220℃; total heating time: 6~9h.

[0033] 5) Controlled rolling:

[0034] ① The initial rolling temperature is 1150~1220℃, and the single-pass reduction is ≥30mm; the intermediate billet cooling rate is 5~6℃ / s, and the waiting time after cooling is 20~60s;

[0035] ②The second-stage rolling temperature is 900~980℃;

[0036] ③ The three-stage initial rolling temperature is 700~750℃, the average reduction rate per pass is 5~15%, and the final rolling temperature is ≥700℃;

[0037] 6) Heat treatment:

[0038] The water immersion temperature is 680~730℃, the red-hot temperature is 380~470℃, and the cooling rate of the core part in the thickness direction of the steel plate is 2~3.5℃ / s.

[0039] Furthermore, in step 1), the molten steel is refined through a converter, LF furnace, RH or VD furnace.

[0040] Furthermore, in step 2), the entire casting process is protected, and the molten steel in the tundish is superheated to 15~20℃.

[0041] Furthermore, in step 3), the continuous casting billet pulling rate is ≤1.2m / min, and the billet is slowly cooled for ≥72h after leaving the production line.

[0042] Furthermore, in step 5), the thickness of the secondary intermediate billet is 1.8 to 2.5 times the thickness of the finished steel plate.

[0043] Furthermore, in step 6), the tempering temperature is 400~550℃.

[0044] Compared with the prior art, the present invention has the following beneficial effects:

[0045] 1. This invention employs an innovative alloy composition system. Excessive carbon (C) content will generate a large amount of hardened microstructure, increasing the likelihood of fatigue cracking during service and significantly reducing low-temperature toughness. Adding appropriate amounts of manganese (Mn) and cobalt (Co) effectively reduces the probability of fatigue crack initiation and resists hydrogen-induced cracking. Ni, combined with molybdenum (Mo) and niobium (Nb), ensures high strength in the steel. Furthermore, a large amount of Ni lowers the ductile-brittle transition temperature, improving fatigue resistance and low-temperature toughness. Cu increases strength and hardness, thus enhancing wear resistance and fatigue resistance. Cu also improves pitting corrosion resistance, reduces fatigue crack initiation, and extends the fatigue cycle. Combined with an innovative three-stage rolling and tempering process, the multi-stage rolling process avoids abnormal microstructure growth in the intermediate billet while ensuring low-temperature toughness and fatigue resistance in the steel core. The synergy of composition and process results in an average grain size of 10-20 μm, leading to excellent yield strength ratio, low-temperature toughness, and fatigue fracture resistance.

[0046] 2. This invention employs a composition design with low C, high Mn, high Ni, and a combination of Mo, Cu, Co, and Nb elements. A certain C content can improve the heat transfer capacity of thick steel plates during heat treatment; the addition of appropriate amounts of Mn and Co elements can effectively improve the elongation properties of thick steel plates. The synergistic effect of Ni, Mo, and Nb elements ensures that the thick steel plate possesses high strength and toughness. Simultaneously, Co and Cu elements can prevent performance deterioration caused by carbide accumulation during tempering. Combining a three-stage rolling process suitable for extra-thick billets, molten steel is refined in a converter, LF furnace, RH furnace, or VD furnace to further reduce the content of phosphorus, sulfur, and non-metallic inclusions. The purpose of the homogenization and holding process is to ensure sufficient homogenization of the high-alloy billet while avoiding abnormal growth of the as-cast microstructure. The first-stage rolling aims to break up the as-cast microstructure by rolling with a large reduction. After rolling to the target thickness, the intermediate billet is rapidly cooled to prevent recrystallization and growth of the internal grains due to prolonged exposure to temperatures above 1150°C. The second-stage rolling process avoids abnormal growth of the intermediate billet microstructure while ensuring a large compression ratio deformation in the core of the intermediate billet, thus guaranteeing the low-temperature toughness and fatigue resistance of the steel plate core. The average reduction per pass and rolling temperature in the third stage are most important for grain refinement and increasing dislocation entanglement. The innovative combination of composition and process enables the maximum thickness of the steel plate to reach 120 mm.

[0047] In summary, this invention combines a low-C, high-Mn, high-Ni composition with Mo, Cu, Co, and Nb elements in a balanced composition design and key production technologies for low-temperature toughness marine engineering steel plates with a fatigue resistance of 550 MPa. It achieves ultra-high strength steel plates with a maximum thickness of 120 mm through a three-stage rolling and tempering process. The innovative alloy composition system and rolling process of this invention guarantee that the steel plate has a yield strength ≥ 550 MPa, tensile strength 640~820 MPa, elongation ≥ 19%, Charpy impact energy ≥ 120 J at -60℃, and a yield strength ratio ≤ 0.84. Cyclic stresses of 0.6 and 0.9 times the yield strength are maintained for 10 cycles. 7 Under the cycle, the steel plate does not experience fatigue fracture. Attached Figure Description

[0048] Figure 1 This is a metallographic diagram of Embodiment 1 of the present invention. Detailed Implementation

[0049] This invention discloses a low-temperature toughness marine engineering steel with fatigue resistance of 550 MPa and its manufacturing method. Those skilled in the art can refer to the content of this document and appropriately modify the process parameters to achieve the desired result. It should be particularly noted that all similar substitutions and modifications are obvious to those skilled in the art and are considered to be included in this invention. The methods and applications of this invention have been described through preferred embodiments, and those skilled in the art can obviously make modifications or appropriate alterations and combinations to the methods and applications described herein without departing from the content, spirit, and scope of this invention to realize and apply the technology of this invention.

[0050] This invention involved extensive and systematic experimental research in several aspects, including alloy element screening and proportioning, steel cleanliness control, and optimization and parameter selection of efficient rolling processes. Ultimately, the alloy element proportions and production processes that meet the objectives of this invention were determined. The chemical composition of the steel of this invention is shown in Table 1, the smelting process in Table 2, the billet heating process in Table 3, the rolling process in Table 4, the heat treatment process in Table 5, the tensile and impact properties in Table 6, and the fatigue properties in Table 7.

[0051] Table 1. Chemical composition of steel in the embodiments of the present invention, wt%.

[0052]

[0053] Table 2 Smelting process of embodiments of the present invention

[0054]

[0055] Table 3. Billet heating process in embodiments of the present invention

[0056]

[0057] Table 4 Rolling process of embodiments of the present invention

[0058]

[0059] Table 5 Heat treatment process of embodiments of the present invention

[0060]

[0061] Table 6 Mechanical properties of steel plates in embodiments of the present invention

[0062]

[0063] Table 7 Fatigue performance of steel plates in embodiments of the present invention

[0064]

[0065] Note: Fatigue testing follows GB / T3075-2008. The test is conducted when the specimen fractures or the cycle life (Nf) reaches 10... 7The experiment is stopped when cycles are reached.

[0066] As shown in Tables 6 and 7, the steel plate of this invention has a yield strength ≥ 550 MPa, tensile strength 640~820 MPa, elongation ≥ 19%, Charpy impact energy at -60℃ ≥ 120 J, and yield strength ratio ≤ 0.84. Cyclic stresses of 0.6 and 0.9 times the yield strength, with 10 cycles... 7 Under certain cycles, the steel plate does not experience fatigue fracture. Its mechanical properties and high service safety performance meet the service conditions of marine engineering equipment. The maximum thickness of the steel plate can reach 120mm, and it has good resistance to fatigue fracture and low-temperature toughness.

[0067] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for manufacturing a low-temperature toughness marine steel with fatigue resistance of 550MPa, characterized in that, The low-temperature toughness marine steel with fatigue resistance of 550MPa is composed of the following chemical composition by weight percentage: C: 0.06%~0.08%, Si: 0.25%~0.45%, Mn: 1.5%~1.92%, P≤0.02%, S≤0.01%, Als: 0.04%~0.06%, Ni: 0.95%~1.94%, Mo: 0.001%~0.15%, Cu: 0.22%~0.28%, Co: 0.001%~0.15%, Nb: 0.03%~0.048%, Ti: 0.009%~0.02%, N: 0.0021%~0.004%, with the remainder being Fe and unavoidable impurities; Its yield strength is ≥550MPa, tensile strength is 640~820MPa, elongation is ≥19%, Charpy impact energy at -60℃ is ≥120J, and yield strength ratio is ≤0.84; Cyclic stresses of 0.6 and 0.9 times the yield strength, with 10 cycles. 7 Under cycles, the steel plate does not experience fatigue fracture; The maximum thickness of the steel plate can reach 120mm; The manufacturing method specifically includes the following steps: 1) Steel refining; 2) Casting of billet; 3) Ingot casting or continuous casting; 4) Heating the billet: The billet is placed in a heating furnace at a temperature of 500~650℃ and held for more than 60 minutes. Heating rate: 3~5℃ / min; heating temperature: 1150~1250℃; heat soaking temperature: 1200~1220℃; total heating time: 6~9h. 5) Controlled rolling: ① The initial rolling temperature is 1150~1220℃, and the single-pass reduction is ≥30mm; the intermediate billet cooling rate is 5~6℃ / s, and the waiting time after cooling is 20~60s; ②The second-stage rolling temperature is 925~980℃; ③ The three-stage initial rolling temperature is 700~750℃, the average reduction rate per pass is 5%~15%, and the final rolling temperature is 702~729℃; 6) Heat treatment: The water immersion temperature is 680~711℃, the red-hot temperature is 380~470℃, the cooling rate of the core position in the thickness direction of the steel plate is 2~3.5℃ / s; the tempering temperature is 400~550℃.

2. The method for manufacturing a low-temperature toughness marine steel with fatigue resistance of 550MPa according to claim 1, characterized in that, In step 1), the molten steel is refined through a converter, LF furnace, RH or VD furnace.

3. The method for manufacturing a low-temperature toughness marine steel with fatigue resistance of 550MPa according to claim 1, characterized in that, In step 2), the entire casting process is protected, and the molten steel in the tundish is superheated to 15~20℃.

4. The method for manufacturing a low-temperature toughness marine steel with fatigue resistance of 550MPa according to claim 1, characterized in that, In step 3), the billet pulling rate is ≤1.2m / min, and the billet is slowly cooled for ≥72h after leaving the production line.

5. The method for manufacturing a low-temperature toughness marine steel with fatigue resistance of 550MPa according to claim 1, characterized in that, In step 5), the thickness of the secondary intermediate billet is 1.8 to 2.5 times the thickness of the finished steel plate.

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