Anti-fatigue 550 Mpa-grade low-temperature toughness marine steel and manufacturing method thereof

Abstract

The invention relates to the technical field of maritime work steel, in particular to anti-fatigue 550 Mpa-grade low-temperature toughness maritime work steel and a manufacturing method thereof. The high-strength steel is composed of the following chemical components in percentage by weight: 0.06%-0.08% of C, 0.25%-0.45% of Si, 1.5%-2.0% of Mn, less than or equal to 0.02% of P, less than or equal to 0.01% of S, 0.04%-0.06% of Als, 0.95%-2.05% of Ni, 0.001%-0.15% of Mo, 0.22%-0.28% of Cu, 0.001%-0.15% of Co, 0.03%-0.05% of Nb, 0.008%-0.02% of Ti, 0.002%-0.004% of N and the balance of Fe and inevitable impurities. The yield strength of the steel plate is larger than or equal to 550 Mpa, the tensile strength ranges from 640 Mpa to 820 Mpa, the ductility is larger than or equal to 19%, the Charpy impact energy at the temperature of-60 DEG C is larger than or equal to 120 J, and the yield ratio is smaller than or equal to 0.84.

Description

Technical Field

[0001] The present invention relates to the technical field of marine engineering steel, and in particular to a fatigue-resistant 550 MPa-grade low-temperature toughness marine engineering steel and a manufacturing method thereof. Background Art

[0002] Since the beginning of the 21st century, the shipbuilding and marine engineering sectors have experienced rapid development, with increasing demand for shipping routes in cold and Arctic waters, placing higher demands on related technologies and equipment. Simultaneously, with the deepening of research on offshore steel materials for use in extreme environments, new offshore steels are emerging to meet increasingly stringent requirements. In recent years, interest in developing shipping routes in cold and Arctic waters has intensified. The Arctic region boasts abundant energy resources such as oil and natural gas, offering enormous development potential. The growth of Arctic shipping has also placed higher demands on the technology and performance of related ships and offshore engineering equipment, including resistance to low temperatures, corrosion, and fatigue. Furthermore, Arctic shipping presents environmental challenges, such as ship emissions and marine pollution, which also require the support of relevant technologies and equipment. To adapt to these changes and challenges, the development of new offshore steels has become a key area of focus. Currently, research on offshore steels focuses on advanced technologies such as microalloying, controlled rolling cooling, and heat treatment processes to enhance their frost resistance, corrosion resistance, and other properties.

[0003] Microalloying technology is also a hot topic in the research and development of offshore 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 heat treatment process optimization are also important technical areas. Using 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 crucial to the development of offshore engineering equipment, significantly improving their reliability, safety, and economic efficiency. The addition of appropriate trace alloying elements and advanced heat treatment processes can effectively enhance the fatigue resistance of steel plates. To meet the new fatigue requirements for offshore steel plates, fatigue crack growth testing, fatigue life testing, and low-cycle fatigue strength testing can be employed.

[0005] When selecting steel materials and processing techniques, factors such as performance, cost, and feasibility must be comprehensively considered. While new marine steel materials are emerging, their prices remain relatively high, so the appropriate choice must be made based on specific application needs and economic conditions. Furthermore, in addition to microalloying and heat treatment techniques, ultrasonic surface treatment can also be used to improve fatigue resistance.

[0006] At present, marine engineering steel can meet most of the market demand in the marine engineering field, but special steel with excellent comprehensive performance of lower ductile-brittle transition temperature and fatigue fracture resistance is still the development goal of countries around the world. High-strength steel plates with high service safety have high scientific research difficulties, strict production processes, high equipment requirements, and great difficulty in development.

[0007] Chinese patent publication number CN113366133B discloses "High-strength steel plate and method for manufacturing the same." This method can produce high-strength steel plate with excellent fatigue strength at resistance spot welds. However, this invention uses a high-Si, high-Mn alloy system and employs only conventional cold rolling and annealing processes, resulting in only thin-gauge steel plate and poor low-temperature toughness. Chinese patent publication number CN111534740A discloses "550 MPa fatigue-resistant high-strength and high-toughness steel plate and method for manufacturing the same." This method utilizes a lower alloy composition and a rolling and tempering production process, similarly limiting the production capacity to 10-60 mm thick steel plates. Summary of the Invention

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

[0009] In order to achieve the above object, the present invention adopts the following technical solutions:

[0010] A fatigue-resistant 550 MPa grade low-temperature toughness marine steel, comprising the following chemical components in percentage by weight:

[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%, and the rest are Fe and unavoidable impurities.

[0012] The present invention adopts a composition design in which C, Mn, Ni, Mo, Cu, Co and microalloying elements Nb and Ti are matched. The effects of selecting the types and contents of the above alloying elements are as follows:

[0013] (1) The C element can improve the yield and tensile strength of steel plates. If the C content is too low, the C solid solution content and carbide content will be reduced, the grain refinement effect will be low, and the steel plate strength will be insufficient. If the C content is too high, a large amount of hardened structure will be produced, which will increase the cracking and fracture of the steel plate due to fatigue cycles during service. The low-temperature toughness will also be significantly reduced. Therefore, the C content in the steel should be precisely controlled. The C content is 0.06%~0.08%.

[0014] (2) Si acts as a reducing agent and deoxidizer during the steelmaking process, which can reduce the oxygen content in the steel and improve the purity of the billet. At the same time, Si can also improve the strength of the steel plate. When the Si content is less than 0.25%, the deoxidation effect is not obvious. When the Si content is greater than 0.45%, the brittle phase of the high-alloy steel plate will increase significantly, and the fatigue resistance and low-temperature toughness and ductility will decrease. The Si content of the present invention is 0.25% to 0.45%.

[0015] (3) Mn can improve the strength of steel plates, but excessive Mn content can cause core segregation during the solidification of thick ingots, affecting the low-temperature toughness of the core of the extra-thick steel plate. Adding an appropriate amount of Mn and Co to the steel can effectively improve the elongation of thick steel plates, reduce the probability of fatigue cracking, and resist hydrogen-induced cracking. The present invention selects an appropriate amount of Mn, with the Mn content being 1.5% to 2.0%.

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

[0017] (5) Al is the main deoxidizing element in steel. When the Al content is too low, the deoxidation effect is poor. 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, and can effectively reduce hydrogen embrittlement and improve corrosion resistance when combined with elements such as Mn in steel, further improving the fatigue resistance of the steel plate. However, if the Al content is too high, large inclusions will be formed. The Als content is 0.04%~0.06%.

[0018] (6) The role of Ni is to improve the strength and toughness of the steel plate, reduce the yield strength ratio of the steel plate, and improve fatigue resistance. The Ni element in the steel, combined with Mo and Nb elements, can ensure that the steel plate has a higher strength. At the same time, adding a large amount of Ni to the steel can reduce the ductile-brittle transition temperature of the steel and improve the fatigue resistance and low-temperature toughness of the 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 the tempering process. Mo, in combination with Cu and Ni, can also improve the pitting corrosion resistance of the steel plate, further increasing the service life of the steel plate under fatigue service conditions. The Mo content is 0.001%~0.15%.

[0020] (8) Cu can increase the strength and hardness of steel, thereby 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 sources, and increase the fatigue cycle. Excessive addition of Cu will increase the tendency of hot cracking. The Cu content is 0.22%~0.28%.

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

[0022] (10) Nb is an important additive element in the present invention. Nb has a strong binding force with C and N elements in steel, forming stable CN compounds. Small 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 elements can also improve the H resistance of steel and improve fatigue resistance. In the present invention, the Nb content is 0.03% to 0.05%.

[0023] (11) As a deoxidizing element in steel, Ti can purify the steel. At the same time, adding a certain amount of Ti to steel can cooperate 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 the steel plate and increase crack sensitivity. The nitrogen content is 0.002%~0.004%.

[0025] The above fatigue-resistant 550Mpa grade low-temperature toughness marine steel has a yield strength ≥550Mpa, a tensile strength of 640~820Mpa, an elongation ≥19%, a -60℃ Charpy impact energy ≥120J, and a yield strength ratio ≤0.84. The cyclic stress of 0.6 and 0.9 times the yield strength, the number of cycles is 10 7 The maximum thickness of the steel plate can reach 120mm.

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

[0027] 1) Refining of molten steel to further reduce the content of P, S and non-metallic inclusions.

[0028] 2) Casting of ingot.

[0029] 3) Die casting or continuous casting.

[0030] 4) Billet heating:

[0031] The ingot is placed in a heating furnace at a temperature of 500-650°C and kept warm for more than 60 minutes; the purpose is to keep the temperature of the ingot consistent in the thickness direction at the low temperature stage and ensure the uniformity of the structure of the extra-thick slab.

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

[0033] 5) Controlled rolling:

[0034] ① The first-stage rolling temperature is 1150~1220℃, and the single-pass reduction is ≥30mm; the intermediate billet cooling rate is 5~6℃ / s, and the temperature is kept at room temperature for 20~60s after cooling;

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

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

[0037] 6) Heat treatment:

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

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

[0040] Furthermore, in step 2), the casting is protected throughout the entire process, and the superheat of the molten steel in the tundish is 15-20°C.

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

[0042] Furthermore, in step 5) (2), 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°C.

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

[0045] 1. This invention utilizes an innovative alloying system. Excessive carbon content will produce a large amount of hardened microstructure, increasing the steel's cracking and fracture risk from fatigue cycling during service, while also significantly reducing its low-temperature toughness. The addition of an appropriate amount of Mn and Co to the steel effectively reduces the incidence of fatigue cracking and resists hydrogen-induced cracking. Ni, combined with Mo and Nb, ensures high strength. Furthermore, the addition of a large amount of Ni lowers the ductile-brittle transition temperature, improving fatigue resistance and low-temperature toughness. Cu increases the steel's strength and hardness, thereby enhancing its wear resistance and fatigue resistance. Cu also improves pitting resistance, reduces fatigue cracking sources, and increases fatigue cycles. Combined with an innovative three-stage rolling and tempering process, this multi-stage rolling process prevents abnormal growth of the intermediate structure while ensuring low-temperature toughness and fatigue resistance in the steel's core. The coordinated combination of components and processes results in an average grain size of 10-20 μm, resulting in excellent yield ratio, low-temperature toughness, and fatigue fracture resistance.

[0046] 2. The present invention utilizes a composition design that combines low carbon, high manganese, and high nickel with the elements Mo, Cu, Co, and Nb. A certain carbon content can improve the heat transfer capacity of thick steel plates during heat treatment; adding an appropriate amount of manganese and cobalt to the steel effectively improves the elongation of thick steel plates. The combined effects of Ni, Mo, and Nb ensure high strength and toughness in thick steel plates. Furthermore, the cobalt and copper elements prevent the accumulation of carbides in thick steel plates during tempering, which can lead to performance degradation. Incorporating a three-stage rolling process suitable for extra-thick billets, the molten steel is refined through a converter, LF furnace, and RH or VD furnace to further reduce the contents of P, S, and non-metallic inclusions. The soaking and holding process ensures full homogenization of the high-alloy billet while preventing abnormal growth of the as-cast structure. The first-stage rolling process uses high-reduction rolling to break up the as-cast structure. After rolling to the target thickness, the interslab is rapidly cooled to prevent recrystallization and growth of grains within the interslab due to prolonged holding at temperatures above 1150°C. The second-stage rolling process prevents abnormal growth while ensuring a high reduction ratio in the core, ensuring low-temperature toughness and fatigue resistance of the steel plate. The average reduction ratio and rolling temperature of the three-stage process are crucial for grain refinement and increased dislocation entanglement. This innovative combination of composition and process enables plate thicknesses up to 120 mm.

[0047] In summary, the present invention combines the composition design of low C, high Mn, high Ni with Mo, Cu, Co, and Nb elements and the key production technology of fatigue-resistant 550Mpa low-temperature toughness marine steel plates, and produces ultra-high strength steel plates with a maximum thickness of 120mm through three-stage rolling + tempering. The innovative alloy composition system and innovative rolling process of the present invention can ensure that the yield strength of the steel plate is ≥550Mpa, the tensile strength is 640~820Mpa, the elongation is ≥19%, the Charpy impact energy at -60℃ is ≥120J, and the yield strength ratio is ≤0.84. The cyclic stress of 0.6 and 0.9 times the yield strength, the number of cycles is 10 7 cycles, the steel plate does not suffer from fatigue fracture. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Figure 1 This is the metallographic organization diagram of Example 1 of the present invention. DETAILED DESCRIPTION

[0049] The present invention discloses a fatigue-resistant 550Mpa-grade low-temperature toughness marine steel and a method for manufacturing the same. Those skilled in the art can refer to the contents of this article and appropriately improve the process parameters for implementation. It is particularly important to point out that all similar replacements and modifications are obvious to those skilled in the art and are considered to be included in the present invention. The methods and applications of the present invention have been described through preferred embodiments, and relevant personnel can obviously modify or appropriately change and combine the methods and applications described herein without departing from the content, spirit and scope of the present invention to implement and apply the technology of the present invention.

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

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

[0052]

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

[0054]

[0055] Table 3 Slab heating process according to the embodiment of the present invention

[0056]

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

[0058]

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

[0060]

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

[0062]

[0063] Table 7 Fatigue properties of steel plates according to embodiments of the present invention

[0064]

[0065] Note: Fatigue test adopts GB / T3075-2008. When the specimen breaks or the cycle life Nf reaches 10 7Stop the test when the number of cycles has expired.

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

[0067] The above description is only a preferred specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any technician familiar with the technical field, within the technical scope disclosed by the present invention, who makes equivalent replacements or changes based on the technical solution and inventive concept of the present invention, should be covered by the scope of protection of the present invention.

Claims

1. A fatigue-resistant 550Mpa grade low-temperature toughness marine steel, characterized in that: It is composed of the following chemical components in percentage by weight: 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%, and the rest are Fe and unavoidable impurities.

2. The fatigue-resistant 550 MPa low-temperature toughness marine steel according to claim 1, characterized in that: 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.

3. The fatigue-resistant 550 MPa low-temperature toughness marine steel according to claim 1, characterized in that: Cyclic stress of 0.6 and 0.9 times the yield strength, number of cycles: 10 7 cycles, the steel plate does not suffer from fatigue fracture.

4. The fatigue-resistant 550 MPa low-temperature toughness marine steel according to claim 1, characterized in that: The maximum thickness of the steel plate can reach 120mm.

5. A method for manufacturing fatigue-resistant 550 MPa low-temperature toughness marine steel according to any one of claims 1 to 4, characterized in that: The manufacturing method specifically comprises the following steps: 1) Molten steel refining; 2) Billet casting; 3) Die casting or continuous casting; 4) Billet heating: Place the ingot into a heating furnace at a temperature of 500-650°C and keep it warm for more than 60 minutes; Heating rate 3~5℃ / min, heating temperature 1150~1250℃, soaking temperature 1150~1220℃, total heating time 6~9h; 5) Controlled rolling: ① The first-stage rolling temperature is 1150~1220℃, and the single-pass reduction is ≥30mm; the intermediate billet cooling rate is 5~6℃ / s, and the temperature is kept at room temperature for 20~60s after cooling; ② The second stage rolling temperature is 900~980℃; ③ The three-stage rolling temperature is 700~750℃, the average reduction rate of each pass is 5%~15%, and the final rolling temperature is ≥700℃; 6) Heat treatment: The water entry temperature is 680~730℃, the red-hot temperature is 380~470℃, and the cooling rate at the core position in the thickness direction of the steel plate is 2~3.5℃ / s.

6. The method for manufacturing fatigue-resistant 550 MPa low-temperature toughness marine steel according to claim 5, characterized in that: In step 1), the molten steel is refined in a converter, LF furnace, RH furnace or VD furnace.

7. The method for manufacturing a fatigue-resistant 550 MPa low-temperature toughness marine steel according to claim 5, characterized in that: In step 2), the casting is protected throughout the whole process, and the superheat of the molten steel in the tundish is 15~20℃.

8. The method for manufacturing fatigue-resistant 550 MPa low-temperature toughness marine steel according to claim 5, characterized in that: In step 3), the continuous casting billet drawing rate is ≤1.2m / min, and the billet is slowly cooled after leaving the line for ≥72h.

9. The method for manufacturing fatigue-resistant 550 MPa low-temperature toughness marine steel according to claim 5, characterized in that: In step 5) of ②, the thickness of the secondary intermediate billet is 1.8 to 2.5 times the thickness of the finished steel plate.

10. The method for manufacturing fatigue-resistant 550 MPa low-temperature toughness marine steel according to claim 5, characterized in that: In step 6), the tempering temperature is 400-550°C.

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