Anti-fatigue super-thick marine steel plate and manufacturing method thereof

Abstract

The present application relates to the technical field of anti-fatigue marine steel plate, in particular to an anti-fatigue heavy marine steel plate and a manufacturing method thereof. The anti-fatigue heavy marine steel plate is composed of the following chemical components with the weight percentage: C: 0.06%-0.11%, Si: 0.15%-0.3%, Mn: 1.45%-1.6%, P≤0.02%, S≤0.01%, Als: 0.02%-0.05%, Co: 0.2%-1.2%, Nb: 0.02%-0.06%, V: 0.04%-0.06%, Ti: 0.007%-0.03%, N: 0.003%-0.008%, and the rest is Fe and inevitable impurities. The anti-fatigue heavy marine steel plate is manufactured by the processes of molten steel refining, casting blank casting, die casting or continuous casting, casting blank heating, controlled rolling and controlled cooling after rolling. The anti-fatigue heavy marine steel plate can produce high-strength low-temperature marine steel with the maximum thickness of 80 mm, and can ensure the production efficiency of the steel plate and obtain excellent anti-fatigue fracture performance.

Description

Technical Field

[0001] This invention relates to the field of fatigue-resistant marine engineering steel plate technology, specifically to a fatigue-resistant extra-thick marine engineering steel plate and its manufacturing method. Background Technology

[0002] The Arctic region possesses abundant energy resources such as oil and natural gas, thus holding immense development potential. The growth of Arctic shipping places higher demands on the technology and performance of related vessels and marine engineering equipment, including resistance to low temperatures, corrosion, and fatigue. Furthermore, the Arctic shipping routes also bring environmental protection issues, such as ship emissions and marine pollution, requiring support from 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 microalloying, 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 must be comprehensively considered. Although new types of steel materials for marine engineering have emerged, their prices remain 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 used.

[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 document CN114277315A discloses "A thick-gauge normalized fatigue-resistant steel plate for wind power and its preparation method," which is a normalized wind power steel plate containing Ni, Cr, and Cu alloys. Its alloy composition is high, and it belongs to the normalized steel alloy composition system, resulting in a high carbon equivalent and poor weldability. Chinese patent CN105452511B discloses "A thick steel plate with excellent fatigue characteristics and its manufacturing method," employing a Ni+Cu alloy composition design combined with conventional TMCP rolling technology, but it can only produce steel plates with a thickness of 18-20 mm. Chinese patent document CN115989327A discloses "A thick steel plate and its manufacturing method," but its chemical composition uses Ni, Cr, Mo, Zr, Cu, and Co elements, resulting in excessively high alloy content and making it impossible to produce large-thickness steel plates. Summary of the Invention

[0008] To overcome the shortcomings of the prior art, the present invention provides an anti-fatigue extra-thick marine engineering steel plate and its manufacturing method. The finished steel plate has a thickness of up to 80 mm and has excellent anti-fatigue properties, seawater corrosion resistance, low-temperature toughness and resistance to environmental brittleness.

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

[0010] A fatigue-resistant, extra-thick marine engineering steel plate is composed of the following chemical composition by weight percentage:

[0011] C: 0.06%~0.11%, Si: 0.15%~0.3%, Mn: 1.45%~1.6%, P≤0.02%, S≤0.01%, Als: 0.02%~0.05%, Co: 0.2%~1.2%, Nb: 0.02%~0.06%, V: 0.04%~0.06%, Ti: 0.007%~0.03%, N: 0.003%~0.008%, with the remainder being Fe and unavoidable impurities.

[0012] The effect of selecting the above alloying elements and their contents:

[0013] (1) Carbon (C) is a basic strengthening element in steel, which improves the strength and hardness of steel plates. Too low a C content will lead to a decrease in C solid solution content and carbide content, reducing the effects of fine grain strengthening and solid solution strengthening, resulting in insufficient steel plate strength. Too high a C content will produce hardened structures, reducing the toughness and fatigue resistance of the steel plate, and resulting in poor weldability. Therefore, the C content must be precisely controlled. The C content is 0.06%~0.11%.

[0014] (2) Si can improve the strength and wear resistance of steel plates. Appropriate addition of Si can improve the fatigue limit and fatigue life of steel. At the same time, Si can reduce the O content. When the Si content is less than 0.15%, the deoxidation effect is not obvious. When the Si content is greater than 0.3%, it will lead to poor low-temperature toughness. The Si content of the invention is 0.16%~0.3%.

[0015] (3) Mn can dissolve in large quantities in the Fe matrix, improving the strength of the steel plate. When the Mn content is less than 1.45%, it contributes little to the strength of thick steel plates. At the same time, Mn is an element that expands the austenite phase region and improves the stability of austenite. The clean smelting method used in this invention can appropriately increase the Mn content, which can improve the fatigue resistance and low-temperature toughness of the steel plate. When the mass percentage of Mn is greater than 1.6%, segregation will result in poor low-temperature toughness in the core of the thick plate. The Mn content is 1.45%~1.6%.

[0016] (4) P and S elements have no benefit to the mechanical properties of steel plates, especially fatigue resistance. 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, microalloying elements such as V and Ti cannot refine the grains due to oxidation. For steel plates with a large thickness that require fatigue resistance, the Al content in the steel needs to be appropriately increased. Conversely, if the Al content is too high, large inclusions will be formed. The Al content is 0.02%~0.05%.

[0018] (6) Co can improve the strength and hardness of steel plates, thereby improving their fatigue resistance. Co can work together with microalloying elements such as Nb and V to refine grains, improve dislocation density, and increase the fatigue strength of steel plates. Adding excessive Co alone can lead to twinning defects in steel plates and reduce their low-temperature toughness. The Co content is 0.2%~1.2%.

[0019] (7) Nb is the main additive element in this invention, which improves the strength and toughness of the steel plate while also enhancing its fatigue resistance. During heating, undissolved Nb C and N compound particles are distributed on the austenite grain boundaries, which can hinder austenite grain growth during heating, refine the grains, and simultaneously improve fatigue resistance and toughness. During controlled cooling of the steel plate, a large amount of Nb (CN) precipitates out and interacts with Co, further promoting dislocation entanglement and refining the grains. In this invention, the Nb content is 0.02%~0.06%.

[0020] (8) V can form V(C,N) particles in the matrix, which can refine the grains. The addition of V can improve the low-temperature impact toughness and fatigue resistance of self-tempered steel plates. The V content is 0.04%~0.06%.

[0021] (9) The addition of Ti is to form TiN with Ti and N, which prevents grain growth in the billet during heating and rolling, and improves the overall performance of the steel plate by interacting with elements such as Co. When Ti is below 0.007%, the effect on strengthening and toughening the steel is not obvious, and when it exceeds 0.03%, the toughness of the steel will deteriorate. Therefore, the Ti content in this invention is 0.007%~0.03%.

[0022] (10) Nitrogen (N) can combine with Nb, Ti, and V to form fine and dispersed nitrogen oxide precipitates, which can effectively promote the growth of ferrite nucleation within the grains and effectively control the growth of the original austenite grains. Increasing the N content can increase the TiN content in the steel. However, when the solid solution N content is too high, the hot plasticity of the steel decreases, the toughness of the steel plate decreases, and a large number of microcracks easily appear on the surface of the steel plate. Therefore, the N content should be 0.003%~0.007%.

[0023] The aforementioned fatigue-resistant extra-thick marine engineering steel plates have a yield strength ≥360MPa, tensile strength 490~630MPa, elongation ≥21%, and core Charpy impact energy at -60℃ ≥100J. The cycle life is 10 cycles at 0.9 times the yield strength. 7 Under fatigue cycles, the steel plate does not fracture. The maximum thickness of the finished product of the extra-thick fatigue-resistant marine engineering steel plate is 80mm.

[0024] The manufacturing method of the aforementioned fatigue-resistant extra-thick marine engineering steel plate specifically includes the following steps:

[0025] 1) Steel refining.

[0026] 2) Casting of billet.

[0027] 3) Ingot casting or continuous casting.

[0028] 4) Heating the billet:

[0029] The billet is placed into the heating furnace at a furnace temperature of 500~650℃ and held for 60~80 minutes. The purpose is to keep the temperature of the extra-thick billet consistent in the thickness direction during the low-temperature stage, so as to avoid the generation of internal defects in the billet caused by uneven heating of Co and Mn elements.

[0030] The heating rate should be 3~5℃ / min to avoid excessive heating of the billet, which could lead to excessive internal and external stress and cause cracks.

[0031] The heating temperature is 1120~1200℃, the soaking temperature is 1150~1180℃, and the total time in the furnace is 6~9h. The purpose of low-temperature soaking and heat preservation is to ensure that the C / N compounds of the microalloy are fully dissolved while avoiding abnormal growth of the as-cast structure and avoiding internal defects caused by high Co and high Mn content in the billet.

[0032] 5) Controlled rolling:

[0033] ① The initial rolling temperature is 1100~1150℃, and the single-pass reduction is ≥60mm. The purpose of the first-stage high-temperature rolling is to take advantage of the high temperature of the billet core and the low surface temperature during the high-temperature stage to achieve obvious hardening. The large reduction during rolling destroys the columnar structure of the core and increases the deformation of the billet core structure.

[0034] The intermediate billet is water-cooled at a cooling rate of 2~6℃ / s at 1 / 4 of its thickness. The purpose of using the intermediate billet cooling process is to improve the production efficiency of steel plate rolling. At the same time, appropriately shortening the intermediate billet cooling time can prevent the core grains of the billet from regrowth, which would weaken the accumulation effect of grain deformation in the core of the billet in the first stage and affect the low-temperature impact toughness and fatigue resistance of the steel plate core.

[0035] ② The initial rolling temperature in the second stage is 700~750℃, the average reduction per pass in the second stage is 6~15mm, and the final rolling temperature is ≥700℃. The purpose of the low-temperature rolling in the second stage is to utilize the greater decrease in surface temperature to increase the deformation of the steel plate core, improve the core grain size, and promote the flattening and refinement of austenite grains. The synergistic effect of Co with microalloying elements such as Nb and V in the steel plays a role in grain refinement, improving dislocation density and increasing the fatigue strength of the steel plate. The single-pass reduction and rolling temperature are the most important factors for grain refinement.

[0036] 6) Controlled cooling after rolling:

[0037] The water temperature for controlled cooling after rolling is 600~730℃, and the temperature for reheating is 385~450℃.

[0038] The purpose of controlled cooling after rolling is to utilize the rapid cooling of the intermediate billet during high-temperature rolling and the deformation energy accumulated during low-temperature rolling, combined with a relatively large supercooling temperature, to promote the nucleation of a large number of acicular ferrites while inhibiting their growth. The effective grain size is 3~12μm, forming a uniform, fine, and dispersed large-angle grain boundary microstructure, ensuring the low-temperature toughness and fatigue resistance of thick steel plates. If the cooling rate is too slow, a large number of polygonal ferrites will form, increasing the carbon and other alloying elements in the austenite and making it impossible to obtain acicular ferrites. If the cooling rate is too fast, a large number of hardened structures will appear in the steel plate, affecting the low-temperature toughness and fatigue resistance of the steel plate.

[0039] Furthermore, in step 1), the molten steel is refined through a converter, LF furnace, RH or VD furnace to further reduce the content of P, S and non-metallic inclusions.

[0040] Furthermore, in step 1), the refined molten steel consists of the following chemical composition by weight percentage:

[0041] C: 0.06%~0.11%, Si: 0.15%~0.3%, Mn: 1.45%~1.6%, P≤0.02%, S≤0.01%, Als: 0.02%~0.05%, Co: 0.2%~1.2%, Nb: 0.02%~0.06%, V: 0.04%~0.06%, Ti: 0.007%~0.03%, N: 0.003%~0.008%, with the remainder being Fe and unavoidable impurities.

[0042] Furthermore, in step 2), the entire casting process is protected.

[0043] Furthermore, in step 2), the superheat of the molten steel in the tundish is 15~25℃.

[0044] Furthermore, in step 6), the water volume ratio of the upper and lower spray pipes is 1.2~1.7, and the roller speed is 0.5~1.5m / s.

[0045] Compared with the prior art, the beneficial effects of the present invention are:

[0046] 1. This invention employs a composition design with low levels of C, Mn, Co, Nb, V, and Ti elements to ensure that the steel plate achieves excellent strength, toughness, and fatigue resistance, with a cycle life of 10 cycles at 0.9 times the yield strength. 7 Under these cycles, the steel plate does not fracture. This invention utilizes C and Mn elements to strengthen the steel plate while intentionally controlling their content to avoid the formation of hardened structures, which reduces the steel plate's toughness and fatigue resistance. Co can work synergistically with microalloying elements such as Nb and V in the steel, refining the grain structure, improving dislocation density, and increasing the fatigue strength of the steel plate. Adding excessive Co alone can lead to twinning defects in the steel plate, reducing its low-temperature toughness. During heating, undissolved Nb C and N compound particles are distributed on the austenite grain boundaries, hindering austenite grain growth during heating, refining the grain size, and simultaneously improving fatigue resistance and toughness. The addition of V can improve the low-temperature impact toughness and fatigue resistance of self-tempered steel plates. To ensure good strength, toughness, and fatigue resistance, this invention precisely controls the compositional combination of C, Mn, Co, Nb, V, and Ti elements. Furthermore, the roles of these elements in this invention are unique, breaking through conventional understanding and possessing original value.

[0047] 2. A billet heating process is adopted that incorporates elements such as C, Mn, Co, Nb, V, and Ti. The billet is placed in the heating furnace at a temperature of 500-650℃ and held for 6-80 minutes. This is to ensure that the billet maintains a consistent temperature along its thickness during the low-temperature stage, preventing uneven heating of Co and Mn elements that could lead to internal defects. During subsequent heating, the heating rate is controlled at 3-5℃ / min to avoid excessively rapid heating, which could cause excessive internal and external stresses and cracking. The heating section temperature is 1120-1200℃, and the soaking temperature is 1150-1180℃, with a total furnace time of 6-9 hours. The purpose of this low-temperature soaking and holding is to ensure the complete dissolution of C / N compounds in the microalloyed structure while preventing abnormal growth of the as-cast microstructure and avoiding internal defects caused by high Co and Mn content in the billet.

[0048] 3. The purpose of the first-stage high-temperature rolling in this invention is to utilize the high core temperature and low surface temperature of the billet during the high-temperature stage, resulting in significant hardening. Large reduction during rolling disrupts the columnar structure of the core, increasing core deformation. The intermediate billet cooling process aims to improve steel plate rolling efficiency. Simultaneously, appropriately shortening the intermediate billet cooling time prevents the core grains from regrowth, thus reducing the cumulative effect of core grain deformation in the first stage and impacting the low-temperature impact toughness and fatigue resistance of the steel plate core. The purpose of the second-stage low-temperature rolling is to utilize the significant decrease in surface temperature to increase core deformation, improve core grain size, and promote austenite grain flattening and refinement. The synergistic effect of Co with microalloying elements such as Nb and V in the steel enhances grain refinement, improves dislocation density, and increases fatigue strength. The single-pass reduction rate and rolling temperature are most crucial for grain refinement. The purpose of controlled cooling after rolling is to utilize the rapid cooling of the intermediate billet during high-temperature rolling and the deformation energy accumulated during low-temperature rolling, combined with a large supercooling temperature, to promote the nucleation of a large number of acicular ferrites while inhibiting their growth. The effective grain size is 3~12μm, forming a uniform, fine, and dispersed large-angle grain boundary microstructure, which ensures the low-temperature toughness and fatigue resistance of thick steel plates.

[0049] In summary, this invention employs a composition design with low levels of C, Mn, Co, Nb, V, and Ti elements to ensure that the steel plate achieves excellent strength, toughness, and fatigue resistance, with a cycle life of 10 cycles at 0.9 times the yield strength. 7 Under these cycles, the steel plate does not fracture. The yield strength is guaranteed to be ≥360MPa, tensile strength 490~630MPa, elongation ≥21%, and core Charpy impact energy at -60℃ ≥100J. A billet heating process with elements in combination with C, Mn, Co, Nb, V, and Ti can produce high-strength low-temperature marine engineering steel with a maximum thickness of 80mm. High-efficiency temperature-controlled rolling ensures excellent fatigue fracture resistance while maintaining high steel plate production efficiency. Attached Figure Description

[0050] Figure 1 This is a metallographic diagram of Embodiment 1 of the present invention. Detailed Implementation

[0051] This invention discloses a fatigue-resistant extra-thick marine engineering steel plate 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.

[0052] To address the compositional and performance requirements of high-strength marine engineering steel in the extremely cold and deep-sea marine environment, this invention utilizes a compositional design combining C, Si, Mn, Co with microalloying elements V, Nb, and Ti, and key production technologies for fatigue-resistant extra-thick marine engineering steel plates. The inventors conducted 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, they determined the alloy element proportions and production processes that meet the objectives of this invention. The chemical composition of the steel plates in this embodiment is shown in Table 1, the smelting and billet heating processes in Table 2, the rolling and post-rolling controlled cooling processes in Table 3, and the mechanical properties of the steel plates in Table 4.

[0053] Table 1 Chemical composition of steel plates in embodiments of the present invention, wt%

[0054]

[0055] Table 2 Smelting and billet heating processes in embodiments of the present invention

[0056]

[0057] Table 3 Rolling and Post-Rolling Controlled Cooling Processes in Embodiments of the Invention

[0058]

[0059] Table 4 Mechanical properties of steel plates in embodiments of the present invention

[0060]

[0061] Note: Fatigue testing follows GB / T3075-2008. The test result is given when the specimen fractures or the cycle life (N) reaches its maximum. f Reaching 10 7 The test is stopped when cycles are reached, with a frequency of 90~92Hz and a stress of 0.9 times the yield strength.

[0062] like Figure 1As shown, the metallographic structure of Example 1 is acicular ferrite at 1 / 2 and 1 / 4 of the thickness of the steel plate, with an effective grain size of 3~12μm and good mechanical properties.

[0063] As shown in Table 4, this invention is a fatigue-resistant, extra-thick marine engineering steel plate suitable for extremely cold marine engineering applications. Its mechanical properties and high service safety performance meet the service requirements of marine engineering equipment. The steel plate has a maximum thickness of 80mm and possesses excellent strength, toughness, and fatigue resistance. Its cycle life at 0.9 times yield strength is 10 cycles. 7 The steel plate does not fracture under cycles. The yield strength of the steel plate is ≥360MPa, the tensile strength is 490~630MPa, the elongation is ≥21%, and the Charpy impact energy of the core at -60℃ is ≥100J.

[0064] 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 an extra-thick, fatigue-resistant marine engineering steel plate, characterized in that, The fatigue-resistant extra-thick marine engineering steel plate is composed of the following chemical composition by weight percentage: C: 0.06%~0.11%, Si: 0.15%~0.3%, Mn: 1.45%~1.6%, P≤0.02%, S≤0.01%, Als: 0.02%~0.05%, Co: 1.03%~1.2%, Nb: 0.02%~0.06%, V: 0.04%~0.06%, Ti: 0.007%~0.029%, N: 0.0031%~0.008%, with the remainder being Fe and unavoidable impurities; Its yield strength is ≥360MPa, tensile strength is 490~630MPa, elongation is ≥21%, and core Charpy impact energy at -60℃ is ≥100J; 0.9 times yield strength, cycle life 10 7 Under cycles, the steel plate does not break; The maximum thickness of the finished product of fatigue-resistant extra-thick marine engineering steel plate is 80mm. 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 into the heating furnace at a furnace temperature of 530~650℃ and held for 60~80 minutes. Heating rate: 3~5℃ / min, heating temperature: 1120~1200℃, soaking temperature: 1150~1180℃, total furnace time: 6~9h; 5) Controlled rolling: ① The initial rolling temperature is 1100~1140℃, and the single-pass reduction is ≥60mm; the intermediate billet is water-cooled at a cooling rate of 2~6℃ / s at 1 / 4 of its thickness. ② The initial rolling temperature of the second stage is 700~750℃, the average reduction rate of the single-pass rolling in the second stage is 6~15mm, and the final rolling temperature is 700~713℃; 6) Controlled cooling after rolling: The water temperature for cooling after rolling is 600~672℃, and the temperature for reheating is 385~450℃; the water volume ratio of the upper and lower nozzles is 1.2~1.7, and the roller speed is 0.5~1.5m / s.

2. The method for manufacturing a fatigue-resistant extra-thick marine engineering steel plate 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 fatigue-resistant extra-thick marine engineering steel plate according to claim 1, characterized in that, In step 1), the refined molten steel has the following chemical composition by weight percentage. composition: C: 0.06%~0.11%, Si: 0.15%~0.3%, Mn: 1.45%~1.6%, P≤0.02%, S≤0.01%, Als: 0.02%~0.05%, Co: 1.03%~1.2%, Nb: 0.02%~0.06%, V: 0.04%~0.06%, Ti: 0.007%~0.029%, N: 0.0031%~0.008%, with the remainder being Fe and unavoidable impurities.

4. The method for manufacturing a fatigue-resistant extra-thick marine engineering steel plate according to claim 1, characterized in that, In step 2), the entire casting process is protected.

5. The method for manufacturing a fatigue-resistant extra-thick marine engineering steel plate according to claim 1, characterized in that, In step 2), the molten steel in the tundish is superheated to 15~25℃.

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