An extra-thick marine engineering steel resistant to fatigue crack propagation and its manufacturing method

Abstract

This invention relates to the field of marine engineering steel technology, specifically to an extra-thick marine engineering steel resistant to fatigue crack propagation and its manufacturing method. The specific steps include: 1) steel refining; 2) electroslag remelting; 3) electroslag ingot heating; loading the electroslag ingot into a heating furnace at a temperature of 600-700℃, holding at low temperature for 1-2 hours; heating rate of 2-6℃ / min, heating temperature of 1200-1250℃, holding for 1-3 hours; 4) controlled rolling; initial rolling temperature of 1180-1230℃, average pass reduction of 6-20%, final rolling temperature of 900-1050℃; 5) quenching and tempering; employing quenching + sub-temperature quenching + high-temperature tempering, with a quenching temperature of 750-900℃, a sub-temperature quenching temperature of 700-820℃, and a tempering temperature of 400-700℃. This invention utilizes a compositional design that combines C, Mn, Ni, Cr, Mo, Cu with microalloying elements such as Co, V, and B. This significantly improves the fatigue crack propagation performance of steel plates, allowing for a maximum plate thickness of 150mm, while exhibiting ultra-high strength and excellent low-temperature toughness.

Description

Technical Field

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

[0002] In recent years, the development of shipping routes in frigid oceans and the Arctic Circle has been gaining momentum. The Arctic region possesses abundant energy resources such as oil and natural gas, thus holding immense development potential. Along with the growth of Arctic shipping, higher demands are being 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, 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 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 CN111621723B discloses "700MPa Grade Low-Temperature Quenched and Tempered Steel Plate with Excellent Weldability and Fatigue Resistance and its Manufacturing Method." This method can produce thin-gauge low-temperature quenched and tempered steel plates with good weldability. The invention employs a low-C, low-Si, and low-Ni composition system. The low-carbon equivalent composition design improves weldability, but it does not combine the combined effects of C, Mn, and Ni elements with quenching and tempering microalloying elements, making it impossible to produce large-thickness steel plates. Increasing the thickness of the rolled steel plate leads to a decrease in strength, low-temperature toughness, and fatigue resistance. Chinese patent CN108624809B discloses "Ultra-High Strength Steel Plate with Excellent Seawater Corrosion Resistance, Fatigue Resistance, and Environmental Brittleness Resistance and its Manufacturing Method," proposing an ultra-high strength marine engineering steel with low C, low Si, low Mn, high Ni, and high Cr. This composition has a high precious metal content, resulting in high cost and difficulty in continuous casting production. Due to the high Cr and Ni content, the surface quality of the steel plate is difficult to control, making large-scale stable production impossible. Summary of the Invention

[0008] To overcome the shortcomings of the prior art, the present invention provides an extra-thick marine engineering steel with fatigue crack propagation resistance and its manufacturing method, which can significantly improve the fatigue crack propagation performance of steel plates, with a maximum thickness of up to 150 mm, ultra-high strength, and excellent low-temperature toughness.

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

[0010] An extra-thick marine engineering steel resistant to fatigue crack propagation, comprising the following chemical composition by weight percentage:

[0011] C: 0.07%~0.12%, Si: 0.15%~0.25%, Mn: 0.8%~1.4%, P≤0.02%, S≤0.01%, Als: 0.03%~0.08%, Ni: 2.0%~3.5%, Cr: 0.3%~0.55%, Mo: 0.4%~0.65%, Cu: 0.1%~0.35%, Co: 0.02%~0.1%, V: 0.03%~0.07%, Ti: 0.005%~0.015%, N: 0%~0.005%, B: 0.001%~0.0015%, with the remainder being Fe and unavoidable impurities.

[0012] This invention utilizes compositional design that combines C, Mn, Ni, Cr, Mo, Cu with microalloying elements such as Co, V, and B, and considers the effects of selecting the types and amounts of these alloying elements:

[0013] (1) As a basic strengthening element in steel, carbon (C) is the main element for ensuring strength and hardness in this invention. 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, insufficient steel plate strength, and inability to propagate fatigue cracks through grain boundary strengthening. Furthermore, too low a C content cannot guarantee the hardenability of extra-thick steel plates. However, too high a C content will produce a large amount of hardened structure, increasing the tendency to crack during deformation and affecting low-temperature impact toughness and fatigue resistance. Therefore, the C content in the steel should be precisely controlled. The C content is 0.07%~0.12%.

[0014] (2) Si can improve the strength of steel plates. At the same time, Si can reduce the O content as a deoxidizer. When the Si content is less than 0.15%, the deoxidation effect is not obvious and the strength is reduced. In addition, Si will control the O content of electroslag ingots to a certain extent during the electroslag remelting process. To avoid segregation and inclusions in thick billets, the Si content should not be greater than 0.25%. The Si content of this invention is 0.15%~0.25%.

[0015] (3) Mn has a similar atomic radius to Fe and, like Co and other elements, dissolves in large quantities in the Fe matrix, increasing the strength of the steel plate and improving its fatigue resistance. Since this invention uses electroslag remelting to prepare electroslag ingots, the Mn content can be appropriately increased to fully utilize the strengthening effect of Mn. When the Mn content is below 0.8%, it contributes little to the fatigue resistance of the steel plate. When the mass percentage of Mn is greater than 1.4%, Mn will reduce the low-temperature toughness of the core of the thick plate. The Mn content is 0.8%~1.4%.

[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 and can also improve the corrosion fatigue resistance of steel plates to a certain extent. When the Al content is too low, the deoxidation effect is not good. Microalloying elements such as Ti cannot achieve the purpose of refining grains due to oxidation. Ultra-high strength steel plates with a thickness greater than 100mm need to appropriately increase the Al content in the steel. On the contrary, if the Al content is too high, large inclusions will be formed. The Al content is 0.03%~0.08%.

[0018] (6) The role of Ni is to improve the hot workability of steel plates and enhance their toughness and fatigue resistance. Ni, along with Mn and Co, dissolves in the matrix in large quantities, which can improve strength and reduce the ductile-brittle transition temperature. At the same time, Ni can improve the corrosion fatigue resistance of steel plates. The Ni content is 2.0%~3.5%.

[0019] (7) Cr is an element that improves the hardenability and strength of extra-thick steel plates. When the C content is low, adding an appropriate amount of Cr can ensure that the steel plate reaches the required strength. However, adding too much Cr will reduce the toughness and fatigue resistance of the material. Cr can improve the corrosion resistance of steel plates to a certain extent. The Cr content is 0.3%~0.55%.

[0020] (8) The addition of Mo can improve the hardenability of steel plate. Adding an appropriate amount of Mo can also improve the temper brittleness of steel plate. Mo can also work with Ni to play a certain corrosion resistance role. These beneficial effects can improve the strength, toughness and fatigue of steel plate. The Mo content is 0.4%~0.65%.

[0021] (9) Cu can improve the strength and hardness of steel, and increase the fatigue strength of steel plates. At the same time, Cu can improve the atmospheric corrosion resistance of steel plates. An appropriate amount of Cu can also improve the fluidity of molten steel during electroslag remelting and improve the filling performance of electroslag ingots. Excessive Cu content will cause hot cracking. The Cu content is 0.1%~0.35%.

[0022] (10) The role of Co is to improve the high-temperature thermal stability of steel. During the quenching and tempering process, Co can organize grain growth. At the same time, Co can effectively improve the strength and fatigue resistance of steel plates. The Co content is 0.02%~0.1%.

[0023] (11) V can form V(C,N) particles in the matrix, which can refine and strengthen the grains. Adding V to heat-treated steel plates can significantly improve the strength and toughness of the steel plates. V can improve the tendency of hot cracking during rolling and tempering, with a V content of 0.03%~0.07%.

[0024] (12) Ti can produce a strong precipitation strengthening effect, prevent the recrystallization and growth of austenite, and refine the grains to improve the yield strength of steel. Ti, together with V, N, C and other elements, can form fine and dispersed C and N compound second phases during quenching, effectively controlling the growth of the original austenite grains, and thus significantly improving the strength and toughness of the steel plate. Ti can effectively improve the fatigue life of steel plates during fatigue, with a Ti content of 0.005%~0.015%.

[0025] (13) N element can play a role in solid solution strengthening and can improve the hardenability of steel plate. When combined with V, Ti and other elements, it can improve the strength and fatigue resistance of steel plate. The N content is 0%~0.005%.

[0026] (14) Bo (B) element can improve the hardenability of steel plates. Especially for the production of extra-thick steel plates, which require the core of the steel plate to have excellent low-temperature toughness and fatigue resistance, it is necessary to increase the content of Bo element. However, excessive Bo element content can easily cause cracks. The Bo content is 0.001%~0.0015%.

[0027] The aforementioned extra-thick marine engineering steel with resistance to fatigue crack propagation has a yield strength ≥620MPa, tensile strength 700~890MPa, elongation ≥20%, and Charpy impact energy ≥120J at -60℃. The maximum thickness of the steel plate can reach 150mm.

[0028] The manufacturing method of the aforementioned fatigue crack propagation resistant extra-thick marine engineering steel specifically includes the following steps:

[0029] 1) Steel refining:

[0030] Molten steel is refined in a converter, LF furnace, RH or VD furnace to further reduce the content of P, S and non-metallic inclusions.

[0031] The refined molten steel consists of the following chemical composition by weight percentage:

[0032] C: 0.07%~0.12%, Si: 0.15%~0.25%, Mn: 0.8%~1.4%, P≤0.02%, S≤0.01%, Als: 0.03%~0.08%, Ni: 2.0%~3.5%, Cr: 0.3%~0.55%, Mo: 0.4%~0.65%, Cu: 0.1%~0.35%, Co: 0.02%~0.1%, V: 0.03%~0.07%, Ti: 0.005%~0.015%, N: 0%~0.005%, B: 0.001%~0.0015%, with the remainder being Fe and unavoidable impurities.

[0033] 2) Electroslag remelting:

[0034] Electroslag remelting superheat ≤15℃, crystallizer cooling water flow rate 45~60m³ / h 3 / h, outlet water temperature ≤45℃. Total argon flow rate ≥20m³ / h. 3 / h.

[0035] 3) Electroslag ingot heating:

[0036] The electroslag ingot is loaded into a heating furnace at a temperature of 600-700℃ and held at low temperature for 1-2 hours. The purpose is to keep the billet at a consistent temperature in the thickness direction during the low-temperature stage, so as to prepare for a uniform microstructure in the high-temperature section.

[0037] The heating rate is 2~6℃ / min, the heating temperature is 1200~1250℃, and the holding time is 1~3h. The purpose of low-temperature homogenization and holding is to ensure that the C / N compounds are fully dissolved while avoiding abnormal growth of the as-cast structure.

[0038] 4) Controlled rolling:

[0039] The initial rolling temperature is 1180~1230℃, the average reduction per pass is 6~20%, and the final rolling temperature is 900~1050℃. The purpose of high-temperature hot rolling is to increase the reduction per pass and improve the as-cast microstructure of the slab. Increasing the reduction per pass in the recrystallization temperature range near Ac3 promotes the flattening and refinement of austenite grains. This prepares the microstructure for tempering treatment. Utilizing the fine austenite microstructure and residual deformation stress after rolling, the steel plate is rapidly and comprehensively transformed into a fine martensite microstructure.

[0040] 5) Conditioning:

[0041] The quenching and tempering process is crucial to the low-temperature impact toughness of steel plates. A combination of quenching, sub-critical quenching, and high-temperature tempering is employed, with quenching temperatures ranging from 750 to 900°C, sub-critical quenching temperatures from 700 to 820°C, and tempering temperatures from 400 to 700°C. By employing quenching, sub-critical quenching in the critical zone, and tempering, a finer effective grain size can be obtained, increasing the number of large-angle grain boundaries, increasing the proportion and uniformity of soft phase distribution in tempered martensite, and further improving the low-temperature toughness and fatigue resistance of the steel plate.

[0042] Compared with the prior art, the beneficial effects of the present invention are:

[0043] 1. This invention utilizes a compositional design that combines C, Mn, Ni, Cr, Mo, Cu with microalloying elements such as Co, V, and B. The synergistic effect of C and Mn ensures the strength and hardness of the 150mm steel plate while preventing fatigue crack propagation through grain boundary strengthening and solid solution strength. The abundant solid solution of Ni, Mn, and Co in the matrix enhances the strength of thick steel plates and lowers the ductile-brittle transition temperature. Ni also improves the steel plate's corrosion fatigue resistance. Mo, in conjunction with Ni, provides some corrosion resistance. These beneficial effects all contribute to improving the steel plate's strength, toughness, and fatigue resistance. Cu enhances the steel plate's resistance to atmospheric corrosion and corrosion fatigue. B improves the steel plate's hardenability, which is particularly important for producing extra-thick steel plates where excellent low-temperature toughness and fatigue resistance are required in the core. This synergistic combination of elements is a targeted design for the strength, toughness, and fatigue resistance of the 150mm steel plate produced in this invention.

[0044] 2. The manufacturing method of this invention employs high-purity and alloying smelting + electroslag remelting + low-temperature heating + high-efficiency rolling + quenching + sub-temperature quenching + high-temperature tempering. Molten steel is refined in a converter, LF furnace, RH or VD furnace to further reduce the content of P, S and non-metallic inclusions. The electroslag ingot is charged into the heating furnace at 600-700℃ and held at low temperature for 1-2 hours to maintain a consistent temperature along the thickness direction of the billet during the low-temperature stage, preparing for a uniform microstructure in the high-temperature section. The purpose of low-temperature homogenization is to ensure sufficient dissolution of C / N compounds while preventing abnormal growth of the as-cast microstructure. High-temperature hot rolling aims to increase the single-pass reduction, improving the as-cast microstructure of the slab. Increasing the single-pass reduction in the recrystallization temperature region near Ac3 promotes the flattening and refinement of austenite grains, preparing the microstructure for tempering treatment. By utilizing the fine austenitic structure and residual deformation stress after rolling, the steel plate can be rapidly and comprehensively transformed into a fine martensite structure. By employing quenching, sub-critical quenching in the critical zone, and tempering, a finer effective grain size can be obtained, the number of large-angle grain boundaries can be increased, the proportion and uniformity of soft phase in tempered martensite can be increased, and the low-temperature toughness and fatigue resistance of the steel plate can be further improved.

[0045] 3. This invention employs a composition design that combines C, Mn, Ni, Cr, Mo, Cu with microalloying elements such as Co, V, and B. It also incorporates an innovative process of alloying smelting, electroslag remelting, low-temperature heating, high-efficiency rolling, quenching, sub-temperature quenching, and high-temperature tempering to enhance the microstructure (the microstructure at 1 / 4 of the steel plate thickness is tempered martensite, with an effective grain size of 4-10 micrometers and a large-angle grain boundary ratio of ≥40%). The combination of composition and process results in an extra-thick steel plate with excellent low-temperature toughness and fatigue resistance.

[0046] In summary, this invention utilizes a compositional design combining C, Mn, Ni, Cr, Mo, Cu, and microalloying elements such as Co, V, and B. The manufacturing method employs high-purity alloying smelting, electroslag remelting, low-temperature heating, high-efficiency rolling, quenching, sub-temperature quenching, and high-temperature tempering. Using this invention, 620MPa ultra-high-strength marine engineering steel with a maximum thickness of 150mm can be produced. Its tensile strength is 700~890MPa, and its Charpy impact energy at -60℃ is ≥120J. It improves the fatigue crack propagation performance of the steel plate; when Lg(ΔK) = 3.5MPa × m0.5, Lg(da / dN) ≤ -2.6mm / cycle. Attached Figure Description

[0047] Figure 1 This is a metallographic image of the tempered state in Embodiment 1 of the present invention. Detailed Implementation

[0048] This invention discloses an extra-thick marine engineering steel resistant to fatigue crack propagation 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 clearly modify or appropriately change and combine 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.

[0049] 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 in the embodiments of this invention is shown in Table 1, the smelting and rolling preheating process is shown in Table 2, the rolling and quenching and tempering process is shown in Table 3, and the mechanical properties of the steel plate are shown in Table 4.

[0050] Table 1. Chemical composition of steel in the embodiments of the present invention, wt%

[0051]

[0052] Table 2 Smelting and Rolling Preheating Processes in Embodiments of the Invention

[0053]

[0054] Table 3 Rolling and tempering processes in embodiments of the present invention

[0055]

[0056] Table 4 Mechanical properties of steel plates in embodiments of the present invention

[0057]

[0058] like Figure 1 As shown, the tempered metallographic structure of Example 1 has tempered martensite at 1 / 4 of the steel plate thickness, with an effective grain size of 4-10 micrometers, a large-angle grain boundary ratio of ≥40%, and good mechanical properties.

[0059] As shown in Table 4, this invention is a thick ultra-high strength steel plate with a yield strength of 620 MPa suitable for marine engineering. Its mechanical properties and high service safety performance meet the service conditions of marine engineering equipment. The yield strength of this invention is ≥620 MPa, tensile strength is 700~890 MPa, and Charpy impact energy at -60℃ is ≥120 J. It improves the fatigue crack propagation performance of the steel plate; when Lg(ΔK) = 3.5 MPa × m0.5, Lg(da / dN) ≤ -2.6 mm / cycle. The maximum thickness of the finished marine engineering steel plate for service in polar marine environments is 150 mm.

[0060] 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 extra-thick marine engineering steel resistant to fatigue crack propagation, characterized in that, Marine engineering steel has the following chemical composition by weight percentage. composition: C: 0.084%~0.12%, Si: 0.16%~0.25%, Mn: 0.8%~0.94%, P≤0.02%, S≤0.01%, Als: 0.03%~0.038%, Ni: 2.73%~3.5%, Cr: 0.51%~0.55%, Mo: 0.4%~0.51%, Cu: 0.1%~0.35%, Co: 0.02%~0.09%, V: 0.03%~0.07%, Ti: 0.005%~0.006%, N: 0%~0.005%, B: 0.001%~0.0015%, with the remainder being Fe and unavoidable impurities; Its yield strength is ≥620MPa, tensile strength is 700~890MPa, elongation is ≥20%, Charpy impact energy at -60℃ is ≥120J; the thickness of the finished steel plate is 80~150mm; The manufacturing method specifically includes the following steps: 1) Steel refining; 2) Electroslag remelting; 3) Electroslag ingot heating: The electroslag ingots are loaded into a heating furnace with a furnace temperature of 600~650℃ and held at low temperature for 1~2 hours. Heating rate: 3.5~6℃ / min; heating temperature: 1200~1250℃; holding time: 1~3h. 4) Controlled rolling: The initial rolling temperature is 1180~1230℃, the average reduction per pass is 6%~20%, and the final rolling temperature is 988~1050℃; 5) Conditioning: The process involves quenching, sub-temperature quenching, and high-temperature tempering. The quenching temperature is 750~900℃, the sub-temperature quenching temperature is 700~820℃, and the tempering temperature is 500~630℃.

2. The method for manufacturing an extra-thick marine engineering steel resistant to fatigue crack propagation 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 an extra-thick marine engineering steel resistant to fatigue crack propagation according to claim 1, characterized in that, In step 2), the electroslag remelting superheat is ≤15℃.

4. The method for manufacturing an extra-thick marine engineering steel resistant to fatigue crack propagation according to claim 1, characterized in that, In step 2), the cooling water flow rate of the crystallizer is 45~60m³ / h. 3 / h, outlet water temperature ≤45℃.

5. The method for manufacturing an extra-thick marine engineering steel resistant to fatigue crack propagation according to claim 1, characterized in that, In step 2), the total argon flow rate is ≥20m³ / h. 3 / h.

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