Extra-thick ocean engineering steel capable of resisting fatigue crack propagation and manufacturing method of extra-thick ocean engineering steel

Abstract

The invention relates to the technical field of steel for ocean engineering, in particular to extra-thick steel capable of resisting fatigue crack propagation and used for ocean engineering and a manufacturing method of the extra-thick steel. The method specifically comprises the following steps: 1) refining molten steel; (2) electroslag remelting; 3) heating the electroslag ingot; the electroslag ingot is loaded into a heating furnace with the furnace temperature being 600-700 DEG C, and the low-temperature heat preservation time is 1-2 h; the heating rate is 2-6 DEG C / min, the heating temperature is 1200-1250 DEG C, and heat preservation is conducted for 1-3 h; (4) controlled rolling; the initial rolling temperature ranges from 1180 DEG C to 1230 DEG C, the average pass reduction rate ranges from 6% to 20%, and the final rolling temperature ranges from 900 DEG C to 1050 DEG C; and 5) hardening and tempering: quenching, subcritical quenching and high-temperature tempering are adopted, the quenching temperature is 750-900 DEG C, the subcritical quenching temperature is 700-820 DEG C, and the tempering temperature is 400-700 DEG C. The component design that C, Mn, Ni, Cr, Mo, Cu, Co, V, B and other microalloy elements are matched is utilized, the fatigue crack propagation performance of the steel plate can be remarkably improved, the maximum thickness of the steel plate can reach 150 mm, and the steel plate has ultrahigh strength and excellent low-temperature toughness.

Description

Technical Field

[0001] The present invention relates to the technical field of marine engineering steel, in particular to a super-thick marine engineering steel resistant to fatigue crack propagation and a manufacturing method thereof. Background Art

[0002] In recent years, interest in developing shipping routes in cold oceans and the Arctic Circle has been growing. The Arctic region, rich in energy resources such as oil and natural gas, holds enormous potential for development. The growth of Arctic shipping has placed higher demands on the technology and performance of related ships and marine 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 steel materials for marine engineering has become a key area of focus. Currently, research on marine steel materials focuses on advanced technologies such as microalloying, controlled rolling cooling, and heat treatment processes to improve 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. 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 steel materials for marine engineering have emerged, their prices remain relatively high, so appropriate choices must be made based on specific 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 No. 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 plate with excellent weldability. This invention utilizes a low-C, low-Si, and low-Ni composition system. The low carbon equivalent composition design improves weldability, but it lacks the combined effects of C, Mn, and Ni elements with quenched and tempered microalloying elements, making it impossible to produce thicker steel plates. Increasing the thickness of the rolled steel plate leads to reduced strength, low-temperature toughness, and fatigue resistance. Chinese Patent Publication No. CN108624809B discloses "Ultra-high-strength steel plate with excellent seawater corrosion resistance, fatigue resistance, and environmental brittleness resistance, and its manufacturing method." It proposes an ultra-high-strength marine 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 steelmaking and continuous casting. The high Cr and Ni content makes the surface quality of the steel plate difficult to control, making large-scale, stable production impossible. Summary of the Invention

[0008] In order to overcome the shortcomings of the existing technology, the present invention provides an extra-thick marine engineering steel that is resistant to fatigue crack propagation and a manufacturing method thereof, which can significantly improve the fatigue crack propagation performance of the steel plate. The maximum thickness of the steel plate can reach 150mm, and it has ultra-high strength and excellent low-temperature toughness.

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

[0010] A fatigue crack growth resistant extra-thick marine engineering steel comprising the following chemical components in percentage by weight:

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

[0012] The present invention utilizes a composition design that matches microalloying elements such as C, Mn, Ni, Cr, Mo, Cu with Co, V, and B. The selection of the types and contents of the above alloying elements has the following effects:

[0013] (1) C is a basic strengthening element in steel and is the main element to ensure strength and hardness in the present invention. 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, the steel plate strength will be insufficient, and fatigue crack propagation through grain boundary strengthening structure will not be possible. In addition, if the C content is too low, the hardenability of the extra-thick steel plate cannot be guaranteed. However, if the C content is too high, a large amount of hardened structure will be produced, and the crack tendency will be greater during deformation, affecting the low-temperature impact toughness and fatigue resistance. Therefore, the C content in steel should be precisely controlled. The C content is 0.07%~0.12%.

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

[0015] (3) The Mn element has a similar atomic radius to that of Fe. Like elements such as Co, it is dissolved in the Fe matrix in large quantities, thereby increasing the strength of the steel plate and improving its fatigue resistance. Since the present invention adopts the electroslag remelting method to prepare the electroslag ingot, the Mn content can be appropriately increased to fully utilize the strengthening effect of the Mn element. When the Mn content is less than 0.8%, the contribution to the fatigue resistance of the steel plate is small. When the mass percentage of the Mn element is greater than 1.4%, the Mn element will reduce the low-temperature toughness of the core of the thick plate. The Mn content is 0.8% to 1.4%.

[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 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 poor, and microalloying elements such as Ti cannot achieve the purpose of grain refinement due to oxidation. Ultra-high strength steel plates with a thickness greater than 100 mm need to appropriately increase the Als content in the steel; on the contrary, if the Al content is too high, large inclusions will be formed. The Als content is 0.03%~0.08%.

[0018] (6) The role of Ni is to improve the hot workability of steel plates, and enhance toughness and fatigue resistance. Ni, Mn, and Co elements are dissolved in the matrix in large quantities, which can increase 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 to the steel will reduce the toughness and fatigue resistance of the material. Cr improves the corrosion resistance of the steel plate to a certain extent. The Cr content is 0.3%~0.55%.

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

[0021] (9) Cu in steel can increase the strength and hardness of steel and improve 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 can 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 to hot cracking during rolling and tempering. The V content is 0.03%~0.07%.

[0024] (12) Ti can produce a strong precipitation strengthening effect, preventing the recrystallization growth of austenite, refining the grains and improving the yield strength of the steel. Ti and elements such as V, N, and C can form fine and dispersed C and N compound second phases during the quenching process, effectively controlling the growth of the original austenite grains, thereby significantly improving the strength and toughness of the steel plate. Ti can effectively improve the fatigue life of the steel plate during the fatigue process. The Ti content is 0.005%~0.015%.

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

[0026] (14) Element B 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 B content. However, too high a B content is prone to cracking. The B content is 0.001%~0.0015%.

[0027] The fatigue crack propagation-resistant, extra-thick marine engineering steel has a yield strength of 620 MPa or higher, a tensile strength of 700-890 MPa, an elongation of 20% or higher, and a Charpy impact energy of 120 J or higher at -60°C. The maximum thickness of the steel plate can reach 150 mm.

[0028] The manufacturing method of the above-mentioned extra-thick marine engineering steel resistant to fatigue crack growth specifically comprises the following steps:

[0029] 1) Molten steel refining:

[0030] 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.

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

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

[0033] 2) Electroslag remelting:

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

[0035] 3) Electroslag ingot heating:

[0036] The electroslag ingot is loaded into a heating furnace with a furnace temperature of 600~700℃ and kept at low temperature for 1~2h. The purpose is to keep the temperature of the billet consistent in the thickness direction at the low temperature stage, so as to prepare for uniform organization 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 uniform heating and holding is to ensure that the C / N compounds are fully dissolved while avoiding abnormal growth of the cast structure.

[0038] 4) Controlled rolling:

[0039] The starting rolling temperature is 1180-1230°C, with an average pass reduction of 6-20%, and the finishing temperature is 900-1050°C. The purpose of high-temperature hot rolling is to increase the reduction per pass and improve the as-cast slab structure. Increasing the reduction per pass in the recrystallization temperature region near the Ac3 temperature promotes flattening and refinement of austenite grains, preparing the structure for quenching and tempering. The fine austenite structure and residual deformation stress after rolling are utilized to rapidly and comprehensively transform the steel plate into a fine martensite structure.

[0040] 5) Tempering:

[0041] The quenching and tempering process is key to the steel's low-temperature impact toughness. Quenching, intermittent quenching, and high-temperature tempering are employed, with quenching temperatures ranging from 750°C to 900°C, intermittent quenching temperatures from 700°C to 820°C, and tempering temperatures from 400°C to 700°C. This combination of quenching, intercritical intermittent quenching, and tempering achieves a finer effective grain size, increases the number of high-angle grain boundaries, and enhances the proportion and uniformity of the soft phase in the tempered martensite, further improving the steel's low-temperature toughness and fatigue resistance.

[0042] Compared with the prior art, the present invention has the following beneficial effects:

[0043] 1. The present invention utilizes a composition design that is coordinated with microalloying elements such as C, Mn, Ni, Cr, Mo, and Cu, as well as Co, V, and B. The present invention utilizes the coordination of C and Mn elements to ensure the strength and hardness of the 150mm steel plate while preventing fatigue crack propagation through grain boundary strengthening and solid solution strength; by dissolving a large amount of Ni, Mn, and Co elements in the matrix, the strength of the thick steel plate can be improved and the ductile-brittle transition temperature can be reduced. At the same time, the Ni element can improve the corrosion fatigue resistance of the steel plate; the Mo element can also play a certain corrosion resistance role in combination with Ni. These beneficial effects can all enhance the strength, toughness, and fatigue resistance of the steel plate; the Cu element can improve the atmospheric corrosion resistance and corrosion fatigue resistance of the steel plate; the B element can improve the hardenability of the steel plate. In particular, for the production of extra-thick steel plates, the core of the steel plate is required to have excellent low-temperature toughness and fatigue resistance. The coordination of the above elements is a targeted design for the strength, toughness, and fatigue resistance of the 150mm steel plate in the present invention.

[0044] 2. The manufacturing method of the present invention utilizes high-cleanliness and alloying smelting, electroslag remelting, low-temperature heating, high-efficiency rolling, quenching, sub-temperature quenching, and high-temperature tempering. The molten steel is refined in a converter, LF furnace, RH, or VD furnace to further reduce the contents of P, S, and non-metallic inclusions. The electroslag ingot is loaded into a heating furnace at a temperature of 600-700°C and held at low temperature for 1-2 hours. This ensures that the temperature of the steel billet through the thickness is consistent at low temperatures, preparing for uniform microstructure in the high-temperature stage. The purpose of low-temperature heat soaking and holding is to ensure the full dissolution of C / N compounds while preventing abnormal growth of the as-cast microstructure. The purpose of high-temperature hot rolling is to increase the single-pass reduction and improve the as-cast microstructure of the slab. Increasing the single-pass reduction in the recrystallization temperature region near the Ac3 temperature promotes flattening and refinement of austenite grains. This prepares the microstructure for quenching and tempering. By utilizing the fine austenite structure and residual deformation stress after rolling, the steel plate is quickly and comprehensively transformed into a fine martensite structure; by adopting quenching, intercritical zone sub-temperature quenching and tempering, a finer effective grain size can be obtained, the number of large-angle grain boundaries can be increased, the proportion and distribution uniformity of the soft phase in the tempered martensite can be increased, and the low-temperature toughness and fatigue resistance of the steel plate can be further improved.

[0045] 3. The present invention adopts a composition design that matches C, Mn, Ni, Cr, Mo, Cu with micro-alloying elements such as Co, V, and B, and combines it with an innovative process of alloying smelting + electroslag remelting + low-temperature heating + high-efficiency rolling + quenching + sub-temperature quenching + high-temperature tempering to strengthen the microstructure (the microstructure at 1 / 4 of the thickness of the steel plate is tempered martensite, the effective grain size is 4-10 microns, and the proportion of high-angle grain boundaries is ≥40%). The combination of composition and process produces an extra-thick steel plate with excellent low-temperature toughness and fatigue resistance.

[0046] In summary, the present invention utilizes a compositional design that coordinates C, Mn, Ni, Cr, Mo, Cu, and microalloying elements such as Co, V, and B. Its manufacturing method employs high-cleanliness alloying smelting followed by electroslag remelting, low-temperature heating, high-efficiency rolling, quenching, sub-temperature quenching, and high-temperature tempering. This method can produce 620 MPa ultra-high-strength marine engineering steel with a maximum thickness of 150 mm. It exhibits a tensile strength of 700-890 MPa and a Charpy impact energy of 120 J or greater at -60°C. It also improves fatigue crack growth resistance, achieving a Lg (da / dN) of ≤-2.6 mm / cycle when Lg (ΔK) = 3.5 MPa × m0.5. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Figure 1 This is the tempered metallographic structure diagram of Example 1 of the present invention. DETAILED DESCRIPTION

[0048] The present invention discloses an extra-thick marine engineering steel that is resistant to fatigue crack propagation 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 to achieve the desired effect. It should be noted that all similar substitutions 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.

[0049] The inventors conducted extensive and systematic experimental research on alloying element selection and proportioning, steel cleanliness control, and efficient rolling process optimization and parameter selection, ultimately determining the alloying element proportioning and production process that meet the objectives of the present invention. The chemical composition of the steel examples of the present invention is shown in Table 1, the smelting and rolling preheating processes are shown in Table 2, the rolling and tempering processes are shown in Table 3, and the mechanical properties of the steel plates are shown in Table 4.

[0050] Table 1 Chemical composition of steel according to the present invention, wt%

[0051]

[0052] Table 2 Smelting and rolling preheating process of the embodiment of the present invention

[0053]

[0054] Table 3 Rolling and tempering process of the embodiment of the present invention

[0055]

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

[0057]

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

[0059] As shown in Table 4, the present invention provides a thick, ultra-high-strength steel plate with a yield strength of 620 MPa suitable for use in marine engineering applications. Its mechanical properties and high service safety meet the requirements of marine engineering equipment. The plate features a yield strength of ≥620 MPa, a tensile strength of 700-890 MPa, and a Charpy impact energy of ≥120 J at -60°C. The plate also exhibits improved fatigue crack growth resistance, achieving Lg (da / dN) ≤-2.6 mm / cycle when Lg (ΔK) = 3.5 MPa × m0.5. The maximum thickness of the finished marine steel plate for service in polar marine environments is 150 mm.

[0060] 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 super thick marine engineering steel with resistance to fatigue crack growth, characterized in that: The chemical composition is as follows: composition: 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%, and the rest are Fe and unavoidable impurities.

2. The fatigue crack growth resistant extra-thick marine engineering steel according to claim 1, characterized in that: Its yield strength is ≥620MPa, tensile strength is 700~890MPa, elongation is ≥20%, and Charpy impact energy at -60℃ is ≥120J.

3. The fatigue crack growth resistant extra-thick marine engineering steel according to claim 1, characterized in that: The maximum thickness of the steel plate can reach 150mm.

4. A method for manufacturing the fatigue crack propagation resistant extra-thick marine engineering steel according to any one of claims 1 to 3, characterized in that: The manufacturing method specifically comprises the following steps: 1) Molten steel refining; 2) Electroslag remelting; 3) Electroslag ingot heating: Place the electroslag ingot into a heating furnace at a temperature of 600-700°C and keep it at low temperature for 1-2 hours; Heating rate: 2~6℃ / min, heating temperature: 1200~1250℃, holding temperature: 1~3h; 4) Controlled rolling: The starting rolling temperature is 1180~1230℃, the average pass reduction rate is 6%~20%, and the finishing rolling temperature is 900~1050℃; 5) Tempering: The process is quenching + sub-temperature quenching + high temperature tempering, the quenching temperature is 750~900℃, the sub-temperature quenching temperature is 700~820℃, and the tempering temperature is 400~700℃.

5. The method for manufacturing fatigue crack propagation resistant extra-thick marine engineering steel according to claim 4, characterized in that: In step 1), the molten steel is refined in a converter, LF furnace, RH furnace or VD furnace.

6. The method for manufacturing fatigue crack propagation resistant extra-thick marine engineering steel according to claim 4, characterized in that: In step 1), the refined molten steel has the following chemical composition by weight percentage: composition: 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%, and the rest are Fe and unavoidable impurities.

7. The method for manufacturing fatigue crack propagation resistant extra-thick marine engineering steel according to claim 4, characterized in that: In step 2), the electroslag remelting superheat is ≤15°C.

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

9. The method for manufacturing fatigue crack propagation resistant extra-thick marine engineering steel according to claim 4, characterized in that: In step 2), the argon flow rate throughout the process is ≥ 20m 3 / h.

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