Extremely cold fatigue-resistant ultra-high strength marine engineering steel and its manufacturing method

Abstract

The present invention relates to an extremely cold, fatigue-resistant, ultra-high-strength marine engineering steel and a manufacturing method thereof. The chemical composition of the steel plate is C: 0.05%-0.08%, Si: 0.15%-0.35%, Mn: 0.95%-1.45%, P≤0.02%, S≤0.01%, Als: 0.015%-0.045%, Ni: 0.5%-2.0%, Cr: 0.1%-0.3%, Mo: 0.2%-0.4%, Cu: 0.2%-0.4%, Co: 0.05%-0.2%, Nb: 0.02%-0.07%, Ti: 0.008%-0.015%, N: 0.002%-0.008%, with the remainder being Fe and impurities. This steel plate utilizes a unique composition design, coupled with a production process for ultra-high-strength marine engineering steel plates with large thickness and high deformation, to ensure that the mechanical properties and service safety performance of the steel plate meet the service requirements of marine engineering equipment.

Description

Technical Field

[0001] The present invention relates to the technical field of marine engineering steel production, and in particular to an extremely cold-resistant fatigue-resistant ultra-high-strength 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. The accelerated development of shipping routes in cold oceans and the Arctic Circle has placed higher demands on related technologies and equipment. Simultaneously, with the deepening of research into offshore steel materials for use in extreme environmental conditions, new offshore steels are emerging to meet increasingly stringent requirements. The Arctic region, rich in resources such as oil and natural gas, holds enormous development potential. Arctic shipping demands not only technical performance in cryogenics, corrosion, and fatigue resistance, but also environmental protection, such as ship emissions and the prevention of marine pollution, which also require relevant technologies and equipment. To adapt to these changes and challenges, research into new offshore steel materials is currently focused on microalloying, controlled rolling, controlled cooling, and heat treatment processes, aiming to improve the materials' frost resistance, corrosion resistance, and other application properties.

[0003] Microalloying technology is currently a hot topic in the research and development of marine 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 an important research direction. The use of controlled rolling technology can effectively control the structure and phase transformation of steel, and improve the toughness and fatigue resistance of steel. The fatigue resistance of steel plates is of great significance to marine engineering equipment, and can significantly improve its reliability, safety and economic benefits. By adding appropriate trace alloying elements and combining with advanced heat treatment processes, the fatigue resistance of steel plates can be effectively improved. In response to the new requirements for the fatigue resistance of marine steel plates, fatigue crack growth tests, fatigue life tests and low-cycle fatigue strength tests can be used for verification.

[0004] When selecting steel materials and processing techniques, factors such as performance, cost, and feasibility must be comprehensively considered. While new marine steel materials are constantly emerging, they are generally expensive, requiring a rational selection based on specific needs and economic viability for specific applications. Furthermore, in addition to micro-alloying and heat treatment processes, emerging technologies such as ultrasonic surface treatment can also be used to enhance material fatigue resistance.

[0005] Existing marine engineering steels can already meet most market demands, but special steels with lower ductile-brittle transition temperatures and excellent overall performance remain a research and development target for countries around the world. The development of high-strength steel plates with high service safety is challenging, requiring rigorous production processes and high equipment requirements. For example, the Chinese invention patent with authorization publication number CN113913695B discloses a "corrosion-resistant and fatigue-resistant underwater oil and gas production and transportation pipeline steel and its production method." The steel plate incorporates alloying elements such as Ni, Cu, and Ca to ensure excellent fatigue resistance in oil and gas corrosive environments. However, the alloying elements and production process make it impossible to produce ultra-thick steel plates with a maximum thickness of 100 mm for extreme cold environments. Chinese invention patent CN113969372B, which discloses a "Low-carbon, Fatigue-resistant Steel Plate for Wind Power Generation and Its Preparation Method," utilizes a high-Mn, low-alloying element composition design and a conventional TMCP rolling process combined with tempering. These composition and production processes are unable to produce a steel plate with a maximum thickness of 100mm, low-temperature toughness at -60°C, and fatigue resistance that meets the required standards. Chinese invention patent CN108411188B, which discloses a "Thick Steel Plate with High Crack Arrest and Fatigue Strength and Its Preparation Method," has a low alloying element content and does not utilize Co to enhance strength and fatigue resistance. Conventional TMCP processes are unable to produce thick, fatigue-resistant steel plates. Summary of the Invention

[0006] The present invention provides an extremely cold fatigue-resistant ultra-high strength marine engineering steel and a manufacturing method thereof. The unique composition design is combined with the production process of ultra-high strength marine engineering steel plates with large thickness and large deformation to ensure that the mechanical properties and service safety performance of the steel plates meet the service conditions of marine engineering equipment. The fatigue crack growth performance of the base material and the welding heat-affected zone is: when Lg(ΔK)=3.5MPa·m 0.5 When, Lg(da / dN)≤-2.6mm / cycle.

[0007] In order to achieve the above object, the present invention adopts the following technical solutions:

[0008] Disclosed is an extremely cold, fatigue-resistant, ultra-high-strength marine steel. The chemical composition of the steel plate is, by weight percentage, C: 0.05%-0.08%, Si: 0.15%-0.35%, Mn: 0.95%-1.45%, P≤0.02%, S≤0.01%, Als: 0.015%-0.045%, Ni: 0.5%-2.0%, Cr: 0.1%-0.3%, Mo: 0.2%-0.4%, Cu: 0.2%-0.4%, Co: 0.05%-0.2%, Nb: 0.02%-0.07%, Ti: 0.008%-0.015%, N: 0.002%-0.008%, and the remainder is Fe and unavoidable impurities.

[0009] The yield strength of the finished steel plate is ≥460MPa, the tensile strength is 540~720MPa, the elongation is ≥19%, and the average Charpy impact energy at -60℃ is ≥120J; the fatigue crack growth performance of the base material and the welding heat affected zone is: when Lg(ΔK)=3.5MPa·m 0.5 When, Lg(da / dN)≤-2.6mm / cycle.

[0010] The maximum thickness of the finished steel plate is 100mm.

[0011] The metallographic structure at 1 / 4 of the thickness of the finished steel plate is acicular ferrite + polygonal ferrite, and the volume ratio of polygonal ferrite is 5% to 15%, and the grain size of the steel plate is 3 to 15 μm.

[0012] A method for manufacturing extremely cold fatigue-resistant ultra-high strength marine engineering steel comprises the following steps:

[0013] 1) Smelting: After the molten steel is smelted in a converter, it is then refined in a LF furnace, RH furnace or VD furnace to further reduce the content of P, S and non-metallic inclusions;

[0014] 2) Electroslag remelting: Cooling water flow rate during electroslag remelting is ≤50m 3 / h, feeding time ≥3h;

[0015] 3) Heating: The blank is loaded into the heating furnace when the furnace temperature is 600-650℃ and kept warm for more than 60 minutes; then the heating rate is controlled to be 3-5℃ / min to increase the temperature to 1150-1250℃, the soaking temperature is 1100-1180℃, and the soaking time is 6-10 hours;

[0016] 4) Rolling: Rough rolling is carried out in two steps; the starting rolling temperature of the first rolling is 950-1100°C, the average pass reduction rate is 10%-15%, the thickness ratio of the slab after the first rolling to the thickness of the finished steel plate is 2.5-5, and the finishing rolling temperature of the first rolling is 700-800°C; the slab heating temperature before the second rolling is 750-950°C, the total heating time is 5-8h, the starting rolling temperature of the second rolling is 750-900°C, and the thickness ratio of the intermediate slab after the second rolling to the thickness of the finished steel plate is 1.5-2.2; the starting rolling temperature of the finishing rolling is 720-800°C, and the finishing rolling temperature of the finishing rolling is 700-735°C;

[0017] 5) Controlled cooling: The water entry temperature of the steel plate is 650-730°C, the red-return temperature is 390-460°C, and the stacking slow cooling time is ≥36h.

[0018] Compared with the prior art, the present invention has the following beneficial effects:

[0019] 1) Adopt low C and low Mn design, combine Ni, Cr, Mo, Cu, Co elements, and cooperate with the production process of low-speed electroslag remelting multi-stage heating + two-stage rolling + controlled cooling treatment to improve the fatigue crack growth performance of the base material and the welding heat affected zone. When Lg (ΔK) = 3.5MPa·m 0.5 When, Lg(da / dN)≤-2.6mm / cycle.

[0020] 2) A unique alloy composition system is used in conjunction with the production process to ensure that the yield strength of the steel plate after controlled cooling treatment is ≥460MPa, the tensile strength is 540~720MPa, and the Charpy impact energy at -60℃ is ≥120J. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is a metallographic photograph of the finished steel plate of the present invention at 1 / 4 of the thickness. DETAILED DESCRIPTION

[0022] The present invention addresses the composition and performance requirements of ultra-high-strength marine steel resistant to extreme cold and fatigue, utilizes a composition design that combines Ni, Cr, Mo, Cu, Co, and microalloying elements, and develops key production technologies for ultra-high-strength marine steel resistant to extreme cold and fatigue. To achieve the objectives of the present invention, the inventors conducted extensive and systematic experimental research on alloy element screening and proportioning, steel cleanliness control, and high-efficiency rolling process optimization and parameter selection, ultimately determining the alloy element proportioning and production process that meet the objectives of the present invention:

[0023] The present invention discloses an extremely cold fatigue-resistant ultra-high strength marine steel, wherein the chemical composition of the steel plate is, by weight percentage, C: 0.05%-0.08%, Si: 0.15%-0.35%, Mn: 0.95%-1.45%, P≤0.02%, S≤0.01%, Als: 0.015%-0.045%, Ni: 0.5%-2.0%, Cr: 0.1%-0.3%, Mo: 0.2%-0.4%, Cu: 0.2%-0.4%, Co: 0.05%-0.2%, Nb: 0.02%-0.07%, Ti: 0.008%-0.015%, N: 0.002%-0.008%, and the remainder is Fe and unavoidable impurities.

[0024] The composition design reasons of the extreme cold fatigue resistant ultra-high strength marine engineering steel described in the present invention are as follows:

[0025] 1) Carbon, a fundamental strengthening element in steel, plays a key role in ensuring strength and hardness in the marine engineering steel described herein. A low carbon content can reduce grain refinement. Excessive carbon content can produce a large amount of hardened microstructure, significantly reducing the steel's toughness, ductility, weldability, and fatigue resistance. Therefore, the carbon content must be precisely controlled. In this invention, the carbon content is controlled to 0.05% to 0.08%.

[0026] 2) Si improves the strength of steel plates and is a major deoxidizing component in the steelmaking process, acting as a deoxidizer and reducing agent. A Si content exceeding 0.35% can lead to the formation of hard phases in the structure, affecting the material's fatigue resistance. Therefore, excessive Si addition is not recommended. The present invention controls the Si content to 0.15% to 0.35%.

[0027] 3) The Mn element has a similar atomic radius to that of Fe and can be dissolved in the Fe matrix in large quantities, thereby increasing the strength of the steel plate. Mn combines with S to form MnS, which can avoid thermal cracks caused by the formation of FeS at the grain boundaries. At the same time, Mn is an element that expands the austenite phase region and can improve the stability of austenite. When the Mn element content is greater than 0.7%, the tendency of the steel plate to transform into martensite is weakened during the quenching process; the present invention adopts the electroslag remelting method to prepare the billet steel ingot, which can avoid the composition segregation caused by the Mn element in the core of the billet when the Mn content is high, so that the core of the thick plate has higher low-temperature toughness and fatigue resistance; the present invention controls the Mn content to 0.95% to 1.45%.

[0028] 4) P and S elements have no benefit on the mechanical properties of the steel plate, especially the elongation. The present invention controls P≤0.02% and S≤0.01%.

[0029] 5) Al is the primary deoxidizing element in steel. Als reacts with metal ions in steel to mitigate corrosion fatigue, improving the steel's corrosion resistance. Low Al content results in poor deoxidation, and thicker ultra-high-strength steel plates require a moderately increased Als content. Excessive Al content can easily lead to the formation of large inclusions. Therefore, the Als content in this invention is controlled to 0.015% to 0.045%.

[0030] 6) Ni improves the toughness and hot workability of the steel plate. Large additions can lower the ductile-brittle transition temperature and increase tensile elongation. Ni also works synergistically with elements like Cu and Co to improve the ability to prevent the initiation and propagation of fatigue sources while maintaining strength. Ni also offers some corrosion resistance; the present invention limits the Ni content to 0.5% to 2.0%.

[0031] 7) The Cr element in steel can effectively improve the strength of the steel plate. When combined with the Ni element, it can better improve the low-temperature toughness of the steel plate and reduce the rolling reduction ratio. Adding an appropriate amount can improve the toughness and fatigue resistance of the core of the extra-thick steel plate. However, too high a Cr content will generate fatigue crack sources. Therefore, the present invention controls the Cr content to 0.1% to 0.3%.

[0032] 8) Mo element can improve the hardenability of steel plate. At the same time, the fine carbides formed by Mo element in steel can effectively improve the strength of steel plate. Mo element combined with Ni and Cr elements can effectively reduce the fatigue crack sensitivity of steel plate. The present invention controls the Mo content to 0.2% to 0.4%.

[0033] 9) Cu in steel can improve the wear resistance and fatigue resistance of the steel plate. At the same time, a certain content of Cu element will exist in the nanoscale phase near the welding heat-affected zone, thereby improving the low-temperature toughness and fatigue resistance of the steel plate weld and heat-affected zone. However, excessive Cu element can cause high-temperature thermal cracking of the steel plate. Therefore, the present invention controls the Cu content to 0.2% to 0.4%.

[0034] 10) Co can be dissolved in the matrix along with Ni and Mn, increasing strength and hardness while also improving the steel's resistance to fatigue crack initiation and growth. However, excessive Co content can reduce the steel's low-temperature toughness; therefore, the Co content in this invention is controlled to 0.05% to 0.2%.

[0035] 11) Nb effectively refines the steel's grain size, improving the balance between material strength and toughness through grain refinement. The addition of Nb effectively improves rolling efficiency and reduces the steel plate's rolling reduction ratio. It also prevents intergranular corrosion in the steel plate, enhancing its fatigue resistance in extremely cold marine environments. The present invention controls the Nb content to 0.02% to 0.07%.

[0036] 12) Ti forms Ti(C,N) particles in the matrix, refining and strengthening the grains. Adding an appropriate amount of Ti to the steel can also reduce the rolling reduction ratio. The present invention controls the Ti content to 0.008% to 0.015%.

[0037] 13) The nitrogen element in steel can improve the strength and hardness of the steel plate, reduce the crack sensitivity of the steel plate, and increase the service life of the steel plate under fatigue environment. The present invention controls the nitrogen content to 0.002% to 0.008%.

[0038] The manufacturing process of the extremely cold fatigue-resistant ultra-high strength marine engineering steel described in the present invention adopts a production process of high cleanliness and alloying smelting + electroslag remelting + multi-stage heating + two-stage rolling + controlled cooling treatment, which specifically includes the following steps:

[0039] (1) High cleanliness and alloy smelting;

[0040] After smelting in a converter, the molten steel is refined in an LF furnace, RH or VD furnace to further reduce the content of P, S and non-metallic inclusions.

[0041] (2) Electroslag remelting;

[0042] Cooling water flow rate during electroslag remelting ≤ 50m 3 / h, shrinkage compensation time ≥3h.

[0043] (3) Heating;

[0044] The billet is loaded into the heating furnace at a temperature of 600-650°C and held at this temperature for at least 60 minutes. This ensures a consistent temperature throughout the thickness of the billet during the low-temperature phase, paving the way for uniform microstructure in the high-temperature phase. The heating rate is then controlled at 3-5°C / min to prevent excessive heating and uneven internal heating. The heating temperature is 1150-1250°C, the soaking temperature is 1100-1180°C, and the soaking time is 6-10 hours. The soaking time should be minimal to prevent abnormal microstructure growth.

[0045] (4) rolling;

[0046] Rough rolling is performed in two passes. The first pass starts at a temperature of 950-1100°C, with an average pass reduction of 10%-15%. The ratio of slab thickness after the first pass to finished product thickness is 2.5-5, and the final temperature of the first pass is 700-800°C. The purpose of the first pass is to maximize the single-pass reduction ratio at a stage of low deformation resistance, thereby improving the slab structure. This allows the large deformation to quickly penetrate the core of the slab, improving the core structure and fatigue resistance.

[0047] The heating temperature of the billet during the second rolling is 750-950°C, and the total heating time is 5-8 hours; the starting rolling temperature of the second rolling is 750-900°C, and the ratio of the thickness of the rolling intermediate billet to the thickness of the finished product is 1.5-2.2.

[0048] The start rolling temperature of the finishing rolling is 720-800°C, and the final rolling temperature is 700-735°C. Low-temperature rolling can effectively increase the internal deformation energy storage of the steel plate. The small and flat grains increase the grain boundary area, which can prevent fatigue crack propagation and improve the fatigue resistance of the steel plate.

[0049] (5) Cooling control;

[0050] The steel plate entering water temperature is 650-730℃, the red-returning temperature is 390-460℃, and the stacking slow cooling time is ≥36 hours.

[0051] The yield strength of the finished steel plate is ≥460MPa, the tensile strength is 540~720MPa, the elongation is ≥19%, the Charpy impact energy at -60℃ is ≥120J, and the fatigue crack growth performance of the base material and the welding heat affected zone is: when Lg(ΔK)=3.5MPa·m 0.5 When Lg(da / dN)≤-2.6mm / cycle. The maximum thickness of the finished steel plate is 100mm.

[0052] like Figure 1 As shown in the figure, the metallographic structure of the finished steel plate at 1 / 4 of the thickness is acicular ferrite + polygonal ferrite (volume content 5%~15%), the grain size is 3~15μm, and the mechanical properties are good.

[0053] In order to more intuitively embody the present invention, the embodiments of the present invention are further described in conjunction with examples. The following examples are only preferred specific embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any technical solution that can be obviously obtained by a person skilled in the art within the technical scope disclosed in the present invention, including simple changes or equivalent replacements, is within the scope of protection of the present invention.

[0054] [Example]

[0055] The chemical composition of the steel in each example is shown in Table 1, the production process parameters are shown in Table 2-1 and Table 2-2, and the properties of the finished steel plate are shown in Table 3.

[0056] Table 1 Chemical composition of steel (weight percentage, %)

[0057]

[0058] Table 2-1 Production process parameters (I)

[0059]

[0060] Table 2-2 Production process parameters (II)

[0061]

[0062] Table 3 Finished steel plate properties

[0063]

[0064] 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. An extremely cold fatigue-resistant ultra-high strength marine engineering steel, characterized in that: The chemical composition of the steel plate includes, by weight percentage: C 0.051%-0.08%, Si 0.15%-0.22%, Mn 1.17%-1.45%, P≤0.02%, S≤0.01%, Als 0.015%-0.045%, Ni 0.88%-2.0%, Cr 0.16%-0.27%, Mo 0.24%-0.4%, Cu 0.31%-0.4%, Co0.08%-0.2%, Nb 0.02%-0.07%, Ti 0.008%-0.015%, N 0.0034%-0.008%, and the rest is Fe and unavoidable impurities; The method for manufacturing the extremely cold fatigue-resistant ultra-high strength marine engineering steel comprises the following steps: 1) Smelting: After the molten steel is smelted in a converter, it is then refined in a LF furnace, RH furnace or VD furnace to further reduce the content of P, S and non-metallic inclusions; 2) Electroslag remelting: Cooling water flow rate during electroslag remelting is ≤50m 3 / h, feeding time ≥3h; 3) Heating: The blank is loaded into the heating furnace when the furnace temperature is 600-650℃ and kept warm for more than 60 minutes; then the heating rate is controlled to be 3-5℃ / min to increase the temperature to 1160-1250℃, the soaking temperature is 1100-1180℃, and the soaking time is 6-10 hours; 4) Rolling: Rough rolling is carried out in two steps; the starting rolling temperature of the first rolling is 950-1100°C, the average pass reduction rate is 10%-15%, the thickness ratio of the slab after the first rolling to the thickness of the finished steel plate is 2.5-5, and the finishing rolling temperature of the first rolling is 700-800°C; the slab heating temperature before the second rolling is 750-950°C, the total heating time is 5-8h, the starting rolling temperature of the second rolling is 750-900°C, and the thickness ratio of the intermediate slab after the second rolling to the thickness of the finished steel plate is 1.5-2.2; the starting rolling temperature of the finishing rolling is 720-800°C, and the finishing rolling temperature of the finishing rolling is 700-735°C; 5) Controlled cooling: The water entry temperature of the steel plate is 650-730°C, the red-return temperature is 390-460°C, and the stacking slow cooling time is ≥36h.

2. The method for manufacturing an extremely cold fatigue-resistant ultra-high strength marine engineering steel according to claim 1, characterized in that: The yield strength of the finished steel plate is ≥460MPa, the tensile strength is 540~720MPa, the elongation is ≥19%, and the average Charpy impact energy at -60℃ is ≥120J; the fatigue crack growth performance of the base material and the welding heat affected zone is: when Lg(ΔK)=3.5MPa·m 0.5 When, Lg(da / dN)≤-2.6mm / cycle.

3. The method for manufacturing an extremely cold fatigue-resistant ultra-high strength marine engineering steel according to claim 1, characterized in that: The thickness of the finished steel plate is 45 to 100 mm.

4. The method for manufacturing an extremely cold fatigue-resistant ultra-high strength marine engineering steel according to claim 1, characterized in that: The metallographic structure at 1 / 4 of the thickness of the finished steel plate is acicular ferrite + polygonal ferrite, and the volume ratio of polygonal ferrite is 5% to 15%, and the grain size of the steel plate is 3 to 15 μm.

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