Fatigue-resistant steel plate in extremely cold deep-sea environment and manufacturing method thereof

Abstract

The present invention relates to a fatigue-resistant steel plate for use in extremely cold deep-sea environments and a method for manufacturing the same. The steel plate comprises the following chemical components: C 0.03% to 0.09%, Si 0.05% to 0.15%, Mn 0.3% to 0.45%, P ≤ 0.02%, S ≤ 0.01%, Al 0.005% to 0.02%, Ni 1.5% to 4.5%, Cr 0.06% to 0.35%, Mo 0.1% to 0.5%, Co 0.5% to 3.5%, Nb 0.05% to 0.06%, Re 0.01% to 0.06%, with the remainder being Fe and impurities. The present invention utilizes a unique composition design combined with a corresponding production process to ensure that the mechanical properties and service safety 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 a fatigue-resistant steel plate in an extremely cold deep-sea environment and a manufacturing method thereof. Background Art

[0002] The 21st century is the century of the ocean. With the advancement of science and technology and the improvement of people's living standards, countries around the world are focusing their attention on the vast resources contained in the ocean. The sustained and rapid development of the offshore equipment industry in recent years has driven an increase in demand for offshore steel and promoted product upgrades. The market urgently needs high-strength, extra-thick, ultra-low-temperature tough offshore steel plates with excellent comprehensive properties.

[0003] As conventional oil and gas resources gradually deplete, the Arctic's abundant energy reserves are attracting increasing attention. A survey conducted by the U.S. Geological Survey indicates that the Arctic Circle holds 90 billion barrels of oil, 47 trillion cubic meters of natural gas, and 44 billion barrels of liquefied natural gas, respectively, representing approximately 22% of the world's total undiscovered energy resources. Furthermore, the global greenhouse effect is exacerbating Arctic warming, and the sea ice cover continues to decline rapidly, making it increasingly favorable for resource development and navigation. In recent years, Russia and Nordic countries have significantly accelerated Arctic oil and gas exploration and development. The growing interest in the Arctic's potential for energy and trade routes is driving the development of large, high-tech polar offshore equipment. This, in turn, is placing higher demands on ultra-low-temperature steels suitable for polar service. High-strength polar offshore steels with excellent low-temperature toughness and weldability are becoming a trend.

[0004] Offshore platforms operate in harsh environments. Besides conventional stresses, they must also consider a variety of factors, including strong winds, surges, tides, ice impacts, and earthquakes. This dictates the unique characteristics of the steel used in offshore platforms, and the materials used in their construction must be adaptable to a variety of sea conditions. Furthermore, steel plates are exposed to humid, high-salinity marine environments for long periods of time, exposed to the effects of moist air, seawater, and the attachment of marine organisms. This can easily lead to paint peeling, surface corrosion, and corrosion fatigue, which reduces the mechanical properties of the steel plates and shortens their service life, severely impacting the normal operation of offshore platforms. Furthermore, offshore platforms are located far from the coast and cannot undergo regular docking for repairs and maintenance like ships. To ensure the safe operation of offshore platforms in extremely cold and complex environments, such as the polar regions, there is an urgent need to develop high-quality, ultra-high-strength steel for offshore engineering, exhibiting excellent comprehensive performance. This steel must possess high strength, ultra-low temperature toughness, fatigue resistance, ease of welding, and resistance to corrosion in marine environments and the attachment of marine organisms.

[0005] Existing marine engineering steels can already meet most market demands, but specialized steels with lower ductile-brittle transition temperatures and superior overall performance remain a research and development target for countries worldwide. High-strength steel plates with high service safety are challenging to develop, require rigorous production processes, and place high demands on equipment. For example, Chinese patent application publication number CN116043120A discloses "A 1000MPa-grade cold-rolled complex-phase steel with excellent formability and its preparation method." While this 1000MPa-grade cold-rolled steel plate utilizes a high-Mn, low-alloy composition, its use of a cold-rolling and continuous annealing process ensures strength and formability down to 4mm. However, without the combined effects of elements like Ni, Cr, and Co, it is difficult to guarantee strength and fatigue resistance when producing thicker plates. A Chinese patent application with application publication number CN116179970A discloses a "steel plate for extremely cold marine environments with a yield strength of 900 MPa and a manufacturing method". The ultra-high-strength low-temperature steel with a yield strength of 900 MPa is designed with an alloy composition of high Ni and high Cu. It does not utilize the effects of elements such as Co and Re on the fatigue resistance of the steel plate, and the strength of the steel plate cannot reach a fatigue limit of more than 400 MPa. Summary of the Invention

[0006] The present invention provides a fatigue-resistant steel plate for an extremely cold deep-sea environment and a manufacturing method thereof. A unique composition design is adopted in conjunction with a corresponding production process to ensure that the mechanical properties and service safety of the steel plate meet the service conditions of marine engineering equipment. The low-temperature impact energy of -60°C is ≥120J, and the fatigue resistance is excellent. According to the ISO11782-1:2017 "Corrosion fatigue testing of metals and alloys - Part 1: Cyclic failure tests" standard, the steel plate corrosion fatigue test is carried out. When the stress ratio is -1 and the loading frequency is 1Hz, the corrosion fatigue strength of the steel plate in a seawater environment is ≥411MPa.

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

[0008] Disclosed is a fatigue-resistant steel plate for use in an extremely cold deep-sea environment. The chemical composition of the steel plate comprises, by weight percentage, the following: C 0.03%-0.09%, Si 0.05%-0.15%, Mn 0.3%-0.45%, P ≤ 0.02%, S ≤ 0.01%, Als 0.005%-0.02%, Ni 1.5%-4.5%, Cr 0.06%-0.35%, Mo 0.1%-0.5%, Co 0.5%-3.5%, Nb 0.05%-0.06%, Re 0.01%-0.06%, and the remainder being Fe and unavoidable impurities.

[0009] The yield strength of the finished steel plate is ≥785MPa, the tensile strength is 865~980MPa, and the elongation is ≥16%; the average Charpy impact energy at -60℃ is ≥120J; the steel plate corrosion fatigue test is carried out in accordance with ISO11782-1:2017 "Corrosion fatigue tests of metals and alloys Part 1: Cyclic failure tests". When the stress ratio is -1 and the loading frequency is 1Hz, the corrosion fatigue strength of the steel plate in seawater environment is ≥411MPa.

[0010] The maximum thickness of the finished steel plate is 80mm.

[0011] The metallographic structure of the finished steel plate at 1 / 2 of the thickness is tempered bainite + tempered martensite, in which the volume content of tempered martensite is 30% to 45%, and the grain size of the original austenite in the tempered structure is 5 to 12 μm.

[0012] A method for manufacturing fatigue-resistant steel plates in an extremely cold deep-sea environment comprises the following steps:

[0013] 1) Smelting and continuous casting: After smelting in a converter, the molten steel undergoes LF refining, RH refining or VD refining to further reduce the content of P, S and non-metallic inclusions; the molten steel is continuously cast to obtain ingots;

[0014] 2) Heating:

[0015] The ingot is loaded into the heating furnace when the furnace temperature is 400-700℃ and kept warm for 2-3.5h. In the subsequent heating process, the heating rate is controlled at 5-6℃ / min, the soaking temperature is 1280-1350℃, and kept warm for 2-3h.

[0016] 3) Rolling: The starting rolling temperature in the rough rolling stage is 1150-1180°C, the single-pass reduction in the rough rolling stage is 15-40 mm, the cumulative reduction rate in the rough rolling stage is 40%-55%, and the deformation rate is <1.5s -1 The starting rolling temperature of the finishing rolling stage is 850-1000℃, the average reduction rate of the finishing rolling stage is 10%-15%, and the single-pass reduction of at least the last three passes is ≤10mm. The final rolling temperature of the finishing rolling stage is 650-720℃.

[0017] 4) Quenching and tempering heat treatment: quenching temperature is 820-950℃, quenching holding time is 0.8-1.6min / mm; quenching cooling rate is 2-5℃ / s; tempering temperature is 350-750℃, tempering holding time is 4-5min / mm.

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

[0019] 1) The unique alloy composition system ensures that the yield strength of the steel plate after quenching and tempering treatment is ≥785MPa, the tensile strength is 865~980MPa, and the Charpy impact energy at -60℃ is ≥120J.

[0020] 2) The steel plate has excellent fatigue resistance. The corrosion fatigue test of the steel plate was carried out in accordance with the ISO11782-1:2017 "Corrosion fatigue testing of metals and alloys - Part 1: Cyclic failure tests". When the stress ratio is -1 and the loading frequency is 1Hz, the corrosion fatigue strength of the steel plate in seawater environment is ≥411MPa.

[0021] 3) By combining the elements of C, Mn, Ni, Cr, Mo, Co, and Re, and coordinating with the production process of large-thickness marine steel plates, ultra-high-strength 785MPa marine steel plates with a maximum thickness of 80mm are produced. The production process of "two-stage controlled rolling + quenching heat treatment + tempering heat treatment" is adopted, which effectively improves the fatigue resistance of the steel plates. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 This is a metallographic photograph of the finished steel plate of the present invention at 1 / 2 of the thickness. DETAILED DESCRIPTION

[0023] The present invention aims to meet the performance requirements of ultra-high-strength marine steel in extremely cold deep-sea marine environments. It utilizes a composition design that combines C, Mn, Ni, Cr, Mo, Co, and Re elements, and combines it with a production process for fatigue-resistant steel plates suitable for extremely cold deep-sea environments. A large number of systematic experimental studies have been conducted on many aspects, including alloy element screening and proportioning, steel cleanliness control, efficient rolling process optimization, and parameter selection, to ultimately determine the alloy element proportioning and production process that can meet the purposes of the present invention.

[0024] The present invention provides an anti-fatigue steel plate for use in an extremely cold deep-sea environment, wherein the chemical composition of the steel plate comprises, by weight percentage, the following: C 0.03% to 0.09%, Si 0.05% to 0.15%, Mn 0.3% to 0.45%, P ≤ 0.02%, S ≤ 0.01%, Als 0.005% to 0.02%, Ni 1.5% to 4.5%, Cr 0.06% to 0.35%, Mo 0.1% to 0.5%, Co 0.5% to 3.5%, Nb 0.05% to 0.06%, Re 0.01% to 0.06%, and the remainder is Fe and unavoidable impurities.

[0025] The composition design reasons of the fatigue-resistant steel plate for extremely cold deep-sea environment described in the present invention are as follows:

[0026] 1) Carbon, a fundamental strengthening element in steel, improves the hardenability of steel during quenching. However, too low a carbon content reduces the carbon solid solution content and carbide content, failing to maintain the steel's strength and hardenability. Excessive carbon content produces a large amount of hardened structure, reducing the steel's elongation and low-temperature impact toughness. Therefore, the carbon content in steel must be precisely controlled. The present invention controls the carbon content to 0.03% to 0.09%.

[0027] 2) Si plays a solid solution strengthening role in steel, but it also creates a hard and brittle phase, reducing the toughness and ductility of the steel plate. This invention uses Re deoxidation to purify the steel, allowing for the use of lower concentrations of Si, Mn, and Al in combination with deoxidation. Therefore, to maximize the fatigue resistance and low-temperature toughness of the steel plate, the Si content needs to be reduced; this invention controls the Si content to 0.05% to 0.15%.

[0028] 3) Mn can be dissolved in large quantities with Ni and Co in the Fe matrix, improving the strength and low-temperature toughness of the steel plate. A Mn content below 0.3% contributes little to the steel plate's strength. Furthermore, Mn expands the austenite phase and improves austenite stability. To improve the fatigue resistance of the steel plate, the present invention aims to reduce the impact of Mn segregation on the performance degradation of the thick plate core. Therefore, the present invention controls the Mn content to 0.3% to 0.45%.

[0029] 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%.

[0030] 5) Al is the main deoxidizing element in steel. However, when the steel contains a large amount of nitrogen, excessive Al content will form large inclusions, affecting the low-temperature impact toughness of the steel plate. In this case, the Al content should be minimized. The present invention controls the AlS content to 0.005% to 0.02%.

[0031] 6) Ni improves the toughness and fatigue resistance of the steel plate. Ni also improves the hardenability of the steel plate. Large amounts of Ni can lower the ductile-brittle transition temperature, reduce intergranular corrosion, and improve corrosion fatigue resistance. The present invention controls the Ni content to 1.5% to 4.5%.

[0032] 7) Adding Cr to steel can improve the strength of the steel plate. Adding a certain amount of Ni and Cr to steel can improve the corrosion resistance and low-temperature toughness of the steel plate, so the Cr content can be appropriately increased. However, too high a Cr content will reduce the impact toughness of the steel plate. The present invention controls the Cr content to 0.06% to 0.35%.

[0033] 8) Mo can improve the hardenability of the steel plate. At the same time, the fine carbides formed by Mo in the steel can effectively improve the strength and heat treatment stability of the steel plate. Adding an appropriate amount of Mo to the heat-treated steel plate can improve the corrosion resistance and pitting corrosion resistance of the steel plate. The present invention controls the Mo content to 0.1% to 0.5%.

[0034] 9) Co is a key element added to the steel of this invention, improving the strength and wear resistance of the steel plate. Co, in conjunction with the Ni element in the steel, enhances the steel plate's fatigue resistance. Excessive Co addition alone results in an increase in the hard phase and a decrease in low-temperature toughness. However, the addition of a large amount of Ni to the steel of this invention reduces the content of hard phase elements such as Mn and Si. Combined with the heating and rolling processes of this invention, this toughness reduction issue can be effectively avoided, maximizing the effectiveness of Co. The present invention controls the Co content to 0.5% to 3.5%.

[0035] 10) Nb is a strong carbonitride-forming element. It improves the strength and hardness of steel plates. Nb effectively delays the recrystallization of deformed austenite, preventing grain growth and improving both strength and toughness through grain refinement. During cooling, large amounts of Nb (CN) precipitate, further promoting dislocation entanglement and grain refinement. Nb also stabilizes CN compounds, improving the corrosion fatigue resistance of the steel plate. Therefore, the present invention limits the Nb content to 0.05% to 0.06%.

[0036] 11) Re added to steel acts as a nucleation point, significantly refining the grain size and improving the steel's strength, tensile toughness, and fatigue resistance. Adding large amounts of Re reacts with elements like O, N, and S in the molten steel, purifying the steel and reducing inclusion content. Re also improves the fatigue resistance of steel by altering the composition of phase transformation products and the grain size structure. The addition of Re also inhibits hydrogen-induced cracking, improving the steel's low-temperature toughness, fatigue fracture resistance, and service life. The present invention controls the Re content to 0.01% to 0.06%.

[0037] The present invention discloses a method for manufacturing fatigue-resistant steel plates for use in extremely cold deep-sea environments, which adopts a production process of "high cleanliness and alloying smelting + two-stage controlled rolling + quenching heat treatment + tempering heat treatment". The specific steps are as follows:

[0038] (1) High cleanliness and alloy smelting;

[0039] After smelting in a converter, the molten steel is refined in an LF furnace, RH furnace or VD furnace to further reduce the content of P, S and non-metallic inclusions to obtain molten steel that meets the composition requirements.

[0040] (2) The ingot obtained by continuous casting is placed in a heating furnace at a furnace temperature of 400-700℃ and kept warm for 2-3.5 hours. The purpose is to keep the temperature of the ingot uniform in the thickness direction in the low temperature stage, so as to prepare for the uniform structure in the high temperature stage. The heating rate of the ingot in the subsequent heating process is controlled at 5-6℃ / min. The purpose of using rapid heating is to avoid brittle defects inside the ingot. The uniform heating temperature is 1280-1350℃ and kept warm for 2-3 hours. The purpose of using the high temperature stage uniform heating is to ensure that the C / N compounds are fully dissolved and the steel plate is uniformly heated in the thickness direction, so that the hardened steel plate formed by elements such as Co is fully softened.

[0041] (3) Two-stage controlled rolling;

[0042] The starting rolling temperature in the rough rolling stage is 1150-1180℃, the single-pass reduction in the rough rolling stage is 15-40mm, the cumulative reduction rate in the rough rolling stage is 40%-55%, and the deformation rate is <1.5s -1 The roughing stage uses high-temperature, high-reduction rolling to quickly reduce the steel plate to the target thickness while the deformation resistance is low. The reason for keeping the deformation rate low is to prevent the large amount of Co in the steel from forming a hard phase in the steel plate, which in turn can cause cracks in the steel plate during rolling.

[0043] The starting rolling temperature during the finishing rolling stage is 850-1000°C, with an average reduction of 10%-15%. The reduction per pass in at least the last three passes is ≤10mm. The final rolling temperature during the finishing rolling stage is 650-720°C. The purpose of controlling the process parameters during the finishing rolling stage is to increase the deformation of the steel plate core by utilizing the significant drop in surface temperature, improve the core grain size, promote the flattening and refinement of austenite grains, and prepare for deformation energy storage during the quenching and tempering process. The reduction per pass in the final few passes (preferably three) near the final rolling temperature is controlled to prevent bending deformation caused by increased hardness and to ensure plate flatness.

[0044] (4) Quenching and tempering heat treatment;

[0045] The quenching temperature is 820-950°C, the quenching holding time is 0.8-1.6 min / mm, and the quenching cooling rate is 2-5°C / s. The tempering temperature is 350-750°C, and the tempering holding time is 4-5 min / mm. Through quenching and tempering, a uniform and fine-scale "tempered bainite + tempered martensite" structure is obtained, with the volume content of tempered martensite being 30%-45% and the original austenite grain size of the tempered bainite being 5-12 μm. This significantly improves the low-temperature impact toughness and fatigue resistance of the steel plate, while also adjusting the strength and hardness of the steel plate and preventing the precipitation of hard and brittle phases.

[0046] The finished steel plate has a yield strength of ≥785 MPa, a tensile strength of 865-980 MPa, an elongation of ≥16%, and an average Charpy impact energy of ≥120 J at -60°C. The finished steel plate also exhibits excellent fatigue resistance. According to ISO 11782-1:2017, "Corrosion fatigue testing of metals and alloys - Part 1: Cyclic failure tests," the corrosion fatigue strength of the steel plate in a seawater environment is ≥411 MPa at a stress ratio of -1 and a loading frequency of 1 Hz. The maximum thickness of the finished steel plate is 80 mm.

[0047] like Figure 1 As shown in the figure, the structure of the finished steel plate at 1 / 2 of the thickness is "tempered bainite + tempered martensite", the tempered martensite content is 30% to 45%, the original austenite grain size of the tempered structure is 5 to 12 μm, and the mechanical properties are good.

[0048] 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.

[0049] [Example]

[0050] The chemical composition of each example steel 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.

[0051] Table 1 Chemical composition of steel (weight percentage, %)

[0052]

[0053] Table 2-1 Production process parameters (I)

[0054]

[0055] Table 2-2 Production process parameters (II)

[0056]

[0057] Table 3 Mechanical properties of finished steel plates

[0058]

[0059] 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 method for manufacturing fatigue-resistant steel plates in extremely cold deep-sea environments, characterized in that: The chemical composition of the steel plate includes, by weight percentage: C 0.03% to 0.09%, Si 0.05% to 0.15%, Mn 0.3% to 0.45%, P≤0.02%, S≤0.01%, Als 0.005% to 0.02%, Ni 1.5% to 4.5%, Cr 0.06% to 0.35%, Mo 0.1% to 0.5%, Co 0.5% to 3.5%, Nb 0.05% to 0.06%, Re 0.01% to 0.06%, and the rest is Fe and unavoidable impurities; The method for manufacturing fatigue-resistant steel plates in an extremely cold deep-sea environment comprises the following steps: 1) Smelting and continuous casting: After smelting in a converter, the molten steel undergoes LF refining, RH refining or VD refining to further reduce the content of P, S and non-metallic inclusions; the molten steel is continuously cast to obtain ingots; 2) Heating: The ingot is loaded into the heating furnace when the furnace temperature is 400-700℃ and kept warm for 2-3.5h. In the subsequent heating process, the heating rate is controlled at 5-6℃ / min, the soaking temperature is 1280-1350℃, and kept warm for 2-3h. 3) Rolling: The starting rolling temperature in the rough rolling stage is 1150-1180°C, the single-pass reduction in the rough rolling stage is 15-40 mm, the cumulative reduction rate in the rough rolling stage is 40%-55%, and the deformation rate is <1.5s -1 The starting rolling temperature of the finishing rolling stage is 850-1000℃, the average reduction rate of the finishing rolling stage is 10%-15%, and the single-pass reduction of at least the last three passes is ≤10mm. The final rolling temperature of the finishing rolling stage is 650-720℃. 4) Quenching and tempering heat treatment: quenching temperature is 820-950℃, quenching holding time is 0.8-1.6min / mm; quenching cooling rate is 2-5℃ / s; tempering temperature is 350-750℃, tempering holding time is 4-5min / mm.

2. The method for manufacturing fatigue-resistant steel plates in extremely cold deep-sea environments according to claim 1, characterized in that: The yield strength of the finished steel plate is ≥785MPa, the tensile strength is 865~980MPa, and the elongation is ≥16%; the average Charpy impact energy at -60℃ is ≥120J; the steel plate corrosion fatigue test is carried out in accordance with ISO11782-1:2017 "Corrosion fatigue tests of metals and alloys Part 1: Cyclic failure tests". When the stress ratio is -1 and the loading frequency is 1Hz, the corrosion fatigue strength of the steel plate in seawater environment is ≥411MPa.

3. The method for manufacturing fatigue-resistant steel plates in extremely cold deep-sea environments according to claim 1, characterized in that: The maximum thickness of the finished steel plate is 80mm.

4. The method for manufacturing fatigue-resistant steel plates in extremely cold deep-sea environments according to claim 1, characterized in that: The metallographic structure of the finished steel plate at 1 / 2 of the thickness is tempered bainite + tempered martensite, in which the volume content of tempered martensite is 30% to 45%, and the grain size of the original austenite in the tempered structure is 5 to 12 μm.

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