A gigapascal-grade offshore steel plate for extremely cold and ultra-deep environments and a manufacturing method thereof

Abstract

The present invention belongs to the technical field of steel material preparation, and specifically relates to a gigapascal-level marine engineering steel plate for extremely cold and ultra-deep environments and a manufacturing method thereof. The steel plate comprises the following components by weight percentage: C: 0.08%-0.15%, Si: 0.15%-0.45%, Mn: 0.6%-1.15%, P≤0.02%, S≤0.01%, Als: 0.005%-0.05%, Ni: 7.2%-14%, Cr: 0.46%-0.64%, Mo: 0.55%-0.79%, Cu: 0.42%-0.76%, Co: 0.1%-10.0%, V: 0.11%-0.23%, Ti: 0.01%-0.5%, N: 0.003%-0.007%, and the rest is Fe and unavoidable impurities. The present invention achieves excellent low-temperature toughness, 1000MPa ultra-high strength, and excellent fatigue resistance by optimizing and selecting alloy element screening and proportioning, smelting, electroslag remelting, forging and blanking, two-stage controlled rolling, and multiple quenching and tempering heat treatment processes. The steel plate has mechanical properties and high service safety performance that can meet the service conditions of marine engineering equipment and is suitable for extremely cold and ultra-deep environments.

Description

Technical Field

[0001] The present invention relates to the technical field of steel material preparation, and in particular to a gigapascal-level marine engineering steel plate for extremely cold and ultra-deep environments and a manufacturing method thereof. Background Art

[0002] With the development of science and technology and the improvement of people's living standards, countries around the world have begun to focus on the vast resources in the ocean. Due to the rapid development of the offshore equipment industry, the market demand for high-strength, extra-thick, ultra-low temperature tough offshore steel plates has continued to increase. Traditional oil and gas energy is becoming increasingly depleted, so the reserves of oil, natural gas and liquefied natural gas in the Arctic region are becoming more and more valuable. Under the influence of the global greenhouse effect, the Arctic is warming up and the sea ice coverage area is decreasing, which has promoted resource development and ship navigation. Russia and Nordic countries have significantly accelerated the exploration and development of oil and gas in the Arctic, and promoted the demand and development of large-scale high-tech polar offshore equipment. Therefore, high-strength polar low-temperature offshore steel has gradually become a development trend. The service environment of offshore engineering platforms is harsh, so it is necessary to consider the influence of multiple factors, and the material selection must be able to adapt to various sea conditions. At the same time, the offshore platform steel plates are in a humid and high-salinity marine environment for a long time, and are affected by humid air, seawater, and marine organisms. The adhesion causes problems such as paint film shedding, steel plate surface corrosion, and corrosion fatigue, which seriously affects the service life. In order to enable offshore engineering platforms to be used safely in complex environments such as the polar regions, it is urgent to develop high-quality ultra-high-strength steel for offshore engineering. This steel plate must have the advantages of high strength, ultra-low temperature toughness, fatigue resistance, easy welding, resistance to marine environmental corrosion, and resistance to marine biological attachment.

[0003] At present, marine engineering steel can meet most of the needs of the marine engineering market, but ultra-high strength special steel with excellent comprehensive performance and lower ductile-brittle transition temperature 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 development difficulties. Patent CN115786813A discloses a low-temperature steel plate with a tensile strength of 3000MPa. A large amount of Ni, Co, and Mo combined with trace Zr and Ce elements are added to the steel plate composition. Although it helps to improve the purity of the steel, the cost is greatly increased. In addition, the composition needs to be hot-rolled, cold-rolled, and aging-treated. The thickness of the steel plate in the steel plant is only 0.5-2mm, and it is impossible to produce a maximum thickness of 100mm for the gigapascal low-temperature marine steel suitable for extremely cold deep-sea environments. Patent CN114622145A discloses a dual-phase maraging steel, which is designed to be free of C, Si, Mn and other alloying elements and can only be produced by electric arc furnace smelting and cold rolling. This composition and process cannot produce steel plates with a maximum thickness of 100 mm, and has poor low-temperature toughness. Summary of the invention

[0004] The purpose of the present invention is to provide a gigapascal marine engineering steel plate for extremely cold and ultra-deep environments and a method for manufacturing the same. Through the optimization and parameter selection of alloy element screening and proportioning, smelting, electroslag remelting, forging and blanking, two-stage controlled rolling and multiple tempering heat treatment processes, the obtained steel plate has excellent low-temperature toughness and 1000MPa ultra-high strength. Its mechanical properties and high service safety performance can meet the service conditions of marine engineering equipment and is suitable for extremely cold and ultra-deep environments.

[0005] In order to achieve the above object, the technical solution of the present invention is as follows:

[0006] In one aspect, the present invention provides a GPa-class marine steel plate for use in extremely cold and ultra-deep environments, wherein the steel plate comprises the following components by weight percentage:

[0007] C: 0.08%-0.15%, Si: 0.15%-0.45%, Mn: 0.6%-1.15%, P≤0.02%, S≤0.01%, Als: 0.005%-0.05%, Ni: 7.2%-14%, Cr: 0.46%-0.64%, Mo: 0.55%-0.79%, Cu: 0.42%-0.76%, Co: 0.1%-10.0%, V: 0.11%-0.23%, Ti: 0.01%-0.5%, N: 0.003%-0.007%, and the rest are Fe and unavoidable impurities.

[0008] The reasons for adopting the above components are as follows:

[0009] C is the most effective strengthening element in steel, increasing the strength and hardness of steel plates. C can be dissolved in the Fe matrix, or it can form carbides with alloy elements, playing a significant role in grain refinement. When the C content in steel is less than 0.08%, the strength of the steel plate is insufficient; when the C content is higher than 0.15%, a large amount of hardened structure will be produced, and the toughness and fatigue resistance will be reduced. Therefore, the content of C in steel should be precisely controlled without affecting its impact toughness and fatigue resistance. The C content is 0.08%-0.15%.

[0010] Si can improve the strength of the steel plate, and as a deoxidizer, it can reduce the O content. The Si content added in the present invention can help the fatigue limit and fatigue life of the steel plate. When the Si content is lower than 0.15%, the deoxidation effect is not obvious, and the inclusion content in the steel is high; and when the Si content is greater than 0.45%, it will cause the structure to coarsen and the toughness to decrease. Therefore, in order to ensure the strength and fatigue resistance of the steel plate, the Si content should be controlled at 0.15%-0.45% in the scheme of the present invention.

[0011] Mn element can be dissolved in Fe matrix in large quantities to improve the strength of steel plate. When the Mn content is lower than 0.6%, it cannot effectively improve the strength of steel plate. When the Mn content is higher than 1.15%, the Mn element will concentrate towards the core of the billet during steelmaking solidification, reducing the low-temperature toughness of the core of the thick plate. The Mn content is 0.6%-1.15%.

[0012] P and S elements have no benefit to the mechanical properties of steel plates, especially fatigue resistance. P should be controlled to be ≤ 0.02% and S ≤ 0.01%.

[0013] Al is the main deoxidizing element in steel. When the Al content is too low, the deoxidation effect is poor. Micro-alloy elements such as V and Ti cannot effectively refine the grains and improve fatigue resistance due to oxidation. Ultra-high strength steel plates need to appropriately increase the Als content in the steel; however, if the Al content is too high, large inclusions will be formed. The Als content is 0.005%-0.05%.

[0014] The role of Ni is to improve the toughness of the steel plate and improve the fatigue resistance of the steel plate. Adding a large amount of Ni can obtain a lower ductile-brittle transition temperature, improve the low-temperature toughness of the steel plate and improve the fatigue resistance. The Ni content is 7.2%-14.0%.

[0015] The Cr element in steel can effectively improve the strength of the steel plate; however, too high a Cr content will reduce the impact toughness of the steel plate. Cr combined with Ni in steel can effectively improve the fatigue resistance of the steel plate. The Cr content is 0.46%-0.64%.

[0016] Mo can form fine carbides in steel, improve the strength of steel plates, and thus improve the fatigue performance of steel plates. Adding a certain amount of Mo to steel can improve the hardenability of extra-thick steel plates and improve the heat treatment effect of extra-thick steel plates. Mo can also play a certain role in corrosion resistance in combination with Ni, and the Mo content is 0.55%-0.79%.

[0017] The main function of Co in steel is to improve the strength and wear resistance of steel. Co can also improve the strength and toughness of steel, and improve the processing performance and welding performance of steel. Co reduces the stacking fault energy of the matrix and inhibits the recovery of dislocation substructure in martensite, promotes the nucleation and precipitation of carbides, thereby improving the strength and fatigue resistance of steel. Co can form carbides and hard alloys that are stable at high temperatures with elements such as C and Cr, thereby improving the oxidation resistance, corrosion resistance and wear resistance of steel. However, the addition amount of Co cannot be too high, otherwise it will increase the cold brittleness of the alloy steel and reduce the toughness. Therefore, the Co content is 0.1%-10.0%.

[0018] Cu element can improve the strength of steel plate in steel, and Cu element can work together with Ni element in steel to reduce the tough-brittle transition temperature of steel plate. Adding excessive Cu element alone will cause hot brittleness. The Cu element content is 0.42%-0.76%.

[0019] The V element can form V (C, N) particles during the smelting process, which plays a role in refining the grains. Adding V elements to steel plates can significantly improve the strength and low-temperature toughness of steel plates. At the same time, the V element can effectively improve the weldability of steel plates. The V content is 0.11%-0.23%.

[0020] Ti and N form TiN, which prevents the growth of grains during heating and rolling of the steel billet and improves the fatigue resistance of the steel plate. A large amount of Ti can form a stable intermetallic phase with Ni, but too high a Ti content will increase the crack sensitivity of the steel and increase the brittle fracture tendency of the steel. Therefore, in the present invention, the Ti content is 0.01%-0.5%.

[0021] The N element can cooperate with the Ti and V elements to form fine dispersed N compounds, which can effectively promote the nucleation of ferrite in the grain and effectively control the growth of the original austenite grains. The increase of N content can increase the TiN in the steel. However, when the solid solution N content is too high, a large number of micro cracks are prone to appear on the surface of the steel plate. Therefore, the N content is 0.003%-0.007%.

[0022] In the above technical solution, further, the steel plate has a yield strength ≥1000MPa, a tensile strength of 1050-1200MPa, a lateral elongation ≥11%, a Charpy impact energy at -80°C ≥90J, and a Charpy impact energy at -196°C ≥60J.

[0023] Another aspect of the present invention provides a method for manufacturing the above-mentioned GPa-grade offshore steel plate for extremely cold and ultra-deep environments, the method comprising the following steps:

[0024] (1) Smelting

[0025] The molten steel is refined through a converter, LF furnace, RH or VD furnace, and the molten steel is cast into steel billets by die casting or continuous casting;

[0026] (2) Electroslag remelting

[0027] The steel billet is used as a melting electrode for electroslag remelting to prepare electroslag billets, which are then homogenized at 1050±50℃ and stacked for slow cooling for ≥72h.

[0028] (3) Forging

[0029] The electroslag billet after slow cooling is forged, the forging heating temperature is 1220-1280°C, and the final forging temperature is ≥950°C to obtain a forging billet;

[0030] (4) Two-stage controlled rolling

[0031] The forging billet is loaded into a heating furnace for heating, and then subjected to two-stage controlled rolling. The first-stage rolling temperature is 1000-1230°C, the final rolling temperature is 900-1050°C, the intermediate billet thickness is 100-200mm, the second-stage rolling temperature is 750-900°C, and the final rolling temperature is 700-800°C.

[0032] (5) Quenching and tempering heat treatment

[0033] The primary quenching temperature is 700-900℃, the heating time is 1.4-2.5min / mm, the secondary quenching temperature is 700-900℃, the heating time is 1.4-2.5min / mm, the tempering temperature is 300-600℃, and the tempering time is 2-4min / mm.

[0034] In the above technical solution, further, in step (3), the ratio of the thickness of the forged billet to the thickness of the finished steel plate is ≥3.5.

[0035] In the above technical solution, further, in step (3), the single pass reduction of forging is ≤30 mm.

[0036] In the above technical solution, further, in step (4), the total heating time is 5.5-9 hours, the aquaporin temperature is 1220-1280°C, and the aquaporin insulation time is 1-3 hours.

[0037] In the above technical solution, further, in step (4), the average pass reduction rate of the first stage rolling is 13-20%, and the average pass reduction rate of the second stage rolling is 5-10%.

[0038] The beneficial effects of the present invention are:

[0039] 1. The present invention combines the coordinated composition design of elements such as C, Mn, Ni, Cr, Mo, Cu, Co, V, Ti and the key production technology of gigapascal-grade marine steel plates for extremely cold and ultra-deep environments, and can produce ultra-high strength 1000MPa marine steel with a maximum thickness of 100mm.

[0040] 2. The present invention achieves a steel plate with a yield strength of ≥1000MPa, a tensile strength of 1050-1200MPa, a transverse elongation of ≥11%, a Charpy impact energy of ≥90J at -80℃, a Charpy impact energy of ≥60J at -196℃, a fatigue stress to yield strength ratio of ≤0.9, and a fatigue cycle of ≥10 7 Second-rate. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 This is the structure of the steel plate of Example 1 at room temperature. DETAILED DESCRIPTION

[0042] The following examples may enable those skilled in the art to more fully understand the present invention, but are not intended to limit the present invention in any way.

[0043] Unless otherwise specified, the materials used in the embodiments of the present invention can be obtained through commercial channels or prepared according to conventional methods well known to those skilled in the art.

[0044] Embodiment 1-40

[0045] The chemical compositions of the steel plates of Examples 1-40 of the present invention are shown in Table 1.

[0046] Table 1 Chemical composition of steel plates of Examples 1-40 (wt%)

[0047]

[0048]

[0049] The manufacturing method of the above-mentioned Gigapascal marine steel plate for extremely cold and ultra-deep environment specifically comprises the following steps:

[0050] (1) Smelting

[0051] The molten steel is refined through a converter, LF furnace, RH or VD furnace, and the content of P, S and non-metallic inclusions is further reduced by controlling the cleanliness of the steel. The molten steel is cast into billets by die casting or continuous casting, and the casting is protected throughout the whole process;

[0052] (2) Electroslag remelting

[0053] The steel billet is used as a melting electrode for electroslag remelting to prepare an electroslag billet, which is then subjected to homogenization annealing at 1050±50℃ and stacking slow cooling for ≥72h. The reason for using electroslag billets to roll steel plates is that the electroslag billets have a uniform and dense structure and a small core segregation, which can ensure the low-temperature impact toughness of the extra-thick steel plate after rolling. The homogenization annealing and stacking slow cooling of the electroslag billets after electroslag can further avoid excessive internal stress and uneven structure caused by the cooling process of the electroslag billets.

[0054] (3) Forging

[0055] The electroslag billet after stacking and slow cooling is forged, the forging heating temperature is 1220-1280℃, the single pass forging reduction is ≤30mm, the final forging temperature is ≥950℃, and the forging billet is obtained, and the ratio of the thickness of the forging billet to the thickness of the finished steel plate is ≥3.5; the purpose of forging billet is to use the multi-directional forging reduction deformation to break the large-scale microstructure of the core of the steel ingot to the maximum extent, prepare the organization for the rolled steel plate, and further improve the low-temperature toughness of the core of the finished steel plate. The purpose of controlling the single pass forging reduction is to avoid cracks on the surface of the steel ingot;

[0056] (4) Two-stage controlled rolling

[0057] The forging billet is loaded into a heating furnace for heating. The total heating time is 5.5-9 hours, the soaking temperature is 1220-1280℃, and the soaking and holding time is 1-3 hours. The purpose of ensuring the total heating time is to make the core of the thick electroslag billet reach the target temperature and avoid excessive soaking temperature. The purpose of controlling the soaking and holding time is to prevent the abnormal growth of the austenite structure of the billet and affect the low-temperature toughness of the rolled steel plate; after heating, two-stage controlled rolling is carried out. The first stage has a starting rolling temperature of 1000-1230℃, an average pass reduction rate of 13-20%, a final rolling temperature of 900-1050℃, and a second stage has a rolling temperature of 1000-1230℃, an average pass reduction rate of 13-20%, and a final rolling temperature of 900-1050℃. The thickness of the intermediate billet is 100-200mm, the second-stage rolling temperature is 750-900℃, the average pass reduction rate is 5-10%, and the final rolling temperature is 700-800℃; the purpose of the first-stage rolling is to maximize the pass reduction rate under the condition of relatively low hardness of the high-temperature steel plate, further crush the cast grains of the billet, increase the deformation energy storage and grain nucleation points, the 100-200mm intermediate billet thickness can ensure sufficient deformation in the low-temperature rolling, and the relatively low second-stage rolling temperature ensures that the steel plate has a large amount of deformation energy storage during the quenching and tempering process, ensuring the performance of the steel plate;

[0058] (5) Quenching and tempering heat treatment

[0059] The primary quenching temperature is 700-900℃, the heating time is 1.4-2.5min / mm, the secondary quenching temperature is 700-900℃, the heating time is 1.4-2.5min / mm, the tempering temperature is 300-600℃, and the tempering time is 2-4min / mm; the quenching and tempering process is a very critical link in the steel plate manufacturing process, which can effectively affect the performance of the steel plate, especially the low-temperature toughness and fatigue resistance of the steel plate. In the two quenching processes, the main purpose of high-temperature quenching is to make the steel plate fully austenitized and ensure that the austenite of the steel plate is neither overheated nor underheated. Under the premise of ensuring no cracking and deformation, the cooling capacity is improved, the organizational transformation is accelerated, and the organization is transformed into martensite in large quantities to increase the hardened organization. Since high-temperature quenching can obtain a more uniform austenite organization, the strength and hardness of the steel plate are improved, so it has a good fatigue resistance and tensile strength. During the tempering stage, high-temperature tempering can adjust the quality of the precipitated second phase while adjusting the matrix structure, thereby optimizing the organizational structure of the steel plate and improving the toughness of the steel plate; at the same time, in the sub-temperature quenching process, the low-temperature toughness of the steel plate can be improved by forming residual austenite; finally, in the process of full tempering, due to the sufficient temperature and time of tempering, the tempering transformation products can be transformed as much as possible, reducing the strength and hardness of the steel plate while improving the toughness and plasticity.

[0060] The process parameters of electroslag remelting and forging are shown in Table 2, the process parameters of two-stage controlled rolling are shown in Table 3, and the process parameters of quenching and tempering heat treatment are shown in Table 4.

[0061] Table 2 Process parameters of electroslag remelting and forging

[0062]

[0063]

[0064] Table 3 Process parameters of two-stage controlled rolling

[0065]

[0066]

[0067] Table 4 Process parameters of quenching and tempering heat treatment

[0068]

[0069]

[0070]

[0071] The tensile and impact properties of the steel plates of Examples 1-40 are shown in Table 5, the fatigue properties are shown in Table 6, and the metallographic structure content and dimensions are shown in Table 7.

[0072] Table 5 Mechanical properties of steel plates of Examples 1-40

[0073]

[0074]

[0075] Table 6 Fatigue properties of steel plates of Examples 1-40

[0076]

[0077]

[0078]

[0079] Table 7 Metallographic structure content and size of steel plate of Example 1-40

[0080]

[0081]

[0082] The structure of the steel plate obtained in Example 1 at room temperature is as follows Figure 1 As shown, it can be found that the matrix structure of the experimental steel at room temperature is tempered troostite and ferrite. It can be seen from Table 7 that the tempered troostite structure and ferrite structure of the steel plate prepared by the present invention are mostly between 3-6μm, small in size and evenly distributed. In addition, the structure contains a large amount of residual austenite, and its content is between 10.5%-16.8%. The morphology of the residual austenite is mainly film-like and block-like (Mao Island), and the film-like residual austenite with higher stability accounts for the vast majority of the residual austenite; the content of the blocky residual austenite with lower stability is low, and the grain size is mostly between 1-2μm.

[0083] The above embodiments are only preferred embodiments of the present invention and are not intended to limit the implementation methods. The protection scope of the present invention shall be subject to the scope defined in the claims. Other different forms of changes or modifications may be made based on the above description. Obvious changes or modifications derived therefrom are still within the protection scope of the present invention.

Claims

1. A gigapascal marine steel plate for extremely cold and ultra-deep environments, characterized in that: The steel plate comprises the following components by weight percentage: C: 0.08%-0.15%, Si: 0.15%-0.45%, Mn: 0.6%-1.15%, P≤0.02%, S≤0.01%, Als: 0.005%-0.05%, Ni: 7.2%-14%, Cr: 0.46%-0.64%, Mo: 0.55%-0.79%, Cu: 0.42%-0.76%, Co: 0.1%-10.0%, V: 0.11%-0.23%, Ti: 0.01%-0.5%, N: 0.003%-0.007%, the rest is Fe and unavoidable impurities; The method for manufacturing the steel plate comprises the following steps: (1) Smelting The molten steel is refined through a converter, LF furnace, RH or VD furnace, and the molten steel is cast into a steel billet by die casting or continuous casting; (2) Electroslag remelting The steel billet is used as a melting electrode for electroslag remelting to prepare electroslag billets, which are then homogenized at 1050±50℃ and stacked for slow cooling for ≥72h. (3) Forging The electroslag billet after slow cooling is forged, the forging heating temperature is 1220-1280°C, and the final forging temperature is ≥950°C to obtain a forging billet; (4) Two-stage controlled rolling The forging billet is loaded into a heating furnace for heating, and then subjected to two-stage controlled rolling. The first-stage rolling temperature is 1000-1230°C, the final rolling temperature is 900-1050°C, the intermediate billet thickness is 100-200mm, the second-stage rolling temperature is 750-900°C, and the final rolling temperature is 700-800°C. (5) Quenching and tempering heat treatment The primary quenching temperature is 700-900℃, the heating time is 1.4-2.5min / mm, the secondary quenching temperature is 700-900℃, the heating time is 1.4-2.5min / mm, the tempering temperature is 300-600℃, and the tempering time is 2-4min / mm.

2. The GPa-grade offshore steel plate for extremely cold and ultra-deep environments according to claim 1, characterized in that: The steel plate has a yield strength of ≥1000MPa, a tensile strength of 1050-1200MPa, a transverse elongation of ≥11%, a Charpy impact energy of ≥90J at -80°C, a Charpy impact energy of ≥60J at -196°C, and a fatigue stress to yield strength ratio of ≤0.

9. 7 Second-rate.

3. The GPa-grade offshore steel plate for extremely cold and ultra-deep environments according to claim 1, characterized in that: In step (3), the ratio of the thickness of the forged billet to the thickness of the finished steel plate is ≥3.

5.

4. The manufacturing method according to claim 1, characterized in that: In step (3), the reduction amount of a single forging pass is ≤30 mm.

5. The GPa-grade offshore steel plate for extremely cold and ultra-deep environments according to claim 1, characterized in that: In step (4), the total heating time is 5.5-9 hours, the absorptive temperature is 1220-1280° C., and the absorptive holding time is 1-3 hours.

6. The GPa-grade offshore steel plate for extremely cold and ultra-deep environments according to claim 1, characterized in that: In step (4), the average pass reduction rate of the first stage rolling is 13-20%, and the average pass reduction rate of the second stage rolling is 5-10%.

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