Gigapascal-grade marine-engineering steel plate for use in extremely cold and ultra-deep environment, and manufacturing method therefor

Abstract

A gigapascal-grade marine-engineering steel plate for use in an extremely cold and ultra-deep environment, and a manufacturing method therefor. The steel plate comprises the following components in percentage by weight: 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%, and N: 0.003%-0.007%, with the balance being Fe and inevitable impurities. By means of the optimization and parameter selection of the processes of alloying element screening and proportioning, smelting, electroslag remelting, cogging for forging, two-stage controlled rolling, and a multi-step quenching and tempering treatment, the prepared steel plate has good low-temperature toughness, an ultra-high strength of 1000 MPa, and good fatigue resistance. The mechanical properties and high service safety performance thereof can meet the service conditions for marine engineering equipment, making the steel plate applicable to an extremely cold and ultra-deep environment.

Description

Gigapascal-grade marine steel plate for extremely cold and ultra-deep environment and manufacturing method thereof TECHNICAL FIELD

[0001] The present application relates to the technical field of steel material preparation, in particular, especially relates to a gigapascal-grade marine steel plate for extremely cold and ultra-deep environment and a manufacturing method thereof. BACKGROUND

[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 large amount of resources in the ocean. Due to the rapid development of the marine equipment industry, the market demand for high-strength, super-thick, and ultra-low-temperature toughness marine steel plates is increasing. 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 global greenhouse effect, the warming of the Arctic is intensifying, and the coverage area of sea ice is continuously decreasing, which promotes resource development and ship navigation. Russia and Nordic countries have significantly accelerated the oil and gas exploration and development in the Arctic, and promoted the demand and development of large high-tech polar marine equipment. Therefore, high-strength grade polar low-temperature marine steel has gradually become a development trend. The service environment of offshore engineering platforms is harsh, so various factors must be considered in material selection to adapt to various sea conditions. At the same time, the marine platform steel plate is in a humid and high-salinity marine environment for a long time, and is affected by humid air, seawater, and marine organism attachment, which causes paint film peeling, steel plate surface corrosion, corrosion fatigue, and other problems, seriously affecting the service life. In order to enable offshore engineering platforms to be safely used in complex environments such as the polar region, it is urgent to develop high-quality ultra-high-strength steel for offshore engineering. Such steel plates must have high strength, ultra-low-temperature toughness, fatigue resistance, easy weldability, resistance to marine environment corrosion, and resistance to marine organism attachment.

[0003] At present, marine engineering steel can meet most of the market demand in the marine field, but ultra-high-strength special steel materials with excellent comprehensive performance and lower ductile-to-brittle transition temperature are still the goal of development in countries around the world. The high-strength steel plate with high service safety has high difficulty in scientific research, strict production process, high requirement for equipment, and great difficulty in development. Patent CN115786813A discloses a low-temperature steel plate with a tensile strength of 3000MPa. A large amount of Ni, Co, Mo, and trace amounts of Zr and Ce elements are added to the composition of the steel plate, which helps to improve the purity of the steel and significantly increases the cost. Moreover, the composition needs to be subjected to hot rolling, cold rolling, and aging treatment. The thickness of the steel plate produced by the steel plant is only 0.5-2mm, and it cannot produce a gigapascal-grade low-temperature marine steel plate with a maximum thickness of 100mm suitable for extremely cold and deep sea environments. Patent CN114622145A discloses a dual-phase martensitic age steel. The composition design is C-free and Si-free, and does not contain alloying elements such as Mn. The steel plate can only be produced by electric arc furnace smelting and cold rolling process. This composition and process cannot produce a steel plate with a maximum thickness of 100mm, and the low-temperature toughness is poor.

[0004] SUMMARY

[0005] The purpose of the present application is to provide a gigapascal-level offshore steel plate for extremely cold and super deep environment and a manufacturing method thereof, by optimizing and selecting parameters of alloy element screening and proportioning, smelting, electroslag remelting, forging breakdown, two-stage controlled rolling, and multiple quenching and tempering heat treatment processes, the prepared steel plate has excellent low-temperature toughness and 1000MPa ultrahigh strength, and the mechanical properties and high service safety performance can meet the service conditions of marine engineering equipment, and is suitable for extremely cold and super deep environment.

[0006] In order to achieve the above-mentioned purpose, the technical scheme of the present application is as follows:

[0007] The present application provides a gigapascal-level offshore steel plate for extremely cold and super deep environment, the steel plate comprises the following components by weight percentage:

[0008] 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 inevitable impurities.

[0009] The reasons for adopting the above component design are as follows:

[0010] C is the most effective strengthening element in steel, which increases the strength and hardness of the steel plate. C element can be solid-solved in Fe matrix, or form carbide with alloying elements to play a significant grain refinement role. When the C element in steel is less than 0.08%, the strength of the steel plate is insufficient; and when the C is higher than 0.15%, a large amount of hardened structure is generated, and the toughness and fatigue resistance are reduced. Therefore, the content of C element in steel should be accurately controlled without affecting the impact toughness and fatigue resistance. The content of C is 0.08%-0.15%.

[0011] Si can improve the strength of the steel plate, and as a deoxidizer, it can reduce the content of O. The addition of Si content in the present application can help the fatigue limit and fatigue life of the steel plate. When the content of Si is less than 0.15%, the deoxidizing effect is not obvious, and the inclusion content in the steel is high; and when the content of Si is greater than 0.45%, the structure is coarsened, and the toughness is reduced. Therefore, in order to ensure the strength and fatigue resistance of the steel plate, the content of Si should be controlled in the range of 0.15%-0.45% in the present application.

[0012] Mn element can be dissolved in Fe matrix in large quantities to improve the strength of the steel plate. When the Mn content is less than 0.6%, the steel plate cannot effectively improve the strength of the steel plate, and when the Mn content is greater than 1.15%, the Mn element will segregate to the core of the billet during the steelmaking solidification process, reducing the low temperature toughness of the core of the thick plate, and the Mn content is 0.6%-1.15%.

[0013] P and S elements have no benefit to the mechanical properties of the steel plate, especially the fatigue resistance, and should be controlled to P≤0.02% and S≤0.01%.

[0014] Al is the main deoxidizing element in steel, and when the Al content is too low, the deoxidizing effect is poor, and V, Ti and other micro-alloying elements cannot effectively refine the grains to improve the fatigue resistance, so the Als content in the steel should be appropriately increased; however, too high Al element forms large inclusions, and the Als content is 0.005%-0.05%.

[0015] The role of Ni is to improve the toughness of the steel plate and improve the fatigue resistance of the steel plate. 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 of the steel plate, and the Ni content is 7.2%-14.0%.

[0016] Cr element can effectively improve the strength of the steel plate in the steel; however, too high Cr content will reduce the impact toughness of the steel plate, and Cr combined with Ni in the steel can effectively improve the fatigue resistance of the steel plate, and the Cr content is 0.46%-0.64%.

[0017] Mo element in the steel can form fine carbides to improve the strength of the steel plate, thereby improving the fatigue performance of the steel plate. Adding a certain amount of Mo element to the steel can improve the hardenability of the ultra-thick steel plate and improve the heat treatment effect of the ultra-thick steel plate. Mo element can also cooperate with Ni to have certain corrosion resistance, and the Mo content is 0.55%-0.79%.

[0018] The main role of Co element 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. By reducing the stacking fault energy of the matrix and inhibiting the recovery of dislocation substructure in martensite, Co promotes the nucleation and precipitation of carbides, thereby improving the strength and fatigue resistance of steel. Co can form stable carbides and hard alloys at high temperatures with C, Cr and other elements, thereby improving the oxidation resistance, corrosion resistance and wear resistance of steel. However, the addition amount of Co should not 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%.

[0019] The Cu element can improve the strength of the steel plate in the steel, and the Cu element can act together with the Ni element in the steel to reduce the brittle-ductile transition temperature of the steel plate. Excessive Cu element alone causes thermal brittleness. The Cu element content is 0.42%-0.76%.

[0020] The V element can form V(C, N) particles during smelting to refine the grains. The addition of the V element in the steel plate can significantly improve the strength and low-temperature toughness of the steel plate. Meanwhile, the V element can effectively improve the weldability of the steel plate, and the V content is 0.11%-0.23%.

[0021] The Ti element forms TiN with N to prevent the grain growth of the billet during heating and rolling, and improve the fatigue resistance of the steel plate. A large amount of Ti element can form a stable intermetallic phase with the Ni element, but excessive Ti content will increase the crack sensitivity of the steel and increase the brittle fracture tendency of the steel. Therefore, in the present application, the Ti content is 0.01%-0.5%.

[0022] The N element can cooperate with the Ti and V elements to form fine and dispersed nitrides, which can effectively promote the nucleation and growth of intracrystalline ferrite, and effectively control the growth of the original austenite grains. The increase of the N content can increase the TiN in the steel. However, when the solid solution N content is too large, a large number of microcracks can easily appear on the surface of the steel plate. Therefore, the N content is 0.003%-0.007%.

[0023] In the above technical solution, further, the yield strength of the steel plate is ≥1000MPa, the tensile strength is 1050-1200MPa, the transverse elongation is ≥11%, the Charpy impact energy at-80℃ is ≥90J, and the Charpy impact energy at-196℃ is ≥60J.

[0024] Another aspect of the present application provides a manufacturing method of the gigapascal-grade offshore steel plate for extremely cold and ultra-deep environments, and the method comprises the following steps:

[0025] (1) Smelting

[0026] The molten steel is refined through a converter, an LF furnace, an RH or a VD furnace, and the molten steel is cast into a billet in a mold casting or continuous casting mode;

[0027] (2) Electroslag remelting

[0028] The billet is used as a melting electrode to perform electroslag remelting to prepare an electroslag billet, and the electroslag billet is discharged to perform 1050±50℃ homogenization annealing and slow cooling for ≥72h;

[0029] (3) Forging and breaking down

[0030] The electroslag billet after slow cooling is subjected to forging and breaking down, the forging heating temperature is 1220-1280℃, and the final forging temperature is ≥950℃, to obtain a forged billet;

[0031] (4) two-stage controlled rolling

[0032] The forged blank is loaded into a heating furnace for heating, and after heating, two-stage controlled rolling is carried out, the first-stage rolling temperature is 1000-1230℃, the finish rolling temperature is 900-1050℃, the intermediate blank thickness is 100-200mm, the second-stage rolling temperature is 750-900℃, and the finish rolling temperature is 700-800℃.

[0033] (5) quenching and tempering heat treatment

[0034] 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.

[0035] In the above technical solution, further, in step (3), the thickness ratio of the forged blank to the finished steel plate is ≥3.5.

[0036] In the above technical solution, further, in step (3), the single-pass reduction of the forged blank is ≤30mm.

[0037] In the above technical solution, further, in step (4), the total heating time is 5.5-9 hours, the soaking temperature is 1220-1280℃, and the soaking holding time is 1-3 hours.

[0038] 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%.

[0039] The beneficial effects of the present application are:

[0040] 1. The present application combines the composition design of elements such as C, Mn, Ni, Cr, Mo, Cu, Co, V, Ti, and the key production technology of gigapascal-level offshore steel plate for extremely cold and deep environment, which can produce ultra-high strength 1000MPa offshore steel with a maximum thickness of 100mm.

[0041] 2. Through the optimization and parameter selection of alloy element screening and proportioning, steel cleanliness control, electroslag remelting, forging breakdown, two-stage controlled rolling, and multiple quenching and tempering heat treatment process, the prepared steel plate has a yield strength ≥1000MPa, a tensile strength 1050-1200MPa, a transverse elongation ≥11%, a -80℃ Charpy impact energy ≥90J, a -196℃ Charpy impact energy ≥60J, and a fatigue stress and yield strength ratio ≤0.9 under stress cycles, and a fatigue cycle ≥10 7 times. BRIEF DESCRIPTION OF DRAWINGS

[0042] Figure 1 is a microstructure of the steel plate of Example 1 at room temperature. DETAILED DESCRIPTION

[0043] The following examples can make the ordinary skilled in the art more fully understand the present application, but in no way limit the present application.

[0044] Unless otherwise specified, the materials used in the examples of the present application can be obtained commercially or prepared according to conventional methods well known to those skilled in the art.

[0045] Examples 1-40

[0046] The chemical composition of the steel plates of Examples 1-40 of the present application is shown in Table 1.

[0047] Table 1 Chemical composition of the steel plates of Examples 1-40 (wt%)

[0048] The method for manufacturing the gigapascal-grade offshore steel plate for extremely cold and ultra-deep environments described above specifically includes the following steps:

[0049] (1) Smelting

[0050] The molten steel is refined by converter, LF furnace, RH or VD furnace, the content of P, S and non-metallic inclusions is further reduced by controlling the cleanliness of the steel, and the molten steel is cast into a billet by mold casting or continuous casting, and the whole process is protected casting.

[0051] (2) Electroslag remelting

[0052] The billet is used as a melting electrode for electroslag remelting to prepare an electroslag billet, the electroslag billet is annealed at 1050±50℃ for homogenization, and is slowly cooled for ≥72h; the reason for using the electroslag billet to roll the steel plate is that the electroslag billet has uniform and dense structure and small core segregation, which can ensure the low-temperature impact toughness of the thick steel plate after rolling, and the homogenization annealing and slow cooling of the electroslag billet after electroslag remelting can further avoid the excessive internal stress and non-uniform structure caused by the cooling process of the electroslag billet;

[0053] (3) Forging breakdown

[0054] The electroslag billet after slow cooling is forged and broken down, the forging heating temperature is 1220-1280℃, the single pass reduction of the forging breakdown is ≤30mm, and the final forging temperature is ≥950℃, obtaining a forged billet, the thickness ratio of the forged billet to the finished steel plate is ≥3.5; the purpose of using forging breakdown is to break the large size microstructure in the core of the ingot by multi-directional deformation of forging to the greatest extent, to prepare the structure for rolling the steel plate, and to further improve the low-temperature toughness of the core of the finished steel plate, and the purpose of controlling the single pass forging reduction is to avoid cracks on the surface of the ingot;

[0055] (4) Two-stage controlled rolling

[0056] The forging blank is charged into a heating furnace for heating, the total heating time is 5.5-9 hours, the soaking temperature is 1220-1280℃, and the soaking time is 1-3 hours. The purpose of ensuring the total heating time is to make the core of the large-thickness electroslag blank reach the target temperature, and to avoid the soaking temperature being too high. The purpose of controlling the soaking time is to prevent abnormal growth of the austenite structure of the blank, which affects the low-temperature toughness of the rolled steel plate. After heating, two-stage controlled rolling is performed. The first-stage rolling temperature is 1000-1230℃, the average pass reduction rate is 13-20%, the finish rolling temperature is 900-1050℃, and the intermediate blank thickness is 100-200mm. The second-stage rolling temperature is 750-900℃, the average pass reduction rate is 5-10%, and the finish rolling temperature is 700-800℃. The purpose of the first-stage rolling is to maximize the pass reduction rate under the condition of relatively high temperature and low hardness of the steel plate, to further crush the as-cast grains of the blank, to increase the deformation storage energy and grain nucleation sites, and to ensure sufficient deformation in the low-temperature rolling stage by the intermediate blank thickness of 100-200mm. The relatively low second-stage rolling temperature ensures a large amount of deformation storage energy in the process of quenching and tempering, and ensures the performance of the steel plate.

[0057] (5) Quenching and tempering heat treatment

[0058] 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 manufacturing process of the steel plate, which can effectively affect the performance of the steel plate, especially the low-temperature toughness and fatigue resistance of the steel plate. In the process of two-stage quenching, the main purpose of high-temperature quenching is to fully austenitize the steel plate and ensure that the austenite of the steel plate is not overheated or underheated. On the premise of ensuring no cracking and deformation, the cooling capacity is improved, the microstructure transformation is accelerated, the microstructure is largely transformed into martensite, and the hardened structure is increased. Since the high-temperature quenching can obtain more uniform austenite structure, the strength and hardness of the steel plate are improved, and therefore the steel plate has good fatigue fracture resistance. In the tempering stage, high-temperature tempering can adjust the quality of the second phase while adjusting the matrix structure, thereby optimizing the microstructure of the steel plate and improving the toughness of the steel plate. At the same time, in the intercritical quenching process, the low-temperature toughness of the steel plate can be improved by forming residual austenite. Finally, in the process of full tempering, the tempering transformation product can be ensured to be transformed as much as possible by sufficient tempering temperature and time, which reduces the strength and hardness of the steel plate while improving the toughness and plasticity.

[0059] The process parameters of the electroslag remelting and open forging are shown in Table 2, the process parameters of the two-stage controlled rolling are shown in Table 3, and the process parameters of the quenching and tempering heat treatment are shown in Table 4.

[0060] Table 2 Process parameters of electroslag remelting and open forging

[0061] Table 3 Process parameters of two-stage controlled rolling

[0062] Table 4 Process parameters of quenching and tempering heat treatment

[0063] 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 metallurgical structure content and size are shown in Table 7.

[0064] Table 5 Mechanical properties of the steel plates of Examples 1-40

[0065] Table 6 Fatigue properties of the steel plates of Examples 1-40

[0066] Table 7 Metallurgical structure content and size of the steel plates of Examples 1-40

[0067] The microstructure of the steel plate prepared in Example 1 at room temperature is shown in Fig. 1, and it can be found that the matrix structure of the experimental steel at room temperature is tempered sorbite and ferrite. As shown in Table 7, the tempered sorbite structure and the ferrite structure of the steel plate prepared in the present application are mostly between 3-6 μm, and the size is small and uniformly distributed. In addition, a large amount of residual austenite is contained in the structure, and the content is between 10.5%-16.8%. The morphology of the residual austenite is mainly film and block (M-A island), and the film residual austenite with high stability accounts for the majority of the residual austenite; the block residual austenite with low stability has a low content, and the grain size is mostly between 1-2 μm.

[0068] The above examples are only preferred examples of the present application, and are not a limitation on the embodiments. The protection scope of the present application should be defined by the scope defined in the claims. Other different forms of changes or variations can be made on the basis of the above description. The changes or variations thus extended are still within the protection scope of the present application.

Claims

1. A gigapascal-grade marine steel plate for use in an extremely cold superdeep environment, characterized by, The steel plate comprises the following components in percentage by weight: 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 inevitable impurities.

2. The gigapascal-grade offshore steel plate for polar superdeep environments according to claim 1, characterized by, The steel sheet has a yield strength of ≥ 1000 MPa, a tensile strength of 1050-1200 MPa, a transverse direction elongation of ≥ 11%, a Charpy impact energy of ≥ 90 J at -80°C, a Charpy impact energy of ≥ 60 J at -196°C, and a fatigue stress to yield strength ratio of ≤ 0.9 for a fatigue cycle of ≥ 10 7 times.

3. A method of manufacturing a gigapascal-grade marine steel plate for an extremely cold superdeep environment according to any one of claims 1 to 2, characterized by, The method comprises the following steps: (1) smelting The molten steel is refined by a converter, an LF furnace, an RH or a VD furnace, and is cast into a steel billet by mold casting or continuous casting; (2) electroslag remelting The steel billet is used as a melting electrode to perform electroslag remelting to prepare an electroslag billet, and the electroslag billet is discharged to perform homogenizing annealing at 1050±50℃ and is stack slow cooled for≥72h; (3) forging breakdown The electroslag billet after stack slow cooling is subjected to forging breakdown, the forging heating temperature is 1220-1280℃, the final forging temperature is≥950℃, and a forged billet is obtained; (4) two-stage controlled rolling The forged billet is loaded into a heating furnace for heating, and after heating, two-stage controlled rolling is performed, the one-stage rolling starting temperature is 1000-1230℃, the final rolling temperature is 900-1050℃, the intermediate billet thickness is 100-200mm, the two-stage rolling starting temperature is 750-900℃, and the final rolling temperature is 700-800℃; (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.

4. The production method according to claim 3, characterized by In step (3), the thickness ratio of the forged billet to the finished steel plate is≥3.

5.

5. The production method according to claim 3, wherein In step (3), the single pass reduction of the forging breakdown is≤30mm.

6. The production method according to claim 3, wherein In step (4), the total heating time is 5.5-9 hours, the soaking temperature is 1220-1280℃, and the soaking holding time is 1-3 hours.

7. The production method according to claim 3, wherein In step (4), the average pass reduction rate of the one-stage rolling is 13-20%, and the average pass reduction rate of the two-stage rolling is 5-10%.

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