Low-temperature container steel plate for low-temperature LNG storage tank and production method of low-temperature container steel plate

Abstract

The invention discloses a low-temperature container steel plate for a low-temperature LNG (Liquefied Natural Gas) storage tank and a production method of the low-temperature container steel plate, the thickness of the steel plate is 8mm-50mm, and the steel plate comprises the following chemical components in percentage by mass: 0.02%-0.06% of C, 0.05%-0.15% of Si, 4.50%-5.5% of Mn, less than or equal to 0.005% of P, less than or equal to 0.002% of S, 0.02%-0.03% of Nb, 1.0%-1.5% of Ni, 0.015%-0.035% of Als, 0.002%-0.03% of Ce, 0.08%-0.14% of Mo and the balance of Fe and residual elements. The structure of the obtained steel plate is tempered martensite and reversed austenite through converter alloy smelting, secondary decarburization and dephosphorization in a steel ladle, refining and P and S removal, pouring, rolling and composite structure regulation and control heat treatment, the yield strength of the steel plate is 470-550 MPa, the tensile strength is 600-680 MPa, the elongation is 20-25%, the impact absorption energy at the temperature of 120 DEG C below zero is 198-282J, and the production cost is reduced.

Description

Technical Field

[0001] The invention belongs to the technical field of medium and thick plate production, and specifically relates to a production method of a low-temperature container steel plate for a low-temperature LNG storage tank. Background Art

[0002] As the trade volume of liquefied natural gas (LNG) continues to grow, the demand for high-quality materials that can withstand extremely low temperatures continues to increase, but the high cost and complex production process restrict the application of such steel plates. How to reduce costs and stably produce low-temperature container steel that meets technical requirements has become a technical problem that current steel companies need to solve.

[0003] Publication number CN117327995A discloses a low-density 5Ni steel and a method for manufacturing the same. The chemical composition of the steel is as follows by mass percentage: C: 0.01-0.10%, Si: 0.15-0.30%, Mn: 0.30-0.80%, Ni: 4.50-5.50%, Al: 3.0-5.0%, S: ≤0.005%, P: ≤0.008%. 4.5-5.5% Ni and 3.0-5.0% Al are added to the steel. Although the performance is stable, the overall alloy cost is very high and there is no cost advantage after mass production.

[0004] Patent document with publication number CN104195428A discloses a V-containing low-carbon high-strength 5Ni steel medium and thick plate and a manufacturing method thereof, wherein the chemical composition is calculated by mass percentage as follows: C: 0.045-0.075%, Si: 0.15-0.25%, Mn: 0.50-0.80%, S: ≤0.005%, P: ≤0.008%, Ni: 4.75-5.25%, V: 0.03%-0.05%, Alt: 0.020-0.050%. The method is to add an appropriate amount of V on the basis of low C and a Ni content of about 5%, and obtain a 5Ni steel through two-stage controlled rolling and tempering heat treatment. The production thickness of the steel plate is between 25mm and 50mm, and the thin plate version below 25mm cannot be controlled, and the overall alloy cost is relatively high. Summary of the invention

[0005] In order to solve the above technical defects, the purpose of the present invention is to provide a low-temperature container steel plate for low-temperature LNG storage tanks, which has a reasonable design and can reduce production costs while ensuring that the low-temperature container steel plate produced is stable and meets technical requirements.

[0006] Another object of the present invention is to provide a method for producing a cryogenic container steel plate for a cryogenic LNG storage tank.

[0007] To achieve the above-mentioned purpose, the technical solution adopted by the present invention is: a low-temperature container steel plate for a low-temperature LNG storage tank, the thickness of the steel plate is 8mm~50mm, and it contains the following chemical components in mass percentage: C: 0.02%~0.06%, Si: 0.05%~0.15%, Mn: 2.0%~2.5%, P: ≤0.005%, S: ≤0.002%, Nb: 0.02%~0.03%, Ni: 3.0%~3.5%, Als: 0.015%~0.035%, Ce: 0.002%~0.03%, Mo: 0.08%~0.14%, and the rest are Fe and residual elements.

[0008] The production method of the cryogenic container steel plate for the cryogenic LNG storage tank includes converter alloy smelting, secondary decarburization in molten steel ladle, refining deP and deS, VD vacuum degassing, pouring, rolling, and composite structure control heat treatment, which are as follows:

[0009] a. Converter alloy smelting: The converter uses alloy or alloy scrap steel for smelting. Nickel plate, nickel iron or nickel-containing scrap steel are added before converter blowing. The converter oxygen blowing time is greater than 10 minutes. The tapping temperature is 1600-1620℃, the tapping P is ≤0.005%, and the final carbon content is ≤0.05%. No deoxidizer and Si, Mn and other alloys are added during tapping.

[0010] b. Secondary decarburization and dephosphorization in the ladle: Argon blowing and stirring are carried out during the tapping of the converter. No deoxidizer is added during the tapping process. After 1 / 4 of the steel is tapped, 300-350kg of iron oxide scale is added to the ladle. Through the impact stirring of the molten steel and the argon blowing stirring of the ladle, the oxygen in the molten steel is used for secondary decarburization. After the tapping is completed, 200-300kg of iron oxide scale is added again, and 250-350kg of lime is added at the same time. Argon blowing and stirring are carried out for 3-5 minutes to carry out secondary decarburization and dephosphorization. This process removes residual oxygen in the steel while decarburizing, effectively reducing the content of oxide inclusions. After the decarburization is completed, the residue on the surface of the ladle is poured out and hoisted to the work station for LF refining and VD degassing;

[0011] c. Refining is divided into two steps: the first step is decarburization and dephosphorization. In the refining process, 250-350 kg of lime is added, the temperature is raised to slag, the temperature is controlled at 1560-1580 ° C, argon is blown and stirred for 3-5 minutes, and decarburization and dephosphorization are performed again; molten steel is slaged and slag is removed, and high basicity and reducing white slag is refined. After slag making, various alloys are added in the order of nickel plate and ferromanganese. Sampling and testing are performed. During the steel refining process, Als is controlled to be greater than 0.020%, and then Nb, Mo, Ce and other alloys are added to adjust the composition to meet the requirements; the second step is VD vacuum degassing. Under a vacuum degree of ≤45Pa, the VD pressure holding time is not less than 20 minutes;

[0012] d. Heating rolling: After the ingot is mechanically polished until the surface is smooth and defect-free, an anti-oxidation coating is sprayed on the surface, and then it is hoisted to the heating furnace for heating. The temperature of the soaking section is controlled at 1180-1200℃, and the holding time is 250-350min. After tapping, it is descaled by high-pressure water, and then two-stage rolling is carried out. The first stage adopts high-permeability rolling, and the single-pass reduction is controlled to be more than 35mm, and the deformation coefficient is controlled to be more than 0.55 and the pass deformation rate is more than 16% for more than 2 passes. The cumulative reduction in one stage is 150-200mm to ensure that the rolling force is effectively transmitted to the core of the steel billet, so as to achieve uniform deformation inside the steel plate. At this time, the center deformation of the steel plate is greater than the surface deformation, so as to achieve the purpose of refining the core grains and uniform organization.

[0013] After the first stage, air-cool the steel plate to a temperature of 820-840°C, and then start the second rolling. The deformation of a single pass is 10-25mm, and the end temperature of rolling is controlled at 780-800°C, which can effectively prevent the grains of the steel plate from growing again. The last pass is to level the steel plate, and the deformation is 1-3mm to prevent the steel plate from buckling. After rolling, controlled cooling is performed, and the surface temperature is required to be 730-750°C when cooling, and the cooling end temperature is <100°C.

[0014] e. Composite structure control heat treatment process: quenching temperature is controlled at 740-760℃, keep warm for 40-130min after the furnace temperature reaches the temperature, cooling temperature is 710-730℃, cool to room temperature; tempering temperature is controlled at 590-610℃, keeping warm for 50-200min, and cool down naturally after leaving the furnace.

[0015] The steel plate structure obtained according to the above scheme is tempered martensite + reversed austenite, with a yield strength of 470-550MPa, a tensile strength of 600-680MPa, an elongation of 20-25%, and an impact absorption energy of 198-282J at -120°C, thereby reducing the amount of nickel alloy added and reducing the production cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The present invention will be further described in detail below in conjunction with the accompanying drawings.

[0017] Figure 1 This is a 500x metallographic structure diagram of a 35 mm thick steel plate at 1 / 4 of the thickness of the present invention.

[0018] Figure 2 This is a 500x metallographic structure diagram of a 35 mm thick steel plate at 1 / 2 thickness of the present invention. DETAILED DESCRIPTION

[0019] Embodiment A cryogenic container steel plate for a cryogenic LNG storage tank is produced, having a thickness of 8 mm to 50 mm, and comprising the following chemical compositions in mass percentage (unit, wt%): C: 0.02% to 0.05%, Si: 0.05% to 0.15%, Mn: 4.5% to 5.5%, P: ≤0.005%, S: ≤0.002%, Nb: 0.02% to 0.03%, Ni: 1.0% to 1.5%, Als: 0.015% to 0.035%, Ce: 0.002% to 0.03%, Mo: 0.08% to 0.14%, and the rest are Fe and residual elements.

[0020] Increasing the C content in the above composition design will significantly increase the ductile-brittle transition temperature, reduce the low-temperature impact of the steel plate, increase the hardness of the steel plate and the crack sensitivity after welding, and affect the welding performance of the steel plate. Therefore, the C content is controlled to be 0.02-0.06%.

[0021] Mn is an austenite stabilizing element, which is beneficial to the formation and stability of austenite. Mn can improve the matrix strength through solid solution strengthening and precipitation strengthening, improve the hardenability of steel, reduce the critical cooling rate of steel, and promote martensite transformation. At the same time, it can reduce the toughness-brittleness transition temperature of steel. Compared with Ni, as an element that improves the low-temperature toughness of steel, it is more economical, so the Mn content is controlled to 4.5-5.5%.

[0022] In this steel, P and S are impurity elements, which tend to be concentrated at grain boundaries, reducing the brittle fracture stress and increasing the brittle transition temperature, so the lower the better.

[0023] Al is a strong deoxidizing element, and the content of Al is controlled within the range of 0.015% to 0.035% in the present invention;

[0024] Nb effectively increases the recrystallization temperature of the steel plate during controlled rolling, expands the temperature range of the non-recrystallization zone, and can refine the grains, playing a role in grain refinement and strengthening. The present invention is designed to control the content within the range of 0.02% to 0.03%.

[0025] Ni can improve the strength and low temperature toughness of steel at the same time, and the present invention is designed to be controlled within the range of 1.0% to 1.5%;

[0026] Ce can deeply deoxidize, desulfurize, and modify inclusions, and at the same time has the effects of improving oxidation resistance and refining grains. The present invention requires 0.002% to 0.03%.

[0027] The above-mentioned cryogenic container steel plate for cryogenic LNG storage tank is produced through the following processes:

[0028] 1) Converter alloy smelting: The converter uses alloy or alloy scrap steel for smelting, and nickel plate, nickel iron or nickel-containing scrap steel are added synchronously before the converter blowing. The converter oxygen blowing time is greater than 10 minutes, the steel tapping temperature is 1600-1620°C, the steel tapping P is ≤0.005%, and the final carbon content is ≤0.05%;

[0029] 2) Secondary decarburization and dephosphorization in the ladle: Argon blowing and stirring are carried out during the tapping of the converter. No deoxidizer is added during the tapping process. After 1 / 4 of the steel is tapped, 300-350 kg of iron oxide scale is added to the ladle. Through the impact stirring of the molten steel and the argon blowing stirring of the ladle, the oxygen in the molten steel is used for secondary decarburization. After the tapping is completed, 200-300 kg of iron oxide scale is added again, and 250-350 kg of lime is added at the same time. Argon blowing and stirring are carried out for 3-5 minutes to carry out secondary decarburization and dephosphorization. This process removes residual oxygen in the steel while decarburizing, effectively reducing the content of oxide inclusions. After the decarburization is completed, the residue on the surface of the ladle is poured out and hoisted to the work station for LF refining and VD degassing;

[0030] 3) LF process: The first step is to decarbonize and dephosphorize. During the refining process, 250-350 kg of lime is added, the temperature is raised to slag, the temperature is controlled at 1560-1580 ° C, and argon is blown and stirred for 3-5 minutes; the molten steel is slaged and slag is removed, and high-basicity, reducing white slag is refined. After slag making, various alloys are added in the order of nickel plate and ferromanganese. Sampling is performed. During the steel refining process, Als is controlled to be greater than 0.020%, and then Nb, Mo, Ce and other alloys are added to adjust the composition to meet the requirements.

[0031] 4) Vacuum refining process: In the vacuum refining process, the holding time at ≤45Pa is controlled to be >20min. After breaking the vacuum, rice husk is added for heat preservation, and pouring begins after soft blowing for 3 to 5 minutes.

[0032] 5) Casting process: In the casting process, the continuous casting speed adopts a constant pulling speed, and the casting superheat is controlled at 5-20°C. After the casting of the billet is completed, it is pile-cooled for 36 hours, and then mechanically polished and the surface is inspected.

[0033] 6) Heating process: After the ingot is mechanically polished until the surface is smooth and defect-free, an anti-oxidation coating is sprayed on the surface, and then it is hoisted into a heating furnace for heating. The temperature of the soaking section is controlled at 1180-1200°C, and the holding time is 250-350 minutes;

[0034] 7) Rolling process: After steel is tapped, it is descaled with high-pressure water, followed by two-stage rolling. In the first stage, high-permeability rolling is used, and the single-pass reduction is controlled to be above 35mm, and the deformation coefficient is controlled to be greater than 0.55 and the pass deformation rate is greater than 16% for more than 2 passes. The cumulative reduction in one stage is 150-200mm to ensure that the rolling force is effectively transmitted to the core of the steel billet to achieve uniform deformation inside the steel plate. At this time, the deformation of the center of the steel plate is greater than the deformation of the surface, achieving the purpose of refining the core grains and uniform organization;

[0035] After the first stage, air-cool the steel plate to a temperature of 820-840°C, and then start the second rolling. The deformation of a single pass is 10-25mm, and the rolling termination temperature is controlled at 780-800°C. This can effectively prevent the grains of the steel plate from growing again. The last pass is to level the steel plate, and the deformation is 1-3mm to prevent the steel plate from buckling.

[0036] After rolling, controlled cooling is carried out. The surface temperature during cooling is required to be 730-750℃, and the cooling termination temperature is less than 100℃.

[0037] 8) Composite structure control heat treatment process: quenching temperature is controlled at 740-760℃, keep warm for 40-130min after the furnace temperature reaches the temperature, cool to 710-730℃, cool to room temperature; tempering temperature is controlled at 590-610℃, keep warm for 50-200min, and cool down naturally after leaving the furnace.

[0038] Through the processes of converter alloy smelting, secondary decarburization in molten steel ladle, LF deP, VD vacuum degassing, continuous casting, billet heating, rolling, controlled cooling, composite structure regulation heat treatment, etc., a 8mm to 50mm thick low-temperature container steel plate for low-temperature LNG storage tanks with the chemical composition described in Table 1 is obtained, and its mechanical properties are shown in Table 2 below.

[0039] Table 1 Chemical composition of 8mm~50mm thick cryogenic container steel plate for cryogenic LNG storage tank (Wt,%)

[0040] Thickness(mm) C Si Mn P S Als Nb Ni Ce Mo 10 0.02 0.09 4.82 0.004 0.001 0.022 0.023 1.03 0.015 0.09 20 0.04 0.13 4.68 0.005 0.002 0.017 0.025 1.10 0.008 0.12 35 0.03 0.11 5.00 0.003 0.001 0.026 0.026 1.22 0.022 0.14 50 0.05 0.10 5.33 0.004 0.001 0.031 0.022 1.38 0.025 0.10

[0041] Table 2 Mechanical properties of low temperature container steel plates for low temperature LNG storage tanks with a thickness of 8mm to 50mm

[0042] Thickness(mm) Batch number Yield strength (MPa) Tensile strength(MPa) Elongation(%) -120℃ impact energy(J) 10 52 492-545 615-672 20-23 228-282 20 35 470-532 600-668 20-22 205-276 35 12 488-522 602-661 20-25 213-280 50 10 479-535 610-680 21-24 198-269

[0043] This time, 52 batches, 35 batches, 12 batches and 10 batches of low-temperature container steel plates for low-temperature LNG storage tanks with thickness of 10mm, 20mm, 35mm and 50mm were trial-produced respectively. Through reasonable chemical composition design and production process control, the structure of the steel plate is martensite + reversed austenite, the yield strength of the steel plate is 470~550MPa, the tensile strength is 600~680MPa, the elongation is 20~25%, and the impact absorption energy at -120℃ is 198-282J, which meets the design requirements.

[0044] The beneficial effect of the above composition design and process design is that the technical solution can simultaneously improve the strength and low-temperature toughness of the steel plate, reduce the number of heat treatment requirements, reduce alloy addition, shorten the production cycle, and reduce production costs.

Claims

1. A low-temperature container steel plate for a low-temperature LNG storage tank, characterized in that: The steel plate has a thickness of 8 mm to 50 mm and contains the following chemical components in mass percentage: C: 0.02% to 0.06%, Si: 0.05% to 0.15%, Mn: 2.0% to 2.5%, P: ≤0.005%, S: ≤0.002%, Nb: 0.02% to 0.03%, Ni: 3.0% to 3.5%, Als: 0.015% to 0.035%, Ce: 0.002% to 0.03%, Mo: 0.08% to 0.14%, and the rest are Fe and residual elements; The steel plate has a structure of tempered martensite + reversed austenite, a yield strength of 470-550 MPa, a tensile strength of 600-680 MPa, an elongation of 20-25%, and an impact absorption energy of 198-282 J at -120°C.

2. The method for producing a low-temperature container steel plate for a low-temperature LNG storage tank according to claim 1 comprises the following steps: alloy smelting in a converter, secondary decarburization in a molten steel ladle, refining and deP and deS, VD vacuum degassing, pouring, rolling, and composite structure control heat treatment, wherein: The following steps are involved: a. Converter alloy smelting: The converter uses alloy or alloy scrap steel for smelting. Nickel plate, nickel iron or nickel-containing scrap steel are added before converter blowing. The converter oxygen blowing time is greater than 10 minutes. The tapping temperature is 1600-1620℃, the tapping P is ≤0.005%, and the final carbon content is ≤0.05%. No deoxidizer and Si, Mn and other alloys are added during tapping. b. Secondary decarburization and dephosphorization in the ladle: Argon blowing and stirring are carried out during the tapping of the converter. No deoxidizer is added during the tapping process. After 1 / 4 of the steel is tapped, 300-350kg of iron oxide scale is added to the ladle. Through the impact stirring of the molten steel and the argon blowing stirring of the ladle, the oxygen in the molten steel is used for secondary decarburization. After the tapping is completed, 200-300kg of iron oxide scale is added again, and 250-350kg of lime is added at the same time. Argon blowing and stirring are carried out for 3-5 minutes to carry out secondary decarburization and dephosphorization. This process removes residual oxygen in the steel while decarburizing, effectively reducing the content of oxide inclusions. After the decarburization is completed, the residue on the surface of the ladle is poured out and hoisted to the work station for LF refining and VD degassing; c. Refining is divided into two steps: the first step is decarburization and dephosphorization. In the refining process, 250-350 kg of lime is added, the temperature is raised to slag, the temperature is controlled at 1560-1580 ° C, argon is blown and stirred for 3-5 minutes, and decarburization and dephosphorization are performed again; molten steel is slaged and slag is removed, and high basicity and reducing white slag is refined. After slag making, various alloys are added in the order of nickel plate and ferromanganese. Sampling and testing are performed. During the steel refining process, Als is controlled to be greater than 0.020%, and then Nb, Mo, Ce and other alloys are added to adjust the composition to meet the requirements; the second step is VD vacuum degassing. Under a vacuum degree of ≤45Pa, the VD pressure holding time is not less than 20 minutes; d. Heating rolling: After the ingot is mechanically polished until the surface is smooth and defect-free, an anti-oxidation coating is sprayed on the surface, and then it is hoisted to the heating furnace for heating. The temperature of the soaking section is controlled at 1180-1200℃, and the holding time is 250-350min. After tapping, it is descaled by high-pressure water, and then two-stage rolling is carried out. The first stage adopts high-permeability rolling, and the single-pass reduction is controlled to be more than 35mm, and the deformation coefficient is controlled to be greater than 0.55 and the pass deformation rate is greater than 16% for more than 2 passes. The cumulative reduction in one stage is 180-200mm to ensure that the rolling force is effectively transmitted to the core of the steel billet, so as to achieve uniform deformation inside the steel plate. At this time, the center deformation of the steel plate is greater than the surface deformation, so as to achieve the purpose of refining the core grains and uniform organization. After the first stage, air-cool the steel plate to a temperature of 820-840°C, and then start the second rolling. The deformation of a single pass is 10-25mm, and the end temperature of rolling is controlled at 780-800°C, which can effectively prevent the grains of the steel plate from growing again. The last pass is to level the steel plate, and the deformation is 1-3mm to prevent the steel plate from buckling. After rolling, controlled cooling is performed, and the surface temperature is required to be 730-750°C when cooling, and the cooling end temperature is <100°C. e. Composite structure control heat treatment process: quenching temperature is controlled at 740-760℃, keep warm for 40-130min after the furnace temperature reaches the temperature, cooling temperature is 710-730℃, cool to room temperature; tempering temperature is controlled at 590-610℃, keeping warm for 50-200min, and cool down naturally after leaving the furnace.

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