An Al-Mg aluminum alloy sheet for an LNG storage tank, a preparation method thereof, and an LNG storage tank

Abstract

The present invention provides an Al-Mg aluminum alloy sheet for an LNG storage tank, a preparation method thereof, and an LNG storage tank. The preparation method of the Al-Mg aluminum alloy sheet for an LNG storage tank provided by the present invention sequentially performs batching, melting, melt purification, grain refinement, casting, ingot soaking, rolling, annealing, and stretching. During the above preparation process, process conditions are controlled, especially the selection of the grain refiner in grain refinement, the casting conditions, the ingot soaking conditions, the finished product annealing conditions, etc. Combining the control of the chemical composition by the present invention, improvements are made from both aspects of chemical composition and preparation process, so that the composition and the process are well adapted and promote each other, thereby improving the strength, corrosion resistance, processing and welding performance, and long-term service performance of the product.

Description

Technical Field

[0001] The present invention relates to the field of metal materials, and particularly to an Al-Mg aluminum alloy sheet for LNG storage tanks, a preparation method thereof, and an LNG storage tank. Background Art

[0002] Liquefied natural gas (LNG) refers to natural gas liquefied under the conditions of a pressure of 0.1 Mpa and a temperature of -161.5 °C. It is recognized as the cleanest fossil energy on earth and is one of the fastest-growing fuels in the world. Since the main natural gas supply regions and demand regions are far apart worldwide and are often separated by oceans, the transportation of liquefied natural gas mainly uses ocean shipping at present. After liquefaction, the volume of natural gas is reduced to 1 / 625 of its original gaseous state, and its mass is only about 45% of that of water of the same volume.

[0003] The service life of an LNG ship is generally 35 to 40 years. Given the special physical and chemical properties of LNG, extremely high requirements are imposed on all aspects of the LNG ship. Due to the existence of cyclic thermal stress, in order to transport LNG more safely and effectively, the LNG carrier needs to have low-temperature reliability and heat insulation. Therefore, generally speaking, an LNG carrier requires a dedicated ship. An LNG carrier is a "super freezer on the sea" and is regarded as the "crown jewel" of world shipbuilding.

[0004] In the design of the storage tank of an LNG carrier, both high strength and high toughness are required. The main factors to be considered are to find materials that can adapt to low-temperature media and to handle volatile and flammable substances properly. In addition to the above requirements for materials, special requirements are also imposed on the formability and welding performance of the shipbuilding processing technology of the materials. Low-temperature equipment should have reliable low-temperature cryogenic performance. In particular, storage equipment should at least meet the requirement of resistance to low temperature below -162 °C, and the design should reach -196 °C. Therefore, the materials used for LNG storage tanks need to meet some basic requirements: have good adaptability to impact loads and cyclic variable loads; have high strength and good elongation plasticity at low temperatures; within the working temperature range (from normal temperature to liquid gas temperature), the expansion coefficient of the tank materials changes little, so as to obtain small temperature difference stresses.

[0005] At present, the top structure of commonly used liquefied natural gas storage tanks adopts 5xxx aluminum alloy. 5xxx series aluminum alloys have excellent corrosion resistance and low-temperature performance, and also have advantages such as low density, high specific strength, high formability, easy surface treatment, and non-combustibility, and have been widely used in many fields. 5xxx series aluminum alloys have good processing performance and can be processed into various plates, bars, and wire rods, and are widely used in fields such as ships, vehicles, aerospace, etc. Al-Mg aluminum alloy for ultra-low temperature has become a widely used material for manufacturing low-temperature storage tanks due to its high strength, good plasticity, corrosion resistance, and workability. Al-Mg aluminum alloy is also widely selected in the manufacture of LNG ships and LNG storage tanks. Even some LNG storage tanks with completely aluminum alloy main wall structures, but most aluminum alloys still become important materials for the top structure of storage tanks because of their unique light weight and corrosion resistance. In European and American countries, aluminum reticulated shells are widely used on the tops of storage tanks, wastewater treatment ponds, and silos in environments with strong corrosion to steel.

[0006] In order to meet the requirements of the product for low temperature resistance, high strength, corrosion resistance, good processing and welding performance, etc., it is necessary to reasonably control the alloy impurity content. At the same time, very high requirements are put forward for the control accuracy and ratio of the main elements, the melt purity, the casting and heat treatment processes. However, for 5xxx aluminum alloy of the Al-Mg system, as the impurity content becomes lower and the composition range narrows, the requirements for industrial production are extremely harsh. Due to the high Mg content, the melt is prone to oxidation and hydrogen absorption, and conventional melt purification is difficult to meet its purity requirements. Existing aluminum alloy melting and casting technologies are difficult to ensure the quality requirements of chemical composition, hydrogen content, and welding performance. Due to the interaction and mutual restriction between the composition and process of the alloy, its development faces great technical challenges. Summary of the Invention

[0007] In view of this, the present invention provides an Al-Mg aluminum alloy plate for LNG storage tanks and its preparation method, as well as an LNG storage tank. The Al-Mg aluminum alloy ingot for LNG storage tanks provided by the present invention has good casting performance, corrosion resistance, processing and welding performance, and the flaw detection meets Grade AA, and can meet the quality requirements of the final product.

[0008] The present invention provides a preparation method for an Al-Mg aluminum alloy plate for LNG storage tanks, comprising the following steps:

[0009] A) Batching, melting, and melt purification:

[0010] After batching the raw materials, carry out in-furnace refining, on-line degassing refining, and melt filtration to obtain an alloy melt;

[0011] B) Grain refinement:

[0012] Mix the alloy melt with a grain refiner for grain refinement to obtain a refined alloy melt;

[0013] C) Casting:

[0014] The refined alloy melt is cast to obtain an ingot;

[0015] D) Ingot soaking:

[0016] The ingot is annealed to obtain an annealed ingot;

[0017] E) Rolling:

[0018] The annealed ingot is hot-rolled to obtain a sheet blank;

[0019] F) Annealing and stretching:

[0020] The sheet blank is annealed and stretched to obtain an Al-Mg aluminum alloy sheet for LNG storage tanks;

[0021] By mass percentage, the chemical composition of the Al-Mg aluminum alloy sheet for LNG storage tanks includes:

[0022] Si: ≤0.10%;

[0023] Fe: ≤0.25%;

[0024] Cu: ≤0.05%;

[0025] Mn: 0.45% - 0.90%;

[0026] Mg: 4.5% - 5.0%;

[0027] Cr: 0.05% - 0.20%;

[0028] Zn: ≤0.08%;

[0029] Ti: ≤0.05%;

[0030] B: 0.0005% - 0.0025%;

[0031] Be: 0.0005% - 0.0030%;

[0032] Na: ≤0.0005%;

[0033] Single impurity content: ≤0.03%;

[0034] Total impurity content: ≤0.10%;

[0035] Al: the balance.

[0036] Preferably, in step B), the grain refiner is at least one of Al-5Ti-B wire, Al-5Ti-0.2B wire, and Al-3Ti-0.15C wire.

[0037] Preferably, the dosage of the Al-5Ti-B wire is 0.5 - 1.5 kg / t;

[0038] the dosage of the Al-5Ti-0.2B wire is 0.8 - 2.0 kg / t;

[0039] the dosage of the Al-3Ti-0.15C wire is 1.2 - 2.5 kg / t.

[0040] Preferably, in step C), the conditions for casting are as follows:

[0041] The casting speed is 42 - 56 mm / min, the steady-state water flow rate for casting is 55 - 120 m 3 / h per root, the initial water flow rate is 70% - 80% of the steady-state water flow rate, the temperature of the molten aluminum at the end of the launder is 690 - 715 °C, and the water temperature is 20 - 30 °C.

[0042] Preferably, in step D), the conditions for the annealing treatment are as follows:

[0043] The heating rate is 40 - 100 °C / h, the annealing temperature is 460 - 530 °C, and the holding time is 5 - 20 h.

[0044] Preferably, in step E), the hot rolling process includes: first heating the ingot to the target temperature and holding it, and then hot rolling the ingot using a hot rolling mill;

[0045] wherein the target temperature is 450 - 480 °C, and the holding time is 3 - 24 h.

[0046] Preferably, in step F), the conditions for the annealing treatment are: temperature 330 - 380 °C, and the holding time is 0.5 - 1.5 h.

[0047] Preferably, in step F), the elongation rate for the stretching treatment is 0.5% - 1.0%.

[0048] The present invention also provides an Al-Mg aluminum alloy sheet for LNG storage tanks prepared by the preparation method described in the above technical solution.

[0049] The present invention also provides an LNG storage tank, the raw materials for the preparation of which include the Al-Mg aluminum alloy sheet for LNG storage tanks described in the above technical solution.

[0050] The preparation method of the Al-Mg aluminum alloy sheet for LNG storage tanks provided by the present invention comprises, in sequence, batching, melting, melt purification, grain refinement, casting, homogenization of the ingot, rolling, annealing, and stretching. During the above preparation process, process conditions are controlled, especially the selection of the grain refiner during grain refinement, the casting conditions, the ingot homogenization conditions, the finished product annealing conditions, etc. Combining the control of the chemical composition by the present invention, improvements are made from both the chemical composition and the preparation process, enabling better adaptation and mutual promotion between the composition and the process, thereby improving the strength, corrosion resistance, processing and welding performance, and long-term serviceability of the product.

[0051] The test results show that the flaw detection level of the Al-Mg aluminum alloy sheet obtained by the present invention reaches Grade AA, the tensile strength reaches above 310 MPa, the yield strength reaches above 170 MPa, and the elongation rate reaches above 16%, showing excellent comprehensive performance. Specific embodiments

[0052] The present invention provides a preparation method of an Al-Mg aluminum alloy sheet for LNG storage tanks, comprising the following steps:

[0053] A) Batching, melting, and melt purification:

[0054] After batching the raw materials, in-furnace refining, on-line degassing refining, and melt filtration are carried out to obtain an alloy melt.

[0055] B) Grain refinement:

[0056] The alloy melt is mixed with a grain refiner for grain refinement to obtain a refined alloy melt.

[0057] C) Casting:

[0058] The refined alloy melt is cast to obtain an ingot.

[0059] D) Homogenization of the ingot:

[0060] The ingot is annealed to obtain an annealed ingot.

[0061] E) Rolling:

[0062] The annealed ingot is hot-rolled to obtain a sheet blank.

[0063] F) Annealing and stretching:

[0064] The sheet blank is annealed and stretched to obtain an Al-Mg aluminum alloy sheet for LNG storage tanks.

[0065] By mass percentage, the chemical composition of the Al-Mg aluminum alloy sheet for LNG storage tanks comprises:

[0066] Si: ≤0.10%;

[0067] Fe: ≤0.25%;

[0068] Cu: ≤0.05%;

[0069] Mn: 0.45% - 0.90%;

[0070] Mg: 4.5% - 5.0%;

[0071] Cr: 0.05% - 0.20%;

[0072] Zn: ≤0.08%;

[0073] Ti: ≤0.05%;

[0074] B: 0.0005% - 0.0025%;

[0075] Be: 0.0005% - 0.0030%;

[0076] Na: ≤0.0005%;

[0077] Single impurity content: ≤0.03%;

[0078] Total impurity content: ≤0.10%;

[0079] Al: the balance.

[0080] Regarding step A) : Batching, melting and melt purification

[0081] After batching the raw materials, carry out in-furnace refining, on-line degassing refining and melt filtration to obtain an alloy melt.

[0082] In the present invention, the raw materials include aluminum ingots and master alloys. The raw materials may further include primary alloy scrap, that is, primary alloy scrap can be used or not. Among the above raw materials, aluminum ingots with higher purity and other raw materials with better uniformity are used to ensure that impurity elements such as Si, Fe, Cu, Zn, and Na are controlled at a lower level. Among them, the aluminum ingot is preferably an aluminum ingot with a grade above Al99.70. The master alloy is an intermediate alloy and a pure metal. The intermediate alloy is preferably at least one of an aluminum-manganese intermediate alloy, an aluminum-chromium intermediate alloy, an aluminum-beryllium intermediate alloy, and an aluminum-titanium intermediate alloy. In some embodiments of the present invention, the intermediate alloys are AlMn15, AlCr4, AlBe3, and AlTi4; in some other embodiments of the present invention, the intermediate alloys are AlMn15, AlCr4, and AlBe3. The pure metal is preferably a magnesium ingot. The primary alloy scrap is preferably at least one of pure aluminum, 3003 alloy, 5052 alloy, 5083 alloy, and 5A06 alloy. Among them, the mass ratio of the primary alloy scrap in all raw materials is preferably ≤50%, more preferably ≤30%. The dosage ratios of the remaining raw materials can be proportioned accordingly according to the proportions of the components in the target product.

[0083] In the present invention, after batching, in-furnace refining is first carried out. In the present invention, the method of in-furnace refining is preferably powder injection refining. In the present invention, the temperature of in-furnace refining is preferably 730-750 °C, specifically 730 °C, 735 °C, 740 °C, 745 °C, 750 °C. The time of in-furnace refining is preferably 30-40 min, specifically 30 min, 35 min, 40 min. The in-furnace refining is preferably carried out in a mixed gas environment, and the mixed gas is a protective gas and chlorine gas; among them, the flow rate of the protective gas is preferably 3-8 m 3 / h, and the flow rate of the chlorine gas is preferably 0.1-0.8 m 3 / h. The present invention has no special restrictions on the type of the protective gas, and any protective gas well-known to those skilled in the art can be used, such as nitrogen or argon, etc., and argon is preferably used.

[0084] After the above in-furnace refining, online degassing refining is carried out. In the present invention, the gas for online degassing refining is preferably an inert gas. The flow rate of the inert gas is preferably 3-6 m 3 m 3 / h. In the present invention, the temperature of online degassing refining is preferably 720-750 °C, specifically 720 °C, 725 °C, 730 °C, 735 °C, 740 °C, 745 °C, 750 °C.. Preferably, in the present invention, online degassing refining is carried out until the hydrogen content of the melt ≤0.15 cm 3 / 100 g Al, and the Na content of the melt ≤0.0005%.

[0085] After the above-mentioned online degassing and refining, melt filtration is carried out, specifically online melt filtration. In the present invention, the melt filtration method is preferably plate filtration or deep bed filtration. Among them, the mesh number of the filtration is preferably 30-50 PPI, specifically 30 PPI, 40 PPI, or 50 PPI. After the above filtration treatment, a pure alloy melt is obtained.

[0086] Regarding step B) : Grain refinement

[0087] The alloy melt is mixed with a grain refiner for grain refinement to obtain a refined alloy melt

[0088] In the present invention, the grain refiner is preferably at least one of Al-5Ti-B wire, Al-5Ti-0.2B wire, and Al-3Ti-0.15C wire. In the present invention, Ti and B are introduced into the alloy to refine the as-cast grains, but if their dosage is too high, it will affect the subsequent welding performance of the product and form aggregated compounds. The present invention preferably controls the dosage of the grain refiner as follows: among them, the dosage of the Al-5Ti-B wire is preferably 0.5-1.5 kg / t (where t is the unit of melt volume, that is, 0.5-1.5 kg grain refiner / t alloy melt), specifically 0.5 kg / t, 0.6 kg / t, 0.7 kg / t, 0.8 kg / t, 0.9 kg / t, 1.0 kg / t, 1.1 kg / t, 1.2 kg / t, 1.3 kg / t, 1.4 kg / t, 1.5 kg / t. The dosage of the Al-5Ti-0.2B wire is preferably 0.8-2.0 kg / t, specifically 0.8 kg / t, 0.9 kg / t, 1.0 kg / t, 1.1 kg / t, 1.2 kg / t, 1.3 kg / t, 1.4 kg / t, 1.5 kg / t, 1.6 kg / t, 1.7 kg / t, 1.8 kg / t, 1.9 kg / t, 2.0 kg / t. The dosage of the Al-3Ti-0.15C wire is preferably 1.2-2.5 kg / t, specifically 1.2 kg / t, 1.3 kg / t, 1.4 kg / t, 1.5 kg / t, 1.6 kg / t, 1.7 kg / t, 1.8 kg / t, 1.9 kg / t, 2.0 kg / t, 2.1 kg / t, 2.2 kg / t, 2.3 kg / t, 2.4 kg / t, 2.5 kg / t. After the above grain refinement, a refined alloy melt is obtained.

[0089] Regarding step C) : Casting

[0090] The refined alloy melt is cast to obtain an ingot.

[0091] In the present invention, the casting speed is preferably 42 to 56 mm / min, specifically it can be 42 mm / min, 43 mm / min, 44 mm / min, 45 mm / min, 46 mm / min, 47 mm / min, 48 mm / min, 49 mm / min, 50 mm / min, 51 mm / min, 52 mm / min, 53 mm / min, 54 mm / min, 55 mm / min, 56 mm / min. The steady-state water flow rate of the casting is preferably 55 to 120 m 3 / h / root, specifically it can be 55 m 3 / h / root, 60 m 3 / h / root, 70 m 3 / h / root, 80 m 3 / h / root, 90 m 3 / h / root, 100 m 3 / h / root, 110 m 3 / h / root, 120 m 3 / h / root; the starting water flow rate of the casting is preferably 70% to 80% of the steady-state water flow rate, specifically it can be 70%, 75%, 80%. The molten aluminum temperature at the end of the flow pan of the casting is preferably 690 to 715 °C, specifically it can be 690 °C, 695 °C, 700 °C, 705 °C, 710 °C, 715 °C. The water temperature of the casting is preferably 20 to 30 °C, specifically it can be 20 °C, 25 °C, 30 °C. By controlling the above casting conditions including the casting speed and the cooling intensity, etc., the present invention can make the ingot grains fine, improve the density of the ingot, refine the size of the primary crystallized substances, reduce the regional segregation, and is beneficial to improving the ingot quality and the comprehensive performance of the product.

[0092] Regarding step D) : Ingot soaking

[0093] Anneal the ingot to obtain an annealed ingot.

[0094] In the present invention, the heating rate of the annealing treatment is preferably 40 - 100 °C / h, and specifically can be 40 °C / h, 50 °C / h, 60 °C / h, 70 °C / h, 80 °C / h, 90 °C / h, 100 °C / h. The heating time of the annealing treatment is preferably 8 - 12 h, and specifically can be 8 h, 9 h, 10 h, 11 h, 12 h. The temperature of the annealing treatment (i.e., the target temperature reached after heating) is preferably 460 - 530 °C, and specifically can be 460 °C, 470 °C, 480 °C, 490 °C, 500 °C, 510 °C, 520 °C, 530 °C. The holding time of the annealing treatment is preferably 5 - 20 h, and specifically can be 5 h, 10 h, 15 h, 20 h. By performing homogenization annealing on the ingot, the present invention can improve the non-uniformity of the chemical composition and structure of the ingot, thereby obtaining a more uniform and stable structure, increasing the plasticity of alloy processing and improving the final properties of the product; one of the main phases of the 5xxx alloy is the β(Mg2Al3) phase, and its overburn temperature is 451 °C. Through the research of the applicant, it is found that if the heating rate is too fast or the local temperature is too high, and if there is an undissolved Mg2Al3 phase in the central part of the ingot, when the temperature of the ingot during heating or rolling is relatively high, rolling cracking, pores, overburning or rejection during flaw detection are likely to occur. Controlling the above-mentioned certain heating program and annealing conditions in the present invention is beneficial to improving the quality of the ingot and enhancing the product performance.

[0095] In the present invention, after the above-mentioned homogenization heat treatment of the ingot, it is preferably followed by surface milling of the ingot, and then the subsequent rolling process is carried out. In the present invention, in the surface milling process, it is preferably controlled that the small surface is milled by 3 - 10 mm and the large surface is milled by 8 - 12 mm. In the present invention, after surface milling, the specifications of the obtained ingot are preferably: thickness 450 - 550 mm, width 1200 - 2500 mm.

[0096] Regarding step E) : Rolling

[0097] The annealed ingot is hot-rolled to obtain a sheet blank.

[0098] In the present invention, the process of the hot rolling includes: first heating the ingot to the target temperature and holding it, and then hot-rolling the ingot using a hot rolling mill. In the present invention, the above-mentioned target temperature is preferably 450 - 480 °C, and specifically can be 450 °C, 460 °C, 470 °C, 480 °C. The holding time is preferably 3 - 24 h, and specifically can be 3 h, 5 h, 10 h, 15 h, 20 h, 24 h. In the present invention, the specifications of the hot rolling are preferably hot-rolling into a hot-rolled blank with a thickness of 15 - 50 mm.

[0099] Regarding step F) : Annealing and stretching

[0100] The sheet blank is subjected to annealing treatment and stretching treatment to obtain an Al-Mg aluminum alloy sheet for LNG storage tanks.

[0101] In the present invention, the temperature of the annealing treatment is preferably 330-380 °C, and specifically can be 330 °C, 340 °C, 350 °C, 360 °C, 370 °C, 380 °C. The heat preservation time of the annealing treatment is preferably 0.5-1.5 h, and specifically can be 0.5 h, 1.0 h, 1.5 h. The state of the material obtained after step E) is the O state. By performing the annealing treatment on it and controlling it at the above annealing temperature and heat preservation time, the material can be completely recrystallized and good plasticity can be ensured, and the yield strength of the sheet is improved. After the above treatment, the tensile strength of the sheet can be ≥310 MPa, the yield strength can be ≥170 MPa, and the elongation can be ≥16%.

[0102] In the present invention, after the above annealing treatment, a stretching treatment is performed. In the present invention, the stretching rate of the stretching treatment is preferably controlled at 0.5%-1.0%, and specifically can be 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%. Controlled under the above stretching conditions, it can ensure that the sheet meets the requirements.

[0103] In the present invention, after the above treatment, an Al-Mg aluminum alloy sheet for LNG storage tanks is obtained. It is subjected to flaw detection, specifically water immersion flaw detection, to determine whether the flaw detection level is qualified. If it is qualified, subsequent shearing to size and packaging can be carried out according to the requirements.

[0104] In the present invention, the obtained Al-Mg aluminum alloy sheet for LNG storage tanks belongs to 5xxx aluminum alloy, and its chemical composition includes:

[0105] Si: ≤0.10%;

[0106] Fe: ≤0.25%;

[0107] Cu: ≤0.05%;

[0108] Mn: 0.45%-0.90%;

[0109] Mg: 4.5%-5.0%;

[0110] Cr: 0.05%-0.20%;

[0111] Zn: ≤0.08%;

[0112] Ti: ≤0.05%;

[0113] B: 0.0005%-0.0025%;

[0114] Be: 0.0005%-0.0030%;

[0115] Na: ≤0.0005%;

[0116] Single impurity content: ≤0.03%;

[0117] Total impurity content: ≤0.10%;

[0118] Al: the balance.

[0119] Among them, the Mn content can specifically be 0.45%, 0.50%, 0.55%, 0.60%, 0.65%, 0.70%, 0.75%, 0.80%, 0.85%, 0.90%. The Mg content is 4.5% - 5.0%, excluding the endpoint 5.0%, and can specifically be 4.5%, 4.6%, 4.7%, 4.8%, 4.9%. The Cr content can specifically be 0.05%, 0.10%, 0.15%, 0.20%. The B content can specifically be 0.0005%, 0.0010%, 0.0015%, 0.0020%, 0.0025%. The Be content can specifically be 0.0005%, 0.0010%, 0.0015%, 0.0020%, 0.0025%, 0.0030%.

[0120] By controlling the Mg content within the above range in the present invention, the tensile strength and yield strength of the material can be improved. Through the applicant's research, it is found that if the Mg content exceeds 5%, the stress corrosion resistance is likely to decrease; in a high-Mg alloy (containing 5% Mg or higher Mg) after quenching or low-temperature (70 - 200 °C) tempering, continuous network-like β (Al3Mg2) phase precipitates along the grain boundaries, and when there is an electrolyte, this β phase will be preferentially corroded; therefore, only in the 5xxx alloy with a Mg content lower than 5% do the matrix and the welded part have high corrosion resistance. The content of Mg in the alloy also affects the tendency to form crystallization cracks during welding, the plasticity, porosity, and corrosion resistance of the welded part. When the Mg content increases from 2.5% to 4.5%, the tendency to form cracks increases, and when the Mg content is higher or lower, the crack tendency decreases significantly. In order to improve the crack resistance of the material during welding, the 5xxx alloy added with Mn, Cr, Ti, and about 4% Mg is the alloy with the best corrosion resistance in high alloying, and is not very sensitive to intermediate annealing, final annealing, and low-temperature heating during the manufacturing process, and can make a better adaptation between the composition and the preparation process, improving the material properties. Therefore, the present invention controls the Mg content to be 4.5% - 5.0%, and does not include the endpoint 5.0%.

[0121] The present invention also reasonably controls the element contents of Mn, Cr, Ti, Be, and B. In the present invention, adding a certain amount of Mn and Cr to the 5xxx aluminum alloy can improve the strength of the matrix material and the welded part, and at the same time can also improve the ability of the material to resist hot cracks during welding (the crack tendency is the smallest when the Mn content is 0.8%), and the ability to resist stress corrosion failure. In addition, adding Mn can also refine the grains. At the same time, the present invention also introduces a certain amount of grain refiner containing Ti and B elements, which can refine the casting structure of the alloy, is beneficial to the formation of a relatively dense weld structure, and can also improve the ability of the material to resist hot cracks during crystallization to a certain extent. In order to prevent the formation of primary Ti compounds, the addition amount of Ti does not exceed 0.05%. If the content of B element is too high, it is easy to agglomerate and affect the tissue uniformity and welding performance, and it is preferably controlled within 0.0025%. In addition, the present invention also controls a certain amount of Be element, which can prevent the oxidation of the alloy during the heating processes such as melting, casting, processing and welding, and can also prevent tearing during the casting process. Therefore, the present invention controls Mn at 0.45% - 0.90%, Cr at 0.05% - 0.20%, Ti ≤ 0.05%, Be at 0.0005% - 0.0030%, and B at 0.0005% - 0.0025%.

[0122] The present invention also reduces the contents of Si, Fe, Cu, Zn, and Na elements. Through research by the applicant, it is found that when the Si content exceeds 0.2%, the mechanical properties of the alloy, especially the elongation, will decrease. Fe not only deteriorates the mechanical properties but also makes the corrosion resistance of the alloy worse, especially more obvious when Mn exists; when the contents of Si and Fe are too high, not only the casting properties become worse, but also the processing properties of the sheet become worse, such as the edge cracking of the sheet is aggravated. The impurity Cu can reduce the corrosion resistance of the 5xxx alloy. The impurity Zn affects the welding performance of the alloy. In the aluminum alloy without Mg, Na does not exist in the free state but in the form of high-melting-point NaAlSi compounds and does not cause sodium embrittlement; in the low-Mg alloy, because the affinity of Mg element for Si is greater than that of Na, Mg and Si preferentially form Mg2Si phase, but the Mg content in the alloy is limited, part of it is dissolved in aluminum (the minimum solubility of Mg in Al at room temperature is about 2.3%), and it also forms compounds with Si in a ratio of 1.73:1. The excess Si reacts with Na to form NaAlSi compounds, so sodium embrittlement rarely occurs; however, in the high-Mg alloy, all the impurity Si is taken away by Mg, making Na exist only in the free state. Even trace amounts of impurity Na can strongly damage the casting and hot deformation properties of the alloy, showing "sodium embrittlement". Therefore, the present invention controls Si ≤ 0.10%, Fe ≤ 0.25%, Cu ≤ 0.05%, Zn ≤ 0.08%, and Na ≤ 0.0005%.

[0123] In some embodiments of the present invention, the chemical composition of the Al-Mg aluminum alloy sheet includes:

[0124] Si: 0.06%;

[0125] Fe: 0.16%;

[0126] Cu: 0.03%;

[0127] Mn: 0.60%;

[0128] Mg: 4.65%;

[0129] Cr: 0.09%;

[0130] Zn: 0.03%;

[0131] Ti: 0.03%;

[0132] B: 0.002%;

[0133] Be: 0.001%;

[0134] Na: 0.0003%;

[0135] Total impurities: 0.05%;

[0136] Al: the balance.

[0137] In some other embodiments of the present invention, the chemical composition of the Al-Mg aluminum alloy sheet includes:

[0138] Si: 0.05%;

[0139] Fe: 0.20%;

[0140] Cu: 0.02%;

[0141] Mn: 0.70%;

[0142] Mg: 4.85%;

[0143] Cr: 0.07%;

[0144] Zn: 0.03%;

[0145] Ti: 0.03%;

[0146] B: 0.0018%;

[0147] Be: 0.0015%;

[0148] Na: 0.0002%;

[0149] Total impurities: 0.06%;

[0150] Al: the balance.

[0151] In some other embodiments of the present invention, the chemical composition of the Al-Mg aluminum alloy sheet includes:

[0152] Si: 0.04%;

[0153] Fe: 0.19%;

[0154] Cu: 0.02%;

[0155] Mn: 0.80%;

[0156] Mg: 4.90%;

[0157] Cr: 0.09%;

[0158] Zn: 0.03%;

[0159] Ti: 0.02%;

[0160] B: 0.002%;

[0161] Be: 0.0013%;

[0162] Na: 0.0002%;

[0163] Total impurities: 0.05%;

[0164] Al: the balance.

[0165] The combination of the above chemical composition and the preparation process of the present invention faced many difficulties during the research and development exploration. For example, in order to improve the welding performance of the product, elements such as Zn and B need to be controlled at a low level. However, if the Zn content is too low, the cost will increase significantly, and B is an element in the grain refiner. If the B element content is too low, the grain refinement requirement cannot be met, resulting in poor tissue uniformity, and if it is too high, the welding performance will be affected; in order to improve the strength of the sheet, elements such as Mg and Mn need to be reasonably controlled; in order to ensure that the sheet has good stability and service life, the melt purity needs to be controlled at an extremely high level. At the same time, as the composition range narrows and the requirements become higher, the casting process window also narrows, and the difficulty of ingot forming increases significantly... etc. The restrictions between other elements and the restrictions between chemical composition and process will not be listed one by one. To meet the comprehensive performance requirements of the product, the present invention needs to control and balance multiple aspects such as chemical composition, grain refinement, melt purification, casting, and heat treatment processes.

[0166] The present invention also provides an Al-Mg aluminum alloy sheet for LNG storage tanks prepared by the preparation method described in the above technical solution.

[0167] The present invention also provides an LNG storage tank, the raw materials for preparing which include the Al-Mg aluminum alloy sheet for LNG storage tanks described in the above technical solution.

[0168] The preparation method of the Al-Mg aluminum alloy sheet for LNG storage tanks provided by the present invention comprises, in sequence, batching, melting, melt purification, grain refinement, casting, homogenization of the ingot, rolling, annealing, and stretching. During the above preparation process, process conditions are controlled, especially the selection of the grain refiner during grain refinement, the casting conditions, the ingot homogenization conditions, the finished product annealing conditions, etc. Combining the control of the chemical composition by the present invention, improvements are made from both the chemical composition and the preparation process, so that the composition and the process are well adapted to each other and promote each other, thereby improving the strength, corrosion resistance, processing and welding performance, and long-term service performance of the product.

[0169] The test results show that the flaw detection level of the Al-Mg aluminum alloy sheet obtained by the present invention reaches grade AA, the tensile strength reaches above 310 MPa, the yield strength reaches above 170 MPa, and the elongation rate reaches above 16%, showing excellent comprehensive performance.

[0170] To further understand the present invention, the preferred embodiments of the present invention will be described below in conjunction with examples. However, it should be understood that these descriptions are only for further explaining the features and advantages of the present invention, rather than limiting the claims of the present invention.

[0171] Example 1

[0172] A) Batching, melting, and melt purification:

[0173] Batching: 89.5% of Al99.70 aluminum ingots, 6% of master alloys (AlMn15, AlCr4, AlBe3, and AlTi4), and 4.5% of pure magnesium ingots.

[0174] Melting: The raw materials are fed into an electric furnace for in-furnace refining. The in-furnace refining temperature is 740 °C, and the refining duration is 30 min; the refining gas is Ar + Cl2, the Ar flow rate is 5 m 3 / h, and the Cl2 flow rate is 0.4 m 3 / h. Then, online degassing refining is carried out. The online degassing refining temperature is 730 °C, the gas is argon, and the argon flow rate is 4 m 3 / h. Until the hydrogen content of the melt ≤ 0.15 cm 3 / 100 gAl, and the Na content of the melt ≤ 0.0005%.

[0175] Melt filtration: The melt is filtered online using a 50 PPI filter plate.

[0176] B) Grain refinement:

[0177] Add the grain refiner Al-5Ti-B to the alloy melt at a dosage of 1.5 kg / t for grain refinement to obtain a refined alloy melt.

[0178] C) Casting:

[0179] The refined alloy melt is cast to obtain an ingot.

[0180] The casting conditions are as follows: casting speed 45 mm / min, steady-state water flow rate 90 m 3 / h / root, the starting water flow rate is 70% of the steady-state water flow rate, the molten aluminum temperature at the end of the launder is 701 °C, and the water temperature is 26 °C.

[0181] D) Homogenization of the ingot:

[0182] The ingot is annealed to obtain an annealed ingot.

[0183] The annealing conditions are as follows: heating rate 60 °C / h, target temperature 470 °C, holding time 10 h.

[0184] Facing treatment: facing 5 mm for small faces and 12 mm for large faces. The obtained ingot specifications are: thickness 480 mm, width 1720 mm.

[0185] E) Rolling:

[0186] The annealed ingot is heated to 470 °C and held for 5 h, and then hot-rolled by a hot rolling mill to obtain a hot-rolled blank with a thickness of 40 mm.

[0187] F) Annealing and stretching:

[0188] Annealing: temperature 360 °C, holding time 1 h.

[0189] Stretching: The annealed ingot is stretched, and the stretching rate is 0.5 - 1.0% to obtain a sheet.

[0190] The chemical composition of the finally obtained Al-Mg aluminum alloy sheet is shown in Table 1.

[0191] Table 1: Chemical composition of the Al-Mg aluminum alloy sheet obtained in Example 1

[0192]

[0193] Example 2

[0194] A) Batching, melting, and melt purification:

[0195] Batching: 76.6% of Al99.70 aluminum ingots, 4.8% of master alloys (AlMn15, AlCr4, AlBe3, and AlTi4), 3.6% of pure magnesium ingots, and 15% of primary alloy scrap 5083 alloy.

[0196] Melting: The raw materials are fed into an electric furnace for in-furnace refining. The in-furnace refining temperature is 750 °C, and the refining duration is 40 min; the refining gas is Ar + Cl2, and the Ar flow rate is 5.5 m 3 / h, the flow rate of Cl2 is 0.4 m 3 / h. Then, degassing and refining are carried out online. The temperature for online degassing and refining is 735 °C, the gas is argon, and the flow rate of argon is 4 m 3 / h. Until the hydrogen content in the melt ≤ 0.15 cm 3 / 100 g Al, and the Na content in the melt ≤ 0.0005%.

[0197] Melt filtration: The melt is filtered online using a 50 PPI filter plate.

[0198] B) Grain refinement:

[0199] The grain refiner Al-5Ti-B is added to the alloy melt at a dosage of 1.2 kg / t for grain refinement to obtain a refined alloy melt.

[0200] C) Casting:

[0201] The refined alloy melt is cast to obtain an ingot.

[0202] The casting conditions are: casting speed 48 mm / min, steady-state water flow rate 100 m 3 / h / root, the starting water flow rate is 75% of the steady-state water flow rate, the temperature of the molten aluminum at the end of the runner is 698 °C, and the water temperature is 24 °C.

[0203] D) Ingot soaking:

[0204] The ingot is annealed to obtain an annealed ingot.

[0205] The annealing conditions are: heating rate 50 °C / h, target temperature 480 °C, and holding time 8 h.

[0206] Milling treatment: The small face is milled by 4 mm and the large face is milled by 10 mm. The obtained ingot specifications are: thickness 480 mm, width 1510 mm.

[0207] E) Rolling:

[0208] The annealed ingot is heated to 460 °C and held for 5 h, and then hot-rolled by a hot rolling mill to obtain a hot-rolled blank with a thickness of 20 mm.

[0209] F) Annealing and stretching:

[0210] Annealing: temperature 350 °C, holding time 1.5 h.

[0211] Stretching: The annealed ingot is stretched, and the stretching rate is 0.5 - 1.0% to obtain a sheet.

[0212] The chemical composition of the finally obtained Al-Mg aluminum alloy sheet is shown in Table 2.

[0213] Table 2: Chemical Composition of the Al-Mg Aluminum Alloy Sheet Obtained in Example 2

[0214]

[0215] Example 3

[0216] A) Charge Preparation, Melting, and Melt Purification:

[0217] Charge Preparation: 62.7% of Al99.70 aluminum ingots, 4.0% of master alloys (AlMn15, AlCr4, and AlBe3), 3.1% of pure magnesium ingots, and 30% of primary alloy scrap 5083 alloy.

[0218] Melting: Feed the raw materials into an electric furnace for in-furnace refining. The in-furnace refining temperature is 745°C, and the refining duration is 35 min. The refining gas is Ar + Cl2, with an Ar flow rate of 6 m 3 / h and a Cl2 flow rate of 0.5 m 3 / h. Then conduct on-line degassing refining. The on-line degassing refining temperature is 740°C, the gas is argon, and the argon flow rate is 4 m 3 / h. Until the hydrogen content in the melt ≤ 0.15 cm 3 / 100 g Al and the Na content in the melt ≤ 0.0005%.

[0219] Melt Filtration: Filter the melt on-line using a 50 PPI filter plate.

[0220] B) Grain Refinement:

[0221] Add grain refiner Al-5Ti-B to the alloy melt at a dosage of 0.8 kg / t for grain refinement to obtain a refined alloy melt.

[0222] C) Casting:

[0223] Cast the refined alloy melt to obtain an ingot.

[0224] Casting conditions are: casting speed 50 mm / min, steady-state water flow rate 70 m 3 / h / root, starting water flow rate is 80% of the steady-state water flow rate, the aluminum liquid temperature at the end of the launder is 705°C, and the water temperature is 23°C.

[0225] D) Ingot Homogenization:

[0226] Anneal the ingot to obtain an annealed ingot.

[0227] Annealing conditions are: heating rate 50°C / h, target temperature 470°C, and holding time 14 h.

[0228] Milling treatment: small face milling 5mm, large face milling 10mm. The specifications of the obtained ingot are: thickness 500mm, width 1960mm.

[0229] E) Rolling:

[0230] The annealed ingot was heated to 460° C. and kept at this temperature for 6 hours, and then the ingot was hot rolled by a hot rolling unit to form a hot-rolled billet with a thickness of 30 mm.

[0231] F) Annealing and stretching:

[0232] Annealing: temperature 340℃, holding time 1.5h.

[0233] Stretching: The annealed ingot is stretched at a stretching rate of 0.5-1.0% to obtain a plate.

[0234] The chemical composition of the Al-Mg aluminum alloy sheet is shown in Table 3.

[0235] Table 3: Chemical composition of Al-Mg aluminum alloy plate obtained in Example 3

[0236]

[0237] Comparative Example 1

[0238] The method is implemented according to Example 1, except that the raw material composition is adjusted during batching so that the chemical composition of the obtained Al-Mg aluminum alloy plate is as shown in Table 4:

[0239] Table 4: Chemical composition of the Al-Mg aluminum alloy plate obtained in Example 4

[0240]

[0241] Comparative Example 2

[0242] The process is implemented according to Example 1, except that the process conditions of casting in step C), the process conditions of soaking the ingot in step D) and the process conditions of annealing in step F) are changed to the following process conditions:

[0243] C) Casting:

[0244] The refined alloy melt is cast to obtain an ingot.

[0245] Casting conditions: casting speed 35mm / min, steady water flow 40m 3 / h / root, the initial water flow rate is 50% of the steady-state water flow rate, the aluminum liquid temperature at the end of the flow plate is 680℃, and the water temperature is 15℃.

[0246] D) Ingot uniform heating:

[0247] The ingot is annealed to obtain an annealed ingot.

[0248] The annealing conditions are as follows: heating rate 120 °C / h, target temperature 480 °C, and holding time 6 h.

[0249] Facing treatment: the same as in Example 1.

[0250] F) Annealing and stretching:

[0251] Annealing: temperature 300 °C, holding time 2 h.

[0252] Stretching: the same as in Example 1.

[0253] Comparative Example 3

[0254] Implemented according to Example 1, except that the process conditions for casting in step C), soaking of the ingot in step D), and annealing in step F) are changed to the following process conditions:

[0255] C) Casting:

[0256] The refined alloy melt is cast to obtain an ingot.

[0257] The casting conditions are as follows: casting speed 70 mm / min, steady-state water flow rate 150 m 3 / h / root, the starting water flow rate is 90% of the steady-state water flow rate, the molten aluminum temperature at the end of the launder is 730 °C, and the water temperature is 35 °C.

[0258] D) Soaking of the ingot:

[0259] The ingot is annealed to obtain an annealed ingot.

[0260] The annealing conditions are as follows: heating rate 30 °C / h, target temperature 450 °C, and holding time 10 h.

[0261] Facing treatment: the same as in Example 1.

[0262] F) Annealing and stretching:

[0263] Annealing: temperature 400 °C, holding time 2 h.

[0264] Stretching: the same as in Example 1.

[0265] Test example: Product performance test

[0266] The products of each example and comparative example are subjected to flaw detection tests, tensile strength tests, yield strength tests, and elongation tests. The test results are shown in Table 4.

[0267] Among them:

[0268] Flaw detection test: Immersion flaw detection.

[0269] The tests for tensile strength, yield strength and elongation were carried out with reference to ASTM B557M (Standard Test Method for Tensile Testing of Wrought and Cast Aluminum and Magnesium Alloy Products).

[0270] Table 4: Product Test Results

[0271] Flaw detection level Tensile strength, MPa Yield strength, MPa Elongation, % Example 1 AA 315 180 21 Example 2 AA 321 188 19.5 Example 3 AA 331 192 18 Comparative example 1 Unqualified 285 150 23 Comparative example 2 Unqualified 280 160 15 Comparative example 3 Unqualified 260 130 25

[0272] As can be seen from the test results in Table 4, the flaw detection level of the Al-Mg aluminum alloy sheets obtained in Examples 1-3 of the present invention reaches Grade AA, the tensile strength reaches above 315 MPa, the yield strength reaches above 180 MPa, and the elongation reaches above 18%, showing excellent comprehensive performance. Compared with the examples, the tensile and yield strengths of Comparative Example 1 are worse and cannot meet the requirements of Grade AA flaw detection, proving that controlling the present invention under certain chemical compositions is beneficial to improving the comprehensive performance of the material. Compared with the examples, the tensile and yield strengths of Comparative Examples 2-3 are worse, and the elongation of Comparative Example 2 is worse. Both of them cannot meet the requirements of Grade AA flaw detection, proving that controlling certain casting conditions and annealing conditions in the present invention is beneficial to improving the comprehensive performance of the material.

[0273] Specific examples are used in this article to elaborate on the principles and implementation modes of the present invention. The description of the above examples is only used to help understand the method and its core idea of the present invention, including the best mode, and also enables any person skilled in the art to practice the present invention, including manufacturing and using any device or system, and implementing any combined method. It should be noted that for those of ordinary skill in the art in the technical field, without departing from the principle of the present invention, several improvements and modifications can be made to the present invention, and these improvements and modifications also fall within the protection scope of the claims of the present invention. The protection scope of the present invention patent is defined by the claims and may include other embodiments that can be conceived by those skilled in the art. If these other embodiments have structural elements similar to the literal description of the claims, or if they include equivalent structural elements that have no substantial difference from the literal description of the claims, then these other embodiments should also be included within the scope of the claims.

Claims

1. A preparation method of an Al-Mg aluminum alloy sheet for an LNG storage tank, characterized in that, It includes the following steps: A) Batching, melting, and melt purification: After batching the raw materials, in-furnace refining, on-line degassing refining, and melt filtration are carried out to obtain an alloy melt. B) Grain refinement: The alloy melt is mixed with a grain refiner for grain refinement to obtain a refined alloy melt. The grain refiner is at least one of Al-5Ti-B wire, Al-5Ti-0.2B wire, and Al-3Ti-0.15C wire. C) Casting: The refined alloy melt is cast to obtain an ingot. The conditions for the casting are as follows: The casting speed is 42 - 56 mm / min, the steady-state water flow rate for casting is 55 - 120 m 3 / h per root, the starting water flow rate is 70% - 80% of the steady-state water flow rate, the molten aluminum temperature at the end of the flow pan is 690 - 715 °C, and the water temperature is 20 - 30 °C; D) Ingot soaking: The ingot is annealed to obtain an annealed ingot. E) Rolling: The annealed ingot is hot-rolled to obtain a sheet blank. F) Annealing and stretching: The sheet blank is annealed and stretched to obtain an Al-Mg aluminum alloy sheet for LNG storage tanks. By mass percentage, the chemical composition of the Al-Mg aluminum alloy sheet for LNG storage tanks includes: Si: ≤0.10%; Fe: ≤0.25%; Cu: <0.05%; Mn: 0.45% - 0.90%; Mg: 4.5% - 5.0%, excluding the endpoint 5.0%; Cr：0.05%~0.20%； Zn: ≤0.08%; Ti: ≤0.05%; B：0.0005%~0.0025%； Be: 0.0005% - 0.0030%; Na: ≤0.0005%; Single impurity content: ≤0.03%; Total impurity content: ≤0.10%; Al: the balance; In step D), the conditions for the annealing treatment are: heating rate 40 - 100 °C / h, annealing temperature 460 - 530 °C, and holding time 5 - 20 h. In step E), the process of the hot rolling includes: first heating the ingot to the target temperature and holding, and then hot-rolling the ingot using a hot rolling mill; wherein, the target temperature is 450 - 480 °C, and the holding time is 3 - 24 h. In step F), the conditions for the annealing treatment are: temperature 330 - 380 °C, and holding time 0.5 - 1.5 h.

2. The preparation method according to claim 1, characterized in that, The dosage of the Al-5Ti-B wire is 0.5 - 1.5 kg / t. The dosage of the Al-5Ti-0.2B wire is 0.8 - 2.0 kg / t. The dosage of the Al-3Ti-0.15C wire is 1.2 - 2.5 kg / t.

3. The preparation method according to claim 1, characterized in that, In step F), the stretching rate of the stretching treatment is 0.5% - 1.0%.

4. An Al-Mg aluminum alloy sheet for LNG storage tanks prepared by the preparation method according to any one of claims 1 to 3.

5. An LNG storage tank, characterized in that, Its preparation raw materials include the Al-Mg aluminum alloy sheet for LNG storage tanks according to claim 4.

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