Steel plate for LNG (liquefied natural gas) transportation tank and manufacturing method thereof

Abstract

The invention provides a steel plate for an LNG transportation tank and a manufacturing method of the steel plate. The steel plate for the LNG transportation tank comprises the following chemical components in percentage by mass: 0.05%-0.25% of C, 0.05%-0.30% of Si, 21.0%-26.0% of Mn, less than or equal to 0.030% of P, less than or equal to 0.008% of S, 1.0%-4.0% of Al and the balance of Fe and inevitable impurities. The microstructure of the steel plate for the LNG transportation tank is recrystallized austenite. The steel plate for the LNG transportation tank realizes the balance of strength, ultralow-temperature toughness and light weight, and has good ultralow-temperature mechanical properties.

Description

Technical Field

[0001] The present application relates to the technical field of steel plate production, and in particular to a steel plate for LNG transport tanks and a manufacturing method thereof. Background Art

[0002] With the increasing global demand for clean energy, the demand for the transportation of liquefied natural gas (LNG) has increased significantly. LNG needs to be stored and transported at extremely low temperatures of -163°C, so the low-temperature performance of the material is extremely high. Although traditional LNG storage tank materials such as 9% nickel steel, aluminum alloy and stainless steel have good low-temperature performance, they have problems such as high cost and limited resources. In particular, nickel resources are scarce and their prices fluctuate greatly, prompting the industry to look for alternative materials. High manganese steel (containing 22%-25% manganese) exhibits excellent toughness, strength and processing properties at extremely low temperatures, and manganese resources are abundant and the cost is low, making it an ideal choice to replace traditional materials.

[0003] On the other hand, LNG tank trucks have strict requirements on the room temperature tensile strength, yield strength, yield strength ratio and elongation of steel used in LNG storage and use. However, existing steel grades are difficult to meet the strict requirements of LNG tank trucks. New steel plates need to be developed to achieve a balance between strength and lightweight to meet the needs of LNG tank trucks. Summary of the Invention

[0004] The present application provides a steel plate for LNG transport tanks and a manufacturing method thereof, aiming to achieve a balance between the strength, low-temperature toughness and lightness of the steel plate for LNG transport tanks by improving the composition of the steel plate, and to improve the low-temperature mechanical properties.

[0005] In the first aspect, an embodiment of the present application provides a steel plate for LNG transport tanks, which comprises the following chemical composition in mass percentage: C: 0.05% to 0.25%, Si: 0.05% to 0.30%, Mn: 21.0% to 26.0%, P≤0.030%, S≤0.008%, Al: 1.0% to 4.0%, and the rest are Fe and unavoidable impurities; the microstructure of the steel plate for LNG transport tanks is recrystallized austenite.

[0006] In some optional embodiments, the thickness of the steel plate for the LNG transport tank is 3 to 12 mm.

[0007] In some optional embodiments, the mechanical properties of the steel plate for LNG transport tanks meet the following requirements: yield strength at room temperature is 220-445 MPa, tensile strength is 580-720 MPa, elongation after fracture is 48.5%-78.5%, yield strength ratio is 0.30-0.65, and impact absorption energy at -196°C is 120-166 J.

[0008] In some optional embodiments, the low-temperature mechanical properties of the steel plate for LNG transport tanks meet the following requirements: the yield strength at -196°C is 360-540 MPa, the tensile strength at -196°C is 700-960 MPa, the elongation at break at -196°C is 52%-82%, and the grain size of the recrystallized austenite is grade 7-9 according to GB / T 6394-2017 "Method for Determination of Average Grain Size of Metals".

[0009] In some optional embodiments, the steel plate for LNG transport tanks includes, in terms of chemical composition percentages: C: 0.17% to 0.23%, Si: 0.19% to 0.27%, Mn: 23.3% to 24.6%, P≤0.011%, S≤0.004%, Al: 1.88% to 2.56%, and the rest is Fe and unavoidable impurities.

[0010] In a second aspect, an embodiment of the present application provides a method for manufacturing a steel plate for an LNG transport tank according to the first aspect, the method comprising:

[0011] The slab is heated, rolled, cooled, coiled, and cross-cut to obtain the steel plate for LNG transport tanks, wherein the slab comprises the following chemical composition by mass percentage: C: 0.05% to 0.25%, Si: 0.05% to 0.30%, Mn: 21.0% to 26.0%, P≤0.030%, S≤0.008%, Al: 1.0% to 4.0%, and the remainder is Fe and unavoidable impurities. The final rolling temperature of the rolling is 890 to 980°C.

[0012] In some optional embodiments, the target temperature of the heating is 1120-1270° C., and the heating time is 3-6 hours.

[0013] In some optional embodiments, the starting rolling temperature is 1050-1150° C., and the total reduction ratio of the rolling is 93%-98.6%.

[0014] In some optional embodiments, during the cooling and coiling, the laminar cooling rate is 40°C / s to 120°C / s, and the coiling temperature is ≤650°C.

[0015] In some optional embodiments, the method further includes continuously casting the molten steel to produce slabs. During the continuous casting process, the continuous casting tundish is overheated by 5 to 15°C, the water inlet insertion depth of the continuous casting tundish is 80 to 90 mm, the water inlet angle of the continuous casting tundish is an upward inclination angle of 16 to 25°, and the corner slag is removed every 10 to 20 minutes during the continuous casting process.

[0016] The steel plate for LNG transport tanks in the embodiment of the present application adopts Mn: 21.0% to 26.0% and Al: 1.0% to 4.0% to achieve a high manganese and high aluminum steel composition design, so that the steel plate for LNG transport tanks has excellent toughness and plasticity and suitable tensile strength, yield strength and yield strength ratio, so that it has good low-temperature mechanical properties and achieves a balance between strength and lightweight of the steel plate for LNG transport tanks. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the present application and, together with the description, serve to explain the principles of the present application.

[0018] Figure 1 A physical picture of the steel plate for LNG transport tanks according to Example 1 of the present application is shown.

[0019] Figure 2 The metallographic structure diagram of the steel plate for LNG transport tank prepared in Example 2 of the present application is shown.

[0020] Figure 3 The tensile curve of the steel plate for LNG transport tank prepared in Example 3 of the present application is shown.

[0021] The above drawings illustrate specific embodiments of the present application, which will be described in more detail below. These drawings and the textual description are not intended to limit the scope of the present application in any way, but rather to illustrate the concepts of the present application to those skilled in the art by reference to specific embodiments. DETAILED DESCRIPTION

[0022] The various embodiments or implementation schemes in this specification are described in a progressive manner, and each embodiment focuses on the differences from other embodiments.

[0023] Throughout this specification, reference to terms such as "one embodiment," "some embodiments," "illustrative embodiments," "examples," "specific examples," or "some examples" means that a specific feature, structure, material, or characteristic described in conjunction with an embodiment or example is included in at least one embodiment or example of the present application. In this specification, illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples.

[0024] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of the technical features being referred to. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of such features. Throughout the description of this application, "plurality" means at least two, for example, two, three, etc., unless otherwise specifically defined.

[0025] As described in the background technology section above, the current steel plates used for LNG transport tanks do not meet delivery requirements. Commonly used stainless steel is too expensive. Using other processes and compositions, Q400GMDR is primarily rolled to a thickness of 12 to 60 mm, primarily on medium plate mills. Hot rolling mills rarely roll high-manganese, high-aluminum steel. Especially when the alloy content is high (13% and above), thin, wide-width steel plates are subject to significant compression during rolling, resulting in high rolling forces. Hot rolling mills often experience excessive rolling current or torque, leading to scrap.

[0026] Research has found that the addition of manganese and an appropriate amount of aluminum to LNG tank steel plates stabilizes the austenite structure, preventing deformation and martensitic transformation at low temperatures, thereby maintaining excellent low-temperature toughness. An appropriate amount of aluminum also improves the material's corrosion resistance and reduces its density. The addition of aluminum further refines the grain size, reduces grain boundary brittleness, and further enhances the material's impact resistance in low-temperature environments.

[0027] The following is a detailed description of a steel plate for LNG transport tanks and a method for manufacturing the same provided in this application.

[0028] In the first aspect, an embodiment of the present application provides a steel plate for LNG transport tanks, which comprises the following chemical composition in mass percentage: C: 0.05% to 0.25%, Si: 0.05% to 0.30%, Mn: 21.0% to 26.0%, P≤0.030%, S≤0.008%, Al: 1.0% to 4.0%, and the rest are Fe and unavoidable impurities; the microstructure of the steel plate for LNG transport tanks is recrystallized austenite.

[0029] The Mn content within the above range is beneficial to austenite stabilization: it expands the γ phase region, inhibits ferrite formation, promotes grain uniformity, and improves the strength-toughness balance; Mn can lower the ductile-brittle transition temperature, with a reduction of ~2°C for every 0.1% Mn, which is suitable for a -50°C environment and is beneficial to improving low-temperature toughness.

[0030] Adding 1.0% to 4.0% Al is beneficial to the alloying of the steel plate, and can also refine the grains and reduce the brittleness of the grain boundaries. When the Al content reaches this range, it is also beneficial to stabilize the austenite structure and improve the impact resistance of the material in a low temperature environment.

[0031] Recrystallized austenite is formed by nucleation and growth of new austenite grains after cold deformation of steel plate when heated above the recrystallization temperature. These grains are free of distortion and equiaxed. Their grain size is generally 7 to 9 levels, with uniform chemical composition. Recrystallization rolling is used to control austenite stability, achieve the TRIP / TWIP effect, optimize elongation, and improve strength, toughness, and formability.

[0032] In summary, the Fe-C-Mn-Al steel plate for LNG transport tanks disclosed herein is a high-manganese, high-aluminum steel with excellent cryogenic mechanical properties, suitable for use in the manufacture of LNG transport tanks. Furthermore, this Fe-C-Mn-Al steel plate for LNG transport tanks enables the development of new materials for LNG transport tanks, replacing expensive stainless steel and achieving weight and cost reductions.

[0033] In some optional embodiments, the thickness of the steel plate for LNG transport tanks is 3 to 12 mm, and the width of the steel plate for LNG transport tanks is ≤ 2080 mm.

[0034] Optionally, the thickness of the steel plate for LNG transport tanks can be any value among 3mm, 4mm, 5mm, 6mm, 7mm, 8mm, 9mm, 10mm, 11mm, 12mm or a range thereof.

[0035] According to the NB / T 47058-2017 "Refrigerated Liquefied Gas Tank Truck" standard, LNG tank trucks impose strict requirements on the tensile strength, yield strength, yield strength ratio, and elongation after fracture of steel used in LNG storage and use. The tensile strength is 490-725MPa, the yield strength is 210-460MPa, the yield strength ratio is ≤0.85, and the elongation after fracture is ≥40%. In related technologies, stainless steel with added elements such as Cr and Ni is used to manufacture LNG tank trucks. The price of stainless steel (such as 304 and 316L) is much higher than that of high-manganese steel, and the prices of alloying elements such as nickel and molybdenum fluctuate greatly. The material cost of using stainless steel to manufacture LNG tank trucks is relatively high.

[0036] Moreover, when stainless steel is used to prepare LNG tankers, the processing cost is high and inert gas shielded welding (such as TIG) is required, which significantly increases energy consumption and labor costs. In addition, the heat-affected zone is prone to chromium carbide precipitation in stainless steel. 23 C6), which leads to intergranular corrosion and low-temperature cracks, and its welding deformation is difficult to control, affecting the sealing performance.

[0037] In some optional embodiments, the mechanical properties of the steel plate for LNG transport tanks meet the following requirements: room temperature yield strength of 220-445 MPa, tensile strength of 580-720 MPa, elongation at break of 48.5-78.5%, yield strength ratio of 0.30-0.65, and impact energy absorption of 120-166 J at -196°C. This avoids the problem of excessive strength of existing high-manganese steel for LNG.

[0038] In some optional embodiments, the low-temperature mechanical properties of the steel plate for LNG transport tanks meet the following requirements: the yield strength at -196°C is 360-540 MPa, the tensile strength at -196°C is 700-960 MPa, and the elongation at break at -196°C is 52-82%; the grain size of the recrystallized austenite is 7-9 levels according to GB / T 6394-2017 "Method for Determination of Average Grain Size of Metals".

[0039] Therefore, the steel plate for LNG transport tanks has good low-temperature performance and can be used to prepare LNG transport tanks to meet the low-temperature transportation requirements of liquefied natural gas, etc.

[0040] In some optional embodiments, the LNG transport tank steel plate comprises, by chemical composition percentage, the following: C: 0.17% to 0.23%, Si: 0.19% to 0.27%, Mn: 23.3% to 24.6%, P ≤ 0.011%, S ≤ 0.004%, Al: 1.88% to 2.56%, with the remainder being Fe and unavoidable impurities. Further controlling the manganese and aluminum content in the LNG transport tank steel plate can further enhance the strength of the high-manganese steel used in LNG transport tanks while also providing excellent low-temperature performance.

[0041] In a second aspect, an embodiment of the present application provides a method for manufacturing a steel plate for an LNG transport tank according to the first aspect, the method comprising:

[0042] The slab is heated, rolled, cooled, coiled, and cross-cut, wherein the slab comprises the following chemical composition by mass percentage: C: 0.05% to 0.25%, Si: 0.05% to 0.30%, Mn: 21.0% to 26.0%, P≤0.030%, S≤0.008%, Al: 1.0% to 4.0%, and the remainder is Fe and unavoidable impurities. The final rolling temperature of the rolling is 890 to 980°C.

[0043] According to the embodiment of the present application, the slab prepared with the above-mentioned manganese and aluminum contents is beneficial to improving the yield rate of steel plates for LNG transport tanks through the selected heating process and rolling, cooling coiling, and cross-cutting.

[0044] In some optional embodiments, the method further includes: continuously casting the refined molten steel to produce a slab, wherein the refined molten steel includes the following chemical components in mass percentage: C: 0.05% to 0.25%, Si: 0.05% to 0.30%, Mn: 21.0% to 26.0%, P≤0.030%, S≤0.008%, Al: 1.0% to 4.0%, and the rest is Fe and unavoidable impurities.

[0045] In some optional embodiments, the method further includes continuously casting the molten steel to produce slabs, and during the continuous casting process, the continuous casting ladle is superheated by 5 to 15°C to ensure rapid solidification of the molten steel; high-aluminum steel protective slag is used to reduce the chemical reaction rate between Al in the molten steel and SiO2 in the protective slag in the crystallizer; the water nozzle is inserted to a depth of 80 to 90 mm, and the water nozzle angle is an upward inclination angle of 16 to 25° to ensure uniform heat flow and good slag formation in the crystallizer; and the corner slag strips are removed every 10 to 20 minutes during the continuous casting process to ensure uniform flow of liquid slag and achieve a lubricating effect.

[0046] In some optional embodiments, the mold flux in the continuous casting tundish comprises the following chemical composition, by mass percentage: SiO2: 32%-38%, Al2O3: 1.5%-3.3%, Li2O: 4%-6%, Na2O: 8.5%-11.3%, C: 6.4%-8.2%, with the remainder being CaO and unavoidable impurities. This mold flux has a low melting point and can reduce the chemical reaction rate between Al in the molten steel and SiO2 in the mold flux within the mold, thereby ensuring stable composition of the molten steel and mold flux, and producing high-quality slabs.

[0047] In some optional embodiments, the target temperature of the heating is 1120-1270° C., and the heating time is 3-6 hours.

[0048] Optionally, the target heating temperature may be 1120°C, 1130°C, 1140°C, 1150°C, 1160°C, 1170°C, 1180°C, 1190°C, 1200°C, 1210°C, 1220°C, 1230°C, 1240°C, 1250°C, 1260°C, 1270°C, etc. The heating time may be 3.0 h, 3.5 h, 4.0 h, 4.5 h, 5.0 h, 5.5 h, 6.0 h, etc. Controlling the target heating temperature is beneficial for controlling the initial temperature of subsequent rough rolling. By controlling the heating time, the steel plate can be heated uniformly, which is beneficial for the subsequent uniform formation of recrystallized austenite within the steel plate.

[0049] In some optional embodiments, the starting rolling temperature is 1050-1150° C., and the total reduction ratio of the rolling is 93%-98.6%.

[0050] By controlling the rolling process, the temperature during the rolling process can be controlled so that recrystallized austenite is generated during the rolling process, the low-temperature strength of the steel plate is reduced and the low-temperature toughness of the steel plate is improved.

[0051] In addition, the rolling process is stable, the thickness fluctuation is much smaller than the thickness error of the medium plate rolling mill, the thickness is well hit, and the plate shape is flat.

[0052] In some optional embodiments, the rolling includes rough rolling, the starting rolling temperature of the rough rolling is 1050-1150°C, the rough rolling is performed for 5 passes, and the final rolling temperature of the rough rolling is 900-980°C. Within the above rolling temperature range, the austenite grains of the invented steel can fully achieve dynamic recrystallization.

[0053] In some optional embodiments, during the cooling and coiling, the laminar cooling rate is 40°C / s to 120°C / s, and the coiling temperature is ≤650°C. Optionally, the laminar cooling rate can be any value selected from 40°C / s, 45°C / s, 50°C / s, 55°C / s, 60°C / s, 65°C / s, 70°C / s, 75°C / s, 80°C / s, 85°C / s, 90°C / s, 95°C / s, 100°C / s, 105°C / s, 110°C / s, 115°C / s, and 120°C / s, or a range thereof. By controlling the cooling rate during the cooling and coiling, the growth of austenite grains can be suppressed, thereby controlling the strength, toughness, and low-temperature impact properties of the steel plate, avoiding excessive strength, and also avoiding cracks during subsequent processing (such as welding and cold bending).

[0054] Alternatively, the coiling temperature may be 550°C, 555°C, 560°C, 565°C, 570°C, 575°C, 580°C, 585°C, 590°C, 595°C, 600°C, 605°C, 610°C, 615°C, 620°C, 625°C, 630°C, 635°C, 640°C, 645°C, 650°C, etc. In some optional embodiments, the coiling temperature is 610°C to 640°C. Thus, by controlling the coiling temperature during cooling coiling, carbides (such as M 23 C6) precipitates along the grain boundaries to avoid deterioration of low-temperature toughness, thereby improving the low-temperature performance of steel plates for LNG transport tanks.

[0055] Furthermore, this coiling temperature avoids the need for water-toughening treatment of existing high-manganese steel for LNG, saving water resources and energy. Furthermore, coiling at this temperature avoids the precipitation of intergranular carbides, ensuring a single austenitic structure and thus maintaining ultra-low temperature toughness.

[0056] The LNG transport tank steel plate produced by this method has good low-temperature strength and toughness, and can be widely used in low-temperature applications. In addition, the LNG transport tank steel plate is conducive to the production of lightweight products.

[0057] Below, the embodiment of the present application is described. The embodiment described below is exemplary and is only used to explain the present application, and is not to be construed as limiting the present application. Where specific techniques or conditions are not specified in the embodiments, the techniques or conditions described in the literature in this area or the product specifications are used. Reagents or instruments used that do not specify the manufacturer are conventional products that can be obtained commercially.

[0058] Example 1

[0059] The embodiment of the present application provides a steel plate for LNG transport tanks, the chemical composition of which is shown in Table 1.

[0060] The present application provides a method for manufacturing a steel plate for an LNG transport tank, comprising:

[0061] 1) Molten iron is smelted to produce refined molten steel; the refined molten steel is continuously cast into slabs with a cross-section of 230×2050 mm. During the continuous casting process, the continuous casting tundish is superheated to 10°C, the nozzle insertion depth is 85 mm, the nozzle angle is upwardly inclined at 19°, and the corner slag is removed every 20 minutes during the continuous casting process. The mold slag in the continuous casting tundish has the following chemical composition, by mass percentage: SiO2: 32%-38%, Al2O3: 1.5%-3.3%, Li2O: 4%-6%, Na2O: 8.5%-11.3%, C: 6.4-8.2%, and the balance is CaO.

[0062] 2) Slab heating: heat the slab to 1256°C and keep it in the furnace for 4.5 hours;

[0063] 3) rolling, with a starting rolling temperature of 1125° C., a finishing rolling temperature of 944° C., and a total rolling reduction rate of 97.4%; the rolling includes rough rolling, which is performed in 5 passes, with a finishing rolling temperature of 944° C.

[0064] 4) Cooling and coiling: 8 sets of laminar cooling headers are used, and the coiling temperature is 526°C;

[0065] 5) Cross-cutting and flattening: the high manganese and high aluminum steel hot-rolled coil for LNG transport tanks is flattened and cut into plates to obtain 6×2070×8000 mm steel plates for LNG transport tanks.

[0066] Example 2

[0067] The embodiment of the present application provides a steel plate for LNG transport tanks, the chemical composition of which is shown in Table 1.

[0068] The present application provides a method for manufacturing a steel plate for an LNG transport tank, comprising:

[0069] 1) Smelting molten iron to produce refined molten steel; continuously casting the refined molten steel into 230×

[0070] The slab has a cross section of 1820mm; the continuous casting tundish is overheated at 5°C, the nozzle insertion depth of the continuous casting tundish is 90mm, the nozzle angle of the continuous casting tundish is an upward angle of 25°, and the corner slag is removed every 10 minutes during the continuous casting process.

[0071] 2) Slab heating: heat the slab to 1208°C for 4.1 hours in the furnace;

[0072] 3) rolling, the starting rolling temperature is 1107°C, the final rolling temperature of the finishing rolling is 921°C, and the total rolling reduction rate is 94.8%;

[0073] 4) Cooling coiling, laminar cooling headers are opened in 5 groups, and the coiling temperature is 636℃;

[0074] 5) Cross-cutting and flattening: the high manganese and high aluminum steel hot-rolled coil for LNG transport tanks is flattened and cut into plates to obtain 12×1850×6000 mm steel plates for LNG transport tanks.

[0075] Example 3

[0076] The embodiment of the present application provides a steel plate for LNG transport tanks, the chemical composition of which is shown in Table 1.

[0077] The present application provides a method for manufacturing a steel plate for an LNG transport tank, comprising:

[0078] 1) Smelting molten iron to produce refined molten steel; continuously casting the refined molten steel into slabs with a cross-section of 230×1590 mm; the continuous casting tundish is superheated to 15°C, the nozzle insertion depth of the continuous casting tundish is 90 mm, the nozzle angle of the continuous casting tundish is an upward angle of 16°, and the corner slag is removed every 20 minutes during the continuous casting process.

[0079] 2) Slab heating: heat the slab to 1188°C for 3.5 hours in the furnace;

[0080] 3) rolling, the starting rolling temperature is 1125°C, the final rolling temperature of the finishing rolling is 895°C, and the total rolling reduction rate is 98.7%;

[0081] 4) Cooling and coiling: 5 sets of laminar cooling headers are opened, and the coiling temperature is 588°C;

[0082] 5) Cross-cutting and flattening: the high manganese and high aluminum steel hot-rolled coil for LNG transport tanks is flattened and cut into plates to obtain 3×1610×6000 mm steel plates for LNG transport tanks.

[0083] Comparative Example 1

[0084] The comparative example of the present application provides a steel plate for LNG transport tanks, the chemical composition of which is shown in Table 1.

[0085] The comparative example of the present application provides a method for manufacturing a steel plate for an LNG transport tank, comprising:

[0086] 1) smelting molten iron to produce refined molten steel; continuously casting the refined molten steel into slabs with a cross-section of 230×2050 mm;

[0087] 2) Slab heating: heat the slab to 1255°C and keep it in the furnace for 5.5 hours;

[0088] 3) rolling, the starting rolling temperature is 1134°C, the final rolling temperature of the finishing rolling is 965°C, and the total rolling reduction rate is 97.4%;

[0089] 4) Cooling and coiling: 8 sets of laminar cooling headers are used, and the coiling temperature is 549°C;

[0090] 5) Cross-cutting and flattening: the high manganese and high aluminum steel hot-rolled coil for LNG transport tanks is flattened and cut into plates to obtain 6×2070×8000 mm steel plates for LNG transport tanks.

[0091] Comparative Example 2

[0092] The comparative example of the present application provides a steel plate for LNG transport tanks, the chemical composition of which is shown in Table 1.

[0093] The comparative example of the present application provides a method for manufacturing a steel plate for an LNG transport tank, comprising:

[0094] 1) smelting molten iron to produce refined molten steel; continuously casting the refined molten steel into slabs with a cross-section of 230×1820 mm;

[0095] 2) Slab heating: heat the slab to 1104°C and keep it in the furnace for 3.8 hours;

[0096] 3) rolling, the starting rolling temperature is 1007°C, the final rolling temperature of the finishing rolling is 809°C, and the total rolling reduction rate is 94.8%;

[0097] 4) Cooling coiling, laminar cooling header opens 1 group, coiling temperature is 763℃;

[0098] 5) Cross-cutting and flattening: the high manganese and high aluminum steel hot-rolled coil for LNG transport tanks is flattened and cut into plates to obtain 12×1850×6000 mm steel plates for LNG transport tanks.

[0099] Comparative Example 3

[0100] The comparative example of the present application provides a steel plate for LNG transport tanks, the chemical composition of which is shown in Table 1.

[0101] The comparative example of the present application provides a method for manufacturing a steel plate for an LNG transport tank, comprising:

[0102] 1) smelting molten iron to produce refined molten steel; continuously casting the refined molten steel into slabs with a cross-section of 230×1590 mm;

[0103] 2) Slab heating: heat the slab to 1176°C and keep it in the furnace for 4.5 hours;

[0104] 3) rolling, the starting rolling temperature is 1060°C, the final rolling temperature of the finishing rolling is 795°C, and the total rolling reduction rate is 98.7%;

[0105] 4) Cooling and coiling: 5 sets of laminar cooling headers are opened, and the coiling temperature is 542°C;

[0106] 5) Cross-cutting and flattening: the high manganese and high aluminum steel hot-rolled coil for LNG transport tanks is flattened and cut into plates to obtain 3×1610×6000 mm steel plates for LNG transport tanks.

[0107] Comparative Example 4

[0108] The difference between this comparative example and Example 1 is that during rolling, the final rolling temperature of the finishing rolling is 850°C.

[0109] Comparative Example 5

[0110] The difference between this comparative example and Example 1 is that during rolling, the final rolling temperature of the finishing rolling is 1010°C.

[0111] Example 4

[0112] The difference between this embodiment and embodiment 1 is that the manufacturing method of the steel plate for LNG transport tank is different. The molten steel is continuously cast to produce the slab. During the continuous casting process, the continuous casting ladle is overheated by 5°C, the water inlet of the continuous casting ladle is inserted to a depth of 60mm, the water inlet angle of the continuous casting ladle is an upward inclination angle of 10°, and the slag at the corners is removed once during the continuous casting process.

[0113] Example 5

[0114] The difference between this embodiment and embodiment 1 is that the manufacturing method of the steel plate for LNG transport tank is different. The molten steel is continuously cast to produce the slab. During the continuous casting process, the continuous casting ladle is overheated by 23°C, the water inlet insertion depth of the continuous casting ladle is 95mm, the water inlet angle of the continuous casting ladle is an upward inclination angle of 28°, and the slag at the corners is removed every 20 minutes during the continuous casting process.

[0115] Example 6

[0116] The difference between this embodiment and Example 1 is that the manufacturing method of the steel plate for LNG transport tanks is different. The molten steel is continuously cast to produce a slab. During the continuous casting process, the protective slag composition of the continuous casting ladle is a conventional composition. The protective slag includes the following chemical components in mass percentage: SiO2: 39%~44%, Al2O3: 3.5%~5.2%, Li2O: 6%~8%, Na2O: 10.8%~12.6%, C: 4.4~6.5%, and the balance is CaO.

[0117] Table 1 Chemical composition of Examples and Comparative Examples by weight percentage / wt.%

[0118]

[0119] Performance testing

[0120] Sampling shall be carried out in accordance with GB / T 2975 “Sampling location and specimen preparation for mechanical properties testing of steel and steel products”.

[0121] Figure 1 A physical picture of the steel plate for LNG transport tanks according to Example 1 of the present application is shown, wherein the steel plate for LNG transport tanks has a thickness of 6 mm and is in a bundled state.

[0122] (1) Metallographic structure observation: Prepare 4% nitric acid alcohol corrosion reagent, drop 1-2 drops of the reagent on the surface of the polished high-strength steel plate prepared in Example 3, and leave it on the sample surface for about 10 seconds until the sample surface turns dark from the mirror image. After rinsing with alcohol, quickly process it on absorbent paper, and finally blow dry it with a hair dryer before determining the structure type under a metallographic microscope.

[0123] The microstructure of the steel plate was observed by a ZEISS metallographic microscope (OM). The sample was placed on the lens of the ZEISS microscope and the focus of the microscope was adjusted. The microstructure of the sample can be clearly displayed on the computer. At the same time, the microscope was adjusted appropriately, and the position of the lens was moved up and down and left and right to observe the structure of the microstructure. The magnification was selected and photographed, and the size was marked. It will be used as the object of future experimental analysis and research. The observation results are shown in Figure 2 .

[0124] Figure 2 The metallographic structure diagram of the steel plate for LNG transport tanks prepared in Example 2 of the present application is shown. It can be seen from the figure that the metallographic structure of the steel plate for LNG transport tanks prepared in Example 2 is a fully recrystallized recrystallized austenite structure.

[0125] (2) Mechanical properties testing at room temperature and low temperature:

[0126] According to GB / T228.1-2010 "Tensile tests on metallic materials - Part 1 - Room temperature test methods", the steel plates of the examples or comparative examples were sampled and tensile tests were carried out using a German Zwick tensile testing machine with a load of 50 to 1500 kN and a displacement speed of 2 mm / min. Test data such as the tensile strength, yield strength, elongation and impact absorption energy at -196°C of the materials at room temperature were obtained by computer graphics. The test results are shown in Table 2.

[0127] Table 2 Mechanical properties of the steel plates of the embodiment and the comparative example

[0128]

[0129] As can be seen from Table 2, the yield strength of the steel plate for LNG transport tanks in the embodiment at room temperature is 220-445 MPa, the tensile strength is 580-720 MPa, the elongation after fracture is 48.5%-78.5%, the yield strength ratio is 0.30-0.65, and the impact absorption energy at -196°C is 120-166 J, while the impact absorption energy at -196°C of the comparative example is lower.

[0130] The room temperature mechanical properties and low temperature mechanical properties of the steel plates for LNG transport tanks in the examples are better, while the strength of the steel plates without Al addition in Comparative Example 1 is too high, and the elongation after fracture and -196°C impact toughness are both low, which cannot meet the performance requirements of the LNG transport tank standard. In Comparative Example 2, the tensile strength and yield strength of the steel plates are too low when heated at a lower temperature, rolled, finished rolled and coiled at a higher temperature. Because the solid solution is insufficient at low temperatures, the austenite grains are not fully recrystallized during low temperature rolling, and are in a flattened grain state. The higher coiling temperature causes the steel coil to undergo growth during the cooling process, thereby reducing the tensile strength and yield strength. In Comparative Example 3, the Al content is not within the required range, and when the final rolling temperature is low, the tensile strength, elongation after fracture, and -196°C impact toughness of the resulting steel plates do not meet the requirements.

[0131] In Comparative Examples 4-5, the final rolling temperature was inappropriate, resulting in a larger grain size and a poor match between strength and toughness.

[0132] In Examples 4-5, due to the chemical composition of the molten steel in the continuous casting ladle, the process during continuous casting was not adapted, resulting in poor heat transfer and lubrication due to the reaction between the molten steel and the protective slag. The quality of the slabs produced by continuous casting was poor, resulting in a large number of scars on the surface of the rolled steel coils.

[0133] In Example 6, due to the chemical composition of the molten steel in the continuous casting tundish, conventional protective slag is used in the continuous casting tundish during continuous casting, causing the molten steel and the protective slag to react, etc., the quality of the slab produced by continuous casting is poor, resulting in a large number of scars on the surface of the rolled steel coil.

[0134] The tensile properties were obtained by referring to GB / T 228.1-2021 "Tensile tests on metallic materials - Part 1 - Room temperature test methods". The results are as follows: Figure 3 .

[0135] Figure 3 The tensile curve of the steel plate for LNG transport tanks prepared in Example 3 of the present application is shown, wherein the horizontal axis is the relative deformation and the vertical axis is the stress. Curve 8-1 in the figure represents the first tensile specimen, and curve 8-2 in the figure represents the second tensile specimen, indicating that the steel plate for LNG transport tanks has good tensile properties.

[0136] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit them. Although the present application has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some or all of the technical features therein. These modifications or replacements do not deviate the essence of the corresponding technical solutions from the scope of the technical solutions of the embodiments of the present application.

Claims

1. A steel plate for LNG transport tank, characterized in that: The steel plate for LNG transport tanks includes the following chemical composition, calculated by mass percentage: C: 0.05% to 0.25%, Si: 0.05% to 0.30%, Mn: 21.0% to 26.0%, P≤0.030%, S≤0.008%, Al: 1.0% to 4.0%, and the remainder is Fe and unavoidable impurities; the microstructure of the steel plate for LNG transport tanks is recrystallized austenite.

2. The steel plate for LNG transport tank according to claim 1, characterized in that: The thickness of the steel plate for LNG transport tank is 3 to 12 mm.

3. The steel plate for LNG transport tank according to claim 1, characterized in that: The mechanical properties of the steel plate for LNG transport tanks meet the following requirements: yield strength at room temperature of 220-445 MPa, tensile strength of 580-720 MPa, elongation after fracture of 48.5%-78.5%, yield strength ratio of 0.30-0.65, and impact absorption energy of -196°C of 120-166 J.

4. The steel plate for LNG transport tank according to claim 1, characterized in that: The ultra-low temperature mechanical properties of the steel plate for LNG transport tanks meet the following requirements: yield strength of 360-540 MPa at -196°C, tensile strength of 700-960 MPa at -196°C, and elongation at break of 52%-82% at -196°C; the grain size of the recrystallized austenite is grade 7-9 according to GB / T6394-2017 "Method for Determination of Average Grain Size of Metals".

5. The steel plate for LNG transport tank according to claim 1, characterized in that: The steel plate for LNG transport tanks includes, in terms of chemical composition percentage, C: 0.17% to 0.23%, Si: 0.19% to 0.27%, Mn: 23.3% to 24.6%, P≤0.011%, S≤0.004%, Al: 1.88% to 2.56%, and the remainder is Fe and unavoidable impurities.

6. A method for manufacturing a steel plate for an LNG transport tank according to any one of claims 1 to 5, characterized in that: The method comprises: The slab is heated, rolled, cooled, coiled, and cross-cut to obtain the steel plate for LNG transport tanks, wherein the slab comprises the following chemical composition by mass percentage: C: 0.05% to 0.25%, Si: 0.05% to 0.30%, Mn: 21.0% to 26.0%, P≤0.030%, S≤0.008%, Al: 1.0% to 4.0%, and the remainder is Fe and unavoidable impurities; the finishing rolling temperature of the rolling is 890 to 980°C.

7. The method according to claim 6, characterized in that The method further includes continuously casting molten steel to produce slabs. During the continuous casting process, the continuous casting tundish is overheated by 5 to 15°C, the nozzle of the continuous casting tundish is inserted to a depth of 80 to 90 mm, the nozzle angle of the continuous casting tundish is an upward inclination angle of 16 to 25 degrees, and slag is removed from the corners every 10 to 20 minutes during the continuous casting process.

8. The method according to claim 6, characterized in that The target temperature of the heating is 1120-1270° C., and the heating time is 3-6 hours.

9. The method according to claim 6, characterized in that The starting rolling temperature is 1050-1150° C., and the total reduction ratio of the rolling is 93%-98.6%.

10. The method according to claim 6, characterized in that During the cooling and coiling, the laminar cooling speed is 40°C / s to 120°C / s, and the coiling temperature is ≤650°C.

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