Steel plate for low-temperature storage tank of liquefied gas carrier and manufacturing method of steel plate

Abstract

The invention discloses a steel plate for a low-temperature storage tank of a liquefied gas carrier and a manufacturing method of the steel plate. The steel plate comprises the following chemical components in percentage by weight: less than or equal to 0.019% of C, less than or equal to 0.09% of Si, 2.50-3.50% of Mn, less than or equal to 0.009% of P, less than or equal to 0.005% of S, 5.00-6.00% of Ni, 0.010-0.035% of Al, less than or equal to 0.006% of N, 0.0009-0.0015% of B and the balance of Fe and other inevitable impurities. A 5.5 Ni-medium Mn-(Mo)-B component system is adopted for the steel plate, a special heat treatment process of one-time tempering and two-time quenching is combined, Mn and Ni are matched to play a role in stabilizing retained austenite and inverted austenite, the concentration gradient of a grain boundary or phase boundary manganese element is remarkably reduced, the stress concentration of the phase boundary and the number of crack sources during follow-up impact deformation are reduced, and the quality of the steel plate is improved. The manganese brittleness phenomenon is improved, the impact toughness of the low-temperature steel plate is remarkably improved, and therefore the low-temperature impact toughness of the steel plate at the temperature of-196 DEG C is guaranteed.

Description

Technical Field

[0001] The present invention relates to the technical field of steel preparation, and particularly relates to a steel plate for a cryogenic storage tank of a liquefied gas carrier and a manufacturing method thereof. Background Art

[0002] Liquefied natural gas (LNG) is mainly formed by purifying natural gas through cryogenic cooling to form a cryogenic liquid mixture of methane, and is a new type of clean fuel. The liquefied natural gas is beneficial for loading, and is more convenient for long-distance transportation and storage. The boiling point of liquid natural gas is determined by its composition components. Under normal atmospheric pressure, its boiling point is about -166 to -157 °C. LNG is transported by sea or water using special professional liquefied gas transport ships.

[0003] In recent years, the demand for LNG ships has shown an increasing trend. Therefore, cryogenic steel plates for LNG tank construction have become the focus of market competition. The lowest service temperature of LNG storage and transportation equipment reaches -196 °C, and the service environment is extremely harsh. The existing mature materials under the -196 °C cryogenic system are mainly austenitic stainless steel and 9% Ni cryogenic steel. Austenitic stainless steel is a commonly used low-temperature alloy steel, with excellent processing and welding properties, and good toughness at low temperatures. However, compared with other low-temperature steels, its strength is low and the alloy content is high, resulting in extremely high costs. 9% Ni cryogenic steel still has good low-temperature toughness at -196 °C, and is mainly used in the manufacture of large containers for storing and transporting cryogenic liquids. However, the price of nickel metal has increased significantly in recent years, resulting in a very high alloy cost for 9Ni steel. High Ni steel is currently the steel type commonly used by shipbuilding users for ultra-low temperature storage tank bodies, and is used for the production of fuel tanks for some special ships or LNG carrier tank bodies. In recent years, other steel mills at home and abroad have developed low-cost Ni-saving or Ni-free steel types for ultra-low temperature medium transport ships.

[0004] On the route of nickel saving, Nippon Steel joined several enterprises to adopt the controlled rolling and controlled cooling process to develop a 7% Ni steel plate with a 20% reduction in Ni content, and its toughness reached the level of 9% Ni. Moreover, the 7% Ni steel plate for LNG storage tanks produced by it has been first put into practical use in the world. Subsequently, the 7% Ni steel plate was incorporated into the Japanese Industrial Standard (JIS), named SL7N590, and was included in the ASTM standard and ASME standard in the same year. The Ni content of the steel is the same as that specified in JIS, which is 6.0% - 7.5%. Moreover, the new Ni-saving type LNG storage tank steel plate obtained the NK and DNV classification society certifications in 2014. Its rolling and heat treatment process route is TMCP (DQ-L-T), where DQ: direct quenching, L: intermediate heat treatment, T: tempering.

[0005] POSCO in South Korea has taken a different technical route from that of Japan. It began researching high manganese steel many years ago. The company, together with Daewoo Shipbuilding & Marine Engineering Co., Ltd. and several other classification societies, jointly established a project called "Joint Development of High Manganese Steel and Welding Materials for Ultra-Low Temperatures". After the new low-temperature high manganese steel developed by POSCO completed the classification society certification for steel and welding materials, it was certified by the Korea National Institute of Standards and Technology. Subsequently, mass production of the low-temperature high manganese steel was achieved and it was applied to the LNG storage tanks manufactured by the Okpo Shipyard of Daewoo Shipbuilding & Marine Engineering Co., Ltd. and the fuel tanks of the LNG-powered bulk carriers it constructed. Subsequently, its low-temperature high manganese steel obtained the standard technical certification issued by ASTM in the United States and the IMO certification. The low-temperature high manganese steel of POSCO can withstand the test of an ultra-low temperature environment of -196°C and is suitable for the storage and transportation of LNG. The yield strength of this steel grade is greater than 400 MPa, the tensile strength is 800 - 970 MPa. Under the ultra-low temperature condition of -196°C, the Charpy V-notch impact energy is greater than 41 J. The cost of the high manganese steel is lower than that of 9Ni steel, and the price is only 70% - 80% of that of nickel alloy steel. POSCO expects the high manganese steel to gradually replace nickel alloy steel in the LNG storage tank market. The Mn content in the chemical composition of POSCO's low-temperature high manganese steel can be as high as over 20%. However, the smelting of high manganese steel requires some improvements in the process equipment of the steel mill, and there are also some problems with relatively large fumes during welding by users.

[0006] Chinese Patent CN 104988404 A discloses a low-nickel steel plate for pressure vessels under the condition of -196°C and its production method. The composition of the steel plate is C≤0.04, Si≤0.05, Mn: 1.60 - 2.50, P≤0.005, S≤0.003, Alt: 0.015 - 0.050, Ni: 6.00 - 8.00, Cu: 0.35 - 0.50, Cr: 0.55 - 1.00, N≤0.004. In addition, it contains one or more of Mo≤0.35, Nb≤0.05, Ti≤0.025, Ca≤0.005, and the balance is Fe and inevitable inclusions. In the composition design of this patent, 7Ni + low manganese is adopted, and the combination of elements such as Cu, Cr, Nb, Ti, and Ca ensures the low-temperature toughness of the steel plate. During rolling, according to the different thicknesses of the steel plate, heat treatment processes such as secondary normalizing + tempering, quenching + tempering, and secondary quenching + tempering are carried out respectively. A steel plate with a relatively low nickel content is obtained, which has a low cost, adopts a feasible production process, is suitable for large-scale production of low-temperature storage tanks for LNG, and is a pressure vessel steel with good low-temperature toughness, particularly good impact performance, and strong welding performance.

[0007] Chinese Patent CN 115747652 A discloses a Nb7Ni cryogenic steel for nickel-saving LNG storage tanks and its heat treatment process. It relates to the field of manufacturing cryogenic energy storage materials. The chemical composition and mass percentage of the Nb7Ni cryogenic steel are as follows: C: 0.04 - 0.08%, Mn: 0.8 - 1.1%, Si: 0.15 - 0.3%, Ni: 6.2 - 7.6%, Nb: 0.02 - 0.06%, S ≤ 0.01%, P ≤ 0.001%, and the rest are Fe and inevitable impurity elements. The invention realizes the reduction of Ni by adding microalloying element Nb and optimizes the heat treatment process of high-temperature quenching + two-phase region quenching + high-temperature tempering (QLT), so that the amount and stability of reversed austenite generated after tempering are comparable to those of 9Ni steel, and it has similar mechanical properties. Especially, the transverse impact energy AKV at -196°C ≥ 110 J, obtaining a low-cost nickel-saving Nb7Ni cryogenic steel to replace 9Ni steel for application in LNG storage tanks, with excellent economic applicability.

[0008] Chinese Patent CN108118255A discloses a high-manganese TWIP cryogenic steel with high impact toughness. The chemical composition of the cryogenic steel in mass percentage is: C: 0.050 - 0.30%, Mn: 25 - 35%, Si: 0.30 - 1.5%, Al: 2.0 - 4.0%, and the rest are Fe and inevitable impurities; the stacking fault energy of the cryogenic steel at -196°C is 25 - 40 mJ / m 2 . The impact energy of the cryogenic steel obtained by this invention is between 200 J and 250 J at liquid nitrogen temperature (-196°C); the impact energy at room temperature is between 300 J and 350 J, the yield strength is between 200 MPa and 300 MPa, the tensile strength is between 600 MPa and 700 MPa, and the elongation after fracture is between 45% and 65%. Its technical route is high-manganese TWIP steel plus appropriate Al. Although the low-temperature impact toughness at -196°C is very high, its yield strength is only 200 - 300 MPa, similar to the technical route of high-manganese steel of POSCO in South Korea. The complete replacement of the high-Ni steel technical route by this type of high-manganese steel is significantly different from the present invention.

[0009] Existing steels that meet the usage requirements at low temperatures and ultra-low temperatures of -101°C to -196°C mostly adopt high-nickel series, high-manganese series, or the method of adding precious metal elements to reduce the Ni content to achieve high low-temperature toughness. Summary of the Invention

[0010] The object of the present invention is to provide a steel plate for cryogenic storage tanks of liquefied gas carriers and a manufacturing method thereof. The composition design adopts a Ni-saving + medium manganese composition design, and good low-temperature toughness can be obtained without adding precious alloy elements such as Nb, V, Ti, etc. The yield strength of the steel plate is ≥630 MPa, the tensile strength is ≥780 MPa, the elongation is ≥22%, the impact energy Akv of the base metal at -196 °C is ≥150 J, fully meeting the performance standard of 9Ni steel; after welding the steel plate, the impact energy Akv of the fusion line and HAZ at -196 °C is ≥41 J, meeting the requirements of the steel plate for cryogenic storage tanks of liquefied gas carriers.

[0011] In order to achieve the above object, the technical solution provided by the present invention is:

[0012] A steel plate for cryogenic storage tanks of liquefied gas carriers, the chemical composition of which by weight percentage is: C ≤ 0.019%, Si ≤ 0.09%, Mn: 2.50 - 3.50%, P ≤ 0.009%, S ≤ 0.005%, Ni: 5.00 - 6.00%, Al: 0.010 - 0.035%, N ≤ 0.006%, B: 0.0009 - 0.0015%, and the balance contains Fe and other inevitable impurities.

[0013] Further, the balance is Fe and other inevitable impurities.

[0014] Furthermore, the chemical composition of the steel plate further includes Mo ≤ 0.50%.

[0015] The microstructure of the steel plate of the present invention is martensite + ferrite + retained austenite + inverse austenite, and the volume ratio of the retained austenite + inverse austenite is ≤ 5%.

[0016] The yield strength of the steel plate of the present invention is ≥630 MPa, the tensile strength is ≥780 MPa, the elongation is ≥22%, the impact energy Akv of the base metal at -196 °C is ≥150 J; the impact energy Akv of the fusion line and HAZ at -196 °C after welding is ≥41 J.

[0017] In the composition design of the steel plate for cryogenic storage tanks of liquefied gas carriers of the present invention:

[0018] The present invention adopts a technical route with low C and low Si, without adding C and Si elements additionally, where Si ≤ 0.09% and C ≤ 0.019%. Carbon is the most important alloying element in steel, which can improve strength. Especially in heat-treated steel, it can significantly improve the strength of steel through solid solution strengthening, precipitation strengthening or phase transformation strengthening, etc. However, it is not conducive to the plasticity and toughness of steel, especially the low-temperature impact toughness. Therefore, for the good impact work at the extremely low temperature of -196°C to be obtained in the present invention, excluding the strengthening effect and toughness loss effect of carbon element, an extremely low carbon content of C ≤ 0.019% is adopted. Similarly, the Si element in steel can also improve strength and elastic limit, but it will also cause a certain degree of damage to the plasticity, low-temperature toughness and weldability of steel. Therefore, Si is not added additionally in the present invention. The Si in steel is generally introduced through ore molten iron and deoxidizer added during smelting, and Si ≤ 0.09% is controlled.

[0019] Ni: Nickel is an important element for stabilizing austenite and has no obvious effect on improving strength. Adding nickel to steel, especially in quenched and tempered steel, can make the finally obtained structure contain a certain amount of retained austenite (or inverse austenite). These austenite structures can greatly improve the low-temperature impact toughness of steel, especially the low-temperature toughness at -196°C. However, due to the sharp increase in the price of nickel metal in recent years, countries are developing nickel-saving or low-nickel or nickel-free steels to replace traditional 9Ni steel. In order to reduce costs, the present invention reduces the nickel element content. Of course, too low Ni content (less than 5.00%) will also cause the structure of the base metal of the steel plate after heat treatment to not meet the standard requirements, and its HAZ after re-welding heat cycle cannot meet the requirement of impact work ≥ 41 J at -196°C. This is because too low Ni content causes a significant reduction in the retained austenite (or inverse austenite) structure in the fusion line or HAZ area structure of the base metal after welding, and its mechanical stability and thermal stability decrease significantly at the extremely low temperature of -196°C, thus rapidly deteriorating the low-temperature impact toughness at -196°C. Therefore, the present invention controls the nickel content at 5.00 - 6.00%.

[0020] Mn: Manganese is also an element that stabilizes austenite. Its ability is second only to the alloying element nickel. It is an inexpensive element for stabilizing austenite and strengthening the alloy. At the same time, manganese increases the hardenability of steel and reduces the critical cooling rate for martensite formation. Manganese also acts together with aluminum in steel to deoxidize. However, manganese has a high segregation tendency and will segregate at grain boundaries or phase boundaries, significantly increasing the brittleness of grain boundaries or phase boundaries (in this invention, it is the martensite-austenite phase interface), commonly known as manganese embrittlement. After experiments in this invention, it was found that adding a certain amount of Mn, making it cooperate with Ni to play a role in stabilizing retained austenite or reverse austenite, and by significantly reducing the concentration gradient of manganese elements at grain boundaries or phase boundaries, reducing the stress concentration at the phase interface and the number of crack sources during subsequent impact deformation, the manganese embrittlement phenomenon is improved, and the impact toughness of low-temperature steel plates is significantly increased. Excessive manganese (exceeding 3.5%, especially after >4.5%) affects the castability of subsequent continuous casting, so its content cannot be too high. Therefore, the content of manganese in this invention should be controlled at 2.50 - 3.50%.

[0021] B: Adding boron in this invention is mainly to improve the hardenability of steel. Because the carbon content in this invention is extremely low, in order to easily obtain martensite-like structures during the first quenching and be more conducive to the reverse transformation of martensite into reverse austenite during the subsequent second partitioning heating and holding process, and finally obtain stable reverse austenite and retained austenite structures in the rapidly cooled steel plates, an appropriate amount of B element is added. Therefore, the content of B in this invention is controlled at 0.0009 - 0.0015%.

[0022] Al: Aluminum is a strong deoxidizing element in steel. In order to ensure that the oxygen content in steel is as low as possible, the content of aluminum is controlled at 0.010 - 0.035%. The excess aluminum after deoxidation and the nitrogen element in steel can form AlN precipitates, reducing the content of N gas elements, increasing strength, and refining the austenite grain size of steel elements during heat treatment heating.

[0023] N: In this invention, the content requirement of the gas element nitrogen is ≤0.006%, and no additional N is added to reduce the solid solution strengthening effect of N, which is not conducive to toughness.

[0024] P, S: As impurity elements, the lower the better. In this invention, it is required to control P≤0.009% and S≤0.005%.

[0025] Mo: Mo can strengthen the matrix, precipitate carbides during the subsequent first tempering process, and the precipitation products during the subsequent partitioning quenching heat treatment can significantly refine the original austenite grains, refine the subsequent martensite-ferrite / austenite grains and improve low-temperature toughness. Therefore, in this invention, Mo element not exceeding 0.50% can be selectively added.

[0026] The present invention uses a lower Ni content to reduce the cost of the steel plate for cryogenic storage tanks, adds a certain amount of Mn, which cooperates with Ni to play a role in stabilizing retained austenite or inverse austenite, avoiding a significant decrease in its mechanical stability and thermal stability at extremely low temperatures of -196°C due to the reduction of Ni content, and significantly reducing the concentration gradient of manganese elements at grain boundaries or phase boundaries, reducing the stress concentration at phase interfaces and the number of crack sources during subsequent impact deformation, improving the phenomenon of manganese embrittlement, significantly enhancing the impact toughness of the cryogenic steel plate, and thus ensuring the impact toughness of the steel plate at -196°C. Further, by adding Mo, the original austenite grains can be significantly refined, and the subsequent martensite-ferrite / austenite grains can be refined, further improving the low-temperature toughness.

[0027] Due to the extremely low C and Si design of the present invention, the hardenability of the steel is improved by increasing the Mn element and adding a very small amount of B element to ensure and supplement the lost strength, so as to comprehensively ensure the strength and toughness of the steel under low-temperature conditions. The yield strength of the steel plate is ≥630 MPa, the tensile strength is ≥780 MPa, the elongation is ≥22%, the impact energy Akv of the base metal at -196°C is ≥150 J, fully meeting the performance standards of 9Ni steel. After welding, the impact energy Akv at -196°C of the fusion line and HAZ meets the requirements of the steel plate for cryogenic storage tanks of liquefied gas carriers, which is ≥41 J.

[0028] The manufacturing method of the steel plate for cryogenic storage tanks of liquefied gas carriers described in the present invention includes the following steps:

[0029] 1) Smelting and refining

[0030] Blow the converter and perform vacuum degassing treatment according to the above composition, and continuously cast into billets or ingots, and then roll them into steel billets after blooming;

[0031] 2) Slab heating

[0032] Heat the billets or steel billets, and the heating temperature is: 1080 - 1150°C;

[0033] 3) Rolling

[0034] The billets or steel billets are rolled through multiple passes to obtain steel plates, the total rolling reduction rate is ≥80%, and the finish rolling temperature is ≥750°C;

[0035] 4) Heat treatment

[0036] Tempering, the tempering temperature is 490 - 510°C, the tempering holding time = 2×T, the unit of the tempering holding time is min, T is the thickness of the steel plate, the unit is mm, and, the tempering holding time is ≥30 min, and then take it out of the furnace and air-cool to room temperature;

[0037] The first quenching: The quenching temperature is 690 - 710°C, the quenching holding time = 1.5×T, the unit of the quenching holding time is min, T is the thickness of the steel plate, the unit is mm, and the quenching holding time ≥ 15 min. Then it is taken out of the furnace and cooled in water to room temperature.

[0038] The second quenching: The quenching temperature is 590 - 610°C, the quenching holding time = 1.5×T, the unit of the quenching holding time is min, T is the thickness of the steel plate, the unit is mm, and the quenching holding time ≥ 15 min. Then it is taken out of the furnace and cooled in water to room temperature.

[0039] In the manufacturing method of the steel plate for low-temperature storage tanks of liquefied gas carriers described in the present invention:

[0040] Converter blowing and vacuum treatment: The purpose is to ensure the basic composition requirements of the molten steel, remove harmful gases such as oxygen and hydrogen in the steel, and add necessary alloying elements such as manganese and nickel to adjust the alloying elements.

[0041] Continuous casting or ingot casting: Ensure the uniform internal composition and good surface quality of the cast billet. The ingots obtained by ingot casting need to be rolled into steel billets.

[0042] Heating and rolling: The continuous casting billet or steel billet is heated at a temperature of 1080 - 1150°C to obtain a uniform austenitized structure. It is rolled into a steel plate through multiple passes within the austenite recrystallization and non-recrystallization temperature ranges. The total reduction ratio is not less than 80%, the finishing rolling temperature is not less than 750°C, and it is air-cooled after rolling. The final steel plate obtains a fine original acicular ferrite structure.

[0043] The analysis of the influence mechanism of the heat treatment process on the composition system of the steel plate described in the present invention is as follows:

[0044] Tempering: The steel plate is first put into a tempering furnace and heated to 490 - 510°C. After tempering and holding for a period of time, it is taken out of the furnace and air-cooled to room temperature. The tempering temperature is lower than the AC1 temperature of the composition system of the present invention. Tempering in this temperature range can ensure the precipitation of some NiAl nano-precipitates and / or fine carbonitrides of Mo, so as to refine the original austenite structure during subsequent heating and austenitization. The final structure obtained after tempering is acicular ferrite + fine nano-precipitates or fine carbonitrides.

[0045] The first quenching: The first quenching temperature of the steel plate is controlled at 690 - 710°C. This temperature range is within the AC1 - AC3 temperature range of the composition system of the present invention. The purpose of quenching in this temperature range is to re-partially austenitize and obtain a re-distribution of elemental compositions between austenite and ferrite, reducing the concentration gradient of Mn element at the phase boundary, and making the concentration gradient tend to be stabilized. After quenching, it is cooled in water to room temperature. During the water-cooling process, the cooling rate is relatively fast, and austenite transforms into martensite. The final structure is martensite structure + ferrite + a small amount of retained austenite.

[0046] Second quenching: The second quenching temperature of the steel plate is controlled at 590 - 610 °C. Quenching is carried out again in this AC1 - AC3 two-phase region. The purpose is to re-austenitize the martensite structure obtained from the first quenching and obtain a small amount of reverse austenite structure transformed from martensite. Then, re-partitioning and stabilization of the composition occur among the phases, further reducing the segregation and concentration gradient of Mn at the phase boundary. After quenching, it is water-cooled to room temperature again. The structure obtained after the second quenching is a more refined martensite structure + ferrite structure and a small amount of more stable retained austenite + reverse austenite mixed structure at low temperature.

[0047] The design difficulty of the low-temperature steel plate of the present invention lies in that the Ni content is reduced to reduce costs, and Mn is used as the austenite stabilizing element. However, the improvement of the low-temperature impact toughness of the steel by Mn is not as obvious as that by Ni, and Mn is prone to segregation, especially forming manganese embrittlement at positions such as grain boundaries and phase boundaries, which is not conducive to the improvement of low-temperature impact toughness. Therefore, on the basis of the composition design, the present invention adopts the above-mentioned unique process of first tempering once and then double-partition quenching, preferentially precipitating nano-particles or carbonitrides to refine the original austenite grains. Through the introduction of different precursor structures, a duplex structure after different partition quenching is obtained. The concentration gradient of Mn at the phase boundary or grain boundary is reduced, greatly alleviating the manganese embrittlement phenomenon. The mechanism is that the martensite and retained austenite in the structure have a K-S relationship, and the Mn element concentration gradient is distributed in the austenite. The discontinuous transformation of austenite at low temperature optimizes the TRIP effect of the material, increasing the product of strength and plasticity at low temperature. The thin-film austenite that divides the martensite structure undergoes secondary reverse transformation during the low-temperature partitioning process and stabilizes as reverse austenite. When subjected to a low-temperature impact at -196 °C subsequently, the stability of the retained austenite and / or reverse austenite mixed structure is extremely high, significantly improving the low-temperature impact toughness and increasing the impact energy. Research shows that in the structure of the steel plate that has only undergone double-partition quenching without tempering, there is a relatively large deviation in the K-S relationship at the interface between martensite and austenite, even a non-coherent relationship, causing relatively large interfacial residual stress, which serves as the crack origin during subsequent impact deformation, significantly deteriorating the low-temperature impact energy.

[0048] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0049] In the composition design of the present invention, low C and low Si are adopted, combined with a composition system of 5.5Ni - medium Mn - (Mo) - B. The cooperation of Mn and Ni plays a role in stabilizing the retained austenite or reverse austenite, significantly reducing the concentration gradient of manganese elements at the grain boundary or phase boundary, reducing the stress concentration at the phase interface and the number of crack sources during subsequent impact deformation, improving the manganese embrittlement phenomenon, significantly increasing the impact toughness of the low-temperature steel plate, thereby ensuring the low-temperature impact toughness of the steel plate at -196 °C and avoiding the need to add other precious alloy elements such as Nb, V, Ti, etc. to make up for the impact on low-temperature toughness due to the reduction of Ni content.

[0050] Based on the composition design, the present invention adopts a unique heat treatment process of one-time tempering + two-time quenching. First, one-time tempering is carried out to preferentially precipitate nano-particles or carbonitrides, refine the original austenite grains, and obtain a structure of martensite + ferrite + a small amount of retained austenite after tempering. Then, after two quenching processes, according to the introduction of different precursor structures after tempering, element partitioning and stabilization are carried out among the phases, reducing the concentration gradient of Mn at the phase boundary and avoiding the reduction of low-temperature toughness caused by manganese embrittlement; and a structure of martensite + ferrite and a small amount of more stable retained austenite + inverse austenite mixed structure at low temperature is obtained. The retained austenite and inverse austenite structures with strong stability at low temperature and a volume fraction of ≤5% are retained in the structure, achieving the perfect combination of the strength and low-temperature toughness of the steel plate.

[0051] The steel plate obtained by the present invention has a thickness of 5 - 50 mm, its yield strength ≥630 MPa, tensile strength ≥780 MPa, elongation ≥22%, and the Akv of the base metal at -196°C ≥150 J, fully meeting the performance standards of 9Ni steel. After welding, the Akv of the fusion line and HAZ at -196°C ≥41 J, meeting the requirements of the steel plate for low-temperature storage tanks of liquefied gas carriers. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] Figure 1 It is a microstructural photograph of the steel plate in Example 1 of the present invention.

[0053] Figure 2 It is a microstructural photograph of the steel plate in Example 2 of the present invention.

[0054] Figure 3 It is a microstructural photograph of the steel plate in Example 10 of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0055] The present invention will be further described below in conjunction with examples and drawings.

[0056] The compositions and process parameters of the examples of the present invention are shown in Table 1 and Table 2, and the steel properties of each example are shown in Table 3.

[0057] Figure 1 , Figure 2 and Figure 3 are the microstructural photographs of Examples 1, 2 and 10 respectively. As can be seen from the figures, the steel plates of the examples of the present invention have obtained a mixed structure of martensite + ferrite and a small amount of retained austenite and inverse austenite as shown in the figures.

[0058] Comparative Example 1 has the composition of traditional 9Ni steel. Compared with it, the present invention has comparable performance, but the Ni content in the present invention is significantly reduced, reducing the production cost of the steel plate.

[0059] In Comparative Example 2, the same components as in Example 5 of the present invention were used, but the conventional process of first quenching once and then tempering was adopted in the process. Although the strength of the finally obtained steel plate could meet the requirements of the present invention, its low-temperature toughness at -196°C was much lower than that of the present invention.

[0060]

[0061]

[0062]

Claims

1. A steel plate for cryogenic storage tanks of liquefied gas carriers, with the chemical composition by weight percentage being: C ≤ 0.019%, Si ≤ 0.09%, Mn: 2.50 - 3.50%, P ≤ 0.009%, S ≤ 0.005%, Ni: 5.00 - 6.00%, Al: 0.010 - 0.035%, N ≤ 0.006%, B: 0.0009 - 0.0015%, and the balance containing Fe and other inevitable impurities.

2. The steel plate for cryogenic storage tank of liquefied gas carrier according to claim 1, characterized in that, The chemical composition of the steel plate further includes Mo ≤ 0.50%.

3. The steel plate for cryogenic storage tank of liquefied gas carrier according to claim 1 or 2, characterized in that, The balance is Fe and other inevitable impurities.

4. The steel plate for cryogenic storage tank of liquefied gas carrier according to claim 1 or 2 or 3, characterized in that, The microstructure of the steel plate is martensite + ferrite + retained austenite + reverse austenite, and the volume ratio of the retained austenite + reverse austenite ≤ 5%.

5. The steel plate for cryogenic storage tank of liquefied gas carrier according to claim 1 or 2 or 3 or 4, characterized in that, The yield strength of the steel plate ≥ 630 MPa, the tensile strength ≥ 780 MPa, the elongation ≥ 22%, the impact energy Akv of the base metal at -196°C ≥ 150 J; the impact energy Akv of the fusion line and HAZ at -196°C after welding ≥ 41 J.

6. The manufacturing method of the steel plate for the cryogenic storage tank of a liquefied gas carrier according to any one of claims 1 to 5, characterized in that, Including the following steps: 1) Smelting and refining Smelt and refine according to the composition described in Claim 1 or 2 or 3, and continuously cast into billets or ingots, and then hot-rolled into steel billets after blooming. 2) Slab heating Heat the billets or steel billets, with the heating temperature: 1080 - 1150°C. 3) Rolling The billets or steel billets are rolled through multiple passes to obtain the steel plate, with the total reduction ratio ≥ 80% and the finishing rolling temperature ≥ 750°C. 4) Heat treatment Tempering, with the tempering temperature 490 - 510°C, the tempering holding time = 2×T, the unit of the tempering holding time is min, T is the thickness of the steel plate, the unit is mm, and the tempering holding time ≥ 30 min, and then air-cooled to room temperature after leaving the furnace. The first quenching, with the quenching temperature 690 - 710°C, the quenching holding time = 1.5×T, the unit of the quenching holding time is min, T is the thickness of the steel plate, the unit is mm, and the quenching holding time ≥ 15 min, and then water-cooled to room temperature after leaving the furnace. The second quenching, with the quenching temperature 590 - 610°C, the quenching holding time = 1.5×T, the unit of the quenching holding time is min, T is the thickness of the steel plate, the unit is mm, and the quenching holding time ≥ 15 min, and then water-cooled to room temperature after leaving the furnace.

7. The manufacturing method according to claim 6, characterized in that, The thickness of the steel plate is 5 - 50 mm.

Images