Low-temperature steel plate for LNG (Liquefied Natural Gas) ship and manufacturing method thereof

Abstract

The invention discloses a low-temperature steel plate for an LNG (Liquefied Natural Gas) ship and a manufacturing method of the low-temperature steel plate. The low-temperature steel plate comprises the following chemical components in percentage by weight: 0.020-0.100% of C, 0.10-0.35% of Si, 1.50-4.50% of Mn, less than or equal to 0.010% of P, less than or equal to 0.005% of S, 5.30-6.30% of Ni, 0.020-0.050% of Al, less than or equal to 0.006% of N, less than or equal to 0.005% of Ca and the balance of Fe and other inevitable impurities. A 5.5 Ni-Mn component system is adopted for the steel plate, the heat treatment process of two times of quenching and one time of tempering is combined, the effect of stabilizing retained austenite or inverted austenite is achieved through cooperation of Mn and Ni, the concentration gradient of the 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 impact toughness influence of the steel plate at the low 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 cryogenic steel plate for LNG ships and a manufacturing method thereof. Background Art

[0002] Liquefied natural gas (LNG) is mainly formed by purifying natural gas through deep cryogenic liquid to form a methane cryogenic liquid mixture, which is a new type of clean fuel. The liquefied natural gas is conducive to loading and is more convenient for long-distance transportation and storage. The boiling point of liquid natural gas is determined by its composition. 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 7% Ni steel plates with a 20% reduction in Ni content, and their toughness reached the level of 9% Ni. Moreover, the 7% Ni steel plates for LNG storage tanks produced by them have been practically applied for the first time in the world. Subsequently, the 7% Ni steel plates were incorporated into the Japanese Industrial Standard (JIS), named SL7N590, and were 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 nickel-saving LNG storage tank steel plates 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 started 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 classification society certifications of the newly developed low-temperature high manganese steel for both steel and welding materials were completed, it passed the certification of 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 Daewoo Shipbuilding & Marine Engineering Co., Ltd.'s Okpo Shipyard 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. POSCO's low-temperature high manganese steel can withstand the test of the 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, and 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 dust during welding by users.

[0006] Chinese Patent CN 104988404 A discloses a low-nickel steel plate for pressure vessels under the -196°C low-temperature condition 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 unavoidable 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 lower nickel content is obtained, which has a lower 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, excellent 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 reverse-transformed austenite after tempering are equivalent 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 use 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 achieve high low-temperature toughness by using high-nickel series, high-manganese series, or by adding precious metal elements to reduce the Ni content. Summary of the Invention

[0010] The object of the present invention is to provide a cryogenic steel plate for LNG ships and its manufacturing method. By adopting the composition design of reducing Ni + medium Mn and 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 ≥770 MPa, the elongation is ≥22%, the impact energy Akv of the base metal at -196 °C is ≥160 J, fully meeting the performance standards of 9Ni steel; after welding, the impact energy Akv of the fusion line and HAZ at -196 °C is ≥55 J, meeting the requirements of the steel plate for cryogenic storage tanks of liquefied gas ships.

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

[0012] A cryogenic steel plate for LNG ships, the chemical composition of which is in weight percentage: C: 0.020 - 0.100%, Si: 0.10 - 0.35%, Mn: 1.50 - 4.50%, P ≤ 0.010%, S ≤ 0.005%, Ni: 5.30 - 6.30%, Al: 0.020 - 0.050%, N ≤ 0.006%, Ca ≤ 0.005%, and the balance contains Fe and other inevitable impurities.

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

[0014] Even further, the chemical composition of the steel plate further includes Cr ≤ 1.00%.

[0015] The microstructure of the steel plate of the present invention is tempered martensite + a small amount of retained austenite and reverse austenite, and the volume ratio of the retained austenite and reverse austenite is ≤5%.

[0016] The yield strength of the steel plate of the present invention is ≥630 MPa, the tensile strength is ≥770 MPa, the elongation is ≥22%, the impact energy Akv of the base metal at -196 °C is ≥160 J; after welding, the impact energy Akv of the fusion line and HAZ at -196 °C is ≥55 J.

[0017] In the composition design of the steel plate of the present invention:

[0018] Carbon: A key and important element to ensure the strength of the steel plate. Since carbon has a high solubility in austenite, it can keep austenite in a high stability. A certain content of carbon is beneficial to obtaining a certain amount of retained austenite + reverse austenite structure. Excessive carbon content can increase the strength and hardness and decrease the plasticity and toughness, which is not conducive to the improvement of low-temperature toughness. Therefore, if the steel plate is to obtain a certain strength and high low-temperature toughness, the carbon content must be considered in combination with other alloy elements such as Ni and Mn contents. In the present invention, the carbon content is controlled at 0.020 - 0.100%. Carbon below 0.020% will greatly increase the smelting cost, and carbon above 0.100% will affect the improvement of low-temperature toughness.

[0019] Silicon: Adding silicon to steel can improve the purity of steel and deoxidation. Silicon plays a role in solid-solution strengthening in steel. Its solubility in austenite is relatively large. Increasing the silicon content is beneficial to improving the strength and hardness of steel. At the same time, silicon can promote the removal of carbon during phase transformation, enriching carbon in the retained austenite to make it stable. Combining with the post-rolling heat treatment process can improve the plasticity and toughness of steel. However, if the silicon content is too high, the viscosity of the oxide scale during steel plate heating will be relatively large, and it is difficult to descale after leaving the furnace, resulting in serious red oxide scale on the surface of the rolled steel plate and poor surface quality. Moreover, high silicon is not conducive to welding performance. Considering the various effects of silicon comprehensively, silicon is not used as the main alloying element in this invention. The silicon content in conventional hot metal or molten steel is adopted, and on the premise of not increasing the smelting cost, the silicon content is controlled within 0.10 - 0.35%.

[0020] Nickel: An important element for stabilizing austenite, which has no obvious effect on increasing 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, in order to reduce costs, the nickel element content in this invention is reduced. It is reduced by about 30% from the nominal content of about 9% in 9Ni steel to no more than 6.30% of nickel element that can be added. Of course, too low Ni content (less than 5.30%) will also cause the HAZ of the finally obtained structure of the steel plate after heat treatment to not meet the requirement of -196°C impact energy ≥ 41J after being re-heated cyclically by welding. 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 zone 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 -196°C low-temperature impact toughness. Therefore, the nickel content in this invention is controlled within 5.30 - 6.30%.

[0021] Manganese: It is also a stable austenite structure. Its ability is second only to the alloying element nickel. It is a cheap element for stabilizing austenite and strengthening alloys. At the same time, manganese increases the hardenability of steel and reduces the critical cooling rate of martensite formation. Manganese also plays a deoxidation role together with aluminum in steel. However, manganese has a high tendency to segregate. It will segregate at the grain boundary or phase boundary, significantly increasing the brittleness of the grain boundary or phase boundary (the interface between martensite and austenite in the present invention) is commonly known as manganese brittleness. After testing, the present invention found that adding a suitable Mn content to stabilize the residual austenite or reverse austenite with Ni, and significantly improving the manganese brittleness of the phase boundary through a suitable two-partition quenching and one-time tempering heat treatment process, so that the manganese element near the phase interface forms a certain concentration gradient, reducing the stress concentration of the phase interface and the number of crack sources during subsequent impact deformation, and significantly improving the low-temperature impact toughness. Finally, the steel plate of the invention obtains the desired structure and has a higher low-temperature toughness. The manganese content of the present invention should be controlled at 1.50-4.50%. And too high manganese (especially >4.5%) affects the castability of subsequent continuous casting, so its content cannot be too high.

[0022] Sulfur and phosphorus: Sulfur combines with manganese and other elements in steel to form plastic inclusions such as manganese sulfide, which is particularly detrimental to the transverse plasticity and toughness of steel, so the sulfur content should be as low as possible. Phosphorus is also a harmful element in steel, which seriously damages the plasticity and toughness of steel plates. For the present invention, sulfur and phosphorus are both unavoidable impurity elements, and the lower the better. Considering the actual control level of impurity elements during steelmaking in steel mills, in order not to significantly increase the smelting cost, the present invention requires P≤0.010% and S≤0.005%.

[0023] Aluminum: A strong deoxidizing element. In order to ensure that the oxygen content in the steel is as low as possible, the aluminum content is controlled at 0.020-0.050%. After deoxidation, the excess aluminum and nitrogen in the steel can form AlN precipitates, reduce the nitrogen gas element content, improve the strength, and refine the austenite grain size of the steel during heat treatment.

[0024] Chromium: Chromium improves the hardenability of steel and increases the tempering stability of steel. Chromium has a high solubility in austenite, stabilizes austenite, dissolves in martensite in large quantities after quenching, and precipitates Cr in the subsequent tempering process. 23 Carbides such as C7 and Cr7C3 can improve the strength and hardness of steel. In order to maintain the strength level of steel, chromium can partially replace manganese to reduce the segregation tendency of high manganese. Therefore, the present invention can choose to add no more than 1.00% chromium.

[0025] Calcium: The main purpose of adding calcium in the present invention is to change the sulfide form and improve the transverse performance and cold bending performance of the steel. Steel with very low sulfur content may not be treated with calcium. The present invention controls the calcium content to ≤0.005%.

[0026] Nitrogen: In the present invention, the content requirement of the gaseous element nitrogen is ≤0.006%, and no additional N is added to reduce the solution strengthening effect of N, which is not conducive to toughness.

[0027] The present invention adopts a lower Ni content to reduce the cost of the steel plate for cryogenic storage tanks, adds a certain amount of Mn, and makes it cooperate 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 manganese embrittlement phenomenon, 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.

[0028] The present invention adopts a composition design of low carbon and appropriate silicon, combined with the cooperation of Mn and Ni elements, to make up for the influence of the reduction of Ni content on low-temperature toughness, enabling the steel plate to maintain good strength and toughness at low temperatures, with a yield strength ≥630 MPa, a tensile strength ≥770 MPa, an elongation ≥22%, a base metal -196°C impact energy Akv ≥160 J, fully meeting the performance standards of 9Ni steel; after welding, the fusion line and HAZ -196°C impact energy Akv ≥55 J, meeting the requirements of the steel plate for cryogenic storage tanks of liquefied gas carriers, meeting the requirements of the steel plate for cryogenic storage tanks of liquefied gas carriers

[0029] The manufacturing method of the cryogenic steel plate for LNG ships according to the present invention includes the following steps:

[0030] 1) Smelting and refining

[0031] 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;

[0032] 2) Slab heating

[0033] Heat the billets or steel billets, with the heating temperature: 1050 - 1120°C;

[0034] 3) Rolling

[0035] The billets or steel billets are rolled through multiple passes to obtain steel plates, with the total rolling reduction rate ≥80% and the finish rolling temperature ≥700°C;

[0036] 4) Heat treatment

[0037] The first quenching, with the quenching temperature 790 - 810°C, the quenching holding time = 1.5×T, the unit of the tempering 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 taken out of the furnace and air-cooled to room temperature;

[0038] The second quenching: The quenching temperature is 680 - 700°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 take it out of the furnace and cool it in water to room temperature;

[0039] Tempering: The tempering temperature is 550 - 570°C, the tempering holding time = 2.0×T, the unit of the quenching holding time is min, T is the thickness of the steel plate, the unit is mm, and the tempering holding time ≥ 30 min. Take it out of the furnace and cool it in water to room temperature.

[0040] In the manufacturing method of the present invention:

[0041] 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.

[0042] Continuous casting or ingot casting: Ensure the uniform internal composition and good surface quality of the ingot. The ingot cast by ingot casting needs to be rolled into a billet.

[0043] Heating and rolling: The continuous casting billet or billet is heated at a temperature of 1050 - 1120°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 700°C, and it is air-cooled after rolling. The final steel plate obtains a fine original acicular ferrite structure.

[0044] The influence of the heat treatment process on the steel plate of the present invention:

[0045] The first quenching: The quenching temperature of the steel plate is controlled at 790 - 810°C. This temperature range is above the complete austenitization temperature A of the composition system of the present invention. Hold the temperature in this range to make the steel plate obtain a fine and uniform original austenitized structure. After quenching, cool it in water to room temperature. During the cooling process, most of the alloying elements are dissolved into the martensite, and the final structure is martensite structure + a small amount of retained austenite. C3 above, hold the temperature in this range to make the steel plate obtain a fine and uniform original austenitized structure. After quenching, cool it in water to room temperature. During the cooling process, most of the alloying elements are dissolved into the martensite, and the final structure is martensite structure + a small amount of retained austenite.

[0046] The second quenching: The re-quenching temperature of the steel plate is controlled at 680 - 700°C. This temperature range is located at A of the composition system of the present invention C1 ~A C3Within the temperature range, heat preservation is carried out within this temperature range to partially re-austenitize the martensite structure obtained during the first quenching process and obtain austenite structure partially transformed from martensite. And an element composition repartition between the two structures is obtained, making the repartition and gradient of C, Ni, and Mn within and near the phase boundaries of the two phases tend to be stable. After the repartition heat preservation is completed, it is then water-cooled to room temperature. The structure obtained after the second quenching is martensite structure and a small amount of more stable retained austenite + inverse austenite structure at low temperature.

[0047] Tempering: The steel plate is tempered at 550 - 570 °C. This temperature range is lower than the AC1 temperature of the composition system of the present invention. The purpose of heating and heat preservation within this temperature range is to remove the internal stress of the structures obtained from the previous two quenching processes, decompose the quenched martensite structure, and make the retained austenite and inverse austenite more stable; after heat preservation for a period of time, it is taken out of the furnace and air-cooled to room temperature. The final structure is tempered martensite + a small amount of retained austenite and inverse austenite.

[0048] The design difficulty of the low-temperature steel of the present invention lies in that in order to reduce costs, the Ni content is reduced, 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.

[0049] Therefore, the present invention adopts two-stage repartition quenching followed by one-stage tempering to refine the original austenite grains, making the retained austenite and inverse austenite divide and refine the martensite structure through the structures obtained from the two-stage repartition quenching, and making them more stable through the subsequent tempering. It still has high mechanical stability and thermal stability at -196 °C. Through the duplex structure after different repartition quenching, the Mn concentration gradient at the phase boundary or grain boundary is significantly 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 partitioned transformation of austenite at low temperature optimizes the TRIP effect of the material, improving the strength-ductility product at low temperature. The thin-film austenite that divides the martensite structure undergoes secondary inversion during the low-temperature repartition process to form and stabilize inverse austenite. During the subsequent low-temperature impact at -196 °C, the mixed structure of retained austenite and inverse austenite has extremely high stability, significantly improving the low-temperature impact toughness and increasing the impact energy. Research shows that in the structure that has only undergone two-stage repartition quenching without tempering, there is a relatively large deviation from the K-S relationship or even a non-coherent relationship at the interface between martensite and austenite, causing relatively large interface residual stress, which serves as the crack origin during subsequent impact deformation, significantly deteriorating the low-temperature impact energy.

[0050] Compared with the prior art, the beneficial effects of the present invention:

[0051] In the composition design of the present invention, low C and appropriate Si are adopted, combined with a 5.5Ni-medium Mn composition system. The cooperation of Mn and Ni is utilized to stabilize retained austenite and inverse austenite, and significantly reduce the concentration gradient of manganese elements at grain boundaries or phase boundaries, reduce the stress concentration at phase interfaces and the number of crack sources during subsequent impact deformation, so as to improve the manganese embrittlement phenomenon, significantly improve the impact toughness of the low-temperature steel plate, and thus ensure the -196 °C low-temperature impact toughness of the steel plate. It is avoided that other precious alloying elements such as Nb, V, and Ti need to be added to make up for the influence of the reduction of Ni content on low-temperature toughness.

[0052] Based on the composition design, the present invention adopts a heat treatment process of two quenches + one tempering. After two quenches, a small amount of retained austenite and inverse austenite that are stable at low temperatures are ensured in the structure, and the element partitioning and stabilization are carried out among various phases, reducing the concentration gradient of Mn at the phase boundary, avoiding the reduction of low-temperature toughness caused by manganese embrittlement, and realizing the perfect combination of the strength and low-temperature toughness of the steel plate.

[0053] The steel plate obtained by the present invention has a thickness of 5 - 50 mm, its yield strength ≥ 630 MPa, tensile strength ≥ 770 MPa, elongation ≥ 22%, the impact energy Akv of the base metal at -196 °C ≥ 160 J, fully meeting the performance standards of 9Ni steel; after welding, the impact energy Akv of the fusion line and HAZ at -196 °C ≥ 55 J, meeting the requirements of the steel plate for low-temperature storage tanks of liquefied gas carriers. Brief Description of the Drawings

[0054] Figure 1 It is a microstructural photograph of the steel plate in Example 1 of the present invention.

[0055] Figure 2 It is a microstructural photograph of the steel plate in Example 2 of the present invention.

[0056] Figure 3 It is a microstructural photograph of the steel plate in Example 3 of the present invention. Detailed Description of the Invention

[0057] The present invention will be further described below in conjunction with examples and drawings.

[0058] The compositions and process parameters of the examples of the present invention are shown in Tables 1 and 2, and the steel properties of each example are shown in Table 3.

[0059] Figure 1 、 Figure 2 and Figure 3 are the typical microstructures of Examples 1, 2, and 3 respectively. As can be seen from the figures, after unique composition design and heat treatment, the steel plates of the examples of the present invention have all obtained a tempered martensite-like microstructure and a mixed microstructure of a small amount of retained austenite and / or inverse austenite as shown in the figures.

[0060] Comparative example 1 has the composition of traditional 9Ni steel. Compared with the present invention, the properties obtained are similar, but the present invention significantly reduces the Ni content and lowers the cost.

[0061] In comparative example 2, the same composition as in example 6 of the present invention is used, but in the process, a heat treatment process of one quenching and one tempering is adopted. Although the strength obtained is similar to that of the present invention, the low-temperature toughness is not as good as that of the present invention.

[0062]

[0063]

[0064]

Claims

1. A cryogenic steel plate for LNG ships, the chemical composition of which is in weight percentage: C: 0.020 - 0.100%, Si: 0.10 - 0.35%, Mn: 1.50 - 4.50%, P ≤ 0.010%, S ≤ 0.005%, Ni: 5.30 - 6.30%, Al: 0.020 - 0.050%, N ≤ 0.006%, Ca ≤ 0.005%, and the balance contains Fe and other inevitable impurities.

2. The cryogenic steel plate for LNG ships according to claim 1, wherein The chemical composition of the steel plate further includes Cr ≤ 1.00%.

3. The cryogenic steel plate for LNG ships according to claim 1 or 2, characterized in that, The balance is Fe and other inevitable impurities.

4. The cryogenic steel plate for LNG ships according to claim 1 or 2 or 3, characterized in that, The microstructure of the steel plate is tempered martensite + a small amount of retained austenite and inverse austenite, and the volume ratio of the retained austenite and inverse austenite ≤ 5%.

5. The cryogenic steel plate for LNG ships 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 ≥ 770 MPa, the elongation ≥ 22%, the impact energy Akv of the base metal at -196 °C ≥ 160 J; the impact energy Akv of the fusion line and HAZ at -196 °C after welding ≥ 55 J.

6. The manufacturing method of the cryogenic steel plate for LNG ships according to any one of claims 1 to 5, characterized in that, It includes the following steps: 1) Smelting and refining Smelt and refine according to the composition described in Claim 1 or 2 or 3, continuously cast into billets or ingots and then hot-rolled into steel billets after blooming. 2) Slab heating Heat the billets or steel billets, the heating temperature: 1050 - 1120 °C; 3) Rolling The billets or steel billets are rolled through multiple passes to obtain the steel plate, the total reduction ratio of rolling ≥ 80%, and the finishing rolling temperature ≥ 700 °C; 4) Heat treatment The first quenching, the quenching temperature is 790 - 810 °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 take it out of the furnace and air-cool to room temperature; The second quenching, the quenching temperature is 680 - 700 °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 take it out of the furnace and water-cool to room temperature; Tempering, the tempering temperature is 550 - 570 °C, the tempering holding time = 2.0 × T, the unit of the quenching holding time is min, T is the thickness of the steel plate, the unit is mm, and the tempering holding time ≥ 30 min, take it out of the furnace and water-cool to room temperature.

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

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