A high-nitrogen low-nickel steel plate for a liquefied carbon dioxide liquid cargo tank with low yield ratio and a manufacturing method thereof

Abstract

This invention discloses a high-nitrogen, low-nickel steel plate for liquefied carbon dioxide cargo tanks with a low yield strength ratio and its manufacturing method, belonging to the field of metal material manufacturing technology. The chemical composition of the steel plate disclosed in this invention, by weight percentage, is as follows: C: 0.12wt%~0.18wt%, Si: 0.02wt%~0.04wt%, Mn: 1.3wt%~1.8wt%, Ni: 1.2wt%~3wt%, V: 0.07wt%~0.12wt%, Ti: 0.04wt%~0.08wt%, S: ≤0.002wt%, P: ≤0.005wt%, N: 0.010wt%~0.018wt%, with the balance being Fe and unavoidable impurity elements, wherein the V / N ratio is 3.8~12. The design employs a low-alloy composition, combining the effects of V and N to achieve both second-phase particle precipitation strengthening and grain refinement. By optimizing the V / N ratio, the best effect of V precipitation is achieved, thereby improving the tensile strength and toughness of the steel plate in the heat-treated state and reducing the yield strength ratio.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of metal material manufacturing, and particularly relates to a high-nitrogen low-nickel steel plate for a low-yield ratio liquefied carbon dioxide liquid cargo tank and a manufacturing method thereof. BACKGROUND

[0002] Since 2016, carbon dioxide capture, utilization and storage has become an important way for global exploration, and liquefied carbon dioxide transport ships are the key infrastructure. In order to meet the design requirements of the liquefied carbon dioxide transport ship, the ultra-high strength low-temperature steel for the liquefied carbon dioxide transport ship is required to be very strict.

[0003] Chinese patent CN 115786806 A discloses a high-strength low-carbon equivalent super-thick steel plate with good low-temperature toughness and a manufacturing method thereof. The method is to add Nb, V and Ti and other strengthening alloy elements to ensure the strength of the steel plate, and add Ni to ensure the low-temperature toughness of the steel plate. A plurality of steel blanks are used as base blanks, the surface of the base blank is milled and ground to remove defects, and then the composite blank is welded after the surface of the base blank is milled and ground to remove defects. The steel plate has a structure of ferrite + pearlite + bainite, which is obtained by austenite recrystallization zone rolling and normalizing fast cooling + tempering treatment. However, although examples 1-4 meet the requirements of the carbon equivalent of P690QL2 steel plate, the strength of the steel plate in the examples is less than 500 MPa, which does not meet the use requirements of the ultra-high strength steel for the liquefied carbon dioxide liquid cargo tank.

[0004] Chinese patent CN 109988968 B discloses a low-carbon equivalent large-thickness Q690E grade high-strength steel plate and a production method thereof. The impact of the steel plate at-40 DEG C is greater than 100 J, but the impact toughness of the steel plate at-60 DEG C is not reflected in the specification, which cannot meet the safety and actual supply requirements of engineering use, and limits the overall application of the steel plate in industrial production.

[0005] The Chinese invention patent CN 118581404 A discloses a low carbon equivalent ultra-high strength 1.5Ni steel plate and a preparation method thereof. The chemical composition of the steel plate is as follows in terms of percentage by weight: C: 0.09% to 0.16%, Si: 0.04% to 0.1%, Mn: 1.0% to 1.5%, Ni: 1.5% to 2.5%, Cr: 0.3% to 0.7%, Mo: 0.2% to 0.4%, V: 0.02% to 0.05%, Ti: 0.010% to 0.015%, S≤0.002%, P≤0.005%, Als: 0.04% to 0.06%, and the balance of Fe and inevitable impurity elements. In addition, the carbon equivalent Ceq is less than 0.60%, and CEV=C+Mn / 6+(Cr+Mo+V) / 5+(Cu+Ni) / 15. The steel plate has a yield strength ratio exceeding 0.9, and is prone to brittle fracture in actual application, and is not suitable for the construction of liquefied carbon dioxide liquid cargo tanks in low temperature environments.

[0006] The DNV ship rules do not have a carbon equivalent limit for ultra-high strength steel. When designing a liquid cargo transport ship, the shipyard considers that the tank body for building a liquid cargo tank needs to be safely served and operated in marine conditions, and proposes a low carbon equivalent requirement for the steel plate. Therefore, from the perspective of engineering application safety, in order to meet the construction requirements, the purpose of the present application is to provide a quenched and tempered steel plate with vanadium and nitrogen strengthening, which meets the design and application of ultra-high strength, contains about 1.5% Ni element, can meet the low temperature impact toughness, and further adds a certain amount of V micro-alloying element to form vanadium carbonitride and vanadium carbide precipitates with N in the steel to improve the strength of the steel plate. The combination of various chemical components is reasonable, the steel plate can be welded, and meets the processing and use requirements of pressure equipment.

[0007] The paper on ultra-high strength and toughness steel for liquefied carbon dioxide liquid cargo tanks also relates to the rolling process, heat treatment process, microstructure and welding process, and strives to better meet the technical requirements of P690QL2 steel in EN10028-6, but cannot meet the high standard requirements of engineering application, and no steel plant in China has successfully developed this steel grade with low alloy content system. SUMMARY

[0008] In view of this, the purpose of the present application is to provide a high-nitrogen and low-nickel steel plate for liquefied carbon dioxide liquid cargo tanks with a low yield strength ratio, a yield strength of ≥770 MPa, a tensile strength of ≥800 MPa, and a KV2 of ≥150 J at -60℃. The microstructure of the steel plate is a tempered sorbite structure composed of TiN, VN, V(C, N) precipitates, polygonal ferrite and granular cementite.

[0009] The present application designs a kind of high-nitrogen low-nickel steel plate for liquefied carbon dioxide liquid cargo tank with low yield ratio, compared with existing composition system, without adding Cr, Mo and other alloy elements, combined with two-stage controlled rolling process, without controlling the reduction rate of the last pass of finish rolling, the effect of rolling force on the yield strength of steel plate is reduced, using quenching + tempering heat treatment process, instead of traditional two-phase zone quenching process, under the strict restriction of carbon equivalent, both the tensile strength of steel plate is improved, and excellent strength and toughness are obtained, and the yield ratio of steel plate is reduced, heat treatment reduces a two-phase zone quenching process, which is economical, and specific chemical composition, controlled rolling process, heat treatment process and temperature control range are designed.The physical level of the steel plate meets the requirements of engineering application, and has been successfully applied to the world's first carbon dioxide transport ship liquid cargo tank built by a domestic shipyard.

[0010] In view of the technical characteristics of the strength and toughness of the steel plate during the construction and service of the existing liquefied carbon dioxide liquid cargo tank, the present application uses VN alloy composition design for 10-80mm thickness 1.5Ni steel plate, uses continuous casting process for steelmaking, produces 250-300mm section continuous casting billet, uses 5500 wide and thick plate rolling mill for high temperature hot rolling, and then carries out quenching + high temperature tempering heat treatment process to obtain 1.5Ni steel plate.

[0011] In order to achieve the above purpose, the present application provides the following technical scheme:

[0012] According to the first aspect of the present application, a high-nitrogen low-nickel steel plate for liquefied carbon dioxide liquid cargo tank with low yield ratio is provided, and the chemical composition is as follows: C: 0.12wt%-0.18wt%, Si: 0.02wt%-0.04wt%, Mn: 1.3wt%-1.8wt%, Ni: 1.2wt%-3wt%, V: 0.07wt%-0.12wt%, Ti: 0.04wt%-0.08wt%, S: ≤0.002wt%, P: ≤0.005wt%, N: 0.010wt%-0.018wt%, and the balance is Fe and unavoidable impurities, wherein the V / N ratio is 3.8-12.

[0013] The effects of each chemical composition of the high-nitrogen low-nickel steel plate for liquefied carbon dioxide liquid cargo tank with low yield ratio are as follows:

[0014] Carbon: in the present application, C is used as a strengthening element for interstitial solid solution to improve the strength and hardness of the steel plate, and too high C content will reduce the corrosion resistance and weldability of the steel, and too high C will affect the low temperature performance of the steel.In the present application, due to the addition of appropriate amount of vanadium element, vanadium carbide and carbonitride will be generated at grain boundary, as a strengthening phase, appropriate reduction of C content can obtain steel plate with good combination of strength and toughness, therefore, the C content in the present application is controlled to be 0.12wt%-0.18wt%.

[0015] Silicon: Si can improve the yield strength of steel and has a strong deoxidizing power. Si is a strong ferrite forming element, which can effectively inhibit the precipitation of cementite and prevent the diffusion of C, thereby slowing down the decomposition of martensite and the growth rate of carbide aggregation. This makes the hardness of steel decrease more slowly during tempering, and significantly improves the tempering stability and strength of steel. However, excessive Si content will reduce the weldability of steel. Therefore, this invention controls its content at 0.02wt%~0.04wt%.

[0016] Manganese (Mn) is a good deoxidizer and desulfurizer. Adding a certain amount of manganese to steel can eliminate or reduce the hot brittleness of steel caused by sulfur. However, when too much Mn is added to steel, it will coarsen the grains, increase the carbon equivalent, and thus affect the weldability of the steel, and cause temper brittleness. Therefore, this invention selects an Mn content of 1.3wt%~1.8wt%.

[0017] Nickel: Ni is an effective element that enables steel to obtain excellent strength and low-temperature toughness. Ni is one of the elements that infinitely expands the austenite region. Therefore, after tempering, nickel-containing steel can obtain a fully refined tempered sorbite structure, and the steel has a good balance of strength and toughness. However, nickel is a scarce resource and expensive. Therefore, the Ni content is controlled at 1.2wt%~3wt%.

[0018] Sulfur: S easily forms FeS and MnS inclusions in steel, causing hot brittleness and significantly reducing the toughness of steel. Therefore, the sulfur content in steel should be reduced as much as possible.

[0019] Phosphorus: P often segregates at grain boundaries in steel, disrupting the continuity of the matrix, significantly reducing the toughness of steel, worsening weldability, and making it prone to cold brittleness. Therefore, the P content in steel should be reduced as much as possible.

[0020] Vanadium (V) has a strong affinity for both carbon (C) and nitrogen (N), forming stable compounds with them. In steel, V primarily exists as carbides, its main function being to refine the steel's microstructure and grain size, increasing its tensile strength and toughness. When dissolved in a solid solution at high temperatures, it produces solid solution strengthening, enhancing the steel's fracture resistance, i.e., tensile strength. V increases the tempering stability of quenched steel and produces a secondary hardening effect. Therefore, this invention controls the V content to be 0.07wt%~0.12wt%.

[0021] Titanium (Ti) primarily functions as a grain refiner and strengthener. Ti has a strong affinity for both N and C, making it an excellent deoxidizer and effective element for fixing C and N. The resulting TiC structure is strongly bonded, resistant to decomposition, and effectively inhibits grain growth. TiN acts as a matrix for VN precipitates within austenite, making it a more stable compound.

[0022] Nitrogen: When combined with elements such as Ti and V, N can reduce their adverse effects. Adding N to steel in a certain proportion with V can promote the precipitation of carbonitrides of V, improve the strength of steel, and can be used as an alloying element for low alloy steel. It can also effectively reduce costs. Therefore, the present invention controls the N content to be 0.009wt%~0.022wt%.

[0023] V / N ratio: Control the V / N ratio to 3.8~12.

[0024] Based on the above technical solution, the steel plate has a carbon equivalent Ceq < 0.60%, a yield strength ≥ 770 MPa, a tensile strength ≥ 800 MPa, a KV2 ≥ 150 J at -60℃, and a yield strength ratio ≤ 0.9.

[0025] The thickness of the steel plate is 10~80mm;

[0026] The microstructure of the steel plate is a tempered sorbite structure composed of titanium nitride, vanadium nitride, vanadium carbide, vanadium carbonitride precipitates, and polygonal ferrite.

[0027] According to a second aspect of the present invention, a method for manufacturing high-nitrogen, low-nickel steel plates for liquefied carbon dioxide cargo tanks with low yield strength ratio is provided, comprising the following process: converter smelting - LF refining - RH refining - continuous casting - slab slow cooling and billet surface treatment - heating - rolling - flaw detection - heat treatment - packaging and warehousing; wherein the rolling process specifically comprises: removing iron oxide scale with high-pressure water before rolling, the continuous casting billet adopting two-stage controlled rolling, the single-pass reduction rate of the last three passes in the finishing rolling stage being less than 22%, the initial rolling temperature of the second stage being 1020~1080℃, and the final rolling temperature being 990~1030℃;

[0028] The heat treatment includes quenching heat treatment and tempering heat treatment. Specifically, the quenching heat treatment involves offline quenching of the rolled steel plate at a quenching temperature A. C3 + (30~50℃), the quenching temperature is specifically 870~890℃, and the holding time is 5~6.5min / mm;

[0029] The specific tempering heat treatment process is as follows: the quenched steel plate is tempered at a tempering temperature of AC1- (30~50℃), specifically 570~580℃, and the holding time is 7~9min / mm.

[0030] Based on the above technical solution, the converter smelting specifically involves: using an oxygen top-blown converter for smelting, ensuring carbon and temperature coordination during the blowing process, and guaranteeing that the P content is ≤0.005 and the S content is ≤0.003.

[0031] Based on the above technical solution, the RH refining specifically involves: degassing the molten steel in an RH vacuum furnace, ensuring that the holding time of the RH furnace is 15~20 min, and ensuring that [H] ≤ 1.5 ppm and [O] ≤ 15 ppm.

[0032] Based on the above technical solution, the slab slow cooling and billet surface treatment are as follows: after the continuous casting billet with a cross-sectional size of 250~300mm is taken off the production line, it is stacked in the slow cooling pit for slow cooling, so that the gas in the billet can be fully diffused and discharged, and the gas content of the billet is reduced to the greatest extent. The slow cooling time is ≥50 hours, and the billet surface treatment is shot blasting or grinding.

[0033] Based on the above technical solution, the heating process is as follows: the billet is fed into a walking beam furnace with an average heating rate of 9~10 min / cm. To ensure high-temperature rolling, it is heated to 1200~1250℃. When the core temperature reaches the surface temperature, the heating is started and the holding time is not less than 1 hour.

[0034] Based on the above technical solution, the microstructure of the high-nitrogen, low-nickel steel plate for the low yield strength ratio liquefied carbon dioxide cargo tank is tempered sorbite + VN precipitation + TIN precipitation, wherein the ferrite content in the tempered sorbite is 72-80%.

[0035] In this invention, the smelting of ultra-high strength and toughness low-nickel steel for liquefied carbon dioxide cargo tanks adopts converter + LF + RH refining, which reduces inclusions and ensures precise control over steel composition and gas content. During continuous casting, the entire pouring and slab slow cooling process is protected, and the top, bottom, and sides of the stacked, slowly cooled slabs are protected with hot slabs to ensure the internal quality of the slabs. After mechanical grinding, the slab surface must be free of any defects, and the edges of the slabs must be chamfered using angle grinders or other tools. Controlled rolling ensures that the final rolling temperature is within the recrystallization zone, aiming to obtain a finer austenitic grain structure to refine the grain structure and improve the strength and toughness of the steel. Post-rolling heat treatment employs a quenching + tempering process to ensure the strength and toughness of the steel plate.

[0036] Compared with the prior art, the present invention has the following beneficial effects:

[0037] (1) This invention employs a low-alloy composition design, combining the effects of V and N to achieve both second-phase particle precipitation strengthening and grain refinement strengthening. Through the optimal V / N ratio, the best effect of V precipitation is achieved, improving the tensile strength and toughness of the steel plate in the heat-treated state, thereby reducing the yield strength ratio. This invention uses Ni alloy toughening design to improve the low-temperature toughness of the steel plate.

[0038] (2) The finished low-nickel steel plate for liquefied carbon dioxide cargo tanks manufactured using this technical solution has a thickness of 10~80mm, a yield strength ≥770MPa, a tensile strength ≥800MPa, and a V-shaped impact energy ≥100J at -60℃.

[0039] (3) The present invention employs a series of methods such as shot blasting of the upper and lower surfaces of the billet, controlling heating during the heating process, high-pressure water descaling, and high-temperature rolling to ensure that the steel plate has good surface quality.

[0040] (4) The heat treatment process selected in this design is quenching + tempering. The quenching holding time is extended to promote the solid solution of V. The purpose is to obtain higher strength for the steel plate after quenching. The quenching temperature is set to A. c3 + (30~50℃) To ensure that the austenite grains are not completely coarsened, tempering is used to reduce the internal stress generated by quenching and improve toughness and microstructure stability; this invention uses A C1 Tempering is performed at a temperature of 30~50℃, and the holding time is extended to ensure the effective precipitation of alloying elements in the steel plate, increase the ferrite content in the tempered sorbite structure, and improve the toughness of the steel plate. Attached Figure Description

[0041] To more clearly illustrate the embodiments of the present invention, the accompanying drawings involved in the embodiments will be briefly described below.

[0042] Figure 1 The image shows the microstructure of the steel plate manufactured in Example 1 after being etched with a 4% nitric acid alcohol solution under an optical microscope. Detailed Implementation

[0043] The present invention will be described in detail below with reference to the embodiments. However, the implementation of the present invention is not limited thereto. Obviously, the embodiments described below are only some embodiments of the present invention. For those skilled in the art, other similar embodiments can be obtained without creative effort and all fall within the protection scope of the present invention.

[0044] The present invention provides a high-nitrogen, low-nickel steel plate for liquefied carbon dioxide cargo tanks with a low yield strength ratio in the specific embodiments section. The chemical composition of the steel plate, by weight percentage, is as follows: C: 0.12wt%~0.18wt%, Si: 0.02wt%~0.04wt%, Mn: 1.3wt%~1.8wt%, Ni: 1.2wt%~3wt%, V: 0.07wt%~0.12wt%, Ti: 0.04wt%~0.08wt%, S: ≤0.002wt%, P: ≤0.005wt%, N: 0.010wt%~0.018wt%, with the balance being Fe and unavoidable impurity elements.

[0045] In this invention, the high-nitrogen, low-nickel steel is smelted using a converter + LF + RH refining process, which reduces inclusions and ensures precise control over the steel composition and gas content. During continuous casting, the entire pouring and slab slow cooling process is protected, with hot slabs used to protect the top, bottom, and sides of the stacked, slowly cooled slabs, ensuring the internal quality of the cast slab. After mechanical grinding, the slab surface must be free of any defects, and the edges must be chamfered using angle grinders or other tools. Controlled rolling ensures the final rolling temperature is within the non-recrystallization zone, aiming to obtain a finer ferrite grain structure to refine the grain structure and improve the steel's strength and toughness. The final pass of finishing rolling does not control the deformation amount to reduce the impact of rolling force on the yield strength of the steel plate. Post-rolling heat treatment employs a quenching + tempering process to ensure the steel plate's strength and toughness.

[0046] The present invention also provides a method for manufacturing high-nitrogen, low-nickel steel plates for liquefied carbon dioxide cargo tanks with low yield strength ratios in the specific embodiments section. The specific process is as follows:

[0047] (1) Converter + LF + RH refining: The LF furnace produces reducing slag for desulfurization, reduces inclusions, and adjusts the composition. Then, the molten steel is degassed in the RH vacuum furnace, ensuring that the holding time in the RH furnace is 15~20min. The H and O contents are measured to ensure that [H]≤1.5ppm and [O]≤15ppm;

[0048] (2) Continuous casting: The vibration parameters of the continuous casting crystallizer are selected as high frequency and small amplitude. The target superheat of the tundish is ≤30℃. The entire process is protected during casting to reduce secondary oxidation during continuous casting, reduce the content of inclusions in the steel, and improve the purity of the steel, so as to ensure that the segregation at the center of the billet is not higher than B0.5 grade.

[0049] (3) Slow cooling and shot blasting process for slabs: After the continuous casting slabs with a cross-sectional size of 250~300mm are delivered, they are stacked in the slow cooling pit for slow cooling, so that the gas in the slab can be fully diffused and discharged, and the gas content of the slab can be reduced to the greatest extent. The slow cooling time is ≥50 hours, and the surface of the slab is cleaned by shot blasting or grinding.

[0050] (4) Heating process: The billet is fed into a walking beam furnace with an average heating rate of 9~10 min / cm. To ensure high-temperature rolling, it is heated to 1200~1250℃. When the core temperature reaches the surface temperature, the heat is held for at least 1 hour to ensure that the alloying elements in the steel are fully dissolved to ensure the uniformity of the composition and properties of the final product.

[0051] (5) Rolling: Before rolling, the iron oxide scale is removed by high-pressure water. The steel ingot adopts two-stage controlled rolling. After the second stage, the single-pass reduction rate of the three passes is ≤22%. The starting rolling temperature of the second stage is 1050±30℃, and the finishing rolling temperature is 1000±30℃. For the first stage rolling which belongs to the recrystallization zone, recrystallization and deformation are carried out alternately to fully refine the austenite grains. For the recrystallization zone rolling at around 1000℃, the influence of alloying elements on recrystallization is minimized, and the deformed austenite can be repeatedly recrystallized during multi-pass deformation to gradually refine the austenite and finally obtain a very fine ferrite grain structure. This is because as the temperature increases, more V can precipitate in the ferrite to produce precipitation strengthening. Controlling the final pass of the finishing rolling (two-stage) rolling is to reduce the influence of rolling force on the yield strength of the steel plate.

[0052] (6) Quenching heat treatment process: After rolling, the steel plate undergoes offline quenching treatment. Alloying elements such as Mn, Ni, and V in the steel can increase the hardenability of the thick plate. The ferrite-austenite equilibrium phase transformation initiation temperature A of the steel of this invention was determined. C1 The equilibrium phase transformation temperature A of ferrite and austenite is 614℃. C3 The quenching temperature is 847℃, and the quenching temperature A is... C3 + (30~50℃), the preferred quenching temperature is 880±10℃, and the holding time is 5~6.5min / mm. The purpose is to make the microstructure uniformly austenitized and the austenite grain size uniform.

[0053] (7) Tempering heat treatment process: After quenching, the steel plate is tempered at temperature A. C1 - (30~50℃), preferably tempering temperature 570~580℃, holding time 7~9min / mm, give enough time to allow carbon and nitrogen in quenched martensite to diffuse fully with other alloying elements, air cool after tempering. The purpose of tempering is to cause the alloying elements to rearrange and generate new element enrichment areas and alloy carbides.

[0054] Example

[0055] The production process of the high-nitrogen, low-nickel steel plate for liquefied carbon dioxide cargo tanks with low yield strength ratio of the present invention is as follows: molten iron pretreatment - converter smelting - ladle refining (LF+RH) - continuous casting - slow cooling in stacks - billet surface treatment - heating - rolling - flaw detection - heat treatment - packaging and warehousing. Table 1 shows the composition involved in each embodiment, Table 2 shows the billet heating, rolling, and cooling processes of the embodiments, Table 3 shows the heat treatment process of the embodiments, and Table 4 shows the comprehensive mechanical properties and microstructure of the embodiments. The microstructure of the steel plate manufactured in Example 1 after etching with a 4% nitric acid alcohol solution under an optical microscope is shown in the figure below. Figure 1 As shown, the microstructure of the steel plate consists of tempered sorbite + VN precipitation + TIN precipitation.

[0056] Table 1. Chemical composition and mass percentage (wt%) of the steel plates in Examples 1-4

[0057]

[0058] Table 2. Slab heating, rolling, and cooling processes in Examples 1-4

[0059]

[0060] Table 3 Heat treatment processes of Examples 1-4

[0061]

[0062] Table 4. Mechanical properties and microstructure of low-nickel steel plates for low yield strength ratio liquefied carbon dioxide cargo tanks described in Examples 1-4

[0063]

[0064] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A high-nitrogen, low-nickel steel plate for liquefied carbon dioxide cargo tanks with a low yield strength ratio, characterized in that, The chemical composition of the steel plate, by weight percentage, is as follows: C: 0.12wt%~0.18wt%, Si: 0.02wt%~0.04wt%, Mn: 1.3wt%~1.8wt%, Ni: 1.2wt%~3wt%, V: 0.07wt%~0.12wt%, Ti: 0.04wt%~0.08wt%, S: ≤0.002wt%, P: ≤0.005wt%, N: 0.010wt%~0.018wt%, with the balance being Fe and unavoidable impurity elements, wherein the V / N ratio is 3.8~12.

2. The high-nitrogen, low-nickel steel plate for liquefied carbon dioxide cargo tanks with low yield strength ratio according to claim 1, characterized in that, The steel plate has a carbon equivalent Ceq < 0.60%, a yield strength ≥ 770 MPa, a tensile strength ≥ 800 MPa, a KV2 ≥ 150 J at -60℃, and a yield strength ratio ≤ 0.90; the thickness of the steel plate is 10~80 mm. The microstructure of the steel plate is a tempered sorbite structure composed of titanium nitride, vanadium nitride, vanadium carbide, vanadium carbonitride precipitates, and polygonal equiaxed ferrite + granular cementite.

3. A method for manufacturing a high-nitrogen, low-nickel steel plate for a low yield strength ratio liquefied carbon dioxide cargo tank as described in any one of claims 1-2, characterized in that, The process includes the following steps: converter smelting - LF refining - RH refining - continuous casting - slab slow cooling and billet surface treatment - heating - rolling - flaw detection - heat treatment - packaging and warehousing; the rolling process specifically involves: removing iron oxide scale with high-pressure water before rolling, and using two-stage controlled rolling for the continuously cast billet. In the finishing rolling stage, the single-pass reduction rate of the last three passes is less than 22%, the initial rolling temperature of the second stage is 1020~1080℃, and the final rolling temperature is 990~1030℃. The heat treatment includes quenching heat treatment and tempering heat treatment. Specifically, the quenching heat treatment involves offline quenching of the rolled steel plate at a quenching temperature A. C3 + (30~50℃), heat preservation time is 5~6.5min / mm; The specific tempering heat treatment process is as follows: the quenched steel plate is tempered at a tempering temperature of AC1 (30~50℃) and a holding time of 7~9 min / mm.

4. The method for manufacturing high-nitrogen, low-nickel steel plate for liquefied carbon dioxide cargo tanks with low yield strength ratio according to claim 3, characterized in that, The converter smelting process specifically involves using an oxygen top-blown converter, ensuring carbon and temperature coordination during the blowing process to guarantee that the P content is ≤0.005% and the S content is ≤0.003%.

5. The method for manufacturing high-nitrogen, low-nickel steel plate for liquefied carbon dioxide cargo tanks with low yield strength ratio according to claim 3, characterized in that, The RH refining process specifically involves degassing the molten steel in an RH vacuum furnace, ensuring that the holding time in the RH furnace is 15-20 minutes, and ensuring that [H] ≤ 1.5 ppm and [O] ≤ 15 ppm.

6. The method for manufacturing high-nitrogen, low-nickel steel plate for liquefied carbon dioxide cargo tanks with low yield strength ratio according to claim 3, characterized in that, The slab slow cooling and billet surface treatment are as follows: After the continuous casting billet with a cross-sectional size of 250~300mm is taken off the production line, it is stacked in the slow cooling pit for slow cooling for ≥50 hours. The billet surface treatment is shot blasting or grinding.

7. The method for manufacturing high-nitrogen, low-nickel steel plate for liquefied carbon dioxide cargo tanks with low yield strength ratio according to claim 3, characterized in that, The heating process involves feeding the billet into a walking beam furnace at an average heating rate of 9-10 min / cm. To ensure high-temperature rolling, the billet is heated to 1200-1250°C. Once the core temperature reaches the surface temperature, the billet is held for at least 1 hour.

8. The method for manufacturing high-nitrogen, low-nickel steel plate for liquefied carbon dioxide cargo tanks with low yield strength ratio according to claim 3, characterized in that, The quenching temperature in the quenching heat treatment process is 870~890℃. The tempering temperature in the tempering heat treatment process is 570~580℃.

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