5Ni steel for liquid ammonia storage tank and manufacturing method of 5Ni steel

Abstract

The invention relates to the technical field of steel for liquid ammonia storage tanks, in particular to 5Ni steel for a liquid ammonia storage tank and a manufacturing method of the 5Ni steel. The invention discloses a high-strength and high-toughness steel which is composed of the following chemical components in percentage by weight: 0.01%-0.12% of C, 0.10%-0.30% of Si, 0.30%-0.80% of Mn, 4.75%-4.99% of Ni, 0.01%-0.10% of Mo, less than or equal to 0.005% of S, less than or equal to 0.008% of P and the balance of Fe and inevitable impurities. Converter + LF + VD refining is adopted for smelting, and accurate control over steel components and the gas content in steel is guaranteed; casting and slab slow cooling are protected in the whole continuous casting process, and the internal quality of a casting blank is guaranteed; refining of an initial structure is guaranteed through controlled rolling; and after-rolling heat treatment adopts a process of offline quenching, two-phase region quenching and tempering, so that the low-temperature toughness of the steel plate is improved, and the yield ratio is effectively reduced. The yield strength is larger than or equal to 390 MPa, the tensile strength is 530-710 MPa, the-120 DEG C V-type impact energy is larger than or equal to 100 J, and the side expansion is larger than or equal to 1.0. The nickel-saving type design is adopted, the Ni content is smaller than 5%, the requirement for stress corrosion prevention of the liquid ammonia storage tank is met, and various mechanical properties are excellent.

Description

Technical Field

[0001] The present invention relates to the technical field of steel for liquid ammonia storage tanks, and in particular to 5Ni steel for liquid ammonia storage tanks and a manufacturing method thereof. Background Art

[0002] Among many alternative fuels such as liquid ammonia, hydrogen and methanol, liquid ammonia, as an environmentally friendly fuel, produces water and nitrogen when completely burned, does not produce pollutants and carbon dioxide, and is easy to liquefy, store and transport, becoming the focus of the shipping industry.

[0003] During the bunkering, supply, and fuel tank maintenance processes, liquid ammonia is contaminated by air. This causes tensile stress and residual weld stress in fuel tank / cargo tank welds due to operating pressure. Under tensile stress, steel is susceptible to stress corrosion cracking in an air-contaminated liquid ammonia environment. O₂, CO₂, and N₂ in the air all promote liquid ammonia corrosion of fuel tank / cargo tank materials. Because liquid ammonia can cause stress corrosion cracking in containers made of carbon-manganese steel or nickel steel, to minimize this risk, the IGC Code and classification society specifications such as DNV, BV, and CCS require that the nickel content of 5Ni steel used in liquid ammonia fuel tanks / cargo tanks must be less than 5%.

[0004] In order to improve utilization efficiency, users hope that transport ships have multifunctionality. Liquid ammonia transport ships built with 5Ni steel with a Ni content of less than 5% can be compatible with the transportation of liquefied ethylene, achieving multiple uses of one ship.

[0005] Chinese patent document with publication number CN105331890A discloses “a method for producing high-toughness 5Ni steel medium and thick plates by online quenching”, Chinese patent document with publication number CN104388838B discloses “a 5Ni steel plate for low-temperature pressure vessels and a production method thereof”, Chinese patent document with publication number CN105331890A discloses “a method for producing high-toughness 5Ni steel medium and thick plates by online quenching”, Chinese patent document with publication number CN114686649A discloses “a method for producing high-toughness 5Ni steel medium and thick plates by online quenching”, and Chinese patent document with publication number CN110863135A discloses “a high-nickel steel for low-temperature containers and a manufacturing method thereof”. Although the performance of the 5Ni steel manufactured by the above method meets the requirements for 5Ni steel in EN10028-4, its Ni weight percentage content is 5.25-6%, which cannot meet the requirement of Ni weight percentage less than 5%. Summary of the Invention

[0006] In order to overcome the shortcomings of the existing technology, the present invention provides a 5Ni steel for liquid ammonia storage tanks and a manufacturing method thereof. The present invention adopts a nickel-saving design with a Ni content of less than 5%, which meets the requirements for preventing stress corrosion of liquid ammonia storage tanks and has excellent mechanical properties.

[0007] In order to achieve the above object, the present invention adopts the following technical solutions:

[0008] A 5Ni steel for liquid ammonia storage tanks is composed of the following chemical components in percentage by weight:

[0009] C: 0.01% to 0.12%, Si: 0.10% to 0.30%, Mn: 0.30% to 0.80%, Ni: 4.75% to 4.99%, Mo: 0.01% to 0.10%, S: ≤0.005%, P: ≤0.008%, and the balance is Fe and unavoidable impurities.

[0010] In the composition design of the present invention:

[0011] (1) Carbon: Carbon is the most effective chemical element to improve the strength of steel, but at the same time, carbon will greatly reduce the toughness of steel and destroy the welding performance of steel. After comprehensive consideration, it is suitable for high nickel steel for low temperature containers to control the carbon content at 0.01% to 0.12%.

[0012] (2) Silicon: Silicon can improve the strength of steel, but is detrimental to the toughness of steel. Its content is controlled at 0.10% to 0.30%;

[0013] (3) Manganese: Manganese can improve the strength and toughness of steel, but when the manganese content is too high, it will promote grain growth and produce temper brittleness. The manganese content should be controlled between 0.30% and 0.80%;

[0014] (4) Nickel: Nickel can improve the strength of steel and enable steel to obtain excellent low-temperature toughness. Nickel is one of the elements that can infinitely expand the austenite zone. Therefore, after quenching and tempering, high-nickel steel can obtain a completely refined tempered martensite structure, and the strength and toughness of the steel are well matched. However, in order to prevent stress corrosion, the Ni content needs to be less than 5%. Therefore, the nickel content should be controlled at 4.75% to 4.99%;

[0015] (5) Molybdenum: Molybdenum can refine the grain size of steel, significantly improve the hardenability of steel, and inhibit the temper brittleness of steel, thereby increasing the strength of steel and improving the toughness of steel. However, the international standard has an upper limit on the amount of Mo added to 5Ni steel. Therefore, the molybdenum content should be controlled at 0.01% to 0.10%;

[0016] (6) Sulfur: Sulfur easily forms FeS and MnS inclusions in steel, causing hot brittleness and significantly reducing the toughness of the steel. Therefore, the sulfur content in steel should be reduced as much as possible;

[0017] (7) Phosphorus: Phosphorus often concentrates at the grain boundaries in steel, destroying the continuity of the matrix, significantly reducing the toughness of the steel, deteriorating the welding performance, and easily causing cold brittleness. Therefore, the phosphorus content in steel should be reduced as much as possible.

[0018] The liquid ammonia storage tank is made of 5Ni steel, with a yield strength of ≥390MPa, a tensile strength of 530-710MPa, a V-type impact energy of ≥100J at -120°C, and a lateral expansion of ≥1.0.

[0019] The manufacturing method of the 5Ni steel for the liquid ammonia storage tank specifically comprises the following steps:

[0020] 1) Converter + LF + VD refining:

[0021] During converter smelting, the amount of Ni added is controlled at 4.75% to 4.80%. The precise control of the final Ni content is completed in the LF furnace, and the Ni content is strictly controlled to be less than 5%.

[0022] 2) Continuous casting:

[0023] Control the continuous casting slab casting superheat at 10-20℃ and the slab pulling speed <1m / min;

[0024] 3) Slab slow cooling:

[0025] Slow cooling time ≥48 hours;

[0026] 4) Rolling:

[0027] The steel ingot adopts two-stage controlled rolling, the first stage rolling temperature is ≥1050℃, the second stage rolling temperature is ≥850℃, the final rolling temperature is 800~840℃, and air cooling is performed after rolling;

[0028] 5) Quenching + two-phase quenching + tempering heat treatment:

[0029] The room temperature steel plate is sent into the heating furnace, kept at 890-920℃ for 2-4min / mm, and quenched; kept at 710-750℃ for 2-4min / mm, and quenched; kept at 610-650℃ for 4-6min / mm, and air-cooled after tempering.

[0030] Furthermore, in step 1), the molten steel is degassed in a VD vacuum furnace, and the pressure holding time of the VD furnace is 15 to 20 minutes.

[0031] Furthermore, in step 1), the gas content in the steel is precisely controlled by the VD process, and [H] is controlled to be ≤ 2 ppm and [O] ≤ 20 ppm.

[0032] Furthermore, in step 2), the casting is protected throughout the entire process, thereby reducing secondary oxidation during the continuous casting process, lowering the inclusion content in the steel, and improving the purity of the steel.

[0033] Furthermore, in step 2), the ratio of the thickness of the continuous casting slab to the thickness of the finished steel plate is ≥6.

[0034] Furthermore, in step 3), the continuous casting billet is put into a slow cooling pit for slow cooling.

[0035] Compared with the prior art, the present invention has the following beneficial effects:

[0036] 1. The Ni content of the present invention is less than 5%. A certain amount of Mo is added to the steel. The addition of Mo element makes up for the reduction of Ni content, improves the hardenability of the steel, and increases the strength of the steel.

[0037] 2. The yield strength of the present invention is ≥390MPa, the tensile strength is 530-710MPa, the elongation is ≥20%, the -120℃ V-type impact energy is ≥100J, and the lateral expansion is ≥1.0; all the properties are excellent and meet the user's requirements for 5Ni steel.

[0038] 3. This invention utilizes a converter + LF + VD refining process. The LF furnace ensures precise control of the final Ni content, while the VD ensures precise control of the gas content in the steel. Protecting the casting process throughout the continuous casting process reduces secondary oxidation, lowers steel inclusion content, and improves steel purity. Controlling the pouring superheat and the continuous casting strand speed effectively reduces strand quality defects. Increasing the compression ratio from the continuous casting strand to the finished steel plate effectively controls grain size. This achieves low carbon, low phosphorus, and low sulfur control, improving strand quality and ultimately enhancing final product performance.

[0039] 4. The continuous casting slab of the present invention is slowly cooled in a slow cooling pit, thereby reducing the gas content inside the slab to the greatest extent, improving the internal quality of the slab, and thus improving the internal quality of the steel plate after rolling.

[0040] 5. Post-rolling heat treatment utilizes an offline quenching + two-phase quenching + tempering process, with quenching at 890-920°C for 2-4 min / mm, quenching at 710-750°C for 2-4 min / mm, tempering at 610-650°C for 4-6 min / mm, and then air cooling. Mo exists as a solid solution in 5Ni steel, enhancing the stability of the crystal structure (including FCC austenite and BCC ferrite / martensite). Therefore, to improve compositional uniformity, both the quenching and tempering temperatures are increased compared to 5Ni steel without Mo addition. This method improves the low-temperature toughness of the steel plate and effectively reduces the yield strength ratio. The purpose of single quenching is to obtain lath martensite with uniform composition; two-phase quenching allows alloy elements and impurity elements to be enriched in the austenite region, improving the stability of austenite while purifying the matrix; the purpose of tempering is to obtain a structure dominated by tempered bainite, and to obtain a certain amount of austenite through martensite reversal, further increasing the toughness of the steel.

[0041] 6. The present invention adopts a nickel-saving design with a Ni content of less than 5%, meeting the requirements for preventing stress corrosion of liquid ammonia storage tanks.

[0042] 7. Liquid ammonia transport ships manufactured using the 5Ni steel of the present invention can also transport liquefied ethylene, achieving multiple uses for one ship. This increases the use of 5Ni steel and can create direct economic benefits. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 It is the organization diagram under the optical microscope of the present invention.

[0044] Figure 2 It is the organization diagram under scanning electron microscope of the present invention. DETAILED DESCRIPTION

[0045] The present invention discloses a 5Ni steel for liquid ammonia storage tanks and a method for manufacturing the same. Those skilled in the art can refer to the contents of this article and appropriately improve the process parameters for implementation. It should be noted in particular that all similar replacements and modifications are obvious to those skilled in the art and are considered to be included in the present invention. The methods and applications of the present invention have been described through preferred embodiments, and relevant personnel can obviously modify or appropriately change and combine the methods and applications described herein without departing from the content, spirit and scope of the present invention to implement and apply the technology of the present invention.

[0046] A 5Ni steel for liquid ammonia storage tanks is composed of the following chemical components in percentage by weight:

[0047] C: 0.01% to 0.12%, Si: 0.10% to 0.30%, Mn: 0.30% to 0.80%, Ni: 4.75% to 4.99%, Mo: 0.01% to 0.10%, S: ≤0.005%, P: ≤0.008%, and the balance is Fe and unavoidable impurities.

[0048] The liquid ammonia storage tank is made of 5Ni steel, with a yield strength of ≥390MPa, a tensile strength of 530-710MPa, a V-type impact energy of ≥100J at -120°C, and a lateral expansion of ≥1.0.

[0049] The manufacturing method of the 5Ni steel for the liquid ammonia storage tank specifically comprises the following steps:

[0050] (1) Converter + LF + VD refining:

[0051] During converter smelting, the nickel addition level is controlled at 4.75% to 4.80%. During LF refining, the nickel alloy is added more frequently and the single addition amount is reduced to strictly control the final nickel content to less than 5%. The molten steel is then degassed in a VD vacuum furnace, maintaining a holding time of 15 to 20 minutes. The hydrogen and oxygen contents are measured to ensure [H] ≤ 2 ppm and [O] ≤ 20 ppm.

[0052] (2) Continuous casting:

[0053] Protect the pouring process throughout, reduce secondary oxidation during the continuous casting process, reduce the inclusion content in the steel, and improve the purity of the steel; the continuous casting billet pouring superheat is 10-20℃, the continuous casting billet thickness / finished steel plate thickness is ≥6, and the billet pulling speed is <1m / min; controlling the pouring superheat and the continuous casting billet pulling speed can effectively reduce billet quality defects; increasing the compression ratio from the continuous casting billet to the finished steel plate can effectively control the grain size.

[0054] (3) Slab slow cooling:

[0055] The continuous casting billet is slowly cooled in the slow cooling pit to allow the gas in the billet to be fully diffused and discharged, thus minimizing the gas content in the billet. The slow cooling time is ≥48 hours.

[0056] (4) Rolling:

[0057] The steel ingots are rolled in two stages: the first stage starts at a temperature ≥1050°C, the second stage starts at a temperature ≥850°C, and the final rolling temperature is 820±20°C, followed by air cooling after rolling. The first stage, rolling in the recrystallization zone above 1050°C, ensures sufficient elongation of the austenite and fully utilizes the strengthening effect of controlled rolling. Rolling in the non-recrystallization zone above 850°C increases the effective nucleation area of ​​ferrite and refines the ferrite grains. The final rolling temperature is controlled at approximately 820±20°C to control the grain size after hot rolling.

[0058] (5) Quenching + two-phase quenching + tempering heat treatment:

[0059] Put the room temperature steel plate into the heating furnace, keep it at 890-920℃ for 2-4min / mm for quenching, keep it at 710-750℃ for 2-4min / mm for quenching, keep it at 610-650℃ for 4-6min / mm for tempering and then air cool.

[0060] The purpose of single quenching is to obtain lath martensite with uniform composition; two-phase quenching allows alloy elements and impurity elements to be enriched in the austenite region, improving the stability of austenite while purifying the matrix; the purpose of tempering is to obtain a structure dominated by tempered bainite, and to obtain a certain amount of austenite through martensite reversal, further increasing the toughness of the steel.

[0061] The Mo element exists in the form of solid solution in 5Ni steel, which enhances the stability of the crystal structure (including austenite in FCC and ferrite / martensite in BCC). Therefore, in order to improve the composition uniformity, the quenching temperature and tempering temperature are increased compared with 5Ni steel without Mo addition.

[0062] The smelting process of the present invention adopts converter + LF + VD refining, which ensures the precise control of steel composition and gas content in the steel; the continuous casting process protects the pouring and slow cooling of the slab throughout, which ensures the internal quality of the ingot; the controlled rolling ensures the refinement of the initial structure; the post-rolling heat treatment adopts the process of offline quenching + two-phase zone quenching + tempering to improve the low-temperature toughness of the steel plate and effectively reduce the yield ratio.

[0063] [Example]

[0064] The chemical composition of the embodiments of the present invention is shown in Table 1; the smelting and continuous casting processes of the corresponding embodiments are shown in Table 2; the rolling processes of the corresponding embodiments are shown in Table 3; the heat treatment system of the corresponding embodiments is shown in Table 4; and the performance of the corresponding embodiments is shown in Table 5.

[0065] Table 1 Chemical composition of the present invention by weight

[0066] Example C Si Mn Ni Mo P S 1 0.11 0.15 0.33 4.95 0.03 0.005 0.002 2 0.08 0.18 0.51 4.83 0.04 0.005 0.002 3 0.07 0.29 0.60 4.75 0.06 0.005 0.002 4 0.03 0.23 0.45 4.80 0.08 0.005 0.002 5 0.06 0.21 0.72 4.90 0.07 0.005 0.002 6 0.05 0.12 0.70 4.78 0.05 0.005 0.002

[0067] Table 2 Smelting and continuous casting process of the embodiment of the present invention

[0068]

[0069] Table 3 Rolling process of the embodiment of the present invention

[0070] Example First stage rolling temperature, ℃ Second stage rolling temperature, ℃ Finish rolling temperature, ℃ 1 1075 880 820 2 1080 880 800 3 1070 875 830 4 1060 870 810 5 1065 870 840 6 1075 855 820

[0071] Table 4 Heat treatment process of the embodiment of the present invention

[0072]

[0073] Table 5 Mechanical properties of the embodiments of the present invention

[0074]

[0075] As shown in Table 5, the present invention exhibits a yield strength of 502 MPa or greater, a tensile strength of 600-638 MPa, an elongation of 25% or greater, a -120°C V-type impact energy of 212 J or greater, and a lateral expansion of 2.3 or greater. The present invention exhibits strong resistance to permanent deformation, fracture, impact resistance, fracture resistance, toughness, plasticity, and other excellent mechanical properties.

[0076] Figure 1 The microstructure of the steel plate produced according to the example scheme corroded by 4% nitric acid alcohol solution under an optical microscope is shown in FIG. Figure 1 As shown in the figure, the microstructure of the steel plate is tempered troostite + a small amount of austenite. Figure 2 This is a microstructure diagram of a steel plate manufactured according to the example scheme and corroded by 4% nitric acid alcohol solution under a scanning electron microscope. The microstructure of the steel plate is tempered bainite + a small amount of austenite.

[0077] Depend on Figure 1 It can be seen that the structure is uniform and fine, and the grain size is within 10 microns. Due to the addition of Mo, Mo-containing carbides precipitate in the crystals, which has a significant strengthening effect on the matrix. Figure 2 The images show that the laths within the grains have been completely broken, indicating sufficient tempering, and micron-sized austenite can be observed. Grain refinement combined with the precipitation of Mo-containing carbides can effectively improve the strength of the steel, while a uniform and fine structure combined with micron-sized austenite can effectively improve the low-temperature toughness of the steel.

[0078] The addition of Mo element in the present invention compensates for the reduction of Ni content, improves the hardenability of steel, and increases the strength of steel. The smelting and continuous casting process scheme of the present invention realizes low carbon, low phosphorus and low sulfur control, improves the quality of the ingot, and thus improves the performance of the final product. The present invention adopts slow cooling of the slab, which can reduce the gas content inside the ingot to the greatest extent, improve the internal quality of the ingot, and thus improve the internal quality of the steel plate after rolling. The present invention adopts quenching + two-phase zone quenching + tempering process to fully improve the low-temperature toughness of steel; the present invention adopts nickel-saving design, and the Ni content is less than 5%, which meets the requirements of preventing stress corrosion of liquid ammonia storage tanks. The liquid ammonia transport ship manufactured using the 5Ni steel of the present invention can be compatible with the transportation of liquefied ethylene, realizing multiple uses of one ship. Increasing the use of 5Ni steel can create direct economic benefits.

[0079] The above description is only a preferred specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any technician familiar with the technical field, within the technical scope disclosed by the present invention, who makes equivalent replacements or changes based on the technical solution and inventive concept of the present invention, should be covered by the scope of protection of the present invention.

Claims

1. A 5Ni steel for liquid ammonia storage tank, characterized in that: It is composed of the following chemical components in percentage by weight: C: 0.01% to 0.12%, Si: 0.10% to 0.30%, Mn: 0.30% to 0.80%, Ni: 4.75% to 4.99%, Mo: 0.01% to 0.10%, S: ≤0.005%, P: ≤0.008%, and the balance is Fe and unavoidable impurities.

2. The 5Ni steel for liquid ammonia storage tank according to claim 1, characterized in that: The liquid ammonia storage tank is made of 5Ni steel, which has a yield strength of ≥390MPa and a tensile strength of 530-710MPa.

3. The 5Ni steel for liquid ammonia storage tank according to claim 1, characterized in that: The liquid ammonia storage tank is made of 5Ni steel, whose -120°C V-type impact energy is ≥100J and lateral expansion is ≥1.

0.

4. A method for manufacturing 5Ni steel for liquid ammonia storage tanks according to any one of claims 1 to 3, characterized in that: The manufacturing method specifically comprises the following steps: 1) Converter + LF + VD refining: During converter smelting, the amount of Ni added is controlled at 4.75% to 4.80%. The precise control of Ni content is completed in the LF furnace, and the Ni content is controlled to be less than 5%; 2) Continuous casting: Control the continuous casting slab casting superheat at 10-20℃ and the slab pulling speed <1m / min; 3) Slab slow cooling: Slow cooling time ≥48 hours; 4) Rolling: The steel ingot adopts two-stage controlled rolling, the first stage rolling temperature is ≥1050℃, the second stage rolling temperature is ≥850℃, the final rolling temperature is 800~840℃, and air cooling is performed after rolling; 5) Quenching + two-phase quenching + tempering heat treatment: The room temperature steel plate is sent into the heating furnace, kept at 890-920℃ for 2-4min / mm, and quenched; kept at 710-750℃ for 2-4min / mm, and quenched; kept at 610-650℃ for 4-6min / mm, and air-cooled after tempering.

5. The method for manufacturing 5Ni steel for liquid ammonia storage tanks according to claim 4, characterized in that: In step 1), the molten steel is degassed in a VD vacuum furnace, and the pressure holding time of the VD furnace is 15 to 20 minutes.

6. The method for manufacturing 5Ni steel for liquid ammonia storage tanks according to claim 4, characterized in that: In step 1), the gas content in the steel is precisely controlled by the VD process, and [H] is controlled to be ≤ 2 ppm and [O] to be ≤ 20 ppm.

7. The method for manufacturing 5Ni steel for liquid ammonia storage tanks according to claim 4, characterized in that: In step 2), the pouring is protected throughout the entire process.

8. The method for manufacturing 5Ni steel for liquid ammonia storage tanks according to claim 4, characterized in that: In step 2), the ratio of the continuous casting slab thickness to the finished steel plate thickness is ≥6.

9. The method for manufacturing 5Ni steel for liquid ammonia storage tanks according to claim 4, characterized in that: In step 3), the continuous casting billet is put into a slow cooling pit for slow cooling.

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