Cr-Mo-containing 5.5 Ni steel plate for LNG storage tank and manufacturing method of Cr-Mo-containing 5.5 Ni steel plate

Abstract

The invention relates to a Cr-Mo-containing 5.5 Ni steel plate for an LNG (Liquefied Natural Gas) storage tank and a manufacturing method of the Cr-Mo-containing 5.5 Ni steel plate. The steel plate comprises the following chemical components in percentage by mass: 0.03-0.05% of C; 0.15% to 0.30% of Si; mn: 1.20% to 1.30%; p: < = 0.005%; s: < = 0.002%; al: greater than or equal to 0.020%; the content of Ni is 5.00 to 6.00 percent; 0.30% to 0.50% of Cr and Mo; h < = 1 ppm; 0.10% or less of As + Sb + Bi + Sn + Pb; and the balance of Fe and inevitable impurities. The invention relates to a 5.5 Ni steel plate for an LNG (Liquefied Natural Gas) storage tank, in particular to a steel plate for a pressure vessel with the thickness of 5-50mm and the temperature of minus 196 DEG C. A converter is used for production, and the delivery-state mechanical properties and process properties of the obtained steel plate meet the following requirements: the tensile strength Rm of transverse and longitudinal samples of the steel plate is 690-820 MPa, the yield strength Rel is greater than or equal to 590 MPa, and the elongation A is greater than or equal to 21%; at the temperature of-196 DEG C, the impact absorption energy KV8 of the transverse and longitudinal impact sample is greater than or equal to 100J; the sample is bent at normal temperature under the condition that D is equal to 2a and b is equal to 2a, the bending angle is 180 degrees, and no crack exists.

Description

Technical Field

[0001] The invention belongs to the technical field of special steel smelting, and particularly relates to a 5.5Ni steel plate for LNG storage tanks containing Cr-Mo and a manufacturing method thereof. Background Art

[0002] Natural gas is currently the most important clean energy, but there are obvious peaks and troughs in its use. In order to stabilize the supply of natural gas, it is necessary to build large-capacity liquefied natural gas (hereinafter referred to as LNG) storage tanks for peak regulation, that is, to liquefy natural gas at -163°C and store it in storage tanks to meet the stable supply of daily production and life. LNG storage tanks generally have a capacity of 80,000-200,000 cubic meters and a design service life of 50 years. In the entire use process, from a design perspective, no inspection and maintenance links are reserved. Therefore, LNG storage tanks have very high requirements for the materials used for the entire tank body, requiring the steel plate to have uniform and stable performance and a large surplus.

[0003] At present, 9Ni steel plates are widely used in the construction of large LNG storage tanks in China. The Ni content in this steel is about 9% (wt%), which can better meet the service requirements of the steel plates under ultra-low temperature conditions of -163°C; but the price of Ni metal is expensive, resulting in high manufacturing costs for 9Ni steel plates.

[0004] The reason why 9Ni steel plate can be widely used in ultra-low temperature environment is that after quenching-sub-quenching-tempering, the 9Ni steel plate contains a certain amount of reversed austenite content. Reverse austenite can effectively prevent the expansion of cracks, thereby improving the toughness of the steel plate. Reverse austenite is mainly obtained by sub-temperature quenching of the steel plate in the two-phase region. After the first step of quenching, the organization is a lath martensite structure; during sub-temperature quenching, part of the martensite is reversed to austenite, and when it is subsequently quenched to room temperature, part of the austenite will remain; in the last step of tempering, the alloy elements further diffuse into the austenite, which improves the stability of the austenite. This part of austenite is the reversed austenite in the steel.

[0005] However, for 5.5Ni steel plate, due to the significant reduction in Ni content, the hardenability of the steel plate will be significantly reduced. However, in order to ensure that the performance of 5.5Ni steel plate is equivalent to that of 9Ni steel plate, the 5.5Ni structure must be similar to that of 9Ni steel, that is, it must be ensured that the lath martensite structure is first obtained when the Ni content is greatly reduced, and then subsequent heat treatment is performed. It is well known that elements such as C, Mn, Cr, and Mo can effectively increase the hardenability of steel plates; strong cooling can also increase the hardenability of steel plates.

[0006] Currently, there is no literature report on the production of 5.5Ni steel plates that can meet the low-temperature impact performance of 9Ni steel plates by using a composition design containing a certain amount of CrMo and an online DQ+sub-temperature quenching+tempering (T) method and a manufacturing method thereof.

[0007] Patent application number CN202210355545.9 involves "a marine 5Ni steel plate with excellent anti-brittle fracture performance and its manufacturing method". The patent uses the DQ-sub-temperature quenching-tempering heat treatment method to produce marine 5Ni steel plates. This heat treatment method is efficient and economical, and better meets the requirements of organizational control and performance. However, this patent is mainly for 5Ni steel plates, and the steel plates produced can only meet the -130℃ impact toughness requirements, and cannot meet the -196℃ impact toughness requirements of 9Ni steel, and cannot replace 9Ni steel.

[0008] Patent application number CN202210862591.8 involves "a steel for ultra-low temperature pressure vessels and its production method". This patent uses a composition containing a certain amount of CrMo alloy to produce 5.5Ni steel plates. The steel plates can basically meet the -196℃ impact toughness requirements and tensile strength requirements of 9Ni steel, but the heat treatment method used is offline quenching + tempering. Although this method can meet the low-temperature impact requirements, it cannot achieve the reverse austenite content formed by the DQ+quenching+tempering heat treatment method, and the impact toughness will be significantly worse than the DQ+quenching+tempering method. Summary of the invention

[0009] The technical problem to be solved by the present invention is to provide a 5.5Ni steel plate containing Cr-Mo for LNG storage tanks and a manufacturing method thereof in view of the above-mentioned prior art. The steel plate can better meet the design requirements of LNG storage tanks, and the steel plate composition meets the requirements of ASMESA-645 / 645M; the tensile strength Rm of the transverse and longitudinal specimens of the steel plate is 690-820MPa, the yield strength Rel≥590MPa, and the elongation A≥20%; and at a temperature of -196°C, the impact absorption energy KV8≥100J of the transverse and longitudinal impact specimens (specimen size 10mm×10mm×55mm)

[0010] The technical solution adopted by the present invention to solve the above problems is: a 5.5Ni steel plate for LNG storage tanks containing Cr-Mo, the steel plate includes the following chemical components by mass percentage: C: 0.03-0.05%; Si: 0.15-0.30%; Mn: 1.20-1.30%; P: ≤0.005%; S: ≤0.002%; Al: ≥0.020%; Ni: 5.00-6.00%; Cr+Mo: 0.30-0.50%; H≤1ppm; As+Sb+Bi+Sn+Pb≤0.10%; the balance is Fe and unavoidable impurities. The 5.5Ni steel plate for LNG storage tanks of the present invention specifically refers to a steel plate for pressure vessels with a thickness of 5-50mm and a guaranteed -196°C.

[0011] The following is a detailed description of the effects and dosage of the components in the present invention: C: It is an element that improves the hardenability of steel plates and ensures the strength of steel plates. It is also the most important element that deteriorates the low-temperature toughness of steel plates. For 5.5Ni steel plates that ensure low-temperature toughness at -196°C, the first thing to consider is the toughness of the steel plates. Therefore, the C content in the patent of this invention is 0.03-0.05%.

[0012] Si: It is a deoxidizing element in steel and improves the strength of steel in the form of solid solution strengthening. When the Si content is lower than 0.15%, the deoxidation effect is poor; when the Si content is higher, the toughness and coating performance will decrease. The Si content of the present invention is controlled to be 0.15-0.30%.

[0013] Mn: It is a weak deoxidizing element in steel and can improve the hardenability of the steel plate, to a certain extent make up for the hardenability effect of the Ni element, and promote the formation of lath martensite. Therefore, the Mn content of the present invention is greatly increased compared with the Mn content of 9Ni steel, and the control range is 1.20 ~ 1.30%.

[0014] Ni: is a beneficial element that can significantly improve low-temperature toughness, mainly in terms of improving the hardenability of the steel plate, improving the stability of austenite, and reducing the tough-to-brittle transition temperature of the steel plate. Therefore, the present invention controls its content within 5.00-6.00%.

[0015] Cr and Mo: Both Cr and Mo elements are elements that increase hardenability. For low-C, low-Ni 5.5Ni steel plates, in order to ensure the hardenability of the steel plates, additional Cr and Mo elements need to be added to make up for the lack of C content and improve the hardenability of the steel plates. At the same time, the alloy elements are diffused into austenite in the later stage to stabilize the austenite structure; but it cannot be ignored that Cr and Mo elements have a certain negative impact on low-temperature impact toughness. Considering that the patent of the present invention adopts direct quenching (DQ) heat treatment after rolling, its quenching ability is much stronger than the conventional offline quenching (RQ) process, and the design values ​​of Cr and Mo content are relatively low, therefore, the present invention controls its content at Cr+Mo: 0.30-0.50%.

[0016] S, P: harmful impurity elements in steel, which are easy to form defects such as segregation and inclusions. As impurity elements, they will have an extremely adverse effect on the toughness of the steel plate (especially the toughness of the core) and the toughness of the welding heat affected zone, and their content should be reduced as much as possible. The present invention controls P ≤ 0.005%, S ≤ 0.002%, and the inclusion morphology must be spheroidized and evenly distributed through Ca treatment technology.

[0017] Another object of the present invention is to provide a method for manufacturing the above-mentioned Cr-Mo-containing 5.5Ni steel plate for LNG storage tanks, wherein the smelting process adopts converter-LF refining outside the furnace-RH vacuum treatment-CC continuous casting into billets-billet pit slow cooling; the rolling process adopts a two-stage controlled rolling process, the first stage is rough rolling, the second stage is finishing rolling, and DQ water cooling is performed after rolling; the heat treatment adopts continuous furnace sub-temperature quenching + high temperature tempering heat treatment.

[0018] The above production process specifically includes the following steps: 1) Prepare smelting raw materials according to the chemical composition, and sequentially undergo KR hot metal pretreatment, converter smelting, LF refining, RH refining and continuous casting. Since the phosphorus content required in 5.5Ni steel is relatively low, sufficient P removal is required during the steelmaking process; unlike conventional steel tapping in a reducing environment, the present invention uses slag tapping in an oxidizing environment; the oxidation reaction of phosphorus is an exothermic reaction, so the two key favorable conditions are sufficient heat and an oxidizing environment. Therefore, in the low-temperature stage (1500-1550°C) of the converter smelting process, continuous large-scale oxygen blowing is used, the oxygen blowing time is ≥20min, and sufficient stirring is used to accelerate the C+O=CO reaction, release a large amount of heat, and promote the rapid increase of the temperature in the steel; in addition, a large amount of oxygen blowing can form an oxidizing environment, and at the same time, no reducing metals such as Al are added, the oxidizing environment is maintained, and the reaction of P2O5 is promoted. By accelerating the oxidation of P element in molten steel into P2O5, the generated P2O5 enters the slag, and the molten steel is dephosphorized, thereby reducing the phosphorus content in the molten steel and achieving the smelting of ultra-low phosphorus content 5.5Ni molten steel.

[0019] 2) The high-purity molten steel produced is cast using an optimized continuous casting process. During the continuous casting process, the superheat is controlled at 10-15°C to promote the formation of more equiaxed crystals in the crystallizer, reduce the temperature gradient during solidification, improve the degree of central segregation of the ingot, and reduce the tendency of surface cracks in the ingot; the continuous casting speed is controlled at 0.7-1.0 m / min to fully solidify the molten steel, reduce the generation of surface cracks and inclusions on the ingot, and help form a denser ingot structure, further reducing central porosity and segregation. The light pressure reduction during the continuous casting process needs to be ≥6 mm, so that the molten steel at the end of solidification can flow evenly and distribute, the solidification shrinkage pores are closed, and the degree of central porosity is reduced. The continuous casting slab with low central segregation and porosity and a thickness of 150 mm is produced by the above process. After continuous casting, the continuous casting billet is hoisted into a slow cooling pit with heating function, and the pit temperature is not less than 850℃; when the slow cooling is to 250℃, the heating function is started to keep the billet cooled to 150℃ at a cooling rate of 2-4℃ / h (the optimal H removal temperature of 5.5Ni billet is 150-250℃). The purpose of this slow cooling step is to reduce the H content in the steel, thereby further improving the core quality of the continuous casting billet to ensure the uniform and stable performance of the steel plate. After the slow cooling is completed, the surface of the continuous casting billet is cleaned with temperature.

[0020] 3) The continuous casting billet is heated to 1180-1280°C and kept warm for 1-2 hours to fully dissolve the alloy elements in the steel to ensure the uniformity of the composition and performance of the final product. After the continuous casting billet is taken out of the furnace, it is descaled with high-pressure water, and then two-stage rolling is carried out. The first stage of rolling is rough rolling, and the rolling temperature is not controlled, but the single-pass reduction rate of the last three passes needs to be controlled to be ≥15% to ensure that the core defects of the continuous casting billet are fully bridged so that the performance of the steel plate is guaranteed; the second stage of rolling is finishing rolling, the starting rolling temperature range is 840-880°C, and the final rolling temperature range is 780-820°C. After the rolling is completed, the steel plate is directly DQed to room temperature using the residual heat after rolling, the DQ water temperature is 760±20°C, and the maximum water volume is used for rapid cooling to ensure that the cooling rate is ≥30°C / s. The super-strong cooling capacity of DQ is used to ensure that there is sufficient supercooling in the process of organizational transformation, which promotes the rapid transformation of supercooled austenite to lath martensite, and finally the steel plate obtains a lath martensite structure.

[0021] 4) The rolled steel plate is subjected to sub-temperature quenching + high-temperature tempering heat treatment. The heat treatment is carried out in a continuous furnace. The sub-temperature quenching heating and insulation temperature is 650~730℃, the furnace time is 3.0-5.0min / mm (not less than 40min), and water cooling is performed after the furnace is taken out; the high-temperature tempering heating and insulation temperature is 560~600℃, the furnace time is 4.0-7.0min / mm (not less than 60min), and air cooling is performed after the furnace is taken out.

[0022] Compared with the prior art, the advantages of the present invention are: (1) The mechanical properties and process properties of 5.5Ni steel plates are specified to meet the construction needs of LNG and other storage tanks; (2) Based on the product usage characteristics and DQ heat treatment process characteristics, the chemical composition of the steel grade was designed, with the Ni content being around 5.5%; (3) Using the residual heat after rolling for DQ heat treatment reduces the offline quenching process, reduces manufacturing costs, and improves production efficiency; Aiming at the urgent need of the current LNG storage tank construction, the present invention adopts reasonable chemical composition design; combines the continuous casting process to produce the continuous casting slab with low center segregation and looseness; cooperates with the large reduction rolling process; optimizes the heat treatment process, and manufactures the low-cost 5.5Ni steel plate for LNG storage tank. DETAILED DESCRIPTION

[0023] The technical solution of the present invention is described in more detail in conjunction with the preferred embodiments of the present invention. However, these embodiments are only descriptions of the preferred implementation methods of the present invention and cannot impose any limitation on the scope of the present invention.

[0024] The steel plate involved in Example 1 has a thickness of 8 mm.

[0025] The production process of 8mm thick steel plate is as follows: The smelting raw materials are configured according to the chemical composition of the above steel plates and are sequentially subjected to KR molten iron pretreatment - converter smelting - LF refining - RH refining - 150mm continuous casting - continuous casting billet slow cooling in the pit - continuous casting billet cleaning - continuous casting billet heating - insulation treatment - high-pressure water descaling - controlled rolling - DQ - sub-temperature quenching - tempering.

[0026] The high-temperature slab coming out of the continuous casting machine is placed in a slow cooling pit with a heating function. The temperature entering the pit is not less than 850℃. After slow cooling to 250℃, the heating function is started to keep the slab cooled to 150℃ at a cooling rate of 2-4℃ / h before leaving the hood. The purpose of this slow cooling step is to reduce the H content in the steel.

[0027] Furthermore, the specific process of heating, controlled rolling and cooling of slabs into steel plates is as follows: the continuous casting slab is heated to 1180-1280°C, kept warm for 1-2 hours, and then rolled in two stages. The first stage rolling (rough rolling) has a starting temperature of 1070°C, 5 rolling passes, and an intermediate slab thickness of 20mm. The second stage rolling is finish rolling, with 6 rolling passes and a final plate thickness of 8mm. After rolling, the steel plate is directly DQed to room temperature using the residual heat after rolling, and the DQ water entry temperature is 763°C.

[0028] The rolled steel plate is subjected to sub-temperature quenching + high-temperature tempering heat treatment. The heat treatment is carried out in a continuous furnace. The sub-temperature quenching heating and insulation temperature is 680°C, the furnace time is 40 minutes, and it is water-cooled to room temperature after being taken out of the furnace; the high-temperature tempering heating and insulation temperature is 580°C, the furnace time is 60 minutes, and it is air-cooled after being taken out of the furnace.

[0029] The finished steel plate formed by the above manufacturing process has excellent comprehensive performance, as shown in Table 1 and Table 2 for details.

[0030] The steel plate involved in Example 2 has a thickness of 16 mm.

[0031] The production process of 16mm thick steel plate is as follows: The steel plate slab steelmaking and slab slow cooling are consistent with Example 1.

[0032] Furthermore, the specific process of heating, controlled rolling and cooling of the slab into steel plate is as follows: the continuous casting slab is heated to 1180~1280℃, kept warm for 1-2 hours, and then rolled in two stages. The first stage rolling (i.e. rough rolling) has a starting temperature of 1070℃, 5 rolling passes, and an intermediate slab thickness of 32mm. The second stage rolling is finish rolling, which is 5 rolling passes with a final plate thickness of 16mm. After rolling, the steel plate is directly DQed to room temperature using the residual heat after rolling, and the DQ water entry temperature is 770℃.

[0033] The rolled steel plates are subjected to sub-temperature quenching + high-temperature tempering heat treatment. The heat treatment is carried out in a continuous furnace. The sub-temperature quenching heating and insulation temperature is 680°C, the furnace time is 64 minutes, and it is water-cooled to room temperature after being taken out of the furnace; the high-temperature tempering heating and insulation temperature is 580°C, the furnace time is 96 minutes, and it is air-cooled after being taken out of the furnace.

[0034] The finished steel plate formed by the above manufacturing process has excellent comprehensive performance, as shown in Table 1 and Table 2 for details.

[0035] The thickness of the steel plate involved in Example 3 is 32 mm.

[0036] The production process of 32mm thick steel plate is as follows: The steel plate slab steelmaking and slab slow cooling are consistent with Example 1.

[0037] Furthermore, the specific process of heating, controlled rolling and cooling of slabs into steel plates is as follows: the continuous casting slab is heated to 1180-1280°C, kept warm for 1-2 hours, and then rolled in two stages. The first stage rolling (rough rolling) has a starting temperature of 1070°C, 5 passes of rolling, and an intermediate slab thickness of 58mm. The second stage rolling is finish rolling, which is performed for 5 passes with a final plate thickness of 32mm. After rolling, the steel plate is directly DQed to room temperature using the residual heat after rolling, and the DQ water entry temperature is 772°C.

[0038] The rolled steel plates are subjected to sub-temperature quenching + high-temperature tempering heat treatment. The heat treatment is carried out in a continuous furnace. The sub-temperature quenching heating and insulation temperature is 690°C, the furnace time is 128min, and it is water-cooled to room temperature after being taken out of the furnace; the high-temperature tempering heating and insulation temperature is 570°C, the furnace time is 192min, and it is air-cooled after being taken out of the furnace.

[0039] The finished steel plate formed by the above manufacturing process has excellent comprehensive performance, as shown in Table 1 and Table 2 for details.

[0040] Table 1 Example smelting chemical composition (wt%) Table 2 Mechanical properties of steel plates produced in Example *

[0041] *: 8mm thickness steel plate impact specimen size 5mmX10mmX55mm Although the preferred embodiments of the present invention are described in detail above, it should be clearly understood that various modifications and variations are possible for those skilled in the art. Any modification, equivalent substitution, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A 5.5Ni steel plate for LNG storage tank containing Cr-Mo, characterized in that: The steel plate includes the following chemical components in mass percentage: C: 0.03-0.05%; Si: 0.15-0.30%; Mn: 1.20-1.30%; P: ≤0.005%; S: ≤0.002%; Al: ≥0.020%; Ni: 5.00-6.00%; Cr+Mo: 0.30-0.50%; H≤1ppm; As+Sb+Bi+Sn+Pb≤0.10%; the balance is Fe and unavoidable impurities.

2. The 5.5Ni steel plate for LNG storage tank containing Cr-Mo according to claim 1, characterized in that: The steel plate has a thickness of 5-50 mm and is a steel plate for pressure vessels that can withstand temperatures of -196°C.

3. The 5.5Ni steel plate for LNG storage tank containing Cr-Mo according to claim 1, characterized in that: The tensile strength Rm of the transverse and longitudinal specimens of the steel plate is 690-820MPa, the yield strength Rel≥590MPa, and the elongation A≥20%; and at a temperature of -196°C, the impact absorption energy KV8 of the transverse and longitudinal impact specimens is ≥100J.

4. A method for manufacturing a 5.5Ni steel plate for LNG storage tanks containing Cr-Mo as claimed in claim 1, characterized in that: The method comprises the following steps: 1) preparing smelting raw materials according to the chemical composition, and sequentially undergoing KR molten iron pretreatment, converter smelting, LF refining and RH refining; 2) The high-purity molten steel produced is cast using an optimized continuous casting process. During the continuous casting process, the superheat is controlled at 10-15°C, the continuous casting speed is controlled at 0.7-1.0m / min, and the light pressure during the continuous casting process is ≥6mm. After the continuous casting is completed, the continuous casting billet is hoisted into a slow cooling pit with a heating function. After the slow cooling is completed, the surface of the continuous casting billet is cleaned with heat; 3) The continuous casting billet is heated to 1180-1280°C and kept at this temperature for 1-2 hours to fully dissolve the alloy elements in the steel to ensure uniform composition and performance of the final product. After the continuous casting billet is taken out of the furnace, it is descaled with high-pressure water and then rolled in two stages to obtain a lath-shaped martensitic structure of the steel plate. 4) The rolled steel plate is subjected to sub-temperature quenching + high-temperature tempering heat treatment. The heat treatment is carried out in a continuous furnace. The sub-temperature quenching heating and insulation temperature is 650~730℃, the furnace time is 3.0-5.0min / mm, not less than 40min, and water cooling is performed after the furnace is taken out; the high-temperature tempering heating and insulation temperature is 560~600℃, the furnace time is 4.0-7.0min / mm, not less than 60min, and air cooling is performed after the furnace is taken out.

5. The method for manufacturing a 5.5Ni steel plate for LNG storage tank containing Cr-Mo according to claim 4, characterized in that: In step (1), during the low temperature stage of 1500-1550° C. in the converter smelting process, a large amount of oxygen is blown continuously for a blowing time of ≥20 min, and sufficient stirring is performed to accelerate the C+O=CO reaction, release a large amount of heat, and promote a rapid increase in the temperature of the steel. By accelerating the oxidation of P element in the molten steel to P2O5, the generated P2O5 enters the slag, and the molten steel is dephosphorized, thereby reducing the phosphorus content in the molten steel and achieving the smelting of ultra-low phosphorus content 5.5Ni molten steel.

6. The method for manufacturing a 5.5Ni steel plate for LNG storage tank containing Cr-Mo according to claim 4, characterized in that: In step (2), the temperature of the continuous casting slab entering the pit is not less than 850°C; when slowly cooled to 250°C, the heating function is started to keep the slab cooled to 150-250°C at a cooling rate of 2-4°C / h.

7. The method for manufacturing a 5.5Ni steel plate for LNG storage tank containing Cr-Mo according to claim 4, characterized in that: In step (3), the first stage of rolling is rough rolling, the rolling temperature is not controlled, and the reduction rate of each of the last three passes is controlled to be ≥15%; the second stage of rolling is finishing rolling, the starting rolling temperature range is 840-880°C, and the final rolling temperature range is 780-820°C. After rolling, the steel plate is directly DQed to room temperature using the residual heat after rolling, the DQ water temperature is 760±20°C, and rapid cooling is performed with the maximum water volume to ensure that the cooling rate is ≥30°C / s. The super-strong cooling capacity of DQ is used to ensure that there is sufficient supercooling during the microstructure transformation process, which promotes the rapid transformation of supercooled austenite into lath martensite.

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