Low-reheat-crack-sensitivity P690QL2 steel plate for liquid cargo tank of LCO2 transport ship and preparation method of low-reheat-crack-sensitivity P690QL2 steel plate

Abstract

The invention discloses a P690QL2 steel plate with low reheat crack sensitivity for a liquid cargo tank of an LCO2 transport ship and a preparation method of the P690QL2 steel plate. The P690QL2 steel plate comprises 0.04%-0.10% of C, 0.05%-0.30% of Si, 1.0%-1.6% of Mn, 1.0%-2.5% of Ni, 0.2%-0.4% of Cr, 0.2%-0.4% of Mo, 0.03%-0.10% of Nb, 0.005%-0.020% of Ti, 0.0005%-0.0020% of B, smaller than or equal to 0.005% of S, smaller than or equal to 0.010% of P, 0.02%-0.05% of Als and the balance Fe and inevitable impurity elements. Through reasonable component design and combination of two-stage rolling and heat treatment processes, the mechanical property meets the technical requirements of the liquid cargo tank of the liquefied carbon dioxide transport ship, and the reheating crack sensitivity of the steel plate in the using process is reduced.

Description

Technical Field

[0001] This invention relates to the field of low-temperature steel plate manufacturing technology for cargo tanks of liquefied carbon dioxide transport ships, and more specifically, to a P690QL2 steel plate for cargo tanks of LCO2 transport ships with low reheat cracking sensitivity and its preparation method. Background Technology

[0002] P690QL2 ultra-high strength low-temperature steel, due to its extremely high strength, good toughness, and especially its excellent low-temperature crack resistance even at -35℃, is used in the construction of cargo tanks for liquefied carbon dioxide carriers. However, precisely because of the high strength of P690QL2 ultra-high strength low-temperature steel, if the operating procedures are not strictly followed during the welding and construction of liquefied carbon dioxide cargo tanks, and if the preheating temperature, welding current, voltage, or interpass temperature is not properly controlled, reheat cracks may occur during post-weld heat treatment.

[0003] Generally, CrMo(V) steels with high alloying elements, such as 2.25Cr1Mo or 2.25Cr1MoV, are more prone to reheat cracking. However, this type of steel has low strength, with a tensile strength generally ≤700MPa. It also lacks good impact toughness at low temperatures, making it difficult to apply to the construction of cargo tanks for liquefied carbon dioxide carriers.

[0004] In the prior art, patent CN115852120A discloses a method for producing P690QL2 steel plates with a thickness ≤50mm. While this method, using TMCP and quenching + tempering, achieves the required low-temperature toughness for P690QL2 steel plates, the TMCP process has low production efficiency, and the steel plate's red-hot temperature is closely related to the amount of water poured in, making it difficult to guarantee uniform microstructure and properties during production. Furthermore, the TMCP process results in poor plate shape for thin-gauge steel plates. Moreover, this technology does not consider reheat cracking sensitivity; within its chemical composition range, calculations show a reheat cracking sensitivity index ΔG > 0. Patent CN115786820A discloses a method for manufacturing P690QL2 marine storage tank steel. This invention also employs TMCP and quenching + tempering processes, and similarly does not consider reheat cracking sensitivity; within its chemical composition range, calculations show a reheat cracking sensitivity index ΔG > 0. Patent CN111621708A discloses a new type of steel plate with higher impact toughness than P690QL2 steel plate used in LPG ship storage tanks and its production method. The invention uses 550-700mm steel ingots to roll finished steel plates with a thickness of 10-50mm. The invention also uses TMCP and quenching + tempering processes, and also does not consider reheat cracking sensitivity. Within its chemical composition range, through calculation, its reheat cracking sensitivity index ΔG > 0.

[0005] Therefore, with the continuous improvement of the performance requirements of P690QL2 low-temperature steel, the existing research is still insufficient in terms of process optimization and microstructure and performance control, making it difficult to meet the application requirements of LCO2 transport ships for ultra-high mold welding and low-temperature performance. Summary of the Invention

[0006] The purpose of this invention is to overcome the aforementioned deficiencies in the existing technology and provide a P690QL2 steel plate with low reheat cracking sensitivity for LCO2 cargo tanks and its preparation method. By controlling elements such as Ni, Cr, Mo, Nb, and Ti, a new solution is provided for the selection of low-temperature materials for LCO2 cargo tanks, and a relatively relaxed process window is provided for welding construction of LCO2 cargo tanks. This results in a P690QL2 steel plate with good strength-toughness matching, low reheat cracking sensitivity, and CTOD performance at -35℃, with a thickness range of 10mm to 50mm. This improves the yield strength of the steel plate, achieves excellent strength, toughness, and low-temperature performance, and is easy to weld, meeting the processing and welding requirements of pressure equipment. To achieve the above objectives, the technical solution of this invention is as follows: A P690QL2 steel plate for cargo tanks of LCO2 transport ships with low reheat cracking sensitivity comprises the following components by weight percentage: C: 0.04%~0.10%, Si: 0.05%~0.30%, Mn: 1.0%~1.6%, Ni: 1.0%~2.5%, Cr: 0.2%~0.4%, Mo: 0.2%~0.4%, Nb: 0.03%~0.10%, Ti: 0.005%~0.020%, B: 0.0005%~0.0020%, S≤0.005%, P≤0.010%, Als: 0.02%~0.05%, with the balance being Fe and unavoidable impurity elements.

[0007] Optionally, the chemical composition weight percentage ratio of Cr, Mo, Nb and Ti in the P690QL2 steel plate conforms to: 1.2%≤Cr+Mo+10Nb+100Ti≤3.8%.

[0008] Optionally, the P690QL2 steel plate has a yield strength ≥690MPa, a tensile strength of 770~940MPa (preferably 770~850MPa), an elongation ≥14%, and a plate thickness of 10mm~50mm; the P690QL2 steel plate has a V-shaped impact energy KV2 ≥180J at -60℃, a V-shaped impact energy KV2 ≥100J at -80℃, and a CTOD δ ≥0.2mm at -35℃; in a corrosive medium of liquefied carbon dioxide at -35℃, the corrosion rate is ≤0.005mm / year; under a heat input of 10~25kJ / cm and a post-weld heat treatment temperature of 550~650℃, there are no reheat cracks in the weld, the weld heat-affected zone, or the base material of the steel plate.

[0009] Optionally, the ferrite-austenite equilibrium phase transformation start temperature AC1 of the P690QL2 steel plate is ≥620℃, and the ferrite-austenite equilibrium phase transformation end temperature AC3 is ≥848℃.

[0010] Optionally, the microstructure of the P690QL2 steel plate is tempered sorbite.

[0011] The present invention also discloses a method for preparing P690QL2 steel plate with low reheat cracking sensitivity for LCO2 transport ship cargo tanks as described above, including smelting, continuous casting, heating, rolling and heat treatment. In the rolling process, the heated billet is descaled by high-pressure water after exiting the furnace. The descaled billet is then rolled into a steel plate in two stages. To ensure that the steel plate obtains a fine hot-rolled microstructure, providing a microstructure basis for subsequent heat treatment and obtaining the corresponding properties, while ensuring the accuracy of the steel plate shape and thickness, the initial rolling temperature of the first stage is ≥1050℃; the initial rolling temperature of the second stage is 870~990℃; and the final rolling temperature of the second stage is 800~940℃. The heat treatment includes quenching heat treatment and tempering heat treatment; wherein, in the quenching heat treatment, the rolled steel plate is subjected to offline quenching heat treatment, the quenching temperature is 50~80℃ higher than the equilibrium transformation end temperature of ferrite-austenite, and the holding time is 2~4min / mm, to obtain the quenched steel plate; in the tempering heat treatment, the quenched steel plate is subjected to tempering heat treatment, the tempering temperature is 10~50℃ lower than the equilibrium transformation start temperature of ferrite-austenite, and the holding time is 4~7min / mm.

[0012] Optionally, the descaled billet is rolled into a steel plate in two stages, and the microstructure of the steel plate is controlled to be a fine granular bainite structure with a grain size of 5~12μm and an irregular square morphology.

[0013] Optionally, in the smelting, continuous casting, and heating processes, the smelting raw materials are sequentially smelted in a converter, refined in an LF furnace, and refined in an RH furnace to obtain molten steel. The molten steel is then continuously cast into a billet and subjected to heat treatment to obtain the heated billet.

[0014] Optionally, the size of the cast billet is 250~300mm.

[0015] Optionally, when the target thickness t of the P690QL2 steel plate is 10mm≤t<16mm, the second-stage initial rolling temperature is 940~990℃. To avoid excessive cooling during the rolling process due to the thinness of the steel plate, which would affect the plate shape and thickness accuracy, the second-stage final rolling temperature is 860~940℃. When the target thickness t of the P690QL2 steel plate is 16mm≤t<30mm, the second-stage initial rolling temperature is 890~960℃, and the second-stage final rolling temperature is 820~880℃. When the target thickness t of the P690QL2 steel plate is 30mm≤t≤50mm, to ensure that the rolled steel plate obtains fine granular bainite and to avoid grain growth during the air cooling process after rolling due to a high final rolling temperature, the second-stage initial rolling temperature is 870~920℃, and the second-stage final rolling temperature is 800~860℃.

[0016] Optionally, the quenching temperature is 915–940℃; the tempering temperature is 605–650℃.

[0017] Implementing the embodiments of the present invention will have the following beneficial effects: (1) The P690QL2 steel plate for the cargo tank of LCO2 transport ship with low reheat cracking sensitivity prepared by the preparation method of the present invention has a finished thickness of 10mm~50mm, yield strength ≥690MPa, tensile strength 770~940MPa, elongation ≥14%, V-type impact energy KV2≥180J at -60℃, V-type impact energy KV2≥100J at -80℃, and CTOD δ≥0.2mm at -35℃. The mechanical properties can meet the technical requirements of the cargo tank of liquefied carbon dioxide transport ship, and the actual tensile strength is not greater than 850MPa, which reduces the possibility of reheat cracking sensitivity during the use of the steel plate.

[0018] (2) The P690QL2 steel plate for the cargo tank of LCO2 transport ship with low reheat cracking sensitivity prepared by the preparation method of the present invention has a Cr content of 0.2%~0.4% and a Mo content of 0.2%~0.4%, and a reheat cracking sensitivity index ΔG<0, which reduces the reheat cracking sensitivity of the steel plate during use.

[0019] (3) The present invention adopts two-stage controlled rolling to control the microstructure of the steel plate into fine granular bainite structure. The grain size of granular bainite is 5~12μm and the morphology of granular bainite is irregular square, which provides a good grain basis for the subsequent heat treatment structure. In particular, the final rolling temperature of the second stage for thin steel plates with a specification of 10mm≤t<16mm is 860~940℃, which ensures the plate shape and thickness accuracy of the rolled steel plate.

[0020] (4) The present invention performs offline quenching heat treatment at 50~80℃ above the ferrite-austenite equilibrium phase transformation end temperature AC3, so that the steel plate can be fully austenitized, the structure is uniform, the alloying elements can be evenly distributed in the austenite grains, and the austenite grains are not too coarse. The steel plate obtains martensitic structure after quenching, which provides a sufficient strength basis for the strength loss of the steel plate after tempering.

[0021] (5) The present invention tempers the steel plate by 10~50°C below the equilibrium phase transformation start temperature AC1 of ferrite-austenite and provides sufficient holding time so that the laths in the quenched martensite can be fully decomposed. After tempering, the steel plate is air-cooled to obtain tempered sorbite structure, which provides sufficient low-temperature toughness for the steel plate.

[0022] (6) The steel plate manufactured using the present invention has a corrosion rate of ≤0.005 mm / year in a corrosive medium of liquefied carbon dioxide at -35°C.

[0023] (7) The steel plate manufactured using the present invention does not produce reheat cracks in the weld, heat-affected zone, or base material of the steel plate under heat input conditions of 10~25kJ / cm and post-weld heat treatment temperature of 550~650℃.

[0024] (8) No V element is added to the steel of the present invention. Combined with the two-stage controlled rolling process and the quenching + tempering heat treatment process, the reheat crack sensitivity index ΔG < 0 is guaranteed, so as to avoid increasing the reheat crack tendency in the steel, and to avoid the preheating temperature being too low, the welding line energy being too high, the weld strength being too high, and the post-weld heat treatment parameters being unreasonable, so as not to form reheat cracks. Attached Figure Description

[0025] Figure 1 The metallographic structure diagram is shown for the steel plate manufactured in Example 1. Detailed Implementation

[0026] The present invention will be further described below with reference to specific embodiments, but this does not limit the present invention in any way.

[0027] This invention discloses a P690QL2 steel plate for cargo tanks of LCO2 transport ships with low reheat cracking sensitivity, comprising the following components by weight percentage: C: 0.04%~0.10%, Si: 0.05%~0.30%, Mn: 1.0%~1.6%, Ni: 1.0%~2.5%, Cr: 0.2%~0.4%, Mo: 0.2%~0.4%, Nb: 0.03%~0.10%, Ti: 0.005%~0.020%, B: 0.0005%~0.0020%, S≤0.005%, P≤0.010%, Als: 0.02%~0.05%, with the balance being Fe and unavoidable impurity elements.

[0028] The chemical composition weight percentage ratio of Cr, Mo, Nb and Ti in P690QL2 steel plate conforms to: 1.2%≤Cr+Mo+10Nb+100Ti≤3.8%.

[0029] Specifically, the reasons for limiting the composition of the P690QL2 steel plate of the present invention are as follows: (1) Carbon: C is the most effective element for improving the strength of steel plates. In this invention, C is used as a strengthening element for interstitial solid solution to improve the strength and hardness of this type of low-temperature steel. When the carbon content is between 0.11% and 0.15%, not only does the tendency for center segregation of the slab increase, but also surface cracks of the continuously cast slab are easily generated due to peritectic reaction. Excessive C content will reduce the toughness of the steel and is not conducive to the welding of steel plates. Therefore, this invention selects to add C content of 0.04% to 0.10%.

[0030] (2) Silicon: Si can improve the strength of steel and can also be used for reduction and deoxidation. However, excessive Si content is detrimental to the weldability of steel. In addition, excessive Si content will significantly promote ferrite coarsening and reduce the plasticity and toughness of steel. Therefore, the present invention controls the Si content at 0.05%~0.30%.

[0031] (3) 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, and S≤0.005% should be controlled.

[0032] (4) Phosphorus: P often accumulates at grain boundaries in steel, which disrupts the continuity of the matrix, significantly reduces the toughness of steel, worsens welding performance, and easily causes cold brittleness. Therefore, the P content in steel should be reduced as much as possible and controlled to P≤0.010%.

[0033] (5) Manganese: Mn is one of the main strengthening elements in this invention. Mn is a strong austenite stabilizing element and an effective element for improving the strength of steel. Mn and Fe are infinitely miscible in steel, and the strength is improved through solid solution strengthening. 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, the Mn content in this invention is controlled at Mn: 1.0%~1.6%.

[0034] (6) Nickel: Ni is the most important element for improving the low-temperature impact toughness of steel plates in this invention. If the Ni content is too low, the low-temperature toughness of the steel plate will be insufficient. However, if the Ni content is too high, exceeding 2.5%, it will not only increase the cost but also increase the hot cracking tendency of the steel plate. Considering the above two factors, this invention controls the Ni content at 1.0%~2.5%. On the one hand, by expanding the austenite region and dissolving infinitely in γ-Fe, nickel-containing steel can obtain a fully refined tempered sorbite structure after quenching and tempering. During the repeated heating and cooling process of steel plate welding, the ductile-brittle transition temperature is reduced, and the low-temperature toughness of the steel plate is improved. On the other hand, the production cost of steel plates is reduced.

[0035] The addition of an appropriate amount of Ni can not only refine the microstructure of steel and reduce galvanic corrosion caused by uneven composition and structure, but also help reduce the precipitation of coarse carbides at grain boundaries or phase boundaries, thereby reducing the susceptibility to intergranular corrosion and stress corrosion cracking.

[0036] (7) Molybdenum: Mo can delay the incubation time of austenite transformation in steel, shift the transformation curves of pearlite and ferrite to the right, promote the transformation at medium temperature, and improve hardenability. At the same time, Mo is a strong carbide forming element. During tempering, the molybdenum dissolved in the matrix can easily form alloy compounds such as MxC, which can ensure the strength of tempering heat treatment. However, if the Mo content is too high, it will lead to the embrittlement of steel. Therefore, the present invention sets the Mo content range to 0.2%~0.4%.

[0037] (8) Chromium: Cr is a strong carbide-forming element. It forms stable carbides with C in steel, which play a role in strength at room temperature and high temperature. In order for this steel to have sufficient strength, the Cr content range is set to 0.2%~0.4% in this invention.

[0038] Furthermore, Ni, along with Cr and Mo, synergistically improves microstructure uniformity and enhances the material's resistance to pitting corrosion. However, excessive Ni addition not only increases costs but also hinders the recombination and escape of hydrogen atoms from the steel surface under acidic conditions, leading to hydrogen penetration into the steel interior and easily inducing hydrogen embrittlement.

[0039] (9) Niobium and Titanium: Nb and Ti are microalloying elements, mainly playing a role in grain refinement and strengthening in this invention. They inhibit austenite deformation and recrystallization and prevent grain growth during the hot rolling process of steel plates, and perform precipitation strengthening of steel through strain-induced precipitation of carbon and nitrides of niobium and titanium. Ti can also fix S and N, improving the strength of steel, while Nb can improve the weldability of steel. Therefore, this invention controls the Nb content to be 0.03%~0.10% and the Ti content to be 0.005%~0.020%.

[0040] Generally, a large amount of Cr and Mo elements are added to Cr-Mo-V series heat-resistant steel. Both are carbide-forming elements and have precipitation strengthening effect. As the content increases, the tendency for reheat cracking increases. This invention patents a P690QL2 low-temperature steel plate for liquefied carbon dioxide transport ships, which is different from Cr-Mo-V series heat-resistant steel. However, considering the high strength of the steel plate, in order to avoid reheat cracking during welding construction, only a small amount of Cr and Mo elements are added to the steel, and no V element is added, and 1.2%≤Cr+Mo+10Nb+100Ti≤3.8%.

[0041] (10) Aluminum: Aluminum mainly plays a role in nitrogen fixation and deoxidation. AlN formed by the combination of Al and N can effectively refine the grains, but too high a content will impair the toughness of the steel, and the billet and steel plate are prone to cracking. Therefore, the present invention controls its content (Als) to be between 0.02% and 0.05%.

[0042] (11) Boron: Trace amounts of boron can significantly improve the hardenability of steel plates. For quenched thick steel plates, the addition of boron can significantly promote the formation of martensite or bainite, thereby improving their strength. However, excessive boron will precipitate at the austenite grain boundaries in the steel and cause hot brittleness. Therefore, the boron content is 0.0005%~0.0020%.

[0043] In one specific embodiment, the P690QL2 steel plate has a yield strength ≥690MPa, a tensile strength of 770~940MPa, and an elongation ≥14%.

[0044] In one specific embodiment, the V-shaped impact energy KV2 of the P690QL2 steel plate is ≥180J at -60℃, ≥100J at -80℃, and CTOD δ is ≥0.2mm at -35℃.

[0045] In one specific embodiment, the thickness of the P690QL2 steel plate is 10mm~50mm.

[0046] In one specific embodiment, at -35°C, in a corrosive medium of liquefied carbon dioxide, the corrosion rate is ≤0.005mm / year; under a heat input of 10~25kJ / cm, and at a post-weld heat treatment temperature of 550~650°C, there are no reheat cracks in the weld, the weld heat-affected zone, or the base material of the steel plate.

[0047] In one specific embodiment, the microstructure of the P690QL2 steel plate is tempered sorbite.

[0048] This invention also discloses a method for preparing P690QL2 steel plates with low reheat cracking sensitivity for LCO2 transport ship cargo tanks, as described above, including smelting, continuous casting, heating, rolling, and heat treatment; specifically including: In S1, during smelting, continuous casting, and heating, the raw materials are sequentially smelted in a converter, refined in an LF furnace, and refined in an RH furnace to obtain molten steel. The molten steel is then continuously cast into billets and subjected to heating treatment to obtain heated billets.

[0049] S2. During rolling, the heated billet is descaled by high-pressure water after exiting the furnace. The descaled billet is rolled into steel plate in two stages to control the microstructure of the steel plate to be fine granular bainite. The granular bainite grain size is 5~12μm and the morphology of the granular bainite is irregular square, which provides a good grain basis for the subsequent heat treatment. The first stage rolling temperature is ≥1050℃; the second stage rolling temperature is 870~990℃; and the second stage final rolling temperature is 800~940℃.

[0050] In one specific embodiment, when the target thickness t of the P690QL2 steel plate is 10mm≤t<16mm, the second-stage initial rolling temperature is 940~990℃, and the second-stage final rolling temperature is 860~940℃; when the target thickness t of the P690QL2 steel plate is 16mm≤t<30mm, the second-stage initial rolling temperature is 890~960℃, and the second-stage final rolling temperature is 820~880℃; when the target thickness t of the P690QL2 steel plate is 30mm≤t≤50mm, the second-stage initial rolling temperature is 870~920℃, and the second-stage final rolling temperature is 800~860℃, ensuring the accuracy of the rolled steel plate's shape and thickness.

[0051] S6. Heat treatment includes quenching heat treatment and tempering heat treatment. In quenching heat treatment, the rolled steel plate undergoes offline quenching heat treatment. Alloying elements such as Cr, Mn, and Ni in the steel can increase the hardenability of the thick plate. The quenching temperature is 50-80℃ higher than the equilibrium transformation temperature of ferrite and austenite, with a preferred quenching temperature of 915-940℃ and a holding time of 2-4 min / mm. This results in a quenched steel plate, the purpose of which is to ensure that the steel plate is fully austenitized, has a uniform microstructure, and that all alloying elements can... The austenite grains are evenly distributed within the austenite grains, ensuring that the austenite grains are not too coarse, resulting in a martensitic structure after quenching of the steel plate. In the tempering heat treatment, the quenched steel plate is subjected to tempering heat treatment at a temperature 10-50°C lower than the equilibrium phase transformation start temperature of ferrite-austenite, with a preferred tempering temperature of 605-650°C and a holding time of 4-7 min / mm. Sufficient holding time is provided to allow the laths in the quenched martensite to fully decompose. After tempering, the plate is air-cooled. The purpose of tempering is to obtain a tempered sorbite structure.

[0052] In one specific embodiment, the size of the cast billet is 250~300mm.

[0053] Specifically, in this invention, the smelting of P690QL2 low-temperature steel for the liquid cargo tanks of LCO2 transport ships adopts converter + LF + RH refining, which ensures precise control of harmful elements and gas content in the steel; the controlled two-stage rolling ensures the refinement of the initial microstructure; the post-rolling heat treatment adopts quenching + tempering process, which can fully guarantee the strength and low-temperature toughness of the steel plate.

[0054] The following are specific embodiments. The preparation method of P690QL2 steel plate with low reheat cracking sensitivity for LCO2 transport ship cargo tanks in this embodiment includes: smelting (converter smelting + LF refining + RH refining) - continuous casting - heating - rolling - heat treatment. Table 1 shows the components and their contents involved in each embodiment; Table 2 shows the equilibrium phase transformation temperature parameters involved in each embodiment; Table 3 shows the rolling process parameters and heat treatment temperature of each embodiment; Table 4 shows the heat treatment process parameters of each embodiment; Table 5 shows the tensile properties of each embodiment; Table 6 shows the low-temperature impact properties and CTOD properties of each embodiment; Table 7 shows the welding heat input, post-weld heat treatment temperature, and crack assessment of each embodiment; and Table 8 shows the laboratory corrosion rate of the steel plate at 35°C.

[0055] Table 1. Components and their contents (wt%) involved in each embodiment.

[0056] Note: A = wt% (Cr + Mo + 10Nb + 100Ti) Table 2 shows the equilibrium phase transition temperatures involved in each embodiment.

[0057] Table 3. Rolling process parameters and granular bainite grain size for each embodiment.

[0058] Table 4 Heat treatment process parameters for each embodiment

[0059] Table 5 Tensile properties of each embodiment

[0060] Table 6 Low-temperature impact performance and CTOD performance of each embodiment

[0061] Table 7 Welding heat input, post-weld heat treatment temperature, and crack assessment for each embodiment.

[0062] Note: Evaluation method for surface cracks in steel plate welds NB / T 47013.

[0063] Table 8. Laboratory corrosion rate of steel plates at -35℃

[0064] Note: Based on NACE TM0169-2000 (Standard for Total Immersion Corrosion Testing of Metals). Carbon dioxide was kept in a liquid state at -35°C using pressurized liquid cooling technology. P690QL2 steel was immersed in this liquid carbon dioxide environment. After immersion for a certain period, the sample was removed and placed in a 10% HNO3 solution at 60°C for 10–30 minutes until all corrosion products on the sample surface were removed. Then, the sample was cleaned, dried, and weighed to calculate the corrosion loss, thereby obtaining the corrosion rate of the P690QL2 steel.

[0065] The P690QL2 steel plate produced using this invention has a yield strength ≥690MPa, a tensile strength of 770~940MPa, an elongation ≥14%, and a plate thickness of 10mm~50mm. The P690QL2 steel plate exhibits a V-shaped impact energy (KV2) ≥180J at -60℃, a V-shaped impact energy (KV2) ≥100J at -80℃, and a CTOD δ ≥0.2mm at -35℃. In a corrosive medium of liquefied carbon dioxide at -35℃, the corrosion rate is ≤0.005mm / year. Under a heat input of 10~25kJ / cm and a post-weld heat treatment temperature of 550~650℃, there are no reheat cracks in the weld, weld heat-affected zone, or base material. Furthermore, the microstructure of the produced P690QL2 steel plate is tempered sorbite.

[0066] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the patent application. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the scope of protection of the present invention. Therefore, the scope of protection of this patent should be determined by the appended claims.

Claims

1. A P690QL2 steel plate for cargo tanks of LCO2 transport ships with low reheat cracking sensitivity, characterized in that, Includes the following components by weight percentage: C: 0.04%~0.10%, Si: 0.05%~0.30%, Mn: 1.0%~1.6%, Ni: 1.0%~2.5%, Cr: 0.2%~0.4%, Mo: 0.2%~0.4%, Nb: 0.03%~0.10%, Ti: 0.005%~0.020%, B: 0.0005%~0.0020%, S≤0.005%, P≤0.010%, Als: 0.02%~0.05%, with the balance being Fe and unavoidable impurity elements.

2. The P690QL2 steel plate with low reheat cracking sensitivity for LCO2 transport ship cargo tanks according to claim 1, characterized in that, The chemical composition weight percentage ratio of Cr, Mo, Nb and Ti in the P690QL2 steel plate conforms to: 1.2%≤Cr+Mo+10Nb+100Ti≤3.8%.

3. The P690QL2 steel plate with low reheat cracking sensitivity for LCO2 transport ship cargo tanks according to claim 1, characterized in that, The P690QL2 steel plate has a yield strength ≥690MPa, a tensile strength of 770~940MPa, an elongation ≥14%, and a thickness of 10mm~50mm. The P690QL2 steel plate exhibits a V-shaped impact energy (KV2) ≥180J at -60℃, a V-shaped impact energy (KV2) ≥100J at -80℃, and a CTOD δ ≥0.2mm at -35℃. In a corrosive medium of liquefied carbon dioxide at -35℃, the corrosion rate is ≤0.005mm / year. Under a heat input of 10~25kJ / cm and a post-weld heat treatment temperature of 550~650℃, there are no reheat cracks in the weld, the weld heat-affected zone, or the base material of the steel plate.

4. The P690QL2 steel plate with low reheat cracking sensitivity for LCO2 transport ship cargo tanks according to claim 1, characterized in that, The microstructure of the P690QL2 steel plate is tempered sorbite.

5. A method for preparing P690QL2 steel plate with low reheat cracking sensitivity for LCO2 transport ship cargo tanks as described in any one of claims 1-4, characterized in that, This includes smelting, continuous casting, heating, rolling, and heat treatment; In the rolling process, the heated billet is descaled by high-pressure water after exiting the furnace. The initial rolling temperature is ≥1050℃; the initial rolling temperature is 870~990℃; and the final rolling temperature is 800~940℃. The heat treatment includes quenching heat treatment and tempering heat treatment; wherein, in the quenching heat treatment, the rolled steel plate is subjected to offline quenching heat treatment, the quenching temperature is 50~80℃ higher than the ferrite-austenite equilibrium phase transformation end temperature, and the holding time is 2~4min / mm, to obtain the quenched steel plate; in the tempering heat treatment, the quenched steel plate is subjected to tempering heat treatment, the tempering temperature is 10~50℃ lower than the ferrite-austenite equilibrium phase transformation start temperature, and the holding time is 4~7min / mm.

6. The method for preparing P690QL2 steel plate with low reheat cracking sensitivity for LCO2 transport ship cargo tanks according to claim 5, characterized in that, After descaling, the billet is rolled into steel plate in two stages. The microstructure of the steel plate is controlled to be fine granular bainite, with a grain size of 5~12μm and an irregular square morphology.

7. The method for preparing P690QL2 steel plate with low reheat cracking sensitivity for LCO2 transport ship cargo tanks according to claim 5, characterized in that, The size of the cast billet is 250~300mm.

8. The method for preparing P690QL2 steel plate with low reheat cracking sensitivity for LCO2 transport ship cargo tanks according to claim 5, characterized in that, When the target thickness t of P690QL2 steel plate is 10mm≤t<16mm, the second-stage initial rolling temperature is 940~990℃, and the second-stage final rolling temperature is 860~940℃. When the target thickness t of P690QL2 steel plate is 16mm≤t<30mm, the second-stage initial rolling temperature is 890~960℃, and the second-stage final rolling temperature is 820~880℃. When the target thickness t of P690QL2 steel plate is 30mm≤t≤50mm, the second-stage initial rolling temperature is 870~920℃, and the second-stage final rolling temperature is 800~860℃.

9. The method for preparing P690QL2 steel plate with low reheat cracking sensitivity for LCO2 transport ship cargo tanks according to claim 5, characterized in that, The quenching temperature is 915~940℃; the tempering temperature is 605~650℃.

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