Ultra-high strength and thickness marine engineering steel plate with excellent low-temperature CTOD performance of welded joints and its preparation method

Abstract

This invention relates to ultra-high strength and thickness marine engineering steel plates with excellent low-temperature CTOD performance for welded joints and their preparation method. The steel composition is as follows: C: 0.03%–0.08%, Si: 0.15%–0.35%, Mn: 2.0%–3.2%, Nb: 0.02%–0.05%, Cr: 0.2%–0.6%, Ti: 0.012%–0.02%, Cu: 0.1%–0.5%, Ni: 1.2%–2.3%, B: 0.0009%–0.0015%, Mo: 0.15%–0.5%, Al: 0.025%–0.065%, Mg: 0.0045%–0.006%, Y: 0.015%–0.035%, S≤0.001%, P≤0.006%, with the remainder being iron and impurities. The advantages are: the use of a four-stage heating + two-stage rolling + quenching + two-phase critical quenching + tempering process reduces stress concentration at the crack tip and greatly improves the low-temperature toughness of the welded joint.

Description

Technical Field

[0001] This invention belongs to the field of marine engineering steel production, and particularly relates to an ultra-high strength and thickness marine engineering steel plate with excellent low-temperature CTOD performance of welded joints and its preparation method. Background Technology

[0002] With the ever-increasing demand for oil and natural gas, the development of polar and deep-sea resources has become a top priority. The trend of marine engineering equipment development towards deep-sea and extremely cold environments will become more pronounced. This necessitates marine engineering materials with higher strength and excellent low-temperature toughness, making the development of deep-sea, thick-walled, low-temperature ultra-high strength and toughness marine engineering steel an urgent priority. Meanwhile, as a crucial processing technology for connecting steel used in marine equipment, welding can significantly reduce production costs, simplify processes, and improve equipment production efficiency. However, because marine engineering equipment operates in low-temperature, high-pressure environments for extended periods and is frequently subjected to external loads such as sea winds, waves, and sea ice, welded joints are highly susceptible to low-temperature brittle fracture and ultimate strength fracture, severely impacting the operational safety of marine engineering equipment. Therefore, there is an urgent need to develop ultra-high strength marine engineering steel with excellent CTOD (Combined Temperature Coefficient) performance in welded joints.

[0003] Patent publication number CN110616300B discloses a low-temperature steel with excellent CTOD properties and its manufacturing method. While achieving excellent low-temperature toughness of the base steel plate, it also exhibits excellent low-temperature toughness and CTOD in the HAZ during high heat input welding. It successfully resolves the contradictions between low carbon equivalent and high strength, high strength and HAZ low-temperature toughness (especially CTOD), and low-cost manufacturing with excellent low-temperature toughness and high heat input weldability. However, its strength level is only 420 MPa, which cannot meet the ultra-high strength requirements for deep-sea equipment construction.

[0004] Patent publication number CN113549826B discloses a marine engineering steel with excellent CTOD performance for welded joints and its manufacturing method. Through a design with low precious metal alloy content and the addition of trace amounts of rare earth element Ce, a coupled design of alloy composition, smelting, deformation, and heat treatment processes is employed. This results in a marine engineering equipment steel plate with a finished thickness of 60-100mm, excellent low-temperature impact toughness at -40℃, and excellent CTOD performance for welded joints at -10℃. However, the lowest temperature evaluation only considers the CTOD performance of the steel plate at -10℃, which cannot meet the operational requirements of marine engineering equipment in extremely cold regions with temperatures as low as -40℃.

[0005] Patent publication number CN113549827B discloses an FH690 grade marine engineering steel with excellent low-temperature toughness and its manufacturing method. The steel plate of this invention possesses ultra-high strength (yield strength ≥ 690 MPa, tensile strength 770–940 MPa, elongation after fracture ≥ 14%), excellent low-temperature toughness (impact energy ≥ 100 J at -60℃), and uniform microstructure. However, this steel plate does not address the CTOD performance of welded joints, and its maximum thickness is only 50 mm, limiting its application range. Comparing the above prior art, it can be seen that currently, marine engineering steels with excellent CTOD performance mainly have the following problems:

[0006] 1. The steel plate has a low strength level, which cannot meet the construction requirements of deep-sea equipment for ultra-high strength materials.

[0007] 2. The low-temperature CTOD performance is poor, and the safe operation of marine engineering equipment cannot be guaranteed in extremely cold environments.

[0008] 3. The product has a relatively small thickness, limiting its application range. Summary of the Invention

[0009] To overcome the shortcomings of the prior art, the purpose of this invention is to provide an ultra-high strength and thickness marine engineering steel plate with excellent low-temperature CTOD performance of welded joints and its preparation method. This marine engineering steel plate has ultra-high strength, high elongation and excellent low-temperature toughness, which solves the problem that the safe operation of marine engineering equipment in polar and deep-sea environments cannot be guaranteed.

[0010] To achieve the above objectives, the present invention provides the following technical solution:

[0011] A high-strength, thick marine engineering steel plate with excellent low-temperature CTOD performance for welded joints, the chemical composition of the steel by weight percentage is as follows:

[0012] C: 0.03%–0.08%, Si: 0.15%–0.35%, Mn: 2.0%–3.2%, Nb: 0.02%–0.05%, Cr: 0.2%–0.6%, Ti: 0.012%–0.02%, Cu: 0.1%–0.5%, Ni: 1.2%–2.3%, B: 0.0009%–0.0015%, Mo: 0.15%–0.5%, Al: 0.025%–0.065%, Mg: 0.0045%–0.006%, Y: 0.015%–0.035%, S≤0.001%, P≤0.006%, with the remainder being iron and unavoidable impurities.

[0013] The microstructure of the marine engineering steel plate consists of tempered sorbite, critical ferrite, and retained austenite. Statistically, based on the planar area method, the tempered sorbite content is 40%–45%; the critical ferrite content is 45%–51%; and the retained austenite content is 7%–12%.

[0014] The aforementioned marine steel plate welded joint has a CTOD ≥ 0.32 mm at -40℃, a yield strength greater than 690 MPa, a tensile strength of 770~860 MPa, an elongation after fracture ≥ 17%, and an impact absorption energy ≥ 120 J at -60℃.

[0015] The thickness of the steel plate is 80-120mm.

[0016] The mechanism of action of each alloy component in the steel of this invention is as follows:

[0017] C: C is an austenite stabilizing element that significantly affects the strength, hardenability, and low-temperature toughness of steel. C can inhibit the formation of high-temperature phase transformation structures such as proeutectoid ferrite and Widmanstätten, thereby improving the strength and hardness of steel. However, excessive C content will increase the cold cracking sensitivity and brittleness tendency of welded joints, and impair low-temperature toughness. In this invention, the C content is controlled within the range of 0.03% to 0.08%.

[0018] Si: Si is a solid solution strengthening element that can significantly improve the strength and hardenability of steel. Within the diffusion-type phase transformation temperature range, Si promotes the diffusion of carbon atoms, causing them to accumulate at dislocations and form stable Cotillard atmospheres. This pins dislocations, increases the shear resistance during ferrite nucleation, and is beneficial for the formation of acicular ferrite and granular bainite. However, excessive Si content can increase the size of the proto-austenite grains in the welded joint, raising the ductile-brittle transition temperature of the steel and negatively impacting low-temperature toughness. In this invention, the Si content is controlled within the range of 0.15% to 0.35%.

[0019] Mn: Mn is an austenitic temperature-regulating element and also a good desulfurizer and deoxidizer, reducing the adverse effects of elements such as O and S on steel. Adding a certain amount of manganese can promote a rightward shift of the CCT curve, lower the brittle transition temperature of steel, promote the formation of low-phase transformation temperature microstructures, and improve hardenability and strength, significantly enhancing the low-temperature toughness of steel. However, excessive manganese content will slow down the diffusion rate of C in steel and hinder the formation of carbides. In this invention, the Mn content is controlled within the range of 2.0% to 3.2%.

[0020] Nb: In steel, Nb combines with C and N to form highly dispersed carbonitride compounds, which fix the austenite grain boundaries, prevent their migration, hinder austenite grain growth, and refine the austenite grains. Deformation in the austenite region causes strain-induced precipitation of the dispersed carbonitrides, delaying recrystallization and thus improving the strength and low-temperature toughness of the steel plate. Simultaneously, Nb has a high fixing effect on elements such as O and S; the resulting microparticles are insoluble at high temperatures, preventing grain coarsening in the heat-affected zone, improving the low-temperature toughness of welded joints, and enhancing the weldability of the steel. In this invention, the Nb content is controlled within the range of 0.02% to 0.05%.

[0021] Cr: Cr is a ferritinizing element that can lower the transformation temperature of austenite to α-ferrite, inhibit the diffusion ability of C atoms, and greatly improve the hardenability of weld joints. However, excessive addition of Cr will promote the formation of MA components, increase the precipitation of carbides at grain boundaries, reduce the number of large-angle grain boundaries, increase the grain size, and worsen the low-temperature toughness. In this invention, the range of Cr is controlled to be 0.2% to 0.6%.

[0022] Ti: Ti is beneficial for the formation of high-melting-point TiO2 inclusions, which are dispersed and precipitated at the weld joint, pinning austenite grain boundaries and refining the grain structure. However, excessive Ti content promotes the formation of MA components and also forms a large number of non-deformable TiC and TiN inclusions. These inclusions cause stress concentration during steel plate deformation, promote crack initiation at the inclusion sites, and reduce the low-temperature toughness at the weld joint. This invention controls the Ti content to be within the range of 0.012% to 0.02%.

[0023] Cu: Cu is a ferrite-strengthening element. Adding a certain amount of Cu during heat treatment can precipitate a Cu-rich ε-phase in the material matrix, resulting in precipitation strengthening and thus improving the strength and toughness of the steel. However, excessive Cu content may cause copper embrittlement during hot working, reducing the low-temperature toughness of welded parts and making them prone to cracking at higher temperatures. This invention controls the Cu content within the range of 0.1% to 0.5%.

[0024] Ni: Ni has a lattice constant similar to γ-Fe, allowing it to form a continuous solid solution in steel. This improves the strength and toughness of steel through grain refinement and solid solution strengthening. It also inhibits carbide precipitation and enhances austenite stability, resulting in a stable austenitic structure in alloy steels. However, Ni is a precious metal, and considering production costs, this invention controls the Ni content to be between 1.2% and 2.3%.

[0025] Boron (B): Boron can improve the strength and hardenability of steel, which is beneficial for improving the density and hot rolling performance of thick steel plates. At the same time, boron tends to segregate at austenite grain boundaries, lowering the grain boundary energy and austenite transformation temperature, promoting the formation of fine acicular ferrite, and thus improving the low-temperature toughness of the weld. In this invention, the boron content is controlled within the range of 0.0009% to 0.0015%.

[0026] Mo: A small amount of Mo can refine the grains and improve the strength and toughness of the welded joint. However, Mo is also a hardening element. If the Mo content is too high, it will easily cause the weld to become brittle during welding, which is not conducive to the low-temperature toughness of the welded joint. The present invention controls the Mo range to be 0.15% to 0.5%.

[0027] Al: Al is the main deoxidizer in steel, forming Al₂O₃ to achieve deoxidation. Simultaneously, it can combine with nitrogen in the steel to form AlN compounds, creating dispersed second-phase particles that hinder grain boundary movement, inhibit austenite grain growth, and refine the inherent grain size of the steel. However, adding excessive Al results in large-particle Al₂O₃ inclusions, causing stress concentration during pressure loading and damaging the low-temperature toughness of the welded area. This invention controls the Al content within the range of 0.025% to 0.065%.

[0028] Mg: Mg is a strong deoxidizing and chalcophilic element. Adding a certain amount of Mg can modify inclusions in steel, inhibit the formation of dendritic MnS, promote the formation of fine, dispersed spherical MnS, form nanoscale precipitates, pin austenite grain boundaries, thereby refining the grains. It also works with Ti to form MgO+TiN composite inclusions on the MgO surface, which helps improve the low-temperature impact toughness and CTOD performance of the weld heat-affected zone. In this invention, the Mg content is controlled within the range of 0.0045% to 0.006%.

[0029] Y (Y): Y can significantly reduce the content of O and S in steel, forming spherical inclusions with higher melting points that are randomly distributed within the grains, leading to a sharp refinement of the grains without causing stress concentration around them. This eliminates the influence of type II sulfides distributed along grain boundaries on the anisotropy of the steel. Simultaneously, Y can significantly reduce the amount of carbon and nitrogen dissolved in steel, inhibiting their segregation at grain boundaries and crystal defects after dissolution, reducing the number of interstitial atoms pinning dislocations, and thus improving the low-temperature toughness of the steel. In this invention, the range of Y is controlled to be 0.015%–0.035%.

[0030] S: S mainly exists in steel as FeS inclusions, exhibiting low plasticity and high brittleness. It easily forms crack initiation sites, severely impairing the low-temperature toughness of the steel. It also easily causes hot brittleness after hot working, which is detrimental to post-weld CTOD performance. This invention controls the S content to ≤0.001%.

[0031] P: P produces solid solution strengthening and work hardening in steel, which increases strength and hardness but greatly impairs the low-temperature toughness of the steel. Simultaneously, it segregates in the steel, increasing temper brittleness and severely damaging post-weld toughness and CTOD performance. This invention controls the range of P to ≤0.006%.

[0032] A method for preparing ultra-high strength and thick marine engineering steel plates with excellent low-temperature CTOD performance of welded joints includes smelting, continuous casting, heating, rolling and cooling, and heat treatment.

[0033] 1) Continuous casting

[0034] Argon gas protection is applied to the connection between the long nozzle and the immersion nozzle during continuous casting; the tundish superheat is 17-24℃; the end of continuous casting is lightly reduced by 6-12mm to obtain a billet thickness of 240-300mm; after the billet is removed from the line, it is stacked and slowly cooled at a rate of 13.8-15.7℃ / h, and the billet temperature is 350-450℃ when it is loaded into the heating furnace after slow cooling.

[0035] 2) Heating

[0036] The continuous casting billet adopts a low-temperature four-stage heating process:

[0037] Preheating temperature: 770–850℃; preheating time: 50–60 min.

[0038] The temperature of heating section one is 900-980℃, and the heating time is 50-60 minutes.

[0039] The temperature of heating section two is 1000-1100℃, and the heating time is 60-70 minutes.

[0040] The temperature of the soaking zone is 970–1100℃, and the heating time is 90–100 min;

[0041] 3) Rolling and cooling

[0042] The rolling process employs a two-stage finished product rolling process with a large reduction:

[0043] The first-stage rolling temperature is 970-1050℃, the first pass reduction is ≥40mm, the reduction rate of the remaining passes is ≥12%, and the thickness of the intermediate billet is 2.2-2.8 times the thickness of the finished steel plate; the intermediate billet is water-cooled at a cooling rate of 6.1-7.4℃ / s.

[0044] The second-stage rolling temperature is 800-860℃, and the single-pass reduction rate in this stage is ≥11%; the final rolling temperature is 740-820℃; after rolling, the product is slowly cooled in a slow cooling tank with an entry temperature of not less than 400℃ and a slow cooling time of not less than 30 hours.

[0045] 4) Quenching

[0046] The steel plate is subjected to two-stage quenching heat treatment. The first stage quenching temperature is 850-880℃, the heating time is 2.0-2.5 min / mm × steel plate thickness, and the holding time is 40-60 min. The second stage quenching temperature is 690-720℃, the heating time is 2.3-2.7 min / mm × steel plate thickness, and the holding time is 60-80 min.

[0047] 5) Tempering

[0048] The steel plate is tempered at a temperature of 580–620℃ for 1.8–2.2 min / mm × steel plate thickness, and then air-cooled after being removed from the furnace.

[0049] The smelting process involves adding pre-desulfurized molten iron into a converter and smelting using a double-slag method. Electrolytic nickel, electrolytic copper, and ferromolybdenum alloy are added first. High-phosphorus slag is discharged in the early stage, and then slag material is added again to form slag. Finally, alloys are added in the converter for deoxidation and alloying.

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

[0051] 1. This invention employs a low-carbon composition design, replacing some of the expensive Ni element with Mn. Through the composite addition of Nb and Ti micro-isomorphic carbonitride forming elements, the grain size is refined, improving the steel's strength, elongation, and low-temperature toughness of the welded joint. The introduction of Mg and Y elements significantly reduces the O and S content in the steel and substantially reduces the amount of C and N dissolved, altering non-metallic inclusions and thus greatly improving the low-temperature CTOD performance of the welded joint of the steel plate.

[0052] 2. This invention adopts a four-stage heating + two-stage rolling + quenching + two-phase critical quenching + tempering process to generate highly stable residual austenite under low temperature conditions, reduce stress concentration at crack tips, greatly improve the low temperature toughness of welded joints, and achieve a CTOD ≥ 0.32 mm at -40℃ for welded joints.

[0053] 3. The ultra-high strength and thickness of the marine engineering steel plate has a mixed microstructure of tempered sorbite, critical ferrite and retained austenite, which has excellent mechanical properties and low-temperature CTOD performance of welded joints. The yield strength is greater than 690MPa, the tensile strength is 770~860MPa, the elongation after fracture is ≥17%, the impact energy absorbed at -60℃ is ≥120J, and the CTOD at -40℃ is ≥0.32mm. Attached Figure Description

[0054] Figure 1 This is a metallographic image (500x) of Example 1. Detailed Implementation

[0055] The present invention will now be described in detail with reference to the accompanying drawings, but it should be noted that the implementation of the present invention is not limited to the following embodiments.

[0056] A high-strength, thick marine engineering steel plate with excellent low-temperature CTOD performance for welded joints. The chemical composition of the steel, by weight percentage, is as follows: C: 0.03%–0.08%, Si: 0.15%–0.35%, Mn: 2.0%–3.2%, Nb: 0.02%–0.05%, Cr: 0.2%–0.6%, Ti: 0.012%–0.02%, Cu: 0.1%–0.5%, Ni: 1.2%–2.3%, B: 0.0009%–0.0015%, Mo: 0.15%–0.5%, Al: 0.025%–0.065%, Mg: 0.0045%–0.006%, Y: 0.015%–0.035%, S≤0.001%, P≤0.006%, with the remainder being iron and unavoidable impurities. The microstructure of marine engineering steel plates consists of tempered sorbite + critical ferrite + retained austenite. Based on the planar area method, the tempered sorbite content is 40%–45%; the critical ferrite content is 45%–51%; and the retained austenite content is 7%–12%.

[0057] A method for preparing an ultra-high strength and thick marine engineering steel plate with excellent low-temperature CTOD performance of welded joints is as follows:

[0058] 1) Smelting process

[0059] a) Double slag dephosphorization: Pre-desulfurized molten iron is added to the converter and smelted using the double slag method. First, electrolytic nickel, electrolytic copper, and ferromolybdenum alloy are added. High-phosphorus slag is discharged in the early stage, and then slag is added again to form slag to reduce the phosphorus content in the steel. Finally, various alloys are added in the converter for deoxidation and alloying.

[0060] b) LF refining: The molten steel is refined by LF. During the LF refining process, a deoxidizer is added to create a reducing slag, remove sulfur from the steel, fine-tune the chemical composition of each element, remove inclusions from the steel, and achieve deep desulfurization.

[0061] c) VD treatment: The molten steel is subjected to VD treatment to ensure vacuum degassing time. The process control of vacuum holding pressure for 10 minutes + weak argon blowing for 15 minutes promotes the floating of inclusions generated during refining into the slag, and reduces the nitrogen and oxygen content to the lowest possible level, thereby further removing inclusions from the steel.

[0062] Purification metallurgical process control is adopted to reduce the content of impurity elements such as S, P, O, and N in steel.

[0063] 2) Continuous casting process: During continuous casting, the connection between the long nozzle and the submerged entry nozzle is protected with argon gas throughout the process to reduce oxygen and nitrogen absorption by the molten steel. The superheat of the tundish is controlled at 17–24℃. A light reduction technique is used at the end of the continuous casting process, with a reduction of 6–12 mm, resulting in a billet thickness of 240–300 mm. After the billets are removed from the line, they are stacked and slowly cooled to eliminate stress within the billets. The stacking and slow cooling rate is 13.8–15.7℃ / h. The temperature of the billets is controlled at 350–450℃ when they are loaded into the heating furnace after slow cooling.

[0064] 3) Heating Process: The continuously cast billet adopts a low-temperature four-stage heating process. Preheating temperature: 770–850℃, preheating time: 50–60 min. Heating stage one temperature: 900–980℃, heating time: 50–60 min. Heating stage two temperature: 1000–1100℃, heating time: 60–70 min. Soaking stage temperature: 970–1100℃, heating time: 90–100 min. Appropriately reducing the heating stage time avoids abnormal growth of the as-cast microstructure during the heating phase.

[0065] 4) Rolling and Cooling Process: The rolling process employs a two-stage rolling process with large reduction. The first stage starts at a rolling temperature of 970–1050℃, with a first-pass reduction ≥40mm and subsequent passes reducing by ≥12%. The intermediate billet thickness is controlled to be 2.2–2.8 times the thickness of the finished steel plate. The intermediate billet is water-cooled at a rate of 6.1–7.4℃ / s to prevent further grain growth and maintain grain refinement. The second stage starts at a rolling temperature of 800–860℃, with a minimum single-pass reduction of ≥11%. The final rolling temperature is 740–820℃. After rolling, the billet is slowly cooled in a slow cooling bath at an entry temperature not lower than 400℃ for at least 30 hours.

[0066] 5) Quenching process: The steel plate is subjected to two-stage quenching heat treatment. The first stage quenching temperature is 850~880℃, the heating time is 2.0~2.5min / mm×steel plate thickness, and the holding time is 40~60min. The second stage quenching temperature is 690~720℃, the heating time is 2.3~2.7min / mm×steel plate thickness, and the holding time is 60~80min.

[0067] The two-stage two-phase region critical quenching method can transform the matrix into some critical ferrites. During tempering, a multiphase structure of critical ferrite, tempered sorbite, and reversed austenite can be formed. At the same time, two-phase region critical quenching can promote the enrichment of alloying elements in the reversed austenite, promote the rapid formation of reversed austenite during tempering, increase the soft phase structure in the steel, improve low-temperature toughness, and reduce stress concentration at crack tips.

[0068] 6) Tempering process: The steel plate is tempered at a temperature of 580-620℃ for 1.8-2.2 min / mm × steel plate thickness, and then air-cooled after being taken out of the furnace.

[0069] The chemical composition of the steel in this embodiment is shown in Table 1; the smelting and billet pretreatment parameters of the steel in this embodiment are shown in Table 2; the slab heating process of the steel plate in this embodiment is shown in Table 3; the rolling and cooling process of the steel plate in this embodiment is shown in Table 4; the heat treatment process of the steel plate in this embodiment is shown in Table 5; the mechanical properties and welding CTOD properties of the steel plate in this embodiment are shown in Table 6; and the metallographic image of Example 1 is shown in Table 6. Figure 1 .

[0070] Table 1. Components of each embodiment

[0071]

[0072]

[0073] Table 2 Steel smelting and billet pretreatment parameters

[0074]

[0075] Table 3 Heating Process

[0076]

[0077]

[0078] Table 4 Rolling and Cooling Processes

[0079]

[0080] Table 5 Heat Treatment Process

[0081]

[0082]

[0083] Table 6 Mechanical properties and welding CTOD performance of steel plates

[0084]

[0085]

[0086] This invention employs a four-stage heating + two-stage rolling + quenching + two-phase critical quenching + tempering process to generate highly stable residual austenite under low-temperature conditions, reduce stress concentration at crack tips, and greatly improve the low-temperature toughness of the welded joint, achieving a CTOD ≥ 0.32 mm at -40℃ for the welded joint.

Claims

1. A high-strength, thick marine engineering steel plate with excellent low-temperature CTOD performance for welded joints, characterized in that, The chemical composition of the steel, expressed as a percentage by weight, is as follows: C: 0.03%~0.08%, Si: 0.15%~0.35%, Mn: 2.0%~3.2%, Nb: 0.02%~0.05%, Cr: 0.2%~0.6%, Ti: 0.012%~0.02%, Cu: 0.1%~0.5%, Ni: 1.2%~2.3%, B: 0.0009%~0.0015%, Mo: 0.15%~0.5%, Al: 0.025%~0.065%, Mg: 0.0045%~0.006%, Y: 0.015%~0.035%, S≤0.001%, P≤0.006%, with the remainder being iron and unavoidable impurities; The microstructure of the marine engineering steel plate consists of tempered sorbite, critical ferrite, and retained austenite. Statistically, using the planar area method, the tempered sorbite content is 40%–45%; the critical ferrite content is 45%–51%; and the retained austenite content is 7%–12%. The thickness of the steel plate is 80~120mm.

2. The ultra-high strength and thickness marine engineering steel plate with excellent low-temperature CTOD performance of welded joints according to claim 1, characterized in that, The aforementioned marine steel plate welded joint has a CTOD ≥ 0.32 mm at -40℃, a yield strength greater than 690 MPa, a tensile strength of 770~860 MPa, an elongation after fracture ≥ 17%, and an impact absorption energy ≥ 120 J at -60℃.

3. A method for preparing an ultra-high strength and thick marine steel plate with excellent low-temperature CTOD performance of the welded joint as described in claim 1 or 2, comprising smelting, continuous casting, heating, rolling and cooling, and heat treatment, characterized in that: 1) Continuous casting Argon gas protection is applied to the connection between the long nozzle and the immersion nozzle during continuous casting; the tundish superheat is 17~24℃; the end of continuous casting is lightly reduced by 6~12mm to obtain a billet thickness of 240~300mm; after the billet is removed from the line, it is stacked and slowly cooled at a rate of 13.8~15.7℃ / h, and the billet temperature is 350~450℃ when it is loaded into the heating furnace after slow cooling. 2) Heating The continuous casting billet adopts a low-temperature four-stage heating process: Preheating temperature 770~850℃, preheating time 50~60min; The temperature of heating section one is 900~980℃, and the heating time is 50~60min; The temperature of heating section two is 1000~1100℃, and the heating time is 60~70min; The temperature of the soaking zone is 970~1100℃, and the heating time is 90~100min; 3) Rolling and cooling The rolling process employs a two-stage finished product rolling process with a large reduction: The first-stage rolling temperature is 970~1050℃, the first pass reduction is ≥40mm, the reduction rate of the remaining passes is ≥12%, and the thickness of the intermediate billet is 2.2~2.8 times the thickness of the finished steel plate; the intermediate billet is water-cooled at a cooling rate of 6.1~7.4℃ / s. The second-stage rolling temperature is 800~860℃, and the single-pass reduction rate in this stage is ≥11%; the final rolling temperature is 740~820℃; after rolling, the roll is slowly cooled in a slow cooling tank with an entry temperature of not less than 400℃ and a slow cooling time of not less than 30 hours. 4) Quenching The steel plate is subjected to two-stage quenching heat treatment. The first stage quenching temperature is 850~880℃, the heating time is 2.0~2.5min / mm×steel plate thickness, and the holding time is 40~60min. The second stage quenching temperature is 690~720℃, the heating time is 2.3~2.7min / mm×steel plate thickness, and the holding time is 60~80min. 5) Tempering The steel plate is tempered at a temperature of 580~620℃ for 1.8~2.2 min / mm×steel plate thickness, and then air-cooled after being taken out of the furnace.

4. The method for preparing an ultra-high strength and thick marine engineering steel plate with excellent low-temperature CTOD performance of welded joints according to claim 3, characterized in that, The smelting process involves adding pre-desulfurized molten iron into a converter and smelting using a double-slag method. Electrolytic nickel, electrolytic copper, and ferromolybdenum alloy are added first. High-phosphorus slag is discharged in the early stage, and then slag material is added again to form slag. Finally, alloys are added in the converter for deoxidation and alloying.

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