Ultrahigh strength and thickness maritime work steel plate with excellent welding joint low-temperature CTOD performance and preparation method of ultrahigh strength and thickness maritime work steel plate

Abstract

The invention relates to an ultrahigh strength and thickness maritime work steel plate with excellent welding joint low-temperature CTOD performance and a preparation method. The steel comprises the following components: 0.03%-0.08% of C, 0.15%-0.35% of Si, 2.0%-3.2% of Mn, 0.02%-0.05% of Nb, 0.2%-0.6% of Cr, 0.012%-0.02% of Ti, 0.1%-0.5% of Cu, 1.2%-2.3% of Ni, 0.0009%-0.0015% of B, 0.15%-0.5% of Mo, 0.025%-0.065% of Al, 0.0045%-0.006% of Mg, 0.015%-0.035% of Y, less than or equal to 0.001% of S, less than or equal to 0.006% of P and the balance of iron and impurities. The method has the advantages that a four-section heating, two-stage rolling, quenching, two-phase region critical quenching and tempering process is adopted, so that stress concentration at the tip of a crack is relieved, and the low-temperature toughness of a welded joint is greatly improved.

Description

Technical Field

[0001] The invention belongs to the field of marine engineering steel production, and in particular relates to an ultra-high strength and thickness marine engineering steel plate with excellent low-temperature CTOD performance of welded joints and a preparation method thereof. Background Art

[0002] With the growing demand for oil and natural gas, the development of polar deep-sea resources has become a top priority. The trend of marine engineering equipment developing towards deep sea and extremely cold environments will become more obvious. This requires marine engineering materials to have higher strength and excellent low-temperature toughness, and the development of deep-sea, large-thickness, low-temperature, ultra-high-strength and tough marine engineering steel is imminent. At the same time, as an important processing technology for connecting steel for marine equipment, welding can greatly reduce production costs, reduce process complexity, and improve equipment production efficiency. However, since marine engineering equipment has been serving in low-temperature and high-pressure environments for a long time, and is often affected by external loads such as sea breeze, waves, and sea ice, it is very easy to cause low-temperature brittle fracture and ultimate strength fracture failure at the welded joints, which seriously affects the service safety of marine engineering equipment. Therefore, it is urgent to develop ultra-high-strength marine engineering steel with excellent CTOD performance in welded joints.

[0003] The patent publication number is CN110616300B, which discloses a low-temperature steel with excellent CTOD characteristics and its manufacturing method. While obtaining excellent low-temperature toughness of the base steel plate, the low-temperature toughness and CTOD of the HAZ during high heat input welding are also excellent; it successfully solves the contradiction between low carbon equivalent and high strength, the contradiction between high strength and HAZ low-temperature toughness, especially CTOD, and the contradiction between low-cost manufacturing and excellent low-temperature toughness and high heat input weldability. However, its strength level is only 420MPa, which cannot meet the construction requirements of deep-sea equipment for ultra-high strength materials.

[0004] The patent publication number is CN113549826B, which discloses a marine engineering steel with excellent CTOD performance of welded joints and its manufacturing method. Through the design of low precious metal alloy content, the addition of trace rare earth element Ce, and the use of alloy composition design-smelting-deformation-heat treatment process coupling design, a finished product thickness of 60-100mm, -40℃ low-temperature impact toughness and -10℃ welded joint CTOD performance of marine equipment steel plate with excellent performance is finally obtained. However, its lowest temperature only evaluates the CTOD performance of the steel plate at -10℃, which cannot meet the working requirements of marine engineering equipment in extremely cold areas with a temperature of -40℃.

[0005] The patent publication number is CN113549827B, which discloses a FH690 grade marine engineering steel with excellent low temperature toughness and its manufacturing method. The steel plate of the present invention has ultra-high strength (yield strength ≥ 690MPa, tensile strength 770-940MPa, elongation after fracture ≥ 14%), excellent low temperature toughness (-60℃ impact energy ≥ 100J), and uniform microstructure and performance. However, the steel plate does not involve the CTOD performance of the welded joint, and the maximum thickness of the steel plate is only 50mm, which limits its scope of use. Compared with the above prior art, it can be seen that the current marine engineering steel with excellent CTOD performance mainly has the following problems:

[0006] 1. The strength level of steel plates is low and cannot meet the construction requirements of deep-sea equipment for ultra-high strength materials.

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

[0008] 3. The product thickness specification is small and the scope of use is narrow. Summary of the invention

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

[0010] To achieve the above object, the present invention is implemented through the following technical solutions:

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

[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%, the rest are iron and unavoidable impurities.

[0013] The offshore steel plate has a microstructure of tempered troostite, critical ferrite and retained austenite. According to the plane area method, the content of tempered troostite is 40% to 45%, the content of critical ferrite is 45% to 51% and the content of retained austenite is 7% to 12%.

[0014] The marine steel plate welded joint has a -40°C CTOD of ≥0.32 mm, a yield strength of greater than 690 MPa, a tensile strength of 770-860 MPa, a post-break elongation of ≥17%, and a -60°C impact absorption energy of ≥120 J.

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

[0016] Among them, the action mechanism of each alloy component in the steel of the present invention is as follows:

[0017] C: C is an austenite stabilizing element and has a great influence on 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 widmanstattenite, and improve the strength and hardness of steel. However, too high a C content will increase the cold crack sensitivity and brittleness tendency of the welded joint, and damage the low temperature toughness. The present invention controls the C content in 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. In the diffusion phase transformation temperature zone, Si can promote the diffusion of C atoms, gather at dislocations, form stable Coriolis gas clusters, pin dislocations, increase the shear resistance during ferrite nucleation, and facilitate the formation of acicular ferrite and granular bainite. However, excessive Si content will increase the original austenite grains in the welded joint, increase the toughness-brittleness transition temperature of the steel, and be detrimental to low-temperature toughness. The present invention controls the range of Si to be 0.15% to 0.35%.

[0019] Mn: Mn is an austenite temperature element, and is 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 the right shift of the CCT curve, reduce the brittle transition temperature of steel, promote the formation of low phase transition temperature structure, and improve hardenability and strength, greatly improving the low-temperature toughness of steel. However, too high manganese content will slow down the diffusion rate of C in steel and hinder the formation of carbides. The present invention controls the range of Mn to 2.0% to 3.2%.

[0020] Nb: Nb combines with C and N in steel to form highly dispersed carbonitrides, which play a role in fixing the austenite grain boundaries, preventing the migration of austenite grain boundaries, hindering the growth of austenite grains, and playing a role in refining austenite grains. Deformation in the austenite region will cause the dispersed carbonitrides to produce strain-induced precipitation, delaying recrystallization, thereby improving the strength and low-temperature toughness of the steel plate. At the same time, the Nb element has a high fixing effect on elements such as O and S. The generated microscopic particles are insoluble at high temperatures, which can prevent the grain coarsening of the heat-affected zone, improve the low-temperature toughness of the steel plate welded joint, and improve the welding performance of the steel. The present invention controls the range of Nb to be 0.02% to 0.05%.

[0021] Cr: Cr is a ferritizing element, which can reduce the transformation temperature of austenite to α-ferrite, inhibit the diffusion capacity of C atoms, and greatly improve the hardenability of the welded joint. However, excessive addition of Cr elements will promote the formation of MA components, increase the precipitation of carbides at the grain boundaries, reduce the number of large-angle grain boundaries, increase the grain size of the organization, and deteriorate the low-temperature toughness. The present invention controls the range of Cr to 0.2% to 0.6%.

[0022] Ti: Ti is conducive to the formation of high melting point TiO2 inclusions, which are dispersed and precipitated at the weld joint, pinning the austenite grain boundary and refining the grain size of the structure. However, too high Ti content will promote the formation of MA components and form a large number of TiC and TiN inclusions that are not easy to deform, which will cause stress concentration during the deformation of the steel plate, promote the generation of crack sources at the inclusions, and reduce the low temperature toughness at the weld joint. The present invention controls the range of Ti to be 0.012% to 0.02%.

[0023] Cu: Cu is a ferrite strengthening element. Adding a certain amount of Cu can precipitate Cu-rich ε-phase in the material matrix during heat treatment, resulting in precipitation strengthening, thereby improving the strength and toughness of the steel. However, excessive Cu content may cause copper brittleness during hot working, reduce the low-temperature toughness of the welding part, and easily generate cracks at high temperatures. The present invention controls the range of Cu to be 0.1% to 0.5%.

[0024] Ni: Ni has a lattice constant similar to that of γ-Fe, and can form a continuous solid solution in steel, thereby improving the strength and toughness of steel through grain refinement and solid solution strengthening. It can also inhibit the precipitation of carbides, improve the stability of austenite, and form a stable austenite structure in alloy steel. However, Ni is a precious metal, and considering the production cost, the present invention controls Ni in the range of 1.2% to 2.3%.

[0025] B: B can improve the strength and hardenability of steel, which is beneficial to improving the compactness and hot rolling performance of thick steel plates. At the same time, the B element is easy to segregate at the austenite grain boundary, reduce the grain boundary energy and austenite transformation phase transition temperature, promote the formation of fine needle-shaped ferrite, and help improve the low-temperature toughness of the weld. The present invention controls the range of B to be 0.0009% to 0.0015%.

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

[0027] Al: Al is the main deoxidizer in steel, forming Al2O3 to achieve deoxidation. At the same time, it can combine with N in steel to form AlN compound dispersed second phase particles to hinder grain boundary movement, inhibit austenite grain growth, and refine the intrinsic grain of steel. However, adding excessive Al will produce large-grained Al2O3 inclusions, which will cause stress concentration during pressure loading and destroy the low-temperature toughness of the welding part. The present invention controls the range of Al to 0.025% to 0.065%.

[0028] Mg: Mg is a strong deoxidizing and sulfur-affinity element. Adding a certain amount of Mg can modify the inclusions in the steel, inhibit the formation of dendritic MnS, promote the formation of fine and dispersed spherical MnS, form nano-scale precipitates, pin the austenite grain boundaries to refine the grains, and work together with the Ti element to form MgO+TiN composite inclusions on the MgO surface, which helps to improve the low-temperature impact toughness and CTOD performance of the welding heat-affected zone. The present invention controls the range of Mg to be 0.0045% to 0.006%.

[0029] Y: Y element can deeply reduce the content of O and S in steel, forming spherical inclusions with high melting point and random distribution in the crystal, leading to rapid refinement of grains without causing stress concentration around them, eliminating the influence of the second type of sulfides distributed along the grain boundaries on the anisotropy of steel. At the same time, Y can significantly reduce the amount of carbon and nitrogen in the steel, inhibit the segregation at the grain boundaries and crystal defects after the dissolution, reduce the number of interstitial atoms pinning dislocations, and thus improve the low-temperature toughness of the steel. The present invention controls the range of Y to be 0.015% to 0.035%.

[0030] S: S mainly exists in the form of FeS inclusions in steel, which has low plasticity and high brittleness, and is easy to form crack sources, seriously damaging the low-temperature toughness of steel. It is easy to produce hot brittleness after hot working, which is not conducive to the CTOD performance after welding. The present invention controls the range of S to ≤0.001%.

[0031] P: P produces solid solution strengthening and cold work hardening in steel, which will increase the strength and hardness while greatly damaging the low-temperature toughness of the steel. At the same time, P segregates in the steel, increases the temper brittleness of the steel, and seriously damages the post-weld toughness and CTOD performance of the steel. The present invention controls the range of P to be ≤0.006%.

[0032] A method for preparing an ultra-high strength and thickness marine engineering steel plate with excellent low-temperature CTOD performance of welded joints, comprising smelting, continuous casting, heating, rolling and cooling, and heat treatment:

[0033] 1) Continuous casting

[0034] During the continuous casting process, the connection between the long nozzle and the immersion nozzle is protected by argon gas; the tundish superheat is 17-24°C; the light reduction at the end of continuous casting is 6-12mm, and the thickness of the cast billet is 240-300mm; the cast billet after rolling off the line is stacked and slowly cooled, with a stacking slow cooling rate of 13.8-15.7°C / h, and the temperature of the slab when loaded into the heating furnace after slow cooling is 350-450°C;

[0035] 2) Heating

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

[0037] Preheating temperature: 770~850℃, preheating time: 50~60min;

[0038] Heating stage 1: temperature 900-980°C, heating time 50-60min;

[0039] The temperature of the second heating section is 1000-1100°C, and the heating time is 60-70 minutes;

[0040] The temperature of the soaking section is 970-1100°C, and the heating time is 90-100 minutes;

[0041] 3) Rolling and cooling

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

[0043] The first stage rolling temperature is 970-1050℃, the first pass reduction is ≥40mm, the other passes reduction is ≥12%, the intermediate billet thickness is 2.2-2.8 times the thickness of the finished steel plate; the intermediate billet is water-cooled, and the cooling rate is 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 steel is put into the slow cooling tank for slow cooling, the temperature of the tank is not less than 400℃, and the slow cooling time is 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°C, the heating time is 2.0-2.5min / mm×steel plate thickness, the holding time is 40-60min, the second-stage quenching temperature is 690-720°C, the heating time is 2.3-2.7min / mm×steel plate thickness, and the holding time is 60-80min;

[0047] 5) Tempering

[0048] The steel plate is tempered at a temperature of 580-620°C and a furnace time of 1.8-2.2 min / mm×steel plate thickness, and then air-cooled after being taken out of the furnace.

[0049] The smelting process is to add pre-desulfurized molten iron into the converter, adopt the double slag method to smelt, first add electrolytic nickel, electrolytic copper, and molybdenum-iron alloy, release the high-phosphorus slag in the early stage, then add slag again to make slag, and then add alloy in the converter for deoxidation and alloying.

[0050] Compared with the prior art, the present invention has the following beneficial effects:

[0051] 1. The present invention adopts a low-carbon component design, replaces part of the expensive Ni element with the Mn element, and grinds and refines the grains through the composite addition of Nb and Ti microisomorphic carbonitride forming elements to improve the strength, elongation and low-temperature toughness of the steel welded joint. The introduction of elements Mg and Y deeply reduces the content of O and S in the steel, and significantly reduces the amount of C and N in the steel. The non-metallic inclusions in the steel are deteriorated, thereby greatly improving the low-temperature CTOD performance of the steel plate welded joint.

[0052] 2. The present 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, alleviate stress concentration at the crack tip, greatly improve the low-temperature toughness of the welded joint, and achieve a welded joint CTOD ≥ 0.32 mm at -40 °C.

[0053] 3. The microstructure of ultra-high strength and thickness marine engineering steel plate is a mixed structure of tempered troostite + critical ferrite + retained austenite, with excellent mechanical properties and low-temperature CTOD performance of welded joints, yield strength greater than 690MPa, tensile strength 770-860MPa, elongation after fracture ≥17%, -60℃ impact absorption energy ≥120J, -40℃ CTOD ≥0.32mm. BRIEF DESCRIPTION OF THE DRAWINGS

[0054] Figure 1 This is the metallographic structure diagram of Example 1 (500 times). DETAILED DESCRIPTION

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

[0056] The invention discloses an ultra-high strength and thickness marine engineering steel plate with excellent low-temperature CTOD performance of a welded joint. The chemical composition of the steel is as follows by weight percentage: 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%, and the rest is iron and unavoidable impurities. The structure of offshore steel plate is tempered troostite + critical ferrite + retained austenite. According to the plane area method, the content of tempered troostite is 40% to 45%; the content of critical ferrite is 45% to 51%; and the content of retained austenite is 7% to 12%.

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

[0058] 1) Smelting process

[0059] a) Double slag method for dephosphorization: Pre-desulfurized molten iron is added into the converter and smelted by the double slag method. Electrolytic nickel, electrolytic copper and molybdenum-iron alloy are added first. The high-phosphorus slag is discharged in the early stage and then the slag is added again to make slag to reduce the phosphorus content in the steel. Various alloys are then added into the converter for deoxidation and alloying.

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

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

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

[0063] 2) Continuous casting process: During the continuous casting process, the long water inlet and the immersion water inlet are fully protected by argon gas to reduce the absorption of oxygen and nitrogen by the molten steel. The superheat of the tundish is controlled at 17-24°C. The light pressure reduction technology is adopted at the end of continuous casting, with a pressure reduction of 6-12mm, and the thickness of the ingot is 240-300mm. The ingots after the line are stacked and slowly cooled to eliminate the stress in the ingots. The stacking slow cooling rate is 13.8-15.7°C / h. After slow cooling, the temperature of the slabs when loaded into the heating furnace is controlled at 350-450°C.

[0064] 3) Heating process: The continuous casting billet adopts a low-temperature four-stage heating process. Preheating temperature 770-850℃, preheating time 50-60min. Heating section 1 temperature 900-980℃, heating time 50-60min. Heating section 2 temperature 1000-1100℃, heating time 60-70min. Soaking section temperature 970-1100℃, heating time 90-100min. Appropriately reduce the heating section time to avoid abnormal growth of the cast structure during the heating stage.

[0065] 4) Rolling and cooling process: The rolling process adopts two-stage finished product rolling with large reduction. The first-stage rolling temperature is 970-1050℃, the first pass reduction is ≥40mm, and the other pass reduction rate is ≥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 with a cooling rate of 6.1-7.4℃ / s to prevent the intermediate billet grains from continuing to grow and play a role in maintaining grain refinement. The second-stage rolling temperature is 800-860℃, and the minimum single-pass reduction rate in this stage is ≥11%. The final rolling temperature is 740-820℃. After rolling, it is slowly cooled in a slow cooling tank, the tank entry temperature is not less than 400℃, and the slow cooling time is not less than 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°C, 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°C, the heating time is 2.3-2.7min / mm×steel plate thickness, and the holding time is 60-80min.

[0067] The matrix can be transformed into some critical ferrite by two-stage two-phase critical quenching, and a complex structure of critical ferrite, tempered troostite and reversed austenite can be formed during tempering. At the same time, two-phase critical quenching can promote the enrichment of alloy elements in reversed austenite, promote the rapid formation of reversed austenite during tempering, increase the soft phase structure in steel, improve low-temperature toughness, and alleviate stress concentration at the crack tip.

[0068] 6) Tempering process: The steel plate is tempered at a temperature of 580-620°C and in the furnace for 1.8-2.2 min / mm × steel plate thickness. It is air-cooled after leaving the furnace.

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

[0070] Table 1 Composition 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 process

[0079]

[0080] Table 5 Heat treatment process

[0081]

[0082]

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

[0084]

[0085]

[0086] The present 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, alleviate stress concentration at the crack tip, greatly improve the low-temperature toughness of the welded joint, and achieve a -40°C CTOD of the welded joint ≥ 0.32mm.

Claims

1. An ultra-high strength and thickness marine steel plate with excellent low temperature CTOD performance of welded joints, It is characterized in that The chemical composition of steel by weight percentage is: 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%, the rest are iron and unavoidable impurities.

2. According to claim 1, an ultra-high strength and thickness marine steel plate with excellent low temperature CTOD performance of welded joints, It is characterized in that The offshore steel plate has a microstructure of tempered troostite, critical ferrite and retained austenite. According to the plane area method, the content of tempered troostite is 40% to 45%, the content of critical ferrite is 45% to 51% and the content of retained austenite is 7% to 12%.

3. The ultra-high strength and thickness marine steel plate with excellent low temperature CTOD performance of welded joints according to claim 1, It is characterized in that The marine steel plate welded joint has a -40°C CTOD of ≥0.32 mm, a yield strength of greater than 690 MPa, a tensile strength of 770-860 MPa, a post-break elongation of ≥17%, and a -60°C impact absorption energy of ≥120 J.

4. The ultra-high strength and thickness marine steel plate with excellent low temperature CTOD performance of welded joints according to claim 1, It is characterized in that The thickness of the steel plate is 80-120 mm.

5. A method for preparing an ultra-high strength and thickness marine engineering steel plate with excellent low temperature CTOD performance of welded joints as claimed in any one of claims 1 to 4, comprising smelting, continuous casting, heating, rolling and cooling, and heat treatment, Features: 1) Continuous casting During the continuous casting process, the connection between the long nozzle and the immersion nozzle is protected by argon gas; the tundish superheat is 17-24°C; the light reduction at the end of continuous casting is 6-12mm, and the thickness of the cast billet is 240-300mm; the cast billet after rolling off the line is stacked and slowly cooled, with a stacking slow cooling rate of 13.8-15.7°C / h, and the temperature of the slab when loaded into the heating furnace after slow cooling is 350-450°C; 2) Heating The continuous casting billet adopts a low-temperature four-stage heating process: Preheating temperature: 770~850℃, preheating time: 50~60min; Heating stage 1: temperature 900-980°C, heating time 50-60min; The temperature of the second heating section is 1000-1100°C, and the heating time is 60-70 minutes; The temperature of the soaking section is 970-1100°C, and the heating time is 90-100 minutes; 3) Rolling and cooling The rolling process uses a large reduction two-stage finished product rolling: The first stage rolling temperature is 970-1050℃, the first pass reduction is ≥40mm, the other passes reduction is ≥12%, the intermediate billet thickness is 2.2-2.8 times the thickness of the finished steel plate; the intermediate billet is water-cooled, and the cooling rate is 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 steel is put into the slow cooling tank for slow cooling, the temperature of the tank is not less than 400℃, and the slow cooling time is 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°C, the heating time is 2.0-2.5min / mm×steel plate thickness, the holding time is 40-60min, the second-stage quenching temperature is 690-720°C, 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°C and a furnace time of 1.8-2.2 min / mm×steel plate thickness, and then air-cooled after being taken out of the furnace.

6. The method for preparing an ultra-high strength and thickness marine steel plate with excellent low temperature CTOD performance of welded joints according to claim 5, It is characterized in that The smelting process is to add pre-desulfurized molten iron into the converter, adopt the double slag method to smelt, first add electrolytic nickel, electrolytic copper, and molybdenum-iron alloy, release the high-phosphorus slag in the early stage, then add slag again to make slag, and then add alloy in the converter for deoxidation and alloying.

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