250mm-260mm thick EH690Z35 steel for ocean engineering and production method of 250mm-260mm thick EH690Z35 steel

Abstract

The invention discloses EH690Z35 steel with the thickness of 250-260 mm for ocean engineering and a production method of the EH690Z35 steel. The EH690Z35 steel is prepared from the following chemical components in percentage by mass (wt%): 0.11-0.13% of C, 0.05-0.15% of Si, 0.5-0.6% of Mn, less than or equal to 0.003% of P, less than or equal to 0.003% of S, 0.020-0.040% of Als, 0.035-0.045% of Nb, 0.035-0.045% of V, 0.020-0.030% of Ti, 0.6-0.65% of Cr, 3.4-3.6% of Ni, 0.25-0.3% of Cu, 0.6-0.65% of Mo, 0.0015-0.0025% of B and the balance of Fe and residual elements. The carbon equivalent CEV is controlled to be 0.68 to 0.72. The adding amount of hardenability elements is reasonably controlled, meanwhile, the content of harmful elements in the steel is reduced through a low-manganese low-phosphorus steel production process, the low-temperature impact toughness of the large-thickness high-strength steel is effectively improved, and the advantages of various heat treatment furnace types are fully played in cooperation with a water-cooling ingot mold and a strict heat treatment process; and it is ensured that the EH690Z35 extra-thick steel obtains an ideal bainite and martensite mixed structure, and all properties meet the use requirements.

Description

Technical Field

[0001] The invention belongs to the field of extra-thick plate production, and specifically relates to 250-260 mm thick EH690Z35 steel for marine engineering and a production method thereof. Background Art

[0002] Self-elevating platforms mostly use rack and pinion lifting, and the lifting racks are mainly made of EH690 / A517 GrQ / ASTMA514 GrQ high-strength and extra-thick rack steel plates. Due to the increasing water depth in the platform working area and the increasingly harsh working environment, the thickness specifications of the lifting rack steel are constantly increasing, all above 100mm. At present, mainstream platform manufacturers generally use 177.8mm thick EH690 steel plates as rack processing parts, and 210mm thick EH690 rack steel is still in the research and development and promotion stage. The research and development of this steel with a larger thickness is of great significance to my country's industrial development.

[0003] Chinese patent CN201710892107 "A 690Mpa grade extra-thick rack steel and its production process" discloses a production method of 178mm thick rack steel using steel ingots, the chemical composition of which is as follows by weight: C: 0.10% to 0.16%, Si: 0.15% to 0.35%, Mn: 0.95% to 1.35%, P ≤ 0.015%, S ≤ 0.010%, Cr: 0.50% to 0.60%, Ni: 2.30%~2.60%, Mo: 0.45%~0.55%, V: 0.03%~0.05%, A1: 0.07%~0.09%, Ti≤0.010%, B: 0.0010%~0.0020%, N≤0.010%, the rest is Fe and unavoidable impurities; carbon equivalent Cep=0.624~0.755, welding sensitivity coefficient 0.2655~0.3404, thickness 152~178mm. This patent uses a higher alloy component to make up for the problem of insufficient cooling rate in the core of thick steel plates. When using a higher Ni alloy content, it can only produce rack steel with a maximum thickness of 178mm.

[0004] Chinese patent CN116103562A discloses a production method of extra-thick rack steel plate, which is a process route of smelting-casting-rolling-rapid cooling-finishing-quenching-tempering, and the chemical composition of the steel is as follows: C: 0.12-0.18%, Si: 0.15-0.40%, Mn: 1.0-1.2%, P≤0.006%, S≤0.002%, Cr: 0.8-1.5%, Nb : 0.005~0.025%, V: 0.03~0.08%, Ni: 0.8~1.5%, Mo: 0.4~0.7%, Ti≤0.01%, A1: 0.05~0.09%, B: 0.0015~0.0025%, the balance is Fe and unavoidable impurities: The maximum thickness of the steel plate that can be produced by this invention is 210mmmm, the carbon equivalent is 0.7~0.77, and the welding performance is relatively poor.

[0005] In view of this, the present invention is proposed. Summary of the invention

[0006] In order to improve the above technical defects, the purpose of the present invention is to provide a 250-260 mm thick EH690Z35 steel for offshore platforms. By reasonably controlling the amount of hardenability elements added, the low-temperature impact toughness of thick high-strength steel is effectively improved. In combination with water-cooled ingot molds and strict heat treatment processes, the EH690Z35 extra-thick steel is ensured to obtain an ideal bainite + martensite mixed structure.

[0007] Another object of the present invention is to provide a method for producing EH690Z35 steel for offshore platforms with a thickness of 250 to 260 mm.

[0008] In order to ensure the high strength, high low-temperature impact toughness, corrosion resistance and other special requirements of EH690Z35, this scheme adopts the design concept of low carbon, low manganese and high strength and toughness. Studies have found that the reasonable ratio of Cu, Cr, Ni, Mo, and B elements has an important contribution to the toughness and hardness of high-strength steel. Manganese segregation affects the uniformity of the performance of the whole plate. Low-carbon design is conducive to improving low-temperature impact toughness. Micro-alloying elements such as Nb, V, and Ti are crucial to toughness, strength and weldability. Various types of M (C, N) nano-precipitates will be formed during the heat treatment process.

[0009] Therefore, the present invention provides an EH690Z35 steel for an offshore platform with a thickness of 250 to 260 mm, which adopts the following chemical components in mass percentage: C: 0.11 to 0.13, Si: 0.05 to 0.15, Mn: 0.5 to 0.6, P≤0.003, S≤0.003, Als: 0.020 to 0.040, Nb: 0.035 to 0.045, V: 0.035 to 0.045, Ti: 0.020 to 0.030, Cr: 0.6 to 0.65, Ni: 3.4 to 3.6, Cu: 0.25 to 0.3, Mo: 0.6 to 0.65, B: 0.0015 to 0.0025, and the remainder is Fe and residual elements; the carbon equivalent CEV is controlled to be 0.68-0.72.

[0010] The production method of the above-mentioned 250-260 mm thick EH690Z35 steel for offshore platforms includes converter smelting, LF dephosphorization, LF desulfurization, VD refining, water-cooled mold casting, billet heating and rolling, secondary heating and rolling, and heat treatment, which is specifically as follows:

[0011] 1) Converter smelting: The converter adopts semi-slag operation. After tapping, 20-40kg / t of carbon powder is added to the converter with reserved slag. The molten iron S≤0.010%, P≤0.050%, and the molten iron temperature is ≥1280℃. The scrap steel added to the converter must be dry high-quality scraps and nickel-containing scrap steel. The converter loading amount is controlled according to 6-8 tons of cast steel. The tapping temperature is ≥1550℃, 0.04%≤C≤0.07%, and P≤0.008%;

[0012] 2) LF dephosphorization: After the molten steel arrives at the station, 300-400 kg of fluorite is added first, and then lime is added in batches. The amount of lime added at a time does not exceed 200 kg, and the ratio of lime to fluorite is controlled at 1.8-2.2. The temperature of the heating process does not exceed 1580°C; if the dephosphorization efficiency is low, 150-300 kg of iron oxide scale is added to promote dephosphorization. After the dephosphorization of the molten steel is below 0.002%, the electrode is lifted to prepare for slag pouring. Before pouring the slag, 3-4 bags of carbonized rice husks need to be added and evenly spread on the surface of the molten steel;

[0013] 3) LF desulfurization: Slag making is carried out according to the large slag process standard, the amount of lime added is 1000-1200 kg, the alkalinity is controlled at 4.0-6.0, after heating for 3 minutes, first add 46-53 kg of calcium carbide, then add 20-40 kg of aluminum particles, and then add 2-4 shovels of aluminum particles to the ladle with a shovel every 2 minutes to ensure that the slag turns white; in the second heating, add 10-30 kg of calcium carbide each time according to the arc submergence effect, and add 1-3 shovels of aluminum particles to the ladle with a shovel every 2 minutes to ensure that the white slag is maintained throughout the second heating process; in the third heating, add deoxidizer according to the color of the slag to maintain the white slag; the white slag is required to be kept for 30-35 minutes during the refining process;

[0014] 4) VD refining: It is required to maintain the pressure for ≥18min under the vacuum degree of ≤67Pa, ensure that the molten steel is well tumbled during the pressure maintenance process, and the end point H≤1.2PPm; add rice husks in time for soft blowing for 5-8min after breaking the vacuum, and then prepare the bag to leave the station, and the temperature of leaving the station is controlled at 1578-1583℃;

[0015] 5) Water-cooled mold casting: The thickness of the ingot mold is 960-990 mm, the taper is 30-33 mm, and an extrusion mechanism is set on the side to extrude the wide side of the ingot in the late stage of solidification to further form a dense structure. The pouring temperature is controlled at 1560-1565 ° C;

[0016] 6) Ingot heating and rolling: furnace temperature ≤750℃ when charging steel, start heating after simmering steel for 2-4 hours, keep warm at 1280-1300℃ for 14-15 hours, turn over steel 4 hours before tapping to ensure even burning of ingot. It should be noted that in order to effectively play the role of alloying elements Ni, Nb and Mo, a sufficiently high heating temperature is required to fully dissolve them, but too high heating temperature will cause the austenite grains to grow and the ferrite grains to coarsen. Coarse ferrite structure will reduce its overall performance. Therefore, combined with the solidification characteristics of the steel ingot produced by the water-cooled copper plate ingot mold device, slow heating, multi-stage heat preservation, and steel turning before steel tapping ensure that the steel ingot is evenly burned through and the surface oxide scale is easy to remove; the opening rolling temperature of the billet rolling is 1050-1100℃, and the pass reduction is 60-70mm, which promotes effective welding of the internal looseness, sufficient crushing of the cast structure, and uniform refinement of the steel plate grains. When it is rolled to 600mm, it is released, and the rolling end temperature is 950-980℃;

[0017] 7) Secondary heating and rolling: After the intermediate billet is cut into four pieces, it is placed in a soaking pit, with a holding temperature of 1220-1240°C and a holding time of 6-7 hours; during the secondary rolling of finished products, there is no need to widen it again, and it can be directly extended to the finished product thickness. The starting rolling temperature is ≥1020°C, the final rolling temperature is ≤950°C, and the pass reduction is ≥30±5mm; during the rolling process, high-pressure water is used for cooling, and rolling can be continued only after the surface of the steel plate is fully red, ensuring that the deformation penetrates to 1 / 2 of the thickness, promoting further grain refinement, and the total rolling time is controlled within 4-5 minutes. After rolling, enter the ACC to quickly reduce the surface and core temperature of the steel plate to the non-recrystallization zone, reduce the temperature difference between the center area of ​​the steel plate thickness and the surface temperature, inhibit the growth and recrystallization of austenite grains, promote grain refinement and grain uniformity, and the water entry temperature is 900-950°C and the red return temperature is 800-830°C;

[0018] 8) Heat treatment: including high temperature quenching + critical quenching + tempering. First, the steel plate is heated by an external mechanized furnace. The heating temperature and insulation temperature are 920-940°C and the insulation time is 2.2-2.6 min / mm. After the steel plate is taken out of the furnace, it is hoisted above the circulating quenching pool within 3 minutes. The surface temperature of the steel plate when entering the water is 790-810°C. The water flow rate of the circulating quenching pool is ≥300m 3 / h, stirring gas flow rate 2500~3000m 3 / h, water temperature 30~40℃, the steel plate is cooled to water temperature in the circulating quenching pool and then lifted out; then heated in a chamber furnace, heating temperature 860-880℃, gas calorific value 1500~1600kCal / Nm 3 The holding time is 1.6~2.4min / mm. After the holding is finished, the steel plate is sent to the quenching machine for quenching. The water volume of the quenching machine is ≥15000m 3 / h, water pressure 0.98~1.02MPa; after quenching, the steel plate is loaded into the roller hearth tempering furnace for heating, the heating temperature is 630±20℃, the insulation time is 3.5~5min / mm, and the steel plate is water-cooled to below 400℃ after being taken out of the furnace, and then cooled naturally.

[0019] It should be noted that the ultra-thick high-strength steel is relatively poor in microstructure uniformity due to the quenching cooling rate and the differences in composition at different positions. In order to effectively improve the strength and ensure low-temperature impact toughness, the EH690 heat treatment adopts a combination of high-temperature quenching, critical quenching, and tempering processes. Among them, the purpose of high-temperature quenching is to uniform the composition and refine the austenite grains. The principle is that the original microstructure of the rolled sample is coarser than that of the quenched sample, and there are some degenerate pearlite and ferrite in the microstructure of the rolled sample. In general, the distribution of carbon elements is not as uniform as that of the quenched sample. The part with a higher carbon content is easy to form austenite nuclei and enter the austenitization state in advance. After high-temperature quenching, the bainite formed will preferentially nucleate at the bainite grain boundary during the reheating process to generate fine austenite independently distributed along the grain boundary. After that, the austenite grows along the grain boundary. During the quenching process, the steel plate is merged into a network. As the temperature continues to rise, fine austenite begins to form in the crystal. Relatively speaking, the number of austenite nucleation points increases, and the speed at which austenite advances into bainite is slower, and the grain growth rate slows down accordingly. The second quenching is quenching at austenitizing temperature + 5 to 20 °C. Through high pressure, large flow cooling water intervals and repeated cooling of the steel plate surface, a large temperature gradient is formed on the cross section of the steel plate to further refine the original austenite grain size. After quenching, the tempering process is used to eliminate the residual stress in the quenched steel and improve the impact toughness and uniformity of the strength of the entire plate.

[0020] In addition, the calorific value of the gas in the external mechanized furnace is relatively low, and the high-temperature quenching temperature rises slowly. With long-term heat preservation, it is conducive to promoting full homogenization of the composition and refinement of the organization; critical quenching heating is carried out in a chamber furnace, using mixed gas as the combustion medium, and the heating efficiency is higher than that of the external machine furnace and lower than that of the roller-type quenching furnace. After leaving the furnace, it quickly enters the high-pressure and large-volume roller-type quenching machine for quenching, realizing precise control of the quenching temperature and maximizing the hardenability; the advantage of the roller-type tempering furnace is that the steel plate continues to move smoothly in the area with the best temperature control in the furnace, eliminating the anisotropy of performance caused by uneven temperature, and quickly enters the water to cool to below 400°C after leaving the furnace, and quickly passes through the brittle temperature range. Give full play to the characteristics of the three types of furnaces, and finally obtain a special thick rack steel with matching strength and toughness and excellent low-temperature impact toughness.

[0021] The beneficial effect of the present invention lies in that, by reasonably controlling the addition amount of hardenability elements and adopting a low-manganese and low-phosphorus steel production process to reduce the harmful element content in the steel, the low-temperature impact toughness of thick high-strength steel is effectively improved, and in combination with a water-cooled ingot mold and a strict heat treatment process, the advantages of various heat treatment furnace types are fully utilized to ensure that the EH690Z35 extra-thick steel obtains an ideal bainite + martensite mixed structure, and various performances meet the use requirements. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The technical features of the present invention are further described below in conjunction with the accompanying drawings and embodiments.

[0023] Figure 1 Schematic diagram of the metallographic structure of the thickness section of an embodiment of the present invention (metallographic microscope, ×100).

[0024] Figure 2 Schematic diagram of the metallographic structure of the thickness section of an embodiment of the present invention (metallographic microscope, ×200).

[0025] Figure 3 Schematic diagram of the metallographic structure of the thickness section of an embodiment of the present invention (metallographic microscope, ×500). DETAILED DESCRIPTION

[0026] In order to make the purpose, technical scheme and advantages of the embodiments of the present invention clearer, the technical scheme in the embodiments of the present invention will be described clearly and completely below. If the specific conditions are not specified in the embodiments, they are carried out according to conventional conditions or conditions recommended by the manufacturer. If the manufacturer of the reagents or instruments used is not specified, they are all conventional products that can be purchased commercially.

[0027] The features and performance of the present invention are further described in detail below in conjunction with the embodiments.

[0028] A 254 mm thick EH690Z35 steel for an offshore platform is produced, using the following chemical composition in mass percentage (unit, wt%): C: 0.11-0.13, Si: 0.05-0.15, Mn: 0.5-0.6, P≤0.003, S≤0.003, Als: 0.020-0.040, Nb: 0.035-0.045, V: 0.035-0.045, Ti: 0.020-0.030, Cr: 0.6-0.65, Ni: 3.4-3.6, Cu: 0.25-0.3, Mo: 0.6-0.65, B: 0.0015-0.0025, the remainder being Fe and residual elements; the carbon equivalent CEV is controlled to be 0.68-0.72.

[0029] The production process is as follows:

[0030] 1) KR hot metal pretreatment: The slag must be cleaned before the hot metal arrives at the station to ensure that the slag layer thickness on the liquid surface is ≤25mm. The sulfur content of the hot metal after KR treatment is controlled below 0.005%, and the desulfurization temperature drop is ≤25℃;

[0031] 2) Converter smelting: The converter adopts semi-slag operation. After tapping, 20-40kg / t of carbon powder is added to the converter with reserved slag. The molten iron S≤0.010%, P≤0.050%, and the molten iron temperature is ≥1280℃. The scrap steel added to the converter must be dry high-quality scraps and nickel-containing scrap steel. The converter loading amount is controlled according to the remaining 6-8 tons of steel casting. The tapping temperature is ≥1550℃, 0.04%≤C≤0.07%, P≤0.008%. The slag cone is used to stop the slag after tapping. If the slag stopping fails, the furnace must be lifted in advance to ensure that the thickness of the converter slag is controlled below 20mm to avoid the slag returning to P;

[0032] 3) LF dephosphorization: After the molten steel arrives at the station, 300-400 kg of fluorite is added first, and then lime is added in batches and small quantities (the amount of lime added each time during the heating process does not exceed 200 kg). The ratio of lime to fluorite is controlled at about 2.0, and the temperature during the heating process shall not exceed 1580°C; if the dephosphorization efficiency is low, 150-300 kg of iron oxide scale is added to promote dephosphorization. After the dephosphorization of the molten steel is below 0.002%, the electrode is lifted to prepare for slag pouring. Before pouring the slag, 3-4 bags of carbonized rice husks need to be added and evenly spread on the surface of the molten steel;

[0033] 4) LF desulfurization: slag making is carried out according to the large slag process standard, the amount of lime added is 1000-1200 kg, the alkalinity is controlled at 4.0-6.0, after heating for 3 minutes, first add 50 kg of calcium carbide, then add 20-40 kg of aluminum particles, and then add 2-4 shovels of aluminum particles to the ladle with a shovel every 2 minutes to ensure that the slag turns white; in the second heating, 10-30 kg of calcium carbide is added each time according to the submerged arc effect, and 1-3 shovels of aluminum particles are added to the ladle with a shovel every 2 minutes to ensure that the white slag is maintained during the entire second heating process; in the third heating, deoxidizer is added according to the color of the slag to maintain the white slag; the white slag is required to be maintained for 30-35 minutes during the refining process.

[0034] 5) VD refining: It is required to maintain the pressure for ≥18min under the vacuum degree of 67Pa, ensure the tumbling effect of molten steel during the pressure maintenance process, and the end point H≤1.2PPm; add rice husks in time for soft blowing for 5-8min after breaking the air, and then prepare the bag to leave the station, and the leaving station temperature is controlled at 1578-1583℃;

[0035] 6) Casting process: The temperature of the insulation cap is greater than 150℃, and a 50-ton fixed width and thickness water-cooled ingot mold is used. The thickness of the ingot body is 960-990mm. The mold swinging process must ensure that all resistant auxiliary materials are dry, and the pouring system is dry and clean. The water-cooled mold used must ensure a taper of 30mm and an extrusion mechanism is set on the side to squeeze the wide side of the ingot in the later stage of solidification to further form a dense structure. Proper insulation measures are taken at the ingot cap. During the solidification process, a shrinkage channel with a large top and a small bottom can be formed inside the ingot, avoiding the generation of shrinkage cavities, reducing the looseness and segregation inside the ingot, and promoting the internal organization to be dense and uniform in composition. The pouring temperature is controlled at 1560-1565℃; the temperature of the molten steel at the station is controlled between the standard upper limit and the upper limit +3℃. If the temperature of the molten steel at the station does not meet the above requirements, soft argon blowing is immediately carried out, and pouring is started 5 minutes after the argon blowing is completed.

[0036] 7) Cleaning and coating of steel ingots: The steel ingots should be cleaned within 24 hours after demoulding, and the cleaning temperature should be ≥200℃. After cleaning, they should be stacked in a slow cooling pit, and slowly cooled to 50-100℃ before spraying anti-oxidation coating.

[0037] 8) Billet heating and rolling: furnace temperature ≤750℃ when loading steel, simmer steel for 2-4 hours to reduce the temperature difference between furnace temperature and ingot, reduce the stress difference caused by the temperature difference between inside and outside of ingot, target insulation temperature of high temperature section 1280-1300℃, insulation time 14-15 hours, turn steel over four hours before rolling to ensure uniform ingot burning, total heating time 15min / cm; rolling temperature 1050-1100℃, pass reduction 60-70mm, release when rolling to 600mm thick, rolling end temperature 950-980℃;

[0038] 9) Secondary heating and rolling: After the intermediate billet is cut into four pieces, it is placed in a soaking pit. The target temperature of the high temperature section is 1220-1240°C and the holding time is 6-7 hours. There is no need to widen it again during the secondary rolling, and it can be directly extended to the finished product thickness. The starting rolling temperature is ≥1020°C, and the pass reduction is ≥30±5mm. High-pressure water is used for cooling during the rolling process. Rolling can only be started after the surface of the steel plate has fully returned to red. The total rolling time is controlled within 4-5 minutes, and the final rolling temperature is ≤950°C. After rolling, ACC is turned on for 2 times of water cooling to quickly reduce the surface and core temperature of the steel plate to the non-recrystallization zone. The water entry temperature is 900-950°C and the red return temperature is 800-830°C.

[0039] 10) Heat treatment: including high temperature quenching + critical quenching + tempering. First, use an external mechanized furnace to heat, the heating temperature is 920 ~ 940 ℃, the holding time is 2.4min / mm, and the steel plate is hoisted into the quenching pool within 3 minutes after it comes out of the furnace and cooled to room temperature by water. The stirring gas flow rate is 2500 ~ 3000m 3 / h, circulating water flow 300m 3 / h, in which the surface temperature of the steel plate when entering the water is 790-810℃, the core temperature is 30-50℃ higher than the surface temperature, the water temperature in the cooling pool is 30-40℃, and the structure obtained by this quenching is mainly bainite; then heated in a chamber furnace, the heating temperature is 860-880℃, and the calorific value of the gas is 1500-1600kCal / Nm 3 , insulation time 2min / mm, after insulation, the steel plate is sent to the quenching machine for quenching. The temperature of the steel plate when it reaches the quenching machine usually drops by 30-50℃, and the water volume of the quenching machine is controlled at 15400m 3 / h, water pressure 0.98~1.02MPa; after quenching, the steel plate is loaded into the roller hearth tempering furnace for heating, the heating temperature is 630±20℃, the insulation time is 4min / mm, and the steel plate is water-cooled to below 400℃ after being taken out of the furnace, and then cooled naturally.

[0040] The steel plates obtained in the example were tested and analyzed, among which: the chemical composition of the steel plates, the sampling location of the mechanical properties test pieces and the preparation of the samples were carried out in accordance with the provisions of the GB / T 2975 mechanical properties test sampling location and sample preparation; the low temperature impact toughness test was carried out in accordance with the GB / T 229 standard; the tensile properties test was carried out in accordance with the GB / T 228 standard; and the various mechanical properties and metallographic structures of the steel plates in the delivery state were comprehensively inspected. The mechanical properties are shown in Table 1:

[0041] Table 1 Mechanical properties of EH690Z35 steel plate

[0042]

[0043] From the above data, we can see that the steel plate has good low-temperature impact toughness, and the difference in mechanical properties between the head and tail of the steel plate is small, and the lamellar tearing resistance is good. All performance and indicators meet the requirements of EH690Z35 steel for marine engineering steel plates.

[0044] The metallographic structure on the thickness section of the steel plate is as follows Figure 1-3 shown.

[0045] Metallographic test results: (1) Inclusions A+B+D ≤ 2.0 level; (2) Structure: Bainite + Martensite mixed structure.

[0046] External inspection and flaw detection: The developed steel plate has a 100% genuine product rate in external inspection, and the final steel plate flaw detection meets the Level I flaw detection requirements of GB / T 2970 "Super-thick steel plate inspection method".

Claims

1. A 250-260 mm thick EH690Z35 steel for marine engineering, characterized in that: The invention comprises the following chemical components in mass percentage (wt%): C: 0.11-0.13, Si: 0.05-0.15, Mn: 0.5-0.6, P≤0.003, S≤0.003, Als: 0.020-0.040, Nb: 0.035-0.045, V: 0.035-0.045, Ti: 0.020-0.030, Cr: 0.6-0.65, Ni: 3.4-3.6, Cu: 0.25-0.3, Mo: 0.6-0.65, B: 0.0015-0.0025, and the balance is Fe and residual elements; the carbon equivalent CEV is controlled to be 0.68-0.

72.

2. The method for producing 250-260 mm thick EH690Z35 steel for marine engineering according to claim 1, characterized in that: Including converter smelting, LF dephosphorization, LF desulfurization, VD refining, water-cooled mold pouring, billet heating and rolling, secondary heating and rolling, heat treatment, as follows: 1) Converter smelting: The converter adopts semi-slag operation. After tapping, 20-40kg / t of carbon powder is added to the converter with reserved slag. The molten iron S≤0.010%, P≤0.050%, and the molten iron temperature is ≥1280℃. The scrap steel added to the converter must be dry high-quality scraps and nickel-containing scrap steel. The loading amount of the converter is controlled according to the remaining 6-8 tons of steel casting. The tapping temperature is ≥1550℃, 0.04%≤C≤0.07%, and P≤0.008%; 2) LF dephosphorization: After the molten steel arrives at the station, 300-400 kg of fluorite is added first, and then lime is added in batches. The amount of lime added at a time does not exceed 200 kg, and the ratio of lime to fluorite is controlled at 1.8-2.

2. The temperature of the heating process does not exceed 1580°C; if the dephosphorization efficiency is low, 150-300 kg of iron oxide scale is added to promote dephosphorization. After the dephosphorization of the molten steel is below 0.002%, the electrode is lifted to prepare for slag pouring. Before pouring the slag, 3-4 bags of carbonized rice husks need to be added and evenly spread on the surface of the molten steel; 3) LF desulfurization: slag making is carried out according to the large slag process standard, the amount of lime added is 1000-1200 kg, the alkalinity is controlled at 4.0-6.0, after heating for 3 minutes, first add 46-53 kg of calcium carbide, then add 20-40 kg of aluminum particles, and then add 2-4 shovels of aluminum particles to the ladle with a shovel every 2 minutes to ensure that the slag turns white; in the second heating, add 10-30 kg of calcium carbide each time according to the arc submergence effect, and add 1-3 shovels of aluminum particles to the ladle with a shovel every 2 minutes to ensure that the white slag is maintained throughout the second heating process; in the third heating, add deoxidizer according to the color of the slag to maintain the white slag; the white slag is required to be kept for 30-35 minutes during the refining process; 4) VD refining: It is required to maintain the pressure for ≥18min under the vacuum degree of ≤67Pa, ensure that the molten steel is well tumbled during the pressure maintenance process, and the end point H≤1.2PPm; add rice husks in time for soft blowing for 5-8min after breaking the vacuum, and then prepare the bag to leave the station, and the temperature of leaving the station is controlled at 1578-1583℃; 5) Water-cooled mold casting: The thickness of the ingot mold is 960-990 mm, the taper is 30-33 mm, and an extrusion mechanism is set on the side. The casting temperature is controlled at 1560-1565 ° C; 6) Billet heating and rolling: furnace temperature ≤750℃ when steel is charged, temperature rises after simmering for 2-4 hours, insulation temperature 1280-1300℃, insulation time 14-15 hours, steel is turned over 4 hours before steel tapping to ensure uniform ingot burning; billet rolling starting temperature 1050-1100℃, pass reduction 60-70mm, release when rolling to 600mm, rolling end temperature 950-980℃; 7) Secondary heating and rolling: After the intermediate billet is cut into four pieces, it is placed in a soaking pit, and the holding temperature is 1220-1240°C for 6-7 hours; during the secondary rolling, there is no need to widen it again, and it can be directly extended to the finished product thickness. The starting rolling temperature is ≥1020°C, the final rolling temperature is ≤950°C, and the pass reduction is ≥30±5mm. High-pressure water is used for cooling during the rolling process. Rolling can only be continued after the surface of the steel plate has fully returned to red. The total rolling time is controlled at 4-5 minutes. After rolling, enter the ACC to quickly reduce the surface and core temperature of the steel plate to the non-recrystallization zone. The water entry temperature is 900-950°C and the red return temperature is 800-830°C; 8) Heat treatment: including high temperature quenching + critical quenching + tempering. First, the steel plate is heated by an external mechanized furnace. The heating temperature and insulation temperature are 920-940°C and the insulation time is 2.2-2.6 min / mm. After the steel plate is taken out of the furnace, it is hoisted above the circulating quenching pool within 3 minutes. The surface temperature of the steel plate when entering the water is 790-810°C. The water flow rate of the circulating quenching pool is ≥300m 3 / h, stirring gas flow rate 2500~3000m 3 / h, water temperature 30~40℃, the steel plate is cooled to the water temperature in the circulating quenching pool and then lifted out; then heated in a chamber furnace, the heating temperature is 860-880℃, the calorific value of the gas is 1500~1600kCal / Nm 3 The holding time is 1.6~2.4min / mm. After the holding is finished, the steel plate is sent to the quenching machine for quenching. The water volume of the quenching machine is ≥15000m 3 / h, water pressure 0.98~1.02MPa; after quenching, the steel plate is loaded into the roller hearth tempering furnace for heating, the heating temperature is 630±20℃, the insulation time is 3.5~5min / mm, and the steel plate is water-cooled to below 400℃ after being taken out of the furnace, and then cooled naturally.

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