Steel plate for marine industry with ultra-high strength and toughness of 690 MPa grade having a thickness of 80 mm and method for producing the same

Abstract

The present invention relates to a steel plate for marine industry having ultra-high strength and toughness of 690 MPa grade with a thickness of 80 mm and a manufacturing method thereof. Its chemical composition, by mass%, is: C: 0.08% - 0.10%, Si: 0.20% - 0.30%, Mn: 1.10% - 1.25%, P ≤ 0.007%, S ≤ 0.002%, Nb: 0.020% - 0.030%, Ti: 0.010% - 0.020%, V: 0.030% - 0.045%, Cr: 0.40% - 0.60%, Ni: 1.40% - 1.50%, Cu: 0.15% - 0.25%, Mo: 0.25% - 0.35%, Als: 0.015% - 0.045%, Pcm ≤ 0.33%, Ceq ≤ 0.64%, and the balance is Fe and inevitable impurity elements. The steel plate for marine industry having ultra-high strength and toughness of 690 MPa grade with a thickness of 80 mm according to the present invention satisfies all the characteristic indexes of the certification requirements of CCS EH690 steel, with a saturated corrosion current density ≤ 1.90 mA / cm2 (-300 mV vs. Ag / AgCl reference electrode) and a density of corrosive active inclusions ≤ 9 pieces / mm2. The product has characteristics such as high strength and toughness, low temperature resistance, and corrosion resistance.

Description

Technical Field

[0001] The present invention belongs to the technical field of the production of ultra-high-strength steels for the marine industry. Specifically, the present invention relates to a steel plate for the marine industry having ultra-high strength and toughness of 690 MPa grade with a thickness specification of 80 mm and a manufacturing method thereof.

[0002] (Related Application) This application claims priority based on Chinese Patent Application No. 2022107478038, entitled "Steel Plate for Marine Industry with Ultra-High Strength and Toughness of 690 MPa Grade with a Thickness of 80 mm and a Manufacturing Method Thereof", filed on June 29, 2022, and all the contents of this Chinese patent application are incorporated herein by reference.

Background Art

[0003] As the exploration of the marine field continues to deepen, the steel for ships and the marine industry with extremely thick specifications and its development and production have become the top priorities. Due to the complex and easily changing marine application environment, the steel for the marine industry with extremely thick specifications is required to have excellent comprehensive properties such as high strength, high toughness, ease of welding, and seawater corrosion resistance.

[0004] The manufacturing technology of conventional steel plates of 690 MPa grade with extremely thick specifications has many drawbacks. In terms of component design, a large amount of alloying elements are added, which increases the production cost and causes difficulties in the production process of the steel plate or subsequent processing and use. In terms of the production process, in many cases, there is a problem that the heat treatment process is long.

[0005] In the ultra-high strength EH690 extra-thick steel plate and its manufacturing method disclosed in Chinese Patent CN111304551B, C: 0.10% - 0.17%, Si: 0.25% - 0.45%, Mn: 0.90% - 1.30%, S ≤ 0.003%, P ≤ 0.010%, V: 0.041% - 0.076%, Als: 0.03% - 0.05%, N: 0.004% - 0.013%, Ni: 1.40% - 1.80%, Cr: 0.60% - 1.00%, Mo: 0.30% - 0.50%, Nb: 0.021% - 0.04%, Cu: 0.43% - 0.50%, Ti ≤ 0.02%, and many elements such as V, Cr, Mo, and Cu are added. In this patent, after rolling, it is necessary to adopt a heat treatment process of quenching + intercritical quenching + tempering.

[0006] Chinese Patent Application Publication CN110846577A proposes a medium manganese steel of 690MPa grade with a low yield ratio and its manufacturing method. The components contain 4.1% - 4.7% of Mn element. The large addition of Mn element brings great difficulties to the steelmaking and continuous casting processes. Continuous casting production is extremely prone to accidents. Also, medium manganese steel has high low-temperature toughness, but the rolled steel plate is extremely prone to problems such as non-qualified flaw detection and corner cracking.

[0007] Chinese Patent Application Publication CN112251670A and Chinese Patent CN102965592B both propose manufacturing methods for 690MPa grade extra-thick specification steel for the marine industry. To improve the strength of the steel plate, 0.001% - 0.0015% of B and 0.001% - 0.003% of B are respectively added to its chemical composition. The addition of B element significantly increases the tendency of the steel plate to crack during welding, which is disadvantageous for subsequent processing. In these manufacturing technologies, in order to obtain the required properties, it is necessary to perform a normalizing heat treatment at 890 - 920°C once before quenching + high-temperature tempering heat treatment, or perform a low-temperature tempering heat treatment at 300°C or below once after quenching + high-temperature tempering heat treatment, which is disadvantageous for the industry's needs of improving production efficiency and reducing costs.

Summary of the Invention

Problems to be Solved by the Invention

[0008] In view of the drawbacks and defects existing in the prior art, an object of the present invention is to provide a steel plate for marine industry having ultra-high strength and toughness of 690 MPa grade with a thickness of 80 mm and a manufacturing method thereof. The steel plate for marine industry having ultra-high strength and toughness of 690 MPa grade with a thickness of 80 mm of the present invention meets the certification requirements of EH690 steel of the China Classification Society in terms of characteristic indexes, and the saturated corrosion current density of -300 mV (with respect to Ag / AgCl reference electrode) is ≤1.90 mA / cm 2 , and the density of corrosion-active inclusions is ≤9 pieces / mm 2 . The product has excellent comprehensive properties such as high strength and toughness, low-temperature resistance, and corrosion resistance.

Means for Solving the Problems

[0009] In order to achieve the above object, a first aspect of the present invention provides a component design of a steel plate for marine industry having ultra-high strength and toughness of 690 MPa grade with a thickness of 80 mm, and adopts the following technical solutions.

[0010] A steel plate for marine industry having ultra-high strength and toughness of 690 MPa grade with a thickness of 80 mm, wherein the chemical components in the steel plate are, by mass%, C: 0.08% - 0.10%, Si: 0.20% - 0.30%, Mn: 1.10% - 1.25%, P ≤ 0.007%, S ≤ 0.002%, Nb: 0.020% - 0.030%, Ti: 0.010% - 0.020%, V: 0.030% - 0.045%, Cr: 0.40% - 0.60%, Ni: 1.40% - 1.50%, Cu: 0.15% - 0.25%, Mo: 0.25% - 0.35%, Als: 0.015% - 0.045%, and the balance is Fe and inevitable impurity elements, and is controlled to Pcm ≤ 0.33% and Ceq ≤ 0.64%.

[0011] Here, Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15, and Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B.

[0012] In the case of a steel plate for marine industry having ultra-high strength and toughness of 690 MPa grade with a thickness of 80 mm, as a preferred embodiment, inevitable impurity elements are, by mass%, H ≦ 0.0002%, O ≦ 0.003%, N ≦ 0.004%, B ≦ 0.0005%, As ≦ 0.006%, Sb ≦ 0.010%, Sn ≦ 0.020%, Pb ≦ 0.010%, Bi ≦ 0.010%.

[0013] The functions of the main alloys in the steel plate for marine industry having ultra-high strength and toughness of 690 MPa grade with a thickness of 80 mm are as follows.

[0014] In the case of a steel plate with an extremely thick specification, Cr can effectively improve hardenability to compensate for strength loss due to thickness, and can also improve the corrosion resistance of the steel plate. On the other hand, if the content is too high, low-melting-point Cr-Mn composite oxides are formed, surface cracking is likely to occur in the hot working process of the steel plate, and the welding properties also deteriorate. Therefore, in the present invention, the content of Cr is controlled to be 0.40% - 0.60%.

[0015] Ni is the element that can most significantly improve the low-temperature toughness of a steel plate with an extremely thick specification. By appropriately adding Ni, the energy of crystal stacking defects can be reduced, which is advantageous for the slip movement of dislocations, improving impact toughness. Also, Ni can promote the formation of a dense protective rust layer on the surface of the steel plate and improve the corrosion resistance of the steel plate. On the other hand, if the content of Ni is too high, it is disadvantageous for ensuring welding properties. Therefore, in the present invention, the content of Ni is controlled to be 1.40% - 1.50%.

[0016] Cu can improve the corrosion resistance and strength of steel, and can also improve welding properties and machining properties, etc. On the other hand, if the content of Cu is too high, the hot brittleness of the steel plate tends to increase. Therefore, the content of Cu in the present invention is controlled to be 0.15% - 0.25%.

[0017] Mo is an element that improves hardenability, expands the γ-phase region, delays the formation of primary ferrite, can effectively improve strength, and can also significantly improve the stability of strength and toughness characteristics in the thickness direction of thick steel plates. On the other hand, if the Mo content is too high, the weldability of the steel plate deteriorates. Therefore, in the present invention, the Mo content is controlled to be 0.25% - 0.35%.

[0018] Nb can effectively achieve the effect of refining crystal grains and can also play a role in precipitation strengthening. On the other hand, due to the limitation of C and the influence of heating temperature, if the content is too high, Nb cannot be fully dissolved. Therefore, in the present invention, the Nb content is controlled to be 0.020% - 0.030%.

[0019] Ti can similarly achieve the effect of refining crystal grains and precipitation strengthening, and can significantly improve the low-temperature impact toughness of the steel plate. On the other hand, if the content is too high, large TiN particles are likely to occur, resulting in the loss of the crystal refinement effect. Therefore, in the present invention, the Ti content is controlled to be 0.010% - 0.020%.

[0020] V is an element that refines crystal grains in steel and also has the effect of precipitation strengthening. When the addition amount is less than 0.02%, the effect is not obvious. When it is greater than 0.05%, the toughness and weldability of the steel decrease. Therefore, in the present invention, the V content is controlled to be 0.030% - 0.045%.

[0021] Al can fix free N in steel, improve the low-temperature toughness of the steel plate and the weld HAZ. Also, the dispersed precipitation of AlN suppresses the growth of austenite crystal grains in the heating process, uniformly refines the size of austenite crystal grains, and can improve impact toughness. On the other hand, if the Al content is too high, the number of inclusions in the steel increases, the size of the inclusions becomes larger, the internal quality of the steel plate deteriorates, and the hot workability, welding characteristics, and cutting process characteristics of the steel are impaired. Therefore, in the present invention, the Al content is controlled to be 0.020% - 0.050%.

[0022] Ceq: Controlling the carbon equivalent is advantageous for ensuring the strength and weldability of the steel plate. In the present invention, Ceq is controlled to be ≤ 0.64%.

[0023] Pcm: Controlling the weld crack susceptibility composition is advantageous for ensuring the welding characteristics of the product. In the present invention, Pcm is controlled to be ≤ 0.33%.

[0024] Impurity elements such as S and P in steel increase the degree of segregation of the continuous casting slab and deteriorate the uniformity of the tissue characteristics in the thickness direction of the steel plate. Therefore, the contents of S and P are controlled to be 0.005% and 0.010% or less, respectively. B is likely to be enriched at grain boundaries, reducing the low-temperature impact characteristics and fatigue characteristics of the steel plate and significantly increasing the tendency of weld cracking. Therefore, B is controlled to be ≤ 0.0005%, and other inevitable impurity elements are controlled as follows: As ≤ 0.006%, Sb ≤ 0.010%, Sn ≤ 0.020%, Pb ≤ 0.010%, Bi ≤ 0.010%, H ≤ 0.0002%, O ≤ 0.003%, N ≤ 0.004%.

[0025] The second aspect of the present invention provides a method for manufacturing a steel plate for marine industry having ultra-high strength and toughness of 690 MPa grade with a thickness of 80 mm, including converter steelmaking, double refining of LF + RH, continuous casting, slab heating, rolling, and heat treatment. Specifically, it is as follows.

[0026] (1) Regarding converter steelmaking Using hot metal after KR treatment, with hot metal S ≤ 0.008%, adding nickel plates, copper plates, ferromolybdenum together with steel scrap, smelting using a double slag deep dephosphorization process, controlling the basicity of the final slag to R = 3.0 - 4.0, achieving early slag formation at the beginning, good slag formation during the process, and complete slag formation of the final slag, alloying with metallic manganese, ferroniobium, ferrovanadium, low-carbon ferromanganese, and ferrosilicon, adding the alloy when 1 / 4 of the molten steel is discharged, completing the alloy addition when 3 / 4 of the molten steel is discharged, and adding aluminum manganese iron according to 3 - 3.5 kg / t steel for deoxidation.

[0027] (2) Regarding LF+RH refining Stir while bottom-blowing argon throughout the whole process, perform slag modification using aluminum granules, calcium carbide, and silicon carbide, adjust the Ti component with titanium wire, adjust the Al component with aluminum wire, control the basicity of the final slag to be 2.5 or more as much as possible, set the LF refining time to ≥50 min, and among them, set the soft blowing time to ≥10 min.

[0028] For RH refining, adopt this processing mode. Ensure that the vacuum degree is 30 Pa or less and the pure degassing time is ≥5 min. After the RH treatment is completed, supply calcium-aluminum wire at 1 - 1.5 m / t, set the soft blowing time to ≥14 min, and the RH refining time to ≥50 min.

[0029] (3) Regarding continuous casting Adopt protective casting throughout the whole process, protect the slag with peritectic steel, set the liquidus temperature to 1510 - 1520 °C, control the superheat degree to 20 °C or less, set the casting speed of the 300 mm thick continuous casting slab used to 0.70 - 0.90 m / min, adopt the soft reduction technology at the solidification end of the slab in the sector zone, put the slab into the pit, stack it, and perform slow cooling for 72 h or more.

[0030] (4) Regarding slab heating Charge the slab into the furnace cold, adopt the multi-stage heating and temperature rising method, set the temperature in the soaking zone to 1190 - 1240 °C, the soaking time to ≥60 min, the tapping temperature to 1200 - 1230 °C, and the total time for heating and temperature rising to 290 - 310 min. On the one hand, ensure that the slab is uniformly and completely burned, and on the other hand, prevent the austenite crystal grains from growing excessively. Before performing rolling treatment on the high-temperature slab, perform high-pressure water dephosphorization treatment.

[0031] (5) Regarding rolling The rolling process is a two-stage rolling process of rough rolling and finish rolling. The rough rolling is recrystallization rolling. The number of passes in rough rolling is set to ≤5, and the reduction ratio of at least 2 passes (the thickness reduction amount per pass / the inlet thickness, the same hereinafter) is ensured to be ≥19%. By achieving a large reduction ratio, the austenite crystal grains are sufficiently refined, providing a tissue guarantee for improving the strength and toughness characteristics of the thick-specification steel plate. After rough rolling, the thickness of the intermediate slab is 120 - 130 mm. The finish rolling is unrecrystallized rolling. The rolling start temperature of the finish rolling is 835 - 865°C, and the number of passes in the finish rolling is set to ≤7. Preferably, by setting the number of passes in the finish rolling to 5, the forced phase transition mechanism caused by the accumulated deformation effect between passes and the internal defects of the deformed austenite crystals is maximally utilized to promote the generation of a large number of deformation zones, twins, and dislocations inside the austenite crystal grains, creating conditions for ferrite phase transition nucleation, and improving the strength and toughness of the steel plate. The thickness of the intermediate slab obtained after rough rolling is controlled, and the reduction amounts of rough rolling and finish rolling are reasonably allocated to improve the characteristics of the steel plate, especially the impact toughness of the central part and the uniformity of the characteristics in the thickness direction.

[0032] After rolling, by controlling the cooling process, the ferrite nucleation rate is increased, and a finely dispersed precipitation phase is formed to further improve the strength and toughness of the steel plate. The cooling start temperature is 800 - 820°C, the cooling end temperature is 550 - 590°C, and the cooling rate is controlled at 6 - 10°C / s. Then, to ensure that the phase transition process is fully carried out and the tissue uniformity of the thick-specification steel plate is improved, the steel plate is transferred to a slow cooling pit or stacked for slow cooling, and the slow cooling time is set to ≥48 h.

[0033] (6) Regarding heat treatment Adopt a heat treatment method of quenching + high-temperature tempering.

[0034] Based on the component design, the Ac3 temperature of the steel plate of the present invention is about 850 °C. To obtain fine and uniform austenite crystal grains, and further ensure obtaining a fine martensite structure after quenching, and reduce the content of the low-hardness ferrite phase in the quenched structure, it is necessary to set the quenching heating temperature 30 - 50 °C or more above Ac3. Also, considering that the components contain strong carbide-forming elements such as Nb, V, and Ti, since the austenite crystal grains can be coarsened and the temperature can be increased, the quenching temperature is set at 920 ± 5 °C, thereby accelerating the dissolution of alloy carbides, enhancing the stability of supercooled austenite, and improving the hardenability of the steel. The heating time is 1.3 - 1.6 min / mm × plate thickness, and the holding time is 30 ± 3 min. After the heating and holding are completed, the steel plate is put into a quenching machine for quenching treatment. What is used in the present invention is an ultra-wide integrated slit-type quenching machine designed by Tohoku University. Its high-pressure spray system of the cooling water system includes two sets of slit nozzles and four sets of high-density type I nozzles, with a length of 3640 mm. The low-pressure spray system includes 18 sets of high-density type II nozzles, with a length of 12600 mm. The water pressure in the high-pressure zone is 0.7 - 0.9 bar, and the water pressure in the low-pressure zone is 0.3 - 0.4 bar. According to the rule of "quenching with low roller speed + large water volume", the roller table speed of the quenching machine is 1.6 - 1.8 m / min, the water volume in the high-pressure zone is 5376 - 6067 m 3 / h, and the water volume in the low-pressure zone is 3499 - 3888 m 3 / h. A suitable process scheme is that the roller table speed of the quenching machine is 1.6 m / min, the water volume in the high-pressure zone is 5376 m 3 / h, and the water volume in the low-pressure zone is 3888 m 3 / h; or the roller table speed of the quenching machine is 1.8 m / min, the water volume in the high-pressure zone is 6067 m 3 / h, and the water volume in the low-pressure zone is 3499 m 3It is to set it as / h. By setting the ratio of the water volume of the upper nozzle to the water volume of the lower nozzle to approximately 1:1.4, the symmetry and uniformity of the steel plate in the quenching process are ensured. In this way, by cooling in the high-pressure zone, the steel plate is completely quenched to complete all phase transitions, and the low-pressure zone further removes the heat conducted from the inside to the surface of the steel plate, prevents tempering due to residual heat, and finally lowers the temperature of the steel plate to room temperature.

[0035] High-temperature tempering can remove the complex internal stress of the steel plate after rapid cooling and quenching, and can endow the steel plate with excellent comprehensive mechanical properties. In the present invention, the tempering heating temperature is 600 ± 5 °C, the heating time is 2 - 2.5 min / mm × plate thickness, and the heat preservation time is 30 ± 3 min. After the steel plate is taken out of the furnace, it is air-cooled to room temperature by blowing cooling air on the cooling bed. In order to prevent the high-temperature tempering brittleness of the steel plate, it is necessary to avoid stacking and storing it in a short time after taking it out of the furnace.

[0036] In this way, a steel plate for marine industry with ultra-high strength and toughness of 690 MPa grade and a thickness of 80 mm is obtained. The main characteristic indexes are: yield strength ≥ 690 MPa, tensile strength 770 - 940 MPa, elongation at break ≥ 16%, transverse impact energy at the center at -40 °C ≥ 100 J, CTOD at -40 °C ≥ 0.15 mm, saturated corrosion current density ≤ 1.90 mA / cm 2 with respect to -300 mV (versus Ag / AgCl reference electrode), and the density of corrosion-active inclusions ≤ 9 pieces / mm 2 is.

Advantages of the Invention

[0037] Compared with the prior art, the present invention has the following advantages.

[0038] (1) Good property uniformity. The present invention provides high-quality slab raw materials by controlling the composition, purity and gas content in the steelmaking process, scientifically designs the heating, rolling and post-rolling cooling control processes, and takes measures such as large reduction in the rough rolling stage and rational allocation of the reduction in the two stages to make the deformation penetrate to the center of the steel plate, optimizing the heat treatment process, and obtaining a high-strength and tough structure throughout the thickness section, with excellent mechanical properties, transverse and longitudinal impact properties, aging impact properties and cold bending properties, as well as uniform properties through the thickness, which fully meets the certification requirements of the Ship Grade Association for EH690 steel.

[0039] (2) Low cost, high production efficiency. By optimizing the composition design and using a reasonable rolling control and cooling control process, the plastic deformation and post-rolling cooling are combined with the solid phase transition to fully play the effects of solid solution strengthening, precipitation strengthening and fine grain strengthening, and the effects of alloying elements are fully played out. The design of the roller table speed of the quenching machine, the water volume of the high pressure and low pressure zones and the ratio of water and drainage are optimized to improve the heat treatment effect of thick steel plates, and after quenching and high temperature tempering, steel plates with excellent properties can be obtained without other heat treatment processes, shortening the production process and reducing production costs.

[0040] (3) Excellent corrosion resistance. The present invention uses high-purity molten steel smelting technology, high-penetration rolling technology, and an optimized heat treatment process to obtain a structure with low inclusions and high homogeneity, which has excellent corrosion resistance, and greatly improves the corrosion potential of the steel plate substrate itself. In addition, the addition of Cr+Cu+Ni alloy elements effectively promotes the formation of a dense and highly adhesive protective rust layer on the surface of the steel plate, which can prevent the penetration of corrosive media such as H2O, O2, and Cl- into the steel substrate, ensuring the safety of the product in the highly corrosive marine environment. [Brief description of the drawings]

[0041]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Mode for Carrying Out the Invention

[0042] Hereinafter, the present invention will be described in more detail with reference to the drawings and specific embodiments.

[0043] Example 1 With the chemical components shown in Table 1, according to the above smelting process, high-purity molten steel was obtained by converter smelting and LF+RH double refining, and a 300-mm-thick continuous casting slab was obtained by casting. The slab was heated with a tapping temperature of 1210 °C and a holding time in the furnace of 300 min, and rough rolling was carried out in 5 passes (the 5th pass was idle), with the reduction ratios in the 3rd and 4th passes being 19.3% and 23.7% respectively. After the completion of rough rolling, the thickness of the intermediate slab was 130 mm, and when the temperature reached 845 °C, the finish rolling stage was started. Finish rolling was carried out in 7 passes (the 7th pass was idle), and cooling control was carried out at a rate of about 8 °C / s, with the cooling start temperature being 815 °C and the cooling end temperature being 590 °C. After rolling, it was slowly cooled in a slow-cooling pit for 48 h or more. The quenching temperature was heated to 920 °C and held for 30 min. The roller table speed of the quenching machine was 1.8 m / min, the water volume in the high-pressure zone was 6067 m 3 / h, the water volume in the low-pressure zone was 3499 m 3 / h, and the ratio of the water volume of the upper nozzle to the water volume of the lower nozzle was 1:1.4. The tempering heating temperature was 600 °C and held for 30 min. As can be seen from FIGS. 1-3, the structure near the surface was mainly tempered sorbite, and as it approached the center, the content of granular bainite, ferrite, and pearlite, which were dispersed and distributed, gradually increased. Inspection was carried out in accordance with the test methods and requirements in the 'Materials and Welding Code' (2021) of the China Classification Society, and the main tensile property and impact property indexes are shown in Table 2, and the corrosion resistance property indexes are shown in Table 3.

[0044] Example 2 With the chemical composition shown in Table 1, high-purity molten steel was obtained by converter steelmaking and LF+RH double refining according to the above smelting process, and a 300-mm-thick continuous casting slab was obtained by casting. The slab was heated, and the tapping temperature for continuous casting slab heating was 1230°C, and the in-furnace time was 290 min. Rough rolling was performed in 5 passes, and the reduction ratios in the 4th and 5th passes were 19.1% and 22.3% respectively. After the rough rolling was completed, the thickness of the intermediate slab was 120 mm. When the temperature reached 860°C, the finish rolling stage was started, and finish rolling was performed in 5 passes. Cooling control was carried out at a rate of about 10°C / s, with the cooling start temperature being 803°C and the cooling end temperature being 559°C. After rolling, it was slowly cooled in a slow cooling pit for 48 h or more. The quenching temperature was heated to 920°C and held for 30 min. The roller table speed of the quenching machine was 1.6 m / min, the water volume in the high-pressure zone was 5376 m 3 / h, and the water volume in the low-pressure zone was 3888 m 3 / h. The ratio of the water volume of the upper nozzle to the water volume of the lower nozzle was 1:1.4. The tempering heating temperature was 600°C and held for 30 min.

[0045] Comparative Example To explain the influence of the parameters of the quenching machine on the properties of the steel plate, the present invention describes an embodiment of the adjustment process of the quenching machine as a comparative example. The smelting and casting embodiments of the comparative example were the same as those of Example 2. The slab was heated, and the tapping temperature for continuous casting slab heating was 1230°C, and the in-furnace time was 290 min. Rough rolling was performed in 5 passes, and the reduction ratios in the 4th and 5th passes were 20.0% and 21.6% respectively. After the rough rolling was completed, the thickness of the intermediate slab was 120 mm. When the temperature reached 850°C, the finish rolling stage was started, and finish rolling was performed in 5 passes. Cooling control was carried out at a rate of about 10°C / s, with the cooling start temperature being 805°C and the cooling end temperature being 562°C. After rolling, it was slowly cooled in a slow cooling pit for 48 h or more. The quenching temperature was heated to 920°C and held for 30 min. The roller table speed of the quenching machine was 2.0 m / min, the water volume in the high-pressure zone was 6255 m 3 / h, and the water volume in the low-pressure zone was 3287 m 3 / h, the ratio of the water volume of the upper nozzle to that of the lower nozzle was set to 1:1.4. The tempering heating temperature was set to 600 °C and held for 30 min.

[0046] Table 2 shows the tensile properties and impact properties of the steel plates obtained in the examples and comparative examples. The steel plates manufactured by the method of the present invention had a yield strength ≥ 690 MPa, a tensile strength of 770 - 940 MPa, an elongation at break ≥ 16%, a transverse impact energy at -40 °C ≥ 100 J, and good thickness-direction property uniformity. On the other hand, the strength and toughness of the comparative examples were clearly lower than those of the examples, and in particular, the impact properties at the 1 / 4 thickness position and the center could not meet the property requirements of E690 steel.

[0047] Table 3 shows the corrosion resistance indexes of the steel plates obtained in the examples and comparative examples. The saturated corrosion current density was measured with an Autolab electrochemical workstation. Using a three-electrode system, with an Ag / AgCl electrode as the reference electrode and a Pt electrode as the auxiliary electrode, the steel plate was sampled to manufacture a working electrode with an exposed area of 1 cm 2 sample. Anodic polarization was performed on the sample at a constant potential of -300 mV in an artificial seawater solution (see Table 4 for the composition), and the change in polarization current and the corrosion current density after its stabilization were recorded. Figure 4 is the -300 mV (vs. Ag / AgCl) constant potential polarization curve of the steel plate obtained in Example 1. When measuring the density of corrosion-active inclusions, the steel plate was sampled to manufacture a 10 mm × 10 mm × 5 mm sample. After polishing, it was immersed in artificial seawater for 20 min. After taking it out, it was rinsed with alcohol and dried with cooling air, and then placed under a 50x optical microscope to continuously take a plurality of photos to calculate the density of corrosion-active inclusions. Figure 5 shows the situation of corrosion-active inclusions in the 50x field of view of the steel plate obtained in Example 1.

[0048] Table 1 Chemical compositions (wt%) of the steel plates in the examples and comparative examples of the present invention

Table 1

[0049] Table 2 Properties of the steel plates in the examples and comparative examples of the present invention

Table 2

[0050] Table 3 Indicators of the corrosion resistance characteristics of the steel plate in the examples of the present invention

Table 3

[0051] Table 4 Composition of artificial seawater

Table 4

[0052] The upper limit value, lower limit value, and interval value of the process parameters (temperature, time, etc.) of the present invention can all be realized by this method, and the examples will not be described one by one here.

[0053] For the content not described in detail in the present invention, any conventional technical knowledge in the relevant field can be adopted.

[0054] It should be noted that the above examples are only used to explain the technical solution of the present invention and do not limit it. Although the present invention has been described in detail with reference to the examples, those skilled in the art should understand that any changes or equivalent substitutions of the technical solution of the present invention do not deviate from the spirit and scope of the technical solution of the present invention and all belong to the scope of the claims of the present invention.

Claims

1. A steel plate for marine industry with ultra-high strength and toughness of 690 MPa grade and a thickness of 80 mm, wherein the chemical composition is, by mass%, C: 0.08% to 0.10%, Si: 0.20% to 0.30%, Mn: 1.10% to 1.25%, P ≤ 0.007%, S ≤ 0.002%, Nb: 0.020% to 0.030%, Ti: 0.010% to 0.020%, V: 0.030% to 0.045%, Cr: 0.40% to 0.60%, Ni: 1.40% to 1.50%, Cu: 0.15% to 0.25%, Mo: 0.25% to 0.35%, Als: 0.015% to 0.045%, Pcm ≤ 0.33%, Ceq ≤ 0.64%, and the balance is Fe and inevitable impurity elements, and it is characterized by a steel plate for marine industry with ultra-high strength and toughness of 690 MPa grade and a thickness of 80 mm.

2. The content of each composition of the inevitable impurity elements is, by mass%, H ≤ 0.0002%, O ≤ 0.003%, N ≤ 0.004%, B ≤ 0.0005%, As ≤ 0.006%, Sb ≤ 0.010%, Sn ≤ 0.020%, Pb ≤ 0.010%, Bi ≤ 0.010%, and it is characterized by the steel plate for marine industry with ultra-high strength and toughness of 690 MPa grade and a thickness of 80 mm according to Claim 1.

3. The main characteristic indicators are: yield strength ≥ 690 MPa, tensile strength 770 - 940 MPa, elongation at break ≥ 16%, transverse impact energy at the center at -40°C ≥ 100 J, CTOD at -40°C ≥ 0.15 mm, saturated corrosion current density at -300 mV vs. Ag / AgCl reference electrode ≤ 1.90 mA / cm ２ , density of corrosion-active inclusions ≤ 9 pieces / mm ２ A steel plate for marine industry with ultra-high strength and toughness of 690 MPa grade and a thickness of 80 mm according to claim 1 or 2, characterized in that it is as described above.

4. A manufacturing method of a steel plate for marine industry with ultra-high strength and toughness of 690 MPa grade and a thickness of 80 mm according to any one of Claims 1 to 3, comprising a converter steelmaking step, an LF + RH double refining step, a continuous casting step, a slab heating step, a rolling step, and a heat treatment step, and it is characterized by a manufacturing method of a steel plate for marine industry with ultra-high strength and toughness of 690 MPa grade and a thickness of 80 mm.

5. In the converter steelmaking step, using hot metal after KR treatment to make the hot metal S ≤ 0.008%, adding nickel plate, copper plate, ferromolybdenum together with steel scrap, smelting using a double slag deep dephosphorization process, alloying with metallic manganese, ferroniobium, ferrovanadium, low-carbon ferrochrome, ferrosilicon, and adding aluminum manganese iron according to 3 to 3.5 kg / t steel for deoxidation, and it is characterized by the manufacturing method according to Claim 4.

6. In the LF+RH double refining step, argon is bottom-blown throughout the entire LF refining process, the refining time is set to ≥50 min, among which the soft blowing time is 10 - 15 min, for RH refining, the vacuum degree is ensured to be 30 Pa or less, and the pure degassing time is ≥5 min. After the RH treatment is completed, calcium-aluminum wire is supplied at 1 - 1.5 m / t, the soft blowing time is 15 - 20 min, and the RH smelting time is ≥50 min. The manufacturing method according to claim 4 is characterized by the above.

7. In the continuous casting step, protective casting is adopted throughout the entire process, the superheat degree is controlled to be 20°C or less, the casting speed of the continuous casting slab with a thickness of 300 mm used is 0.70 - 0.90 m / min, and the obtained slab is slowly cooled for 72 hours or more. The manufacturing method according to claim 4 is characterized by the above.

8. In the slab heating step, a multi-stage heating and temperature-rising method is adopted, the temperature of the soaking zone is 1190 - 1240°C, the soaking time is ≥60 min, the tapping temperature is 1200 - 1230°C, and the total time for heating and temperature-rising is 290 - 310 min. The manufacturing method according to claim 4 is characterized by the above.

9. The rolling step includes two-stage rolling of rough rolling and finish rolling. The rough rolling is recrystallization rolling, the number of passes in rough rolling is set to ≤5, and at least two passes have a reduction rate of ≥19% ensured. After rough rolling, the thickness of the intermediate slab is 120 - 130 mm. The finish rolling is unrecrystallized rolling, the rolling start temperature of the finish rolling is 835 - 865°C, the number of rolling passes in the finish rolling is set to ≤7, the cooling start temperature of the rapid cooling process after rolling is 800 - 820°C, the cooling end temperature is 550 - 590°C, and the cooling rate is 6 - 10°C / s. The manufacturing method according to claim 4 is characterized by the above.

10. In the heat treatment step, an oil quenching + high-temperature tempering process is adopted. The oil quenching temperature is 920 ± 5 °C, the heating time is 1.3 - 1.6 min / mm × plate thickness, the holding time is 30 ± 3 min, the water pressure in the high-pressure zone of the oil quenching machine is 0.7 - 0.9 bar, the water pressure in the low-pressure zone is 0.3 - 0.4 bar, the roller table speed is 1.6 - 1.8 m / min, the water volume in the high-pressure zone is 5376 - 6067 m ３ / h, the water volume in the low-pressure zone is 3499 - 3888 m ３ / h, the ratio of the water volume of the upper nozzle to the water volume of the lower nozzle is about 1:1.4, the tempering heating temperature is 600 ± 5 °C, the heating time is 2 - 2.5 min / mm × plate thickness, the holding time is 30 ± 3 min, and after taking it out of the furnace, it is air-cooled. The manufacturing method according to claim 4, characterized in that.

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