High-strength super-thick steel for ocean engineering and production method thereof

Abstract

The application discloses a high-strength and super-thick marine engineering steel and a production method thereof. The chemical components of the steel are as follows: C: 0.02-0.07%, Si: 0.10-0.25%, Mn: 1.75-2.5%, P: 0.022-0.026%, S: ≤0.008%, Nb: 0.035-0.040%, V: 0.035-0.045%, Ti: 0.02-0.03%, Cr: 0.45-0.55%, Ni: 0.45-0.55%, Co: 0.07-0.10%, Als: 0.035-0.045%, N: 0.010-0.012%, Cu: 0.25-0.33%, Sn: 0.03-0.05% and Sb: 0.03-0.05%. After smelting, slab continuous casting, casting blank heating, rolling and cooling, the steel has the following properties: Rp0.2 is 485-525 MPa, A is greater than or equal to 25%, impact energy at-80 DEG C is greater than or equal to 200 J, corrosion resistance is less than 0.080 mm / a, fatigue life under a peak stress of 240 MPa is greater than 2 million times, the maximum thickness can reach 150 mm, and Z-direction performance is greater than or equal to 45%. The application solves the problems of low strength, poor low-temperature impact toughness, insufficient corrosion resistance and insufficient fatigue resistance of the existing marine engineering steel plate.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of metal material preparation, and particularly relates to a high-strength and thick marine engineering steel and a production method thereof. BACKGROUND

[0002] Marine equipment manufacturing steels are subjected to complex marine waves, temperature, humidity, salinity and other complex environments, and higher requirements are put forward for the performance of the steels, especially the corrosion and fatigue performance. For example, the steels are required to have high strength, large thickness, low-temperature impact toughness, good Z-direction performance, and also good corrosion resistance and fatigue resistance. The existing steels are difficult to meet the above performance requirements at the same time.

[0003] Patent No. 202110729039.7 discloses a small-compression-ratio high-homogeneity low-yield-strength-ratio 500MPa-grade high-strength steel and a manufacturing method thereof. The yield strength of the steel plate is above 500MPa, the tensile strength is above 600MPa, the yield strength ratio is not greater than 0.85, and the core grain size is above 6.5. The product has the characteristics of low yield ratio, high homogeneity, and high strength and toughness. However, the embodiments only involve the impact toughness at -20℃, and do not involve the corrosion resistance problem.

[0004] Patent No. 202010557893.5 discloses a 550MPa-grade weather-resistant steel plate with excellent weldability and a manufacturing method thereof. The TMCP process is optimized, the yield strength is ≥460MPa, the tensile strength is ≥570MPa, the Charpy impact energy at -40℃ (single value) is ≥120J, the impact toughness at -40℃ is KV2≥100J, and the heat-affected zone (HAZ) at -40℃ Akv is ≥100J. However, the strength is relatively low, the low-temperature toughness is only at -40℃, and the corrosion resistance and fatigue resistance problems are not solved.

[0005] Patent No. CN201410036368.3 discloses a corrosion-resistant steel plate for resisting the South China Sea marine environment and a production process thereof. The production process includes a converter smelting process, an LF refining process, a vacuum degassing process, a continuous casting process, a controlled rolling and controlled cooling process, etc. The microstructure type of the steel plate is theoretically single-phase polygonal ferrite fine structure (average grain size 10.17μm), and in the actual industrial production, it inevitably contains a small amount of pearlite structure. Compared with the conventional ship structure steel EH36, the corrosion resistance of the steel plate to the marine environment (marine atmosphere, tidal range, full immersion, etc.) is improved by more than 50%, and the steel plate has good strength-toughness matching and welding performance. However, the strength is relatively low, the low-temperature toughness is insufficient, and the fatigue resistance is not evaluated.

[0006] The patent with the application number 201910712227.1 discloses a yield strength 345 MPa grade high fatigue structural steel and a manufacturing method thereof. The chemical composition of the steel is: C 0.13%~0.16%, Mn 1.30%~1.60%, Nb 0.020%~0.050%, Alt 0.020%~0.030%, Ti≤0.010%, Si≤0.12%, P≤0.010%, S≤0.005%, and the balance is iron and inevitable impurities. By adopting large reduction + controlled cooling process, the obtained steel plate has good comprehensive mechanical properties and better surface quality. However, the strength of the steel plate is low, and its corrosion resistance is not evaluated, only the impact toughness at-20℃ is evaluated, which cannot meet the use requirements in different marine environments.

[0007] The patent with the application number 202110068169.0 discloses an engineering anti-corrosion fatigue steel and a preparation method thereof. The steel is based on the main elements of E690 steel (C 0.04%~0.07%, Si 0.20%~0.26%, Mn 1.45%~1.60%, P≤0.01%, S≤0.015%, Cr 0.44%~0.50%), and the elements are controlled and characteristic elements are added, Cu 0.28%~0.66%, Ni 0.76%~1.55%, Sb 0.03%~0.12%, and the rest is Fe and inevitable impurities. The corrosion fatigue strength is improved by up to 52%, but the low temperature toughness of the steel is not evaluated. SUMMARY

[0008] In view of the deficiencies of the prior art, the purpose of the present application is to provide a high-strength thick marine engineering steel and a production method thereof. The steel is characterized by low carbon and low alloying, combined with the addition of multi-alloy strengthening elements such as Cr, Ni, Co, and V, N precipitation strengthening elements, and the addition of P, Al, and Sb, Sn as the basic characteristics, to improve the strength, low temperature toughness, and good corrosion resistance and fatigue resistance of the steel plate. The problems of low strength, poor low temperature impact toughness, insufficient corrosion resistance and fatigue resistance of the existing marine engineering steel plate are solved.

[0009] To achieve the above purpose, the technical scheme adopted by the present application is as follows:

[0010] The present application is characterized by a steel plate with excellent low-temperature toughness, high strength, corrosion resistance and fatigue resistance. In the present application, the chemical composition of the steel plate (wt%) is as follows: C: 0.02% to 0.07%, Si: 0.10% to 0.25%, Mn: 1.75% to 2.5%, P: 0.022% to 0.026%, S: ≤0.008%, Nb: 0.035% to 0.040%, V: 0.035% to 0.045%, Ti: 0.02% to 0.03%, Cr: 0.45% to 0.55%, Ni: 0.45% to 0.55%, Co: 0.07% to 0.10%, Als: 0.035% to 0.045%, N: 0.010% to 0.012%, Cu: 0.25% to 0.33%, Sn: 0.03% to 0.05%, Sb: 0.03% to 0.05%, wherein 0.06 ≥ (Nb+V+Ti) / Mn ≥ 0.04, 7 ≥ Si / Al ≥ 2.5, (Cu+P+Co) / Cr ≥ 0.70, 8 ≥ Cr / Co ≥ 4, Ni / Cr ≥ 0.8, 36Nb / Mn ≥ 0.60, (Nb+V+Ti+Als) / N ≥ 11, (Nb+V+Ti) / (Sn+Sb) ≥ 0.9, Ni / Cu ≥ 1.4, and the rest is Fe and inevitable impurities.

[0011] The above alloying elements and their contents are selected because of their roles in improving the strength, toughness, corrosion resistance and fatigue resistance of the steel plate for marine engineering:

[0012] C: C and Cr can form alloy cementite (Fe·Cr)3C, and can also form carbides such as Cr7C3, Cr 23 C6, etc. The melting point, hardness, wear resistance and stability of these carbides are higher than those of Fe3C, which can improve the strength of the steel. C and strong carbide-forming elements such as V, Nb and Ti can form carbides such as VC, NbC and TiC, which have the highest stability and the highest melting point, hardness and wear resistance. Therefore, C is the most effective element for improving the strength of the steel plate, and a content lower than 0.02% will significantly reduce the strength of the steel plate. However, C has a great influence on the low-temperature toughness, elongation and weldability of the steel. From the perspective of improving the toughness, corrosion resistance, fatigue resistance and weldability of the steel, the C content in the steel should be appropriately low. Therefore, the C content in the present application is selected to be 0.02% to 0.07%.

[0013] Si: Si is necessary for steelmaking deoxidation, has a certain solid solution strengthening effect, although Si can improve the strength of the steel plate, but Si reduces the critical cooling rate of martensite transformation, seriously damages the low temperature toughness, elongation and weldability of high strength steel plate, Si not only promotes the formation of M-A island, but also the size of the formed M-A island is relatively large and unevenly distributed, seriously damages the toughness of the welding heat affected zone (HAZ), therefore the Si content in the steel should be controlled as low as possible. A certain Si content can effectively improve the marine corrosion resistance of steel, and the combined addition of Si and Al can improve the corrosion resistance and high temperature oxidation resistance. The content of Si in the application is controlled at 0.10% to 0.25%, and 7≥Si / Al≥2.5.

[0014] Mn: Mn is the main element for improving strength and toughness, can significantly improve the hardenability of steel, and is very low in cost, which is the main additive element in steel. When the C content is low, a higher Mn content can effectively improve the hardenability of the steel, and improve the strength of the steel plate while having excellent low temperature toughness through microstructure refinement and promoting bainite transformation; Mn expands the austenite region, promotes grain growth, and needs to be combined with the composite addition of grain refining elements Nb, V and Ti to promote grain refinement and improve the fatigue performance of the steel; but Mn is easy to segregate during the solidification of molten steel, which aggravates the segregation and porosity in the center of the casting blank, resulting in low low temperature toughness of high strength steel plate and cracks in the welded joint, which needs to be improved by optimizing the continuous casting process and heating process. The content of Mn in the application is selected at 1.75% to 2.50%, and 0.06≥(Nb+V+Ti) / Mn≥0.04.

[0015] P: P has strong solid solution strengthening effect in steel, as an alloying element added to low alloy structural steel, it can improve the strength and corrosion resistance of the steel, and its content greater than or equal to 0.02% can significantly improve the corrosion resistance. But when its content is too large, it will have an adverse effect on the low temperature toughness of the base material and the toughness of the welding heat affected zone, so its content should be controlled in a reasonable range as much as possible. The content of P in the application is controlled at 0.022% to 0.026%.

[0016] S: S is severely segregated in steel, which deteriorates the quality of the steel. S is an inclusion forming element, which forms FeS, MnS and other inclusions, thereby reducing the ductility of the steel, and the inclusions will become the source of corrosion, which is not conducive to the corrosion resistance of the steel plate. And FeS is easy to melt at the grain boundary due to its low melting point, which weakens the bonding force between the grains, resulting in the thermal brittleness of the steel, therefore a certain amount of Mn element should be added to form MnS with high melting point and deformation ability. Therefore, the content of S in the application is controlled to be less than or equal to 0.008%.

[0017] Co: Co mainly plays a solid solution strengthening role in the steel, improves the oxidation resistance and the ability of corrosion resistance of the steel, Co can increase the interaction between Fe atoms, reduces the critical concentration of Cr atoms forming clusters, and further improves the stability of Cr atom clusters, when Co and Cr atoms act on the steel at the same time, the surface forms a smooth passivation film with high structural stability, which can well protect the matrix, thus has excellent corrosion resistance. Co promotes precipitation hardening by promoting more (Ti, Al) Ni3 nucleation sites, improves strength and toughness, the addition of Co has little effect on the precipitation ratio of Ni and Ti, but can reduce the size of the precipitated phase, Co increases the nucleation rate of Ni3Ti, and increases the number density of precipitated phase, that is, Co can make the distribution of precipitated phase more dispersed, improve the fatigue resistance of the steel, and enhance the precipitation strengthening effect of precipitated phase. Therefore, the content of the present application is controlled at 0.07% to 0.10%, and 8≥Cr / Co≥4.

[0018] Cr: Cr can improve the strength and hardness of the steel. Cr is a ferrite forming element, which helps to increase the ferrite content in the steel and thus improve the low temperature toughness of the steel. Cr is an element that improves the corrosion resistance of the steel, but adding Cr alone sometimes reduces the corrosion resistance, even worse than that of ordinary carbon steel, and needs to be used in combination with other corrosion-resistant alloy elements such as Cu, P, Co, etc. The corrosion resistance will be significantly improved. The content of the present application is controlled at 0.45% to 0.55%, and (Cu+P+Co) / Cr≥0.70.

[0019] Ni: Ni strengthens ferrite and refines pearlite in the steel, and the overall effect is to improve the strength, and has no significant effect on plasticity. Ni can improve the fatigue resistance of the steel and reduce the sensitivity of the steel to notches, so Ni can improve the fatigue performance of the steel. Ni does not form carbides, and plays a role in strengthening ferrite by forming a simple substitution solid solution, reduces the ductile-brittle transition temperature of the steel, and improves the low temperature toughness of the steel; a certain Ni content can ensure that the steel plate has sufficient hardenability, uniform performance in the thickness direction, and at the same time, ensure the matching of strength and toughness of the steel plate and low temperature toughness. Adding Ni to the steel can also reduce the copper brittleness of Cu-containing steel, reduce intergranular cracking during hot rolling, improve the atmospheric corrosion resistance of the steel plate, and Ni and Cr can significantly improve the corrosion resistance of the steel. The content of the present application is controlled at 0.45% to 0.55%, and Ni / Cr≥0.8, Ni / Cu≥1.4.

[0020] Cu: Cu is the most main and most commonly used alloying element in corrosion-resistant steel. Cu can activate the cathode, promote the passivation of steel anode to slow down corrosion; a layer of copper-rich phase will be formed on the surface of steel during corrosion process, and there is a dense and strongly adherent intermediate layer between the corrosion layer on the surface of steel and the copper-rich layer, which further relieves the corrosion of steel, especially when used with P, it can significantly improve the performance of resistance to marine atmospheric corrosion and seawater corrosion; Cu and Ni composite addition can not only reduce the copper brittleness of copper-containing steel and reduce the intergranular cracking during hot rolling, but more importantly, Cu and Ni are both austenite stabilizing elements, and Cu and Ni composite addition can greatly reduce Ar3 and improve the driving force of austenite to ferrite phase transition; at the same time, Cu can accelerate the strain-induced precipitation of carbonitride niobium at high temperature, and the recrystallization stop temperature is increased, which is beneficial to the non-recrystallization zone controlled rolling to refine the phase transition product and improve the fatigue resistance of the material. The content of Cu in the application is controlled at 0.25% to 0.33%.

[0021] Nb: Nb is an important element in controlled rolling and controlled cooling steel, Nb is a strong carbide forming element, and the NbC and NbN two-phase particles formed by C and N are important elements in controlled rolling and controlled cooling steel, which can effectively refine the grain, thereby improving the strength and low temperature impact toughness. The composite addition of Nb and Mn can effectively inhibit the recovery, recrystallization and other processes of austenite during rolling, on the one hand, it can increase the austenite recrystallization temperature, thereby increasing the rolling temperature and reducing the rolling unit load; on the other hand, it can effectively refine the phase transition structure of the steel plate, thereby improving the strength and low temperature impact toughness; it can prevent intergranular corrosion of the steel by oxidizing medium. Nb can increase the solid solution amount of rare earth in steel, thereby improving the corrosion resistance of the steel, and the content of Nb in the application is controlled at 0.035% to 0.040%, and 36Nb / Mn≥0.60.

[0022] V: V has great affinity with O and N, and is a strong carbide forming element; generally, the dispersion degree of VC is very high and very stable, so it can obtain a dense and fine-grained structure by deoxidizing and degassing, improve plasticity, toughness and high strength, and the impact performance and fatigue strength are higher than those of steel without V, and it has high strength and toughness at high temperature and low temperature (<0℃). Since the high dispersion of VC prevents the grain of the weld from growing too large, the weldability of the steel can be improved, but the steel will grow strongly when heated to the VC dissolution temperature. When dissolved in the solid solution at high temperature, the hardenability is increased; otherwise, if it exists in the form of carbide, the hardenability is reduced. V increases the tempering stability of quenched steel and produces a secondary hardening effect. V can increase the solid solution amount of rare earth in steel, thereby improving the corrosion resistance of the steel, and the content of V in the application is controlled at 0.035% to 0.045%.

[0023] Ti: Ti has very strong affinity with N, O and C, and stronger affinity with S than Fe. Therefore, it is a good deoxidizing and degassing agent and an effective element for fixing N and C. Ti is a strong carbonitride forming element, and a trace amount of Ti can combine with N in steel to form TiN, preventing the growth of austenite grains during soaking, and also preventing the growth of austenite grains in the heat-affected zone of welding, thereby improving weldability. TiC and TiN have strong binding force, are stable and not easy to decompose, and can only slowly dissolve into the solid solution in steel when heated to above 1000℃, thus significantly controlling the growth of grains in the heat-affected zone of welding and improving the weldability of the material. Since Ti fixes N and S and forms TiN, the plasticity and impact toughness of the steel can be significantly improved. Ti can increase the solid solution amount of rare earth in steel, thereby improving the corrosion resistance of the steel. However, Ti has very strong affinity with N and O and is prone to form TiN and TiO2, resulting in a large number of non-metallic inclusions and subcutaneous porosity and other defects at a relatively low temperature. The content of Ti in the present application is controlled at 0.02% to 0.03%.

[0024] N: N and C can be dissolved in Fe to form interstitial solid solution, N expands the austenite phase region of steel, and is a very strong element for forming and stabilizing austenite, and can replace a part of Ni within a certain limit, and N on the surface of steel can combine with elements such as Nb, Al, V and Ti to form extremely stable nitrides, becoming surface hardening and strengthening elements, and improving the corrosion resistance of the steel. However, too high residual nitrogen content in steel can lead to loose macrostructure or porosity, so a certain amount of Al needs to be added to form stable AlN to avoid nitrogen escaping to form porosity and other defects during solidification. Therefore, the content of N in the present application is controlled at less than or equal to 0.010% to 0.012%, and (Nb+V+Ti+Als) / N≥11.

[0025] Al: Al is mainly used for deoxidization and grain refinement, Al generates effective fine dispersions with N or O to inhibit the grain growth of steel during heating, promotes the decomposition of austenite during cooling of the steel to improve the hardenability of the steel, and becomes a nucleation site for recrystallization, promoting the nucleation of ferrite and refining the grains to improve the fatigue resistance of the steel. AlN itself has high stability during heating, thus improving the thermal stability of the steel, which is beneficial to reducing the tendency of overheating of the steel, and improving the oxidation resistance of the steel; Al generates effective surface hardening layer through low-temperature diffusion of N (nitriding) to improve the oxidation resistance and corrosion resistance of the steel; when deoxidizing with Al, a certain amount of Si is appropriately added to significantly improve the deoxidizing property of Al; however, if the amount of Al is too large, abnormal structure of the steel and tendency of graphitization of the steel will be generated. Therefore, the content of Als in the present application is controlled at 0.035% to 0.045%.

[0026] Sn and Sb: At austenitic temperatures, antimony (Sb) in steel precipitates at MnS inclusions and along the original austenite grain boundaries, thus inhibiting the enrichment and precipitation of MnS inclusions at the grain boundaries. Sb can also refine the grain size of secondary recrystallization. When added simultaneously with Nb, V, and Ti, it can further refine the steel's microstructure and improve toughness, thereby enhancing the steel's fatigue resistance and corrosion resistance. Sn and Sb help improve the material's corrosion resistance; both adding Sn alone and adding Sn and Sb in combination significantly improve the material's corrosion resistance. Sn and Sb form a corrosion-resistant oxide film of SnO2 and Sb2O5 on the steel surface, effectively preventing the interaction between the matrix and the corrosive medium, thus inhibiting the corrosion of the steel in the corrosive medium. Sn and Sb, after hydration, yield Sn... 2+ Sb 3+ The precipitates formed in the anodic micro-regions fill corrosion cracks or cavities, enhancing the resistance to corrosion. - The ability to penetrate; Sn and Sb can also be absorbed by Cl - Hydration under environmental media inhibits Fe 3+ Hydrolysis produces H + The process improves the pH value of the corrosion micro-area and alleviates the anodic dissolution process. Sn and Sb, as corrosion inhibitors, change the anode and cathode reaction process and can significantly improve the corrosion resistance of steel in marine environments. In this invention, their contents are controlled at 0.03% to 0.05%, and (Nb+V+Ti) / (Sn+Sb)≥0.9.

[0027] The above describes the content range and function of various added elements. The manufacturing method of this invention for producing high-strength, extra-thick marine engineering steel includes:

[0028] 1. The steel is smelted according to the above composition, and the process includes:

[0029] 1) During converter smelting, the contents of elements such as C, Si, Mn, P, and S are adjusted to be within the range of this invention. During the smelting process, the molten iron is first desulfurized and pretreated. After desulfurization, the S content in the molten iron is ≤0.0025%. A combined top and bottom blowing process is adopted, and the converter tapping temperature is 1630~1650℃. Then, Si-Ca wire feeding treatment is carried out, and the Ca content is controlled at 0.0015%~0.0025%.

[0030] 2) Refine the molten steel. The RH treatment time is 30-35 minutes. Nitrogen is blown throughout the RH treatment. The [H] in the steel is controlled to be ≤1.0 ppm and [O] is controlled to be ≤20 ppm. The net circulation time before removal is 6-10 minutes.

[0031] 3) Add Sn and Sb elements before the RH treatment is completed. The amount added should be 1.2 to 1.3 times the target control amount to ensure that their content can be controlled within the target range.

[0032] 2. Casting the molten steel obtained in step 1 into a desired continuous casting billet, in order to control the content of intermediate axis crystal in the continuous casting billet, the tundish superheat is 20-30°C, the lower superheat can reduce the solidification time of the molten steel, and can reduce the segregation of C, Mn and other elements in the center of the billet, reduce the porosity, shrinkage and other defects of the billet, and ensure the Z-direction performance and fatigue resistance of the steel plate; adopting full protection pouring, the continuous casting billet drawing speed is controlled at 1.1-1.3 m / min, the specific water quantity of the secondary cooling water is 0.80-0.90 m 3 / t, so that the equiaxed crystal ratio of the continuous casting billet is > 28.0%, and the electromagnetic stirring is adopted at the solidification end to make the molten steel stirring uniform and then realize high strength and high density solidification as soon as possible, and the heavy press-down is adopted in the continuous casting process, and the press-down amount is 15.0-20.0 mm.

[0033] 3. In order to control the grain size of the continuous casting billet to be not greater than 500 μm, the continuous casting billet is rapidly cooled, the open cooling temperature is 960-990°C, the cooling speed is 8.5-9.5°C / s, and after cooling to 685-725°C, it enters the slow cooling pit for slow cooling, and then is cooled to below 150°C at a cooling speed of 4.0-22.0°C / h.

[0034] 4. The continuous casting billet obtained in step 3 is sent into a heating furnace for heating. The heating adopts a segmented heating process, enters the furnace at a furnace temperature of 660-760°C, and is kept for 1.5-2.5 h to release the internal stress of the billet; below 950°C, a slow heating process is adopted to further release the internal stress caused by the cooling and heavy press-down of the continuous casting billet, and to prevent new temperature stress caused by too fast heating, the heating speed is controlled at 8-10°C / min, and is heated to 950°C and kept for 20-35 min; above 950°C, a process of rapid heating and appropriately prolonging the holding time is adopted to prevent the coarsening of austenite grains and make the alloying elements in the billet fully diffuse and dissolve, further reducing the element segregation, the heating speed is controlled at 12-15°C / min, and is heated to 1190-1210°C for uniform heating, and the holding time is 3.5-4.5 h.

[0035] 5. The cast blank is rolled into finished steel plate by three stages, in the first stage, in order to fully break the columnar crystal of the continuous casting blank, a high temperature slow rolling and large reduction process is adopted, the cast blank is directly rolled after descaling, the rolling speed is 0.80-1.20 m / s, the reduction of the first three passes is >45 mm, the billet is cooled by rolling mill cooling water between each pass, the cooling time is 5-7 s, the final rolling temperature is 1000-1050℃, the thickness of the warm billet is 1.5-2.5 times the thickness of the finished product, in order to suppress the growth of the intermediate billet grains, the warm billet is cooled by water spraying, the cooling speed is 7.0-9.0℃ / s, and the temperature is cooled to 10-25℃ above the second stage rolling temperature; the second stage rolling temperature is 910-940℃, the rolling speed is 1.5-2.0 m / s, the final rolling temperature is 825-855℃, and the thickness of the warm billet is 1.5-2.0 times the thickness of the finished product; the third stage rolling temperature is 770-820℃, and the final rolling temperature is 720-740℃.

[0036] 6. The rolled steel plate adopts a rapid cooling (ACC) process, the cooling speed is 6.0-14.0℃ / s, the steel plate red temperature is 540-610℃, the fine grains after rolling can be maintained, and the grain growth is prevented.

[0037] Compared with the prior art, the present application has the following beneficial effects:

[0038] The present application adopts low carbon and high manganese content design, controls the range of composite addition of alloying elements 0.06≥(Nb+V+Ti) / Mn≥0.04, 7≥Si / Al≥2.5, (Cu+P+Co) / Cr≥0.70, 8≥Cr / Co≥4, Ni / Cr≥0.8, 36Nb / Mn≥0.60, (Nb+V+Ti+Als) / N≥11, Ni / Cu≥1.4, (Nb+V+Ti) / (Sn+Sb)≥0.9, improves the strength of the steel while ensuring that the steel plate has excellent low temperature toughness, corrosion resistance and fatigue resistance. By optimizing the continuous casting process to control the influence of element segregation on the fatigue performance and low temperature performance of the steel plate, slow and long time segmented heating process is adopted for heating, three stage process of high temperature slow speed large reduction, medium temperature rapid recrystallization zone rolling and low temperature rolling is adopted for rolling, and ACC controlled cooling process is adopted for subsequent process, relying on fine grain strengthening, dislocation strengthening, solid solution strengthening and second phase strengthening to ensure the strength of the steel plate; relying on grain refinement to ensure the good low temperature toughness of the steel plate; relying on the oxides formed by Cr, Ni, Co and other elements to ensure that the steel plate has good corrosion resistance; improving the fatigue resistance of the steel plate by fine and dispersed two-phase particles control and element segregation control; by adding a small amount of Sn, Sb and P elements, reducing the segregation of P element, and improving the corrosion resistance of the steel plate.

[0039] 1. The application proposes to add Co, Ni and Cr elements in combination, so that the steel plate has good comprehensive mechanical properties, i.e. yield strength of 485-525 MPa, elongation of ≥25%, and impact energy at -80℃ of ≥200 J;

[0040] 2. The application proposes to add a small amount of Sn, Sb and P elements in combination, so as to improve the corrosion resistance of the steel plate, and the corrosion resistance rate of the steel plate to marine atmosphere is less than 0.080 mm / a;

[0041] 3. The steel plate has good fatigue resistance, and the fatigue life under peak stress of 240 MPa is more than 2 million times;

[0042] 4. The steel plate with a maximum thickness of 150 mm and Z-direction performance of ≥45% can be produced. DETAILED DESCRIPTION

[0043] The application will be described in detail through the following examples.

[0044] According to the above chemical composition and production process, the actual smelting composition of the application is shown in Table 1, the actual process parameters of the application are shown in Tables 2-6, and the actual performance is shown in Table 7.

[0045] Table 1 Smelting composition, Wt%

[0046]

[0047]

[0048] Table 2 Steelmaking process parameters

[0049] No. Hppm Oppm RH treatment time / min Net cycle time / min 1 0.5 0.5 2 0.5 0.5 2 0.5 1 0.85 15 32 9 2 0.90 12 30 6 3 0.80 18 34 10 4 0.80 13 35 8 5 0.88 15 35 8 6 0.95 12 30 8 7 0.76 14 32 9 8 0.82 15 35 9 9 0.80 16 35 8 10 0.90 18 30 8

[0050] Table 3 Continuous casting process parameters

[0051]

[0052] Table 4 Heating process parameters

[0053]

[0054]

[0055] Table 5 One-stage rolling process parameters

[0056]

[0057] Table 6 Two-stage and three-stage rolling and post-rolling cooling process parameters

[0058]

[0059] Table 7 Actual performance

[0060]

[0061]

[0062] As can be seen from Table 7, the yield strength of the example steels of the present application is 486-520 MPa, all of which reach the design strength of 485 MPa or more, the elongation is greater than or equal to 25%, the Z-direction performance is greater than 50%, and the impact energy at -80°C is greater than 200 J, indicating that the strength and toughness of the example steels meet the design requirements and have a certain margin. The fatigue performance test was carried out using an Instron 8802 fatigue testing machine under normal environment, the loading mode was tension-compression fatigue, the stress ratio Rs was -1, the test frequency was 20 Hz, and the fatigue life number under a peak stress of 240 MPa was greater than 2 million times. According to the test method specified in GBT 19746-2005, the inventive steel was subjected to 168-hour periodic immersion rapid corrosion evaluation test, the corrosion resistance of the inventive steel was evaluated by weight loss method, and the annual average corrosion rate was converted.

[0063] It is indicated herein that the above examples are only for illustrating the technical concept and characteristics of the present application, and are not a limitation of the present application. Any equivalent replacement, modification or improvement without departing from the essence of the present application falls within the protection scope of the present application.

Claims

1. A high-strength ultra-heavy marine engineering steel, characterized in that, The steel has the following chemical composition in weight percent: C: 0.02%~0.07%, Si: 0.10%~0.25%, Mn: 1.75%~2.5%, P: 0.022%~0.026%, S: ≤0.008%, Nb: 0.035%~0.040%, V: 0.035%~0.045%, Ti: 0.02%~0.03%, Cr: 0.45%~0.55%, Ni: 0.45%~0.55%, Co: 0.07%~0.10%, Als: 0.035%~0.045%, N: 0.010%~0.012%, Cu: 0.25%~0.33%, Sn: 0.03%~0.05%, Sb: 0.03%~0.05%, wherein 0.06≥(Nb+V+Ti) / Mn≥0.04, 7≥Si / Al≥2.5, (Cu+P+Co) / Cr≥0.70, 8≥Cr / Co≥4, Ni / Cr≥0.8, 36Nb / Mn≥0.60, (Nb+V+Ti+Als) / N≥11, (Nb+V+Ti) / (Sn+Sb)≥0.9, Ni / Cu≥1.4, and the rest is Fe and inevitable impurities; the production method comprises smelting, slab continuous casting, casting blank heating, rolling and cooling, and the casting blank is rolled into finished steel plate in three stages, the casting blank is directly rolled after descaling after being discharged, the rolling speed is 0.80~1.20 m / s, the reduction of the first three passes is >45 mm, the blank is cooled by rolling mill cooling water between each pass, the cooling time is 5~7 s, the finish rolling temperature is 1000~1050 ℃, the thickness of the warm blank is 1.5~2.5 times the thickness of the finished product, the warm blank is cooled by water spraying, the cooling speed is 7.0~9.0 ℃ / s, and the temperature is cooled to 10~25 ℃ above the second stage rolling temperature; the second stage rolling temperature is 910~940 ℃, the rolling speed is 1.5~2.0 m / s, the finish rolling temperature is 825~855 ℃, and the thickness of the warm blank is 1.5~2.0 times the thickness of the finished product; the third stage rolling temperature is 770~820 ℃, and the finish rolling temperature is 720~740 ℃; the steel plate is cooled by a rapid cooling process after rolling, the cooling speed is 6.0~14.0 ℃ / s, and the red temperature of the steel plate is 540~610 ℃. The steel has a yield strength of 485~525 MPa, an elongation of ≥25%, and an impact energy of ≥200 J at -80 ℃.

2. The high-strength super-thick steel for ocean engineering according to claim 1, characterized in that, The steel has a corrosion rate of <0.080 mm / a in marine atmosphere.

3. The high-strength super-thick steel for ocean engineering according to claim 1, characterized in that, The fatigue life under a peak stress of 240 MPa is >200 million times.

4. The high-strength super-thick steel for ocean engineering according to claim 1, characterized in that, The thickness of the steel plate is 60~150 mm, and the Z-direction performance is ≥45%.

5. The high-strength super-thick steel for ocean engineering according to claim 1, characterized in that, ​ 6. A method for producing the high-strength ultra-heavy marine engineering steel according to any one of claims 1 to 5, comprising smelting, slab continuous casting, casting blank heating, rolling and cooling, characterized in that: The slab is cooled rapidly in the continuous casting process, the open cooling temperature is 960-990 ℃, the cooling speed is 8.5-9.5 ℃ / s, and the slab is cooled to 685-725 ℃ and then enters the slow cooling pit for slow cooling, and then cooled to below 150 ℃ at a cooling speed of 4.0-22.0 ℃ / h; the continuous casting slab is sent into a heating furnace for heating, the heating adopts a staged heating process, the slab is put into the furnace at a furnace temperature of 660-760 ℃, and is kept for 1.5-2.5 h; the heating speed is controlled at 8-10 ℃ / min below 950 ℃, and the slab is kept for 20-35 min after being heated to 950 ℃; the heating speed is controlled at 12-15 ℃ / min above 950 ℃, the slab is kept for 3.5-4.5 h after being heated to 1190-1210 ℃, and the slab is rolled into finished steel plate through three stages, the slab is directly rolled after being descaled after being discharged from the furnace, the rolling speed is 0.80-1.20 m / s, the reduction per pass is >45 mm in the first three passes, the slab is cooled by rolling mill cooling water between passes, the cooling time is 5-7 s, the final rolling temperature is 1000-1050 ℃, and the thickness of the slab after being kept is 1.5-2.5 times the thickness of the finished product, the slab after being kept is cooled by water spraying, the cooling speed is 7.0-9.0 ℃ / s, and the slab is cooled to 10-25 ℃ above the second stage open rolling temperature; the second stage open rolling temperature is 910-940 ℃, the rolling speed is 1.5-2.0 m / s, the final rolling temperature is 825-855 ℃, and the thickness of the slab after being kept is 1.5-2.0 times the thickness of the finished product; the third stage open rolling temperature is 770-820 ℃, the final rolling temperature is 720-740 ℃, and the steel plate is rapidly cooled after rolling at a cooling speed of 6.0-14.0 ℃ / s, and the steel plate is red temperature of 540-610 ℃.

7. The method of producing a high-strength super-thick steel for ocean engineering according to claim 6, characterized in that, In the smelting process, the molten iron is pretreated by desulfurization, the S content in the molten iron after desulfurization is ≤0.0025%, a top and bottom combined blowing process is adopted, the converter tapping temperature is 1630-1650 ℃, and then Si-Ca wire feeding treatment is carried out, and the Ca content is controlled at 0.0015%-0.0025%; the molten steel is refined, the RH treatment time is 30-35 min, nitrogen is blown throughout the RH treatment, and the net circulation time before moving out is 6-10 min.

8. The method of producing a high-strength super-thick steel for ocean engineering according to claim 7, characterized in that, Sn and Sb elements are added before the end of the RH treatment, and the addition amount is 1.2-1.3 times the target control amount.

9. The method of producing a high-strength super-thick steel for ocean engineering according to claim 6, characterized in that, In the slab continuous casting process, the tundish superheat is 20-30 ℃, the whole process is protected casting, the continuous casting slab drawing speed is controlled at 1.1-1.3 m / min, the secondary cooling water ratio is 0.80-0.90 m3 / t, and heavy reduction is adopted in the continuous casting process, and the reduction amount is 15.0-20.0 mm.

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