A GPa-grade ultra-high-strength fatigue-resistant marine engineering steel and its production method

Abstract

This invention discloses a GPa-grade ultra-high strength fatigue-resistant marine engineering steel and its production method. The chemical composition (wt%) is: C: 0.02–0.07, Si: 0.10–0.25, Mn: 1.75–2.50, P: 0.022–0.026, S: ≤0.008, Nb: 0.03–0.04, V: 0.09–0.15, Ti: 0.045–0. .055, Mo: 0.50~0.80, Cr: 2.00~2.40, Ni: 8.0~10.0, Co: 8.0~12.0, Als: 0.035~0.045, N: 0.013~0.014, Cu: 0.58~0.62, Sb: 0.03~0.05, RE: 0.03~0.05, B: 0.0008~0.001. The process includes smelting, continuous casting, rapid cooling, billet heating, rolling, and heat treatment. The maximum thickness of the steel plate is 100mm, Rp0.2 is 1080~1120MPa, A≥18%, fatigue life at peak stress of 560MPa >2 million cycles, impact energy at -60℃ ≥150J, and marine atmospheric corrosion resistance rate <0.10mm / a. This invention not only improves the strength of steel and enhances the impact toughness of steel plates at low temperatures, but also improves the corrosion resistance and fatigue resistance of steel.

Description

Technical Field

[0001] This invention belongs to the field of metal material preparation technology, and specifically relates to a GPa-grade ultra-high strength fatigue-resistant marine engineering steel and its production method. Background Technology

[0002] Steel used in marine equipment manufacturing requires higher corrosion resistance and fatigue resistance when facing complex marine environments such as waves, temperature, humidity, and salinity. Existing steels often struggle to simultaneously meet these performance requirements, necessitating a high-strength steel plate with excellent low-temperature impact toughness, corrosion resistance, and fatigue resistance.

[0003] The patent "A 1GPa Grade Marine Engineering Steel Plate with Excellent Low-Temperature Toughness and Its Preparation Method", application number 202011576945.X, discloses a 1GPa grade marine engineering steel plate with excellent low-temperature toughness and its preparation method. The chemical composition of the plate is combined with controlled rolling and multi-step heat treatment processes to achieve the production of steel plates with high strength, low yield strength ratio and excellent low-temperature toughness. The resulting steel plate has high yield, stable strength and low-temperature toughness, and features ultra-high strength (yield strength ≥ 1GPa), low yield strength ratio (< 0.94), and excellent low-temperature toughness (impact energy ≥ 150J at -80℃). However, it can only produce thicknesses of 10-50mm and does not address corrosion resistance and fatigue resistance.

[0004] The patent "960MPa grade quenched and tempered steel plate with excellent toughness and plasticity and its manufacturing method", patent number ZL201010227961.8, discloses a steel plate with relatively high comprehensive mechanical properties, tensile strength ≥980MPa, yield strength ≥890MPa, and Charpy transverse impact energy (single value) ≥47J at -60℃, but its strength is relatively low and does not involve corrosion performance and fatigue performance evaluation.

[0005] The patent "An FH690 grade marine engineering steel with excellent low-temperature toughness and its manufacturing method", application number 202110788240.2, discloses an FH690 grade marine engineering steel with a maximum thickness of 50mm and excellent low-temperature toughness and its manufacturing method. The steel plate has a yield strength ≥690MPa, tensile strength 770~940MPa, elongation after fracture ≥14%, and low-temperature toughness -60℃ impact energy ≥100J. However, its maximum thickness is only 50mm, and the corrosion resistance problem has not been solved, which cannot meet the needs of marine engineering construction.

[0006] The patent "A corrosion-resistant steel plate for use in the South China Sea marine environment and its production process", application number: CN201410036368.3, provides a corrosion-resistant steel plate for use in the South China Sea marine environment and its production process. The production process includes a converter smelting process, an LF refining process, a vacuum degassing process, a continuous casting process, and a controlled rolling and controlled cooling process. Theoretically, the microstructure of this steel plate is a single-phase polygonal ferrite fine microstructure (average grain size 10.17μm). In actual industrial production, it inevitably contains a very small amount of pearlite microstructure. Compared with conventional ship hull structural steel EH36, its corrosion resistance to marine environments (marine atmosphere, tidal range, full immersion, etc.) is improved by more than 50%, and it has good strength and toughness matching and weldability. However, its strength is low, its low-temperature toughness is insufficient, and its fatigue resistance has not been evaluated.

[0007] The patent "A High Fatigue Structural Steel with a Yield Strength of 345MPa and its Manufacturing Method", application number: 201910712227.1, discloses a high fatigue structural steel with a yield strength of 345MPa. Its chemical composition 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%, with the balance being iron and unavoidable impurities. The steel plate obtained by using a high-pressure reduction + controlled cooling process has good comprehensive mechanical properties and good surface quality. However, the steel plate has low strength, and its corrosion performance is not evaluated. Only the impact toughness at -20℃ is evaluated, which is far from meeting the requirements for use in different marine environments.

[0008] The patent "An Engineering Corrosion-Resistant Fatigue Steel and Its Preparation Method", application number: 202110068169.0, discloses an engineering corrosion-resistant fatigue steel. This steel, 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%), undergoes elemental regulation and the addition of characteristic elements. Specifically, Cu: 0.28%–0.66%, Ni: 0.76%–1.55%, Sb: 0.03%–0.12%, with the remainder being Fe and unavoidable impurities. Its corrosion fatigue strength can be increased by up to 52%, but the low-temperature toughness of the steel is not evaluated. Summary of the Invention

[0009] To address the shortcomings of existing technologies, the present invention aims to propose a GPa-grade ultra-high strength fatigue-resistant marine engineering steel and its production method. This steel is characterized by low carbon and low alloying, and incorporates multi-element alloying strengthening elements such as Mo, Cr, Ni, and Co, as well as precipitation strengthening elements such as V and N. The addition of P, Al, Sb, and RE further enhances the steel plate's strength and low-temperature toughness, while also exhibiting good corrosion resistance and fatigue resistance. This invention solves the problems of low strength, poor low-temperature impact toughness, and insufficient corrosion resistance and fatigue resistance in existing marine engineering steel plates.

[0010] This invention is based on a steel plate with excellent low-temperature toughness, high strength, corrosion resistance, and fatigue resistance. The chemical composition of the steel plate in this invention is as follows: C: 0.02%–0.07%, Si: 0.10%–0.25%, Mn: 1.75%–2.50%, P: 0.022%–0.026%, S: ≤0.008%, Nb: 0.03%–0.04%, V: 0.09%–0.15%, Ti: 0.045%–0.055%, Mo: 0.50%–0.80%, Cr: 2.00%–2.40%, Ni: 8.0%–10.0%, Co: 8.0%–12.0%, Als: 0.035%–0.045%, N: 0. 0.013%~0.014%, Cu: 0.58%~0.62%, Sb: 0.03%~0.05%, RE: 0.03%~0.05%, B: 0.0008%~0.001%, of which 0.08≤(Nb+V+Ti) / Mn≤0.12, 2.5≤Si / Al≤7.0, 3.5≤(Cu+Co) / Cr≤5.5, 12Cr / Co≥2.2, 3.2≤Ni / Cr≤4.5, 36Nb / Mn≥0.5, (Nb+V+Ti+Als) / N≥15, Ni / Cu≥13, (Nb+V+Ti) / RE≥4.2, RE / P≥1.10, and the remainder is Fe and unavoidable impurities.

[0011] The above-mentioned alloying elements and their contents were selected in this invention because of their respective roles in improving the strength, toughness, corrosion resistance, and fatigue resistance of marine engineering steel.

[0012] C: C and Cr can form alloy cementite (Fe·Cr)3C, and can also form carbides, such as Cr7C3, Cr 23C6 and other carbides have higher melting points, hardness, wear resistance, and stability than Fe3C, thus improving steel strength. C, along with strong carbide-forming elements such as V, Nb, and Ti, preferentially forms VC, NbC, and TiC carbides, which have the highest stability, melting point, hardness, and wear resistance. C is the most effective element for increasing steel plate strength; however, its content below 0.02% significantly reduces the steel plate's strength. But C greatly affects the low-temperature toughness, elongation, and weldability of steel. Therefore, from the perspective of improving steel toughness, corrosion resistance, fatigue resistance, and weldability, the C content in steel should be controlled to a moderately low level. Thus, the C content in this invention is selected to be between 0.02% and 0.07%.

[0013] Si: Si is an essential element for deoxidation in steelmaking and has a certain solid solution strengthening effect. Although Si can improve the strength of steel plates, it reduces the critical cooling rate of martensitic transformation, severely impairing the low-temperature toughness, elongation, and weldability of ultra-high strength steel plates. Si not only promotes the formation of martensitic islands, but also results in larger and unevenly distributed martensitic islands, severely damaging the toughness of the weld heat-affected zone (HAZ). Therefore, the Si content in steel should be controlled as low as possible. A certain Si content can effectively improve the steel's resistance to marine corrosion, and the combined addition of Si and Al can improve corrosion resistance and high-temperature oxidation resistance. In this invention, the Si content is controlled at 0.10%–0.25%, with 2.5 ≤ Si / Al ≤ 7.0.

[0014] Mn: Mn is a key element for improving strength and toughness, significantly enhancing steel hardenability, and is very inexpensive, making it a major additive element in steel. When the carbon content is low, a higher Mn content can effectively improve the hardenability of steel, increasing the strength of the steel plate through microstructure refinement and promoting bainite transformation, while also exhibiting excellent low-temperature toughness. Mn expands the austenite region and promotes grain enlargement; therefore, it is necessary to add grain-refining elements such as Nb, V, and Ti to further refine the grains and improve the fatigue performance of the steel. However, Mn is prone to segregation during the solidification process of molten steel, exacerbating segregation and porosity in the center of the billet, leading to low low-temperature toughness of ultra-high-strength steel plates and cracks in welded joints. This needs to be improved by optimizing the continuous casting and heating processes. In this invention, the Mn content is selected to be between 1.75% and 2.50%, with 0.08 ≤ (Nb + V + Ti) / Mn ≤ 0.12.

[0015] P (Polyphosphate) has a strong solid solution strengthening effect in steel. When added to low-alloy structural steel as an alloying element, it can improve its strength and atmospheric corrosion resistance. A P content ≥0.02% can significantly improve corrosion resistance. However, excessive P content can adversely affect the low-temperature toughness of the base metal and the toughness of the weld heat-affected zone. Therefore, its content should be controlled within a reasonable range as much as possible. In this invention, the P content is controlled at 0.022%–0.026%.

[0016] S: S segregation in steel is severe, deteriorating the steel's quality. S is an inclusion-forming element, forming inclusions such as FeS and MnS, which reduces the ductility of the steel. Furthermore, the vicinity of these inclusions becomes a corrosion initiation site, negatively impacting the corrosion resistance of the steel plate. FeS, due to its low melting point, easily melts at grain boundaries, weakening the intergranular bonding force and leading to hot brittleness in the steel. Therefore, a certain amount of Mn should be added, which can form MnS with a higher melting point and deformability. Thus, the S content in this invention should be ≤0.008%.

[0017] Co (Co) primarily functions as a solid solution strengthener in steel, improving its oxidation resistance and corrosion resistance. Co increases the interaction between Fe atoms, lowers the critical concentration for Cr atom cluster formation, and thus enhances the stability of Cr atom clusters. When Co and Cr atoms act simultaneously in steel, a smooth passivation film forms on its surface, exhibiting high structural stability and effectively protecting the matrix, resulting in excellent corrosion resistance. Co promotes the nucleation and hardening of more (Ti, Al)Ni3 precipitates, improving strength and toughness. The addition of Co has little effect on the Ni / Ti precipitation ratio, but it can reduce the size of the precipitated phases. Co increases the nucleation rate of Ni3Ti and the number density of precipitated phases, meaning Co can make the distribution of precipitated phases more dispersed, improving the fatigue resistance of steel and enhancing the precipitation strengthening effect. In this invention, the Co content is controlled at 8%–12%, with 12Cr / Co ≥ 2.2.

[0018] Mo: In quenched and tempered steel, Mo enables deeper and more thorough quenching of parts with larger cross-sections, improves the steel's resistance to tempering or tempering stability, allows parts to be tempered at higher temperatures, thereby more effectively eliminating (or reducing) residual stress and improving plasticity. Mo can improve resistance to organic acids (such as formic acid, acetic acid, oxalic acid, etc.) as well as hydrogen peroxide, sulfuric acid, sulfurous acid, sulfates, acidic dyes, bleaching powder solutions, etc. The addition of Mo prevents pitting corrosion caused by the presence of chloride ions and can improve the resistance of Cr and Ni steels to intergranular corrosion. However, excessive Mo will cause the steel to become embrittled, reduce strength and plasticity, and deteriorate toughness. Therefore, the Mo content in this invention is controlled at 0.50% to 0.80%.

[0019] Cr: Cr can improve the strength and hardness of steel. Cr is a ferrite-forming element, which helps increase the ferrite content in steel, thereby improving its low-temperature toughness. Cr is an element that improves the corrosion resistance of steel; however, adding Cr alone can sometimes reduce corrosion resistance, even making it worse than ordinary carbon steel. It needs to be used in combination with other corrosion-resistant alloying elements, such as Cu, P, and Co, to significantly improve corrosion resistance. In this invention, the Cr content is controlled at 2.0%–2.4%, and 3.5 ≤ (Cu+Co) / Cr ≤ 5.5.

[0020] Ni: Ni strengthens ferrite and refines pearlite in steel, with the overall effect of increasing strength, but its effect on plasticity is not significant. Ni can improve the fatigue resistance of steel and reduce its sensitivity to notches; therefore, Ni can improve the fatigue performance of steel. Ni does not form carbides but can strengthen ferrite by forming simple substitution solid solutions. It also lowers the ductile-brittle transition temperature of steel and improves its low-temperature toughness. A certain Ni content can ensure that the steel plate has sufficient hardenability, uniform properties in the thickness direction, and a good balance of strength and toughness, as well as low-temperature toughness. Adding Ni to steel can also reduce copper embrittlement in Cu-containing steel, reduce intergranular cracking during hot rolling, and improve the atmospheric corrosion resistance of the steel plate. The combination of Ni and Cr can significantly improve the corrosion resistance of steel. In this invention, the Ni content is controlled at 8.0%–10.0%, 3.2 ≤ Ni / Cr ≤ 4.5, and Ni / Cu ≥ 13.

[0021] Cu: Cu is the most important and widely used alloying element in corrosion-resistant steel. Cu can activate the cathode, promote anodic passivation, and slow down corrosion. During corrosion, a copper-rich phase forms on the steel surface. Between the corrosion layer and the copper-rich layer, there is a dense and strongly adherent intermediate layer, which further alleviates corrosion. In particular, when used in combination with phosphorus (P), it can significantly improve the resistance to marine atmospheric corrosion and seawater corrosion. The combined addition of Cu and Ni not only reduces copper embrittlement in copper-containing steel and mitigates intergranular cracking during hot rolling, but more importantly, both Cu and Ni are austenite stabilizing elements. The combined addition of Cu and Ni can significantly reduce Ar3, increase the driving force for the austenite-to-ferrite phase transformation, and accelerate the high-temperature strain-induced precipitation of niobium carbonitride and raise the recrystallization stopping temperature. This is beneficial for controlled rolling in the non-recrystallization zone, refining phase transformation products and improving the fatigue resistance of the material. In this invention, the Cu content is controlled at 0.58%–0.62%.

[0022] Nb: Nb is an important element in controlled-rolled and controlled-cooled steel. As a strong carbide-forming element, Nb forms NbC and NbN two-phase particles with C and N, which are crucial elements in controlled-rolled and controlled-cooled steel. This effectively refines the grain structure, thereby simultaneously improving strength and low-temperature impact toughness. The combined addition of Nb and Mn effectively inhibits austenite recovery and recrystallization during rolling. On one hand, it increases the austenite recrystallization temperature, thus increasing the rolling temperature and reducing the load on the rolling mill; on the other hand, it effectively refines the phase transformation structure of the steel plate, thereby simultaneously improving strength and low-temperature impact toughness and preventing intergranular corrosion by oxidizing media. Nb can increase the solid solution content of rare earth elements in steel, thereby improving the steel's corrosion resistance. In this invention, the Nb content is controlled at 0.03%–0.04%, and 36Nb / Mn ≥ 0.5.

[0023] Vanadium (VC) has a strong affinity for both oxygen (O) and nitrogen (N), making it a strong carbide-forming element. VC generally has high dispersion and is extremely stable, thus facilitating deoxidation and degassing to achieve a dense, fine-grained structure, improving the plasticity, toughness, and strength of steel. Its impact performance and fatigue strength are higher than vanadium-free steel, exhibiting high strength and toughness at both high and low temperatures (<0°C). The high dispersion of VC prevents coarse grain growth in the weld, improving the weldability of the steel. However, heating to the VC melting temperature causes strong grain growth in the steel. When dissolved in a solid solution at high temperatures, it increases hardenability; conversely, if present in carbide form, it reduces hardenability. VC increases the tempering stability of quenched steel and produces a secondary hardening effect. VC can increase the solid solution content of rare earth elements in steel, thereby improving the corrosion resistance of the steel. In this invention, the VC content is controlled at 0.09%–0.15%.

[0024] Ti: Ti has a strong affinity for N, O, and C, and its affinity for S is stronger than that for Fe. Therefore, it is an excellent deoxidizer and degassing agent, as well as an effective element for fixing N and C. Ti is a strong carbonitride forming element. Trace amounts of Ti can combine with N in steel to form TiN, preventing the growth of austenite grains during homogenization. It can also prevent the growth of austenite grains in the weld heat-affected zone, thereby improving weldability. TiC and TiN have strong, stable bonding and are not easily decomposed. In steel, they can only slowly dissolve into the solid solution when heated to above 1000℃, which can significantly control the growth of grains in the weld heat-affected zone and improve the weldability of the material. Therefore, Ti fixes N and S and forms TiN, which can significantly improve the plasticity and impact toughness of steel. Ti can increase the solid solution content of rare earth elements in steel, thereby improving the corrosion resistance of the steel. However, Ti has a strong affinity for N and O, and easily forms TiN and TiO2. At lower temperatures, steel ingots will form more non-metallic inclusions and subcutaneous pores and other defects. Therefore, the O content of titanium-containing steel must be controlled. In this invention, the Ti content is controlled at 0.045% to 0.055%.

[0025] Nitrogen (N): Like carbon (C), nitrogen (N) can dissolve in fe to form interstitial solid solutions. N expands the austenite phase region of steel and is a strong austenite-forming element, capable of replacing a portion of nitrogen (Ni) within certain limits. N penetrating the steel surface can combine with elements such as Nb, Al, V, and Ti to form highly stable nitrides, thereby improving the corrosion resistance of the steel. However, excessive residual N content in the steel can lead to a loose macrostructure or the formation of pores. Therefore, a certain amount of Al needs to be added to nitrogen-containing steel to form stable AlN, preventing nitrogen from escaping and forming pores during solidification. Thus, in this invention, the N content is controlled at 0.013%–0.014%, and (Nb+V+Ti+Als) / N ≥ 15.

[0026] Al: Al is mainly used for deoxidation and grain refinement. Al reacts with N or O to form effective fine dispersions, inhibiting grain growth during steel heating. During cooling, it promotes austenite decomposition, thus improving hardenability. It also acts as a nucleation point for recrystallization, promoting ferrite nucleation and refining grains, thereby improving fatigue resistance. AlN itself exhibits high stability during heating, thus improving the thermal stability of steel, reducing overheating tendency, and enhancing oxidation resistance. Al forms an effective surface hardening layer through lower-temperature diffusion (nitriding) of N, improving oxidation and corrosion resistance. Adding a certain amount of Si during Al deoxidation can significantly improve its deoxidizing properties; however, excessive Al content can lead to abnormal microstructures and graphitization tendency. Therefore, the Al content in this invention is controlled at 0.035%–0.045%.

[0027] Sb: At austenitic temperatures, 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, refining the steel's microstructure and improving toughness, thereby enhancing the steel's fatigue resistance and corrosion resistance. Sb helps improve the corrosion resistance of materials; both single and compound additions of Sb to steel significantly improve its corrosion resistance. Sb forms a corrosion-resistant Sb₂O₅ oxide film on the steel surface, effectively preventing the interaction between the matrix and the corrosive medium, and inhibiting the corrosion of the steel in the corrosive medium. Sb obtained through hydration... 3+ The precipitates formed in the anodic micro-regions fill corrosion cracks or cavities, enhancing the resistance to corrosion. - Sb can also penetrate through Cl. - Hydration under environmental media inhibits Fe 3+ Hydrolysis produces H + The process improves the pH value of the corrosion micro-zone and alleviates the anodic dissolution process. Sb, as a corrosion inhibitor, alters the anode and cathode reaction processes, significantly improving the steel's resistance to marine corrosion. In this invention, the Sb content is controlled at 0.03%–0.05%.

[0028] Rare earth elements (RE) are highly reactive and have strong binding properties. Adding RE to steel can improve solidification structure, alter solid-state phase transformation structure, form harmless low-melting-point inclusions, strengthen interfaces through segregation, and passivate surface rust layers. RE can increase the self-corrosion potential and polarization resistance of weathering steel, thereby inhibiting the anodic reaction, increasing the resistance of the entire electrochemical reaction, and significantly reducing the corrosion rate of the steel. RE is enriched at grain boundaries through diffusion mechanisms, inhibiting the segregation of inclusions at grain boundaries, and improving the low-temperature performance and corrosion resistance of steel. Adding RE to phosphorus-containing steel can reduce macroscopic segregation, reducing the segregation of phosphorus at grain boundaries and ferrite interfaces, making the distribution of phosphorus in the steel more reasonable, thereby significantly improving the toughness, corrosion resistance, and fatigue resistance of the steel. However, rare earth elements are scarce resources, and their addition must be controlled. In this invention, the RE content is controlled at 0.03% to 0.05%, (Nb+V+Ti) / RE ≥ 4.2, and RE / P ≥ 1.10.

[0029] B: B has a strengthening effect on grain boundaries, reducing precipitates along grain boundaries and improving grain boundary conditions. Adding B ensures the hardenability of the steel plate without compromising its weldability, HAZ toughness, or slab surface quality, significantly improving the hardenability of the steel. Nb and B have a strong interaction; their combined addition can raise the recrystallization temperature of the steel to over 950℃. Simultaneously, the synergistic effect of Ti and B is crucial for improving the impact toughness of the material. Ti is used to fix N in the steel, allowing B to dissolve in the steel. In this invention, the B content is controlled at 0.0008%–0.001%.

[0030] The above describes the content range and function of various added elements. The manufacturing method of this invention for producing high-performance fatigue-resistant steel plates includes:

[0031] 1. The steel is smelted according to the above composition, and the process includes:

[0032] (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%. The furnace top and bottom composite blowing process is adopted. The converter tapping temperature is 1630~1650℃. Then, Si-Ca wire feeding treatment is carried out, and the Ca content is controlled between 0.0015% and 0.0025%.

[0033] (2) Refine the molten steel. 7-9 minutes before the end of vacuum treatment in the LF furnace, add 1.95-3.25 kg / ton of steel containing 20% ​​rare earth alloy to the LF furnace and then blow argon for 5-10 minutes. Then carry out RH treatment for 30-35 minutes. Nitrogen is blown throughout the RH treatment process. Control the [H] in the steel to be ≤1.0 ppm and [O] to be ≤20 ppm. The net circulation time before removal is 6-10 minutes.

[0034] (3) Add Sb before the RH treatment is finished, ensuring that the amount added is 1.2 to 1.3 times the target control amount, so as to ensure that its endpoint content can be controlled within the target range.

[0035] 2. The molten steel obtained in step 1 is cast into the required continuously cast billets. To control the content of isoaxial crystals in the continuously cast billets, the superheating temperature of the tundish is 20-30℃. Lower superheating reduces the solidification time of the molten steel and decreases the segregation of elements such as C and Mn in the center of the billet, reducing defects such as porosity and shrinkage cavities, thus ensuring the Z-axis properties and fatigue resistance of the steel plate. Full-process protective casting is adopted, and the billet pulling speed is controlled at 0.8-1.2 m / min, with a secondary cooling water ratio of 0.80-1.0 m³ / min. 3 / t, so that the equiaxed crystal ratio of the continuously cast billet is >30.0%, electromagnetic stirring is used at the end of solidification to make the molten steel uniform and achieve high strength and high density solidification as soon as possible, and heavy reduction is used in the continuous casting process, with a reduction of 15.0~20.0mm.

[0036] 3. In order to control the grain size of the continuously cast billet, the billet is rapidly cooled with an initial cooling temperature of 950-1000℃ and a cooling rate of 9.0-12.0℃ / s. After cooling to 720-750℃, it is placed in a slow cooling pit for slow cooling. In order to reduce the internal stress of the billet, the billet is cooled to below 150℃ at a cooling rate of 3.0-10.0℃ / h.

[0037] 4. The continuously cast billet obtained in step 3 is fed into a heating furnace for heating. A segmented heating process is adopted. The billet is fed into the furnace at a temperature of 600–650℃ and held for 2.0–3.0 hours to release internal stress. Below 950℃, a slow heating process is used to further release internal stress caused by cooling and heavy pressure, and to prevent temperature stress caused by excessively rapid heating. The heating rate is controlled at 6–8℃ / min, and the billet is heated to 950℃ and held for 30–40 minutes. Above 950℃, a rapid heating process with appropriately extended holding time is adopted to prevent austenite grain coarsening while allowing alloying elements in the billet to fully diffuse and dissolve, further reducing element segregation. The heating rate is controlled at 12–20℃ / min, and the billet is heated to 1200℃–1220℃ for homogenization, with a holding time of 2.0–4.0 hours.

[0038] 5. The billet is rolled into finished steel plate in two stages. In the first stage, in order to fully break the columnar crystals of the continuously cast billet, a high-temperature slow rolling process with a large reduction is adopted. After removing phosphorus from the furnace, the billet is directly rolled at a rolling speed of 0.80-1.00 m / s. The reduction in each of the first three passes is ≥35 mm. During the rolling process, the billet is cooled with mill cooling water between each pass. The final rolling temperature is 1000-1030℃. The thickness of the billet waiting to be heated is 1.5-2.0 times the thickness of the finished product. In order to suppress the growth of grains in the intermediate billet, the billet waiting to be heated is cooled by water spray at a cooling rate of 8.0-10.0℃ / s, cooling to 15-25℃ above the starting rolling temperature of the second stage. The starting rolling temperature of the second stage is 880-900℃, the rolling speed is 1.2-1.6 m / s, the final rolling temperature is 810-840℃, and then it is air-cooled to room temperature.

[0039] 6. The rolled steel plate undergoes two quenching and tempering heat treatments. The first quenching temperature is 870-900℃, and the holding time is 1.2-1.5 min / mm. The second quenching temperature is 840-860℃, and the holding time is 1.5-2.0 min / mm. The tempering temperature is 540-580℃, and the holding time is 2.5-3.5 min / mm. The effects of the two quenching processes are as follows: (1) The first quenching can only make the surface of the steel reach a certain hardness, while the internal hardness of the steel can also reach the same level through reheating. This makes the steel have better wear resistance and tensile strength, and improves the quality of the product; (2) After the first quenching, a large amount of residual stress will be generated inside the steel, which is prone to deformation or cracks during use. The second quenching can eliminate these residual stresses, improve the structure of the steel, and make it more uniform and dense. This not only improves the service life of the product, but also reduces the adverse effects caused by stress; (3) The second quenching can effectively control the consistency of the internal structure and performance of the steel, and ensure the stability of the product quality; (4) Through the second quenching, finer grains will be formed inside the steel, thereby increasing the grain boundary area of ​​the steel and improving the low-temperature toughness of the steel.

[0040] This invention employs a low-carbon and high-manganese content design, controlling the range of composite alloying elements as follows: 0.08≤(Nb+V+Ti) / Mn≤0.12, 2.5≤Si / Al≤7.0, 3.5≤(Cu+Co) / Cr≤5.5, 12Cr / Co≥2.2, 3.2≤Ni / Cr≤4.5, 36Nb / Mn≥0.5, (Nb+V+Ti+Als) / N≥15, Ni / Cu≥13, (Nb+V+Ti) / RE≥4.2, RE / P≥1.10. This improves the steel's strength while ensuring excellent low-temperature toughness, corrosion resistance, and fatigue resistance. The continuous casting process is optimized to control the impact of elemental segregation on the fatigue and low-temperature performance of the steel plate. A slow, long-duration segmented heating process is used for heating, and a two-stage rolling process involving high-temperature, slow-speed, high-reduction rolling in the uncrystallized zone is employed, followed by two quenching and tempering heat treatments. The strength of the steel plate is ensured by grain refinement, dislocation strengthening, solid solution strengthening, and second-phase strengthening; the low-temperature toughness of the steel plate is ensured by grain refinement; the oxides formed by elements such as Cr, Ni, and Co ensure the good corrosion resistance of the steel plate; the fatigue resistance of the steel plate is improved by controlling the fine and dispersed two-phase particles and element segregation; and the corrosion resistance of the steel plate is further improved by adding a small amount of Sb, RE, and P elements to reduce the segregation of P elements.

[0041] 1. A composite addition of Mo, Co, Ni, and Cr elements is proposed, resulting in steel plates with good comprehensive mechanical properties: yield strength 1080~1120MPa, elongation ≥18%, and impact energy ≥150J at -60℃;

[0042] 2. A composite method is proposed to improve the corrosion resistance of steel plates by adding small amounts of Sb, RE, and P elements, with a marine atmospheric corrosion resistance rate of <0.10 mm / a;

[0043] 3. The steel plate has good fatigue resistance, with a fatigue life of more than 2 million cycles under a peak stress of 560 MPa.

[0044] 4. It can produce a wide range of thicknesses, with a maximum thickness of 100mm. Detailed Implementation

[0045] Based on the above chemical composition and production process, the actual smelting composition of the present invention is shown in Table 1, the actual process parameters of the present invention are shown in Tables 2 to 6, and the actual properties are shown in Table 7.

[0046] Table 1 Smelting composition, Wt%

[0047]

[0048] Table 2 Steelmaking Process Parameters

[0049]

[0050]

[0051] Table 3 Continuous Casting Process Parameters

[0052]

[0053] Table 4 Heating process parameters

[0054]

[0055] Table 5 Rolling process parameters

[0056]

[0057]

[0058] Table 6 Heat Treatment Process

[0059]

[0060] Table 7 Physical Performance

[0061]

[0062] As shown in Table 7, the yield strength of the steel plates with a thickness of 25–100 mm in the embodiments of the present invention ranges from 1082 to 1118 MPa, all exceeding the design strength of 1000 MPa. The elongation ranges from 18.5% to 22.5%, and the impact energy at -60℃ ranges from 152 to 215 J. This indicates that the strength and toughness of the steel in each embodiment not only meet the design requirements but also have a certain margin. Fatigue performance tests were conducted using an Instron 8802 fatigue testing machine under normal conditions. The loading method was tensile-compressive fatigue, the stress ratio Rs = -1, and the test frequency was 20 Hz. The fatigue life at a peak stress of 560 MPa exceeded 2 million cycles. Following the test methods specified in GB / T 19746-2005, a 168-hour cyclic immersion rapid corrosion evaluation test was conducted on the invented steel. The corrosion resistance performance of the invented steel was evaluated using the weight loss method, and the annual average corrosion rate was calculated to be 0.085–0.096 mm / a.

[0063] It is hereby clarified that the listed embodiments are intended to illustrate the technical concept and main features of the present invention, and are not intended to be a limiting description of the invention. Any equivalent substitutions or improvements that do not depart from the essence of the present invention should be considered as included within the scope of protection of the present invention.

Claims

1. A GPa-grade ultra-high strength fatigue-resistant marine engineering steel, characterized in that, The chemical composition of the steel plate is as follows: C: 0.02%–0.07%, Si: 0.10%–0.25%, Mn: 1.75%–2.50%, P: 0.022%–0.026%, S: ≤0.008%, Nb: 0.03%–0.04%, V: 0.09%–0.15%, Ti: 0.045%–0.055%, Mo:

0. 50%~0.80%, Cr: 2.00%~2.40%, Ni: 8.0%~10.0%, Co: 8.0%~12.0%, Als: 0.035%~0. 045%, N: 0.013% ~ 0.014%, Cu: 0.58% ~ 0.62%, Sb: 0.03% ~ 0.05%, RE: 0.03% ~ 0.05%, B: 0.0008%~0.001%, of which 0.08≤(Nb+V+Ti) / Mn≤0.12, 2.5≤Si / Al≤7.0, 3.5≤(Cu+Co) / Cr≤5.5, 12Cr / Co≥2.2, 3.2≤Ni / Cr≤4.5, 36Nb / Mn≥0.5, (Nb+V+Ti+Als) / N≥15, Ni / Cu≥13, (Nb+V+Ti) / RE≥4.2, RE / P≥1.10, with the remainder being Fe and unavoidable impurities; the production method includes smelting, continuous casting of slabs, rapid cooling of slabs, heating of slabs, rolling, double quenching and tempering heat treatment, wherein the slab casting speed is controlled at 0.8~1.2m / min, and the secondary cooling water ratio is 0.80~1.0m. 3 / t, reduction of 15.0~20.0mm; the continuous casting billet rapid cooling start temperature is 950~1000℃, cooling rate is 9.0~12.0℃ / s, after cooling to 720~750℃, it enters the slow cooling pit for slow cooling, and then cools to below 150℃ at a cooling rate of 3.0~10.0℃ / h; the billet heating adopts a segmented heating process, entering the furnace at 600~650℃, holding for 2.0~3.0h, below 950℃ a slow heating process is used, the heating rate is controlled at 6~8℃ / min, heating to 950℃ and holding for 30~40min, above 950℃ a rapid heating process with appropriately extended holding time, heating rate controlled at 12~20℃ / min, heating to 1200~1220℃ for homogenization, holding for 2.0~4.0h; the billet is rolled into finished steel plate in two stages, the first stage adopts a high temperature slow rolling process with large reduction. The rolling speed is 0.80–1.00 m / s, with a reduction of ≥35 mm per pass in the first three passes. Cooling is achieved between each pass using mill cooling water. The final rolling temperature is 1000–1030℃. The thickness of the billet waiting to be heated is 1.5–2.0 times the finished product thickness. The cooling rate is 8.0–10.0℃ / s, cooling to 15–25℃ above the second-stage rolling temperature. The second-stage rolling temperature is 880–900℃. The rolling speed is 1… The rolling speed is 0.2–1.6 m / s, the final rolling temperature is 810–840℃, and then air-cooled to room temperature; after rolling, the steel plate undergoes two quenching and tempering heat treatments. The first quenching temperature is 870–900℃, and the holding time is 1.2–1.5 min / mm. The second quenching temperature is 840–860℃, and the holding time is 1.5–2.0 min / mm. The tempering temperature is 540–580℃, and the holding time is 2.5–3.5 min / mm.

2. The GPa-grade ultra-high strength fatigue-resistant marine engineering steel according to claim 1, characterized in that, The maximum thickness of the finished steel plate can reach 100mm.

3. The GPa-grade ultra-high strength fatigue-resistant marine engineering steel according to claim 1, characterized in that, The steel has a yield strength between 1080 and 1120 MPa, an elongation of ≥18%, a peak stress of 560 MPa, a fatigue life of more than 2 million cycles, and an impact energy of ≥150 J at -60℃.

4. The GPa-grade ultra-high strength fatigue-resistant marine engineering steel according to claim 1, characterized in that, Resistance to marine atmospheric corrosion rate <0.10 mm / a.

5. The GPa-grade ultra-high strength fatigue-resistant marine engineering steel according to claim 1, characterized in that, During the smelting process, 1.95-3.25 kg / ton of steel containing 20% ​​rare earth alloy is added to the LF furnace 7-9 minutes before the vacuum treatment ends, followed by argon blowing for 5-10 minutes; then RH treatment is performed for 30-35 minutes, with nitrogen blowing throughout the RH treatment process, controlling [H] ≤ 1.0 ppm and [O] ≤ 20 ppm in the steel, with a net circulation time of 6-10 minutes before removal. Sb element is added before the RH treatment ends, ensuring that the amount added is 1.2-1.3 times the target control amount.

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