High-performance fatigue-resistant steel for marine engineering and method for producing same

Abstract

The application discloses a high-performance anti-fatigue 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%-2.50%, P: 0.022%-0.026%, S: ≤0.008%, Nb: 0.065%-0.070%, V: 0.070%-0.075%, Ti: 0.035%-0.042%, Cr: 1.25%-1.45%, Ni: 1.25%-1.45%, Co: 0.30%-0.35%, Als: 0.035%-0.045%, N: 0.0150%-0.0160%, Cu: 0.45%-0.50%, Sb: 0.03%-0.05% and RE: 0.03%-0.05%. After smelting, slab continuous casting, casting blank heating, rolling, cooling and heat treatment, the steel has the following properties: Rp0.2 is 690-750 MPa, A is greater than or equal to 20%, impact energy at-80 DEG C is greater than or equal to 200 J, corrosion resistance is less than 0.090 mm / a, fatigue life under peak stress 350 MPa load is greater than 2 million times, the maximum thickness can reach 150 mm, and Z-direction performance is greater than or equal to 40%. 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] This invention belongs to the field of metal material preparation technology, and specifically relates to a high-performance fatigue-resistant marine engineering steel and its production method. Background Technology

[0002] Steel used in marine equipment manufacturing faces complex marine environments with varying waves, temperatures, humidity, and salinity, placing higher demands on its performance, particularly its corrosion and fatigue resistance. Steels require higher strength, greater thickness, low-temperature impact toughness, good Z-axis properties, and simultaneously good corrosion resistance and fatigue resistance. Existing steels struggle to meet all these performance requirements simultaneously.

[0003] Patent application number 202011611587.1 discloses a 690MPa grade marine engineering steel plate and its manufacturing method. The steel plate has a yield strength Rp0.2≥690MPa (e.g., 710~762MPa), tensile strength Rm≥750MPa (e.g., 760~793MPa), elongation after fracture A≥16% (e.g., more than 19%, 19.0%~23.0%), reduction of area Z≥60% (e.g., more than 68%, 68.0%~79.0%), and impact energy KV2≥150J at -40℃. However, deep sea and polar regions require lower temperature impact toughness, and the corrosion resistance problem has not been solved.

[0004] Patent application number 202110788240.2 discloses an FH690 grade marine engineering steel with 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.

[0005] Patent application number 202110729125.8 discloses a 690MPa grade high-strength steel and its manufacturing method. The method involves slab heating, high-pressure descaling and water cooling, double-stand reversible rolling, rapid cooling, and heat treatment to produce high-strength steel with a yield strength of 690MPa. The product has a carbon equivalent of no more than 0.43%, a yield strength exceeding 690MPa, and a tensile strength exceeding 800MPa. The product exhibits characteristics such as low carbon equivalent, high strength, high toughness, low cost, and low internal stress in the steel plate. However, its low-temperature impact toughness is only -20℃, which does not meet the requirements of deep-sea and extremely cold environments.

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

[0007] Patent application number 201910712227.1 discloses a high-fatigue structural steel with a yield strength of 345MPa and its manufacturing method. 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%, with the balance being iron and unavoidable impurities. By employing a high-pressure reduction + controlled cooling process, the obtained steel plate has good comprehensive mechanical properties and good surface quality. However, the steel plate has low strength, and its corrosion performance was not evaluated; only the impact toughness at -20℃ was evaluated, which is far from meeting the requirements for use in different marine environments.

[0008] Patent application number 202110068169.0 discloses an engineering corrosion-resistant fatigue steel and its preparation method. 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%), element regulation and characteristic element addition are carried out, 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 high-performance fatigue-resistant marine engineering steel and its production method. This steel is characterized by low carbon and low alloying, and incorporates composite additions of Cr, Ni, and Co multi-element alloying strengthening elements and V and N precipitation strengthening elements. The addition of P, Al, Sb, and RE is also a key feature, improving the steel plate's strength and low-temperature toughness while 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] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0011] This invention is based on a steel plate with excellent low-temperature toughness, ultra-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: 0.065%–0.070%, V: 0.070%–0.075%, Ti: 0.03%–0.04%, Cr: 1.25%–1.45%, Ni: 1.25%–1.45%, Co: 0.30%–0.35%, Als: 0.035%–0.04%. 5%, N: 0.0150%~0.0160%, Cu: 0.45%~0.50%, Sb: 0.03%~0.05%, RE: 0.03%~0.05%, of which 0.12≥(Nb+V+Ti) / Mn≥0.07, 6.0≥Si / Al≥2.0, (Cu+P+Co) / Cr≥0.54, 5≥Cr / Co≥3, Ni / Cr≥0.8, 36Nb / Mn≥0.90, (Nb+V+Ti+Als) / N≥13, Ni / Cu≥2.2, (Nb+V+Ti) / RE≥3.5, RE / P≥1.3, the remainder being Fe and unavoidable impurities.

[0012] 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 plates.

[0013] 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. Therefore, C is the most effective element for improving steel plate strength; however, its content below 0.02% will significantly reduce the strength of the steel plate. But C greatly affects the low-temperature toughness, elongation, and weldability of steel. From the perspective of improving the toughness, corrosion resistance, fatigue resistance, and weldability of steel, the C content in steel should be controlled to be appropriately low. Therefore, the C content in this invention is selected to be between 0.02% and 0.07%.

[0014] 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 6.0 ≥ Si / Al ≥ 2.0.

[0015] 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 and enhance the strength of the steel plate by refining the microstructure and promoting bainite transformation, while also exhibiting excellent low-temperature toughness. Mn expands the austenite region and promotes grain enlargement, requiring the combined addition of grain-refining elements 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 process and heating process. In this invention, the Mn content is selected to be between 1.75% and 2.50%, and 0.12 ≥ (Nb + V + Ti) / Mn ≥ 0.07.

[0016] P: P has a strong solid solution strengthening effect in steel. When added as an alloying element to low-alloy structural steel, it can improve its strength and atmospheric corrosion resistance. A content of ≥0.02% can significantly improve corrosion resistance. However, excessive P content can adversely affect the low-temperature toughness of the base material and the toughness of the weld heat-affected zone. Therefore, its content should be controlled within a reasonable range. In this invention, the P content is controlled at 0.022%–0.026%.

[0017] S: S segregates severely in steel, deteriorating its 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 bonding force between grains 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, in this invention, its content is controlled to be less than or equal to 0.008%.

[0018] Co primarily functions as a solid solution strengthener in steel, improving its oxidation 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 precipitation hardening by fostering more nucleation sites for (Ti, Al)Ni3, thereby improving strength and toughness. While the addition of Co has little effect on the Ni / Ti precipitation ratio, it can reduce the size of the precipitated phases. Co increases the nucleation rate of Ni3Ti and simultaneously increases the number density of precipitated phases, meaning Co makes the distribution of precipitated phases more dispersed, improving the fatigue resistance of steel and enhancing the precipitation strengthening effect. Therefore, in this invention, its content is controlled at 0.30%–0.35%, with 5 ≥ Cr / Co ≥ 3.

[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 1.25%–1.45%, and (Cu+P+Co) / Cr ≥ 0.54.

[0020] Ni: Ni strengthens ferrite and refines pearlite in steel, resulting in increased strength overall, with little effect on plasticity. Ni can improve the fatigue resistance of steel and reduce its notch sensitivity, thus improving the fatigue performance of steel. Ni does not form carbides; instead, it strengthens ferrite by forming simple substitutional solid solutions, lowering the ductile-brittle transition temperature and improving the low-temperature toughness of steel. A certain Ni content ensures sufficient hardenability and uniform properties in the thickness direction of the steel plate, while also ensuring a balance between strength and toughness and low-temperature toughness. Adding Ni to steel can also reduce copper embrittlement in Cu-containing steel, mitigate 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 1.25%–1.45%, Ni / Cr ≥ 0.8, and Ni / Cu ≥ 2.2.

[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 surface of the steel. 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 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 addition of Cu and Ni can significantly reduce Ar3, increasing the driving force for the austenite-to-ferrite phase transformation. Simultaneously, Cu can accelerate the high-temperature strain-induced precipitation of niobium carbonitride, raise the recrystallization stopping temperature, and facilitate controlled rolling in the non-recrystallization zone to refine phase transformation products, thereby improving the fatigue resistance of the material. In this invention, its content is controlled at 0.45%–0.50%.

[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 the 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; it also prevents intergranular corrosion of the steel by oxidizing media. Nb can increase the solid solution content of rare earth elements in steel, thereby improving the corrosion resistance of the steel. In this invention, its content is controlled at 0.065%–0.070%, with 36Nb / Mn ≥ 0.90.

[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 plasticity, toughness, and strength. Its impact performance and fatigue strength are higher than vanadium-free steel, exhibiting high strength and good toughness at both high and low temperatures (<0°C). Because the high dispersion of VC prevents coarse grain growth in the weld, it improves the weldability of the steel. However, heating to the VC melting temperature will cause 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 decreases 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, its content is controlled at 0.070%–0.075%.

[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, and 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 and also preventing austenite grain growth in the weld heat-affected zone, thereby improving weldability. TiC and TiN have strong, stable, and non-decomposing bonding forces. In steel, they only slowly dissolve into the solid solution when heated to above 1000℃, thus significantly controlling grain growth in the weld heat-affected zone and improving the weldability of the material. Because Ti fixes N and S and forms TiN, the plasticity and impact toughness of steel can be significantly improved. Ti can increase the solid solution content of rare earth elements in steel, thereby improving the corrosion resistance of steel. However, Ti has a strong affinity for N and O and easily forms TiN and TiO2, resulting in more non-metallic inclusions and subcutaneous porosity defects at lower temperatures. The content of this invention is controlled at 0.035% to 0.042%.

[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 and stabilizing element. Within certain limits, it can replace some nitrogen (Ni). N that penetrates the steel surface can combine with elements such as Nb, Al, V, and Ti to form extremely stable nitrides, improving the corrosion resistance of the steel. However, excessive residual nitrogen in steel can lead to a loose macrostructure or porosity. Therefore, a certain amount of Al needs to be added to nitrogen-containing steel to form stable AlN, preventing N from escaping and forming defects such as porosity during solidification. Therefore, in this invention, the N content is controlled at 0.015%–0.016%, and (Nb+V+Ti+Als) / N ≥ 13.

[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 steel cooling, it promotes austenite decomposition, improving hardenability. It also acts as a nucleation point for recrystallization, promoting ferrite nucleation and refining grains, thus improving fatigue resistance. AlN itself has high stability during heating, thereby improving the thermal stability of steel, reducing overheating tendency, and improving oxidation resistance. Al forms an effective surface hardening layer through the lower-temperature diffusion (nitriding) of N, improving oxidation and corrosion resistance. Adding a certain amount of Si during Al deoxidation can significantly improve the deoxidation effect of Al; however, excessive Al content can lead to abnormal structures and promote graphitization. Therefore, the Al content in this invention is controlled at 0.035%–0.045%.

[0027] 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, refining the steel's microstructure and improving its toughness, thereby enhancing the steel's fatigue resistance and 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 after 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 corrosion resistance of steel in marine environments. In this invention, its content is controlled at 0.03%–0.05%.

[0028] Rare earth elements (REs) are highly reactive and have strong binding properties. Adding REs 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. REs can increase the self-corrosion potential and polarization resistance of weathering steel, thereby inhibiting anodic reactions, increasing the resistance to the entire electrochemical reaction, and significantly reducing the corrosion rate of the steel. REs are 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 REs 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, REs are a scarce resource, and their addition must be controlled. In this invention, the RE content is controlled at 0.03%–0.05%, (Nb+V+Ti) / RE ≥ 3.5, and RE / P ≥ 1.3.

[0029] The above describes the content range and function of various added elements. The manufacturing method of this invention for producing high-performance fatigue-resistant marine engineering steel includes:

[0030] 1. The steel is smelted according to the above composition, and the process includes:

[0031] 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%.

[0032] 2) Refine the molten steel and adjust the content of other alloying elements such as Co, Cr, and Ni to the range of this invention.

[0033] 3) 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 purge with argon for 5-10 minutes; then perform RH treatment for 30-35 minutes. Nitrogen is purged throughout the RH treatment process, and the [H] in the steel is controlled to be ≤1.0 ppm and [O] ≤20 ppm. The net circulation time before removal is 6-10 minutes.

[0034] 4) Add RE and Sb elements before the RH treatment is completed, ensuring that the amount added is 1.2 to 1.3 times the target control amount, so as to ensure that the endpoint content can be controlled within the target range.

[0035] 2. Cast the molten steel obtained in step 1 into the required continuous casting billets. To control the content of isometric crystals in the continuous casting billets, the tundish is superheated to 20-30°C. Lower superheat can reduce the solidification time of the molten steel and reduce the segregation of elements such as carbon and manganese in the center of the billet, thereby reducing defects such as porosity and shrinkage cavities, and 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 1.1-1.3 m / min, with a secondary cooling water ratio of 0.90-1.00 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 to be no more than 500μm, the continuously cast billet is rapidly cooled. The initial cooling temperature is 980~1000℃, the cooling rate is 9.0~11.0℃ / s, and after cooling to 690~730℃, it is put into a slow cooling pit for slow cooling, and then cooled to below 150℃ at a cooling rate of 3.0~20.0℃ / 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 600–700℃ 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 8–10℃ / min, and the billet is heated to 950℃ and held for 25–40 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 10–15℃ / min, and the billet is heated to 1200–1220℃ for homogenization, with a holding time of 2.5–3.5 hours.

[0038] 5. The billet is rolled into finished steel plate in three 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 descaling, the billet is directly rolled at a rolling speed of 0.80-1.10 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 for 5-7 seconds. The final rolling temperature is 1050-1080℃, and the thickness of the billet after cooling is 2.0-2 mm. To suppress grain growth in the intermediate billet, the billet is cooled by water spray at a rate of 6.0–8.0 °C / s to a thickness 5 times that of the finished product. The intermediate billet is cooled to 15–25 °C above the second-stage rolling temperature. The second-stage rolling temperature is 900–920 °C, the rolling speed is 1.2–1.6 m / s, the final rolling temperature is 810–840 °C, and the thickness at the intermediate billet is 1.5–2.0 times that of the finished product. The third-stage rolling temperature is 750–800 °C, and the final rolling temperature is 720–740 °C.

[0039] 6. The rolled steel plate adopts the rapid cooling (ACC) process with a cooling rate of 10.0~15.0℃ / s and a steel plate reddening temperature of 500~550℃, which can maintain fine grains after rolling and prevent grain growth.

[0040] 7. The steel plate is subjected to quenching and tempering heat treatment. The quenching temperature is 900~920℃ and the holding time is 1.0~1.3min / mm; the tempering temperature is 580~600℃ and the holding time is 2.0~2.5min / mm.

[0041] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0042] This invention employs a low-carbon and high-manganese content design, controlling the range of composite alloying elements as follows: 0.12≥(Nb+V+Ti) / Mn≥0.07, 6.0≥Si / Al≥2.0, (Cu+P+Co) / Cr≥0.54, 5≥Cr / Co≥3, Ni / Cr≥0.8, 36Nb / Mn≥0.90, (Nb+V+Ti+Als) / N≥13, Ni / Cu≥2.2, (Nb+V+Ti) / RE≥3.5, RE / P≥1.3; thereby improving 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 impact of elemental segregation on the fatigue and low-temperature performance of steel plates, a segmented heating process with slow speed and long duration is adopted. The rolling process employs a three-stage process: high-temperature slow-speed high-reduction rolling, medium-temperature rapid recrystallization zone rolling, and low-temperature rolling. Combined with subsequent ACC controlled cooling process and quenching + tempering heat treatment, the strength of the steel plate is guaranteed by grain refinement, dislocation strengthening, solid solution strengthening, and second-phase strengthening. The low-temperature toughness of the steel plate is guaranteed by grain refinement. The oxides formed by elements such as Cr, Ni, and Co ensure 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 elemental segregation. The corrosion resistance of the steel plate is improved by adding a small amount of Sb and RE and P elements to reduce the segregation of P element.

[0043] 1. A composite addition of Co, Ni, and Cr elements is proposed, which results in a steel plate with good comprehensive mechanical properties, yield strength of 690-750 MPa, elongation ≥20%, and impact energy at -80℃ greater than or equal to 200 J.

[0044] 2. A composite method is proposed to add small amounts of Sb, RE, and P elements to improve the corrosion resistance of steel plates, with a marine atmospheric corrosion rate of less than 0.090 mm / a;

[0045] 3. The steel plate has good fatigue resistance, with a fatigue life of more than 2 million cycles under a peak stress of 350MPa.

[0046] 4. It can produce a wide range of thicknesses, with a maximum thickness of 150mm, and the Z-axis performance is greater than or equal to 40%. Detailed Implementation

[0047] The present invention will be described in more detail below through examples.

[0048] 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 7, and the actual properties are shown in Table 8.

[0049] Table 1 Smelting composition, Wt%

[0050]

[0051] Table 2 Steelmaking Process Parameters

[0052]

[0053]

[0054] Table 3 Continuous Casting Process Parameters

[0055]

[0056] Table 4 Heating process parameters

[0057]

[0058] Table 5. First-stage rolling process parameters

[0059]

[0060]

[0061] Table 6. Process parameters for the second and third stages of rolling and post-rolling cooling.

[0062]

[0063] Table 7 Heat Treatment Process

[0064]

[0065] Table 8 Physical Performance

[0066]

[0067]

[0068] As shown in Table 8, the yield strength of the steel in the embodiments of the present invention ranges from 694 to 748 MPa, all exceeding the design strength of 690 MPa. The elongation is greater than 20%, the Z-axis performance is greater than 40%, and the impact energy at -80℃ is greater than 200 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 tension-compression fatigue, the stress ratio Rs = -1, and the test frequency was 20 Hz. The fatigue life at a peak stress of 350 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 steel. The corrosion resistance of the steel was evaluated using the weight loss method, and the calculated annual average corrosion rate was not less than 0.09 mm / a.

[0069] It is hereby noted that the above embodiments are only for illustrating the technical concept and features of the present invention, and are not intended to limit the present invention. Any equivalent substitutions or modifications made without departing from the essence of the present invention fall within the protection scope of the present invention.

Claims

1. A high-performance, fatigue-resistant, marine engineering steel, characterized in that, The steel has the following chemical composition in weight percent: C: 0.02% to 0.07%, Si: 0.10% to 0.25%, Mn: 1.75% to 2.50%, P: 0.022% to 0.026%, S: ≤0.008%, Nb: 0.065% to 0.070%, V: 0.070% to 0.075%, Ti: 0.035% to 0.042%, Cr: 1.25% to 1.45%, Ni: 1.25% to 1.45%, Co: 0.30% to 0.35%, Als: 0.035% to 0.045%, N: 0.0150% to 0.0160%, Cu: 0.45% to 0.50%, Sb: 0.03% to 0.05%, RE: 0.03% to 0.05%, wherein 0.11 ≥ (Nb+V+Ti) / Mn ≥ 0.07, 6.0 ≥ Si / Al ≥ 2.0, (Cu+P+Co) / Cr ≥ 0.54, 5 ≥ Cr / Co ≥ 3, Ni / Cr ≥ 0.8, 36Nb / Mn ≥ 0.90, (Nb+V+Ti+Als) / N ≥ 13, Ni / Cu ≥ 2.2, (Nb+V+Ti) / RE ≥ 3.5, RE / P ≥ 1.3, and the rest is Fe and inevitable impurities; the production method comprises smelting, slab continuous casting, casting blank heating, rolling and cooling, heat treatment, three-stage rolling of the casting blank into finished steel plate, first-stage casting blank discharging, descaling and direct rolling, rolling speed 0.80 to 1.10 m / s, each pass reduction of the first three passes > 35 mm, cooling of the blank by rolling mill cooling water between each pass, cooling time 5 to 7 s, finish rolling temperature 1050 to 1080℃, warm blank thickness 2.0 to 2.5 times the finished product thickness, water spray cooling of the warm blank, cooling speed 6.0 to 8.0℃ / s, cooling to 15 to 25℃ above the second-stage rolling start temperature; second-stage rolling start temperature 900 to 920℃, rolling speed 1.2 to 1.6 m / s, finish rolling temperature 810 to 840℃, warm thickness 1.5 to 2.0 times the finished product thickness; third-stage rolling start temperature 750 to 800℃, finish rolling temperature 720 to 740℃; fast cooling process for the rolled steel plate, cooling speed 10.0 to 15.0℃ / s, steel plate re-red temperature 500 to 550℃; quenching + tempering heat treatment of the steel plate, quenching temperature 900 to 920℃, holding time 1.0 to 1.3 min / mm; tempering temperature 580 to 600℃, holding time 2.0 to 2.5 min / mm. The steel has yield strength 690 to 750 MPa, elongation ≥ 20%, and -80℃ impact energy ≥ 200 J.

2. The high-performance, fatigue-resistant, marine engineering steel according to claim 1, characterized in that, The steel has a marine atmospheric corrosion resistance rate < 0.090 mm / a.

3. The high-performance, fatigue-resistant, marine engineering steel according to claim 1, characterized in that, The fatigue life under peak stress 350 MPa load is > 20 million times.

4. The high-performance, fatigue-resistant, marine engineering steel according to claim 1, characterized in that, The maximum thickness of the steel plate can reach 150 mm, and the Z-direction performance is ≥ 40%.

5. The high-performance, fatigue-resistant, marine engineering steel according to claim 1, characterized in that, ​ 6. A method of producing the high-performance fatigue-resistant steel for ocean engineering according to any one of claims 1 to 5, comprising melting, slab continuous casting, casting slab heating, rolling, and cooling, characterized in that, The slab is rapidly cooled in the continuous casting process, the cooling temperature is 980-1000℃, the cooling speed is 9.0-11.0℃ / s, and the slab is cooled to 690-730℃ and then enters the slow cooling pit for slow cooling, and then the cooling speed is 3.0-20.0℃ / h to cool to below 150℃; the slab is heated by the sectional heating process, and enters the furnace when the furnace temperature is 600-700℃, and the holding time is 2.0-3.0h; the heating speed is controlled to be 8-10℃ / min when the temperature is below 950℃, and the holding time is 25-40min when the temperature is 950℃; the heating speed is controlled to be 10-15℃ / min when the temperature is above 950℃, and the slab is uniformly heated to 1200-1220℃, and the holding time is 2.5-3.5h; the slab is rolled into finished steel plate by three-stage rolling, the slab is directly rolled after descaling after discharging, the rolling speed is 0.80-1.10m / s, the reduction per pass is >35mm in the first three passes, the slab is cooled by rolling mill cooling water between passes, the cooling time is 5-7s, the final rolling temperature is 1050-1080℃, the thickness of the slab after waiting is 2.0-2.5 times the thickness of the finished product, the slab after waiting is cooled by water spraying, the cooling speed is 6.0-8.0℃ / s, and the temperature is cooled to 15-25℃ above the second-stage rolling temperature; the second-stage rolling temperature is 900-920℃, the rolling speed is 1.2-1.6m / s, the final rolling temperature is 810-840℃, and the thickness of the slab after waiting is 1.5-2.0 times the thickness of the finished product; the third-stage rolling temperature is 750-800℃, and the final rolling temperature is 720-740℃; the steel plate is rapidly cooled after rolling, the cooling speed is 10.0-15.0℃ / s, and the red temperature of the steel plate is 500-550℃; the steel plate is subjected to quenching+tempering heat treatment, the quenching temperature is 900-920℃, the holding time is 1.0-1.3min / mm, the tempering temperature is 580-600℃, and the holding time is 2.0-2.5min / mm.

7. A method of producing a high-performance fatigue-resistant 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 combined blowing process is used, the tapping temperature of the converter is 1630-1650℃, and then Si-Ca wire feeding treatment is performed, the Ca content is controlled to be 0.0015%-0.0025%, and the free O content should be controlled to be below 5ppm; 20% rare earth alloy is added into the LF furnace at 7-9min before the vacuum treatment of the LF furnace is completed, the amount of the rare earth alloy is 1.95-3.25kg / ton of steel, and then argon blowing is performed for 5-10min; then RH treatment is performed, the RH treatment time is 30-35min, nitrogen is blown during the whole RH treatment, and the net circulation time before removal is 6-10min.

8. A method of producing a high-performance fatigue-resistant steel for ocean engineering according to claim 6, characterized in that, In the continuous casting process, the superheat of the tundish is 20-30℃, the whole process is protected, the casting speed of the continuous casting slab is controlled to be 1.1-1.3m / min, the specific water consumption of the secondary cooling water is 0.90-1.00m3 / t, and heavy reduction is used in the continuous casting process, and the reduction is 15.0-20.0mm.

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