Ultra-high-strength and high-toughness steel for marine engineering and production method thereof

Abstract

This invention discloses an ultra-high strength and high toughness 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: 0.06–0.08, V: 0.08–0.085, Ti: 0.045–0.05. 5. Mo: 0.20~0.35, Cr: 1.60~1.80, Ni: 1.60~1.80, Co: 0.40~0.52, Als: 0.035~0.045, N: 0.015~0.016, Cu: 0.52~0.55, Sn: 0.03~0.05, Sb: 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 is 100mm, Z-axis properties ≥35%, Rp0.2 is 950~985MPa, A ≥20%, fatigue life at peak stress of 470MPa >2 million cycles, impact energy at -60℃ ≥150J, and marine atmospheric corrosion resistance rate <0.09mm / a. This invention improves the strength of steel while solving the problems of poor low-temperature impact toughness, insufficient corrosion resistance and fatigue resistance of steel plates.

Description

Technical Field

[0001] This invention belongs to the field of metal material preparation technology, and specifically relates to an ultra-high strength and high toughness marine engineering steel and its production method. Background Art

[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 980MPa grade hot-rolled ferritic bainitic dual-phase steel and its manufacturing method", application number 201610450203.X, discloses a microstructure of ferrite + bainite, with an average ferrite grain size of 5-10μm and an equivalent bainite grain size of ≤20μm. The steel has a yield strength ≥600MPa, tensile strength ≥980MPa, and elongation ≥15%. It exhibits excellent strength, plasticity, and toughness, while having a low yield strength ratio. It can be applied to parts such as wheels that require good formability and high strength thinning. However, the general specifications for automobile wheels are relatively thin, and the low-temperature toughness is insufficient. It also does not involve corrosion resistance and cannot meet the requirements for use in deep-sea environments.

[0004] 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.

[0005] The patent "A 690MPa Grade High-Strength Steel and Its Manufacturing Method", application number 202110729125.8, discloses a steel plate manufacturing method as follows: slab heating, high-pressure descaling and water cooling, double-stand reversible rolling, rapid cooling, and heat treatment. This produces high-strength steel with a yield strength of 690MPa, a carbon equivalent of no more than 0.43%, a yield strength exceeding 690MPa, and a tensile strength exceeding 800MPa. The product possesses 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] 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

[0007] To address the shortcomings of existing technologies, the present invention aims to propose an ultra-high strength and high toughness 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 V and N precipitation strengthening elements. It also features the addition of P, Al, Sb, and Sn, thereby improving the steel plate's strength and low-temperature toughness. Furthermore, it exhibits good corrosion resistance and fatigue resistance, solving the problems of low strength, poor low-temperature impact toughness, and insufficient corrosion resistance and fatigue resistance in existing marine engineering steel plates.

[0008] 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: 0.06%–0.08%, V: 0.08%–0.085%, Ti: 0.045%–0.055%, Mo: 0.20%–0.35%, Cr: 1.60%–1.80%, Ni: 1.60%–1.80%, Co: 0.40%–0.52%, Als: 0.035%. %~0.045%, N: 0.015%~0.016%, Cu: 0.52%~0.55%, Sn: 0.03%~0.05%, Sb: 0.03%~0.05%, B: 0.0008%~0.001%, of which 0.08≤(Nb+V+Ti) / Mn≤0.12, 2.0≤Si / Al≤7.0, (Cu+P+Co) / Cr≥0.55, 3.2≤Cr / Co≤4.5, Ni / Cr≥0.93, 36Nb / Mn≥0.95, (Nb+V+Ti+Als) / N≥14, Ni / Cu≥3, Ti / B≥45, and the remainder is Fe and unavoidable impurities.

[0009] 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.

[0010] C: C and Cr can form alloy cementite (Fe·Cr)3C, and can also form carbides, such as Cr7C3, Cr 23 C6 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%.

[0011] Si (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 2.0 ≤ Si / Al ≤ 7.0.

[0012] 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.

[0013] 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%.

[0014] 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 manganese should be added to form MnS, which has a higher melting point and deformability. Therefore, the S content in this invention should be ≤0.008%.

[0015] 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 0.20% to 0.35%.

[0016] 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 precipitation, 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 that 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 0.40%–0.52%, with 3.2 ≤ Cr / Co ≤ 4.5.

[0017] 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 1.60%–1.80%, and (Cu+P+Co) / Cr ≥ 0.55.

[0018] 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 but strengthens ferrite by forming simple substitutional solid solutions. It also lowers the ductile-brittle transition temperature and improves the low-temperature toughness of steel. A certain Ni content ensures 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, 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 1.60%–1.80%, Ni / Cr ≥ 0.93, and Ni / Cu ≥ 3.0.

[0019] 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, raising 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 0.52%–0.55%.

[0020] 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 corrosion resistance of the steel. In this invention, the Nb content is controlled at 0.06%–0.08%, and 36Nb / Mn ≥ 0.95.

[0021] 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 resistance 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 0.08%–0.085%.

[0022] 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%.

[0023] 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 0.015%–0.016%, and (Nb+V+Ti+Als) / N ≥ 14.

[0024] 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%.

[0025] Sn and 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 its toughness, thereby enhancing the steel's fatigue resistance and corrosion resistance. Sn and Sb help improve the corrosion resistance of materials. Adding Sn alone or in combination with Sb significantly improves the corrosion resistance of the material. 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 and 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-zone and alleviates the anodic dissolution process. Sn and Sb act as corrosion inhibitors, altering the anode and cathode reaction processes and significantly improving the steel's resistance to marine corrosion. In this invention, the Sn and Sb contents are controlled at 0.03% to 0.05%.

[0026] 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%, and the Ti / B ratio is ≥45.

[0027] 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:

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

[0029] (1) 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, the 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 min. Nitrogen is blown throughout the RH treatment. The [H] content in the steel is controlled to be ≤1.0 ppm and [O] content to be ≤20 ppm. The net circulation time before removal is 6-10 min. Add Sb and Sn elements 10 min before the end of the RH treatment. Ensure that the amount added is 1.2-1.3 times the target control amount to ensure that the final content can be controlled within the target range.

[0031] 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 to 20.0 mm.

[0032] 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.

[0033] 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.

[0034] 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.

[0035] 6. The rolled steel plate undergoes two quenching and tempering heat treatments. The first quenching temperature is 880-910℃, and the holding time is 1.2-1.5 min / mm. The second quenching temperature is 850-870℃, and the holding time is 1.5-2.0 min / mm. The tempering temperature is 550-600℃, 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.

[0036] 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, 2.0≤Si / Al≤7.0, (Cu+P+Co) / Cr≥0.55, 3.2≤Cr / Co≤4.5, Ni / Cr≥0.93, 36Nb / Mn≥0.95, (Nb+V+Ti+Als) / N≥14, Ni / Cu≥3, Ti / B≥45. 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 non-crystallized 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 Sn, Sb, and P elements to reduce the segregation of P elements.

[0037] 1. A composite addition of Co, Ni, and Cr elements is proposed, resulting in steel plates with good comprehensive mechanical properties: yield strength 950–985 MPa, elongation ≥20%, and impact energy ≥150 J at -60℃;

[0038] 2. A composite method of adding small amounts of Sn, Sb and P elements is proposed to improve the corrosion resistance of steel plates, with a marine atmospheric corrosion resistance rate of <0.09mm / a;

[0039] 3. The steel plate has good fatigue resistance, with a fatigue life of more than 2 million cycles under a peak stress of 470 MPa.

[0040] 4. It can produce a wide range of thicknesses, with a maximum thickness of 100mm and a Z-axis performance of ≥35%. Detailed Implementation

[0041] 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.

[0042] Table 1 Smelting composition, Wt%

[0043]

[0044]

[0045] Table 2 Steelmaking Process Parameters

[0046] Serial Number Argon blowing time / min RH processing time / min Net cycle time / min 1 10 30 8 2 5 35 6 3 10 35 8 4 8 30 6 5 8 30 7 6 8 30 8 7 9 34 7 8 9 35 10 9 6 32 10 10 5 34 6

[0047] Table 3 Continuous Casting Process Parameters

[0048]

[0049] Table 4 Heating process parameters

[0050]

[0051]

[0052] Table 5 Rolling process parameters

[0053]

[0054] Table 6 Heat Treatment Process

[0055]

[0056] Table 7 Physical Performance

[0057]

[0058]

[0059] 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 is 953–983 MPa, the elongation is 21%–24.2%, the Z-direction performance is 38%–46%, and the impact energy at -60℃ is 153–310 J, indicating that the strength and toughness of the steel in each embodiment not only meet the design requirements. 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 470 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 0.065–0.088 mm / a.

[0060] 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 type of ultra-high strength and high toughness 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: 0.06%–0.08%, V: 0.08%–0.085%, Ti: 0.045%–0.055%, Mo: 0.20%–0.35%, Cr: 1.60%–1.80%, Ni: 1.60%–1.80%, Co: 0.40%–0.52%, Als: 0.035%–0.045%, N: 0.015%– 0.016%, Cu: 0.52%~0.55%, Sn: 0.03%~0.05%, Sb: 0.03%~0.05%, B: 0.0008%~0.001%, of which 0.08≤(Nb+V+Ti) / Mn≤0.12, 2.0≤Si / Als≤7.0, (Cu+P+Co) / Cr≥0.55, 3.2≤Cr / Co≤4.5, Ni / Cr≥0.93, 36Nb / Mn≥0.95, (Nb+V+Ti+Als) / N≥14, Ni / Cu≥3, Ti / B≥45, with the remainder being Fe and unavoidable impurities.

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

3. The ultra-high strength and high toughness marine engineering steel according to claim 1, characterized in that, The Z-axis performance of the steel plate is ≥35%.

4. The ultra-high strength and high toughness marine engineering steel according to claim 1, characterized in that, The steel has a yield strength between 950 and 985 MPa, an elongation of ≥20%, a peak stress of 470 MPa, a fatigue life of more than 2 million cycles, and an impact energy of ≥150 J at -60℃.

5. The ultra-high strength and high toughness marine engineering steel according to claim 1, characterized in that, Resistance to marine atmospheric corrosion rate <0.09 mm / a.

6. A method for producing ultra-high strength and high toughness marine engineering steel according to any one of claims 1 to 5, comprising smelting, continuous casting of slabs, rapid cooling of slabs, heating of slabs, rolling, double quenching, and tempering heat treatment, characterized in that, The billet pulling speed of continuous casting is controlled at 0.8~1.2 m / min, and the secondary cooling water flow rate is 0.80~1.0 m³ / min. 3 / t, the reduction under heavy pressure is 15.0~20.0mm; the rapid cooling start-up temperature of the continuously cast billet is 950~1000 ℃, the 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 ℃ and holding for 2.0~3.0 h, a slow heating process is used below 950 ℃, the heating rate is controlled at 6~8 ℃ / min, and it is heated to 950 ℃ and held for 30~40 min, and a rapid heating process is used above 950 ℃, the heating rate is controlled at 12~20 ℃ / min, and it is heated to 1200~1220 ℃ for homogenization, and the holding time is 2.0~4.

0. h; The billet is rolled into finished steel plate in two stages. The first stage adopts a high-temperature slow rolling process with a large reduction. The rolling speed is 0.80~1.00 m / s, and the reduction in each of the first three passes is ≥35 mm. During the rolling process, the mill cooling water is used for cooling 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. The cooling rate is 8.0~10.0 ℃ / s. It is cooled to the second stage starting rolling temperature + (15~25 ℃). The second stage starting rolling temperature is 880~900 ℃, the rolling speed is 1.2~1.6 m / s, and the final rolling temperature is 810~840 ℃. Then it is air-cooled to room temperature.

7. The method for producing ultra-high strength and high toughness marine engineering steel according to claim 6, characterized in that, 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%. Then, RH treatment is carried out for 30~35 min. Nitrogen is blown throughout the RH treatment. The net circulation time before unloading is 6~10 min. Before the end of RH treatment, Sb and Sn elements are added to ensure that the amount added is 1.2~1.3 times the target control amount.

8. The production method of ultra-high strength and high toughness marine engineering steel according to claim 6, wherein the rolled steel plate is subjected to two quenching and tempering heat treatments, the first quenching temperature is 880~910 ℃, the holding time is 1.2~1.5 min / mm, the second quenching temperature is 850~870 ℃, the holding time is 1.5~2.0 min / mm, the tempering temperature is 550~600 ℃, and the holding time is 2.5~3.5 min / mm.

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