An 890mpa-grade high-toughness marine steel suitable for extremely cold environments and a preparation method thereof

Abstract

The application relates to the technical field of offshore engineering steel, and discloses a 890MPa-grade high-toughness offshore engineering steel suitable for an extremely cold environment and a preparation method thereof. The chemical components of the offshore engineering steel include C, Si, Mn, P, S, Ni, Cr, Mo, V, Nb, Ti, Cu, RE, Als, and the balance of Fe and inevitable impurities. The preparation method comprises the following steps: smelting, continuous casting, casting blank heating, preliminary rolling, heat treatment and warm rolling process. The thickness of the finished offshore engineering steel prepared by the application is 25-45 mm, the yield strength is 890-940 MPa, the tensile strength is 990-1070 MPa, the elongation rate is 17.5-20.2%, the KV2 at-60 DEG C is 320-380 J, and the KV2 at-196 DEG C is 250-315 J. The application solves the technical problems that the high-strength offshore engineering steel has insufficient low-temperature toughness and the strength and toughness are difficult to be simultaneously improved, and is especially suitable for offshore engineering structures under extremely cold conditions.

Description

Technical Field

[0001] This invention relates to the field of marine engineering steel technology, specifically providing an 890MPa grade high-toughness marine engineering steel suitable for extremely cold environments and its preparation method, which is particularly suitable for cold ocean environments such as polar regions. Background Technology

[0002] With the continuous advancement of global offshore oil and gas exploration and development, ocean navigation operations and polar resource development are gradually covering high-latitude sea areas such as the Arctic, making the service environment faced by marine engineering equipment increasingly harsh. Polar icebreakers and marine operation platforms are subjected to coupled conditions of extreme low temperatures, strong storms, repeated sea ice impacts, and freeze-thaw corrosion in the seawater splash zone for extended periods.

[0003] Existing conventional high-strength marine engineering steels typically only meet the requirements for use in low-temperature environments ranging from -40℃ to -60℃. In ultra-low temperature environments of -60℃, traditional marine structural steels are prone to decreased impact toughness, ductile-brittle transition, and insufficient fracture resistance. Long-term service can lead to structural fatigue, weld cracking, and localized failures, making it difficult to meet the long-term structural strength and low-temperature stability requirements of high-end polar equipment. Therefore, existing polar marine structural steels generally require a yield strength of 890MPa or higher, while also possessing high impact toughness at -60℃, good weldability, and fracture resistance. Their conventional design service life typically needs to exceed 30 years.

[0004] However, for marine engineering steel, as the strength level increases, its low-temperature impact toughness often tends to decrease, a phenomenon particularly pronounced under extremely cold service conditions. For example, the Charpy V-notch impact energy of traditional quenched and tempered 690MPa grade and above marine engineering steel at -60℃ is generally below 150J, and the ductile-brittle transition temperature is usually above -80℃, far from meeting the safety requirements for low-temperature brittle fracture prevention in polar engineering. The FH690 steel plate manufactured using three-stage controlled rolling technology in Chinese patent application CN113549827A, entitled "A Marine Engineering Steel with Excellent Low-Temperature Toughness and Its Manufacturing Method Thereof," has an impact energy of only over 100J at -60℃; while the 690MPa grade steel plate prepared in Chinese patent CN109161791A, entitled "Steel with Excellent Low-Temperature Toughness for Ships and Marine Engineering and Its Manufacturing Method Thereof," has an impact energy of over 140J at -60℃. The impact energy level of the aforementioned existing marine steel products at -60℃ is significantly lower than the high and low temperature toughness required for engineering projects in extremely cold regions.

[0005] For next-generation marine engineering steels designed for higher performance in the future, impact toughness at -196℃ has become an internationally recognized indicator for evaluating the ultimate toughness of materials. Excellent impact toughness at -196℃ signifies a higher safety margin and resistance to brittle fracture at low-temperature service temperatures such as -60℃. However, the low-temperature performance limits of existing 890MPa-grade conventional marine engineering steels are primarily adapted to -60℃ environments. Under -196℃ testing conditions, they undergo a significant ductile-brittle transition, with a substantial decrease in plasticity and impact toughness. This results in insufficient safety margin at -60℃ service temperatures, and the structural stability and weld reliability cannot meet the assembly and service requirements of cryogenic marine equipment. Currently, a marine structural steel material system that combines ultra-high strength, excellent -196℃ cryogenic impact toughness, high weld stability, and long service life is still incomplete. Existing materials struggle to simultaneously meet the high-strength structural load-bearing requirements of 890MPa-grade steels and the resistance to brittle fracture under cryogenic extreme conditions.

[0006] To address the issue of insufficient low-temperature toughness, existing technologies typically employ increasing the Ni content to lower the ductile-brittle transition temperature. For example, Chinese patent application CN116179968A discloses a 785MPa marine engineering steel plate with good impact toughness at -80℃ and its manufacturing method, with Ni content increased to 5.3%–8.6% and Charpy impact energy at -80℃ ≥90J. Chinese patent application CN116179970A discloses a steel plate with a yield strength of 900MPa for extremely cold marine environments, with Ni content as high as 7%–10% and V content as high as 0.1%–0.3%, produced using electroslag remelting and a double quenching + high-temperature tempering process, achieving a Charpy impact energy at -80℃ ≥70J. Chinese patent application CN118639141A discloses a gigapascal-grade marine engineering steel plate for extremely cold and ultra-deep environments, further increasing the Ni content to 7.2%–14%, thereby raising the Charpy impact energy at -196℃ to 60J. However, such high-Ni schemes have the following shortcomings when applied to 890MPa grade thick marine steel: (1) The high Ni content leads to a significant increase in alloy cost and increases the tendency of the weld heat-affected zone to harden and cold crack, which is not conducive to high heat input welding; (2) The V content (0.1%~0.3%) is much higher than the microalloying level of conventional marine steel (the classification society standard usually requires V≤0.12%). The high V content is prone to forming coarse carbonitride precipitation, which will damage the low temperature toughness. In addition, it requires electroslag remelting and complex quenching and tempering processes, which are long and energy-intensive, and are not suitable for large-scale industrial continuous casting and rolling production.

[0007] Besides the aforementioned high-Ni low-temperature steels, current low-temperature steels exhibiting excellent impact toughness at -196℃ are mainly concentrated in the austenitic steel field. This is primarily due to the fact that the face-centered cubic structure of austenite does not exhibit low-temperature sensitivity, and its toughness does not decrease rapidly with decreasing temperature. However, such steels typically require the addition of high contents of elements such as Mn, Cr, and Ni to form an austenitic structure. For example, Chinese patent application CN111373066A discloses an ultra-low-temperature steel and its manufacturing method, using a Ni-based composition system with a content of 8.9% to 9.3%. Although it can achieve good toughness at -196℃, its composition and process are specifically designed for cryogenic storage tanks and are not suitable for marine engineering steels. Furthermore, among the steel grades with an impact energy of over 200J at -196℃, Chinese patent application CN102586683A discloses a Ni-based low-temperature steel and its manufacturing method, as well as hulls for liquefied natural gas storage tanks and transport ships. Its Ni content reaches 8.5% to 9.5%, resulting in extremely high alloy costs. Additionally, Chinese patent application CN118756060A discloses a high-strength, high-toughness, high-manganese steel plate for ultra-low temperatures and its preparation method. This steel plate has a Mn content of 21.0% to 25.5%, also exhibiting high alloy costs. Moreover, both of these steel grades are primarily suitable for applications such as liquefied natural gas storage tanks, and their composition systems and preparation methods are difficult to directly apply to the production of thick marine engineering steel plates that require a balance of high strength, excellent low-temperature toughness, and low alloy costs.

[0008] In summary, existing technologies have not yet effectively solved the technical challenge of simultaneously achieving high impact toughness at extremely low temperatures of -60℃ and -196℃ while maintaining a yield strength of 890MPa. Therefore, developing a marine engineering steel that can simultaneously adapt to ultra-low temperature environments of -60℃ and -196℃, and possesses ultra-high strength, excellent low-temperature toughness, and weldability, can effectively compensate for the deficiencies of existing technologies and meet the practical application requirements of next-generation ultra-low temperature marine engineering equipment. Summary of the Invention

[0009] The purpose of this invention is to address the technical challenge of achieving high impact toughness at extremely low temperatures of -60℃ and -196℃ simultaneously while maintaining a yield strength of 890MPa. This invention provides an 890MPa-grade high-toughness marine engineering steel suitable for extremely cold environments and its preparation method. The marine engineering steel provided by this invention achieves a yield strength ≥890MPa while significantly improving the low-temperature impact toughness of the steel plate, enabling its safe application in extremely cold marine engineering fields.

[0010] This invention discloses an 890MPa grade high-toughness marine engineering steel suitable for extremely cold environments. The marine engineering steel comprises the following chemical composition by mass fraction: C: 0.080–0.118%, Si: 0.22–0.38%, Mn: 1.12–1.56%, P≤0.008%, S≤0.0003%, Ni: 1.48–2.00%, Cr: 0.42–0.77%, Mo: 0.45–0.70%, V: 0.033–0.069%, Nb: 0.024–0.052%, Ti: 0.014–0.026%, Cu: 0.18–0.42%, RE: 0.010–0.018%, Als: 0.028–0.065%, with the balance being Fe and unavoidable impurities. Furthermore, the contents of the above elements must simultaneously satisfy the following relationship: (1)1.35≤(Ni+Cr) / (C+Mn) ≤1.72; (2)0.68≤(Mn+Mo) / (Ni+Cr+Cu)≤0.85; (3)0.072≤V+Ti+Nb≤0.14; (4)0.14≤(V+Ti+Nb) / Mo≤0.24; (5)0.51≤(Mn+C+Nb) / (V+Ti+Mo+Ni)≤0.88.

[0011] Preferably, the present invention provides an 890MPa grade high-toughness marine engineering steel suitable for extremely cold environments, wherein the marine engineering steel comprises the following chemical composition by mass fraction: C: 0.080-0.110%, Si: 0.26-0.32%, Mn: 1.15-1.48%, P≤0.008%, S≤0.0003%, Ni: 1.48-1.86%, Cr: 0.42-0.69%, Mo: 0.45-0.64%, V: 0.042-0.059%, Nb: 0.026-0.040%, Ti: 0.015-0.022%, Cu: 0.21-0.37%, RE: 0.011-0.015%, Als: 0.028-0.055%, with the remainder being Fe and unavoidable impurities.

[0012] The RE is added using RE cored wire, which contains the following chemical composition by mass fraction: Ce: 50-65%, La: 10-25%, with the balance being Fe and unavoidable impurities. The RE cored wire is added in 2-3 batches during the LF refining process, with an interval of 8-15 minutes between each batch, to control the target RE content in the molten steel to be 0.010%-0.018%.

[0013] The finished marine steel produced by this invention has a thickness of 25-45 mm, a yield strength of 890-940 MPa, a tensile strength of 990-1070 MPa, an elongation of 17.5-20.2%, a KV2 of 320-380 J at -60℃, and a KV2 of 250-315 J at -196℃.

[0014] Furthermore, the microstructure of the finished marine steel obtained by this invention consists of 30-45% layered ferrite, 40-60% layered martensite, and 15-25% equiaxed recrystallized ferrite by volume; wherein the aspect ratio of the layered ferrite and layered martensite grains is ≥4.3, and more than 80% of the layered ferrite lamellar width is ≤320nm, and the layered martensite lamellar width is ≤200nm; the average size of the Nb, V, Ti and other nano-precipitates formed inside the layered ferrite grains is ≤130nm.

[0015] Furthermore, the finished marine steel prepared by this invention has a high-angle grain boundary (HAGB) density with an orientation difference greater than 15° in its microstructure, and a density ≥ 2.12 × 10⁻⁶. 6 m / m 2 The density of small-angle grain boundaries (LAGBs) with an orientation difference in the range of 5–15° is ≥1.05 × 10⁻⁶. 6 m / m 2 Microstructure dislocation density ≥2.3×10 16 m / m 3 .

[0016] Furthermore, the average length of the delamination crack formed by the finished marine steel V-notch impact specimen prepared by the present invention after impact at -196℃ is ≥5.5mm, and the Vickers hardness increment (hardness of the deformed area minus the hardness of the undeformed area) at the impact fracture surface is ≥54.8HV.

[0017] The present invention discloses a method for preparing 890MPa grade high-toughness marine engineering steel suitable for extremely cold environments, comprising: smelting → continuous casting → billet heating → preliminary rolling → heat treatment → warm rolling process, wherein: the warm rolling process is as follows: the heat-treated steel plate is subjected to warm rolling treatment, the steel plate is heated from room temperature to 610-650℃ at a heating rate of 20-30℃ / min, held at this temperature for 1.5-2h and then warm rolling begins, the reduction rate per pass during warm rolling is 6-9%, and the total number of passes is controlled at 8-11; after warm rolling, the steel plate is tempered at 550-600℃ and held at this temperature for 1-1.2h, and then cooled to room temperature in the furnace.

[0018] The present invention discloses a method for preparing 890MPa grade high-toughness marine engineering steel suitable for extremely cold environments, comprising smelting → continuous casting → billet heating → preliminary rolling → heat treatment → warm rolling process, wherein: (1) Smelting: High-quality smelting raw materials are selected, and the production process is carried out using deep desulfurization pretreatment of molten iron, deep dephosphorization smelting using the double slag retention method in converter, LF refining, RH vacuum degassing, and continuous casting. The smelting composition is controlled according to the target value, and P≤0.008%, S≤0.0003%, and the content of residual elements are strictly controlled. During the LF refining process, RE cored wire is added in batches to modify inclusions, and the target RE content is controlled to be 0.010%~0.018%. The qualified molten steel contains the following chemical composition by mass percentage: C: 0.080~0.118%, Si: 0.22~0.38%, Mn: 1.12~1.56%. %, P≤0.008%, S≤0.0003%, Ni: 1.48~2.00%, Cr: 0.42~0.77%, Mo: 0.45~0.70%, V: 0.033~0.069%, Nb: 0.024~0.052%, Ti: 0.014~0.026%, Cu: 0.18~0.42%, RE: 0.010~0.018%, Als: 0.028~0.065%, balance being Fe and unavoidable impurities; (2) Continuous casting: The continuous casting process adopts electromagnetic stirring and dynamic light reduction process. The superheat is controlled at 15-25℃. After continuous casting, a continuous casting billet with a thickness of 400mm is obtained. After the continuous casting billet is removed from the line, it is stacked and cooled slowly for ≥58h. (3) Heating the billet: First heat the billet to 1140-1200℃ and heat it for more than 45 minutes to reduce the deformation resistance and create conditions for rolling with a large reduction rate. (4) Preliminary rolling: Rolling is carried out in two stages: During rough rolling, the surface temperature of the billet is reduced to 1110-1135℃, and the core temperature is 40-60℃ higher than the surface temperature. Rough rolling is carried out in the austenite recrystallization zone, and the final rolling temperature is ≥980℃. The average reduction rate per pass is 11.5-14.0%, and the thickness of the intermediate billet is 140-180mm. Finish rolling is completed in the austenite non-recrystallization zone, which is 90-120℃ higher than the Ac3 line. The final rolling temperature is 50-70℃ higher than the Ac3 line, and the total rolling compression ratio is ≥3.2:1. The thickness of the steel plate after preliminary rolling is 100-125mm. (5) Heat treatment: The critical zone quenching temperature is 770-800℃, the heating rate is 23-32℃ / min, the holding time is 1-1.2h, and the steel plate is rapidly cooled to room temperature by water after being taken out of the furnace; the high temperature tempering temperature is 630-660℃, the holding time is 2.2-2.6h, and the steel plate is air cooled to room temperature after being taken out of the furnace; the yield strength of the heat-treated steel plate is 480-525MPa, the tensile strength is 555-640MPa, the elongation is 26.5-29.5%, the KV2 at -60℃ is 170-200J, and the KV2 at -196℃ is 18-26J; (6) Warm rolling process: The heat-treated steel plate is warm rolled. The steel plate is heated from room temperature to 610-650℃ at a heating rate of 20-30℃ / min. After holding at the temperature for 1.5-2h, warm rolling begins. During the warm rolling process, the reduction rate of each pass is 6-9%, and the total number of passes is controlled at 8-11. After the warm rolling is completed, the steel plate is tempered at 550-600℃ and held at the temperature for 1-1.2h, and then cooled to room temperature with the furnace.

[0019] Furthermore, the total reduction rate in the warm rolling process is controlled at 60% to 80%, and the thickness of the finished marine steel is 25 to 45 mm.

[0020] The application of the marine engineering steel described in this invention in low-temperature or extremely cold marine engineering structural components.

[0021] To address the challenge of achieving a balance between strength and low-temperature toughness in existing marine engineering steels, this invention provides an ultra-low temperature, high-toughness, high-strength marine steel plate. Firstly, a low-Ni-Cr composition system is employed in the design, combined with microalloying. Simultaneously, rare earth elements (REs) are added to implement composite deoxidation and inclusion modification treatments, achieving deep purification of the molten steel. Building upon this, a high-reduction-rate warm rolling process is further implemented within a suitable temperature range to refine the microstructure, forming a uniformly distributed layered structure that further enhances the steel plate's strength and toughness.

[0022] The design principles of each alloy component in this invention are as follows: Carbon (C): Carbon is the most basic strengthening element in steel, improving its strength through solid solution strengthening and precipitation strengthening. Too low a carbon content results in insufficient strength, while too high a carbon content reduces the steel's low-temperature impact toughness and deteriorates its weldability. This invention controls the carbon content between 0.080% and 0.118%, balancing strength and low-temperature toughness.

[0023] Si (Si) is a deoxidizing element and also has a certain solid solution strengthening effect. Too low a Si content results in insufficient deoxidation, while too high a Si content leads to coarse grains and reduces the steel's plasticity and low-temperature toughness. This invention controls the Si content at 0.22–0.38%, ensuring effective deoxidation while avoiding adverse effects on toughness.

[0024] Mn is an important solid solution strengthening element that can also expand the austenite phase region and improve the hardenability of steel. Too low a Mn content results in insufficient strengthening, while too high a content can easily cause segregation, reducing the low-temperature toughness of the core of thick plates. This invention controls the Mn content at 1.12–1.56%, working synergistically with elements such as Ni and Cr to ensure both the strength and hardenability of the steel.

[0025] P and S elements: P and S are harmful impurity elements in steel. P easily causes cold brittleness, and S easily forms MnS inclusions, reducing the plasticity and low-temperature toughness of steel. This invention strictly controls P ≤ 0.008% and S ≤ 0.0003%, effectively reducing the harmful effects of impurity elements on low-temperature toughness.

[0026] Ni (Ni) is a core element for improving the low-temperature toughness of steel. It can significantly reduce the ductile-brittle transition temperature and increase low-temperature impact energy. Too low a Ni content results in limited improvement in low-temperature toughness, while too high a content significantly increases alloy cost and enhances susceptibility to cold cracking during welding. This invention controls the Ni content at 1.48–2.00%, ensuring excellent low-temperature toughness while avoiding the cost and weldability issues associated with high Ni content.

[0027] Cr element: Cr can improve the hardenability and tempering stability of steel, while also possessing a certain degree of corrosion resistance. Too low a Cr content results in insufficient hardenability, while too high a content reduces low-temperature toughness. This invention controls the Cr content at 0.42–0.77%, working synergistically with elements such as Ni and Mo to ensure the hardenability and microstructure uniformity of thick plates.

[0028] Mo (Mo): Mo significantly improves the hardenability and tempering stability of steel, suppresses temper brittleness, and forms fine carbides to increase strength while inhibiting the coarsening of carbonitridium precipitates such as V and Ti. Too low a content results in insufficient effect, while too high a content increases cost and reduces toughness. This invention controls the molybdenum content at 0.45–0.70%, ensuring the hardenability and tempering stability of the core of the thick plate.

[0029] V (Volume): V is a strong carbonitride forming element. During tempering, it can form fine and dispersed V(C,N) precipitates, producing a secondary hardening effect and improving the strength of the steel. If the content is too low, the precipitation strengthening effect is insufficient; if the content is too high, coarse precipitates are easily formed, impairing low-temperature toughness. This invention controls the V content at 0.033–0.069%, ensuring strength while avoiding adverse effects on low-temperature toughness.

[0030] Nitrogen (Nb): Nitrogen is a strong carbonitride forming element. During rolling and heat treatment, it effectively refines grains, improving the strength and low-temperature toughness of steel. Too low a content results in insufficient grain refinement, while too high a content easily leads to the formation of coarse precipitates, which conversely impairs toughness. This invention controls the content at 0.024–0.052%, working synergistically with Ti and V to achieve both grain refinement and precipitation strengthening.

[0031] Ti: Ti is a strong carbonitride forming element, which can effectively fix free nitrogen in steel, refine grains, and improve the toughness and weldability of steel. Too low a content results in insufficient grain refinement, while too high a content easily leads to the formation of coarse inclusions. This invention controls the content at 0.014–0.026%, working synergistically with Nb and V to achieve grain refinement and precipitation strengthening.

[0032] Cu (Cu): Cu can improve the strength and corrosion resistance of steel, and its synergistic effect with Ni can improve low-temperature toughness. However, excessive Cu content can easily cause Cu embrittlement. This invention controls the Cu content at 0.18-0.42%, ensuring strength while avoiding the risk of Cu embrittlement.

[0033] RE: The RE in this invention is added using RE cored wire, which contains the following chemical composition by mass fraction: Ce: 50-65%, La: 10-25%, with the balance being Fe and unavoidable impurities. Rare earth elements Ce and La can combine with oxygen and sulfur in steel to form stable rare earth oxysulfides, causing elongated MnS inclusions to spheroidize, reducing inclusion size, and improving the purity and low-temperature impact toughness of the steel. Simultaneously, rare earth elements can refine the solidification structure and inhibit the segregation of harmful elements at grain boundaries. This invention controls the rare earth content at 0.010-0.018%, effectively improving the low-temperature toughness and isotropy of the steel.

[0034] Al (Al) is a major deoxidizing element in steel. It can combine with nitrogen to form AlN, refining the grain size. Too low an aluminum content results in insufficient deoxidation, while too high a content easily leads to the formation of large inclusions, reducing the steel's toughness and fatigue performance. This invention controls the acid-soluble aluminum content at 0.028–0.065%, ensuring effective deoxidation while avoiding inclusion coarsening.

[0035] The innovation of this invention lies not only in the control of a single element, but also in achieving synergistic optimization of multiple performance characteristics through the following key proportional relationships: (1) 1.35≤(Ni+Cr) / (C+Mn)≤1.72; This relationship is used to control the hardenability of steel plates to ensure the uniformity of the microstructure of the thick plate section, thereby ensuring that the steel plate obtains stable strength and low-temperature toughness. (2) 0.68≤(Mn+Mo) / (Ni+Cr+Cu)≤0.85; This relationship is used to control the ratio between elements that resist temper brittleness and elements that provide low-temperature toughness. When the ratio is below 0.68, the resistance to temper brittleness is insufficient, and toughness is prone to decrease during tempering; when the ratio is above 0.85, the elements that provide low-temperature toughness and corrosion resistance are relatively insufficient. (3) 0.072≤V+Ti+Nb≤0.14 and (4) 0.14≤(V+Ti+Nb) / Mo≤0.24; Equation (3) is used to control the total amount of microalloying elements to ensure that a sufficient number of fine carbonitride precipitates are obtained in the steel to achieve fine grain strengthening and precipitation strengthening; Equation (4) is used to suppress the coarsening of Nb, Ti and V precipitates during tempering. When the ratio is higher than 0.24, the ability of Mo to suppress the coarsening of precipitates is weakened, resulting in a decrease in strength and low-temperature toughness; (5) 0.51≤(Mn+C+Nb) / (V+Ti+Mo+Ni)≤0.88; This relationship is the core control formula for achieving excellent low-temperature toughness through the warm rolling process in this invention. It is used to ensure that the steel plate has sufficient dynamic recrystallization ability and thermoplasticity during the warm rolling process, and to avoid warm rolling cracking. In the formula, Mn, C, and Nb will promote the stability of austenite and dynamic recrystallization, which is conducive to the smooth progress of warm rolling deformation. V, Ti, Mo, and Ni will inhibit dislocation accumulation and local stress concentration caused by excessive recrystallization, which will lead to warm rolling cracking. By controlling this ratio between 0.51 and 0.88, a balance between dynamic recrystallization and microstructure refinement is achieved during the warm rolling process. This ensures the feasibility of the warm rolling process and obtains an ultra-fine grain structure, providing microstructure guarantee for excellent low-temperature toughness.

[0036] The reasons for setting each step and process parameter in the preparation method of this invention are as follows: (1) Smelting and continuous casting process: Deep desulfurization of molten iron and deep dephosphorization of double slag retention method are adopted to reduce harmful impurity elements such as P and S to extremely low levels, and avoid their grain boundary segregation leading to low-temperature brittle fracture; RE cored wire is added in batches during LF refining to allow rare earth elements to fully react with oxygen and sulfur in molten steel, transforming long strip MnS inclusions into spherical rare earth composites, reducing the size of inclusions and improving their distribution morphology; Electromagnetic stirring and dynamic light pressure are adopted throughout the continuous casting process to effectively reduce center segregation and shrinkage porosity and improve the internal quality of the billet; The billet thickness is designed to be 400mm to provide sufficient compression ratio for subsequent rolling.

[0037] (2) Rolling process: Before rough rolling, the temperature difference between the surface and core of the billet is controlled at 40-60℃, so that the core can maintain a low deformation resistance at a higher temperature to facilitate large reduction deformation, thereby achieving uniform grain refinement; the average reduction rate per pass of rough rolling is controlled at 11.5%-14.0% to ensure sufficient deformation to break the as-cast structure; the thickness of the intermediate billet is controlled at 140-180mm to provide sufficient cumulative deformation for the finishing rolling stage; the total rolling compression ratio is ≥3.2:1 to ensure that the as-cast structure is fully broken and the core grains are effectively refined.

[0038] (3) Heat treatment: The critical zone quenching temperature is controlled at 770-800℃ to prepare a dual-phase structure of martensite and ferrite with different volume fractions. By activating the heterogeneous deformation-induced hardening effect during deformation, the low-temperature toughness and resistance to brittle fracture of the steel are improved. The holding time is 1-1.2h to ensure uniform temperature in the thickness direction of the steel plate and to allow austenitization to proceed fully. The high-temperature tempering temperature is controlled at 630-660℃. Tempering in this temperature range reduces the strength of the steel plate and facilitates subsequent warm rolling.

[0039] (4) Warm rolling process: The warm rolling temperature is controlled at 610-650℃. The purpose is to ensure that no phase transformation occurs during the rolling process, and at the same time, to allow the martensite to soften sufficiently within this temperature range, so that it can undergo plastic deformation simultaneously with the ferrite and achieve microstructure refinement. If the warm rolling temperature is too high, the degree of recrystallization increases and the grain refinement effect weakens; if the warm rolling temperature is too low, the ferrite will bear the main plastic deformation, and the martensite will be difficult to refine effectively. The reduction rate per pass is controlled at 6%-9%, and the total number of passes is 8-11. While ensuring the total deformation, the excessive deformation in a single pass is avoided, which may lead to rolling cracks. High dislocation density and subgrain boundaries are introduced into the ferrite grains to form a layered ultrafine grain structure. After rolling, the temperature is held at 550-600℃ for 1-1.2 hours to allow the dislocations generated by deformation to recover and recrystallize, and at the same time, to eliminate some internal stress.

[0040] Compared with the prior art, the present invention has the following beneficial effects: (1) The finished marine steel produced by this invention has a thickness of 25-45 mm, a yield strength of 890-940 MPa, a tensile strength of 990-1070 MPa, an elongation of 17.5-20.2%, a Charpy impact energy KV2 of 320-380 J at -60℃, and a Charpy impact energy KV2 of 250-315 J at -196℃, achieving a synergistic improvement in high strength and extremely low temperature toughness.

[0041] (2) The present invention adopts a low-cost composition system of “low Ni-Cr + micro-alloying” and controls the content of P (≤0.008%) and S (≤0.0003%) at extremely low levels through ultra-pure smelting processes such as rare earth (RE) composite deoxidation, which effectively reduces the cost of alloy.

[0042] (3) This invention constructs a soft-hard interwoven multi-layered heterogeneous structure composed of equiaxed recrystallized ferrite and layered martensite and ferrite through a process combining "critical zone quenching + warm rolling large deformation". During low-temperature impact, this structure significantly improves the work hardening ability and low-temperature toughness of the material through multiple mechanisms such as the coordination of plastic deformation of recrystallized grains, heterogeneous deformation-induced hardening, grain boundary inhibition of dislocation slip at large and small angles, and dislocation emission from dislocation sources.

[0043] (4) The present invention uses a layered structure design to cause the steel plate to delaminate during the impact process, changing the local stress state from plane strain to plane stress state, increasing the critical stress for cleavage fracture, and enabling the material to maintain stable crack propagation and energy dissipation efficiency at extreme low temperatures.

[0044] In summary, this invention, through meticulous component design, complete purification smelting process, and innovative warm rolling technology, has successfully overcome the technical challenge of synergistically improving the strength and low-temperature toughness of high-strength marine engineering steel in extremely cold environments. Attached Figure Description

[0045] Figure 1 These are SEM images of the microstructure of marine steel in Embodiment 1 of the present invention before and after the warm rolling process. Figure 2 These are SEM images of the microstructure of the finished marine steel products of Comparative Examples 1-2 of this invention; Figure 3 This is a distribution diagram of large-angle and small-angle grain boundaries in the microstructure of the finished marine steel of Embodiment 1 of the present invention; Figure 4 This is a microstructure distribution diagram of nano-precipitated phases in the finished marine steel of Example 1 of the present invention; Figure 5 This is a distribution diagram of RE-containing inclusions (La and Ce) in the finished marine steel of Embodiment 1 of the present invention; Figure 6 This is the morphology of the finished marine steel Charpy impact sample of Example 1 of the present invention after impact at -60℃; Figure 7 This is the morphology of the finished marine steel Charpy impact sample of Example 1 of the present invention after impact at -196℃. Detailed Implementation

[0046] To better explain the technical solution of the present invention, the technical solution of the present invention will be further described below with reference to specific embodiments. The following embodiments are merely illustrative of the technical solution of the present invention and do not limit the present invention in any way. The sequence numbers of the following embodiments are merely for description and do not represent the superiority or inferiority of the embodiments.

[0047] Table 1 below lists the chemical composition values ​​of marine steel in various embodiments and comparative examples of the present invention; Table 2 below is a list of values ​​for the chemical composition ratios of marine steels in various embodiments and comparative examples of the present invention; Table 3 below lists the main process parameters for each embodiment and comparative example of marine steel of the present invention; Table 4 below lists the performance test and analysis results of marine engineering steel in various embodiments and comparative examples of the present invention before and after the warm rolling process.

[0048] A method for preparing 890MPa grade high-toughness marine engineering steel suitable for extremely cold environments according to various embodiments of the present invention includes smelting → continuous casting → billet heating → preliminary rolling → heat treatment → warm rolling process, wherein: (1) Smelting: High-quality smelting raw materials are selected, and the production process is carried out by deep desulfurization pretreatment of molten iron, deep dephosphorization smelting by converter double slag retention method, LF refining, RH vacuum degassing and continuous casting process; the smelting composition is controlled according to the target value, and P≤0.008%, S≤0.0003% and residual element content are strictly controlled; RE cored wire is added in batches during the LF refining process to modify inclusions, and the target RE content is controlled to be 0.010%~0.018%. The qualified molten steel contains the following chemical composition by mass percentage: C: 0.080~0.118%, Si: 0. 22~0.38%, Mn: 1.12~1.56%, P≤0.008%, S≤0.0003%, Ni: 1.48~2.00%, Cr: 0.42~0.77%, Mo: 0.45~0.70%, V: 0.033~0.069%, Nb: 0.024~0.052%, Ti: 0.014~0.026%, Cu: 0.18~0.42%, RE: 0.010~0.018%, Als: 0.028~0.065%, balance being Fe and unavoidable impurities; (2) Continuous casting: The continuous casting process adopts electromagnetic stirring and dynamic light reduction process. The superheat is controlled at 15-25℃. After continuous casting, a continuous casting billet with a thickness of 400mm is obtained. After the continuous casting billet is removed from the line, it is stacked and cooled slowly for ≥58h. (3) Heating the billet: First heat the billet to 1140-1200℃ and heat it for more than 45 minutes to reduce the deformation resistance and create conditions for rolling with a large reduction rate. (4) Preliminary rolling: Rolling is carried out in two stages: During rough rolling, the surface temperature of the billet is reduced to 1110-1135℃, and the core temperature is 40-60℃ higher than the surface temperature. Rough rolling is carried out in the austenite recrystallization zone, and the final rolling temperature is ≥980℃. The average reduction rate per pass is 11.5-14.0%, and the thickness of the intermediate billet is 140-180mm. Finish rolling is completed in the austenite non-recrystallization zone, which is 90-120℃ higher than the Ac3 line. The final rolling temperature is 50-70℃ higher than the Ac3 line, and the total rolling compression ratio is ≥3.2:1. The thickness of the steel plate after preliminary rolling is 100-125mm. (5) Heat treatment: The critical zone quenching temperature is 770-800℃, the heating rate is 23-32℃ / min, the holding time is 1-1.2h, and the steel plate is rapidly cooled to room temperature by water after being taken out of the furnace; the high temperature tempering temperature is 630-660℃, the holding time is 2.2-2.6h, and the steel plate is air cooled to room temperature after being taken out of the furnace. (6) Warm rolling process: The heat-treated steel plate is warm rolled. The steel plate is heated from room temperature to 610-650℃ at a heating rate of 20-30℃ / min. After holding at the temperature for 1.5-2h, warm rolling begins. During the warm rolling process, the reduction rate of each pass is 6-9%, and the total number of passes is controlled at 8-11. After the warm rolling is completed, the steel plate is tempered at 550-600℃ and held at the temperature for 1-1.2h, and then cooled to room temperature with the furnace.

[0049] Furthermore, the total reduction rate in the warm rolling process is controlled at 60% to 80%, and the thickness of the finished marine steel is 25 to 45 mm.

[0050] Furthermore, in step (5), the initial rolling temperature of the finishing rolling is 875-920℃, and the final rolling temperature is 835-860℃.

[0051] Table 1. List of chemical composition values ​​for marine steel in various embodiments and comparative examples of the present invention. Table 2. List of values ​​for the chemical composition ratios of marine steels in various embodiments and comparative examples of the present invention. Table 3. List of main process parameters for marine steel in various embodiments and comparative examples of the present invention. Table 4. Performance testing and analysis results of marine engineering steel in various embodiments and comparative examples of the present invention before and after the warm rolling process. Note: Samples for the Charpy impact test are taken along the rolling direction of the finished steel plate. Before conducting the -196℃ Charpy impact test, the surface of the impact sample needs to be sanded and polished step by step with sandpaper to eliminate the influence of surface defects such as burrs generated during processing on the experimental results. The sample is placed in liquid nitrogen for 15-20 minutes before the impact test. The impact test for each example is performed 5 times, and the average value is taken as the impact energy measurement value.

[0052] As shown in Table 4 above, the 890MPa grade high- and low-temperature toughness marine steel with a thickness of 25–45 mm prepared by this invention exhibits a yield strength ≥893MPa, tensile strength ≥997MPa, elongation ≥17.5%, Charpy impact energy KV2 ≥327J at -60℃, and Charpy impact energy KV2 ≥257J at -196℃, demonstrating high strength and excellent ultra-low temperature impact toughness. Furthermore, the marine steel of this invention uses a low-Ni, Cr alloy system, effectively reducing alloy costs and demonstrating good economic efficiency.

[0053] The superior strength and low-temperature toughness stem from meticulous composition design, purified smelting, and innovative warm rolling processes. Regarding composition, ultra-pure smelting processes such as RE composite deoxidation control minimize the content of P (≤0.008%) and S (≤0.0003%), transforming elongated MnS inclusions into RE-rich spherical inclusions, thus reducing their adverse effects on toughness. Simultaneously, by controlling the proportions of microalloying elements to meet the requirements of 0.072≤V+Ti+Nb≤0.14 and 0.14≤(V+Ti+Nb) / Mo≤0.24, a large number of nanoscale precipitates are formed during tempering. Mo inhibits the coarsening of these precipitates, preventing reduced strengthening effects and strain concentration-induced deterioration of low-temperature toughness. In addition, by controlling the Ni, Cr, and Cu contents within the range of 1.35≤(Ni+Cr) / (C+Mn)≤1.72 and 0.68≤(Mn+Mo) / (Ni+Cr+Cu)≤0.85, the hardenability of the thick plate section is ensured while improving the low-temperature toughness, thus avoiding insufficient strength.

[0054] In terms of process, a combination of "critical zone quenching + warm rolling with large deformation" is used to form a multi-layered structure composed of equiaxed recrystallized ferrite and layered martensite and ferrite. During deformation, multi-stage strain hardening and delamination behavior occur, thereby significantly improving low-temperature toughness. To optimize the warm rolling effect, the element ratio is controlled at 0.51≤(Mn+C+Nb) / (V+Ti+Mo+Ni)≤0.88 to ensure that the steel plate has sufficient dynamic recrystallization capacity and thermoplasticity during warm rolling, avoiding warm rolling cracking. This ensures both process feasibility and the acquisition of an ultrafine-grained structure, providing microstructural guarantee for excellent low-temperature toughness.

[0055] As can be seen from Tables 1-4, before the warm rolling treatment, the steel plates of Examples 1-6 had coarse grains, low dislocation density, and low proportion of large and small angle grain boundaries. After preliminary rolling and heat treatment, the yield strength of the steel plates was 480-525 MPa, the tensile strength was 555-640 MPa, the elongation was 26.5-29.5%, the KV2 at -60℃ was 170-200 J, and the KV2 at -196℃ was 18-26 J. However, after the warm rolling process, the strength and toughness of the steel plates of the present invention were greatly changed, and a marine engineering steel plate with high strength and excellent ultra-low temperature impact toughness was obtained with a yield strength ≥893 MPa, tensile strength ≥997 MPa, elongation ≥17.5%, Charpy impact energy KV2 at -60℃ ≥327 J, and Charpy impact energy KV2 at -196℃ ≥257 J. In Comparative Example 1, the Ni content was relatively low, resulting in a (Ni+Cr) / (C+Mn) ratio of only 1.052, which is lower than the lower limit of this invention (≥1.35), indicating insufficient toughening effect. The (Mn+C+Nb) / (V+Ti+Mo+Ni) ratio was 0.957, which is higher than the upper limit of this invention (0.88), thus requiring a lower rolling temperature (580℃). However, the lower temperature limits the reduction rate, resulting in incomplete lamellarization and poor uniformity of the microstructure. Insufficient dynamic recovery and recrystallization lead to a higher dislocation density, which deteriorates the ductility and low-temperature toughness. In Comparative Example 2, the increased Ni content resulted in a (Ni+Cr) / (C+Mn) ratio of 1.947, higher than the upper limit of this invention (1.72); the (Mn+C+Nb) / (V+Ti+Mo+Ni) ratio was 0.496, lower than the lower limit of this invention (0.51). Therefore, a higher warm rolling temperature (700℃) was used, leading to increased dynamic recrystallization, decreased dislocation density, and a strength less than 890 MPa. Thus, although Comparative Examples 1 and 2 used the same composition system and process flow, the different specific component values ​​and alloy element ratios resulted in differences in warm rolling process parameters, making the resulting steels unsuitable for extremely cold environments.

[0056] Figure 1The figures show the microstructure of the steel plate from Example 1 of this invention before (left side) and after (right side) warm rolling. As can be seen from the figures, after critical zone quenching, the microstructure consists of approximately 60% ferrite and 40% martensite by volume. The ferrite is blocky and lath-shaped, with an average size of 12.4 μm; the martensite is granular, with an average size of 2.2 μm. After warm rolling, the microstructure of the finished marine steel obtained in Example 1 consists of approximately 40% layered ferrite, 44% layered martensite, and 16% equiaxed recrystallized ferrite by volume. In Examples 2, 4, and 5, due to differences in critical zone quenching temperature and a reduction in total reduction rate to 70%, 64%, and 60%, respectively. The microstructure of the finished steel prepared in Example 2 consists of approximately 31% layered ferrite, 51% layered martensite, and 18% equiaxed recrystallized ferrite by volume fraction; the microstructure of the finished steel prepared in Example 4 consists of approximately 43% layered ferrite, 40% layered martensite, and 17% equiaxed recrystallized ferrite by volume fraction; and the microstructure of the finished steel prepared in Example 5 consists of approximately 38% layered ferrite, 42% layered martensite, and 20% equiaxed recrystallized ferrite by volume fraction.

[0057] Figure 2 These are SEM images of the marine steel products obtained after warm rolling processes in Comparative Example 1 (left image) and Comparative Example 2 (right image) of this invention. As shown in the images, Comparative Example 1, due to a reduction rate of only 50%, which is lower than the minimum limit (60%) specified in this invention, failed to completely transform its microstructure into a uniform layered structure, resulting in a large number of coarse ferrite grains. This makes it prone to brittle fracture during low-temperature impact, leading to a decrease in toughness. Comparative Example 2, on the other hand, benefited from a higher warm rolling temperature (700℃), which enhanced its dynamic recrystallization ability. Although the reduction rate reached 75%, excessive dynamic recrystallization damaged the integrity of the layered structure, weakening the layered toughening effect and ultimately reducing its low-temperature toughening capability.

[0058] Figure 3 and Figure 4 The figures show the grain boundary distribution and nano-precipitate distribution of the finished marine steel produced in Example 1 of this invention. As can be seen from the figures, the large deformation warm rolling reduction rate (80%) completely lamellarizes the microstructure, resulting in significantly refined grains. Specifically, the aspect ratio of the layered grains is 6.2, over 80% of the layered ferrite lamellars have a width ≤260nm, the layered martensite lamellars have a width ≤170nm, and the average size of the nano-precipitates formed within the ferrite lamellar grains is ≤130nm. Furthermore, the layered design leads to the formation of high-density HAGBs and LAGBs (4.32×10⁻⁶) in the microstructure. 6 m / m 2 and 1.08×10 6 m / m 2In Examples 2, 5, and 6, the aspect ratios of the layered grains decreased slightly to 5.6, 4.3, and 5.3, respectively, due to the reduction in the total reduction rate to 70%, 60%, and 65%. Nevertheless, since the total reduction rate still reached 60%, in Example 5, over 80% of the layered ferrite lamellars had a width ≤315 nm, and the layered martensite lamellars had a width ≤195 nm. Furthermore, the densities of HAGBs and LAGBs remained at 2.48 × 10⁻⁶. 6 m / m 2 and 2.32×10 6 m / m 2 This ensures that the microstructure can promote coordinated sliding and strain distribution between adjacent hard phase layers through large-scale plastic deformation of recrystallized grains during low-temperature impact, activating heterogeneous deformation-induced hardening and improving the work hardening capability of the sample under low-temperature conditions. Simultaneously, HAGBs and LAGBs, on the one hand, hinder dislocation slip and improve strain hardening capability; on the other hand, they can also act as active dislocation sources, continuously emitting dislocations into the matrix, providing driving force for plastic deformation, thus ensuring that the material possesses both high strength and excellent low-temperature toughness.

[0059] Figure 5 This is a SEM image of typical inclusions in the finished marine steel produced in Example 1 of this invention. As can be seen from the image, through ultra-pure smelting processes such as rare earth (RE) composite deoxidation, the contents of P (≤0.008%) and S (≤0.0003%) are controlled at extremely low levels. Furthermore, elongated MnS and other inclusions are transformed into spherical composite inclusions rich in RE (La, Ce), with a size controlled to ≤1.8 μm and a density controlled to ≤3.4 inclusions / mm². 2 By rounding the shape, the interfacial gaps at the edges of irregular inclusions are eliminated, the stress concentration between inclusions and the matrix is ​​reduced, and the possibility of crack initiation is decreased, thereby improving the low-temperature impact toughness of the steel.

[0060] Figure 6 and Figure 7The images show photographs of the samples from Example 1 after impact at -60℃ and -196℃, respectively. As can be seen from the figures, both samples underwent significant plastic deformation. The hardness of the samples before impact was measured to be 375.8 HV, and the hardness near the fracture surface after impact reached 445.2 HV and 436.0 HV, respectively, with hardness increments of 69.4 HV and 60.2 HV, indicating that the samples possess excellent strain hardening capabilities under low-temperature conditions. The images also show that the layered structure design causes the steel plate to delaminate during impact, dividing the sample into multiple thin layers. This weakens the triaxial constraint at the crack tip, transforming the local stress state from plane strain to a plane stress state more conducive to plastic deformation, increasing the critical stress for cleavage fracture, and enabling the steel plate to maintain stable crack propagation even at extreme low temperatures. Furthermore, as the temperature decreases, the brittleness of the layered grain boundaries between microstructures increases, and the delamination phenomenon becomes more pronounced. The average length of the delaminated crack increases to 5.9 mm, thereby improving energy release efficiency and further optimizing the low-temperature impact toughness.

[0061] The above embodiments are merely specific examples for explaining the present invention and do not limit the present invention in any way. Any non-substantial changes made by any person based on the above content and form that do not depart from the scope of protection of the claims of the present invention should be considered to fall within the scope of protection of the claims of the present invention.

Claims

1. A high-toughness marine steel of 890MPa grade suitable for extremely cold environments, characterized in that... The marine steel comprises the following chemical composition by mass fraction: C: 0.080–0.118%, Si: 0.22–0.38%, Mn: 1.12–1.56%, P≤0.008%, S≤0.0003%, Ni: 1.48–2.00%, Cr: 0.42–0.77%, Mo: 0.45–0.70%, V: 0.033–0.069%, Nb: 0.024–0.052%, Ti: 0.014–0.026%, Cu: 0.18–0.42%, RE: 0.010–0.018%, Als: 0.028–0.065%, with the balance being Fe and unavoidable impurities; and the contents of the above elements must simultaneously satisfy the following relationship: (1)1.35≤(Ni+Cr) / (C+Mn)≤1.72; (2)0.68≤(Mn+Mo) / (Ni+Cr+Cu)≤0.85; (3)0.072≤V+Ti+Nb≤0.14; (4)0.14≤(V+Ti+Nb) / Mo≤0.24; (5)0.51≤(Mn+C+Nb) / (V+Ti+Mo+Ni)≤0.88; The preparation method of the marine engineering steel includes smelting → continuous casting → billet heating → preliminary rolling → heat treatment → warm rolling process. The warm rolling process is as follows: the heat-treated steel plate is subjected to warm rolling treatment, the steel plate is heated from room temperature to 610-650℃ at a heating rate of 20-30℃ / min, held at this temperature for 1.5-2 hours and then warm rolling begins. During the warm rolling process, the reduction rate per pass is 6-9%, and the total number of passes is controlled at 8-11. After the warm rolling is completed, the steel plate is tempered at 550-600℃ and held at this temperature for 1-1.2 hours, and then cooled to room temperature in the furnace. The finished marine steel has a thickness of 25–45 mm, a yield strength of 890–940 MPa, a tensile strength of 990–1070 MPa, an elongation of 17.5–20.2%, a KV2 of 320–380 J at -60℃, and a KV2 of 250–315 J at -196℃.

2. The 890MPa grade high-toughness marine steel suitable for extremely cold environments according to claim 1, characterized in that... The marine steel comprises the following chemical composition by mass fraction: C: 0.080–0.110%, Si: 0.26–0.32%, Mn: 1.15–1.48%, P≤0.008%, S≤0.0003%, Ni: 1.48–1.86%, Cr: 0.42–0.69%, Mo: 0.45–0.64%, V: 0.042–0.059%, Nb: 0.026–0.040%, Ti: 0.015–0.022%, Cu: 0.21–0.37%, RE: 0.011–0.015%, Als: 0.028–0.055%, with the remainder being Fe and unavoidable impurities.

3. The 890MPa grade high-toughness marine steel suitable for extremely cold environments according to claim 1, characterized in that: The RE is added using RE cored wire, which contains the following chemical composition by mass fraction: Ce: 50-65%, La: 10-25%, with the balance being Fe and unavoidable impurities. The RE cored wire is added in 2-3 batches during the LF refining process, with an interval of 8-15 minutes between each batch, to control the target RE content in the molten steel to be 0.010%-0.018%.

4. A high-toughness marine steel of 890MPa grade suitable for extremely cold environments according to claim 1 or 2, characterized in that: The microstructure of the finished marine steel consists of 30-45% layered ferrite, 40-60% layered martensite, and 15-25% equiaxed recrystallized ferrite by volume. Among them, the aspect ratio of the layered ferrite and layered martensite grains is ≥4.3, and the lamellar width of more than 80% of the layered ferrite is ≤320nm, and the lamellar width of the layered martensite is ≤200nm. The average size of the Nb, V, and Ti nanoprecipitates formed inside the layered ferrite grains is ≤130nm.

5. The 890MPa grade high-toughness marine steel suitable for extremely cold environments according to claim 4, characterized in that: The density of high-angle grain boundaries (HAGBs) with a microstructure orientation difference greater than 15° is ≥2.12 × 10⁻⁶. 6 m / m 2 The density of small-angle grain boundaries (LAGBs) with an orientation difference in the range of 5–15° is ≥1.05 × 10⁻⁶. 6 m / m 2 Microstructure dislocation density ≥2.3×10 16 m / m 3 .

6. A high-toughness marine steel of 890MPa grade suitable for extremely cold environments according to claim 1 or 2, characterized in that: The average length of the delamination crack formed on the finished marine steel V-notch impact specimen after impact at -196℃ is ≥5.5mm, and the Vickers hardness increment at the impact fracture surface is ≥54.8HV.

7. The method for preparing 890MPa grade high-toughness marine steel suitable for extremely cold environments according to claim 1, characterized in that: In the warm rolling process, the total reduction rate is controlled at 60-80%, and the thickness of the finished marine steel is 25-45 mm.

8. A method for preparing 890MPa grade high-toughness marine steel suitable for extremely cold environments according to claim 1, comprising smelting → continuous casting → billet heating → preliminary rolling → heat treatment → warm rolling process, characterized in that: (1) Smelting: Select high-quality smelting raw materials and adopt the following processes for production: deep desulfurization pretreatment of molten iron, deep dephosphorization smelting using converter double slag retention method, LF refining, RH vacuum degassing and continuous casting. The smelting composition is controlled according to target values, with strict control over P ≤ 0.008%, S ≤ 0.0003%, and residual element content. During the LF refining process, RE cored wire is added in batches for inclusion modification treatment, controlling the target RE content to 0.010%–0.018%. The qualified molten steel contains the following chemical composition by mass percentage: C: 0.080–0.118%, Si: 0.22–0.38%, Mn: 1.12–1.56%, P ≤ 0.008%. %, S≤0.0003%, Ni: 1.48~2.00%, Cr: 0.42~0.77%, Mo: 0.45~0.70%, V: 0.033~0.069%, Nb: 0.024~0.052%, Ti: 0.014~0.026%, Cu: 0.18~0.42%, RE: 0.010~0.018%, Als: 0.028~0.065%, balance being Fe and unavoidable impurities; (2) Continuous casting: The continuous casting process adopts electromagnetic stirring and dynamic light reduction process. The superheat is controlled at 15-25℃. After continuous casting, a continuous casting billet with a thickness of 400mm is obtained. After the continuous casting billet is removed from the line, it is stacked and cooled slowly for ≥58h. (3) Heating the billet: First heat the billet to 1140-1200℃ and heat it for more than 45 minutes to reduce the deformation resistance and create conditions for rolling with a large reduction rate. (4) Preliminary rolling: Rolling is carried out in two stages: During rough rolling, the surface temperature of the billet is reduced to 1110-1135℃, and the core temperature is 40-60℃ higher than the surface temperature. Rough rolling is carried out in the austenite recrystallization zone, and the final rolling temperature is ≥980℃. The average reduction rate per pass is 11.5-14.0%, and the thickness of the intermediate billet is 140-180mm. Finish rolling is completed in the austenite non-recrystallization zone, which is 90-120℃ higher than the Ac3 line. The final rolling temperature is 50-70℃ higher than the Ac3 line, and the total rolling compression ratio is ≥3.2:

1. The thickness of the steel plate after preliminary rolling is 100-125mm. (5) Heat treatment: The critical zone quenching temperature is 770-800℃, the heating rate is 23-32℃ / min, the holding time is 1-1.2h, and the steel plate is rapidly cooled to room temperature by water after being taken out of the furnace; the high temperature tempering temperature is 630-660℃, the holding time is 2.2-2.6h, and the steel plate is air cooled to room temperature after being taken out of the furnace; the yield strength of the heat-treated steel plate is 480-525MPa, the tensile strength is 555-640MPa, the elongation is 26.5-29.5%, the KV2 at -60℃ is 170-200J, and the KV2 at -196℃ is 18-26J.

9. The application of marine steel according to claim 1 or 2 in low-temperature or extremely cold marine engineering structural components.

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