A 960mpa-grade corrosion-resistant marine steel suitable for tropical marine climate and a preparation method thereof

Abstract

The present application relates to the technical field of high-strength corrosion-resistant steel material for tropical marine climate, and particularly relates to a 960MPa-grade corrosion-resistant marine steel suitable for tropical marine climate and a preparation method thereof; the marine steel contains chemical components of C, Si, Mn, P, S, Ni, Mo, Cr, V, Ti, Cu, Sb, Sn, Als, the rest being Fe and inevitable impurities, and the content of the above elements must simultaneously satisfy the following relationships: 1.28 <= (Mn+Cr) / Mn <= 1.62; 0.062% <= V+Ti <= 0.089%; 0.11 <= (V+Ti) / Cr <= 0.24; 0.70 <= (Ni+C) / (Mo+Mn) <= 1.23; 0.085 <= (Sb+Sn) / (Cr+Mo+Ni) <= 0.13; the yield strength of the marine steel prepared by the present application is >=960MPa, the tensile strength is >=1049MPa, the elongation is >=18%, the KV2 at -60 DEG C is >=205J, and the saturation current density is <=5.5 mA / cm 2 ; the technical problem that the strength and toughness and corrosion resistance of the super-high-strength marine steel are difficult to be simultaneously improved is solved, and the marine steel is particularly suitable for tropical extreme marine environment.

Description

Technical Field

[0001] This invention relates to the field of steel technology for marine engineering, and more particularly to the field of high-strength corrosion-resistant steel for tropical marine climates under conditions of high temperature, high humidity, high salt spray, and strong radiation. Specifically, it provides a 960MPa grade corrosion-resistant marine engineering steel suitable for tropical marine climates and its preparation method. Background Technology

[0002] With the expanding application scope of marine engineering steel (such as deep-sea drilling platforms, large floating structures, and cross-sea channels), the research and development of marine engineering steel has become a hot research topic in the industry. Tropical marine climates are characterized by high temperature, high humidity, high salt spray, and strong radiation. Their harsh corrosive environment poses a severe challenge to the long-term safe service of marine engineering structural steel, mainly manifested as rapid uniform corrosion, severe localized pitting and crevice corrosion, and a tendency for stress corrosion cracking under high stress. Traditional commercial marine engineering steels (such as EH420 / EH460 grades) often suffer from high maintenance costs, shortened structural lifespan, and even sudden failures in tropical marine climates due to excessively rapid corrosion rates and insufficient strength reserves. To meet the urgent needs of deep-sea engineering for lightweight structures, high safety, and ultra-long service life (typically exceeding 30 years), developing a new generation of marine engineering steel with both ultra-high strength and excellent corrosion resistance has become a pressing issue for the industry.

[0003] In recent years, many researchers have primarily improved steel performance through alloying design (such as adding corrosion-resistant and strengthening elements like Cu, Ni, Cr, and Mo) combined with advanced controlled rolling and cooling (TMCP) and quenching and tempering (QT) processes. For example, Chinese patent application CN118685711A discloses "A 460MPa grade marine engineering steel with excellent fracture resistance and its preparation method," and Chinese patent application CN118147520A discloses "Ultra-high strength marine engineering steel with high ductility and resistance to ice load corrosion and its manufacturing method." The reported advanced marine engineering steels have yield strengths reaching 460 and 550 MPa, respectively. However, in the pursuit of higher strengths (such as 960 MPa), a severe challenge of the mutual constraint between strength, toughness, and corrosion resistance is often encountered. While Chinese patent applications CN117660837A, CN121575326A, and CN121046730A, titled "High-Ductility, Seawater Corrosion-Resistant, Ultra-High-Strength Marine Steel and Its Manufacturing Method," "A Corrosion-Resistant, Fatigue-Resistant, Ultra-High-Strength Marine Steel and Its Preparation Method," and "An Ultra-High-Strength, Tough, and Wear-Resistant Marine Steel Plate Based on Gradient Microstructure Design and Its Manufacturing Method," achieve strengthening by increasing alloy content, this often deteriorates low-temperature toughness and leads to selective corrosion due to uneven microstructure, making it difficult to meet the long-term corrosion resistance requirements of tropical marine climates. In Chinese patent application CN114959443A, "A Seawater Corrosion-Resistant High-Strength Steel, Round Bar, and Its Manufacturing Method" is disclosed. This invention achieves a high yield strength of 1000 MPa and relatively excellent corrosion resistance by increasing the C and Ni contents to 0.24%~0.34% and 2.0%~4.2%, respectively. However, high carbon content leads to a sharp deterioration in impact toughness (only 88J of impact energy at room temperature), and high Ni content undoubtedly increases the cost of steel, thus hindering large-scale industrial production. Another approach, such as Chinese patent applications CN116179970A and CN112746224A, discloses "A steel plate with a yield strength of 900MPa for extremely cold marine environments and its manufacturing method" and "A steel plate with a yield strength of 690MPa for marine engineering and its manufacturing method." While these inventions improve toughness, their smelting process lacks precise control over inclusions, resulting in insufficient steel cleanliness and uncertainties regarding corrosion resistance, especially in harsh tropical marine climates.

[0004] In summary, existing technologies have not yet effectively solved the problem of simultaneously achieving ultra-high strength of 960MPa or higher and long-term corrosion resistance in extreme tropical marine climates while ensuring good service performance and processability. Summary of the Invention

[0005] This invention aims to overcome the technical bottlenecks commonly found in existing ultra-high strength marine engineering steels, such as the difficulty in achieving synergistic effects of strength, toughness, and corrosion resistance, high alloy costs, and complex production processes. It provides a 960MPa-grade corrosion-resistant marine engineering steel suitable for tropical marine climates and its preparation method. By employing low-carbon and Sb-Sn composite alloying designs, this invention achieves 960MPa-grade ultra-high strength while significantly improving the steel's low-temperature toughness and seawater corrosion resistance. Furthermore, it features controllable alloy costs, stable process flow, and suitability for large-scale industrial production.

[0006] To achieve the above objectives, the present invention employs the following technical solution: This invention discloses a 960MPa grade corrosion-resistant marine steel suitable for tropical marine climates. The marine steel comprises the following chemical composition by mass percentage: C: 0.09–0.135%, Si: 0.25–0.36%, Mn: 1.10–1.60%, P≤0.010%, S≤0.002%, Ni: 1.22–1.55%, Mo: 0.36–0.72%, Cr: 0.32–0.66%, V: 0.045–0.068%, Ti: 0.012–0.021%, Cu: 0.15–0.38%, Sb: 0.084–0.116%, Sn: 0.092–0.118%, Als: 0.025–0.045%, with the remainder being Fe and unavoidable impurities. Furthermore, the content of the above elements must simultaneously satisfy the following relationship: (1)1.28≤(Mn+Cr) / Mn≤1.62; (2) 0.062%≤V+Ti≤0.089%; (3)0.11≤(V+Ti) / Cr≤0.24; (4)0.70≤(Ni+C) / (Mo+Mn)≤1.23; (5)0.085≤(Sb+Sn) / (Cr+Mo+Ni)≤0.13.

[0007] Preferably, the marine steel comprises the following chemical composition by mass percentage: C: 0.105-0.130%, Si: 0.26-0.34%, Mn: 1.16-1.43%, P≤0.010%, S≤0.002%, Ni: 1.26-1.43%, Mo: 0.40-0.56%, Cr: 0.41-0.59%, V: 0.045-0.058%, Ti: 0.012-0.018%, Cu: 0.18-0.35%, Sb: 0.084-0.110%, Sn: 0.094-0.115%, Als: 0.025-0.037%, with the remainder being Fe and unavoidable impurities.

[0008] The finished marine steel produced by this invention has a thickness of 90-120 mm, a yield strength of 960-1050 MPa, a tensile strength of 1049-1150 MPa, an elongation of 18-21%, and a KV2 of 205-285 J at -60℃.

[0009] The corrosion-active inclusions in the finished marine steel prepared by this invention have a density size ≤2.3μm and a density ≤5 inclusions / mm². 2 The saturation current density under a constant electrode potential of -300mV is ≤5.5 mA / cm². 2 .

[0010] The original austenite grain size in the finished marine steel prepared by this invention is ≤20μm, and the microstructure is composed of lath martensite with a volume fraction of 30-70%, strip bainite with a volume fraction of 20-60%, and granular bainite with a volume fraction of 10-20%. Moreover, the width of more than 85% of the martensite laths is ≤0.92μm, and the aspect ratio of the granular bainite is ≤2.4μm.

[0011] Furthermore, a high density of (Ti,V) is formed within the laths of the lath martensite and the grain boundaries. X (C,N) Y The average size of the carbonitrides is ≤200 nm, and the number density reaches (5.1~6.3)×10⁻⁶. 13 pcs / m 2 The dislocation density in the tissue is (5.4~7.6)×10 16 / m 2 The Vickers hardness of the microstructure reaches 390~420HV.

[0012] The present invention discloses a method for preparing 960MPa grade corrosion-resistant marine steel suitable for tropical marine climates, comprising the following steps: (1) Converter smelting: The double-slag ultra-low phosphorus smelting process is adopted, and the temperature of the incoming molten iron is controlled in the range of 1280~1340℃. Top and bottom blowing is carried out throughout the process, the lance position is 1.64~1.74m, the slag basicity is 2.1~2.3, and the oxygen supply intensity is 3.45~3.60m. 3 / t·min, tapping time ≥5min, Al-containing materials shall not be used for deoxidation and alloying after the converter; (2) Ladle refining: After the ladle enters the LF furnace, it is heated to 1580~1620℃. After oxygen determination, [O] ≤ 36ppm. Subsequently, Sb, Sn, V and Ti are added for deoxidation and alloying. The holding time is ≥ 25min to ensure that the alloying elements are fully dissolved. Then the ladle is hoisted into the RH device for vacuum treatment. The ultimate vacuum degree is controlled at 30~40Pa. The ultimate vacuum holding time is ≥ 18min. After alloy fine-tuning, the circulation time is ≥ 8min. The final hydrogen content [H] is controlled to ≤ 1.2ppm. The qualified molten steel contains the following chemical composition by mass percentage: C: 0.09~0.1 35%, Si: 0.25~0.36%, Mn: 1.10~1.60%, P≤0.010%, S≤0.002%, Ni: 1.22~1.55%, Mo: 0.36~0.72%, Cr: 0.32~0.66%, V: 0.045~0.068%, Ti: 0.012~0.021%, Cu: 0.15~0.38%, Sb: 0.084~0.116%, Sn: 0.092~0.118%, Als: 0.025~0.045%, with the remainder being Fe and unavoidable impurities; (3) Continuous casting process: control the superheat to 10~15℃, cast at a low superheat and constant casting speed of 0.45~0.72m / min, and the current intensity of electromagnetic stirring is 380~410A. The casting produces a 360~460mm high homogeneous continuous casting billet. The billet is stacked on-site for slow cooling after leaving the line, and the slow cooling time is ≥80h. (4) Heating of billet: The billet is uniformly heated in the heating furnace to 1180~1210℃ and held for 40~60min; (5) Rolling process: First, the billet is pre-cooled in a gradient. Specifically, the surface temperature of the billet is reduced to 1120~1140℃, while the core temperature is controlled at 1160~1190℃. Then, two-stage controlled rolling is carried out. Rough rolling is carried out in the austenite recrystallization zone with an average pass reduction of 11~14%. The thickness of the intermediate billet is H+(40~60)mm, where H is the thickness of the finished steel plate in mm. Finish rolling is completed in the austenite non-recrystallization zone at 60~110℃ above Ac3. The total rolling compression ratio is ≥3.5:1. (6) Quenching process: The quenching temperature is 930~950℃, the heating rate is 25℃ / min, the holding time is (1.0~1.25)·H min, and the steel plate is rapidly cooled to room temperature by water after being taken out of the furnace. The cooling rate is 50~80℃ / s. (7) Tempering process: The tempering temperature is 640~680℃, the tempering holding time is (2.35~2.85)·H min, and the steel plate is air-cooled to room temperature after being taken out of the furnace.

[0013] Preferably, in the above rolling process, the surface temperature of the billet is reduced to 1120~1140℃ by water spray cooling.

[0014] Preferably, in the above rolling process, the roughing rolling start temperature is 1125~1150℃ and the finishing rolling temperature is 1050~1075℃; the finishing rolling start temperature is 855~880℃ and the finishing rolling temperature is 835~848℃.

[0015] Application of the marine steel prepared by this invention in tropical marine engineering structural components.

[0016] To address the challenge of achieving a balance between strength, toughness, and corrosion resistance in existing high-strength marine engineering steels, this invention presents a 960MPa-grade high-strength corrosion-resistant marine engineering steel. The core of this invention is the adoption of a low-carbon and Sb-Sn composite alloying design concept. The design principles for each chemical component in the alloy are as follows: Carbon (0.09–0.135%): Carbon is the core element ensuring strength, contributing to strength through solid solution strengthening and the formation of carbide precipitates with microalloying elements. Controlling the carbon content at a low level aims to ensure high strength while significantly improving the weldability, low-temperature toughness, and crack resistance of the steel, avoiding the deterioration of the weld heat-affected zone performance caused by excessive carbon content.

[0017] Si (0.25~0.36%): As a major solid solution strengthening element, Si, while ensuring strength, utilizes its property of inhibiting cementite precipitation. Combined with gradient precooling, it promotes the formation of bainite / ferrite structures during post-rolling cooling, thus ensuring toughness while improving strength. Controlling this content within the range of 0.25~0.36% achieves the optimal balance between strengthening effect and hot-rolled surface quality and weldability.

[0018] Mn (1.10–1.60%): Mn is a key element for stabilizing austenite, improving hardenability, and strengthening through solid solution. Its lower limit (1.10%) ensures that a uniform bainitic / martensite structure is obtained at an appropriate cooling rate; its upper limit (1.60%) prevents excessive content from inhibiting phase transformation points and increasing the risk of central segregation, thereby impairing toughness and thickness properties.

[0019] Ni (1.22–1.55%): Ni is a key element for improving the corrosion resistance of steel in acidic or chloride-containing environments, and it is also an excellent toughening element. Ni can stabilize austenite, lower the ductile-brittle transition temperature of steel, and significantly improve the low-temperature toughness of steel plates, especially the weld heat-affected zone.

[0020] Mo (0.36–0.72%): Mo is a highly effective element for significantly enhancing resistance to pitting and crevice corrosion. It significantly strengthens localized corrosion resistance through enrichment in the passivation film and adsorption at corrosion points. This content range ensures its effectiveness in harsh environments, such as tropical marine climates.

[0021] Cr (0.32–0.66%): Cr is the most fundamental and crucial element determining the corrosion resistance of steel. Its mechanism of action is to promote the formation of an extremely thin, dense, and well-adhered Cr-rich oxide film on the surface of the steel. This film effectively blocks direct contact between the corrosive medium and the steel substrate, significantly improving the ability to resist uniform corrosion and pitting corrosion initiation.

[0022] Cu (0.15–0.38%): Cu is an important element for improving the weather resistance and corrosion resistance of steel in marine atmospheres. However, excessively high copper content can lead to precipitation at grain boundaries during high-temperature heating, resulting in "hot brittleness" and deteriorating hot workability. This invention strictly limits the Cu content to a narrow range of 0.15–0.38%, aiming to achieve a significant improvement in corrosion resistance brought by Cu while completely avoiding the risk of hot brittleness and its adverse effects on low-temperature toughness.

[0023] V (0.045–0.068%): V is a strong C / N compound forming element. During tempering, fine V(C,N) particles precipitate, producing a significant precipitation strengthening effect, which is key to achieving ultra-high strength of 960 MPa. Its content needs to be precisely controlled; too low a content results in insufficient strengthening, while too high a content easily leads to the formation of coarse precipitates that impair toughness.

[0024] Ti (0.012~0.021%): Ti acts as a stabilizer for nitrogen, forming fine, high-temperature stable TiN particles. These particles effectively pin austenite grain boundaries and inhibit grain coarsening during billet heating and hot rolling, resulting in fine proto-austenite grains in the final microstructure. This directly improves the steel's strength, low-temperature toughness, and resistance to brittle fracture.

[0025] Sb (0.084~0.116%) and Sn (0.092~0.118%): This invention precisely designs the content of Sb and Sn, as well as their proportions with other alloying elements. The composite addition of Sb and Sn refines the solidification structure, and the residual trace dissolved elements can optimize the rust layer structure and improve the stability of the passivation film during corrosion. Sb and Sn can effectively block the penetration of chloride ions, significantly improving the long-term weather resistance of steel in the high-salt and high-humidity environment of tropical marine climates. This invention limits the Sb and Sn content to 0.084~0.116% and 0.092~0.118% respectively because when the content is below the minimum value, the above-mentioned segregation inhibition effect and rust layer optimization effect are not obvious; while when the content is too high, it is easy to form a low-melting-point eutectic phase at the grain boundary, which deteriorates the hot working performance and weldability of the steel.

[0026] Sulfur (≤0.002%) and phosphorus (≤0.010%): P and S are extremely harmful impurities in steel. P easily leads to grain boundary segregation, causing temper brittleness; S forms sulfide inclusions, which are the source of pitting corrosion initiation and severely impair toughness. Adopting extremely low P and S design is a prerequisite for eliminating the tendency of temper brittleness at the source, reducing pitting corrosion initiation sources, and obtaining high purity and excellent toughness.

[0027] Als (0.025~0.037%): By controlling the Als content in steel, fine AlN particles can be formed, which can effectively suppress the coarsening of austenite grains during preheating and gradient precooling before rolling, providing initial conditions for grain refinement in the subsequent two-stage large deformation. On the other hand, this content avoids the increase of inclusions caused by excessive Als, ensuring the cleanliness and toughness of the steel.

[0028] The innovation of this invention lies not only in the control of a single element, but also in the synergistic optimization of multiple performance characteristics through the following key proportional relationships: (1) 1.28≤(Mn+Cr) / Mn≤1.62: This ratio comprehensively controls the stability and hardenability of austenite, ensuring the uniformity of the microstructure of the thick plate section.

[0029] (2) 0.062%≤V+Ti≤0.089% and (3) 0.11≤(V+Ti) / Cr≤0.24: The former ensures sufficient microalloying elements for precipitation strengthening; the latter precisely balances the ratio of precipitation strengthening elements (V,Ti) to passivation film forming elements (Cr), aiming to optimize the balance between the size distribution of precipitated phases and the corrosion resistance of the matrix, so as to obtain high strength while avoiding excessive carbides from destroying the corrosion resistance continuity of the matrix.

[0030] (4) 0.70≤(Ni+C) / (Mo+Mn)≤1.23 and (5) 0.085≤(Sb+Sn) / (Cr+Mo+Ni)≤0.13: This is the core relationship formula for achieving synergistic improvement of strength, toughness and corrosion resistance in this invention. The former formula controls the ratio of the main alloy system for toughness and corrosion resistance. The latter formula precisely controls the synergistic ratio between special corrosion-resistant elements (Sb, Sn) and the main corrosion-resistant alloy elements (Cr, Mo, Ni), aiming to ensure that Sb and Sn can effectively play the role of optimizing the rust layer without having an adverse effect on the main alloy elements, thereby maximizing corrosion resistance under ultra-high strength.

[0031] The reasons for setting each step and process parameter in the preparation method of this invention are as follows: (1) Smelting and refining: In the LF refining process, after the oxygen determination is completed, Sb and Sn are added for composite deoxidation, and V and Ti alloys are added simultaneously for microalloying. The use of aluminum-containing materials for deoxidation is strictly prohibited throughout the process to avoid the formation of high-melting-point Al2O3 cluster inclusions. These inclusions are difficult to remove and will seriously damage the toughness of the steel. Subsequently, RH vacuum treatment is carried out (ultimate vacuum holding time ≥18min, endpoint [H] ≤1.2ppm) for deep dehydrogenation, which is a necessary guarantee to prevent hydrogen-induced cracks (white spots) in thick plates.

[0032] (2) Continuous casting process: High homogeneity continuous casting technology is adopted to control macroscopic segregation and central porosity of the billet. Low superheat (≤15℃) and constant casting speed (≤0.72m / min) are used for casting. Low superheat can increase the proportion of equiaxed crystals and reduce central segregation; constant casting speed is conducive to the stability of the solidification process and is a prerequisite for obtaining a uniform solidification structure. Dynamic light reduction + butterfly magnetic field electromagnetic stirring is used to promote the transformation of columnar crystals to equiaxed crystals, refine the solidification structure, homogenize the composition, and apply an appropriate reduction at the end of solidification to reduce central segregation.

[0033] (3) Rolling and Cooling: The rolling process of "gradient precooling + two-stage large deformation" is adopted. Gradient precooling before rolling enhances the deformation penetration; low-speed, large-reduction rolling is carried out in the austenite recrystallization zone with a large compression ratio (≥3.5:1) and two-stage rough rolling, with the average reduction rate per pass controlled at 11%~14%. Large deformation can fully induce dynamic / static recrystallization of austenite, repeatedly breaking and refining the original grains. Rapid cooling after rolling is to fix the deformed austenite state and promote the formation of a microstructure dominated by martensite and bainite.

[0034] (4) Heat treatment: Quenching and tempering (930-950℃ quenching + 640-680℃ tempering) finally obtains a strong and toughened structure mainly composed of martensite and bainite. During the tempering process, the dispersed precipitation of V and Ti carbides further contributes to the strength.

[0035] (5) The finished marine steel produced by the method of the present invention using the above-mentioned components and processes has a thickness of 90-120 mm, a yield strength of 960-1050 MPa, a tensile strength of 1049-1150 MPa, an elongation of 18-21%, and a KV2 of 205-285 J at -60℃; the density of corrosion-active inclusions in the finished marine steel is ≤2.3 μm in size and ≤5 inclusions / mm. 2 The saturation current density under a constant electrode potential of -300mV is ≤5.5 mA / cm². 2 The marine steel produced by this invention has an original austenite grain size ≤20μm, and a microstructure consisting of 30-70% lath martensite, 20-60% strip bainite, and 10-20% granular bainite. More than 85% of the martensite laths have a width ≤0.92μm, and the granular bainite has an aspect ratio ≤2.4μm. High-density (Ti,V) is formed within the martensite and bainite laths and at the grain boundaries. X (C,N) Y The average size of the carbonitrides is ≤200 nm, and the number density reaches (5.1~6.3)×10⁻⁶. 13 pcs / m 2 The dislocation density in the tissue is (5.4~7.6)×10 16 / m 2 The Vickers hardness of the microstructure reaches 390~420HV.

[0036] The core of this invention lies in employing a composition system of "low carbon + Sb-Sn composite alloying" and controlling the morphology of inclusions at the metallurgical source through an ultra-pure smelting process, thereby reducing corrosion activity sources. Simultaneously, a "gradient pre-cooling combined with two-stage large deformation" rolling process introduces high-density dislocations into the steel plate and refines the grains, providing abundant nucleation sites for subsequent second-phase precipitation. During the quenching and tempering heat treatment, these deformation-introduced defects are effectively utilized, promoting the preferential precipitation of nanoscale C and N compounds at dislocations and grain boundaries. The dispersed precipitates and refined grains contribute significantly to the strengthening effect. Furthermore, the precipitates pinned at grain boundaries and subgrain boundaries purify the interface region, effectively hindering the intergranular diffusion of corrosive media.

[0037] Furthermore, through the synergistic design of low-carbon and Sb-Sn composite alloying, a composite passivation film with a two-layer structure can be formed in the steel during the early stages of corrosion: the inner layer is a stable oxide enriched with Sn / Sb, and the outer layer is a Cr / Fe oxide. This passivation film has a dense structure, and the high thermodynamic stability of its inner layer significantly inhibits Cl corrosion. - The eroded film dissolves. When the passivation film is affected by Cl... -When localized damage occurs due to erosion, the ultra-high density, uniformly dispersed nanoscale precipitates in the matrix can act as local "sacrificial anodes," preferentially undergoing trace dissolution and releasing Ti. 4+ V 3+ Plasma and OH in the medium - By combining these technologies, a new and stable hydroxide / oxide repair layer is dynamically generated, thereby rapidly repairing film defects and blocking pitting corrosion propagation. In summary, through full-process composition and process control, this invention achieves ultra-high strength of 960 MPa while obtaining a structurally stable passivation film with excellent self-healing capabilities, thus significantly improving the corrosion resistance of steel in tropical marine climates and overcoming the technical challenge of synergistically improving high strength and high corrosion resistance. Attached Figure Description

[0038] Figure 1 This is a diagram showing the original austenite grain distribution of the finished marine steel produced in Example 3 of this invention; Figure 2 This is a metallographic diagram of the finished marine steel produced in Example 3 of the present invention; Figure 3 It is the nano-precipitated phase in the finished marine steel produced in Example 3 of this invention; Figure 4 This is a diagram showing the dislocation distribution in the finished marine steel produced in Embodiment 3 of the present invention; Figure 5 This is a schematic diagram of Sn and Sb inclusions in the finished marine steel produced in Example 3 of this invention. Detailed Implementation

[0039] 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.

[0040] Table 1 below is a list of chemical composition values ​​for each embodiment and comparative example of the present invention; Table 2 below is a list of values ​​for the chemical composition ratios of various embodiments and comparative examples of the present invention; Table 3 below lists the main process parameter values ​​for each embodiment and comparative example of the present invention; Table 4 below lists the main performance test results of the finished marine steel products obtained from various embodiments and comparative examples of the present invention.

[0041] A method for preparing 960MPa grade corrosion-resistant marine steel suitable for tropical marine climates according to various embodiments of the present invention includes the following steps: (1) Converter smelting: The double-slag ultra-low phosphorus smelting process is adopted, and the temperature of the incoming molten iron is controlled in the range of 1280~1340℃. Top and bottom blowing is carried out throughout the process, the lance position is 1.64~1.74m, the slag basicity is 2.1~2.3, and the oxygen supply intensity is 3.45~3.60m. 3 / t·min, tapping time ≥5min, Al-containing materials shall not be used for deoxidation and alloying after the converter; (2) Ladle refining: After the ladle enters the LF furnace, it is heated to 1580~1620℃. After oxygen determination, [O] ≤ 36ppm. Sb, Sn, V and Ti are then added for deoxidation and alloying. The holding time is ≥ 25min to ensure that the alloying elements are fully dissolved. The ladle is then hoisted into the RH device for vacuum treatment. The ultimate vacuum degree is controlled at 30~40Pa. The ultimate vacuum holding time is ≥ 18min. After alloy fine-tuning, the circulation time is ≥ 8min. The final hydrogen content [H] is controlled to ≤ 1.2ppm. (3) Continuous casting process: control the superheat to 10~15℃, cast at a low superheat and constant casting speed of 0.45~0.72m / min, and the current intensity of electromagnetic stirring is 380~410A. The casting produces a 360~460mm high homogeneous continuous casting billet. The billet is stacked on-site for slow cooling after leaving the line, and the slow cooling time is ≥80h. (4) Heating of billet: The billet is uniformly heated in the heating furnace to 1180~1210℃ and held for 40~60min; (5) Rolling process: First, the billet is pre-cooled in a gradient, and then two-stage controlled rolling is carried out. The rough rolling is carried out in the austenite recrystallization zone with an average reduction rate of 11-14% per pass. The intermediate billet thickness is H+(40-60) mm, where H is the thickness of the finished steel plate in mm. The finish rolling is completed in the austenite non-recrystallization zone at 60-110℃ above Ac3, with a total rolling compression ratio ≥3.5:1. (6) Quenching process: The quenching temperature is 930~950℃, the heating rate is 25℃ / min, the holding time is (1.0~1.25)·H min, and the steel plate is rapidly cooled to room temperature by water after being taken out of the furnace. The cooling rate is 50~80℃ / s. (7) Tempering process: The tempering temperature is 640~680℃, the tempering holding time is (2.35~2.85)·H min, and the steel plate is air-cooled to room temperature after being taken out of the furnace.

[0042] In the above rolling process, the surface temperature of the billet is reduced to 1120~1140℃ by water spray cooling, while the core temperature is still controlled at 1160~1190℃.

[0043] In the above rolling process, the roughing temperature is 1125~1150℃ and the final rolling temperature is 1050~1075℃.

[0044] In the above rolling process, the initial rolling temperature for finishing rolling is 855~880℃, and the final rolling temperature is 835~848℃.

[0045] Table 1. List of chemical components (wt, %) of various embodiments and comparative examples of the present invention Table 2. Chemical composition values ​​(wt, %) of each embodiment and comparative example of the present invention for the corresponding equations. Table 3. List of main process parameter values ​​for each embodiment and comparative example of the present invention. Table 4. Performance test and analysis results of the finished marine steel products obtained from various embodiments and comparative examples of the present invention. Note: The saturation current density test was conducted at room temperature using a three-electrode system on an electrochemical workstation. The working electrode was the steel sample to be tested, the counter electrode was a platinum electrode, and the reference electrode was an AgCl electrode. The electrolyte was a simulated seawater medium prepared by dissolving 9.926 g NaCl + 3.416 g MgCl2 in 1 liter of ultrapure water. Natural aeration was continuously maintained during the test to keep the solution stable. Before the test, the sample surface was sanded to 2000# with sandpaper and polished, then ultrasonically cleaned with acetone and anhydrous ethanol, and dried with cold air. Before the formal potentiostatic test, an open-circuit potential test was performed. After the open-circuit potential stabilized, a potentiostatic polarization test was performed at -300 mV (relative to the open-circuit potential), and the saturation current density was recorded.

[0046] As shown in Table 4 above, the 960MPa grade high-strength and high-corrosion-resistant marine steel prepared by this invention has a yield strength ≥960MPa, tensile strength ≥1049 MPa, impact energy KV2 ≥205 J at -60℃, elongation ≥18%, and corrosion current density ≤5.5 mA / cm² at a constant potential of -300mV. 2 This invention successfully achieves a synergistic effect of high strength, excellent low-temperature toughness, and outstanding corrosion resistance. The low-carbon and microalloying design (C: 0.09–0.135%, 0.062% ≤ V + Ti ≤ 0.089%) also ensures that the marine steel produced by this invention has excellent machinability in subsequent use.

[0047] The superior performance is attributed to the innovative composition and process of this invention. In terms of composition, ultra-low P and S (S≤0.002%, P≤0.010%) purifies the steel from the source, and the Cu-Sb-Sn composite corrosion-resistant system, combined with a key ratio of 0.08≤(Sb+Sn) / (Cr+Mo+Ni)≤0.13, optimizes the rust layer and passivation film. In terms of process, Sb / Sn and Ti / V microalloying, along with "gradient pre-cooling + two-stage large deformation rolling," achieves a fine-grained structure (strip width ≤0.92μm), ensuring the high strength of the marine engineering steel. Simultaneously, precisely controlling the ratio of 0.70≤(Ni+C) / (Mo+Mn)≤1.23 is crucial for balancing strength and toughness. This invention, through this design, maintains high strength while ensuring an impact energy ≥205 J at -60℃, thus systematically solving the industry challenge of simultaneously achieving "strength-toughness-corrosion resistance" in ultra-high-strength marine engineering steel.

[0048] As can be seen from Tables 1, 2, and 4 above, Comparative Example 1 adopted a design with higher C, Mn, and V+Ti contents, which improved the yield strength. However, its toughening element Ni content (0.75%) was lower than the reasonable ratio range of this invention (1.22%~1.55%), resulting in lower impact toughness. Simultaneously, excessive V and Ti additions easily caused coarsening of precipitates and micro-alloying element segregation in the core of thick plates, forming micron-sized V and Ti inclusions, which became preferential sites for crack and corrosion initiation, further impairing toughness and corrosion resistance. Regarding the key ratio for corrosion resistance, the (Sn+Sb) / (Cr+Mo+Ni) ratio was only 0.047, lower than the lower limit of this invention (≥0.08). This imbalance weakened the steel's ability to form a stable passivation film in corrosive media, manifested as a significant increase in corrosion current density and a decrease in seawater corrosion resistance.

[0049] Comparative Example 2 improved impact toughness by increasing the content of toughening elements such as Ni and Cr, but this resulted in the ratios of (Mn+Cr) / Mn and (Ni+C) / (Mo+Mn) (2.029 and 1.468, respectively) exceeding the upper limit of the reasonable range of this invention (1.62 and 1.23). The excessively high ratios led to insufficient hardenability, making it difficult to form ≥30% volume fraction of martensite and fine lath structure during heat treatment. The final yield strength was only 914 MPa, failing to meet the 960 MPa strength requirement for high-performance marine steel.

[0050] Conclusion: Although Comparative Example 1 and Comparative Example 2 both use the same composition system and production process as this invention, the different specific component values, alloy element ratios, and synergistic effects result in the steel produced by Comparative Example 1 and Comparative Example 2 being completely unsuitable for tropical marine engineering.

[0051] See Figure 1 , Figure 2, Figure 1 and Figure 2 The images shown are metallographic images and microstructure images of the original austenite grain distribution of the marine steel prepared in Example 3 of this invention. Figure 1 It can be seen that the original austenite grain size of the marine steel prepared in Example 3 is 19.6 μm. Figure 2 The data shows that after quenching and tempering heat treatment, the microstructure of the steel mainly consists of lath martensite and fine strip bainite, with a small amount of granular bainite. The volume fractions of lath martensite and strip bainite are about 40% and 50% respectively, and the remainder is granular bainite. More than 85% of the martensite laths have a width ≤0.92μm, and the aspect ratio of the granular bainite is ≤2.4μm.

[0052] Figure 3 and Figure 4 These figures show the high-density nano-precipitates and dislocation distribution dispersed within the martensite and bainite laths of the marine steel prepared in Example 3 of this invention. The tempered martensite structure provides excellent comprehensive mechanical properties, while the bainite structure contributes to a good balance between strength and toughness. In the harsh corrosive environment of tropical marine climates with high temperature, high humidity, high salt spray, and strong radiation, the thermal and chemical stability of the microstructure is crucial for the long-term corrosion resistance of the material. This invention, through precise control of tempering process parameters (temperature and time), induces precipitation with an average size ≤200 nm and a number density of 5.8 × 10⁻⁶ within the martensite and bainite laths and at grain boundaries. 13 pcs / m 2 High density (Ti,V) X (C,N) Y Carbonitride-like compounds. Simultaneously, this process increases the dislocation density in the microstructure to 6.6 × 10⁻⁶. 16 / m 2 This process achieves a uniform and dispersed distribution of dislocations and nano-precipitates. This microstructure not only effectively improves the steel's resistance to pitting corrosion and stress corrosion cracking, but also ensures that the material's excellent weldability and low-temperature toughness are not compromised.

[0053] Figure 5 This is a scanning electron microscope (SEM) image of typical inclusions in the steel of Example 3 of the present invention. As shown in the figure, after the Sb-Sn composite alloying refining process, the oxide inclusions in the steel are modified into fine, nearly spherical Sn and Sb-containing composite oxides. Furthermore, the purification smelting process resulted in an average inclusion size of 2.1 μm and a density controlled at 4.2 inclusions / mm². 2 The spherical Sn / Sb composite oxides, through morphological rounding, eliminate interfacial gaps at the edges of irregular inclusions, thus reducing the corrosive medium (Cl) in seawater. -The doping of Sn and Sb elements significantly enhances the chemical stability of the inclusions, suppressing their tendency to dissolve at the interface at the corrosion potential. This dual mechanism of "physical isolation + chemical inertness" makes these inclusions less likely to become a preferential source of pitting corrosion, thus maintaining a low saturation current density (5.38 mA / cm²). 2 This improves the overall corrosion resistance.

[0054] The above embodiments are merely specific examples exemplified to explain 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. The present invention is not limited to the specific embodiments described above.

Claims

1. A 960MPa grade corrosion-resistant marine steel suitable for tropical marine climates, characterized in that... The marine steel comprises the following chemical composition by mass percentage: C: 0.09–0.135%, Si: 0.25–0.36%, Mn: 1.10–1.60%, P≤0.010%, S≤0.002%, Ni: 1.22–1.55%, Mo: 0.36–0.72%, Cr: 0.32–0.66%, V: 0.045–0.068%, Ti: 0.012–0.021%, Cu: 0.15–0.38%, Sb: 0.084–0.116%, Sn: 0.092–0.118%, Als: 0.025–0.045%, with the remainder being Fe and unavoidable impurities; and the contents of the above elements must simultaneously satisfy the following relationship: (1)1.28≤(Mn+Cr) / Mn≤1.62; (2) 0.062%≤V+Ti≤0.089%; (3)0.11≤(V+Ti) / Cr≤0.24; (4)0.70≤(Ni+C) / (Mo+Mn)≤1.23; (5)0.085≤(Sb+Sn) / (Cr+Mo+Ni)≤0.13; The original austenite grain size in the finished marine steel is ≤20μm, and the microstructure is composed of lath martensite with a volume fraction of 30-70%, strip bainite with a volume fraction of 20-60%, and granular bainite with a volume fraction of 10-20%. Moreover, the width of more than 85% of the martensite laths is ≤0.92μm, and the aspect ratio of the granular bainite is ≤2.4μm. High-density (Ti,V) is formed within the laths of the lath martensite and the grain boundaries. X (C,N) Y The average size of the carbonitrides is ≤200 nm, and the number density reaches (5.1~6.3)×10 13 pcs / m 2 The dislocation density in the tissue is (5.4–7.6) × 10⁻⁶. 16 / m 2 The Vickers hardness of the microstructure reaches 390-420 HV.

2. The 960MPa grade corrosion-resistant marine steel suitable for tropical marine climates according to claim 1, characterized in that... The marine steel comprises the following chemical composition by mass percentage: C: 0.105–0.130%, Si: 0.26–0.34%, Mn: 1.16–1.43%, P≤0.010%, S≤0.002%, Ni: 1.26–1.43%, Mo: 0.40–0.56%, Cr: 0.41–0.59%, V: 0.045–0.058%, Ti: 0.012–0.018%, Cu: 0.18–0.35%, Sb: 0.084–0.110%, Sn: 0.094–0.115%, Als: 0.025–0.037%, with the remainder being Fe and unavoidable impurities.

3. A 960MPa grade corrosion-resistant marine steel suitable for tropical marine climates according to claim 1 or 2, characterized in that: The finished marine steel has a thickness of 90–120 mm, a yield strength of 960–1050 MPa, a tensile strength of 1049–1150 MPa, an elongation of 18–21%, and a KV2 of 205–285 J at -60℃.

4. A 960MPa grade corrosion-resistant marine steel suitable for tropical marine climates according to claim 1 or 2, characterized in that: The resulting marine steel contains corrosion-active inclusions with a density of ≤2.3μm in size and ≤5 inclusions / mm². 2 The saturation current density under a constant electrode potential of -300mV is ≤5.5 mA / cm². 2 .

5. A method for preparing 960MPa grade corrosion-resistant marine steel suitable for tropical marine climates, characterized in that... Includes the following steps: (1) Converter smelting: The double-slag ultra-low phosphorus smelting process is adopted, and the temperature of the incoming molten iron is controlled in the range of 1280-1340℃. Top and bottom blowing is carried out throughout the process, the lance position is 1.64-1.74m, the slag basicity is 2.1-2.3, and the oxygen supply intensity is 3.45-3.60m. 3 / t·min, tapping time ≥5min, Al-containing materials shall not be used for deoxidation and alloying after the converter; (2) Ladle refining: After the ladle enters the LF furnace, it is heated to 1580-1620℃. After oxygen determination, [O] ≤ 36ppm. Sb, Sn, V and Ti are then added for deoxidation and alloying. The holding time is ≥ 25min to ensure that the alloying elements are fully dissolved. Subsequently, the ladle is hoisted into the RH device for vacuum treatment. The ultimate vacuum degree is controlled at 30-40Pa. The ultimate vacuum holding time is ≥ 18min. After alloy fine-tuning, the circulation time is ≥ 8min. The final hydrogen content [H] is controlled to ≤ 1.2ppm. The molten steel that has been smelted to a qualified state contains the following chemical composition by mass percentage: C: 0.09-0. 0.135%, Si: 0.25-0.36%, Mn: 1.10-1.60%, P≤0.010%, S≤0.002%, Ni: 1.22-1.55%, Mo: 0.36-0.72%, Cr: 0.32-0.66%, V: 0.045-0.068%, Ti: 0.012-0.021%, Cu: 0.15-0.38%, Sb: 0.084-0.116%, Sn: 0.092-0.118%, Als: 0.025-0.045%, with the remainder being Fe and unavoidable impurities; (3) Continuous casting process: control the superheat to 10-15℃, cast at a low superheat and constant speed of 0.45-0.72m / min, and the current intensity of electromagnetic stirring is 380-410A. Casting yields a 360-460mm high homogeneous continuous casting billet. The billet is stacked on-site for slow cooling after leaving the line, with a slow cooling time of ≥80h. (4) Heating of billet: The billet is uniformly heated in the heating furnace to 1180-1210℃ and held for 40-60 minutes; (5) Rolling process: First, the billet is pre-cooled in a gradient. Specifically, the surface temperature of the billet is reduced to 1120-1140℃, while the core temperature is controlled at 1160-1190℃. Then, two-stage controlled rolling is carried out. Rough rolling is carried out in the austenite recrystallization zone with an average pass reduction of 11-14%. The thickness of the intermediate billet is H+(40-60) mm, where H is the thickness of the finished steel plate in mm. Finish rolling is completed in the austenite non-recrystallization zone at 60-110℃ above Ac3. The total rolling compression ratio is ≥3.5:

1. (6) Quenching process: The quenching temperature is 930~950℃, the heating rate is 25℃ / min, the holding time is (1.0~1.25)·H min, and the steel plate is rapidly cooled to room temperature by water after being taken out of the furnace. The cooling rate is 50~80℃ / s. (7) Tempering process: The tempering temperature is 640~680℃, the tempering holding time is (2.35~2.85)·H min, and the steel plate is air-cooled to room temperature after being taken out of the furnace.

6. The method for preparing a 960MPa grade corrosion-resistant marine steel suitable for tropical marine climates according to claim 5, characterized in that: The rolling process reduces the surface temperature of the billet to 1120–1140°C by using water spray cooling.

7. The method for preparing a 960MPa grade corrosion-resistant marine steel suitable for tropical marine climates according to claim 5, characterized in that: In the rolling process, the roughing rolling temperature is 1125-1150℃ and the finishing rolling temperature is 1050-1075℃; the finishing rolling temperature is 855-880℃ and the finishing rolling temperature is 835-848℃.

8. The application of marine steel according to claim 1 in tropical marine engineering structural components.

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