High-strength and high-toughness steel for marine engineering equipment with excellent work hardening capacity and manufacturing method thereof

Abstract

This invention discloses a high-strength, high-toughness steel for marine engineering equipment with excellent work hardening ability and its manufacturing method. The steel's chemical composition by weight percentage is: C = 0.08%–0.15%, Si = 0.10%–0.20%, Mn = 0.50%–1.00%, Ni = 0.30%–0.80%, Cr = 0.50%–1.50%, Mo = 0.10%–0.50%, Cu = 0.10%–0.50%, Nb or V = 0.02%–0.04%, and Al = 0.05%–0.08%. The composition is as follows: B = 0.0005%~0.0015%, Ti ≤ 0.01%, N ≤ 0.005%, P ≤ 0.010%, S ≤ 0.006%, with the remainder being Fe and unavoidable impurities. The technical solution of this invention adopts direct quenching after hot rolling and tempering heat treatment to obtain a multiphase structure of tempered martensite, restored ferrite and carbides. The work hardening rate during the deformation stage from yield strength to tensile strength is higher than 5000MPa, the yield strength ratio is lower than 0.90, the yield strength is higher than 690MPa, and the Charpy impact energy at -60℃ is higher than 150J.

Description

Technical Field

[0001] This invention belongs to the field of metal material manufacturing technology, and relates to a high-strength and high-toughness steel for marine engineering equipment with excellent work hardening ability and its manufacturing method. Background Technology

[0002] In international classification society (ICS) standards, the yield strength range of ultra-high strength marine engineering steel is from 420 MPa to 960 MPa. It employs a weldable, low-carbon, low-alloy composition system, typically with added alloying elements such as Ni, Cr, Mo, Cu, and V. Grades of 690 MPa and above require a quenching followed by high-temperature tempering heat treatment. High-temperature tempering causes martensite recovery, diffusion of supersaturated C in the distorted lattice, and the formation of carbides with Cr, Mo, and V, reducing dislocation density. This significantly reduces high-density dislocation strengthening and solid solution carbon strengthening, resulting in a simultaneous decrease in yield strength and tensile strength. The tensile stress-strain curve becomes flatter, significantly reducing work hardening capacity. The work hardening rate drops immediately after over-yielding until necking fracture occurs. Therefore, the yield strength (YS) to tensile strength (UTS) ratio of quenched and tempered ultra-high strength steel is typically higher than 0.95, indicating limited strength reserve after plastic deformation. This makes it prone to localized plastic instability and fracture after forming, increasing the service safety risks of marine engineering structures.

[0003] Therefore, the steel selection specifications for marine structure design clearly stipulate that the yield strength ratio of steel with a strength of 420~960MPa should be ≤0.94. However, how to reduce the yield strength ratio of ultra-high strength steel, especially steel with a strength of 690MPa and above, has become a technical challenge. The key to controlling the yield strength ratio above 690MPa lies in improving the work hardening capacity after yielding, optimizing alloy composition and processes to regulate the contribution of various strengthening mechanisms, and increasing tensile strength by stabilizing the high work hardening rate after yielding while ensuring yield strength.

[0004] Chinese patent CN116516252A discloses a 1200MPa ultra-high strength and ductility hot-rolled Mn-TRIP steel with a yield strength of 850-950MPa, tensile strength of 1200-1400MPa, elongation after fracture of 30%-35% or more, and hole expansion rate ≥60%, as well as its preparation method. The steel is designed with a composition of C: 0.23%-0.33%, Si: 0.45%-1.55%, Mn: 6.8%-96%, and Al: 2.8%-4.8%. However, its carbon equivalent (Ceq) far exceeds the specifications for marine engineering steel, making it unsuitable for welded marine engineering structures.

[0005] Chinese patent CN115181886A discloses a method for manufacturing 980MPa grade low-carbon low-alloy duplex steel and rapid heat treatment. By rapidly heat treating to alter the recovery, recrystallization, and austenitic phase transformation processes of the deformed microstructure, the prepared duplex steel exhibits a yield strength of 598–749 MPa, a tensile strength of 1030–1090 MPa, an elongation of 10.6–16.6%, a strength-ductility product of 109–174 GPa%, and a strain hardening index n90 value greater than 0.21. However, the method provided by this patent requires hot rolling, coiling, cold rolling, and rapid heat treatment, limiting the width and thickness of the steel plate. This makes it unsuitable for medium-thick steel plate production processes and equipment, and thus unsuitable for marine engineering structures.

[0006] Chinese patent CN111448332B discloses a method for manufacturing high-strength steel plates with a tensile strength of 780 MPa or higher, low yield strength ratio, excellent ductility, and work hardening index, exhibiting excellent processing performance. However, this technology targets high-strength steel plates for automotive structural components, employing a high-Si, high-Mn, and high-Al alloy composition, and is produced through hot-rolled coils, cold rolling, and continuous annealing processes, which does not meet the requirements for use in marine engineering structures. Summary of the Invention

[0007] This invention aims to provide a high-strength and high-toughness steel for marine engineering equipment with excellent work hardening ability and its manufacturing method. By optimizing the alloy composition, controlling rolling and cooling, and using high-temperature tempering processes, the proportion of bainite, martensite, and alloy carbides in the multiphase microstructure is controlled, thereby improving the work hardening rate during the deformation stage after yielding and solving the technical problems of formability and local plastic failure of ultra-high strength marine engineering steel.

[0008] To achieve the above-mentioned objectives, the present invention adopts the following technical solution:

[0009] A high-strength, high-toughness steel for marine engineering equipment with excellent work hardening ability. The steel's chemical composition by weight percentage is: C = 0.08%–0.15%, Si = 0.10%–0.20%, Mn = 0.50%–1.00%, Ni = 0.30%–0.80%, Cr = 0.50%–1.50%, Mo = 0.10%–0.50%, Cu = 0.10%–0.50%, Nb or V = 0.02%–0.04%, Al = 0.05%–0.08%, B = 0.0005%–0.0015%, Ti ≤ 0.01%, N ≤ 0.005%, P ≤ 0.010%. S≤0.006%, the remainder being Fe and unavoidable impurities; the volume percentage of tempered martensite in the microstructure of the steel plate is 75%~85%, the volume percentage of recovered polygonal ferrite is 13.5%~23.5%, and the volume percentage of carbides is 0.5%~1.5%.

[0010] A method for manufacturing high-strength and high-toughness steel for marine engineering equipment with excellent work hardening ability, comprising the following steps:

[0011] 1) Hot metal pretreatment: First, the hot metal is desulfurized by mixing magnesium desulfurizing agent with hot metal at a temperature of not less than 1300℃ to ensure that the sulfur content of the hot metal is not higher than 0.002%.

[0012] 2) Converter refining: The pretreated molten iron is poured into the converter for smelting. The treatment time is 25-30 minutes to remove excess carbon, phosphorus and sulfur from the molten iron, so that C≤0.12%, P≤0.010% and S≤0.008%, and the steel is tapped when the temperature reaches 1650±30℃.

[0013] 3) Ladle refining: During the tapping process, the converter adds a pre-calculated FeMn, FeSi, FeNi, FeAl, FeCr, Cu, and Mo alloy; the ladle is transported to the LF ladle refining station, bottom blowing argon is turned on, graphite electrodes are used for heating, lime and aluminum are added to adjust the slag and further deoxidize it, and after treatment for 25 to 30 minutes, the temperature of the molten steel is adjusted to 1600±10℃;

[0014] 4) Vacuum degassing: After refining in the LF furnace, the molten steel is sent to the RH / VD vacuum treatment station and ferroboron is added to the molten steel. Then, vacuum degassing is carried out for no less than 10 minutes in a vacuum environment of no more than 0.5tor. When the temperature of the molten steel reaches 1560±10℃, heat preservation agent is added.

[0015] 5) Tundish casting: The ladle after vacuum treatment is hoisted to the continuous casting station and the molten steel is diverted from the bottom of the ladle to the tundish. The flow rate and temperature of the molten steel are controlled. The temperature of the molten steel in the tundish is controlled at 1535-1550℃. Argon gas sealing and covering agent protection are carried out throughout the casting process. The molten steel flows into the crystallizer through the submerged entry nozzle.

[0016] 6) Slab continuous casting: Molten steel from the tundish is poured into a cooling rectangular copper plate crystallizer through an immersion nozzle. The casting superheat is controlled at 15-25°C. The molten steel in contact with the copper wall of the crystallizer solidifies into a slab shell. The slab is removed from the crystallizer by the crystallizer vibration and the dummy bar and enters the foot roll area clamping roller table, the secondary cooling zone roller table, the fan-shaped bending roller and the straightening roller. It is then flame-cut to obtain continuous casting slabs of various lengths and thicknesses of 200-320mm.

[0017] 7) Slab heating: The continuously cast slab is sent to the walking beam furnace and heated to 1150-1230℃ in stages, and held in the soaking zone for 1-3 hours to control the austenite grain size at 50-100μm;

[0018] 8) Controlled rolling: The continuously cast slab is pushed out of the heating furnace and sent to the descaling machine via conveyor rollers. The surface iron oxide scale is removed by high pressure water, and then sent to the four-high rolling mill for the first stage of controlled rolling in the high temperature zone of 1050~950℃. The intermediate slab is cooled to 850~900℃ on the waiting roller table for the second stage of controlled rolling.

[0019] 9) Online linear quenching: After final rolling, the steel plate is conveyed by roller conveyor and enters the laminar flow cooling zone for two-stage accelerated cooling;

[0020] 10) Tempering heat treatment: The steel plate after direct quenching is transported to a tempering furnace for high-temperature tempering heat treatment. The tempering temperature is 550~650℃ and the holding time is 2.5 times the plate thickness mm×min / mm.

[0021] Further, step 8) Controlled rolling: In the first stage of controlled rolling, the average single-pass reduction in the high-temperature section above 950℃ is more than 30mm, and the intermediate billet thickness reaches 2 to 5 times the target thickness of the finished product; in the second stage of controlled rolling, the cumulative reduction reaches 50% to 80%, and the steel plate is rolled to the target size and shape requirements of the finished product.

[0022] Further, in step 9), online linear quenching: In the first stage, for the first 1 / 3 of the cooling zone, a weak cooling mode is used to control the cooling rate of the steel plate at 5-10℃ / s, and the steel plate temperature reaches 620-660℃, so that the portion with a volume percentage not exceeding 25% undergoes proeutectoid ferrite phase transformation; in the second stage, for the last 2 / 3 of the cooling zone, a strong cooling direct quenching mode is used to control the cooling rate of the steel plate at 15-30℃ / s, until the final cooling temperature is below 300℃, ensuring that the remaining austenite undergoes martensitic phase transformation and has a content of more than 75%.

[0023] The mechanism of action of each alloy component in the ultra-high strength marine engineering steel with excellent work hardening ability manufactured by this invention is as follows:

[0024] Carbon (C) is an essential element for controlling the phase transformation structure and improving strength during quenching. It remains in a solid solution state within austenite and, after direct quenching, forms high-dislocation-density martensite with severe lattice distortion, providing strong dislocation strengthening. During tempering, it precipitates carbide particles along with Fe, Cr, Mo, and V, ensuring strength stability. However, excessively high C content not only easily leads to banded structures, which are detrimental to plasticity and toughness, but also tends to form martensite in the weld heat-affected zone, reducing the toughness of the welded area. Considering overall performance, a C content of 0.08–0.15% is preferred.

[0025] Si is a deoxidizing element in the ladle after converter refining and can also play a certain role in solid solution strengthening. At the same time, Si can inhibit the precipitation and coarsening of cementite, thereby improving toughness. However, it is easy to accumulate on the surface during the slab heating process to form ferroolitic, which affects the descaling effect of iron oxide scale. Therefore, the preferred Si content is 0.10 to 0.20%.

[0026] Mn is the key element for ensuring martensite formation and providing solid solution strengthening. Excessive Mn content inhibits the austenite-to-ferrite transformation, hindering the formation of proeutectoid ferrite during the first stage of accelerated online cooling after rolling. Insufficient Mn content prevents sufficient martensite formation during the second stage of rapid cooling and direct quenching. Therefore, the Mn content ranges from 0.50% to 1.20%, with a preferred range of 0.70% to 1.00%.

[0027] Ni is a key element for improving the toughness and hardenability of steel, and also plays a role in solid solution strengthening. In steel, Ni lowers the activation energy of dislocation movement and enhances dislocation mobility during deformation, thereby improving the plasticity and low-temperature toughness of the steel. However, Ni is also a high-cost alloy; therefore, the Ni content in this invention is no higher than 0.30–0.80%, preferably in the range of 0.50–0.70%.

[0028] Cr is a key element for improving the hardenability of steel and promotes martensite formation during the quenching process. Simultaneously, Cr is also an austenite-refining element and a strong carbide-forming element, which can significantly improve resistance to temper brittleness and enhance the low-temperature toughness of tempered steel plates. Therefore, the Cr content in this invention ranges from 0.50% to 1.50%, with a preferred range of 0.80% to 1.20%.

[0029] Mo is a key element for improving the hardenability of steel. Together with C, it forms nano-sized carbides MoC, which have a significant precipitation strengthening effect. Therefore, the Mo content in this invention ranges from 0.10% to 0.50%, with a preferred range of 0.20% to 0.40%.

[0030] During tempering, Cu precipitates a nano-sized Cu-rich phase, which plays a role in precipitation strengthening. Therefore, the Cu content of this invention ranges from 0.15% to 0.50%, with a preferred range of 0.20% to 0.30%.

[0031] Nb or V: strong carbide-forming microalloying elements that precipitate nano-sized NbC or VC during tempering, providing effective precipitation strengthening. Therefore, the present invention uses either Nb or V, with a content ranging from 0.02% to 0.04%.

[0032] V possesses precipitation strengthening, grain refinement, and improved resistance to HIC and sulfide stress cracking. In steel, it forms nano-sized VC and V(C,N) particles with C and N, thereby increasing strength and hindering the austenite-to-ferrite phase transformation, achieving grain refinement and toughening effects. Simultaneously, it acts as a hydrogen trap, enhancing resistance to hydrogen-induced cracking and sulfide stress cracking in acidic environments. Therefore, the V content in this invention ranges from 0.005% to 0.04%, preferably from 0.01% to 0.03%.

[0033] Ti has the function of refining austenite grain size and improving weldability. During high-temperature refining, it forms TiN particles with free N in the molten steel, which acts as a solid N, protecting the added B in a solid solution state and improving hardenability. During the slab heating process, it effectively inhibits austenite grain growth, and during the welding process, the grains in the heat-affected zone coarsen, improving the toughness of the weld. Therefore, the Ti content of this invention ranges from 0.008% to 0.020%, with a preferred range of 0.010% to 0.015%.

[0034] Al plays a strong role in deoxidation and denitrogenation during the refining process, forming Al2O3 and AlN inclusions. This ensures that the free oxygen content in the molten steel is no higher than 10 ppm and the free nitrogen content is no higher than 40 ppm, thus ensuring that the added boron does not form BN in the steel and remains in a solid solution state. Therefore, the Al content in this invention ranges from 0.05% to 0.08%, with a preferred range of 0.055% to 0.065%.

[0035] Boron (B) is a strong hardenability element. Even trace amounts dissolved and segregated at the austenite grain boundaries in steel can effectively enhance the martensite formation ability during the quenching process. Boron is also a strong nitride-forming element; therefore, it must be used in combination with Al in this invention, with a content ranging from 0.0005% to 0.0015%, preferably from 0.0008% to 0.0012%.

[0036] Nitrogen (N) is an unavoidable gaseous element in steel. During the steelmaking process, it readily forms sharp-angled nitride particles with Ti, Al, and B, which agglomerate at the austenite grain boundaries, negatively impacting the steel's toughness. Therefore, the N content in this invention is no higher than 0.005%, preferably no higher than 0.003%.

[0037] P is an element that is detrimental to the low-temperature toughness and ductility of ultra-high strength marine engineering steel. It tends to agglomerate in the center of the slab during continuous casting and segregate at the original austenite grain boundaries, reducing the crystallization bonding force and impairing toughness. In this invention, P is controlled to be no higher than 0.010%, preferably no higher than 0.005%.

[0038] S is a harmful element that is detrimental to toughness and plasticity. It forms large-sized MnS inclusions in the core segregation and Mn segregation zone of the billet. During the rolling stage, it is transformed into a long strip shape with sharp ends, which becomes a crack source. Therefore, the S content of the material of this invention is controlled to be no higher than 0.006%, and preferably no higher than 0.003%.

[0039] The beneficial effects of this invention are as follows: 1) Low cost and simple alloy composition: Using low C and Mn+Cr as the two main alloys and Nb or V as a single microalloying element, it has the advantages of low carbon equivalent, simple composition and low cost, reducing the difficulty of refining and other processes; 2) High-efficiency preparation process: Using controlled rolling and online direct quenching process, the traditional offline reheating and quenching process of tempered ultra-high strength marine engineering steel is eliminated, improving production efficiency and reducing energy consumption in the production process; 3) Effectively improve work hardening ability: Using a two-stage controlled cooling process of weak at the beginning and strong at the end, a two-phase structure of a small amount of proeutectoid ferrite soft phase and a main martensite hard phase is obtained in the wide and thick plate, which effectively improves the work hardening ability during plastic deformation and its stability after over-yielding, and obtains a low yield strength ratio that is difficult to achieve with traditional offline quenching and tempering ultra-high strength marine engineering steel, thereby improving the service safety of marine engineering structures. Attached Figure Description

[0040] Figure 1 A schematic diagram of the online direct quenching process for controlled rolling and two-stage controlled cooling.

[0041] Figure 2 To compare the scanning electron microscope images of the fully martensitic structure of the steel plate.

[0042] Figure 3 This is a photograph of the microstructure of the steel plate in Example 1.

[0043] Figure 4 This is a photograph of the microstructure of the steel plate in Example 2.

[0044] Figure 5 Example 3: Micrograph of the steel plate.

[0045] In the diagram: 1-Heating furnace; 2-Slab; 3-High-pressure water descaling; 4-Four-roll reversible hot rolling; 5-Laminar flow weak cooling; 6-Strong cold direct quenching; 7-Multi-roll straightening; 8-Finished steel plate. Detailed Implementation

[0046] The following examples are used to illustrate the point by comparing them with comparative steel.

[0047] The chemical composition (mass percentage) of the steels used in the examples and comparisons is shown in Table 1; the controlled rolling and controlled cooling direct quenching or process of the steels used in the examples and comparisons is shown in Table 2.

[0048] The test results of the mechanical properties and HIC resistance of the steel in the examples and comparisons are shown in Table 3.

[0049] Compared to steel:

[0050] A method for manufacturing steel for marine engineering equipment includes the following production steps: After treatment in an LF furnace and an RH furnace for 25 minutes and 15 minutes respectively, the steel is cast in a tundish at a superheat of 24°C, continuously cast into a 260mm thick slab. The slab is homogenized at 1210°C and held for 120 minutes. After exiting the furnace, it undergoes high-pressure water descaling. The initial rolling temperature is 1080°C, rough rolling to a 130mm thick intermediate slab with an average single-pass reduction of 22mm. A second-stage finish rolling is then performed at 880°C, with a cumulative reduction of 92mm. The final rolling temperature is 850°C, followed by ACC accelerated cooling at a rate of 25°C / s to 260°C. After straightening, the steel is transported to a cooling bed. After cooling, the steel plate is sent to a heating furnace for tempering heat treatment at 600°C ± 10°C for 95 minutes. Figure 2 and Figure 3 To compare the microstructure of the steel plate, the martensite volume percentage was 100%. Example 1

[0051] A high-strength, high-toughness steel for marine engineering equipment with excellent work hardening ability and its manufacturing method are disclosed. The production process includes the following steps: after treatment in an LF furnace for 22 minutes and an RH furnace for 16 minutes, the steel is cast in a tundish at a superheat of 23°C, continuously cast into a 260mm thick slab. The slab is homogenized at 1210°C and held for 120 minutes. After exiting the furnace, it undergoes high-pressure water descaling. The initial rolling temperature is 1080°C, rough rolling to an 80mm thick intermediate slab with an average single-pass reduction of 25mm. A second-stage finish rolling is then performed at 820°C with a cumulative reduction of 65mm. The final rolling temperature is 780°C. The steel is then accelerated cooling to 270°C at a rate of 36°C / s using ACC cooling. After straightening, it is transported to a cooling bed. After cooling, the steel plate is sent to a heating furnace for tempering heat treatment at 600°C ± 10°C for 40 minutes. Figure 4 The microstructure of the core of Example 1 is shown, with tempered martensite accounting for 84%, ferrite accounting for 15.5%, and carbide accounting for 0.5%.

[0052] Figure 1 As shown: The microstructure of the steel plate in Example 1 is 84% ​​martensite + 15.5% polygonal ferrite. Example 2

[0053] A high-strength and high-toughness marine engineering equipment steel with excellent work hardening ability and its manufacturing method. The production process steps are as follows: after being treated in an LF furnace and an RH furnace for 21 min and 23 min respectively, the superheating temperature is 21℃ during casting in a ladle, and a 360mm thick slab is formed by continuous casting. The macrosegregation level in the core is Class B 0.5. After being homogenized at 1220℃ and held for 210 minutes, the slab was descaled by high-pressure water after exiting the furnace. The initial rolling temperature was 1082℃, and the slab was rough rolled to a thickness of 180mm. The average single-pass reduction was 38mm. The second stage of finishing rolling was carried out at 790℃, with a cumulative reduction of 120mm. The final rolling temperature was 780℃. The slab was then cooled to 310℃ at an ACC accelerated cooling rate of 18℃ / s. After straightening, the slab was transported to a cooling bed. After cooling, the slab was sent to a heating furnace for tempering heat treatment at 630℃±10℃ for 100 minutes. Figure 4 The microstructure of the core of Example 2 is shown, with tempered martensite accounting for 76%, ferrite accounting for 22.5%, and carbide accounting for 1.5%.

[0054] Figure 2 As shown: The microstructure of the steel plate in Example 2 is 76% martensite + 22.5% polygonal ferrite. Example 3

[0055] A high-strength, high-toughness steel for marine engineering equipment with excellent work hardening ability and its manufacturing method are disclosed. The production process includes the following steps: after treatment in an LF furnace and an RH furnace for 23 min and 24 min respectively, the steel is cast in a tundish with a superheat of 18°C, continuously cast into a 300 mm thick slab with a core macrosegregation level of Class B, 0.5. The slab is homogenized at 1210°C and held for 160 min. After exiting the furnace, it undergoes high-pressure water descaling. The initial rolling temperature is 1078°C, rough rolling to a 150 mm thick intermediate slab with an average single-pass reduction of 30 mm. A second-stage finish rolling is then performed at 792°C with a cumulative reduction of 120 mm. The final rolling temperature is 780°C. The steel is then accelerated cooling to 270°C at a rate of 26°C / s using ACC, followed by straightening and transport to a cooling bed. After cooling, the steel plate is sent to a heating furnace for tempering heat treatment at 610°C ± 10°C for 75 min. Figure 4 The microstructure of the core of Example 3 is shown, with tempered martensite accounting for 79%, ferrite accounting for 20%, and carbide accounting for 1.0%.

[0056] Figure 3 As shown: The microstructure of the steel plate in Example 3 is 79% martensite + 20% polygonal ferrite.

[0057] Table 3 lists the mechanical property test results of the comparative steel and the steel produced in Examples 1-3 of this invention. Table 3 shows that, after adopting the alloy composition, steelmaking and controlled rolling and controlled cooling direct quenching and tempering process of this invention, the microstructure of the examples, which is mainly martensite and supplemented by polygonal ferrite, has a high work hardening rate and has significant advantages in tensile properties, low yield strength ratio and Charpy impact energy at -60℃, meeting the requirements for safe service of marine engineering equipment.

[0058] Table 1. Chemical composition (mass percentage) of the steels used in the embodiments of the present invention and the comparative steels.

[0059] .

[0060] Table 2. Controlled rolling and controlled cooling direct quenching or process in the embodiments of the present invention.

[0061] .

[0062] Table 3. Test results of mechanical properties and HIC resistance of the steel in the embodiments of the present invention.

[0063] .

Claims

1. A method for manufacturing a high-strength, high-toughness steel for marine engineering equipment with excellent work hardening ability, characterized in that: The steel's chemical composition by weight percentage is: C = 0.08%–0.15%, Si = 0.10%–0.20%, Mn = 0.50%–1.00%, Ni = 0.30%–0.80%, Cr = 0.50%–1.50%, Mo = 0.10%–0.50%, Cu = 0.10%–0.50%, Nb or V = 0.02%–0.04%, Al = 0.05%–0.08%, B = 0.0005%–0.0015%, Ti ≤ 0.01%, N ≤ 0.005%, P ≤ 0.010%, S ≤ 0.006%, with the remainder being Fe and unavoidable impurities. The microstructure of the steel plate contains 75%–85% tempered martensite, 13.5%–23.5% polygonal ferrite, and 0.5%–1.5% carbides. The processing steps include: 1) Hot metal pretreatment: The hot metal is desulfurized by mixing magnesium desulfurizing agent with hot metal at a temperature of not less than 1300°C to ensure that the sulfur content of the hot metal is not higher than 0.002%.

2. Converter refining: The pretreated molten iron is poured into the converter for smelting. The treatment time is 25-30 minutes to remove excess carbon, phosphorus and sulfur from the molten iron, so that C≤ 0.12%, P≤ 0.010% and S≤ 0.008%. The steel is tapped when the temperature reaches 1650 ± 30℃. 3) Ladle refining: FeMn, FeSi, FeNi, FeAl, FeCr, Cu, and Mo alloys are added to the converter during the tapping process in a pre-calculated amount; the ladle is transported to the LF ladle refining station, bottom blowing argon gas is turned on, graphite electrodes are used for heating, lime and aluminum are added to adjust the slag and further deoxidize it, and after treatment for 25 to 30 minutes, the temperature of the molten steel is adjusted to 1600 ± 10℃; 4) Vacuum degassing: After refining in the LF furnace, the molten steel is sent to the RH / VD vacuum treatment station and ferroboron is added to the molten steel. Then, vacuum degassing is carried out for no less than 10 minutes in a vacuum environment of no more than 0.5tor. When the temperature of the molten steel reaches 1560 ±10℃, heat preservation agent is added. 5) Tundish casting: The ladle after vacuum treatment is hoisted to the continuous casting station and the molten steel is diverted from the bottom of the ladle to the tundish. The flow rate and temperature of the molten steel are controlled. The temperature of the molten steel in the tundish is controlled at 1535-1550℃. Argon gas sealing and covering agent protection are carried out throughout the casting process. The molten steel flows into the crystallizer through the submerged entry nozzle. 6) Slab continuous casting: Molten steel from the tundish is poured into a cooling rectangular copper plate crystallizer through an immersion nozzle. The casting superheat is controlled at 15-25°C. The molten steel in contact with the copper wall of the crystallizer solidifies into a slab shell. The slab is removed from the crystallizer by the crystallizer vibration and the dummy bar and enters the foot roll area clamping roller table, the secondary cooling zone roller table, the fan-shaped bending roller and the straightening roller. It is then flame-cut to obtain a continuous casting slab with a thickness of 200-320 mm. 7) Slab heating: The continuously cast slab is sent to the walking beam furnace and heated to 1150-1230℃ in stages, and held in the soaking zone for 1-3 hours to control the austenite grain size at 50-100μm; 8) Controlled rolling: The continuously cast slab is pushed out of the heating furnace and sent to the descaling machine via conveyor rollers. The surface iron oxide scale is removed by high pressure water, and then sent to the four-high rolling mill for the first stage of controlled rolling in the high temperature zone of 1050~950℃. The intermediate slab is cooled to 850~900℃ on the waiting roller table for the second stage of controlled rolling. 9) Online linear quenching: After final rolling, the steel plate is conveyed by roller conveyor and enters the laminar flow cooling zone for two-stage accelerated cooling. In the first stage, in the first 1 / 3 of the length of the cooling zone, a weak cooling mode is used to control the cooling rate of the steel plate at 5-10℃ / s, and the steel plate temperature reaches 620-660℃, so that the portion with a volume percentage not exceeding 25% undergoes proeutectoid ferrite phase transformation. In the second stage, in the last 2 / 3 of the length of the cooling zone, a strong cooling direct quenching mode is used to control the cooling rate of the steel plate at 15-30℃ / s, until the final cooling temperature is below 300℃, ensuring that the remaining austenite undergoes martensitic phase transformation and has a content of more than 75%. 10) Tempering heat treatment: The steel plate after direct quenching is transported to a tempering furnace for high-temperature tempering heat treatment. The tempering temperature is 550~650℃ and the holding time is 2.5 times the plate thickness mm×min / mm.

2. The method for manufacturing a high-strength, high-toughness marine engineering equipment steel with excellent work hardening ability according to claim 1, characterized in that... Step 8) Controlled rolling: In the first stage of controlled rolling, the average single-pass reduction in the high-temperature zone of 1050~950℃ is more than 30 mm, and the intermediate billet thickness reaches 2 to 5 times the target thickness of the finished product; in the second stage of controlled rolling, the cumulative reduction reaches 50% to 80%, and the steel plate is rolled to the target size and shape of the finished product.

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