Low carbon equivalent high toughness ultra-high strength fh550 steel for marine engineering and method for producing the same

Abstract

This invention discloses a low-carbon equivalent, high-toughness, ultra-high-strength marine engineering steel, FH550, and its production method, belonging to the field of metallic materials technology. The chemical composition of this steel, by mass percentage, is: C: 0.06~0.12%, Si: 0.20~0.40%, Mn: 1.30~1.70%, P≤0.015%, S≤0.003%, Al: 0.02~0.05%, Nb: 0.030~0.06%, V: 0.010~0.040%, Ti: 0.010~0.025%, Cr: 0.10~0.30%, B: ≤0.0005%, with the balance being Fe and impurities, and a carbon equivalent (Ceq) ≤0.44%. The production method includes: hot metal pretreatment → BOF smelting → LF refining → RH vacuum treatment → thick slab continuous casting → slow cooling with cover → billet heating → two-stage rolling → accelerated cooling → flaw detection → inspection. The two-stage rolling adopts a strong reduction process, and accelerated cooling after rolling replaces the quenching and tempering treatment. The FH550 steel plate of this invention has a yield strength ≥550MPa, an average KV2 impact energy ≥260J at -60℃, and combines ultra-high strength, low-temperature high toughness, and excellent weldability. The production process is simple, has a short cycle, and low cost, and can be widely used in deep-sea marine engineering, bridges, and other fields.

Description

Technical Field

[0001] This invention relates to the field of metallic materials technology, specifically to a low-carbon equivalent, high-toughness, ultra-high-strength FH550 steel plate for marine engineering, and a method for producing the steel plate. This steel plate can be widely used in the manufacturing of oil drilling platforms, marine engineering structures, bridges, large steel structures, engineering machinery, containers, and other engineering applications in deep-sea, low-temperature environments. Background Technology

[0002] 550MPa grade ultra-high strength steel is a core structural material for marine engineering equipment, mainly used in oil drilling platforms, engineering machinery, bridges, ship plates, and other fields. As my country's marine engineering develops towards deep-sea areas, marine engineering equipment is showing a trend towards larger size and lighter weight, which puts forward higher technical requirements for the strength, low-temperature toughness, and weldability of structural materials. Traditional 420MPa and 500MPa grade medium strength steels are gradually unable to meet the needs of engineering applications.

[0003] In existing technologies, 550MPa grade ultra-high strength marine engineering steel generally adopts a technical solution of high carbon content + precious alloy addition + quenching and tempering treatment. For example, the EH550 extra-thick steel plate for marine engineering disclosed in patent CN109112419A uses a high carbon content design and adds precious alloy Ni to improve toughness. At the same time, the steel plate needs to undergo quenching and tempering treatment. This type of technical solution has many technical defects: High carbon content and high carbon equivalent lead to poor weldability of steel plates, large fluctuations in low-temperature toughness of welded joints, and defects such as welding cracks, which pose safety hazards to the construction and use of marine engineering equipment. Adding precious alloys such as Ni significantly increases raw material costs, which is detrimental to the market promotion of the product. The quenching and tempering process is complex, has a long production cycle, and requires specialized heat treatment equipment, which further increases manufacturing costs. Existing processes cannot produce steel plates that simultaneously achieve ultra-high strength, high toughness at low temperatures, and excellent weldability, resulting in poor performance matching.

[0004] Therefore, developing a 550MPa grade marine engineering steel with low carbon equivalent, high toughness, ultra-high strength, excellent weldability, and simple and low production process has become an urgent technical problem to be solved in this field. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a low-carbon equivalent, high-toughness, ultra-high-strength FH550 steel for marine engineering, which is in contrast to the above-mentioned prior art. This steel plate has ultra-high strength, ultra-low temperature toughness, good plasticity and excellent welding performance. It has a low carbon equivalent and is suitable for marine engineering applications in the low-temperature environment of deep sea. Another objective of the present invention is to provide a production method for this steel plate. This method has a simple process, short production cycle, low manufacturing cost, no need for heat treatment, and can realize continuous industrial production.

[0006] The technical solution adopted by this invention to solve the above problems is as follows: a low-carbon equivalent, high-toughness, ultra-high-strength FH550 steel for marine engineering, whose chemical composition by mass percentage is: C: 0.06~0.12%, Si: 0.20~0.40%, Mn: 1.30~1.70%, P≤0.015%, S≤0.003%, Al: 0.02~0.05%, Nb: 0.030~0.06%, V: 0.010~0.040%, Ti: 0.010~0.025%, Cr: 0.10~0.30%, B: ≤0.0005%, with the balance being Fe and unavoidable impurities; at the same time, the content of each element satisfies the carbon equivalent formula Ceq=[C]+[Mn] / 6+([Cr]+[Mo]+[V]) / 5+([Ni]+[Cu]) / 15≤0.44%, wherein Mo, Ni, and Cu The mass percentage is 0 or the content of unavoidable impurities.

[0007] The FH550 steel plate has a thickness of ≤35mm, with an internal structure mainly of bainite and a surface layer of ferrite + bainite, and fine and uniform grains. Its mechanical properties meet the following requirements: yield strength ≥550MPa, tensile strength 670~820MPa, elongation ≥16%, yield-to-tensile ratio ≤0.85, and Charpy V-type impact energy KV2 at -60℃ with an average value ≥260J and a single value ≥200J.

[0008] The FH550 steel plate of this invention adopts a low carbon equivalent + composite microalloying composition design. The role of each element and the principle of content limitation are as follows: Carbon (C) is an economical strengthening element in steel, forming dispersed alloy carbides that increase the strength of steel plates. However, excessively high C content leads to an increase in carbon equivalent, resulting in poor weldability and reduced ductility and toughness. Therefore, this invention controls the C content to 0.06~0.12%, minimizing the carbon equivalent while ensuring strength and improving weldability.

[0009] Silicon (Si): A solid solution strengthening element, it dissolves in ferrite to increase the strength of steel plates. In low-alloy steel, an increase of 0.10% in Si can increase the tensile strength of hot-rolled steel by approximately 8 MPa. However, when the Si content exceeds 0.50%, it leads to a decrease in the impact toughness and reduction of area of ​​the steel plate, and the surface is prone to the formation of iron oxide scale, affecting surface quality. Therefore, this invention controls the Si content to be between 0.20% and 0.40%.

[0010] Manganese (Mn) is a solid solution strengthening element that can refine grains, lower the ductile-brittle transition temperature of steel, and promote the solid solution of microalloying elements such as Nb, V, and Ti into austenite. However, excessive Mn content can lead to reduced toughness and segregation in steel. Therefore, this invention controls the Mn content at 1.30~1.70%.

[0011] Chromium (Cr): It can improve the hardenability of steel, has a solid solution strengthening effect, and enhances the strength and hardness of steel; however, excessive Cr content will lead to a decrease in the toughness of steel and an increase in temper brittleness. Therefore, this invention controls the Cr content to be between 0.10% and 0.30%.

[0012] Aluminum (Al): As a deoxidizer, it removes oxygen from molten steel and simultaneously forms fine AlN particles, which are dispersed throughout the steel and pin the austenite grain boundaries, refining the grains and improving the strength and low-temperature toughness of the steel plate. Therefore, this invention controls the Al content to 0.02~0.05%.

[0013] Niobium (Nb) has a dual function of grain refinement and precipitation strengthening. Undissolved Nb (C,N) can prevent austenite grain growth, while Nb (C,N) precipitated during cooling can pin grain boundaries, further refining the grains. Therefore, the Nb content in this invention is controlled at 0.030~0.06%.

[0014] Vanadium (V): Combines with C and N to form stable carbonitrides, which can refine grains, prevent austenite grain coarsening, and improve the strength and toughness of steel plates. Therefore, the V content is controlled at 0.010~0.040% in this invention.

[0015] Titanium (Ti): Combines with C and N to form high-melting-point TiC, TiN, and Ti(CN) particles, which pin grain boundaries at high temperatures, inhibiting austenite grain growth and refining the microstructure. Simultaneously, titanium carbonitrides remain stable during welding, preventing grain coarsening in the weld zone and improving the low-temperature toughness of the weld. Therefore, this invention controls the Ti content to 0.010~0.025%.

[0016] Boron (B): Trace amounts of B can improve the hardenability of steel and promote the formation of bainite structure; however, excessive B content will lead to a decrease in the hot plasticity of steel and make it prone to cracking. Therefore, this invention controls the B content to ≤0.0005%.

[0017] Phosphorus (P) and sulfur (S) are harmful elements in steel. P easily leads to cold brittleness in steel, while S easily forms sulfide inclusions, causing center segregation and hot brittleness, while also reducing the mechanical and weldability of steel plates. Therefore, this invention strictly controls P ≤ 0.015% and S ≤ 0.003% to minimize the impact of these harmful elements.

[0018] Carbon equivalent (Ceq): Carbon equivalent is a core indicator for evaluating the weldability of steel. The higher the Ceq, the worse the weldability. This invention ensures that Ceq is ≤0.44% through precise control of the content of each element, thus ensuring that the steel plate has excellent weldability.

[0019] This invention also provides a production method for the aforementioned low-carbon equivalent high-toughness ultra-high-strength marine engineering steel FH550, comprising the following steps: hot metal pretreatment → BOF converter smelting → LF refining → RH vacuum treatment → thick slab continuous casting → continuous casting slab with cover for slow cooling → slab heating → two-stage rolling → accelerated cooling → flaw detection → inspection. The specific process requirements for each step are as follows: Hot metal pretreatment: Hot metal is pre-desulfurized using the KR method. After treatment, the [S] in the hot metal is ≤0.02%, removing harmful impurities such as sulfur and phosphorus from the hot metal and ensuring the purity of the hot metal. BOF converter smelting: The pretreated molten iron is added to the converter for smelting. The tapping temperature is controlled at 1620~1700℃. During the tapping process, aluminum wire is used for deoxidation. After deoxidation, the [O] in the molten steel is ≤20ppm. At the same time, alloys such as ferrovanadium, ferrochrome, ferroniobium, and ferrotitanium are added to fine-tune the composition and ensure that the alloy elements are uniformly dissolved. LF refining + RH vacuum treatment: Molten steel enters the LF refining furnace for precise composition control and slag system optimization, and then enters the RH vacuum treatment furnace for degassing and inclusion removal. After vacuum treatment, the H content in the molten steel is ≤0.9ppm. After the RH vacuum treatment, calcium wire is fed into the molten steel for calcium treatment to modify the morphology of inclusions and prevent crack defects caused by inclusions. Thick slab continuous casting: During the continuous casting process, the superheat of the molten steel is strictly controlled between 10 and 30°C. A light reduction process matching the composition and superheat of the molten steel is adopted. The casting speed is reasonably adjusted according to the thickness of the slab to produce a thick slab with a suitable thickness. The quality of the slab meets the following requirements: center segregation equal to or better than Class C 0.5, center porosity equal to or better than 0.5, and no defects such as intermediate cracks, triangular cracks, or surface cracks. Slow cooling of continuously cast billets: The continuously cast billets are sent into a slow cooling hood for slow cooling. After slow cooling, the billet surface temperature is ≤150℃ and then it is removed from the hood. During the slow cooling process, the cooling rate is controlled to prevent the billet from generating internal stress and cracks due to a sudden drop in temperature. After removing the billet from the hood, the surface of the billet is polished and cleaned to remove surface defects such as iron oxide scale, cracks, and slag inclusions. Billet heating: The cleaned thick billet is sent into the heating furnace for heating. The heating temperature is controlled at 1180~1250℃ and the holding time is ≥3 hours to ensure that the alloy elements inside the billet are fully dissolved and the austenite grains are homogenized, so as to avoid undissolved carbides from affecting the subsequent rolling performance. Two-stage rolling: After the heated billet exits the furnace, it undergoes high-pressure water descaling to remove iron oxide scale from the surface. This is followed by two-stage controlled rolling. Before each rolling pass, the billet surface is cooled and descaled using high-pressure descaling water. The first stage of rolling is for the austenite recrystallization zone, with a total reduction rate of ≥70%. A high reduction rolling process is adopted, with a single-pass reduction rate of ≥17% in the last three passes. The final rolling temperature is controlled at 960~1050℃. The austenite grain refinement is achieved through a large reduction rate. The second stage of rolling is for the non-recrystallized austenite region. The initial rolling temperature is controlled at 840~920℃, and the total reduction rate is ≥50%. Through deformation accumulation, the austenite grains are further refined, laying the foundation for the formation of fine bainite structure in subsequent cooling. Accelerated cooling: After the second stage of rolling is completed, the steel plate is immediately subjected to online accelerated cooling treatment. The initial cooling temperature is controlled at 730~800℃, the reddening temperature is controlled at 430~500℃, and the cooling rate is controlled at 10~20℃ / s. Rapid cooling inhibits the precipitation of pearlite and ferrite, promotes the formation of fine bainite structure, and achieves the strengthening and toughening of the steel plate. Flaw detection + inspection: After accelerated cooling, the steel plate is naturally cooled to room temperature and then subjected to ultrasonic flaw detection to check for defects such as shrinkage cavities, porosity, and cracks inside the steel plate. Subsequently, samples are taken for chemical composition analysis and mechanical property testing (tensile, impact, hardness). Once the test is qualified, it is a finished steel plate.

[0020] Compared with the prior art, the present invention has the following beneficial effects: 1. The FH550 steel plate of the present invention has a yield strength ≥550MPa, tensile strength 670~820MPa, average KV2 impact energy ≥260J at -60℃, yield strength ratio ≤0.85, elongation ≥16%, and simultaneously possesses ultra-high strength, ultra-low temperature toughness, good plasticity and excellent welding performance. It solves the technical problem of difficulty in balancing strength, toughness and welding performance in the prior art, and is suitable for marine engineering applications in the deep-sea low-temperature environment. 2. This invention controls the carbon equivalent to ≤0.44% through a low-carbon content + composite micro-alloying composition design. This eliminates the need for complex preheating and post-heating processes during steel plate welding, resulting in crack-free welded joints and excellent low-temperature toughness, which significantly reduces the difficulty of engineering construction. 3. This invention abandons the traditional quenching and tempering treatment, and adopts online accelerated cooling after rolling to replace heat treatment. It combines an integrated process of molten iron pretreatment - double refining - controlled rolling and controlled cooling, which simplifies the process, greatly shortens the production cycle, and is suitable for industrial continuous production. 4. This invention eliminates the need to add precious alloys such as Ni, thus reducing raw material costs; it also eliminates the need for quenching and tempering, saving on investment and operating costs for heat treatment equipment; and by using conventional smelting and rolling equipment, there is no need to add any special equipment, further reducing manufacturing costs and enhancing the market competitiveness of the product. Attached Figure Description

[0021] Figure 1 The image shows the metallographic structure of the surface layer of the 35mm thick FH550 steel plate in Embodiment 1 of the present invention (magnified 100 times). The structure consists of ferrite and bainite with fine and uniform grains. Figure 2 The image shows the metallographic structure (100x magnification) at 1 / 4 thickness of a 35mm thick FH550 steel plate in Embodiment 1 of the present invention. The structure is bainite with no coarse grains. Figure 3 The image shows the metallographic structure (100x magnification) at half the thickness of a 35mm thick FH550 steel plate in Embodiment 1 of the present invention. The structure is bainite, uniform, and free of inclusions. Detailed Implementation

[0022] The technical solution of the present invention will be described in more detail below with reference to preferred embodiments. However, these embodiments are merely descriptions of preferred implementations of the present invention and should not be construed as limiting the scope of the present invention. Example 1

[0023] A low-carbon-equivalent, high-toughness, ultra-high-strength FH550 steel for marine engineering, with a thickness of 35 mm, has the following chemical composition by mass percentage: C: 0.09%, Si: 0.30%, Mn: 1.50%, P: 0.012%, S: 0.001%, Al: 0.030%, Nb: 0.040%, V: 0.026%, Ti: 0.015%, Cr: 0.23%, B: 0.0003%, with the balance being Fe and unavoidable impurities; the calculated carbon equivalent Ceq = 0.38% ≤ 0.44%.

[0024] The method for producing this steel plate includes the following steps: Hot metal pretreatment: KR method pre-desulfurization, after treatment [S]=0.018%; BOF converter smelting: tapping temperature 1650℃, after aluminum wire deoxidation [O]=18ppm, ferrovanadium, ferrochrome, ferroniobium and ferrotitanium are added to adjust the alloy composition; LF refining + RH vacuum treatment: After LF refining, the composition is precisely controlled. After RH vacuum treatment, [H] = 0.8ppm. Calcium is then fed through a calcium feeding line for calcium treatment. Thick slab continuous casting: The molten steel is superheated to 20℃ and a light reduction process is used to cast a 280mm thick slab. The center segregation of the slab is grade C0.5, the center porosity is grade 0.5, and there are no crack defects. Slow cooling of continuously cast billets under a cover: Slow cooling until the surface temperature of the billet reaches 120℃ before removing it from the cover and grinding and cleaning the surface; Billet heating: heating temperature 1220℃, holding for 3.5 hours; Two-stage rolling: High-pressure water descaling after exiting the furnace; the total reduction rate of the first stage rolling is 75%, and the reduction rates of the next three passes are 18%, 19%, and 17% respectively, with a final rolling temperature of 1000℃; the starting rolling temperature of the second stage rolling is 880℃, with a total reduction rate of 55%, and high-pressure descaling before each rolling pass. Accelerated cooling: initial cooling temperature 760℃, reddening temperature 460℃, cooling rate 15℃ / s; Flaw detection + inspection: Ultrasonic testing revealed no internal defects, and samples were taken for mechanical property testing.

[0025] Mechanical property test results: yield strength 609MPa, tensile strength 731MPa, elongation 20.0%, yield strength ratio 0.83; Charpy V-type impact energy at -60℃: near the table 310J, 317J, 316J (average 314J), at T / 4 305J, 291J, 313J (average 303J), at T / 2 325J, 227J, 335J (average 296J), all meeting the requirement of average ≥260J.

[0026] Metallographic structure: The surface layer is ferrite + bainite, and the T / 4 and T / 2 areas are both bainite. The grains are fine and uniform, with no coarse structure or inclusions. Example 2

[0027] A low-carbon-equivalent, high-toughness, ultra-high-strength FH550 steel for marine engineering, with a thickness of 20 mm, has the following chemical composition by mass percentage: C: 0.06%, Si: 0.20%, Mn: 1.30%, P: 0.010%, S: 0.002%, Al: 0.020%, Nb: 0.030%, V: 0.010%, Ti: 0.010%, Cr: 0.10%, B: 0.0002%, with the balance being Fe and unavoidable impurities; carbon equivalent Ceq = 0.32% ≤ 0.44%.

[0028] The production method of this steel plate, and the key parameters of each process are as follows: after KR pre-desulfurization, [S]=0.015%, converter tapping temperature 1620℃, after deoxidation, [O]=15ppm, after RH vacuum treatment, [H]=0.7ppm; continuous casting superheat 10℃, billet center segregation C class 0.5 grade; billet heating temperature 1180℃, holding for 3 hours; first stage rolling total reduction rate 70%, the reduction rate of the last three passes 17%, final rolling temperature 960℃; second stage rolling starting temperature 840℃, total reduction rate 50%; accelerated cooling initial temperature 730℃, reheating temperature 430℃, cooling rate 10℃ / s.

[0029] Mechanical property test results: yield strength 565MPa, tensile strength 682MPa, elongation 18.5%, yield strength ratio 0.83; average impact energy at -60℃ KV2 285J, meeting technical requirements. Example 3

[0030] A low-carbon-equivalent, high-toughness, ultra-high-strength FH550 steel for marine engineering, with a thickness of 30 mm, has the following chemical composition by mass percentage: C: 0.12%, Si: 0.40%, Mn: 1.70%, P: 0.015%, S: 0.003%, Al: 0.050%, Nb: 0.060%, V: 0.040%, Ti: 0.025%, Cr: 0.30%, B: 0.0005%, with the balance being Fe and unavoidable impurities; carbon equivalent Ceq = 0.44%.

[0031] The production method of this steel plate, and the key parameters of each process are as follows: after KR pre-desulfurization, [S]=0.020%, converter tapping temperature 1700℃, after deoxidation, [O]=20ppm, after RH vacuum treatment, [H]=0.9ppm; continuous casting superheat 30℃, billet center segregation C class 0.5 grade; billet heating temperature 1250℃, holding for 4 hours; first stage rolling total reduction rate 80%, the reduction rate of the last three passes 20%, final rolling temperature 1050℃; second stage rolling starting temperature 920℃, total reduction rate 60%; accelerated cooling initial temperature 800℃, reddening temperature 500℃, cooling rate 20℃ / s.

[0032] Mechanical property test results: yield strength 598MPa, tensile strength 815MPa, elongation 16.5%, yield strength ratio 0.73; average impact energy at -60℃ KV2 272J, meeting technical requirements.

[0033] The above embodiments demonstrate that the production method of the present invention can stably produce FH550 steel plates with low carbon equivalent, high toughness, and ultra-high strength. The composition, microstructure, and properties of the steel plates all meet the design requirements, and the production process is simple and inexpensive.

[0034] The above are merely specific embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A low-carbon equivalent, high-toughness, ultra-high-strength FH550 steel for marine engineering, characterized in that, The chemical composition by mass percentage is as follows: C: 0.06~0.12%, Si: 0.20~0.40%, Mn: 1.30~1.70%, P≤0.015%, S≤0.003%, Al: 0.02~0.05%, Nb: 0.030~0.06%, V: 0.010~0.040%, Ti: 0.010~0.025%, Cr: 0.10~0.30%, B: ≤0.0005%, with the balance being Fe and unavoidable impurities. The content of each element satisfies the carbon equivalent formula Ceq=[C]+[Mn] / 6+([Cr]+[Mo]+[V]) / 5+([Ni]+[Cu]) / 15≤0.44%, where Mo, Ni, and Cu are 0 or unavoidable impurities.

2. The low-carbon-equivalent high-toughness ultra-high-strength FH550 steel for marine engineering according to claim 1, characterized in that, The FH550 steel has a thickness of ≤35mm, and its internal structure is mainly bainite, while the surface structure is ferrite + bainite.

3. The low-carbon equivalent, high-toughness, ultra-high-strength FH550 steel for marine engineering according to claim 1 or 2, characterized in that, The mechanical properties of the FH550 steel meet the following requirements: yield strength ≥ 550 MPa, tensile strength 670~820 MPa, elongation ≥ 16%, yield strength ratio ≤ 0.85, and Charpy V-type impact energy KV2 average value ≥ 260 J at -60℃.

4. A method for producing low-carbon equivalent high-toughness ultra-high-strength FH550 steel for marine engineering as described in any one of claims 1 to 3, characterized in that, The process includes the following steps: molten iron pretreatment → BOF converter smelting → LF refining → RH vacuum treatment → thick slab continuous casting → continuous casting billet with cover for slow cooling → billet heating → two-stage rolling → accelerated cooling → flaw detection → inspection.

5. The method of producing a low carbon equivalent high toughness ultra-high strength FH550 steel for marine engineering according to claim 4, characterized in that, The molten iron pretreatment adopts the KR method for pre-desulfurization, and the [S] in the molten iron after treatment is ≤0.02%; the tapping temperature of the BOF converter smelting is 1620~1700℃, and aluminum wire deoxidation is adopted. After deoxidation, the [O] in the molten steel is ≤20ppm, and ferrovanadium, ferrochrome, ferroniobium and ferrotitanium are added to adjust the alloy composition.

6. The method for producing low-carbon equivalent high-toughness ultra-high-strength FH550 steel for marine engineering according to claim 4, characterized in that, The H content in the molten steel after RH vacuum treatment is ≤0.9ppm. After the RH vacuum treatment is completed, calcium wire is fed into the molten steel for calcium treatment. The superheat of the molten steel for the continuous casting of the thick slab is 10~30℃. A light reduction process is adopted. The quality of the slab meets the following requirements: central segregation equal to or better than Class C 0.5 grade, central porosity equal to or better than 0.5 grade, no intermediate cracks, and no triangular cracks.

7. The method of producing a low carbon equivalent high toughness ultra-high strength FH550 steel for marine engineering according to claim 4, characterized in that, The continuously cast billet is covered and slowly cooled until the surface temperature of the billet is ≤150℃ before being removed from the cover. After being removed from the cover, the surface of the billet is polished and cleaned. The heating temperature of the billet is 1180~1250℃, and the holding time is ≥3 hours.

8. A method of producing a low carbon equivalent high toughness ultra-high strength FH550 steel for marine engineering according to claim 4, characterized in that, Before the two-stage rolling process, the billet undergoes high-pressure water descaling, and high-pressure descaling water cooling is used for descaling before each rolling pass; the two-stage rolling process includes: First stage rolling: total reduction rate ≥70%, single-pass reduction rate of the last three passes ≥17%, final rolling temperature 960~1050℃; Second stage rolling: the initial rolling temperature is 840~920℃, and the total reduction rate is ≥50%.

9. The production method of FH550 steel for marine engineering with low carbon equivalent, high toughness, and ultra-high strength according to claim 4, characterized in that, The accelerated cooling is online accelerated cooling, with an initial cooling temperature of 730~800℃, a reddening temperature of 430~500℃, and a cooling rate of 10~20℃ / s.

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