High-strength high-low-temperature-toughness marine structural steel and method for manufacturing same

Abstract

The application relates to the technical field of preparation of marine steel materials, in particular to a high-strength high-low-temperature-toughness marine structural steel and a preparation method thereof. The application provides a high-strength high-low-temperature-toughness marine structural steel, which comprises the following elements in percentage by mass: C: 0.08-0.12%, Si: 0.18-0.22%, Mn: 0.48-0.58%, Cr: 0.48-0.58%, Ni: 4.3-4.6%, V: 0.04-0.06%, Mo: 0.35-0.45%, Ti: 0.03-0.10%, P: <=0.008%, S: <=0.008%, O: <=0.001%, and the balance is Fe. The marine structural steel has high structural steel strength and high-low-temperature toughness.

Description

Technical Field

[0001] This invention relates to the field of marine steel material preparation technology, and in particular to a high-strength, high-low-temperature toughness marine structural steel and its preparation method. Background Technology

[0002] With the continuous development of marine engineering structures and the deepening exploration of deep-sea and open ocean areas, the requirements for the use of marine structural steel are becoming increasingly stringent. Ships must not experience brittle fracture under extremely cold conditions and strong impact loads. This places higher demands on the strength and low-temperature impact toughness of marine structural steel. Therefore, the development of marine structural steel with ultra-high strength and high low-temperature toughness has become a practical necessity. Summary of the Invention

[0003] The purpose of this invention is to provide a high-strength, high-low temperature toughness marine structural steel and its preparation method. The marine structural steel of this invention has both high structural steel strength and high-low temperature toughness.

[0004] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0005] This invention provides a high-strength, high- and low-temperature toughness marine structural steel, comprising the following elements by mass percentage: C: 0.08–0.12%, Si: 0.18–0.22%, Mn: 0.48–0.58%, Cr: 0.48–0.58%, Ni: 4.3–4.6%, V: 0.04–0.06%, Mo: 0.35–0.45%, Ti: 0.03–0.10%, P: ≤0.008%, S: ≤0.008%, O: ≤0.001%, with the balance being Fe.

[0006] Preferably, the main phase of the high-strength, high-low temperature toughness marine structural steel is tempered martensite, with a hardness of 25-35 HRC, a yield strength of 840-900 MPa, a tensile strength of 860-930 MPa, and a ductile-brittle transition temperature of -120 to -100℃.

[0007] This invention also provides a method for preparing the marine structural steel described in the above technical solution, comprising the following steps:

[0008] According to the elemental composition of the marine structural steel described in the above technical solution, the reaction raw materials are melted and cast in sequence to obtain steel ingots;

[0009] The steel ingot is subjected to high-temperature homogenization treatment, high-temperature diffusion annealing, forging, hot rolling, first heat treatment and second heat treatment in sequence to obtain high-strength and high-low temperature toughness marine structural steel.

[0010] Preferably, the smelting process includes: mixing metallic nickel, metallic molybdenum, metallic chromium and metallic iron, melting them, and then sequentially adding graphite carbon, vanadium, manganese and titanium under vacuum conditions for smelting and deoxidation.

[0011] The method of adding graphite carbon and then performing smelting and deoxidation is called vacuum carbon deoxidation.

[0012] Preferably, the high-temperature homogenization treatment is carried out at a temperature of 1100–1200°C for 9–12 hours.

[0013] Preferably, the high-temperature diffusion annealing temperature is 1200℃ and the time is 2 hours.

[0014] Preferably, the forging process uses a 750 air hammer to forge the original billet obtained by high-temperature diffusion annealing into a square billet;

[0015] The forging temperature is 950–1150°C, and the forging method is free forging.

[0016] Preferably, the hot rolling process is as follows: the forged billet is heated to 1200°C and held for 2 hours, and then hot rolled in multiple passes into a hot-rolled plate with a thickness of 12 mm.

[0017] The initial rolling temperature of the multi-pass hot rolling is 1150℃, and the final rolling temperature is 950℃.

[0018] Preferably, the temperature of the first heat treatment is 830-900℃, and the holding time is 3h.

[0019] Preferably, the temperature of the second heat treatment is 620-650°C, and the holding time is 2 hours.

[0020] This invention provides a high-strength, high- and low-temperature toughness marine structural steel, characterized by comprising the following elements by mass percentage: C: 0.08–0.12%, Si: 0.18–0.22%, Mn: 0.48–0.58%, Cr: 0.48–0.58%, Ni: 4.3–4.6%, V: 0.04–0.06%, Mo: 0.35–0.45%, Ti: 0.03–0.10%, P: ≤0.008%, S: ≤0.008%, O: ≤0.001%, with the balance being Fe. The high-strength, high- and low-temperature toughness marine structural steel of this invention, by adding Ti, can precipitate nano-sized Ti carbonitrides at grain boundaries, effectively refining the grains. This is beneficial for improving the welding and hot-working properties of the marine structural steel, helping to increase the yield and economic benefits. Meanwhile, the microalloying of Ti strengthens marine structural steel without increasing crack sensitivity, which helps to produce easy-to-weld ultra-high strength high and low temperature toughness marine structural steel.

[0021] This invention also provides a method for preparing marine structural steel as described in the above technical solution, comprising the following steps: according to the elemental composition of the marine structural steel described in the above technical solution, the reaction raw materials are sequentially smelted and cast to obtain steel ingots; the steel ingots are sequentially subjected to high-temperature homogenization treatment, high-temperature diffusion annealing, forging, hot rolling, first heat treatment and second heat treatment to obtain high-strength and high-low-temperature toughness marine structural steel. Compared with the prior art, the preparation method of this invention has the following beneficial effects: 1) In this invention, the addition of Ti element to the alloy for microalloying can precipitate nano-scale carbonitrides during high-temperature austenitization, thereby precipitating at large-angle grain boundaries, thus delaying the recrystallization and growth of grains during austenitization, playing a role in grain refinement, and grain refinement is the only strengthening method that can improve both strength and toughness; 2) The microalloying of Ti strengthens the marine structural steel without increasing crack sensitivity, which helps to produce easy-to-weld ultra-high strength and high-low-temperature toughness marine structural steel; 3) This invention has the following advantages: The dual hot working treatment of forging and hot rolling for marine structural steel can further refine the grains and enhance the strengthening effect of marine structural steel; 4) The marine structural steel prepared by the above preparation method has a hardness of 25-35 HRC, a yield strength of 840-900 MPa, a tensile strength of 860-930 MPa, and a ductile-brittle transition temperature (DBTT) of -120 to -100℃; the oxygen content is ≤10 ppm. The extremely low oxygen content helps to reduce the formation of large-sized oxide inclusions, which is beneficial to subsequent material processing, improves the yield of material processing, and improves economic benefits. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the microstructure of the ultra-high strength high and low temperature toughness marine structural steel described in Example 1;

[0023] Figure 2 This is a schematic diagram of the microstructure of the ultra-high strength high and low temperature toughness marine structural steel described in Example 2. Detailed Implementation

[0024] This invention provides a high-strength, high- and low-temperature toughness marine structural steel, comprising the following elements by mass percentage: C: 0.08–0.12%, Si: 0.18–0.22%, Mn: 0.48–0.58%, Cr: 0.48–0.58%, Ni: 4.3–4.6%, V: 0.04–0.06%, Mo: 0.35–0.45%, Ti: 0.03–0.10%, P: ≤0.008%, S: ≤0.008%, O: ≤0.001%, with the balance being Fe.

[0025] The marine structural steel of the present invention comprises 0.08-0.12% C by mass percentage, preferably 0.09-0.11%, and more preferably 0.10%. In the present invention, the C can form carbides with alloying elements, playing a role in solid solution and precipitation strengthening.

[0026] The marine structural steel of the present invention comprises 0.18-0.22% Si by mass percentage, preferably 0.19-0.21%, and more preferably 0.20%. In the present invention, the Si can form a solid solution with iron atoms, thereby playing a role in solid solution strengthening, and at the same time, it can improve the corrosion resistance of the steel by combining with oxygen atoms.

[0027] The marine structural steel of the present invention comprises 0.48-0.58% Mn by mass percentage, preferably 0.50-0.56%, and more preferably 0.52-0.54%. In the present invention, the Mn compensates for the strength reduction caused by the reduction in carbon content, and plays a role in solid solution strengthening.

[0028] The marine structural steel of the present invention comprises 0.48-0.58% Cr by weight, preferably 0.50-0.56%, and more preferably 0.52-0.54%. In the present invention, the Cr can enhance the oxidation resistance and corrosion resistance of the steel, as well as enhance the strength and wear resistance of the steel.

[0029] The marine structural steel of the present invention comprises 4.3-4.6% Ni by mass percentage, preferably 4.35-4.55%, and more preferably 4.4-4.5%. In the present invention, the Ni can significantly strengthen the ferrite matrix and also has the effect of significantly reducing the ductile-brittle transition temperature.

[0030] The marine structural steel of the present invention comprises 0.04-0.06% V by mass percentage, preferably 0.045-0.055%, and more preferably 0.05%. In the present invention, the V can promote the nucleation of carbides and improve the hardenability of the steel.

[0031] The marine structural steel of the present invention comprises 0.35-0.45% Mo by mass percentage, preferably 0.36-0.44%, and more preferably 0.38-0.42%. In the present invention, the Mo can inhibit the coarsening of TiC, thereby refining the grains and improving the strength, while also improving the corrosion resistance of the steel.

[0032] The marine structural steel of the present invention comprises 0.03-0.10% Ti by mass percentage, preferably 0.04-0.08%, and more preferably 0.05-0.06%. In the present invention, the Ti can form nano-carbides, thereby improving the toughness and weldability of the steel plate.

[0033] The marine structural steel of the present invention comprises P ≤ 0.008% by mass, preferably 0.001 to 0.006%, and more preferably 0.003 to 0.005%. In the present invention, P can improve the corrosion resistance of low alloy high strength steel.

[0034] The marine structural steel of the present invention, by mass percentage, comprises S ≤ 0.008%, preferably 0.001–0.006%, and more preferably 0.003–0.005%. In the present invention, the S content can control the content of O, S, and Ca elements in the steel, thereby reducing the impact of inclusions on welding quality.

[0035] The marine structural steel of the present invention comprises, by mass percentage, O ≤ 0.001%, preferably 0.0002% to 0.0008%, more preferably 0.0004% to 0.0006%. In the present invention, the O forms oxides with non-metallic elements, and reducing their size can refine the austenite grains, thereby improving toughness and increasing strength.

[0036] The marine structural steel of this invention also includes a balance of Fe by weight percentage.

[0037] In this invention, the main phase of the high-strength, high-low-temperature toughness marine structural steel is preferably tempered martensite, the hardness is preferably 25-35 HRC, the yield strength is preferably 840-900 MPa, the tensile strength is preferably 860-930 MPa, and the ductile-brittle transition temperature (DBTT) is preferably -120 to -100℃.

[0038] This invention also provides a method for preparing the marine structural steel described in the above technical solution, comprising the following steps:

[0039] According to the elemental composition of the marine structural steel described in the above technical solution, the reaction raw materials are melted and cast in sequence to obtain steel ingots;

[0040] The steel ingot is subjected to high-temperature homogenization treatment, high-temperature diffusion annealing, forging, hot rolling, first heat treatment and second heat treatment in sequence to obtain high-strength and high-low temperature toughness marine structural steel.

[0041] In this invention, unless otherwise specified, all raw materials used in the preparation are commercially available products well known to those skilled in the art.

[0042] According to the elemental composition of the marine structural steel described in the above technical solution, the reaction raw materials are smelted and cast sequentially to obtain steel ingots.

[0043] In this invention, the smelting process preferably includes: mixing metallic nickel, metallic molybdenum, metallic chromium and metallic iron, melting them, and then sequentially adding graphite carbon, vanadium, manganese and titanium under vacuum conditions for smelting and deoxidation; the method of adding graphite carbon for smelting and deoxidation is preferably vacuum carbon deoxidation.

[0044] In this invention, the nickel is preferably a nickel plate with a purity ≥99.9 wt%; the molybdenum is preferably a molybdenum bar with a purity ≥99.9 wt%; the chromium is preferably ≥99.9 wt%; and the iron is preferably industrial pure iron. In this invention, the mixing is preferably performed by sequentially adding the nickel, molybdenum, chromium, and iron to a vacuum induction furnace. In this invention, the melting temperature is preferably 1510 ± 20 °C; the melting time is not specifically limited, and a time known to those skilled in the art is sufficient to ensure complete melting of the aforementioned metals. In this invention, the vanadium is preferably electrolytic vanadium with a purity ≥99.9 wt%; the manganese is preferably electrolytic manganese with a purity ≥99.9 wt%; and the titanium is preferably sponge titanium with a purity ≥99.9 wt%. In this invention, the deoxidation method after adding graphite carbon is preferably vacuum carbon deoxidation; in this invention, the time interval between adding graphite carbon, vanadium, manganese, and titanium is preferably 2–4 minutes, more preferably 3 minutes. In this invention, the graphite carbon, vanadium, manganese and titanium are preferably added from an external feed hopper.

[0045] In this invention, the casting is preferably carried out in an inert atmosphere, preferably an argon atmosphere; the casting temperature is preferably 1510±20℃.

[0046] After obtaining the steel ingot, the present invention sequentially performs high-temperature homogenization treatment, high-temperature diffusion annealing, forging, hot rolling, first heat treatment and second heat treatment to obtain high-strength and high-low temperature toughness marine structural steel.

[0047] In this invention, the temperature of the high-temperature homogenization treatment is preferably 1100–1200℃, more preferably 1120–1180℃, and most preferably 1140–1160℃; the holding time is preferably 9–12 h, more preferably 10–11 h; the heating rate to the temperature of the high-temperature homogenization treatment is preferably ≤6℃ / min, more preferably 2–5℃ / min, and most preferably 3–4℃ / min. After the high-temperature homogenization treatment is completed, the invention preferably includes cooling; the cooling method is preferably furnace cooling, and the furnace door is in a closed state.

[0048] In this invention, the high-temperature homogenization treatment reduces internal stress in the ingot and improves the machinability of the material.

[0049] In this invention, the temperature of the high-temperature diffusion annealing is preferably 1150–1250°C, more preferably 1180–1220°C, and most preferably 1200°C; the time is preferably 1–3 hours, more preferably 1.5–2.5 hours, and most preferably 2 hours. Before performing the high-temperature annealing, this invention also preferably includes cutting off the head and tail of the cast ingot.

[0050] In this invention, the high-temperature diffusion annealing improves the problems of macroscopic segregation and uneven distribution of inclusions in the ingot.

[0051] In this invention, the forging is preferably performed using a 750 air hammer to forge the billet obtained from high-temperature diffusion annealing into a square billet; the forging temperature is preferably 950–1150°C, more preferably 980–1100°C, and most preferably 1000–1060°C; the forging time is preferably 2–4 hours, more preferably 2.5–3.5 hours, and most preferably 3 hours; the forging method is preferably free forging; after the forging is completed, the invention also preferably includes cooling; the cooling is preferably performed in air to room temperature.

[0052] In this invention, the forging process can eliminate defects such as casting porosity generated during the smelting process of metal and optimize the microstructure.

[0053] In this invention, the hot rolling process is preferably as follows: the forged billet is heated to 1200°C and held for 2 hours, then hot-rolled in multiple passes to form a hot-rolled plate with a thickness of 12 mm. The initial rolling temperature of the multiple hot rolling passes is preferably 1150°C, and the final rolling temperature is preferably 950°C. In this invention, the single-pass reduction of the multiple hot rolling passes is preferably ≤35%, and the total reduction of the multiple hot rolling passes is preferably ≤66%. After the hot rolling is completed, this invention also preferably includes cooling. This invention does not impose any special limitations on the cooling process; cooling in air, as is well known to those skilled in the art, is acceptable.

[0054] In this invention, the hot rolling can reduce or eliminate casting defects, transform the as-cast structure into a deformed structure, and improve the processing performance of the alloy.

[0055] In this invention, the temperature of the first heat treatment is preferably 830–900°C, more preferably 840–890°C, and most preferably 860–880°C; the holding time is preferably 3 hours. The heating rate to the temperature of the first heat treatment is preferably ≤6°C / min, more preferably 1–5°C / min, and most preferably 2–4°C / min. After the first heat treatment is completed, this invention also preferably includes water cooling. This invention does not have any special limitations on the water cooling process, and any process well known to those skilled in the art can be used.

[0056] In this invention, the first heat treatment causes the matrix to form as much martensite as possible, thereby improving the hardness and wear resistance of the material.

[0057] In this invention, the temperature of the second heat treatment is preferably 620–650°C, more preferably 630–640°C, and most preferably 633–636°C; the holding time is preferably 2 hours. The heating rate to the temperature of the second heat treatment is preferably ≤6°C / min, more preferably 1–5°C / min, and most preferably 2–4°C / min. After the second heat treatment is completed, this invention also preferably includes air cooling. This invention does not have any special limitations on the air cooling process, and any process well known to those skilled in the art can be used.

[0058] In this invention, the second heat treatment can eliminate or reduce internal stress, reduce brittleness, prevent deformation and cracking, stabilize the microstructure, stabilize the size and shape, and ensure the accuracy and performance of the parts.

[0059] The following detailed description, in conjunction with embodiments, illustrates the high-strength, high-low-temperature toughness marine structural steel and its preparation method provided by the present invention. However, these descriptions should not be construed as limiting the scope of protection of the present invention.

[0060] Example 1

[0061] The elemental composition (by mass percentage) of 785MPa grade marine structural steel is as follows: C: 0.09%, Si: 0.20%, Mn: 0.55%, Cr: 0.56%, Ni: 4.49%, V: 0.05%, Mo: 0.39%, Ti: 0.05%, P: 0.004%, S: 0.003%, O: 0.00099%, with the balance being Fe.

[0062] Preparation method of 785MPa grade marine structural steel:

[0063] First, metallic nickel plates (purity ≥99.9%), metallic molybdenum bars (purity ≥99.9%), metallic chromium (purity ≥99.9%), and industrial pure iron are sequentially added to a vacuum induction furnace and smelted at 1510±20℃ until the furnace charge is fully melted. Then, graphite carbon is added sequentially from the external charging bin and smelting continues for 60 minutes (vacuum carbon deoxidation is used for smelting). Next, electrolytic vanadium (purity ≥99.9%) is added and smelted for 2 minutes, followed by electrolytic manganese (purity ≥99.9%) and sponge titanium (purity ≥99.9%), and then smelting continues until the furnace charge is fully melted. Finally, the furnace charge is cast (casting is carried out in an argon atmosphere at a temperature of 1505℃) into steel ingots.

[0064] The steel ingot is heated (heating rate is 6℃ / min) to 1100℃ and held at that temperature for 10h for high-temperature homogenization treatment. Then it is cooled to room temperature in the furnace with the furnace door closed.

[0065] After the head and tail of the homogenized ingot are cut off, it is placed at 1200℃ for 2 hours for high-temperature diffusion annealing. The original billet is forged into a square billet using a 750 air hammer at a forging temperature of 1050℃ and a forging method of free forging. Then it is cooled to room temperature in air.

[0066] The forged billet was heated to 1200℃ and held for 2 hours. It was then hot-rolled into a 12mm thick hot-rolled plate through multiple passes using a rolling mill. The initial rolling temperature was 1150℃ and the final rolling temperature was 950℃ (maximum reduction per pass was 33%, and total reduction was 64%). It was then cooled to room temperature and cooled in air.

[0067] The rolled slab was heated (heating rate 6℃ / min) to 880℃ and held for 3 hours, then water-cooled to room temperature; then heated (heating rate 6℃ / min) to 630℃ and held for 2 hours, and finally cooled to room temperature in air to obtain ultra-high strength high-low temperature toughness marine structural steel (microstructure is tempered martensite, hardness is 29.53HRC, yield strength is 876.32MPa, tensile strength is 920.18MPa, ductile-brittle transition temperature (DBTT) is -115℃, which meets the requirements);

[0068] Figure 1 This is a schematic diagram of the microstructure of the ultra-high strength, high and low temperature toughness marine structural steel. Figure 1 It is known that the microstructure of the ultra-high strength high and low temperature toughness marine structural steel includes tempered martensite and ferrite.

[0069] Example 2

[0070] The elemental composition (by mass percentage) of 785MPa grade marine structural steel is as follows: C: 0.11%, Si: 0.19%, Mn: 0.55%, Cr: 0.56%, Ni: 4.34%, V: 0.05%, Mo: 0.39%, Ti: 0.10%, P: 0.003%, S: 0.003%, O: 0.00065%, with the balance being Fe.

[0071] Preparation method of 785MPa grade marine structural steel:

[0072] First, metallic nickel plates (purity ≥99.9%), metallic molybdenum bars (purity ≥99.9%), metallic chromium (purity ≥99.9%), and industrial pure iron are sequentially added to a vacuum induction furnace and smelted at 1510±20℃ until the furnace charge is fully melted. Then, graphite carbon is added sequentially from the external charging bin and smelting continues for 60 minutes (vacuum carbon deoxidation is used for smelting). Next, electrolytic vanadium (purity ≥99.9%) is added and smelted for 3 minutes, followed by electrolytic manganese (purity ≥99.9%) and sponge titanium (purity ≥99.9%), and then smelted for 3 minutes. Smelting continues until the furnace charge is fully melted. Finally, the furnace charge is cast (casting is carried out in an argon atmosphere at a temperature of 1530℃) into steel ingots.

[0073] The steel ingot is heated (heating rate of 6℃ / min) to 1150℃ and held at that temperature for 10 hours for high-temperature homogenization treatment. Then it is cooled to room temperature in the furnace with the furnace door closed.

[0074] After the head and tail of the homogenized ingot are cut off, it is placed at 1200℃ for 2 hours for high-temperature diffusion annealing. The original billet is forged into a square billet using a 750 air hammer at a forging temperature of 1150℃ and a forging method of free forging. Then it is cooled to room temperature in air.

[0075] The forged billet was heated to 1200℃ and held for 2 hours. It was then hot-rolled into a 12mm thick hot-rolled plate through multiple passes using a rolling mill. The initial rolling temperature was 1150℃, and the final rolling temperature was 950℃ (maximum reduction per pass 35%, total reduction 66%). The billet was then heated (heating rate 6℃ / min) to 880℃ and held for 3 hours, followed by water cooling to room temperature. The temperature was then raised again (heating rate 6℃ / min) to 6300℃ and held for 2 hours. Finally, it was cooled in air to room temperature to obtain ultra-high strength high-low temperature toughness marine structural steel (microstructure is tempered martensite, hardness is 27.73HRC, yield strength is 841.95MPa, tensile strength is 888.28MPa, ductile-brittle transition temperature (DBTT) is -110.12℃, which meets the requirements).

[0076] Figure 2 This is a schematic diagram of the microstructure of the ultra-high strength, high and low temperature toughness marine structural steel. Figure 2 It is known that the microstructure of the ultra-high strength high and low temperature toughness marine structural steel includes tempered martensite and ferrite.

[0077] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A high-strength, high- and low-temperature toughness marine structural steel, characterized in that, The composition, by mass percentage, consists of the following elements: C: 0.08~0.12%, Si: 0.18~0.22%, Mn: 0.48~0.58%, Cr: 0.48~0.58%, Ni: 4.3~4.6%, V: 0.04~0.06%, Mo: 0.35~0.45%, Ti: 0.03~0.10%, P: ≤0.008%, S: ≤0.008%, O: ≤0.001%, with the balance being Fe; The main phase of the high-strength, high-low temperature toughness marine structural steel is tempered martensite, with a hardness of 25~35HRC, a yield strength of 840~900MPa, a tensile strength of 860~930MPa, and a ductile-brittle transition temperature of -120~-100℃. The method for preparing the marine structural steel includes the following steps: According to the elemental composition of the marine structural steel, the reaction raw materials are melted and cast in sequence to obtain steel ingots; The steel ingot is subjected to high-temperature homogenization treatment, high-temperature diffusion annealing, forging, hot rolling, first heat treatment and second heat treatment in sequence to obtain high-strength and high-low temperature toughness marine structural steel. The high-temperature homogenization treatment is carried out at a temperature of 1100~1200℃ for 9~12 hours. The high-temperature diffusion annealing was performed at 1200°C for 2 hours. The forging process uses a 750 air hammer to forge the original billet obtained by high-temperature diffusion annealing into a square billet. The forging temperature is 950~1150℃, and the forging method is free forging; The hot rolling process is as follows: the forged billet is heated to 1200℃ and held for 2 hours, and then hot rolled in multiple passes into a hot-rolled plate with a thickness of 12mm. The initial rolling temperature of the multi-pass hot rolling is 1150℃, and the final rolling temperature is 950℃. The temperature of the first heat treatment is 830~900℃, and the holding time is 3h; The second heat treatment temperature is 620~650℃, and the holding time is 2h.

2. The method for preparing marine structural steel according to claim 1, characterized in that, Includes the following steps: According to the elemental composition of the marine structural steel as described in claim 1, the reaction raw materials are sequentially smelted and cast to obtain steel ingots; The steel ingot is subjected to high-temperature homogenization treatment, high-temperature diffusion annealing, forging, hot rolling, first heat treatment and second heat treatment in sequence to obtain high-strength and high-low temperature toughness marine structural steel. The high-temperature homogenization treatment is carried out at a temperature of 1100~1200℃ for 9~12 hours. The high-temperature diffusion annealing was performed at 1200°C for 2 hours. The forging process uses a 750 air hammer to forge the original billet obtained by high-temperature diffusion annealing into a square billet. The forging temperature is 950~1150℃, and the forging method is free forging; The hot rolling process is as follows: the forged billet is heated to 1200℃ and held for 2 hours, and then hot rolled in multiple passes into a hot-rolled plate with a thickness of 12mm. The initial rolling temperature of the multi-pass hot rolling is 1150℃, and the final rolling temperature is 950℃. The temperature of the first heat treatment is 830~900℃, and the holding time is 3h; The second heat treatment temperature is 620~650℃, and the holding time is 2h.

3. The preparation method according to claim 2, characterized in that, The smelting process includes: mixing metallic nickel, metallic molybdenum, metallic chromium and metallic iron, melting them, and then sequentially adding graphite carbon, vanadium, manganese and titanium under vacuum conditions for smelting and deoxidation. The method of adding graphite carbon and then performing smelting and deoxidation is called vacuum carbon deoxidation.

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