460 mpa-grade marine engineering steel having excellent fracture resistance and manufacturing method therefor

Abstract

A 460 MPa-grade marine engineering steel having excellent fracture resistance and a manufacturing method therefor, said steel comprising: C: 0.05%-0.10%; Si: 0.15%-0.40%; Mn: 1.40%-2.00%; P≤0.008%; S≤0.002%; Cu: 0.15%-0.45%; Ni: 0.50%-1.00%; Cr: 0.15%-0.40%; Nb: 0.015%-0.05%; Ti: 0.007%-0.02%; Alt: 0.015%-0.055%; O≤0.001%; N≤0.005%; and the balance being Fe and unavoidable impurities, wherein Pcm≤0.24%. By means of optimization of raw material composition screening and proportioning, a preparation process, microstructure strength and toughness matching, etc., a 460 MPa-grade marine engineering steel having excellent fracture resistance and a maximum thickness of 100 mm is obtained.

Description

A 460MPa grade marine steel with excellent fracture resistance and its preparation method Technical Field

[0001] This invention relates to the fields of materials and metallurgy, and more specifically, to a 460MPa grade marine steel with excellent fracture resistance and its preparation method. Background Technology

[0002] With the rapid development of science and technology and the rapid expansion of oil and gas field development, research on large marine engineering equipment such as jacket platforms is gradually extending to deep-water areas. These large marine engineering devices are submerged in seawater year-round, operating in extremely harsh environments. They are subjected not only to their own weight but also to the effects of service conditions. Considering water depth, seawater corrosion, and extreme weather events such as submarine earthquakes, typhoons, and storms, existing 355MPa and 420MPa grade materials are no longer sufficient to meet the demands of future large-scale deep-water jacket platforms. There is an urgent need to develop ultra-high-strength marine engineering steel with superior fracture resistance.

[0003] Patent CN102732781A discloses a steel for offshore platforms with a CTOD of ≥2 mm at -40℃ and its manufacturing method. However, its maximum finished product thickness is only 60 mm, and it does not address the CTOD performance of welded joints, limiting its application. Patent CN101578384A discloses a steel with excellent CTOD characteristics in the welded heat-affected zone and its manufacturing method, employing a CR / TMCP / DQ process. However, its strength level is only 420 MPa, which cannot meet the strength requirements of future deep-water jacket platforms, and its maximum thickness is only 60 mm, further limiting its application. Patent CN113549826A discloses a marine engineering steel with excellent CTOD performance for welded joints and its manufacturing method. It employs a low-C, low-Mn composition design, combined with TMCP rolling and tempering processes, producing a steel plate with a microstructure of tempered bainite or tempered martensite. However, the yield strength of the steel plate cannot meet the tensile strength requirements of 460 MPa grade steel plates, failing to meet the requirements of ultra-high strength marine engineering steel for offshore engineering platform equipment. Patent CN108474089A discloses a thick steel plate with excellent low-temperature toughness and resistance to hydrogen-induced cracking, and its manufacturing method. However, it uses a quenching and tempering process, which is complex, and the yield strength of the steel plate still cannot meet the tensile strength requirements of 460 MPa grade steel plates, failing to meet the requirements of ultra-high strength marine engineering steel for offshore engineering platform equipment.

[0004] Therefore, current marine engineering steel products have limitations such as small thickness specifications, low strength levels, narrow application range, and lack of post-weld performance considerations, failing to meet the requirement of marine engineering steel to achieve excellent post-weld performance.

[0005] Summary of the Invention

[0006] The purpose of this invention is to overcome the aforementioned defects in the existing technology. This invention has conducted extensive and systematic experimental research on several aspects, including alloy element screening and proportioning, steel cleanliness control, continuous casting process, coupled design optimization of heating and low-overheat austenitic rolling process, and microstructure strength and toughness matching. Finally, a composition system and its manufacturing process that can meet the purpose of this invention were determined. By optimizing the preparation process parameters, a 460MPa grade marine engineering steel with excellent fracture resistance and a maximum thickness of up to 100mm was obtained, along with its preparation method. It has the characteristics of ultra-high strength and plasticity, excellent low-temperature toughness, excellent Z-axis properties, excellent fracture resistance, and uniform microstructure.

[0007] To achieve the above objectives, the technical solution of the present invention is as follows:

[0008] A 460MPa grade marine steel with excellent fracture resistance comprises the following components by mass percentage: C: 0.05%–0.10%, Si: 0.15%–0.40%, Mn: 1.40%–2.00%, P≤0.008%, S≤0.002%, Cu: 0.15%–0.45%, Ni: 0.50%–1.00%, Cr: 0.15%–0.40%, Nb: 0.015%–0.00%. 0.05%, Ti: 0.007%~0.02%, Alt: 0.015%~0.055%, O≤0.001%, N≤0.005%, balance being Fe and unavoidable impurities, the weld crack sensitivity index Pcm of the 460MPa grade marine steel is ≤0.24%; where Pcm=C+Si / 30+Mn / 20+Cu / 20+Ni / 60+Cr / 20+Mo / 15+V / 10+5B.

[0009] The present invention also discloses a method for preparing 460MPa grade marine steel with excellent fracture resistance as described above, comprising the following steps: smelting, continuous casting, primary rolling, primary cooling, secondary rolling, secondary cooling, and slow cooling;

[0010] After the molten steel is smelted, it is continuously cast into billets to obtain billets;

[0011] The billet is rolled once, with a heating temperature of 1150℃~1190℃, a soaking temperature of 1130℃~1170℃, and a soaking time of 30min~50min; the initial rolling temperature is 1000℃~1050℃, and the reduction rate of a single longitudinal rolling pass is ≥15%, except for the widening pass.

[0012] After one rolling process, the billet is cooled once using a DQ and ACC rapid cooling system with an average cooling rate of ≥3℃ / s, and the final cooling temperature is ≤300℃.

[0013] The billet after one cooling is rolled twice, with a heating temperature of 930℃~980℃, a soaking temperature of 900℃~950℃, and a soaking time of 40min~60min; the initial rolling temperature is 810℃~860℃, the final rolling temperature is 790℃~840℃, and the minimum reduction rate per pass in the two stages is ≥12%.

[0014] The billet after the second rolling is subjected to a second cooling. After the second rolling, the initial cooling temperature is 760℃~800℃. A DQ and ACC rapid cooling system with an average cooling rate of 6℃ / s~10℃ / s is used. The final cooling temperature is 450℃~500℃. After slow cooling, the 460MPa grade marine steel is obtained.

[0015] Implementing the embodiments of the present invention will have the following beneficial effects:

[0016] (1) This invention designs the optimal alloy composition system and optimizes the controlled rolling and cooling process to produce steel plates with good comprehensive mechanical properties. It solves the problem of the difficulty in matching and harmonizing the strength, plasticity, low temperature toughness (-60℃) and fracture resistance of 460MPa grade steel plates. The produced steel plates have a yield strength ≥460MPa, tensile strength 540MPa~720MPa, elongation after fracture ≥19%, impact absorption energy of the base material at -60℃ and after welding ≥100J, Z-direction reduction of area ≥60%, and CTOD value of the base material at -10℃ and after welding ≥0.46mm. The steel plates have the characteristics of uniform microstructure and properties.

[0017] (2) The preparation method of the present invention has the characteristics of low production cost and strong process applicability. It gives full play to the technical equipment advantages of the wide and thick plate rolling mill, and the maximum thickness of the ultra-high strength and high toughness marine engineering steel thick plate can reach 100mm.

[0018] (3) The microstructure of the steel plate of the present invention is ferrite + bainite. Attached Figure Description

[0019] Figure 1(a) is a cross-sectional 1 / 4 metallographic morphology diagram of a 100mm thick steel plate of Embodiment 2 of the present invention.

[0020] Figure 1(b) is a half-section metallographic morphology of a 100mm thick steel plate of Embodiment 2 of the present invention. Detailed Implementation

[0021] The present invention will be further described below with reference to specific embodiments, but this does not limit the present invention in any way.

[0022] I. Chemical composition and mechanical properties

[0023] This invention discloses a 460MPa grade marine engineering steel with excellent fracture resistance, comprising the following components by mass percentage: C: 0.05%–0.10%, Si: 0.15%–0.40%, Mn: 1.40%–2.00%, P≤0.008%, S≤0.002%, Cu: 0.15%–0.45%, Ni: 0.50%–1.00%, Cr: 0.15%–0.40%, Nb: 0.015%–0.05%, Ti: 0.007%–0.02%, Alt: 0.015%–0.055%, O≤0.001%, N≤0.005%, with the balance being Fe and unavoidable impurities. The weld crack sensitivity index Pcm of the 460MPa grade marine engineering steel is ≤0.24%.

[0024] Wherein, Pcm=C+Si / 30+Mn / 20+Cu / 20+Ni / 60+Cr / 20+Mo / 15+V / 10+5B.

[0025] Specifically, the mechanism of action of each alloy component in the steel of this invention is as follows:

[0026] Carbon (C) is an essential element for ensuring strength and hardenability. It plays a significant role in improving the strength of steel through solid solution strengthening and precipitation strengthening. However, an increase in carbon content seriously affects the weldability and low-temperature toughness of steel. From the perspective of product performance, the C content should be controlled between 0.05% and 0.10%.

[0027] Si is a solid solution strengthening element and a deoxidizing element. Although Si is beneficial to improving the strength of steel plates and their oxidation resistance at high temperatures, Si promotes the coarsening of packet dimensions, which seriously impairs the low-temperature toughness, elongation, fracture resistance and weldability of ultra-high strength steel plates. Considering the economics and operability of steelmaking, the Si content is controlled at 0.15% to 0.40%.

[0028] Mn, as the most important alloying element in steel, not only improves the strength of steel plates, but also expands the austenite phase region, lowers the Ar3 point temperature, and refines ferrite grains, thereby improving the low-temperature toughness of steel plates. However, when the quality of Mn is too high, Mn segregation and strip-shaped MnS are easily formed, resulting in poor fracture resistance and low-temperature toughness of the core of thick plates, and a decrease in the performance of the weld heat-affected zone. Therefore, the Mn content is controlled at 1.40% to 2.00%.

[0029] P is an element that adversely affects fracture resistance. It can segregate in the center of the slab and accumulate at grain boundaries, thus damaging the base material and post-weld properties. The P content should be controlled to be no higher than 0.008%.

[0030] S is an element that adversely affects fracture resistance. It can form sulfide inclusions, which can become crack initiation sites. The S content should be controlled to be no higher than 0.002%.

[0031] Cu (Cu) precipitation strengthening can significantly improve the strength of steel; appropriate amounts of Cu can increase strength without compromising impact toughness. Cu is also an austenite-forming element, expanding the austenite phase region and promoting austenite formation and stability during controlled cooling and self-tempering. Using Cu and Ni together can not only significantly reduce Ar3 content but also prevent hot brittleness. However, excessive Cu content can cause hot brittleness, which is detrimental to the properties of the base metal and the heat-affected zone; therefore, the Cu content range is 0.15%–0.45%.

[0032] Ni dissolves in austenite, inhibiting austenite recrystallization, refining austenite grains, and improving the low-temperature toughness and fracture resistance of steel plates. Ni can reduce the diffusion rate of various elements in steel, thus delaying the decomposition and transformation of austenite and improving the steel's permeability. However, excessive Ni content will increase Pcm (partial weld compensator) and affect weldability; therefore, the Ni content should be controlled between 0.50% and 1.00%.

[0033] Cr can improve the hardenability and strength of steel plates. Cr can also inhibit the transformation of proeutectoid ferrite and pearlite, which is beneficial for obtaining acicular ferrite structure. Cr has a similar solid solution strengthening effect to Mn and is less prone to segregation. However, excessive Cr content increases the tendency for temper brittleness and increases welding difficulty, while excessively low content cannot effectively exert its strengthening effect. Therefore, the Cr content should be controlled at 0.15% to 0.40%.

[0034] The addition of nitrogen (Nb) promotes grain refinement of the steel rolling microstructure, simultaneously improving strength and toughness. Niobium effectively refines the microstructure during controlled rolling by inhibiting austenite recrystallization and strengthens the matrix through precipitation. Furthermore, the presence of Nb precipitates increases the density of effective hydrogen traps, resulting in smaller hydrogen-induced cracks. However, when the Nb content exceeds a certain range, MA islands will form in the welded hardened area (HAZ), which is detrimental to toughness and fracture resistance. Therefore, the Nb content is preferably controlled between 0.015% and 0.05%.

[0035] Ti inhibits excessive austenite grain growth during slab heating. Its trace amounts lead to the formation of nitrides, carbides, or carbonitrides, resulting in grain refinement and improved toughness of the base metal. However, excessive Ti content can cause excess Ti to precipitate as TiC on bainite laths and grain boundaries, severely degrading the low-temperature toughness and fracture resistance of the steel plate. Therefore, the Ti content is controlled between 0.007% and 0.02%.

[0036] Al is a deoxidizing and grain-refining element. At high temperatures, Al forms fine AlN precipitates, which inhibit austenite grain growth during the austenitization process of slabs / steel plates, thus refining the austenite grains and improving the toughness of the steel at low temperatures. Excessive Al content leads to the formation of larger Al oxides, reducing the low-temperature impact resistance of the steel plate. Furthermore, it makes the slab prone to edge and corner cracks during continuous casting. Therefore, the preferred Al content is controlled between 0.015% and 0.055%.

[0037] O is an element that negatively affects impact toughness. It combines with other elements in steel to form non-metallic inclusions, which become crack initiation sites. The O content should be controlled to be no higher than 0.001%.

[0038] Nitrogen (N) reacts with aluminum (AL) to form coarse AlN crystals that precipitate along the original austenite grain boundaries, affecting the hardenability and low-temperature impact toughness of the steel. In this invention, the N content is controlled to be no higher than 0.005%.

[0039] Furthermore, the matrix microstructure of the 460MPa grade marine steel with excellent fracture resistance of the present invention is ferrite and bainite.

[0040] The mechanical properties of the 460MPa grade marine steel with excellent fracture resistance of the present invention are as follows: the thickness of the 460MPa grade marine steel with excellent fracture resistance of the present invention is 80mm to 100mm; the yield strength of the 460MPa grade marine steel is ≥460MPa, the tensile strength is 540MPa to 720MPa, the elongation after fracture is ≥19%, the impact absorption energy of the base metal at -60℃ and after welding is ≥100J, the Z-direction reduction of area is ≥60%, and the CTOD of the base metal at -10℃ and after welding is ≥0.46mm.

[0041] II. Production Process Technology

[0042] The present invention also discloses a method for preparing 460MPa grade marine steel with excellent fracture resistance as in any embodiment of the present invention, comprising the following steps: smelting, continuous casting, primary rolling, primary cooling, secondary rolling, secondary cooling, and slow cooling.

[0043] Based on the above technical solution, the preparation method further includes the following steps:

[0044] (1) After smelting the molten steel, it is continuously cast into billets to obtain billets.

[0045] In one specific embodiment, molten steel is pretreated, smelted, and refined sequentially, followed by vacuum treatment, and then continuously cast into billets to obtain cast billets. In the molten iron pretreatment, the molten iron undergoes desulfurization, resulting in an S content of ≤0.012%. In the converter smelting, the furnace charge includes molten iron pretreated with iron and scrap steel. The molten steel after being tapped from the converter undergoes LF refining for 20-40 minutes, followed by RH treatment for 20-40 minutes, with nitrogen blowing throughout the RH treatment to control O, H, and N content. In continuous casting, the superheat of the molten steel in the ladle is 23°C-28°C, with continuous protective casting. Electromagnetic stirring is activated, combined with a light reduction process of 6-9 mm, resulting in a billet thickness of 300-360 mm. After the billets are removed from the casting line, they are stacked for slow cooling at a stacking temperature ≥650°C at a rate of 5-6°C / h.

[0046] (2) The billet is rolled once, with a heating temperature of 1150℃~1190℃, a soaking temperature of 1130℃~1170℃, and a soaking time of 30min~50min. After the billet is removed from the furnace, it is descaled by high pressure water for 2 to 3 times to remove the iron oxide scale on the surface and reduce the temperature of the continuous casting billet. The initial rolling temperature is 1000℃~1050℃. Except for the widening pass, the reduction rate of the longitudinal rolling single pass is ≥15% to improve the as-cast structure of the slab. The thickness of the steel plate after the initial rolling is 1.8 to 2.4 times the thickness of the finished steel plate.

[0047] (3) After the billet is rolled once, it is cooled once by using the DQ and ACC rapid cooling system with an average cooling rate of ≥3℃ / s and a final cooling temperature of ≤300℃.

[0048] (4) The billet after initial cooling is subjected to secondary rolling. The heating temperature is 930℃~980℃, the soaking temperature is 900℃~950℃, and the soaking time is 40min~60min. After soaking and exiting the furnace, the billet is descaled by high-pressure water for 1 to 3 passes. This removes the surface iron oxide scale and forms a temperature gradient in the thickness direction of the steel plate, ensuring that the rolling penetrates to the core of the steel plate. The initial rolling temperature is 810℃~860℃, and the final rolling temperature is 790℃~840℃. The minimum reduction rate per pass in the second stage is ≥12%. The purpose is to fully deform the austenite grains, provide energy storage and location for the phase deformation nuclei, increase the phase deformation nucleation rate, and further reduce the ferrite grains in the two-phase region, ultimately achieving the purpose of grain refinement.

[0049] (5) The billet after the second rolling is subjected to a second cooling. After the second rolling, the initial cooling temperature is 760℃~800℃. The DQ and ACC rapid cooling systems with an average cooling rate of 6℃ / s~10℃ / s are used. The final cooling temperature is 450℃~500℃. After slow cooling, 460MPa grade marine steel is obtained.

[0050] (6) After secondary cooling, place it in a slow cooling tank. The temperature of the tank should not be lower than 350℃ and the slow cooling time should be ≥24h.

[0051] Specifically, this invention has conducted extensive and systematic experimental research on several aspects, including alloy element screening and proportioning, steel cleanliness control, continuous casting process, coupled design optimization of heating and low-overheat austenitic rolling process, and microstructure strength and toughness matching. Finally, a composition system and manufacturing process that can meet the purpose of this invention were determined. By optimizing the preparation process parameters, a 460MPa grade marine engineering steel with excellent fracture resistance and a maximum thickness of up to 100mm was finally obtained. It has the characteristics of ultra-high strength and plasticity, excellent low-temperature toughness, excellent Z-axis properties, excellent fracture resistance, and uniform microstructure.

[0052] The following are specific embodiments.

[0053] The chemical composition and welding crack sensitivity coefficient Pcm of the steel in Examples 1-10 of this invention are shown in Table 1. The smelting, continuous casting, and slow cooling processes of the billet in Examples 1-10 of this invention are shown in Table 2. The primary rolling and primary cooling processes in Examples 1-10 of this invention are shown in Table 3. The secondary rolling and secondary cooling processes in Examples 1-10 of this invention are shown in Table 4. The slow cooling processes in Examples 1-10 of this invention are shown in Table 5. The mechanical properties of the steel plate base material in Examples 1-10 of this invention are shown in Table 6. The welding performance of the steel plates in Examples 1-10 of this invention is shown in Table 7. The CTOD performance of the welded joints of the steel in Examples 1-10 of this invention is shown in Table 8. The metallographic morphology of a 1 / 4 cross-section of a 100mm thick steel plate in Example 2 of this invention is shown in Figure 1(a). The metallographic morphology of a 1 / 2 cross-section of a 100mm thick steel plate in Example 2 of this invention is shown in Figure 1(b).

[0054] Table 1. Chemical composition of steel and welding crack sensitivity coefficient (%) of Examples 1-10 of the present invention

[0055] Table 2. Smelting, continuous casting, and billet slow cooling processes of Examples 1-10 of the present invention.

[0056] Table 3. Single rolling and single cooling processes of Examples 1-10 of the present invention.

[0057] Table 4. Secondary rolling and secondary cooling processes in Examples 1-10 of the present invention.

[0058] Table 5. Slow cooling process of Examples 1-10 of the present invention

[0059] Table 6 Mechanical properties of the steel plate base material in Examples 1-10 of the present invention

[0060] Table 7 Welding performance of steel plates in Examples 1-10 of the present invention

[0061] Table 8. CTOD performance of steel welded joints in Examples 1-10 of the present invention

[0062] Based on the above results, it can be seen that this invention has conducted extensive and systematic experimental research on several aspects, including alloy element screening and proportioning, steel cleanliness control, continuous casting process, coupled design optimization of heating and low-overheat austenitic rolling process, and microstructure strength and toughness matching. Ultimately, a compositional system and manufacturing process that meet the objectives of this invention were determined. By optimizing the preparation process parameters, a 460MPa grade marine engineering steel with excellent fracture resistance and a maximum thickness of 100mm was obtained. The steel plate has a ferrite + bainite microstructure, exhibiting ultra-high strength and plasticity, excellent low-temperature toughness, excellent Z-axis properties, excellent fracture resistance, and uniform microstructure. The produced steel plate has a yield strength ≥460MPa, tensile strength 540MPa~720MPa, elongation after fracture ≥19%, impact absorption energy of the base metal at -60℃ and after welding ≥100J, Z-axis reduction of area ≥60%, and CTOD of the base metal at -10℃ and after welding ≥0.46mm.

[0063] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the patent application. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the scope of protection of the present invention. Therefore, the scope of protection of this patent should be determined by the appended claims.

Claims

1. A 460MPa grade marine steel with excellent fracture resistance, characterized in that, Includes the following components by mass percentage: C: 0.05%–0.10%, Si: 0.15%–0.40%, Mn: 1.40%–2.00%, P≤0.008%, S≤0.002%, Cu: 0.15%–0.45%, Ni: 0.50%–1.00%, Cr: 0.15%–0.40%, Nb: 0.015%–0.05%, Ti: 0.007%–0.02%, Alt: 0.015%–0.055%, O≤0.001%, N≤0.005%, with the balance being Fe and unavoidable impurities. The weld crack sensitivity index Pcm of the 460MPa grade marine steel is ≤0.24%. Wherein, Pcm=C+Si / 30+Mn / 20+Cu / 20+Ni / 60+Cr / 20+Mo / 15+V / 10+5B.

2. The 460MPa grade marine steel with excellent fracture resistance according to claim 1, characterized in that, The thickness of the 460MPa grade marine steel is 80mm to 100mm.

3. The 460MPa grade marine steel with excellent fracture resistance according to claim 1, characterized in that, The matrix microstructure of the 460MPa grade marine steel consists of ferrite and bainite.

4. The 460MPa grade marine steel with excellent fracture resistance according to claim 1, characterized in that, The 460MPa grade marine steel has a yield strength ≥460MPa, tensile strength 540MPa~720MPa, elongation after fracture ≥19%, impact absorption energy of the base metal at -60℃ and after welding ≥100J, Z-direction reduction of area ≥60%, and CTOD of the base metal at -10℃ and after welding ≥0.46mm.

5. A method for preparing 460MPa grade marine steel with excellent fracture resistance as described in any one of claims 1-4, characterized in that, Includes the following steps: Smelting, continuous casting, primary rolling, primary cooling, secondary rolling, secondary cooling, slow cooling; After the molten steel is smelted, it is continuously cast into billets to obtain billets; The cast billet is subjected to a single rolling process, with a heating temperature of 1150℃~1190℃, a soaking temperature of 1130℃~1170℃, and a soaking time of 30min~50min; the initial rolling temperature is 1000℃~1050℃, except for the widening pass. The single-pass reduction rate in longitudinal rolling is ≥15%; After one rolling process, the billet is cooled once using a DQ and ACC rapid cooling system with an average cooling rate of ≥3℃ / s, and the final cooling temperature is ≤300℃. The billet after one cooling is rolled twice, with a heating temperature of 930℃~980℃, a soaking temperature of 900℃~950℃, and a soaking time of 40min~60min; the initial rolling temperature is 810℃~860℃, the final rolling temperature is 790℃~840℃, and the minimum reduction rate per pass in the two stages is ≥12%. The billet after the second rolling is subjected to a second cooling. After the second rolling, the initial cooling temperature is 760℃~800℃. A DQ and ACC rapid cooling system with an average cooling rate of 6℃ / s~10℃ / s is used. The final cooling temperature is 450℃~500℃. After slow cooling, the 460MPa grade marine steel is obtained.

6. The method for preparing 460MPa grade marine steel with excellent fracture resistance according to claim 5, characterized in that, In the aforementioned single rolling process, after the billet is hot-sinked and exits the furnace, it undergoes two to three high-pressure water descaling processes; the thickness of the steel plate after billet preparation is 1.8 to 2.4 times the thickness of the finished steel plate. In the secondary rolling process, after the billet is heated and exited from the furnace, it undergoes one to three high-pressure water descaling processes.

7. The method for preparing 460MPa grade marine steel with excellent fracture resistance according to claim 5, characterized in that, The smelting process includes deep desulfurization of molten iron, converter smelting, ladle refining, and vacuum treatment.

8. The method for preparing 460MPa grade marine steel with excellent fracture resistance according to claim 7, characterized in that, In the hot metal pretreatment, the hot metal is desulfurized, and the sulfur content of the hot metal after pre-desulfurization is ≤0.012%. In the converter smelting, the furnace charge includes the hot metal after the hot metal pretreatment and scrap steel. The molten steel after being tapped from the converter smelting is refined by LF for 20 min to 40 min, and the LF-refined molten steel is treated by RH for 20 min to 40 min. Nitrogen is blown throughout the RH treatment to control the O, H and N content in the steel.

9. The method for preparing 460MPa grade marine steel with excellent fracture resistance according to claim 5, characterized in that, In the continuous casting process, the superheat of the molten steel in the ladle is 23℃~28℃, the casting is carried out under full protection, the light reduction is 6mm~9mm, the thickness of the continuous casting billet is 300mm~360mm, the continuous casting billet is stacked and slowly cooled after leaving the line, the stacking temperature is ≥650℃, and the stacking slow cooling rate is 5℃ / h~6℃ / h.

10. The method for preparing 460MPa grade marine steel with excellent fracture resistance according to claim 5, characterized in that, In the slow cooling process, after secondary cooling, the product is placed in a slow cooling tank with an inlet temperature of not less than 350°C and a slow cooling time of ≥24 hours.

Images