355mpa grade super-thick marine steel against bauschinger effect and manufacturing method thereof

Abstract

The present application relates to the field of steel material preparation, in particular to a 355MPa grade super-thick marine steel plate with anti-Bauschinger effect and a manufacturing method thereof.The steel plate is composed of the following chemical components by weight percentage: C: 0.05%~0.085%, Si: 0.12%~0.25%, Mn: 1.05%~1.45%, P≤0.02%, S≤0.01%, Als: 0.03%~0.05%, Ni: 0.2%~0.7%, Cr: 0.2%~0.4%, Mo: 0.05%~0.3%, Co: 0.1%~0.4%, Nb: 0.01%~0.05%, V: 0.01%~0.05%, Ti: 0.005%~0.01%, N: 0.004%~0.008%, Zr: 0.05%~0.3%, Sb: 0.05%~0.15%, and the rest is Fe and inevitable impurities.The thickness of the finished steel plate can reach 120mm, and the mechanical properties and high service safety performance can meet the service conditions of polar marine engineering equipment, and the steel plate has excellent anti-Bauschinger effect.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of steel material preparation, in particular to a 355MPa grade super-thick offshore steel with anti-Bauschinger effect and a manufacturing method thereof. BACKGROUND

[0002] Since the 21st century, the rapid development of ships and ocean engineering, the demand for cold and Arctic routes has grown, driving the demand for high-performance offshore steel with high low-temperature resistance, corrosion resistance, and anti-Bauschinger effect. The Arctic has huge potential for resource development, but also faces environmental challenges.

[0003] The research and development of new offshore steel focuses on micro-alloying, controlled rolling and controlled cooling, and optimization of heat treatment processes to improve low-temperature resistance, corrosion resistance, and anti-Bauschinger effect. Micro-alloying enhances steel plate performance by adding trace elements, while innovative rolling and heat treatment processes can optimize the organization, improve low-temperature toughness and fatigue resistance. Anti-Bauschinger effect can significantly improve the reliability and safety of equipment, and needs to be combined with material pre-stretching detection and other methods to meet high service safety requirements.

[0004] Currently, offshore steel can meet most of the needs, but special steel with ultra-low brittle transition temperature, excellent anti-Bauschinger effect, and corrosion resistance is still the global research focus. However, such high-end steel production costs are high, and the process is complex, and performance, cost, and feasibility need to be balanced. In addition, new technologies such as ultrasonic surface modification are also being explored to further improve material performance.

[0005] The Chinese patent application with application number CN202210834931.6 "High-strength low-carbon equivalent super-thick steel plate with good low-temperature toughness and manufacturing method thereof" proposes a normalized super-thick steel plate with high C and Mn low alloy. The C and Mn content is relatively high, combined with normalizing + tempering process, which belongs to the normalizing steel alloy composition system, the high carbon equivalent of the steel plate leads to high dislocation density during deformation, and the steel plate cannot meet the-60℃ low-temperature toughness while obtaining excellent anti-Bauschinger effect. The Chinese patent application with application number CN202311405045.2 "Production method of thick gauge offshore wind power pipe pile steel" proposes a normalized steel plate for-20℃ environment, which adopts high C high Mn low Ni alloy composition design combined with conventional normalizing heat treatment process, and can only produce steel plates for service use at room temperature. The composition system and production process determine that the invention does not have the ability to resist Bauschinger effect. SUMMARY

[0006] In order to overcome the deficiencies of the prior art, the present application provides a 355MPa grade extra-thick marine steel resistant to Bauschinger effect and a manufacturing method thereof, the thickness of the finished steel plate can reach 120mm, the low-temperature impact energy of the core of the steel plate is greater than or equal to 150J at-60℃, the yield strength reduction of the steel plate is less than or equal to 10% at 2% residual strain, the steel plate is excellent in Bauschinger effect resistance, and the uniform elongation is greater than or equal to 12%.

[0007] In order to achieve the above-mentioned purpose, the present application adopts the following technical scheme:

[0008] A 355MPa grade extra-thick marine steel resistant to Bauschinger effect is composed of the following chemical components by weight percentage:

[0009] C: 0.05%~0.085%, Si: 0.12%~0.25%, Mn: 1.05%~1.45%, P≤0.02%, S≤0.01%, Als: 0.03%~0.05%, Ni: 0.2%~0.7%, Cr: 0.2%~0.4%, Mo: 0.05%~0.3%, Co: 0.1%~0.4%, Nb: 0.01%~0.05%, V: 0.01%~0.05%, Ti: 0.005%~0.01%, N: 0.004%~0.008%, Zr: 0.05%~0.3%, Sb: 0.05%~0.15%, and the rest is Fe and inevitable impurities.

[0010] The roles of the above alloy element types and contents are as follows:

[0011] 1. C is the basic strengthening element in steel, which improves the strength and hardness of the steel plate, and the C content is too low, which will reduce the solid solution content and carbide content of C, reduce the fine grain strengthening effect, and reduce the strength of the steel plate and the resistance to Bauschinger effect. If the C content is too high, hardening structure will be produced, and the low-temperature toughness of the steel plate will be reduced, so the C content is accurately controlled. Therefore, the C content is accurately controlled to be 0.05%~0.085% in the present application.

[0012] 2. Si can improve the strength and Bauschinger effect resistance of the steel plate, and appropriate addition of Si element can improve the strength and fatigue life of the steel, and Si can reduce the O content, and when the Si content is less than 0.12%, the deoxidization effect is not obvious, and when the Si content is greater than 0.25%, the low-temperature toughness will be reduced. Therefore, the Si content is accurately controlled to be 0.12%~0.25% in the present application.

[0013] 3. Mn can dissolve extensively in the Fe matrix, increasing the strength of the steel plate. When the Mn content is below 1.05%, its contribution to the strength of thick steel plates is relatively small. This patent utilizes a continuous casting heavy-pressure process to appropriately increase the Mn content, thereby improving the steel plate's resistance to the Bouschinger effect and its low-temperature toughness. However, when the Mn mass percentage exceeds 1.45%, segregation leads to poor low-temperature toughness in the core of the thick plate. Therefore, this invention precisely controls the Mn content between 1.05% and 1.45%.

[0014] 4. P and S elements have no benefit to the mechanical properties of steel plates, especially elongation. P should be controlled to ≤0.02% and S to ≤0.01%.

[0015] 5. Al is the main deoxidizing element in steel. When the Al content is too low, microalloying elements such as V and Ti are oxidized and cannot achieve the purpose of refining grains. For thicker steel plates that require resistance to the Bauschinger effect, the Al content in the steel needs to be appropriately increased. Conversely, if the Al content is too high, large inclusions will form. Therefore, this invention precisely controls the Al content at 0.03%~0.05%.

[0016] 6. The role of Ni is to improve the toughness and hot workability of steel plates. Adding a large amount can achieve a lower ductile-brittle transition temperature, improve the toughness and plasticity of the steel plate, and Ni has a certain resistance to pitting corrosion, improving the resistance of the steel plate to the Bauschinger effect under marine service conditions. Therefore, this invention precisely controls the Ni content to 0.2%~0.7%.

[0017] 7. Cr can effectively improve the strength and stiffness of steel plates; appropriately increasing the Cr content can improve the uniform deformation ability of steel plates, but excessive Cr content will reduce the impact toughness of steel plates. Therefore, this invention precisely controls the Cr content to 0.2%~0.4%.

[0018] 8. Mo can form fine carbides in steel, effectively improving the yield strength of steel plates. Furthermore, adding an appropriate amount of Mo to heat-treated steel plates can enhance their low-temperature toughness. Therefore, this invention precisely controls the Mo content to 0.05%~0.3%.

[0019] 9. Co can improve the strength and hardness of steel plates, thereby enhancing their fatigue resistance. Co can work synergistically with microalloying elements such as Nb and V to refine grain structure, improve dislocation density, and enhance the steel plate's resistance to the Bauschinger effect. However, adding excessive Co alone can lead to twinning defects in the steel plate, reducing its low-temperature toughness. Therefore, this invention precisely controls the Co content to 0.1%~0.4%.

[0020] 10、Nb is the main additive element of the present application, which improves the strength and toughness of the steel plate and the fatigue resistance of the steel plate. The undissolved Nb C, N compound particles distribute on the austenite grain boundaries during heating, which can hinder the growth of austenite grains during heating, refine the grains, and improve the low-temperature toughness; during the controlled cooling process of the steel plate, a large amount of Nb(CN) is precipitated, which further promotes dislocation tangling, grain refinement, and resistance to Bauschinger effect by acting with Co element. Therefore, the content of Nb is accurately controlled to be 0.01% to 0.05% in the present application.

[0021] 11、The V element can form V(C, N) particles in the matrix, which can refine the grains, and the addition of the V element can improve the low-temperature impact toughness and the Bauschinger effect performance of the tempered steel plate. Therefore, the content of V is accurately controlled to be 0.01% to 0.05% in the present application.

[0022] 12、The addition of Ti element is to form TiN by Ti and N to prevent the growth of grains during heating and rolling, and to improve the comprehensive performance of the steel plate by acting with Co and other elements. When Ti is less than 0.005%, the effect of strengthening and toughening the steel is not obvious, and when Ti is more than 0.01%, the toughness of the steel will deteriorate. Therefore, the content of Ti is accurately controlled to be 0.005% to 0.01% in the present application.

[0023] 13、The N element can cooperate with Nb, Ti, and V elements to form fine and dispersed N compound precipitates, which can effectively promote the nucleation and growth of intracrystalline ferrite, and effectively control the growth of original austenite grains. The increase of N content can increase the TiN in the steel. However, when the solid solution N content is too large, the toughness of the steel plate decreases, and a large number of microcracks easily appear on the surface of the steel plate. Therefore, the content of N is accurately controlled to be 0.004% to 0.008% in the present application.

[0024] 14、The Zr element has a strong degassing effect, which can remove harmful elements such as H and O in the steel, purify the steel, and thus improve the purity and quality of the steel. The Zr element can refine the grains of the steel and improve the mechanical properties of the steel. Refining the grains can increase the strength and toughness of the steel, and also help to improve the impact toughness and low-temperature performance of the steel. Therefore, the content of Zr is accurately controlled to be 0.05% to 0.3% in the present application.

[0025] 15、The Sb element can effectively improve the tensile strength, hardness, and stiffness of the steel plate, thereby improving the Bauschinger effect resistance of the steel plate. At the same time, an appropriate amount of Sb element can also improve the low-temperature toughness of the steel plate and the microstructure stability during hot working, which is beneficial to the grain refinement and strengthening and toughening during the rolling process of the steel plate. The addition of an appropriate amount of Sb element in the steel is beneficial to reducing the segregation phenomenon in the core of the thick steel plate, improving the low-temperature toughness of the core of the steel plate, and improving the corrosion resistance of the steel plate. Therefore, the content of Sb is accurately controlled to be 0.05% to 0.15% in the present application.

[0026] The 355MPa grade extra-thick marine steel plate with excellent Bauschinger effect resistance has a yield strength of ≥355MPa, a tensile strength of 490-630MPa, a transverse elongation of ≥30%, and a Charpy impact energy of ≥150J at the core of the steel plate at -60℃. The yield strength of the steel plate is reduced by ≤10% at 2% residual strain, the steel plate has excellent Bauschinger effect resistance, a uniform elongation of ≥12%, and a carbon equivalent Ceq of 0.37-0.45. The maximum thickness of the finished steel plate is 120mm.

[0027] The microstructure of the steel plate is ferrite (grain size ≥10) + pearlite, and the carbon nitride size is 5-12nm.

[0028] The manufacturing method of the 355MPa grade extra-thick marine steel plate with excellent Bauschinger effect resistance specifically comprises the following steps:

[0029] 1) Steel refining:

[0030] The molten steel is refined by a converter, an LF furnace, an RH or VD furnace, and further reduced in P, S and non-metallic inclusion content.

[0031] 2) Continuous casting:

[0032] The molten steel is fully protected during casting, and the overheat degree of the molten steel in the tundish is 15-25℃. Two sets of compression rollers at the solid-liquid two-phase zone position of the core of the continuously cast slab are selected to implement heavy pressing down, and the pressing down amount of the two sets of compression rollers is ≥10mm.

[0033] 3) Rolling:

[0034] The billet is loaded into a heating furnace at a furnace temperature of 600-800℃ and is kept for 30-90min, so that the temperature of the billet in the low-temperature stage is uniform in the thickness direction of the extra-thick billet, and internal defects of the billet caused by uneven heating of Co, Mn and Zr elements are avoided. The heating rate of the continuously cast billet in the subsequent heating process is controlled at 6-8℃ / min, so that the internal and external stresses of the billet caused by too fast heating of the billet are avoided. The heating section temperature is 1050-1120℃, the soaking temperature is 1020-1100℃, and the soaking time is 60-90min. The purpose of low-temperature soaking is to ensure that the C / N compounds of the micro-alloy are fully dissolved, while avoiding abnormal growth of the as-cast structure and internal defects caused by high Co and Zr contents in the billet.

[0035] The first stage rolling temperature is 1000-1080℃, and the intermediate blank thickness is 1.5-2.5 times of the thickness of the finished steel plate. The purpose of the first stage rolling is to increase the core deformation while keeping the core temperature of the blank high, to reduce the rolling temperature as much as possible, to avoid recrystallization after the steel plate is rolled, to harden the surface obviously, to roll with large reduction, to destroy the columnar structure in the core of the blank, and to increase the deformation of the core structure of the blank. The second stage rolling temperature is 800-850℃, the average reduction of the second stage single pass rolling is 10-14mm, and the finish rolling temperature is 750-800℃. The purpose of the second stage low temperature rolling is to increase the deformation of the core of the steel plate by using the surface temperature drop, to improve the grain size of the core of the steel plate, and to promote the flattening and fine grain of the austenite grains. The elements Co and Zr in the steel work together with the micro-alloy elements Nb and V to improve the dislocation density, to improve the yield strength and elastic deformation uniformity of the steel plate, and the single pass reduction and rolling temperature are most important for grain refinement and uniform precipitation of C / N compounds.

[0036] 4) Controlled cooling:

[0037] The water entry temperature after rolling is 630-730℃, the red temperature is 450-500℃, and the cooling rate of the surface of the steel plate is 4-7℃ / s. The purpose of the controlled cooling after rolling is to use the deformation energy accumulated by low temperature rolling, to promote the fine and dispersed distribution of a large number of carbonitride to pin the grain boundary, to promote the further refinement and nucleation of ferrite structure, to inhibit its growth, to control the grain size to be above 10, to form a uniform, fine and dispersed large angle grain boundary microstructure, and to ensure the low temperature toughness and anti-Bauschinger effect performance of the thick steel plate. Slow cooling rate will lead to the growth of ferrite pearlite structure, and fast cooling rate will lead to a large number of hardened structures in the steel plate, which will affect the low temperature toughness and anti-Bauschinger effect performance of the steel plate.

[0038] 5) Tempering:

[0039] The water entry temperature after rolling is 630-730℃, the red temperature is 450-500℃, and the cooling rate of the surface of the steel plate is 4-7℃ / s.

[0040] The purpose of quenching is to use the deformation energy accumulated by low temperature rolling, to promote the fine and dispersed distribution of a large number of carbonitride to pin the grain boundary, to promote the further refinement and nucleation of ferrite structure, to inhibit its growth, to control the grain size to be above 10, to form a uniform, fine and dispersed large angle grain boundary microstructure, and to ensure the low temperature toughness and anti-Bauschinger effect performance of the thick steel plate.

[0041] Tempering temperature 450~550℃, tempering time 1.5~2min / mm. The purpose of tempering heat treatment is to further optimize the microstructure of the steel plate by regulating the size and distribution state of the dispersed carbonitride in the steel, to replace dislocation strengthening with fine-grain strengthening and solid solution strengthening to improve the strength and toughness of the steel plate, and to weaken the strength and toughness decline caused by dislocation migration after the steel plate is pre-deformed.

[0042] Compared with the prior art, the present application has the following advantages:

[0043] 1. The present application adopts the composition design of C, Mn, Ni, Cr, Mo, Co, Nb, V, Ti, Zr and Sb elements to ensure that the steel plate has excellent strength and toughness and anti-bauschinger effect performance. The yield strength of the steel plate decreases by ≤10% at 2% residual strain. The Ni element has a certain anti-point corrosion effect, which improves the anti-bauschinger effect of the steel plate in marine service conditions. The addition of Cr element effectively improves the strength and stiffness of the steel plate. The addition of appropriate amount of Mo element improves the low temperature toughness of the steel plate and effectively improves the yield strength of the steel plate. The Co element improves the strength and hardness of the steel plate, thereby improving the fatigue resistance of the steel plate. The Co element cooperates with Nb and V micro-alloying elements in the steel to improve the dislocation density and improve the resistance of the steel plate to bauschinger effect. The addition of Ti element is to form TiN with Ti and N to prevent the growth of grains during heating and rolling, and to improve the comprehensive performance of the steel plate together with Co and other elements. The Zr element has a strong degassing effect and can remove harmful elements such as H and O in the steel to purify the steel, thereby improving the purity and quality of the steel. The Sb element can effectively improve the tensile strength, hardness and stiffness of the steel plate, thereby improving the anti-bauschinger effect of the steel plate.

[0044] 2. The innovative alloy composition system adopted in the present application can ensure that the yield strength of the steel plate is ≥355MPa, the tensile strength is 490~630MPa, the elongation is ≥30%, the Charpy impact energy at -60℃ of the core is ≥150J, the uniform elongation is ≥12%, and the carbon equivalent Ceq is 0.37~0.45.

[0045] 3. The present application adopts the C, Mn, Ni, Cr, Mo, Co, Nb, V, Ti, Zr and Sb elements to prepare high-quality heavy-pressed continuous casting billets and anti-bauschinger effect super-thick marine steel plates, which can produce high-strength low-temperature marine steel plates with a maximum thickness of 120mm. The high-efficiency temperature control rolling and tempering heat treatment can ensure the production efficiency of the steel plate while obtaining excellent anti-bauschinger effect performance.

[0046] 4. The microstructure of the steel plate of the present application is ferrite (grain size ≥10 levels) and pearlite structure, and the carbonitride size is 5~12nm, which has good mechanical properties.

[0047] 5、The present application controls cooling after rolling, utilizes the deformation energy storage accumulated by low temperature rolling, combines with a larger supercooling temperature, promotes a large amount of carbonitride to be distributed in a small and dispersed manner to pin the grain boundary, promotes the ferrite structure to be further refined and nucleated, and inhibits the growth, so that the grain size is controlled to be above 10, a uniform, small and dispersed large-angle grain boundary microstructure is formed, and the low temperature toughness and the anti-Bauschinger effect performance of the large thickness steel plate are ensured.

[0048] 6、The present application adjusts the size and distribution state of the dispersed carbonitride in the steel through tempering, further optimizes the microstructure of the steel plate, replaces dislocation strengthening with fine grain strengthening and solid solution strengthening to replace the effect of strength and toughness of the steel plate, and weakens the problem of strength and toughness reduction caused by dislocation migration after the steel plate is pre-deformed. BRIEF DESCRIPTION OF DRAWINGS

[0049] Figure 1 is a metallographic structure diagram of the embodiment 1 of the present application.

[0050] Figure 2 is a carbon complex diagram of the embodiment 1 of the present application. DETAILED DESCRIPTION

[0051] The present application discloses a 355MPa grade super-thick marine steel plate with anti-Bauschinger effect and a manufacturing method thereof. Those skilled in the art can refer to the content herein and appropriately improve the process parameters to realize. It is particularly pointed out that all similar replacements and changes are obvious to those skilled in the art, and they are regarded as included in the present application. The method and application of the present application have been described by the preferred embodiments, and the related personnel can obviously make changes or appropriate changes and combinations to the method and application described herein without departing from the content, spirit and scope of the present application, to realize and apply the present application technology.

[0052] In view of the composition and performance requirements of high-strength marine steel in extremely cold deep sea environment, the composition design of C, Si, Mn, Co, Zr and micro-alloy elements V, Nb and Ti and the key production technology of anti-Bauschinger effect super-thick marine steel plate are used to realize the purpose of the present application. Through a large number of systematic experimental researches in the aspects of alloy element screening and ratio, high-purity core heavy pressing control during continuous casting, efficient rolling process optimization and parameter selection, the alloy element ratio and production process which can meet the purpose of the present application are finally determined. The chemical composition of the steel plate of the embodiment of the present application is shown in Table 1, the continuous casting process of the embodiment of the present application is shown in Table 2, the rolling process of the embodiment of the present application is shown in Table 3, the controlled cooling and tempering process of the embodiment of the present application is shown in Table 4, and the mechanical properties of the steel plate of the embodiment of the present application are shown in Table 5.

[0053] Table 1 Chemical composition of the steel plate of the embodiment of the present application (wt%)

[0054]

[0055] Table 2 Continuous casting process of embodiments of the present invention

[0056]

[0057] Table 3 Rolling process of embodiments of the present invention

[0058]

[0059] Table 4 Controlled cooling tempering process in embodiments of the present invention

[0060]

[0061] Table 5 Mechanical properties of steel plates in embodiments of the present invention

[0062]

[0063] like Figure 1 , Figure 2 As shown, the metallographic structure of Example 1 is ferrite (grain size ≥ 10) and pearlite at 1 / 2 and 1 / 4 of the thickness of the steel plate, with an average carbonitride size of 5~12nm and good mechanical properties.

[0064] As shown in Table 5, this invention is a 355MPa grade extra-thick marine engineering steel resistant to the Bauschinger effect, with a yield strength ≥355MPa, tensile strength 490~630MPa, elongation ≥30%, and Charpy impact energy of the steel plate core at -60℃ ≥150J. At 2% residual strain, the yield strength decreases by ≤10%, exhibiting excellent resistance to the Bauschinger effect, uniform elongation ≥12%, carbon equivalent Ceq of 0.37~0.45, and the finished steel plate thickness can reach 120mm.

[0065] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A 355 MPa grade extra thick offshore steel against Bauschinger effect, characterized in that, consists of the following chemical components by weight percentage: C: 0.05%~0.085%, Si: 0.12%~0.25%, Mn: 1.05%~1.45%, P≤0.02%, S≤0.01%, Als: 0.03%~0.05%, Ni: 0.2%~0.7%, Cr: 0.2%~0.4%, Mo: 0.05%~0.3%, Co: 0.1%~0.4%, Nb: 0.01%~0.05%, V: 0.01%~0.05%, Ti: 0.005%~0.01%, N: 0.004%~0.008%, Zr: 0.05%~0.3%, Sb: 0.05%~0.15%, the rest being Fe and inevitable impurities; The microstructure of the steel plate is ferrite + pearlite structure, the ferrite grain size is ≥10 grade, and the carbon nitride size is 5~12 nm; The manufacturing method of the steel plate specifically comprises the following steps: 1) steel refining; 2) continuous casting: full-protective casting, the superheat degree of the molten steel in the tundish is 15~25℃, and the two groups of compression rollers at the solid-liquid two-phase zone position of the core of the continuous casting billet are implemented with heavy reduction; 3) two-stage controlled rolling: the first-stage rolling temperature is 1000~1080℃, and the intermediate billet thickness is 1.5~2.5 times the thickness of the finished steel plate; the second-stage open rolling temperature is 800~850℃, the average reduction rate of the second-stage single pass rolling is 10~14mm, and the finish rolling temperature is 750~800℃; 4) controlled cooling: the water entry temperature after rolling is controlled to be 630~730℃, the red temperature is 450~500℃, and the surface cooling rate of the steel plate is 4~7℃ / s; 5) tempering: the tempering temperature is 450~550℃, and the tempering time is 1.5~2min / mm.

2. The 355MPa grade super-thick marine steel plate against Bauschinger effect according to claim 1, wherein the yield strength is ≥355MPa, the tensile strength is 490~630MPa, the elongation is ≥30%, the Charpy impact energy of the steel plate core at-60℃ is ≥150J, the yield strength reduction of the steel plate at 2% residual strain is ≤10%, the uniform elongation is ≥12%, and the carbon equivalent Ceq is 0.37~0.

45.

3. The 355MPa grade super-thick marine steel plate against Bauschinger effect according to claim 1, wherein the maximum thickness of the finished steel plate is 120mm. The manufacturing method specifically comprises the following steps: 1) steel refining; 2) continuous casting:

4. A method of manufacturing a 355 MPa grade extra thick marine steel against the Bauschinger effect according to any one of claims 1 to 3, characterized in that, full-protective casting, the superheat degree of the molten steel in the tundish is 15~25℃, and the two groups of compression rollers at the solid-liquid two-phase zone position of the core of the continuous casting billet are implemented with heavy reduction; 3) two-stage controlled rolling: the first-stage rolling temperature is 1000~1080℃, and the intermediate billet thickness is 1.5~2.5 times the thickness of the finished steel plate; the second-stage open rolling temperature is 800~850℃, the average reduction rate of the second-stage single pass rolling is 10~14mm, and the finish rolling temperature is 750~800℃; 4) controlled cooling: the water entry temperature after rolling is controlled to be 630~730℃, the red temperature is 450~500℃, and the surface cooling rate of the steel plate is 4~7℃ / s; 5) tempering: ​ ​ ​ tempering temperature 450~550℃, tempering time 1.5~2min / mm.

5. The method according to claim 4, wherein the 355 MPa grade extra-thick marine steel is manufactured by, 1) refining the molten steel by a converter, a LF furnace, a RH or a VD furnace.

6. The method according to claim 4, wherein the 355 MPa grade extra-thick marine steel is manufactured by, 2) the compression roller of the two groups has a compression reduction of ≥10 mm.

7. The method according to claim 4, wherein the 355 MPa grade extra-thick marine steel is manufactured by, 3) the blank is loaded into a heating furnace at a furnace temperature of 600~800℃ and is kept for 30~90 min; the heating temperature is 1050~1120℃, the heating rate is controlled at 6~8℃ / min, the soaking temperature is 1020~1100℃, and the soaking time is 60~90 min.

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