High-uniformity extra-thick 420MPa-grade maritime work steel plate for extremely low-temperature environment and manufacturing method thereof

Abstract

The invention discloses a high-uniformity extra-thick 420MPa-grade maritime work steel plate for an extremely low temperature environment and a manufacturing method thereof, and belongs to the field of ferrous metallurgy. The steel plate comprises the following chemical components in percentage by mass: 0.03%-0.07% of C, 0.15%-0.35% of Si, 1.40%-1.70% of Mn, less than or equal to 0.002% of S, less than or equal to 0.010% of P, 0.015%-0.045% of Als, 0.03%-0.05% of Nb, 0.10%-0.30% of Cr, 0.010%-0.020% of Ti and the balance of Fe and inevitable impurities, and the carbon equivalent CEV is less than or equal to 0.36%. The manufacturing process comprises the steps of smelting, continuous casting, blank heating, controlled rolling, controlled cooling and stacking slow cooling. The thickness of the obtained steel plate is 50-100 mm, the yield strength is larger than or equal to 420 MPa, the impact energy at the temperature of-60 DEG C is larger than or equal to 200 J, the strength difference of the whole plate is smaller than or equal to 40 MPa, and the steel plate has high strength, high toughness, good weldability and high uniformity and meets the requirement for the deep and far sea extremely-low-temperature environment.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of steel metallurgy, and particularly relates to a high-uniformity super-thick 420MPa-grade marine steel plate for an extremely low-temperature environment and a manufacturing method thereof, which is suitable for manufacturing marine structural members such as offshore platforms, drilling platforms and marine oil pipelines in deep-sea extremely low-temperature (-60℃ and below) environments, and can meet the stringent requirements of the structural members on high strength, high toughness, good weldability, excellent low-temperature adaptability and uniformity of the whole plate. BACKGROUND

[0002] With the continuous extension of marine resource development to deep-sea areas, the working environments of various marine structures are becoming more and more harsh, the thickness of the steel plates used is becoming larger and larger, and the steel plates are required to have high strength, high toughness, good weldability and low-temperature environmental adaptability, and at the same time, the steel plates are required to have good uniformity of the whole plate organization performance to adapt to the complex operating environment in deep-sea areas. In order to ensure the uniformity of the whole plate organization performance of the steel plate, a quenched and tempered process is often selected for the design of the steel plate, but this will prolong the delivery cycle and increase the process cost. In addition, in order to meet the strength requirements of the 420MPa-grade steel plate, more alloying elements need to be added to the quenched and tempered process steel plate, which inevitably leads to a decrease in weldability.

[0003] In recent years, the TMCP process has been widely studied by researchers in the field and has achieved certain results, because the TMCP process can realize organization performance regulation through controlled rolling and controlled cooling, simultaneously meet the high strength and high toughness targets, and has the comprehensive advantages of efficient production, controllable cost and excellent welding performance. However, the researches are usually focused on higher strength and lower ductile-brittle transition temperature, and there is no research on the uniformity of the whole plate organization and performance of the high-strength super-thick TMCP steel plate for an extremely low-temperature environment, which may lead to greater welding stress concentration, thereby bringing the risk of structural weak points in the operation process of deep-sea marine structures, and affecting the safe operation of marine structures.

[0004] The invention patent with the publication number CN115838902A "TMCP process super-thick steel plate and production method thereof" contains C: 0.02-0.08%, Si: 0.15-0.35%, Mn: 1.40%-2.00%, P≤0.012%, S≤0.003%, Nb: 0.01%-0.03%, Ti: 0.008%-0.02%, Al: 0.015%-0.05%, and produces a maximum 150mm-thick 355-460MPa-grade super-thick steel plate by adopting a continuous casting billet twice heating and TMCP process, realizing high strength, high toughness and high weldability; the billet twice heating has large energy consumption and long cycle, the low-temperature toughness is only -40℃, and the steel plate near-surface low-temperature impact and the uniformity of the whole plate organization performance are not involved, which cannot meet the needs of the steel for deep-sea extremely low-temperature environments, and is essentially different from the present application.

[0005] Invention patent "FH420 ultra-high strength steel plate with excellent low-temperature fracture toughness and low-temperature elongation and manufacturing method" with publication number CN 118668145 A, which contains C: 0.04%~0.10%, Si: 0.08%~0.25%, Mn: 1.20%~1.80%, S≤0.002%, P≤0.008%, Als: 0.015%~0.045%, N: 0.003%~0.015%, Nb: 0.04%~0.08%, V: 0.02%~0.05%, Cu: 0.10%~0.30%, Cr: 0%~0.30%, Ni: 0.40%~1.00%, Mo: 0.10%~0.25%, Ti: 0.008%~0.018%, provides a TMCP state FH420 ship steel plate with a maximum thickness of 100mm produced by continuous casting billet, which has high strength, low-80℃ low-temperature impact resistance, -20℃ low-temperature elongation≥20%, and the low-temperature fracture CTOD characteristic value of the base material and the heat affected zone is >0.2mm. The low-temperature toughness of the steel plate near the surface is not involved, and the research on the uniformity of the whole plate structure performance is not mentioned, which is essentially different from the present invention. Nb: 0.04%~0.08%, V: 0.02%~0.05%, Cu: 0.10%~0.30%, Cr: 0%~0.30%, Ni: 0.40%~1.00%, Mo: 0.10%~0.25%, Ti: 0.008%~0.018%, provides a TMCP state FH420 ship steel plate with a maximum thickness of 100mm produced by continuous casting billet, which has high strength, low-80℃ low-temperature impact resistance, -20℃ low-temperature elongation≥20%, and the low-temperature fracture CTOD characteristic value of the base material and the heat affected zone is >0.2mm. The low-temperature toughness of the steel plate near the surface is not involved, and the research on the uniformity of the whole plate structure performance is not mentioned, which is essentially different from the present invention.

[0006] Invention patent "420MPa low-carbon weldable marine structure steel plate and manufacturing method thereof" with publication number CN 111394653 A, which contains C: 0.05%~0.11%, Si: 0.10%~0.60%, Mn: 1.00%~1.90%, P<0.015%, S<0.010%, Cr: 0~0.50%, Ni: 0~1.20%, Mo: 0~0.65%, Cu: 0~0.40%, Nb: 0~0.060%, V: 0~0.15%, Ti: 0~0.12%, B: 0~0.0030%, Al: 0.010%~0.060%, provides a production method for producing 10-100mm thick 420MPa grade steel plate by TMCP+tempering process of continuous casting billet; its low-temperature toughness is only-40℃, and it does not involve the low-temperature impact of the steel plate near the surface and the plate thickness 1 / 4, and the uniformity of the whole plate structure performance, which is essentially different from the present invention.

[0007] The invention patent with the publication number CN 118979189 A "Production method of high-homogeneity high-toughness S355MLO offshore steel thick plate", which contains C: 0.03%-0.07%, Si: 0.15%-0.35%, Mn: 1.45%-1.65%, P≤0.015%, S≤0.005%, Alt: 0.025%-0.045%, Nb: 0.025%-0.040%, Ni: 0.25%-0.50%, Cr: 0.10%-0.25%, N≤50ppm, H≤2ppm, provides a production method for producing 120mm-thick high-homogeneity high-toughness offshore thick plate by adopting continuous casting billet; the low-temperature toughness thereof is only-50℃, and the performance high homogeneity only involves the thickness direction impact toughness of a single position, without involving the measures for controlling the strength uniformity of the whole plate, which is essentially different from the present application.

[0008] In summary, the prior art has the following defects: ① insufficient toughness at very low temperature (-60℃), especially large performance difference between the core and the near surface of the super-thick plate; ② poor microstructure and performance uniformity of the whole plate (head / middle / tail, edge / middle, thickness direction), which is easy to cause welding risk; ③ dependent on alloying of precious metals or complex process, high cost and low efficiency. Therefore, it is necessary to develop a super-thick 420MPa-grade offshore steel plate without precious alloy, with simplified process, and with very low temperature adaptability and high uniformity, which becomes the key to solve the demand for deep-sea resource development equipment. SUMMARY

[0009] The technical problem to be solved by the present application is to provide a high-uniformity super-thick 420MPa-grade offshore steel plate for very low temperature environment and a manufacturing method thereof, which realizes the following objectives through "low-alloy component design + precise TMCP process control": the yield strength of the steel plate is ≥420MPa, the tensile strength is ≥520MPa, the impact energy at-60℃ (near surface / plate thickness 1 / 4 / plate thickness 1 / 2) is ≥200J; the strength difference of the same steel plate in the head / middle / tail, edge / middle, and thickness direction is ≤40MPa, the microstructure uniformity is excellent; the carbon equivalent CEV is ≤0.36%, the CTOD characteristic value of the welding heat-affected zone at-20℃ is ≥0.20mm, the welding adaptability is good; no Ni, Cu, Mo, or other precious alloys are added, the process is simplified (single heating, no tempering), and the production cost and delivery cycle are reduced.

[0010] The technical solution adopted by the present application to solve the above problems is: a high-uniformity super-thick 420MPa-grade offshore steel plate for very low temperature environment, the chemical composition of the steel plate is as follows in terms of mass percentage: C: 0.03%-0.07%; Si: 0.15%-0.35%; Mn: 1.40% to 1.70%; S≤0.002%; P≤0.010%; Als: 0.015% to 0.045%; Nb: 0.03% to 0.05%; Cr: 0.10% to 0.30%; Ti: 0.010% to 0.020%; the balance being Fe and inevitable impurities; a carbon equivalent CEV≤0.36%, the calculation formula being: CEV = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 (in the present application, the contents of Mo, V, Ni and Cu are 0, so it is simplified as CEV = C + Mn / 6 + Cr / 5).

[0011] The roles of each element and the design basis are as follows: C (0.03% to 0.07%): low-carbon design can reduce the sensitivity to welding cracks and low-temperature brittleness, and at the same time ensure the strength through interstitial solid solution strengthening; if C < 0.03%, the strength is insufficient; if C > 0.07%, the low-temperature toughness and weldability decrease significantly.

[0012] Si (0.15% to 0.35%): as a deoxidizer and solid solution strengthening element, it compensates for the strength loss caused by low carbon; if Si > 0.35%, it is easy to form martensite-austenite islands (M-A components), which damages the low-temperature toughness; if Si < 0.15%, the deoxidization is insufficient, and the inclusions increase.

[0013] Mn (1.40% to 1.70%): it refines ferrite grains and improves hardenability, and at the same time balances the strength and toughness through solid solution strengthening; if Mn > 1.70%, the center segregation of continuous casting billets is aggravated, forming banded structures, which leads to a decrease in the impact toughness of the core; if Mn < 1.40%, the hardenability is insufficient, and soft ferrite is easy to appear in the core of thick plates, and the strength is low.

[0014] S (≤0.002%), P (≤0.010%): harmful elements are strictly controlled, S is easy to form MnS inclusions, which leads to the deterioration of the toughness of the core; P increases cold brittleness and worsens the weldability, so it needs to be controlled at a very low content.

[0015] Als (0.015% to 0.045%): deep deoxidization, avoiding the oxidation of Ti, Nb and other micro-alloy elements; at the same time, AlN is formed to refine austenite grains; if Als < 0.015%, the deoxidization is insufficient; if Als > 0.045%, it is easy to produce hot cracks in the continuous casting billets.

[0016] Nb (0.03%~0.05%): significantly increase the austenite recrystallization temperature (make the non-recrystallization zone temperature range expand to 800-950℃), ensure that the near-surface and core of the super-thick plate are rolled in the non-recrystallization zone, form refined ferrite + bainite structure, and improve the uniformity of the whole plate; if Nb<0.03%, the recrystallization temperature is not high enough; if Nb>0.05%, coarse NbC is easily formed, which reduces the toughness.

[0017] Cr (0.10%~0.30%): improve the hardenability, delay the bainite transformation, and reduce the organization unevenness caused by the difference in cooling speed in the thickness direction; at the same time, refine the bainite grains and improve the low-temperature toughness; if Cr>0.30%, coarse carbides are easily formed, which reduces the impact energy; if Cr<0.10%, the hardenability improvement effect is not obvious.

[0018] Ti (0.010%~0.020%): combine with N to form fine TiN, pin the austenite grain boundary, and prevent grain coarsening during heating and welding; at the same time, Ti (C, N) is precipitated during rolling, producing precipitation strengthening; if Ti<0.010%, the grain refinement effect is not enough; if Ti>0.020%, large-size TiN inclusions are easily formed, which become weak points of toughness.

[0019] The manufacturing method of the present application includes six key processes of smelting, continuous casting, billet heating, controlled rolling, controlled cooling, and stacking and slow cooling, and the specific control parameters are as follows: 1. Molten steel smelting and continuous casting Smelting process: KR molten iron pretreatment (desulfurization to S≤0.002%)→ converter smelting (end point C≤0.06%)→ LF refining (composition fine adjustment, temperature rise)→ RH vacuum degassing (vacuum degree≤1mbar, pressure holding time≥15min, H≤2ppm)→ Ca treatment (Ca wire feeding amount 0.2-0.4kg / t steel, control inclusion shape to be spherical); Continuous casting process: adopt special thick slab continuous casting machine (slab thickness 300-400mm, slab width 2000-2500mm), low superheat protection casting (casting temperature = liquidus temperature + 10~22℃), dynamic soft reduction (reduction amount 15-25mm, reduction position is in the 1 / 3-2 / 3 region of the solidification end); Casting blank quality requirement: center segregation≤C0.5 level, porosity≤0.5 level, no internal cracks and shrinkage.

[0020] 2. Billet heating Heating system: cold billet into the furnace (room temperature to 500℃, heating rate≤150℃ / h to avoid thermal stress cracks), 500-1120℃, heating rate≤200℃ / h, final heating temperature 1120~1190℃; Soaking control: the soaking time in high temperature section (1120-1190℃) is ≥240min, to ensure the micro-alloy elements such as Nb and Ti are fully solid-solved (solid-solution rate ≥95%) and to avoid austenite grain coarsening (grain size ≤50μm).

[0021] 3. Controlled rolling (two-stage rolling) Coarse rolling stage (recrystallization zone rolling): Starting rolling temperature 1000-1100℃ (to ensure austenite is in the recrystallization zone); Deformation control: total deformation rate 50%-70%, among which single pass deformation rate >16% (to break coarse austenite grains) and single pass deformation rate >10% (to promote recrystallization refinement) in 3 passes; Intermediate blank thickness: determined according to the finished product thickness, intermediate blank thickness / finished product thickness = 3-5 (to reserve sufficient deformation amount for finish rolling).

[0022] Finish rolling stage (non-recrystallization zone rolling): Starting rolling temperature 750-840℃ (to ensure austenite is in the non-recrystallization zone, relying on the recrystallization inhibition effect of Nb); Dynamic process adjustment: calculate the number of passes (8-12 passes) according to the intermediate blank length (2-6m), finished product thickness (50-100mm), and finished product width (2000-3000mm), and single pass deformation rate 7%-11% (to avoid poor plate shape caused by excessive deformation); Final rolling control: the temperature difference between the head and tail of the steel plate in the final rolling pass is <30℃ (achieved by compensating for heating at the head), and the final rolling temperature is 750-820℃ (to ensure that the austenite does not change phase after final rolling, preparing for subsequent controlled cooling); Total deformation in finish rolling accounts for 30%-50% (to ensure uniform deformation in the thickness direction and avoid insufficient deformation in the core).

[0023] 4. Controlled cooling (two-stage cooling) First stage (weak cooling, early phase of phase change): Cooling method: laminar cooling, with the head and tail 600-1000mm range shielded (to avoid overcooling at the head and tail), and the water amount at the width direction both sides = the central water amount ×40%-60% (to compensate for the problem of fast heat dissipation at the edges); Temperature control: cool to the whole plate temperature 650-720℃ (to ensure entering the bainite transformation zone and avoid excessive precipitation of pro-eutectoid ferrite); Cooling speed: 3-8℃ / s (slow cooling to ensure uniform temperature in the thickness direction).

[0024] Second stage (strong cooling, main phase of phase change): Cooling mode: accelerated laminar cooling, head and tail 400-600mm range shielding (shorten the length of shielding, balance the head and tail and the middle cooling), width direction both sides water quantity = middle water quantity ×60%-80%; Cooling speed: 10-25℃ / s (fast cooling, promote fine bainite and acicular ferrite formation); Final cooling temperature: 400-460℃ (avoid cooling to martensite transformation zone, while ensuring sufficient bainite transformation, reducing internal stress).

[0025] 5, stack slow cooling Slow cooling timing: immediately stack when the temperature of the steel plate is ≥350℃ after controlled cooling (avoid insufficient internal stress release due to too low temperature); Stacking mode: align the same specification steel plates, stack height ≤8 layers, interlayer cushion wood (ensure uniform heat dissipation); Slow cooling time: ≥36h (make the internal hydrogen of the steel plate fully diffuse to avoid hydrogen-induced cracking; at the same time, release the rolling and cooling internal stress to improve the dimensional stability).

[0026] The thickness of the steel plate of the present application is 50-100mm, and the performance meets the following requirements: Tensile properties: yield strength ReH≥420MPa, tensile strength Rm≥520MPa, elongation A≥25%; the ReH difference of the same steel plate head, middle, tail, plate width 1 / 4 (W / 4) and plate width 1 / 2 (W / 2) position, and plate thickness 1 / 4 (T / 4) and plate thickness 3 / 4 (3T / 4) position is ≤40MPa, the Rm difference is ≤35MPa; Low temperature impact toughness: transverse -60℃ impact energy Akv at W / 4 position (representative position) near the surface, T / 4, T / 2 of the head, middle and tail is ≥200J (average value of 3 samples, single value ≥180J); Z-direction performance: Z-direction reduction of area Z≥65% (anti-laminated tearing capacity, meet the welding requirements of marine structure); Fracture toughness: base material -20℃ CTOD characteristic value ≥0.20mm, welding heat affected zone (linear energy 35kJ / cm) -20℃ CTOD characteristic value ≥0.20mm; Microstructure uniformity: near-surface microstructure is fine proeutectoid ferrite + bainite (grain size ≤10μm), T / 4 position is acicular ferrite + strip bainite (grain size ≤8μm), T / 2 position is acicular ferrite + block bainite (grain size ≤12μm); the grain size difference of the same thickness position of the head / tail is ≤3μm.

[0027] Compared with the prior art, the present application has the following beneficial effects: 1. Component design advantages: No noble alloy dependence: Replace Ni, Mo, etc. noble metals with Nb-Ti-Cr complex micro-alloying, reduce alloy cost by 15%-20%; Low welding crack sensitivity: CEV≤0.36%, far lower than the requirement of 420MPa marine steel in GB / T 712-2019 CEV≤0.40%, can adapt to large heat input welding (maximum heat input 50kJ / cm).

[0028] 2. Process innovation advantages: Single heating instead of twice heating: Compared with the twice heating process of CN115838902A, energy consumption is reduced by 25%-30%, and heating cycle is shortened by 40%; Dynamic temperature compensation controlled rolling: Through dynamic calculation of roughing temperature and control of the temperature difference at the end of finish rolling, the problem of large temperature difference between head and tail and edge undercooling of thick plates is solved, and the strength difference of the whole plate is ≤40MPa, which is better than the existing technology (usually ≥60MPa); Two-stage controlled cooling: The weak cooling stage balances the temperature in the thickness and width directions, and the strong cooling stage precisely controls the phase change, realizing synchronous improvement of near-surface and core toughness (-60℃ impact energy ≥200J), breaking through the bottleneck of poor core toughness of existing technology.

[0029] 3. Performance advantages: Very low temperature adaptability: -60℃ impact energy ≥200J, far exceeding the -40℃ / -50℃ toughness level of existing technology, covering polar regions and other very low temperature sea areas; Whole plate uniformity: Control performance from three dimensions of "head / middle / tail", "edge / middle", and "thickness direction", avoid welding stress concentration, and improve the service safety of marine structures by more than 30%; Comprehensive cost performance: Consider high strength, high toughness, high uniformity and low cost, compared with quenched and tempered steel plate, delivery cycle is shortened by 50%, and comprehensive cost is reduced by 20%-25%. BRIEF DESCRIPTION OF DRAWINGS

[0030] Figures 1-3 The 200 times microstructure of the near surface at the 1 / 4 position of the plate width, the 1 / 4 position of the plate thickness and the 1 / 2 position of the plate thickness of the head of the steel plate in Example 2 of the application is shown.

[0031] Figures 4-6 The 200 times microstructure of the near surface at the 1 / 4 position of the plate width, the 1 / 4 position of the plate thickness and the 1 / 2 position of the plate thickness of the tail of the steel plate in Example 2 of the application is shown. DETAILED DESCRIPTION

[0032] The technical solutions of the present application will be described in more detail below in combination with preferred embodiments of the present application. However, the embodiments are merely descriptions of preferred embodiments of the present application and should not impose any limitation on the scope of the present application.

[0033] The chemical components of the steel plates in each embodiment are shown in Table 1, the blank heating, controlled rolling and controlled cooling process parameters are shown in Table 2, the conventional mechanical properties of the steel plates are shown in Table 3, and the low-temperature fracture toughness of the base material and the welded joint is shown in Table 4.

[0034] Table 1 Chemical components of the steel in each embodiment (wt%)

[0035] Table 2 Blank heating, controlled rolling and controlled cooling process parameters of each embodiment

[0036] Table 3 Conventional mechanical properties of the finished steel plates in each embodiment

[0037]

[0038] Table 4 Low-temperature fracture toughness of the base material and the welded joint in each embodiment

[0039] Figures 1-6 Typical microstructure photos of the near-surface, 1 / 4 of the plate thickness and 1 / 2 of the plate thickness at the head and tail of the finished steel plate in Example 2 are given, and the results show that the microstructure type and grain size at the same thickness position of the head and tail are similar, the near-surface is fine proeutectoid ferrite + bainite, the 1 / 4 of the plate thickness is acicular ferrite + banded bainite, and the 1 / 2 of the plate thickness is acicular ferrite + massive bainite.

[0040] As can be seen from the above, on the basis of reasonable chemical component design, through reasonable heating temperature and high-temperature stage time, as well as temperature compensation control in the two-stage controlled rolling and two-stage controlled cooling process, the 420MPa grade ultra-high strength steel plate with a maximum thickness of 100mm is realized to have high strength, high toughness, good weldability, low-temperature environmental adaptability and good uniformity of the whole plate microstructure and performance. The yield strength is ≥420MPa, the tensile strength is ≥520MPa, and the strength difference of the T / 4 and 3T / 4 positions of the 1 / 4 of the plate width and the 1 / 2 of the plate width of the head, the middle and the tail of the same steel plate is ≤40MPa; the-60℃ impact energy of the near-surface, 1 / 4 of the plate thickness and 1 / 2 of the plate thickness at the 1 / 4 of the plate width of the head, the middle and the tail of the steel plate is ≥200J; the Z-direction reduction of area is ≥65%; and the CTOD characteristic value of the base material and the heat-affected zone under-20℃ condition is ≥0.20.

[0041] The above merely illustrates the specific embodiments of the present application, but the protection scope of the present application is not limited thereto, any person skilled in the art can easily think of the changes or replacements within the technical range disclosed by the present application, which should be covered in the protection scope of the present application. Therefore, the protection scope of the present application should be subject to the protection scope of the claims.

Claims

1. A high uniformity extra-thick 420 MPa grade marine steel plate for use in a cryogenic environment, characterized in that, The chemical composition includes, by mass percentage, C: 0.03%-0.07%; Si: 0.15%-0.35%; Mn: 1.40%-1.70%; S≤0.002%; P≤0.010%; Als: 0.015%-0.045%; Nb: 0.03%-0.05%; Cr: 0.10%-0.30%; Ti: 0.010%-0.020%; the balance being Fe and inevitable impurities; the carbon equivalent CEV calculated from the analysis composition of smelting is ≤0.36%, and the carbon equivalent calculation formula is CEV=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15.

2. The high uniformity extra thick 420 MPa grade marine steel plate for extremely low temperature environment according to claim 1, characterized by, The thickness of the steel plate is 50-100mm.

3. The high uniformity extra thick 420 MPa grade marine steel plate for extremely low temperature environment according to claim 1, characterized in that, The yield strength of the steel plate is ≥420MPa, the tensile strength is ≥520MPa, and the strength difference of the T / 4 and 3T / 4 positions of the plate width 1 / 4 and plate width 1 / 2 of the head, middle and tail of the same steel plate is ≤40MPa.

4. The high uniformity extra thick 420 MPa grade marine steel plate for extremely low temperature environment according to claim 1, characterized by, The -60℃ impact energy of the near-surface, plate thickness 1 / 4 and plate thickness 1 / 2 positions of the plate width 1 / 4 positions of the head, middle and tail of the steel plate is ≥200J.

5. The high uniformity extra thick 420 MPa grade marine steel plate for extremely low temperature environment according to claim 1, characterized by, The Z-direction section shrinkage of the steel plate is ≥65%; the CTOD characteristic value of the base material and heat-affected zone under the condition of -20℃ is ≥0.20mm.

6. A method of manufacturing a high-uniformity extra-thick 420 MPa-grade marine steel plate for use in an extremely low temperature environment, for manufacturing the marine steel plate according to any one of claims 1 to 5, characterized by, The process includes smelting, continuous casting, billet heating, controlled rolling, controlled cooling and stacking and slow cooling process, and the specific steps are as follows: 1) molten steel smelting and continuous casting: KR molten iron pretreatment, converter smelting, LF refining, RH vacuum degassing and Ca treatment are used to produce high-purity molten steel; thick continuous casting slabs are produced by thick slab production process, and low superheat protection casting and dynamic soft reduction control are used in the casting process, the casting temperature is controlled at 10-22℃ above the liquidus, and the center segregation of the casting slab is C0.5 level or better, and the porosity is 0.5 level or better; 2) billet heating: cold billet is heated to 1120-1190℃, and the high-temperature section time is ≥240min; 3) controlled rolling: two-stage controlled rolling is adopted, and the total deformation ratio of the finishing rolling stage is 30%-50%; the rough rolling opening temperature is 1000-1100℃, ensuring that the single pass deformation rate of one pass is >16% and the single pass deformation rate of three passes is >10%; the opening temperature of the finishing rolling stage is 750-840℃, the finishing rolling opening temperature and pass number are dynamically calculated according to the intermediate billet length, steel plate thickness, width and final product length, the single pass deformation rate is 7%-11%, and the temperature difference between the head and tail of the steel plate at the final rolling pass is <30℃, and the head and tail temperature of the steel plate at the final rolling pass is 750-820℃. 4) Controlled cooling: two-stage controlled cooling is adopted, the first stage is weak cooling, the head and tail 600-1000mm range is shielded and the water amount on both sides of the steel plate width is adjusted to 40%-60% in the middle part to realize the whole plate temperature of 650-720℃; the second stage is strong cooling, the cooling speed is 10-25℃ / s, the head and tail 400-600mm range is shielded and the water amount on both sides of the steel plate width is adjusted to 60%-80% in the middle part, and the final cooling temperature of the whole plate is 400-460℃; 5) Pile-up slow cooling: the steel plate after controlled cooling is subjected to pile-up slow cooling treatment, the slow cooling starting temperature is ≥350℃, and the slow cooling time is ≥36 hours.

7. The method of producing a high-uniformity extra-thick 420 MPa-grade marine steel plate for an extremely low temperature environment according to claim 6, characterized by In the RH vacuum degassing treatment in step 1), the vacuum degree is ≤1mbar, the pressure maintaining time is ≥15min, and the H content in the molten steel is ≤2ppm.

8. The method of producing a high-uniformity extra-thick 420 MPa-grade marine steel plate for an extremely low temperature environment according to claim 6, characterized by, In step 2), after the slab is put into the furnace, the temperature rising rate in the room temperature to 500℃ stage is ≤150℃ / h, and the temperature rising rate in the 500-1120℃ stage is ≤200℃ / h.

9. The method of producing a high-uniformity extra-thick 420 MPa-grade marine steel plate for an extremely low temperature environment according to claim 6, characterized in that, In step 3), the total deformation rate in the rough rolling stage is 50%-70%, and the ratio of the intermediate billet thickness to the finished product thickness is 3-5.

10. The method of producing a high-uniformity extra-thick 420 MPa-grade marine steel plate for an extremely low temperature environment according to claim 6, characterized in that, In step 4), the cooling speed in the first stage weak cooling is 3-8℃ / s; in step 5), the same specification steel plates are aligned and stacked when stacking, and the stack height is ≤8 layers, and a spacer is arranged between the layers.

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