High-rupture-resistance and easy-to-weld super-thick YP550 grade steel plate and manufacturing method thereof

Abstract

A high-crack-arrest, easy-to-weld super-thick YP550 grade steel plate and a manufacturing method thereof, the composition of the steel plate is as follows: C 0.050-0.085%, Si≤0.12%, Mn 1.80-2.20%, P≤0.013%, S≤0.003%, Cu 0.15-0.40%, Ni 0.55-0.95%, Cr 0.05-0.25%, Mo 0.05-0.15%, Nb 0.030-0.055%, Ti 0.008-0.016%, Als 0.025-0.060%, N≤0.0050%, Ca 0.0010-0.0040%, the balance containing Fe and inevitable inclusions; and the following conditions are met: a welding cold crack sensitivity index Pcm≤0.24%; a BCC crystal structure dislocation low-temperature mobility index θ≥0.79; Ca treatment, a Ca / S ratio is 1-3, (%Ca)×(%S)≤1.5×10 ‑3 While obtaining high strength, excellent low-temperature toughness, high crack-arrest property and high elongation of the super-thick base material steel plate, the low-temperature toughness of the fusion line and HAZ during large heat input welding is also excellent, and the steel plate is particularly suitable for large-power offshore wind pile legs, offshore platforms, cross-sea bridge steel tower structures, port machinery and engineering machinery, etc.

Description

Technical Field

[0001] This invention relates to the field of high-strength steel technology, and in particular to a high-crack-arresting, easily weldable ultra-thick YP550 grade steel plate and its manufacturing method. Background Technology

[0002] As is well known, low-carbon (high-strength) low-alloy steel is one of the most important engineering structural materials, widely used in oil and gas pipelines, offshore platforms, shipbuilding, hydropower projects, bridge structures, boiler containers, building structures, automotive industry, railway transportation, and machinery manufacturing. The performance of low-carbon (high-strength) low-alloy steel depends on its chemical composition and manufacturing process. Among these, strength, toughness, plasticity, weldability, and the matching between them are the most important properties of low-carbon (high-strength) low-alloy steel, which are ultimately determined by the microstructure and dislocation substructure of the finished steel.

[0003] With the continuous advancement of metallurgical technology, higher demands are being placed on the toughness, plasticity, weldability, and low anisotropy of high-strength steel. Specifically, the steel plate must possess resistance to brittle fracture and ductile instability fracture (i.e., crack arrest characteristics) at extremely low temperatures (-50℃), while simultaneously achieving fracture elongation and uniform elongation at the level of steel plates with a tensile strength of 500MPa. Furthermore, the steel plate must exhibit excellent weldability and low anisotropy, capable of withstanding high heat input welding (welding heat input ≥50kJ / cm). Moreover, under conditions of lower alloy content, especially precious metal content, and lower manufacturing costs, the comprehensive mechanical and performance properties of the steel plate must be significantly improved. This reduces the amount of alloy used in the steel, saving costs. High strength and lightweight construction reduce the weight, stability, and safety of steel components. More importantly, it further improves the cold / hot workability of steel components and their safety and reliability during service.

[0004] Currently, Japan, South Korea, the European Union, North America, and my country have launched a research boom in developing next-generation high-performance steel materials. They are striving to achieve better microstructure matching, ultra-fine metallographic microstructure and substructure (dislocation configuration, packet) fine structure through alloy combination optimization design, control of submicroscopic fine structure and innovative manufacturing process technology, so that high-strength steel can obtain better strength and plasticity matching, toughness and plasticity matching, weldability and low anisotropy.

[0005] When manufacturing thick steel plates with a yield strength ≥420MPa and a low-temperature impact toughness ≥34J at -60℃ using existing technologies related to this invention, a certain amount of Ni or Cu+Ni elements (≥0.30%) is generally added to the steel (see The Firth (1986) International Symposium and Exhibit on Offshore Mechanics and Arctic Engineering, 1986, Tokyo, Japan, 354; “DEVELOPMENTS IN MATERIALS FOR ARCTICOFFSHORE STRUCTURES”; “Structural Steel Plates for Arctic Use Produced by Multipurpose Accelerated Cooling System” (Japanese), Kawasaki Steel Technology News, 1985, No. 1 68~72; “Application of Accelerated Cooling For Producing 360MPa Yield Strength Steel plates of up to 150mm in Thickness with Low Carbon Equivalent”, Accelerated Cooling Rolled Steel, 1986, 209~219; "High Strength Steel PlatesFor Ice-Breaking Vessels Produced by Thermo-Mechanical Control Process", Accelerated Cooling Rolled Steel, 1986, 249~260; "420MPa Yield Strength SteelPlate with Superior Fracture Toughness for Arctic Offshore Structures", "Kawasaki steel technical report", 1999, No.40, 56; "420MPa and 500MPa YieldStrength Steel Plate with High HAZ toughness Produced by TMCP for OffshoreStructure", "Kawasaki steel technical report", 1993, No. 29, 54; "ToughnessImprovement in Bainite Structure by Thermo-Mechanical Control Process (Japanese) Sumitomo Metal, Vol. 50, No. 1 (1998), 26; "Steel Plates for Offshore Platform Structures Used in Icy Sea Regions" (Japanese), Iron and Steel Research, 1984, No. 314, 19-43), to ensure that the base steel plate has excellent low-temperature toughness, when welding with a heat input of <50KJ / cm, the toughness of the heat-affected zone (HAZ) can also reach -60℃ Akv≥34J; however, when welding with an ultra-high heat input (≥100KJ / cm), the low-temperature toughness of the weld heat-affected zone (HAZ) is generally difficult to achieve, and the low-temperature toughness of the heat-affected zone (HAZ) is severely degraded. Numerous patent documents primarily describe how to achieve low-temperature toughness in the base steel plate, with limited explanation of how to obtain excellent low-temperature toughness in the heat-affected zone (HAZ) under welding conditions. This is especially true for welding with extremely high heat input, where ensuring low-temperature toughness in the HAZ is extremely difficult. Furthermore, to guarantee low-temperature toughness, a certain amount of Ni or Cu+Ni elements is typically added to the steel. Even with extremely high heat input welding, the low-temperature toughness of the HAZ in the steel plate rarely reaches -60℃ (see Japanese Patents Sho 63-93845, Sho 63-79921, Sho 60-258410, Japanese Patent Application Publication No. 4-285119, Japanese Patent Application Publication No. 4-308035, Japanese Patent Application Publication No. 3-264614, Japanese Patent Application No. 2-250917, Japanese Patent Application No. 4-143246; US Patents 4855106, 5183198, and 4137104).

[0006] Currently, only Nippon Steel Corporation of Japan employs oxide metallurgy technology (see US Patent 4629505 and WO 01 / 59167A1) to improve the low-temperature toughness of the heat-affected zone (HAZ) of ultra-high heat input welded steel plates. This technology involves TiN particles dissolving and losing their function near the fusion line during high-temperature welding due to prolonged exposure to high temperatures. Ti2O3, being more stable than TiN, does not dissolve even at the steel's melting point. Ti2O3 particles can serve as nucleation sites for acicular ferrite within austenite grains, promoting the nucleation of acicular ferrite (AF), effectively dividing austenite grains, refining the HAZ structure, and forming a high-strength, high-toughness acicular ferrite structure.

[0007] Furthermore, Baosteel has developed a series of high-strength steel plates capable of high-heat-input welding at low temperatures (see Chinese patents ZL201410300713.X, ZL201310244712.3, ZL201310244706.8, ZL201310124065.2, ZL201310244713.8, ZL201210209637.2, ZL201410815614.5 and Chinese patent applications 201710183350.X and 201910149978.7). To ensure the low-temperature toughness of the heat-affected zone during high-heat-input welding, a certain amount of precious alloying elements Cu and N are added to the steel plates (especially ultra-thick steel plates). i. When the yield strength YP reaches 460MPa, it is especially necessary to add a small amount of Mo. Although the welding processability of the steel plate is good under high heat input, the toughness of the weld heat-affected zone, especially the low-temperature toughness of the weld heat-affected zone of thick steel plates, is not very stable (under high heat input welding conditions, Mo promotes the formation of coarse upper bainite, which deteriorates the toughness of the weld coarse-grained heat-affected zone). It cannot stably meet the requirement of impact toughness at -40℃, and the manufacturing cost of thick steel plates is also high. More importantly, none of the TMCP steel plates developed above involve the research and control of the crack arrest characteristic Kca of steel plates. The developed steel plates cannot fully meet the design safety requirements of special large-scale heavy steel structures (such as large-capacity offshore wind turbine leg structures, marine engineering structures, and giant ship floating cranes).

[0008] Secondly, the steel plates produced by the aforementioned patents do not meet the strength requirements of yield strength ≥550MPa and tensile strength ≥650MPa. To achieve these strength levels, offline quenching and tempering (Q+T) processes are generally used in production (see Chinese patents ZL201910150314.2, ZL201610463494.6, ZL201010113835.X, ZL200810042088.8, etc.). This not only results in long manufacturing cycles and high manufacturing costs, failing to meet the green and low-carbon concept, but more importantly, the microstructure of the steel plates is a single bainitic structure. The relatively large size of the bulk grains (around 20 μm) results in poor crack arrest properties and low elongation of the steel plate. To obtain steel plates with high crack arrest properties and high elongation, a two-stage quenching and tempering process (Q+Q'+T) must be used to obtain a fine and uniform ferrite + bainite multiphase structure. This will result in excessively long steel plate manufacturing processes and high manufacturing costs, failing to meet the requirements of low-carbon and green concepts and the increasingly fierce market competition. In addition, the offline quenching and tempering process requires a high Mo content and micro-B treatment, which not only increases the alloy cost but also leads to the deterioration of the weldability of the steel plate, failing to meet the requirements of high heat input welding. Summary of the Invention

[0009] The purpose of this invention is to provide a high-crack-arrest, easily weldable ultra-thick YP550 grade steel plate and its manufacturing method. While achieving high strength, excellent low-temperature toughness, high crack-arrest properties, and high elongation in the ultra-thick base steel plate, the low-temperature toughness of the fusion line and HAZ is also excellent during high heat input welding. The steel plate has a yield strength ≥ 550 MPa, tensile strength ≥ 650 MPa, elongation at break δ5 ≥ 18%, Charpy impact energy KV2 (single value) ≥ 100 J at -50℃, and Kca (-10℃) ≥ 6000 N / mm². 3 / 2 It has excellent weldability, requires no preheating before welding, can withstand welding heat input of ≥50kJ / cm, and has an impact energy of KV2 (single value) of ≥60J in the heat-affected zone at -50℃. It is particularly suitable for high-power offshore wind turbine legs, offshore platforms, large floating cranes, icebreakers, cross-sea bridge steel tower structures, hydropower metal structure components, port machinery and engineering machinery, etc., and can achieve low-cost, stable mass industrial production.

[0010] High-strength, high-toughness, and high-crack-arrest ultra-thick steel plates are among the most challenging types of thick plates. This is because these steel plates not only require low carbon content, low phosphorus content (Pcm), high strength, excellent low-temperature toughness, and high crack-arrest properties, but also must be able to withstand high heat input welding (welding heat input ≥ 50 KJ / cm). The weld fusion line and heat-affected zone of the steel plate must possess high strength, excellent low-temperature impact toughness, and high crack-arrest properties. However, it is very difficult to meet these performance requirements simultaneously.

[0011] A) The relationship between low C, low Pcm and large thickness, high strength, and high elongation;

[0012] B) Balancing low-temperature toughness, high crack arrest, and large thickness with low manufacturing cost (low Ni content);

[0013] C) The combination of high strength, high crack arrest properties and excellent weldability, especially high heat input weldability.

[0014] The aforementioned characteristics conflict and are difficult to reconcile in composition design and TMCP process design: When reducing C content and Pcm, it is difficult to achieve high strength, high crack arrest properties, and high elongation in ultra-thick steel plates (excessive strengthening will result in low elongation); while improving the low-temperature toughness, crack arrest properties, and elongation of ultra-thick steel plates, it is difficult to achieve low C, low Pcm, and low manufacturing costs (mainly due to low Ni content); when achieving high strength, high crack arrest properties, and high elongation in ultra-thick steel plates, weldability, especially for steel plates with high heat input, is difficult to guarantee. Balancing large thickness, low C / low Pcm, high strength, high toughness, high crack arrest properties, high plasticity, and high heat input weldability is one of the biggest challenges of this product and a key core technology.

[0015] To achieve the above objectives, the technical solution of the present invention is as follows:

[0016] A high-crack-arrest, easily weldable, ultra-thick YP550 grade steel plate, the weight percentage of its components is as follows:

[0017] C: 0.050~0.085%

[0018] Si: ≤0.12%

[0019] Mn: 1.80~2.20%

[0020] P: ≤0.013%

[0021] S: ≤0.003%

[0022] Cu: 0.15–0.40%

[0023] Ni: 0.55%–0.95%

[0024] Cr: 0.05–0.25%

[0025] Mo: 0.05–0.15%

[0026] Nb: 0.030–0.055%

[0027] Ti: 0.008~0.016%

[0028] Als: 0.025–0.060%

[0029] N: ≤0.0050%

[0030] Ca: 0.0010~0.0040%

[0031] The balance includes Fe and other unavoidable inclusions; and the following relationship must be satisfied simultaneously:

[0032] The cold cracking sensitivity index Pcm for welding is ≤0.24%.

[0033] Pcm=(%C)+(%Si) / 30+[(%Mn)+(%Cu)+(%Cr)] / 20+(%Ni)

[0034] / 60+(%Mo) / 15+(%V) / 10+5(%B), in %;

[0035] (%C)×[(%Mn)+7.76(%Si)+10.23(%P)+21.47(%S)]≤0.216;

[0036] Als / (N-0.292Ti)≥28;

[0037] The low-temperature mobility index of dislocations in BCC crystal structures is θ≥0.79;

[0038] θ=1.25(%Ni)+[1.57(%Mn)-0.69(%Mn) 2 +1.69 (% Cu)

[0039] -3.51(%Si)-0.46[(%Cr)+(%Mo)]-2.43[(%Al)+(%Nb)+

[0040] (%Ti)];

[0041] Ca treatment was performed, with the Ca / S ratio controlled between 1 and 3, and (%Ca) × (%S) ≤ 1.5 × 10⁻⁶. -3 .

[0042] Furthermore, the balance consists of Fe and other unavoidable inclusions.

[0043] The microstructure of the steel plate described in this invention consists of a small amount of uniform fine ferrite and low-carbon bainite, with an average grain size of less than 20 μm.

[0044] The steel plate described in this invention has a yield strength ≥ 550 MPa, tensile strength ≥ 650 MPa, elongation at break δ5 ≥ 18%, Charpy impact energy (single value) ≥ 100 J, and Kca (-10℃) ≥ 6000 N / mm². 3 / 2 It has excellent weldability, requires no preheating before welding, can withstand welding heat input of ≥50kJ / cm, and has an impact energy of KV2 (single value) of ≥60J in the heat-affected zone at -50℃.

[0045] In the composition design of the steel plate described in this invention:

[0046] Carbon (C) has a significant impact on the strength, low-temperature toughness, crack arrest properties, elongation, and weldability of TMCP ultra-thick steel plates, especially on weldability with high heat input. From the perspective of improving the low-temperature toughness and weldability with high heat input of ultra-thick steel plates, it is desirable to control the C content in the steel to be relatively low. However, from the perspective of the strength of ultra-thick steel plates, microstructure control during production, and manufacturing costs, the C content should not be controlled too low. Too low a C content can easily lead to excessively high grain boundary migration, resulting in coarse grains in the microstructure of the base steel plate and the weld HAZ, which are prone to mixed crystal formation. Moreover, too low a C content in the steel can cause grain boundary weakening, which seriously deteriorates the crack arrest properties of the base steel plate and the low-temperature toughness of the weld HAZ. Therefore, for ultra-thick steel plates with a tensile strength ≥650MPa, the reasonable range of C content is 0.050~0.085%.

[0047] Si promotes deoxidation of molten steel and can improve the strength of steel plates. However, when using Al-deoxidized molten steel, the deoxidation effect of Si is not significant. Although Si can improve the strength of steel plates, it severely impairs the low-temperature toughness, crack arrest properties, elongation, and weldability of weathering steel plates. Especially with the increase in strength and high alloy content of high-strength ultra-thick steel plates, during high heat input welding, Si not only promotes the formation of weathering islands, but also results in large and unevenly distributed weathering islands, which severely impairs the low-temperature toughness and crack arrest properties of the weld heat-affected zone (HAZ) of the steel plate. Therefore, the Si content in steel should be controlled as low as possible. Considering that Si has a certain weathering effect and the economy and operability of the steelmaking process, the Si content in this invention is controlled below 0.12%.

[0048] In addition to increasing the strength of steel plates, Mn, as the most important alloying element, also has the functions of expanding the austenite phase region, lowering the Ar3 point temperature, refining the grains of TMCP steel plates to increase the strength of steel plates (fine grain strengthening effect), improving the low-temperature toughness and crack arrest properties of steel plates (fine grain toughening), and promoting the formation of low-temperature phase transformation structures (phase transformation strength effect) to increase the strength of steel plates. However, Mn is prone to segregation during the solidification process of molten steel, especially when the Mn content is high. This not only makes casting operations difficult, but Mn also tends to undergo conjugate segregation with elements such as C, P, and S. In particular, when the C content in the steel is high, it aggravates segregation, porosity, and shrinkage cavities in the center of the billet. Severe segregation in the central region of the billet can easily form abnormal structures during subsequent TMCP and welding processes, leading to low low-temperature toughness of the steel plate and cracks and embrittlement of the welded joints. In addition, for high-strength TMCP ultra-thick steel plates, excessively high Mn content not only causes a sharp deterioration in the low-temperature toughness, crack arrest characteristics, and elongation of the steel plate, but also leads to a severe deterioration in the weldability of the steel plate, especially the weldability under high heat input. Furthermore, when the Mn content is too high, the low-temperature mobility of ferrite dislocations decreases (PN force increases), leading to a deterioration in the intrinsic plasticity, toughness, and crack arrest characteristics of the steel plate. Therefore, the suitable Mn content for this invention is 1.80% to 2.20%.

[0049] Phosphorus (P) is a harmful inclusion in steel that has a significant detrimental effect on the mechanical properties of steel plates, especially low-temperature impact toughness, crack arrest characteristics, elongation, and weldability. Theoretically, the lower the content, the better. However, considering the operability and cost of steelmaking, for TMCP ultra-thick steel plates that require high heat input welding, high toughness (toughness at -50℃), high crack arrest characteristics (Kca at -10℃), high strength, and high heat input welding, the P content needs to be controlled at ≤0.013%.

[0050] Sulfur (S), as a harmful inclusion in steel, significantly impairs the low-temperature toughness, crack arrest properties, elongation, and weldability of steel plates. More importantly, S combines with manganese (Mn) in steel to form MnS inclusions. During hot rolling, the plasticity of MnS causes it to extend along the rolling direction, forming MnS inclusion bands along the rolling direction. This not only severely damages the low-temperature impact toughness, crack arrest properties, elongation, Z-axis properties, and fatigue resistance of the steel plate, but also causes a serious deterioration in the weldability of the steel plate (especially weldability under high heat input). At the same time, S is also the main element that causes hot brittleness during hot rolling, and theoretically, the lower the content, the better. However, considering the principles of steelmaking operability, steelmaking cost, and smooth logistics, for TMCP ultra-thick steel plates that require high heat input welding, high toughness (toughness at -50℃), high crack arrest properties (Kca at -10℃), high strength, and high heat input welding, the S content needs to be controlled at ≤0.003%.

[0051] Cu is primarily an austenite stabilizing element and also improves the low-temperature mobility of ferrite dislocations. Adding Cu can lower the Ar1 and Ar3 point temperatures, refine the microstructure of TMCP weathering steel plates, and improve their low-temperature toughness and crack arrest properties. However, excessive Cu addition, exceeding 0.40%, can easily lead to copper embrittlement, surface cracking of cast billets, internal cracking, and especially affect the impact load fracture characteristics (i.e., crack arrest properties) of thick steel plates. Furthermore, it severely degrades the low-temperature toughness and crack arrest properties of welded joints. Considering that Cu is a relatively expensive alloying element, from a cost-effectiveness perspective, the upper limit of Cu should be controlled at 0.40%. Insufficient Cu addition, below 0.15%, has less of the aforementioned effects; therefore, the Cu content should be controlled between 0.15% and 0.40%.

[0052] Adding Ni not only reduces the lattice friction force (PN force) of dislocations in the BCC crystal structure, improves the low-temperature mobility of ferrite dislocations, promotes dislocation cross-slip, and improves the intrinsic ductility and toughness of ferrite; in addition, as a strong austenite stabilizing element, Ni significantly lowers the Ar1 and Ar3 point temperatures, increases the driving force for the austenite-to-ferrite phase transformation, causing austenite to undergo phase transformation at lower temperatures, significantly refines the microstructure of TMCP steel plates, increases the resistance to crack propagation through ferrite grains, and significantly improves the low-temperature toughness of TMCP steel plates. Therefore, Ni has the effect of simultaneously improving the strength, low-temperature toughness, crack arrest properties, and elongation of TMCP steel plates without reducing elongation (i.e., ductility and toughness); adding Ni to steel can also reduce copper embrittlement in copper-containing steels, alleviate intergranular cracking during hot rolling, and improve the atmospheric corrosion resistance of steel plates. Therefore, theoretically, the higher the Ni content in steel within a certain range, the better. However, excessively high Ni content will not only harden the heat-affected zone of the weld and negatively affect the weldability of the steel plate and the low-temperature toughness of welded joints with large heat input, but also significantly increase the manufacturing cost of the steel plate (Ni is a precious alloying element). Therefore, for ultra-thick, crack-arresting, and highly workable TMCP steel plates with tensile strength ≥650MPa, the Ni content should be controlled between 0.55% and 0.95%.

[0053] For ultra-thick, crack-arresting, and highly machinable TMCP steel plates with a tensile strength ≥650MPa, adding a certain amount of Cr (≤0.25%) can improve the strength of thick steel plates without compromising their low-temperature toughness, bending cold working, and high-input welding. However, if the Cr content is too low (<0.05%), Cr contributes little to the strength of the steel plate and fails to meet the performance requirements. If too much Cr is added (>0.25%), it impairs the low-temperature toughness, crack-arresting properties, and high machinability of the steel plate, especially its weldability and high-heat-input welding properties. The low-temperature toughness and crack-arresting properties of the weld heat-affected zone are severely deteriorated (coarse, feathery upper bainite structure forms in the weld heat-affected zone). Therefore, the Cr content in this invention is controlled between 0.05% and 0.25%.

[0054] Adding Mo can significantly improve the hardenability of steel plates, promote the formation of bainite / martensite low-temperature phase transformation structures, increase the strength of steel plates, inhibit the segregation of P and S at grain boundaries, and improve the strength-toughness balance of steel plates. However, as a strong carbide-forming element, excessive Mo not only severely damages the low-temperature toughness, crack arrest characteristics, and elongation of steel plates (due to the inhibition of ferrite + bainite multiphase structures and the promotion of single bainite structures), but also significantly deteriorates the weldability of steel plates, especially high heat input weldability, promotes the formation of M / A islands, and severely degrades the weld fusion line, low-temperature toughness of the heat-affected zone, and crack arrest characteristics of steel plates. Therefore, considering the phase transformation strengthening effect of Mo and its influence on the low-temperature toughness, crack arrest characteristics, elongation, and weldability of the base steel plate, the Mo content should be controlled between 0.05% and 0.15%.

[0055] The purpose of adding trace amounts of nitrogen (Nb) to steel is to control rolling without recrystallization, refine the grain size of the steel plate, and improve the strength and toughness of TMCP steel plates. For ultra-thick steel plates, when the Nb content is below 0.030%, it not only fails to effectively control rolling but also negatively impacts the strength-toughness balance and crack-arresting properties of TMCP ultra-thick steel plates. When the Nb content exceeds 0.055%, it not only deteriorates the low-temperature toughness and crack-arresting properties of the steel plate (excessive Nb content easily leads to mixed crystals) but also increases the alloy cost of the steel plate (Nb...). The content of Nb (also a precious alloying element) remains high; moreover, under high heat input welding conditions, the formation of upper bainite (Bu) and the secondary precipitation and embrittlement of Nb (C,N) are induced, which seriously impairs the low-temperature toughness and crack arrest characteristics of the heat-affected zone (HAZ) of ultra-thick steel plates under high heat input welding. Therefore, the Nb content is controlled between 0.030 and 0.055% to obtain the best controlled rolling effect, achieve the strength-toughness / strength-plasticity matching of TMCP ultra-thick steel plates, high crack arrest characteristics, and high elongation, while not impairing the weldability of ultra-thick steel plates under high heat input.

[0056] The purpose of adding trace amounts of Ti to steel is to combine with N in the steel to generate highly stable TiN particles, which inhibit grain growth in the steel plate and the weld HAZ region. The Ti content added to the steel must match the N content in the steel. The matching principle is that TiN cannot precipitate in liquid steel but must precipitate in the solid phase. Therefore, the precipitation temperature of TiN must be ensured to be below 1400℃. When the amount of Ti added to the steel is too small (<0.008%), the number of TiN particles formed is insufficient to inhibit the growth of austenite grains during TMCP and welding thermal cycling, thus failing to improve the low-temperature toughness and weldability of ultra-thick steel plates. When the Ti content is too large (>0.016%), the TiN precipitation temperature exceeds 1400℃, and some TiN particles precipitate large-sized TiN particles during the solidification of the molten steel. These large-sized TiN particles not only fail to inhibit grain growth but also become the starting point for crack initiation. Therefore, the Ti content in this invention is controlled within the range of 0.008% to 0.016%.

[0057] Al in steel plates can fix free nitrogen in the steel, reduce free nitrogen in the weld heat-affected zone (HAZ), and improve the low-temperature impact toughness and crack arrest characteristics of welded HAZ with large heat input. However, adding too much Al to the steel will not only form a large number of dispersed needle-like Al2O3 inclusions in the steel, but also impair the low-temperature impact toughness and weldability of the steel plate. According to the composition system analysis of ultra-thick steel plates, the Al content of this invention is controlled between 0.025% and 0.060%.

[0058] The control range of nitrogen (N) corresponds to that of titanium (Ti), which is crucial for controlling steel grain size, improving low-temperature toughness, crack arrest properties, and weldability. If the N content is too low, the resulting TiN particles are few in number and large in size, failing to control the grain size and thus hindering the improvement of low-temperature toughness and weldability; in fact, it can be detrimental. However, if the N content is too high, the amount of free nitrogen ([N]) in the steel increases, especially under high heat input welding conditions, where the free nitrogen content in the heat-affected zone (HAZ) increases dramatically, severely damaging the HAZ's low-temperature toughness, crack arrest properties, and bending cold workability, thus deteriorating the steel's processing and usability. Therefore, in this invention, the N content is controlled at ≤0.0050%.

[0059] Ca treatment of steel serves two purposes: firstly, it further purifies the molten steel; secondly, it modifies the sulfides in the steel, transforming them into non-deformable, stable, fine spherical sulfides; thirdly, it suppresses the hot brittleness of sulfur; fourthly, it improves the low-temperature toughness, crack arrest properties, elongation, and Z-axis properties of ultra-thick steel plates; and fifthly, it improves the weldability of the steel plate. In addition, Ca treatment improves the castability of high-acid-soluble aluminum steel. More importantly, Ca(O,S) particles cannot agglomerate and grow in either liquid or solid steel, but are distributed in a fine and dispersed state. These high-melting-point Ca(O,S) particles pin the growth of austenite grains in the heat-affected zone during welding with high heat input, refine the grains in the weld heat-affected zone, and greatly improve the low-temperature ductility and toughness of the heat-affected zone with high heat input. The amount of Ca added depends on the sulfur content in the steel. Too little Ca will have little effect; too much Ca will result in excessively large Ca(O,S) inclusions, increasing brittleness and potentially becoming crack initiation points. This reduces the steel's low-temperature toughness, crack arrest characteristics, elongation, and weldability under high heat input, while also lowering steel purity and contaminating the molten steel. Generally, the Ca content is controlled according to ESSP = (%Ca)[1-1.25(%O)] / 1.25(%S), where ESSP is the sulfide inclusion shape control index, preferably ranging from 1.00 to 3.00. Therefore, the appropriate range for controlling the Ca content in this invention is 0.0010–0.0040%.

[0060] In particular, the present invention must simultaneously satisfy the following relationships:

[0061] C≤0.085%, Pcm≤0.24%;

[0062] Wherein, Pcm is the welding cold cracking sensitivity index, Pcm=(%C)+(%Si) / 30+[(%Mn)+(%Cu)+(%Cr)] / 20+(%Ni) / 60+(%Mo) / 15+(%V) / 10+5(%B), in percentage.

[0063] Ensuring excellent weldability of the steel plate under low carbon and low Pcm conditions; more importantly, the ultra-thick steel plate can be welded with high heat input under high Nb content conditions (0.030-0.055%); the weld heat-affected zone not only forms non-embrittled low carbon bainite with low M / A island content, but also has a small degree of secondary precipitation embrittlement of Nb(C,N) during multi-layer and multi-pass welding; thus ensuring the low-temperature toughness and crack arrest characteristics of the weld heat-affected zone of the ultra-thick steel plate.

[0064] (%C)×[(%Mn)+7.76(%Si)+10.23(%P)+21.47(%S)]≤0.216;

[0065] 1) It inhibits the degree of conjugate segregation of C, Mn, Si, P and S in the high manganese composition system, improves the three properties of the slab (integrity, homogeneity and purity), especially inhibits the center segregation, porosity and shrinkage of the continuous casting slab, and improves the strength and toughness matching, crack arrest characteristics, high machinability and weldability of the steel plate.

[0066] 2) Inhibit the formation of coarse martensite in high-manganese systems (increase the critical cooling rate of martensitic phase transformation), promote the formation of lower bainite, inhibit the precipitation of M / A islands during welding thermal cycling, and improve the crack arrest characteristics and elongation of steel plates.

[0067] 3) Promotes the separation transformation of ferrite and bainite in the high manganese composition system and the formation of ferrite + bainite (especially lower bainite) multiphase structure; while greatly improving the strength of the steel plate, the steel plate obtains excellent low-temperature toughness, crack arrest characteristics and high machinability; this is one of the key core technologies of this invention.

[0068] Als / (N-0.292Ti)≥28; This ensures that during the multi-layer, multi-pass welding process with a large heat input, the solid solution [N] is completely formed into fine and dispersed AlN, eliminating the strong embrittlement effect of solid solution [N] on the weld heat-affected zone and improving the low-temperature toughness and crack arrest characteristics of the weld heat-affected zone of the ultra-thick steel plate.

[0069] The low-temperature mobility index of dislocations in BCC crystal structures is θ≥0.79;

[0070] θ=1.25(%Ni)+[1.57(%Mn)-0.69(%Mn) 2 +1.69% Cu -3.51% Si

[0071] -0.46[(%Cr)+(%Mo)]-2.43[(%Al)+(%Nb)+(%Ti)].

[0072] Under low-temperature conditions, this invention reduces the lattice friction force PN (i.e., homogenizes the dsp electron cloud distribution of the atomic hybrid orbitals outside the atomic nucleus in the BCC crystal structure, reduces the directionality of the dsp electron cloud distribution, increases metallic bond characteristics, and reduces covalent bond characteristics, thus reducing the lattice friction force of dislocation slip), promotes dislocation cross-slip, improves the low-temperature mobility of dislocations in the BCC crystal structure, expands the plastic deformation field at the crack tip, increases the plastic deformation work (i.e., the proliferation and slip velocity of FR source dislocations at the crack tip), improves the intrinsic plasticity and toughness of the BCC crystal structure, and enhances the crack arrest properties of the steel plate. This is one of the key core technologies of this invention.

[0073] Ca treatment was performed, with the Ca / S ratio controlled between 1 and 3, and (%Ca) × (%S) ≤ 1.5 × 10⁻⁶. -3 ;

[0074] While ensuring spheroidization of sulfurization and minimizing the reduction of low-temperature toughness by Ca(O,S) particles, the Ca(O,S) particles are uniformly and finely distributed in the steel, which inhibits the growth of austenite grains in the heat-affected zone of welding with large heat input, improves the low-temperature toughness and crack arrest characteristics of the heat-affected zone of welding with large heat input in the steel plate, and improves the Z-direction properties of ultra-thick steel plates.

[0075] The component data in the above formulas are calculated as percentages. For example, if the carbon content is 0.10%, simply substitute 0.10 into the formula for calculation.

[0076] The manufacturing method of the high crack arrest and easy-to-weld ultra-thick YP550 grade steel plate of the present invention includes the following steps:

[0077] 1) Smelting and casting

[0078] The above-mentioned components are smelted and cast into slabs;

[0079] 2) Heating the slab, with the temperature controlled between 1100 and 1180℃;

[0080] 3) Controlled rolling

[0081] The first stage is ordinary rolling, which is continuous rolling using the maximum rolling capacity of the rolling mill;

[0082] The second stage employs controlled rolling without recrystallization, with the initial rolling temperature controlled between 710 and 750°C.

[0083] Rolling pass reduction rate ≥7%, cumulative reduction rate ≥45%, final rolling temperature 700~740℃;

[0084] Furthermore, the following relationship must also be satisfied:

[0085] [(%Nb)×Au×ξ] / {H×[(%Cr)+1.46(%Mo)]×(T 终轧 )}≥4.41×10 -4 ;

[0086] in,

[0087] Au is the austenite stabilization index, Au = 2.54 + 40.53[(%C) +

[0088] (%N)]+0.43[(%Cu)+(%Ni)+(%Mn)]-0.22(%Al)-2.64[(%P)

[0089] +(%S)]-1.26[(%Cr)+(%Mo)]-(%Si), unit %;

[0090] ξ represents the cumulative reduction rate of non-recrystallization / critical zone controlled rolling, in percentage (%).

[0091] H represents the thickness of the finished steel plate, in mm;

[0092] T 终轧 The final rolling temperature for non-recrystallization / critical zone controlled rolling, in °C;

[0093] 4) Control cooling

[0094] After controlled rolling, the steel plate is immediately transported to the accelerated cooling equipment for rapid cooling. The initial cooling temperature is 690–730℃, the cooling rate is ≥4℃ / s, and the final cooling temperature is 250–400℃. Furthermore, the following conditions must be met simultaneously:

[0095] [DI OL ×(T 开冷 )×(V c )] / [(T 停冷 [)×H]≥6.26;

[0096] in,

[0097] DI OL For online hardenability index,

[0098] DI OL =0.51C 0.5 [(1+0.7(%Si)][(1+3.33(%Mn)][(1+0.35

[0099] (%Cu)][(1+0.36(%Ni)][(1+2.16(%Cr)][(1+3(%Mo)][(1

[0100] +1.75(%V)][(1+1.77(%Al)]×25.4(mm), unit mm;

[0101] T 开冷 The starting temperature for accelerated cooling, measured in °C;

[0102] T 停冷 The stopping temperature for accelerated cooling, in °C;

[0103] V c The unit for accelerating the cooling rate of the steel plate is ℃ / s;

[0104] H represents the thickness of the finished steel plate, in mm.

[0105] Preferably, in step 1) continuous casting, the superheat of the tundish is controlled at 8-25℃, the casting speed is controlled at 0.6-1.0m / min, and the liquid level fluctuation in the crystallizer is controlled at ≤5mm.

[0106] Preferably, when the steel plate thickness is ≥80mm, the steel plate is subjected to slow cooling. The slow cooling process involves holding the plate at a temperature of not less than 300℃ for at least 24 hours, followed by natural air cooling to room temperature.

[0107] In the manufacturing method described in this invention:

[0108] According to the composition system of the steel and the requirements for strength, plasticity and low-temperature toughness of the steel plate, the manufacturing process design scheme is as follows: continuous casting is adopted, the superheat of the tundish is controlled at 8-25℃ (high manganese composition system must avoid casting with high superheat), the casting speed is controlled at 0.6-1.0m / min, and the liquid level fluctuation in the crystallizer is controlled at ≤5mm.

[0109] Based on the above-mentioned C, Mn, Nb and Ti content ranges, the slab heating temperature is controlled between 1100℃ and 1180℃ to ensure that Nb in the steel is completely dissolved into austenite during the slab heating process, while the austenite grains in the slab do not undergo abnormal growth.

[0110] Controlled rolling

[0111] The first stage is ordinary rolling, which uses the maximum rolling capacity of the rolling mill for continuous rolling. This maximizes the production capacity of the rolling line while ensuring that the deformed steel billet recrystallizes and refines the austenite grains.

[0112] The second stage adopts non-recrystallization controlled rolling. Based on the above-mentioned Nb element content range in steel and the requirements for texture strengthening and toughening, in order to ensure the effect of non-recrystallization controlled rolling and critical zone controlled rolling, the starting rolling temperature is controlled at 710-750℃, the rolling pass reduction rate is ≥7%, the cumulative reduction rate is ≥45%, and the final rolling temperature is 700-740℃.

[0113] After controlled rolling, the steel plate is immediately transported to the accelerated cooling equipment for rapid cooling. The initial cooling temperature is 690–730℃ to ensure the precipitation of a small amount of fine proeutectoid ferrite, which is beneficial for subsequent separation-type phase transformation. The cooling rate is ≥4℃ / s, and the cooling stop temperature is 250–400℃. When the steel plate thickness is ≥80mm, the steel plate undergoes slow cooling, which involves holding at a temperature of at least 300℃ for at least 24 hours, followed by natural air cooling to room temperature.

[0114] In addition to meeting the above requirements, the following relationship must also be satisfied:

[0115] [(%Nb)×Au×ξ] / {H×[(%Cr)+1.46(%Mo)]×(T 终轧 )}≥4.41×10 -4 ;

[0116] Where Au is the austenite stabilization index, Au = 2.54 + 40.53[(%C) + (%N)] + 0.43[(%Cu) + (%Ni) + (%Mn)] - 0.22(%Al) - 2.64[(%P) + (%S)] - 1.26[(%Cr) + (%Mo)] - (%Si), in %; ξ is the cumulative reduction rate of non-recrystallization / critical zone controlled rolling, in %; H is the thickness of the finished steel plate, in mm; T 终轧 The final rolling temperature is the temperature of the non-recrystallized / critical zone controlled rolling process, in °C.

[0117] As can be seen from the formula: with the increase of niobium content (%Nb), the controlled rolling effect increases, the degree of austenite recrystallization is greatly suppressed, the austenite flattening effect is enhanced, and the dislocation density and deformation band density inside the austenite grains increase; secondly, with the increase of the cumulative reduction rate ξ of controlled rolling in the non-recrystallized / critical zone, the dislocation density and deformation band density inside the flattened austenite grains also increase significantly; leading to an increase in the position of ferrite phase deformation nuclei and an increase in the driving force of austenite to ferrite phase transformation. During the accelerated cooling process of thick steel plates containing Cr / Mo, the coarse feather-like bainite structure and the Widmanstätten phase transformation structure of side laths are suppressed, while the fine equiaxed ferrite structure and lath bainite structure increase. In addition, with the increase of the cumulative reduction rate ξ of controlled rolling in the recrystallized / critical zone, the deformation texture (100) <011> The enhancement of texture and toughness is achieved through several factors. First, as the austenite stabilization index Au increases, the anisotropy of the electron cloud in the outer dsp hybrid orbitals of the alloy atoms decreases, the covalent bond characteristics weaken while the metallic bond characteristics strengthen, and the low-temperature mobility of BCC dislocations improves the intrinsic ductility and toughness of the steel plate. At the same time, the Ar3 point temperature decreases, refining the grain structure of the ultra-thick steel plate. The combined effect of these results ensures that the ultra-thick steel plate has excellent low-temperature toughness and crack arrest characteristics. However, as the steel plate thickness H and the final rolling temperature increase, the controlled rolling effect weakens, the austenite flattening effect deteriorates, the dislocation density / deformation band density inside the austenite grains is low, and there are fewer nucleation sites, increasing the stability of austenite. Combined with a relatively low cooling rate (due to the plate thickness) and the presence of Cr / Mo alloys, this promotes the development of coarse feather-like upper bainite structure and side lath Widmanstätten structure, deteriorating the low-temperature toughness and crack arrest characteristics of the ultra-thick steel plate.

[0118] In summary, when all indicators meet the above formula, the ultra-thick steel plate is guaranteed to have excellent low-temperature toughness and crack-arresting properties. This is one of the key core technologies of this invention.

[0119] [DI OL ×(T 开冷 )×(V c )] / [(T 停冷 [)×H]≥6.26;

[0120] Among them, DI OL DI is the online hardenability index. OL =0.51C 0.5 [(1+0.7(%Si)][(1+3.33(%Mn)][(1+0.35(%Cu)][(1+0.36(%Ni)][(1+2.16(%Cr)][(1+3(%Mo)][(1+1.75(%V)][(1+1.77(%Al)]×25.4(mm), unit mm; T 开冷 The initial temperature for accelerated cooling, measured in °C (T). 停冷 The stopping temperature for accelerated cooling, measured in °C; V cH represents the accelerated cooling rate of the steel plate, in °C / s; H represents the thickness of the finished steel plate, in mm.

[0121] As can be seen from the formula: with the online hardenability index DI... OL Increase the cooling temperature T 开冷 Heating and cooling rates V c As the temperature increases, the content of low-temperature phase transformation structures (such as bainite, especially lower bainite) in the steel plate increases, leading to higher steel plate strength. Conversely, as the cooling temperature increases and the steel plate thickness increases (resulting in a lower cooling rate, especially in the center of ultra-thick steel plates), the content of low-temperature phase transformation structures (such as bainite, especially lower bainite) decreases, resulting in lower steel plate strength. In summary, when all indicators satisfy the above formula, it ensures that the ultra-thick steel plate possesses high strength, high toughness, and high crack-arresting properties, while also exhibiting excellent weldability under high heat input. This successfully eliminates the inherent contradiction between the low carbon equivalent of ultra-thick steel plates and their high strength, high toughness, high crack-arresting properties, and excellent weldability. This is one of the key core technologies of this invention.

[0122] The component data in the above formulas are calculated as percentages. For example, if the carbon content is 0.10%, simply substitute 0.10 into the formula for calculation.

[0123] The beneficial effects of this invention are:

[0124] This invention relates to a steel plate that, through a matched combination design of main alloying elements, microalloying elements, and inclusion elements, and in conjunction with a new generation of TMCP manufacturing technology, produces a high-performance TMCP-type ultra-thick crack-arresting steel plate at low cost. This ultra-thick crack-arresting steel plate not only possesses high strength, high toughness, and high crack-arresting properties, but also exhibits excellent weldability, particularly high heat input welding, significantly shortening the manufacturing cycle of steel structures and creating substantial value for construction companies. It achieves low-carbon, green, and environmentally friendly manufacturing and use of the steel plate. The high performance and high added value of the steel plate are concentrated in its large thickness (≥80mm), high strength, high toughness, high crack-arresting properties, and excellent weldability, especially the perfect match of high heat input weldability, successfully solving the following problems:

[0125] A) The relationship between low carbon content, low carbon equivalent (Pcm), and large thickness and high strength;

[0126] B) Balancing low-temperature toughness, high crack arrestability, and large thickness with low manufacturing cost;

[0127] C) The contradiction between high strength, high crack arrest properties, and excellent weldability, especially high heat input weldability, which is difficult to reconcile in composition design and TMCP process design, greatly improves the safety, stability, and durability of large heavy steel structures. The good weldability (especially high heat input welding) saves users' steel structure manufacturing costs and shortens the manufacturing time, not only creating huge value for users, but more importantly, reducing carbon emissions (no preheating before welding, no heat treatment after welding, and high-efficiency welding with ultra-high heat input). Therefore, this type of steel plate is not only a high-value-added product with green and environmentally friendly features throughout its entire life cycle. Attached Figure Description

[0128] Figure 1 This is a photograph of the microstructure (1 / 4 thickness) of the steel in Example 3 of the present invention. Detailed Implementation

[0129] The present invention will be further described below with reference to the embodiments and accompanying drawings.

[0130] The composition of the embodiments of the present invention is shown in Table 1, with the balance being Fe and unavoidable impurities. Tables 2 and 3 show the manufacturing process parameters of the embodiments of the present invention. Table 4 shows the performance parameters of the embodiments of the present invention.

[0131] Depend on Figure 1 It can be seen that the microstructure of the finished steel plate of the present invention is a small amount of uniform and fine ferrite (less than 40%) + low carbon bainite, and the average size of the microstructure is less than 20 μm; thus the steel plate obtains high strength, high toughness, and high crack arrest properties, and the steel plate has excellent weldability, especially high heat input welding.

[0132] In summary, the ultra-thick steel plate of this invention is mainly used in ship structures, marine engineering structures, bridge engineering structures, energy development projects, offshore wind power projects, and the manufacturing of engineering machinery, mining machinery, and heavy vehicles, and can achieve low-cost, stable, and mass industrial production.

[0133] With the rigid constraints of my country's national economic development and carbon neutrality environmental targets, and the requirement to build a green, resource-saving, and harmonious society, infrastructure construction (such as large-scale cross-sea railway and highway bridge projects), large-scale shipbuilding (such as VLCCs), marine development, high-power offshore wind power projects, and major equipment manufacturing have been placed on the agenda. The world-renowned large-capacity offshore power project in southeastern my country uses ultra-thick EH40 / EH47 steel, while European and Middle Eastern marine engineering projects require 75-120mm ultra-thick EQ51 and EQ55 steels, and turbine base ring plates use 100-200mm S500QL1 and S550QL1 steels, which are YP550MPa grade high crack arrest steel plates. Currently, my country's basic energy construction (hydropower, offshore wind power, etc.), marine engineering development, and major equipment manufacturing are booming. YP550 grade ultra-thick steel plates, with their high toughness, high crack arrest properties, and excellent weldability, are key materials for basic energy construction, marine engineering development, and related equipment manufacturing industries, and have broad market prospects.

[0134]

[0135]

[0136]

Claims

1. A high crack-arrest, easy-to-weld ultra-thick YP550 grade steel plate, having the following composition in weight percent: C：0.050～0.085％ Si: ≤0.12% Mn: 1.80-2.20% P：≤0.013％ S：≤0.003％ Cu: 0.15-0.40% Ni: 0.55-0.95% Cr：0.05～0.25％ Mo: 0.05-0.15% Nb: 0.030-0.055% Ti: 0.008-0.016% Als: 0.025-0.060% N：≤0.0050％ Ca: 0.0010-0.0040% the balance being Fe and other unavoidable impurities; and must simultaneously satisfy the following relationships: welding cold crack sensitivity index Pcm ≤ 0.24%; Pcm = (%C) + (%Si) / 30 + [(%Mn) + (%Cu) + (%Cr)] / 20 + (%Ni) / 60 + (%Mo) / 15 + (%V) / 10 + 5(%B) in %; (%C) x [(%Mn) + 7.76(%Si) + 10.23(%P) + 21.47(%S)] ≤ 0.216; Als / (N-0.292Ti) ≥ 28; BCC crystal structure dislocation low temperature mobility index θ ≥ 0.79; θ = 1.25 (% Ni) + [1.57 (% Mn) - 0.69 (% Mn) + 1.69 (% Cu) 2 + 0.69 (% Mn) + 1.69 (% Cu) -3.51(%Si) - 0.46[(%Cr) + (%Mo)] - 2.43[(%Al) + (%Nb) + (%Ti)]; Ca treatment, Ca / S ratio is controlled at 1-3, and (%Ca) x (%S) < 1.5 x 10 -3 ; the microstructure of the steel plate is a small amount of uniform fine ferrite + low carbon bainite, and the average grain size of the microstructure is below 20 μm; The steel sheet has a yield strength of ≥550 MPa, a tensile strength of ≥650 MPa, a fracture elongation δ5 of ≥18%, a Charpy impact energy at -50°C of ≥100 J, and a Kca at -10°C of ≥6000 N / mm 3 / 2 The steel sheet has excellent weldability, does not require preheating before welding, can withstand a welding heat input of ≥50 kJ / cm, and has an impact energy KV2 at -50°C of the heat affected zone of ≥60 J.

2. The method of producing a high-riεt-terrace, easily-weldable super-thick YP550 grade steel sheet as claimed in claim 1, characterized by, comprising the following steps: 1) smelting and casting smelting and casting into a slab according to the composition of claim 1; 2) slab heating, the temperature being controlled at 1100-1180°C; 3) controlled rolling the first stage is ordinary rolling, using the maximum rolling capacity of the rolling mill to continuously roll; the second stage uses non-recrystallization controlled rolling, the rolling temperature being controlled at 710-750°C, the rolling pass reduction being ≥7%, the cumulative reduction being ≥45%, and the finish rolling temperature being 700-740°C; and, must simultaneously satisfy the following relationship: [(%Nb) x Au x ξ] / {H x [(%Cr) + 1.46(%Mo)] x (T 终轧 )} ≥ 4.41 x 10 -4 ; wherein, Au is the austenite stabilization index, Au = 2.54 + 40.53[(%C) + (%N)] + 0.43[(%Cu) + (%Ni) + (%Mn)] - 0.22(%Al) - 2.64[(%P) + (%S)] - 1.26[(%Cr) + (%Mo)] - (%Si) in %; ξ is the non-recrystallization / critical zone controlled rolling cumulative reduction, in %; H is the thickness of the finished steel plate, in mm; T 终轧 final rolling temperature for non-recrystallization / critical region controlled rolling, unit: °C; 4) controlled cooling after the controlled rolling, the steel plate is immediately sent to an accelerated cooling device, and then the steel plate is immediately subjected to accelerated cooling, the open cooling temperature of the steel plate being 690-730°C, the cooling speed being ≥4°C / s, and the stop cooling temperature being 250-400°C; and, the controlled cooling must simultaneously satisfy the following relationship: [DI OL ×(T 开冷 )×(V c )] / [(T 停冷 )×H]≥6.26; wherein, DI OL is the online hardenability index, DI OL = 0.51C 0.5 [(1 + 0.7 (% Si)][(1 + 3.33 (% Mn)][(1 + 0.35 [(1 + 0.36(%Ni)] [(1 + 2.16(%Cr)] [(1 + 3(%Mo)] [(1 + 1.75(%V)] [(1 + 1.77(%Al)] x 25.4, in mm; T 开冷 temperature for the start of accelerated cooling, in °C; T 停冷 temperature for the acceleration of cooling, unit: °C; V c to accelerate the cooling rate of the steel sheet, in °C / s; H is the thickness of the finished steel plate, in mm.

3. The manufacturing method of the high crack arrest and easy-to-weld ultra-thick YP550 grade steel plate as described in claim 2, characterized in that, Step 1) During the continuous casting process, the tundish pouring superheat is controlled at 8-25°C, the casting speed is controlled at 0.6-1.0 m / min, and the crystallizer liquid level fluctuation is controlled at ≤5 mm.

4. The manufacturing method of the high crack arrest and easy-to-weld ultra-thick YP550 grade steel plate as described in claim 2, characterized in that, Step 4) After the accelerated cooling is finished, when the thickness of the steel plate is ≥80 mm, the steel plate is slowly cooled, the slow cooling process is not less than 300°C for more than 24 hours, and then the steel plate is naturally air cooled to room temperature.

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