High-crack-arrest easy-to-weld super-thick YP550-grade steel plate and manufacturing method thereof

Abstract

The invention discloses a high-crack-arrest and easy-to-weld super-thick YP550-grade steel plate and a manufacturing method thereof. The high-crack-arrest and easy-to-weld super-thick YP550-grade steel plate comprises the following components in percentage by weight: 0.050-0.085% of C, less than or equal to 0.12% of Si, 1.80-2.20% of Mn, less than or equal to 0.013% of P, less than or equal to 0.003% of S, 0.15-0.40% of Cu, 0.55-0.95% of Ni, 0.05-0.25% of Cr, 0.05-0.15% of Mo, 0.030-0.055% of Nb, 0.008-0.016% of Ti, 0.025-0.060% of Als, less than or equal to 0.0050% of N, 0.0010-0.0040% of Ca and the balance of Fe and The welding cold crack sensitivity index Pcm is less than or equal to 0.24%; the dislocation low-temperature mobility index theta of the BCC crystal structure is greater than or equal to 0.79; and in Ca treatment, the Ca / S ratio is 1-3, and (% Ca) * (% S) is less than or equal to 1.5 * 10 <-3 >. While high strength, excellent low-temperature toughness, high crack arrest characteristic and high elongation of the ultra-thick base metal steel plate are obtained, the low-temperature toughness of a fusion line and the low-temperature toughness of HAZ are also excellent during high heat input welding, and the ultra-thick base metal steel plate is particularly suitable for high-power offshore wind power pile legs, ocean platforms, cross-sea bridge steel tower structures, port machinery, engineering machinery and the like.

Description

Technical Field

[0001] The invention relates to the technical field of high-strength steel, and in particular to an ultra-thick YP550 grade steel plate with high crack arrest and easy welding, and a manufacturing method thereof. Background Art

[0002] As we all know, low carbon (high strength) low alloy steel is one of the most important engineering structural materials and is widely used in oil and gas pipelines, offshore platforms, shipbuilding, hydropower projects, bridge structures, boiler containers, building structures, automobile 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 which strength, toughness, plasticity, weldability and their matching are the most important properties of low carbon (high strength) low alloy steel, which is ultimately determined by the microstructure and dislocation substructure of the finished steel.

[0003] With the continuous development of metallurgical technology, people have put forward higher requirements on the toughness, plasticity, especially weldability and low anisotropy of high-strength steel, that is, the steel plate has the ability to resist brittle fracture and plastic instability fracture (i.e. crack arrest characteristics) under extremely low temperature conditions (-50℃), while the elongation at fracture and uniform elongation reach the level of steel plates with tensile strength of 500MPa. The steel plate has excellent weldability and low anisotropy of performance, and can withstand large heat input welding (welding heat input ≥50kJ / cm); and under the conditions of lower alloy content, especially precious metal content, and lower manufacturing cost, the comprehensive mechanical properties and performance of the steel plate are greatly improved to reduce the alloy content of steel and save costs. High strength and lightweight reduce the weight, stability and safety of steel components. More importantly, it is to further improve the cold / hot processability of steel components and the safety and reliability during service.

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

[0005] When using the prior art related to the present invention to manufacture thick steel plates with a yield strength of ≥420MPa and a low-temperature impact toughness of ≥34J at -60°C, a certain amount of Ni or Cu+Ni elements (≥0.30%) are 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 ARCTIC OFFSHORE STRUCTURES"; "Structural Steel Plates for Arctic Use Produced by Multipurpose Accelerated Cooling System" (Japanese), "Kawasaki Steel Technology Report", 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 Plates, 1986 ... 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 Plate for Offshore Platform Structure Used in Ice Sea Areas" (Japanese), "Steel Research", 1984, No.314, 19-43), to ensure that the parent steel plate has excellent low-temperature toughness. When the heat input welding is <50KJ / cm, the toughness of the heat affected zone (HAZ) can also reach -60℃Akv≥34J; however, when ultra-large heat input (≥100KJ / cm) is used for welding, the low-temperature toughness of the heat affected zone (HAZ) is generally difficult to achieve, and the low-temperature toughness of the heat affected zone (HAZ) is seriously deteriorated. A large number of patent documents only explain how to achieve low-temperature toughness of the base steel plate, but there are few descriptions on how to obtain excellent low-temperature toughness of the heat affected zone (HAZ) under welding conditions, especially how to ensure the low-temperature toughness of the heat affected zone (HAZ) when ultra-large heat input welding is used. In order to ensure the low-temperature toughness of the steel plate, a certain amount of Ni or Cu+Ni elements are generally added to the steel, and the low-temperature toughness of the heat affected zone (HAZ) of the steel plate ultra-large heat input welding is rarely able to reach -60°C (see Japanese Patents Sho 63-93845, Sho 63-79921, Sho 60-258410, Te Ping Kai 4-285119, Te Ping Kai 4-308035, Ping 3-264614, Ping 2-250917, Ping 4-143246, US Patent 4855106, US Patent 5183198, US Patent 4137104). .

[0006] At present, only Japan's Nippon Steel Corporation has improved the low-temperature toughness of the heat-affected zone (HAZ) of ultra-high heat input welded steel plates by using oxide metallurgy technology (see US Patent 4629505, WO 01 / 59167A1), that is, during high heat input welding, near the fusion line, due to long-term high temperature, TiN particles dissolve and lose their function, while Ti2O3 is more stable than TiN and will not dissolve even when the melting point of steel is reached. Ti2O3 particles can become the nucleation site of acicular ferrite in austenite crystals, promote the nucleation of acicular ferrite (AF) in austenite crystals, effectively divide austenite grains, refine HAZ structure, and form high-strength and high-toughness acicular ferrite structure.

[0007] In addition, Baosteel has developed a series of low-temperature high-strength steel plates that can be welded with high heat input (see Chinese patents ZL201410300713.X, ZL201310244712.3, ZL201310244706.8, ZL201310124065.2, ZL201310244713.8, ZL201210209637.2, ZL201410815614.5 and Chinese patent application No. 201710183350.X, 201910149978.7). In order to ensure the low-temperature toughness of the heat-affected zone of high heat input welding, a certain amount of precious alloy 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 particularly necessary to add a small amount of Mo. Although the high heat input welding processability of the steel plate is good, the toughness of the welding heat affected zone, especially the low-temperature toughness of the thick steel plate welding heat affected zone is not stable (under high heat input welding conditions, Mo promotes the formation of coarse upper bainite and deteriorates the toughness of the welding coarse grain heat affected zone), and the requirement of -40℃ impact toughness cannot be stably met. The manufacturing cost of thick steel plates is also high; more importantly, the TMCP steel plates developed above do not involve the research and control of the steel plate crack arrest characteristics Kca, and the developed steel plates cannot fully meet the design safety requirements of special large and heavy steel structures (such as large-capacity offshore wind power pile leg structures, marine engineering structures and giant ship floating cranes, etc.);

[0008] Secondly, the strength level of the steel plate of the above patents cannot reach the yield strength ≥550MPa and the tensile strength ≥650MPa; to achieve this strength level, the offline quenching and tempering process Q+T is generally used for production (see Chinese patents ZL201910150314.2, ZL201610463494.6, ZL201010113835.X, ZL200810042088.8, etc.), which not only leads to a long steel plate manufacturing cycle, high manufacturing cost, and does not meet the green and low-carbon concept, but more importantly, the steel plate microstructure is a single bainite structure. The size of the bulk crystal cluster is large (about 20μm), resulting in poor crack arrest properties and low elongation of the steel plate. In order to obtain a steel plate with high crack arrest properties and high elongation, it is necessary to use a two-stage tempering process of Q+Q'+T to obtain a fine and uniform ferrite + bainite complex structure. This will cause the steel plate manufacturing process to be too long and the manufacturing cost to remain high, which cannot meet the low-carbon green concept and the increasingly fierce market competition environment. In addition, the offline tempering process component system requires a higher Mo content and micro-B treatment, which not only causes high alloy costs, but also leads to deterioration of the weldability of the steel plate, which cannot meet the requirements of high heat input welding. Summary of the invention

[0009] The object of the present invention is to provide a high crack arrest, easy-to-weld ultra-thick YP550 grade steel plate and a manufacturing method thereof, which can obtain high strength, excellent low-temperature toughness, high crack arrest characteristics and high elongation of the ultra-thick base steel plate, and at the same time, the low-temperature toughness of the fusion line and the HAZ during high heat input welding is also excellent; the yield strength of the steel plate is ≥550MPa, the tensile strength is ≥650MPa, the elongation at break δ5 is ≥18%, the Charpy impact energy KV2 (single value) of the -50°C steel plate is ≥100J, and Kca (-10°C) is ≥6000N / mm 3 / 2 , excellent weldability, no need for preheating before welding, can withstand welding heat input ≥50kJ / cm, heat affected zone -50℃ impact energy KV2 (single value) ≥60J; particularly suitable for high-power offshore wind power pile legs, offshore platforms, large floating cranes, icebreakers, cross-sea bridge steel tower structures, hydropower metal structural components, port machinery and engineering machinery, etc., and can achieve low-cost and stable mass industrial production.

[0010] Ultra-thick steel plates with high strength, high toughness and high crack arrest are one of the most difficult types of thick plate products. The reason is that this type of steel plate not only requires low C, low Pcm, high strength, excellent low-temperature toughness and high crack arrest characteristics, but also the steel plate must be able to withstand large heat input welding (welding heat input ≥ 50KJ / cm). The steel plate welding fusion line and heat-affected zone have high strength, excellent low-temperature impact toughness and high crack arrest characteristics. However, these performance requirements are difficult to meet at the same time:

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

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

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

[0014] The above characteristics conflict with each other in the composition design and TMCP process design and are difficult to reconcile: when reducing the C content and Pcm, it is difficult to achieve high strength, high crack arrest characteristics and high elongation of ultra-thick steel plates (excessive strengthening will cause low elongation); while improving the low-temperature toughness, crack arrest characteristics and elongation of ultra-thick steel plates, it is difficult to achieve low C, low Pcm and low manufacturing cost of ultra-thick steel plates (mainly low Ni content); when ultra-thick steel plates obtain high strength, high crack arrest characteristics and high elongation, the weldability of the steel plates, especially the steel plates with large heat input, is difficult to guarantee. How to balance large thickness, low C / low Pcm, high strength, high toughness, high crack arrest characteristics, high plasticity and large heat input weldability is one of the biggest difficulties of this product and also the key core technology.

[0015] To achieve the above object, the technical solution of the present invention is:

[0016] A high crack arrest, easy to weld, ultra-thick YP550 grade steel plate, the weight percentage of its components is:

[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 must satisfy the following relationship at the same time:

[0032] Welding cold crack sensitivity index Pcm≤0.24%;

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

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

[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 structure 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, Ca / S ratio is controlled between 1 and 3, and (%Ca)×(%S)≤1.5×10 -3 .

[0042] Furthermore, the balance is Fe and other inevitable inclusions.

[0043] The microstructure of the steel plate of the present invention is a small amount of uniform fine ferrite + low-carbon bainite, and the average grain size of the microstructure is less than 20 μm.

[0044] The steel plate of the present invention has a yield strength of ≥550MPa, a tensile strength of ≥650MPa, a fracture elongation δ5 of ≥18%, a Charpy impact energy (single value) of the steel plate at -50°C of ≥100J, and a Kca (-10°C) of ≥6000N / mm 3 / 2 ; Excellent weldability, no preheating required before welding, can withstand welding heat input ≥50kJ / cm, and the impact energy KV2 (single value) of the heat affected zone at -50℃ is ≥60J.

[0045] In the composition design of the steel plate of the present invention:

[0046] C has a great influence on the strength, low-temperature toughness, crack arrest characteristics, elongation and weldability of TMCP ultra-thick steel plates, especially the high heat input weldability. From the perspective of improving the low-temperature toughness and high heat input weldability of ultra-thick steel plates, it is hoped that the C content in the steel can be controlled relatively low; however, from the perspective of the strength of ultra-thick steel plates and the microstructure control and manufacturing cost during the production process, the C content should not be controlled too low; too low C content can easily lead to excessively high grain boundary mobility, coarse grains of the microstructure of the parent steel plate and the welded HAZ, and easy generation of mixed crystals, and too low C content in the steel can cause grain boundary weakening, seriously deteriorating the crack arrest characteristics of the parent steel plate and the low-temperature toughness of the welded 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 the deoxidation of molten steel and can improve the strength of steel plates. However, for molten steel deoxidized by Al, the deoxidation effect of Si is not significant. Although Si can improve the strength of the steel plate, Si seriously damages the low-temperature toughness, crack arrest properties, elongation and weldability of weathering steel plates. Especially for high-strength and ultra-thick steel plates with increased strength and higher alloy content, when welding with large heat input, Si not only promotes the formation of MA islands, but also forms large MA islands with uneven distribution, which seriously damages the low-temperature toughness and crack arrest properties of the 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 the present invention is controlled below 0.12%.

[0048] As the most important alloying element in steel, Mn not only improves the strength of the steel plate, but also has the functions of expanding the austenite phase region, lowering the Ar3 point temperature, refining the TMCP steel plate grains to increase the strength of the steel plate (fine grain strengthening effect), improving the low-temperature toughness and crack arrest characteristics of the steel plate (fine grain toughening), and promoting the formation of low-temperature phase transformation structure (phase transformation strength effect) to increase the strength of the steel plate. However, Mn is prone to segregation during the solidification of molten steel, especially when the Mn content is high, which not only makes the casting operation difficult, but also Mn is prone to conjugate segregation with elements such as C, P, and S. In particular, when the C content in the steel is high, the segregation, looseness and shrinkage cavity in the center of the ingot are aggravated. Severe segregation in the center of the ingot is prone to form abnormal tissue during the subsequent TMCP and welding process, resulting in 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, when the Mn content is too high, not only the low-temperature toughness, crack arrest characteristics, and elongation of the steel plate are sharply deteriorated, but also the weldability of the steel plate, especially the high heat input weldability, is seriously deteriorated; in addition, when the Mn content is too high, the low-temperature mobility of ferrite dislocations is reduced (PN force is increased), resulting in deterioration of the intrinsic plastic toughness and crack arrest characteristics of the steel plate; therefore, the suitable Mn content of the present invention is 1.80-2.20%.

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

[0050] As a harmful inclusion in steel, S has a great damaging effect on the low-temperature toughness, crack arrest characteristics, elongation and weldability of the steel plate. More importantly, S combines with Mn in the steel to form MnS inclusions. During the hot rolling process, the plasticity of MnS causes MnS to extend along the rolling direction to form a MnS inclusion belt along the rolling direction, which not only seriously damages the low-temperature impact toughness, crack arrest characteristics, elongation, Z-direction performance and fatigue resistance of the steel plate, but also causes the weldability of the steel plate (especially the large heat input weldability) to be seriously deteriorated; at the same time, S is also the main element that produces hot brittleness during hot rolling, and theoretically the lower the better; but considering the operability of steelmaking, steelmaking cost and the principle of smooth logistics, for TMCP ultra-thick steel plates that require large heat input welding, high toughness (toughness of -50°C), high crack arrest characteristics (Kca of -10°C), high strength and large heat input welding, the S content needs to be controlled at ≤0.003%.

[0051] Cu is mainly an austenite stabilizing element and an element that improves the low-temperature mobility of ferrite dislocations. Adding Cu can also reduce the Ar1 and Ar3 point temperatures, refine the microstructure of TMCP weathering steel plates, and improve the low-temperature toughness and crack arrest properties of TMCP steel plates. However, if Cu is added too much, higher than 0.40%, it will not only easily cause copper brittleness, surface cracking of the ingot, internal cracking problems, and especially the impact load fracture characteristics (i.e., crack arrest characteristics) of thick steel plates; it will also cause serious deterioration of the low-temperature toughness and crack arrest characteristics of welded joints. At the same time, considering that Cu is a relatively precious alloying element, from a cost-effectiveness perspective, it is appropriate to control the upper limit of Cu to 0.40%. If Cu is added too little, lower than 0.15%, the above effects are small; therefore, the Cu content is controlled between 0.15 and 0.40%.

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

[0053] For ultra-thick, crack-arresting, high-machinability TMCP steel plates with a tensile strength of ≥650MPa, adding a certain amount of Cr (≤0.25%) can improve the strength of the thick steel plates without damaging the low-temperature toughness, bending cold working and large-input welding of the steel plates; but if the Cr content is too little (<0.05%), the contribution of Cr to the strength of the steel plates is small and the performance index requirements cannot be met; if too much Cr is added (>0.25%), the low-temperature toughness, crack-arresting properties and high machinability of the steel plates are damaged, especially the weldability of the steel plates, especially the high-heat-input weldability, and the low-temperature toughness and crack-arresting properties of the welding heat-affected zone are seriously deteriorated (coarse feather-like upper bainite structure is formed in the welding heat-affected zone); therefore, the Cr content of the present invention is controlled between 0.05 and 0.25%.

[0054] Adding Mo can greatly improve the hardenability of the steel plate, promote the formation of bainite / martensite low-temperature phase transformation structure, improve the strength of the steel plate, inhibit the segregation of P and S on the grain boundary, and improve the strength and toughness matching of the steel plate; however, as a strong carbide-forming element, when Mo is added too much, it not only seriously damages the low-temperature toughness, crack arrest properties and elongation of the steel plate (due to inhibiting the ferrite + bainite complex phase structure and promoting the formation of a single bainite structure); but also greatly deteriorates the weldability of the steel plate, especially the high heat input weldability, promotes the formation of M / A islands, and seriously deteriorates the low-temperature toughness and crack arrest properties of the steel plate welding fusion line and heat-affected zone; therefore, considering the phase transformation strengthening effect of Mo and its influence on the low-temperature toughness, crack arrest properties, elongation and weldability of the parent steel plate, the Mo content is controlled between 0.05 and 0.15%.

[0055] The purpose of adding trace amounts of Nb elements to steel is to carry out non-recrystallization controlled rolling, refine the grain size of the steel plate, and improve the strength and toughness of the TMCP steel plate. For ultra-thick steel plates, when the amount of Nb added in the steel is less than 0.030%, in addition to the inability to effectively play the role of controlled rolling, it is also unfavorable for the strengthening and toughening matching and crack arrest characteristics of the TMCP ultra-thick steel plate; when the amount of Nb added exceeds 0.055%, it not only causes the low-temperature toughness and crack arrest characteristics of the steel plate to deteriorate (too high Nb content is prone to mixed crystals) and the alloy cost of the steel plate (Nb It is also a precious alloying element) remains high; and under high heat input welding conditions, it induces the formation of upper bainite (Bu) and secondary precipitation embrittlement of Nb (C, N), which seriously damages the low-temperature toughness and crack arrest properties of the heat affected zone (HAZ) of ultra-thick steel plates in 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 toughness / plasticity matching of TMCP ultra-thick steel plates, high crack arrest properties, and high elongation, without damaging the high heat input weldability of ultra-thick steel plates.

[0056] The purpose of adding a trace amount of Ti to the steel is to combine with N in the steel to generate highly stable TiN particles to inhibit the growth of steel plate grains and grains in the welding HAZ area; the Ti content added to the steel must match the N content in the steel, and the matching principle is that TiN cannot precipitate in liquid molten steel but must precipitate in the solid phase; therefore, the precipitation temperature of TiN must be ensured to be lower than 1400°C; 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 cycles and improve the low-temperature toughness and weldability of ultra-thick steel plates; when the Ti content is too much (>0.016%), the TiN precipitation temperature exceeds 1400°C, and some TiN particles precipitate large-sized TiN particles during the solidification of the molten steel. Such large-sized TiN particles not only cannot inhibit grain growth, but instead become the starting point for crack initiation; therefore, the control range of the Ti content of the present invention is 0.008-0.016%.

[0057] Als in the steel plate can fix the free [N] in the steel, reduce the free [N] in the welding heat affected zone (HAZ), and improve the low-temperature impact toughness and crack arrest characteristics of the HAZ with large heat input welding; however, adding excessive Als to the steel will not only form a large number of dispersed needle-shaped Al2O3 inclusions in the steel, but also damage the low-temperature impact toughness and weldability of the steel plate. According to the analysis of the component system of the ultra-thick steel plate, the Als content of the present invention is controlled between 0.025 and 0.060%.

[0058] The control range of N corresponds to the control range of Ti, which is important for controlling the grain of steel plates, improving the low-temperature toughness, crack arrest characteristics and weldability of steel plates; if the N content is too low, the number of TiN particles generated is small and the size is large, which cannot play the role of controlling the grain of steel plates and improving the low-temperature toughness and weldability of steel plates, but is harmful to the low-temperature toughness and weldability of steel plates; however, when the N content is too high, the free [N] in the steel increases, especially under the condition of large heat input welding, the free [N] content in the heat affected zone (HAZ) increases sharply, which seriously damages the low-temperature toughness, crack arrest characteristics and bending cold workability of HAZ, and deteriorates the processing and use characteristics of steel. Therefore, the N content of the present invention is controlled to be ≤0.0050%.

[0059] Ca treatment of steel can, on the one hand, further purify the molten steel, and on the other hand, denature the sulfides in the steel, turning them into non-deformable, stable and fine spherical sulfides, inhibiting the hot brittleness of S, improving the low-temperature toughness, crack arrest characteristics, elongation and Z-direction properties of ultra-thick steel plates, and improving the weldability of steel plates. In addition, Ca treatment is used to improve the pourability of high-acid-soluble aluminum molten steel; more importantly, Ca (O, S) particles cannot condense and grow in liquid molten steel or solid steel, and are distributed in the steel in a fine and dispersed state. Such high-melting-point Ca (O, S) particles pin the growth of austenite grains in the heat-affected zone during welding with large heat input, refine the grains in the welding heat-affected zone, and greatly improve the low-temperature plastic toughness of the heat-affected zone with large heat input. The amount of Ca added depends on the S content in the steel. If the amount of Ca added is too low, the treatment effect is not great; if the amount of Ca added is too high, the size of Ca(O,S) formed is too large, the brittleness is also increased, and it can become the starting point of fracture cracks, reducing the low-temperature toughness, crack arrest characteristics, elongation of the steel and the large heat input weldability of the steel plate, while also reducing the purity of the steel and polluting 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, and the value range is preferably between 1.00 and 3.00. Therefore, the suitable range of the Ca content controlled by the present invention is 0.0010 to 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 crack sensitivity index, Pcm=(%C)+(%Si) / 30+[(%Mn)+(%Cu)+(%Cr)] / 20+(%Ni) / 60+(%Mo) / 15+(%V) / 10+5(%B), and the unit is %.

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

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

[0065] 1) Inhibit the degree of conjugate segregation of high manganese component system C, Mn, Si, P, S, improve the three properties of slab internal quality (soundness, homogeneity and purity), especially inhibit the central segregation, looseness and shrinkage of continuous casting slab, improve the strength and toughness matching, crack arrest characteristics, high mechanical processing characteristics and weldability of steel plate;

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

[0067] 3) Promote the separation phase transformation of ferrite and bainite and the formation of ferrite + bainite (especially lower bainite) complex phase structure under high manganese component system; while greatly improving the strength of the steel plate, the steel plate obtains excellent low-temperature toughness, crack arrest properties and high machinability; this is one of the key core technologies of the present invention.

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

[0069] The low-temperature mobility index of dislocations in BCC crystal structure 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, the lattice friction force PN of dislocation slip in the BCC crystal structure is reduced (i.e., the distribution of the electron hybrid orbital dsp electron cloud outside the nucleus of the alloy system is homogenized, the directionality of the distribution of the electron hybrid orbital dsp electron cloud is reduced, the metal bond characteristics are increased, the covalent bond characteristics are reduced, and thus the dislocation slip lattice friction force is reduced), the dislocation cross slip is promoted, the low-temperature mobility of the dislocation in the BCC crystal structure is improved, the plastic deformation field at the crack tip is expanded, the plastic deformation work (i.e., the value-added and slip velocity of the dislocation at the crack tip FR source) is increased, the intrinsic plastic toughness of the BCC crystal structure is increased, and the crack arrest characteristics of the steel plate are improved. This is one of the key core technologies of the present invention.

[0073] Ca treatment, Ca / S ratio is controlled between 1 and 3, and (%Ca)×(%S)≤1.5×10 -3 ;

[0074] While ensuring sulfide spheroidization and minimizing the effect of Ca(O,S) particles on low-temperature toughness, the Ca(O,S) particles are evenly and finely distributed in the steel, inhibiting the growth of austenite grains in the heat-affected zone of large heat input welding, improving the low-temperature toughness and crack arrest characteristics of the heat-affected zone of large heat input welding of the steel plate, and improving the Z-direction performance of the ultra-thick steel plate.

[0075] The composition data in the above relationship is calculated as a percentage. For example, if the carbon content is 0.10%, just substitute 0.10 when calculating the relationship.

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

[0077] 1) Smelting and casting

[0078] Smelting and casting into slabs according to the above ingredients;

[0079] 2) Slab heating, the temperature is controlled at 1100~1180℃;

[0080] 3) Controlled rolling

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

[0082] The second stage adopts non-recrystallization controlled rolling, and the rolling temperature is controlled at 710-750℃.

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

[0084] And, 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 is the cumulative reduction rate of controlled rolling in the non-recrystallization / critical region, unit: %;

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

[0092] T 终轧 is the final rolling temperature of non-recrystallization / critical region controlled rolling, unit: °C;

[0093] 4) Controlled cooling

[0094] After controlled rolling, the steel plate is immediately transported to the accelerated cooling equipment, and then the steel plate is accelerated cooled. The steel plate cooling temperature is 690-730℃, the cooling rate is ≥4℃ / s, and the cooling stop temperature is 250-400℃; and the following relationship must also be met at the same time:

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

[0096] in,

[0097] DI OL is the 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 is the starting temperature of accelerated cooling, in °C;

[0102] T 停冷 is the stop temperature of accelerated cooling, in °C;

[0103] V c is the accelerated cooling rate of the steel plate, in °C / s;

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

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

[0106] Preferably, when the thickness of the steel plate is ≥80 mm, the steel plate is slowly cooled, and the slow cooling process is to keep the temperature at not less than 300° C. for more than 24 hours, and then the steel plate is naturally air-cooled to room temperature.

[0107] In the manufacturing method of the present invention:

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

[0109] According to the above-mentioned C, Mn, Nb and Ti content ranges, the slab heating temperature is controlled between 1100°C and 1180°C to ensure that all Nb in the steel is dissolved into the austenite during the slab heating process and that the austenite grains of the slab do not grow abnormally.

[0110] Controlled rolling

[0111] The first stage is ordinary rolling, which uses the maximum rolling capacity of the rolling mill for continuous rolling, maximizing the rolling line capacity while ensuring the recrystallization of the deformed steel billet and refining the austenite grains.

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

[0113] After the controlled rolling, the steel plate is immediately transported to the accelerated cooling equipment, and then the steel plate is accelerated cooled; the steel plate cooling temperature is 690 ~ 730 ℃, to ensure that a small amount of fine proeutectoid ferrite is precipitated, which is conducive to the subsequent separation phase transformation; the cooling rate is ≥ 4 ℃ / s, and the stop cooling temperature is 250 ~ 400 ℃. When the thickness of the steel plate is ≥ 80mm, the steel plate is slowly cooled, and the slow cooling process is to keep the temperature at not less than 300 ℃ for more than 24 hours, and then the steel plate is naturally air-cooled to room temperature.

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

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

[0116] 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), unit %; ξ is the cumulative reduction rate of non-recrystallization / critical region controlled rolling, unit %; H is the thickness of the finished steel plate, unit mm; T 终轧 It is the final rolling temperature of non-recrystallization / critical zone controlled rolling, unit is ℃.

[0117] It can be seen from the formula that as the niobium content (% Nb) increases, the controlled rolling effect increases, the degree of austenite recrystallization inhibition is large, the austenite flattening effect is enhanced, and the dislocation density and deformation band density inside the austenite grains increase; secondly, as the cumulative reduction rate ξ of controlled rolling in the non-recrystallization / critical zone increases, the dislocation density and deformation band density inside the flat austenite grains also increase significantly; resulting in an increase in the position of the ferrite phase deformation nucleus and an increase in the driving force for the austenite to ferrite phase transformation. During the accelerated cooling of thick, Cr / Mo-containing steel plates, the coarse feathery bainite structure and the side lath Widmanstatten phase transformation structure are inhibited, and the fine equiaxed ferrite structure and lath bainite structure increase. In addition, as the cumulative reduction rate ξ of controlled rolling in the recrystallization / critical zone increases, the deformation texture (100) <011> The effect of texture strengthening and toughening is enhanced; thirdly, with the increase of austenite stabilization index Au, the anisotropy of the electron cloud of the outer dsp hybrid orbit of the alloy atom decreases, the covalent bond characteristics weaken and the metal bond characteristics enhance, the low-temperature mobility of BCC dislocations increases, and the intrinsic plastic toughness of the steel plate is improved. At the same time, the Ar3 point temperature decreases, and the grain structure of the ultra-thick steel plate is refined; the above results are superimposed on each other, ensuring that the ultra-thick steel plate has excellent low-temperature toughness and crack arrest properties. However, with the increase of steel plate thickness H and final rolling temperature, the controlled rolling effect weakens, the austenite flattening effect deteriorates, the dislocation density / deformation band density inside the austenite grain is low, the nucleation position is small, the austenite stability increases, and the superimposed small cooling rate (caused by the plate thickness) and the presence of Cr / Mo alloy promote the development of coarse feather-like upper bainite structure and side plate strip Widmanstatten structure, which deteriorates the low-temperature toughness and crack arrest properties of the ultra-thick steel plate.

[0118] In summary, when all indicators meet the above formula, it is ensured that the ultra-thick steel plate has excellent low-temperature toughness and crack arrest properties. This is one of the key core technologies of the present invention.

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

[0120] Among them, DI OL is the online hardenability index, DI 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 T is the starting temperature of accelerated cooling, in °C; 停冷 is the stop temperature of accelerated cooling, unit: °C; V cis the accelerated cooling rate of the steel plate, in °C / s; H is the thickness of the finished steel plate, in mm.

[0121] It can be seen from the formula: With the online hardenability index DI OL Increase, cool down temperature T 开冷 Rising and cooling speed V c As the temperature increases, the content of low-temperature phase transformation structure (such as bainite, especially lower bainite) of the steel plate increases, and the strength of the steel plate increases; and as the cooling temperature increases and the thickness of the steel plate increases (the effect is that the cooling rate is low, especially the cooling rate in the center of the ultra-thick steel plate is extremely low), the content of low-temperature phase transformation structure (such as bainite, especially lower bainite) of the steel plate is lower, and the strength of the steel plate is lower; in summary, when all indicators meet the above formula, it ensures that the ultra-thick steel plate has high strength, high toughness, and high crack arrest characteristics, while the steel plate has excellent high heat input weldability, and successfully eliminates the contradiction between low C, low carbon equivalent and high strength, high toughness, high crack arrest characteristics and excellent weldability of ultra-thick steel plates, which are mutually opposed and difficult to be compatible. This is one of the key core technologies of the present invention.

[0122] The composition data in the above relationship is calculated as a percentage. For example, if the carbon content is 0.10%, just substitute 0.10 when calculating the relationship.

[0123] Beneficial effects of the present invention:

[0124] The steel plate of the present invention is designed by matching the main alloy elements, microalloy elements and inclusion elements, and combined with the new generation TMCP manufacturing process, to produce TMCP type ultra-thick crack-arresting steel plates with excellent comprehensive performance at low cost. The ultra-thick crack-arresting steel plates of the invention not only have high strength, high toughness, and high crack-arresting properties, but also have excellent weldability, especially large heat input welding, which greatly shortens the manufacturing cycle of steel structures, creates huge value for construction companies, and realizes low-carbon, green and environmental protection in the manufacturing and use of steel plates. The high performance and high added value of the steel plate are concentrated in the large thickness (≥80mm), high strength, high toughness, high crack-arresting properties and excellent weldability of the steel plate, especially the perfect match of large heat input welding, which successfully solves:

[0125] A) Between low C, low carbon equivalent (Pcm) and large thickness and high strength;

[0126] B) Low temperature toughness, high crack arrest and large thickness, low manufacturing cost;

[0127] C) The conflict between high strength, high crack arrest characteristics and excellent weldability, especially large heat input weldability, in the composition design and TMCP process design is difficult to reconcile, which greatly improves the safety, stability and durability of large heavy steel structures; good weldability (especially large heat input welding) saves the cost of steel structure manufacturing for user companies and shortens the time of steel structure manufacturing for users, which not only creates huge value for users, but more importantly reduces carbon emissions (no preheating before welding, no heat treatment after welding, ultra-large heat input and high-efficiency welding), so this type of steel plate is not only a high value-added, full-life green and environmentally friendly product. BRIEF DESCRIPTION OF THE DRAWINGS

[0128] Figure 1 This is a photo of the microstructure (1 / 4 thickness) of the steel in Example 3 of the present invention. DETAILED DESCRIPTION

[0129] The present invention will be further described below in conjunction with the embodiments and drawings.

[0130] The composition of the embodiment of the present invention is shown in Table 1, and the balance is Fe and unavoidable impurities. Tables 2 and 3 are the manufacturing process parameters of the embodiment of the present invention. Table 4 is the performance parameters of the embodiment 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 grain size of the microstructure is less than 20 μm; thereby the steel plate obtains high strength, high toughness, and high crack arrest properties, and the steel plate has excellent weldability, especially large heat input welding.

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

[0133] Infrastructure construction (such as large-scale cross-sea highway and railway bridge projects), large-scale shipbuilding (such as VLCC), marine development, high-power offshore wind power projects and major equipment manufacturing have been put on the daily agenda. The world-renowned large-capacity offshore power projects in the southeast coast of my country use ultra-thick EH40 / EH47, and European and Middle Eastern marine projects require 75-120mm ultra-thick EQ51, EQ55 and turbine seat ring plates use 100-200mm S500QL1, S550QL1, which is YP550MPa grade high crack arrest steel plate; At present, my country's basic energy construction (hydropower, offshore wind power, etc.), marine engineering development and major equipment manufacturing are in the ascendant; The key materials for basic energy construction, marine engineering development and related equipment manufacturing industries---YP550 grade ultra-thick steel plates with high toughness, high crack arrest properties and excellent weldability have broad market prospects.

[0134]

[0135]

[0136]

Claims

1. A high crack arrest, easy to weld, ultra-thick YP550 grade steel plate, the weight percentage of its components is: 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 includes Fe and other unavoidable inclusions; and must satisfy the following relationship at the same time: 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)×[(%Mn)+7.76(%Si)+10.23(%P)+21.47(%S)]≤0.216; Als / (N-0.292Ti)≥28; The low-temperature mobility index of dislocations in BCC crystal structure θ≥0.79; θ=1.25(%Ni)+[1.57(%Mn)-0.69(%Mn) 2 ]+1.69(%Cu) -3.51(%Si)-0.46[(%Cr)+(%Mo)]-2.43[(%Al)+(%Nb)+(%Ti)]; Ca treatment, Ca / S ratio is controlled between 1 and 3, and (%Ca)×(%S)≤1.5×10 -3 .

2. The high crack arrest, easy to weld, ultra-thick YP550 grade steel plate according to claim 1, characterized in that: The balance is Fe and other inevitable inclusions.

3. The high crack arrest, easy to weld ultra-thick YP550 grade steel plate according to claim 1 or 2, characterized in that: 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 less than 20 μm.

4. The high crack arrest, easy to weld ultra-thick YP550 grade steel plate as claimed in claim 1, 2 or 3, characterized in that: The yield strength of the steel plate is ≥550MPa, the tensile strength is ≥650MPa, the elongation at break δ5 is ≥18%, the Charpy impact energy (single value) of the -50°C steel plate is ≥100J, and Kca (-10°C) is ≥6000N / mm 3 / 2 , excellent weldability, no preheating required before welding, can withstand welding heat input ≥50kJ / cm, and the impact energy KV2 (single value) of the heat affected zone at -50℃ is ≥60J.

5. The method for manufacturing the high crack arrest, easy-to-weld ultra-thick YP550 grade steel plate according to claim 1, 2, 3 or 4, characterized in that: The steps include: 1) Smelting and casting Smelting and casting into slabs according to the composition of claim 1 or 2; 2) The slab is heated and the temperature is controlled at 1100-1180°C; 3) Controlled rolling The first stage is ordinary rolling, which uses the maximum rolling capacity of the rolling mill for continuous rolling; The second stage adopts non-recrystallization 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℃; Furthermore, the following relationship must also be satisfied: [(%Nb)×Au×ξ] / {H×[(%Cr)+1.46(%Mo)]×(T 终轧 )}≥4.41×10 -4 ; in, 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), unit %; ξ is the cumulative reduction rate of controlled rolling in the non-recrystallization / critical region, unit: %; H is the thickness of the finished steel plate, in mm; T 终轧 is the final rolling temperature of non-recrystallization / critical region controlled rolling, unit: °C; 4) Controlled cooling After controlled rolling, the steel plate is immediately transported to the accelerated cooling equipment, and then the steel plate is accelerated cooled. The steel plate cooling temperature is 690-730℃, the cooling rate is ≥4℃ / s, and the cooling stop temperature is 250-400℃. In addition, controlled cooling must also meet the following relations: [DI OL ×(T 开冷 )×(V c )] / [(T 停冷 )×H]≥6.26; in, DI OL is the online hardenability index, OF 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 is the starting temperature of accelerated cooling, in °C; T 停冷 is the stop temperature of accelerated cooling, in °C; V c is the accelerated cooling rate of the steel plate, in °C / s; H is the thickness of the finished steel plate, in mm.

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

7. The method for manufacturing the high crack arrest, easy to weld ultra-thick YP550 grade steel plate as claimed in claim 5, characterized in that: Step 4) After the accelerated cooling is completed, when the thickness of the steel plate is ≥80 mm, the steel plate is slowly cooled. The slow cooling process is to keep the temperature at 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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