Low-compression-ratio high-toughness extra-thick Q550-grade high-strength steel plate and manufacturing method thereof

Abstract

The invention relates to a low-compression-ratio high-toughness extra-thick Q550-grade high-strength steel plate and a manufacturing method thereof. The steel plate comprises the following chemical components in percentage by weight: 0.10 to 0.15 percent of C, 0.20 to 0.40 percent of Si, 1.10 to 1.40 percent of Mn, 0.020 to 0.040 percent of Nb, 0.025 to 0.050 percent of V, 0.015 to 0.030 percent of Ti, 0.020 to 0.050 percent of Al, 0.50 to 0.90 percent of Ni, less than or equal to 0.30 percent of Cu, 0.40 to 0.60 percent of Cr, 0.30 to 0.50 percent of Mo, 0.0010 to 0.0020 percent of B, 0.0010 to 0.0030 percent of Ca, less than or equal to 0.012 percent of P, less than or equal to 0.002 percent of S, less than or equal to 0.0020 percent of O, less than or equal The thickness of the steel plate is 160-200 mm, the yield strength is larger than or equal to 550 MPa, the tensile strength is 670-770 MPa, and the ductility is larger than or equal to 18%; charpy impact energy at the low temperature of-40 DEG C is greater than or equal to 120J And d is equal to 3a, no crack exists in 180-degree cold bending, the excellent comprehensive mechanical property is achieved, and the requirement of engineering mechanical equipment for the extra-thick high-strength steel plate is met.

Description

Technical Field

[0001] The present invention belongs to the technical field of special steel smelting, and in particular relates to a low compression ratio, high toughness, extra-thick Q550 grade high-strength steel plate and a manufacturing method thereof. Background Art

[0002] Q550-grade steel plate, characterized by high strength, excellent toughness, and superior processability, is widely used in engineering machinery, bridge construction, and other fields. In recent years, with the rise of large-scale and ultra-large projects both domestically and internationally, construction machinery and equipment have become increasingly larger, leading to a significant increase in demand for thicker, extra-thick, high-strength steel plates. Currently, the maximum thickness of Q550-grade steel, as defined by the national standard GB / T16270-2009, is 150mm, which no longer fully meets the demand for extra-thick steel plates required for key components in some specialized engineering equipment.

[0003] The main difficulties in producing Q550 grade extra-thick steel plates are: (1) Due to the thickness restrictions of the continuous casting slabs of each steel mill, the thicker the finished steel plate, the smaller the compression ratio, resulting in coarse grains and difficulty in ensuring the strength and toughness of the steel plate; (2) The thicker the steel plate, the larger the temperature gradient will be from the surface to the core of the steel plate, and the uneven cooling in the thickness direction will result in the steel plate strength decreasing closer to the core; (3) Q550 grade steel plates tend to use martensitic structure design, which has high strength and is more sensitive to hydrogen-induced delayed cracking. Because the thicker the steel plate, the longer the diffusion path of hydrogen atoms will be, making it difficult for them to escape, resulting in delayed cracking during subsequent cutting, welding, and even use of the steel plate, affecting equipment safety. How to overcome the above difficulties and develop extra-thick Q550 grade steel plates with high toughness of more than 150mm is an urgent problem that steel mills need to solve.

[0004] Chinese patent CN 116179950 A discloses a 550MPa grade extra-thick steel plate and its manufacturing method. The invention employs two-stage rolling. To meet the reduction ratio requirements for crushing the interior of the continuous casting ingot to the core cast grains and the looseness of the weld center, the thickness of the steel plate after rough rolling is required to be 1.2-1.6 times the thickness of the finished product. After rolling, the steel plate is cooled using a combination of DQ+ACC. The steel plate contains 1.10-1.70% Mn. The increase in Mn content can compensate for the decrease in strength caused by the decrease in C content. However, Mn is a segregating element, and excessive Mn content can easily lead to increased segregation in the center of the continuous casting ingot. Subsequent cutting and welding of the steel plate can easily cause cracks, especially in the extra-thick steel plate, which in turn affects the safety of the extra-thick steel plate in key components of engineering equipment. Furthermore, although this invention uses an offline quenching method for production, the steel plate thickness is 100-180mm. Compared to the conventional method, the present invention firstly reduces and controls the Mn content to a certain extent, which significantly improves the center segregation of the continuous casting slab to a certain extent. Secondly, by using a heavy reduction technology at the end of the continuous casting slab solidification, the center segregation and center porosity of the continuous casting slab are further reduced. This significantly reduces the risk of cracks caused by center segregation and porosity during subsequent cutting and welding operations of the finished steel plate. Furthermore, compared to the timely addition of a hydrogen expansion annealing step after the rolling mill rolls off the production line, the present invention significantly further releases the hydrogen content within the extra-thick steel plate, greatly reducing the risk of hydrogen-delayed cracking after subsequent cutting and welding of the extra-thick steel plate, and significantly improving its efficiency.

[0005] Chinese patents CN 114682746 A and CN 115094316 B disclose a method for producing extra-thick steel plates, and CN 115094316 B disclose an extra-thick steel plate with excellent low-temperature core impact toughness and its manufacturing method. Both inventions utilize a two-stage rolling process: the former utilizes ACC and UFC water cooling, while the latter utilizes NAC followed by tempering after water cooling. The steel plate thicknesses range from 180-200mm to 170-200mm, respectively. However, the steel grades disclosed in both patents are 300MPa, representing different steel grades.

[0006] Chinese patent CN 114182172 B discloses a method for producing extra-thick steel plates with a strength greater than 550 MPa. This invention uses an offline quenching and high-temperature tempering process, but the maximum thickness of the steel plates is 100-150 mm.

[0007] Chinese patent CN 109881086 B discloses a 300mm thick, high-strength tempered steel plate Q550EZ35 and its production method. This invention uses water-cooled steel ingots, followed by high-temperature tempering, normalizing, quenching, and tempering. The steel plate exhibits tempered bainite microstructure, yield strengths of 557 / 576 MPa, tensile strengths of 670 / 668 MPa, longitudinal impact energy at -40°C ≥190 J, Z-axis impact energy ≥60%, and a bending d of 3a, all of which are acceptable. Although the steel plate thickness is greater than that of the present invention, its disadvantages are the low yield rate of water-cooled steel ingots, typically only around 70%, compared to the yield rate of continuous casting billets, which is typically above 88%. Furthermore, the addition of high-temperature tempering and normalizing steps before the traditional quenching and tempering process results in a long and cumbersome production cycle, increasing both production costs and process costs. Under actual large-scale production conditions, production efficiency is significantly reduced, product delivery times are also affected to a certain extent, and the overall process is complex and involved. The present invention has obvious advantages over the conventional method. It eliminates high-temperature tempering, normalizing and other processes, and adopts continuous casting production. The production efficiency is much higher, the cost is obviously competitive, and the product delivery time can be guaranteed or even advanced.

[0008] In summary, current production technologies for high-toughness, extra-thick Q550-grade high-strength steel plate primarily rely on quenching and tempering or NAC+tempering. To ensure sufficient core penetration during rolling, most utilize methods such as ensuring adequate finishing deformation or performing low-temperature finishing rolling. The maximum thickness of finished plate from continuous casting slabs is 180mm. Production of extra-thick, high-toughness steel plates with low compression ratios exceeding 180mm is not addressed. Furthermore, no solution has been proposed for the hydrogen-induced delayed cracking problem in these extra-thick, high-strength steel plates. Summary of the Invention

[0009] The technical problem to be solved by the present invention is to provide a low compression ratio, high toughness, 160-200mm ultra-thick Q550 grade high-strength steel plate and its production method in response to the above-mentioned existing technology. The steel plate has excellent hardenability, ensuring that the ultra-thick steel plate is refined in grain size during large deformation rolling in a high temperature zone, thereby improving strength and toughness.

[0010] The technical solution adopted by the present invention to solve the above problems is: a low compression ratio, high toughness, extra thick Q550 grade high-strength steel plate, the chemical composition and weight percentage of which are: C: 0.10% to 0.15%, Si: 0.20% to 0.40%, Mn: 1.10% to 1.40%, Nb: 0.020% to 0.040%, V: 0.025% to 0.050%, Ti: 0.015% to 0.030%, Al: 0.020~0.050%, Ni: 0.50~0.90%, Cu: ≤0.30%, Cr: 0.40~0.60%, Mo: 0.30~0.50%, B: 0.0010~0.0020%, Ca: 0.0010~0.0030%, P: ≤0.012%, S: ≤0.002%, O: ≤0.0020%, N: ≤0.0040%, H: ≤0.00015%, Ca / S: 1.5~3.0, the balance is Fe and unavoidable impurity elements.

[0011] The thickness of the Q550 grade high-strength steel plate of the present invention is 160-200 mm.

[0012] The reasons for limiting the steel composition in the present invention are as follows: Carbon: As a solid solution element, carbon significantly improves the strength of steel plates, but it also negatively impacts their plasticity, toughness, cold bending, and weldability. Furthermore, carbon is a readily segregating element; excessive levels can exacerbate central segregation in continuous castings, further impacting the internal quality of the ingot. To ensure a balanced strength-toughness balance and cold bending performance, the carbon content in this invention is controlled to 0.10-0.15%.

[0013] Si: It plays a role in solid solution strengthening in the steel. However, excessive Si content will deteriorate the toughness of martensitic high-strength steel and increase the cold crack sensitivity of the steel plate, especially for very thick steel plates. In the present invention, the Si content is controlled between 0.20% and 0.40%.

[0014] Mn: In the steel, it improves hardenability, promotes martensitic transformation, and increases strength. However, Mn is a major segregating element. Excessive Mn content can cause centerline segregation of MnS in the continuous casting slab, adversely affecting the toughness, lamellar tearing resistance, and weldability of the steel plate. The present invention specifies that the Mn addition amount be within the range of 1.10% to 1.40%.

[0015] Nb: It significantly refines grains, strengthens by precipitation, and strengthens the austenite. Nb dissolved in austenite improves hardenability, while its carbide precipitation refines grains and increases matrix strength. In this invention, the Nb content is controlled to 0.020-0.050%.

[0016] V: A grain-refining element; it provides some precipitation strengthening during tempering. Excessive V content can deteriorate the toughness and weldability of the steel plate. Therefore, the V content in this invention is controlled to 0.025-0.050%.

[0017] Ti: Ti(C / N) has a high dissolution temperature, typically above 1400°C. At high temperatures, it has a strong pinning effect on grain boundaries, preventing the coarsening of austenite grains during heating. The present invention requires high-temperature rolling, with a Ti content controlled to 0.015-0.030%.

[0018] Al: a deoxidizing and grain-refining element. The present invention stipulates that the Al content is 0.02-0.05%.

[0019] Cr: An element that improves the hardenability of steel, inhibits the formation of polygonal ferrite and pearlite, promotes the transformation of low-temperature microstructures such as bainite or martensite, and increases the strength of the steel. However, excessive Cr content can affect the toughness of the steel and reduce the weldability of the steel plate. In the present invention, the Cr content is controlled within a range of 0.40% to 0.60%.

[0020] Mo: An element that improves the hardenability of steel, promoting the formation of bainite or martensite during cooling. Adding a certain amount of Mo to a steel grade increases the strength of the steel without affecting its cryogenic impact properties. Mo also increases the steel's tempering resistance, maintaining strength at higher temperatures. In the present invention, the Mo content is controlled between 0.30% and 0.50%.

[0021] Ca: An inclusion-modifying element, it reacts with elongated MnS to form spherical CaS, changing the anisotropy of the steel plate. It also modifies Al2O3 inclusions produced by Al deoxidation into spherical, low-melting-point inclusions, promoting their removal and improving the steel plate's plasticity and toughness. In this invention, the Ca content is controlled to 0.0010-0.0030%.

[0022] P: harmful element in steel, easy to segregate, and has an adverse effect on the plasticity and toughness of the material. A high P content increases the brittleness of the steel plate. The present invention stipulates that P: ≤ 0.012% S: A harmful element in steel grades, prone to segregation. Mn easily forms MnS segregation inclusions, leading to delamination and cracking of the steel plate. The present invention stipulates that S: ≤ 0.002%.

[0023] O, N, and H: Harmful gas elements. High content of these elements results in a high number of inclusions, reduces the plasticity and toughness of the steel plate, and increases the risk of cracking. The present invention strictly controls the O content to ≤ 0.0020%; the N content to ≤ 0.00040%; and the H content to ≤ 0.00020%.

[0024] CEV: This paper uses the carbon equivalent formula CEV = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15. The carbon equivalent significantly affects the strength and weldability of steel. A high CEV increases strength but reduces weldability. To ensure the hardenability of the steel plate, this paper controls the CEV to 0.58% ≤ CEV ≤ 0.68%.

[0025] The present invention further provides the above-mentioned low compression ratio, high toughness, 160-200mm thick Q550 grade high-strength steel plate and its production method, the specific process is as follows: Smelting process: The molten iron is pretreated and smelted in an electric furnace or converter. It is then sent to the LF refining furnace for refining and subjected to VD or RH vacuum treatment. After degassing, the molten steel is treated with trace amounts of Ca and allowed to stand for 15 to 30 minutes while soft-blowing with argon.

[0026] Continuous casting process: A 450mm billet continuous casting machine is used for low superheat and full argon protection casting, and the superheat of the molten steel is controlled at 10-25°C. Due to the low compression ratio of the finished steel plate, the light reduction process must be optimized during the continuous casting stage to ensure the quality of the core of the continuous casting billet. The present invention adopts a heavy reduction process at the end of solidification of the billet to fully bridge the defects such as center segregation, center pores, and looseness of the billet. The present invention controls the amount of heavy reduction to be between 13 and 19 mm; the center segregation of the billet is not higher than C1.0 level, and the center looseness is not higher than 1.0 level. See the low-magnification picture of the slab. Figure 2 .

[0027] Hot Rolling Process: The continuous casting slab enters a walking-beam furnace and is heated to 1220-1270°C for 8-13 min / cm2. This allows the alloying elements in the steel to fully dissolve, ensuring uniform composition and properties of the final product. After exiting the furnace, the slab undergoes high-pressure water descaling and then undergoes a two-stage controlled rolling process: rough rolling and finishing rolling. To increase deformation in the core of the steel plate, rough rolling is performed in a high-temperature zone, with the final rough rolling temperature ≥1050°C. The ratio of the final plate thickness after rough rolling to the finished plate thickness is controlled to be less than 1.2 times, and the number of passes with a reduction of 30-35 mm is ≥3. The starting temperature for finishing rolling is 860-900°C, and the final rolling temperature is ≥780°C. To prevent grain growth and minimize excessive stress, the steel plate is cooled using ACC after rolling, with the final cooling temperature at 600-680°C.

[0028] Steel plate hydrogen expansion process: After rolling off the production line, the steel plate is loaded into an electric furnace with a furnace temperature ≥300°C and a heating rate of 30-50°C / h. The steel plate is heated to 550-650°C and kept in the furnace for ≥72 hours. This promotes the homogenization of the core structure and properties of the steel plate, reduces the H content and internal stress inside the steel plate, greatly improves the toughness of the steel plate, and reduces the risk of hydrogen-induced delayed cracking in high-toughness, extra-thick, high-strength steel plates.

[0029] Heat Treatment: After hydrogen expansion in an electric furnace, the steel plate is quenched at a temperature of 890-930°C. The heat transfer rate in the furnace is controlled at 1.8-2.2 min / mm. To ensure uniform heating of the entire steel plate, the temperature is controlled to an accuracy of ±10°C. The steel plate is tempered at a temperature of 600-650°C. Once the furnace reaches the desired temperature, the heat transfer rate in the furnace is controlled at 3.0-4.0 min / mm.

[0030] Compared with the prior art, the advantages of the present invention are: The invention provides a high-toughness, ultra-thick Q550-grade high-strength steel plate with a compression ratio of ≤2.8, a thickness of 160-200 mm, and a manufacturing method thereof. The steel plate has excellent hardenability.

[0031] The present invention uses a 450mm continuous casting machine for low superheat and argon protection throughout the casting process. Different from the traditional dynamic light reduction process (reduction of 5-10mm), the present invention uses a heavy reduction process at the end of solidification of the continuous casting billet, with a heavy reduction of between 13-19mm, which fully bridges the defects of the continuous casting billet such as center segregation, core porosity, and looseness. The center segregation of the continuous casting billet will not exceed C1.0 level, and the center looseness will not exceed 1.0 level. See the low-magnification picture of the slab. Figure 2 .

[0032] The present invention increases the core deformation of the steel plate by rough rolling in the high temperature zone, and the final rolling temperature is ≥1050°C. The present invention fully utilizes the Nb / V / Ti microalloying effect to ensure grain refinement and improve strength and toughness of the extra-thick steel plate during large deformation rolling in the high temperature zone.

[0033] Unlike conventional two-stage rolling to ensure sufficient reduction in the finishing stage, this method achieves the primary deformation during the roughing stage. The ratio of the steel plate thickness after roughing to the finished product thickness is controlled to be less than 1.2 times, and the number of passes with a reduction of 30-35 mm is ≥3.

[0034] In order to solve the problem of hydrogen-induced delayed cracking of high-toughness, extra-thick, high-strength steel plates during subsequent flame cutting, welding and application, the present invention adopts a steel plate hydrogen expansion process with a hydrogen expansion temperature of 550-650°C and a furnace time of ≥72 hours.

[0035] The present invention's 160-200mm thick, high-toughness, extra-thick Q550-grade steel plate boasts a yield strength Re ≥ 550 MPa, a tensile strength Rm 670-770 MPa, an elongation A ≥ 18%, and a -40°C low-temperature impact strength Akv ≥ 120 J. The steel plate exhibits excellent cold bending properties, with a d=3a and no cracking after a 180° cold bend. The steel plate's microstructure primarily consists of fine lower bainite and tempered bainite, with tempered bainite accounting for ≥ 60%, meeting the application requirements of Q550-grade high-strength steel plate for engineering machinery. The method of the present invention can be extended and applied to other high-strength steel plates, such as steel for engineering machinery, steel for high-rise buildings, steel for bridges, high-strength steel for offshore shipbuilding, steel for pressure vessels, etc. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 (X500) is a typical microstructure image of the test steel of Example 1 of the present invention. The steel plate matrix structure is fine lower bainite and tempered bainite, of which the tempered bainite accounts for ≥60%, and the microstructure grain size is level 7.

[0037] Figure 2 This is a low-magnification image of a 450mm continuous casting slab using the present invention's heavy reduction process (reduction 13-19mm) at the end of continuous casting solidification. The slab exhibits a central segregation of C 0.5 and a central porosity of 0.5. In contrast, the slab using conventional dynamic soft reduction (reduction 5-10mm) exhibits severe central segregation of B 1.5 and a central porosity of 1.0. Heavy reduction at the end of continuous casting significantly improves central segregation in the slab compared to conventional soft reduction, thereby effectively preventing cracks caused by severe central segregation during the cutting and welding of extra-thick steel plates. DETAILED DESCRIPTION

[0038] The technical solution of the present invention will be described in more detail with reference to the preferred embodiments of the present invention. However, these embodiments are merely descriptions of preferred implementations of the present invention and do not limit the scope of the present invention in any way.

[0039] The production process of the Q550 grade low compression ratio high toughness extra thick high strength steel of the present invention is: smelting->refining outside the furnace->vacuum degassing->Ca treatment->continuous casting->heating->rolling cooling->steel plate hydrogen expansion->quenching and tempering.

[0040] The method for producing the low compression ratio, high toughness, and extra-thick Q550 grade high-strength steel plate of Examples 1-2 of the present invention comprises the following steps: (1) Smelting: The steel is smelted in a converter, then sent to an LF furnace for refining and RH vacuum degassing. The steel is then broken through for trace Ca treatment and the steel is kept still for 15 to 30 minutes with soft argon blowing. The final composition of the steel is shown in Table 1.

[0041] (2) Continuous casting: Molten steel is cast into 450mm thick continuous casting billets. The casting temperature is controlled at 10-25℃ above the liquidus line and argon protection is used throughout the casting process. During the casting process, heavy pressure is applied at the end of the solidification of the continuous casting billet, and the amount of heavy pressure reduction is between 13 and 19mm. The center segregation of the billet is controlled not to exceed C1.0 level, and the center porosity is controlled not to exceed 1.0 level. The relevant process parameters are shown in Table 2, and the low-magnification pictures of the slab are shown in Figure 2 .

[0042] (3) Rolling and cooling: The continuous casting billet obtained in step (1) is loaded into a step-beam heating furnace and heated to 1220-1270°C for 8-13 min / cm, so that the alloy elements in the steel are fully dissolved to ensure the uniformity of the composition and performance of the final product. After the billet is removed from the furnace, it is descaled with high-pressure water and then subjected to two-stage controlled rolling of rough rolling + finishing rolling. In order to increase the rolling deformation of the core of the steel plate, the rough rolling is carried out in the high temperature zone, and the rough rolling final rolling temperature is ≥1050°C. The ratio of the thickness of the steel plate after rough rolling to the thickness of the finished product is controlled to be less than 1.2 times, and the number of passes with a reduction of 30-35 mm is ≥3. The starting rolling temperature of the steel plate finishing rolling is 860-900°C, and the final rolling temperature is ≥780°C. In order to avoid grain growth and not generate large stress, the steel plate is cooled by ACC control after rolling, and the final cooling temperature is 600-680°C. The relevant process parameters are shown in Table 3.

[0043] (4) Steel plate hydrogen expansion process: After rolling off the production line, the steel plate is loaded into an electric furnace with a furnace temperature of ≥300°C and a heating rate of 30-50°C / h. The steel plate is heated to 550-650°C and kept in the furnace for ≥72 hours to homogenize the core structure and properties of the steel plate, reduce the hydrogen content and internal stress in the steel plate, greatly improve the toughness of the steel plate, and reduce the risk of hydrogen-induced delayed cracking in high-toughness, extra-thick, high-strength steel plates. The relevant process parameters are shown in Table 4.

[0044] (5) Heat treatment process: After the steel plate is hydrogenated in an electric furnace, it is quenched at a temperature of 890-930°C. The heat transfer rate in the furnace is controlled at 1.8-2.2 min / mm. To ensure uniform heating of the steel plate as a whole, the temperature control accuracy is ±10°C. The steel plate is tempered at a temperature of 600-650°C. After the furnace temperature reaches the desired temperature, the heat transfer rate in the furnace is controlled at 3.0-4.0 min / mm. The relevant process parameters are shown in Table 4.

[0045] The specific composition and process parameters are shown in Tables 1 to 5. The corresponding properties of the steel plates of each example are shown in Table 5.

[0046] Figure 1 (X500) shows the microstructure of the test steel in Example 1. The steel plate matrix structure is composed of fine lower bainite and tempered bainite, of which the tempered bainite accounts for ≥ 60%. The grain size is ≥ 7, ensuring the steel plate has excellent strength and toughness.

[0047] Figure 2This is a low-magnification image of a 450mm continuous casting slab produced using the present invention's heavy reduction process (reduction 13-19mm) at the end of continuous casting solidification. The slab exhibits a central segregation of C 0.5 and a central porosity of 0.5. In contrast, the slab produced using conventional dynamic light reduction (reduction 5-10mm) exhibits severe central segregation of B 1.5 and a central porosity of 1.0. Heavy reduction at the end of continuous casting significantly improves central segregation compared to conventional light reduction, thereby reducing the likelihood of cracks in the center of extra-thick steel plates during cutting and welding, and improving the microstructure and properties at half the thickness of the plate.

[0048] The present invention adopts a high-cleanliness steelmaking and continuous casting process, controlled rolling and cooling, steel plate hydrogen expansion and tempering process, and controls from the perspectives of chemical composition design, parent material structure, rolling deformation, heat treatment temperature and time, etc., to ensure that the steel plate has high strength, high toughness and excellent cold bending performance. It can be widely used in the manufacture of large-scale engineering machinery and equipment or steel structures such as offshore platforms, high-rise buildings, bridges, etc. that have strict requirements on formability and weldability.

[0049] Table 1 Chemical composition of the super-strong steel plate of the embodiment (wt%) Table 2 Continuous casting process control Table 3 Rolling process control

[0050] Table 4 Heat treatment process control Table 5 Tensile, impact and bending properties of the embodiments of the present invention

[0051] Although the preferred embodiments of the present invention have been described in detail above, it should be clearly understood that various modifications and variations are possible for those skilled in the art. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention are intended to be included within the scope of protection of the present invention.

Claims

1. A Q550 grade high-strength steel plate with low compression ratio, high toughness and extra thickness, characterized by: The chemical composition and weight percentage of the steel plate are: C: 0.15-0.30%, Si: 0.15-0.35%, Mn: 1.20-1.50%, P: ≤0.010%, S: ≤0.001%, Cr: 1.20-1.80%, Mo: 0.25-0.65%, Ni: 0.9-1.4%, Al: 0.02-0.10%, V: 0.1-0.25%, N: ≤0.007%, B: ≤0.002%, Cu: ≤0.15%, and the balance is iron and unavoidable impurity elements.

2. The low compression ratio, high toughness, extra-thick Q550 grade high-strength steel plate according to claim 1, characterized in that: The carbon equivalent of the steel plate CEV=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15, and is controlled to be 0.58%≤CEV≤0.68%.

3. The low compression ratio, high toughness, extra thick Q550 grade high strength steel plate according to claim 1, characterized in that: The thickness of the steel plate is 160-200 mm.

4. The low compression ratio, high toughness, extra thick Q550 grade high strength steel plate according to claim 1, characterized in that: The steel plate has a yield strength Re≥550MPa, a tensile strength Rm 670-770MPa, an elongation A≥18%; a -40°C low-temperature impact Akv≥120J, and has excellent cold bending performance, with no cracks after 180° cold bending.

5. The low compression ratio, high toughness, extra thick Q550 grade high strength steel plate according to claim 1, characterized in that: The structure of the steel plate is mainly composed of fine lower bainite and tempered bainite, wherein the tempered bainite accounts for ≥60%.

6. A method for manufacturing the low compression ratio, high toughness, extra thick Q550 grade high strength steel plate according to claim 1, characterized in that: The method mainly includes: 1) Smelting: Pre-treat the molten iron, smelt it in an electric furnace or converter, then send it to the LF refining furnace for refining and undergo VD or RH vacuum treatment; 2) Continuous casting: A 450mm billet continuous casting machine is used for low superheat and full argon protection casting, and the superheat of the molten steel is controlled at 10-25°C; 3) Heating and rolling: The continuous casting billet enters the walking beam heating furnace and is heated to 1220-1270℃ for 8-13min / cm. After the billet is removed from the furnace, it is descaled by high-pressure water and then subjected to two-stage controlled rolling of rough rolling and finishing rolling. After rolling, the steel plate is cooled by ACC control, and the final cooling temperature is 600-680℃. 4) Steel plate hydrogen expansion: After rolling off the production line, the steel plate is loaded into the electric furnace with a furnace temperature of ≥300°C, a heating rate of 30-50°C / h, and the steel plate is heated to 550-650°C with a furnace time of ≥72 hours; 5) Heat treatment: After the steel plate is hydrogenated in an electric furnace, it is quenched at a temperature of 890-930°C. The heat transmittance in the furnace is controlled at 1.8-2.2 min / mm. The steel plate is tempered at a temperature of 600-650°C.

7. The method for manufacturing a Q550 grade high-strength steel plate with low compression ratio, high toughness and extra thickness according to claim 6, characterized in that: In step 1), after the molten steel is degassed, a trace amount of Ca is treated and the molten steel is allowed to stand for 15 to 30 minutes while being soft-blown with argon.

8. The method for manufacturing a Q550 grade high-strength steel plate with low compression ratio, high toughness and extra thickness according to claim 6, characterized in that: In step 2), a heavy reduction process is adopted at the end of solidification of the ingot, and the heavy reduction is controlled between 13 and 19 mm; the center segregation of the ingot is not higher than C1.0 level, and the center porosity is not higher than 1.0 level.

9. The method for manufacturing a Q550 grade high-strength steel plate with low compression ratio, high toughness and extra thickness according to claim 6, characterized in that: Step 3) The medium and rough rolling is carried out in a high temperature zone, the rough rolling finishing temperature is ≥1050°C, the ratio of the steel plate thickness after rough rolling to the finished product thickness is controlled to be less than 1.2 times, and the number of passes with a reduction of 30-35 mm is ≥3, the steel plate finishing rolling starting temperature is 860-900°C, and the finishing temperature is ≥780°C.

10. The method for manufacturing a Q550 grade high-strength steel plate with low compression ratio, high toughness and extra thickness according to claim 6, characterized in that: Step 5) After the tempering furnace reaches the desired temperature, the heat transmittance in the furnace is controlled at 3.0 to 4.0 min / mm.

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