FH460 ultra-high strength steel plate with excellent low-temperature fracture toughness and elongation and its manufacturing method

Abstract

This invention relates to an FH460 ultra-high strength steel plate with excellent low-temperature fracture toughness and elongation, and its manufacturing method. The chemical composition of the steel plate is: C: 0.05%~0.14%; Si: 0.06%~0.20%; Mn: 1.40%~1.95%; S≤0.002%; P≤0.008%; Als: 0.015%~0.045%; N: 0.003%~0.015%; Nb: 0.03%~0.06%; V: 0.03%~0.06%; Cu: 0.20%~0.40%; Cr: 0.15%~0.40%; Ni: 0.50%~1.00%; Mo: 0.15%~0.30%; Ti: 0.008%~0.018%; with the balance being Fe and impurities. Steel plates have good uniformity of microstructure, tear resistance, and resistance to low-temperature fracture across their full thickness cross-section.

Description

Technical Field

[0001] This invention relates to the field of shipbuilding steel preparation technology, and in particular to a TMCP-state FH460 ultra-high strength and thickness steel plate with excellent low-temperature fracture toughness and low-temperature elongation, and its manufacturing method. Background Technology

[0002] Due to the vast resource reserves, the development and utilization of the Arctic and deep-sea areas has become an inevitable trend for future development. Simultaneously, the opening of Arctic shipping routes has made resource extraction and transportation even more crucial, thus promoting the rapid development of high-end equipment such as polar vessels and drilling platforms. Preliminary statistics indicate that the number of polar vessels and related platforms being built is increasing at a rate of 4% annually. The construction of polar vessels relies heavily on key materials such as cryogenic steel capable of withstanding the harsh polar operating environment. High-strength, high-performance steel with good low-temperature toughness and easy weldability is fundamental to the safe navigation of polar vessels, requiring steel plates to possess certain strength, as well as excellent low-temperature fracture toughness, low-temperature elongation, and processing performance. Correspondingly, the demand for cryogenic steel for extremely cold environments is further increasing, with the annual demand for steel for polar vessels expected to reach 400,000 tons, and the annual demand for steel for polar platforms reaching 150,000 tons. Currently, these products are produced using the TMCP process both domestically and internationally. To ensure the required low-temperature performance, strict control over the heating temperature of the billet, the rolling temperature, and the rapid cooling process after rolling is necessary, making production extremely challenging.

[0003] Chinese invention patent application number CN202211393065.8 discloses "An economical method for controlling the cooling uniformity of steel plates for 460MPa-level engineering structures," with the following chemical composition and mass percentages: C: 0.13%–0.16%, Si: 0.25%–0.35%, Mn: 1.55%–1.65%, P≤0.025%, S≤0.02%, Nb: 0.01%–0.025%, Ti: 0.015%–0.025%. 0.02%, Al: 0.015%~0.045%, N: 0.003%~0.006%, balance is Fe and unavoidable impurities, O≤0.0050%, and the total amount of other impurity elements is less than 0.05%. It is used to improve the cooling uniformity of 460MPa grade ultra-long steel plates with a thickness of 620mm and a length of 4255m after rolling. However, the present invention is aimed at improving the low temperature toughness, fracture toughness and other properties of the product. Therefore, the two are fundamentally different.

[0004] Chinese invention patent application number CN202211270942.2 discloses a "corrosion-resistant 460MPa grade steel plate and its production method". The chemical composition of the steel plate, by mass percentage, includes: C: 0.06-0.09%, Si: 0.2-0.3%, Mn: 0.9-1.0%, Cr: 0.5-0.6%, Ni: 0.30-0.40%, Cu: 0.40-0.45%, Mo: 0.05-0.10%, Nb: 0.02-0.04%, Al... The composition of the metals is 0.02-0.04%, P: 0.015-0.025%, rare earth elements: 0.015-0.025%, and the remainder is Fe and unavoidable impurities. Some impurity elements include, by mass percentage: S≤0.002%, O≤0.002%, N≤0.004%. It produces thick steel plates through the TMCP process, and the product has corrosion resistance. However, the evaluation temperature of its product for low-temperature toughness is only -50℃, and there is no low-temperature CTOD fracture toughness evaluation, which is fundamentally different from the product of this invention.

[0005] Chinese invention patent application CN202211052107.1 discloses "a hot-rolled angle steel with a yield strength of 460MPa and a resistance to -20℃ and its production method." The composition of this hot-rolled angle steel includes: C: 0.05%~0.11%, Si: 0.25%~0.55%, Mn: 1.20%~1.65%, P: ≤0.020%, S: ≤0.015%, V: 0.020%~0.080%, Nb: 0.020%~ 0.080%, Cr: 0.015%~0.050%, Als: 0.010%~0.035%, H≤0.0002%, N≤0.020%, the remainder being Fe and unavoidable impurities. It belongs to the category of structural steel products, and the evaluation temperature for low-temperature toughness is only -20℃. At the same time, the product does not have excellent low-temperature CTOD characteristics and good low-temperature elongation, and cannot fully meet the actual needs of marine equipment for high serviceability steel. It is fundamentally different from the present invention.

[0006] Chinese patent application CN202210804234.6 discloses a "460MPa grade extra-thick steel plate for service in extremely cold regions and its preparation method." The chemical composition of the steel plate, by mass percentage, is as follows: C: 0.08%–0.10%, Si: 0.20%–0.40%, Mn: 1.50%–1.65%, P≤0.013%, S≤0.005%, Nb: 0.030%–0.040%, Ti: 0.010%– The composition is as follows: 0.020% V: 0.030%–0.045%, Cr: 0.15%–0.25%, Ni: 0.20%–0.30%, Al: 0.020%–0.050%, with the remainder being Fe and unavoidable impurities. CEV ≤ 0.43% and Pcm ≤ 0.22%. The resulting extra-thick 460MPa grade steel plate possesses advantages such as high toughness, low-temperature resistance, and ease of welding. The thickness of the finished steel plate is 80–100 mm. However, its low-temperature toughness evaluation temperature is only -40℃, and it lacks good low-temperature elongation and low-temperature fracture characteristics, which is fundamentally different from the present invention. Summary of the Invention

[0007] This invention provides an FH460 ultra-high strength steel plate with excellent low-temperature fracture toughness and elongation, and a manufacturing method thereof. By combining reasonable composition design with TMCP multi-stage rolling process, the obtained steel plate has good uniformity of microstructure, tear resistance and low-temperature fracture resistance in the full thickness section, which can meet the requirements of marine steel plates for harsh and demanding marine environments.

[0008] To achieve the above objectives, the present invention employs the following technical solution:

[0009] An FH460 ultra-high strength steel plate with excellent low-temperature fracture toughness and elongation is disclosed. The chemical composition of the steel plate, by mass percentage, is as follows: C: 0.05%–0.14%; Si: 0.06%–0.20%; Mn: 1.40%–1.95%; S≤0.002%; P≤0.008%; Als: 0.015%–0.045%; N: 0.003%–0.015%; Nb: 0.03%–0.06%; V: 0.03%–0.06%; Cu: 0.20%–0.40%; Cr: 0.15%–0.40%; Ni: 0.50%–1.00%; Mo: 0.15%–0.30%; Ti: 0.008%–0.018%; with the balance being Fe and unavoidable impurities.

[0010] Furthermore, the microstructure of the finished steel plate is polygonal ferrite + bainite, wherein the volume percentage of polygonal ferrite is 35% to 50%.

[0011] Furthermore, the finished steel plate has a yield strength ≥460MPa, tensile strength ≥570MPa, impact energy at -80℃ ≥180J, Z-direction properties ≥60%; low temperature elongation at -20℃ ≥18%; and a low temperature fracture CTOD characteristic value at -30℃~-70℃ >0.2mm.

[0012] Furthermore, the thickness of the finished steel plate is 60–100 mm.

[0013] A method for manufacturing FH460 ultra-high strength steel plate with excellent low-temperature fracture toughness and elongation, the process includes steel smelting and casting, billet heating, multi-stage controlled rolling, controlled cooling, and slow cooling in a stack; wherein the following processes are controlled:

[0014] 1) Heating of steel billets;

[0015] Heating temperature 1130~1180℃, heat spread temperature 1100~1160℃, heat spread time 60~90min;

[0016] 2) Multi-stage controlled rolling;

[0017] At least three stages of controlled rolling are employed; the initial rolling temperature of the first stage is 1080–1120℃, with a single-pass deformation rate ≥16%; the initial rolling temperature of the second stage is 860–940℃, with a single-pass deformation rate ≥12%; the intermediate billet thickness is 1.5–2.0 times the finished steel plate thickness; the initial rolling temperature of the third stage is 780–840℃, with a single-pass deformation rate ≥10%; and the final rolling temperature is 750–820℃.

[0018] 3) Control cooling;

[0019] The average cooling rate is 15-25℃ / s, the starting cooling temperature is 670-730℃, and the final cooling temperature is 380-450℃.

[0020] 4) Stacking and slow cooling: The steel plates after controlled cooling are stacked and slow cooled for a time of ≥36 hours to obtain finished steel plates.

[0021] Furthermore, during the steel smelting and casting process, the processing time for both LF refining and RH refining is 25-50 minutes, and the superheat of the molten steel in the tundish is ≤25℃; the continuously cast slab undergoes slow cooling treatment by stacking, with a processing time ≥24 hours.

[0022] Furthermore, during the steel smelting and casting process, the molten steel is protected during the entire casting process, and the continuous casting billet forming process adopts light pressure and electromagnetic stirring.

[0023] Furthermore, during the steel smelting and casting process, the inclusions in the refined steel meet the following requirements: Class A inclusions ≤ 0.5 grade, Class B inclusions ≤ 1.0 grade, Class C inclusions ≤ 1.0 grade, and Class D inclusions ≤ 0.5 grade.

[0024] Furthermore, in step 3), the cooling is controlled using the ACC laminar flow fully automatic cooling mode, with the steel plate's head and tail shielded throughout the process.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] The beneficial effects of this invention are:

[0027] 1) The chemical composition is designed reasonably. By adding appropriate amounts of alloys and using low P and S pure steel for smelting, the purity of the steel is improved, ensuring that FH460 ultra-high strength steel plate has excellent low-temperature fracture performance and excellent low-temperature elongation.

[0028] 2) By adopting a low-temperature heating process and rationally designing the homogenization temperature and the holding time during the homogenization temperature stage, the grain size of the original austenite structure of the steel plate is refined, which ensures the smooth rolling of the steel plate and the uniformity of the structure, and provides an organizational basis for improving the strength and toughness of the steel plate.

[0029] 3) By combining multi-stage controlled rolling and water cooling (TMCP) processes, the microstructure of FH460 ultra-high strength and thickness steel plate is strengthened and toughened, the phase transformation microstructure is made more uniform and refined, the morphology and grain size are controlled, and the dimensional accuracy and surface quality are controlled to a higher level.

[0030] 4) The TMCP-state FH460 marine steel plate with a maximum thickness of 100mm has high strength (yield strength ≥460MPa, tensile strength ≥570MPa), low temperature resistance (impact energy ≥180J at -80℃), Z-direction performance ≥60%, low temperature elongation ≥18% at -20℃, and characteristic indexes of low temperature fracture (-30℃, -50℃, -70℃) CTOD (crack tip opening displacement) characteristic value of the base material and heat-affected zone >0.2mm. Its good microstructure uniformity, fracture resistance, low temperature elongation and other characteristics can meet the technical requirements of marine steel plates for harsh marine environments. Attached Figure Description

[0031] Figure 1 This is a metallographic photograph of the finished steel plate prepared in Example 1 of the present invention. Detailed Implementation

[0032] The present invention discloses an FH460 ultra-high strength steel plate with excellent low-temperature fracture toughness and elongation. The chemical composition of the steel plate, by mass percentage, is as follows: C: 0.05%–0.14%; Si: 0.06%–0.20%; Mn: 1.40%–1.95%; S≤0.002%; P≤0.008%; Als: 0.015%–0.045%; N: 0.003%–0.015%; Nb: 0.03%–0.06%; V: 0.03%–0.06%; Cu: 0.20%–0.40%; Cr: 0.15%–0.40%; Ni: 0.50%–1.00%; Mo: 0.15%–0.30%; Ti: 0.008%–0.018%; with the balance being Fe and unavoidable impurities.

[0033] The microstructure of the finished steel plate is polygonal ferrite + bainite, with the volume percentage of polygonal ferrite being 35% to 50%.

[0034] The finished steel plate has a yield strength ≥460MPa, tensile strength ≥570MPa, impact energy at -80℃ ≥180J, Z-direction properties ≥60%, low temperature elongation at -20℃ ≥18%, and low temperature fracture CTOD characteristic value at -30℃~-70℃ >0.2mm.

[0035] The thickness of the finished steel plate is 60-100mm.

[0036] The present invention discloses a method for manufacturing FH460 ultra-high strength steel plate with excellent low-temperature fracture toughness and elongation. The process includes steel smelting and casting, billet heating, multi-stage controlled rolling, controlled cooling, and slow cooling in a stack; wherein the controlled processes are as follows:

[0037] 1) Steel smelting and casting;

[0038] The processing time for both LF refining and RH refining is 25–50 min, with the superheat of the molten steel in the ladle ≤25℃. The continuously cast slabs undergo slow cooling in a stacked manner for ≥24 h. The molten steel is poured under full protective conditions, and the continuous casting slab forming process employs light reduction and electromagnetic stirring. The inclusions in the refined steel meet the following requirements: Class A inclusions ≤0.5 grade, Class B inclusions ≤1.0 grade, Class C inclusions ≤1.0 grade, and Class D inclusions ≤0.5 grade.

[0039] 2) Heating the steel billet;

[0040] Heating temperature 1130~1180℃, heat spread temperature 1100~1160℃, heat spread time 60~90min;

[0041] 3) Multi-stage controlled rolling;

[0042] At least three stages of controlled rolling are employed; the initial rolling temperature of the first stage is 1080–1120℃, with a single-pass deformation rate ≥16%; the initial rolling temperature of the second stage is 860–940℃, with a single-pass deformation rate ≥12%; the intermediate billet thickness is 1.5–2.0 times the finished steel plate thickness; the initial rolling temperature of the third stage is 780–840℃, with a single-pass deformation rate ≥10%; and the final rolling temperature is 750–820℃.

[0043] 4) Control cooling;

[0044] The average cooling rate is 15-25℃ / s, the starting cooling temperature is 670-730℃, and the final cooling temperature is 380-450℃. The cooling is controlled by an ACC laminar flow fully automatic cooling mode, with the steel plate completely shielded at both ends.

[0045] 5) Stacking and slow cooling: The steel plates after controlled cooling are stacked and slow cooled for ≥36 hours to obtain finished steel plates.

[0046] The mechanism of action of each alloy component in the FH460 ultra-high strength steel plate with excellent low-temperature fracture toughness and elongation of this invention is as follows:

[0047] C is an inexpensive element that effectively improves the strength of steel plates. However, as the carbon content increases, the plasticity, low-temperature toughness and resistance to welding cracks of the steel plate will be significantly reduced. An appropriate amount of carbon can be used in conjunction with cooling control to ensure the strength and toughness of the steel plate. From the perspective of improving the low-temperature fracture performance of the product, this invention controls the C content to 0.05% to 0.14%.

[0048] Si (Si) is a major deoxidizing component in steelmaking. It acts as both a deoxidizer and a reducing agent, contributing to increased steel plate strength. However, when its content exceeds 0.5%, it promotes the formation of Mao islands, impairing weldability and low-temperature toughness. Appropriate Si addition can improve the elastic limit and yield strength of steel, as well as its oxidation resistance at high temperatures. For marine steel plates, a lower Si content can improve the surface quality. This invention aims to achieve excellent low-temperature fracture resistance in the product; therefore, the Si content should not be too high. This invention controls the Si content to be between 0.06% and 0.20%.

[0049] Manganese (Mn) is an essential element for ensuring the strength and toughness of steel. Mn combines with sulfur (S) to form MnS, which prevents hot cracking caused by FeS formation at grain boundaries. Mn is also a good deoxidizer. Appropriate amounts of manganese can improve the strength and toughness of steel, but excessive content can lead to segregation in the cast billet, resulting in banded structures that are difficult to eliminate after rolling, reducing the transverse properties and resistance to lamellar tearing of the steel plate. To improve the strength-toughness balance of the product, this invention adds Mn along with nickel, which enhances the product's low-temperature fracture and low-temperature elongation properties. Therefore, this invention controls the Mn content to be 1.40%–1.95%.

[0050] P: It is an element that has an adverse effect on the impact value and can impair low-temperature toughness by segregation in the center of the slab and agglomeration at grain boundaries. In this invention, the P content is controlled to be no higher than 0.008%.

[0051] S: It is an element that has an adverse effect on the impact value and can form sulfide inclusions that become crack initiation sites. This invention controls the S content to be no higher than 0.002%.

[0052] Als: As a necessary deoxidizing and grain-refining element added in this invention, its content is above 0.01%, but when the content exceeds 0.08%, it is easy to cause hot cracking of the billet, and the toughness of the steel will decrease. Therefore, this invention controls its content to be 0.015% to 0.045%.

[0053] Nb: Adding Nb to steel can effectively refine the grain size and improve its strength and toughness. However, the effect is not obvious when the addition amount is less than 0.01%, therefore, the Nb content in this invention is controlled at 0.03-0.06%.

[0054] Vanadium (VC) exhibits a significant precipitation strengthening effect. Fine, dispersed VC particles can precipitate from the martensitic or ferrite matrix, thus significantly strengthening the steel. However, when the content is too high, the precipitation strengthening effect is not significantly improved, and the cost is also high. Therefore, this invention controls the vanadium content in the steel to be 0.03%–0.06%.

[0055] Mo is a key element in expanding the γ-phase region, delaying the formation of ferrite that precipitates first during the γ→α phase transformation, and promoting the formation of acicular ferrite. It plays a crucial role in controlling the phase transformation microstructure, effectively improving material strength, lowering the phase transformation temperature, and reducing the critical cooling rate for bainite transformation. This facilitates the promotion of bainite transformation over a wider cooling rate range, giving thick steel plates better process adaptability and effectively improving the stability of strength and toughness in the thickness direction of the steel plate. Therefore, this invention controls the Mo content to be between 0.15% and 0.30%.

[0056] Cu: Adding Cu to steel can improve its corrosion resistance and strength, as well as its weldability, formability, and machinability. Adding Cu and Ni simultaneously can also prevent hot brittleness. In this invention, the Cu content is controlled at 0.20%–0.40%.

[0057] Cr: An important element for improving the hardenability of steel. For thick-gauge shipbuilding and offshore platform steel, adding a higher Cr content can effectively improve hardenability to compensate for the strength loss caused by thickness and improve the uniformity of performance in the thickness direction. Therefore, the Cr content in this invention is controlled at 0.15% to 0.40%.

[0058] Ni has a solid solution strengthening effect, which can promote the formation of stable austenitic structure in alloy steel. It has the characteristics of minimizing the Ar3 point and the increase of carbon equivalent or cold crack sensitivity coefficient Pcm. It can improve the strength and toughness of steel and improve the hot brittleness caused by Cu in steel. Therefore, the present invention controls the Ni content to be between 0.50% and 1.00%.

[0059] Nitrogen (N) combines with elements such as Al, Ti, and Nb to form nitrides, which are elements that refine the microstructure of the base material. To achieve this effect, the N content needs to be above 0.002%; however, excessive dissolved N deteriorates the toughness of the HAZ (heat-affected zone), while properly controlling the N content can refine the grains. Therefore, this invention controls the N content to be between 0.003% and 0.015%.

[0060] Ti: As a component added to improve the toughness of steel and the toughness of welded parts, it exists in the form of TiN and plays a role. However, when its content exceeds 0.04%, it is easy to form large TiN particles and lose its due effect. Therefore, the present invention controls the Ti content to be between 0.008% and 0.018%.

[0061] The present invention discloses a method for manufacturing FH460 ultra-high strength steel plate with excellent low-temperature fracture properties and elongation. The production process includes steel smelting and casting → billet heating → multi-stage controlled rolling → controlled cooling → stacking and slow cooling, etc. The key steps of the preparation process are as follows:

[0062] 1) Steel smelting and casting process: Select high-quality raw materials, primarily molten iron or high-quality recycled steel, with P and S content as low as possible. Control the smelting composition according to target values, strictly control residual element content, and avoid exceeding the carbon equivalent limit. Treat the steel in both the LF and RH refining furnaces for 25-50 minutes each. Maintain a superheat of ≤25℃ in the tundish. Perform full-process protective casting. Inclusions of types A, B, C, and D in the steel must meet the following requirements: Type A inclusions ≤0.5 grade, Type B inclusions ≤1.0 grade, Type C inclusions ≤1.0 grade, and Type D inclusions ≤0.5 grade. The continuous casting billet forming process employs a light reduction technique and incorporates electromagnetic stirring equipment, effectively improving center segregation and grain size uniformity. Continuously cast slabs require slow cooling in a stacked state for at least 24 hours.

[0063] 2) Steel billet heating process: A new low-temperature heating system is adopted, with a heating temperature of 1130~1180℃, a heat soaking temperature of 1100~1160℃, and a heat soaking time of 60~90min.

[0064] 3) Multi-stage controlled rolling process: Three-stage controlled rolling technology is adopted. The first stage rolling temperature is 1080-1120℃, and the single-pass deformation rate is ≥16%; the second stage rolling temperature is 860-940℃, the single-pass deformation rate is ≥12%, and the intermediate billet thickness is 1.5-2.0 times the thickness of the finished steel plate; the third stage rolling temperature is 780-840℃, the single-pass deformation rate is ≥10%, and the final rolling temperature is 750-820℃.

[0065] This invention combines large deformation rolling process to refine the original microstructure, control and adjust the composition, uniformity and refinement of phase transformation microstructure and multiphase particle precipitation behavior, so as to ensure the uniformity of grain size on the full thickness section of the thick steel plate, and improve the low temperature elongation and low temperature fracture resistance (CTOD) performance of the steel plate.

[0066] 4) Controlled cooling process: The average cooling rate of the steel plate is 15℃~25℃ / s, the initial cooling temperature is 670~730℃, and the final cooling temperature is 380~450℃. The cooling process adopts ACC laminar flow fully automatic controlled cooling mode, and the steel plate is shielded at both ends throughout the process to ensure the uniformity and stability of the performance of different positions of the steel plate.

[0067] 5) Stacking and slow cooling process: After controlled cooling, the steel plates are stacked and slow cooled for a time of ≥36 hours to obtain the finished steel plate.

[0068] The following embodiments are implemented based on the technical solution of the present invention, and provide detailed implementation methods and specific operation processes. However, the scope of protection of the present invention is not limited to the following embodiments.

[0069]

Example

[0070] The chemical composition of the FH460 ultra-high strength marine steel plate (hereinafter referred to as steel plate) prepared in each embodiment is shown in Table 1, the steelmaking process parameters are shown in Table 2, the rolling and cooling process parameters of the steel plate are shown in Table 3, the conventional mechanical properties of the finished steel plate are shown in Table 4, and the low temperature fracture properties of the finished steel plate are shown in Table 5.

[0071] Table 1 Chemical composition of steel plates in each embodiment

[0072]

[0073] Table 2. Smelting process parameters of molten steel in each embodiment.

[0074]

[0075] Table 3 Rolling and cooling process parameters of steel plates in each embodiment

[0076]

[0077]

[0078] Note: t is the thickness of the finished steel plate, in mm.

[0079] Table 4. Conventional mechanical properties of finished steel plates from each embodiment.

[0080]

[0081] Table 5 Low-temperature fracture properties of finished steel plates from each embodiment

[0082]

[0083] Figure 1 The image shows the microstructure of the finished steel plate obtained in Example 1. It can be seen that the microstructure of the finished steel plate is basically polygonal ferrite + bainite. The grain boundaries of the steel plate are clear and the grains are very fine, thus achieving the excellent low-temperature fracture toughness and low-temperature elongation of FH460 with ultra-high strength and thickness.

[0084] The above description is only a preferred specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any technician familiar with the technical field, within the technical scope disclosed by the present invention, who makes equivalent replacements or changes based on the technical solution and inventive concept of the present invention, should be covered by the scope of protection of the present invention.

Claims

1. An FH460 ultra-high strength steel plate with excellent low-temperature fracture toughness and elongation, characterized in that, The chemical composition of the steel plate, by mass percentage, is: C: 0.05%–0.14%; Si: 0.06%–0.20%; Mn: 1.40%–1.95%; S≤0.002%; P≤0.008%; Als: 0.015%–0.045%. N：0.003%～0.015%； Nb: 0.03%~0.06%; V: 0.03%~0.06%; Cu: 0.20%~0.40%; Cr: 0.15%~0.40%; Ni: 0.50%~1.00%; Mo: 0.15%~0.30%; Ti: 0.008%~0.018%; the balance is Fe and unavoidable impurities; the microstructure of the finished steel plate is polygonal ferrite + bainite, wherein the volume percentage of polygonal ferrite is 35%~50%.

2. The FH460 ultra-high strength steel plate with excellent low-temperature fracture toughness and elongation according to claim 1, characterized in that, The finished steel plate has a yield strength ≥460MPa, tensile strength ≥570MPa, impact energy at -80℃ ≥180J, Z-direction performance ≥60%, low temperature elongation at -20℃ ≥18%, and low temperature fracture CTOD characteristic value at -30℃~-70℃ >0.2mm.

3. The FH460 ultra-high strength steel plate with excellent low-temperature fracture toughness and elongation according to claim 1, characterized in that, The thickness of the finished steel plate is 60-100mm.

4. The method for manufacturing an FH460 ultra-high strength steel plate with excellent low-temperature fracture toughness and elongation according to claim 1, characterized in that, The process includes steel smelting and casting, billet heating, multi-stage controlled rolling, controlled cooling, and slow cooling in stacks; among which the following processes are controlled: 1) Heating of steel billets; Heating temperature 1130~1180℃, heat spread temperature 1100~1160℃, heat spread time 60~90min; 2) Multi-stage controlled rolling; At least three stages of controlled rolling are adopted; the first stage rolling temperature is 1080-1120℃, and the single-pass deformation rate is ≥16%; the second stage rolling temperature is 860-940℃, and the single-pass deformation rate is ≥12%; the intermediate billet thickness is 1.5-2.0 times the finished steel plate thickness; the third stage rolling temperature is 780-840℃, and the single-pass deformation rate is ≥10%; the final rolling temperature is 750-820℃. 3) Control cooling; The average cooling rate is 15-25℃ / s, the starting cooling temperature is 670-730℃, and the final cooling temperature is 380-450℃. 4) Stacking and slow cooling: The steel plates after controlled cooling are stacked and slow cooled for ≥36 hours to obtain finished steel plates.

5. The method for manufacturing an FH460 ultra-high strength steel plate with excellent low-temperature fracture toughness and elongation according to claim 4, characterized in that, During the steel smelting and casting process, the processing time for both LF refining and RH refining is 25-50 min, and the superheat of the molten steel in the tundish is ≤25℃; the continuously cast slabs undergo slow cooling treatment by stacking, with a processing time ≥24 h.

6. The method for manufacturing an FH460 ultra-high strength steel plate with excellent low-temperature fracture toughness and elongation according to claim 4, characterized in that, During the steel smelting and casting process, the molten steel is protected during the entire casting process, and the continuous casting billet forming process adopts light pressure and electromagnetic stirring.

7. The method for manufacturing an FH460 ultra-high strength steel plate with excellent low-temperature fracture toughness and elongation according to claim 4, characterized in that, During the steel smelting and casting process, the inclusions in the refined steel meet the following requirements: Class A inclusions ≤ 0.5 grade, Class B inclusions ≤ 1.0 grade, Class C inclusions ≤ 1.0 grade, and Class D inclusions ≤ 0.5 grade.

8. The method for manufacturing an FH460 ultra-high strength steel plate with excellent low-temperature fracture toughness and elongation according to claim 4, characterized in that, In step 3), the cooling is controlled by ACC laminar flow fully automatic cooling mode, and the steel plate is shielded at both ends throughout the process.

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