FH550 ultra-high strength steel plate with low yield strength ratio and excellent low-temperature fracture properties and its manufacturing method

Abstract

This invention relates to an FH550 ultra-high strength steel plate with low yield strength ratio and excellent low-temperature fracture properties, and its manufacturing method. The chemical composition of the steel plate is: C: 0.05%–0.13%; Si: 0.05%–0.18%; Mn: 1.60%–2.00%; S≤0.002%; P≤0.008%; Als: 0.015%–0.045%; N: 0.003%–0.0 15%; Nb: 0.02%–0.05%; V: 0.03%–0.08%; Cu: 0.20%–0.45%; Cr: 0.30%–0.60%; Ni: 0.80%–1.50%; Mo: 0.30%–0.60%; Ti: 0.008%–0.018%; Ce: 0.01%–0.03%, with the balance being Fe and impurities. Through a combination of rational composition design and TMCP multi-stage rolling technology, the obtained steel plate exhibits good uniformity of microstructure and properties, a good balance of strength and toughness across the entire thickness section, as well as excellent low-temperature CTOD fracture performance and Z-axis properties.

Description

Technical Field

[0001] This invention relates to the field of steel material preparation technology for ships, and in particular to a thick TMCP state FH550 ultra-high strength steel plate with low yield strength ratio and excellent low-temperature fracture properties, and its manufacturing method. Background Technology

[0002] Currently, the development of the marine sector has risen to an unprecedented strategic level. Improving my country's marine resource development capabilities means moving from shallow waters to deep seas, from nearshore areas to the open ocean, and from territorial waters to the high seas, as well as expanding to both the north and south. High-end marine equipment such as cryogenic icebreakers, polar transport ships, and ultra-deepwater drilling platforms are important guarantees, and high-performance steel materials for extremely cold environments required to manufacture these equipment are the key basic conditions.

[0003] The construction of polar vessels relies heavily on key materials such as cryogenic steel adapted to the harsh polar operating environment. High-strength, high- and low-temperature toughness, and easy-to-weld high-performance steel are fundamental guarantees for the safe navigation of polar vessels. This requires steel plates to possess comprehensive properties such as certain strength, a low yield strength ratio, good low-temperature toughness, resistance to lamellar tearing, good weldability, and machinability. Currently, these products are manufactured using the TMCP process both domestically and internationally. To ensure the low-temperature performance requirements, strict control over the billet heating temperature, rolling temperature, and post-rolling rapid cooling process is necessary, making production extremely challenging.

[0004] Chinese patent application CN202211365304.9 discloses "an easily weldable normalized pressure vessel steel plate and its manufacturing method." The chemical composition and mass percentage of the steel plate are as follows: C: 0.16%–0.19%, Si: 0.15%–0.40%, Mn: 1.45%–1.65%, P≤0.010%, S0.003%–0.010%, Ni: 0.40%–0.80%, V: 0.13%–0.20%, Nb: 0.015%–0.040%, N: 0.0120%– 0.0200%, Alt: 0.010%~0.040%, Cu≤0.10%, B: 0.0010%~0.0050%, O≤0.0020%, Ce: 0.005%~0.045%, La: 0.005%~0.045%, with the balance being Fe and unavoidable impurities; the pressure vessel steel plate produced by the normalizing process has a maximum thickness of only 30mm and a low-temperature toughness grade of only -40℃, which cannot meet the higher requirements of marine equipment for the strength and thickness of carbon-manganese low-temperature steel, and is fundamentally different from the present invention.

[0005] Chinese patent application CN202211295829.X discloses "A 700MPa grade high-strength, low yield strength ratio hot-rolled steel bar and its production method," with the following chemical composition: C: 0.26–0.30%, Si: 0.65–0.80%, Mn: 1.45–1.60%, V: 0.150–0.180%, Nb: 0.008–0.020%, Cr: 0.20–0.35%, Ti: 0.016–0.024%, S: ≤0.015%, P: ≤0.015%, with the balance being Fe and unavoidable impurities. This product is a 700MPa grade high-strength, low yield strength ratio hot-rolled steel bar, and only its strength index is evaluated, without addressing its low-temperature toughness and fracture properties. It belongs to the wire rod product category, not the medium-thick plate category, and cannot meet the strength and thickness requirements of carbon-manganese-based low-temperature steel for marine equipment; the two are fundamentally different.

[0006] Chinese patent application CN202211249039.8 discloses a method for preparing Q350EWR1 railway car steel with low yield strength ratio treated with rare earth elements. The composition is as follows: C: 0.02–0.07%, Si: 0.12–0.50%, Mn: 0.15–1.10%, P≤0.015%, S≤0.010%, Cu: 0.30–0.55%, Cr: 3.0–5.5%, Ni: 0.12–0.65%, N≤0.003%, O≤0.002%, RE: ≥0.0010%, with the remainder being iron and other unavoidable impurities. However, the yield strength of the steel produced is only 350 MPa, and the maximum thickness of the finished steel plate is only 17 mm. It is a hot-rolled coil product, and no relevant indicators of low-temperature toughness are recorded. Therefore, it cannot meet the strength and thickness requirements of carbon-manganese low-temperature steel for marine equipment, and is fundamentally different from this invention.

[0007] Chinese invention patent application CN201710493766.1 discloses a "thick plate of FH550 grade with excellent low-temperature toughness and its manufacturing method," with the following composition: C: 0.04%–0.10%, Si: 0.3%–0.5%, Mn: 1.0%–1.6%, Cu: 0.5%–0.7%, Ni: 0.5%–0.7%, Cr: 0.2%–0.4%, Mo: 0.2%–0.3%, P≤0.01%, S ≤0.01%, Als: 0.01%~0.05%, Nb: 0.02%~0.05%, Ti≤0.005%~0.03%, V: 0.04%~0.06%, N≤0.005%, the remainder being Fe and unavoidable impurities; although its steel plate has a yield strength of 550MPa, its low-temperature toughness grade is only F (-60℃), and it does not involve the CTOD low-temperature fracture index, nor does it have the characteristics of low yield strength ratio, which is fundamentally different from the present invention.

[0008] Chinese patent application CN202211506158.7 discloses "a method for producing P690QL2 steel plate with a thickness ≤50mm", with the following composition: C: 0.06%~0.09%, Si: 0.10%~0.30%, Mn: 1.3%~1.5%, P≤0.015%, S≤0.002%, Nb≤0.060%, V≤0.080%, Ti: 0.008%~0.020%, Ni: 1 0.20%–1.70%, Cr: 0.30%–0.50%, Mo: 0.40%–0.60%, Cu≤0.30%, Al: 0.030%–0.070%, Ca: 0.0010%–0.0050%, N≤0.0050%, H≤0.0003%, with the balance being Fe and unavoidable impurities; the maximum thickness of the steel plate is only 50mm, using a traditional quenching and tempering process, and the product does not have a low yield strength ratio characteristic, which is fundamentally different from the present invention. Summary of the Invention

[0009] This invention provides an FH550 ultra-high strength steel plate with low yield strength ratio and excellent low-temperature fracture properties, and its manufacturing method. By combining reasonable composition design with TMCP multi-stage rolling process, the obtained steel plate has good uniformity of microstructure and toughness across the entire thickness section, as well as good low-temperature CTOD fracture performance and Z-direction properties. Thus, the steel plate has good low-temperature fracture resistance and resistance to lamellar tearing, which can meet the requirements of marine steel plates used in harsh marine and low-temperature environments.

[0010] To achieve the above objectives, the present invention employs the following technical solution:

[0011] An FH550 ultra-high strength steel plate with low yield strength ratio and excellent low-temperature fracture properties has the following chemical composition by mass percentage: C: 0.05%–0.13%; Si: 0.05%–0.18%; Mn: 1.60%–2.00%; S≤0.002%; P≤0.008%; Als: 0.015%–0.045%; N: 0.003%–0.015%; N b: 0.02%–0.05%; V: 0.03%–0.08%; Cu: 0.20%–0.45%; Cr: 0.30%–0.60%; Ni: 0.80%–1.50%; Mo: 0.30%–0.60%; Ti: 0.008%–0.018%; Ce: 0.01%–0.03%, with the balance being Fe and unavoidable impurities.

[0012] Furthermore, the microstructure of the finished steel plate is acicular ferrite + lath bainite, wherein the volume percentage of acicular ferrite is 30% to 45%.

[0013] Furthermore, the finished steel plate has a yield strength ≥550MPa, tensile strength ≥640MPa; impact energy at -70℃ ≥200J; Z-direction properties ≥60%; yield strength ratio ≤0.85; and CTOD characteristic value of low-temperature fracture in the base material and heat-affected zone at -10℃~-50℃ >0.2mm.

[0014] Furthermore, the thickness of the finished steel plate is 55–80 mm.

[0015] A method for manufacturing FH550 ultra-high strength steel plate with low yield strength ratio and excellent low-temperature fracture properties, 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:

[0016] 1) Heating of steel billets;

[0017] Heating temperature 1220~1270℃, heat spread temperature 1170~220℃, heat spread time 40~80min;

[0018] 2) Multi-stage controlled rolling;

[0019] At least three stages of controlled rolling are employed; the first stage rolling temperature is 1120–1190℃, with a single-pass deformation rate ≥18%; the second stage rolling temperature is 940–1000℃, with a single-pass deformation rate ≥15%; the intermediate billet thickness is 1.8–2.5 times the finished steel plate thickness; the third stage rolling temperature is 830–880℃, with a single-pass deformation rate ≥10%; the final rolling temperature is 810–860℃.

[0020] 3) Control cooling;

[0021] The average cooling rate is 15-25℃ / s, the starting cooling temperature is 760-820℃, and the final cooling temperature is 440-520℃.

[0022] 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.

[0023] Furthermore, during the steel smelting and casting process, the processing time for both LF refining and RH refining is 25-30 minutes, and the superheat of the molten steel in the tundish is ≤30℃; the continuously cast slab undergoes slow cooling treatment by stacking, with a processing time ≥24 hours.

[0024] 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.

[0025] 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.

[0026] Furthermore, in step 3), the cooling control adopts the ACC laminar flow + DQ ultra-fast cooling fully automatic cooling control mode, and the steel plate is shielded at both ends throughout the process.

[0027] Compared with the prior art, the beneficial effects of the present invention are:

[0028] 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 the FH550 ultra-high strength and thickness steel plate has excellent low-temperature fracture performance.

[0029] 2) By rationally designing the homogenization temperature and the holding time during the homogenization stage when heating the billet, the grain size of the original austenite structure of the steel plate can be refined, ensuring the smooth rolling of the steel plate and the uniformity of the structure, thus providing a structural basis for improving the strength and toughness of the steel plate.

[0030] 3) By combining multi-stage controlled rolling and water cooling (TMCP) processes, the FH550 ultra-high strength and thickness steel plate is strengthened and toughened, the phase transformation structure is made more uniform and refined, the morphology and grain size are controlled, and the dimensional accuracy and surface quality are controlled.

[0031] 4) The TMCP-state FH550 marine steel plate with a maximum thickness of 80mm has high strength (yield strength ≥550MPa, tensile strength ≥640MPa), low temperature resistance (impact energy ≥200J at -70℃), Z-direction properties ≥60%, yield strength ratio ≤0.85, and characteristic values ​​of CTOD (crack tip opening displacement) of the base material and heat-affected zone at low temperatures (-10℃, -30℃, -50℃) >0.2mm. Its good microstructure uniformity, fracture resistance, and excellent surface quality can meet the technical requirements of marine steel plates in harsh and demanding marine environments. Attached Figure Description

[0032] Figure 1 This is a metallographic photograph of the finished steel plate prepared in Example 1 of the present invention. Detailed Implementation

[0033] The FH550 ultra-high strength steel plate with low yield strength ratio and excellent low-temperature fracture properties described in this invention has the following chemical composition by mass percentage: C: 0.05%–0.13%; Si: 0.05%–0.18%; Mn: 1.60%–2.00%; S≤0.002%; P≤0.008%; Als: 0.015%–0.045%; N: 0.003%–0.015%. %; Nb: 0.02%–0.05%; V: 0.03%–0.08%; Cu: 0.20%–0.45%; Cr: 0.30%–0.60%; Ni: 0.80%–1.50%; Mo: 0.30%–0.60%; Ti: 0.008%–0.018%; Ce: 0.01%–0.03%, with the balance being Fe and unavoidable impurities.

[0034] The microstructure of the finished steel plate is acicular ferrite + lath bainite, with the volume percentage of acicular ferrite being 30% to 45%.

[0035] The finished steel plate has a yield strength ≥550MPa, tensile strength ≥640MPa; impact energy at -70℃ ≥200J; Z-direction properties ≥60%; yield strength ratio ≤0.85; and CTOD characteristic value of low temperature fracture in the base material and heat-affected zone at -10℃~-50℃ >0.2mm.

[0036] The thickness of the finished steel plate is 55-80mm.

[0037] The present invention discloses a method for manufacturing FH550 ultra-high strength steel plate with low yield strength ratio and excellent low-temperature fracture properties. 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:

[0038] 1) Steel smelting and casting;

[0039] The processing time for both LF refining and RH refining is 25–30 min, and the superheat of the molten steel in the ladle is ≤30℃. The continuously cast slabs undergo slow cooling in a stack for ≥24 h. The molten steel is poured under full protection, and the continuous casting slab forming process uses 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.

[0040] 2) Heating the steel billet;

[0041] Heating temperature 1220~1270℃, heat spread temperature 1170~220℃, heat spread time 40~80min;

[0042] 3) Multi-stage controlled rolling;

[0043] At least three stages of controlled rolling are employed; the first stage rolling temperature is 1120–1190℃, with a single-pass deformation rate ≥18%; the second stage rolling temperature is 940–1000℃, with a single-pass deformation rate ≥15%; the intermediate billet thickness is 1.8–2.5 times the finished steel plate thickness; the third stage rolling temperature is 830–880℃, with a single-pass deformation rate ≥10%; the final rolling temperature is 810–860℃.

[0044] 4) Control cooling;

[0045] The average cooling rate is 15-25℃ / s, the starting cooling temperature is 760-820℃, and the final cooling temperature is 440-520℃. The controlled cooling adopts the ACC laminar flow + DQ ultra-fast cooling fully automatic controlled cooling mode, and the steel plate is shielded at both ends throughout the process.

[0046] 5) 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.

[0047] This invention addresses multiple aspects, including alloy element selection and proportioning, steel cleanliness control, heating process improvement, rolling process optimization, cooling process control, and microstructure strength and toughness matching. It employs an appropriate "carbon + manganese" and low-alloy composition system to achieve the design of ultra-high strength and thickness F-grade low-temperature steel plates. The production process utilizes a multi-stage controlled rolling process (TMCP), using 230–360 mm cross-section continuous casting billets to produce TMCP-state FH550 marine steel plates with a maximum thickness of 80 mm. The finished steel plates exhibit excellent strength and toughness matching across the entire thickness section, along with good low-temperature CTOD fracture performance, Z-axis properties, and good low-temperature fracture resistance and lamellar tear resistance, ensuring the performance requirements of low-temperature steel plates in harsh environments.

[0048] The FH550 ultra-high strength steel plate with low yield strength ratio and excellent low-temperature fracture properties described in this invention has the following mechanism of action of various alloy components in the steel:

[0049] 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 weld 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 performance of the product, this invention controls the C content to 0.05% to 0.13%.

[0050] 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, yield strength, and yield ratio of steel, as well as its oxidation resistance at high temperatures. For marine steel plates, a lower Si content can also improve surface quality. This invention aims to achieve a low yield strength ratio, therefore the Si content should not be too high; thus, the Si content is controlled at 0.05%–0.18%.

[0051] 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 together with nickel, which enhances the product's fracture performance; therefore, the Mn content is controlled at 1.60%–2.00%.

[0052] P: It is an element that has an adverse effect on impact value and will impair low-temperature toughness by segregating in the center of the slab and agglomerating at grain boundaries. In this invention, the P content is controlled to be no higher than 0.008%.

[0053] 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%.

[0054] Als: As a necessary deoxidizing and grain-refining element added in this invention, its content is above 0.01%; however, when the content exceeds 0.08%, it is easy to cause hot cracking in the cast billet, and at the same time, the toughness of the steel decreases. Therefore, this invention controls its content range to be 0.015% to 0.045%.

[0055] 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%; while the toughness and weldability of the steel decrease when the addition amount is greater than 0.05%; therefore, this invention controls the Nb content to be 0.02-0.05%.

[0056] 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%–0.08%.

[0057] 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 the critical cooling rate for bainite transformation, which is beneficial for promoting bainite transformation over a wider cooling rate range, giving thick steel plates better process adaptability, and effectively improving the stability of the strength and toughness properties in the thickness direction of the steel plate. Therefore, this invention controls the Mo content to be between 0.30% and 0.60%.

[0058] 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%–0.45%.

[0059] 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 the thickness and improve the uniformity of performance in the thickness direction. An appropriate amount of chromium can greatly improve the corrosion resistance of steel plates. Therefore, the Cr content in this invention is controlled at 0.30% to 0.60%.

[0060] 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 at 0.80% to 1.50%.

[0061] 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%.

[0062] 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 original effect. Therefore, the present invention controls the Ti content to be between 0.008% and 0.018%.

[0063] Ce: As an important rare earth element, Ce plays an important role in iron and steel metallurgy. An appropriate amount of Ce can improve the strength and toughness of steel. At the same time, rare earth Ce can also act as a deoxidizer and desulfurizer in steel, effectively removing oxygen and sulfur from steel and improving the purity and quality of steel. Taking into account the difficulty and cost of adding it, this invention controls the Ce content to be 0.01% to 0.03%.

[0064] This invention discloses a method for manufacturing FH550 ultra-high strength steel plate with low yield strength ratio and excellent low-temperature fracture properties. The production process includes steel smelting and casting → billet heating → multi-stage controlled rolling → controlled cooling → stacking and slow cooling. The key steps in its preparation process are as follows:

[0065] 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. Perform LF refining and RH refining for 20-30 minutes each. The superheat of the molten steel in the ladle should be ≤30℃. Protect the casting process throughout. Inclusions of types A, B, C, and D in the steel must meet the following requirements: Type A inclusions ≤0.5%, Type B inclusions ≤1.0%, Type C inclusions ≤1.0%, and Type D inclusions ≤0.5%. 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.

[0066] 2) Steel billet heating process: A new heating system is adopted, with a heating temperature of 1220~1270℃, a heat soaking temperature of 1170~1220℃, and a heat soaking time of 40min~80min.

[0067] 3) Multi-stage controlled rolling process: Three-stage controlled rolling technology is adopted. The first stage rolling temperature is 1120~1190℃, and the single-pass deformation rate is ≥18%. The second stage rolling temperature is 940~1000℃, and the single-pass deformation rate is ≥15%. The thickness of the intermediate billet is 1.8~2.5 times the thickness of the finished steel plate. The third stage rolling temperature is 830~880℃, the single-pass deformation rate is ≥10%, and the final rolling temperature is 810~860℃.

[0068] 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 strength, toughness and resistance to low temperature fracture (CTOD) of the steel.

[0069] 4) Controlled cooling process: The average cooling rate of the steel plate is 15℃~25℃ / S, the starting cooling temperature is 760~820℃, and the final cooling temperature is 450~520℃. The cooling process adopts the ACC laminar flow + DQ ultra-fast cooling fully automatic controlled cooling mode. 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.

[0070] 5) Stacking and slow cooling process: After controlled cooling, the steel plates are stacked and slow cooled for ≥36 hours to obtain finished steel plates.

[0071] The chemical composition of the FH550 ultra-high strength steel plate described in this invention is rationally designed. It is smelted using low-P and S pure steel, which improves the purity of the steel and ensures that the thick TMCP-state FH550 marine steel plate has excellent low-temperature toughness and fracture performance. The billet heating process optimizes the homogenization temperature and the holding time at the homogenization temperature stage, which lays the foundation for refining the original austenite grain size of the steel plate and ensuring the uniformity of the steel plate structure after subsequent rolling, as well as improving the strength and toughness of the steel plate. Through multi-stage controlled rolling + water cooling (TMCP) process, the structural strength and toughness control of the FH550 steel plate is achieved.

[0072] 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.

[0073]

Example

[0074] The chemical composition of the FH550 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 steel plate are shown in Table 4, and the low temperature fracture properties of the steel plate are shown in Table 5.

[0075] Table 1. Chemical composition (mass percentage) of steel in each example.

[0076]

[0077] Table 2 Steelmaking process parameters in each embodiment

[0078]

[0079] Table 3. Steel plate rolling and cooling process parameters in each embodiment.

[0080]

[0081]

[0082] Note: t is the thickness of the finished steel plate, in mm.

[0083] Table 4. Conventional mechanical properties of the finished steel plates in each example.

[0084]

[0085] Table 5 Low-temperature fracture properties of finished steel plates in each embodiment

[0086]

[0087] 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 acicular ferrite + lath bainite. The grain boundaries of the steel plate are clear and the grains are very fine, thus achieving the good low-temperature fracture toughness of FH550 ultra-high strength, thick thickness and low yield strength ratio steel plate.

[0088] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A low yield strength ratio and excellent low-temperature fracture properties of FH550 ultra-high strength steel plate, characterized in that, The chemical composition, by mass percentage, is: C: 0.05%–0.13%; Si: 0.05%–0.18%; Mn: 1.60%–2.00%; S≤0.002%; P≤0.008%; Als: 0.015%–0.045%. N: 0.003% ~ 0.015%; Nb: 0.02% ~ 0.05%; V: 0.03% ~ 0.08%; Cu: 0.20% ~ 0.45%; Cr: 0.30% ~ 0.60%; Ni: 0.80% ~ 1.50%; Mo: 0.30% ~ 0.60%; Ti: 0.008% ~ 0.018%; Ce: 0.01%~0.03%, balance being Fe and unavoidable impurities; the microstructure of the finished steel plate is acicular ferrite + lath bainite, wherein the volume percentage of acicular ferrite is 30%~45%.

2. The FH550 ultra-high strength steel plate with low yield strength ratio and excellent low-temperature fracture properties according to claim 1, characterized in that, The finished steel plate has a yield strength ≥550MPa, tensile strength ≥640MPa; impact energy at -70℃ ≥200J; Z-direction properties ≥60%; yield strength ratio ≤0.85; and CTOD characteristic value of low-temperature fracture in the base material and heat-affected zone at -10℃~-50℃ >0.2mm.

3. The FH550 ultra-high strength steel plate with low yield strength ratio and excellent low-temperature fracture properties according to claim 1, characterized in that, The thickness of the finished steel plate is 55-80mm.

4. The method for manufacturing the FH550 ultra-high strength steel plate with low yield strength ratio and excellent low-temperature fracture properties as described in any one of claims 1 to 3, 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 1220~1270℃, heat spread temperature 1170~220℃, heat spread time 40~80min; 2) Multi-stage controlled rolling; At least three stages of controlled rolling are adopted; the first stage rolling temperature is 1120-1190℃, and the single-pass deformation rate is ≥18%; the second stage rolling temperature is 940-1000℃, and the single-pass deformation rate is ≥15%; the intermediate billet thickness is 1.8-2.5 times the finished steel plate thickness; the third stage rolling temperature is 830-880℃, and the single-pass deformation rate is ≥10%; the final rolling temperature is 810-860℃. 3) Control cooling; The average cooling rate is 15-25℃ / s, the starting cooling temperature is 760-820℃, and the final cooling temperature is 440-520℃. 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 FH550 ultra-high strength steel plate with low yield strength ratio and excellent low-temperature fracture properties 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 25-30 minutes, and the superheat of the molten steel in the tundish is ≤30℃; the continuously cast slabs undergo slow cooling treatment by stacking, with a processing time ≥24 hours.

6. The method for manufacturing an FH550 ultra-high strength steel plate with low yield strength ratio and excellent low-temperature fracture properties 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 FH550 ultra-high strength steel plate with low yield strength ratio and excellent low-temperature fracture properties 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 FH550 ultra-high strength steel plate with low yield strength ratio and excellent low-temperature fracture properties according to claim 4, characterized in that, In step 3), the cooling is controlled by ACC laminar flow + DQ ultra-fast cooling fully automatic cooling mode, and the steel plate is shielded at both ends throughout the process.

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