960MPa-class ultra-high strength steel plate with anti-fouling properties for marine organisms and its manufacturing method

Abstract

The present invention relates to a 960 MPa class ultra-high strength steel plate having marine biofouling inhibitory ability, characterized in that its chemical components are, in weight percentage, 0.03% to 0.12% C, 0.05% to 0.20% Si, 0.50% to 2.00% Mn, P≦0.015%, S≦0.005%, Cu 1.60% to 3.00%, Cr 0.10% to 1.00%, Ni 2.0% to 6.0%, Mo 0.10% to 1.00%, Nb≦0.10%, V≦0.10%, Ti≦0.02%, Al≦0.04%, and the balance being Fe and unavoidable impurity elements. The present invention does not use Sb, Sn, does not add boron, but uses Cu and V to strengthen the martensite matrix through complex precipitation, and uses Cu-rich clusters to achieve better biofouling inhibition, which can meet the material selection requirements for steels used in the main parts of ships or marine engineering structures. Secondary heating is not required after rolling, and the direct quenching process is used for production, which saves energy and improves production efficiency.

Description

[Technical Field]

[0001] The present invention relates to the technical field of ultra-high strength structural steel manufacturing, and more particularly to a 960 MPa class ultra-high strength steel plate having the ability to inhibit the adhesion of marine organisms and a method for manufacturing the same. [Background technology]

[0002] The 21st century is the century of the ocean, and countries around the world are actively developing materials for ships and marine engineering to improve the convenience of maritime transport and maximize the use of the ocean's abundant resources. Among these materials, steel holds a leading position, with the strength grade of the current mainstream steel reaching 690 MPa. The use of high-grade steel in ships and marine engineering allows for thinner steel walls. Reducing wall thickness not only reduces the difficulty of welding and improves welding efficiency, but also reduces weight, shifting the center of gravity of steel structures downward and improving safety.

[0003] The surfaces of offshore ships, drilling platforms, port buildings, and other underwater facilities in contact with seawater are prone to the attachment of marine organisms and are subject to fouling by marine organisms. In the case of ships, the attachment of marine organisms significantly increases the surface friction resistance, leading to increased fuel consumption and reduced ship speed. The attachment of marine organisms also causes adhesive corrosion, posing a significant threat to the safety and lifespan of steel materials. In the case of platforms, their movement is restricted and they are located far from land, making maintenance inconvenient. To address the adverse effects of marine organisms, the development of materials capable of inhibiting marine organism fouling is the optimal technological solution.

[0004] Currently, the technical solutions that can achieve marine biofouling control mainly include coating layers, polymer composite materials, and steel materials with the addition of specific element combinations. In the patent document with application number 201810949527.7, "Method for Producing a Multifunctional Metal-Based Protective Coating Layer with Corrosion Resistance, Antibacterial Properties, and Biofouling Inhibition," nanosilver is in situ synthesized on graphene oxide-OH functional groups through a chemical reaction, followed by ball milling to form a metal-graphene-nanosilver composite powder. The composite powder is then deposited onto a protective coating layer using cold spray technology, achieving its excellent corrosion resistance, antibacterial properties, and biofouling inhibition properties. This coating layer is essentially a physical barrier and does not alter the biofouling inhibition properties of the protected material. In contrast, the solution of the present invention achieves biofouling inhibition through a specific composition of Cu-rich nanoclusters without polluting the environment. This material can be used as a structural load-bearing material in marine environments and belongs to the category of materials with integrated structure and function. Previous materials lack the potential for use as structural materials.

[0005] Patent document No. 201810281214.9, "Nylon-copper composite material for biofouling prevention and manufacturing method thereof," describes the production of a nylon-based polymer composite using a copper component, a coupling agent, and nylon. The copper component dissolves in seawater to prevent marine biofouling. The copper component is a copper compound. Polymer materials have low strength and cannot be used as structural components. While similar to coating layers, they have the advantage of not containing toxic substances and not polluting the environment compared to coating layers. Patent document No. 201810607711.3, "Nylon-copper composite material for long-term marine biofouling prevention and manufacturing method thereof," describes the production of a nylon-based polymer material using metallic copper powder, a coupling agent, sodium carboxymethylcellulose powder, and nylon powder. While this material has good biofouling prevention and corrosion resistance, it is also a polymer material and has low strength, making it unusable as a structural component. On the other hand, compared to the above two technologies, the solution of the present invention provides a structural steel material for marine environments rather than a polymer composite material, and the material that performs the biofouling prevention function in the material is a Cu-rich nanocluster of a specific composition, making it possible to realize a high-strength steel material that has the ability to prevent biofouling itself while also having strength that is far greater than that of the polymer materials described in the prior art.

[0006] Patent document No. 202011054693.3, "Corrosion-Resistant and Biofouling-Inhibiting EH690 Steel Plate and Its Manufacturing Method," describes a steel plate produced using a two-stage controlled rolling and tempering process. After tempering, an accelerated cooling rate of 5-15°C / s is used. This material's corrosion-inhibiting properties in marine environments are achieved by the elements Sb (0.005%-0.3%) and Sn (0.005%-0.3%), with a Cu content of 0.5%-1.5%, which effectively inhibits the growth and adhesion of microorganisms. When heat-treating the steel plate, it must be packaged in thin sheets and then placed in a heating furnace. This technology achieves its technical objective through the interaction of the elements Cu, Ni, Mo, Sb, and Sn. Summary of the Invention [Problem to be solved by the invention]

[0007] The problem to be solved by the present invention is to provide a 960 MPa class ultra-high strength steel plate with marine biofouling resistance that achieves better biofouling resistance by using Cu-rich clusters and meets the material selection requirements for steel used in main parts of ships or marine engineering structures. [Means for solving the problem]

[0008] In order to achieve the above object, the present invention employs the following means. The chemical composition of the 960 MPa-class ultra-high strength steel plate with marine biofouling prevention capabilities is, by weight percentage, C 0.03% to 0.12%, Si 0.05% to 0.20%, Mn 0.50% to 2.00%, P≦0.015%, S≦0.005%, Cu 1.60% to 3.00%, Cr 0.10% to 1.00%, Ni 2.0% to 6.0%, Mo 0.10% to 1.00%, Nb≦0.10%, V≦0.10%, Ti≦0.02%, Al≦0.04%, and the remainder being Fe and unavoidable impurity elements.

[0009] The 960MPa-class ultra-high-strength steel plate with the ability to inhibit marine biofouling has a yield strength Rp0.2≧960MPa, a tensile strength≧980MPa, an elongation≧12%, a Charpy impact strength in the transverse direction at -40℃≧70J, a Charpy impact strength in the longitudinal direction at -40℃≧100J, and a 12-month marine biofouling rate of 15% or less.

[0010] The functions and ranges of the main alloying elements are explained as follows: Carbon (C) is the second most important element in steel after iron, and directly affects the strength, plasticity, toughness, weldability, and other properties of steel. C is highly effective in increasing the strength of steel through solid solution strengthening and precipitation strengthening, but an increase in the C content adversely affects the plasticity, toughness, and weldability of steel. For this reason, the present invention specifies a C content range of 0.03 to 0.12%.

[0011] Regarding silicon (Si), Si is an important reducing agent and deoxidizer in the steelmaking process. It dissolves in ferrite and austenite to increase the hardness and strength of steel. Increasing the Si content can suppress the tendency for Fe3C to precipitate. An excessively high Si content favors the formation of martensite-austenite islands, significantly reducing the plasticity, toughness, and weldability of steel. Therefore, in the present invention, the Si content range is set to 0.05 to 0.20%.

[0012] Manganese (Mn) can improve the hardenability of steel and is beneficial to the strength of steel; it can eliminate the effects of S (sulfur) and improve the hot workability of steel. Mn is relatively inexpensive and can be infinitely dissolved with Fe, so it increases the strength of steel while having a relatively small effect on plasticity. Therefore, Mn is widely used as a strengthening element in steel. An excessively high Mn content can cause problems in continuous casting. Slabs This accelerates the segregation of Mn, increases the grade of the banded structure of the steel plate, reduces the uniformity of the structure, and is detrimental to the lamellar tear resistance, plasticity, low-temperature toughness, and weldability of the steel. For this reason, in the present invention, the Mn content range is set to 0.50 to 2.00%.

[0013] Niobium (Nb) is one of the most important microalloying elements. It partially dissolves in solid solution and exhibits the function of solid solution strengthening. When present in the form of carbides, nitrides, or oxide particles, Nb can improve the tempering stability of steel and has a secondary hardening effect. A trace amount of Nb can increase the strength of steel without affecting its plasticity or toughness. Its grain refinement effect can improve the impact toughness of steel and lower its brittle transition temperature. In the controlled rolling process, dissolving Nb in solid solution significantly increases the recrystallization temperature of steel, allowing the steel to be rolled at a higher temperature range, thereby reducing internal stress in the steel. In the present invention, the Nb content is set to 0.10% or less.

[0014] Vanadium (V) has a very strong affinity with C, N, and O, forming corresponding stable compounds. V exists mainly in the form of carbides in steel, refining the structure and grain size, increasing strength and toughness, improving weldability, and reducing overheating susceptibility. Vanadium can improve the tempering stability of hardened steel and also brings about a secondary hardening effect; in tempered steel, it mainly increases the strength and yield ratio of the steel. In the present invention, the V content is set to 0.10% or less.

[0015] Titanium (Ti) has a very strong affinity with C, N, and O, forming corresponding stable compounds with these elements, making it one of the most important N-fixing elements. Ti-containing precipitate phases have strong bonding strength, are stable, and are resistant to decomposition, suppressing the tendency of steel grains to grow at high temperatures and improving the weldability of steel. Fixing N and S with Ti is advantageous for increasing the strength and plasticity of steel. Increasing the Ti content causes the Ti-containing precipitate phase to coarsen, adversely affecting performance. In the present invention, the Ti content is set to 0.02% or less.

[0016] Copper (Cu) increases the strength and yield ratio of steel without adversely affecting weldability. When the copper content exceeds a certain amount, aging strengthening occurs after solution treatment and aging. Isothermal heat treatment can form Cu-containing clusters.100-x (Mn2Ni1Fe1) x (x=2-10) is generated, and these clusters have strong bioincompatibility and are advantageous for improving the ability to inhibit marine biofouling. If the Cu content is less than 1.6%, the precipitation of copper-rich clusters cannot be achieved, and the effect is more pronounced when it exists as ε-Cu and its volume fraction is 0.5% or more. If the content is low, its effect is similar to that of nickel but weaker. If the content is high, it is disadvantageous for hot deformation processing, and Cu dissolved during hot deformation processing causes copper embrittlement. In the present invention, the Cu content range is 1.60-3.00%.

[0017] Chromium (Cr) can improve the hardenability of steel and has a secondary hardening effect, increasing the strength, hardness, and wear resistance of steel without embrittlement, but it can also reduce elongation and area reduction. The main role of Cr in the tempered structure is to improve hardenability, thereby imparting good overall mechanical properties to the steel after quenching and tempering. If too much Cr is added, Cr-containing carbides precipitate at the original austenite grain boundaries during tempering and welding thermal cycle processes, coagulate, and grow, significantly impairing the low-temperature toughness and weldability of the steel sheet. In the present invention, the Cr content range is 0.10 to 1.00%.

[0018] Nickel (Ni) stabilizes austenite and improves hardenability. Adding a certain amount of Ni to steel can improve strength, toughness, and corrosion resistance, and lower the ductile-brittle transition temperature. Ni-containing steel is generally resistant to overheating, preventing grain growth at high temperatures and maintaining a fine grain structure. However, taking cost factors into consideration, the present invention specifies the Ni content range as 2.0 to 6.0%, and preferably the Ni content is twice the Cu content.

[0019] Molybdenum (Mo) improves the hardenability and hot strength of steel and prevents temper embrittlement. Mo increases the hardenability and hot strength of large-section steel members in tempered steel, improving the tempering resistance or tempering stability of the steel, allowing the steel members to be tempered at higher temperatures. This more effectively removes (or reduces) residual stress and enhances plasticity. The addition of Mo is beneficial for converting V from a solid solution state to a precipitation state. Since the solid solution strengthening effect of V is significantly lower than its precipitation strengthening effect, the formation of (V,Mo)C-type carbides can significantly improve the utilization effect of V. In the present invention, the Mo content range is set to 0.10 to 1.00%.

[0020] Aluminum (Al) is added to steel as a deoxidizer or alloying element, and its deoxidizing ability is much greater than that of silicon or manganese. The primary function of aluminum in steel is to refine grain size and fix nitrogen in the steel, thereby significantly improving the impact toughness of the steel and reducing its tendency toward cold brittleness and aging. Aluminum can also improve the corrosion resistance of steel, particularly when used in combination with elements such as molybdenum, copper, silicon, and chromium. A disadvantage of aluminum is that it affects the hot workability, weldability, and machinability of steel. In this invention, the Al content range is set to 0.04% or less.

[0021] Regarding phosphorus (P), P is introduced into steel from ores and, like S, is a harmful element. P can increase the strength and hardness of steel, but it also causes a significant decrease in plasticity and impact toughness, and significantly embrittles steel, especially at low temperatures. The higher the P content, the greater the cold brittleness. Removing P to a low level significantly increases the cost of steelmaking. In the present invention, the P content range is set to 0.015% or less.

[0022] Regarding sulfur (S), S is derived from steelmaking ores and fuel coke. It is one of the most common harmful elements in steel and is detrimental to the ductility, toughness, weldability, and corrosion resistance of steel. If S exists in steel in the form of FeS, it may cause "hot embrittlement" during hot working. In the present invention, the S content range is set to 0.005% or less.

[0023] The manufacturing process for 960MPa-class ultra-high strength steel plates with marine biofouling prevention capabilities is as follows: smelting → continuous casting → Slabs The process involves the following steps: reheating → descaling with high-pressure water → rolling in the recrystallized region → rolling in the non-recrystallized region → accelerated cooling → tempering heat treatment, and the specific steps are as follows: 1) Slabs Reheating: heating temperature T F The temperature is set to 1150℃ to 1250℃, and the total time in the furnace is t F is set to 4-6h; 2) Descaling with high-pressure water: Ensures effective descaling and ensures continuous casting after descaling Slabs Temperature T s ≥ 1120°C; 3) Recrystallization region rolling: This is the first stage of rolling, and the final rolling temperature T Rf ≧980℃, cumulative reduction rate ε R ≥ 50%; 4) Unrecrystallized region rolling: This is the second stage of rolling, and the rolling start temperature T Fs ≦920℃, final rolling temperature T Ff ≧860℃, cumulative reduction rate ε F ≥ 60% and the height of unrecrystallized austenite is 20 μm or less; 5) Accelerated cooling: After the steel sheet is rolled, it is cooled directly to room temperature. Cs is 820-860℃; 6) Tempering heat treatment: After water cooling, the steel plate is subjected to tempering heat treatment at a tempering temperature T T Controlled between 500 and 700°C, tempering and heat retention time t T is set to h*(1.5~3.5)min depending on the thickness h of the finished steel plate, and Cu-containing cluster Cu 100-x (Mn2Ni1Fe1) x (x=2~10) volume fraction V fis to be 0.5% or more.

[0024] In this invention, composite precipitation strengthening with elements such as Cu and V is adopted, and continuous casting Slabs By controlling the heating temperature between 1150 and 1250°C and the total time in the furnace between 4 and 6 hours, it is possible to ensure that the precipitated phases of the alloying elements are sufficiently redissolved into austenite, and to fully exert the beneficial effects of recrystallization suppression, solid solution strengthening, precipitation strengthening, and grain refinement in the subsequent controlled rolling process, and to prepare the components and temperature to obtain the final structure. If the temperature and time are lower than the selected range, solid solution will be insufficient, which will affect the final steel sheet strength, and if the time and temperature are higher than the selected range, the continuous casting Slabs The original austenite grains are likely to become coarse, which is disadvantageous in controlling the toughness of the steel plate.

[0025] continuous casting Slabs After removing the steel from the furnace, it is first descaled using high-pressure water to ensure the quality of the rolled surface of the steel plate. If the temperature is lowered below 1120°C after descaling, the rolling load during the rolling stage will increase. The rolling is performed in two stages. The first stage is rolling in the austenite recrystallization temperature range, and the rolling is completed before 980°C to avoid entering the partial recrystallization temperature range and causing uneven grain size. Completing the rolling in a high temperature range improves the deformation conditions of the rolled part and increases the pass reduction. The cumulative reduction rate in the first stage is set to 50% or more to ensure that the initial equiaxed austenite grains rolled in the second stage are sufficiently refined. The size of the austenite grains at this time is the initial height of the austenite.

[0026] The second stage of rolling is performed in the non-recrystallized region, with the rolling temperature range being 860 to 920°C. If the temperature is higher than 920°C, the rolled material may enter the partial recrystallization region, resulting in non-uniform crystal grains. On the other hand, if the temperature is lower than 860°C, it becomes difficult to ensure the start temperature required for the subsequent direct accelerated cooling. The cumulative reduction rate in the second stage is set to 50% or more in order to sufficiently flatten the recrystallized austenite crystal grains. This is advantageous for the subsequent structural transformation, structural refinement, and performance control. In particular, sufficiently flattened austenite crystal grains have numerous dislocations inside, and Cu-containing clusters Cu 100-x (Mn2Ni1Fe1) x (x=2~10) provides favorable conditions for the formation of solid-state phase transitions.

[0027] After the steel sheet has been rolled, it is rapidly cooled directly from a temperature of 820 to 860°C to room temperature so that the steel sheet transforms into a martensite structure. After the accelerated cooling of the steel plate is completed, tempering heat treatment is performed. If the tempering temperature is higher than 700°C, the strength of the steel plate will be significantly reduced, which is unfavorable to the balance between the final strength and toughness of the steel plate. In this case, Cu-containing clusters Cu 100-x (Mn2Ni1Fe1) x (x=2~10) has a relatively low equilibrium content and is easy to grow, so the cluster point density decreases, which is unfavorable for suppressing marine biofouling. On the other hand, when the temperature is lower than 500°C, the tempering of the quenched martensite becomes insufficient, which is unfavorable for low-temperature toughness, and the Cu-containing cluster Cu 100-x (Mn2Ni1Fe1) x A longer holding time is required to achieve the content of (x=2~10). If the tempering time is too long, the strength will decrease, and if the tempering time is too short, the toughness will be insufficient, so the heating coefficient is set to 1.5~3.5min / mm. [Effects of the Invention]

[0028] The present invention has the following advantageous effects compared to the prior art. 1) The present invention does not use Sb or Sn, does not add boron, and instead uses Cu and V to complex precipitation strengthen the martensite matrix, and uses Cu-rich clusters to achieve better biofouling suppression, thereby meeting the material selection requirements for steels used in the main parts of ships or marine engineering structures. 2) Cu-containing clusters formed during isothermal heat treatment 100-x (Mn2Ni1Fe1) x (x=2~10) is the key to inhibiting marine biofouling. The composition design employs relatively high Mn and Ni contents, and the Ni and Fe contents are higher than the Cu content to provide a high thermodynamic driving force and promote cluster formation. 3) Secondary heating is not required after rolling, and production is carried out using a direct quenching process, which saves energy and improves production efficiency. DETAILED DESCRIPTION OF THE INVENTION

[0029] The following provides a clear and complete description of the technical means in the embodiments of the present invention based on the embodiments of the present invention, but it is clear that these embodiments are only a part of the embodiments of the present invention, not all of them. Any other embodiments that can be obtained by those skilled in the art based on the embodiments of the present invention without creative efforts are all included in the protection scope of the present invention. The chemical composition of the 960 MPa-class ultra-high strength steel plate with marine biofouling prevention capabilities is, by weight percentage, C 0.03% to 0.12%, Si 0.05% to 0.20%, Mn 0.50% to 2.00%, P≦0.015%, S≦0.005%, Cu 1.60% to 3.00%, Cr 0.10% to 1.00%, Ni 2.0% to 6.0%, Mo 0.10% to 1.00%, Nb≦0.10%, V≦0.10%, Ti≦0.02%, Al≦0.04%, and the remainder being Fe and unavoidable impurity elements.

[0030] The manufacturing method of 960 MPa-class ultra-high strength steel plate with marine biofouling resistance includes the processes of steelmaking, refining, casting, rolling, cooling and heat treatment, and the specific steps are as follows: 1) Steelmaking and continuous casting: In the smelting process of molten steel, the content of each element is strictly controlled according to the composition requirements of the steel plate; the smelting process is from hot metal pretreatment to converter smelting to furnace refining; in the continuous casting process, the whole process is protected casting, and one or more of electromagnetic stirring, light reduction and heavy reduction are carried out; Slabs Strictly control the internal and external quality. 2) Slabs Reheating: heating temperature T F The temperature is set to 1150℃ to 1250℃, and the total time in the furnace is t F is set to 4-6h; 3) Descaling by high-pressure water: Ensures effective descaling and ensures continuous casting after descaling Slabs Temperature T s ≥ 1120°C; 4) Recrystallization region rolling: This is the first stage of rolling, and the final rolling temperature T Rf ≧980℃, cumulative reduction rate ε R ≥ 50%; 5) Unrecrystallized region rolling: This is the second stage of rolling, and the rolling start temperature T Fs ≦920℃, final rolling temperature T Ff ≧860℃, cumulative reduction rate ε F ≥ 60% and the height of unrecrystallized austenite is 20 μm or less; 6) Accelerated cooling: After the steel sheet is rolled, it is cooled directly to room temperature. Cs is 820-860℃; 7) Tempering heat treatment: After water cooling, the steel plate is subjected to tempering heat treatment at a tempering temperature T T Controlled between 500 and 700°C, tempering and heat retention time t T is set to h*(1.5~3.5)min depending on the thickness h of the finished steel plate, and Cu-containing cluster Cu 100-x (Mn2Ni1Fe1) x (x=2~10) volume fraction V f is to be 0.5% or more. [Example]

[0031] Table 1 shows the chemical compositions of the steel materials used in the examples of the present invention. [Table 1]

[0032] Table 2 shows the rolling and heat treatment process parameters of the steel in the examples of the present invention. [Table 2]

[0033] Table 3 shows the mechanical properties of the steels in the examples of the present invention. [Table 3]

[0034] From the data in Tables 1, 2 and 3, it can be seen that the steel plate manufactured by the technical means of the present invention has a yield strength of 965 MPa or more, a tensile strength of 1005 MPa or more, an elongation of 15% or more, and a Charpy impact energy at -40°C of 70 J or more, and has an excellent balance of strength and toughness, as well as an ability to inhibit marine biofouling, and the performance variation of the steel plate is small throughout the entire process window.

[0035] Although examples of the present invention have been described, it should be understood by those skilled in the art that various changes, modifications, substitutions and variations can be made to these examples without departing from the principles and spirit of the present invention, and that the scope of the present invention is limited by the appended claims and their equivalents.

Claims

1. A steel sheet having a yield strength of 960 MPa or more, and having a chemical composition, by weight percentage, of C 0.03% to 0.12%, Si 0.05% to 0.20%, Mn 0.50% to 2.00%, P≦0.015%, S≦0.005%, Cu 1.60% to 3.00%, Cr 0.10% to 1.00%, Ni 2.0% to 6.0%, Mo 0.10% to 1.00%, Nb≦0.10%, V≦0.10%, Ti≦0.02%, Al≦0.04%, and the remainder being Fe and unavoidable impurity elements, and containing Cu clusters Cu １００－ｘ (Mn ２ Ni １ Fe １ ) ｘ (x=2 to 10) are formed, and the volume fraction V f of the Cu-containing clusters Cu 100-x (Mn 2 Ni 1 Fe 1 ) x (x=2 to 10) is 0.5% or more.

2. The steel plate according to claim 1, characterized in that it has a tensile strength of ≧980 MPa and an elongation of ≧12%.

3. 2. The steel plate according to claim 1, wherein the weight percentage of Ni is 2.0% to 4.2%.

4. 2. The steel plate according to claim 1, wherein the weight percentage of C is 0.06% to 0.12%.

5. 2. Steel plate according to claim 1, characterized in that it is used in ships or marine engineering structures.

6. The method for producing a steel sheet according to any one of claims 1 to 5, characterized in that it comprises the steps of smelting → continuous casting → slab reheating → descaling with high-pressure water → recrystallization region rolling → non-recrystallization region rolling → accelerated cooling → tempering heat treatment, and the specific steps are as follows: 1) Slab reheating: heating temperature T Ｆ is set to 1150°C to 1250°C, and the total time in the furnace is t Ｆ is set to 4-6h; 2) Descaling by high-pressure water: Ensure the descaling effect and reduce the temperature T of the continuous casting slab after descaling. ｓ ≧1120°C; 3) Recrystallization region rolling: This is the first stage of rolling, and the final rolling temperature T Ｒｆ ≧980℃, cumulative reduction rate ε Ｒ ≧50%; 4) Unrecrystallized region rolling: This is the second stage of rolling, and the rolling start temperature T Ｆｓ ≦920°C, final rolling temperature T Ｆｆ ≧860℃, cumulative reduction rate ε Ｆ ≧50%; 5) Accelerated cooling: After the rolling of the steel sheet is completed, the steel sheet is immediately quenched to room temperature. Ｃｓ is 820 to 860°C; 6) Tempering heat treatment: After water cooling, the steel plate is subjected to tempering heat treatment at a tempering temperature T Ｔ The tempering temperature is controlled between 500 and 700°C, and the tempering time per unit thickness of the finished steel plate is t Ｔ / h is set to 1.5 to 3.5 min / mm, and Cu-containing cluster Cu １００－ｘ (Mn ２ Ni １ Fe １ ) ｘ Volume fraction V (x = 2 to 10) ｆ is set to 0.5% or more.

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