Low yield ratio marine steel with yield strength ≥ 750MPa and its manufacturing method

Abstract

The present invention relates to a low yield ratio marine steel having a yield strength of ≧750 MPa, the chemical composition of which is 0.06%-0.10% C, 0.1%-0.2% Si, 0.60%-1.0% Mn, P≦0.015%, S≦0.005%, 0.62%-1.20% Cu, 0.20%-0.50% Cr, 0.50%-1.20% Ni, 0.30%-0.70%, Nb≦0.06%, 0.02%-0.05% V, Ti≦0.02%, Al≦0.04%, and the balance being Fe and impurities, and a method for producing the same. In this invention, by using Cu-Mo-Nb-V-Ti composite strengthening and adjusting the controlled rolling and controlled cooling parameters, the microstructure of the steel plate is a mixed structure of martensite-bainite-nanoscale precipitates, which has high strain strengthening ability, low yield ratio, and good weldability. In this invention, the TMCP process is adopted, which has low manufacturing cost and high efficiency.

Description

[Technical Field]

[0001] The present invention relates to the technical field of marine steel, in particular to a low yield ratio marine steel with a yield strength of ≥ 750 MPa and a method for producing the same. [Background technology]

[0002] Steel is an important structural material for marine engineering equipment and is widely used in facilities such as offshore oil drilling platforms, offshore wind power plants, and submarine pipelines. Steel for marine structures is used in harsh environments and must withstand not only gravity loads but also wind loads, wave loads, ice loads, and earthquake loads. Furthermore, marine engineering equipment is difficult to maintain, so its use period is long, placing high demands on steel materials. Currently, steel plates for marine structures are becoming stronger, thicker, and larger. As the strength of steel increases, the ratio of its yield strength to its tensile strength (yield ratio) tends to increase. As the yield ratio of a material increases, the stress between the point at which plastic deformation occurs (yield point) and the point at which failure occurs does not change significantly. This means that offshore facilities do not have much time to absorb energy through deformation and prevent failure. Ensuring the safety of offshore facilities when subjected to large external forces such as earthquakes and tsunamis becomes difficult. Therefore, steel for offshore construction equipment structures must simultaneously satisfy both high strength and a low yield ratio. In light of this situation, the present inventors have developed a low-yield-ratio marine steel with a yield strength of ≥ 750 MPa, a tensile strength of ≥ 1050 MPa, a yield ratio of ≤ 0.72, and a transverse impact energy of ≥ 100 J at -40°C.

[0003] A Chinese patent application with application number 202010235198.7 discloses a "low-yield-ratio, high-strength steel plate with a yield strength of 690 MPa and a method for manufacturing the same." This method involves a tempered sorbite-bainite composite microstructure obtained by secondary quenching, resulting in a yield strength of ≥ 690 MPa, a tensile strength of ≥ 770 MPa, and a yield ratio of ≤ 0.88. This method requires a secondary quenching process in which the steel plate is heated twice, resulting in high energy consumption and low manufacturing efficiency. Furthermore, the carbon content is controlled to 0.1%-0.2%, making it difficult to meet the welding requirements for marine steel, resulting in high welding difficulties. While the yield ratio of this steel plate is ≤ 0.88, as can be seen from the examples, the yield ratios of the manufactured steel plates are all higher than 0.86, significantly different from the yield ratio of the steel plate of the present invention (≤ 0.72), making it difficult to ensure that marine facilities can absorb a large amount of energy during plastic deformation.

[0004] The Chinese patent application with application number 202110035527.8 discloses "Low yield ratio marine steel plate with excellent low-temperature toughness and manufacturing method thereof," in which a steel plate with a yield ratio of ≦0.8 is obtained through the processes of controlled rolling, controlled cooling, and slow cooling of the steel plate. The Si content is controlled to 0.2% to 0.4%, and if the content is too high, the plasticity and toughness of the heat-affected zone of the steel plate will decrease. The Mn content is controlled to 1.45% to 1.65%, and if the content is too high, the continuous casting Slabs The segregation of Cu is accelerated, resulting in a decrease in the uniformity of the structure. The Cu content is controlled to 0.15% to 0.3%, which is too low to form nano-sized Cu-rich precipitates in the steel sheet, and Cu is not effective in improving the strength and strain strengthening ability of the steel sheet. The steel sheet manufactured by this method has a yield strength of 420 MPa or more, which is two or more grades lower than the steel sheet with a yield strength of 750 MPa or more manufactured by the method of the present invention, making it difficult to meet the demand for high strength.

[0005] The Chinese patent application with application number 201780071626.3 discloses "Low yield ratio, ultra-high strength steel material and its manufacturing method," in which the steel plate is first r3Cool to below temperature and then B s Steel plates with a yield ratio of ≦0.85 and a tensile strength of ≧800 MPa can be obtained by two cooling rates, cooling below the temperature. The controlled cooling process in this method is relatively complicated, and it is difficult to accurately control the end temperature of the primary cooling in actual on-site production. Furthermore, the cooling rate of the secondary cooling must be more than 30°C / s, which places high demands on equipment capacity and makes the process less applicable. Furthermore, the high Mn content in this steel makes it difficult to achieve high yields in continuous casting. Slabs The segregation of Cu is accelerated, resulting in a decrease in the uniformity of the structure. Because the amount of Cu added is small, it is not possible to form a nanoscale Cu-rich phase to enhance the strength and strain strengthening ability of the steel sheet. Furthermore, the impact energy of steel sheets manufactured by this method is verified only at -5°C, and impact energy at -40°C is not reflected, limiting its application range.

[0006] The Chinese patent application with application number 202111254001.5 discloses a "method for manufacturing a medium-thickness steel plate with high strength and low yield ratio and a yield strength of 690 MPa." In this method, the plate material after hot rolling is pre-heated at 300-650°C for 60 minutes or more, and then further heated in the two-phase region for 30-120 minutes, followed by water quenching. Finally, the plate material is heated to 200-450°C and tempered at a medium to low temperature, thereby obtaining a steel plate with a yield strength of 690 MPa or more and a yield ratio of 0.85 or less. Because the steel contains a large amount of Mn, continuous casting Slabs The segregation of Cu is accelerated, resulting in a decrease in the uniformity of the structure. Because the amount of Cu added is low, the effect of Cu in improving the strength and strain strengthening ability of the steel sheet is low. Because the amount of V added is high, the size of V-containing precipitates is too large, which weakens their ability to inhibit dislocation motion and makes it impossible to improve the strain strengthening ability of the steel sheet. Furthermore, an increase in V content deteriorates the impact toughness of the heat-affected zone of the steel sheet, resulting in an increase in alloy costs. Furthermore, since this method requires the sheet material to be heated twice after hot rolling, the steel sheet manufacturing cycle is long and the manufacturing costs are high.

[0007] The Chinese patent application with application number 201210348440.7 discloses "Ultra-high strength and high toughness steel plate for marine construction and its manufacturing method," and in this method, A c3The above process is then subjected to austenitic phase quenching at a quenching temperature of 900-920°C, followed by tempering at 600-630°C to obtain ultra-high strength, high toughness steel plates for marine construction. These steel plates have a yield strength of 710-800 MPa, a tensile strength of 770-840 MPa, and a transverse impact energy of ≥ 90 J at -40°C. This method uses a tempering process, which requires the steel plate to be heated twice, resulting in high energy consumption and low manufacturing efficiency. Furthermore, the small amount of Cu added makes it impossible to form nanoscale Cu-rich phases to enhance the strength and strain strengthening ability of the steel plate.

[0008] A Chinese patent application with application number 202111253774.1 discloses a "960 MPa-class low yield ratio marine steel plate and its manufacturing method." This method uses a dual-phase annealing, full austenitizing quenching, and medium-low temperature tempering process to produce a high-toughness, low-yield ratio medium-thickness steel plate with a yield strength of 960 MPa, a tensile strength of 1100 MPa, and an impact toughness of 69 J at -40°C. This method requires multiple heating steps, resulting in a complex manufacturing process, high energy consumption, and low manufacturing efficiency. Furthermore, the small amount of Cu added prevents the formation of nanoscale Cu-rich phases to enhance the strength and strain strengthening ability of the steel plate.

[0009] A Chinese patent application with application number 202210648955.2 discloses a "marine atmosphere corrosion-resistant high-strength steel and its manufacturing method," which combines micro-magnesium treatment, titanium micro-alloying, controlled rolling, and controlled cooling to obtain marine atmosphere corrosion-resistant steel with high strength, a low yield ratio, and excellent toughness. The steel has a yield strength of 600-700 MPa, a tensile strength of 750-850 MPa, and an impact energy of ≥ 100 J at -20°C. The method requires a Si content of 0.6%-0.8%, which is too high and reduces the plasticity and toughness of the weld metal. The Mn content is controlled to 1.4%-1.7%, which is too high and reduces the plasticity and toughness of the weld metal during continuous casting. SlabsThe segregation of Cu is accelerated, resulting in a decrease in the uniformity of the structure. Because the amount of Cu added is low, the effect of Cu in improving the strength and strain strengthening ability of the steel sheet is low. Summary of the Invention [Problem to be solved by the invention]

[0010] This invention provides a marine steel with a low yield ratio and a yield strength of ≥ 750 MPa, as well as a manufacturing method for the same. The steel plate manufactured in this invention has a microstructure consisting of a mixed structure of martensite (hard phase), bainite (soft phase), and nanoscale precipitates, achieved through composite strengthening with Cu-Mo-Nb-V-Ti and adjustment of controlled rolling and controlled cooling parameters. This steel plate possesses high strain strengthening capability, a low yield ratio, and excellent weldability. This invention employs the TMCP process, which does not require complex tempering processes, has low manufacturing costs, and is highly efficient. [Means for solving the problem]

[0011] In order to achieve the above object, the present invention employs the following means. Low yield ratio marine steel with a yield strength of 750 MPa or higher has a chemical composition, by weight, of the steel plate: C 0.06%-0.10%, Si 0.1%-0.2%, Mn 0.60%-1.0%, P≦0.015%, S≦0.005%, Cu 0.62%-1.20%, Cr 0.20%-0.50%, Ni 0.50%-1.20%, Mo 0.30%-0.70%, Nb≦0.06%, V 0.02%-0.05%, Ti≦0.02%, Al≦0.04%, with the balance being Fe and unavoidable impurities.

[0012] Furthermore, the microstructure of the steel sheet is a mixed structure of martensite, bainite, and nanoscale precipitates, of which the martensite structure accounts for 35% to 45% and the bainite structure accounts for 55% to 65%; the martensite structure is uniformly distributed in the bainite matrix, and the nanoscale precipitates are uniformly dispersed throughout the microstructure. Furthermore, the steel plate has a yield strength of ≥ 750 MPa, a tensile strength of ≥ 1050 MPa, a yield ratio of ≤ 0.72, and an impact energy in the transverse direction at -40°C of ≥ 100 J.

[0013] The manufacturing method of low yield ratio marine steel with yield strength ≥ 750MPa is as follows: Slabs Slow cooling, Slabs The process involves the following steps: reheating, controlled rolling, controlled cooling, and stacked slow cooling. 1) Slabs Reheating and descaling: continuous casting after cooling Slabs Reheat to a heating temperature of T F The temperature is set to 1150℃ to 1250℃, and the total time in the furnace is t F After heating, descale with high-pressure water, and then perform continuous casting after descale. Slabs Temperature T s ≥ 1120°C; 2) Rough rolling: After descaling, the first stage of rough rolling is performed at the final rolling temperature T Rf ≥ 1000°C; 3) Finish rolling: After rough rolling is completed, the second stage of finish rolling is performed, and the rolling start temperature of the finish rolling is T Fs ≦900℃, final rolling temperature T Ff ≥ 850°C; 4) Laminar flow cooling: Direct laminar flow cooling is performed after steel plate rolling is completed; cooling start temperature T Cs The temperature is set to 820-850°C, and the cooling rate R C is controlled at 10℃ / s to 20℃ / s, and the self-tempering temperature T Cf Control the temperature to 300-350℃; 5) Stacked slow cooling: After the steel plate has finished air cooling, it is immediately placed in a slow cooling pit and slowly cooled to room temperature in stacks, with the stacked slow cooling time tC ≥ 12 hours. [Effects of the Invention]

[0014] The present invention has the following advantageous effects compared to the prior art. 1) The key to lowering the yield ratio of steel sheets is to adjust the ratio of soft and hard phases in the steel sheet's microstructure and to give the structure high strain hardening ability. Furthermore, to ensure high yield strength, high tensile strength, and high toughness in steel sheets, it is also necessary to precisely adjust and control the strength of each of the soft and hard phases in the steel and the balance between the phases. In the present invention, by using Cu-Mo-Nb-V-Ti composite strengthening and adjusting the controlled rolling and controlled cooling parameters, the steel plate produced has a microstructure with a mixed structure of martensite (hard phase), bainite (soft phase), and nanoscale precipitates, of which martensite accounts for 35% to 45% and bainite accounts for 55% to 65%. The martensite structure is uniformly distributed in the bainite matrix, and the nanoscale precipitates are uniformly dispersed throughout the microstructure. This steel plate has high strain strengthening ability, a low yield ratio, and good weldability. The steel plate has a yield strength of ≥ 750 MPa, a tensile strength of ≥ 1050 MPa, a yield ratio of ≤ 0.72, and a transverse impact energy of ≥ 100 J at -40°C. 2) The present invention employs the TMCP process, which does not require a complicated tempering process, has low manufacturing costs, and is highly efficient. 3) The present invention is not limited to marine steel, but can also be applied to other high-strength steel plates, such as steel for high-rise buildings, steel for bridges, steel for construction machinery, steel for pressure vessels, etc. DETAILED DESCRIPTION OF THE INVENTION

[0015] The low yield ratio marine steel of the present invention has a yield strength of 750 MPa or more and has a chemical composition, in weight percentages, of the steel plate: C 0.06% to 0.10%, Si 0.1% to 0.2%, Mn 0.60% to 1.0%, P≦0.015%, S≦0.005%, Cu 0.62% to 1.20%, Cr 0.20% to 0.50%, Ni 0.50% to 1.20%, Mo 0.30% to 0.70%, Nb≦0.06%, V 0.02% to 0.05%, Ti≦0.02%, Al≦0.04%, and the balance being Fe and unavoidable impurity elements. Furthermore, the microstructure of the steel sheet is a mixed structure of martensite, bainite, and nanoscale precipitates, of which the martensite structure accounts for 35% to 45% and the bainite structure accounts for 55% to 65%; the martensite structure is uniformly distributed in the bainite matrix, and the nanoscale precipitates are uniformly dispersed throughout the microstructure. Furthermore, the steel plate has a yield strength of ≥ 750 MPa, a tensile strength of ≥ 1050 MPa, a yield ratio of ≤ 0.72, and an impact energy in the transverse direction at -40°C of ≥ 100 J.

[0016] The method for producing a low yield ratio marine steel having a yield strength of 750 MPa or more according to the present invention comprises the steps of smelting, continuous casting, casting, Slabs Slow cooling, Slabs The process involves the following steps: reheating, controlled rolling, controlled cooling, and stacked slow cooling. 1) Slabs Reheating and descaling: continuous casting after cooling Slabs Reheat to a heating temperature of T F The temperature is set to 1150℃ to 1250℃, and the total time in the furnace is t F After heating, descale with high-pressure water, and then perform continuous casting after descale. Slabs Temperature T s ≥ 1120°C; 2) Rough rolling: After descaling, the first stage of rough rolling is performed at the final rolling temperature T Rf ≥ 1000°C; 3) Finish rolling: After rough rolling is completed, the second stage of finish rolling is performed, and the rolling start temperature of the finish rolling is T Fs ≦900℃, final rolling temperature T Ff ≥ 850°C; 4) Laminar flow cooling: Direct laminar flow cooling is performed after steel plate rolling is completed; cooling start temperature T Cs The temperature is set to 820-850°C, and the cooling rate R C is controlled at 10℃ / s to 20℃ / s, and the self-tempering temperature T Cf Control the temperature to 300-350℃; 5) Stacked slow cooling: After the steel plate has finished air cooling, it is immediately placed in a slow cooling pit and slowly cooled to room temperature in stacks, with the stacked slow cooling time tC ≥ 12 hours.

[0017] The functions and ranges of the main alloying elements in the low yield ratio marine steel with a yield strength of 750 MPa or more according to the present invention are selected as follows: Regarding carbon (C), C is the second most important element in steel after iron and directly affects the strength, plasticity, toughness, weldability, and other properties of steel sheet. C can effectively increase the strength and hardenability of steel sheet, but an excessively high C content adversely affects the plasticity, toughness, and weldability of steel sheet. For this reason, in the present invention, the C content range is set to 0.06% to 0.10%. Regarding silicon (Si), Si is an important reducing agent and deoxidizer in the steelmaking process, and can increase the hardness and strength of steel sheets through solid solution strengthening. However, if the Si content is too high, it reduces the plasticity and toughness of the weld metal. Therefore, in the present invention, the Si content range is set to 0.1% to 0.2%. Manganese (Mn) can form an infinite solid solution with Fe, which can increase the strength of steel plates while ensuring that the steel has sufficient plasticity and toughness. Therefore, Mn is widely used as a strengthening element in steel. Mn can react with S in steel to form MnS, which can eliminate the harmful effects of S. However, too high a 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 sheet, reduces the uniformity of the steel sheet structure, and is detrimental to the lamellar tear resistance, plasticity, low-temperature toughness, and weldability of the steel sheet. For this reason, in the present invention, the Mn content range is set to 0.6% to 1.0%.

[0018] Regarding niobium (Nb), Nb is one of the most important microalloying elements, and some of it dissolves in the matrix to enhance solid-solution strengthening. During controlled rolling, the dissolved Nb significantly increases the recrystallization temperature of the steel sheet, allowing the steel sheet to be rolled at a higher temperature, thereby reducing the internal stress of the steel sheet. The remaining Nb forms fine carbides and nitrides, inhibiting austenite recrystallization and maintaining strain effects to refine ferrite grains, thereby improving the strength and impact toughness of the steel sheet and lowering its brittle transition temperature. Nanoscale Nb-containing precipitates inhibit dislocation motion and enhance the strain strengthening ability of the steel sheet. In the present invention, the Nb content is set to 0.06% or less. Regarding vanadium (V), V is a strong carbonitride-forming element that refines the structure and crystal grains, increases strength and toughness, improves weldability, and reduces overheat susceptibility. Nanoscale V-containing precipitates inhibit dislocation motion and can enhance the strain strengthening ability of steel sheets. However, if the V content is too high, the V-containing precipitates become large in size, adversely affecting the strain strengthening ability of the steel sheet and deteriorating the impact toughness of the weld heat-affected zone. Therefore, in the present invention, the V content range is set to 0.02% to 0.05%.

[0019] Regarding titanium (Ti), Ti is a strong carbonitride-forming element. Ti-containing precipitate phases can effectively pin grain boundaries, hinder austenite growth, refine grains, and improve the strength and low-temperature toughness of steel sheets. Nanoscale Ti-containing precipitate phases can inhibit dislocation motion and improve the strain strengthening ability of steel sheets. However, if the Ti content is too high, the Ti-containing precipitate phases will coarsen, adversely affecting the performance of the steel sheets. Therefore, in the present invention, the Ti content is set to 0.02% or less. Copper (Cu) can increase the strength and hardenability of steel sheets, suppress ferrite transformation during the cooling process, and do not adversely affect weldability. When the Cu content exceeds a certain amount, nano-sized Cu-rich phases are formed in the steel sheet, increasing the strength of the steel sheet and inhibiting dislocation motion during deformation, thereby improving the strain strengthening ability of the steel sheet. However, an excessively high Cu content is disadvantageous for hot deformation processing and can cause copper embrittlement during hot deformation processing. Therefore, in the present invention, the Cu content range is set to 0.62% to 1.20%. Regarding chromium (Cr), Cr can improve the hardenability, strength, hardness, and wear resistance of steel sheet, but it also reduces the elongation and area reduction rate. If too much Cr is added, Cr-containing carbides precipitate at prior austenite grain boundaries during the welding heat cycle process, coagulate, and grow, significantly impairing the low-temperature toughness and weldability of the steel sheet. Therefore, in the present invention, the Cr content range is set to 0.20% to 0.50%.

[0020] 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 less susceptible to overheating, prevents grain growth at high temperatures, and maintains a fine grain structure. However, taking cost factors into consideration, the present invention specifies a Ni content range of 0.50% to 1.20%. Molybdenum (Mo) improves the hardenability and hot strength of steel, suppresses ferrite transformation during the cooling process of steel sheets, increases the dislocation density within crystal grains over a wider cooling range, and improves the strain strengthening ability of steel sheets. However, if the Mo content is too high, weldability deteriorates and alloy costs increase. In the present invention, the Mo content range is set to 0.30% to 0.70%.

[0021] Regarding aluminum (Al), Al is an essential deoxidizing element, which refines the grain size and fixes the N in the steel, thereby significantly improving the impact toughness of the steel plate and reducing the tendency to cold brittleness and aging.Al can also improve the corrosion resistance of the steel, and this effect is even better when used in combination with elements such as Mo, Cu, Si, and Cr.However, if the Al content is too high, the casting Slabs Therefore, in the present invention, the Al content range is set to 0.04% or less. 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 plates, but it also causes a significant decrease in plasticity and impact toughness, and significantly embrittles steel materials, especially at low temperatures. The higher the P content, the greater the cold brittleness. However, removing P to a low level significantly increases the cost of steelmaking. Therefore, in the present invention, the P content range is set to 0.015% or less. 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.

[0022] The control range of the main manufacturing process parameters of the low yield ratio marine steel with yield strength ≧ 750 MPa according to the present invention is due to the following reasons: In the present invention, composite strengthening of steel is achieved by elements such as Cu, Mo, Nb, V, and Ti, and the steel is then continuously cast. SlabsBy controlling the heating temperature between 1150 and 1250°C and the total time in the furnace between 3 and 6 hours, it is possible to ensure that the precipitated phases of the alloying elements are fully redissolved into austenite, and in the subsequent controlled rolling process, the effective effects of suppressing recrystallization, solid solution strengthening, precipitation strengthening, grain refinement, and improving the strain strengthening ability of the steel sheet are fully exerted, and the composition and temperature are prepared to obtain the final structure. If the heating temperature and heating time are lower than the selected range, solid solution will be insufficient, affecting the final steel sheet strength and strain strengthening ability of the steel sheet. If the heating time and heating temperature are higher than the selected range, the continuous casting Slabs The prior austenite grains are likely to become coarse, which is disadvantageous in controlling the toughness of the steel plate.

[0023] continuous casting Slabs After the steel sheet is removed from the furnace, it is first descaled using high-pressure water to ensure the quality of the rolled surface. If the temperature after descaling is lowered below 1120°C, the rolling load during the rolling stage increases and the austenite recrystallization effect decreases, affecting the refinement of grains. The refinement of austenite grains ensures that the untransformed austenite in the steel sheet has sufficient grain boundaries for martensite nucleation during the laminar cooling process and after the bainite transformation is complete, ultimately resulting in a uniform distribution of martensite in the bainite. Rolling is carried out in two stages. The rough rolling stage is rolling in the austenite recrystallization temperature range, and the reason for completing the rolling at 1000°C or higher is to avoid entering the partial recrystallization temperature range and causing the crystal grain size to become uneven. By completing the rolling in a high temperature range, the deformation conditions of the rolled material become good and the amount of reduction in each pass is improved. The intermediate thickness is more than twice the thickness of the finished steel plate. Slabs This is to ensure a sufficient cumulative reduction in the second rolling stage and to sufficiently flatten the recrystallized austenite grains, which is advantageous for the subsequent structural transformation and grain refinement. The finish rolling stage is rolling in the non-recrystallized region, and the rolling temperature range is 850 to 900°C. If the temperature is higher than 900°C, the rolled part may enter the partial recrystallization region and the crystal grains may become non-uniform. On the other hand, if the temperature is lower than 850°C, it is difficult to ensure the starting temperature required for the subsequent direct accelerated cooling.

[0024] After rolling, accelerated cooling is initiated at 820-850°C at a cooling rate of 10-20°C / s, using laminar cooling to ensure that the steel sheet transforms into a martensite-bainite mixed structure, with martensite accounting for 35-45% and bainite accounting for 55-65%, and that the martensite structure is uniformly distributed throughout the bainite matrix. If the cooling rate is too fast, the martensite content in the steel sheet will be too high, resulting in a high yield ratio and reduced low-temperature toughness. If the cooling rate is too slow, the steel sheet will not be supercooled sufficiently, making it difficult for martensite to form, ultimately resulting in low tensile strength. The self-tempering temperature of the steel plate is controlled between 300 and 350°C. The steel plate is stacked in a cooling pit and slowly cooled to room temperature. This low-temperature tempering reduces the carbon content in martensite and bainite, releases residual stress in the steel, and produces uniformly dispersed Cu-rich nanoscale precipitates throughout the microstructure. Higher tempering temperatures than the selected temperature make it difficult to ensure complete transformation, affecting the balance between strength and toughness of the final steel plate and microstructural control. Lower tempering temperatures and slow cooling times result in insufficient low-temperature tempering, resulting in insufficient Cu-rich nanoscale precipitates dispersed throughout the steel plate. This results in the steel plate's workability being insufficient, the residual stress in the steel being unable to be released, and the steel plate's toughness being reduced. The smelting process includes hot metal pretreatment, converter smelting, extra-furnace refining, and vacuum treatment. The continuous casting process involves full-scale protected casting, and one or more of electromagnetic stirring, soft reduction, and heavy reduction are performed. [Example]

[0025] The following examples are carried out according to the technical means of the present invention, and show detailed embodiments and specific operation procedures, but do not limit the protection scope of the present invention. The methods used in the following examples are conventional methods unless otherwise specified. Table 1 shows the chemical composition of the steel sheet in this example, Table 2 shows the rolling and heat treatment process parameters of the steel sheet in this example, and Table 3 shows the mechanical properties of the steel sheet in this example.

[0026] [Table 1]

[0027] [Table 2]

[0028] [Table 3]

[0029] From the data in Tables 1, 2 and 3, it can be seen that the steel sheets manufactured by the manufacturing method of the present invention have a yield strength of ≥ 750 MPa, a tensile strength of ≥ 1050 MPa, a yield ratio of ≤ 0.72, and an impact energy in the transverse direction at -40°C of ≥ 100 J, and therefore have a low yield ratio and small variation over the entire range of the process window. The above are merely preferred embodiments of the present invention, and the scope of protection of the present invention is not limited thereto. Any equivalent replacements or modifications made by those skilled in the art based on the technical means and inventive ideas of the present invention within the technical scope disclosed in the present invention are all included in the scope of protection of the present invention.

Claims

1. A method for producing marine steel having a yield strength ≥ 750 MPa and a yield ratio ≤ 0.72, and having chemical components, in mass percentage, of C 0.06% to 0.10%, Si 0.1% to 0.2%, Mn 0.60% to 1.0%, P ≤ 0.015%, S ≤ 0.005%, Cu 0.62% to 1.20%, Cr 0.20% to 0.50%, Ni 0.50% to 1.20%, Mo 0.30% to 0.70%, Nb ≤ 0.06%, V 0.02% to 0.05%, Ti ≤ 0.02%, Al ≤ 0.04%, and the balance being Fe and unavoidable impurity elements, A method for producing marine steel, the production process including smelting, continuous casting, slow cooling of the cast slab, slab reheating, controlled rolling and controlled cooling, and slow cooling in a stack, and characterized in having the following steps: 1) Slab reheating and descaling: the continuously cast slab after cooling is reheated to a heating temperature TF of 1150°C to 1250°C, and the total time tF in the furnace is 3 to 6 hours; after heating is completed, descaling is performed using high-pressure water, and the temperature of the continuously cast slab after descaling Ts is 1120°C or higher; 2) Rough rolling: After descaling, the first stage of rough rolling is performed, and the final rolling temperature of the rough rolling is TRf≧1000°C; 3) Finish rolling: After rough rolling is completed, second-stage finish rolling is performed, and the rolling start temperature of the finish rolling is set to TFs≦900°C and the final rolling temperature TFf≧850°C; 4) Laminar cooling: After the steel plate is rolled, laminar cooling is carried out directly; the cooling start temperature TCs is set to 820-850°C, the cooling rate RC is controlled to 10°C / s-20°C / s, and the self-tempering temperature TCf is controlled to 300-350°C; 5) Slow cooling in layers: After air cooling of the steel sheets is completed, they are immediately placed in a slow cooling pit and slowly cooled in layers to room temperature, with the time for slow cooling in layers being tC ≥ 12 hours.

2. 2. The method for manufacturing marine steel according to claim 1, wherein the marine steel has a tensile strength of ≥ 1050 MPa and an impact energy in the transverse direction at -40°C of ≥ 100 J.

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