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, marine 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. In addition, marine engineering equipment is difficult to maintain, so it has a long service life and high requirements for steel materials. Currently, steel plates for marine structures are becoming stronger, thicker, and larger. As the strength of steel material increases, the ratio of its yield strength to its tensile strength (yield ratio) tends to increase. When the yield ratio of a material increases, the stress from the time when plastic deformation occurs (yield point) to the time when destruction occurs does not change much. In other words, there is not much time for marine facilities to absorb energy by deformation and prevent destruction. When a huge external force such as an earthquake or tsunami acts on marine facilities, it becomes difficult to ensure the safety of the facilities. Therefore, steel for marine construction equipment structures needs to simultaneously satisfy both the requirements of high strength and low yield ratio. In view of this situation, the present invention has 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 impact energy in the lateral direction 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 manufacturing method thereof", in which a composite structure of tempered sorbite + bainite is obtained by secondary quenching, and the yield strength is ≧690 MPa, the tensile strength is ≧770 MPa, and the yield ratio is ≦0.88. This method adopts a secondary quenching process in which the steel plate is heated twice, so the energy consumption in the manufacturing process is large and the manufacturing efficiency is low. In addition, since the C content is controlled to 0.1% to 0.2%, it is difficult to meet the requirements for welding of marine steel, and the welding difficulty of the steel plate is high. Although the yield ratio of the steel plate is ≦0.88, as is clear from the examples, the yield ratios of the manufactured steel plates are all higher than 0.86, which is significantly different from the yield ratio (≦0.72) of the steel plate according to the present invention, and it is difficult to ensure that the marine facility absorbs a lot of energy in the process of 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 ≦0.8 is obtained by the process of controlled rolling, controlled cooling and slow cooling of the steel plate. The Si content is controlled to 0.2%-0.4%, and the content is too high, which reduces the plasticity and toughness of the heat-affected zone of the steel plate. The Mn content is controlled to 1.45%-1.65%, and the content is too high, which accelerates the segregation of the continuous casting billet and reduces the uniformity of the structure. The Cu content is controlled to 0.15%-0.3%, and the content is too low, which prevents the formation of nano-sized Cu-rich precipitation phase in the steel plate, and the effect of Cu in improving the strength and strain strengthening ability of the steel plate is low. The steel plate produced by this method has a yield strength of 420 MPa or more, which is two or more grades lower than the steel plate produced by the method according to the present invention, which has a yield strength of 750 MPa or more, making it difficult to satisfy the requirement for high strength.

[0005] The Chinese patent application with application number 201780071626.3 discloses a "low yield ratio, ultra-high strength steel material and its manufacturing method", in which the steel plate is first cut into A r3 Cool to below temperature and then Bs The steel plate has a yield ratio ≦0.85 and a tensile strength ≧800MPa by two cooling rates, which are below the temperature. The controlled cooling process of this method is relatively complicated, and it is difficult to accurately control the end temperature of the primary cooling in actual on-site production; the cooling rate of the secondary cooling needs to be more than 30℃ / s, which requires high requirements for equipment capacity and poor process applicability. In addition, the steel contains a large amount of Mn, which accelerates the segregation of the continuous casting billet and reduces the uniformity of the structure. The amount of Cu added is small, so it is not possible to form a nano-scale Cu-rich phase to increase the strength and strain strengthening ability of the steel plate. In addition, the steel plate produced by this method is only verified for impact energy at -5℃, and does not reflect the impact energy at -40℃, which limits its application range.

[0006] A Chinese patent application with application number 202111254001.5 discloses a "method for producing a medium-thickness steel plate with high strength and low yield ratio of 690MPa-grade yield strength", in which the plate material after hot rolling is pre-heated at 300-650°C for 60min or more, further heated in the two-phase region for 30-120min, water quenched, and finally heated to 200-450°C for medium-low temperature tempering to obtain a steel plate with a yield strength of ≥ 690MPa and a yield ratio of ≤ 0.85. The high Mn content in the steel accelerates the segregation of the continuous casting billet and reduces the uniformity of the structure. The low Cu content reduces the effect of Cu in improving the strength and strain strengthening ability of the steel plate. Due to the high amount of V added, the size of V-containing precipitates is too large, and the dislocation motion inhibition ability is weak, so the strain strengthening ability of the steel sheet cannot be improved, and the impact toughness of the heat-affected zone of the steel sheet is deteriorated with an increase in V, and the alloy cost increases. In addition, the method requires heating the plate material twice after hot rolling, so the manufacturing cycle of the steel sheet is long and the manufacturing cost is 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", which includes A c3The above is subjected to austenitic phase quenching treatment, the quenching temperature is set to 900-920°C, and the quenching is tempered at 600-630°C to obtain a steel plate for marine engineering with ultra-high strength and high toughness. The steel plate has a yield strength of 710-800MP, a tensile strength of 770-840MPa, and a transverse impact energy of ≧90J at -40°C. This method uses a tempering process and heats the steel plate twice, so the energy consumption in the manufacturing process is large and the manufacturing efficiency is low. In addition, because the amount of Cu added is small, it is not possible to form a nano-scale Cu-rich phase to increase the strength and strain strengthening ability of the steel plate.

[0008] A Chinese patent application with application number 202111253774.1 discloses a "low yield ratio marine steel plate with a yield strength of 960 MPa and its manufacturing method", in which a high toughness and 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 is manufactured by a process of dual-phase annealing + full austenitizing quenching + medium-low temperature tempering treatment. This method requires heating the steel plate multiple times, which results in a complicated manufacturing process, high energy consumption during the manufacturing process, and low manufacturing efficiency. In addition, the amount of Cu added is small, so it is not possible to form a nanoscale Cu-rich phase 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 atmospheric corrosion-resistant high-strength steel and its manufacturing method", in which a marine atmospheric corrosion-resistant steel with high strength, low yield ratio and excellent toughness is obtained by a process combining micro-magnesium treatment, titanium micro-alloying, and controlled rolling and controlled cooling. 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 has a Si content of 0.6%-0.8%, which is too high, resulting in a decrease in the plasticity and toughness of the weld metal. The Mn content is controlled to 1.4%-1.7%, which is too high, resulting in an acceleration of segregation of the continuous casting billet and a decrease in the uniformity of the structure. The Cu content is low, resulting in a low effect of Cu in improving the strength and strain strengthening ability of the steel plate. Summary of the Invention [Problem to be solved by the invention]

[0010] The present invention provides a marine steel with a low yield ratio and a yield strength of ≥ 750MPa and a manufacturing method thereof. In the present invention, the steel plate manufactured by composite strengthening with Cu-Mo-Nb-V-Ti and adjusting the controlled rolling and controlled cooling parameters has a microstructure of martensite (hard phase) - bainite (soft phase) - nanoscale precipitates mixed structure, and has high strain strengthening ability, low yield ratio, and good weldability. The present invention adopts the TMCP process, which does not require a complicated tempering process, 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 ≥ 750MPa has the following chemical composition by weight: 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%, and the balance being Fe and unavoidable impurity elements.

[0012] Furthermore, the microstructure of the steel plate is a mixed structure of martensite + bainite + 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 distributed 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 has the following steps: the manufacturing process includes smelting, continuous casting, casting billet slow cooling, billet reheating, controlled rolling, controlled cooling and stacking slow cooling. 1) Billet reheating and descaling: The cooled continuous casting billet is reheated at 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, descaling is performed using high-pressure water. The temperature of the descaled continuous casting billet is T s ≧1120℃; 2) Rough rolling: After descaling, the first stage of rough rolling is performed at the final rolling temperature T Rf ≧1000℃; 3) Finish rolling: After rough rolling is completed, the second stage of finish rolling is performed at the rolling start temperature T Fs ≦900℃, final rolling temperature T Ff ≧850℃; 4) Laminar flow cooling: Direct laminar flow cooling is performed after steel sheet rolling is completed; cooling start temperature T Cs The temperature is set to 820 to 850°C, and the cooling rate R C is controlled to 10℃ / s~20℃ / s, and the self-tempering temperature T Cf Control the temperature to 300-350℃; 5) Stacked cooling: After air cooling of the steel plates is completed, they are immediately placed in a cooling pit and cooled slowly in stacks to room temperature. The stacked cooling time tC is ≥ 12 hours. Effect 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 plate is to adjust the ratio of soft and hard phases in the microstructure of the steel plate and to give the structure high strain hardening ability. In addition, to ensure high yield strength, high tensile strength and high toughness of the steel plate, 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 microstructure of the steel plate produced is a mixed structure of martensite (hard phase)-bainite (soft phase)-nanoscale precipitates, of which martensite accounts for 35%-45% and bainite accounts for 55%-65%, the martensite structure is uniformly distributed in the bainite matrix, and the nanoscale precipitates are uniformly distributed throughout the microstructure. The steel plate has high strain strengthening ability, low yield ratio, and good weldability, and the steel plate has a yield strength of ≧750MPa, a tensile strength of ≧1050MPa, a yield ratio of ≦0.72, and an impact energy in the transverse direction at -40℃ of ≧100J. 2) The present invention adopts 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 PREFERRED EMBODIMENTS

[0015] The low yield ratio marine steel having a yield strength of ≥ 750 MPa according to the present invention has chemical compositions, by weight percentage, of a steel plate as follows: 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%, and the balance being Fe and unavoidable impurity elements. Furthermore, the microstructure of the steel plate is a mixed structure of martensite + bainite + 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 distributed 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 manufacturing method of the low yield ratio marine steel having a yield strength of ≧750 MPa according to the present invention includes a manufacturing process of smelting, continuous casting, slowly cooling the cast billet, reheating the billet, controlled rolling, controlled cooling and slowly cooling in a stack, and has the following steps: 1) Billet reheating and descaling: The cooled continuous casting billet is reheated at 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, descaling is performed using high-pressure water. The temperature of the descaled continuous casting billet is T s ≧1120℃; 2) Rough rolling: After descaling, the first stage of rough rolling is performed at the final rolling temperature T Rf ≧1000℃; 3) Finish rolling: After rough rolling is completed, the second stage of finish rolling is performed at the rolling start temperature T Fs ≦900℃, final rolling temperature T Ff ≧850℃; 4) Laminar flow cooling: Direct laminar flow cooling is performed after steel sheet rolling; cooling start temperature T Cs The temperature is set to 820 to 850°C, and the cooling rate R C is controlled to 10℃ / s~20℃ / s, and the self-tempering temperature T Cf Control the temperature to 300-350℃; 5) Stacked cooling: After air cooling of the steel plates is completed, they are immediately placed in a cooling pit and cooled slowly in stacks to room temperature. The stacked cooling time tC is ≥ 12 hours.

[0017] The functions and ranges of the main alloying elements in the low yield ratio marine steel with yield strength ≧750 MPa according to the present invention are explained 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 the steel plate. C can effectively increase the strength and hardenability of the steel plate, but an excessively high C content adversely affects the plasticity, toughness, and weldability of the steel plate. 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 deoxidizing agent in the steelmaking process, and can increase the hardness and strength of steel sheets through solid solution strengthening, but 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%. Regarding manganese (Mn), Mn can be infinitely dissolved with Fe, and can ensure that the steel has sufficient plasticity and toughness while increasing the strength of the steel plate. Therefore, Mn is widely used as a strengthening element in steel. Mn can react with S element in steel to form MnS, which can eliminate the harmful effect of S. However, too high Mn content accelerates the segregation of continuous casting billet, increases the grade of the banded structure of the steel plate, reduces the uniformity of the steel plate structure, and is disadvantageous to the lamellar tear resistance, plasticity, low temperature toughness and weldability of the steel plate. For this reason, in the present invention, the Mn content range is 0.6% to 1.0%.

[0018] Regarding niobium (Nb), Nb is one of the most important micro-alloying elements, and a part of Nb dissolves in the matrix to play the role of solid solution strengthening. In the controlled rolling process, the dissolved Nb significantly increases the recrystallization temperature of the steel plate, and the rolling process of the steel plate can be performed at a higher temperature range, thereby reducing the internal stress of the steel plate. The remaining Nb forms fine carbides and nitrides, inhibits the recrystallization of austenite, and keeps the strain effect to refine the ferrite grains, thereby increasing the strength and impact toughness of the steel plate and lowering its brittle transition temperature. The nano-scale Nb-containing precipitate phase can inhibit dislocation motion and increase the strain strengthening ability of the steel plate. In the present invention, the content of Nb is set to be 0.06% or less. Vanadium (V) is a strong carbonitride forming element, and has the effect of refining the structure and crystal grains, increasing strength and toughness, improving weldability, and reducing overheating sensitivity. Nanoscale V-containing precipitate phases can inhibit dislocation motion and increase 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 sheets and worsening the impact toughness of the welded 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. The Ti-containing precipitate phase can effectively pin the grain boundary, hinder the growth of austenite, refine the grains, and increase the strength and low-temperature toughness of the steel plate. The nanoscale Ti-containing precipitate phase can inhibit dislocation motion and increase the strain strengthening ability of the steel plate. However, if the Ti content is too high, the Ti-containing precipitate phase will become coarse, which will adversely affect the performance of the steel plate. Therefore, in the present invention, the Ti content is set to 0.02% or less. Copper (Cu) can increase the strength and hardenability of the steel sheet, inhibit ferrite transformation during the cooling process of the steel sheet, and do not adversely affect the weldability. When the Cu content exceeds a certain amount, nano-sized Cu-rich phases are generated in the steel sheet, increasing the strength of the steel sheet, inhibiting dislocation motion during deformation, and improving the strain strengthening ability of the steel sheet. However, an excessively high Cu content is disadvantageous to hot deformation processing and causes 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 increase the hardenability, strength, hardness, and wear resistance of the steel sheet, but it reduces the elongation and area reduction rate. If the amount of Cr added is too much, Cr-containing carbides precipitate at the prior austenite grain boundaries during the welding heat cycle process, coagulate and grow, and the low-temperature toughness and weldability of the steel sheet are significantly impaired. Therefore, in the present invention, the Cr content range is set to 0.20% to 0.50%.

[0020] Nickel (Ni) has the effect of stabilizing austenite and enhancing hardenability. Adding a certain amount of Ni to steel can increase strength, toughness, and corrosion resistance, and can lower the ductile-brittle transition temperature. Ni-containing steel is generally less susceptible to overheating, can prevent grain growth at high temperatures, and can maintain a fine grain structure. However, in consideration of cost factors, the present invention specifies the Ni content range as 0.50% to 1.20%. Molybdenum (Mo) improves the hardenability and hot strength of steel, inhibits ferrite transformation during the steel sheet cooling process, increases the dislocation density in crystal grains over a wider cooling range, and enhances the strain strengthening ability of the steel sheet. However, if the Mo content is too high, the weldability decreases and the alloy cost increases. In the present invention, the Mo content range is 0.30% to 0.70%.

[0021] Regarding aluminum (Al), Al is an essential deoxidizing element, and by refining crystal grains and fixing N in steel, it can significantly improve the impact toughness of the steel plate and reduce the tendency of cold brittleness and aging. In addition, Al can improve the corrosion resistance of steel, and the 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, heat cracking is likely to occur in the cast billet. Therefore, in the present invention, the Al content range is set to 0.04% or less. Phosphorus (P) is introduced into steel from ores and is a harmful element, similar to S. 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, and 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 is present 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 ≧750MPa according to the present invention is due to the following reasons: In the present invention, the composite strengthening of steel is achieved by elements such as Cu, Mo, Nb, V, and Ti, and the heating temperature of the continuous casting billet is controlled between 1150 and 1250 ° C, and the total time in the furnace is controlled between 3 and 6 h, thereby ensuring that the precipitation phase of the alloying elements is sufficiently redissolved into austenite, and 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 in the subsequent controlled rolling process, and the components and temperature are prepared to obtain the final structure. If the heating temperature and heating time are lower than the selected range, the solid solution will be insufficient, which will affect the final steel sheet strength and the strain strengthening ability of the steel sheet, and if the heating time and heating temperature are higher than the selected range, the prior austenite grains of the continuous casting billet are likely to become coarse, which is disadvantageous for controlling the toughness of the steel sheet.

[0023] After the continuous casting billet is removed from the furnace, it is first descaled by high pressure water to ensure the quality of the rolled surface of the steel plate. If the temperature after descaling is lower than 1120℃, the rolling load in the rolling stage will increase and the austenite recrystallization effect will decrease, which will affect the refinement of the grains. The refinement of the austenite grains can ensure that the untransformed austenite in the steel plate has enough grain boundaries for martensite nucleation during the laminar cooling process and after the bainite transformation is completed, and finally the martensite is uniformly distributed in the bainite. Two stages of rolling are performed. The rough rolling stage is rolling in the austenite recrystallization temperature range, and the rolling is completed at 1000°C or higher to avoid entering the partial recrystallization temperature range and causing the grain size to become uneven. Completing the rolling at a high temperature range improves the deformation conditions of the rolled material and improves the pass reduction. The intermediate billet is made at least twice the thickness of the finished steel plate in order to ensure the cumulative reduction in the second stage of rolling and to sufficiently flatten the recrystallized austenite grains. This 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 member 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 start temperature required for the subsequent direct accelerated cooling.

[0024] After the rolling of the steel plate is completed, in order to ensure that the steel plate transforms into a martensite-bainite mixed structure, with martensite accounting for 35%-45%, bainite accounting for 55%-65%, and the martensite structure is uniformly distributed in the bainite matrix, accelerated cooling is started at a temperature of 820-850°C, the cooling rate is 10°C / s-20°C / s, and laminar cooling is used as the cooling method. If the cooling rate is too fast, the martensite content in the steel plate will be too high, and the final material will have a high yield ratio and low-temperature toughness. If the cooling rate is too slow, the steel plate will not be supercooled enough to make it difficult to form martensite, and the tensile strength of the steel plate will ultimately be too low. The self-tempering temperature of the steel plate is controlled to 300-350°C, and the quenched structure is tempered at a low temperature during the process of the steel plate being stacked and cooled slowly to room temperature in the cooling pit, thereby reducing the carbon content in martensite and bainite, releasing the residual stress in the steel, and generating Cu-rich nanoscale precipitates uniformly distributed throughout the microstructure. If the self-tempering temperature is higher than the selected one, it is difficult to ensure the completeness of the transformation, which affects the balance of the strength and toughness of the final steel plate and the structure control. If the self-tempering temperature and the slow cooling time are lower than the selected one, the low-temperature tempering effect cannot be obtained, the Cu-rich nanoscale precipitates distributed in the steel plate are insufficient, the processing strengthening ability of the steel plate cannot be ensured, and the residual stress in the steel cannot be released, resulting in a decrease in the toughness of the steel plate. The smelting process includes hot metal pretreatment, converter smelting, furnace refining and vacuum treatment. The continuous casting process includes full-process protected casting, and one or more of electromagnetic stirring, soft reduction and heavy reduction are performed. EXAMPLES

[0025] The following examples are carried out according to the technical means of the present invention, showing detailed embodiments and specific operation procedures, but are not intended to limit the scope of protection 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 plate in this example, Table 2 shows the rolling and heat treatment process parameters of the steel plate in this example, and Table 3 shows the mechanical properties of the steel plate 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 plate produced by the production method of the present invention 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, and has 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 replacement or modification made by a person 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 is included in the scope of protection of the present invention.

Claims

1. The chemical composition of the steel plate, by mass percentage, is 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 is Fe and inevitable impurity elements. The marine steel is characterized by this.

2. The microstructure of the steel plate is a mixed structure of martensite + bainite + nano-scale precipitates; the martensite structure is uniformly distributed in the bainite matrix, and the nano-scale precipitates are uniformly dispersed throughout the microstructure. The marine steel according to Claim 1 is characterized by this.

3. The steel plate has a yield strength ≥ 750 MPa, a tensile strength ≥ 1050 MPa, a yield ratio ≤ 0.72, and a transverse impact energy at -40°C ≥ 100 J. The marine steel according to Claim 1 is characterized by this.

4. The manufacturing process includes steelmaking, continuous casting, slow cooling of the casting slab, reheating of the slab, controlled rolling and controlled cooling, and step-by-step slow cooling, and has the following steps. The manufacturing method of the marine steel according to any one of Claims 1 to 3 is characterized by this. 1) Slab reheating and scale removal: Reheat the continuously cast slab after cooling, and set the heating temperature T Ｆ to 1150°C to 1250°C, and set the total time t Ｆ in the furnace to 3 to 6 h; after the heating is completed, perform scale removal with high-pressure water, and ensure that the temperature T ｓ of the continuously cast slab after scale removal is ≥ 1120°C; 2) Rough rolling: After scale removal, perform the first-stage rough rolling, and set the final rolling temperature T of the rough rolling Ｒｆ ≥ 1000°C; 3) Finish rolling: After the rough rolling is completed, perform the second-stage finish rolling, and the rolling start temperature T of the finish rolling Ｆｓ ≤ 900 °C, and the final rolling temperature T Ｆｆ ≥ 850 °C; 4) laminar flow cooling: After the steel plate rolling is completed, laminar flow cooling is directly carried out; the cooling start temperature T Ｃｓ is set to 820 to 850 °C, and the cooling rate R Ｃ is controlled to be 10 °C / s to 20 °C / s, and the self-tempering temperature T Ｃｆ is controlled to be 300 to 350 °C; 5) Step-by-step slow cooling: After the air cooling of the steel plate is completed, immediately put it into a slow cooling pit and perform step-by-step slow cooling to room temperature, and make the time tC of step-by-step slow cooling ≥ 12 h.