WO2024082997A1 - 一种屈服强度≥750MPa的低屈强比海工钢及其生产工艺 - Google Patents

一种屈服强度≥750MPa的低屈强比海工钢及其生产工艺 Download PDF

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WO2024082997A1
WO2024082997A1 PCT/CN2023/123725 CN2023123725W WO2024082997A1 WO 2024082997 A1 WO2024082997 A1 WO 2024082997A1 CN 2023123725 W CN2023123725 W CN 2023123725W WO 2024082997 A1 WO2024082997 A1 WO 2024082997A1
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steel plate
steel
yield strength
temperature
cooling
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English (en)
French (fr)
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李天怡
刘文月
王超逸
王传军
安涛
臧岩
耿志宇
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鞍钢集团北京研究院有限公司
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C33/00—Making ferrous alloys
    • C22C33/04—Making ferrous alloys by melting
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/002—Bainite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/004—Dispersions; Precipitations
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P10/00—Technologies related to metal processing
    • Y02P10/20—Recycling

Definitions

  • the present invention relates to the technical field of offshore steel, and in particular to a low yield strength ratio offshore steel with a yield strength of ⁇ 750MPa and a production process thereof.
  • marine structural steel As a key structural material for marine engineering equipment, steel is widely used in offshore oil drilling platforms, offshore wind power, submarine pipelines and other facilities.
  • the service environment of marine structural steel is harsh. In addition to gravity loads, it also has to withstand wind loads, wave loads, ice loads, and earthquake loads. In addition, due to the difficulty of maintenance, the service life of marine engineering equipment is also long, and the requirements for steel are higher.
  • marine structural steel plates are gradually developing in the direction of high strength, large thickness, and large specifications.
  • yield ratio the ratio of its yield strength to tensile strength
  • the present invention has developed a low yield ratio marine steel with a yield strength ⁇ 750MPa, a tensile strength ⁇ 1050MPa, a yield ratio ⁇ 0.72, and a -40°C lateral impact energy ⁇ 100J.
  • the Chinese patent application with application number 202010235198.7 discloses "a kind of high-strength steel plate with a yield strength of 690MPa and a low yield ratio and a method for manufacturing the same".
  • a tempered bainite + bainite complex structure is obtained, with a yield strength of ⁇ 690MPa, a tensile strength of ⁇ 770MPa, and a yield ratio of ⁇ 0.88.
  • This method adopts a secondary quenching process to heat the steel plate twice, which consumes a lot of energy during the production process and has low production efficiency.
  • its C content is controlled at 0.1%-0.2%, which is difficult to meet the requirements of marine steel for welding, and the welding of steel plates is difficult.
  • the yield ratio of its steel plate is ⁇ 0.88, but it can be seen from the examples that the yield ratio of the steel plates produced by it is higher than 0.86, which is significantly different from the yield ratio of the steel plate of the present invention ( ⁇ 0.72), and it is difficult to ensure that the marine facilities absorb more energy during plastic deformation.
  • the Chinese patent application with application number 202110035527.8 discloses "a low yield ratio marine steel plate with excellent low temperature toughness and its manufacturing method".
  • a steel plate with a yield ratio ⁇ 0.8 is obtained.
  • Its Si content is controlled at 0.2%-0.4%. If the content is too high, the plasticity and toughness of the heat affected zone of the steel plate will be reduced.
  • Its Mn content is controlled at 1.45%-1.65%. If the content is too high, it will aggravate the segregation of the continuous casting billet and reduce the uniformity of the organization.
  • Its Cu content is controlled at 0.15%-0.3%.
  • the yield strength of the steel plate produced by this method is above 420MPa, which is more than two steel grades lower than the yield strength of the steel plate produced by the method of the present invention with a yield strength of more than 750MPa, and it is difficult to meet the high strength requirements.
  • the Chinese patent application with application number 201780071626.3 discloses "a low yield ratio ultra-high strength steel and its manufacturing method", which uses two cooling rates, that is, cooling the steel plate to below the A r3 temperature first and then cooling it to below the B s temperature to obtain a steel plate with a yield ratio ⁇ 0.85 and a tensile strength ⁇ 800MPa.
  • the cooling process of this method is relatively complicated, and it is difficult to accurately control the termination temperature of the first cooling in actual on-site production; and the cooling rate of the second cooling needs to be greater than 30°C/s, which has high requirements on equipment capacity and poor process applicability.
  • the addition of Mn to the steel will aggravate the segregation of the continuous casting billet and reduce the uniformity of the structure.
  • the Chinese patent application with application number 202111254001.5 discloses "a method for preparing medium and thick steel plates with high strength and toughness and low yield ratio of 690MPa yield strength".
  • the hot-rolled plate is pre-heated at 300-650°C for more than 60min, then water-quenched after heat preservation in the two-phase region for 30-120min, and finally heated to 200-450°C for medium-low temperature tempering, and a steel plate with a yield strength ⁇ 690MPa and a yield ratio ⁇ 0.85 is obtained.
  • the addition of Mn in the steel will aggravate the segregation of the continuous casting billet and reduce the uniformity of the structure.
  • the addition of Cu is low, and the improvement of the strength and strain strengthening ability of the steel plate by Cu is limited.
  • V is high, the size of the V-containing precipitates is too large, and the ability to hinder dislocation movement is weak, and the strain strengthening ability of the steel plate cannot be improved; and the large amount of V deteriorates the impact toughness of the heat-affected zone of the steel plate, and will increase the alloy cost.
  • this method heats the hot-rolled plate twice, and the production cycle of the steel plate is long and the production cost is high.
  • the Chinese patent application with application number 201210348440.7 discloses "an ultra-high strength and high toughness steel plate for marine engineering and its production method".
  • the ultra-high strength and high toughness steel plate for marine engineering is obtained by quenching in the austenite phase above A c3 , the quenching temperature is 900-920°C, and tempering at 600-630°C after quenching.
  • the yield strength of the steel plate is 710-800MP, the tensile strength is 770-840MPa, and the transverse impact energy at -40°C is ⁇ 90J.
  • This method adopts a quenching and tempering process to heat the steel plate twice, which consumes a lot of energy in the production process and has low production efficiency; Cu is added in small amounts, and it is impossible to improve the strength and strain strengthening ability of the steel plate by forming a nano-scale Cu-rich phase.
  • the Chinese patent application with application number 202111253774.1 discloses "a low yield ratio marine steel plate with a yield strength of 960MPa and a preparation method thereof".
  • a high-strength and toughness low yield ratio medium and thick steel plate with a compressive strength ⁇ 960MPa, a tensile strength ⁇ 1100MPa, and -40°C impact toughness ⁇ 69J is prepared through two-phase annealing + complete austenitization quenching + medium and low temperature tempering treatment process.
  • This method heats the steel plate multiple times, the production process is complicated, the energy consumption in the production process is high, and the production efficiency is low; the amount of Cu added is small, and the strength and strain strengthening ability of the steel plate cannot be improved by forming a nano-scale Cu-rich phase.
  • the Chinese patent application with application number 202210648955.2 discloses "A high-strength steel resistant to marine atmospheric corrosion and its production method".
  • a marine atmospheric corrosion-resistant steel with high strength, low yield ratio and excellent toughness is obtained.
  • the yield strength of the steel is 600-700MPa
  • the tensile strength is 750-850MPa
  • the impact energy at -20°C is ⁇ 100J.
  • the Si content in this method is 0.6%-0.8%. If the content is too high, the plasticity and toughness of the weld metal will be reduced.
  • the Mn content is controlled at 1.4%-1.7%. If the content is too high, the segregation of the continuous casting billet will be aggravated and the uniformity of the organization will be reduced.
  • the addition of Cu is low, and the improvement of the strength and strain strengthening ability of the steel plate by Cu is limited.
  • the present invention provides a low yield ratio marine engineering steel with a yield strength of ⁇ 750MPa and a production process thereof.
  • the microstructure of the produced steel plate is a mixed structure of martensite (hard phase)-bainite (soft phase)-nanoscale precipitates, which has higher strain strengthening ability and lower yield ratio, and has good weldability.
  • the TMCP process is adopted, and no complicated quenching and tempering process is required, so the production cost is low and the efficiency is high.
  • a low yield ratio marine steel with a yield strength ⁇ 750MPa the chemical composition of the steel plate is as follows by weight percentage: 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 is Fe and unavoidable impurity elements.
  • the microstructure of the steel plate is a mixed structure of martensite + bainite + nanoscale precipitates, wherein the martensite structure accounts for 35% to 45% and the bainite structure accounts for 55% to 65%; the martensite structure is evenly distributed in the bainite matrix, and the nanoscale precipitates are evenly dispersed in the entire microstructure.
  • the steel plate has a yield strength of ⁇ 750 MPa, a tensile strength of ⁇ 1050 MPa, a yield strength ratio of ⁇ 0.72, and a transverse impact energy of ⁇ 100 J at -40°C.
  • a production process of low yield strength ratio marine engineering steel with a yield strength of ⁇ 750MPa comprising: smelting, continuous casting, slow cooling of ingot, reheating of ingot, controlled rolling and controlled cooling, and stacking and slow cooling; wherein:
  • the present invention has the following beneficial effects:
  • the key to reducing the yield strength ratio of the steel plate is to adjust the ratio of the soft phase and the hard phase in its microstructure, and make the structure have a higher strain hardening ability; at the same time, in order to ensure the high yield strength, high tensile strength and high toughness of the steel plate, the strength of the soft and hard phases in the steel and the matching between the phases also need to be precisely controlled; the present invention uses Cu-Mo-Nb-V-Ti composite strengthening and adjusts the controlled rolling and controlled cooling parameters to make the microstructure of the produced steel plate a mixed structure of martensite (hard phase)-bainite (soft phase)-nanoscale precipitates, wherein the martensite accounts for 35%-45%, the bainite accounts for 55%-65%, the martensite structure is uniformly distributed in the bainite matrix, and the nanoscale precipitates are uniformly dispersed in the entire microstructure, which has a higher strain hardening ability and a lower yield strength ratio, and has good weldability; the yield strength of the steel plate
  • the present invention adopts the TMCP process, which does not require a complicated quenching and tempering process, has low production cost and high efficiency;
  • the present invention is not only applicable to marine engineering steel, but also to other high-strength steel plates, such as steel for high-rise buildings, steel for bridges, steel for engineering machinery, steel for pressure vessels, etc.
  • the low yield ratio marine steel with a yield strength of ⁇ 750MPa described in the present invention has the following chemical compositions 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 remainder is Fe and unavoidable impurity elements.
  • the microstructure of the steel plate is a mixed structure of martensite + bainite + nanoscale precipitates, wherein the martensite structure accounts for 35% to 45% and the bainite structure accounts for 55% to 65%; the martensite structure is evenly distributed in the bainite matrix, and the nanoscale precipitates are evenly dispersed in the entire microstructure.
  • the yield strength of the steel plate is ⁇ 750MPa
  • the tensile strength is ⁇ 1050MPa
  • the yield strength ratio is ⁇ 0.72
  • the transverse impact energy at -40°C is ⁇ 100J.
  • the production process of a low yield strength ratio marine engineering steel with a yield strength of ⁇ 750MPa described in the present invention comprises: smelting, continuous casting, slow cooling of ingots, reheating of ingots, controlled rolling and controlled cooling, and stacking and slow cooling; wherein:
  • Carbon C is the main element in steel after Fe, which directly affects the strength, plasticity, toughness and weldability of steel plates. C can effectively improve the strength and hardenability of steel plates, but too high a C content will have a negative impact on the plasticity, toughness and weldability of steel plates. Therefore, the present invention sets the C content range to 0.06% to 0.10%.
  • Si is an important reducing agent and deoxidizer in the steelmaking process, and can improve the hardness and strength of the steel plate through solid solution strengthening. However, if the Si content is too high, the plasticity and toughness of the weld metal will be reduced. Therefore, the present invention sets the Si content range to 0.1% to 0.2%.
  • Mn can be infinitely dissolved with Fe, which can improve the strength of the steel plate while ensuring that the steel has sufficient plasticity and toughness. Therefore, Mn is a strengthening element widely used in steel. Mn can react with the S element in steel to form MnS, which can eliminate the harmful effects of S. However, too high a Mn content will aggravate the segregation of the continuous casting billet, increase the grade of the banded structure of the steel plate, and cause the uniformity of the steel plate structure to deteriorate, which is unfavorable to the lamellar tearing resistance, plasticity, low-temperature toughness and welding performance of the steel plate. For this reason, the present invention sets the Mn content range to 0.6% to 1.0%.
  • Niobium Nb is one of the most important microalloying elements. Part of Nb is dissolved in the matrix to play a role in solid solution strengthening. During the controlled rolling process, the solid solution Nb significantly increases the recrystallization temperature of the steel plate, which can complete the rolling process of the steel plate in a higher temperature range, thereby reducing the internal stress of the steel plate; the remaining part of Nb forms fine carbides and nitrides, inhibits the recrystallization of austenite, maintains the deformation effect, and refines the ferrite grains, which can improve the strength and impact toughness of the steel plate and reduce The brittle transition temperature is lowered.
  • the nano-scale Nb-containing precipitate phase can hinder dislocation movement and improve the strain strengthening ability of the steel plate.
  • the present invention selects the Nb content to be no higher than 0.06%.
  • Vanadium V is a strong carbonitride-forming element, which has the effects of refining the structure and grains, improving strength and toughness, improving welding performance, and reducing overheat sensitivity. Nano-scale V-containing precipitates can hinder dislocation movement and improve the strain strengthening ability of steel plates. However, when the V content is too high, the V-containing precipitates are large in size, which is harmful to the strain strengthening ability of the steel plate and will deteriorate the impact toughness of the welding heat-affected zone. Therefore, the present invention selects a V content range of 0.02% to 0.05%.
  • Titanium 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 improve the toughness and low-temperature toughness of the steel plate.
  • the nano-scale Ti-containing precipitate phase can hinder the dislocation movement and improve the strain strengthening ability of the steel plate.
  • the present invention selects a Ti content of no more than 0.02%.
  • Copper Cu can improve the strength and hardenability of the steel plate, and can inhibit the ferrite transformation during the cooling process of the steel plate, and has no adverse effect on the welding performance.
  • the Cu content exceeds a certain amount, a nano-sized Cu-rich phase will be generated in the steel plate, which will increase the strength of the steel plate, hinder the dislocation movement during deformation, and improve the strain strengthening ability of the steel plate.
  • the present invention selects a Cu content range of 0.62% to 1.20%.
  • Chromium Cr can increase the hardenability, strength, hardness and wear resistance of the steel plate, but it will reduce the elongation and cross-sectional shrinkage. If too much Cr is added, during the welding thermal cycle, Cr-containing carbides will precipitate and aggregate and grow at the original austenite grain boundaries, seriously damaging the low-temperature toughness and weldability of the steel plate. Therefore, the present invention selects the Cr content range of 0.20% to 0.50%.
  • Nickel Ni has the function of stabilizing austenite and improving hardenability. Adding a certain amount of Ni to steel can improve strength, toughness, corrosion resistance, and reduce the tough-brittle transition temperature. Ni-containing steel is generally not easy to overheat. It can prevent the growth of grains at high temperatures and keep the fine grain structure. However, considering the cost factor, the present invention selects the Ni content range of 0.50% to 1.20%.
  • Molybdenum can improve the hardenability and heat resistance of steel and inhibit the ferrite transformation during the cooling process of the steel plate. It can increase the dislocation density inside the grains within a larger cooling range and improve the strain strengthening ability of the steel plate. However, when the Mo content is too high, it is not good for weldability and will increase the alloy cost.
  • the present invention selects the Mo content range of 0.30% to 0.70%.
  • Aluminum Al is a necessary deoxidizing element, and can refine grains and fix N in steel, thereby significantly improving the impact toughness of steel plates and reducing cold brittleness and aging tendencies; Al can also improve the corrosion resistance of steel, especially when used in combination with Mo, Cu, Si, Cr and other elements, the effect is better; but too high Al content is prone to hot cracks in the casting. Therefore, the Al content range selected in the present invention is not higher than 0.04%.
  • Phosphorus P is brought into steel from ores and is one of the harmful elements, similar to S. Although P can increase the strength and The hardness increases, but the plasticity and impact toughness decrease significantly. Especially at low temperatures, it makes the steel material significantly brittle. The higher the P content, the greater the cold brittleness. However, removing P to a lower level will significantly increase the steelmaking cost. Therefore, the present invention selects a P content range of no more than 0.015%.
  • Sulfur S comes from steelmaking ore and fuel coke, and is one of the most common harmful elements in steel. It is detrimental to the ductility, toughness, weldability and corrosion resistance of steel. If S exists in the form of FeS in steel, it can also cause hot brittleness during hot working.
  • the present invention selects the S content range to be no higher than 0.005%.
  • the present invention adopts elements such as Cu, Mo, Nb, V, and Ti to realize the composite strengthening of steel, controls the heating temperature of the continuous casting billet between 1150 and 1250°C, and the total furnace time between 3 and 6 hours, which can ensure that the precipitation phase of the alloy elements is fully dissolved back into the austenite, and fully exerts the beneficial effects of inhibiting recrystallization, solid solution strengthening, precipitation strengthening, grain refinement, and improving the strain strengthening ability of the steel plate in the subsequent controlled rolling process, so as to prepare the composition and temperature for obtaining the final organizational structure.
  • the solid solution will be insufficient, which will affect the final strength and strain strengthening ability of the steel plate; if the heating time and heating temperature are higher than the selected range, the original austenite grains of the continuous casting billet are prone to be too coarse, which is not conducive to the control of the toughness of the steel plate.
  • the continuous casting billet After the continuous casting billet is discharged from the furnace, it is first descaled with high-pressure water to ensure the surface quality of the steel plate after rolling. If the temperature after descaling is lower than 1120°C, the rolling load in the rolling stage will increase, and the austenite recrystallization effect will be reduced, affecting the grain refinement. Finer austenite grains can ensure that during the laminar cooling process and after the bainite transformation is completed, the untransformed austenite in the steel plate still has enough grain boundaries for martensite nucleation, and finally the martensite is evenly distributed in the bainite.
  • the rough rolling stage is rolling in the austenite recrystallization temperature zone.
  • the rolling is completed above 1000°C to avoid entering the partial recrystallization temperature zone and causing uneven grain size.
  • the rolling is completed in a higher temperature zone, and the deformation conditions of the rolled piece are good, which is conducive to increasing the pass reduction.
  • the intermediate billet with a thickness not less than 2 times that of the finished steel plate is selected to ensure the cumulative reduction of the second stage rolling, so that the recrystallized austenite grains are fully flattened, which is conducive to subsequent organizational transformation and grain refinement.
  • the finishing rolling stage is rolling in the non-recrystallization zone, and the rolling temperature range is selected to be 850-900°C.
  • the temperature is higher than 900°C, the rolled piece may enter the partial recrystallization zone, resulting in uneven grains.
  • the temperature is lower than 850°C, it is difficult to ensure the starting temperature required for subsequent direct accelerated cooling.
  • the cooling method adopts laminar cooling to ensure that the steel plate is transformed into a mixed structure of martensite and bainite, ensuring that martensite accounts for 35% to 45%, bainite accounts for 55% to 65%, and the martensite structure is evenly distributed in the bainite matrix.
  • the cooling rate is too high, the martensite content in the steel plate is too high, and the final material has a high yield strength ratio and poor low-temperature toughness; if the cooling rate is too low, the steel plate is not undercooled enough, and martensite is difficult to produce, which ultimately leads to low tensile strength of the steel plate.
  • the red-return temperature of the steel plate is controlled at 300-350°C.
  • the fast-cooling structure is low-temperature tempered to reduce the carbon content in martensite and bainite, release the residual stress in the steel, and produce Cu-rich nano-precipitates uniformly dispersed throughout the microstructure.
  • the temperature is higher than the selected red-return temperature, it is difficult to ensure the integrity of the phase transformation, which affects the strength and toughness matching and organizational regulation of the final steel plate. If the temperature is lower than the selected red-returning temperature and the selected slow cooling time, the low-temperature tempering effect cannot be achieved, the dispersed Cu-rich nano-precipitates in the steel plate are insufficient, the processing strengthening ability of the steel plate cannot be guaranteed, and the residual stress in the steel cannot be released, resulting in poor toughness of the steel plate.
  • the smelting process includes molten iron pretreatment, converter smelting, refining outside the furnace and vacuum treatment.
  • the continuous casting is fully protected and cast, and one or more of electromagnetic stirring, light pressure or heavy pressure is put into use.
  • Table 1 shows the chemical composition of the steel plate in this embodiment
  • Table 2 shows the rolling and heat treatment process parameters of the steel plate in this embodiment
  • Table 3 shows the mechanical properties of the steel plate in this embodiment.
  • the steel plate prepared by the production process of the present invention has a yield strength of ⁇ 750MPa, a tensile strength of ⁇ 1050MPa, a yield strength ratio of ⁇ 0.72, and a transverse impact energy of -40°C ⁇ 100J. It has a lower yield strength ratio and fluctuates less within the entire process window.

Abstract

一种屈服强度≥750MPa的低屈强比海工钢及其生产工艺，钢板化学成分为：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％，余量为Fe和杂质。通过Cu-Mo-Nb-V-Ti复合强化及调整控轧控冷参数，使钢板的微观组织为马氏体-贝氏体-纳米尺度析出物的混合组织，具有较高的应变强化能力和较低的屈强比，兼具良好的焊接性；采用TMCP工艺，生产成本低、效率高。

Description

一种屈服强度≥750MPa的低屈强比海工钢及其生产工艺 技术领域
本发明涉及海工钢技术领域，尤其涉及一种屈服强度≥750MPa的低屈强比海工钢及其生产工艺。
背景技术
钢铁作为海洋工程装备的关键结构材料，广泛应用于海上石油钻井平台、海上风电、海底管线等设施。海洋结构用钢的服役环境恶劣，除重力载荷外，还要承受风载荷、波浪载荷、冰载荷、地震载荷。此外，由于维护困难，海洋工程装备的服役期也较长，对钢材的要求也就更高。目前，海洋结构用钢板逐渐向高强度、大厚度、大规格化的方向发展。
当钢材强度提高时，其屈服强度和拉伸强度之比(屈强比)往往呈现上升趋势。如果材料的屈强比上升，则其从发生塑性变形的时刻(屈服点)至发生破坏的时刻的应力相差不大，即海洋设施通过变形吸收能量防止破坏的时间不多。当地震、海啸等巨大外力作用于海洋设施时，难以确保设施的安全。因此，海洋工程装备结构用钢需要同时满足高强度和低屈强比的要求。针对此种情况，本发明开发出了一种屈服强度≥750MPa、抗拉强度≥1050MPa、屈强比≤0.72、-40℃横向冲击功≥100J的低屈强比海工钢。
申请号为202010235198.7的中国专利申请公开了“一种屈服强度690MPa级低屈强比高强钢板及其制造方法”，通过二次淬火，得到回火索氏体+贝氏体复相组织，屈服强度≥690MPa、抗拉强度≥770MPa，屈强比≤0.88。该方法采用二次淬火工艺，将钢板两次加热，生产过程中能耗大，生产效率低；此外，其C含量控制在0.1％-0.2％，难以满足海工用钢对于焊接的要求，钢板焊接难度大；其钢板的屈强比≤0.88，但由实施例可以看出，其生产的钢板屈强比均高于0.86，与本发明钢板的屈强比(≤0.72)有显著差异，难以确保设海洋施在塑性变形过程中吸收较多能量。
申请号为202110035527.8的中国专利申请公开了“一种优良低温韧性的低屈强比海工钢板及其制造方法”，通过控轧控冷、钢板缓冷工艺，得到屈强比≤0.8的钢板。其Si含量控制在0.2％-0.4％，含量过高，会降低钢板热影响区的塑性、韧性。其Mn含量控制在1.45％-1.65％，含量过高，会加剧连铸坯偏析，降低组织均匀性。其Cu含量控制在0.15％-0.3％，含量过低， 无法在钢板中生成纳米尺寸富Cu析出相，Cu对钢板的强度及应变强化能力提高有限。该方法生产的钢板屈服强在420MPa以上，与采用本发明所述方法生产的屈服强度在750MPa以上的钢板，强度相差两个钢级以上，难以满足高强度的需求。
申请号为201780071626.3的中国专利申请公开了“一种低屈强比超高强度钢材及其制造方法”，通过两种冷却速度，即将钢板先冷却到Ar3温度以下，再冷却到Bs温度以下，得到屈强比≤0.85、抗拉强度≥800MPa的钢板。该方法控冷工艺较为复杂，现场实际生产中难以精确控制第一次冷却的终止温度；且第二次冷却的冷速需要大于30℃/s，对设备能力要求高，工艺适用性差。此外，其钢中添加的Mn多，会加剧连铸坯偏析、降低组织均匀性。Cu添加的少，无法通过形成纳米级富Cu相提高钢板强度及应变强化能力。另外，该方法生产的钢板仅检验-5℃时的冲击功，未体现出-40℃时的冲击功，限制了其适用范围。
申请号为202111254001.5的中国专利申请公开了“一种屈服强度690MPa级高强韧性低屈强比中厚钢板的制备方法”。通过将热轧后的板材在300-650℃预保温60min以上，再在两相区保温30-120min后水淬，最后再将板材加热至200-450℃进行中低温回火，得到了屈服强度≥690MPa、屈强比≤0.85的钢板。其钢中添加的Mn多，会加剧连铸坯偏析、降低组织均匀性。Cu的添加量低，Cu对钢板的强度及应变强化能力提高有限。V的添加量高，含V析出物尺寸过大，阻碍位错运动能力弱，无法提高钢板应变强化能力；且较多的V对钢板热影响区冲击韧性有所恶化，并会提高合金成本。此外，该方法将热轧后的板材两次加热，钢板生产周期长、生产成本高。
申请号为201210348440.7的中国专利申请公开了“一种超高强高韧性海洋工程用钢板及其生产方法”。通过在Ac3之上进行奥氏体相区淬火处理，淬火温度在900-920℃，淬火后在600-630℃回火，得到超高强高韧性海洋工程用钢板。钢板屈服强度为710-800MP，抗拉强度为770-840MPa，-40℃横向冲击功≥90J。该方法采用调质工艺，将钢板二次加热，生产过程中能耗大，生产效率低；Cu添加的少，无法通过形成纳米级富Cu相提高钢板强度及应变强化能力。
申请号为202111253774.1的中国专利申请公开了“一种屈服强度960MPa级低屈强比海工用钢板及其制备方法”。通过两相区退火+完全奥氏体化淬火+中低温回火处理工艺制备出欺负强度≥960MPa，抗拉强度≥1100MPa，-40℃冲击韧性≥69J的高强韧性低屈强比中厚钢板。该方法将钢板多次加热，生产工艺复杂，生产过程中能耗大，生产效率低；Cu添加的少，无法通过形成纳米级富Cu相提高钢板强度及应变强化能力。
申请号为202210648955.2的中国专利申请公开了“一种耐海洋大气腐蚀高强钢及其生产方法”。通过微镁处理结合钛微合金化配合控轧控冷工艺，获得高强度、低屈强比和优良韧性的耐海洋大气腐蚀钢，钢的屈服强度为600-700MPa，抗拉强度为750-850MPa，-20℃冲击功≥100J。该方法Si含量在0.6％-0.8％，含量过高，会降低焊缝金属的塑性、韧性。Mn含量控制在1.4％-1.7％，含量过高，会加剧连铸坯偏析，降低组织均匀性。Cu的添加量低，Cu对钢板的强度及应变强化能力提高有限。
发明内容
本发明提供了一种屈服强度≥750MPa的低屈强比海工钢及其生产工艺，通过Cu-Mo-Nb-V-Ti复合强化及调整控轧控冷参数，使所生产钢板的微观组织为马氏体(硬相)-贝氏体(软相)-纳米尺度析出物的混合组织，具有较高的应变强化能力和较低的屈强比，且兼具良好的焊接性；采用TMCP工艺，无需复杂的调质工艺，生产成本低、效率高。
为了达到上述目的，本发明采用以下技术方案实现：
一种屈服强度≥750MPa的低屈强比海工钢，钢板化学成分按照重量百分比计为：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％，余量为Fe和不可避免的杂质元素。
进一步的，所述钢板的微观组织为马氏体+贝氏体+纳米尺度析出物的混合组织，其中马氏体组织占比35％～45％，贝氏体组织占比55％～65％；马氏体组织均匀分布在贝氏体基体中，纳米尺度析出物均匀弥散在整个微观组织中。
进一步的，所述钢板的屈服强度≥750MPa、抗拉强度≥1050MPa、屈强比≤0.72、-40℃横向冲击功≥100J。
一种屈服强度≥750MPa的低屈强比海工钢的生产工艺，生产过程包括：冶炼、连铸、铸坯缓冷、坯料再加热、控轧控冷及堆垛缓冷；其中：
1)坯料再加热与除鳞：将冷却后的连铸坯再次加热，加热温度TF为1150℃～1250℃，在炉总时长tF为3～6h；加热完成后进行高压水除鳞，除鳞后的连铸坯温度Ts≥1120℃；
2)粗轧：除鳞后进行第一阶段粗轧，粗轧终轧温度TRf≥1000℃；
3)精轧：粗轧结束后进行第二阶段精轧，精轧开轧温度TFs≤900℃，终轧温度TFf≥850℃；
4)层流冷却：钢板轧制完成后直接进行层流冷却；冷却开始温度TCs为820～850℃，冷却速度RC控制在10℃/s～20℃/s，返红温度TCf控制在300～350℃；
5)堆垛缓冷：钢板空冷结束后立即放入缓冷坑堆垛缓冷至室温，堆垛缓冷时间tC≥12h。
与现有技术相比，本发明的有益效果是：
1)降低钢板屈强比的关键在于调整其微观组织中软相和硬相的比例，并使组织具有较高的应变硬化能力；同时，为了保证钢板高屈服强度、高抗拉强度和高韧性，钢中软硬相各自的强度以及相之间的匹配也需精确调控；本发明通过Cu-Mo-Nb-V-Ti复合强化，以及调整控轧控冷参数，使生产出的钢板的微观组织为马氏体(硬相)-贝氏体(软相)-纳米尺度析出物的混合组织，其中马氏体占比35％-45％，贝氏体占比55％-65％，马氏体组织均匀分布在贝氏体基体中，纳米尺度析出物均匀弥散在整个微观组织中，具有较高的应变强化能力和较低的屈强比，且兼具良好的焊接性；钢板的屈服强度≥750MPa、抗拉强度≥1050MPa、屈强比≤0.72、-40℃横向冲击功≥100J。
2)本发明采用TMCP工艺，无需复杂的调质工艺，生产成本低、效率高；
3)本发明不仅适用于海工钢，也适用于其他高强度钢板，例如高层建筑用钢、桥梁用钢、工程机械用钢、压力容器用钢等。
具体实施方式
本发明所述一种屈服强度≥750MPa的低屈强比海工钢，钢板化学成分按照重量百分比计为：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％，余量为Fe和不可避免的杂质元素。
进一步的，所述钢板的微观组织为马氏体+贝氏体+纳米尺度析出物的混合组织，其中马氏体组织占比35％～45％，贝氏体组织占比55％～65％；马氏体组织均匀分布在贝氏体基体中，纳米尺度析出物均匀弥散在整个微观组织中。
进一步的，所述钢板的屈服强度≥750MPa、抗拉强度≥1050MPa、屈强比≤0.72、-40℃横向冲击功≥100J。
本发明所述一种屈服强度≥750MPa的低屈强比海工钢的生产工艺，生产过程包括：冶炼、连铸、铸坯缓冷、坯料再加热、控轧控冷及堆垛缓冷；其中：
1)坯料再加热与除鳞：将冷却后的连铸坯再次加热，加热温度TF为1150℃～1250℃，在炉总时长tF为3～6h；加热完成后进行高压水除鳞，除鳞后的连铸坯温度Ts≥1120℃；
2)粗轧：除鳞后进行第一阶段粗轧，粗轧终轧温度TRf≥1000℃；
3)精轧：粗轧结束后进行第二阶段精轧，精轧开轧温度TFs≤900℃，终轧温度TFf≥850℃；
4)层流冷却：钢板轧制完成后直接进行层流冷却；冷却开始温度TCs为820～850℃，冷却速度RC控制在10℃/s～20℃/s，返红温度TCf控制在300～350℃；
5)堆垛缓冷：钢板空冷结束后立即放入缓冷坑堆垛缓冷至室温，堆垛缓冷时间tC≥12h。
本发明所述一种屈服强度≥750MPa的低屈强比海工钢中主要合金元素的作用和选择范围的理由说明如下：
碳C：C是钢中仅次于Fe的主要元素，它直接影响钢板的强度、塑性、韧性和可焊接性等性能。C可以有效提高钢板的强度及淬透性，但C含量过高会对钢板的塑性、韧性和焊接性有负面影响。因此，本发明将C含量范围设定为0.06％～0.10％。
硅Si：Si是炼钢过程中重要的还原剂和脱氧剂，能通过固溶强化提高钢板的硬度和强度。但Si含量过高，会降低焊缝金属的塑性、韧性。因此，本发明将Si含量范围设定为0.1％～0.2％。
锰Mn：Mn能与Fe无限固溶，可以在提高钢板强度的同时保证钢具有足够的塑性和韧性，因此，Mn是广泛用于钢中的强化元素。Mn能够与钢中S元素反应形成MnS，可以消除S的有害作用。但是Mn含量过高会加剧连铸坯偏析，增加钢板带状组织等级，导致钢板组织均匀性变差，对钢板的抗层状撕裂性能、塑性、低温韧性和焊接性能不利。为此，本发明将Mn含量范围设定为0.6％～1.0％。
铌Nb：Nb是最主要的微合金化元素之一，部分Nb溶于基体中起到固溶强化作用，在控轧过程中，固溶Nb显著提高钢板的再结晶温度，可以使钢板的轧制过程在更高的温度范围内完成，从而降低钢板的内应力；剩余的部分Nb形成细小碳化物和氮化物，抑制奥氏体再结晶，保持形变效果从而细化铁素体晶粒，能够提高钢板的强度和冲击韧性，并降 低其脆性转变温度。纳米级含Nb析出相可以阻碍位错运动，提高钢板应变强化能力。本发明选取Nb含量不高于0.06％。
钒V：V是强碳氮化物形成元素，有细化组织和晶粒、提高强度和韧性、改善焊接性能、降低过热敏感性的作用。纳米级含V析出相可以阻碍位错运动，提高钢板应变强化能力。但V含量过高时，含V析出物尺寸较大，对钢板应变强化能力有害，且会恶化焊接热影响区冲击韧性。因此，本发明选取V含量范围0.02％～0.05％。
钛Ti：Ti是一种强碳氮化物形成元素。含Ti的析出相可以有效钉扎晶界、阻碍奥氏体长大、细化晶粒、提高钢板的强韧性及低温韧性。纳米级含Ti析出相可以阻碍位错运动，提高钢板的应变强化能力。但Ti含量过高时含Ti析出相会粗化，对钢板性能造成不利影响。因此本发明选取Ti含量不高于0.02％。
铜Cu：Cu能提高钢板的强度、淬透性，并能够抑制钢板冷却过程中的铁素体转变，且对焊接性能没有不利的影响。Cu含量超过一定量时，钢板中会产生纳米尺寸的富Cu相，提高钢板强度，并在变形时阻碍位错运动，提高钢板的应变强化能力。但是Cu含量过高，会对热变形加工不利，在热变形加工时导致铜脆现象。因此本发明选取Cu含量范围为0.62％～1.20％。
铬Cr：Cr能增加钢板的淬透性、强度、硬度和耐磨性，但会降低伸长率和断面收缩率。如果Cr添加过多，在焊接热循环过程中，含Cr碳化物在原奥氏体晶界析出并聚集长大，严重损害钢板低温韧性及焊接性。因此本发明选取Cr含量范围为0.20％～0.50％。
镍Ni：Ni有稳定奥氏体、提高淬透性的作用。钢中添加一定量的Ni可以提高强度、韧性、耐腐蚀性，降低韧脆转变温度。含Ni钢一般不易过热，它可阻止高温时晶粒的增长，使其保持细晶粒组织。但考虑到成本因素，本发明选取Ni含量范围为0.50％～1.20％。
钼Mo：Mo在钢中能提高淬透性和热强性，抑制钢板冷却过程中的铁素体转变。提高在较大冷却范围内晶粒内部位错密度，提高钢板应变强化能力。但Mo含量过高时对焊接性不利，且会提高合金成本。本发明选取Mo含量范围0.30％～0.70％。
铝Al：Al是必须的脱氧元素，且可以细化晶粒、固定钢中的N，从而显著提高钢板的冲击韧性，降低冷脆倾向和时效倾向性；Al还可提高钢的抗腐蚀性能，特别是与Mo、Cu、Si、Cr等元素配合使用时，效果更好；但Al含量过高容易产生铸坯热裂纹。因此本发明选取Al含量范围不高于0.04％。
磷P：P是由矿石带入钢中的，和S类似，是有害元素之一。P虽能使钢板的强度、 硬度增高，但会引起塑性、冲击韧性显著降低。特别是在低温时，它使钢材显著变脆，含P量愈高，冷脆性愈大。但脱P至较低水平会显著增加炼钢成本。因此本发明选取P含量范围不高于0.015％。
硫S：S来源于炼钢的矿石与燃料焦炭，是钢中最常见的有害元素之一，对钢的延展性、韧性、焊接性、耐腐蚀性不利。若S以FeS形态存在于钢中还可在热加工时产生热脆。本发明选取S含量范围不高于0.005％。
本发明所述一种屈服强度≥750MPa的低屈强比海工钢的主要生产工艺参数控制范围原因如下：
本发明采用了Cu、Mo、Nb、V、Ti等元素实现钢的复合强化，控制连铸坯加热温度在1150～1250℃之间，在炉总时长3～6h之间，可保证合金元素的析出相充分回溶到奥氏体中，在后续的控轧过程中充分发挥抑制再结晶、固溶强化、析出强化、细化晶粒、提高钢板应变强化能力等有利作用，为获得最终的组织结构做好成分与温度准备。低于所选加热温度和加热时间范围，固溶将不充分，影响最终的钢板强度和钢板应变强化能力；高于所选定的加热时间和加热温度范围，则连铸坯原始奥氏体晶粒易过于粗大，不利于钢板韧性的控制。
连铸坯出炉后，先进行高压水除鳞，以保证钢板轧制表面质量。除鳞后的温度若低于1120℃，会增加轧制阶段的轧制负荷，并会降低奥氏体再结晶效果，影响晶粒细化。较细的奥氏体晶粒能够保证在层流冷却过程中、贝氏体转变完成后，钢板中尚未转变的奥氏体还拥有足够多的晶界用于马氏体形核，最终使马氏体均匀分布在贝氏体中。
采用两阶段轧制，粗轧阶段为奥氏体再结晶温区轧制，在1000℃之上完成轧制，是为了避免进入部分再结晶温区，出现晶粒尺寸不均匀现象；在较高温区完成轧制，轧件的变形条件良好，有利于提高道次压下量。选择不低于成品钢板厚度2倍的中间坯，是为了保证第二阶段轧制的累积压下量，使再结晶奥氏体晶粒充分扁平化，有利于后续的组织转变和晶粒细化。
精轧阶段为未再结晶区轧制，选择轧制温度区间为850～900℃，温度高于900℃时，轧件有可能进入部分再结晶区，造成晶粒不均匀。而温度低于850℃时，难以保证后续直接加速冷却所需要的开始温度。
钢板轧制完成后，在820～850℃温度开始加速冷却，冷速介于10℃/s～20℃/s之间， 冷却方式采用层流冷却，保证钢板转变为马氏体-贝氏体混合组织，保证马氏体占比35％～45％，贝氏体占比55％～65％，并使马氏体组织均匀分布在贝氏体基体中。如冷速过大，钢板中马氏体含量过高，最终材料屈强比高、低温韧性差；如冷速过低，钢板过冷度不够，马氏体难以产生，最终导致钢板抗拉强度过低。钢板返红温度控制在300～350℃，钢板在缓冷坑中堆垛缓冷至室温的过程中对快冷组织进行低温回火，降低马氏体、贝氏体中碳含量，释放钢中残余应力，并且产生均匀弥散在整个微观组织中的富Cu纳米析出物。高于所选的返红温度时，难以保证相变的完整性，影响最终钢板的强韧性匹配以及组织调控。低于所选的返红温度及所选缓冷时间，则起不到低温回火的效果，钢板中弥散分布的富Cu纳米析出物不足，不能保证钢板加工强化能力，且无法释放钢中残余应力，钢板韧性差。
所述冶炼的工艺路线包括铁水预处理、转炉冶炼、炉外精炼及真空处理。所述连铸全程保护浇铸，投入电磁搅拌、轻压下或重压下的一种或多种。
以下实施例在以本发明技术方案为前提下进行实施，给出了详细的实施方式和具体的操作过程，但本发明的保护范围不限于下述的实施例。下述实施例中所用方法如无特别说明均为常规方法。
【实施例】
表1为本实施例中钢板的化学成分；表2为本实施例中钢板的轧制与热处理工艺参数；表3为本实施例中钢板的力学性能。
表1钢板的化学成分wt％
表2钢板的轧制与热处理工艺参数
表3钢板的力学性能
从表1、表2与表3的数据可以看出，采用本发明所述生产工艺制备的钢板屈服强度≥750MPa，抗拉强度≥1050MPa，屈强比≤0.72，-40℃横向冲击功≥100J，具备较低的屈强比，且在全工艺窗口范围内波动较小。
以上所述，仅为本发明较佳的具体实施方式，但本发明的保护范围并不局限于此，任何熟悉本技术领域的技术人员在本发明揭露的技术范围内，根据本发明的技术方案及其发明构思加以等同替换或改变，都应涵盖在本发明的保护范围之内。

Claims (4)

  1. 一种屈服强度≥750MPa的低屈强比海工钢，其特征在于，钢板化学成分按照重量百分比计为：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％，余量为Fe和不可避免的杂质元素。
  2. 根据权利要求1所述的一种屈服强度≥750MPa的低屈强比海工钢，其特征在于，所述钢板的微观组织为马氏体+贝氏体+纳米尺度析出物的混合组织，其中马氏体组织占比35％～45％，贝氏体组织占比55％～65％；马氏体组织均匀分布在贝氏体基体中，纳米尺度析出物均匀弥散在整个微观组织中。
  3. 根据权利要求1所述的一种屈服强度≥750MPa的低屈强比海工钢，其特征在于，所述钢板的屈服强度≥750MPa、抗拉强度≥1050MPa、屈强比≤0.72、-40℃横向冲击功≥100J。
  4. 如权利要求1～3任意一种所述屈服强度≥750MPa的低屈强比海工钢的生产工艺，其特征在于，生产过程包括：冶炼、连铸、铸坯缓冷、坯料再加热、控轧控冷及堆垛缓冷；其中：
    1)坯料再加热与除鳞：将冷却后的连铸坯再次加热，加热温度TF为1150℃～1250℃，在炉总时长tF为3～6h；加热完成后进行高压水除鳞，除鳞后的连铸坯温度Ts≥1120℃；
    2)粗轧：除鳞后进行第一阶段粗轧，粗轧终轧温度TRf≥1000℃；
    3)精轧：粗轧结束后进行第二阶段精轧，精轧开轧温度TFs≤900℃，终轧温度TFf≥850℃；
    4)层流冷却：钢板轧制完成后直接进行层流冷却；冷却开始温度TCs为820～850℃，冷却速度RC控制在10℃/s～20℃/s，返红温度TCf控制在300～350℃；
    5)堆垛缓冷：钢板空冷结束后立即放入缓冷坑堆垛缓冷至室温，堆垛缓冷时间tC≥12h。
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