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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/18—Hardening; Quenching with or without subsequent tempering
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  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
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  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/005—Heat treatment of ferrous alloys containing Mn
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  • C21D6/00—Heat treatment of ferrous alloys
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  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
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  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0226—Hot rolling
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  • 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
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  • 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0263—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following 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
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
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  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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  • 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
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  • 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
  • C22C38/44—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
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  • 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
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  • 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
  • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
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  • 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
  • C21D2211/008—Martensite

Definitions

• the present disclosure relates to a high strength steel plate and a manufacturing method therefor, and more particularly, to a high strength steel plate having excellent tensile strength and impact toughness, particularly suitable for a nuclear reactor containment container, and a manufacturing method therefor.
• a nuclear reactor containment vessel utilizes a steel material, andA516-70 steel produced by a normalizing process is mainly used as a thick steel plate material.
• A516-70 steel has insufficient tensile strength (about 500 Mpa) to ensure the nuclear power plant's safety, and, thus, a range of use thereof may be extremely limited. That is, since the A516-70 steel has relatively low tensile strength, when used to manufacture a nuclear reactor containment vessel, there may be a risk of damage or explosion due to the failure to withstand the high pressure inside. Accordingly, there may be an urgent need to develop a material suitable for a nuclear reactor containment container while having tensile strength of a certain level or more.
• Patent Document 1 discloses a high strength steel plate with improved tensile strength, which may be a high strength steel plate that may be used for a nuclear reactor containment vessel.
• the steel plate disclosed in Patent Document 1 has a level of tensile strength that may be used as a steel plate for a nuclear reactor containment vessel, but may not be suitable as a material for a nuclear reactor containment vessel due to deteriorations in low-temperature toughness and nil-ductility transition temperature properties.
• Patent Document 1 Korea Patent Publication No. 10-2010-0076745 (published on July 6, 2010 )
• a high strength steel plate having excellent tensile strength, low-temperature toughness, and nil-ductility transition temperature properties particularly suitable for a nuclear reactor containment container of a nuclear power plant, and a manufacturing method therefor, may be provided.
• a high strength steel plate includes, by weight: 0.05 to 0.20% of C, 0.15 to 0.55% of Si, 0.9 to 1.75% of Mn, 0.001 to 0.05% of Al, 0.03% or less of P, 0.03% or less of S, 0.05 to 0.3% of Cr, 0.05 to 0.6% of Ni, 0.005 to 0.35% of Cu, 0.05 to 0.2% of Mo, 0.005 to 0.07% of V, 0.005 to 0.04% of Nb, 0.0005 to 0.005% of Ca, 0.005 to 0.025% of Ti, 0.002 to 0.006% of N, less than 0.0005% of B, and a balance of Fe, with inevitable impurities, satisfies relationships of Cu + Ni + Cr + Mo: 1.5% or less, Cr + Mo: 0.4% or less, V + Nb: 0.1% or less, and Ca/S: 1.0 or higher, and includes a combined structure of tempered martensite and tempered bainite as a microstructure.
• the tempered martensite may be included in 30 to 60 area% in the microstructure, the tempered bainite may be included in 40 to 70 area% in the microstructure, and the sum of the tempered martensite and the tempered bainite may be 100 area%.
• the tempered martensite may be included in 40 to 60 area% in the microstructure, and the tempered bainite may be included in 40 to 60 area% in the microstructure.
• a nil-ductility transition temperature of the steel plate may be -50°C or lower.
• Tensile strength of the steel plate may be 600 MPa or more.
• Charpy impact toughness of the steel plate may be 250 J or more at -60°C.
• a grain aspect ratio (a long axis/short axis ratio) of the microstructure may be 1.1 to 2.5.
• a method of manufacturing a high strength steel plate includes: reheating a steel slab at 1050 to 1250°C, the steel slab comprising, by weight: 0.05 to 0.20% of C, 0.15 to 0.55% of Si, 0.9 to 1.75% of Mn, 0.001 to 0.05% of Al, 0.03% or less of P, 0.03% or less of S, 0.05 to 0.3% of Cr, 0.05 to 0.6% of Ni, 0.005 to 0.35% of Cu, 0.05 to 0.2% of Mo, 0.005 to 0.07% of V, 0.005 to 0.04% of Nb, 0.0005 to 0.005% of Ca, 0.005 to 0.025% of Ti, 0.002 to 0.006% of N, less than 0.0005% of B, and a balance of Fe, with inevitable impurities, satisfying relationships of Cu + Ni + Cr + Mo: 1.5% or less, Cr + Mo: 0.4% or less, V + Nb: 0.1% or less, and Ca/S: 1.0 or higher, rolling the steel slab comprising, by weight:
• An accumulated reduction amount of the rolling may be 50 to 90%.
• a grain aspect ratio (a long axis/short axis ratio) of a microstructure of the steel plate by the rolling may be controlled to have a range of 1.1 to 2.5.
• the austenizing may be performed for a time period of 1.6 ⁇ t (where, t denotes a thickness (mm) of the steel plate) + (10 to 30 minutes).
• the tempering may be performed for a time period of 2.4 ⁇ t (where, t denotes a thickness (mm) of the steel plate) + (10 to 30 minutes).
• a high strength steel plate securing a tensile strength of 600 MPa or more, a Charpy impact toughness of 250 J or more at -60°C, and a nil-ductility transition temperature of -50°C or lower, to be particularly suitable for a nuclear reactor containment container of a nuclear power plant, and a manufacturing method therefor, may be provided.
• the present disclosure relates to a high strength steel plate and a manufacturing method therefor, and, hereinafter, preferred embodiments of the present disclosure will be described.
• Embodiments of the present disclosure may be modified in various forms, and the scope of the present disclosure should not be construed as being limited to embodiments to be described below. These embodiments may be provided to those skilled in the art to further detail the present disclosure.
• a high strength steel plate may include, by weight: 0.05 to 0.2% of C, 0.15 to 0.55% of Si, 0.9 to 1.75% of Mn, 0.001 to 0.05% of Al, 0.03% or less of P, 0.03% or less of S, 0.05 to 0.3% of Cr, 0.05 to 0.6% of Ni, 0.005 to 0.35% of Cu, 0.05 to 0.2% of Mo, 0.005 to 0.07% of V, 0.005 to 0.04% of Nb, 0.0005 to 0.005% of Ca, 0.005 to 0.025% of Ti, 0.002 to 0.006% of N, less than 0.0005% of B, and a balance of Fe, with inevitable impurities.
• carbon (C) may be an effective element for securing strength
• the present disclosure may limit a lower limit of the carbon (C) content to 0.05% to prevent a decrease in strength on a matrix phase.
• carbon (C) is excessively added, toughness and weldability may be deteriorated, making it unsuitable for use for a nuclear reactor containment container, and the present disclosure may limit an upper limit of the carbon (C) content to 0.2%. Therefore, the carbon (C) content of the present disclosure may be 0.05 to 0.2%, and a more preferable carbon (C) content may be 0.08 to 0.15%.
• Silicon (Si) may be an element added for a deoxidation effect, a solid solution strengthening effect, and an impact transition temperature increasing effect. Therefore, the present disclosure may limit a lower limit of the silicon (Si) content to 0.15% to achieve this effect.
• a preferred lower limit of the silicon (Si) content may be 0.2%, and a more preferred lower limit of the silicon (Si) content may be 0.3%.
• the present disclosure may limit an upper limit of the silicon (Si) content to 0.55%.
• a preferred upper limit of the silicon (Si) content may be 0.5%, and a more preferred upper limit of the silicon (Si) content may be 0.4%.
• Manganese (Mn) may be an effective element for securing strength, and the present disclosure may limit a lower limit of the manganese (Mn) content to 0.9% to achieve this effect.
• a preferred lower limit of the manganese (Mn) content may be 1.0%, and a more preferred lower limit of the manganese (Mn) content may be 1.2%.
• Manganese (Mn) may be combined with sulfur (S) to form a non-metallic inclusion such as MnS. When manganese (Mn) is added excessively, elongation at room temperature and low-temperature toughness may be deteriorated. Therefore, the present disclosure may limit an upper limit of the manganese (Mn) content to 1.75%.
• a preferred upper limit of the manganese (Mn) content may be 1.7%, and a more preferred upper limit of the manganese (Mn) content may be 1.6%.
• the present disclosure may limit a lower limit of the aluminum (Al) content to 0.001% for a deoxidation effect in a steelmaking process. When aluminum (Al) is added excessively, the deoxidation effect may be saturated, but manufacturing costs may be increased.
• the present disclosure may limit an upper limit of the aluminum (Al) content to 0.05%.
• the aluminum (Al) content is more preferably 0.01 to 0.04%.
• Phosphorus (P) may be an element that impairs low-temperature toughness. Therefore, P may be desirable to have its content managed to be as low as possible. Since Phosphorus (P) may be an element that may be inevitably contained in a steelmaking process, may take excessive cost to completely remove it, the present disclosure may limit an upper limit of the phosphorus (P) content to 0.03%. A preferred upper limit of the phosphorus (P) content may be 0.02%, and a more preferred upper limit of the phosphorus (P) content may be 0.01%.
• S may be also an element that adversely affects low-temperature toughness, together with phosphorus (P). Therefore, S may be desirable to have its content managed to be as low as possible. Since sulfur (S) may be an element that may be inevitably contained in a steelmaking process, like phosphorus (P), and may take excessive cost to completely remove it, the present disclosure may limit an upper limit of the sulfur (S) content to 0.03%. A preferred upper limit of the sulfur (S) content may be 0.02%, and a more preferred upper limit of the sulfur (S) content may be 0.01%.
• chromium (Cr) may be an element contributing to an increase in strength
• the present disclosure may limit a lower limit of the chromium (Cr) content to 0.05% to achieve this effect.
• Chromium (Cr) may be an expensive element. When Cr is added excessively, it is not preferable from a viewpoint of economic efficiency. Therefore, the present disclosure may limit an upper limit of the chromium (Cr) content to 0.3%. Therefore, the chromium (Cr) content of the present disclosure may be 0.05 to 0.3%, and is more preferably 0.05 to 0.2%.
• Nickel (Ni) may be an effective element for improving low-temperature toughness. Therefore, the present disclosure may limit a lower limit of the nickel (Ni) content to 0.05% to achieve this effect. Nickel (Ni) may be an expensive element. When Ni is excessively added, an increase in production cost may occur. Therefore, the present disclosure may limit an upper limit of the nickel (Ni) content to 0.6%. Therefore, the nickel (Ni) content of the present disclosure may be 0.05 to 0.6%, and is more preferably 0.2 to 0.6%.
• Copper (Cu) may be an effective element for increasing strength. Therefore, the present disclosure may limit a lower limit of the copper (Cu) content to 0.005% to achieve this effect. Copper (Cu) may be an expensive element. When Cu is excessively added, an increase in production cost may occur. Therefore, the present disclosure may limit an upper limit of the copper (Cu) content to 0.35%. Therefore, the copper (Cu) content of the present disclosure may be 0.005 to 0.35%, and is more preferably 0.01 to 0.3%.
• Molybdenum (Mo) may be an alloy element effective for improving strength, and may be an element that prevents crack generation caused by sulfide. Therefore, the present disclosure may limit a lower limit of the molybdenum (Mo) content to 0.05% to achieve this effect. Molybdenum (Mo) may be also an expensive element. When Mo is excessively added, an increase in production cost may occur. Therefore, the present disclosure may limit an upper limit of the molybdenum (Mo) content to 0.2%. Therefore, the molybdenum (Mo) content of the present disclosure may be 0.05 to 0.2%, and is more preferably 0.1 to 0.2%.
• Vanadium (V) may be an effective element for improving low-temperature toughness. Therefore, the present disclosure may limit a lower limit of the vanadium (V) content to 0.005% to achieve this effect. Vanadium (V) may be also an expensive element. When V is excessively added, an increase in production cost may occur. Therefore, the present disclosure may limit an upper limit of the vanadium (V) content to 0.07%. Therefore, the vanadium (V) content of the present disclosure may be 0.005 to 0.07%, and is more preferably 0.01 to 0.07%.
• Niobium (Nb) may be an element that may be dissolved in austenite to increase hardenability of the austenite.
• niobium (Nb) may be an element that is precipitated as carbonitride (Nb(C,N)) matching with a matrix, together with titanium (Ti), and may be a major element for obtaining a tensile strength of 600 MPa or more for which the present disclosure seeks . Therefore, the present disclosure may limit a lower limit of the niobium (Nb) content to 0.005% to achieve this effect.
• HIC hydrogen-induced cracking
• the present disclosure may limit an upper limit of the niobium (Nb) content to 0.04%. Therefore, the niobium (Nb) content of the present disclosure may be 0.005 to 0.04%, and is more preferably 0.01 to 0.03%.
• Calcium (Ca) may be combined with sulfur (S) to form a CaS precipitate, and may thus be an effective element for suppressing formation of MnS. Therefore, the present disclosure may limit a lower limit of the calcium (Ca) content to 0.0005% to achieve this effect.
• Ca When calcium (Ca) is excessively added, Ca may react with oxygen in steel to produce CaO, which may be a non-metallic inclusion. Therefore, the present disclosure may limit an upper limit of the calcium (Ca) content to 0.005%. Therefore, the calcium (Ca) content of the present disclosure may be 0.0005 to 0.005%, and is more preferably 0.001 to 0.003%.
• An appropriate content of titanium (Ti) may be fluidly limited according to the content of nitrogen (N).
• an amount of TiN produced may be relatively small, which may be disadvantageous for fine-graining.
• titanium (Ti) is added in an excessive amount, TiN may become coarse during a heating operation to reduce an effect of inhibiting grain growth. Therefore, in consideration of the content (e.g., 0.002 to 0.006%) of nitrogen (N), the content of titanium (Ti) of the present disclosure may be 0.005 to 0.025%, and is more preferably 0.01 to 0.02%.
• Nitrogen (N) may be widely known as an element that plays a role in increasing toughness of a base material and impact toughness of a heat-affected zone (HAZ) by forming a TiN precipitate with titanium (Ti) to refine grains.
• nitrogen (N) may be an element that should be added to achieve the purpose of grain refinement. Therefore, the present disclosure may limit a lower limit of the nitrogen (N) content to 0.002% to achieve this effect.
• the nitrogen (N) content is excessively added, an amount of TiN may be excessively increased and low-temperature toughness may be reduced. Therefore, the present disclosure may limit an upper limit of the nitrogen (N) content to 0.006%. Therefore, the nitrogen (N) content of the present disclosure may be 0.002 to 0.006%, and is more preferably 0.002 to 0.004%.
• the content of boron (B) may be actively suppressed, but excessive cost may be consumed to completely remove boron (B), which may be inevitably introduced during a steelmaking process. Therefore, the present disclosure may limit the boron (B) content to less than 0.0005%.
• a preferred boron (B) content is 0.0002% or less, and a more preferred boron (B) content is 0.0001% or less.
• a high strength steel plate according to an aspect of the present disclosure may satisfy relationships of Cu + Ni + Cr + Mo: 1.5% or less, Cr + Mo: 0.4% or less, V + Nb: 0.1% or less, and Ca/S: 1.0 or higher.
• the relationships of Cu + Ni + Cr + Mo, Cr + Mo, and V + Nb may be values that are respectively limited in the basic specification (ASTMA20) regarding steel for a pressure vessel, and the content of Cu + Ni + Cr + Mo may be limited to 1.5% or less, the content of Cr + Mo may be limited to 0.4% or less, and the content of V + Nb may be limited to 0.1% or less.
• a ratio of Ca/s may be an essential composition ratio of spheroidizing an MnS inclusion to improve hydrogen-induced crack resistance. When the ratio of Ca/s is less than 1.0, it may be difficult to expect the effect, the ratio may be limited to satisfy 1.0 or more.
• a high strength steel plate according to an aspect of the present disclosure may include a combined structure of tempered martensite and tempered bainite as a microstructure.
• Microstructure combined structure of tempered martensite and tempered bainite
• a microstructure of the steel material may have a microstructure of tempered martensite and tempered bainite.
• the tempered martensite and the tempered bainite may include 30 to 60 area% and 40 to 70 area%, respectively, and a tensile strength of 600 MPa, a nil-ductility transition temperature of -50°C or lower, and a Charpy impact toughness of 250 J or more at -60°C may be effectively secured.
• a preferred area fraction of tempered martensite is 40 to 60%, and a preferred area fraction of tempered bainite is 40 to 60%.
• the sum of area fractions of the tempered martensite and the tempered bainite may be 100%.
• a grain aspect ratio (a ratio of long axis/short axis) may be controlled within a certain range, and the grain aspect ratio may be controlled by a rolling (a recrystallization control rolling) process.
• a rolling a recrystallization control rolling
• the grain aspect ratio is less than 1.1, a shape of the grain may be rounded, surface energy thereof may become small, and it may be difficult to expect refinement of the grain. Therefore, it may be difficult to secure sufficient impact toughness and strength.
• the grain aspect ratio exceeds 2.5, a rolling load for forming the grain becomes too high, and impact toughness may be lowered, which is not preferable. Therefore, the present disclosure may limit the grain aspect ratio (the ratio of long axis/short axis) to have a range of 1.1 to 2.5.
• a high strength steel plate may be manufactured by reheating a steel slab at 1050 to 1250°C, the steel slab including, by weight: 0.05 to 0.20% of C, 0.15 to 0.55% of Si, 0.9 to 1.75% of Mn, 0.001 to 0.05% of Al, 0.03% or less of P, 0.03% or less of S, 0.05 to 0.3% of Cr, 0.05 to 0.6% of Ni, 0.005 to 0.35% of Cu, 0.05 to 0.2% of Mo, 0.005 to 0.07% of V, 0.005 to 0.04% of Nb, 0.0005 to 0.005% of Ca, 0.005 to 0.025% of Ti, 0.002 to 0.006% of N, less than 0.0005% of B, and a balance of Fe, with inevitable impurities, satisfying relationships of Cu + Ni + Cr + Mo: 1.5% or less, Cr + Mo: 0.4% or less, V + Nb: 0.1% or less, and Ca/S: 1.0 or higher, rolling the slab in a temperature range of
• a description of the alloy composition and the content of the slab of the present disclosure may be replaced with the description of the alloy composition and the content of the steel plate described above.
• a slab provided with the above-described alloy composition may be reheated at a temperature range of 1050 to 1250°C. This is because, when a reheating temperature thereof is less than 1050°C, it may be difficult to sufficiently dissolve solute atoms, and when a reheating temperature thereof exceeds 1250°C, an austenite grain size may be excessively coarsened and properties of a steel plate may be deteriorated.
• Recrystallization Control Rolling Operation a temperature range of Tnr to (Tnr + 100°C), an accumulated reduction amount of 50 to 90% at a rolling reduction ratio of 10% or more per rolling pass
• the recrystallization control rolling operation refers to a rolling operation to be performed at a temperature, equal to or higher than an unrecrystallized temperature.
• the unrecrystallized temperature Tnr may be derived by the following Equation 1, which has been already known.
• Equation 1 a unit of each alloy element is weight%.
• Tnr °C 887 ⁇ 464 ⁇ C + 890 ⁇ Ti+363 ⁇ Al-357 ⁇ Si+ 6445 ⁇ Nb-644 ⁇ Nb 1/2 + 732 ⁇ V-230 ⁇ V 1/2
• the rolling operation may be performed in a temperature range of Tnr to Tnr + 100°C.
• a rolling reduction ratio of 10% or more may be applied per rolling pass, to finally perform the rolling operation in an accumulated reduction amount of 50 to 90%.
• Such a reduction amount may be provided to control an average size (30 ⁇ m or less) of a microstructure required in the present disclosure and a grain aspect ratio (a long axis/short axis ratio) to 1.1 to 2.5. Therefore, when the accumulated reduction amount is less than 50%, it may be difficult to expect a refinement effect of the microstructure and a control effect of the grain aspect ratio. When the accumulated reduction amount exceeds 90%, a rolling load may be excessively applied, which may cause a problem in process.
• the quenching operation may be an important process for obtaining a combined structure of tempered martensite and tempered bainite, and may be necessary to strictly control process conditions to form a microstructure capable of securing a tensile strength of 600 MPa or more, a -60°C Charpy impact toughness of 250J or more, and a nil-ductility transition temperature property of -50°C or lower.
• the austenizing operation may be performed for a time period of 1.6 ⁇ t (where, t denotes a thickness (mm) of the steel plate) + (10 to 30 minutes) in a temperature range of 870 to 950°C.
• the austenizing operation may be a heat treatment for austenizing the structure before the quenching operation.
• a temperature range of the heat treatment is less than 870°C, it may be difficult to re-solidify the solute elements, and thus it may be difficult to secure strength.
• a temperature range of the heat treatment exceeds 950°C, growth of grains may occur and coarse grains may occur, to impair low-temperature toughness. Therefore, a temperature range of the austenizing operation of the present disclosure may be limited to a temperature range of 870 to 950°C.
• the austenizing operation may be performed for a time period of 1.6 ⁇ t (where, t denotes a thickness (mm) of the steel plate) + (10 to 30 minutes).
• a time period of the austenizing operation is excessively short, an effect of sufficient austenizing may not be expected due to insufficient heating time, and it may be difficult to homogenize the structure.
• a time period of the austenizing operation is excessively long, production time may be prolonged and productivity may be deteriorated. Therefore, a time period of the austenizing operation of the present disclosure may be limited to 1.6 ⁇ t (where, t denotes a thickness (mm) of the steel plate) + (10 to 30 minutes) .
• 10 to 30 minutes may be set as a maintenance time period to perform the austenizing operation.
• the steel plate after the austenizing operation, may be quenched, preferably water-cooled, to be transformed to a combined structure of martensite and bainite.
• Conditions for the quenching operation in the present disclosure is not particularly limited, and any rapid quenching method including a water cooling operation may be applied to the quenching operation of the present disclosure.
• the steel plate may be cooled to a temperature range of 300°C or lower.
• Tempering Operation 2.4 ⁇ t (where, t denotes a thickness (mm) of the steel plate) + (10 to 30 minutes) in a temperature range of 595 to 700°C
• a tempering operation of the quenched steel material to 300°C or lower may be used to remove residual stress in a structure thereof. Therefore, tempered martensite and tempered bainite may be formed.
• a temperature range of the tempering operation of the present disclosure may be limited to 595 to 700°C. This may be because, when a temperature range of the tempering operation is less than 595°C, carbides and the like may be not smoothly precipitated, and when a temperature range of the tempering operation exceeds 700°C, strength of the steel material may be lowered.
• the tempering operation of the present disclosure may be carried out for a time period of 2.4 ⁇ t (where, t denotes a thickness (mm) of the steel plate) + (10 to 30 minutes) to obtain a sufficient tempering effect.
• t denotes a thickness (mm) of the steel plate
• 10 to 30 minutes may be set as a maintenance time period to perform the tempering operation.
• Test pieces were prepared by performing a reheating operation, a recrystallization control rolling operation, an austenizing operation, a quenching operation, and a tempering operation using respective slabs made of the compositions of Inventive Steel and Comparative Steel, as illustrated in Table 2 below. Properties such as strength, low-temperature toughness, and nil-ductility transition temperature were evaluated, and the results therefrom are illustrated in Table 3 below.
• the low-temperature impact toughness may be evaluated as a Charpy impact energy value obtained by performing a Charpy impact test on a specimen having a V notch at -60°C.
• the nil-ductility transition temperature may be a result value according to the drop-weight test transition temperature set by the ASTM E208-06 method.
• Table 2 Condition Steel Plate Thickness (mm) Slab Reheating Temp. (°C) Rolling Temp. (°C) Recrystallization Control Rolling Cumulative Reduction Amount (%) Grain Aspect Ratio* Austenizing Temp. (°C) Tempering Temp.
• Inventive Examples 1 to 7 have microstructures of 30 to 60% of tempered martensite and 40 to 70% of tempered bainite, and secure a tensile strength of 600 MPa or more, an impact toughness of 300 J or more at -60°C, and a nil-ductility transition temperature property of -50°C or lower.
• Comparative Example 1 it can be seen that, since a steel composition satisfies the defined steel composition of the present disclosure, but an accumulated reduction amount of a recrystallization control rolling operation does not satisfy the defined scope of the present disclosure, the area fractions of a microstructure defined by the present disclosure are not satisfied, and a nil-ductility transition temperature property at -50°C or lower is, thus, not secured.
• a microstructure has 80 area% or more of tempered martensite and 20 area% or less of tempered bainite, and tensile strength, impact toughness, and nil-ductility transition temperature properties are thus deteriorated.
• a steel plate according to an embodiment of the present disclosure may control a steel composition, a microstructure, and manufacturing operations under optimal conditions, to secure tensile strength of 600 MPa or more, Charpy impact toughness of 250 J or more at -60°C, and a nil-ductility transition temperature of -50°C or lower, a high strength steel plate having properties suitable for a nuclear reactor containment vessel may be provided.

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