Classifications

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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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  • 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
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/02—Hardening articles or materials formed by forging or rolling, with no further heating beyond that required for the formation
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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
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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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  • 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
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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
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
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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
  • 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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  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/50—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for welded joints
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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
  • C22C38/002—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
  • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
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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
  • C22C38/02—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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  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
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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
  • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
• • 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
  • C22C38/14—Ferrous alloys, e.g. steel alloys containing 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
  • C22C38/42—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
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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
  • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
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  • 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
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  • 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
  • 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
  • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
• • 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
  • C21D2211/001—Austenite
• • 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
  • C21D2211/008—Martensite

Definitions

• the present disclosure relates to a steel product.
• Steel products can be used in welded structures such as buildings, bridges, ships, pipelines, offshore structures, pressure vessels and tanks. Steel products having excellent strength and adaptability to low-temperature toughness are effective in low-temperature applications.
• Cryogenic steel is used in cryogenic pressure vessels such as storage tanks for liquefied gases.
• Al-killed steel, nickel steel, high-Mn steel, austenitic stainless steel and the like exist in a cryogenic steel, in accordance with the usage temperature.
• nickel steel such as 3.5% Ni steel is used as a material for tanks that store liquefied ethane or liquefied ethylene whose usage temperatures are around -100°C.
• Patent Document 1 proposes a nickel-containing steel product for low temperatures that has excellent toughness and has a specific chemical composition containing 2.7% or more and 5.0% or less Ni, wherein the prior austenite grain diameter at the time of quenching heating is 20 ⁇ m or less, and the effective crystal grain diameter after a heat treatment is 12 ⁇ m or less, and the tensile strength is 450 MPa or more and 690 MPa or less.
• a cryogenic pressure vessel is manufactured by welding a steel product, and, in order to eliminate residual stress that arises due to the welding, there are cases in which a post weld heat treatment (called PWHT upon occasion) is carried out. Recently, the demand for low-temperature toughness of steel products after PWHT has increased even more.
• the gist of the present disclosure is as follows.
• a steel product that is suited to low-temperature applications and that has good low-temperature toughness regardless of whether before or after a post weld heat treatment can be provided.
• Fig. 1 is a diagram illustrating an example of results of discriminating the microstructure.
• post weld heat treatment in the present disclosure means a post weld heat treatment based on the contents prescribed in JIS Z 3700:2009 “Method of post weld heat treatment Method", unless otherwise specified.
• the "steel product” and the “base metal” in the present disclosure mean the steel product portion that does not include a surface treated layer such as a plating layer or a coated film. However, a surface treated layer such as a plating layer or a coated film may be formed on the surface of the steel product relating to the present disclosure.
• the "base metal" in a welded joint means the steel product portion that is not affected by welding, in contrast to the welded portion (the welded joint or the heat affected zone).
• step is not only an independent step and includes steps that, even in a case in which that step cannot be clearly distinguished from another step, achieve the intended object of that step.
• the inventors of the present disclosure carried out studies in order to improve the strength of steel products.
• the tensile strength of a steel product is ensured by the configuration of the microstructure.
• the inventors of the present disclosure took samples from a 1/4t portion (t: thickness of the steel product), which is the region that is 1/4 of the thickness in the thickness direction from the surface of a steel product after hot rolling and accelerated cooling, and carried out tensile test thereon, and observed the microstructures.
• the inventors of the present disclosure carried out studies in order to improve the toughness of steel products.
• the toughness of a steel product is ensured by the configuration of the microstructure.
• the inventors of the present disclosure took samples from the 1/4t portion of a steel product after hot rolling and accelerated cooling, and carried out Charpy impact test, and observed the microstructures.
• the total of the area ratios of lower bainite and martensite is 15.0% or more. Note that the total of the area ratios of lower bainite and martensite was measured by using EBSD.
• the inventors of the present disclosure carried out studies in order to ensure the toughness of steel products.
• the toughness of a steel product is ensured by making the region, at which the difference in crystal orientation is 15° or more and which is surrounded by a high angle grain boundary, small.
• the inventors of the present disclosure took samples from the 1/4t portion of a steel product manufactured by controlling the cooling rate and the cooling stoppage temperature after hot rolling, and measured the circle equivalent diameter of the region surrounded by a high angle grain boundary by EBSD.
• the circle equivalent diameter of the region surrounded by a high angle grain boundary is called the crystal grain diameter hereinafter.
• the sample was subjected to mechanical polishing and electrolytic polishing, and analysis was carried out on a 4 mm 2 region by an EBSD device equipped with an FE-SEM (field emission scanning electron microscope).
• C is an element that improves the strength of the steel product. From the standpoint of ensuring the strength of the steel product that is used in a structure, in the present disclosure, the C content is 0.03% or more. The C content is preferably 0.05% or more, or 0.07% or more. On the other hand, C is an element that reduces toughness, and, from the standpoint of ensuring the toughness of the heat affected zone (hereinafter called "HAZ" upon occasion), in the present disclosure, the C content is 0.20% or less. The C content is preferably 0.16% or less, 0.14% or less, or 0.12% or less.
• the Si is an element that is used as a deoxidizing agent, and further, that dissolves into the steel and increases the strength. From the standpoint of controlling the O concentration contained in molten steel, in the present disclosure, the Si content is 0.01% or more. The Si content is preferably 0.03% or more, 0.05% or more, or 0.10% or more. On the other hand, if the Si content is excessive, there are cases in which a hard phase forms in the HAZ, and the toughness decreases. Accordingly, from the standpoint of ensuring the toughness of the HAZ, in the present disclosure, the Si content is 0.50% or less. The Si content is preferably 0.30% or less, or 0.20% or less.
• Mn is an element that is used as a deoxidizing agent, and further, that improves the hardenability of the steel and contributes to increasing the strength. From the standpoint of controlling the O concentration contained in molten steel, in the present disclosure, the Mn content is 0.10% or more. Moreover, due to Mn in an amount of 0.10% or more, by forming MnS, the solid-solution S is reduced, and hot cracking is prevented. From the standpoint of ensuring the strength of the steel product and the toughness of the HAZ, the Mn content is preferably 0.30% or more, or 0.50% or more.
• the Mn content is 1.65% or less.
• the Mn content is preferably 1.50% or less, 1.25% or less, or 1.10% or less.
• the P is an impurity element.
• the P content may be 0.001 % or more.
• the P content is 0.025% or less.
• the P content is preferably 0.016% or less, 0.012% or less, or 0.008% or less.
• the S content is an impurity element.
• the S content may be 0.0001% or more.
• the S content is 0.0250% or less.
• the S content is preferably 0.0100% or less or 0.0050% or less.
• Ni is an element that is effective in improving the hardenability and toughness of the steel. Therefore, in the present disclosure, the Ni content is 2.65% or more.
• the Ni content is preferably 3.00% or more or 3.20% or more.
• Ni is an expensive element, and, from the standpoint of cost reduction, in the present disclosure, the Ni content is 4.45% or less.
• the Ni content is preferably 4.10% or less, or 3.80% or less.
• Al is an element that is effective in deoxidation, and is an element that, by forming a nitride, refines the crystal grain diameter at the time of quenching. Therefore, in the present disclosure, the Al content is 0.001% or more. However, if Al is excessively contained, there is the concern that the Al will generate a coarse nitride, and the toughness of the steel product and the HAZ will decrease. Accordingly, the Al content is 0.100% or less.
• the Al content is preferably 0.080%, or 0.050% or less.
• the O content is an impurity element.
• the lower limit of the O content is not limited, from the standpoint of the manufacturing cost, in the present disclosure, the O content may be 0.0001% or more.
• the O content is excessive, there are cases in which a coarse oxide is generated, and the toughness and ductility of the steel product and the HAZ deteriorate.
• the O content is 0.0100% or less.
• the O content is preferably 0.0060% or less, or 0.0040% or less.
• N is an impurity element.
• the N content may be 0.0001% or more. From the standpoint of ensuring the properties of the steel product and the toughness of the HAZ, in the present disclosure, the N content is 0.0100% or less.
• the N content is preferably 0.0050% or less, or 0.0040% or less.
• the steel product relating to the present disclosure may contain other elements (optional elements) instead of some of the Fe.
• the following optional elements are given as examples, but the contents of these elements may be 0%.
• the steel product relating to the present disclosure may be made to contain one or two or more of the optional elements Cu, Cr, Mo, and B that are described hereinafter and have the effect of improving the hardenability.
• Cu is an element that is sometimes mixed into the steel product in the manufacturing process.
• the lower limit value of the Cu content is not limited and may be 0%.
• Cu has little adverse effect on the weldability and on the toughness of the HAZ, and has the effect of improving the hardenability of steel, and therefore, is an element that improves the strength of the steel product.
• the Cu content may be 0.01% or more.
• the Cu content is preferably 0.10% or more.
• the Cu content is 1.50% or less.
• the Cu content is preferably 1.00% or less, 0.80% or less, 0.60% or less, or 0.50% or less.
• the Cr content is an element that is sometimes mixed into a steel product in the manufacturing process.
• the lower limit value of the Cr content is not limited, and may be 0%.
• Cr is also an element that improves the strength of a steel product because it has the effect of increasing the hardenability of the steel. Therefore, in the present disclosure, the Cr content may be 0.01% or more.
• the Cr content is preferably 0.10% or more.
• the Cr content is 3.00% or less.
• the Cr content is preferably 2.20% or less, 1.40% or less, or 0.80% or less.
• Mo is an element that is sometimes mixed into a steel product in the manufacturing process.
• the lower limit value of the Mo content is not limited, and may be 0%.
• Mo is also an element that improves the strength of a steel product because it has the effect of increasing the hardenability of the steel. Therefore, in the present disclosure, the Mo content may be 0.01% or more.
• the Mo content is preferably 0.05% or more, 0.10% or more, 0.20% or more or 0.30% or more.
• the Mo content is 2.00% or less.
• the Mo content is preferably 1.20% or less, or 0.80% or less.
• B is an element that is sometimes mixed into a steel product in the manufacturing process.
• the lower limit value of the B content is not limited, and may be 0%.
• B is also an element that exhibits a marked effect of increasing the hardenability of steel and improves the strength of a steel product. Therefore, in the present disclosure, the B content may be 0.0003% or more.
• the B content is 0.0050% or less.
• the B content is preferably 0.0030% or less, or 0.0020% or less.
• the steel product relating to the present disclosure may be made to contain one or two or more of the optional elements Nb, Ti, and V that are described hereinafter and have the effect of increasing the strength of the steel product by precipitates such as carbides or nitrides.
• Nb is an element that is sometimes mixed into a steel product in the manufacturing process.
• the lower limit value of the Nb content is not limited, and may be 0%.
• Nb is also an element that forms a carbide or a nitride, and has the effect of refining the microstructure, and improves the strength of the steel product. Therefore, in the present disclosure, the Nb content may be 0.001% or more.
• the Nb content is 0.050% or less.
• the Nb content is preferably 0.040% or less, or 0.030% or less. In particular, from the standpoint of ensuring the toughness of the steel product after PWHT, the Nb content may be 0.004% or less.
• Ti is an element that is sometimes mixed into a steel product in the manufacturing process.
• the lower limit value of the Ti content is not limited, and may be 0%.
• Ti is also an element that forms a carbide or a nitride, and has the effect of refining the microstructure, and improves the strength of the steel product. Therefore, in the present disclosure, the Ti content may be 0.001% or more.
• the Ti content is 0.050% or less.
• the Ti content is preferably 0.040% or less, or 0.030% or less. In particular, from the standpoint of ensuring the toughness of the steel product after PWHT, the Ti content may be 0.004% or less, or 0.002% or less.
• V is an element that is sometimes mixed into a steel product in the manufacturing process.
• the lower limit value of the V content is not limited, and may be 0%.
• V is also an element that forms a carbide or a nitride, and improves the strength of the steel product. Therefore, in the present disclosure, the V content may be 0.01% or more.
• the V content is 0.10% or less.
• the V content is preferably 0.08% or less, or 0.05% or less.
• the steel product relating to the present disclosure may be made to contain one or two or more of the optional elements Mg, Ca, and REM that are described hereinafter.
• Mg is an element that is sometimes mixed into a steel product in the manufacturing process.
• the lower limit value of the Mg content is not limited, and may be 0%.
• Mg is also an element that forms an oxide and improves the toughness of the heat affected zone. Therefore, in the present disclosure, the Mg content may be 0.0003% or more, 0.0006% or more, or 0.0010% or more.
• the Mg content is 0.0200% or less.
• the Mg content is preferably 0.0100% or less, 0.0060% or less, or 0.0040% or less.
• Ca is an element that is sometimes mixed into a steel product in the manufacturing process.
• the lower limit value of the Ca content is not limited, and may be 0%.
• Ca is also an element that, by spheroidizing the sulfide within the steel product, mitigates the effect of the MnS that decreases the toughness of the steel product and the heat affected zone. Therefore, in the present disclosure, the Ca content may be 0.0003% or more, 0.0006% or more, or 0.0010% or more.
• the Ca content is if the Ca content is excessive, there are cases in which the Ca forms a coarse oxide and decreases the toughness of the steel. Accordingly, from the standpoint of ensuring the toughness, in the present disclosure, the Ca content is 0.0200% or less.
• the Ca content is preferably 0.0100% or less, 0.0060% or less, or 0.0040% or less.
• Rare earth metal is a collective term for a total of 17 elements that are the two elements of Sc and Y and fifteen lanthanoid elements such as La, Ce, Nd.
• the REM content means the total content of the aforementioned 17 elements.
• REMs are elements that are sometimes mixed into a steel product in the manufacturing process.
• the lower limit value of the REM content is not limited, and may be 0%.
• REMs are also elements that form oxides and improve the toughness of the heat affected zone. Therefore, in the present disclosure, the REM content may be 0.0003% or more, 0.0006% or more, or 0.0010% or more.
• the REM content is 0.0200% or less.
• the REM content is preferably 0.0100% or less, 0.0060% or less, or 0.0040% or less.
• the balance of the chemical composition of the steel product relating to the present disclosure is iron (Fe) and impurities.
• Impurities mean components that are mixed due to raw materials such as ore and scrap, and other factors, at the time of industrially manufacturing the steel product.
• [C], [Si], [Mn], [Cu], [Ni], [Cr] and [Mo] are the contents (mass%) of C, Si, Mn, Cu, Ni, Cr and Mo in the steel. In a case in which a given element is not contained, zero is substituted in. Note that ⁇ C] has the same meaning as [C] 1/2 .
• the range of value ⁇ is 4.0 - 16.0. This is an index expressing the hardenability of the steel product.
• the greater the value ⁇ the more that lower bainite and martensite microstructures having a superior balance of strength and toughness can be formed.
• ⁇ is in the appropriate range, in the microstructure of the HAZ as well, the ratio of the lower bainite and martensite microstructures having a superior balance of strength and toughness becomes high, and the HAZ toughness also can be ensured.
• ⁇ is 4.0 or more, the hardenability of the base metal is ensured, and the ratio of the lower bainite and martensite having a favorable balance of strength and toughness increases, and a deterioration in toughness is suppressed.
• value ⁇ is preferably 4.5 or more or 5.0 or more. Further, value ⁇ is preferably 15.5 or less or 15.0 or less.
• the microstructure of the steel product relating to the present disclosure is described next.
• the microstructure of the region that is 1/4 of the thickness in the thickness direction from the surface of the steel product relating to the present disclosure contains lower bainite and martensite. Further, in addition to lower bainite, upper bainite also may be contained as bainite.
• Boinite is a microstructure containing bainitic ferrite ( ⁇ °B) that has a substructure within the grain, and is a collective term for upper bainite and lower bainite.
• Upper bainite is one of or both of upper bainite that contains residual austenite or an MA phase (martensiteaustenite mixed phase) between the laths, and upper bainite that contains a carbide between the laths.
• Lower bainite is lath-shaped lower bainite containing a carbide within the laths.
• Lath martensite is a microstructure that is composed of packets and blocks formed from groups of laths having a specific arrangement, and in which a single austenite grain is divided into plural packets.
• Lower bainite and martensite are hard phases and increase the toughness of the steel product. From the standpoint of ensuring the toughness of the steel product, the area ratio of lower bainite and martensite at the 1/4t portion is 15.0% or more. The area ratio of lower bainite and martensite at the 1/4t portion is preferably 20.0% or more, or 30.0% or more. The total of the area ratio of the lower bainite and the area ratio of the martensite at the 1/4t portion may be 100%.
• the total of the area ratios of upper bainite and lower bainite and martensite at the 1/4t portion is 90.0% or more.
• the total of the area ratios of upper bainite, lower bainite and martensite at the 1/4t portion may be 100%.
• the upper bainite of the 1/4t portion may be 1.0% or more.
• Observation of the microstructure of the steel product is carried out by using a sample in which a 1/4t portion of the steel product serves as the observed surface.
• Two types of samples on which (a) electrolytic polishing or (b) nital etching is carried out are prepared.
• Each sample of (a) and (b) is measured at three places by the following method, and the average value of the three places is used as the area ratio of the microstructure of that steel product. Note that three of each sample of (a) and (b) may be prepared, and the averages of the respective samples may be computed. Or, three places of one sample of each may be measured in a visual field, and the average of each computed.
• CI value The confidence index
• the determination of ferrite, and upper bainite, lower bainite and martensite, is carried out by setting the threshold value of the Grain Average Misorientation (hereinafter called "GAM") to 0.5.
• GAM Grain Average Misorientation
• the GAM value is an index defined in OIM Analysis (EBSD crystal orientation analyzing software manufactured by TSL, United States).
• OIM Analysis EBSD crystal orientation analyzing software manufactured by TSL, United States.
• the region in which the GAM is 0.5 or less is ferrite.
• the region in which the GAM exceeds 0.5 is upper bainite, lower bainite or martensite.
• the upper bainite, lower bainite and martensite in the present disclosure are determined by using the GAM of EBSD as the threshold value, not only upper bainite, lower bainite and martensite, but also tempered upper bainite, tempered lower bainite and tempered martensite are included. Comparing the microstructures of direct quenching (DQ) and (DQT) that is carried out thereafter up to tempering (T), although dissolution of the MA and coarsening of carbide occur after tempering, the way of looking at the microstructure does not vary greatly.
• DQ direct quenching
• DQT direct quenching
• T tempering
• Measurement of the area ratio of the upper bainite by SEM observation is carried out by using a nital etched sample.
• the measurement magnification is 500 ⁇ , and measurement of a range of 360 ⁇ m ⁇ 480 ⁇ m is carried out.
• the portion, which has a clear lath structure and at which carbide and MA are generated along the lath boundary, is upper bainite.
• An example of the results of discriminating the microstructures is shown in Fig. 1.
• (A) and (B) are SEM images of the same region of a steel product that is manufactured by DQT and whose value ⁇ is 9.9.
• the regions surrounded by the white lines are upper bainite (Bu), and the other regions are lower bainite + martensite (BL+M).
• upper bainite (Bu) carbides appearing white are sparsely distributed, and regions with varying density are present.
• lower bainite + martensite (BL+M) the carbides are densely and uniformly distributed.
• the total of the area ratios of the lower bainite and martensite is determined by subtracting the area ratio of the upper bainite from the total of the area ratios of the upper bainite, lower bainite, and martensite, which were measured as described above.
• the average crystal grain diameter (effective crystal grain diameter) of the 1/4t portion of the steel product is preferably 20.0 ⁇ m or less. This is because it was learned that, if the average crystal grain diameter of the 1/4t portion of the steel product is 20.0 ⁇ m or less, the toughness of the steel product tends to improve even more regardless of whether before or after PWHT. However, the average crystal grain diameter of the 1/4t portion of the steel product may exceed 20.0 ⁇ m. The smaller the average crystal grain diameter of the steel product, the more preferable, and therefore, the lower limit value thereof is not limited. Usually, the average crystal grain diameter is 10 ⁇ m or more. The effective crystal grain diameter is determined by a weighted average.
• Effective crystal grain diameter D area that is determined by a weighted average is calculated by the following formula by using, of the crystal grain diameters that are measured in a 4 mm 2 region, area Si and grain diameter d i of the ith crystal grain detected at the time of measurement.
• D area ⁇ S i • d i / ⁇ S i
• the form of the prior austenite crystal grains (called prior austenite grains or prior ⁇ grains upon occasion) of the steel product of the present disclosure may be a form that is flat in the rolling direction. If the prior austenite grains of the region that is 1/4 of the thickness in the thickness direction from the surface of the steel product are made to be flat grains of an aspect ratio of 1.5 or more, an even greater improvement in the toughness of the steel product is possible. This is because, by increasing the grain boundary area by making the prior austenite grains flat, there is a substantial refining of the austenite grains, and this is effective in refining the average crystal grain diameter.
• the aspect ratio of prior austenite grains is usually 4.0 or less, and may be 3.5 or less.
• the aspect ratio of the prior austenite crystal grains of the 1/4t portion may be less than 1.5.
• the aspect ratio of the prior austenite crystal grains of the 1/4t portion may be 1.4 or less, or 1.3 or less.
• the aspect ratio of the prior austenite crystal grains (hereinafter called prior austenite grains upon occasion) of the steel product is determined as follows.
• An L cross-section (a cross-section parallel to the rolling direction and the thickness direction of the steel product) of the region that is 1/4 of the thickness in the thickness direction from the surface of the steel product is mirror polished, and corrosion is carried out by a corrosive liquid of a saturated solution base of 2 - 4% picric acid, and the prior austenite grain boundaries of an arbitrary region of 1.0 mm in the rolling direction ⁇ 0.5 mm in the thickness direction are made to appear.
• the long diameters and short diameters of the individual prior austenite grains are measured, and the aspect ratio of each prior austenite grain is calculated by long diameter ⁇ short diameter.
• the arithmetic mean of the calculated aspect ratios of all of the prior austenite grains is determined as the "aspect ratio of the prior austenite grains". Note that the maximum length of the prior austenite grain is used as the long diameter, and the maximum interval between two lines, which contact the grain and are parallel to the long diameter direction, is used as the short diameter.
• the steel product relating to the present disclosure has mechanical properties that are such that the steel product has both strength and low-temperature toughness. In addition to having excellent toughness at -100°C in particular, the steel product can exhibit excellent low-temperature toughness after PWHT as well.
• the tensile strength of the steel product is 590 - 930 MPa.
• a steel product that can ensure strength of a structure even if the thickness is thin is necessary. Because steel products that are selected as steel products to be used in such applications usually are steel products having the aforementioned tensile strength, the steel product relating to the present disclosure also is manufactured to have the aforementioned tensile strength.
• the Charpy impact absorption energy at -100°C of the steel product of the present disclosure is preferably 150 J or more. Due to the steel product of the present disclosure having low-temperature toughness of a Charpy impact absorption energy at -100°C of 150 J or more, a transport tank formed from the steel product of the present disclosure can be suitably used for transporting liquid carbon dioxide. Note that the Charpy impact absorption energy at -100°C is a numerical value measured by using a sample taken from a position of 1/4 of the thickness.
• the Charpy impact absorption energy at -100°C is preferably 150 J or more.
• the Charpy impact absorption energy at -100°C after PWHT may be 100 J or more.
• the Charpy impact absorption energy at -100°C after PWHT also is a numerical value measured by using a sample taken from a position of 1/4 of the thickness.
• the Charpy impact absorption energy at -100°C after thermal cycling is preferably 50 J or more. Due to the steel product of the present disclosure having low-temperature toughness that is such that the Charpy impact absorption energy at -100°C after thermal cycling is 50 J or more, a transport tank formed from the steel product of the present disclosure can be suitably used for transporting liquid carbon dioxide for example.
• the Charpy impact absorption energy at -100°C after thermal cycling may be 40 J or more.
• the Charpy impact absorption energy at -100°C after thermal cycling is a numerical value measured by using a sample taken from a position of 1/4 of the thickness of the steel product as a thermal cycling test piece, and providing it with a thermal history of raising the temperature at 60°C/s to 1350°C, and, after maintenance at 1350°C for 1 s, cooling at 20°C/s to room temperature, and thereafter, taking a Charpy test piece therefrom.
• PWHT is carried out on the welded portions of low temperature tanks after being assembled into transport tanks, in order to prevent breakage in advance.
• PWHT in which the rate of temperature increase and the rate of temperature decrease are 55°C/h in the temperature region of 425°C or more, and the steel product is held for 2 hours at 600°C, is carried out on the steel product of the present disclosure, and thereafter, a Charpy test piece is taken, and measurement is carried out.
• the Charpy impact absorption energy at -100°C is preferably 50 J or more.
• the Charpy impact absorption energy at -100°C of the portion at which PWHT is carried out after the above-described thermal cycling test may be 40 J or more.
• the tensile strength (TS) and the yield strength (YS) in the Examples are measured in accordance with a tensile test that is based on JIS Z2241:2011.
• the tensile test uses a JIS14A test piece that has been taken from a position of 1/4 of the thickness and whose length direction is the direction (the C direction) parallel to the width direction of the steel product.
• TS and YS are computed by using three test pieces and taking the averages thereof.
• the yield ratio (YR, %) is computed by (YS/TS) ⁇ 100, on the basis of the respective average values of TS and YS.
• the Charpy impact absorption energy is measured by a Charpy impact test at -100°C on the basis of the prescriptions of JIS Z2242:2018 and by using an impact blade of a radius of 2 mm.
• the Charpy impact absorption energy is calculated by measuring by using three test pieces, and taking the average thereof.
• the Charpy impact absorption energy test uses a V-notched test piece that has been taken from a position of 1/4 of the thickness of the steel product and whose length direction is the direction (the C direction) parallel to the width direction of the steel product.
• the form of the steel product relating to the present disclosure is not particularly limited, and examples are steel plates, steel strips, structural steel and steel pipes.
• steel pipes and structural steel include steel products in which steel plates are joined, e.g., in addition to welded steel pipes and welded structural steel, structural steel joined by rivets, and the like.
• the thickness of the steel product such as steel plates, steel strips, structural steel and steel pipes (the thickness of the flanges in the case of structural steel) is not particularly limited, and usually is 3 mm or more and 150 mm or less.
• the thickness of the steel product may be 6 mm or more, 10 mm or more, 15 mm or more, or 30 mm or more. Further, the thickness of the steel product may be 100 mm or less, 80 mm or less, or 60 mm or less.
• the steel product relating to the present disclosure has mechanical properties such that the steel product has both strength and low-temperature toughness, and, in particular, can exhibit excellent low-temperature toughness even after PWHT. Therefore, the steel product relating to the present disclosure can be suitably used as a tank that stores and transports liquefied gasses, and liquid carbon dioxide in particular.
• the method of manufacturing the steel product relating to the present disclosure is not particularly limited, but, for example, after melting a steel that satisfies the above-described chemical composition, a steel slab is manufactured by continuous casting.
• the steel slab is subjected to either direct quenching (DQ), in which it is heated, hot rolled, and then directly water-cooled, or reheat quenching (RQ), in which it is hot rolled, air-cooled, reheated, and then water-cooled, to made into a steel product.
• DQ direct quenching
• RQ reheat quenching
• the hot rolled steel does not necessarily have to be air-cooled before reheating, but water cooling may be used.
• an intermediate heat treatment (L) and tempering (T) may also be carried out.
• the manufacturing process after the hot rolling is selected from combinations of the above-described DQ, RQ, L and T, and is, for example, DQT, RQT, DQLT, or RQLT.
• DQT is preferable in the manufacturing of the steel product relating to the present disclosure.
• An example of a preferable manufacturing process is given hereinafter.
• the heating temperature of the steel slab on which the hot rolling is carried out is Ac 3 or more.
• the heating temperature of the steel slab is preferably 1000°C or more.
• the heating temperature of the hot rolling is 1250°C or less.
• the heating temperature of the hot rolling is preferably 1200°C or less.
• the element symbols in the formula mean the content (mass%) of each element contained in the steel slab.
• the hot rolling is structured by rolling in a temperature range in which recrystallization occurs (rolling in the recrystallization temperature range) and rolling in a temperature range in which recrystallization is suppressed (rolling in the non-recrystallization temperature range).
• Rolling in the recrystallization temperature range is hot rolling carried out with the temperature of the rolled steel during rolling being 900°C or more.
• the cumulative rolling reduction ratio of the rolling in the recrystallization temperature range is preferably 20% or more, and more preferably 30% or more.
• Rolling in the non-recrystallization temperature range is hot rolling carried out with the temperature of the rolled steel during rolling being less than 900°C.
• the cumulative rolling reduction ratio of the rolling in the non-recrystallization temperature range is preferably 20% or more, and more preferably 30% or more.
• the cumulative rolling reduction ratio of the rolling in the non-recrystallization temperature range is determined from the difference between the thickness of the rolled steel at 900°C and the thickness of the steel product after rolling ends.
• Cumulative rolling reduction ratio (%) of rolling in non-recrystallization temperature range 100 ⁇ ([thickness of rolled steel at 900°C] - [thickness of steel product after rolling ends]) / [thickness of rolled steel at 900°C]
• the end temperature of the hot rolling is Ar 3 or more.
• accelerated cooling such as water cooling is carried out on the steel product.
• the start temperature of the accelerated cooling is Ar 3 or more.
• the element symbols in the formula mean the content (mass%) of each element contained in the steel product, and t means the thickness (mm) of the steel product.
• the cooling rate is 1.0°C/s or more.
• the cooling rate of the accelerated cooling is preferably 5.0°C/s or more, or 10.0°C/s or more.
• the cooling rate is a value obtained by calculating the cooling rate at a position of 1/4 of the thickness by simulation in accordance with thermal transfer calculation.
• the stoppage temperature of the accelerated cooling is 400°C or less.
• the stoppage temperature of the accelerated cooling is preferably 350°C or less. Accelerated cooling may be carried out to room temperature. From the standpoint of dehydrogenation of the steel product, the stoppage temperature is preferably 100°C or more.
• a tempering treatment may be carried out on the steel product.
• the heating temperature of the tempering treatment is preferably 650°C or less, 620°C or less, or 590°C or less.
• the heating temperature of the tempering treatment is preferably 350°C or more, or 400°C or more.
• the steel product relating to the present disclosure is manufactured by RQ
• the effects of the heating temperature and the rolling reduction ratio of the rolled steel at the time of the hot rolling on the mechanical properties of the steel product are small.
• the heating temperature of the rolled steel is preferably 1000°C or more.
• the rolling reduction ratio is insufficient, there are cases in which initial stage defects from the time of manufacturing the steel slab remain at the thickness central portion, and the quality of the steel product decreases. Therefore, the total of the rolling reduction ratios of hot rolling (also called cumulative rolling reduction ratio) is preferably 35% or more.
• the steel slab may, as is, be water-cooled or air-cooled.
• the reheating temperature of the steel product is Ac 3 or more, because quenching is carried out from an austenite single-phase microstructure. From the standpoint of ensuring the homogeneity of the microstructure, the reheating temperature of the steel product is preferably 750°C or more, 850°C or more, 880°C or more, or 900°C or more.
• the upper limit temperature of the reheating temperature is not particularly stipulated, there are cases in which excessive heating to a high temperature leads to coarsening of the austenite grains and a decrease in toughness, and therefore, the upper limit temperature of the reheating temperature is preferably 1000°C or less, 950°C or less, or 930°C or less.
• a tempering treatment may be carried out on the steel product.
• the heating temperature of the tempering treatment is preferably 660°C or less, or 640°C or less.
• the heating temperature of the tempering treatment is preferably 400°C or more, 450°C or more, or 500°C or more.
• the steel product relating to the present disclosure is described concretely hereinafter by way of Examples.
• the conditions in the following Examples are examples of conditions that are employed in order to confirm the feasibility and the effects of the present disclosure, and the steel product relating to the present disclosure is not limited to the following Examples.
• slabs having the chemical compositions shown in Table 1 were cast by continuous casting.
• the balance, which is other than the components listed in Table 1, is Fe and impurities. Further, blank cells mean that the alloy elements were not intentionally added in the steelmaking process.
• Tempor heat treatment is the heating temperature in the tempering treatment after the quenching.
• Table 2 No. plate thickness (mm) Ac 3 [°C] rolling conditions Ar 3 [°C] direct quenching conditions Temper heat treatment [°C] heating temperature [°C] cumulative rolling reduction ratio [%] at 900°C or more cumulative rolling reduction ratio [%] at less than 900°C end temperature [°C] cooling after rolling start temperature [°C] stoppage temperature [°C] cooling rate [°C/s] 1A 30 754 1100 38 59 840 water cooling 534 830 350 10.5 600 2A 45 754 1120 19 52 810 water cooling 540 800 280 4.2 500 3A 40 779 1140 48 45 840 water cooling 548 830 50 3.8 440 4A 25 732 1080 55 57 790 water cooling 525 780 25 6.7 610 5A 25 732 1040 45 48 860 water cooling 485 850 150 7.2 600 6A 45 820
• microstructures and mechanical properties of the obtained steel products were measured by the above-described methods. The results are shown in Table 3. The meanings of the symbols of the microstructures are as follows. Note that the remainders of the microstructures were pearlite, MA phase, and ferrite.
• Nos. 1A - 22A and 101A - 109A are Examples of the present invention, and Nos. 23A, 26A and 110A - 114A are Comparative Examples.
• value ⁇ was less than the lower limit value of the present disclosure, and the hardenability was insufficient, and the strength was insufficient. Sufficient low-temperature toughness also was not obtained.
• value ⁇ exceeded the upper limit value of the present disclosure, and the hardenability was too high, and the strength was excessive.
• slabs having the chemical compositions shown in Table 4 were cast by continuous casting.
• the balance, which is other than the components listed in Table 4, is Fe and impurities.
• blank cells mean that the alloy elements were not intentionally added in the steelmaking process.
• the underlines mean that the value is outside of the scope of the present disclosure.
• Tempor heat treatment is the heating temperature in the tempering treatment after the quenching.
• Table 5 No. plate thickness (mm) rolling conditions
• AC 3 [°C] reheat quenching conditions
• Temper heat treatment [°C] heating temperature [°C] hot rolling cumulative rolling reduction ratio [%] cooling after hot rolling reheat quenching temperature [°C] cooling after reheating 1B 40 1120 45 water cooling 754 920 water cooling 580 2B 35 1090 50 water cooling 754 1010 water cooling 600 3B 30 1100 48 air cooling 764 980 water cooling 480 4B 25 1200 55 water cooling 766 950 water cooling - 5B 30 1150 68 air cooling 743 870 water cooling 500 6B 45 1140 55 water cooling 799 900 water cooling 640 7B 30 1080 60 air cooling 812 920 water cooling 600 8B 35 1060 52 water cooling 796 910 water cooling 620 9B 40 1130 59 air cooling 825 930 water cooling 590 10B 35 1160 47
• microstructures and mechanical properties of the obtained steel products were measured by the above-described methods. The results are shown in Table 6. The meanings of the symbols of the microstructures are as follows. Note that the remainders of the microstructures were pearlite, MA phase, and ferrite.
• KV2 the average value of the Charpy impact absorption energy at - 100°C
• Nos. 1B - 21B and 101B - 109B are Examples of the present invention, and Nos. 22B - 25B and 110B - 113B are Comparative Examples.
• value ⁇ was less than the lower limit value of the present disclosure, and the hardenability was insufficient, and the strength was insufficient. Sufficient low-temperature toughness also was not obtained.
• value ⁇ exceeded the upper limit value of the present disclosure, and the hardenability was too high, and the strength was excessive.
• the steel products relating to the present disclosure can be used mainly for transport tanks of liquefied carbon dioxide. Further, the steel products relating to the present disclosure can also be used in other welded structures such as buildings, bridges, ships, pipelines, offshore structures, pressure vessels and tanks.

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