EP4793381A1 - Steel material and pressure vessel - Google Patents

Steel material and pressure vessel

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Publication number
EP4793381A1
EP4793381A1 EP24877318.6A EP24877318A EP4793381A1 EP 4793381 A1 EP4793381 A1 EP 4793381A1 EP 24877318 A EP24877318 A EP 24877318A EP 4793381 A1 EP4793381 A1 EP 4793381A1
Authority
EP
European Patent Office
Prior art keywords
steel product
less
steel
temperature
toughness
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Pending
Application number
EP24877318.6A
Other languages
German (de)
French (fr)
Inventor
Tetsuya Uchiyama
Daiki NAKANISHI
Hirokazu USUKI
Hiroyuki Shirahata
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Nippon Steel Corp
Original Assignee
Nippon Steel Corp
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Nippon Steel Corp filed Critical Nippon Steel Corp
Publication of EP4793381A1 publication Critical patent/EP4793381A1/en
Pending legal-status Critical Current

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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
    • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
    • 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 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
    • 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
    • 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
    • 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
    • C21D9/50—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for welded joints
    • 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
    • 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
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • 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
    • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
    • 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
    • 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
    • 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
    • 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
    • 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

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Mechanical Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Heat Treatment Of Steel (AREA)

Abstract

A steel product having a specific chemical composition, wherein α is from 5.0 to 16.0 and β is 0.017 or less, the steel product having a tensile strength of from 615 MPa to 930 MPa, and having a microstructure at a portion that is 1/4 of a thickness from a surface of the steel product comprising lower bainite and martensite, wherein a total area ratio of the lower bainite and the martensite is 15.0% or more, a total area ratio of upper bainite, the lower bainite, and the martensite is 90.0% or more, and an area ratio of retained austenite is less than 1.7%. α = 0.50 × √[C] × (1 + 0.64[Si]) × (1 + 4.10[Mn]) × (1 + 0.27[Cu]) × (1 + 0.52[Ni]) × (1 + 2.33[Cr]) × (1 + 3.14[Mo]), β = [Mn] × [P] - [Mo]/100

Description

    Technical Field
  • The present disclosure relates to a steel product and a pressure vessel.
    • Background Art
  • Steel products can be used in welded structures such as buildings, bridges, ships, pipelines, offshore structures, and pressure vessels (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. For example, 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.
  • As with this 3.5% Ni steel, steel products that require ensured low-temperature toughness-exemplified by those used in cryogenic pressure vessels-are often made to contain Ni.
  • For example, Patent Document 1 proposes a cryogenic nickel steel plate having a specific chemical composition containing 3.0% or more but less than 5.0% of Ni, an alumina cluster index of 0.030 or less, an effective crystal grain size of 12.0 µm or less, an average value of tensile strength of from 540 MPa to 610 MPa, and a Charpy impact absorption energy at - 140°C of 150 J or more.
  • Further, Patent Document 2 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 of Ni, wherein the prior austenite grain size at the time of quenching heating is 20 µm or less, and the effective crystal grain size after a heat treatment is 12 µm or less, and the tensile strength is 450 MPa or more and 690 MPa or less.
  • Patent Document 3 discloses a method for manufacturing a high-strength and high-toughness steel, in which a steel slab having a specific chemical composition containing 1.0 to 8.0% of Ni is heated at 1000 to 1250°C, then hot-rolled at a reduction of 20 to 60% in a temperature range where austenite recrystallizes in hot rolling, and then at a reduction of 30 to 70% in a temperature range where austenite does not recrystallize, the rolling was completed at 650°C or higher, the slab is then subjected to quenching in which water cooling was started at a temperature of Ar3 or higher and ended at a temperature of 150°C or lower, the slab is further reheated up to a temperature between Ac3 and Ac3 + 100°C, quenched, and then tempered at a temperature of Ac1 or lower.
    • Patent Document 1: Japanese Patent Publication ( JP-B) No. 6610352
    • Patent Document 2: JP-B No. 6984319
    • Patent Document 3: Japanese Patent Application Laid-Open ( JP-A) No. H01-230713
    SUMMARY OF INVENTION Technical Problem
  • Having both high strength and low-temperature toughness is desirable for cryogenic steels used in cryogenic pressure vessels. Further, 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.
  • An object of the present disclosure is to provide a steel product and a pressure vessel which have high tensile strength, which exhibit good low-temperature toughness regardless of whether before or after a post weld heat treatment, and which are suited to low-temperature applications.
    • Solution to Problem
  • The gist of the present disclosure is as follows.
    • <1> A steel product having a chemical composition comprising, in mass%,
      • C: from 0.03% to 0.20%,
      • Si: from 0.01% to 0.50%,
      • Mn: from 0.10% to 2.00%,
      • P: 0.025% or less,
      • S: 0.0250% or less,
      • Ni: 4.51% or more and less than 6.10%,
      • Al: from 0.001% to 0.100%,
      • O: 0.0100% or less,
      • N: 0.0100% or less,
      • Cu: 0 to 1.50%,
      • Cr: 0 to 3.00%,
      • Mo: 0 to 2.00%,
      • B: 0 to 0.0050%,
      • Nb: 0 to 0.050%,
      • Ti: 0 to 0.050%,
      • V: 0 to 0.10%,
      • Mg: 0 to 0.0200%,
      • Ca: 0 to 0.0200%,
      • REM: 0 to 0.0200%, and
      • balance: Fe and impurities,
      • and in which α represented by following Formula (1) is from 5.0 to 16.0, and β represented by following Formula (2) is 0.017 or less,
      • wherein:
        • a tensile strength is from 615 MPa to 930 MPa,
        • a microstructure of a portion that is 1/4 of a thickness in a thickness direction from a surface of the steel product comprises lower bainite and martensite, a total of area ratios of the lower bainite and the martensite is 15.0% or more, a total of area ratios of upper bainite, the lower bainite, and the martensite is 90.0% or more, and an area ratio of retained austenite is less than 1.7%, α = 0.50 × √[C] × (1 + 0.64[Si]) × (1 + 4.10[Mn]) × (1 + 0.27[Cu]) × (1 + 0.52[Ni]) × (1 + 2.33[Cr]) × (1 + 3.14[Mo]) β = Mn × P − Mo / 100
        • wherein in a symbol of an element in square brackets in Formulae (1) and (2) represents a content (mass%) of a corresponding element contained in the steel product, provided that, zero is substituted when the corresponding element is not contained.
    • <2> The steel product according to <1>, wherein the chemical composition comprises the following Group A:
      [Group A]:
      one or two or more selected from the group consisting of:
      • Cu: from 0.01% to 1.50%,
      • Cr: from 0.01% to 3.00%,
      • Mo: from 0.01% to 2.00%, and
      • B: from 0.0003% to 0.0050%.
    • <3> The steel product according to <1> or <2>, wherein the chemical composition comprises the following Group B:
      [Group B]:
      one or two or more selected from the group consisting of:
      • Nb: from 0.001% to 0.050%,
      • Ti: from 0.001% to 0.050%, and
      • V: from 0.01% to 0.10%.
    • <4> The steel product according to any one of <1> to <3>, wherein the chemical composition comprises the following Group C:
      [Group C]:
      one or two or more selected from the group consisting of:
      • Mg: from 0.0003% to 0.0200%,
      • Ca: from 0.0003% to 0.0200%, and
      • REM: from 0.0003% to 0.0200%.
    • <5> The steel product according to any one of <1> to <4>, wherein, in the microstructure of the portion that is 1/4 of the thickness, in the thickness direction, from the surface of the steel product, an average crystal grain size is 20.0 µm or less.
    • <6> The steel product according to any one of <1> to <5>, wherein an aspect ratio of a prior austenite grain in the portion that is 1/4 of the thickness, in the thickness direction, from the surface of the steel product is 1.5 or more.
    • <7> The steel product according to any one of <1> to <6>, wherein a Charpy impact absorption energy at -110°C in the portion that is 1/4 of the thickness is 150 J or more.
    • <8> The steel product according to any one of <1> to <7>, wherein, when a heat treatment, in which a rate of temperature increase and a rate of temperature decrease in a temperature region of 425°C or more are 55°C/h and the temperature is held for 2 hours at 600°C, is carried out on the steel product, a Charpy impact absorption energy at -110°C of the heat-treated portion is 150 J or more.
    • <9> The steel product according to any one of <1> to <8>, wherein when a thermal cycle simulating welding equivalent to 2 kJ/mm is applied to the portion that is 1/4 of the thickness, and a heat treatment is further carried out on the steel product, in which a rate of temperature increase and a rate of temperature decrease in a temperature region of 425°C or more are 55°C/h, and the temperature is held for 2 hours at 600°C, a P concentration at prior austenite grain boundaries is 3.0% or less in atomic percent concentration, and a grain boundary carbide coverage is 40% or less.
    • <10> The steel product according to any one of <1> to <9>, wherein when a thermal cycle simulating welding equivalent to 2 kJ/mm is applied to the portion that is 1/4 of the thickness, and a heat treatment is further carried out on the steel product, in which a rate of temperature increase and a rate of temperature decrease in a temperature region of 425°C or more are 55°C/h, and the temperature is held for 2 hours at 600°C, a Charpy impact absorption energy at -110°C of the heat-treated portion is 50 J or more.
    • <11> A pressure vessel, comprising the steel product according to any one of <1> to <10>.
    Advantageous Effects of Invention
  • According to the present disclosure, there can be provided a steel product and a pressure vessel which have high tensile strength, which exhibit good low-temperature toughness regardless of whether before or after a post weld heat treatment, and which are suited to low-temperature applications. The low temperature herein means a temperature range, for example, from -10 to -110°C.
    • BRIEF DESCRIPTION OF DRAWINGS
  • Fig. 1 is a diagram illustrating an example of results of discriminating the microstructure.
    • DESCRIPTION OF EMBODIMENTS
  • The present disclosure is described in detail hereinafter.
  • The "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", "base metal of 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 according to the present disclosure.
  • In the present disclosure, numerical ranges expressed by using "-" mean ranges in which the numerical values listed before and after the "-" are included as the lower limit value and upper limit value, respectively. However, numerical value ranges in which "exceeds" or "less than" is appended to the numerical value listed before or after the "-" mean a range in which these numerical values are not included as the lower limit value or the upper limit value.
  • With respect to the contents of the elements in a chemical composition, "%" means "mass%".
  • The term "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.
  • A steel product according to an embodiment of the present disclosure is described hereinafter. The new knowledge, which was obtained as the result of studies by the inventors of the present disclosure and which was arrived at in completing the steel product according to the present disclosure, is described in detail first.
  • 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 portion 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. As a result, it was learned that, at the microstructure of the 1/4t portion of a steel product having a tensile strength of 615 MPa or more and 930 MPa or less, the area ratio of ferrite is less than 10.0%, and the total of the area ratios of upper bainite, lower bainite and martensite is 90.0% or more. Note that the total of the area ratios of upper bainite, lower bainite and martensite was measured by using electron back scatter diffraction (hereinafter called "EBSD").
  • Further, 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. As a result, it was found that, in a steel product exhibiting a Charpy impact absorption energy of 150 J or more at -110°C, the total of the area ratios of lower bainite and martensite is 15.0% or more, and the area ratio of retained austenite is less than 1.7%. Note that the total of the area ratios of lower bainite and martensite was measured by using EBSD. Note that the area ratio of the retained austenite was measured by X-ray diffraction. The volume ratio of the retained austenite measured by the X-ray diffraction may be considered to be the area ratio.
  • Moreover, 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 size hereinafter. The sample was subjected to mechanical polishing and electrolytic polishing, and analysis was carried out on a 4 mm2 region by an EBSD device equipped with an FE-SEM (field emission scanning electron microscope). The value calculated by an area weighted average that was weighted by the area per crystal grain among the crystal grain sizes measured in the 4 mm2 region, was used as the average crystal grain size (also called "effective crystal grain size" upon occasion). It was learned that, if the average crystal grain size of a 1/4t portion of a steel product is 20.0 µm or less, there is the tendency for the toughness of the steel product to improve more regardless of whether before or after a post weld heat treatment.
  • In addition, in a case where a sample was collected from the 1/4t portion in the thickness direction from the surface of the steel product, and a thermal cycle simulating welding equivalent to 2 kJ/mm was applied, the steel product having a Charpy impact absorption energy of 50 J or more at -110°C, regardless of before and after post weld heat treatment, was found to have a P concentration at the prior austenite grain boundaries of 3.0% or less in atomic percent concentration, and a grain boundary carbide coverage of 40% or less, after going through the thermal cycle and the post weld heat treatment.
  • Further, the inventors of the present disclosure have found that similar results can be obtained not only with steel products after hot rolling and accelerated cooling, but also with steel products after reheat quenching.
    • <Chemical Composition>
  • Alloy elements that constitute the chemical composition of the steel product according to the present disclosure are described next. Note that, in the following description of the alloy elements, the "%" of the content means "mass%".
    • (C: 0.03% or more and 0.20% or less)
  • 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.
    • (Si: 0.01% or more and 0.50% or less)
  • Si is used as a deoxidizer, and is also an element that forms a solid solution in the steel and increases the strength. In the present disclosure, the Si content is 0.01% or more, from the standpoint of controlling the concentration of O contained in the molten steel. The Si content is preferably 0.03% or more, 0.05% or more, 0.10% or more, or 0.12% or more. On the other hand, if the Si content is excessive, a hard phase may be formed in the HAZ, and the toughness may be reduced. Meanwhile in a case where the PWHT is applied to the heat-affected zone after welding, the grain boundary coverage by carbide precipitated at the prior austenite grain boundaries would increase, and the toughness after PWHT after welding would decrease. Thus, the Si content in the present disclosure is 0.50% or less, from the standpoint of ensuring the HAZ toughness before and after PWHT. The Si content is preferably 0.30% or less, or 0.20% or less.
    • (Mn: 0.10% or more and 2.00% 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. On the other hand, if the Mn content is excessive, there are cases in which the toughness after PWHT decreases due to the Mn segregating at the grain boundary at the time of PWHT. Accordingly, from the standpoint of ensuring the toughness of the steel product after PWHT, in the present disclosure, the Mn content is 2.00% or less. The Mn content is preferably 1.80% or less, or 1.50% or less.
    • (P: 0.025% or less)
  • P is an impurity element. Although the lower limit of the P content is not limited, from the standpoint of the manufacturing cost, in the present disclosure, the P content may be 0.001% or more. On the other hand, if the P content is excessive, there are cases in which the toughness after PWHT decreases due to the P segregating at the grain boundary at the time of PWHT. Accordingly, in the present disclosure, 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.
    • (S: 0.0250% or less)
  • S is an impurity element. Although the lower limit of the S content is not limited, from the standpoint of the manufacturing cost, in the present disclosure, the S content may be 0.0001% or more. On the other hand, if the S content is excessive, there are cases in which elongated MnS is generated at the center segregation region, and the toughness and ductility of the steel product and the HAZ deteriorate. From the standpoint of ensuring the toughness and ductility of the steel product and the HAZ, the S content is 0.0250% or less. The S content is preferably 0.0100% or less or 0.0050% or less.
    • (Ni: 4.51% or more and less than 6.10%)
  • Ni is an element that is effective in improving the hardenability and toughness of the steel. Therefore, in the present embodiment, the Ni content is 4.51% or more. The Ni content is preferably 5.00% or more, or 5.25% or more. However, Ni is an expensive element, and, from the standpoint of cost reduction, in the present disclosure, the Ni content is less than 6.10%. The Ni content is preferably 6.00% or less, or 5.75% or less.
    • (Al: 0.001% or more and 0.100% or less)
  • Al is an element that is effective in deoxidation, and is an element that, by forming a nitride, refines the crystal grain 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 coarse nitride particles, 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 less, 0.070% or less, 0.060% or less, or 0.050% or less.
    • (O: 0.0100% or less)
  • O is an impurity element. Although 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. On the other hand, if the O content is excessive, there are cases in which coarse oxide particles are generated, and the toughness and ductility of the steel product and the HAZ deteriorate. From the standpoint of ensuring the toughness and ductility of the steel product and the HAZ, the O content is 0.0100% or less. The O content is preferably 0.0060% or less, 0.0050% or less, or 0.0040% or less.
    • (N: 0.0100% or less)
  • N is an impurity element. Although the lower limit of the N content is not limited, from the standpoint of the manufacturing cost, in the present disclosure, 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 according to the present disclosure may contain other elements (optional elements) instead of some of the Fe. The following optional elements in Groups A to C are given as examples, but the contents of these elements may be 0%.
    • [Group A]
  • In order to improve the strength and toughness, as needed, the steel product according 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: 1.50% or less)
  • Cu is an element that is sometimes mixed into the steel product in the manufacturing process. However, the lower limit value of the Cu content is not limited and may be 0%. Further, 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. Thus, in the present disclosure, the Cu content may be 0.01% or more. The Cu content is preferably 0.10% or more. However, from the standpoint of suppressing the occurrence of Cu cracking at the time of hot rolling of the steel product, in the present disclosure, 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.
    • (Cr: 3.00% or less)
  • Cr is an element that is sometimes mixed into a steel product in the manufacturing process. However, the lower limit value of the Cr content is not limited, and may be 0%. Further, 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. However, from the standpoint of suppressing deterioration in the toughness and weldability of the HAZ, in the present disclosure, 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: 2.00% or less)
  • Mo is an element that is sometimes mixed into a steel product in the manufacturing process. However, the lower limit value of the Mo content is not limited, and may be 0%. Further, 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, and also segregates at the grain boundaries of the steel to improve grain boundary strength. 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. However, from the standpoints of suppressing deterioration in the toughness and weldability of the HAZ, and suppressing an increase in the alloy cost, in the present disclosure, the Mo content is 2.00% or less. The Mo content is preferably 1.20% or less, or 0.80% or less.
    • (B: 0.0050% or less)
  • B is an element that is sometimes mixed into a steel product in the manufacturing process. However, the lower limit value of the B content is not limited, and may be 0%. Further, 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. However, from the standpoint of suppressing deterioration in the surface quality of a steel slab manufactured in continuous casting, in the present disclosure, the B content is 0.0050% or less. The B content is preferably 0.0030% or less, or 0.0020% or less.
    • [Group B]
  • In order to improve the strength, as needed, the steel product according 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: 0.050% or less)
  • Nb is an element that is sometimes mixed into a steel product in the manufacturing process. However, the lower limit value of the Nb content is not limited, and may be 0%. Further, 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. However, from the standpoint of suppressing deterioration in the toughness and weldability of the HAZ, 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: 0.050% or less)
  • Ti is an element that is sometimes mixed into a steel product in the manufacturing process. However, the lower limit value of the Ti content is not limited, and may be 0%. Further, 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. However, from the standpoint of suppressing deterioration in the toughness and weldability of the HAZ, the Ti content is 0.050% or less. The Ti content is preferably 0.040% or less, 0.030% or less, or 0.020% 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: 0.10% or less)
  • V is an element that is sometimes mixed into a steel product in the manufacturing process. However, the lower limit value of the V content is not limited, and may be 0%. Further, 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. However, from the standpoints of suppressing deterioration in the toughness and weldability of the HAZ, and suppressing an increase in the alloy cost, the V content is 0.10% or less. The V content is preferably 0.08% or less, or 0.05% or less.
    • [Group C]
  • In order to improve the toughness of the HAZ, as needed, the steel product according 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: 0.0200% or less)
  • Mg is an element that is sometimes mixed into a steel product in the manufacturing process. However, the lower limit value of the Mg content is not limited, and may be 0%. Further, 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. On the other hand, if the Mg content is excessive, there are cases in which the Mg forms coarse oxide particles and decreases the toughness of the steel. Accordingly, from the standpoint of ensuring the toughness, in the present disclosure, 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: 0.0200% or less)
  • Ca is an element that is sometimes mixed into a steel product in the manufacturing process. However, the lower limit value of the Ca content is not limited, and may be 0%. Further, 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. On the other hand, if the Ca content is excessive, there are cases in which the Ca forms coarse oxide particles 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.
    • (REM: 0.0200% or less)
  • Rare earth metal (REM) 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. However, the lower limit value of the REM content is not limited, and may be 0%. Further, 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. On the other hand, if the REM content is excessive, there are cases in which the REMs form coarse oxide particles and decrease the toughness of the steel. Accordingly, from the standpoint of ensuring the toughness, in the present disclosure, 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.
    • (Balance: Fe and Impurities)
  • The balance of the chemical composition of the steel product according 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.
  • In addition to the limits on the contents of the respective elements, in the present disclosure, the ranges of values α and β are limited as follows, respectively.
    • (Value α: 5.0 or more and 16.0 or less)
  • Value α is computed by following Formula (1). α = 0.50 × √[C] × (1+0.64[Si]) × (1+4.10[Mn]) × (1+0.27[Cu]) × (1+0.52[Ni]) × (1+2.33[Cr]) × (1+3.14[Mo])
  • Wherein [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.
  • In the present disclosure, the range of value α is 5.0 - 16.0. This is an index expressing the hardenability of the steel product. The larger the value α, the more that lower bainite and martensite microstructures having a superior balance of strength and toughness can be formed. When α 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. When α is 5.0 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. Further, in the microstructure of the HAZ as well, it is easy for the ratio of the lower bainite and martensite to increase, and the HAZ toughness also improves. On the other hand, if value α is 16.0 or less, toughness can be ensured without the strength of the steel product becoming too high. Further, if value α is 16.0 or less, the toughness after PWHT also can be ensured. Moreover, the HAZ toughness also is ensured without the HAZ becoming too hard.
  • Due to the aforementioned numerical value range relating to value α being satisfied, there can be provided a nickel-containing steel product for low temperatures that has excellent strength and toughness. Value α is preferably 5.5 or more, 6.0 or more, or 7.0 or more. Further, value α is preferably 15.5 or less or 15.0 or less.
    • (Value β: 0.017 or less)
  • The value β is computed by the following Formula (2). β = Mn × P − Mo / 100 wherein, [Mn], [P], and [Mo] are the contents (mass%) of Mn, P, and Mo in the steel. In a case in which a given element is not contained, zero is substituted in.
  • In the present disclosure, the range of the value β is set to 0.017 or less. The value β is an index expressing the toughness of the steel product, after application of a thermal cycle simulating welding equivalent to 2 kJ/mm, and further after application of PWHT. The larger the value β, the more the toughness degrades after application of a thermal cycle simulating welding equivalent to 2 kJ/mm, and further after application of PWHT. When the value β is 0.017 or less, grain boundary fracture can be suppressed in the Charpy test after the thermal cycle and after PWHT, and the toughness can be ensured.
  • Due to the aforementioned numerical value range relating to value β being satisfied, there can be provided a cryogenic nickel steel product having excellent toughness after thermal cycle and after PWHT. The value β is preferably 0.016 or less, or 0.015 or less.
  • The lower limit of the value β is not particularly limited, and may be -0.010 or more, -0.005 or more, or 0.000 or more from the standpoint of cost.
    • <Microstructure>
  • The microstructure of the steel product according to the present disclosure is described next. The microstructure of the portion that is 1/4 of the thickness in the thickness direction from the surface of the steel product according to the present disclosure contains lower bainite and martensite. Further, in addition to lower bainite, upper bainite also may be contained as bainite.
  • "Bainite" is a collective term for upper bainite and lower bainite. "Upper bainite" is one of or both of upper bainite that contains retained austenite or an MA phase (martensite-austenite constituent) 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.
  • There are four forms of "martensite" that are lath, butterfly, lenticular and plate-shaped, but mainly lath martensite is generated in the components of the present disclosure. 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.
    • (Total of Area Ratios of Lower Bainite and Martensite: 15.0% or more)
  • 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%.
    • (Total of Area Ratios of Upper Bainite, Lower Bainite, and Martensite: 90.0% or more)
  • From the standpoint of ensuring the strength of the steel product, the total of the area ratios of upper bainite, 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%. Further, the upper bainite of the 1/4t portion may be 1.0% or more.
    • (Area Ratio of Retained Austenite: less than 1.7%)
  • From the standpoint of ensuring the toughness of the steel product, the area ratio of the retained austenite of the 1/4t portion is less than 1.7%. The area ratio of the retained austenite of the 1/4t portion is preferably 1.0% or less, and may be 0.0%. This is thought to be because, in the Ni-containing steel plate of the present disclosure, the Ni content is lower than conventional 9% Ni steel, and therefore, even if retained austenite were to exist at -110°C, it would be unstable, and, if the steel structure were to incur plastic deformation at the crack tip, the retained austenite would change into martensite due to plasticity-induced martensitic transformation. Thus, the retained austenite at room temperature is made to be less than 1.7% as a volume ratio (area ratio).
  • Further, the higher the Ni content, the easier it is for the volume ratio of the retained austenite to increase as well, and an increase in the Ni content is also not preferable in that it leads to an increase in cost.
  • 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.
  • By using an electrolytically polished sample that, after mirror finishing by mechanical polishing, is subjected to electrolytic polishing that removes the strained layers arising due to the mechanical polishing, measurement of the total of the area ratios of the upper bainite, lower bainite, martensite and retained austenite is carried out by EBSD. The measurement magnification is 200×, and measurement of a range of 400 µm × 400 µm is carried out at a pitch of 0.4 µm. The measuring is carried out in a state in which the beam diameter of the electron beam is 0.4 µm or less. The confidence index (hereinafter called "CI value") is set to 0.1 or more. 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. Note that the GAM value is an index defined in 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, martensite or retained austenite. Because the upper bainite, lower bainite, martensite and retained austenite in the present disclosure are determined by using the GAM of EBSD as the threshold value, not only upper bainite, lower bainite, martensite and retained austenite, 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.
  • 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 µm × 480 µm 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. The microstructure of the region, at which the internal structure of the microstructure is relatively coarse, and the density of carbide is low and both sparse and dense, 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 α is 9.9. In (B), the regions surrounded by the white lines are upper bainite (Bu), and the other regions are lower bainite + martensite (BL+M). In the portions identified as upper bainite (Bu), carbides appearing white are sparsely distributed, and regions with varying density are present. On the other hand, at the portions identified as lower bainite + martensite (BL+M), the carbides are densely and uniformly distributed. The total of the area ratios of the lower bainite, martensite and retained austenite is determined by subtracting the area ratio of the upper bainite from the total of the area ratios of the upper bainite, lower bainite, martensite and retained austenite, which were measured as described above. Moreover, the area ratio of the retained austenite is determined by the measuring method described hereinafter, and the total of the area ratios of the lower bainite and martensite is determined by subtracting the area ratio of the retained austenite from the total of the area ratios of the lower bainite, martensite and retained austenite.
  • When the total of area ratios of lower bainite and martensite is more than 0% according to the above structure discrimination, lower bainite and martensite are usually included. Lower bainite and martensite can be distinguished by a SEM or TEM (transmission electron microscope), and the presence of each structure can be found.
    • (Area Ratio of Retained Austenite)
  • The area ratio of retained austenite is measured by an X-ray diffraction method. The area ratio of retained austenite is measured using a sample in which a portion of 1/4 of the thickness in the thickness direction from a surface of the steel product (in the present specification, also referred to as a "1/4t portion") is taken as a measured area. The sample is a test piece having a thickness of 2 mm; the sample is taken from a position of 1/4 of the width from an end portion in the width direction (a direction perpendicular to the rolling direction and the thickness direction) of the steel product, is subjected to chemical polishing, and is used for measurement of the volume ratio of retained austenite by an X-ray diffraction method using a Mo tube. Quantification was performed based on the ratio of the integrated intensities of the (200) and (211) diffraction peaks of the ferrite phase to the integrated intensities of the (200), (220), and (311) diffraction peaks of the austenite phase, and the average value of six combinations was taken. The integrated intensity of the diffraction peak is obtained by fitting the background on the basis of the signals before and after the peak and subtracting the signals. The volume ratio measured by the X-ray diffraction method is regarded as the area ratio.
    • (Average Crystal Grain Size of 1/4t Portion of Steel Product)
  • In the present disclosure, the average crystal grain size (effective crystal grain size) of the 1/4t portion of the steel product is preferably 20.0 µm or less. This is because it was learned that, if the average crystal grain size of the 1/4t portion of the steel product is 20.0 µm 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 size of the 1/4t portion of the steel product may exceed 20.0 µm. The smaller the average crystal grain size of the steel product, the more preferable, and therefore, the lower limit value thereof is not limited. Usually, the average crystal grain size is 10 µm or more. The effective crystal grain size is determined by a weighted average. Effective crystal grain size Darea that is determined by a weighted average is calculated by the following formula by using, of the crystal grain sizes that are measured at a magnification of 500× in a 4 mm2 region, area Si and grain size di of the ith crystal grain detected at the time of measurement. D area = ΣS i · d i / ΣS i
    • (Aspect Ratio of Prior Austenite Grains of 1/4t Portion of Steel Product)
  • The form of the prior austenite grains (called prior austenite 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 portion 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 effective crystal grain size. 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 grains (hereinafter sometimes referred to as prior austenite grains) 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 portion that is 1/4 of the thickness in the thickness direction from the surface of the steel product is mirror polished, and etching is carried out by an etchant of a saturated aqueous 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.
  • Next, a region of 0.5 mm2 is observed at a magnification of 500×, 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.
    • <Mechanical Properties>
  • The steel product according 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 -110°C in particular, the steel product can exhibit excellent low-temperature toughness after PWHT as well.
    • (Tensile Strength: 615 MPa or more and 930 MPa or less)
  • In the present disclosure, the tensile strength of the steel product is 615 - 930 MPa. In order to reduce the weight of large welded structures such as transport tanks, 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 present disclosure also is manufactured to have the aforementioned tensile strength.
    • (Yield Ratio)
  • The yield ratio (YR (%) = yield strength / tensile strength × 100) of the steel product according to the present disclosure is not particularly limited, and is preferably 90% or less. In a case in which there is no yield point, the yield strength is determined by using 0.2% proof stress.
    • (Charpy Impact Absorption Energy at -110°C)
  • In order to ensure high toughness at low temperatures, the Charpy impact absorption energy at -110°C 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 -110°C of 150 J or more, a tank manufactured using the steel product of the present disclosure can be suitably used for transporting liquid carbon dioxide. The steel product according to the present disclosure may exhibit low-temperature toughness with a Charpy impact absorption energy of 100 J or more at -110°C. Note that the Charpy impact absorption energy at -110°C is a numerical value measured by using a sample taken from a position of 1/4 of the thickness.
    • (Charpy Impact Absorption Energy at -110°C after PWHT)
  • There are cases in which 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. At this time, not only the welded portions, but also the base metal portion (also simply called base metal) of the steel product that is not affected by welding are heated. If the time over which the base metal is heated in the temperature range of 425°C or more is long, the toughness of the base metal tends to decrease. In the steel product of the present disclosure, in a case in which PWHT, in which the holding temperature is 600°C, and the holding time is 2 hours, and the rate of temperature increase and the rate of temperature decrease are 55°C/h in the temperature region of 425°C or more, is carried out on the steel product, with regard to the toughness of the portion where the PWHT has been carried out, the Charpy impact absorption energy at -110°C is preferably 150 J or more. The Charpy impact absorption energy at -110°C after PWHT may be 100 J or more. The Charpy impact absorption energy at -110°C after PWHT also is a numerical value measured by using a sample taken from a position of 1/4 of the thickness.
    • (P Concentration at Prior Austenite Grain Boundary at 1/4t Portion of Steel Product after Thermal Cycle and after PWHT)
  • The steel product according to the present disclosure preferably has a P concentration at the prior austenite grain boundaries of 3.0% or less in atomic percent concentration, after the thermal cycle at the portion of 1/4 of the thickness in the thickness direction from the surface of the steel product, and after PWHT. In the measurement of P concentration at the prior austenite grain boundaries, a periphery of a region including the prior austenite grain boundaries is milled with use of focused ion beam (FIB), and then the region including the prior austenite grain boundaries in a size of about 100 µm × 100 µm is fixed to the tip of a nanoneedle with the aid of Pt deposition, picked up, and further thinned to 100 nm or less. Thereafter, by scanning transmission electron microscopy with use of an aberration-corrected transmission electron microscope, a region of 20 nm × 20 nm is scanned at 1,800,000× magnification and at 0.35 nm/pixel, under a condition that the electron beam is incident in parallel to the prior austenite grain boundaries, and the emitted characteristic X-rays are detected by energy dispersive X-ray fluorescence spectroscopy (EDS) for elemental analysis. Signals are integrated at a 2-pixel width in a direction parallel to the grain boundary, to quantify signals attributable to P and Fe, whereby a line profile of P in the vicinity of the grain boundary is acquired. The P concentration determined by the signal integration just above the grain boundary, and the element quantification is taken as the amount of P segregation at the grain boundary. The unit is expressed in atomic percent concentration. Data are acquired at three grain boundaries in total, and an average value is determined for evaluation. The smaller the amount of P segregation at the grain boundary, the more the grain boundary fracture is suppressed, and the HAZ toughness after the thermal cycle and after PWHT can be ensured. The concentration is preferably 2.5% or less in atomic percent concentration, and is more preferably 2.0% or less.
    • (Grain Boundary Carbide Coverage in Prior Austenite Grain Boundary at Portion that is 1/4 of Thickness in Thickness Direction from Surface of Steel Product after Thermal Cycle and after PWHT)
  • The steel product according to the present disclosure preferably has a grain boundary carbide coverage at the prior austenite grain boundaries of 40% or less at the portion that is 1/4 of the thickness in the thickness direction from the surface of the steel product after thermal cycle and after PWHT. The grain boundary carbide coverage at the prior austenite grain boundaries is measured by subjecting a sample taken from a portion that is 1/4 of the thickness of the steel product, after thermal cycle and after PWHT, to electrolytic polishing, and observing a secondary electron image along prior austenite grain boundaries using a scanning electron microscope having a field emission-type electron gun. The magnification is appropriately changed depending on the prior austenite grain size. For example, the image is acquired at 10,000× magnification. The secondary electron image is acquired for one grain boundary, the length of the entire grain boundary and the length of a region where the carbide is present are measured, the length of the region where the carbide is present is divided by the length of the entire grain boundary, and the quotient is multiplied by 100, to calculate the grain boundary carbide coverage. Data are obtained for five grain boundaries, and evaluation is performed using an average value thereof as a grain boundary carbide coverage. The lower the grain boundary carbide coverage, the more the grain boundary fracture is suppressed, and the HAZ toughness after thermal cycle and after PWHT can be ensured. The coverage is preferably 37% or less, and is more preferably 35% or less.
    • (Charpy Impact Absorption Energy at -110°C after Thermal Cycling)
  • In the steel product of the present disclosure, in order to ensure high toughness after a thermal cycling test that simulates the welded portions at a low temperature, the Charpy impact absorption energy at -110°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 -110°C after thermal cycling is 50 J or more, a tank manufactured using the steel product of the present disclosure can be suitably used for transporting liquid carbon dioxide for example. The Charpy impact absorption energy at -110°C after thermal cycling may be 40 J or more. Note that the Charpy impact absorption energy at -110°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.
    • (Charpy Impact Absorption Energy at -110°C after Thermal Cycling and PWHT)
  • There are cases in which 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. After the above-described thermal cycling test, 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. In this case, with regard to the toughness of the portion where the PWHT has been carried out, the Charpy impact absorption energy at -110°C is preferably 50 J or more. The Charpy impact absorption energy at -110°C of the portion at which PWHT is carried out after the above-described thermal cycling test may be 40 J or more.
  • Note that there are cases in which the toughness of the steel product decreases due to PWHT. Although the reason for this is not clear, it is assumed that P (phosphorus) and Mn diffuse to the grain boundaries, and carbides are formed at the grain boundaries, thereby reducing toughness. A decrease in the toughness due to PWHT is suppressed by limiting the contents of P and Mn and making the average crystal grain size of the steel product small, and also a decrease in toughness after the thermal cycle and after PWHT is suppressed by limiting the contents of P and Mn, by satisfying the correlation formula β, by controlling the P concentration at the prior austenite grain boundary to 3.0% or less in atomic percent concentration, and by controlling the grain boundary carbide coverage to 40% or less.
  • The tensile strength (TS) and the yield strength (YS) 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 (the direction perpendicular to the rolling direction and the thickness 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 -110°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 V-notch of the test piece is formed such that the longitudinal direction of the notch is the thickness direction of the steel product and the depth direction of the notch is the rolling direction of the steel product.
  • The form of the steel product according to the present disclosure is not particularly limited, and examples are steel plates, steel strips, structural steel and steel pipes. However, 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.
  • Further, although the application of the steel product according to the present disclosure also is not particularly limited, the steel product according 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 according to the present disclosure can be suitably used as a constituent material for a pressure vessel, specifically a tank that stores and transports liquefied gasses, and liquid carbon dioxide in particular.
    • (Method of Manufacturing Steel Product)
  • The method of manufacturing the steel product according 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 be made into a steel product. Note that, in the case of RQ, the hot rolled steel does not necessarily have to be air-cooled before reheating, but water cooling may be used. Moreover, tempering (T) may also be carried out.
    1. (1) DQT: direct quenching (DQ) and tempering (T)
    2. (2) RQT: air cooling or water cooling, reheat quenching (RQ), and tempering (T)
  • From the standpoint of the manufacturing cost, DQT is preferable in the manufacturing of the steel product according to the present disclosure. An example of a preferable manufacturing process is given hereinafter.
  • When the steel product according to the present disclosure is produced by DQ, from the standpoint of carrying out hot rolling in a temperature range in which the microstructure of the rolled steel is austenite, the heating temperature of the steel slab on which the hot rolling is carried out is Ac3 or more. From the standpoint of decreasing the deformation resistance, the heating temperature of the steel slab is preferably 1000°C or more. On the other hand, from the standpoint of suppressing coarsening of the heated γ grains, 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. Note that Ac3 is a value computed by the following formula. Ac3 = 937.2 - 436.5C + 56Si - 19.7Mn - 16.3Cu - 26.6Ni - 4.9Cr + 38.1Mo + 124.8V + 136.3Ti - 19.1Nb + 198.4Al + 3315B
  • The element symbols in the formula mean the content (mass%) of each element contained in the steel slab.
  • There are cases in which 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. From the standpoint of refining the austenite grain size of the steel product, the cumulative rolling reduction ratio of the rolling in the recrystallization temperature range is preferably 20% or more, and more preferably 30% or more. The cumulative rolling reduction ratio of the rolling in the recrystallization temperature range is determined from the difference between the thickness of the steel slab before hot rolling and the thickness of the rolled steel at 900°C. Cumulative rolling reduction ratio (%) of rolling in recrystallization temperature range = 100 × ([thickness of steel slab] - [thickness of steel to be rolled at 900°C]) / [thickness of steel slab]
  • Rolling in the non-recrystallization temperature range is hot rolling carried out with the temperature of the steel to be rolled during rolling being less than 900°C. From the standpoint of refining the average crystal grain size of the steel product, 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 steel to be rolled 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 steel to be rolled at 900°C] - [thickness of steel product after rolling ends]) / [thickness of steel to be rolled at 900°C]
  • From the standpoint of suppressing the generation of ferrite that decreases strength, the end temperature of the hot rolling is Ar3 or more. After hot rolling ends, accelerated cooling such as water cooling is carried out on the steel product. From the standpoint of suppressing the generation of ferrite that decreases strength, the start temperature of the accelerated cooling is Ar3 or more. Note that Ar3 is a value computed by the following formula. Ar 3 = 910 − 310 C − 80 Mn − 20 Cu − 15 Cr − 55 Ni − 80 Mo + 0.35 t − 8
  • 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.
  • From the standpoint of promoting bainitic transformation and martensitic transformation, 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 faster the cooling rate of the accelerated cooling, the more preferable, but from standpoints such as cost and homogeneity of the accelerated cooling, the cooling rate is preferably 50.0°C/s or less, or 30.0°C/s or less. 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.
  • From the standpoint of improving the strength of the steel product by ensuring the upper bainite, lower bainite and martensite, 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.
  • After accelerated cooling, a tempering treatment may be carried out on the steel product. From the standpoint of suppressing a decrease in strength, the heating temperature of the tempering treatment is preferably 650°C or less, 620°C or less, or 590°C or less. On the other hand, from the standpoint of improving the toughness, the heating temperature of the tempering treatment is preferably 350°C or more, or 400°C or more.
    • EXAMPLES
  • The steel product according to the present disclosure is described concretely hereinafter by way of Examples. However, 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 present disclosure is not limited to the following Examples.
    • [Manufacturing of Steel Product]
  • First, 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. The underlines mean that the value is outside of the scope of the present disclosure.
  • Next, steel products were manufactured from these slabs under the manufacturing conditions listed in Table 2. "Temper heat treatment" is the heating temperature in the tempering treatment after the quenching. [Table 2]
    No. Plate thickness (mm) Ac3 [°C] Hot rolling conditions Ar3 [°C] Direct quenching (accelerated cooling) conditions Temper heat treatment [°C]
    Heating temperature [°C] Cumulative rolling reduction ratio at 900°C or higher [%] 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]
    1 40 703 1200 40 76 810 water-cooling 456 790 280 11.3 530
    2 35 698 1150 27 80 830 water-cooling 408 810 163 7.9 560
    3 40 725 1100 55 55 800 water-cooling 495 780 194 5.7 610
    4 20 708 1080 70 70 775 water-cooling 452 750 134 6.6 620
    5 55 706 1000 40 53 820 water-cooling 447 796 334 5.3 560
    6 35 739 1150 56 68 785 water-cooling 488 770 267 13.3 450
    7 55 774 1200 37 45 810 water-cooling 512 785 310 2.6 610
    8 40 713 1100 23 67 820 water-cooling 446 805 157 4.8 470
    9 30 685 1150 29 83 750 water-cooling 409 740 221 5.1 550
    10 50 773 1230 57 59 810 water-cooling 506 790 143 3.6 610
    11 60 734 1050 33 50 800 water-cooling 527 775 343 20.1 360
    12 45 742 1080 46 68 790 water-cooling 495 765 310 5.2 550
    13 55 783 1200 65 35 870 water-cooling 492 850 207 10.3 610
    14 60 772 1150 39 40 810 water-cooling 495 795 246 3.9 500
    15 25 780 1230 68 70 820 water-cooling 497 790 105 5.1 615
    16 55 753 1150 66 38 810 water-cooling 479 785 176 6.8 400
    17 80 701 1200 57 37 780 water-cooling 450 770 245 4.3 530
    18 45 733 1100 47 48 830 water-cooling 468 790 398 7.2 470
    19 65 805 1050 56 45 810 water-cooling 447 800 320 5.4 560
    20 45 749 1150 42 50 820 water-cooling 496 805 154 2.6 440
    21 30 746 1230 60 70 840 water-cooling 468 820 237 8.1 500
    22 55 754 1150 38 43 780 water-cooling 503 765 210 4.8 590
    23 60 706 1100 13 44 770 water-cooling 477 750 376 5.3 530
    24 70 712 1200 63 15 810 water-cooling 468 795 298 3.8 460
    25 50 762 1150 14 48 800 water-cooling 532 785 389 8.8 430
    26 60 741 1100 56 13 820 water-cooling 439 800 287 3.2 480
    27 25 762 1050 55 76 790 water-cooling 456 775 201 6.3 520
    28 35 713 1000 57 63 800 water-cooling 371 780 369 5.4 500
    29 40 712 1200 45 73 780 water-cooling 455 765 220 4.3 490
    30 45 708 1100 50 65 810 water-cooling 440 790 148 6.3 400
    31 35 756 1050 70 60 800 water-cooling 511 780 336 7.6 560
    32 50 730 1100 33 68 820 water-cooling 454 800 298 4.1 520
    33 40 747 1200 56 58 790 water-cooling 532 495 220 5.5 580
    34 45 746 1050 49 60 810 water-cooling 408 790 276 8.4 510
    35 50 762 1200 45 60 820 water-cooling 508 800 176 0.3 490
    • [Measurement and Evaluation]
  • The microstructures and mechanical properties at a portion 1/4t 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. For the region determined as lower bainite + martensite (BL + M), the presence of lower bainite and martensite was found by SEM observation. The balance of the microstructure is pearlite, an MA phase, or ferrite.
    • Bu: upper bainite
    • BL: lower bainite
    • M: martensite
    • Retained γ: retained austenite
  • The toughness was measured for a case where the steel product (base metal) was not subjected to heat treatment, and for cases where heat treatment processes of (2) to (4) below were carried out, with use of a sample taken from the portion that is 1/4 of the thickness for each case.
    1. (1) Base metal toughness
      Average value (KV2) of Charpy impact absorption energy at -110°C
    2. (2) Base metal toughness after PWHT
      Average value of Charpy impact absorption energy at -110°C after PWHT in which the holding temperature was 600°C, the holding time was 2 hours, and each of the rate of temperature increase and the rate of temperature decrease in a temperature range of 425°C or higher was 55°C/h
    3. (3) Thermal Cycle toughness
      Average value of Charpy impact absorption energy at -110°C, after application of thermal history (thermal cycle) in which temperature is raised to 1350°C at 60°C/s, held at 1350°C for 1 s, and then cooled down to room temperature at 20°C/s
    4. (4) Toughness after Thermal Cycle and PWHT
      Average value of Charpy impact absorption energy at -110°C, after application of thermal history (thermal cycle) in which temperature is raised to 1350°C at 60°C/s, held at 1350°C for 1 s, and then cooled down to room temperature at 20°C/s; and after PWHT in which the holding temperature was 600°C, the holding time was 2 hours, and each of the rate of temperature increase and the rate of temperature decrease in a temperature range of 425°C or higher was 55°C/h.
    [Table 3]
    No. Microstructure (area ratio) Average aspect ratio of prior γ grain Average crystal grain size [µm] YS [MPa] TS [MPa] YR [%] Base metal toughness KV2 [J] Base metal toughness after PWHT KV2 [J] Amount of P segregation at prior γ grain boundary after thermal cycle and PWHT [at.%] Carbide coverage at prior γ grain boundary after thermal cycle and PWHT [%] Toughness after thermal cycle KV2 [J] Toughness after thermal cycle and PWHT KV2 [J] Notes
    BL+M [%] Bu+BL+M [%] Retained γ [%]
    1 23.6 92.2 0.0 2.8 17.7 539 617 87 175 171 0.2 27 134 110 Examples of present invention
    2 28.8 93.1 0.3 2.9 16.9 556 653 85 183 169 ND 23 138 112
    3 23.3 92.6 0.3 2.7 17.5 532 640 83 174 160 ND 23 131 114
    4 19.4 92.0 0.2 2.8 17.5 519 614 85 173 159 ND 24 135 117
    5 20.3 92.6 0.2 3.0 17.4 528 632 84 175 172 ND 22 137 134
    6 33.8 93.2 0.3 3.1 16.8 563 664 85 189 173 ND 24 142 130
    7 37.1 92.7 0.4 3.2 16.5 569 667 85 193 189 ND 28 143 140
    8 38.7 92.6 0.3 2.7 16.4 582 674 86 195 194 ND 29 144 143
    9 39.1 92.8 0.5 3.1 15.7 580 668 87 192 190 ND 28 146 142
    10 56.7 94.7 0.6 2.9 14.2 625 728 86 287 280 ND 24 188 183
    11 56.4 94.4 0.7 2.8 12.9 639 758 84 284 285 ND 26 184 180
    12 59.7 94.9 0.6 3.0 12.6 651 783 83 282 264 ND 23 186 175
    13 61.1 95.2 0.5 3.2 11.7 659 771 85 310 291 ND 25 195 178
    14 64.8 95.7 0.6 3.4 12.8 678 770 88 231 210 0.2 23 185 164
    15 67.2 95.8 0.5 2.9 13.0 680 782 87 251 232 ND 28 182 167
    16 67.8 96.3 0.6 2.7 13.6 675 778 87 262 260 ND 26 185 183
    17 88.1 97.1 0.9 3.3 14.2 728 825 88 254 234 ND 22 170 153
    18 91.2 97.7 1.1 2.9 14.6 744 854 87 241 218 0.4 29 167 153
    19 93.8 98.0 1.0 2.7 15.1 756 862 88 256 237 ND 22 164 148
    20 94.7 97.8 1.2 3.2 15.4 759 876 87 220 203 0.3 26 158 144
    21 95.1 98.2 1.1 3.0 15.8 765 880 87 253 234 ND 21 155 138
    22 97.9 97.9 1.4 2.4 16.1 772 902 86 241 219 ND 27 154 137
    23 21.8 92.1 0.3 2.9 26.9 526 641 82 122 117 0.2 22 134 130
    24 21.3 92.5 0.2 1.6 28.5 522 638 82 118 102 0.2 23 139 122
    25 28.2 93.6 0.3 2.8 28.3 551 654 84 110 96 0.4 21 133 118
    26 96.3 97.2 1.2 1.7 28.9 763 889 86 135 113 0.3 26 159 141
    27 39.1 94.3 0.6 3.2 16.2 583 670 87 133 88 0.2 47 76 25 Comparative Examples
    28 90.2 98.1 1.0 2.7 15.5 740 849 87 129 46 0.2 21 72 22
    29 28.3 93.4 0.3 3.2 17.2 555 651 85 110 67 4.1 25 57 21
    30 56.0 94.2 0.6 3.0 14.8 589 675 87 112 86 0.4 23 77 33
    31 4.3 66.2 0.2 3.2 39.5 501 582 86 71 66 0.2 25 41 45
    32 98.2 98.2 1.5 2.9 16.8 889 996 89 142 121 0.2 21 42 44
    33 22.2 68.1 0.0 2.7 15.2 408 512 80 158 145 0.2 25 133 114
    34 97.6 97.6 2.1 3.0 17.1 929 1089 85 111 95 ND 24 32 24
    35 9.1 82.3 0.1 2.9 17.7 519 642 81 72 65 ND 23 168 166
  • In Table 3, "Amount of P segregation at prior γ grain boundary after thermal cycle and PWHT [at.%]" means the P concentration (atomic %) at the prior austenite grain boundaries after the thermal cycle and after the post weld heat treatment (PWHT), wherein "ND" means below the detection limit. "Carbide coverage at prior γ grain boundary after thermal cycle and PWHT [%]" means the carbide coverage at the prior austenite grain boundaries after the thermal cycle, and after the post weld heat treatment (PWHT).
  • Nos. 1 to 26 are Examples of the present invention, and Nos. 27 to 35 are Comparative Examples.
  • In No. 27, the amount of Si went over the upper limit, so that the grain boundary carbide coverage at the prior austenite grain boundaries after the thermal cycle and after PWHT exceeded 40%, and sufficient low-temperature toughness was not obtained.
  • In No. 28, the amount of Mn went over the upper limit, so that also the value β was out of the upper limit, and sufficient low-temperature toughness was not obtained after the thermal cycle and after PWHT.
  • In No. 29, the amount of P went over the upper limit, so that the amount of P segregation at the prior austenite grain boundaries after the thermal cycle and after PWHT exceeded 3%, and sufficient low-temperature toughness was not obtained.
  • In No. 30, the value β went over the upper limit, so that sufficient low-temperature toughness was not obtained after the thermal cycle and after PWHT.
  • In No. 31, the value α was below the lower limit value of the present disclosure, so that the hardenability was insufficient, and the strength was insufficient. Sufficient low-temperature toughness was not obtained either.
  • In No. 32, the value α exceeded the upper limit value of the present disclosure, so that the hardenability was too high, and the strength was excessive.
  • In No. 33, the start temperature of direct quenching was too low, so that the total area ratio of the upper bainite, the lower bainite, and the martensite was insufficient, and the strength was insufficient.
  • In No. 34, the value α exceeded the upper limit value of the present disclosure, the hardenability was too high, and the strength was excessive. Also the retained γ was excessive, so that sufficient low-temperature toughness was not obtained.
  • In No. 35, the cooling rate in direct quenching was low, so that the total area ratio of the lower bainite and the martensite was insufficient, and sufficient low-temperature toughness was not obtained.
  • In contrast to the Comparative Examples, in all of the Examples of the present invention (Nos. 1- 26), the chemical compositions and microstructures of the steel products were controlled appropriately, and the tensile strengths were in the appropriate range of 615 MPa or more and 930 MPa or less. In addition, a low-temperature toughness of 100 J or more at -110°C was obtained regardless of whether before or after PWHT. In particular, in Nos. 1 to 22 having an average crystal grain size of 20.0 µm or less at the 1/4t portion, a low-temperature toughness of 150 J or more was obtained at -110°C regardless of before or after PWHT, and also the toughness after thermal cycle and the toughness after thermal cycle and PWHT were found to be given by a low-temperature toughness of 50 J or more at -110°C.
    • Industrial Applicability
  • The steel products according to the present disclosure can be used mainly for transport tanks of liquefied carbon dioxide, for example. Further, the steel products according to the present disclosure can also be used in other welded structures such as buildings, bridges, ships, pipelines, offshore structures, and pressure vessels (tanks).
  • The disclosures of Japanese Patent Application No. 2023-176326 filed on October 11, 2023 are, in their entireties, incorporated by reference into the present specification. All publications, patent applications, and technical standards mentioned in the present specification are incorporated by reference into the present specification to the same extent as if such individual publication, patent application, or technical standard was specifically and individually put forth herein.

Claims (11)

  1. A steel product having a chemical composition comprising, in mass%,
    C: from 0.03% to 0.20%,
    Si: from 0.01% to 0.50%,
    Mn: from 0.10% to 2.00%,
    P: 0.025% or less,
    S: 0.0250% or less,
    Ni: 4.51% or more and less than 6.10%,
    Al: from 0.001% to 0.100%,
    O: 0.0100% or less,
    N: 0.0100% or less,
    Cu: 0 to 1.50%,
    Cr: 0 to 3.00%,
    Mo: 0 to 2.00%,
    B: 0 to 0.0050%,
    Nb: 0 to 0.050%,
    Ti: 0 to 0.050%,
    V: 0 to 0.10%,
    Mg: 0 to 0.0200%,
    Ca: 0 to 0.0200%,
    REM: 0 to 0.0200%, and
    balance: Fe and impurities,
    and in which α represented by following Formula (1) is from 5.0 to 16.0, and β represented by following Formula (2) is 0.017 or less,
    wherein:
    a tensile strength is from 615 MPa to 930 MPa,
    a microstructure of a portion that is 1/4 of a thickness in a thickness direction from a surface of the steel product comprises lower bainite and martensite, a total of area ratios of the lower bainite and the martensite is 15.0% or more, a total of area ratios of upper bainite, the lower bainite, and the martensite is 90.0% or more, and an area ratio of retained austenite is less than 1.7%, α = 0.50 × √[C] × (1 + 0.64[Si]) × (1 + 4.10[Mn]) × (1 + 0.27[Cu]) × (1 + 0.52[Ni]) × (1 + 2.33[Cr]) × (1 + 3.14[Mo]) β = Mn × P − Mo / 100
    wherein in a symbol of an element in square brackets in Formulae (1) and (2) represents a content (mass%) of a corresponding element contained in the steel product, provided that, zero is substituted when the corresponding element is not contained.
  2. The steel product according to claim 1, wherein the chemical composition comprises the following Group A:
    [Group A]:
    one or two or more selected from the group consisting of:
    Cu: from 0.01% to 1.50%,
    Cr: from 0.01% to 3.00%,
    Mo: from 0.01% to 2.00%, and
    B: from 0.0003% to 0.0050%.
  3. The steel product according to claim 1 or 2, wherein the chemical composition comprises the following Group B:
    [Group B]:
    one or two or more selected from the group consisting of:
    Nb: from 0.001% to 0.050%,
    Ti: from 0.001% to 0.050%, and
    V: from 0.01% to 0.10%.
  4. The steel product according to any one of claims 1 to 3, wherein the chemical composition comprises the following Group C:
    [Group C]:
    one or two or more selected from the group consisting of:
    Mg: from 0.0003% to 0.0200%,
    Ca: from 0.0003% to 0.0200%, and
    REM: from 0.0003% to 0.0200%.
  5. The steel product according to any one of claims 1 to 4, wherein, in the microstructure of the portion that is 1/4 of the thickness, in the thickness direction, from the surface of the steel product, an average crystal grain size is 20.0 µm or less.
  6. The steel product according to any one of claims 1 to 5, wherein an aspect ratio of a prior austenite grain in the portion that is 1/4 of the thickness, in the thickness direction, from the surface of the steel product is 1.5 or more.
  7. The steel product according to any one of claims 1 to 6, wherein a Charpy impact absorption energy at -110°C at the portion that is 1/4 of the thickness is 150 J or more.
  8. The steel product according to any one of claims 1 to 7, wherein, when a heat treatment, in which a rate of temperature increase and a rate of temperature decrease in a temperature region of 425°C or more are 55°C/h and the temperature is held for 2 hours at 600°C, is carried out on the steel product, a Charpy impact absorption energy at -110°C at the portion that is 1/4 of the thickness of the heat-treated portion is 150 J or more.
  9. The steel product according to any one of claims 1 to 8, wherein when a thermal cycle simulating welding equivalent to 2 kJ/mm is applied to the portion that is 1/4 of the thickness of the steel product, and a heat treatment is further carried out on the steel product, in which a rate of temperature increase and a rate of temperature decrease in a temperature region of 425°C or more are 55°C/h, and the temperature is held for 2 hours at 600°C, a P concentration at prior austenite grain boundaries is 3.0% or less in atomic percent concentration, and a grain boundary carbide coverage is 40% or less.
  10. The steel product according to any one of claims 1 to 9, wherein when a thermal cycle simulating welding equivalent to 2 kJ/mm is applied to the portion that is 1/4 of the thickness of the steel product, and a heat treatment is further carried out on the steel product, in which a rate of temperature increase and a rate of temperature decrease in a temperature region of 425°C or more are 55°C/h, and the temperature is held for 2 hours at 600°C, a Charpy impact absorption energy at -110°C of the heat-treated portion is 50 J or more.
  11. A pressure vessel, comprising the steel product according to any one of claims 1 to 10.
EP24877318.6A 2023-10-11 2024-10-11 Steel material and pressure vessel Pending EP4793381A1 (en)

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JPH01230713A (en) 1988-03-08 1989-09-14 Nippon Steel Corp Production of high-strength and high-toughness steel having excellent stress corrosion cracking resistance
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WO2009072303A1 (en) * 2007-12-05 2009-06-11 Honda Motor Co., Ltd. High-strength steel sheet, automotive strengthening member comprising the same, and process for producing automotive strengthening member
JP6610352B2 (en) 2016-03-11 2019-11-27 日本製鉄株式会社 Low temperature nickel-containing steel sheet with excellent tensile strength and toughness and method for producing the same
JP6984319B2 (en) 2017-10-31 2021-12-17 日本製鉄株式会社 Nickel-containing steel sheet for low temperature with excellent toughness and its manufacturing method
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