EP2871255B1 - Steel, process for manufacturing the same, and lng tank - Google Patents

Steel, process for manufacturing the same, and lng tank Download PDF

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Publication number
EP2871255B1
EP2871255B1 EP13887092.8A EP13887092A EP2871255B1 EP 2871255 B1 EP2871255 B1 EP 2871255B1 EP 13887092 A EP13887092 A EP 13887092A EP 2871255 B1 EP2871255 B1 EP 2871255B1
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Prior art keywords
steel
amount
retained
lower limit
average
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German (de)
English (en)
French (fr)
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EP2871255A1 (en
EP2871255A4 (en
Inventor
Tomoya Kawabata
Takayuki KAGAYA
Takahiro Kamo
Hironori WAKAMATSU.
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Nippon Steel Corp
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Nippon Steel and Sumitomo Metal Corp
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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
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • 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
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/001—Heat treatment of ferrous alloys containing Ni
    • 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 by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
    • C—CHEMISTRY; METALLURGY
    • 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
    • C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
    • 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
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
    • 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
    • C22C38/16—Ferrous alloys, e.g. steel alloys containing 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
    • C22C38/42—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
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F17—STORING OR DISTRIBUTING GASES OR LIQUIDS
    • F17C—VESSELS FOR CONTAINING OR STORING COMPRESSED, LIQUEFIED OR SOLIDIFIED GASES; FIXED-CAPACITY GAS-HOLDERS; FILLING VESSELS WITH, OR DISCHARGING FROM VESSELS, COMPRESSED, LIQUEFIED, OR SOLIDIFIED GASES
    • F17C1/00—Pressure vessels, e.g. gas cylinder, gas tank, replaceable cartridge
    • F17C1/12—Pressure vessels, e.g. gas cylinder, gas tank, replaceable cartridge with provision for thermal insulation

Definitions

  • the present invention relates to a steel for very low-temperature use which is excellent in toughness, a method for manufacturing the steel, and an LNG tank to which the steel is applied.
  • the term "for very-low temperature use” represents use of a liquefied petroleum gas (LPG), a liquefied natural gas (LNG), and the like at a temperature region in which the LPG, LNG, and the like exist in a liquid state, that is, use in a very low-temperature environment of -60°C or lower.
  • LPG liquefied petroleum gas
  • LNG liquefied natural gas
  • use in the vicinity of -165°C which is a temperature environment in which the LNG is stored in a liquid state, is set as a main target.
  • a steel which is used to manufacture a very-low-temperature storage tank that stores a liquefied gas such as LPG and LNG is demanded to have excellent fracture toughness from the viewpoint of securement of stability.
  • brittle crack propagation arrest characteristics (hereinafter, referred to as "arrest characteristics") of a base metal and a welded joint in the vicinity of -165°C, which is an LNG temperature environment and the like are demanded for 9% Ni steel that is used in an LNG tank (in the specification, “%” represents “mass%” unless otherwise stated).
  • the arrest characteristics are demanded as important characteristics without limitation to the LNG tank.
  • a fracture-resistant performance is a very high level characteristic.
  • steel corresponding to the demand include 9% Ni steel.
  • the 9% Ni steel When the 9% Ni steel is subjected to the above-described three-stage heat treatment method (quenching (Q), two-phase region quenching(lamellarizing) (L), and tempering (T)), the 9% Ni steel can have the target performance.
  • the 9% Ni steel in which a large amount of an expensive alloy element such as Ni is added, is expensive, and thus there is an economical problem. Accordingly, to suppress the cost of a steel, a steel in which an amount of Ni is suppressed has been developed.
  • Patent Document 1 discloses a steel in which the amount of Ni is decreased to less than 8%.
  • Patent Document 1 discloses a finding that the brittle crack propagation arrest characteristics of a steel plate are improved by increasing an amount of retained ⁇ . Accordingly, the present inventors have performed various fracture toughness tests with respect to steel plates with satisfactory V-notch Charpy absorbed energy v E- 196 , which are disclosed in Patent Document 1 (particularly, Test Nos. 1-a to 1-h, 4-12, and 22 to 35 described in Patent Document 1).
  • a dynamic tear (DT) energy (a fracture characteristic evaluation parameter obtained by a DT test) at -196°C does not satisfy 1500 J, or absorption energy at - 196°C, which is obtained by a pre-crack Charpy test with respect to a test specimen having a plate thickness of less than 15 mm, does not satisfy 100 J/cm 2 .
  • the present inventors have obtained a finding that an effect of improving the brittle crack propagation arrest characteristics are limited only with a simple increase in the amount of retained ⁇ , and in a case where the increased retained ⁇ is not stable, martensite after transformation may become a cause of deterioration in the brittle crack propagation arrest characteristics.
  • Patent Document 1 does not disclose a necessity for stabilization of the retained ⁇ , and also does not disclose a method of stabilizing the retained ⁇ . When the retained ⁇ is not stabilized, it is considered that the brittle crack propagation arrest characteristics are not sufficiently improved.
  • Patent Document 2 discloses a technology of securing a large amount of retained ⁇ (retained austenite) with high stability by performing rolling with a defined cumulative reduction, and by performing an off-line quenching and tempering (QT) or direct-quenching and tempering (DQT) treatment.
  • QT off-line quenching and tempering
  • DQT direct-quenching and tempering
  • Patent Document 2 contains 9% or more Ni in terms of mass%, but there are only two examples in which the retained ⁇ is secured in an amount of 4% or more that are defined in the present invention.
  • the two examples include an example that is manufactured by DQT, but a heating temperature during DQT is as high as 1200°C, and thus it is recognized that the example is different from the invention from the viewpoint of a process. This implies that the invention disclosed in Patent Document 2 does not have high fracture-resistant characteristics due to the reason to be described later.
  • Patent Document 3 discloses processes of so-called controlled rolling, direct-quenching, and tempering (CR-DQT) or controlled rolling, direct-quenching, lamellarizing, and tempering (CR-DQLT) in which low-temperature reduction is performed after low-temperature heating, water cooling is performed to a temperature of 200°C or lower immediately after the low-temperature reduction, and a heat treatment is performed.
  • CR-DQT controlled rolling, direct-quenching, and tempering
  • CR-DQLT controlled rolling, direct-quenching, lamellarizing, and tempering
  • Patent Document 3 does not disclose a cooling rate after tempering and a concentration of Ni or Mn in the retained ⁇ which greatly relate to characteristics of a steel. This implies that the invention disclosed in Patent Document 3 does not have high fracture-resistant characteristics.
  • Patent Document 4 has paid attention to a segregation ratio of Ni, but in Patent Document 4, it is necessary to perform a crack diffusion treatment so as to decrease the segregation ratio. This is not preferable from the viewpoints of economic efficiency or lead time.
  • Patent Document 4 relates to the processes of so-called CR-DQT or CR-DQLT. These processes themselves are the same as those in the invention.
  • Patent Document 4 does not disclose a cooling rate after tempering at all, and does not define a concentration of Ni or Mn in the retained ⁇ . Accordingly, it can be said that the technology disclosed in Patent Document 4 is not a technology capable of stably satisfying very high fracture-resistant characteristics.
  • WO 2013/046357 discloses a steel plate for use in LNG application requiring low temperature toughness comprising more than 7.5-10.0 Ni.
  • the invention has been made in consideration of the above-described circumstances, and an object thereof is to provide a steel excellent in economic efficiency and fracture-resistant characteristics by realizing compatibility between provision of very high fracture-resistant characteristics and suppression of the cost of the steel, a method for manufacturing the steel, and an LNG tank.
  • the present inventors have employed steel in which an amount of Ni, which is effective to secure low-temperature toughness, is in a range of 6.6% to 8.0% in terms of mass%, and have performed various experimental tests in the above-described range to examine correspondence with characteristics. As a result, the present inventors have obtained the following findings (a) to (h).
  • the invention has been completed on the basis of the above-described findings, and the gist of the invention relates to a steel in (1) to (4) to be described below, a method for manufacturing a steel in (5) to (8) to be described below, and an LNG tank to which the steel is applied in (9) to (10).
  • C is an element that is necessary to secure strength of a base metal.
  • an amount of C is less than 0.01%, it is difficult to secure necessary strength, and formation of lath martensite in a fusion line (FL) becomes insufficient during welding, and thus toughness of a heat-affected zone (HAZ) in the vicinity of FL also decreases. Accordingly, it is necessary for the lower limit of the amount of C to be set to 0.01 %.
  • the amount of C exceeds 0.12%, deterioration in toughness of the HAZ, particularly, the HAZ in the vicinity of FL becomes significant. Accordingly, the amount of C is set to 0.01% to 0.12%.
  • the lower limit of the amount of C may be set to 0.02%, 0.03%, or 0.04%.
  • the upper limit of the amount of C may be set to 0.10%, 0.08%, 0.07%, or 0.06%.
  • Si is an element that is necessary as a deoxidizing agent. To attain a deoxidation effect, it is necessary for the lower limit of an amount of Si to be set to 0.01%.
  • Si and a tempering of as-quenched martensite are greatly relevant to each other, and thus when the amount of Si exceeds 0.30%, Si suppresses a decomposition precipitation reaction of C with respect to cementite from martensite, in which C is solid-dissolved in a supersaturated state, during a weld cooling. Due to the suppression of the decomposition precipitation reaction of C, self-tempering is delayed, and thus toughness of a welded portion decreases.
  • the amount of Si contained in an amount of more than 0.30% increases martensite-austenite constituent, thereby decreasing the toughness of the welded portion. Accordingly, the amount of Si is set to 0.01% to 0.30%. In addition, it is preferable that the amount of Si be as small as possible from the viewpoint of an improvement in the toughness of the welded portion, and the upper limit of the amount of Si may be set to 0.20%, 0.15%, or 0.10% so as to improve the toughness of the welded portion. The lower limit of the amount of Si may be set to 0.02%, 0.03%, or 0.04% so as to reliably perform deoxidation.
  • Mn is a deoxidizing agent, and is an element that is necessary to secure the strength and the toughness of the base metal, and the hardenability of the HAZ.
  • an amount of Mn is less than 0.4%, these effects are not obtained, and a ferrite side plate is generated in the HAZ, and thus formation of the lath martensite becomes insufficient. Therefore, the toughness of the welded portion decreases, and thus the lower limit of the amount of Mn is set to 0.4%.
  • the amount of Mn exceeds 2.0%, ununiformity in base metal characteristics may be caused in a plate thickness direction due to central segregation of Mn. Accordingly, the amount of Mn is set to 0.4% to 2.0%.
  • the lower limit of the amount of Mn may be set to 0.50%, 0.60%, or 0.70% so as to secure hardenability and improve the toughness of the welded portion.
  • the upper limit of the amount of Mn may be set to 1.5%, 1.2%, 1.0%, or 0.9% so as to prevent ununiformity in the base metal characteristics in the plate thickness direction.
  • P exists in steel as an impurity.
  • P segregates at a grain boundary, and thus P becomes a cause of decreasing toughness.
  • the amount of P is limited to 0.05% or less.
  • the upper limit of the amount of P may be set to 0.03%, 0.02%, 0.01%, 0.008%, or 0.006%. It is not necessary to particularly define the lower limit of the amount of P, and the lower limit thereof is 0%. However, a decrease in P more than necessary leads to an increase in the cost during refining, and thus the lower limit of the amount of P may be set to 0.0001% or 0.0005%.
  • the amount of S exists in steel as an impurity. S that excessively exists promotes central segregation, or becomes a cause of generating a large amount of MnS having a stretched shape that becomes a cause of a brittle fracture.
  • the amount of S is set to 0.008% or less.
  • the upper limit of the amount of S may be set to 0.006%, 0.004%, 0.003%, or 0.002% to improve the mechanical properties of the base metal and the HAZ. It is preferable that the amount of S be as small as possible, and thus it is not necessary to define the lower limit of the amount of S, and the lower limit is 0%.
  • the lower limit of the amount of S may be set to 0.0001% or 0.0003% from the viewpoint of the refining cost.
  • Ni is the most basic element that is necessary to secure toughness for a steel for low-temperature use. It is necessary for Ni to be contained in an amount of 6.6% or more so as to secure the toughness for the steel for low-temperature use. The more an amount of Ni increases, the higher low-temperature toughness is obtained. However, the more the amount of Ni increases, the more the cost increases, and thus the upper limit of the amount of Ni is set to 8.0% . Accordingly, a target of the amount of Ni is 6.6% to 8.0%.
  • the amount of Ni be 6.7% or more from the viewpoint of securing low-temperature toughness, and the lower limit of the amount of Ni may be set to 6.8%, 6.9%, or 7.0% as necessary.
  • the upper limit of the amount of Ni may be set to 7.8%, 7.6%, or 7.4% from the viewpoint of suppressing an increase in the cost.
  • the amount of Ni exceeds 8.0%, characteristics demanded for the steel for low-temperature use are obtained.
  • the upper limit of the amount of Ni is 7.4%.
  • Al is an element that is typically contained as a deoxidizing agent.
  • Al has a function of delaying self-tempering of martensite. Accordingly, it is preferable that an amount of Al be as small as possible.
  • Al suppresses the decomposition precipitation reaction of C with respect to cementite from martensite, in which C is solid-dissolved in a supersaturated state, during a weld cooling. Accordingly, Al may decrease the toughness of the welded portion.
  • the amount of Al is less than 0.002%, it is difficult to obtain a sufficient deoxidizing effect.
  • the amount of Al is set to 0.002% to 0.080%.
  • the lower limit of the amount of Al may be set to 0.005%, 0.010%, 0.015%, or 0.020% so as to reliably perform deoxidization.
  • the upper limit of the amount of Al may be set to 0.060%, 0.050%, or 0.040% so as to improve the toughness of the welded portion.
  • N exists in steel as an impurity, and becomes a cause of deterioration in the toughness of the HAZ through an increase in solid-dissolved N or generation of a precipitate, and thus it is preferable that an amount of N be small so as to secure the toughness of the HAZ.
  • the upper limit of the amount of N may be set to 0.0045% or 0.0040% to improve the toughness of the HAZ. It is not necessary to define the lower limit of the amount of N, and the lower limit is 0%. However, the lower limit of the amount of N may be set to 0.0001% or 0.0010% from the viewpoint of the cost during refining.
  • the steel according to this embodiment includes the above-described components, and remainder includes Fe and impurities.
  • the impurities represent ore or scrap as a raw material when the steel is industrially manufactured, or a component that is unavoidably mixed in due to various factors of a manufacturing, and the impurities are permitted in a range having no effect on the invention.
  • the steel according to this embodiment may contain one or more kinds of elements selected from Cu, Cr, Mo, V, B, Nb, Ti, Sn, Ca, Mg, and REM in addition to the above-described components. It is not necessary to particularly define the lower limit of the amount of these components, and the lower limit is 0%. In addition, even though these alloy elements are intentionally added to the steel according to this embodiment, or even when these alloy elements are mixed-in to the steel as impurities, if the amount of these alloy elements is in a defined range, it is interpreted that the steel is within the claims of the invention.
  • Cu may be contained as necessary. When Cu is contained, it is possible to improve the strength of the base metal. However, when an amount of Cu exceeds 1.00%, the toughness of the HAZ that is heated to a temperature of Ac 3 point or lower may deteriorate, and thus the upper limit of the amount of Cu is set to 1.00%. It is preferable that the upper limit of the amount of Cu be 0.80% or 0.60%, and more preferably 0.30%. In addition, in the case of desiring to obtain the effect of improving the strength of the base metal due to Cu, the lower limit of the amount of Cu may be set to 0.10%.
  • Cr may be contained as necessary. When Cr is contained, carbon dioxide gas corrosion resistance is improved, and hardenability is improved. As a result, strength can be improved. However, when an amount of Cr exceeds 1.00%, it is difficult to suppress hardening of the HAZ, and the effect of improving the carbon dioxide gas corrosion resistance becomes saturated, and thus the upper limit of the amount of Cr is set to 1.00%.
  • the upper limit of the amount of Cr may be set to 0.80%, 0.60%, or 0.50% so as to suppress hardening of the HAZ. It is not necessary to define the lower limit of the amount of Cr, and the lower limit is 0%.
  • the lower limit of the amount of Cr may be set to 0.05%.
  • the lower limit of the amount of Cr may be set to 0.10% so as to reliably obtain the effect of improving the hardenability. More preferably, the lower limit of the amount of Cr is 0.20%.
  • the lower limit of the amount of Cr may be set to 0.30% to 0.40% as necessary.
  • Mo may be contained as necessary. When Mo is contained, it is possible to obtain an effect of improving the strength and the toughness of the base metal. However, when an amount of Mo exceeds 0.50%, hardness of the HAZ increases, and thus toughness and SSC resistance may be damaged. Accordingly, the upper limit of the amount of Mo is set to 0.50%, and preferably 0.30%. The upper limit of the amount of Mo may be set to 0.20%, 0.15%, or 0.12% so as to improve the toughness and the SSC resistance. It is not necessary to define the lower limit of the amount of Mo, and the lower limit is 0%.
  • the lower limit of the amount of Mo be set to 0.05%.
  • the lower limit of the amount of Mo may be set to 0.06% or 0.07% as necessary.
  • V 0% to 0.10%
  • V may be contained as necessary. When V is contained, it is possible to obtain an effect of improving the strength of the base metal mainly due to precipitation of carbonitrides during tempering. However, when an amount of V exceeds 0.10%, the effect of improving the strength of the base metal may be saturated, and deterioration in toughness may be caused, and thus the upper limit of the amount of V is set to 0.10%. It is not necessary to define the lower limit of the amount of V, and the lower limit is 0%. The upper limit of the amount of V may be set to 0.08%, 0.06%, or 0.04% so as to improve the toughness. In addition, in the case of desiring to obtain the effect of improving the strength of the base metal due to V, the lower limit of the amount of V may be set to 0.015% or 0.02%.
  • B may be contained as necessary. When B is contained, it is possible to obtain an effect of improving the strength of the base metal. However, when an amount of B exceeds 0.0050%, precipitation of coarse boron compounds is caused, and thus the toughness may deteriorate, and thus the upper limit of the amount of B is set to 0.0050%.
  • the upper limit of the amount of B may be set to 0.0040%, 0.0030%, or 0.0020% so as to prevent deterioration in the toughness. It is not necessary to define the lower limit of the amount of B, and the lower limit is 0%.
  • the lower limit of the amount of B be set to 0.0003%, and more preferably 0.0005% or 0.0010%.
  • the upper limit of the amount of B may be set to 0.0010%, 0.0005%, 0.0003%, or 0.0002%.
  • Nb may be contained as necessary. When Nb is contained, a structure is made fine, and thus it is possible to obtain an effect of improving low-temperature toughness. However, when an amount of Nb exceeds 0.10%, coarse carbides or nitrides may be formed, and thus the toughness may deteriorate. Accordingly, the upper limit of the amount of Nb is set to 0.10%. It is not necessary to define the lower limit of the amount of Nb, and the lower limit is 0%. The upper limit of the amount of Nb may be set to 0.08%, 0.06%, or 0.04% so as to prevent a decrease in the toughness. In addition, in the case of desiring to obtain the effect of improving the low-temperature toughness due to Nb, the lower limit of the amount of Nb may be set to 0.01% or 0.02%.
  • Ti may be contained as necessary. Ti is mainly used as a deoxidizing element, and also forms an oxide phase including Al, Ti, and Mn, and thus Ti has an effect of making a structure fine. However, when an amount of Ti exceeds 0.10%, an oxide that is formed becomes a Ti oxide or a Ti-Al oxide, and thus a dispersion density decreases. Particularly, an effect of making a structure of a heat-affected zone of a small-heat-input welded portion fine may be lost. Accordingly, the upper limit of the amount of Ti is set to 0.10%, and preferably 0.07% or 0.05%. It is not necessary to define the lower limit of the amount of Ti, and the lower limit is 0%. In addition, in the case of desiring to obtain the effect of making a structure fine due to Ti, the lower limit of the amount of Ti may be set to 0.02% or 0.03%.
  • Sn may be contained as necessary.
  • Sn is converted into Sn 2+ , and is dissolved in a material that adheres to a surface of the steel, and has an effect of suppressing corrosion due to an inhibitor effect in an acidic chloride solution.
  • Sn rapidly reduces Fe 3+ , and has an effect of decreasing a concentration of Fe 3+ as an oxidizing agent. Accordingly, Sn suppresses a corrosion-promoting effect of Fe 3+ , and thus weather resistance in a high floating salinity environment is improved.
  • an amount of Sn exceeds 0.50%, the above-described effects are saturated, and thus the upper limit of the amount of Sn is set to 0.50%, and preferably 0.20%.
  • the upper limit of the amount of Sn may be limited to 0.10%, 0.05%, or 0.01% to decrease the cost of an alloy. It is not necessary to define the lower limit of the amount of Sn, and the lower limit is 0%. In addition, in the case of desiring to obtain the effect of corrosion resistance and weather resistance due to Sn, the lower limit of the amount of Sn may be set to 0.03% or 0.05%.
  • Ca may be contained as necessary.
  • Ca reacts with S in steel to form an oxysulfide in molten steel.
  • the oxysulfide does not extend in a rolling direction by rolling differently from MnS, and thus the oxysulfide has a spherical shape even after rolling.
  • the spherical oxysulfide has an effect of suppressing a welding crack or a hydrogen-induced crack in which a front end and the like of a stretched inclusion serve as a crack origin.
  • an amount of Ca exceeds 0.004%, deterioration in toughness may be caused, and thus the upper limit of the amount of Ca is set to 0.004%.
  • the upper limit of the amount of Ca may be set to 0.003% so as to reliably avoid a decrease in toughness. It is not necessary to define the lower limit of the amount of Ca, and the lower limit is 0%. In addition, in the case of desiring to obtain the effect of suppressing the welding crack or the hydrogen-induced crack due to Ca, the lower limit of the amount of Ca may be set to 0.0003% or 0.0005%.
  • Mg may be contained as necessary.
  • Mg When Mg is contained, a fine Mg-containing oxide is generated, and thus Mg is effective for miniaturization of a grain size of ⁇ .
  • the upper limit of the amount of Mg is set to 0.0020%, and preferably 0.0010%. It is not necessary to define the lower limit of the amount of Mg, and the lower limit is 0%.
  • the lower limit of the amount of Mg be set to 0.0002%, and more preferably 0.0004%.
  • REM rare-earth element
  • REM ultraviolet-earth element
  • the upper limit of the amount of REM is set to 0.0020%, and more preferably 0.0010%.
  • the lower limit of the amount of REM it is not necessary to define the lower limit of the amount of REM, and the lower limit is 0%.
  • the lower limit of the amount of REM it is preferable that the lower limit of the amount of REM be set to 0.0002%, and more preferably 0.0003%.
  • REM is a general term of a total of 17 elements including 15 elements of lanthanoid, Y, and Sc, and one or more kinds of these elements may be contained.
  • the term of the amount of REM represents a total amount of these elements.
  • the steel according to this embodiment contains the above-described components, and remainder includes iron and impurities.
  • a weld steel according to this embodiment may contain the following alloy elements to further improve strength, toughness, and the like of the steel itself, or as impurities from an auxiliary raw material such as scrap.
  • the upper limit of an amount of Sb may be set to 0.03%.
  • the upper limit of the amount of Sb may be set to 0.01 %, 0.005%, 0.003%, or 0.001% so as to improve the toughness of the HAZ.
  • an upper limit of an amount of As may be set to 0.02%.
  • the upper limit of the amount of As may be set to 0.005%, 0.003, or 0.001 % as necessary.
  • the upper limit of an amount of each of Pb, Zr, Zn, and W may be set to 0.1%, 0.01%, or 0.005% so as to improve the strength and the toughness. It is not necessary to particularly determine the lower limit of the amount of these elements, and the lower limit is 0%.
  • Co may be contained in Ni as an impurity. Co damages the toughness of the HAZ, and thus the upper limit of an amount of Co may be set to 0.5%, 0.3%, 0.1%, or 0.05%. It is not necessary to particularly determine the lower limit of the amount of Co, and the lower limit is 0%.
  • the retained ⁇ in a steel contributes to an improvement in brittle crack propagation arrest characteristics of the steel. As a result, it is possible to expect an effect of improving toughness under a low-temperature environment. To obtain this effect, it is necessary for the lower limit of the amount of the retained ⁇ at the (1/4)t location regarding the plate thickness t of the steel to be 4.0 vol%.
  • the lower limit of the amount of retained ⁇ may be set to 4.5 vol%, 5.0 vol%, 5.5 vol%, 6.0 vol%, or 6.5 vol% so as to improve the toughness.
  • the upper limit of the amount of retained ⁇ is not particularly defined, but when the retained ⁇ excessively exists, there is a concern that a yield strength may decrease.
  • the upper limit of the amount of the retained ⁇ may be set to 20.0 vol% or 15.0 vol%.
  • evaluation of the amount of the retained ⁇ at the (1/4)t location regarding the plate thickness t is performed for evaluation at a mean location over the entire region in a plate thickness direction.
  • Ac 1 is defined by the following Expression (4).
  • Ac 1 712 + 20.1 ⁇ Si ⁇ 17.8 ⁇ Mn ⁇ 19.1 ⁇ Ni + 11.9 ⁇ Cr ⁇ 9.8 ⁇ Mo
  • a symbol of an element in Expression represents an amount (mass%) of each element in a steel.
  • FIG. 1 is a graph illustrating a relationship between the tempering temperature and the amount of the retained ⁇ in various steels manufactured by heating a slab having a chemical composition of Steel No. 1 described in Table 1 at 950°C, performing rolling of attaining a cumulative reduction of 70% at 850°C or lower, performing water cooling to room temperature immediately after the rolling, performing tempering at various temperatures, and performing water cooling.
  • the cumulative reduction represents a percentage ((t1-t2)/t1 ⁇ 100) of a value obtained by dividing a difference between a plate thickness t1 at the time of initiating rolling and a plate thickness t2 at the time of completing the rolling by the plate thickness t1 at the time of initiating the rolling. As shown in FIG.
  • the retained ⁇ in an ⁇ structure is in a metastable state, and when the retained ⁇ is subjected to plastic deformation, martensite transformation tends to occur. It is necessary for the retained ⁇ to be dispersed so as to improve brittle fracture initiation characteristics or brittle fracture propagation arrest characteristics. If the retained ⁇ is lost when an earthquake occurs, desired fracture-resistant characteristics are not exhibited. Even when an applied amount of macro plastic deformation is constant, deformation that is applied to retained ⁇ grains greatly varies in accordance with a distribution type of the retained ⁇ . The more the retained ⁇ grains are relatively fine and are close to a spherical shape, the more a deformation distribution rate decreases.
  • the upper limit of the average value of the aspect ratio of the retained ⁇ which is obtained with observation of a cross-section
  • the upper limit of the average value of the major axis of the retained ⁇ grains which is obtained with observation of a cross-section
  • the upper limit of the average value of the aspect ratio may be set to 2.3 or 2.0.
  • the upper limit of the average value of the major axis may be set to 0.80 ⁇ m or 0.75 ⁇ m. It is not necessary to define the lower limit of the average value of the major axis, but the lower limit is typically 0.05 ⁇ m.
  • the [Mn] retained ⁇ indicates the average Mn concentration in the retained ⁇
  • the [Mn] ⁇ indicates an average Mn concentration in ferrite
  • the [Ni] retained ⁇ indicates the average Ni concentration in the retained ⁇
  • the [Ni] ⁇ indicates an average Ni concentration in ferrite.
  • Ni and Mn which are austenite stabilization elements, are elements that lower a transformation point of ⁇ to ⁇ , and it is known that Ni and Mn have an effect of stabilizing the retained ⁇ .
  • FIG. 2 is a graph illustrating a relationship between a cumulative reduction at 850°C or lower and concentration rates ([M] ⁇ /[M] ⁇ ) of Ni and Mn in various steels manufactured by heating a slab having a chemical composition of Steel No. 1 described in Table 1 at 960°C, performing rolling with various cumulative reductions, performing water cooling to room temperature immediately after the rolling, and performing tempering at 570°C (with water cooling after tempering).
  • the concentration rates of Ni and Mn represent values that are obtained by dividing [Mn] retained ⁇ and [Ni] retained ⁇ by [Mn] ⁇ and [Ni] ⁇ , respectively. From FIG. 2 , it can be seen that particularly, when the lower limit of the cumulative reduction is set to 50%, a concentration ratio of 1.4 or more is obtained, and thus Expression (1) and Expression (2) can be satisfied.
  • FIG. 3 illustrates a relationship between the concentration rates of Ni and Mn and dynamic tear (DT) energy that is a representative fracture characteristic evaluation parameter.
  • DT energy is more than 1500 J.
  • the DT energy is more than 1500 J.
  • the lower limits of the concentrations rates of Ni and Mn are set to 1.4, the DT energy is more than 1500 J.
  • the lower limits of the concentration rates are set to 1.5 or 1.6, higher DT energy is obtained, and thus this case is preferable.
  • the concentration rates of Ni and Mn It is not necessary to particularly define the upper limits of the concentration rates of Ni and Mn. However, the concentration rates of Ni and Mn hardly exceed 10 or 5, and thus the upper limits may be set to 10 or 5.
  • the steel according to this embodiment can be manufactured through the following. However, there is no limitation to the following manufacturing methods.
  • a slab casting conditions thereof are not particularly defined.
  • a slab obtained through ingot-making and blooming may be used, or a continuously cast slab may be used. From the viewpoints of manufacturing efficiency, yield rate, and energy saving, it is preferable to use the continuously cast slab.
  • the plate thickness of the steel that is manufactured is set to 3 mm to 100 m, and mainly 6 mm to 50 mm.
  • the steel that is manufactured may be referred to as a steel plate.
  • a slab heating temperature is controlled to 920°C to 980°C. It is preferable that the lower limit of the slab heating temperature be set to 920°C so as to obtain desired fracture-resistant characteristics by allowing solid-dissolution of AlN to progress to suppress coarsening of crystal grains during the subsequent heat treatment.
  • the upper limit of the slab heating temperature is set to 970°C in order for ⁇ grains not to be excessively coarsened and in order for the fracture-resistant characteristics not to be damaged.
  • a heated slab is rolled.
  • rolling may be performed by dividing the rolling into rough rolling and finish rolling.
  • a slab thickness at the time of terminating the rough rolling becomes 3 times to 8 times a product thickness (steel thickness).
  • a product thickness steel thickness
  • reduction is performed in order for the slab thickness after termination of the rough rolling to be 3 or more times the plate thickness of the product, it is possible to perform sufficient reduction in the subsequent finish rolling. As a result, it is possible to improve the toughness of the steel that is a product.
  • a finish rolling temperature a temperature when the finish rolling is terminated
  • the finish rolling reduction is continuously performed with respect to the slab subjected to the rough rolling as described above without performing cooling, thereby obtaining a production having a predetermined plate thickness.
  • the lower limit of the cumulative reduction at 850°C or lower is set to 50%.
  • introduction of a deformation band is positively performed, and thus the retained ⁇ that is finally formed remains in a large amount.
  • this case is effective to make the average aspect ratio of the retained ⁇ small. This is because when the reduction is large, the retained ⁇ that is stretched is divided.
  • a finish rolling initiation temperature be set as low as possible in order for the final rolling temperature (finish rolling temperature) during the finish rolling to be 700°C to 730°C.
  • the steel after the finish rolling be subjected to accelerated cooling.
  • the cooling rate in the accelerated cooling after the rolling be fast.
  • the lower limit of the cooling rate at the central portion of the plate thickness t of the steel, that is, at a (1/2)t location regarding the plate thickness t is set to 3 °C/s.
  • the lower limit of the cooling rate is set to 10 °C/s.
  • the upper limit of the cooling rate at the (1/2)t location regarding the plate thickness t is not particularly defined, but may be set to 50 °C/s in consideration of facility capacity.
  • the lower limit of a cooling initiation temperature is set to 660°C so as to convert a structure of the steel into a sufficiently quenched structure, and to obtain a concentration ratio of 1.4 or more with fine retained ⁇ by the subsequent tempering treatment and the like.
  • the accelerated cooling be performed until a surface temperature of the steel reaches 250°C or lower.
  • a cooling stop temperature is higher than 250°C, transformation into a martensite structure becomes incomplete, or a phenomenon in which dislocation in the martensite structure is recovered due to auto-tempering effect occurs. As a result, fine retained ⁇ is not effectively generated at the subsequent heat treatment, and thus a possibility of deficiency in strength increases.
  • the upper limit of the cooling stop temperature be set to 200°C or 150°C.
  • the lower limit of the cooling stop temperature is not particularly defined, but may be set to 50°C or room temperature in consideration of facility capacity.
  • DQT direct-quenching and tempering
  • DQLT direct-quenching, lamellarizing, and tempering
  • a generation site of ⁇ fails, and thus the amount of ⁇ does not increase sufficiently.
  • work strain which is introduced to ⁇ during rolling, is maintained before quenching, and thus it is possible to very finely adjust a martensite structure after quenching.
  • ⁇ which is generated from the fine martensite structure during the subsequent heat treatment, is fine and exists in a large amount.
  • the L treatment in which heating is performed in a temperature range of 620°C to 720°C and then a water cooling treatment is performed, may be performed as necessary.
  • the lower limit of the heating temperature is set to 620°C
  • an increase in the retained ⁇ can be expected.
  • the upper limit of the heating temperature is set to 720°C, coarsening of a structure can be prevented.
  • a preferable heating temperature range in the L treatment is 640°C to 700°C.
  • the tempering is very important to realize the invention, and is essential in which detailed control is necessary. In a case where a tempering temperature is too low, an amount of the retained ⁇ generated becomes deficient, and thus the amount of the retained ⁇ itself becomes small. In addition, when the tempering temperature is too low, there is a possibility that tempering embrittlement occurs, and as a result, fracture-resistant characteristics are damaged. In contrast, when the tempering temperature is too high, the amount of ⁇ during heating increases, but the concentration of Ni and Mn in the retained ⁇ decreases. In this case, the retained ⁇ is mostly transformed during the subsequent cooling, or even when transformation does not occur during cooling, the retained ⁇ is transformed only when being exposed to a very low temperature and is lost.
  • a tempering temperature range depends on a thermodynamic equilibrium behavior and varies in accordance with a chemical composition of a steel. Specifically, it is necessary to set the lower limit and the upper limit of the tempering temperature T to 3.8 ⁇ Ni-33+Ac 1 and 6.3 ⁇ Ni-0.4+Ac 1 , respectively. That is, it is necessary to satisfy the following Expression (3). Here, coefficients described in Expression (3) are obtained by multiple regression of experiment results. It is preferable that the tempering temperature be set to be higher than Ac 1 .
  • the lower limit of the cooling rate When the lower limit of the cooling rate is set to 0.5 °C/s, a density of dislocation in martensite which occurs due to the martensite transformation can be increased, and a dynamic restriction effect is added to the retained ⁇ adjacent to the martensite, and thus it can be assumed that stability of the retained ⁇ can be improved. Accordingly, after performing heating in the tempering, it is necessary to set the lower limit of the cooling rate at the central portion in a plate thickness direction to 0.5 °C/s until a surface temperature reaches 300°C or lower.
  • the upper limit of the cooling rate after the heating in the tempering is not particularly defined, but may be set to 50 °C/s in consideration of the maximum facility capacity.
  • the lower limit of the cooling stop temperature is not particularly defined, but may be set to 50°C or room temperature in consideration of facility capacity.
  • Ac 1 is defined by the following Expression (4).
  • Ac 1 712 + 20.1 ⁇ Si ⁇ 17.8 ⁇ Mn ⁇ 19.1 ⁇ Ni + 11.9 ⁇ Cr ⁇ 9.8 ⁇ Mo
  • a symbol of an element in Expression represents an amount (mass%) of each element in a steel.
  • Patent Document 1 the reason why the DT test and pre-crack Charpy test were not satisfactory in steel plates (particularly, Test Nos. 1-a to 1-h, 4-12, and 22-35) with satisfactory V-notch Charpy absorbed energy v E- 196 and the like is that the accelerated cooling was not performed with respect to all of the steel plates after tempering, or the cooling rate was set to less than 0.5 °C/s.
  • V-notch test specimen (a full-size test specimen) defined in JIS Z 2242 was collected along the rolling direction.
  • steel types having a plate thickness of less than 10 mm it was impossible to collect the V-notch test specimen having a width of 10 mm and a plate thickness of 10 mm, and thus a sub-size test specimen was collected.
  • a tensile test at room temperature and a Charpy impact test at -196°C were performed to examine a tensile strength TS (MPa), a yield strength YS (MPa), and V-notch Charpy absorbed energy v E- 196 (J) (an average value of three values). Absorbed energy was converted into absorbed energy per 1 cm 2 for easy comparison between a sub-size test specimen and a full-size test specimen.
  • a dynamic tear (DT) test defined in ASTM E604 was performed, at -196°C, and the absorbed energy DT -196 (J) was evaluated.
  • YS 585 MPa or higher, TS: 690 MPa or higher, V-notch Charpy absorbed energy value v E -196 per unit area: 150 J/cm 2 or more, and absorbed energy DT- 196 (J) in a DT test: 1500 J or more were determined as "passing".
  • the DT test could not be performed to evaluate a material having a plate thickness less than 15 mm, and thus a pre-crack Charpy test was performed with respect to a test specimen having a plate thickness of less than 15 mm.
  • the test specimen for pre-crack Charpy test has a crack depth of 2 mm with respect to a test specimen width of 10 mm, but a V-notch depth in the crack depth was limited to 1 mm, and a fatigue crack was introduced as the remaining 1 mm.
  • a crack easily occurred. According to this, pre-crack Charpy test results and brittle crack propagation arrest characteristics had a satisfactory correlation.
  • a criterion for determination of good or bad brittle crack propagation characteristics with the pre-crack Charpy test was absorbed energy at -196°C similar to the V-notch Charpy, and a test specimen in which absorbed energy per 1 cm 2 was 100J/cm 2 or more was determined as "passing".
  • a method of evaluating the amount of the retained ⁇ was as follows. A test specimen for measurement of the retained ⁇ was collected from a (1/4)t location regarding the plate thickness t of the steel, and the amount (vol%) of the retained ⁇ was measured with X-ray diffraction. A cross-section that was measured was set to an L cross-section (plane that is parallel with a rolling direction and is perpendicular to a surface of a steel plate). In addition, a shape of the retained ⁇ was evaluated by thin film observation with a transmission electron microscope. Twenty or more of retained ⁇ grains were observed, and an average aspect ratio and an average value of the major axis of the grain samples were measured, and an average value in the sample was calculated.
  • concentration rates of Mn and Ni in the retained ⁇ were evaluated by the following method.
  • An average Mn concentration and an average Ni concentration in the retained ⁇ were measured with energy dispersive X-ray spectrometry (EDX) quantitative analysis, and the average concentrations were compared with an average Mn concentration and an average Ni concentration in ferrite, respectively, and then evaluation of whether or not the following Expression (1) and Expression (2) were satisfied was performed.
  • the average Mn concentration and the average Ni concentration in ferrite were set to a bulk value (chemical analysis result) of a corresponding steel.
  • the [Mn] retained ⁇ indicates the average Mn concentration in the retained ⁇
  • the [Mn] ⁇ indicates the average Mn concentration in ferrite
  • the [Ni] retained ⁇ indicates the average Ni concentration in the retained ⁇
  • the [Ni] ⁇ indicates the average Ni concentration in ferrite.
  • the steel according to the invention in which the amount of Ni is 6.6% to 8.0% in terms of mass% is excellent in economic efficiency and fracture-resistant characteristics.
  • This steel is suitable for use in an inner tank member or an annular plate of an LNG tank.

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EP2871255A1 (en) 2015-05-13
KR20150020258A (ko) 2015-02-25
WO2014203347A1 (ja) 2014-12-24
EP2871255A4 (en) 2016-01-06
KR101572786B1 (ko) 2015-11-27
JPWO2014203347A1 (ja) 2017-02-23
CN104520461B (zh) 2016-06-15
CN104520461A (zh) 2015-04-15
JP5561442B1 (ja) 2014-07-30

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