Classifications

• • 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
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  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/001—Heat treatment of ferrous alloys containing Ni
• • 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
  • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0226—Hot rolling
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
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  • 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
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  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • 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
  • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
• • 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
  • C22C38/16—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
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  • 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
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  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/44—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
  • F17C2203/00—Vessel construction, in particular walls or details thereof
  • F17C2203/06—Materials for walls or layers thereof; Properties or structures of walls or their materials
  • F17C2203/0634—Materials for walls or layers thereof
  • F17C2203/0636—Metals
  • F17C2203/0639—Steels
• • 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
  • F17C2203/00—Vessel construction, in particular walls or details thereof
  • F17C2203/06—Materials for walls or layers thereof; Properties or structures of walls or their materials
  • F17C2203/0634—Materials for walls or layers thereof
  • F17C2203/0636—Metals
  • F17C2203/0648—Alloys or compositions of metals
• • 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
  • F17C2203/00—Vessel construction, in particular walls or details thereof
  • F17C2203/06—Materials for walls or layers thereof; Properties or structures of walls or their materials
  • F17C2203/0634—Materials for walls or layers thereof
  • F17C2203/0658—Synthetics
  • F17C2203/0675—Synthetics with details of composition

Definitions

• the present invention relates to a thick steel plate that is used as a material of structural member requiring cryogenic-temperature properties, for a storage tank for LNG (liquefied natural gas) and so on, particularly a thick steel plate having excellent HAZ toughness at cryogenic temperatures.
• a natural gas contains methane as a main ingredient and is liquefied at cryogenic temperatures under atmospheric pressure. On that occasion, its volume is decreased to about 1/600. For that reason, a way of storing or transporting the natural gas in the form of a liquid rather than a gas is advantageous. Meanwhile, it is necessary to hold the natural gas at cryogenic temperatures, and therefore, a material having excellent cryogenic-temperature properties is required for an LNG storage tank and so on.
• a thick steel plate that is used for an LNG storage tank and so on is ferrite-based steel.
• this ferrite-based steel becomes brittle at low temperatures and possibly results in fracture as in ceramics.
• Ni is an expensive element
• a reduction of the Ni content is always required. From the standpoint of a balance between those matters, it is the present situation that 9% Ni steel is used as the material of structural member requiring excellent cryogenic-temperature properties, for an LNG storage tank and so on.
• an object of the present invention is to provide a thick steel plate having excellent HAZ toughness at cryogenic temperatures, that is capable of ensuring HAZ toughness at cryogenic temperatures while minimizing the addition amount of expensive Ni as far as possible.
• the thick steel plate having HAZ toughness at a cryogenic temperature in the present invention includes, in terms of mass %, 0.02 to 0.10% of C, 0.40% or less (not including 0%) of Si, 0.5 to 2.0% of Mn, 0.007% or less (not including 0%) of P, 0.007% or less (not including 0%) of S, 0.005 to 0.05% of Al, 5.0 to 7.5% of Ni, 0.025% or less (not including 0%) of Ti, and 0.010% or less (not including 0%) of N, with the remainder being iron and inevitable impurities, and a Di value determined according to the following formula is 2.5 or more and 5.0 or less: ([C]/10) 0.5 ⁇ (1 + 0.7 ⁇ [Si]) ⁇ (1 + 3.33 ⁇ [Mn]) ⁇ (1 + 0.35 ⁇ [Cu]) ⁇ (1 + 0.36 ⁇ [Ni]) ⁇ (1 + 2.16 ⁇ [Cr]) ⁇ (1 + 3 ⁇ [Mo]) ⁇ (1
• N parameter is 20 ppm or less
• an Ni-Ti balance is ⁇ 0.0024 ⁇ ([Ni] - 7.5) 2 + 0.010 - [Ti] ⁇ ⁇ 0, and a grain size after heating at 700°C for 5 seconds and cooling from 700°C to 500°C over 19 seconds is 4.0 ⁇ m or less, provided that in each of the formulae, [ ] expresses mass %.
• the thick steel plate further includes, in terms mass %, one or two or more of 1.0% or less (not including 0%) of Cu, 1.2% or less (not including 0%) of Cr, and 1.0% or less (not including 0%) of Mo in terms of mass %.
• the thick steel plate further includes, in terms mass %, one or two or more of 0.1% or less (not including 0%) of Nb, 0.5% or less (not including 0%) of V, 0.005% or less (not including 0%) of B, and 0.005% or less (not including 0%) of Zr in terms of mass %.
• the thick steel plate further includes, in terms mass %, one or two of 0.003% or less (not including 0%) of Ca and 0.005% or less (not including 0%) of REM in terms of mass %.
• cryogenic temperature as intended in the present invention refers to from -165°C to -196°C.
• a heat affected zone (HAZ) where the microstructure formed by heat treatment vanishes by a heat cycle
• HZ heat affected zone
• it is an effective method of adopting refinement of the microstructure size or reduction of low-temperature YS.
• an attention has been paid to control of solute N that is a cause of bringing an increase of YS due to the Cottrell atmosphere and the Ni quantity that is said to be able to reduce low-temperature YS of the matrix.
• the Di value is 2.5 or more and 5.0 or less
• the Di value that is an indicator of hardenability during quenching can be determined according to the following formula: C / 10 0.5 ⁇ 1 + 07 ⁇ Si ⁇ 1 + 3.33 ⁇ Mn ⁇ 1 + 0.35 ⁇ Cu ⁇ 1 + 0.36 ⁇ Ni ⁇ ⁇ 1 + 2.16 ⁇ Cr ⁇ 1 + 3 ⁇ Mo ⁇ 1 + 1.75 ⁇ V ⁇ 1 + 200 ⁇ B ⁇ ⁇ 1.7 ⁇ 0.09 ⁇ 6.5
• the Di value In order to obtain a microstructure of a fine size, it is effective to prescribe the Di value for the sake of convenience. In the case where the Di value is less than 2.5, the microstructure becomes rough, and vE -196 in the Charpy impact absorption test is decreased. On the other hand, when the Di value is more than 5.0, the hardness increases, and in this case, vE -196 in the Charpy impact absorption test is decreased, too. Therefore, an appropriate range of the Di value that is an indicator of hardenability during quenching is set to 2.5 or more and 5.0 or less.
• Examples of the element fixing solute N may include Al, B, Nb, Ti, and the like.
• HAZ is influenced by the heat cycle, even when N fixation is made on the base metal, an N compound that is thermally instable is dissolved during the heat cycle in HAZ.
• Ti capable of forming a thermally stable N compound.
• the Ti compound is used as an indicator of the N fixation.
• An appropriate range of the sol. N parameter that can be determined according to the following formula is 20 ppm or less in terms of a mass ratio.
• a lower limit value of the sol. N parameter is not particularly prescribed, it is preferably set to -40 ppm or more because there is a concern that when Ti is excessive relative to N, reduction of the toughness following an increase of the hardness is brought.
• compound type Ti refers to a Ti content contained in the Ti compound.
• the mass (unit: ppm) of the compound type Ti can be determined by measuring a Ti concentration (insol. Ti amount) at which the compound is formed by means of electrolytic extraction from a t/4 position of the thick steel plate (t: plate thickness).
• the extraction may be performed by the iodine methanol method, and the mass of the compound type Ti can be determined by filtering an electrolytic solution after extraction with a filter having a pore size of 0.1 ⁇ m and quantitating the Ti amount in the extraction residue remained on the filter by inductively coupled plasma (ICP) emission spectrum analysis.
• ICP inductively coupled plasma
• Ni-Ti balance at which the above-described effect by the addition of Ti can be obtained was determined through experiments. It may be considered that the effect by the addition of Ti mainly relies upon the above-described sol. N fixation. However, besides, it may also be considered that the refinement of microstructure size by the Ti compound and the like may be considered to be effective, and apart from a Ti-N balance, it is also necessary to control the Ni-Ti balance.
• Ni-Ti balance so as to satisfy a relationship: ⁇ 0.0024 ⁇ ([Ni] - 7.5) 2 + 0.010 - [Ti] ⁇ ⁇ 0.
• an upper limit value by this formula is not particularly prescribed, a desired upper value thereof is, for example, 0.0180.
• the grain size after heating at 700°C for 5 seconds and cooling from 700°C to 500°C over 19 seconds is 4.0 ⁇ m or less
• the grain size of HAZ fine, the low-temperature toughness of HAZ is improved.
• the grain size of HAZ is influenced by some factors, such as a strain within the microstructure, etc., in addition to the base metal microstructure and the grain size of the base metal, and therefore, it is insufficient to prescribe the grain size of HAZ only by the base metal microstructure.
• the grain size after a heat cycle of heating at 700°C for 5 seconds and further cooling from 700°C to 500°C over 19 seconds is prescribed.
• the microstructure after such a heat cycle may be said to be a microstructure of an HAZ-corresponding part.
• the component composition of the thick steel plate is prescribed.
• the component composition is described in detail.
• the content of each of elements (chemical components) is hereunder described merely in terms of %, but all of them are mass %.
• C is effective for lowering an Ms point to obtain a microstructure of a fine size.
• C In order to effectively exhibit such an action, C must be contained in an amount of at least 0.02% or more.
• a lower limit of the content of C is preferably 0.03%, and more preferably 0.04%.
• an upper limit thereof is controlled to 0.10%.
• the upper limit of the content of C is preferably 0.08%, and more preferably 0.06%.
• Si 0.40% or less (not including 0%)
• Si is a useful element as a deoxidizer. Si has an action to prevent Ti from consumption for deoxidation and assist the N fixation. However, when Si is excessively added, a hard martensite island is promoted, leading to reduction of the cryogenic-temperature toughness. Thus, an upper limit thereof is controlled to 0.40%.
• the upper limit of the content of Si is preferably 0.35%, and more preferably 0.20%. Though a lower limit of the content of Si is not particularly prescribed, it is preferably 0.01%.
• Mn is effective for lowering an Ms point to obtain a microstructure of a fine size.
• Mn must be contained in an amount of at least 0.5% or more.
• a lower limit of the content of Mn is preferably 0.6%, and more preferably 0.7%.
• temper brittleness is brought, whereby the desired cryogenic-temperature toughness cannot be ensured.
• an upper limit thereof is controlled to 2.0%.
• the upper limit of the content of Mn is preferably 1.5%, and more preferably 1.3%.
• P is an impurity element causing reduction of the toughness, and therefore, its content is preferably low as far as possible.
• the content of P is needed to be controlled to 0.007% or less and is preferably controlled to 0.005% or less. Though it is desirable that the content of P is low as far as possible, it is industrially difficult to decrease the content of P in the steel to 0%.
• S is an impurity element causing reduction of the toughness, and therefore, its content is preferably low as far as possible.
• the content of S is needed to be controlled to 0.007% or less and is preferably controlled to 0.005% or less. Though it is desirable that the content of S is low as far as possible, it is industrially difficult to decrease the content of S in the steel to 0%.
• Al is a useful element as a deoxidizer.
• Al has an action to prevent Ti from consumption for deoxidation and assist the N fixation.
• Al promotes desulfurization.
• a lower limit thereof is controlled to 0.005%.
• the lower limit of the content of Al is preferably 0.010%, and more preferably 0.015%.
• oxides, nitrides, and so on are coarsened, and the cryogenic-temperature toughness is reduced, too.
• an upper limit thereof is controlled to 0.05%.
• the upper limit of the content of Al is preferably 0.045%, and more preferably 0.04%.
• Ni is an effective element for improving the cryogenic-temperature toughness. In order to effectively exhibit such an action, Ni must be contained in an amount of at least 5.0% or more. A lower limit of the content of Ni is preferably 5.2%, and more preferably 5.4%. However, when Ni that is an expensive element is excessively added, an increase of costs of raw material is brought. Thus, an upper limit thereof is controlled to 7.5%. The upper limit of the content of Ni is preferably 6.5%, more preferably 6.2%, and still more preferably 6.0%.
• Ti is an effective element for fixation of solute N.
• a lower limit thereof is preferably 0.003%, and more preferably 0.005%.
• a preferred upper limit of the content of Ti is controlled to 0.025%.
• the upper limit of the content of Ti is more preferably 0.018%, and still more preferably 0.015%.
• N 0.010% or less (not including 0%)
• solute N When a large quantity of N is present as solute N, the HAZ toughness is reduced. Even if the solute N could be fixed by some method, from the viewpoint of solubility product, all of N activities are preferably small. Thus, an upper limit thereof is controlled to 0.010%.
• the upper limit of the content of N is preferably 0.006%, and more preferably 0.004%. Though it is desirable that the content of N is low as far as possible, it is industrially difficult to decrease the content of N in the steel to 0%.
• Cu, Cr, and Mo are each an effective element for lowering an Ms point to obtain a microstructure of a fine size. These elements may be added singly or in combination of two or more thereof.
• its content is preferably 0.05% or more; in the case of adding Cr, its content is preferably 0.05% or more, and in the case of adding Mo, its content is preferably 0.01% or more.
• each element is excessively added, an excessive improvement of strength is brought, so that the desired cryogenic-temperature toughness cannot be ensured.
• its content is needed to be controlled to 1.0% or less, preferably 0.8% or less, and more preferably 0.7% or less.
• Nb 0.1% or less (not including 0%)
• V 0.5% or less (not including 0%)
• B 0.005% or less (not including 0%)
• Zr 0.005% or less (not including 0%)
• Nb, V, B, and Zr are each an effective element for fixation of solute N. These elements may be added singly or in combination of two or more thereof.
• Nb its content is preferably 0.005% or more
• V its content is preferably 0.005% or more
• B its content is 0.0005% or more
• Zr its content is 0.0005% or more.
• each element is excessively added, an excessive improvement of strength is brought, or coarse inclusions are formed to reduce the toughness.
• Nb its content is needed to be controlled to 0.1% or less, preferably 0.05% or less, and more preferably 0.02% or less.
• V its content is needed to be controlled to 0.5% or less, preferably 0.3% or less, and more preferably 0.2% or less.
• B its content is needed to be controlled to 0.005% or less, preferably 0.003% or less, and more preferably 0.002% or less.
• Zr its content is needed to be controlled to 0.005% or less and preferably 0.004% or less.
• Ca and REM are each an element that fixes solute sulfur and further makes sulfides harmless. These elements may be added singly or in combination of two or more thereof. When the content of each of these elements is insufficient, the concentration of solute sulfur in the steel increases, and the toughness is reduced. Thus, in the case of adding Ca, its content is preferably controlled to 0.0005% or more, and in the case of adding REM, its content is preferably controlled to 0.0005% or more. However, when each element is excessively added, sulfides, oxides, nitrides, and so on are coarsened, so that the toughness is reduced, too. Thus, in the case of adding Ca, its content is needed to be controlled to 0.003% or less and preferably 0.0025% or less. In addition, in the case of adding REM, its content is needed to be controlled to 0.005% or less and preferably 0.004% or less.
• the REM rare earth element
• the REM means a group of elements including Sc (scandium) and Y (yttrium) in addition to lanthanide elements (15 elements of from La (atomic No. 57) to Lu (atomic No. 71) in the periodic table), and these elements can be used singly or in combination of two or more thereof.
• the content of REM means a content of a sole element in the case where only one REM is contained or a total content in the case where two or more REMs are contained.
• Sc and Y are low in an atomic weight as compared with other REMs.
• an inexpensive misch metal containing plural lanthanide elements is used, but Sc and Y may also be used.
• Sc and Y may also be used.
• they are added so as to satisfy the following formula. 2 / 3 ⁇ 1 / 88 ⁇ 226 ⁇ 1 / 4.8 ⁇ REM Sc Y + 2 / 3 ⁇ 1 / 140 ⁇ 327 ⁇ 1 / 7 ⁇ REM others ⁇ 0.0015
• [REM (Sc, Y)] represents an addition amount (mass %) of Sc and Y; and [REM (others)] represents an addition amount (mass %) of REMs other than Sc and Y.
• REM Ce and La are a preferred element.
• the addition form of REM is not particularly limited, REM may be added in a form of misch metal mainly containing Ce and La (for example, Ce: about 70% and La: about 20 to 30%), or may be added in a form of a simple substance of Ce or La, or the like.
• the thick steel plate of the present invention can be obtained through steps of melting steel satisfying the above-described component composition by a usual melting method to prepare a slab, followed by performing usual heating, hot rolling (rough rolling and finish rolling), and cooling. However, by carrying out the heat treatment of the base metal under the following condition, the thick steel plate capable of surely satisfying the requirements of the present invention can be manufactured.
• the heat treatment of the base metal is carried out in a temperature region of 630°C to Ac3 (two-phase region).
• the microstructure of the HAZ part after welding can be subjected to grain refining. That is, in the present invention, the grain size after a heat cycle of heating at 700°C for 5 seconds and cooling from 700°C to 500°C over 19 seconds can be regulated to 4.0 ⁇ m or less.
• the grain size after the above-described heat cycle becomes coarse, so that the predetermined toughness cannot be satisfied.
• Thick steel plates having respective component compositions shown in Tables 1 and 2 were used, and from a t/4 position (t: plate thickness) of each of those thick steel plates, a small specimen having a size of 12.5t ⁇ 55W ⁇ 33L in parallel to the plate width direction was taken. Thereafter, from the small specimen to which a heat treatment described in each of Tables 3 and 4 had been applied, every two Charpy impact test specimens (JIS Z2242 V-notch test specimens) were taken and measured for absorption energy at -196°C according to the essentials described in JIS Z2242.
• the grain size was measured as follows. That is, in the microstructure just beneath the fractured surface photographed by an optical microscope, with respect to the range of 150 ⁇ m substantially in the notch vertical direction and 200 ⁇ m in the notch horizontal direction, a site divided by a segment of black contrast having a width of 0.5 ⁇ m or less was defined as a microstructure unit, 50 or more structure units were measured by the segment method relative to the notch horizontal direction, and an average thereof was defined as the grain size.
• Nos. 1 to 21 are concerned with an invention example satisfying the requirements of the present invention; and in Nos. 1 to 21, all of the average values of the absorption energy at -196°C were 41 J or more, and the relationship of (vE -196 ⁇ 41 J) was satisfied. From these test results, it can be said that all of the invention examples of Nos. 1 to 21 satisfying the requirements of the present invention are a thick steel plate having excellent HAZ toughness at cryogenic temperatures.
• Nos. 22 to 39 are concerned with a comparison example not satisfying any one of the requirements of the present invention. and in Nos. 22 to 39, all of the average values of the absorption energy at -196°C were less than 41 J, and the relationship of (vE -196 ⁇ 41 J) could not be satisfied, and thus, the thorough HAZ toughness at cryogenic temperatures could not be ensured.
• a single bevel groove (root gap: 6 mm, groove angle: 30°) was applied, and a joint was fabricated under the following condition.
• the design is performed in such a manner that by applying a multi-pass X groove, a low-toughness HAZ is not substantially included, and from the standpoint of shape, a crack develops only in the low-toughness HAZ part.
• the single bevel groove was applied.
• the thick steel plate of the present invention has excellent HAZ toughness at cryogenic temperatures and is useful as a structural member requiring cryogenic-temperature properties, for an LNG storage tank and so on.

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• 2014
  • 2014-04-08 JP JP2014079378A patent/JP6196929B2/ja not_active Expired - Fee Related
• 2015
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