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  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
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  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
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  • C22C—ALLOYS
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  • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
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  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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  • C22C—ALLOYS
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  • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
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  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
• • 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
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  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
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  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
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  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
• • C—CHEMISTRY; METALLURGY
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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
  • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
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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
  • C22C38/54—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

Definitions

• the present invention relates to steel plates having excellent cryogenic toughness (toughness at cryogenic temperatures). Specifically, the present invention relates to a steel plate having good toughness [in particular, toughness in a crosswise direction (width direction; C-direction)] at cryogenic temperatures of -196°C or lower even when having a reduced Ni content of about 5.0% to about 7.5%.
• LNG liquefied natural gas
• Such steel plates are represented by those for use in storage tanks and transport ships. It should be noted, however, that the present invention is not limited to steel plates of this type and is applicable to all steel plates for use in applications where the steel plates are exposed to cryogenic temperatures of -196°C or lower.
• LNG tank-use steel plates are to be used in liquefied natural gas (LNG) storage tanks and require not only high strengths but also such high toughness as to endure a cryogenic temperature of -196°C.
• LNG liquefied natural gas
• steels are known to have better hardness-toughness balance especially at low temperatures by the addition of Ni.
• steel plates having a Ni content of about 9% (9% Ni steel plates) have been used as steel plates for the use.
• Nonpatent Literature 1 describes the effects of a heat treatment of 6% Ni steel in the ferrite-austenite two-phase region on low-temperature toughness.
• a heat treatment (L treatment; lamellarizing) in the ferrite-austenite two-phase region (between A c1 and A c3 ) is added prior to a tempering treatment of a conventional process.
• L treatment lamellarizing
• the resulting 6% Ni steel can ensure cryogenic toughness at -196°C of equal to or better than the Cryogenic toughness of a 9% Ni steel that has undergone regular quenching and tempering treatments.
• the 6% Ni steel has excellent cryogenic toughness in the L-direction, but, unfortunately, tends to generally have inferior cryogenic toughness in a crosswise direction (C-direction) as compared with the cryogenic toughness in a rolling direction (longitudinal direction; L-direction).
• C-direction crosswise direction
• L-direction longitudinal direction
• the literature fails to describe percent brittle fracture.
• Patent Literature 1 describes a method. This method uses a steel having a Ni content of 4.0% to 10% and having austenite grain size and other factors controlled within predetermined ranges. In the method, the steel is subjected to a specific treatment one or more times and tempered at a temperature equal to or lower than the A c1 transformation temperature. In the treatment, the steel is hot-rolled, heated to a temperature between A c1 and A c3 , and then cooled. This treatment corresponds to the L treatment in Nonpatent Literature 1. Patent Literature 2 describes another method.
• This method employs a steel having a Ni content of 4.0% to 10% and having a particle size of AlN particles before hot rolling of 1 ⁇ m or less.
• the method subjects the steel to heat treatments (L treatment and subsequent tempering treatment) as in Patent Literature 1.
• the values of impact energy at -196°C (vE -196 ) described in these methods are probably those in the L-direction, and the values as toughness in the C-direction are not found therein.
• the methods fail to consider the strength and to describe the percent brittle fracture.
• Nonpatent Literature 2 describes development of a 6% Ni steel for LNG tanks, in which the L treatment (two-phase region quenching treatment) and a thermal-mechanical control process (TMCP) are employed in combination. This literature describes that the resulting steel has a satisfactory value of toughness in the rolling direction (L-direction), but fails to describe the toughness value in the crosswise direction (C-direction).
• Patent Literature 3 describes a steel plate for cryogenic temperature use, where the steel plate has a reduced Ni content, and a method for producing the steel plate.
• the steel plate employs a Ni steel plate having a Ni content of greater than 5.0% to less than 8.0% and having a yield strength of 590 MPa or more at room temperature. Even in use environments, the steel plate has excellent safety against fracture equivalent to that of 9% Ni steels.
• a steel ingot is heated at a low temperature for a short time in a heating process, and the heated steel ingot in a rolling process is subjected to rough rolling to such a reduction that the steel ingot upon the completion of rough rolling has a thickness 3 to 8 times as much as the thickness of the product (thickness of the steel plate after finish rolling).
• the technique is based on a finding that a steel plate can have better safety against fracture (namely, can have higher toughness in a low-temperature environment) when the steel plate is allowed to surely have a higher yield point in an environment at low temperatures (cryogenic temperatures), where the resulting steel plate is used in such cryogenic temperature environment.
• the workpiece (slab) is rolled from a slab thickness of 300 mm down to a finish thickness of 50 mm or less (mostly to a finish thickness of less than 50 mm).
• the resulting steel plate as surely having a relatively high rolling reduction, has both retained austenite in a certain fraction and a finely dispersed matrix phase and achieves cryogenic toughness at a level equal to the 9% Ni steels.
• the steel plate disclosed in Patent Literature 3 has a tensile strength TS at room temperature of at largest 741 MPa.
• Patent Literature 3 mentions the absorbed energy in the C-direction, but fails to describe the percent brittle fracture.
• the steel plate disclosed in Patent Literature 3 has a tensile strength TS at room temperature of at largest about 741 MPa.
• the percent brittle fracture refers to a peiroentage of brittle fracture occurring upon the application of a load in a Charpy impact test. In a region where brittle fracture occurs, energy to be absorbed by the steel until the fracture occurs is remarkably lowered, and this causes the fracture to easily proceed. To prevent this and to provide better cryogenic toughness, the steel should very importantly have not only a better general Charpy impact value (vE -196 ), but also a percent brittle fracture of 10% or less.
• vE -196 general Charpy impact value
• the present invention has been made in consideration of these circumstances. It is an object of the present invention to provide a high-strength steel plate that includes a Ni steel having a Ni content of about 5.0% to about 7.5%, has excellent cryogenic toughness (in particular, cryogenic toughness in the C-direction) at -196°C, and achieves a percent brittle fracture of equal to or less than 10%. It is another object of the present invention to provide a method for producing the steel plate.
• the present invention has achieved the objects and provides, in one embodiment (first embodiment), a steel plate having excellent cryogenic toughness.
• the steel plate contains, in mass percent, C in a content of 0.02% to 0.10%, Si in a content of 0.40% or less (excluding 0%), Mn in a content of 0.50% to 2.0%, P in a content of 0.007% or less (excluding 0%), S in a content of 0.007% or less (excluding 0%), Al in a content of 0.005% to 0.050%, Ni in a content of 5.0% to 7.5%, N in a content of 0.010% or less (excluding 0%), and at least one element selected from the group consisting of Cr in a oontent of 1.20% or less (excluding 0%) and Mo in a content of 1.0% or less (excluding 0%) with the remainder consisting of iron and inevitable impurities.
• the steel plate has a Di value as specified by Formula (1) of 2.5 or more.
• the steel plate includes a retained austenite phase (retained ⁇ ) existing at -196°C in a volume fraction of 2.0% to 12.0%.
• the steel plate has a retained austenite stabilization parameter as specified by Formula (2) of 3.1 or more.
• Formula (2) is defined by chemical compositions contained in the retained austenite and is expressed as follows:
• the steel plate may have a retained austenite phase volume fraction-retained austenite stabilization parameter as specified by Formula (3) of 40 or less.
• the element contents, Di value, and retained austenite volume fraction of the steel plate may be controlled within narrower ranges (more specified ranges), and the Mn content in the retained austenite may be controlled instead of the retained austenite stabilization parameter.
• This steel plate can have still higher base metal strengths and can offer satisfactory cryogenic toughness.
• the present invention provides, in another embodiment (second embodiment), a steel plate having excellent cryogenic toughness.
• the steel plate contains, in mass percent, C in a content of 0.02% to 0.10%, Si in a content of 0.40% or less (excluding 0%), Mn in a content of 0.6% to 2.0%, P in a content of 0.007% or less (excluding 0%), S in a content of 0.007% or less (excluding 0%), Al in a content of 0.005% to 0.050%, Ni in a content of 5.0% to 7.5%, N in a content of 0.010% or less (excluding 0%), Mo in a content of 0.30% to 1.0%, and Cr in a content of 1.20% or less (excluding 0%) with the remainder consisting of iron and inevitable impurities.
• the steel plate has a Di value as specified by Formula (1) of greater than 5.0.
• the steel plate includes a retained austenite phase (retained ⁇ ) existing at -196°C in a volume fraction of 2.0% to 5.0%.
• the retained austenite phase (retained ⁇ ) existing at -196°C has a Mn content of 1.05% or more.
• the steel has Mn and Ni contents (in mass percent) meeting a condition specified by Formula (4): Mn ⁇ 0.31 ⁇ 7.20 ⁇ Ni + 0.50 where [Mn] and [Ni] are contents (in mass percent) respectively of Mn and Ni in the steel.
• the steel plate may further contain Cu in a content of 1.0% or less (excluding 0%).
• the steel plate may further contain at least one selected from the group consisting of Ti in a content of 0.025% or less (excluding 0%), Nb in a content of 0.100% or less (excluding 0%), and V in a content of 0.50% or less (excluding 0%).
• the steel plate may further contain B in a content of 0.0050% or less (excluding 0%).
• the steel plate may further contain at least one selected from the group consisting of Ca in a content of 0.0030% or less (excluding 0%) and at least one rare-earth element (REM) in a content of 0.0050% or less (excluding 0%).
• REM rare-earth element
• the steel plate may further contain Zr in a content of 0.005% or less (excluding 0%).
• the present invention also achieves the objects and further provide, in yet another embodiment (third embodiment), a method for producing the steel plate according to the first embodiment of the present invention.
• the method includes the steps a) and b).
• a steel plate is formed firom a steel and subjected to a heat treatment (L treatment) in a ferrite-austenite two-phase region (between A c1 and A c3 ).
• the steel has such controlled chemical compositions and the L treatment is performed at such a conttrolled temperature (L treatment temperature) that an L parameter as specified by Formula (5) is 0.25 to 0.45 and a ⁇ L parameter as specified by Formula (6) is 7 or less.
• the step b) is performed after the L treatment, in which the steel plate is water-cooled down to room temperature and subjected to a tempering treatment (T treatment) at a temperature equal to or lower than the A c1 temperature for 10 to 60 minutes.
• T treatment a tempering treatment
• Formula (5) is defined by the L treatment temperature and the A c1 and A c3 temperatures in the steel.
• Formula (6) is defined by the L parameter and the steel chemical compositions.
• the present invention achieves the objects and provides, in still another embodiment (fourth embodiment), a method for producing the steel plate according to the second embodiment of the present invention.
• the L treatment temperature and the steel chemical compositions are adjusted so that an L parameter as specified by Formula (5) is 0.6 to1.1 and a ⁇ L parameter as specified by Formula (6) is 0 or less.
• the present invention can provide high-strength steel plates each including a Ni steel having a Ni content of about 5.0% to about 7.5%.
• the steel plates have high base metal strengths, specifically, have a tensile strength TS of greater than 741 MPa and a yield strength YS of greater than 590 MPa, and preferably have a tensile strength TS of equal to or greater than 830 MPa and a yield strength YS of equal to or greater than 690 MPa.
• the steel plates have excellent cryogenic toughness (in particular, cryogenic toughness in the C-direction) at temperatures of -196°C or lower and have a percent brittle fracture of equal to or less than 10% at -196°C, and preferably have a percent brittle fracture of equal to or less than 50% at -233°C.
• the present inventors made investigations so as to provide a steel plate that has a Ni content of 7.5% or less and, when subjected to a Charpy impact test in the C-direction, has a percent brittle fracture of 10% or less at -196°C, a tensile strength TS of greater than 741 MPa, and a yield strength YS of greater than 590 MPa.
• a production method according to the present invention was designed to attain cryogenic toughness at the same level as compared with a 9% Ni steel even without strictly controlling rolling and cooling oonditions after T treatment as in the techniques disclosed in Patent Literatures 1 and 3.
• the chemical compositions of a material steel were designed in consideration of the case where such rolling reduction as in Patent Literature 3 is not obtained.
• Rolling in the present invention was designed on the assumption that the rolling reduction at a temperature of 830°C or higher is controlled to about 50% or less, and the rolling reduction at a temperature of 700°C or higher is controlled to about 85% or less; and that cooling after the tempering treatment (T treatment) is performed not by water cooling, but by air cooling.
• the rolling reduction (%) was calculated as: 100 ⁇ [(Thickness before rolling)-(Thickness after rolling)]/(Thickness before rolling).
• the steel (plate) was designed to have cryogenic toughness as evaluated in the C-direction, in which direction toughness is tend to be hardly ensured as compared with the L-direction.
• the cryogenic toughness was to be evaluated not by absorbed energy, but by percent brittle fracture so as to surely provide toughness at a certain level.
• the steel plate herein was also designed to have a tensile strength (TS) of greater than 741 MPa. This is because a higher tensile strength TS within specifications is better in consideration of safety in designing of pressure vessels to be used at cryogenic temperatures.
• the present inventors made intensive investigations so as to provide a steel plate that is produced under the production conditions, has a percent brittle fracture at -196°C of equal to or less than 10% in the C-direction in a Charpy impact test, a tensile strength TS of greater than 741 MPa, and a yield strength YS of greater than 590 MPa.
• control conditions (A) and (B)] and ⁇ L parameter [control oondition (C)] as follows; and have found that the controls allow the steel plate to include stable retained austenite at a certain level and to have excellent cryogenic toughness, where the stable retained austenite does not transform into martensite, but plastically deforms during the Charpy impact test.
• the controls are expressed as follows:
• the steel plate according to the embodiment of the present invention contains, in mass percent, C in a content of 0.02% to 0.10%, Si in a content of 0.40% or less (excluding 0%), Mn in a content of 0.50% to 2.0%, P in a content of 0.007% or less (excluding 0%), S in a content of 0.007% or less (excluding 0%), Al in a content of 0.005% to 0.050%, Ni in a content of 5.0% to 7.5%, N in a content of 0.010% or less (excluding 0%), and at least one element selected from the group consisting of Cr in a content of 1.20% or less (excluding 0%) and Mo in a content of 1.0% or less (excluding 0%), with the remainder consisting of iron and inevitable impurities.
• the steel plate has a Di value as specified by Formula (1) of 2.5 or more, a volume fraction of retained austenite phase (retained ⁇ ) existing at -196°C of 2.0% to 12.0%, and a retained austenite stabilization parameter as specified by Formula (2) of 3.1 or more.
• Formula (1) is defined by the steel chemical compositions.
• Formula (2) is defined by chemical compositions contained in the retained austenite existing at -196°C.
• Carbon (C) is essential to ensure strengths and retained austenite at certain levels. To have such activities effectively, the carbon content is controlled in its lower limit to be 0.02% or more, preferably 0.03% or more, and more preferably 0.04% or more. However, carbon, if added in excess, may cause the steel plate to have excessively high strengths and to have inferior cryogenic toughness contrarily. To prevent this, the carbon content is controlled in its upper limit to be 0.10% or less, preferably 0.08% or less, and more preferably 0.06% or less.
• Silicon (Si) is useful as a deoxidizer. However, silicon, if added in excess, may accelerate the formation of hard martensite islands and may cause the steel plate to have inferior cryogenic toughness. To prevent this, the Si content is controlled in its upper limit to be 0.40% or less, preferably 0.35% or less, and more preferably 0.20% or less.
• Manganese (Mn) stabilizes austenite (y) and contributes to a larger amount of retained austenite.
• the Mn content is controlled in its lower limit to be 0.50% or more, preferably 0.6% or more, and more preferably 0.7% or more.
• Mn if added in excess, may cause temper embrittlement and cause the steel plate to fail to surely have desired cryogenic toughness.
• the Mn content is controlled in its upper limit to be 2.0% or less, preferably 1.5% or less, and more preferably 1.3% or less.
• Phosphorus (P) is an impurity element and causes grain boundary fracture. To ensure desired cryogenic toughness, the phosphorus content is controlled in its upper limit to be 0.007% or less, and preferably 0.005% or less. The phosphorus content is preferably minimized, but it is difficult to industrially control the phosphorus content to be 0%.
• S is an impurity element and causes grain boundary fracture, as with phosphorus.
• the sulfur content is controlled in its upper limit to be 0.007% or less.
• a steel has an increasing percent brittle fracture with an increasing sulfur content and fails to achieve desired cryogenic toughness (percent brittle fracture at -196°C of equal to or less than 10%).
• the sulfur content is preferably controlled in its upper limit to be 0.005% or less.
• the sulfur content is preferably minimized, but it is difficult to industrially control the sulfur content to be 0%.
• the steel plate if having an insufficient Al content, may have increased contents typically of solute sulfur and solute nitrogen to thereby have inferior cryogenic toughness.
• the Al content is controlled in its lower limit to be 0.005% or more, preferably 0.010% or more, and more preferably 0.015% or more.
• Al if added in excess, may cause oxides, nitrides, and other particles to coarsen and may also cause the steel plate to have inferior cryogenic toughness.
• the Al content is controlled in its upper limit to be 0.050% or less, preferably 0.045% or less, and more preferably 0.04% or less.
• Nickel (Ni) is essential to allow the steel plate to surely include retained austenite (retained ⁇ ) that is useful for better cryogenic toughness.
• the Ni content is controlled in its lower limit to be 5.0% or more, preferably 5.2% or more, and more preferably 5.4% or more.
• the Ni content is controlled in its upper limit to be 7.5% or less, preferably 7.0% or less, more preferably 6.5% or less, furthermore preferably 6.2% or less, and still more preferably 6.0% or less.
• Nitrogen (N) causes strain aging and causes the steel plate to have inferior cryogenic toughness. To prevent this, the nitrogen content is controlled in its upper limit to be 0.010% or less, preferably 0.006% or less, and more preferably 0.004% or less.
• Chromium (Cr) and molybdenum (Mo) each contribute to higher strengths. Each of these elements may be added alone or in combination. To have the activities effectively, the Cr content is controlled to be 0.05% or more, and the Mo content is controlled to be 0.01% or more. However, each of the elements, if added in excess, may cause the steel plate to have excessively high strengths and to fail to ensure desired cryogenic toughness. To prevent this, the Cr content is controlled in its upper limit to be 1.20% or less, preferably 1.1% or less, more preferably 0.9% or less, and furthermore preferably 0.5% or less; and the Mo content is controlled in its upper limit to be 1.0% or less, preferably 0.8% or less, and more preferably 0.6% or less.
• the steel plate according to the present invention contains the chemical compositions as basic compositions with the remainder consisting of iron and inevitable impurities.
• the steel plate according to the present invention may further contain one or more of following selective chemical compositions so as to further have one or more properties.
• Cu is preferably contained in a content of 0.05% or more.
• the Cu content is preferably controlled in its upper limit to be 1.0% or less, more preferably 0.8% or less, and furthermore preferably 0.7% or less.
• Titanium (Ti), niobium (Nb), and vanadium (V) each precipitate as carbonitrides and allows the steel to have higher strengths.
• Each of these elements may be added alone or in combination.
• the Ti, Nb, and V contents are each preferably controlled to be 0.005% or more.
• each of these elements, if added in excess, may cause the steel plate to have excessively high strengths and to fail to ensure desired cryogenic toughness.
• the Ti content is preferably controlled in its upper limit to be 0.025% or less, more preferably 0.018% or less, and furthermore preferably 0.015% or less.
• the Nb content is preferably controlled in its upper limit to be 0.100% or less, more preferably 0.05% or less, and furthermore preferably 0.02% or less.
• the V content is preferably controlled in its upper limit to be 0.50% or less, more preferably 0.3% or less, and furthermore preferably 0.2% or less.
• Boron (B) element allows the steel to have better hardenability and to thereby have higher strengths.
• the boron content is preferably controlled to be 0.0005% or more.
• boron if added in excess, may cause the steel plate to have excessively high strengths and to fail to ensure desired cryogenic toughness.
• the boron content is preferably controlled in its upper limit to be 0.0050% or less, more preferably 0.0030% or less, and furthermore preferably 0.0020% or less.
• Ca calcium
• REM rare-earth element
• Ca Calcium
• REM rare-earth elements fix solute sulfur and make sulfides harmless.
• Each of these elements may be added alone or in combination.
• Each of these elements, if present in an insufficient content, may cause the steel plate to have a higher solute sulfur content and to have inferior toughness.
• the Ca content is preferably conttrolled to be 0.0005% or more; and the REM content is preferably controlled to be 0.0005% or more.
• REM content' refers to the content of one rare-earth element when the one rate-earth element alone as selected from the rare-earth elements mentioned below is contained; and refers to the total content of two or more rate-earth elements when the two or more rate-earth elements are contained.
• each of Ca and REM may cause particles such as sulfides, oxides, and nitrides to coarsen and may also cause the steel plate to have inferior toughness.
• the Ca content is preferably controlled in its upper limit to be 0.0030% or less, and more preferably 0.0025% or less.
• the REM content is preferably controlled in its upper limit to be 0.0050% or less, and more preferably 0.0040% or less.
• the term "REM (rare-earth element)" refers to the group of elements including lanthanoid elements as well as Sc (scandium) and Y (yttrium).
• the lanthanoid elements are fifteen elements from La with atomic number 57 to Lu with atomic number 71 in the periodic table of elements.
• the steel may contain each of these elements alone or in combination.
• Ce and La are preferred.
• the REM may be added in a form not limited.
• the REM may be added in the form of a misch metal or in the form of a single element such as Ce or La.
• the misch metal mainly contains Ce and La and may for example contain about 70% of Ce and about 20% to about 30% of La.
• Zirconium (Zr) fixes nitrogen.
• the steel plate if having an insufficient Zr content, may have an increased solute nitrogen content and have inferior toughness.
• the Zr content is preferably controlled to be 0.0005% or more.
• Zr if added in excess, may cause particles such as oxides and nitrides to coarsen and may cause the steel plate to have inferior toughness.
• the Zr content is preferably controlled in its upper limit to be 0.005% or less, and more preferably 0.0040% or less.
• the steel plate according to the present invention contains a retained austenite phase existing at -196°C in a volume fraction of 2.0% to 12.0% (preferably 4.0% to 12.0%).
• the volume fraction of the retained austenite phase is controlled to 2.0% or more of the total microstructure (all phases) existing at -196°C.
• the retained austenite is relatively soft as compared with the matrix phase and, if present in an excessively large amount, may cause the steel plate to fail to have a yield strength YS at a predetermined level.
• the retained austenite phase volume fraction is controlled in its upper limit to be 12.0% or less.
• the retained austenite phase volume fraction is preferably 4.0% or more, and more preferably 6.0% or more in its lower limit; and is preferably 11.5% or less, and more preferably 11.0% or less in its upper limit
• the retained austenite phase is importantly controlled in its volume fraction in the steel plate according to the present invention.
• the other phases than the retained austenite are not limited and may be those generally present in steel plates.
• the other phases than retained austenite are exemplified by bainite, martensite, cementite, and other carbides.
• the steel plate according to the present invention also has a Di value as specified by Formula (1) of 2.5 or more.
• Formula (1) relates to the Di value indicating hardenability and is described as Grossmann's equation in Trans. Metall. Soc. AIME, 150 (1942), p. 227 .
• the steel plate With increasing amounts of the alloy elements defining Formula (1) as the Di value, the steel plate is more readily hardened (has a higher Di value) and more readily include a finely dispersed microstructure.
• the steel plate With an increasing Di value, the steel plate has higher strengths and more surely has strengths at desired levels.
• the present inventors have found that there is a correlation between the Di value and the microstructure size after rolling; and that the Di value may be controlled to 2.5 or more so as to obtain a finely dispersed microstructure after rolling and to obtain high strengths at desired levels.
• the Di value is found to be a parameter as a useful index so as to obtain a finely dispersed rolling microstructure even with a low rolling reduction in a non-crystallization region. This leads to the formation of retained austenite in a sufficient volume fraction as a result of the subsequent heat treatment and thereby ensures stable retained austenite that is useful for better cryogenic toughness.
• the Di value is also an effective parameter so as to ensure good properties even when a reduced process load is applied.
• the process load herein is reduced by mitigating production conditions described in Patent Literature 3, such as a lowered rolling reduction at low temperatures in a non-recrystallization region, and time restriction until cooling start.
• the Di value is controlled to be 2.5 or more.
• the steel plate if having a Di value of less than 2.5, may fail to obtain a finely dispersed microstructure after rolling and fail to obtain retained austenite in a predetermined amount.
• this steel plate may fail to control the after-mentioned retained austenite stabilization parameter and retained austenite volume fraction-retained austenite stabilization parameter at predetermined levels and may thereby fail to include stable retained austenite phase and to ensure desired cryogenic toughness.
• the Di value is preferably 3.0 or more.
• the upper limit of the Di value is not limited from the above-mentioned viewpoints, but is preferably about 5.0 or less. This is preferred from the viewpoints typically of cost and in consideration that the current strength specification range for LNG tank steels is 830 MPa or less.
• the steel plate according to the present invention also has a retained austenite stabilization parameter as specified by Formula (2) controlled to be 3.1 or more. This allows the steel plate to have desired cryogenic toughness.
• a steel if having a reduced Ni content of 7.5% or less as in the present invention, generally has inferior hardenability, thereby includes a coarsened microstructure after rolling, and fails to ensure the retained austenite volume fraction obtained after the heat treatment, or fails to obtain a Di value at a certain level. According to the present invention, however, these parameters or conditions can be appropriately controlled by appropriately controlling the retained austenite stabilization parameter, where the parameter is determined in consideration of balance among chemical compositions in the retained austenite.
• the retained austenite stabilization parameter is derived with reference to an equation relating to the Ms point (martensite transformation start temperature).
• the retained austenite stabilization parameter is controlled in its lower limit to be 3.1 or more, preferably 3.3 or more, more preferably 3.5 or more, and furthermore preferably 3.7 or more.
• the retained austenite stabilization parameter is not critical in its upper limit from the viewpoint of providing better cryogenic toughness.
• the steel plate according to the present invention preferably has a retained austenite volume fraction-retained austenite stabilization parameter as specified by Formula (3) as controlled to be 40 or less so as to ensure still better cryogenic toughness.
• the parameter is defined by the retained austenite volume fraction and the retained austenite stabilization parameter.
• the present inventors have conceived that the distribution of retained austenite significantly affects the improvement of cryogenic toughness and have defined the parameter as above, because the retained austenite plastically deforms during the impact test at a cryogenic temperature and effectively contributes to better toughness.
• retained austenite grains are present at small intervals and are finely dispersed, and these grains do not transform into martensite, but plastically deform even at cryogenic temperature.
• the steel plate has good cryogenic toughness.
• the retained austenite volume fraction-retained austenite stabilization parameter is preferably 35 or less, and more preferably 30 or less. From the viewpoint of better cryogenic toughness, the parameter is preferably minimized.
• the parameter is not critical in its lower limit in relation to the cryogenic toughness, but is preferably about 10 or more in consideration of the chemical compositions specified in the present invention.
• the steel plate when being controlled to have a retained austenite volume fraction-retained austenite stabilization parameter within a more appropriate range, can have a percent brittle fracture at a good level of 50% or less even at a temperature of -233°C, lower than the temperature of - -196°C. Specifically, when the retained austenite volume fraction-retained austenite stabilization parameter is minimized in its upper limit (about 30 or less), the steel plate can have a percent brittle fracture at -233°C as reduced to 50% or less.
• the above-described configuration according to the present invention can give a steel plate that has a percent brittle fracture at -196°C of equal to or less than 10% in the C-direction in a Charpy impact test and has a tensile strength TS of greater than 741 MPa and a yield strength YS of greater than 590 MPa.
• the present inventors made further investigations so as to give a steel plate that has high base metal strengths and still has satisfactory cryogenic toughness.
• the present inventors made investigations in order to provide a steel plate that has a percent brittle fracture at -196°C of 10% or less in the C-direction in a Charpy impact test, a tensile strength TS of greater than 830 MPa, and a yield strength YS of greater than 690 MPa.
• steel plate can have higher base metal strengths as above and still have satisfactory cryogenic toughness when the element contents, Di value, and retained austenite volume fraction in the steel plate are controlled within more specific ranges, and the Mn content in the retained austenite is controlled instead of the retained austenite stabilization parameter.
• the element contents, Di value, and retained austenite volume fraction in the steel plate are controlled within the more specific ranges as follows.
• the steel plate when meeting the condition (f) below, can have a percent brittle fracture at -196°C of 10% or less, a tensile strength TS of greater than 830 MPa, and a yield strength YS of greater than 690 MPa even without controlling the retained austenite stabilization parameter.
• the condition (f) is expressed as follows:
• the steel plate according to the first embodiment of the present invention has a percent brittle fracture at -196°C of equal to or less than 10% in the C-direction in a Charpy impact (absorption) test, a tensile strength TS of greater than 741 MPa, and a yield strength YS of greater than 590 MPa.
• the steel plate (according to the second embodiment) meeting the conditions (a) to (e) (preferably further meeting the condition (f)) has a configuration different from the steel plate according to the first embodiment.
• the conditions (a) to (f) will be described below.
• the Mn content in the retained austenite existing at -196°C is preferably 1.40% or more, and more preferably 1.75% or more.
• the upper limit of the preferred Mn content in the retained austenite is not critical in relation with the activities, but is preferably about 2.50% or less in consideration typically of the Mn content range in the steel.
• At least one of the factors (i) the retained austenite volume fraction, (ii) the Mn content in the retained austenite, and (iii) the ⁇ L parameter is controlled within a more appropriate range.
• the steel plate according to the second embodiment can have a percent brittle fracture at a good level of 50% or less even at -233°C, lower than -196°C. This was demonstrated in after-mentioned Experimental Example 4. Specifically, (i) the retained austenite volume fraction is contrrolled to be about 3.5% to about 4.8%, (ii) the Mn content in the retained austenite is controlled to be about 1.40% to about 2.5%, and/or (iii) the ⁇ L parameter is controlled to be about -10 or less.
• At least one of these controls allows the steel to have better toughness at -233°C.
• at least two of the conditions (i) to (iii) are controlled within the ranges, and/or (i) the Mn content in the retained austenite is further controlled to be 1.75% to 2.50%.
• the steel plate according to this embodiment can have still better toughness at -233°C.
• the method for producing the steel plate according to the first embodiment of the present invention includes the steps a) and b).
• a steel plate is formed from a steel and is subjected to a heat treatment (L treatment) in a ferrite-austenite two-phase region (between A c1 and A c3 ).
• the steel has such chemical compositions, and the L treatment is performed at such a temperature (L treatment temperature) that an L parameter as specified by Formula (5) is 0.25 to 0.45 and a ⁇ L parameter as specified by Formula (6) is 7 or less.
• Formula (5) is defined by the L treatment temperature and the A c1 and A c3 temperatures in the steel.
• Formula (6) is defined by the L parameter and the steel chemical compositions.
• the step b) is performed after the L treatment.
• the steel plate is water-cooled down to room temperature and subjected to a tempering treatment (T treatment) at a temperature equal to or lower than the A c1 temperature for 10 to 60 minutes.
• T treatment tempering treatment
• the production method according to the third embodiment of the present invention appropriately controls a rolling process and a subsequent tempering treatment (T treatment) so as to produce the steel plate meeting the conditions.
• T treatment tempering treatment
• a steel making process herein is not limited and can be performed by a generally employed procedure.
• Steps (processes) including the rolling process and subsequent processes that feature the present invention will be sequentially illustrated in detail below.
• the heating temperature is controlled to be about 900°C to about 1100°C
• the finish rolling temperature (FRT) is controlled to be about 700°C to about 900°C
• the start cooling temperature (SCT) is controlled to be about 650°C to about 800°C.
• the start cooling temperature (SCT) is preferably controlled within the range within 60 seconds after the finish rolling. This gives a finely dispersed microstructure after rolling and subsequent cooling, where the finely dispersed microstructure usefully contributes to better toughness.
• the workpiece is cooled in a temperature range from 800°C to 500°C at an average cooling rate of about 10°C/s or more.
• the average cooling rate in the temperature range is controlled herein so as to give a finely dispersed microstructure after cooling.
• the average cooling rate is not critical in its upper limit.
• the cooling herein is preferably performed at an average cooling rate of about 10°C/s or more at least in the temperature range.
• the cooling at the average cooling rate is preferably stopped at a temperature of 200°C or lower. This can reduce the amount of untransformed austenite and can give a finely dispersed and homogeneous microstructure.
• the workpiece After the hot rolling, the workpiece is heated to and held at a temperature in a ferrite ( ⁇ )-austenite ( ⁇ ) two-phase region between the A c1 and A c3 temperatures and then water-cooled.
• This process is referred to as an "L treatment”
• the heating and holding temperature in this process is referred to as an "L treatment temperature”.
• the L treatment temperature and chemical compositions in the steel are appropriately controlled so that the L parameter specified by Formula (5) and the ⁇ L parameter specified by Formula (6) fall within the predetermined ranges.
• This control is performed so as to control the retained austenite volume fraction and the retained austenite stabilization parameter (preferably retained austenite volume fraction-retained austenite stabilization parameter) within the ranges specified herein.
• the L treatment temperature after the hot rolling is preferably controlled to be within the range of A c1 to (A c1 +A c3 )/2.
• the L treatment if performed at a temperature of lower than the A c1 temperature or higher than [(A c1 +A c3 )/2], may eventually fail to allow the steel plate to have a sufficient volume fraction of retained austenite existing at -196°C and/or sufficient retained austenite stability (see Sample Nos. 29 and 30 in Table 2B of after-mentioned Experimental Example 1).
• the L treatment temperature is preferably from about 620°C to about 650°C.
• the A c1 and A c3 temperatures herein are calculated based on formulae below (see “ Koza Gendai no Kinzoku-gaku, Zairyo-hen 4, Tekko Zairyo” (in Japanese), The Japan Institute of Metals and Materials ).
• a c 1 temperature 723 ⁇ 10.7 ⁇ Mn ⁇ 16.9 ⁇ Ni + 29.1 ⁇ Si + 16.9 ⁇ Cr + 290 ⁇ As + 6.38 ⁇ W
• a c 3 temperature 910 ⁇ 203 ⁇ C 1 / 2 ⁇ 15.2 ⁇ Ni + 44.7 ⁇ Si + 104 ⁇ V + 31.5 ⁇ Mo ⁇ 30 ⁇ Mn + 11 ⁇ Cr + 20 ⁇ Cu
• [Mn], [Ni], [Si], [Cr], [As], [W], [C], [V], [Mo], and [Cu] are contents (in mass percent) respectively of alloy elements Mn, Ni, Si, Cr, As, W, C, V, Mo, and Cu in the steel.
• the steel plate according to the present invention does not contain As and W as steel chemical compositions, and the calculation according to the formula is performed while defining [As] and [W] each as 0%.
• the heating at a temperature in the two-phase region is preferably performed for a time (holding time) of about 10 to about 50 minutes.
• the heating if performed for a time shorter than 10 minutes, may fail to allow alloy elements to sufficiently concentrate in the austenite phase. In oontrast, the heating, if performed for a time longer than 50 minutes, may cause the ⁇ phase to be annealed and cause the steel plate to have lower strengths.
• the heating time is preferably 30 minutes in its upper limit.
• the L parameter specified by Formula (5) is herein controlled to be 0.25 to 0.45, where Formula (5) is defined by individual chemical compositions.
• the L parameter is defined to efficiently use the alloy element concentration during the L treatment so as to allow the steel plate to finally have a retained austenite volume fraction and retained austenite stability both at certain levels.
• a steel plate having an L parameter out of the range fails to have a desired retained austenite volume fraction and/or sufficient retained austenite stability, as demonstrated in after-mentioned Experimental Examples.
• the L parameter is preferably 0.28 to 0.42, and more preferably 0.30 to 0.40.
• the ⁇ L parameter is herein controlled to be 7 or less.
• the ⁇ L parameter is defined by the contents of Mn, Cr, and Mo, and the L parameter, as specified by Formula (6).
• the ⁇ L parameter is defined so as to restrain adverse effects of temper embrittlement occurring in a portion where Mn and Cr are excessively concentrated, where the Mn and Cr concentration is caused typically by phosphorus segregation at prior austenite grain boundaries during the L treatment. It is difficult to directly measure the content of phosphorus segregated at prior austenite grain boundaries. Accordingly, the ⁇ L parameter can act as, so to speak, an alternate parameter for the content of phosphorus segregated at prior austenite grain boundaries.
• a steel including a smaller amount of phosphorus segregated at prior austenite grain boundaries has a lower ⁇ L parameter.
• the ⁇ L parameter is preferably 0.0 or less, and more preferably -10.0 or less.
• the ⁇ L parameter is not critical in its lower limit, but is preferably about -30 or more. This is preferred in synthetic consideration that the amount of Mo to be added is preferably minimized from the viewpoint of cost; and that the contents and the L parameter preferably fall within the specific ranges.
• Sample Nos.1, 2, and 25 (all according to the present invention) in Table 1A in after-mentioned Experimental Example 1 are compared. These samples have retained austenite volume fractions and retained austenite stabilization parameters at the same levels. Specifically, Sample No. 1 has a retained austenite volume fraction of 8.0% and a retained austenite stabilization parameter of 3.7. Sample No.
• Sample No. 2 has a retained austenite volume fraction of 9.4% and a retained austenite stabilization parameter of 3.8.
• Sample No. 25 has a retained austenite volume fraction of 7.9% and a retained austenite stabilization parameter of 3.7.
• these samples have significantly different ⁇ L parameters of -6.8 (Sample No. 1), -10.9 (Sample No. 2), and 5.2 (Sample No. 25). Accordingly, among the three samples, Sample No. 2 having the lowest ⁇ L parameter is most excellent in cryogenic toughness.
• the workpiece is water-cooled down to room temperature and subjected to a tempering treatment (T treatment).
• T treatment a tempering treatment
• the tempering treatment is performed at a temperature of equal to or lower than the A c1 temperature for 10 to 60 minutes.
• Such low-temperature tempering allows carbon to concentrate in the metastable retained austenite and thereby further stabilizes the metastable retained austenite phase to give a retained austenite phase that stably exists at -196°C.
• the low-temperature tempering helps the steel plate to have a low Ms temperature.
• the tempering if performed at a temperature higher than the A c1 temperature, may cause the metastable retained austenite phase to decompose into a ferrite ( ⁇ ) phase and a cementite phase and may cause the steel plate to fail to include the retained austenite phase existing at -196°C in a sufficient volume fraction, where the metastable retained austenite phase is formed during holding in the two-phase region.
• the tempering if performed at a temperature lower than 540°C or for a time shorter than 10 minutes, may cause carbon to fail to sufficiently concentrate into the metastable retained austenite phase and may cause the steel plate to fail to have a desired volume fraction of retained austenite existing at -196°C.
• the tempering if performed for a time longer than 60 minutes, may cause excessive reduction of ⁇ phase dislocation density and may thereby cause the steel plate to fail to surely have predetermined strengths (TS and YS) (see Sample No. 33 of Table 2B in Experimental Example 1).
• the tempering treatment is preferably performed at a temperature of 540°C to 560°C for a time of 15 minutes to 45 minutes (more preferably 35 minutes or shorter, and furthermore preferably 25 minutes or shorter).
• the workpiece After undergoing the tempering treatment as above, the workpiece is cooled down to room temperature.
• the cooling after the tempering is performed not by water cooling, but by air cooling. This is because carbon is concentrated into the retained austenite during air cooling, and the steel plate cooled by air cooling has a higher retained austenite stabilization parameter as compared with a steel plate cooled by water cooling.
• This production method includes the steps a') and b).
• the L treatment temperature and the steel chemical compositions are adjusted so that an L parameter as specified by Formula (5) is 0.6 to1.1 and a ⁇ L parameter as specified by Formula (6) is 0 or less.
• Formula (6) is defined by the L parameter and the steel chemical compositions.
• the step b) is performed after the L treatment, in which the steel plate is water-cooled down to room temperature and subjecting to a tempering treatment (T treatment) at a temperature equal to or lower than the A c1 temperature for 10 to 60 minutes.
• T treatment tempering treatment
• the method for producing the steel plate according to the second embodiment of the present invention controls the L parameter specified by Formula (5) to be 0.6 to 1.1.
• the L parameter is defined so as to allow the final steel plate to have a sufficient retained austenite volume fraction and satisfactory retained austenite stability (in particular one determined by the Di value and the Mn content in the retained austenite).
• the L parameter is specified in its upper limit to be 1.1 or less firom the viewpoints of the chemical compositions and desired microstructure conditions of the steel plate according to the embodiment of the present invention.
• the L treatment increases the retained austenite stability (namely, allows Mn to be concentrated into the retained austenite). Conversely, the L treatment causes the Mn content in the matrix (in the steel) to be reduced.
• the L parameter is controlled in its lower limit (0.6 or more) in the embodiment of the present invention.
• the L parameter is preferably 0.7 to 1.0.
• the ⁇ L parameter is herein controlled to be 0 or less.
• the ⁇ L parameter is defined by the L parameter and the contents of Mn, Cr, and Mo in the steel, as specified by Formula (6).
• the ⁇ L parameter is defined so as to restrain adverse effects of temper embrittlement occurring in a portion where Mn and Cr are excessively concentrated, where the Mn and Cr concentration is caused typically by phosphorus segregation at prior austenite grain boundaries during the L treatment, as is described above. It is difficult to directly measure the content of phosphorus segregated at prior austenite grain boundaries. Accordingly, the ⁇ L parameter can act as, so to speak, an alternate parameter for the content of phosphorus segregated at prior austenite grain boundaries.
• a steel including a smaller amount of phosphorus segregated at prior austenite grain boundaries has a lower ⁇ L parameter.
• the ⁇ L parameter is preferably -10.0 or less.
• the ⁇ L parameter is not critical in its lower limit, but is preferably about -30 or more. This is preferred in synthetic consideration that the amount of Mo to be added is preferably minimized firom the viewpoint of cost; and that the contents and the L parameter preferably fall within the specific ranges.
• the workpiece is water-cooled down to room temperature and subjected to a tempering treatment (T treatment).
• T treatment a tempering treatment
• the tempering treatment is performed at a temperature of equal to or lower than the A c1 temperature for 10 to 60 minutes.
• Such low-temperature tempering allows carbon to be concentrated in the metastable retained austenite and thereby further stabilizes the metastable retained austenite phase to give a retained austenite phase that stably exists at -196°C, as is described above.
• the low-temperature tempering helps the steel plate to have a low Ms temperature.
• the tempering if performed at a temperature higher than the A c1 temperature, may cause the metastable retained austenite phase to decompose into a ferrite ( ⁇ ) phase and a cementite phase and may cause the steel plate to fail to include the retained austenite phase at -196°C in a sufficient volume fraction, where the metastable retained austenite phase is formed during holding in the two-phase region.
• the tempering if performed for a time shorter than 10 minutes, may cause carbon to fail to be sufficiently concentrated into the metastable retained austenite phase and may cause the steel plate to fail to have a desired volume fraction of retained austenite existing at - -196°C.
• the tempering if performed for a time longer than 60 minutes, may cause excessive reduction of ⁇ phase dislocation density and may thereby cause the steel plate to fail to surely have predetermined strength (TS) (see Sample No. 7 of Table 2B in after-mentioned Experimental Example 3).
• the tempering is preferably performed for a time of 15 minutes to 45 minutes, and more preferably 20 minutes to 35 minutes.
• the tempering is performed at a temperature equal to or lower than the A c1 temperature, and preferably at a temperature of 510°C to 520°C.
• the workpiece After undergoing the tempering treatment as above, the workpiece is cooled down to room temperature.
• the cooling after the tempering is performed not by water cooling, but by air cooling. This is because carbon is concentrated into the retained austenite during air cooling, and the steel plate cooled by air cooling has higher stability of retained austenite as compared with a steel plate cooled by water cooling.
• This experimental example relates to steel plates having a percent brittle fracture at -196°C of equal to or less than 10%, a tensile strength TS of greater than 741 MPa, and a yield strength YS of greater than 590 MPa.
• Molten steels as test samples having chemical compositions given in Table 1 were made using a vacuum induction furnace (150-kg VIF). The molten steels were cast, subjected to hot forging, and yielded ingots of a size of 150 mm by 150 mm by 600 mm.
• REM used in this experimental example was a misch metal containing about 50% of Ce and about 25% of La.
• the ingots were heated to 1100°C and rolled at a temperature of 830°C or higher to a thickness of 75 mm.
• the workpieces were rolled at a finish rolling temperature (FRT) of 700°C and water-cooled from a start cooling temperature (SCT) of 650°C within 60 seconds after the finish rolling.
• FRT finish rolling temperature
• SCT start cooling temperature
• the workpieces were rolled down to a thickness of 25 mm with a rolling reduction of 83%.
• the cooling in the range from 800°C down to 500°C was performed at an average cooling rate of 19°C/s, and the cold rolling was performed to a stop temperature of 200°C or lower to give steel plates.
• the above-prepared steel plates were each subjected to an L treatment by heating to and holding at an L treatment temperature given in Table 2 for 30 minutes, followed by water cooling.
• the steel plates were further subjected to a T treatment (tempering) at a temperature (T treatment temperature) for a time (T time) given in Table 2, and air-cooled down to room temperature.
• the prepared steel plates were evaluated on the amount (volume fraction) of retained austenite phase existing at -196°C, retained austenite stabilization parameter, tensile properties (tensile strength TS and yield strength YS), and cryogenic toughness (percent) brittle fracture in the C-direction at -196°C or -233°C) in the following manner.
• a test specimen of a size of 10 mm by 10 mm by 55 mm was sampled from each steel plate at a position one-fourth the thickness, held at the liquid nitrogen temperature (-196°C) for 5 minutes, and subjected to X-ray diffractometry using a two-dimensional micro-diffraction X-ray diffractometer (RINT-RAPID II) supplied by Rigaku Corporation.
• the contents of chemical compositions in the retained austenite existing at -196°C to define Formula (2) were measured. Specifically, ⁇ C>, ⁇ Mn>, ⁇ Al>, ⁇ Cu>, ⁇ Ni>, ⁇ Cr>, ⁇ Mo>, and ⁇ V> as contents (in mass percent) respectively of C, Mn, Al, Cu, Ni, Cr, Mo, and V were measured in manners as follows.
• calibrator silicon (Si) was applied onto each test sample steel, and a precise ⁇ -Fe lattice constant [ao (in angstrom)] was determined with angle correction with the Si peak.
• the carbon (C) content in the retained austenite was determined by inverse calculation from the precisely determined ⁇ -Fe lattice constant and the contents of following chemical compositions excluding carbon.
• Test specimens of a size of 10 mm by 10 mm by 55 mm were sampled from each steel plate at a position one-fourth the thickness, held at the liquid nitrogen temperature (-196°C) for 5 minutes, and subjected to Ni content measurement using an electron probe microanalyzer (EPMA) JXA-8500F supplied by JEOL Ltd. at an aoceleration voltage of 15 kV and an applied current of 50 nA with a minimum beam diameter. The measurement was performed three times per each sample (steel plate), and the maximum among the measured values was defined as the Ni content in the retained austenite.
• EPMA electron probe microanalyzer
• the Al content in the retained austenite was determined as zero (0) on the assumption that all Al formed oxides and/or nitrides and existed therein.
• the alloy element contents ⁇ Mn>, ⁇ Cu>, ⁇ Cr>, ⁇ Mo>, and ⁇ V> after the L treatment and the subsequent T treatment were considered to be proportional to the measured Ni content ⁇ Ni> as measured by the measurement (2-2) and were calculated in a manner as follows.
• the Ni content in the heat treatments i.e., the L treatment and the T treatment may behave (vary) as specified by the formula: Constant in each heat treatment ⁇ Driving force of austenite reverse transformation ⁇ Diffusion constant of each alloy element
• the term “diffusion constant of each alloy element” in the formula was calculated based on the temperature and holding time in each heat treatment using a value found in " Diffusion in Solid Metals and Alloys", H. Mehrer, 1990 .
• the term "constant in each heat treatment” was experimentally determined in the following manner.
• the measured Ni content after the L treatment and the subsequent T treatment is specified by the formula as the product of [(Constant in L treatment) ⁇ (Driving force of austenite reverse transformation) ⁇ (Ni diffusion constant in L treatment)] and [(Constant in T treatment) ⁇ (Driving force of austenite reverse transformation) ⁇ (Ni diffusion constant in L treatment)].
• the measured Ni content after the L treatment and the subsequent T treatment includes both the terms “constant in L treatment” and “constant in T treatment”, and the "constant in T treatment” varies with the "oonstant in L treatment”.
• the constants in the individual heat treatments ["constant in L treatment” and “constant in T treatment”] were recursively determined so that the product be most approximal to the measured Ni content after the L treatment and the subsequent T treatment.
• the alloy element contents ⁇ Mn>, ⁇ Cu>, ⁇ Cr>, ⁇ Mo>, and ⁇ V> were calculated.
• a JIS Z2241 No. 4 test specimen was sampled from each steel plate at a position one-fourth the thickness in a direction parallel to the C-direction and subjected to a tensile test by the method prescribed in JIS Z 2241 to measure the tensile strength TS and yield strength YS.
• a sample having a tensile strength TS of greater than 740 MPa and a yield strength YS of greater than 590 MPa was evaluated as having satisfactory base metal strengths.
• V-notched test specimens according to JIS Z 2242 Three Charpy impact test specimens (V-notched test specimens according to JIS Z 2242) were sampled from each steel plate in a direction parallel to the C-direction each at a position one-fourth the plate thickness and one-fourth the plate width and at a position one-fourth the plate thickness and half the plate width.
• the percents brittle fracture at -196°C (%) of the test specimens at the two positions were measured by the method prescribed in JIS Z2242 and were averaged independently. Of the two averages thus calculated, one indicating inferior properties (namely, one with a higher percent brittle fracture) was employed A sample having an employed average of 10% or less was evaluated as having excellent cryogenic toughness in this experimental example.
• Sample Nos. 1 to 25 in Table 2A are samples meeting all the conditions specified in the present invention. These samples could provide steel plates having excellent cryogenic toughness at - 196°C even though having high base metal strengths. Specifically, the samples each had an average of percent brittle fracture in the C-direction of equal to or less than 10%.
• Sample Nos. 26 to 45 in Table 2B are comparative examples not meeting one or more of the conditions specified in the present invention, because the samples did not meet either one of the steel chemical compositions and the preferred production conditions specified in the present invention. The samples failed to have desired property or properties.
• Sample No. 26 had a Di value not meeting the condition specified in the present invention.
• the sample failed to have a desired retained austenite volume fraction and had a low retained austenite stabilization parameter.
• the sample had a retained austenite volume fraction-retained austenite stabilization parameter of greater than the predetermined range.
• the sample had a high percent brittle fracture and failed to achieve desired cryogenic toughness at -196°C.
• the sample had a low Di value and therefore had a low tensile strength TS.
• Sample No. 27 employed Steel No. 27 in Table 1B having an excessively high carbon content and had inferior cryogenic toughness.
• Sample No. 28 employed Steel No. 28 in Table 1B having an excessively high phosphorus content.
• the sample failed to have a desired retained austenite volume fraction and had a low retained austenite stabilization parameter.
• the sample had a retained austenite volume fraction-retained austenite stabilization parameter of greater than the predetermined range and, as a result, had inferior cryogenic toughness.
• Sample No. 29 employed Steel No. 29 in Table 1B having chemical compositions meeting the conditions specified in the present invention, but underwent heating at a temperature lower than the two-phase region temperature (L treatment temperature), and had a low L parameter.
• the sample therefore contained retained austenite in an insufficient volume fraction and had a low retained austenite stabilization parameter.
• the sample had a retained austenite volume fraction-retained austenite stabilization parameter of greater than the predetermined range. As a result, the sample had inferior cryogenic toughness.
• Sample No. 30 employed Steel No. 30 in Table 1B having an excessively high Si content, underwent heating at a temperature higher than the two-phase region temperature (L treatment temperature), and had excessively high L parameter and ⁇ L parameter.
• the sample therefore contained retained austenite in an insufficient volume fraction, a low retained austenite stabilization parameter, and a retained austenite volume fraction-retained austenite stabilization parameter of greater than the predetermined range. As a result, the sample had inferior cryogenic toughness.
• Sample No. 31 employed Steel No. 31 in Table 1B having chemical compositions meeting the conditions specified in the present invention, but underwent tempering (T treatment) at an excessively low temperature.
• the sample therefore contained retained austenite in an insufficient volume fraction and had a low retained austenite stabilization parameter.
• the sample had a retained austenite volume fraction-retained austenite stabilization parameter of greater than the predetermined range. As a result, the sample had inferior cryogenic toughness.
• Sample No. 32 employed Steel No. 32 in Table 1B having an excessively high Mn content and had an excessively high ⁇ L parameter. The sample therefore had inferior cryogenic toughness.
• Sample No. 33 employed Steel No. 33 in Table 1B having chemical compositions meeting the conditions specified in the present invention, but underwent tempering for an excessively long time (T time). As a result, the sample had low strengths (TS and YS).
• Sample No. 34 employed Steel No. 34 in Table 1B having an excessively low Mn content and had an excessively low Di value.
• the sample failed to have a desired retained austenite volume fraction and had a low retained austenite stabilization parameter.
• the sample had a retained austenite volume fraction-retained austenite stabilization parameter of greater than the predetermined range.
• the sample had a high percent brittle fracture and failed to achieve desired cryogenic toughness at -196°C.
• the sample also had a low tensile strength TS due to the low Di value.
• Sample No. 35 employed Steel No. 35 in Table 1B having an excessively high sulfur content. As a result, the sample had a high percent brittle fracture and failed to achieve desired cryogenic toughness.
• Sample No. 36 employed Steel No. 36 in Table 1B having chemical compositions meeting the conditions specified in the present invention, but had an excessively high L parameter.
• the sample therefore contained retained austenite in an insufficient volume fraction and had a retained austenite volume fraction-retained austenite stabilization parameter of greater than the predetermined range. As a result, the sample had inferior cryogenic toughness.
• Sample No. 37 employed Steel No. 37 in Table 1B having a low carbon content, a high Al content, and a low Ni content.
• the sample therefore contained retained austenite in an insufficient volume fraction and had a low retained austenite stabilization parameter.
• the sample had a retained austenite volume fraction-retained austenite stabilization parameter of greater than the predetermined range.
• the sample had inferior cryogenic toughness and also had a low tensile strength TS.
• Sample No. 38 employed Steel No. 38 in Table 1B having a low Al content and a high nitrogen content and therefore had inferior cryogenic toughness.
• Sample No. 39 employed Steel No. 39 in Table 1B having excessively high contents of selective compositions Cu and Ca and therefore had inferior cryogenic toughness.
• Sample No. 40 employed Steel No. 40 in Table 1B having excessively high contents of selective compositions Cr and Zr and therefore had inferior cryogenic toughness.
• Sample No. 41 employed Steel No. 41 in Table 1B having excessively high contents of selective compositions Nb and REM and therefore had inferior cryogenic toughness.
• Sample No. 42 employed Steel No. 42 in Table 1B having an excessively high content of selective composition Mo, had a high Di value, and therefore had inferior cryogenic toughness.
• Sample No. 43 employed Steel No. 43 in Table 1B having an excessively high content of selective composition Ti and therefore had inferior cryogenic toughness.
• Sample No. 44 employed Steel No. 44 in Table 1B having an excessively high content of selective composition V and therefore had inferior cryogenic toughness.
• Sample No. 45 employed Steel No. 45 in Table 1B having an excessively high content of selective composition boron (B) and therefore had inferior cryogenic toughness.
• each three test specimens were sampled from each of samples given in Table 3 at a position one-fourth the plate thickness and one-fourth the plate width, subjected to a Charpy impact test at -233°C by a method mentioned below, an average of measured percent brittle fracture values was calculated and evaluated.
• the sample numbers in Table 3 correspond to the sample numbers (steel numbers) in Tables 1A and 2A.
• Sample Nos. 10 and 16 had higher retained austenite volume fraction-retained austenite stabilization parameter of about 35 and therefore had higher percent brittle fracture at -233°C as compared with the above-mentioned samples.
• This experimental example relates to steel plates having a percent brittle fracture at -196°C of equal to or less than 10%, a tensile strength TS of greater than 830 MPa, and a yield strength YS of greater than 690 MPa.
• Molten steels as test samples having chemical compositions given in Table 4 were made using a vacuum induction furnace (150-kg VIF). The molten steels were cast, subjected to hot forging, and yielded ingots of a size of 150 mm by 150 mm by 600 mm.
• REM used in this experimental example was a misch metal containing about 50% of Ce and about 25% of La.
• the ingots were heated to 1100°C and rolled at a temperature of 830°C or higher to a thickness of 75 mm.
• the workpieces were rolled at a finish rolling temperature (FRT) of 700°C and water-cooled from a start cooling temperature (SCT) of 650°C within 60 seconds after the finish rolling.
• FRT finish rolling temperature
• SCT start cooling temperature
• the workpieces were rolled to a thickness of 25 mm with a rolling reduction of 85%.
• the cooling in the range from 800°C down to 500°C was performed at an average cooling rate of 19°C/s, and the cold rolling was performed to a stop temperature of 200°C or lower to give steel plates.
• the above-prepared steel plates were each subjected to an L treatment by heating to and holding at an L treatment temperature given in Table 5 for 30 minutes, followed by water cooling.
• the steel plates were further subjected to a T treatment (tempering) at a temperature (T treatment temperature) for a time (T time) given in Table 2, and air-cooled down to room temperature.
• the above-prepared steel plates were examined and evaluated on the amount (volume fraction) of retained austenite phase existing at -196°C, Mn content in the retained austenite phase, tensile properties (tensile strength TS and yield strength YS), and cryogenic toughness (percent brittle fracture in the C-direction at -196°C or -233°C).
• the amount (volume fraction) of retained austenite phase existing at 196°C, tensile properties (tensile strength TS and yield strength YS), and cryogenic toughness (percent brittle fracture in the C-direction) were measured by the procedure of Experimental Example 1. How the Mn content in the retained austenite phase existing at -196°C was measured will be described below.
• An average Mn content in the retained austenite phase was measured by transmission electron microscopy-energy dispersive X-ray spectroscopy (TEM-EDX) and calculated by a procedure as follows. The calculation was performed assuming that the retained austenite phase includes Fe, Mn, and Ni as chemical compositions.
• An actual retained austenite phase may include other elements such as C and Si, in addition to Fe, Mn, and Ni. However, these elements are present in small amounts and are approximately trivial in this experimental example.
• a test specimen of a size of 10 mm by 10 mm by 55 mm was sampled from each steel plate at a position one-fourth the thickness, held at the liquid nitrogen temperature (-196°C) for 5 minutes, cut to a size of 10 mm by 10 mm by 2 mm, mechanically polished to reduce the thickness "t" from 2 mm to 0.1 mm, blanked into a disc having a size of 3 mm in diameter, electrically polished, and yielded a thin-film specimen.
• the above-prepared thin-film specimen was analyzed using a transmission electron microscope H-800 supplied by Hitachi, Ltd, based on which an austenite phase was identified using a transition image and a reciprocal lattice, and the Mn content in the austenite phase was measured using an EDX analyzer EMAX7000 supplied by HORIBA, Ltd
• the measurement using the EDX was performed at an acceleration voltage of 200 kV and a 75000-fold observation magnification on five points per sample. The measured values at the five points were averaged, and the average was defined as the Mn content in the retained austenite.
• Sample Nos. 1 to 21 in Table 5A respectively employed Steel Nos. 1 to 21 in Table 4A having chemical compositions meeting the conditions specified in the present invention and were prepared under the production conditions specified in the present invention. These samples could provide steel plates having excellent cryogenic toughness at -196°C even though having high base metal strengths. Specifically, the samples each had an average percent brittle fracture in the C-direction of equal to or less than 10%.
• Sample Nos. 1 to 21 in Table 5B are comparative examples not meeting one or more of the conditions specified in the present invention, including the steel chemical compositions and production conditions, and failed to have desired properties.
• Sample No. 1 in Table 5B employed Steel No. 1 in Table 4B having chemical compositions meeting the conditions specified in the present invention, but had a Di value not meeting the condition specified in the present invention.
• the sample failed to have a desired retained austenite volume fraction. As a result, the sample had a high percent brittle fracture and failed to achieve desired cryogenic toughness at -196°C.
• Sample No. 2 in Table 5B employed Steel No. 2 in Table 4B having a high carbon content and a low Mo content and had inferior cryogenic toughness.
• Sample No. 3 in Table 5B employed Steel No. 3 in Table 4B having an excessively high phosphorus content and had inferior cryogenic toughness.
• Sample No. 4 in Table 5B employed Steel No. 4 in Table 4B having chemical compositions meeting the conditions specified in the present invention, but underwent heating at a temperature lower than the two-phase region temperature (L treatment temperature), and had a low L parameter.
• the sample therefore contained retained austenite in an insufficient volume fraction and had inferior cryogenic toughness.
• Sample No. 5 in Table 5B employed Steel No. 5 in Table 4B having excessively high Si and Mo contents, underwent heating at a temperature higher than the two-phase region temperature (L treatment temperature), and had excessively high L parameter and ⁇ L parameter.
• the sample therefore contained retained austenite in an insufficient volume fraction and had inferior cryogenic toughness.
• Sample No. 6 in Table 5B employed Steel No. 6 in Table 4B having a high Mn content and a low Mo content, underwent tempering at an excessively high temperature (T treatment temperature), had an excessively high ⁇ L parameter, and failed to have a desired retained austenite volume fraction. As a result, the sample had inferior cryogenic toughness.
• Sample No. 7 in Table 5B employed Steel No. 7 in Table 4B having chemical compositions meeting the conditions specified in the present invention, but underwent tempering for an excessively long time (T time).
• the sample had a Ni-Mn balance as specified by Formula (2) lower than the preferred range.
• the sample had inferior low-temperature toughness (cryogenic toughness) and also had a low strength (TS).
• Sample No. 8 in Table 5B employed Steel No. 8 in Table 4B having an excessively low Mn content.
• the sample had a Ni-Mn balance as specified by Formula (2) lower than the preferred range, had a low Mn content in the retained austenite, and contained retained austenite in an insufficient volume fraction. As a result, the sample had inferior cryogenic toughness.
• Sample No. 9 in Table 5B employed Steel No. 9 in Table 4B having an excessively high sulfur content. As a result, the sample had a high percent brittle fracture and failed to achieve desired cryogenic toughness.
• Sample No. 10 in Table 5B employed Steel No. 10 in Table 4B having an excessively low carbon content, an excessively high Al content, and an excessively low Ni content and having a Ni-Mn balance specified by Formula (2) lower than the preferred range.
• the sample contained retained austenite in a low volume fraction because of excessively low contents of C and Ni that are useful for ensuring retained austenite in a sufficient volume fraction.
• the sample had inferior cryogenic toughness, although having a yield strength YS at good level.
• the steel had a low tensile strength TS because of excessively low contents of C and Ni that are effective for higher strength.
• Sample No. 11 in Table 5B employed Steel No. 11 in Table 4B having excessively low Al and Mo contents and an excessively high nitrogen content and had an excessively high ⁇ L parameter. The sample therefore had inferior cryogenic toughness.
• Sample No. 12 in Table 5B employed Steel No. 12 in Table 4B having excessively high contents of selective compositions Cu and Ca The sample therefore had inferior cryogenic toughness.
• Sample No. 13 in Table 5B employed Steel No. 13 in Table 4B having an excessively low content of Mo and excessively high contents of Cr and Zr each added as selective compositions and had an excessively high ⁇ L parameter. The sample therefore had inferior cryogenic toughness.
• Sample No. 14 in Table 5B employed Steel No. 14 in Table 4B having excessively high contents of selective compositions Nb and REM. The sample therefore had inferior cryogenic toughness.
• Sample No. 15 in Table 5B employed Steel No. 15 in Table 4B having an excessively high content of selective composition Mo. The sample therefore had inferior cryogenic toughness.
• Sample No. 16 in Table 5B employed Steel No. 16 in Table 4B having an excessively high content of elective composition Ti. The sample therefore had inferior cryogenic toughness.
• Sample No. 17 in Table 5B employed Steel No. 17 in Table 4B having an excessively high content of selective composition vanadium (V). The sample therefore had inferior cryogenic toughness.
• Sample No. 18 in Table 5B employed Steel No. 18 in Table 4B having an excessively high content of selective composition boron (B). The sample therefore had inferior cryogenic toughness.
• Sample No. 19 in Table 5B employed Steel No. 19 in Table 4B having chemical compositions meeting the conditions specified in the present invention, but had an excessively high L parameter, and underwent an L treatment at an excessively high temperature.
• the sample therefore had an excessively low Mn content in the retained austenite, contained the retained austenite in an insufficient volume fraction, and had inferior cryogenic toughness.
• Sample No. 20 in Table 5B employed Sample No. 20 in Table 5B having chemical compositions meeting the conditions specified in the present invention, but underwent tempering at an excessively high temperature (T treatment temperature), and had a Ni-Mn balance specified by Formula (2) lower than the preferred range.
• T treatment temperature excessively high temperature
• the sample failed to have a desired retained austenite volume fraction and had a low Mn content in the retained austenite.
• the sample had a high percent brittle fracture, failed to achieve desired cryogenic toughness at -196°C, and was inferior in yield strength YS and tensile strength TS.
• Sample No. 21 in Table 5B employed Steel No. 21 in Table 4B having an excessively low Mo content and had an excessively high L parameter and an excessively high ⁇ L parameter. As a result, the sample had a high percent brittle fracture and failed to achieve desired cryogenic toughness at -196°C.
• each three test specimens were sampled from each of samples given in Table 6 at a position one-fourth the plate thickness and one-fourth the plate width, subjected to a Charpy impact test at -233°C by a method described below, and an average of measured percent brittle fracture values was calculated and evaluated.
• the sample numbers in Table 6 correspond to the sample numbers (steel numbers) in Tables 4A and 5A.
• a sample having a percent brittle fracture equal to or less than 50% was evaluated as being excellent in percent brittle fracture at -233°C.
• " Cryogenic-Temperature Impact Test of Austenitic Stainless Cast Steel" Journal of the High Pressure Gas Safety Institute of Japan, vol. 24, p. 181 .
• Table 6 indicates data of the (i) retained austenite volume fraction, (ii) Mn content in retained austenite, and (iii) ⁇ L parameter as abstracted from Table 5A Details of them are as follows. [Table 6] No.
• Sample No. 1 to 3, 5 to 14, and 17 to 20 in Table 6 respectively employed Steel Nos. 1 to 3, 5 to 14, and 17 to 20 in Table 5A meeting at least one of the preferred conditions (i) to (iii).
• the samples each had a good percent brittle fracture at -233°C of 50% or less.
• Sample Nos. 4,15,16, and 21 in Table 6 respectively employed Steel Nos. 4,15,16, and 21 in Table 5A meeting none of the preferred conditions (i) to (iii).
• the samples failed to have desired toughness at -233°C.
• Sample Nos. 1 to 3 in Table 6 respectively employed Steel Nos.1 to 3 in Table 5A and had a good percent brittle fracture at -233°C of 50%.
• Sample No. 4 in Table 6 employed Steel No. 4 in Table 5A meeting none of the preferred conditions (i) to (iii) and failed to have desired toughness at -233°C.
• Sample No. 5 in Table 6 employed Steel No. 5 in Table 5A meeting all the preferred conditions (i) to (iii) and having a Mn content in the retained austenite controlled within the more preferred range of 1.75% to 2.50% in the condition (ii).
• the sample could have still better toughness (lower percent of brittle fracture) at -233°C of 15%.
• Sample No. 6 in Table 6 employed Steel No. 6 in Table 5A meeting the preferred conditions (i) and (iii).
• the sample could have still better toughness (lower percent of brittle fracture) at -233°C of 40%.
• Sample No. 7 in Table 6 employed Steel No. 7 in Table 5A meeting the preferred condition (iii) and had a good percent brittle fracture at -233°C of 50%.
• Sample No. 8 in Table 6 employed Steel No. 8 in Table 5A meeting the preferred conditions (i) and (ii) and having a Mn content in the retained austenite controlled within the more preferred range of 1.75% to 2.50% in the condition (ii).
• the sample could have still better toughness (lower percent of brittle fracture) at -233°C of 25%.
• Sample No. 9 in Table 6 employed Steel No. 9 in Table 5A meeting the preferred conditions (i) and (iii) and could have still better toughness (lower percent of brittle fracture) at -233°C of 40%.
• Sample No. 10 in Table 6 employed Steel No. 10 in Table 5A meeting the preferred conditions (ii) and (iii) and could have still better toughness (lower percent of brittle fracture) at -233°C of 40%.
• Sample No. 11 in Table 6 employed Steel No. 11 in Table 5A meeting the preferred condition (ii) and had a good percent brittle fracture at -233°C of 50%.
• Sample No. 12 in Table 6 employed Steel No. 12 in Table 5A meeting the preferred conditions (ii) and (iii) and could have still better toughness (lower percent of brittle fracture) at -233°C of 40%.
• Sample No. 13 in Table 6 employed Steel No. 13 in Table 5A meeting all the preferred conditions (i) to (iii) and having a Mn content in the retained austenite controlled within the more preferred range of 1.75% to 2.50% in the condition (ii).
• the sample could have still better toughness (lower percent of brittle fracture) at -233°C of 15%.
• Sample No. 14 in Table 6 employed Steel No. 14 in Table 5A meeting the preferred condition (ii) and had a good percent brittle fracture at -233°C of 50%.
• Sample Nos. 15 and 16 in Table 6 respectively employed Steel Nos. 15 and 16 in Table 5A meeting none of the preferred conditions (i) to (iii) and failed to have desired toughness at -233°C.
• Sample No. 17 in Table 6 employed Steel No. 17 in Table 5A meeting the preferred condition (iii) and had a good percent brittle fracture at -233°C of 50%.
• Sample No. 18 in Table 6 employed Steel No. 18 in Table 5A meeting the preferred condition (i) and had a good percent brittle fracture at -233°C of 50%.
• Sample No. 19 in Table 6 employed Steel No. 19 in Table 5A meeting the preferred condition (ii) and had a good percent brittle fracture at -233°C of 50%.
• Sample No. 20 in Table 6 employed Steel No. 20 in Table 5A meeting the preferred condition (i) and had a good percent brittle fracture at -233°C of 50%.
• Sample No. 21 in Table 6 employed Steel No. 21 in Table 5A meeting none of the preferred conditions (i) to (iii) and failed to have desired toughness at -233°C.
• the steel plates according to the present invention are useful as steel plates that are in contact with substances at cryogenic temperatures, such as in liquefied natural gas storage tanks.

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