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  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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  • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
  • C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
  • C21C7/04—Removing impurities by adding a treating agent
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  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
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  • C22C—ALLOYS
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  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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  • C22C—ALLOYS
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  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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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
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/20—Ferrous alloys, e.g. steel alloys containing chromium 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
  • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium 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
  • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium 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
  • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium 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
  • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium 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
  • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of 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
  • C22C38/18—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
  • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties 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
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following 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
  • C21D9/08—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for tubular bodies or pipes
  • C21D9/14—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for tubular bodies or pipes wear-resistant or pressure-resistant pipes

Definitions

• the present invention relates to a thick steel plate and a welded joint, and more specifically relates to a thick steel plate suitable as a material steel plate of a structural component for energy field such as a line pipe and a marine structure, and a welded joint using the thick steel plate.
• Steel for a pipe line is one of the steel material for energy field and is used for transportation of oil and natural gas, and not only the mechanical properties such as strength and toughness as a structural component but also resistance to oil and natural gas passing through the pipe is required for the steel.
• HIC resistance hydrogen-induced cracking resistance
• the T-cross weld joint that receives two kinds of thermal histories of seam welding in working a thick steel plate into a pipe and girth welding in joining pipes with each other is subjected to complicated thermal histories such as rapid heating and rapid cooling, and therefore, the strength, namely hardness, increases and a cracking called sulfide stress corrosion cracking is liable to occur in the welding heat-affected zone: HAZ.
• the sulfide stress corrosion cracking is hereinafter also referred to as SSCC. Therefore, to realize high strength line pipe steel, the SSCC resistance in the T-cross weld joint is also one of problems.
• Patent Literature 1 describes the technique that a block bainite structure which is considered to be harmful to the HIC resistance is reduced, and uniform upper bainite or acicular ferrite structure is developed, thereby X70 grade high strength thick steel plate of API standard could be obtained while securing the HIC resistance of a base plate.
• Patent Literature 2 describes a technique that precipitation strengthening by fine Nb and V carbonitride is utilized and high strength of 56 kgf/mm 2 or more of tensile strength can be achieved.
• this Patent Literature does not describe the HIC resistance of a base plate, and only HAZ in seam welding is taken into consideration with respect to the SSCC resistance.
• the test conditions described in the Examples are that dipping time in a solution simulating the sour environment, namely the environment containing many H 2 S, is 21 days, and are therefore not sufficiently severe conditions.
• Patent Literature 3 such a composition as suppressing increase of the hardness which is deemed to deteriorate the SSCC resistance of the T-cross weld joint is described.
• the SSCC resistance itself is not evaluated, and the HIC resistance of the base plate is not described.
• the present invention has been made to overcome the above conventional problems, and its object is to provide a thick steel plate with excellent sour resistance, particularly HIC resistance. Further, other objects of the present invention are to provide a thick steel plate that can achieve a welded joint with excellent SSCC resistance of the T-cross weld joint and a welded joint with excellent SSCC resistance of the T-cross weld joint.
• the thick steel plate of the present invention contains, in terms of mass %, C: 0.01 to 0.12%, Si: 0.02 to 0.50%, Mn: 0.6 to 2.0%, P: more than 0% and 0.030% or less, S: more than 0% and 0.004% or less, Al: 0.010 to 0.080%, Cr: 0.10 to 1.50%, Nb: 0.002 to 0.050%, REM: 0.0002 to 0.05%, Zr: 0.0003 to 0.01%, Ca: 0.0003 to 0.006%, N: more than 0% and 0.010% or less, and O: more than 0% and 0.0040% or less, with the remainder being iron and inevitable impurities, in which the steel includes an inclusion having a width of 1 ⁇ m or more, in which the inclusion has a composition satisfying that Zr amount in the inclusion is 1 to 40%, REM amount therein is 5 to 50%, Al amount therein is 3 to 30%, and Ca amount therein is 5 to 60%.
• S amount in the inclusion is preferably more than 0% and 20% or less.
• [Cr]/[Nb] is preferably 10 or more, provided that [ ] in the formula indicates mass %.
• the thick steel plate of the present invention preferably further contains, in terms of mass %, one kind or two or more kinds of Mg: more than 0% and 0.005% or less, Ti: 0.003 to 0.030%, Ni: 0.01 to 1.50%, Cu: 0.01 to 1.50%, Mo: 0.01 to 1.50%, V: 0.003 to 0.08%, and B: 0.0002 to 0.0032%, in which [Cr]+[Mo]+[Ni]+[Cu] is 2.1 or less, provided that [ ] in the formula indicates mass %.
• the thick steel plate of the present invention preferably contains, in terms of mass %, Ni: 0.01 to 1.50%, in which 0.25 ⁇ [Cr]+[Ni] is 0.10 to 1.50, provided that [ ] in the formula indicates mass %.
• a welded joint of the present invention includes any one of the above thick steel plates of the present invention and a girth weld metal.
• the welded joint of the present invention preferably has an immersion potential difference ⁇ E between the thick steel plate and the girth weld metal, obtained by the following formula, being 25 mV or less.
• ⁇ E Immersion potential mV after 1 hour of girth weld metal ⁇ immersion potential mV after 1 hour of thick steel plate
• a thick steel plate with excellent sour resistance can be provided.
• a welded joint with excellent SSCC resistance of T-cross weld joint can be provided by using the thick steel plate of the present invention.
• the present inventors repeated intensive researches and studies from the viewpoint of controlling inclusions in steel in addition to a component composition of a steel, which becomes the basis in exerting properties of a thick steel plate.
• a thick steel plate with excellent sour resistance is obtained by holding coarse inclusions with 1 ⁇ m or more width in a predetermined component composition, and the present invention has been completed.
• the inclusions in the present invention mean coarse precipitates formed during melting and solidification, and specifically mean particles by oxides, carbides, sulfides, nitrides, and the like of alloy components in steel.
• HIC resistance of steel can be improved and secured by converting the coarse inclusions of 1 ⁇ m or more which become a cause of the hydrogen-induced cracking from inclusions having a coefficient of thermal expansion larger than that of steel into inclusions having a coefficient of thermal expansion smaller than that of steel, and by making such a inclusions.
• inclusions having a coefficient of thermal expansion smaller than that of steel specifically oxides of Zr, Al and REM and the like are effective.
• the weld metal used in the T-cross weld joint will be explained in detail below including the reason of the determination.
• all of % which is an expression unit of the composition means mass %.
• C is an indispensable element for securing the strength of the thick steel plate, and should be contained in an amount of 0.01% or more. It is preferably 0.02% or more, and more preferably 0.03% or more. However, when the C amount is excessive, island martensite is liable to be formed in the base plate, this becomes the origin of hydrogen-induced cracking, and HIC resistance of the base plate deteriorates. Therefore, the C amount should be 0.12% or less. It is preferably 0.10% or less, and more preferably 0.08% or less.
• Si is effective in deoxidation.
• Si amount is 0.02% or more. It is preferably 0.04% or more, and more preferably 0.06% or more.
• the Si amount should be suppressed to 0.50% or less. It is preferably 0.45% or less, and more preferably 0.35% or less.
• Mn is an indispensable element for securing the strength of the thick steel plate, and should be contained in an amount of 0.6% or more. It is preferably 0.8% or more, and more preferably 1.0% or more. However, when the Mn amount is excessive, MnS is formed, and HIC resistance deteriorates. Therefore, the upper limit of the Mn amount is 2.0%. It is preferably 1.9% or less, and more preferably 1.8% or less.
• the P is an element inevitably included in a steel material. When its content exceeds 0.030%, the HIC resistance and the SSCC resistance are adversely affected. Therefore, in the present invention, the P amount is suppressed to 0.030% or less. It is preferably 0.020% or less, and more preferably 0.010% or less.
• the upper limit of the S amount is 0.004%. It is preferably 0.003% or less, more preferably 0.0025% or less, and still more preferably 0.0020% or less.
• Al is effective in reducing voids against the matrix phase of steel by decreasing a coefficient of thermal expansion of inclusions, and improving the HIC resistance. Also, inclusions containing an appropriate amount of Al promote the formation of intragranular bainite, and therefore good SSCC resistance is obtained. In order to exert the effect, it should be contained in an amount of at least 0.010% or more.
• the Al amount is preferably 0.020% or more, and more preferably 0.025% or more. However, when the Al amount is excessive, Al oxide is formed in a cluster shape and becomes the origin of the hydrogen-induced cracking. Therefore, the Al amount should be 0.080% or less.
• the Al amount is preferably 0.060% or less, and more preferably 0.050% or less.
• the Cr is an indispensable element for securing strength. Also, it contributes to the improvement of SSCC resistance by suppressing soft ferrite in the T-cross weld joint. In order to exert such an effect, it should be contained in an amount of at least 0.10% or more.
• the Cr amount is preferably 0.15% or more, more preferably 0.17% or more, and still more preferably 0.20% or more.
• the Cr amount is preferably 1.00% or less, and more preferably 0.80% or less.
• Nb is an indispensable element for securing strength, and further contributes to the improvement of the SSCC resistance by suppressing soft ferrite in the T-cross weld joint. In order to exert such an effect, it should be contained in an amount of at least 0.002% or more.
• the Nb amount is preferably 0.005% or more, and more preferably 0.010% or more.
• the Nb amount is preferably 0.033% or less, and more preferably 0.030% or less.
• REM Radar Earth Metal
• the inclusions containing an appropriate amount of REM accelerate the formation of intragranular bainite, and therefore good SSCC resistance can be obtained.
• REM should be contained in an amount of 0.0002% or more.
• the REM amount is preferably 0.0005% or more, and more preferably 0.0010% or more.
• the upper limit of the REM amount is 0.05%.
• the REM means 15 elements from La to Lu in the periodic table, namely the lanthanoid elements, Sc and Y.
• Zr reduces voids against the matrix phase of steel by reducing a coefficient of thermal expansion of inclusions, and improves the HIC resistance. Inclusions containing an appropriate amount of Zr accelerate the formation of intragranular bainite, and as a result, good SSCC resistance can be obtained. In order to exert such an effect, Zr should be contained in an amount of 0.0003% or more.
• the Zr amount is preferably 0.0005% or more, and more preferably 0.0010% or more.
• the upper limit of the Zr amount is 0.01%.
• the Zr amount is preferably 0.007% or less, and more preferably 0.005% or less.
• Ca has an action of forming CaS to fix S and reducing the amount of MnS formed, thereby improving the SSCC resistance. Also, inclusions containing an appropriate amount of Ca accelerate the formation of intragranular bainite, and therefore good SSCC resistance can be obtained. In order to exert such an effect, Ca should be contained in an amount of 0.0003% or more.
• the Ca amount is preferably 0.0005% or more, and more preferably 0.0010% or more.
• the upper limit of the Ca amount is 0.006%.
• the Ca amount is preferably 0.005% or less, and more preferably 0.004% or less.
• the upper limit of the N amount is 0.010%.
• the N amount is preferably 0.008% or less, and more preferably 0.006% or less.
• O oxygen
• Coarse oxides are excessively formed by the excessive addition thereof, and hydrogen-induced cracking is generated from those as the origin. Therefore, the upper limit of the O amount is 0.0040%.
• the O amount is preferably 0.0030% or less, and more preferably 0.0020% or less.
• the thick steel plate of the present invention is that the component composition described before is satisfied, and in addition to this, [Cr]/[Nb] is preferably 10 or more.
• [ ] indicates mass %.
• the component composition of the steel material of the thick steel plate of the present invention is described as above, and the remainder is iron and inevitable impurities. Also, by further containing, in addition to the elements described above, at least one kind or two or more kinds selected from the group consisting of Mg, Ti, Ni, Cu, Mo, V, and B in the amount described below, the HIC resistance, the SSCC resistance and the like can be improved. Those elements will be explained below.
• Mg has an action of forming MgS and finely dispersing the sulfide, thereby improving the SSCC resistance of the base plate.
• the upper limit of the Mg amount is 0.005%. It is more preferably 0.004% or less, and still more preferably 0.003% or less.
• Ti is an element contributing to the improvement of the strength of the thick steel plate by precipitation strengthening. In order to exert this action, it is preferred to be contained in an amount of 0.003% or more. It is more preferably 0.004% or more, and still more preferably 0.005% or more. On the other hand, when the Ti content is excessive, the amount of hard martensite in the T-cross weld joint increases, thereby deteriorating the SSCC resistance. Therefore, it is preferred to be 0.030% or less. It is more preferably 0.025% or less, and more preferably 0.020% or less.
• Ni is an element contributing to the improvement of strength of the thick steel plate. In order to exert the action, it is preferred to be contained in an amount of 0.01% or more. It is more preferably 0.05% or more, and still more preferably 0.10% or more. On the other hand, when the Ni content is excessive, the amount of hard martensite in the T-cross weld joint increases, and the SSCC resistance is deteriorated. Therefore, it is preferred to be 1.50% or less. It is more preferably 1.00% or less, and still more preferably 0.50% or less.
• Cu is an element contributing to the improvement of strength of the thick steel plate. In order to exert this action, it is preferred to be contained in an amount of 0.01% or more. It is more preferably 0.05% or more, and still more preferably 0.10% or more. On the other hand, when the Cu content is excessive, the amount of hard martensite in the T-cross weld joint is increased, and the SSCC resistance is deteriorated. Therefore, it is preferred to be 1.50% or less. It is more preferably 1.00% or less, and still more preferably 0.50% or less.
• Mo is an element contributing to the improvement of strength of the thick steel plate. In order to exert this action, it is preferred to be contained in an amount of 0.01% or more. It is more preferably 0.05% or more, and still more preferably 0.10% or more. On the other hand, when the Mo content is excessive, the amount of hard martensite in the T-cross weld joint is increased, and the SSCC resistance is deteriorated. Therefore, it is preferred to be 1.50% or less. It is more preferably 1.00% or less, and still more preferably 0.50% or less.
• V is an element contributing to the improvement of strength of the thick steel plate. In order to exert this action, it is preferred to be contained in an amount of 0.003% or more. It is more preferably 0.005% or more, and still more preferably 0.010% or more. On the other hand, when the V content is excessive, the amount of hard martensite in the T-cross weld joint is increased, and the SSCC resistance is deteriorated. Therefore, it is preferred to be 0.08% or less. It is more preferably 0.07% or less, and still more preferably 0.05% or less.
• B is an element contributing to the improvement of strength of the thick steel plate. In order to exert this action, it is preferred to be contained in an amount of 0.0002% or more. It is more preferably 0.0005% or more, and still more preferably 0.0010% or more. On the other hand, when the B content is excessive, the amount of hard martensite in the T-cross weld joint is increased, and the SSCC resistance is deteriorated. Therefore, it is preferred to be 0.0032% or less. It is more preferably 0.0030% or less, and still more preferably 0.0025% or less.
• [Cr]+[Mo]+[Ni]+[Cu] is preferably 2.1 or less, in addition to satisfying the component composition described before.
• [ ] in the formula indicates mass %.
• [Cr]+[Mo]+[Ni]+[Cu] is 2.1 or less. It is more preferably 1.9 or less, and still more preferably 1.7 or less.
• the immersion potential difference ⁇ E of the thick steel plate and girth weld metal after 1 hour when immersed in solutions is 25 mV or less. It is more preferably 20 mV or less, and still more preferably 15 mV or less.
• ⁇ E Immersion potential mV after 1 hour of girth weld metal ⁇ immersion potential mV after 1 hour of thick steel plate
• Electrode potential appearing when a metal is immersed in a solution is sometimes defined as corrosion potential or mixed potential, but this is defined as “immersion potential” in the present invention.
• 0.25 ⁇ [Cr]+[Ni] is preferably 0.10 to 1.50, in addition to satisfying the component composition described before, particularly the condition that Ni content is 0.10 to 1.50%.
• [ ] in the formula indicates mass %.
• the value obtained from 0.25 ⁇ [Cr]+[Ni] is preferably 0.10 or more, more preferably 0.15 or more, and still more preferably 0.20 or more.
• the value obtained from 0.25 ⁇ [Cr]+[Ni] is excessive, the potential of the steel plate is greatly increased than the potential of the weld metal, selective corrosion of the weld metal by galvanic corrosion makes progress, and the SSCC resistance deteriorates. Therefore, the upper limit of the value obtained from 0.25 ⁇ [Cr]+[Ni] is 1.50. The upper limit is more preferably 1.00 or less, and still more preferably 0.70 or less.
• the metal used in girth welding has the following component composition for securing strength and toughness of the weld metal and improving corrosion resistance. Specifically, it is preferred to contain, in terms of mass %, C: 0.02 to 0.10%, Si: 0.10 to 0.60%, Mn: 0.90 to 2.50%, and Ni: 0.20 to 1.00%. It is allowed to further contain P: 0.015% or less, S: 0.010% or less, Cu: 1.0% or less, Mo: 1.0% or less, Nb: 0.5% or less, V: 0.3% or less, Ti: 0.05% or less, and Al: 0.1% or less as components other than the above. It is desirable that components other than those are iron and inevitable impurities. The reasons for limiting the component composition of the weld metal are described below.
• C is an element necessary in securing strength of the weld metal.
• predetermined strength is not obtained.
• grain boundary carbide is coarsened, leading to deterioration of toughness. Therefore, it is 0.10% or less.
• Si is an element necessary in securing strength the weld metal.
• the Si content is less than 0.10%, predetermined strength is not obtained.
• excessive Si content leads to deterioration of toughness. Therefore, it is 0.60% or less.
• Mn is an element necessary in securing the balance between strength and toughness of the weld metal.
• the Mn content should be 0.90% or more.
• it should be 2.50% or less.
• Ni exerts the effect of increasing the potential of the weld metal and improving corrosion resistance. Further, it is an element effective in securing strength by increasing quenchability, and improving low temperature toughness. In order to obtain this effect, the Ni content should be 0.20% or more. On the other hand, when the Ni content is excessive, high temperature cracking is likely induced, and further the potential of the weld metal is increased excessively, leading to the generation of selective corrosion of the base plate. Therefore, the upper limit is 1.00% or less.
• Zr in the inclusions with 1 ⁇ m or more width is mainly present as an oxide.
• the Zr oxide has a coefficient of thermal expansion smaller than that of steel. Therefore, when the Zr amount in the inclusions is secured, voids against the surrounding steel matrix phase can be reduced, thereby improving the HIC resistance. Also, the oxide containing an appropriate amount of Zr accelerates the formation of intragranular bainite, and as a result, good SSCC resistance can be obtained.
• the Zr content in the inclusions is 1 to 40%. When the Zr amount is less than 1% or exceeds 40%, the HIC resistance of the base plate or the SSCC resistance of the T-cross weld joint becomes insufficient.
• the REM in the inclusions with 1 ⁇ m or more width is present as an oxide, oxysulfide and the like.
• REM oxide has a coefficient of thermal expansion smaller than that of steel. Therefore, when the REM amount in the inclusions is secured, voids against the surrounding steel matrix phase can be reduced, thereby improving the HIC resistance.
• S is fixed, and the formation of a sulfide such as MnS adversely affecting the HIC resistance can be suppressed.
• those REM inclusions accelerate the formation of intragranular bainite, and as a result, good SSCC resistance can be obtained.
• the REM amount in the inclusions is 5 to 50%. When the REM amount is less than 5% or exceeds 50%, the HIC resistance of the base plate or the SSCC resistance of the T-cross weld joint becomes insufficient.
• Al in the inclusions with a width of 1 ⁇ m or more is mainly present as an Al oxide.
• the Al oxide has a coefficient of thermal expansion smaller than that of steel. Therefore, when the Zr amount in the inclusions is secured, voids against the surrounding steel matrix phase can be reduced, and this is effective to improve the HIC resistance. Also, the oxide containing an appropriate amount of Al accelerates the formation of intragranular bainite, and as a result, good SSCC resistance can be obtained.
• the Al amount in the inclusions is 3 to 30%. When the Al amount is less than 3% or exceeds 30%, the HIC resistance of the base plate or the SSCC resistance of the T-cross weld joint becomes insufficient.
• Ca in the inclusions with 1 ⁇ m or more width contributes to the formation of the fine microstructure of steel in the T-cross weld joint during welding, and accelerates the formation of intragranular bainite structure originated from the inclusions.
• the microstructure of steel in the T-cross weld joint after welding becomes fine, and good SSCC resistance can be obtained.
• the Ca amount in the inclusions is 5 to 60%. When the Ca amount is less than 5% or exceeds 60%, the SSCC resistance in the T-cross weld joint cannot be improved.
• S amount in the inclusions with 1 ⁇ m or more width can be reduced by limiting S content in a steel plate and the content of alloy components such as Zr and REM which refine and disperse sulfur inclusions, to the component composition described above, and additionally controlling the composition of the inclusions as described above.
• the S amount in the inclusions exceeds 20%, coarse sulfide becomes excessive, and as a result, the HIC resistance of the base plate or the SSCC resistance of the T-cross weld joint becomes insufficient.
• a steel plate having the S amount in the inclusions controlled to 20% or less good HIC resistance and SSCC resistance are obtained.
• the S amount in the inclusions is better as being smaller. However, when it is 0%, it is considered that S cannot entirely be fixed by the inclusions and the HIC resistance of the base plate or the SSCC resistance of the T-cross weld joint becomes insufficient.
• the total number of the inclusions is not particularly limited so long as the effect of the present invention is not remarkably impaired, but it is preferred that about 500 to 5,000 /cm 2 are dispersed in a steel plate.
• 500 /cm 2 it is considered that the origin of intragranular bainite becomes insufficient, sufficient effect of formation of fine microstructure is not obtained, and as a result, the SSCC resistance deteriorates.
• the inclusions act as the origin of fracture, and there is a possibility that both HIC resistance and SSCC resistance deteriorate.
• a slag satisfying Fe 0.1 to 10% is used and S is set to 0.004% or less.
• REM and Zr added after desulfurization and deoxidizing can form oxides preferentially without being dissolved in the molten steel.
• the Fe concentration in the slag is 0.1% or more.
• the Fe concentration in the slag is preferably 0.5% or more, and more preferably 1.0% or more.
• the Fe concentration in the slag exceeds 10%, oxides are formed excessively, and the oxides become the origin of hydrogen-induced cracking. Therefore, the Fe concentration in the slag is 10% or less. It is preferably 8% or less, and more preferably 5% or less. Also, in adding Ca, by sufficiently executing desulfurization in the slag and suppressing S to 0.004% or less, CaS can be prevented from being formed excessively when Ca is added after adding REM, the composition of inclusions can be prevented from deviating from a predetermined range, and as a result, the HIC resistance and the SSCC resistance can be secured.
• the CaO concentration in the slag is 10% or more.
• CaO in the slag reacts with dissolved S in the molten steel and changes into CaS, thereby reduction of S in the molten steel, namely desulfurization, can be executed sufficiently.
• the CaO concentration in the slag is preferably 15% or more, and more preferably 20% or more.
• the upper limit is approximately 80%.
• the dissolved oxygen concentration "Of" of the molten steel is 10 or less in terms of the ratio relative to the S concentration of the molten steel (Of/S). REM forms, when added to the molten steel, oxides at the same time of forming the sulfides thereof.
• Of/S is 10 or less as described above.
• the Of/S is preferably 5 or less, more preferably 3.5 or less, and still more preferably 2 or less.
• the lower limit of the Of/S is approximately 0.1. Adjusting Of/S to 10 or less as described above can be achieved by executing at least one deoxidation of deoxidation by an RH degassing apparatus and deoxidation by feeding deoxidizing elements such as Mn, Si and Ti.
• Al is first added, and Zr and REM are then added.
• Zr and REM are then added.
• the deoxidizing power of Zr and REM is stronger than that of Al
• the Al amount in inclusions cannot be made to be a desired value. Therefore, Al should be added prior to Zr and REM.
• the desulfurizing powder of REM is weaker than that of Ca, therefore, if Ca is added before adding REM, a large amount of CaS is formed, the composition of the inclusions deviates from the predetermined range, and as a result, the HIC resistance and the SSCC resistance are deteriorated.
• REM should be added before adding Ca, and therefore, the adding order of Al, Zr, REM, and Ca should be Al ⁇ (Zr and REM) ⁇ Ca.
• the time after adding REM until adding Ca should be 4 min or more.
• the time after adding REM until adding Ca is preferably 5 min or more, and more preferably 8 min or more. From the viewpoint of the productivity, the upper limit of the time after adding REM until adding Ca is approximately 60 min.
• the deoxidizing capacity of Zr, REM and Ca when the deoxidizing capacity of Zr, REM and Ca is compared, it is generally considered that the deoxidizing power of Ca is strongest and the deoxidizing power is in the order of Ca>REM>Zr. Thus, Zr is lowest. Therefore, in order to contain Zr in the inclusions, namely in order to form ZrO 2 as oxide-based inclusions, Zr should be added prior to adding Ca and REM whose deoxidizing power is stronger than that of Zr. For this reason, the adding order of Al, Zr, REM, and Ca should be Al ⁇ Zr ⁇ REM ⁇ Ca. However, because the deoxidizing capacity of REM is smaller as compared with Ca, even if REM is added simultaneously with Zr, it is possible to contain Zr in the inclusions. Therefore, the adding order of those may also be Al ⁇ (Zr and REM) ⁇ Ca.
• the steel plate having each desired element amount only has to be obtained, and such a method can be cited for example to add Zr so as to become 3 to 100 ppm in terms of the concentration in the molten steel, to add REM thereafter or simultaneously so as to become 2 to 500 ppm in terms of the concentration in the molten steel, and, after 4 min or more elapses thereafter, to add Ca so as to become 3 to 60 ppm in terms of the concentration in the molten steel.
• Casting is started quickly, for example, in 80 min or less after Ca is added as described above, and casting is executed so that the time after adding Ca until completion of solidification is 200 min or less.
• the reason of this is as follows. Because Ca is an element that is high in both desulfurization capacity and deoxidizing capacity, the composition of oxides and sulfides is liable to convert to stable CaS and CaO with the lapse of time after adding Ca, and the composition of inclusions cannot be made to fall within the predetermined range. Therefore, in the present invention, the time after adding Ca until completion of solidification is 200 min or less. It is preferably 180 min or less, and more preferably 160 min or less. The lower limit of the time is approximately 4 min from the viewpoint of homogenizing Ca.
• the cooling time at t/4 position of 1,300°C to 1,200°C slab in casting is 270 to 460 sec.
• the cooling time exceeds the upper limit, complex formation of the sulfide-based secondary inclusions on the inclusions is mainly promoted, and the composition of the inclusions deviates from the predetermined range. As a result, the SSCC resistance is deteriorated.
• the cooling time is lower than the lower limit, the cooling load significantly increases, which is not preferable practically.
• hot rolling is executed according to an ordinary method, and the thick steel plate can be manufactured. Also, by using the steel plate, a steel pipe for a line pipe can be manufactured by a method generally executed.
• steps of rolling and thereafter are not particularly limited, it is preferable, for example, to heat a cast slab to 1,100°C or above to execute hot rolling with the compression reduction of 40% or more at a recrystallization temperature region, followed by accelerated cooling in a cooling rate of 10 to 20°C/s from 780°C. The conditioning thereafter is not necessary.
• a welded joint using the thick steel plate of the present invention is provided.
• the welded joint includes the thick steel plate and a girth weld metal, and is obtained by welding the edge of the thick steel plate with a girth weld metal.
• the welded joint of the present invention preferably has immersion potential difference ⁇ E between the thick steel plate and the girth weld metal, obtained by the following formula, being 25 mV or less.
• ⁇ E Immersion potential mV after 1 hour of girth weld metal ⁇ immersion potential mV after 1 hour of thick steel plate
• the immersion potential difference ⁇ E is 25 mV or less, and the deterioration of the SSCC resistance in the weld joint can be suppressed.
• the immersion potential difference ⁇ E is more preferably 20 mV or less, and still more preferably 15 mV or less.
• the weld metals described above are preferably used as the metal to be used in the girth welding.
• a welding method for forming the welded joint is not particularly limited, and can be performed by conventional methods. Examples thereof include arc welding, laser welding and electron beam welding.
• the present invention is descried in more detail below by reference to Examples. However, the present invention is not limited by the following Examples and can be performed by adding appropriate modifications in a range capable of conforming to the gist of the present invention, and those are included in the technical scope of the present invention.
• the molten steel refined in a 240t converter by an ordinary method was subjected to processing such as desulfurizing, deoxidizing, composition regulating, and inclusion controlling by using an LF furnace, various kinds of molten steel having the steel composition and the composition of the inclusions in steel shown in Tables 1 to 4 were prepared as slabs by the continuous casting method, those were subjected to hot rolling and then accelerated cooling, and thick steel plates with 40 mm thickness and 3,500 mm width were manufactured. By using the thick steel plates obtained and girth weld metals, welded joints described after were manufactured. Tables 5 and 6 show the main process conditions in the molten steel processing, continuous casting and accelerated cooling described above. Tables 7 and 8 show the various properties of each steel plate thus obtained.
• the analyzing method for the composition of the inclusions shown in Tables 3 and 4, and the measuring method and the evaluating method of each property of Tables 7 and 8 will be explained below.
• the cross section in the plate thickness direction of the as-rolled material was observed focusing the plate thickness central part by using EPMA-8705 manufactured by Shimadzu Corporation.
• 3 cross sections were observed with 400 observation magnification and approximately 50 mm 2 field of view, and the component composition at the inclusion central part was quantitatively analyzed by wavelength dispersion spectrometry of the characteristic X-ray for the inclusions with 1 ⁇ m or more width.
• the observation field was the range of 7 mm in the plate thickness direction and 7 mm in the plate width direction such that the plate thickness center part became the central.
• the elements of the analyzing object were Al, Mn, Si, Mg, Ca, Ti, Zr, S, REM, and Nb.
• the REM used herein means La, Ce, Nd, Dy, and Y.
• the relationship between the X-ray strength and the element concentration of each element was obtained beforehand as an analytical curve by using known substances, and the element concentration of the inclusions was then quantitatively determined from the X-ray strength obtained from the inclusions and the analytical curve described above. Furthermore, the average value of the content of each element described above of the inclusions with 1 ⁇ m or more width in 3 cross sections described above was obtained, and was defined as the composition of inclusions.
• test and evaluation were executed according to the method defined in NACE standard TM0248-2003. Specifically, a specimen was dipped in 25°C 5 mass % NaCl+0.5 mass % CH 3 COOH aqueous solution saturated with 1 atm hydrogen sulfide for 96 hours. With respect to evaluation of the HIC test, each specimen was cut at 10 mm pitch in the longitudinal direction, the cut surface was polished, all cross sections were observed with 100 magnifications by using an optical microscope, and the presence or absence of the cracking with 1 mm or more of the cracking length of HIC was confirmed.
• the thick steel plate having the steel composition shown in Tables 1 and 2 was worked to have an X groove of 75°, and welding was executed by 2-pass submerged arc welding, thereby manufacturing the pipe.
• the heat input during welding was first pass: 3.7 kJ/mm and second pass: 5.4 kJ/mm.
• 1 pass bead-on-plate welding by gas shield arc welding was executed so as to orthogonally cross the seam weld line by reference to " Practical application to UOE steel pipe excellent in SSCC resistance, Matsuyama et al., Welding Technology, September 1988, p. 58 ", thereby manufacturing the welded joint.
• Lincolnweld LA-81 (manufactured by Lincoln) was used as the welding metal in seam welding
• MX-A55Ni1 manufactured by Kobe Steel, Ltd. was used as the welding metal in girth welding.

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• 2016
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