Classifications

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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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  • 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
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21C—MANUFACTURE OF METAL SHEETS, WIRE, RODS, TUBES OR PROFILES, OTHERWISE THAN BY ROLLING; AUXILIARY OPERATIONS USED IN CONNECTION WITH METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL
  • B21C37/00—Manufacture of metal sheets, bars, wire, tubes or like semi-manufactured products, not otherwise provided for; Manufacture of tubes of special shape
  • B21C37/06—Manufacture of metal sheets, bars, wire, tubes or like semi-manufactured products, not otherwise provided for; Manufacture of tubes of special shape of tubes or metal hoses; Combined procedures for making tubes, e.g. for making multi-wall tubes
  • B21C37/08—Making tubes with welded or soldered seams
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  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/56—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering characterised by the quenching agents
  • C21D1/60—Aqueous agents
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  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/005—Heat treatment of ferrous alloys containing Mn
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  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/10—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies
  • C21D8/105—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies of ferrous alloys
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  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/08—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for tubular bodies or pipes
  • C21D9/085—Cooling or quenching
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/50—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for welded joints
  • C21D9/505—Cooling thereof
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  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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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
  • C22C38/002—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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  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
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  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
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  • C22C—ALLOYS
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  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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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
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  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
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  • 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
  • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
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  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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  • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
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  • 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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  • 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
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/003—Cementite
• • C—CHEMISTRY; METALLURGY
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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
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/005—Ferrite
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/009—Pearlite

Definitions

• the present disclosure relates to an electric resistance welded steel pipe for a linepipe.
• a pipeline is a system constructed on the ground, seabed, and the like, to transfer oil or gas.
• a pipeline is formed by joining a plurality of linepipes together.
• An electric resistance welded steel pipe may be used for such linepipes.
• a sour environment means an acid environment containing H 2 S which is a corrosive gas.
• SSC Sulfide Stress Cracking
• Patent Document 1 Japanese Patent Application Laid-Open ( JP-A) No. H06-41684 (Patent Document 1) and JP-A No. H06-235045 (Patent Document 2) disclose a technique for improving the SCC resistance of an electric resistance welded steel pipe for a linepipe.
• the electric resistance welded steel pipe disclosed in Patent Document 1 is produced using a steel containing from 0.05 to 0.35% of C, from 0.02 to 0.50% of Si, from 0.30 to 2.00% of Mn, from 0.0005 to 0.0080% of Ca, and from 0.005 to 0.100% of Al and the balance composed of Fe.
• contents of S, O, and Ca satisfy the formula (1.0 ⁇ (%Ca) ⁇ 1-72 (%O) ⁇ /1.25(%S) ⁇ 2.5), the deoxidation product is a composite inclusion of (CaO) m (Al 2 O 3 ) n , m/n ⁇ 1, and the maximum value of hardness measured within 30 mm on both sides centered on an electric resistance welded abutting surface is 250 or less in the Vickers hardness, and the difference between the maximum and minimum values is 30 or less in the Vickers hardness.
• Paragraph 0063 of Patent Document 1 describes that the above-described electric resistance welded steel pipe has high strength and excellent sulfide stress corrosion cracking resistance characteristics even in low pH and severe environments.
• the electric resistance welded steel pipe disclosed in Patent Document 2 is produced using a steel containing, % by weight, from 0.05 to 0.35% of C, from 0.02 to 0.50% of Si, from 0.30 to 2.00% of Mn, 0.030% or less of P, 0.005% or less of S, from 0.0005 to 0.0080% of Ca, and from 0.005 to 0.100% of Al, wherein contents of S, O, and Ca satisfy the formula (1.0 ⁇ (%Ca) ⁇ 1-72 (%O) ⁇ /1.25 (%S) ⁇ 2.5), the relationship between the amount of O and the amount of Ca satisfies (%Ca)/(%O) ⁇ 0.55, and the balance is consisting of Fe and unavoidable impurities.
• the maximum value of hardness measured within 30 mm on both sides centered on an electric resistance welded abutting surface is 250 or less in Vickers hardness, and the difference between the maximum value and the minimum value is 30 or less in Vickers hardness.
• Paragraph 0059 of Patent Document 2 describes that the above-described electric resistance welded steel pipe does not degrade the SSC properties even in low pH and severe environments.
• a bulge extending in the direction of the pipe axis of the electric resistance welded steel pipe may occur near the surface of the electric resistance welded steel pipe.
• a plurality of blisters and minute internal cracks in the electric resistance welded steel pipe connect in the direction of the thickness of the electric resistance welded steel pipe, resulting in SOHIC.
• Patent Documents 1 and 2 do not describe SOHIC resistance. Therefore, even when the techniques disclosed in Patent Documents 1 and 2 are applied to an electric resistance welded steel pipe for a linepipe used in sour environments, excellent SOHIC resistance may not be achieved.
• An object of the present disclosure is to provide an electric resistance welded steel pipe for a linepipe having excellent SSC resistance and SOHIC resistance.
• Means for solving the problem described above includes the following aspects.
• an electric resistance welded steel pipe for a linepipe having excellent SSC resistance and SOHIC resistance is provided.
• a numerical range expressed by "x to y" herein includes the values of x and y in the range as the minimum and maximum values, respectively.
• the content of a component (element) expressed by “%” herein means “% by mass”.
• C carbon
• C content The content of another element may be expressed similarly.
• step herein encompasses not only an independent step but also a step of which the desired object is achieved even in a case in which the step is incapable of being definitely distinguished from another step.
• the electric resistance welded steel pipe for a linepipe of the present disclosure (hereinafter, also simply referred to as “electric resistance welded steel pipe of the present disclosure”) is an electric resistance welded steel pipe for a linepipe, the steel pipe comprising a base metal portion and an electric resistance welded portion, wherein: a chemical composition of the base metal portion consists of, in terms of % by mass: from 0.030 to 0.090% of C, from 0.01 to 0.50% of Si, from 0.50 to 1.50% of Mn, from 0 to 0.020% of P, from 0 to 0.0020% of S, from 0.005 to 0.060% of Nb, from 0.005 to 0.030% of Ti, from 0.0001 to 0.0040% of Ca, from 0 to 0.050% of Al, from 0.0010 to 0.0080% of N, from 0 to 0.0030% of O, from 0 to 0.500% of Cu, from 0 to 0.500% of Ni, from 0 to 0.0020% of B, from 0 to 0.100% of V
• base metal portion refers to a portion other than the electric resistance welded portion and a heat affected zone in the electric resistance welded steel pipe.
• heat affected zone (sometimes referred to as “HAZ”) refers to a zone which is present in the vicinity of the electric resistance welded portion, and which has been affected by heat due to electric resistance welding and the seam heat treatment.
• the electric resistance welded steel pipe of the present disclosure have excellent SSC resistance and SOHIC resistance.
• the present inventors investigated the SSC resistance and the SOHIC resistance of an electric resistance welded steel pipe satisfying all the requirements in the present disclosure except the requirements for ⁇ YS and ⁇ Hv (namely, ⁇ YS is from 0 to 80 MPa and ⁇ Hv is from 0 to 25Hv) (hereinafter, referred to as "electric resistance welded steel pipe X").
• the SSC resistance and/or the SOHIC resistance at an electric resistance welded portion may decrease in an electric resistance welded steel pipe X.
• the present inventors first considered a reason for the decrease in SSC resistance as follows.
• an electric resistance welded portion of an electric resistance welded steel pipe is rapidly heated and rapidly cooled during electric resistance welding, it is generally considered that the strength of an electric resistance welded portion of an electric resistance welded steel pipe is higher than that of the base metal portion (namely, ⁇ YS is a negative value).
• ⁇ YS is a negative value
• SSC is likely to occur in the electric resistance welded portion of the electric resistance welded steel pipe.
• a heat treatment applied to the electric resistance welded portion hereinafter, also referred to as "seam heat treatment"
• the present inventors examined in more detail the relationship between SSC at an electric resistance welded portion of an electric resistance welded steel pipe X and ⁇ YS in the electric resistance welded steel pipe X.
• the SSC resistance at an electric resistance welded portion decreases when the yield strength of the electric resistance welded portion is too high, the SSC resistance at an electric resistance welded portion may decrease also when the yield strength of the electric resistance welded portion is too low for the following reasons.
• SSC semiconductor stress cracking
• SSC sulfide stress cracking in electric resistance welded steel pipes is a cracking that can occur when the internal pressure of an electric resistance welded steel pipe increases during use in a sour environment, thereby loading stress in the circumferential direction of the pipe.
• the yield strength of an electric resistance welded portion in an electric resistance welded steel pipe is too low, the internal pressure of the electric resistance welded steel pipe, which increases during use, causes plastic strain (namely, strain in the plastic range) on the electric resistance welded portion.
• the present inventors found that SSC in an electric resistance welded portion can be suppressed (namely, SSC resistance in an electric resistance welded portion can be improved) by bringing the yield strength of the electric resistance welded portion close to that of a base metal portion without decreasing the yield strength of the welded portion too much, specifically by adjusting the ⁇ YS to be from 0 to 80 MPa.
• the present inventors investigated the occurrence of SOHIC in detail, focusing on an electric resistance welded portion of an electric resistance welded steel pipe.
• the present inventors have investigated the Vickers hardness of electric resistance welded portions of electric resistance welded steel pipes X in aspects in which ⁇ YS is from 0 to 80 MPa. As a result, it has been found that when AHv, which is the value obtained by subtracting the Vickers hardness of the inner surface layer of an electric resistance welded portion from the Vickers hardness of the outer surface layer of an electric resistance welded portion, exceeds 25Hv, SOHIC occurs in the electric resistance welded portion.
• the present inventors have found that even in cases in which ⁇ YS is small, when ⁇ Hv is too large, the SOHIC resistance at an electric resistance welded portion decreases.
• the reason for this is considered to be that SOHIC is a cracking caused by a plurality of blisters and minute internal cracks in an electric resistance welded portion that are connected in the thickness direction of the electric resistance welded portion.
• the present inventors have found that the SSC resistance and the SOHIC resistance of an electric resistance welded portion can be improved in an electric resistance welded steel pipe X when the ⁇ YS is from 0 to 80 MPa and the ⁇ Hv is from 0 to 25 Hv (namely, when the steel pipe falls under the electric resistance welded steel pipe of the present disclosure).
• the present inventors have found that the electric resistance welded steel pipe of the present disclosure described above have excellent SSC resistance and SOHIC resistance.
• C (carbon) is an element that increases the strength of steel.
• C when the C content is too high, C may form a carbide with an alloying element in steel, which may decrease the SSC resistance and the SOHIC resistance of the steel material. Furthermore, when the C content is too high, the strength of a steel material may become too high, and ⁇ YS and ⁇ Hv may become too large, resulting in a decrease in the SSC resistance and the SOHIC resistance of the steel material.
• the C content is from 0.030 to 0.090%.
• the lower limit of the C content is preferably 0.035%, and more preferably 0.040%.
• the upper limit of the C content is preferably 0.080%, and more preferably 0.070%.
• Si from 0.01% to 0.50%.
• Si is an element that deoxidizes steel.
• the Si content is from 0.01 to 0.50%.
• the lower limit of the Si content is preferably 0.02%, and more preferably 0.05%.
• the upper limit of the Si content is preferably 0.40%, and more preferably 0.35%.
• Mn from 0.50 to 1.50%
• Mn is an element that deoxidizes steel.
• Mn is also an element that increases the strength of steel. When the Mn content is too low, the above-described effects may not be obtained.
• the Mn content is from 0.50 to 1.50%.
• the lower limit of the Mn content is preferably 0.60%, more preferably 0.80%, and still more preferably 1.00 %.
• the upper limit of the Mn content is preferably 1.40%, and more preferably 1.35%.
• P phosphorus
• the P content may be 0% or may exceed 0%.
• P may segregate at the grain boundary and decrease the SSC resistance and/or the SOHIC resistance of a steel material.
• the P content is from 0 to 0.020%.
• the upper limit of the P content is preferably 0.015%, and more preferably 0.013%.
• the P content is preferably as low as possible. However, extreme reduction of the P content may considerably increase the manufacturing cost of a steel material. Therefore, when industrial production is considered, the lower limit of the P content is preferably 0.001%, and more preferably 0.005%.
• S (sulfur) is an impurity.
• the S content may be 0%, or may exceed 0%.
• S may segregate at the grain boundary, and may decrease the SSC resistance and/or the SOHIC resistance of a steel material.
• the S content is from 0 to 0.0020%.
• the upper limit of the S content is preferably 0.0015%, more preferably 0.0010%, and still more preferably 0.0008%.
• the S content is preferably 0.0015%, more preferably 0.0010%, and still more preferably 0.0008%.
• the S content is preferably as low as possible. However, extreme reduction of the S content may considerably increase the manufacturing cost of a steel material. Therefore, when industrial production is considered, the lower limit of the S content is preferably 0.0001%, and more preferably 0.0002%.
• Nb from 0.005 to 0.060%
• Nb is an element that bonds with C (carbon) and/or N (nitrogen) to form a carbide, a nitride, or a carbonitride (hereinafter, referred to as "carbonitride or the like").
• the carbonitride or the like refinines the sub-structure of a steel material by a pinning effect, and may increase the SSC resistance and/or the SOHIC resistance of a steel material. When the Nb content is too low, the above-described effect may not be obtained.
• the Nb content is from 0.005 to 0.060%.
• the lower limit of the Nb content is preferably 0.008%, and more preferably 0.010%.
• the upper limit of the Nb content is preferably 0.055%, and more preferably 0.050%, and still more preferably 0.045%.
• Ti is an element that bonds with N (nitrogen) to form a nitride.
• the nitride refines crystal grains by a pinning effect. As a result, the SSC resistance and/or the SOHIC resistance of a steel material is increased. When the Ti content is too low, the above-described effect may not be obtained.
• the Ti content is from 0.005 to 0.030%.
• the lower limit of the Ti content is preferably 0.006%, more preferably 0.007%, and still more preferably 0.008%.
• the upper limit of the Ti content is preferably 0.025%, more preferably 0.020%, and still more preferably 0.017%.
• Ca is an element that controls the shape of a sulfide in a steel material and increases the SSC resistance and/or the SOHIC resistance of the steel material. When the Ca content is too low, the above-described effect may not be obtained.
• the Ca content is from 0.0001 to 0.0040%.
• the lower limit of the Ca content is preferably 0.0005%, and more preferably 0.0010%.
• the upper limit of the Ca content is preferably 0.0035%, and more preferably 0.0030%.
• Al 0 to 0.050% Al
• Al is an optional element.
• the Al content may be 0% or more than 0%.
• the Al content is from 0 to 0.050%.
• the upper limit of the Al content is preferably 0.045%, and more preferably 0.040%.
• Al is an element that deoxidizes steel.
• the lower limit of the Al content is preferably 0.0005%, more preferably 0.0010%, and still more preferably 0.0015%.
• N nitrogen
• N is an element that bonds with Ti to form a fine nitride to refine the crystal grains of a steel material, thereby improving the SSC resistance and the SOHIC resistance of the steel material.
• the N content is too low, the above-described effect may not be obtained.
• a nitride may coarsen and the SSC resistance and/or the SOHIC resistance of the steel material may decrease.
• the N content is from 0.0010 to 0.0080%.
• the lower limit of the N content is preferably 0.0015% and more preferably 0.0020%.
• the upper limit of the N content is preferably 0.0070%, more preferably 0.0060%, and still more preferably 0.0050%.
• O (oxygen) is an impurity.
• the O content may be 0% or more than 0%.
• O is an element that forms a coarse oxide and decreases the SSC resistance and/or the SOHIC resistance of a steel material.
• the O content is 0.0030% or less.
• the upper limit of the O content is preferably 0.0028%, and more preferably 0.0025%.
• the O content is preferably as low as possible. However, extreme reduction of the O content considerably increases the manufacturing cost of a steel material. Therefore, considering industrial production, the lower limit of the O content is preferably 0.0001%, more preferably 0.0003%, still more preferably 0.0010%, and still more preferably 0.0015%.
• Cu is an optional element.
• the Cu content may be 0% or more than 0%.
• the strength of the steel material becomes too high, and the SSC resistance and SOHIC resistance of the steel material may decrease.
• the Cu content is 0 to 0.500%.
• the upper limit of Cu content is preferably 0.450%, and more preferably 0.400%.
• Cu is an element that solid-dissolves in steel to increase the strength of the steel.
• the lower limit of Cu content is preferably more than 0%, more preferably 0.010%, more preferably 0.020%, and even more preferably 0.030%.
• Ni from 0 to 0.500%.
• Ni is an optional element.
• the Ni content may be 0% or more than 0%.
• the strength of a steel material becomes too high, and the SSC resistance and the SOHIC resistance of the steel material may decrease, and the electric resistance weldability of the steel material may also decrease.
• the Ni content is from 0 to 0.500%.
• the upper limit of the Ni content is preferably 0.450%, and more preferably 0.400%.
• Ni is an element that solid-dissolves in a steel material to increase the strength of the steel material.
• the lower limit of the Ni content is preferably more than 0%, more preferably 0.010%, still more preferably 0.050%, and still more preferably 0.100%.
• B (boron) is an optional element.
• the B content may be 0% or more than 0%.
• the B content is from 0 to 0.0020%.
• the upper limit of the B content is preferably 0.0015%, and more preferably 0.0012%.
• B is an element that solid-dissolves in a steel material to increase the strength of the steel material.
• the lower limit of the B content is preferably more than 0%, more preferably 0.0001%, still more preferably 0.0002%, and still more preferably 0.0003%.
• V from 0 to 0.100%.
• V vanadium
• the V content may be 0% or more than 0%.
• the low temperature toughness of the steel may decrease.
• the V content is from 0 to 0.100%.
• the upper limit of the V content is preferably 0.090%, and more preferably 0.080%.
• V is an element that forms a carbonitride or the like and increases the strength of a steel material.
• the lower limit of the V content is preferably more than 0%, more preferably 0.001%, still more preferably 0.005%, and still more preferably 0.010%.
• the Cr content may be 0% or more than 0%.
• the Cr content is from 0 to 0.500%.
• the upper limit of the Cr content is preferably 0.450%, and more preferably 0.400%.
• Cr is an element that increases the strength of a steel material by forming a carbide.
• the lower limit of the Cr content is preferably more than 0%, more preferably 0.010%, still more preferably 0.050%, and still more preferably 0.100%.
• Mo is an optional element.
• the Mo content may be 0% or more than 0%.
• the Mo content is from 0 to 0.500%.
• the upper limit of the Mo content is preferably 0.450%, and more preferably from 0.5 to 0.400%.
• Mo is an element that increases the strength of steel by forming a carbide.
• the lower limit of the Mo content is preferably more than 0%, and more preferably 0.010%, still more preferably 0.050%, and still more preferably 0.100%.
• W is an optional element.
• the W content may be 0% or more than 0%.
• the W content is from 0 to 0.500%.
• the upper limit of the W content is preferably 0.450%, and more preferably 0.400%.
• W is an element that increases the strength of a steel material. Furthermore, W is also an element that forms a protective corrosion coating in a hydrogen sulfide environment, inhibits hydrogen ingress, and thus increases the SSC resistance and the SOHIC resistance of the steel material. From the viewpoint of obtaining these effects, the lower limit of the W content is preferably more than 0%, more preferably 0.001%, still more preferably 0.050%, and still more preferably 0.100%.
• the Zr content may be 0%, or more than 0%.
• the Zr content is from 0 to 0.0500%.
• the upper limit of the Zr content is preferably 0.0400%, and more preferably 0.0300%.
• Zr is an element that refines a sulfide in a steel material and improves the SSC resistance and the SOHIC resistance of the steel material.
• the lower limit of the Zr content is preferably more than 0%, more preferably 0.0001%, still more preferably 0.0003%, and still more preferably 0.0005%.
• Ta from 0 to 0.0500%.
• Ta is an optional element.
• the Ta content may be 0%, or more than 0%.
• the Ta content is from 0 to 0.0500%.
• the upper limit of the Ta content is preferably 0.0400%, and more preferably 0.0300%.
• Ta is an element that refines a sulfide in a steel material and improves the SSC resistance and the SOHIC resistance of the steel material.
• the lower limit of the Ta content is preferably more than 0%, more preferably 0.0001%, still more preferably 0.0003%, and still more preferably 0.0005%.
• Mg from 0 to 0.0050%.
• Mg is an optional element.
• the Mg content may be 0%, or more than 0%.
• the Mg content is from 0 to 0.0050%.
• the upper limit of the Mg content is preferably 0.0045%, and more preferably 0.0040%.
• Mg is an element that detoxifies S in a steel material as a sulfide and increases the SSC resistance and the SOHIC resistance of the steel material.
• the lower limit of the Mg content is preferably more than 0%, more preferably 0.0001%, still more preferably 0.0003%, and still more preferably 0.0005%.
• An REM is an optional element.
• the REM content may be 0%, or more than 0%.
• the REM means a rare earth element, or at least one element selected from the group consisting of Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu.
• the REM content means the total content of rare earth elements.
• the REM content is from 0 to 0.0050%.
• the REM is an element that controls the shape of a sulfide in a steel material to improve the SSC resistance and the SOHIC resistance of the steel material.
• the REM bonds with P in a steel material to suppress segregation of P at the grain boundary, thereby suppressing decrease in low temperature toughness of the steel material caused by the segregation of P.
• the lower limit of the REM content is preferably more than 0%, more preferably 0.0001%, still more preferably 0.0003%, and still more preferably 0.0005%.
• Hf from 0 to 0.0050%.
• Hf is an optional element.
• the Hf content may be 0%, or more than 0%.
• the Hf content is from 0 to 0.0050%.
• the upper limit of the Hf content is preferably 0.0045%, and more preferably 0.0040%.
• Hf is an element that controls the shape of a sulfide in a steel material to enhance the SSC resistance and the SOHIC resistance of the steel material.
• the lower limit of the Hf content is preferably more than 0%, more preferably 0.0001%, still more preferably 0.0003%, and still more preferably 0.0005%.
• Re is an optional element.
• the Re content may be 0%, or more than 0%.
• the Re content is from 0 to 0.0050%.
• the upper limit of the Re content is preferably 0.0045%, and more preferably 0.0040%.
• Re is an element that controls the shape of a sulfide in a steel material and enhances the SSC resistance and the SOHIC resistance of the steel material.
• the lower limit of the Re content is preferably more than 0%, more preferably 0.0001%, still more preferably 0.0003%, and still more preferably 0.0005%.
• the balance excluding the respective elements described above is Fe and impurities.
• impurities refers to components which are contained in raw materials (such as ores and scraps), or components which are mixed-in during production steps, and are not intentionally incorporated into the steel.
• Examples of the impurities include all elements other than the elements described above.
• impurities Only one kind, or two or more kinds of elements may be contained as the impurities.
• impurities include Sb, Sn, Co, As, Pb, Bi, and H (hydrogen).
• any of Sb, Sn, Co and As can be contained, for example, in a content of 0.1% or less, any of Pb and Bi can be contained, for example, in a content of 0.005% or less, and H can be contained, for example, in a content of 0.0004% or less, as the impurities.
• the contents of other elements need not be particularly controlled, as long as the contents are within usual ranges.
• the chemical composition of the base metal portion may include one kind, or two or more kinds selected from the group consisting of: more than 0% but equal to or less than 0.500% of Cu, more than 0% but equal to or less than 0.500% of Ni, more than 0% but equal to or less than 0.0020% of B, more than 0% but equal to or less than 0.100% of V, more than 0% but equal to or less than 0.500% of Cr, more than 0% but equal to or less than 0.500% of Mo, more than 0% but equal to or less than 0.500% of W, more than 0% but equal to or less than 0.0500% of Zr, more than 0% but equal to or less than 0.0500% of Ta, more than 0% but equal to or less than 0.0050% of Mg, more than 0% but equal to or less than 0.0050% of REM, more than 0% but equal to or less than 0.0050% of Hf, and more than 0% but equal to or less than 0% but equal to or less than
• the electric resistance welded steel pipe of the present disclosure have a polygonal ferrite fraction (hereinafter, also referred to as "F fraction") of from 80% to less than 100% in the metallographic structure of the inner surface layer of a base metal portion, and the balance contains degenerated-pearlite and cementite.
• F fraction polygonal ferrite fraction
• the above-described metallographic microstructure of the inner surface layer of a base metal portion is a metallographic microstructure that is a prerequisite for obtaining an effect of sour resistance (the SSC resistance and the SOHIC resistance in detail) due to the requirements of the ⁇ YS and the ⁇ Hv under the restriction that the chemical composition of the base metal portion is the chemical composition A described above, and the tensile strength of the base metal portion is in a range of from 515 to 650 MPa.
• the F fraction in the metallographic microstructure of the inner surface layer of a base metal portion is less than 80%, the strength of the base metal portion may become too high, the ⁇ YS and the ⁇ Hv may become too large, and as a result, the SSC resistance and the SOHIC resistance of a steel material may decrease. Therefore, the F fraction in the metallographic structure of the inner surface layer of a base metal portion is 80% or more, preferably 83% or more, more preferably 85% or more, and still more preferably 90% or more.
• the F fraction in the metallographic microstructure of the inner surface layer of a base metal portion is 100%, the tensile strength and the yield strength of the base metal portion may become too low. Therefore, the F fraction in the metallographic microstructure of the inner surface layer of a base metal portion is less than 100%, preferably 98% or less, and still preferably 95% or less.
• the balance excluding polygonal ferrite contains degenerated-pearlite and cementite. This facilitates adjustment of the tensile strength of the base metal portion to within the range described above.
• the balance is preferably composed of degenerated-pearlite and cementite.
• measurement of the F fraction and confirmation of the type of the balance in the metallographic microstructure of the inner surface layer of a base metal portion are performed as follows.
• the L-section (namely, a section parallel to the pipe axial direction and the thickness direction of an electric resistance welded steel pipe) of a base metal 180° position (namely, a position 180° away from the electric resistance welded portion in a pipe circumferential direction) in an electric resistance welded steel pipe is subjected to nital etching.
• a micrograph of the metallographic microstructure of the electric resistance welded steel pipe at a depth of 1 mm from the inner surface of the pipe in the L-section after nital etching is taken using a scanning electron microscope (SEM) at a magnification of 200 times.
• SEM scanning electron microscope
• Image processing is performed using, for example, LUZEX AP, a compact general-purpose image analyzer manufactured by NIRECO CORPORATION.
• the tensile strength (TS) of a base metal portion is from 515 to 650 MPa.
• the TS of the base metal portion means the tensile strength in the pipe axial direction.
• the TS of a base metal portion is 515 MPa or higher, it is easier to meet a strength required for an electric resistance welded steel pipe for a linepipe.
• the TS of a base metal portion is preferably 520 MPa or higher, and more preferably 530 MPa or higher.
• the SSC resistance and the SOHIC resistance of the base metal portion are improved.
• the TS of the base metal portion is preferably 640 MPa or less, and more preferably 630 MPa or less.
• the yield strength (YS) of a base metal portion is 410 MPa or higher.
• the YS of a base metal portion means the yield strength in the pipe axial direction.
• the YS of a base metal portion is 410 MPa or higher, a strength required for electric resistance welded steel pipes for a linepipe is more easily satisfied.
• the YS of a base metal portion is preferably 430 MPa or higher, and more preferably 450 MPa or higher.
• the YS of a base metal portion is preferably 630 MPa or less.
• the SSC resistance and the SOHIC resistance of the base metal portion are advantageous.
• the bending deformation or the buckling control is advantageous when laying a pipeline formed using an electric resistance welded steel pipe for a linepipe.
• the YS of the base metal portion is more preferably 620 MPa or less, still more preferably 610 MPa or less, and still more preferably 600 MPa or less.
• the TS and the YS of a base metal portion are measured in accordance with ASTM E8 (2013) by the following method.
• a round bar test piece with a diameter of 6 mm and a parallel length of 35 mm is obtained from the center of the thickness of an electric resistance welded steel pipe at the base metal 180° position, with the longitudinal direction of a parallel portion of the round bar test piece parallel to the pipe axis direction of the electric resistance welded steel pipe.
• a tensile test (namely, a tensile test in the direction of the pipe axis) is performed at room temperature (25°C) in air, in accordance with ASTM E8 (2013).
• the maximum stress during uniform elongation in the above-described tensile test is defined as the TS (MPa) of the base metal portion.
• the 0.2% proof stress in the above-described tensile test is defined as the YS (MPa) of the base metal portion.
• a pipe axial direction tensile test for example, the tensile test described above.
• substantially no yield elongation is observed means that the yield elongation is less than 1%.
• an as-rolled electric resistance welded steel pipe means an electric resistance welded steel pipe which has not been subjected to any heat treatment other than the seam heat treatment, after pipe-making.
• the YR of a base metal portion means the yield ratio in the pipe axial direction.
• the YR of the base metal portion is preferably 0.97 or less, and more preferably 0.96 or less.
• Examples of the lower limit of the YR of a base metal portion include 0.85 and 0.86.
• ⁇ YS (namely, a value obtained by subtracting a yield strength of the electric resistance welded portion from a yield strength of the base metal portion) is from 0 to 80 MPa.
• the ⁇ YS is 80 MPa or less, and preferably 70 MPa or less.
• the yield strength of an electric resistance welded portion has conventionally been considered to be higher than that of a base metal portion.
• the yield strength of the electric resistance welded portion is equal to or lower than that of a base metal portion (specifically, the ⁇ YS is 0 MPa or higher).
• a method of measuring the yield strength (YS) of a base metal portion to calculate the ⁇ YS is as described above.
• the yield strength (YS) of an electric resistance welded portion for calculating the ⁇ YS is measured in accordance with ASTM E8 (2013) by the following method.
• a round bar test piece with a diameter of 6 mm and a length of parallel portion of 35 mm is obtained from an area including an electric resistance welded portion at the inner surface side of an electric resistance welded steel pipe in such a manner that the length direction of the parallel portion of the round bar test piece is parallel to the circumferential direction of the electric resistance welded steel pipe. More precisely, the above-described round bar test piece is obtained in such a manner that the center in the longitudinal direction of the parallel portion of the above-described round bar test piece and the electric resistance welding abutting line of the electric resistance welded steel pipe are approximately aligned.
• “approximately aligned” means that the center in the longitudinal direction of the parallel portion and the electric resistance welding abutting line of the electric resistance welded steel pipe are perfectly aligned, or that the gap therebetween is 1 mm or less.
• a tensile test (namely, circumferential tensile test) is performed at room temperature (25°C) in air, in accordance with ASTM E8 (2013).
• the 0.2% proof stress in the above-described tensile test is defined as the YS (MPa) of the electric resistance welded portion.
• the ⁇ Hv (namely, the value obtained by subtracting the Vickers hardness of the inner surface layer of an electric resistance welded portion from the Vickers hardness of the outer surface layer of the electric resistance welded portion) is from 0 to 25Hv.
• the SOHIC may occur at an electric resistance welded portion when the ⁇ Hv is more than 25 Hv. Therefore, the ⁇ Hv is 25 Hv or less, preferably 20 Hv, more preferably 18 Hv, and still more preferably 15 Hv.
• the Vickers hardness of the outer surface layer of an electric resistance welded portion is determined as follows.
• the Vickers hardness of the inner surface layer of an electric resistance welded portion is determined as follows.
• five points at a pitch of 0.5 mm are specified as measuring points on a line corresponding to a depth of 1 mm from the inner surface of the electric resistance welded steel pipe, and within a circumferential range of 2 mm centered on the electric resistance welding abutting line.
• Vickers hardness is measured under a load of 100 gf in accordance with JIS Z 2244 (2009).
• the arithmetic mean of the measured values at the five measuring points is defined as "Vickers hardness of the inner surface layer of an electric resistance welded portion".
• the ⁇ Hv is obtained by subtracting the "Vickers hardness of the inner surface layer of an electric resistance welded portion" obtained as described above from the “Vickers hardness of the outer surface layer of an electric resistance welded portion” obtained as described above.
• the requirements of ⁇ YS and the ⁇ Hv ensure excellent sour resistance (in detail, the SSC resistance and the SOHIC resistance. The same applies hereafter) at the electric resistance welded portion, even when the thickness is 13 mm or more. In other words, when the thickness is 13 mm or more, an improvement in sour resistance due to the requirements of the ⁇ YS and the ⁇ Hv is more effective.
• the thickness of the electric resistance welded steel pipe of the present disclosure is preferably 13 mm or more, and more preferably 14 mm or more, from the viewpoint that improvement in sour resistance due to requirements of the ⁇ YS and the ⁇ Hv is highly effective.
• the thickness of the electric resistance welded steel pipe of the present disclosure is preferably 25 mm or less, more preferably 23 mm or less, and still more preferably 20 mm or less, from the viewpoint of further suppressing occurrence of SSC in an electric resistance welded portion due to a large thickness.
• the outer diameter of the electric resistance welded steel pipe of the present disclosure is not particularly limited, and is preferably from 300 to 650 mm.
• the outer diameter is 300 mm or more, performance required for a linepipe, namely, oil or gas transfer efficiency, is excellent.
• the outer diameter is more preferably 330 mm or more, and still more preferably 350 mm or more.
• the electric resistance welded steel pipe is more suitable for manufacturing.
• the outer diameter is preferably 630 mm or less, and more preferably 610 mm or less.
• the SSC resistance and the SOHIC resistance of an electric resistance welded steel pipe can be evaluated by a four-point bending test.
• NACE solution A a mixed aqueous solution of 5.0% by mass sodium chloride and 0.5% by mass acetic acid
• a rectangular test piece of 120 mm long, 10 mm wide, and 2 mm thick is obtained from an area including an electric resistance welded portion at the inner surface side of an electric resistance welded steel pipe.
• the above-described rectangular test piece is obtained in a direction where the length direction of the test piece is in the pipe circumferential direction of the electric resistance welded steel pipe, the width direction of the test piece is in the pipe axial direction of the electric resistance welded steel pipe, and the thickness direction of the test piece is in the thickness direction of the electric resistance welded steel pipe.
• the test sample is obtained in such a manner that the center of the above-described rectangular test piece in the longitudinal direction and the electric resistance welding abutting line of the electric resistance welded steel pipe are approximately aligned.
• approximately aligned means that the center of the above-described rectangular test piece in the longitudinal direction and the electric resistance welding abutting line of the electric resistance welded steel pipe are perfectly aligned, or that a gap between them is within 1 mm.
• a four-point bending stress is applied to the obtained rectangular test piece using a four-point bending test jig in accordance with ASTM G39-99 (2011).
• This 4-point bending stress is applied with the distance between support points (namely, the distance between the outer support points) as 100 mm and the distance between load points (namely, the distance between the inner support points) as 40 mm.
• the 4-point bending stress applied to the test piece is a stress equivalent to 90% of the YS of the base metal portion in the axial direction of the pipe.
• test piece under 4-point bending stress is sealed in a test container with the 4-point bending test jig.
• the test bath described above is injected, leaving the gas phase portion, and the test piece is immersed in the test bath.
• the test bath is saturated with H 2 S gas by continuously ventilating the test bath with 1 atm H 2 S gas and agitating the test bath.
• the test bath in which the test piece is immersed is maintained at 24°C for 720 hours, and then the test piece is removed.
• the removed test piece is observed to determine whether SSC and SOHIC have occurred or not.
• test piece When neither SSC nor SOHIC occurs, the test piece can be judged to have excellent SSC resistance and SOHIC resistance.
• SSC and SOHIC are distinguished by the shape of a crack.
• a crack that extends in both the axial direction and the thickness direction is considered SOHIC, and a crack that extends in the thickness direction but not in the axial direction is considered SSC.
• production method A One example of the method of producing the electric resistance welded steel pipe according to the present disclosure (hereinafter, referred to as "production method A”) will now be described.
• the following production method A is a method of producing the electric resistance welded steel pipes of Examples to be described below.
• the production method A includes:
• a raw pipe means an electric resistance welded steel pipe before a seam heat treatment is applied to an electric resistance welded portion.
• the production method A enables to produce the electric resistance welded steel pipe according to the present disclosure.
• the slab preparation step in the production method A is a step of preparing a slab having the above described chemical composition.
• the step of preparing a slab may be a step of producing a slab, or may be a step of simply preparing a slab which has been produced in advance.
• a molten steel having the above described chemical composition is produced, and the thus produced molten steel is used to produce the slab.
• the slab may be produced by a continuous casting method, or alternatively, the slab may be produced by forming an ingot using the molten steel, and subjecting the ingot to blooming.
• the hot rolling step in the production method A is a step of heating the slab prepared above, and hot rolling the heated slab to obtain a hot-rolled steel sheet.
• the slab is heated at a slab heating temperature of from 1,100°C to 1,250°C.
• the heating temperature is 1,100°C or higher, grain refinement during hot rolling and precipitation strengthening after hot rolling are more easily progressed, and as a result, the strength of a steel is more easily improved.
• the heating temperature is 1,250°C or lower, coarsening of austenite grains can be more easily suppressed, which makes it easier to refine crystal grains, and as a result, the strength of a steel can be more easily improved.
• a slab is heated, for example, by a furnace.
• the slab heating temperature means the temperature of the outer surface of a slab.
• the slab heated as described above is hot-rolled to obtain a hot-rolled steel sheet.
• Hot rolling is preferably performed under conditions where a finish rolling finishing temperature (hereinafter, also referred to as “finish rolling temperature”) is from 780 to 930°C.
• finish rolling temperature a finish rolling finishing temperature
• Hot rolling is generally performed using a roughing mill and a finishing mill. Both the roughing mill and the finishing mill are generally equipped with a number of rolling stands in a row, each of which is equipped with a roll pair.
• the finish rolling temperature (namely, finish rolling finishing temperature) is the surface temperature of a hot-rolled steel sheet at the exit of the final stand of the finish rolling mill.
• the finish rolling temperature is 780°C or higher, the rolling resistance of steel sheets can be reduced, resulting in higher productivity.
• finish rolling temperature is 780°C or higher, a phenomenon of rolling in the two-phase region of ferrite and austenite is suppressed, and formation of a layered structure and decrease in mechanical properties associated with this phenomenon can be suppressed.
• the water cooling step of the hot-rolled steel sheet is a step in which the hot-rolled steel sheet is cooled with water until the temperature of the outer surface of the hot-rolled steel sheet reaches a coiling temperature of from 450 to 625°C.
• the coiling temperature (namely, the temperature at which the cooling of the outer surface of the hot-rolled steel sheet ends) is 450°C or higher, the productivity of the product is ensured.
• the coiling temperature is preferably 500°C or higher.
• the coiling temperature is 625°C or lower, coarsening of crystal grains can be more suppressed, thereby increasing the strength of the hot-rolled steel sheet.
• the coiling temperature is preferably 600°C or lower.
• the pipe-making step is a step in which the hot-rolled steel sheet is uncoiled from the hot coil, the uncoiled hot-rolled steel sheet is roll-formed to prepare an open pipe, and the abutting portion in the thus prepared open pipe is subjected to electric resistance welding to form an electric resistance welded portion, thereby obtaining a raw pipe (namely, the electric resistance welded steel pipe before seam heat treatment is applied to the electric resistance welded portion).
• the seam heat treatment step in the production method A is a step of performing a seam heat treatment on the electric resistance welded portion of the raw pipe (namely, the electric resistance welded steel pipe before the electric resistance welded portion is subjected to the seam heat treatment).
• the seam heat treatment in the production method A is a treatment in which an electric resistance welded portion of a raw pipe is heated to a heating temperature of from 900 to 1,000°C, soaked at the above-described heating temperature for at least 1 second, and then water cooled at a cooling rate of from 5 to 20°C/second to a cooling stop temperature of from 300 to 580°C.
• the electric resistance welded portion After water cooling, the electric resistance welded portion is air cooled until the temperature of the electric resistance welded portion reaches room temperature.
• the seam heat treatment in the production method A is carried out by performing heating and cooling, in this order, on the electric resistance welded portion before being subjected to the seam heat treatment, from the outer surface side of the electric resistance welded portion.
• the seam heat treatment in the production method A is carried out as follows.
• the electric resistance welded portion before being subjected to the seam heat treatment is heated, by induction heating, from the outer surface side until the temperature of the outer surface reaches a heating temperature within the range of from 900 to 1,000°C, and soaked for a soaking time in the range of 1 second or more (preferably from 1 to 5 seconds), in a state in which the temperature of the outer surface is kept at the heating temperature in the above-described range.
• the electric resistance welded portion after soaking is water cooled from the outer surface side at a cooling rate in the range of from 5 to 20°C/second to a cooling stop temperature in the range of from 300 to 580°C.
• examples of means to achieve a cooling rate in the range of from 5 to 20°C/second include misting water cooling shower, adjusting the flow rate of water cooling shower, and adjusting the angle of water cooling shower.
• the heating temperature refers to the temperature of the outer surface of an electric resistance welded portion
• the cooling rate refers to the cooling rate on the outer surface of the electric resistance welded portion
• the cooling stop temperature refers to the recuperative temperature measured on the outer surface of an electric resistance welded portion after water cooling has been stopped, and is the highest temperature measured within 1 minute after water cooling has been stopped.
• the heating temperature in the seam heat treatment step of the production method A is 900°C or higher.
• the heating temperature in the seam heat treatment step in the production method A is 1000°C or less.
• the cooling stop temperature in the seam heat treatment step of the production method A is 300°C or higher.
• the cooling stop temperature in the seam heat treatment step of the production method A is 580°C or lower.
• the cooling rate in the seam heat treatment step of production method A is 5°C/second or higher.
• the cooling rate in the seam heat treatment step of the production method A is 20°C/second or less.
• the production method A may include other steps other than the steps described above.
• Examples of the other steps include a step of adjusting the shape of the electric resistance welded steel pipe using sizing rolls, after the seam heat treatment step.
• Test Nos. 1 to 22 are Examples, and Test Nos. 23 to 43 are Comparative Examples.
• the above-described slab is heated in a furnace, the heated slab is hot-rolled using a plurality of hot-rolling mills to produce a hot-rolled steel sheet, the resulting hot-rolled steel sheet is water-cooled, and the water-cooled hot-rolled steel sheet is coiled to obtain a hot coil composed of the hot-rolled steel sheet.
• the heating temperature when heating the slab was 1,200°C
• the finish rolling temperature in hot rolling was set at from 790°C to 930°C
• the coiling temperature was set at from 500°C to 600°C.
• Each hot-rolled steel sheet was uncoiled from the hot coil, the uncoiled hot-rolled steel sheet was roll-formed to prepare an open pipe, and the abutting portion in the thus prepared open pipe was subjected to electric resistance welding to form an electric resistance welded portion, thereby obtaining a raw pipe.
• the soaking time in the seam heat treatment was adjusted to be from 1 second to 5 seconds.
• the soaking time was controlled by adjusting the timing between the end of heating and the start of showering.
• the cooling rate in the seam thermal treatment was controlled by tuning a water-cooling shower into mist, and adjusting the flow rate of the water-cooling shower and/or the angle of the water-cooling shower.
• the cooling stop temperature in the seam thermal treatment was controlled by adjusting the timing of stopping the shower.
• the F fraction (namely, polygonal ferrite fraction) in the inner surface layer of the base metal portion was measured and the type of the balance was confirmed by the above-described method.
• P + C means degenerated-pearlite and cementite.
• the obtained electric resistance welded steel pipes were evaluated for the SSC resistance and the SOHIC resistance of the electric resistance welded portions (namely, the presence or absence of SSC and SOHIC after a four-point bending test) by the above-described method.
• the electric resistance welded steel pipe of Test No. 23 had an appropriate chemical composition (namely, the above-described chemical composition A) in the base metal portion, the heating temperature in the seam heat treatment step was too high. As a result, the YS of the electric resistance welded portion became too high, resulting in a ⁇ YS of less than 0 MPa. Furthermore, the Vickers hardness of the outer surface layer of the electric resistance welded portion became too hard, and ⁇ Hv exceeded 25Hv. As a result, SSC and SOHIC were confirmed in the four-point bending test. In other words, excellent SSC resistance and SOHIC resistance were not obtained.
• an appropriate chemical composition namely, the above-described chemical composition A
• the electric resistance welded steel pipe of Test No. 24 had an appropriate chemical composition (namely, the above-described chemical composition A) in the base metal portion, the heating temperature in the seam heat treatment step was too low. As a result, the YS of the electric resistance welded portion became too low, resulting in a ⁇ YS of more than 80 MPa. Furthermore, the Vickers hardness of the inner surface layer of the electric resistance welded portion was insufficient, and ⁇ Hv exceeded 25Hv. As a result, SSC and SOHIC were confirmed in the four-point bending test. In other words, excellent SSC resistance and SOHIC resistance were not obtained.
• an appropriate chemical composition namely, the above-described chemical composition A
• the electric resistance welded steel pipe of Test No. 25 had an appropriate chemical composition (namely, the above-described chemical composition A) in the base metal portion, the cooling rate in the seam heat treatment step was too high. As a result, the Vickers hardness of the outer surface layer became too hard, and ⁇ Hv exceeded 25Hv. As a result, SOHIC was confirmed in the four-point bending test. In other words, excellent SOHIC resistance were not obtained.
• the electric resistance welded steel pipe of Test No. 26 had an appropriate chemical composition (namely, the above-described chemical composition A) in the base metal portion, the cooling rate in the seam heat treatment step was too low. As a result, the YS of the electric resistance welded portion became too low, resulting in a ⁇ YS of more than 80 MPa. Furthermore, the Vickers hardness of the inner surface layer of the electric resistance welded portion was insufficient, and ⁇ Hv exceeded 25Hv. As a result, SSC and SOHIC were confirmed in the four-point bending test. In other words, excellent SSC resistance and SOHIC resistance were not obtained.
• an appropriate chemical composition namely, the above-described chemical composition A
• the electric resistance welded steel pipe of Test No. 27 had an appropriate chemical composition (namely, the above-described chemical composition A) in the base metal portion, the cooling stop temperature in the seam heat treatment step was too high. As a result, the YS of the electric resistance welded portion became too low, resulting in a ⁇ YS of more than 80 MPa. Furthermore, the Vickers hardness of the inner surface layer of the electric resistance welded portion was insufficient, and ⁇ Hv exceeded 25Hv. As a result, SSC and SOHIC were confirmed in the four-point bending test. In other words, excellent SSC resistance and SOHIC resistance were not obtained.
• an appropriate chemical composition namely, the above-described chemical composition A
• the electric resistance welded steel pipe of Test No. 28 had an appropriate chemical composition (namely, the above-described chemical composition A) in the base metal portion, the cooling stop temperature in the seam heat treatment step was too low. As a result, the YS of the electric resistance welded portion became too high, resulting in a ⁇ YS of less than 0 MPa. Furthermore, the Vickers hardness of the outer surface layer of the electric resistance welded portion became too hard, and ⁇ Hv exceeded 25Hv. As a result, SSC was confirmed in the four-point bending test. As a result, SSC and SOHIC were confirmed in the four-point bending test. In other words, excellent SSC resistance and SOHIC resistance were not obtained.
• an appropriate chemical composition namely, the above-described chemical composition A
• the electric resistance welded steel pipe of Test No. 29 had too low a C content in the chemical composition of the base metal portion.
• the YS of the base metal portion was less than 410 MPa and the TS of the base metal portion was less than 515 MPa.
• the electric resistance welded steel pipe of Test No. 30 had too high a C content in the chemical composition of the base metal portion.
• the F fraction in the metallographic microstructure of the inner surface layer of the base metal portion was less than 80%, and the TS of the base metal portion exceeded 650 MPa.
• the YS of the base metal portion became too high, and the ⁇ YS exceeded 80 MPa.
• the Vickers hardness of the outer surface layer of the electric resistance welded portion became too hard, and ⁇ Hv exceeded 25Hv.
• SSC and SOHIC were confirmed in the four-point bending test. In other words, excellent SSC resistance and SOHIC resistance were not obtained.
• the electric resistance welded steel pipe of Test No. 31 had too low Mn content in the chemical composition of the base metal portion.
• the YS of the base metal portion was less than 410 MPa, and the TS of the base metal portion was less than 515 MPa.
• the electric resistance welded steel pipe of Test No. 32 had too high an Mn content in the chemical composition of the base metal portion.
• the TS of the base metal portion exceeded 650 MPa.
• the YS of the base metal portion became too high, and the ⁇ YS exceeded 80 MPa.
• the Vickers hardness of the outer surface layer of the electric resistance welded portion became too hard, and ⁇ Hv exceeded 25Hv.
• SSC and SOHIC were confirmed in the four-point bending test. In other words, excellent SSC resistance and SOHIC resistance were not obtained.
• the electric resistance welded steel pipe of Test No. 33 had too high a P content in the chemical composition of the base metal portion. As a result, SSC and SOHIC were confirmed in the four-point bending test. In other words, excellent SSC and SOHIC resistance were not obtained.
• the electric resistance welded steel pipe of Test No. 34 had too high an S content in the chemical composition of the base metal portion. As a result, SOHIC were confirmed in the four-point bending test. In other words, excellent SOHIC resistance was not obtained.
• the electric resistance welded steel pipe of Test No. 35 had too high an Si content in the chemical composition of the base metal portion. As a result, SSC and SOHIC were confirmed in the four-point bending test. In other words, excellent SSC and SOHIC resistance were not obtained.
• the electric resistance welded steel pipe of Test No. 36 had too low an Nb content in the chemical composition of the base metal portion. As a result, SOHIC were confirmed in the four-point bending test. In other words, excellent SOHIC resistance was not obtained.
• the electric resistance welded steel pipe of Test No. 37 had too high an Nb content in the chemical composition of the base metal portion. As a result, SSC and SOHIC were confirmed in the four-point bending test. In other words, excellent SSC and SOHIC resistance were not obtained.
• the electric resistance welded steel pipe of Test No. 38 had too low a Ti content in the chemical composition of the base metal portion. As a result, SOHIC were confirmed in the four-point bending test. In other words, excellent SOHIC resistance was not obtained.
• the electric resistance welded steel pipe of Test No. 39 had too high a Ti content in the chemical composition of the base metal portion. As a result, SOHIC were confirmed in the four-point bending test. In other words, excellent SOHIC resistance was not obtained.
• the electric resistance welded steel pipe of Test No. 40 had too high a Ca content in the chemical composition of the base metal portion. As a result, SOHIC were confirmed in the four-point bending test. In other words, excellent SOHIC resistance was not obtained.
• the electric resistance welded steel pipe of Test No. 41 had too high an Al content in the chemical composition of the base metal portion. As a result, SOHIC were confirmed in the four-point bending test. In other words, excellent SOHIC resistance was not obtained.
• the electric resistance welded steel pipe of Test No. 42 had too high an N content in the chemical composition of the base metal portion. As a result, SOHIC were confirmed in the four-point bending test. In other words, excellent SOHIC resistance was not obtained.
• the electric resistance welded steel pipe of Test No. 43 had too high an O content in the chemical composition of the base metal portion. As a result, SOHIC was confirmed in the four-point bending test. In other words, excellent SOHIC resistance was not obtained.

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• 2020
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