EP3026140B1 - Tôle d'acier pour tube de canalisation et tube de canalisation - Google Patents

Tôle d'acier pour tube de canalisation et tube de canalisation Download PDF

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Publication number
EP3026140B1
EP3026140B1 EP14829550.4A EP14829550A EP3026140B1 EP 3026140 B1 EP3026140 B1 EP 3026140B1 EP 14829550 A EP14829550 A EP 14829550A EP 3026140 B1 EP3026140 B1 EP 3026140B1
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EP
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Prior art keywords
less
plate thickness
steel plate
content
line pipe
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EP14829550.4A
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German (de)
English (en)
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EP3026140A4 (fr
EP3026140A1 (fr
Inventor
Yasuhiro Shinohara
Takuya Hara
Taishi Fujishiro
Naoki Doi
Naoshi AYUKAWA
Eiichi Yamashita
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Nippon Steel Corp
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Nippon Steel and Sumitomo Metal Corp
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
    • C—CHEMISTRY; METALLURGY
    • 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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/002—Bainite
    • 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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite
    • 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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese

Definitions

  • the present invention relates to a steel plate for a line pipe, and a line pipe.
  • sour resistance is apt to be demanded for a line pipe or a steel plate for a line pipe as a material for a line pipe.
  • the sour resistance means hydrogen-induced cracking resistance (HIC resistance) and sulfide stress cracking resistance (SSC resistance) in a corrosive environment containing hydrogen sulfide.
  • a submarine line pipe laying in the deep sea beyond the water depth of 2000 m has been tried. In the deep sea a line pipe is easily collapsed by the water pressure. Therefore, for a submarine line pipe, a steel pipe having generally a wall thickness of 25 mm or more, and having a high compressive strength in the circumferential direction may be demanded.
  • a favorable toughness evaluation result especially the same by a drop weight tear test (DWTT) (this toughness evaluation result is hereinafter also referred to as "DWTT property") may not be secure easily due to insufficient rolling reduction in a recrystallization region and a non-recrystallization region.
  • DWTT drop weight tear test
  • the invention was made under such circumstances with an object to provide a steel plate for a line pipe which is superior in HIC resistance (especially HIC. resistance in a sour environment of pH 5.0 or higher) and satisfies both compressive strength and DWTT property, as well as a line pipe produced using the steel plate for a line pipe.
  • a steel plate for a line pipe superior in HIC resistance especially HIC resistance in a sour environment of pH 5.0 or higher
  • satisfies both compressive strength and DWTT property as well as a line pipe produced with the steel plate for a line pipe
  • Fig. 1 is an optical micrograph (magnification 500-fold) of a cross-section of a steel plate of Inventive Example 10 at a position of 1/2 of the plate thickness (cross-section after polishing and corrosion with a LePera reagent).
  • 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.
  • a "position of 1/2 of the plate thickness” herein means a position corresponding to 1/2 of the plate thickness of a steel plate, namely a center part in a thickness direction of a steel plate.
  • a "position of 1/4 of the plate thickness” herein means a position that is apart from the center part in a thickness direction of a steel plate (position of 1/2 of the plate thickness) by a distance in the direction of the plate thickness equivalent to 1/4 of the plate thickness.
  • C carbon
  • C content Another element may be expressed similarly.
  • a steel plate for a line pipe according to the invention is a steel plate for a line pipe, the steel plate having a plate thickness of 25 mm or more and containing in terms of mass%: 0.040 to 0.080% of C, 0.05 to 0.40% of Si, 1.60 to 2.00% of Mn, 0.020% or less of P, 0.0025% or less of S, 0.05 to 0.20% of Mo, 0.0011 to 0.0050% of Ca, 0.060% or less of Al, 0.010 to 0.030% of Nb, 0.008 to 0.020% of Ti, 0.0015 to 0.0060% of N, and 0.0040% or less of O, wherein a content ratio of Ca to S [Ca/S] is from 0.90 to 2.70, and a content ratio of Ti to N [Ti/N] is 2.20 or higher, a remainder consisting of Fe and unavoidable impurities, wherein Ceq, which is defined by the following Formula (1), is from 0.
  • the steel plate according to the invention can improve HIC resistance (especially HIC resistance in a sour environment of pH 5.0 or higher) and satisfy compressive strength and DWTT property owing to the above constitution.
  • the invention was made based on the following investigation results.
  • the inventors investigated conditions, which a steel material should fulfill in order to inhibit occurrence of hydrogen-induced cracking (HIC) in a sour environment of pH 5.0 or higher, using various steel plates with different compositions.
  • HIC hydrogen-induced cracking
  • the sour resistance was evaluated according to the invention by examining occurrence or nonoccurrence of HIC, and a HIC crack area ratio (hereinafter referred to as "CAR in HIC test").
  • the evaluation was conducted by immersing a steel plate in a pH 5.0-solution saturated with a hydrogen sulfide gas (for example, "Solution B” according to NACE TM0284) and examining a HIC crack area ratio (CAR in HIC test) after 96 hours. When a HIC crack area ratio is 5% or less, the sour resistance was rated as good.
  • a hydrogen sulfide gas for example, "Solution B” according to NACE TM0284
  • elongated MnS elongated MnS present at a position of 1/2 of the plate thickness (hereinafter referred to as "elongated MnS", or also simply as “MnS "), and that a length of the elongated MnS exceeds 1.00 mm.
  • the S content should be 0.0025% or less, and the content ratio [Ca/S] should be from 0.90 to 2.70.
  • the inventors found that in a case in which the content ratio [Ca/S] is less than 0.90, the length of MnS may not be able to be controlled to 1.00 mm or less. Further, the inventors found that in a case in which the content ratio [Ca/S] is beyond 2.70 a coarse aggregate of Ca-based oxides is formed and HIC may occasionally occur originating from the aggregate.
  • HIC in a sour environment of pH 5.0 or higher can be suppressed by making the hardness of a steel plate at the position of 1/2 of the plate thickness to 400 Hv or less.
  • the inventors investigated in detail a relationship at the position of 1/2 of the plate thickness between the hardness and the ferrite fraction.
  • the inventors found that in a case in which the ferrite fraction of a structure at the position of 1/2 of the plate thickness is higher than 60%, the hardness of the steel plate may exceed 400 Hv. This is presumably because, when ferrite is formed at the position of 1/2 of the plate thickness, the C amount is concentrated in the remainder and as the result bainite or martensite with a high C content is formed.
  • the hardness at the position of 1/2 of the plate thickness becomes 400 Hv or less.
  • a center segregation zone means a zone where the Mn concentration is highest, in a case in which the Mn concentration distribution in the thickness direction of the steel plate is measured by an EPMA (Electron Probe Micro Analyzer).
  • a compressive strength is highly correlated with the ferrite fraction (F1), and when the fraction of soft ferrite at the position of 1/4 of the plate thickness becomes higher, the compressive strength decreases.
  • the ferrite fraction (F1) and the ferrite fraction (F2) exceed 60%, the compressive strength decreases remarkably.
  • the steel plate according to the invention shows high compressive strength due to both the ferrite fraction (F1) and the ferrite fraction (F2) being 60% or less.
  • the DWTT property of a steel plate is enhanced, in a case in which a ferrite fraction of the steel plate becomes higher. It was found that the ferrite fraction (F1) of the steel plate is required to be 20% or higher, and the ferrite fraction (F2) thereof is required to be 5% or higher in order to exert such an effect.
  • the steel plate according to the invention satisfies both compressive strength and DWTT property due to the ratio (F1/F2) being 1.00 or higher.
  • the ratio (F1/F2) is less than 1.00, especially the DWTT property deteriorates (for example, refer to Comparative Example 6 below).
  • the ratio (F1/F2) is decided to be 1.00 or higher in the invention.
  • the ratio (F1/F2) was decided to be 5.00 or less in the invention.
  • the ratio (F1/F2) of an ordinary steel plate is less than 1.00 due to the following reason.
  • the cooling rate in a cooling process after rolling for obtaining a steel plate is ordinarily slowest at the position of 1/2 of the plate thickness (center part in the thickness direction of the plate). Therefore, in an ordinary steel plate the ferrite fraction is highest at the position of 1/2 of the plate thickness in the plate thickness direction. Consequently, in an ordinary steel plate, the ratio (F1/F2) is less than 1.00 (for example, refer to Comparative Example 6 below).
  • the inventors succeeded in making the ratio (F1/F2) to 1.00 or higher, by making a cooling rate (V1) at the position of 1/4 of the plate thickness slower than a cooling rate (V2) at the position of 1/2 of the plate thickness in a temperature range between 600 to 700°C, where ferrite is formed.
  • the ratio (F1/F2) of the steel plate according to the invention is required to be from 1.00 to 5.00, and there is no particular restriction on a production method thereof (for example, cooling method after rolling).
  • the remainder at the position of 1/4 of the plate thickness of the steel plate according to the invention is a structure of bainite. As the result, occurrence of HIC is suppressed. In a case in which the remainder at the position of 1/4 of the plate thickness is pearlite, HIC occurs.
  • the remainder at the position of 1/2 of the plate thickness of the steel plate according to the invention is a structure of bainite or a structure of bainite and martensite. As the result, occurrence of HIC is suppressed. In a case in which the remainder at the position of 1/2 of the plate thickness is pearlite, HIC occurs.
  • the steel plate is formed into a steel pipe (line pipe) (pipe making), the steel pipe is then subjected to heating in coating for anti-corrosion, and then the compressive strength in the circumferential direction of the steel pipe is measured for evaluation; or the steel plate is subjected to treatments corresponding to the pipe making and the heating in coating, and then the compressive strength of the steel plate is measured for evaluation as in Examples below.
  • the compressive strength in the circumferential direction of a steel pipe decreases remarkably by a Bauschinger effect due to pipe making, the compressive strength recovers during the heating in coating.
  • the recovery occurs due to a so-called static strain aging, by which C (carbon) diffuses during the heating in coating into a dislocation formed during pipe making to form a Cottrell atmosphere.
  • the inventors diligently investigated alloy elements, which exhibit sufficiently static strain aging, so as to enhance the compressive strength of a steel plate. As the result, it was found that Mo is effective as such an alloy element.
  • the Mo content is set at 0.05% or higher in the invention.
  • the upper limit of the Mo content is preferably 0.20%, because when the Mo content is too high, the hardness at the position of 1/2 of the plate thickness (center part in the thickness direction of the plate) becomes extremely high.
  • the steel plate for a line pipe contains 0.040 to 0.080% of C (carbon), 0.05 to 0.40% of Si (silicon), 1.60 to 2.00% of Mn (manganese), 0.020% or less of P (phosphorus), 0.0025% or less of S (sulfur), 0.05 to 0.20% of Mo (molybdenum), 0.0011 to 0.0050% of Ca (calcium), 0.060% or less of Al (aluminum), 0.010 to 0.030% of Nb (niobium), 0.008 to 0.020% of Ti (titanium), 0.0015 to 0.0060% of N (nitrogen), and 0.0040% or less of O (oxygen); wherein the content ratio of Ca to S [Ca/S] is from 0.90 to 2.70, and the content ratio of Ti to N [Ti/N] is 2.20 or higher; the remainder consists of Fe (iron) and unavoidable impurities; and the Ceq is from 0.380 to 0.480.
  • C carbon
  • Si silicon
  • the C is an element to improve the steel strength. From a viewpoint of such an effect, the lower limit of the C content is 0.040%. Meanwhile, when the C content exceeds 0.080%, generation of a carbide is promoted and the HIC resistance is impaired. Therefore, the upper limit of the C content is set at 0.080%. Further, for suppression of decrease in HIC resistance, weldability, and toughness, a preferable upper limit of the C content is 0.060%.
  • the Si is a deoxidizing element. From a viewpoint of such an effect, the lower limit of the Si content is 0.05%. Meanwhile, when the Si content exceeds 0.40%, the toughness of a heat affected zone (HAZ) (hereinafter also referred to as "HAZ toughness”) decreases. Therefore, the upper limit of the Si content is set at 0.40%.
  • HZ heat affected zone
  • Mn is an element to improve strength and toughness. From a viewpoint of such effects, the lower limit of the Mn content is 1.60%. Meanwhile, when the Mn content exceeds 2.00%, the HAZ toughness decreases. Therefore, the upper limit of the Mn content is set at 2.00%. For suppressing HIC, the Mn content is preferably less than 1.75%.
  • P is an impurity, and when the content exceeds 0.020%, the HIC resistance is impaired, and the HAZ toughness decreases. Therefore, the P content is limited to 0.020% or less.
  • the P content is preferably as low as possible, and there is no particular restriction on the lower limit of the P content.
  • the P content is preferably 0.001 % or higher.
  • S is an element to form MnS elongating during hot rolling in the rolling direction, which decreases the HIC resistance. Therefore, in the invention, it is necessary to reduce the S content, and the S content is limited to 0.0025% or less. Since the S content is preferably as low as possible, and there is no particular restriction on the lower limit of the S content. However, from viewpoints of the production cost for secondary refining and production constraint, the S content may be 0.0008% or higher.
  • Mo is an element to improve hardenability and at the same time to improve strength by forming a carbonitride. Further, in the invention, Mo is contained from a viewpoint of securing a high compressive strength by promoting static strain aging during the heating in coating after making a steel pipe (line pipe), as described above. For obtaining such effects, in the invention, the lower limit of the Mo content is set at 0.05%.
  • the upper limit of the Mo content is set at 0.20%.
  • Ca is an element, which forms a sulfide CaS to suppress formation of MnS elongating in the rolling direction, and contributes remarkably to improvement of the HIC resistance.
  • the Ca content is less than 0.0011% the above effects cannot be obtained, and therefore the lower limit of the Ca content is set at 0.0011 % in the invention.
  • the Ca content exceeds 0.0050%, an oxide accumulates to impair the HIC resistance, and therefore the upper limit of the Ca content is set at 0.0050% or less.
  • Ca is contained in the steel plate to form CaS.
  • S is immobilized. Therefore the content ratio of Ca to S [Ca/S] is an important index in the invention.
  • the content ratio [Ca/S] is less than 0.90, MnS is formed and elongated MnS is formed during rolling. As the result, the HIC resistance is deteriorated.
  • the content ratio [Ca/S] exceeds 2.70, Ca-based oxides aggregate to deteriorate the HIC resistance.
  • the content ratio [Ca/S] is limited to from 0.90 to 2.70 according to the invention.
  • Al is an element contained ordinarily as a deoxidizing element.
  • the upper limit of the Al content is 0.060%.
  • Al is further an element to promote formation of a mixed structure of martensite-austenite (MA).
  • MA martensite-austenite
  • the Al content is preferably 0.008% or less. When the Al content is 0.008% or less, it is advantageous for enhancement of the HAZ toughness.
  • the Al content is preferably 0.0002% or higher.
  • Al is not only contained intentionally in a steel, but may also be mixed into a steel as an impurity.
  • the Al content is preferably as low as possible, and therefore there is no particular restriction on the lower limit of the Al content.
  • Nb is an element to form a carbide or a nitride contributing to improvement of the strength.
  • the Nb content is 0.010% or higher in the invention.
  • the Nb content is set at 0.030% or less in the invention. Further, the Nb content is preferably 0.020% or less.
  • Ti is an element, which is utilized ordinarily as a deoxidizing agent or a nitride forming element for micronizing a crystal grain. For obtaining the effect, the Ti content is set at 0.008% or higher according to the invention. However, Ti is also an element to decrease the toughness by forming a coarse carbonitride, when Ti is contained excessively. Therefore, the Ti content is limited to 0.020% or less in the invention.
  • N nitrogen
  • TiN titanium
  • NbN nitrogen
  • the N content is set at 0.0015% or higher in the invention.
  • the upper limit of the N content is set at 0.0060% in the invention.
  • the content ratio of Ti to N [Ti/N] is important.
  • the content ratio [Ti/N] is less than 2.20, sufficient TiN precipitation does not occur, and micronization of austenite cannot be achieved. Therefore, the content ratio [Ti/N] is 2.20 or higher in the invention.
  • the content ratio [Ti/N] is preferably 3.00 or higher.
  • the content ratio [Ti/N] is 5.00 or less, and preferably 4.00 or less.
  • O is an impurity element.
  • the O content is limited to 0.0040% or less in the invention. Since O is preferably as low level as possible, there is no particular restriction on the lower limit of the O content. However, from viewpoints of production cost and production constraint, the O content may be also 0.0010% or higher.
  • C, Mn, Ni, Cu, Cr, Mo, and V represent respectively the contents (mass%) of elements of C (carbon), Mn (manganese), Ni (nickel), Cu (copper), Cr (chromium), Mo (molybdenum), and V (vanadium).
  • Ni, Cu, Cr, and V are optional elements, and each of them may be also 0%. Preferable contents of the optional elements are described below.
  • Ceq defined by the Formula (1) is limited to from 0.380 to 0.480 in the invention.
  • Ceq is less than 0.380, the strength of a line pipe to be obtained by the steel plate in the invention decreases.
  • the line pipe cannot satisfy a required tensile strength (520 MPa or higher) corresponding to the strength grade X60 or higher.
  • Ceq exceeds 0.480, the toughness (for example, DWTT property) and the sour resistance (for example, HIC resistance) deteriorate.
  • Ceq is limited to from 0.380 to 0.480 in the invention.
  • an unavoidable impurity means a component contained in a source material or a component mixed into a steel in a production process, and not a component contained intentionally in a steel.
  • an unavoidable impurity examples include Sb (antimony), Sn (tin), W (tungsten), Co (cobalt), As (arsenic), Pb (lead), Bi (bismuth), B (boron), and H (hydrogen).
  • the steel plate according to the invention may contain one or more of 0.50% or less of Ni (nickel), 0.50% or less of Cr (chromium), 0.50% or less of Cu (copper), 0.0050% or less of Mg (magnesium), 0.0050% or less of REM (rare earth element), and 0.100% or less of V (vanadium).
  • the steel plate according to the invention may contain one or more of 0.50% or less of Ni, 0.50% or less of Cr, and 0.50% or less of Cu. Further, it may contain one or more of 0.0050% or less of Mg, 0.0050% or less of REM, and 0.100% or less of V
  • These elements may be mixed into a steel as unavoidable impurities besides intentional containing in a steel. Therefore, there is no particular restriction on the lower limits of the contents of the elements.
  • Ni nickel is an element effective for improving toughness and strength.
  • the Ni content is preferably 0.50% or less.
  • the Ni content is preferably 0.05% or higher.
  • Cr chromium
  • the Cr content is preferably 0.50% or less.
  • the Cr content is preferably 0.05% or higher.
  • Cu is an element effective for enhancing the strength without decreasing the toughness.
  • the Cu content is preferably 0.50% or less.
  • the Cu content is preferably 0.05% or higher.
  • Mg is an element effective as a deoxidizing agent and a desulfurization agent, and especially an element which contributes also to improvement of the HAZ toughness by generating a fine oxide.
  • the Mg content is preferably 0.0050% or less.
  • the Mg content is preferably 0.0001 % or higher.
  • REM means herein a rare earth element, and a general term for 17 kinds of elements of Sc (scandium), Y (yttrium), La (lanthanum), Ce (cerium), Pr (praseodymium), Nd (neodymium), Pm (promethium), Sm (samarium), Eu (europium), Gd (gadolinium), Tb (terbium), Dy (dysprosium), Ho (holmium), Er (erbium), Tm (thulium), Yb (ytterbium), and Lu (lutetium). Further, "0.0050% or less of REM” means that the total content of the 17 kinds of elements is 0.0050% or less.
  • REM is elements effective as a deoxidizing agent and a desulfurization agent.
  • the REM content is preferably 0.0050% or less.
  • the REM content is preferably 0.0001 % or higher.
  • V is an element to form a carbide or a nitride contributing to enhancement of the strength.
  • the V content is preferably 0.100% or less.
  • the V content is preferably 0.010% or higher.
  • the steel plate according to the invention due to the ferrite fraction (F1) in a structure at the position of 1/4 of the plate thickness being 20% or higher, and the ferrite fraction (F2) in a structure at the position of 1/2 of the plate thickness being 5% or higher, DWTT property is improved. In at least one of a case in which the ferrite fraction (F1) is less than 20%, and a case in which the ferrite fraction (F2) is less than 5%, DWTT property deteriorates.
  • the ferrite fraction (F1) being 60% or less, and the ferrite fraction (F2) is 60% or less, compressive strength is improved.
  • the compressive strength decreases.
  • the ratio (F1/F2) is from 1.00 to 5.00, it is preferably more than 1.00 but 5.00 or less, and more preferably from 1.05 to 5.00.
  • the hardness at the position of 1/2 of the plate thickness is 400 Hv or less, and the length of MnS at the position of 1/2 of the plate thickness is 1.00 mm or less.
  • the HIC resistance is improved. Further, the above is favorable for the DWTT property.
  • the length of MnS at the position of 1/2 of the plate thickness is 1.00 mm or less as described above, the same is more preferably within a range satisfying the following Formula (2) from a viewpoint of improvement of the HIC resistance.
  • Length of MnS at the position of 1 / 2 of the plate thickness ⁇ 10 1350 ⁇ X / 350 / 1000 wherein, in Formula (2), X is a hardness (Hv) at the position of 1/2 of the plate thickness, having a value of 400 (Hv) or less.
  • Examples of a method for making the length of MnS at the position of 1/2 of the plate thickness satisfy the Formula (2) include a method by which a slab with a maximum Mn segregation degree in a center segregation zone of the slab of 2.2 or less and a thickness of the center segregation zone of 1.0 mm or less is subjected successively to processings of reheating, heavy plate rolling (rough rolling and finish rolling), and cooling to produce the steel plate. Preferable embodiments of respective processings will be described below.
  • the average grain diameter of ferrite at the position of 1/4 of the plate thickness is from 2.0 to 15.0 ⁇ m.
  • the average grain diameter of ferrite at the position of 1/4 of the plate thickness is 15.0 ⁇ m or less, the DWTT property is improved.
  • the average grain diameter of ferrite at the position of 1/4 of the plate thickness is 2.0 ⁇ m or more, increase in a rolling load is suppressed, which is advantageous in terms of a production cost.
  • the average grain diameter of ferrite at the position of 1/2 of the plate thickness is from 5.0 to 20.0 ⁇ m.
  • the average grain diameter of ferrite at the position of 1/2 of the plate thickness is 20.0 ⁇ m or less, the DWTT property is improved.
  • the plate thickness of the steel plate according to the invention is 25 mm or more.
  • the plate thickness is preferably beyond 25 mm, more preferably 30 mm or more, further preferably 32 mm or more, and especially preferably 35 mm or more.
  • the plate thickness may be for example 45 mm or less.
  • the steel plate according to the invention can be produced by producing a slab in a steelmaking process by melting followed by continuous casting, and thereafter subjecting the slab to reheating, heavy plate rolling, and cooling successively.
  • the thickness of the slab is preferably 300 mm or more, because a steel plate with a plate thickness of 25 mm or more can be obtained easily.
  • the reheating temperature in reheating the slab is preferably 950°C or more from a viewpoint of further improvement of the HIC resistance.
  • the reheating temperature is preferably 1150°C or less, from a viewpoint of further suppression of deterioration of the DWTT property.
  • rough rolling with an average rolling reduction of 10% or more per 1 pass to 120 mm or more in a recrystallization temperature range is preferable.
  • the average rolling reduction of 10% or more per 1 pass is advantageous, because recrystallization of austenite is promoted so that the grain size can be made fine.
  • rough rolling only to 120 mm or more is advantageous, because a cumulative rolling reduction can be enlarged in the succeeding rolling in the non-recrystallization region. Namely, in a case in which a cumulative rolling reduction in the rolling in the non-recrystallization region is enlarged, many dislocations can be introduced in austenite grains. Since the dislocations introduced in austenite grains can constitute nucleation sites for transformation to ferrite in the succeeding cooling process, they contribute to micronization of the grain size.
  • a rolling is performed preferably in a non-recrystallization region (for example, a temperature range between 750 and 900°C) down to a final plate thickness of 25 mm or more.
  • a non-recrystallization region for example, a temperature range between 750 and 900°C
  • Cooling after the heavy plate rolling is preferably performed with a cooling start temperature of 700 to 820°C.
  • the cooling start temperature of 700°C or more is advantageous, because the ferrite fraction (F2) at the position of 1/2 of the plate thickness can be easily made to 60% or less, and the maximum hardness at the position of 1/2 of the plate thickness can be easily made to 400 Hv or less.
  • the cooling start temperature of 820°C or less is advantageous, because the ferrite fraction (F2) can be easily adjusted to 5% or higher, and the DWTT property can be easily improved.
  • the cooling rate during the cooling is preferably 10°C/s or more from a viewpoint of further improvement of the strength.
  • the cooling stop temperature is preferably 200°C or more from a viewpoint of further suppression of HIC at the position of 1/2 of the plate thickness so as to further suppress deterioration of the toughness.
  • the cooling stop temperature is preferably 450°C or less from a viewpoint of further improvement of the strength.
  • the cooling rate (V1) at the position of 1/4 of the plate thickness is preferably slower than the cooling rate (V2) at the position of 1/2 of the plate thickness (V2) in a temperature range between 600 and 700°C.
  • the ferrite formation amount at the position of 1/4 of the plate thickness can be made higher than the ferrite formation amount at the position of 1/2 of the plate thickness, and therefore the ratio (F1/F2) can be easily adjusted to 1.00 or higher.
  • the cooling rate (V1) is higher than the cooling rate (V2) as described above, and therefore the ratio (F1/F2) of the obtained steel plate is less than 1.00.
  • the cooling rate in a temperature range of 600°C or less is preferably 15°C/s or more.
  • a line pipe according to the invention is a steel pipe produced using the steel plate for a line pipe according to the invention.
  • the line pipe according to the invention is superior in HIC resistance (especially HIC resistance in a sour environment of pH 5.0 or higher) and satisfies both compressive strength and DWTT property.
  • the line pipe according to the invention can be produced using the steel plate for a line pipe according to the invention as a source material by a publicly known pipe making method.
  • Examples of a publicly known pipe making method include UOE forming method, and JCOE forming method.
  • Steels having a chemical composition set forth in the following Table 1 (steel No. 1 to steel No. 15) were produced by melting, and slabs with a thickness (slab thickness) shown in the following Table 2 were produced by continuous casting. In continuous casting, soft reduction was conducted during the final solidification so as to suppress segregation of Mn in a center segregation zone.
  • components of a steel (the remainder) other than the components shown in the following Table 1 are Fe and unavoidable impurities.
  • REM in steel No. 6 is specifically Ce
  • REM in steel No. 9 is specifically La.
  • the thus obtained slab was heated to from 950 to 1150°C (exceptionally 1180°C in Comparative Example 2), then rough rolling was conducted above 900°C with an average rolling reduction of 10% or higher (exceptionally 8% in Comparative Example 3) down to a thickness of 120 mm or more (exceptionally 100 mm in Comparative Example 4), and thereafter finish rolling was conducted in a non-recrystallization temperature range of 900°C or less (exceptionally 930°C in Comparative Example 5) down to the final plate thickness.
  • the accelerated cooling (water cooling) was regulated such that the cooling rate (V1) at the position of 1/4 of the plate thickness was slower than the cooling rate (V2) at a position of 1/2 of the plate thickness in a temperature range of from 600 to 700°C where ferrite is formed.
  • V1 the cooling rate at the position of 1/4 of the plate thickness
  • V2 the cooling rate at a position of 1/2 of the plate thickness in a temperature range of from 600 to 700°C where ferrite is formed.
  • a water cooling zone where a steel plate after finish rolling passed, was segmented to sprinkling zones and non-sprinkling zones, so that a steel plate could be cooled with water intermittently.
  • cooling and heat-recuperation of a surface of the steel plate are regulated properly such that the V1 was made slower than the V2
  • the ferrite fraction (ferrite area ratio) and the ferrite grain size (average grain diameter of ferrite) were measured, and further the remainder structure was identified.
  • a cross-section of the steel plate was polished, and corroded with a LePera reagent, and a photograph of a structure of the same was taken using an optical microscope at a magnification of 500-fold.
  • magnification 500-fold Through image processing of the obtained optical micrograph (magnification 500-fold), the ferrite fraction (ferrite area ratio) and the ferrite grain size (average grain diameter of ferrite) were determined, and further the remainder structure was identified.
  • the image processing was carried out using a small size multi-purpose image analyzer LUZEX AP (produced by Nireco Corporation).
  • an average grain diameter of ferrite was determined by measuring equivalent circle diameters for 30 grains of ferrite, and calculating a simple average of the 30 equivalent circle diameters.
  • the ferrite fraction at 1/4 of plate thickness F1 the ferrite fraction at 1/2 of plate thickness F2, the ferrite grain size at 1/4 of plate thickness, and the ferrite grain size at 1/2 of plate thickness as shown in the following Table 3 were determined respectively, and the remainder structure at 1/4 of plate thickness and the remainder structure at 1/2 of plate thickness as shown in the following Table 3 were identified respectively.
  • FIG. 1 An optical micrograph (magnification 500-fold) of a cross-section (cross-section after polishing and corrosion with a LePera reagent) at the position of 1/2 of the plate thickness of the steel plate of Inventive Example 10 is shown in Fig. 1 .
  • a ratio [F1/F2] was determined based on a ferrite fraction at the position of 1/4 of the plate thickness (F1), and a ferrite fraction at the position of 1/2 of the plate thickness (F2) measured as above.
  • the steel plate obtained as above was cut along the plate thickness direction, and the appeared cross-section was mirror polished. According to JIS Z 2244 (2009), a Vickers hardness test was conducted on the mirror polished cross-section with a 25 g load.
  • the Vickers hardness test was performed for 400 points at the position of 1/2 of the plate thickness.
  • the maximum value among the 400 measurement results was defined as the "Hardness at 1/2 of plate thickness” (the following Table 3).
  • a macro-specimen was sampled from the steel plate, and a corrosion test was conducted on the sampled macro-specimen according to NACE TM0284. By this, cracking caused by elongated MnS was forcibly induced on the macro-specimen, which was then forcibly fractured in liquid nitrogen. By this, elongated MnS were exposed on the fractured surface. The fractured surface was observed with a scanning electron microscope, and the lengths of all the recognized elongated MnS were measured. From the length measurement results, the lengths of elongated MnS existing at the position of 1/2 of the plate thickness were selected, and the maximum value among the selected values (lengths) was defined as "MnS length at 1/2 of plate thickness" (the following Table 3).
  • API 5L A specimen for a tensile test was sampled from the steel plate such that the longitudinal direction of the specimen is parallel to the width direction of the steel plate.
  • the shape of a specimen was a flat plate shape according to the American Petroleum Institute specification: API 5L (hereinafter referred to simply as "API 5L").
  • a tensile test was conducted on the sampled specimen at room temperature according to API 5L. The tensile strength was determined based on the maximum load in the tensile test.
  • Compressive strength was measured by the following method in order to evaluate a property in the circumferential direction of a steel pipe, the steel pipe being made of the steel plate as a line pipe followed by being subjected to the heating in coating for anti-corrosion.
  • a large width specimen (full thickness specimen) was sampled from the steel plate such that the longitudinal direction of the specimen was parallel to the width direction of the steel plate.
  • a strain correspond to pipe making, a 2% pre-strain was applied to the sampled large width specimen.
  • the compression test specimen was in a cylindrical shape with diameter 22 mm ⁇ length 66 mm, and taken such that the center part in the direction of the steel plate was included and the longitudinal direction of the compression test specimen (test direction of the compression test) was parallel to the width direction of the steel plate.
  • the taken compression test specimen was heat-treated at 220°C for 5 min in a salt bath, and then the heat-treated compression test specimen was subjected to a compression test according to ASTM E9-09.
  • a 0.5% offset yield strength in the compression test was defined as a yield strength (compressive strength).
  • a DWTT specimen was taken out from the steel plate such that the longitudinal direction of the DWTT specimen was parallel to the width direction of the steel plate.
  • the DWTT specimen was a full thickness specimen with a pressed notch.
  • a DWTT test was conducted on the taken out DWTT specimen at -20°C according to API 5L to measure the ratio of a ductile fracture area to the total fracture area (DWTT fracture area ratio (%)).
  • a specimen (full thickness specimen) for a HIC resistance evaluation was taken out of the steel plate.
  • the taken specimen was immersed in a "Solution B" according to NACE TM0284 for 96 hours, and the specimen after the immersion was analyzed with an ultrasonic flaw detector for existence or nonexistence of occurrence of HIC. Based on the analysis results, a crack area ratio (CAR) was determined.
  • CAR crack area ratio
  • the compressive strength in the Table 3 is a compressive strength after application of a 2% pre-strain followed by a heat treatment at 220°C.
  • the steel plate of Inventive Example 10 was subjected to pipe making by the UOE forming method to yield a line pipe 1 with the outer diameter and the wall thickness set forth in Table 4.
  • tensile strength, DWTT fracture area ratio (-20°C), and CAR in a HIC test were measured similarly as the respective measurements with respect to the steel plates above.
  • Yield strength, compressive strength, HAZ toughness, and WM toughness were measured as follows.
  • the yield strength in the longitudinal direction of the line pipe was measured according to ASTM E9-09. In this regard, a 0.5% under load proof strength was defined as a yield strength.
  • the compressive strength in the circumferential direction of a line pipe was measured according to ASTM E9-09. In this regard, a 0.5% extension under load yield strength was defined as a compressive strength.
  • a Charpy test specimen with a V-notch was taken from a position 2 mm-deep from the outer peripheral surface of the line pipe.
  • the V-notch of the specimen was provided such that a fracture after a Charpy impact test includes HAZ and WM at an area ratio of 50%/50%.
  • a Charpy impact test was conducted according to JIS Z2242 (2005) under the temperature condition of -20°C, and a Charpy absorbed energy (J) was defined as a HAZ toughness (J).
  • a Charpy test specimen with a V-notch was taken from a position 2 mm-deep from the outer peripheral surface of a line pipe.
  • the V-notch of the specimen was provided such that the center of the V-notch was positioned at the center of a WM.
  • a Charpy impact test was conducted according to JIS Z2242 (2005) under the temperature condition of -20°C, and a Charpy absorbed energy (J) was defined as a WM toughness (J).
  • a steel plate was prepared identically with the steel plate of Inventive Example 10 except that the plate thickness was changed to 45 mm.
  • the prepared 45 mm-thick steel plate was subjected to pipe making by the JCOE forming method to obtain a line pipe 2 with the outer diameter and the wall thickness set forth in Table 4.

Claims (4)

  1. Tôle en acier pour tube de canalisation, la tôle en acier ayant une épaisseur de tôle de 25 mm ou plus et comprenant en termes de % en poids de :
    0,040 à 0,080 % de C,
    0,05 à 0,40 % de Si,
    1,60 à 2,00 % de Mn,
    0,020 % ou moins de P,
    0,0025 % ou moins de S,
    0,05 à 0,20 % de Mo,
    0,0011 à 0,0050 % de Ca,
    0,060 % ou moins d'Al,
    0,010 à 0,030 % de Nb,
    0,008 à 0,020 % de Ti,
    0,0015 à 0,0060 % de N,
    0,0040 % ou moins de O, et
    éventuellement en termes de % en poids, un ou plusieurs de :
    0,50 % ou moins de Ni,
    0,50 % ou moins de Cr,
    0,50 % ou moins de Cu,
    0,0050 % ou moins de Mg,
    0,0050 % ou moins de REM,
    0,100 % ou moins de V, et
    dans laquelle un rapport de teneurs de Ca à S [Ca/S] est de 0,90 à 2,70, et un rapport de teneurs de Ti à N [Ti/N] est de 2,20 à 5,00,
    le reste étant constitué de Fe et d'inévitables impuretés,
    où Ceq, qui est défini par la Formule (1) suivante, est de 0,380 à 0,480 : Ceq = C + Mn / 6 + Ni + Cu / 15 + Cr + Mo + V / 5
    Figure imgb0008
     dans laquelle, dans la Formule (1), C, Mn, Ni, Cu, Cr, Mo et V représentent des teneurs desdits éléments respectifs (% en poids),
    et dans laquelle :
    à une position de 1/4 de l'épaisseur de tôle, une fraction de ferrite (F1) est de 20 à 60 % et le reste est une structure de bainite,
    à une position de 1/2 de l'épaisseur de tôle, une fraction de ferrite (F2) est de 5 à 60 % et le reste est une structure de bainite ou une structure de bainite et martensite,
    un rapport (F1/F2) de la fraction de ferrite (F1) à la fraction de ferrite (F2) est de 1,00 à 5,00,
    à une position de 1/4 de l'épaisseur de tôle, un diamètre de grain moyen de ferrite est de 2,0 à 15,0 µm, et à une position de 1/2 de l'épaisseur de tôle, le diamètre de grain moyen de ferrite est de 5,0 à 20,0 µm, et
    une dureté à une position de 1/2 de l'épaisseur de tôle est de 400 Hv ou moins, où le test de dureté Vickers a été mis en oeuvre sous une charge de 25 g, et
    une longueur de MnS à une position de 1/2 de l'épaisseur de tôle est de 1,00 mm ou moins.
  2. Tôle en acier pour tube de canalisation selon la revendication 1, dans laquelle une teneur en Al est de 0,008 % ou moins en termes de % en poids.
  3. Tôle en acier pour tube de canalisation selon la revendication 1 ou 2, comprenant, en termes de % en poids, un ou plusieurs de :
    0,50 % ou moins de Ni,
    0,50 % ou moins de Cr,
    0,50 % ou moins de Cu,
    0,0050 % ou moins de Mg,
    0,0050 % ou moins de REM, et
    0,100 % ou moins de V.
  4. Tube de canalisation produit à l'aide de la tôle en acier pour tube de canalisation selon l'une quelconque des revendications 1 à 3.
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