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EP3686305B1 - Steel pipe and steel plate - Google Patents

Steel pipe and steel plate Download PDF

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Publication number
EP3686305B1
EP3686305B1 EP17926174.8A EP17926174A EP3686305B1 EP 3686305 B1 EP3686305 B1 EP 3686305B1 EP 17926174 A EP17926174 A EP 17926174A EP 3686305 B1 EP3686305 B1 EP 3686305B1
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Prior art keywords
microstructure
steel plate
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content
surface layer
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German (de)
English (en)
French (fr)
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EP3686305A4 (en
EP3686305A1 (en
Inventor
Yasuhiro Shinohara
Takuya Hara
Kiyoshi Ebihara
Kazuteru TSUTSUI
Yutaka Hattori
Akira Hashimoto
Nozomu ABE
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Nippon Steel Corp
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Nippon Steel Corp
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • 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
    • C21D8/10—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/08—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for tubular bodies or pipes
    • C21D9/085—Cooling or quenching
    • 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
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
    • 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
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
    • 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
    • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • 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
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C21D1/19—Hardening; Quenching with or without subsequent tempering by interrupted 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
    • 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
    • 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

Definitions

  • An object of the present invention is to provide: a steel pipe that is suitable for a line pipe and has a strength of API X52 to X70 grade and satisfactory SSC resistance and HIC resistance; and a steel plate that is used as a base metal of the steel pipe.
  • steel pipe having satisfactory sulfide stress cracking resistance (SSC resistance) and hydrogen induced cracking resistance (HIC resistance) and a steel plate that is used as a base metal of the steel pipe.
  • the steel pipe having satisfactory sour resistance (SSC resistance and HIC resistance) is suitable as a line pipe that transports petroleum, natural gas, or the like.
  • a steel pipe according to one embodiment of the present invention (hereinafter, also referred to as the steel pipe according to the embodiment) includes:
  • the base metal that is, the steel plate according to the embodiment
  • the steel pipe according to the embodiment will be described.
  • C is an element that is required to improve the strength of the steel.
  • the C content is 0.030% or more.
  • the C content is preferably 0.040% or more.
  • Mn is an element contributing to the strength and toughness of the steel.
  • the Mn content is 1.05% or more.
  • the Mn content is preferably 1.15% or more.
  • Nb is an element that refines crystal grains by widening non-recrystallization temperature range, form a carbide or a nitride and contributes to improvement of the strength of the steel.
  • the Nb content is less than 0.005%, the effect cannot be sufficiently obtained. Therefore, the Nb content is 0.005% or more.
  • the Nb content is preferably 0.010% or more.
  • the Nb content is more than 0.045%, a coarse carbide or nitride is formed, and HIC resistance deteriorates. Therefore, the Nb content is 0.045% or less.
  • the Nb content is 0.035% or less.
  • Ca is an element that is bonded to S to form CaS, suppresses formation of MnS stretched in a rolling direction, and thus contributes improvement of HIC resistance.
  • the Ca content is less than 0.0010%, the effect cannot be sufficiently obtained. Therefore, the Ca content is 0.0010% or more.
  • the Ca content is preferably 0.0020% or more.
  • N is an element that forms a nitride and contributes to suppressing the coarsening of austenite grains during heating.
  • the N content is less than 0.0015%, the effect cannot be sufficiently obtained. Therefore, the N content is 0.0015% or more.
  • the N content is preferably 0.0020% or more.
  • the chemical composition of the base metal (the steel plate according to the embodiment) of the steel pipe according to the embodiment may include one or more selected from the group consisting of Ni, Mo, Cr, Cu, V, Mg, and REM instead of a part of Fe within a range where the characteristics of the steel plate according to the embodiment do not deteriorate.
  • These elements are optional elements and are not necessarily included. That is, the lower limits of the amounts of the elements are 0%.
  • Mo is an element that contributes to improvement of the hardenability of the steel.
  • the Mo content is preferably 0.05% or more.
  • the Mo content is more preferably 0.10% or more.
  • the Mo content is preferably 0.35% or less.
  • the Cr content is an element that contributes to improvement of the strength of the steel.
  • the Cr content is preferably 0.05% or more.
  • the Cr content is more preferably 0.10% or more.
  • the Cr content is preferably 0.35% or less.
  • the Cu is an element that contributes to improvement of the strength of the steel and improvement of corrosion resistance.
  • the Cu content is preferably 0.05% or more.
  • the Cu content is more preferably 0.10% or more.
  • the Cu content is preferably 0.35% or less.
  • V is an element that forms a carbide and/or a nitride and contributes improvement of the strength of the steel.
  • the V content is preferably 0.010% or more.
  • the V content is more preferably 0.030% or more.
  • the V content is preferably 0.100% or less.
  • the V content is preferably 0.080% or less.
  • the Mg content is more than 0.0100%, an oxide aggregates and is coarsened, and HIC resistance and toughness deteriorate. Therefore, even when Mg is included, the Mg content is 0.0100% or less.
  • the Mg content is preferably 0.0050% or less.
  • the REM is an element that contributes to improvement of SSC resistance, HIC resistance, and toughness by controlling the form of a sulfide inclusion.
  • the REM content is preferably 0.0001 % or more.
  • the REM content is more preferably 0.0010% or more.
  • the REM content when the REM content is more than 0.0100%, an oxide is formed, the cleanliness of the steel deteriorates, and HIC resistance and toughness deteriorate. Therefore, even when REM is included, the REM content is 0.0100% or less.
  • the REM content is preferably 0.0060% or less.
  • REM refers to rare earth elements and is a collective term for 17 elements including Sc, Y, and lanthanoids.
  • the REM content refers to the total amount of the 17 elements.
  • the base metal (the steel plate according to the embodiment) of the steel pipe according to the embodiment basically has the chemical composition including the above-described essential elements and the remainder consisting of Fe and impurities.
  • the base metal may have a chemical composition including the above-described essential elements, the above-described optional elements, and the remainder consisting of Fe and impurities.
  • P, S, O, Sb, Sn, Co, As, Pb, Bi, and H are controlled to be in ranges described below.
  • P is an impurity element, and the less the P content, the better.
  • the P content is more than 0.015%, HIC resistance significantly deteriorates. Therefore, the P content is 0.015% or less.
  • the P content is preferably 0.010% or less.
  • the lower limit of the P content in the actual steel plate is substantially 0.003%.
  • S is an element that forms MnS stretched in a rolling direction during hot rolling. This stretched MnS deteriorates HIC resistance. When the S content is more than 0.0015%, HIC resistance significantly deteriorates. Therefore, the S content is 0.0015% or less. The S content is preferably 0.0010% or less.
  • the lower limit thereof may be 0%.
  • the upper limit of the S content in the actual steel plate is substantially 0.0001%.
  • O is an element that avoidably remains after deoxidation.
  • the Q content is more than 0.0040%, a large amount of an oxide is formed, and HIC resistance significantly deteriorates. Therefore, the O content is 0.0040% or less.
  • the O content is preferably 0.0030% or less.
  • the lower limit of the O content in the actual steel plate is substantially 0.0010%.
  • the content each of Sb, Sn, Co, and As is 0.10% or less, the amount of each of Pb and Bi is 0.005% or less, and the H content is preferably 0.0005% or less.
  • the chemical composition of the steel plate used for the base metal of the steel pipe satisfies not only the amount of each of the elements, but also Ceq (carbon equivalent) defined by the following Expression (1) is 0.400 or less.
  • Ceq C + Mn / 6 + Ni + Cu / 15 + Cr + Mo + V / 5
  • [C], [Mn], [Ni], [Cu], [Cr], [Mo], and [V] represent the contents (mass%) of C, Mn, Ni, Cu, Cr, Mo, and V.
  • Ceq is 0.400 or lower.
  • Ceq is preferably 0.350 or lower. In order to secure a predetermined strength, the lower limit of Ceq is 0.300 or higher.
  • microstructure (the microstructure and the hardness thereof) of the base metal of the steel pipe according to the embodiment will be described.
  • the surface layer of the steel plate is more rapidly cooled as compared to the inside of the steel plate. This implies that a difference in mechanical properties is generated due to a difference between the microstructure of the surface layer of the steel plate and the microstructure of the inside of the steel plate. In particular, the hardness of the surface layer of the steel plate is higher than that of the inside of the steel plate.
  • SSC resistance is poor in a range (surface layer) up to 1.0 mm from the surface in the depth direction (through-thickness direction).
  • the present inventors found that, by using recuperating during controlled cooling of the steel plate, the microstructure of the surface layer of the steel plate and the microstructure of the inside of the steel plate can be controlled, and thus an increase in the hardness of the surface layer of the steel plate can be suppressed.
  • the microstructure of the steel plate of the base metal is divided into (i) a microstructure (surface layer microstructure) in a range up to 1.0 mm from the surface of the steel plate in the depth direction (through-thickness direction) and (ii) a microstructure (internal microstructure) in a range up to a thickness center from an area positioned at a distance of more than 1.0 mm from the surface of the base metal in the depth direction.
  • a microstructure surface layer microstructure
  • an internal microstructure in a range up to a thickness center from an area positioned at a distance of more than 1.0 mm from the surface of the base metal in the depth direction.
  • the range up to 1.0 mm from the surface of the steel plate as the base metal in the depth direction will be referred to as "surface layer” (hereinafter, also simply referred to as “the surface layer of the steel plate).
  • the surface layer microstructure is defined as the microstructure in a range up to 1.0 mm from the steel plate surface in the depth direction.
  • the surface layer microstructure in a range up to a depth of 1.0 mm from the surface of the steel plate as the base metal includes polygonal ferrite and granular bainite, the area fraction of polygonal ferrite is 0% to 70%, the total area fraction of polygonal ferrite and granular bainite is 50% or more, and the maximum hardness is 270 Hv or lower.
  • the area fraction of polygonal ferrite in the surface layer is more than 70%, a high concentration of C accumulates on the remainder, a hardening region is formed, and thus SSC resistance deteriorates. Therefore, the area fraction of polygonal ferrite is 70% or less.
  • the area fraction of polygonal ferrite is preferably 50% or less.
  • the total area fraction of polygonal ferrite and granular bainite is 50% or more.
  • the remainder of the surface layer microstructure may include one or more selected from the group consisting of bainite and pseudo pearlite. However, the remainder is not necessarily included. That is, the total area fraction of polygonal ferrite and granular bainite may be 100%.
  • the maximum hardness of the surface layer microstructure is 270 Hv or lower.
  • the maximum hardness of the surface layer microstructure is preferably 250 Hv. From the viewpoint of SSC resistance, although the lower limit of the maximum hardness of the surface layer microstructure is not necessarily determined, the maximum hardness of the surface layer microstructure is substantially 160 Hv or higher.
  • the area fraction of each of the microstructures can be obtained by observing the microstructure with a scanning electron microscope (SEM), for example, at a magnification of 1000-fold.
  • SEM scanning electron microscope
  • the surface layer microstructure can be obtained by observing positions of 0.1 mm, 0.2 mm, and 0.5 mm from the surface of the steel plate and obtaining the average of the area fractions at the respective positions.
  • polygonal ferrite is a microstructure that is observed as a massive microstructure not including a coarse precipitate such as coarse cementite or MA in grains.
  • Bainite is a microstructure in which a prior austenite grain boundary is clear, a fine lath structure is developed in grains, and a fine carbide and an austenite-martensite constituent mixture are scattered in and between laths.
  • bainite also includes tempered bainite.
  • Granular bainite is a microstructure that is formed at an intermediate transformation temperature between acicular ferrite and bainite, the acicular ferrite being a microstructure in which a prior austenite grain boundary is not clear and acicular-shaped ferrite (a carbide and an austenite-martensite constituent are not present) is formed in a random crystal orientation in grains.
  • a prior austenite grain boundary partially appears, a coarse lath structure is present in grains, and a portion where a fine carbide and an austenite-martensite constituent are scattered in and between laths and a portion of acicular or amorphous ferrite where a prior austenite grain boundary is not clear are mixed.
  • Pseudo pearlite is pearlite in which parallel row of cementite is arranged.
  • FIG. 4 shows an example of a microstructure (observed with a scanning electron microscope at a magnification of 1000-fold) at a distance of 0.5 mm from the surface of the steel plate.
  • a portion which is surrounded by a smooth curve and in which internal portion is smooth, is polygonal ferrite, and a portion where white spots are present in internal portion is granular bainite.
  • the maximum hardness of the surface layer microstructure is measured as follows.
  • 300 mm ⁇ 300 mm steel plates are cut out by gas cutting from positions of 1/4, 1/2, and 3/4 of the width of the steel plate (positions of 3 o'clock, 6 o'clock, and 9 o'clock when the weld of the steel pipe is 0 o'clock) from a width-direction end portion (corresponding to the seam portion in the case of the steel pipe) of the steel plate in the width direction of the steel plate.
  • Block test pieces having a length of 20 mm and a width of 20 mm are collected by mechanical cutting from the centers of the cut steel plates and are polished by mechanical polishing.
  • the hardness is measured using a Vickers hardness meter (load: 100 g) at 100 points in total that are obtained by setting a point of 0.1 mm from the surface as a starting point, setting 10 pints from the starting point in a through-thickness direction at an interval of 0.1 mm, and setting 10 points at the same depth at an interval of 1.0 mm in a width direction.
  • a Vickers hardness meter load: 100 g
  • FIGS. 3A to 3C show the results of measuring the hardness of the surface layer microstructure at three positions corresponding to 3 O'clock, 6 O'clock, and 9 O'clock when the weld of the steel pipe is at a 0 O'clock position.
  • the hardness of the surface layer microstructure is measured under a load of 100 g by setting every 10 measurement points at the same depth at an interval of 0.1 mm in a region from a depth of 0.1 mm to a depth of 1.0 mm from the surface layer. It can be seen that, at all the points, the maximum hardness is 270 Hv or lower and SSC resistance is excellent.
  • the microstructure in a range up to a thickness center from an area positioned at a distance of more than 1.0 mm from the surface of the steel plate as the base metal in the depth direction: the area fraction of polygonal ferrite is 40% or less, the maximum hardness is 248 Hv or lower, and the average hardness is 150 to 220 Hv.
  • the area fraction of polygonal ferrite in the internal microstructure is more than 40%, it is difficult to secure a required strength and HIC resistance. Therefore, the area fraction of polygonal ferrite is 40% or less.
  • the area fraction of polygonal ferrite is preferably 30% or less and more preferably 25% or less.
  • the remainder of the internal microstructure consists of one or more selected from the group consisting of granular bainite, bainite, and pseudo pearlite.
  • the maximum hardness in the internal microstructure is higher than 248 Hv, HIC resistance deteriorates. Therefore, the maximum hardness is 248 Hv or lower.
  • the average hardness is lower than 150 Hv, required mechanical properties cannot be secured. Therefore, the average hardness is 150 Hv or higher. the average hardness is 160 Hv or higher.
  • the average hardness is 220 Hv or lower.
  • the average hardness is preferably 210 Hv or lower.
  • the microstructural fraction (area fraction) of the internal microstructure can be obtained by observing a 1/4 thickness (t/4) position from the surface of the steel plate with a scanning electron microscope (SEM), for example, at a magnification of 1000-fold.
  • SEM scanning electron microscope
  • FIG. 5 shows an example of the microstructure of the t/4 position (observed with a scanning electron microscope at a magnification of 1000-fold).
  • a portion which is surrounded by a smooth curve and in which internal portion is smooth is polygonal ferrite.
  • a portion where white spots or a white line appears is granular bainite or pseudo pearlite, and a portion that is surrounded by a jagged white line and where a thin pattern appears is bainite.
  • the maximum hardness and the average hardness in the internal microstructure can be measured using the following method.
  • 300 mm ⁇ 300 mm steel plates are cut out by gas cutting from positions of 1/4, 1/2, and 3/4 (positions of 3 o'clock, 6 o'clock, and 9 o'clock when the weld of the steel pipe is 0 o'clock) from a width-direction end portion (corresponding to the seam portion in the case of the steel pipe) of the steel plate in the width direction of the steel plate.
  • Block test pieces having a length of 20 mm and a width of 20 mm are collected by mechanical cutting from the centers of the cut steel plates and are polished by mechanical polishing.
  • a high hardness value (abnormal value) may appear locally. However, even when this abnormal value appears, HIC resistance can be secured. On the other hand, when two or more measurement points having a hardness of higher than 248 Hv are continuously present in the through-thickness direction, HIC resistance deteriorates, which is not allowable. Accordingly, in the embodiment, even when one measurement point having a hardness of higher than 248 Hv is present, unless two or more measurement points do not continuously appear in the through-thickness direction, this point as an abnormal point is not adopted, and the second highest value is obtained as the maximum hardness. On the other hand, when two or more measurement points having a hardness of higher than 248 Hv are continuously present in the through-thickness direction, the highest value is adopted as the maximum hardness.
  • the average hardness is calculated by obtaining the average value of the hardnesses at all the measurement points.
  • the steel pipe according to the embodiment can be obtained by processing the steel plate according to the embodiment into a pipe shape, making opposite end portions (width-direction end portions of the steel pipe) of the cylindrical steel plate abut against each other, and welding the end portions. Therefore, as shown in FIG. 1 , the steel pipe 1 according to the embodiment includes the weld 3 that is provided in the seam portion of the steel plate 2 and extends in the longitudinal direction of the steel plate. Typically, the weld 3 is continuously provided in a range from one end portion of the steel plate 2 in the longitudinal direction to another end portion thereof.
  • the steel pipe according to the embodiment has the above-described configuration, the effects thereof can be obtained irrespective of the manufacturing method thereof.
  • a manufacturing method including the following processes is preferable because the steel pipe according to the embodiment can be stably obtained.
  • the steel plate according to the embodiment can be obtained using a manufacturing method including:
  • the steel pipe according to the embodiment is obtained using a manufacturing method including (i) and (ii) described above and further including:
  • a slab that is manufactured by casting molten steel having the same chemical composition as that of the base metal of the steel pipe according to the embodiment is heated to 1050°C to 1250°C and subjected to hot rolling.
  • the casting of the molten steel and the manufacturing of the slab before hot rolling may be performed using an ordinary method.
  • the slab heating temperature is preferably 1050°C or higher.
  • the slab heating temperature is more preferably 1100°C or higher.
  • the slab heating temperature is higher than 1250°C, the crystal grain size increases, and low-temperature toughness deteriorates.
  • the austenite grain size increases, and hardenability excessively increases.
  • the slab heating temperature is preferably 1250°C or lower.
  • the slab heating temperature is more preferably higher than 1200°C or lower.
  • the slab heated to the above-described temperature is hot-rolled at a typical rolling reduction ratio to obtain a steel plate.
  • the plate thickness may be set depending on the required thickness of a line pipe and thus is not particularly limited.
  • the rolling finishing temperature is 830°C to 1000°C.
  • the finish rolling temperature is preferably 830°C or higher.
  • the finish rolling temperature is more preferably 850°C or higher.
  • the rolling finishing temperature is preferably 1000°C or lower.
  • the rolling finishing temperature is more preferably 900°C or lower.
  • accelerated cooling is performed on the steel plate after finishing hot rolling from a surface temperature range of 750°C to 950°C to a surface temperature range of 400°C to 650°C at an average cooling rate of 15 to 100 °C/sec such that two or more times of recuperating where an increase in temperature from the start of cooling to the end of cooling is 5°C to 65°C is included.
  • the accelerated cooling including recuperating in the middle can be performed by adjusting the amount of cooling water that is sprayed to the steel plate per cooling zone in a cooling facility in which a plurality of divided cooling zones are arranged in a longitudinal direction of the steel plate (conveyance direction).
  • FIG. 2 shows an example of cooling curves of the steel plate.
  • Four cooling curves include a cooling curve of the thickness middle portion (1/2 thickness portion), a cooling curve of a 1/4 thickness position (t/4 portion) from the surface, a cooling curve of a portion of a depth of 1.0 mm from the surface, and a cooling curve of the steel plate surface. Accelerated cooling is performed on the entire steel plate from the cooling start temperature (Ts) of 830°C to about 620°C for about 10 seconds such that recuperating is performed three times in the middle.
  • Ts cooling start temperature
  • the cooling start temperature Ts and the cooling stop temperature Tf are points shown in the drawing, and the average cooling rate Vc can be obtained by dividing a temperature change ⁇ T (cooling start temperature Ts-cooling stop temperature Tf) by a cooling time ⁇ t (the time for which water cooling is performed).
  • the temperature of the steel plate surface during cooling is temporarily increased by recuperating due to sensible heat inside the steel plate as a result of adjusting the amount of cooling water sprayed per cooling zone.
  • the cooling curve of the steel plate surface and the cooling curve of the portion of a depth of 1.0 mm from the surface are affected by recuperating.
  • the cooling curve of the thickness middle portion (1/2 thickness portion) and the cooling curve of the 1/4 thickness portion are not affected by recuperating, and it can be seen that the inside of the steel plate is cooled at a substantially constant cooling rate.
  • the cooling start temperature Ts is lower than 750°C, in the surface layer microstructure, coarse ferrite is formed after rolling, and a microstructure having a high hardness such as martensite is formed as the remainder. As a result, SSC resistance deteriorates.
  • the cooling start temperature Ts is preferably 750°C or higher.
  • the cooling start temperature Ts is more preferably 780°C or higher.
  • the cooling start temperature Ts is preferably 950°C or lower.
  • the cooling start temperature Ts is more preferably 880°C or lower.
  • the cooling stop temperature Tf is preferably 400°C or higher.
  • the cooling stop temperature Tf is more preferably 480°C or higher.
  • the cooling stop temperature Tf is preferably 650°C or lower.
  • the cooling stop temperature Tf is more preferably 580°C or lower.
  • the average cooling rate Vc is slower than 15 °C/sec, polygonal ferrite having an area fraction of more than 70% is formed in the surface layer microstructure. In addition, in the internal microstructure polygonal ferrite having an area fraction of more than 40% is formed. In this case, the strength as a line pipe cannot be secured. Therefore, the average cooling rate Vc is preferably 15 °C/sec or faster. The average cooling rate Vc is more preferably 25 °C/sec or faster.
  • the average cooling rate Vc is faster than 100 °C/sec, martensite transformation occurs, the hardness of the surface layer microstructure is higher than 270 Hv, and SSC resistance deteriorates. In addition, the maximum hardness of the internal microstructure is higher than 248 Hv, and HIC resistance deteriorates. Therefore, the average cooling rate Vc is preferably 100 °C/sec or slower. The average cooling rate Vc is more preferably 80 °C/sec or slower.
  • the number of times of recuperating where the recuperated temperature during accelerated cooling is in a predetermined range is one or less, the hardness of the surface layer microstructure is higher than 270 Hv, and SSC resistance deteriorates. Therefore, the number of times of recuperating is two or more.
  • FIG. 2 shows the cooling curve when the number of times of recuperating is three.
  • the number of times of recuperating may be appropriately determined between the cooling start temperature and the cooling stop temperature depending on the kind of steel or the plate threading speed.
  • cooling is performed in a film boiling state in order to form a predetermined microstructure.
  • recuperating is not completed during water cooling, and an increase in the temperature caused by recuperating is 65°C or lower.
  • the width of the temperature increase caused by recuperating is preferably 5°C to 65°C.
  • the width of the temperature increase caused by recuperating is preferably 10°C to 65°C.
  • the width of the temperature increase caused by recuperating is not necessarily 5°C to 65°C.
  • the first recuperating is performed such that the steel plate surface temperature after recuperating is 500°C or higher. Even when the steel plate surface temperature after the first recuperating is lower than 500°C, the surface layer microstructure having satisfactory SSC resistance and the internal microstructure having satisfactory HIC resistance can be secured. However, in order to stably secure the surface layer microstructure having satisfactory SSC resistance and the internal microstructure having satisfactory HIC resistance, it is preferable that the first recuperating is performed such that the steel plate surface temperature after recuperating is 500°C or higher.
  • a temperature difference between the surface temperature and the center temperature is eliminated.
  • the surface temperature surface temperature
  • the inside of the steel plate center temperature
  • the average cooling rate is 0.5 °C/sec to 5.0 °C/sec, air cooling may be performed.
  • the average cooling rate is slower than 0.5 °C/sec, the predetermined strength cannot be obtained.
  • the average cooling rate is faster than 5.0 °C/sec, the toughness of the center portion deteriorates.
  • the formation of the steel plate according to the embodiment into the steel pipe is not limited to a specific forming method. Warm working can also be used, but cold working is preferable from the viewpoint of dimensional accuracy.
  • Welding is not limited to a specific welding method, but submerged arc welding (SAW) is preferable.
  • Welding conditions may be well-known conditions depending on the plate thickness and the like.
  • a heat treatment may be performed such that a microstructure (ferrite and pearlite having an area fraction of more than 10%) that deteriorates the toughness of the weld is not formed.
  • the heat treatment temperature may be a typical temperature range and is preferably in a range of 300°C to the Ac 1 point.
  • the microstructure of the base metal is the same as the microstructure of the steel plate according to the embodiment.
  • the base metal of the steel pipe according to the embodiment has the same microstructure as that of the steel plate according to the embodiment, and thus mechanical properties for use in a line pipe and satisfactory local weldability.
  • the weldability of the steel plate according to the embodiment is satisfactory, the weld of the steel pipe according to the embodiment has satisfactory mechanical properties. Accordingly, the steel pipe according to the embodiment is suitable as a steel pipe for a line pipe.
  • a slab having a chemical composition and Ceq shown in Table 1 was hot-rolled and cooled under conditions shown in Table 2. As a result, a steel plate was manufactured.
  • the number of times of recuperating is the number of times of recuperating where a temperature increase was 5°C or higher.
  • the maximum width of recuperating temperature is the width of a temperature increase during recuperating where the width of the temperature increase was the maximum.
  • a test piece was collected from the manufactured steel plate, the surface layer microstructure (positions of 0.1 mm, 0.2 mm, and 0.5 mm) and the internal microstructure (t/4 position) were observed with a SEM at a magnification of 1000-fold and the fractions (area fractions) of polygonal ferrite, granular bainite, and the remainder were calculated.
  • the remainder of the surface layer microstructure consisted of one or more selected from the group consisting of bainite and pseudo pearlite
  • the remainder of the internal microstructure consisted of one or more selected from the group consisting of granular bainite, bainite, and pseudo pearlite.
  • JIS No.5 tensile test piece was prepared, and a tensile test according to JIS Z 2241 was performed to measure a yield strength and a tensile strength.
  • the hardness was measured using a Vickers hardness meter.
  • the hardness of the surface layer microstructure was measured under a load of 100 g by setting every 10 points at the same depth at an interval of 0.1 mm in a region from a depth of 0.1 mm to a depth of 1.0 mm from the surface layer.
  • the hardness of the internal microstructure was measured under a load of 1 kg by setting every 10 points at the same depth at an interval of 0.2 mm in a region from a depth of 1.2 mm from the surface layer to the thickness center. Based on the results, the maximum hardness of the surface layer microstructure was obtained, and the maximum hardness and the average hardness of the internal microstructure were obtained.
  • test piece was collected from the manufactured steel plate, and the following test was performed to evaluate HIC resistance and SSC resistance.
  • HIC hydrogen induced cracking
  • the NACE test is a test in which hydrogen sulfide gas is saturated in a solution including 5% NaCl solution+0.5 acetic acid and having a pH of 2.7 and the steel plate is dipped in the solution for 96 hours to observe whether or not cracking occur.
  • a full thickness test piece having a width of 15 mm and a length of 115 mm was collected from the steel plate in a width direction, and SSC resistance was evaluated in a 4 point bending test according to TM0284m ASTM (American Society for Testing and Materials) G39 of NACE.
  • the test piece to which a stress corresponding to 90% of 0.2% proof stress derived from the tensile test was applied was dipped for 720 hours in an aqueous solution including 5% sodium chloride +0.5 acetic acid at normal temperature (24°C) and having a pH of 2.7 in which hydrogen sulfide gas of 1 atm was saturated, and the test piece surface was observed at a magnification of 10-fold to determine whether or not SSC occurred.
  • the steel plate shown in Table 3 was formed into a pipe shape by C-press, U-press, and O-press, end surfaces were temporarily welded, main welding was performed from internal and external surfaces, and the steel pipe was expanded. As a result, a steel pipe for a line pipe was obtained. As the main welding, submerged arc welding was adopted.
  • Manufacturing No. of the steel plate relates to Manufacturing No. of the steel pipe.
  • the steel pipe of Manufacturing No. T1 was manufactured using the steel plate of Manufacturing No. S 1.
  • the steel pipe of Manufacturing No. T2 was manufactured using the steel plate of Manufacturing No. S2.
  • a test piece was collected from the manufactured steel plate, the surface layer microstructure (positions of 0.1 mm, 0.2 mm, and 0.5 mm) and the internal microstructure (t/4 position) were observed with a scanning electron microscope at a magnification of 1000-fold to calculate the fractions (area fractions) of polygonal ferrite, granular bainite, and the remainder.
  • JIS No.5 tensile test piece was prepared, and a tensile test according to JIS Z 2241 was performed to measure a yield strength and a tensile strength.
  • the hardness was measured using a Vickers hardness meter.
  • the hardness of the surface layer microstructure was measured under a load of 100 g by setting every 10 points at the same depth at an interval of 0.1 mm in a region from a depth of 0.1 mm to a depth of 1.0 mm from the surface layer.
  • the hardness of the internal microstructure was measured under a load of 1 kg by setting every 10 points at the same depth at an interval of 0.2 mm in a region from a depth of 1.2 mm from the surface layer to the thickness center.
  • test piece was collected from the manufactured steel plate, and the following test was performed to evaluate HIC resistance and SSC resistance.
  • HIC hydrogen induced cracking
  • the NACE test is a test in which hydrogen sulfide gas is saturated in a solution including 5% NaCl solution+0.5 acetic acid and having a pH of 2.7 and the steel plate is dipped in the solution for 96 hours to observe whether or not cracking occur.
  • a full thickness test piece having a width of 15 mm and a length of 115 mm was collected from the steel plate in a width direction (direction perpendicular to a rolling direction), and SSC resistance was evaluated in a 4 point bending test according to TM0284m ASTM (American Society for Testing and Materials) G39 of NACE.
  • test piece to which a stress corresponding to 90% of 0.2% proof stress derived from the tensile test was applied was dipped for 720 hours in an aqueous solution including 5% sodium chloride +0.5 acetic acid at normal temperature (24°C) and having a pH of 2.7 in which hydrogen sulfide gas of 1 atm was saturated, and the test piece surface was observed at a magnification of 10-fold to determine whether or not SSC occurred.
  • a test piece where SSC did not occur was evaluated as "Pass (OK)", and a test piece where SSC occurred was evaluated as "Fail (NG)".
  • Pass (OK) Pass
  • NG Fail
  • a steel pipe for a line pipe that is suitable for a line pipe and has a strength of API X52 to X70 grade and satisfactory SSC resistance and HIC resistance; and a steel plate having satisfactory SSC resistance and HIC resistance that is used as a base metal of the steel pipe. Accordingly, the present invention is highly applicable to the steel plate manufacturing industry and the energy industry.

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WO2021144953A1 (ja) 2020-01-17 2021-07-22 日本製鉄株式会社 鋼板および鋼管
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JPWO2019058420A1 (ja) 2019-11-07
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KR20200039738A (ko) 2020-04-16
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