EP3816311B1 - Tuyau d'acier et tôle d'acier - Google Patents

Tuyau d'acier et tôle d'acier Download PDF

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EP3816311B1
EP3816311B1 EP18923989.0A EP18923989A EP3816311B1 EP 3816311 B1 EP3816311 B1 EP 3816311B1 EP 18923989 A EP18923989 A EP 18923989A EP 3816311 B1 EP3816311 B1 EP 3816311B1
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steel plate
microstructure
steel
hardness
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EP3816311A4 (fr
EP3816311A1 (fr
Inventor
Yasuhiro Shinohara
Takuya Hara
Kiyoshi Ebihara
Kazuteru Tsutsui
Yutaka Hattori
Nozomu ABE
Akira Hashimoto
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Nippon Steel Corp
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Nippon Steel Corp
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
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    • 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
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    • 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
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    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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Definitions

  • the present invention relates to a steel pipe and a steel plate that is used for the base material portion of the steel pipe.
  • Steel pipes having excellent HIC resistance have been thus far manufactured by employing a technique, for example, an increase in the purity of steel, a decrease in an inclusion, the control of the shape of a sulfide-based inclusion generated by addition of Ca, or a decrease in center segregation caused by light rolling reduction or accelerated cooling during casting as disclosed in Patent Documents 1 and 2.
  • Patent Document 3 discloses a method for manufacturing a thin sour-resistant steel plate having a plate thickness of 15 mm or less.
  • the manufacturing method of Patent Document 3 defines the conditions of finish rolling from the viewpoint of improving low temperature toughness.
  • accelerated cooling is carried out on a steel plate, and there is a problem in that the surface layer of the steel plate is hardened. As a result of the present inventors' inspection, it was found that steel plates having a hardened surface layer have a concern that the SSC resistance may degrade.
  • Non-Patent Document 1 In addition, conventionally, in a case where the plate thickness was thin, as described in Non-Patent Document 1, there was a case where accelerated cooling was not applicable and steel plates were manufactured by air cooling after rolling. However, in a case where steel plates were manufactured by air cooling, in some cases, ferrite (polygonal ferrite) was generated, and the SSC resistance degraded.
  • Non-Patent Document 1 ISIJ International, Vol. 33 (1993), p. 1190 to 1195
  • an object of the present invention is to provide a steel pipe without using V, Cu, Ni, Mo, and/or the like, which are expensive elements that are easily segregated, as essential elements for ensuring strength and to provide a steel plate that serves as a material of the steel pipe.
  • the steel pipe has a strength of X60 grade in terms of the API standards, has excellent DWTT characteristics at -30°C, furthermore, is excellent in terms of SSC resistance and HIC resistance, and has a plate thickness of the steel plate in the base material portion (a thickness of the steel pipe) of 15 mm or less.
  • the present inventors carried out intensive studies regarding a method for solving the above-described problems. As a result, it was found that, when a hot-rolled steel plate obtained by the hot rolling of a steel piece having a predetermined chemical composition at a finish rolling temperature of 830°C to 1000°C is accelerated-cooled in two separate steps and then recuperated up to a necessary temperature, it is possible to manufacture a steel pipe that has a strength of X60 to X70 in terms of the API standards, is excellent in terms of DWTT characteristics, SSC resistance, and HIC resistance, and has a thickness of 15 mm or less.
  • a steel pipe according to the present embodiment can be provided with predetermined strength, DWTT characteristics, SSC resistance, and HIC resistance by maintaining a low Ceq in a steel plate having a plate thickness of 15 mm or less, which is used as a material of the base material portion, and then controlling rolling and cooling conditions in a steel plate manufacturing step.
  • This is significantly different concept from a technique in which a steel pipe is manufactured by As roll (as rolled) or normalizing, while adding a large amount of an alloying element.
  • a steel pipe that has a strength of X60 to X70 in terms of the API standards (tensile strength of 520 MPa to 760 MPa), has excellent DWTT characteristics, is also excellent in terms of sulfide stress cracking resistance and hydrogen-induced cracking resistance, and has a thickness of 15 mm or less even without using additive elements of V, Cu, Ni, Mo, and/or the like and to provide a steel plate that is used as a base metal of the steel pipe, has excellent DWTT characteristics, and is also excellent in terms of sulfide stress cracking resistance and hydrogen-induced cracking resistance.
  • a high-strength steel plate for a line pipe that is excellent in terms of DWTT characteristics, sulfide stress cracking resistance, and hydrogen-induced cracking resistance, which is preferred as a line pipe configured to transport petroleum, natural gas, and the like, and to provide a steel pipe for a line pipe that includes the steel plate as the base metal and is excellent in terms of DWTT characteristics, sulfide stress cracking resistance, and hydrogen-induced cracking resistance.
  • a steel pipe according to an embodiment of the present invention (hereinafter, the steel pipe according to the present embodiment) has a base material portion composed of a tubular steel plate and a welded part that is provided at a butt portion of the steel plate and extends in a longitudinal direction of the steel plate, wherein the steel plate has a chemical composition containing, by mass%, C: 0.030% to 0.070%, Si: 0.05% to 0.50%, Mn: 1.05% to 1.65%, Al: 0.010% to 0.070%, Ti: 0.005% to 0.020%, Nb: 0.005% to 0.045%, Ca: 0.0010% to 0.0050%, and N: 0.0010% to 0.0050%, as necessary, one or more selected from the group consisting of Ni: 0.50% or less, Mo: 0.50% or less, Cr: 0.50% or less, Cu: 0.50% or less, V: 0.100% or less, Mg: 0.0100% or less, and REM: 0.0100% or less, P: limited to 0.01
  • the steel plate according to the present embodiment is used for the base material portion of the steel pipe according to the present embodiment.
  • C is an element that improves the strength of steel.
  • the C content is set to 0.030% or more.
  • the C content is preferably 0.040% or more.
  • the C content is set to 0.070% or less. From the viewpoint of preventing the degradation of weldability, toughness, or the like, the C content is preferably 0.060% or less.
  • Si is an element that functions as a deoxidizing agent during steelmaking. When a Si content is less than 0.05%, this effect cannot be sufficiently obtained. Therefore, the Si content is set to 0.05% or more.
  • the Si content exceeds 0.50%, the toughness of a welded heat-affected zone (HAZ) decreases. Therefore, the Si content is set to 0.50% or less.
  • the Si content is preferably 0.35% or less.
  • Mn is an element that contributes to an improvement in the strength and toughness of steel.
  • the Mn content is set to 1.05% or more.
  • the Mn content is preferably 1.15% or more.
  • Mn is also an element that forms MnS and degrades the HIC resistance.
  • the Mn content exceeds 1.65%, the HIC resistance degrades, and thus the Mn content is set to 1.65% or less.
  • the Mn content is preferably 1.50% or less.
  • Al is an element that functions as a deoxidizing agent. When an Al content is less than 0.010%, this effect cannot be sufficiently obtained. Therefore, the Al content is set to 0.010% or more. The Al content is preferably 0.020% or more.
  • the Al content is set to 0.070% or less.
  • the Al content is preferably 0.040% or less and more preferably 0.030% or less.
  • Ti is an element that forms a nitride and contributes to the refinement of crystal grains.
  • the Ti content is set to 0.005% or more.
  • the Ti content is preferably 0.008% or more.
  • the Ti content exceeds 0.020%, a coarse nitride is generated, and the HIC resistance degrades. Therefore, the Ti content is set to 0.020% or less.
  • the Ti content is preferably 0.015% or less.
  • Nb is an element that contributes to an improvement in the strength of steel by expanding the non-recrystallization temperature range, refining crystal grains, and forming a carbide or a nitride.
  • the Nb content is set to 0.005% or more.
  • the Nb content is preferably 0.010% or more.
  • the Nb content exceeds 0.045%, a coarse carbide or nitride is generated, and the HIC resistance degrades. In addition, the elongation and the toughness also decrease. Therefore, the Nb content is set to 0.045% or less.
  • the Nb content is preferably 0.035% or less.
  • Ca is an element that contributes to an improvement in the HIC resistance by forming CaS and preventing the formation of MnS that extends in a rolling direction.
  • the Ca content is set to 0.0010% or more.
  • the Ca content is preferably 0.0020% or more.
  • the Ca content is set to 0.0050% or less.
  • the Ca content is preferably 0.0040% or less.
  • N is an element that contributes to the refinement of the microstructure by forming a nitride that prevents the coarsening of austenite grains during heating.
  • the N content is set to 0.0010% or more.
  • the N content is preferably 0.0020% or more.
  • the N content exceeds 0.0050%, a coarse nitride is generated, and the HIC resistance degrades. Therefore, the N content is set to 0.0050% or less.
  • the N content is preferably 0.0040% or less.
  • the base material portion of the steel pipe according to the present embodiment may contain, in addition to the above-described elements, one or more of Ni, Mo, Cr, Cu, V, Mg, and REM as necessary in the following ranges in order to improve strength, toughness, and other characteristics.
  • the lower limits thereof are 0%.
  • Ni is an element that contributes to an improvement in the toughness, strength, and corrosion resistance of steel.
  • the Ni content is preferably set to 0.05% or more.
  • the Ni content is more preferably 0.10% or more.
  • the Ni content exceeds 0.50%, the hardness of the base material portion exceeds 248 Hv, and the HIC resistance deteriorates. Therefore, even in a case where Ni is contained, the Ni content is set to 0.50% or less.
  • the Ni content is preferably 0.35% or less.
  • Mo is an element that contributes to an improvement in the hardenability of steel.
  • the Mo content is preferably set to 0.05% or more.
  • the Mo content is more preferably 0.10% or more.
  • the Mo content exceeds 0.50%, the hardness of the base material portion exceeds 248 Hv, and the HIC resistance deteriorates. Therefore, even in a case where Mo is contained, the Mo content is set to 0.50% or less.
  • the Mo content is preferably 0.35% or less.
  • the Cr is an element that contributes to an improvement in the strength of steel.
  • the Cr content is preferably set to 0.05% or more.
  • the Cr content is more preferably 0.10% or more.
  • the Cr content exceeds 0.50%, the strength excessively increases, and the toughness degrades. Therefore, even in a case where Cr is contained, the Cr content is set to 0.50% or less.
  • the Cr content is preferably 0.35% or less.
  • the Cu is an element that contributes to an increase in the strength of steel and an improvement in the corrosion resistance.
  • the Cu content is preferably set to 0.05% or more.
  • the Cu content is more preferably 0.10% or more.
  • the Cu content exceeds 0.50%, the maximum hardness of the base material portion exceeds 248 Hv, and the HIC resistance deteriorates. Therefore, even in a case where Cu is contained, the Cu content is set to 0.50% or less.
  • the Cu content is preferably 0.35% or less.
  • V 0% to 0.100%
  • V is an element that forms a carbide or a nitride and contributes to an improvement in the strength of steel.
  • the V content is preferably set to 0.010% or more.
  • the V content is more preferably 0.030% or more.
  • the V content exceeds 0.100%, the toughness of steel degrades. Therefore, the V content is set to 0.100% or less.
  • the V content is preferably 0.080% or less.
  • Mg is an element that forms a fine oxide that contributes to an improvement in toughness by preventing the coarsening of crystal grains.
  • the Mg content is preferably set to 0.0001% or more.
  • the Mg content is more preferably 0.0010% or more.
  • the Mg content exceeds 0.0100%, oxides agglomerate and coarsen, and the HIC resistance or the toughness degrades. Therefore, even in a case where Mg is contained, the Mg content is set to 0.0100% or less.
  • the Mg content is preferably 0.0050% or less.
  • the REM is an element that contributes to an improvement in the toughness by controlling the form of a sulfide-based inclusion.
  • the REM content is preferably set to 0.0001% or more.
  • the REM content is more preferably 0.0010% or more.
  • the REM content when the REM content exceeds 0.0100%, an oxide is generated, the cleanliness of steel decreases, and consequently, the toughness degrades. Therefore, even in a case where REM is contained, the REM content is set to 0.0100% or less.
  • the REM content is preferably 0.0060% or less.
  • REM means rare earth elements and is a collective term of 17 elements of Sc, Y, and lanthanoid, and the REM content indicates a total amount of these 17 elements.
  • the base material portion of the steel pipe according to the present embodiment (the steel plate according to the present embodiment) basically contains the above-described essential elements and contains the above-described optional elements as necessary, and the remainder includes Fe and impurities.
  • the impurities mean components that are incorporated from a raw material such as an ore or a scrap or from a variety of environments of a manufacturing process during the industrial manufacturing of a steel material and are allowed to be contained as long as the impurities do not adversely affect the characteristics of steel.
  • P, S, O, Sb, Sn, Co, As, Pb, Bi, and H are preferably controlled to the ranges described below.
  • P is an impurity element.
  • the P content is set to 0.015% or less.
  • the P content is preferably 0.010% or less.
  • the content is preferably small, and thus the lower limit includes 0%.
  • the practical lower limit of the P content is 0.003%.
  • S is an element that degrades the HIC resistance by generating MnS that extends in the rolling direction during hot rolling.
  • a S content exceeds 0.0015%, the HIC resistance significantly degrades. Therefore, the S content is set to 0.0015% or less.
  • the S content is preferably 0.0010% or less.
  • the S content is preferably small, and thus the lower limit includes 0%. However, when the S content is decreased to less than 0.0001 %, the manufacturing costs significantly increase. Therefore, the practical lower limit of the S content is 0.0001%.
  • O is an element that inevitably remains in steel after deoxidation.
  • an O content exceeds 0.0040%, an oxide is generated, and the HIC resistance degrades. Therefore, the O content is set to 0.0040% or less.
  • the O content is preferably 0.0030% or less.
  • the O content is preferably small, and thus the lower limit includes 0%.
  • the manufacturing costs significantly increase, and thus the practical lower limit of the O content is 0.0010% in consideration of the fact that the steel plate is a practical steel plate.
  • impurities for example, 0.10% or less of Sb, Sn, Co, or As may remain in the steel plate, 0.005% or less of Pb or Bi may remain in the steel plate, and 0.0005% or less of H may remain in the steel plate.
  • the amount of each element in the above-described range it is necessary to control the amount of each element in the above-described range and then control the Ceq in a predetermined range.
  • the Ceq is calculated from the amounts of the components as described below.
  • the Ceq (carbon equivalent) is an index that indicates the hardenability of the steel plate. In order to secure a necessary strength in the steel pipe according to the present embodiment, the Ceq is set to 0.250 to 0.350.
  • the Ceq is defined by Expression (1).
  • Ceq C + Mn / 6 + Ni + Cu / 15 + Cr + Mo + V / 5
  • [C], [Mn], [Ni], [Cu], [Cr], [Mo], and [V] in Expression (1) are respectively the amounts of C, Mn, Ni, Cu, Cr, Mo, and V in the steel plate in terms of mass%.
  • the Ceq is set to 0.250 or more.
  • the Ceq is preferably 0.260 or more.
  • the Ceq exceeds 0.350, the hardenability becomes too high, the maximum hardness in the internal microstructure exceeds 248 Hv, and/or the maximum hardness of the surface layer area microstructure exceeds 250 Hv. As a result, the HIC resistance and/or the SSC resistance degrades. Therefore, the Ceq is set to 0.350 or less.
  • the Ceq is preferably 0.340 or less and more preferably 0.330 or less.
  • a microstructure in a range from a position deep over 1.0 mm apart from the surface of the steel plate in the base material portion to the plate thickness center in the depth direction (thickness direction) includes 85% or more of one or both of granular bainite and bainite in terms of the total area ratio and has an area ratio of an MA being 1.0% or less.
  • the microstructure in the range from a position deep over 1.0 mm from the surface of the steel plate to the plate thickness center in the depth direction (hereinafter, simply referred to as "internal microstructure” in some cases) is set to the microstructure containing 85% or more of one or both of granular bainite and bainite in terms of the total area ratio.
  • the total of the area ratios of granular bainite and/or bainite is set to 85% or more.
  • the total of the area ratios is preferably 90% or more. Since the area ratio depends on a kind of steel and a cooling rate, the upper limit of the area ratio may be 100%, but the practical upper limit is 95%.
  • the area ratio of a martensite-austenite constituent (MA) is set to 1.0% or less.
  • the MA may be 0%.
  • the remainder of the internal microstructure may be ferrite.
  • a microstructure (surface layer area microstructure) in a range of 1.0 mm from the surface of the steel plate in the depth direction includes 95% or more of one or both of granular bainite and tempered bainite in terms of the area ratio.
  • the SSC resistance improves, which is preferable.
  • the area ratios in the microstructure can be measured by observing the microstructure using a scanning electron microscope at a magnification of, for example, 1000 times.
  • the microstructure at the 1/4 position of the plate thickness (t/4) from the surface of the steel plate shows a typical microstructure of the internal microstructure. Therefore, in the present embodiment, when the microstructure is observed at t/4 of the base material portion (steel plate) of the steel pipe, and the microstructure at t/4 is the above-described microstructure, the internal microstructure is determined to be in the above-described range.
  • the microstructure of the surface layer area is obtained by measuring the positions 0.1 mm, 0.2 mm, and 0.5 mm apart from the surface of the steel plate and averaging the area ratios at the respective positions.
  • the bainite is a microstructure in which prior austenite grain boundaries are clear, fine lath microstructures are developed in the grains, and a fine carbide and the MA are scattered in the laths and between the laths.
  • the tempered bainite is a microstructure having a lath shape in which a carbide is dispersed in laths and lath boundaries.
  • the granular bainite is generated at a transformation temperature between the transformation temperatures of acicular ferrite and bainite and has intermediate microstructural characteristics.
  • the granular bainite is a microstructure in which A-part and B-part are present in a mixed form.
  • the A-part is a part in which prior austenite grain boundaries are partially visible, coarse lath microstructures are present in the grains, and a fine carbide and an austenite -martensite constituent are dispersed in the laths and between the laths.
  • the B-part is an acicular or irregular ferrite, and in the B-part prior austenite grain boundaries are not clear.
  • the ferrite is a microstructure in which an internal microscopic structure is rarely present in the grains and the grains have a flat inside.
  • the ferrite is a microstructure that appears white in the case of being observed with an optical microscope.
  • the MA is colored by Le Pera etching and is thus identifiable.
  • FIG. 3A shows an example of the microstructure captured at the t/4 position of the steel plate, which is the base material portion of the steel pipe according to the present embodiment, with a scanning electron microscope
  • FIG. 3B shows an example of the microstructure captured at the position 0.5 mm from the surface of the steel plate, which is the base material portion of the steel pipe according to the present embodiment, with the scanning electron microscope.
  • the maximum hardness is set to 248 Hv or less, and the average hardness is set to 170 to 220 Hv.
  • the maximum hardness exceeds 248 Hv, the HIC resistance degrades, and thus the maximum hardness is set to 248 Hv or less.
  • the maximum hardness is preferably 230 Hv.
  • the average hardness is less than 170 Hv, it is not possible to secure a necessary strength, and thus the average hardness is set to 170 Hv or more.
  • the average hardness is preferably 180 Hv or more.
  • the average hardness exceeds 220 Hv, the HIC resistance and the toughness degrade. Therefore, the average hardness is set to 220 Hv or less.
  • the average hardness is preferably 210 Hv or less.
  • the maximum hardness of the surface layer area microstructure is set to 250 Hv or less.
  • the maximum hardness is preferably 240 Hv or less.
  • the maximum hardness and the average hardness in the internal microstructure can be measured by the following method.
  • Hardness is measured with a Vickers hardness meter (load: 100 g) at points from a depth position of 1.1 mm from the surface of the steel plate as a starting point to the plate thickness center at intervals of 0.1 mm in the plate thickness direction and at 20 points for each of the corresponding depth at intervals of 1.0 mm in the width direction.
  • load 100 g
  • the maximum hardness of the internal microstructure is determined to be Hv 248 or less.
  • the average hardness is calculated by averaging the hardness values of all measurement points.
  • the maximum hardness of the surface layer area microstructure from the surface of the steel plate to a depth of 1.0 mm is measured as described below.
  • 300 mm ⁇ 300 mm steel plates are cut out by gas cutting from the 1/4 position, the 1/2 position, and the 3/4 position in the width direction of the steel plate (in the steel pipe, the three o'clock position, the six o'clock position, and the nine o'clock position respectively in a case where the welded part is regarded to be at the zero o'clock position) from an end portion of the steel plate in the width direction (in the case of the steel pipe, equivalent to the butt portion), and block test pieces having a length of 20 mm and a width of 20 mm are collected by mechanical cutting from the center of the cut-out steel plates and polished by mechanical polishing.
  • hardness is measured with a Vickers hardness meter (load: 100 g) at a total of 100 points (10 points at intervals of 0.1 mm in the plate thickness direction from the position 0.1 mm from the surface as a starting position ⁇ 10 points at intervals of 1.0 mm in the width direction at each of the corresponding depths). That is, in three block test pieces, hardness is measured at a total of 300 points.
  • the maximum hardness of the surface layer area is determined to be 250 Hv or less.
  • the integration degree of ⁇ 100 ⁇ 110> is 1.5 or more.
  • the steel plate according to the present embodiment is manufactured through steps such as hot rolling, cooling, and recuperating without being subjected to a quenching and tempering treatment. Therefore, the internal microstructure has a texture as described above. When the steel plate has a texture, the DWTT characteristics of the steel plate improve.
  • the texture can be obtained by the following method.
  • Plate thickness of steel plate in base material portion 15 mm or less
  • the steel pipe according to the present embodiment is a steel pipe for which a steel plate that is manufactured without carrying out a quenching and tempering treatment (as rolled and cooled) and has a plate thickness of 15 mm or less is used as the base material portion so as to have DWTT characteristics, SSC resistance, and HIC resistance, which are difficult to satisfy at the same time in the related art.
  • the steel pipe according to the present embodiment has excellent SSC resistance and excellent HIC resistance even when the steel plate has a plate thickness of 12 mm or less.
  • the target strength of the base material portion (the steel plate according to the present embodiment) of the steel pipe according to the present embodiment is a strength corresponding to 5L-X60 to X70 in terms of the API standards (tensile strength of 520 MPa to 760 MPa) in order to reliably secure a strength necessary for steel pipes.
  • the upper limit of the tensile strength necessary for steel pipes is preferably a tensile strength (TS) of 650 MPa or less in order to secure overmatching in the welded part during on-site welding at the time of using the steel pipe as a structural member.
  • the steel pipe according to the present embodiment is obtained by processing the steel plate according to the present embodiment into a tubular shape and butting and welding both end portions of the tubular steel plate. Therefore, the steel pipe has a welded part that is provided at the butt portion of the steel plate and extends in the longitudinal direction of the steel plate.
  • the welded part is formed to be thicker than the base material portion.
  • the weld metal is a higher alloy than the base metal and also has high corrosion resistance. Therefore, the welded part rarely serves as a starting point of fracture. Therefore, the welded part of the steel pipe according to the present embodiment is not particularly limited as long as the weld is obtained by SAW welding or the like under ordinary conditions.
  • the steel pipe is capable of obtaining the effects regardless of the manufacturing method.
  • the steel pipe is capable of stably obtaining the effects, which is preferable.
  • the steel pipe according to the present embodiment is obtained by a manufacturing method including
  • the above-described temperatures are controlled based on the surface temperature.
  • Heating temperature of steel piece 1050°C to 1250°C
  • the steel piece having the above-described chemical composition is heated.
  • the heating temperature of the steel piece is lower than 1050°C, a coarse non-solid-soluted carbonitride of Nb and Ti is generated, and the HIC resistance degrades. Therefore, the heating temperature of the steel piece is preferably set to 1050°C or higher.
  • the heating temperature of the steel piece is more preferably 1100°C or higher.
  • the heating temperature of the steel piece is preferably set to 1250°C or lower.
  • the heating temperature of the steel piece is more preferably 1200°C or lower.
  • the casting of molten steel and the manufacturing of the steel piece prior to the hot rolling step may be carried out according to ordinary methods.
  • Finish rolling temperature 830°C to 1000°C
  • the heated steel piece is hot-rolled to a steel plate of 15 mm or less.
  • the finish rolling temperature is preferably set to 830°C to 1000°C.
  • the finish rolling temperature is preferably 850°C or higher.
  • the finish rolling temperature is preferably set to 1000°C or lower.
  • the finish rolling temperature is more preferably 980°C or lower.
  • the steel plate having a surface temperature of a temperature Ts (cooling start temperature) which is in a temperature range of higher than 750°C to 950°C is cooled to a temperature Tm (cooling stop temperature) which is in a temperature range of 660°C to 750°C at an average cooling rate Vc1 of 25 to 50 °C/second.
  • Ts cooling start temperature
  • Tm cooling stop temperature
  • the cooling start temperature Ts is 750°C or lower in terms of the surface temperature, the area ratio of ferrite exceeds 15%. In this case, the area ratio of one or both of granular bainite and bainite becomes less than 85%, and the HIC resistance degrades. Therefore, the cooling start temperature Ts is preferably higher than 750°C in terms of the surface temperature. The cooling start temperature Ts is more preferably 800°C or higher.
  • the cooling start temperature Ts exceeds 950°C, crystal grains coarsen, and the low-temperature toughness degrades. In addition, there is a case where the maximum hardness of the surface layer area becomes too high. Therefore, the cooling start temperature Ts is preferably set to 950°C or lower in terms of the surface temperature. The cooling start temperature Ts is more preferably 930°C or lower.
  • the average cooling rate Vc1 is preferably set to 25 °C/second or more.
  • the average cooling rate is more preferably 30 °C/second or more.
  • the average cooling rate Vc1 exceeds 100 °C/second, the maximum hardness exceeds 248 Hv in the internal microstructure, and thus the HIC resistance degrades. Therefore, the average cooling rate Vc1 is preferably set to 100 °C/second or less. The average cooling rate is more preferably 50 °C/second or less and still more preferably 45°C/second or less.
  • the cooling stop temperature Tm in the first cooling step is lower than 660°C in terms of the surface temperature, a large amount of ferrite is generated, it is not possible to obtain 85% or more of one or both of granular bainite and bainite in terms of the area ratio, and the SSC resistance and the HIC resistance degrade. Therefore, the cooling stop temperature Tm is preferably set to 660°C or higher. The cooling stop temperature is more preferably 680°C or higher. On the other hand, when the cooling stop temperature Tm exceeds 750°C, there is a concern that the surface layer area may be hardened and the SSC resistance may degrade. Therefore, the cooling stop temperature Tm is preferably set to 750°C or lower. The cooling stop temperature is more preferably 720°C or lower.
  • the steel plate is cooled from the cooling stop temperature Tm of the first stage of 660°C to 750°C to a cooling stop temperature Tf of 400°C or lower at an average cooling rate of more than 50 °C/second.
  • the average cooling rate Vc2 is preferably set to more than 50 °C/second.
  • the average cooling rate is more preferably 60 °C/second or more.
  • the upper limit of the average cooling rate Vc2 is not particularly limited, but the cooling power of a cooling facility becomes a practical upper limit, and thus the upper limit is approximately 200 °C/second in the current status.
  • the cooling stop temperature Tf is preferably set to 400°C or lower.
  • the cooling stop temperature is more preferably 380°C or lower.
  • the cooling stop temperature Tf is determined depending on the kind of steel or the cooling rate, and the lower limit is not particularly set. However, from the viewpoint of obtaining a necessary microstructure or hardness by sufficiently recuperating the steel plate, the cooling stop temperature is preferably 250°C or higher.
  • cooling can be carried out by, in a cooling facility in which a cooling zone is divided into a plurality of sections and disposed in the longitudinal direction (conveyance direction) of the steel plate, adjusting the amount of cooling water sprayed to the steel plate in each section of the cooling zone.
  • the cooling rate is obtained by dividing the temperature difference between the cooling start temperature and the cooling stop temperature by the cooling time.
  • the steel plate After the steel plate is accelerated-cooled to the cooling stop temperature Tf of 400°C or lower as described above, the steel plate is recuperated at a recuperating rate Vr of 50 °C/second or more until the steel plate surface temperature Tr reaches higher than 550°C to 650°C.
  • the recuperating rate Vr is less than 50 °C/second, since there is a concern that the surface layer area may be hardened and the SSC resistance may degrade, the recuperating rate is set to 50 °C/second or more. Since the recuperating rate may be appropriately set in consideration of the time necessary for the surface temperature of the steel plate to reach higher than 550°C to 650°C, the upper limit is not particularly limited.
  • the recuperating rate is obtained by dividing the recuperating temperature width by the time necessary for recuperating.
  • the steel plate surface temperature after recuperating is preferably set to higher than 550°C.
  • the steel plate surface temperature after recuperating is more preferably 580°C or higher.
  • the steel plate surface temperature after recuperating exceeds 650°C, the average hardness does not reach 170 Hv. Therefore, the steel plate surface temperature after recuperating is preferably set to 650°C or lower.
  • the steel plate surface temperature after recuperating is more preferably 620°C or lower.
  • the recuperating rate and the amount of recuperating vary with the temperature difference between the surface and the inside of the steel plate when the cooling is stopped.
  • the temperature difference between the surface and the inside of the steel plate is not simply determined by the cooling rate, but varies with the sprayed water density, the impact pressure, or the like in water cooling. Therefore, the cooling conditions need to be determined such that the recuperating rate reaches 50 °C/second or more and the surface temperature after recuperating reaches higher than 550°C to 650°C.
  • Appropriate conditions can be set by, for example, carrying out an experiment for determining the conditions in advance.
  • FIG. 2 schematically shows an example of the cooling curve of the steel plate after finish rolling (a change in the steel plate surface temperature in the first cooling step, the second cooling step, and the recuperating step).
  • the steel plate after the recuperating step is preferably cooled to 300°C or lower at an average cooling rate of 0.01 °C/second or more.
  • the average cooling rate is less than 0.01 °C/second, it becomes impossible to obtain a target strength.
  • the steel plate that is used for the base material portion of the steel pipe according to the present embodiment can be manufactured by the above-described steps. That is, the steel plate according to the present embodiment is non-heat treated steel.
  • the steel plate according to the present embodiment obtained by the above-described steps is formed into a tubular shape, and the butt portion of the tubular steel plate (both end portions of the steel plate in the width direction) is welded to produce a steel pipe.
  • the forming of the steel plate according to the present embodiment into the steel pipe is not limited to specific forming.
  • the forming may be warm working, but is preferably cold working from the viewpoint of the dimensional accuracy.
  • the welding is also not limited to specific welding, but is preferably submerged arc welding. As the welding conditions, well-known conditions may be used depending on the thickness of the steel plate or the like.
  • a heat treatment (seam heat treatment) may be carried out on the welded part.
  • the heat treatment temperature may be in an ordinary temperature range, but is particularly preferably in a range of 300°C to Ac 1 point.
  • the pipe according to the present embodiment is a steel pipe having sufficient mechanical properties as a steel pipe for a line pipe in both the base material portion and the welded part.
  • a test piece was collected from a manufactured steel plate, and the internal microstructure was determined by observing the microstructure at the 1/4 position (t/4) of the plate thickness from the surface of the steel plate using a scanning electron microscope at a magnification of 1000 times.
  • the microstructure of the surface layer area was obtained by observing and measuring the positions 0.1 mm, 0.2 mm, and 0.5 mm apart from the surface of the steel plate and averaging the area ratios at the respective positions.
  • JIS No. 5 tensile test piece was produced, a tensile test prescribed in JIS Z 2241 was carried out, and the yield strength and the tensile strength were measured.
  • the hardness of the internal microstructure and the hardness of the surface layer area microstructure were measured with a Vickers hardness meter.
  • hardness was measured with a Vickers hardness meter (load: 100 g) at points from a depth position of 1.1 mm from the surface of the steel plate as a starting point to the plate thickness center at intervals of 0.1 mm in the plate thickness direction and at 20 points for each of the corresponding depth at intervals of 1.0 mm in the width direction.
  • a Vickers hardness meter load: 100 g
  • the highest value was regarded as the maximum hardness.
  • the average hardness was calculated by averaging the hardness values of all measurement points.
  • a 300 mm ⁇ 300 mm steel plate was cut out by gas cutting from an end portion of the steel plate in the width direction, and block test pieces having a length of 20 mm and a width of 20 mm were collected by mechanical cutting from the center of the cut-out steel plate and polished by mechanical polishing.
  • hardness was measured with a Vickers hardness meter (load: 100 g) at a total of 100 points (10 points at intervals of 0.1 mm in the plate thickness direction from the position 0.1 mm from the surface as a starting position ⁇ 10 points at intervals of 1.0 mm in the width direction at each of the corresponding depths). That is, in three block test pieces, hardness was measured at a total of 300 points.
  • HIC hydrogen-induced cracking
  • the NACE test is a test in which hydrogen sulfide gas is saturated in a solution (pH: 2.7) of a 5% NaCl solution and 0.5% acetic acid, the steel plate is immersed in the solution, and whether or not cracking occurs, is observed after 96 hours.
  • the DWTT characteristics (ductile fracture surface ratio at -30°C) were evaluated by the following method.
  • a DWTT test piece was collected from the steel plate such that the width direction of the steel plate became parallel to the longitudinal direction of the test piece.
  • the sample collection position was set to the 1/4 position of the steel plate in the width direction.
  • the DWTT test piece was an overall thickness test piece with a press notch.
  • a DWTT test was carried out on this test piece at -30°C based on API 5L, and the ductile fracture surface ratio to the entire fracture surface was measured. As the numerical value of the fracture surface ratio (%) increased, the DWTT characteristics became more excellent. In the present invention, in a case where the ductile fracture surface ratio was 85% or more, the DWTT characteristics were determined to be excellent.
  • the steel plates shown in Tables 1 to 3 were formed into a tubular shape by C press, U press, and O press, the end surfaces were tack-welded together, and final welding was carried out from the inner and outer surfaces. After that, the pipes were expanded to obtain steel pipes for a line pipe. As the final welding, submerged arc welding was applied.
  • Test pieces were collected from the base material portions of the manufactured steel pipes, and the fractions (area ratios) of each microstructure of the surface layer area microstructure and the internal microstructure were calculated. Specifically, the internal microstructure was determined by observing the microstructure at the 1/4 position (t/4) of the plate thickness from the surface of the steel plate using a scanning electron microscope at a magnification of 1000 times. The microstructure of the remainder not shown in the tables was ferrite. The microstructure of the surface layer area was obtained by measuring the positions 0.1 mm, 0.2 mm, and 0.5 mm apart from the surface of the steel plate and averaging the area ratios at the respective positions.
  • JIS No. 5 tensile test piece was produced, a tensile test prescribed in JIS Z 2241 was carried out, and the yield strength and the tensile strength were measured.
  • the hardness of the internal microstructure and the hardness of the surface layer area microstructure were measured with a Vickers hardness meter.
  • hardness was measured with a Vickers hardness meter (load: 100 g) at points from a depth position of 1.1 mm from the surface of the steel plate as a starting point to the plate thickness center at intervals of 0.1 mm in the plate thickness direction and at 20 points for each of the corresponding depth at intervals of 1.0 mm in the width direction.
  • a Vickers hardness meter load: 100 g
  • the highest value was regarded as the maximum hardness.
  • the average hardness was calculated by averaging the hardness values of all measurement points.
  • 300 mm ⁇ 300 mm steel plates were cut out by gas cutting from the three o'clock position, the six o'clock position, and the nine o'clock position respectively in a case where the welded part from the butt portion of the steel pipe was regarded to be at the zero o'clock position, and block test pieces having a length of 20 mm and a width of 20 mm were collected by mechanical cutting from the center of the cut-out steel plates and polished by mechanical polishing.
  • hardness was measured with a Vickers hardness meter (load: 100 g) at a total of 100 points (10 points at intervals of 0.1 mm in the plate thickness direction from the position 0.1 mm from the surface as a starting position ⁇ 10 points at intervals of 1.0 mm in the width direction at each of the corresponding depths). That is, in three block test pieces, hardness was measured at a total of 300 points.
  • test piece was collected from the base material portion of the manufactured steel pipe, and the following tests were carried out, thereby evaluating the HIC resistance and the SSC resistance.
  • HIC hydrogen-induced cracking
  • the NACE test is a test in which hydrogen sulfide gas is saturated in a solution (pH: 2.7) of a 5% NaCl solution and 0.5% acetic acid, the steel plate is immersed in the solution, and whether or not cracking occurs is observed after 96 hours.
  • the DWTT characteristics (ductile fracture surface ratio at -30°C) were evaluated by the following method.
  • a DWTT test piece was collected from the steel pipe such that the circumferential direction of the steel pipe became parallel to the longitudinal direction of the test piece.
  • the sample collection position was set to the 90° position from the seam position of the steel pipe.
  • the DWTT test piece was an overall thickness test piece with a press notch.
  • a DWTT test was carried out on this test piece at -30°C based on API 5L, and the ductile fracture surface ratio to the entire fracture surface was measured. As the numerical value of the fracture surface ratio (%) increased, the DWTT characteristics became more excellent. In the present invention, in a case where the ductile fracture surface ratio was 85% or more, the DWTT characteristics were determined to be excellent.
  • the present invention it is possible to provide a steel pipe that has a strength of X60 or higher in terms of the API standards, is excellent in terms of sulfide stress cracking resistance and hydrogen-induced cracking resistance, and has a thickness of 15 mm or less even without using additive elements of V, Cu, Ni, Mo, and/or the like and to provide a steel plate that is used as a base metal of the steel pipe and is excellent in terms of sulfide stress cracking resistance and hydrogen-induced cracking resistance. Therefore, the present invention is highly applicable in the steel pipe manufacturing industry and the energy industry.

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Claims (4)

  1. Tuyau en acier comprenant :
    une portion de matériau de base constituée d'une tôle d'acier tubulaire ; et
    une partie soudée qui est fournie à une portion d'aboutement de la tôle d'acier et s'étend dans une direction longitudinale de la tôle d'acier,
    dans lequel la tôle d'acier présente une composition chimique contenant, en % en masse,
    C : 0,030 % à 0,070 %,
    Si : 0,05 % à 0,50 %,
    Mn : 1,05 % à 1,65 %,
    Al : 0,010 % à 0,070 %,
    Ti : 0,005 % à 0,020 %,
    Nb : 0,005 % à 0,045 %,
    Ca : 0,0010 % à 0,0050 %,
    N : 0,0010 % à 0,0050 %,
    Ni : 0 % à 0,50 %,
    Mo : 0 % à 0,50 %,
    Cr : 0 % à 0,50 %,
    Cu : 0 % à 0,50 %,
    V : 0 % à 0,100 %,
    Mg : 0 % à 0,0100 %,
    REM : 0 % à 0,0100 %,
    P : limité à 0,015 % ou inférieur,
    S : limité à 0,0015 % ou inférieur,
    O : limité à 0,0040 % ou inférieur, et 0,10 % ou moins de Sb, Sn, Co, ou 0,005 % ou moins de Pb ou Bi et 0,0005 % ou moins de H,
    un reste : Fe et impuretés,
    la tôle d'acier présente un Ceq de 0,250 à 0,350, le Ceq étant défini par l'expression (1) suivante,
    une microstructure interne, qui est une microstructure dans un intervalle à partir d'une position profonde de plus de 1,0 mm à partir d'une surface de la portion de matériau de base jusqu'à un centre d'épaisseur de tôle dans une direction de profondeur, inclut 85 % ou plus d'un ou des deux de bainite granulaire et bainite en termes d'un rapport de surface totale et inclut 1,0 % ou moins d'un constituant de martensite-austénite en termes de rapport de surface,
    dans la microstructure interne, une dureté maximale est de 248 Hv ou inférieure et une dureté moyenne est de 170 à 220 Hv,
    la portion de matériau de base présente une texture ayant un degré d'intégration de {100}<110> de 1,5 ou supérieur dans un plan parallèle à une surface de tôle à ¼ de position d'une épaisseur de tôle à partir de la surface dans un direction d'épaisseur de tôle,
    une microstructure de région de couche de surface, qui est une microstructure dans un intervalle de 1,0 mm à partir de la surface de la portion de matériau de base dans la direction de profondeur, inclut 95 % ou plus d'un ou des deux de bainite granulaire et bainite trempée en termes d'un rapport de surface totale,
    dans la microstructure de région de couche de surface, une dureté maximale est de 250 Hv ou inférieure ; la portion de matériau de base présente une résistance à la traction de 520 à 760 MPa ;
    une épaisseur de tôle de la tôle d'acier est de 15 mm ou inférieure,
    dans lequel les dureté maximale, dureté moyenne et résistance à la traction sont mesurées par les procédés décrits dans la description ; Ceq = C + Mn / 6 + Ni + Cu / 15 + Cr + Mo + V / 15
    Figure imgb0015
    où [C], [Mn], [Ni], [Cu], [Cr], [Mo], et [V] dans l'expression (1) sont respectivement les quantités de C, Mn, Ni, Cu, Cr, Mo, et V dans la tôle d'acier en termes de % en masse ; les propriétés mécaniques et la microstructure ont été déterminées selon la description.
  2. Tuyau en acier selon la revendication 1,
    dans lequel la composition chimique contient, en % en masse, un ou plusieurs choisis dans le groupe consistant en
    Ni : 0,05 % à 0,50 %,
    Mo : 0,05 % à 0,50 %,
    Cr : 0,05 % à 0,50 %,
    Cu : 0,05 % à 0,50 %,
    V : 0,010 % à 0,100 %,
    Mg : 0,0001 % à 0,0100 %, et
    REM : 0,0001 % à 0,0100 %.
  3. Tuyau en acier selon la revendication 1 ou 2,
    dans lequel un reste de la microstructure interne est de la ferrite.
  4. Tôle d'acier qui est utilisée pour la portion de matériau de base du tuyau en acier selon l'une quelconque des revendications 1 à 3,
    dans laquelle la tôle d'acier présente une composition chimique contenant, en % en masse,
    C : 0,030 % à 0,070 %,
    Si : 0,05 % à 0,50 %,
    Mn : 1,05 % à 1,65 %,
    Al : 0,010 % à 0,070 %,
    Ti : 0,005 % à 0,020 %,
    Nb : 0,005 % à 0,045 %,
    Ca : 0,0010 % à 0,0050 %,
    N : 0,0010 % à 0,0050 %,
    Ni : 0 % à 0,50 %,
    Mo : 0 % à 0,50 %,
    Cr : 0 % à 0,50 %,
    Cu : 0 % à 0,50 %,
    V : 0 % à 0,100 %,
    Mg : 0 % à 0,0100 %,
    REM : 0 % à 0,0100 %,
    P : limité à 0,015 % ou inférieur,
    S : limité à 0,0015 % ou inférieur,
    O : limité à 0,0040 % ou inférieur, et
    un reste : Fe et impuretés,
    la tôle d'acier présente un Ceq de 0,250 à 0,350, le Ceq étant défini par l'expression (1) suivante,
    une microstructure interne, qui est une microstructure dans un intervalle à partir d'une position profonde de plus de 1,0 mm à partir d'une surface de la tôle d'acier jusqu'à un centre d'épaisseur de tôle dans une direction de profondeur, inclut 85 % ou plus d'un ou des deux de bainite granulaire et bainite en termes d'un rapport de surface totale et inclut 1,0 % ou moins d'un MA en termes de rapport de surface,
    dans la microstructure interne, une dureté maximale est de 248 Hv ou inférieure et une dureté moyenne est de 170 à 220 Hv,
    la tôle d'acier présente une texture ayant un degré d'intégration de {100}<110> de 1,5 ou supérieur dans un plan parallèle à une surface de tôle à ¼ de position d'une épaisseur de tôle à partir de la surface dans un direction d'épaisseur de tôle,
    une microstructure de région de couche de surface, qui est une microstructure dans un intervalle de 1,0 mm à partir de la surface de la tôle d'acier dans la direction de profondeur, inclut 95 % ou plus d'un ou des deux de bainite granulaire et bainite trempée en termes d'un rapport de surface totale,
    dans la microstructure de région de couche de surface, une dureté maximale est de 250 Hv ou inférieure ; la tôle d'acier présente une résistance à la traction de 520 à 760 MPa ;
    une épaisseur de tôle de la tôle d'acier est de 15 mm ou inférieure,
    dans lequel les dureté maximale, dureté moyenne et résistance à la traction sont mesurées par les procédés décrits dans la description ; Ceq = C + Mn / 6 + Ni + Cu / 15 + Cr + Mo + V / 15
    Figure imgb0016
    où [C], [Mn], [Ni], [Cu], [Cr], [Mo], et [V] dans l'expression (1) sont respectivement les quantités de C, Mn, Ni, Cu, Cr, Mo, et V dans la tôle d'acier en termes de % en masse ; les propriétés mécaniques et la microstructure ont été déterminées selon la description.
EP18923989.0A 2018-06-29 2018-06-29 Tuyau d'acier et tôle d'acier Active EP3816311B1 (fr)

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