WO2014054287A1 - 厚肉鋼管の製造方法 - Google Patents

厚肉鋼管の製造方法 Download PDF

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WO2014054287A1
WO2014054287A1 PCT/JP2013/005900 JP2013005900W WO2014054287A1 WO 2014054287 A1 WO2014054287 A1 WO 2014054287A1 JP 2013005900 W JP2013005900 W JP 2013005900W WO 2014054287 A1 WO2014054287 A1 WO 2014054287A1
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Prior art keywords
pipe
steel pipe
flow
water
axial flow
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PCT/JP2013/005900
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English (en)
French (fr)
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勝村 龍郎
啓之 福田
菅野 康二
和俊 石川
石黒 康英
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Jfeスチール株式会社
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Priority to BR112015007331A priority Critical patent/BR112015007331A2/pt
Priority to EP13844288.4A priority patent/EP2905347B1/en
Priority to US14/433,727 priority patent/US9506132B2/en
Priority to JP2014539616A priority patent/JP5896036B2/ja
Priority to MX2015003780A priority patent/MX2015003780A/es
Publication of WO2014054287A1 publication Critical patent/WO2014054287A1/ja

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    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/18Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/54Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING 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
    • C21D1/00General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING 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
    • C21D1/00General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18Hardening; Quenching with or without subsequent tempering
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    • C21METALLURGY OF IRON
    • C21DMODIFYING 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
    • C21D1/00General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/56General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering characterised by the quenching agents
    • C21D1/60Aqueous agents
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    • C21DMODIFYING 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
    • C21D6/00Heat treatment of ferrous alloys
    • C21D6/001Heat treatment of ferrous alloys containing Ni
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    • C21DMODIFYING 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
    • C21D6/00Heat treatment of ferrous alloys
    • C21D6/004Heat treatment of ferrous alloys containing Cr and Ni
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    • C21METALLURGY OF IRON
    • C21DMODIFYING 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
    • C21D6/00Heat treatment of ferrous alloys
    • C21D6/005Heat treatment of ferrous alloys containing Mn
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    • C21METALLURGY OF IRON
    • C21DMODIFYING 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
    • C21D6/00Heat treatment of ferrous alloys
    • C21D6/008Heat treatment of ferrous alloys containing Si
    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING 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
    • C21D8/00Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/10Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies
    • C21D8/105Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies of ferrous alloys
    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING 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
    • C21D9/00Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/08Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for tubular bodies or pipes
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    • C21METALLURGY OF IRON
    • C21DMODIFYING 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
    • C21D9/00Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/08Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for tubular bodies or pipes
    • C21D9/085Cooling or quenching
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    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/001Ferrous alloys, e.g. steel alloys containing N
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    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/02Ferrous alloys, e.g. steel alloys containing silicon
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    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/04Ferrous alloys, e.g. steel alloys containing manganese
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    • C22C38/06Ferrous alloys, e.g. steel alloys containing aluminium
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    • C22C38/08Ferrous alloys, e.g. steel alloys containing nickel
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    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/12Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
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    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/14Ferrous alloys, e.g. steel alloys containing titanium or zirconium
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    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/18Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/42Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
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    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/18Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/44Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
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    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/18Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/46Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
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    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/18Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/48Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
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    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/18Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/50Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium

Definitions

  • the target strength mentioned here means a yield point, specifically, a 0.2% proof stress, a 0.7% proof strength, or a yield strength.
  • the inner / outer surface immersion axial quenching method which is performed, is advantageous in that it has a large cooling capacity and a simple structure of the apparatus (see Patent Document 1 [0002]).
  • Patent Document 1 [0002] In order to minimize the difference in cooling history in the circumferential direction of pipe, the steel pipe is immersed in water in the water tank while rotating, and the water injected from the nozzle in the water (nozzle)
  • a rotary quenching equipment for steel pipe that sprays and quenches the inner and outer surfaces of the steel pipe is installed in the final heat treatment line of the carbon steel pipe (Patent Documents 2 [0002] to [0003]. reference).
  • the steel pipe having the composition A disclosed in Patent Document 3 is the thick steel pipe, it is stably adjusted to the target strength (surface hardness / center) in one QT. It is difficult to adjust the hardness ratio to 1.00 to 1.05. Therefore, in such a case, conventionally, one or both of repeating Q (quenching) a plurality of times or increasing the amount of alloy that contributes to improving the hardenability in the composition A is included. Measures were adopted. However, the former measure has the disadvantage of increasing heat treatment costs, and the latter measure is limited because the weldability and corrosion resistance (particularly corrosion resistance in hydrogen sulfide environment (hydrogen sulfide corrosion resistance)) are impaired. In addition, there is a disadvantage that the alloy cost is increased. Therefore, in the background art, the thick steel pipe is stably adjusted to the target strength by one QT (surface hardness / center hardness ratio: adjusted to 1.00 to 1.05). There was a problem that it was difficult.
  • the present inventors diligently studied to solve the above-mentioned problems, and as a result, immersed in water while rotating around the tube axis while supporting a high-temperature steel pipe, water flows on the inner and outer surfaces of the steel pipe while continuing the rotation.
  • the cooling step of imparting a cooling property by adopting a specific cooling condition, the cooling capacity is improved so that even the thick steel pipe having the composition A is sufficiently hardened to the center of the thickness.
  • the target strength can be stably adjusted (adjusted to the ratio of surface hardness / center hardness: 1.00 to 1.05) by one QT, and the present invention has been made.
  • the present invention supports a steel pipe having a thickness of 1/2 inch or more heated in the ⁇ range (austenite region) and is immersed in water while rotating around the tube axis and rotating in the water.
  • the axial flow that is the water flow in the direction of the pipe axis on the inner surface (inside surface of a pipe)
  • the collision flow that is the water flow that collides with the outer surface of the tube (outer surface of a pipe)
  • the rotation has a pipe peripheral speed (circumferential velocity of pipe) of 4 m / s or more
  • the application of the axial flow and the collision flow is as follows: Starting within 1.1 s after immersion of the entire steel pipe and continuing until the steel pipe reaches 150 ° C.
  • the pipe flow velocity (pipe flow velocity) is 7 m / s or more
  • the discharge flow velocity of the collision flow Production of thick-walled steel pipes characterized by a discharge velocity of 9 m / s or more. It is a method.
  • the cooling capacity during quenching is improved to the range of 7500 to 8000 kcal / m 2 ⁇ h ⁇ ° C. in terms of the heat-transfer coefficient at the inner and outer surfaces of the steel pipe, Even the thick steel pipe of A can be fully fired to the center of the thickness, and can be stably adjusted to the target strength by one QT.
  • FIG. 1 is a schematic diagram showing an example of a cooling process according to the present invention.
  • a steel pipe 1 having a thickness of 1/2 inch or more (preferably 2 inches or less) heated to the ⁇ region for quenching is supported and rotated 2 around the pipe axis.
  • the axial flow 5 which is the water flow in the axial direction of the tube, is directed to the tube inner surface side, toward the tube outer surface side.
  • a collision flow 6 which is a water flow that collides with the outer surface of the tube.
  • the support and rotation means (support and rotary means) of the steel pipe 1 are a plurality of pipes having a rotation axis parallel to the pipe axis on the pipe outer periphery at a plurality of places (at least two places) in the pipe axis direction of the steel pipe 1.
  • the steel pipe 1 is supported by abutting (at least two places) on the rollers 10, and the steel pipe 1 is rotated 2 by driving and rotating any one (at least one place) of the plurality of rollers 10.
  • the plurality of rollers 10 can be moved in and out of the water 3 by being supported and lifted by support lifting means (not shown).
  • the water temperature of the water 3 is preferably 50 ° C. or less.
  • the axial flow 5 is given by water injection (water injection) from a nozzle 11 disposed on one end surface side of the steel pipe 1 in the tube axis direction.
  • the collision flow 6 is applied by water injection from a plurality of nozzles 12 arranged in the pipe axis direction on both sides of the steel pipe 1 in the pipe radial direction.
  • the nozzles 11 and 12 are supported and lifted by the support lifting / lowering means (not shown) and can enter and exit the water 3.
  • VLC 7 m / s or more
  • the pipe peripheral speed VR of the rotation 2 is less than the VCR (4 m / s)
  • a plastic strain plastic due to a difference in cooling history at a position in the pipe peripheral direction and a difference in transformation behavior associated therewith. Since strain) increases and the steel pipe deforms, VR ⁇ VRC (4 m / s).
  • the pipe peripheral speed VR is 5 m / s or more.
  • the upper limit value of VR is 8 m / s or less because of the steel pipe popping out due to eccentricity.
  • t1C 1.1 s
  • bubbles generated on the inner surface side of the tube are more stable. It evolves into a film (water vapor film) and adheres to the inner surface of the tube, and the attached water vapor film is difficult to be detached from the inner surface of the tube even by the application of the axial flow 7, and the cooling capacity does not improve, so t1 ⁇ t1C (1 .1s).
  • t1 is 0.9 s or less.
  • T1C 150 ° C.
  • T1 is a value measured when the axial flow 5 and the collision flow 6 are stopped and held in water for about 10 seconds, extracted into the atmosphere, and further held for about 10 seconds.
  • T1 is 100 ° C. or lower.
  • the lower limit of T1 is 50 ° C. because the cooling time is required and the productivity is lowered as the temperature is lowered.
  • VL of the axial flow 5 is less than the VLC (7 m / s), it is difficult to remove bubbles generated on the inner surface of the tube, and the cooling ability of the inner surface of the tube is not improved. Therefore, VL ⁇ VLC (7 m / s) ).
  • the pipe flow velocity VL is 10 m / s or more.
  • the upper limit of VL is 20 m / s from the reason of equipment cost.
  • the discharge flow velocity VT of the collision flow 6 is less than the VTC (9 m / s), it is difficult to remove bubbles generated on the outer surface of the tube, and the cooling capacity of the outer surface of the tube is not improved. Therefore, VT ⁇ VTC (9 m / s) ).
  • the discharge flow velocity VT of the collision flow 6 is 12 m / s or more.
  • the upper limit of VT is 30 m / s for reasons of equipment costs.
  • the steel composition of the steel pipe to which the present invention is applied has a thin target (thickness less than 1/2 inch), and a predetermined target strength can be obtained stably even if the cooling conditions specified in the present invention are not met.
  • the predetermined target strength is stabilized by the method of the present invention.
  • the said composition A is mentioned, for example.
  • a seamless steel pipe having the chemical composition (unit: mass%, balance is Fe and inevitable impurities) and size (wall thickness t x outer diameter D x length L) shown in Table 1 is tempered only once (QT). Processed.
  • the cooling process of the Q treatment was the cooling process of the form illustrated in FIG.
  • the T treatment (tempering treatment) was performed under normal tempering conditions (conditions of heating to the normal tempering temperature inside the furnace and then allowing to cool outside the furnace).
  • Table 2 shows the processing conditions of the QT process.
  • the steel pipe after the T treatment was examined for the yield point (target strength: abbreviation TS) and the hardness of the surface part and the central part in the thickness direction.
  • the difference in hardness between the surface layer portion and the center portion is reduced (the surface / center hardness ratio is within 1.00 to 1.05), and a homogeneous material can be obtained.

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  • Materials Engineering (AREA)
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Abstract

 従来技術では厚肉鋼管を1回のQ-Tで目標強度95~140ksi(=TS:655~965MPa)に安定して調整するのは困難である。 具体的には、γ域に加熱した肉厚1/2インチ以上の鋼管を支持して管軸周りに回転させつつ水中に浸漬し、前記水中で回転中の鋼管に対し、管内面側へは管軸方向の水流である軸流、管外面側へは管外面に衝突する水流である衝突流を付与する冷却工程を有する厚肉鋼管の製造方法であって、前記回転は管周速を4m/s以上とし、前記軸流および、衝突流の付与は、前記鋼管全体の浸漬後1.1s以内に開始して前記鋼管が150℃以下となるまで継続させ、前記軸流の管内流速を7m/s以上、前記衝流の吐出流速を9m/s以上とする。

Description

厚肉鋼管の製造方法
 本発明は、厚肉鋼管(heavy wall steel pipe or steel tube)の製造方法に関し、詳しくは、肉厚(wall thickness)が1/2インチ(inch)(=12.7mm)以上の鋼管である厚肉鋼管を熱処理(heat treatment)、特に1回の焼入れ焼戻し(quenching and tempering)(Q-T)により目標強度(target strength)95~140ksi(=TS:655~965MPa)に調整しうる厚肉鋼管の製造方法に関する。なお、ここで言う目標強度とは、降伏点(yield point)を意味し、具体的には、 0.2%耐力、0.7%耐力あるいは、降伏強度(yield strength)である。
 鋼管の焼入れ技術として次の事項が知られている。
1) 管端(pipe end)を含む多点拘束(multiple constraint)に鋼管の回転を付加した浸漬式内外面焼入法(both side dip quenching of steel pipes)は、焼入歪(quench distortion)の防止に著効があり、しかも、冷却能力(cooling capacity)も向上するので、継目無鋼管(seamless steel pipe)や電縫鋼管(electric resistance welded steel pipe)、特に、厚肉鋼管の熱処理(Q-T)に適している(非特許文献1参照)。
2) 加熱された鋼管を水槽に投入して浸漬し、鋼管の内外両面にその軸線方向(direction of axis)に沿う冷却水流(cooling water flow)(軸流(axial stream))を与えて焼入れを行う方式である内外面浸漬軸流焼入方式は、冷却能力が大きく、装置(equipment)の構造が単純であると云う点で有利である(特許文献1[0002]参照)。
3) 管周方向(circumferential direction of pipe)の冷却履歴(cooling history)の差を最小にする為に鋼管を回転させながら水槽の水に浸漬し、水中のノズル(nozzle)から噴射させた水を鋼管の内外面に吹き付け、急冷する鋼管の回転焼入れ装置(rotary quenching equipment)が、炭素鋼管の最終熱処理ライン(final heat treatment line)内に設置されている(特許文献2[0002]~[0003]参照)。
 一方、薄肉(thin-walled)(肉厚1インチ未満)の鋼管では、Q-Tにより安定して前記目標強度に調整できるものとして、質量%で、C:0.15~0.50%、Si:0.1~1.0%、Mn:0.3~1.0%、P:0.015%以下、S:0.005%以下、Al:0.01~0.1%、N:0.01%以下、Cr:0.1~1.7%、Mo:0.40~1.1%、V:0.01~0.12%、Nb:0.01~0.08%、B:0.0005~0.003%を含有し、あるいはさらに、Cu:1.0%以下、Ni:1.0%以下、Ti:0.03%以下、W:2.0%以下、Ca:0.001~0.005%のうちの1種又は2種以上を含有し、残部がFe及び不可避的不純物である組成(組成Aと称する)とされた鋼管が知られている(特許文献3参照)。
特開平7-90378号公報 特開2008-231487号公報 特開2011-246798号公報
村田ら:「鋼管の浸漬式内外面焼入法」鐵と鋼、’82-S1226(562)
 しかし、前記背景技術では、前記特許文献3に開示された組成Aの鋼管は前記厚肉鋼管とされた場合、1回のQ-Tでは前記目標強度に安定して調整(表面硬さ/中心硬さの比:1.00~1.05に調整)することが困難である。そこで、かかる場合、従来では、Q(quenching)を複数回繰り返すか、前記組成Aにおいて焼入れ性(quench hardenability)の向上に寄与する合金(alloy)の添加量を増やすかの何れか一方又は両方の対策が採用されていた。しかし、前者の対策では熱処理コストが嵩む不利を伴い、後者の対策では溶接性(weldability)や耐食性(corrosion resistance)(特に硫化水素環境中での耐食性(hydrogen sulfide corrosion resistance))が損なわれるため限界があり又合金コストも嵩む不利を伴う。したがって、前記背景技術では前記厚肉鋼管を1回のQ-Tで前記目標強度に安定して調整(表面硬さ/中心硬さの比:1.00~1.05に調整)するのは困難であると云う課題があった。
 本発明者らは前記課題を解決する為に鋭意検討し、その結果、高温の鋼管を支持して管軸周りに回転させつつ水中に浸漬し、前記回転の続行下で鋼管の内外面に水流を付与する冷却工程において、特定の冷却条件(cooling condition)を採用することにより、冷却能力が向上して、前記組成Aの厚肉鋼管であっても肉厚中心部まで十分に焼きが入るようになり、1回のQ-Tで前記目標強度に安定して調整(表面硬さ/中心硬さの比:1.00~1.05に調整)できることを見出し、本発明を成した。
 即ち本発明は、γ域(gamma range:オーステナイト域:austenite region)に加熱した肉厚1/2インチ以上の鋼管を支持して管軸周りに回転させつつ水中に浸漬し、前記水中で回転中の鋼管に対し、管内面側(inside surface of a pipe)へは管軸方向の水流である軸流、管外面側(outer surface of a pipe)へは管外面に衝突する水流である衝突流(impinging stream)を付与する冷却工程を有する厚肉鋼管の製造方法であって、前記回転は管周速(circumferential velocity of pipe)を4m/s以上とし、前記軸流および、衝突流の付与は、前記鋼管全体の浸漬後1.1s以内に開始して前記鋼管が150℃以下となるまで継続させ、前記軸流の管内流速(pipe flow velocity)を7m/s以上、前記衝突流の吐出流速(discharge flow velocity)を9m/s以上とすることを特徴とする厚肉鋼管の製造方法である。
 本発明によれば、焼入れ時の冷却能力(cooling capacity)が、鋼管内外面における熱伝達率(heat-transfer coefficient)でみて7500~8000kcal/m・h・℃の範囲まで向上し、前記組成Aの厚肉鋼管であっても肉厚中心部まで十分に焼きが入るようになり、1回のQ-Tで前記目標強度に安定して調整できる。
本発明に係る冷却工程の1例を示す模式図である。
 図1は、本発明に係る冷却工程の1例を示す模式図である。図示の如く、本発明に係る冷却工程では、焼入れするためにγ域に加熱した肉厚1/2インチ以上(好ましくは、2インチ以下)の鋼管1を支持して管軸周りに回転2させつつ水(冷媒(cooling medium))3中に浸漬4し、前記水3中で回転2中の鋼管1に対し、管内面側へは管軸方向の水流である軸流5、管外面側へは管外面に衝突する水流である衝突流6を付与する。本例では、鋼管1の支持および回転手段(support and rotary means)は、鋼管1の管軸方向の複数箇所(少なくとも2箇所)で管外周部に、管軸と平行な回転軸を有した複数(少なくとも2箇所)のローラ10を当接させて鋼管1を支持し、前記複数のローラ10のうちの何れか(少なくとも1箇所)を駆動回転させて鋼管1を回転2させる構成とした。尚、前記複数のローラ10は図示しない支持昇降手段(support and elevating means)で支持および昇降されて水3中に出入りできる。尚、此処で、水3の水温は50℃以下が好ましい。
 又、本例においては、軸流5は鋼管1の管軸方向の一端面側に配置したノズル11からの水噴射(water injection)で付与される。一方、衝突流6は鋼管1の管径方向の両側で管軸方向に複数配列されたノズル12からの水噴射で付与される。前記ノズル11,12は前記複数のローラ10と同様、前記図示しない支持昇降手段で支持および昇降されて水3中に出入りできる。
 前記冷却工程において、前記回転2は管周速VRを該VRの臨界値(critical value)VCR=4m/s以上とし、前記軸流5および、衝突流6の付与は、前記鋼管1全体の浸漬4後の時間の臨界値t1C=1.1s以内に開始して前記鋼管1が温度の臨界値T1C=150℃以下となるまで継続させ、前記軸流5の管内流速VLを該VLの臨界値VLC=7m/s以上、前記衝突流6の吐出流速VTを該VTの臨界値VTC=9m/s以上とする。
 前記回転2の管周速VRが前記VCR(4m/s)未満であると、管周方向の位置での冷却履歴の差とこれに伴う変態挙動(transformation behavior)の差とによる塑性ひずみ(plastic strain)が大きくなって鋼管が変形するため、VR≧VRC(4m/s)とする。又、このようにすることで、焼入れ時の管内外面両側からの気泡(gas bubble)の離脱を促進し、熱伝達率を高める効果もある。
好ましくは、管周速VRは5m/s以上である。なお、VRの上限値は、偏心による鋼管の飛び出しの理由から、8m/s以下である。
 前記鋼管1全体の浸漬4から前記軸流5および、衝突流6の付与開始までの時間t1が前記t1C(1.1s)超であると、特に管内面側に生じた気泡がより安定な水蒸気膜(water vapor film)へと進化して管内面に付着し、該付着した水蒸気膜は前記軸流7の付与によっても管内面から離脱させ難く、冷却能力が向上しないため、t1≦t1C(1.1s)とする。好ましくは、t1は0.9s以下である。
 前記軸流5及び前記衝突流6の付与停止時の鋼管温度T1が前記T1C(150℃)超であると、肉厚方向の深部まで十分に焼入れ硬化(quenching and hardening)し難いため、T1≦T1C(150℃)とする。尚、ここでT1は軸流5及び衝突流6を停止して10秒程度水中に保持した後、大気中に抽出してさらに10秒程度保持した時点で測温した値を用いるものとする。好ましくは、T1は100℃以下である。なお、T1の下限値は、低温にするほど冷却時間が必要となり生産性が低下する理由から、50℃である。
 前記軸流5の管内流速VLが前記VLC(7m/s)未満であると、管内面側に生じた気泡を除去し難く、管内面の冷却能が向上しないため、VL≧VLC(7m/s)とする。
好ましくは、管内流速VLは10m/s以上である。なお、VLの上限値は、設備コストの理由から、20m/sである。
 前記衝突流6の吐出流速VTが前記VTC(9m/s)未満であると、管外面側に生じた気泡を除去し難く、管外面の冷却能が向上しないため、VT≧VTC(9m/s)とする。
好ましくは、衝突流6の吐出流速VTは12m/s以上である。なお、VTの上限値は、設備コストの理由から、30m/sである。
 本発明を適用する鋼管の鋼組成は、薄肉(肉厚1/2インチ未満)では本発明に規定された冷却条件を外れても所定の目標強度が安定して得られるが、厚肉(肉厚1/2インチ以上、好ましくは2インチ以下)では、従来の冷却方法では、所定の目標強度が安定して得られない鋼組成であっても、本発明方法により、所定の目標強度が安定して得られるれる。このような鋼組成としては、例えば前記組成Aが挙げられる。
 表1に示す化学組成(単位:質量%、残部はFe及び不可避的不純物)及びサイズ(肉厚t×外径D×長さL)の継目無鋼管を1回のみ焼入れ焼戻し(Q-T)処理した。前記Q処理の冷却工程は図1に例示した形態の冷却工程とした。前記T処理(tempering treatment)は通常の焼戻し条件(炉内(inside of furnace)で通常の焼戻し温度(tempering temperature)に加熱後、炉外で放冷する条件)で行った。前記Q-T処理の処理条件を表2に示す。
 前記T処理後の鋼管について、降伏点(目標強度:略号TS)、及び、肉厚方向の表層部(surface part)と中心部(central portion)の硬さを調査した。
 前記調査の結果を表2に示す。表2より、本発明例では比較例に比べて、肉厚中心でのTSが前記目標強度95~140ksi(=655~965MPa)に達したことが分る。加えて表層部と中心部の硬さの差が小さくなり(表面/中心硬さ比:1.00~1.05に収まり)均質な材質が得られることが認められる。
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000002
1     鋼管
2     回転
3     水(冷媒)
4     浸漬
5     軸流
6     衝突流
10    ローラ
11,12 ノズル

Claims (1)

  1.  γ域に加熱した肉厚1/2インチ以上の鋼管を支持して管軸周りに回転させつつ水中に浸漬し、前記水中で回転中の鋼管に対し、管内面側へは管軸方向の水流である軸流、管外面側へは管外面に衝突する水流である衝突流を付与する冷却工程を有する厚肉鋼管の製造方法であって、前記回転は管周速を4m/s以上とし、前記軸流および、衝突流の付与は、前記鋼管全体の浸漬後1.1s以内に開始して前記鋼管が150℃以下となるまで継続させ、前記軸流の管内流速を7m/s以上、前記衝流の吐出流速を9m/s以上とすることを特徴とする厚肉鋼管の製造方法。
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