JP6354789B2 - Manufacturing method of steel plate for high strength and high toughness steel pipe and steel plate for high strength and high toughness steel pipe - Google Patents
Manufacturing method of steel plate for high strength and high toughness steel pipe and steel plate for high strength and high toughness steel pipe Download PDFInfo
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- 229910000831 Steel Inorganic materials 0.000 title claims description 90
- 239000010959 steel Substances 0.000 title claims description 90
- 238000004519 manufacturing process Methods 0.000 title claims description 16
- 238000001816 cooling Methods 0.000 claims description 60
- 238000005096 rolling process Methods 0.000 claims description 51
- 229910000859 α-Fe Inorganic materials 0.000 claims description 42
- 239000000463 material Substances 0.000 claims description 30
- 239000000203 mixture Substances 0.000 claims description 21
- 238000001953 recrystallisation Methods 0.000 claims description 21
- 229910001563 bainite Inorganic materials 0.000 claims description 17
- 230000009467 reduction Effects 0.000 claims description 14
- 230000001186 cumulative effect Effects 0.000 claims description 9
- 239000012535 impurity Substances 0.000 claims description 9
- 229910052717 sulfur Inorganic materials 0.000 claims description 7
- 229910052799 carbon Inorganic materials 0.000 claims description 6
- 229910052802 copper Inorganic materials 0.000 claims description 6
- 229910052750 molybdenum Inorganic materials 0.000 claims description 6
- 229910052759 nickel Inorganic materials 0.000 claims description 6
- 229910052758 niobium Inorganic materials 0.000 claims description 6
- 229910052760 oxygen Inorganic materials 0.000 claims description 6
- 229910052698 phosphorus Inorganic materials 0.000 claims description 6
- 229910052748 manganese Inorganic materials 0.000 claims description 5
- 229910052757 nitrogen Inorganic materials 0.000 claims description 5
- 229910052742 iron Inorganic materials 0.000 claims description 3
- 229910001566 austenite Inorganic materials 0.000 description 14
- 230000000694 effects Effects 0.000 description 14
- VNWKTOKETHGBQD-UHFFFAOYSA-N methane Chemical compound C VNWKTOKETHGBQD-UHFFFAOYSA-N 0.000 description 12
- 230000009466 transformation Effects 0.000 description 12
- 238000010438 heat treatment Methods 0.000 description 11
- 239000002245 particle Substances 0.000 description 10
- 239000007789 gas Substances 0.000 description 9
- 229910000734 martensite Inorganic materials 0.000 description 9
- 238000000034 method Methods 0.000 description 9
- 230000007423 decrease Effects 0.000 description 7
- 238000009863 impact test Methods 0.000 description 5
- 239000003345 natural gas Substances 0.000 description 5
- 238000010521 absorption reaction Methods 0.000 description 4
- 230000002411 adverse Effects 0.000 description 4
- 230000000052 comparative effect Effects 0.000 description 4
- 238000000926 separation method Methods 0.000 description 4
- 238000003466 welding Methods 0.000 description 4
- 238000005266 casting Methods 0.000 description 3
- 238000002844 melting Methods 0.000 description 3
- 230000008018 melting Effects 0.000 description 3
- 150000004767 nitrides Chemical class 0.000 description 3
- 239000002994 raw material Substances 0.000 description 3
- 238000005728 strengthening Methods 0.000 description 3
- 229910000885 Dual-phase steel Inorganic materials 0.000 description 2
- 238000007796 conventional method Methods 0.000 description 2
- 230000001771 impaired effect Effects 0.000 description 2
- 230000006872 improvement Effects 0.000 description 2
- 238000001556 precipitation Methods 0.000 description 2
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- 239000006104 solid solution Substances 0.000 description 2
- 239000000126 substance Substances 0.000 description 2
- 238000009864 tensile test Methods 0.000 description 2
- HMUNWXXNJPVALC-UHFFFAOYSA-N 1-[4-[2-(2,3-dihydro-1H-inden-2-ylamino)pyrimidin-5-yl]piperazin-1-yl]-2-(2,4,6,7-tetrahydrotriazolo[4,5-c]pyridin-5-yl)ethanone Chemical compound C1C(CC2=CC=CC=C12)NC1=NC=C(C=N1)N1CCN(CC1)C(CN1CC2=C(CC1)NN=N2)=O HMUNWXXNJPVALC-UHFFFAOYSA-N 0.000 description 1
- LDXJRKWFNNFDSA-UHFFFAOYSA-N 2-(2,4,6,7-tetrahydrotriazolo[4,5-c]pyridin-5-yl)-1-[4-[2-[[3-(trifluoromethoxy)phenyl]methylamino]pyrimidin-5-yl]piperazin-1-yl]ethanone Chemical compound C1CN(CC2=NNN=C21)CC(=O)N3CCN(CC3)C4=CN=C(N=C4)NCC5=CC(=CC=C5)OC(F)(F)F LDXJRKWFNNFDSA-UHFFFAOYSA-N 0.000 description 1
- 229910001035 Soft ferrite Inorganic materials 0.000 description 1
- ATJFFYVFTNAWJD-UHFFFAOYSA-N Tin Chemical compound [Sn] ATJFFYVFTNAWJD-UHFFFAOYSA-N 0.000 description 1
- 239000000654 additive Substances 0.000 description 1
- 230000000996 additive effect Effects 0.000 description 1
- 239000010953 base metal Substances 0.000 description 1
- 230000015572 biosynthetic process Effects 0.000 description 1
- 229910052804 chromium Inorganic materials 0.000 description 1
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- 239000000470 constituent Substances 0.000 description 1
- 238000009749 continuous casting Methods 0.000 description 1
- 238000005336 cracking Methods 0.000 description 1
- 230000006866 deterioration Effects 0.000 description 1
- 238000011161 development Methods 0.000 description 1
- 238000005098 hot rolling Methods 0.000 description 1
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- 229920006395 saturated elastomer Polymers 0.000 description 1
- 229930195734 saturated hydrocarbon Natural products 0.000 description 1
- 229910052720 vanadium Inorganic materials 0.000 description 1
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 1
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Description
本発明は、高変形能を備え靭性に優れ、高い強度を有する高強度高靭性鋼管用鋼板及びその製造方法に関する。特に、本発明の製造方法は、強度レベルがAPI X80以上で、板厚20mm以下のUOEまたはプレスベンド法によって製造されるラインパイプ用の鋼板の製造方法として好適である。 The present invention relates to a steel sheet for high strength and high toughness steel pipe having high deformability, excellent toughness and high strength, and a method for producing the same. In particular, the production method of the present invention is suitable as a production method of a steel plate for a line pipe produced by a UOE or press bend method having a strength level of API X80 or more and a plate thickness of 20 mm or less.
天然ガス供給地の遠隔化に伴い、天然ガス輸送用パイプラインの長距離化が進み、輸送効率向上のため、操業ガス圧の高圧化やパイプの大径化が図られ、このために厚肉材や高強度グレード材が使用される。 As the natural gas supply areas become remote, the pipeline for natural gas transportation has become longer, and the operating gas pressure has been increased and the pipe diameter has been increased to improve transportation efficiency. Materials and high-strength grade materials are used.
厚肉材では現地においてパイプ同士をつなげる際の多層盛溶接のパス数が増大してしまいコスト高となってしまうことや、一定の厚み以上では円周溶接後のPWHT(POST WELD HEAT TREATMENT)が必要となることから、薄肉の高強度グレード材の需要が高まっている。 For thicker materials, the number of passes of multi-layer welding when connecting pipes in the field increases, resulting in high costs, and PWHT (POST WELD HEAT TREATMENT) after circumferential welding at a certain thickness or more. The demand for thin-walled high-strength grade materials is increasing because of the necessity.
天然ガス輸送用パイプラインにおいて、高圧操業もしくはリッチガスの輸送の場合、ガスバースト発生時の不安定延性破壊における亀裂伝播防止の観点から、シャルピー衝撃試験における高吸収エネルギー化が求められている。なお、本発明における高圧操業とは、ガス圧が10MPa以上のものを指す。また、リッチガスとは飽和炭化水素ガスにおいてメタン(化学式:CH4)より重分子のガスを指す。 In the case of high-pressure operation or rich gas transportation in a pipeline for natural gas transportation, high absorption energy in the Charpy impact test is required from the viewpoint of preventing crack propagation in unstable ductile fracture when a gas burst occurs. The high-pressure operation in the present invention refers to a gas pressure of 10 MPa or more. The rich gas refers to a gas having a higher molecular weight than methane (chemical formula: CH 4 ) in the saturated hydrocarbon gas.
不安定延性破壊における亀裂の伝播の防止には、亀裂伝播速度よりもガスの流出速度が大きく、亀裂先端のガス圧が十分低下する必要がある。また、亀裂伝播速度は吸収エネルギーが大きいほど遅い傾向を有する。高圧操業を目的とする場合、亀裂伝播の駆動力が大きくなるため、より高い吸収エネルギーが求められることとなる。 In order to prevent the propagation of cracks in unstable ductile fracture, the gas outflow rate must be greater than the crack propagation rate, and the gas pressure at the crack tip must be sufficiently reduced. The crack propagation speed tends to be slower as the absorbed energy is larger. When aiming at high-pressure operation, since the driving force for crack propagation increases, higher absorbed energy is required.
例えば、特許文献1には、高速延性破壊伝播停止性能を向上させるために、制御圧延、加速冷却により微細ベイナイト組織として、鋼板のシャルピー吸収エネルギーを向上させる方法が開示されている。 For example, Patent Document 1 discloses a method for improving Charpy absorbed energy of a steel sheet as a fine bainite structure by controlled rolling and accelerated cooling in order to improve high-speed ductile fracture propagation stopping performance.
また、天然ガス輸送用ラインパイプの素材として用いられる鋼板は高靭性であることも求められ、この観点からも吸収エネルギーを高めることが求められる。 Moreover, the steel plate used as a raw material of a natural gas transportation line pipe is also required to have high toughness, and from this viewpoint, it is also required to increase the absorbed energy.
一方、天然ガス輸送用パイプラインの場合、大地震や凍土地帯における地盤変動により、大変形が生じても、亀裂の発生防止が可能な高変形能を備えることも要求されるようになってきた。 On the other hand, in the case of pipelines for natural gas transportation, it has become necessary to have a high deformability that can prevent cracks even if large deformations occur due to ground deformation in a large earthquake or frozen land zone. .
鋼材の高変形能の指標として、降伏強度を引張強度で割った降伏比(YR(%))が使われ、低YR化されるほどパイプ座屈発生の限界歪が向上する。 The yield ratio (YR (%)) obtained by dividing the yield strength by the tensile strength is used as an index of the high deformability of the steel material. The lower the YR, the higher the critical strain of pipe buckling.
鋼材のミクロ組織を軟質なフェライト相と、硬質なベイナイトやマルテンサイトなどが適度に分散した硬質相の2相組織とすることで、低YRとなることが知られている。例えば特許文献2、3および4には、ミクロ組織をフェライト+ベイナイト、あるいはフェライト+マルテンサイト、またはフェライト+ベイナイト+マルテンサイトとしている。 It is known that a low YR is achieved by making the microstructure of a steel material a soft ferrite phase and a hard phase in which hard bainite, martensite and the like are appropriately dispersed. For example, in Patent Documents 2, 3 and 4, the microstructure is ferrite + bainite, ferrite + martensite, or ferrite + bainite + martensite.
しかしながら、特許文献1記載の均質単相組織鋼では、2相組織鋼でないために低YR特性を得ることが出来ない。 However, the homogeneous single phase steel described in Patent Document 1 cannot obtain low YR characteristics because it is not a dual phase steel.
特許文献2のように、2相域で圧延を実施したのちに加速冷却を実施すると低YRかつ低温靭性が向上できることが知られているが、第2相であるフェライトの分率が高くなる傾向となり、吸収エネルギーは大きく低下しているものと考えらえる。 As in Patent Document 2, it is known that low YR and low temperature toughness can be improved by performing accelerated cooling after rolling in a two-phase region, but the fraction of ferrite as the second phase tends to increase. Thus, it can be considered that the absorbed energy is greatly reduced.
また、特許文献3のような2相組織鋼においては高強度かつ低YRの特性が得られるものの、吸収エネルギーについては均質単相組織鋼と比較して劣る。 Moreover, although the high strength and low YR characteristics are obtained in the dual phase steel as in Patent Document 3, the absorbed energy is inferior to that of the homogeneous single phase steel.
特許文献4において、靭性に悪影響を及ぼす粗大フェライト析出を回避する製造条件を提案しているものの、強度レベルは630MPaと低く、高強度域での適用に懸念がある。 Although Patent Document 4 proposes a manufacturing condition for avoiding coarse ferrite precipitation that adversely affects toughness, the strength level is as low as 630 MPa, and there is concern about application in a high strength region.
そこで、本発明は、吸収エネルギーに悪影響を及ぼすことなく低YR化を達成する高強度高靭性鋼管用鋼板およびその製造方法を提供することを目的とする。 Therefore, an object of the present invention is to provide a high-strength and high-toughness steel pipe steel sheet that achieves a low YR without adversely affecting the absorbed energy and a method for producing the same.
本発明者等は、上記課題を解決するため、鋼組織における第2相の形態に及ぼす圧延時の圧下量及び冷却条件に着目して鋭意検討した結果、再結晶温度域での圧延後、未再結晶温度域の圧延での圧下量を制御し、(Ar3−50℃)以上Ar3未満からの加速冷却の冷却条件を制御することにより、優れた吸収エネルギー、高強度および低YR特性が得られることを見出した。ここで、Ar3変態点は以下の式で表される。式中の元素記号は各元素の含有量(質量%)を意味し、含有しない元素については0とする。 In order to solve the above-mentioned problems, the present inventors have intensively studied paying attention to the rolling amount and the cooling condition during rolling on the form of the second phase in the steel structure, and as a result, after rolling in the recrystallization temperature range, By controlling the amount of reduction in rolling in the recrystallization temperature range and controlling the cooling conditions for accelerated cooling from (Ar 3 -50 ° C.) to less than Ar 3 , excellent absorbed energy, high strength and low YR characteristics can be obtained. It was found that it can be obtained. Here, the Ar 3 transformation point is represented by the following formula. The element symbol in a formula means content (mass%) of each element, and is set to 0 about the element which does not contain.
Ar3(℃)=910−310C−80Mn−20Cu−15Cr−55Ni−80Mo
具体的には、下記1、2を見出した。
Ar 3 (° C.) = 910-310C-80Mn-20Cu-15Cr-55Ni-80Mo
Specifically, the following 1 and 2 were found.
1.再結晶温度域における圧延により均一なオーステナイト粒を得た後、未再結晶温度域での圧延を900℃以下の低温領域において所定の累積圧下率で実施することにより、オーステナイト粒に均一な多量のひずみを導入することが可能となる。 1. After obtaining uniform austenite grains by rolling in the recrystallization temperature region, rolling in the non-recrystallization temperature region is performed at a predetermined cumulative rolling reduction in a low temperature region of 900 ° C. or less, thereby obtaining a large amount of uniform austenite grains. Strain can be introduced.
2.(Ar3−50℃)以上Ar3未満を開始温度とする加速冷却により、均一かつ多量のひずみが導入されたオーステナイト粒を、ベイナイト主体の組織中に均質微細なフェライトが微量形成された2相組織にできることが見出された。 2. (Ar 3 -50 ° C.) Two phases in which a homogeneous and fine ferrite is formed in a small amount of homogeneous austenite grains in which a large amount of strain is introduced by accelerated cooling with an Ar temperature of less than Ar 3 as a starting temperature. It was found that the organization can do it.
本発明は以下のものを提供する。 The present invention provides the following.
[1]質量%で、C:0.02〜0.09%、Si:0.01〜0.30%、Mn:1.0〜3.0%、sol.Al:0.003〜0.050%、Nb:0.005〜0.10%、Ti:0.005〜0.05%、N:0.0010〜0.0060%を含有し、P:0.030%以下、S:0.010%以下、O:0.004%未満に制限し、さらに、質量%でCu:0.01〜1.0%、Ni:0.01〜1.0%、Cr:0.01〜1.0%、Mo:0.01〜1.0%、のうちから選ばれる1種または2種以上を任意成分として含有し、下記(1)式で計算されるCeq値が0.40〜0.60であり、残部がFeおよび不可避的不純物からなる成分組成を有する鋼素材を、表面温度:1000℃以上1250℃以下に加熱し、再結晶温度域において第一圧延し、該第一圧延後、900℃以下の未再結晶温度域において累積圧下率40%以上75%以下の条件で第二圧延し、該第二圧延後、下記の冷却開始温度及び冷却停止温度の冷却条件で冷却を行うことを特徴とする高強度高靭性鋼管用鋼板の製造方法。 [1] By mass%, C: 0.02 to 0.09%, Si: 0.01 to 0.30%, Mn: 1.0 to 3.0%, sol. Al: 0.003 to 0.050%, Nb: 0.005 to 0.10%, Ti: 0.005 to 0.05%, N: 0.0010 to 0.0060%, P: 0 0.030% or less, S: 0.010% or less, O: Restricted to less than 0.004%, and Cu: 0.01 to 1.0%, Ni: 0.01 to 1.0% in mass% , Cr: 0.01 to 1.0%, Mo: 0.01 to 1.0%, one or more selected from among the optional components, calculated by the following formula (1) A steel material having a component composition comprising a Ceq value of 0.40 to 0.60 and the balance consisting of Fe and unavoidable impurities is heated to a surface temperature of 1000 ° C. or more and 1250 ° C. or less. Rolled, and after the first rolling, the cumulative rolling reduction is 40% or more and 75 in the non-recrystallization temperature range of 900 ° C. or less. And second rolling under the following conditions, after said second rolling, the method of producing a high strength and high toughness steel pipe for a steel sheet which is characterized in that the cooling in the cooling conditions of cooling start temperature and cooling stop temperature below.
冷却開始温度: (Ar3−50℃)以上Ar3未満
冷却停止温度: 250〜500℃
Ceq(%)=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5 (1)
各元素は質量%での値とする。
Cooling start temperature: (Ar 3 −50 ° C.) or more and less than Ar 3 Cooling stop temperature: 250 to 500 ° C.
Ceq (%) = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 (1)
Each element is a value in mass%.
[2]前記成分組成は、さらに、質量%で、Ca:0.0005〜0.0100%を含有することを特徴とする[1]に記載の高強度高靭性鋼管用鋼板の製造方法。 [2] The method for producing a steel sheet for high strength and high toughness steel pipe according to [1], wherein the component composition further contains Ca: 0.0005 to 0.0100% by mass%.
[3]質量%で、C:0.02〜0.09%、Si:0.01〜0.30%、Mn:1.0〜3.0%、sol.Al:0.003〜0.050%、Nb:0.005〜0.10%、Ti:0.005〜0.05%、N:0.0010〜0.0060%を含有し、P:0.030%以下、S:0.010%以下、O:0.004%未満に制限し、さらに、質量%でCu:0.01〜1.0%、Ni:0.01〜1.0%、Cr:0.01〜1.0%、Mo:0.01〜1.0%、のうちから選ばれる1種または2種以上を任意成分として含有し、下記(1)式で計算されるCeq値が0.40〜0.60であり、残部がFeおよび不可避的不純物からなる成分組成と、ベイナイトを主体とし、フェライトを含有し、該フェライトの平均粒径が5μm未満、面積率でのフェライト分率が5%未満である鋼組織とを有することを特徴とする高強度高靭性鋼管用鋼板。
Ceq(%)=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5 (1)
各元素は質量%での値とする。
[4]前記成分組成は、さらに、質量%で、Ca:0.0005〜0.0100%を含有することを特徴とする[3]に記載の高強度高靭性鋼管用鋼板。
[3] By mass%, C: 0.02 to 0.09%, Si: 0.01 to 0.30%, Mn: 1.0 to 3.0%, sol. Al: 0.003 to 0.050%, Nb: 0.005 to 0.10%, Ti: 0.005 to 0.05%, N: 0.0010 to 0.0060%, P: 0 0.030% or less, S: 0.010% or less, O: Restricted to less than 0.004%, and Cu: 0.01 to 1.0%, Ni: 0.01 to 1.0% in mass% , Cr: 0.01 to 1.0%, Mo: 0.01 to 1.0%, one or more selected from among the optional components, calculated by the following formula (1) Ceq value is 0.40 to 0.60, the balance is composed of Fe and inevitable impurities, the main component is bainite, ferrite is contained, the average particle size of the ferrite is less than 5 μm, and the area ratio For high strength and high toughness steel pipes characterized by having a steel structure with a ferrite fraction of less than 5% Plate.
Ceq (%) = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 (1)
Each element is a value in mass%.
[4] The high strength and high toughness steel pipe steel plate according to [3], wherein the component composition further contains Ca: 0.0005 to 0.0100% by mass%.
本発明の高強度高靭性鋼管用鋼板は、圧延−加速冷却プロセスの最適化により特定の鋼組織に調整され、この特定の鋼組織と特定の成分組成との組み合わせで降伏比80%以下の低降伏比特性を備える。そして、本発明の高強度高靭性鋼管用鋼板は、従来の高変形能材と比較して、延性破壊領域において250J以上の優れた吸収エネルギーを示す。250J以上の吸収エネルギーであれば、高靭性であると評価できる。また、強度レベルは、降伏強度555MPa以上および引張強度700MPa以上である。 The steel sheet for high strength and high toughness steel pipe of the present invention is adjusted to a specific steel structure by optimizing the rolling-accelerated cooling process, and has a low yield ratio of 80% or less with a combination of this specific steel structure and a specific component composition. Has yield ratio characteristics. And the steel plate for high strength and high toughness steel pipes of this invention shows the outstanding absorbed energy of 250J or more in a ductile fracture area | region compared with the conventional high deformability material. If the absorbed energy is 250 J or more, it can be evaluated as having high toughness. The strength levels are a yield strength of 555 MPa or more and a tensile strength of 700 MPa or more.
特に、本発明の高強度高靭性鋼管用鋼板は、板厚20mm以下で上記の特性を有することから、産業上極めて有用である。 In particular, the steel sheet for high-strength and high-toughness steel pipe of the present invention is extremely useful industrially because it has the above-mentioned characteristics with a plate thickness of 20 mm or less.
以下、本発明の実施形態について説明する。なお、本発明は以下の実施形態に限定されない。 Hereinafter, embodiments of the present invention will be described. In addition, this invention is not limited to the following embodiment.
<高強度高靭性鋼管用鋼板の製造方法>
本発明の製造方法では、以下の成分組成を有する鋼素材を用いて、高強度高靭性鋼管用鋼板を製造する。先ず、鋼素材について説明する。
<Method for producing steel sheet for high strength and high toughness steel pipe>
In the production method of the present invention, a high strength and high toughness steel pipe steel plate is produced using a steel material having the following component composition. First, the steel material will be described.
鋼素材は、質量%で、C:0.02〜0.09%、Si:0.01〜0.30%、Mn:1.0〜3.0%、sol.Al:0.003〜0.050%、Nb:0.005〜0.10%、Ti:0.005〜0.05%、N:0.0010〜0.0060%を含有し、P:0.030%以下、S:0.010%以下、O:0.004%未満に制限し、さらに、質量%でCu:0.01〜1.0%、Ni:0.01〜1.0%、Cr:0.01〜1.0%、Mo:0.01〜1.0%、のうちから選ばれる1種または2種以上を任意成分として含有し、下記(1)式で計算されるCeq値が0.40〜0.60であり、残部がFeおよび不可避的不純物からなる成分組成を有する。なお、成分含有量の質量%を単に%と記載する場合がある。なお、P含有量、S含有量及びO含有量の「制限し」とは、製造時にこれらの含有量を制限する工程が必須になることを意味するものではなく、これらの元素の含有量が上記範囲にあれば本発明範囲内である。
Ceq(%)=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5 (1)
(1)式中の各元素は質量%での値とする。
The steel material is mass%, C: 0.02 to 0.09%, Si: 0.01 to 0.30%, Mn: 1.0 to 3.0%, sol. Al: 0.003 to 0.050%, Nb: 0.005 to 0.10%, Ti: 0.005 to 0.05%, N: 0.0010 to 0.0060%, P: 0 0.030% or less, S: 0.010% or less, O: Restricted to less than 0.004%, and Cu: 0.01 to 1.0%, Ni: 0.01 to 1.0% in mass% , Cr: 0.01 to 1.0%, Mo: 0.01 to 1.0%, one or more selected from among the optional components, calculated by the following formula (1) The Ceq value is 0.40 to 0.60, and the balance has a component composition consisting of Fe and inevitable impurities. In addition, mass% of component content may be described as simply%. In addition, “restricting” the P content, the S content, and the O content does not mean that a step of restricting these contents is essential at the time of production, and the content of these elements is not limited. Within the above range, it is within the scope of the present invention.
Ceq (%) = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 (1)
(1) Each element in the formula is a value in mass%.
また、上記成分組成は、さらに、質量%で、Ca:0.0005%〜0.0100%を含有してもよい。 Moreover, the said component composition may contain Ca: 0.0005%-0.0100% further by the mass%.
C:0.02〜0.09%
CはAPI X80以上の強度を確保するために有効な成分である。そのためには、C含有量を少なくとも0.02%にすることが必要である。一方、C含有量が0.09%を超えると加速冷却後に形成される硬質相がマルテンサイトとなり、母材シャルピー吸収エネルギーが低下する。そこで、C含有量は0.02%以上0.09%以下(以下、0.02〜0.09%)とする。後述する固溶Nbの活用および、優れた母材吸収エネルギーを得るためには、C含有量は0.03以上0.06%以下とすることが好ましい。
C: 0.02 to 0.09%
C is an effective component for ensuring the strength of API X80 or higher. For this purpose, the C content needs to be at least 0.02%. On the other hand, if the C content exceeds 0.09%, the hard phase formed after accelerated cooling becomes martensite, and the base material Charpy absorbed energy decreases. Therefore, the C content is set to 0.02% or more and 0.09% or less (hereinafter 0.02 to 0.09%). In order to utilize solid solution Nb described later and to obtain excellent base material absorption energy, the C content is preferably 0.03 to 0.06%.
Si:0.01〜0.30%
Siは脱酸に必要な元素である。Si含有量が0.01%未満ではその効果は少ない。また、Si含有量が0.30%を超えると溶接性および母材部のシャルピー吸収エネルギーが著しく低下する。そこで、Si含有量は0.01〜0.30%とする。母材部のシャルピー吸収エネルギーをさらに向上させたい場合には、Si含有量を0.01〜0.10%とすることが好ましい。
Si: 0.01-0.30%
Si is an element necessary for deoxidation. The effect is small when the Si content is less than 0.01%. On the other hand, if the Si content exceeds 0.30%, the weldability and the Charpy absorbed energy of the base metal part are significantly reduced. Therefore, the Si content is set to 0.01 to 0.30%. In order to further improve the Charpy absorbed energy of the base material portion, the Si content is preferably 0.01 to 0.10%.
Mn:1.0〜3.0%
MnはCと同様に鋼板の強度を確保するために必要であり、API X80以上の強度を確保するためには、Mn含有量を1.0%以上にすることが必要である。一方、Mn含有量が3.0%を超えると、鋳造時に不可避的に形成される偏析部に特に濃化し、セパレーションを誘発し、吸収エネルギーの低下をまねく原因となる。そこで、Mn含有量は1.0〜3.0%とする。高強度と吸収エネルギーをより改善したい場合は、Mn含有量を1.5〜2.2%とすることが好ましい。ここで述べるセパレーションとは、シャルピー衝撃試験時に破面において観察される亀裂伝播方向に対して垂直に生じる割れのことを指す。
Mn: 1.0-3.0%
Mn is necessary to ensure the strength of the steel sheet in the same manner as C, and in order to ensure the strength of API X80 or higher, the Mn content needs to be 1.0% or higher. On the other hand, if the Mn content exceeds 3.0%, the segregation part inevitably formed at the time of casting is particularly concentrated and induces separation, leading to a decrease in absorbed energy. Therefore, the Mn content is set to 1.0 to 3.0%. When it is desired to further improve the high strength and the absorbed energy, the Mn content is preferably 1.5 to 2.2%. The separation described here refers to cracks that occur perpendicular to the crack propagation direction observed on the fracture surface during the Charpy impact test.
P:0.030%以下、S:0.010%以下
P、Sは不純物として鋼中に不可避的に含まれる元素であり、鋼母材や、溶接熱影響部の靭性を劣化させるため、これらの含有量は経済性を考慮して可能な範囲で低減することが好ましい。本発明では、P含有量を0.030%以下、S含有量を0.010%以下とする。母材の中心偏析部の靭性を特に向上させたい場合は、P含有量を0.008%以下、S含有量を0.001%以下とすることが好ましい。
P: 0.030% or less, S: 0.010% or less P and S are elements inevitably contained in steel as impurities, and these deteriorate the toughness of the steel base material and the weld heat affected zone. The content of is preferably reduced as much as possible in consideration of economy. In the present invention, the P content is 0.030% or less, and the S content is 0.010% or less. When it is particularly desired to improve the toughness of the center segregation part of the base material, the P content is preferably 0.008% or less and the S content is 0.001% or less.
Al:0.003〜0.050%
Alは脱酸元素であり、Al含有量が0.003%未満ではその効果は十分ではなく、過剰に添加すると靭性の劣化をもたらす。そこで、Al含有量は0.003〜0.050%とする。特に、母材において優れたシャルピー吸収エネルギーを確保するためには、0.01〜0.04%とすることが好ましい。なお、Alはsol.Al(酸可溶性Al)とする。
Al: 0.003 to 0.050%
Al is a deoxidizing element, and if the Al content is less than 0.003%, the effect is not sufficient, and if added in excess, toughness is deteriorated. Therefore, the Al content is set to 0.003 to 0.050%. In particular, in order to ensure excellent Charpy absorbed energy in the base material, the content is preferably 0.01 to 0.04%. Al is sol. Let Al (acid-soluble Al).
Nb:0.005〜0.10%
Nbはオーステナイト未再結晶温度域を高温側に拡大する働きをする。後述するオーステナイト未再結晶温度域での40%以上の累積圧下率を十分確保するために、少なくともNbを0.005%含有する必要がある。また、Nbは固溶強化元素としても有用であり、API X80グレードの強度を確保するためにも不可欠である。しかし、Nb含有量が0.10%を超えると組織が過剰に硬化し、母材のシャルピー吸収エネルギーが低下する。そこで、Nb含有量は0.005〜0.10%とする。母材の、高強度と優れたシャルピー吸収エネルギーを両立させる観点からは、Nb含有量を0.010〜0.07%とすることが好ましい。
Nb: 0.005 to 0.10%
Nb functions to expand the austenite non-recrystallization temperature range to the high temperature side. In order to sufficiently secure a cumulative rolling reduction of 40% or more in the austenite non-recrystallization temperature range described later, it is necessary to contain at least 0.005% of Nb. Nb is also useful as a solid solution strengthening element and is indispensable for securing the strength of API X80 grade. However, when the Nb content exceeds 0.10%, the structure is excessively hardened and the Charpy absorbed energy of the base material is lowered. Therefore, the Nb content is set to 0.005 to 0.10%. From the viewpoint of achieving both high strength and excellent Charpy absorbed energy of the base material, the Nb content is preferably set to 0.010 to 0.07%.
Ti:0.005〜0.05%
Tiは鋼中で窒化物を形成する。Ti含有量が0.005%以上であると、窒化物のピンニング効果でオーステナイト粒の粗大化を防げる。このため、Ti含有量を0.005%以上にすることは、母材の靭性確保や溶接熱影響部での靭性確保の観点から有効である。Ti含有量が0.05%を超えると、TiCの析出強化により靭性が著しく低下する。そこで、Ti含有量は0.005〜0.05%とする。オーステナイト粒をさらに微細化し、あるいは、析出硬化をさらに抑制するためには、Ti含有量を0.008〜0.016%とすることが好ましい。
Ti: 0.005 to 0.05%
Ti forms nitrides in the steel. When the Ti content is 0.005% or more, coarsening of austenite grains can be prevented by the pinning effect of nitride. For this reason, making Ti content 0.005% or more is effective from a viewpoint of ensuring toughness of a base material and toughness ensuring in a welding heat affected zone. When the Ti content exceeds 0.05%, the toughness is remarkably lowered due to precipitation strengthening of TiC. Therefore, the Ti content is set to 0.005 to 0.05%. In order to further refine the austenite grains or further suppress precipitation hardening, the Ti content is preferably set to 0.008 to 0.016%.
N:0.0010〜0.0060%
NはTiと窒化物を形成してオーステナイトの粗大化を抑制し、靭性の向上に寄与する。このようなピンニング効果を得るため、Nを0.0010%以上含有する。一方、N含有量が0.0060%を超えると、溶接部、特に溶融線近傍で1450℃以上に加熱された溶接熱影響部でTiNが分解した場合、固溶Nに起因した溶接熱影響部の靭性が劣る場合がある。したがって、N含有量はN:0.0010〜0.0060%とする。溶接熱影響部の靭性に対する要求レベルが高い場合には、N含有量は0.0010〜0.0050%とすることが好ましい。
N: 0.0010 to 0.0060%
N forms a nitride with Ti, suppresses the coarsening of austenite, and contributes to the improvement of toughness. In order to obtain such a pinning effect, N is contained in an amount of 0.0010% or more. On the other hand, when the N content exceeds 0.0060%, when TiN decomposes in the weld zone, particularly in the weld heat affected zone heated to 1450 ° C. or higher in the vicinity of the melting line, the weld heat affected zone caused by the solute N May have poor toughness. Therefore, the N content is N: 0.0010 to 0.0060%. When the required level for the toughness of the weld heat affected zone is high, the N content is preferably 0.0010 to 0.0050%.
O:0.004%未満
Oは鋼中で酸化物を形成し、母材の吸収エネルギーや靭性を低下させる要因となる。O含有量が0.004%以上になると吸収エネルギーが著しく低下するため、O含有量は0.004%未満とする。酸化物形成を抑制し、吸収エネルギーをさらに高めるためにはO含有量を0.003%以下とすることが好ましい。
O: Less than 0.004% O forms an oxide in the steel and causes the absorbed energy and toughness of the base material to decrease. When the O content is 0.004% or more, the absorbed energy is remarkably lowered, so the O content is less than 0.004%. In order to suppress oxide formation and further increase the absorption energy, the O content is preferably 0.003% or less.
鋼素材は、強度調整の観点から必要に応じてCu、Ni、Cr、Moのうちから選ばれる1種または2種以上を任意元素として含有することができる。 A steel raw material can contain 1 type, or 2 or more types chosen from Cu, Ni, Cr, and Mo as an arbitrary element as needed from a viewpoint of intensity | strength adjustment.
Cu:0.01〜1.0%
Cuは強度を増加させるための元素である。その効果を得るためにはCu含有量を0.01%以上とすることが好ましい。Cu含有量が1.0%を超えると熱間脆性により鋼板表面の性状が劣化する。そこで、Cuを添加する場合は、その含有量を0.01〜1.0%とする。
Cu: 0.01 to 1.0%
Cu is an element for increasing the strength. In order to obtain the effect, the Cu content is preferably 0.01% or more. If the Cu content exceeds 1.0%, the surface properties of the steel sheet deteriorate due to hot brittleness. Therefore, when adding Cu, the content is set to 0.01 to 1.0%.
Ni:0.01〜1.0%
Niは母材の強度を増加させつつ靭性も向上させる。この効果を得るためにはNi含有量を0.01%以上にする。Ni含有量が1.0%を超えると効果が飽和し経済性を損なう。そこで、Niを添加する場合は、その含有量を0.01〜1.0%とする。
Ni: 0.01 to 1.0%
Ni improves the toughness while increasing the strength of the base material. In order to obtain this effect, the Ni content is set to 0.01% or more. If the Ni content exceeds 1.0%, the effect is saturated and the economy is impaired. Therefore, when adding Ni, the content is set to 0.01 to 1.0%.
Cr:0.01〜1.0%
Crは強度を増加するのに有効であり、Cr含有量が0.01%以上でその効果を発揮する。Cr含有量が1.0%を超えると靭性が劣化する。そこで、Crを添加する場合は、Cr含有量を0.01〜1.0%とする。
Cr: 0.01 to 1.0%
Cr is effective for increasing the strength, and exhibits its effect when the Cr content is 0.01% or more. If the Cr content exceeds 1.0%, the toughness deteriorates. Therefore, when adding Cr, the Cr content is set to 0.01 to 1.0%.
Mo:0.01〜1.0%
Moは強度を増加するのに有効であり、Mo含有量が0.01%以上でその効果を発揮し、Mo含有量が1.0%を超えると著しく靭性が劣化するとともに経済性が損なわれる。そこで、Moを添加する場合は、その含有量を0.01〜1.0%とする。
Mo: 0.01 to 1.0%
Mo is effective for increasing the strength, and the effect is exhibited when the Mo content is 0.01% or more. When the Mo content exceeds 1.0%, the toughness is remarkably deteriorated and the economic efficiency is impaired. . Therefore, when adding Mo, the content is set to 0.01 to 1.0%.
Ceq(%):0.40〜0.60
Ceq(%)はC+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5)で表され、式中の各元素記号は各元素の含有量(質量%)を意味する(含まないものは0とする)。板厚20mm以下で、API X80以上の強度を達成するため、Ceqを0.40以上とする。一方、Ceq(%)が0.60を超える場合、溶接性が劣化し特にパイプの円周溶接時の低温割れを防止できない。そこで、Ceqの上限を0.60とする。なお、含有しない元素は0とする。
Ceq (%): 0.40 to 0.60
Ceq (%) is represented by C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5), and each element symbol in the formula means the content (mass%) of each element (the one not included is 0) ). In order to achieve a strength of API X80 or more at a plate thickness of 20 mm or less, Ceq is set to 0.40 or more. On the other hand, when Ceq (%) exceeds 0.60, the weldability deteriorates, and in particular, it is not possible to prevent cold cracking during circumferential welding of pipes. Therefore, the upper limit of Ceq is set to 0.60. The element not contained is 0.
また、上記の通り、本発明では上記成分組成がCaを含んでもよい。 Further, as described above, in the present invention, the component composition may contain Ca.
Ca:0.0005〜0.0100%
Caは鋼中のSを固定して鋼板の靭性を向上させる働きがあり、この効果を発揮させるためには0.0005%以上含有することが好ましく、0.0010%以上含有することがさらに好ましい。一方、0.0100%を超えてCaを含有すると鋼中の介在物が増加し、靭性を劣化させる場合がある。したがって、Caを含有させる場合には、Caの含有量を0.0100%以下とすることが好ましく、0.0080%以下とすることがさらに好ましい。
Ca: 0.0005 to 0.0100%
Ca has a function of fixing S in steel and improving the toughness of the steel sheet. In order to exert this effect, 0.0005% or more is preferable, and 0.0010% or more is more preferable. . On the other hand, if Ca exceeds 0.0100%, inclusions in the steel increase and the toughness may be deteriorated. Therefore, when Ca is contained, the Ca content is preferably 0.0100% or less, and more preferably 0.0080% or less.
上記した成分以外の残部は、Feおよび不可避的不純物からなる。なお、不可避的不純物には、製造プロセスにおいて不可避的に混入するする成分の他、本発明の効果を害さない範囲で、他の添加成分とともに不可避的に含有される成分を含む。なお、上記任意成分の含有量が下限値未満であっても本発明の効果は害されないため、上記任意成分の含有量が上記下限値未満の場合には上記任意成分を不可避的不純物として含有するものとする。 The balance other than the components described above consists of Fe and inevitable impurities. The unavoidable impurities include components inevitably contained in addition to other additive components within a range not impairing the effects of the present invention, in addition to components inevitably mixed in the manufacturing process. In addition, since the effect of this invention is not spoiled even if content of the said arbitrary component is less than a lower limit, when content of the said arbitrary component is less than the said lower limit, the said arbitrary component is contained as an unavoidable impurity. Shall.
上記の成分組成を有する鋼素材の製造方法は特に限定されない。例えば、上記成分組成を有する溶鋼を、転炉、電気炉等の溶製手段で常法により溶製し、連続鋳造法または造塊〜分塊法等で常法によりスラブ等の鋼素材とする。なお、溶製方法、鋳造方法については上記した方法に限定されるものではない。その後、特定の成分組成を有する上記鋼素材を再結晶温度域で圧延後、900℃以下の未再結晶温度域で圧延した後、所定の温度範囲内から加速冷却を開始することにより、ベイナイト組織中にフェライトが均質微細かつ微量分散した組織とすることができる。以下、製造条件について、スラブ加熱、圧延条件、加速冷却の順で説明する。下記の製造条件における温度は表面温度を意味する。 The manufacturing method of the steel raw material which has said component composition is not specifically limited. For example, molten steel having the above component composition is melted by a conventional method using a melting means such as a converter or an electric furnace, and is made into a steel material such as a slab by a conventional method using a continuous casting method or an ingot-bundling method. . Note that the melting method and the casting method are not limited to the methods described above. Then, after rolling the steel material having a specific component composition in a recrystallization temperature range, rolling in a non-recrystallization temperature range of 900 ° C. or less, and then starting accelerated cooling from within a predetermined temperature range, a bainite structure A structure in which ferrite is homogeneously fine and finely dispersed can be obtained. Hereinafter, manufacturing conditions will be described in the order of slab heating, rolling conditions, and accelerated cooling. The temperature in the following production conditions means the surface temperature.
1.スラブ加熱
鋳造後、スラブ温度(鋼素材の温度)が室温まで低下してからあるいは高温の状態で、加熱炉に装入して1000℃以上に加熱する。
1. Slab heating After casting, the slab temperature (temperature of the steel material) is lowered to room temperature or in a high temperature state and charged in a heating furnace and heated to 1000 ° C or higher.
加熱温度は、靭性確保の観点からはより低温が好ましいが、1000℃未満ではスラブ厚中央の未厚着ザクが残存して板厚中心でのシャルピー特性を劣化させる可能性がある。また、Nb、Vなどを十分に固溶させるため、加熱温度を1000℃以上とする。より好ましくは、Nbを十分に固溶させるため以下の式から算出される温度Tsol.Nb以上にすることが望ましい。
Tsol.Nb=6770/(2.26−Log(Nb(C+0.857N)))−273
式中の各元素記号は各元素の含有量(質量%)とする。
The heating temperature is preferably lower from the viewpoint of securing toughness, but if it is less than 1000 ° C., there is a possibility that unthickened zack at the center of the slab thickness remains and deteriorates the Charpy characteristics at the center of the plate thickness. Further, in order to sufficiently dissolve Nb, V, etc., the heating temperature is set to 1000 ° C. or higher. More preferably, in order to sufficiently dissolve Nb, the temperature T sol. Nb or more is desirable.
T sol. Nb = 6770 / (2.26-Log (Nb (C + 0.857N)))-273
Each element symbol in the formula is the content (% by mass) of each element.
一方、1250℃を超える温度に加熱すると、加熱段階でオーステナイト粒が粗大化し、圧延終了時において微細なオーステナイト粒を得にくくなり靭性が劣化する。そこで、加熱温度の上限を1250℃とする。また、シャルピー吸収エネルギーを向上させるという理由で1200℃以下がより好ましい。 On the other hand, when heated to a temperature exceeding 1250 ° C., the austenite grains become coarse in the heating stage, and it becomes difficult to obtain fine austenite grains at the end of rolling, and toughness deteriorates. Therefore, the upper limit of the heating temperature is set to 1250 ° C. Moreover, 1200 degrees C or less is more preferable for the reason of improving Charpy absorbed energy.
2.圧延条件
再結晶温度域の圧延(第一圧延)は、スラブ等の鋼素材を、所望の形状とし、再結晶による組織の均質化を図るために行う。引き続き、900℃以下の未再結晶温度域で累積圧下率40%以上の圧延(第二圧延)を行う。
2. Rolling conditions Rolling in the recrystallization temperature range (first rolling) is performed to make a steel material such as a slab into a desired shape and to homogenize the structure by recrystallization. Subsequently, rolling (second rolling) with a cumulative reduction rate of 40% or more is performed in a non-recrystallization temperature range of 900 ° C. or lower.
再結晶温度域の圧延は、加熱時のオーステナイト粒をある程度まで均一微細化するのに必要であり、1パス以上、好ましくは累積で20%以上の圧下率で圧延を行う。 Rolling in the recrystallization temperature range is necessary to uniformly refine the austenite grains at the time of heating to a certain degree, and rolling is performed at a rolling reduction of 1 pass or more, preferably 20% or more cumulatively.
また、900℃以下の未再結晶温度域圧延は、圧下率が小さいと、その後の変態においてミクロ組織の微細化がなされず、強度の確保が困難となる。また、900℃より高温で圧延を実施すると、部分再結晶によりオーステナイトが十分に微細化されず靭性の低下を招く。そこで、900℃以下の未再結晶温度域において40%以上の累積圧下率を確保する。この未再結晶温度域の圧延では、圧下率(累積圧下率)は高い方が好ましいが、圧下率を高くしすぎて集合組織が発達しすぎるとセパレーションを誘発し、吸収エネルギーの低下をまねく。そこで、上記累積圧下率の上限を75%以下とした。 Further, in the non-recrystallization temperature range rolling at 900 ° C. or less, if the rolling reduction is small, the microstructure is not refined in the subsequent transformation, and it is difficult to ensure the strength. In addition, when rolling is performed at a temperature higher than 900 ° C., austenite is not sufficiently refined by partial recrystallization, resulting in a decrease in toughness. Therefore, a cumulative rolling reduction of 40% or more is ensured in the non-recrystallization temperature range of 900 ° C. or lower. In rolling in this non-recrystallization temperature range, it is preferable that the rolling reduction (cumulative rolling reduction) is high. However, if the rolling reduction is excessively high and the texture is developed too much, separation is induced and the absorbed energy is reduced. Therefore, the upper limit of the cumulative rolling reduction is set to 75% or less.
また、板厚が薄い場合は板厚中心における冷却速度が速く、圧延開始から冷却開始までの温度低下が著しい。したがって、冷却開始温度を制御する観点から、第二圧延の圧延開始温度は750℃超であることが好ましい。なお、板厚が15mmを下回る場合は800℃以上であることがさらに好ましい。 Further, when the plate thickness is thin, the cooling rate at the plate thickness center is fast, and the temperature drop from the start of rolling to the start of cooling is remarkable. Therefore, from the viewpoint of controlling the cooling start temperature, the rolling start temperature of the second rolling is preferably higher than 750 ° C. In addition, when plate | board thickness is less than 15 mm, it is still more preferable that it is 800 degreeC or more.
また、Ar3変態点〜900℃の間の未再結晶温度域において上記未再結晶温度域圧延を施すことにより、オーステナイト粒に効果的にひずみを導入することができる。このひずみ導入は、加速冷却後の変態組織を微細化させ、強度や靭性の向上に寄与する。 Moreover, by performing the non-recrystallization temperature range rolling in the non-recrystallization temperature range between the Ar 3 transformation point and 900 ° C., strain can be effectively introduced into the austenite grains. This strain introduction contributes to the improvement of strength and toughness by miniaturizing the transformed structure after accelerated cooling.
一方、Ar3変態点を下回ってオーステナイト−フェライト2相域での圧延を施した場合、変態生成したフェライトが加工により伸展し、集合組織が過度に発達することから圧延終了温度はAr3変態点より高温とすることが好ましい。 On the other hand, when rolling is performed in the austenite-ferrite two-phase region below the Ar 3 transformation point, the transformation-generated ferrite is extended by processing and the texture is excessively developed, so that the rolling end temperature is Ar 3 transformation point. A higher temperature is preferred.
3.加速冷却
加速冷却は、API X80以上の強度を確保しつつ、鋼組織をベイナイト主体とし微細フェライトを含有する2相組織化とする目的で、冷却開始温度を(Ar3−50℃)以上Ar3未満、冷却停止温度を250℃以上500℃以下の条件とする。なお、加速冷却とは冷却速度が10℃/s以上100℃/s未満であることを意味する。
3. Accelerated cooling Accelerated cooling has a cooling start temperature of (Ar 3 -50 ° C.) or more and Ar 3 for the purpose of making the steel structure a bainite main component and a two-phase structure containing fine ferrite while ensuring a strength of API X80 or higher. The cooling stop temperature is set to 250 ° C. or more and 500 ° C. or less. The accelerated cooling means that the cooling rate is 10 ° C./s or more and less than 100 ° C./s.
(Ar3変態点−50℃)より低い温度から加速冷却を開始した場合、圧延終了から加速冷却開始までのオーステナイト−フェライト二相域での保持時間が長くなるためにフェライトが不規則に変態生成し、この段階で十分成長した粗大なフェライトと、加速冷却中あるいは加速冷却終了後に生成する変態初期のフェライトとが混在し、均質微細なフェライトが得られず吸収エネルギーの低下をまねく。 When accelerated cooling is started from a temperature lower than (Ar 3 transformation point-50 ° C), since the holding time in the austenite-ferrite two-phase region from the end of rolling to the start of accelerated cooling becomes longer, the ferrite is irregularly formed. At this stage, coarse ferrite that is sufficiently grown and ferrite in the early stage of transformation generated during accelerated cooling or after completion of accelerated cooling are mixed, resulting in a decrease in absorbed energy because homogeneous and fine ferrite cannot be obtained.
一方、Ar3変態点以上の温度から加速冷却を開始した場合、組織はベイナイト単相となるため、YRが所定の値を満たすことが不可能である。したがって、冷却開始温度は(Ar3−50℃)以上Ar3未満の温度範囲内とすることが不可欠である。 On the other hand, when accelerated cooling is started from a temperature equal to or higher than the Ar 3 transformation point, the structure becomes a bainite single phase, so that YR cannot satisfy a predetermined value. Therefore, it is indispensable that the cooling start temperature is in the temperature range of (Ar 3 -50 ° C.) or more and less than Ar 3 .
加速冷却の冷却速度はベイナイト主体の組織にするために10℃/s以上とする。10℃/s未満の場合、板厚中央部でフェライト変態が生じて強度が低下する。一方、冷却速度が100℃/s以上ではマルテンサイトが形成され靭性に悪影響を及ぼすことから、加速冷却の冷却速度は、10℃/s以上100℃/s未満とする。加速冷却の冷却速度は、高変形能、高靭性、高強度の全てを満たす観点から40℃/s以上であることが好ましい。なお、冷却速度は平均冷却速度を意味する。 The cooling rate of accelerated cooling is 10 ° C./s or more in order to obtain a bainite-based structure. When the temperature is less than 10 ° C./s, ferrite transformation occurs in the center portion of the plate thickness and the strength is lowered. On the other hand, when the cooling rate is 100 ° C./s or more, martensite is formed and adversely affects toughness. Therefore, the cooling rate for accelerated cooling is 10 ° C./s or more and less than 100 ° C./s. The cooling rate of accelerated cooling is preferably 40 ° C./s or more from the viewpoint of satisfying all of high deformability, high toughness, and high strength. The cooling rate means an average cooling rate.
加速冷却停止温度が500℃より高温の場合、変態強化の効果が小さくなり、API X80グレードの強度を確保することが困難となる。一方、加速冷却停止温度を250℃より低温とした場合、マルテンサイト変態が生じ、靭性の劣化をまねく恐れがある。そこで、加速冷却の停止温度は250〜500℃とする。高強度材を得るためにより好ましい条件として、加速冷却の停止温度は250〜400℃とする。なお、加速冷却停止後は、水冷や強制空冷ではなく、放冷(自然空冷)とすることが好ましい。 When the accelerated cooling stop temperature is higher than 500 ° C., the effect of transformation strengthening becomes small, and it becomes difficult to secure the strength of API X80 grade. On the other hand, if the accelerated cooling stop temperature is lower than 250 ° C., martensitic transformation occurs, which may lead to deterioration of toughness. Therefore, the stop temperature of accelerated cooling is set to 250 to 500 ° C. As a more preferable condition for obtaining a high-strength material, the stop temperature of accelerated cooling is 250 to 400 ° C. In addition, after accelerating cooling stop, it is preferable to set it as cooling (natural air cooling) instead of water cooling or forced air cooling.
<高強度高靭性鋼管用鋼板>
本発明の高強度高靭性鋼管用鋼板の成分組成は、上記鋼素材の成分組成と同様であり、各成分の技術的意義も同様であるため、説明を省略する。
<Steel sheet for high strength and high toughness steel pipe>
The component composition of the steel sheet for high strength and high toughness steel pipe of the present invention is the same as the component composition of the steel material, and the technical significance of each component is also the same, and thus the description thereof is omitted.
本発明の高靭性鋼管用鋼板の鋼組織は、ベイナイトを主体とし、フェライトを含有し、該フェライトの平均粒径が5μm未満、面積率でのフェライト分率が5%未満である。 The steel structure of the steel sheet for high toughness steel pipe of the present invention is mainly composed of bainite, contains ferrite, and has an average particle size of less than 5 μm and a ferrite fraction in area ratio of less than 5%.
「ベイナイトを主体とし」とは、面積率でのベイナイト分率が80%以上であることを意味する。ベイナイト主体とすることで、所定の強度を確保しつつ優れた靭性を得ることができる。 “Mainly containing bainite” means that the bainite fraction in area ratio is 80% or more. By using bainite as a main component, excellent toughness can be obtained while ensuring a predetermined strength.
本発明の高強度高靭性鋼管用鋼板の鋼組織は、第2相として、フェライトを含有し、フェライトの平均粒径が5μm未満、面積率でのフェライト分率が5%未満である。本発明の高強度高靭性鋼管用鋼板が、高変形特性を併せ持つためには2相組織とする必要がある。この2相組織による効果を十分に得るためにはフェライト分率を2.0%以上にすることが好ましい。また、第2相の分率が高まるにつれ吸収エネルギーは低下する傾向を有するため、形成されるフェライトの分率を面積率で5%未満とする。また、組織の均一性を保つためにもその平均粒径を5μm未満とする。また、平均粒径は塑性変形時の変形を担うために1.0μm以上が好ましい。 The steel structure of the steel sheet for high strength and high toughness steel pipe of the present invention contains ferrite as the second phase, the ferrite has an average particle diameter of less than 5 μm, and the ferrite fraction in area ratio is less than 5%. In order for the steel sheet for high strength and high toughness steel pipe of the present invention to have high deformation characteristics, it is necessary to have a two-phase structure. In order to sufficiently obtain the effect of this two-phase structure, the ferrite fraction is preferably set to 2.0% or more. Further, since the absorbed energy tends to decrease as the fraction of the second phase increases, the fraction of ferrite formed is made less than 5% in terms of area ratio. In order to maintain the uniformity of the structure, the average particle size is set to less than 5 μm. Further, the average particle diameter is preferably 1.0 μm or more in order to bear the deformation at the time of plastic deformation.
なお、ベイナイト、フェライト以外の相を面積率で10%以下含んでもよい。その他の相としてはマルテンサイト、MA(Martensite−Austenite Constituent)、パーライト等が挙げられる。 A phase other than bainite and ferrite may be included in an area ratio of 10% or less. Examples of other phases include martensite, MA (Martensite-Austenite Constituent), pearlite, and the like.
また、本発明では、特定の成分組成を有する鋼素材を特定の条件で圧延した後に行われる加速冷却の冷却開始温度が(Ar3−50℃)以上Ar3未満であるため、微量のフェライトを有する鋼組織になる。 In the present invention, since the cooling start temperature of accelerated cooling performed after rolling a steel material having a specific component composition under specific conditions is (Ar 3 -50 ° C.) or higher and lower than Ar 3 , a small amount of ferrite is added. It has a steel structure.
本発明の高強度高靭性鋼管用鋼板は、板厚が薄くても、高変形能、高靭性、高強度の全てを満たすことができる。「板厚が薄い」とは厚みが20mm以下を意味する。より具体的には10〜20mmである。 The steel sheet for high strength and high toughness steel pipe of the present invention can satisfy all of high deformability, high toughness and high strength even if the plate thickness is thin. “Thin plate thickness” means a thickness of 20 mm or less. More specifically, it is 10-20 mm.
表1に示す組成の鋳片を、表2に示すスラブ加熱条件、熱間圧延条件、加速冷却条件により12〜18mm厚の鋼板とした(なお、再結晶温度域での累積圧延率は70〜85%とした(具体的な条件は表2に示した。)。)。 The slab having the composition shown in Table 1 was made into a steel plate having a thickness of 12 to 18 mm according to the slab heating conditions, hot rolling conditions, and accelerated cooling conditions shown in Table 2 (Note that the cumulative rolling rate in the recrystallization temperature range is 70 to 70 mm. (The specific conditions are shown in Table 2.)
ここで、Ar3変態点は以下の式で計算した値を用いた。式中の元素記号は各元素の含有量(質量%)を意味し、含有しない元素については0とする。 Here, the value calculated by the following formula was used as the Ar 3 transformation point. The element symbol in a formula means content (mass%) of each element, and is set to 0 about the element which does not contain.
Ar3(℃)=910−310C−80Mn−20Cu−15Cr−55Ni−80Mo
得られた鋼板について、板厚中心から6mmφの引張試験片を採取し、圧延直角方向(C方向)に引っ張る引張試験を実施し、降伏強度(YS)、引張強度(YS)および降伏比(降伏強度と引張強度の比、YR)を求めた。結果を表3に示した。
Ar 3 (° C.) = 910-310C-80Mn-20Cu-15Cr-55Ni-80Mo
About the obtained steel plate, a tensile test piece of 6 mmφ is taken from the center of the plate thickness, a tensile test is performed by pulling in the direction perpendicular to the rolling direction (C direction), yield strength (YS), tensile strength (YS), and yield ratio (yield). The ratio of strength to tensile strength, YR) was determined. The results are shown in Table 3.
シャルピー衝撃試験は、板厚方向1/2の位置からJIS Z 2202(1998改訂版)に準拠したVノッチ標準寸法のシャルピー衝撃試験片を採取して、JIS Z 2242(1998改訂版)に準拠して−20℃でシャルピー衝撃試験を実施し、吸収エネルギー(vE−20℃)を求めた。 In the Charpy impact test, a V-notch standard size Charpy impact test piece conforming to JIS Z 2202 (1998 revised version) was sampled from a position in the thickness direction 1/2 and conformed to JIS Z 2242 (1998 revised version). Then, a Charpy impact test was conducted at -20 ° C to determine the absorbed energy (vE- 20 ° C ).
主相の確認および第2相の分率の測定には、得られた鋼板について3%ナイタールで腐食し、鋼板表面から1/4厚みの位置を観察位置とし、500倍のSEM観察視野において観察されるフェライトの平均断面積率をフェライトの組織分率(分率)とした。結果を表3に示した。また、この観察によりフェライト以外は面積率で80%以上のベイナイトおよび合計の面積率が10%以下のマルテンサイトおよび/またはMAであることが確認された(マルテンサイトやMAを含まない開発例もあった。)。 For confirmation of the main phase and measurement of the fraction of the second phase, the obtained steel sheet was corroded with 3% nital, and the position of 1/4 thickness from the steel sheet surface was taken as the observation position, and observed in a 500 times SEM observation field. The average cross-sectional area ratio of the ferrite formed was defined as the structure fraction (fraction) of the ferrite. The results are shown in Table 3. This observation also confirmed that bainite with an area ratio of 80% or more other than ferrite and martensite and / or MA with a total area ratio of 10% or less (development examples that do not include martensite or MA). there were.).
フェライト粒径Rは、フェライト粒を等方的な球形ととらえ、観察視野における断面積と同じ面積の円の直径を粒径とした。すなわちフェライト粒径Rは以下の式で求められる。結果を表3に示した。 The ferrite particle size R was obtained by considering the ferrite particles as isotropic spheres and using the diameter of a circle having the same area as the cross-sectional area in the observation field as the particle size. That is, the ferrite particle size R is obtained by the following formula. The results are shown in Table 3.
R = 2×√(A/π) (A:測定したフェライトの断面積)
本発明に適合した発明例は、いずれもフェライトの平均粒径が5μm未満、分率が5%未満かつ、降伏強度555MPa以上、引張強度700MPa以上、降伏比80%以下、−20℃におけるシャルピー吸収エネルギーが250J以上と優れた特性が認められた。
R = 2 × √ (A / π) (A: measured cross-sectional area of ferrite)
In all of the inventive examples suitable for the present invention, the average particle diameter of ferrite is less than 5 μm, the fraction is less than 5%, the yield strength is 555 MPa or more, the tensile strength is 700 MPa or more, the yield ratio is 80% or less, and the Charpy absorption at −20 ° C. Excellent characteristics with an energy of 250 J or more were recognized.
また、発明例3−1、発明例3−2から、スラブ加熱温度をやや低く設定することで、シャルピー吸収エネルギーを向上させられることが確認された。 Moreover, it was confirmed from Invention Example 3-1 and Invention Example 3-2 that the Charpy absorbed energy can be improved by setting the slab heating temperature slightly low.
一方、比較例7は、鋼の化学組成は適合しているものの、圧延条件が最適でないため、セパレーションの発生、組織の不均一性からエネルギーの低下につながっている。 On the other hand, in Comparative Example 7, although the chemical composition of the steel is suitable, the rolling conditions are not optimal, and this leads to a reduction in energy due to the occurrence of separation and the unevenness of the structure.
比較例8、9は、冷却条件が適切でないため、フェライト分率が所定の割合にならず、エネルギーの低下を招いている、もしくはYRが高い値となっている。 In Comparative Examples 8 and 9, since the cooling conditions are not appropriate, the ferrite fraction does not reach a predetermined ratio, leading to a decrease in energy or a high YR value.
比較例10は、Ceq値が下限を下回っており、目標とする強度レベルを満足できなかった。 In Comparative Example 10, the Ceq value was below the lower limit, and the target strength level could not be satisfied.
比較例11〜14は吸収エネルギーに悪影響を及ぼす元素が上限値を超えたため、吸収エネルギーの目標値を下回っている。 In Comparative Examples 11 to 14, the element that adversely affects the absorbed energy exceeded the upper limit value, and thus the target value of the absorbed energy was below.
Claims (4)
冷却開始温度: (Ar3−50℃)以上Ar3未満
冷却停止温度: 250〜500℃
Ceq(%)=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5 (1)
各元素は質量%での値とする。
なお、前記Ar 3 は、以下の式で表される。
Ar 3 (℃)=910−310C−80Mn−20Cu−15Cr−55Ni−80Mo In mass%, C: 0.02 to 0.09%, Si: 0.01 to 0.30%, Mn: 1.0 to 3.0%, sol. Al: 0.003 to 0.050%, Nb: 0.005 to 0.10%, Ti: 0.005 to 0.05%, N: 0.0010 to 0.0060%, P: 0 0.030% or less, S: 0.010% or less, O: Restricted to less than 0.004%, and Cu: 0.01 to 1.0%, Ni: 0.01 to 1.0% in mass% , Cr: 0.01 to 1.0%, Mo: 0.01 to 1.0%, one or more selected from among the optional components, calculated by the following formula (1) A steel material having a component composition comprising a Ceq value of 0.40 to 0.60 and the balance consisting of Fe and unavoidable impurities is heated to a surface temperature of 1000 ° C. or more and 1250 ° C. or less. Rolled, and after the first rolling, the cumulative rolling reduction is 40% or more and 75 in the non-recrystallization temperature range of 900 ° C. or less. And second rolling under the following conditions, after said second rolling, and performing accelerated cooling at a cooling condition of the cooling start temperature and cooling stop temperature below fraction of an area ratio of 80% or more bainite And having a steel structure containing ferrite, the average grain size of which is less than 5 μm, and the ferrite fraction in area ratio is less than 5%, the yield ratio is 80% or less, and the plate thickness is 10 to 20 mm. A manufacturing method of a steel sheet for a certain high strength and high toughness steel pipe.
Cooling start temperature: (Ar 3 −50 ° C.) or more and less than Ar 3 Cooling stop temperature: 250 to 500 ° C.
Ceq (%) = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 (1)
Each element is a value in mass%.
The Ar 3 is represented by the following formula.
Ar 3 (° C.) = 910-310C-80Mn-20Cu-15Cr-55Ni-80Mo
面積率での分率が80%以上のベイナイトと、フェライトを含有し、該フェライトの平均粒径が5μm未満、面積率でのフェライト分率が5%未満である鋼組織とを有し、降伏比が80%以下、板厚が10〜20mmであることを特徴とする高強度高靭性鋼管用鋼板。
Ceq(%)=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5 (1)
各元素は質量%での値とする。 In mass%, C: 0.02 to 0.09%, Si: 0.01 to 0.30%, Mn: 1.0 to 3.0%, sol. Al: 0.003 to 0.050%, Nb: 0.005 to 0.10%, Ti: 0.005 to 0.05%, N: 0.0010 to 0.0060%, P: 0 0.030% or less, S: 0.010% or less, O: Restricted to less than 0.004%, and Cu: 0.01 to 1.0%, Ni: 0.01 to 1.0% in mass% , Cr: 0.01 to 1.0%, Mo: 0.01 to 1.0%, one or more selected from among the optional components, calculated by the following formula (1) A component composition having a Ceq value of 0.40 to 0.60 and the balance of Fe and inevitable impurities;
Fraction is 80% or more of bainite by area ratio, and contains a ferrite, the average grain size of the ferrite possess a steel structure is less than 5 [mu] m, the ferrite fraction of an area ratio less than 5%, the yield the ratio of 80% or less, high strength and high toughness steel pipe for steel plate thickness is characterized 10~20mm der Rukoto.
Ceq (%) = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 (1)
Each element is a value in mass%.
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