WO2013105396A1 - 低合金鋼 - Google Patents
低合金鋼 Download PDFInfo
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- WO2013105396A1 WO2013105396A1 PCT/JP2012/082608 JP2012082608W WO2013105396A1 WO 2013105396 A1 WO2013105396 A1 WO 2013105396A1 JP 2012082608 W JP2012082608 W JP 2012082608W WO 2013105396 A1 WO2013105396 A1 WO 2013105396A1
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- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
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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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- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
Definitions
- the present invention relates to a low alloy steel.
- the present invention relates to a low alloy steel in which a weld heat affected zone after heat treatment after welding has excellent resistance against embrittlement caused by hydrogen such as stress corrosion cracking in a wet hydrogen sulfide environment.
- Patent Document 1 proposes a steel that is substantially free of Ni, Cu, and Ca, and that has excellent HIC resistance by specifying the thermal history and heat treatment conditions during production.
- Patent Document 2 proposes a steel having HIC resistance and SSC resistance by making Cr, Ni and Cu essential additions.
- Patent Document 3 proposes a steel having improved HIC resistance and SSC resistance by defining the amounts of C, Ti, N, V and O within specific ranges.
- Non-Patent Document 1 it is widely known that the SSC sensitivity increases with an increase in hardness.
- HZ welding heat affected zone
- Patent Document 4 by reducing the amount of C and Mn and containing 0.5% or more of Mo, the hardening of the weld heat affected zone is suppressed, and the base material and A high-strength steel has also been proposed in which HAZ has both HIC resistance and SSC resistance.
- Patent Document 5 proposes a low alloy steel containing 0.5% or more of Cr on the premise that PWHT is performed for one hour per one inch of thickness.
- PWHT has a certain effect, but in the case of line pipe laying, since efficiency is important, such as welding on an offshore ship, generally PWHT is not implemented or applied In that case, it is desired to apply PWHT for a very short time.
- An object of the present invention is to provide a low alloy steel in which PWHT, particularly HAZ subjected to short time PWHT, has excellent hydrogen embrittlement resistance in a wet hydrogen sulfide environment.
- the present inventors first investigated the hydrogen embrittlement of the as-welded HAZ in order to clarify the necessary conditions in order to enhance the hydrogen embrittlement resistance of the HAZ of the steel material subjected to PWHT. As a result, hydrogen embrittlement of HAZ is considered to occur by the following mechanism.
- HAZ is heated to a high temperature due to the thermal history of welding, rapidly cooled, and becomes an as-quenched martensite or bainite structure, so that dislocations in which hydrogen is trapped as compared to a tempered base material and The pores are present at high density, and the cementite is dispersed. Therefore, it is considered that HAZ is more susceptible to hydrogen embrittlement than the base material.
- the present inventors tried to optimize the alloy elements in order to improve the hydrogen embrittlement resistance of HAZ to which PWHT was applied. As a result, it has been found that it is effective to contain at least one of Ti, V and Nb in order to increase the hydrogen embrittlement susceptibility of HAZ subjected to PWHT. The reason is considered as follows.
- MX type fine carbides have a greater affinity for carbon than iron, and form MX type fine carbides in the process of PWHT. Since the MX type carbide has better consistency with the parent phase than cementite, the lattice strain at the interface with the substrate is small, and the amount of diffusible hydrogen in the carbide is large. For this reason, when hydrogen invades due to a corrosion reaction, the accumulation sites of diffusible hydrogen are dispersed, thereby suppressing significant hydrogen accumulation and the generation of embrittlement starting points, thereby reducing the embrittlement.
- the present invention has been made on the basis of such knowledge and has the following [1] to [6].
- % for the content means “% by mass”.
- C 0.01 to 0.15%
- C is an element effective in increasing the hardenability of steel and increasing the strength. In order to acquire the effect, it is necessary to make it contain 0.01% or more. However, when the content exceeds 0.15%, a large amount of cementite is precipitated when PWHT is applied, and the hydrogen embrittlement susceptibility of HAZ is increased. Therefore, the C content is set to 0.01 to 0.15%.
- the lower limit of the C content is preferably 0.03%.
- the C content is preferably 0.12% or less.
- Si 3% or less Si is an element effective for deoxidation, but if it is excessively contained, toughness is reduced. For this reason, Si content shall be 3% or less.
- the Si content is preferably 2% or less.
- the lower limit is not particularly defined, even if the Si content is reduced, the deoxidation effect is lowered, the cleanliness of the steel is deteriorated, and excessive reduction leads to an increase in production cost. For this reason, it is preferable that Si content shall be 0.01% or more.
- Mn 3% or less Mn is an element effective for deoxidation, like Si, and is an element that contributes to improvement of strength by enhancing the hardenability of steel. However, if it is contained excessively, the HAZ is markedly cured and the hydrogen embrittlement resistance is increased. Therefore, the Mn content is 3% or less.
- the lower limit is not particularly defined, it is preferable to contain 0.2% or more in order to obtain the effect of improving the strength of Mn. A more preferred lower limit is 0.4%, and a preferred upper limit is 2.8%.
- Al 0.08% or less
- Al is an element effective for deoxidation, but its effect is saturated even if it is contained excessively, and the toughness is reduced. Therefore, the Al content is set to 0.08% or less.
- a preferable content is 0.06% or less.
- Al is preferably contained in an amount of 0.001% or more.
- the Al content of the present invention refers to acid-soluble Al (so-called “sol.Al”).
- One or more selected from Ti, V and Nb a range satisfying the following formula (1) 0.1 ⁇ [C (%)] ⁇ [Ti (%)] + [V (%)] + 0.5 ⁇ [Nb (%)] ⁇ 0.2 (1)
- each element symbol in the mathematical formula means the content (% by mass) of each element.
- These elements form MX type fine carbides in the process of PWHT, and improve hydrogen embrittlement resistance. In order to obtain this effect, it is necessary to set “[Ti (%)] + [V (%)] + 0.5 ⁇ [Nb (%)]” to 0.1 ⁇ [C (%)] or more. .
- “[Ti (%)] + [V (%)] + 0.5 ⁇ [Nb (%)]” needs to be 0.2% or less.
- a preferable upper limit is 0.18%, and a more preferable upper limit is 0.15%.
- the low alloy steel according to the present invention contains each of the above elements, with the balance being Fe and impurities.
- An impurity means the component mixed by raw materials and other factors, such as an ore and a scrap, when manufacturing steel materials industrially.
- impurities the following elements need to be strictly limited in content.
- N 0.01% or less N is present in steel as an impurity, but if fine carbonitride is formed, it causes embrittlement and lowers toughness even when dissolved. Therefore, it is necessary to limit the content to 0.01% or less.
- the content is preferably 0.008% or less.
- the lower limit of the N content is preferably 0.0001%.
- P 0.05% or less P is present in the steel as an impurity, but segregates at grain boundaries in HAZ, leading to a decrease in toughness. Therefore, the content is limited to 0.05% or less. There is no particular lower limit, but excessive reduction leads to a significant increase in manufacturing costs. Therefore, the lower limit of the P content is preferably 0.001%.
- S 0.03% or less S is present in steel as an impurity like P, but forms sulfides in the steel, and the interface with the substrate acts as a hydrogen accumulation site, increasing hydrogen embrittlement susceptibility. Also, the HAZ toughness is reduced. Therefore, the content is stricter than P and limited to 0.03% or less. There is no particular lower limit, but excessive reduction leads to a significant increase in manufacturing costs. Therefore, the lower limit of the S content is preferably 0.0001%.
- O 0.03% or less O is present in the steel as an impurity, but when it is contained in a large amount, it generates a large amount of oxide, which deteriorates workability and ductility. Therefore, it is necessary to make it 0.03% or less. Desirably, it is 0.025% or less. There is no particular need to provide a lower limit, but excessive reduction leads to a significant increase in manufacturing costs. Therefore, it is desirably 0.0005% or more.
- the low alloy steel according to the present invention may contain the following elements instead of a part of Fe.
- Cr and / or Mo 1.5% or less in total
- These elements may increase the hardenability and contribute to improving the strength, and therefore may be contained.
- the content when the content is excessive, it precipitates as carbides, inhibits precipitation of carbides such as Ti, and increases hydrogen embrittlement sensitivity. Therefore, when Cr and / or Mo is contained, the content is made 1.5% or less in total.
- a preferable minimum is 0.02%, More preferably, it is 0.05%.
- a preferable upper limit is 1.2%.
- Ni and / or Cu 0.8% or less in total
- these elements may be contained because they enhance the hardenability and contribute to improving the strength. However, even if contained excessively, the effect is saturated and the cost is increased. Therefore, when it contains Ni and / or Cu, the content shall be 0.8% or less in total.
- the preferable minimum in the case of adding is 0.02%, More preferably, it is 0.05%. A more preferred upper limit is 0.7%.
- Ca and / or Mg 0.05% or less in total
- these elements may be contained in order to improve the hot workability of steel. However, if its content is excessive, it may combine with oxygen, significantly reducing cleanliness, and possibly degrading hot workability. Therefore, when it contains 1 or more types of these elements, the content shall be 0.05% or less in total.
- a preferable minimum is 0.0005%, More preferably, it is 0.001%.
- a preferable upper limit is 0.03%.
- each element symbol in the mathematical formula means the content (% by mass) of each element.
- B segregates at the grain boundary, suppresses the precipitation of ferrite from the grain boundary, indirectly increases the hardenability, and contributes to improving the strength.
- excessive inclusion may precipitate as boride during the PWHT process, or may be substituted with C to form a solid solution in cementite, thereby increasing the lattice strain with the substrate and reducing hydrogen embrittlement resistance.
- fills (2) Formula The desirable lower limit is 0.0001%, and more desirably 0.0005%.
- the low alloy steel according to the present invention is particularly suitable when PWHT is applied that satisfies the following formula (3). Exhibits excellent effects. 8000 ⁇ T ⁇ ⁇ 20 + log (t / 3600) ⁇ ⁇ 15000 (3)
- T is the processing temperature (° C.) of the heat treatment after welding
- t is the processing time (seconds) of the heat treatment after welding.
- T ⁇ ⁇ 20 + log (t / 3600) ⁇ is less than 8000, the hydrogen embrittlement resistance of the HAZ of the steel material made of the low alloy steel according to the present invention may not be improved.
- the PWHT time is more preferably 300 seconds or less.
- the low alloy steel of the present invention preferably has a yield strength (YS) of 552 MPa or more.
- YS yield strength
- the reason is that low-strength steel with high strength has a remarkable decrease in strength of the steel including the welded portion due to PWHT, and the advantage of improving hydrogen embrittlement resistance by PWHT in a short time is more easily obtained.
- test material was produced by machining a 12 mm thick low alloy steel plate having a chemical composition shown in Table 1 into a 12 mm square and a length of 100 mm.
- This test material was subjected to a HAZ reproducible welding heat cycle that was heated to 1350 ° C., a temperature at which HAZ was markedly cured by high-frequency induction heating, for 3 seconds and then rapidly cooled.
- the following tests were conducted using this test material.
- ⁇ SSC resistance test> A test piece having a thickness of 2 mm, a width of 10 mm, and a length of 75 mm was taken from the obtained test material, and the SSC resistance was evaluated by a four-point bending test in accordance with EFC16 defined by European Federation of Corrosion.
- EFC16 European Federation of Corrosion.
- a stress corresponding to 50% of the 0.2% proof stress derived from the tensile test was applied to the collected specimen by 4-point bending, and then 5% of room temperature (24 ° C.) saturated with 1 atm hydrogen sulfide gas. It was immersed in a salt + 0.5% acetic acid aqueous solution for 336 hours to examine whether SSC was generated. And the thing in which SSC did not generate
- a low alloy steel having excellent hydrogen embrittlement resistance in a wet hydrogen sulfide environment or the like in PHAT, particularly HAZ subjected to short time PWHT.
- This low alloy steel is most suitable as a material for steel pipes for transporting crude oil or natural gas.
Abstract
Description
0.1×[C(%)]≦[Ti(%)]+[V(%)]+0.5×[Nb(%)]≦0.2 (1)
ただし、数式中の各元素記号は、各元素の含有量(質量%)を意味する。
0.1×[C(%)]≦[Ti(%)]+[V(%)]+0.5×[Nb(%)]≦0.2 (1)
ただし、数式中の各元素記号は、各元素の含有量(質量%)を意味する。
[B(%)]<0.1×[C(%)] (2)
ただし、数式中の各元素記号は、各元素の含有量(質量%)を意味する。
8000≦T×{20+log(t/3600)}≦15000 (3)
ただし、Tは、溶接後熱処理の処理温度(℃)であり、tは、溶接後熱処理の処理時間(秒)である。
Cは、鋼の焼入れ性を高めて強度を高めるのに有効な元素である。その効果を得るためには0.01%以上含有させる必要がある。しかし、その含有量が0.15%を超えると、PWHTを施した際に多量のセメンタイトを析出させ、HAZの水素脆化感受性を高める。よって、C含有量は、0.01~0.15%とする。C含有量の下限は、0.03%とするのが好ましい。C含有量は、0.12%以下とするのが好ましい。
Siは、脱酸に有効な元素であるが、過剰に含有させると靭性の低下を招く。このため、Si含有量は3%以下とする。Si含有量は2%以下とするのが好ましい。下限は特に定めないが、Si含有量を低減しても、脱酸効果が低下し、鋼の清浄度を劣化させ、過度な低減は製造コストの増大を招く。このため、Si含有量は、0.01%以上とするのが好ましい。
Mnは、Siと同様、脱酸に有効な元素であり、また、鋼の焼入れ性を高めて強度の向上に寄与する元素である。しかし、過剰に含有させると、HAZの著しい硬化を招き、耐水素脆化感受性を高めてしまう。このため、Mn含有量は3%以下とする。下限は特に定めないが、Mnの強度向上効果を得る場合には、0.2%以上含有するのが好ましい。より好ましい下限は、0.4%であり、好ましい上限は、2.8%である。
Alは、脱酸に有効な元素であるが、その効果は、過剰に含有させても飽和し、また、靭性の低下を招く。よって、Al含有量は、0.08%以下とする。好ましい含有量は、0.06%以下である。下限は特に定めないが、過度の低減は、脱酸効果が十分に得られず鋼の清浄度を劣化させるとともに、製造コストの増大を招く。そのため、Alは0.001%以上含有させるのが好ましい。本発明のAl含有量とは、酸可溶Al(所謂「sol.Al」)を指す。
0.1×[C(%)]≦[Ti(%)]+[V(%)]+0.5×[Nb(%)]≦0.2 (1)
ただし、数式中の各元素記号は、各元素の含有量(質量%)を意味する。
これら元素は、PWHTの過程でMX型の微細な炭化物を形成し、耐水素脆化性を高める。この効果を得るためには、「[Ti(%)]+[V(%)]+0.5×[Nb(%)]」を0.1×[C(%)]以上とする必要がある。しかし、その含有量が過剰な場合、炭化物が粗大となり、却って水素脆化感受性を高めるとともに、靭性の低下を招く。よって、「[Ti(%)]+[V(%)]+0.5×[Nb(%)]」を0.2%以下とする必要がある。好ましい上限は0.18%であり、より好ましい上限は0.15%である。
Nは、不純物として鋼中に存在するが、微細な炭窒化物を形成すると脆化を招き、固溶した場合でも靭性を低下させる。そのため、その含有量を0.01%以下に制限する必要がある。その含有量は0.008%以下とするのが好ましい。下限は特に定めないが、過度の低減は、製造コストの著しい増大を招く。そのため、N含有量の下限は、0.0001%とするのが好ましい。
Pは、不純物として鋼中に存在するが、HAZにおいて粒界に偏析し、靭性の低下を招く。そのため、その含有量を0.05%以下に制限する。下限は特に定めないが、過度の低減は、製造コストの著しい増大を招く。そのため、P含有量の下限は、0.001%とするのが好ましい。
Sは、Pと同様に不純物として鋼中に存在するが、鋼材中で硫化物を形成し、基質との界面が水素の集積サイトとして働き、水素脆化感受性を高め、また、HAZ靭性の低下も招く。そのため、その含有量をPよりも厳しく、0.03%以下に制限する。下限は特に定めないが、過度の低減は、製造コストの著しい増大を招く。そのため、S含有量の下限は、0.0001%とするのが好ましい。
Oは、不純物として鋼中に存在するが、多量に含まれる場合には、多量の酸化物を生成し、加工性や延性を劣化させる。そのため、0.03%以下とする必要がある。望ましくは0.025%以下である。特に下限は設ける必要はないが、過度の低減は製造コストの著しい増大を招く。そのため、望ましくは0.0005%以上とする。
これら元素は、いずれも焼入れ性を高めて強度向上に寄与するので、含有させてもよい。しかし、その含有量が過剰な場合、炭化物として析出し、Ti等の炭化物の析出を阻害し、水素脆化感受性を高める。よって、Crおよび/またはMoを含有させる場合には、その含有量を合計で1.5%以下とする。なお、好ましい下限は0.02%,さらに好ましくは0.05%である。好ましい上限は1.2%である。
これら元素は、焼入れ性を高めて強度向上に寄与するので、含有させてもよい。しかし、過剰に含有させても、その効果が飽和するだけで、コスト増大を招く。よって、Niおよび/またはCuを含有させる場合には、その含有量を合計で0.8%以下とする。尚,添加する場合の好ましい下限は0.02%、さらに好ましくは0.05%である。より好ましい上限は0.7%である。
これらの元素は、いずれも鋼の熱間加工性を改善するため、含有させてもよい。しかし、その含有量が過剰な場合、酸素と結合し、清浄を著しく低下させ、却って熱間加工性を劣化させるおそれがある。よって、これらの元素の1種以上を含有させる場合には、その含有量を合計で0.05%以下とする。なお、好ましい下限は0.0005%、さらに好ましくは0.001%である。好ましい上限は0.03%である。
[B(%)]<0.1×[C(%)] (2)
ただし、数式中の各元素記号は、各元素の含有量(質量%)を意味する。
Bは、粒界に偏析し、粒界からのフェライトの析出を抑制して間接的に焼入れ性を高め、強度向上に寄与するため、含有させてもよい。しかし、過剰な含有は、PWHTの過程で、ホウ化物として析出するか、Cと置換してセメンタイト中に固溶し、基質との格子歪をより大きくし、耐水素脆化性を低下させるおそれがある。よって、Bを含有させる場合には、その含有量を(2)式を満たす範囲とするのが好ましい。なお、望ましい下限は0.0001%であり、更に望ましくは0.0005%である。
8000≦T×{20+log(t/3600)}≦15000 (3)
ただし、Tは、溶接後熱処理の処理温度(℃)であり、tは、溶接後熱処理の処理時間(秒)である。
「T×{20+log(t/3600)}」が8000未満の場合、本発明に係る低合金鋼からなる鋼材のHAZの耐水素脆化性を高めることができないおそれがある。一方、「T×{20+log(t/3600)}」が15000を超えるとTi等からなるMX型の微細な炭化物の粗大化が進行し、十分な耐水素脆化性を得られなくなるとともに、溶接部を含む鋼の強度低下が顕著となる。よって、本発明に係る低合金鋼に施されるPWHTは、上記(3)式を満たす条件で行なわれるものであることが好ましい。
JIS Z2241に準拠し、得られた試験材から平行部径6mm、平行部長さ10mmの丸棒引張試験片を採取し、常温での引張試験を行った。
得られた試験材から厚さ2mm、幅10mm、長さ75mmの試験片を採取し、European Federation of Corrosionが定めるEFC16に則った4点曲げ試験により、耐SSC性を評価した。試験は、採取した試験片に4点曲げにより引張試験から導出した0.2%耐力の50%に相当する応力を付加した後、1atm硫化水素ガスを飽和させた常温(24℃)の5%食塩+0.5%酢酸水溶液に336時間浸漬し、SSCの発生有無を調べた。そして、SSCが発生しなかったものを合格、SSCが発生したものを不合格とした。
Claims (3)
- 溶接後熱処理が施される低合金鋼であって、
質量%で、C:0.01~0.15%、Si:3%以下、Mn:3%以下およびAl:0.08%以下、Ti、VおよびNbから選択される1種以上の元素:下記の(1)式を満足する範囲、残部がFeおよび不純物からなり、
不純物としてのNが0.01%以下、Pが0.05%以下、Sが0.03%以下、Oが0.03%以下であることを特徴とする低合金鋼。
0.1×[C(%)]≦[Ti(%)]+[V(%)]+0.5×[Nb(%)]≦0.2 (1)
ただし、数式中の各元素記号は、各元素の含有量(質量%)を意味する。
- 溶接後熱処理が施される低合金鋼であって、
質量%で、C:0.01~0.15%、Si:3%以下、Mn:3%以下およびAl:0.08%以下、
Ti、VおよびNbから選択される1種以上の元素:下記の(1)式を満足する範囲、
下記(A)~(D)に掲げる元素群から選択される1種以上の元素、残部がFeおよび不純物からなり、
不純物としてのNが0.01%以下、Pが0.05%以下、Sが0.03%以下、Oが0.03%以下であることを特徴とする低合金鋼。
(A)Crおよび/またはMo:合計で1.5%以下
(B)Niおよび/またはCu:合計で0.8%以下
(C)Caおよび/またはMg:合計で0.05%以下
(D)B:下記の(2)式を満足する範囲
0.1×[C(%)]≦[Ti(%)]+[V(%)]+0.5×[Nb(%)]≦0.2 (1)
[B(%)]<0.1×[C(%)] (2)
ただし、数式中の各元素記号は、各元素の含有量(質量%)を意味する。
- 溶接後熱処理が、下記の(3)式を満たす条件で行なわれるものであることを特徴とする請求項1または2記載の低合金鋼。
8000≦T×{20+log(t/3600)}≦15000 (3)
ただし、Tは、溶接後熱処理の処理温度(℃)であり、tは、溶接後熱処理の処理時間(秒)である。
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BR112014017219A BR112014017219A8 (pt) | 2012-01-12 | 2012-12-17 | aço de baixa liga |
CA2861740A CA2861740C (en) | 2012-01-12 | 2012-12-17 | Low alloy steel |
CN201280066898.1A CN104040005A (zh) | 2012-01-12 | 2012-12-17 | 低合金钢 |
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US14/371,044 US20150047749A1 (en) | 2012-01-12 | 2012-12-17 | Low alloy steel |
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CN105466129A (zh) * | 2015-12-19 | 2016-04-06 | 丹阳市宸兴环保设备有限公司 | 一种冰箱后背板用钢板 |
BR102016001063B1 (pt) * | 2016-01-18 | 2021-06-08 | Amsted Maxion Fundição E Equipamentos Ferroviários S/A | liga de aço para componentes ferroviários, e processo de obtenção de uma liga de aço para componentes ferroviários |
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US20150047749A1 (en) | 2015-02-19 |
CA2861740C (en) | 2016-09-06 |
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JP5370503B2 (ja) | 2013-12-18 |
CA2861740A1 (en) | 2013-07-18 |
AU2012365129B2 (en) | 2015-11-05 |
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AU2012365129A1 (en) | 2014-07-17 |
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