JP4233628B2 - Austenitic stainless steel for hydrogen generator with excellent scale peeling resistance - Google Patents

Description

【００１２】
表２は表１の各種成分鋼を用い、加熱−冷却を１０００サイクルまで実施したときの、スケ−ルの剥離状態を観察した結果であり、表中の数字は剥離が開始したサイクル数である。ここで、１０００サイクル超は全加熱時間が１０００時間となる１０００サイクル後でもスケ−ル剥離が無いことを示す。また、同表には燃焼雰囲気および大気中で温度勾配が無い状態での同様の熱サイクルを繰り返した際のスケール剥離開始までのサイクル数も参考として記載した。
【００１３】
【表２】

【００１４】
ＳＵＳ３１６Ｌは温度勾配が無く均一加熱の場合、大気中加熱では４８７サイクルまでスケール剥離が発生しなかったが、メタン燃焼雰囲気中では１２４サイクルで剥離した。さらに、温度勾配がある場合には５５サイクルでスケ−ルが剥離した。また、ＳＵＳ３１６ＬとＣｒ量以外は同成分であるＡ、Ｂ鋼はＣｒ量の増加とともに耐スケール剥離性が改善するものの、温度勾配が存在する条件ではＡ鋼では１０６サイクル、Ｂ鋼では８６７サイクルで剥離し、目標の１０００サイクルを達成することが出来なかった。
【００１５】
これらＳＵＳ３１６Ｌ、Ａ鋼およびＢ鋼のスケ−ル剥離箇所を観察すると、均一加熱条件下での加熱では剥離は全面でランダムに発生しているのに対し、温度勾配を持つ条件下では約６００℃程度の低温部から剥離が発生し、温度の増加とともに剥離面積が増大した。この結果から、
▲１▼燃焼雰囲気中とはいえ、通常はスケ−ル剥離がしないと考えられる６００℃程度の低温域でも温度勾配が有る場合にはスケ−ル剥離が進行する。
▲２▼Ｃｒの増加のみでは耐スケ−ル剥離性を改善できない
ことが明らかになった。
【００１６】
一方、Ａ鋼とほぼ同Ｃｒ量のＤ鋼、Ｆ鋼、Ｇ鋼、またＢ鋼とほぼ同Ｃｒ量のＣ鋼、Ｅ鋼ではいずれの雰囲気、あるいは温度勾配の有無に拘わらず、１０００サイクル後でもスケール剥離が認められず、より長時間の使用に耐えることがわかった。この原因を明らかにするために、剥離が発生したＳＵＳ３１６Ｌ、および表１のＡ鋼、Ｂ鋼では温度勾配の存在する条件下で加熱した時のスケ−ル剥離直前のサイクルのパイプおよびスケ−ル剥離直後のパイプ、そして剥離が発生しなかったＣ〜Ｇ鋼では１０００サイクル終了後のパイプをガス入側の低温部からガス出側の高温部にかけて１０ｍｍずつ切り出し、それらの断面を光学顕微鏡観察およびＥＰＭＡによる元素分布測定を行った。
【００１７】
その結果、以下のことが明らかになった。
▲１▼５５サイクルでスケールが剥離したＳＵＳ３１６のスケールは、下地金属側の内層が（Ｃｒ、Ｆｅ）酸化物、外層がＦｅ酸化物の２層構造であり、この内層スケールのＣｒ量は加熱温度の増加とともに減少し、６００℃以上の高温部ではＣｒ量の少ない保護性の無い状態である。
▲２▼Ａ鋼およびＢ鋼は、ＳＵＳ３１６Ｌに比べて内層スケール中のＣｒ量が多い（Ｃｒ、Ｆｅ）酸化物であり、スケール厚もＳＵＳ３１６Ｌよりは薄く、保護皮膜としてある程度作用しているが、温度域が６００℃以上の部位より局所的に瘤状の膨れが発生する。この瘤状スケールは平滑部に比べてスケール厚が厚く、内層部もＦｅ量が増加した状態であり、その部位からスケール剥離が進行する。 ▲３▼１０００サイクル後も剥離しなかったＣ鋼〜Ｇ鋼の内層スケールは、低温部から高温部にかけてＣｒ量が減少した酸化物となるものの、瘤状の膨れは認められない。また、スケールと下地金属との界面にはＣ鋼およびＤ鋼ではＳｉＯ₂ が点状あるいは層状に形成しており、Ｅ鋼とＦ鋼は界面に金属状Ｎｉが濃化した領域が存在し、さらにはＧ鋼ではＡｌ₂ Ｏ₃ が点状あるいは層状に存在する。
【００１８】
すなわち、温度勾配のある環境下での耐スケール剥離向上にはＮｉ、Ｓｉ、また必要に応じて添加するＡｌが極めて重要な役割を果たすことが明らかになった。そこで、Ｃｒ量をＡ鋼、Ｄ鋼、Ｇ鋼と同様の１９％として、Ｎｉ量を１１〜２１％、Ｓｉ量を０．４〜５．５％、Ａｌ量を０．０１〜３．０％に変化させ、他のＳ、Ｐ等はＳＵＳ３１６Ｌと同量としたオ−ステナイト系ステンレス鋼を真空溶解し、１ｍｍ厚の冷延焼鈍板を作製した後、各種長さの直径２０ｍｍφのパイプを製造した。これを前述と同様のプロパンガスを使用した水素発生器の状況下でのガス入側温度を１００℃〜５００℃、ガス出側温度を８００℃から１２００℃として、１時間加熱、３０分間放冷のサイクルを１０００サイクル実施した。
【００１９】
図１は横軸に｛２．１（Ｃｒ−１７）＋３．５（Ｎｉ−１２）＋６．８（Ｓｉ−０．５）＋３．９Ａｌ｝／｛０．０１２Ｔ＋α｝とするＣｒ、Ｎｉ、Ｓｉ、Ａｌの濃度、および使用時の最高温度Ｔ（Ｋ）、パイプに存在する温度勾配α（℃／ｍｍ）を関数とした因子を、縦軸にスケール剥離が開始までのサイクル数をまとめたものである。同図より明らかなように、｛２．１（Ｃｒ−１７）＋３．５（Ｎｉ−１２）＋６．８（Ｓｉ−０．５）＋３．９Ａｌ｝／｛（０．０１２Ｔ＋α｝が１以上ではスケール剥離は発生せず、良好な特性を持つことがわかる。
【００２０】
すなわち、温度勾配のある燃焼雰囲気中での形成するスケールの剥離を抑制するためには、使用する最高温度と温度勾配によりＣｒやＮｉ、Ｓｉ、また必要に応じて添加するＡｌの量を考慮する必要がある。また、Ｃｒ量の係数は他の元素に比べて小さく、このことが前述したＣｒの増加ではスケール剥離を抑制できなかった理由であったことを示唆している。
【００２１】
さらに発明者らは、このＮｉ、ＳｉおよびＡｌの効果について検討するため、上記試験材の断面観察を行った。その結果、Ｎｉ、Ｓｉ、また必要に応じて添加するＡｌはスケール／下地界面に点状あるいは層状濃化しており、それらの濃化量は含有するＳｉ、ＮｉあるいはＡｌ量の増加とともに増加することが明らかになった。一般的に、Ｎｉ、Ｓｉ、Ａｌの効果については、それらが酸化物あるいは金属状態でスケール／下地界面に濃化することにより、金属イオンの拡散係数を低下させ、スケール生成を抑制することで耐酸化性を向上させると考えられる。しかしながら、表２から明らかなように、温度勾配の有る場合にはスケール厚が薄い低サイクル数でも剥離し、温度勾配が無い場合にはそれ以上のスケール厚となってから剥離が発生する。したがって、発明者らはＮｉ、Ｓｉ、Ａｌの効果は従来知見のような単にスケールの成長を抑制する役割ではなく、他の作用により耐スケ−ル剥離性向上に寄与しているとの知見を得た。
【００２２】
そこで、スケールと下地金属界面を詳細に調査した結果、界面に濃化したＳｉやＡｌの酸化物、あるいは金属状Ｎｉは母材が温度勾配により長手方向に異なる膨張率で膨張する、あるいは冷却時に収縮する場合に、表層スケールに与える応力を緩和するように作用し、スケール剥離を抑制していることを見出した。さらに、Ｃｒを含めたこれらＮｉ、Ｓｉ、また必要に応じて添加するＡｌの添加量は使用する最高温度Ｔ（Ｋ）、温度勾配α（℃／ｍｍ）に下記(1) 式の関係があり、その条件を満足する場合にのみスケ−ル剥離が発生しないとの結論を得た。
{2.1(Cr-17) ＋3.5(Ni-12)＋6.8(Si-0.5)+3.9Al}／{(0.012T+ α} ≧1
・・・(1)
【００２３】
次に、本発明の限定理由について説明する。
ＣはＣｒ炭化物を形成し易く、耐食性を劣化させるので低い方がよいが、水素発生器のように装置として固定した状態で使用する場合には高温強度も必要な特性となるため、０．０１％以上、０．１５以下とした。
【００２４】
Ｓｉは前述したように、加熱時でスケ−ル／下地金属界面に濃化し、温度勾配により板面内に発生する応力分布を緩和し、耐スケール剥離性を向上させる元素である。一方、多量の添加は靱性を著しく低下するため、０．５％以上、３．５１％以下とした。
【００２５】
Ｍｎは脱酸元素として有効であるのである程度必要である。しかし、耐食性、加工性の点から限界があるので０．２％〜２．０％とした。
【００２６】
Ｐは粒界偏析し易い元素であり、熱間加工性、耐食性を劣化させ、さらには加熱使用時にスケ−ル／下地界面に濃化し易く、スケール剥離の原因ともなるため０．０４％以下とした。
【００２７】
Ｓは粒界偏析し易く、熱間加工性を著しく劣化し、熱間加工時の粒界割れを生じてヘゲ疵の主原因となる。また、ＳはＰと同様に、使用加熱時にスケール／下地金属界面に濃化し、スケ−ル剥離の原因ともなるため０．０２％以下とした。
【００２８】
Ｎｉはオ−ステナイト系ステンレス鋼の主要元素である。温度勾配のある環境下では耐スケ−ル剥離性を向上させる作用を持ち、多くの添加が好ましいが、経済性の面から２２％以下とした。また、下限については、スケール剥離抑制として作用する最低量として１２％とした。
【００２９】
Ｃｒは耐酸化性を確保する上で１７％以上が必要であるが、熱間加工性、脆性を考慮して上限を２６％とした。
【００３０】
Ａｌは耐スケール剥離性に効果があるものの、過剰な添加は製造性を著しく悪くするため、必要に応じて添加するものであり、その添加量は０．０１％以上、２．７４％以下とした。
【００３２】
さらに本発明では、Ｃｕ，Ｍｏ，Ｃａ，Ｙ，Ｃｅ，Ｐｒ，Ｎｄの少なくとも１種を必要に応じて添加する。
【００３３】
Ｃｕは耐食性を向上させ、水素発生器として海浜地区等の環境下で使用する場合、あるいは触媒担体から出る腐食性物質が問題となる場合には有効な元素である。しかし、多量の添加は熱間加工性を著しく低下させるため、０．１％から２．５％とした。
【００３４】
ＭｏはＣｕと同様に耐食性を向上し、さらに高温強度の増加には有効な元素であるが、原料コストとδ−フェライト量の観点から０．１〜６．５％とした。
【００３５】
【００３６】
Ｌａ、Ｙ、Ｃｅ、Ｐｒ、Ｎｄはスケ−ルの剥離性に有効な元素であるが、多くの添加はコストの増加と製造性の著しい低下の原因となるため、それぞれ０．００１〜０．１％とした。
【００３７】
【実施例】
表３には各種成分のオーステナイト系ステンレス鋼を用いて、各種板厚のパイプを製造し、触媒担体を充填した後、各種炭素系ガスの燃焼雰囲気下で加熱して水素発生を各種温度勾配、温度、時間で実施した時の全加熱時間が１０００時間までのスケール剥離状態を観察した結果を示す。
【００３８】
実験は水素発生器の使用寿命としての全加熱時間１０００時間を目安として、１０００時間までのサイクルで実施した。Ａ鋼、Ｂ鋼およびＣ鋼の次式の(1) 式から計算されるＡ値が１以下の場合には１０００時間以下で剥離が発生するが、Ａ値が１以上となるＣ鋼〜Ｌ鋼では全加熱時間が１０００時間でもスケ−ル剥離が無く、それ以上での使用に十分耐えることが明らかである。
A＝{2.1(Cr-17)+3.5(Ni-12)+ 6.8(Si-0.5)+3.9Al}/{0.012T+ α} ≧1・・・(1)
【００３９】
【表３】

【００４０】
【発明の効果】
本発明により、炭素系ガスの燃焼雰囲気中で加熱−冷却が繰り返され、かつその温度分布に温度勾配がある水素発生器に於いて、全加熱時間が１０００時間を超えてもスケール剥離の無い、オーステナイト系ステンレス鋼の提供が可能となった。水素発生器は次世代のエネルギー源として非常に重要であり、このための材料を提供できる本願発明の工業的価値は極めて高いものであるといえる。
【図面の簡単な説明】
【図１】耐スケール剥離性因子{2.1(Cr-17)+3.5(Ni-12)+ 6.8(Si-0.5)+3.9Al}/{0.012T+ α} と燃焼雰囲気下におけるスケ−ル剥離までの断続加熱時間の関係を示す図である。[0001]
BACKGROUND OF THE INVENTION
The present invention introduces hydrocarbon-based gas and water into the catalyst filled in the pipe, heats the catalyst part using the combustion heat of the hydrocarbon-based gas, and generates hydrogen by catalytic reaction at high temperature. The present invention relates to an austenitic stainless steel having excellent scale peeling resistance for a hydrogen generator to be produced.
[0002]
[Prior art]
A hydrogen generator using a catalytic reaction of a hydrocarbon-based gas needs to heat the reaction part from the outside in order to start the reaction between the catalyst and the introduced gas and further improve the efficiency. As this heating method, the presence of electricity is indispensable in the electric heating of a heater or the like, and the use environment is limited. For this reason, a combustion heating method using a hydrocarbon-based gas combustion burner or the like as a raw material gas is used.
[0003]
Research on the oxidation of heating in the combustion atmosphere of this hydrocarbon-based gas has been advanced, and accelerated oxidation occurs due to about 5 to 15% of water present in the combustion atmosphere, and the amount of oxidation is significantly increased compared to the atmosphere. It is well known to do. This increase in the amount of oxidation due to accelerated oxidation causes thinning of the material, and if the thickly formed scale is peeled off during use, not only the appearance is impaired, but also the efficiency of the catalytic reaction as a hydrogen generator material is reduced. Cause a big problem.
[0004]
In order to suppress such accelerated oxidation by water vapor, austenitic stainless steels having excellent oxidation resistance with increased Cr addition amount have been developed.
Further, JP-A-57-39159 and JP-A-60-92454 disclose an austenitic stainless steel to which Ca, Mg and rare earth elements are added in order to suppress delamination, and JP-A-55-43498. Discloses an austenitic stainless steel to which Al is added in an amount of 4.5% to 6% in order to form an Al ₂ O ₃ film stable at a high temperature. Further, AISI 302B (18Cr-9Ni-2.5Si) and AISI 314 (25Cr-20Ni-2Si) to which Si is added have been developed as austenitic stainless steels for heat-resistant members.
[0005]
[Problems to be solved by the invention]
However, although the above-mentioned known austenitic stainless steel has excellent oxidation resistance under continuous heating for a long time, the austenitic stainless steel has a high thermal expansion coefficient under intermittent heating and repeated heating. There is a drawback that the scale is easily peeled off due to poor consistency with the material.
Therefore, in the case of repeated use in a cycle in which heating is performed within a certain period of time requiring hydrogen and then allowed to cool, as in the hydrogen generator applied in the present invention, the purpose is simply to reduce the oxidation rate. The conventional austenitic stainless steel cannot be applied.
[0006]
Furthermore, the hydrogen generator has a high temperature of 400 ° C. or lower in the steady state in the gas inlet side where the hydrocarbon gas and water as raw materials are introduced, and a temperature of 600 to 1200 ° C. or higher in the site where they react with the catalyst. The temperature gradient is 0.2 ° C./mm or more in the same pipe surface.
That is, the hydrogen generator
(1) High temperature of 600 ° C or higher in a combustion atmosphere containing moisture (2) Heating / cooling thermal cycle (3) Excellent resistance to scale peeling in harsh environments with a temperature gradient of 0.2 ° C / mm or higher Materials are required.
[0007]
However, there is a problem that austenitic stainless steel that satisfies these three elements does not exist and cannot be applied. On the other hand, ferritic stainless steel is more advantageous than austenitic stainless steel in terms of coefficient of thermal expansion, but is considered difficult to apply in terms of high-temperature strength.
[0008]
[Means for Solving the Problems]
The inventors have studied an alloy index for satisfying the above three conditions, and have completed the present invention to complete an austenitic stainless steel excellent in scale peel resistance based on the index. That is, the present invention is an austenite system that has a temperature gradient and is excellent in scale peeling resistance as a material for a hydrogen generator that is used by repeatedly repeating a heating-cooling cycle under a combustion atmosphere of a hydrocarbon gas. Stainless steel is provided, the gist of which is as follows.
(1) In mass %,
C: 0.01 to 0.15%, Si: 0.5 to 3.51 %,
Mn: 0.2-2%, P: 0.04% or less,
S: 0.02% or less, Ni: 12-22%,
Cr: 17-26%
The balance of Fe and inevitable impurities, and satisfying the following formula (1) between the amount of element addition and the maximum heating temperature T (K) and temperature gradient α (° C./mm) to be used An austenitic stainless steel for hydrogen generators with excellent scale peeling resistance.
{2.1 (Cr-17) +3.5 (Ni-12) +6.8 (Si-0.5)
+ 3.9Al} / {0.012T + α} ≧ 1 (1)
(2) In addition to the components described in the preceding paragraph, in addition to the mass, Al: 0.01-2.74% is contained, and hydrogen generation excellent in scale peel resistance described in ( 1 ) above Austenitic stainless steel for dexterity .
(3) In addition to the components set forth in the preceding paragraph, wherein in addition mass%,
Cu : 0.1-2.5%, Mo: 0.1-6.5%,
La: 0.001 to 0.1%, Y: 0.001 to 0.1%,
Ce: 0.001 to 0.1%, Pr: 0.001 to 0.1%,
The austenitic stainless steel for a hydrogen generator having excellent scale peel resistance as described in (1) or (2) above, containing one or more of Nd: 0.001 to 0.1%.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, further details including the background to the present invention will be described.
The inventors have changed the Cr, Ni, Mn, and Si, with a focus on SUS316L (17Cr-13Ni-2.5Mo) austenitic stainless steel, which is widely used from the viewpoint of high-temperature strength and oxidation resistance. Austenitic stainless steel of 1 component is melted in a vacuum of 300 kg, a thin plate having a thickness of 0.7 mm is produced by a normal process to obtain a pipe having a diameter of about 15 mm, and then the pipe is cut into a length of 200 mm; A catalyst carrying a noble metal was filled inside, and a mixed gas of methane gas, air, and water was introduced into the tube, and heated from the outside with a methane gas combustion burner.
[0010]
The pipe temperature on the gas inlet side in the steady state at this time is 200 ° C., the pipe temperature on the gas outlet side is 1050 ° C., and a temperature gradient of about 4 ° C./mm exists in a 200 mm short pipe. Moreover, in order to generate hydrogen, the cycle which hold | maintains high temperature for 1 hour, 30 minutes after that was allowed to cool, and was heated again was repeated. Regarding the number of cycles, the total high temperature heating time was 1000 hours as a general service life of a hydrogen generator, and here 1000 cycles was taken as a standard.
[0011]
[Table 1]

[0012]
Table 2 shows the results of observing the peeled state of the scale when the various component steels shown in Table 1 were used for heating and cooling up to 1000 cycles, and the numbers in the table are the number of cycles at which peeling started. . Here, more than 1000 cycles indicates that there is no scale peeling even after 1000 cycles in which the total heating time is 1000 hours. In the same table, the number of cycles until the start of scale peeling when a similar thermal cycle in a state where there is no temperature gradient in the combustion atmosphere and the atmosphere is repeated is also described for reference.
[0013]
[Table 2]

[0014]
In the case of SUS316L with no temperature gradient and uniform heating, scale peeling did not occur until 487 cycles when heated in the atmosphere, but peeled after 124 cycles in the methane combustion atmosphere. Further, when there was a temperature gradient, the scale peeled off in 55 cycles. In addition, SUS316L and the A and B steels, which are the same components except the Cr amount, improve the scale peel resistance as the Cr amount increases, but under conditions where a temperature gradient exists, the A steel has 106 cycles and the B steel has 867 cycles. It peeled off and the target 1000 cycles could not be achieved.
[0015]
When observing the scale peeling locations of these SUS316L, A steel and B steel, peeling occurred randomly on the entire surface when heated under uniform heating conditions, whereas it was about 600 ° C under conditions with a temperature gradient. Peeling occurred from the low temperature part, and the peeling area increased with increasing temperature. from this result,
{Circle around (1)} Although in the combustion atmosphere, the scale peeling proceeds if there is a temperature gradient even in a low temperature range of about 600 ° C., which is normally considered not to cause the scale peeling.
(2) It has been clarified that the scale peel resistance cannot be improved only by increasing Cr.
[0016]
On the other hand, D steel, F steel, G steel with approximately the same Cr amount as steel A, and C steel and E steel with approximately the same Cr amount as steel B, regardless of the atmosphere or temperature gradient, after 1000 cycles However, scale peeling was not recognized, and it was found that it could withstand longer use. In order to clarify this cause, SUS316L in which peeling occurred, and pipes and scales in the cycle immediately before peeling when heated under conditions where a temperature gradient exists in steels A and B in Table 1. Pipes immediately after exfoliation, and in C to G steel where exfoliation did not occur, the pipes after 1000 cycles were cut from the low temperature part on the gas inlet side to the high temperature part on the gas outlet side by 10 mm, and their cross sections were observed with an optical microscope. Element distribution was measured by EPMA.
[0017]
As a result, the following became clear.
(1) The scale of SUS316, which has peeled off in 55 cycles, has a two-layer structure in which the inner layer on the base metal side is (Cr, Fe) oxide and the outer layer is Fe oxide. As the temperature increases, the amount of Cr is small and there is no protective property at high temperatures of 600 ° C. or higher.
(2) Steels A and B are oxides with a large amount of Cr in the inner layer scale compared to SUS316L (Cr, Fe), and the scale thickness is also thinner than SUS316L, which acts as a protective film to some extent. A bump-like bulge is locally generated from a region where the temperature range is 600 ° C. or higher. The scale-like scale is thicker than the smooth part, and the inner layer part is also in an increased amount of Fe, and scale peeling progresses from that part. (3) The inner scale of C steel to G steel, which did not peel after 1000 cycles, became an oxide with a reduced Cr content from the low temperature portion to the high temperature portion, but no swollen swelling was observed. Further, in the steel C and D steel, SiO ₂ is formed in a dotted or layered form at the interface between the scale and the base metal, and the E steel and the F steel have a region where metallic Ni is concentrated at the interface, Further, Al ₂ O ₃ is present in the form of dots or layers in G steel.
[0018]
That is, it has been clarified that Ni, Si, and Al added as necessary plays an extremely important role in improving the scale peeling resistance in an environment with a temperature gradient. Therefore, the Cr amount is 19% similar to the A steel, D steel, and G steel, the Ni amount is 11 to 21%, the Si amount is 0.4 to 5.5%, and the Al amount is 0.01 to 3.0. %, And other S, P, etc. were austenitic stainless steel with the same amount as SUS316L, vacuum-melted to produce cold-rolled annealed plates with a thickness of 1 mm, and then pipes with diameters of 20 mmφ of various lengths. Manufactured. This was heated for 1 hour and allowed to cool for 30 minutes with the gas inlet side temperature set to 100 to 500 ° C. and the gas outlet side temperature set to 800 to 1200 ° C. in the situation of a hydrogen generator using propane gas as described above. This cycle was performed 1000 times.
[0019]
FIG. 1 shows Cr, Ni, Si with {2.1 (Cr-17) +3.5 (Ni-12) +6.8 (Si-0.5) +3.9 Al} / {0.012T + α} on the horizontal axis. , The concentration of Al, the maximum temperature T (K) at the time of use, and a factor as a function of the temperature gradient α (° C / mm) existing in the pipe, the vertical axis summarizes the number of cycles until scale peeling starts It is. As is clear from the figure, when {2.1 (Cr-17) +3.5 (Ni-12) +6.8 (Si-0.5) + 3.9Al} / {(0.012T + α} is 1 or more, It can be seen that scale peeling does not occur and that the film has good characteristics.
[0020]
Consider Namely, in order to suppress the peeling of the scale forming in the combustion atmosphere of a temperature gradient, Cr and Ni by the maximum temperature and temperature gradients to be used, Si, also the amount of Al to be added as required There is a need. Further, the coefficient of Cr amount is smaller than that of other elements, suggesting that this was the reason why scale peeling could not be suppressed by the increase in Cr described above.
[0021]
Furthermore, in order to examine the effects of Ni, Si and Al, the inventors performed cross-sectional observation of the test material. As a result, Ni, Si, and Al added as needed are concentrated in the form of dots or layers at the scale / underlying interface, and the concentration increases as the amount of Si, Ni or Al contained increases. Became clear. In general, the effects of Ni, Si and Al are concentrated in the oxide / metal state at the scale / underlying interface, thereby reducing the diffusion coefficient of metal ions and suppressing the formation of scale. It is thought to improve the chemical properties. However, as is apparent from Table 2, peeling occurs even with a low cycle number where the scale thickness is thin when there is a temperature gradient, and peeling occurs after a larger scale thickness when there is no temperature gradient. Accordingly, the inventors have found that the effects of Ni, Si, and Al are not simply the role of suppressing the growth of scale as in the conventional knowledge, but contribute to the improvement of the scale peel resistance by other actions. Obtained.
[0022]
Therefore, as a result of detailed investigation of the interface between the scale and the underlying metal, Si or Al oxide or metallic Ni concentrated at the interface expands with a different expansion coefficient in the longitudinal direction due to the temperature gradient, or during cooling It has been found that when it shrinks, it acts to relieve stress applied to the surface scale and suppresses scale peeling. Furthermore, the amount of Ni, Si including Cr, and the amount of Al added as necessary is related to the maximum temperature T (K) and temperature gradient α (° C./mm) using the following formula (1). It was concluded that scale peeling does not occur only when the conditions are satisfied.
{2.1 (Cr-17) +3.5 (Ni-12) +6.8 (Si-0.5) + 3.9Al} / {(0.012T + α} ≧ 1
... (1)
[0023]
Next, the reason for limitation of the present invention will be described.
Since C is easy to form Cr carbide and deteriorates corrosion resistance, it is better to be low. However, when used in a state of being fixed as a device like a hydrogen generator, high temperature strength is also a necessary characteristic. % Or more and 0.15 or less.
[0024]
As described above, Si is an element that concentrates at the scale / underlying metal interface during heating, relaxes the stress distribution generated in the plate surface due to the temperature gradient, and improves the scale peel resistance. On the other hand, addition of a large amount significantly reduces toughness, so the content was made 0.5% to 3.51 %.
[0025]
Since Mn is effective as a deoxidizing element, it is necessary to some extent. However, there is a limit in terms of corrosion resistance and workability, so 0.2% to 2.0% was set.
[0026]
P is an element that easily segregates at the grain boundary, deteriorates hot workability and corrosion resistance, and further tends to concentrate at the scale / underlying interface during heating, and also causes scale peeling, and is 0.04% or less. did.
[0027]
S is easily segregated at the grain boundary, remarkably deteriorates hot workability, and causes grain boundary cracking during hot working, which is a main cause of whipping. Further, S, like P, is concentrated at the scale / underlying metal interface during use heating and causes scale peeling, so it was made 0.02% or less.
[0028]
Ni is a main element of austenitic stainless steel. In an environment with a temperature gradient, it has the effect of improving the scale peel resistance, and many additions are preferable, but it was made 22% or less from the viewpoint of economy. The lower limit was set to 12% as the minimum amount that acts as a scale peeling inhibitor.
[0029]
In order to ensure oxidation resistance, Cr needs to be 17% or more, but the upper limit was made 26% in consideration of hot workability and brittleness.
[0030]
Although Al is effective in resistance to scale peeling, excessive addition significantly deteriorates manufacturability, so it is added as necessary, and the addition amount is 0.01% or more and 2.74 % or less. did.
[0032]
Furthermore, in the present invention, at least one of Cu , Mo, Ca, Y, Ce, Pr, and Nd is added as necessary .
[0033]
Cu improves corrosion resistance, and is an effective element when used in an environment such as a beach area as a hydrogen generator, or when a corrosive substance coming out of a catalyst carrier becomes a problem. However, a large amount of addition significantly reduces the hot workability, so the content was made 0.1% to 2.5%.
[0034]
Mo, like Cu, improves corrosion resistance and is an effective element for increasing high-temperature strength. However, it was made 0.1 to 6.5% from the viewpoint of raw material cost and δ-ferrite content.
[0035]
[0036]
La, Y, Ce, Pr, and Nd are effective elements for the releasability of the scale, but many additions cause an increase in cost and a significant decrease in manufacturability. 1%.
[0037]
【Example】
In Table 3, various austenitic stainless steels of various components are used to manufacture pipes with various plate thicknesses, and after filling with a catalyst carrier, heating is performed in a combustion atmosphere of various carbon-based gases to generate various temperature gradients. The result of having observed the scale peeling state to the total heating time up to 1000 hours when implemented by temperature and time is shown.
[0038]
The experiment was carried out in a cycle of up to 1000 hours, with a total heating time of 1000 hours as the service life of the hydrogen generator as a guide. Peeling occurs in 1000 hours or less when the A value calculated from the following formula (1) of steel A, steel B and steel C is 1 or less, but steel C having an A value of 1 or more to L It is clear that steel does not have scale peeling even when the total heating time is 1000 hours, and is sufficiently resistant to use beyond that.
A = {2.1 (Cr-17) +3.5 (Ni-12) + 6.8 (Si-0.5) + 3.9Al} / {0.012T + α} ≧ 1 ... (1)
[0039]
[Table 3]

[0040]
【The invention's effect】
According to the present invention, in a hydrogen generator in which heating and cooling are repeated in a combustion atmosphere of a carbon-based gas and the temperature distribution thereof has a temperature gradient, there is no scale peeling even when the total heating time exceeds 1000 hours. Austenitic stainless steel can be provided. The hydrogen generator is very important as a next-generation energy source, and it can be said that the industrial value of the present invention capable of providing a material for this purpose is extremely high.
[Brief description of the drawings]
[Figure 1] Scale peeling resistance factor {2.1 (Cr-17) + 3.5 (Ni-12) + 6.8 (Si-0.5) + 3.9Al} / {0.012T + α} and scale peeling in a combustion atmosphere It is a figure which shows the relationship of the intermittent heating time of.

Claims (3)

% By mass
C: 0.01 to 0.15%,
Si: 0.5 to 3.51 %,
Mn: 0.2-2%
P: 0.04% or less,
S: 0.02% or less,
Ni: 12-22%,
Cr: 17 to 26%,
The balance of Fe and inevitable impurities, and satisfying the following formula (1) between the amount of element addition and the maximum heating temperature T (K) and temperature gradient α (° C./mm) to be used An austenitic stainless steel for hydrogen generators with excellent scale peeling resistance.
{2.1 (Cr-17) +3.5 (Ni-12) +6.8 (Si-0.5)
+ 3.9Al} / (0.012T + α) ≧ 1 (1) The austenitic stainless steel for a hydrogen generator excellent in scale peel resistance according to claim 1, wherein the steel component further contains Al: 0.01-2.74% by mass% . As a steel component, further in mass %,
Cu : 0.1 to 2.5%,
Mo: 0.1 to 6.5%,
La: 0.001 to 0.1%,
Y: 0.001 to 0.1%
Ce: 0.001 to 0.1%,
Pr: 0.001 to 0.1%,
Nd: 0.001 to 0.1%
The austenitic stainless steel for a hydrogen generator excellent in scale peel resistance according to claim 1 or 2 , wherein the austenitic stainless steel has excellent scale peeling resistance.

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