JPWO2007091535A1 - Ferritic heat resistant steel - Google Patents

Ferritic heat resistant steel Download PDF

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JPWO2007091535A1
JPWO2007091535A1 JP2007557837A JP2007557837A JPWO2007091535A1 JP WO2007091535 A1 JPWO2007091535 A1 JP WO2007091535A1 JP 2007557837 A JP2007557837 A JP 2007557837A JP 2007557837 A JP2007557837 A JP 2007557837A JP WO2007091535 A1 JPWO2007091535 A1 JP WO2007091535A1
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steel
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利夫 藤田
利夫 藤田
佐藤 恭
恭 佐藤
田村 広治
広治 田村
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    • FMECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F22STEAM GENERATION
    • F22BMETHODS OF STEAM GENERATION; STEAM BOILERS
    • F22B37/00Component parts or details of steam boilers
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    • F22STEAM GENERATION
    • F22BMETHODS OF STEAM GENERATION; STEAM BOILERS
    • F22B37/00Component parts or details of steam boilers
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    • C21D2211/00Microstructure comprising significant phases
    • C21D2211/008Martensite

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Abstract

重量%で、C0.01〜0.10%、Si0.30〜1.0%、P0.020%以下、S0.010%以下、Mn0.2〜1.2%、Ni0.3%以下、Cr8.0〜11.0%、Mo0.1〜1.2%、W1.0〜2.5%、V0.10〜0.30%、Nb0.02〜0.12%、Co0.01〜4.0%、N0.01〜0.08%、B0.001以上で0.010%未満、Cu0.3%以下、Al0.010%以下、Mo%+0.5×W%1.0〜1.6、C%+N%0.02〜0.15%で、調質熱処理により得られる焼戻しマルテンサイト単相組織からなるフェライト系耐熱鋼である。650℃付近の蒸気温度で用いられても、長時間クリープ破断強度に優れ、かつ水蒸気酸化性にも優れる。または、さらにAl%+0.1×Niを%0.02以下とすれば、クリープ強度の安定化が向上する。% By weight, C0.01-0.10%, Si0.30-1.0%, P0.020% or less, S0.010% or less, Mn0.2-1.2%, Ni0.3% or less, Cr8 0.0 to 11.0%, Mo 0.1 to 1.2%, W 1.0 to 2.5%, V 0.10 to 0.30%, Nb 0.02 to 0.12%, Co 0.01 to 4. 0%, N0.01-0.08%, B0.001 or more and less than 0.010%, Cu0.3% or less, Al0.010% or less, Mo% + 0.5 × W% 1.0-1.6 C% + N% 0.02 to 0.15%, and a ferritic heat resistant steel made of a tempered martensite single phase structure obtained by tempering heat treatment. Even when used at a steam temperature around 650 ° C., it has excellent long-term creep rupture strength and excellent steam oxidization. Alternatively, if Al% + 0.1 × Ni is set to 0.02 or less, the stabilization of creep strength is improved.

Description

本発明はフェライト系耐熱鋼に係り、特に発電効率を向上させた超々臨界圧の火力プラントに好適なボイラ鋼管用の高強度鋼に関する。   The present invention relates to a ferritic heat-resistant steel, and more particularly to a high-strength steel for boiler steel pipes suitable for a super-supercritical pressure thermal power plant with improved power generation efficiency.

近年、火力発電プラントではCO2排出量削減等、地球規模の環境問題を背景としてプラント効率の向上のために蒸気条件の高温高圧化が進められている。そして、現在得られる最高の主蒸気温度である600℃程度の蒸気温度から、さらに650℃、究極的には700℃程度の蒸気温度を達成できるプラントの開発研究が国内外で進められている。このような蒸気温度の上昇に伴い、ボイラの高温耐圧部にはクリープ破断強度の高い耐熱鋼が必要となる。そのため、ボイラの伝熱管には、耐食性とクリープ破断強度の優れたオーステナイト系の耐熱鋼が多く使われるようになってきた。In recent years, high-temperature and high-pressure steam conditions have been promoted in thermal power plants to improve plant efficiency against the background of global environmental problems such as CO 2 emission reduction. Research and development of a plant that can achieve a steam temperature of about 650 ° C., and finally about 700 ° C., from a steam temperature of about 600 ° C., which is the highest main steam temperature that can be obtained at present, is underway in Japan and overseas. As the steam temperature rises, heat resistant steel with high creep rupture strength is required for the high temperature pressure resistant part of the boiler. For this reason, austenitic heat-resistant steels having excellent corrosion resistance and creep rupture strength have been widely used for boiler heat transfer tubes.

一方、管寄せや配管のような大径で厚肉管の場合は、これらオーステナイト系耐熱鋼を用いた場合、フェライト系耐熱鋼に比べて線膨張係数が高く、熱伝達率が小さい。したがって、プラントの起動時や停止時には、これらの管寄せや配管に大きな熱応力が発生して熱疲労による損傷を受けやすいという問題がある。また材料費や加工費の上昇による経済的な問題もあった。このためクリープ破断強度が高く、耐食性も良好な新しいフェライト系耐熱鋼の開発が望まれていた。このようなフェライト系耐熱鋼の例としては、従来の9%Cr1%MoNbV鋼をベースにCrの割合を増加し、WとCo等の合金元素を添加してクリープ破断強度の改善を図った材料が提案されている(例えば特許第2528767号)。   On the other hand, in the case of large-diameter and thick-walled pipes such as headers and pipes, when these austenitic heat resistant steels are used, the linear expansion coefficient is higher and the heat transfer coefficient is smaller than that of ferritic heat resistant steels. Therefore, when starting or stopping the plant, there is a problem that large thermal stresses are generated in these headers and pipes and are easily damaged by thermal fatigue. There were also economic problems due to rising material costs and processing costs. Therefore, development of a new ferritic heat resistant steel having high creep rupture strength and good corrosion resistance has been desired. As an example of such a ferritic heat resistant steel, a material in which the ratio of Cr is increased based on the conventional 9% Cr1% MoNbV steel, and an alloying element such as W and Co is added to improve the creep rupture strength. Has been proposed (for example, Japanese Patent No. 2528767).

しかしながら、例えば650℃付近の蒸気温度となるボイラでフェライト系耐熱鋼を使用する場合、フェライト系耐熱鋼は多くのWやその他の合金元素を含有するため、長時間使用していると脆弱な金属間化合物あるいは炭化物の凝集粗大化を生じる。そのため、数万時間以上の長時間の使用でクリープ破断強度が低下することが分かってきた。特に600℃を大きく超えた、650℃付近の高温においては、数万時間前後でクリープ強度が急激に低下する、いわゆる腰折れ現象が高Cr鋼開発の大きな障壁となっている(例えば、非特許文献1)。   However, when ferritic heat resistant steel is used in a boiler with a steam temperature of, for example, around 650 ° C., the ferritic heat resistant steel contains a lot of W and other alloy elements. Aggregate coarsening of intermetallic compounds or carbides. For this reason, it has been found that the creep rupture strength decreases when used for a long time of tens of thousands of hours or more. In particular, at a high temperature near 650 ° C., which greatly exceeds 600 ° C., the so-called hip-fold phenomenon, in which the creep strength rapidly decreases around tens of thousands of hours, is a large barrier for the development of high Cr steel (for example, non-patent literature). 1).

また、これら管寄せや配管は、ボイラ鋼管の内部流体である高温水蒸気に長期間晒される。高Crフェライト鋼が650℃付近の高温で使用される場合には水蒸気による酸化スケールの生成が顕著となるため(例えば、前記ワークショップ前刷集 p153)、スケールの成長やスケールの剥離、そしてスケールの下流への飛散も問題となる。この問題を改善するために貴金属を添加する特殊な方法(非特許文献2)も提案されているが、材料費が著しく上昇するため、実用化には至っていない。   Further, these headers and pipes are exposed to high-temperature steam that is an internal fluid of the boiler steel pipe for a long period of time. When high Cr ferritic steel is used at a high temperature around 650 ° C., the generation of oxide scale due to water vapor becomes significant (for example, the preprint of the workshop p153), scale growth, scale peeling, and scale Scattering downstream is also a problem. In order to improve this problem, a special method of adding a noble metal (Non-Patent Document 2) has also been proposed, but it has not been put into practical use because the material cost increases remarkably.

そして、非特許文献3によれば、9〜12%のCrを含有した高クロム鋼に、VやNb、N、Mo、W、Bなどの成分を加えてクリープ破断強度の強化を図った構成が記載されている。前記非特許文献3の高クロム鋼の中には長時間における600℃付近のクリープ破断強度が確保できているものもあるが、650℃付近のクリープ破断強度は、600℃における強度に比べてかなり低下する。   And according to Non-Patent Document 3, the composition is intended to enhance the creep rupture strength by adding components such as V, Nb, N, Mo, W, and B to high chromium steel containing 9 to 12% Cr. Is described. Although some of the high chromium steels of Non-Patent Document 3 have secured a creep rupture strength near 600 ° C. for a long time, the creep rupture strength near 650 ° C. is considerably higher than the strength at 600 ° C. descend.

また、本発明者らは先に650℃付近のクリープ破断強度が高いフェライト系耐熱鋼の開発を行い特許出願を行った(特開2005−23378号公報)。
特許第2528767号公報 特開2005−23378号公報 R.Viswanathan et al. "Materials for Ultrasupercritical Coal-fired Power Boilers"(p146〜p157); R.Blum et al. "Materials Development in Thermie Project for 700℃ USC Plant" (p158〜p176) 第8回超鉄鋼ワークショップ委員会 独立行政法人 物質・材料研究機構 超鉄鋼研究センター 2004年7月22日 春山他 「9Crフェライト鋼の水蒸気酸化挙動に及ぼすPb添加量の影響」 材料とプロセス 日本鉄鋼協会 2003年3月 第16巻 第3号 p.648 Gabrel J et al. "Status of development of the VM12 steel for tubular application in advanced power plants" Proceedings of the 8th Liege Conference Part II, Materials for Advanced Power Engineering 2006, Forschungszentrum Julich GmbH, September 2006, p1068〜1075
The inventors previously developed a ferritic heat resistant steel having a high creep rupture strength near 650 ° C. and filed a patent application (Japanese Patent Laid-Open No. 2005-23378).
Japanese Patent No. 2528767 JP 2005-23378 A R.Viswanathan et al. "Materials for Ultrasupercritical Coal-fired Power Boilers"(p146-p157); R.Blum et al. "Materials Development in Thermie Project for 700 ° C USC Plant" (p158-p176) Workshop Committee National Institute for Materials Science Super Steel Research Center July 22, 2004 Haruyama et al. “Effect of Pb addition on steam oxidation behavior of 9Cr ferritic steel” Materials and Processes Japan Iron and Steel Institute March 2003 Vol. 16 No. 3 p. 648 Gabrel J et al. "Status of development of the VM12 steel for tubular application in advanced power plants" Proceedings of the 8th Liege Conference Part II, Materials for Advanced Power Engineering 2006, Forschungszentrum Julich GmbH, September 2006, p1068-1075

本発明者らが先に発明した特許文献2記載のフェライト系耐熱鋼は650℃付近のクリープ破断強度が高いが、長時間にわたる強度が不足し、また耐水蒸気酸化性(管内の水蒸気で酸化する性質が改善される)にも改善の余地があった.。このように従来提案された合金では650℃付近で使用する材料の特性としてはまだ不十分である。さらに650℃付近のクリープ破断強度が高く、しかも長時間にわたって強度が安定し、同時に耐水蒸気酸化性に優れたフェライト系耐熱鋼が必要とされている。   The ferritic heat resistant steel described in Patent Document 2 previously invented by the present inventors has a high creep rupture strength at around 650 ° C., but lacks the strength over a long period of time, and is resistant to steam oxidation (oxidizes with steam in the pipe) There is also room for improvement). Thus, the conventionally proposed alloys are still insufficient as the characteristics of the material used near 650 ° C. Further, there is a need for a ferritic heat resistant steel having a high creep rupture strength near 650 ° C., a stable strength over a long period of time, and at the same time excellent steam oxidation resistance.

本発明の課題は、従来の材料に比べてさらに650℃付近で使用される場合の長時間クリープ破断強度に優れ、かつ水蒸気酸化性にも優れた高強度フェライト系耐熱鋼を提供することである。   An object of the present invention is to provide a high-strength ferritic heat resistant steel that is superior in long-term creep rupture strength when used near 650 ° C. as compared with conventional materials, and also excellent in steam oxidation. .

上記本発明の課題は、以下の解決手段により達成される。
請求項1記載の発明は、重量%で、炭素(C):0.01〜0.10%、ケイ素(Si):0.30〜1.0%、リン(P):0.020%以下、硫黄(S):0.010%以下、マンガン(Mn):0.2〜1.2%、ニッケル(Ni):0.3%以下、クロム(Cr):8.0〜11.0%、モリブデン(Mo):0.1〜1.2%、タングステン(W):1.0〜2.5%、バナジウム(V):0.10〜0.30%、ニオブ(Nb):0.02〜0.12%、コバルト(Co):0.01〜4.0%、窒素(N):0.01〜0.08%、ホウ素(B):0.001以上で0.010%未満、銅(Cu):0.3%以下、アルミニウム(Al):0.010%以下、さらに(Mo%+0.5×W%)の量を1.0〜1.6に制限し、さらに(C%+N%)の量を0.02〜0.15%に制限した成分で、調質熱処理により得られる焼戻しマルテンサイト単相組織からなるフェライト系耐熱鋼である。
The object of the present invention is achieved by the following means.
The invention according to claim 1 is, by weight, carbon (C): 0.01 to 0.10%, silicon (Si): 0.30 to 1.0%, phosphorus (P): 0.020% or less. , Sulfur (S): 0.010% or less, manganese (Mn): 0.2 to 1.2%, nickel (Ni): 0.3% or less, chromium (Cr): 8.0 to 11.0% Molybdenum (Mo): 0.1 to 1.2%, Tungsten (W): 1.0 to 2.5%, Vanadium (V): 0.10 to 0.30%, Niobium (Nb): 0. 02 to 0.12%, cobalt (Co): 0.01 to 4.0%, nitrogen (N): 0.01 to 0.08%, boron (B): 0.001 or more and less than 0.010% , Copper (Cu): 0.3% or less, aluminum (Al): 0.010% or less, and further, the amount of (Mo% + 0.5 × W%) is limited to 1.0 to 1.6, Et a (C% + N%) the amount of a component which is limited to from .02 to .15 percent, a ferritic heat-resistant steel consisting of tempered martensite single phase structure obtained by the refining heat treatment.

請求項2記載の発明は、重量%で(Al%+0.1×Ni%)の量を0.02%以下に制限した成分である請求項1記載のフェライト系耐熱鋼である。   The invention according to claim 2 is the ferritic heat resistant steel according to claim 1, which is a component in which the amount of (Al% + 0.1 × Ni%) is limited to 0.02% or less.

請求項3記載の発明は、30重量%以下のδフェライトを含有する請求項1又は2記載のフェライト系耐熱鋼である。   The invention according to claim 3 is the ferritic heat resistant steel according to claim 1 or 2 containing 30 wt% or less of δ ferrite.

炭素(C)は、高Crフェライト系耐熱鋼の強化に寄与する炭化物(M23C6、M6C、M7C3等)を形成するのに重要な元素である。従来の実用鋼では、炭素は0.1〜0.12%程度必要とされてきたが、0.10%を超えると炭化物の凝集・粗大化を促進してクリープ破断強度を低下させるため、本発明では0.10%以下として、長時間クリープ強度を安定させる。Cの含有量は低いほどクリープ破断強度はよくなるが、0.01%未満では靭性が悪くなるため、実用鋼として0.01〜0.10%とする。Cの含有量の細かな制御は製鋼上高度な技術を要するが、特にCの含有量を従来の鋼の0.1%前後から0.08%以下に減少すれば、Ac1点(変態点)を大幅に高め、長時間クリープ破断強度をより高めることができる。   Carbon (C) is an important element for forming carbides (M23C6, M6C, M7C3, etc.) that contribute to strengthening of high Cr ferritic heat resistant steel. In conventional practical steel, about 0.1 to 0.12% of carbon has been required. However, if it exceeds 0.10%, the agglomeration and coarsening of carbides are promoted and the creep rupture strength is reduced. In the invention, the creep strength is stabilized for a long time by setting it to 0.10% or less. The lower the C content, the better the creep rupture strength. However, if it is less than 0.01%, the toughness becomes worse, so 0.01 to 0.10% for practical steel. Fine control of the C content requires advanced technology in steelmaking, but if the C content is reduced from around 0.1% to 0.08% or less of conventional steel, the Ac1 point (transformation point) The creep rupture strength can be further increased for a long time.

ケイ素(Si)は、脱酸剤として鋼を製造する際に必要な元素であった。しかし、近年は真空脱酸処理が可能となり、真空脱酸処理により低Si鋼が得られるようになって、高Cr系耐熱鋼にも利用されるようになってきた。Siは耐酸化性を向上させる元素で、600℃級のボイラ材として必要な耐水蒸気酸化性を確保するためには最低0.30%は必要である。なお、650℃級のボイラ用材料として十分な耐水蒸気酸化性を確保するためには、一般的にスケール厚さが200μm以下であることが望ましい。   Silicon (Si) was an element necessary for producing steel as a deoxidizer. However, in recent years, vacuum deoxidation treatment has become possible, and low Si steel has been obtained by vacuum deoxidation treatment, and has been used for high Cr heat resistant steel. Si is an element that improves the oxidation resistance, and at least 0.30% is necessary to ensure the steam oxidation resistance necessary for a 600 ° C. class boiler material. In order to ensure sufficient steam oxidation resistance as a 650 ° C. class boiler material, it is generally desirable that the scale thickness is 200 μm or less.

一方、Siを1.0%を超えて多量に添加すると、タングステン(W)のラーベス相等の生成が促進され、また粒界の偏析等によって延性を低下させる。したがって、クリープ強度を重視する場合はSiの含有量が低く抑えられる傾向があり、650℃付近で鋼を使う場合の障害となっていた。   On the other hand, when Si is added in a large amount exceeding 1.0%, the formation of a Laves phase of tungsten (W) or the like is promoted, and the ductility is lowered due to segregation of grain boundaries or the like. Therefore, when the creep strength is regarded as important, the Si content tends to be kept low, which has been an obstacle when using steel near 650 ° C.

しかし本発明では、後述するアルミニウム(Al)のM23C6炭化物の凝集粗大化の低減効果によりSiの含有量を高めても高いクリープ強度が得られることが見出された。したがって、650℃級のボイラ用材料として十分な耐水蒸気酸化性を確保するため、Si含有量は0.30〜1.0%とする。また、鋼の延性をより重視する場合は、Siの含有量が高いと延性が低下することから、Siの含有量を0.30〜0.8%とすればよい(図1)。   However, in the present invention, it has been found that high creep strength can be obtained even if the Si content is increased due to the effect of reducing the aggregation and coarsening of M23C6 carbide of aluminum (Al) described later. Therefore, in order to ensure sufficient steam oxidation resistance as a 650 ° C. class boiler material, the Si content is set to 0.30 to 1.0%. In the case where the ductility of steel is more important, the ductility is lowered when the Si content is high, so the Si content may be 0.30 to 0.8% (FIG. 1).

マンガン(Mn)は、脱酸剤として0.2%以上必要であり、同時にオーステナイト生成元素としてδフェライトの生成を抑制する有益な元素である。しかし、1.2%を超えて添加するとAc1変態点が低下し、クリープ強度が低下する。従ってMn含有量は0.2〜1.2%に限定する。   Manganese (Mn) is a useful element that needs 0.2% or more as a deoxidizer and at the same time suppresses the formation of δ ferrite as an austenite generating element. However, if added over 1.2%, the Ac1 transformation point is lowered and the creep strength is lowered. Therefore, the Mn content is limited to 0.2 to 1.2%.

リン(P)及び硫黄(S)は、低融点元素であるため、含有量が多いとクリープ破断強度に悪影響を及ぼすので、その含有量は低いほどよい。しかし、PとSを完全に除くことは難しく、その含有量を極端に低くすると材料価格の上昇を招くため極端に低い組成にする必要はなく、Pは0.020%以下、Sは0.010%以下に制限すれば良い。   Since phosphorus (P) and sulfur (S) are low melting point elements, if the content is large, the creep rupture strength is adversely affected. Therefore, the lower the content, the better. However, it is difficult to completely remove P and S. If the content is extremely low, the material price is increased, so there is no need to make the composition extremely low, P is 0.020% or less, and S is 0.2%. It may be limited to 010% or less.

クロム(Cr)は、鋼の耐酸化性、耐水蒸気酸化性を与える重要な元素であるが、その含有量が11%を超えると、δフェライトを形成して靭性が低下し、またM23C6型炭化物等の析出と析出による成長粗大化も顕著になって長時間クリープ強度を低下させる。したがって、含有量は11%以下にする必要がある。また本発明ではSiを多く添加して耐水蒸気酸化性を向上させているが、Crが8%未満ではその効果が十分でないため、8〜11%とする。   Chromium (Cr) is an important element that imparts oxidation resistance and steam oxidation resistance of steel. However, if its content exceeds 11%, δ ferrite is formed and the toughness is reduced. Also, M23C6 type carbide Such as precipitation and growth coarsening due to precipitation become prominent, and the creep strength is lowered for a long time. Therefore, the content needs to be 11% or less. Further, in the present invention, a large amount of Si is added to improve the steam oxidation resistance. However, if Cr is less than 8%, the effect is not sufficient, so 8 to 11%.

モリブデン(Mo)は、炭化物の微細析出によりクリープ破断強度を高める有効な元素である。したがって、モリブデンの含有量は炭化物の析出による強化のためには少なくとも0.1%以上必要であるが、1.2%を超えて添加し、さらにWを0.1%以上含有するとδフェライトが生成し、またMoを含むM23C6型炭化物の凝集粗大化が生じてクリープ強度の低下につながる。したがって、Moの含有量は0.1〜1.2%とする。   Molybdenum (Mo) is an effective element that increases the creep rupture strength by fine precipitation of carbides. Therefore, the molybdenum content is required to be at least 0.1% or more for strengthening by precipitation of carbides. However, if it exceeds 1.2% and further contains 0.1% or more of W, δ-ferrite is formed. In addition, M23C6 type carbide containing Mo is agglomerated and coarsened, resulting in a decrease in creep strength. Therefore, the Mo content is set to 0.1 to 1.2%.

タングステン(W)は、炭化物の析出強化と母地への固溶強化作用により本鋼のクリープ破断強度を高めるために最も重要な元素である。従来は3%前後の添加で効果があって、4%以上添加するとWを含むM23C6型炭化物やラーベス相(Fe2W)が凝集粗大化してクリープ破断強度を低下させると言われていた。しかし、3%程度の添加でも長時間クリープ破断強度を低下させることが分かったため、含有量を低めに抑え、且つ650℃2万時間で100N/mm以上のクリープ破断強度が得られる2.5%以下とする(図2)。また、含有量が1.0%未満では図2に示すデータから推測すると、クリープ破断強度の向上が図れないため、1.0〜2.5%とする。また、WはMoと複合してクリープ強度に影響するので、W単独の含有率を規定するとともに、(Mo%+0.5×W%)の値をクリープ破断強度の向上に有効な1.0〜1.6%とする。Tungsten (W) is the most important element for increasing the creep rupture strength of the steel by the precipitation strengthening of carbides and the solid solution strengthening action on the matrix. Conventionally, it is said that the addition of about 3% is effective, and if added over 4%, the M23C6 type carbide containing W and the Laves phase (Fe2W) are agglomerated and coarsened to reduce the creep rupture strength. However, since it was found that the creep rupture strength is lowered for a long time even with addition of about 3%, the content is kept low, and a creep rupture strength of 100 N / mm 2 or more is obtained at 650 ° C. for 20,000 hours. % Or less (FIG. 2). Further, if the content is less than 1.0%, it is assumed that the creep rupture strength cannot be improved if estimated from the data shown in FIG. In addition, since W is combined with Mo and affects the creep strength, the content of W alone is specified, and the value of (Mo% + 0.5 × W%) is 1.0 which is effective for improving the creep rupture strength. ˜1.6%.

コバルト(Co)は、オーステナイト生成元素であり、Ac1変態点をあまり低下させずにδフェライトの生成を防止するため、重要な元素である。δフェライト量は、添加する他の合金元素量によって変動するが、コバルト(Co)は、少なくとも0.01%、多くても4.0%添加すれば十分にδフェライトを防止することができる。   Cobalt (Co) is an austenite-generating element and is an important element for preventing the formation of δ ferrite without significantly reducing the Ac1 transformation point. The amount of δ ferrite varies depending on the amount of other alloy elements to be added. However, if cobalt (Co) is added at least 0.01% and at most 4.0%, δ ferrite can be sufficiently prevented.

バナジウム(V)は、Vの炭化物を生成して比較的少量で有効にクリープ破断強度を高める元素であり、Vの炭化物の生成には少なくとも0.10%の添加が必要である。しかし、含有量が0.30%を超えると生成したVの炭化物が凝集粗大化して逆にクリープ破断強度を低下させるため、0.10〜0.30%とする。   Vanadium (V) is an element that generates V carbide and effectively increases the creep rupture strength in a relatively small amount, and at least 0.10% of addition is necessary for formation of V carbide. However, if the content exceeds 0.30%, the generated carbide of V is agglomerated and coarsened, and conversely, the creep rupture strength is lowered, so the content is made 0.10 to 0.30%.

ニオブ(Nb)は、安定な炭窒化物であるNb(C、N)(ニオブの炭窒化物)を形成して少量でもクリープ破断強度を高めることができるが、含有量が0.12%を超えると短時間の強度は向上するが長時間強度はよくない。また、0.02%未満では析出するNb(C、N)が不足して十分に強化されないため、0.02〜0.12%とする。   Niobium (Nb) can form Nb (C, N) (niobium carbonitride), which is a stable carbonitride, to increase the creep rupture strength even in a small amount, but the content is 0.12%. If it exceeds, the strength for a short time is improved, but the strength for a long time is not good. Further, if it is less than 0.02%, the precipitated Nb (C, N) is insufficient and is not sufficiently strengthened, so 0.02 to 0.12%.

窒素(N)は、Vの窒化物形成による析出強化及び自身の固溶強化によってクリープ破断強度を高める。但し0.08%を超える窒素は窒化物を多く形成し過ぎて凝集粗大化が起こり、クリープ破断強度が低下するため、0.08% 以下とする。また窒素の含有量が0.01%未満ではクリープ破断強度を高める効果が十分に得られないため、0.01〜0.08%とする。Nによるクリープ破断強度はC量と密接な相互関係があり、(C%+N%)を0.02〜0.15%に制限した場合に最も高いクリープ破断強度が得られる。   Nitrogen (N) increases the creep rupture strength by precipitation strengthening due to V nitride formation and by its solid solution strengthening. However, if it exceeds 0.08%, too much nitride is formed and agglomeration and coarsening occur, and the creep rupture strength decreases, so the content is made 0.08% or less. Further, if the nitrogen content is less than 0.01%, the effect of increasing the creep rupture strength cannot be obtained sufficiently, so 0.01 to 0.08%. The creep rupture strength due to N has a close correlation with the amount of C, and the highest creep rupture strength is obtained when (C% + N%) is limited to 0.02 to 0.15%.

ニッケル(Ni)は、靭性の向上とδフェライトの生成を抑える有力な元素であり、従来のボイラ用鋼では0.5%程度の範囲で特に制限されることなく添加されていた。しかし、ニッケルの添加が、Ac1変態点を著しく低下させて長時間クリープ強度に悪影響を及ぼすことが分かったため、クリープ破断強度の観点からは0.1%以下に極力低減することが好ましい。しかし、そのためには製鋼の際に屑鉄、炉壁、取鍋等からニッケルが混入する量をできるだけ少なくする必要があり、製鋼技術上の制限が大きくなるので、実用鋼として0.3%を上限とする。なお、Niは含有されなくてもよい。   Nickel (Ni) is a powerful element that suppresses toughness and suppresses the formation of δ ferrite, and has been added to conventional boiler steels without particular limitation in the range of about 0.5%. However, it has been found that addition of nickel significantly lowers the Ac1 transformation point and adversely affects the creep strength for a long time. Therefore, it is preferable to reduce it to 0.1% or less from the viewpoint of creep rupture strength. However, in order to do so, it is necessary to reduce the amount of nickel mixed from scrap iron, furnace walls, ladle, etc. as much as possible during steelmaking, and the limit on steelmaking technology increases. And Ni may not be contained.

アルミニウム(Al)は、従来、脱酸剤及び結晶粒微細化剤として添加されている。しかし、0.010%以上の過剰なAlはクリープ強度の向上に有効な窒素をAl窒化物として捕らえたり、またM23C6型炭化物の表面に濃化してCrの拡散を促進し、M23C6型炭化物の凝集粗大化を早める。また、本発明者らはAlの含有率が一定値を超えると微量でも650℃付近の数万時間の長時間クリープ強度を大きく低下させることを発見した。従来の実用鋼ではAlを0.03%程度まで添加して靭性の向上を図っているが、本発明の耐熱鋼はAl量を0.01%以下に抑えることにより、650℃の長時間クリープ強度を著しく高めている。   Aluminum (Al) is conventionally added as a deoxidizer and a grain refiner. However, excess Al of 0.010% or more catches nitrogen effective for improving the creep strength as Al nitride, or concentrates on the surface of M23C6 type carbide to promote Cr diffusion, and agglomeration of M23C6 type carbide Speed up coarsening. In addition, the present inventors have found that if the Al content exceeds a certain value, the creep strength for a long time of several tens of thousands of hours near 650 ° C. is greatly reduced even at a minute amount. In conventional practical steels, Al is added to about 0.03% to improve toughness. However, the heat-resistant steel of the present invention keeps the amount of Al to 0.01% or less, and thus creeps at 650 ° C for a long time. Strength is remarkably increased.

Alの含有率をこのように極低レベルに抑えることは、従来は非常に困難であった。しかし、近年は真空炭素脱酸法で極低レベルのAl鋼を製造することが可能になった。本発明の鋼はSi量を増加させたことも特徴の一つであり、Alの脱酸作用が失われても、Siによる脱酸作用が利用できるので、鋼中の酸素量を低くすることができる。なお、Alは含有されなくてもよい。   In the past, it was very difficult to suppress the Al content to such an extremely low level. However, in recent years, it has become possible to produce an extremely low level of Al steel by vacuum carbon deoxidation. One feature of the steel of the present invention is that the amount of Si is increased. Even if the deoxidation effect of Al is lost, the deoxidation effect due to Si can be used, so that the oxygen content in the steel is reduced. Can do. Al may not be contained.

Al量とNi量を種々変化させた材料における650℃でのクリープ破断強度縦軸に、横軸にAl量を表した結果を図3に示す。650℃10万時間のクリープ破断強度として100N/mm前後を得るためには、2万時間で100N/mmを超える強度が必要である。そのためには例えばNi量が約0.3%の場合、Al量を0.005%程度以下に抑えればよい。Ni量が0.1%以下であれば、Al量の許容範囲を広げることができる。FIG. 3 shows the results of expressing the amount of Al on the vertical axis of creep rupture strength at 650 ° C. and the horizontal axis of the material with various amounts of Al and Ni varied. In order to obtain a creep rupture strength of about 100 N / mm 2 at 650 ° C. for 100,000 hours, a strength exceeding 100 N / mm 2 is required in 20,000 hours. For this purpose, for example, when the amount of Ni is about 0.3%, the amount of Al may be suppressed to about 0.005% or less. If the Ni content is 0.1% or less, the allowable range of the Al content can be expanded.

本発明の鋼では、クリープ強度の安定化のために特に有害な元素であるAlとNiを低減したことを特徴としており、両者の低減が不可欠である。図3と同じ試験結果を横軸のAl量をAl量+(0.1×Ni量)にして書き換えたものを図4に示す。これらの試験結果から、確実に100N/mmを超えるクリープ破断強度が得られるAl量とNi量の範囲として、(Al%+0.1×Ni%)を0.02%以下に制限する。The steel of the present invention is characterized by the reduction of Al and Ni, which are particularly harmful elements, for stabilizing the creep strength, and the reduction of both is essential. FIG. 4 shows a rewrite of the same test results as in FIG. 3 with the amount of Al on the horizontal axis changed to Al amount + (0.1 × Ni amount). From these test results, (Al% + 0.1 × Ni%) is limited to 0.02% or less as a range of Al amount and Ni amount that can surely obtain a creep rupture strength exceeding 100 N / mm 2 .

銅(Cu)は、不純物として鋼に混入すると、Coと同様にδフェライトの生成を抑制する作用を有する。しかし、Cuの混入は600℃以上で長時間クリープ破断強度を低下させることがあるので、0.3%以下に制限する。なお、Cuは含有されなくてもよい。   Copper (Cu), when mixed in steel as an impurity, has the effect of suppressing the formation of δ ferrite, like Co. However, since Cu contamination may reduce the creep rupture strength for a long time at 600 ° C. or higher, it is limited to 0.3% or less. Cu may not be contained.

ホウ素(B)は、粒界強化元素(結晶粒界を強化する元素)であり、微量でも著しくクリープ破断強度を高める。またM23C6型炭化物中に固溶し、M23C6型炭化物の凝集粗大化を抑制する作用によりクリープ破断強度を高める作用があるので、少なくとも0.001%添加する。しかし、Bを0.010%以上添加すると鋼の溶接性が著しく悪化するため、Bの添加量は0.010%未満とする。   Boron (B) is a grain boundary strengthening element (an element that strengthens the crystal grain boundary), and significantly increases the creep rupture strength even in a small amount. Moreover, since it has the effect | action which raises a creep rupture strength by the solid solution in a M23C6 type carbide | carbonized_material and the effect | action which suppresses the aggregation coarsening of a M23C6 type carbide | carbonized_material, it adds at least 0.001%. However, if B is added in an amount of 0.010% or more, the weldability of the steel is remarkably deteriorated, so the addition amount of B is less than 0.010%.

本発明のフェライト系耐熱鋼の主要化学成分範囲は上記の通りであるが、不純物として次の元素をそれぞれに重量%で記した含有割合未満の量で含むこともある。   The main chemical component ranges of the ferritic heat resistant steel of the present invention are as described above. However, the following elements may be included as impurities in amounts less than the content ratios described in weight%.

Ta<0.2%、Ti<0.1%、Zr<0.2%、La<0.1%、
Ce<0.1%、Pd<0.2%、Re<0.5%、Hf<0.3%
これらの元素にも強度向上の効果があり、その作用は以下の通りである。
Ta <0.2%, Ti <0.1%, Zr <0.2%, La <0.1%,
Ce <0.1%, Pd <0.2%, Re <0.5%, Hf <0.3%
These elements also have an effect of improving the strength, and the action is as follows.

Ta:TaCを形成し、基地を強化する。
Ti:TiCを形成し、基地を強化する。
Zr:ZrCを形成し、基地を強化する。
La、Ce:鋼中の酸素の割合を低下させてクリープ破断強度を高める。
Pd:クリープ破断強度、耐酸化性(耐水蒸気酸化性)を向上させる。
Re:基地を強化する。
Hf:HfCを形成し、基地を強化する。
Ta: TaC is formed and the base is strengthened.
Ti: TiC is formed and the base is strengthened.
Zr: ZrC is formed and the base is strengthened.
La, Ce: Increase the creep rupture strength by reducing the proportion of oxygen in the steel.
Pd: Improves creep rupture strength and oxidation resistance (water vapor oxidation resistance).
Re: Strengthen the base.
Hf: HfC is formed and the base is strengthened.

本発明のフェライト系耐熱鋼は溶解、鍛造後に1,050〜1,100℃の温度での焼きならし及び750〜800℃での焼戻しを行い、焼戻しマルテンサイト組織として使用する。靱性の確保の観点からは、焼戻しマルテンサイト組織の単相とすることが望ましい。しかし、高温用ボイラ部材として用いる際に、ある程度の靱性の低下が許容される場合は、CrやSi等のフェライト形成元素を上記制限範囲内で多めに設定してδフェライトを析出させてもよい。また、靱性とクリープ破断強度の点からもδフェライトは体積率が35%を超えると強度と靱性が低下することが知られているため、30%以下に抑えるように限定する。   The ferritic heat resistant steel of the present invention is used as a tempered martensite structure after melting and forging, normalizing at a temperature of 1,050 to 1,100 ° C. and tempering at 750 to 800 ° C. From the viewpoint of securing toughness, it is desirable to have a single phase of a tempered martensite structure. However, when using it as a high-temperature boiler member, if some reduction in toughness is allowed, δ ferrite may be precipitated by setting a larger amount of ferrite-forming elements such as Cr and Si within the above limit range. . Further, from the viewpoint of toughness and creep rupture strength, δ ferrite is known to decrease in strength and toughness when the volume ratio exceeds 35%, so it is limited to 30% or less.

本発明の鋼は、従来の高Cr耐熱鋼の思想に対して、Cを半分程度に低減し、AlとNiを極力低く抑え、Siを増加させたことが特徴である。そして、これらの複合作用によって、初めてクリープ強度の安定性を向上させ、同時に耐酸化性(耐水蒸気酸化性)を改善させることにより、650℃まで使用可能な高Crフェライト系耐熱鋼を達成できる。鋼の使用目的に応じて種々の製造方法を採ることが可能であり、鋼管のみならず鋼板としても使用できる。   The steel of the present invention is characterized in that C is reduced to about half, Al and Ni are suppressed as low as possible, and Si is increased compared to the concept of conventional high Cr heat resistant steel. And by these combined actions, it is possible to achieve a high Cr ferritic heat resistant steel that can be used up to 650 ° C. by improving the stability of creep strength for the first time and improving the oxidation resistance (water vapor oxidation resistance) at the same time. Various production methods can be adopted depending on the purpose of use of the steel, and it can be used not only as a steel pipe but also as a steel plate.

また、本発明によるフェライト系耐熱鋼は従来のフェライト系耐熱鋼に比べて著しくクリープ破断強度が向上し、かつ長時間の使用においても安定した強度と延性を有する。したがって、超々臨界圧ボイラの高温耐圧部に適用すれば蒸気温度を650℃前後に高めることが可能となって火力発電プラントのプラント効率を向上できる。また水蒸気酸化スケールの成長や剥離、そして飛散による機器の損傷を軽減できる。このため、プラントの耐久性も向上し、火力発電プラントにおける石炭等の燃料消費量低減及びCO2排出量削減に顕著な効果が得られる。Further, the ferritic heat resistant steel according to the present invention has a significantly improved creep rupture strength as compared with the conventional ferritic heat resistant steel, and has a stable strength and ductility even when used for a long time. Therefore, if it is applied to the high-temperature pressure-resistant part of the ultra-supercritical boiler, the steam temperature can be increased to around 650 ° C., and the plant efficiency of the thermal power plant can be improved. In addition, it can reduce the damage of equipment due to the growth and peeling of the steam oxidation scale and the scattering. For this reason, the durability of the plant is also improved, and a remarkable effect can be obtained in the reduction of fuel consumption and CO 2 emission of coal or the like in the thermal power plant.

本発明の実施例を以下に実例を用いて説明する。
表1に示す化学組成を有する本実施例の耐熱鋼と比較鋼を真空誘導溶解炉において溶製し、各々50kgのインゴットに鋳造した。比較鋼Aは公称9Cr1MoNbV鋼、比較鋼B及びCは公称9Cr0.5Mo1.8WNbV鋼で、いずれもボイラ用鋼として実用化されている。熱間鋳造によって厚さ20mmの鋼板とした後、1,050℃×60分の焼きならし及び780℃×60分の焼戻しを施し、クリープ破断試験を実施した。また、鋼鈑から小型の板状試験片を加工し、650℃において水蒸気による酸化試験を実施した。
Embodiments of the present invention will be described below using actual examples.
The heat-resistant steel and comparative steel of this example having the chemical composition shown in Table 1 were melted in a vacuum induction melting furnace and cast into 50 kg ingots. The comparative steel A is a nominal 9Cr1MoNbV steel, the comparative steels B and C are nominal 9Cr0.5Mo1.8WNbV steel, both of which are put into practical use as boiler steel. After forming a steel plate having a thickness of 20 mm by hot casting, normalization at 1,050 ° C. × 60 minutes and tempering at 780 ° C. × 60 minutes were performed, and a creep rupture test was performed. Moreover, a small plate-shaped test piece was processed from a steel plate, and an oxidation test using water vapor was performed at 650 ° C.

Figure 2007091535
Figure 2007091535

本実施例の鋼及び比較鋼のクリープ破断試験の結果を各温度毎に応力−破断時間線図としてプロットし、長時間側へ外挿して推定した600及び650℃における10万時間クリープ破断強度を表2に示す。   The results of the creep rupture test of the steel of this example and the comparative steel are plotted as stress-rupture time diagrams for each temperature and extrapolated to the long time side to estimate the 100,000 hour creep rupture strength at 600 and 650 ° C. It shows in Table 2.

Figure 2007091535
Figure 2007091535

本発明の鋼A及びBは、650℃×10万時間のクリープ破断強度が、従来のボイラ用耐熱鋼として長年使用されてきた比較鋼Aに対して約2倍、さらに高強度の比較鋼B及びCに対して約1.5倍であり、画期的な強度を有する。また、本実施例の鋼及び比較鋼の水蒸気による酸化試験結果を表3に示すが、比較鋼に対して水蒸気による酸化スケールの成長が抑制されており、650℃の蒸気温度でも十分に使用できると考えられる。   Steels A and B according to the present invention have a creep rupture strength of 650 ° C. × 100,000 hours, which is about twice that of the comparative steel A that has been used for many years as a conventional heat-resistant steel for boilers. And about 1.5 times that of C, it has a breakthrough strength. Moreover, although the oxidation test result by the water vapor | steam of the steel of a present Example and a comparative steel is shown in Table 3, the growth of the oxidation scale by water vapor | steam is suppressed with respect to a comparative steel, and it can fully use also at the steam temperature of 650 degreeC. it is conceivable that.

Figure 2007091535
Figure 2007091535

なお、本発明では焼きならし温度を1,050℃として実験を行ったが、焼ならし温度を上げることによって、更に高いクリープ破断強度を得られる。しかし同時に靭性が低下するので、焼きならし温度は1,100℃までの温度範囲が好ましい。   In the present invention, the experiment was carried out at a normalizing temperature of 1,050 ° C., but higher creep rupture strength can be obtained by raising the normalizing temperature. However, since the toughness is lowered at the same time, the normalizing temperature is preferably in the temperature range up to 1,100 ° C.

本発明におけるフェライト系耐熱鋼は、特に蒸気温度が650℃前後の超々臨界圧ボイラの過熱器の管寄せや主蒸気管の材料に好適である。また、厚肉で大径の管材のみならず小径の伝熱管材として用いることもできる。   The ferritic heat resistant steel in the present invention is particularly suitable for a superheater header of a super supercritical pressure boiler having a steam temperature of around 650 ° C. and a material for a main steam pipe. Moreover, it can be used not only as a thick and large-diameter tube material but also as a small-diameter heat transfer tube material.

本発明のフェライト系耐熱鋼は、特に蒸気温度が650℃前後の超々臨界圧ボイラの過熱器の管寄せや主蒸気管の材料及び厚肉で大径の管材のみならず小径の伝熱管材として産業上の利用可能性が高い。   The ferritic heat-resisting steel of the present invention is used not only as a superheater for a super-supercritical boiler with a steam temperature of around 650 ° C., but as a material for main steam pipes and thick and large-diameter pipes as well as small-diameter heat transfer pipes. Industrial applicability is high.

本発明に関わる9CrWCo系鋼でSi量を変化させた場合の水蒸気による酸化試験結果を示した図である。It is the figure which showed the oxidation test result by water vapor | steam at the time of changing Si amount with 9CrWCo type steel in connection with this invention. 本発明に関わる9CrWCo系鋼でW量を変化させた場合のクリープ破断試験結果を示した図である。It is the figure which showed the creep rupture test result at the time of changing W amount with 9CrWCo type steel in connection with this invention. 本発明に関わる9CrWCo系鋼でAlとNi量を変化させた場合の横軸をAl量としてクリープ破断試験結果を示した図である。It is the figure which showed the creep rupture test result by making the horizontal axis | shaft at the time of changing Al and Ni amount with 9CrWCo type steel in connection with this invention into Al amount. 図3と同じ試験結果を横軸のAl量を(Al+0.1×Ni)量にして書き換えた図である。It is the figure which rewritten the same test result as FIG. 3 by making the amount of Al of a horizontal axis into the amount of (Al + 0.1 * Ni).

Claims (3)

重量%で、炭素(C):0.01〜0.10%、ケイ素(Si):0.30〜1.0%、リン(P):0.020%以下、硫黄(S):0.010%以下、マンガン(Mn):0.2〜1.2%、ニッケル(Ni):0.3%以下、クロム(Cr):8.0〜11.0%、モリブデン(Mo):0.1〜1.2%、タングステン(W):1.0〜2.5%、バナジウム(V):0.10〜0.30%、ニオブ(Nb):0.02〜0.12%、コバルト(Co):0.01〜4.0%、窒素(N):0.01〜0.08%、ホウ素(B):0.001以上で0.010%未満、銅(Cu):0.3%以下、アルミニウム(Al):0.010%以下、さらに(Mo%+0.5×W%)の量を1.0〜1.6に制限し、さらに(C%+N%)の量を0.02〜0.15%に制限した成分で、調質熱処理により得られる焼戻しマルテンサイト単相組織からなることを特徴とするフェライト系耐熱鋼。   Carbon (C): 0.01 to 0.10%, Silicon (Si): 0.30 to 1.0%, Phosphorus (P): 0.020% or less, Sulfur (S): 0.0. 010% or less, manganese (Mn): 0.2 to 1.2%, nickel (Ni): 0.3% or less, chromium (Cr): 8.0 to 11.0%, molybdenum (Mo): 0.0. 1 to 1.2%, tungsten (W): 1.0 to 2.5%, vanadium (V): 0.10 to 0.30%, niobium (Nb): 0.02 to 0.12%, cobalt (Co): 0.01 to 4.0%, nitrogen (N): 0.01 to 0.08%, boron (B): 0.001 or more and less than 0.010%, copper (Cu): 0.0. 3% or less, aluminum (Al): 0.010% or less, and further, the amount of (Mo% + 0.5 × W%) is limited to 1.0 to 1.6, and (C% + N%) The restriction ingredients to 0.02 to 0.15%, ferritic heat resistant steel characterized by consisting of tempered martensite single phase structure obtained by the refining heat treatment. 重量%で(Al%+0.1×Ni%)の量を0.02%以下に制限した成分であることを特徴とする請求項1記載のフェライト系耐熱鋼。   2. The ferritic heat resistant steel according to claim 1, wherein the ferritic heat resistant steel is a component in which the amount of (Al% + 0.1 × Ni%) is limited to 0.02% or less. 30重量%以下のδフェライトを含有することを特徴とする請求項1又は2記載のフェライト系耐熱鋼。   The ferritic heat resistant steel according to claim 1 or 2, characterized by containing 30 wt% or less of δ ferrite.
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