JPH0369984B2

JPH0369984B2 -

Info

Publication number: JPH0369984B2
Application number: JP19263988A
Authority: JP
Inventors: Yoshitaka Iwabuchi; Junji Itagaki
Original assignee: Japan Steel Works Ltd
Current assignee: Japan Steel Works Ltd
Priority date: 1988-08-03
Filing date: 1988-08-03
Publication date: 1991-11-06
Also published as: JPH0243347A

Description

[Detailed description of the invention]

〔産業上の利用分野〕本発明は超々臨界圧蒸気タービンのケーシン
グ、フランジ、バルブ等に用いられる高温高圧用
鋳鋼に関する。〔従来の技術〕近年、大型火力発電プラントの高効率化のた
め、主蒸気温度が上昇し、約650℃に及ぶ超々臨
界圧タービンが開発されている。このような超々臨界圧下で用いられるケーシン
グ、フランジ、バルブ等の蒸気タービン用鋳鋼品
は苛酷な使用環境に耐え得るために、高温特性は
勿論のこと、高靭性で経年劣化の少ないことが要
求される。従来、Cr−Mo鋳鋼またはCr−Mo−
Ｖ鋳鋼において、Si含有量を0.10％以下にして、
靭性および高温特性の改善を図つた高温高圧用鋳
鋼が特開昭52−112220および、特開昭58−221264
に開示されている。〔発明が解決しようとする課題〕しかし、特開昭52−112220および特開昭58−
221264に開示されている高温高圧用鋳鋼では、高
温特性および靭性の改善という課題は、Si含有量
を0.10％以下に低減することにより解決される
が、もう一つの経年劣化の防止という課題は解決
されていない。本発明は、上記の課題に鑑みてなされたもの
で、高温特性および靭性に優れ、且つ経年劣化の
少ない新規な高温高圧用鋳鋼を提供することを目
的とするものである。〔課題を解決するための手段〕上記の目的達成の具体的な手段は、Cr−Mo鋳
鋼またはCr−Mo−Ｖ鋳鋼において、Siに替えて
Tiにより脱酸を行ない、SiおよびMnを添加する
ことなく、SiおよびMnを含めた不可避不純物量
を極力低減した高純度鋼とすることにより、高温
特性および靭性を改善し、さらに経年劣化を防止
しようとするものである。すなわち、本発明は(1)
重量パーセントでC0.1〜0.3％、Ni0.2〜2.0％、
Cr1.0〜3.0％、Mo0.25〜2.0％およびTi0.1％以下
を含有し、残部がFeおよびSiを0.1％以下、Mnを
0.1％以下、（Ｐ＋As＋Sn＋Sb）を0.013％以下に
抑えた不可避不純物から成る耐経年劣化に優れた
高温高圧用鋳鋼。(2)(1)の組成にさらにV0.3％以
下を含有して成る耐経年劣化に優れた高温高圧用
鋳鋼である。〔作用〕本発明鋳鋼の組成限定理由を以下に述べる。Ｃ：0.1〜0.3％Ｃは本発明鋳鋼の所定強度確保のために不可欠
な元素である。その含有量が0.1％未満では、所要強度が得ら
れず、0.3％を超えて含有させると、ラプチヤー
強度および切欠き靭性が劣化し、また、溶接割れ
感受性を増大せしめる。 Ni：0.2〜2.0％ NiはSiおよびMnを不可避不純物として0.1％以
下に抑えたことによつて生じる焼入れ性の低下を
補償するために、0.2％以上必要であるが、2.0％
を超えて含有させると、高温強度が低下するの
で、その含有量を0.2〜2.0％に限定した。 Cr：1.0〜3.0％ Crは本発明鋳鋼の基本成分であり、所要の高
温強度を確保するためには不可欠な元素である。
その含有量が1.0％未満では、所要の高温強度を
確保できず、3.0％を超えて含有させると、Cr炭
化物の析出が著しくなり、切欠き靭性および溶接
性が劣化するので、その含有量を1.0〜3.0％に限
定した。 Mo：0.25〜2.0％ MoはCrと同様に、本発明鋳鋼の基本成分であ
り、所要の高温強度を確保するためには、不可欠
な元素である。0.25％以上Moを含有させると、
焼戻し軟化抵抗を高めて、クリープ強度を改善
し、焼戻し脆化感受性を抑制するが、2.0％を超
えて含有させると、むしろMo炭化物の析出が著
しくなり、切欠き靭性が低下し、また溶接割れ感
受性を増大せしめるので、その含有量を0.25〜
2.0％に限定した。 Ti：0.1％以下 Tiは本発明鋳鋼においてSiにかわる脱酸剤と
して使用され、脱酸後に一部鋼中に残留する。ま
たTiは高温特性を改善する作用があるが、0.1％
を超えて含有させると、過剰のTiは酸化物ある
いは炭化物として存在し、延性および切欠き靭性
を低下させるのでその含有量を0.1％以下に制限
した。Ｖ：Ｖは高温強度およびクリープラプチヤー強度を
高めるために添加されるが、0.3％を超えて含有
させると、Ｖ炭化物の析出が著しくなり、靭性が
低下し、溶接割れ感受性が増大するので、その含
有量を0.3％以下に制限することが好ましい。不可避不純物（Si：0.1％以下、Mn：0.1％以
下、Ｐ＋As＋Su＋Sb：0.013％以下）本発明鋳鋼の如きCr−Mo鋳鋼あるいはCr−
Mo−Ｖ鋳鋼においては、SiおよびMnの多量の
含有は焼戻脆性を助長する。従つて、超々臨界圧
条件下で使用される本発明鋳鋼では、Si、Mnを
多量に含有すると使用中に焼戻脆性を生じ、経年
劣化を起す。このため、経年劣化を防止するため
にはSi、Mnを合金元素として添加せず、不可避
不純物として、極力その含有量を抑えることが望
ましい。しかし現状の精錬技術レベルでは、Siお
よびMnのある程度の含有は避けられず、一方、
（Si＋Mn）含有量が0.2％以下であれば、焼戻脆
性がほとんど生じないことから、SiおよびMnの
不可避的含有量をそれぞれ0.1％以下に抑えた。
また、SiおよびMnと共存して焼戻脆性感受性を
助長する元素として、Ｐ、Sn、As、Sbがあげら
れる。これらの元素は、原材料から不可避的に混
入し、Ｐを除くSn、As、Sbは精錬によつて除去
することは困難であるため、原材料の厳選による
ところが大きく、個々にその許容量を限定せず、
（Si＋Mn）含有量が0.2％以下のもとでは、（Ｐ＋
Sn＋As＋Sb）含有量を0.013％以下に抑えれば焼
戻脆性は殆ど生じない。この他、鋼質を劣化させ
る不純物としてCu、Ｓがあげられる。本発明鋳
鋼において、Cu含有量0.1％、Ｓ含有量0.005％を
超えると、切欠き靭性が劣化するので、これらの
値以下に不可避的含有量を抑えることが望まし
い。〔実施例〕第１表に示す組成の本発明の鋳鋼および従来鋳
鋼を真空溶解炉にて溶解し、砂型に鋳込んだ。こ
れらの鋳塊に焼準、焼戻しの調質熱処理を施して
供試材とした。なお各供試材の一部に、さらに
450℃×10⁴時間の脆化処理を施した。これらの供
試材の材料試験結果を第２表に示す。第２表から
明らかなように、本発明の鋳鋼においては、脆化
処理後のFATT（破面遷移温度）が脆化処理前の
ものとほとんどかわらず、脆化は起きていない。
即ち、本発明の鋳鋼は高温環境で長期間使用して
も、ほとんど焼戻脆性は起きず、耐経年劣化に優
れた材料と言える。 [Industrial Application Field] The present invention relates to high-temperature, high-pressure cast steel used for casings, flanges, valves, etc. of ultra-supercritical pressure steam turbines. [Prior Art] In recent years, in order to improve the efficiency of large thermal power plants, ultra-supercritical pressure turbines have been developed in which the main steam temperature rises to about 650°C. Cast steel products for steam turbines, such as casings, flanges, and valves used under ultra-supercritical pressure, must not only have high-temperature properties but also high toughness and minimal deterioration over time in order to withstand the harsh operating environment. Ru. Conventionally, Cr-Mo cast steel or Cr-Mo-
In V cast steel, the Si content is reduced to 0.10% or less,
High-temperature, high-pressure cast steel with improved toughness and high-temperature properties was published in JP-A-52-112220 and JP-A-58-221264.
has been disclosed. [Problem to be solved by the invention] However, Japanese Patent Application Laid-open No. 52-112220 and Japanese Patent Application Laid-open No. 1987-
221264, the problem of improving high-temperature properties and toughness is solved by reducing the Si content to 0.10% or less, but the other problem of preventing aging deterioration is also solved. It has not been. The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a novel high-temperature, high-pressure cast steel that has excellent high-temperature properties and toughness, and has little deterioration over time. [Means for solving the problem] The specific means for achieving the above purpose is to replace Si in Cr-Mo cast steel or Cr-Mo-V cast steel.
By deoxidizing with Ti and creating a high-purity steel with the amount of unavoidable impurities including Si and Mn reduced as much as possible without adding Si or Mn, high-temperature properties and toughness are improved, and deterioration over time is prevented. This is what I am trying to do. That is, the present invention (1)
C0.1~0.3%, Ni0.2~2.0%, by weight percentage
Contains 1.0 to 3.0% Cr, 0.25 to 2.0% Mo, and 0.1% or less of Ti, with the balance being 0.1% or less of Fe and Si, and Mn.
High-temperature, high-pressure cast steel with excellent aging resistance, consisting of unavoidable impurities with (P+As+Sn+Sb) below 0.1% and below 0.013%. (2) This is a high-temperature, high-pressure cast steel with excellent aging resistance that further contains V0.3% or less in the composition of (1). [Operation] The reasons for limiting the composition of the cast steel of the present invention will be described below. C: 0.1-0.3% C is an essential element for ensuring the specified strength of the cast steel of the present invention. If the content is less than 0.1%, the required strength cannot be obtained, and if the content exceeds 0.3%, the rupture strength and notch toughness will deteriorate, and the susceptibility to weld cracking will increase. Ni: 0.2 to 2.0% Ni is required to be 0.2% or more in order to compensate for the decrease in hardenability caused by suppressing Si and Mn as unavoidable impurities to 0.1% or less, but 2.0%
If the content exceeds 0.2%, the high temperature strength will decrease, so the content was limited to 0.2% to 2.0%. Cr: 1.0 to 3.0% Cr is a basic component of the cast steel of the present invention, and is an essential element to ensure the required high temperature strength.
If the content is less than 1.0%, the required high-temperature strength cannot be secured, and if the content exceeds 3.0%, precipitation of Cr carbides will become significant and the notch toughness and weldability will deteriorate. Limited to 1.0-3.0%. Mo: 0.25 to 2.0% Mo, like Cr, is a basic component of the cast steel of the present invention, and is an essential element in order to ensure the required high temperature strength. When Mo is contained more than 0.25%,
It increases temper softening resistance, improves creep strength, and suppresses temper embrittlement susceptibility, but if Mo content exceeds 2.0%, precipitation of Mo carbides becomes significant, notch toughness decreases, and weld cracking occurs. Since it increases sensitivity, its content should be reduced from 0.25 to
Limited to 2.0%. Ti: 0.1% or less Ti is used as a deoxidizing agent in place of Si in the cast steel of the present invention, and a portion remains in the steel after deoxidizing. Also, Ti has the effect of improving high temperature characteristics, but 0.1%
If the Ti content exceeds 0.1%, excessive Ti exists as an oxide or carbide and reduces ductility and notch toughness, so the content was limited to 0.1% or less. V: V is added to increase high-temperature strength and creep-rupture strength, but if it is added in excess of 0.3%, precipitation of V carbides becomes significant, reducing toughness and increasing weld cracking susceptibility. It is preferable to limit its content to 0.3% or less. Unavoidable impurities (Si: 0.1% or less, Mn: 0.1% or less, P+As+Su+Sb: 0.013% or less) Cr-Mo cast steel such as the cast steel of the present invention or Cr-
In Mo-V cast steel, the content of large amounts of Si and Mn promotes temper embrittlement. Therefore, in the cast steel of the present invention used under ultra-supercritical pressure conditions, if it contains a large amount of Si or Mn, it will develop temper brittleness during use and cause deterioration over time. Therefore, in order to prevent aging deterioration, it is desirable not to add Si and Mn as alloying elements, but to suppress their content as unavoidable impurities as much as possible. However, with the current level of refining technology, it is unavoidable to contain some amount of Si and Mn.
If the (Si + Mn) content is 0.2% or less, temper embrittlement hardly occurs, so the unavoidable contents of Si and Mn were each suppressed to 0.1% or less.
In addition, P, Sn, As, and Sb are elements that coexist with Si and Mn and promote temper embrittlement susceptibility. These elements are unavoidably mixed in from raw materials, and it is difficult to remove Sn, As, and Sb other than P by refining, so it depends largely on the careful selection of raw materials, and the permissible amount cannot be limited individually. figure,
When the (Si+Mn) content is 0.2% or less, (P+
If the Sn+As+Sb) content is kept below 0.013%, temper embrittlement will hardly occur. Other impurities that degrade steel quality include Cu and S. In the cast steel of the present invention, if the Cu content exceeds 0.1% and the S content exceeds 0.005%, the notch toughness deteriorates, so it is desirable to suppress the unavoidable content to below these values. [Example] Cast steel of the present invention and conventional cast steel having the compositions shown in Table 1 were melted in a vacuum melting furnace and cast into sand molds. These ingots were subjected to refining heat treatment of normalizing and tempering to obtain test materials. Furthermore, some of the sample materials also contain
An embrittlement treatment was performed at 450°C for 10 ⁴ hours. Table 2 shows the material test results for these test materials. As is clear from Table 2, in the cast steel of the present invention, the FATT (fracture surface transition temperature) after the embrittlement treatment is almost different from that before the embrittlement treatment, and no embrittlement has occurred.
That is, even when the cast steel of the present invention is used for a long period of time in a high-temperature environment, it hardly develops temper brittleness and can be said to be a material with excellent aging resistance.

【表】【table】

〔Effect of the invention〕

Cr−Mo鋳鋼およびCr−Mo−Ｖ鋳鋼において、
不可避不純物を極力低減し、超高純度化すること
により本発明の耐経年劣化に優れた高温高圧用鋳
鋼を得ることができた。 In Cr-Mo cast steel and Cr-Mo-V cast steel,
By reducing unavoidable impurities as much as possible and achieving ultra-high purity, it was possible to obtain the high-temperature, high-pressure cast steel of the present invention which has excellent aging resistance.

Claims

[Claims] 1. C0.1~0.3%, Ni0.2~2.0% by weight
%, Cr1.0~3.0%, Mo0.25~2.0% and Ti0.1%
Contains the following, with the balance being less than 0.1% Fe and Si,
Mn 0.1% or less, (P+As+Sn+Sb) 0.013%
High-temperature, high-pressure cast steel with excellent aging resistance and containing the following unavoidable impurities: 2. A high-temperature, high-pressure cast steel with excellent aging resistance, which further contains 0.3% or less of V in the composition of claim 1.