JP4296303B2 - High Cr ferritic iron alloy with excellent toughness and method for producing the same - Google Patents
High Cr ferritic iron alloy with excellent toughness and method for producing the same Download PDFInfo
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Description
本発明は、高Ｃｒフェライト系鉄合金に関し、特に板厚が厚くともシグマ（σ）脆化や４７５℃脆化を起こすことのない特性を有する靭性に優れる高Ｃｒフェライト系鉄合金とその製造方法に関するものである。 The present invention relates to a high Cr ferritic iron alloy, and in particular, a high Cr ferritic iron alloy excellent in toughness having characteristics that do not cause sigma (σ) embrittlement or 475 ° C. embrittlement even when the plate thickness is thick, and a method for producing the same It is about.
高Ｃｒ鉄合金は、一般に耐食性や耐酸化性に優れる他、加工性や高温強度に優れた材料として知られており、各種の高Ｃｒ合金が提案されている。例えば、特許文献１には、Ｆｅ＋Ｃｒ＋Ａｌが９９．９８ｗｔ％以上の耐食性、加工性、耐酸化性に優れた高純度合金が、特許文献２には、３〜６０ｗｔ％のＣｒを含有し、Ｔｉ＋Ｎｂ＋Ｚｒ＋Ｖ＋Ｔａ＋Ｗ＋５０Ｂが０．０１〜６ｗｔ％である加工性、高温強度、耐食性に優れた合金が、特許文献３には、５〜６０ｗｔ％のＣｒ、０．５〜６ｗｔ％のＭｏを含有し、Ｃ＋Ｎ＋Ｏ＋Ｐ＋Ｓが１００ｐｐｍ以下である加工性と耐孔食性に優れた高純度合金が、特許文献４には、２０〜５０ｗｔ％のＣｒ、５ｗｔ％以下のＭｏを含有し、Ｃ＋Ｎが３０ｐｐｍ以下で、（Ｎｂ＋２Ｔｉ）≧１０（Ｃ＋Ｎ）を満足する極低炭素窒素高Ｃｒ合金が提案されている。
High Cr iron alloys are generally known as materials having excellent corrosion resistance and oxidation resistance, as well as excellent workability and high temperature strength, and various high Cr alloys have been proposed. For example,
また、ＢＣＣ構造を有する高Ｃｒフェライト系鉄合金は、ＦＣＣ構造を有するオーステナイト系鉄合金やＮｉ基合金と比較して、熱伝導性が高く、熱膨張係数が小さいこと、また、耐応力腐食割れ性にも優れていることが知られている。しかしながら、高Ｃｒフェライト系鉄合金は、上記優れた特性を有する反面、溶体化処理などの高温加熱処理後の降温時に徐冷を受けると、σ相やχ相といった金属間化合物の析出に起因するシグマ（σ）脆化や、高Ｃｒ相と低Ｃｒ相への分離に起因する４７５℃脆化を起こし易く、延性や靭性が著しく低下することが知られている。そこで、高Ｃｒフェライト系鉄合金の熱処理後の冷却は、σ脆化や４７５℃脆化が起こる温度域を水冷して急冷し、延性や靭性の低下を避けることが行われている。 Moreover, the high Cr ferritic iron alloy having the BCC structure has higher thermal conductivity and a smaller thermal expansion coefficient than the austenitic iron alloy and Ni-based alloy having the FCC structure, and stress corrosion cracking resistance. It is known that it is also excellent in performance. However, while the high Cr ferritic iron alloy has the above-mentioned excellent characteristics, it is caused by precipitation of intermetallic compounds such as σ phase and χ phase when subjected to slow cooling when the temperature is lowered after high temperature heat treatment such as solution treatment. It is known that sigma (σ) embrittlement and 475 ° C. embrittlement due to separation into a high Cr phase and a low Cr phase are likely to occur, and ductility and toughness are significantly reduced. Therefore, cooling after heat treatment of the high Cr ferritic iron alloy is performed by water-cooling rapidly in a temperature range where σ embrittlement or 475 ° C. embrittlement occurs to avoid a decrease in ductility and toughness.
しかし、溶体化処理等の熱処理後の冷却を水冷としても、板厚が厚い場合には、板厚中心部は冷却速度が遅くなるためσ脆化や４７５℃脆化が問題となることがある。そのため、厚板部材や大径部材で、耐食性や耐酸化性の他、靭性が要求される分野には、高Ｃｒフェライト系鉄合金を適用することができず、熱伝導性や耐応力腐食割れ性、価格面等でフェライト系鉄合金よりも劣るオーステナイト系鉄合金（例えば、オーステナイト系ステンレス鋼）が用いられているのが実情である。そこで、耐食性に優れるだけでなく、極厚でも靭性に優れる高Ｃｒフェライト系鉄合金の開発が望まれている。 However, even if the cooling after the heat treatment such as solution treatment is water cooling, if the plate thickness is large, the cooling rate is slow at the center of the plate thickness, so σ embrittlement or 475 ° C embrittlement may be a problem. . For this reason, high Cr ferritic iron alloys cannot be applied to fields that require toughness in addition to corrosion resistance and oxidation resistance, such as thick plate members and large-diameter members. Actually, an austenitic iron alloy (for example, austenitic stainless steel), which is inferior to a ferritic iron alloy in terms of properties and price, is used. Therefore, development of a high Cr ferritic iron alloy that is not only excellent in corrosion resistance but also excellent in toughness even at a very thick thickness is desired.
また、靭性の低下を避けるには、Ｃ，Ｎ，Ｏ等の不純物成分を極微量域まで低減し、超高純度化することも有効な手段である（特許文献１〜４参照）。超高純度合金を溶製する方法については、例えば、特許文献５や６に、高周波真空溶解装置を用いた高純度金属の溶解技術が開示されている。しかし、これらの技術は、１×１０−８ｔｏｒｒ以下の超高真空下で、水冷された銅製のスカルるつぼを用いて溶解を行う技術であり、溶解量には限界がある。また、特許文献７には、真空誘導溶解炉において、ＣａＯ耐火材製のるつぼに高純度原材料を装入し、高真空下で高純度原料が溶解しない温度でできるだけ高温度まで加熱、保持した後、溶解炉内を速やかにＡｒ雰囲気として、高純度原料を溶解する高純度合金の製造方法が開示されているが、この技術も溶解量は８０ｋｇ程度に過ぎない。
Further, in order to avoid a decrease in toughness, it is also an effective means to reduce impurity components such as C, N, and O to a very small amount and to achieve ultrahigh purity (see
一方、工業的規模での溶解技術としては、特許文献８に、Ｎが０．００１体積％以下、Ｈ２Ｏが０．０１体積％以下の高純度不活性ガス雰囲気中で、不純物成分の合計含有量が１０質量％以下の耐火物製容器内で金属又は合金原料を溶解し、金属または合金中に混入するＣ＋Ｎ＋Ｓ＋Ｐの合計量を０．００５質量ｍａｓｓ％以下に規制する高純度金属又は合金の溶解方法が提案されている。しかしながら、この技術は、ＭｏやＷ等の高融点金属を含有する合金を溶解する場合、溶解に長時間を要するため、Ｓｉをはじめとする種々の不純物がるつぼから混入して、合金の純度が低下してしまうという問題がある。特に、Ｓｉなどのような真空精錬で除去できない成分のピックアップは、延性や靭性低下の原因になる。そのため、工業的規模でも短時間で高純度の高Ｃｒフェライト系鉄合金を製造できる技術の開発が望まれている。
高純度化することにより、靭性をある程度改善することは可能である。しかし、特許文献１〜４の技術は、いずれも熱延板厚が５ｍｍ以下の薄板を対象としており、厚板材で問題となるσ脆化や４７５℃脆化については何ら考慮していない。また、特許文献５〜８の技術は、高純度金属の溶解技術に関するものであり、やはり、溶解後の合金特性については検討していない。結局、高Ｃｒフェライト系鉄合金に関する従来技術は、いずれも、板厚が厚い用途で問題となる、溶体化処理等の冷却時におけるσ脆化や４７５℃脆化を示すことのない、靭性に優れる高純度の高Ｃｒフェライト系鉄合金を、工業的規模でかつ安価に製造する技術を開示するものではない。
It is possible to improve toughness to some extent by increasing the purity. However, all of the techniques of
本発明の目的は、耐食性や耐酸化性に優れるだけでなく、極厚でもσ脆化や４７５℃脆化を示すことがなく、かつ、靭性にも優れる高純度の高Ｃｒフェライト系鉄合金を提供すると共に、そのような高純度の高Ｃｒフェライト系鉄合金を工業的規模で提供することを可能とする製造方法を提案することにある。 An object of the present invention is to provide a high-purity high Cr ferritic iron alloy that not only has excellent corrosion resistance and oxidation resistance, but also does not exhibit σ embrittlement or 475 ° C embrittlement even at an extremely thick thickness, and has excellent toughness. And providing a manufacturing method capable of providing such a high purity high Cr ferritic iron alloy on an industrial scale.
発明者らは、上記課題を達成すべく鋭意研究を行った。その結果、Ｃ，Ｎ，Ｏを極微量に低減して超高純度化した上で、Ｃｒ含有量を３０ｍａｓｓ％以下かつ溶体化処理時の板厚に応じてある特定の関係を満たよう制御すれば、板厚が３００ｍｍの極厚であっても、溶体化処理後の延性と靭性に優れる高Ｃｒフェライト系鉄合金が得られること、また、この高Ｃｒフェライト系鉄合金を溶製する際に、耐火性るつぼへの原材料の装入を、原材料の有する融点の高い順で行い、その後、溶解を開始すれば、るつぼと溶融金属との接触時間を短縮化し、不純物のピックアップを防止できるので、高純度の高Ｃｒフェライト系鉄合金を工業的規模で製造できることを見出し、本発明を完成させた。 The inventors have intensively studied to achieve the above problems. As a result, after ultra-high purity is achieved by reducing C, N, O to a very small amount, the Cr content is controlled to satisfy a specific relationship according to the plate thickness at 30 mass% or less and the solution treatment. For example, even when the plate thickness is 300 mm, a high Cr ferritic iron alloy having excellent ductility and toughness after solution treatment can be obtained, and when this high Cr ferritic iron alloy is melted If the raw material is charged into the refractory crucible in descending order of the melting point of the raw material and then the melting is started, the contact time between the crucible and the molten metal can be shortened, so that the pick-up of impurities can be prevented. The present inventors have found that a high-purity, high Cr ferritic iron alloy can be produced on an industrial scale.
すなわち、本発明は、Ｃｒを１３〜３０ｍａｓｓ％かつ製品板厚ｔとの間で下記式；
Ｃｒ含有量≦７（ｌｏｇｔ−２．５）２＋２０ ただし、ｔ：板厚（ｍｍ）
を満たして含有するとともに、Ｃ＋Ｎ＋Ｏを０．００５ｍａｓｓ％以下、Ａｌ，ＴｉおよびＮｂの１種以上を０．００５〜０．５ｍａｓｓ％、Ｓｉを０．１５ｍａｓｓ％以下含有し、残部がＦｅおよび不可避的不純物からなり、シグマ脆化および４７５℃脆化を起こさない、製品板厚が２０ｍｍ以上３００ｍｍ以下の高Ｃｒフェライト系鉄合金である。
That is, according to the present invention, Cr is 13-30 mass% and the product sheet thickness t is the following formula:
Cr content ≦ 7 (logt−2.5) 2 +20 where t: plate thickness (mm)
In addition, C + N + O is contained at 0.005 mass% or less, at least one of Al, Ti and Nb is contained at 0.005 to 0.5 mass%, Si is contained at 0.15 mass% or less, and the balance is Fe and inevitable. A high Cr ferritic iron alloy having a product plate thickness of 20 mm or more and 300 mm or less, which is made of impurities and does not cause sigma embrittlement and 475 ° C. embrittlement.
また、本発明は、９９．９９ｍａｓｓ％以上の超高純度Ｃｒ、９９．９９ｍａｓｓ％以上の超高純度Ｆｅを原材料とし、真空誘導溶解炉で耐火性るつぼを用いて溶解し、超高純度高Ｃｒフェライト系鉄合金を製造する方法において、上記耐火性るつぼへの原材料の装入を、るつぼ下部に高融点の超高純度Ｃｒ、その超高純度Ｃｒの上部に低融点の超高純度Ｆｅの順で行い、その後の溶解を、まず上部の低融点金属、次いで下部の高融点金属の順で行うことにより、Ｃｒを１３〜３０ｍａｓｓ％かつ製品板厚ｔとの間で下記式；
Ｃｒ含有量≦７（ｌｏｇｔ−２．５）２＋２０ ただし、ｔ：板厚（ｍｍ）
を満たして含有するとともに、Ｃ＋Ｎ＋Ｏを０．００５ｍａｓｓ％以下、Ａｌ、ＴｉおよびＮｂの１種以上を０．００５〜０．５ｍａｓｓ％、Ｓｉを０．１５ｍａｓｓ％以下含有し、残部がＦｅおよび不可避的不純物からなり、製品板厚が２０ｍｍ以上３００ｍｍ以下の鉄合金を得ることを特徴とする超高純度高Ｃｒフェライト系鉄合金の製造方法を提案する。
In addition, the present invention uses ultrahigh purity Cr of 99.99 mass% or more and ultrahigh purity Fe of 99.99 mass% or more as raw materials, and is melted by using a refractory crucible in a vacuum induction melting furnace. In the method for producing a ferritic iron alloy, the raw materials are charged into the above-mentioned refractory crucible, and the high melting point ultra-high purity Cr is placed at the bottom of the crucible and the low melting point ultra-high purity Fe is placed above the ultra-high purity Cr. And the subsequent melting is performed in the order of the upper low melting point metal and then the lower high melting point metal in this order, so that Cr is 13-30 mass% and the product sheet thickness t is expressed by the following formula:
Cr content ≦ 7 (logt−2.5) 2 +20 where t: plate thickness (mm)
In addition, C + N + O is contained in 0.005 mass% or less, one or more of Al, Ti, and Nb is contained in 0.005-0.5 mass%, Si is contained in 0.15 mass% or less, and the balance is Fe and inevitable. The present invention proposes a method for producing an ultra-high purity high Cr ferritic iron alloy comprising impurities and obtaining an iron alloy having a product plate thickness of 20 mm to 300 mm.
本発明によれば、３００ｍｍの極厚材でもσ脆化や４７５℃脆化を起こすことのない靭性に優れる高Ｃｒフェライト系鉄合金を得ることができるので、厚板部材や大径部材で耐食性が要求される分野に好適に用いることができる。 According to the present invention, a high Cr ferritic iron alloy having excellent toughness that does not cause σ embrittlement or 475 ° C. embrittlement even with an extremely thick material of 300 mm can be obtained. Can be suitably used in a field where is required.
本発明の高Ｃｒフェライト系鉄合金の成分組成を、上記範囲に制限する理由について説明する。
Ｃｒ：１３〜３０ｍａｓｓ％
Ｃｒは、耐食性を向上させる元素であり、１３ｍａｓｓ％未満では耐食性が不足する。一方、３０ｍａｓｓ％超になると、超高純度化しても溶体化からの冷却時にσ脆化、４７５℃脆化を生じやすくなり、延性と靭性が低下する。よって、Ｃｒは１３〜３０ｍａｓｓ％の範囲に制限する。好ましくは、１３〜２５ｍａｓｓ％の範囲である。
The reason why the component composition of the high Cr ferritic iron alloy of the present invention is limited to the above range will be described.
Cr: 13-30 mass%
Cr is an element that improves the corrosion resistance. If it is less than 13 mass%, the corrosion resistance is insufficient. On the other hand, if it exceeds 30 mass%, σ embrittlement and 475 ° C embrittlement are liable to occur during cooling from solution formation, and ductility and toughness decrease. Therefore, Cr is limited to a range of 13 to 30 mass%. Preferably, it is the range of 13-25 mass%.
Ｃｒ含有量と板厚ｔとの関係：Ｃｒ含有量≦７（ｌｏｇｔ−２．５）２＋２０
Ｃｒは、上述したように脆性に大きく影響する元素であり、同じ板厚では、Ｃｒ含有量が多いほど脆化しやすい。一方、板厚中心部における冷却速度は、板厚の増加に伴い低下するため、同じＣｒ含有量では、板厚が厚いほど、溶体化処理等の熱処理時の冷却速度が遅くなり、脆化しやすくなる。したがって、板厚の増加に伴い、脆化を起こさないＣｒ含有量の上限値は減少する。
発明者らは、水冷で冷却した時の、板厚ｔ（ｍｍ）と板厚中心部が脆化を生じないＣｒ含有量（ｍａｓｓ％）との関係を調査した結果、下記（１）式；
Ｃｒ含有量（ｍａｓｓ％）≦７（ｌｏｇｔ−２．５）２＋２０ ・・・・・・ （１）
の関係があることがわかった。すなわち、製品の板厚に応じて、Ｃｒ含有量を（１）式を満たすよう制御すれば、極厚材でも板厚中心部においてもσ脆性や４７５℃脆性等の脆化を起こすことがないことを見出した。
Relationship between Cr content and sheet thickness t: Cr content ≦ 7 (logt−2.5) 2 +20
As described above, Cr is an element that greatly affects brittleness. With the same plate thickness, the more the Cr content, the more likely it becomes brittle. On the other hand, the cooling rate at the center of the plate thickness decreases with an increase in the plate thickness, so with the same Cr content, the thicker the plate thickness, the slower the cooling rate during heat treatment such as solution treatment, and the more likely it becomes embrittlement. Become. Therefore, as the plate thickness increases, the upper limit of the Cr content that does not cause embrittlement decreases.
As a result of investigating the relationship between the plate thickness t (mm) and the Cr content (mass%) at which the central portion of the plate thickness does not cause embrittlement when cooled by water cooling, the following formula (1);
Cr content (mass%) ≦ 7 (logt−2.5) 2 +20 (1)
It was found that there is a relationship. That is, if the Cr content is controlled so as to satisfy the formula (1) according to the thickness of the product, neither embrittlement such as σ brittleness or 475 ° C. brittleness will occur in the extremely thick material or the central portion of the thickness. I found out.
なお、ここで、「脆化を生じない」とは、脆性を破面遷移温度（Ｆｒａｃｔｕｒｅ ａｐｐｅａｒａｎｃｅ ｔｒａｎｓｉｔｉｏｎ ｔｅｍｐｅｒａｔｕｒｅ；ＦＡＴＴ）で評価した時に、板厚（すなわち冷却速度）によってＦＡＴＴが変化しないことを意味し、一方、「脆化する」とは、板厚（冷却速度）によってＦＡＴＴが変化することを意味する。 Here, “does not cause embrittlement” means that when the brittleness is evaluated by the fracture surface transition temperature (FATT), the FATT does not change depending on the plate thickness (that is, the cooling rate). On the other hand, “become embrittled” means that the FATT changes depending on the plate thickness (cooling rate).
Ｃ＋Ｎ＋Ｏ≦０．００５ｍａｓｓ％
Ｃ，Ｎ，Ｏは、粒界に偏析して、Ｃｒとナノサイズのクラスターを形成することで、Ｃｒ濃度の揺らぎを生成し、そのＣｒ濃度の揺らぎがσ脆化や４７５℃脆化を促進する。またＣ，Ｎは、マトリックスを固溶硬化して延性や靭性を低下させたり、多量の炭化物や窒化物の析出物を形成して延性や靭性を低下させたりする。そのため、これらの元素は極力低減することが望ましく、Ｃ，Ｎ，Ｏの合計で０．００５ｍａｓｓ％以下に制限する。好ましくは、０．００４５ｍａｓｓ％以下である。
因みに、従来技術（例えば、特許文献３）には、Ｃ＋Ｎ＋Ｏ＋Ｐ＋Ｓ≦０．０１ｍａｓｓ％の高純度合金が開示されている。しかし、従来の分析技術は、分析精度が悪く、Ｃ＋Ｎ＋Ｏ＋Ｐ＋Ｓ≦０．０１ｍａｓｓ％の意味するところは、Ｃ＋Ｎ＋Ｏ≦０．００５ｍａｓｓ％という低レベルも物までを含むものではなかった。この点、発明者らは、その分析精度の向上に努めた結果、本発明において規定しているＣ＋Ｎ＋Ｏ≦０．００５ｍａｓｓ％は、従来のＣ＋Ｎ＋Ｏ＋Ｐ＋Ｓ≦０．０１ｍａｓｓ％よりも、より高い純度に相当するものである。
C + N + O ≦ 0.005 mass%
C, N, and O segregate at the grain boundaries to form Cr and nano-sized clusters, thereby generating fluctuations in Cr concentration, which fluctuations promote σ embrittlement and 475 ° C embrittlement. To do. C and N reduce the ductility and toughness by solid solution hardening of the matrix, or reduce the ductility and toughness by forming a large amount of carbide and nitride precipitates. Therefore, it is desirable to reduce these elements as much as possible, and the total amount of C, N, and O is limited to 0.005 mass% or less. Preferably, it is 0.0045 mass% or less.
Incidentally, a high-purity alloy of C + N + O + P + S ≦ 0.01 mass% is disclosed in the prior art (for example, Patent Document 3). However, the conventional analysis technique has poor analysis accuracy, and the meaning of C + N + O + P + S ≦ 0.01 mass% does not include even the low level of C + N + O ≦ 0.005 mass%. In this regard, the inventors have sought to improve the analysis accuracy, and as a result, C + N + O ≦ 0.005 mass% defined in the present invention corresponds to higher purity than the conventional C + N + O + P + S ≦ 0.01 mass%. Is.
Ａｌ，Ｔｉ，Ｎｂのうちの１種または２種以上：それぞれ０．００５〜０．５ｍａｓｓ％
ＡｌおよびＴｉは、脱酸効果を有するとともに、耐食性を向上させる。また、Ｔｉ，Ｎｂは、Ｃ，Ｎとの結合力が強いため、不可避的に混入したＣ，Ｎを固着し、固溶しているＣ，Ｎを低減することにより延性や靭性、耐食性の向上に寄与する。それらの作用効果を得るためには、Ａｌ，Ｔｉ，Ｎｂのうちの１種または２種以上を、それぞれ０．００５ｍａｓｓ％以上添加する必要がある。一方、添加量が０．５ｍａｓｓ％を超えると、余剰のＡｌ，ＴｉおよびＮｂによる固溶強化によって延性や靭性が低下する。よって、Ａｌ，ＴｉおよびＮｂは、１種または２種以上をそれぞれ０．００５〜０．５ｍａｓｓ％の範囲で添加する。好ましくは、０．００５〜０．１ｍａｓｓ％である。
One or more of Al, Ti, and Nb: 0.005 to 0.5 mass% each
Al and Ti have a deoxidizing effect and improve corrosion resistance. In addition, Ti and Nb have strong bonding strength with C and N, so that inevitably mixed C and N are fixed, and the solid solution C and N are reduced to improve ductility, toughness, and corrosion resistance. Contribute to. In order to obtain these functions and effects, it is necessary to add one or more of Al, Ti, and Nb in an amount of 0.005 mass% or more. On the other hand, when the addition amount exceeds 0.5 mass%, ductility and toughness are reduced by solid solution strengthening with excess Al, Ti, and Nb. Therefore, Al, Ti, and Nb are added in the range of 0.005 to 0.5 mass%, respectively, as one kind or two or more kinds. Preferably, it is 0.005-0.1 mass%.
Ｓｉ：０．１５ｍａｓｓ％以下
Ｓｉは、溶解時に、耐火性のるつぼから混入してくる不可避的不純物であり、マトリックスを固溶硬化して延性や靭性を低下させたり、σ脆化および４７５℃脆化を促進したりする。従って、耐脆化感受性を損なわないためには、Ｓｉは極力低減する必要があり、上限を０．１５ｍａｓｓ％に制限する。好ましくは、０．０５ｍａｓｓ％以下である。
Si: 0.15 mass% or less Si is an unavoidable impurity mixed in from a refractory crucible at the time of melting, and the matrix is solid-solution hardened to reduce ductility and toughness, σ embrittlement and 475 ° C brittleness. Or promote Therefore, in order not to impair the embrittlement resistance, Si needs to be reduced as much as possible, and the upper limit is limited to 0.15 mass%. Preferably, it is 0.05 mass% or less.
なお、本発明の高Ｃｒフェライト系鉄合金は、上記成分組成以外の残部は、Ｆｅおよび不可避的不純物である。なお、不可避的不純物として含まれる、Ｓ，Ｐは、延性、靭性および耐食性を低下させるため、Ｓ：０．００３０ｍａｓｓ％以下、Ｐ：０．００３０ｍａｓｓ％以下に制限することが望ましい。 In the high Cr ferritic iron alloy of the present invention, the balance other than the above component composition is Fe and inevitable impurities. S and P contained as inevitable impurities are desirably limited to S: 0.0030 mass% or less and P: 0.0030 mass% or less in order to reduce ductility, toughness, and corrosion resistance.
次に、本発明の高Ｃｒフェライト系鉄合金の板厚を制限する理由について説明する。
本発明は、σ脆化や４７５℃脆化を防止するために、板厚の増加に応じてＣｒ含有量を低減することを特徴としている。しかし、板厚が３００ｍｍを超えると４７５℃脆化のため、Ｃｒ量を低下させても脆化が起こるので、板厚の上限を３００ｍｍと規定する。一方、板厚が、２０ｍｍ未満になると、Ｃｒ含有量が１３〜３０ｍａｓｓ％の範囲であれば、通常行われる水冷処理ではσ脆化や４７５℃脆化を起こすことがない。従って、板厚範囲は、２０ｍｍ以上３００ｍｍ以下とする。なお、本発明の効果は、板厚が３０ｍｍ以上、特に４０ｍｍ以上で大きい。
Next, the reason for limiting the plate thickness of the high Cr ferritic iron alloy of the present invention will be described.
The present invention is characterized in that the Cr content is reduced as the plate thickness increases in order to prevent σ embrittlement and 475 ° C embrittlement. However, if the plate thickness exceeds 300 mm, 475 ° C. embrittles, and therefore, even if the Cr content is reduced, embrittlement occurs. Therefore, the upper limit of the plate thickness is defined as 300 mm. On the other hand, when the plate thickness is less than 20 mm, σ embrittlement or 475 ° C. embrittlement will not occur in the water cooling treatment that is usually performed if the Cr content is in the range of 13 to 30 mass%. Therefore, the plate thickness range is 20 mm or more and 300 mm or less. The effect of the present invention is significant when the plate thickness is 30 mm or more, particularly 40 mm or more.
ここで、上記板厚を板厚中心部における冷却速度は、下記（２）式；
Ｖ＝１０（４．８２５−１．５３１×ｌｏｇｔ） ・・・・・・ （２）
ただし、Ｖ：冷却速度（℃／ｍｉｎ）、ｔ：板厚（ｍｍ）
で表すことができる。
そこで、上記板厚範囲（２０〜３００ｍｍ）を冷却速度に換算すると、約１０〜６８０℃／ｍｉｎの冷却速度に相当する。したがって、本発明の効果は、板厚中心部の冷却速度が約１０〜６８０℃／ｍｉｎの範囲であれば得ることができる、即ち、脆化を示さないことになる。
Here, the cooling rate at the center of the plate thickness is the following equation (2):
V = 10 (4.825-1.531 × logt) (2)
Where V: cooling rate (° C./min), t: plate thickness (mm)
Can be expressed as
Therefore, when the plate thickness range (20 to 300 mm) is converted into a cooling rate, it corresponds to a cooling rate of about 10 to 680 ° C./min. Therefore, the effect of the present invention can be obtained if the cooling rate at the central portion of the plate thickness is in the range of about 10 to 680 ° C./min, that is, does not show embrittlement.
次に、本発明の高Ｃｒフェライト系鉄合金の製造方法について説明する。
本発明の製造方法は、超高純度の大型合金塊を溶製する際のるつぼからの不純物のピックアップを抑制するために、溶解時間をできるだけ短くして耐火性るつぼと溶解金属との接触時間を短縮するところに特徴がある。そのために、本発明では、９９．９９ｍａｓｓ％以上の超高純度Ｃｒ、９９．９９ｍａｓｓ％以上の超高純度Ｆｅを原材料とし、耐火性るつぼを用いて真空誘導溶解炉で超高純度高Ｃｒフェライト系鉄合金を溶解するに当たり、上記耐火性るつぼへの原料の装入を、るつぼ最下部に最も高融点の金属の原材料を装入し、その上部に、融点が高い金属の順に原材料を装入し、最上部には、最も低融点の金属の原材料を装入して、その後、溶解する。このような溶解を行うことによって、溶解加熱時には、先ず、最上部の最も低融点の金属の原材料が溶解してるつぼ底部に溜まり、この溜まった低融点の溶解金属が、下部の高融点金属を包囲・接触し、反応して低融点の合金を形成し、溶解が著しく促進される。その結果、るつぼと溶解金属との接触時間が短縮されるので、１トン以上の合金を溶解する場合においても、溶解金属がるつぼから汚染されるのを最小限に抑制できる。
Next, the manufacturing method of the high Cr ferritic iron alloy of this invention is demonstrated.
The production method of the present invention reduces the contact time between the refractory crucible and the molten metal by reducing the melting time as much as possible in order to suppress the picking up of impurities from the crucible when melting a large alloy lump of ultra high purity. It is characterized by shortening. Therefore, in the present invention, the ultra-high purity high Cr ferrite system using ultra high purity Cr of 99.99 mass% or more and ultra high purity Fe of 99.99 mass% or more in a vacuum induction melting furnace using a refractory crucible. In melting the iron alloy, the raw material is charged into the above refractory crucible, the raw material of the highest melting point metal is charged at the bottom of the crucible, and the raw material is charged into the upper part in the order of the metal having the highest melting point. At the top, the lowest melting point metal raw material is charged and then melted. By carrying out such melting, at the time of melting and heating, first, the raw material of the lowest melting point metal at the top is melted and collected at the bottom of the crucible, and the collected melting metal with the low melting point becomes the lower melting point metal. Surrounding, contacting, reacting to form a low melting point alloy, and melting is significantly accelerated. As a result, since the contact time between the crucible and the molten metal is shortened, contamination of the molten metal from the crucible can be minimized even when an alloy of 1 ton or more is melted.
具体的には、主成分がＣｒ、Ｆｅである場合には、るつぼ下部に高融点であるＣｒ原材料（ｍ．ｐ．１８９０℃）を、その上部に低融点であるＦｅ原材料（ｍ．ｐ．１５３６℃）を装入する。 Specifically, when the main components are Cr and Fe, a Cr raw material (mp 1890 ° C.) having a high melting point is provided at the lower part of the crucible, and an Fe raw material (mp. 1536 ° C.).
高Ｃｒフェライト系鉄合金を溶製するに当たって用いる各金属の原材料は、できるだけ高純度のものを用いることが好ましく、例えば、ＦｅやＣｒ原材料としては、９９．９９ｍａｓｓ％以上の純度のものを用いることが必要である。また、ＡｌやＴｉ，Ｎｂの原材料も高純度であることが好ましく、９９．５ｍａｓｓ％以上、さらに好ましくは９９．８ｍａｓｓ％以上の純度を有する原材料を用いることが望ましい。 It is preferable to use materials of high purity as much as possible as raw materials of each metal used for melting high Cr ferritic iron alloys. For example, as materials of Fe and Cr, use materials having a purity of 99.99 mass% or more. is required. In addition, the raw materials of Al, Ti, and Nb are also preferably highly pure, and it is desirable to use raw materials having a purity of 99.5 mass% or more, more preferably 99.8 mass% or more.
また、高Ｃｒフェライト系鉄合金を溶製する溶解炉としては、真空誘導加熱炉を好適に用いることができるが、他の溶解炉であっても、真空中あるいは不活性ガス雰囲気中で溶解できる溶解炉であれば用いることができる。また、耐火性るつぼの材質は、特に限定しないが、溶解合金の汚染を防止する観点からは、脱Ｓ能や脱Ｐ能が比較的高いＭｇＯ・Ａｌ２Ｏ３（スピネル）製やＭｇＯ製、ＣａＯ製であることが好ましい。 Moreover, as a melting furnace for melting a high Cr ferritic iron alloy, a vacuum induction heating furnace can be preferably used, but even other melting furnaces can be melted in a vacuum or in an inert gas atmosphere. Any melting furnace can be used. In addition, the material of the refractory crucible is not particularly limited, but from the viewpoint of preventing contamination of the molten alloy, it is made of MgO.Al 2 O 3 (spinel), MgO, which has relatively high de-S capacity and de-P capacity. It is preferably made of CaO.
上記本発明の製造方法によれば、工業的規模でも、Ｃ＋Ｎ＋Ｏ≦０．００５ｍａｓｓ％の超高純度の高Ｃｒフェライト系鉄合金を製造することができる。
なお、溶解した高Ｃｒフェライト鉄合金は、その後、鋳造して鋳塊としてから、熱間鍛造や熱間圧延等の熱間加工を行い、溶体化処理等を施して製品とすることが好ましい。上記熱間加工を行う際の加熱条件や加工条件は、特に制限されるものではないが、例えば、鋳塊の加熱温度は１２８０〜１０００℃、鍛造・圧延温度は１２８０〜７００℃の範囲とするのが好ましい。その他、１回または複数回の加熱と熱間加工を繰り返す場合には、加熱保持時間の１回以上を数時間から数十時間に設定し、加工ひずみや拡散の効果を利用し、偏析低減（成分均質化）を図ることが好ましい。また、溶体化処理は、８５０〜１０５０℃で、板厚中心部を含めた部材全体がほぼ均一な温度になるよう加熱保持を行った後、水冷することが好ましい。
According to the production method of the present invention, an ultra-high purity high Cr ferritic iron alloy with C + N + O ≦ 0.005 mass% can be produced even on an industrial scale.
In addition, after melt | dissolving the high Cr ferritic iron alloy after casting and making it as an ingot, it is preferable to perform hot working, such as hot forging and hot rolling, and to give a solution treatment etc. to make a product. The heating conditions and processing conditions for performing the hot working are not particularly limited. For example, the ingot heating temperature is 1280 to 1000 ° C., and the forging / rolling temperature is 1280 to 700 ° C. Is preferred. In addition, when repeating one or more times of heating and hot working, set one or more heating holding times to several hours to several tens of hours to reduce segregation by utilizing the effects of processing strain and diffusion ( It is preferable to achieve homogenization. The solution treatment is preferably performed at 850 to 1050 [deg.] C., after heating and holding so that the entire member including the center portion of the plate thickness has a substantially uniform temperature, and then water cooling.
表１に示す符号１〜５の成分組成を有する合金を真空誘導加熱炉（Ｖａｃｕｕｍ ｉｎｄｕｃｔｉｏｎ ｍｅｌｔｉｎｇ ｆｕｒｎａｃｅ；ＶＩＭ炉）で溶製し、鋳造して７０ｋｇの鋳塊とし、１２００℃に加熱後、熱間鍛造し、板厚３５ｍｍの板に加工した。その後、これらの鍛造板から３５ｍｍ厚×４５ｍｍ幅×１３０ｍｍのブロック材または約１５ｍｍ□の角棒を切り出し、これらを加熱炉に装入して９３０〜１１４０℃の各温度まで昇温し、１ｈｒ保持する溶体化処理を施し、ブロック材は、上記溶体化温度から中心部の冷却速度が１６．７℃／ｍｉｎ、５．０℃／ｍｉｎ、１．６７℃／ｍｉｎとなるように約３０℃まで冷却し、また、約１５ｍｍ□の角棒は、上記溶体化温度から、水冷にて板厚６．５ｍｍの幅広板材を水冷した場合に相当する中心部の冷却条件（約３８００℃／ｍｉｎ）で約３０℃まで冷却し、その後、これらを脆性試験に供した。脆性試験は、ＪＩＳ Ｚ ２２０２に規定された１０ｍｍ□×５５ｍｍで２ｍｍＶノッチ（旧ＪＩＳ５号試験片）を鍛造方向に平行に採取し、ＪＩＳ Ｚ ２２４２に準拠してシャルピー衝撃試験を行い、遷移曲線から延性破面率５０％の破面遷移温度（ＦＡＴＴ）を求めた。なお、各合金とも１つの冷却条件で６本ずつの試験を行った。 An alloy having a component composition of 1 to 5 shown in Table 1 is melted in a vacuum induction heating furnace (VIM furnace), cast into a 70 kg ingot, heated to 1200 ° C., and hot Forged and processed into a plate with a thickness of 35 mm. Thereafter, a block material of 35 mm thickness × 45 mm width × 130 mm or a square bar of about 15 mm □ is cut out from these forged plates, charged in a heating furnace, heated to each temperature of 930 to 1140 ° C., and held for 1 hr. The block material is subjected to the solution treatment, and the block material is heated from the solution temperature to about 30 ° C. so that the cooling rate at the center is 16.7 ° C./min, 5.0 ° C./min, 1.67 ° C./min. The square bar of about 15 mm □ is cooled under the cooling condition (about 3800 ° C./min) in the central portion corresponding to the case where a wide plate material having a thickness of 6.5 mm is cooled by water cooling from the above solution temperature. They were cooled to about 30 ° C., after which they were subjected to a brittleness test. In the brittleness test, 10 mm □ × 55 mm specified in JIS Z 2202 and a 2 mm V notch (former JIS No. 5 test piece) were taken in parallel to the forging direction, Charpy impact test was conducted in accordance with JIS Z 2242, and the transition curve was used. The fracture surface transition temperature (FATT) with a ductile fracture surface ratio of 50% was determined. Each of the alloys was subjected to six tests under one cooling condition.
図１は、下横軸に板厚（上横軸に水冷時の板厚中心部の冷却速度）、縦軸に破面遷移温度（ＦＡＴＴ）をとり、板厚（冷却速度）とＦＡＴＴとの関係を示したものである。図１において、ＦＡＴＴが変化しない板厚（冷却速度）域は、脆化が生じていないことを、また、ＦＡＴＴが上昇している板厚（冷却速度）域は、脆化が生じていることを示す。図１から、Ｃｒ含有量が２５ｍａｓｓ％以上の合金では、板厚が４０ｍｍよりも薄くなければ脆化が生じること、およびＣｒ含有量が３０ｍａｓｓ％以上の合金では、板厚が２０ｍｍよりも薄くなければ脆化が生じることが判る。 In FIG. 1, the lower horizontal axis is the plate thickness (upper horizontal axis is the cooling rate at the center of the plate thickness during water cooling), and the vertical axis is the fracture surface transition temperature (FATT). It shows the relationship. In FIG. 1, the plate thickness (cooling rate) region where FATT does not change does not cause embrittlement, and the plate thickness (cooling rate) region where FATT increases indicates that embrittlement occurs. Indicates. From FIG. 1, it can be seen that an alloy with a Cr content of 25 mass% or more causes embrittlement unless the plate thickness is less than 40 mm, and an alloy with a Cr content of 30 mass% or more must have a plate thickness of less than 20 mm. It can be seen that embrittlement occurs.
図２は、図１から求められる脆化が生じない上限のＣｒ含有量と板厚（および水冷時の板厚中心の冷却速度）との関係を実線で示したものである。この図から、脆化が生じないＣｒ含有量は、先述した（１）式；
Ｃｒ含有量（ｍａｓｓ％）≦７（ｌｏｇｔ−２．５）２＋２０ ・・・・・・ （１）
で表されることがわかる。
なお、図２中には参考として、本発明において、耐食性から規定されるＣｒ含有量の下限値と、板厚の範囲を、それぞれ破線および一点差線で示した。これらの線で囲まれた領域に板厚およびＣｒ含有量を調整することによって、脆化を生ずることのない高Ｃｒフェライト系鉄合金を得ることができる。
FIG. 2 shows the relationship between the upper limit Cr content at which embrittlement does not occur and the plate thickness (and the cooling rate at the center of the plate thickness at the time of water cooling) obtained from FIG. 1 by a solid line. From this figure, the Cr content at which embrittlement does not occur is the formula (1) described above;
Cr content (mass%) ≦ 7 (logt−2.5) 2 +20 (1)
It can be seen that
In FIG. 2, for reference, in the present invention, the lower limit value of the Cr content defined by the corrosion resistance and the range of the plate thickness are indicated by a broken line and a one-point difference line, respectively. By adjusting the plate thickness and Cr content in the region surrounded by these lines, a high Cr ferritic iron alloy that does not cause embrittlement can be obtained.
実施例１で用いた、表１の符号３〜５の合金から得た供試材を、実施例１と同じ条件で溶体化処理し、その後、板厚６．５ｍｍの板材を水冷した場合の板厚中心部の冷却速度に相当する約３８００℃／ｍｉｎで約３０℃まで冷却し、引張試験および衝撃試験に供した。なお、ここで、板厚６．５ｍｍの冷却速度としたのは、この板厚では、板厚の変化すなわち冷却速度の変化による機械的特性変化が小さいため、脆化を考慮せずに各合金の強度と靭性を比較することができるからである。引張試験は、上記供試材から長さ１２０ｍｍ、ゲージ部１０ｍｍφ×５０ｍｍＬの丸棒つばつき引張試験片を鍛造方向に平行に採取し、ＪＩＳ Ｚ ２２４１に準拠して室温で引張強さ（ＴＳ）を測定した。このときのひずみ速度は０．２％耐力までは０．３％／ｍｉｎ（５．０×１０−４ｓ−１）、その後、破断するまでは３．８ｍｍ／ｍｉｎ（１．２７×１０−３ｓ−１）とした。また、脆性の評価は、実施例１と同じ要領でシャルピー衝撃試験を行い、遷移曲線から５０％破面遷移温度（ＦＡＴＴ）を求めた。 When the sample material obtained from the alloy of the code | symbol 3-5 of Table 1 used in Example 1 was solution-treated on the same conditions as Example 1, the board | plate material with a plate thickness of 6.5 mm was water-cooled after that. The plate was cooled to about 30 ° C. at about 3800 ° C./min corresponding to the cooling rate at the center of the plate thickness, and subjected to a tensile test and an impact test. Here, the cooling rate of 6.5 mm is used because the change in plate thickness, that is, the change in mechanical properties due to the change in cooling rate is small. This is because the strength and toughness can be compared. In the tensile test, a round bar collar tensile test piece having a length of 120 mm and a gauge part of 10 mmφ × 50 mmL was taken in parallel to the forging direction from the above test material, and tensile strength (TS) at room temperature in accordance with JIS Z 2241. Was measured. Strain rate 0.2% and yield strength 0.3% / min in this case (5.0 × 10 -4 s -1) , then until the break 3.8mm / min (1.27 × 10 - 3 s −1 ). Moreover, the brittleness evaluation evaluated the 50% fracture surface transition temperature (FATT) from the transition curve by performing the Charpy impact test in the same manner as in Example 1.
図３は、上記測定の結果を、強度−靭性（ＦＡＴＴ）バランスとして示したものであり、実線は発明合金（合金符号３）、一点鎖線は比較合金（合金符号４，５）を示す。図３から、Ｃｒが３０％を超えると、ＦＡＴＴが大きく上昇するため、Ｃｒの上限は３０ｍａｓｓ％以下とする必要があることがわかる。なお、Ｃｒ含有量が高い比較合金４，５は、発明合金３と一直線上にあり、ほぼ同じ強度−靭性バランスを有している。これは、本実施例の試験が、板厚６．５ｍｍの板材を水冷した場合の板厚中心部の冷却速度に相当する冷却を施した材料を用いているために低速冷却による脆化を考慮する必要がないからである。 FIG. 3 shows the result of the above measurement as a strength-toughness (FATT) balance, where the solid line indicates the invention alloy (alloy code 3) and the alternate long and short dash line indicates the comparative alloy (alloy codes 4 and 5). From FIG. 3, it can be seen that when Cr exceeds 30%, FATT greatly increases, so the upper limit of Cr needs to be 30 mass% or less. The comparative alloys 4 and 5 having a high Cr content are in line with the invention alloy 3 and have substantially the same strength-toughness balance. This is because the test of this example uses a material that has been cooled corresponding to the cooling rate of the central part of the plate thickness when a 6.5 mm thick plate is water-cooled, so that embrittlement due to low-speed cooling is considered. Because there is no need to do.
本発明により製造される高純度高Ｃｒフェライト系鉄合金は、厳しい耐食性と靭性が要求される工業用大型部材全般に適用することができる。 The high purity high Cr ferritic iron alloy produced according to the present invention can be applied to all large industrial members requiring severe corrosion resistance and toughness.
Claims (2)
Ｃ＋Ｎ＋Ｏを０．００５ｍａｓｓ％以下、
Ａｌ，ＴｉおよびＮｂの１種以上を０．００５〜０．５ｍａｓｓ％、
Ｓｉを０．１５ｍａｓｓ％以下含有し、
残部がＦｅおよび不可避的不純物からなり、シグマ脆化および４７５℃脆化を起こさない、製品板厚が２０ｍｍ以上３００ｍｍ以下の高Ｃｒフェライト系鉄合金。
記
Ｃｒ含有量≦７（ｌｏｇｔ−２．５）２＋２０ ただし、ｔ：板厚（ｍｍ） While containing 13-30 mass% Cr and the product sheet thickness t satisfying the following formula,
C + N + O is 0.005 mass% or less,
0.005 to 0.5 mass% of one or more of Al, Ti and Nb,
Si is contained 0.15 mass% or less,
A high Cr ferritic iron alloy having a product thickness of 20 mm or more and 300 mm or less, the balance being made of Fe and unavoidable impurities and not causing sigma embrittlement and 475 ° C embrittlement.
Cr content ≦ 7 (logt−2.5) 2 +20 where t: plate thickness (mm)
記
Ｃｒ含有量≦７（ｌｏｇｔ−２．５）２＋２０ ただし、ｔ：板厚（ｍｍ） Using ultra high purity Cr of 99.99 mass% or more and ultra high purity Fe of 99.99 mass% or more as raw materials, melting with a refractory crucible in a vacuum induction melting furnace to produce an ultra high purity high Cr ferritic iron alloy In this method, the raw materials are charged into the refractory crucible in the order of high melting point ultra high purity Cr at the bottom of the crucible and low melting point ultra high purity Fe at the top of the ultra high purity Cr, followed by dissolution. In the order of the low melting point metal at the top and then the high melting point metal at the bottom, Cr is contained in an amount of 13 to 30 mass% and the product sheet thickness t satisfying the following formula, and C + N + O is 0.1. 005 mass% or less, containing one or more of Al, Ti and Nb in an amount of 0.005 to 0.5 mass%, Si containing 0.15 mass% or less, the balance being Fe and inevitable impurities, Method for producing ultra-high purity and high Cr ferritic iron alloy, characterized in that to obtain a 300mm less iron alloy than 20 mm.
Cr content ≦ 7 (logt−2.5) 2 +20 where t: plate thickness (mm)
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