JP3982193B2 - Method of determining material for martensitic stainless steel pipe - Google Patents

Description

【０００４】
ところで、この５つのグレードに対応する鋼管を製造するため、鋼管の製造業者は、例えば表１に示したような３つの成分規格を設け、各グレードに応じて、鋼管の形状、成分、熱処理温度を個々に設計して、鋼管を製造しているのが現状である。また、特開平１１−３４３５１９号公報に開示されているように、鋼中のＭｏ含有量と、鋼管の熱処理条件との関係で降伏強度Ｙｓを満足させる技術もある。
【０００５】
しかしながら、成分規格が多いと、それに合うように素材を溶製する必要があり、溶製方法が異なるためにコストの増大を招く。また、特開平１１−３４３５１９号公報記載の技術では、今一歩Ｙｓの達成精度が良好でないのが現状である。
【０００６】
【発明が解決しようとする課題】
本発明は、かかる事情に鑑み、従来の成分規格数を少なく統合しても、顧客の要求機械特性を満足するマルテンサイト系ステンレス鋼管の製造が可能な素材の決定方法を提供することを目的としている。
【０００７】
【課題を解決するための手段】
発明者は、上記目的を達成するため鋭意研究し、その成果を本発明に具現化した。
【０００８】
すなわち、本発明は、Ｃ：０．１５〜０．２２質量％，Ｓｉ：１．０質量％以下，Ｍｎ：０．２５〜１．００質量％，Ｐ：０．０２質量％以下，Ｓ：０．０１質量％以下，Ｃｒ：１２．０〜１４．０質量％，Ｃｕ：０．２５質量％以下，Ｎｉ：０．５０質量％以下、Ｖ：０．２００質量％以下，Ｎｂ：０．０５０質量％以下，Ｂ：２０ｐｐｍ以下を含有し、残部はＦｅ及び不可避不純物からなる鋼管を、その製造工程の最終で焼戻し処理を施して製造するに際し、
製品鋼管の目標製品特性である降伏強度と、それに影響を与える操業因子との間で重回帰分析を行い、下記（３）式で示す重回帰式を求めると共に、該（３）式を用い、製品鋼管の外径、肉厚及び目標製品特性である降伏強度の値に応じて、素材中のＶ，Ｎｂ，Ｂの値を、下記（１）式で定義する成分指標の値が当初値の±２０の範囲に収まるように調整し、定めることを特徴とするマルテンサイト系ステンレス鋼管用素材の決定方法。
Ｙｓ（Ｎ／ｍｍ ^２ ）＝２０２２．４１３＋０．２９６・ＯＤ−１．２１０・ＷＴ−０．０８２・Ｔ・Ｐａｒａｍｅｔｅｒ＋６．０３４・１０ ^−２ ・Ｃ＋０．６２８・１０ ^−３ ・Ｖ＋
７．９９２・１０ ^−３ ・Ｎｂ＋０．７３０・１０ ^−４ ・Ｂ （３）
ここで、Ｙｓは、製品鋼管の目標降伏強度（Ｎ／ｍｍ ^２ ）、ＯＤは製品鋼管の外径（ｍｍ）、ＷＴは製品鋼管の肉厚（ｍｍ）、Ｔ・Ｐａｒａｍｅｔｅｒは下記（２）式で示す焼戻し指標である。
成分指標＝Ｖ(×１０^−３)＋１０Ｎｂ（×１０^−３）＋２Ｃ（×１０^−２）＋２．５Ｂ（×１０^−４）、単位：質量％ （１）
焼戻し指標＝{焼戻し温度（℃）＋２７３}×{２０＋ｌｏｇ（均熱時間（分）／６０）} （２）
また、本発明は、前記製品鋼管の目標製品特性の降伏強度及び（３）式に代え、目標製品特性が引張り強度で、（３）式が下記（４）式であることを特徴とするマルテンサイト系ステンレス鋼管用素材の決定方法である。
Ｔｓ（Ｎ／ｍｍ ^２ ）＝２０８２．５０６＋０．２９４・ＯＤ−１．０６８・ＷＴ−０．０７７・Ｔ・Ｐａｒａｍｅｔｅｒ＋７．９４０・１０ ^−２ ・Ｃ＋０．４４９・１０ ^−３ ・Ｖ＋
６．４４８・１０ ^−３ ・Ｎｂ＋０．６９０・１０ ^−４ ・Ｂ （４）
ここで、Ｔｓは、製品鋼管の目標引張り強度（Ｎ／ｍｍ ^２ ）である。
さらに、本発明は、前記製品鋼管の目標製品特性の降伏強度及び（３）式に代え、目標製品特性がロックウエル硬度で、（３）式が下記（５）式であることを特徴とするマルテンサイト系ステンレス鋼管用素材の決定方法である。
ＨＲＣ（−）＝８４．１３１＋０．００４・ＯＤ−０．０８５・ＷＴ−０．００４・Ｔ・Ｐａｒａｍｅｔｅｒ＋０．２８１・１０ ^−２ ・Ｃ＋０．０１４・１０ ^−３ ・Ｖ＋０．３４７・１０ ^−３ ・Ｎｂ−０．０２９・１０ ^−４ ・Ｂ （５）
ここで、ＨＲＣは、製品鋼管の目標ロックウエル硬度（―）である。
【０００９】
この場合、前記鋼管は、継目無鋼管、電縫鋼管あるいは鍛接鋼管のいずれであっても良い。
【００１０】
本発明によれば、成分規格を外れる素材であっても、焼戻し条件の調整によって、目標鋼管特性を達成できるようになる。その結果、従来より成分規格数が低減でき、素材の溶製負荷が低減するばかりでなく、素材の製造コストが低減する。また、焼戻しにおける熱エネルギーの省力が達成される。
【００１１】
【発明の実施の形態】
以下、発明をなすに至った経緯をまじえ、本発明の実施の形態を説明する。
【００１２】
まず、本発明の対象であるマルテンサイト系ステンレス鋼管の主な化学組成は、Ｃ：０．１５〜０．２２質量％以下、Ｓｉ：１．０質量％以下、Ｍｎ：０．２５〜１．００質量％、Ｐ：０．０２質量％以下、Ｓ：０．０１質量％以下、Ｃｒ：１２．０〜１４．０質量％、Ｃｕ：０．２５質量％以下、Ｎｉ：０．５０質量％以下とする。これは、前記ＡＰＩ規格でＬ８０グレード（１３Ｃｒ）として規定されている組成である。本発明では、このＬ８０の組成にさらにＶ、Ｎｂ、Ｂを添加したものを素材とすることにした。それらの添加理由は、下記の通りである。
Ｖ：０．２００質量％以下、
Ｖは、Ｃ又はＮと化合し、オーステナイト粒界に析出して結晶の粒成長を抑止することで、鋼の強度を向上させる。しかし、過度な添加は、粒界の脆化につながり、特に連続鋳造機での曲げ加工時に鋳片に欠陥を発生させるので、上限を０．２００質量％に抑える。
Ｎｂ：０．０５０質量％以下
Ｎｂは、Ｖと同様、Ｃ又はＮと化合し、オーステナイト粒界に析出して鋼に析出強化の効果を持つ。また、その析出温度はＶよりも高いため、Ｖに比べ微量添加で同等の効果を呈する。そこで、経済的な観点より、上限０．０５０質量％とした。
Ｂ：０．００２０質量％以下
Ｂは、微量添加で鋼材の焼入れ硬化性を増すが、過剰に添加すると、鋼が脆化するので、上限を０．００２０質量％とした。
そして、これらの成分を前記５つのグレードに対応するように、規格化して製鋼工場で素材を溶製している（表１参照）。しかしながら、前記したように、多種類の素材を溶製することは、コストの増大を招くので、できるだけ規格数を減らすことが好ましい。つまり、同一規格の素材でも特性の異なる製品鋼管が入手できれば良い。
【００１３】
そこで、発明者は、製品特性に影響を及ぼす要因について検討した。その際、製品鋼管の降伏強度、引張り強度及び硬度について重相関分析を行ったところ、表２に示すような関係を得た。つまり、降伏強度の例で説明するならば、該強度と選択した要因との間には、下記（３）式で示す重相関関係が成立することがわかった。
Ｙｓ（Ｎ／ｍｍ²）＝２０２２．４１３＋０．２９６・ＯＤ−１．２１０・ＷＴ−０．０８２Ｔ・Ｐａｒａｍｅｔｅｒ＋６．０３４×１０^-2・Ｃ＋０．６２８×１０^-3・Ｖ＋７．９９２×１０^-3・Ｎｂ＋０．７３０×１０^-4・Ｂ （３）
ここで、ＯＤは製品鋼管の外径（ｍｍ）、ＷＴは肉厚（ｍｍ），Ｔ・Ｐａｒａｍｅｔｅｒは、焼戻し指標であり、一般に（２）式で表す。
【００１４】
焼戻し指標＝[焼戻し温度（℃）＋２７３]×[２０＋ｌｏｇ（均熱時間（分）／６０）] （２）
【００１５】
【表２】

【００１６】
これは、重相関係数が０．９３と非常に大きい関係であったので、従来のＹｓ実績データと上記式で計算したＹｓとの関係を調べると、図１に示すように、計算値と実績値が一致していた。なお、引張り強度Ｔｓ及び硬度(この場合、ロックウエル硬度ＨＲＣを採用)についても、図２及び図３にそれぞれ示すように非常に良い結果になった。
【００１７】
この結果は、製品鋼管の外径及び肉厚が決定されると、成分がある程度変動しても、焼戻し指標を変化させることで相殺できることを示している。つまり、前記成分規格が若干異なっても、製品鋼管のＹｓ，ＴｓあるいはＨＲＣを同一にできる。他言すれば、焼戻し指標を予め決めれば、素材の成分を逆に決定できることになる。そこで、発明者は、重要成分であるＣ，Ｖ，Ｎｂ及びＢを一括して下記（１）式に示す成分指標を作成し，その値が変動しても、焼戻し指標で補える範囲を見出すことにした。その結果、その値が±２０の範囲内であることがわかり、本発明を完成させたのである。
成分指標＝Ｖ(×１０^-3)＋１０Ｎｂ（×１０^-3）＋２Ｃ（×１０^-2）＋２．５Ｂ（×１０^-4）、単位：質量％ （１）
さらに言えば、Ｃｒ鉱石の品位によりＶ，Ｂの鋼材汚染量は増減し、所定の機械的特性を得るには焼戻し温度を変更する必要がある。その際、炉の処理能力に余裕があると言っても、経済的損失を少しでも回避するため、炉温変更によるロスタイムを減らしたい。そのためには、Ｖ，Ｂの最大汚染量を想定し、汚染が少ない場合には、Ｖ，Ｂを積極的に添加すれば良い。しかしながら、積極添加を行うと、かえって経済的損失が大きくなる。そこで、本発明では、前記成分指標で、各元素の機械的特性との相関を指数化し、比較的相関の強いＮｂを微量添加することで調整するようにしたのである。
【００１８】
以下、実施例において、本発明を具体的に説明する。
【００１９】
（実施例１）
マルテンサイト系ステンレス鋼でＹｓが５５０〜６５０ＭＰａの継目無鋼管を製造した。しかも、その生産量が少なかったので、焼戻し温度や焼戻し時間を自由に設定できる余裕があった。そこで、従来は表３の比較例１に示すような成分規格に対応する素材を、転炉で高価なフェロクロムを添加して溶製していたが、その規格からＢ，Ｖが高い方に外れる成分の素材でもって前記焼戻し指標の値で調整し、製造することにした。
【００２０】
そのため、前記（２）式の焼戻し温度を従来の７００℃より７２５℃へ上昇し、焼戻し時間（つまり、（２）式の均熱時間のこと）を従来の３０分より長い５０分に設定した。その結果、上記（１）式の成分指標の値を，（２）式の焼戻し指標とのバランスにより、従来より７１４だけ大きくしても良いことがわかった。つまり、その大きくなった分だけ、表３の本発明１及び１´に示したように、素材が含有するＢ，Ｖを高くできる。
【００２１】
このことを実際に確認するため、安価で低純度（Ｂ，Ｖが多い）のＣｒ鉱石を転炉で溶融還元してステンレス鋼製造用の母溶湯を製造し、その後転炉で脱炭してマルテンサイト系ステンレス鋼を溶製した。そして、得られた溶鋼を連続鋳造して鋼鋳片にしてから、分塊工場で丸ビレットとし、該ビレットを素材にマンドレル・ミル方式で継目無鋼管を製造した。その際の焼戻しは、前記設定した温度で５０分間行い、後に製品鋼管の降伏強度Ｙｓを測定した。その結果、従来の成分規格の素材で製造したものとほぼ同程度の６０２ＭＰａという値を得た。
【００２２】
【表３】

【００２３】
（実施例２）
マルテンサイト系ステンレス鋼でＹｓが５５０〜６５０ＭＰａの電縫鋼管を製造した。その生産量が多かったので、焼戻し温度や焼戻し時間（つまり、（２）の均熱時間）を自由に設定できる余裕がない。そのため，上記焼戻し時間を極力短くすべく素材のＢ，Ｖ，Ｎｂを低下させ、さらに、前記（２）式の焼戻し温度を炉の最大能力である７５５℃に設定した。その結果、上記（１）式の成分指標の値が、（２）式の焼戻し指標とのバランスで、従来より４３０だけ小さくしても良いことがわかった。つまり、その小さくなった分だけ、表４の本発明２に示したように、焼戻し時間を５分まで短くできる。
【００２４】
このことを実際に確認するため、転炉で、安価で低純度（Ｂ，Ｖが多い）のＣｒ鉱石を用いるが、その投入量を規制し、Ｖ，Ｂの溶鋼汚染を減らしてマルテンサイト系ステンレス鋼を溶製した。そして、該溶鋼を連続鋳造して鋼鋳片にしてから、圧延工場で熱間圧延鋼帯とし、該鋼帯を素材に電縫鋼管を製造した。その際の焼戻しは、前記設定した温度で５分行い、後に製品鋼管の降伏強度Ｙｓを測定した。その結果、従来の成分規格の素材で製造したものとほぼ同程度の６０８ＭＰａという値を得た。
【００２５】
【表４】

【００２６】
また、これらの結果は、素材の溶製にＶのばらつき防止のため使用できなかった安価原料のＣｒ鉱石の溶融還元が適用できることを示しており、素材製造コストの大幅な低減となる。さらに、表５に示すように、従来は製品のグレード別に有していた成分規格の幾つかを統合し、成分規格数の低減も達成できた。さらに、焼戻し時間の短縮もできるようになり、省エネルギーにもなった。
【００２７】
【表５】

【００２８】
【発明の効果】
以上述べたように、本発明により、成分規格数を従来より少なくしても、ＡＰＩのグレードを満足するマルテンサイト系ステンレス鋼管が製造できるようになる。その結果、従来は素材の溶製に使用できなかったＣｒ鉱石の溶融還元法が採用できるようになり、素材製造コストの低減が達成できた。また、製品のグレード別に有していた成分規格の幾つかを統合し、成分規格数の低減も達成できたばかりでなく、焼戻し時間の短縮もできるようになり、省エネルギーにもなった。
【図面の簡単な説明】
【図１】本発明をなす基礎となった鋼管の降伏強度Ｙｓの計算値と実績値との関係を示す図である。
【図２】本発明をなす基礎となった鋼管の引張り強度Ｔｓの計算値と実績値との関係を示す図である。
【図３】本発明をなす基礎となった鋼管のロックウエル硬度ＨＲＣの計算値と実績値との関係を示す図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for determining a martensitic stainless steel pipe material.
[0002]
[Prior art]
Martensitic stainless steel pipes are effectively used even under harsh environmental conditions such as oil well drilling. This is because the oil well environment requires corrosion resistance of the steel pipe when it contains carbon dioxide gas. API (American Petroleum Institute, American Petroleum Institute) currently has only registered L80 as a martensitic stainless steel pipe containing 13% by mass of Cr. Some users demand higher mechanical properties. As shown in Table 1, the mechanical properties required for the steel pipe can be roughly classified into five grades when the yield strength Ys is in the range of 80 to 125 ksi. In addition, the steel pipe said here contains not only a seamless steel pipe but an electric-resistance-welded steel pipe and a forging steel pipe.
[0003]
[Table 1]

[0004]
By the way, in order to manufacture steel pipes corresponding to these five grades, a steel pipe manufacturer provides, for example, three component standards as shown in Table 1, and according to each grade, the shape, components, and heat treatment temperature of the steel pipe. At present, steel pipes are manufactured by individually designing the pipes. In addition, as disclosed in Japanese Patent Application Laid-Open No. 11-343519, there is a technique that satisfies the yield strength Ys in relation to the Mo content in steel and the heat treatment conditions of the steel pipe.
[0005]
However, when there are many component specifications, it is necessary to melt a raw material so as to meet it, and the melting method is different, which causes an increase in cost. Further, in the technology described in Japanese Patent Application Laid-Open No. 11-343519, the current accuracy of achieving Ys one step is not good at present.
[0006]
[Problems to be solved by the invention]
In view of such circumstances, an object of the present invention is to provide a method for determining a material capable of producing a martensitic stainless steel pipe that satisfies a customer's required mechanical characteristics even if the number of conventional component specifications is reduced. Yes.
[0007]
[Means for Solving the Problems]
The inventor diligently studied to achieve the above object, and the results were embodied in the present invention.
[0008]
That is, the present invention includes C: 0.15-0.22 mass%, Si: 1.0 mass% or less, Mn: 0.25-1.00 mass%, P: 0.02 mass% or less, S: 0.01% by mass or less, Cr: 12.0 to 14.0% by mass, Cu: 0.25% by mass or less, Ni: 0.50% by mass or less, V: 0.200% by mass or less, Nb: 0. When producing a steel pipe containing 050 mass% or less, B: 20 ppm or less, and the balance being Fe and inevitable impurities, by tempering at the end of the production process,
The multiple regression analysis is performed between the yield strength, which is the target product characteristic of the product steel pipe, and the operating factors that affect it, and the multiple regression equation shown in the following equation (3) is obtained, and the equation (3) is used, Depending on the outer diameter and thickness of the product steel pipe and the value of the yield strength, which is the target product characteristic, the values of the component indices defined by the following formula (1) are the values of V, Nb, and B in the material. A method for determining a material for martensitic stainless steel pipe, wherein the material is adjusted and determined so as to be within a range of ± 20 .
Ys (N / mm ² ) = 2022.413 + 0.296 · OD−1.210 · WT−0.082 · T · Parameter + 6.034 · 10 ⁻² · C + 0.628 · 10 ⁻³ · V +
7.992 · 10 ⁻³ · Nb + 0.730 · 10 ⁻⁴ · B (3)
Here, Ys is the target yield strength (N / mm ² ) of the product steel pipe, OD is the outer diameter (mm) of the product steel pipe, WT is the wall thickness (mm) of the product steel pipe, and T · Parameter is the following equation (2) It is a tempering index shown by.
Component index = V (× 10 ⁻³ ) +10 Nb (× 10 ⁻³ ) +2 C (× 10 ⁻² ) +2.5 B (× 10 ⁻⁴ ), unit: mass% (1)
Tempering index = {tempering temperature (° C.) + 273} × {20 + log (soaking time (min) / 60)} (2)
Further, the present invention provides a martensite characterized in that, instead of the yield strength of the target product characteristic of the product steel pipe and the formula (3), the target product characteristic is the tensile strength, and the formula (3) is the following formula (4). This is a method for determining a material for a site-based stainless steel pipe.
^{Ts (N / mm 2) =} 2082.506 + 0.294 · OD-1.068 · WT-0.077 · T · Parameter + 7.940 · 10 -2 · C + 0.449 · 10 -3 · V +
6.448 · 10 ⁻³ · Nb + 0.690 · 10 ⁻⁴ · B (4)
Here, Ts is the target tensile strength (N / mm ² ) of the product steel pipe .
Furthermore, the present invention provides a martensite characterized in that, instead of the yield strength of the target product characteristic of the product steel pipe and the formula (3), the target product characteristic is Rockwell hardness, and the formula (3) is the following formula (5). This is a method for determining a material for a site-based stainless steel pipe.
HRC (-) = 84.131 + 0.004 · OD-0.085 · WT-0.004 · T · Parameter + 0.281 · 10 -2 · C + 0.014 · 10 -3 · V + 0.347 · 10 -3 · Nb -0.029 · 10 ^-4 · B (5)
Here, HRC is the target Rockwell hardness (-) of the product steel pipe.
[0009]
In this case, the steel pipe is a seamless steel pipe, it may be either electric resistance welded steel pipe or butt-welded steel pipe.
[0010]
According to this invention, even if it is a raw material which deviates from a component specification, a target steel pipe characteristic can be achieved by adjustment of tempering conditions. As a result, the number of component specifications can be reduced as compared to the conventional method, and not only the melting load of the material is reduced, but also the production cost of the material is reduced. Moreover, the labor saving of the heat energy in tempering is achieved.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below based on the circumstances leading to the invention.
[0012]
First, the main chemical composition of the martensitic stainless steel pipe that is the subject of the present invention is C: 0.15-0.22 mass% or less, Si: 1.0 mass% or less, Mn: 0.25-1. 00 mass%, P: 0.02 mass% or less, S: 0.01 mass% or less, Cr: 12.0 to 14.0 mass%, Cu: 0.25 mass% or less, Ni: 0.50 mass% The following. This is a composition defined as L80 grade (13Cr) in the API standard. In the present invention, the material obtained by further adding V, Nb, and B to the composition of L80 is used. The reason for adding them is as follows.
V: 0.200 mass% or less,
V combines with C or N and precipitates at austenite grain boundaries to suppress crystal grain growth, thereby improving the strength of the steel. However, excessive addition leads to embrittlement of grain boundaries, and in particular causes defects in the slab during bending with a continuous casting machine, so the upper limit is kept to 0.200% by mass.
Nb: 0.050% by mass or less Nb, like V, combines with C or N and precipitates at the austenite grain boundaries and has the effect of precipitation strengthening in the steel. Moreover, since the deposition temperature is higher than V, the same effect is exhibited by addition of a small amount compared to V. Therefore, from the economical viewpoint, the upper limit was made 0.050% by mass.
B: 0.0020% by mass or less B increases the quenching hardenability of the steel material by adding a small amount, but if added excessively, the steel becomes brittle, so the upper limit was made 0.0020% by mass.
These components are standardized so as to correspond to the above five grades, and the raw materials are melted at a steelmaking factory (see Table 1). However, as described above, melting many kinds of materials increases the cost, so it is preferable to reduce the number of standards as much as possible. In other words, it is only necessary to obtain a product steel pipe with different characteristics even with the same standard material.
[0013]
Therefore, the inventor examined factors affecting product characteristics. At that time, when a multiple correlation analysis was performed on the yield strength, tensile strength and hardness of the product steel pipe, the relationship shown in Table 2 was obtained. That is, it will be understood from the example of the yield strength that a multiple correlation represented by the following equation (3) is established between the strength and the selected factor.
^{Ys (N / mm 2) =} 2022.413 + 0.296 · OD-1.210 · WT-0.082T · Parameter + 6.034 × 10 -2 · C + 0.628 × 10 -3 · V + 7.992 × 10 -3 · Nb + 0.730 × 10 ⁻⁴ · B (3)
Here, OD is the outer diameter (mm) of the product steel pipe, WT is the wall thickness (mm), and T · Parameter is a tempering index, and is generally expressed by equation (2).
[0014]
Tempering index = [tempering temperature (° C.) + 273] × [20 + log (soaking time (min) / 60)] (2)
[0015]
[Table 2]

[0016]
This is a very large relationship with a multiple correlation coefficient of 0.93. Therefore, when the relationship between the conventional Ys performance data and Ys calculated by the above equation is examined, as shown in FIG. The actual values were consistent. The tensile strength Ts and the hardness (in this case, the Rockwell hardness HRC was adopted) were very good as shown in FIGS. 2 and 3, respectively.
[0017]
This result shows that once the outer diameter and thickness of the product steel pipe are determined, even if the components fluctuate to some extent, they can be offset by changing the tempering index. That is, even if the component specifications are slightly different, Ys, Ts or HRC of the product steel pipe can be made the same. In other words, if the tempering index is determined in advance, the ingredients of the material can be determined in reverse. Therefore, the inventor creates C, V, Nb, and B, which are important components, collectively as a component index shown in the following formula (1), and finds a range that can be compensated by the tempering index even if the value fluctuates. I made it. As a result, it was found that the value was within the range of ± 20, and the present invention was completed.
Component index = V (× 10 ⁻³ ) +10 Nb (× 10 ⁻³ ) +2 C (× 10 ⁻² ) +2.5 B (× 10 ⁻⁴ ), unit: mass% (1)
Furthermore, the amount of contamination of V and B steel materials increases or decreases depending on the grade of Cr ore, and it is necessary to change the tempering temperature in order to obtain predetermined mechanical characteristics. At that time, even if the furnace has sufficient processing capacity, we want to reduce the loss time due to the furnace temperature change in order to avoid any economic loss. For this purpose, the maximum contamination amount of V and B is assumed, and when the contamination is small, V and B may be added positively. However, the positive addition increases the economic loss. Therefore, in the present invention, the correlation with the mechanical characteristics of each element is indexed with the component index, and adjustment is made by adding a small amount of Nb having a relatively strong correlation.
[0018]
Hereinafter, the present invention will be specifically described in Examples.
[0019]
Example 1
A seamless steel pipe having a Ys of 550 to 650 MPa was manufactured from martensitic stainless steel. Moreover, since the production amount was small, there was a margin for freely setting the tempering temperature and tempering time. Therefore, in the past, materials corresponding to the component specifications as shown in Comparative Example 1 in Table 3 were melted by adding expensive ferrochrome in the converter, but the B and V deviated from those standards to the higher one. It was decided to manufacture by adjusting the value of the tempering index with the component materials.
[0020]
Therefore, setting the expression (2) of the tempering temperature to rise to 72 5 ° C. than conventional 700 ° C., tempering time (i.e., (2) that the soaking time) long 50 minutes from the conventional 30 minutes did. As a result, it was found that the value of the component index in the above formula (1) may be increased by 714 from the conventional value due to the balance with the tempering index in the formula (2). That is, as shown in the present inventions 1 and 1 ' in Table 3, B and V contained in the material can be increased by the increased amount.
[0021]
In order to actually confirm this, inexpensive or low-purity (a lot of B and V) Cr ore is smelted and reduced in a converter to produce a molten metal for stainless steel production, and then decarburized in the converter. Martensitic stainless steel was melted. Then, the obtained molten steel was continuously cast into a steel slab, and then a round billet was formed at a lump factory, and a seamless steel pipe was manufactured by the mandrel mill method using the billet as a raw material. Tempering at that time was performed for 50 minutes at the set temperature, and the yield strength Ys of the product steel pipe was measured later. As a result, a value of 602 MPa was obtained, which was almost the same as that manufactured with a material having a conventional component standard.
[0022]
[Table 3]

[0023]
(Example 2)
An electric resistance welded steel pipe made of martensitic stainless steel and having a Ys of 550 to 650 MPa was manufactured. Since the production amount was large, there is no room for freely setting the tempering temperature and the tempering time (that is, the soaking time of (2)) . Therefore, the material in order to minimize short the tempering time B, V, reducing the Nb, further set the (2) 7 5 5 ℃ tempering temperature is the maximum capacity of the furnace of Formula. As a result, it has been found that the value of the component index in the above formula (1) may be made smaller by 430 than the conventional value in balance with the tempering index in the formula (2). That is, the tempering time can be shortened to 5 minutes as shown in the present invention 2 in Table 4 by the smaller amount.
[0024]
In order to actually confirm this, the converter uses cheap or low purity (a lot of B and V) Cr ore, but the input amount is regulated to reduce the V and B molten steel contamination, thereby reducing the martensite system. Stainless steel was melted. Then, the molten steel was continuously cast into a steel slab, and then a hot-rolled steel strip was formed at a rolling mill, and an electric resistance steel pipe was manufactured using the steel strip as a raw material. Tempering at that time was performed at the set temperature for 5 minutes, and the yield strength Ys of the product steel pipe was measured later. As a result, a value of 608 MPa was obtained, which was almost the same as that manufactured with a material having a conventional component standard.
[0025]
[Table 4]

[0026]
In addition, these results show that smelting reduction of low-cost raw material Cr ore that could not be used to prevent the variation of V can be applied to the melting of the raw material, which greatly reduces the raw material manufacturing cost. Furthermore, as shown in Table 5, it was possible to integrate some of the component specifications that were conventionally provided for each product grade and to reduce the number of component specifications. In addition, the tempering time can be shortened, saving energy.
[0027]
[Table 5]

[0028]
【The invention's effect】
As described above, according to the present invention, a martensitic stainless steel pipe satisfying the API grade can be manufactured even if the number of component standards is smaller than that of the conventional one. As a result, the smelting reduction method of Cr ore, which could not be used for melting of the material, can be adopted, and the material production cost can be reduced. In addition, by integrating some of the component specifications that had been provided for each product grade, not only could the number of component specifications be reduced, but also the tempering time could be shortened, saving energy.
[Brief description of the drawings]
FIG. 1 is a diagram showing the relationship between the calculated value and the actual value of the yield strength Ys of a steel pipe that forms the basis of the present invention.
FIG. 2 is a diagram showing the relationship between the calculated value and the actual value of the tensile strength Ts of the steel pipe that forms the basis of the present invention.
FIG. 3 is a diagram showing the relationship between the calculated value and the actual value of the Rockwell hardness HRC of the steel pipe that forms the basis of the present invention.

Claims (4)

C: 0.15-0.22 mass%, Si: 1.0 mass% or less, Mn: 0.25-1.00 mass%, P: 0.02 mass% or less, S: 0.01 mass% or less , Cr: 12.0 to 14.0 mass%, Cu: 0.25 mass% or less, Ni: 0.50 mass% or less, V: 0.200 mass% or less, Nb: 0.050 mass% or less, B When the steel pipe containing 20 ppm or less and the balance being made of Fe and inevitable impurities is subjected to a tempering process at the end of the manufacturing process,
The multiple regression analysis is performed between the yield strength, which is the target product characteristic of the product steel pipe, and the operating factors that affect it, and the multiple regression equation shown in the following equation (3) is obtained, and the equation (3) is used, the outer diameter of the product steel pipe, according to the value of the yield strength of a thickness and the target product properties, V Filling, Nb, the value of B, the following (1) initially value the value of the component index defined in formula The method for determining the material for martensitic stainless steel pipes is characterized by adjusting and setting so as to fall within the range of ± 20 .
Ys (N / mm ² ) = 2022.413 + 0.296 · OD−1.210 · WT−0.082 · T · Parameter + 6.034 · 10 ⁻² · C + 0.628 · 10 ⁻³ · V +
7.992 · 10 ⁻³ · Nb + 0.730 · 10 ⁻⁴ · B (3)
Here, Ys is the target yield strength (N / mm ² ) of the product steel pipe, OD is the outer diameter (mm) of the product steel pipe, WT is the wall thickness (mm) of the product steel pipe, and T · Parameter is the following equation (2) It is a tempering index shown by.
Component index = V (× 10 ⁻³ ) +10 Nb (× 10 ⁻³ ) +2 C (× 10 ⁻² ) +2.5 B (× 10 ⁻⁴ ), unit: mass% (1)
Tempering index = [tempering temperature (° C.) + 273] × [20 + log (soaking time (min) / 60)] (2) The martensite according to claim 1, wherein the yield strength of the target product characteristic of the product steel pipe and the target product characteristic is tensile strength, and the formula (3) is the following formula (4) instead of the formula (3). To determine the material for stainless steel pipes
^{Ts (N / mm 2) =} 2082.506 + 0.294 · OD-1.068 · WT-0.077 · T · Parameter + 7.940 · 10 -2 · C + 0.449 · 10 -3 · V +
6.448 · 10 ⁻³ · Nb + 0.690 · 10 ⁻⁴ · B (4)
Here, Ts is the target tensile strength (N / mm ² ) of the product steel pipe . The martensite according to claim 1 , wherein the target product characteristic is Rockwell hardness instead of the yield strength of the target product characteristic of the product steel pipe and the expression (3), and the expression (3) is the following expression (5). To determine the material for stainless steel pipes
HRC (-) = 84.131 + 0.004 · OD-0.085 · WT-0.004 · T · Parameter + 0.281 · 10 -2 · C + 0.014 · 10 -3 · V + 0.347 · 10 -3 · Nb -0.029 · 10 ^-4 · B (5)
Here, HRC is the target Rockwell hardness (-) of the product steel pipe.   The method for determining a material for a martensitic stainless steel pipe according to any one of claims 1 to 3, wherein the steel pipe is any one of a seamless steel pipe, an ERW steel pipe, and a forged steel pipe.

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