JP4155300B2 - Duplex stainless steel and manufacturing method thereof - Google Patents

Duplex stainless steel and manufacturing method thereof Download PDF

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JP4155300B2
JP4155300B2 JP2005512929A JP2005512929A JP4155300B2 JP 4155300 B2 JP4155300 B2 JP 4155300B2 JP 2005512929 A JP2005512929 A JP 2005512929A JP 2005512929 A JP2005512929 A JP 2005512929A JP 4155300 B2 JP4155300 B2 JP 4155300B2
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JPWO2005014872A1 (en
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朋彦 大村
聡 松本
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Sumitomo Metal Industries Ltd
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
    • C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
    • C21C7/04—Removing impurities by adding a treating agent
    • C21C7/064—Dephosphorising; Desulfurising
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
    • C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
    • C21C7/0087—Treatment of slags covering the steel bath, e.g. for separating slag from the molten metal
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
    • C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
    • C21C7/04—Removing impurities by adding a treating agent
    • C21C7/06—Deoxidising, e.g. killing
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten

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  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Mechanical Engineering (AREA)
  • Analytical Chemistry (AREA)
  • Treatment Of Steel In Its Molten State (AREA)
  • Heat Treatment Of Steel (AREA)
  • Cleaning And De-Greasing Of Metallic Materials By Chemical Methods (AREA)

Description

本発明は、海水中で優れた耐食性を有する二相ステンレス鋼に関する。この鋼は、熱交換用配管、化学プラント用の配管もしくは構造物、ラインパイプ、油井用もしくはガス井用のケーシングもしくはチュービング、または、アンビリカルチューブ（海底油井の制御用配管）等の鋼管もしくは鋼板等に用いられる。  The present invention relates to a duplex stainless steel having excellent corrosion resistance in seawater. This steel can be used for heat exchange pipes, chemical plant pipes or structures, line pipes, oil well or gas well casings or tubing, or umbilical tubes (submarine oil well control pipes). Used for.

従来、海底油井等から採掘される原油や天然ガスは作業環境が過酷なため、敬遠されてきたが、近年のエネルギー事情の逼迫に伴い、これらの原油や天然ガスを活用せざるを得ない情勢となってきている。このため、海水中で使用される鋼管または構造物等の材料として、耐孔食性に優れたステンレス鋼、特に二相ステンレス鋼の需要が高まっている。  Conventionally, crude oil and natural gas mined from subsea wells have been avoided because of the harsh working environment. However, with the recent tightening of energy situation, these crude oil and natural gas have to be used. It has become. For this reason, as a material such as a steel pipe or a structure used in seawater, there is an increasing demand for stainless steel excellent in pitting corrosion resistance, particularly duplex stainless steel.

特許文献１には、一般に二相ステンレス鋼の耐孔食性を向上に有効なＣｒ、ＭｏおよびＮ（窒素）の含有量を調整するとともに、Ｗを含有させて耐孔食性を高めた、いわゆるスーパー二相ステンレス鋼が開示されている。この文献には、二相ステンレス鋼の耐孔食性を表す指標として、一般に知られている下記の（Ａ）式の耐孔食性指標ＰＲＥ（Ｐｉｔｔｉｎｇ Ｒｅｓｉｓｔａｎｃｅ Ｅｑｕｉｖａｌｅｎｔ）のほか、Ｗを含む下記の（Ｂ）式のＰＲＥＷが提案されている。  Patent Document 1 generally describes a so-called super structure in which the contents of Cr, Mo, and N (nitrogen), which are effective in improving the pitting corrosion resistance of duplex stainless steels, are adjusted, and W is included to improve the pitting corrosion resistance. A duplex stainless steel is disclosed. In this document, in addition to the commonly known pitting corrosion resistance index PRE (Pitting Resistance Equivalent) of the following formula (A) as an index representing the pitting corrosion resistance of duplex stainless steel, the following (B) ) PREW has been proposed.

通常の二相ステンレス鋼では、耐孔食性指数ＰＲＥまたはＰＲＥＷを３５以上となるように調整され、更に、スーパー二相ステンレス鋼では４０以上となるように調整される。従来の耐孔食性の向上技術は、この耐孔食性指数ＰＲＥまたはＰＲＥＷをどれだけ高めるかに基づいて行われてきた。  For ordinary duplex stainless steel, the pitting corrosion resistance index PRE or PREW is adjusted to be 35 or more, and for super duplex stainless steel, it is adjusted to 40 or more. Conventional techniques for improving pitting corrosion resistance have been performed based on how much the pitting corrosion resistance index PRE or PREW is increased.

ＰＲＥ＝Ｃｒ＋３．３Ｍｏ＋１６Ｎ（窒素） （Ａ）
ＰＲＥＷ＝Ｃｒ＋３．３（Ｍｏ＋０．５Ｗ）＋１６Ｎ（窒素） （Ｂ）
なお、上記の（Ａ）式および（Ｂ）式中の各元素記号は、それぞれの元素の含有量（質量％）を示す。PRE = Cr + 3.3Mo + 16N (nitrogen) (A)
PREW = Cr + 3.3 (Mo + 0.5 W) +16 N (nitrogen) (B)
In addition, each element symbol in said (A) type | formula and (B) type | formula shows content (mass%) of each element.

二相ステンレス鋼においては、非金属介在物の耐孔食性に及ぼす影響は検討されていない。しかし、オーステナイト系ステンレス鋼の耐孔食性に関しては、非特許文献１に記載されるように、Ｍｎ硫化物が耐孔食性には最も有害であり、酸化物は無害であることが知られている。  In duplex stainless steel, the effect of nonmetallic inclusions on pitting corrosion resistance has not been studied. However, regarding pitting corrosion resistance of austenitic stainless steel, as described in Non-Patent Document 1, it is known that Mn sulfide is most harmful to pitting corrosion resistance and oxide is harmless. .

ステンレス鋼中に含まれる酸化物系介在物は、一般的には、Ａｌ酸化物（Ａｌ２Ｏ３）、Ｓｉ酸化物（ＳｉＯ２）、Ｃｒ酸化物（Ｃｒ２Ｏ３）等の酸化物からなる複合酸化物である。これらは水溶液中で溶けにくい性質、いわゆる不溶性を有するため、孔食に影響しないと考えられてきた。一方、鋼材中の不純物元素であるＣａおよびＭｇ、更にはＳが酸化物中に含有される可能性があるが、これらの元素が耐孔食性に及ぼす影響について調べた例は今までに無い。The oxide inclusions contained in stainless steel are generally from oxides such as Al oxide (Al 2 O 3 ), Si oxide (SiO 2 ), and Cr oxide (Cr 2 O 3 ). A composite oxide. It has been considered that these do not affect pitting corrosion because they are difficult to dissolve in an aqueous solution, so-called insolubility. On the other hand, Ca and Mg, which are impurity elements in steel, and S may be contained in the oxide, but there has been no example of examining the influence of these elements on pitting corrosion resistance.

特開平５−１３２７４１号公報JP-A-5-132741

Ｊ．Ｅ．Ｃａｓｔｌｅ、外１名、”Ｓｔｕｄｉｅｓ ｂｙ Ａｕｇｅｒ Ｓｐｅｃｔｒｏｓｃｏｐｙ ｏｆ ＰｉｔＩｎｉｔｉａｔｉｏｎ ａｔ ｔｈｅ ｓｉｔｅ ｏｆ Ｉｎｃｌｕｓｉｏｎｓ ｉｎ Ｓｔａｉｎｌｅｓｓ Ｓｔｅｅｌ”、Ｃｏｒｒｏｓｉｏｎ Ｓｃｉｅｎｃｅ、Ｖｏｌｕｍｅ３０、Ｎｏ．４／５、第４０９頁J. et al. E. Castle, 1 other, “Studies by Auger Spectroscopy of PitInitiation at the site of Infusions in Stainless Steel”, Corrosion Science, Volume 30, No. 1 4/5, page 409

近年では高温の海水環境等の過酷な腐食環境への二相ステンレス鋼の適用が増している。このような過酷な条件を模擬した腐食試験、例えば８０℃の塩化第二鉄試験等では、スーパー二相ステンレス鋼といえども十分な耐孔食性が安定して得られるわけではない。また、Ｃｒ、ＭｏおよびＮ（窒素）、更にはＷ等の含有量の調整だけでは耐孔食性の改善が不十分な場合がある。更に、二相ステンレス鋼でも、オーステナイト系ステンレス鋼と同様に、鋼中のＭｎ硫化物を低減することにより耐孔食性をある程度改善できるが、完全に孔食を防止することができるわけではない。  In recent years, the application of duplex stainless steel to severe corrosive environments such as high-temperature seawater environments has increased. In a corrosion test simulating such severe conditions, for example, a ferric chloride test at 80 ° C., sufficient pitting corrosion resistance is not stably obtained even with super duplex stainless steel. Moreover, improvement of pitting corrosion resistance may be insufficient only by adjusting the content of Cr, Mo, N (nitrogen), and W. Further, even with duplex stainless steel, pitting corrosion resistance can be improved to some extent by reducing Mn sulfide in the steel, as with austenitic stainless steel, but pitting corrosion cannot be completely prevented.

本発明は、これらの問題を解決するためになされたものであり、良好な耐孔食性が安定して得られる二相ステンレス鋼およびその製造方法を提供することを目的とする。  The present invention has been made to solve these problems, and an object of the present invention is to provide a duplex stainless steel that can stably obtain good pitting corrosion resistance and a method for producing the same.

本発明者らは、二相ステンレス鋼の耐孔食性に影響する冶金因子を詳細に調査した結果、前述した従来の孔食発生の要因だけでなく、溶製過程で生成する酸化物系介在物であっても、ＣａおよびＭｇを含有するもの、ならびにＳを含有するものは、耐孔食性に大きな影響を及ぼしうることを見出した。本発明者らの研究のよって得られた知見は、下記のとおりである。  As a result of detailed investigation of metallurgical factors affecting the pitting corrosion resistance of duplex stainless steel, the present inventors have found that not only the conventional pitting corrosion occurrence factors described above, but also oxide inclusions generated in the melting process Even so, it has been found that those containing Ca and Mg and those containing S can greatly affect pitting corrosion resistance. The knowledge obtained by the inventors' research is as follows.

Ｃａの含有量が０．０００５質量％未満の場合、またはＭｇの含有量が０．０００１質量％未満の場合に鋼中に形成される酸化物系介在物は、不溶性のＡｌ２Ｏ３を主体とするものであり、孔食が発生することはない。また、ＣａまたはＭｇの含有量が０．００５質量％を超える場合に鋼中に形成される酸化物系介在物は、（Ｃａ、Ｍｇ）Ｏを主体とするものであり、このような酸化物は、孔食の起点となりにくい。When the Ca content is less than 0.0005 mass%, or the Mg content is less than 0.0001 mass%, the oxide inclusions formed in the steel are mainly composed of insoluble Al 2 O 3 . And no pitting corrosion occurs. The oxide inclusions formed in the steel when the Ca or Mg content exceeds 0.005% by mass are mainly composed of (Ca, Mg) O, and such oxides. Is unlikely to be the starting point of pitting corrosion.

しかし、Ｃａを０．０００５〜０．００５質量％含み、且つＭｇを０．０００１〜０．００５質量％含む場合には、鋼中の形成される酸化物系介在物は、Ａｌ２Ｏ３と（Ｃａ、Ｍｇ）Ｏとが共存する状態となり、これらの酸化物系介在物が隣接して形成された場合には、孔食の起点となりやすくなる。However, when Ca is contained in 0.0005 to 0.005 mass% and Mg is contained in 0.0001 to 0.005 mass%, the oxide inclusions formed in the steel are Al 2 O 3 and When (Ca, Mg) O coexists, and these oxide inclusions are formed adjacent to each other, they tend to be the starting point of pitting corrosion.

このため、本発明者らは、Ｃａを０．０００５〜０．００５質量％含み、且つＭｇを０．０００１〜０．００５質量％含む二相ステンレス鋼の孔食発生原因について研究を重ねた結果、鋼中に形成された酸化物系介在物の大きさおよび個数によって、孔食が発生する場合と発生しない場合があることを見出した。  For this reason, the present inventors have conducted research on the cause of pitting corrosion of duplex stainless steel containing 0.0005 to 0.005 mass% of Ca and 0.0001 to 0.005 mass% of Mg. The present inventors have found that pitting corrosion may or may not occur depending on the size and number of oxide inclusions formed in the steel.

Ｓは、鋼中に不可避的に存在する元素であり、現在の製鋼技術ではその含有量を完全にゼロとすることはできない。Ｓは、鋼中に形成される酸化物系介在物中に多量に含まれると耐孔食性を劣化させるが、本発明者らの研究により、このような酸化物系介在物であっても、その大きさおよび個数を調整することによって孔食の発生を抑制することができることが判明した。  S is an element that inevitably exists in steel, and the content cannot be completely zero by current steelmaking technology. When S is contained in a large amount in the oxide inclusions formed in the steel, the pitting corrosion resistance is deteriorated. It has been found that the occurrence of pitting corrosion can be suppressed by adjusting the size and the number.

従来の方法による溶製や加工熱処理では、所望の酸化物系介在物状態の二相ステンレス鋼を製造することはできない。本発明者らは、種々の検討の結果、（α）還元処理時のスラグ塩基度、（β）取鍋でのキリング温度と時間、（γ）鋳造後の総加工比を最適の組合せに制御することにより、所望の酸化物系介在物状態が得られ、今までに無い高清浄鋼が製造できることを見出した。  A duplex stainless steel in a desired oxide inclusion state cannot be produced by melting or thermomechanical treatment by a conventional method. As a result of various studies, the present inventors have controlled (α) slag basicity during reduction treatment, (β) killing temperature and time in a ladle, and (γ) total processing ratio after casting to an optimal combination. As a result, it was found that a desired oxide-based inclusion state can be obtained, and an unprecedented highly clean steel can be produced.

このように本発明は、二相ステンレス鋼としての性能を確保できる鋼材の化学組成、耐孔食性を大きく向上させる酸化物系介在物状態、および高清浄化を達成するための製造条件に基づき完成させた。  Thus, the present invention has been completed based on the chemical composition of the steel material capable of ensuring the performance as a duplex stainless steel, the oxide inclusion state that greatly improves the pitting corrosion resistance, and the production conditions for achieving high cleaning. It was.

本発明は、下記の（ａ）ならびに（ｂ）に示す二相ステンレス鋼および下記の（ｃ）に示す二相ステンレス鋼の製造方法を要旨とする。  The gist of the present invention is a duplex stainless steel shown in the following (a) and (b) and a method for producing the duplex stainless steel shown in (c) below.

（ａ）質量％で、Ｃ：０．０３％以下、Ｓｉ：０．０１〜２％、Ｍｎ：０．１〜２％、Ｐ：０．０５％以下、Ｓ：０．００１％以下、Ａｌ：０．００３〜０．０５％、Ｎｉ：４〜１２％、Ｃｒ：１８〜３２％、Ｍｏ：０．２〜５％、Ｎ（窒素）：０．０５〜０．４％、Ｏ（酸素）：０．０１％以下、Ｃａ：０．０００５〜０．００５％、Ｍｇ：０．０００１〜０．００５％、Ｃｕ：０〜２％、Ｂ：０〜０．０１％およびＷ：０〜４％を含有し、残部がＦｅおよび不純物からなる二相ステンレス鋼であって、その中に含まれる介在物のうち、ＣａおよびＭｇの合計含有量が２０〜４０質量％であり、且つ長径が７μｍ以上である酸化物系介在物が加工方向に垂直な断面１ｍｍ２あたり１０個以下であることを特徴とする二相ステンレス鋼。(A) In mass%, C: 0.03% or less, Si: 0.01-2%, Mn: 0.1-2%, P: 0.05% or less, S: 0.001% or less, Al : 0.003-0.05%, Ni: 4-12%, Cr: 18-32%, Mo: 0.2-5%, N (nitrogen): 0.05-0.4%, O (oxygen) ): 0.01% or less, Ca: 0.0005 to 0.005%, Mg: 0.0001 to 0.005%, Cu: 0 to 2%, B: 0 to 0.01%, and W: 0 to 0 It is a duplex stainless steel containing 4% with the balance being Fe and impurities, and among the inclusions contained therein, the total content of Ca and Mg is 20 to 40% by mass, and the major axis is A duplex stainless steel characterized in that oxide inclusions having a size of 7 μm or more are 10 or less per 1 mm 2 in cross section perpendicular to the processing direction.

（ｂ）質量％で、Ｃ：０．０３％以下、Ｓｉ：０．０１〜２％、Ｍｎ：０．１〜２％、Ｐ：０．０５％以下、Ｓ：０．００１％以下、Ａｌ：０．００３〜０．０５％、Ｎｉ：４〜１２％、Ｃｒ：１８〜３２％、Ｍｏ：０．２〜５％、Ｎ（窒素）：０．０５〜０．４％、Ｏ（酸素）：０．０１％以下、Ｃａ：０．０００５〜０．００５％、Ｍｇ：０．０００１〜０．００５％、Ｃｕ：０〜２％、Ｂ：０〜０．０１％およびＷ：０〜４％を含有し、残部がＦｅおよび不純物からなる二相ステンレス鋼であって、その中に含まれる介在物のうち、ＣａおよびＭｇの合計含有量が２０〜４０質量％であり、且つ長径が７μｍ以上である酸化物系介在物が加工方向に垂直な断面１ｍｍ２あたり１０個以下であるとともに、Ｓの含有量が１５質量％以上であり、且つ長径が１μｍ以上である酸化物系介在物が加工方向に垂直な断面０．１ｍｍ２あたり１０個以下であることを特徴とする二相ステンレス鋼。(B) In mass%, C: 0.03% or less, Si: 0.01-2%, Mn: 0.1-2%, P: 0.05% or less, S: 0.001% or less, Al : 0.003-0.05%, Ni: 4-12%, Cr: 18-32%, Mo: 0.2-5%, N (nitrogen): 0.05-0.4%, O (oxygen) ): 0.01% or less, Ca: 0.0005 to 0.005%, Mg: 0.0001 to 0.005%, Cu: 0 to 2%, B: 0 to 0.01%, and W: 0 to 0 It is a duplex stainless steel containing 4% with the balance being Fe and impurities, and among the inclusions contained therein, the total content of Ca and Mg is 20 to 40% by mass, and the major axis is with oxide inclusions is 7μm or more is 10 or less the cross section perpendicular 1 mm 2 per the working direction, with the content of S 15 mass% or more Ri, and the major axis is duplex stainless steel, wherein the oxide-based inclusions is 1μm or more and 10 or less the cross section perpendicular 0.1 mm 2 per the working direction.

なお、上記（ａ）または（ｂ）に記載の鋼は、そのＣｕ、ＢおよびＷの含有量がそれぞれ質量％で０．２〜２％、０．００１〜０．０１％および０．１〜４％であるのが望ましい。また、下記の（１）式で表される耐孔食性指数ＰＲＥＷが４０以上であるのが望ましい。但し、（１）式中の各元素記号は、それぞれの元素の含有量（質量％）を意味する。

Figure 0004155300
In addition, as for the steel as described in said (a) or (b), the content of Cu, B, and W is 0.2-2% by mass%, 0.001-0.01%, and 0.1-0.1%, respectively. 4% is desirable. Moreover, it is desirable that the pitting corrosion resistance index PREW represented by the following formula (1) is 40 or more. However, each element symbol in the formula (1) means the content (% by mass) of each element.
Figure 0004155300

（ｃ）下記の（２）式で表されるスラグ塩基度が０．５〜３．０となる条件で還元し、出鋼した溶鋼に、１５００℃以上の温度で５分以上のキリングを実施した後に鋳造し、得られた鋳片を下記の（３）式で表される総加工比Ｒが１０以上となる条件で加工することを特徴とする上記（ａ）または（ｂ）に記載の二相ステンレス鋼の製造方法。但し、（２）式中の各化合物は、それぞれの化合物のスラグ中濃度（質量％）を意味する。また、（３）式中のＡ０ｎは塑性変形工程での変形前の断面積、Ａｎは塑性変形工程での変形後の断面積を意味し、それぞれの添字ｎ（１、２、…ｉ）は、塑性変形工程の各スタンド順を意味する。

Figure 0004155300
(C) The slag basicity represented by the following formula (2) is reduced under the condition that the basicity of the slag is 0.5 to 3.0, and the molten steel produced is killed for 5 minutes or more at a temperature of 1500 ° C. After casting, the obtained slab is processed under the condition that the total processing ratio R represented by the following formula (3) is 10 or more, as described in (a) or (b) above A method for producing duplex stainless steel. However, each compound in the formula (2) means the concentration (% by mass) in the slag of each compound. Further, the cross-sectional area before deformation in A0 n plastic deformation process in (3), A n denotes the cross-sectional area after deformation in the plastic deformation process, each subscript n (1, 2, ... i ) Means the order of each stand in the plastic deformation process.
Figure 0004155300

本発明によれば、良好な耐孔食性を有する二相ステンレス鋼が安定して得られる。このため、例えば、熱交換用配管、化学プラント用の配管もしくは構造物、ラインパイプ、油井用もしくはガス井用のケーシングもしくはチュービング、または、アンビリカルチューブ（海底油井の制御用配管）等の鋼管もしくは鋼板等に最適な二相ステンレス鋼を提供することができる。  According to the present invention, a duplex stainless steel having good pitting corrosion resistance can be obtained stably. For this reason, for example, steel pipes or steel plates such as heat exchange pipes, chemical plant pipes or structures, line pipes, oil well or gas well casings or tubing, or umbilical tubes (submarine oil well control pipes). It is possible to provide the optimum duplex stainless steel.

１．化学組成
鋼材の化学組成は、二相ステンレス鋼としての十分な耐孔食性を確保するためには、下記の範囲とする必要がある。以下の説明において、含有量についての「％」は「質量％」を意味する。1. Chemical composition The chemical composition of the steel material must be in the following range in order to ensure sufficient pitting corrosion resistance as a duplex stainless steel. In the following description, “%” for the content means “% by mass”.

Ｃ：０．０３％以下
Ｃは、鋼中に不可避的に存在する。その含有量が０．０３％を超えると炭化物が析出し易くなり耐孔食性が低下する。従って、Ｃの含有量を０．０３％以下とした。C: 0.03% or less C is unavoidably present in steel. If the content exceeds 0.03%, carbides are liable to precipitate and pitting corrosion resistance is reduced. Therefore, the content of C is set to 0.03% or less.

Ｓｉ：０．０１〜２％
Ｓｉは、鋼の脱酸に有効な元素であり、０．０１％以上含有させる必要がある。しかし、その含有量が２％を超えると、金属間化合物の生成を促進し、耐孔食性を低下させる。従って、Ｓｉの含有量を０．０１〜２％とした。Si: 0.01-2%
Si is an element effective for deoxidation of steel and needs to be contained by 0.01% or more. However, if its content exceeds 2%, the formation of intermetallic compounds is promoted and the pitting corrosion resistance is lowered. Therefore, the Si content is set to 0.01 to 2%.

Ｍｎ：０．１〜２％
Ｍｎは、Ｎｉと同様にオーステナイト相を安定化させるのに有効であり、０．１％以上含有させる必要がある。一方、２％を超えるＭｎを含有させると耐孔食性を低下させる。従って、Ｍｎの含有量を０．１〜２％とした。Mn: 0.1 to 2%
Mn is effective for stabilizing the austenite phase like Ni, and needs to be contained in an amount of 0.1% or more. On the other hand, when Mn exceeding 2% is contained, the pitting corrosion resistance is lowered. Therefore, the Mn content is set to 0.1 to 2%.

Ｐ：０．０５％以下
Ｐは、不純物として鋼中に不可避的に存在し、活性溶解して耐孔食性を低下させる。含有量が０．０５％を超えるとこの影響が顕著となるため、その含有量は０．０５％以下にする必要がある。Ｐの含有量は、できるだけ低くすることが望ましい。P: 0.05% or less P is unavoidably present in steel as an impurity, and is actively dissolved to reduce pitting corrosion resistance. If the content exceeds 0.05%, this effect becomes significant, so the content needs to be 0.05% or less. The content of P is desirably as low as possible.

Ｓ：０．００１％以下
ＳもＰと同様に鋼中に不可避的に存在し、溶解し易い硫化物を生成することで耐孔食性を低下させる。含有量が０．００１％を超えるとこの影響が顕著となる。また後述のように、０．００１％以下の含有量でも、酸化物系介在物中に含有された場合に孔食発生を助長するため、Ｓの含有量は、この範囲でできるだけ低くすることが望ましい。S: 0.001% or less S, like P, is unavoidably present in steel, and lowers the pitting corrosion resistance by forming a sulfide that is easily dissolved. When the content exceeds 0.001%, this effect becomes significant. Further, as will be described later, even if the content is 0.001% or less, the content of S can be made as low as possible within this range in order to promote the occurrence of pitting corrosion when contained in oxide inclusions. desirable.

Ａｌ：０．００３〜０．０５％
Ａｌは、鋼の脱酸に必要な元素であり、０．００３％以上含有させる必要がある。一方、過剰に含有させるとＡｌ窒化物が析出し耐孔食性向上に有効な元素であるＮ（窒素）を吸収し、耐孔食性を低下させる。従って、Ａｌの含有量を０．００３〜０．０５％とした。なお、Ａｌは「ｓｏｌ．Ａｌ（酸可溶Ａｌ）」を意味する。Al: 0.003-0.05%
Al is an element necessary for deoxidation of steel and should be contained by 0.003% or more. On the other hand, if it is contained excessively, Al nitride precipitates and absorbs N (nitrogen), which is an effective element for improving pitting corrosion resistance, and lowers pitting corrosion resistance. Therefore, the Al content is set to 0.003 to 0.05%. Al means “sol.Al (acid-soluble Al)”.

Ｎｉ：４〜１２％
Ｎｉは、オーステナイト相を安定化する元素であり、４％未満ではその効果が不十分である。一方、１２％を超えるとオーステナイト相が過多となり二相ステンレス鋼としての機械的性質が損なわれる。従って、Ｎｉの含有量を４〜１２％とした。Ni: 4-12%
Ni is an element that stabilizes the austenite phase, and its effect is insufficient when it is less than 4%. On the other hand, if it exceeds 12%, the austenite phase becomes excessive and the mechanical properties as a duplex stainless steel are impaired. Therefore, the Ni content is 4 to 12%.

Ｃｒ：１８〜３２％
Ｃｒは、耐孔食性の向上に有効であり、その含有量が１８％未満では耐孔食性が不十分となる。一方、その含有量が３２％を超えるとフェライト相が過多となり二相ステンレス鋼としての機械的性質が損なわれる。従って、Ｃｒの含有量を１８〜３２％とした。Cr: 18-32%
Cr is effective for improving the pitting corrosion resistance, and if its content is less than 18%, the pitting corrosion resistance becomes insufficient. On the other hand, if its content exceeds 32%, the ferrite phase becomes excessive and the mechanical properties as a duplex stainless steel are impaired. Therefore, the Cr content is set to 18 to 32%.

Ｍｏ：０．２〜５％
ＭｏもＣｒと同様に耐孔食性を高める元素であり、０．２％未満ではその効果が十分でない。一方、５％を超えると金属間化合物が析出して、逆に耐孔食性を低下させる。従って、Ｍｏの含有量を０．２〜５％とした。Mo: 0.2 to 5%
Mo, like Cr, is an element that increases pitting corrosion resistance, and its effect is not sufficient when it is less than 0.2%. On the other hand, when it exceeds 5%, an intermetallic compound is precipitated, and conversely, the pitting corrosion resistance is lowered. Therefore, the Mo content is set to 0.2 to 5%.

Ｎ（窒素）：０．０５〜０．４％
Ｎ（窒素）は、Ｎｉと同様に、オーステナイト相を安定化させる作用を持つ元素である。Ｎ（窒素）は、ＣｒやＭｏと同様に耐孔食性を高める効果を有する元素でもある。しかし、その含有量が０．０５％未満ではこれらの効果が不十分である。一方、０．４％を越えて含有させると熱間加工性が低下する。従って、Ｎ（窒素）の含有量を０．０５〜０．４％とした。N (nitrogen): 0.05 to 0.4%
N (nitrogen) is an element having an action of stabilizing the austenite phase, similarly to Ni. N (nitrogen) is also an element having an effect of enhancing pitting corrosion resistance like Cr and Mo. However, if the content is less than 0.05%, these effects are insufficient. On the other hand, if the content exceeds 0.4%, the hot workability decreases. Therefore, the content of N (nitrogen) is set to 0.05 to 0.4%.

Ｏ（酸素）：０．０１％以下
Ｏ（酸素）もＳと同様に鋼中に不可避的に存在し、酸化物系介在物の状態で存在する。酸化物は、後述のように組成によっては孔食起点となり耐孔食性を低下させ、特に、その含有量が０．０１％を超えると粗大な酸化物が増し、この傾向が顕著となる。従って、Ｏ（酸素）は０．０１％以下に制限する必要がある。Ｏ（酸素）の含有量はできるだけ低くすることが望ましい。O (oxygen): 0.01% or less O (oxygen) is inevitably present in the steel as in S, and is present in the form of oxide inclusions. As will be described later, the oxide serves as a starting point for pitting corrosion and lowers pitting corrosion resistance. In particular, when the content exceeds 0.01%, coarse oxide increases, and this tendency becomes remarkable. Therefore, O (oxygen) needs to be limited to 0.01% or less. It is desirable that the content of O (oxygen) be as low as possible.

Ｃａ：０．０００５〜０．００５％、Ｍｇ：０．０００１〜０．００５％
ＣａおよびＭｇは、いずれもＳを硫化物として固定することにより、鋼の熱間加工性を改善する効果を有する元素である。しかし、前述のように、Ｃａ：０．０００５〜０．００５％およびＭｇ：０．０００１〜０．００５％を含む二相ステンレス鋼においては、Ａｌ２Ｏ３と（Ｃａ、Ｍｇ）Ｏとが共存し、これらが隣接して形成された場合、耐孔食性に悪影響を及ぼす。従って、ＣａおよびＭｇの含有量は、それぞれ耐孔食性が劣化しやすい範囲である０．０００５〜０．００５％および０．０００１〜０．００５％と規定した。本発明の二相ステンレス鋼の耐孔食性は、後段で説明するように、酸化物系介在物状態を規制することにより改善される。Ca: 0.0005 to 0.005%, Mg: 0.0001 to 0.005%
Ca and Mg are both elements that have the effect of improving the hot workability of steel by fixing S as sulfides. However, as described above, in the duplex stainless steel containing Ca: 0.0005 to 0.005% and Mg: 0.0001 to 0.005%, Al 2 O 3 and (Ca, Mg) O are contained. When they coexist and are formed adjacent to each other, the pitting corrosion resistance is adversely affected. Therefore, the contents of Ca and Mg are defined as 0.0005 to 0.005% and 0.0001 to 0.005%, which are ranges where the pitting corrosion resistance is likely to deteriorate, respectively. The pitting corrosion resistance of the duplex stainless steel of the present invention is improved by regulating the oxide inclusion state as will be described later.

本発明の二相ステンレス鋼は、上記の化学組成を有し、残部がＦｅおよび不純物からなる鋼である。また、本発明の二相ステンレス鋼は、任意の添加元素として、Ｃｕ、ＢおよびＷの一種以上を含有するものであってもよい。  The duplex stainless steel of the present invention is a steel having the chemical composition described above, with the balance being Fe and impurities. Moreover, the duplex stainless steel of this invention may contain 1 or more types of Cu, B, and W as arbitrary addition elements.

Ｃｕ：０〜２％
Ｃｕは、Ｎｉと同様にオーステナイト相を安定化させる。また、硫化水素環境で硫化物皮膜を安定化して耐孔食性を向上させる。従って、必要に応じてＣｕを含有させてもよい。上記の効果を得るためには０．２％以上含有させるのが望ましいが、２％を越えて含有させると熱間加工性が低下する。従って、Ｃｕを含有させる場合には、その含有量を０．２〜２％とするのが望ましい。Cu: 0 to 2%
Cu stabilizes the austenite phase like Ni. It also stabilizes the sulfide film in a hydrogen sulfide environment to improve pitting corrosion resistance. Therefore, you may contain Cu as needed. In order to acquire said effect, it is desirable to contain 0.2% or more, but when it contains exceeding 2%, hot workability will fall. Therefore, when Cu is contained, the content is desirably 0.2 to 2%.

Ｂ：０〜０．０１％
Ｂは、熱間加工性の改善に有効な元素であるため、必要に応じて含有させてもよい。この効果を得るためにはその含有量を０．００１％以上とするのが望ましいが、０．０１％を越えて含有させてもその効果は飽和する。従って、Ｂを含有させる場合には、その含有量を０．００１〜０．０１％とするのが望ましい。B: 0 to 0.01%
B is an element effective for improving hot workability, and may be contained as necessary. In order to obtain this effect, the content is desirably 0.001% or more, but even if the content exceeds 0.01%, the effect is saturated. Therefore, when B is contained, the content is desirably 0.001 to 0.01%.

Ｗ：０〜４％
Ｗは、ＣｒやＭｏと同様に耐孔食性の向上に有効な元素であるため、必要に応じて含有させてもよい。これらの効果は、その含有量が０．１％以上の場合に顕著となる。しかし、４％を越えて含有させると金属間化合物が析出し耐孔食性がかえって低下する。従って、Ｗを含有させる場合には、その含有量を０．１〜４％とするのが望ましい。W: 0-4%
W is an element effective for improving the pitting corrosion resistance like Cr and Mo, and may be contained as necessary. These effects become significant when the content is 0.1% or more. However, if the content exceeds 4%, an intermetallic compound is precipitated and the pitting corrosion resistance is lowered. Therefore, when it contains W, it is desirable to make the content into 0.1 to 4%.

２．耐孔食性指数
本発明の二相ステンレス鋼は、上記の化学組成を有し、且つ、下記に定義される耐孔食性指数が４０以上のスーパー二相ステンレス鋼であるのが望ましい。但し、（１）式中の各元素記号は、それぞれの元素の含有量（質量％）を意味する。

Figure 0004155300
2. Pitting Corrosion Resistance Index The duplex stainless steel of the present invention is preferably a super duplex stainless steel having the above chemical composition and having a pitting corrosion index defined below of 40 or more. However, each element symbol in the formula (1) means the content (% by mass) of each element.
Figure 0004155300

３．酸化物系介在物の条件
本発明者らは、酸化物系介在物が耐孔食性に及ぼす影響を以下の手法で調査した。3. Conditions for oxide inclusions The inventors investigated the effect of oxide inclusions on pitting corrosion resistance by the following method.

後述の表３および４に示す化学組成を有する溶鋼を種々の条件で加工し、肉厚１．４〜１６（ｍｍ）の二相ステンレス鋼管を作製した。これらの鋼管を扁平した後、管肉厚×１０ｍｍ×１０ｍｍの試験片を切り出した。この試験片の加工方向と垂直な断面（図１に示す「観察面」）方向に樹脂を埋込んだ後、この断面を鏡面研磨した。研磨表面を走査電子顕微鏡（ＳＥＭ）を用いて観察し、酸化物系介在物の長径および化学組成を測量した。  Molten steel having the chemical composition shown in Tables 3 and 4 to be described later was processed under various conditions to produce a duplex stainless steel pipe having a wall thickness of 1.4 to 16 (mm). After flattening these steel tubes, a test piece having a tube thickness of 10 mm × 10 mm was cut out. After embedding resin in the direction of the cross section (“observation surface” shown in FIG. 1) perpendicular to the processing direction of the test piece, this cross section was mirror-polished. The polished surface was observed using a scanning electron microscope (SEM), and the major axis and chemical composition of the oxide inclusions were measured.

酸化物系介在物の長径とは、図２に示すように、母材と介在物との界面上の異なる二点を結んだ直線のうち、最も長くなる直線の長さ（ａ１またはａ２）を意味する。また、酸化物系介在物の組成は、介在物の中心部近傍（図２に示す例ではｂ１またはｂ２）、即ち、介在物の断面形状の重心部近傍をＥＤＸ（エネルギー分散型Ｘ線分析）を用いて、Ｏ（酸素）以外の合金元素の含有量を求めた。  As shown in FIG. 2, the major axis of the oxide-based inclusion is the length of the longest straight line (a1 or a2) among two straight lines connecting two different points on the interface between the base material and the inclusion. means. The composition of oxide inclusions is EDX (energy dispersive X-ray analysis) near the center of the inclusion (b1 or b2 in the example shown in FIG. 2), that is, near the center of gravity of the cross-sectional shape of the inclusion. Was used to determine the content of alloy elements other than O (oxygen).

酸化物系介在物を観察した後、８０℃の６％塩化第二鉄水溶液中に６時間浸漬し、酸化物系介在物周辺の腐食状況を観察した結果、一部の試験片に酸化物系介在物を起点とした孔食が観察された。孔食を起こした酸化物系介在物は、Ａｌ２Ｏ３と（Ｃａ、Ｍｇ）Ｏからなる複合酸化物であり、（Ｃａ、Ｍｇ）Ｏの部分が優先的に溶出し、母材との間に隙間を形成し、そこから孔食に進展していた。After observing oxide inclusions, they were immersed in a 6% aqueous ferric chloride solution at 80 ° C. for 6 hours, and the corrosion conditions around the oxide inclusions were observed. Pitting corrosion starting from inclusions was observed. The oxide inclusions causing pitting corrosion are composite oxides composed of Al 2 O 3 and (Ca, Mg) O, and the (Ca, Mg) O portion is preferentially eluted, A gap was formed between them, and progressed from there to pitting corrosion.

そこで、生成した酸化物系介在物のそれぞれをＳＥＭで観察し、酸化物介在物と孔食の有無との関係について調査した。  Therefore, each of the generated oxide inclusions was observed by SEM, and the relationship between the oxide inclusions and the presence or absence of pitting corrosion was investigated.

図３は、酸化物系介在物の長径とＣａおよびＭｇの合計含有量との関係を示す図である。なお、図３中の「×」は孔食の起点となった酸化物系介在物、「○」は孔食の起点とならなかった酸化物系介在物を意味する。  FIG. 3 is a diagram showing the relationship between the major axis of oxide inclusions and the total content of Ca and Mg. In FIG. 3, “x” means an oxide inclusion that is the starting point of pitting corrosion, and “◯” means an oxide inclusion that does not become the starting point of pitting corrosion.

図３に示すように、ＣａおよびＭｇの含有量が合計で２０〜４０％であり、且つ長径が７μｍ以上である酸化物が孔食の起点となった。しかし、ＣａおよびＭｇの含有量が合計で２０％未満の酸化物はＡｌ酸化物主体であり、溶出しにくく孔食の起点とならなかった。また、ＣａおよびＭｇの含有量が合計で４０％を越えた酸化物は完全に溶出するが母材との間の隙間の形成効果が小さく、孔食へは進展しなかった。更に、ＣａおよびＭｇの含有量が合計で２０〜４０％である酸化物系介在物であっても、長径が７μｍ未満のものは、隙間が十分な大きさとならず、酸化物が溶出しても孔食へは進展しなかった。  As shown in FIG. 3, an oxide having a total content of Ca and Mg of 20 to 40% and a major axis of 7 μm or more became the starting point of pitting corrosion. However, oxides with a total content of Ca and Mg of less than 20% are mainly Al oxides, and are difficult to elute and do not become the starting point of pitting corrosion. Further, oxides with a total content of Ca and Mg exceeding 40% were completely eluted, but the effect of forming a gap with the base material was small and did not progress to pitting corrosion. Furthermore, even if the oxide inclusions have a total content of Ca and Mg of 20 to 40%, those having a major axis of less than 7 μm do not have a sufficiently large gap and the oxide is eluted. However, it did not progress to pitting corrosion.

このため、ＣａおよびＭｇの含有量が合計で２０〜４０％であり、且つ長径が７μｍ以上である酸化物系介在物に着目し、耐孔食温度を調査した。なお、臨界孔食温度は、５℃毎に温度を変化させた３５℃〜８０℃の６％塩化第二鉄水溶液に２４時間浸漬し、孔食の発生しなかった最高温度を意味する。その結果、ＣａおよびＭｇの含有量が合計で２０〜４０％であり、且つ長径が７μｍ以上である酸化物系介在物の個数が加工方向に垂直な断面１ｍｍ２あたりに１０個を超えると臨界孔食温度が著しく低下し、上記の過酷な腐食環境での耐食性が不十分となることが判明した。For this reason, the pitting corrosion temperature was investigated paying attention to the oxide type inclusion whose content of Ca and Mg is 20 to 40% in total and whose major axis is 7 μm or more. The critical pitting corrosion temperature means the maximum temperature at which pitting corrosion did not occur when immersed in a 6% ferric chloride aqueous solution at 35 ° C. to 80 ° C. with the temperature changed every 5 ° C. for 24 hours. As a result, if the total content of Ca and Mg is 20 to 40% and the number of oxide inclusions having a major axis of 7 μm or more exceeds 10 per 1 mm 2 cross section perpendicular to the processing direction, it is critical. It was found that the pitting corrosion temperature was remarkably lowered and the corrosion resistance in the above severe corrosive environment was insufficient.

従って、ＣａおよびＭｇの含有量が合計で２０〜４０％であり、且つ長径が７μｍ以上である酸化物系介在物が加工方向に垂直な断面１ｍｍ２あたり１０個以下であることを条件とした。また、種々の酸化物系介在物について、ＣａおよびＭｇの場合と同様に、孔食の発生傾向を整理した。Thus, 20 to 40% in total content of Ca and Mg, and the major axis is with the proviso that oxide inclusions is 7μm or more is 10 or less per square cross section perpendicular 1mm in working direction . Moreover, about the various oxide inclusions, the occurrence tendency of pitting corrosion was arranged as in the case of Ca and Mg.

図４は、酸化物系介在物の長径とＳの含有量との関係を示す図である。なお、図４中の「×」および「○」の意味は図３と同様である。  FIG. 4 is a diagram showing the relationship between the major axis of oxide inclusions and the S content. The meanings of “x” and “◯” in FIG. 4 are the same as those in FIG.

図４に示すように、Ｓ含有量が１５％以上であり、且つ長径が１μｍ以上である酸化物系介在物は孔食起点となった。Ｓを含有する酸化物系介在物は、微小で孔食試験後に完全に溶出しているが、溶出後に発生する硫化水素が腐食を促進し、孔食に進展した。一方、長径が１μｍ未満の酸化物系介在物、およびＳ含有量が１５％未満の酸化物系介在物は、孔食の起点とならなかった。  As shown in FIG. 4, the oxide inclusions having an S content of 15% or more and a major axis of 1 μm or more became pitting corrosion starting points. The oxide inclusions containing S are fine and completely eluted after the pitting corrosion test, but hydrogen sulfide generated after the elution promoted corrosion and progressed to pitting corrosion. On the other hand, oxide inclusions having a major axis of less than 1 μm and oxide inclusions having an S content of less than 15% did not become the starting point of pitting corrosion.

このため、Ｓの含有量が１５％以上であり、且つ長径が１μｍ以上である酸化物系介在物に着目して、上記と同様の臨界孔食温度を調査したところ、この介在物が加工方向に垂直な断面０．１ｍｍ２あたり１０個以下の場合に、耐孔食性が向上することが分かった。For this reason, paying attention to oxide inclusions having a S content of 15% or more and a major axis of 1 μm or more, the same critical pitting temperature as above was investigated. It was found that the pitting corrosion resistance is improved when the number of cross-sections is 0.1 or less per 0.1 mm 2 perpendicular to.

従って、Ｓの含有量が１５％以上であり、且つ長径が１μｍ以上である酸化物系介在物が加工方向に垂直な断面０．１ｍｍ２あたり１０個以下とするのが望ましい。Therefore, it the content of S is 15% or more, and major axis is desirable that oxide inclusions is 1μm or more and 10 or less the cross section perpendicular 0.1 mm 2 per the working direction.

４．本発明の二相ステンレス鋼の製造方法
二相ステンレス鋼中の酸化物系介在物の組成を制御する製造方法を詳細に検討した。その結果、特に、（α）還元処理、（β）キリングおよび（γ）鋳造後の加工のそれぞれの製造工程を最適化することで、今までに無い高清浄度二相ステンレス鋼が得られることは分かった。以下、それぞれの製造工程について説明する。4). 2. Manufacturing method of duplex stainless steel of the present invention A manufacturing method for controlling the composition of oxide inclusions in duplex stainless steel was examined in detail. As a result, unprecedented high cleanliness duplex stainless steel can be obtained by optimizing each manufacturing process of (α) reduction treatment, (β) killing and (γ) processing after casting. I understand. Hereinafter, each manufacturing process will be described.

（α）還元処理
還元処理は、下記の（２）式で表されるスラグ塩基度が０．５〜３．０となる条件で行う。但し、（２）式中の各化合物は、それぞれの化合物のスラグ中濃度（質量％）を意味する。

Figure 0004155300
((Alpha)) Reduction process A reduction process is performed on the conditions from which the slag basicity represented by following (2) Formula becomes 0.5-3.0. However, each compound in the formula (2) means the concentration (% by mass) in the slag of each compound.
Figure 0004155300

電気炉等で原料を溶解して得られるステンレス粗溶鋼には、ＡＯＤやＶＯＤ等の二次精錬炉で、酸素を溶鋼に吹き込んで脱炭した後、脱炭時に酸化されたクロムを回収するため金属アルミ等の脱酸材と石灰石等の脱硫材を投入し、還元と称される処理が行われる。この還元期において、これらと結合した酸素および硫黄は、Ａｌ２Ｏ３、ＣａＳ等としてスラグ中へと移行することで溶鋼から除去される。For crude stainless steel obtained by melting raw materials in an electric furnace, etc., in order to recover chromium oxidized during decarburization after decarburization by blowing oxygen into the molten steel in a secondary refining furnace such as AOD or VOD A deoxidizing material such as metallic aluminum and a desulfurizing material such as limestone are added, and a process called reduction is performed. In this reduction phase, oxygen and sulfur combined with these are removed from the molten steel by moving into slag as Al 2 O 3 , CaS, or the like.

本発明の特徴である低酸素および低硫黄を達成するためには、上記の（２）式で表されるスラグ塩基度を０．５以上とする必要がある。特に、酸化物系介在物中のＳ含有量を極力低減するには、スラグ塩基度を１．０以上とするのが望ましい。一方、スラグ塩基度が高すぎると、融点の上昇に伴い流動性が乏しくなることに加え、ＣａおよびＭｇの合計含有量が２０〜４０％である酸化物系介在物が鋼中に残留し易くなり、鋼材の耐孔食性が低下する。この観点からその上限値を３．０とする必要がある。酸化物系介在物中のＣａ含有量およびＭｇ含有量を十分に低減するには、スラグ塩基度は２．５以下とするのが望ましい。  In order to achieve the low oxygen and low sulfur characteristic of the present invention, the slag basicity represented by the above formula (2) needs to be 0.5 or more. In particular, in order to reduce the S content in the oxide inclusions as much as possible, it is desirable to set the slag basicity to 1.0 or more. On the other hand, if the slag basicity is too high, the fluidity becomes poor as the melting point increases, and oxide inclusions having a total content of Ca and Mg of 20 to 40% tend to remain in the steel. Thus, the pitting corrosion resistance of the steel material is reduced. From this viewpoint, the upper limit value needs to be 3.0. In order to sufficiently reduce the Ca content and the Mg content in the oxide inclusions, the slag basicity is desirably 2.5 or less.

また、上記スラグ塩基度での還元処理は通常は１回のみ行うが、酸素および硫黄をさらに低減するためには、この還元期を２回以上繰り返すほうが望ましい。この時、１回目の還元処理で生じたスラグは、二次精錬炉体を傾け、適当な治具で掻き出すことで、２回目の還元を実施する前に炉外へ排出される。これは１回目の還元期で生じた硫黄を多量に含んだスラグを除去することで、２回目の還元期での脱硫能を高めるために重要である。  The reduction treatment with the slag basicity is usually performed only once, but it is desirable to repeat this reduction period twice or more in order to further reduce oxygen and sulfur. At this time, the slag generated by the first reduction treatment is discharged outside the furnace before the second reduction is performed by tilting the secondary refining furnace body and scraping it out with an appropriate jig. This is important for enhancing the desulfurization ability in the second reduction period by removing slag containing a large amount of sulfur generated in the first reduction period.

（β）キリング
還元処理後のキリングは、１５００℃以上の温度で５分以上行う。(Β) Killing Killing after the reduction treatment is performed at a temperature of 1500 ° C. or more for 5 minutes or more.

上記の（α）に示す還元処理の後、所定の成分への微調整により二次精錬を終了した溶鋼は、取鍋に出鋼され鋳造される。出鋼された溶鋼は鋳込みまでの間、溶鋼上に浮いているスラグと再度混ざり合わないように静置あるいは鋳込み場所へ移動される。この処理をキリングと称するが、キリングの間、溶鋼中に懸濁した酸化物の一部は、その比重差により浮上し、スラグ中に吸収分離される。二相ステンレス鋼に所望の酸化物系介在物の状態を与えるためには、粗大な酸化物を浮上分離させる必要があり、このためには、キリング温度を１５００℃以上とし、かつキリング時間を５分以上確保する必要がある。また、これら酸化物の浮上除去をさらに促進するには、キリング温度を１５５０℃以上、キリング時間を１０分以上とするのが望ましい。  After the reduction treatment shown in (α) above, the molten steel that has been subjected to secondary refining by fine adjustment to a predetermined component is put into a ladle and cast. The discharged steel is left stationary or moved to the casting site so as not to be mixed with the slag floating on the molten steel until casting. This treatment is called killing. During killing, part of the oxide suspended in the molten steel floats due to the difference in specific gravity, and is absorbed and separated in the slag. In order to give a desired state of oxide inclusions to the duplex stainless steel, it is necessary to float and separate coarse oxides. For this purpose, the killing temperature is set to 1500 ° C. or more and the killing time is set to 5 ° C. It is necessary to secure more than minutes. In order to further promote the floating removal of these oxides, it is desirable to set the killing temperature to 1550 ° C. or more and the killing time to 10 minutes or more.

（γ）鋳造後の加工
鋳造後の加工は、下記の（３）式で表される総加工比Ｒが１０以上となる条件で行う。但し、（３）式中のＡ０ｎは塑性変形工程での変形前の断面積、Ａｎは塑性変形工程での変形後の断面積を意味し、それぞれの添字ｎ（１、２、…ｉ）は、塑性変形工程の各スタンド順を意味する。

Figure 0004155300
(Γ) Processing after casting The processing after casting is performed under the condition that the total processing ratio R represented by the following formula (3) is 10 or more. However, (3) the cross-sectional area before deformation in A0 n plastic deformation process in the formula, A n denotes the cross-sectional area after deformation in the plastic deformation process, each subscript n (1, 2, ... i ) Means the order of each stand in the plastic deformation process.
Figure 0004155300

鋳造された鋳片は、鍛造や熱間圧延といった熱間加工や、冷間圧延といった冷間加工を施された後、所定の製品寸法に成形される。この際、加工による材料の加工方向への変形に伴い、酸化物系介在物は破砕され微細化する。二相ステンレス鋼に所望の酸化物系介在物の状態を与えるためには、鋳片から最終製品までの総加工比Ｒを１０以上とする必要がある。  The cast slab is subjected to hot working such as forging or hot rolling or cold working such as cold rolling, and then formed into a predetermined product size. At this time, the oxide inclusions are crushed and refined as the material is deformed in the processing direction. In order to give the duplex stainless steel a desired state of oxide inclusions, the total processing ratio R from the slab to the final product needs to be 10 or more.

なお、塑性変形工程には、切削工程その他の延伸を伴わない加工工程を含まない。従って、塑性変形工程の間に、切削工程が含まれている場合でも、上記の（３）式の計算は、この切削工程による断面積の変化は考慮せずに行う。  The plastic deformation process does not include a cutting process or other processing process that does not involve stretching. Therefore, even when a cutting process is included in the plastic deformation process, the calculation of the above equation (3) is performed without considering the change in the cross-sectional area due to the cutting process.

表１に示す組成の二相ステンレス鋼（耐孔食指数ＰＲＥＷが４０以上のスーパー二相ステンレス鋼）を５００ｋｇの誘導溶解炉で溶解した後、ＡＯＤ炉に移し、二次精練を行った。この際、還元期のスラグ塩基度を２．０とした。スラグおよび溶鋼は、それぞれ還元期終了後にサンプリングした。また、取鍋に出鋼された溶鋼を直ちにサーモカップルで測温した後、鋳込み開始までの時間を測定した。  A duplex stainless steel having the composition shown in Table 1 (super duplex stainless steel with a pitting corrosion resistance PREW of 40 or more) was melted in a 500 kg induction melting furnace, then transferred to an AOD furnace, and subjected to secondary scouring. At this time, the slag basicity during the reduction period was set to 2.0. Slag and molten steel were sampled after the reduction period. Moreover, after measuring the temperature of the molten steel delivered to the ladle with a thermocouple immediately, the time until the start of casting was measured.

この時、鋳込み開始のためにレードルクレーンで吊り上げるまでの間、取鍋は一定位置で静置して振動を与えないようにしてキリングを実施した。この際のキリング条件は表２に示すとおりである。  At this time, the ladle was left standing at a fixed position until it was lifted by a ladle crane to start casting, and killing was performed so as not to give vibration. The killing conditions at this time are as shown in Table 2.

Figure 0004155300
Figure 0004155300

Figure 0004155300
Figure 0004155300

溶鋼は、下注法により平均寸法で一辺１６０ｍｍの鋼塊に、あるいは連続鋳造法により外径１８０ｍｍの丸鋳片に鋳造した。鋳造した鋼片を、鍛造、熱間押出、冷間圧延により、種々の加工度を加え、外径１６〜２８０ｍｍ、肉厚１．４〜１６ｍｍの寸法の継目無鋼管に形成した後、１１００℃で３分保持後水冷の固溶化熱処理を行った。  Molten steel was cast into a steel ingot having an average dimension of 160 mm on the average dimension by the sub-casting method, or a round slab having an outer diameter of 180 mm by the continuous casting method. After forming the cast steel slab into a seamless steel pipe having an outer diameter of 16 to 280 mm and a wall thickness of 1.4 to 16 mm by adding various degrees of processing by forging, hot extrusion, and cold rolling, 1100 ° C. After 3 minutes, water-cooled solution heat treatment was performed.

上記の管材を切断扁平後、管肉厚×１０ｍｍ×１０ｍｍの寸法の試験片を各２個切り出した後、管断面方向に樹脂を埋込んだ後、この断面を鏡面研磨した。その後、長径７μｍ以上の酸化物系介在物については５０倍の倍率で各５視野、長径１μｍ以上の酸化物系介在物については２００倍の倍率で各５視野、ＳＥＭ観察を行った。  After cutting and flattening the above-mentioned tube material, two test pieces each having dimensions of tube thickness × 10 mm × 10 mm were cut out, and after embedding resin in the tube cross-sectional direction, this cross section was mirror-polished. Thereafter, SEM observation was performed for five oxide visual inclusions at a magnification of 50 times for oxide inclusions having a major axis of 7 μm or more, and five visual fields for an oxide inclusion having a major axis of 1 μm or more at a magnification of 200 times.

酸化物系介在物の長径は、図２の定義に従って測定し、酸化物系介在物の中心部近傍（図２のｂ１またはｂ２）をＥＤＸ（エネルギー分散型Ｘ線分析）により組成分析した。分析時にはＯ（酸素）の測定値は精度上の信頼性が低いことから、Ｏ（酸素）を除いたＡｌ、Ｃａ、Ｍｇ、Ｓ、Ｍｎの質量比を測定した。  The major axis of the oxide inclusions was measured according to the definition shown in FIG. 2, and the composition of the vicinity of the central portion of the oxide inclusions (b1 or b2 in FIG. 2) was analyzed by EDX (energy dispersive X-ray analysis). At the time of analysis, the measured value of O (oxygen) has low accuracy in accuracy, so the mass ratio of Al, Ca, Mg, S, and Mn excluding O (oxygen) was measured.

また、管材を１０ｍｍ長さに輪切り切断後、切断端面を６００番エメリー紙で研磨し、孔食試験に供した。５℃毎に温度を変化させた３５℃〜８０℃の６％塩化第二鉄水溶液に２４時間浸漬し、孔食の発生しなかった最高温度を測定した。１つの試験管につき５個の試験片を用いて測定し、そのうち最も低い値を臨界孔食温度とし、耐孔食性の目安とした。  Further, the tube material was cut into 10 mm lengths, and then the cut end surface was polished with No. 600 emery paper and subjected to a pitting corrosion test. It was immersed in a 6% ferric chloride aqueous solution at 35 ° C. to 80 ° C. with the temperature changed every 5 ° C. for 24 hours, and the maximum temperature at which pitting corrosion did not occur was measured. Measurements were made using five test pieces per test tube, and the lowest value was taken as the critical pitting corrosion temperature, which was used as a measure of pitting corrosion resistance.

表２に示すように、同じ組成を有する鋼でもキリング条件により耐孔食性が異なる。即ち、本発明例１〜３では、キリングの開始温度が１５００℃以上で、５分以上保持したため、介在物のうち、ＣａおよびＭｇの合計含有量が２０〜４０％であり、且つ長径が７μｍ以上である酸化物系介在物が加工方向に垂直な断面１ｍｍ２あたり１０個以下となって良好な耐孔食性が得られた。特に、本発明例１および２では、Ｓの含有量が１５％以上であり、且つ長径が１μｍ以上である酸化物系介在物が加工方向に垂直な断面０．１ｍｍ２あたり１０個以下の条件をも満たすので、臨界孔食温度８０℃と、極めて良好な耐孔食性を示した。As shown in Table 2, pitting corrosion resistance varies depending on killing conditions even in steels having the same composition. That is, in Invention Examples 1 to 3, since the killing start temperature was 1500 ° C. or higher and held for 5 minutes or more, among the inclusions, the total content of Ca and Mg was 20 to 40% and the major axis was 7 μm. The above oxide inclusions were 10 or less per 1 mm 2 in cross section perpendicular to the processing direction, and good pitting corrosion resistance was obtained. In particular, in Inventive Examples 1 and 2, the oxide inclusions having an S content of 15% or more and a major axis of 1 μm or more are 10 or less per 0.1 mm 2 in cross section perpendicular to the processing direction. Therefore, the critical pitting corrosion temperature of 80 ° C. and extremely good pitting corrosion resistance were exhibited.

一方、キリング温度および保持時間の一方又は双方が本発明で規定される範囲を外れる比較例１〜３では、粗大な酸化物系介在物の個数が増大して、耐孔食性が劣化した。  On the other hand, in Comparative Examples 1 to 3 in which one or both of the killing temperature and the holding time deviate from the range defined in the present invention, the number of coarse oxide inclusions increased and the pitting corrosion resistance deteriorated.

表３および４に示す組成の二相ステンレス鋼を５００ｋｇの誘導溶解炉で溶解した後、ＡＯＤ炉に移し、二次精錬を行った。この際、還元期のスラグ塩基度を種々変化させた。スラグおよび溶鋼は、それぞれ還元期終了後および還元後の成分微調整直後にサンプリングし、それぞれ化学分析によりその組成分析を行った。また、取鍋に出鋼された溶鋼を直ちにサーモカップルで測温した後、鋳込み開始までの時間を測定した。  After duplex stainless steels having the compositions shown in Tables 3 and 4 were melted in a 500 kg induction melting furnace, they were transferred to an AOD furnace and subjected to secondary refining. At this time, the slag basicity during the reduction period was variously changed. Slag and molten steel were sampled after completion of the reduction period and immediately after fine adjustment of the components after reduction, respectively, and their compositions were analyzed by chemical analysis. Moreover, after measuring the temperature of the molten steel delivered to the ladle with a thermocouple immediately, the time until the start of casting was measured.

Figure 0004155300
Figure 0004155300

Figure 0004155300
Figure 0004155300

この時、鋳込み開始のためにレードルクレーンで吊り上げるまでの間、取鍋は一定位置で静置して振動を与えないようにした。溶鋼は下注法により平均寸法で一辺１６０ｍｍの鋼塊に、あるいは連続鋳造法により外径１８０ｍｍの丸鋳片に鋳造した。鋳造した鋼片を、鍛造、熱間押出、冷間圧延により、種々の加工度を加え、外径１６〜２８０ｍｍ、肉厚１．４〜１６ｍｍの寸法の継目無鋼管に形成した後、１１００℃で３分保持後水冷の固溶化熱処理を行った。還元期のスラグ塩基度、キリング条件および総加工比を表５および６に示す。  At this time, the ladle was left at a fixed position so as not to give vibration until it was lifted by a ladle crane to start casting. Molten steel was cast into a steel ingot with an average dimension of 160 mm on the average dimension by the drop casting method, or a round slab having an outer diameter of 180 mm by the continuous casting method. After forming the cast steel slab into a seamless steel pipe having an outer diameter of 16 to 280 mm and a wall thickness of 1.4 to 16 mm by adding various degrees of processing by forging, hot extrusion, and cold rolling, 1100 ° C. After 3 minutes, water-cooled solution heat treatment was performed. Tables 5 and 6 show the slag basicity, killing conditions, and total processing ratios during the reduction phase.

上記の管材を切断扁平後、管肉厚×１０ｍｍ×１０ｍｍの寸法の試験片を各２個切り出した後、管断面方向に樹脂を埋込んだ後、この断面を鏡面研磨した。その後、長径７μｍ以上の酸化物系介在物については５０倍の倍率で各５視野、長径１μｍ以上の酸化物系介在物については２００倍の倍率で各５視野、ＳＥＭ観察を行った。酸化物系介在物の長径は、図２の定義に従って測定し、酸化物系介在物の中心部近傍（図２のｂ１またはｂ２）をＥＤＸ（エネルギー分散型Ｘ線分析）により組成分析した。分析時にはＯ（酸素）の測定値は精度上の信頼性が低いことから、Ｏ（酸素）を除いたＡｌ、Ｃａ、Ｍｇ、Ｓ、Ｍｎの質量比を測定した。その結果を表５および６に併記する。  After cutting and flattening the above-mentioned tube material, two test pieces each having dimensions of tube thickness × 10 mm × 10 mm were cut out, and after embedding resin in the tube cross-sectional direction, this cross section was mirror-polished. Thereafter, SEM observation was performed for five oxide visual inclusions at a magnification of 50 times for oxide inclusions having a major axis of 7 μm or more, and five visual fields for an oxide inclusion having a major axis of 1 μm or more at a magnification of 200 times. The major axis of the oxide inclusions was measured according to the definition shown in FIG. 2, and the composition of the vicinity of the central portion of the oxide inclusions (b1 or b2 in FIG. 2) was analyzed by EDX (energy dispersive X-ray analysis). At the time of analysis, the measured value of O (oxygen) has low accuracy in accuracy, so the mass ratio of Al, Ca, Mg, S, and Mn excluding O (oxygen) was measured. The results are also shown in Tables 5 and 6.

また、管材を１０ｍｍ長さに輪切り切断後、切断端面を６００番エメリー紙で研磨し、孔食試験に供した。５℃毎に温度を変化させた３５℃〜８０℃の６％塩化第二鉄水溶液に２４時間浸漬し、孔食の発生しなかった最高温度を測定した。１つの試験管につき５個の試験片を用いて測定し、そのうち最も低い値を臨界孔食温度とし、耐孔食性の目安とした。  Further, the tube material was cut into 10 mm lengths, and then the cut end surface was polished with No. 600 emery paper and subjected to a pitting corrosion test. It was immersed in a 6% ferric chloride aqueous solution at 35 ° C. to 80 ° C. with the temperature changed every 5 ° C. for 24 hours, and the maximum temperature at which pitting corrosion did not occur was measured. Measurements were made using five test pieces per test tube, and the lowest value was taken as the critical pitting corrosion temperature, which was used as a measure of pitting corrosion resistance.

なお、耐孔食性の目標値は、耐孔食指数ＰＲＥ（またはＰＲＥＷ）が４０未満の通常の二相ステンレス鋼（表３および４に記載の鋼Ｎｏ．１〜８、１０、２１〜２７、４２、４３および４６）では臨界孔食温度３５℃、耐孔食指数ＰＲＥ（またはＰＲＥＷ）が４０以上のスーパー二相ステンレス鋼（表３および４に記載の鋼Ｎｏ．９、１１〜２０、２８〜４１、４４、４５、４７および４８）では臨界孔食温度７０℃とした。その結果を表５および６に併記する。  The target value of pitting corrosion resistance is a normal duplex stainless steel having a pitting corrosion index PRE (or PREW) of less than 40 (steel Nos. 1 to 8, 10, 21 to 27 described in Tables 3 and 4). 42, 43 and 46) super duplex stainless steels having a critical pitting temperature of 35 ° C. and a pitting resistance index PRE (or PREW) of 40 or more (steel Nos. 9, 11 to 20, 28 described in Tables 3 and 4). In 41, 44, 45, 47 and 48), the critical pitting temperature was set to 70 ° C. The results are also shown in Tables 5 and 6.

Figure 0004155300
Figure 0004155300

Figure 0004155300
Figure 0004155300

本発明例４〜２３は、化学組成、および、ＣａならびにＭｇの合計含有量が２０〜４０％であり、且つ長径が７μｍ以上である酸化物系介在物の個数が本発明で規定される範囲にあった。このため、通常のステンレス鋼でもスーパーステンレス鋼でも上記の目標値以上の優れた耐孔食性が得られた。特に、Ｓの含有量が１５％以上であり、且つ長径が１μｍ以上である酸化物系介在物が加工方向に垂直な断面０．１ｍｍ２あたり１０個以下であった本発明例４〜７、１２、１３、１５〜１８、２２および２３では、通常のステンレス鋼でもスーパーステンレス鋼でも更に優れた耐孔食性が得られた。Examples 4 to 23 of the present invention have a chemical composition and a range in which the total content of Ca and Mg is 20 to 40% and the number of oxide inclusions whose major axis is 7 μm or more is defined by the present invention. It was in. For this reason, excellent pitting corrosion resistance exceeding the above-mentioned target value was obtained with both normal stainless steel and super stainless steel. In particular, Invention Examples 4 to 7 in which the content of S was 15% or more and the number of oxide inclusions having a major axis of 1 μm or more was 10 or less per 0.1 mm 2 cross section perpendicular to the processing direction, In 12, 13, 15-18, 22 and 23, further excellent pitting corrosion resistance was obtained with both normal stainless steel and super stainless steel.

一方、化学組成が本発明で規定される範囲を外れる比較例２０〜３１は、二相ステンレス鋼として十分な耐食性能を確保できなかった。また、本発明で規定される化学組成の範囲内の鋼であるが、製造条件が適当でない比較例４〜１９は、孔食に有害な酸化物系介在物が多く残留したため、耐孔食性が良くなかった。  On the other hand, Comparative Examples 20 to 31 in which the chemical composition deviates from the range defined in the present invention could not secure sufficient corrosion resistance performance as a duplex stainless steel. Moreover, although it is steel within the range of the chemical composition prescribed | regulated by this invention, since many comparative oxides 4-19 in which manufacturing conditions are not suitable remain harmful to pitting corrosion, pitting corrosion resistance is low. It was not good.

本発明によれば、良好な耐孔食性を有する二相ステンレス鋼が安定して得られる。このため、例えば、熱交換用配管、化学プラント用の配管もしくは構造物、ラインパイプ、油井用もしくはガス井用のケーシングもしくはチュービング、または、アンビリカルチューブ（海底油井の制御用配管）等の鋼管もしくは鋼板等に最適な二相ステンレス鋼を提供することができる。  According to the present invention, a duplex stainless steel having good pitting corrosion resistance can be obtained stably. For this reason, for example, steel pipes or steel plates such as heat exchange pipes, chemical plant pipes or structures, line pipes, oil well or gas well casings or tubing, or umbilical tubes (submarine oil well control pipes). It is possible to provide the optimum duplex stainless steel.

［図１］酸化物系介在物の観察面を示す図である。
［図２］酸化物系介在物の長径および組成の測定箇所を定義する図である。
［図３］酸化物系介在物の長径とＣａおよびＭｇの合計含有量との関係を示す図である。
［図４］酸化物系介在物の長径とＳの含有量との関係を示す図である。FIG. 1 is a view showing an observation surface of oxide inclusions.
[FIG. 2] It is a figure which defines the measurement part of the major axis and composition of an oxide inclusion.
FIG. 3 is a diagram showing the relationship between the major axis of oxide inclusions and the total content of Ca and Mg.
FIG. 4 is a diagram showing the relationship between the major axis of oxide inclusions and the S content.

符号の説明Explanation of symbols

１．鋼板（または鋼管）1. Steel plate (or steel pipe)

Claims (7)

質量％で、Ｃ：０．０３％以下、Ｓｉ：０．０１〜２％、Ｍｎ：０．１〜２％、Ｐ：０．０５％以下、Ｓ：０．００１％以下、Ａｌ：０．００３〜０．０５％、Ｎｉ：４〜１２％、Ｃｒ：１８〜３２％、Ｍｏ：０．２〜５％、Ｎ（窒素）：０．０５〜０．４％、Ｏ（酸素）：０．０１％以下、Ｃａ：０．０００５〜０．００５％、Ｍｇ：０．０００１〜０．００５％、Ｃｕ：０〜２％、Ｂ：０〜０．０１％およびＷ：０〜４％を含有し、残部がＦｅおよび不純物からなる二相ステンレス鋼であって、その中に含まれる介在物のうち、ＣａおよびＭｇの合計含有量が２０〜４０質量％であり、且つ長径が７μｍ以上である酸化物系介在物が加工方向に垂直な断面１ｍｍ２あたり１０個以下であることを特徴とする二相ステンレス鋼。In mass%, C: 0.03% or less, Si: 0.01-2%, Mn: 0.1-2%, P: 0.05% or less, S: 0.001% or less, Al: 0.00%. 003 to 0.05%, Ni: 4 to 12%, Cr: 18 to 32%, Mo: 0.2 to 5%, N (nitrogen): 0.05 to 0.4%, O (oxygen): 0 0.01% or less, Ca: 0.0005 to 0.005%, Mg: 0.0001 to 0.005%, Cu: 0 to 2%, B: 0 to 0.01% and W: 0 to 4% It is a duplex stainless steel that contains Fe and impurities in the balance, and among the inclusions contained therein, the total content of Ca and Mg is 20 to 40% by mass, and the major axis is 7 μm or more. A duplex stainless steel characterized in that there are 10 or less oxide inclusions per 1 mm 2 in cross section perpendicular to the processing direction. 質量％で、Ｃ：０．０３％以下、Ｓｉ：０．０１〜２％、Ｍｎ：０．１〜２％、Ｐ：０．０５％以下、Ｓ：０．００１％以下、Ａｌ：０．００３〜０．０５％、Ｎｉ：４〜１２％、Ｃｒ：１８〜３２％、Ｍｏ：０．２〜５％、Ｎ（窒素）：０．０５〜０．４％、Ｏ（酸素）：０．０１％以下、Ｃａ：０．０００５〜０．００５％、Ｍｇ：０．０００１〜０．００５％、Ｃｕ：０〜２％、Ｂ：０〜０．０１％およびＷ：０〜４％を含有し、残部がＦｅおよび不純物からなる二相ステンレス鋼であって、その中に含まれる介在物のうち、ＣａおよびＭｇの合計含有量が２０〜４０質量％であり、且つ長径が７μｍ以上である酸化物系介在物が加工方向に垂直な断面１ｍｍ２あたり１０個以下であるとともに、Ｓの含有量が１５質量％以上であり、且つ長径が１μｍ以上である酸化物系介在物が加工方向に垂直な断面０．１ｍｍ２あたり１０個以下であることを特徴とする二相ステンレス鋼。In mass%, C: 0.03% or less, Si: 0.01-2%, Mn: 0.1-2%, P: 0.05% or less, S: 0.001% or less, Al: 0.00%. 003 to 0.05%, Ni: 4 to 12%, Cr: 18 to 32%, Mo: 0.2 to 5%, N (nitrogen): 0.05 to 0.4%, O (oxygen): 0 0.01% or less, Ca: 0.0005 to 0.005%, Mg: 0.0001 to 0.005%, Cu: 0 to 2%, B: 0 to 0.01% and W: 0 to 4% It is a duplex stainless steel that contains Fe and impurities in the balance, and among the inclusions contained therein, the total content of Ca and Mg is 20 to 40% by mass, and the major axis is 7 μm or more. with some oxide inclusions is 10 or less the cross section perpendicular 1 mm 2 per the working direction, the content of S is not less than 15 wt%, One major axis duplex stainless steel, wherein the oxide inclusions is 1μm or more is 10 or less per square cross section perpendicular 0.1mm in processing direction. 質量％で０．２〜２％のＣｕを含むことを特徴とする請求項１または２に記載の二相ステンレス鋼。The duplex stainless steel according to claim 1 or 2, comprising 0.2 to 2% by mass of Cu. 質量％で０．００１〜０．０１％のＢを含むことを特徴とする請求項１から３までのいずれかに記載の二相ステンレス鋼。The duplex stainless steel according to any one of claims 1 to 3, characterized by containing 0.001 to 0.01% B by mass%. 質量％で０．１〜４％のＷを含むことを特徴とする請求項１から４までのいずれかに記載の二相ステンレス鋼。The duplex stainless steel according to any one of claims 1 to 4, characterized by containing 0.1 to 4% W by mass%. 下記の（１）式で表される耐孔食性指数ＰＲＥＷが４０以上であることを特徴とする請求項１から５までのいずれかに記載の二相ステンレス鋼。
Figure 0004155300
但し、（１）式中の各元素記号は、それぞれの元素の含有量（質量％）を意味する。6. The duplex stainless steel according to claim 1, wherein the pitting corrosion resistance index PREW represented by the following formula (1) is 40 or more.
Figure 0004155300
However, each element symbol in the formula (1) means the content (% by mass) of each element. 下記の（２）式で表されるスラグ塩基度が０．５〜３．０となる条件で還元し、出鋼した溶鋼に、１５００℃以上の温度で５分以上のキリングを実施した後に鋳造し、得られた鋳片を下記の（３）式で表される総加工比Ｒが１０以上となる条件で加工することを特徴とする請求項１から６までのいずれかに記載の二相ステンレス鋼の製造方法。
Figure 0004155300
但し、（２）式中の各化合物は、それぞれの化合物のスラグ中濃度（質量％）を意味する。また、（３）式中のＡ０ｎは塑性変形工程での変形前の断面積、Ａｎは塑性変形工程での変形後の断面積を意味し、それぞれの添字ｎ（１、２、…ｉ）は、塑性変形工程の各スタンド順を意味する。Casting after carrying out killing for 5 minutes or more at a temperature of 1500 ° C. or higher on the molten steel reduced and produced under the condition that the slag basicity represented by the following formula (2) is 0.5 to 3.0 The two-phase according to any one of claims 1 to 6, wherein the obtained slab is processed under a condition that the total processing ratio R represented by the following formula (3) is 10 or more: Stainless steel manufacturing method.
Figure 0004155300
However, each compound in the formula (2) means the concentration (% by mass) in the slag of each compound. Further, the cross-sectional area before deformation in A0 n plastic deformation process in (3), A n denotes the cross-sectional area after deformation in the plastic deformation process, each subscript n (1, 2, ... i ) Means the order of each stand in the plastic deformation process.
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NO336117B1 (en) 2015-05-18
BRPI0406423A (en) 2005-10-04
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