JP5777387B2 - Bright annealing method for duplex stainless steel - Google Patents
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本発明は、二相ステンレス鋼を、表層部の組織と鋼の耐孔食性を損なうことなく光輝焼鈍を行う方法に関する。 The present invention relates to a method for performing bright annealing of a duplex stainless steel without impairing the structure of the surface layer portion and the pitting corrosion resistance of the steel.
二相ステンレス鋼は、フェライト相とオーステナイト相が混在した組織を有し、海水環境での耐孔食性、耐すき間腐食性及び耐応力腐食割れ性に優れ、さらに、単位重量に対する強度がオーステナイト系ステンレス鋼よりも優れている。このため、近年、海水環境、油井関連の構造物及び海水淡水化装置の熱交換器など、高い耐孔食性が求められる環境に用いる材料として用途が広がっている。 Duplex stainless steel has a structure in which a ferrite phase and an austenite phase coexist, has excellent pitting corrosion resistance, crevice corrosion resistance, and stress corrosion cracking resistance in a seawater environment. Better than steel. For this reason, in recent years, the use is expanding as a material used for the environment where high pitting corrosion resistance is required, such as a seawater environment, oil well-related structures, and a heat exchanger of a seawater desalination apparatus.
二相ステンレス鋼の耐孔食性は、鋼全体としての成分組成の他に、オーステナイト相とフェライト相のそれぞれに分配される成分組成が変化するためオーステナイト相とフェライト相の比率によっても、耐孔食性が大きく変化する。このため、二相ステンレス鋼は、鋼全体の成分組成と、熱処理によるオーステナイト相とフェライト相の比率と、をそれぞれ適正に保つよう制御される必要がある。 The pitting corrosion resistance of duplex stainless steel is different from the composition of the steel as a whole, and the component composition distributed to each of the austenite phase and the ferrite phase changes. Changes significantly. For this reason, the duplex stainless steel needs to be controlled so that the component composition of the entire steel and the ratio of the austenite phase to the ferrite phase by heat treatment are maintained appropriately.
例えば、二相ステンレス鋼の冷延品は、通常素材のスラブを熱間圧延、中間焼鈍、冷間圧延の各工程を経て、所定の形状と寸法にした後、製品焼鈍を行って製造する。また、二相ステンレス鋼の冷延品は、製品焼鈍後に、さらに、用途に応じて光輝焼鈍される場合がある。従来、一般的に、オーステナイト系及びフェライト系ステンレス鋼の光輝焼鈍では、アンモニア（ＮＨ3）を分解したガス、すなわち、水素ガス濃度が７５体積％、窒素ガス濃度２５体積％の混合ガスの雰囲気中で焼鈍する方法が用いられている。 For example, a cold rolled product of duplex stainless steel is manufactured by subjecting a normal material slab to a predetermined shape and dimensions through hot rolling, intermediate annealing, and cold rolling processes, and then product annealing. Moreover, the cold rolled product of duplex stainless steel may be further brightly annealed depending on the application after product annealing. Conventionally, generally, in bright annealing of austenitic and ferritic stainless steels, in an atmosphere of a gas obtained by decomposing ammonia (NH 3 ), that is, a mixed gas having a hydrogen gas concentration of 75% by volume and a nitrogen gas concentration of 25% by volume. An annealing method is used.
二相ステンレス鋼の光輝焼鈍において、アンモニアを分解したガスなど窒素分圧が過多である雰囲気ガスの中で焼鈍すると、雰囲気ガス中の窒素が二相ステンレス鋼の表面から二相ステンレス鋼中へ吸収されていく(以下、単に「吸窒」という場合がある)ことがある。窒素は強いオーステナイト生成作用を有する元素であるので、吸窒現象が生じると、二相ステンレス鋼の表層で窒素含有量が増加する。すると、表層の部分でオーステナイト相が増加しフェライト相が減少して、耐応力腐食割れ性が低下する。また、ステンレス鋼の窒素含有量が増加すると耐孔食性が向上するものの、窒素含有量が過度に増加すると、固容しきれなくなった窒素が窒化物として析出し、却って耐孔食性が低下する。 In bright annealing of duplex stainless steel, when annealing is performed in an atmosphere gas with excessive nitrogen partial pressure, such as a gas that decomposes ammonia, nitrogen in the atmosphere gas is absorbed from the surface of the duplex stainless steel into the duplex stainless steel. (Hereinafter sometimes simply referred to as “nitrogen”). Nitrogen is an element having a strong austenite forming action, so when a nitrogen absorption phenomenon occurs, the nitrogen content increases in the surface layer of the duplex stainless steel. Then, the austenite phase increases and the ferrite phase decreases in the surface layer portion, and the stress corrosion cracking resistance decreases. Further, although the pitting corrosion resistance is improved when the nitrogen content of the stainless steel is increased, when the nitrogen content is excessively increased, nitrogen that cannot be solidified is precipitated as a nitride, and the pitting corrosion resistance is lowered.
一方で、水素ガス単体の雰囲気ガス中で長時間の光輝焼鈍を行なうと、二相ステンレス鋼の表面から二相ステンレス鋼中の窒素が放出される(以下、単に「脱窒」という場合がある)ことがある。脱窒現象により二相ステンレス鋼の表層で窒素含有量が減少すると、表層の部分でフェライト相が増加しオーステナイト相が減少して、耐応力腐食割れ性が向上する。しかし、窒素は耐孔食性を向上させる元素であるので、表層の窒素含有量が低下することで耐孔食性が劣化する。 On the other hand, when bright annealing is performed for a long time in an atmosphere gas of hydrogen gas alone, nitrogen in the duplex stainless steel is released from the surface of the duplex stainless steel (hereinafter, sometimes simply referred to as “denitrification”). )Sometimes. When the nitrogen content decreases in the surface layer of the duplex stainless steel due to the denitrification phenomenon, the ferrite phase increases and the austenite phase decreases in the surface layer portion, and the stress corrosion cracking resistance is improved. However, since nitrogen is an element that improves the pitting corrosion resistance, the pitting corrosion resistance deteriorates as the nitrogen content in the surface layer decreases.
従って、二相ステンレス鋼のオーステナイト相とフェライト相の比率や鋼の窒素含有量などは、要求される性能に応じて最適範囲に管理する必要がある。そして、所望の二相ステンレス鋼の性能を安定して得るためには、最終段階である光輝焼鈍時に、オーステナイト相とフェライト相の比率や窒化物の析出といった結晶組織の変化のおそれ及び耐孔食性の劣化のおそれがある吸窒や脱窒を十分に抑制することが必要となる。 Therefore, it is necessary to manage the ratio of the austenite phase and the ferrite phase of the duplex stainless steel, the nitrogen content of the steel, and the like within the optimum range according to the required performance. In order to obtain the desired duplex stainless steel performance stably, there is a risk of changes in crystal structure such as the ratio of austenite phase to ferrite phase and precipitation of nitride during bright annealing, which is the final stage, and pitting corrosion resistance. Therefore, it is necessary to sufficiently suppress nitrogen absorption and denitrification that may cause deterioration.
そこで、特許文献１では、窒素含有量が(９N−０．５)〜(９N＋０．５)体積％、残部が実質的に水素ガスからなる混合ガスの雰囲気中で焼鈍することで、表層での吸窒現象や脱窒現象を生じずに二相ステンレス鋼の光輝焼鈍を行えることが記載されている。しかしながら、特許文献１の光輝焼鈍方法では、吸窒現象や脱窒現象を防止するために、光輝焼鈍を行う二相ステンレス鋼の窒素含有量に応じて焼鈍雰囲気中の窒素ガス濃度を厳密に制御する必要がある。従って、特許文献１の光輝焼鈍方法では、多様な種類の二相ステンレス鋼を安定して連続的かつ容易に光輝焼鈍することはできないという問題がある。 Therefore, in Patent Document 1, the nitrogen content is (9N−0.5) to (9N + 0.5)% by volume, and the annealing is performed in a mixed gas atmosphere in which the balance is substantially made of hydrogen gas. It describes that bright annealing of duplex stainless steel can be performed without causing nitrogen absorption or denitrification. However, in the bright annealing method of Patent Document 1, the nitrogen gas concentration in the annealing atmosphere is strictly controlled according to the nitrogen content of the duplex stainless steel performing bright annealing in order to prevent the nitrogen absorption phenomenon and the denitrification phenomenon. There is a need to. Therefore, the bright annealing method of Patent Document 1 has a problem that various types of duplex stainless steels cannot be brightly annealed stably and continuously.
上記事情に鑑み、本発明の目的は、二相ステンレス鋼表層からの脱窒と吸窒を防止して、品質安定性を損なうことなく多様な種類の二相ステンレス鋼を連続的かつ容易に光輝焼鈍する方法を提供することである。 In view of the above circumstances, the object of the present invention is to prevent denitrification and nitrogen absorption from the surface layer of the duplex stainless steel, and to continuously and easily brighten various types of duplex stainless steel without losing quality stability. It is to provide a method of annealing.
本発明の態様は、窒素を０．１６〜０．３２質量％含有する、フェライト相・オーステナイト相の組織を有する二相ステンレス鋼の光輝焼鈍方法であって、水素ガス１００体積％からなる雰囲気中または水素ガスと１．０体積％以下の希ガスとからなる混合ガスの雰囲気中で、温度が１０３０〜１１００℃、焼鈍時間が２０〜１２０秒で焼鈍することを特徴とする二相ステンレス鋼の光輝焼鈍方法である。この態様では、光輝焼鈍を水素ガス雰囲気中で実施する。従って、光輝焼鈍の雰囲気ガス中に窒素ガスは実質的にまたは完全に含まれない。明細書中、「水素ガス雰囲気」とは、純水素ガスの雰囲気、すなわち、水素ガス１００体積％の雰囲気の他に、水素ガスとアルゴンガス等の窒素ガス以外の希ガスとからなる混合ガスの雰囲気が含まれる。 An aspect of the present invention is a bright annealing method for duplex stainless steel having a structure of ferrite phase / austenite phase containing 0.16 to 0.32% by mass of nitrogen, in an atmosphere composed of 100% by volume of hydrogen gas Or, in a mixed gas atmosphere consisting of hydrogen gas and 1.0% by volume or less of a rare gas , the temperature is 1030 to 1100 ° C. and the annealing time is 20 to 120 seconds. This is a bright annealing method. In this embodiment, the bright annealing is performed in a hydrogen gas atmosphere. Therefore, nitrogen gas is not substantially or completely contained in the bright annealing atmosphere gas. In the specification, the term “hydrogen gas atmosphere” refers to an atmosphere of pure hydrogen gas, that is, a mixed gas composed of hydrogen gas and a rare gas other than nitrogen gas such as argon gas in addition to an atmosphere of 100% by volume of hydrogen gas. The atmosphere is included.
本発明の態様は、前記フェライト相・オーステナイト相の組織を有する二相ステンレス鋼の厚さが、０．９〜４．０ｍｍであることを特徴とする二相ステンレス鋼の光輝焼鈍方法である。 An aspect of the present invention is the bright annealing method for duplex stainless steel, wherein the duplex stainless steel having a ferrite phase / austenite structure has a thickness of 0.9 to 4.0 mm.
本発明の態様は、前記水素ガス１００体積％からなる雰囲気中または水素ガスと１．０体積％以下の希ガスとからなる混合ガスの雰囲気中の露点が、−３５℃以下であることを特徴とする二相ステンレス鋼の光輝焼鈍方法である。 Aspect of the present invention, wherein a dew point in the hydrogen atmosphere consisting of gas 100 vol% or hydrogen gas and less 1.0% by volume of the mixed gas of a rare gas atmosphere, at -35 ° C. or less This is a bright annealing method for duplex stainless steel.
本発明の態様は、前記二相ステンレス鋼が、C：０．００１〜０．０３０質量％、Si：０．０５〜１．００質量％、Mn：０．１〜２．０質量％、Cr：２３〜２９質量％、Ni：５．０〜９．０質量％、Mo：２．０〜５．０質量％、N：０．１６〜０．３２質量％、残部がFeおよび不可避的不純物からなり、フェライト相・オーステナイト相の組織を有し、前記フェライト相が３０〜７０体積％であることを特徴とする二相ステンレス鋼の光輝焼鈍方法である。 In an embodiment of the present invention, the duplex stainless steel is C: 0.001 to 0.030 mass%, Si: 0.05 to 1.00 mass%, Mn: 0.1 to 2.0 mass%, Cr : 23-29% by mass, Ni: 5.0-9.0% by mass, Mo: 2.0-5.0% by mass, N: 0.16-0.32% by mass, the balance being Fe and inevitable impurities It is a bright annealing method for duplex stainless steel, characterized in that it has a structure of ferrite phase / austenite phase, and the ferrite phase is 30 to 70% by volume.
本発明の態様によれば、光輝焼鈍を水素ガス雰囲気中で実施するので、雰囲気ガス成分の混合比率の調整が不要となる。水素ガス雰囲気中でも脱窒を抑制できるので、品質安定性に優れた光輝焼鈍を容易に実施することができる。また、光輝焼鈍の対象となる二相ステンレス鋼の成分組成（特に、窒素の含有量）が変化しても、雰囲気ガス成分の再調整が不要なので、多様な種類の二相ステンレス鋼を連続的に光輝焼鈍でき、生産性が向上する。さらに、雰囲気ガスの成分を調整する工程と設備が不要となるので、光輝焼鈍工程を迅速化でき、さらに生産コストを低減できる。 According to the aspect of the present invention, since bright annealing is performed in a hydrogen gas atmosphere, it is not necessary to adjust the mixing ratio of atmospheric gas components. Since denitrification can be suppressed even in a hydrogen gas atmosphere, bright annealing with excellent quality stability can be easily performed. In addition, even if the component composition of the duplex stainless steel subject to bright annealing (especially the nitrogen content) changes, there is no need to readjust the atmospheric gas components, so various types of duplex stainless steel can be used continuously. Can be brightly annealed to improve productivity. Furthermore, since the process and equipment for adjusting the components of the atmospheric gas are not required, the bright annealing process can be speeded up and the production cost can be further reduced.
次に、本発明の光輝焼鈍方法について詳細に説明する。本発明の光輝焼鈍方法は、窒素を０．１６〜０．３２質量％含有する二相ステンレス鋼の光輝焼鈍方法であって、水素ガス雰囲気中で、温度が１０３０〜１１００℃、焼鈍時間が２０〜１２０秒で焼鈍する。 Next, the bright annealing method of the present invention will be described in detail. The bright annealing method of the present invention is a bright annealing method of duplex stainless steel containing 0.16 to 0.32% by mass of nitrogen, in a hydrogen gas atmosphere at a temperature of 1030 to 1100 ° C. and an annealing time of 20 Annealing in ~ 120 seconds.
本発明の光輝焼鈍方法は、水素ガス雰囲気中で焼鈍する。以下に詳述するように、本発明では、光輝焼鈍の条件、すなわち、光輝焼鈍の温度と時間を調整することにより、水素ガス雰囲気中の光輝焼鈍であっても、脱窒を十分に抑制できる。水素ガス雰囲気とは、水素ガス１００体積％の雰囲気でもよく、水素ガス１００体積％でなくとも、水素ガスとアルゴンガス等の希ガスとからなる混合ガスの雰囲気でもよいことを意味する。水素ガスに希ガスが混合している場合には、希ガスは不可避的不純物として混入している点から、希ガスはなるべく低い濃度が好ましく、例えば、１．０体積％以下の濃度が特に好ましい。 The bright annealing method of the present invention is performed in a hydrogen gas atmosphere. As described in detail below, in the present invention, denitrification can be sufficiently suppressed even by bright annealing in a hydrogen gas atmosphere by adjusting the conditions of bright annealing, that is, the temperature and time of bright annealing. . The hydrogen gas atmosphere may be an atmosphere of 100% by volume of hydrogen gas, or may be an atmosphere of a mixed gas composed of hydrogen gas and a rare gas such as argon gas, instead of 100% by volume of hydrogen gas. In the case where a rare gas is mixed with hydrogen gas, the rare gas preferably has a concentration as low as possible, for example, a concentration of 1.0% by volume or less is particularly preferred because the rare gas is mixed as an inevitable impurity. .
二相ステンレス鋼とは、オーステナイト相とフェライト相とが混在した組織で構成されている鋼である。本発明の光輝焼鈍方法では、二相ステンレス鋼の鋼種は、二相ステンレス鋼からの脱窒を防止して窒素含有量を確実に維持する点から、窒素を０．１６〜０．３２質量％含有する二相ステンレス鋼が特に有効である。窒素を０．１６〜０．３２質量％含有する二相ステンレス鋼の鋼種は、特に限定されないが、例えば、ＳＵＳ３２９Ｊ３Ｌ、ＳＵＳ３２９Ｊ４Ｌ、ＵＮＳ Ｓ３２７５０、ＵＮＳＳ３２７６０、ＵＮＳ Ｓ３２２０３、ＵＮＳ Ｓ３２１０１等が挙げられる。また、オーステナイト相とフェライト相との比率は特に限定されないが、本発明の光輝焼鈍方法は、オーステナイト相が３０〜７０体積％混在する二相ステンレス鋼に特に有効である。 The duplex stainless steel is a steel composed of a structure in which an austenite phase and a ferrite phase are mixed. In the bright annealing method of the present invention, the steel type of the duplex stainless steel has a nitrogen content of 0.16 to 0.32% by mass from the viewpoint of preventing the denitrification from the duplex stainless steel and ensuring the nitrogen content. The contained duplex stainless steel is particularly effective. The steel type of the duplex stainless steel containing 0.16 to 0.32% by mass of nitrogen is not particularly limited, and examples thereof include SUS329J3L, SUS329J4L, UNS S32750, UNSS32760, UNS S32203, UNS S32101, and the like. The ratio of the austenite phase to the ferrite phase is not particularly limited, but the bright annealing method of the present invention is particularly effective for a duplex stainless steel in which 30 to 70% by volume of the austenite phase is mixed.
二相ステンレス鋼の形態は特に限定されず、例えば、板状、帯状、管状、バネ状等が挙げられ、また、形鋼でもよい。 The form of the duplex stainless steel is not particularly limited, and examples thereof include a plate shape, a belt shape, a tubular shape, a spring shape, and the like, and a shape steel may be used.
光輝焼鈍の温度の下限値は、光輝焼鈍前および光輝焼鈍時の加熱過程で生じるσ相等の有害な金属間化合物を固容して無害化する点から１０３０℃であり、短時間の光輝焼鈍においても組織を再結晶させる点から１０４０℃が好ましく、短時間の光輝焼鈍でも上記再結晶を確実にする点から１０５０℃が特に好ましい。一方で、光輝焼鈍の温度の上限値は、水素ガス雰囲気中の光輝焼鈍において二相ステンレス鋼表層からの脱窒を防止して耐孔食性を維持する点から１１００℃であり、フェライト相の割合が著しく増加して組織の結晶粒が粗大化するのを確実に防止する点から１０９０℃が好ましく、二相ステンレス鋼表層からの脱窒を確実に防止する点から１０８０℃が特に好ましい。 The lower limit of the temperature of bright annealing is 1030 ° C. from the point of solidifying and detoxifying harmful intermetallic compounds such as σ phase generated during the heating process before and during bright annealing. However, 1050 ° C. is preferable from the viewpoint of recrystallization of the structure, and 1050 ° C. is particularly preferable from the viewpoint of ensuring the above-mentioned recrystallization even in short-time bright annealing. On the other hand, the upper limit of the temperature of bright annealing is 1100 ° C. from the point of preventing denitrification from the surface layer of the duplex stainless steel in bright annealing in a hydrogen gas atmosphere and maintaining pitting corrosion resistance, and the ratio of the ferrite phase Is preferably 9090 ° C. from the viewpoint of reliably preventing the crystal grains from coarsening and coarsening the crystal grains of the structure, and is particularly preferably 1080 ° C. from the viewpoint of reliably preventing denitrification from the surface layer of the duplex stainless steel.
光輝焼鈍時間の下限値は、二相ステンレス鋼の内部まで熱処理効果を得る点から２０秒であり、鋼内部まで確実に熱処理の効果を得る点から２５秒が好ましい。一方で、光輝焼鈍時間の上限値は、熱処理による二相ステンレス鋼表層からの脱窒を防止する点で１２０秒であり、フェライト相の割合が著しく増加して組織の結晶粒が粗大化するのを確実に防止する点から１００秒が好ましく、二相ステンレス鋼表層からの脱窒を確実に防止する点から９０秒が特に好ましい。 The lower limit of the bright annealing time is 20 seconds from the point of obtaining the heat treatment effect up to the inside of the duplex stainless steel, and preferably 25 seconds from the point of obtaining the heat treatment effect as far as the inside of the steel. On the other hand, the upper limit of the bright annealing time is 120 seconds in terms of preventing denitrification from the surface layer of the duplex stainless steel due to the heat treatment, and the proportion of the ferrite phase is remarkably increased and the grains of the structure are coarsened. 100 seconds is preferable from the viewpoint of reliably preventing the denitrification, and 90 seconds is particularly preferable from the viewpoint of reliably preventing denitrification from the surface layer of the duplex stainless steel.
光輝焼鈍する二相ステンレス鋼の厚さや径は特に限定されず、使用条件や用途に応じて適宜選択可能である。例えば、二相ステンレス鋼の形態が板状、帯状または管状の場合、厚さ（肉厚）の上限値は、二相ステンレス鋼の内部まで熱処理の効果を確実に及ぼす点から６．０ｍｍが好ましく、σ相の析出を確実に防止する点から４．０ｍｍが特に好ましい。一方で、その下限値は、熱容量が小さくなって二相ステンレス鋼表層から窒素が拡散するのを防止する点から０．３ｍｍが好ましく、前記窒素の拡散を確実に防止する点から０．９ｍｍが特に好ましい。また、二相ステンレス鋼の形態が丸材の場合、その直径の好ましい上限値と下限値は、それぞれ、上記した板状、帯状または管状の場合と同様である。 The thickness and diameter of the duplex stainless steel to be brightly annealed are not particularly limited, and can be appropriately selected according to use conditions and applications. For example, when the form of the duplex stainless steel is plate-shaped, strip-shaped, or tubular, the upper limit of the thickness (thickness) is preferably 6.0 mm from the viewpoint of reliably exerting the heat treatment effect up to the interior of the duplex stainless steel. From the viewpoint of reliably preventing the precipitation of the σ phase, 4.0 mm is particularly preferable. On the other hand, the lower limit is preferably 0.3 mm from the point of preventing the diffusion of nitrogen from the surface of the duplex stainless steel due to a decrease in heat capacity, and 0.9 mm from the point of reliably preventing the diffusion of nitrogen. Particularly preferred. When the duplex stainless steel is round, the preferred upper limit and lower limit of the diameter are the same as those in the case of the plate, strip, or tube, respectively.
水素ガス雰囲気中の露点は、光輝焼鈍中に鋼の表面が過度に酸化または過度に還元されるのを抑制する範囲で適宜選択可能である。例えば、露点の上限値は、Ｆｅの過度な酸化による着色が二相ステンレス鋼表層に現れて表面外観が損なわれるのを防止する点から−３５℃が好ましく、二相ステンレス鋼表層の過度な酸化により脱クロム層が生じ、耐孔食性が劣化するのを防止する点から−４０℃が特に好ましい。一方で、露点の下限値は、過度な還元を防止して若干の酸化皮膜の形成により脱窒現象を抑制する点から−５５℃が好ましく、前記脱窒現象の抑制を十分にする点から−５０℃が特に好ましい。 The dew point in the hydrogen gas atmosphere can be appropriately selected within a range that suppresses excessive oxidation or excessive reduction of the steel surface during bright annealing. For example, the upper limit of the dew point is preferably −35 ° C. from the viewpoint of preventing coloring due to excessive oxidation of Fe from appearing on the surface of the duplex stainless steel and damaging the surface appearance, and excessive oxidation of the duplex stainless steel surface. −40 ° C. is particularly preferable from the viewpoint of preventing a chrome removal layer from being generated and deteriorating pitting corrosion resistance. On the other hand, the lower limit of the dew point is preferably −55 ° C. from the viewpoint of preventing excessive reduction and suppressing the denitrification phenomenon by forming a slight oxide film, and from the viewpoint of sufficiently suppressing the denitrification phenomenon − 50 ° C. is particularly preferred.
上記以外の光輝焼鈍条件は、特に限定されるものではなく、σ相の析出防止のための熱処理後の急速冷却等、従来の二相ステンレス鋼の光輝焼鈍時に用いられている常用の条件にて本発明の光輝焼鈍方法を行うことができる。このように、焼鈍温度、焼鈍時間を適正に管理することにより、水素雰囲気中においても、二相ステンレス鋼からの脱窒現象を効果的に抑制した光輝焼鈍を実施できる。 The bright annealing conditions other than the above are not particularly limited, and are the usual conditions used during bright annealing of conventional duplex stainless steel, such as rapid cooling after heat treatment to prevent precipitation of σ phase. The bright annealing method of the present invention can be performed. As described above, by appropriately controlling the annealing temperature and the annealing time, bright annealing in which the denitrification phenomenon from the duplex stainless steel is effectively suppressed can be performed even in a hydrogen atmosphere.
本発明で光輝焼鈍する二相ステンレス鋼には、強力なオーステナイト生成元素であると共に耐孔食性を向上させる効果を有する元素である窒素を０．１６〜０．３２質量％含有するものが特に有効である。一方で、その他の成分の含有量は、鋼の用途に応じて適宜選択可能である。以下に、二相ステンレス鋼が有する窒素以外の成分組成の例について説明する。 For the duplex stainless steel to be brightly annealed in the present invention, it is particularly effective to contain 0.16-0.32% by mass of nitrogen, which is a powerful austenite forming element and an element having an effect of improving pitting corrosion resistance. It is. On the other hand, the content of other components can be appropriately selected according to the use of steel. Below, the example of component compositions other than nitrogen which a duplex stainless steel has is demonstrated.
Ｃは、耐孔食性を低下させる元素なので、含有量の上限値は０．０３０質量％が好ましく、０．０２５質量％が特に好ましい。一方で、下限値は、強度の低下を防止する点で０．００１質量％が好ましい。 Since C is an element that lowers pitting corrosion resistance, the upper limit of the content is preferably 0.030% by mass, and particularly preferably 0.025% by mass. On the other hand, the lower limit is preferably 0.001% by mass in terms of preventing the strength from being lowered.
Ｓｉは、脱酸剤として添加される成分である。Ｓｉの含有量の上限値は、延性の低下を防止し、またσ相などの金属間化合物の析出を抑えて耐孔食性の低下を防止する点から１．００質量％が好ましく、下限値は、脱酸剤としての効果を発揮する点で０．０５質量％が好ましい。 Si is a component added as a deoxidizer. The upper limit of the Si content is preferably 1.00% by mass from the viewpoint of preventing the ductility from decreasing and preventing the precipitation of intermetallic compounds such as the σ phase to prevent the pitting corrosion resistance from decreasing. Moreover, 0.05 mass% is preferable at the point which exhibits the effect as a deoxidizer.
Ｍｎは、オーステナイト生成元素であるため、オーステナイト相とフェライト相の比率を調整し、二相ステンレス鋼の耐孔食性を改善するのに有効な成分である。Ｍｎの含有量の上限値は、σ相などの金属間化合物の析出を抑えて耐孔食性の低下を防止する点から２．０質量％が好ましく、下限値は、所定のオーステナイト相の比率を得る点から０．１質量％が好ましい。 Since Mn is an austenite generating element, Mn is an effective component for adjusting the ratio of the austenite phase and the ferrite phase and improving the pitting corrosion resistance of the duplex stainless steel. The upper limit of the Mn content is preferably 2.0% by mass from the viewpoint of preventing the precipitation of intermetallic compounds such as the σ phase and preventing the decrease in pitting corrosion resistance, and the lower limit is the predetermined austenite phase ratio. From the point of obtaining, 0.1% by mass is preferable.
Ｃｒは、フェライト生成元素であり、また耐孔食性を向上させる元素でもある。Ｃｒの含有量の上限値は、σ相などの金属間化合物の析出を抑える点から２９質量％が好ましく、フェライト相の過度の増加を防止して二相組織を維持する点から２６質量％が特に好ましい。一方、Ｃｒの含有量の下限値は、所期の耐孔食性を得る点から２３質量％が好ましい。 Cr is a ferrite-forming element and also an element that improves pitting corrosion resistance. The upper limit of the Cr content is preferably 29% by mass from the viewpoint of suppressing precipitation of intermetallic compounds such as the σ phase, and 26% by mass from the point of preventing excessive increase of the ferrite phase and maintaining a two-phase structure. Particularly preferred. On the other hand, the lower limit of the Cr content is preferably 23% by mass from the viewpoint of obtaining the desired pitting corrosion resistance.
Ｎｉは、オーステナイト生成元素であり、また耐孔食性を向上させる元素でもある。Ｎｉの含有量の上限値は、過度にオーステナイト相が増加してフェライト相が減少するのを防止する点から９．０質量が好ましく、下限値は、所期の耐孔食性を得る点から５．０質量％が好ましい。 Ni is an austenite-forming element and also an element that improves pitting corrosion resistance. The upper limit of the Ni content is preferably 9.0 mass from the viewpoint of preventing the austenite phase from excessively increasing and the ferrite phase from decreasing, and the lower limit is 5 from the viewpoint of obtaining the desired pitting corrosion resistance. 0.0 mass% is preferable.
Ｍｏは、耐孔食性を向上させる元素である。Ｍｏの含有量の上限値は、σ相などの金属間化合物の析出を防止する点から５．０質量％が好ましく、下限値は、その効果を得る点から２．０質量％が好ましい。 Mo is an element that improves pitting corrosion resistance. The upper limit of the Mo content is preferably 5.0% by mass from the viewpoint of preventing the precipitation of intermetallic compounds such as the σ phase, and the lower limit is preferably 2.0% by mass from the viewpoint of obtaining the effect.
次に、本発明の実施例を説明するが、本発明はその趣旨を超えない限り、これらの例に限定されるものではない。 Next, examples of the present invention will be described. However, the present invention is not limited to these examples as long as the gist thereof is not exceeded.
下記表１に示す３種類の化学組成の鋼を、常法に従って、電気炉にて溶製後、AOD、CTS工程により精錬を行い、連続鋳造で造塊した。その後、常法に従って、熱間圧延にて板厚７．０ｍｍまで圧延を行い熱延鋼板とした。次いで、この熱延鋼板を冷間圧延、製品焼鈍、酸洗工程を経て、板厚が０．３〜６．０ｍｍまでの二相ステンレス鋼の冷帯とした。 The steels having the three chemical compositions shown in Table 1 below were melted in an electric furnace in accordance with a conventional method, then refined by AOD and CTS processes, and ingot was formed by continuous casting. Then, according to a conventional method, it rolled by hot rolling to plate | board thickness 7.0mm, and was set as the hot-rolled steel plate. Next, this hot-rolled steel sheet was subjected to cold rolling, product annealing, and pickling processes to form a duplex stainless steel cold zone having a thickness of 0.3 to 6.0 mm.
なお、表１中、各元素の化学組成の含有量は質量％、フェライト相とオーステナイト相の比率は体積％で示す。また、ＣＰＴとは、臨界孔食発生温度であり、表１中の単位は℃である。 In Table 1, the content of the chemical composition of each element is represented by mass%, and the ratio of the ferrite phase to the austenite phase is represented by volume%. CPT is the critical pitting corrosion temperature, and the unit in Table 1 is ° C.
上記のようにして得られた冷帯に、通常の雰囲気熱処理炉を用いて光輝焼鈍を施した。光輝焼鈍は、焼鈍温度、焼鈍時間及び焼鈍雰囲気の露点をそれぞれ変化させて、水素ガス１００体積％の雰囲気ガス中にて行なった。光輝焼鈍後の冷帯について、表面外観、表層部の窒素含有量、表層組織および耐孔食性を評価した。また、比較のために、表１に示す３種類の化学組成の鋼について、それぞれ、上記と同様の条件にて、造塊、熱間圧延、冷間圧延、製品焼鈍及び酸洗を経て得られた冷帯について、その表層を＃２０００番手のサンドペーパーで湿式研磨を行って比較用試験材(以下、「研磨仕上材」)を作製し、該研磨仕上材についても耐孔食性を評価した。詳細な評価方法は、以下の通りである。 The cold zone obtained as described above was subjected to bright annealing using a normal atmospheric heat treatment furnace. Bright annealing was performed in an atmosphere gas of 100% by volume of hydrogen gas by changing the annealing temperature, the annealing time, and the dew point of the annealing atmosphere. About the cold zone after bright annealing, surface appearance, nitrogen content of the surface layer part, surface layer structure, and pitting corrosion resistance were evaluated. In addition, for comparison, steels having three kinds of chemical compositions shown in Table 1 were obtained through ingot forming, hot rolling, cold rolling, product annealing, and pickling under the same conditions as above. The surface of the cold zone was wet-polished with # 2000 sandpaper to produce a comparative test material (hereinafter referred to as “polishing finish”), and the pitting corrosion resistance of the polishing finish was also evaluated. The detailed evaluation method is as follows.
（１）表面外観
光輝焼鈍後の冷帯の表面外観を目視により判定した。光輝焼鈍前の表面外観を保ち、異常が認められなかった場合を良好として「○」、僅かでも外観に異常が見られた場合を不良として「×」と評価した。
(1) Surface appearance The surface appearance of the cold zone after bright annealing was determined visually. The surface appearance before bright annealing was maintained, and the case where no abnormality was observed was evaluated as “good”, and the case where even a slight abnormality was observed was evaluated as “bad”.
（２）表層の窒素含有量（質量％）
マーカス型高周波グロー放電発光表面分析装置（（株）ＨＯＲＩＢＡ製、「ＧＤ‐ｐｒｏｆｉｌｅｒ２」）を用いて、光輝焼鈍後の冷帯について表層（冷帯の表面から深さ１４０μｍまでの間）の窒素含有量を測定した。上記表１の鋼種の窒素含有量と比較して、光輝焼鈍後の窒素含有量の変化量が±１５質量％未満の場合を良好として「○」、光輝焼鈍後の窒素含有量の変化量が±１５質量％以上、±２０質量％未満の場合を良（許容範囲）として「△」、光輝焼鈍後の窒素含有量の変化量が±２０質量％以上である場合を不良として「×」と評価した。
(2) Nitrogen content (% by mass) in the surface layer
Nitrogen content in the surface layer (between the surface of the cold zone and a depth of 140 μm) with respect to the cold zone after bright annealing using a Marcus type high-frequency glow discharge luminescence surface analyzer (“GD-profiler2” manufactured by HORIBA) The amount was measured. Compared with the nitrogen content of the steel types in Table 1 above, the case where the amount of change in nitrogen content after bright annealing is less than ± 15% by mass is “◯”, and the amount of change in nitrogen content after bright annealing is The case where the difference is ± 15% by mass or more and less than ± 20% by mass is regarded as “good” (acceptable range), and the case where the change in the nitrogen content after bright annealing is ± 20% by mass or more is regarded as defective. evaluated.
（３）表層組織
光輝焼鈍後の冷帯を、圧延方向に直角に切り出した小片にＳｔｒｕｅｒｓ（株）製、「テヌポール‐３」にて電解研磨を行った後、電解放出型走査型電子顕微鏡（日本電子（株）製、「ＪＳＭ‐７００１Ｆ」）を用いて、冷帯表層（冷帯の表面から深さ２００μｍまでの間）の組織観察と析出物の同定を行った。また、表層組織（冷帯の表面から深さ１８０μｍまでの間）のフェライト相とオーステナイト相の比率は、光輝焼鈍後の冷帯を用いて後方散乱電子回折装置（ＴＳＬソリューションズ（株）製、「ＥＢＳＤ、解析ソフトＯＩＭＡｎａｌｙｓｉｓ ５．１」）にて測定した。表層組織のフェライト相が３０〜７０体積％の比率であり、有害な析出物であるσ相とクロム窒化物が見られない場合を良好として「〇」、表層組織のフェライト相が３０〜７０体積％の比率であり、有害な析出物であるσ相とクロム窒化物の面積率が０％超、０．１％以下の場合を良（許容範囲）として「△」、表層組織のフェライト相の比率が３０〜７０体積％の範囲を外れる場合または有害な析出物であるσ相とクロム窒化物の面積率が０．１％を超えて析出した場合を不良として「×」と評価した。
(3) Surface layer structure After the bright annealing, the cold zone was electropolished on a small piece cut out at right angles to the rolling direction with “Tnupole-3” manufactured by Struers Co., Ltd., and then the field emission scanning electron microscope ( Using JEOL Co., Ltd., “JSM-7001F”), the structure of the cold surface layer (between the surface of the cold zone and the depth of 200 μm) was observed and the precipitates were identified. The ratio of the ferrite phase and austenite phase in the surface layer structure (between the surface of the cold zone and the depth of 180 μm) was measured using a backscattered electron diffractometer (TSL Solutions Co., Ltd.), using the cold zone after bright annealing. EBSD, analysis software OIMA Analysis 5.1 "). The case where the ferrite phase of the surface layer structure is 30 to 70% by volume, and the case where no σ phase and chromium nitride which are harmful precipitates are found is good, “◯”, and the ferrite phase of the surface layer structure is 30 to 70% by volume. %, The case where the area ratio of σ phase and chromium nitride, which are harmful precipitates, is more than 0% and 0.1% or less is “good” (acceptable range), “△”, the ferrite phase of the surface layer structure When the ratio is out of the range of 30 to 70% by volume or when the area ratio of the σ phase and chromium nitride, which are harmful precipitates, exceeds 0.1%, it is evaluated as “Poor”.
（４）耐孔食性
塩化第二鉄水溶液浸漬試験法にて実施した。
試験片：光輝焼鈍後の冷帯
試験溶液：６質量％FeCl3＋１質量％HCl水溶液
表面処理：光輝焼鈍後のまま(光輝焼鈍後の研磨なし、光輝焼鈍後の酸洗処理なし)
試験温度：１５℃〜７５℃
浸漬時間：２４時間
試験片数：各２個
各試験片について孔食深さが２５μｍ以上となる臨界孔食発生温度(CPT)の値を求め、この平均値が研磨仕上材のCPTと同じ場合を良好として「○」、研磨仕上材と比較したCPTの低下が０℃を超え、５℃未満の場合を良（許容範囲）として「△」、研磨仕上材と比較したCPTの低下が５℃以上の場合を不良として「×」と評価した。
(4) Pitting corrosion resistance It carried out by the ferric chloride aqueous solution immersion test method.
Test piece: Cold zone test solution after bright annealing: 6% by mass FeCl 3 + 1% by mass aqueous HCl surface treatment: As after bright annealing (no polishing after bright annealing, no pickling after bright annealing)
Test temperature: 15 ° C to 75 ° C
Immersion time: 24 hours Number of test pieces: 2 each When the value of critical pitting corrosion temperature (CPT) at which the pitting depth is 25 μm or more is obtained for each test piece, and this average value is the same as the CPT of the polished finish "C" for good finish, and CPT drop compared to abrasive finishes over 0 ° C and less than 5 ° C for good (acceptable range) "△", CPT drop compared to abrasive finishes at 5 ° C The above case was evaluated as “x” as a failure.
評価結果を、下記表２に示す。 The evaluation results are shown in Table 2 below.
表２に示すように、水素ガス雰囲気（水素ガス１００体積％、窒素ガス０体積％）下、温度が１０３０℃以上１１００℃以下、焼鈍時間が２０秒以上１２０秒以下の範囲にて光輝焼鈍を行なった試験番号１、３、４、６、８、１０、１１、１３、１４、１５、１８は、表面外観、表層の窒素含有量、表層組織のいずれも問題なく、耐孔食性も優れていた。これに対し、光輝焼鈍時の熱処理温度が１０３０℃よりも低い試験番号７は、有害な金属間化合物であるσ相が面積率で０．２％析出して表層組織が劣化した。さらに、試験番号７は、研磨仕上材と比較したCPTの低下が１５℃（鋼種Cの研磨仕上材のCPTは７５℃、試験番号７のCPTは６０℃）となり、耐孔食性も劣化した。光輝焼鈍時の熱処理温度が１１００℃よりも高く焼鈍時間も１２０秒より長い試験番号１６、１７は、表層組織のフェライト相の割合が７０体積％を超え、さらにクロム窒化物が面積率で０．２％析出して表層組織が劣化した。さらに、試験番号１６、１７は、表層の窒素含有量が２０質量％以上低下した。また、研磨仕上材と比較したCPTの低下が、試験番号１６では３５℃、試験番号１７では５５℃（鋼種Cの研磨仕上材のCPTは７５℃、試験番号１６のCPTは４０℃、試験番号１７のCPTは２０℃）となり、耐孔食性も劣化した。耐孔食性の劣化は、表層の窒素含有量が２０質量％以上低下し、かつクロム窒化物が析出し、周囲にクロムの欠乏領域を生成したことが原因と考えられる。 As shown in Table 2, bright annealing is performed in a hydrogen gas atmosphere (hydrogen gas 100 volume%, nitrogen gas 0 volume%) in a temperature range of 1030 ° C. to 1100 ° C. and an annealing time of 20 seconds to 120 seconds. Test Nos. 1, 3, 4, 6, 8, 10, 11, 13, 14, 15, and 18 were conducted without any problems in surface appearance, surface nitrogen content, and surface structure, and excellent in pitting corrosion resistance. It was. On the other hand, in test number 7 where the heat treatment temperature during bright annealing was lower than 1030 ° C., the σ phase, which is a harmful intermetallic compound, was precipitated by 0.2% in area ratio, and the surface layer structure deteriorated. Furthermore, in Test No. 7, the decrease in CPT compared to the polishing finish was 15 ° C. (CPT of the polishing finish of Steel Type C was 75 ° C., CPT of Test No. 7 was 60 ° C.), and the pitting corrosion resistance was also deteriorated. In Test Nos. 16 and 17, in which the heat treatment temperature during bright annealing is higher than 1100 ° C. and the annealing time is longer than 120 seconds, the proportion of the ferrite phase in the surface layer structure exceeds 70% by volume, and chromium nitride has an area ratio of 0.00. 2% precipitated and the surface layer structure deteriorated. In Test Nos. 16 and 17, the nitrogen content of the surface layer decreased by 20% by mass or more. In addition, the decrease in CPT compared with the polished finish was 35 ° C. for test number 16 and 55 ° C. for test number 17 (75 ° C. for the polished finish of steel type C, 40 ° C. for CPT of test number 16 and test number 17 CPT was 20 ° C.), and the pitting corrosion resistance was also deteriorated. The deterioration of the pitting corrosion resistance is considered to be caused by the fact that the nitrogen content of the surface layer is reduced by 20% by mass or more, and chromium nitrides are precipitated and a chromium-deficient region is generated in the surrounding area.
光輝焼鈍時の熱処理温度が１０３０〜１１００℃であるものの焼鈍時間が１２０秒より長い試験番号２、５、９、１２では、表層組織は良好であったが、表層の窒素含有量が２０質量％以上低下した。また、研磨仕上材と比較したCPTの低下が、試験番号２、５、９、１２で、それぞれ、５℃、５℃、５℃、１５℃（研磨仕上材のCPTは、それぞれ、鋼種Aで５０℃、鋼種Bで５５℃、鋼種Cで７５℃、試験番号２、５、９、１２のCPTは、それぞれ、４５℃、５０℃、７０℃、６０℃）となり、耐孔食性も劣化した。耐孔食性の劣化は、表層の窒素含有量が２０質量％以上低下したことが原因と考えられる。 In Test Nos. 2, 5, 9, and 12 in which the heat treatment temperature during bright annealing was 1030 to 1100 ° C. and the annealing time was longer than 120 seconds, the surface layer structure was good, but the nitrogen content of the surface layer was 20% by mass. More than that. Also, the decrease in CPT compared to the abrasive finish was 5 ° C, 5 ° C, 5 ° C, and 15 ° C for test numbers 2, 5, 9, and 12 (CPT of the abrasive finish was steel grade A, respectively. 50 ° C, 55 ° C for steel type B, 75 ° C for steel type C, and CPT of test numbers 2, 5, 9, and 12 were 45 ° C, 50 ° C, 70 ° C, and 60 ° C, respectively. . The deterioration of pitting corrosion resistance is considered to be caused by a decrease in the nitrogen content of the surface layer by 20% by mass or more.
光輝焼鈍時の焼鈍時間が２０秒以上１２０秒以下の範囲であるものの熱処理温度が１１００℃よりも高い試験番号１９では、表層組織のフェライト相の割合が７０体積％を超え、さらにクロム窒化物が面積率で０．１５％析出して表層組織が劣化した。さらに、試験番号１９は、表層の窒素含有量が２０質量％以上低下した。また、研磨仕上材と比較したCPTの低下が、試験番号１９では２５℃（鋼種Cの研磨仕上材のCPTは７５℃、試験番号１９のCPTは５０℃）となり、耐孔食性も劣化した。耐孔食性の劣化は、表層の窒素含有量が２０質量％以上低下し、かつクロム窒化物が析出し、周囲にクロムの欠乏領域を生成したことが原因と考えられる。 In the test number 19 where the annealing time during bright annealing is in the range of 20 seconds to 120 seconds but the heat treatment temperature is higher than 1100 ° C., the proportion of the ferrite phase in the surface layer structure exceeds 70% by volume, and chromium nitride The surface layer structure was deteriorated by precipitation of 0.15% by area ratio. In Test No. 19, the nitrogen content of the surface layer was reduced by 20% by mass or more. In addition, the decrease in CPT compared to the polished finish was 25 ° C. in Test No. 19 (CPT of the polished finish of Steel Type C was 75 ° C., CPT of Test No. 19 was 50 ° C.), and the pitting corrosion resistance was also deteriorated. The deterioration of the pitting corrosion resistance is considered to be caused by the fact that the nitrogen content of the surface layer is reduced by 20% by mass or more, and chromium nitrides are precipitated and a chromium-deficient region is generated in the surrounding area.
板厚が０．９ｍｍより薄い試験番号１４（板厚０．３ｍｍ）では、表層の窒素含有量は低下したものの、その低下量は１６質量％と、１５質量％以上２０質量％未満の許容範囲にとどまった。また、試験番号１４では、CPTが低下したものの、低下量は２．５℃と許容範囲にとどまり耐孔食性の低下を抑制できた。板厚が４ｍｍより厚い試験番号１５（板厚６．０ｍｍ）では、σ相の析出が面積率で０．０５％にとどまり、表層組織の劣化を抑制できた。また、試験番号１５ではCPTが低下したものの、その低下量は２．５℃と許容範囲にとどまり耐孔食性の低下を抑制できた。 In test number 14 (thickness 0.3 mm) where the plate thickness is less than 0.9 mm, the nitrogen content of the surface layer was reduced, but the fall amount was 16% by mass, and the allowable range was 15% by mass or more and less than 20% by mass. I stayed at. In Test No. 14, although CPT decreased, the decrease amount was 2.5 ° C., which was within an allowable range, and the decrease in pitting corrosion resistance could be suppressed. In the test number 15 (plate thickness 6.0 mm) where the plate thickness is larger than 4 mm, the precipitation of the σ phase was only 0.05% in terms of the area ratio, and the deterioration of the surface layer structure could be suppressed. Moreover, although CPT fell in the test number 15, the fall amount stayed at an allowable range of 2.5 ° C., and the reduction of pitting corrosion resistance could be suppressed.
本発明は、鋼表層からの脱窒と吸窒を防止して、品質安定性を損なうことなく多様な種類の鋼を連続的かつ容易に光輝焼鈍できるので、二相ステンレス鋼等のステンレス鋼の分野で利用価値が高い。 The present invention prevents denitrification and nitrogen absorption from the steel surface layer, and can continuously and easily brightly anneal various types of steel without losing quality stability. High utility value in the field.
Claims (4)
水素ガス１００体積％からなる雰囲気中または水素ガスと１．０体積％以下の希ガスとからなる混合ガスの雰囲気中で、温度が１０３０〜１１００℃、焼鈍時間が２０〜１２０秒で焼鈍することを特徴とする二相ステンレス鋼の光輝焼鈍方法。 A bright annealing method for duplex stainless steel having a structure of ferrite phase / austenite phase containing 0.16 to 0.32% by mass of nitrogen,
In an atmosphere of 100% by volume of hydrogen gas or a mixed gas atmosphere of hydrogen gas and 1.0% by volume or less of rare gas , annealing is performed at a temperature of 1030 to 1100 ° C. and an annealing time of 20 to 120 seconds. A bright annealing method for duplex stainless steel characterized by
C：０．００１〜０．０３０質量％
Si：０．０５〜１．００質量％
Mn：０．１〜２．０質量％
Cr：２３〜２９質量％
Ni：５．０〜９．０質量％
Mo：２．０〜５．０質量％
N：０．１６〜０．３２質量％
残部がFeおよび不可避的不純物からなり、フェライト相・オーステナイト相の組織を有し、前記フェライト相が３０〜７０体積％であることを特徴とする請求項１に記載の二相ステンレス鋼の光輝焼鈍方法。 The duplex stainless steel is
C: 0.001 to 0.030 mass%
Si: 0.05-1.00 mass%
Mn: 0.1 to 2.0% by mass
Cr: 23 to 29% by mass
Ni: 5.0-9.0 mass%
Mo: 2.0-5.0 mass%
N: 0.16-0.32 mass%
The bright annealing of the duplex stainless steel according to claim 1, wherein the balance is composed of Fe and inevitable impurities, has a structure of a ferrite phase / austenite phase, and the ferrite phase is 30 to 70% by volume. Method.
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