JP6482074B2 - Duplex stainless steel sheet and its manufacturing method - Google Patents
Duplex stainless steel sheet and its manufacturing method Download PDFInfo
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- 238000009835 boiling Methods 0.000 claims description 4
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Description
本発明は、化学プラント等、極めて優れた耐粒界腐食性が要求される環境において使用される二相ステンレス鋼板とその製造方法に関するものである。 The present invention relates to a duplex stainless steel sheet used in an environment where extremely excellent intergranular corrosion resistance is required, such as a chemical plant, and a method for producing the same.
二相ステンレス鋼は、強度と耐食性を兼ね備えた優れた材料であり、海水などのように高い濃度で塩化物が存在する環境下や、油井や化学プラントなどのように厳しい腐食を受ける環境下でも優れた耐食性を有する材料として知られている。そのため、二相ステンレス鋼は、化学プラントの中でも特に厳しい腐食を受ける尿素プラントにも使用されており、尿素生産量の増大にともない、需要が年々増加する傾向にある。 Duplex stainless steel is an excellent material that combines strength and corrosion resistance, even in environments where chlorides are present at high concentrations such as seawater, or in environments subject to severe corrosion such as oil wells or chemical plants. It is known as a material having excellent corrosion resistance. Therefore, duplex stainless steel is also used in urea plants that are subjected to particularly severe corrosion among chemical plants, and the demand tends to increase year by year as the production amount of urea increases.
二相ステンレス鋼の耐食性を向上させる方法には、合金成分を適正化する方法、介在物を制御する方法、固溶化熱処理温度と冷却速度を適正に制御する方法等を挙げることができる。例えば、二相ステンレス鋼の合金成分を適正範囲に制御する方法としては、特許文献１には、Ｎｄを適正量添加することに加えて、Ｎｄ，Ｐ，Ｓ，ＡｌおよびＭｏの含有量が所定の関係を満たすよう添加することによって、相界面におけるＰ，Ｓを効果的に補足し、さらに、σ相の生成を抑制して耐粒界腐食性を飛躍的に向上させた二相ステンレス鋼が開示されている。
Examples of methods for improving the corrosion resistance of the duplex stainless steel include a method for optimizing alloy components, a method for controlling inclusions, and a method for appropriately controlling the solution heat treatment temperature and cooling rate. For example, as a method for controlling the alloy components of duplex stainless steel within an appropriate range,
また、介在物制御により耐食性を向上させる方法として、例えば、特許文献２には、鋼中に含まれるＣａ濃度を０．０００５ｍａｓｓ％以下に低減し、個々のＣａＯ含有介在物におけるＣａＯ濃度を４０ｍａｓｓ％以下にするとともに、全非金属介在物に占めるＣａＯ含有介在物の個数比率を４０％以下、全非金属介在物におけるＣａＯ濃度を１０ｍａｓｓ％以下にすることで耐孔食性を向上させた二相ステンレス鋼が開示されている。
As a method for improving corrosion resistance by inclusion control, for example,
また、固溶化熱処理温度と冷却速度を適正に制御する方法として、例えば、特許文献３には、連続鋳造にて製造したスラブを１１００℃〜１３００℃の範囲で２時間以上加熱した後、熱間圧延し、その後１０５０℃以上１３００℃未満の範囲で固溶化熱処理を実施し、直ちに３℃／ｓ以上で冷却をすることで、σ相の生成を抑制し、耐孔食性を向上した二相ステンレス鋼を得る方法が、また、特許文献４には、１１００℃以上の温度で鍛造または圧延後、直ちに冷却速度１００℃／分以上で８００℃以下まで急冷してフェライト相内に炭窒化物を析出させた後、固溶化熱処理温度９５０〜１１００℃に加熱し、急冷することで耐食性に優れた二相系ステンレス鋼を得る方法が開示されている。
In addition, as a method for appropriately controlling the solution heat treatment temperature and the cooling rate, for example, in
しかしながら、上記の特許文献１に開示の技術のように、合金成分を適正化する方法では、Ｎｄのような高価な元素を添加することが多く、原料コストの増加を伴うことが多い。また、特許文献２に開示の技術のように、介在物を制御する方法では、耐孔食性は向上できるが、耐粒界腐食性をも向上することはできない。また、特許文献３や特許文献４に開示の技術は、開示された熱処理条件や冷却条件で二相ステンレス鋼板を製造し、σ相や炭化物、窒化物が観察されないようにしても、依然として耐食性に劣る場合があった。
However, in the method of optimizing the alloy components as in the technique disclosed in
本発明は、従来技術が抱える上記問題点に鑑みてなされたものであり、その目的は、高価な合金元素を添加することなく、耐粒界腐食性に優れる二相ステンレス鋼板を安価に提供するとともに、その有利な製造方法を提案することにある。 The present invention has been made in view of the above-mentioned problems of the prior art, and the object thereof is to provide a duplex stainless steel sheet excellent in intergranular corrosion resistance at low cost without adding an expensive alloy element. At the same time, an advantageous manufacturing method is proposed.
発明者らは、上記課題の解決に向けて、二相ステンレス鋼板に施す固溶化熱処理条件が、α／γ相界面（粒界）近傍の成分濃度分布に及ぼす影響に着目して鋭意検討を重ねた。その結果、耐粒界腐食性に劣る二相ステンレス鋼板は、α／γ相界面のγ相側にＣｒ濃度の低い領域が存在しており、これが、耐粒界腐食性を低下させている原因であること、そして、そのＣｒが欠乏した領域の生成を防止するためには、二相ステンレス鋼板に施す固溶化熱処理における焼鈍温度および冷却速度を適正化することが有効であることを見出し、本発明を開発するに至った。 In order to solve the above problems, the inventors have made extensive studies focusing on the effect of the solution heat treatment conditions applied to the duplex stainless steel sheet on the component concentration distribution near the α / γ phase interface (grain boundary). It was. As a result, the duplex stainless steel sheet inferior in intergranular corrosion resistance has a region with a low Cr concentration on the γ phase side of the α / γ phase interface, which is the cause of decreasing intergranular corrosion resistance. In order to prevent the formation of the Cr-deficient region, it has been found effective to optimize the annealing temperature and cooling rate in the solution heat treatment applied to the duplex stainless steel sheet. Invented the invention.
すなわち、本発明は、Ｃ：０．０５ｍａｓｓ％以下、Ｓｉ：０．１０〜１．００ｍａｓｓ％、Ｍｎ：０．３〜２．０ｍａｓｓ％、Ｐ：０．０１０〜０．０５０ｍａｓｓ％、Ｓ：０．０００１〜０．０２ｍａｓｓ％、Ａｌ：０．００１〜０．０５ｍａｓｓ％、Ｎ：０．０５〜０．４ｍａｓｓ％、Ｎｉ：４〜９ｍａｓｓ％、Ｃｒ：２０〜２７ｍａｓｓ％、Ｍｏ：２〜５ｍａｓｓ％、Ｃｕ：０．０１〜０．３０ｍａｓｓ％、Ｗ：０．０１〜０．４ｍａｓｓ％、Ｂ：０．０００１〜０．００１ｍａｓｓ％およびＣａ：０．０００６〜０．０１ｍａｓｓ％を含有し、残部がＦｅおよび不可避的不純物からなる成分組成を有し、金属組織内にσ相が存在せず、かつ、α／γ相界面からγ相側に０．５μｍまでの領域の最低Ｃｒ濃度と、同一γ相内で、α／γ相界面からγ相側に０．５μｍより離れた領域の平均Ｃｒ濃度の差が１．５ｍａｓｓ％未満であることを特徴とする二相ステンレス鋼板である。 That is, the present invention is C: 0.05 mass% or less, Si: 0.10 to 1.00 mass%, Mn: 0.3 to 2.0 mass%, P: 0.010 to 0.050 mass%, S: 0 0.0001-0.02 mass%, Al: 0.001-0.05 mass%, N: 0.05-0.4 mass%, Ni: 4-9 mass%, Cr: 20-27 mass%, Mo: 2-5 mass% Cu: 0.01-0.30 mass%, W: 0.01-0.4 mass%, B: 0.0001-0.001 mass%, and Ca: 0.0006-0.01 mass%, the balance being It has a composition composed of Fe and inevitable impurities, has no σ phase in the metal structure, and has the same γ as the minimum Cr concentration in the region from the α / γ phase interface to 0.5 μm on the γ phase side. Within the phase, α / γ phase boundary It is a duplex stainless steel sheet characterized in that the difference in average Cr concentration in a region separated from the surface to the γ phase side by more than 0.5 μm is less than 1.5 mass%.
本発明の二相ステンレス鋼板は、上記成分組成に加えてさらに、Ｖ：０．００３〜０．５ｍａｓｓ％およびＮｂ：０．００３〜０．５ｍａｓｓ％のうちから選ばれる１種または２種を含有することを特徴とする。 In addition to the above component composition, the duplex stainless steel sheet of the present invention further contains one or two selected from V: 0.003 to 0.5 mass% and Nb: 0.003 to 0.5 mass%. It is characterized by doing.
また、本発明の二相ステンレス鋼板は、７０ｍａｓｓ％沸騰硝酸による腐食速度が０．３０ｇ／ｍ２・ｈｒ未満であることを特徴とする。 Further, the duplex stainless steel sheet of the present invention is characterized in that the corrosion rate by 70 mass% boiling nitric acid is less than 0.30 g / m 2 · hr.
また、本発明は、上記のいずれかに記載の二相ステンレス鋼板を製造するに際し、常法に従って製造した熱延鋼板あるいは冷延鋼板を１０００〜１１００℃の温度に加熱した後８００℃以上の温度まで冷却速度３℃／ｓ以上で冷却し、その後、直ちに６℃／ｓ以上で冷却する固溶化熱処理を施すことを特徴とする二相ステンレス鋼板の製造方法を提案する。 Moreover, when manufacturing the duplex stainless steel sheet in any one of said, this invention heats the hot-rolled steel plate or cold-rolled steel plate manufactured in accordance with the conventional method to the temperature of 1000-1100 degreeC, Then, the temperature of 800 degreeC or more We propose a method for producing a duplex stainless steel sheet, characterized in that it is cooled at a cooling rate of 3 ° C./s or more and then subjected to solution heat treatment immediately after cooling at 6 ° C./s or more.
本発明によれば、耐粒界腐食性に優れる二相ステンレス鋼板を安価に提供することができるので、化学プラントなど、粒界腐食の発生が懸念される環境下で使用される耐食性材料として好適に用いることができる。 According to the present invention, a duplex stainless steel sheet having excellent intergranular corrosion resistance can be provided at a low cost, which is suitable as a corrosion resistant material used in an environment where the occurrence of intergranular corrosion is a concern, such as a chemical plant. Can be used.
まず、本発明の基本的な技術思想について説明する。
前述したように、同一成分で、かつ、図１に示した写真のように、α／γ相界面（結晶粒界）や結晶粒内にσ相や炭化物、窒化物等の析出物が全く観察されない二相ステンレス鋼板においても、粒界腐食試験における腐食速度に大きな差異が生じることが多々認められた。
そこで、発明者らは、上記耐粒界腐食性の差は、粒界近傍における何らかの成分濃度の差異によるものと考え、α／γ相界面近傍の成分分析を行った。具体的には、耐粒界腐食性に劣る二相ステンレス鋼板と耐粒界腐食性が良好な二相ステンレス鋼板から試料を採取し、圧延方向に垂直な断面を観察面とする透過電子顕微鏡（ＴＥＭ）用の試料を作製し、ＴＥＭに付属のエネルギー分散Ｘ線分光分析（ＥＤＸ分析）装置を用いて、α／γ相界面を基準にし、α相側およびγ相側に、それぞれ６０ｎｍ間隔で点分析を行った。
First, the basic technical idea of the present invention will be described.
As described above, precipitates such as σ phase, carbide, nitride, etc. are completely observed in the same component and in the α / γ phase interface (grain boundary) and in the crystal grains as shown in the photograph in FIG. It was often found that even in the duplex stainless steel sheet that was not used, a large difference in the corrosion rate in the intergranular corrosion test occurred.
Therefore, the inventors considered that the difference in intergranular corrosion resistance was due to a difference in some component concentration in the vicinity of the grain boundary, and conducted component analysis in the vicinity of the α / γ phase interface. Specifically, samples were taken from a duplex stainless steel sheet with poor intergranular corrosion resistance and a duplex stainless steel sheet with good intergranular corrosion resistance, and a transmission electron microscope with a cross section perpendicular to the rolling direction as the observation surface ( TEM) sample is prepared, and using the energy dispersive X-ray spectroscopic analysis (EDX analysis) apparatus attached to the TEM, the α / γ phase interface is used as a reference, and the α phase side and the γ phase side are respectively spaced by 60 nm. Point analysis was performed.
その結果、耐粒界腐食性が良好な二相ステンレス鋼板では、図２に示したように、α／γ相界面に対してγ相側のα／γ相界面近傍にはＣｒ濃度分布に大きな変動は認められなかったが、耐粒界腐食性に劣る二相ステンレス鋼板では、図３に示したように、α／γ相界面に対してγ相側のα／γ相界面近傍には、γ相の内部よりＣｒ濃度の低い領域（以降、本発明では、この領域を「Ｃｒ欠乏領域」という）が存在しており、これが、耐粒界腐食性を低下させている原因であること、特に、上記のα／γ相界面近傍のＣｒ欠乏領域の最低Ｃｒ濃度と、上記Ｃｒ欠乏領域が存在するγ相内部の平均Ｃｒ濃度の差が、１．５ｍａｓｓ％以上になると、耐粒界腐食性が大きく低下することが明らかとなった。なお、この「Ｃｒ欠乏領域」は、クロム炭化物の形成を伴わないことから、従来から知られている「Ｃｒ欠乏層」とは全く異なるものである。 As a result, in the duplex stainless steel sheet having good intergranular corrosion resistance, as shown in FIG. 2, the Cr concentration distribution is large in the vicinity of the α / γ phase interface on the γ phase side with respect to the α / γ phase interface. In the duplex stainless steel sheet inferior in intergranular corrosion resistance, as shown in FIG. 3, in the vicinity of the α / γ phase interface on the γ phase side relative to the α / γ phase interface, There is a region where the Cr concentration is lower than the inside of the γ phase (hereinafter, this region is referred to as “Cr-deficient region” in the present invention), and this is the cause of reducing the intergranular corrosion resistance, In particular, when the difference between the minimum Cr concentration in the Cr-deficient region near the α / γ phase interface and the average Cr concentration in the γ-phase in which the Cr-deficient region exists is 1.5 mass% or more, the intergranular corrosion resistance It became clear that the nature decreased greatly. The “Cr-deficient region” is completely different from the conventionally known “Cr-deficient layer” because it does not involve the formation of chromium carbide.
そこで、上記Ｃｒ欠乏領域が形成される原因を究明するため、耐粒界腐食性に劣る二相ステンレス鋼板と耐粒界腐食性が良好な二相ステンレス鋼板の製造履歴を詳細に調査した。その結果、両鋼板の間には、固溶化熱処理における焼鈍温度や冷却速度に違いがあり、特に、Ｃｒ欠乏領域が認められた鋼板は、Ｃｒ欠乏領域が認めらないあるいは軽度であった鋼板と比較して、冷却速度が遅い傾向があることがわかった。 Therefore, in order to investigate the cause of the formation of the Cr-deficient region, the production history of a duplex stainless steel sheet with poor intergranular corrosion resistance and a duplex stainless steel sheet with good intergranular corrosion resistance was investigated in detail. As a result, there is a difference in the annealing temperature and cooling rate in the solution heat treatment between the two steel plates. In particular, the steel plate in which the Cr-deficient region is recognized is the steel plate in which the Cr-deficient region is not recognized or mild In comparison, it was found that the cooling rate tends to be slow.
以上の結果から、発明者らは、上記Ｃｒ欠乏領域が形成されるメカニズムについて、以下のように推定している。
固溶化熱処理において二相域の温度に加熱した二相ステンレス鋼を冷却すると、フェライト相（α相）とオーステナイト相（γ相）は、それぞれ平衡状態における相比になろうとしてα／γ相界面（粒界）が移動すると同時に、α相中およびγ相中のＣｒ濃度も平衡濃度になろうとしてＣｒの拡散が起こる。
本発明の成分組成を有する二相ステンレス鋼の場合、状態図上では、固溶化熱処理温度から低下するほど、γ相の比率が増加し、平衡Ｃｒ濃度は低下する。そのため、本発明の二相ステンレス鋼は、固溶化熱処理温度からの冷却中に、α／γ相界面がα相側に移動してγ相の領域が拡大するとともに、新たにγ相となった領域は、従来からあるγ相と比較してのＣｒ濃度が低くなる。
From the above results, the inventors estimate the mechanism by which the Cr-depleted region is formed as follows.
When the duplex stainless steel heated to a temperature in the two-phase region is cooled in the solution heat treatment, the ferrite phase (α phase) and the austenite phase (γ phase) become α / γ phase interfaces in an attempt to reach the phase ratio in the equilibrium state. Simultaneously with the movement of the (grain boundaries), Cr diffusion in the α phase and the γ phase tends to reach an equilibrium concentration.
In the case of the duplex stainless steel having the composition of the present invention, on the phase diagram, the proportion of the γ phase increases and the equilibrium Cr concentration decreases as the temperature decreases from the solution heat treatment temperature. Therefore, during the cooling from the solution heat treatment temperature, the duplex stainless steel of the present invention moved to the α phase side and the region of the γ phase expanded and became a new γ phase. In the region, the Cr concentration is lower than that of the conventional γ phase.
ここで、上記の冷却速度が遅く、Ｃｒの拡散時間が十分に確保できる場合には、Ｃｒ濃度が低い領域にＣｒが供給されて、上記Ｃｒ濃度の低い領域は解消される。しかし、固溶化熱処理において通常行われているような緩速冷却では、上記のようなＣｒ拡散によるＣｒ濃度の均一化効果を期待することはできないため、局所的な「Ｃｒ欠乏領域」が残存してしまう。そこで、本発明は、固溶化熱処理温度から急速冷却し、冷却中におけるα／γ相界面の移動を防止することで、「Ｃｒ欠乏領域」の形成を抑止することとした。
本発明は、上記の新規な知見に、さらに検討を加えてなされたものである。
Here, when the cooling rate is slow and a sufficient Cr diffusion time can be secured, Cr is supplied to a region having a low Cr concentration, and the region having a low Cr concentration is eliminated. However, in the slow cooling that is normally performed in the solution heat treatment, it is not possible to expect the effect of uniforming the Cr concentration by the Cr diffusion as described above, so a local “Cr deficient region” remains. End up. Therefore, in the present invention, the formation of the “Cr-deficient region” is suppressed by rapidly cooling from the solution heat treatment temperature and preventing the movement of the α / γ phase interface during the cooling.
The present invention has been made by further studying the above-described novel findings.
次に、本発明の二相ステンレス鋼板が有すべき成分組成について説明する。
Ｃ：０．０５ｍａｓｓ％以下
Ｃは、Ｃｒとクロム炭化物を形成し、耐食性の向上に必要なＣｒの濃度を低下させ、いわゆる「Ｃｒ欠乏層」を生成させる元素であるので、できる限り低減するのが望ましい。しかし、Ｃの過度の低減は、鋼の強度低下を招いたり、製造コストの上昇を招いたりする。よって、Ｃの含有量は０．０５ｍａｓｓ％以下とする。好ましくは０．００５〜０．０４０ｍａｓｓ％、より好ましくは０．０１０〜０．０３５ｍａｓｓ％の範囲である。
Next, the component composition that the duplex stainless steel sheet of the present invention should have will be described.
C: 0.05 mass% or less C is an element that forms Cr and chromium carbide, lowers the concentration of Cr necessary for improving corrosion resistance, and generates a so-called “Cr-deficient layer”. Is desirable. However, excessive reduction of C causes a decrease in steel strength or an increase in manufacturing cost. Therefore, the C content is 0.05 mass% or less. Preferably it is 0.005-0.040 mass%, More preferably, it is the range of 0.010-0.035 mass%.
Ｓｉ：０．１０〜１．００ｍａｓｓ％
Ｓｉは、脱酸材として添加される元素であり、０．１０ｍａｓｓ％以上含有させる必要がある。しかし、Ｓｉの過剰添加は、上記効果が飽和する他、延性の低下や強度の上昇を招き、さらには、σ相やχ相などの金属間化合物の析出を助長して耐食性を低下させる。よって、Ｓｉの含有量は０．１０〜１．００ｍａｓｓ％の範囲とする。好ましくは０．１５〜０．７５ｍａｓｓ％、より好ましくは０．２０〜０．６０ｍａｓｓ％の範囲である。
Si: 0.10 to 1.00 mass%
Si is an element added as a deoxidizing material and needs to be contained in an amount of 0.10 mass% or more. However, excessive addition of Si not only saturates the above effects, but also reduces ductility and strength, and further promotes precipitation of intermetallic compounds such as σ phase and χ phase, thereby reducing corrosion resistance. Therefore, the Si content is in the range of 0.10 to 1.00 mass%. Preferably it is 0.15-0.75 mass%, More preferably, it is the range of 0.20-0.60 mass%.
Ｍｎ：０．３〜２．０ｍａｓｓ％
Ｍｎは、オーステナイト生成元素であり、同じオーステナイト生成元素である高価なＮｉの代替となり得るものであるので、０．３ｍａｓｓ％以上添加する。しかし、過度の添加は、耐食性を低下させるσ相やχ相などの金属間化合物の析出を促進するので、上限は２．０ｍａｓｓ％とする。好ましくは０．４〜１．５ｍａｓｓ％、より好ましくは０．５〜１．２ｍａｓｓ％の範囲である。
Mn: 0.3 to 2.0 mass%
Mn is an austenite-forming element and can be an alternative to expensive Ni, which is the same austenite-forming element, so 0.3 mass% or more is added. However, excessive addition promotes the precipitation of intermetallic compounds such as σ phase and χ phase that lower the corrosion resistance, so the upper limit is made 2.0 mass%. Preferably it is 0.4-1.5 mass%, More preferably, it is the range of 0.5-1.2 mass%.
Ｐ：０．０１０〜０．０５０ｍａｓｓ％
Ｐは、不可避的に混入してくる不純物元素であり、結晶粒界に偏析し易く、耐食性や熱間加工性を低下させるので、できる限り低減するのが望ましい。しかし、Ｐは、耐粒界腐食性を大きく低下させるＢと競合して粒界に偏析するため、一定量のＰを含有させることで、Ｂの粒界への偏析を軽減することができる。また、一定量のＰを粒界に偏析させることで、冷却中の相比変化によるα／γ相界面の移動が抑制されるので、Ｃｒ欠乏領域の形成を抑制する効果もある。上記の効果は、０．０１０ｍａｓｓ％以上の添加で得られる。しかし、０．０５０ｍａｓｓ％を超える添加は、耐粒界腐食性を著しく低下させる。よって、Ｐの含有量は０．０１０〜０．０５０ｍａｓｓ％の範囲とする。好ましくは０．０１５〜０．０４０ｍａｓｓ％の範囲である。
P: 0.010-0.050 mass%
P is an impurity element that is inevitably mixed in, and is easily segregated at the crystal grain boundaries, and lowers corrosion resistance and hot workability, so it is desirable to reduce it as much as possible. However, P competes with B, which greatly reduces intergranular corrosion resistance, and segregates at the grain boundary. Therefore, by containing a certain amount of P, segregation of B to the grain boundary can be reduced. In addition, since a certain amount of P is segregated at the grain boundaries, the movement of the α / γ phase interface due to the change in the phase ratio during cooling is suppressed, so there is also an effect of suppressing the formation of the Cr-deficient region. Said effect is acquired by addition of 0.010 mass% or more. However, addition exceeding 0.050 mass% significantly reduces the intergranular corrosion resistance. Therefore, the P content is in the range of 0.010 to 0.050 mass%. Preferably it is the range of 0.015-0.040 mass%.
Ｓ：０．０００１〜０．０２ｍａｓｓ％
Ｓは、Ｐと同様、不可避的に混入してくる不純物元素であり、結晶粒界に偏析し易く、耐食性や熱間加工性を低下させるので、上限を０．０２ｍａｓｓ％とする。しかし、Ｓは、Ｐと同様、一定量を含有させることで、冷却中のα／γ相界面の移動を抑制し、Ｃｒ欠乏領域の生成を抑止する効果がある。そこで、本発明では、Ｓの含有量は０．０００１〜０．０２ｍａｓｓ％の範囲とする。好ましくは０．０００３〜０．０１ｍａｓｓ％の範囲である。
S: 0.0001 to 0.02 mass%
S, like P, is an impurity element that is inevitably mixed in, and is easily segregated at the crystal grain boundaries, and lowers corrosion resistance and hot workability. Therefore, the upper limit is set to 0.02 mass%. However, S, like P, has the effect of suppressing the movement of the α / γ phase interface during cooling and suppressing the generation of Cr-deficient regions by containing a certain amount. Therefore, in the present invention, the S content is in the range of 0.0001 to 0.02 mass%. Preferably it is the range of 0.0003-0.01 mass%.
Ａｌ：０．００１〜０．０５ｍａｓｓ％
Ａｌは、強力な脱酸材であり、０．００１ｍａｓｓ％以上添加する必要がある。しかし、０．０５ｍａｓｓ％を超えて添加しても、その効果が飽和するだけでなく、鋼板の表面品質（外観）や耐食性に悪影響を及ぼす巨大介在物の形成を助長し、さらには、Ｎと結合してＡｌＮを形成し、耐食性に有効なＮを低減する。よって、Ａｌは０．００１〜０．０５ｍａｓｓ％の範囲とする。好ましくは０．００５〜０．０４ｍａｓｓ％の範囲である。
Al: 0.001 to 0.05 mass%
Al is a strong deoxidizing material, and it is necessary to add 0.001 mass% or more. However, even if added over 0.05 mass%, the effect is not only saturated, but also promotes the formation of giant inclusions that adversely affect the surface quality (appearance) and corrosion resistance of the steel sheet. Bonds to form AlN and reduces N effective for corrosion resistance. Therefore, Al is set to a range of 0.001 to 0.05 mass%. Preferably it is the range of 0.005-0.04 mass%.
Ｎ：０．０５〜０．４ｍａｓｓ％
Ｎは、強力なオーステナイト生成元素であり、また、ＣｒやＭｏと同様、耐食性を向上するとともに、金属間化合物の析出を抑制するのに有効な元素であるので、０．０５ｍａｓｓ％以上含有させる必要がある。しかし、０．４ｍａｓｓ％を超えて添加すると、熱間変形抵抗が上昇して熱間加工性を害するだけでなく、二相組織を維持することが困難になる。よって、Ｎは０．０５〜０．４ｍａｓｓ％の範囲とする。好ましくは０．０６〜０．３ｍａｓｓ％、より好ましくは０．０８〜０．２５ｍａｓｓ％の範囲である。
N: 0.05 to 0.4 mass%
N is a strong austenite-forming element and, like Cr and Mo, is an element effective for improving corrosion resistance and suppressing the precipitation of intermetallic compounds, so it is necessary to contain 0.05 mass% or more. There is. However, if added over 0.4 mass%, the hot deformation resistance is increased and the hot workability is impaired, and it is difficult to maintain a two-phase structure. Therefore, N is set to a range of 0.05 to 0.4 mass%. Preferably it is 0.06-0.3 mass%, More preferably, it is the range of 0.08-0.25 mass%.
Ｎｉ：４〜９ｍａｓｓ％
Ｎｉは、オーステナイト生成元素であり、フェライト組織との二相組織を維持するためには、４ｍａｓｓ％以上含有させる必要がある。しかし、９ｍａｓｓ％を超える添加は、オーステナイト組織が過剰になると共に、過不動態腐食の加速因子となり、耐食性を低下させる。よって、Ｎｉの含有量は４〜９ｍａｓｓ％の範囲とする。好ましくは５．０〜８．５ｍａｓｓ％、より好ましくは５．５〜８．０ｍａｓｓ％の範囲である。
Ni: 4-9 mass%
Ni is an austenite-forming element and needs to be contained in an amount of 4 mass% or more in order to maintain a two-phase structure with the ferrite structure. However, the addition exceeding 9 mass% causes an austenite structure to be excessive and an acceleration factor for overpassive corrosion, thereby reducing the corrosion resistance. Therefore, the Ni content is in the range of 4-9 mass%. Preferably it is the range of 5.0-8.5 mass%, More preferably, it is the range of 5.5-8.0 mass%.
Ｃｒ：２０〜２７ｍａｓｓ％
Ｃｒは、耐食性を向上させる元素であり、その効果を得るためには２０ｍａｓｓ％以上含有させる必要がある。しかし、２７ｍａｓｓ％を超えて添加すると、σ相やχ相などの金属間化合物の形成を助長し、かえって耐食性を低下させる。また、Ｃｒは、フェライト生成元素であり、過剰な添加は二相組織の維持を困難とする。よって、Ｃｒの含有量は２０〜２７ｍａｓｓ％の範囲とする。好ましくは２２〜２６．５ｍａｓｓ％、より好ましくは２３〜２６ｍａｓｓ％の範囲である。
Cr: 20-27 mass%
Cr is an element that improves the corrosion resistance. In order to obtain the effect, it is necessary to contain 20 mass% or more. However, when added in excess of 27 mass%, formation of intermetallic compounds such as σ phase and χ phase is promoted, and the corrosion resistance is reduced. Further, Cr is a ferrite-forming element, and excessive addition makes it difficult to maintain a two-phase structure. Therefore, the Cr content is in the range of 20 to 27 mass%. Preferably it is 22-26.5 mass%, More preferably, it is the range of 23-26 mass%.
Ｍｏ：２〜５ｍａｓｓ％
Ｍｏは、耐食性の向上に有効な元素である。その効果を得るためには２ｍａｓｓ％以上添加する必要がある。しかし、５ｍａｓｓ％を超えて添加すると、金属間化合物の析出を助長し、かえって耐食性を低下させるので、上限は５ｍａｓｓ％とする。好ましくは２．５〜４．３ｍａｓｓ％、より好ましくは３．０〜４．０ｍａｓｓ％の範囲である。
Mo: 2-5 mass%
Mo is an element effective for improving the corrosion resistance. In order to acquire the effect, it is necessary to add 2 mass% or more. However, if added over 5 mass%, the precipitation of intermetallic compounds is promoted and the corrosion resistance is lowered, so the upper limit is made 5 mass%. Preferably it is 2.5-4.3 mass%, More preferably, it is the range of 3.0-4.0 mass%.
Ｃｕ：０．０１〜０．３０ｍａｓｓ％
Ｃｕは、一般的な耐食性の向上に有効な元素である。上記効果を得るためには０．０１ｍａｓｓ％以上含有させる必要がある。一方、尿素プラント等の特定の腐食環境においては、却って腐食を進行させる元素となるので、上限は０．３０ｍａｓｓ％に制限する必要がある。よって、Ｃｕは０．０１〜０．３０ｍａｓｓ％の範囲とする。好ましくは０．０５〜０．２５ｍａｓｓ％、より好ましくは０．０８〜０．２０ｍａｓｓ％の範囲である。
Cu: 0.01-0.30 mass%
Cu is an element effective for improving general corrosion resistance. In order to acquire the said effect, it is necessary to contain 0.01 mass% or more. On the other hand, in a specific corrosive environment such as a urea plant, it becomes an element that causes corrosion to proceed. Therefore, the upper limit needs to be limited to 0.30 mass%. Therefore, Cu is set to a range of 0.01 to 0.30 mass%. Preferably it is 0.05-0.25 mass%, More preferably, it is the range of 0.08-0.20 mass%.
Ｗ：０．０１〜０．４ｍａｓｓ％
Ｗは、Ｍｏとの共存下において、二相ステンレス鋼の耐食性を向上させる元素であり、０．０１ｍａｓｓ％以上の添加が必要である。しかし、０．４ｍａｓｓ％を超える添加は、σ相やχ相などの金属間化合物の析出を助長し、耐食性を低下させる。よって、Ｗは０．０１〜０．４ｍａｓｓ％の範囲とする。好ましくは０．０５〜０．３ｍａｓｓ％、より好ましくは０．０８〜０．２ｍａｓｓ％の範囲である。
W: 0.01-0.4 mass%
W is an element that improves the corrosion resistance of the duplex stainless steel in the presence of Mo and needs to be added in an amount of 0.01 mass% or more. However, addition exceeding 0.4 mass% promotes precipitation of intermetallic compounds such as σ phase and χ phase, and lowers corrosion resistance. Therefore, W is in the range of 0.01 to 0.4 mass%. Preferably it is 0.05-0.3 mass%, More preferably, it is the range of 0.08-0.2 mass%.
Ｂ：０．０００１〜０．００１ｍａｓｓ％
Ｂは、熱間加工性の向上に極めて有効な元素であり、上記効果は極微量の添加でも得られる。しかし、Ｂは、粒界に偏析し、耐粒界腐食性を大きく低下させる元素でもある。よって、Ｂの含有量は０．０００１〜０．００１ｍａｓｓ％の範囲とする。
B: 0.0001 to 0.001 mass%
B is an extremely effective element for improving hot workability, and the above effect can be obtained even by adding a very small amount. However, B is also an element that segregates at grain boundaries and greatly reduces intergranular corrosion resistance. Therefore, the B content is in the range of 0.0001 to 0.001 mass%.
Ｃａ：０．０００６〜０．０１ｍａｓｓ％
Ｃａは、熱間加工性に有害なＳと結合してＣａＳを形成し、熱間加工性を改善するのに有効な元素であり、上記効果を得るためには０．０００６ｍａｓｓ％以上含有させる必要がある。しかし、０．０１ｍａｓｓ％を超える添加は、ＣａＯを含有する介在物を形成し、かえって耐食性を低下させる。よって、Ｃａは０．０００６〜０．０１ｍａｓｓ％の範囲とする。好ましくは０．０００７〜０．００５ｍａｓｓ％の範囲である。
Ca: 0.0006 to 0.01 mass%
Ca is an element effective to improve hot workability by combining with S harmful to hot workability to form CaS. In order to obtain the above effect, it is necessary to contain 0.0006 mass% or more. There is. However, addition exceeding 0.01 mass% forms inclusions containing CaO, and rather reduces the corrosion resistance. Therefore, Ca is set to a range of 0.0006 to 0.01 mass%. Preferably it is the range of 0.0007-0.005 mass%.
本発明の二相ステンレス鋼は、上記必須とする成分以外の残部は、Ｆｅおよび不可避的不純物である。ただし、必要に応じて、ＶおよびＮｂのうちから選ばれる１種または２種を下記の範囲で含有してもよい。
Ｖ：０．００３〜０．５ｍａｓｓ％、Ｎｂ：０．００３〜０．５ｍａｓｓ％
ＶおよびＮｂは、耐食性の向上に有効な元素であり、その効果を得るためには、それぞれ０．００３ｍａｓｓ％以上添加するのが好ましい。しかし、０．５ｍａｓｓ％を超えて添加すると、σ相やχ相などの金属間化合物の析出を助長し、耐食性を低下させたり、熱間加工性を害したりする。よって、上記元素を添加する場合には、それぞれの上記範囲で添加するのが好ましい。より好ましくは、それぞれ０．００３〜０．２ｍａｓｓ％の範囲である。
In the duplex stainless steel of the present invention, the balance other than the essential components is Fe and inevitable impurities. However, you may contain 1 type or 2 types chosen from V and Nb in the following range as needed.
V: 0.003-0.5 mass%, Nb: 0.003-0.5 mass%
V and Nb are effective elements for improving the corrosion resistance. In order to obtain the effect, V and Nb are each preferably added in an amount of 0.003 mass% or more. However, if added in excess of 0.5 mass%, precipitation of intermetallic compounds such as σ phase and χ phase is promoted, corrosion resistance is reduced, and hot workability is impaired. Therefore, when adding the said element, it is preferable to add in each said range. More preferably, it is the range of 0.003-0.2 mass%, respectively.
次に、本発明の二相ステンレス鋼板の製造方法について説明する。
本発明の二相ステンレス鋼板（熱延鋼板、冷延鋼板）は、従来公知の方法・条件で製造すればよく、特に制限はない。例えば、電気炉や転炉等で鋼を溶製し、二次精錬して上述した成分組成の鋼に調整した後、連続鋳造法あるいは造塊−分解圧延法で鋼素材（スラブ）とする。次いで、熱延鋼板は、上記スラブを再加熱し、熱間圧延し、固溶化熱処理を施した後、酸洗して製品板とする。一方、冷延鋼板は、上記スラブを再加熱し、熱間圧延し、必要に応じて適宜の条件で熱処理を施し、酸洗して熱延焼鈍板とし、さらに、上記熱延焼鈍板を冷間圧延し、固溶化熱処理を施した後、酸洗して製品板とする。
Next, the manufacturing method of the duplex stainless steel sheet of this invention is demonstrated.
The duplex stainless steel sheet (hot-rolled steel sheet, cold-rolled steel sheet) of the present invention may be produced by a conventionally known method / condition, and is not particularly limited. For example, steel is melted in an electric furnace, a converter, or the like, subjected to secondary refining and adjusted to the steel having the above-described composition, and then made into a steel material (slab) by a continuous casting method or an ingot-decomposition rolling method. Next, the hot-rolled steel sheet is obtained by reheating the slab, hot-rolling, subjecting it to a solution heat treatment, and pickling it to obtain a product plate. On the other hand, the cold-rolled steel sheet is obtained by reheating the slab, hot-rolling, heat-treating it under appropriate conditions as necessary, pickling it to obtain a hot-rolled annealed sheet, and further cooling the hot-rolled annealed sheet. After hot rolling and solution heat treatment, pickling is performed to obtain a product plate.
ただし、上記熱延鋼板および冷延鋼板の製造工程における固溶化熱処理は、以下の条件で行うことが必要である。
固溶化熱処理温度：１０００〜１１００℃
固溶化熱処理は、鋼板中の炭化物を固溶化するために行う熱処理であり、この熱処理温度（焼鈍温度）は１０００〜１１００℃の範囲とする必要がある。１０００℃未満では、固溶化処理温度に平衡するγ相のＣｒ濃度が低下するため、冷却中に形成されるＣｒ欠乏領域の最低Ｃｒ濃度との差は小さくなる。しかし、１０００℃未満では、σ相の析出を招く他、熱間圧延時に析出したσ相が、固溶化熱処理によっても消失せずに残存し、耐食性や靭性の低下を招く。一方、熱処理温度が１１００℃を超えると、元素の拡散が活発になり過ぎるため、後述する１次冷却中にＣｒ欠乏領域が形成され易くなり、耐食性の低下を招く。よって、固溶化熱処理温度は１０００〜１１００℃の範囲とする。好ましくは１０３０〜１０８０℃の範囲である。
なお、上記温度に保持する時間は３０〜６００秒とするのが好ましい。３０秒未満では、固溶化の効果が十分に得られず、一方、６００秒を超えると、上記効果が飽和する他、生産性を阻害するようになるからである。
However, it is necessary to perform the solution heat treatment in the manufacturing process of the hot-rolled steel sheet and the cold-rolled steel sheet under the following conditions.
Solution heat treatment temperature: 1000-1100 ° C
The solution heat treatment is a heat treatment performed to solidify the carbide in the steel sheet, and the heat treatment temperature (annealing temperature) needs to be in the range of 1000 to 1100 ° C. If the temperature is lower than 1000 ° C., the Cr concentration in the γ phase that is in equilibrium with the solution treatment temperature decreases, and therefore the difference from the minimum Cr concentration in the Cr-deficient region formed during cooling becomes small. However, when the temperature is lower than 1000 ° C., the σ phase is precipitated, and the σ phase precipitated during hot rolling remains without disappearing even by the solution heat treatment, thereby causing a decrease in corrosion resistance and toughness. On the other hand, when the heat treatment temperature exceeds 1100 ° C., element diffusion becomes excessively active, so that a Cr-deficient region is likely to be formed during the primary cooling described later, leading to a decrease in corrosion resistance. Therefore, the solution heat treatment temperature is in the range of 1000 to 1100 ° C. Preferably it is the range of 1030-1080 degreeC.
The time for maintaining the temperature is preferably 30 to 600 seconds. If the time is less than 30 seconds, the effect of solid solution cannot be obtained sufficiently. On the other hand, if the time exceeds 600 seconds, the above effect is saturated and productivity is inhibited.
上記熱処理後の冷却は、８００℃以上の温度まで冷却速度３℃／ｓ以上で冷却（以降、この冷却を「１次冷却」という）し、その後、直ちに６℃／ｓ以上で冷却する（以降、この冷却を「２次冷却」という）必要がある。
１次冷却：８００℃以上の温度まで、冷却速度３℃／ｓ以上で冷却
固溶化熱処理後の１次冷却速度が３℃／ｓ未満では、冷却中に元素が拡散する時間が確保され、Ｃｒ欠乏領域の形成が進行するため、耐食性低下を招く。また、σ相や窒化物、炭化物が析出して、耐食性の低下を招くこともある。上記１次冷却の冷却速度は、好ましくは４℃／ｓ以上、より好ましくは５℃／ｓ以上である。なお、１次冷却の冷却速度の上限は、特に制限はないが、２０℃／ｓ以下が好ましい。また、上記１次冷却の冷却方法は、上記冷却速度が確保できれば特に制限はないが、ガス冷却とするのが好ましい。
The cooling after the heat treatment is performed at a cooling rate of 3 ° C./s or higher to a temperature of 800 ° C. or higher (hereinafter, this cooling is referred to as “primary cooling”), and then immediately cooled at 6 ° C./s or higher (hereinafter referred to as “cooling”). This cooling is called “secondary cooling”).
Primary cooling: Cooling at a cooling rate of 3 ° C./s or higher to a temperature of 800 ° C. or higher If the primary cooling rate after the solution heat treatment is less than 3 ° C./s, the time for the element to diffuse during cooling is secured, Cr Since the formation of the deficient region proceeds, the corrosion resistance is reduced. In addition, the σ phase, nitrides, and carbides may precipitate, leading to a decrease in corrosion resistance. The cooling rate of the primary cooling is preferably 4 ° C./s or more, more preferably 5 ° C./s or more. The upper limit of the cooling rate of primary cooling is not particularly limited, but is preferably 20 ° C./s or less. The primary cooling method is not particularly limited as long as the cooling rate can be secured, but gas cooling is preferable.
また、上記１次冷却の終了温度を８００℃以上とする理由は、Ｃｒの平衡濃度は、固溶化熱処理温度（１０００〜１１００℃）から低くなるほど、α相においては高くなり、γ相においては低くなる。特に、８００℃未満の温度においては、その傾向が顕著となる。Ｃｒ欠乏領域は、冷却中に生成したγ相に相当するため、８００℃未満まで冷却した際に形成されるＣｒ欠乏領域は、Ｃｒ濃度が低く、耐食性に劣る。従って、耐食性に劣るＣｒ欠乏領域の生成を抑制するため、１次冷却の冷却終了温度は８００℃以上とした。好ましくは９００℃以上である。なお、冷却終了温度の上限は、上記固溶化熱処理温度（１０００〜１１００℃）以下であればよく、特に制限はない。 The reason for setting the end temperature of the primary cooling to 800 ° C. or higher is that the equilibrium concentration of Cr is higher in the α phase and lower in the γ phase as the solution temperature is lower than the solution heat treatment temperature (1000 to 1100 ° C.). Become. In particular, the tendency becomes remarkable at a temperature of less than 800 ° C. Since the Cr-deficient region corresponds to the γ phase generated during cooling, the Cr-deficient region formed when cooled to less than 800 ° C. has a low Cr concentration and is inferior in corrosion resistance. Therefore, in order to suppress the generation of a Cr-deficient region inferior in corrosion resistance, the cooling end temperature of primary cooling is set to 800 ° C. or higher. Preferably it is 900 degreeC or more. In addition, the upper limit of cooling completion temperature should just be below the said solution heat treatment temperature (1000-1100 degreeC), and there is no restriction | limiting in particular.
２次冷却速度：６℃／ｓ以上
上記１次冷却に続く２次冷却は、冷却速度を６℃／ｓ以上として行う必要がある。２次冷却速度が６℃／ｓ未満では、やはり、Ｃｒ欠乏領域の生成が促進されたり、σ相や炭化物が析出したりするため、耐食性の低下を招く。また、４７５℃付近を緩冷却すると、４７５℃脆化が起こり、製造性の低下を招く。好ましくは７℃／ｓ以上、より好ましくは８℃／ｓ以上である。なお、上記２次冷却の冷却方法は、上記冷却速度が確保できれば特に制限はなく、例えば、走行する鋼板表面に冷却ガスを吹き付けるガスジェット冷却方式、冷却水を吹き付ける注水冷却方式、ガスと冷却水を混合して吹き付けるミスト冷却方式、鋼板を水中に浸漬して冷却する水冷方式、水冷ロールを鋼板に接触させて冷却するロール冷却方式等、いずれの方法を用いてもよい。
Secondary cooling rate: 6 ° C./s or more The secondary cooling following the primary cooling needs to be performed at a cooling rate of 6 ° C./s or more. If the secondary cooling rate is less than 6 ° C./s, the formation of Cr-deficient regions is promoted and the σ phase and carbides are precipitated, leading to a decrease in corrosion resistance. In addition, if the temperature near 475 ° C. is slowly cooled, 475 ° C. embrittlement occurs, resulting in a decrease in manufacturability. Preferably it is 7 degrees C / s or more, More preferably, it is 8 degrees C / s or more. The cooling method of the secondary cooling is not particularly limited as long as the cooling rate can be ensured. For example, a gas jet cooling method in which cooling gas is blown onto the traveling steel plate surface, a water injection cooling method in which cooling water is blown, gas and cooling water Any method may be used, such as a mist cooling method in which the steel plate is mixed and sprayed, a water cooling method in which the steel plate is immersed in water and cooled, and a roll cooling method in which the water cooling roll is brought into contact with the steel plate to cool.
なお、上記の説明では、焼鈍温度からの冷却過程を１次冷却と２次冷却とに分けているが、上記の冷却速度を確保できれば、２つに分ける必要はなく、例えば、全冷却過程を６℃／ｓ以上で冷却するようにしてもよいことは勿論である。 In the above description, the cooling process from the annealing temperature is divided into primary cooling and secondary cooling. However, if the above cooling rate can be secured, it is not necessary to divide into two, for example, the entire cooling process Of course, the cooling may be performed at 6 ° C./s or more.
また、冷延鋼板を製造する場合、熱延板に施す熱処理は、熱間圧延時の歪を除去し、軟化すること目的として行うものであり、上記目的を達成できればその条件については特に制限しないが、例えば、１０００〜１１５０℃の温度で、３０〜３００秒間均熱保持した後、６℃／ｓ以上の冷却速度で冷却する条件とするのが好ましい。 Moreover, when manufacturing a cold-rolled steel sheet, the heat treatment applied to the hot-rolled sheet is performed for the purpose of removing and softening the strain during hot rolling, and the conditions are not particularly limited as long as the above-described object can be achieved. However, it is preferable to set it as the conditions cooled at a cooling rate of 6 degrees C / s or more, after hold | maintaining soaking for 30 to 300 seconds at the temperature of 1000-1150 degreeC, for example.
次に、本発明の二相ステンレス鋼板について説明する。
上記成分組成を有する鋼板に上記条件の固溶化熱処理を施した本発明の二相ステンレス鋼板は、金属組織内にσ相が存在していないことが必要である。σ相はＣｒとＭｏを主体とするものであるため、σ相近傍に局所的なＣｒとＭｏの欠乏領域が形成されて耐食性が低下したり、σ相は硬質で脆いため靭性が低下したりするからである。なお、このσ相の存在は、透過型電子顕微鏡やエッチング後の光学顕微鏡観察等で容易に確認することができる。
Next, the duplex stainless steel sheet of the present invention will be described.
The duplex stainless steel sheet of the present invention obtained by subjecting a steel sheet having the above composition to a solution heat treatment under the above conditions needs to have no σ phase in the metal structure. Since the σ phase is mainly composed of Cr and Mo, a local Cr and Mo deficient region is formed in the vicinity of the σ phase and the corrosion resistance is lowered, or the toughness is lowered because the σ phase is hard and brittle. Because it does. The existence of this σ phase can be easily confirmed by observation with a transmission electron microscope or an optical microscope after etching.
また、本発明の二相ステンレス鋼板は、金属組織内にσ相が存在していないことに加えて、α／γ相界面からγ相側に０．５μｍまでの領域の最低Ｃｒ濃度と、同一γ相内で、α／γ相界面からγ相側に０．５μｍより離れた領域の平均Ｃｒ濃度の差が１．５ｍａｓｓ％未満であることが必要である。上記Ｃｒ濃度差が１．５ｍａｓｓ%以上となると、低Ｃｒ濃度領域と高Ｃｒ濃度領域との間で局部電池を形成し粒界腐食が顕著となるからである。また、最低Ｃｒ濃度の測定領域をα／γ相界面からγ相側に０．５μｍの範囲とした理由は、図３からもわかるように、α／γ相界面の移動によって形成される低Ｃｒ領域はα／γ相界面から０．５μｍ程度にまで及ぶからである。なお、上記Ｃｒ濃度差は、好ましくは１．３ｍａｓｓ%以下、より好ましくは１．２ｍａｓｓ％以下である。 Further, the duplex stainless steel sheet of the present invention has the same minimum Cr concentration in the region from the α / γ phase interface to the γ phase side to 0.5 μm in addition to the absence of the σ phase in the metal structure. Within the γ phase, it is necessary that the difference in average Cr concentration in a region separated from the α / γ phase interface to the γ phase side by more than 0.5 μm is less than 1.5 mass%. This is because when the Cr concentration difference is 1.5 mass% or more, a local battery is formed between the low Cr concentration region and the high Cr concentration region, and intergranular corrosion becomes remarkable. The reason why the measurement region of the minimum Cr concentration is in the range of 0.5 μm from the α / γ phase interface to the γ phase side, as can be seen from FIG. 3, is the low Cr formed by the movement of the α / γ phase interface. This is because the region extends from the α / γ phase interface to about 0.5 μm. The Cr concentration difference is preferably 1.3 mass% or less, more preferably 1.2 mass% or less.
ここで、上記Ｃｒ濃度の測定方法については、特に制限はないが、微小部分のＣｒ濃度を精度よく測定する観点からは、ＴＥＭやＳＥＭ等に付属のエネルギー分散Ｘ線分光分析（ＥＤＸ分析）装置を用いることが好ましく、その際、より測定精度を高める観点から、線分析より点分析することが好ましい。また、Ｃｒ濃度を測定するα／γ相界面（粒界）は、試料表面（観察面）に対して垂直な界面であることが好ましい。界面が傾斜していると、α／γ相界面からの距離が不正確となってしまうからである。 Here, the method for measuring the Cr concentration is not particularly limited, but from the viewpoint of accurately measuring the Cr concentration in a minute portion, an energy dispersive X-ray spectroscopic analysis (EDX analysis) apparatus attached to a TEM, SEM or the like. In this case, it is preferable to perform point analysis from line analysis from the viewpoint of further improving measurement accuracy. The α / γ phase interface (grain boundary) for measuring the Cr concentration is preferably an interface perpendicular to the sample surface (observation surface). This is because if the interface is inclined, the distance from the α / γ phase interface becomes inaccurate.
上記したα／γ相界面からγ相側に０．５μｍまでの領域の最低Ｃｒ濃度と、同一γ相内で、α／γ相界面からγ相側に０．５μｍより離れた領域の平均Ｃｒ濃度の差が１．５ｍａｓｓ％未満である二相ステンレス鋼板は、耐粒界腐食性に極めて優れたものとなる。そのため、例えば、ＡＳＴＭ Ａ２６２ PracticeＣ（Huey Test）に準拠して、７０ｍａｓｓ％沸騰硝酸を腐食液とし、４８時間を１バッチとする浸漬試験を各バッチで腐食液を更新しながら５バッチ行う粒界腐食試験における腐食速度を０．３０ｇ／ｍ２・ｈｒ未満とすることができる。 The minimum Cr concentration in the region from the α / γ phase interface to 0.5 μm on the γ phase side and the average Cr in the region separated from the α / γ phase interface to the γ phase side by 0.5 μm in the same γ phase. A duplex stainless steel sheet having a concentration difference of less than 1.5 mass% is extremely excellent in intergranular corrosion resistance. Therefore, for example, in accordance with ASTM A262 Practice C (Huey Test), intergranular corrosion in which 70 mass% boiling nitric acid is used as a corrosive solution and an immersion test with 48 batches as one batch is performed for 5 batches while renewing the corrosive solution in each batch. The corrosion rate in the test can be less than 0.30 g / m 2 · hr.
表１に示した種々の成分組成を有するＦｅ−Ｎｉ−Ｃｒ−Ｍｏ系二相ステンレス鋼を常法の精錬プロセスで溶製し、連続鋳造法でスラブとし、１１００〜１２００℃の温度に再加熱した後、熱間圧延して厚さが４〜６ｍｍの熱延板とした。
次いで、上記熱延板から以下の方法で熱延鋼板および冷延鋼板を製造した。
まず、熱延鋼板は、上記熱延板に表２に示した種々の焼鈍温度、冷却速度で固溶化熱処理を施した後、酸洗して製造した。
また、冷延鋼板は、上記熱延板に１０００〜１１５０℃の温度で熱処理を施し、酸洗し、冷間圧延して厚さが３〜４ｍｍの冷延板とした後、表２に示した種々の焼鈍温度、冷却速度の固溶化熱処理を施した後、酸洗して製造した。
Fe-Ni-Cr-Mo type duplex stainless steels having various composition shown in Table 1 are melted by a conventional refining process, made into a slab by a continuous casting method, and reheated to a temperature of 1100 to 1200 ° C. Then, hot rolling was performed to obtain a hot rolled sheet having a thickness of 4 to 6 mm.
Subsequently, a hot-rolled steel sheet and a cold-rolled steel sheet were produced from the hot-rolled sheet by the following method.
First, the hot-rolled steel sheet was manufactured by subjecting the hot-rolled sheet to solution heat treatment at various annealing temperatures and cooling rates shown in Table 2, and then pickling.
The cold-rolled steel sheet is subjected to heat treatment at a temperature of 1000 to 1150 ° C., pickled, cold-rolled to obtain a cold-rolled sheet having a thickness of 3 to 4 mm, and then shown in Table 2. Further, it was manufactured by subjecting it to a solution heat treatment at various annealing temperatures and cooling rates, followed by pickling.
上記のようにして得た各種の熱延鋼板および冷延鋼板について、以下の評価試験に供した。
＜α／γ相界面近傍のＣｒ濃度分布測定＞
上記の熱延鋼板および冷延鋼板から圧延方向に対して垂直な断面を有する試料を採取し、該試料から、電解研磨により、上記断面を観察面とし、厚さが５０ｎｍ程度の領域が存在するＴＥＭ−ＥＤＸ分析用試料を作製した。
次いで、上記ＴＥＭ−ＥＤＸ分析用試料をＴＥＭで観察して、電子ビームと平行する、すなわち、観察面にほぼ垂直なα／γ相界面を探し出し、該α／γ相界面のγ相側を、α／γ相界面から垂直方向に６０ｎｍ間隔で点分析を行った。なお、分析精度を確保するため、１点あたりのビーム照射時間は２００秒とした。
次いで、上記測定の結果から、α／γ相界面から垂直方向に０．５μｍ以内の領域の最低Ｃｒ濃度と、０．５μｍより離れた領域の平均Ｃｒ濃度の差ΔＣｒを求めた。
＜σ相の存在有無＞
上記Ｃｒ濃度分布測定に用いた試料を用いて、ＴＥＭでσ相の存在有無を確認した。
＜粒界腐食試験＞
上記の熱延鋼板および冷延鋼板から、板厚×２０ｍｍ×２５ｍｍの試験片を採取し、該試験片の表面を＃１２０まで湿式研磨し、腐食試験片とした。
上記腐食試験片に対して、ＡＳＴＭ Ａ２６２ PracticeＣ（Huey Test）に準拠して、７０ｍａｓｓ％沸騰硝酸を腐食液とし、４８時間を１バッチとする浸漬試験を、各バッチで腐食液を更新しながら５バッチ行う粒界腐食試験を行い、試験前後の質量差から腐食減量を求めて、腐食速度を算出し、上記腐食速度が０．３０ｇ／ｍ２・ｈｒ未満のものを耐粒界腐食性が優（○）、０．３０ｇ／ｍ２・ｈｒ以上０．３３ｇ／ｍ２・ｈｒ未満のものを耐粒界腐食性が良（△）、０．３３ｇ／ｍ２・ｈｒ以上のものを耐粒界腐食性が劣（×）と評価した。
The various hot-rolled steel sheets and cold-rolled steel sheets obtained as described above were subjected to the following evaluation tests.
<Measurement of Cr concentration distribution near α / γ phase interface>
A sample having a cross section perpendicular to the rolling direction is taken from the hot-rolled steel sheet and the cold-rolled steel sheet, and an area having a thickness of about 50 nm exists from the sample by electropolishing the cross-section as an observation surface. A sample for TEM-EDX analysis was prepared.
Next, the TEM-EDX analysis sample is observed with a TEM to find an α / γ phase interface parallel to the electron beam, that is, substantially perpendicular to the observation surface, and the γ phase side of the α / γ phase interface is Point analysis was performed at intervals of 60 nm in the vertical direction from the α / γ phase interface. In order to ensure analysis accuracy, the beam irradiation time per point was set to 200 seconds.
Next, from the measurement results, a difference ΔCr between the lowest Cr concentration in the region within 0.5 μm in the vertical direction from the α / γ phase interface and the average Cr concentration in the region separated from 0.5 μm was obtained.
<Existence of sigma phase>
Using the sample used for the Cr concentration distribution measurement, the presence or absence of the σ phase was confirmed by TEM.
<Intergranular corrosion test>
A test piece having a thickness of 20 mm × 25 mm was taken from the hot-rolled steel plate and the cold-rolled steel plate, and the surface of the test piece was wet-polished to # 120 to obtain a corrosion test piece.
In accordance with ASTM A262 Practice C (Huey Test), an immersion test using 70 mass% boiling nitric acid as a corrosive liquid and 48 batches as one batch is performed while updating the corrosive liquid in each batch. Perform intergranular corrosion test in batches, calculate the corrosion weight loss from the mass difference before and after the test, calculate the corrosion rate, and those with the above corrosion rate less than 0.30 g / m 2 · hr have excellent intergranular corrosion resistance. (◯), 0.30 g / m 2 · hr or more and less than 0.33 g / m 2 · hr with good intergranular corrosion resistance (Δ), 0.33 g / m 2 · hr or more with grain resistance The interfacial corrosion was evaluated as inferior (x).
上記評価試験の結果を表２中に併記した。
この結果から、本発明に適合する成分組成を有する熱延鋼板または冷延鋼板に対して、本発明に適合する条件の固溶化熱処理を施した鋼板は、いずれもα／γ相界面におけるＣｒ濃度差が１．５ｍａｓｓ％未満で、粒界腐食速度が０．３０ｇ／ｍ２・ｈｒ未満であり、耐粒界腐食性に優れている。
これに対して、本発明の成分組成を満たしていても、本発明に適合しない固溶化熱処理を施した鋼板は、α／γ相界面におけるＣｒ濃度差が１．５ｍａｓｓ％以上、粒界腐食速度も０．３０ｇ／ｍ２・ｈｒ以上で、耐粒界腐食性に劣っている。
また、本発明の成分組成を満たしていても、本発明より低い温度で固溶化熱処理を施した鋼板は、金属組織内にσ相が生成し、耐粒界腐食性が低下する傾向にある。
また、本発明の成分組成を満たしていない鋼板は、例え本発明に適合する条件で固溶化熱処理を施してＣｒ濃度差ΔＣｒを１．５ｍａｓｓ％未満としても、鋼板自体の耐食性に劣るため、粒界腐食速度は０．３０ｇ／ｍ２・ｈｒ以上となっている。
The results of the evaluation test are also shown in Table 2.
From this result, the steel sheet subjected to the solution heat treatment under the conditions suitable for the present invention with respect to the hot-rolled steel sheet or the cold-rolled steel sheet having the component composition suitable for the present invention are both Cr concentration at the α / γ phase interface. The difference is less than 1.5 mass%, the intergranular corrosion rate is less than 0.30 g / m 2 · hr, and the intergranular corrosion resistance is excellent.
On the other hand, even if the component composition of the present invention is satisfied, the steel plate subjected to the solution heat treatment not conforming to the present invention has a Cr concentration difference of 1.5 mass% or more at the α / γ phase interface, and the intergranular corrosion rate. Is 0.30 g / m 2 · hr or more, and the intergranular corrosion resistance is inferior.
Further, even if the composition of the present invention is satisfied, a steel sheet that has been subjected to a solution heat treatment at a temperature lower than that of the present invention tends to produce a σ phase in the metal structure and reduce intergranular corrosion resistance.
In addition, the steel sheet that does not satisfy the component composition of the present invention is inferior in corrosion resistance of the steel sheet itself even if it is subjected to solution heat treatment under the conditions suitable for the present invention and the Cr concentration difference ΔCr is less than 1.5 mass%. The field corrosion rate is 0.30 g / m 2 · hr or more.
本発明の二相ステンレス鋼板は、耐粒界腐食性に優れているため、化学プラント以外の厳しい腐食性環境下で使用される高耐食材料として好適に用いることができる。 Since the duplex stainless steel sheet of the present invention is excellent in intergranular corrosion resistance, it can be suitably used as a high corrosion resistance material used in severe corrosive environments other than chemical plants.
Claims (4)
In manufacturing the duplex stainless steel sheet according to any one of claims 1 to 3, after heating a hot-rolled steel sheet or a cold-rolled steel sheet manufactured according to a conventional method to a temperature of 1000 to 1100 ° C, the temperature is 800 ° C or higher. A method for producing a duplex stainless steel sheet, wherein the solution is cooled to a temperature at a cooling rate of 3 ° C./s or more, and then subjected to a solution heat treatment that immediately cools at a temperature of 6 ° C./s or more.
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