JP2008231542A - Ferritic stainless steel having excellent weld zone workability and crevice corrosion resistance and reduced surface flaw - Google Patents

Ferritic stainless steel having excellent weld zone workability and crevice corrosion resistance and reduced surface flaw Download PDF

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JP2008231542A
JP2008231542A JP2007075688A JP2007075688A JP2008231542A JP 2008231542 A JP2008231542 A JP 2008231542A JP 2007075688 A JP2007075688 A JP 2007075688A JP 2007075688 A JP2007075688 A JP 2007075688A JP 2008231542 A JP2008231542 A JP 2008231542A
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corrosion resistance
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stainless steel
workability
crevice corrosion
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JP5111910B2 (en
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Misa Sakitani
美茶 崎谷
Toru Matsuhashi
透 松橋
Akihiko Takahashi
明彦 高橋
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Nippon Steel Stainless Steel Corp
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Nippon Steel and Sumikin Stainless Steel Corp
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Abstract

<P>PROBLEM TO BE SOLVED: To provide a ferritic stainless steel having excellent weld zone workability and crevice corrosion resistance and reduced surface flaws. <P>SOLUTION: The ferritic stainless steel having excellent weld zone workability and crevice corrosion resistance and reduced surface flaws has a composition comprising, by mass, ≤0.020% C, ≤0.025% N, ≤0.25% Si, ≤0.20% Mn, ≤0.035% P, ≤0.010% S, 16 to 24% Cr, 0.50 to 2.00% Mo, 0.05 to 0.20% Ti and 0.05 to 0.40% Nb, and the balance Fe with inevitable impurities, and satisfying inequalities (A) and (B). If required, one or more kinds selected from ≤2.0% Ni, ≤1.0% Cu, ≤0.2% V, ≤0.2% Zr and 0.005% B may be incorporated therein; wherein, 21.5≤Cr+3.3Mo≤26 and Ti-0.28Mo≥-0.25 (A), and (Ti+Nb)/(C+N)≥16 and 0.2≤Ti/(Ti+Nb)≤0.8 (B) are satisfied. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

本発明は、溶接部加工性と耐すき間腐食性に優れた表面疵の少ないフェライト系ステンレス鋼に関するものである。 The present invention relates to a ferritic stainless steel having excellent surface workability and crevice corrosion resistance and having a small surface flaw.

オーステナイト系ステンレス鋼は一般に耐食性および伸び、張り出し性等の加工性が優れているため、用途は広範囲にわたっている。しかしながら、近年、需要増大・供給不足懸念に起因するNi等の原料高騰が著しく、Niを含まず経済的に有利なフェライト系ステンレス鋼の代替・適用が拡大しており、高耐食性のみならず、高成形性および優れた表面品位も要望されている。   Since austenitic stainless steels are generally excellent in corrosion resistance and workability such as elongation and stretchability, they have a wide range of applications. However, in recent years, there has been a marked increase in raw materials such as Ni due to concerns about increased demand and supply shortages, and the substitution and application of ferritic stainless steel that does not contain Ni and is economically advantageous has expanded, not only high corrosion resistance, High moldability and excellent surface quality are also demanded.

ステンレス鋼の孔食電位や耐候性等の耐食性はCrとMoの含有量によって高めることができ、成分設計によってはSUS304、SUS316以上の耐食性が得られる。更にフェライト系ステンレス鋼は耐応力腐食割れ性も優れるというオーステナイト系ステンレス鋼にはない特長を有しており、より広範囲な用途に、複雑化・多様化した構造物へ使用される機会が増えている。一方で、複雑な形状や溶接部等のすき間構造を有する構造物への適用の場合、すき間腐食が問題となる。その場合、耐すき間腐食性の寄与が大きいMoの添加が不可欠である。さらに溶接部耐食性の向上には特許文献１で示されているように、C,N量の規定、TiやNbなどの安定化元素の添加が提案されている。これはC量やN量を低減し、かつTiおよびNb添加によりCおよびNと炭・窒化物を形成することでCr炭化物の粒界析出を防止し、耐粒界腐食性を高めたものである。   Corrosion resistance such as pitting potential and weather resistance of stainless steel can be increased by Cr and Mo content, and depending on the component design, corrosion resistance higher than SUS304 and SUS316 can be obtained. In addition, ferritic stainless steel has an advantage not found in austenitic stainless steel that it has excellent resistance to stress corrosion cracking, and there are more opportunities to use it in more complex and diversified structures for a wider range of applications. Yes. On the other hand, in the case of application to a structure having a gap structure such as a complicated shape or a welded portion, gap corrosion becomes a problem. In that case, it is indispensable to add Mo, which has a large contribution to crevice corrosion resistance. Furthermore, for improving the corrosion resistance of welded parts, as shown in Patent Document 1, the provision of C and N amounts and the addition of stabilizing elements such as Ti and Nb have been proposed. This is because the amount of C and N is reduced, and by adding Ti and Nb, carbon and nitride are formed with C and N to prevent grain boundary precipitation of Cr carbide and to improve intergranular corrosion resistance. is there.

一方、フェライト系ステンレス鋼の加工性は、ｒ値は高いため絞り加工は良好なものの、SUS304等のオーステナイト系ステンレス鋼と比べると、伸び、張り出し性が劣る。特に、溶接部では結晶粒の粗大化にともなう靭性・延性の低下により、加工性の劣化が顕著である。問題解決のため、特許文献2や特許文献3等に示されているように、TiN析出による凝固組織の微細化、粒界純化による再結晶集合組織の発達を促すTi量の最適化等の技術が提案されている。    On the other hand, the workability of ferritic stainless steel is high because the r value is high, but the drawing process is good, but the elongation and the stretchability are inferior compared to austenitic stainless steel such as SUS304. In particular, in welds, workability is significantly deteriorated due to a decrease in toughness and ductility accompanying the coarsening of crystal grains. In order to solve the problem, as shown in Patent Document 2 and Patent Document 3, etc., technologies such as refinement of the solidification structure by TiN precipitation and optimization of Ti amount that promotes the development of recrystallization texture by grain boundary purification, etc. Has been proposed.

またTi量を多量に添加すると、製造時に表面欠陥が多数発生し、圧延時にTi系硬質介在物を起因とする表面疵を生じ、表面品位の劣化をもたらすとともに、これを起点に銹が発生し耐食性も低下するという問題が生じる。加工性悪化の一因としてCr、Mo等の固溶強化が挙げられるが、これらCr量およびMo量を低減すると用途上必要な耐食性を下回る場合もあるため、耐食性を担保し、かつ加工性を向上させる必要がある。
特公昭55-21102号公報 特開2002-12955号公報 特開2002-275590号公報 If a large amount of Ti is added, many surface defects will occur during production, and surface defects will occur due to Ti hard inclusions during rolling, resulting in surface quality deterioration and defects starting from this. There arises a problem that the corrosion resistance is also lowered. One of the causes of deterioration in workability is solid solution strengthening of Cr, Mo, etc. However, if these Cr and Mo contents are reduced, the corrosion resistance required for applications may be lower, so corrosion resistance is ensured and workability is improved. There is a need to improve.
Japanese Patent Publication No.55-21102 Japanese Patent Laid-Open No. 2002-12955 JP 2002-275590 A

そこで、本発明では、用途上必要耐食性を担保しつつ、従来の加工性よりも向上したフェライト系ステンレス鋼の開発を検討した。溶接部加工性の向上にはTiの添加とCr,Moの低減により効果が得られるということが知られている。本発明者らは、溶接部加工性の劣化へ及ぼすMo量の寄与を見出し、必要Mo量に応じたTi量の添加最適条件を探ることにより溶接部加工性の向上を可能にした。また、その検討過程で、本発明成分の鋼においては溶接部加工性の劣化に及ぼすCr量の寄与はMo量と比較すると小さく、溶接部のエリクセン値を加工性の指標として数十〜数百分の一の寄与であったため、Crを除くMo量およびTi量の最適バランスの数値化により溶接部加工性の向上を図った。これにより、Mo添加フェライト系ステンレス鋼において、従来よりも加工性を向上することが可能となることを知見し、溶接部加工性および耐すき間腐食性に優れた表面疵の少ないフェライト系ステンレス鋼を発明した。   Therefore, in the present invention, the development of ferritic stainless steel that has improved the conventional workability while ensuring the corrosion resistance necessary for the application was examined. It is known that the improvement of weldability can be achieved by adding Ti and reducing Cr and Mo. The present inventors have found the contribution of the amount of Mo to the deterioration of weld zone workability, and have made it possible to improve the weld portion workability by searching for the optimum conditions for the addition of Ti according to the required amount of Mo. In addition, in the examination process, the contribution of the Cr amount to the deterioration of the weldability in the steel of the present invention component is small compared to the Mo amount. Because it contributed a fraction, the workability of the weld zone was improved by quantifying the optimal balance of Mo and Ti contents excluding Cr. As a result, we have found that it is possible to improve the workability of the Mo-added ferritic stainless steel compared to the conventional method, and we have developed a ferritic stainless steel with low surface flaws and excellent weldability and crevice corrosion resistance. Invented.

上記課題を解決するため、本発明は、Mo添加フェライト系ステンレス鋼の溶接部加工性と耐食性の指標がMo量とTi量のバランスによって整理されることを見出し、更に安定化元素のTi量とNb量のバランスの最適化を加えたことにより、溶接加工性および耐食性に優れた表面疵の少ないフェライト系ステンレス鋼を提供するものであり、その要旨とするところは特許請求の範囲に記載した通りの下記内容である。
本発明は、その結果に基づくものであり、以下の構成を要旨とする。即ち本発明は、（1）質量%で、
C：0.020%以下、
N：0.025%以下、
Si：0.25%以下、
Mn：0.20%以下、
P：0.035%以下、
S：0.010%以下、
Cr：16〜24%、
Mo：0.50〜2.00%、
Ti：0.05〜0.20%、
Nb：0.05〜0.40%を含有し、残部はFeおよび不可避的不純物からなり、下記(A)式および(B)式を満足する組成を有する。 In order to solve the above-mentioned problems, the present invention has found that the weld processability and corrosion resistance index of the Mo-added ferritic stainless steel are arranged by the balance between the Mo amount and the Ti amount, and further, the Ti amount of the stabilizing element and By optimizing the balance of the Nb amount, the present invention provides a ferritic stainless steel having excellent surface workability and corrosion resistance and low surface flaws, the gist of which is as described in the claims. It is the following contents.
The present invention is based on the results and has the following configuration. That is, the present invention is (1) mass%,
C: 0.020% or less,
N: 0.025% or less,
Si: 0.25% or less,
Mn: 0.20% or less,
P: 0.035% or less,
S: 0.010% or less,
Cr: 16-24%
Mo: 0.50-2.00%
Ti: 0.05-0.20%
Nb: 0.05 to 0.40% is contained, the balance is composed of Fe and inevitable impurities, and has a composition satisfying the following formulas (A) and (B).

ここに、
21.5≦Cr+3.3Mo≦26 かつ Ti-0.28Mo≧-0.25・・・ (A)
（Ti+Nb）/（C+N）≧16 かつ 0.2≦Ti/（Ti+Nb）≦0.8・・・ (B)
（2）さらに、質量％で、
Ｎｉ：2.0％以下、
Ｃｕ：1.0％以下、の一種又は二種以上を含むことを特徴とする、（1）に記載の、溶接部加工性および耐すき間腐食性に優れた表面疵の少ないフェライト系ステンレス鋼。
（3）さらに、質量％で、
Ｖ：0.2％以下、
Ｚｒ：0.2％以下、の一種又は二種以上を含むことを特徴とする、（1）または（2）に記載の、記載の、溶接部加工性および耐すき間腐食性に優れた表面疵の少ないフェライト系ステンレス鋼。
（4）さらに、質量％で、
Ｂ：0.005％以下を含有することを特徴とする、（1）乃至（3）のいずれか一項に記載の、溶接部加工性および耐すき間腐食性に優れた表面疵の少ないフェライト系ステンレス鋼。 here,
21.5 ≦ Cr + 3.3Mo ≦ 26 and Ti-0.28Mo ≧ -0.25 ... (A)
(Ti + Nb) / (C + N) ≧ 16 and 0.2 ≦ Ti / (Ti + Nb) ≦ 0.8 ... (B)
(2) Furthermore, in mass%,
Ni: 2.0% or less,
The ferritic stainless steel having low surface flaws and excellent weldability and crevice corrosion resistance according to (1), characterized by containing one or more of Cu: 1.0% or less.
(3) Furthermore, in mass%,
V: 0.2% or less,
Zr: 0.2% or less, including one or more of Zr: described in (1) or (2), having low surface flaws with excellent weldability and crevice corrosion resistance Ferritic stainless steel.
(4) Furthermore, in mass%,
B: Ferritic stainless steel with low surface defects and excellent weldability and crevice corrosion resistance according to any one of (1) to (3), characterized by containing 0.005% or less .

以上の説明から明らかなように、本発明によれば溶接部加工性および耐すき間腐食性に優れた表面疵の少ないフェライト系ステンレス鋼を提供することできる。本発明のフェライト系ステンレス鋼は、溶接部加工性に優れ、70℃以下の低温の給湯温度における耐すき間腐食性を有することから、溶接部を有する構造物、例えば、貯湯温度に適した貯湯タンク材料や貯水槽、貯湯槽用材料として提供することができる。また家庭用厨房機器、シンクなど広い用途への応用が期待される。   As is apparent from the above description, according to the present invention, it is possible to provide a ferritic stainless steel having a small surface flaw and excellent weldability and crevice corrosion resistance. Since the ferritic stainless steel of the present invention has excellent weldability and has crevice corrosion resistance at a low hot water supply temperature of 70 ° C. or less, a structure having a weld, for example, a hot water storage tank suitable for hot water storage temperature It can be provided as a material, water storage tank, hot water storage tank material. It is also expected to be applied to a wide range of uses such as household kitchen equipment and sinks.

本発明者らは、従来、考慮されていなかった、Mo量とTi量のバランスが溶接部加工性と耐すき間腐食性を併せ持つ適正範囲の指標として有効であることを見出し、本発明に到った。さらに安定化元素であるTi量とNb量のバランスのこの鋼ならではの数値化を図ることで、本発明は溶接部の加工性・耐食性および表面疵の少ないフェライト系ステンレス鋼を提供することが可能となる。   The present inventors have found that the balance between the Mo amount and the Ti amount, which has not been considered in the past, is effective as an index of an appropriate range having both weldability and crevice corrosion resistance, and has reached the present invention. It was. In addition, by quantifying the balance of Ti and Nb, which are stabilizing elements, this steel can provide ferritic stainless steel with less workability and corrosion resistance of welds and less surface flaws. It becomes.

ステンレス鋼の一般耐食性を改善することにはCr量を高めること、Moを添加することが有効である。しかしながら、上述したように多量のCr,Mo添加は加工性の低下を招く。一方、TiはC、N安定化元素であり、多いほど加工性は向上するが、多すぎるとTiを主体とした介在物が粗大となり、製造時に表面疵を生じやすくなる。本発明では鋭意検討の結果、両者の短所を補い、耐食性を担保しつつ溶接部加工性の向上ができる、Mo量とTi量の最適バランスを見出した。また、本発明過程において本成分範囲ではCrがMoに比較して溶接部加工性の劣化に及ぼす寄与が小さいことも見出した。   In order to improve the general corrosion resistance of stainless steel, it is effective to increase the Cr content and to add Mo. However, as described above, addition of a large amount of Cr and Mo causes a decrease in workability. On the other hand, Ti is a C and N stabilizing element, and as the amount increases, the workability improves. However, when the amount is too large, inclusions mainly composed of Ti become coarse, and surface flaws are likely to occur during production. In the present invention, as a result of intensive studies, the inventors have found an optimal balance between the Mo amount and the Ti amount that can compensate for the shortcomings of both and improve the weldability while ensuring corrosion resistance. Further, in the process of the present invention, it has also been found that Cr contributes less to the deterioration of workability of welded parts than Mo in the range of this component.

以下に、その詳細を図により説明する。   The details will be described below with reference to the drawings.

図1にCr-Moと孔食電位の関係および耐すき間腐食性の有無を示す。孔食電位は、質量％で、C:0.003〜0.012%、N:0.006〜0.015%、Si:0.12〜0.20、Mn:0.10〜0.13%、P:0.024〜0.030%、S:0.0004〜0.0070%、Ti：0.16〜0.20％、Nb:0.05〜0.39％、Cr:16.4〜22.0％、Mo：0.5〜1.9％の組成を有する鋼材を用いて、JIS G 0577に従い測定したV’c100をプロットしている。耐すき間腐食性は、孔食電位測定で用いた組成を有する板厚0.7〜1.0mmの2枚の同組成のステンレス鋼を重ね合わせてスポット溶接したものを用い、600ppmCl-および20ppmCu2+を含有した水溶液中に浸漬し、酸素を吹き込んだ状態で70℃に各々14日間保持した後のすき間腐食の有無で評価した。すき間腐食の最大孔食深さが40μｍ以上の場合をすき間腐食が生じたとし、すき間腐食が生じた場合を（×）、すき間腐食が生じなかった場合を（○）として図中に示した。これにより、Cr+3.3Moが少ない場合には孔食電位の低下やすき間腐食発生など耐食性が劣り、Cr+3.3Mo=21.5以上で高耐食性を有することが分かる。またCr+3.3Mo=26超になると、靭性の劣化が生じ、製造性が低下する。従って、21.5≦Cr+3.3Mo≦26とした。 Figure 1 shows the relationship between Cr-Mo and pitting potential and the presence or absence of crevice corrosion resistance. Pitting corrosion potential is mass%, C: 0.003-0.012%, N: 0.006-0.015%, Si: 0.12-0.20, Mn: 0.10-0.13%, P: 0.024-0.030%, S: 0.0004-0.0070%, Plotting V'c100 measured according to JIS G 0577 using steel materials with composition of Ti: 0.16-0.20%, Nb: 0.05-0.39%, Cr: 16.4-22.0%, Mo: 0.5-1.9% . Crevice corrosion resistance is 600 ppm Cl - and 20 ppm Cu 2+ using spot-welded two stainless steels of the same thickness 0.7 to 1.0 mm with the composition used in the pitting potential measurement. It was evaluated by the presence or absence of crevice corrosion after being immersed in each aqueous solution and kept at 70 ° C. for 14 days in a state of blowing oxygen. The case where the maximum pitting corrosion depth of crevice corrosion is 40 μm or more is shown in the figure as crevice corrosion, (x) when crevice corrosion occurs, and (◯) when crevice corrosion does not occur. This shows that when Cr + 3.3Mo is small, the corrosion resistance is inferior, such as a decrease in pitting potential and the occurrence of crevice corrosion, and Cr + 3.3Mo = 21.5 or more has high corrosion resistance. On the other hand, when Cr + 3.3Mo = 26 is exceeded, the toughness is deteriorated and the productivity is lowered. Therefore, 21.5 ≦ Cr + 3.3Mo ≦ 26.

溶接部加工性は以下の手法により評価した。溶接試験片を質量％で、C:0.003〜0.012%、N:0.006〜0.015%、Si:0.12〜0.20、Mn:0.10〜0.13%、P:0.024〜0.030%、S:0.0004〜0.0070%、Ti：0.16〜0.20％、Nb:0.05〜0.39％、Cr:16.4〜22.0％、Mo：0.5〜1.9％の組成を有する板厚約0.8mmのフェライト系ステンレス鋼を用い、入熱550〜650J/cmで同鋼種TIG突合せ溶接して作製した。得られた溶接試験片を用いて、JIS Z 2247に準拠して、エリクセン試験機で張出加工を5回行い、平均エリクセン値を測定した結果、エリクセン高さ5mm以上を良（○）5mm未満を不良（△）として、図2に溶接部加工性に及ぼすMo量とTi量の影響を示した。また、Ti量が0.25%超ではTi系酸化物およびTi系窒化物に起因した表面疵が発生する。一方、Tiが0.05%未満ではC,Nの固定が不十分となり、加工性と耐食性が劣化し、0.05％以上でこれらの効果が生じる。溶接部加工性は良好であったが、耐すき間腐食性試験においてすき間腐食が生じた場合を（×）として図２に示した。上記、TiとCrの最適バランスの数値化検討によりTi-0.28Mo≦-0.25を得た。   The weldability was evaluated by the following method. Weld specimens in mass%, C: 0.003-0.012%, N: 0.006-0.015%, Si: 0.12-0.20, Mn: 0.10-0.13%, P: 0.024-0.030%, S: 0.0004-0.0070%, Ti : 0.16-0.20%, Nb: 0.05-0.39%, Cr: 16.4-22.0%, Mo: Ferritic stainless steel with a thickness of about 0.8mm with a composition of 0.5-1.9%, heat input 550-650J / cm The same steel grade TIG butt welded. Based on JIS Z 2247, the resulting welded test piece was subjected to overhanging 5 times with an Erichsen testing machine, and the average Erichsen value was measured. As a result, the Erichsen height was 5 mm or more, and good (○) less than 5 mm. Fig. 2 shows the effects of Mo content and Ti content on weldability. On the other hand, when the Ti content exceeds 0.25%, surface defects due to Ti-based oxides and Ti-based nitrides are generated. On the other hand, when Ti is less than 0.05%, C and N are not sufficiently fixed, workability and corrosion resistance deteriorate, and these effects occur when 0.05% or more. Although the weldability was good, the case where crevice corrosion occurred in the crevice corrosion resistance test is shown as (x) in FIG. Ti-0.28Mo ≦ -0.25 was obtained by the above numerical examination of the optimal balance of Ti and Cr.

Ti-Nbの複合添加の効果を次に示す。粒界へのＣｒ炭化物、あるいはＣｒ窒化物の析出を抑制することを目的として、安定化元素であるＴｉ，Ｎｂをするが、添加溶接部および母材HAZ部の耐粒界腐食性におよぼすTi、Nbの効果は（Ti+Nb）/（C+N）≧16で公知である。これをTiのみで十分安定化するためにはTiの添加を多量に要するため、上述したように製鋼段階で硬質介在物が生じ易く、製品表面疵を発生し易い。一方、Nb単独添加する場合、母材に対して溶接部の延性を低下させ、また溶接部の結晶粒粗大化を抑制する効果もTiより劣る。これに対して、Ti+Nbの複合添加の場合、表面疵に対するTiの悪影響、溶接部の延性に対するNbの悪影響を軽減し、表面疵の少ない母材に対して溶接部の延性低下のない鋼材を得ることができる。介在物を起因とする耐食性を考慮すると、Ｔｉ／（Ｎｂ＋Ｔｉ）が0.2を下回ると、水溶性介在物を基点としたすき間腐食が発生する。またＴｉ／（Ｎｂ＋Ｔｉ）が0.8をこえると、圧延時にＴｉ系の介在物を起因とした表面疵を生じ、外観上問題があるだけでなく、この疵部を基点としたすき間腐食も発生する場合があった。本発明は、Ti-Nb複合添加の効果に加え、Mo-Tiバランスの最適化をすることにより、溶接部加工性および耐食性の向上をすることができる。以上より、溶接部加工性および耐食性に優れた表面疵の少ないフェライト系ステンレス鋼を提供でき得る知見を得た。   The effect of the combined addition of Ti-Nb is shown below. Ti and Nb are used as stabilizing elements for the purpose of suppressing precipitation of Cr carbide or Cr nitride on the grain boundary, but Ti has an effect on the intergranular corrosion resistance of the additive weld and the base metal HAZ. The effect of Nb is known as (Ti + Nb) / (C + N) ≧ 16. In order to sufficiently stabilize this with only Ti, a large amount of Ti is required. Therefore, as described above, hard inclusions are likely to occur at the steelmaking stage, and product surface flaws are likely to occur. On the other hand, when Nb is added alone, the effect of reducing the ductility of the welded portion relative to the base material and suppressing the grain coarsening of the welded portion is inferior to Ti. On the other hand, in the case of combined addition of Ti + Nb, steel material that reduces the adverse effect of Ti on surface defects and the negative effect of Nb on the ductility of welds and does not reduce the ductility of welds against the base metal with less surface defects Can be obtained. Considering the corrosion resistance due to inclusions, when Ti / (Nb + Ti) is less than 0.2, crevice corrosion based on water-soluble inclusions occurs. In addition, when Ti / (Nb + Ti) exceeds 0.8, surface flaws caused by Ti-based inclusions occur during rolling, and there are not only problems in appearance, but also crevice corrosion based on this flaw part also occurs. was there. The present invention can improve weldability and corrosion resistance by optimizing the Mo-Ti balance in addition to the effects of Ti-Nb composite addition. From the above, we have obtained knowledge that can provide ferritic stainless steel with excellent surface weldability and corrosion resistance, and with less surface flaws.

以下にその他化学組成について説明する。
ＣおよびＮは、フェライト系ステンレス鋼ではその固溶限が小さいため、多すぎる場合または溶接部ではＣｒの炭窒化物を析出し、特に粒に局部的なＣｒ欠乏部を生じてしまい、耐食性が劣化する。このためＣ、Ｎはそれぞれ0.015％以下、0.025%以下とし、さらに安定化元素ＮｂおよびＴｉを添加して、前述の(B)式を満たす条件が必要である。さらに、経済性と特性を考慮すると、ＣおよびＮの好ましい範囲は0.002〜0.015％である。 Other chemical compositions will be described below.
Since C and N have a small solid solubility limit in ferritic stainless steel, if they are too much or if they are welded, Cr carbonitride is precipitated, and in particular, local Cr-deficient parts are formed in the grains, resulting in corrosion resistance. to degrade. Therefore, it is necessary that C and N are 0.015% or less and 0.025% or less, respectively, and that the stabilizing elements Nb and Ti are added to satisfy the above-described equation (B). Further, considering the economy and characteristics, the preferable range of C and N is 0.002 to 0.015%.

Ｓｉは、製鋼における脱酸材として使用されるため混入する場合があり、微量では耐食性を向上させることもあるが、多すぎると加工性が低下するために上限を0.25％とした。さらに、材質特性を考慮すると、Ｓｉの好ましい範囲は0.05〜0.20％である。   Since Si is used as a deoxidizing material in steelmaking, it may be mixed in, and a trace amount may improve the corrosion resistance. However, if it is too much, the workability deteriorates, so the upper limit was made 0.25%. Furthermore, considering the material characteristics, the preferable range of Si is 0.05 to 0.20%.

Ｍｎは母材の強度や溶接部の靱性を向上するが、多すぎると耐食性の低下を引き起こすため、上限を0.20％とした。さらに、経済性と材質特性を考慮するとＭｎの好ましい範囲は0.05〜0.15％である。   Mn improves the strength of the base metal and the toughness of the weld, but if it is too much, the corrosion resistance decreases, so the upper limit was made 0.20%. Furthermore, considering the economy and material properties, the preferable range of Mn is 0.05 to 0.15%.

Ｐ，Ｓは不可避的不純物であるが、熱間加工性や耐食性を低下させる元素であるため低い方が望ましい。このためＰは0.035％以下、Ｓは0.010％以下とした。
さらに、経済性を考慮すると、Ｐ、Ｓの好ましい範囲を各々0.010〜0.030％、0.001〜0.008％である。 P and S are unavoidable impurities, but are preferably lower because they are elements that reduce hot workability and corrosion resistance. Therefore, P is set to 0.035% or less, and S is set to 0.010% or less.
Furthermore, considering the economy, the preferable ranges of P and S are 0.010 to 0.030% and 0.001 to 0.008%, respectively.

Crは耐食性を高める主元素であり、濃度が高いほど耐食性が向上する。しかし、Cr量を高めると、靭性の低下から製造性が困難となる上、鋼材が硬質化し加工性が低下する。このため、上限を24％とした。また低すぎると耐食性が低下する為、下限を16%とした。さらに、耐食性と加工性を考慮すると、Crの好ましい範囲は18〜20％である。   Cr is a main element that enhances corrosion resistance, and the higher the concentration, the better the corrosion resistance. However, if the Cr content is increased, manufacturability becomes difficult due to a decrease in toughness, and the steel material becomes hard and workability decreases. For this reason, the upper limit was made 24%. Moreover, since corrosion resistance will fall if too low, the minimum was made into 16%. Further, considering the corrosion resistance and workability, the preferable range of Cr is 18 to 20%.

MoはCrとともに耐食性を向上させ、再不働態化能を高める。この効果は本発明が対象とするCr含有量では、Moが0.5%以上で得られる。しかしながら、Mo含有量が多すぎると、加工性および溶接時の溶け込み性が低下し、鋼の価格変動も大きくなる。このため、上限を2.0％として、溶接部加工性の向上のため式(A)の関係を満たす。さらに、経済性と耐食性を考慮すると、Moの好ましい範囲は0.8〜1.5％である。   Mo, together with Cr, improves corrosion resistance and enhances repassivation ability. This effect is obtained when the Cr content targeted by the present invention is Mo 0.5% or more. However, if the Mo content is too large, the workability and the penetration during welding are lowered, and the price fluctuation of the steel also increases. For this reason, the upper limit is set to 2.0%, and the relationship of the formula (A) is satisfied to improve weldability. Furthermore, considering economic efficiency and corrosion resistance, the preferable range of Mo is 0.8 to 1.5%.

Tiは本発明を構成する上で、重要な元素であり、C,N安定化のため添加されるが、多すぎると、製造時に表面疵を生じやすくなり、耐食性および溶接部の強度を下げる為、上限を0.20％とした。Tiを添加せずNbのみを添加した場合には、溶接部の延性が大幅に劣るため、Ti とNbは（B）式の関係を満足するように同時に添加されなければならない。加えて、加工性確保のため、下限を0.05%とした。さらに、Ti-0.28Mo≧-0.25のバランスで最適な加工性が得られる範囲となる。   Ti is an important element in constructing the present invention, and is added for C and N stabilization. However, if it is too much, it tends to cause surface flaws during production, in order to reduce corrosion resistance and weld strength. The upper limit was made 0.20%. When only Nb is added without adding Ti, the ductility of the weld is greatly inferior, so Ti and Nb must be added simultaneously to satisfy the relationship of formula (B). In addition, the lower limit was made 0.05% to ensure processability. Further, the optimum workability is obtained with a balance of Ti-0.28Mo ≧ −0.25.

NbはTiと同様に、CおよびNの安定化のために添加される。多いほど、C,Nの安定化能は高いが、多すぎると強度が上がり、加工性を低下させる為、上限を0.40%とした。またNbを添加せずTiのみを添加させた場合、表面疵が溶接部の強度低下を引き起こすため、Nbの下限を0.05%とした。   Nb is added to stabilize C and N, as is Ti. The greater the amount, the higher the stabilizing ability of C and N. However, if the amount is too large, the strength increases and the workability decreases, so the upper limit was made 0.40%. In addition, when only Ti was added without adding Nb, surface flaws caused a decrease in the strength of the weld, so the lower limit of Nb was set to 0.05%.

選択元素について以下に説明する。   The selective element will be described below.

ＣｕおよびＮｉはＣｒ，Ｍｏに加えて添加することにより、耐孔食性や耐すき間腐食性を向上させることができる。ただしＣｕおよびＮｉの添加は加工性を低下させるほか、応力腐食割れの懸念が生じるため、上限をＣｕは1.0％、Ｎｉは2.0％とする。さらに、経済性と特性を考慮すると、好ましい範囲は、Ｃｕが0.05〜0.40％、Ｎｉは0.1〜1.0％とする。   By adding Cu and Ni in addition to Cr and Mo, pitting corrosion resistance and crevice corrosion resistance can be improved. However, addition of Cu and Ni lowers workability and raises the concern of stress corrosion cracking, so the upper limit is made 1.0% for Cu and 2.0% for Ni. Further, in consideration of economy and characteristics, preferable ranges are 0.05 to 0.40% for Cu and 0.1 to 1.0% for Ni.

ＶはＣｒ，Ｍｏに加えて添加することによりフェライト系ステンレス鋼の弱点である耐銹性や耐すき間腐食性が改善され，適切な組合せによりＳＵＳ３０４と同等以上の耐食性が得られるだけでなく，Ｃｒ，Ｍｏの使用を最小限にしてＶを添加すれば伸びや平均ｒ値の低下も小さく，耐食性と合わせて優れた加工性を確保することができる。Ｖの過度の添加はやはり加工性を低下させる上，耐食性向上効果も飽和するため，Ｖの上限を０．２％とする。さらに、経済性と特性を考慮すると、Ｖの好ましい範囲は0.05〜0.15％である。   When V is added in addition to Cr and Mo, the corrosion resistance and crevice corrosion resistance, which are weak points of ferritic stainless steel, are improved, and not only the corrosion resistance equivalent to or higher than SUS304 can be obtained by appropriate combination, but also Cr. If V is added while minimizing the use of Mo, the elongation and the average r value decrease are small, and excellent workability can be secured together with the corrosion resistance. Excessive addition of V lowers workability and also saturates the effect of improving corrosion resistance, so the upper limit of V is 0.2%. Further, considering the economy and characteristics, the preferable range of V is 0.05 to 0.15%.

Ｚｒは不動態皮膜の強化や介在物の組成制御を通じて，耐銹性や耐すき間腐食性の改善に効果を発揮する。しかし，過度の添加は，伸びの低下をもたらすとともに，製造工程で鋳造が困難になったりするため，Ｚｒの添加量は，0.2％以下とする。さらに、耐食性と加工性を考慮すると、Ｚｒの好ましい範囲は0.05〜0.15％である。  Zr is effective in improving weather resistance and crevice corrosion resistance by strengthening the passive film and controlling the composition of inclusions. However, excessive addition causes a decrease in elongation and makes casting difficult in the manufacturing process. Therefore, the amount of Zr added should be 0.2% or less. Furthermore, considering the corrosion resistance and workability, the preferred range of Zr is 0.05 to 0.15%.

Ｂは高純度フェライト系ステンレス鋼の二次加工脆性改善に有効な粒界強化元素であり，このような効果は０．０００２％以上で安定する。しかし，過度の添加はフェライトを固溶強化して延性低下の原因になるので，上限を０．００５％とする。好ましい範囲は0.0005〜0.0020％である。
以上の元素に加えて本発明では，耐食性のさらなる向上や加工性，表面特性の改善を意図して，Ｓｎ，Ｍｇ、Ａｌのうち１種または２種以上を目的に応じて適宜添加してもよい。特にAlは脱酸元素として有効である。 B is a grain boundary strengthening element effective for improving the secondary work brittleness of high purity ferritic stainless steel, and such an effect is stable at 0.0002% or more. However, excessive addition causes solid solution strengthening of ferrite and causes a decrease in ductility, so the upper limit is made 0.005%. A preferred range is 0.0005 to 0.0020%.
In addition to the above elements, in the present invention, one or more of Sn, Mg, and Al may be appropriately added depending on the purpose in order to further improve corrosion resistance, improve workability, and improve surface characteristics. Good. In particular, Al is effective as a deoxidizing element.

本発明の実施例を以下に記す。   Examples of the present invention will be described below.

表1に記す成分組成のフェライト系ステンレス鋼を実験室の真空溶解炉で溶製、鋳造した。これを実験室で熱間圧延、熱延板焼鈍・酸洗、冷延、冷延板焼鈍・酸洗を実施し、0.8mmの冷延板を作製した。なお冷延板焼鈍の温度は、各々の鋼材の再結晶温度に基づき950〜1050℃の間で調整した。   Ferritic stainless steel having the composition shown in Table 1 was melted and cast in a laboratory vacuum melting furnace. This was subjected to hot rolling, hot-rolled sheet annealing / pickling, cold rolling, cold-rolled sheet annealing / pickling in a laboratory to produce a 0.8 mm cold-rolled sheet. The temperature of cold-rolled sheet annealing was adjusted between 950 and 1050 ° C. based on the recrystallization temperature of each steel material.

この冷延板から20mm×50mmおよび20mm×20mmの板を切断し、2枚を重ね合わせてその中央部をスポット溶接してすき間腐食試験片とした。スポット溶接条件はスポット径8mm、電流値は約3.8ｋAとした。耐すき間腐食性試験は、600ｐｐｍＣｌ−と20ｐｐｍ Ｃｕ２＋を含む水溶液を用い、試験片のすき間部に試験液を十分浸透させてから試験片を浸漬したまま１４日間放置し、取り出した後のすき間腐食の有無で評価した。試験温度は７０℃とした。すき間腐食が進展する成長すき間は孔食深さが40μm以上とされており、40μm以下ではすき間腐食が進展しないとされていることから、すき間腐食の最大孔食深さが40μｍ以上の場合をすき間腐食が生じたとし、すき間腐食が生じた場合を（×）、すき間腐食が生じなかった場合を（○）として表中に示した。実施例24では、Cr+3.3Moが18.6と21.5以下であり、本実験において耐すき間腐食性を示さなかった。実施例39は、Ｔｉ／（Ｎｂ＋Ｔｉ）が0.2以下で水溶性介在物を基点としたすき間腐食の発生が見られた。   20 mm × 50 mm and 20 mm × 20 mm plates were cut from this cold-rolled plate, and the two sheets were overlapped and spot welded at the center to obtain a crevice corrosion test piece. The spot welding conditions were a spot diameter of 8 mm and a current value of about 3.8 kA. In the crevice corrosion resistance test, an aqueous solution containing 600 ppm Cl- and 20 ppm Cu 2+ was used. After the test solution was sufficiently infiltrated into the crevice portion of the test piece, the test piece was left immersed for 14 days, and the crevice corrosion after taking it out was measured. Evaluated by the presence or absence. The test temperature was 70 ° C. Growing crevice where crevice corrosion progresses has a pitting depth of 40 μm or more, and crevice corrosion does not progress below 40 μm. Therefore, crevice corrosion has a maximum pitting depth of 40 μm or more. When corrosion occurred, crevice corrosion occurred (×), and no crevice corrosion occurred (◯). In Example 24, Cr + 3.3Mo was 18.6 and 21.5 or less, and no crevice corrosion resistance was exhibited in this experiment. In Example 39, Ti / (Nb + Ti) was 0.2 or less, and the occurrence of crevice corrosion based on water-soluble inclusions was observed.

表面疵の評価は、サンプルの圧延時の表面疵発生状況を目視により観察し、ランク付けを行った。疵ランクA、Bまでが表面の美観を損ねないレベルで、C、Dは表面疵が発生したことを示している。実施例36では、Ｔｉ／（Ｎｂ＋Ｔｉ）が0.8を越え、圧延時にＴｉ系の介在物を起因とした表面疵が確認された。   Evaluation of surface defects was performed by visually observing the state of occurrence of surface defects during rolling of the samples and ranking.ま で Ranks A and B are at a level that does not impair the aesthetics of the surface, and C and D indicate that surface creases have occurred. In Example 36, Ti / (Nb + Ti) exceeded 0.8, and surface defects due to Ti inclusions were observed during rolling.

溶接部加工性は、JIS Z 2247に準拠したエリクセン試験にて行った。溶接試験片に板厚0.8mmのフェライト系ステンレス鋼を用い、入熱550〜650J/cmで同鋼種TIG突合せ溶接して作製した。得られた溶接試験片を用いて、JIS Z 2247に準拠して、エリクセン試験機で張出加工を5回行い、平均エリクセン値を測定した結果、エリクセン高さ5mm以上を良（○）5mm未満を不良（×）として評価した。例えば、Ti-0.28Moが-0.25以下となる実施例41では、平均エリクセン値が基準に満たなかった。   The weldability was measured by an Erichsen test based on JIS Z 2247. Ferritic stainless steel with a plate thickness of 0.8 mm was used for the weld specimen, and the same steel type TIG butt welding was performed at a heat input of 550 to 650 J / cm. Based on JIS Z 2247, the resulting welded test piece was subjected to overhanging 5 times with an Erichsen testing machine, and the average Erichsen value was measured. As a result, the Erichsen height was 5 mm or more, and good (○) less than 5 mm. Was evaluated as defective (x). For example, in Example 41 in which Ti-0.28Mo is −0.25 or less, the average Erichsen value did not meet the standard.

Figure 2008231542
Figure 2008231542

Cr+3.3Moと耐食性の関係を示す図である。It is a figure which shows the relationship between Cr + 3.3Mo and corrosion resistance. 溶接部加工性に及ぼすMo量とTi量の関係を示す図である。It is a figure which shows the relationship between Mo amount and Ti amount which influences weldability.

Claims (4)

質量%で、
C：0.020%以下、
N：0.025%以下、
Si：0.25%以下、
Mn：0.20%以下、
P：0.035%以下、
S：0.010%以下、
Cr：16〜24%、
Mo：0.50〜2.00%、
Ti：0.05〜0.20%、
Nb：0.05〜0.40%を含有し、残部はFeおよび不可避的不純物からなり、下記(A)式および(B)式を満足することを特徴とする、溶接部加工性および耐すき間腐食性に優れた表面疵の少ないフェライト系ステンレス鋼。
ここに、
21.5≦Cr+3.3Mo≦26 かつ Ti-0.28Mo≧-0.25・・・ (A)
（Ti+Nb）/（C+N）≧16 かつ 0.2≦Ti/（Ti+Nb）≦0.8・・・(B) % By mass
C: 0.020% or less,
N: 0.025% or less,
Si: 0.25% or less,
Mn: 0.20% or less,
P: 0.035% or less,
S: 0.010% or less,
Cr: 16-24%
Mo: 0.50-2.00%
Ti: 0.05-0.20%
Contains Nb: 0.05 to 0.40%, the balance consists of Fe and inevitable impurities, and satisfies the following formulas (A) and (B). Excellent weldability and crevice corrosion resistance Ferritic stainless steel with less surface flaws.
here,
21.5 ≦ Cr + 3.3Mo ≦ 26 and Ti-0.28Mo ≧ -0.25 ... (A)
(Ti + Nb) / (C + N) ≧ 16 and 0.2 ≦ Ti / (Ti + Nb) ≦ 0.8 (B) さらに、質量％で、
Ｎｉ：2.0％以下、
Ｃｕ：1.0％以下、の一種又は二種以上を含むことを特徴とする、請求項１に記載の、溶接部加工性および耐すき間腐食性に優れた表面疵の少ないフェライト系ステンレス鋼。 Furthermore, in mass%,
Ni: 2.0% or less,
The ferritic stainless steel with low surface flaws and excellent weldability and crevice corrosion resistance according to claim 1, characterized by containing one or more of Cu: 1.0% or less. さらに、質量％で、
Ｖ：0.2％以下、
Ｚｒ：0.2％以下、の一種又は二種以上を含むことを特徴とする、請求項１または請求項２に記載の、溶接部加工性および耐すき間腐食性に優れた表面疵の少ないフェライト系ステンレス鋼。 Furthermore, in mass%,
V: 0.2% or less,
Zr: 0.2% or less, one or more of Zr: ferritic stainless steel with low surface defects and excellent weldability and crevice corrosion resistance according to claim 1 or claim 2 steel. さらに、質量％で、
Ｂ：０．００５％以下を含有することを特徴とする、請求項１乃至請求項３のいずれか一項に記載の、溶接部加工性および耐すき間腐食性に優れた表面疵の少ないフェライト系ステンレス鋼。 Furthermore, in mass%,
B: 0.005% or less of the ferrite system according to any one of claims 1 to 3, excellent in weld zone workability and crevice corrosion resistance and having a low surface flaw Stainless steel.
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JP2010202973A (en) * 2009-02-09 2010-09-16 Nippon Steel & Sumikin Stainless Steel Corp Ferritic stainless steel with low black spot generation
JP2010248625A (en) * 2009-03-27 2010-11-04 Nippon Steel & Sumikin Stainless Steel Corp Ferritic stainless steel having excellent local corrosion resistance
JP2011184731A (en) * 2010-03-08 2011-09-22 Nippon Steel & Sumikin Stainless Steel Corp Ferritic stainless steel having excellent corrosion resistance in condensed water environment generated from hydrocarbon combustion exhaust gas
CN102251086A (en) * 2010-05-19 2011-11-23 宝山钢铁股份有限公司 Molybdenum-based ferrite stainless steel and preparation method thereof
JP2012036444A (en) * 2010-08-06 2012-02-23 Nippon Steel & Sumikin Stainless Steel Corp Ferritic stainless steel with low blackspot generation
JP2012117691A (en) * 2010-11-29 2012-06-21 Noritz Corp Apparatus for latent heat recovery type hot water generation and method of manufacturing the same
DE112022003529T5 (en) 2021-07-13 2024-05-23 Nippon Yakin Kogyo Co., Ltd. Ni-Cr-Mo alloy for welded pipes with superior processability and corrosion resistance

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JP2006257544A (en) * 2005-02-15 2006-09-28 Nippon Steel & Sumikin Stainless Steel Corp Ferritic stainless steel having excellent crevice corrosion resistance

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JP2005089828A (en) * 2003-09-17 2005-04-07 Nisshin Steel Co Ltd Ferritic stainless steel sheet improved in crevice corrosion resistance
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JP2010202973A (en) * 2009-02-09 2010-09-16 Nippon Steel & Sumikin Stainless Steel Corp Ferritic stainless steel with low black spot generation
KR101370205B1 (en) * 2009-02-09 2014-03-05 닛폰 스틸 앤드 스미킨 스테인레스 스틸 코포레이션 Ferrite stainless steel with low black spot generation
US8894924B2 (en) 2009-02-09 2014-11-25 Nippon Steel & Sumikin Stainless Steel Corporation Ferrite stainless steel with low black spot generation
JP2010248625A (en) * 2009-03-27 2010-11-04 Nippon Steel & Sumikin Stainless Steel Corp Ferritic stainless steel having excellent local corrosion resistance
JP2011184731A (en) * 2010-03-08 2011-09-22 Nippon Steel & Sumikin Stainless Steel Corp Ferritic stainless steel having excellent corrosion resistance in condensed water environment generated from hydrocarbon combustion exhaust gas
CN102251086A (en) * 2010-05-19 2011-11-23 宝山钢铁股份有限公司 Molybdenum-based ferrite stainless steel and preparation method thereof
JP2012036444A (en) * 2010-08-06 2012-02-23 Nippon Steel & Sumikin Stainless Steel Corp Ferritic stainless steel with low blackspot generation
JP2012117691A (en) * 2010-11-29 2012-06-21 Noritz Corp Apparatus for latent heat recovery type hot water generation and method of manufacturing the same
DE112022003529T5 (en) 2021-07-13 2024-05-23 Nippon Yakin Kogyo Co., Ltd. Ni-Cr-Mo alloy for welded pipes with superior processability and corrosion resistance

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