JP4201370B2 - Stainless steel for sewage treatment equipment - Google Patents
Stainless steel for sewage treatment equipment Download PDFInfo
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- JP4201370B2 JP4201370B2 JP36433797A JP36433797A JP4201370B2 JP 4201370 B2 JP4201370 B2 JP 4201370B2 JP 36433797 A JP36433797 A JP 36433797A JP 36433797 A JP36433797 A JP 36433797A JP 4201370 B2 JP4201370 B2 JP 4201370B2
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Description
【0001】
【発明の属する技術分野】
本発明は下水処理分野における下水処理設備の構成材料として、優れた耐食性を有する下水処理設備用ステンレス鋼に関するものである。
【0002】
【従来の技術】
下水処理場では沈砂池、沈殿池、汚泥処理槽、消毒槽などのセクションに分かれており、各セクションにおいて、汚泥を分別したり、汚水を撹拌したりする種々の下水処理機械機器がある。しかし、下水中には硫化物や塩素イオンをはじめとする腐食性イオン種が多量に存在するため、金属材料に対しては厳しい腐食環境となる。従来、下水処理機器の構成材料としては、主に重塗装を施した表面処理鋼が用いられている。
【0003】
【発明が解決しようとする課題】
下水中では流路の底や下水の流速の低下している部分などに汚濁物が堆積し、その堆積物内では酸素の供給が断たれ、嫌気化することにより硫酸塩還元菌が活性化し、下水中に含有されている硫酸塩を還元して硫化水素が生成される。生成された硫化水素は下水中の鉄などの物質と反応し、沈降または下水中に溶解したり、一部は気相中に放出される。下水処理機器では臭気が逸散しないようにケーシングされるが、ケーシング材の内面気相部においては水蒸気が滞留し、結露する。その結露水の中に硫化水素が溶け込み、イオウ酸化菌の作用により硫酸が生成され、硫酸イオンが濃縮することにより、極めて厳しい腐食環境となる。また、下水には塩化物も多く含まれ、特に海岸に近い下水処理場では塩化物イオン濃度が極めて高くなる。
【0004】
重塗装を施した鋼板といえども、こういった過酷な腐食環境においては塗膜が劣化したり、疵付き部や塗装面に存在するピンホールから腐食性イオンが浸透することにより、塗膜下腐食を生じる。腐食による強度低下や孔あきは下水処理機器の機能性を損なうものである。したがって、近年、下水処理分野に対してステンレス鋼の適用が検討されている。しかし、過酷な腐食環境においては、ステンレス鋼の種類によっては腐食する場合がある。ステンレス鋼の場合は孔食を起こすために、いったん腐食が進行すると機能性を損なう可能性がある。また、最近では下水処理場の美化も下水道整備計画にくみこまれており、下水処理機器をとりまくケーシング材などが赤さびを呈し、見栄えを損なうことは好ましくない。
【0005】
【課題を解決するための手段】
これらの問題を解決するために、本発明はステンレス鋼の化学的組成が質量で、C:0.05%以下、Mn:2.0%以下、P:0.04%以下、S:0.01%以下、Si:1.00%以下、Cr:16〜35%、N:0.05%以下、Mo:2.5〜4.0%を主成分としてNb:0.1〜1.0%、Ti:0.05〜0.3%、Al:0.01〜0.5%の一種または一種以上を含むフェライト系ステンレス鋼およびC:0.05%以下、Mn: 2.0%以下、P:0.04%以下、S:0.01%以下、Cr:16〜25%、N:0.3%以下、Si:1.00%以下、 Ni:6〜15%、Mo:2.5〜4.0%を含み、残部Feおよび不可避的不純物からなるオーステナイト系およびフェライト・オーステナイトの二相系ステンレス鋼であることを特徴とし、下水処理環境において優れた耐食性を有する下水処理設備用ステンレス鋼を提供するものである。
【0006】
発明者らは硫化物および塩素イオンの単独あるいは共存する環境下において、各ステンレス鋼の耐食性を調査し、耐食性に及ぼす合金元素の影響を検討した。その結果、この環境ではCr含有量が16%以上の鋼ではMo添加が有効であり、Mo量とCr量を適正範囲に規定することにより、ステンレス鋼の孔食および隙間腐食を改善できる成分領域を見出した。
【0007】
【発明の実施の形態】
以下、本発明のステンレス鋼を構成する成分の作用効果と、その適正合金量を前記のように定めた理由について説明する。
【0008】
C:Cはオーステナイト生成元素であり、耐隙間腐食性には大きな影響を与えないが、溶接部などでの粒界腐食感受性を高める。フェライト系鋼においては、過剰に添加すると加工性を損ない、溶接部の耐粒界腐食性を低下させるために、極低下が望ましいが、製造コストが上がるためにその上限を0.05%とした。一方、オーステナイト系および二相鋼においても、過剰に存在すると鋭敏化を起こし、溶接部などでの粒界腐食感受性を高めるため、その上限を0.05%とした。
【0009】
Mn:MnもCと同様にオーステナイト生成元素であり、鋼の脱硫、脱酸作用および熱間加工性改善効果がある。また、孔食や隙間腐食の防止に有効なNの固溶度を上げる元素で積極的に添加する必要がある。しかし、過剰な添加は、腐食の起点となりやすい硫化物を形成しやすくするために、上限を2.0%とした。
【0010】
P:本発明鋼では金属組織にかかわらず、Pの過剰な添加は耐食性を低下させるとともに、鋼を硬質にして成形性および溶接部の靱性を損ねる。しかし、ステンレス鋼などのCr含有鋼を脱Pすることは困難であり、極低下することは製造コストの上昇を招く。0.04%までは耐食性や加工性の低下は認められないことから、0.04%を上限とした。
【0011】
S:本発明鋼では金属組織にかかわらず、Sの過剰な添加は鋼中のMnと硫化物を形成し、耐隙間腐食性や耐孔食性に有害である。したがって、製造コストが上がらない範囲で出来る限り低い方がよく、その上限を0.01%とした。
【0012】
Si:Siは金属組織にかかわらず、製鋼時に脱酸剤として添加される。鋼中に過剰に存在すると、鋼を硬質にし、溶接部の靱性などを低下させるので、上限を1.00%とした。
【0013】
Cr:Crはステンレス鋼においては必要不可欠元素であり、塩素イオンの存在する下水環境では耐孔食性の観点から16%以上必要である。Cr量が多ければ多いほど耐食性は向上するが、添加量にともないコストは高くなる。したがって、フェライト系ステンレス鋼では上限を35%とした。一方、オーステナイト系および二相鋼においては、Cr量を増やすと、オーステナイト相を維持するためのNi等の添加量を増やす必要があり、また製造性や加工性が損なわれるので上限を25%とした。
【0014】
Mo:Moは硫化物および塩化物の存在する環境下における耐隙間腐食性や耐孔食性の改善に極めて有効な元素である。その効果を発現するためには2.5%以上添加する必要がある。しかし、過剰に添加すると加工性を損なうとともに製造コストが上がるため、上限を4.0%とした。ここで、Moの効果はCrが16%以上の成分域で特に顕著である。
【0015】
N:Nは強力なオーステナイト生成元素である。フェライト系鋼においては、過剰に添加すると加工性を損ない、溶接部の耐粒界腐食性を低下させるために、上限を0.05%とした。一方、オーステナイト系および二相鋼においては、孔食や隙間腐食を防止するのに有効な元素であり、製鋼上の問題から上限を0.3%とした。
【0016】
Ni:Niはオーステナイト相を保持するための主要元素である。フェライト系鋼の場合にはとくに添加する必要はないが、オーステナイト系鋼あるいは二相鋼の場合には耐隙間腐食性などを考慮して、6%は必要である。15%を超えるとコスト的に不利となるので組織調整のために上限を15%とした。
【0017】
Nb、Ti、Al:Nb、Tiは共に本発明で規制したCr量のフェライト系ステンレス鋼において粒界腐食を防止する。本発明のフェライト系ステンレス鋼はNb、Ti、Alの1種または2種以上を任意成分として含むことができる。TiとAlはNbと共に複合添加する時、酸洗によってステンレス鋼表面に形成される不動態皮膜に占めるMoの割合を高くする。その結果、硫化物や塩素イオンの存在する環境でも優れた耐食性が得られる。
【0018】
Nbは本発明で規制したC量レベルのフェライト系ステンレス鋼において粒界腐食を防止する。この作用を得るため、0.1%以上のNb含有量が必要である。しかし、過剰のNb添加によって溶接部の靱性を阻害する。そのため、上限を1.0%に規定した。
【0019】
Tiは不動態皮膜を強固にし、比較的低いCr量およびMo量の組成であっても優れた耐食性が得られる。また、粒界腐食を抑制し、C,Nを固定する作用も有する。このような効果を得るためには0.005%以上の Tiを含有することが必要である。しかし、過剰に添加すると、表面品質を低下させ、局部的な腐食を強める傾向がみられる。そのため、上限を0.3%に規定した。
【0020】
Alは脱酸剤として添加される成分であるが、不動態皮膜を緻密化する作用も有する。このような効果を得るためには0.01%以上のAlを含有することが必要である。しかし、過剰に添加すると、表面品質を低下させ、かつ溶接性が低下する。そのため、上限を0.5%に規定した。
【0021】
【実施例】
以下、実施例によって本発明を具体的に説明する。
【0022】
【実施例1】
SS400にタールエポキシ塗装を施した試験片と本発明鋼(1tmm)を用い、硫化物イオンおよび塩素イオンの存在する環境における耐食性を促進試験により検討した。 試験液には2,000ppmCl-の上水を用い、H2Sガスを吹き込むことによりHS-濃度を200ppmに調整した。 試験液に1時間浸漬後、60℃で3時間乾燥することを1サイクルとして、200サイクル後の腐食状態を調査した。表1に供試材の化学成分と試験結果を示す。
【0023】
【表1】
【0024】
SS400にタールエポキシ塗装を施した試験片では塗膜下腐食が生じ、全面に赤さびが発生していた。SUS304鋼は全面的に赤さびが生じていた。本発明鋼であるNo.1鋼、No.2鋼およびNo.3鋼は腐食しておらず、硫化物と塩化物の共存環境でも優れた耐食性を示した。比較鋼のうち、本請求項記載のCr量でMo量を下げたNo.4鋼、No.5鋼およびNo.6鋼は部分的に赤さびが生じていた。本試験のような環境では、Moが耐食性に対して有効に作用すると考える。 本実験で用いたNo.1鋼〜No.6鋼の200サイクル後の最大侵食深さを測定した。図1に最大侵食深さに及ぼすMo量の影響を示す。フェライト系、オーステナイト系、二相系によらずMo量を2.5%以上添加した本発明鋼は侵食がなく、耐孔食性が著しく改善されたことがわかる。硫化水素や塩素イオンの存在する環境下においては、2.5%以上のMoの添加が有効である。
【0025】
比較鋼のうち、本請求項記載のMo量でありながら、Cr量を下げたNo.7鋼およびNo.8鋼も部分的に赤さびが生じていた。本試験条件のように硫化水素や塩素イオンの存在する環境下においては一定量以上のCrの添加が必要であることがわかる。また、No.9鋼はMoおよびCr量は満たしているものの、部分的に赤さびを生じた。これはNb、TiおよびAlが添加されていないためであり、本発明におけるフェライト系鋼に対してはNb、TiおよびAlの耐食性改善作用があることがわかる。本実施例の結果から、硫化水素や塩素イオンの存在する環境下においては本発明に記述しているとおり、化学成分を調整することにより、優れた耐食性を得られることがわかった。
【0026】
【発明の効果】
本発明により、硫化水素や塩素イオンの存在する下水処理環境下において用いる設備、例えば沈砂掻き揚げ機などのケーシング材などに、耐孔食性ならびに耐隙間腐食性を有するフェライト系あるいはオーステナイト系のステンレス鋼の提供が可能となり、機能性の向上および設備機器の景観の維持に有効となる。
【図面の簡単な説明】
【図1】最大侵食深さに及ぼすMo量の影響[0001]
BACKGROUND OF THE INVENTION
The present invention relates to stainless steel for sewage treatment facilities having excellent corrosion resistance as a constituent material of sewage treatment facilities in the field of sewage treatment.
[0002]
[Prior art]
The sewage treatment plant is divided into sections such as a sand basin, a sedimentation basin, a sludge treatment tank, and a disinfection tank. In each section, there are various sewage treatment machinery equipment that separates sludge and stirs sewage. However, since sewage contains a large amount of corrosive ion species such as sulfides and chlorine ions, it becomes a severe corrosive environment for metal materials. Conventionally, as a constituent material of sewage treatment equipment, surface-treated steel subjected to heavy coating has been mainly used.
[0003]
[Problems to be solved by the invention]
In the sewage, pollutants accumulate on the bottom of the flow path and the part where the flow rate of the sewage is decreasing, and the supply of oxygen is cut off in the sediment, and the sulfate-reducing bacteria are activated by anaerobic, Hydrogen sulfide is produced by reducing sulfate contained in sewage. The produced hydrogen sulfide reacts with substances such as iron in the sewage and settles or dissolves in the sewage, or a part is released into the gas phase. In the sewage treatment equipment, casings are made so that odors do not dissipate, but water vapor stays in the gas phase portion on the inner surface of the casing material, causing condensation. Hydrogen sulfide dissolves in the condensed water, sulfuric acid is generated by the action of sulfur oxidizing bacteria, and sulfate ions concentrate, resulting in an extremely severe corrosive environment. In addition, sewage contains a large amount of chloride, and the chloride ion concentration becomes extremely high particularly at a sewage treatment plant near the coast.
[0004]
Even in steel plates with heavy coating, the coating film deteriorates in such a severe corrosive environment, and corrosive ions permeate from pinholes on the ridges and the painted surface. Causes corrosion. Strength reduction and perforation due to corrosion impair the functionality of sewage treatment equipment. Therefore, in recent years, application of stainless steel to the sewage treatment field has been studied. However, in a severe corrosive environment, corrosion may occur depending on the type of stainless steel. In the case of stainless steel, pitting corrosion occurs, and once corrosion progresses, the functionality may be impaired. In addition, recently, beautification of sewage treatment plants has been included in the sewerage maintenance plan, and it is not preferable that casing materials and the like surrounding sewage treatment equipment exhibit red rust and deteriorate the appearance.
[0005]
[Means for Solving the Problems]
In order to solve these problems, according to the present invention, the chemical composition of stainless steel is, by mass, C: 0.05% or less, Mn: 2.0% or less, P: 0.04% or less, S: 0.00. 01% or less, Si: 1.00% or less, Cr: 16 to 35%, N: 0.05% or less, Mo: 2.5 to 4.0% as a main component, Nb: 0.1 to 1.0 %, Ti: 0.05 to 0.3%, Al: Ferritic stainless steel containing one or more of 0.01 to 0.5% and C: 0.05% or less, Mn: 2.0% or less , P: 0.04% or less, S: 0.01% or less, Cr: 16-25%, N: 0.3% or less, Si: 1.00% or less, Ni: 6-15%, Mo: 2 Austenitic and ferrite austenitic two-phase stainless steel containing 0.5 to 4.0% and the balance Fe and inevitable impurities Characterized in that it is a less steel, there is provided a sewage treatment facility for stainless steel having excellent corrosion resistance in sewage treatment environments.
[0006]
The inventors investigated the corrosion resistance of each stainless steel in an environment where sulfides and chloride ions are present alone or coexist, and examined the influence of alloying elements on the corrosion resistance. As a result, in this environment, the addition of Mo is effective for steels with a Cr content of 16% or more, and by defining the Mo amount and Cr amount within an appropriate range, the component region that can improve pitting corrosion and crevice corrosion of stainless steel I found.
[0007]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the reason why the effects of the components constituting the stainless steel of the present invention and the appropriate alloy amount are determined as described above will be described.
[0008]
C: C is an austenite-forming element and does not significantly affect crevice corrosion resistance, but increases intergranular corrosion susceptibility in welds and the like. In ferritic steels, if added excessively, the workability is impaired and the intergranular corrosion resistance of the weld is reduced, so an extremely low reduction is desirable, but the upper limit is set to 0.05% because the manufacturing cost increases. . On the other hand, austenitic and duplex stainless steels are also sensitized when they are present in excess, and the upper limit is made 0.05% in order to increase the intergranular corrosion susceptibility in welds and the like.
[0009]
Mn: Mn is an austenite-forming element like C, and has an effect of improving the desulfurization, deoxidation and hot workability of steel. Further, it is necessary to positively add an element that increases the solid solubility of N, which is effective for preventing pitting corrosion and crevice corrosion. However, the upper limit is set to 2.0% in order to make it easy to form a sulfide that tends to cause corrosion when excessively added.
[0010]
P: Regardless of the metal structure in the steel of the present invention, excessive addition of P lowers the corrosion resistance and makes the steel hard and impairs the formability and the toughness of the weld. However, it is difficult to remove Cr-containing steel such as stainless steel, and the extreme reduction causes an increase in manufacturing cost. Up to 0.04%, no deterioration in corrosion resistance and workability is observed, so 0.04% was made the upper limit.
[0011]
S: In the steel of the present invention, regardless of the metal structure, excessive addition of S forms sulfides with Mn in the steel, which is harmful to crevice corrosion resistance and pitting corrosion resistance. Therefore, it is better to be as low as possible without increasing the manufacturing cost, and the upper limit is set to 0.01%.
[0012]
Si: Si is added as a deoxidizing agent during steelmaking regardless of the metal structure. If excessively present in the steel, the steel is hardened and the toughness of the welded portion is reduced, so the upper limit was made 1.00%.
[0013]
Cr: Cr is an indispensable element in stainless steel, and in a sewage environment where chlorine ions are present, 16% or more is necessary from the viewpoint of pitting corrosion resistance. As the amount of Cr increases, the corrosion resistance improves, but the cost increases with the amount of addition. Therefore, the upper limit is set to 35% for ferritic stainless steel. On the other hand, in austenitic and duplex stainless steels, increasing the amount of Cr requires increasing the amount of Ni and the like to maintain the austenitic phase, and also impairs manufacturability and workability, so the upper limit is 25%. did.
[0014]
Mo: Mo is an extremely effective element for improving crevice corrosion resistance and pitting corrosion resistance in an environment where sulfides and chlorides are present. In order to exhibit the effect, it is necessary to add 2.5% or more. However, if added excessively, the workability is impaired and the manufacturing cost increases, so the upper limit was made 4.0%. Here, the effect of Mo is particularly remarkable in the component region where Cr is 16% or more.
[0015]
N: N is a strong austenite forming element. In ferritic steel, the upper limit was made 0.05% in order to reduce workability when added in excess and to reduce intergranular corrosion resistance of the weld. On the other hand, austenitic and duplex stainless steels are effective elements for preventing pitting corrosion and crevice corrosion, and the upper limit was set to 0.3% due to problems in steelmaking.
[0016]
Ni: Ni is a main element for maintaining the austenite phase. In the case of ferritic steel, it is not necessary to add, but in the case of austenitic steel or duplex steel, 6% is necessary in consideration of crevice corrosion resistance. If it exceeds 15%, it is disadvantageous in terms of cost, so the upper limit was made 15% for organizational adjustment.
[0017]
Nb, Ti, Al: Nb and Ti together prevent intergranular corrosion in the ferritic stainless steel with Cr content regulated in the present invention. The ferritic stainless steel of the present invention can contain one or more of Nb, Ti and Al as optional components. When Ti and Al are added together with Nb, the proportion of Mo in the passive film formed on the stainless steel surface by pickling is increased. As a result, excellent corrosion resistance can be obtained even in an environment where sulfides and chlorine ions are present.
[0018]
Nb prevents intergranular corrosion in ferritic stainless steel of the C amount level regulated in the present invention. In order to obtain this effect, an Nb content of 0.1% or more is necessary. However, excessive Nb addition impairs the toughness of the weld. Therefore, the upper limit is defined as 1.0%.
[0019]
Ti strengthens the passive film, and excellent corrosion resistance can be obtained even with a relatively low composition of Cr and Mo. It also has the effect of suppressing intergranular corrosion and fixing C and N. In order to obtain such an effect, it is necessary to contain 0.005% or more of Ti. However, when added in excess, there is a tendency to reduce surface quality and increase local corrosion. Therefore, the upper limit is specified as 0.3%.
[0020]
Al is a component added as a deoxidizer, but also has an effect of densifying the passive film. In order to obtain such an effect, it is necessary to contain 0.01% or more of Al. However, when it adds excessively, surface quality will fall and weldability will fall. Therefore, the upper limit is specified as 0.5%.
[0021]
【Example】
Hereinafter, the present invention will be described specifically by way of examples.
[0022]
[Example 1]
Using a test piece of SS400 coated with tar epoxy and the steel of the present invention (1 tmm), corrosion resistance in an environment where sulfide ions and chlorine ions are present was examined by an accelerated test. The test solution 2,000PpmCl - with clean water, HS by blowing H2S gas - was adjusted to a concentration of 200 ppm. The corrosion state after 200 cycles was investigated by immersing in the test solution for 1 hour and then drying at 60 ° C. for 3 hours as one cycle. Table 1 shows the chemical components of the test materials and the test results.
[0023]
[Table 1]
[0024]
In the specimens with tar epoxy coating on SS400, corrosion under the coating film occurred and red rust was generated on the entire surface. SUS304 steel had red rust on the entire surface. The No. 1 steel, No. 2 steel and No. 3 steel, which are the steels of the present invention, were not corroded and exhibited excellent corrosion resistance even in an environment where sulfides and chlorides coexist. Among the comparative steels, the No. 4 steel, No. 5 steel, and No. 6 steel in which the Mo amount was lowered by the Cr amount described in this claim partially had red rust. In an environment such as this test, Mo is considered to act effectively on corrosion resistance. The maximum erosion depth after 200 cycles of No. 1 steel to No. 6 steel used in this experiment was measured. FIG. 1 shows the effect of Mo amount on the maximum erosion depth. It can be seen that the steel of the present invention added with 2.5% or more of Mo regardless of ferrite, austenite, or two-phase system has no erosion and the pitting corrosion resistance is remarkably improved. In an environment where hydrogen sulfide and chlorine ions are present, addition of 2.5% or more of Mo is effective.
[0025]
Among the comparative steels, the No. 7 steel and the No. 8 steel with the reduced Cr content partially produced red rust even though they had the Mo content described in the present claims. It can be seen that a certain amount or more of Cr needs to be added in an environment where hydrogen sulfide and chlorine ions are present as in this test condition. In addition, No. 9 steel partially produced red rust although the amount of Mo and Cr was satisfied. This is because Nb, Ti and Al are not added, and it can be seen that the ferritic steel in the present invention has an effect of improving the corrosion resistance of Nb, Ti and Al. From the results of this example, it was found that excellent corrosion resistance can be obtained by adjusting chemical components as described in the present invention in an environment where hydrogen sulfide and chlorine ions are present.
[0026]
【The invention's effect】
According to the present invention, ferritic or austenitic stainless steel having pitting corrosion resistance and crevice corrosion resistance for equipment used in a sewage treatment environment in the presence of hydrogen sulfide or chlorine ions, for example, casing materials such as a sand scourer. Can be provided, which is effective for improving functionality and maintaining the landscape of equipment.
[Brief description of the drawings]
Fig. 1 Effect of Mo content on maximum erosion depth
Claims (1)
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JP36433797A JP4201370B2 (en) | 1997-12-19 | 1997-12-19 | Stainless steel for sewage treatment equipment |
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JP36433797A JP4201370B2 (en) | 1997-12-19 | 1997-12-19 | Stainless steel for sewage treatment equipment |
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JPH11181555A JPH11181555A (en) | 1999-07-06 |
JP4201370B2 true JP4201370B2 (en) | 2008-12-24 |
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JP5676896B2 (en) * | 2009-03-27 | 2015-02-25 | 新日鐵住金ステンレス株式会社 | Ferritic stainless steel with excellent local corrosion resistance |
JP2018028236A (en) * | 2016-08-19 | 2018-02-22 | 日新製鋼株式会社 | Stainless steel for metal panel for sewage pipe lining and method for lining to sewage pipe using the same |
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