WO2024024236A1 - Microbiologically assisted cracking-resistant low-alloy steel - Google Patents

Microbiologically assisted cracking-resistant low-alloy steel Download PDF

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WO2024024236A1
WO2024024236A1 PCT/JP2023/019060 JP2023019060W WO2024024236A1 WO 2024024236 A1 WO2024024236 A1 WO 2024024236A1 JP 2023019060 W JP2023019060 W JP 2023019060W WO 2024024236 A1 WO2024024236 A1 WO 2024024236A1
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corrosion
content
microbial
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PCT/JP2023/019060
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French (fr)
Japanese (ja)
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至 寒澤
純二 嶋村
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Ｊｆｅスチール株式会社
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur

Definitions

  • the present invention relates to a microbial stress corrosion cracking resistant steel material suitable for structural members such as automobile parts, power generation equipment parts, chemical plant parts, architectural parts, machine parts, ship parts, and oil field incidental equipment parts. .
  • Microbial corrosion is a highly localized corrosion phenomenon, and microbial corrosion that occurs in structures can become the starting point for through holes and stress corrosion cracking, which can lead to serious accidents.
  • MAC Computerobiologically Assisted Cracking
  • stress corrosion cracking assisted by microbial corrosion is one of the stress corrosion cracking phenomena that is a concern in real environments where microorganisms exist.
  • Common countermeasures against microbial corrosion include reducing the number of microorganisms present in the environment using disinfectants and physically removing microorganisms attached to materials (cleaning with brushes, etc.).
  • microbial corrosion there are limits to the reduction of microbial corrosion by these means. For example, it is difficult to physically clean the outer surface of a pipeline buried in soil, and it is also difficult to actively use disinfectants due to the impact on the soil environment. Therefore, there is increasing interest in approaches that increase the microbial corrosion resistance of materials. In line with this trend, attempts have been made in the field of steel materials to impart resistance to microbial corrosion.
  • a layer containing zinc is provided between a steel material and an epoxy resin coating, and the layer containing zinc is a layer consisting only of zinc, or a layer containing 85% by mass or more of zinc, and other configurations.
  • a method for preventing corrosion and paint peeling of a steel material has been reported, which is characterized by a layer made of an alloy containing any one of nickel, aluminum, magnesium, and iron as an element.
  • Patent Document 2 an outer layer consisting of a Cr(III)-Fe(III)-based hydroxide and an inner layer of a film mainly composed of a Cr(III)-based oxide and/or hydroxide are formed on the surface.
  • a stainless steel with excellent microbial corrosion resistance characterized by having a two-layer coating has been reported.
  • Patent Document 3 in mass %, Cu: 0.010% or more and less than 2.000%, Ni: 0.010% or more and 2.000% or less, Mo: 0.010% or more and 1.000% or less, By containing one or more selected from W: 0.010% to 1.000% and Sn: 0.010% to 0.500%, antibacterial properties and microbial corrosion resistance properties are enhanced. Steel materials have been proposed.
  • Non-Patent Document 1 reports that corrosion acceleration by SRB is based on the direct extraction of electrons from steel materials by SRB. That is, in the metabolic activity of SRB, SRB present on the surface of the steel material directly oxidizes Fe, thereby promoting dissolution of the steel material. S 2- ions produced as a result of this metabolic activity combine with Fe 2+ ions produced as a result of steel material corrosion, and a hardly soluble FeS film is formed on the steel surface. Since this FeS coating acts as a corrosion-resistant coating, it is generally believed that it contributes to reducing corrosion of steel materials.
  • the above conventional technology has the following problems.
  • Patent Document 1 The method for preventing corrosion of steel described in Patent Document 1 is considered to have sufficient antibacterial properties, but it requires painting and forming an alloy film, making it very expensive, so it requires particularly high corrosion resistance. This will result in excessive performance for applications other than those in severe applications. In other words, it is not practical to use it for parts to which low-alloy steel is originally applied due to cost considerations. Furthermore, if the surface is scratched due to impact, cuts, etc., the durability of the scratched portion cannot be expected, making it difficult to obtain long-term effects.
  • Patent Document 2 the technology described in Patent Document 2 is also considered to be effective as a measure to improve the microbial corrosion resistance of stainless steel materials, but it is not suitable for cost reasons for parts for which inexpensive low-alloy steel materials are expected to be applied. is difficult. Further, as in Patent Document 1, resistance to microbial corrosion cannot be ensured at locations where the surface is scratched and the coating structure effective in microbial corrosion resistance is lost.
  • microbial corrosion resistance is evaluated based on local corrosion in an actual seawater environment containing sulfate-reducing bacteria, which are corrosive bacteria. In real environments, microbial corrosion becomes apparent in areas where corrosive bacteria become locally active and the bacteria concentration is high. Not expected.
  • the technology described in Patent Document 3 is simply the result of evaluating local corrosion in a seawater corrosive environment, and is particularly uncertain as a technology for reducing stress corrosion cracking induced by microbial corrosion. .
  • stress corrosion cracking in the present invention refers to the MAC phenomenon in a broad sense, which includes not only a static stress environment but also a dynamically changing stress environment, that is, a stress environment equivalent to low cycle corrosion fatigue. say.
  • the inventors have made extensive studies to solve the above problems.
  • SRB stress corrosion cracking phenomenon induced by sulfate-reducing bacteria
  • Non-Patent Document 1 we obtained the following findings. That is, in a microbial corrosion environment where SRB exists, a film of FeS, which is a corrosion product, is formed on the surface of the steel material as a result of the metabolic reaction of SRB and the corrosion reaction of the steel material. Since this FeS film has conductivity, it has the property of promoting the cathode reaction, and on the other hand, since it acts as a physical protective film, it suppresses the anodic dissolution reaction.
  • SRB stress corrosion cracking phenomenon induced by sulfate-reducing bacteria
  • the microbial corrosion environment is essentially an environment with high local corrosivity. Furthermore, if this FeS coating is mechanically destroyed due to the presence of external stress, galvanic corrosion occurs, with the FeS coating fractured part as the anode site and the FeS coating remaining part as the cathode site, resulting in a significant local corrosion phenomenon.
  • This locally corroded area becomes an oxygen-deficient environment, which increases the activity of SRB, which is an anaerobic bacterium, and causes the direct oxidation (iron dissolution) reaction due to the metabolism of SRB to proceed at an accelerated pace.
  • the component composition is a microbial stress corrosion cracking resistant low alloy steel material further containing one or more of the following groups A to E in mass %.
  • Group A Ni: 0.01-4.00%, Group B; Cr: 0.01-4.00%, Sb: 0.01-0.50%, Sn: 0.01-0.50%, Mo: 0.01-2.00% and W: 0
  • Group D selected from Ti: 0.005-0.100%, Zr: 0.005-0.100%, Nb: 0.005-0.100% and V: 0.005-0.100%
  • the present invention when used for structural members such as automobile parts, power generation equipment parts, chemical plant parts, architectural parts, machine parts, ship parts, and oil field incidental equipment parts, compared to conventional ones, It is possible to obtain inexpensive low-alloy steel materials that are resistant to microbial stress corrosion cracking. Further, the present invention is extremely useful industrially.
  • % representing the content of each component element means “% by mass” unless otherwise specified.
  • C 0.30% or less C is an element necessary to ensure the strength of steel, and if the content exceeds 0.30%, workability and weldability will deteriorate significantly, so it is limited to 0.30% or less. Preferably it is 0.25% or less, more preferably 0.20% or less. Note that the lower limit is not particularly limited, but is preferably 0.02% or more.
  • Mn 0.10-3.00% Mn is added to improve strength and toughness, but if it is less than 0.10%, the effect is not sufficient, and if it exceeds 3.00%, weldability deteriorates, so the Mn content is 0.10%. 3.00% or less. Preferably it is 0.20% or more and 2.50% or less, more preferably 0.30% or more and 2.00% or less.
  • P 0.030% or less
  • the P content increases, toughness and weldability deteriorate, so the P content was suppressed to 0.030% or less.
  • it is 0.025% or less. Since it is difficult to reduce the content to less than 0.002% in industrial scale production, a content of 0.002% or more is permissible.
  • N 0.0100% or less Since N is a harmful element that reduces toughness, it is desirable to reduce it as much as possible. In particular, when the amount of N exceeds 0.0100%, the decrease in toughness becomes large. Therefore, the N content is set to 0.0100% or less. Preferably it is 0.0080% or less. More preferably, it is 0.0070% or less. Note that the lower limit is not particularly limited, but it is preferably 0.0005% or more, since excessive reduction in N content leads to an increase in cost.
  • Si 0.02 ⁇ 1.00% Si is a very important element from the viewpoint of improving microbial stress corrosion cracking resistance. That is, Si is eluted to the surface of the steel material as Si 2+ ions as the steel material corrodes in an environment where SRB exists. A reaction to form FeS occurs in a corrosive environment due to the reaction between S 2 ⁇ ions and Fe 2+ ions that occur along with the metabolic reaction of SRB. These liberated Si 2+ ions are incorporated into the crystal structure of FeS to form a local SiS structure within the macroscopic FeS structure. This Si-substituted FeS has a non-uniform crystal structure, so its conductivity is significantly reduced.
  • the Si content is 0.02% or more, preferably 0.03% or more, and more preferably 0.05% or more.
  • the formation reaction of this Si-substituted FeS is strongly influenced by the amount of S, O, and Al in the steel material.
  • the content is set to 1.00% or less, preferably 0.80% or less, and more preferably 0.70% or less.
  • S 0.0002-0.0100% 8 ⁇ Si(%)/S(%) ⁇ 1200
  • S in the steel material is eluted onto the surface of the steel material as it corrodes. If the eluted Si 2+ ions remain as they are, they do not have sufficient affinity with FeS, and therefore, the incorporation of Si into the FeS structure does not proceed sufficiently.
  • S eluted from the steel material quickly reacts with Si 2+ ions to form a core structure of SiS.
  • the SiS core structure has a high affinity with the FeS structure and promotes the incorporation of Si into the FeS structure.
  • the S content required to obtain this effect is 0.0002% or more. Preferably it is 0.0003% or more.
  • S is a harmful element that deteriorates the toughness and weldability of steel. In particular, when the S content exceeds 0.0100%, the deterioration of base metal toughness and weld zone toughness increases. Therefore, the S content is 0.0100% or less, preferably 0.008% or less, and more preferably 0.007% or less.
  • Al 0.003-0.500%
  • Al is an element added as a deoxidizer, and the Al content is 0.003% or more. However, when the Al content exceeds 0.500%, the toughness of the steel decreases. Therefore, the Al content is 0.003 to 0.500%, preferably 0.003 to 0.300%. Further, Al is an element that influences the expression of the effect of improving microbial stress corrosion cracking resistance due to Si. For this reason, it is necessary to appropriately manage the ratio between the amount of Al and the amount of O, as described below.
  • O 0.0005-0.0050% 3 ⁇ Al(%)/O(%)
  • O is an important element that must be controlled in order to obtain the effect of suppressing galvanic corrosion by Si. That is, like Si, O in the steel material is eluted onto the surface of the steel material as it corrodes. If this eluted O is present in excess, it combines with Si to form SiO 2 . As a result, the formation of SiS nuclei is inhibited, and the formation of Si-substituted FeS does not proceed. This SiO 2 formation becomes obvious when the O content exceeds 0.0050%, so the O content is set to 0.0050% or less. Preferably it is 0.0045% or less, more preferably 0.0040% or less.
  • the lower limit is not particularly limited in view of the characteristics of the steel material, but is 0.0005% or more because excessive reduction in the amount of O will lead to an increase in manufacturing costs in the steel manufacturing process.
  • it is 0.0006% or more, more preferably 0.0008% or more.
  • the content ratio of O (%) to Al (%) must be increased. Needs to be managed appropriately. That is, Al combines with O to quickly form Al 2 O 3 . The O liberated with the dissolution of the base material is quickly consumed by the Al liberated in this way, thereby suppressing the SiO 2 formation reaction and progressing the Si-substituted FeS formation reaction. The formation of Si-substituted FeS due to the rapid formation of Al 2 O 3 from Al and O becomes obvious when Al (%) / O (%) is 3 or more.
  • %) shall be 3 or more.
  • the upper limit is not particularly determined, but as mentioned above, increasing Al (%)/O (%) will reduce the amount of O excessively, which will reduce the manufacturing cost in the steelmaking process. Since it leads to an increase, it is preferable to set it to 150 or less.
  • Cu and Ag has a high resistance to resistance in the low alloy steel material of this embodiment. It is an essential element to obtain microbial stress corrosion cracking resistance.
  • Cu and Ag are liberated from the steel as Cu 2+ ions and Ag + ions, respectively, as the steel material elutes. These free ions are taken up by microorganisms on the surface of the steel material and strongly bind to -SH group-containing amino acids and proteins present in the enzyme systems of the microorganisms, thereby inhibiting the metabolic activities of the microorganisms.
  • the effect of suppressing localized corrosion as microbial stress corrosion cracking based on this microbial metabolic inhibition effect can be achieved by containing 0.02% or more of Cu, or by containing 0.01% or more of Ag, or by containing 0.01% or more of Cu. It can be obtained by containing 0.02% or more of Ag and 0.01% or more of Ag.
  • the Cu content is 0.02 to 3.00%. Preferably it is 0.05 to 2.00%, more preferably 0.10 to 1.50%. Further, the content of Ag is 0.01 to 0.50%. Preferably it is 0.01 to 0.30%, more preferably 0.02 to 0.20%.
  • the Ag content is less than 0 to 0.01%. Also, when the Ag content is 0.01 to 0.50%, the Cu content is 0 to less than 0.02%.
  • the basic components of this embodiment have been explained above.
  • the remainder other than the above components is Fe and inevitable impurities.
  • the Ni content is less than 0.01%
  • the Cr content is less than 0.01%
  • the Sb content is less than 0.01%
  • the Sn content is less than 0.01%
  • the Mo content is 0.01%.
  • W content is less than 0.01%
  • Ca content is less than 0.0001%
  • Mg content is less than 0.0001%
  • REM content is less than 0.001%
  • Ti content is 0.005%
  • a range in which the Zr content is less than 0.005%, the Nb content is less than 0.005%, the V content is less than 0.005%, and the B content is less than 0.0001% is treated as an unavoidable impurity.
  • the following elements may be contained as appropriate.
  • Ni 0.01-4.00% Ni can be added to improve the manufacturability of steel sheets. In order to obtain this effect, the Ni content is 0.01% or more. On the other hand, excessive Ni content causes deterioration in weldability and increase in manufacturing cost. Therefore, the Ni content is 4.00% or less. Preferably it is 3.00% or less, more preferably 2.00% or less. More preferably, it is 1.5% or less.
  • Cr 0.01 ⁇ 4.00%
  • Sb 0.01 ⁇ 0.50%
  • Sn 0.01 ⁇ 0.50%
  • Mo 0.01 ⁇ 2.00%
  • W 0.01 ⁇ 2.00% of one or more selected from among Cr, Sb, Sn, Mo, and W are elements that improve corrosion resistance in a wet environment where microbial corrosion occurs, especially in a seawater environment. Therefore, apart from microbial corrosion, one or more types can be contained for the purpose of improving seawater corrosion resistance. However, if the addition amount is large, it will cause deterioration of the toughness of the welded part and increase the manufacturing cost. 01 to 0.50%, Mo: 0.01 to 2.00%, and W: 0.01 to 2.00%.
  • Ca, Mg, and REM are One or more types selected from Ca: 0.0001 to 0.0100%, Mg: 0.0001 to 0.0200%, and REM: 0.001 to 0.200%
  • Ca, Mg, and REM are One or more types can be contained for the purpose of ensuring toughness. However, if the amount added is large, it will cause deterioration of the toughness of the welded part and increase manufacturing cost, so the Ca content should be 0.0001% or more and 0.0100% or less, and the Mg content should be 0.0001% or more and 0.01% or less. 0.0200% or less, and the REM content is 0.001% or more and 0.200% or less.
  • Ti 0.005 to 0.100%
  • Zr 0.005 to 0.100%
  • Nb 0.005 to 0.100%
  • V 0.005 to 0.100%
  • Ti, Zr, Nb, and V can be contained in order to ensure the desired strength. However, if too much of either is contained, the toughness and weldability will deteriorate, so the content should be 0.005% or more and 0.100% or less. Preferably it is 0.005% or more and 0.05% or less.
  • B 0.0001-0.0300%
  • B is an element that improves the hardenability of steel materials. Moreover, B can be contained for the purpose of ensuring the strength of the steel material. However, if it is contained in excess, the toughness will be significantly deteriorated. The strength improvement effect is poor when the B content is less than 0.0001%, and the deterioration of toughness becomes significant when it exceeds 0.0300%. Therefore, the B content is 0.0001% or more. 0300% or less.
  • Molten steel having the above-mentioned composition is melted in a known furnace such as a converter or an electric furnace, and is made into a steel material such as a slab or billet by a known method such as a continuous casting method or an ingot forming method.
  • a known furnace such as a converter or an electric furnace
  • a steel material such as a slab or billet
  • a known method such as a continuous casting method or an ingot forming method.
  • vacuum degassing refining or the like may be performed upon melting.
  • a known steel smelting method may be used to adjust the composition of the molten steel.
  • the heating temperature is preferably 1030°C or higher.
  • heating above 1350°C causes surface marks and increases scale loss and fuel consumption, so it is preferably 1350°C or lower. More preferably it is 1050 to 1300°C.
  • the temperature of the steel material is originally in the range of 1030 to 1350° C., it may be directly subjected to hot rolling without being heated.
  • reheating treatment, pickling, and cold rolling may be performed to obtain a cold rolled sheet having a predetermined thickness.
  • the finish rolling end temperature be 600°C or higher. If the finish rolling end temperature is less than 600° C., the rolling load increases due to an increase in deformation resistance, making it difficult to carry out rolling. Cooling after finish rolling in hot rolling is preferably performed by air cooling or accelerated cooling at a cooling rate of 150° C./s or less, but this does not apply when heat treatment is performed in a subsequent step.
  • test piece was taken from the above-mentioned hot-rolled steel sheet, and a corrosion fatigue test was performed on the test piece using a sulfate-reducing bacteria culture solution (SRB culture solution) to evaluate microbial stress corrosion cracking characteristics.
  • SRB culture solution a sulfate-reducing bacteria culture solution
  • vulgaris NBRC104121 was added to a screw cap test tube containing 5 mL of MB medium, and cultured at 37° C. for 4 days. Approximately 2.5 mL of the culture solution was then added to a sterile centrifuge tube containing fresh MB medium (1 L) in an anaerobic glove box.
  • a tensile test piece with a parallel portion of 6 mm ⁇ x 25 mm was taken so that the C direction (width direction) of the steel plate was the tensile direction of the test piece.
  • a tensile test was conducted at room temperature in accordance with the provisions of JIS Z 2241, and the yield strength (YS) of the steel plate was determined in order to calculate the stress to be applied in the corrosion fatigue test described below.
  • the steel plate was processed into a round bar of 130 mm x 6.35 mm ⁇ , and both ends were threaded. This round bar was machined to a diameter of 3.81 mm by 12.7 mm from the center to both ends to create a corrosion fatigue test piece with a parallel portion having a length of 25.4 mm.
  • This parallel portion corresponds to the C direction (sheet width direction) of the steel plate.
  • This corrosion fatigue test piece was subjected to ultrasonic degreasing in acetone for 5 minutes, and then attached to a corrosion fatigue tester.
  • NS4 aqueous solution containing 95% by mass of .181 g/L CaCl 2 .2H 2 O
  • an anaerobic gas (95 vol.% N 2 + 5 vol.
  • test pieces were taken out after the test, and the appearance was observed using a microscope with a 500x field of view to confirm the presence or absence of cracks.
  • test pieces in which cracks were confirmed the cross section was observed, the maximum crack length in the cross section was measured, and the crack propagation distance was calculated.
  • Microbial stress corrosion cracking resistance was evaluated based on the following criteria. For cracks with a length of less than 30 ⁇ m, crack propagation was slow and the risk of corrosion destruction was determined to be low.
  • the unit of volume "L" herein represents 10 ⁇ 3 m 3 .

Abstract

Provided is a microbiologically assisted cracking-resistant low-alloy steel that is practical in terms of production. The microbiologically assisted cracking-resistant low-alloy steel comprises C: 0.30% or less, Mn: 0.10-3.00%, P: 0.030% or less, N: 0.0100% or less, Si: 0.02-1.00%, S: 0.0002-0.0100%, Al: 0.003-0.500%, O: 0.0005-0.0050%, and further comprises one or two selected from Cu: 0.02-3.00% and Ag: 0.01-0.50%, with the remainder being a component composition including Fe and unavoidable impurities. The content ratio of Si to S, Si (%)/S (%), is 8-1200, and the content ratio of Al to O, Al (%)/O (%), is at least 3.

Description

耐微生物応力腐食割れ低合金鋼材Microbial stress corrosion cracking resistant low alloy steel
　本発明は、自動車用部材、発電設備部材、化学プラント部材、建築用部材、機械部材、船舶部材、油田付帯設備用部材などの構造用部材に好適な、耐微生物応力腐食割れ鋼材に関するものである。 The present invention relates to a microbial stress corrosion cracking resistant steel material suitable for structural members such as automobile parts, power generation equipment parts, chemical plant parts, architectural parts, machine parts, ship parts, and oil field incidental equipment parts. .
　従前、大腸菌、サルモネラ菌に代表される病原性細菌、および硫酸塩還元菌、硫黄酸化細菌に代表される腐食性細菌などのような、我々の社会生活の中で、衛生的および工業的側面から、その増殖を忌避すべき細菌が知られている。特に近年では、分析および生物学分野の著しい進歩に伴い、細菌による社会生活への負の影響が広く認知されてきている。その一つが微生物腐食現象である。微生物腐食は非常に局部腐食性が高い腐食現象であり、構造物に生じた微生物腐食部が貫通孔や応力腐食割れの起点となり、深刻な事故を招く危険性がある。特に、後者は、ＭＡＣ（Ｍｉｃｒｏｂｉｏｌｏｇｉｃａｌｌｙ　Ａｓｓｉｓｔｅｄ　Ｃｒａｃｋｉｎｇ）「微生物腐食により支援された応力腐食割れ」として、微生物が存在する実環境で懸念される応力腐食割れ現象の一つとして、広く認知されてきている。微生物腐食に対する対策は、殺菌剤により環境中に存在する微生物を低減すること、材料に付着した微生物を物理的に除去する(ブラシ等による清掃)ことが一般的である。しかしながら、これら手段による微生物腐食の低減には限界がある。例えば、土壌に埋設されたパイプラインの外面に対して、物理的な清掃は困難であり、また土壌環境に対する影響から、殺菌剤を積極的に使用することも難しい。従って、材料側の微生物腐食耐性を高めるアプローチにも関心が高まっている。このような流れから、鋼材分野においても、微生物腐食に対する抵抗性を付与する試みが行われている。 Previously, pathogenic bacteria such as Escherichia coli and Salmonella, and corrosive bacteria such as sulfate-reducing bacteria and sulfur-oxidizing bacteria have been used in our social lives from a sanitary and industrial perspective. Bacteria whose proliferation should be avoided are known. Particularly in recent years, with significant advances in the fields of analysis and biology, the negative impact of bacteria on social life has become widely recognized. One of these is the phenomenon of microbial corrosion. Microbial corrosion is a highly localized corrosion phenomenon, and microbial corrosion that occurs in structures can become the starting point for through holes and stress corrosion cracking, which can lead to serious accidents. In particular, the latter has been widely recognized as MAC (Microbiologically Assisted Cracking) "stress corrosion cracking assisted by microbial corrosion" as one of the stress corrosion cracking phenomena that is a concern in real environments where microorganisms exist. Common countermeasures against microbial corrosion include reducing the number of microorganisms present in the environment using disinfectants and physically removing microorganisms attached to materials (cleaning with brushes, etc.). However, there are limits to the reduction of microbial corrosion by these means. For example, it is difficult to physically clean the outer surface of a pipeline buried in soil, and it is also difficult to actively use disinfectants due to the impact on the soil environment. Therefore, there is increasing interest in approaches that increase the microbial corrosion resistance of materials. In line with this trend, attempts have been made in the field of steel materials to impart resistance to microbial corrosion.
　例えば、特許文献１では、鉄鋼材料とエポキシ樹脂塗膜の間に亜鉛を含む層を設け、前記亜鉛を含む層が、亜鉛のみからなる層、若しくは、亜鉛を８５質量％以上含み、他の構成元素としてニッケル、アルミニウム、マグネシウム、鉄のいずれかを含む合金からなる層であることを特徴とする鉄鋼材料の腐食及び塗膜剥離の防止方法が報告されている。 For example, in Patent Document 1, a layer containing zinc is provided between a steel material and an epoxy resin coating, and the layer containing zinc is a layer consisting only of zinc, or a layer containing 85% by mass or more of zinc, and other configurations. A method for preventing corrosion and paint peeling of a steel material has been reported, which is characterized by a layer made of an alloy containing any one of nickel, aluminum, magnesium, and iron as an element.
　また、特許文献２では、表面に、Ｃｒ（ＩＩＩ）－Ｆｅ（ＩＩＩ）系の水酸化物からなる外層とＣｒ（ＩＩＩ）系の酸化物および／または水酸化物を主体とする皮膜の内層の２層の皮膜を有することを特徴とする耐微生物腐食性に優れたステンレス鋼が報告されている。 Furthermore, in Patent Document 2, an outer layer consisting of a Cr(III)-Fe(III)-based hydroxide and an inner layer of a film mainly composed of a Cr(III)-based oxide and/or hydroxide are formed on the surface. A stainless steel with excellent microbial corrosion resistance characterized by having a two-layer coating has been reported.
　さらに、特許文献３では、質量％で、Ｃｕ：０．０１０％以上２．０００％未満、Ｎｉ：０．０１０％以上２．０００％以下、Ｍｏ：０．０１０％以上１．０００％以下、Ｗ：０．０１０％以上１．０００％以下およびＳｎ：０．０１０％以上０．５００％以下のうちから選ばれる１種または２種以上を含有させることで抗菌性と耐微生物腐食特性を高めた鋼材が提案されている。 Further, in Patent Document 3, in mass %, Cu: 0.010% or more and less than 2.000%, Ni: 0.010% or more and 2.000% or less, Mo: 0.010% or more and 1.000% or less, By containing one or more selected from W: 0.010% to 1.000% and Sn: 0.010% to 0.500%, antibacterial properties and microbial corrosion resistance properties are enhanced. Steel materials have been proposed.
　一方、微生物腐食の主な原因菌として知られている硫酸塩還元菌（ＳＲＢ：Ｓｕｌｆａｔｅ－ｒｅｄｕｃｉｎｇ　ｂａｃｔｅｒｉａ）に関して、その鋼材腐食促進メカニズムについても、近年新たな知見が得られている。 On the other hand, new knowledge has been obtained in recent years regarding the mechanism of sulfate-reducing bacteria (SRB), which is known to be the main cause of microbial corrosion, and its mechanism of accelerating steel corrosion.
　例えば、非特許文献１において、ＳＲＢによる腐食促進がＳＲＢによる鋼材からの電子の直接的な引き抜き作用に基づくことが報告されている。すなわち、ＳＲＢの代謝活動において、鋼材表面に存在するＳＲＢがＦｅを直接酸化することで、鋼材の溶解が促進される。この代謝活動の結果として生成されるＳ２－イオンは、鋼材腐食の結果生じるＦｅ２＋イオンと結びつき、難溶性ＦｅＳ被膜が鋼材表面に形成される。このＦｅＳ被膜は腐食抵抗被膜として作用するため、一般的に鋼材腐食の低減に寄与するとされている。
　しかしながら、ＳＲＢによる微生物腐食現象においては、ＦｅＳが比較的高い導電性を有しているために、ＦｅＳの表面に付着した硫酸塩還元菌は、ＦｅＳを介しても母材(鋼材)の電子引き抜き(直接酸化)が可能となる。そのため、ＦｅＳ被膜の形成が不十分な箇所では、局部腐食が促進することとなる。すなわち、例えば、塑性応力により、物理的に鋼材表面に新生面が生じた箇所では、ＦｅＳによる表面保護が作用しないばかりか、ＦｅＳが存在した箇所とのガルバニック効果により、孔食が生じ、応力腐食割れが顕在化する。 For example, Non-Patent Document 1 reports that corrosion acceleration by SRB is based on the direct extraction of electrons from steel materials by SRB. That is, in the metabolic activity of SRB, SRB present on the surface of the steel material directly oxidizes Fe, thereby promoting dissolution of the steel material. S 2- ions produced as a result of this metabolic activity combine with Fe 2+ ions produced as a result of steel material corrosion, and a hardly soluble FeS film is formed on the steel surface. Since this FeS coating acts as a corrosion-resistant coating, it is generally believed that it contributes to reducing corrosion of steel materials.
However, in the microbial corrosion phenomenon caused by SRB, since FeS has relatively high conductivity, sulfate-reducing bacteria attached to the surface of FeS can withdraw electrons from the base material (steel material) even through FeS. (direct oxidation) becomes possible. Therefore, local corrosion will be accelerated in areas where the FeS coating is insufficiently formed. That is, for example, in areas where new surfaces have physically formed on the steel surface due to plastic stress, surface protection by FeS will not work, and due to the galvanic effect with areas where FeS was present, pitting corrosion will occur, resulting in stress corrosion cracking. becomes apparent.
特開２０１０－２２２６０６号公報Japanese Patent Application Publication No. 2010-222606 特開平７－２６３９５号公報Japanese Patent Application Publication No. 7-26395 特開２０１７－１９０５２２号公報JP 2017-190522 Publication
　上記の従来技術には以下のような問題がある。 The above conventional technology has the following problems.
　特許文献１に記載された鋼材の防食方法は、抗菌性は十分に有していると考えられるが、塗装および合金被膜形成が必要であり、非常に高価となるため、特別に高い耐食性が要求されるような過酷な用途以外では過剰性能となる。すなわち、本来低合金鋼材が適用されているような部材に対しては、コスト上、使用が現実的ではない。また、衝撃や切創等により、表面に傷がついてしまった場合には、傷がついた部分の耐性は期待できず、長期的な効果を得ることは難しい。 The method for preventing corrosion of steel described in Patent Document 1 is considered to have sufficient antibacterial properties, but it requires painting and forming an alloy film, making it very expensive, so it requires particularly high corrosion resistance. This will result in excessive performance for applications other than those in severe applications. In other words, it is not practical to use it for parts to which low-alloy steel is originally applied due to cost considerations. Furthermore, if the surface is scratched due to impact, cuts, etc., the durability of the scratched portion cannot be expected, making it difficult to obtain long-term effects.
　また、特許文献２に記載の技術についても、ステンレス鋼材の微生物腐食抵抗性の向上策として有効と考えられるが、安価な低合金鋼材の適用が想定されている部材に対しては、コスト上適用が困難である。また、特許文献１と同様に表面に傷がつき、耐微生物腐食に有効な被膜構造が失われた箇所に対しては、微生物腐食に対する抵抗が担保できない。 In addition, the technology described in Patent Document 2 is also considered to be effective as a measure to improve the microbial corrosion resistance of stainless steel materials, but it is not suitable for cost reasons for parts for which inexpensive low-alloy steel materials are expected to be applied. is difficult. Further, as in Patent Document 1, resistance to microbial corrosion cannot be ensured at locations where the surface is scratched and the coating structure effective in microbial corrosion resistance is lost.
　さらに、特許文献３に記載の鋼材については、腐食性細菌である硫酸塩還元菌を含む実海水環境での局部腐食に基づき、微生物腐食耐性を評価している。実環境での微生物腐食は、局所的に腐食性細菌が活性となり菌濃度が高い場所において顕在化するが、特許文献３に開示されている鋼材は、この微生物腐食の実態が反映された環境を想定していない。
　すなわち、特許文献３に記載の技術は、単に海水腐食環境中での局部腐食性を評価した結果に過ぎず、特に、微生物腐食により誘起された応力腐食割れに対する低減技術としては、不確実である。 Furthermore, regarding the steel material described in Patent Document 3, microbial corrosion resistance is evaluated based on local corrosion in an actual seawater environment containing sulfate-reducing bacteria, which are corrosive bacteria. In real environments, microbial corrosion becomes apparent in areas where corrosive bacteria become locally active and the bacteria concentration is high. Not expected.
In other words, the technology described in Patent Document 3 is simply the result of evaluating local corrosion in a seawater corrosive environment, and is particularly uncertain as a technology for reducing stress corrosion cracking induced by microbial corrosion. .
　このように、低合金鋼材分野において、安価に且つ、長期的な耐微生物腐食特性を担保する観点から、好適な技術は確立されておらず、材料そのものの耐微生物腐食性を向上させる技術が望まれるものの、技術的に確立されていない。特に、応力が作用した箇所において微生物腐食が進行することで生じる応力腐食割れ現象に関して、その低減技術は検討されていない。 As described above, in the field of low-alloy steel materials, from the viewpoint of ensuring long-term microbial corrosion resistance at low cost, no suitable technology has been established, and technology that improves the microbial corrosion resistance of the material itself is desired. However, it has not been technically established. In particular, no technology has been studied to reduce stress corrosion cracking, which occurs when microbial corrosion progresses at locations where stress is applied.
　本発明は、かかる従来技術の問題を解決し、製造上実用的な、耐微生物応力腐食割れ低合金鋼材を提供することを目的とする。なお、本発明における応力腐食割れは、静的な応力環境のみならず、動的に変動する応力環境、すなわち低サイクルの腐食疲労相当の応力環境をも包含した、広義の意味でのＭＡＣ現象をいう。 The purpose of the present invention is to solve the problems of the prior art and provide a microbial stress corrosion cracking resistant low alloy steel material that is practical for manufacturing. Note that stress corrosion cracking in the present invention refers to the MAC phenomenon in a broad sense, which includes not only a static stress environment but also a dynamically changing stress environment, that is, a stress environment equivalent to low cycle corrosion fatigue. say.
　発明者らは、上記の課題を解決すべく、鋭意検討を重ねた。まず、発明者らは、非特許文献１を参考に微生物腐食の代表的な原因菌である硫酸塩還元菌（ＳＲＢ）のより誘起される応力腐食割れ現象と、その機構を詳細に検討したところ、以下の知見を得た。
　すなわち、ＳＲＢが存在する微生物腐食環境では、ＳＲＢの代謝反応と鋼材の腐食反応の結果、鋼材表面に腐食生成物であるＦｅＳの被膜が形成される。このＦｅＳ被膜は導電性を有することからカソード反応を促進する性質を有する一方、物理的な保護被膜として作用することから、アノード溶解反応を抑制する。このＦｅＳ被膜が完全に均一な性状で形成させることは困難であるため、本質的に微生物腐食環境は高い局部腐食性を有した環境である。さらに、このＦｅＳ被膜が外部応力の存在により機械的に破壊された場合、ＦｅＳ被膜破壊部をアノードサイト、ＦｅＳ被膜残存部をカソードサイトとしたガルバニック腐食が発現し、著しい局部腐食現象が生じる。この局部腐食部は、酸素が欠乏した環境となることで、嫌気性細菌であるＳＲＢの活性が高まり、ＳＲＢの代謝による直接的な酸化(鉄溶解)反応も加速度的に進行することとなる。このガルバニックカップリングと、ＳＲＢの代謝活性化を駆動力とした選択的な局部腐食の進行の結果、応力腐食割れに到る。
　このように、ＳＲＢを含む微生物腐食環境下における応力腐食割れを低減するためには、導電性ＦｅＳ被膜の存在によるガルバニック腐食と、ＳＲＢの代謝による鉄の直接酸化(溶解)の両面から、対策を講じる必要がある。
　そこで、発明者らは、上記の知見に基づき、微生物応力腐食割れに耐性を示す鋼材の開発に向けて鋭意研究を重ねた。
　その結果、導電性ＦｅＳ被膜の存在によるガルバニック腐食を抑制するためには、Ｓｉ、Ｓ、ＡｌおよびＯの含有量と割合を適正量に制御することが有効であることを知見した。
　また、ＳＲＢの代謝による鉄の直接酸化(溶解)を抑制するためには、ＣｕやＡｇの添加が有効であるとわかった。
　本発明は、上記の新規な知見に基づき、さらに検討を重ねた末に完成されたもので、その要旨構成は、以下の通りである。 The inventors have made extensive studies to solve the above problems. First, the inventors conducted a detailed study of the stress corrosion cracking phenomenon induced by sulfate-reducing bacteria (SRB), which is a typical causative agent of microbial corrosion, and its mechanism, with reference to Non-Patent Document 1. , we obtained the following findings.
That is, in a microbial corrosion environment where SRB exists, a film of FeS, which is a corrosion product, is formed on the surface of the steel material as a result of the metabolic reaction of SRB and the corrosion reaction of the steel material. Since this FeS film has conductivity, it has the property of promoting the cathode reaction, and on the other hand, since it acts as a physical protective film, it suppresses the anodic dissolution reaction. Since it is difficult to form this FeS film with completely uniform properties, the microbial corrosion environment is essentially an environment with high local corrosivity. Furthermore, if this FeS coating is mechanically destroyed due to the presence of external stress, galvanic corrosion occurs, with the FeS coating fractured part as the anode site and the FeS coating remaining part as the cathode site, resulting in a significant local corrosion phenomenon. This locally corroded area becomes an oxygen-deficient environment, which increases the activity of SRB, which is an anaerobic bacterium, and causes the direct oxidation (iron dissolution) reaction due to the metabolism of SRB to proceed at an accelerated pace. As a result of this galvanic coupling and the progress of selective local corrosion driven by the metabolic activation of SRB, stress corrosion cracking occurs.
In this way, in order to reduce stress corrosion cracking in a microbial corrosion environment containing SRB, countermeasures must be taken from both galvanic corrosion due to the presence of a conductive FeS film and direct oxidation (dissolution) of iron due to the metabolism of SRB. It is necessary to take measures.
Therefore, based on the above knowledge, the inventors conducted extensive research toward the development of a steel material that is resistant to microbial stress corrosion cracking.
As a result, it was found that in order to suppress galvanic corrosion due to the presence of the conductive FeS film, it is effective to control the contents and proportions of Si, S, Al, and O to appropriate amounts.
It was also found that addition of Cu or Ag is effective in suppressing direct oxidation (dissolution) of iron due to SRB metabolism.
The present invention was completed after further studies based on the above-mentioned novel findings, and the gist and structure of the present invention is as follows.
[１]質量％で、Ｃ：０．３０％以下、Ｍｎ：０．１０～３．００％、Ｐ：０．０３０％以下、Ｎ：０．０１００％以下、Ｓｉ：０．０２～１．００％、Ｓ：０．０００２～０．０１００％、Ａｌ：０．００３～０．５００％、Ｏ：０．０００５～０．００５０％を含有すると共に、Ｃｕ：０．０２～３．００％、Ａｇ：０．０１～０．５０％のうちから選ばれる１種又は２種を含有し、残部はＦｅおよび不可避的不純物からなる成分組成を有し、ＳｉとＳとの含有量の比Ｓｉ（％）／Ｓ（％）が８以上１２００以下、ＡｌとＯとの含有量の比Ａｌ（％）／Ｏ（％）が３以上の耐微生物応力腐食割れ低合金鋼材である。 [1] In mass %, C: 0.30% or less, Mn: 0.10 to 3.00%, P: 0.030% or less, N: 0.0100% or less, Si: 0.02 to 1. 00%, S: 0.0002 to 0.0100%, Al: 0.003 to 0.500%, O: 0.0005 to 0.0050%, and Cu: 0.02 to 3.00%. , Ag: contains one or two selected from 0.01 to 0.50%, the remainder is Fe and unavoidable impurities, and the ratio of Si to S content is (%) / S (%) is 8 or more and 1200 or less, and the ratio of Al and O content Al (%) / O (%) is 3 or more, and is a microbial stress corrosion cracking resistant low alloy steel material.
[２]上記の[１]において、前記成分組成は、さらに、質量％で、下記のＡ群～Ｅ群のうちの１群以上を含有する耐微生物応力腐食割れ低合金鋼材である。
Ａ群；Ｎｉ：０．０１～４．００％、
Ｂ群；Ｃｒ：０．０１～４．００％、Ｓｂ：０．０１～０．５０％、Ｓｎ：０．０１～０．５０％、Ｍｏ：０．０１～２．００％およびＷ：０．０１～２．００％のうちから選ばれる１種以上、
Ｃ群；Ｃａ：０．０００１～０．０１００％、Ｍｇ：０．０００１～０．０２００％およびＲＥＭ：０．００１～０．２００％のうちから選ばれる１種以上、
Ｄ群；Ｔｉ：０．００５～０．１００％、Ｚｒ：０．００５～０．１００％、Ｎｂ：０．００５～０．１００％およびＶ：０．００５～０．１００％のうちから選ばれる１種以上、
Ｅ群；Ｂ：０．０００１～０．０３００％。 [2] In [1] above, the component composition is a microbial stress corrosion cracking resistant low alloy steel material further containing one or more of the following groups A to E in mass %.
Group A; Ni: 0.01-4.00%,
Group B; Cr: 0.01-4.00%, Sb: 0.01-0.50%, Sn: 0.01-0.50%, Mo: 0.01-2.00% and W: 0 One or more types selected from .01 to 2.00%,
Group C; one or more selected from Ca: 0.0001 to 0.0100%, Mg: 0.0001 to 0.0200% and REM: 0.001 to 0.200%,
Group D: selected from Ti: 0.005-0.100%, Zr: 0.005-0.100%, Nb: 0.005-0.100% and V: 0.005-0.100% One or more types of
Group E; B: 0.0001-0.0300%.
　本発明によれば、自動車用部材、発電設備部材、化学プラント部材、建築用部材、機械部材、船舶部材、油田付帯設備用部材などの構造用部材に使用した場合に、従来と比較してより安価な耐微生物応力腐食割れ低合金鋼材を得ることが可能である。また、本発明は産業上も極めて有用である。 According to the present invention, when used for structural members such as automobile parts, power generation equipment parts, chemical plant parts, architectural parts, machine parts, ship parts, and oil field incidental equipment parts, compared to conventional ones, It is possible to obtain inexpensive low-alloy steel materials that are resistant to microbial stress corrosion cracking. Further, the present invention is extremely useful industrially.
　以下、本発明の一実施形態に係る低合金鋼材について説明する。まず、低合金鋼材の成分組成の限定理由について述べる。なお、本明細書において、各成分元素の含有量を表す「％」は、特に断らない限り「質量％」を意味する。 Hereinafter, a low alloy steel material according to an embodiment of the present invention will be described. First, the reasons for limiting the composition of low-alloy steel materials will be described. In this specification, "%" representing the content of each component element means "% by mass" unless otherwise specified.
Ｃ：０．３０％以下
　Ｃは、鋼の強度確保に必要な元素であり、０．３０％を超える含有では加工性、溶接性が著しく劣化するため、０．３０％以下に制限する。好ましくは０．２５％以下であり、より好ましくは０．２０％以下である。なお、下限については特に限定しないが、好ましくは０．０２％以上である。 C: 0.30% or less C is an element necessary to ensure the strength of steel, and if the content exceeds 0.30%, workability and weldability will deteriorate significantly, so it is limited to 0.30% or less. Preferably it is 0.25% or less, more preferably 0.20% or less. Note that the lower limit is not particularly limited, but is preferably 0.02% or more.
Ｍｎ：０．１０～３．００％
　Ｍｎは、強度、靭性を改善するために添加するが、０．１０％未満ではその効果が十分でなく、３．００％を超えると溶接性が劣化するため、Ｍｎ含有量は０．１０％以上３．００％以下である。好ましくは０．２０％以上２．５０％以下であり、より好ましくは０．３０％以上２．００％以下である。 Mn: 0.10-3.00%
Mn is added to improve strength and toughness, but if it is less than 0.10%, the effect is not sufficient, and if it exceeds 3.00%, weldability deteriorates, so the Mn content is 0.10%. 3.00% or less. Preferably it is 0.20% or more and 2.50% or less, more preferably 0.30% or more and 2.00% or less.
Ｐ：０．０３０％以下
　Ｐは、含有量が多くなると、靭性及び溶接性を劣化させるため、Ｐの含有量は０．０３０％以下に抑制するものとした。好ましくは０．０２５％以下である。０．００２％未満とするのは工業的規模の製造では難しいため、０．００２％以上の含有は許容される。 P: 0.030% or less When P content increases, toughness and weldability deteriorate, so the P content was suppressed to 0.030% or less. Preferably it is 0.025% or less. Since it is difficult to reduce the content to less than 0.002% in industrial scale production, a content of 0.002% or more is permissible.
Ｎ：０．０１００％以下
　Ｎは、靭性を低下させる有害な元素であるので、極力低減させることが望ましい。特に、Ｎ量が０．０１００％を超えると、靭性の低下が大きくなる。従って、Ｎ含有量は０．０１００％以下とする。好ましくは０．００８０％以下である。より好ましくは０．００７０％以下である。なお、下限については特に限定しないが、過剰なＮ含有量の低減はコストの増加を招くため、０．０００５％以上とすることが好ましい。 N: 0.0100% or less Since N is a harmful element that reduces toughness, it is desirable to reduce it as much as possible. In particular, when the amount of N exceeds 0.0100%, the decrease in toughness becomes large. Therefore, the N content is set to 0.0100% or less. Preferably it is 0.0080% or less. More preferably, it is 0.0070% or less. Note that the lower limit is not particularly limited, but it is preferably 0.0005% or more, since excessive reduction in N content leads to an increase in cost.
Ｓｉ：０．０２～１．００％
　Ｓｉは、耐微生物応力腐食割れ性の向上の観点から、非常に重要な元素である。すなわち、Ｓｉは、ＳＲＢが存在する環境での鋼材の腐食に伴って鋼材表面に、Ｓｉ２＋イオンとして溶出する。ＳＲＢの代謝反応に伴って生じるＳ２－イオンとＦｅ２＋イオンとの反応により、腐食環境中ではＦｅＳの形成反応が生じる。この遊離したＳｉ２＋イオンはＦｅＳの結晶構造に取り込まれて、巨視的なＦｅＳ構造中にあって、局所的なＳｉＳ構造を形成する。このＳｉ置換型ＦｅＳは、不均一な結晶構造のために、導電性が著しく低下する。
　導電性が低下したＦｅＳ被膜は、外部応力の作用により、ＦｅＳ被膜破壊部が形成されたとしても、その被膜破壊部と被膜健全部との間に強いカップリングをもたらさず、ガルバニック腐食の進行が停滞する。そのため、応力腐食割れ（ＳＣＣ）の起点である局部腐食の形成が著しく抑制され、鋼材の耐応力腐食割れ性が向上する。
　このような効果を発現させるため、Ｓｉ含有量は０．０２％以上であり、好ましくは０．０３％以上、より好ましくは０．０５％以上である。
　しかしながら、このＳｉ置換型ＦｅＳの形成反応は、鋼材中のＳ量、Ｏ量、Ａｌ量の影響を強く受ける。Ｓｉによるガルバニック腐食抑制効果を安定的に得るために、後述する鋼材中のＳ量との含有量の比を適切に管理するとともに、Ａｌ量とＯ量の比を適切に管理する必要がある。
　なお、Ｓｉは過剰な添加により、靭性や溶接性の劣化を招くため、含有量を１．００％以下とし、好ましくは０．８０％以下であり、より好ましくは０．７０％以下である。 Si: 0.02~1.00%
Si is a very important element from the viewpoint of improving microbial stress corrosion cracking resistance. That is, Si is eluted to the surface of the steel material as Si 2+ ions as the steel material corrodes in an environment where SRB exists. A reaction to form FeS occurs in a corrosive environment due to the reaction between S 2− ions and Fe 2+ ions that occur along with the metabolic reaction of SRB. These liberated Si 2+ ions are incorporated into the crystal structure of FeS to form a local SiS structure within the macroscopic FeS structure. This Si-substituted FeS has a non-uniform crystal structure, so its conductivity is significantly reduced.
Even if a FeS film breakdown part is formed due to the action of external stress, a FeS film with reduced conductivity does not create strong coupling between the film breakdown part and a healthy part of the film, and the progress of galvanic corrosion is prevented. Stagnant. Therefore, the formation of localized corrosion, which is the starting point of stress corrosion cracking (SCC), is significantly suppressed, and the stress corrosion cracking resistance of the steel material is improved.
In order to exhibit such an effect, the Si content is 0.02% or more, preferably 0.03% or more, and more preferably 0.05% or more.
However, the formation reaction of this Si-substituted FeS is strongly influenced by the amount of S, O, and Al in the steel material. In order to stably obtain the effect of suppressing galvanic corrosion by Si, it is necessary to appropriately manage the ratio of the content to the S content in the steel material, which will be described later, and the ratio of the Al content to the O content.
Note that excessive addition of Si causes deterioration of toughness and weldability, so the content is set to 1.00% or less, preferably 0.80% or less, and more preferably 0.70% or less.
Ｓ：０．０００２～０．０１００％
８≦Ｓｉ（％）／Ｓ（％）≦１２００
　Ｓは、ＳｉのＦｅＳ被膜への取り込み挙動に影響し、Ｓｉによるガルバニック腐食抑制効果を得るために重要な元素である。すなわち、鋼材中のＳはＳｉ同様に、腐食に伴って鋼材表面に、溶出する。溶出したＳｉ２＋イオンのままでは、ＦｅＳとの十分な親和性を有していないために、ＦｅＳ構造中にＳｉの取り込みが十分に進まない。
　ここで、鋼材から溶出したＳは、速やかにＳｉ２＋イオンと反応して、ＳｉＳの核構造を形成する働きをする。ＳｉＳ核構造は、ＦｅＳ構造と高い親和性を有し、ＦｅＳ構造へＳｉの取り込みを促進する。その結果、Ｓｉ置換型ＦｅＳ形成によるガルバニック腐食抑制効果に起因した耐応力腐食割れ性が向上する効果が顕在化する。
　この効果を得るために必要なＳ含有量は０．０００２％以上である。好ましくは、０．０００３％以上である。一方、Ｓは、鋼の靭性および溶接性を劣化させる有害元素であり、特に、Ｓの含有量が０．０１００％を超えると、母材靭性および溶接部靭性の劣化が大きくなる。よって、Ｓ含有量は０．０１００％以下であり、好ましくは０．００８％以下、さらに好ましくは０．００７％以下である。 S: 0.0002-0.0100%
8≦Si(%)/S(%)≦1200
S affects the behavior of Si to be incorporated into the FeS film, and is an important element in order to obtain the effect of suppressing galvanic corrosion by Si. That is, like Si, S in the steel material is eluted onto the surface of the steel material as it corrodes. If the eluted Si 2+ ions remain as they are, they do not have sufficient affinity with FeS, and therefore, the incorporation of Si into the FeS structure does not proceed sufficiently.
Here, S eluted from the steel material quickly reacts with Si 2+ ions to form a core structure of SiS. The SiS core structure has a high affinity with the FeS structure and promotes the incorporation of Si into the FeS structure. As a result, the effect of improving stress corrosion cracking resistance due to the galvanic corrosion inhibiting effect due to the formation of Si-substituted FeS becomes apparent.
The S content required to obtain this effect is 0.0002% or more. Preferably it is 0.0003% or more. On the other hand, S is a harmful element that deteriorates the toughness and weldability of steel. In particular, when the S content exceeds 0.0100%, the deterioration of base metal toughness and weld zone toughness increases. Therefore, the S content is 0.0100% or less, preferably 0.008% or less, and more preferably 0.007% or less.
　併せて、上記の耐応力腐食割れ性に対するＳの効果を発現するためには、さらにＳｉとＳの含有量の比を適切に管理する必要がある。
　Ｓｉ（％）に対するＳ（％）の比率が高い場合、鋼材から溶出したＳｉとＳによるＳｉＳの形成が継続的に進行し、ＳｉＳがＦｅＳ構造に取り込まれることなく、単独の化合物として析出し、Ｓｉ置換型ＦｅＳが十分に形成されなくなる。この現象は、Ｓｉ（％）／Ｓ（％）が８未満の場合に顕在化することから、Ｓｉ（％）／Ｓ（％）は８以上であり、好ましくは１０以上であり、より好ましくは１２以上である。
　一方、Ｓｉ（％）に対するＳ（％）の比率が小さい場合、過剰なＳｉの存在により、エレメンタルＳｉの形成が熱力学的に安定となり、ＳｉＳ核が十分に形成されず、Ｓｉ置換型ＦｅＳによるガルバニック腐食抑制作用が発現しなくなる。この現象は、Ｓｉ（％）／Ｓ（％）が１２００を超えた場合に顕在化することから、Ｓｉ（％）／Ｓ（％）は１２００以下であり、好ましくは１１００以下、より好ましくは１０００以下である。 In addition, in order to exhibit the effect of S on the stress corrosion cracking resistance described above, it is necessary to appropriately control the ratio of the Si and S contents.
When the ratio of S (%) to Si (%) is high, the formation of SiS by Si and S eluted from the steel progresses continuously, and SiS is not incorporated into the FeS structure but precipitates as a single compound. Si-substituted FeS is not sufficiently formed. This phenomenon becomes apparent when Si(%)/S(%) is less than 8, so Si(%)/S(%) is 8 or more, preferably 10 or more, and more preferably It is 12 or more.
On the other hand, when the ratio of S (%) to Si (%) is small, the formation of elemental Si becomes thermodynamically stable due to the presence of excess Si, and SiS nuclei are not sufficiently formed, resulting in the formation of Si-substituted FeS. Galvanic corrosion inhibiting effect is no longer expressed. This phenomenon becomes apparent when Si (%) / S (%) exceeds 1200, so Si (%) / S (%) is 1200 or less, preferably 1100 or less, more preferably 1000 or less. It is as follows.
Ａｌ：０．００３～０．５００％
　Ａｌは、脱酸剤として添加される元素であり、Ａｌ含有量は０．００３％以上とする。しかし、Ａｌ含有量が０．５００％を超えると、鋼の靭性が低下する。このため、Ａｌ含有量は０．００３～０．５００％であり、好ましくは０．００３～０．３００％である。
　また、Ａｌは、前述のＳｉによる耐微生物応力腐食割れ性が向上する効果の発現に影響する元素である。そのために、後述のようにＡｌ量とＯ量の比を適切に管理する必要がある。 Al: 0.003-0.500%
Al is an element added as a deoxidizer, and the Al content is 0.003% or more. However, when the Al content exceeds 0.500%, the toughness of the steel decreases. Therefore, the Al content is 0.003 to 0.500%, preferably 0.003 to 0.300%.
Further, Al is an element that influences the expression of the effect of improving microbial stress corrosion cracking resistance due to Si. For this reason, it is necessary to appropriately manage the ratio between the amount of Al and the amount of O, as described below.
Ｏ：０．０００５～０．００５０％
３≦Ａｌ（％）／Ｏ（％）
　Ｏは、Ｓｉによるガルバニック腐食抑制効果を得るために管理すべき重要な元素である。すなわち、鋼材中のＯは、Ｓｉ同様に腐食に伴って鋼材表面に溶出する。この溶出したＯが過剰に存在する場合、Ｓｉと結びつくことで、ＳｉＯ２が形成される。その結果、ＳｉＳ核の形成が阻害され、Ｓｉ置換型ＦｅＳの形成が進行しなくなる。
　このＳｉＯ２形成は、Ｏ含有量が０．００５０％を超えると顕在化するため、Ｏ含有量は０．００５０％以下とする。好ましくは０．００４５％以下であり、より好ましくは０．００４０％以下である。一方、下限については、鋼材の特性上は特に制限されるものではないが、過剰なＯ量の低減は製鋼工程における製造コストの増加を招くため、０．０００５％以上である。好ましくは０．０００６％以上、より好ましくは０．０００８％以上である。 O: 0.0005-0.0050%
3≦Al(%)/O(%)
O is an important element that must be controlled in order to obtain the effect of suppressing galvanic corrosion by Si. That is, like Si, O in the steel material is eluted onto the surface of the steel material as it corrodes. If this eluted O is present in excess, it combines with Si to form SiO 2 . As a result, the formation of SiS nuclei is inhibited, and the formation of Si-substituted FeS does not proceed.
This SiO 2 formation becomes obvious when the O content exceeds 0.0050%, so the O content is set to 0.0050% or less. Preferably it is 0.0045% or less, more preferably 0.0040% or less. On the other hand, the lower limit is not particularly limited in view of the characteristics of the steel material, but is 0.0005% or more because excessive reduction in the amount of O will lead to an increase in manufacturing costs in the steel manufacturing process. Preferably it is 0.0006% or more, more preferably 0.0008% or more.
　一方、ＯによるＳｉＯ２形成を抑制し、Ｓｉ置換型ＦｅＳの形成によるガルバニック腐食抑制効果を安定的に得るためには、上記に加えて、さらにＡｌ（％）に対するＯ（％）の含有比を適切に管理する必要がある。すなわち、Ａｌは、Ｏと結びつくことで、Ａｌ２Ｏ３が速やかに形成する。母材溶解に伴って遊離したＯが、このように遊離したＡｌにより速やかに消費されることで、ＳｉＯ２の形成反応が抑制されて、Ｓｉ置換型ＦｅＳの形成反応が進行する。このＡｌとＯの速やかなＡｌ２Ｏ３の形成によるＳｉ置換型ＦｅＳの形成は、Ａｌ（％）／Ｏ（％）が３以上の場合に顕在化することから、Ａｌ（％）／Ｏ（％）は３以上とする。なお、上限については、特に定めるものではないが、上述のとおり、Ａｌ（％）／Ｏ（％）を高くすることは、Ｏ量を過剰に低減することになり、それは製鋼工程における製造コストの増加につながるため、１５０以下とすることが好ましい。 On the other hand, in addition to the above, in order to suppress the formation of SiO 2 due to O and to stably obtain the galvanic corrosion suppressing effect due to the formation of Si-substituted FeS, in addition to the above, the content ratio of O (%) to Al (%) must be increased. Needs to be managed appropriately. That is, Al combines with O to quickly form Al 2 O 3 . The O liberated with the dissolution of the base material is quickly consumed by the Al liberated in this way, thereby suppressing the SiO 2 formation reaction and progressing the Si-substituted FeS formation reaction. The formation of Si-substituted FeS due to the rapid formation of Al 2 O 3 from Al and O becomes obvious when Al (%) / O (%) is 3 or more. %) shall be 3 or more. The upper limit is not particularly determined, but as mentioned above, increasing Al (%)/O (%) will reduce the amount of O excessively, which will reduce the manufacturing cost in the steelmaking process. Since it leads to an increase, it is preferable to set it to 150 or less.
Ｃｕ：０．０２～３．００％、Ａｇ：０．０１～０．５０％のうちから選ばれる１種または２種
　ＣｕとＡｇの少なくとも１種は、本実施形態の低合金鋼材において、耐微生物応力腐食割れ性を得るために必須の元素である。ＣｕとＡｇは、鋼材の溶出に伴って、鋼中からそれぞれＣｕ２＋イオン、Ａｇ＋イオンとして遊離する。この遊離イオンが鋼材表面の微生物に取り込まれ、微生物の酵素系に存在する－ＳＨ基を有するアミノ酸、タンパク質と強く結合し、微生物の代謝活動を阻害する。
　環境中に酸素が多く存在する場合に、Ｃｕ２＋イオンとＡｇ＋イオンは、酸化物(ＣｕＯ、Ａｇ２Ｏ）に速やかに変化してしまうため、この微生物の代謝活動を阻害する効果の発現可能性が低くなる。一方、応力腐食割れ過程において局部腐食部では、酸素が欠乏した環境になるため、このＣｕおよびＡｇによる微生物の代謝活動を阻害する効果は、十分に発現する。この結果、鋼材局部腐食部でのＳＲＢに代表される腐食性微生物の代謝に基づいた局部腐食の進行が抑制される。 One or two types selected from Cu: 0.02 to 3.00% and Ag: 0.01 to 0.50% At least one of Cu and Ag has a high resistance to resistance in the low alloy steel material of this embodiment. It is an essential element to obtain microbial stress corrosion cracking resistance. Cu and Ag are liberated from the steel as Cu 2+ ions and Ag + ions, respectively, as the steel material elutes. These free ions are taken up by microorganisms on the surface of the steel material and strongly bind to -SH group-containing amino acids and proteins present in the enzyme systems of the microorganisms, thereby inhibiting the metabolic activities of the microorganisms.
When there is a lot of oxygen in the environment, Cu 2+ ions and Ag + ions quickly change to oxides (CuO, Ag 2 O), which can inhibit the metabolic activity of microorganisms. sex becomes lower. On the other hand, in the stress corrosion cracking process, locally corroded areas become oxygen-deficient environments, so that the effect of Cu and Ag on inhibiting the metabolic activity of microorganisms is fully expressed. As a result, the progress of local corrosion based on the metabolism of corrosive microorganisms such as SRB in locally corroded parts of the steel material is suppressed.
　この微生物の代謝阻害作用に基づいた、微生物応力腐食割れとしての局部腐食の抑制効果を、Ｃｕを０．０２％以上含有することにより、若しくはＡｇを０．０１％以上含有することにより、又はＣｕを０．０２％以上及びＡｇを０．０１％以上含有することにより得ることができる。
　しかし、ＣｕとＡｇを過剰に含有すると、溶接性や鋼板製造性が劣化する。よって、Ｃｕの含有量は、０．０２～３．００％である。好ましくは０．０５～２．００％、より好ましくは０．１０～１．５０％である。
　また、Ａｇの含有量は、０．０１～０．５０％である。好ましくは０．０１～０．３０％、より好ましくは０．０２～０．２０％である。
　また、Ｃｕ、Ａｇのいずれか１種が添加されている場合、つまり、Ｃｕの含有量が０．０２～３．００％のときは、Ａｇの含有量は、０～０．０１％未満であり、また、Ａｇの含有量が０．０１～０．５０％のときは、Ｃｕの含有量は０～０．０２％未満である。 The effect of suppressing localized corrosion as microbial stress corrosion cracking based on this microbial metabolic inhibition effect can be achieved by containing 0.02% or more of Cu, or by containing 0.01% or more of Ag, or by containing 0.01% or more of Cu. It can be obtained by containing 0.02% or more of Ag and 0.01% or more of Ag.
However, when Cu and Ag are contained excessively, weldability and steel plate manufacturability deteriorate. Therefore, the Cu content is 0.02 to 3.00%. Preferably it is 0.05 to 2.00%, more preferably 0.10 to 1.50%.
Further, the content of Ag is 0.01 to 0.50%. Preferably it is 0.01 to 0.30%, more preferably 0.02 to 0.20%.
Furthermore, when either one of Cu or Ag is added, that is, when the Cu content is 0.02 to 3.00%, the Ag content is less than 0 to 0.01%. Also, when the Ag content is 0.01 to 0.50%, the Cu content is 0 to less than 0.02%.
　以上、本実施形態の基本成分について説明した。上記成分以外の残部はＦｅおよび不可避的不純物である。
　なお、Ｎｉ含有量が０．０１％未満、Ｃｒ含有量が０．０１％未満、Ｓｂ含有量が０．０１％未満、Ｓｎ含有量が０．０１％未満、Ｍｏ含有量が０．０１％未満、Ｗ含有量が０．０１％未満、Ｃａ含有量が０．０００１％未満、Ｍｇ含有量が０．０００１％未満、ＲＥＭ含有量が０．００１％未満、Ｔｉ含有量が０．００５％未満、Ｚｒ含有量が０．００５％未満、Ｎｂ含有量が０．００５％未満、Ｖ含有量が０．００５％未満、Ｂ含有量が０．０００１％未満の範囲を不可避的不純物として扱う。
　その他にも必要に応じて、以下の元素を適宜含有することができる。 The basic components of this embodiment have been explained above. The remainder other than the above components is Fe and inevitable impurities.
In addition, the Ni content is less than 0.01%, the Cr content is less than 0.01%, the Sb content is less than 0.01%, the Sn content is less than 0.01%, and the Mo content is 0.01%. W content is less than 0.01%, Ca content is less than 0.0001%, Mg content is less than 0.0001%, REM content is less than 0.001%, Ti content is 0.005% A range in which the Zr content is less than 0.005%, the Nb content is less than 0.005%, the V content is less than 0.005%, and the B content is less than 0.0001% is treated as an unavoidable impurity.
In addition, the following elements may be contained as appropriate.
Ｎｉ：０．０１～４．００％
　Ｎｉは、鋼板の製造性を改善するために添加することができる。この効果を得るためには、Ｎｉ含有量は０．０１％以上である。一方、Ｎｉを過剰に含有すると、溶接性の劣化や、製造コスト上昇を招く。このため、Ｎｉ含有量は４．００％以下である。好ましくは３．００％以下、より好ましくは２．００％以下である。さらに好ましくは１．５％以下である。 Ni: 0.01-4.00%
Ni can be added to improve the manufacturability of steel sheets. In order to obtain this effect, the Ni content is 0.01% or more. On the other hand, excessive Ni content causes deterioration in weldability and increase in manufacturing cost. Therefore, the Ni content is 4.00% or less. Preferably it is 3.00% or less, more preferably 2.00% or less. More preferably, it is 1.5% or less.
Ｃｒ：０．０１～４．００％、Ｓｂ：０．０１～０．５０％、Ｓｎ：０．０１～０．５０％、Ｍｏ：０．０１～２．００％およびＷ：０．０１～２．００％のうちから選ばれる１種以上
　Ｃｒ、Ｓｂ、Ｓｎ、Ｍｏ、Ｗは微生物腐食が生じる湿潤環境のうち、特に海水環境における耐食性を高める元素である。従って、微生物腐食とは別に、海水耐食性を向上させる目的に１種以上を含有することができる。しかしながら、添加量が多い場合には、溶接部の靱性劣化や製造コストの増加を招くため、Ｃｒ：０．０１～４．００％、Ｓｂ：０．０１～０．５０％、Ｓｎ：０．０１～０．５０％、Ｍｏ：０．０１～２．００％およびＷ：０．０１～２．００％である。より好ましくは、Ｃｒ：０．０２～３．００％、Ｓｂ：０．０２～０．３０％、Ｓｎ：０．０２～０．３０％、Ｍｏ：０．０２～１．５０％およびＷ：０．０２～１．５０％である。さらに好ましくは、Ｃｒ：０．０３～２．００％、Ｓｂ：０．０３～０．２０％、Ｓｎ：０．０３～０．２０％、Ｍｏ：０．０３～１．００％およびＷ：０．０３～１．００％の範囲である。 Cr: 0.01~4.00%, Sb: 0.01~0.50%, Sn: 0.01~0.50%, Mo: 0.01~2.00% and W: 0.01~ 2.00% of one or more selected from among Cr, Sb, Sn, Mo, and W are elements that improve corrosion resistance in a wet environment where microbial corrosion occurs, especially in a seawater environment. Therefore, apart from microbial corrosion, one or more types can be contained for the purpose of improving seawater corrosion resistance. However, if the addition amount is large, it will cause deterioration of the toughness of the welded part and increase the manufacturing cost. 01 to 0.50%, Mo: 0.01 to 2.00%, and W: 0.01 to 2.00%. More preferably, Cr: 0.02-3.00%, Sb: 0.02-0.30%, Sn: 0.02-0.30%, Mo: 0.02-1.50% and W: It is 0.02 to 1.50%. More preferably, Cr: 0.03-2.00%, Sb: 0.03-0.20%, Sn: 0.03-0.20%, Mo: 0.03-1.00% and W: It is in the range of 0.03 to 1.00%.
Ｃａ：０．０００１～０．０１００％、Ｍｇ：０．０００１～０．０２００％およびＲＥＭ：０．００１～０．２００％のうちから選ばれる１種以上
　Ｃａ、Ｍｇ、ＲＥＭは、溶接部の靱性を確保する目的で、１種以上含有することができる。しかしながら、添加量が多い場合には、溶接部の靱性劣化や製造コストの増加を招くため、Ｃａ含有量は０．０００１％以上０．０１００％以下、Ｍｇ含有量は０．０００１％以上０．０２００％以下、ＲＥＭの含有量は０．００１％以上０．２００％以下である。 One or more types selected from Ca: 0.0001 to 0.0100%, Mg: 0.0001 to 0.0200%, and REM: 0.001 to 0.200% Ca, Mg, and REM are One or more types can be contained for the purpose of ensuring toughness. However, if the amount added is large, it will cause deterioration of the toughness of the welded part and increase manufacturing cost, so the Ca content should be 0.0001% or more and 0.0100% or less, and the Mg content should be 0.0001% or more and 0.01% or less. 0.0200% or less, and the REM content is 0.001% or more and 0.200% or less.
Ｔｉ：０．００５～０．１００％、Ｚｒ：０．００５～０．１００％、Ｎｂ：０．００５～０．１００％およびＶ：０．００５～０．１００％のうちから選ばれる１種以上
　Ｔｉ、Ｚｒ、Ｎｂ、Ｖは、目的とする強度を確保するために、１種以上を含有することができる。しかし、いずれも多く含有すると、靱性と溶接性が劣化することから、０．００５％以上０．１００％以下である。好ましくは０．００５％以上０．０５％以下である。 One type selected from Ti: 0.005 to 0.100%, Zr: 0.005 to 0.100%, Nb: 0.005 to 0.100% and V: 0.005 to 0.100% One or more of Ti, Zr, Nb, and V can be contained in order to ensure the desired strength. However, if too much of either is contained, the toughness and weldability will deteriorate, so the content should be 0.005% or more and 0.100% or less. Preferably it is 0.005% or more and 0.05% or less.
Ｂ：０．０００１～０．０３００％
　Ｂは鋼材の焼入性を向上させる元素である。また、鋼材の強度を確保する目的でＢを含有することができる。しかしながら、過剰に含有する場合、靱性の大幅な劣化を招く。強度の向上効果は、Ｂの含有量が０．０００１％未満では乏しく、靱性の劣化は、０．０３００％を超えた場合に顕著となるため、Ｂの含有量は０．０００１％以上０．０３００％以下である。 B: 0.0001-0.0300%
B is an element that improves the hardenability of steel materials. Moreover, B can be contained for the purpose of ensuring the strength of the steel material. However, if it is contained in excess, the toughness will be significantly deteriorated. The strength improvement effect is poor when the B content is less than 0.0001%, and the deterioration of toughness becomes significant when it exceeds 0.0300%. Therefore, the B content is 0.0001% or more. 0300% or less.
　なお、本発明の効果を損なわない範囲内であれば、上記以外の成分の含有を拒むものではない。 Incidentally, the inclusion of components other than those mentioned above is not prohibited as long as the effects of the present invention are not impaired.
　次に、本実施形態に係る低合金鋼材の製造条件について説明する。
　上記した成分組成を有する溶鋼を、転炉や電気炉等の公知の炉で溶製し、連続鋳造法や造塊法等の公知の方法でスラブやビレット等の鋼素材とする。なお、溶製に際して、真空脱ガス精錬等を実施しても良い。溶鋼の成分調整方法は、公知の鋼製錬方法に従えばよい。 Next, manufacturing conditions for the low alloy steel material according to this embodiment will be explained.
Molten steel having the above-mentioned composition is melted in a known furnace such as a converter or an electric furnace, and is made into a steel material such as a slab or billet by a known method such as a continuous casting method or an ingot forming method. Incidentally, upon melting, vacuum degassing refining or the like may be performed. A known steel smelting method may be used to adjust the composition of the molten steel.
　次に、上記の鋼素材を所望の寸法形状に熱間圧延する際には、１０３０～１３５０℃の温度に加熱する。加熱温度が１０３０℃未満では変形抵抗が大きく、熱間圧延が難しくなるため加熱温度は１０３０℃以上が好ましい。一方、１３５０℃を超える加熱は、表面痕の発生原因となることや、スケールロスや燃料原単位が増加するため１３５０℃以下が好ましい。より好ましくは１０５０～１３００℃である。なお、鋼素材の温度が、もともと１０３０～１３５０℃の範囲の場合には、加熱せずに、そのまま熱間圧延に供してもよい。なお、熱間圧延後、再加熱処理、酸洗、冷間圧延を施し、所定板厚の冷延板としてもよい。 Next, when hot rolling the above steel material into a desired size and shape, it is heated to a temperature of 1030 to 1350°C. If the heating temperature is less than 1030°C, the deformation resistance will be large and hot rolling will be difficult, so the heating temperature is preferably 1030°C or higher. On the other hand, heating above 1350°C causes surface marks and increases scale loss and fuel consumption, so it is preferably 1350°C or lower. More preferably it is 1050 to 1300°C. Note that if the temperature of the steel material is originally in the range of 1030 to 1350° C., it may be directly subjected to hot rolling without being heated. In addition, after hot rolling, reheating treatment, pickling, and cold rolling may be performed to obtain a cold rolled sheet having a predetermined thickness.
　熱間圧延では、仕上圧延終了温度を６００℃以上とすることが好ましい。仕上圧延終了温度が６００℃未満では、変形抵抗の増大により圧延荷重が増加し、圧延の実施が困難となる。熱間圧延における仕上圧延終了後の冷却は、空冷または冷却速度：１５０℃／ｓ以下の加速冷却とすることが好ましいが、後工程において熱処理を施す場合はこの限りではない。 In hot rolling, it is preferable that the finish rolling end temperature be 600°C or higher. If the finish rolling end temperature is less than 600° C., the rolling load increases due to an increase in deformation resistance, making it difficult to carry out rolling. Cooling after finish rolling in hot rolling is preferably performed by air cooling or accelerated cooling at a cooling rate of 150° C./s or less, but this does not apply when heat treatment is performed in a subsequent step.
　その他の製造条件は、低合金鋼材の一般的な製造方法に従えばよい。 Other manufacturing conditions may follow general manufacturing methods for low alloy steel materials.
　次に、本発明の実施例について説明する。なお、本発明は以下の実施例のみに限定されるものではない。
　表１－１～１－３に示す成分組成の溶鋼を、溶製、鋳造してスラブ（鋼素材）とした。ついで、スラブを１２００℃に加熱後、熱間圧延により板厚２０ｍｍの熱延板とした。表１－１～１－３の成分組成の欄で「－」は元素を添加していないことを表す。 Next, examples of the present invention will be described. Note that the present invention is not limited only to the following examples.
Molten steel having the composition shown in Tables 1-1 to 1-3 was melted and cast to form a slab (steel material). Next, the slab was heated to 1200° C. and then hot-rolled into a hot-rolled plate having a thickness of 20 mm. In the component composition columns of Tables 1-1 to 1-3, "-" indicates that no element is added.
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000002
Figure JPOXMLDOC01-appb-T000002
Figure JPOXMLDOC01-appb-T000003
Figure JPOXMLDOC01-appb-T000003
　上記熱延鋼板から試験片を採取し、硫酸塩還元菌培養液（ＳＲＢ培養液）により試験片の腐食疲労試験を行い、微生物応力腐食割れ特性を評価した。評価手順と方法を以下に示す。 A test piece was taken from the above-mentioned hot-rolled steel sheet, and a corrosion fatigue test was performed on the test piece using a sulfate-reducing bacteria culture solution (SRB culture solution) to evaluate microbial stress corrosion cracking characteristics. The evaluation procedure and method are shown below.
　まず、高濃度ＳＲＢ培養液を調整した。微生物株にはＤｅｓｕｌｆｏｖｉｂｒｉｏ　ｖｕｌｇａｒｉｓ　ｓｕｂｓｐ．　ｖｕｌｇａｒｉｓ　ＮＢＲＣ　１０４１２１ （＝ＡＴＣＣ　２９５７９）を用いた。培養用培地にはＡＴＣＣ　Ｍｅｄｉｕｍ　１２４９　Ｍｏｄｉｆｉｅｄ　Ｂａａｒ‘ｓ（ＭＢ）Ｍｅｄｉｕｍ　Ｆｏｒ　Ｓｕｌｆａｔｅ　Ｒｅｄｕｃｅｒｓを用いた。ＭＢ培地において継代培養しているＤ．　ｖｕｌｇａｒｉｓ　ＮＢＲＣ１０４１２１を５ｍＬのＭＢ培地を添加したねじ口試験管に添加し、３７℃で４日間培養した。その後、嫌気グローブボックス内で培養液約２．５ｍＬを新鮮なＭＢ培地（１Ｌ）入りの滅菌遠沈管に添加した。滅菌遠沈管をガスパック１００嫌気システムにセットして、嫌気状態とし、２１℃で３日間培養した。培養後、嫌気下で、１０倍段階希釈液０．１ｍＬを計数用培地に塗布し、３７℃で４日間培養した。培養後、３０～３００個程度の黒色コロニーが認められた希釈段階について計数を行い（ｎ数＝３）、ＳＲＢ濃度が２．４×１０７～２．９×１０７（細菌数／ｍＬ）であることを確認した。 First, a high concentration SRB culture solution was prepared. Microbial strains include Desulfovibrio vulgaris subsp. Vulgaris NBRC 104121 (=ATCC 29579) was used. ATCC Medium 1249 Modified Baar's (MB) Medium For Sulfate Reducers was used as the culture medium. D. subcultured in MB medium. vulgaris NBRC104121 was added to a screw cap test tube containing 5 mL of MB medium, and cultured at 37° C. for 4 days. Approximately 2.5 mL of the culture solution was then added to a sterile centrifuge tube containing fresh MB medium (1 L) in an anaerobic glove box. The sterilized centrifuge tube was set in a Gas Pack 100 anaerobic system to create an anaerobic condition and cultured at 21° C. for 3 days. After culturing, 0.1 mL of the 10-fold serial dilution was applied to the counting medium under anaerobic conditions, and cultured at 37°C for 4 days. After culturing, counts were performed at dilution stages where approximately 30 to 300 black colonies were observed (n number = 3), and the SRB concentration was 2.4 × 10 7 to 2.9 × 10 7 (number of bacteria/mL). It was confirmed that
　次いで、鋼板のＣ方向(幅方向)が試験片の引張方向になるように平行部６ｍｍφ×２５ｍｍの寸法の引張試験片を採取した。ＪＩＳ　Ｚ　２２４１の規定に準拠して、室温で引張試験を行い、後述の腐食疲労試験で付与する応力を算出するために、鋼板の降伏強度（ＹＳ）を求めた。
　また、鋼板を１３０ｍｍ×６．３５ｍｍφの丸棒に加工し、両端にねじ切り加工を施した。この丸棒の中心部から両端に向けて１２．７ｍｍずつを３．８１ｍｍφに加工し、長さ２５．４ｍｍの平行部を設けた腐食疲労試験片を作成した。この平行部は、鋼板のＣ方向(板幅方向)に相当する。
　本腐食疲労試験片を、アセトン中で超音波の脱脂を５分間行い、腐食疲労試験機に取り付けた。この試験片を覆うセル中へ、高濃度ＳＲＢ培養液５質量％と、ＮＳ４水溶液（０．１３１ｇ／Ｌ　ＭｇＳＯ４・７Ｈ２Ｏ，０．４８３ｇ／Ｌ　ＮａＨＣＯ３，０．１２２ｇ／Ｌ　ＫＣｌおよび０．１８１ｇ／Ｌ　ＣａＣｌ２・２Ｈ２Ｏ）９５質量％とを混合した溶液を充填した。嫌気ガス（９５ｖｏｌ．％Ｎ２＋５ｖｏｌ．％ＣＯ２）雰囲気下において、試験前に測定した降伏強度（ＹＳ）をもとに、試験片の引張軸方向に、最大応力を降伏強度×１００％、最小応力を降伏強度×８０％とする変動応力を、１．１×１０－３Ｈｚの周期で最長１６８時間まで加えた。 Next, a tensile test piece with a parallel portion of 6 mmφ x 25 mm was taken so that the C direction (width direction) of the steel plate was the tensile direction of the test piece. A tensile test was conducted at room temperature in accordance with the provisions of JIS Z 2241, and the yield strength (YS) of the steel plate was determined in order to calculate the stress to be applied in the corrosion fatigue test described below.
Further, the steel plate was processed into a round bar of 130 mm x 6.35 mmφ, and both ends were threaded. This round bar was machined to a diameter of 3.81 mm by 12.7 mm from the center to both ends to create a corrosion fatigue test piece with a parallel portion having a length of 25.4 mm. This parallel portion corresponds to the C direction (sheet width direction) of the steel plate.
This corrosion fatigue test piece was subjected to ultrasonic degreasing in acetone for 5 minutes, and then attached to a corrosion fatigue tester. Into the cell covering this test piece, 5% by mass of high-concentration SRB culture solution, NS4 aqueous solution (0.131 g/L MgSO 4 .7H 2 O, 0.483 g/L NaHCO 3 , 0.122 g/L KCl and 0 A solution containing 95% by mass of .181 g/L CaCl 2 .2H 2 O) was filled. Under an anaerobic gas (95 vol.% N 2 + 5 vol. % CO 2 ) atmosphere, based on the yield strength (YS) measured before the test, the maximum stress in the tensile axis direction of the test piece was calculated as yield strength x 100%, A varying stress with a minimum stress equal to 80% of the yield strength was applied at a frequency of 1.1×10 −3 Hz for up to 168 hours.
　まず、試験期間中での試験片の破断の有無を確認した。また、破断しなかった鋼材については、試験後に試験片を取り出し、顕微鏡による５００倍視野での外観観察を実施し、クラックの有無を確認した。クラックが確認された試験片については、断面を観察し、断面における最大クラック長さを測定し、クラック進展距離を算出した。耐微生物応力腐食割れ性は、以下の基準で評価した。クラック長さ３０μm未満については、クラック進展が遅く、腐食破壊が生じるリスクは低いと判断した。クラックなし（◎）、クラック長さ３０μｍ未満（○）を合格とし、クラック長さ３０μｍ以上（△）、破断（×）を不合格とした。
　　　◎　：クラックなし
　　　○　：クラック長さ３０μｍ未満
　　　△　：クラック長さ３０μｍ以上
　　　×　：破断
　得られた結果を表２－１～２－３に併記する。 First, the presence or absence of breakage of the test piece during the test period was confirmed. For steel materials that did not break, test pieces were taken out after the test, and the appearance was observed using a microscope with a 500x field of view to confirm the presence or absence of cracks. For test pieces in which cracks were confirmed, the cross section was observed, the maximum crack length in the cross section was measured, and the crack propagation distance was calculated. Microbial stress corrosion cracking resistance was evaluated based on the following criteria. For cracks with a length of less than 30 μm, crack propagation was slow and the risk of corrosion destruction was determined to be low. No cracks (◎) and a crack length of less than 30 μm (○) were considered acceptable, and cracks with a length of 30 μm or more (△) and breakage (×) were considered a failure.
◎: No crack ○: Crack length less than 30 μm △: Crack length 30 μm or more ×: Fracture The obtained results are also listed in Tables 2-1 to 2-3.
　表２－１～２－３に示したとおり、発明例は全て十分な耐微生物応力腐食割れ性を有している。これに対して、比較例はいずれも耐微生物応力腐食割れ性が不十分であり、耐微生物応力腐食割れ低合金鋼材として不適である。 As shown in Tables 2-1 to 2-3, all of the invention examples have sufficient microbial stress corrosion cracking resistance. On the other hand, all of the comparative examples have insufficient microbial stress corrosion cracking resistance and are unsuitable as low alloy steel materials resistant to microbial stress corrosion cracking.
　本明細書中で容積の単位「Ｌ」は１０－３ｍ３を表す。 The unit of volume "L" herein represents 10 −3 m 3 .
Figure JPOXMLDOC01-appb-T000004
Figure JPOXMLDOC01-appb-T000004
Figure JPOXMLDOC01-appb-T000005
Figure JPOXMLDOC01-appb-T000005
Figure JPOXMLDOC01-appb-T000006
Figure JPOXMLDOC01-appb-T000006

Claims (2)

  1. 質量％で、
    Ｃ：０．３０％以下、
    Ｍｎ：０．１０～３．００％、
    Ｐ：０．０３０％以下、
    Ｎ：０．０１００％以下、
    Ｓｉ：０．０２～１．００％、
    Ｓ：０．０００２～０．０１００％、
    Ａｌ：０．００３～０．５００％、
    Ｏ：０．０００５～０．００５０％
    を含有すると共に、
    Ｃｕ：０．０２～３．００％、
    Ａｇ：０．０１～０．５０％
    のうちから選ばれる１種又は２種を含有し、残部はＦｅおよび不可避的不純物からなる成分組成を有し、ＳｉとＳとの含有量の比Ｓｉ（％）／Ｓ（％）が８以上１２００以下、ＡｌとＯとの含有量の比Ａｌ（％）／Ｏ（％）が３以上である耐微生物応力腐食割れ低合金鋼材。     In mass%,
    C: 0.30% or less,
    Mn: 0.10-3.00%,
    P: 0.030% or less,
    N: 0.0100% or less,
    Si: 0.02-1.00%,
    S: 0.0002-0.0100%,
    Al: 0.003-0.500%,
    O: 0.0005-0.0050%
    In addition to containing
    Cu: 0.02-3.00%,
    Ag: 0.01~0.50%
    Contains one or two selected from the following, with the remainder consisting of Fe and unavoidable impurities, and the ratio of Si (%) / S (%) of Si and S content is 8 or more A microbial stress corrosion cracking resistant low alloy steel material having a content ratio of Al (%)/O (%) of Al and O (%) of 3 or more.
  2. 前記成分組成は、さらに、質量％で、下記のＡ群～Ｅ群のうちの１群以上を含有する請求項１に記載の耐微生物応力腐食割れ低合金鋼材。
    Ａ群；
    Ｎｉ：０．０１～４．００％、
    Ｂ群；
    Ｃｒ：０．０１～４．００％、
    Ｓｂ：０．０１～０．５０％、
    Ｓｎ：０．０１～０．５０％、
    Ｍｏ：０．０１～２．００％および
    Ｗ：０．０１～２．００％
    のうちから選ばれる１種以上、
    Ｃ群；
    Ｃａ：０．０００１～０．０１００％、
    Ｍｇ：０．０００１～０．０２００％および
    ＲＥＭ：０．００１～０．２００％
    のうちから選ばれる１種以上、
    Ｄ群；
    Ｔｉ：０．００５～０．１００％、
    Ｚｒ：０．００５～０．１００％、
    Ｎｂ：０．００５～０．１００％および
    Ｖ：０．００５～０．１００％
    のうちから選ばれる１種以上、
    Ｅ群；
    Ｂ：０．０００１～０．０３００％。

          The microbial stress corrosion cracking resistant low alloy steel material according to claim 1, wherein the component composition further contains, in mass %, one or more of the following groups A to E.
    Group A;
    Ni: 0.01 to 4.00%,
    Group B;
    Cr: 0.01-4.00%,
    Sb: 0.01 to 0.50%,
    Sn: 0.01-0.50%,
    Mo: 0.01-2.00% and W: 0.01-2.00%
    One or more types selected from
    Group C;
    Ca: 0.0001-0.0100%,
    Mg: 0.0001-0.0200% and REM: 0.001-0.200%
    One or more types selected from
    Group D;
    Ti: 0.005-0.100%,
    Zr: 0.005-0.100%,
    Nb: 0.005-0.100% and V: 0.005-0.100%
    One or more types selected from
    Group E;
    B: 0.0001 to 0.0300%.
PCT/JP2023/019060 2022-07-29 2023-05-23 Microbiologically assisted cracking-resistant low-alloy steel WO2024024236A1 (en)

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CN103741056A (en) * 2014-01-26 2014-04-23 北京科技大学 Corrosion resistant steel plate for resisting marine environment of South China Sea and production process of corrosion resistant steel plate
JP2017190522A (en) * 2016-04-11 2017-10-19 Ｊｆｅスチール株式会社 Steel material
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WO2019131035A1 (en) * 2017-12-26 2019-07-04 Ｊｆｅスチール株式会社 Low alloy high strength seamless steel pipe for oil wells
WO2020203931A1 (en) * 2019-03-29 2020-10-08 日鉄ステンレス株式会社 Duplex stainless steel welded joint and method for manufacturing same
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* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2005179699A (en) * 2003-12-16 2005-07-07 Nippon Steel Corp Method for inhibiting potential of stainless steel from becoming noble
CN103741056A (en) * 2014-01-26 2014-04-23 北京科技大学 Corrosion resistant steel plate for resisting marine environment of South China Sea and production process of corrosion resistant steel plate
JP2017190522A (en) * 2016-04-11 2017-10-19 Ｊｆｅスチール株式会社 Steel material
WO2019035329A1 (en) * 2017-08-15 2019-02-21 Ｊｆｅスチール株式会社 High strength stainless seamless steel pipe for oil wells, and method for producing same
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