Microbial stress corrosion cracking resistant low alloy steel

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

[Technical field]

[0001] The present invention relates to a microbial stress corrosion cracking resistant steel material suitable for structural members such as automobile members, power generation facility members, chemical plant members, building members, machine members, ship members, and oil field auxiliary facility members. [Background technology]

[0002] In the past, there were known bacteria whose proliferation should be avoided from the hygienic and industrial perspectives in our social life, such as pathogenic bacteria such as Escherichia coli and Salmonella, and corrosive bacteria such as sulfate-reducing bacteria and sulfur-oxidizing bacteria. In particular, in recent years, with the remarkable progress in the fields of analysis and biology, the negative impact of bacteria on social life has been widely recognized. One of these is the phenomenon of microbial corrosion. Microbiologically assisted corrosion is a corrosion phenomenon with a very high localized corrosiveness, and there is a risk that the microbially assisted corrosion that occurs in structures can become the starting point of penetration holes and stress corrosion cracking, leading to serious accidents. In particular, the latter, known as MAC (Microbiologically Assisted Cracking), has been widely recognized as one of the stress corrosion cracking phenomena of concern in real environments where microorganisms exist. Countermeasures against microbially assisted corrosion are generally to reduce the number of microorganisms in the environment using germicides and to physically remove microorganisms attached to materials (cleaning with a brush, etc.). However, there are limitations to the reduction of microbially assisted corrosion by these means. For example, physical cleaning of the outer surface of a pipeline buried in soil is difficult, and the active use of fungicides is also difficult due to the impact on the soil environment. Therefore, there is growing interest in approaches to increase the resistance of materials to microbial corrosion. In light of this trend, attempts are being made to impart resistance to microbial corrosion in the steel industry as well.

[0003] For example, Patent Document 1 reports a method for preventing corrosion of steel materials and peeling of a coating film, in which a zinc-containing layer is provided between a steel material and an epoxy resin coating film, and the zinc-containing layer is a layer consisting of zinc alone, or a layer consisting of an alloy containing 85 mass% or more of zinc and any one of nickel, aluminum, magnesium, and iron as another constituent element.

[0004] Furthermore, Patent Document 2 reports a stainless steel with excellent resistance to microbial corrosion, characterized by having a two-layer coating on its surface: an outer layer made of Cr(III)-Fe(III) hydroxide and an inner layer made mainly of Cr(III) oxides and / or hydroxides.

[0005] Furthermore, Patent Document 3 proposes a steel material with enhanced antibacterial and microbial corrosion resistance properties by containing, by mass%, one or more elements selected from 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, W: 0.010% or more and 1.000% or less, and Sn: 0.010% or more and 0.500% or less.

[0006] Meanwhile, regarding sulfate-reducing bacteria (SRB), which are known to be the main cause of microbial corrosion, new findings have been obtained in recent years regarding their mechanism of promoting corrosion of steel materials.

[0007] For example, Non-Patent Document 1 reports that the promotion of corrosion by SRB is based on the direct extraction of electrons from steel by SRB. In other words, in the metabolic activity of SRB, the SRB present on the steel surface directly oxidizes Fe, promoting the dissolution of the steel. S, which is generated as a result of this metabolic activity, 2- The ions are the Fe 2+ When it bonds with ions, a sparingly soluble FeS film is formed on the surface of the steel. This FeS film acts as a corrosion-resistant film and is generally considered to contribute to reducing steel corrosion. However, in the microbial corrosion phenomenon caused by SRB, since FeS has a relatively high electrical conductivity, sulfate-reducing bacteria attached to the surface of FeS can extract electrons (direct oxidation) from the base material (steel) through FeS. Therefore, in areas where the formation of the FeS film is insufficient, local corrosion is promoted. That is, for example, in areas where a new surface is physically created on the steel surface due to plastic stress, not only does the surface protection by FeS not work, but pitting corrosion occurs due to the galvanic effect with the areas where FeS was present, and stress corrosion cracking becomes apparent. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] JP 2010-222606 A [Patent Document 2] Japanese Patent Application Publication No. 7-26395 [Patent Document 3] JP 2017-190522 A [Non-patent literature]

[0009] [Non-Patent Document 1] X. Deng et al, Angew. Chem. 132, (2020), p6051 Summary of the Invention [Problem to be solved by the invention]

[0010] The above-mentioned conventional techniques have the following problems.

[0011] The corrosion prevention method for steel materials described in Patent Document 1 is considered to have sufficient antibacterial properties, but it requires painting and alloy coating formation, which is very expensive, and therefore has excessive performance except for severe applications where particularly high corrosion resistance is required. In other words, it is not realistic to use it for components that are originally made of low-alloy steel materials due to cost. In addition, if the surface is scratched by impact or cut, the scratched part cannot be expected to have resistance, and it is difficult to obtain a long-term effect.

[0012] 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 difficult to apply it to members for which inexpensive low-alloy steel materials are expected to be used due to cost reasons. Also, as in Patent Document 1, resistance to microbial corrosion cannot be guaranteed in areas where the surface is scratched and the coating structure effective in resisting microbial corrosion has been lost.

[0013] Furthermore, the steel material described in Patent Document 3 is evaluated for its microbial corrosion resistance based on localized corrosion in an actual seawater environment that contains sulfate-reducing bacteria, which are corrosive bacteria. Microbially influenced corrosion in an actual environment becomes evident in places where corrosive bacteria are locally activated and the bacterial concentration is high, but the steel material disclosed in Patent Document 3 does not assume an environment that reflects the actual state of this microbial corrosion. In other words, the technology described in Patent Document 3 is merely the result of evaluating localized corrosiveness in a seawater corrosive environment, and is uncertain as a technology for reducing stress corrosion cracking induced by microbial corrosion, in particular.

[0014] Thus, in the field of low-alloy steel, no suitable technology has been established from the viewpoint of ensuring low-cost and long-term microbial corrosion resistance, and although a technology for improving the microbial corrosion resistance of the material itself is desired, no such technology has been established. In particular, no technology has been considered for reducing the phenomenon of stress corrosion cracking, which occurs when microbial corrosion progresses at a location where stress is applied.

[0015] The present invention aims to solve the problems of the conventional techniques and provide a low-alloy steel material resistant to microbial stress corrosion cracking that is practical for manufacturing. Note that the stress corrosion cracking in the present invention refers to the MAC phenomenon in a broad sense, which includes not only static stress environments but also dynamically fluctuating stress environments, i.e., stress environments equivalent to low-cycle corrosion fatigue. [Means for solving the problem]

[0016] The inventors have conducted extensive research to solve the above problems. First, the inventors have conducted a detailed study of the stress corrosion cracking phenomenon induced by sulfate-reducing bacteria (SRB), a typical causative bacterium of microbial corrosion, with reference to Non-Patent Document 1, and have obtained the following findings. That is, in a microbial corrosion environment where SRB exists, a film of FeS, a corrosion product, is formed on the surface of the steel as a result of the metabolic reaction of SRB and the corrosion reaction of the steel. This FeS film has the property of promoting the cathodic reaction because it is conductive, while it acts as a physical protective film and 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 localized corrosiveness. Furthermore, if this FeS film is mechanically destroyed by the presence of external stress, galvanic corrosion occurs with the destroyed part of the FeS film as the anode site and the remaining part of the FeS film as the cathode site, resulting in a significant localized corrosion phenomenon. This localized corrosion area becomes an oxygen-deficient environment, which increases the activity of SRB, an anaerobic bacterium, and the direct oxidation (iron dissolution) reaction due to the metabolism of SRB also progresses at an accelerated rate. As a result of this galvanic coupling and the progression of selective localized corrosion driven by the metabolic activation of SRB, stress corrosion cracking occurs. Thus, in order to reduce stress corrosion cracking in a microbial corrosion environment containing SRB, it is necessary to take measures against both galvanic corrosion due to the presence of the conductive FeS coating and the direct oxidation (dissolution) of iron due to the metabolism of SRB. Based on the above findings, the inventors have therefore conducted extensive research to develop a steel material that exhibits resistance to microbial stress corrosion cracking. As a result, it was found that controlling the content and ratio of Si, S, Al and O to appropriate amounts is effective in suppressing galvanic corrosion caused by the presence of a conductive FeS coating. It was also found that the addition of Cu or Ag was effective in suppressing the direct oxidation (dissolution) of iron by SRB metabolism. The present invention was completed based on the above-mentioned novel findings and as a result of further investigation, and the gist of the present invention is as follows.

[0017] [1] A low-alloy steel material resistant to microbial stress corrosion cracking, comprising, by 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%, one or two selected from Cu: 0.02 to 3.00% and Ag: 0.01 to 0.50%, with the balance being Fe and unavoidable impurities, in which the ratio of Si to S content Si(%) / S(%) is 8 to 1,200 and the ratio of Al to O content Al(%) / O(%) is 3 or more.

[0018] [2] In the above [1], the composition further contains, by mass%, one or more of the following groups A to E. Group A: Ni: 0.01-4.00%, Group B: one or more selected from 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%; 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: one or more selected from Ti: 0.005-0.100%, Zr: 0.005-0.100%, Nb: 0.005-0.100%, and V: 0.005-0.100%, Group E; B: 0.0001~0.0300%. Effect of the Invention

[0019] According to the present invention, it is possible to obtain a low-alloy steel material resistant to microbial stress corrosion cracking that is less expensive than conventional steels when used for structural members such as automobile members, power generation facility members, chemical plant members, building members, machine members, ship members, and oil field auxiliary facility members. The present invention is also extremely useful industrially. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] Hereinafter, a low alloy steel according to an embodiment of the present invention will be described. First, the reasons for limiting the composition of the low alloy steel will be described. In this specification, "%" representing the content of each component element means "mass%" unless otherwise specified.

[0021] C: 0.30% or less C is an element necessary for ensuring the strength of steel, and since a content exceeding 0.30% significantly deteriorates workability and weldability, it is limited to 0.30% or less. It is preferably 0.25% or less, and more preferably 0.20% or less. There is no particular lower limit, but it is preferably 0.02% or more.

[0022] Mn: 0.10-3.00% Mn is added to improve strength and toughness, but if it is less than 0.10%, the effect is insufficient, and if it exceeds 3.00%, weldability deteriorates, so the Mn content is 0.10% to 3.00%, preferably 0.20% to 2.50%, and more preferably 0.30% to 2.00%.

[0023] P:0.030% or less Since a high P content deteriorates toughness and weldability, the P content is restricted to 0.030% or less, and preferably 0.025% or less. Since it is difficult to reduce the P content to less than 0.002% in industrial-scale production, a P content of 0.002% or more is permitted.

[0024] 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, if the N content exceeds 0.0100%, the toughness is significantly reduced. Therefore, the N content is set to 0.0100% or less. It is preferably set to 0.0080% or less. More preferably, it is set to 0.0070% or less. There is no particular lower limit, but since excessive reduction of the N content leads to an increase in costs, it is preferable to set it to 0.0005% or more.

[0025] Silicon: 0.02 to 1.00% Silicon is a very important element from the viewpoint of improving resistance to microbial stress corrosion cracking. In other words, silicon is a metal that forms on the steel surface as the steel corrodes in an environment where SRBs are present. 2+ The SRB is dissolved as an ion. 2- Ions and Fe 2+ In a corrosive environment, the reaction with the free Si ions results in the formation of FeS. 2+ The ions are incorporated into the crystal structure of FeS, forming local SiS structures within the macroscopic FeS structure. This Si-substituted FeS has a significantly reduced electrical conductivity due to its inhomogeneous crystal structure. Even if the FeS coating with reduced electrical conductivity is destroyed by external stress, it does not provide strong coupling between the destroyed part and the healthy part of the coating, and the progress of galvanic corrosion stagnates. 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 is improved. In order to achieve such effects, the Si content is 0.02% or more, preferably 0.03% or more, and more preferably 0.05% or more. However, the reaction of forming this Si-substituted FeS is strongly influenced by the amounts of S, O, and Al in the steel. In order to stably obtain the effect of inhibiting galvanic corrosion by Si, it is necessary to properly control the ratio of the content of Si to the amount of S in the steel, which will be described later, as well as the ratio of the amount of Al to the amount of O. Incidentally, since excessive addition of Si leads to deterioration of toughness and weldability, the content is set to 1.00% or less, preferably 0.80% or less, and more preferably 0.70% or less.

[0026] S: 0.0002~0.0100% 8≦Si(%) / S(%)≦1200 S affects the incorporation behavior of Si into the FeS film, and is an important element for obtaining the effect of inhibiting galvanic corrosion by Si. That is, S in steel, like Si, dissolves onto the steel surface as corrosion occurs. Dissolved Si 2+ In the ionized state, Si does not have sufficient affinity with FeS, and therefore the incorporation of Si into the FeS structure does not proceed satisfactorily. Here, the S dissolved from the steel material is quickly converted to Si. 2+ It reacts with 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 inhibition effect of the formation of Si-substituted FeS becomes apparent. The S content required to obtain this effect is 0.0002% or more, and preferably 0.0003% or more. On the other hand, S is a harmful element that deteriorates the toughness and weldability of steel, and when the S content exceeds 0.0100%, the deterioration of the base metal toughness and the weld toughness becomes particularly large. Therefore, the S content is 0.0100% or less, preferably 0.008% or less, and more preferably 0.007% or less.

[0027] Additionally, in order to obtain the effect of S on the stress corrosion cracking resistance, 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 material continues, and SiS precipitates as a single compound without being incorporated into the FeS structure, resulting in insufficient formation of Si-substituted FeS. This phenomenon becomes evident when Si(%) / S(%) is less than 8, so Si(%) / S(%) is 8 or more, preferably 10 or more, and more preferably 12 or more. On the other hand, when the ratio of S(%) to Si(%) is small, the presence of excess Si makes the formation of elemental Si thermodynamically stable, and SiS nuclei are not sufficiently formed, so that the galvanic corrosion inhibition effect of Si-substituted FeS is not expressed. This phenomenon becomes evident when Si(%) / S(%) exceeds 1200, so Si(%) / S(%) is 1200 or less, preferably 1100 or less, and more preferably 1000 or less.

[0028] Al: 0.003 to 0.500% Al is an element added as a deoxidizer, and the Al content is set to 0.003% or more. However, if the Al content exceeds 0.500%, the toughness of the steel decreases. Therefore, the Al content is set to 0.003 to 0.500%, and preferably 0.003 to 0.300%. In addition, Al is an element that affects the expression of the effect of improving microbial stress corrosion cracking resistance due to Si, and therefore, it is necessary to appropriately control the ratio of Al content to O content, as described later.

[0029] O: 0.0005 to 0.0050% 3≦Al(%) / O(%) O is an important element that must be controlled in order to obtain the effect of inhibiting galvanic corrosion by Si. That is, O in steel materials dissolves onto the steel surface as corrosion occurs, just like Si. When there is an excess of this dissolved O, it bonds 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 2Since the formation of Cr becomes evident when the O content exceeds 0.0050%, the O content is set to 0.0050% or less. It is preferably 0.0045% or less, and more preferably 0.0040% or less. On the other hand, the lower limit is not particularly limited in terms of the properties of the steel material, but since excessive reduction in the O content leads to an increase in manufacturing costs in the steelmaking process, it is set to 0.0005% or more. It is preferably 0.0006% or more, and more preferably 0.0008% or more.

[0030] On the other hand, SiO 2 In order to suppress the formation of Si-substituted FeS and stably obtain the galvanic corrosion inhibition effect due to the formation of Si-substituted FeS, in addition to the above, it is necessary to properly control the content ratio of O (%) to Al (%). 2 O 3 The O released during the dissolution of the base material is quickly consumed by the released Al, forming SiO 2 The reaction of forming Si-substituted FeS is suppressed, and the reaction of forming Si-substituted FeS proceeds. 2 O 3 Since the formation of Si-substituted FeS due to the formation of O becomes evident when the Al(%) / O(%) is 3 or more, the Al(%) / O(%) is set to 3 or more. Although there is no particular upper limit, as described above, increasing the Al(%) / O(%) will result in an excessive reduction in the O content, which will lead to an increase in the manufacturing costs in the steelmaking process, so it is preferable to set the Al(%) / O(%) to 150 or less.

[0031] One or two selected from Cu: 0.02-3.00%, Ag: 0.01-0.50% At least one of Cu and Ag is an essential element for obtaining microbial stress corrosion cracking resistance in the low alloy steel material of this embodiment. Cu and Ag are dissolved from the steel material and are dissolved in the steel material. 2+ Ion, Ag +These free ions are taken up by microorganisms on the steel surface and bind strongly to amino acids and proteins with -SH groups present in the enzyme systems of the microorganisms, inhibiting their metabolic activity. When there is a lot of oxygen in the environment, Cu 2+ Ions and Ag + The ions are oxides (CuO, Ag 2 O), making it less likely that the effect of inhibiting the metabolic activity of microorganisms will be realized. On the other hand, in the localized corroded area during the stress corrosion cracking process, an oxygen-deficient environment is created, so the effect of Cu and Ag inhibiting the metabolic activity of microorganisms is fully realized. As a result, the progression of localized corrosion based on the metabolism of corroding microorganisms, such as SRB, is suppressed in the localized corroded area of ​​steel.

[0032] The effect of suppressing localized corrosion as microbial stress corrosion cracking based on the metabolic inhibition of this microorganism can be obtained by containing 0.02% or more of Cu, or by containing 0.01% or more of Ag, or by containing 0.02% or more of Cu and 0.01% or more of Ag. However, excessive Cu and Ag content deteriorates weldability and steel sheet manufacturability, so the Cu content is 0.02 to 3.00%, preferably 0.05 to 2.00%, and more preferably 0.10 to 1.50%. The Ag content is 0.01 to 0.50%, preferably 0.01 to 0.30%, and more preferably 0.02 to 0.20%. 。

[0033] The basic components of this embodiment have been described above. The remainder other than the above components is Fe and unavoidable impurities. In addition, the following ranges are treated as unavoidable impurities: Ni content less than 0.01%, Cr content less than 0.01%, Sb content less than 0.01%, Sn content less than 0.01%, Mo content less than 0.01%, W content less than 0.01%, Ca content less than 0.0001%, Mg content less than 0.0001%, REM content less than 0.001%, Ti content less than 0.005%, Zr content less than 0.005%, Nb content less than 0.005%, V content less than 0.0001% and B content less than 0.0001%. In addition, the following elements may be appropriately contained as necessary.

[0034] Ni: 0.01 to 4.00% Ni can be added to improve the manufacturability of steel sheets. To obtain this effect, the Ni content is 0.01% or more. On the other hand, excessive Ni content leads to deterioration of weldability and an increase in manufacturing costs. For this reason, the Ni content is 4.00% or less. It is preferably 3.00% or less, more preferably 2.00% or less, and even more preferably 1.5% or less.

[0035] One or more selected from 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% Cr, Sb, Sn, Mo, and W are elements that enhance corrosion resistance, particularly in seawater environments, among the wet environments in which microbial corrosion occurs. Therefore, one or more of them can be contained for the purpose of improving seawater corrosion resistance, apart from microbial corrosion. However, if the amount of addition is large, it leads to deterioration of the toughness of the weld and an increase in manufacturing costs, so 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%. More preferably, Cr: 0.02-3.00%, Sb: 0.02-0.30%, Sn: 0.02-0.30%, Mo: 0.02-1.50%, and W: 0.02-1.50%. More preferably, the contents are in the ranges of Cr: 0.03 to 2.00%, Sb: 0.03 to 0.20%, Sn: 0.03 to 0.20%, Mo: 0.03 to 1.00%, and W: 0.03 to 1.00%.

[0036] One or more selected from Ca: 0.0001-0.0100%, Mg: 0.0001-0.0200%, and REM: 0.001-0.200% Ca, Mg, and REM may be contained in one or more types in order to ensure the toughness of the welded part. However, if the amount added is large, it will cause a deterioration in the toughness of the welded part and an increase in manufacturing costs, so the Ca content is 0.0001% to 0.0100%, the Mg content is 0.0001% to 0.0200%, and the REM content is 0.001% to 0.200%.

[0037] One or more selected from Ti: 0.005-0.100%, Zr: 0.005-0.100%, Nb: 0.005-0.100%, and V: 0.005-0.100% In order to ensure the desired strength, one or more of Ti, Zr, Nb, and V may be contained. However, if any of them is contained in a large amount, toughness and weldability are deteriorated, so the content is 0.005% or more and 0.100% or less. Preferably, the content is 0.005% or more and 0.05% or less.

[0038] B: 0.0001 to 0.0300% B is an element that improves the hardenability of steel. In addition, B can be contained in order to ensure the strength of the steel. However, excessive B content leads to a significant deterioration in toughness. The effect of improving strength is poor when the B content is less than 0.0001%, and the deterioration of toughness becomes significant when the B content exceeds 0.0300%, so the B content is 0.0001% or more and 0.0300% or less.

[0039] Incidentally, the inclusion of ingredients other than those mentioned above is not prohibited so long as it does not impair the effects of the present invention.

[0040] Next, the manufacturing conditions of the low alloy steel material according to this embodiment will be described. 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 a billet by a known method such as a continuous casting method or an ingot casting method. In addition, vacuum degassing refining or the like may be performed during the melting. The composition of the molten steel may be adjusted according to a known steel smelting method.

[0041] Next, when the above steel material is hot-rolled to 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 is large and hot rolling becomes difficult, so the heating temperature is preferably 1030°C or higher. On the other hand, heating to a temperature higher than 1350°C may cause surface marks and increase scale loss and fuel consumption, so the heating temperature is preferably 1350°C or lower. More preferably, it is 1050 to 1300°C. In addition, when the temperature of the steel material is originally in the range of 1030 to 1350°C, it may be subjected to hot rolling as it is without heating. In addition, after hot rolling, it may be subjected to reheating, pickling, and cold rolling to form a cold-rolled sheet of a predetermined thickness.

[0042] In hot rolling, the finish rolling temperature is preferably 600° C. or higher. If the finish rolling 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 the finish rolling in hot rolling is preferably air-cooled 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 process.

[0043] Other manufacturing conditions may be the same as those for a general manufacturing method of low alloy steel materials. EXAMPLES

[0044] Next, examples of the present invention will be described, but the present invention is not limited 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). The slab was then heated to 1200°C and hot-rolled to form a hot-rolled sheet having a thickness of 20 mm. In the composition columns of Tables 1-1 to 1-3, "-" indicates that no element was added.

[0045] [Table 1-1]

[0046] [Table 1-2]

[0047] [Table 1-3]

[0048] Test pieces were taken from the above hot-rolled steel sheets, and corrosion fatigue tests were performed on the test pieces using sulfate-reducing bacteria culture medium (SRB culture medium) to evaluate the microbial stress corrosion cracking properties. The evaluation procedures and methods are as follows.

[0049] First, a high-concentration SRB culture solution was prepared. The microbial strain used was Desulfovibrio vulgaris subsp. vulgaris NBRC 104121 (=ATCC 29579). The culture medium used was ATCC Medium 1249 Modified Baar's (MB) Medium For Sulfate Reducers. D. vulgaris NBRC104121 subcultured in MB medium was added to a screw-top test tube containing 5 mL of MB medium and cultured at 37°C for 4 days. Then, in an anaerobic glove box, about 2.5 mL of the culture solution was added to a sterile centrifuge tube containing fresh MB medium (1 L). The sterile centrifuge tube was set in a Gas Pack 100 anaerobic system to create an anaerobic state, and cultured at 21°C for 3 days. After culture, 0.1 mL of the 10-fold serial dilution solution was applied to the counting medium under an anaerobic condition, and cultured at 37°C for 4 days. After incubation, the dilution steps in which 30 to 300 black colonies were observed were counted (n=3), and the SRB concentration was 2.4 × 10 7 ~2.9×10 7 (bacterial count / mL).

[0050] Next, tensile test pieces with dimensions of 6 mmφ x 25 mm were taken so that the C direction (width direction) of the steel plate was the tensile direction of the test piece. In accordance with the provisions of JIS Z 2241, a tensile test was performed at room temperature, and the yield strength (YS) of the steel plate was obtained in order to calculate the stress to be applied in the corrosion fatigue test described later. The steel plate was then machined into a 130mm x 6.35mmφ round bar, with both ends threaded. The bar was machined from the center to both ends by 12.7mm each to 3.81mmφ, creating a corrosion fatigue test specimen with a parallel section 25.4mm long. This parallel section corresponds to the C direction (width direction) of the steel plate. The corrosion fatigue test specimen was ultrasonically degreased in acetone for 5 minutes and then attached to the corrosion fatigue tester. A cell covering the specimen was filled with 5% by mass of high-concentration SRB culture solution and an aqueous solution of NS4 (0.131 g / L MgSO 4 7H 2 O, 0.483g / L NaHCO 3 , 0.122g / L KCl and 0.181g / L CaCl2 2H 2 The tank was filled with a solution of anaerobic gas (95 vol.% N 2 +5% vol. CO 2 ) atmosphere, based on the yield strength (YS) measured before the test, a fluctuating stress of 1.1 x 10 was applied in the tensile axial direction of the test piece, with the maximum stress being the yield strength x 100% and the minimum stress being the yield strength x 80%. -3 The pulse was applied at a frequency of 100 Hz for up to 168 hours.

[0051] First, the presence or absence of fracture of the test specimens during the test period was confirmed. For steel materials that did not fracture, the test specimens were removed after the test and the appearance was observed under a microscope at a magnification of 500 times to confirm the presence or absence of cracks. For test specimens 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. The resistance to microbial stress corrosion cracking was evaluated according to the following criteria. For crack lengths of less than 30 μm, it was determined that the crack propagation was slow and the risk of corrosion destruction was low. No cracks (◎) and crack lengths of less than 30 μm (○) were considered to be pass, while crack lengths of 30 μm or more (△) and fracture (×) were considered to be fail. ◎: No cracks ○: Crack length less than 30 μm △: Crack length 30μm or more ×: Break The results are shown in Tables 2-1 to 2-3.

[0052] As shown in Tables 2-1 to 2-3, all of the inventive examples have sufficient microbial stress corrosion cracking resistance. In contrast, 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.

[0053] In this specification, the unit of volume "L" is 10 -3 m 3 Represents.

[0054] [Table 2-1]

[0055]

Table 2-2

[0056]

Table 2-3

Claims

1. In mass percent, 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 to 0.0050%, and Ag: 0.05-0.50% Contains Optionally, Cu: 0.02-3.00% Contains The balance of the composition is Fe and unavoidable impurities, and the ratio of the Si content to the S content, Si(%) / S(%), is 8 or more and 1,200 or less, and the ratio of the Al content to the O content, Al(%) / O(%), is 3 or more.

2. The microbial stress corrosion cracking resistant low alloy steel material according to claim 1, wherein the 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 to 2.00% and W: 0.01~2.00% One or more selected from Group C; Ca: 0.0001-0.0100%, Mg: 0.0001 to 0.0200% and REM: 0.001~0.200% One or more selected from Group D; Ti: 0.005-0.100%, Zr: 0.005-0.100%, Nb: 0.005 to 0.100% and V:0.005~0.100% One or more selected from Group E; B: 0.0001 to 0.0300%.

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