Microbial corrosion-resistant steel sheet and method for manufacturing the same

Abstract

To provide an inexpensive, microbially corrosion-resistant steel sheet that has long-term resistance to microbial corrosion against Shewanella chilikensis and is suitable for use in transport pipelines. [Solution] The composition of the steel sheet is such that, in mass%, it contains C: 0.01% to 0.30%, Si: 0.01% to 1.00%, Mn: 0.10% to 2.50%, P: 0.030% or less, S: 0.0100% or less, N: 0.0100% or less, Al: 0.010% to 0.300%, Cu: 0.01% to 0.15%, and Cr: 0.05% to 1.50%, with the remainder being Fe and unavoidable impurities, the content of solid-solution Cr is 0.03% or more in mass%, and the mass ratio of the content of solid-solution Cr to the content of Cr in the composition is 0.30 or more.

Description

Technical Field

[0001] The present invention relates to a steel sheet resistant to microbiological corrosion and a method for producing the same, which are suitable for use in a transportation pipeline.

Background Art

[0002] Microbially influenced corrosion refers to a phenomenon in which the corrosion of a metallic material is promoted by the vital activities of microorganisms. Bacteria that induce microbially influenced corrosion include anaerobic sulfate-reducing bacteria, aerobic sulfur-oxidizing bacteria, and oxidizing bacteria. These bacteria inhabit every place in nature, and when the environment is suitable, they multiply and produce metabolites that are corrosive to metallic materials. One of the characteristics of microbially influenced corrosion is that it induces local corrosion at the site of colonies where microorganisms adhering and multiplying on the surface of a metallic material gather. In particular, in a steel pipeline used in a transportation pipeline, when pitting corrosion due to microbially influenced corrosion progresses to form a through-hole, there is a risk of causing a serious accident and significant economic loss due to fluid leakage.

[0003] As a countermeasure against microbially influenced corrosion in a transportation pipeline, it is effective to prevent the adhesion and multiplication of microorganisms that cause corrosion. As countermeasures that act directly on microorganisms, there are methods such as physically removing microorganisms adhering to the surface of a transportation pipeline using a brush or the like, and methods of killing microorganisms using a bactericide. However, these countermeasures require continuous costs, and there are concerns about the impact on the ecosystem due to the use of bactericides. Therefore, attempts have been made to enhance the microbiological corrosion resistance of steel materials as an alternative to these countermeasures.

[0004] For example, Patent Document 1 discloses a technique for providing a zinc-containing layer between a steel material and an epoxy resin coating to prevent microbial corrosion by anaerobic bacteria, specifically iron-corrosive methane-producing bacteria and sulfate-reducing bacteria. Patent Document 2 also discloses a technique for providing a two-layer coating on the surface of stainless steel, consisting of an outer layer made of Cr(III)-Fe(III) hydroxide and an inner layer mainly composed of Cr(III) oxides and / or hydroxides, to prevent microbial corrosion by microorganisms contained in natural seawater. These techniques enhance microbial corrosion resistance by providing a coating on the surface of the steel material.

[0005] On the other hand, technologies to improve the microbial corrosion resistance of steel materials themselves have also been reported. For example, Patent Document 3 discloses a technology to release aluminum ions from the surface of steel materials by adding a predetermined amount of Al to the steel material in order to prevent microbial corrosion caused by seawater containing sulfate-reducing bacteria, thereby reducing the enzymatic activity of microorganisms. Cu, Ni, Mo, W, and Sn, which release ions that inhibit microbial metabolism, are also added to this steel material at the same time. In addition, Patent Document 4 discloses a technology to release silver ions from the surface of low-alloy steel materials that inhibit microbial metabolism, in order to prevent microbial corrosion caused by sulfate-reducing bacteria. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2010-222606 [Patent Document 2] Japanese Patent Application Publication No. 7-26395 [Patent Document 3] Japanese Patent Publication No. 2017-190522 [Patent Document 4] Japanese Patent Publication No. 2023-140295 [Non-patent literature]

[0007] [Non-Patent Document 1] SJ Salgar-Chaparro et al., “Corrosion of carbon steel by Shewanella chilikensis DC57 under thiosulfate and nitrate reducing conditions”, Frontiers in Bioengineering and Biotechnology, 10 March 2022, Vol. 10, Article 825776. [Overview of the project] [Problems that the invention aims to solve]

[0008] The steel materials disclosed in Patent Documents 1 and 2 both achieve extremely high microbial corrosion resistance by applying a coating to their surface. However, these steel materials are very expensive due to the cost of surface treatment. Furthermore, they are over-engineered except for harsh applications that require exceptionally high corrosion resistance. Therefore, applying the steel materials disclosed in Patent Documents 1 and 2 to components used in applications where low-alloy steel materials are normally used is not cost-effective. In addition, once the surface of the steel material is damaged by impact, cuts, etc., it is impossible to maintain microbial corrosion resistance in the damaged area. For this reason, it is difficult to obtain long-term effects through surface coating alone.

[0009] In the steel materials disclosed in Patent Documents 3 and 4, the action of metal ions leaching from the surface of the steel material into the microbial habitat reduces the enzyme activity of sulfate-reducing bacteria or inhibits their metabolism. However, the types of enzymes that microorganisms use in metabolic reactions and the effect of the presence of specific metal ions on microbial metabolism vary depending on the type of microorganism. Therefore, it is unclear whether the techniques disclosed in Patent Documents 3 and 4 are also effective against microbial corrosion caused by microorganisms other than sulfate-reducing bacteria.

[0010] Conventional research has primarily focused on sulfate-reducing bacteria as the causative agents of microbial corrosion in anaerobic environments. However, it is becoming increasingly clear that sulfate-reducing bacteria are not the only ones significantly influencing microbial corrosion of steel. For example, Non-Patent Literature 1 reports that a strain called DC57 of the species Shewanella chilikensis (hereinafter sometimes referred to as "SC bacteria"), a microorganism commonly found in marine environments, causes microbial corrosion of steel placed in anaerobic environments in the laboratory. This strain was isolated from corroded structures in a floating oil production system in Western Australia.

[0011] As mentioned above, bacteria that induce microbial corrosion inhabit all kinds of natural environments. SC bacteria are thought to be present not only in marine environments but also in the production water contained in gas and crude oil extracted from gas and oil wells. When gas or crude oil pipelines are not in operation, gas or crude oil can accumulate inside the pipelines, creating an anaerobic environment that is favorable for the proliferation of SC bacteria. The proliferating SC bacteria can form colonies, and these colonies can remain in specific locations inside the pipelines, potentially inducing microbial corrosion of the pipelines.

[0012] This invention has been made in view of the above problems, and aims to provide a microbial corrosion-resistant steel sheet and a method for manufacturing the same that have long-term resistance to microbial corrosion against SC bacteria, which have not received much attention in the past, and are suitable for use in transport pipelines and inexpensive. [Means for solving the problem]

[0013] The inventors diligently conducted research to solve the above problems. First, they investigated the effects of various additive elements, which are added in the prior art to release ions that inhibit the metabolism of microorganisms, on the metabolism of SC bacteria. As a result, among the metal ions that are said to inhibit the metabolism of sulfate-reducing bacteria in Patent Document 3, copper ions (Cu 2+) was found to actually activate the vital activities of SC bacteria. And it was found that by restricting the copper content in the steel plate, the microbiological corrosion resistance of the steel plate was significantly improved.

[0014] Next, when chromium ions (Cr 3+ ) are released from the steel plate, the formation of iron sulfide, a conductive corrosion product that promotes local corrosion, is inhibited, and the formation of iron oxide species such as iron oxide (II, III) with low conductivity is promoted, whereby it was found that local corrosion is suppressed. And it was found that by setting the content of solid-solved Cr in the steel plate to a certain ratio or more, the release of chromium ions is promoted, and the microbiological corrosion resistance of the steel plate can be significantly improved.

[0015] Furthermore, in order to set the content of solid-solved Cr in the steel plate to a certain ratio or more, it was found that it is important to appropriately control the heating time when heating the steel slab before hot rolling, the holding temperature at that heating temperature, as well as the cooling rate and cooling stop temperature of the steel plate after hot rolling. As a result, it was found that a steel plate showing excellent microbiological corrosion resistance can be obtained without performing heat treatment such as tempering after hot rolling.

[0016] The present invention was completed after further studies based on the above novel findings, and its main configuration is as follows.

[0017] [1] In mass%, C: 0.01% or more and 0.30% or less, Si: 0.01% or more and 1.00% or less, Mn: 0.10% or more and 2.50% or less, P: 0.030% or less, S: 0.0100% or less, N: 0.0100% or less, Al: 0.010% or more and 0.300% or less, Cu: 0.01% or more and 0.15% or less, and Cr: 0.05% or more and 1.50% or less contains a component composition consisting of the balance being Fe and inevitable impurities, The content of solid-solved Cr is 0.03% or more by mass%, and a microbiological corrosion-resistant steel sheet in which the mass ratio of the content of solid-solved Cr to the content of Cr in the component composition is 0.30 or more.

[0018] [2] The component composition further includes, by mass%, [Group A] One or more selected from the group consisting of Ni: 1.00% or less, Mo: 1.00% or less, W: 1.00% or less, Sb: 0.20% or less, and Sn: 0.20% or less, [Group B] One or more selected from the group consisting of Ca: 0.0100% or less, Mg: 0.020% or less, and REM: 0.200% or less, [Group C] One or more selected from the group consisting of Ti: 0.100% or less, Zr: 0.100% or less, Nb: 0.100% or less, and V: 0.100% or less, and [Group D] B: 0.0300% or less The microbiological corrosion-resistant steel sheet according to [1] above, containing one or more groups selected from the group consisting of.

[0019] [3] The arithmetic mean roughness Ra of the surface of the steel sheet is 60 μm or less, and the microbiological corrosion-resistant steel sheet according to [1] or [2] above.

[0020] [4] By mass%, C: 0.01% or more and 0.30% or less, Si: 0.01% or more and 1.00% or less, Mn: 0.10% or more and 2.50% or less, P: 0.030% or less, S: 0.0100% or less, N: 0.0100% or less, Al: 0.010% or more and 0.300% or less, Cu: 0.01% or more and 0.15% or less, and Cr: 0.05% or more and 1.50% or less heating a steel slab having a component composition containing the above and having the balance consisting of Fe and inevitable impurities, and at that time, setting the heating temperature to 1000 °C or more and the holding time at the heating temperature to 20 min or more; Next, a hot rolling process is performed on the steel billet to form a steel plate, Next, a cooling step is performed on the steel plate under the conditions that the cooling rate is 5°C / s or more in the temperature range of 750°C to 650°C at the center of the plate thickness, and the cooling stop temperature is 650°C or lower. A method for manufacturing a microbial corrosion-resistant steel sheet having the properties of a steel sheet.

[0021] [5] The above component composition is further, in mass%, [Group A] One or more elements selected from the group consisting of Ni: 1.00% or less, Mo: 1.00% or less, W: 1.00% or less, Sb: 0.20% or less, and Sn: 0.20% or less. [Group B] One or more selected from the group consisting of Ca: 0.0100% or less, Mg: 0.020% or less, and REM: 0.200% or less. [Group C] One or more selected from the group consisting of Ti: 0.100% or less, Zr: 0.100% or less, Nb: 0.100% or less, and V: 0.100% or less, and [Group D] B: 0.0300% or less A method for producing a microbial corrosion-resistant steel sheet according to [4] above, comprising one or more selected from the group consisting of the above.

[0022] [6] The method for producing a microbial corrosion-resistant steel sheet according to [4] or [5] above, wherein the moisture content of the atmosphere inside the heating furnace when heating the steel billet in the heating step is 50% by volume or less, and the reduction ratio of the final rolling pass in the hot rolling step is 4% or more.

[0023] [7] A method for manufacturing a microbial corrosion-resistant steel sheet according to any one of [4] to [6] above, wherein the heating rate in the range of 700°C to 800°C when heating the steel billet is 0.3°C / min or more. [Effects of the Invention]

[0024] According to the present invention, it is possible to obtain an inexpensive, microbially corrosion-resistant steel sheet that has long-term resistance to SC bacteria and is suitable for use in transport pipelines, making it extremely useful in industry. [Modes for carrying out the invention]

[0025] 1. Shewanella chilichensis First, we will describe Shewanella chilichensis (SC bacterium), which this invention assumes to be the causative agent of microbial corrosion. As mentioned above, SC bacterium is an anaerobic microorganism commonly found in marine environments and belongs to the genus Shewanella. Several other species are known to belong to the genus Shewanella besides SC bacterium. SC bacterium is a relatively new species, discovered in the Chilicara Lagoon in India and reported in 2009. SC bacterium is a completely different species from bacteria classified as sulfate-reducing bacteria.

[0026] As mentioned above, the strain of SC bacterium called DC57, described in Non-Patent Literature 1, was isolated from a corroded structure in a floating oil production system in Western Australia, and its discovery was reported in 2020. Genomic analysis performed on DC57 showed an ANI value of 98.86%, indicating genetic sequence homology with JC5, a known strain of SC bacterium.

[0027] The mechanism of microbial corrosion by SC bacteria is not well understood. However, according to Non-Patent Literature 1, thiosulfate ions (S2O3 2- It was confirmed that pitting corrosion due to microbial corrosion occurred on the surface of steel samples when the DC57 strain of SC bacteria was placed in an anaerobic environment where ) was present. From this experimental fact, it is presumed that the metabolic reaction of SC bacteria that causes pitting corrosion in steel is probably the following reaction.

[0028] SC bacteria reduce thiosulfate ions to produce hydrogen sulfide. Since this hydrogen sulfide acts as an oxidizing agent for iron, the hydrogen evolution reaction from hydrogen sulfide is called the cathode reaction, and iron(II) ions (Fe) from steel are released. 2+ A corrosion reaction occurs in which the dissolution reaction of ) is the anodic reaction.

[0029] Furthermore, the eluted iron(II) ions react with hydrogen sulfide in the environment to produce iron sulfide (FeS), a corrosion product. As corrosion progresses, a conductive film composed of this iron sulfide forms on the surface of the steel plate. If the iron sulfide film is uneven, the galvanic effect due to the potential difference is thought to drive preferential local corrosion and pitting corrosion at areas where the film is weaker.

[0030] As described later, the present invention aims to prevent microbial corrosion caused by SC bacteria by suppressing the formation of iron sulfide on the surface of steel plates. The steel plates according to the present invention are presumed to have resistance to microbial corrosion by other strains of SC bacteria that have high genetic sequence homology with strain DC57, as well as by species other than SC bacteria included in the genus Shewanella.

[0031] 2. Content of essential components and solid-solution Cr Next, embodiments for carrying out the present invention will be described. In one embodiment, the present invention is a microbial corrosion-resistant steel sheet having a component composition in mass%, containing C: 0.01% to 0.30%, Si: 0.01% to 1.00%, Mn: 0.10% to 2.50%, P: 0.030% or less, S: 0.0100% or less, N: 0.0100% or less, Al: 0.010% to 0.300%, Cu: 0.01% to 0.15%, and Cr: 0.05% to 1.50%, with the remainder being Fe and unavoidable impurities, having a solid-solution Cr content of 0.03% or more in mass%, and a mass ratio of the solid-solution Cr content to the Cr content in the component composition of 0.30 or more. In this specification, "%" representing the content of each component element means mass percentage unless otherwise specified.

[0032] [Essential ingredients] C: 0.01% or more and 0.30% or less Carbon (C) is an element that increases the strength of steel plates. However, if the C content is too low, sufficient strength cannot be ensured. Therefore, the C content should be 0.01% or more, preferably 0.02% or more. On the other hand, if the C content is too high, the workability and weldability of the steel plate deteriorate significantly. Therefore, the C content should be 0.30% or less, preferably 0.25% or less, and more preferably 0.20% or less.

[0033] Si: 0.01% or more and 1.00% or less Si is an element added to steel materials for the purpose of deoxidizing them. However, if the Si content is too low, a sufficient deoxidation effect cannot be obtained. Therefore, the Si content should be 0.01% or more, preferably 0.02% or more, and more preferably 0.03% or more. On the other hand, if the Si content is too high, the toughness and weldability of the steel sheet deteriorate. Therefore, the Si content should be 1.00% or less, preferably 0.80% or less, and more preferably 0.70% or less.

[0034] Mn: 0.10% or more and 2.50% or less Mn is an element added to steel sheets to improve their strength and toughness. However, if the Mn content is too low, its effect is insufficient. Therefore, the Mn content should be 0.10% or more, preferably 0.30% or more, more preferably 0.50% or more, and even more preferably more than 0.65%. On the other hand, if the Mn content is too high, the weldability of the steel sheet deteriorates. Therefore, the Mn content should be 2.50% or less, preferably 2.00% or less.

[0035] P:0.030% or less If the phosphorus (P) content is excessive, the toughness and weldability of the steel plate deteriorate. Therefore, the P content should be 0.030% or less, preferably 0.025% or less. On the other hand, there is no particular lower limit to the P content, but since it is difficult to reduce it to less than 0.002% in industrial-scale manufacturing, a content of 0.002% or more is acceptable.

[0036] S: 0.0100% or less Since sulfur (S) is a harmful element that reduces the toughness and weldability of steel plates, it is desirable to reduce its content as much as possible. If the S content is excessive, the toughness of the base material and the toughness of the heat-affected zone (HAZ) of the weld will deteriorate significantly. For this reason, the S content should be 0.0100% or less, preferably 0.0080% or less, and more preferably 0.0070% or less. On the other hand, there is no particular lower limit to the S content, but since it is difficult to reduce it to less than 0.0002% in industrial-scale manufacturing, a content of 0.0002% or more is acceptable.

[0037] N: 0.0100% or less Since nitrogen (N) is a harmful element that reduces the toughness of steel sheets, it is desirable to reduce its content as much as possible. If the N content is excessive, the reduction in the toughness of the steel sheet will be significant. Therefore, the N content should be 0.0100% or less, preferably 0.0080% or less, and more preferably 0.0070% or less. On the other hand, there is no particular lower limit to the N content, but it is preferable to have a lower limit of 0.0005% or more.

[0038] Al: 0.010% or more and 0.300% or less Al, like Si, is an element added to steel for the purpose of deoxidizing it. However, if the Al content is too low, a sufficient deoxidation effect cannot be obtained. Therefore, the Al content should be 0.010% or more, preferably 0.015% or more. On the other hand, if the Al content is too high, the toughness of the steel sheet will decrease. Therefore, the Al content should be 0.300% or less, preferably 0.250% or less.

[0039] Cu: 0.01% or more and 0.15% or less Since Cu is an element that promotes microbial corrosion of steel plates by SC bacteria, its content needs to be appropriately limited. Steel plates containing Cu will release copper ions (Cu) from the base material during the corrosion process. 2+ ) is eluted. As described in Patent Document 3, copper ions are known to exert antibacterial activity by binding to the thiol group of the enzyme system of sulfate-reducing bacteria and inhibiting their metabolism. However, quite the opposite, copper ions have the effect of promoting microbial corrosion against SC bacteria.

[0040] Although the exact effects of copper ions on SC bacteria are not fully understood, the inventors believe the following: When copper ions are present above a certain concentration on the surface of a steel plate, the SC bacteria are stressed, causing them to enter a state of starvation. In this state of starvation, the SC bacteria activate their biological activities to obtain energy through metabolism. As a result of metabolism by the SC bacteria, the formation of iron sulfide, a protective corrosion product, is promoted. In addition, some copper ions react with hydrogen sulfide produced by the SC bacteria to form copper sulfide (CuS). The presence of iron sulfide and copper sulfide on the surface of the steel plate increases the non-uniformity of the protective film on the surface. As a result, it is thought that the progression of pitting corrosion, driven by the galvanic effect due to the potential difference, is promoted.

[0041] These effects of copper ions become particularly pronounced when the Cu content in the steel sheet exceeds 0.15%. Therefore, the Cu content should be 0.15% or less, preferably 0.12% or less, more preferably 0.10% or less, and even more preferably 0.09% or less. On the other hand, even when Cu is not intentionally added, at least 0.01% Cu is usually present. Therefore, the Cu content should be 0.01% or more. Furthermore, when Cu is intentionally included to increase material strength, the Cu content should preferably be 0.03% or more.

[0042] Cr: 0.05% or more and 1.50% or less Cr is an element added to steel sheets to improve their resistance to microbial corrosion by SC bacteria. For this reason, Cr is positioned as an important additive element in the steel sheet according to the present invention. As mentioned above, SC bacteria placed in a humid environment form a film on the surface of the steel sheet consisting of iron sulfide, which is a conductive corrosion product. However, in the presence of chromium ions, the formation of iron sulfide is inhibited, and instead, the formation of iron oxide species such as iron(II,III)(Fe3O4), which has low conductivity, is promoted. As a result, electrical conductivity in the film is hindered, so the "galvanic effect caused by the non-uniformity of the film," which is the driving force of localized corrosion, does not act strongly, and localized corrosion is suppressed.

[0043] The exact reason why the formation of iron(II,III) oxides is promoted in the presence of chromium ions is not fully understood, but the inventors believe the following: In a humid environment, chromium ions leached from the matrix phase of the steel sheet react with hydroxide ions (OH) in the environment. - It reacts with chromium to rapidly form chromium oxide clusters (molecular ionic species). These chromium oxide clusters act as nuclei for the formation of iron(II,III) oxides when iron ions dissolved in the environment combine with oxygen. Therefore, in the presence of chromium ions, iron ions deposit on the surface of the steel plate not only as iron sulfide but also as iron(II,III) oxides. As a result, it is thought that a film with lower conductivity than an iron sulfide film is formed.

[0044] If the Cr content is too low, the above-mentioned local corrosion suppression effect cannot be obtained. Therefore, the Cr content should be 0.05% or more, preferably 0.06% or more, and more preferably 0.07% or more. On the other hand, if the Cr content is too high, the weldability and manufacturability of the steel plate will decrease, which is disadvantageous from a cost standpoint. Therefore, the Cr content should be 1.50% or less, preferably 1.20% or less, and more preferably 1.00% or less.

[0045] Fe and unavoidable impurities: remainder The remaining components of the steel sheet, other than those mentioned above, consist of Fe and unavoidable impurities. Unavoidable impurities refer to impurities that are inevitably introduced from raw materials, manufacturing processes, or manufacturing equipment. Here, raw materials refer to iron ore, reduced iron, or scrap iron, etc. Unavoidable impurities are permissible to be included in the steel sheet to the extent that they do not hinder the purpose of the present invention.

[0046] [Solid solution Cr content] Solid-solution Cr content: 0.03% or more As mentioned above, in a humid environment, chromium ions leach from the matrix phase (main phase) of the steel sheet. The chromium ions that leach in a humid environment are those that have been dissolved as solid-solution Cr in the matrix phase. Of the Cr contained in the steel sheet, non-solid-solution Cr, which is not dissolved in the matrix phase but is included as a chromium-based compound, does not directly contribute to the leaching of chromium ions. Therefore, in the steel sheet according to the present invention, the content of solid-solution Cr dissolved in the matrix phase is specified in order to reliably obtain the effect of suppressing microbial corrosion by adding Cr.

[0047] If the solid-solution Cr content is too low, the effect of suppressing local corrosion cannot be obtained. Therefore, the solid-solution Cr content should be 0.03% or more, preferably 0.04% or more, and more preferably 0.05% or more. To achieve a solid-solution Cr content of 0.03% or more, as mentioned above, the Cr content in the steel sheet should be 0.05% or more. On the other hand, there is no particular upper limit to the solid-solution Cr content, but as mentioned above, since the Cr content of the steel sheet is limited to 1.50% or less, the solid-solution Cr content will not exceed 1.50%.

[0048] In this specification, "solid-solution Cr content" refers to the mass ratio of solid-solution Cr in the matrix phase to the total mass of the steel sheet. The solid-solution Cr content is expressed as a mass percentage, similar to the component composition. To determine the solid-solution Cr content, first, a steel sheet sample is electrolyzed using an electrolyte to extract Cr precipitates containing non-solid-solution Cr, and the non-solid-solution Cr content in the steel sheet sample is determined by quantitative analysis of the obtained Cr precipitates. Next, the solid-solution Cr content is determined by subtracting the non-solid-solution Cr content from the Cr content. The method for determining the solid-solution Cr content will be explained in more detail in the examples described later.

[0049] Mass ratio of solid-solution Cr content to total Cr content: 0.30 or higher As mentioned above, non-solid-soluble Cr does not directly contribute to the elution of chromium ions. In other words, non-solid-soluble Cr is released into the humid environment as the matrix phase corrodes and has no effect on microbial corrosion. Specific examples of non-solid-soluble Cr include Cr-based oxides, such as Cr-Al-Mg oxides.

[0050] However, if the amount of non-soluble Cr released into a humid environment is excessive, the formation of iron(II,III) oxides by the aforementioned chromium oxide clusters will be inhibited, and the local corrosion suppression effect of soluble Cr will not be obtained. This is because precipitates containing non-soluble Cr act as nuclei for the formation of chromium oxide crystals from chromium oxide clusters, and the amount of chromium oxide clusters that serve as nuclei for the formation of iron(II,III) oxides decreases. Therefore, in the steel sheet according to the present invention, the mass ratio of the content of soluble Cr to the content of Cr in the component composition is specified.

[0051] If the mass ratio of the solid-solution Cr content to the Cr content in the component composition is too low, the local corrosion suppression effect of solid-solution Cr cannot be obtained. Therefore, the mass ratio of the solid-solution Cr content to the Cr content should be 0.30 or higher, preferably 0.40 or higher. The higher this mass ratio, the greater the local corrosion suppression effect, so there is no particular upper limit, and the mass ratio may be 1.00.

[0052] To achieve a mass ratio of solid-solution chromium to total chromium content of 0.30 or higher, the manufacturing conditions of the steel sheet should be appropriately controlled, as described later. Among the manufacturing conditions, it is effective to appropriately control the heating temperature of the steel billet before hot rolling, the holding time at that heating temperature, the cooling rate in the cooling process after hot rolling, and the cooling stop temperature.

[0053] 3.Optional ingredients In a preferred embodiment, the microbial corrosion-resistant steel sheet according to the present invention is The component composition is further expressed in mass percent. [Group A] One or more elements selected from the group consisting of Ni: 1.00% or less, Mo: 1.00% or less, W: 1.00% or less, Sb: 0.20% or less, and Sn: 0.20% or less. [Group B] One or more selected from the group consisting of Ca: 0.0100% or less, Mg: 0.020% or less, and REM: 0.200% or less. [Group C] One or more selected from the group consisting of Ti: 0.100% or less, Zr: 0.100% or less, Nb: 0.100% or less, and V: 0.100% or less, and [Group D] B: 0.0300% or less It contains one or more groups selected from the group consisting of the following.

[0054] One or more elements selected from the group (Group A) consisting of Ni: 1.00% or less, Mo: 1.00% or less, W: 1.00% or less, Sb: 0.20% or less, and Sn: 0.20% or less. Ni, Mo, W, Sb, and Sn are elements that enhance the corrosion resistance of steel sheets in humid environments provided by gas extracted from gas and oil wells, and by productive water (brine) contained in crude oil. These elements can be included in quantities of one or more to improve corrosion resistance to brine, separate from the purpose of improving microbial corrosion resistance. However, excessive content can degrade the toughness of the HAZ (High-Area Zone) and increase costs. Therefore, when included, the respective concentrations should be Ni: 1.00% or less, Mo: 1.00% or less, W: 1.00% or less, Sb: 0.20% or less, and Sn: 0.20% or less. Preferably, the concentrations should be Ni: 0.6% or less, Mo: 0.6% or less, W: 0.6% or less, Sb: 0.12% or less, and Sn: 0.12% or less. Furthermore, the lower limit for inclusion is preferably Ni: 0.01% or more, Mo: 0.01% or more, W: 0.01% or more, Sb: 0.01% or more, and Sn: 0.01% or more. The above content levels can be independently selected for each element in Group A.

[0055] One or more elements selected from the group consisting of Ca: 0.0100% or less, Mg: 0.020% or less, and REM: 0.200% or less (Group B). Ca, Mg, and REM may be included in one or more amounts to ensure the toughness of the HAZ. However, if the content is excessive, it may degrade the toughness of the HAZ or increase costs. Therefore, when included, the amounts should be Ca: 0.0100% or less, Mg: 0.020% or less, and REM: 0.200% or less, respectively. Preferably, Ca: 0.0060% or less, Mg: 0.012% or less, and REM: 0.120% or less. Furthermore, the lower limits for content should preferably be Ca: 0.0001% or more, Mg: 0.0001% or more, and REM: 0.001% or more. The above content can be independently selected for each element in Group B.

[0056] One or more elements selected from the group consisting of Ti: 0.100% or less, Zr: 0.100% or less, Nb: 0.100% or less, and V: 0.100% or less (Group C). Ti, Zr, Nb, and V may be included in one or more quantities to ensure the strength of the steel sheet. However, if the content is excessive, it will degrade the toughness and weldability of the steel sheet. Therefore, when included, the content of each should be 0.100% or less, preferably 0.050% or less. Furthermore, the lower limit for the content of each is preferably 0.005% or more. The above content can be independently selected for each element in the C group.

[0057] B: 0.0300% or less (D group) B is an element that improves the hardenability of steel sheets. It can also be included to ensure the strength of the steel sheet. However, excessive B content leads to a significant deterioration of toughness. Therefore, the B content should be 0.0300% or less, preferably 0.0200% or less. Furthermore, the lower limit for B content should preferably be 0.0001% or more.

[0058] 4. Surface roughness In a preferred embodiment, the microbial corrosion-resistant steel sheet according to the present invention has an arithmetic mean surface roughness Ra of 60 μm or less.

[0059] In order to further improve the microbial corrosion resistance of the microbial corrosion-resistant steel sheet according to the present invention, it is preferable to appropriately control the surface roughness of the steel sheet. If the surface roughness of the steel sheet is excessive, SC bacteria can easily adhere to the surface of the steel sheet and continue their life activities at the location where they adhere. When an environment suitable for the proliferation of SC bacteria, such as an anaerobic environment, is established, the SC bacteria will proliferate and form colonies, and microbial corrosion of the steel sheet by local corrosion will be promoted at the location of the colonies. Although the initial surface roughness of the steel sheet changes as microbial corrosion progresses, the initial surface roughness of the steel sheet has a considerable influence on the formation of pitting corrosion in the initial stages of microbial corrosion.

[0060] Therefore, in a preferred embodiment, the arithmetic mean roughness Ra of the steel sheet surface is set to 60 μm or less, more preferably to 50 μm or less. There is no particular lower limit to the arithmetic mean roughness Ra of the steel sheet, but since excessively reducing surface roughness would impose a significant industrial cost burden, it is preferable that the arithmetic mean roughness Ra of the steel sheet be 0.1 μm or more. It is preferable that the arithmetic mean roughness Ra of the steel sheet surface is 60 μm or less on both sides of the steel sheet.

[0061] In this preferred embodiment, the arithmetic mean roughness Ra of the steel sheet surface can be determined using the evaluation method specified in Japanese Industrial Standard JIS B 0601 (2013), using the contour line of the steel sheet surface measured with a stylus-type surface roughness meter. It is preferable to measure the contour line of the steel sheet surface over a reference length of 5.0 mm in the width direction of the steel sheet.

[0062] The surface roughness of the steel sheet according to the present invention varies greatly depending on the manufacturing conditions of the steel sheet. In particular, since Cr contained in the steel sheet according to the present invention is an element that enhances the adhesion of scale to the steel sheet, if the manufacturing conditions of the steel sheet are not properly controlled, the surface roughness may become significantly rough. Therefore, in order to control the arithmetic mean surface roughness Ra of the steel sheet to 60 μm or less, it is important to properly control the hot rolling conditions of the steel billet among the manufacturing conditions of the steel sheet, as described below, and it is especially important to apply a reduction amount of a certain amount or more in the final rolling pass of hot rolling. In addition, it is also effective to properly control the moisture concentration of the atmosphere inside the heating furnace during the heating process of the steel billet before hot rolling.

[0063] 5.Applications The steel plate according to the present invention can be suitably used in the manufacture of line pipes. Line pipes are steel pipes used in transport pipelines for transporting flammable liquids or gases such as petroleum and gas. Transport pipelines are facilities made by continuously joining line pipes and are installed on land, underground, underwater, or below the seabed. As mentioned above, SC bacteria are thought to exist not only in the marine environment but also in the gas extracted from gas and oil wells and in the production water contained in crude oil. Therefore, line pipes manufactured using the steel plate according to the present invention can be suitably used in transport pipelines installed underwater or below the seabed, or in transport pipelines incorporated into floating petroleum production systems installed on the sea. However, the use of the steel plate according to the present invention is not limited to line pipes, and it can also be used in structures installed in any environment in which SC bacteria can live.

[0064] The method for manufacturing line pipes from steel plates according to the present invention is not particularly limited. For example, line pipes made of spiral steel pipes, UEO steel pipes, or electric resistance welded steel pipes, etc., which have excellent microbial corrosion resistance to SC bacteria and are suitable for transporting crude oil and natural gas, can be manufactured by forming the steel plates into a tubular shape using press bending, roll forming, or UEO forming, and then welding the butt joints.

[0065] Of these line pipes, UOE steel pipes are manufactured by beveling the ends of steel plates, forming them into a steel pipe shape using C-press, U-press, and O-press, then seam welding the butt joints by internal and external welding, and further expanding the pipe as needed. Any welding method is acceptable as long as sufficient mechanical strength and toughness can be obtained for the welded joint, but from the viewpoint of excellent welding quality and manufacturing efficiency, submerged arc welding is preferred.

[0066] 6. Manufacturing method In another embodiment, the present invention relates to a method for manufacturing a microbial corrosion-resistant steel sheet, comprising: a heating step of heating a steel billet having the above-described component composition, wherein the heating temperature is 1000°C or higher and the holding time at the heating temperature is 20 min or higher; a hot rolling step of hot rolling the steel billet to form a steel sheet; and a cooling step of controlled cooling of the steel sheet under the conditions that the temperature of the steel sheet at the center of the sheet thickness is in the temperature range of 750°C to 650°C, with a cooling rate of 5°C / s or higher and a cooling stop temperature of 650°C or lower. The reasons for limiting the component composition have already been explained, so the explanation will be omitted here.

[0067] [Steel piece] In the method for producing microbial corrosion-resistant steel sheets according to the present invention, molten steel having the aforementioned component composition is melted in a known furnace such as a converter or electric furnace, and formed into steel billets such as slabs or billets by a known method such as continuous casting or ingot forming. Vacuum degassing refining may be performed during melting. The component composition of the molten steel can be adjusted according to known steel refining methods.

[0068] [Heating process] Next, the steel billet is heated, with a heating temperature of 1000°C or higher and a holding time of 20 minutes or more. Here, the "heating temperature" when heating the steel billet refers to the furnace setting temperature of the heating furnace used to heat the steel billet, and the steel billet is heated to this furnace setting temperature all the way to the center. If the heating temperature and holding time are insufficient, the Cr inclusions present in the steel billet will maintain their form as Cr inclusions thermodynamically or kinetically. In that case, it will not be possible to sufficiently dissolve Cr in the steel billet, and the mass ratio of the solid-solution Cr content to the Cr content in the final steel plate product cannot be stably set to 0.30 or higher. For this reason, the heating temperature should be 1000°C or higher, and the holding time should be 20 minutes or more. The heating temperature of the steel billet is preferably 1030°C or higher, more preferably 1070°C or higher. The holding time of the steel billet at the heating temperature is preferably 60 minutes or more, more preferably 120 minutes or more.

[0069] However, if the heating temperature of the steel billet is too high, it can cause surface marks, increase scale loss and fuel consumption. Therefore, the heating temperature of the steel billet is preferably 1350°C or lower, and more preferably 1300°C or lower. Furthermore, there is no particular upper limit to the holding time at the heating temperature of the steel billet, but from the viewpoint of productivity, it is preferable to keep it at 1000 min or less.

[0070] From the viewpoint of stably maintaining a mass ratio of solid-solution Cr content to Cr content of 0.30 or more in the final steel sheet product, it is preferable that the heating rate of the steel billet in the range of 700°C to 800°C is not excessively slow. The temperature range of 700°C to 800°C is the temperature range in which Cr inclusions become thermodynamically stable. If the heating rate in this temperature range is excessively slow, the precipitation of thermodynamically stable Cr inclusions will proceed significantly during the heating process, which may result in the mass ratio of solid-solution Cr content to Cr content in the steel sheet becoming less than 0.30. Therefore, the heating rate of the steel billet in the range of 700°C to 800°C is preferably 0.3°C / min or more, more preferably 0.4°C / min or more, and even more preferably 0.5°C / min or more. There is no particular upper limit to the preferred heating rate when the heating temperature of the steel billet is in the range of 700°C to 800°C, and it may be less than or equal to the highest heating rate achievable in the heating furnace used (e.g., 20°C / min or less).

[0071] Furthermore, from the viewpoint of achieving an arithmetic mean roughness Ra of 60 μm or less on the surface of the steel sheet, it is preferable that the moisture concentration of the atmosphere inside the heating furnace when heating the steel billet in the heating process be between 5 volume% and 50 volume%. If the moisture concentration of the atmosphere inside the heating furnace is too low, the detachability of scale formed on the surface of the steel billet during the heating process will decrease. This makes it difficult to isolate the scale from the steel billet, and the surface roughness of the steel sheet may become rougher. For this reason, it is preferable that the moisture concentration of the atmosphere inside the heating furnace be 5 volume% or more, and more preferably 10 volume% or more. On the other hand, if the moisture concentration of the atmosphere inside the heating furnace is too high, scale loss may increase. For this reason, it is preferable that the moisture concentration of the atmosphere inside the heating furnace be 50 volume% or less, and more preferably 40 volume% or less. The moisture concentration of the atmosphere inside the heating furnace can be controlled by selecting and using a combustion gas with an appropriate moisture concentration.

[0072] [Hot rolling process] Next, a hot rolling process is performed on the steel billet heated in the heating process to form a steel plate. If the finishing rolling temperature in the hot rolling process is too low, the rolling load increases due to the increased deformation resistance, placing a heavy burden on the rolling process. Therefore, it is preferable that the finishing rolling temperature be 680°C or higher. On the other hand, if the finishing rolling temperature is too high, the desired strength may not be obtained. Therefore, it is preferable that the finishing rolling temperature be 930°C or lower.

[0073] In a preferred embodiment, the method for manufacturing a microbial corrosion-resistant steel sheet according to the present invention involves reducing the reduction ratio of the final rolling pass in the hot rolling process by 4% or more. As described above, the steel sheet according to the present invention contains Cr, which enhances the adhesion of scale to the steel sheet. Therefore, it is important to ensure that the reduction ratio of the final rolling pass, which determines the final surface properties of the steel sheet in hot rolling, is above a certain level. Accordingly, in a preferred embodiment, the reduction ratio of the final rolling pass in the hot rolling process is set to 4% or more. This promotes scale removal and allows the arithmetic mean roughness Ra of the steel sheet surface to be controlled to 60 μm or less. The reduction ratio (%) of the final rolling pass is calculated by (100 - ((thickness after the final rolling pass) / (thickness before the final rolling pass) × 100)). Preferably, the reduction ratio of the final rolling pass in the hot rolling process is 5% or more.

[0074] While there is no particular upper limit to the reduction ratio of the final rolling pass in the hot rolling process, it is preferable to keep the reduction ratio at 25% or less from the standpoint of maintaining the rolling equipment.

[0075] [Cooling process] Next, the hot-rolled steel sheet is subjected to a controlled cooling process under the conditions that the cooling rate is 5°C / s or more in the temperature range of 750°C to 650°C at the center of the sheet thickness, and the cooling stop temperature is 650°C or lower. If the cooling rate is too low or the cooling stop temperature is too high, the content of non-solid-solution Cr increases, and the desired solid-solution Cr content and the mass ratio of solid-solution Cr content to Cr content cannot be obtained. Therefore, the cooling rate in the temperature range of 750°C to 650°C should be 5°C / s or more, and the cooling stop temperature should be 650°C or lower. Preferably, the cooling rate in the temperature range of 750°C to 650°C should be 10°C / s or more. On the other hand, if the cooling rate is too high or the cooling stop temperature is too low, the toughness of the steel sheet may decrease or distortion may occur in the shape of the steel sheet. Therefore, it is preferable that the cooling rate in the temperature range from 750°C to 650°C be 100°C / s or less, and the cooling stop temperature be 300°C or higher. More preferably, the cooling rate in the temperature range from 750°C to 650°C is 50°C / s or less, and even more preferably less than 20°C / s.

[0076] The cooling rate referred to here is the cooling rate in the temperature range of 750°C to 650°C, and the temperature refers to the temperature of the steel plate at the center of its thickness. Although the temperature of the steel plate at the center of its thickness cannot be measured directly by physical means, it can be calculated in real time from the temperature distribution within the cross-section of the plate thickness by differential calculation using a process computer, for example, based on the surface temperature of the steel plate at the start of cooling and the surface temperature of the steel plate at the end of cooling, as measured by a radiation thermometer. In this specification, "temperature of the steel plate at the center of its thickness" refers to the temperature obtained in this manner.

[0077] The method of controlled cooling in the cooling process is not particularly limited, as long as it satisfies the above conditions. The method of controlled cooling may be any of the following: air cooling, accelerated cooling, etc.

[0078] Furthermore, after hot rolling, the material may be subjected to reheating, pickling, and cold rolling as needed to obtain cold-rolled steel sheets of a specified thickness. Other manufacturing conditions are not particularly limited and should be followed according to conventional methods. [Examples]

[0079] Next, embodiments of the present invention will be described. However, the present invention is not limited to the following embodiments.

[0080] Molten steel having the component composition shown in Table 1 was melted using commonly known methods and continuously cast to form steel billets. The remainder of the component composition shown in Table 1 consisted of Fe and unavoidable impurities. Steel grades No. 1 to 27, and steel grades No. 35 and 36 are suitable steels having the component composition specified in the present invention. Steel grades No. 28 to 34 are comparative steels whose Cu or Cr content is outside the range of the present invention.

[0081] [Table 1]

[0082] Next, the obtained steel billet was heated, followed by a hot rolling process to obtain a steel plate with a thickness of 25 mm by hot rolling the steel billet at a finish rolling completion temperature of 810°C, and then a cooling process to perform controlled cooling of the steel plate by water cooling, thereby producing the microbial corrosion-resistant steel plates shown in Table 2. The moisture concentration of the atmosphere inside the heating furnace during the heating process, the heating rate from 700°C to 800°C, the heating temperature and the holding time at the heating temperature, the reduction ratio of the final rolling pass during the hot rolling process, and the cooling rate and cooling stop temperature in the temperature range from 750°C to 650°C during the cooling process are shown in Table 2. Here, the temperature during the cooling process is the temperature of the steel plate at the center of the plate thickness. Steel plates No. 1 to 27 and steel plates No. 44 to 51 are examples of the invention that satisfy the component composition and manufacturing conditions specified in the present invention. Steel plates No. 28 to 34 are comparative examples in which the Cu or Cr content is outside the range of the present invention. Steel plates No. 35 to 43 are comparative examples whose manufacturing conditions fall outside the scope of the present invention.

[0083] Next, the surface roughness and solid solution Cr content were measured using a portion of the obtained steel sheet. For surface roughness measurement, first, the contour line of the steel sheet surface was measured using a stylus-type surface roughness measuring instrument over a reference length of 5.0 mm in the width direction of the steel sheet. Then, using the obtained contour line, the arithmetic mean roughness Ra of the steel sheet surface was determined according to the evaluation method specified in Japanese Industrial Standard JIS B 0601:2013. The obtained arithmetic mean roughness Ra of the steel sheet surface is shown in Table 2.

[0084] To measure the content of solid-solution Cr, first, the oxide film on the surface of the obtained steel sheet was removed, and then a sample for analysis was taken that included the entire thickness of the steel sheet. Next, constant current electrolysis was performed on the collected sample to extract Cr precipitates, including non-solid-solution Cr. The electrolyte used was a solution of methanol with 10 vol% acetylacetone and 1 mass% tetramethylammonium chloride added. Next, the Cr precipitate extracted into the electrolyte was collected using a filter with a pore size of 0.1 μm, and the collected Cr precipitate was dissolved with acid. Then, inductively coupled plasma atomic emission spectroscopy was performed on the obtained solution to determine the amount of Cr in the Cr precipitate contained in the collected sample for analysis, i.e., the content of non-solid-solution Cr. Subsequently, the content of solid-solution Cr was determined by subtracting the obtained content of non-solid-solution Cr from the Cr content in the composition of the steel billet. The obtained content of solid-solution Cr and the mass ratio of the content of solid-solution Cr to the total Cr content are shown in Table 2.

[0085] Next, the remaining unused steel plates were used to evaluate their resistance to microbial corrosion by SC bacteria. First, evaluation samples with a square shape of 15 mm on each side and a thickness of 3 mm were cut from the steel plates, and the entire surface of the samples was coated with cationic electrodeposition paint. Next, using 320-grit sandpaper, only one side of the square surface of the sample was polished to remove the coating, and then the surface was polished to a finish using 600-grit sandpaper to create the test specimens. However, for steel plates No. 1 to 4 and steel plates No. 40 to 50, one side of the square surface of the sample was masked before applying the cationic electrodeposition paint. After that, the masking was removed and polishing was not performed to maintain the arithmetic mean roughness Ra of the steel plate at the time of manufacture on the surface of the sample specimens. Furthermore, all test specimens were immersed in 70% ethanol, removed from the ethanol, the surface of the test specimens was wiped with a sterile cloth, air-dried under ultraviolet irradiation, and then subjected to testing.

[0086] Table 2 shows the arithmetic mean roughness Ra of the test specimens measured by the method described above. As shown in Table 2, the arithmetic mean roughness Ra of the test specimens that underwent finish polishing was 1 μm. On the other hand, the arithmetic mean roughness Ra of the test specimens that were not subjected to finish polishing was no different from the arithmetic mean roughness Ra of the surface during steel plate manufacturing.

[0087] Next, a simulated production water solution was prepared as a culture medium for SC bacteria. The simulated production water solution is an artificially created solution that mimics the components of production water contained in gas and crude oil extracted from gas and oil wells where SC bacteria are thought to inhabit. The simulated production water solution was prepared by mixing pure water with 20 g / L of sodium chloride, 20 mol / L of sodium lactate, 20 mol / L of sodium acetate, 9 mol / L of sodium pyruvate, 9 mol / L of ammonium chloride, 4 mol / L of sodium bicarbonate, 10 mol / L of sodium thiosulfate, and 1% by mass of casamino acid. The pH of the prepared simulated production water solution was 7.2.

[0088] Next, a highly concentrated solution of SC bacteria was prepared using the following procedure. First, 5 mL of a simulated production water solution, which had been sterilized in advance by autoclaving, was sealed in a screw-cap test tube. SC bacteria strain DC57 was added using a syringe, and the SC bacteria were cultured at 37°C for 4 days. The SC bacteria added were those that had been subcultured in the simulated production water solution medium. Next, 1 L of a simulated production water solution, which had been sterilized in advance by autoclaving, was placed in a sterilized centrifuge tube, and 2.5 mL of the culture solution cultured earlier was added to the centrifuge tube in an anaerobic glove box. Next, the centrifuge tube containing the SC bacteria was placed in a GasPak 100 anaerobic system ("GasPak" is a registered trademark of Becton Dietkinson & Company) to create an anaerobic environment, and the temperature was maintained at 21°C for 3 days to obtain approximately 1 L of a highly concentrated culture solution of SC bacteria. Next, 100 mL was taken from the obtained high-concentration culture solution of SC bacteria and centrifuged at 3000 revolutions per minute for 5 minutes. Then, 90 mL of the supernatant that did not contain SC bacteria was removed from the high-concentration culture solution after centrifugation to obtain 10 mL of ultra-high-concentration solution of SC bacteria.

[0089] Next, the microbial corrosion resistance of the test specimens was evaluated using the obtained test specimens and the ultra-high concentration solution of SC bacteria. First, all equipment provided for the test was sterilized in advance by autoclaving. Next, to avoid contamination of the test equipment and test specimens with contaminants, the test cell was assembled in a clean bench, and the previously prepared test specimens were placed inside the test cell. Next, 5 L of production water simulation solution, which had been sterilized in advance by autoclaving, was added to the test cell. Then, the test was started by adding the entire 10 mL of the ultra-high concentration solution of SC bacteria prepared by the above method to the test cell. The period during which the test specimens were immersed in the production water simulation solution was 28 days. During the test, the temperature of the test cell was maintained at 40°C, and the production water simulation solution inside the test cell was replaced with fresh production water simulation solution at a rate of 30 mL per hour using a pump connected to the test cell. Furthermore, during the test, an anaerobic environment was maintained by blowing nitrogen gas, which was filtered through a bacterial collection filter to prevent contamination by unwanted bacteria, into the simulated production water solution at a flow rate of 300 mL per minute.

[0090] After the immersion period, the test specimen was removed from the test cell, and the corrosion products adhering to the surface were wiped off with a sponge or similar material. The corrosion products were then completely removed by immersing the specimen in an acid containing an inhibitor. Next, the specimen was washed with pure water, then in ethanol, and air-dried. Subsequently, the depth of pitting corrosion was measured using a three-dimensional laser microscope across the entire exposed surface of the specimen where the coating had been removed. The maximum value among the obtained measurements was defined as the maximum pitting depth. The laser wavelength of the three-dimensional laser microscope was 658 nm, and the measurement pitch was 0.5 μm. Furthermore, the evaluation of microbial corrosion resistance was as follows: "Excellent" if the maximum pitting depth was less than 20 μm, "Good" if it was between 20 μm and 40 μm, and "Poor" if it was 40 μm or more. The obtained maximum pitting depths and evaluation results are shown in Table 2.

[0091] [Table 2]

[0092] As shown in Table 2, the inventive examples that satisfy all of the following conditions for the microbial corrosion-resistant steel sheet according to the present invention—the component composition of the steel sheet, the content of solid-solution Cr, the mass ratio of the content of solid-solution Cr to the total Cr content, and the manufacturing conditions for the method of producing the microbial corrosion-resistant steel sheet according to the present invention—all show excellent or good evaluations of microbial corrosion resistance. In particular, the inventive examples of steel sheets No. 1 to 27, and steel sheets No. 49 and 50, which have an arithmetic mean surface roughness Ra of 60 μm or less, all show excellent evaluations of microbial corrosion resistance. In contrast, the comparative examples that do not satisfy any of the above conditions all show unacceptable evaluations of microbial corrosion resistance and are unsuitable as microbial corrosion-resistant steel sheets.

Claims

1. In mass percent, C: 0.01% or more and 0.30% or less, Si: 0.01% or more and 1.00% or less, Mn: 0.10% or more and 2.50% or less, P: 0.030% or less, S: 0.0100% or less, N: 0.0100% or less, Al: 0.010% or more and 0.300% or less, Cu: 0.01% or more and 0.15% or less, Cr: 0.05% or more and 1.50% or less It contains, with the remainder being Fe and unavoidable impurities, and has a component composition. The solid-solution Cr content is 0.03% or more by mass, and A microbial corrosion-resistant steel sheet, wherein the mass ratio of the solid-solution Cr content to the Cr content in the aforementioned component composition is 0.30 or more.

2. The aforementioned component composition is further expressed in mass%, [Group A] One or more elements selected from the group consisting of Ni: 1.00% or less, Mo: 1.00% or less, W: 1.00% or less, Sb: 0.20% or less, and Sn: 0.20% or less. [Group B] One or more selected from the group consisting of Ca: 0.0100% or less, Mg: 0.020% or less, and REM: 0.200% or less. [Group C] One or more selected from the group consisting of Ti: 0.100% or less, Zr: 0.100% or less, Nb: 0.100% or less, and V: 0.100% or less, and [Group D] B: 0.0300% or less The microbial corrosion-resistant steel sheet according to claim 1, comprising one or more selected from the group consisting of the above.

3. The microbial corrosion-resistant steel sheet according to claim 1 or 2, wherein the arithmetic mean roughness Ra of the surface of the steel sheet is 60 μm or less.

4. In mass percent, C: 0.01% or more and 0.30% or less, Si: 0.01% or more and 1.00% or less, Mn: 0.10% or more and 2.50% or less, P: 0.030% or less, S: 0.0100% or less, N: 0.0100% or less, Al: 0.010% or more and 0.300% or less, Cu: 0.01% or more and 0.15% or less, Cr: 0.05% or more and 1.50% or less A heating step is performed in which a steel billet containing a component composition consisting of Fe and unavoidable impurities is heated, the heating temperature is set to 1000°C or higher, and the holding time at the heating temperature is 20 min or higher. Next, a hot rolling process is performed on the steel billet to form a steel plate, Next, a cooling step is performed on the steel plate under the conditions that the cooling rate is 5°C / s or more in the temperature range of 750°C to 650°C at the center of the plate thickness, and the cooling stop temperature is 650°C or lower. A method for manufacturing a microbial corrosion-resistant steel sheet having the properties of a steel sheet.

5. The aforementioned component composition is further expressed in mass%, [Group A] One or more elements selected from the group consisting of Ni: 1.00% or less, Mo: 1.00% or less, W: 1.00% or less, Sb: 0.20% or less, and Sn: 0.20% or less. [Group B] One or more selected from the group consisting of Ca: 0.0100% or less, Mg: 0.020% or less, and REM: 0.200% or less. [Group C] One or more selected from the group consisting of Ti: 0.100% or less, Zr: 0.100% or less, Nb: 0.100% or less, and V: 0.100% or less, and [Group D] B: 0.0300% or less A method for producing a microbial corrosion-resistant steel sheet according to claim 4, comprising one or more selected from the group consisting of the above.

6. A method for producing a microbial corrosion-resistant steel sheet according to claim 4 or 5, wherein the moisture content of the atmosphere inside the heating furnace when heating the steel billet in the heating step is 50% by volume or less, and the reduction ratio of the final rolling pass in the hot rolling step is 4% or more.

7. A method for manufacturing a microbial corrosion-resistant steel sheet according to claim 4 or 5, wherein the heating rate in the range of 700°C to 800°C when heating the steel billet is 0.3°C / min or more.

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