steel

Abstract

To provide a steel that enables formation of an oxide layer with low electric conductivity on a surface layer, when used in an environment with concerns about galvanic corrosion, and can prevent the occurrence of galvanic corrosion.SOLUTION: A steel disclosed herein contains, in mass%, C: 0.005-0.200%, Si: 0.03-1.50%, Mn: 0.05-3.00%, P: 0.035% or less, S: 0.035% or less, Cr: 0.80-10.00%, Cu: 0.001-0.500%, Ni: 0.001-0.500%, Mo: 0.001-1.500%, Sn: 0.001-0.500%, Al: 0-1.500%, W: 0-2.00%, with the balance being Fe and impurities, satisfying formula (1): 2.20≤(2Cr+0.5Ni+0.5Mn) / (Cu+1)≤18.50. In the formula (1), each element symbol has a content of the corresponding element in mass% assigned thereto.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present disclosure relates to steel materials, and more particularly to steel materials having excellent resistance to galvanic corrosion. [Background technology]

[0002] Steel is widely used in a variety of structures. However, structural steel faces the issue of corrosion when exposed to the natural environment. This means that structural steel is required to have excellent corrosion resistance. Specifically, methods that have been studied to prevent corrosion of structural steel include applying rust- and corrosion-resistant coatings and adding alloying elements to steel to improve its corrosion resistance.

[0003] Specifically, Patent Document 1 (Japanese Patent Laid-Open No. 2012-177168), Patent Document 2 (Japanese Patent Laid-Open No. 2017-35877), and Patent Document 3 (Japanese Patent Laid-Open No. 2015-218383) disclose steel materials with excellent corrosion resistance.

[0004] The steel material described in Patent Document 1 is a steel material for ships, and has a chemical composition, in mass %, containing C: 0.01 to 0.20%, Si: 0.01 to 2.5%, Mn: 0.1 to 2.0%, P: 0.03% or less, S: 0.01% or less, Al: 0.005 to 0.3%, and N: 0.008% or less, and further containing one or two selected from group A elements: Mo: 0.005 to 3.0%, W: 0.005 to 3.0%, and group B elements: Cu: 0.005 to 2.0%. The steel material contains a combination of two or more elements selected from Groups A to C, including 0.005%, 0.005-5.0% Cr, 0.005-5.0% Ni, and one or two elements selected from Group C, Sb: 0.005-1.0%, and Sn: 0.005-1.0%, with the balance consisting of Fe and impurities, and has a coating of an aqueous zinc primer formed on the surface of the steel material, and an epoxy-based coating further on top of that coating. Patent Document 1 states that this steel material exhibits excellent corrosion resistance to coating even in a corrosive seawater environment, eliminating the need for repair painting until the ship's design life of 25 years.

[0005] The steel material described in Patent Document 2 is a corrosion-protective coated steel material that has an inorganic zinc-based paint composition-containing layer containing magnesium and granular zinc and having a thickness of 10 μm or more on the surface of the steel material, and in the cross section of the inorganic zinc-based paint composition-containing layer, the area ratio of the region where the magnesium concentration is 0.2 mass% or more relative to the total element amount of the inorganic zinc-based paint composition-containing layer is 5 to 55%. Patent Document 2 also describes that this steel material can improve corrosion resistance against chlorides without using zinc alloy powder, compared to the addition of special pigments or inhibitors.

[0006] The steel material described in Patent Document 3 is a high-strength 13Cr stainless steel thick plate containing, by mass%, 0.015-0.050% C, 0.15-0.50% Si, 0.20-0.50% Mn, 0.030% or less P, 0.012% or less S, 11.50-13.50% Cr, less than 0.20% Ni, less than 0.20% Mo, less than 0.0080% O, and 0.010-0.045% N, with the balance being Fe and impurities. The steel material has a two-phase structure consisting of 30-70% by volume ferrite phase and the balance being martensite phase, with the maximum grain size of ferrite crystal grains being 100 μm or less. Patent Document 3 also describes that this steel material has stable corrosion resistance in ordinary corrosive environments (e.g., atmospheric environments, submerged environments, etc.). [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2012-177168 [Patent Document 2] Japanese Patent Application Laid-Open No. 2017-35877 [Patent Document 3] Japanese Patent Application Laid-Open No. 2015-218383 Summary of the Invention [Problem to be solved by the invention]

[0008] Galvanic corrosion can occur in structural steel. Galvanic corrosion refers to a phenomenon in which steel comes into contact with an electrical conductor whose natural potential is more noble than that of the steel, resulting in accelerated corrosion of the steel. Here, the electrical conductor in contact with the steel may be metallic or non-metallic. For example, in belt conveyors, rails, yard moving machines (e.g., stackers, reclaimers, wheel loaders), etc. at steel mills, coke powder may come into contact with the steel. Here, coke is a non-metal but an electrical conductor, and its natural potential is more noble than that of the steel. Therefore, when the steel comes into contact with coke powder, corrosion of the steel is accelerated. In particular, when moisture is added to the contact point between the steel and the coke, etc., a battery is formed between the steel, the coke, etc., and the moisture, further accelerating corrosion of the steel.

[0009] Therefore, structural steel materials used in environments where they come into contact with electrical conductors whose natural potential is more noble than that of the steel material are required to have not only excellent corrosion resistance of the steel material itself but also excellent galvanic corrosion resistance. In this specification, "excellent galvanic corrosion resistance" means that galvanic corrosion is unlikely to occur even when the steel material comes into contact with an electrical conductor whose natural potential is more noble than that of the steel material.

[0010] According to the above Patent Documents 1 to 3, steel materials having excellent corrosion resistance in atmospheric environments and seawater corrosive environments can be obtained. However, the above Patent Documents 1 to 3 do not consider galvanic corrosion at all.

[0011] An object of the present disclosure is to provide a steel material that can form an oxide layer with low electrical conductivity on its surface during use in an environment where galvanic corrosion is a concern, thereby suppressing the occurrence of galvanic corrosion. [Means for solving the problem]

[0012] The steel material according to the present disclosure is In mass%, C: 0.005~0.200%, Si: 0.03 to 1.50% Mn: 0.05 to 3.00%, P: 0.035% or less, S: 0.035% or less, Cr: 0.80~10.00%, Cu: 0.001 to 0.500%, Ni: 0.001 to 0.500%, Mo: 0.001 to 1.500%, Sn: 0.001 to 0.500%, Al: 0 to 1.500%, W: 0 to 2.00%, and the balance being Fe and impurities, Satisfies equation (1). 2.20≦(2Cr+0.5Ni+0.5Mn) / (Cu+1)≦18.50 (1) Here, the element symbols in formula (1) are substituted with the contents of the corresponding elements in mass %. [Effects of the Invention]

[0013] The steel material according to the present disclosure can form an oxide layer with low electrical conductivity on the surface when used in an environment where galvanic corrosion is a concern, thereby making it possible to suppress the occurrence of galvanic corrosion. DETAILED DESCRIPTION OF THE INVENTION

[0014] The present inventors have conducted extensive research into improving the galvanic corrosion resistance of steel materials. As described above, galvanic corrosion occurs when a steel material comes into contact with an electrical conductor that has a more noble natural potential than the steel material. In other words, they thought that the galvanic corrosion resistance of steel materials could be improved by blocking electrical contact between the steel material and the electrical conductor. That is, as described in Patent Documents 1 and 2, forming a corrosion-resistant coating with low electrical conductivity on the surface of a steel material can block electrical contact between the steel material and the electrical conductor. In this way, it is possible to improve the galvanic corrosion resistance of steel materials.

[0015] On the other hand, even if a corrosion-protective coating with low electrical conductivity is formed on the surface of a steel material to enhance the galvanic corrosion resistance of the steel material, if a defect occurs in part of the corrosion-protective coating, galvanic corrosion may occur in the steel material. In this case, significant localized corrosion thinning may occur at the defected part. Specifically, if a defect occurs in part of the corrosion-protective coating, an electrical conductor may enter between the corrosion-protective coating and the steel material from the defected part, raising the concern that galvanic corrosion may spread over a wide area. Therefore, even if a corrosion-protective coating is formed, it is preferable that the steel material have excellent galvanic corrosion resistance.

[0016] Therefore, the present inventors investigated various methods for improving the galvanic corrosion resistance of steel materials, assuming that a portion of the corrosion protective coating is damaged. As a result, the present inventors thought that the galvanic corrosion resistance of steel materials could be improved by forming an oxide layer with low electrical conductivity on the surface of the steel material. Specifically, an oxide layer is formed on the surface of the steel material while the steel material is being used in an environment where galvanic corrosion is a concern. A method for forming the oxide layer is, for example, air oxidation. If the oxide layer formed here has low electrical conductivity, it can inhibit electrical contact between the base material and an electrical conductor. In this way, even if a portion of the corrosion protective coating is damaged, the galvanic corrosion resistance of the steel material can be improved by the oxide layer with low electrical conductivity.

[0017] Next, the inventors focused on the chemical composition of steel and investigated the formation of an oxide layer with low electrical conductivity on the surface of the steel. As a result, it was found that if the chromium (Cr) content in the chemical composition of the steel is increased, an oxide layer with relatively low electrical conductivity can be formed on the surface of the steel. Therefore, the steel according to this embodiment has a Cr content of 0.80 to 10.00%.

[0018] On the other hand, detailed studies by the present inventors revealed that when the Cr content is increased to 0.80 to 10.00%, localized corrosion is more likely to occur in steel. Therefore, the present inventors conducted detailed studies on methods for suppressing localized corrosion in steel with an increased Cr content of 0.80 to 10.00%. As a result, the present inventors found that in steel with an increased Cr content of 0.80 to 10.00%, localized corrosion can be suppressed by increasing the tin (Sn) content to 0.001 to 0.500%.

[0019] That is, if a steel material contains, in mass%, C: 0.005 to 0.200%, Si: 0.03 to 1.50%, Mn: 0.05 to 3.00%, P: 0.035% or less, S: 0.035% or less, Cr: 0.80 to 10.00%, Cu: 0.001 to 0.500%, Ni: 0.001 to 0.500%, Mo: 0.001 to 1.500%, Sn: 0.001 to 0.500%, Al: 0 to 1.500%, W: 0 to 2.00%, and the balance being Fe and impurities, the galvanic corrosion resistance of the steel material may be improved.

[0020] On the other hand, even for steel materials having the above-mentioned chemical composition, there are cases where the electrical conductivity of the oxide layer formed on the surface of the steel material does not decrease stably. If the electrical conductivity of the oxide layer formed on the surface of the steel material cannot be decreased stably, electrical contact with an electrical conductor cannot be sufficiently inhibited. As a result, when a defect occurs in the corrosion protective coating, the occurrence of galvanic corrosion due to electrical contact between the steel material and an electrical conductor cannot be sufficiently suppressed.

[0021] Therefore, the inventors have investigated various methods for reducing the electrical conductivity of the oxide layer formed during use of a steel material having the above-mentioned chemical composition, and have found that the electrical conductivity of the oxide layer formed during use can be stably reduced by satisfying the above-mentioned chemical composition and further satisfying the following formula (1): 2.20≦(2Cr+0.5Ni+0.5Mn) / (Cu+1)≦18.50 (1) Here, the element symbols in formula (1) are substituted with the contents of the corresponding elements in mass %.

[0022] Fn1 is defined as (2Cr+0.5Ni+0.5Mn) / (Cu+1). Fn1 is an index showing the electrical conductivity of an oxide layer formed on the surface of a steel material having the above-described chemical composition during use in an environment where galvanic corrosion is a concern. If Fn1 is too low, the electrical conductivity of the oxide layer formed during use cannot be stably reduced. As a result, galvanic corrosion cannot be sufficiently suppressed when damage occurs to the corrosion-protective coating. On the other hand, if Fn1 is too high, a passive film is likely to be formed on the surface of the steel material, resulting in a significant reduction in electrical conductivity. If Fn1 is too high, localized corrosion (pitting corrosion) is more likely to occur in the steel material. As a result, the corrosion resistance of the steel material is significantly reduced. Therefore, the steel material according to this embodiment has the above-described chemical composition, and Fn1 is set to 2.20 to 18.50. As a result, the steel material according to this embodiment has excellent galvanic corrosion resistance.

[0023] The details of why adjusting Fn1 to 2.20 to 18.50 can stably reduce the electrical conductivity of the oxide layer formed on the surface of a steel material during use in an environment where galvanic corrosion is a concern are not clear. However, the examples described below demonstrate that the above-mentioned chemical composition and Fn1 of 2.20 to 18.50 can stably reduce the electrical conductivity of the oxide layer formed on the surface of a steel material.

[0024] The gist of the steel material according to this embodiment, which was completed based on the above findings, is as follows.

[0025] [1] In mass%, C: 0.005~0.200%, Si: 0.03 to 1.50% Mn: 0.05 to 3.00%, P: 0.035% or less, S: 0.035% or less, Cr: 0.80~10.00%, Cu: 0.001 to 0.500%, Ni: 0.001 to 0.500%, Mo: 0.001 to 1.500%, Sn: 0.001 to 0.500%, Al: 0 to 1.500%, W: 0 to 2.00%, and the balance being Fe and impurities, Satisfying equation (1), Steel material. 2.20≦(2Cr+0.5Ni+0.5Mn) / (Cu+1)≦18.50 (1) Here, the element symbols in formula (1) are substituted with the contents of the corresponding elements in mass %.

[0026] [2] [1] The steel material according to Al: 0.001 to 1.500%, and W: Contains one or more elements selected from the group consisting of 0.01 to 2.00% Steel material.

[0027] [3] The steel material according to [1] or [2], A corrosion-resistant coating is provided on the surface of the steel material. Steel material.

[0028] [4] [3] The steel material according to The corrosion-resistant coating is Contains one or more resins selected from the group consisting of epoxy resins and modified epoxy resins, Steel material.

[0029] [5] [3] or [4], The corrosion-resistant coating is The thickness is 300 μm or more, Steel material.

[0030] The steel material according to this embodiment will be described in detail below. Unless otherwise specified, "%" for elements means mass %.

[0031] [Chemical composition] The chemical composition of the steel material according to this embodiment contains the following elements.

[0032] C: 0.005 to 0.200% Carbon (C) is an element necessary to ensure the strength of steel. If the C content is too low, sufficient strength cannot be obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the C content is too high, even if the contents of other elements are within the ranges of this embodiment, the amount of carbides becomes too large, significantly reducing the corrosion resistance and weldability of the steel. Therefore, the C content is 0.005 to 0.200%. The preferred lower limit of the C content is 0.008%, more preferably 0.010%, even more preferably 0.015%, and even more preferably 0.020%. The preferred upper limit of the C content is 0.180%, more preferably 0.160%, and even more preferably 0.150%.

[0033] Si: 0.03 to 1.50% Silicon (Si) is an element necessary for deoxidizing steel. If the Si content is too low, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Si content is too high, the toughness of the steel material will decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the Si content is 0.03 to 1.50%. A preferred lower limit for the Si content is 0.05%, more preferably 0.10%, and even more preferably 0.20%. A preferred upper limit for the Si content is 1.30%, more preferably 1.10%, and even more preferably 1.00%.

[0034] Mn: 0.05 to 3.00% Manganese (Mn) is a low-cost element that has the effect of increasing the strength of steel. If the Mn content is too low, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Mn content is too high, weldability will deteriorate even if the contents of other elements are within the ranges of this embodiment. If the Mn content is too high, sulfide-based inclusions (MnS), which serve as corrosion initiation sites, will be formed, reducing the corrosion resistance of the steel. Therefore, the Mn content is 0.05 to 3.00%. The preferred lower limit of the Mn content is 0.06%, more preferably 0.08%, and even more preferably 0.10%. The preferred upper limit of the Mn content is 2.80%, more preferably 2.50%, even more preferably 2.20%, and even more preferably 2.00%.

[0035] P:0.035% or less Phosphorus (P) is an impurity. That is, the lower limit of the P content is greater than 0%. If the P content is too high, weldability will decrease even if the contents of other elements are within the ranges of this embodiment. If the P content is too high, P will segregate at grain boundaries, reducing the corrosion resistance of the steel. Therefore, the P content is 0.035% or less. A preferred upper limit of the P content is 0.030%, more preferably 0.025%, and even more preferably 0.020%. The P content should be as low as possible. However, an extreme reduction in the P content significantly increases manufacturing costs. Therefore, considering industrial production, a preferred lower limit of the P content is 0.001%, more preferably 0.002%, and even more preferably 0.003%.

[0036] S: 0.035% or less Sulfur (S) is an impurity. That is, the lower limit of the S content is greater than 0%. If the S content is too high, even if the contents of other elements are within the ranges of this embodiment, sulfide-based compounds (MnS) that act as corrosion initiation sites are formed, thereby reducing the corrosion resistance of the steel material. Therefore, the S content is 0.035% or less. A preferred upper limit of the S content is 0.030%, more preferably 0.025%, and even more preferably 0.020%. The S content should be as low as possible. However, an extreme reduction in the S content significantly increases production costs. Therefore, considering industrial production, a preferred lower limit of the S content is 0.001%, more preferably 0.002%, and even more preferably 0.003%.

[0037] Cr: 0.80~10.00% Chromium (Cr) is an element that improves the hardenability of steel. Furthermore, in a corrosive environment, Cr forms an oxide layer on the surface of the steel, inhibiting electrical contact between the steel and an electrical conductor. As a result, the galvanic corrosion resistance of the steel is improved. If the Cr content is too low, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Cr content is too high, localized corrosion is likely to occur, even if the contents of other elements are within the ranges of this embodiment, and the corrosion resistance of the steel is reduced. Therefore, the Cr content is 0.80 to 10.00%. The preferred lower limit of the Cr content is 1.00%, more preferably 1.50%, even more preferably 2.00%, even more preferably 2.50%, even more preferably 3.00%, even more preferably 3.50%, even more preferably 4.00%, even more preferably 4.50%, and even more preferably 5.00%. The upper limit of the Cr content is preferably 9.00%, more preferably 8.00%, even more preferably 7.00%, even more preferably 6.50%, and still more preferably 6.00%.

[0038] Cu: 0.001 to 0.500% Copper (Cu) is an element that suppresses anodic dissolution of steel and enhances its corrosion resistance. If the Cu content is too low, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Cu content is too high, the above effects saturate. If the Cu content is too high, embrittlement may occur even if the contents of other elements are within the ranges of this embodiment. Therefore, the Cu content is 0.001 to 0.500%. The preferred lower limit of the Cu content is 0.005%, more preferably 0.010%, even more preferably 0.020%, and even more preferably 0.050%. The preferred upper limit of the Cu content is 0.450%, more preferably 0.400%, and even more preferably 0.350%.

[0039] Ni: 0.001 to 0.500% Nickel (Ni) is an element that suppresses anodic dissolution and improves the corrosion resistance of steel. If the Ni content is too low, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Ni content is too high, the cost of the steel increases significantly even if the contents of other elements are within the ranges of this embodiment. If the Ni content is too high, localized corrosion will be further promoted, and the corrosion resistance of the steel will actually decrease. Therefore, the Ni content is 0.001 to 0.500%. The lower limit of the Ni content is preferably 0.005%, more preferably 0.010%, even more preferably 0.020%, and even more preferably 0.050%. The upper limit of the Ni content is preferably 0.450%, even more preferably 0.400%, and even more preferably 0.350%.

[0040] Mo: 0.001 to 1.500% Molybdenum (Mo) is an element that improves the corrosion resistance of steel materials. If the Mo content is too low, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Mo content is too high, the above effects saturate. If the Mo content is too high, the production cost of steel materials increases significantly even if the contents of other elements are within the ranges of this embodiment. Therefore, the Mo content is 0.001 to 1.500%. The preferred lower limit of the Mo content is 0.005%, more preferably 0.010%, even more preferably 0.020%, and even more preferably 0.050%. The preferred upper limit of the Mo content is 1.300%, more preferably 1.000%, and even more preferably 0.800%.

[0041] Sn: 0.001 to 0.500% Tin (Sn) is an element that suppresses the deterioration of the corrosion resistance of steel materials caused by Cr and improves the corrosion resistance of steel materials. If the Sn content is too low, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Sn content is too high, Sn segregates at grain boundaries, even if the contents of other elements are within the ranges of this embodiment, and the corrosion resistance of the steel material decreases. If the Sn content is too high, the toughness of the steel material may also decrease. Therefore, the Sn content is 0.001 to 0.500%. The preferred lower limit of the Sn content is 0.002%, more preferably 0.003%, and even more preferably 0.005%. The preferred upper limit of the Sn content is 0.450%, more preferably 0.400%, even more preferably 0.350%, even more preferably 0.300%, and even more preferably 0.250%.

[0042] The balance of the chemical composition of the steel material according to the present embodiment is composed of Fe and impurities. Here, the impurities refer to substances that are mixed in from raw materials such as ore and scrap, or the manufacturing environment, during industrial production of the steel material, and are acceptable within a range that does not adversely affect the steel material according to the present embodiment.

[0043] In the steel material according to this embodiment, impurities include all kinds of elements. There may be only one type of impurity, or two or more types. Examples of impurities include V, Nb, Ti, B, N, O, Ca, Mg, Zr, rare earth elements (REM), Sb, Se, Hf, Ga, Sr, Co, and As. The following contents of these elements are permitted as impurities: V: 0-0.30%, Nb: 0-0.10%, Ti: 0-0.10%, N: 0.070% or less, O: 0.05% or less, Ca: 0-0.01%, Mg: 0-0.01%, Zr: 0-0.05%, rare earth elements (REM): 0-0.05%, B: 0-0.0100%, Sb: 0-0.08%, Se: 0-0.010%, Hf: 0-0.010%, Ga: 0-0.010%, Sr: 0-0.010%, Co: 0-0.010%, and As: 0-0.010% (However, 0.002% is excluded) .

[0044] [Optional element] The steel material according to this embodiment may further contain Al instead of a portion of Fe.

[0045] Al: 0 to 1.500% Aluminum (Al) is an optional element and may not be contained. That is, the Al content may be 0%. When contained, Al deoxidizes the steel. Even if even a small amount of Al is contained, the above effect can be obtained to some extent. However, if the Al content is too high, even if the contents of other elements are within the range of this embodiment, coarse nitrides are formed, reducing the toughness of the steel. In this case, the corrosion resistance of the steel may also be reduced. Therefore, the Al content is 0 to 1.500%. The preferred lower limit of the Al content is more than 0%, more preferably 0.001%, even more preferably 0.005%, and even more preferably 0.010%. The preferred upper limit of the Al content is 1.200%, even more preferably 1.000%. In this specification, the Al content means the total Al content.

[0046] The steel material according to this embodiment may further contain W instead of a portion of Fe.

[0047] W: 0~2.00% Tungsten (W) is an optional element and does not necessarily need to be contained. That is, the W content may be 0%. When contained, W enhances the corrosion resistance of the steel material. Even if even a small amount of W is contained, the above effect can be obtained to a certain extent. However, if the W content is too high, the above effect saturates even if the contents of other elements are within the ranges of this embodiment. In this case, the manufacturing cost of the steel material will increase significantly. Therefore, the W content is 0 to 2.00%. The preferred lower limit of the W content is more than 0%, more preferably 0.01%, even more preferably 0.05%, and even more preferably 0.10%. The preferred upper limit of the W content is 1.50%, even more preferably 1.20%, and even more preferably 1.00%.

[0048] [Formula (1)] The steel material according to this embodiment satisfies the above-mentioned chemical composition and further satisfies the following formula (1). 2.20≦(2Cr+0.5Ni+0.5Mn) / (Cu+1)≦18.50 (1) Here, the element symbols in formula (1) are substituted with the contents of the corresponding elements in mass %.

[0049] Fn1 (=(2Cr+0.5Ni+0.5Mn) / (Cu+1)) is an index that indicates the electrical conductivity of an oxide layer that forms on the surface of a steel material having the above-described chemical composition when the steel material is used in an environment where galvanic corrosion is a concern. If Fn1 is too low, the electrical conductivity of the oxide layer that forms during use cannot be stably reduced, and galvanic corrosion cannot be sufficiently suppressed. On the other hand, if Fn1 is too high, a passive film is likely to be formed on the surface of the steel material, which significantly reduces the electrical conductivity. If Fn1 is too high, localized corrosion (pitting corrosion) is more likely to occur in the steel material. As a result, the corrosion resistance of the steel material is significantly reduced. Therefore, the steel material according to this embodiment has the above-described chemical composition, and furthermore, Fn1 is set to 2.20 to 18.50. As a result, the steel material according to this embodiment has excellent galvanic corrosion resistance.

[0050] The upper limit of Fn1 is preferably 18.0, more preferably 17.0, even more preferably 16.0, and even more preferably 15.0. The lower limit of Fn1 is preferably 2.25, more preferably 2.50, even more preferably 3.00, even more preferably 3.50, even more preferably 4.00, even more preferably 5.00, even more preferably 6.00, and even more preferably 6.50.

[0051] [Electrical conductivity of oxide layer] In this embodiment, the steel material has the above-mentioned chemical composition, and furthermore, Fn1 is 2.20 to 18.50. As a result, when used in an environment where galvanic corrosion is a concern, the steel material according to this embodiment can form an oxide layer with low electrical conductivity on the surface layer, thereby suppressing the occurrence of galvanic corrosion. In this embodiment, the level of electrical conductivity of the oxide layer formed on the surface layer can be evaluated by the following method.

[0052] Specifically, a plate-shaped test piece is prepared from the steel material according to this embodiment. The size of the plate-shaped test piece is not particularly limited as long as it is larger than the axial cross section of the test cell described below. The size of the plate-shaped test piece is, for example, 40 mm long x 40 mm wide x 3 mm thick. A conductor for resistance measurement is connected to the underside of the prepared plate-shaped test piece. The connection method is not particularly limited, and the conductor may be connected by, for example, solder welding. The plate-shaped test piece may be placed on a stainless steel plate without directly connecting the conductor to the underside. In other words, as long as conductivity is ensured for performing the measurements described below, the method for ensuring conductivity is not limited. After conductivity is ensured, a test cell is placed on the plate-shaped test piece. The test cell is cylindrical and made of an insulator, and the axial direction of the test cell corresponds to the thickness direction of the plate-shaped test piece. The cross section of the test cell is, for example, a circle with a diameter of 32 mm.

[0053] Coke is introduced into the test cell and layered on the plate-shaped test piece. The thickness of the coke is not particularly limited, but for example, 10 mm. The particle size of the coke is not particularly limited, but for example, 0.7 mm or less. Furthermore, ion-exchanged water is added to the test cell to saturate the coke with water. A stainless steel electrode with the same shape as the cross section of the test cell is pressed against the coke. The pressure with which the stainless steel electrode is pressed is not particularly limited, but for example, 100 N. The stainless steel electrode is made of SUS304 as specified in JIS G 4304 (2012), for example.

[0054] The initial resistance value R0 (Ω) is measured between the plate-shaped test piece and a stainless steel electrode pressed against the coke from above. The test environment is then maintained at 40°C for 90 days. After 90 days, the resistance value R (Ω) is measured between the plate-shaped test piece and a stainless steel electrode pressed against the coke from above. The ratio of the determined resistance value R to the initial resistance value R0 is defined as the resistance change rate.

[0055] In this embodiment, if the resistance change rate obtained by the above method is 10.0 or more, it is determined that an oxide layer with low electrical conductivity has been formed on the surface during use in an environment where galvanic corrosion is a concern.

[0056] [Anti-corrosion coating] The steel material according to this embodiment may have a corrosion-resistant coating on its surface. When a corrosion-resistant coating is formed on the surface of the steel material, the galvanic corrosion resistance of the steel material is further improved. Furthermore, the environmental isolation effect of the corrosion-resistant coating can be expected to have the effect of extending the period until galvanic corrosion occurs. If the electrical conductivity of the corrosion-resistant coating is low, the galvanic corrosion resistance of the steel material can be effectively improved. Therefore, the corrosion-resistant coating is preferably an insulator. From the viewpoint of galvanic corrosion resistance and the corrosion resistance of the steel material itself, the thickness of the corrosion-resistant coating is preferably 300 μm or more.

[0057] The anticorrosion coating is not particularly limited, and any known anticorrosion coating can be used. The anticorrosion coating may contain, for example, a resin. In this case, the resin may be, for example, one or more selected from the group consisting of epoxy resins and modified epoxy resins.

[0058] The thickness of a corrosion-protective film can be measured using the following method. The probe of an electromagnetic induction type film thickness gauge is placed in contact with the steel material on which the corrosion-protective film has been formed. The probe has an electromagnet, and when a magnetic body is brought close, electromagnetic induction occurs, and the voltage changes depending on the distance between the probe and the magnetic body. The thickness of the corrosion-protective film is determined from the change in voltage. Measurements are taken at 10 random locations, and the arithmetic average of the measurements is taken as the thickness of the corrosion-protective film.

[0059] [Steel use] The steel material according to this embodiment is not limited in its applications and can be widely used in environments where galvanic corrosion is a concern. For example, it can be used as a construction steel material for belt conveyors, rail sections, etc. in steelworks. For example, it can also be used in heavy machinery such as yard movers (stackers, reclaimers, wheel loaders, etc.). For example, it can also be used in conveyor scales, hopper scales, and load cells used therein. Furthermore, it can be widely used in environments where galvanic corrosion of electrical conductors other than coke is a concern. For example, it can be used in environments where galvanic corrosion of carbon fiber as an electrical conductor is a concern. In this case, it can be used in places where it comes into contact with carbon fiber sheets used as reinforcing materials, etc.

[0060] [Manufacturing method] A method for manufacturing a steel material according to this embodiment will be described. Below, a method for manufacturing a steel plate will be described as an example of the steel material according to this embodiment. The method for manufacturing a steel plate includes a step of preparing a material (material preparation step) and a step of hot working the material to manufacture a steel plate (hot working step). Note that the manufacturing method according to this embodiment is not limited to the manufacturing method described below. Each step will be described in detail below.

[0061] [Material preparation process] In the material preparation step, a material is produced using molten steel having the above-mentioned chemical composition. The method for producing the material is not particularly limited and may be a well-known method. Specifically, a cast piece (slab, bloom, or billet) may be produced using the molten steel by a continuous casting method. An ingot may be produced using the molten steel by an ingot casting method. If necessary, the slab, bloom, or ingot may be subjected to blooming to produce a billet. The material (slab, bloom, or billet) is produced by the above steps.

[0062] [Hot processing process] In the hot working process, the prepared material is hot worked to produce steel. Specifically, the material is first heated in a heating furnace. The heating temperature is not particularly limited, but is, for example, 1100 to 1300°C. The material extracted from the heating furnace is hot worked to produce steel plate. The hot working method is not particularly limited and may be a well-known method. For example, hot rolling or hot forging may be performed.

[0063] According to the above manufacturing method, the steel material according to this embodiment can be manufactured. In the above manufacturing method, a method for manufacturing a steel plate has been described as an example. However, the steel material according to this embodiment may be a steel bar or other shape. Similar to the above manufacturing method, a manufacturing method for a shape other than a steel plate also includes, for example, a material preparation step and a hot working step. However, the above manufacturing method is only an example, and the steel material may be manufactured by other manufacturing methods. Hereinafter, the present disclosure will be described more specifically with reference to examples. [Example]

[0064] Molten steels having the chemical compositions shown in Tables 1 and 2 were produced. The Fn1 value for each test number was determined from the chemical composition of the molten steel for each test number and the above-mentioned definition of Fn1 (=(2Cr+0.5Ni+0.5Mn) / (Cu+1)). The determined Fn1 values ​​are shown in Tables 1 and 2. Note that "-" in Tables 1 and 2 indicates that the content of each element is at the impurity level. Furthermore, the molten steel for each test number contained impurities not listed in Tables 1 and 2 within the ranges described in this specification. Specifically, in Test No. 14, in addition to the element contents listed in Tables 1 and 2, impurities including 0.15% V, 0.04% Nb, 0.0010% B, and 0.03% N were contained. Furthermore, in Test No. 26, in addition to the element contents listed in Table 1, 0.20% V, 0.02% Nb, and 0.0005% B were contained. Furthermore, in test number 43, in addition to the element contents shown in Table 1, V: 0.18% and Nb: 0.06% were contained.

[0065] [Table 1]

[0066] [Table 2]

[0067] The molten steel of each test number listed in Tables 1 and 2 was cast to produce a raw material ingot. The raw material ingot of each test number was heated to 1050 to 1180°C, and then hot-rolled using a continuous rolling mill to produce a steel plate of each test number. The steel plate of each test number produced in this example had a plate thickness of approximately 6 mm.

[0068] [Evaluation test] The steel sheets having the respective test numbers manufactured as described above were subjected to a resistance change rate measurement test and a pitting corrosion evaluation test, which will be described below.

[0069] [Resistance change rate measurement test] Plate-shaped test specimens were prepared from the steel plates of each test number, and the resistance change rate was measured. Specifically, plate-shaped test specimens measuring 40 mm in length, 40 mm in width, and 3 mm in thickness were prepared from the steel plates of each test number and placed on a SUS304 stainless steel plate. Furthermore, a test cell made of polyvinyl chloride resin (cylindrical shape with a cross-sectional diameter of 32 mm) was placed on top of the plate-shaped test specimen of each test number. Coke was layered to a depth of 10 mm in the test cell, and the coke was saturated with ion-exchanged water. The particle size of the coke was 0.7 mm or less.

[0070] A SUS304 stainless steel electrode was pressed against the coke in the test cell with a pressure of 100 N, and the initial resistance R0 (Ω) was measured between the plate test piece and the stainless steel electrode. Furthermore, after maintaining the test environment at 40°C for 90 days, the resistance R (Ω) was measured between the plate test piece and the stainless steel electrode. The ratio of the resistance R to the initial resistance R0 for each test number was defined as the resistance change rate. The obtained resistance change rates for each test number are shown in Table 1.

[0071] [Pitting corrosion evaluation test] The plate test pieces with each test number used in the resistance change rate measurement test described above were evaluated for the occurrence of pitting corrosion. Specifically, in the resistance change rate measurement test described above, the plate test pieces were removed after being held at 40°C for 90 days. The plate test pieces with each test number were visually inspected to evaluate the occurrence of pitting corrosion. The occurrence of pitting corrosion for each test number is shown in the "Pitting Corrosion" column of Table 2.

[0072] [Evaluation results] Referring to Tables 1 and 2, the steel sheets of test numbers 1 to 32 had appropriate chemical compositions and Fn1 values ​​of 2.20 to 18.50. As a result, the resistance change rate was 10.0 or more, indicating that an oxide layer with low electrical conductivity can be formed on the surface during use in an environment where galvanic corrosion is a concern. Furthermore, no pitting corrosion was confirmed in the pitting corrosion evaluation test. In other words, the steel sheets of test numbers 1 to 32 were able to suppress the occurrence of galvanic corrosion.

[0073] On the other hand, the steel sheets of test numbers 33 to 35 had too low Fn1. As a result, the resistance change rate was less than 10.0, and an oxide layer with low electrical conductivity could not be formed on the surface during use in an environment where galvanic corrosion is a concern. In other words, the steel sheets of test numbers 33 to 35 were not able to suppress the occurrence of galvanic corrosion.

[0074] The steel sheets of test numbers 36 and 37 had too high a Cr content. Furthermore, Fn1 was too high. As a result, the resistance change rate was less than 10.0, and an oxide layer with low electrical conductivity could not be formed on the surface during use in an environment where galvanic corrosion was a concern. Furthermore, the occurrence of pitting corrosion was confirmed. In other words, the steel sheets of test numbers 36 and 37 were not able to suppress the occurrence of galvanic corrosion, and pitting corrosion also occurred.

[0075] The steel plate of test number 38 had too high a Cr content. Furthermore, the Ni content was too high. Furthermore, the Fn1 was too high. As a result, the resistance change rate was less than 10.0, and an oxide layer with low electrical conductivity could not be formed on the surface during use in an environment where galvanic corrosion was a concern. Furthermore, the occurrence of pitting corrosion was confirmed. In other words, the steel plate of test number 38 was not able to suppress the occurrence of galvanic corrosion, and pitting corrosion also occurred.

[0076] The steel plate of test number 39 had too high Fn1. As a result, the resistance change rate was less than 10.0, and an oxide layer with low electrical conductivity could not be formed on the surface during use in an environment where galvanic corrosion is a concern. Furthermore, the occurrence of pitting corrosion was confirmed. In other words, the steel plate of test number 39 was not able to suppress the occurrence of galvanic corrosion, and pitting corrosion also occurred.

[0077] The steel plate with test number 40 had too high Fn1, which resulted in the occurrence of pitting corrosion.

[0078] The steel plate with test number 41 had too high a Ni content. Furthermore, Fn1 was too high. As a result, pitting corrosion was observed.

[0079] The steel sheet of test number 42 had a too low Cr content. Furthermore, Fn1 was too low. As a result, the resistance change rate was less than 10.0, and an oxide layer with low electrical conductivity could not be formed on the surface during use in an environment where galvanic corrosion was a concern. In other words, the steel sheet of test number 42 was not able to suppress the occurrence of galvanic corrosion.

[0080] The Sn content of the steel plates of test numbers 43 and 44 was too low, and as a result, pitting corrosion was observed.

[0081] The embodiments of the present disclosure have been described above. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and can be implemented by appropriately modifying the above-described embodiments within the scope of the present disclosure.

Claims

1. In mass%, C: 0.005-0.200%, Si: 0.03 to 1.50%, Mn: 0.05-3.00%, P: 0.035% or less, S: 0.035% or less, Cr: 0.80-7.00%, Cu: 0.001 to 0.500%, Ni: 0.001 to 0.500%, Mo: 0.001 to 1.500%, Sn: 0.001 to 0.500%, Al: 0-1.500%, W: 0 to 2.00%, and the balance being Fe and impurities; Satisfying formula (1), Steel material. 5.96≦(2Cr+0.5Ni+0.5Mn) / (Cu+1)≦18.50 (1) Here, the element symbols in formula (1) are substituted with the contents of the corresponding elements in mass %.

2. The steel material according to claim 1, Al: 0.001 to 1.500%, and W: Contains one or more elements selected from the group consisting of 0.01 to 2.00%; Steel material.

3. The steel material according to claim 1 or claim 2, A corrosion-resistant coating is provided on the surface of the steel material. Steel material.

4. The steel material according to claim 3, The corrosion-resistant coating is Contains one or more resins selected from the group consisting of epoxy resins and modified epoxy resins, Steel material.

5. The steel material according to claim 3 or claim 4, The corrosion-resistant coating is The thickness is 300 μm or more. Steel material.

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