Steel material for civil engineering and construction, and structure

Abstract

Provided is a steel material for civil engineering and construction, and having excellent coating film durability and excellent corrosion resistance under a coating film. This steel material for civil engineering and construction has a component composition which contains, in mass%, 0.020-0.200% of C, 0.05-1.00% of Si, 0.20-2.00% of Mn, not more than 0.050% of P, not more than 0.0100% of S, 0.001-0.100% of Al, 0.030-1.00% of Cu, 0.010-1.00% of Ni, and 0.010-0.200% of Sb, and in which the remaining portion is Fe and unavoidable impurities, and the ratio Cu / Sb of the Cu content (mass%) with respect to the Sb content (mass%) is more than 3.33.

Description

Civil engineering and construction steel materials and structures

[0001] The present invention relates to a steel material for civil engineering and construction and a structure using the same.

[0002] In steelworks, mineral resources such as coal and ore, and materials such as coke and sintered ore produced from mineral resources, are used as raw materials for iron production. Some structures in steelworks and the like are used in environments where the above-mentioned raw materials are used. Examples include unloaders that unload coal and ore, belt conveyors that transport coal, ore, coke, or sintered ore, and hoppers that store and transport coal, ore, coke, or sintered ore. Furthermore, some structures are used in environments where coal, ore, coke, and sintered ore dust is blown in and accumulates. Furthermore, in thermal power plants, coal and the like are used as raw materials for thermal power generation. Examples of structures used in environments where raw materials are used in thermal power plants include unloaders that unload coal transported by large bulk carriers, belt conveyors that transport coal, and coal storage buildings.

[0003] However, in environments where coal (coking coal), ore, coke, sintered ore, etc. are present, the steel materials used in structures are susceptible to corrosion. Therefore, there have been reports of facility accidents due to corrosion deterioration of the above-mentioned structures. Furthermore, if rust layers formed in storage tanks, such as in hoppers that store and transport coal, peel off and get mixed in with the coal, depending on the amount, this could lead to operational problems in refining facilities and plants that use coal as a raw material.

[0004] Generally, a coating is applied to the surface of steel materials for civil engineering and construction used in corrosive environments. The presence of a coating can extend the period until corrosion of the steel begins. However, if the coating's durability is low, the effect will not last, and frequent painting for repairs will be required. Furthermore, even if a coating is applied, if corrosion progresses beneath the coating, the strength will not be maintained due to thinning. Therefore, it is necessary to improve the coating durability of steel materials for civil engineering and construction and to suppress under-coat corrosion.

[0005] As steel materials for use in corrosive environments, for example, Patent Documents 1 and 2 propose corrosion-resistant steel materials for use in the holds of coal and ore carriers in the presence of coal, etc. Patent Document 1 describes that the inside of the holds of coal and ore carriers is a corrosive environment in which the pH value of condensed water drops to about 2 to 3, and that when the corrosion-resistant steel material contains Sn as an alloy element, the corrosion resistance of the painted parts in that corrosive environment is significantly improved.

[0006] Furthermore, Patent Document 2 describes that Sn is contained in the steel material in order to ensure corrosion resistance, and that the Cu content is set to 0.05% or less in order to prevent cracks during rolling due to the inclusion of Sn.

[0007] Patent Document 3 proposes a corrosion-resistant steel material for coal unloading, storage, and transportation facilities that is inhibited from rusting in a coal corrosive environment and prevents rust peeling. Specifically, in a coal corrosive environment, dilute sulfuric acid is not produced, and Cl － In such an environment, the addition of Cr and Al to steel is considered to be effective in preventing localized corrosion, rusting, and rust peeling.

[0008] JP 2007-262555 A JP 2008-174768 A JP 2012-092437 A

[0009] However, it has been found that when the corrosion-resistant steel materials for coal and ore carrier holds proposed in Patent Documents 1 and 2 are used in civil engineering and construction applications in severely corrosive environments such as thermal power plants and steelworks, sufficient coating durability and corrosion resistance beneath the coating are not obtained.

[0010] Similarly, when the steel material described in Patent Document 3 is used for civil engineering and construction applications in severely corrosive environments such as thermal power plants and steelworks, it has been found that sufficient coating durability and corrosion resistance beneath the coating cannot be obtained. Furthermore, the steel material described in Patent Document 3 has a high content of alloy elements, resulting in high manufacturing costs.

[0011] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a steel material for civil engineering and construction that has excellent coating film durability and corrosion resistance beneath the coating film even in extremely severe corrosive environments where one or both of fossil fuels and ores are used.

[0012] As a result of extensive research, the present inventors have found that the above object can be achieved by employing the following configuration, and have completed the present invention.

[0013] (1) A steel material for civil engineering and construction having a chemical composition containing, by mass%, C: 0.020 to 0.200%, Si: 0.05 to 1.00%, Mn: 0.20 to 2.00%, P: 0.050% or less, S: 0.0100% or less, Al: 0.001 to 0.100%, Cu: 0.030 to 1.00%, Ni: 0.010 to 1.00%, and Sb: 0.010 to 0.200%, with the balance consisting of Fe and unavoidable impurities, and in which the ratio of the Cu content (% by mass) to the Sb content (% by mass), Cu / Sb, exceeds 3.33.

[0014] (2) The steel material for civil engineering and construction according to (1), wherein the composition further contains, in mass%, at least one selected from the group consisting of Cr: 0.20% or less, Sn: 0.020% or less, Mo: 0.200% or less, W: 0.200% or less, Nb: 0.100% or less, and Ti: 0.100% or less.

[0015] (3) A steel material for civil engineering and construction according to (1) or (2) above, in which Cu / Sb is 6.66 or less.

[0016] (4) A steel material for civil engineering and construction according to any one of (1) to (3) above, which is for factory equipment that uses one or both of fossil fuels and ores.

[0017] (5) A steel material for civil engineering and construction according to any one of (1) to (4) above, having a coating film on the surface.

[0018] (6) The coating resistance after the saltwater immersion test in which the coating was immersed in a 3 wt% sodium chloride aqueous solution for 48 hours was 10 ７ Ω cm ２ The steel material for civil engineering and construction according to (5) above.

[0019] (7) The steel material for civil engineering and construction according to (5) or (6), wherein the coating film has a corrosion-resistant base layer, a primer layer, and a top coat layer, the corrosion-resistant base layer is a layer using zinc-rich paint, the primer layer is a layer using epoxy resin paint, and the top coat layer is a layer using polyurethane resin paint or fluororesin paint.

[0020] (8) A structure using the steel material for civil engineering and construction described in any one of (1) to (7).

[0021] The steel material of the present invention has excellent coating durability and corrosion resistance under the coating, and exhibits excellent performance even in extremely severe corrosive environments such as those in which fossil fuels and / or ores are used. Therefore, the steel material for civil engineering and construction of the present invention can be suitably used as a material for various structures, and in particular, can be suitably used as a steel material for factory equipment in which fossil fuels and / or ores are used.

[0022] Figure 1 shows the SO2 emissions from coking coal, coke, ore, and sinter. ４ ２－ Figure 2 is a graph showing the amount of Cl leaching from coking coal, coke, ore and sinter. － 3 is a graph showing the amount of leaching of SO ４ ２－ 1 is a graph showing the relationship between the amount of leaching and the average amount of corrosion.

[0023] As described above, conventional steel materials such as those proposed in Patent Documents 1 to 3 did not have sufficient coating durability and corrosion resistance under the coating in a corrosive environment where fossil fuels or ores are used. The inventors therefore clarified the reasons for this through the following experiments.

[0024] (Experiment 1) In order to investigate the corrosive environment in which factory equipment that handles fossil fuels and ores is placed, the amount of ions leaching from dust generated in factory equipment that handles these materials was investigated using the following procedure.

[0025] First, dust was collected from the ore-fall section of a steelworks line that handles raw coal, coke, ore, and sintered ore. Next, 20 g of each collected dust was immersed in 200 ml of distilled water for 72 hours. After filtering out the solid content, the filtrate was adjusted to a constant volume of 250 ml using distilled water. Then, SO contained in the filtrate was analyzed by ion chromatography. ４ ２－ and Cl － The amount of various ions such as SO from each dust was measured. ４ ２－ and Cl － The amounts of leached ions are shown in Figures 1 and 2, respectively. Here, the amounts of leached ions are shown as values ​​converted per kg of dust. ４ ２－ and Cl － The ion with the highest leaching amount was SO ４ ２－ It was found that. ４ ２－ and Cl － Analysis was also conducted for ions other than those listed above, but the amount of each ion in the filtrate was below the detection limit or at the same level as in tap water.

[0026] (Experiment 2) Next, SO from dust ４ ２－ The relationship between the amount of leaching and the amount of corrosion of steel material was investigated. General structural steel was used as the steel material, and the dust was collected in the same manner as in Experiment 1.

[0027] The dust was piled up to a height of 10 mm on the surface of the steel material. In this state, a wet-dry cycle test was conducted in which wet conditions of 25°C and 95% RH and dry conditions of 35°C and 60% RH were alternately repeated. The test period was 28 days, and distilled water was poured once a week until the piled up dust was submerged.

[0028] After the test, rust was removed with a pickling solution containing an inhibitor, and the mass of the steel material was then measured to determine the difference in mass before and after the test. The mass difference was then divided by the area of ​​the steel material surface where dust had accumulated, and the density of the steel was calculated as 7.87 g / cm. ３ The value obtained by dividing by 1 / 2 was taken as the average corrosion amount.４ ２－ The amount of leaching of SO was measured in the same manner as in Experiment 1. ４ ２－ The relationship between the amount of leaching and the average amount of corrosion is shown in Figure 3.

[0029] As can be seen from FIG. ４ ２－ In the region where the leaching amount of SO ４ ２－ The amount of corrosion of steel increased with the increase in the amount of leaching of SO. ４ ２－ In the region where the leaching amount was 100 mg / kg or more, two types of dust were present: dust showing a relatively high average corrosion amount of around 0.15 mm (plotted with black squares), and dust showing a relatively low average corrosion amount of around 0.05 mm (plotted with black circles).

[0030] Therefore, we investigated the differences between these dusts and found that dust with a high average corrosion amount had a higher electrical conductivity than dust with a low average corrosion amount. Specifically, the electrical conductivity of dust with an average corrosion amount of around 0.15 mm was about 1 S / m, and the electrical conductivity of dust with an average corrosion amount of around 0.05 mm was about 0.01 S / m.

[0031] From this, the inventors have concluded that if fossil fuels or ores have high electrical conductivity due to the manufacturing process, SO ４ ２－ In addition to corrosion caused by the dust particles, it is believed that galvanic corrosion occurred between the dust particles and the steel, accelerating the corrosion of the steel. In this case, the surface area of ​​the dust particles is much larger than that of the steel, and they are responsible for the reduction of oxygen, which is the reaction that dissolves iron, on their surface, so the corrosion rate is determined by the rate at which the iron dissolves.

[0032] The difference in electrical conductivity is thought to be due to the difference in the components contained in fossil fuels and ores. For example, in the case of coal and coke, the higher the coalification level and the more graphitized the material is, the higher the electrical conductivity. In the case of ores, Fe ３ O ４ The conductivity varies depending on the content of iron and metal.

[0033] From the above, to use steel in a corrosive environment, it is necessary to use it in an acidic environment, especially in SO ４ ２－ and Cl － In addition, corrosion resistance is required against galvanic corrosion that can occur between the highly electrically conductive dust and steel.

[0034] (Component Composition) The component composition of the steel material according to the present invention will be described below. In the following description, "%" as a unit of content represents "mass %" unless otherwise specified.

[0035] C: 0.020 to 0.200% C is an element that has the effect of increasing the strength of steel. To achieve this effect, the C content is set to 0.020% or more, preferably 0.050% or more. On the other hand, if the C content is higher than 0.200%, the weldability and toughness of the welded joint decrease. Therefore, the C content is set to 0.200% or less, preferably 0.150% or less.

[0036] Si: 0.05 to 1.00% Si is an element that acts as a deoxidizer in the steelmaking process. Si also has the effect of increasing the strength of steel. Furthermore, by including Si, the corrosion products formed on the steel surface have a high corrosion resistance effect, improving the corrosion resistance of the steel in an acidic environment. Therefore, the Si content is set to 0.05% or more, preferably 0.20% or more. On the other hand, if the Si content is higher than 1.00%, the toughness of the steel deteriorates. Therefore, the Si content is set to 1.00% or less, preferably 0.70% or less, and more preferably 0.40% or less.

[0037] Mn: 0.20 to 2.00% Mn is an element that has the effect of increasing the strength of steel at low cost. Furthermore, Mn also has the effect of preventing hot embrittlement. Therefore, the Mn content is set to 0.20% or more, preferably 0.80% or more. On the other hand, if the Mn content is higher than 2.00%, the toughness and weldability of the steel will decrease. Therefore, the Mn content is set to 2.00% or less, preferably 1.40% or less.

[0038] P: 0.050% or less P is a harmful element that segregates at grain boundaries, degrading not only the toughness of steel but also the weldability and toughness of welds. Therefore, the P content is set to 0.050% or less, preferably 0.030% or less, and more preferably 0.025% or less. On the other hand, since it is desirable to reduce P as much as possible, the lower limit of the P content is not particularly limited and may be 0%. However, excessive reduction leads to increased manufacturing costs and reduced productivity. Therefore, from the viewpoint of industrial production, it is preferable that the P content be 0.001% or more.

[0039] S: 0.0100% or less S is a harmful element that not only deteriorates the toughness and weldability of steel materials, but also forms MnS, which acts as a starting point for localized corrosion, thereby reducing localized corrosion resistance. Therefore, the S content is set to 0.0100% or less, preferably 0.0070% or less, and more preferably 0.0050% or less. On the other hand, since it is desirable to reduce S as much as possible, the lower limit of the S content is not particularly limited and may be 0%. However, excessive reduction leads to increased manufacturing costs and reduced productivity. Therefore, from the viewpoint of industrial production, it is preferable that the S content be 0.0010% or more.

[0040] Al: 0.001 to 0.100% Al is an element that acts as a deoxidizer and is widely used in molten steel deoxidation processes. To obtain the deoxidizing effect, the Al content is set to 0.001% or more, preferably 0.005% or more, and more preferably 0.010% or more. On the other hand, if the Al content is higher than 0.100%, the toughness of the weld decreases. Therefore, the Al content is set to 0.100% or less, preferably 0.050% or less.

[0041] Cu: 0.030 to 1.00% Cu precipitates as metallic Cu at the interface between the corrosion product and the base steel in a corrosive environment, thereby suppressing the dissolution reaction of the base steel. In addition, Cu forms CuFeO below the S-enriched portion of the corrosion product layer. ４By precipitating as Cu, the diffusion of S into the base steel side is suppressed and corrosion by S is prevented. Therefore, the Cu content is set to 0.030% or more, preferably more than 0.050%, and more preferably 0.060% or more. On the other hand, if the Cu content is higher than 1.00%, the toughness and weldability of the steel material decrease. Therefore, the Cu content is set to 1.00% or less, preferably 0.50% or less, and more preferably 0.35% or less.

[0042] Ni: 0.010 to 1.00% Ni densifies the corrosion products formed on the surface of the steel material and prevents H from being transferred to the base steel. ２ O, O ２ , S.O. ４ ２－ , Cl － It is an element that has the effect of suppressing the diffusion of H. ２ O and O ２ is essential for corrosion, and SO ４ ２－ and Cl － promotes corrosion. Therefore, suppressing the diffusion of these chemical species improves the corrosion resistance of steel. To achieve this effect, the Ni content is set to 0.010% or more. Furthermore, from the viewpoint of preventing cracking due to Cu, the Ni content is preferably set to 1 / 3 or more of the Cu content, and more preferably 1 / 2 or more of the Cu content. On the other hand, if the Ni content is higher than 1.00%, the effect saturates and the cost increases. Therefore, the Ni content is set to 1.00% or less, preferably 0.50% or less.

[0043] Sb: 0.010 to 0.200% Sb is an element that is concentrated near the interface between corrosion products and the base steel in a low pH environment (acidic environment). In addition, Sb has a large hydrogen overvoltage, so when Sb precipitates, the hydrogen generation reaction at the precipitated portion is suppressed, improving corrosion resistance in an acidic environment. In addition, in the present invention, it is important that the steel material contains both Sb and Cu. By including Sb together with Cu, the corrosion products become dense, and the hydrogen generation reaction at the base steel is suppressed. ２ O, O ２ , S.O. ４ ２－ , Cl －The corrosion products function as a physical barrier, so even if the number of potential cathode sites increases due to corrosion factors such as dust, the number of potential anode sites on the steel substrate is limited. As a result, corrosion resistance against galvanic corrosion is improved. In other words, the dense corrosion products prevent H ２ This prevents O from diffusing into the base steel and prevents Fe from eluting from the base steel, improving corrosion resistance against galvanic corrosion. Furthermore, if a coating film is present on the surface of the steel, moisture that permeates the coating film is retained for a long time. In this case, Sb acts as an inhibitor and suppresses corrosion. To achieve these effects, the Sb content is set to 0.010% or more. On the other hand, if the Sb content is higher than 0.200%, the toughness of the steel decreases. Furthermore, surface cracking is also promoted. Therefore, the Sb content is set to 0.200% or less, preferably 0.100% or less.

[0044] The composition of one embodiment of the present invention is composed of the above elements, with the balance being Fe and inevitable impurities. The inevitable impurities are impurities that are inevitably mixed in from raw materials, the manufacturing process, or manufacturing equipment, and are allowed to be present to the extent that they do not impair the objectives of the present invention. Examples of the raw materials include iron ore, reduced iron, and scrap. Examples of the inevitable impurities include O, N, H, Zn, Pb, As, and Bi.

[0045] Cu / Sb: More than 3.33 In order to obtain excellent coating durability and corrosion resistance under the coating in a corrosive environment where fossil fuels or ores are used, the ratio of the Cu content (mass%) to the Sb content (mass%), Cu / Sb, needs to be more than 3.33. The reason for this will be explained below.

[0046] As explained above, the coexistence of Cu and Sb can improve corrosion resistance. In particular, when the coating film is scratched and exposed to repeated dry and wet cycles in an acidic environment, Cu and Sb combine to form an intermetallic compound, Cu. ２ Sb is formed, which results in improved corrosion resistance. However, if the ratio of Cu content to Sb content is low, a sufficient amount of Cu is not formed. ２ Sb cannot be formed.２ In order to obtain the above-mentioned effect by forming Sb, the ratio of the Cu content (mass %) to the Sb content (mass %), Cu / Sb, is set to more than 3.33.

[0047] On the other hand, although there is no particular upper limit for the Cu / Sb ratio, from the viewpoint of suppressing the occurrence of spot-like blisters, a Cu / Sb ratio of 6.66 or less is preferable. The reason for this is as follows: moisture that permeates the coating film is retained beneath the coating film, so the base steel / coating film interface remains wet for a long period of time. In such an environment, protection by dense rust is difficult, so corrosion must be suppressed by the inhibitor action of Sb. However, if the amount of Cu relative to Sb is excessive, Cu will bond with Sb, reducing the amount of Sb that can act as an inhibitor. However, if the Cu / Sb ratio is 6.66 or less, the inhibitor action of Sb can effectively suppress the occurrence of spot-like blisters.

[0048] In another embodiment of the present invention, the above-mentioned composition may optionally further contain at least one selected from the group consisting of Cr: 0.20% or less, Sn: 0.020% or less, Mo: 0.200% or less, W: 0.200% or less, Nb: 0.100% or less, and Ti: 0.100% or less.

[0049] Cr: 0.20% or less Cr is Cl － In addition, from the viewpoint of production costs, the addition of a large amount of SO should be avoided. ４ ２－ In fossil fuel or mineral environments where leaching of Cr is high, Cr causes acidification and significantly reduces corrosion resistance. Therefore, the Cr content is set to 0.20% or less, preferably 0.10% or less. On the other hand, since it is desirable to reduce Cr as much as possible, the lower limit of the Cr content is not particularly limited and may be 0%.

[0050] Sn: 0.020% or less Sn is an element that concentrates near the interface between corrosion products and the base steel in low-pH environments (acidic environments). Furthermore, because Sn has a large hydrogen overvoltage, its precipitation suppresses the hydrogen generation reaction at the precipitated area, improving corrosion resistance in acidic environments. Therefore, Sn can be added at any amount depending on the desired corrosion resistance. However, while Sn alone has the same effect as Sb, it does not form an intermetallic compound with Cu like Sb, which improves corrosion resistance, and therefore does not exhibit a synergistic effect. Therefore, sufficient corrosion resistance cannot be exhibited in strongly acidic environments. In addition, Sn has the effect of promoting surface cracking caused by Cu. Therefore, when Sn is contained, the Sn content is set to 0.020% or less, preferably less than 0.005%, and more preferably 0.004% or less. On the other hand, the lower limit of the Sn content is not particularly limited and may be 0%.

[0051] Mo: 0.200% or less Mo generates oxyacids when eluted from steel. The generated oxyacids electrically repel anions, preventing the anions from penetrating the surface of the base steel. As a result, corrosion resistance in acidic to neutral environments is improved. Furthermore, Mo has the structure of FeMoO ４ It also has the effect of improving corrosion resistance by generating sparingly soluble corrosion products such as those mentioned above. Therefore, Mo can be added at any amount depending on the required corrosion resistance. However, if the Mo content exceeds 0.200%, not only will the above effect saturate, but costs will also increase. Therefore, when Mo is added, the Mo content is set to 0.200% or less, preferably 0.100% or less. On the other hand, the lower limit of the Mo content is not particularly limited and may be 0%. However, from the viewpoint of fully obtaining the corrosion resistance improving effect of Mo, it is preferable that the Mo content be 0.010% or more.

[0052] W: 0.200% or less When eluted from steel, W generates oxyacids. The generated oxyacids electrically repel anions, preventing the anions from penetrating the surface of the base steel. As a result, corrosion resistance in acidic to neutral environments is improved. Furthermore, W is a component of FeWO ４ It also has the effect of improving corrosion resistance by producing insoluble corrosion products such as WO ４２－ By including ４ ２－ and Cl － Therefore, W can be optionally contained depending on the required corrosion resistance. However, if the W content exceeds 0.200%, not only will the above effect saturate, but costs will also increase. Therefore, when W is contained, the W content is set to 0.200% or less, preferably 0.100% or less. On the other hand, the lower limit of the W content is not particularly limited and may be 0%. However, from the viewpoint of fully obtaining the effect of W in improving corrosion resistance, it is preferable that the W content be 0.010% or more.

[0053] Nb: 0.100% or less Nb is an element that increases the strength of steel. Therefore, Nb can be added at any amount depending on the required strength. However, if the Nb content is higher than 0.100%, the toughness of the steel decreases. Therefore, when Nb is added, the Nb content is set to 0.100% or less, preferably 0.020% or less. On the other hand, the lower limit of the Nb content is not particularly limited and may be 0%, but in order to fully obtain the above effects of Nb, the Nb content is set to 0.0010% or more, preferably 0.0050%.

[0054] Ti: 0.100% or less Like Nb, Ti is an element that increases the strength of steel. Therefore, Ti can be contained arbitrarily depending on the required strength. However, if the Ti content is higher than 0.100%, the toughness of the steel decreases. Therefore, when Ti is contained, the Ti content is set to 0.100% or less, preferably 0.020% or less. On the other hand, the lower limit of the Ti content is not particularly limited and may be 0%, but in order to fully obtain the above effects of Ti, the Ti content is set to 0.0010% or more, preferably 0.0050% or more.

[0055] The steel material for civil engineering and construction of the present invention having the above-mentioned component composition is excellent in coating durability and corrosion resistance under the coating.

[0056] Generally, coatings have the function of insulating and protecting the steel surface from a corrosive environment. However, the coatings become scratched with use, and this function deteriorates. In particular, in factories, including steelworks, coatings are easily scratched by mechanical impacts such as falling ore. Then, highly conductive dust comes into contact with and accumulates on the scratched areas, accelerating corrosion. Therefore, even if a coating is applied, it is not enough to ensure sufficient corrosion resistance.

[0057] Therefore, in the present invention, the composition of the steel material is controlled as described above, and in particular, the Cu / Sb ratio is controlled within a specific range, thereby enabling the formation of a dense rust layer. The dense rust layer reduces the number of corrosion active sites by suppressing the diffusion of water to the steel surface, thereby suppressing corrosion. In addition, the dense rust layer has the effect of suppressing galvanic corrosion.

[0058] As a result, the steel material for civil engineering and construction of the present invention has excellent coating durability and corrosion resistance under the coating even in extremely severe corrosive environments where fossil fuels and / or ores are used.

[0059] Therefore, the steel material of the present invention can be suitably used as a steel material for civil engineering and construction. Here, "for civil engineering and construction" means that it can be used for either or both of civil engineering and construction.

[0060] The steel material for civil engineering and construction of the present invention can be particularly suitably used as a material (structural steel material) for factory facilities that use either or both of fossil fuels and ores. Typical examples of factory facilities that use either or both of fossil fuels and ores include facilities in steelworks.

[0061] In steelworks, there are points and areas where the pH of water drops below 1, creating a much more severe corrosive environment than that of ships. For example, SO from unburned powder (powder of unburned material remaining after coal combustion) ４ ２－ The amount of leaching can be more than 10,000 mg per kg, and the SO ４ ２－This is significantly higher than the leaching amount (approximately 1000 mg per kg). Therefore, the steel material of the present invention is particularly suitable for use in steelworks equipment, and is most suitable for use in equipment that uses unburned powder. An example of equipment that uses unburned powder is a belt conveyor line that collects unburned powder.

[0062] The tensile strength of the steel material for civil engineering and construction of the present invention is not particularly limited and may be any value. However, steel materials with a strength of 400 MPa or higher, such as SS400 (400 MPa) and SM490 (490 MPa), are generally used as steel materials for civil engineering and construction. Therefore, the tensile strength of the steel material is preferably 400 MPa or higher, and more preferably 490 MPa or higher. On the other hand, the upper limit of the tensile strength is not particularly limited, and may be, for example, 610 MPa or lower, or 510 MPa or lower. The tensile strength of the steel material can be measured in accordance with JIS Z 2241:2011.

[0063] The shape of the steel material for civil engineering and construction of the present invention is not particularly limited and may be any shape. For example, the steel material for civil engineering and construction may be a steel plate, a section steel, or a steel bar. Examples of the section steel include H-beams, I-beams, T-beams, angle steel, and channel steel. Examples of the steel bar include round steel and square steel.

[0064] [Coating film] The steel material in one embodiment of the present invention has a coating film on its surface. By providing the coating film, the steel material surface can be insulated from a corrosive environment, thereby further improving corrosion resistance. The coating film is not particularly limited, and any coating film can be used.

[0065] Furthermore, there is no particular limitation on the properties of the coating film. However, from the viewpoint of preventing breakage of the coating film and occurrence of red rust even in long-term use, it is preferable that the coating film resistance after a saltwater immersion test in which the coating film is immersed in a 3 wt % aqueous solution of sodium chloride for 48 hours is 10 ７ Ω cm ２ The above is preferable, and the reason for this will be explained below.

[0066] As mentioned above, in the present invention, the composition of the steel material is controlled, and in particular, the Cu / Sb ratio is controlled within a specific range, thereby enabling the formation of a dense rust layer, and as a result, excellent coating durability and corrosion resistance beneath the coating are realized.

[0067] However, after long-term use, the coating film may be destroyed by corrosion, causing red rust. This is because when the coating film is exposed to water, such as during rainfall, the water diffuses through the coating film and reaches the interface between the coating film and the steel material, where it reacts with the oxygen and SO contained in the water. ４ ２－ This is because corrosion occurs due to the water penetration. When the weather is fine, moisture diffuses into the atmosphere, forming a rust layer at the interface between the paint film and the steel. Although this corrosion due to water penetration is slight, if it occurs repeatedly over a long period of time, eventually "rust bumps" will form. The resulting stress will then destroy the paint film, causing red rust to form.

[0068] Therefore, by using a coating film with low water permeability, it is possible to prevent the destruction of the coating film due to such corrosion and the occurrence of red rust.

[0069] In order to use the coated steel sheet stably, it is desirable that the coating does not peel off due to corrosion for as long as possible. Specifically, it is desirable that the coating does not break down and red rust does not leach out through a one-year test in which the weather changes through the four seasons. To satisfy this condition, it is necessary to ensure that the coating resistance is 10% or less after a saltwater immersion test in which the steel sheet is immersed in a 3 wt% aqueous solution of sodium chloride for 48 hours. ７ Ω cm ２ It is preferable to use a coating film having the above properties.

[0070] By combining a steel material that satisfies the requirements of the present invention with a coating that satisfies the above requirements, excellent coating durability and corrosion resistance under the coating can be obtained, and the occurrence of red rust can be suppressed for a long period of time, making it extremely suitable for use as a steel material for civil engineering and construction.

[0071] To obtain a coating film whose coating film resistance satisfies the above conditions, the type of resin constituting the coating film, the film thickness, etc., can be adjusted. In other words, the coating film resistance is basically expressed as the product of the volume resistivity (Ω cm), which is a physical property inherent to the coating film, and the film thickness (cm). Therefore, the coating film resistance can be adjusted by selecting a paint that can form a coating film with a high volume resistivity or by increasing the film thickness.

[0072] The coating resistance can be measured by AC impedance measurement based on the two-point frequency method. More specifically, it can be measured by the method described in the examples.

[0073] The coating film may, for example, have a corrosion-resistant base layer, a primer layer, and a topcoat layer. Usually, the coating film has the corrosion-resistant base layer, the primer layer, and the topcoat layer in this order from the steel sheet side. An intermediate coating layer may also be present between the primer layer and the topcoat layer.

[0074] - Corrosion-resistant primer layer: The corrosion-resistant primer layer is preferably a layer using zinc-rich paint (zinc-rich paint layer). Either inorganic or organic zinc-rich paint can be used as the zinc-rich paint. That is, the corrosion-resistant primer layer may be either an inorganic zinc-rich paint layer or an organic zinc-rich paint layer. Because the zinc-rich paint layer contains Zn, the sacrificial corrosion protection effect of the Zn improves paint film durability, and rust formation on steel can be suppressed for a long period of time as long as the Zn remains. Furthermore, when the area of ​​paint blister is small, the corrosion protection effect is provided to the entire blistered area, thereby reducing the corrosion depth. In the case of an inorganic zinc-rich paint layer, applying a mist coat after the applied inorganic zinc-rich paint has dried can fill voids formed by the volatile components of the inorganic zinc-rich paint. Therefore, inorganic zinc-rich paint is preferred from the perspective of further improving corrosion resistance. However, applying the mist coat increases the number of work steps and increases costs, so organic zinc-rich paint is preferred from a cost perspective. From the standpoint of corrosion resistance and cost, the contractor can choose either inorganic zinc-rich paint or organic zinc-rich paint as the paint for the corrosion-resistant base layer.

[0075] The thickness of the corrosion-resistant base layer is not particularly limited, but is preferably 10 μm or more, more preferably 25 μm or more, in order to prevent a shortening of the period during which the sacrificial corrosion protection effect of Zn is obtained and further enhance the corrosion resistance improvement effect of the corrosion-resistant base layer. On the other hand, if the thickness of the corrosion-resistant base layer exceeds 100 μm, cracks may occur in the corrosion-resistant base layer, and the sacrificial corrosion protection effect may be reduced. In addition, dripping is likely to occur when applying the paint for the corrosion-resistant base layer, making it difficult to control the film thickness. Therefore, the thickness of the corrosion-resistant base layer is preferably 100 μm or less.

[0076] Furthermore, when applying a coating for the anticorrosion base layer, it is preferable to clean the steel surface to a Class 1 clean surface in the case of new construction. Specifically, shot blasting or sand blasting can be used. This creates a rough surface, enhancing the anchoring effect of the anticorrosion base layer. On the other hand, in the case of repairs, it is preferable to clean the surface to a Class 3 clean surface in order to remove the brittle rust layer.

[0077] Undercoat layer: The undercoat layer is preferably a layer using an epoxy resin paint (epoxy resin coating film). By forming an epoxy resin coating film on the anticorrosion base layer, the durability of the coating film can be improved.

[0078] The thickness of the primer layer is not particularly limited, but from the viewpoint of further improving corrosion resistance, it is preferably 60 μm or more. Furthermore, if the thickness is 60 μm or more, it is easy to control the thickness. On the other hand, if the thickness is thicker than 150 μm, the cost increases. This is because a primer layer with a thickness exceeding 150 μm must be applied in three or more coats, which increases the number of steps. Therefore, from the viewpoint of cost, it is preferable that the thickness of the primer layer is 150 μm or less.

[0079] Topcoat layer: The topcoat layer is preferably a layer using a polyurethane resin paint or a fluororesin paint. That is, the topcoat layer is preferably a polyurethane resin coating film or a fluororesin coating film. By forming the topcoat layer on the undercoat layer, the durability of the coating film can be further improved. Furthermore, deterioration of the coating film due to ultraviolet rays can also be suppressed.

[0080] The thickness of the topcoat layer is not particularly limited, but from the viewpoint of corrosion resistance and workability, it is preferably 60 to 150 μm. By setting the thickness within this range, corrosion resistance equivalent to that of the anticorrosion base layer can be achieved. The topcoat layer may be formed by one or multiple coats. When multiple coats are applied, corrosion from pinholes formed in the topcoat layer can be suppressed, but the number of days for application increases, so the installer can decide the number of coats from the viewpoint of corrosion resistance and cost.

[0081] The thickness of each of the anticorrosion base layer, the undercoat layer, and the topcoat layer can be measured using an electromagnetic film thickness meter.

[0082] [Structure] The structure according to the present invention is a structure using the above-mentioned steel material, and may be a structure used in a corrosive environment, and may be a civil engineering structure or an architectural structure. The structure has a low risk of equipment accidents and operational troubles due to corrosion deterioration, and also requires less frequent painting for repairs.

[0083] [Manufacturing conditions] Next, suitable manufacturing conditions for the steel material of the present invention will be described. The steel material is preferably manufactured by hot rolling a steel material having the above-mentioned composition obtained by continuous casting or the like, either as is or after cooling, followed by reheating, and then cooling the material.

[0084] First, a steel material can be obtained using molten steel having the above-mentioned composition. The method for obtaining the steel material is not limited, and continuous casting or the like can be used. Next, the steel material may be subjected to hot rolling as is, or may be reheated after cooling and then subjected to hot rolling. It is preferable to appropriately control the reduction ratio of the hot rolling from the viewpoint of ensuring mechanical properties. Furthermore, the finishing temperature of the hot rolling is preferably 750°C or higher, because a temperature lower than 750°C increases deformation resistance and causes shape defects. Furthermore, it is preferable to perform controlled cooling after hot rolling, specifically, to cool the steel to 600°C or lower at a cooling rate of 150°C / min or higher. Furthermore, heat treatment may be performed to obtain the mechanical properties required for steel materials for civil engineering and construction.

[0085] The present invention will be described below using examples, but the present invention is not limited to the following examples.

[0086] Example 1 First, molten steel having the chemical composition shown in Table 1 was melted and a steel slab was obtained by continuous casting. The steel slab was then charged into a heating furnace and heated to 1200°C, after which it was hot-rolled at a finishing temperature of 800°C to obtain a steel material (hot-rolled steel sheet). The steel material was then cooled to 600°C or less at a cooling rate of 150°C / min or more. Steel Material No. 21 corresponds to SM490A rolled steel sheet for welded structures as specified in JIS G 3106. As described below, the pass / fail judgment was made based on the results of Steel Material No. 21. Of the obtained steel materials, Steel Material No. 28 exhibited surface cracks and was therefore not subjected to further evaluation.

[0087]

[0088] Next, test materials were prepared from the steel material in the following manner.

[0089] First, test pieces measuring 2 mm thick x 70 mm wide x 150 mm long were cut out from the steel material, and the test pieces were shot blasted to remove surface scale and oil. The test pieces were then coated with the types of coatings shown in Table 2 to prepare the test materials. In Table 2, epoxy and polyurethane refer to epoxy resin paint and polyurethane resin paint, respectively. Spray coating was used to form the coating, and the coating was allowed to harden for one month after application. The coating thickness of the resulting coating was measured using an electromagnetic coating thickness meter. The resulting coating thicknesses are shown in Table 2.

[0090] (Corrosion Test) Next, a corrosion test was carried out on the test material to evaluate the corrosion resistance of the steel material under the coating film and the durability of the coating film.

[0091] Prior to conducting the corrosion test, the conditions under which the corrosion test should be conducted were first considered. Specifically, 100 locations in a steelworks that use at least one material selected from coal, coke, ore, and sintered ore were selected, and steel material (SM490A) was installed at each location. After one year had passed, the amount of corrosion (annual corrosion amount) of the steel material was measured. The location with the largest annual corrosion amount was a location where dust containing impurities with a large amount of corrosive ions accumulates in the steelmaking process. This dust was dust derived from coke (hereinafter referred to as coke dust). Therefore, it was decided to conduct the corrosion test by depositing coke dust collected from the location on the test material for a long period of time. Note that at the location where the coke dust was collected, SO ４ ２－ The amount of leaching was so large that when pH test paper was used on the wet dust after rainfall, the pH showed less than 1. Furthermore, coke is electrically conductive, which causes galvanic corrosion.

[0092] A corrosion test was conducted under the conditions determined as described above. Specifically, a scribe measuring 70 mm in length and 0.45 to 0.55 mm in width was made in the center of the test material parallel to the long side using a cutter, exposing the steel substrate. The collected coke dust was spread to a height of 10 mm in a container box with drainage holes, and the test material was then placed on top of it. The coke dust was then further covered on top of the test material to a height of 35 mm. To prevent depletion of leached ions, the coke dust was replaced every two months. After replacement, the coke dust was collected and the amount of ions leached from the coke dust was analyzed using inductively coupled plasma (ICP) atomic emission spectroscopy. The results confirmed that the amount of leached ions had decreased by approximately 20% since the start of the test. The corrosion test lasted for one year.

[0093] After the corrosion test, the test specimens were collected and evaluated for corrosion depth and blister area of ​​the coating film according to the following procedure.

[0094] Corrosion depth: After the corrosion test, the surface coating was removed from the test material using a coating remover (Neo River S-911). Then, the rust remaining on the surface of the test material was removed using an acid pickling solution (HCl and H) specified in ISO 8407. ２O was mixed in a 1:1 ratio and removed with a pickling solution containing 3.5 g / L of hexamethylenetetramine as an inhibitor. Then, the depth of the corroded area (corrosion depth) on the surface of the test material was measured at 100 μm intervals in both the vertical and horizontal directions using a laser displacement meter. From the measurement results, the volume of metal loss due to corrosion (corrosion volume) was calculated, and the area of ​​the region where the corrosion depth exceeded a predetermined threshold was calculated as the corrosion area using commercially available image software such as Adobe Photoshop (registered trademark). The average corrosion depth was calculated by dividing the corrosion volume by the corrosion area. The maximum corrosion depth was defined as the corrosion depth at the deepest point in the corroded area.

[0095] Blister Area: The recovered test material after the corrosion test was photographed in a dark room with a light shining on one side to highlight the blister area, and the blister area was calculated using commercially available image software such as Adobe Photoshop (registered trademark).

[0096] The results are shown in Table 2. The measurement results for the reference test material No. 21 were as follows: Average corrosion depth: 718 μm / y or less Maximum corrosion depth: 1517 μm / y or less Blister area: 166.9 mm ２ / 10mm / y or less

[0097] Therefore, the pass / fail of the "corrosion resistance under the coating film" and "coating film durability" of each test material was judged based on 90% of the measured values ​​for the above test material No. 21. Specifically, if the average corrosion depth was 646 μm / y or less and the maximum corrosion depth was 1365 μm / y or less, the "corrosion resistance under the coating film" was judged to be pass. In addition, if the blister area was 150.2 mm ２ If the corrosion resistance was 10 mm / y or less, the "coating durability" was judged to be acceptable. All of the steel materials that satisfied the conditions of the present invention had excellent corrosion resistance under the coating and coating durability.

[0098] - Point-like blisters In addition to corrosion occurring at the scribed portions of these test pieces, point-like blisters occurring due to corrosion caused by moisture permeating through the coating surface were also evaluated. Specifically, first, the area where point-like blisters occurred was calculated using Adobe Photoshop (registered trademark) in the same manner as the blister area described above. Next, the point-like blisters occurrence area rate (%) was calculated by dividing the area where point-like blisters occurred by the total area excluding the blister area at the scribed portion. If the point-like blisters occurrence area rate (%) was less than 10%, it was determined that the occurrence of point-like blisters was suppressed. The occurrence of point-like blisters was suppressed in test materials with a Cu / Sb ratio of 6.66 or less.

[0099]

[0100] (Example 2) Next, the following test was conducted to evaluate the effect of coating resistance on the generation of red rust. First, test pieces measuring 2 mm thick x 70 mm wide x 150 mm long were cut out from the steel No. 11 used in Example 1 above, and the surfaces of the test pieces were shot blasted to remove surface scale and oil. Then, coatings of the types shown in Table 3 were formed on the surfaces of the test pieces, and these were used as test materials.

[0101] The types of coatings in Table 3 represent coatings formed using the following paints: Epoxy: Epoxy resin paint Epoxy + aluminum: Epoxy aluminum paint (epoxy paint filled with aluminum flakes) Polyurethane: Polyurethane resin paint Slag lead: Paint made by compounding alkaline powder containing blast furnace slag with a special rust inhibitor of calcium nitrite and a modified synthetic resin emulsion as a binder Fluorine: Fluorine resin paint Silicon epoxy: Silicon modified epoxy resin Phthalic acid: Phthalic acid (alkyd) resin

[0102] The coating was formed by spray coating, and the coating was left to harden for one month.

[0103] Coating Resistance The coating resistance of each of the obtained test materials after a saltwater immersion test was measured using the following method. First, the test materials on which the coating film had been formed were immersed in a 3 wt % aqueous sodium chloride solution for 48 hours. Thereafter, the coating resistance was measured by AC impedance measurement based on the two-point frequency method. A corrosion monitor SICM-714B (Shrinks Corporation) was used for the measurement. The measurement conditions were a high frequency of 10 kHz and a low frequency of 10 mHz. The measurement was performed after 48 hours of immersion so that the coating film had sufficiently absorbed water and the resistance value would stabilize.

[0104] Meanwhile, a corrosion test was carried out for each of the test materials in a coke dust environment for one year under the same conditions as in Example 1. During the corrosion test, the presence or absence of red rust was checked every month, and the time until red rust appeared was determined.

[0105] In a 3% saltwater immersion test, the coating resistance after 48 hours was 10 ７ Ω cm ２ The test materials exceeding this value did not experience corrosion to the extent that the coating was destroyed, and no red rust developed, even when exposed to the coke dust environment for one year.

[0106]

[0107] (Example 3) Next, coating films of the types and thicknesses shown in Table 4 were formed on each of the steel materials Nos. 1, 14, 21, and 22 used in Example 1. In Table 4, epoxy, polyurethane, fluorine, inorganic Zn, and organic Zn refer to epoxy resin paint, polyurethane resin paint, fluororesin paint, inorganic zinc-rich paint, and organic zinc-rich paint, respectively. Corrosion resistance and coating durability were evaluated in the same manner as described above. The evaluation results are also shown in Table 4.

[0108] Compared to a test material using steel No. 21 and the same type of coating, if both the average corrosion depth and the maximum corrosion depth were 10% or more smaller, the corrosion resistance was evaluated as excellent, and if the blister area was 10% or more smaller, the coating durability was evaluated as excellent. Even when the coating was changed, the steel material according to the present invention had excellent corrosion resistance and coating durability. Furthermore, by using zinc-rich paint as the corrosion-resistant primer layer, the corrosion resistance and coating durability were further improved.

[0109]

Claims

1. A steel material for civil engineering and construction having a chemical composition containing, by mass%, C: 0.020 to 0.200%, Si: 0.05 to 1.00%, Mn: 0.20 to 2.00%, P: 0.050% or less, S: 0.0100% or less, Al: 0.001 to 0.100%, Cu: 0.030 to 1.00%, Ni: 0.010 to 1.00%, and Sb: 0.010 to 0.200%, with the balance consisting of Fe and unavoidable impurities, and in which the ratio of the Cu content (mass%) to the Sb content (mass%), Cu / Sb, exceeds 3.

33.

2. A steel material for civil engineering and construction as described in claim 1, wherein the chemical composition further contains, in mass percent, at least one selected from the group consisting of Cr: 0.20% or less, Sn: 0.020% or less, Mo: 0.200% or less, W: 0.200% or less, Nb: 0.100% or less, and Ti: 0.100% or less.

3. Steel material for civil engineering and construction according to claim 1 or 2, in which Cu / Sb is 6.66 or less.

4. The steel material for civil engineering and construction according to any one of claims 1 to 3, which is for factory equipment that uses one or both of fossil fuels and ores.

5. A steel material for civil engineering and construction according to any one of claims 1 to 4, having a coating film on the surface.

6. After the saltwater immersion test in which the coating film is immersed in a 3 wt% sodium chloride aqueous solution for 48 hours, the coating film resistance is 10 ７ Ω cm ２ The steel material for civil engineering and construction according to claim 5 .

7. A steel material for civil engineering and construction according to claim 5 or 6, wherein the coating film has a corrosion-resistant base layer, a primer layer, and a top coat layer, the corrosion-resistant base layer being a layer using zinc-rich paint, the primer layer being a layer using epoxy resin paint, and the top coat layer being a layer using polyurethane resin paint or fluororesin paint.

8. A structure using the steel material for civil engineering and construction according to any one of claims 1 to 7.

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