steel

Abstract

To provide a steel that exhibits corrosion resistance in a weak acid environment.SOLUTION: A steel has a chemical composition consisting of, in mass%, C: 0.20% or less, Si: 1.0% or less, Mn: 3.0% or less, P: 0.050% or less, S: 0.030% or less, In: 0.005-0.20%, Al: 0.10% or less, with the balance being Fe and impurities.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a steel material. [Background technology]

[0002] In the flue gas systems of thermal power plant boilers and waste incineration facilities, a drop in the temperature of the exhaust gas causes condensed water to form from the water vapor contained in the gas. This condensed water is a weakly acidic aqueous solution containing dissolved sulfur oxides, hydrogen chloride, and carbon dioxide contained in the exhaust gas, and corrosion caused by this weak acidity can be a problem.

[0003] Furthermore, these components contained in exhaust gas react with oxygen and water vapor in the air to produce sulfuric acid and nitric acid, which dissolve in rainwater and lower its pH, resulting in acid rain. Water saturated with carbon dioxide has a pH of 5.6, and rain with a pH lower than this is called acid rain. The annual average pH of rainwater across Japan is below 5. Therefore, although not as severe as an acid dew point environment, there are concerns about corrosion by weak acids in environments where rainwater directly splashes onto the steel used in rainwater drainage facilities.

[0004] Meanwhile, in recent years, the development of infrastructure in Southeast Asia and subtropical regions has led to a strong demand for steel for civil engineering and construction. Examples of steel for civil engineering and construction include steel sheet piles and steel pipe piles, which are partially buried in the soil as foundations for civil engineering and construction. The soil found in these regions is weakly acidic, containing acid sulfates that are known to be highly corrosive to steel, raising concerns about corrosion of steel by the weak acid.

[0005] As steel materials having excellent corrosion resistance in such acid corrosion environments, for example, Patent Document 1 discloses a steel material having a coating metal on a surface exposed to the corrosive environment, and Patent Document 2 discloses a steel material having an Sn-containing layer on the surface of the steel material. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2001-164336 [Patent Document 2] Japanese Patent Application Publication No. 2017-14577 Summary of the Invention [Problem to be solved by the invention]

[0007] The steel material disclosed in Patent Document 1 has excellent corrosion resistance at worked parts in various corrosive environments, but there is still room for improvement in corrosion resistance in acid dew-point corrosion environments. Also, the steel material disclosed in Patent Document 2 has excellent corrosion resistance in strong acid environments with a pH of less than 2 and weak acid environments with a pH of 2 to 4, but since it requires application of a coating containing Sn, there is still room for improvement from the viewpoint of manufacturability.

[0008] Furthermore, conventional steel materials with excellent acid dew-point corrosion resistance exhibit excellent corrosion resistance against dew-point corrosion that occurs in strongly acidic environments with a pH of 2 or less, but in weakly acidic environments with a pH of 3 to 5, there is no significant difference in corrosion resistance compared to ordinary steel.

[0009] An object of the present invention is to solve the above problems and to provide a steel material that exhibits excellent corrosion resistance in a weakly acidic environment. [Means for solving the problem]

[0010] The present invention has been made to solve the above problems, and the gist of the present invention is the following steel material.

[0011] (1) Chemical composition, in mass%, C: 0.20% or less, Si: 1.0% or less, Mn: 3.0% or less, P: 0.050% or less, S: 0.030% or less, In: 0.005~0.20%, Al: 0.10% or less, The balance is Fe and impurities. Steel material.

[0012] (2) The chemical composition contains, in mass %, a part of the Fe replaced by Cu: 1.0% or less, Ni: 1.0% or less, Cr: 1.0% or less, Mo: 1.0% or less W: 1.0% or less, Sb: 0.30% or less, Co: 1.0% or less, As: 0.30% or less, Ce: 0.50% or less, Bi: 0.10% or less, Se: 0.50% or less, Pb: 0.50% or less, Hf: 0.20% or less, Zn: 0.10% or less, Ga: 0.10% or less, Sr: 0.020% or less, Ba: 0.020% or less, Ge: 0.10% or less, Sc: 0.010% or less, and Sm: 0.010% or less, It contains one or more selected from The steel material described in (1) above.

[0013] (3) The chemical composition contains, in mass %, a part of the Fe replaced by Ti: 0.20% or less, Zr: 0.20% or less, Nb: 0.10% or less, V: 0.50% or less, B: 0.010% or less, Ta: 0.10% or less, Te: 0.50% or less, Y: 0.10% or less, La: 0.10% or less, Nd: 0.010% or less, Ca: 0.010% or less, Mg: 0.010% or less, and REM: 0.0150% or less, It contains one or more selected from The steel material according to (1) or (2) above.

[0014] (4) At least a part of the surface of the steel material has been subjected to anticorrosion treatment. The steel material according to any one of (1) to (3) above. [Effects of the Invention]

[0015] According to the present invention, a steel material exhibiting excellent corrosion resistance in a weakly acidic environment can be obtained. DETAILED DESCRIPTION OF THE INVENTION

[0016] In a weakly acidic environment, corrosion of steel proceeds through the following anodic reaction: Fe → Fe 2+ +2e - (Fe dissolution reaction)

[0017] Furthermore, since the protective effect of the rust layer cannot be expected in a weakly acidic environment, slowing down the anodic dissolution reaction of the steel itself is effective in improving corrosion resistance. In other words, it is important to suppress the anodic dissolution reaction in a weakly acidic aqueous solution.

[0018] Furthermore, the following reduction reaction of hydrogen ions proceeds, accelerating the anodic reaction of Fe dissolution: 2H + +2e - →H2

[0019] In order to suppress the anodic dissolution reaction, it is effective to add Sn to steel. However, while Sn is very effective in a strongly acidic environment with a pH of less than 2, there is still room for improvement in suppressing the anodic dissolution reaction in a weakly acidic environment.

[0020] Based on this corrosion mechanism, the present inventors have conducted detailed research into the relationship between various metal elements and anodic dissolution reactions in order to improve corrosion resistance in weakly acidic environments, and have obtained the following findings (a) to (c).

[0021] (a) Indium forms cations in a corrosive environment. 3+ It dissolves as a fluorine-containing compound and acts as an inhibitor in acid chloride solutions, suppressing corrosion.

[0022] (b) The rate of hydrogen evolution reaction on an In surface is slower than that on an Fe surface. 3+ By forming an extremely thin metallic In layer on the steel surface in the operating environment through underpotential deposition (UPD), the anodic dissolution reaction of Fe can be significantly suppressed. This can significantly improve corrosion resistance even with trace amounts of In.

[0023] (c) Such In 3+ Unlike Sn, the effect of UPD on improving corrosion resistance is most effectively exhibited in the pH range of 3 to 5. Therefore, by adding In, it is possible to improve corrosion resistance in a weakly acidic environment.

[0024] The present invention was made based on the above findings. Each of the features of the present invention will be described in detail below.

[0025] (A) Chemical composition The reasons for limiting the content of each element are as follows: In the following description, "%" in the content means "% by mass."

[0026] C: 0.20% or less C is an effective element for ensuring the strength of the material. However, excessive content significantly reduces weldability. Furthermore, as the C content increases, the amount of cementite that acts as a cathode and promotes corrosion in a pH-decreasing environment increases, reducing corrosion resistance. Therefore, the C content is set to 0.20% or less. The C content is preferably 0.18% or less, and more preferably 0.16% or less. To achieve the above effects, the C content is preferably 0.02% or more, and more preferably 0.05% or more.

[0027] Si: 1.0% or less Silicon is an effective element for deoxidation. However, excessive content of silicon impairs the toughness of the base material and welded joints. Therefore, the silicon content is set to 1.0% or less. The silicon content is preferably set to 0.80% or less, and more preferably set to 0.60% or less. To achieve the above effect, the silicon content is preferably set to 0.03% or more, and more preferably set to 0.05% or more.

[0028] Mn:3.0% or less Mn is a low-cost element that increases the strength of steel. However, excessive Mn content deteriorates weldability and joint toughness. Therefore, the Mn content is set to 3.0% or less. The Mn content is preferably 2.5% or less, and more preferably 2.0% or less. To achieve the above effects, the Mn content is preferably 0.20% or more, and more preferably 0.40% or more.

[0029] P:0.050% or less P is an element present as an impurity in steel. It reduces acid resistance and, in chloride corrosion environments, reduces the pH of the corrosion interface, reducing corrosion resistance. Furthermore, it reduces weldability and the toughness of the weld heat-affected zone, so the lower the P content, the better. Therefore, the P content is set to 0.050% or less. The P content is preferably 0.030% or less, and more preferably 0.010% or less, less than 0.0050%. There is no need to specify a lower limit for the P content; in other words, the P content may be 0%, but excessive reductions increase steelmaking costs. Therefore, the P content may be set to 0.0001% or more.

[0030] S: 0.030% or less S is an element present as an impurity in steel. S forms MnS in steel, which acts as a starting point for corrosion. Excessive S content significantly reduces corrosion resistance. Therefore, the S content is set to 0.030% or less. The S content is preferably set to 0.025% or less, and more preferably set to 0.020% or less. There is no need to specify a lower limit for the S content; in other words, the S content may be 0%, but excessive reductions lead to increased steelmaking costs. Therefore, the S content may be set to 0.0001% or more.

[0031] In: 0.005 to 0.20% In in an acidic environment, 3+ It dissolves as In and acts as an inhibitor in acidic chloride solutions, suppressing corrosion. 3+ Indium precipitates as a monoatomic layer on the steel surface by UPD, significantly suppressing the anodic dissolution reaction of steel, and even a small amount can significantly improve corrosion resistance. However, excessive In content not only saturates this effect, but also deteriorates the toughness of the base material. Therefore, the In content is set to 0.005 to 0.20%. The In content is preferably 0.008% or more, and more preferably 0.010% or more. The In content is also preferably 0.15% or less, and more preferably 0.10% or less.

[0032] Al: 0.10% or less Al is an effective element for deoxidizing steel. However, excessive Al content not only reduces corrosion resistance in low pH environments, thereby reducing corrosion resistance in chloride corrosion environments, but also reduces toughness due to the coarsening of nitrides. Therefore, the Al content is set to 0.10% or less. The Al content is preferably 0.080% or less, and more preferably 0.060% or less. To obtain the deoxidizing effect of Al, the Al content is preferably 0.005% or more, more preferably 0.010% or more, and even more preferably 0.030% or more.

[0033] The steel material according to the present invention has the above-mentioned chemical composition, with the balance being Fe and impurities. Here, the impurities refer to components that are mixed in due to various factors in the manufacturing process, including raw materials such as ores and scraps, when industrially manufacturing steel material, and are acceptable within a range that does not adversely affect the present invention.

[0034] In the chemical composition of the steel material of the present invention, one or more elements selected from the following may be contained in place of a portion of Fe within the ranges shown below. The reasons for limiting each element will be explained below.

[0035] Cu: 1.0% or less Cu has the effect of improving corrosion resistance by suppressing the anodic dissolution of steel in a low pH environment, so it can be added as needed. However, excessive addition not only saturates the effect but also causes embrittlement. Therefore, the Cu content is set to 1.0% or less. To stably obtain the above effect, the Cu content is preferably set to 0.02% or more, and more preferably 0.03% or more.

[0036] Ni: 1.0% or less Like Cu, Ni has the effect of suppressing the anodic dissolution of steel in a low pH environment, thereby improving corrosion resistance, and can be added as needed. However, excessive Ni content not only saturates the effect, but also leads to a significant increase in costs. Therefore, the Ni content is set to 1.0% or less. The Ni content is preferably set to 0.80% or less. To stably obtain the above effect, the Ni content is preferably set to 0.01% or more, and more preferably 0.02% or more.

[0037] Cr:1.0% or less Cr has the effect of improving corrosion resistance, so it can be added as needed. However, excessive addition reduces acid resistance, which may result in reduced corrosion resistance in chloride-rich environments. On the other hand, a Cr content of 1.0% or less does not result in a reduction in acid resistance, so the Cr content is set to 1.0% or less. The Cr content is preferably 0.80% or less. To stably obtain the effect of improving corrosion resistance, the Cr content is preferably 0.01% or more, and more preferably 0.02% or more.

[0038] Mo: 1.0% or less Mo dissolves and forms oxyanion MoO4 2- Mo is an element that adsorbs to corrosion products on the steel in the form of Mo and has the effect of inhibiting chloride ions from reaching the steel surface, so it can be added as needed. However, if added in excess, not only will the effect saturate, but the cost of the steel will increase significantly. Therefore, the Mo content is set to 1.0% or less. The Mo content is preferably set to 0.70% or less. To stably obtain the above effect, the Mo content is preferably set to 0.01% or more, and more preferably 0.02% or more.

[0039] W: 1.0% or less W, like Mo, dissolves and forms oxyanion WO4 2- W exists in the form of tungsten and is an element that has the effect of inhibiting chloride ions from reaching the steel surface, so it can be added as needed. However, if it is added in excess, not only will the effect saturate, but the cost of the steel will increase significantly. Therefore, the W content is set to 1.0% or less. The W content is preferably set to 0.70% or less. To stably obtain the above effect, the W content is preferably set to 0.01% or more, and more preferably 0.02% or more.

[0040] Sb: 0.30% or less Sb is an element that improves corrosion resistance in acidic environments. It suppresses the anodic dissolution reaction of steel in low pH environments and also inhibits the hydrogen gas generation reaction and Fe 3+Sb can be added as needed because it improves corrosion resistance in chloride environments by suppressing the reduction reaction of Sb. However, excessive Sb content significantly deteriorates toughness. Therefore, the Sb content is set to 0.30% or less. The Sb content is preferably set to 0.15% or less. To stably obtain the above effects, the Sb content is preferably set to 0.05% or more, and more preferably set to 0.08% or more.

[0041] Co: 1.0% or less Co is an element that improves corrosion resistance in acidic environments, so it can be added as needed. However, if it is added in excess, not only will the effect saturate, but the cost of the steel will increase significantly. Therefore, the Co content is set to 1.0% or less. The Co content is preferably set to 0.70% or less. To stably obtain the above effects, the Co content is preferably set to 0.01% or more, and more preferably 0.02% or more.

[0042] As: 0.30% or less Although the effect of As is not as significant as that of Sb, it is an element that is effective in improving corrosion resistance in acidic environments, and therefore may be added as needed. However, excessive addition of As reduces hot workability. Therefore, the As content is set to 0.30% or less. The As content is preferably set to 0.20% or less. To stably obtain the above effects, the As content is preferably set to 0.02% or more, and more preferably set to 0.05% or more.

[0043] Ce: 0.50% or less Ce is a compound that is highly reactive in corrosive environments. 3+ Ce is an element that dissolves as a cation and acts as an inhibitor in chloride solutions to suppress the anodic dissolution reaction of steel, so it can be added as needed. However, excessive Ce content can cause rolling cracks. Therefore, the Ce content should be 0.50% or less. The Ce content should preferably be 0.150% or less. To stably obtain the above effects, the Ce content should preferably be 0.005% or more, and more preferably 0.010% or more.

[0044] Bi:0.10% or less Although the effect of Bi is not as significant as that of Sb, it is an element that improves corrosion resistance in acidic environments, so it can be added as needed. However, excessive addition of Bi reduces hot workability. Therefore, the Bi content is set to 0.10% or less. The Bi content is preferably set to 0.050% or less. To stably obtain the above effects, the Bi content is preferably set to 0.002% or more, and more preferably 0.005% or more.

[0045] Se: 0.50% or less Pb: 0.50% or less Se and Pb are elements effective in improving corrosion resistance in acidic environments, so they can be added as needed. However, excessive addition will deteriorate hot workability. Therefore, the Se and Pb contents are each set to 0.50% or less. The Se and Pb contents are preferably set to 0.150% or less. To stably obtain the above effects, the Se and Pb contents are each preferably set to 0.005% or more, and more preferably 0.010% or more.

[0046] Hf: 0.20% or less Hf is an element that forms oxides on the surface of steel material to improve corrosion resistance, so it can be added as needed. Therefore, the Hf content is set to 0.20% or less. The Hf content is preferably set to 0.10% or less. In order to stably obtain the above effects, the Hf content is preferably set to 0.002% or more, and more preferably set to 0.005% or more.

[0047] Zn: 0.10% or less Ga: 0.10% or less Zn and Ga are elements that suppress the cathodic reaction on the steel surface in an acidic environment and improve corrosion resistance, so they can be added as needed. However, excessive addition of these elements will deteriorate the toughness and weldability of the base metal. Therefore, the Zn and Ga contents are each set to 0.10% or less. The Zn and Ga contents are preferably set to 0.080% or less. To stably obtain the above effects, the Zn and Ga contents are preferably set to 0.002% or more, and more preferably 0.005% or more.

[0048] Sr: 0.020% or less Ba:0.020% or less Sr and Ba have the effect of suppressing the decrease in pH at the interface in the corrosion reaction zone and suppressing the acceleration of corrosion, so they can be added as needed. However, excessive addition may reduce the toughness of the base material. Therefore, the Sr and Ba contents are each set to 0.020% or less. The Sr and Ba contents are preferably set to 0.010% or less. To stably obtain the above effects, the Sr and Ba contents are preferably set to 0.0005% or more, and more preferably 0.0010% or more.

[0049] Ge: 0.10% or less Ge has the effect of improving corrosion resistance, so it can be added as needed. However, excessive addition of Ge will reduce the mechanical properties of the base material. Therefore, the Ge content should be 0.10% or less. The Ge content is preferably 0.080% or less. To stably obtain the above effects, the Ge content should preferably be 0.002% or more, and more preferably 0.005% or more.

[0050] Sc:0.010% or less Sc is an element that forms oxides on the surface of steel material to improve corrosion resistance, so it can be added as needed. However, excessive addition is not preferable because it reduces low-temperature toughness. Therefore, the Sc content is set to 0.010% or less. To stably obtain the above effects, the Sc content is preferably set to 0.0001% or more.

[0051] Sm: 0.010% or less Sm has the effect of improving corrosion resistance, so it can be added as needed. However, excessive Sm content reduces the mechanical properties of the base material. Therefore, the Sm content is set to 0.010% or less. The Sm content is preferably set to 0.0060% or less. To stably obtain the above effects, the Sm content is preferably set to 0.0002% or more, and more preferably set to 0.0005% or more.

[0052] Ti: 0.20% or less Ti is an element that has the effect of suppressing the formation of MnS, which forms sulfides and becomes the starting point for corrosion, so it can be added as needed. However, if it is added in excess, not only will the effect saturate but the cost of the steel will increase. Therefore, the Ti content is set to 0.20% or less. The Ti content is preferably set to 0.150% or less. To stably obtain the above effect, the Ti content is preferably set to 0.001% or more, and more preferably 0.005% or more.

[0053] Zr: 0.20% or less Like Ti, Zr has the effect of suppressing the formation of MnS, which is the starting point for corrosion, by forming sulfides, so it can be added as needed. However, if it is added in excess, not only will the effect saturate, but the cost of the steel will increase. Therefore, the Zr content is set to 0.20% or less. The Zr content is preferably set to 0.150% or less. To stably obtain the above effect, the Zr content is preferably set to 0.001% or more, and more preferably 0.005% or more.

[0054] Nb: 0.10% or less Nb is an element that increases the strength of steel, so it can be added as needed. However, if it is added in excess, not only will the effect saturate, but the toughness of the HAZ will decrease. Therefore, the Nb content should be 0.10% or less. The Nb content should preferably be 0.050% or less. To stably obtain the above effects, the Nb content should preferably be 0.001% or more, and more preferably 0.003% or more.

[0055] V: 0.50% or less Like Nb, V is an element that increases the strength of steel. However, if it is added in excess, not only does the effect saturate, but costs also increase significantly. Therefore, the V content is set to 0.50% or less. The V content is preferably set to 0.30% or less. To stably obtain the above effects, the V content is preferably set to 0.005% or more, and more preferably set to 0.010% or more.

[0056] B: 0.010% or less B is an element that improves hardenability and increases strength, so it can be added as needed. However, if it is added in excess, the effect of increasing strength saturates, and there is a tendency for the toughness of both the base material and the HAZ to deteriorate significantly. Therefore, the B content is set to 0.010% or less. To stably obtain the above effects, it is preferable that the B content be 0.0003% or more.

[0057] Ta: 0.10% or less Ta is an element that contributes to improving the strength of steel materials and can be added as needed. It has also been found that Ta contributes to improving corrosion resistance, although the mechanism is not entirely clear. However, excessive Ta content not only saturates the effect but also increases costs. Therefore, the Ta content is set to 0.10% or less. The Ta content is preferably set to 0.060% or less. To stably obtain the above effects, the Ta content is preferably set to 0.001% or more, and more preferably set to 0.005% or more.

[0058] Te: 0.50% or less Te is an element that contributes to improving the strength of steel and can be added as needed. However, excessive addition reduces toughness and weldability. Therefore, the Te content is set to 0.50% or less. The Te content is preferably set to 0.40% or less. To stably obtain the above effects, the Te content is preferably set to 0.0005% or more, and more preferably set to 0.0010% or more.

[0059] Y: 0.10% or less La: 0.10% or less Y and La are elements that are effective in controlling the morphology of inclusions, improving ductility, and improving HAZ toughness in high-heat-input welded joints, so they can be added as needed. However, excessive addition causes inclusions to coarsen, adversely affecting mechanical properties, particularly ductility and toughness. Therefore, the Y and La contents are each set to 0.10% or less. The Y and La contents are preferably set to 0.060% or less. To stably obtain the above effects, the Y and La contents are preferably set to 0.0001% or more, and more preferably 0.0050% or more.

[0060] Nd: 0.010% or less Nd is an element that contributes to improving toughness by refining the structure, and can be added as needed. It has also been found that Nd contributes to improving corrosion resistance, although the mechanism is not entirely clear. However, excessive Nd content not only saturates the effect but also increases costs. Therefore, the Nd content should be 0.010% or less. The Nd content is preferably 0.0080% or less. To consistently achieve the above effects, the Nd content should preferably be 0.0001% or more, and more preferably 0.0005% or more.

[0061] Ca: 0.010% or less Ca is an element mainly used to control the morphology of sulfides and can be added as needed. It also has the effect of suppressing the decrease in pH at the interface in the corrosion reaction zone and suppressing the acceleration of corrosion. However, excessive Ca content may impair mechanical properties. Therefore, the Ca content is set to 0.010% or less. The Ca content is preferably 0.0050% or less. To stably obtain the above effects, the Ca content is preferably 0.0002% or more, and more preferably 0.0005% or more.

[0062] Mg: 0.010% or less Like Ca, Mg suppresses the decrease in pH at the interface in the corrosion reaction zone, so it can be added as needed. However, if added in excess, the effect will saturate. Therefore, the Mg content should be 0.010% or less. The Mg content should preferably be 0.0050% or less. To stably obtain the above effect, the Mg content should preferably be 0.0002% or more, and more preferably 0.0005% or more.

[0063] REM: 0.0150% or less REM (rare earth elements), with the exception of Y, Sc, La, Ce, Nd, and Sm, have the effect of improving the weldability of steel, and can be added as needed. However, if added in excess, the effect saturates, so the REM content is set to 0.0150% or less. The REM content is preferably set to 0.0100% or less. To stably obtain the above effect, the REM content is preferably set to 0.0002% or more, and more preferably 0.0005% or more.

[0064] Here, REM is a collective term for 17 elements, including 15 lanthanoid elements plus Y and Sc. However, in the present invention, Y, Sc, La, Ce, Nd, and Sm are separately defined as described above, and therefore the REM content refers to the content of one or the total content of two or more of the elements excluding Y, Sc, La, Ce, Nd, and Sm from REM.

[0065] (B) Anticorrosive coating The steel material of the present invention described above exhibits good corrosion resistance even when used as is. However, when its surface is subjected to a corrosion prevention treatment, specifically when the surface is covered with a corrosion-resistant coating made of an organic resin or metal, the durability of the corrosion-resistant coating improves compared to conventional steel materials, and the corrosion resistance is further improved.

[0066] Examples of corrosion-resistant coatings made of organic resins include vinyl butyral-based, epoxy-based, urethane-based, and phthalic acid-based resin coatings. Examples of corrosion-resistant coatings made of metals include plated coatings of Zn, Al, Zn-Al, etc., and thermally sprayed coatings of Zn, Al, Al-Mg, etc.

[0067] The improved durability of the corrosion-protective coating is believed to be due to the fact that corrosion of the underlying steel material of the present invention is significantly suppressed, thereby suppressing swelling or peeling of the corrosion-protective coating due to corrosion of the underlying steel material from defective areas in the corrosion-protective coating.

[0068] (C) Manufacturing method There are no particular limitations on the method for producing the steel material according to the present invention. For example, the steel material includes steel plates, steel pipes, etc., which are produced by hot rolling an ingot having the above-mentioned chemical composition, and further cold rolling it as needed. There are no particular limitations on the heating conditions when hot rolling, and ordinary conditions may be used.

[0069] When producing steel materials, steel is melted in a conventional manner, and after adjusting the components, the resulting cast steel billet is hot-rolled and, if necessary, cold-rolled. After hot-rolling, the steel may be water-cooled directly or air-cooled, and then reheated and quenched. After hot-rolling, the steel may be wound into a coil. After hot-rolling, the steel may be cold-rolled and then further heat-treated.

[0070] When producing steel pipes, steel plates may be formed into a tubular shape and welded, and can be used to produce UO steel pipes, electric resistance welded steel pipes, forged steel pipes, spiral steel pipes, etc. Seamless steel pipes produced by hot extrusion or piercing rolling of steel billets are also included in the steel materials of the present invention.

[0071] The treatment for covering with the above-mentioned anticorrosion coating may be carried out by a conventional method. Furthermore, it is not necessarily required to apply an anticorrosion coating to the entire surface of the steel material, and it is sufficient to apply an anticorrosion treatment to only one side of the steel material that is exposed to a corrosive environment, or in the case of a steel pipe, only the outer or inner surface, i.e., only at least a part of the steel material surface.

[0072] The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples. [Example]

[0073] Steels having the chemical compositions shown in Tables 1 to 3 were melted and formed into 50 kg ingots, which were then hot forged in a conventional manner to produce blocks with a thickness of 60 mm. The blocks were then heated at 1120°C for 1 hour, hot rolled, and finished at 850°C to a thickness of 20 mm, and then allowed to cool to room temperature in the air to produce steel plates.

[0074] [Table 1]

[0075] [Table 2]

[0076] [Table 3]

[0077] Two test pieces, each 25 mm wide, 25 mm long, and 4 mm thick, were then taken from each steel plate, and one test piece was subjected to the following corrosion test simulating a weakly acidic environment. The other test piece was spray-painted with a modified epoxy paint to form a corrosion-resistant coating of approximately 200 μm over the entire surface of the test piece, and a cross-shaped scratch was made in the corrosion-resistant coating to expose part of the base metal, before being subjected to the same corrosion test.

[0078] Corrosion resistance was evaluated by an immersion test in a sulfuric acid solution adjusted to a pH of 3. Test pieces were immersed in the solution at 60°C for 6 hours, and the amount of thickness reduction was measured. For corrosion-protected steel, the maximum corrosion depth in the scratches in the corrosion-protective coating was measured using a confocal laser microscope.

[0079] The test results are shown in Table 4. The "corrosion weight loss" in this table is the average thickness loss of the test specimen, calculated using the weight loss before and after the test and the surface area of ​​the test specimen. The "corrosion depth" is the maximum depth of the paint flaw from the steel surface.

[0080] [Table 4]

[0081] As is clear from the results in Table 4, the steel material of test steel No. 4, which is a comparative example, does not contain In, and therefore the corrosion weight loss was 2.1 g / m 2 / hr, and the corrosion depth exceeded 25.0 μm.

[0082] On the other hand, in the steel materials of test steels Nos. 1 to 3 and 5 to 31, which are examples of the present invention, all satisfy the component contents specified in the present invention, and therefore the corrosion weight loss was 1.5 g / m 2 / hr or less, and the corrosion depth was 18.0 μm or less. [Industrial Applicability]

[0083] The steel material according to the present invention can be used as a corrosion-resistant steel that exhibits excellent corrosion resistance in a weakly acidic environment.

Claims

1. The chemical composition, in mass%, is C: 0.20% or less, Si: 1.0% or less, Mn: 3.0% or less, P: 0.050% or less, S: 0.030% or less, In: 0.005 to 0.20%, Al: 0.10% or less, The balance is Fe and impurities. Used in a weakly acidic environment with a pH of 3 to 5. Steel material.

2. The chemical composition contains, in mass %, replacing a part of the Fe, Cu: 1.0% or less, Ni: 1.0% or less, Cr: 1.0% or less, Mo: 1.0% or less, W: 1.0% or less, Sb: 0.30% or less, Co: 1.0% or less, As: 0.30% or less, Ce: 0.50% or less, Bi: 0.10% or less, Se: 0.50% or less, Pb: 0.50% or less, Hf: 0.20% or less, Zn: 0.10% or less, Ga: 0.10% or less, Sr: 0.020% or less, Ba: 0.020% or less, Ge: 0.10% or less, Sc: 0.010% or less, and Sm: 0.010% or less, It contains one or more selected from The steel material according to claim 1.

3. The chemical composition contains, in mass %, replacing a part of the Fe, Ti: 0.20% or less, Zr: 0.20% or less, Nb: 0.10% or less, V: 0.50% or less, B: 0.010% or less, Ta: 0.10% or less, Te: 0.50% or less, Y: 0.10% or less, La: 0.10% or less, Nd: 0.010% or less, Ca: 0.010% or less, Mg: 0.010% or less, and REM: 0.0150% or less, It contains one or more selected from The steel material according to claim 1 or 2.

4. At least a part of the surface of the steel material has been subjected to anticorrosion treatment. The steel material according to any one of claims 1 to 3.

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