steel

Abstract

To provide a steel that exhibits excellent corrosion resistance in a dry-wet repeated environment including a large amount of chloride.SOLUTION: A steel has a chemical composition comprising, in mass%, C: 0.01-0.20%, Si: 0.01-1.0%, Mn: 0.05-3.00%, P: 0.050% or less, S: 0.030% or less, Al: 0.005-0.100%, N: 0.001-0.010%, Sn: 0.01-0.50%, In: 0.001-0.20%, and the balance being Fe and impurities.SELECTED DRAWING: None

Description

[Technical Field]

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

[0002] It is well known that chlorides have a significant effect on the acceleration of steel corrosion. This is particularly true for structures such as bridges in coastal areas, steel sheet piles and steel pipe piles used in port facilities, ship hulls, ballast tanks, marine structures, and offshore wind power generation facilities, which are directly exposed to seawater splashes and are also exposed to repeated dry and wet environments, resulting in extremely severe corrosion.

[0003] Corrosion is also severe in seawater, although not as severe as in environments where dryness and wetting occur repeatedly. In coastal areas, there is no seawater splashing, but the arrival of sea salt particles accelerates corrosion. Even in inland areas, chloride-based corrosion is a common problem, with roads being sprayed with antifreeze agents containing chlorides in winter to prevent them from freezing.

[0004] Furthermore, even though the tanks of ore carriers and crude oil tankers are not directly exposed to seawater, they are washed with seawater, and corrosion caused by the chlorides that remain after washing can be a problem. Crude oil tankers also have a severely corrosive environment, with drain water containing a highly concentrated chloride solution. Corrosion caused by chlorides is also a problem in oil sands drilling and transportation equipment.

[0005] For these reasons, steel materials are painted when used in environments where corrosion by chlorides is a problem, but because corrosion occurs and progresses due to deterioration of the paint film and in areas where the paint film is thin, such as the edges of the steel, maintenance (repainting) is essential if the structure is to be used for a long period of time.

[0006] In such cases, problems arise, such as the need to install scaffolding depending on the structure, which can result in enormous maintenance costs, and the fact that painting generates large amounts of VOCs (volatile organic compounds), which are considered harmful to humans. For these reasons, there has been a strong desire to develop steel materials that have good corrosion resistance even without painting, or steel materials that can extend the interval between repainting.

[0007] As steel materials having excellent corrosion resistance in such chloride environments, for example, Patent Document 1 discloses a steel material with an increased Cr content, and Patent Document 2 discloses a steel material with an increased Ni content.

[0008] On the other hand, as examples of steels that do not increase the Cr or Ni content, Patent Document 3 discloses a steel material that contains P, Ni, and Mo as essential elements and that contains Sb and / or Sn, and Patent Document 4 discloses a steel material that contains P, Cu, Ni, and Sb as essential elements. Furthermore, Patent Document 5 discloses a steel material that contains Cu as an essential element and that contains Sb and / or Sn, and Patent Document 6 discloses a steel material that contains Sn as an essential element. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Publication No. 9-176790 [Patent Document 2] Japanese Patent Application Publication No. 5-51668 [Patent Document 3] Japanese Patent Application Publication No. 10-251797 [Patent Document 4] Japanese Patent Application Laid-Open No. 2002-53929 [Patent Document 5] Japanese Patent Application Publication No. 9-25536 [Patent Document 6] Japanese Patent Application Laid-Open No. 2012-255184 [Non-patent literature]

[0010] [Non-Patent Document 1] "Environmental Materials Science", Hiroo Nagano, Masato Yamashita, Hitoshi Uchida, Kyoritsu Shuppan (2004) p.74 Summary of the Invention [Problem to be solved by the invention]

[0011] Cr and Ni are elements that generally contribute to the corrosion resistance of steel materials. However, the steel materials disclosed in Patent Documents 1 and 2 have room for improvement in terms of corrosion resistance in extremely severe chloride environments. In addition, since Cr and Ni are expensive elements, increasing the Cr and Ni contents also poses a problem in terms of cost.

[0012] Furthermore, the steel material for welded structures disclosed in Patent Document 3 contains a large amount of P, which inhibits weldability, and therefore has problems with weldability. On the other hand, the steel material disclosed in Patent Document 4 is only said to have good weather resistance in an environment with an airborne salt content of 0.8 mdd, but has the problem of insufficient weather resistance in a severe airborne salt environment exceeding that level.

[0013] Furthermore, the steel material disclosed in Patent Document 5 is a steel material that has corrosion resistance to combustion exhaust gases emitted when heavy oil or the like is burned, and is a steel material that is used in an environment that is significantly different from a chloride environment. Therefore, such a steel material cannot necessarily be used in a chloride environment.

[0014] The steel material disclosed in Patent Document 6 is a steel material with excellent corrosion resistance that can be used in an environment containing chlorides and undergoing repeated dry and wet cycles. However, there is still room for improvement for use in even harsher environments.

[0015] The present invention aims to solve the above problems and to provide a steel material that exhibits excellent corrosion resistance in an environment containing a large amount of chlorides and undergoing repeated dry and wet cycles. [Means for solving the problem]

[0016] The present invention has been made to solve the above problems, and the gist of the present invention is the following steel material.

[0017] (1) Chemical composition, in mass%, C: 0.01 to 0.20%, Si: 0.01 to 1.0%, Mn: 0.05 to 3.00%, P: 0.050% or less, S: 0.030% or less, Al: 0.005 to 0.100%, N: 0.001 to 0.010%, Sn: 0.01 to 0.50% In: 0.001 to 0.20%, The balance is Fe and impurities. Steel material.

[0018] (2) The chemical composition contains, in mass %, a part of the Fe replaced by Cu: 1.0% or less, Ni: 1.0% or less, and Cr: 1.0% or less, It contains one or more selected from The steel material described in (1) above.

[0019] (3) The chemical composition contains, in mass %, a part of the Fe replaced by Mo: 1.0% or less, and W: 1.0% or less, It contains one or more selected from The steel material according to (1) or (2) above.

[0020] (4) The chemical composition contains, in mass %, a part of the Fe replaced by 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 any one of (1) to (3) above.

[0021] (5) 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 any one of (1) to (4) above.

[0022] (6) 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 (5) above. [Effects of the Invention]

[0023] According to the present invention, a steel material can be obtained that exhibits excellent corrosion resistance in an environment containing a large amount of chlorides and undergoing repeated dry and wet cycles. DETAILED DESCRIPTION OF THE INVENTION

[0024] In chloride-rich environments, the FeCl3 solution undergoes repeated drying and wetting cycles, and Fe 3+ The pH of the corrosion interface is reduced by the hydrolysis of Fe 3+ acts as an oxidizing agent, accelerating corrosion.

[0025] The corrosion reaction at this time is as follows: Cathodic reaction: Fe 3+ +e - →Fe 2+ (Fe 3+ reduction reaction) Anode reaction: Fe → Fe 2+ +2e - (Fe dissolution reaction)

[0026] Therefore, the overall reaction of corrosion is as shown in equation (i) below. 2Fe 3+ +Fe → 3Fe 2+ (i)

[0027] The Fe produced by the reaction of formula (i) above 2+ is converted to Fe by air oxidation. 3+ The resulting Fe 3+ Again, Fe acts as an oxidant and accelerates corrosion. 2+ The reaction rate of air oxidation of Fe is generally slow in low pH environments, but is accelerated in concentrated chloride solutions. 3+ This cyclic reaction causes the corrosion resistance of steel to deteriorate significantly in environments with very high airborne salt content.

[0028] The acidic pH of the corrosion interface is neutralized by rainwater or seawater that adheres to the steel surface, but then decreases again due to the above-mentioned reaction. In this way, in an environment where dry and wet conditions are repeated, the pH of the corrosion interface changes continuously from acidic to neutral.

[0029] Furthermore, in environments with very high chloride concentrations, the protective effect of the rust layer cannot be expected, so slowing down the anodic dissolution reaction of the steel itself is effective in improving corrosion resistance. In other words, in environments with very high chloride concentrations, it is important to suppress the anodic dissolution reaction in acidic to neutral chloride solutions.

[0030] Furthermore, in a low pH environment, the following hydrogen ion reduction reaction proceeds, promoting the anodic reaction of Fe dissolution: 2H + +2e - →H2

[0031] In order to suppress the anodic dissolution reaction, it is effective to include Sn in the steel. Sn reacts with the cation Sn 2+ It dissolves as Sn and acts as an inhibitor in acidic chloride solutions, thereby suppressing corrosion. 3+ It quickly reduces Fe as an oxidizing agent. 3+ By reducing the concentration, 3+ The corrosion-promoting action of the metal can be suppressed.

[0032] It is also known that the rate of the hydrogen evolution reaction on a Sn surface is slower than that on an Fe surface, so forming a Sn layer on the steel surface can suppress the hydrogen evolution reaction, and as a result, the anodic dissolution of the steel can be suppressed.

[0033] Specifically, Sn 2+ Underpotential deposition (UPD) of Sn improves corrosion resistance by forming an extremely thin metallic Sn layer on the surface of steel in the operating environment.

[0034] Thus, Sn can suppress anodic dissolution reactions and improve corrosion resistance. However, while Sn is very effective in strongly acidic environments with a pH of less than 2, there is still room for improvement in suppressing anodic dissolution reactions in weakly acidic environments.

[0035] Based on the effect of Sn in such a salt environment, the present inventors have conducted detailed research into the relationship between various metal elements and the anodic dissolution reaction in order to further improve corrosion resistance in an environment where the pH fluctuates, and have obtained the following findings (a) to (c).

[0036] (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.

[0037] (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.

[0038] (c) Sn 2+ The UPD of In occurs in a strongly acidic environment with a pH of less than 2. 3+ UPD occurs in a weakly acidic environment of pH 3 to 5. Therefore, by simultaneously containing Sn and In, a better corrosion inhibition effect can be obtained against changes in pH caused by repeated dry and wet cycles than by containing either element alone.

[0039] The present invention was made based on the above findings. Each of the features of the present invention will be described in detail below.

[0040] (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."

[0041] C: 0.01 to 0.20% C is an element necessary to ensure the strength of the material. However, excessive C 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 0.01 to 0.20%. The C content is preferably 0.02% or more, and more preferably 0.03% or more. The C content is preferably 0.18% or less, and more preferably 0.16% or less.

[0042] Si: 0.01 to 1.0% Silicon is an element necessary for deoxidation. However, excessive content of silicon impairs the toughness of the base material and welded joints. Therefore, the silicon content is set to 0.01 to 1.0%. The silicon content is preferably 0.03% or more, and more preferably 0.05% or more. The silicon content is preferably 0.80% or less, and more preferably 0.60% or less.

[0043] Mn: 0.05 to 3.00% 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 0.05 to 3.00%. The Mn content is preferably 0.20% or more, and more preferably 0.40% or more. The Mn content is preferably 2.50% or less, and more preferably 2.00% or less.

[0044] P:0.050% or less P is an element present as an impurity in steel. P deteriorates the acid resistance of steel and reduces corrosion resistance in chloride-rich corrosive environments, which lower the pH of the corrosion interface. P also deteriorates weldability and toughness in the weld heat-affected zone. Therefore, the P content is set to 0.050% or less. The P content is preferably 0.030% or less, and more preferably less than 0.010%. 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 result in increased steelmaking costs. Therefore, the P content may be set to 0.0001% or more.

[0045] 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.

[0046] Al: 0.005 to 0.100% Al is an effective element for deoxidizing steel. However, excessive Al content not only deteriorates corrosion resistance in corrosive environments with low pH and high chloride content, but also causes nitrides to coarsen, resulting in a deterioration in toughness. Therefore, the Al content is set to 0.005 to 0.100%. The Al content is preferably 0.080% or less, and more preferably 0.060% or less. Furthermore, to stably obtain the deoxidizing effect of Al, the Al content is preferably 0.010% or more, and more preferably 0.030% or more.

[0047] N: 0.001 to 0.010% N is an element that forms nitrides and contributes to improving mechanical properties by refining crystal grains. However, excessive N content causes deterioration of mechanical properties due to nitrides. Therefore, the N content is set to 0.001 to 0.010%. To suppress the formation of coarse ferrite, the N content is preferably 0.002% or more, more preferably 0.003% or more. Furthermore, the N content is preferably 0.008% or less, more preferably 0.006% or less.

[0048] Sn: 0.01 to 0.50% Sn is in a corrosive environment 2+ It dissolves as a tin oxide and acts as an inhibitor in acidic chloride solutions to suppress corrosion. 2+Sn has the effect of significantly suppressing the anodic dissolution reaction of steel through underpotential deposition (UPD), and therefore even a small amount can significantly improve corrosion resistance. However, if added in excess, not only does this effect saturate, but the toughness of the base material and high-heat-input welded joints deteriorates. Therefore, the Sn content is set to 0.01 to 0.50%. The Sn content is preferably 0.05% or more, and more preferably 0.10% or more. Furthermore, the Sn content is preferably 0.40% or less, and more preferably 0.30% or less.

[0049] In: 0.001 to 0.20% In in a corrosive environment 3+ It dissolves as a fluorine-containing compound and acts as an inhibitor in acidic chloride solutions to suppress corrosion. 3+ Since In has the effect of significantly suppressing the anodic dissolution reaction of steel through UPD, even a small amount can significantly improve corrosion resistance. However, if it is added in excess, not only does this effect saturate, but the toughness of the base material deteriorates. Therefore, the In content is set to 0.001 to 0.20%. The In content is preferably 0.010% or more, and more preferably 0.020% or more. Furthermore, the In content is preferably 0.15% or less, and more preferably 0.10% or less.

[0050] Here, we will explain why the simultaneous inclusion of In and Sn can provide excellent corrosion resistance in a chloride-containing, dry-wet, cyclic environment. As described above, in a chloride-containing, dry-wet, cyclic environment, the pH of the corrosion interface changes continuously from acidic to neutral.

[0051] When steel is exposed to an environment where the pH of the corrosion interface is less than 2, Sn in the steel 2+ Since Sn is more stable as an ion, it is extracted from the base material and 2+ Then, when the potential reaches a certain level, Sn precipitates as a monoatomic layer on the steel surface due to UPD, suppressing the anodic dissolution reaction of the steel. When the pH of the corrosion interface rises to about 3, Sn 2+Since Sn can no longer exist stably, some of the Sn becomes an oxide. As a result, a monoatomic layer of Sn and / or a Sn oxide layer is formed on the surface of the steel. The Sn oxide layer also has a corrosion-inhibiting effect on steel, so steel exhibits excellent corrosion resistance even in acidic to neutral corrosive environments.

[0052] On the other hand, when steel is exposed to an environment with a pH of 3 to 5 at the corrosion interface, Sn 2+ No ion elution occurs. However, In 3+ Since In can exist stably as an ion, it is extracted from the base material and 3+ When the potential reaches a certain level, In is precipitated as a monoatomic layer on the steel surface by UPD, suppressing the anodic dissolution reaction of the steel. After that, when the pH of the corrosion interface drops below 2, In dissolves, but Sn remains as Sn 2+ This dissolves as a monoatomic layer on the surface of the steel material, suppressing the anodic dissolution reaction of the steel material.

[0053] By incorporating In and Sn simultaneously as described above, in a chloride-containing, dry-wet repeated environment, In and / or Sn form a monoatomic layer on the surface of the steel material in accordance with changes in pH, thereby effectively improving the corrosion resistance of the steel material.

[0054] 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.

[0055] 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.

[0056] 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.

[0057] Ni: 1.0% or less Ni has the effect of improving corrosion resistance by suppressing the anodic dissolution of steel in high-chloride environments where protective rust formation cannot be expected, so it can be added as needed. However, excessive addition not only saturates the effect but also leads to increased 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.

[0058] Cr:1.0% or less Cr has the effect of improving corrosion resistance, so it can be added as needed. However, excessive addition may deteriorate acid resistance, and corrosion resistance may deteriorate in chloride-rich environments. Therefore, the Cr content is set to 1.0% or less. The Cr content is preferably set to 0.80% or less. In order to stably obtain the above effects, the Cr content is preferably set to 0.01% or more, and more preferably 0.02% or more.

[0059] Mo: 1.0% or less Mo dissolves and forms oxyanion MoO4 2-Mo is an element that adsorbs to rust in the form of and has the effect of inhibiting the permeation of chloride ions through the rust layer, 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 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.

[0060] 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 suppressing the permeation of chloride ions through rust layers, 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.

[0061] 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.

[0062] 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.

[0063] As: 0.30% or less Although the effect of As is not as significant as that of Sb and Sn, 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.

[0064] Ce: 0.50% or less Ce is in a corrosive environment 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 is preferably 0.15% 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.

[0065] Bi:0.10% or less Although the effect of Bi is not as significant as that of Sb and Sn, 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.

[0066] 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 0.15% 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.

[0067] Hf: 0.20% or less Hf is an element that densifies the rust layer formed on the surface of the steel material and improves 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.

[0068] 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.

[0069] 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.

[0070] 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.

[0071] Sc:0.010% or less Sc is an element that is incorporated into the rust layer formed by corrosion, forming a dense rust layer and suppressing general corrosion of the steel material, 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 effect, the Sc content is preferably set to 0.0001% or more.

[0072] 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.

[0073] 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 0.15% or less. To stably obtain the above effect, the Ti content is preferably set to 0.001% or more, and more preferably to 0.005% or more.

[0074] Zr: 0.20% or less Like Ti, Zr has the effect of suppressing the formation of MnS, which is the starting point of 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 0.15% 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.

[0075] 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.

[0076] V: 0.50% or less Like Nb, V is an element that increases the strength of steel. Like Mo and W, V dissolves and exists in the form of oxygen ions, inhibiting the permeation of chloride ions through rust layers. Therefore, V can be added as needed. However, excessive V content not only saturates the effect but also significantly increases costs. Therefore, the V content should be 0.50% or less. The V content is preferably 0.30% or less. To consistently achieve the above effects, the V content should preferably be 0.005% or more, and more preferably 0.010% or more.

[0077] 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.

[0078] 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.

[0079] 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.

[0080] 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.

[0081] 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.

[0082] 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.

[0083] 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.

[0084] 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.

[0085] 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.

[0086] (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.

[0087] 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.

[0088] 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.

[0089] (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.

[0090] 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.

[0091] 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.

[0092] 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.

[0093] 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]

[0094] Steel having the chemical composition shown in Table 1 was melted and formed into a 50 kg ingot, which was then hot forged in a conventional manner to produce a 60 mm thick block. The block was 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 a steel plate.

[0095] [Table 1]

[0096] [Table 2]

[0097] [Table 3]

[0098] Two test pieces, each 60 mm wide, 100 mm long, and 3 mm thick, were then taken from each steel plate, and one test piece was subjected to the following corrosion test simulating a chloride environment. For the other test piece, a modified epoxy paint was spray-painted to form a corrosion-resistant coating of approximately 200 μm over the entire surface, and a cross-shaped scratch was made in the corrosion-resistant coating to expose part of the base metal, before the same corrosion test was performed.

[0099] The corrosion test was performed using the SAE (Society of Automotive Engineers) J2334 test. The J2334 test is an accelerated test consisting of one cycle (24 hours total) of 6 hours of wetting (50°C, 100% RH), 0.25 hours of salt deposition (immersion in a 0.5% NaCl, 0.1% CaCl2, and 0.075% NaHCO3 aqueous solution), and 17.75 hours of drying (60°C, 50% RH). The corrosion pattern is said to be similar to that of a chloride-containing corrosive environment (see Non-Patent Document 1). While the salt deposition solution is neutral, the thickness and salt concentration of the water film attached to the steel surface change during the wetting / drying process. As a result, it is possible to simulate a repeated wet-dry environment with fluctuating pH.

[0100] After 120 cycles of the SAE J2334 test, the rust layer on the surface of each test piece was removed and the thickness loss was measured.For corrosion-protected steel materials, the maximum corrosion depth in the corrosion-protective coating defects was measured.

[0101] 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.

[0102] [Table 4]

[0103] As is clear from the results in Table 4, the comparative steel No. 7 does not contain Sn or In, and therefore both the corrosion weight loss and corrosion depth exceed 1.00 mm. Furthermore, the comparative steels Nos. 5 and 6 contain only Sn or In, and therefore have greater corrosion weight loss and corrosion depth than the steels containing both Sn and In.

[0104] On the other hand, in the case of the steel materials of test steels Nos. 1 to 4 and 8 to 36, which are examples of the present invention, all satisfy the component contents specified in the present invention, and therefore the corrosion weight loss was small, at 0.43 mm or less, and the corrosion depth was small, at 0.44 mm or less. [Industrial Applicability]

[0105] The steel material according to the present invention can be used as a corrosion-resistant steel having excellent corrosion resistance, which is used in an environment containing a large amount of chlorides and undergoing repeated dry and wet cycles.

Claims

1. The chemical composition, in mass%, is C: 0.01-0.20%, Si: 0.01-1.0%, Mn: 0.05-3.00%, P: 0.050% or less, S: 0.030% or less, Al: 0.005-0.100%, N: 0.001 to 0.010%, Sn: 0.01-0.50%, In: 0.001 to 0.20%, The balance is Fe and impurities. 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, and Cr: 1.0% 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, Mo: 1.0% or less, and W: 1.0% or less, It contains one or more selected from The steel material according to claim 1 or 2.

4. The chemical composition contains, in mass %, replacing a part of the Fe, 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 any one of claims 1 to 3.

5. 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 any one of claims 1 to 4.

6. 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 5.

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