steel

Abstract

To provide a steel that exhibits excellent corrosion resistance in alternating wet and dry conditions with high chloride content.SOLUTION: A steel consists of, 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%, Sn: 0.01-0.30%, In: 0.001-0.200%, with the balance being Fe and impurities. The number density of In particles, each with a diameter of 5 μm or more, measured on the steel surface, is 50 / mm2 or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] This invention relates to steel materials. [Background technology]

[0002] It is well known that chlorides have a very significant impact on accelerating the corrosion of steel materials. In particular, structures such as bridges in coastal areas, steel sheet piles and steel pipe piles used in port facilities, ship plating, ballast tanks, offshore structures, and offshore wind power generation facilities are directly exposed to seawater spray and are also subjected to repeated wetting and drying cycles, resulting in extremely high corrosion rates.

[0003] Furthermore, corrosion is significant even in seawater, though not as severe as in environments with repeated wetting and drying cycles. In coastal areas, although there is no seawater spray, corrosion is accelerated by the airborne presence of sea salt particles. Inland areas also pose a problem for chloride-induced corrosion, as evidenced by the application of de-icing agents containing chlorides to prevent road freezing in winter.

[0004] Furthermore, even tanks on ore carriers or crude oil tankers that are not directly exposed to the seawater environment but undergo cleaning with seawater are susceptible to corrosion from residual chlorides after cleaning. Crude oil tankers also present a harsh corrosive environment due to the presence of drain water, which is a highly concentrated chloride solution. Additionally, corrosion from chlorides is a problem in oil sands drilling and transportation equipment.

[0005] For these reasons, steel materials are painted, especially in environments where chloride corrosion is a problem. However, due to the deterioration of the paint film, and because corrosion occurs and progresses from areas with thin paint film, such as the edges of the steel materials, maintenance (repainting) is essential when structures are used for a long period of time.

[0006] In such cases, maintenance costs become enormous due to the need to erect scaffolding depending on the structure, and painting generates large amounts of VOCs (volatile organic compounds) that are harmful to human health. For these reasons, there has been a strong demand for the development of steel materials that have good corrosion resistance without painting, or steel materials that allow for extended intervals between repainting.

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

[0008] On the other hand, as steels that do not increase Cr or Ni, for example, Patent Document 3 discloses a steel material in which P, Ni, and Mo are essential elements and Sb and / or Sn are added, and Patent Document 4 discloses a steel material in which P, Cu, Ni, and Sb are essential elements. Furthermore, Patent Document 5 discloses a steel material in which Cu is an essential element and Sb and / or Sn are added, and Patent Document 6 discloses a steel material in which Sn is 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 Publication No. 2002-53929 [Patent Document 5] Japanese Patent Application Publication No. 9-25536 [Patent Document 6] Japanese Patent Publication No. 2012-255184 [Overview of the project] [Problems that the invention aims to solve]

[0010] 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 still have room for improvement in terms of corrosion resistance in a very severe chloride environment. In addition, since Cr and Ni are expensive elements, an increase in the content of Cr and Ni also poses a problem in terms of cost.

[0011] Moreover, the steel material for welded structures disclosed in Patent Document 3 contains a large amount of P that inhibits weldability, so there is a problem in terms of weldability. On the other hand, the steel material disclosed in Patent Document 4 only claims to have good weather resistance in an environment with a flying chloride content of 0.8 mdd, and there is a problem that the weather resistance is not sufficient in a severe chloride flying environment exceeding this.

[0012] Furthermore, the steel material disclosed in Patent Document 5 is a steel material having corrosion resistance against combustion exhaust gas discharged when burning heavy oil or the like, and is a steel material used in an environment significantly different from that under a chloride environment. Therefore, it is not always possible to apply such a steel material in a chloride environment.

[0013] And the steel material disclosed in Patent Document 6 is a steel material excellent in corrosion resistance used in a dry-wet repeated environment containing chlorides. However, there is still room for improvement for use in a more severe environment.

[0014] An object of the present invention is to solve the above problems and provide a steel material that exhibits excellent corrosion resistance in a dry-wet repeated environment containing a large amount of chlorides.

Means for Solving the Problems

[0015] The present invention has been made to solve the above problems, and the gist is the following steel material.

[0016] (1) The chemical composition of the steel material is, by mass%, C: 0.01 to 0.20%, [[ID=3I]] 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 - 0.100%, Sn: 0.01 - 0.30%, In: 0.001 - 0.200%, The balance: Fe and impurities, The number density of In particles with a diameter of 5 μm or more measured on the surface of the steel material is 50 / mm , ,

[0018] , , , , , , , , , , , , , , , , , , , , , , ,

[0019] , or less, Steel material.

[0017] (2) Instead of a part of the Fe, in mass%, <00​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​According to the present invention, a steel material can be obtained that exhibits excellent corrosion resistance in a repeatedly wet and dry environment containing a large amount of chloride. [Modes for carrying out the invention]

[0020] The invention described in Japanese Patent Application No. 2022-23029 states that by simultaneously incorporating Sn and In, a superior corrosion suppression effect against pH changes caused by repeated wetting and drying was obtained compared to incorporating each of them individually.

[0021] However, further research by the inventors revealed that corrosion pits are prone to forming in weakly acidic environments in steel materials containing Sn and In. Furthermore, it was found that the corrosion pits originate from coarse In particles measured on the steel surface.

[0022] Since In particles are generated during the molten steel stage, it is difficult to suppress their generation. Therefore, the inventors have found that by applying heat treatment under appropriate conditions after hot rolling, the number density of coarse In particles can be reduced, thereby suppressing the occurrence of corrosion pits. This has been shown to improve corrosion resistance in repeatedly wet and dry environments containing a large amount of chloride.

[0023] This invention is based on the above findings. The requirements of this invention will be described in detail below.

[0024] (A) Chemical composition The reasons for the limitations on each element are as follows. Note that in the following explanation, "%" for content refers to "mass%".

[0025] C: 0.01~0.20% Carbon (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 produced, which acts as a cathode and promotes corrosion in environments with lower pH, also increases, thus reducing corrosion resistance. For this reason, the C content should be 0.01 to 0.20%. Preferably, the C content is 0.02% or more, and more preferably 0.05% or more. Also, preferably, the C content is 0.18% or less, and more preferably 0.16% or less.

[0026] Si: 0.01~1.0% Si is an element necessary for deoxidation. However, excessive Si content impairs the toughness of the base material and welded joints. Therefore, the Si content should be between 0.01% and 1.0%. Preferably, the Si content is 0.05% or more, and more preferably 0.10% or more. Furthermore, preferably, the Si content is 0.80% or less, and more preferably 0.60% or less.

[0027] Mn: 0.05~3.00% Mn is an element that increases the strength of steel at a low cost. However, excessive amounts of Mn deteriorate both weldability and joint toughness. Therefore, the Mn content should be between 0.05% and 3.00%. Preferably, the Mn content is 0.20% or more, and more preferably 0.40% or more. Furthermore, preferably, the Mn content is 2.50% or less, and more preferably 2.00% or less.

[0028] P:0.050% or less P is an element present as an impurity in steel. P degrades the acid resistance of steel and reduces corrosion resistance in chloride-rich corrosive environments where the pH of the corrosion interface decreases. P also degrades weldability and toughness in the heat-affected zone of welds. Therefore, the P content should be 0.050% or less. Preferably, the P content is 0.030% or less, and more preferably less than 0.010%. There is no particular need to specify a lower limit for the P content; in other words, the P content may be 0%, but extreme reduction will lead to increased steelmaking costs. Therefore, the P content may be 0.0001% or more.

[0029] S: 0.030% or less S is an element that exists as an impurity in steel. S forms MnS, which is the starting point for corrosion in steel, and if its content is excessive, the corrosion resistance will decrease significantly. For this reason, the S content should be 0.030% or less. Preferably, the S content should be 0.025% or less, and more preferably 0.020% or less. There is no particular need to specify a lower limit for the S content; in other words, the S content may be 0%, but extreme reduction will lead to an increase in steelmaking costs. For this reason, the S content may be 0.0001% or more.

[0030] Al: 0.005~0.100% Al is an effective element for deoxidizing steel. However, excessive amounts of Al not only degrade corrosion resistance in chloride-rich corrosive environments with low pH, but also cause toughness degradation due to the coarsening of nitrides. Therefore, the Al content should be between 0.005% and 0.100%. Preferably, the Al content is 0.080% or less, and more preferably 0.060% or less. Furthermore, in order to stably obtain the deoxidizing effect of Al, the Al content is preferably 0.010% or more, and more preferably 0.030% or more.

[0031] Sn: 0.01~0.30% Sn in corrosive environments 2+ It dissolves as an inhibitor and has the effect of suppressing corrosion through its inhibitory action in acidic chloride solutions. Furthermore, Sn 2+ Because it significantly suppresses the anodic dissolution reaction of steel through underpotential precipitation (UPD), even a small amount can greatly improve corrosion resistance. However, if it is included in excess, the aforementioned effect will not only saturate, but the toughness of the base material and high heat input welded joints will also deteriorate. For this reason, the Sn content should be 0.01 to 0.30%. The Sn content is preferably 0.05% or more, and more preferably 0.10% or more. Furthermore, the Sn content is preferably 0.25% or less, and more preferably 0.20% or less.

[0032] In: 0.001~0.200% In corrosive environments. 3+ It dissolves as an inhibitor and has the effect of suppressing corrosion through its inhibitory action in acidic chloride solutions. Furthermore, In 3+ Because UPD significantly suppresses the anodic dissolution reaction of steel, even a small amount can greatly improve corrosion resistance. However, if it is included in excess, the aforementioned effect will not only saturate but the toughness of the base material will also deteriorate. Therefore, the In content should be 0.001 to 0.200%. The In content is preferably 0.010% or more, and more preferably 0.020% or more. Furthermore, the In content is preferably 0.150% or less, and more preferably 0.100% or less.

[0033] Furthermore, the higher the In content, the greater the number density of In particles tends to be. Therefore, the number density of In particles with a diameter of 5 μm or more is set to 50 / mm². 2 To achieve the following, the heat treatment conditions are adjusted as appropriate according to the In content of the steel, as described below.

[0034] The steel material according to the present invention has the above chemical composition, with the remainder being Fe and impurities. Here, impurities refer to components that are mixed in during the industrial production of steel material due to various factors in the manufacturing process, including raw materials such as ore and scrap, and are acceptable within a range that does not adversely affect the present invention.

[0035] In the chemical composition of the steel material of the present invention, one or more elements selected from the following elements may be included in place of a portion of Fe, within the range shown below. The reasons for limiting each element will be explained below.

[0036] Cu: 1.0% or less Cu has the effect of improving corrosion resistance by suppressing anodic dissolution of steel in low pH environments, and can therefore be included as needed. However, excessive inclusion will not only saturate the effect but also cause embrittlement. Therefore, the Cu content should be 1.0% or less. To stably obtain the above effect, it is preferable to have a Cu content of 0.02% or more, and more preferable to have a Cu content of 0.03% or more.

[0037] Ni: 1.0% or less Ni has the effect of improving corrosion resistance by suppressing anodic dissolution of steel in high-chloride environments where protective rust formation cannot be expected, and can therefore be included as needed. However, excessive amounts will not only saturate the effect but also lead to increased costs. Therefore, the Ni content should be 1.0% or less. Preferably, the Ni content should be 0.80% or less. To stably obtain the above effect, it is preferable that the Ni content be 0.01% or more, and more preferably 0.02% or more.

[0038] Cr:1.0% or less Cr has the effect of improving corrosion resistance, so it can be included as needed. However, if included in excess, acid resistance will deteriorate, and corrosion resistance may deteriorate in environments with high chloride content. Therefore, the Cr content should be 1.0% or less. Preferably, the Cr content should be 0.80% or less. Furthermore, in order to stably obtain the above effect, it is preferable that the Cr content be 0.01% or more, and more preferably 0.02% or more.

[0039] Mo: 1.0% or less Mo dissolves into oxygen ions MoO4. 2-Since it is an element that adsorbs rust in the form of [element] and has the effect of suppressing the permeation of chloride ions in the rust layer, it can be contained as needed. However, if it is contained excessively, not only will the effect saturate, but the cost of the steel material will increase significantly. Therefore, the Mo content should be 1.0% or less. The Mo content is preferably 0.70% or less. In order to stably obtain the above effect, the Mo content is preferably 0.01% or more, and more preferably 0.02% or more.

[0040] W: 1.0% or less Similar to Mo, W dissolves and exists in the form of oxygen acid ion WO4 2- Since it is an element that exists in the form of [element] and has the effect of suppressing the permeation of chloride ions in the rust layer, it can be contained as needed. However, if it is contained excessively, not only will the effect saturate, but the cost of the steel material will increase significantly. Therefore, the W content should be 1.0% or less. The W content is preferably 0.70% or less. In order to stably obtain the above effect, the W content is preferably 0.01% or more, and more preferably 0.02% or more.

[0041] Sb: 0.30% or less Sb is an element that has the effect of improving corrosion resistance in an acidic environment. It suppresses the anodic dissolution reaction of steel in a low pH environment and improves corrosion resistance in a chloride environment by suppressing the hydrogen gas generation reaction and the reduction reaction of Fe 3+ Since it can be contained as needed, however, if it is contained excessively, the toughness will deteriorate significantly. Therefore, the Sb content should be 0.30% or less. The Sb content is preferably 0.15% or less. In order to stably obtain the above effect, the Sb content is preferably 0.05% or more, and more preferably 0.08% or more.

[0042] Co: 1.0% or less Co is an element that improves corrosion resistance in acidic environments, so it can be included as needed. However, excessive amounts will not only saturate the effect but also significantly increase the cost of the steel. Therefore, the Co content should be 1.0% or less. Preferably, the Co content is 0.70% or less. To stably obtain the above effects, it is preferable that the Co content be 0.01% or more, and more preferably 0.02% or more.

[0043] As: 0.30% or less Although the effect of As is not as pronounced as that of Sb and Sn, it is an effective element for improving corrosion resistance in acidic environments, and can be included as needed. However, excessive amounts will reduce hot workability. Therefore, the As content should be 0.30% or less. Preferably, the As content is 0.20% or less. To stably obtain the above effects, it is preferable to have an As content of 0.02% or more, and more preferably 0.05% or more.

[0044] Pb: 0.30% or less Since lead (Pb) is an effective element for improving corrosion resistance in acidic environments, it can be included as needed. However, excessive amounts will degrade hot workability. Therefore, the Pb content should be 0.30% or less. Preferably, the Pb content is 0.15% or less. To stably obtain the above effects, it is preferable that the Pb content be 0.005% or more, and more preferably 0.010% or more.

[0045] Ti: 0.200% or less Since titanium (Ti) is an element that has the effect of suppressing the formation of manganese (MnS), which is the starting point of corrosion, by forming sulfides, it can be included as needed. However, if it is included in excess, not only will the effect saturate but the cost of the steel will also increase. Therefore, the Ti content should be 0.200% or less. Preferably, the Ti content should be 0.150% or less. In order to stably obtain the above effect, it is preferable that the Ti content be 0.001% or more, and more preferably 0.005% or more.

[0046] Zr: 0.20% or less Zr, like Ti, has the effect of suppressing the formation of MnS, which is the initiation site of corrosion, by forming sulfides, and can be included as needed. However, if it is included in excess, not only will the effect saturate but the cost of the steel will also increase. Therefore, the Zr content should be 0.20% or less. Preferably, the Zr content is 0.15% or less. In order to stably obtain the above effect, it is preferable that the Zr content be 0.001% or more, and more preferably 0.005% or more.

[0047] Nb: 0.10% or less Since Nb is an element that increases the strength of steel, it can be included as needed. However, if it is included in excess, not only will the effect saturate, but the toughness of the heat-affected zone (HAZ) will decrease. Therefore, the Nb content should be 0.10% or less. Preferably, the Nb content should be 0.050% or less. In order to stably obtain the above effect, it is preferable that the Nb content be 0.001% or more, and more preferably 0.003% or more.

[0048] V: 0.50% or less V, like Nb, is an element that increases the strength of steel. Also, like Mo and W, it exists in the form of dissolved oxygen ions and has the effect of suppressing the permeation of chloride ions in the rust layer, so it can be included as needed. However, if it is included in excess, not only will the effect saturate but the cost will also increase significantly. Therefore, the V content should be 0.50% or less. Preferably, the V content should be 0.30% or less. In order to stably obtain the above effect, it is preferable that the V content be 0.005% or more, and more preferably 0.010% or more.

[0049] B: 0.010% or less Since B is an element that improves hardenability and increases strength, it can be included as needed. However, if it is included in excess, the effect of increasing strength will saturate, and both the base material and the HAZ will show a significant tendency towards toughness deterioration. Therefore, the B content should be 0.010% or less. To stably obtain the above effect, it is preferable to have a B content of 0.0003% or more.

[0050] Ca: 0.010% or less Ca is an element primarily used to control the form of sulfides and can be included as needed. It also has the effect of suppressing the decrease in pH at the interface in the corrosion reaction area, thereby suppressing the acceleration of corrosion. However, excessive inclusion may impair mechanical properties. Therefore, the Ca content should be 0.010% or less. Preferably, the Ca content is 0.0050% or less. To stably obtain the above effects, it is preferable that the Ca content be 0.0002% or more, and more preferably 0.0005% or more.

[0051] Mg: 0.010% or less Like calcium, magnesium (Mg) suppresses the decrease in pH at the interface in the corrosion reaction area, and can therefore be included as needed. However, excessive amounts will saturate the effect. Therefore, the Mg content should be 0.010% or less. Preferably, the Mg content is 0.0050% or less. To stably obtain the above effect, it is preferable that the Mg content be 0.0002% or more, and more preferably 0.0005% or more.

[0052] REM: 0.0150% or less Rare earth elements (REMs) have the effect of improving the weldability of steel, and can therefore be included as needed. However, if they are included in excess, the effect will saturate, so the REM content should be 0.0150% or less. Preferably, the REM content is 0.0100% or less. To stably obtain the above effect, it is preferable that the REM content be 0.0002% or more, and more preferably 0.0005% or more.

[0053] Here, REM is a collective term for 17 elements including Sc, Y, and lanthanides, and the REM content represents the total amount of these elements. Note that lanthanides are added industrially in the form of mischmetal.

[0054] (B)In particles The steel material according to the present invention has a number density of In particles with a diameter of 5 μm or more, as measured on the surface of the steel material, of 50 / mm². 2 The following applies. Note that In particles with a diameter of less than 5 μm have little effect on the formation of corrosion pits; therefore, this invention focuses on In particles with a diameter of 5 μm or more. In the following description, In particles with a diameter of 5 μm or more will also be simply referred to as In particles.

[0055] As described above, in the steel material of the present invention, In particles are generated during the molten steel stage. However, coarse In particles measured on the surface of the steel material become the starting point for corrosion and degrade corrosion resistance in a repeatedly wet-and-dry environment containing a large amount of chloride. Therefore, the number density of In particles with a diameter of 5 μm or more measured on the surface of the steel material should be 50 / mm². 2 The following applies:

[0056] The number density of In particles is measured by the following method. First, the surface of the steel material is mirror-polished, and then a mapping image of In particles on the steel material surface is obtained using an electron beam microanalyzer (EPMA). Then, the number of In particles with a diameter of 5 μm or more is counted from the obtained mapping image, and the number density is determined by dividing this number by the field of view area.

[0057] EPMA performs elemental analysis up to a depth of approximately 1 μm from the surface of the steel material, so it counts In particles that are not visible on the surface of the steel material. However, In particles present at a depth of approximately 1 μm from the surface of the steel material can easily become exposed to the surface when the steel material corrodes and thins, and can therefore become the starting point for corrosion in a repeatedly wet and dry environment containing a large amount of chloride. For this reason, this invention focuses on In particles measured by EPMA on the surface of the steel material.

[0058] Furthermore, In particles are identified using the In content obtained from EPMA quantitative analysis, and particles with an In content more than twice as high as the average content across the entire analysis field are identified as In particles. In this invention, measurements are performed under the following conditions: acceleration voltage: 15kV, beam diameter: 100nm or less, and measurement pitch: 0.2μm.

[0059] (C) Manufacturing method The steel manufacturing method according to the present invention is characterized by the heat treatment conditions after hot rolling, as described later, but there are no particular restrictions on other manufacturing methods. For example, it includes steel plates, steel pipes, etc., manufactured by hot rolling an ingot having the above-mentioned chemical composition, and then cold rolling it as necessary. There are no particular restrictions on the heating conditions when performing hot rolling, and ordinary conditions may be used.

[0060] When manufacturing steel materials, steel is melted using conventional methods, its composition is adjusted, and the resulting steel billets are hot-rolled and heat-treated. Further cold-rolling is performed as needed.

[0061] The solid solubility limit of In in the Fe-In binary system is 0.57%, but since various other elements are also dissolved in steel, the solid solubility limit of In in steel is lower than that in the Fe-In binary system. In that does not dissolve in steel exists in the steel as In particles.

[0062] Therefore, after hot rolling, a heat treatment is performed by holding the material at 1000-1200°C for 20-40 minutes. By performing heat treatment within this temperature range, the In particles are finely dispersed or dissolved in the matrix phase, and the number density of coarse In particles measured on the surface of the steel material is reduced to 50 / mm². 2 The following is possible:

[0063] Furthermore, by holding the material in the above temperature range for 20 minutes or more, the number density of coarse In particles measured on the surface of the steel material can be increased to 50 / mm³. 2 The following is possible. On the other hand, if the holding time at the above holding temperature is 40 minutes or more, the manufacturing cost will increase.

[0064] The number density of in particles can be adjusted by the in content of the steel and the heat treatment conditions. Specifically, the higher the in content of the steel, the higher the number density of in particles tends to be. On the other hand, the higher the holding temperature during heat treatment, or the longer the holding time, the lower the number density of in particles tends to be. Therefore, by adjusting the heat treatment conditions within the above range according to the in content of the steel, the number density of in particles can be set to 50 / mm². 2 The following is possible:

[0065] After heat treatment, the material may be cooled with water, or it may be cooled with air and then reheated and hardened. After heat treatment, it may be wound into a coil. After heat treatment, it may be cold-rolled and then subjected to further heat treatment.

[0066] When manufacturing steel pipes, steel plates may be formed into tubular shapes and then welded, resulting in UO steel pipes, electric resistance welded steel pipes, forge-welded steel pipes, spiral steel pipes, and the like. Seamless steel pipes manufactured by hot extrusion or perforation rolling of steel billets are also included in the steel materials of the present invention.

[0067] The present invention will be described more specifically below with reference to examples, but the present invention is not limited to these examples. [Examples]

[0068] Steel having the chemical composition shown in Table 1 was melted and formed into a 50 kg ingot. This ingot was then hot-forged using a conventional method to produce a 60 mm thick block. Next, the block was heated at 1120°C for 1 hour, then hot-rolled to a thickness of 20 mm at 850°C. Finally, it was heated under the conditions shown in Table 2 and then water-cooled to produce a steel plate.

[0069] [Table 1]

[0070] [Table 2]

[0071] Then, two test pieces with a width of 25 mm, a length of 25 mm, and a thickness of 4 mm were taken from the surface layer of each steel plate. For one of the test pieces, the number density of In particles was measured. The other test piece was subjected to the following corrosion test that simulated a chloride environment.

[0072] <Measurement of the number density of In particles> The number density of In particles was measured by the following method. First, after mirror-polishing the surface of the steel material, an In mapping image of the steel material surface was obtained by EPMA. Then, the number of In particles with a diameter of 5 μm or more was counted and divided by the field area to obtain the number density. The measurement conditions by EPMA were an acceleration voltage of 15 kV, a beam diameter of 100 nm or less, and a measurement pitch of 0.2 μm.

[0073] <Corrosion test> The evaluation of corrosion resistance was performed by an immersion test in a sulfuric acid aqueous solution adjusted to a pH of 3. The test pieces were immersed in the solution at 60 °C for 24 hours, and the reduction amount of the plate thickness was measured respectively.

[0074] The test results are shown in Table 2. The "corrosion loss" in the same table is the average reduction amount of the plate thickness of the test piece, which was calculated using the weight loss before and after the test and the surface area of the test piece. Also, the "number of corrosion pits" is the number of pits with a visible size observed on one side of 25 mm × 25 mm on the surface of the test piece after the test.

[0075] As is clear from the results in Table 2, in Test No. 3 which is a comparative example, the heat treatment was insufficient, and in Test No. 4, no heat treatment was performed. Therefore, the number density of In particles exceeded 50 / mm 2 and the number of corrosion pits became 15 or more. Also, in Test No. 7 which is a comparative example, although both the number density of In particles and the number of corrosion pits were zero, since the steel No. 4 used in the test did not contain In, the corrosion loss became as large as 2.1 g / m 2 / h.

[0076] On the other hand, in Tests No. 1, 2, 5, 6, and 8-25, which are examples of the present invention, the component content specified in the present invention is satisfied in all cases, and therefore the corrosion loss is 1.5 g / m². 2 The rate was less than / h, and the number of corrosion pits was small, at 6 or less. [Industrial applicability]

[0077] The steel material according to the present invention can be used as a corrosion-resistant steel with excellent corrosion resistance in environments with repeated wetting and drying cycles that contain a large amount of chloride.

Claims

1. The chemical composition of the steel material, in mass percent, 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%, Sn: 0.01-0.30%, In: 0.001 to 0.200%, The remainder consists of Fe and impurities. The number density of In particles with a diameter of 5 μm or more, as measured on the surface of the aforementioned steel material, is 50 / mm². ２ The following is: Steel material.

2. The aforementioned chemical composition, in place of a portion of the Fe, is expressed in mass % as follows: Cu: 1.0% or less, Ni: 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, Pb: 0.30% or less, It contains one or more selected from the following: The steel material according to claim 1.

3. The aforementioned chemical composition, in place of a portion of the Fe, is expressed in mass % as follows: Ti: 0.200% or less, Zr: 0.20% or less, Nb: 0.10% or less, V: 0.50% or less, B: 0.010% or less, Ca: 0.010% or less, Mg: 0.010% or less, REM: 0.0150% or less, It contains one or more selected from the following: The steel material according to either claim 1 or claim 2.

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