hot rolled steel

Abstract

To provide a steel material having excellent corrosion resistance under a sulfuric acid corrosive environment of high temperature and high concentration.SOLUTION: A hot rolled steel material having an oxide scale on at least a part of a surface of a base material, wherein a chemical composition of the base material is, in mass%, C: 0.010 to 0.20%, Si: 0.04 to 1.00%, Mn: 0.20 to 2.00%, Cu: 0.05 to 1.00%, Al: 0.005 to 0.10%, Cr: 0.40 to 3.00%, Ti:0.010 to 0.20%, Ni: 0.01 to 0.50%, Sb: 0.0001 to 0.01%, P:0.020% or less, S: 0.020% or less, N: 0.0100% or less, O: 0.0035% or less, remainder: Fe and impurities, with a Cr-rich layer on an interface side between the base material and the oxide scale in the oxide scale, and in a cross-section perpendicular to a surface of the base material, a maximum length of the Sb rich layer in the oxide scale in the direction parallel to the surface of the base metal is 5.0 μm or less.SELECTED DRAWING: None

Description

[Technical Field]

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

[0002] Boiler furnaces and incinerators at waste incineration facilities generate exhaust gases containing water vapor, sulfur oxides, hydrogen chloride, etc. When this exhaust gas is cooled in the exhaust gas chimney, it condenses into sulfuric acid, which causes severe corrosion of the steel materials that make up the exhaust gas passage, a phenomenon known as sulfuric acid dew-point corrosion.

[0003] To address these problems, sulfuric acid dew-point corrosion-resistant steels and highly corrosion-resistant stainless steels have been proposed. For example, Patent Documents 1 to 5 propose steel materials with excellent sulfuric acid dew-point corrosion resistance that contain added elements such as Cu, Sb, Co, and Cr. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2001-164335 [Patent Document 2] Japanese Patent Application Laid-Open No. 2003-213367 [Patent Document 3] Japanese Patent Application Laid-Open No. 2007-239094 [Patent Document 4] Japanese Patent Application Laid-Open No. 2012-57221 [Patent Document 5] International Publication No. 2021 / 095185 Summary of the Invention [Problem to be solved by the invention]

[0005] Steel materials containing Cu, Sb, Cr, etc. exhibit excellent corrosion resistance in sulfuric acid corrosive environments such as exhaust gas chimneys. However, steel materials used in boiler air preheaters and other equipment may be used in sulfuric acid dew-point corrosion environments exceeding 100°C. Furthermore, in such sulfuric acid dew-point corrosion environments exceeding 100°C, extremely high concentrations of sulfuric acid (over 70%) may be generated, and the steel materials used must therefore have excellent corrosion resistance against high-temperature, high-concentration sulfuric acid.

[0006] An object of the present invention is to solve the above problems and to provide a hot-rolled steel material having excellent corrosion resistance in a high-temperature, high-concentration sulfuric acid corrosive environment. [Means for solving the problem]

[0007] The present invention has been made to solve the above-mentioned problems, and is summarized as the following hot-rolled steel material.

[0008] (1) A hot-rolled steel material having an oxide scale on at least a part of the surface of a base material, The chemical composition of the base material is, in mass%, C: 0.010~0.20%, Si: 0.04 to 1.00%, Mn: 0.20 to 2.00%, Cu: 0.05 to 1.00%, Al: 0.005 to 0.10%, Cr: 0.40~3.00%, Ti: 0.010 to 0.20% Ni: 0.01 to 0.50% Sb: 0.0001 to 0.01%, P: 0.020% or less, S: 0.020% or less, N: 0.0100% or less, O: 0.0035% or less, The balance is Fe and impurities. The oxide scale has a Cr-enriched layer on the interface side between the base material and the oxide scale, In a cross section perpendicular to the surface of the base material, the maximum length of the Sb-enriched layer in the oxide scale in a direction parallel to the surface of the base material is 5.0 μm or less. Hot rolled steel.

[0009] (2) The chemical composition contains, in mass %, a part of the Fe replaced by Mo: 0.10% or less W: 0.10% or less, Sn: 0.30% or less, As: 0.30% or less, Co: 0.30% or less, and Bi: 0.30% or less, It contains one or more selected from The hot-rolled steel material according to (1) above.

[0010] (3) The chemical composition contains, in mass %, a part of the Fe replaced by Nb: 0.10% or less, V: 0.10% or less, Ta: 0.050% or less, and B: 0.010% or less, It contains one or more selected from The hot-rolled steel material according to (1) or (2) above.

[0011] (4) The chemical composition contains, in mass %, a part of the Fe replaced by Ca: 0.010% or less, Mg: 0.010% or less, and REM: 0.010% or less, It contains one or more selected from The hot-rolled steel material according to any one of (1) to (3) above. [Effects of the Invention]

[0012] According to the present invention, it is possible to provide a hot-rolled steel material having excellent corrosion resistance in a high-temperature and high-concentration sulfuric acid corrosive environment. DETAILED DESCRIPTION OF THE INVENTION

[0013] In order to solve the above-mentioned problems, the present inventors have conducted detailed research into the corrosion resistance of steel materials, and have come to the following findings.

[0014] In the invention described in Patent Document 5, a layer of concentrated Si, Cu, and Sb is formed between the base material and the oxide scale formed on the surface of the base material, thereby exerting a barrier effect against sulfuric acid and hydrochloric acid, thereby improving corrosion resistance in an acid corrosion environment.

[0015] However, further research by the present inventors has revealed that the above-mentioned barrier effect may not be fully exhibited in a high-temperature, high-concentration sulfuric acid corrosion environment. After investigating the reasons for this, it was found that there are areas in which Sb is locally concentrated as metal Sb in the Si, Cu, and Sb-enriched layer. It was then thought that in a high-temperature, high-concentration sulfuric acid corrosion environment, the oxide scale peels off from the locally concentrated Sb areas, which prevents the barrier effect from being fully exhibited. On the other hand, since Sb is an element that has the effect of significantly improving the corrosion resistance of steel, it is not preferable to completely omit Sb.

[0016] Therefore, the present inventors further investigated a method for suppressing localized concentration of Sb, which is the starting point of corrosion, while containing Sb. As a result, they found that localized concentration of Sb can be suppressed by adding a certain amount of Cr or more and appropriately controlling the hot rolling conditions and coiling conditions. As a result, it was found that corrosion resistance in a high-temperature, high-concentration sulfuric acid corrosive environment can be improved while containing Sb.

[0017] The present invention was made based on the above findings. Each of the requirements of the present invention will be described in detail below.

[0018] (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."

[0019] C: 0.010 to 0.20% C is an element that improves the strength of steel. However, if excessive C is contained, it deteriorates the weld heat affected zone. Therefore, the C content is set to 0.010 to 0.20%. The C content is preferably 0.050% or more. Furthermore, the C content is preferably 0.15% or less, and more preferably 0.10% or less.

[0020] Si: 0.04 to 1.00% Si is an element that contributes to deoxidation and strength improvement and controls the morphology of oxides. However, excessive Si content reduces toughness. Therefore, the Si content is set to 0.04 to 1.00%. The Si content is preferably 0.10% or more, and more preferably 0.20% or more. The Si content is preferably 0.80% or less, and more preferably 0.60% or less.

[0021] Mn: 0.20 to 2.00% Mn is an element that improves strength and toughness. However, excessive Mn content deteriorates mechanical properties. Therefore, the Mn content is set to 0.20 to 2.00%. The Mn content is preferably 0.50% or more, and more preferably 0.80% or more. The Mn content is preferably 1.70% or less, more preferably 1.50% or less, and even more preferably 1.30% or less.

[0022] Cu: 0.05 to 1.00% Cu is an element that, when contained together with Sb, significantly improves corrosion resistance against sulfuric acid. However, excessive Cu content reduces hot workability and impairs productivity. Therefore, the Cu content is set to 0.05 to 1.00%. The Cu content is preferably 0.10% or more, more preferably 0.20% or more, and even more preferably 0.30% or more. Furthermore, the Cu content is preferably 0.85% or less, and more preferably 0.70% or less.

[0023] Al: 0.005 to 0.10% Al is added as a deoxidizer. However, if Al is contained in excess, it deteriorates the toughness of the weld metal. Therefore, the Al content is set to 0.005 to 0.10%. The Al content is preferably 0.030% or more. Furthermore, the Al content is preferably 0.07% or less.

[0024] Cr: 0.40~3.00% Cr is an element that has the effect of improving hardenability, improving strength, and improving sulfuric acid resistance. In the present invention, a Cr-enriched layer can be formed by adding 0.40% or more of Cr and performing finish rolling and coiling in the temperature range described below. However, excessive Cr content may reduce weldability and toughness. Therefore, the Cr content is set to 0.40 to 3.00%. The Cr content is preferably 0.50% or more, more preferably 0.70% or more, and even more preferably 1.00% or more. The Cr content is preferably 2.70% or less, and more preferably 2.50% or less.

[0025] Ti: 0.010 to 0.20% Ti is an element that forms nitrides and contributes to refining crystal grains and improving strength. However, excessive Ti content increases the amount of nitrides that cause corrosion, resulting in deterioration of mechanical properties. Therefore, the Ti content is set to 0.010 to 0.20%. The Ti content is preferably 0.050% or more, and more preferably 0.080% or more. Furthermore, the Ti content is preferably 0.15% or less, and more preferably 0.10% or less.

[0026] Ni: 0.01 to 0.50% Ni is an element that improves corrosion resistance in acid corrosion environments and also has the effect of improving manufacturability in steels containing Cu. Cu is highly effective in improving corrosion resistance, but it is prone to segregation, and its inclusion alone may promote cracking after casting. In contrast, Ni has the effect of reducing Cu surface segregation. The inclusion of Ni not only suppresses Cu segregation and cast slab cracking, but also suppresses the occurrence of localized corrosion due to segregation, thereby improving corrosion resistance. However, Ni is an expensive element, and adding a large amount increases steelmaking costs. Therefore, the Ni content is set to 0.01 to 0.50%. The Ni content is preferably 0.05% or more, more preferably 0.08% or more, and even more preferably 0.10% or more. The Ni content is preferably 0.40% or less, less than 0.29%, 0.25% or less, 0.20% or less, and more preferably 0.15% or less.

[0027] Sb: 0.0001 to 0.01% Sb is an element that, when contained together with Cu, significantly improves corrosion resistance against sulfuric acid. However, if Sb is contained in excess, it becomes difficult to suppress localized concentration of Sb in the oxide scale. Furthermore, hot workability deteriorates, impairing productivity. Therefore, the Sb content is set to 0.0001 to 0.01%. The Sb content is preferably 0.0005% or more, more preferably 0.0010% or more, and even more preferably 0.0020% or more. Furthermore, the Sb content is preferably 0.008% or less, and more preferably 0.005% or less.

[0028] P:0.020% or less P is an impurity that reduces the mechanical properties and productivity of steel. Therefore, the upper limit of the P content is set to 0.020% or less. The P content is preferably 0.017% or less, and more preferably 0.015% or less. It is preferable to reduce the P content as much as possible, that is, the content may be 0%, but an extreme reduction increases the steelmaking cost. Therefore, the P content may be set to 0.001% or more.

[0029] S: 0.020% or less S is an impurity that reduces the mechanical properties and productivity of steel. Therefore, the upper limit of the S content is set to 0.020% or less. The S content is preferably 0.017% or less, and more preferably 0.015% or less. It is preferable to reduce the S content as much as possible, that is, the content may be 0%, but an extreme reduction will increase the steelmaking cost. Therefore, the S content may be set to 0.001% or more.

[0030] N: 0.0100% or less N is an impurity that reduces the mechanical properties and productivity of steel. Therefore, the upper limit of the N content is set to 0.0100% or less. The N content is preferably 0.0080% or less, and more preferably 0.0060% or less. Although the N content may be 0%, an extreme reduction leads to an increase in steelmaking costs. Therefore, the N content may be 0.0010% or more. Furthermore, N has the effect of contributing to the improvement of mechanical properties, etc. by precipitating as fine nitrides. If this effect is desired, the N content may be 0.0020% or more.

[0031] O: 0.0035% or less O is an impurity that forms coarse oxides that act as starting points for corrosion in an acid corrosion environment. Therefore, the upper limit of the O content is set to 0.0035% or less. The O content is preferably 0.0030% or less, and more preferably 0.0025% or less. It is preferable to reduce the O content as much as possible, that is, the O content may be 0%, but an extreme reduction increases the steelmaking cost. Therefore, the O content may be 0.0005% or more, or 0.0010% or more.

[0032] In addition to the above elements, the chemical composition of the steel of the present invention may further contain one or more elements selected from Mo, W, Sn, As, Co, and Bi in the ranges shown below to improve corrosion resistance in acid corrosion environments. Note that these elements are not necessarily essential for the steel material, so the lower limit of their content is 0%. The reasons for limiting each element will be explained below.

[0033] Mo: 0.10% or less Mo is an element that improves corrosion resistance in acidic environments when contained together with Cu, Sb, and Cr, and may therefore be contained as needed. However, because Mo is an expensive element, excessive inclusion of Mo leads to reduced economic efficiency. Therefore, the Mo content is set to 0.10% or less. The Mo content is preferably 0.09% or less, and more preferably 0.08% or less. To more reliably obtain the above effects, the Mo content is preferably 0.01% or more, more preferably 0.02% or more, and even more preferably 0.03% or more.

[0034] W: 0.10% or less Like Mo, W is an element that improves corrosion resistance in acidic environments when contained together with Cu, Sb, and Cr, and therefore may be contained as needed. However, W is also an expensive element, and excessive inclusion of W reduces economic efficiency. Therefore, the W content is set to 0.10% or less. The W content is preferably 0.09% or less, and more preferably 0.08% or less. To ensure the above effects, the W content is preferably 0.01% or more, more preferably 0.02% or more, and even more preferably 0.03% or more.

[0035] Sn: 0.30% or less Sn is an element that improves corrosion resistance in acid corrosion environments when contained together with Cu, and may be contained as needed. However, excessive Sn content reduces hot workability. Therefore, the Sn content is set to 0.30% or less. The Sn content is preferably 0.25% or less, more preferably 0.20% or less, and even more preferably 0.15% or less. To ensure the above effects, the Sn content is preferably 0.01% or more, more preferably 0.02% or more, and even more preferably 0.05% or more.

[0036] As: 0.30% or less Although As does not have a significant effect compared to Sb and Sn, it is an element that is effective in improving corrosion resistance in acid corrosion environments and may be added as needed. However, excessive As content reduces hot workability. Therefore, the As content is set to 0.30% or less. The As content is preferably 0.20% or less, and more preferably 0.10% or less. To ensure the above effects, the As content is preferably 0.01% or more, more preferably 0.02% or more, and even more preferably 0.05% or more.

[0037] Co:0.30% or less Although Co does not have a significant effect compared to Sb and Sn, it is an element that improves corrosion resistance in an acid corrosion environment, so it may be added as needed. However, if Co is added in excess, economic efficiency decreases. Therefore, the Co content is set to 0.30% or less. The Co content is preferably 0.20% or less, and more preferably 0.10% or less. To ensure the above effects, the Co content is preferably 0.01% or more, more preferably 0.02% or more, and even more preferably 0.05% or more.

[0038] Bi:0.30% or less Although Bi does not have a significant effect compared to Sb and Sn, it is an element that improves corrosion resistance in acidic environments and may be added as needed. However, if Bi is added in excess, hot workability deteriorates. Therefore, the Bi content is set to 0.30% or less. The Bi content is preferably 0.20% or less, and more preferably 0.10% or less. To ensure the above effects, the Bi content is preferably 0.001% or more, more preferably 0.002% or more, and even more preferably 0.005% or more.

[0039] In addition to the above elements, the chemical composition of the steel of the present invention may further contain one or more elements selected from Nb, V, Ta, and B within the ranges shown below in order to improve mechanical properties, etc. Note that these elements are not necessarily essential for the steel material, and therefore the lower limit of their content is 0%. The reasons for limiting each element will be explained below.

[0040] Nb: 0.10% or less Like Ti, Nb forms nitrides and contributes to refining crystal grains and improving strength, so it may be added as needed. However, if Nb is added in excess, the nitrides become coarse and mechanical properties deteriorate. Therefore, the Nb content is set to 0.10% or less. The Nb content is preferably 0.09% or less, more preferably 0.08% or less, and even more preferably 0.07% or less. To ensure the above effects, the Nb content is preferably 0.005% or more, more preferably 0.010% or more, and even more preferably 0.015% or more.

[0041] V: 0.10% or less Like Ti and Nb, V is an element that forms nitrides and contributes to refining crystal grains and improving strength, so it may be added as needed. However, if V is added in excess, the nitrides become coarse and mechanical properties deteriorate. Therefore, the V content is set to 0.10% or less. The V content is preferably 0.08% or less, more preferably 0.06% or less, and even more preferably 0.04% or less. To more reliably obtain the above effects, the V content is preferably 0.005% or more.

[0042] Ta:0.050% or less Ta is an element that contributes to improving strength and, although the mechanism is not entirely clear, also contributes to improving corrosion resistance, so it may be added as needed. However, Ta is an expensive element, and adding a large amount of it increases steelmaking costs. Therefore, the Ta content is set to 0.050% or less. The Ta content is preferably 0.040% or less, more preferably 0.030% or less, and even more preferably 0.020% or less. To more reliably obtain the above effects, the Ta content is preferably 0.001% or more, and more preferably 0.005% or more.

[0043] B: 0.010% or less B is an element that improves hardenability and increases strength, so it may be added as needed. However, if excessive B is added, the effect saturates and the toughness of the base material and HAZ may decrease. Therefore, the B content is set to 0.010% or less. The B content is preferably 0.008% or less, more preferably 0.006% or less, and even more preferably 0.004% or less. To more reliably obtain the above effects, the B content is preferably 0.0003% or more, and more preferably 0.0005% or more.

[0044] In addition to the above elements, the chemical composition of the steel of the present invention may further contain one or more elements selected from Ca, Mg, and REM within the ranges shown below for the purpose of deoxidation and inclusion control. Note that these elements are not necessarily essential for the steel material, so the lower limit of their content is 0%. The reasons for limiting each element will be explained below.

[0045] Ca: 0.010% or less Ca is an element mainly used to control the morphology of sulfides, and may be added as needed to form fine oxides. However, excessive Ca content may impair mechanical properties. Therefore, the Ca content is set to 0.010% or less. The Ca content is preferably 0.005% or less. To more reliably obtain the above effects, the Ca content is preferably 0.00005% or more, 0.0001% or more, or 0.0005% or more, more preferably 0.0010% or more, and even more preferably 0.0020% or more.

[0046] Mg: 0.010% or less Mg may be added as needed to form fine oxides. However, adding excessive Mg increases steelmaking costs. Therefore, the Mg content is set to 0.010% or less. The Mg content is preferably 0.005% or less, and more preferably 0.003% or less. To more reliably obtain the above effects, the Mg content is preferably 0.0001% or more, more preferably 0.0003% or more, and even more preferably 0.0005% or more.

[0047] REM: 0.010% or less REM (rare earth elements) are elements mainly used for deoxidation and may be added as needed to form fine oxides. However, adding excessive REM increases steelmaking costs. Therefore, the REM content is set to 0.010% or less. The REM content is preferably 0.005% or less, and more preferably 0.003% or less. To more reliably obtain the above effects, the REM content is preferably 0.0001% or more, more preferably 0.0003% or more, and even more preferably 0.0005% or more.

[0048] Here, REM is a general term for 17 elements in total: Sc, Y, and lanthanides, and the REM content refers to the total amount of the above elements. Note that lanthanides are industrially added in the form of misch metals.

[0049] The chemical composition of the hot-rolled steel material of the present invention is such that the balance is Fe and impurities, which are components that are mixed in from raw materials such as ores and scraps during industrial production of steel material and other factors, and which are acceptable within a range that does not adversely affect the steel material of the present invention.

[0050] (B) Oxide scale The hot-rolled steel material of the present invention has an oxide scale on at least a portion of the surface of a base material, and a Cr-enriched layer in the oxide scale at the interface between the base material and the oxide scale. Furthermore, in a cross section perpendicular to the surface of the base material, the maximum length of the Sb-enriched layer in the oxide scale in a direction parallel to the surface of the base material is 5.0 μm or less. By suppressing the local concentration of Sb, corrosion resistance in a high-temperature, high-concentration sulfuric acid corrosive environment is improved.

[0051] Here, "the interface between the base material and the oxide scale in the oxide scale" means the interface side of the oxide scale from the center position in the thickness direction. The Cr-enriched layer is defined as a region in the oxide scale where the Cr content is at least twice as high as the content in the base material.

[0052] The Sb-enriched layer is defined as a region in the oxide scale where the Sb content is at least twice as high as the content in the base material. If the length of the Sb-enriched layer in the direction parallel to the surface of the base material continuously exceeds 5.0 μm in the direction parallel to the surface of the base material, it becomes a starting point for oxide scale spalling and corrosion. Therefore, the maximum length of the Sb-enriched layer in the oxide scale in the direction parallel to the surface of the base material is set to 5.0 μm or less.

[0053] In the present invention, the mechanism by which localized Sb enrichment can be suppressed is unclear, but is presumed to be as follows. First, when Cr is not contained in the base material, Fe is oxidized on the surface of the base material. Then, Fe migrates to the surface of the base material and is oxidized one after another, causing the oxide scale to grow. As a result, the Fe content at the interface between the base material and the oxide scale becomes low, and conversely, Sb does not migrate, so the relative Sb content becomes high. It is believed that Sb enriches locally in this way.

[0054] On the other hand, by adding a certain amount of Cr or more and performing finish rolling and coiling at high temperatures as in the present invention, Cr, which is more easily oxidized than Fe, is oxidized first on the surface of the base material, forming a Cr-enriched layer. The presence of this Cr-enriched layer suppresses the migration of Fe to the surface of the base material compared to when Cr is not added. Therefore, the decrease in the Fe content at the interface between the base material and the oxide scale in the oxide scale is suppressed, and the increase in the relative Sb content is suppressed. As a result, it is believed that localized enrichment of Sb is also suppressed.

[0055] The Cr-enriched layer in the oxide scale is identified, and the maximum length of the Sb-enriched layer in the oxide scale in a direction parallel to the surface of the base material is measured using the following method. Specifically, Cr and Sb mapping images are obtained using an electron probe microanalyzer (EPMA) for a cross section perpendicular to the surface of the base material, parallel to the rolling direction, and including the interface between the base material and the oxide scale. The Cr-enriched layer in the oxide scale is then identified from the obtained Cr mapping image. The Sb-enriched layer in the oxide scale is also identified from the obtained Sb mapping image, and the length of the Sb-enriched layer in a direction parallel to the surface of the base material is measured. The longest of the measured Sb-enriched layers is then defined as the maximum length of the Sb-enriched layer.

[0056] In the present invention, measurements are performed under the following conditions: acceleration voltage: 15 kV, beam diameter: 100 nm, irradiation time: 20 ms, and measurement pitch: 80 nm. The resolution of the mapping image is 0.08 μm (80 nm) per pixel in both the X and Y directions. In the present invention, if Sb-enriched layers are separated by two or more pixels, i.e., 0.16 μm (160 nm) or more, in the direction parallel to the surface of the base material, the Sb-enriched layers are determined to be discontinuous in the Sb mapping image.

[0057] It is desirable that a Ni-enriched layer be formed closer to the base material than the Cr-enriched layer in the oxide scale. The Ni-enriched layer may contain an Sb-enriched layer, as long as the maximum length in the direction parallel to the surface of the base material is 5.0 μm or less. The presence of the Ni-enriched layer makes it possible to further improve corrosion resistance.

[0058] (C) Manufacturing method A method for producing a hot-rolled steel material according to one embodiment of the present invention will be described. The steel material according to this embodiment includes steel plates, shaped steel, steel pipes, etc., produced by hot rolling. The steel material is preferably a thick steel plate having a plate thickness of 3 mm or more, more preferably 6 mm or more.

[0059] The steel material according to this embodiment is produced by melting steel in a conventional manner, adjusting the composition, and then hot-rolling the resulting steel billet. To oxidize Cr, which is more easily oxidized than Fe, and form a Cr-enriched layer in the oxide scale at the interface between the base metal and the oxide scale, it is important to set the finish rolling start temperature and coiling temperature to relatively high temperatures. To achieve this, it is also important to set the heating temperature before hot rolling to a high temperature, specifically, 1220 to 1400°C.

[0060] If the heating temperature before hot rolling exceeds 1400°C, energy is wasted and production costs increase. On the other hand, by setting the heating temperature before hot rolling to 1220°C or higher, the finish rolling start temperature can be set to 830°C or higher and the coiling temperature to 620°C or higher.

[0061] The finish rolling start temperature is set to 830 to 950°C, and the coiling temperature is set to 620 to 670°C. By setting the temperature range within this range, it is possible to promote the growth of a Cr-enriched layer in the oxide scale, suppress localized concentration of Sb, and improve corrosion resistance in a high-temperature, highly concentrated sulfuric acid corrosive environment.

[0062] When producing steel pipes from the obtained hot-rolled steel plates, the steel plates are formed into tubular shapes and welded, and can be made into, for example, UO steel pipes, electric resistance welded steel pipes, forged steel pipes, spiral steel pipes, etc.

[0063] The present invention will be described in more detail below with reference to examples. Note that the conditions in the examples shown below are examples adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to these examples. Furthermore, various conditions can be adopted in the present invention as long as they do not deviate from the gist of the present invention and the object of the present invention is achieved. [Example]

[0064] Steels (A to V) having the chemical compositions shown in Table 1 were melted, and the steel ingots were subjected to cooling simulating hot rolling and coiling under the conditions shown in Table 2 to produce hot-rolled steel sheets with a thickness of 5 mm. Here, the coiling temperature in Table 2 refers to the cooling start temperature of the cooling simulating coiling.

[0065] [Table 1]

[0066] [Table 2]

[0067] Test specimens for EPMA measurement were cut from each steel sheet obtained, so that the cross section perpendicular to the surface of the base material, parallel to the rolling direction, and including the interface between the base material and the oxide scale served as the measurement surface. The measurement surface was then polished. Then, mapping images of Cr and Sb were obtained using the EPMA, and the presence or absence of a Cr-enriched layer in the oxide scale was determined from the obtained Cr mapping image. Furthermore, Sb-enriched layers in the oxide scale were identified from the obtained Sb mapping image, and the length of the Sb-enriched layer in the direction parallel to the surface of the base material was measured. The longest of the measured Sb-enriched layers was then recorded as the maximum length of the Sb-enriched layer.

[0068] The EPMA measurement conditions were an acceleration voltage of 15 kV, a beam diameter of approximately 100 nm, an irradiation time of 20 ms, and a measurement pitch of 80 nm. The resolution of the mapping image was 0.08 μm (80 nm) per pixel in both the X and Y directions. Furthermore, in the Sb mapping image, the Sb-enriched layer was determined to be discontinuous if the distance between adjacent pixels was greater than two pixels, i.e., greater than 0.16 μm (160 nm), in the direction parallel to the surface of the base material.

[0069] Furthermore, the resulting steel sheets were subjected to the following sulfuric acid immersion test.

[0070] <Sulfuric acid resistance> A test piece measuring 3 mm thick, 25 mm wide, and 25 mm long was taken from the center of each steel plate and polished with a wet #400 polisher to prepare a test piece for evaluating sulfuric acid resistance. The sulfuric acid resistance was evaluated using a sulfuric acid immersion test. In the sulfuric acid immersion test, the test piece was immersed in an 80% sulfuric acid aqueous solution at 140°C for 6 hours.

[0071] Thereafter, the corrosion rate was calculated from the corrosion weight loss of the test piece in the sulfuric acid immersion test. In this example, the corrosion rate in the sulfuric acid immersion test was 10.0 mg / cm 2 / h or less, it was determined that the sulfuric acid resistance was excellent.

[0072] Table 3 shows the presence or absence of a Cr-enriched layer, the maximum length of the Sb-enriched layer, and the evaluation results of the sulfuric acid immersion test.

[0073] [Table 3]

[0074] As shown in Table 3, Test Nos. 1 to 22, which satisfied all of the requirements of the present invention, showed excellent results in the sulfuric acid immersion test. In contrast, Test Nos. 23 to 27, which are comparative examples, showed poor sulfuric acid resistance. [Industrial Applicability]

[0075] The steel material of the present invention can be used in smoke exhaust systems for boilers that burn fossil fuels such as heavy oil and coal, gas fuels such as liquefied natural gas, general waste such as municipal waste, industrial waste such as waste oil, plastics and exhaust tires, and sewage sludge. Specifically, the steel material can be suitably used for flue ducts, casings and heat exchangers of smoke exhaust systems, gas-gas heaters consisting of two heat exchangers (a heat recovery unit and a reheater), desulfurization equipment, electrostatic precipitators, induced draft fans, basket materials and heat transfer element plates of rotary regenerative air preheaters, etc.

Claims

1. A hot-rolled steel material having an oxide scale on at least a portion of a surface of a base material, The chemical composition of the base material is, in mass%, C: 0.010-0.20%, Si: 0.04-1.00%, Mn: 0.20-2.00%, Cu: 0.05-1.00%, Al: 0.005-0.10%, Cr: 0.40-3.00%, Ti: 0.010 to 0.20%, Ni: 0.01-0.50%, Sb: 0.0001 to 0.01%, P: 0.020% or less, S: 0.020% or less, N: 0.0100% or less, O: 0.0035% or less, The balance is Fe and impurities. The oxide scale has a Cr-enriched layer on the interface side between the base material and the oxide scale, In a cross section perpendicular to the surface of the base material, the maximum length of the Sb-enriched layer in the oxide scale in a direction parallel to the surface of the base material is 5.0 μm or less. Hot rolled steel.

2. The chemical composition contains, in mass %, replacing a part of the Fe, Mo: 0.10% or less, W: 0.10% or less, Sn: 0.30% or less, As: 0.30% or less, Co: 0.30% or less, and Bi: 0.30% or less, It contains one or more selected from The hot-rolled steel material according to claim 1.

3. The chemical composition contains, in mass %, replacing a part of the Fe, Nb: 0.10% or less, V: 0.10% or less, Ta: 0.050% or less, and B: 0.010% or less, It contains one or more selected from The hot-rolled steel material according to claim 1 or 2.

4. The chemical composition contains, in mass %, replacing a part of the Fe, Ca: 0.010% or less, Mg: 0.010% or less, and REM: 0.010% or less, It contains one or more selected from The hot-rolled steel material according to any one of claims 1 to 3.

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