steel

Abstract

The present invention provides a steel material that exhibits excellent corrosion resistance in both strongly acidic and weakly acidic environments, as well as high tensile strength, even when the Ni content is reduced. [Solution] The chemical composition is as follows (in mass%): C: 0.01% or more and less than 0.20%, Si: 0.10-0.50%, Mn: 0.30-1.20%, Cu: 0.10-0.50%, Cr: 0.10-1.00%, Ti: 0.01-0.15%, Ni: 0.03-0.10%, Al: 0.005-0.10%, V: 0.001-0.050%, P: 0.025% or less, S: 0.0005-0.0150%, N: 0.0005-0.0080%, O: 0.0005-0.0035%, remainder: Fe and impurities, BI is 33.0-81.0, and 1.00 ≤ (Ti / 48) × 10 3 +(V / 51)×10 4 Steel material that satisfies ≤10.0.

Description

Technical Field

[0001] The present invention relates to steel materials.

Background Art

[0002] In boilers' fireboxes and incinerators in waste incineration facilities, exhaust gases containing water vapor, sulfur oxides, hydrogen chloride, etc. are generated. When this exhaust gas is cooled in an exhaust gas chimney or the like, it condenses into sulfuric acid and hydrochloric acid, and as known as acid dew point corrosion, it causes significant corrosion to the steel materials constituting the exhaust gas flow path.

[0003] Regarding such problems, sulfuric acid dew point corrosion-resistant steel and highly corrosion-resistant stainless steel have been proposed. For example, in Patent Documents 1 to 5, steel materials with excellent sulfuric acid dew point corrosion resistance added with Cu, Sb, Co, Cr, etc. have been proposed.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Summary of the Invention

Problems to be Solved by the Invention

[0005] In acid dew point corrosion environments such as exhaust gas chimneys, when the exhaust gas temperature drops, water vapor in the exhaust gas first condenses on the surface of the steel, and sulfur oxides and hydrogen chloride dissolve in it. The high concentrations of sulfuric acid and hydrochloric acid produced in this way are highly reactive. Steel containing Cu, Sb, Cr, etc., has excellent corrosion resistance in such highly acidic environments.

[0006] On the other hand, in environments around 50°C where the amount of condensed moisture increases further, low concentrations of sulfuric acid and hydrochloric acid are generated. When exhaust gas temperatures fluctuate in the environment in which steel materials are used, or when some parts of the steel are exposed to high-temperature exhaust gas and other parts to low-temperature exhaust gas, the steel is exposed to both strongly acidic and weakly acidic environments. Therefore, it is required that steel exhibit good corrosion resistance in both strongly acidic and weakly acidic environments. However, while conventional steel has excellent corrosion resistance in strongly acidic environments, there is still room for improvement in its corrosion resistance in weakly acidic environments.

[0007] Incidentally, conventionally, a certain amount of nickel (Ni) has been included to ensure corrosion resistance in strongly acidic and weakly acidic environments. However, since Ni is an expensive element, it is desirable to reduce its content as much as possible. Furthermore, steel materials used in exhaust gas chimneys and the like also require strength.

[0008] The present invention aims to solve the above problems and provide a steel material that has excellent corrosion resistance in strongly acidic and weakly acidic environments, as well as high tensile strength, even when the Ni content is reduced. [Means for solving the problem]

[0009] This invention was made to solve the above problems, and its essence is the following steel material.

[0010] (1) The chemical composition is expressed in mass%, C: 0.01% or more and less than 0.20% Si: 0.10~0.50%, Mn: 0.30~1.20%, Cu: 0.10~0.50%, Cr: 0.10 to 1.00%, Ti: 0.01 to 0.15%, Ni: 0.03 to 0.10%, Al: 0.005 to 0.10%, V: 0.001 to 0.050%, P: 0.025% or less, S: 0.0005 to 0.0150%, N: 0.0005 to 0.0080%, O: 0.0005 to 0.0035%, The balance: Fe and impurities, BI defined by the following formula (i) is 33.0 to 81.0, satisfies the following formula (ii), Steel material. BI = (Cr / 52) / (N / 14) ···(i) 1.00 ≤ (Ti / 48)×10 3 +(V / 51)×10 4 ≤ 10.0 ···(ii) However, the element symbols in the above formulas represent the content (mass%) of each element contained in the steel material.

[0011] (2) The chemical composition is, in mass%, C: 0.01% or more and less than 0.20%, Si: 0.10 to 0.50%, Mn: 0.30 to 1.20%, Cu: 0.10 to 0.50%, Cr: 0.10 to 1.00%, Ti: 0.01 to 0.15%, Ni: 0.03 to 0.10%, Al: 0.005 to 0.10%, V: 0.001 to 0.05%, P: 0.025% or less, S: 0.0005 to 0.0150%, N: 0.0005 to 0.0080%, O: 0.0005 to 0.0035%, and further contains one or more selected from the group consisting of the following Group A, Group B, and Group C, The remainder consists of Fe and impurities. The BI defined by the following formula (i) is 33.0 to 81.0, The following equation (ii) is satisfied: Steel material. BI = (Cr / 52) / (N / 14) ... (i) 1.00 ≤ (Ti / 48) × 10 3 +(V / 51)×10 4 ≤10.0 ···(ii) However, the element symbols in the above formula represent the content (mass %) of each element contained in the steel material. [Group A] One or more elements selected from the group consisting of Sn: 0.30% or less, Sb: 0.020% or less, Mo: 0.10% or less, W: 0.10% or less, In: less than 0.03%, As: 0.10% or less, Co: 0.30% or less, Bi: 0.10% or less, Te: 0.10% or less, Pb: 0.10% or less, Zn: 0.10% or less, Ga: 0.10% or less, Ge: 0.10% or less, and Hf: 0.10% or less. [Group B] One or more selected from the group consisting of Nb: ≤0.10%, Zr: ≤0.050%, Ta: ≤0.05%, and B: ≤0.010%. [Group C] One or more elements selected from the group consisting of: Ca and / or Mg combined: 0.010% or less, Sr and / or Ba combined: 0.010% or less, and REM: 0.010% or less.

[0012] (3) The steel material according to (2) above, wherein the chemical composition contains one or more elements selected from group A.

[0013] (4) The steel material according to (2) above, wherein the chemical composition contains one or more elements selected from group B.

[0014] (5) The steel material according to (2) above, wherein the chemical composition contains one or more elements selected from the C group. [Effects of the Invention]

[0015] According to the present invention, even when the Ni content is reduced, a steel material can be obtained that has excellent corrosion resistance in strongly acidic and weakly acidic environments, as well as high tensile strength. [Modes for carrying out the invention]

[0016] In order to solve the aforementioned problems, the inventors conducted a detailed investigation into the corrosion resistance and tensile strength of steel materials and obtained the following findings.

[0017] By incorporating elements such as Cu into steel materials, corrosion resistance in highly acidic environments is improved. On the other hand, as mentioned above, this invention reduces the Ni content in order to lower manufacturing costs. Therefore, the effect of improving corrosion resistance in weakly acidic and highly acidic environments by including a certain amount of Ni or more is insufficient. It was found that there is room for improvement in order to maintain corrosion resistance in highly acidic environments while improving corrosion resistance in weakly acidic environments with a pH of approximately 0 to 2, even when the Ni content is reduced.

[0018] The inventors conducted a detailed investigation into the effects of varying elemental ratios on corrosion resistance in strongly acidic and weakly acidic environments using various steel materials. The results showed that while chromium (Cr) is effective in improving the corrosion resistance of steel, excessive Cr content forms nitrides, becoming a starting point for corrosion. Therefore, they found that by appropriately adjusting the Cr content relative to the N content to avoid excessive Cr, it is possible to maintain resistance to strongly acidic environments while improving resistance to weakly acidic environments by ensuring a certain level of Cr relative to N content. In other words, they found that by setting the acid corrosion index BI = (Cr / 52) / (N / 14) between 33.0 and 81.0, it is possible to maintain corrosion resistance in strongly acidic environments while improving corrosion resistance in weakly acidic environments.

[0019] Within the aforementioned BI range, corrosion resistance in both strongly acidic and weakly acidic environments can be ensured by incorporating Cr and N. However, as mentioned above, steel materials used in exhaust gas chimneys and the like also require strength.

[0020] Therefore, the inventors conducted further investigations into methods for achieving both excellent corrosion resistance and high tensile strength. As a result, they found that to improve the strength of steel, it is effective to include 0.03% or more Ni, as well as Ti and V in amounts that satisfy predetermined conditions. In this way, they found that even with a reduced amount of Ni, it is possible to achieve both excellent corrosion resistance and high tensile strength while suppressing manufacturing costs.

[0021] In this way, even when the Ni content is reduced, excellent corrosion resistance in both strongly acidic and weakly acidic environments, as well as high tensile strength, can be obtained.

[0022] This invention is based on the above findings. The requirements of this invention will be described in detail below.

[0023] (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%".

[0024] C: 0.01% or more and less than 0.20% Carbon (C) is an element that improves the strength of steel. However, if C is present in excess, it degrades the heat-affected zone during welding. Therefore, the C content should be between 0.01% and less than 0.20%. Preferably, the C content should be 0.03% or more, and more preferably 0.05% or more. Furthermore, preferably, the C content should be 0.18% or less, and more preferably 0.16% or less.

[0025] Si: 0.10~0.50% Si is an element that contributes to deoxidation and strength improvement, and controls the morphology of oxides. However, if Si is present in excess, it reduces toughness. Therefore, the Si content should be 0.10 to 0.50%. Preferably, the Si content is 0.15% or more, and more preferably 0.20% or more. Furthermore, preferably, the Si content is 0.45% or less, and more preferably 0.40% or less.

[0026] Mn: 0.30~1.20% Mn is an element that improves strength and toughness. However, if Mn is present in excess, the mechanical properties deteriorate. Therefore, the Mn content should be between 0.30 and 1.20%. Preferably, the Mn content is 0.50% or more, and more preferably 0.70% or more. Furthermore, preferably, the Mn content is 1.00% or less, and more preferably 0.80% or less.

[0027] Cu: 0.10~0.50% Cu is an element that exhibits remarkable corrosion resistance in highly acidic environments. However, if Cu is present in excess, hot workability decreases, impairing productivity. Therefore, the Cu content should be between 0.10% and 0.50%. Preferably, the Cu content is 0.15% or more, more preferably 0.20% or more, and even more preferably 0.25% or more. Furthermore, preferably, the Cu content is 0.45% or less, and more preferably 0.40% or less.

[0028] Cr: 0.10~1.00% Cr is an element that contributes to improved corrosion resistance in strongly acidic and weakly acidic environments. It is also an element that enhances hardenability and improves strength. However, if Cr is present in excess, it may degrade weldability and toughness. Therefore, the Cr content should be between 0.10 and 1.00%. Preferably, the Cr content is 0.80% or less, more preferably 0.70% or less. Furthermore, preferably 0.15% or more, more preferably 0.20% or more, and even more preferably 0.25% or more.

[0029] Ti: 0.01~0.15% Ti is an element that forms nitrides, contributing to grain refinement and improved strength. However, if Ti is present in excess, the nitrides become coarse, and the mechanical properties deteriorate. Therefore, the Ti content should be 0.01 to 0.15%. Preferably, the Ti content should be 0.03% or more, and more preferably 0.05% or more. Furthermore, preferably, the Ti content should be 0.13% or less, and more preferably 0.10% or less.

[0030] Ni: 0.03~0.10% Ni is an element that improves strength. It also has the effect of improving manufacturability in steel containing Cu. Cu is very effective in improving corrosion resistance in highly acidic environments, but it is prone to segregation and, when included alone, may promote cracking after casting. In contrast, Ni has the effect of reducing surface segregation of Cu. By including Ni, the effect of suppressing Cu segregation and cast slab cracking can be obtained. However, as mentioned above, Ni is an expensive element, so in this invention, its content is reduced. Therefore, the Ni content is set to 0.03 to 0.10%. The Ni content is preferably 0.05% or more, and more preferably 0.07% or more. Furthermore, the Ni content is preferably 0.09% or less, and more preferably 0.08% or less.

[0031] Al: 0.005~0.10% Al is added as a deoxidizing agent. However, if Al is present in excess, it will degrade the toughness of the weld metal. Therefore, the Al content should be 0.005 to 0.10%. Preferably, the Al content is 0.010% or more, and more preferably 0.020% or more. Furthermore, preferably, the Al content is 0.080% or less, and more preferably 0.050% or less.

[0032] V: 0.001~0.050% V is an element that forms nitrides and contributes to grain refinement and improved strength. However, if V is present in excess, the nitrides become coarse, and the mechanical properties deteriorate. Therefore, the V content should be between 0.001% and 0.050%. Preferably, the V content is 0.005% or more, more preferably 0.010% or more, and even more preferably 0.015% or more. Furthermore, preferably, the V content is 0.040% or less, and more preferably 0.030% or less.

[0033] P:0.025% or less P is an impurity that reduces the mechanical properties and productivity of steel. Therefore, an upper limit is set on the P content to 0.025% or less. Preferably, the P content is 0.020% or less, and more preferably 0.015% or less. It is preferable to reduce the P content as much as possible, meaning it may be 0%, but extreme reduction will lead to an increase in steelmaking costs. Therefore, the P content may be 0.001% or more, or 0.003% or more.

[0034] S: 0.0005~0.0150% S is generally an impurity that reduces the mechanical properties and productivity of steel. However, in this invention, S, when included together with Cu, has the effect of improving corrosion resistance in highly acidic environments. Therefore, the S content is set to 0.0005 to 0.0150%. Preferably, the S content is 0.0010% or more, and more preferably 0.0030% or more. Furthermore, preferably, the S content is 0.0130% or less, and more preferably 0.0100% or less.

[0035] N: 0.0005~0.0080% N contributes to improving mechanical properties by precipitating as fine nitrides. However, if N is present in excess, the nitrides become coarse, and the mechanical properties and corrosion resistance deteriorate. Therefore, the N content should be between 0.0005% and 0.0080%. Preferably, the N content is 0.0020% or more, and more preferably 0.0030% or more. Furthermore, preferably, the N content is 0.0070% or less, and more preferably 0.0060% or less.

[0036] O: 0.0005~0.0035% O is an element that, by bonding with MnS, detoxifies MnS and prevents deterioration of corrosion resistance and mechanical properties in strongly acidic and weakly acidic environments. However, if O is present in excess, it generates coarse oxides that become the starting point for corrosion in strongly acidic and weakly acidic environments. Therefore, the O content should be between 0.0005% and 0.0035%. Preferably, the O content is 0.0010% or more, and more preferably 0.0015% or more. Furthermore, preferably, the O content is 0.0030% or less, and more preferably 0.0025% or less.

[0037] In the chemical composition of the steel material of the present invention, the remainder is Fe and impurities. Here, impurities refer to components that are mixed in during the industrial production of steel material due to raw materials such as ore and scrap, or other factors, and are acceptable within a range that does not adversely affect the steel material of the present invention.

[0038] BI = (Cr / 52) / (N / 14) ... (i) As described above, in order to improve resistance to weak acids while maintaining resistance to strong acids, it is necessary to control the Cr content relative to the N content. If the Cr content is too high relative to the N content, corrosion resistance in a strongly acidic environment cannot be maintained. On the other hand, if the Cr content is too low relative to the N content, corrosion resistance in a weakly acidic environment cannot be improved. Therefore, the BI value defined in equation (i) above should be between 33.0 and 81.0. The element symbols in equation (i) above represent the content (mass%) of each element contained in the steel material.

[0039] The above formula (i) consists of the number of Cr and N atoms. That is, Cr / 52 and N / 14 are terms obtained by dividing the content of Cr and N by the mass number of each element, respectively. The value of the above formula (i) is preferably 35.0 or more, and more preferably 40.0 or more. Furthermore, the value of the above formula (i) is preferably 75.0 or less, and more preferably 70.0 or less.

[0040] 1.00 ≤ (Ti / 48) × 10 3 +(V / 51)×10 4 ≤10.0 ···(ii) As described above, in order to ensure tensile strength by precipitating nitrides and / or carbonitrides containing Ti and / or V, the middle value of equation (ii) above must be 1.00 or higher. On the other hand, even if the middle value of equation (ii) above exceeds 10.0 and Ti and V are included, the effect of improving tensile strength saturates. Also, since Ti and V are expensive elements, manufacturing costs increase. Therefore, the middle value of equation (ii) above should be between 1.00 and 10.0. The element symbols in equation (ii) above represent the content (mass %) of each element contained in the steel material.

[0041] The above formula (ii) consists of the number of Ti and V atoms. That is, Ti / 48 and V / 51 are terms obtained by dividing the Ti and V content by the mass number of each element, respectively. The middle value of the above formula (ii) is preferably 2.00 or more, and more preferably 3.00 or more. Furthermore, the middle value of the above formula (ii) is preferably 9.00 or less, and more preferably 8.00 or less.

[0042] In the chemical composition of the steel of the present invention, in order to improve corrosion resistance in strongly acidic and weakly acidic environments, one or more elements selected from Sn, Sb, Mo, W, In, As, Co, Bi, Te, Pb, Zn, Ga, Ge, and Hf may be included within the ranges shown below. Since these elements are not necessarily essential in steel, the lower limit of their content is 0%. The reasons for limiting each element are explained below.

[0043] Sn: 0.30% or less Sn (Sn) is an element that improves corrosion resistance in highly acidic environments, and therefore may be included as needed. However, if Sn is included in excess, hot workability will decrease. For this reason, the Sn content should be 0.30% or less. Preferably, the Sn content is 0.25% or less, and more preferably 0.20% or less. Furthermore, if the above effects are to be obtained more reliably, the Sn content is preferably 0.01% or more, and more preferably 0.05% or more.

[0044] Sb: 0.020% or less Sb (Sulfur) is an element that exhibits remarkable corrosion resistance in highly acidic environments when included together with Cu (Cu), and therefore may be included as needed. However, if Sb is included in excess, hot workability decreases, impairing productivity. For this reason, the Sb content should be 0.020% or less. Preferably, the Sb content is 0.015% or less, and more preferably 0.010% or less. Furthermore, if the above effects are to be obtained more reliably, the Sb content is preferably 0.003% or more, and more preferably 0.005% or more.

[0045] Mo: 0.10% or less Mo (Mo) is an element that improves corrosion resistance in highly acidic environments when included together with Cu (Cu), and therefore may be included as needed. However, since Mo is an expensive element, excessive inclusion leads to a decrease in economic efficiency. For this reason, the Mo content should be 0.10% or less. Preferably, the Mo content is 0.08% or less, and more preferably 0.06% or less. Furthermore, if the above effects are to be obtained more reliably, the Mo content is preferably 0.01% or more, and more preferably 0.03% or more.

[0046] W: 0.10% or less Water (W), when included simultaneously with copper (Cu), improves corrosion resistance in both strongly acidic and weakly acidic environments; therefore, it may be included as needed. However, since W is an expensive element, excessive inclusion leads to a decrease in economic efficiency. For this reason, the W content should be 0.10% or less. Preferably, the W content is 0.08% or less, and more preferably 0.06% or less. Furthermore, if the above effects are to be obtained more reliably, the W content is preferably 0.01% or more, and more preferably 0.03% or more.

[0047] In: Less than 0.03% In (in) is an element effective in improving corrosion resistance in weakly acidic environments, and may be included as needed. However, if excessive amounts of In are included, hot workability decreases. Therefore, the In content should be less than 0.03%. Preferably, the In content is 0.02% or less, and more preferably 0.01% or less. Furthermore, if the above effects are to be obtained more reliably, the In content is preferably 0.001% or more, more preferably 0.002% or more, and even more preferably 0.005% or more.

[0048] As: 0.10% or less As is an element effective in improving corrosion resistance in weakly acidic environments, and may be included as needed. However, if As is included in excess, toughness will decrease. Therefore, the As content should be 0.10% or less. Preferably, the As content is 0.08% or less, and more preferably 0.06% or less or 0.03% or less. Furthermore, if the above effects are to be obtained more reliably, the As content is preferably 0.01% or more, and more preferably 0.02% or more.

[0049] Co:0.30% or less Co is an element that forms oxides and improves corrosion resistance in strongly acidic and weakly acidic environments, so it may be included as needed. However, if Co is included in excess, the economic efficiency decreases. Therefore, the Co content should be 0.30% or less. Preferably, the Co content is 0.25% or less, and more preferably 0.20% or less. Furthermore, if the above effects are to be obtained more reliably, the Co content is preferably 0.01% or more, more preferably 0.02% or more, and even more preferably 0.05% or more.

[0050] Bi:0.10% or less Bi is an effective element for improving corrosion resistance in weakly acidic environments, and may be included as needed. However, if Bi is included in excess, toughness will decrease. Therefore, the Bi content should be 0.10% or less. Preferably, the Bi content is 0.08% or less, and more preferably 0.05% or less. If the above effects are to be obtained more reliably, the Bi content is preferably 0.001% or more, more preferably 0.002% or more, and even more preferably 0.005% or more.

[0051] Te: 0.10% or less Te is an element that is effective in improving corrosion resistance in weakly acidic environments, and therefore may be included as needed. However, if Te is included in excess, toughness will decrease. For this reason, the Te content should be 0.10% or less. Preferably, the Te content is 0.08% or less, and more preferably 0.05% or less. Furthermore, if the above effects are to be obtained more reliably, the Te content is preferably 0.001% or more, more preferably 0.002% or more, and even more preferably 0.005% or more.

[0052] Pb: 0.10% or less Since lead (Pb) forms sulfides with sulfur and is an effective element for improving corrosion resistance in weakly acidic environments, it may be included as needed. However, if Pb is included in excess, toughness deteriorates. Therefore, the Pb content should be 0.10% or less. Preferably, the Pb content is 0.08% or less, and more preferably 0.05% or less. Furthermore, if the above effect is to be obtained more reliably, the Pb content should be 0.005% or more, and more preferably 0.01% or more.

[0053] Zn: 0.10% or less Ga: 0.10% or less Zn and Ga form sulfides with S and are effective elements for improving corrosion resistance in weakly acidic environments, so they may be included as needed. However, if Zn and Ga are included in excess, toughness deteriorates. Therefore, the content of Zn and Ga should be 0.10% or less each. Preferably, the content of Zn and Ga should be 0.08% or less each, and more preferably 0.05% or less each. If the above effect is to be obtained more reliably, it is preferable that the content of Zn and Ga be 0.005% or more each, and more preferably 0.01% or more each.

[0054] Ge: 0.10% or less Ge forms sulfides with S and is an effective element for improving corrosion resistance in weakly acidic environments, so it may be included as needed. However, if Ge is included in excess, toughness deteriorates. Therefore, the Ge content should be 0.10% or less. Preferably, the Ge content is 0.08% or less, and more preferably 0.05% or less. If the above effect is to be obtained more reliably, it is preferable that the Ge content be 0.005% or more, and more preferably 0.01% or more.

[0055] Hf: 0.10% or less Since Hf is an element that forms oxides and improves corrosion resistance, it may be included as needed. However, if Hf is included in excess, the economic efficiency decreases. Therefore, the Hf content should be 0.10% or less. Preferably, the Hf content is 0.08% or less, and more preferably 0.06% or less. Furthermore, if the above effects are to be obtained more reliably, the Hf content should be 0.002% or more, and more preferably 0.005% or more.

[0056] In the chemical composition of the steel of the present invention, one or more elements selected from Nb, Zr, Ta, and B may be further included within the ranges shown below in order to improve mechanical properties, etc. Since these elements are not necessarily essential in steel, the lower limit of their content is 0%. The reasons for limiting each element are explained below.

[0057] Nb: 0.10% or less Nb is an element that forms nitrides and contributes to grain refinement and improved strength, so it may be included as needed. However, if Nb is included in excess, the nitrides become coarse and the mechanical properties deteriorate. For this reason, the Nb content should be 0.10% or less. Preferably, the Nb content is 0.09% or less, more preferably 0.08% or less, and even more preferably 0.07% or less. If the above effects are to be obtained more reliably, the Nb content is preferably 0.001% or more, more preferably 0.005% or more, and even more preferably 0.01% or more.

[0058] Zr: 0.050% or less Zr, like Ti, is an element that forms nitrides and contributes to grain refinement and improved strength, so it may be included as needed. However, if Zr is included in excess, the nitrides become coarse, and the mechanical properties deteriorate. For this reason, the Zr content should be 0.050% or less. Preferably, the Zr content is 0.045% or less. Furthermore, if the above effects are to be obtained more reliably, the Zr content should be 0.0010% or more, and more preferably 0.0050% or more.

[0059] Ta: 0.05% or less Ta is an element that contributes to improving strength, and although the mechanism is not entirely clear, it also contributes to improving corrosion resistance in strongly acidic and weakly acidic environments, so it may be included as needed. However, Ta is an expensive element, and including large amounts will lead to increased steelmaking costs. Therefore, the Ta content should be 0.05% or less. Preferably, the Ta content is 0.04% or less, more preferably 0.03% or less, and even more preferably 0.02% or less. If the above effects are to be obtained more reliably, the Ta content is preferably 0.001% or more, and more preferably 0.005% or more.

[0060] B: 0.010% or less B is an element that improves hardenability and increases strength, so it may be included as needed. However, if B is included in excess, the effect may saturate, and the toughness of the base material and HAZ may decrease. For this reason, the B content should be 0.010% or less. Preferably, the B content is 0.008% or less, more preferably 0.006% or less, and even more preferably 0.004% or less. If the above effects are to be obtained more reliably, the B content is preferably 0.0003% or more, and more preferably 0.0005% or more.

[0061] In the chemical composition of the steel of the present invention, one or more elements selected from Ca, Mg, Sr, Ba, and REM may be further included within the ranges shown below for the purpose of deoxidation and control of inclusions. Since these elements are not necessarily essential in steel, the lower limit of their content is 0%. The reasons for limiting each element are explained below.

[0062] Total of either Ca or Mg, or both: 0.010% or less Ca and Mg may be included as needed to form fine oxides. However, adding excessive amounts of Ca and Mg will increase steelmaking costs. Therefore, the total content of Ca and Mg should be 0.010% or less. Ca and Mg may be included individually or both simultaneously. The total content of Ca and Mg is preferably 0.008% or less, and more preferably 0.005% or less. Furthermore, if the above effects are to be obtained more reliably, the total content of Ca and Mg is preferably 0.0001% or more, more preferably 0.0003% or more, and even more preferably 0.0005% or more.

[0063] Furthermore, when Ca and Mg are included individually, the content of Ca and Mg is preferably 0.008% or less, and more preferably 0.005% or less. In addition, the content of Ca and Mg is preferably 0.0001% or more, and more preferably 0.0003% or more.

[0064] Sr and Ba (either one or both combined): 0.010% or less Sr and Ba may be included as needed to form fine oxides. However, adding excessive amounts of Sr and Ba will increase steelmaking costs. Therefore, the total content of Sr and Ba should be 0.010% or less. Sr and Ba may be included individually or both simultaneously. The total content of Sr and Ba is preferably 0.008% or less, and more preferably 0.005% or less. Furthermore, if the above effects are to be obtained more reliably, the total content of Sr and Ba is preferably 0.0001% or more, more preferably 0.0003% or more, and even more preferably 0.0005% or more.

[0065] Furthermore, when either Sr or Ba is included individually, the content of Sr and Ba is preferably 0.008% or less, and more preferably 0.005% or less. In addition, the content of Sr and Ba is preferably 0.0001% or more, and more preferably 0.0003% or more.

[0066] REM: 0.010% or less Rare earth elements (REMs) are primarily used for deoxidation and may be included as needed to form fine oxides. However, excessive addition of REMs increases steelmaking costs. Therefore, the REM content should be 0.010% or less. Preferably, the REM content is 0.0050% or less, and more preferably 0.0030% 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.

[0067] 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 industrially added in the form of mischmetal.

[0068] Furthermore, when one or more elements selected from Ca, Mg, Sr, Ba, and REM are included, the total content is preferably 0.010% or less, or 0.008% or less, and preferably 0.0003% or more, or 0.0005% or more.

[0069] (B) Corrosion prevention treatment The steel material of the present invention described above exhibits good corrosion resistance even when used as is. However, in order to further improve corrosion resistance in strongly acidic and weakly acidic environments, acid-resistant paint, heat-resistant paint, heat-resistant and acid-resistant paint, etc., may be applied to the surface of the steel material.

[0070] Examples of corrosion-resistant coatings made from organic resins include resin coatings made from vinyl resin, epoxy resin, silicone resin, and phenolic resin acid-resistant paints.

[0071] (C) Tensile strength The tensile strength of the steel material of the present invention is preferably 450 MPa or more, more preferably 470 MPa or more, and even more preferably 480 MPa or more.

[0072] (D) Manufacturing method There are no particular limitations on the method for manufacturing steel materials according to the present invention. For example, steel plates, steel pipes, etc., are produced by hot rolling an ingot having the above-mentioned chemical composition, and then cold rolling it as necessary. There are no particular limitations on the heating conditions when performing hot rolling; ordinary conditions may be used.

[0073] When manufacturing steel materials, steel is melted using conventional methods, its composition is adjusted, and the resulting steel billet is hot-rolled and, if necessary, cold-rolled. After hot-rolling, it may be cooled in water or air-cooled, then reheated and quenched. After hot-rolling, it may be wound into a coil. After hot-rolling, it may be cold-rolled and then heat-treated.

[0074] When manufacturing steel pipes, steel plates may be formed into tubular shapes and then welded, resulting in 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.

[0075] Furthermore, the process of covering the material with the aforementioned corrosion-preventive coating can be carried out using the usual methods. Moreover, it is not always necessary to apply the corrosion-preventive coating to the entire surface of the steel material; it is sufficient to treat only one side of the steel material exposed to the corrosive environment—for example, the outer or inner surface of a steel pipe—that is, at least a portion of the steel material's surface.

[0076] The present invention will be described more specifically below with reference to examples, but the present invention is not limited to these examples. [Examples]

[0077] Steel having the chemical compositions shown in Tables 1 and 2 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, and finally cooled to room temperature in the atmosphere to form a steel plate.

[0078] [Table 1]

[0079] [Table 2]

[0080] <Corrosion resistance> Two test pieces, each 3 mm thick, 25 mm wide, and 25 mm long, were taken from the center of each steel plate and finished with wet #600 polishing to serve as test pieces for corrosion resistance evaluation. One of these test pieces underwent a hydrochloric acid immersion test, immersing it in a 3% HCl aqueous solution at 55°C for 6 hours to simulate a strong acid environment. The remaining test piece underwent a hydrochloric acid immersion test, immersing it in a 0.14% HCl aqueous solution at 65°C for 6 hours to simulate a weak acid environment.

[0081] Subsequently, the corrosion rate was calculated from the corrosion loss of the test specimens in the hydrochloric acid immersion test. In this example, the corrosion rate with a 3% HCl aqueous solution at 55°C was 1.5 mg / cm². 2 The corrosion rate is less than / h and the corrosion rate with a 0.14% HCl aqueous solution at 65°C is 5.0 mg / cm³. 2 We determined that the corrosion resistance was excellent when the value was less than / h.

[0082] <Tensile strength> Tensile test specimens (No. 5) were prepared in accordance with JIS Z 2241:2023, and tensile tests were conducted to determine the tensile strength. Specimens with a tensile strength of 450 MPa or higher were marked with ○, and those with a tensile strength of less than 450 MPa were marked with ×.

[0083] Table 3 summarizes the results of the hydrochloric acid immersion test and tensile strength evaluation. In Table 3, "Corrosion rate (Condition 1)" refers to the corrosion rate using a 3% HCl aqueous solution at 55°C, and "Corrosion rate (Condition 2)" refers to the corrosion rate using a 0.14% HCl aqueous solution at 65°C.

[0084] [Table 3]

[0085] As shown in Table 3, tests No. 1 to 37, which fully satisfied the provisions of the present invention, showed excellent results in both corrosion resistance and tensile strength. In contrast, comparative examples No. 38 to 41 showed deterioration in either corrosion resistance or tensile strength. [Industrial applicability]

[0086] The steel material of the present invention can be used in the flue gas equipment of boilers that burn fossil fuels such as heavy oil and coal, gaseous fuels such as liquefied natural gas, general waste such as municipal solid waste, industrial waste such as waste oil, plastics, and discarded tires, and sewage sludge. Specifically, it can be suitably used in flue gas equipment such as flue ducts, casings, heat exchangers, gas-gas heaters consisting of two heat exchangers (heat recovery unit and reheater), desulfurization units, electrostatic precipitators, induced draft fans, basket materials and heat transfer element plates of rotary regenerative air preheaters.

Claims

1. The chemical composition is expressed in mass percent. C: 0.01% or more and less than 0.20% Si: 0.10 to 0.50%, Mn: 0.30-1.20%, Cu: 0.10-0.50%, Cr: 0.10-1.00%, Ti: 0.01 to 0.15%, Ni: 0.03 to 0.10%, Al: 0.005-0.10%, V: 0.001-0.050%, P: 0.025% or less, S: 0.0005-0.0150%, N: 0.0005-0.0080%, O: 0.0005-0.0035%, The remainder consists of Fe and impurities. The BI defined by the following formula (i) is between 33.0 and 81.

0. The following equation (ii) is satisfied: Steel material. BI=(Cr / 52) / (N / 14)...(i) 1.00≦(Ti / 48)×10 ３ + (V / 51)×10 ４ ≦10.0 ・・・(ii) However, the elemental symbols in the above formula represent the content (mass %) of each element contained in the steel material.

2. The chemical composition is expressed in mass percent. C: 0.01% or more and less than 0.20% Si: 0.10 to 0.50%, Mn: 0.30-1.20%, Cu: 0.10-0.50%, Cr: 0.10-1.00%, Ti: 0.01 to 0.15%, Ni: 0.03 to 0.10%, Al: 0.005-0.10%, V: 0.001-0.050%, P: 0.025% or less, S: 0.0005-0.0150%, N: 0.0005-0.0080%, O: 0.0005-0.0035%, Furthermore, it contains one or more selected from the groups A, B, and C listed below. The remainder consists of Fe and impurities. The BI defined by the following formula (i) is between 33.0 and 81.

0. The following equation (ii) is satisfied: Steel material. BI=(Cr / 52) / (N / 14)...(i) 1.00≦(Ti / 48)×10 ３ + (V / 51)×10 ４ ≦10.0 ・・・(ii) However, the elemental symbols in the above formula represent the content (mass %) of each element contained in the steel material. [Group A] One or more elements selected from the group consisting of Sn: 0.30% or less, Sb: 0.020% or less, Mo: 0.10% or less, W: 0.10% or less, In: less than 0.03%, As: 0.10% or less, Co: 0.30% or less, Bi: 0.10% or less, Te: 0.10% or less, Pb: 0.10% or less, Zn: 0.10% or less, Ga: 0.10% or less, Ge: 0.10% or less, and Hf: 0.10% or less. [Group B] One or more selected from the group consisting of Nb: 0.10% or less, Zr: 0.050% or less, Ta: 0.05% or less, and B: 0.010% or less. [Group C] One or more elements selected from the group consisting of: Ca and / or Mg total of 0.010% or less, Sr and / or Ba total of 0.010% or less, and REM: 0.010% or less.

3. The steel material according to claim 2, wherein the chemical composition contains one or more elements selected from group A.

4. The steel material according to claim 2, wherein the chemical composition contains one or more elements selected from group B.

5. The steel material according to claim 2, wherein the chemical composition contains one or more elements selected from the C group.

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