Seamless steel tube

Abstract

A seamless steel pipe having an inner surface and an outer surface, the seamless steel pipe having a chemical composition comprising, in mass%, 0.06% or less of C, 0.55% or less of Si, 0.70-1.40% of Mn, 0.020% or less of P, 0.0005-0.020% of S, 0.005% or less of N, 0.0005-0.0035% of O, 0.25-0.45% of Cu, 0.50% or less of Ni, 0.20% or less of Mo, and 0.05-0.15% of Sb, the remainder being Fe and impurities, the metallographic structure comprising, in area%, 90.0% or more of ferrite, the average crystal grain diameter of the ferrite in the surface layer of the inner surface of the pipe being 3.0-20.0 [mu] m, and the average crystal grain diameter of the ferrite in the surface layer of the inner surface being 3.0-20.0 [mu] the average grain diameter of ferrite in the surface layer of the tube outer surface is 3.0-25.0 [mu] m, MnS and MnS oxide are contained in the surface layer of the tube inner surface and in the surface layer of the tube outer surface, the number density of MnS is less than 100 / mm2, and the ratio of the number density of MnS oxide to the number density of MnS is 0.10 or more.

Description

Technical Field

[0001] The present invention relates to a seamless steel pipe. Background Art

[0002] Boiler furnaces and incinerators of waste incineration facilities generate exhaust gas containing water vapor, sulfur oxides, hydrogen chloride, etc. When the exhaust gas is cooled in the exhaust chimney, it condenses into sulfuric acid and hydrochloric acid, causing significant corrosion to the steel pipes that constitute the exhaust gas flow path. This phenomenon is called sulfuric acid dew point corrosion and hydrochloric acid dew point corrosion.

[0003] To address this problem, sulfuric acid / hydrochloric acid dew point corrosion resistant steel and highly corrosion resistant stainless steel have been proposed. For example, in Patent Documents 1 to 4, steel materials with excellent sulfuric acid dew point corrosion resistance to which Cu, Sb, Co, Cr, etc. are added are proposed. In addition, in Patent Document 5, highly corrosion resistant stainless steel to which Cr, Ni, etc. are added is proposed.

[0004] Prior art literature

[0005] Patent Literature

[0006] Patent Document 1: Japanese Patent Application Publication No. 2001-164335

[0007] Patent Document 2: Japanese Patent Application Publication No. 2003-213367

[0008] Patent Document 3: Japanese Patent Application Publication No. 2007-239094

[0009] Patent Document 4: Japanese Patent Application Publication No. 2012-57221

[0010] Patent Document 5: Japanese Patent Application Laid-Open No. 7-316745 Summary of the invention

[0011] Problem that the invention aims to solve

[0012] However, in the air preheater of coal-fired power plant boilers, gas-gas heaters, etc., and air preheater / desulfurization devices of waste incineration equipment, etc., the outside of the steel pipe is exposed to exhaust gas containing moisture and SOx, HCl, etc., and the inside of the steel pipe becomes an environment in contact with high-temperature boiler water. Steel materials containing Cu, Sb, etc. are also used in seamless steel pipes because of their excellent corrosion resistance. However, the corrosion resistance of seamless steel pipes used in the above environment still has room for further improvement.

[0013] The present invention aims to solve the above-mentioned problems and to provide a seamless steel pipe having excellent corrosion resistance in an environment where the outer surface is exposed to sulfuric acid corrosion and the inner surface is exposed to high-temperature water.

[0014] Solutions for solving problems

[0015] The present invention has been made to solve the above-mentioned technical problems, and the gist of the present invention is the following seamless steel pipe.

[0016] (1) A seamless steel pipe having an inner surface and an outer surface,

[0017] The chemical composition of the seamless steel pipe is expressed in mass % as follows:

[0018] C: 0.06% or less,

[0019] Si: 0.55% or less,

[0020] Mn: 0.70~1.40%,

[0021] P: 0.020% or less,

[0022] S: 0.0005~0.020%,

[0023] N: 0.005% or less,

[0024] O: 0.0005~0.0035%,

[0025] Cu: 0.25-0.45%,

[0026] Ni: 0.50% or less,

[0027] Mo: 0.20% or less,

[0028] Sb: 0.05-0.15%,

[0029] Balance: Fe and impurities,

[0030] The metallographic structure contains ferrite by area %: 90.0% or more,

[0031] The average grain size of ferrite in the inner surface layer of the tube is 3.0 to 20.0 μm.

[0032] The average grain size of ferrite in the outer surface layer of the tube is 3.0 to 25.0 μm.

[0033] The inner surface layer of the tube contains MnS and MnS oxides, and the number density of MnS with a maximum length of more than 2.0 μm is less than 100 / mm 2 , and the ratio of the number density of MnS oxides having a maximum length of 2.0 μm or more to the number density of MnS having a maximum length of 2.0 μm or more is 0.10 or more,

[0034] The outer surface layer of the tube contains MnS and MnS oxides, and the number density of MnS with a maximum length of more than 2.0 μm is less than 100 / mm2 , and the ratio of the number density of MnS oxides having a maximum length of 2.0 μm or more to the number density of MnS having a maximum length of 2.0 μm or more is 0.10 or more.

[0035] (2) The seamless steel pipe according to (1) above, wherein the chemical composition contains, in mass %, selected from

[0036] Al: 0.050% or less,

[0037] Cr: 0.70% or less,

[0038] Nb: 0.100% or less,

[0039] Ta: 0.100% or less,

[0040] V: 0.100% or less,

[0041] Ti: 0.100% or less,

[0042] W: 1.00% or less,

[0043] Ca: 0.0100% or less,

[0044] Mg: 0.0100% or less,

[0045] REM: 0.0100% or less,

[0046] B: 0.0050% or less,

[0047] Sn: 0.30% or less,

[0048] Pb: 0.30% or less,

[0049] Se: 0.100% or less,

[0050] Te: 0.100% or less,

[0051] Bi: 0.100% or less,

[0052] Ag: 0.500% or less, and

[0053] One or more kinds selected from the group consisting of Pd: 0.100% or less are used to replace a part of the Fe.

[0054] (3) The seamless steel pipe according to (1) or (2) above, wherein the number density of MnS with a maximum length of 2.0 μm or more in the outer surface layer of the pipe is less than 50 / mm 2 , and the ratio of the number density of MnS oxides having a maximum length of 2.0 μm or more to the number density of MnS having a maximum length of 2.0 μm or more is 0.50 or more.

[0055] Effects of the Invention

[0056] According to the present invention, it is possible to provide a seamless steel pipe having excellent corrosion resistance in an environment where the outer surface is exposed to sulfuric acid corrosion and the inner surface is exposed to high-temperature water. DETAILED DESCRIPTION

[0057] The present inventors have conducted a detailed investigation on the corrosion resistance of seamless steel pipes in order to solve the above-mentioned technical problems, and as a result, have obtained the following findings.

[0058] The present inventors have studied a method for improving the corrosion resistance of a seamless steel pipe in an environment where the outer surface is exposed to sulfuric acid corrosion and the inner surface is exposed to high-temperature water.

[0059] In steel materials that exhibit corrosion resistance by containing Cu and Sb, the larger the grain size, the better the corrosion resistance. Therefore, it is necessary to control the grain size of the outer surface and inner surface of the steel pipe according to the environment in which it is exposed.

[0060] In addition, although Mn is an element necessary for ensuring the strength and toughness of the steel pipe, it forms MnS, which deteriorates the corrosion resistance in a corrosive environment. In the present invention, S has the effect of improving corrosion resistance by being contained together with Cu and Sb, so extreme reduction is not preferred.

[0061] The present inventors have found through previous studies that MnS can be rendered harmless by making it fine and combining it with oxygen to form MnS oxides. Therefore, it is also important to control the inclusions on the outer and inner surfaces of the steel pipe according to the exposure environment.

[0062] However, unlike the case of manufacturing steel plates, when manufacturing steel pipes by hot working, the inner surface of the steel pipe is kept at a high temperature due to the influence of working heat and scale formation, making it difficult to refine MnS.

[0063] Therefore, the present inventors have conducted further studies and found that by using a scale modifier in the hot working process, it is possible to suppress working heat and scale formation even on the inner surface of the steel pipe, refine MnS, and render it harmless by forming MnS oxides.

[0064] The present invention has been made based on the above findings. Hereinafter, each element of the present invention will be described in detail.

[0065] (A) Chemical composition

[0066] The reasons for limiting the content of each element are as follows. In the following description, "%" related to the content means "mass %".

[0067] C: 0.06% or less

[0068] It is inevitable to contain carbon (C). That is, the C content is greater than 0%. C improves the strength of steel. This effect can be achieved to a certain extent as long as C is contained in a small amount. However, if the C content is greater than 0.06%, the sulfuric acid corrosion resistance of the steel will decrease even if the content of other elements is within the range of the present embodiment. Therefore, the C content is less than 0.06%. Excessive reduction in C content will increase production costs. Therefore, considering industrial production, the preferred lower limit of the C content is 0.0001%, and more preferably 0.0005%. If the strength of the steel is to be improved more effectively, the preferred lower limit of the C content is 0.001%, and more preferably 0.01%. The preferred upper limit of the C content is 0.055%, and more preferably 0.05%.

[0069] Si: 0.55% or less

[0070] Silicon (Si) is inevitably contained. That is, the Si content is greater than 0%. Si deoxidizes the steel. Si also dissolves in ferrite to increase the strength of the steel. This effect can be achieved to a certain extent as long as a small amount of Si is contained. However, if the Si content is greater than 0.55%, the weldability and toughness of the steel will decrease even if the content of other elements is within the range of this embodiment. Therefore, the Si content is less than 0.55%. Excessive reduction in Si content will increase production costs. Therefore, considering industrial production, the preferred lower limit of the Si content is 0.0001%, and more preferably 0.0005%. If the strength of the steel is more effectively improved, the preferred lower limit of the Si content is 0.05%, and more preferably 0.10%. The preferred upper limit of the Si content is 0.50%, more preferably 0.45%, and more preferably 0.40%.

[0071] Mn: 0.70~1.40%

[0072] Manganese (Mn) improves the strength of steel. If the Mn content is less than 0.70%, sufficient strength cannot be obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Mn content is greater than 1.40%, the toughness of the steel decreases. In addition, coarse MnS is generated, which deteriorates corrosion resistance and mechanical properties. Therefore, the Mn content is 0.70 to 1.40%. The preferred lower limit of the Mn content is 0.80%, and more preferably 0.90%. The preferred upper limit of the Mn content is 1.35%, more preferably 1.30%, more preferably 1.25%, and more preferably 1.20%.

[0073] P: 0.020% or less

[0074] Phosphorus (P) is an impurity that is inevitably contained. That is, the P content is greater than 0%. P segregates at the grain boundaries, reducing the sulfuric acid corrosion resistance of the steel. If the P content is greater than 0.020%, even if the contents of other elements are within the range of the present embodiment, sufficient sulfuric acid corrosion resistance cannot be obtained. Therefore, the P content is less than 0.020%. The P content is preferably as low as possible. However, an excessive reduction in the P content will increase production costs. Therefore, considering industrial production, the preferred lower limit of the P content is 0.0001%, more preferably 0.0005%, and more preferably 0.001%.

[0075] S: 0.0005~0.020%

[0076] Sulfur (S) is an impurity that is inevitably contained. However, in the present invention, S has the effect of improving corrosion resistance in an acid corrosion environment by being contained together with Cu and Sb. Therefore, the S content is 0.0005% or more. On the other hand, S reduces the hot workability of steel. If the S content is greater than 0.020%, sufficient hot workability cannot be obtained even if the contents of other elements are within the range of the present embodiment. Therefore, the S content is 0.020% or less. The preferred lower limit of the S content is 0.001%, and more preferably 0.005%. The preferred upper limit of the S content is 0.018%, and more preferably 0.016%.

[0077] N: 0.005% or less

[0078] N is an impurity that reduces the mechanical properties and productivity of steel. Therefore, an upper limit is set for the N content, which is set to 0.005% or less. The N content is preferably 0.004% or less. It should be noted that the N content can be 0%, but an extreme reduction will lead to an increase in steelmaking costs. Therefore, the N content can be set to 0.001% or more. In addition, N has the effect of helping to improve mechanical properties by precipitating in the form of fine nitrides. If this effect is desired, the N content can be set to 0.002% or more.

[0079] O: 0.0005~0.0035%

[0080] O is an element that has the effect of making MnS harmless and preventing deterioration of corrosion resistance and mechanical properties by combining with MnS. However, if too much O is contained, coarse oxides that become the starting point of corrosion are generated in an acid corrosion environment. Therefore, the O content is 0.0005 to 0.0035%. The O content is preferably 0.0010% or more, and more preferably 0.0015% or more. In addition, the O content is preferably 0.0030% or less, and more preferably 0.0025% or less.

[0081] Cu: 0.25~0.45%

[0082] Copper (Cu) improves the sulfuric acid corrosion resistance of steel. If the Cu content is less than 0.25%, even if the other element contents are within the scope of the present embodiment, sufficient sulfuric acid corrosion resistance cannot be obtained. On the other hand, if the Cu content is greater than 0.45%, even if the other element contents are within the scope of the present embodiment, the weldability and hot workability of the steel will also decrease. Therefore, the Cu content is 0.25 to 0.45%. The preferred lower limit of the Cu content is 0.26%, and more preferably 0.27%. The preferred upper limit of the Cu content is 0.40%, more preferably 0.38%, and more preferably 0.36%.

[0083] Ni: 0.50% or less

[0084] Nickel (Ni) is inevitably contained. That is, the Ni content is greater than 0%. Ni improves the hydrochloric acid corrosion resistance of steel. This effect can be achieved to a certain extent as long as Ni is contained in a small amount. However, if the Ni content is greater than 0.50%, the weldability and hot workability of the steel decrease. Therefore, the Ni content is below 0.50%. Excessive reduction in Ni content will increase production costs. Therefore, considering industrial production, the preferred lower limit of Ni content is 0.01%, and more preferably 0.02%. If considering a more effective improvement in hydrochloric acid corrosion resistance, the preferred lower limit of Ni content is 0.05%, and more preferably 0.10%. The preferred lower limit of Ni content is 0.44%, more preferably 0.40%, and more preferably 0.35%.

[0085] Mo: 0.20% or less

[0086] Molybdenum (Mo) is inevitably contained. That is, the Mo content is greater than 0%. Mo improves the sulfuric acid corrosion resistance of steel. This effect can be achieved to a certain extent as long as a small amount of Mo is contained. On the other hand, if the Mo content is greater than 0.20%, the corrosion resistance of the steel decreases. Therefore, the Mo content is below 0.20%. Excessive reduction in the Mo content will increase production costs. Therefore, the preferred lower limit of the Mo content is 0.001%, and more preferably 0.005%. If the sulfuric acid corrosion resistance is further improved, the preferred lower limit of the Mo content is 0.01%, more preferably 0.02%, and more preferably 0.03%. The preferred upper limit of the Mo content is 0.19%, more preferably 0.18%, and more preferably 0.17%.

[0087] Sb: 0.05~0.15%

[0088] Antimony (Sb) improves the sulfuric acid corrosion resistance of steel. If the Sb content is less than 0.05%, even if the contents of other elements are within the scope of the present embodiment, sufficient sulfuric acid corrosion resistance cannot be obtained. On the other hand, if the Sb content is greater than 0.15%, the hot workability and weldability of the steel will decrease even if the contents of other elements are within the scope of the present embodiment. Therefore, the Sb content is 0.05 to 0.15%. The preferred lower limit of the Sb content is 0.06%, more preferably 0.07%, and more preferably 0.08%. The preferred upper limit of the Sb content is 0.14%, and more preferably 0.13%.

[0089] In the chemical composition of the seamless steel pipe of the present embodiment, the balance is Fe and impurities. Here, impurities refer to components that are mixed from raw materials such as ore and scrap and other factors during industrial steel production and are allowed within a range that does not adversely affect the seamless steel pipe of the present invention.

[0090] The chemical composition of the seamless steel pipe of the present embodiment may further contain one or more selected from the group consisting of Al: 0.050% or less, Cr: 0.70% or less, Nb: 0.100% or less, Ta: 0.100% or less, V: 0.100% or less, Ti: 0.100% or less, and W: 1.00% or less to replace part of Fe. Al may be added as a deoxidizer. In addition, among the above elements, elements other than Al all increase the high temperature strength of steel.

[0091] Al: 0.050% or less

[0092] Aluminum (Al) is an arbitrary element and may not be contained. That is, the Al content may be 0%. When Al is contained, Al acts as a deoxidizer. The above-mentioned effect can be obtained to a certain extent as long as a small amount of Al is contained. However, if too much Al is contained, the corrosion resistance will be impaired due to the increase in inclusions. Therefore, the Al content is less than 0.050%. The Al content is preferably more than 0.005%, more preferably more than 0.010%, and further preferably more than 0.020%. In addition, the Al content is preferably less than 0.045%, more preferably less than 0.040%.

[0093] Cr: 0.70% or less

[0094] Chromium (Cr) is an arbitrary element and may not be contained. That is, the Cr content may be 0%. When Cr is contained, Cr improves hardenability and strength. The above-mentioned effects can be obtained to a certain extent as long as a small amount of Cr is contained. However, although Cr is an element that improves weather resistance, it sometimes reduces corrosion resistance in an acid corrosion environment. Therefore, the Cr content is less than 0.70%. The Cr content is preferably more than 0.01%, more preferably more than 0.02%, and further preferably more than 0.05%. The Cr content is preferably less than 0.50%, more preferably less than 0.30%, and further preferably less than 0.10%.

[0095] Nb: 0.100% or less

[0096] Niobium (Nb) is an arbitrary element and may not be contained. That is, the Nb content may be 0%. When Nb is contained, Nb forms carbides or carbonitrides in the steel, which improves the high temperature strength of the seamless steel pipe. The above-mentioned effect can be obtained to a certain extent as long as a small amount of Nb is contained. However, when the Nb content is too high, its effect will be saturated. Therefore, the Nb content is less than 0.100%. The preferred lower limit of the Nb content is greater than 0%, more preferably 0.005%, and more preferably 0.010%. The preferred upper limit of the Nb content is 0.090%, and more preferably 0.080%.

[0097] Ta: 0.100% or less

[0098] Tantalum (Ta) is an arbitrary element and may not be contained. That is, the Ta content may be 0%. When Ta is contained, Ta generates carbides or carbonitrides in the steel, which improves the high temperature strength of the seamless steel pipe. The above-mentioned effect can be obtained to a certain extent as long as Ta is contained in a small amount. However, when the Ta content is too high, its effect will be saturated. Therefore, the Ta content is less than 0.100%. The preferred lower limit of the Ta content is greater than 0%, more preferably 0.005%, and more preferably 0.010%. The preferred upper limit of the Ta content is 0.090%, and more preferably 0.080%.

[0099] V: 0.100% or less

[0100] Vanadium (V) is an arbitrary element and may not be contained. That is, the V content may be 0%. When V is contained, V forms carbides or carbonitrides in the steel, which improves the high temperature strength of the seamless steel pipe. The above-mentioned effect can be obtained to a certain extent as long as a small amount of V is contained. However, when the V content is too high, its effect will be saturated. Therefore, the V content is less than 0.100%. The preferred lower limit of the V content is greater than 0%, more preferably 0.005%, and more preferably 0.010%. The preferred upper limit of the V content is 0.090%, and more preferably 0.080%.

[0101] Ti: 0.100% or less

[0102] Titanium (Ti) is an arbitrary element and may not be contained. That is, the Ti content may be 0%. When Ti is contained, Ti generates carbides or carbonitrides in the steel, thereby improving the high temperature strength of the seamless steel pipe. The above-mentioned effects can be obtained to a certain extent as long as a small amount of Ti is contained. However, when the Ti content is too high, the effect will be saturated. Therefore, the Ti content is less than 0.100%. The preferred lower limit of the Ti content is greater than 0%, more preferably 0.005%, and more preferably 0.010%. The preferred upper limit of the Ti content is 0.090%, and more preferably 0.080%.

[0103] W: 1.00% or less

[0104] Tungsten (W) is an arbitrary element and may not be contained. That is, the W content may be 0%. When W is contained, W improves the high temperature strength of the seamless steel pipe. The above-mentioned effect can be obtained to a certain extent as long as a small amount of W is contained. However, when the W content is too high, its effect will be saturated. Therefore, the W content is less than 1.00%. The preferred lower limit of the W content is greater than 0%, more preferably 0.05%, and more preferably 0.10%. The preferred upper limit of the W content is 0.90%, and more preferably 0.80%.

[0105] The chemical composition of the seamless steel pipe of the present embodiment may further contain one or more selected from the group consisting of Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, and B: 0.0050% or less in place of a portion of Fe. These elements improve the cleanliness of steel.

[0106] Ca: 0.0100% or less

[0107] Calcium (Ca) is an arbitrary element and may not be contained. That is, the Ca content may be 0%. When Ca is contained, the decrease in sulfuric acid corrosion resistance is suppressed and the cleanliness of the steel is improved. The above-mentioned effects can be obtained to a certain extent as long as a small amount of Ca is contained. However, when the Ca content is too high, its effect will be saturated. Therefore, the Ca content is 0.0100% or less. The preferred lower limit of the Ca content is greater than 0%, more preferably 0.0002%, and more preferably 0.0010%. The preferred upper limit of the Ca content is 0.0090%, and more preferably 0.0080%.

[0108] Mg: 0.0100% or less

[0109] Magnesium (Mg) is an arbitrary element and may not be contained. That is, the Mg content may be 0%. When Mg is contained, Mg suppresses the decrease in sulfuric acid corrosion resistance and improves the cleanliness of steel. The above-mentioned effects can be obtained to a certain extent as long as a small amount of Mg is contained. However, when the Mg content is too high, its effect will be saturated. Therefore, the Mg content is less than 0.0100%. The preferred lower limit of the Mg content is greater than 0%, more preferably 0.0002%, and more preferably 0.0010%. The preferred upper limit of the Mg content is 0.0090%, and more preferably 0.0080%.

[0110] REM: 0.0100% or less

[0111] Rare earth elements (REM) are arbitrary elements and may not be contained. That is, the REM content may be 0%. When REM is contained, the decrease in sulfuric acid corrosion resistance is suppressed and the cleanliness of the steel is improved. The above-mentioned effects can be obtained to a certain extent as long as a small amount of REM is contained. However, when the REM content is too high, the effect will be saturated. Therefore, the REM content is less than 0.0100%. The preferred lower limit of the REM content is greater than 0%, more preferably 0.0002%, and more preferably 0.0010%. The preferred upper limit of the REM content is 0.0090%, and more preferably 0.0080%.

[0112] Here, REM is a general term for Sc, Y and lanthanide elements, a total of 17 elements, and the content of REM refers to the total amount of the above elements. It should be noted that lanthanide is added in the form of mixed rare earth in industry.

[0113] B: 0.0050% or less

[0114] Boron (B) is an arbitrary element and may not be contained. That is, the B content may be 0%. When B is contained, the decrease in sulfuric acid corrosion resistance is suppressed and the cleanliness of the steel is improved. The above-mentioned effects can be obtained to a certain extent as long as a small amount of B is contained. However, when the B content is too high, its effect will be saturated. Therefore, the B content is 0.0050% or less. The preferred lower limit of the B content is greater than 0%, more preferably 0.0002%, and more preferably 0.0010%. The preferred upper limit of the B content is 0.0040%, and more preferably 0.0030%.

[0115] The chemical composition of the seamless steel pipe of the present embodiment may further contain one or more selected from the group consisting of Sn: 0.30% or less and Pb: 0.30% or less in place of a portion of Fe. These elements improve the machinability of steel.

[0116] Sn: 0.30% or less

[0117] Tin (Sn) is an arbitrary element and may not be contained. That is, the Sn content may be 0%. When Sn is contained, the machinability of the steel is improved. The above-mentioned effect can be obtained to a certain extent as long as a small amount of Sn is contained. However, when the Sn content is too high, the hot workability of the steel decreases. Therefore, the Sn content is 0.30% or less. The preferred lower limit of the Sn content is greater than 0%, more preferably 0.01%, and more preferably 0.10%. The preferred upper limit of the Sn content is 0.25%, and more preferably 0.20%.

[0118] Pb: 0.30% or less

[0119] Lead (Pb) is an arbitrary element and may not be contained. That is, the Pb content may be 0%. When Pb is contained, the machinability of the steel is improved. The above-mentioned effect can be obtained to a certain extent as long as a small amount of Pb is contained. However, when the Pb content is too high, the hot workability of the steel decreases. Therefore, the Pb content is 0.30% or less. The preferred lower limit of the Pb content is greater than 0%, more preferably 0.01%, and more preferably 0.10%. The preferred upper limit of the Pb content is 0.25%, and more preferably 0.20%.

[0120] The chemical composition of the seamless steel pipe of the present embodiment may further contain one or more selected from the group consisting of Se: 0.100% or less, Te: 0.100% or less, and Bi: 0.100% or less in place of a portion of Fe. These elements improve the sour resistance of steel.

[0121] Se: 0.100% or less

[0122] Selenium (Se) is an arbitrary element and may not be contained. That is, the Se content may be 0%. When Se is contained, the acid resistance of the steel is improved. The above-mentioned effect can be obtained to a certain extent as long as a small amount of Se is contained. However, when the Se content is too high, the manufacturability of the steel decreases and the production cost increases. Therefore, the Se content is less than 0.100%. The preferred lower limit of the Se content is greater than 0%, more preferably 0.001%, and more preferably 0.010%. The preferred upper limit of the Se content is 0.090%, more preferably 0.080%, and more preferably 0.070%.

[0123] Te: 0.100% or less

[0124] Tellurium (Te) is an arbitrary element and may not be contained. That is, the Te content may be 0%. When Te is contained, the acid resistance of the steel is improved. The above-mentioned effect can be obtained to a certain extent as long as a small amount of Te is contained. However, when the Te content is too high, the manufacturability of the steel decreases and the production cost increases. Therefore, the Te content is less than 0.100%. The preferred lower limit of the Te content is greater than 0%, more preferably 0.001%, and more preferably 0.010%. The preferred upper limit of the Te content is 0.090%, more preferably 0.080%, and more preferably 0.070%.

[0125] Bi: 0.100% or less

[0126] Bismuth (Bi) is an arbitrary element and may not be contained. That is, the Bi content may be 0%. When Bi is contained, the acid resistance of the steel is improved. The above-mentioned effect can be obtained to a certain extent as long as a small amount of Bi is contained. However, when the Bi content is too high, the manufacturability of the steel decreases and the production cost increases. Therefore, the Bi content is 0.100% or less. The preferred lower limit of the Bi content is greater than 0%, more preferably 0.001%, and more preferably 0.010%. The preferred upper limit of the Bi content is 0.090%, more preferably 0.080%, and more preferably 0.070%.

[0127] The chemical composition of the seamless steel pipe of the present embodiment may further contain one or more selected from the group consisting of Ag: 0.500% or less and Pd: 0.100% or less in place of a portion of Fe. These elements both improve the sulfuric acid corrosion resistance at high temperatures.

[0128] Ag: 0.500% or less

[0129] Silver (Ag) is an arbitrary element and may not be contained. That is, the Ag content may be 0%. When Ag is contained, the sulfuric acid corrosion resistance of steel at high temperatures is improved. The above effect can be obtained to a certain extent as long as Ag is contained in a small amount. However, when the Ag content is too high, the hot workability of the steel decreases. Therefore, the Ag content is 0.500% or less. The preferred lower limit of the Ag content is greater than 0%, more preferably 0.001%, and more preferably 0.010%. The preferred upper limit of the Ag content is 0.400%, more preferably 0.250%, and more preferably 0.100%.

[0130] Pd: 0.100% or less

[0131] Palladium (Pd) is an arbitrary element and may not be contained. That is, the Pd content may be 0%. When Pd is contained, the sulfuric acid corrosion resistance of steel at high temperatures is improved. The above-mentioned effect can be obtained to a certain extent as long as a small amount of Pd is contained. However, when the Pd content is too high, the hot workability of the steel decreases. Therefore, the Pd content is 0.100% or less. The preferred lower limit of the Pd content is greater than 0%, more preferably 0.001%, and more preferably 0.010%. The preferred upper limit of the Pd content is 0.090%, more preferably 0.080%, and more preferably 0.070%.

[0132] (B) Metallographic structure

[0133] In the metallographic structure of the seamless steel pipe of the present embodiment, the area ratio of ferrite is 90.0% or more. In the metallographic structure, the remainder other than ferrite is pearlite. That is, the metallographic structure (matrix: parent phase) of the seamless steel pipe of the present embodiment is a structure composed of ferrite and pearlite. In the metallographic structure of the seamless steel pipe of the present embodiment, if the area ratio of ferrite is less than 90.0%, stress corrosion cracking is likely to occur. If the area ratio of ferrite is 90.0% or more, stress corrosion cracking is not likely to occur. Therefore, the area ratio of ferrite is 90.0% or more. The preferred lower limit of the ferrite area ratio is 92.0%, and more preferably 95.0%.

[0134] The area ratio of ferrite is determined by the following method. A sample is collected from the central part of the wall thickness of the seamless steel pipe. The size of the sample is not particularly limited as long as the field of view for metallographic structure observation described later can be ensured. The section of the sample surface perpendicular to the length direction of the seamless steel pipe is used as the observation surface. The observation surface is mirror-polished. The mirror-polished sample is immersed in a nitric acid ethanol etching solution to reveal the structure by etching. The etched observation surface is observed using a scanning electron microscope (SEM) using a secondary electron image. Each field of view is set to 200×200μm 2 Observe 5 fields of view at 500 times magnification. In each field of view, determine the structure (ferrite, pearlite, etc.) by contrast. The arithmetic mean of the area ratio of ferrite determined in each field of view (5 fields of view in total) is defined as the area ratio (%) of ferrite.

[0135] In the metallographic structure of the seamless steel pipe of the present embodiment, the average grain size of ferrite in the pipe inner surface layer is 3.0 to 20.0 μm, and the average grain size of ferrite in the pipe outer surface layer is 3.0 to 25.0 μm.

[0136] As described above, in steel materials that exhibit corrosion resistance by containing Cu and Sb, the larger the grain size, the better the corrosion resistance. When the grains are refined, the grain boundary area increases, and the segregation element Sb segregates / disperses at the grain boundaries, resulting in the existence position of Sb effective for sulfuric acid corrosion resistance becoming the grain boundaries. Cu, another element effective for corrosion resistance, exists inside the grains, so when Sb segregates at the grain boundaries, it is difficult to obtain the composite effect of Cu and Sb for exhibiting corrosion resistance. Therefore, the average grain diameter of ferrite in the inner surface layer of the tube and the outer surface layer of the tube is 3.0μm or more.

[0137] On the other hand, if the crystal grains are excessively coarse, the strength required as a structure cannot be obtained, and therefore it is necessary to set an upper limit on the average crystal grain size of ferrite.

[0138] It should be noted that the seamless steel pipe of the present embodiment may be a steel pipe that remains in a hot-worked state (hot-rolled finished product) or may be a steel pipe that is subsequently cold-worked (cold-rolled finished product).

[0139] The average grain diameter of ferrite in seamless steel pipes is determined by the following method. Samples are collected from the surface layer of the inner surface and the surface layer of the outer surface of the seamless steel pipe. It should be noted that the surface layer refers to the area from the inner surface and the outer surface to 2.0 mm in the depth direction. The collected samples are embedded in resin and the surface of the samples is polished. The polished sample surface (observation surface) is set as a cross-section perpendicular to the length direction of the seamless steel pipe. After polishing the observation surface of the sample embedded in the resin, the sample is immersed in nitric acid ethanol etching solution to make the grain boundaries of the ferrite on the surface appear. In any 5 fields of view of the observation surface after corrosion, the circular equivalent diameter of the ferrite in each field of view is calculated, and the average value is taken as the average grain diameter.

[0140] (C) Inclusions

[0141] The seamless steel pipe of this embodiment contains MnS and MnS oxides in the inner surface layer and the outer surface layer of the pipe. The number density of MnS with a maximum length of 2.0 μm or more in the inner surface layer and the outer surface layer of the pipe is less than 100 / mm. 2 Furthermore, the ratio of the number density of MnS oxides having a maximum length of 2.0 μm or more to the number density of MnS having a maximum length of 2.0 μm or more is 0.10 or more.

[0142] It should be noted that MnS with a maximum length of less than 2.0 μm has almost no effect on the corrosion resistance of steel materials. Therefore, in the present invention, inclusions with a maximum length of 2.0 μm or more are taken as the object. In the following description, MnS with a maximum length of 2.0 μm or more is referred to as MnS, and MnS oxide with a maximum length of 2.0 μm or more is referred to as MnS oxide.

[0143] As described above, in the seamless steel pipe of this embodiment, the formation of MnS is inevitable. However, MnS will become the starting point of corrosion in both sulfuric acid corrosion environment and high temperature water environment, and deteriorate the corrosion resistance. Therefore, it is necessary to limit the number density of MnS to less than 100 / mm 2 The number density of MnS is preferably 90 / mm 2 Below, more preferably 80 / mm 2 the following.

[0144] On the other hand, in the seamless steel pipe of the present embodiment, from the perspective of improving strength, toughness and corrosion resistance, an extreme reduction in the content of Mn and S is not preferred. In order to take both into account, MnS needs to be made harmless. When MnS combines with oxygen to form MnS oxide, it is harmless and is unlikely to become a starting point for corrosion. Therefore, in the present invention, the ratio of the number density of MnS oxide to the number density of MnS is set to 0.10 or more. The above ratio is preferably 0.12 or more, and more preferably 0.15 or more.

[0145] Here, as described above, in the seamless steel pipe of this embodiment, the outer surface may be required to have better corrosion resistance, so it is preferable to make the ferrite in the outer surface layer of the pipe relatively coarse-grained. However, in the manufacturing process of the seamless steel pipe, it is sometimes difficult to control the grain size of the outer surface.

[0146] Therefore, from the perspective of ensuring the corrosion resistance of the outer surface of the tube even with relatively fine particles, the number density of MnS in the outer surface layer of the tube is preferably set to less than 50 / mm 2 , and the ratio of the number density of MnS oxides to the number density of MnS is set to be greater than 0.50.

[0147] The number density of MnS and the number density of MnS oxides were measured by energy dispersive X-ray analysis (EDS) with a scanning electron microscope (SEM). The measurement magnification was set to 1000 times, and the maximum length of MnS and MnS oxides detected in the field of view was measured. In addition, the number of inclusions with a maximum length of 2.0 μm or more was counted and divided by the field of view area to obtain the number density.

[0148] Inclusions were identified by EDS, and inclusions with a total content of Mn and S of 90 mass % or more were judged to be MnS. Furthermore, inclusions with a peak of O detected and a total content of Mn, S and O of 90 mass % or more were judged to be MnS oxides.

[0149] (D) Mechanical properties

[0150] The mechanical properties of the seamless steel pipe of the present embodiment are not particularly limited. However, when used as a seamless steel pipe for sulfuric acid corrosion environment represented by air preheaters, gas-gas heaters, etc. of coal-fired power plant boilers, air preheaters / desulfurization devices of waste incineration equipment, etc., preferably, the tensile strength is 380MPa or more and the yield stress is 230MPa or more. The preferred lower limit of the tensile strength is 390MPa, and more preferably 400MPa. The preferred upper limit of the tensile strength is not particularly limited, for example, 600MPa, and can be 580MPa. The preferred lower limit of the yield stress is 240MPa, and more preferably 250MPa. The upper limit of the yield stress is not particularly limited, for example, 450MPa, and can be 440MPa.

[0151] (E) Oxide scale

[0152] When the seamless steel pipe is a hot-rolled finished product, it is preferred that at least a portion of the outer surface of the steel pipe base material has an oxide scale, and an enriched layer of Si, Cu and Sb is present at the interface between the steel pipe base material and the oxide scale. The enriched layer of these elements can play a barrier role against sulfuric acid and hydrochloric acid, further improving the corrosion resistance in an acid corrosion environment.

[0153] Here, the enriched layer of Si, Cu and Sb refers to a layer formed by the diffusion of Si, Cu and Sb in the steel material during heat treatment and enrichment at the interface between the base material and the oxide scale. Specifically, a cross section perpendicular to the surface of the steel material and including the interface between the base material and the oxide scale is subjected to line analysis using an electron probe microanalyzer (EPMA), and the region where the content of Si, Cu and Sb is more than twice that of the base material is defined as an enriched layer. In the present invention, the measurement is performed under the conditions of acceleration voltage: 15 kV, beam diameter: ~100 nm, irradiation time: 20 ms, and measurement interval: 80 nm.

[0154] It should be noted that, when the base material contains Ni, it is preferred that a Ni-rich layer be formed closer to the base material than Si-, Cu-, and Sb-rich layers. By having a Ni-rich layer, corrosion resistance can be further improved.

[0155] (F) Application

[0156] The application of the seamless steel pipe of this embodiment is not particularly limited, and is suitable for use in an environment where the outer surface is exposed to sulfuric acid corrosion and the inner surface is exposed to high-temperature water. More specifically, the seamless steel pipe of this embodiment is suitable for air preheaters, gas-gas heaters, etc. of coal-fired power plant boilers, air preheaters / desulfurization devices of waste incineration equipment, etc.

[0157] (G) Manufacturing method

[0158] A method for producing a seamless steel pipe according to an embodiment of the present invention will be described. The method for producing a seamless steel pipe according to the present embodiment includes a billet preparation step, a hot working step, and a heat treatment step.

[0159] [Blank material preparation process]

[0160] In the billet preparation process, the billet is manufactured by casting using molten steel having the above chemical composition. The billet may be a slab or bloom manufactured by continuous casting using molten steel, or a round billet manufactured by continuous casting. A round billet is a billet having a circular cross section perpendicular to the length direction. In addition, the billet may be an ingot manufactured by ingot casting using molten steel.

[0161] [Hot working process]

[0162] In the hot working step, a billet produced by a casting method is hot worked to produce a seamless steel pipe.

[0163] For example, when the billet is a bloom, a slab or an ingot, the following hot working is performed to produce a seamless steel pipe. First, the billet is heated in a heating furnace. The heating temperature is not particularly limited, and is, for example, 1000 to 1300° C. The heated billet is subjected to initial rolling using a initial rolling mill to produce a round billet.

[0164] Next, the round billet is subjected to the Mannesmann process to produce a hollow shell. First, a scale modifier containing a Si compound is applied to at least one end face of the round billet (the end face on the piercing and rolling side). Here, the scale modifier containing a Si compound refers to a mixture obtained by mixing, in addition to the Si compound, oxides such as Al2O3, Fe2O, CaO, MgO, Na2O, K2O, B2O3, and ZrO2, a binder, water, and an optional compound such as CrB.

[0165] Examples of Si compounds include SiO2 and SiC. In particular, it is preferred to use a compound containing 50% or more SiO2 by mass or 15% or more SiC by mass. By applying the scale modifier to the end face of the round billet in advance, the scale modifier is applied to the inner surface of the billet during piercing and rolling. As a result, the processing heat and scale formation caused by piercing and rolling are suppressed, thereby promoting cooling of the inner surface of the steel pipe and achieving MnS refinement.

[0166] It should be noted that several of the oxides such as Al2O3, Fe2O, CaO, MgO, Na2O, K2O, B2O3, ZrO2, compounds such as C, B, etc. are mixed with the Si compound and mixed into the oxide scale modifier in the form of glass powder. The mixing ratio thereof is adjusted so that the viscosity of the oxide scale modifier reaches a suitable range when heated before piercing and rolling. In addition, in order to improve the operability during coating, the viscosity of the oxide scale modifier at room temperature can also be within a suitable range by adjusting the mixing ratio of glass powder, binder and water.

[0167] The coating method of the scale modifier is not limited, for example, the scale modifier can be applied to at least one end surface of the room temperature steel material by spraying or brushing. The coating amount of the scale modifier can be set to 50 to 100 mg / cm 2 .

[0168] Next, the round billet coated with the scale modifier is heated in a heating furnace. The heating temperature is 1170-1300°C. Next, the round billet is taken out of the heating furnace, and the round billet after being taken out is pierced and rolled using a piercing machine to produce a tube billet. The tube billet after piercing and rolling can be further stretched and rolled using a mandrel seamless tube mill.

[0169] Furthermore, the shell tube that has been reheated after elongation rolling (hot working) using a mandrel mill may be subjected to sizing rolling (sizing) using a sizing mill or a stretch reducer.

[0170] The produced tube blank is cooled. The cooling method is, for example, air cooling. Through the above hot working, a seamless steel tube is produced.

[0171] When the billet prepared in the billet preparation step is a round billet, the above-mentioned initial rolling step is omitted. That is, the prepared billet (round billet) is subjected to the Mannesmann process to produce a tube billet. The produced tube billet is cooled to produce a seamless steel pipe.

[0172] [Heat treatment process]

[0173] In the method for producing a seamless steel pipe of the present embodiment, heat treatment after pipe making may be omitted, but heat treatment may be performed as necessary in consideration of stability of quality such as mechanical properties.

[0174] During the heat treatment, the heat treatment temperature is set to be above the Ac3 transformation point. The heat treatment temperature in the heat treatment is, for example, above 900°C, and more preferably 900-920°C. The holding time at the above heat treatment temperature is not particularly limited, and is, for example, 5-10 minutes. The average grain size of ferrite is adjusted by the heat treatment temperature and holding time in the heat treatment. After the holding time at the above heat treatment temperature has passed, the seamless steel pipe is formed using a stretch reducer. When manufacturing cold-rolled finished products, after forming using a stretch reducer, cold working is performed, and then heat treatment is performed again.

[0175] Heat treatment after hot working and controlled cooling after reheat treatment after cold working have an impact on corrosion resistance. By increasing the cooling rate after heat treatment and reheat treatment, corrosion resistance is improved. Although the detailed mechanism is not clear, it can be considered that this is because by accelerating the cooling rate, Sb segregation to grain boundaries during cooling can be suppressed, ensuring the Sb concentration inside the grains that contributes to corrosion resistance. Therefore, the faster the cooling rate, the better. The cooling rate from the heat treatment temperature to 500°C is preferably set to 15.0°C / minute or more, and more preferably set to 20.0°C / minute or more.

[0176] Hereinafter, the present invention will be further specifically described by way of examples. It should be noted that the conditions in the following examples are one conditional example adopted to confirm the feasibility and effect of the present invention, and the present invention is not limited to this one conditional example. Furthermore, the present invention may adopt various conditions as long as it does not depart from the gist of the present invention and achieves the purpose of the present invention.

[0177] Example

[0178] Steel having the chemical composition shown in Table 1 was melted and round billets were manufactured by continuous casting. The round billets were heated to 1200-1250°C in a heating furnace. Then, the heated round billets were pierced and rolled using a piercer to manufacture tube billets. At this time, except for test numbers 24 and 25, a scale modifier containing a Si compound was applied to the end surface of the piercing and rolling side. As the scale modifier, a mixture containing SiO2: 50.0%, Al2O3: 24.0%, and B2O3: 11.9% was used.

[0179] In addition, the tube blank was subjected to stretch rolling using a mandrel mill and sizing rolling using a stretch reducer or a sizing mill, and then cold working and heat treatment were performed. It should be noted that test numbers 22 and 23 were not cold worked, but heat treated after tube making (sizing rolling). Seamless steel pipes shown in Table 2 were manufactured.

[0180] [Table 1]

[0181]

[0182] [Table 2]

[0183]

[0184] Using each of the obtained steel pipes, various performance evaluation tests described below were carried out.

[0185] [Microstructure observation test]

[0186] Samples were collected from the center of the wall thickness of the seamless steel pipe of each test number. The section of the sample surface perpendicular to the length direction of the seamless steel pipe was used as the observation surface. The observation surface was mirror-polished. The mirror-polished sample was immersed in nitric acid ethanol etching solution to reveal the structure by etching. The etched observation surface was observed using a SEM with a secondary electron image. Each field of view was set to 400μm 2 Observe 5 fields of view at 5000 times magnification. In each field of view, the phase (ferrite, pearlite) is determined by contrast. The arithmetic mean of the area ratio of ferrite determined in each field of view (5 fields of view in total) is defined as the area ratio (%) of ferrite.

[0187] The obtained results are shown in Table 2. "F" in the "Microstructure" column of Table 2 indicates that the ferrite area ratio is 90.0% or more and the balance is pearlite. As shown in Table 2, the microstructures in all examples are composed of ferrite and pearlite, and the ferrite area ratio is 90.0% or more.

[0188] Test Nos. 22 and 23 are hot-rolled finished products that remain in a hot-worked state. Oxide scale is formed on the surface of the steel, and a Si, Cu and Sb enriched layer is formed at the oxide scale / steel plate interface, and a Ni enriched layer is formed on the parent material side of the enriched layer.

[0189] [Ferrite grain size measurement]

[0190] The ferrite grain diameter is measured from the cross-sectional structure of the seamless steel pipe of each test number. Samples are cut from the steel pipe, embedded in resin and polished. The polished sample surface (observation surface) is set as a cross-section perpendicular to the length direction of the seamless steel pipe. After polishing the observation surface of the sample embedded in resin, the sample is immersed in nitric acid ethanol etching solution to make the grain boundaries of the ferrite on the surface visible. In any 5 fields of view of the observation surface after corrosion, the grain diameter of the ferrite in each field of view is calculated. For the area from the inner surface and outer surface of the steel pipe to 40μm in the depth direction, the area of ​​each field of view is set to 200×200μm 2 The average value of the grain sizes evaluated in five fields of view on each of the inner and outer surfaces of the tube was defined as the grain size of the ferrite grains.

[0191] [Sulfuric acid corrosion test]

[0192] From the seamless steel pipe of each test number, an arc-shaped test piece including the inner and outer surfaces of the steel pipe is collected. The width of the test piece is 10 mm, the thickness is the thickness of the steel pipe, and the length is 40 mm.

[0193] The two test pieces were used to perform the "6-hour sulfuric acid corrosion test" as shown below. In the 6-hour sulfuric acid corrosion test, a 50% sulfuric acid solution was prepared using special grade sulfuric acid (density about 1.84) specified in JIS K 8951 and distilled water. The test piece was immersed in a 70°C sulfuric acid solution for 6 hours under atmospheric pressure. The specific liquid volume was set to 22.7 cm 3 / cm 2 After 6 hours, the test piece was taken out from the sulfuric acid solution, washed with water and dried. The corrosion products on the surface of the dried test piece were removed, the mass of the test piece was measured, and the corrosion weight loss was calculated.

[0194] The corrosion rate (mg·cm -2 ·h -1 The average value of the two test pieces is defined as the corrosion rate (mg·cm -2 ·h -1 ). It should be noted that in Table 2, the corrosion rate is 20 mg·cm -2 ·h -1 The following are recorded as ◎, greater than 20 mg·cm -2 ·h -1 and 30 mg cm -2 ·h -1 The value below 0 is recorded as 0, and the value greater than 30 mg·cm -2 ·h -1 and 50 mg cm -2 ·h -1 The following is recorded as △, greater than 50 mg·cm -2 ·h -1 It is recorded as ×.

[0195] Industrial Applicability

[0196] According to the present invention, a seamless steel pipe having excellent corrosion resistance in an environment where the outer surface is exposed to sulfuric acid corrosion and the inner surface is exposed to high-temperature water can be provided. Therefore, the seamless steel pipe according to the present invention is suitable for air preheaters, gas-gas heaters, etc. of coal-fired power plant boilers, air preheaters / desulfurization devices of garbage incineration equipment, etc.

Claims

1. A seamless steel pipe having an inner surface and an outer surface, The chemical composition of the seamless steel pipe is C: 0.06% or less, Si: 0.55% or less, Mn: 0.70~1.40%, P: 0.020% or less, S：0.0005～0.020％、 N: 0.005% or less, O：0.0005～0.0035％、 Cu: 0.25-0.45%, Ni: 0.50% or less, Mo: 0.20% or less, Sb: 0.05-0.15%, Balance: Fe and impurities, The metallographic structure contains ferrite by area %: 90.0% or more, The average grain size of ferrite in the inner surface layer of the tube is 3.0 to 20.0 μm. The average grain size of ferrite in the outer surface layer of the tube is 3.0 to 25.0 μm. The inner surface layer of the tube contains MnS and MnS oxides, and the number density of MnS with a maximum length of more than 2.0 μm is less than 100 / mm 2 , and the ratio of the number density of MnS oxides having a maximum length of 2.0 μm or more to the number density of MnS having a maximum length of 2.0 μm or more is 0.10 or more, The outer surface layer of the tube contains MnS and MnS oxides, and the number density of MnS with a maximum length of more than 2.0 μm is less than 100 / mm 2 , and the ratio of the number density of MnS oxides having a maximum length of 2.0 μm or more to the number density of MnS having a maximum length of 2.0 μm or more is 0.10 or more.

2. The seamless steel pipe according to claim 1, wherein: The chemical composition contains, in mass%, selected from Al: 0.050% or less, Cr: 0.70% or less, Nb: 0.100% or less, Ta: 0.100% or less, V: 0.100% or less, Ti: 0.100% or less, W: 1.00% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, B: 0.0050% or less, Sn: 0.30% or less, Pb: 0.30% or less, Se: 0.100% or less, Te: 0.100% or less, Bi: 0.100% or less, Ag: 0.500% or less, and One or more kinds selected from the group consisting of Pd: 0.100% or less are used to replace a part of the Fe.

3. The seamless steel pipe according to claim 1 or claim 2, wherein: In the outer surface layer of the tube, the number density of MnS with a maximum length of 2.0 μm or more is less than 50 / mm 2 , and the ratio of the number density of MnS oxides having a maximum length of 2.0 μm or more to the number density of MnS having a maximum length of 2.0 μm or more is 0.50 or more.

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