SEAMLESS STEEL PIPE HAVING A DESIRABLE RESISTANCE TO CORROSION BY SULFURIC ACID DEWING POINT AND METHOD FOR MANUFACTURING THE SAME

Abstract

The present invention aims to provide a seamless steel tube and a method for manufacturing the same. A seamless steel tube of the present invention is a seamless steel tube having desirable resistance to corrosion by sulfuric acid condensation point, the seamless steel tube having a composition that includes, in % by mass, C: 0.01 to 0.12%, Si: 0.01 to 0.8%, Mn: 0.10 to 2.00%, P: 0.050% or less, S: 0.040% or less, Al: 0.010 to 0.100%, Cu: 0.03 to 0.80%, Ni: 0.01 to 0.50%, Mo: 0.01 to 0.20%, Sb: 0.002 to 0.50%, Cr: 0.004% or less, W: 0.0.02% or less, and the remainder Fe and incidental impurities, and a structure including a ferrite phase with an area percentage of 50 to 65%, a pearlite phase with an area percentage of 2% or less, and one or both bainite phases and a martensitic phase representing the remainder, the seamless steel tube having an elongation strength of 230 MPa or more and a tensile strength of 380 MPa or more.

Description

The present invention relates to a seamless steel pipe having desirable resistance to sulfuric acid condensation point corrosion and to a method for manufacturing the same. Specifically, the present invention relates to a seamless steel pipe suitable for piping systems in a corrosive sulfuric acid condensation point environment created by the combustion exhaust gas of boilers, gasification melting furnaces, and the like; in particular, a seamless steel pipe for piping systems exhibiting desirable resistance to sulfuric acid condensation point corrosion in a heat recovery steam generator, and to a method for manufacturing the seamless steel pipe. BACKGROUND OF THE INVENTION In the exhaust gases of boilers, thermal power plants, and other similar devices or installations that burn sulfur-containing fuels, such as heavy oil and coal, the sulfur oxides in the exhaust gases condense into sulfuric acid as the temperature drops. This causes severe corrosion known as sulfuric acid condensation corrosion. Several proposals have been made to reduce sulfuric acid condensation point corrosion. For example, PTL 1 describes a steel containing 0.001 to 0.2% carbon by mass, to which suitable amounts of Si, Mn, P, and S are added, and furthermore, Cu: 0.1 to 1% by mass, Mo: 0.001 to 1% by mass, and Sb: 0.01 to 0.2% by mass, to obtain a sulfuric acid condensation point corrosion-resistant steel containing controlled amounts of Sb, C, and Mo that satisfy a specific ratio. PTL 2 describes a steel of a composition (hereafter referred to as the “basic composition”) comprising, by mass percent, 0.050 to 0.150% carbon, suitable amounts of Si, Mi, P, and S, and Cu 0.20 to 0.50%, Ni 0.10 to 0.80%, Cr 0.20 to 1.50%, Sb 0.050 to 0.300%, and Ti 0.005 to 0.050%, and in which the contents of S, Cu, and Sb, and the contents of Cu, Ni, and Sb are specified by specific ratios, and the percentage area of ​​a ferrite phase, the percentage area of ​​a pearlite phase, and the percentage total area of ​​structures other than the ferrite and pearlite phases in the steel structure are controlled to obtain a steel resistant to acid condensation point corrosion sulfuric. PTL 3 describes a steel of the above basic composition comprising Co: 0.002 to 0.020% and in which the contents of S, Cu and Sb, and the contents of Cu, Ni, Sb and Co are specified by concrete ratios, and the percentage area of ​​a ferrite phase, the percentage area of ​​a pearlite phase and the total percentage area of ​​structures other than the ferrite phase and the pearlite phase in the steel structure are controlled to obtain a steel resistant to corrosion by sulfuric acid condensation point. PTL 4 describes a steel of the above basic composition comprising Co: 0.002 to 0.020% and W: 0.005 to 0.200%, and in which the contents of S, Cu, Sb and W, and the contents of Cu, Ni, Sb and Co are specified by specific ratios, and the percentage area of ​​a ferrite phase, QZfrnnn / zznz / B / YiAi The percentage area of ​​a pearlite phase and the percentage total area of ​​structures other than the ferrite phase and the pearlite phase in the steel structure are controlled to obtain a steel resistant to corrosion by sulfuric acid condensation point. PTL 5 describes a steel of the above basic composition comprising Co: 0.002 to 0.020% and Sn: 0.005 to 0.100%, and in which the contents of S, Cu, Sn and Sb, and the contents of Cu, Ni, Sn, Sb and Co are specified by specific ratios, and the percentage area of ​​a ferrite phase, the percentage area of ​​a pearlite phase and the total percentage area of ​​structures other than the ferrite phase and the pearlite phase in the steel structure are controlled to obtain a steel resistant to corrosion by sulfuric acid condensation point. PTL 6 describes a steel comprising, by mass percent, from 0.001 to 0.15% carbon, with suitable amounts of Si, Mi, P, and S added, and furthermore, Cu: 0.10 to 1.00%, Ni: 0.50% or less, Cr: 0.05 to 0.25%, Mo: 0.01 to 0.08%, Al: 0.100% or less, Ti, Nb, and V: 0 to 0.20% or less in total, B: 0 to 0.010%, and Sb and Sn: 0 to 0.10% in total, and having a single-phase ferrite structure, or a structure containing a total of 30% by volume or less of at least one of the phases of cementite, pearlite, bainite, and martensite, and wherein the remainder is a ferrite phase, with crystal grains of ferrite having a controlled average crystal grain size of 12.0 pm or less to obtain a steel sheet that has a desirable resistance to corrosion by sulfuric acid condensation point. List of appointments Patent literature PTL 1: JP-A-2003-213367 PTL 2: WO2018 / 038198 PTL 3: WO2018 / 038195 PTL 4: WO2018 / 038196 PTL 5: WO2018 / 038197 PTL 6: JP-A-2017-160544 BRIEF DESCRIPTION OF THE INVENTION Technical problem All the techniques described in PTL 1 to PTL 6 are intended to reduce the sulfuric acid condensation corrosion rate or the hydrochloric acid condensation corrosion rate, and are likely effective in inhibiting the formation of sulfuric acid condensation corrosion products, which cause problems in applications such as heat recovery steam generators. However, it is difficult to sufficiently inhibit sulfuric acid condensation corrosion in a more severe environment with a sulfuric acid concentration as high as 70% by mass.Previous related technical documents also do not contain detailed descriptions related to the manufacture of a seamless steel tube suitable for piping systems of heat recovery steam generators, and do not provide optimal conditions that guarantee both resistance to corrosion from the condensation point of sulfuric acid and the possibility of manufacturing a seamless steel tube. The present invention was developed in accordance with these circumstances and it is an objective of the present invention to provide a seamless steel tube having a desirable strength to QZfrnnn / zznz / B / YiAi sulfuric acid condensation point corrosion and that it is suitable for piping systems in a sulfuric acid condensation point corrosive environment, such as in a heat recovery steam generator, while it is desirable that it can be manufactured. Another objective of the present invention is to provide a suitable method for manufacturing the seamless steel tube. Solution to the problem To find a solution to the above problems, the present inventors experimentally produced seamless steel tubes of the compositions and steel structures (microstructures) shown in Table 1, with dimensions of 140 mm outside diameter and 10.5 mm wall thickness. The microstructures of the test steel tubes (steel tubes numbers 1-1 to IV-2) shown in Table 1 were analyzed as follows. First, an observation sample was taken from arbitrarily selected longitudinal and circumferential locations of the steel tube. A cross-section orthogonal to the longitudinal direction of the steel tube was polished to a mirror finish to obtain an observation surface. To observe the microstructure, the surface was etched with a 5% nitric acid and alcohol solution. After etching, four randomly selected fields at the center of the wall thickness of the observation sample were photographed using a light microscope (400x). The microstructure type of each steel was then determined from the micrographs. The microstructures differ due to the variations in the heat treatment applied to the steel tube, specifically the normalizing heat treatment and the subsequent quenching method. A corrosion test sample (30 mm long x 20 mm wide x 5 mm thick) was taken, including one outer surface of the steel tube. A corresponding surface of the outer surface of the steel tube was ground to 0.5 mm to remove unwanted components, such as scale. A corrosion test was performed in a sulfuric acid condensation-point environment, as described below. The test environment was created by simulating an environment with a sulfuric acid condensation-point concentration of 70% by mass at approximately 50°C, a temperature range in which the most severe corrosion should occur on a heat recovery tube of a heat recovery steam generator.Specifically, an aqueous sulfuric acid solution, adjusted to a concentration of 70% by mass, was poured into a container, and the corrosion test sample was immersed in the solution after being heated and maintained at 50°C using an external thermostatic bath. The sample was immersed for two different times: 8 hours and 96 hours. After each immersion, the aqueous sulfuric acid solution was drained from the container, and the corrosion test sample was dried and carefully removed to measure its weight. For each corrosion test performed with different immersion times, the corrosion rate was calculated from the surface area and weight of the corrosion test sample measured before the corrosion test, the weight of the corrosion test sample measured after immersion, and the immersion time. The results are presented in Table 1. QZfrnnn / zznz / B / YiAi & ha ω ω μ νj-1C 0Ü10U10U10U1|\ [Table 1]J Steel No. Composition (% by mass) Steel Tube No. Steel Tube Heat Treatment Microstructure Corrosion Rate in Corrosion Test* with 8 hours immersion (mg / cm2 / h) Corrosion Rate in Corrosion Test* with 96 hours immersion (mg / cm2 / h) Remarks C Si Mn PS Al Cu Ni Sb Mo Cr w 1 0.11 0.3 0.56 0.012 0.003 0.023 0.3 0.13 0.06 1-1 Normalizing followed by air quenching Ferrite+knoll 14 34 I-2 Normalizing followed by accelerated quenching Ferrite+knoll + bainite 13 26 II 0.11 0.2 0.54 0.012 0.003 0.031 0.2 0.12 0.06 0.16 11-1 Normalization followed by air quenching Ferrite+knob + bainite 19 24 II-2 Normalization followed by accelerated quenching Ferrite+bainite 17 18 III 0.10 0.3 0.55 0.011 0.002 0.028 0.3 0.13 0.06 - 0.10 - 111-1 Normalization followed by air quenching Ferrite+knob + bainite 15 22 a ω ω μ μ ο οι ο οι ο σι ο σι III-2 Normalizing followed by accelerated cooling Ferrite+bainite 16 18 Seasonal cracks in steel tubes IV 0.11 0.2 0.54 0.013 0.003 0.026 0.2 0.13 0.06 0.034 IV-1 Normalizing followed by air cooling Ferrite+pearlite 12 27 IV-2 Normalizing followed by accelerated cooling Ferrite+pearlite + bainite 11 21 Seasonal cracks in steel tubes 'Immersion in an aqueous solution of sulfuric acid at 50°C, 70% by mass > & h C hhccc J ho As shown in Table 1, the corrosion rate was higher in the corrosion test with a 96-hour immersion time than in the corrosion test with an 8-hour immersion time. Regarding the steel microstructure, the corrosion rate in the 96-hour corrosion test was found to be relatively lower in steel tubes with a ferrite, pearlite, and bainite structure (ferrite + pearlite + bainite) than in steel tubes with a ferrite and pearlite structure (ferrite + pearlite), and even lower in steel tubes with a ferrite and bainite structure (ferrite + bainite). Cross-sectional observations of the corrosion test sample after the test revealed that corrosion was more concentrated in the regions where the pearlite structure was most exposed on the surface.The result suggests that increasing the percentage area of ​​the pearlite structure increases the corrosion rate over a long immersion time. After further studies, the present inventors found combinations of steel compositions and manufacturing conditions for seamless steel pipes that can inhibit the formation of the pearlite structure that leads to the development of corrosion, particularly during prolonged immersion, and satisfactory ranges of compositions and manufacturing conditions to prevent various defects that occur during the manufacture of a seamless steel pipe. The present invention was completed based on these findings and the essence of the present invention is as follows. [1] A seamless steel tube having desirable resistance to sulfuric acid condensation point corrosion, the seamless steel tube having a composition that includes, in mass %, C: 0.01 to 0.12%, Si: 0.01 to 0.8%, Mn: 0.10 to 2.00%, P: 0.050% or less, S: 0.040% or less, Al: 0.010 to 0.100%, Cu: 0.03 to 0.80%, Ni: 0.01 to 0.50%, Mo: 0.01 to 0.20%, Sb: 0.002 to 0.50%, Cr: 0.004% or less, W: 0.002% or less, the remainder being Fe and incidental impurities, and a structure that includes a ferrite phase with a percentage area of 50 to 65%, a pearlite phase with an area percentage of 2% or less, and one or both bainite phases and a martensitic phase making up the remainder, the seamless steel tube has an elongation resistance of 230 MPa or more, and a tensile strength of 380 MPa or more. [2] Seamless steel pipe having a desirable resistance to corrosion by sulfuric acid condensation point according to [1], wherein the composition further includes, in % by mass, one or two groups selected from the following group A and group B, group A: one or both of Sn: 0.005 to 0.50% and Co: 0.005 to 0.20%, group B: Ti: 0.005 to 0.050%. A method for manufacturing seamless steel tubing having desirable resistance to corrosion by sulfuric acid condensation point according to [1] or [2], the method includes: heating a steel pipe material of the composition to 1,100 to 1,300°C, hot rolling the heated steel pipe material to 800°C or more to obtain a seamless steel pipe of a predetermined shape, and cooling the seamless steel pipe to room temperature; and QZfrnnn / zznz / e / YiAi Heat the seamless steel tube to a normalizing temperature of 850 to 1,050°C in a normalizing heat treatment, followed by accelerated cooling to a cooling stop temperature of 500°C or less at an average cooling rate of 10 to 50°C / s. [4] The method according to [3], which includes a heat treatment for tempering in which the seamless steel tube, after accelerated cooling, is cooled to room temperature and reheated to a tempering temperature of 400 to 700°C. In the present invention, the percentage area of ​​the ferrite phase in the steel structure (microstructure) refers to a value calculated by dividing the sum of the ferrite phase areas in an observed field by the field area when observing a cross-section orthogonal to the longitudinal direction of a steel tube under a light microscope. Similarly, the percentage area of ​​the pearlite phase in the steel microstructure refers to a value calculated by dividing the sum of the pearlite phase areas in an observed field by the field area when observing a cross-section orthogonal to the longitudinal direction of a steel tube under a light microscope. In the present invention, "desirable resistance to sulfuric acid condensation point corrosion" means a corrosion rate of 20 mg / cm2 / h or less, measured when a corrosion test sample taken from the outer surface of a seamless steel tube is immersed for 96 hours in a 70% by mass aqueous sulfuric acid solution heated and maintained at 50°C in a corrosion test. Advantageous effects of the invention The present invention can provide a seamless steel tube that has desirable resistance to sulfuric acid condensation corrosion and is suitable for piping systems in environments corrosive to sulfuric acid condensation, such as in a heat recovery steam generator, while also being desirably manufacturable. The present invention can also provide a suitable method for manufacturing the seamless steel tube. A seamless steel tube of the present invention has desirable resistance to corrosion from the condensation point of sulfuric acid, and the reduction in service life and damage due to corrosion can be inhibited even in a severe environment with a sulfuric acid concentration as high as 70% by mass, such as in a heat recovery steam generator. A seamless steel tube of the present invention also has predetermined elongation and tensile strength and is suitable for piping systems. A seamless steel tube of the present invention has desirable manufacturability with good formability and reduced cracking after heat treatment during the manufacturing process. Description of the modalities The present invention is described in more detail below. First, the reasons for limiting the composition of a seamless steel tube having desirable resistance to sulfuric acid condensation point corrosion of the present invention are described below. As used herein, the “%” used as a unit of component content means “% by mass”, a QZfrnnn / zznz / e / YiAi unless specifically stated otherwise. A seamless steel tube having desirable resistance to sulfuric acid condensation point corrosion of the present invention shall also be referred to simply as the “seamless steel tube of the present invention”. C: 0.01 to 0.12%. Carbon (C) is an element that increases the strength of steel. In the present invention, a carbon content of 0.01% or more is required to satisfy the required elongation and tensile strength, especially when seamless steel tubing is used in piping systems. For this reason, the carbon content is 0.01% or more. The carbon content is preferably 0.02% or more. An oxygen content greater than 0.12% tends to cause seasonal cracking due to the tempering of the steel. The upper limit for the carbon content is 0.12% because a carbon content greater than 0.12% causes noticeable seasonal cracking when the steel tubing is subjected to accelerated cooling after the normalizing heat treatment described below. The carbon content is preferably 0.08% or less, more preferably 0.06% or less. Yes: 0.01 to 0.8%. Silicon (Si) is an element that acts as a deoxidizing agent and increases the strength of steel by forming a solid solution within it. In the present invention, a Si content of 0.01% or more is required to satisfy the necessary elongation and tensile strength, especially when seamless steel tubing is used in piping systems. For this reason, the Si content is 0.01% or more. The Si content is preferably 0.1% or more, and more preferably 0.2% or more. The upper limit for the Si content is 0.8% because a Si content above 0.8% has adverse effects on resistance to sulfuric acid condensation point corrosion. The Si content is preferably 0.6% or less, and more preferably 0.4% or less. Mn: 0.10 to 2.00% Manganese (Mn) is an element that increases the strength of steel by improving its hardenability. In the present invention, a manganese content of 0.10% or more is required to satisfy the necessary elongation and tensile strength, particularly when seamless steel tubing is used in piping systems. For this reason, the manganese content is 0.10% or more. The manganese content is preferably 0.50% or more, and more preferably 0.70% or more. The upper limit for the manganese content is 2.00% because a manganese content above 2.00% tends to cause seasonal cracking due to steel tempering, as is the case with carbon. The manganese content is preferably 1.80% or less, and more preferably 1.40% or less. P: 0.050% or less Phosphorus (P) causes severe segregation along the centerline during continuous casting and leads to internal defects at the point of perforation during the hot rolling of a seamless steel tube. It also has adverse effects on resistance to sulfuric acid condensation point corrosion. For this reason, it is preferable in the present invention that the P content be reduced as much as possible. However, a P content of at most 0.050% is acceptable. Therefore, the P content is 0.050% or less. The P content is preferably 0.030% or less, more preferably 0.015% or less. The lower limit of the P content is not particularly limited. However, the preferred lower limit of the P content is approximately QZfrnnn / zznz / e / YiAi 0.001%, since excessive dephosphorization leads to an increase in manufacturing costs. S: 0.040% or less Sulfur (S) also causes severe segregation along the centerline during continuous casting and leads to internal defects at the point of perforation during the hot rolling of a seamless steel tube. A high S content has adverse effects on resistance to sulfuric acid condensation point corrosion. For this reason, it is desirable in the present invention to reduce the S content as much as possible. However, an S content of at most 0.040% is acceptable. Therefore, the S content is 0.040% or less. The S content is preferably 0.010% or less, more preferably 0.003% or less. The lower limit of the S content is not particularly limited. However, the preferred lower limit of the S content is around 0.0005% because excessive desulfurization leads to an increase in manufacturing costs. Al: 0.010 to 0.100% Aluminum (Al) acts as a deoxidizing agent. An Al content of 0.010% or more is required to reduce oxygen in the solid solution and prevent undesirable effects such as a decrease in the effective amount of manganese (Mn) due to the formation of manganese oxide. For this reason, the Al content is 0.010% or more. The Al content is preferably 0.015% or more, and more preferably 0.020% or more. The upper limit for the Al content is 0.100% because an Al content above 0.100% produces large amounts of Al₂O₃ in the steel and has adverse effects on resistance to sulfuric acid condensation point corrosion. The Al content is preferably 0.080% or less, and more preferably 0.040% or less. Cu: 0.03 to 0.80% Copper (Cu) is an effective element for preventing corrosion of steel in a sulfuric acid condensation point environment. The effect of improving resistance to sulfuric acid condensation point corrosion becomes even more pronounced when Cu is added with antimony (Sb). A Cu content of 0.03% or more is needed to achieve these effects. For this reason, the Cu content is 0.03% or more. The Cu content is preferably 0.10% or more, and more preferably 0.20% or more. It is well known that Cu decreases the high-temperature ductility of steel, and therefore the upper limit for the Cu content is 0.80%, since a Cu content above 0.80% produces noticeable defects on the outer surface during hot rolling. The Cu content is preferably 0.60% or less, and more preferably 0.50% or less. Ni: 0.01 to 0.50%. Nickel (Ni) is an element that inhibits the high-temperature ductility decrease of copper (Cu) when added to a copper-containing steel. A Ni content of 0.01% or more is required to achieve this effect. For this reason, the Ni content is 0.01% or more. The Ni content is preferably 0.05% or more, and more preferably 0.10% or more. The upper limit for the Ni content is 0.50% because Ni is an expensive element to add, and the effect becomes saturated even at concentrations above 0.50%. The Ni content is preferably 0.30% or less, and more preferably 0.25% or less. Mo: 0.01 to 0.20%. Molybdenum (Mo) is an element known to interfere with sulfuric acid condensation point corrosion resistance. The pearlite phase in the steel microstructure influences the QZfrnnn / zznz / B / YiAi long-term sulfuric acid condensation point corrosion resistance (described below), and is effective in reducing the pearlite phase to improve long-term sulfuric acid condensation point corrosion resistance. Previous studies by the present inventors revealed that the addition of minimal amounts of Mo effectively prevents the formation of the pearlite phase. A Mo content of 0.01% or more is required to provide the percentage of area needed for the pearlite structure in the microstructure of a steel tube, and to improve long-term sulfuric acid condensation point corrosion resistance. For this reason, the Mo content is 0.01% or more. The Mo content is preferably 0.10% or more, more preferably 0.14% or more. The upper limit of the Mo content is 0.20% because a Mo content greater than 0.01% is detrimental.A 20% concentration causes a serious deterioration in resistance to sulfuric acid condensation point corrosion. The Mo content is preferably 0.18% or less, more preferably 0.16% or less. Sb: 0.002 to 0.50%. Antimony (Sb), like copper (Cu), is an effective element in preventing corrosion of steel in sulfuric acid condensation-corrosive environments, and it greatly improves resistance to sulfuric acid condensation-corrosion when added with copper. An Sb content of 0.002% or more is required to achieve these effects. For this reason, the Sb content is 0.002% or more. The Sb content is preferably 0.02% or more, and more preferably 0.05% or more. Antimony also decreases the high-temperature ductility of steel, and the upper limit for the Sb content is 0.50% because an Sb content above 0.50% can lead to pipe failure due to cracking that occurs during the rolling of the steel pipe material. The Sb content is preferably 0.20% or less, and more preferably 0.09% or less. Cr: 0.004% or less Chromium (Cr) does not significantly affect corrosion resistance based on the condensation point of sulfuric acid itself. However, Cr improves the hardenability of steel and tends to induce seasonal cracking when accelerated cooling is performed after the normalizing heat treatment of a steel tube (described later). The Cr content must be 0.004% or less to inhibit seasonal cracking. There is no specific lower limit for Cr content. However, a Cr content of 0.001% or more is preferable because a Cr content below 0.001% leads to increased manufacturing costs. W: 0.002% or less Water (W) is an effective element for improving resistance to sulfuric acid condensation point corrosion. However, W enhances the hardenability of steel, as does chromium (Cr), and tends to cause seasonal cracking when accelerated cooling is performed after the normalizing heat treatment of a steel tube (described later). A W content of 0.002% or less is required to inhibit seasonal cracking. There is no specific lower limit for W content. However, 0.0003% or more is preferable, as W content below 0.0003% increases manufacturing costs. In addition to the above preferred basic components, the present invention may QZfrnnn / zznz / B / YiAi may optionally contain one or both of Sn: 0.005 to 0.50% and Co: 0.005 to 0.20%, and / or Ti: 0.005 to 0.050%. Sn: 0.005 to 0.50% Tin (Sn) may be added to further reduce corrosion in a corrosive environment with a sulfuric acid condensation point. A tin content of 0.005% or more is needed to sufficiently produce the effect of tin addition. For this reason, tin, when added, is present at 0.005% or more. The tin content is preferably 0.02% or more, more preferably 0.05% or more. The upper limit for tin content, when added, is 0.50% because tin decreases the high-temperature ductility of steel, as does antimony (Sb). The tin content is preferably 0.30% or less, more preferably 0.15% or less. Co: 0.005 to 0.20%. Cobalt (Co) may be included to further reduce corrosion in a corrosive environment with a sulfuric acid condensation point. A Co content of 0.005% or more is required to sufficiently produce the desired effect. For this reason, Co, when included, is present at a concentration of 0.005% or more. The Co content is preferably 0.01% or more, and more preferably 0.03% or more. The upper limit for Co content, when included, is 0.20% because Co decreases the high-temperature ductility of steel, as does tin (Sn). The Co content is preferably 0.08% or less, and more preferably 0.06% or less. Ti: 0.005 to 0.050%. Titanium (Ti) forms nitrides in steel and inhibits the growth of austenite grains during shot peening when the steel is transformed into the austenitic phase, especially at high temperatures. Since this allows the formation of fine ferrite grains in the subsequent ferrite transformation, Ti can be added to improve elongation resistance by enhancing grain refinement. A Ti content of 0.005% or more is required to achieve sufficient grain refinement. For this reason, when Ti is included, it is in an amount of 0.005% or more. The Ti content is preferably 0.008% or more, and more preferably 0.010% or more. The upper limit for the Ti content, when included, is 0.050% because a Ti content above 0.050% results in coarse titanium nitride, which has adverse effects on resistance to sulfuric acid condensation point corrosion.The Ti content is preferably 0.040% or less, more preferably 0.030% or less. In the above composition, the remainder is Fe and incidental impurities. Specific examples of incidental impurities are H, O, As, Zr, Ag, Ta, and Pb. The maximum acceptable limits for these incidental impurities are H: 0.0005%, O: 0.004%, As: 0.006%, Zr: 0.0004%, Ag: 0.001%, Ta: 0.004%, and Pb: 0.005%. The reasons for limiting the steel structure (microstructure) of a seamless steel tube of the present invention are described below. Percentage of ferrite phase area: 50 to 65% The lower limit for the ferrite phase area percentage is 50% because, with a ferrite phase area percentage below 50%, seasonal cracking tends to occur in a steel tube during accelerated cooling following the normalizing heat treatment of a steel tube to achieve a pearlite phase area percentage of 2% or less. The ferrite phase area percentage is preferably 55% or more. QZfrnnn / zznz / e / YiAi The ferrite phase area percentage is preferably 65% ​​because, with a ferrite phase area percentage greater than 65%, the strength of the steel, particularly the elongation resistance, decreases, and the steel cannot meet the elongation resistance required for pipes. The ferrite phase area percentage is preferably 60% or less. Perlite phase area percentage: 2% or less When the microstructure of a seamless steel tube contains a pearlite phase, the corrosion rate at the sulfuric acid condensation point increases when the steel tube is immersed in an aqueous sulfuric acid solution (solution temperature: 50°C, concentration: 70% by mass) for extended periods, as shown in the test results in Table 1. Extensive studies by the present inventors have determined that the percentage area of ​​the pearlite phase in the microstructure must be 2% or less to prevent an increase in the corrosion rate. The percentage area of ​​the pearlite phase is preferably 1% or less, and more preferably 0%.As an example of a method that limits the percentage area of ​​the pearlite phase in the controlled target range, Mo is added to the steel and accelerated cooling is performed after the normalizing heat treatment of a steel tube. In the steel microstructure of a seamless steel tube of the present invention, one or both bainite phases and a martensitic phase represent the remaining structures other than the ferrite and pearlite phases. Specifically, a seamless steel tube of the present invention has a steel microstructure with a ferrite phase comprising 50% to 65% of the area and a pearlite phase comprising 2% or less, with either a bainite phase or a martensitic phase, or both, representing structures other than the ferrite and pearlite phases. The bainite and martensitic phases are structures that develop during accelerated cooling following the normalizing heat treatment of a steel tube. In the present invention, the area percentages of the bainite and martensitic phases are not specifically limited.The percentage of the total area of ​​the remaining structure represented by one or both bainite and martensitic phases is preferably 48% or less. A seamless steel tube of the present invention has an elongation strength of 230 MPa or more, and a tensile strength of 380 MPa or more to ensure sufficient strength in the piping system. The elongation strength is preferably 250 MPa or more. The tensile strength is preferably 400 MPa or more. The elongation strength and tensile strength can be measured using the methods described in the Examples below. A seamless steel tube of the present invention has desirable resistance to corrosion by sulfuric acid condensation point. Specifically, a seamless steel tube of the present invention has a corrosion rate of 20 mg / cm² / h or less, as measured by immersion for 96 hours in a 70% by mass aqueous sulfuric acid solution at 50°C in a corrosion test as described above. In this way, the reduction in service life and damage due to corrosion can be inhibited even in a harsh environment, such as in a heat recovery steam generator. The corrosion rate is preferably 15 mg / cm² / h or less, and more preferably 10 mg / cm² / h or less. QZfrnnn / zznz / B / YiAi The following describes a method for manufacturing a seamless steel tube of the present invention. In the present invention, the process for manufacturing steel is not particularly limited. For example, molten steel of the above composition can be produced using an ordinary steelmaking process such as a converter, an electric furnace, or a vacuum melting furnace. For cost reasons, the molten steel is preferably melted by continuous casting.Continuous casting can be a process that continuously pours molten steel into a common casting having a rectangular cross-section, such as a slab or block, and hot rolls the casting into a steel tube material of a circular cross-section suitable for steel tube rolling (hereinafter, a steel tube material produced by this process shall also be referred to as “billet-rolled steel tube material”), or a process that continuously melts molten steel directly into a steel tube material having a circular cross-section (hereinafter, a steel tube material produced by this process shall also be referred to as “direct-cast steel tube material”). Either of these processes can be used in the present invention. Steel pipe material (billon-rolled steel pipe material or direct-cast steel pipe material) is used to form a seamless steel pipe of a predetermined shape (pipe manufacturing process). In the pipe manufacturing process, the steel pipe material can be formed into a seamless steel pipe of a predetermined shape by hot rolling the preheated steel pipe material (piercing followed by mandrel rolling or closed-rolling on a mandrel to obtain a predetermined wall thickness and rolling to obtain a suitably reduced diameter). The heating temperature and hot rolling temperature of the steel pipe material are as follows.In the present invention, the temperatures of the steel tube material and the temperatures of the steel tube (such as the heating temperature and the hot rolling temperature of a steel tube material, and the normalizing temperature and the cooling stop temperature of a steel tube) are surface temperatures of the steel tube materials and of the steel tubes (outer surface temperature in the case of a steel tube), unless specifically stated otherwise, and can be measured with, for example, a radiation thermometer. Heating temperature of the steel tube material: 1,100 to 1,300°C In the pipe manufacturing process, the steel pipe material is heated and hot-rolled to form a seamless steel pipe of a predetermined shape. From the perspective of rolling load, the lower limit for heating the steel pipe material is 1,100°C. Below this temperature, forming a seamless steel pipe may not be possible due to excessive rolling load. To prevent defects on the inner side of the pipe during punching, the lower limit for heating is preferably 1,150°C, and more preferably 1,200°C.When heated to a temperature above 1,300°C, the steel tube material may not possess the microstructure fractions required for the present invention, even after the heat treatment of the steel tube (normalizing heat treatment followed by accelerated cooling) described below. As a result, the resistance to sulfuric acid condensation point corrosion is negatively affected. For this reason, the upper limit of the heating temperature for the steel tube material is 1,300°C. The heating temperature for the steel tube material is preferably 1,290°C or lower, and more preferably 1,280°C or lower. Hot rolling temperature: 800°C or more The lower limit for hot rolling temperatures is 800°C because temperatures below 800°C can overload the rolling process and prevent the formation of a seamless steel tube. Therefore, the final hot rolling temperature is 800°C or higher. For example, when hot rolling begins with punching, followed by mandrel rolling or closed-over-mandrel rolling, and ends with diameter reduction rolling, the final diameter reduction rolling temperature is 800°C or higher. To prevent defects on the inner and outer surfaces of a steel tube during hot rolling, the rolling temperature is preferably 830°C or higher, and more preferably 850°C or higher. After hot rolling, the tubes are cooled to ambient temperature. In the present invention, "ambient temperature" means 25°C. The cooling method is not particularly limited. Typically, cooling is achieved by air cooling with, for example, a cooling bed. However, cooling can be achieved by weak water cooling in order to reduce the cooling time to ambient temperature and to increase the number of tubes rolled per hour. As used herein, "air cooling" means natural cooling that occurs without the use of any cooling medium. Typically, the average rate of air cooling is 1°C / s or less. Seamless steel pipe, cooled to room temperature after the pipe-making process, undergoes a normalizing heat treatment followed by accelerated cooling (steel pipe heat treatment process). The steel pipe heat treatment process may include an optional tempering heat treatment performed after accelerated cooling. The purpose of the normalizing heat treatment is to adjust the temper of the seamless steel pipe to a predetermined strength suitable for the piping system.In the present invention, the normalizing heat treatment is followed by accelerated cooling to control the microstructure of the steel, in order to inhibit the formation of a pearlite structure, which corrodes preferentially in an aqueous solution of sulfuric acid at 50°C and 70% by mass, created to simulate a sulfuric acid condensation point corrosion environment that causes the most severe corrosion in a steel tube, in particular in a heat recovery tube of a heat recovery steam generator. Normalization temperature: 850 to 1,050°C In the normalizing heat treatment, the steel tube is preferably heated to a temperature range where the steel transforms into an austenitic phase, in order to regulate the grain size in the tube's microstructure. At normalizing temperatures below 850°C, the steel does not fully transform into an austenitic phase, and a partially untransformed ferrite phase remains, causing a decrease in elongation resistance. For this reason, the normalizing temperature is 850°C or higher. The normalizing temperature is preferably 880°C or higher, and more preferably 900°C or higher. When heated above 1050°C, severe oiling of the austenite crystal grains occurs, resulting in a coarse ferrite phase after transformation during accelerated cooling, which further reduces elongation resistance.For this reason, the upper limit of the normalization temperature is 1,050°C. The normalization temperature is preferably 1,000°C or less, more preferably 950°C or less. Average cooling rate of accelerated cooling: 10 to 50°C / s The normalizing heat treatment is followed by accelerated cooling from the normalizing temperature to a cooling stop temperature. Accelerated cooling after the normalizing heat treatment is carried out to inhibit the formation of a pearlite phase in the microstructure of the steel tube. As used herein, “average accelerated cooling rate” means the average cooling rate on an outer surface of a steel tube over a temperature range from the normalizing temperature to the cooling stop temperature.When steel pipe is air-cooled without accelerated cooling, or the average accelerated cooling rate is less than 10°C / s, the pearlite phase area exceeds 2%, and the corrosion rate increases in a long-duration sulfuric acid condensation point corrosion test. The average accelerated cooling rate is preferably 12°C / s or higher, and more preferably 15°C / s or higher. The upper limit for the average accelerated cooling rate is 50°C / s because an average cooling rate above 50°C / s causes severe seasonal cracking after accelerated cooling. The average accelerated cooling rate is preferably 30°C / s or lower, and more preferably 25°C / s or lower. Accelerated cooling stop temperature: 500°C or less The accelerated quenching stop temperature is 500°C or less to inhibit the formation of a pearlite phase in the microstructure of the seamless steel tube. Preferably, the accelerated quenching stop temperature is 450°C or less. There is no specific lower limit for the quenching stop temperature. However, from the perspective of preventing seasonal cracking after accelerated quenching, the quenching stop temperature is preferably 200°C or higher. When the content of alloying elements such as carbon, manganese, copper, nickel, and molybdenum is high, and the steel has particularly high tensile strength, the content of these elements can be adjusted by subjecting the steel tube to a tempering heat treatment after cooling it to room temperature following rapid cooling. When performed, the tempering heat treatment is carried out at the following temperatures. Tempering temperature: 400 to 700°C During the tempering process, the steel tube, cooled to room temperature after rapid quenching, is reheated to a tempering temperature of 400 to 700°C. At tempering temperatures below 400°C, it is difficult to reduce tensile strength, and in some cases, seasonal cracking can occur when the steel tube, after rapid quenching, has high tensile strength. For this reason, the steel tube, when QZfrnnn / zznz / e / YiAi undergoes tempering heat treatment, quenched at a tempering temperature of 400°C or higher. The tempering temperature is preferably 450°C or higher, and more preferably 500°C or higher. When tempered at a tempering temperature above 700°C, the steel partially transforms into an austenitic phase, and the transformation into a ferrite phase occurs during a subsequent cooling process, resulting in a decrease in the elongation resistance of the steel tube. For this reason, the upper limit of the tempering temperature is 700°C when performing tempering heat treatment. The tempering temperature is preferably 650°C or lower, and more preferably 600°C or lower. Examples The cast steels of the compositions indicated in Tables 2 and 3 were produced using a converter method and each was cast into a casting by continuous casting. In the continuous casting process, some steels were cast into castings with a rectangular cross-section (300 mm thick x 400 mm wide), while the others were cast into castings with a circular cross-section (190 mm in diameter). The castings with a rectangular cross-section are substantially square columns, while the castings with a circular cross-section are substantially cylindrical. The casting with a rectangular cross-section was hot-rolled to obtain a steel tube (billet steel tube stock) with a diameter of 190 mm or 140 mm. Seamless steel tubes manufactured from this steel tube stock are referred to as “billet rolled” in the “Type of steel tube stock” section of Tables 4 to 7. Seamless steel tubes manufactured directly from castings (steel tube stock or straight-cast steel tube stock) having a circular cross-section with a diameter of 190 mm, prepared by continuous casting, are referred to as “Straight-cast” in the “Type of steel tube stock” section of Tables 4 to 7. These steel pipe materials were heated and hot-rolled to produce seamless steel pipes with the wall thicknesses and outside diameters specified in Tables 4 through 7 (Pipe Manufacturing Process). The hot rolling of the preheated steel pipe material was carried out using a process that began with piercing, followed by mandrel rolling, and concluded with diameter reduction rolling. The heating temperature and the final hot-rolling temperature of the steel pipe material are specified in Tables 4 through 7 under "Pipe Conditions." Steel tube materials are indicated as “Failed tube making” in the “Remarks” section of Tables 4 to 7 when the tube making process was forced to stop during hot rolling due to difficulties such as overloading, and the indication “Failed” is used in the “Formability” section of Tables 8 to 11. Steel tube materials are indicated as “Successful tube making” in the “Remarks” section of Tables 4 to 7 when the tube making process successfully produced seamless steel tubes without stopping. After hot rolling, the steel tube was allowed to cool to room temperature and subjected to non-destructive testing to check for defects on its inner and outer surfaces. The presence or absence of defects, as well as the success or failure of their removal, were also assessed. The conformability of the same after repair is indicated in the "Conformability" section of Tables 8 to 11. The result is "Excellent" when no defects were observed in any non-destructive test, "Satisfactory" when defects were observed in a non-destructive test, but the required criteria were met after repair, and "Unsatisfactory" when defects were observed in a non-destructive test, and repair was impossible or the required criteria were not met even after repair. Conformability was determined to be desirable when the evaluation result was "Excellent" or "Satisfactory," with the former being more desirable. The term "repair" refers to the removal of scratches and other defects using, for example, a cutting device. Subsequently, the steel tubes were subjected to heat treatment for steel tubes (normalizing heat treatment followed by accelerated cooling, optionally followed by tempering heat treatment) under the heat treatment conditions for steel tubes indicated in Tables 4 to 7. In Tables 4 to 7, the symbol means no treatment. The steel tube, after accelerated cooling (after air cooling for air-cooled steel tubes), was allowed to stand for 48 hours after the steel tube's temperature had cooled to ambient temperature. It was then subjected to a non-destructive test for cracks on the outer surface of the tube in order to assess post-cooling cracking. The presence or absence of seasonal cracking, as well as the success or failure of crack removal after repair, is indicated in the section "Post-cooling Seasonal Cracking" in Tables 8 to 11. Steel tubes marked "Unsatisfactory" and "Failed" in the section "Conformability" were not evaluated for seasonal cracking, and the symbol is used in the section "Post-cooling Seasonal Cracking" in Tables 8 to 11.The evaluation result is “Excellent” when no seasonal cracking was observed in a non-destructive test, “Satisfactory” when seasonal cracking was observed in a non-destructive test, but the required criteria were met after repair, and “Failed” when seasonal cracking was observed in a non-destructive test, and repair was impossible or the required criteria were not met even after repair. Steel pipes with “Excellent” or “Satisfactory” results were considered highly resistant to seasonal cracking, with “Excellent” being the most desirable. Samples for structure observation, tensile test samples, and corrosion test samples in a sulfuric acid condensation point environment were taken from seamless steel tubes produced in the manner described above. An observation sample was taken from arbitrarily chosen longitudinal and circumferential locations on the steel tube, and a cross-section orthogonal to the longitudinal direction of the steel tube was polished to a mirror finish to obtain an observation surface. To observe the microstructure, the surface was etched with a 5% nitric acid and alcohol solution. After etching, four randomly selected fields at the center of the wall thickness of the observation sample were photographed using a light microscope (400x). For each micrograph, the area percentages of the ferrite and pearlite phases were calculated using image processing.The remaining structure other than the ferrite phase and the pearlite phase in the microstructure was also specified, and the percentage of total area of ​​the remaining structure was determined by subtracting the percentages of area of ​​the ferrite phase and the pearlite phase from the percentage of. QZfrnnn / zznz / e / YiAi total area (100%) of all phases. Tables 8 through 11 show the ferrite phase area percentage, the pearlite phase area percentage, and the type and total area percentage of the remaining microstructure for each seamless steel tube. A seamless steel tube of the present invention has a structure with an acceptable ferrite phase area percentage of 50 to 65%, preferably 55 to 60%. A seamless steel tube of the present invention has a structure with an acceptable pearlite phase area percentage of 2% or less, preferably 0%. In Tables 8 through 11, for example, the remaining structure type “Bainite + Martensite” means that the remaining structure consists of a bainite phase and a martensitic phase. A tensile test sample was taken from arbitrarily selected longitudinal and circumferential points of the steel pipe. Steel pipes with an outside diameter less than 170 mm were prepared as test samples according to JIS Z2241 12B, while steel pipes with an outside diameter of 170 mm or greater were prepared as test samples according to JIS Z2241 12C. The tensile test was performed in accordance with JIS Z2241. The elongation and tensile strength obtained in the tensile test are shown in Tables 8 through 11. Steel pipes were considered acceptable when the elongation was equal to or greater than 230 MPa and the tensile strength was equal to or greater than 380 MPa. For the corrosion test performed in a sulfuric acid condensation environment, a corrosion test sample (30 mm long x 20 mm wide x 5 mm thick) was taken, including one outer surface of the steel tube. A corresponding surface of the outer surface of the steel tube was ground to 0.5 mm to remove unwanted components, such as scale. The corrosion test was carried out in a sulfuric acid condensation environment, as described below. The test environment was created to simulate an environment with a sulfuric acid condensation concentration of 70% by mass at approximately 50°C, a temperature range in which the most severe corrosion should occur in a heat recovery tube of a heat recovery steam generator.Specifically, an aqueous sulfuric acid solution, adjusted to a concentration of 70% by mass, was poured into a container, and the corrosion test sample was immersed in the solution after being heated and maintained at 50°C using an external thermostatic bath. The sample was immersed for 96 hours. After 96 hours of immersion, the aqueous sulfuric acid solution was drained from the container, and the corrosion test sample was dried and carefully removed for weight measurement. The corrosion rate was calculated from the surface area and weight of the corrosion test sample measured before the corrosion test, the weight of the corrosion test sample measured after immersion, and the immersion time. The results are presented in Tables 8 through 11. In the corrosion test, samples were considered acceptable when the corrosion rate was 20 mg / cm² / h or less.The corrosion rate is preferably 15 mg / cm2 / ho less, more preferably 10 mg / cm2 / ho less. In Tables 8 to 11, the samples with the symbol are samples that were not subjected to structure observation, tensile testing and corrosion testing in an environment with a sulfuric acid condensation point. QZfrnnn / zznz / B / YiAi [Table 2] Steel No. Composition (% by mass) Group C Si Mn PS Al Cu Ni Mo Sb Cr W Sn Co Ti A 0.05 0.2 0.71 0.009 0.002 0.021 0.40 0.25 0.15 0.06 0.002 0.001 - - Example of conformity B 0.02 0.4 1.38 0.011 0.001 0.033 0.29 0.19 0.14 0.09 0.001 0.002 - - Example of conformity C 0.08 0.1 0.75 0.013 0.002 0.038 0.31 0.16 0.14 0.08 0.002 0.001 - - Example of conformity D 0.03 0.6 1.24 0.025 0.007 0.029 0.42 0.22 0.16 0.07 0.001 0.001 - - - Example of conformity E 0.01 0.3 1.77 0.014 0.002 0.057 0.50 0.23 0.15 0.05 0.002 0.002 - - Example of conformity F 0.11 0.2 0.54 0.012 0.003 0.031 0.22 0.12 0.16 0.06 0.003 0.002 - - Example of conformity G 0.04 0.3 1.02 0.013 0.014 0.015 0.33 0.14 0.14 0.07 0.003 0.001 - - Example of conformity H 0.03 0.4 1.16 0.015 0.003 0.036 0.60 0.24 0.18 0.08 0.004 0.002 - - - Example of conformity 1 0.02 0.4 1.08 0.012 0.002 0.021 0.14 0.29 0.15 0.03 0.002 0.001 - - - Example of conformity J 0.06 0.3 0.88 0.011 0.001 0.024 0.11 0.13 0.16 0.17 0.003 0.001 - - Example of conformity K 0.03 0.2 1.36 0.008 0.003 0.034 0.20 0.07 0.11 0.09 0.001 0.001 - - Example of conformity L 0.02 0.5 1.93 0.013 0.001 0.012 0.47 0.41 0.07 0.28 0.002 0.002 - - Example of conformity M 0.05 0.1 0.24 0.008 0.004 0.093 0.78 0.50 0.13 0.31 0.001 0.002 - - - Example of conformity N 0.04 0.2 0.75 0.011 0.002 0.024 0.36 0.22 0.14 0.07 0.004 0.001 0.15 - - Example of conformity 0 0.05 0.3 0.77 0.012 0.003 0.023 0.39 0.23 0.15 0.08 0.003 0.001 - 0.06 - Example of conformity P 0.04 0.3 0.76 0.011 0.001 0.022 0.41 0.24 0.14 0.08 0.002 0.001 0.14 0.04 - Example of conformity Q 0.02 0.3 1.35 0.014 0.003 0.029 0.33 0.23 0.13 0.08 0.001 0.002 - 0.029 Example of conformity R 0.03 0.2 1.33 0.012 0.002 0.033 0.32 0.22 0.13 0.07 0.003 0.002 0.15 - 0.027 Example of conformity S 0.02 0.3 1.34 0.013 0.003 0.031 0.33 0.24 0.14 0.08 0.001 0.002 0.05 0.05 0.022 Example of conformity. [TABLES] Steel No. Composition (% by mass) Grouping C Si Mn PS Al Cu Ni Mo Sb Cr W Sn Co Ti T 0.13 0.2 0.55 0.011 0.002 0.028 0.20 0.13 0.15 0.06 0.002 0.001 - - Comparative Example U 0.002 0.1 0.53 0.012 0.003 0.027 0.22 0.12 0.14 0.07 0.004 0.002 - - Comparative Example V 0.09 0.9 0.58 0.014 0.004 0.029 0.21 0.13 0.14 0.06 0.003 0.001 - - Comparative Example W 0.10 0.3 2.12 0.011 0.002 0.025 0.21 0.13 0.13 0.07 0.002 0.002 Comparative Example X 0.11 0.2 0.04 0.012 0.001 0.026 0.23 0.12 0.14 0.07 0.004 0.002 Comparative Example Y 0.12 0.3 0.54 0.053 0.003 0.027 0.29 0.14 0.13 0.08 0.003 0.001 Comparative Example Z 0.09 0.2 0.56 0.009 0.044 0.031 0.22 0.12 0.13 0.07 0.002 0.002 Comparative Example AA 0.10 0.3 0.55 0.012 0.002 0.160 0.31 0.13 0.14 0.06 0.004 0.001 Comparative Example AB 0.12 0.2 0.57 0.013 0.002 0.029 0.93 0.50 0.13 0.07 0.003 0.002 Comparative Example AC 0.11 0.3 0.55 0.011 0.003 0.024 0.02 0.25 0.12 0.08 0.001 0.001 Comparative Example AD 0.10 0.3 0.54 0.008 0.001 0.022 0.22 0.003 0.13 0.06 0.002 0.001 Comparative Example AE 0.12 0.2 0.58 0.011 0.002 0.026 0.32 0.14 0.26 0.07 0.004 0.002 Comparative Example AF 0.11 0.3 0.56 0.012 0.003 0.023 0.31 0.13 0.004 0.06 0.001 0.001 Comparative Example AG 0.11 0.3 0.55 0.013 0.002 0.025 0.29 0.15 0.14 0.80 0.002 0.001 Comparative Example AH 0.09 0.2 0.53 0.010 0.001 0.021 0.23 0.12 0.14 0.001 0.003 0.002 Comparative Example Al 0.10 0.2 0.57 0.011 0.003 0.024 0.31 0.16 0.13 0.07 0.005 0.001 Comparative Example AJ 0.09 0.3 0.56 0.012 0.002 0.023 0.21 0.15 0.14 0.06 0.004 0.003 Comparative Example AK 0.12 0.3 0.54 0.013 0.003 0.026 0.32 0.17 0.001 0.26 0.002 0.036 Comparative Example AL 0.11 0.2 0.53 0.011 0.002 0.024 0.22 0.18 0.06 0.01 0.080 0.001 Comparative Example AM ​​0.06 0.3 0.88 0.010 0.003 0.023 0.31 0.14 0.002 0.27 0.060 0.002 - 0.12 Comparative Example AN 0.12 0.2 0.55 0.011 0.002 0.028 0.20 0.16 0.001 0.27 0.221 0.002 - - 0.008 Comparative Example AO 0.09 0.3 0.55 0.012 0.003 0.025 0.22 0.16 0.001 0.28 0.213 0.001 - 0.14 0.011 Comparative example AP 0.13 0.2 0.56 0.011 0.002 0.026 0.21 0.15 0.002 0.26 0.218 0.014 0.009 Comparative example AQ 0.10 0.3 0.57 0.013 0.002 0.027 0.28 0.14 0.001 0.27 0.205 0.001 0.04 0.013 Comparative example. hhc [Table 4] c Steel Tube No. Steel No. Steel Tube Material Type Wall Thickness (mm) Outside Diameter (mm) Tube Manufacturing Conditions Steel Tube Heat Treatment Conditions Remarks Grouping Steel Tube Material Heating Temp. (°C) Hot Rolling Final Temp. (°C) Normalizing Temperature (°C) Average Cooling Rate (°C / s) Accelerated Cooling Stop Temperature (°C) Tempering Temperature (°C) 1-1 A Direct Casting 10.5 140 1265 866 921 25 446 - manufacturable Example of conformity 1-2 A Direct Casting 10.5 140 1261 864 920 24 340 - manufacturable Example of conformity 1-3 A Direct Casting 10.5 140 1262 858 920 25 220 505 manufacturable Example of conformity 1-4 A Direct casting 10.5 140 1263 862 919 Normalizing heat treatment followed by air quenching manufacturable Comparative example 1-5 A Direct casting 10.5 140 1271 863 918 53 344 - manufacturable Comparative Example 1-6 A Direct Casting 10.5 140 1267 851 919 4 205 - manufacturable Comparative Example 1-7 A Direct Casting 10.5 140 1266 856 921 24 513 - manufacturable Comparative Example 1-8 A Direct Casting 10.5 140 1263 857 919 25 218 722 manufacturable Comparative Example 1-9 A Direct Casting 10.5 140 1266 852 1061 25 437 - manufacturable Comparative Example 1-10 A Direct Casting 10.5 140 1264 858 833 24 434 - Manufactureable Comparative Example. 1-11 A Direct Casting 10.5 140 1314 892 922 24 431 - manufacturable Comparative Example 1-12 B Billet Rolled 8.6 114 1278 853 949 24 303 - manufacturable Conformity Example 1-13 C Direct Casting 10.5 140 1271 867 911 21 214 551 manufacturable Conformity Example [Table 5] Steel Tube No. Steel No. Steel Tube Material Type Wall Thickness (mm) Outside Diameter (mm) Tube Manufacturing Conditions Steel Tube Heat Treatment Conditions Remarks Grouping Steel Tube Material Heating Temp. Final Hot Rolled Temperature (°C) Normalizing Temperature (°C) Average Cooling Rate (°C / s) Accelerated Cooling Stop Temperature (°C) Quenching Temperature (°C) 1-14 D Direct Casting 10.5 140 1269 878 941 22 455 manufacturable Example of conformity 1-15 E Billet Rolled 8.6 114 1284 836 967 29 309 - manufacturable Example of conformity 1-16 F Direct Casting 10.5 140 1264 863 918 23 438 - manufacturable Example of conformity 1-17 G Direct casting 10.5 140 1267 861 937 24 433 - manufacturable Example of conformity 1-18 H Direct casting 12.7 179 1244 886 882 18 444 - manufacturable Example of conformity 1-19 1 Direct casting 8.6 114 1287 851 943 32 312 - manufacturable Example of conformity 1-20 J Direct casting 10.5 140 1269 871 933 26 281 576 manufacturable Example of conformity 1-21 K Direct casting 10.5 140 1263 868 944 26 434 - manufacturable Example of conformity 1-22 L Direct casting 8.6 114 1281 811 958 33 314 - manufacturable Example of conformity. ω > & hci\ hccc 4 ho 1-23 M Direct Casting 12.7 179 1295 893 887 11 428 - fabricadle Example of conformity 1-24 N Direct Casting 10.5 140 1261 861 923 25 429 - fabricadle Example of conformity 1-25 O Direct Casting 10.5 140 1263 862 922 24 431 - fabricadle Example of conformity 1-26 R Billet Rolled 10.5 140 1262 859 924 25 211 507 fabricadle Example of conformity 1-27 Q Direct Casting 8.6 114 1277 851 947 25 331 - fabricadle Example of conformity 1-28 R Casting Direct 10.5 140 1258 862 939 24 229 512 manufacturable Example of conformity 1-29 S Billet roll 10.5 140 1261 858 940 25 221 504 manufacturable Example of conformity hhc [Table 6] c Steel Tube No. Steel No. Steel Tube Material Type Wall Thickness (mm) Outside Diameter (mm) Tube Manufacturing Conditions Steel Tube Heat Treatment Conditions Remarks Grouping Steel Tube Material Heating Temperature (°C) Final Hot Rolling Temperature (°C) Normalizing Temperature (°C) Average Cooling Rate (°C / s) Accelerated Cooling Stop Temperature (°C) Tempering Temperature (°C) 1-30 T Direct Cast 10.5 140 1259 871 926 23 447 - manufacturable Comparative Example 1-31 u Direct Cast 10.5 140 1262 869 924 24 439 - manufacturable Comparative Example 1-32 V Direct Cast 10.5 140 1261 872 923 24 440 - manufacturable Comparative Example 1-33 w Direct casting 10.5 140 1262 868 925 23 441 • manufacturable Comparative example 1-34 X Direct casting 10.5 140 1259 872 924 24 438 - manufacturable Comparative example 1-35 Y Direct casting 10.5 140 1261 871 922 24 442 - manufacturable Comparative example 1-36 z Direct casting 10.5 140 1260 867 927 23 444 - manufacturable Comparative example 1-37 AA Direct casting 10.5 140 1262 869 926 24 445 - manufacturable Comparative example 1-38 AB Direct casting 10.5 140 1279 873 - - - - manufacturable Comparative example. 1-39 AC Direct Casting 10.5 140 1258 870 924 23 443 - manufacturable Comparative Example 1-40 AD Direct Casting 10.5 140 1277 871 • manufacturable Comparative Example 1-41 AE Direct Casting 10.5 140 1257 874 923 24 438 - manufacturable Comparative Example 1-42 AF Direct Casting 10.5 140 1259 871 926 23 440 - manufacturable Comparative Example 1-43 AG Direct Casting 10.5 140 1279 877 - - - - Hot Rolling Failure Comparative Example 1-44 AH Direct Casting 10.5 140 1262 867 921 24 437 - manufacturable Comparative Example 1-45 Al Direct Casting 10.5 140 1261 871 923 23 449 - manufacturable Comparative Example 1-46 AJ Direct Casting 10.5 140 1258 874 926 24 448 • manufacturable Comparative Example [Table 7] > ·». Steel Tube No. Steel No. Steel Tube Material Type Wall Thickness (mm) Outside Diameter Tube Manufacturing Conditions Steel Tube Heat Treatment Conditions Remarks Grouping Steel Tube Material Heating Temp. (°C) Final Hot Rolled Temperature (°C) Normalizing Temperature (°C) Average Cooling Rate (°C / s) Tempering Temperature Cooling Stop Temperature (°C) Accelerated Cooling Temperature (°C) 1-47 AK Direct Casting 10.5 140 1260 875 925 Normalizing Heat Treatment followed by Air Quenching (Fabricable) Comparative Example 1-48 AK Direct Casting 10.5 140 1264 878 921 23 445 (Fabricable) Comparative Example 1-49 AL Direct Casting 10.5 140 1258 881 919 Normalizing Heat Treatment followed by Air Quenching (Fabricable) Comparative example 1-50 AL Direct casting 10.5 140 1261 879 920 24 443 manufacturable Comparative example 1-51 AM Direct casting 10.5 140 1262 869 923 Normalizing heat treatment followed by air quenching manufacturable Comparative example 1-52 AM Direct casting 10.5 140 1259 866 922 24 439 manufacturable Comparative example 1-53 AN Direct casting 10.5 140 1257 861 928 Normalizing heat treatment followed by air quenching manufacturable Comparative example 1-54 AN Direct casting 10.5 140 1256 859 929 25 438 manufacturable Comparative example. & hchhccc J ho ω ω ο σι ο M M Ο σι ο01 1-55 AO Direct Casting 10.5 140 1263 871 924 Normalizing Heat Treatment followed by Air Cooling manufacturable Comparative Example 1-56 AO Direct Casting 10.5 140 1261 868 926 24 442 manufacturable Comparative Example 1-57 AP Direct Casting 10.5 140 1255 872 922 Normalizing Heat Treatment followed by Air Cooling manufacturable Comparative Example 1-58 AP Direct Casting 10.5 140 1257 874 924 24 441 manufacturable Comparative Example 1-59 AQ Direct Casting 10.5 140 1252 878 929 Normalizing Heat Treatment followed by Air Cooling manufacturable Comparative Example 1-60 AQ Direct Casting 10.5 140 1249 876 927 23 439 manufacturable Comparative example > & h C hhccc J ho co [Table 8] Steel tube number Steel number Formability Seasonal cracks after Cooling Structure Tensile Properties Corrosion Rate in 96-Hour Immersion Corrosion Test'2 (mg / cm2 / h) Grouping Percentage of Ferrite Phase Area (%) Percentage of Pearlite Phase Area (%) Type and Percentage of Total Area*1 of Remaining Structure (%) Elongation Strength (MPa) Tensile Strength (MPa) 1-1 A Excellent Excellent 56 0 Bainite (44) 288 551 8 Example of Compliance 1-2 A Excellent Excellent 57 0 Bainite+Martensite (43) 292 573 9 Example of Compliance 1-3 A Excellent Excellent 52 0 Martensite (48) 457 594 8 Example of Compliance 1-4 A Excellent Excellent 64 19 Bainite (17) 263 508 28 Comparative Example 1-5 A Excellent Failure 3 0 Bainite+Martensite (97) 311 668 8 Comparative Example 1-6 A Excellent Excellent 61 12 Bainite (27) 285 524 23 Comparative Example 1-7 A Excellent Excellent 60 6 Bainite (34) 287 533 22 Comparative Example1-8 A Excellent Excellent 83 0 Bainite (17) 211 513 9 Comparative Example 1-9 A Excellent Excellent 51 0 Bainite (49) 221 566 9 Comparative Example 1-10 A Excellent Excellent 38 9 Bainite (53) 272 586 32 Comparative Example 1-11 A Excellent Excellent 63 4 Bainite (33) 282 539 21 Comparative Example 1-12 B Excellent Excellent 58 0 Bainite (42) 274 522 10 Conformance Example 1-13 C Excellent Excellent 52 1 Martensite (47) 499 629 14 Conformance Example *1 The number in parentheses is the percentage of total area of ​​layers other than the ferrite base and the pearlite phase. *2 Immersion in an aqueous solution of sulfuric acid 70% by mass at 50°C Α ω ω ο σι o [Table 9] Steel Tube No. Steel No. Formability Seasonal Cracking Post-Cooling Structure Tensile Properties Corrosion Rate in 96-Hour Immersion Corrosion Test*2 (mg / cm² / h) Grouping Percentage of Ferrite Phase Area (%) Percentage of Knob Phase Area (%) Type and Percentage of Total Area*1 of Remaining Structure (%) Elongation Strength (MPa) Tensile Strength (MPa) 1-14 D Excellent Satisfactory 57 0 Bainite (43) 258 471 11 Example of Conformance 1-15 E Satisfactory Satisfactory 64 0 Bainite (36) 259 497 16 Example of Conformance 1-16 F Excellent Satisfactory 51 2 Bainite (47) 332 614 18 Example of Conformance 1-17 G Satisfactory Excellent 61 0 Bainite (39) 278 526 14 Example of conformity 1-18 H Satisfactory Excellent 59 0 Bainite (41) 263 507 10 Example of conformity 1-19 I Excellent Excellent 63 0 Bainite (37) 242 518 13 Example of conformity 1-20 J Satisfactory Excellent 55 Bainite+martensite(44) 413 609 7 Example of conformity 1-21 K Satisfactory Excellent 57 1 Bainite (42) 275 513 14 Example of conformity > & hchhccc J ho 1-22 L Satisfactory Satisfactory 56 0 Bainite(44) 295 534 6 Example of conformity 1-23 M Satisfactory Excellent 52 2 Bainite(46) 304 512 19 Example of conformity 1-24 N Excellent Excellent 55 0 Bainite (45) 283 566 9 Example of conformity 1-25 0 Excellent Excellent 56 0 Bainite(44) 272 549 8 Example of conformity 1-26 P Excellent Excellent 54 0 Bainite+martensite (46) 466 617 9 Example of conformity 1-27 Q Excellent Excellent 59 0 Bainite+martensite (41) 288 524 7 Example of conformity 1-28 R Excellent Excellent 56 0 Bainite (44) 298 512 7 Example of conformity 1-29 S Excellent Excellent 57 0 Bainite (43) 287 523 6 Example of conformity Ί The number in parentheses is the percentage of total area of ​​the layers other than the ferrous phase and the pearlite phase *2 Immersion in a 70% by mass aqueous sulfuric acid solution at 50°C hhc [Table 10] c Steel Tube No. Steel No. Formability Seasonal cracking after cooling Structure Tensile properties Corrosion rate in 96-hour immersion corrosion test (mg / cm² / h) Grouping Percentage of ferrite phase area (%) Percentage of pearlite phase area (%) Type and percentage of remaining total structure area (%) Elongation strength (MPa) Tensile strength (MPa) 1-30 T Excellent Failure 50 1 Bainite (49) 343 658 13 Comparative example 1-31 U Excellent Excellent 66 0 Bainite (34) 204 379 19 Comparative example 1-32 V Excellent Excellent 53 2 Bainite (45) 374 618 23 Comparative example 1-33 W Excellent Failure 52 0 Bainite (48) 329 698 11 Comparative Example 1-34 X Excellent Excellent 76 1 Bainite (23) 211 359 19 Comparative Example 1-35 Y Satisfactory Satisfactory 51 0 Bainite (49) 336 659 28 Comparative Example 1-36 Z Satisfactory Excellent 50 1 Bainite (49) 299 608 25 Comparative Example 1-37 AA ExcellentExcellent 50 0 Bainite (50) 303 617 22 Comparative Example 1-38 AB Unsatisfactory - - - - - - Comparative Example 1-39 AC Excellent Excellent 53 0 Bainite (47) 283 599 39 Comparative Example 1-40 AD Unsatisfactory - - - - - - Comparative Example 1-41 AE Excellent Excellent 54 0 Bainite (46) 314 642 31 Comparative Example 1-42 AF Excellent Excellent 50 6 Bainite (44) 270 559 28 Comparative Example 1-43 AG Fails - - - - - - Comparative Example 1-44 AH Excellent Excellent 52 0 Bainite (48) 329 613 33 Comparative Example 1-45 Al Satisfactory Fails 53 0 Bainite (47) 337 667 14 Comparative Example 1-46 AJ Satisfactory Fails 55 2 Bainite (43) 328 611 17 Comparative Example The number in parentheses is the percentage of total area of ​​layers other than the ferrite and pearlite phases. *2 Immersion in an aqueous solution of sulfuric acid 70% by mass at 50°C > & hci\ hccchohhc [Table 11] c Steel Tube No. Steel No. Formability Seasonal cracking after cooling Structure Tensile properties Corrosion rate in 96-hour immersion corrosion test*2 (mg / cm² / h) Grouping Percentage of ferrite phase area (%) Percentage of pearlite base area (%) Type and percentage of total area*1 of remaining structure (%) Elongation strength (MPa) Tensile strength (MPa) 1-47 AK Excellent Excellent 73 11 Bainite (16) 224 455 34 Comparative example 1-48 AK Excellent Failure 44 1 Bainite (55) 311 562 18 Comparative example 1-49 AL Excellent Excellent 49 7 Bainite (44) 276 422 29 Comparative example 1-50 AL Excellent Failure 42 0 Bainite (58) 299 581 13 Comparative Example 1-51 AM Excellent Excellent 78 5 Bainite (17) 229 418 24 Comparative Example 1-52 AM Excellent Fault 47 2 Bainite (51) 278 554 14 Comparative Example 1-53 AN Excellent Excellent 84 13 Bainite (3) 228 414 38 Comparative Example 1-54 AN Excellent Fault 452 Bainite (53) 335 551 14 Comparative Example 1-55 AO Excellent Excellent 82 14 Bainite (4) 211 382 29 Comparative Example 1-56 AO Excellent Fault 39 1 Bainite (60) 304 582 11 Comparative Example 1-57 AP Excellent Excellent 77 20 Bainite(3) 222 417 41 Comparative Example 1-58 AP Excellent Fault 41 0 Bainite (59) 298 579 13 Comparative Example 1-59 AQ Excellent Excellent 83 12 Bainite (5) 208 374 27 Comparative Example 1-60 AQ Satisfactory Fault 43 i Bainite (56) 301 588 12 Comparative Example The number in parentheses is the percentage of total area of ​​the layers other than the ferrite phase and the knob phase. Immersion in an aqueous solution of sulfuric acid 70% by mass at 50°C co > & hci\ hccc J ho In the present examples (steel tube numbers 1-1 to 1-3, and steel tube numbers 1-12 to 1-29), where the steel compositions, manufacturing conditions, and the results of the steel microstructure observation were within the ranges of the present invention, no defects were observed on the inner and outer surfaces of the steel tube in the non-destructive tests performed to detect defects that occur in the tube during its formation. Even when defects were present, they were minor, and the steel tubes passed the test after repair.The steel tubes in these examples also showed no delayed cracks (seasonal cracks) on their outer surfaces in the non-destructive tests performed after the heat treatment of the steel tubes to detect seasonal cracks (normalizing heat treatment followed by accelerated cooling). Even when cracks were present, they were minor, and the steel tubes passed the test after repair.The steel pipes in the present examples also met the elongation and tensile strength required as seamless steel pipes for piping systems, and had a desirable resistance to sulfuric acid condensation point corrosion with a corrosion rate of 20 mg / cm2 / ho less in the corrosion test carried out under a severe corrosive environment at the sulfuric acid condensation point, specifically, by immersing the steel pipe in an aqueous solution of sulfuric acid 70% by mass at 50°C for 96 hours. Conversely, in the comparative examples (steel tube no. 1-4) in which the steel tubes were air-cooled after normalizing heat treatment, the percentage area of ​​the pearlite phase exceeded the upper limit of the range of the present invention and the corrosion rate in a sulfuric acid condensation point environment was 28 mg / cm2 / h, failing to meet the target range. In the comparative examples (steel tube nos. 1-6, 1-7, 1-10 and 1-11) in which the heat treatment conditions of the steel tube or the tube manufacturing conditions did not fall within the ranges of the present invention, the percentage area of ​​the pearlite phase also exceeded the upper limit of the range of the present invention and the corrosion rate in a sulfuric acid condensation point environment did not satisfy the target range. In the comparative example (steel tube no. 1-5), where the average accelerated cooling rate after normalizing heat treatment exceeded the upper limit of the range of the present invention, the ferrite phase area percentage was less than the lower limit of the range of the present invention, and the total area percentage of the bainite and martensitic phases was 97%, resulting in seasonal cracking. The cracks could not be eliminated even after repair. In the comparative example (steel tube no. 1-8), where the tempering temperature of the quenching heat treatment performed after accelerated cooling followed by normalizing heat treatment was higher than the upper limit of the range of the present invention, the steel tube failed to achieve the desired elongation strength of 230 MPa or more. The desired elongation strength of 230 MPa or more was also not achieved in the comparative example (steel tube no. 1-9), where the normalizing temperature of the normalizing heat treatment was higher than the upper limit of the range of the present invention. The corrosion rate in a sulfuric acid condensation point environment does not QZfrnnn / zznz / e / YiAi met the target range in all comparative examples (steel pipe no. 1-32) in which the Si content in the steel exceeded the upper limit of the range of the present invention, the comparative example (steel pipe no. 1-35) in which the P content exceeded the upper limit of the range of the present invention, the comparative example (steel pipe no. 1-36) in which the S content exceeded the upper limit of the range of the present invention, and the comparative example (steel pipe no. 1-37) in which the Al content exceeded the upper limit of the range of the present invention. The corrosion rate in a sulfuric acid condensation point environment did not meet the target range also in the comparative example (steel pipe no. 1-39), in which the Cu content in the steel was below the lower limit of the range of the present invention, and in the comparative example (steel pipe no. 1-44) in which the Sb content was below the lower limit of the range of the present invention. The corrosion rate in a sulfuric acid condensation point environment did not meet the target range also in the comparative example (steel pipe no. 1-41), in which the Mo content in the steel exceeded the upper limit of the range of the present invention. In the comparative example (steel pipe no. 1-42) in which the Mo content in the steel was below the lower limit of the range of the present invention, the percentage area of ​​the pearlite phase was greater than the upper limit of the range of the present invention, and the corrosion rate in a sulfuric acid condensation point environment did not meet the target range. Seasonal cracking occurred after the heat treatment of the steel tube (normalizing heat treatment and subsequent accelerated cooling), and the cracks could not be eliminated even after repair in the comparative example (steel tube no. 1-30) in which the C content in the steel exceeded the upper limit of the range of the present invention, the comparative example (steel tube no. 1-33) in which the Mn content exceeded the upper limit of the range of the present invention, and the comparative example (steel tube no. 1-45) in which the Cr content exceeded the upper limit of the range of the present invention, and the comparative example (steel tube no. 1-46) in which the W content exceeded the upper limit of the range of the present invention. The steel tubes exhibited defects on the outer surfaces in the non-destructive tests performed after tube manufacturing, and the defects could not be eliminated even after repair in the comparative example (steel tube no. 1-38) in which the Cu content in the steel exceeded the upper limit of the range of the present invention, and in the comparative example (steel tube no. 1-40) in which the Ni content was less than the lower limit of the range of the present invention. In the comparative example (steel tube no. 1-43) in which the Sb content in the steel exceeded the upper limit of the range of the present invention, the tube manufacturing process was stopped due to cracks observed on the outer surface of the tube during hot rolling in the tube manufacturing process and it was not possible to manufacture a seamless steel tube. The ferrite phase area percentage exceeded the upper limit of the range of the present invention, and the elongation resistance and tensile strength did not reach the desired ozfrnnn / zznz / e / YiAi values ​​in the comparative example (steel pipe no. 1-31) in which the C content in the steel was below the lower limit of the range of the present invention, and in the comparative example (steel pipe no. 1-34) in which the Mn content was below the lower limit of the range of the present invention. The percentage area of ​​the pearlite phase exceeded the upper limit of the range of the present invention, and the corrosion rate in a sulfuric acid condensation point environment did not meet the target range in the comparative examples (steel pipe no. 1-47 and no. 157) in which steel no. AK with a Mo content below the lower limit of the range of the present invention, and a W content greater than the upper limit of the range of the present invention, or steel no. AP with both C and W contents exceeding the upper limits of the ranges of the present invention, was air-cooled after normalizing heat treatment of the steel pipe. In the comparative examples (steel pipe no. 1-48 and no.158) in which the steel tube was subjected to accelerated cooling after normalizing heat treatment, the percentage area of ​​the pearlite phase was within the range of the present invention, and the corrosion rate in a sulfuric acid condensation point environment met the target range. However, seasonal cracking was observed in non-destructive testing, and the cracks could not be eliminated even after repair. The percentage area of ​​the pearlite phase exceeded the upper limit of the range of the present invention, and the corrosion rate in a sulfuric acid condensation-point environment did not meet the target range, also in the comparative example (steel pipe no. 1-49) in which AL steel with a Cr content greater than the upper limit of the range of the present invention was air-cooled after normalizing heat treatment. In the comparative example (steel pipe no. 1-50) in which the steel pipe was subjected to accelerated cooling after normalizing heat treatment, the percentage area of ​​the pearlite phase was within the range of the present invention, and the corrosion rate in a sulfuric acid condensation-point environment met the target range.However, seasonal cracking was observed in non-destructive testing and the cracks could not be eliminated even after repair. The percentage area of ​​the pearlite phase exceeded the upper limit of the range of the present invention, and the corrosion rate in a sulfuric acid condensation point environment did not meet the target range in the comparative examples (Steel Pipe Nos. 1-51, 1-53, 1-55, and 1-59) in which steel Nos. AM, AN, AO, or AQ with a Mo content below the lower limit of the range of the present invention and a Cr content above the upper limit of the range of the present invention was air-cooled after normalizing heat treatment of the steel pipe. In the comparative examples (No.of steel pipe 1-52, 1-54, 1-56 and 1-60) in which the steel pipe was subjected to accelerated cooling after normalizing heat treatment, the percentage area of ​​the pearlite phase was within the range of the present invention and the corrosion rate in a sulfuric acid condensation point environment met the target range. However, seasonal cracking was observed in non-destructive testing and the cracks could not be eliminated even after repair.

Claims

1. A seamless steel tube, characterized in that it has a desirable resistance to corrosion by sulfuric acid condensation point, the seamless steel tube having a composition comprising, in % by mass, C: 0.01 to 0.12%, Si: 0.01 to 0.8%, Mn: 0.10 to 2.00%, P: 0.050% or less, S: 0.040% or less, Al: 0.010 to 0.100%, Cu: 0.03 to 0.80%, Ni: 0.01 to 0.50%, Mo: 0.01 to 0.20%, Sb: 0.002 to 0.50%, Cr: 0.004% or less, W: 0.002% or less, the remainder being Fe and incidental impurities, and a structure comprising a ferrite phase with a percentage area 50 to 65%, a pearlite phase with an area percentage of 2% or less, and one or both bainite phases and a martensitic phase making up the remainder, the seamless steel tube having an elongation strength of 230 MPa or more and a tensile strength of 380 MPa or more.

2. The seamless steel tube having a desirable resistance to corrosion by sulfuric acid condensation point according to claim 1, further characterized in that the composition additionally comprises, in % by mass, one or two groups selected from the following group A and group B, group A: one or both of Sn: 0.005 to 0.50% and Co: 0.005 to 0.20%, group B: Ti: 0.005 to 0.050%.

3. A method for manufacturing seamless steel tubing having desirable resistance to sulfuric acid condensation point corrosion according to claim 1 or 2, the method being characterized in that it comprises: heating a steel tubing material of the composition to 1,100 to 1,300°C, hot rolling the heated steel tubing material to 800°C or more to obtain a seamless steel tubing of a predetermined shape, and cooling the seamless steel tubing to ambient temperature; and heating the seamless steel tubing to a normalizing temperature of 850 to 1,050°C in a normalizing heat treatment, followed by accelerated cooling to a cooling stop temperature of 500°C or less at an average cooling rate of 10 to 50°C / s.

4. The method according to claim 3, further characterized in that it comprises a heat treatment for tempering in which the seamless steel tube, after accelerated cooling, is cooled to ambient temperature and reheated to a tempering temperature of 400 to 700°C.