JP6578809B2 - Seamless steel pipe manufacturing method - Google Patents

Description

  The present invention relates to a method for manufacturing a seamless steel pipe.

  Steel pipes used for extraction in oil wells and gas wells (hereinafter, oil wells and gas wells are collectively referred to as oil wells) are used in a high-temperature environment containing carbon dioxide gas or hydrogen sulfide gas. Therefore, the steel pipe is required to have corrosion resistance and strength according to the use environment. Conventionally, as a stainless steel pipe for oil wells, a stainless steel pipe for oil wells of steel containing about 13% of Cr having excellent carbon dioxide corrosion resistance (hereinafter also referred to as 13Cr steel) has been used. International Publication No. 2005/115650 (Patent Document 1) discloses a method for producing a seamless steel pipe containing 10.50 to 14.00% of Cr.

  In recent years, along with the deep well formation of oil wells, there is a demand for stainless steel pipes for oil wells that have better corrosion resistance than 13Cr steel. Then, application of the stainless steel pipe for oil wells of steel containing about 17% of Cr (hereinafter also referred to as 17Cr steel) is being studied. Japanese Patent Laying-Open No. 2005-336595 (Patent Document 2) discloses a method for producing a martensitic stainless steel pipe containing 15.5 to 18% of Cr.

  JP-A-2001-279392 (Patent Document 3) discloses a method for producing a martensitic stainless steel pipe containing 12 to 20% of Cr. In this manufacturing method, the following is performed. Steel heated to a temperature range of 1100 to 1250 ° C. is hot-worked and formed into a tubular shape, and then seam welded to obtain a product (welded steel pipe) as it is. This hot rolling is finished in a temperature range of 900 to 1050 ° C.

  Japanese Patent Laying-Open No. 2013-249516 (Patent Document 4) discloses a method for producing a high-strength stainless steel seamless pipe for oil wells having a wall thickness of more than 25.4 mm. In the seamless pipe manufacturing method, the following is carried out. Hot rolling including piercing and rolling is performed on a steel material heated to a heating temperature of 1100 to 1300 ° C. Let this hot rolling be rolling which is 30% or more in the total reduction in the temperature range of 1100 to 900 ° C.

International Publication No. 2005/115650 JP 2005-336595 A JP 2001-279392 A JP 2013-249516 A

  In the seamless steel pipe of 17Cr steel, there was a tendency that wrinkles were likely to be generated on the inner surface and the outer surface, as compared with the case of 13Cr steel, which is similarly classified as martensitic stainless steel. As a result of investigation by the present inventor, it has been found that this wrinkle is generated from a process of piercing and rolling a billet.

  However, none of the manufacturing methods of Patent Documents 1 to 4 make any mention of the occurrence and countermeasures of wrinkles on the outer surface and the inner surface during pipe making using piercing and rolling.

  The objective of this invention is a manufacturing method of the seamless steel pipe of 17Cr steel, Comprising: It is providing the manufacturing method which suppresses generation | occurrence | production of the flaw in the outer surface and inner surface of a steel pipe.

In the method for producing a seamless steel pipe according to an embodiment of the present invention, the chemical composition of the steel pipe is, by mass%, C: 0.06% or less, Si: 1.0% or less, Mn: 0.01-2. 0%, P: 0.05% or less, S: 0.005% or less, Al: 0.001 to 0.1%, Cr: 15.5 to 18.0%, Ni: 2.5 to 6.0 %, Cu: 1.0 to 3.5%, N: 0.06% or less, 0 to 0.3%, Ca: 0 to 0.05%, and B: 0 to 0.01%, Further, Mo and W satisfy the formula (1), and further contain one or two selected from the group consisting of Ti: 0 to 0.3% and Nb: 0 to 0.3%, and the balance: Fe And inevitable impurities. In this heating method, the billet is heated at a soaking temperature of 1260 to 1300 ° C. and an in-furnace time of 8 hours or less in a heating furnace in which the oxygen concentration in the furnace atmosphere is 0.2 vol% or less. Piercing and rolling the billet.
1.0 ≦ (Mo + 0.5 × W) ≦ 3.5 (1)
Here, the contents of mass% of these elements are substituted for Mo and W in the formula (1).

  ADVANTAGE OF THE INVENTION According to this invention, the seamless steel pipe of 17Cr steel which suppressed generation | occurrence | production of the flaw in the outer surface and inner surface of a steel pipe can be manufactured.

FIG. 1 is a plan view showing an example of a scaly ridge generated on the outer surface of a seamless steel pipe. FIG. 2 is a plan view showing an example of a bowl-shaped ridge generated on the outer surface of the seamless steel pipe. FIG. 3 is a plan view showing an example of a bowl-shaped rod formed on the inner surface of the seamless steel pipe. FIG. 4 is a flowchart illustrating a manufacturing method according to an embodiment of the present invention. FIG. 5 is a plan view of a heating furnace according to an embodiment of the present invention.

  In order to solve the above-mentioned problems, the present inventors investigated in detail the outer surface and inner surface flaws of a 17Cr steel seamless steel pipe. As a result, the present inventors obtained the following knowledge.

  When observing the outer surface and inner surface of the 17Cr steel seamless pipe, the outer surface of the surface is a scaly-shaped surface and the surface of the surface. there were. As shown in FIG. 1, cracks were observed at the grain boundary between the austenite phase and the ferrite phase at the site where the scaly wrinkles on the outer surface occurred. FIG. 1 is a diagram schematically showing a microstructure image around the outer surface of the steel pipe in a cross section perpendicular to the pipe axis direction. Therefore, it is considered that the occurrence of scaly wrinkles can be suppressed by piercing and rolling in a temperature region where the steel has a single structure. In fact, it has been found that, in 13Cr steel, for example, when piercing and rolling is performed at 1200 to 1250 ° C., similar flaws are hardly generated. This is presumably because 13Cr steel substantially becomes an austenite single phase in the temperature range of 1200 to 1250 ° C. However, 17Cr steel becomes two phases of an austenite phase and a ferrite phase in a wide temperature range of 1100 to 1350 ° C. Therefore, it is considered difficult to apply the concept and temperature range of 13Cr steel to 17Cr steel as they are.

  In the 17Cr steel, the portion where the ridge-like ridges were formed on the outer surface, as shown in FIG. 2, had a shape in which the irregularities on the outer surface were folded when rolled. FIG. 2 is a diagram schematically showing a microstructure image of the periphery of the outer surface of the steel pipe in a cross section perpendicular to the pipe axis direction. Furthermore, the generation | occurrence | production state of wrinkles is large with the billet which was applied with the scale inhibitor (it is also called inhibitor) which suppresses scale formation and growth, and the billet heated without applying the inhibitor. Different. Specifically, the billet coated with the inhibitor was prone to wrinkles. Further, when the inhibitor was applied unevenly, wrinkles were noticeably generated. This is presumably because when the billet is heated, the outer surface is oxidized non-uniformly and unevenness due to the scale tends to occur. Therefore, it was found that the billet scale and the presence or absence of application of the inhibitor affect the generation of wrinkles.

  As shown in FIG. 3, 17Cr steel was found to have ridges on the inner surface when rolled. FIG. 3 is a view schematically showing a microstructure image of the periphery of the inner surface of the steel pipe in a cross section perpendicular to the pipe axis direction. This wrinkle is considered to occur because steel having two phases having different deformability is rolled. Therefore, by raising the temperature at which the billet is heated to reduce the difference in deformability between the two phases, it is considered that the difference in the degree of workability between the two phases when rolled can be reduced and wrinkles can be suppressed.

  As a result of investigations based on these, 17Cr steel was heated in a temperature range higher than the heating temperature at the time of pipe production in 13Cr steel, and the atmosphere and heating time of the heating furnace were set to appropriate conditions. It has been found that the occurrence of can be suppressed.

  The present inventors have completed the present invention based on the aforementioned findings. First, an outline of an embodiment of the present invention will be described.

The manufacturing method of the seamless steel pipe is such that the chemical composition of the steel pipe is mass%, C: 0.06% or less, Si: 1.0% or less, Mn: 0.01 to 2.0%, P: 0.00. 05% or less, S: 0.005% or less, Al: 0.001 to 0.1%, Cr: 15.5 to 18.0%, Ni: 2.5 to 6.0%, Cu: 1.0 -3.5%, N: 0.06% or less, V: 0-0.3%, Ca: 0-0.05%, and B: 0-0.01%, and Mo and W are formulas It satisfies (1), and further contains one or two selected from the group consisting of Ti: 0 to 0.3% and Nb: 0 to 0.3%, and the balance: Fe and inevitable impurities . In this heating method, the billet is heated at a soaking temperature of 1260 to 1300 ° C. and an in-furnace time of 8 hours or less in a heating furnace in which the oxygen concentration in the furnace atmosphere is 0.2 vol% or less. Piercing and rolling the billet.
1.0 ≦ (Mo + 0.5 × W) ≦ 3.5 (1)
Here, the contents of mass% of these elements are substituted for Mo and W in the formula (1).

  This manufacturing method can manufacture a seamless steel pipe of 17Cr steel that suppresses wrinkles on the outer surface and the inner surface that occur during pipe manufacturing. Specifically, the manufacturing method can reduce the oxidizability of the atmosphere in the heating furnace by heating the billet using a heating furnace in which the oxygen concentration in the furnace atmosphere is 0.2 vol% or less. . Therefore, the scale of the billet surface is suppressed in formation and growth, and even if the scale inhibitor is not applied to the surface, the scale becomes uniform and thin. As a result, the outer surface of the steel pipe is restrained from generating hook-like wrinkles due to the unevenness of the scale during pipe making. In the manufacturing method, the billet can be pierced and rolled in a temperature range with a high degree of workability by heating at a soaking temperature of 1260 to 1300 ° C. and 8 hours or more in a heating furnace. As a result, the outer surface of the steel pipe is suppressed from generating scale-like wrinkles during pipe making. Furthermore, the billet heated on the above-mentioned conditions has a small difference in deformability between the two phases. As a result, the inner surface of the steel pipe is restrained from generating hook-like wrinkles during pipe making.

  Preferably, Ca / S is 5.0 or more.

  This production method can further suppress the occurrence of scaly wrinkles on the outer surface during pipe making by adjusting the Ca content ratio Ca / S.

[Production method]
An example of the manufacturing method of the seamless steel pipe of this embodiment is demonstrated. As shown in FIG. 4, the manufacturing method includes a step S <b> 1 for preparing the billet 1, a step S <b> 2 for heating the prepared billet 1, and a step S <b> 3 for piercing and rolling the heated billet 1. The manufactured 17Cr steel pipe has the following chemical composition. Hereinafter,% related to elements means mass%.

[Chemical composition]
C: 0.06% or less Carbon (C) increases the strength of steel. However, if there is too much C content, the hardness after tempering will become high too much and SSC resistance will fall. Furthermore, in the chemical composition of the present embodiment, the Ms point decreases as the C content increases. Therefore, as the C content increases, the retained austenite increases and the yield strength tends to decrease. Therefore, the C content is 0.06% or less. The C content is preferably 0.05% or less, and more preferably 0.01% or less. On the other hand, the C content is 0.001% or more in consideration of the steelmaking cost for the decarburization treatment. The C content is preferably 0.002 or more, and more preferably 0.005% or more.

Si: 1.0% or less Silicon (Si) deoxidizes steel. However, if there is too much Si content, the toughness and hot workability of steel will fall. If the Si content is too large, the amount of ferrite produced further increases and the yield strength tends to decrease. Therefore, the Si content is 1.0% or less. The Si content is preferably 0.8% or less, and more preferably 0.5% or less. The Si content is preferably 0.05% or more, and more preferably 0.1% or more.

Mn: 0.01 to 2.0%
Manganese (Mn) deoxidizes and desulfurizes steel and improves hot workability. If the Mn content is too small, the above effect cannot be obtained effectively. On the other hand, if the Mn content is too high, the toughness of the steel is lowered and austenite is easily generated. Therefore, the Mn content is 0.01 to 2.0%. The Mn content is preferably 1.8% or less, and more preferably 1.5% or less. The Mn content is preferably 0.05% or more, more preferably 0.07% or more.

P: 0.05% or less Phosphorus (P) is an impurity. P decreases the SSC resistance of the steel. Therefore, it is preferable that the P content is as small as possible. The P content is 0.05% or less. The P content is preferably 0.04% or less, more preferably 0.03% or less. It is preferable to reduce the P content as much as possible without causing an excessive increase in steelmaking costs.

S: 0.005% or less Sulfur (S) is an impurity. S combines with Mn to form sulfide inclusions and segregates at the grain boundaries between austenite and ferrite. When sulfide inclusions are formed in the billet 1, the steel pipe is likely to cause grain boundary cracking at the time of step S3 or the like (also referred to as pipe making). Therefore, it is preferable that the S content is as small as possible. S content is 0.005% or less. The S content is preferably 0.003% or less, and more preferably 0.002% or less. The S content is preferably reduced as much as possible without causing an excessive increase in steelmaking costs.

Al: 0.001 to 0.1%
Aluminum (Al) deoxidizes steel. However, when there is too much Al content, the inclusion in steel will increase and the toughness of steel will fall. Therefore, the upper limit of the Al content is 0.1%. The Al content is preferably 0.08% or less, and more preferably 0.05% or less. The Al content is preferably 0.002% or more, and more preferably 0.003% or more.

Cr: 15.5 to 18.0%
Chromium (Cr) increases the corrosion resistance of steel. Specifically, Cr lowers the corrosion rate and increases the SCC resistance of the steel. If the C content is too small, the above effect cannot be obtained effectively. On the other hand, if there is too much Cr content, the ferrite fraction in steel will increase and the strength of steel will fall. Therefore, the Cr content is 15.5 to 18.0%. The Cr content is preferably 17.8% or less, and more preferably 17.5% or less. The Cr content is preferably 16.0% or more, and more preferably 16.3% or more.

Ni: 2.5-6.0%
Nickel (Ni) increases the toughness of the steel. Ni further increases the strength of the steel. If the Ni content is too small, the above effect cannot be obtained effectively. On the other hand, if the Ni content is too large, a large amount of austenite is generated, and as a result, the strength of the steel decreases. Therefore, the Ni content is 2.5 to 6.0%. The Ni content is preferably less than 6.0%, and more preferably 5.9% or less. The Ni content is preferably 3.0% or more, and more preferably 3.5% or more.

Cu: 1.0 to 3.5%
Copper (Cu) increases the martensite fraction and increases the strength of the steel. Furthermore, Cu precipitates as Cu particles during tempering and further increases its strength. If the Cu content is too small, the above effect cannot be obtained effectively. On the other hand, if there is too much Cu content, the hot workability of steel will fall. Therefore, the Cu content is 1.0 to 3.5%. Cu content becomes like this. Preferably it is 3.3% or less, More preferably, it is 3.0% or less. Cu content becomes like this. Preferably it is 1.3% or more, More preferably, it is 1.5% or more.

N: 0.06% or less Nitrogen (N) is an impurity. If there is too much N content, austenite will produce | generate excessively and the inclusion in steel will also increase. As a result, the toughness of the steel is reduced. Therefore, the N content is 0.06% or less. N content is 0.05% or less, More preferably, it is 0.03% or less. On the other hand, N has the effect of increasing the strength of the steel. If necessary for this purpose, the N content may be 0.001% or more, and if necessary, may be 0.002% or more.

1.0 ≦ (Mo + 0.5W) ≦ 3.5
Molybdenum (Mo) and tungsten (W) both improve the SCC resistance of the steel. These elements may contain both or only one. However, the atomic weight of W is approximately twice that of Mo, and the effect per content in mass% is approximately half that of Mo. On the other hand, if there is too much content of these elements, the effect will be saturated and cost will rise because it is an expensive component. Therefore, Mo and W satisfy the following formula (1). In Mo + 0.5 × W, it is more preferably 1.8 or more. In Mo + 0.5 × W, it is more preferably 3.3%. The Mo content is preferably 3.5% or less. The Mo content is preferably 0.5% or more. The W content is preferably 0 to 3.5%. The W content is more preferably 0.001% or more.
1.0 ≦ (Mo + 0.5 × W) ≦ 3.5 (1)
Here, the contents of mass% of these elements are substituted for Mo and W in the formula (1).

  The chemical composition of the steel pipe according to the present embodiment may contain the following selective elements. That is, none of the following elements may be contained in the steel pipe according to the present embodiment. Moreover, only a part may be contained.

V: 0 to 0.3%
Vanadium (V) is a selective element. V increases the strength and toughness of the steel. However, if there is too much V content, toughness will fall. Therefore, the V content is 0 to 0.3%. Furthermore, in order to sufficiently obtain the above effects, the V content is preferably 0.005 to 0.3%. V content becomes like this. Preferably it is 0.25% or less, More preferably, it is 0.2% or less. V content becomes like this. Preferably it is 0.006% or more, More preferably, it is 0.007% or more.

Ca: 0 to 0.05%,
Calcium (Ca) is a selective element. Ca combines with S to make sulfide inclusions spherical and suppress intergranular cracking during pipe making. If Ca is contained even a little, the above effect can be obtained to some extent. However, if there is too much Ca content, it will combine with oxygen to significantly reduce the cleanliness of the alloy and reduce hot workability. Therefore, the Ca content is 0 to 0.05%. Moreover, in order to fully acquire the said effect, it is preferable that Ca content is 0.0005 to 0.05%. The Ca content is preferably 0.04% or less, and more preferably 0.03% or less. The Ca content is preferably 0.0006% or more, and more preferably 0.0008% or more. Furthermore, it is preferable that ratio (Ca / S) of Ca content with respect to S content is 5.0 or more which can obtain stably the effect which suppresses the generation | occurrence | production of the damage | wound of an outer surface. Ca / S is preferably 7.0 or more.

B: 0 to 0.01%
Boron (B) is a selective element. B is an element that easily precipitates at the grain boundary, but suppresses the grain boundary segregation of MnS by segregation, and as a result, suppresses the grain boundary cracking during pipe making. If B is contained even a little, the above effect can be obtained to some extent. However, if there is too much B content, the toughness of steel will be reduced. Therefore, the B content is 0 to 0.01%. Moreover, in order to fully acquire the said effect, it is preferable that B content is 0.0003 to 0.01%. The B content is preferably 0.009% or less, and more preferably 0.008% or less. The B content is preferably 0.0005% or more, and more preferably 0.0008% or more.

Nb: 0 to 0.3%, Ti: 0 to 0.3%
Niobium (Nb) and titanium (Ti) are elements that can be substituted for each other, and may contain both or only one. These elements are selective elements. These elements increase the strength and toughness of the steel. These elements improve the pitting corrosion resistance and SCC resistance of steel. If these elements are contained even a little, the above effect can be obtained. However, if there is too much content of these elements, the toughness of steel will fall. Therefore, the Nb content is 0 to 0.3%, and the Ti content is 0 to 0.3%. Further, in order to sufficiently obtain the above effect, it may contain one or two selected from the group consisting of Nb: 0.01 to 0.3% and Ti: 0.01 to 0.3%. preferable. The Nb content is preferably 0.25% or less, and more preferably 0.23% or less. The Nb content is preferably 0.02% or more, more preferably 0.05% or more. The Ti content is preferably 0.25% or less, and more preferably 0.23% or less. The Ti content is preferably 0.02% or more, and more preferably 0.05% or more.

  Note that the balance of the chemical composition of the steel pipe according to the present embodiment is Fe and inevitable impurities. The inevitable impurities here mean elements mixed from ores and scraps used as raw materials when manufacturing steel pipes industrially, or elements mixed from the environment of the manufacturing process.

  In step S2, billet 1 prepared in step S1 is placed in heating furnace 2 and heated. Billet 1 is put into heating furnace 2 without applying a scale inhibitor. In the heating furnace 2, the oxygen concentration in the furnace atmosphere is 0.2% by volume or less. The billet 1 is soaked at a heating temperature of 1260 to 1300 ° C. (also referred to as being heated at a soaking temperature). When the billet 1 is heated using the heating furnace 2 at, for example, a soaking temperature of 1270 ° C., the temperature of the billet 1 is substantially equal to the soaking temperature and becomes 1270 ° C. The billet 1 becomes a two-phase structure of an austenite phase and a ferrite phase by being heated at a soaking temperature in the above range. The soaking temperature is preferably 1265 ° C. or higher, more preferably 1280 ° C. or higher. Furthermore, the billet 1 is heated in the in-furnace time of 8 hours or less. The in-furnace time (h) is a cumulative value (h) of the heating time in the heating furnace 2. The in-furnace time is preferably 7 hours or less, and more preferably 6.5 hours or less. The in-furnace time is preferably 1.5 hours or longer, more preferably 2 hours or longer.

  Specifically, the heating furnace 2 may be divided into a plurality of zones. For example, as shown in FIG. 5, the heating furnace 2 is divided into a pre-tropical zone 21, a heating zone 22, and a soaking zone 23. Each band is arranged in a line. In this case, the billet 1 is heated while moving in the order of the pre-tropical zone 21, the heating zone 22, and the soaking zone 23 as indicated by the arrows in FIG. 5. At this time, the billet 1 is heated to the soaking temperature using the pre-tropical zone 21 and the heating zone 22 and then heated at the soaking temperature using the soaking zone 23. In this case, the oxygen concentration of the heating furnace 2 is, for example, the oxygen concentration of the gas discharged from the soaking zone 23, and the oxygen concentration of the gas is 0.2% by volume or less. Here, the gas supplied to the heating furnace 2 is, for example, a mixture of fuel and air. For example, liquefied natural gas (LNG) or coke oven exhaust gas is used as the fuel, but is not particularly limited. The soaking temperature (° C.) is the heating temperature (° C.) in the soaking zone 23. The in-furnace time is a cumulative value of the heating time in each zone. In the soaking zone 23, the time for holding the billet 1 is preferably 1 hour or more. In addition, the heating furnace 2 may omit the heating using the pretropical zone 21, and the division | segmentation of a some belt | band | zone is not specifically limited. In addition, the heating furnace 2 is not limited to one in which the bands are arranged in a line, and each band may be arranged in an annular shape, such as a rotary hearth type heating furnace, and is not particularly limited.

  In step S3, the billet 1 heated to the soaking temperature in step S2 is immediately pierced and rolled. Specifically, for example, a Mannesmann method is performed on the billet 1 after step S2 using a punching machine to produce a tubular intermediate material.

  The manufacturing method further includes, for example, a step S4 of stretching and rolling the intermediate material immediately after the step S3, a step S5 of constant-rolling the stretched intermediate material, and a step S6 of heat-treating the intermediate-rolled intermediate material. It is preferable to include. In step S4, for example, a mandrel mill is used. In step S5, a reducer, a sizing mill, or the like is used. In other words, in steps S4 and S5, for example, an intermediate material pierced and rolled from billet 1 (also referred to as an intermediate material manufactured in step S3) is hot-rolled to obtain a steel pipe having a predetermined shape. The temperature of the intermediate material at the end of step S5 (hot rolling end temperature) is preferably 1100 ° C. or higher. In addition, process S3, process S4, and process S5 are also collectively called the process of manufacturing a pipe.

  In step S6, the steel pipe that has been subjected to constant diameter rolling in step S5 is heat-treated. Step S56 carries out a quenching process and a tempering process. The quenching treatment may be performed in-line with the step of pipe production following the drawing and rolling in step S5. In step S6, the quenching process and the tempering process may be performed a plurality of times. The steel pipe heat-treated in step S6 obtains a structure mainly composed of tempered martensite. Specifically, this steel pipe obtains a structure having tempered martensite as a main component (also referred to as a first phase) and ferrite as a second phase. This steel pipe may have a structure containing a small amount of retained austenite or bainite. In addition, process S4-process S6 are selective processes.

  As mentioned above, the manufacturing method can manufacture the 17Cr steel seamless steel pipe by implementing these processes, suppressing the wrinkles of the outer surface and inner surface which arise in a pipe making process. Furthermore, a manufacturing method can further suppress generation | occurrence | production of the flaw of the outer surface at the time of pipe making by making the value of Ca / S into 5.0 or more in the chemical composition of a seamless steel pipe.

  No. Each of the billets 1 to 9 was heated in the heating furnace 2. Table 1 shows the presence / absence of the inhibitor, the soaking temperature, the oxygen concentration of the heating furnace 2 and the in-furnace time. In this example, since the billet 1 was heated in the heating furnace 2 for a sufficient time, the temperature of the billet 1 was equal to the soaking temperature. No. No. 3 billet 1 is coated with an inhibitor, and the other billets 1 are not coated with the inhibitor.

  The heated billet 1 is taken out from the heating furnace 2 and is quickly pierced and rolled in step S3, subsequently stretched and rolled in step S4, constant-diameter rolled in step S5, and quenched and tempered in step S6. And the steel pipe of No. 1-9 was manufactured. No. manufactured Table 2 shows the chemical compositions of the steel pipes 1-9. Among these steel pipes, each of the steel pipes of Nos. 1 to 5 was manufactured from the billet 1 having a diameter of 305 mm under the condition of the dimension shape X1. The dimension shape X1 has an outer diameter D of 273 mm and a wall thickness t of 26 mm. Each steel pipe of Nos. 6 to 8 was manufactured from the billet 1 having a diameter of 221 mm under the conditions of the dimension shape X2. The dimension shape X2 has an outer diameter D of 178 mm and a wall thickness t of 11.5 mm. Each steel pipe of No. 9 was manufactured from the billet 1 having a diameter of 187 mm under the condition of the dimension shape X3. The dimension shape X3 has an outer diameter D of 89 mm and a wall thickness t of 6.5 mm. In the chemical composition shown in Table 2, the content of each element is mass%, and the balance is Fe and inevitable impurities.

[Observation test of dredging situation]
Each steel pipe was observed for the presence or absence of flaws on the inner surface and the outer surface from the appearance. Furthermore, the steel pipe having a ridge on the outer surface was cut in a direction perpendicular to the tube axis direction at a portion having the ridge on the outer surface. The depth of the cut steel pipe from the surface of the ridge in the thickness direction (the ridge depth d: mm) was measured from a microphotograph obtained by photographing the periphery of the ridge on the outer surface of the cut surface.

[Test results]
The test results are shown in Table 1. In the wrinkle situation, the evaluation of the inner surface was ◎ without wrinkle and × with wrinkle. Evaluation of the outer surface is as follows: ◎ is no wrinkle or wrinkle depth d is 0.1 mm or less, ○ is wrinkle depth d is more than 0.1 and less than 0.3 mm, and Δ is wrinkle depth d is 0.3 mm. More than 0.5 mm, and x indicates that the heel depth d exceeds 0.5 mm.

  Each of the steel pipes of Nos. 2 to 9 had no wrinkles on the inner surface. In each of these steel pipes, the soaking temperature was 1260 ° C. or higher and the oxygen concentration was less than 0.2 vol%. Furthermore, the steel pipes of Nos. 4 to 9 had no wrinkles on the outer surface or the wrinkle depth d was 0.3 mm or less. Further, each of the steel pipes of Nos. 5 to 9 had no wrinkles on the outer surface or a wrinkle depth d of 0.1 mm or less. On the other hand, the steel pipes of Nos. 2 and 3 had a ridge depth d exceeding 0.3 mm. It was thought that both steel pipes had a long in-furnace time. Furthermore, the steel pipe of No. 3 to which the inhibitor was applied had an outer surface depth d exceeding 0.5 mm. In addition, the number 1 steel pipe was wrinkled on the inner surface and the outer surface. This was considered because the soaking temperature and oxygen concentration of billet 1 were not appropriate.

  As mentioned above, although one Embodiment of this invention was described, embodiment mentioned above is only the illustration for implementing this invention. Therefore, the present invention is not limited to the above-described embodiment, and can be implemented by appropriately modifying the above-described embodiment without departing from the spirit thereof.

  According to the present invention, it can be used for steel pipes for oil wells.

1: Billet 2: Heating furnace 21: Pre-tropical 22: Heating zone 23: Soaking

Claims (1)

A method of manufacturing a seamless steel pipe,
The chemical composition of the steel pipe is mass%,
C: 0.06% or less,
Si: 1.0% or less,
Mn: 0.01 to 2.0%,
P: 0.05% or less,
S: 0.005% or less,
Al: 0.001 to 0.1%,
Cr: 15.5 to 18.0%,
Ni: 2.5-6.0%,
Cu: 1.0 to 3.5%,
N: 0.06% or less,
V: 0 to 0.3%
Ca: 0 to 0.05%, and B: 0 to 0.01%, Mo and W satisfy the formula (1), Ti: 0 to 0.3%, and Nb: 0 to 0.00. Containing one or two selected from the group consisting of 3%,
The balance: Fe and inevitable impurities,
The manufacturing method includes:
Heating the billet at a soaking temperature of 1260 to 1300 ° C. and an in-furnace time of 8 hours or less in a heating furnace in which the oxygen concentration in the furnace atmosphere is 0.2 vol% or less;
And a step of piercing and rolling the heated billet.
1.0 ≦ (Mo + 0.5 × W) ≦ 3.5 (1)
Here, the contents of mass% of these elements are substituted for Mo and W in formula (1) ,
The steel pipe has a Ca / S of 5.0 or more .

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