JP4682450B2 - Seamless steel pipe manufacturing method and seamless steel pipe excellent in internal smoothness - Google Patents

Description

表１の結果から明らかなように、本発明に規定した条件に適合する場合（No.1〜9）には、全て引抜加工した後のシワ疵の深さが0.05mm以下となり、優れた内面平滑性を有する継目無鋼管が得られた。これに対し、本発明の規定を満たさない条件の場合（No.10）には、シワ疵の深さが0.06mmを超え、自動車の駆動部品に適用することができない継目無鋼管もあった。
【００４７】
【発明の効果】
本発明の製造方法によれば、素管の製管種別にかかわらず、１回の引抜加工によっても、内面平滑性に優れた冷間仕上の継目無鋼管を製造できるので、製造上、必要な工程数を増やすことがなく、製造コストが上昇することもない。これにより、自動車用のパワーステアリングやドライブシャフト等の駆動部品に最適な継目無鋼管を提供することができる。
【図面の簡単な説明】
【図１】ストレート型ダイスの断面形状を示す図である。
【図２】試験１で用いた試験用鋼管の引抜加工の前後の形状を模式的に示した図である。
【図３】ダイス角度が１５°〜２５°で引抜加工した場合の六角形穴の頂角位置における肉厚加工度と角張り底深さとの関係を示す図である。
【図４】空引き加工したときの、外径加工度と増肉率の関係を示した図であり、(a)は、ダイス角度２５°のときの外径加工度と増肉率の関係を、(b)は、肉厚／外径の比（Ｔ／Ｄ）が30％のときの外径加工度と増肉率の関係を示している。
【図５】ストレート型ダイスおよび円筒プラグを用いた引抜加工の状況を模式的に示した図であり、(a)はダイス角度が大きい場合を、(b)はダイス角度が小さい場合を示している。
【図６】ダイス角度が１０°〜３０°で引抜加工した場合の肉厚加工度とシワ疵の深さとの関係を示す図である。
【図７】上記図５の結果から、ダイス角度（°）と引抜加工後の管内面のシワ疵の深さが0.05mm以下となる肉厚加工度（％）との関係を示した図である。
【図８】引抜加工を化成皮膜潤滑またはオイル潤滑油で行った場合の肉厚加工度とシワ疵の深さとの関係を示したものである。
【符号の説明】
１ ダイス
２ プラグ
３ 素管[0001]
BACKGROUND OF THE INVENTION
TECHNICAL FIELD The present invention relates to a method for manufacturing a seamless steel pipe that is cold-finished by drawing a base pipe that has been hot-formed, and more specifically for a chemical plant, a power plant pipe, a machine structure, or an automobile. The present invention relates to a method for producing a cold-finished seamless steel pipe excellent in inner surface smoothness suitable for various applications such as parts.
[0002]
[Prior art]
Seamless steel pipes that are cold-finished are mainly used for special purposes, for example, steel pipes for chemical and power plant piping, steel pipes for machine structures, and the like. The seamless steel pipe used for these applications is obtained by hot pipe making by Mannesmann pipe method or Eugene pipe method, but in general, Mannesmann drills a steel piece by Mannesman pipe method, Manufactured by rolling with a constant or reduced diameter with a sizer or reducer. Next, the manufactured raw pipe is finished by cold drawing for the purpose of improving the dimensional accuracy of the outer diameter, inner diameter, and thickness, improving the smoothness of the surface, and ensuring the mechanical strength.
[0003]
In recent years, with the increasing awareness of environmental protection, the adoption of seamless steel pipes as auto parts has attracted attention. In other words, the automobile industry is currently developing technology to improve fuel economy by reducing the weight of the vehicle body, and it has been jointly used for driving parts such as power steering parts and drive shafts of automobiles that have been manufactured from steel bars. Attempts have been made to reduce the weight by hollowing parts using steelless pipes.
[0004]
However, there is a technical problem in adopting seamless steel pipes instead of steel bars. Since these parts are used in a harsh environment, the surface property of the inner surface of the pipe becomes a problem. If there are surface wrinkles such as wrinkles on the inner surface of the tube, cracks due to stress concentration may occur in the use environment starting from the surface and being mounted on the body.
[0005]
When using seamless steel pipes for power steering parts and drive shafts such as the above-mentioned automobiles, the thickness of the steel pipe must be 20% or more. It is used a lot. Furthermore, the allowable wrinkle on the inner surface of the steel pipe is preferably as shallow as possible due to the processing restrictions for use from the steel pipe to the part or the restrictions on use, and in the above-mentioned automotive drive part, the wrinkles generated on the inner surface of the pipe are preferable. The depth needs to be 0.05mm or less.
[0006]
In order to reduce the wrinkles generated on the inner surface of the pipe, it is only necessary to correct the angularity generated in the raw pipe during hot pipe making and prevent the wrinkles from remaining as wrinkles. Here, angularity is caused by irregularities in the inner and outer surfaces of pipes produced in the process of shrinking and sizing, such as reducers or sizers, in seamless steel pipes manufactured by roll rolling such as the Mannesmann pipe method. This means the non-circularity of the inner surface of the pipe. In addition, wrinkle refers to a streak defect with a depth of 0.005 to 0.5 mm that exists on the inner and outer surfaces of a seamless steel pipe. If wrinkles occur due to angling, even after cold working It may remain in the thin-walled portion of the angular corner portion.
[0007]
Conventionally, in order to reduce surface wrinkles such as wrinkles on the inner surface of a pipe, a method of performing many drawing processes in cold working and further improving by machining such as grinding has been adopted. Japanese Patent No. 3085980 discloses a method of manufacturing a stainless steel pipe in which the roughness of the inner surface of the tube is R _max of 1.0 μm or less in order to improve the surface properties of the inner surface of the tube. According to this manufacturing method, a plug having a ring-shaped protrusion or recess is used as a plug for drawing, and the inner surface of the tube is ironed to concentrate shear plastic deformation, thereby reducing the roughness of the inner surface of the tube. You can do that. However, these methods have a problem that the manufacturing man-hours for cold-finished steel pipes are increased and the manufacturing cost is significantly increased.
[0008]
[Problems to be solved by the invention]
As described above, many of the cold-working raw pipes are pierced and rolled by the Mannesmann pipe manufacturing method, which is more efficient than the Eugene pipe manufacturing method, which is hot extrusion. However, with the sizer and reducer in the Mannesmann tube method, the inner diameter of the inner tube is not constrained and the outer diameter is constant or reduced, so the inner tube is less round than the outer diameter. Thickness tends to be uneven, and uneven thickness such as inner hexagonal tension tends to increase.
[0009]
Therefore, drawing is performed to improve the surface properties of the inner surface of the tube, but the inner surface of the tube is squared and the like, and with a single drawing, it is possible to ensure good roundness and reduce wrinkles. It becomes difficult to plan. Therefore, usually, the drawing process is performed twice or more, and further, cutting or electrochemical polishing is performed to improve the properties of the inner surface of the pipe. In particular, when processing a thick steel pipe having a T / D of 20% or more, the surface properties of the inner surface of the pipe are not good, and angulation is likely to occur, so that a number of drawing processes must be performed. For this reason, improvement of the inner surface property of a thick steel pipe having a T / D of 20% or more causes an increase in manufacturing steps and an increase in manufacturing cost.
[0010]
The present invention has been made in view of the above problems, and when performing a cold finish by drawing using a straight die or a cylindrical plug, the die angle (°) of the die and the raw tube By appropriately selecting the processing conditions specified by the thickness processing degree (%), it is guaranteed that the inner surface wrinkles will be reduced by one drawing process, especially that the depth of wrinkles on the inner surface of the pipe will be 0.05 mm or less. An object of the present invention is to provide a method for producing a seamless steel pipe excellent in inner surface smoothness.
[0011]
[Means for Solving the Problems]
In order to find out the processing conditions for applying the cold drawn steel pipe to driving parts such as automobile power steering parts and drive shafts, the present inventor specifically, In order to examine the processing conditions for the wrinkle depth to be 0.05 mm or less, trial and error of the following (Test 1) to (Test 3) were repeated.
[0012]
(Test 1)
In Test 1, it was decided to qualitatively confirm the relationship between the correction of squareness generated on the inner surface of the blank tube, the thickness processing degree, and the die angle. The dies used are all straight type dies, and the cross-sectional shape thereof is shown in FIG. 1, but the die angle of the straight type dies is an angle X formed by the side surface of the conical portion of the die as shown in FIG. . The thickness processing degree (Y) is expressed by (TT) / T × 100 (%), where T is the thickness before drawing and T ′ is the thickness after drawing. .
[0013]
FIG. 2 is a diagram schematically showing the shape before and after the drawing of the test steel pipe used in Test 1. FIG. In order to confirm straightness correction by drawing, first, a test steel pipe having a large squareness on the inner surface was prepared. The test steel pipe is made by drilling a hexagonal hole with a diagonal length of 17.5 mm in a steel bar (S45C) with an outer diameter of 30.5 to 33.1 mm, and this test steel pipe is drawn using a die and a plug. did. In the drawing process, the die angle X of the straight die is set to 15 °, 20 ° and 25 ° so that the thickness processing degree at the apex position of the hexagonal hole where the angularity tends to remain is 5 to 20%. Drawing was performed to obtain a cold-worked steel pipe having an outer diameter of 22.49 mm and a wall thickness of 6.22 mm.
[0014]
In the cold-worked steel pipe, since the angularity remains in the portion corresponding to the apex angle of the hexagonal hole, draw the squared bottom depth and draw the inner surface circumference curve with a contact-type roundness meter after drawing. Measured by. The square bottom depth originally means the square corner depth from the inner diameter, and here, when a plurality of square corners remain, the deepest square corner depth is arranged as the square bottom depth.
[0015]
FIG. 3 is a diagram showing the relationship between the thickness processing degree and the angled bottom depth at the apex position of the hexagonal hole when the die angle is drawn at 15 ° to 25 °. The square bottom depth becomes shallower as the thickness processing degree becomes larger, and becomes shallower as the die angle becomes smaller. From this, it can be seen that the angled bottom depth generated on the inner surface of the pipe at the time of drawing is related to the thickness processing degree and the die angle.
[0016]
Furthermore, in order to confirm the effect of the die angle of the straight die on the actual wall thickness processing, the wall thickness behavior of the steel pipe drawn by the empty drawing without plug was investigated.
[0017]
FIG. 4 is a diagram showing the relationship between the outer diameter processing degree and the wall thickness increase rate when blanking is performed, and FIG. 4 (a) shows the outer diameter processing degree and the wall thickness increasing rate when the die angle is 25 °. FIG. 4 (b) shows the relationship between the outer diameter processing degree and the thickness increase rate when the thickness / outer diameter ratio (T / D) is 30%. Here, the outer diameter processing degree (%) is expressed as (dd ′) / d × 100 (%) where d is the outer diameter before drawing and d ′ is the outer diameter after drawing. Refers to the value to be set. The thickness increase rate (%) is expressed as (t′−t) / t × 100 (%) where t is the thickness before empty drawing and t ′ is the thickness after empty drawing. Refers to the value to be set.
[0018]
FIG. 4 (a) shows that when a thick steel pipe having a T / D of 20% or more is machined in the blanking process, the thickness of the steel pipe generally tends to decrease after machining. . This means that the substantial thickness reduction is reduced in the plug drawing. Further, from FIG. 4B, it can be seen that this thinning tendency can be reduced by reducing the die angle. This means that by reducing the die angle, it is possible to minimize a reduction in the thickness processing degree.
[0019]
To summarize the above, when drawing a thick steel pipe with a T / D of 20% or more with a straight die, it is possible to suppress the tendency of thinning in the machining process by reducing the die angle. Therefore, reducing the die angle is effective for correcting the angulation of the inner surface of the tube and improving wrinkles. This can be explained as follows.
[0020]
FIG. 5 is a diagram schematically showing a drawing process using a straight die and a cylindrical plug. (A) shows a case where the die angle is large, and (b) shows a case where the die angle is small. Yes. In the drawing process, as the die angle becomes smaller, the processing area of the raw tube 3 sandwiched between the die 1 and the plug 2, that is, the contact area with the tool increases, so-called “squeezing effect” increases, The effect of correction and wrinkle improvement will increase.
[0021]
From the above (Test 1), the die angle greatly contributes to straightening and wrinkle improvement. Therefore, in (Test 2) and after, the processing conditions were examined focusing on the die angle.
[0022]
(Test 2)
In order to clarify the relationship between the thickness of the actual machine, the die angle, and the wrinkle depth, pipes were manufactured under the same conditions as the actual steel pipe production and a series of tests were conducted. The raw material was prepared by subjecting a material of S45C to a hot mandrel mill made of Mannesmann tube, descaling with sulfuric acid without annealing, and applying a rust prevention treatment. At this time, the raw tube had an outer diameter of 31.8 mm, an average thickness of 6.7 to 7.5 mm, and an uneven thickness due to angulation was about 0.5 mm.
[0023]
The drawing process uses five straight dies with a die angle of 10 °, 15 °, 20 °, 25 °, and 30 ° and one cylindrical plug, and lubrication is oil (grease) lubrication, and viscosity at 20 ° C. Used was a 590CST sulfurized oil and fat added with an oiliness improver and a thickener. It was processed into a steel pipe with an outer diameter of 22.49mm and a wall thickness of 6.22mm by a single drawing process. At this time, the thickness processing degree is 7 to 17% as calculated by the average thickness. The depth of the inner wrinkle after drawing was measured by observing the inner surface by cross-sectional microphotographing.
[0024]
FIG. 6 is a diagram showing the relationship between the thickness processing degree and the wrinkle depth when the die angle is drawn at 10 ° to 30 °. The horizontal line in the figure shows the wrinkle depth (0.05 mm), which is the standard for superiority or inferiority of the inner surface smoothness in the present invention, and the vertical line in the figure shows the meat where the wrinkle depth is 0.05 mm or less. Indicates the thickness processing degree. Since the vertical line in the figure moves closer to the right as the die angle increases, it turns out that the greater the die angle, the greater the depth of wrinkles can be made without increasing the wall thickness processing degree. It was.
[0025]
FIG. 7 is a diagram showing the relationship between the die angle (°) and the wall thickness working degree (%) at which the depth of the wrinkle on the inner surface of the pipe after drawing is 0.05 mm or less based on the result of FIG. is there. From FIG. 7, the relationship between the die angle X (°) and the wall thickness processing degree Y (%) can be expressed by an exponential function, and can be approximately expressed by the following equation (a).
[0026]
Y ≧ 5.5 · exp (0.036 · X) (a)
(Test 3)
In the lubrication treatment used in cold drawing of steel pipes, in addition to the oil lubrication used in Test 2 above, after applying a zinc phosphate coating as a base treatment, sodium stearate is reacted as a soap treatment, So-called chemical film lubrication, in which a metal soap lubrication layer is formed on the boundary surface, is often used. Therefore, in Test 3, the relationship between the die angle and the wall thickness processing degree was similarly examined in the case of chemical film lubrication.
[0027]
As in the case of Test 2 above, the raw tube is made of a material of S45C hot by a mandrel mill made of Mannesmann, descaled with sulfuric acid without being annealed, and subjected to rust prevention treatment. Prepared. At this time, the raw tube had an outer diameter of 31.8 mm and an average wall thickness of 6.8 to 7.2 mm.
[0028]
For drawing, a straight die with a die angle of 20 ° and a cylindrical plug were used. Lubrication was performed with a chemical conversion coating. The soap was treated with soda to form a metal soap lubrication layer and processed into a steel pipe having an outer diameter of 22.49 mm and a wall thickness of 6.22 mm by a single drawing process. For comparison, the same die and plug were used to perform a drawing process by oil lubrication with a sulfurized fat having a viscosity at 20 ° C. of 590 CST.
[0029]
FIG. 8 shows the relationship between the thickness processing degree and the wrinkle depth when the drawing process is performed with chemical film lubrication (a) or oil lubricant (b). In both cases, the same blank tube was drawn under the same processing conditions. However, when oil-lubricated, the wrinkle depth is 0.02 mm overall, which is smoother than oil-lubricated. High tube inner surface. In other words, it is difficult to make the wrinkle depth 0.05mm or less when lubricated with a chemical film, compared with oil (grease) lubrication. The thickness of 0.05mm or less is 11.5% and 13%, respectively. When lubricated with a chemical conversion film, a wall thickness workability of 1.5% or more is required compared with oil (grease) lubrication. . In addition, the wrinkle depth is 0.05 mm or less in both cases when the thickness processing degree is 10% or more, but in order to ensure that the wrinkle depth is 0.05 mm or less, the wrinkle depth is Based on the maximum thickness processing degree of 0.05 mm, the thickness processing degree at which the wrinkle depth was 0.05 mm or less was judged.
[0030]
Therefore, from the result of FIG. 8 and the relationship of the above formula (a), when lubricated with a chemical conversion film, the relationship between the die angle X (°) and the thickness processing degree Y (%) is approximately the following (b) It is expressed by an expression.
[0031]
Y ≧ 1.5 + 5.5 · exp (0.036 · X) (b)
The present invention has been completed based on the examination results of Test 1 to Test 3 described above, and the following (1) and (2) methods for producing seamless steel pipes with excellent inner surface smoothness and (3) The gist is seamless steel pipe.
[0032]
(1) A method of manufacturing a seamless steel pipe in which the ratio of the thickness / outer diameter (T / D) of the steel pipe that has been cold-finished is 20% or more. A method for producing a seamless steel pipe having excellent inner surface smoothness, characterized by performing cold drawing using a die and a plug under conditions satisfying the following expression (a).
[0033]
Y ≧ 5.5 · exp (0.036 · X) (a)
However, Y indicates the thickness processing degree (%), and X indicates the die angle (°) .
(2) A method for producing a seamless steel pipe having a wall thickness / outer diameter (T / D) ratio of 20% or more of the cold-finished steel pipe, wherein b) A method for producing a seamless steel pipe having excellent inner surface smoothness, characterized by performing cold drawing using a die and a plug under conditions satisfying the formula.
[0034]
Y ≧ 1.5 + 5.5 · exp (0.036 · X) (b)
However, Y is the thickness processing degree (%) and X is the die angle (°). (3) Manufactured by the method for producing a seamless steel pipe according to (1) 1 or (2) above, Seamless steel pipe with a depth of 0.05mm or less.
[0035]
DETAILED DESCRIPTION OF THE INVENTION
The present invention relates to a method for producing a seamless steel pipe excellent in inner surface smoothness. The raw tube targeted by the method of the present invention may be produced by hot roll rolling using the Mannesmann production method, or may be produced by hot extrusion using the Eugene production method. It may be. Usually, the raw tube produced by the Mannesmann tube method has a lower roundness of the inner diameter and the thickness tends to be uneven compared to the Eugene tube method. However, if the method of the present invention is used, 1 Even with a single cold drawing process, it is possible to produce a seamless steel pipe with excellent inner surface smoothness.
[0036]
Here, as described above, the excellent smoothness of the inner surface is considered to be applied to driving parts such as power steering parts and drive shafts. Means 0.05 mm or less. In the method of the present invention, it is assumed that cold drawing of a seamless steel pipe having a T / D of 20% or more is assumed. However, when a seamless steel pipe is used as a driving part of an automobile, the thickness of the same T / D dimension is used. This is because meat steel pipes are frequently used.
[0037]
In the production method of the present invention, the die used is preferably a straight type. This is because the straight die is more stably supplied with oil lubricant to the processing interface than other dies, for example, the R die.
[0038]
On the other hand, the plug used for the drawing process is not limited, and any of a cylindrical plug, a float plug, and a semi-float plug may be used.
[0039]
In the method of the present invention, the processing conditions differ depending on the lubrication treatment applied to the inner and outer surfaces of the raw tube during cold drawing. That is, when oil (oil) lubrication is performed, it is necessary to satisfy the following formula (a), where Y is the thickness processing degree (%) and X is the die angle (°).
[0040]
Y ≧ 5.5 · exp (0.036 · X) (a)
Here, the oil lubrication may be a commonly used one, and for example, an oil (oil / fat) lubricant having a viscosity of 20 to 3000 CST at 20 ° C. may be used, which is prepared based on natural fats and oils, synthetic oils or mineral oils. At this time, you may contain extreme pressure additives, such as S, P, and Cl.
[0041]
On the other hand, when chemical conversion film lubrication is performed, it is necessary to satisfy the following formula (b), where Y is the thickness processing degree (%) and X is the die angle (°).
[0042]
Y ≧ 1.5 + 5.5 · exp (0.036 · X) (b)
In the chemical conversion film lubrication, in addition to the surface treatment with the exemplified zinc phosphate film, a surface treatment for generating a crystal film of oxalate on the surface of the raw tube may be used. After the base treatment, sodium stearate or the like is reacted as a soap treatment to form a metal soap lubricating layer on the boundary surface with the base.
[0043]
If cold drawing is performed in accordance with the above processing conditions, a cold-finished seamless steel pipe having excellent inner surface smoothness can be manufactured by one drawing regardless of the type of pipe making.
[0044]
【Example】
In order to confirm the effect of the present invention, each of the raw pipes (materials and dimensions) produced by the Mannesmann mandrel mill was each subjected to one cold drawing process. When drawing, straight type dies and cylindrical plugs were used, and lubrication was divided into oil lubrication and chemical film lubrication. The oil lubrication used was obtained by adding an oiliness improver and a thickener to a sulfurized fat having a viscosity at 20 ° C. of 590 CST. For chemical conversion film lubrication, a zinc phosphate film was formed on the surface of the steel pipe with zinc phosphate, phosphoric acid and sodium nitrite as a base treatment, followed by soap treatment with sodium stearate to form a metal soap lubrication layer.
[0045]
Under these conditions, after drawing 50 to 80 times under the same conditions, the depth of wrinkles on the inner surface of the tube was measured. Table 1 shows the measurement results of the material and dimensions of the raw pipe, the finished dimensions of the processed steel pipe, the processing conditions, and the wrinkle depth.
[0046]
[Table 1]

As is clear from the results in Table 1, when the conditions specified in the present invention are met (No. 1 to 9), the depth of the wrinkle after drawing all becomes 0.05 mm or less, and the excellent inner surface A seamless steel pipe having smoothness was obtained. On the other hand, in the case where the conditions of the present invention were not satisfied (No. 10), there were also seamless steel pipes in which the depth of the wrinkles exceeded 0.06 mm and could not be applied to automobile driving parts.
[0047]
【The invention's effect】
According to the production method of the present invention, a cold-finished seamless steel pipe having excellent inner surface smoothness can be produced by a single drawing process regardless of the pipe making type of the raw pipe. The number of processes is not increased, and the manufacturing cost does not increase. As a result, it is possible to provide a seamless steel pipe that is optimal for driving components such as power steering and drive shafts for automobiles.
[Brief description of the drawings]
FIG. 1 is a view showing a cross-sectional shape of a straight die.
FIG. 2 is a diagram schematically showing the shape of a test steel pipe used in Test 1 before and after drawing.
FIG. 3 is a diagram showing a relationship between a thickness processing degree and a square bottom depth at a vertex angle position of a hexagonal hole when a die angle is drawn at 15 ° to 25 °.
[4] sinking processed when a view showing the relationship between the outer diameter working ratio and thickness increase rate, (a) represents the die angle 25 ° relationship of the outer diameter working ratio and thickness increase rate when the (B) shows the relationship between the outer diameter working degree and the thickness increase rate when the thickness / outer diameter ratio (T / D) is 30%.
FIGS. 5A and 5B are diagrams schematically showing a drawing process using a straight die and a cylindrical plug. FIG. 5A shows a case where the die angle is large, and FIG. 5B shows a case where the die angle is small. Yes.
FIG. 6 is a diagram showing the relationship between the thickness processing degree and the wrinkle depth when the die angle is 10 ° to 30 ° and the drawing is performed.
FIG. 7 is a diagram showing the relationship between the die angle (°) and the wall thickness working degree (%) at which the depth of the wrinkle on the inner surface of the pipe after drawing is 0.05 mm or less based on the result of FIG. is there.
FIG. 8 shows the relationship between the thickness processing degree and the wrinkle depth when drawing is performed with chemical film lubrication or oil lubricant.
[Explanation of symbols]
1 Dice 2 Plug 3 Base tube

Claims (3)

A method of manufacturing a seamless steel pipe having a wall thickness / outer diameter (T / D) ratio of 20% or more of a cold-finished steel pipe, wherein the raw pipe subjected to oil (oil) lubrication is represented by the following (a ) A method for producing a seamless steel pipe with excellent inner surface smoothness, characterized by performing cold drawing using a die and a plug under a condition satisfying the formula.
Y ≧ 5.5 · exp (0.036 · X) (a)
However, Y indicates the thickness processing degree (%), and X indicates the die angle (°) . A method for producing a seamless steel pipe having a wall thickness / outer diameter (T / D) ratio of 20% or more of a cold-finished steel pipe, wherein a raw pipe subjected to chemical conversion film lubrication is represented by the following formula (b) A method for producing a seamless steel pipe with excellent inner surface smoothness, characterized by performing cold drawing using a die and a plug under conditions satisfying
Y ≧ 1.5 + 5.5 · exp (0.036 · X) (b)
However, Y indicates the thickness processing degree (%), and X indicates the die angle (°) . A seamless steel pipe manufactured by the method for manufacturing a seamless steel pipe according to claim 1 or 2, wherein the depth of wrinkles on the inner surface of the pipe is 0.05 mm or less.

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