JP4103082B2 - Manufacturing method for seamless pipes using a three-roll mandrel mill - Google Patents

Description

  The present invention relates to a method for producing a seamless pipe using a three-roll mandrel mill.

  In the manufacture of seamless pipes by the Mannesmann-Mandrel mill method, first, a round billet or square billet of a material is heated to 1200 to 1260 ° C. in a rotary hearth type heating furnace, and then pierced and rolled with a plug and a rolling roll with a piercing machine. Thus, a hollow shell is manufactured. Next, a mandrel bar is inserted into the hollow inner tube in the form of a skewer, and the outer surface is constrained by a perforated rolling roll with a mandrel mill usually consisting of 5 to 8 stands, so that the predetermined thickness is reached. Reduce meat. Then, after extracting a mandrel bar, the thinned tube material is formed and rolled to a predetermined outer diameter with a drawing mill to obtain a product.

  Here, as a mandrel mill, conventionally, a pair of perforated rolling rolls opposed to each stand is provided, and a two-roll mandrel in which the rolling rolls are pressed alternately by 90 ° in the rolling direction between adjacent stands. A mill is often used. In such a two-roll mandrel mill, seizure flaws at locations corresponding to the flange portion caused by an excessive peripheral speed difference between the groove bottom portion and the flange portion of the rolling roll and excessive biting of the pipe material From the viewpoint of preventing wrinkles caused by the above, it is usual to design the rolling roll so that the curvatures at both ends of the hole mold are increased. In this case, since the portion corresponding to the flange portion of the rolling roll of the tube material is not restricted by the rolling roll or the mandrel bar, but only the longitudinal tension acts, it is possible to manage the deformation (overhang) in the circumferential direction. It is difficult, and there is also a problem that a perforated defect or the like is likely to occur in a material having low hot deformability such as stainless steel. In order to solve such a problem of the two-roll type mandrel mill, each stand is provided with three perforated rolling rolls having an angle of 120 ° in the rolling direction, and the rolling direction of the rolling roll is set between adjacent stands. The introduction of a three-roll mandrel mill arranged alternately by 60 ° has been studied.

  However, in general, in a three-roll mandrel mill, the contact range (location) between the inner surface of the tube material after rolling and the mandrel bar is larger than that in the case of a two-roll method because of the geometric properties of the hole shape of the rolling roll. Will increase. For this reason, as a result of an increase in the load required for pulling out the mandrel bar, serious mandrel bar pull-out failure after rolling and scratching flaws due to the mandrel bar on the inner surface of the pipe material are likely to occur. . Therefore, it is extremely difficult to put a 3-roll mandrel mill into practical use.

In order to solve the problems in such a three-roll mandrel mill, the following methods have been proposed conventionally. That is,
(1) A method of adjusting the stress acting on the pipe material between the stands by adjusting the number of rotations of the rolling rolls of the adjacent stands and controlling the cross-sectional shape of the pipe material (for example, see Non-Patent Document 1),
(2) Although it is an example of a 2 roll type mandrel mill, by setting the hole type perimeter of the first stand to the third stand constituting the mandrel mill to a predetermined multiple or more of the finish perimeter of the mill outlet side pipe material, A method for producing an appropriate clearance between the inner surface of the tube material on the mill exit side and the mandrel bar (for example, see Patent Document 1),
(3) A method of creating a clearance between the mandrel bar and the tube material without reducing the thickness of the tube material by installing a sizing stand on the final stand constituting the mandrel mill (see, for example, Patent Document 2),
Each has been proposed.
"Basic load characteristics and deformation characteristics" (Proceedings of the 1984 Spring Meeting of Plastic Processing, p545-548) Japanese Patent Laid-Open No. 5-185112 JP-A-7-214110

  However, in the conventional method of (1) as disclosed in Non-Patent Document 1, although the shape control of the central portion of the pipe material that is simultaneously biting into a plurality of stands can be performed, the compression force between the stands does not sufficiently act. The shape of both pipe ends cannot be controlled, and both pipe ends are known to have a so-called underfill tendency in which the entire circumference closely adheres to the mandrel bar, and the mandrel bar after rolling is poorly drawn. In addition, the problem that scratches caused by the mandrel bar on the inner surface of the pipe material easily occur cannot be solved.

  Further, the deformation phenomenon of the tube material in the rolling roll in the three-roll type mandrel mill is greatly different from that of the two-roll type mandrel mill, as in the conventional method of (2) disclosed in Patent Document 1, Simply setting the perforation lengths of the first to third stands is not sufficient to solve the problem of poor pulling of the mandrel bar after rolling and scratching flaws due to the mandrel bar on the inner surface of the tube material. is there. That is, in the three-roll type mandrel mill, due to the characteristics of the hole shape, it is difficult to ensure overhanging due to large deformation in the longitudinal direction of the tube material, and it is not possible to ensure sufficient clearance after rolling depending on how the subsequent stand is rolled. In some cases, a problem that the mandrel bar cannot be pulled out, or even if it is pulled out, there is a problem that the load is remarkably increased as compared with the two-roll mandrel mill.

  Further, the conventional method of (3) as disclosed in Patent Document 2 is sufficient when the clearance between the mandrel bar and the tube material has already disappeared before processing with the sizing stand. There is a problem that the effect cannot be obtained.

  As described above, since the deformation phenomenon of the pipe material in the rolling roll in the three-roll mandrel mill is greatly different from that of the two-roll mandrel mill, the knowledge in the two-roll mandrel mill cannot be used as it is. Therefore, the above proposals are not proposals that can effectively solve the problems of the three-roll type mandrel mill, such as defective drawing of the mandrel bar after rolling and generation of scratches caused by the mandrel bar on the inner surface of the pipe material. The situation where it is difficult to put a 3-roll mandrel mill into practical use continues.

  The present invention has been made to solve such a problem of the prior art, effectively suppressing the occurrence of mandrel bar drawing failure after rolling and the occurrence of scratching flaws due to the mandrel bar on the inner surface of the pipe material, and thus the actual operation. It is an object of the present invention to provide a method for producing a seamless pipe using a possible three-roll mandrel mill.

  In order to solve the above-mentioned problems, the present invention is a method for producing a seamless pipe by a three-roll mandrel mill, and among the stands constituting the mandrel mill, the final two stands that perform wall thickness processing on the pipe material The present invention provides a method for producing a seamless pipe by a three-roll mandrel mill, characterized in that the ratio of the inner peripheral length of the pipe material and the outer peripheral length of the mandrel bar is set in the range of 1.07 to 1.17. .

  Preferably, the ratio of the inner circumferential length of the pipe material and the outer circumferential length of the mandrel bar is set in a range of 1.10 to 1.17.

  Preferably, the ratio between the outer diameter of the tube material on the outlet side of the mandrel mill and the groove bottom diameter of the rolling roll in the last two stands is set to 0.25 or less.

  More preferably, the ratio of the tube outer diameter on the outlet side of the mandrel mill and the groove bottom diameter of the rolling roll in the last two stands is set to 0.20 or less.

  Preferably, the ratio between the bar clearance on the mandrel mill entry side and the tube inner diameter on the mandrel mill entry side is set to 0.04 to 0.12.

  More preferably, the ratio between the bar clearance on the inlet side of the mandrel mill and the inner diameter of the pipe material on the inlet side of the mandrel mill is set to 0.06 to 0.12.

  Preferably, a sizing stand is installed after the last two stands, and the degree of processing at the groove bottom portion of the rolling roll in the sizing stand is 5% or more.

  According to the present invention, the ratio of the inner peripheral length of the pipe material to the outer peripheral length of the mandrel bar is set to 1.07 to 1.17 in the final two stands that perform the wall thickness processing on the pipe material among the stands constituting the mandrel mill. By setting this range, the circumferential thickness difference (uneven thickness) of the tube material after rolling does not become too large, the pulling failure of the mandrel bar after rolling, and the occurrence of scratching flaws due to the mandrel bar on the inner surface of the tube material A seamless pipe manufacturing method using a three-roll mandrel mill that can be effectively suppressed and can be actually operated is provided.

Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings as appropriate.
Effective in the production of seamless pipes using a mandrel mill, it is effective between the tube material after rolling and the mandrel bar in order to suppress the pulling failure of the mandrel bar after rolling and the generation of scratches caused by the mandrel bar on the inner surface of the tube. In particular, in the case of a three-roll type mandrel mill, the inventor of the present invention, in the case of the final wall thickness processing, has an inner portion of the tube material with respect to the outer peripheral length of the mandrel bar. It was found that it depends greatly on how the circumference is set. In other words, whether or not the effective clearance occurs is determined by the inner circumference of the pipe material, which is determined by the hole shape of the rolling roll in the last two stands (the last stand and the stand immediately preceding it) where the pipe material is thickened. It has been found that the ratio of the length and the outer circumference of the mandrel bar has a great influence.

  Therefore, the inventors of the present invention, in each of the stands constituting the mandrel mill, in the final two stands that perform the wall thickness processing on the tube material, the ratio of the tube material inner circumference length to the mandrel bar outer circumference length (hereinafter referred to as appropriate). Rolling tests were conducted on 10 pipes each made of carbon steel and 9% Cr steel while varying the circumference ratio), and the influence of the circumference ratio on the pullability of the mandrel bar after rolling was investigated. The result is shown in FIG. FIG. 1 is a diagram showing the influence of the circumference ratio, where (a) shows the influence on the pullability of the mandrel bar, (b) shows the influence on the inner surface quality of the pipe, and (c) shows the deviation of the pipe. The influence on the thickness (circumferential thickness difference of the tube material after rolling) is shown respectively. The vertical axis in FIG. 1 (a) indicates that “2” is good for both carbon steel and 9% Cr steel (the case where all 10 pieces can be pulled out is “good”). "" Means that only carbon steel is good, and "0" means that both carbon steel and 9% Cr steel are bad. In addition, the vertical axis of FIG. 1B indicates that “3” indicates that there is no inner surface flaw (scratching flaw by a mandrel bar) in the pipe material, and “2” indicates that the occurrence rate is 10% or less. Means an occurrence rate of 20% or less, and “0” means an occurrence rate of more than 20%. Furthermore, the vertical axis in FIG. 1C indicates that “2” indicates that the thickness deviation of the pipe material is less than 15%, and “1” indicates that it is 15% or more.

  As shown in FIGS. 1 (a) and 1 (b), by setting the circumference ratio to 1.07 or more, the clearance between the tube material and the mandrel bar can be secured, and both the pullability of the bar and the inner surface quality of the tube material are both It was found that relatively good results were obtained. However, as shown in FIG. 1 (c), when the circumference ratio is larger than 1.17, good results are obtained with respect to the pullability of the bar and the inner surface quality of the pipe (FIGS. 1 (a) and (b)). However, it has been found that there is a quality problem that the uneven thickness of the pipe material becomes too large.

  Therefore, by setting the circumference ratio in the range of 1.07 to 1.17, the circumferential thickness difference (uneven thickness) of the tube material after rolling does not become too large, and the mandrel bar is drawn after rolling. It has been found that defects and scratches caused by mandrel bars on the inner surface of the pipe can be effectively suppressed. As shown in FIG. 1A, it is preferable to set the circumference ratio in the range of 1.10 to 1.17 in order to further suppress the pulling failure of the mandrel bar.

  2A and 2B are explanatory views for explaining the inner peripheral length of the pipe material, wherein FIG. 2A is an arrangement view of each rolling roll, and FIG. 2B is an enlarged view of a portion surrounded by a wavy line in FIG. . As shown in FIG. 2 (b), the inner circumferential length of the pipe material is divided into six in the circumferential direction based on the hole center C by the hole profile from the groove bottom B to one end E of each rolling roll R. The length of the inner surface shape (curve B1E1) of the six divided pipes calculated by determining the outer surface shape (curve BE ′) of the pipe material and correcting the outer surface shape with the groove bottom wall thickness t. It is a value obtained by integration. Here, in order to determine the outer shape (curve BE ′) of the pipe material by the hole profile (curve BE), for example, an arc forming the hole profile near the end E forms an angle of 60 ° with the straight line BC. It is conceivable that the line extends to a straight line CC ′ and an intersection of the extended arc and the straight line CC ′ is a point E ′. Further, correcting the outer surface shape (curve BE ′) with the groove bottom thickness t means that each point constituting the curve BE ′ is moved inward in the normal direction of each point by the thickness t. To do. The groove bottom thickness t is a value determined by a rolling schedule, and the mandrel bar outer circumferential length is a value calculated from the outer diameter of the mandrel bar determined by the rolling schedule.

  As described above, by setting the circumference ratio to a predetermined range, it is possible to effectively suppress the pulling failure of the mandrel bar, etc., but depending on the material of the pipe material, rolling conditions, etc. The close contact with the mandrel bar without filling the hole mold may reduce the pullability and may cause scratches due to the mandrel bar. The inventors of the present invention have made various studies to prevent a drop in pullability by extending the tube material so as to follow the hole shape of the rolling roll as much as possible. As a result, the groove bottom diameter of the rolling roll (FIG. 2 ( By setting the length of the line segment BB ′ connecting the groove bottom B ′ and the opposite groove bottom B ′ shown in a) to be larger than the outer diameter of the tube on the outlet side of the mandrel mill, as shown in FIG. Since the contact length between the groove bottom of the rolling roll and the tube is increased, the deformation resistance in the rolling direction is increased. As a result, the deformation of the rolling roll toward the flange side is promoted, and the overhang that follows the hole shape of the rolling roll Thought that would be encouraged.

  Therefore, the inventors of the present invention variously changed the ratio of the tube outer diameter on the outlet side of the mandrel mill and the groove bottom diameter of the rolling roll in the last two stands (hereinafter referred to as “finish diameter / roll groove bottom diameter” as appropriate). However, a rolling test was performed on 10 pipe materials each made of carbon steel and 9% Cr steel, and the influence of the finished diameter / roll groove bottom diameter on the pullability of the mandrel bar after rolling was investigated. The result is shown in FIG. FIG. 4 is a diagram showing the influence of the ratio between the outer diameter of the tube material on the outlet side of the mandrel mill and the groove bottom diameter of the rolling roll in the last two stands, and (a) shows the influence on the pullability of the mandrel bar, respectively. (B) shows the influence on the inner surface quality of the pipe material, and (c) shows the influence on the uneven thickness of the pipe material. In addition, since the meaning of the numerical value of the vertical axis | shaft of Fig.4 (a)-(c) is the same as that of the case of FIG. 1, description is abbreviate | omitted. In this rolling test, the above-mentioned circumference ratio is set to 1.07 included in the above-mentioned range, and the ratio between the bar clearance on the mandrel mill entry side and the tube inner diameter on the mandrel mill entry side described later is 0.04. It was.

  As shown in FIGS. 4 (a) and 4 (b), by setting the finishing diameter / roll groove bottom diameter to 0.25 or less (more preferably 0.20 or less), the pullability of the bar and the inner surface quality of the pipe material are obtained. Both have been found to give relatively good results. However, when the finishing diameter / roll groove bottom diameter is less than 0.1, good results can be obtained with respect to the pullability of the bar and the inner surface quality of the pipe material, but it is too practical because it becomes too large. Accordingly, the finishing diameter / roll groove bottom diameter is preferably 0.1 to 0.25, and more preferably 0.1 to 0.2. Note that the outer diameter of the pipe material on the outlet side of the mandrel mill is a value determined by a rolling schedule.

  In addition, the inventors of the present invention extend the tube material so as to follow the hole shape of the rolling roll by adjusting the ratio between the bar clearance on the mandrel mill entry side and the tube inner diameter on the mandrel mill entry side. Therefore, it was considered possible to prevent a decrease in pullability. Therefore, a tube material made of carbon steel and 9% Cr steel while variously changing the ratio of the bar clearance on the inlet side of the mandrel mill and the inner diameter of the pipe material on the inlet side of the mandrel mill (hereinafter referred to as “input side bar clearance / finishing diameter” as appropriate). A rolling test was carried out for 10 bars each, and the influence of the entry side bar clearance / finished diameter on the pullability of the mandrel bar after rolling was investigated. The result is shown in FIG. FIG. 5 is a diagram showing the influence of the ratio between the bar clearance on the inlet side of the mandrel mill and the inner diameter of the pipe material on the inlet side of the mandrel mill, and (a) shows the influence on the pullability of the mandrel bar, respectively (b). Indicates the effect on the inner surface quality of the tube material, and (c) indicates the effect on the uneven thickness of the tube material. In addition, since the meaning of the numerical value of the vertical axis | shaft of Fig.5 (a)-(c) is the same as that of the case of FIG. 1, description is abbreviate | omitted. In this rolling test, the above-mentioned circumference ratio was set to 1.07 included in an appropriate range, and the above-mentioned finishing diameter / roll groove bottom diameter was set to 0.25 included in an appropriate range.

  As shown in FIGS. 5 (a) and 5 (b), by setting the entry side bar clearance / finished diameter to 0.04 or more (more preferably 0.06 or more), the pullability of the bar and the quality of the inner surface of the pipe material are improved. Both have been found to give relatively good results. However, when the entry-side bar clearance / finishing diameter is larger than 0.12, the outer peripheral length of the tube material becomes excessive, and rolling may be impossible due to excessive biting, which is not practical. . Accordingly, the entry side bar clearance / finishing diameter is preferably 0.04 to 0.12, and more preferably 0.06 to 0.12. The bar clearance is defined by (mandrel mill entry side tube inner diameter−mandrel bar outer diameter) and is a value determined by a rolling schedule.

  Furthermore, the inventors of the present invention install a sizing stand after the final two stands that perform the wall thickness processing on the pipe material, and by performing processing at a predetermined level or more on the groove bottom portion of the rolling roll in the sizing stand, Unlike the case where a sizing stand is simply installed, the pipe material is deformed in the flange direction, and an effective clearance is generated, so that the pullability and the like can be further improved. Therefore, a rolling test was performed on 10 pipes each made of carbon steel and 9% Cr steel while changing the degree of processing at the groove bottom of the rolling roll in the sizing stand, and the degree of processing was the pullability of the mandrel bar after rolling, etc. The effects on the environment were investigated. The result is shown in FIG. FIG. 6 is a diagram showing the influence of the processing degree at the groove bottom of the rolling roll in the sizing stand, where (a) shows the influence on the pullability of the mandrel bar, and (b) shows the inner surface quality of the pipe material. (C) shows the influence exerted on the uneven thickness of the pipe material. The meanings of the numerical values on the vertical axis in FIGS. 6A to 6C are the same as those in FIG. In this rolling test, the above-mentioned circumference ratio is set to 1.07 included in an appropriate range, and the above-mentioned finishing diameter / roll groove bottom diameter is set to 0.25 included in an appropriate range. The entry side bar clearance / finished diameter was set to 0.04 which is included in an appropriate range.

As shown in FIGS. 6 (a) and 6 (b), it was found that relatively good results were obtained with respect to the pullability of the bar and the inner surface quality of the tube material by setting the working degree to 5% or more. In addition, as shown in FIG. 7, the said process degree is the long diameter (equivalent to line segment CE 'of FIG.2 (b)) in the last stand (N-1 stand) which performs thickness processing with respect to a pipe material. When A _{N-1 is} assumed and B _N is the short diameter (corresponding to the line segment BC in FIG. 2B) at the sizing stand (Nth stand), it is a value calculated by the following equation (1). is there.
Degree of processing = (A _N−1 / B _N −1) × 100 (%) (1)

  Hereinafter, the features of the present invention will be further clarified by showing examples.

  According to the conditions (Examples 1 to 16 and Comparative Examples 1 and 2) showing the numerical values in FIG. 8, a rolling test was performed for each of the ten pipe members made of carbon steel and 9% Cr steel. Here, in Examples 1 to 11 and Comparative Examples 1 and 2, the rolling test was performed on a mandrel mill consisting of a total of 5 stands (the 4th stand and the 5th stand were the final 2 stands where the tube material was thickened). Examples 12 to 16 were carried out using a mandrel mill in which a sizing stand (sixth stand) was installed after the last two stands. In addition, the abbreviation “exit side t / D” shown in FIG. 8 is the wall thickness / outer diameter of the tube material on the mandrel mill output side, and “final wall thickness processing 2Std circumference ratio” is the wall thickness relative to the tube material. The ratio of the inner peripheral length of the pipe material to the outer peripheral length of the mandrel bar in the final two stands (fourth stand and fifth stand) to be processed, and “sizing Std groove bottom pressure” is the ratio of the rolling roll in the sizing stand (sixth stand). It means the degree of processing at the groove bottom.

  The results of the rolling test were evaluated from the viewpoints of (1) pullability of the mandrel bar, (2) tube material inner surface quality, and (3) wall thickness unevenness of the tube material. In addition, since the numerical value shown in FIG. 8 about the evaluation result of each item has the same meaning as the numerical value shown on the vertical axis in FIG.

  As shown in FIG. 8, the pullability and the inner surface quality of Examples 1 to 16 are all “3”, “2” or “1”, and according to the present invention, the mandrel bar after rolling is pulled out. It has been found that defects and scratches caused by mandrel bars on the inner surface of the pipe can be effectively suppressed. Furthermore, the uneven thickness of the pipe material was all “1”, and it was also confirmed that the problem of uneven thickness did not occur. In particular, when the ratio of the inner peripheral length of the pipe material and the outer peripheral length of the mandrel bar is 1.10 to 1.17 (Examples 2, 3, 6, 7, 10, 11, 15, 16), the drawability is improved. All were “2”, and a remarkable effect was obtained.

  Further, if Examples 4 and 5 are compared or Examples 6 and 7 are compared, even if the circumference ratio is the same, the outer diameter of the tube material on the mandrel mill outlet side and the rolling rolls in the final two stands It can be seen that the drawability and / or the quality of the inner surface of the pipe material is improved by setting the ratio to the groove bottom diameter to 0.2.

  Further, if Examples 8 and 9 are compared, or Examples 10 and 11 are compared, the ratio of the circumference ratio or the tube outer diameter on the mandrel mill outlet side to the groove bottom diameter of the rolling roll in the last two stands. Even when the same, the ratio of the bar clearance on the mandrel mill entry side and the tube inner diameter on the mandrel mill entry side is 0.06, which indicates that the pullability and / or the tube inner surface quality is improved.

  Further, when Examples 12, 14 and 15 are compared with Examples 13 and 16, the circumference ratio, the ratio of the tube material outer diameter on the mandrel mill exit side and the groove bottom diameter of the rolling roll in the final two stands, the mandrel Even if the ratio between the bar clearance on the mill entry side and the tube inner diameter on the mandrel mill entry side is the same, by making the degree of processing at the groove bottom of the rolling roll in the sizing stand 5% or more, the drawability and / or Or it turns out that the quality of the inner surface of the pipe material is improved.

  On the other hand, in Comparative Example 1, the circumference ratio was less than 1.07 (1.05), and it was found that both the pullability of the mandrel bar and the quality of the inner surface of the tube were poor. On the other hand, in Comparative Example 2, the circumference ratio was larger than 1.17 (1.19), but good results were obtained with respect to both the pullability of the mandrel bar and the quality of the inner surface of the pipe material. However, the uneven thickness ratio is 15% or more (“0”), which indicates that it cannot be adopted as a method of manufacturing a seamless pipe that can be a product.

Fig. 1 shows the effect of the ratio of the inner peripheral length of the pipe and the outer peripheral length of the mandrel bar on the pullability of the mandrel bar, etc. FIG. It is explanatory drawing for demonstrating the pipe material inner peripheral length set in the manufacturing method of the seamless pipe concerning this invention. FIG. 3 is an explanatory view for explaining the influence of the groove bottom diameter of the rolling roll set in the seamless pipe manufacturing method according to the present invention. FIG. 4 is a diagram showing the influence of the ratio of the tube material outer diameter on the outlet side of the mandrel mill and the groove bottom diameter of the rolling roll in the last two stands on the pullability of the mandrel bar. FIG. 5 is a diagram showing the influence of the ratio between the bar clearance on the mandrel mill entry side and the tube inner diameter on the mandrel mill entry side on the pullability of the mandrel bar. FIG. 6 is a diagram showing the influence of the degree of processing at the groove bottom of the rolling roll in the sizing stand on the pullability of the mandrel bar. FIG. 7 is an explanatory diagram for explaining the degree of processing at the groove bottom portion of the rolling roll in the sizing stand set in the seamless pipe manufacturing method according to the present invention. FIG. 8 is a diagram showing conditions and evaluation results of an example of a method for manufacturing a seamless pipe according to the present invention.

Explanation of symbols

R: Rolling roll B: Groove bottom C: Hole center t: Groove bottom thickness

Claims (7)

A method of manufacturing a seamless pipe using a three-roll mandrel mill,
Of the two stands constituting the mandrel mill, the ratio of the pipe inner peripheral length to the mandrel bar outer peripheral length is set in the range of 1.07 to 1.17 in the final two stands where the tube material is thickened. A method for producing a seamless pipe using a three-roll mandrel mill.   2. The method for producing a seamless pipe by a three-roll mandrel mill according to claim 1, wherein a ratio between the inner peripheral length of the pipe material and the outer peripheral length of the mandrel bar is set in a range of 1.10 to 1.17.   The three-roll type mandrel mill according to claim 1 or 2, wherein the ratio of the tube outer diameter on the outlet side of the mandrel mill and the groove bottom diameter of the rolling roll in the last two stands is set to 0.25 or less. A method for producing seamless pipes.   4. The joint by a three-roll mandrel mill according to claim 3, wherein the ratio of the tube outer diameter on the outlet side of the mandrel mill and the groove bottom diameter of the rolling roll in the last two stands is set to 0.20 or less. Tubeless manufacturing method.   The three-roll type according to any one of claims 1 to 4, wherein a ratio between a bar clearance on the mandrel mill entry side and a tube inner diameter on the mandrel mill entry side is set to 0.04 to 0.12. A seamless pipe manufacturing method using a mandrel mill.   The seamless ratio by the three-roll mandrel mill according to claim 5, wherein the ratio of the bar clearance on the inlet side of the mandrel mill and the inner diameter of the pipe material on the inlet side of the mandrel mill is set to 0.06 to 0.12. A method of manufacturing a tube.   The three rolls according to any one of claims 1 to 6, wherein a sizing stand is installed after the last two stands, and a processing degree at a groove bottom portion of the rolling roll in the sizing stand is set to 5% or more. A method for manufacturing seamless pipes using a mandrel mill.