EP1679136A1

EP1679136A1 - Method of manufacturing seamless tube by three-roll mandrel mill

Info

Publication number: EP1679136A1
Application number: EP04792128A
Authority: EP
Inventors: Takamitsu c/o Sumitomo Metal Ind. Ltd. INAGE; Shigeru c/o Sumitomo Metal Ind. Ltd. KIDANI; Hirotsugu c/o Sumitomo Metal Ind. Ltd. NAKAIKE
Original assignee: Sumitomo Metal Industries Ltd
Current assignee: Nippon Steel Corp
Priority date: 2003-10-07
Filing date: 2004-10-07
Publication date: 2006-07-12
Anticipated expiration: 2024-10-07
Also published as: JP4103082B2; EP1679136B1; JP2005111518A; RU2309015C1; EP1679136A4; DE602004022574D1; WO2005035154A1; CN1863608A; CN100368102C

Abstract

A method for manufacturing seamless tubes by a three-roll mandrel mill according to the present invention, wherein the ratio of the inner circumferential length of the tube material to the outer circumferential length of the mandrel bar is set in the range of 1.07 to 1.17 in the last two stands, that constitute said mandrel mill, in which the tube material is subjected to a wall thickness reduction process, makes it possible to effectively suppress the unsuccessful withdrawal of the mandrel bar as well as the inner surface scratch defects of the tube material by the mandrel bar without excessively increasing the wall thickness difference in the circumferential direction (the wall thickness eccentricity) after rolling, whereby a production run by said three-roll mandrel mill can be performed. Thus, the method for manufacturing seamless tubes by a Mannesmann-Mandrel Mill process can be widely applied.

Description

TECHNICAL FILED

The present invention relates to a method of manufacturing a seamless tube by a three-roll mandrel mill.

BACKGROUND ART

In manufacturing seamless tubes by a Mannesmann-Mandrel Mill process, firstly, a round billet or angular billet as a work material to be rolled is heated at 1200 to 1260°C in a rotary hearth heating furnace, and then is subjected to a piercing process employing a plug and rolling rolls to thereby obtain a hollow shell. Next, a mandrel bar is inserted into the bore of the above hollow shell in a skewered manner, and an elongating process is applied to reduce the wall thickness to the predetermined dimension, while restraining the outer surface of said hollow shell by the grooved rolling rolls mounted on five to eight stands, normally, that constitute the mandrel mill. Then, the mandrel bar is withdrawn and the tube material thus reduced in wall thickness is subjected to a reducer mill, whereby rolling to the predetermined diameter is carried out to obtain a product.
Here, as the mandrel mill, there has been applied the two-roll mandrel mill in many cases wherein a pair of the grooved rolling rolls are disposed at each of the two stands as opposed to each other and the reduction orientation at the adjacent stand is configured to alternately be out of phase by 90°. In such a two-roll mandrel mill, the rolling roll is designed to have a larger curvature at both side edge portions including the flange portion in a groove profile so as to avoid the seizure defect on the tube material at the location corresponding to the roll flange attributable to the excessive peripheral velocity difference between the groove bottom and the flange, as well as to avoid the groove-overfilled defects due to the excessive overfill of the tube material.
Under this circumstance, since only tension in the longitudinal direction is exerted at the location in the tube material corresponding to the roll flange, being without any restraint by rolling rolls or mandrel bar, it is difficult to control the deformation in the circumferential direction (outstretch), whereby there arises an issue that through-wall-hole defects etc. are likely to be caused for a stainless steel etc. having less hot workability.
To solve such a problem in the case of the two-roll mandrel mill, it has been considered to install a three-roll mandrel mill wherein three grooved rolling rolls are disposed to be oriented 120° with respect to each other in terms of reduction orientation and further the rolling rolls at the adjacent stand alternates reduction orientation by 60° in succession.
However, the contact area (contact location) subsequent to rolling between the inner surface of the tube material and the mandrel bar in the case of three-roll mandrel mill should be increased compared to the two-roll mandrel mill because of the geometrical characteristic of the groove profile of the rolling rolls. Thus, the force required for withdrawing the mandrel bar should increase to thereby cause unsuccessful withdrawal of the mandrel bar very likely after rolling and the scratches on the inner surface of the tube material thereby should be generated very likely, which are of serious issues in terms of production run and quality.
In order to solve this problem in three-roll mandrel mill, there are proposed following methods (1) to (3) as below.

(1) A method for controlling the cross-sectional shape of the tube material, wherein, by regulating the rotation rate of the rolling rolls at the adjacent stand, the exerted stress onto the tube material moving between each of stands is adjusted (for example, see "Base Load Characteristic and Deformation Characteristic", p545 - 548 in the Proceeding of Plastic Deformation Convention in Spring term).
(2) A method for providing an adequate clearance between the inner surface of the tube material and the mandrel bar at the exit of the mill, although being the case of the two-roll mandrel mill, by setting the ratio of circumferential length of grooved profile to that of the cross sectional shape of the tube material at the exit of the mill to be a predetermined number (for example, see Japanese Patent Application Publication No. 5-185112).
(3) A method for providing a clearance between the tube material and the mandrel bar without accompanying the wall thinning of the tube material by disposing a sizing stand at the last stand of the mandrel mill (for example, see Japanese Patent Application Publication No.7-214110).

However, in the method shown in the above (1), although it is possible to control the tube material shape at the mid length where the tube material is engaged with a plurality of stands simultaneously, it is not possible to control the shape at both tube ends where an adequate compression between stands cannot be exerted. Consequently, the circumferential inner surface at both tube ends tends to closely contact with the mandrel bar, which is the so-called underfill, whereby it is not possible to solve such problem that the mandrel-withdrawal-related defects after rolling and scratch defects on the inner surface of the tube material by the mandrel bar are likely to occur.
Also, the deformation characteristic of the tube material between the rolling rolls in the three-roll mandrel mill is significantly distinguished from the two-roll mandrel mill, thus by merely setting the circumferential length of grove profile at No.1 stand to No.3 stand as shown in the above (2) that is described in Japanese Patent Application Publication No.5-185112, it is not enough to solve such a problem that the mandrel-withdrawal-related defects after rolling and the scratch defects on the inner surface of the tube material by the mandrel bar are likely to occur.
Namely, in the three-roll mandrel mill, it is difficult to secure the circumferentially outstretched deformation due to heavy deformation of tube material in longitudinal direction, and the adequate clearance cannot be secured after rolling depending on the rolling condition of rear stands. Thus, the problem that the mandrel bar cannot be withdrawn, or otherwise, although possible to withdraw, a problem that the withdrawal force required increases much more than that of two-roll mandrel mill should arise.
Further, in the conventional method of the above (3) as disclosed in Japanese Patent Application Publication No.7-214110, there arises a problem that a satisfactory effect cannot be expected in the case that the clearance between the tube material and the mandrel bar already happens to disappear shortly before rolling at the sizing stand.

DISCLOSURE OF THE INVENTION

As afore-mentioned, since the tube material deformation characteristic in the rolling rolls of the three-roll mandrel mill is significantly distinguished from that of the two-roll mandrel mill, the findings in the two-roll mandrel mill cannot be applied as it is. Consequently, the methods proposed up to date cannot effectively solve the problem in the three-roll mandrel mill such that the withdrawal failure of the mandrel bar after rolling and the scratch defects on the inner surface of the tube material by the mandrel bar occur, thus the difficult situation that the three-roll mandrel mill cannot be put into practice still continues.
The present invention is made to solve this problem encountered in the prior art, and the object thereof is to provide a method of manufacturing seamless tubes by a three-roll mandrel mill, wherein the withdrawal failure of the mandrel bar as well as the generation of the scratch defects on the inner surface of the tube material can be suppressed effectively to thereby enable the production run by said three-roll mandrel mill.
To accomplish the above object, the present invention provides a method of manufacturing seamless tubes by a three-roll mandrel mill, wherein the ratio of the inner circumferential length of the tube material to the outer circumferenctial length of the mandrel bar is set in the range of 1.07 to 1.17 in the last two stands, that constitute said mandrel mill, in which the tube material is subjected to a wall thickness reduction process.
It is preferable that said ratio of the inner circumferential length of the tube material to the outer circumferenctial length of the mandrel bar is set in the range of 1.10 to 1.17.
Further, the ratio of the outside diameter of the tube material at the exit of said mandrel mill to the groove bottom diameter of the rolling roll in the last two stands is preferably set to be not greater than 0.25.
And further, the ratio of the outside diameter of the tube material at the exit of said mandrel mill to the groove bottom diameter of the rolling roll in the last two stands is more preferably set to be not greater than 0.20.
In addition, the ratio of the bar clearance at the entrance of said mandrel mill to the inside diameter of the tube material at the entrance of said mandrel mill is preferably set in the range of 0.04 to 0.12.
And again, the ratio of the bar clearance at the entrance of said mandrel mill to the inside diameter of the tube material at the entrance of said mandrel mill is more preferably set in the range of 0.06 to 0.12.
Also, it is preferable that a sizing stand is disposed behind the last two stands, wherein the reduction rate at the groove bottom of the rolling roll at said sizing stand is not less than 5%.
The present invention provides a method of manufacturing seamless tubes by a three-roll mandrel mill, wherein the ratio of the inner circumferential length of the tube material to the outer circumferential length of the mandrel bar is set within the range of 1.07 to 1.17 in the last two stands, that constitute said mandrel mill, in which the tube material is subjected to a wall thickness reduction process, and wherein the unsuccessful withdrawal of the mandrel bar as well as the inner surface scratches of the tube material by the mandrel bar can be effectively suppressed without excessively increasing the wall thickness difference in the circumferential direction (wall thickness eccentricity) after rolling, whereby a production run by said three-roll mandrel mill can be practically performed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing how the ratio of an inner circumferential length of the tube material to an outer circumferential length of the mandrel bar in the last two stands affects the following parameters, whereas (a) shows the influence on the withdrawability of the mandrel bar, (b) shows the influence on the inner surface quality of the tube material, and (c) shows the influence on the wall thickness eccentricity (wall thickness difference in the circumferential direction after rolling).

FIG. 2 is a diagram explaining the inner circumferential length of the tube material, whereas (a) shows a layout of each rolling roll, and (b) shows an enlarged view of the area enclosed by a broken line in the above (a).
FIG. 3 is a diagram explaining how the groove bottom diameter of the rolling roll, as defined in the method of manufacturing seamless steel tubes according to the present invention, affects the tube material.
FIG. 4 is a diagram showing how the ratio of an outside diameter of the tube material at the exit of said mandrel mill to the groove bottom diameter of the rolling roll in the last two stands affects the following parameters, whereas (a) shows an influence on the with draw ability of the mandrel bar, (b) shows an influence on the inner surface quality of the tube material, and (c) shows an influence on the wall thickness eccentricity of the tube material.
FIG. 5 is a diagram showing how the ratio of a bar clearance at the entrance of said mandrel mill to an inside diameter of the tube material at the entrance of said mandrel mill affects the following parameters, whereas (a) shows an influence on the withdrawability of the mandrel bar, (b) shows an influence on the inner surface quality of the tube material, and (c) shows an influence on the wall thickness eccentricity of the tube material.
FIG. 6 is a diagram showing how the radius reduction rate at the groove bottom of the rolling roll in a sizing stand affects the following parameters, whereas (a) shows an influence on the withdrawability of the mandrel bar, (b) shows an influence on the inner surface quality of the tube material, and (c) shows an influence on the wall thickness eccentricity of the tube material.
FIG. 7 is a diagram explaining said radius reduction rate at the groove bottom of the rolling roll in the sizing stand in the method of manufacturing seamless tubes according to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, one of the embodiments of the present invention is recited while making reference to the accompanying drawings where necessary.
In a method of manufacturing seamless tubes by a mandrel mill, in order to suppress the unsuccessful withdrawal of a mandrel bar after rolling and/or the inner surface scratch defects of the tube material by the mandrel bar, it is necessary to incorporate the effective clearance between the tube material and the mandrel bar after rolling. In this regard, the present inventors discovered that the above parameter greatly depends on what ratio of an inner circumferential length of tube material to an outer circumferential length of the mandrel bar is set at the final wall thickness reduction rolling in the 3-roll mandrel mill.
In other words, it is made clear that the ratio of an inner circumferential length of the tube material to an outer circumferential length of the mandrel bar, being determined by the groove profile of the rolling rolls in the last two stands where the wall thickness reduction rolling is performed, i.e. the last stand and the second stand to the last, greatly affects whether or not said effective clearance can be secured.
Then, the present inventors made an investigation of the influence of the ratio of the inner circumferential length of the tube material to the outer circumferential length of the mandrel bar (hereinafter, referred to as the circumferential length ratio) on the withdrawability of the mandrel bar after rolling in such a way that the rolling test with 10 lengths of the tube material of carbon steel and 9% Cr steel for variance of the circumferential length ratio in the last two stands, in which the wall thickness reduction of the tube material is performed.
FIG. 1 is the diagram showing how the ratio of the inner circumferential length of the tube material to the outer circumferential length of the mandrel bar in last two stands affects the following parameters, whereas (a) shows the influence on the withdrawability of the mandrel bar, (b) shows the influence on the inner surface quality of the tube material, and (c) shows on the wall thickness eccentricity (the wall thickness difference in the circumferential direction after rolling).
In the vertical axis in FIG. 1(a), the Level "2" denotes the succesful withdrawal for both carbon steel and 9% Cr steel (being defined as "excellent" when all of ten lengths of the tube material can be successfully withdrawn), while the Level "1" denotes the successful withdrawal for carbon steel only, and the Level "0" denotes the unsuccessful withdrawal both carbon steel and 9% Cr steel.
And in the vertical axis in FIG. 1(b), the Level "3" denotes no inner surface defect (scratch by the mandrel bar) on the tube material, while the Level "2" denotes that the generation rate of the defect is not greater than 10%, the level "1" denotes the generation rate is above 10% and not greater than 20%, and the Level "0" denotes the generation rate is greater than 20%, respectively.
Further in the vertical axis in FIG. 1 (c), the Level "2" denotes that the rate of wall thickness eccentricity of the tube material is less than 15%, while the Level "1" denotes the case of 15% or above.
As seen from FIGs. 1(a) and (b), it turns out that, by setting the circumferential length ratio to be not less than 1.07, the clearance between the tube material and the mandrel bar can be secured to thereby lead up to obtaining comparatively satisfactory results for the bar withdrawability as well as the inner surface quality. However, as seen from FIG.1(c), it reveals that, by setting the circumferential length ratio to be greater than 1.17, the problem that the wall thickness eccentricity gets worse arises, although the fair result for the bar withdrawability as well as the inner surface quality can be obtained (FIGs. (a) and (b)).
Thus, it is found that, by setting the circumferential length ratio to be in the range of 1.07 to 1.17, while the wall thickness difference in the circumferential direction (the wall thickness eccentricity) of the tube material after rolling can be controlled not to be excessive, the unsuccessful withdrawal of the mandrel bar and the generation of the inner surface scratch defects by the mandrel bar can be effectively suppressed. Further, it is preferable that the above circumferential length ratio is set in the range of 1.10 to 1.17 in order to decrease much more the unsuccessful withdrawal of the mandrel bar.
FIG. 2 is the diagram explaining the inner circumferential length of the tube material, whereas (a) shows the layout of each rolling roll, and (b) shows the enlarged view of the area enclosed by a broken line in the above (a). As shown in FIG. 2(b), the above inner circumferential length of the tube material is obtained in such a way that the outer contour (curvature BE') is firstly determined by equally dividing the tube material into six segments in the circumferential direction with respect to the groove profile center C of the groove profile outlined by the curvature from the groove bottom B to the edge E of each rolling roll R, and then the inner contour is derived by compensating the outer contour with the wall thickness t at the groove bottom, thus the above inner circumferential length of the tube material is calculated by summing each inner length of the six contour segments (represented by curvature B1E1).
In this regard, in order to determine the outer contour (curvature BE') in light of the groove profile (curvature BE), the arc nearby the edge E, for instance, which is part of the groove profile, is extended so as to intersect with the line CC' that makes 60° in angle with respect to the line BC to thereby result in the intersection point E'.
Further, the intent of compensating the outer contour (curvature BE') with the wall thickness t at the groove bottom is defined that each point constituting the curvature BE' is shifted inwardly by an amount of t in normal direction at the relevant point. Here, the above wall thickness t at the groove bottom is determined by the rolling schedule, and the outer circumferential length of the mandrel bar is calculated from the outside diameter of the mandrel bar that is determined by the rolling schedule.
As recited above, although it is generally possible to effectively suppress the unsuccessful withdrawal of the mandrel bar etc. by setting the circumferential length ratio within the predetermined range, there likely occurs the case that the tube material does not follow the roll groove profile and sticks to the mandrel bar instead depending on the material grade of the tube material and/or the rolling parameters, thereby resulting in the reduction of the withdrawability and/or the generation of the scratch defects by the mandrel bar.
The present inventors focused attention on the fact that, based on various investigations for preventing the reduction of the with draw ability, etc. by incorporating circumferentially outstretched deformation of the tube material so as to follow the groove profile contour of the rolling roll as much as possible, when the groove bottom diameter of the rolling roll, i.e. the line segment BB' that is given by connecting the groove bottom B with the opposing groove bottom B' in the above FIG. 2(a), is set large compared to the outside diameter of the tube material at the exit of the mandrel mill, the contact length between the groove bottom region and the tube material becomes longer.
FIG. 3 is the diagram explaining how the groove bottom diameter of the rolling roll affects the tube material for the above case. As seen from FIG. 3, when the groove bottom diameter of the rolling roll is set large compared to the outside diameter of the tube material at the exit of mandrel mill, the contact length between the groove bottom region and the tube material becomes longer. Consequently, the deformation resistance in the reduction rolling direction increases, thus promoting the deformation toward the flange portion of the rolling roll, whereby the circumferentially outstretched deformation following the groove profile contour is promoted.
And then, the present inventors performed rolling test using ten lengths of the tube material in each for carbon steel and 9% Cr steel for variance of the ratio of the outside diameter of the tube material at the exit of the mandrel mill to the groove bottom diameter of the rolling roll in the last two stands (hereinafter, referred to as "finish-diameter/roll-groove-bottom-diameter" where necessary) and investigated the influence of the "finish-diameter/roll-groove-bottom-diameter" on the withdrawability of the mandrel bar and the like after rolling.
FIG. 4 is the diagram showing how the ratio of the outside diameter of the tube material at the exit of the mandrel mill to the groove bottom diameter of the rolling roll in the last two stands affects the following parameters, whereas (a) shows an influence on the withdrawability of the mandrel bar, (b) shows an influence on the inner surface quality of the tube material, and (c) shows an influence on the wall thickness eccentricity of the tube material.
As the Level in each vertical axis in FIGs. 4(a) - (c) exactly means the same extent as the case of the above FIG. 1, the explanation is omitted. In this rolling test, the above circumferential length ratio is set to be 1.07 which is within the above predetermined range, and the ratio of the bar clearance at the entrance of the mandrel mill to the inside diameter of the tube material at the entrance of the mandrel mill, which is described later on, is set to be 0.04.
As shown in FIGs. 4(a) and (b), it turns out that, by setting the "finish-diameter/roll-groove-bottom-diameter" to be not greater than 0.25 (more preferably not greater than 0.20), a comparatively fair result for the withdrawability of the mandrel bar as well as the inner surface quality can be obtained. However, when the "finish-diameter/roll-groove-bottom-diameter" is set to be less than 0.1, the roll becomes too big which is not practical, although a good result for the withdrawability of the mandrel bar as well as the inner surface quality can be obtained. Thus, the "finish-diameter/roll-groove-bottom-diameter" is preferably set to be in the range of 0.1 to 0.25, more preferably in the range of 0.1 to 0.2. Herein, the above tube material diameter at the exit of the mandrel bar is determined by the rolling schedule.
Further, the present inventors thought that, by adjusting the ratio of the bar clearance at the entrance of the mandrel mill to the inside diameter of the tube material at the entrance of the mandrel mill, it becomes also possible to subject the tube material to the circumferentially outstretched deformation to follow the groove profile contour of the rolling roll, thereby enabling the reduction of the withdrawability to be prevented. And, the present inventors performed the rolling test using ten lengths of the tube material in each for carbon steel and 9% Cr steel for variance of the ratio of the bar clearance at the entrance of the mandrel mill to the inside diameter of the tube material at the entrance of mandrel mill (hereinafter, referred to "entrance-bar-clearance/finish-inside-diameter" where necessary) and investigated the influence of "entrance-bar-clearance/finish-inside-diameter" on the withdrawability of the mandrel bar and the like after rolling.
FIG. 5 is the diagram showing how the ratio of the bar clearance at the entrance of said mandrel mill to the inside diameter of the tube material at the entrance of said mandrel mill affects the following parameters, whereas (a) shows an influence on the withdrawability of the mandrel bar, (b) shows an influence on the inner surface quality of the tube material, and (c) shows an influence on the wall thickness eccentricity of tube material.
As the "Level number" in each vertical axis in FIGs. 5(a) - (c) exactly means the same extent as the case of the above FIG. 1, the explanation is omitted. In this rolling test, the circumferential length ratio concerned is set to be 1.07 that is within the proper range and the "finish-diameter/roll-groove-bottom-diameter" is set to be 0.25 that is within the proper range.
As shown in FIGs. 5(a) and (b), it turns out that, by setting the "entrance-bar-clearance/fin.ish-inside-diameter" to be not less than 0.04 (more preferably, not less than 0.06), a comparatively fair result for the withdrawability of the mandrel bar as well as the inner surface quality can be obtained. However, when the "entrance-bar-clearance/finish-inside-diameter" is set to be more than 0.12, the outer circumferential length of the tube material becomes too big, which likely results in excessive overfill to thereby disable the rolling operation, whereby this is not practical.
Therefore, the "entrance-bar-clearance/finish-inside-diameter" is preferably set to be in the range of 0.04 to 0.12, and more preferably in the range of 0.06 to 0.12. Herein, the above bar clearance is defined to be obtained by the equation (the inside diameter of the tube material at the entrance of the mandrel mill - the outside diameter of the mandrel mill), that is determined by the rolling schedule.
Further, the present inventors thought that, by disposing the sizing stand behind the last two stands where the wall thickness reduction rolling is carried out and by configuring an adequate reduction rolling to be equal or greater than the predetermined radius reduction rate at the groove bottom of the rolling rolls at the sizing stand, the tube material likely deforms towards the flange to generate the effective clearance to thereby enhance the with drawability, etc.
And, the present inventors performed the rolling test using ten lengths of the tube material in each for carbon steel and 9% Cr steel for variance of the radius reduction rate at the groove bottom of the rolling rolls at the sizing stand, and investigated the influence of the radius reduction rate on the withdrawability of the mandrel bar and the like after rolling.
FIG. 6 is a diagram showing how the radius reduction rate at the groove bottom of the rolling roll in the sizing stand affects the following parameters, whereas (a) shows an influence on the withdrawability of the mandrel bar, (b) shows an influence on the inner surface quality of the tube material, and (c) shows an influence on the wall thickness eccentricity of the tube material.
As the "Level number" in each vertical axis in FIGs. 6(a) - (c) exactly means the same extent as the case of the above FIG. 1, the explanation is omitted. In this rolling test, the above circumferential length ratio is set to be 1.07 that is within the proper range, the above "finish-diameter/roll-groove-bottom-diameter" is set to be 0.25 that is within the proper range, and the above "entrance-bar-clearancelfinish-inside-diameter" is set to be 0.04 that is also within the proper range.
As shown in FIGs. 6(a) and (b), it turns out that, by setting the radius reduction rate to be not less than 5%, a comparatively fair result for the withdrawability of the mandrel bar as well as the inner surface quality can be obtained.
FIG. 7 is the diagram explaining said radius reduction rate at the groove bottom of the rolling roll in the sizing stand in the method of manufacturing seamless tubes according to the present invention. As shown in FIG. 7, said radius reduction rate is defined by the equation (1) below, given that the major radius (corresponding to the line segment CE' in FIG. 2(b)) at the last stand (designated by No. N-1 stand), where the wall thickness reduction rolling is carried out for the tube material, is A_N-1, and the minor radius (corresponding to the line segment BC in FIG. 2(b)) at the sizing stand (designated by No. N stand) is B_N: $Radius reduction rate = (A_{N - 1} / B_{N - 1}) \times 100 (%)$

EXAMPLES

In the followings, by showing EXAMPLES, the feature of the present invention is more clearly delineated. The parameters in EXAMPLES of the method for manufacturing seamless tubes according to the present invention and the evaluation result are listed in Table 1.
By applying the parameters (Inventive Example Nos. 1 - 16 and Comparative Example Nos. 1 - 2) shown in Table 1, the rolling test using ten lengths of the tube material in each for carbon steel and 9% Cr steel is performed. In this rolling test, the mandrel mill that is constituted by five stands (Nos. 4 and 5 stands correspond to the last two stands where the wall thickness reduction process is performed for the tube material) is used for Inventive Example Nos. 1 - 11 and for Comparative Example Nos. 1 - 2, the mandrel mill that is provided with the sizing stand (No. 6 stand) behind the above last two stands is used for Inventive Example Nos. 12 - 16.
Meanwhile, "Exit t/D", the shortened designation in Table 1, means the "wall thickness of tube material/outside diameter" at the exit of the mandrel mill, likewise "Circumferential Length Ratio* Wall Thickness Reduction*Last 2Std" denotes the ratio of the inner circumferential length of the tube material to the outer circumferential length of the mandrel mill at the last two stands where the wall thickness reduction process is performed for the tube material, and "Radius Reduction Rate *Sizing Std*Groove Bottom" denotes the radius reduction rate at the groove bottom of the rolling roll in the sizing stand (No. 6 stand).
In the above rolling test, in view of (1) the withdrawability of the mandrel bar, (2) the inner surface quality of the tube material, and (3) the wall thickness eccentricity of the tube material, the evaluation is made. Herein, as the "Level" which designates the extent of the evaluation result in Table 1 for each of the withdrawability of the bar, the inner surface quality of the tube material, and the wall thickness eccentricity of the tube material exactly designates the same extent as the Level in the vertical axis in the above FIG. 1, the detail explanation is omitted.
As shown in Table 1, the withdrawability as well as the inner surface quality of the tube material on each of Inventive Example Nos. 1 - 16 is scored as Level "3", Level "2", or Level "1", exhibiting that the unsuccessful withdrawal of the mandrel bar and/or the inner surface scratch defects by the mandrel bar after rolling can be effectively controlled according to the present invention.
Further, the wall thickness eccentricity of the tube material is scored as Level "1" for all of them, confirming that the wall thickness eccentricity is of no issue at all. In particular, when the ratio of the inner circumferential length of the tube material to the outer circumferential length of the mandrel bar is set in the range of 1.10 to 1.17 (Inventive Example Nos. 2, 3, 6, 7, 10, 11, 15 and 16), the withdrawability is scored as Level "2", exhibiting remarkable effect is obtained.
Besides, by comparing the Inventive Example No. 4 with the Inventive Example No. 5, or by comparing the Inventive Example No. 6 with the Inventive Example No. 7, it reveals that, when the ratio of the outside diameter of the tube material at the exit of the mandrel mill to the roll-groove-bottom-diameter at the last two stands is set to be 0.2, the withdrawability and/or the inner surface quality of tube material are improved.
Further, by comparing the Inventive Example No. 8 with the Inventive Example No. 9, or by comparing the Inventive Example No. 10 with the Inventive Example No. 11, it reveals that, even if the circumferential length ratio as well as the ratio of the outside diameter of the tube material at the exit of the mandrel mill to the roll-groove-bottom-diameter at the last two stands is identical, the withdrawability and/or the inner surface quality of the tube material are improved by setting the ratio of the bar clearance at the entrance of the mandrel mill to the inside diameter of the tube material at the entrance of the mandrel mill to be 0.06.
Further again, by comparing the Inventive Example Nos. 12, 14 and 15 with each other, and by comparing the Inventive Example No. 13 with the Inventive Example No. 16, it reveals that, even if all of the circumferential length ratio, the ratio of the outside diameter of the tube material at the exit of the mandrel mill to the roll-groove-bottom-diameter at the last two stands and the ratio of the bar clearance at the entrance of the mandrel mill to the inside diameter of the tube material at the entrance of the mandrel mill are identical, the withdrawability and/or the inner surface quality of tube material are improved by setting the radius reduction rate at the groove bottom of the rolling roll in the sizing stand to be not less than 5%.
As opposed to the above, the circumferential length ratio in the Comparative Example No. 1 is set to be less than 1.07 (1.05), resulting in poor performance in terms of the withdrawability of the mandrel bar as well as the inner surface quality. Meanwhile, the circumferential length ratio in the Comparative Example No. 2 is set to be more than 1.17 (1.19), showing satisfactory performance in terms of both the withdrawability of the mandrel bar and the inner surface quality. However, the wall thickness eccentricity exhibits 15% or more (Level "0"), whereby this cannot be applied as the method for manufacturing seamless tubes to be final products.

INDUSTRIAL APPLICABILITY

The present invention provides a method of manufacturing seamless tubes by a three-roll mandrel mill, wherein the ratio of the inner circumferential length of the tube material to the outer circumferential length of the mandrel bar is set within the range of 1.07 to 1.17 in the last two stands, amongst whole stands that constitute said mandrel mill, in which the tube material is subjected to a wall thickness reduction process, and wherein the unsuccessful withdrawal of the mandrel bar as well as the inner surface scratch defects of the tube material by the mandrel bar can be effectively suppressed without excessively increasing the wall thickness difference in the circumferential direction (the wall thickness eccentricity) after rolling, whereby a commercial operation by said three-roll mandrel mill can be practically performed. Thus, this can be widely used as the method for manufacturing seamless tubes by a Mannesmann-Mandrel Mill process.

Claims

A method for manufacturing seamless tubes by a three-roll mandrel mill, characterized in that the ratio of the inner circumferential length of the tube material to the outer circumferential length of the mandrel bar is set in the range of 1.07 to 1.17 in the last two stands, that constitute said mandrel mill, in which the tube material is subjected to a wall thickness reduction process.
A method for manufacturing seamless tubes by a three-roll mandrel mill according to claim 1, characterized in that said ratio of the inner circumferential length of the tube material to the outer circumferential length of the mandrel bar is set in the range of 1.10 to 1.17.
A method for manufacturing seamless tubes by a three-roll mandrel mill according to claim 1 or claim 2, characterized in that the ratio of the outside diameter of the tube material at the exit of said mandrel mill to the groove bottom diameter of the rolling roll in said last two stands is set to be not greater than 0.25.
A method for manufacturing seamless tubes by a three-roll mandrel mill according to claim 3, characterized in that the ratio of the outside diameter of the tube material at the exit of said mandrel mill to the groove bottom diameter of the rolling roll in said last two stands is set to be not greater than 0.20.
A method for manufacturing seamless tubes by a three-roll mandrel mill according to any of claims 1 - 4, characterized in that the ratio of the bar clearance at the entrance of said mandrel mill to the inside diameter of the tube material at the entrance of said mandrel mill is set in the range of 0.04 to 0.12.
A method for manufacturing seamless tubes by a three-roll mandrel mill according to claim 5, characterized in that the ratio of the bar clearance at the entrance of said mandrel mill to the inside diameter of the tube material at the entrance of said mandrel mill is set in the range of 0.06 to 0.12.
A method for manufacturing seamless tubes by a three-roll mandrel mill according to any of claims 1 - 6, characterized in that a sizing stand is disposed behind said last two stands, and wherein the radius reduction rate at the groove bottom of rolling roll at said sizing stand is not less than 5%.