EP2366466B1

EP2366466B1 - Rolling control method, rolling control apparatus and control program for a mandrel mill, and a method of manufacturing a seamless tube or pipe

Info

Publication number: EP2366466B1
Application number: EP11165541A
Authority: EP
Inventors: Akihito Yamane; Kenichi Sasaki; Kouji Nakaike
Original assignee: Sumitomo Metal Industries Ltd
Current assignee: Nippon Steel Corp
Priority date: 2004-06-30
Filing date: 2005-06-30
Publication date: 2012-09-12
Anticipated expiration: 2025-06-30
Also published as: CN1980751B; CN1980751A; EP2193855B1; EP2366467A1; EP2193855A1; EP2366467B1; CN101264483A; EP2366468B1; EP1779939B1; EP2366468A1; EP1779939A4; WO2006003975A1; JP2006015353A; EP2366466A1; MXPA06014866A; JP4370572B2; CN101264483B; EP1779939A1

Description

Technical Field

This invention relates to a rolling control method, a rolling control apparatus and a control program for a mandrel mill, which make it possible to accurately roll a pierced blank for the entire length or a portion thereof in the lengthwise directions such as a tube end portion to a desired wall thickness when manufacturing a seamless tube or pipe (hereinafter simply referred to as a seamless tube) using a mandrel mill without deteriorating the surface properties of the resulting rolled blank, as well as a seamless tube manufactured by this rolling control method.
A rolling control method for a mandrel mill in accordance with the preamble of claim 1 is e.g. known from JP-A 6 323 8906 .

Background Art

In the manufacture of seamless tubes using the Mannesmann mandrel mill technique, first, a starting material in the form of a round billet or a rectangular billet is heated to 1200 - 1260° C in a rotary hearth heating furnace. The heated round or rectangular billet is then pierced by a piercing mill having a plug and rolls to prepare a hollow pierced blank. Next, the wall thickness of the hollow pierced blank is reduced to a predetermined value by inserting a mandrel bar into the interior of the pierced blank and subjecting the hollow pierced blank to elongation rolling to form a rolled blank using a mandrel mill which usually includes 5 - 8 rolling stands while gripping the outer surface of the pierced blank with grooved rolls in each stand. Then, the mandrel bar is withdrawn from the rolled blank having a decreased wall thickness, and the rolled blank is passed through a reducing mill for sizing into a predetermined outer diameter, thereby producing a desired product in the form of a seamless tube.
A pair of opposing grooved rolls has conventionally been installed in each rolling stand of a mandrel mill. A 2-roll type mandrel mill in which grooved rolls are alternatingly disposed such that the roll-reducing directions of the rolls changes by 90° between adjoining stands is particularly common. A 4-roll type mandrel mill in which four grooved rolls are provided in each stand with the roll-reducing directions forming angles of 90° is also sometimes used. In addition, a 3-roll type mandrel mill in which three grooved rolls are provided in each stand such that the roll-reducing directions of the rolls form angles of 120° and such that the roll-reducing directions varies by 60° between adj oining stands has also been proposed.
In general, during sizing of a rolled blank having a particularly large wall thickness using a reducing mill, the wall thinning phenomenon in which the wall thickness of the end portions in the axial directions of the rolled blank becomes thinner than the wall thickness of the central portion is known to occur. In order to prevent this wall thinning phenomenon at the end portions from occurring during sizing, Patent Document 1, for example, discloses a rolling control method in which an elongation rolling step using a mandrel mill, which is a step prior to a sizing step, is controlled so that the wall thickness of the end portions of the resulting rolled blank is increased, specifically by setting the gap between the grooved rolls installed in the finishing stand or stands of the mandrel mill such that the wall thickness of the end portions of the rolled blank after passing through the finishing stand or stands of the mandrel mill cancels out the decrease in the wall thickness of the end portions in the axial directions of the rolled blank which takes place in the sizing step.
However, when the present inventors carried out a rolling test on a pierced blank in accordance with the method described in Patent Document 1, they found that sometimes the end portions in the axial directions of the rolled blank could not be accurately given a predetermined wall thickness and the surface properties of the rolled blank deteriorated.
Patent Document 2 describes a method of manufacturing a seamless tube which is accurately rolled to a desired wall thickness by controlling the opening size of the gap between grooved rolls installed in the finishing stand or stands of a mandrel mill in accordance with the actual wall thickness of a rolled blank measured on the exit side of the mandrel mill. Specifically, the wall thickness of a rolled blank on the exit side of a mandrel mill is measured using a thickness gauge installed on the exit side of the mandrel mill, and when the measured value of the wall thickness of the rolled blank on the exit side of the mandrel mill is smaller than a desired wall thickness, the opening size of the gap between the grooved rolls is adjusted so as to increase in accordance with the difference from the desired wall thickness, i.e., the rolling positions (rolling force-acting positions) of grooved rolls are moved outwards with respect to their initial rolling positions, whereby the accuracy of the wall thickness of a seamless tube can be increased.
However, when the present inventors carried out a rolling test on a pierced blank in accordance with the method described in Patent Document 2, they found that there were cases in which the opening size of the gap between the grooved rolls continue to increase endlessly with continuing outwards movement of the grooved rolls endlessly, and a desired wall thickness could not be obtained.

Patent Document 1: JP 06-190406 A1
Patent Document 2: JP 08-71616 A1

Disclosure of the Invention

Problem Which the Invention is to Solve

The present invention was made in order to solve the above-described problems of the prior art, and its object is to provide a rolling control method, a rolling control apparatus and control program which make it possible to accurately perform rolling a pierced blank to a desired wall thickness for the entire length or a portion thereof, such as the end portions, when manufacturing a seamless tube using a mandrel mill, and a seamless tube manufactured by the rolling control method.

Means for Solving the Problem

The present inventors performed diligent research concerning why a pierced blank could not be accurately rolled to a desired wall thickness and concerning the cause of a deterioration in the surface properties of a rolled blank by the method described in Patent Document 1. As a result, they discovered the following phenomenon. Normally, in a finishing stand of a mandrel mill (a stand in which grooved rolls which lastly contact a plurality of portions which have the same circumferential position of a pierced blank are installed), the reduction achieved by the grooved rolls in this stand is set to a small value in order to obtain good surface properties of the resulting rolled blank formed by finish rolling. Under such rolling conditions, when it becomes necessary to increase the opening of the gap between the grooved rolls installed in the finishing stand, namely, when it becomes necessary to move the rolling positions of the grooved rolls outwards, if the rolling positions of the grooved rolls installed in upstream stands located on the upstream side in the directions of transport of the blank being rolled with respect to the finishing stand are not in any way adjusted and the rolling positions initially set based on the pass schedule remain unchanged, the phenomenon known as idle rolling takes place in the bottom portion of grooves in the grooved rolls.
The idle rolling phenomenon which may occur in the bottom portions of the grooves in grooved rolls is a situation in which the wall thickness of a pierced blank in the position opposing the bottom portions of the grooves of grooved rolls on the entrance side of the finishing stand of a mandrel mill (the wall thickness of a pierced blank in the position opposing the bottom portions of the grooves of grooved rolls on the entrance side of a finishing stand means the wall thickness of the pierced blank in the position opposing the flange portions of grooved rolls in the rolling stand one stand upstream of the finishing stand, where the flange portions mean the portions positioned at 90° with respect to the bottom portions of the grooves of grooved rolls in the case of a 2-roll mandrel mill or the portions midway between the bottom portions of the grooves of adjoining grooved rolls in the case of a 3- or higher mandrel mill) becomes smaller than the gap between the mandrel bar and the grooved rolls in the finishing stand so that rolling does not take place in the finishing stand of the mandrel mill.
The reason why the idle rolling phenomenon occurs in the bottom portions of the grooves of grooved rolls will be explained. In the case of rolling of a plate-shaped material, the roll-reducing directions are limited to the directions perpendicular to the plate-shaped material, and if the gap between the rolls installed in an upstream stand is set so as to be at least as large as the gap between the rolls installed in a downstream stand, idle rolling does not occur at the time of reduction using the rolls installed in the downstream stand. In contrast, in a mandrel mill for rolling a tube, in the case of any of the above-described 2-roll type, 3-roll type, or 4-roll type, the positions at which rolling force is applied to the tube, i.e., the positions of the bottom portions of the grooves of grooved rolls, differs between adjoining stands in the circumferential directions of a pierced blank to be rolled, and not only the wall thickness of the portions of the pierced blank opposing the bottom portions of the grooves of the grooved rolls but also the wall thickness of the portions of the pierced blank, which oppose the flange portions of the grooved rolls and to which a rolling force is not directly applied, reduces to a certain extent. Since a rolling force is not directly imparted to the portions of a pierced blank which oppose the flange portions of grooved rolls, it is difficult to control the amount of reduction in wall thickness in these portions of the pierced blank, and the wall thickness of these portions after rolling can only be estimated. Therefore, if the reduction in wall thickness of the portions of a pierced blank opposing the flange portions of grooved rolls becomes larger than estimated, even if the gap between the grooved rolls installed in the upstream stand is set to be at least the gap between the grooved rolls installed in the downstream finishing stand, when the portions of a pierced blank which underwent a large reduction in wall thickness which opposed the flange portions of the grooved rolls in the upstream stand are rolled in the bottom portions of the grooves of the grooved rolls of the finishing stand, the wall thickness in these portions of the pierced blank sometimes becomes smaller than the gap between the mandrel bar and the grooved rolls in the finishing stand. As a result, the idle rolling phenomenon occurs in the bottom portions of the grooves of the grooved rolls.
If such idle rolling phenomenon develops in the bottom portions of the grooves of the grooved rolls in the finishing stand, the pierced blank can no longer be accurately rolled to a desired wall thickness in the finishing stand. In other words, if the wall thicknesses of the portions of the pierced blank opposing the flange portions of the grooved rolls installed in the upstream stand excessively decreases to such an extent that idle rolling occurs in the finishing stand, the wall thickness of the resulting rolled blank on the exit side of the finishing stand ends up becoming smaller than a desired wall thickness. The primary purpose of rolling in the finishing stand of a mandrel mill is to make the inner and outer surfaces of a rolled blank smooth and regular by applying a light rolling force with a small amount of rolling reduction. However, if the idle rolling phenomenon manifests in the bottom portions of the grooves in the finishing stand, there end up being portions which do not undergo rolling force at all in that stand, and as a result, the surface properties of the resulting seamless tube worsen.
Thus, the present inventors discovered that the reason why a pierced blank cannot be accurately rolled to a desired wall thickness by elongation rolling and why the surface properties of the resulting rolled blank worsens resides in the idle rolling phenomenon occurring in the bottom portions of the grooves of the grooved rolls in a finishing stand of a mandrel mill. They performed further diligent investigations concerning a method capable of elongation rolling which does not produce the idle rolling phenomenon in the bottom portions of the grooves of grooved rolls. As a result, they found that when moving the rolling positions of grooved rolls installed in a finishing stand outwards at the time of finish rolling, if not only these grooved rolls but also the grooved rolls installed in the closest upstream stand to the finishing stand having the same roll-reducing directions (as the finishing stand) are similarly moved outwards, the occurrence of the idle rolling phenomenon in the bottom portions of the grooves of grooved rolls in the finishing stand can be prevented. This invention was achieved based on this finding.
The present invention is a rolling control method for a mandrel mill having a plurality of rolling stands including a finishing stand by moving the rolling positions of first grooved rolls installed in the finishing stand outwards when rolling a pierced blank in the finishing stand, characterized in that a thickness gauge for measuring the wall thickness of the resulting rolled blank in the roll-reducing directions at each stand is installed on the exit side of the finishing stand, a previously set target wall thickness for the rolled blank in the finishing stand is compared with the actual wall thickness of the rolled blank measured by the thickness gauge, and when the actual wall thickness is smaller than the target wall thickness, movement of the rolling positions of the first grooved rolls is terminated for the next pierced blank to be rolled.
According to the present invention, a thickness gauge is installed at the exit of a finishing stand, and in the finishing stand a previously set target wall thickness for a rolled blank is compared with the actual wall thickness of the rolled blank measured by the thickness gauge. When the actual wall thickness is smaller than the target wall thickness, there is the possibility that the idle rolling phenomenon is occurring in the bottom portions of the grooves of first grooved rolls installed in this stand. Accordingly, in this case, it is determined that the idle rolling phenomenon is occurring in the bottom portions of the grooves, and movement of the rolling positions of the first grooved rolls is terminated for the next pierced blank to be rolled. Therefore, for the next pierced blank to be rolled, the outward movement of the first grooved rolls is terminated, and since the rolling reduction by the first grooved rolls is not decreased, the occurrence of the idle rolling phenomenon in the bottom portions of the grooves of the first grooved rolls in the finishing stand can be prevented.
In the present invention, the expression "movement of the rolling positions of the first grooved rolls is terminated" includes the following two cases (i) and (ii).

(i) Basically, the amount of outwards movement of the first grooved rolls is a value which is previously set regardless of the actual value measured by the thickness gauge. In this case, "movement of the rolling positions is terminated" means (a) when rolling a portion (e. g., an end portion) of a pierced blank, the rolling positions of the first grooved rolls are maintained at the same positions as when rolling the remaining portion (e.g., the central portion) of the pierced blank, or (b) they are kept at the same rolling positions as the rolling positions for the first grooved rolls for the pierced blank which was rolled this time.
(ii) The amount of outward movement of the first grooved rolls is previously set as mentioned in the above, but the set value can be changed in accordance with the actual wall thickness measured by the thickness gauge (for example, when the actual wall thickness is larger than a target wall thickness, the amount of movement is decreased by the difference), and this value is employed for the next pierced blank to be rolled. In this case, "movement of the rolling positions is terminated" means that the set value is not changed in accordance with the actual wall thickness measured by the thickness gauge, and for the next pierced blank to be rolled, the first grooved rolls are moved outwards by the value set for the pierced blank rolled this time.

In the case of a system in which the set value for the amount of outwards movement of the first grooved rolls is varied in accordance with the actual wall thickness measured by the thickness gauge, when the idle rolling phenomenon occurs in the bottom portions of the grooves of the first grooved rolls and the actual wall thickness becomes smaller than the target wall thickness, there is the possibility of the outwards movement of the grooved rolls endlessly continuing. However, according to the present invention, as described in paragraph (ii), the outward movement of the first grooved rolls is terminated, (changing of the set value in accordance with the actual wall thickness measured by the thickness gauge is terminated), so this problem can be solved.
From another standpoint, the present invention is a rolling control apparatus a rolling control apparatus for a mandrel mill having a plurality of rolling stands including a finishing stand and comprising a rolling position adjusting unit for adjusting the rolling positions of grooved rolls installed in the finishing stand and an arithmetic and control unit which instructs the rolling position adjusting unit as to an amount of adjustment of rolling positions for the grooved rolls, characterized in that the arithmetic and control unit is connected to a thickness gauge which is installed on the exit side of the finishing stand and which measures the wall thickness of the resulting rolled blank in the roll-reducing directions at each stand, and in that it carries out a rolling control method as set forth in claim 1 by instructing the rolling position adjusting unit to terminate adjustment of rolling position depending on the output of the thickness gauge.
From another standpoint, the present invention is a rolling control program for operating an arithmetic and control unit which is connected to a rolling position adjusting unit for adjusting the rolling positions of grooved rolls installed in a finishing stand among a plurality of rolling stands constituting a mandrel mill and to a thickness gauge installed on the exit side of the finishing stand for measuring the wall thickness of a rolled blank in the roll reducing directions at each stand and which instructs the rolling position adjusting unit as to an amount of adjustment of rolling positions for the grooved rolls, characterized in that the arithmetic and control unit is operated so as to carry out a rolling control method as set forth in claim 1 by instructing the rolling position adjusting unit to terminate adjustment of rolling position depending on the output of the thickness gauge.

Effects of the Invention

According to the present invention, when manufacturing a seamless tube using a mandrel mill, it is possible to accurately roll a pierced blank for the eitire length of a portion in the lengthwise directions such as an end portion to a desired wall thickness without worsening the surface conditions or properties of the resulting rolled blank.

Brief Description of the Drawings

Figure 1 is a block diagram schematically showing the structure of a rolling control apparatus for carrying out a rolling control method for a mandrel mill not in accordance with the present invention.
Figure 2 is a block diagram schematically showing the structure of a rolling control apparatus for carrying out a rolling control method for a mandrel mill according to a first embodiment.
Figure 3 is a schematic diagram showing one example of a pattern of movement of the rolling positions of grooved rolls installed in a finishing stand.

Best Mode for Carrying Out the Invention

The best mode for carrying out the present invention will be explained while referring to the accompanying drawings. The following explanation will be directed to an embodiment, which is given as an example, in which the rolling positions of grooved rolls installed in a finishing stand are moved outwards when rolling the end portions of a pierced blank in a finishing stand of a 2-roll type mandrel mill. However, this is a mere example of the present invention, and the present, invention can be similarly applied to an embodiment in which the rolling positions of grooved rolls installed in a finishing stand are moved outwards over the entire length of a pierced blank or an embodiment using a mandrel mill other than a 2-roll type,
Figure 1 is block diagram schematically showing the structure of a rolling control apparatus for carrying out a rolling control method for a mandrel mill.
As shown in Figure 1, a rolling control apparatus 1 has a first rolling position adjusting unit 11, a second rolling position adjusting unit 12, and an arithmetic and control unit 13.
The first rolling position adjusting unit 11 is constituted by a cylinder or similar device for adjusting the rolling positions of grooved rolls installed in a finishing stand #i among a plurality of stands constituting a mandrel mill M. The second rolling position adjusting unit 12 is constituted by a cylinder or similar device for adjusting the rolling positions of grooved rolls installed in the closest upstream stand to the finishing stand #i in the mill having the same roll-reducing directions (as the finishing stand), i.e., in stand #(i-2) positioned just two stands upstream of the finishing stand #i. The arithmetic and control unit 13 is connected to the first rolling position adjusting unit 11 and the second rolling position adjusting unit 12 and is constructed so as to instruct the first rolling position adjusting unit 11 and the second rolling position adjusting unit 12 as to an appropriate amount of adjustment of rolling positions based on the current location of the end portions of a pierced blank S being rolled.
Each of the first rolling position adjusting unit 11 and the second rolling position adjusting unit 12 have a mechanism which can move the rolling positions of the grooved rolls installed in the stand during rolling in accordance with the portion (the leading end portion, the central portion, or the trailing end portion) of a single pierced blank S which is being rolled in that stand.
The arithmetic and control unit 13 is constituted by a computer equipped with hardware such as a CPU, a memory, an external storage device, and an external input/output interface. By suitably operating this hardware in accordance with a control program stored therein, it functions as a first rolling position setting part 131 and a second rolling position setting part 132.
An end portion sensing signal which indicates that an end portion of the pierced blank S was detected, for example, by a sensor (not shown) installed on the entrance side of the mandrel mill M is input to the first rolling position setting part 131. In addition, the distance between the sensor and stand #i, the traveling speed of the pierced blank S, the elongation percentage of the pierced blank S in the mandrel mill M and similar parameters are input to this part from an upper level process computer (not shown), for example. Based on the signals and data which were input, the first rolling position setting part 131 calculates the current position of the end portions of the pierced blank S. Specifically, it calculates the timing with which the end portions (leading end portion and trailing end portion) of the pierced blank S reach and leave the finishing stand #i.
Based on the timing which is calculated in this manner, the first rolling position setting part 131 determines an amount of adjustment of rolling positions for the first grooved rolls installed in the finishing stand #i, and it transmits the amount of adjustment of rolling positions which was determined to the first rolling position adjusting unit 11. Specifically, the rolling positions A of the first grooved rolls at the time of rolling of the end portions of the pierced blank S in the finishing stand #i and the rolling positions B of the first grooved rolls at the time of rolling the central portion of the pierced blank S are stored in the first rolling position setting part 131. The first rolling position setting part 131 sets the difference (A - B) as the amount of adjustment of rolling positions so that the first grooved rolls will move from rolling positions B to rolling positions A by the time that the leading end portion of the pierced blank S reaches the finishing stand #i, and it transmits this value to the first rolling position adjusting unit 11.
In the same manner, the first rolling position setting part 131 sets the difference (B - A) as the amount of adjustment of rolling positions so that the first grooved rolls will move from rolling positions A to rolling positions B as soon as the leading end portion of the pierced blank S leaves the finishing stand #i, and it transmits this value to the first rolling position adjusting unit 11. Thereafter, it again sets the difference (A - B) as the amount of adjustment of rolling positions when the trailing end portion of the pierced blank S reaches the finishing stand #1 and it transmits this value to the first rolling position adjusting unit 11.
In addition, it sets the difference (B - A) as the amount of adjustment of rolling positions so that the first grooved rolls will move from rolling positions A to rolling positions B in the period from when the trailing end portion of the pierced blank S leaves the finishing stand #1 until the leading end portion of the next pierced blank S to be rolled reaches the finishing stand #1, and it transmits this value to the first rolling position adjusting unit 11.
Therefore, the rolling positions of the first grooved rolls installed in the finishing stand #i can be moved outwards, i.e., to rolling positions A, when the end portions of a pierced blank S are rolled in the finishing stand #i. Rolling positions A and B vary depending on the dimensions, the wall thickness., the material, and the like of the pierced blank S being rolled, so a plurality of combinations of rolling positions A and B corresponding to various dimensions, wall thickness, material, and the like are stored in the first rolling position adjusting unit 11, and appropriate rolling positions A and B are selected in accordance with the dimensions, wall thickness, material, and the like of the pierced blank S which are input from the upper level process computer, for example.
In the same manner as for the first rolling position setting part 131, end portion sensing signals and the like are input from the outside to the second rolling position setting part 132, and the timing with which the end portions (leading end portion and trailing end portion) of the pierced blank S reach and leave the upstream stand #(i-2) is calculated.
The second rolling position setting part 132 sets the amount of adjustment of rolling positions for the second grooved rolls installed in the upstream stand #(i-2) based on the calculated timing in the same manner as for the first rolling position setting part 131 and transmits the amount of adjustment of rolling positions which was set to the second rolling position adjusting unit 12. The amount of adjustment of rolling positions which is transmitted to the second rolling position adjusting unit 12, i.e., an amount corresponding to the amount of movement when moving the rolling positions of the second grooved rolls installed in the upstream stand #(i-2) outwards when rolling the end portions of the pierced blank S in the upstream stand #(i-2) need not be set to the same value as the amount of adjustment of rolling positions transmitted to the first rolling position adjusting unit 11. For example, it can be set to a value resulting from multiplying by a predetermined coefficient greater than 0 and smaller than 1 (such as 0.8).
In a rolling control apparatus 1, when rolling the end portions of a pierced blank S not only in the finishing stand #i but also in the upstream stand #(i-2), the rolling positions of the second grooved rolls installed in the upstream stand #(i-2) are moved outwards. Therefore, the rolling reduction in the bottom portions of the grooves when rolling the pierced blank S in the second grooved rolls is decreased, and an excessive decrease in the wall thickness of the portions opposing the flange portions of the grooved rolls installed in the next stand #(i-1) is eliminated. Therefore, the occurrence of the idle rolling phenomenon in the bottom portions of the grooves at the time of rolling with the first grooved rolls installed in the finishing stand #i is prevented. Accordingly, the end portions of a pierced blank S can be accurately rolled to a desired wall thickness, and a deterioration in the surface properties of the resulting rolled blank S can be prevented.
An example is given of the case in which a mechanism for moving the rolling positions in the course of rolling of a single pierced blank S is provided in not only the first rolling position adjusting unit 11 but also in the second rolling position adjusting unit 12. However, some mandrel mills M have such a mechanism only in the finishing stand #i. Therefore, in such mandrel mills, instead of moving the rolling positions in accordance with the position of the pierced blank S during rolling, the rolling positions of the grooved rolls of upstream stand #(i-2) can be previously moved outwards prior to rolling the pierced blank S.
The second rolling position setting part 132 sets the amount of adjustment of rolling positions, namely, the amount of adjustment necessary to modify the rolling positions to the previously stored outwards positions when the signal indicating that the leading end portion of the pierced blank S has been sensed is input from the exterior, and it transmits the set amount of adjustment of rolling positions to the second rolling position adjusting unit 12. The second rolling position adjusting unit 12 acts to move the rolling positions of the second grooved rolls outwards based on the transmitted amount of adjustment of rolling positions. The modified rolling positions are maintained at the same value throughout rolling of the pierced blank S.

First embodiment

Figure 2 is a block diagram schematically showing the structure of a rolling control apparatus for carrying out a rolling control method for a mandrel mill according to a first embodiment.
As shown in Figure 2, a rolling control apparatus 2 according to this embodiment has a rolling position adjusting unit 21 and an arithmetic and control unit 22.
The rolling position adjusting unit 21 has a mechanism which is constituted by a cylinder or similar device for adjusting the rolling positions of grooved rolls installed in the finishing stand #i among a plurality of rolling stands constituting a mandrel mill M and which can move the rolling positions of the grooved rolls in accordance with the portion being rolled (leading end portion, central portion, or trailing end portion) of a single pierced blank S during rolling.
The arithmetic and control unit 22 is constituted by a computer having hardware such as a CPU in the same manner as in the first embodiment, but it differs from the example of Figure 1 in that it is connected to a thickness gauge I which measures the wall thickness of the resulting rolled blank S in the roll-reducing directions at each stand (for example, the wall thickness in 4 directions when the mandrel mill M is a 2-roll type) and which is installed on the exit side of the finishing stand #i (in this embodiment, on the exit side of the mandrel mill M).
The arithmetic and control unit 22 suitably drives hardware in accordance with a control program stored therein, whereby it functions as a rolling position setting part 221 and an idle rolling determining part 222. Depending on the output of the thickness gauge I, it instructs the rolling position adjusting unit 21 to stop adjustment of the rolling positions. In the following paragraph, it will be explained more concretely.
In the same manner as with the first rolling position setting part 131 described with respect to the example of Figure 1, an end portion sensing signal for the pierced blank S and the like are input to the rolling position setting part 221, and it calculates the timing when the end portions (the leading end portion and the trailing end portion) of the pierced blank S reach and leave the finishing stand #i. In addition, in the same manner as in the example of Figure 1, the rolling position setting part 221 stores the rolling positions of the grooved rolls when rolling the end portions and the central portion of the pierced blank S in the finishing stand #i. The rolling position setting part 221 sets an amount of adjustment of rolling positions for the grooved rolls installed in the finishing stand #i based on the calculated timing and the stored rolling positions of the grooved rolls, and it transmits the set amount of adjustment of rolling positions to the rolling position adjusting unit 21. As a result, when the end portions of the pierced blank S are rolled in the finishing stand #i, the rolling positions of the grooved rolls installed in the finishing stand #i can be moved outwards by an amount corresponding to the amount of adjustment of rolling positions.
The output of the thickness gauge I (the actual wall thickness of the end portions of the rolled blank S) is input to the idle rolling determining part 222. The target wall thickness of the end portions of the rolled blank S is previously stored in the idle rolling determining part 222. The idle rolling determining part 222 compares the target wall thickness and the actual wall thickness of the rolled blank S, and when the actual wall thickness is smaller than the target wall thickness, namely, when at least one values of the actual wall thickness measured in the roll-reducing directions for the finishing stand #i is smaller than the target wall thickness, it is determined that the idle rolling phenomenon is occurring in the bottom portions of the grooves, and in other cases, it is determined that the phenomenon is not occurring. The target wall thickness of the end portions of the rolled blank S is a value which varies in accordance with the dimensions of the pierced blank S being rolled, the target wall thickness of the central portion, the material of the pierced blank, and the like, so a plurality of target wall thicknesses corresponding to the dimensions, the wall thickness of the central portion, the material, and the like are stored in the first rolling position adjusting unit 11. For example, the target wall thickness is suitably selected in accordance with the dimensions of the pierced blank S, the wall thickness of the central portion, the material, and the like input from an upper level process computer.
When the idle rolling determining part 222 determines that the idle rolling phenomenon is occurring in the bottom portions of the grooves of the grooved rolls, the rolling position setting part 221 issues an instruction to terminate outwards movement of the rolling positions of the grooved rolls for the next pierced blank S to be rolled. Namely, when the end portions of the pierced blank S are rolled, it transmits an instruction to the rolling position adjusting unit 21 to maintain the rolling positions the same as when rolling the central portion of the pierced blank S. Specifically, it sets the amount of adjustment of rolling positions when rolling the end portions of the pierced blank S to zero, and it transmits this value to the rolling position adjusting unit 21.
On the other hand, when the idle rolling determining part 222 determines that the idle rolling phenomenon is not occurring in the bottom portions of the grooves, for the next pierced blank S to be rolled as well, amounts of adjustment of rolling positions are set in accordance with the previously stored rolling positions of the grooved rolls when rolling the end portions and the central portion of the pierced blank S, and these values are transmitted to the rolling position adjusting unit 21.
With a rolling control apparatus 2 according to this embodiment, movement of the grooved rolls outwards can be terminated for the next pierced blank S to be rolled so that the rolling reduction by the grooved rolls is not decreased, and the occurrence of the idle rolling phenomenon in the bottom portions of the grooves in the finishing stand #i can be prevented. As a result, the end portions of the pierced blank S can be accurately rolled to a desired wall thickness, and worsening of the surface properties of the rolled blank S can be prevented.
In the above explanation, an example is given of a mode in which the amount of outwards movement of the grooved rolls for the next pierced blank S when the idle rolling phenomenon did not occur in the bottom portions of the grooves is made a previously set value regardless of the actual wall thickness of the end portions of the rolled blank S measured by the thickness gauge I, i.e., a mode in which when the idle rolling determining part 222 determines that the idle rolling phenomenon did not take place in the bottom portions of the grooves, for the next pierced blank S to be rolled, the amount of adjustment of rolling positions in the finishing stand is set in accordance with previously stored rolling positions for the grooved rolls of this stand when rolling the end portions and the central portion of the pierced blank S, and this value is transmitted to the rolling position adjusting unit 21.

Reference Example 1

The present invention will be characterized still clearer by the following examples and comparative examples.
A rolling control method was applied to as 2-roll type mandrel mill having 6 stands (in which the finishing stands were the #5 and #6 stands), and a rolling test was carried out under the following conditions.

(1) Dimensions of the pierced blank at the entrance of the mill: outer diameter of 190 mm, wall thickness of 16 mm, length of 4600 mm
(2) Dimensions of a rolled blank at the exit of the mill: outer diameter of 168 mm, wall thickness of 5 mm, length of 15000 mm
(3) Outer diameter of mandrel bar: 158 mm
(4) Initially set target wall thickness at the exit of each stand (wall thickness at the bottom portions of the grooves of the grooved rolls) based on the pass schedule:
- # 1 stand: 10 mm,
- #2 stand: 9 mm,
- #3 stand: 6 mm,
- #4 stand: 5.5 mm,
- #5 stand: 5 mm,
- #6 stand: 5 mm

Reference Example 1-1 (not in the scope of the invention)

In the #5 and #6 stands, when rolling the end portions of the pierced blank, the rolling positions of the grooved rolls were moved outwards according to the pattern shown in Figure 3. In the #3 and #4 stands as well, rolling was carried out with the rolling positions of the grooved rolls moved outwards according to the same pattern. In the #1 and #2 stands, rolling was carried out with the initially set rolling positions.

Reference Example 1-2 (not in the scope of the invention)

In the #5 and #6 stands, rolling was carried out with the rolling positions of the grooved rolls moved outwards according to the pattern shown in Figure 3 in the same manner as in Reference Example 1-1. In the #3 and #4 stands, rolling was carried out with the amount of movement reduced to 0.8 times that of the pattern shown in Figure 3 (namely, to a maximum of 0.4 mm). In the #1 and #2 stands, rolling was carried out with the initially set rolling positions.

Reference Example 1-3 (not in the scope of the invention)

In the #5 and #6 stands, in the same manner as in Reference Example 1-1, rolling was carried out with the rolling positions of the grooved rolls moved outwards according to the pattern shown in Figure 3. In the #3 and #4 stands, before rolling the pierced blank, the rolling positions of the grooved rolls were moved outwards so that the target wall thicknesses became 6.5 mm (for the #3 stand) and 6 mm (for the #4 stand), and then rolling was carried out. In the # 1 and #2 stands, rolling was carried out with the initially set rolling positions.

Comparative Example 1

For only the #5 and #6 stands, when rolling the end portions of the pierced blank, rolling was carried out with the rolling positions of the grooved rolls moved outwards according to the, pattern shown in Figure 3. In the #1 - #4 stands, rolling was carried out with the initially set rolling positions.

Evaluation

The rate of wall thickness defects in the resulting rolled blank was evaluated for Reference Examples 1-1 through 1-3 and Comparative Example 1. When there is a location of measurement at the end portions of the, after rolling where the difference between the measured wall thickness and the target value of the wall thickness (for example, in the case of Example 1, the target value of the wall thickness of the end portions is 5.5 mm) exceeds ± 2% of the target value of the wall thickness, it is determined that there is a wall thickness defect. The rate of wall thickness defects is defined by the following equation: $Rate of wall thickness defects (%) = [(number of tubes having a wall Thickness defect) / (total number of rolled tubes)] x 100.$
The results of evaluation are compiled in Table 1. Table 1

Rate of wall Thickness defects

Reference Example 1-1 0%

Reference Example 1-2 0%

Reference Example 1-3 0%

Comparative Example 1 10%
As shown in Table 1, it can be seen that in contrast to Comparative Example 1, the end portions of the pierced blanks for all of Reference Examples 1-1 through 1-3 could be accurately rolled to a desired wall thickness.

Example 2

Example 2-1

In the #5 and #6 stands, when rolling the end portions of the next pierced blank, the pattern shown in Figure 3 was modified based on the actual wall thickness of the rolled blank which was rolled lastly measured by the thickness gauge, and the rolling positions of the grooved rolls were moved outwards based on the modified pattern. Namely, in order to make the actual wall thickness approach the target wall thickness, the set value (amount of outwards movement of rolling positions) was modified by an amount corresponding to the difference between the measured wall thickness and the target wall thickness. However, when the actual wall thickness was smaller than target wall thickness, it was determined that the idle rolling phenomenon was occurring in the bottom portions of the grooves, modification of the set value in accordance with the actual wall thickness (the above-described modification of the pattern) was terminated, and rolling of the next pierced blank to be rolled was carried out with the currently set values. In the #1 - #4 stands, rolling was carried out with the initially set rolling positions.

Reference Example 2-2 (not in the scope of the invention)

In the #5 and #6 stands, rolling was carried out in the same manner as in Example 2-1. It was determined whether there was occurrence of the idle rolling phenomenon in the bottom portions of the grooves by comparing the amount of movement of the rolling positions of the grooved rolls and the amount of change of the actual wall thickness measured by the thickness gauge of the rolled blank in the directions of movement of the grooved rolls, and if the amount of change was not greater than 0.5 times the amount of movement, it was determined that the idle rolling phenomenon was occurring in the bottom portions of the grooves.

Reference Example 2-3 (not in the scope of the invention)

In the #5 and #6 stands, when rolling the end portions of the next pierced blank to be rolled, the pattern shown in Figure 3 was modified based on the actual wall thickness measured by the thickness gauge, and rolling was carried out by moving the rolling positions of the grooved rolls outwards based on the modified pattern. In the #3 and #4 stands as well, rolling was carried out with the rolling positions of the grooved rolls moved outwards using the same pattern as for the #5 and #6 stands. In the #1 and #2 stands, rolling was carried out with the initially set rolling positions.

Reference Example 2-4 (not in the scope of the invention)

In the #5 and #6 stands, rolling was carried out in the same manner as in Reference Example 2-3. In the #3 and #4 stands, rolling was carried out with the amount of movement decreased to 0.8 times that of the pattern for the #5 and #6 stands. In the #1 and #2 stands, rolling was carried out with the initially set rolling positions.

Reference Example 2-5 (not in the scope of the invention)

In the #5 and #6 stands, rolling was carried out in the same manner as in Reference Example 2-3. In the #3 and #4 stands, the rolling positions of the grooved rolls were moved outwards prior to rolling the pierced blank so that the target wall thicknesses became 6.5 mm (#3 stand) and 6 mm (#4 stand), and then rolling was carried out. In the #1 and #2 stands, rolling was carried out with the initially set rolling positions.

Reference Example 2-6 (not in the scope of the invention)

In the #5 and #6 stands, rolling was carried out in the same manner as in Reference Example 2-3. When the actual wall thickness was smaller than the target wall thickness, it was determined that the idle rolling phenomenon was occurring in the bottom portions of the grooves, and for the closest upstream stands to the finishing stands (#5 and #6 stands) having roll-reducing directions which are the same as the directions of wall thickness measurement for which the actual wall thickness was smaller than the target wall thickness (at least one of the #3 and #4 stands), rolling was carried out with the rolling positions of the grooved rolls moved outwards according to the same pattern as for the #5 and #6 stands. The #1 and #2 stands and one of the #3 and #4 stands, if any, for which the rolling positions of the grooved rolls were not moved, rolling was carried out with the initially set rolling positions.

Reference Example 2-7 (not in the scope of the invention)

Rolling was carried out in the same manner as in Reference Example 2-6. However, it was determined whether the idle rolling phenomenon was occurring in the bottom portions of the grooves by comparing the amount of movement of the rolling positions of the grooved rolls with the amount of change of the actual wall thickness of the resulting rolled blank measured by the thickness gauge in the directions of movement of the grooved rolls. If the amount of change was at most 0.5 times the amount of movement, it was determined that the idle rolling phenomenon was occurring in the bottom portions of the grooves.

Comparative Example 2

In the #5 and #6 stands only, when rolling the end portions of the next pierced blank to be rolled, the pattern shown in Figure 3 was modified based on the actual wall thickness measured by the thickness gauge, and the rolling positions of the grooved rolls were moved outwards according to the modified pattern. In the #1 - #4 stands, rolling was carried out using the initially set values.

Evaluation

For Example 2-1 and Reference Examples 2-2 through 2-7 and Comparative Example 2, the rate of wall thickness defects in the resulting rolled blanks after: rolling was evaluated. The rate of wall thickness defects was calculated using the above-described equation. The frequency with which the rolling positions of the grooved rolls installed in the #5 and #6 stands which are finishing stands were moved outwards by at least 1 mm from the initially set values was also evaluated. This frequency is an evaluation index corresponding to the frequency with which the grooved rolls in the finishing stands move endlessly outwards when the idle rolling phenomenon is occurring in the bottom portions of the grooves and the actual wall thickness has become smaller than the target wall thickness.

The results of evaluation are shown in Table 2.

Table 2

	Rate of wall Thickness defects	Frequency of movement of rolling positions
Example 2-1	4%	4%
Reference Example 2-2	4%	0%
Reference Example 2-3	0%	0%
Reference Example 2-4	0%	0%
Reference Example 2-5	0%	0%
Reference Example 2-6	1%	0%
Reference Example 2-7	1%	0%
Comparative Example 2	5%	0%

It can be seen from Table 2 that for Example 2-1 and Reference Examples 2-2 through 2-7, compared to Comparative Example2, the end portions of a pierced blank could be accurately rolled to a desired wall thickness. In contrast to Comparative Example 2, endless outward movement of the grooved rolls of the finishing stands was prevented.

IndustrialApplicability

When manufacturing a seamless tube using a mandrel mill, a pierced blank can be accurately rolled to a desired wall thickness for the entire length or a portion thereof such as the end portions.

Claims

A rolling control method for a mandrel mill (M) having a plurality of rolling stands including a finishing stand (#i) by moving the rolling positions of first grooved rolls installed in the finishing stand (#i) outwards when rolling a pierced blank (S) in the finishing stand, characterized in that a thickness gauge (I) for measuring the wall thickness of the resulting rolled blank in the roll-reducing directions at each stand is installed on the exit side of the finishing stand (#i), a previously set target wall thickness for the rolled blank in the finishing stand (#i) is compared with the actual wall thickness of the rolled blank measured by the thickness gauge (I), and when the actual wall thickness is smaller than the target wall thickness, movement of the rolling positions of the first grooved rolls is terminated for the next pierced blank to be rolled.
A rolling control apparatus (1) for a mandrel mill (M) having a plurality of rolling stands including a finishing stand (#i) and comprising a rolling position adjusting unit (21) for adjusting the rolling positions of grooved rolls installed in the finishing stand (#i) and an arithmetic and control unit (22) which instructs the rolling position adjusting unit (21) as to an amount of adjustment of rolling positions for the grooved rolls, characterized in that the arithmetic and control unit (22) is connected to a thickness gauge (I) which is installed on the exit side of the finishing stand (#i) and which measures the wall thickness of the resulting rolled blank in the roll-reducing directions at each stand, and in that it carries out a rolling control method as set forth in claim 1 by instructing the rolling position adjusting unit (21) to terminate adjustment of rolling position depending on the output of the thickness gauge (I).
A rolling control program for operating an arithmetic and control unit (22) which is connected to a rolling position adjusting unit (21) for adjusting the rolling positions of grooved rolls installed in a finishing stand (#i) among a plurality of rolling stands constituting a mandrel mill (M) and to a thickness gauge (I) installed on the exit side of the finishing stand (#i) for measuring the wall thickness of a rolled blank in the roll reducing directions at each stand and which instructs the rolling position adjusting unit (21) as to an amount of adjustment of rolling positions for the grooved rolls, characterized in that the arithmetic and control unit (22) is operated so as to carry out a rolling control method as set forth in claim 1 by instructing the rolling position adjusting unit (21) to terminate adjustment of rolling position depending on the output of the thickness gauge (I).
A method of manufacturing a seamless tube using a mandrel mill (M) which carries out the rolling control method of claim 1.