JP4888252B2 - Seamless pipe cold rolling method - Google Patents

Description

  The present invention relates to a cold rolling method using a seamless pipe as a raw pipe, and more particularly, to manage wall thickness fluctuations such as so-called square tension occurring on the inner surface of a pipe end of a raw pipe used for cold rolling. The present invention relates to a seamless pipe cold rolling method for preventing the occurrence of cracks at the end of the pipe accompanying cold working.

  Usually, cold drawing methods using a draw bench and cold rolling methods using a pilger mill are commonly used as cold working methods for metal tubes. In particular, the cold rolling method using a pilger mill is capable of cold-working a raw pipe at a high workability compared to the cold drawing method. Therefore, in the production of seamless pipes that require a high workability, the pilger mill (Pilgar mill) is generally used. Cold rolling method by rolling) is used.

  FIG. 1 is a perspective view illustrating the entire configuration of a pair of roll dies used in the cold rolling method. In the cold rolling method, a pair of upper and lower roll dies 2 having a hole mold 2 a formed on the peripheral surface is disposed, and a mandrel having a taper whose diameter decreases toward the tip is provided between the roll dies 2. The roll die 2 is supported on the roll stand 4 by a rotating shaft provided at the shaft center. A pinion gear 5 having a rotation diameter substantially equal to the outer diameter of the roll die 2 is provided at one end of the rotation shaft in mesh with the horizontal rack gear 6.

  The roll die 2 reciprocates in the direction of arrow B as the rack gear 6 reciprocates in the direction of arrow A through the pinion gear 5. At this time, the hole mold 2 a formed on the peripheral surface of the roll die 2 rolls a raw tube to be processed as the roll die 2 reciprocates.

  FIG. 2 is a cross-sectional view for explaining a processing mechanism for rolling an element pipe by a cold rolling method. The hole die 2a formed on the peripheral surface of the roll die 2 constitutes a processed portion in which the hole diameter decreases continuously from the start of processing to the end of processing. Between the pair of roll dies 2, a mandrel 3 having a taper whose diameter decreases toward the tip is provided on the inner surface of the pipe. A pair of upper and lower roll dies 2 provided on the outer surface side of the pipe and having a hole mold 2 a formed on the peripheral surface thereof reciprocate along the mandrel 3, thereby rolling the raw tube 1 as a workpiece.

  On the other hand, the raw tube 1 is processed while being reduced in diameter and reduced in thickness while being rotated by a predetermined angle while being fed by a predetermined processing length while the roll die 2 is reciprocatingly rotated. At this time, the cold-rolled blank 1 is drawn according to the rolling elongation and the rolling feed amount, and rolled to a target product size.

  Since the cold rolling method is composed of the rolling mechanism shown in FIGS. 1 and 2 above, a high workability can be applied to the workpiece, and as described above, the high workability is higher than that of the cold drawing method. Can be cold worked. However, when cold rolling is performed using a seamless pipe as a raw pipe, cracks may occur at the end of the pipe on the rolling start side.

  Usually, the Mannesmann tube method is often used for the seamless pipes used as cold-rolling blanks, and the drawing and drawing mills (stretch reducer or sizer) such as piercing and mandrel mill rolling are used. It is manufactured by performing constant diameter rolling. At this time, a wall thickness variation called so-called square tension occurs on the inner surface of the tube after the rolling with a plurality of roll stands.

  For example, in the production of seamless pipes by the Mannesmann pipe manufacturing method, when outer diameter drawing is performed using a three-roll type drawing mill, wall thickness fluctuations occur due to hexagonal inner surface angulation. In this way, the wall thickness variation that occurs on the inner surface of the seamless pipe constitutes a small local dimensional variation and shape variation, and the local thin wall portion due to the wall thickness variation acts as a notch. It is presumed that the cracks start from cracks starting from cracks.

  Conventionally, when performing cold rolling, various measures have been proposed in order to prevent tube end cracks from occurring in the raw pipe accompanying rolling. First, as measures against a seamless pipe used as a raw pipe, Patent Documents 1 and 2 propose a drawing mill and a wall thickness control method for suppressing angular tension that causes pipe end cracking.

  In patent document 1, for a drawing rolling mill in which a roll stand including three rolling rolls is arranged in a plurality of stages along the rolling force direction, a pair of roll stands in the first stage and the second stage are arranged upside down. The other pair of roll stands in the third stage and the fourth stage are also arranged upside down, and the other pair of arrangement reference lines are changed by a certain angle with respect to the one pair of roll stands. Specifically, each roll stand is arranged upside down in the first stage, the second stage, the third stage is arranged with an angle of 30 ° relative to the first stage stand, and the fourth stage is 3 A configuration is proposed in which the roll stands of the stages are reversed upside down and the third stage and the fourth stage are installed as a pair.

  Further, in the thickness control method of Patent Document 2, the roll gap of at least the last two stands of the three-roll mandrel mill is changed, and the inner hexagonal shape of the inner surface of the tube center portion generated by the three-roll type drawing mill is changed to the center portion of the mother tube. The internal hexagonal cornering that offsets the squareness is added, and the negative end phase hexagonal squareness of the negative end of the pipe end that occurs in the three-roll type drawing mill is offset at the mother pipe end. It has been proposed to provide an internal hexagonal squareness of the phase.

  The pipe end cracks that occur with the start of cold rolling are attracted by microscopic and local wall thickness fluctuations of the raw pipe, and therefore the drawing rolling proposed in Patent Documents 1 and 2 Even if a seamless pipe obtained by a machine or a wall thickness control method is used as a raw pipe, even a minute local wall thickness fluctuation cannot be suppressed, and it has not yet been possible to effectively prevent a pipe end crack.

  Furthermore, as a method for preventing pipe end cracking of a cold rolling raw pipe, Patent Document 3 discloses that the end of the rolled material is prevented from being cracked by heating the end of the raw pipe before cold rolling. An end crack prevention method is disclosed. However, in order to carry out this crack prevention method, it is necessary to install a heating device at the end of the raw tube, and the cost increase due to the installation of the device and the temperature control of the end of the raw tube before cold rolling are complicated. Therefore, the crack prevention method of Patent Document 3 is not practical in the cold rolling operation.

JP 62-199210 A Japanese Patent Application Laid-Open No. 07-246415 JP 2006-136937 A

  As described above, if there is a wall thickness fluctuation on the inner surface of a seamless pipe used as a cold rolling raw pipe, a crack at the pipe end will occur starting from that during cold rolling. Although a drawing mill and a wall thickness control method for suppressing the cornering that is a factor have been proposed, pipe end cracking has not been sufficiently prevented in cold rolling. Further, as a method for preventing cracking of the tube end of the material to be rolled, a method for preventing the end of the raw tube from being heated before cold rolling has been proposed, but it is not practical in the actual operation of cold rolling.

  The present invention has been made in view of the above-described problem of pipe end cracking of the raw pipe that occurs with cold rolling, and the wall thickness variation generated on the inner surface of the pipe end of the raw pipe used for cold rolling. As shown in FIG. 4 to be described later, by managing the development angle b (rad) and the wall thickness difference d (mm), and further chamfering the tube end of the raw tube as necessary, It aims at providing the cold rolling method of the seamless pipe which can prevent effectively generation | occurrence | production of the pipe end crack accompanying a process.

  In order to solve the above-mentioned problems, the present inventor details about the occurrence of squareness due to drawing rolling, which causes the wall thickness variation that occurs on the inner surface of a seamless pipe used as a raw pipe for cold rolling. A study was added.

  FIG. 3 is a diagram schematically showing a cross-sectional shape of an inner hexagonal square formed when an outer diameter drawing process is performed using a three-roll type stretch reducer. Rolling at each stand of a three-roll type stretch reducer has a roll groove bottom portion that has a greater elongation in the rolling direction than an edge portion, and the thickness in the pipe circumferential direction is subject to uneven deformation. Therefore, in rolling with a continuous stand, the roll groove bottom part and the edge part are repeatedly plastically deformed alternately for each stand, so that they have almost the same thickness, but the intermediate position between the roll groove bottom part and the edge part (that is, the roll groove) When the bottom is set to the 0 ° position, a portion where the thickness fluctuates occurs at the 30 ° rotation position), and hexagonal inner surface angularity occurs.

  The hexagonal tension phenomenon as shown in FIG. 3 tends to increase the angular tension as the drawing ratio in the tube length direction in the drawing rolling process increases and the pipe becomes thicker and smaller in diameter, and the wall thickness fluctuation is conspicuous accordingly. become.

  FIG. 4 is a detailed view of the portion E in FIG. 3 showing the detailed configuration of the wall thickness variation. As a result of the inventor's investigation, the structure of the wall thickness variation generated on the inner surface of the seamless pipe used as the raw tube can be arranged by the development angle b (rad) and the wall thickness difference d (mm), and the wall thickness variation It becomes clear that the influence of the pipe on the pipe end crack can be managed by the development angle b (rad) and the wall thickness difference d (mm). That is, it can be understood that the smaller the development angle b (rad) and the larger the wall thickness difference d (mm), the more likely the tube end cracks are generated.

The present invention has been completed on the basis of the above findings, and the gist thereof is the seamless pipe cold rolling method of the following (1) and (2).
(1) When performing cold rolling using a seamless pipe as a raw pipe, the fluctuation in thickness in the pipe circumferential direction on the inner surface of the pipe end on the cold rolling start side is expressed as a development angle b (rad) and a thickness difference d (mm). This is a seamless pipe cold rolling method characterized by using a raw pipe in which the maximum value of the wall thickness fluctuation in the entire circumference of the pipe inner surface satisfies the following formula (1).
d / b ≦ 0.29 (1)
(2) When performing cold rolling using a seamless pipe as a raw pipe, the fluctuation in the pipe circumferential direction on the inner surface of the pipe end on the cold rolling start side is expressed as a development angle b (rad) and a thickness difference d (mm). When the ratio is expressed by the ratio, after performing the chamfering process on the inner surface side of the pipe end portion, a raw pipe whose maximum thickness variation on the entire inner surface of the pipe satisfies the above formula (1) is used. This is a cold rolling method of seamless pipes.

  In the cold rolling method of the present invention, it is excellent when the raw pipe used as a material to be rolled is a seamless pipe manufactured by piercing rolling, stretching rolling and constant diameter rolling by a drawing mill (stretch reducer or sizer). It is possible to effectively prevent cracks in the pipe end caused by cold working.

  The “deployment angle b (rad)” and “thickness difference d (mm)” defined in the present invention are numerical values indicating the configuration of local thickness variation as shown in FIG. In the case of applying, for example, the thickness in the pipe circumferential direction can be continuously measured and grasped using a three-dimensional shape measuring instrument. In addition, as shown in the said FIG. 4, an expansion | deployment angle means the width | variety in the pipe | tube circumferential direction of the part in which the local thin part at the time of developing and displaying a measurement result in the pipe | tube circumferential direction, It is a value represented by an angle (rad) about the axis. Then, d / b is calculated from the continuous measurement values, and a cold rolling blank applicable to the present invention is determined by the maximum d / b value.

  According to the cold rolling method of a seamless pipe of the present invention, the fluctuation of the wall thickness in the pipe circumferential direction on the inner surface of the raw pipe is grasped by the development angle b (rad) and the wall thickness difference d (mm), and is represented by the ratio thereof. By controlling the maximum value of the d / b in the entire inner circumference of the pipe, and further chamfering the pipe end of the raw pipe as necessary, and managing the maximum value thereafter, the pipe accompanying cold working Generation of end cracks can be prevented, and seamless pipes can be manufactured with high yield without causing an increase in manufacturing cost.

  In the cold rolling method of the present invention, when performing cold rolling using a seamless pipe as a raw pipe, the fluctuation of the wall thickness in the pipe circumferential direction on the inner surface of the pipe end on the cold rolling start side is determined as the development angle b (rad) and the wall thickness. When expressed by a ratio d / b of thickness difference d (mm), a raw tube satisfying a maximum value of wall thickness fluctuation of 0.29 (mm / rad) or less around the entire inner surface of the tube is used. .

  In order to study the maximum value of the d / b value of the wall thickness variation applicable to the cold rolling method of the present invention, using a billet with a material of SUS304 and an outer diameter of 187 mm, a three-roll mandrel mill following piercing and rolling A hollow shell was produced by rolling. Thereafter, heating was performed in a reheating furnace, drawing rolling was performed with a three-roll type stretch reducer, and a cold rolling blank having two types of pipe dimensions (outer diameter and wall thickness) was manufactured.

  The obtained cold rolling blank was continuously measured at its end using a three-dimensional shape measuring instrument, and the development angle b (rad) and the wall thickness difference d (mm) were measured. Thereafter, cold rolling was performed with a Pilger mill to produce a cold seamless pipe having a predetermined size, and the occurrence of cracks at the end of the pipe was investigated. Each cold rolling schedule is set to Sch. A and Sch. B is shown in Table 1.

  However, the cross-sectional reduction rate (%) in the table is represented by {1- (cross-sectional area after cold rolling / cross-sectional area before cold rolling)} × 100.

  FIG. 5 is a diagram showing the occurrence of pipe end cracks associated with cold rolling in relation to the development angle b (rad) and the wall thickness difference d (mm) of the cold rolling element pipe. According to the results shown in FIG. 5, as described above, the pipe end crack occurs as the development angle b (rad) is smaller and the thickness difference d (mm) is larger in the wall thickness variation of the inner surface of the raw pipe. It turns out that it is easy.

From the same figure, in order to prevent the occurrence of cracks at the end of the pipe due to cold rolling, the thickness variation of the cold rolling element pipe is determined by the ratio d of the expansion angle b (rad) and the thickness difference d (mm). When expressed by / b, it is understood that the maximum value of the wall thickness fluctuation around the entire inner surface of the pipe needs to satisfy the following expression (1).
d / b ≦ 0.29 (1)
Furthermore, when the cold rolling method of the present invention performs cold rolling using a seamless pipe as a raw pipe, the fluctuation of the wall thickness in the pipe circumferential direction on the inner surface of the pipe end portion on the cold rolling start side is developed at an angle b (rad). And the ratio of wall thickness difference d (mm), after chamfering the inner surface of the tube end, the maximum value of the wall thickness variation on the entire inner surface of the tube satisfies the above formula (1). It is characterized by using an element tube.

  As shown in FIG. 5, the local wall thickness variation present at the tube end of the cold rolling blank tube becomes the starting point of the tube end cracking, but the d / b value of the wall thickness variation is kept below a certain value. If managed, pipe end cracks will not occur. Therefore, even when the d / b value of the wall thickness fluctuation exceeds the control value, the wall thickness fluctuation in the pipe circumferential direction on the inner surface of the pipe end portion is removed, and the wall thickness fluctuation d / b value is kept below a certain value. If it manages so that it may become, generation | occurrence | production of a pipe end crack can be prevented.

  For this reason, in the cold rolling method of the present invention, when necessary, the maximum value of d / b of the wall thickness fluctuation on the entire inner surface of the cold rolling blank satisfies the above formula (1). Thus, cold rolling is performed after chamfering is performed on the inner surface side of the tube end on the cold rolling start side.

  Examples of the chamfering process that can be applied to the present invention include a bevel process by cutting on the inner surface side of the pipe end, a crushing process by inserting a conical cone, and a process by grinder polishing. Further, the chamfering may be performed on the entire circumference on the inner surface side of the pipe end, for example, by chamfering partially only at a location where the thickness variation d / b exceeds 0.29 by grinder polishing, Pipe end cracking can be prevented. In this case, it is desirable to perform chamfering with a thickness of 1.0 mm or more in order to sufficiently remove fluctuations in the thickness of the inner surface of the pipe end portion in consideration of operation stability of cold rolling.

  In the cold rolling method of the present invention, it is desirable that a target for managing the maximum value of the wall thickness variation is a seamless tube having a wall thickness of 10 mm or less. For example, even in the case of cold rolling with a workability as large as 70 to 85% in cross-section reduction ratio, the ratio of thickness difference d / thickness Wt is the ratio of the thickness difference d / thickness Wt. Since it is an extremely small value, the wall thickness difference d does not act as a harmful notch that causes pipe end cracking, and pipe end cracking does not occur even if the raw pipe does not satisfy the formula (1). Because there are cases.

  In the cold rolling method of the present invention, the target raw pipe is a seamless pipe manufactured by Mannesmann pipe manufacturing method, for example, piercing rolling, stretching rolling, and constant diameter rolling with a drawing mill (stretch reducer or sizer). Is desirable.

  In this case, the steel tube type is not limited to ordinary steel, low alloy steel, chrome steel, stainless steel, chrome-nickel-iron alloy, etc., but especially stainless steel containing sulfur (S) is used. In the case of a pipe, it is desirable to apply the cold rolling method of the present invention because MnS precipitated in the steel tends to cause cracks at the end of the cold rolling.

  In order to confirm the effect of the cold rolling method of the present invention, a hollow shell was formed by three-roll mandrel mill rolling following piercing rolling using a billet made of SUS304 and having an outer diameter of 187 mm. Then, it heated in the reheating furnace, drawn-rolled with the 3 roll type | mold stretch reducer, and manufactured the raw tube for cold rolling which consists of 2 types of pipe dimensions.

  The obtained cold rolling blank was continuously subjected to wall thickness measurement using a three-dimensional shape measuring instrument at the end of the tube, and the development angle b (rad) and the wall thickness difference d (mm) were measured. . Thereafter, cold rolling was performed by a pilger mill to produce a cold seamless pipe having a predetermined size. The cold rolling schedule at this time is the same as the schedule shown in Table 1, and Sch. A and Sch. Classified by B.

  In order to observe the effects on pipe end cracking, the cold rolling blanks were divided into cases with and without chamfering. When chamfering was performed, the chamfering process was performed on the inner surface of the pipe end to change the amount of chamfering. These cold-rolling blanks are manufactured according to Sch. A and Sch. Cold rolling was performed with B. Table 2 shows the wall thickness fluctuation and the chamfering state in which d / b before the cold rolling becomes the maximum value at this time, and the pipe end crack occurrence state after the cold rolling.

  However, in the table, when the chamfering amount a (mm) exceeds the wall thickness difference d (mm) due to the wall thickness variation, the (da) value is expressed as “0 (zero)”.

  As shown in Table 2, without chamfering, when cold rolling was performed using a blank tube having a maximum d / b value exceeding 0.29 indicating wall thickness fluctuation before cold rolling, the end of the tube Cracks occurred (Test Nos. 1 and 9), and no pipe end cracks occurred when a raw tube having a maximum d / b value of 0.29 or less was used (Test Nos. 2 and 10).

  On the other hand, with chamfering, when the chamfering amount is smaller than the wall thickness difference d, a thin-walled portion with a d / b value exceeding 0.29 remained, so that pipe end cracking occurred during cold rolling (Test No. 1). 8 and 14). However, when the chamfering amount a (mm) exceeds the wall thickness difference d, the thin wall portion where d / b exceeds 0.29 is removed, and d / b is also measured by the three-dimensional shape measuring instrument after chamfering. Since there was no location where the maximum value exceeded 0.29, tube end cracking did not occur during cold rolling (Test Nos. 3 to 7 and 11 to 13).

  According to the cold rolling method of a seamless pipe of the present invention, the fluctuation of the wall thickness in the pipe circumferential direction on the inner surface of the raw pipe is grasped by the development angle b (rad) and the wall thickness difference d (mm), and is represented by the ratio thereof. By controlling the maximum value of the d / b in the entire inner circumference of the pipe, and further chamfering the pipe end of the raw pipe as necessary, and managing the maximum value thereafter, the pipe accompanying cold working The generation of end cracks can be prevented, and a seamless pipe can be manufactured at a high yield without causing an increase in manufacturing cost. Thereby, in particular, it can be widely applied as a cold rolling method using a seamless element tube by the Mannesmann pipe manufacturing method.

It is a perspective view explaining the whole structure of a pair of roll die used for the cold rolling method. It is sectional drawing explaining the processing mechanism which rolls an element pipe by the cold rolling method. It is a figure which shows typically the cross-sectional shape of the inner side hexagon square tension which arises when performing outside diameter drawing using a 3 roll type stretch reducer. It is a detailed view in the E section of the above-mentioned FIG. It is a figure which shows the generation | occurrence | production condition of the pipe end crack accompanying cold rolling by the relationship between the expansion | deployment angle b (rad) and the wall thickness difference d (mm) of the cold rolling element tube.

Explanation of symbols

1. 1. Tube, Roll die 2a. 2. hole type; 3. Mandrel 4. roll stand; Pinion gear Rack gear

Claims (3)

When performing cold rolling using a seamless pipe as a raw pipe, the thickness fluctuation in the pipe circumferential direction on the inner surface of the pipe end on the cold rolling start side is expressed as a ratio of the development angle b (rad) and the thickness difference d (mm). A cold rolling method for seamless pipes, characterized in that, when expressed, a raw pipe is used in which the maximum value of the wall thickness variation in the entire inner circumference of the pipe satisfies the following formula (1).
d / b ≦ 0.29 (1) When performing cold rolling using a seamless pipe as a raw pipe, the thickness fluctuation in the pipe circumferential direction on the inner surface of the pipe end on the cold rolling start side is expressed as a ratio of the development angle b (rad) and the thickness difference d (mm). In this case, after the chamfering process is performed on the inner surface side of the pipe end portion, a seam pipe in which the maximum value of the wall thickness variation on the entire inner surface of the pipe satisfies the following expression (1) is used. Tubeless cold rolling method.
d / b ≦ 0.29 (1)   The seamless pipe cold rolling method according to claim 1 or 2, wherein the raw pipe is a seamless pipe manufactured by constant diameter rolling by piercing rolling, drawing rolling and drawing rolling.

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