JP2012187625A - Method of cold-rolling seamless tube - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of cold-rolling a seamless tube, by which cracks produced in end parts of a tube stock, which are taken as the starting side and terminating side of cold rolling when performing cold rolling, are suppressed and the cold rolling is stably performed.SOLUTION: In the method of cold-rolling the seamless tube, when performing the cold rolling with a seamless tube composed of an austenitic alloy containing, by mass, 21-31% Cr and 43-60% Ni as a tube stock, the tube stock 1 on which chamfering is performed is used on the sides of the outside 1a of the ends to be the starting side and the terminating side of the cold rolling so that the wall thickness Lt (mm) of the ends after chamfering and the bevel length Lb (mm) in the longitudinal direction satisfy following formula (1) 0.5t≤Lt≤0.75t and formula (2) -4Lt+4.5t≤Lb, where, t is the wall thickness (mm) of the tube stock.

Description

  The present invention relates to a method of performing cold rolling using a seamless pipe made of a high Cr-high Ni austenitic alloy as a base pipe. More specifically, it is possible to suppress cracks occurring at the ends of the cold pipe at the cold rolling start side and end side when cold rolling the blank pipe, and to stabilize by preventing rolling stop due to this pipe end crack. The present invention relates to a seamless pipe cold rolling method capable of performing cold rolling.

Unless otherwise stated, the definitions of terms in this specification are as follows.
“Cross-section reduction rate”: used as an index for evaluating the degree of work in cold rolling. The cross-section reduction rate Rd (%) is the cross-sectional area S1 (mm ² ) of the raw pipe and the cross-sectional area of the pipe after cold rolling. It can be calculated by the following formula (3) from S2 (mm ² ).
Rd = (1-S2 / S1) × 100 (3)

  As cold working methods for metal tubes, a cold drawing method using a draw bench and a cold rolling method using a pilger mill are commonly used. In the cold drawing method using a draw bench, a plug, a floating plug or a mandrel is inserted into the raw tube, and the raw product is drawn through a die to finish the target product dimensions.

  Such a cold drawing method is difficult to apply to cold working of small diameter materials because it is difficult to perform cold drawing at a high workability by increasing the cross-section reduction rate. Further, in the cold drawing method, it is necessary to form a chemical conversion film typified by iron oxalate on the surface of the raw tube for lubrication. When the raw tube material is a high Cr—high Ni austenitic alloy (hereinafter, also simply referred to as “high Cr—high Ni alloy”) containing Cr: 21-31% and Ni: 43-60% by mass%, Since it is difficult to form a chemical conversion film on the tube surface, lubrication is poor and cold drawing is difficult.

  On the other hand, the cold rolling method using a pilger mill can increase the cross-sectional reduction rate compared to the cold drawing method, and can cold-work the raw tube with high workability. For this reason, in the manufacture of seamless pipes that require a high degree of work, a cold rolling method using a pilger mill (Pilger rolling) is generally used.

  In cold rolling by pilger rolling, a pair of upper and lower hole rolls having a hole shape formed on the peripheral surface is used, and a mandrel having a taper whose diameter decreases toward the tip is provided between the hole rolls. This hole-type roll is supported by the roll stand with a rotating shaft provided at the axis.

  When cold rolling is performed on the raw pipe by pilger rolling, the hollow roll supported by the roll stand moves back and forth along the mandrel, thereby rolling the raw pipe as the workpiece while reciprocatingly rotating. The raw tube 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 during the step of reciprocating the perforated roll. At this time, the cold-rolled blank tube is drawn according to the rolling elongation and the rolling feed amount, and rolled to the target product size.

  When cold rolling is continuously performed on a plurality of raw pipes by this Pilger rolling, the end face on the rolling start side of the subsequent raw pipe is aligned with the end face on the rolling end side of the cold rolled raw pipe. Supply the raw tube. As a result, as the subsequent raw pipe is sent out, the end face on the rolling start side of the subsequent raw pipe is extruded from the end face on the end side of the raw pipe to be cold rolled. A tube is sent.

  When performing cold rolling by such pilger rolling, if a seamless pipe made of a high Cr-high Ni alloy is blanked, the raw pipe made of a high Cr-high Ni alloy is a difficult-to-work material. Cracks occur at the ends of the raw pipes on the rolling start side and end side. In this pipe end crack, a substantially linear crack is generated over a length of about 10 to 50 mm in the longitudinal direction from the pipe end, and a plurality of such cracks are generated in the circumferential direction. If a crack at the end of the tube occurs during cold rolling and a part of the end of the tube is missing, it is necessary to stop rolling because defective pieces may cause product defects or equipment damage. In addition, if a pipe end crack occurs during cold rolling, it is necessary to cut down the cracked pipe end in a subsequent process even if rolling can continue without stopping. Yield decreases.

  The number of times cold rolling is performed by reducing the workability per cold rolling in order to suppress the occurrence of pipe end cracks when cold rolling is performed on a raw pipe made of a high Cr-high Ni alloy. It is also conceivable to employ a method of ensuring a predetermined degree of processing by increasing the amount of. However, in this method, the number of times of cold rolling increases and the number of times of softening heat treatment on the element tube also increases, so that the production efficiency is remarkably deteriorated. For this reason, the method of ensuring a predetermined workability by decreasing the workability per cold rolling and increasing the number of times of cold rolling is not practical.

  Various proposals have heretofore been made with respect to the cold rolling method of tubes, for example, Patent Documents 1 and 2. In Patent Document 1, when cold rolling is performed on a raw pipe in which wall thickness variation has occurred due to hexagonal inner surface angulation, a local thin wall portion due to wall thickness variation acts as a notch so that the end on the rolling start side It aims at suppressing the crack which generate | occur | produces in a part. In the cold rolling method described in Patent Document 1, when performing cold rolling using a seamless pipe as a raw pipe, the fluctuation in thickness on the inner surface of the end portion on the rolling start side is expressed as a development angle b (rad) and a thickness. An element tube that uses the difference d (mm) and manages the maximum value of the ratio d / b is used. Further, when the maximum value of d / b exceeds the management range, the maximum value of d / b is managed by chamfering the inner surface side of the end portion that is the rolling start side.

  The cold rolling method described in Patent Document 1 uses a raw pipe in which the maximum value of d / b is controlled, so that when the cold rolling is performed, pipe end cracks due to the internal angle are generated in the raw pipe. It is said that it can be suppressed. However, in cold rolling of a raw tube made of a high Cr-high Ni alloy, which is a difficult-to-work material, cracks at the end of the tube occur even when no wall thickness variation is observed due to internal angular stress. For this reason, it is difficult to suppress pipe end cracking that occurs when cold rolling is performed on a raw pipe made of a high Cr-high Ni alloy by the cold rolling method described in Patent Document 1.

  Patent Document 2 describes a method of performing cold rolling on a clad steel base tube made of a base material and a clad material. The cold rolling method described in Patent Document 2 uses a clad steel base tube that has been chamfered so as to satisfy a predetermined conditional expression on the base metal side of the end. Accordingly, it is possible to prevent the base material from popping out at the end due to the deformation resistance difference between the base material and the clad material, and the base material and the clad material from being separated at the end. Since the cold rolling method described in Patent Document 2 is intended for clad steel base pipes, pipe end cracks that occur when cold rolling is performed using a seamless pipe made of a high Cr-high Ni alloy as the base pipe. Has not been studied.

JP 2009-6384 A JP 2006-346726 A

  As described above, when cold working by cold drawing is performed on a raw pipe made of a high Cr-high Ni austenitic alloy, the degree of workability is reduced, so that it is difficult to apply to small diameter materials, Since it is difficult to form a chemical conversion film on the surface, lubrication is poor and cold drawing is difficult. Further, when cold working by cold rolling is performed on a raw pipe made of a high Cr-high Ni austenitic alloy, pipe end cracking occurs in the raw pipe, which causes a problem of reduction in product yield and rolling stop.

  The present invention has been made in view of such a situation, and when cold rolling is performed on a raw tube made of a high Cr-high Ni austenitic alloy, end portions on the cold rolling start side and end side are provided. An object of the present invention is to provide a seamless pipe cold rolling method that can suppress the occurrence of cracking and can stably perform cold rolling by preventing rolling stop due to pipe end cracking. .

  In order to solve the above problems, the present inventor conducted various tests, and as a result of intensive studies, the raw tube in which chamfering processing was performed on the outer surface side of the end portions serving as the cold rolling start side and the end side It has been found that the occurrence of pipe end cracking can be suppressed by performing cold rolling using. Therefore, in order to obtain an end shape that can suppress the occurrence of cracks at the end of the tube, a test was performed in which cold rolling was performed on an element tube that had been chamfered on the outer surface side of the end.

In this test, a seamless tube obtained by the following procedure was used as a raw tube.
(1) A hollow billet made of a high Cr-high Ni alloy was hot-made by the Eugene Sejurune method to obtain a seamless pipe.
(2) The seamless tube was heated to 1185 ° C. and held for 2 minutes or more, and then subjected to a heat treatment for water cooling.
(3) Chamfering was performed on the outer surface side of both ends of the seamless pipe subjected to heat treatment.

  FIG. 1 is a cross-sectional view showing the shape of an end portion of a raw pipe whose outer surface is chamfered. As shown in the figure, the end shape of the chamfered blank tube 1 can be defined by the bevel length Lb (mm) in the longitudinal direction and the end wall thickness Lt (mm) after chamfering. In this test, the bevel length Lb (mm) in the longitudinal direction and the end wall thickness Lt (mm) after the chamfering process were changed by the chamfering process (3).

Cold rolling by pilger rolling was performed using the seamless tube thus obtained as a raw tube. In this test, the material of the base tube was steel type A or B. These steel types A and B are high Cr-high Ni alloys, and their representative compositions are as follows. Here,% of content of each element is mass%.
Steel type A: 30% Cr-50% Ni-4% W,
Steel type B: 22% Cr-54% Ni-12% Co-9% Mo-1% Al

Moreover, the processing schedule of the cold rolling in this test is as follows.
Seamless pipe as a raw pipe: outer diameter 86.00 mm, wall thickness 18.00 mm,
Tube after cold rolling: outer diameter 45.10 mm, wall thickness 9.45 mm,
Cross-sectional reduction rate: 72.4%

About the pipe | tube which carried out the cold rolling by said schedule, the edge part made into the cold rolling start side and the completion | finish side was observed visually, and the pipe end crack generation | occurrence | production condition was confirmed. In the pipe end crack occurrence situation shown in FIGS. 2 and 3 to be described later, the meanings of the symbols are as follows:
◯: It indicates that the occurrence of pipe end cracking was not observed at either end of the cold rolling start side and end side.
X: It shows that generation | occurrence | production of the pipe end crack was recognized by any one or both of the edge part made into the cold rolling start side and completion | finish side.

  FIG. 2 is a diagram showing a pipe end crack occurrence state in cold rolling using a raw pipe made of steel type A. FIG. In the figure, the vertical axis represents Lb / t, which is the ratio of the bevel length Lb (mm) in the longitudinal direction to the wall thickness t (mm) of the tube, and the horizontal axis represents the wall thickness t (mm) of the tube. Lt / t, which is the ratio of the end wall thickness Lt (mm) after chamfering. From the figure, in cold rolling using a blank tube made of steel type A, the end wall thickness Lt and the bevel length Lb in the longitudinal direction are Lt / t ≦ 0.75 and Lb / t ≧ −2Lt / t + 3. It became clear that the tube end cracking can be suppressed by setting it to 0.

  FIG. 3 is a diagram showing a pipe end crack occurrence state in cold rolling using a raw pipe made of steel type B. FIG. In FIG. 3, the vertical axis is Lb / t and the horizontal axis is Lt / t, as in FIG. From FIG. 3, in cold rolling using a blank tube made of steel type B, the end wall thickness Lt and the longitudinal bevel length Lb are Lt / t ≦ 0.75 and Lb / t ≧ −4Lt / t + 4. It became clear that the pipe end crack can be suppressed by setting 5.

  From the above results, the end wall thickness Lt and the length of the bevel Lb in the longitudinal direction are Lt / t ≦ 0.75, regardless of whether a raw tube made of a high Cr—high Ni alloy of steel type A or B is used. And it became clear that pipe end cracking can be suppressed by setting Lb / t ≧ −4Lt / t + 4.5.

  The present invention has been completed based on these findings, and the gist thereof is the following seamless pipe cold rolling method.

When cold rolling is performed using a seamless tube made of an austenitic alloy containing Cr: 21 to 31% and Ni: 43 to 60% in mass% as the raw tube, the ends on the cold rolling start side and the end side The chamfering process is performed on the outer surface side so that the end wall thickness Lt (mm) and the bevel length Lb (mm) in the longitudinal direction after the chamfering process satisfy the following expressions (1) and (2): A method of cold rolling a seamless tube, characterized by using a tube.
0.5t ≦ Lt ≦ 0.75t (1)
−4Lt + 4.5t ≦ Lb (2)
Here, t is the tube thickness (mm).

The seamless pipe cold rolling method of the present invention has the following remarkable effects.
(1) The cold rolling method for seamless pipes according to the present invention is such that when cold rolling is performed on a raw pipe made of a high Cr-high Ni austenitic alloy, the end portions on the cold rolling start side and end side are arranged. An element pipe that is chamfered so as to satisfy the expressions (1) and (2) on the outer surface side is used.
(2) By said (1), it can suppress that a crack generate | occur | produces in the edge part made into the cold rolling start side and an end side.
(3) Due to the above (2), rolling stop due to pipe end cracking can be prevented, and cold rolling can be performed stably.

It is sectional drawing which shows the edge part shape of the raw pipe by which the chamfering process was given to the outer surface side. It is a figure which shows the pipe end crack generation | occurrence | production situation in the cold rolling using the raw pipe which consists of steel types A. It is a figure which shows the pipe end crack generation | occurrence | production situation in the cold rolling using the raw pipe which consists of steel types B.

As described above, the method of cold rolling a seamless pipe according to the present invention is cold using a seamless pipe made of an austenitic alloy containing Cr: 21-31% and Ni: 43-60% by mass as a base pipe. When performing the rolling, the end wall thickness Lt (mm) after chamfering and the bevel length Lb (mm) in the longitudinal direction on the outer surface side of the end portion that is the cold rolling start side and the end side are the following (1). It is characterized by using an element tube that has been chamfered so as to satisfy the equations (2) and (2).
0.5t ≦ Lt ≦ 0.75t (1)
−4Lt + 4.5t ≦ Lb (2)
Here, t is the tube thickness (mm).

  Below, the reason and preferable range which prescribed | regulated the cold rolling method of the seamless pipe of this invention as mentioned above are demonstrated.

  In the present invention, it is specified that the element tube is made of an austenitic alloy containing Cr: 21-31% and Ni: 43-60% by mass%. This is because it is intended to suppress pipe end cracking that occurs in the pipe when cold rolling is performed on the pipe made of a high Ni alloy. The reason for limiting the contents of Cr and Ni is as follows. In the following description, “%” notation of the content of each element means “mass%”.

Cr: 21-31%
Cr is an important element for ensuring oxidation resistance, steam oxidation resistance and corrosion resistance. In order to ensure corrosion resistance at high temperatures, a content of at least 21% is required. The corrosion resistance increases as the Cr content increases, but if the content exceeds 31%, the structural stability is lowered and the creep strength is impaired. Moreover, not only is it necessary to increase the expensive Ni content in order to stabilize the austenite structure, but also the weldability is reduced. Therefore, the Cr content is 21 to 31%.

Ni: 43-60%
Ni is an element that stabilizes the austenite structure, and is an alloy element that is also important for ensuring corrosion resistance. From the balance with the above Cr amount, Ni should be 43% or more. On the other hand, excessive Ni causes not only an increase in cost but also a decrease in creep strength, so the upper limit is made 60%.

  The seamless pipe cold rolling method of the present invention uses a chamfering shape applied to the outer surface side of the end portion of the raw tube which is the cold rolling start side and the end side as Lt according to the above formulas (1) and (2). ≦ 0.75t and −4Lt + 4.5t ≦ Lb. As a result, the seamless pipe cold rolling method of the present invention can suppress the occurrence of cracks at the ends of the rolling start side and the end side as shown in the test results of FIGS.

  Here, in the conventional seamless tube cold rolling method, cold rolling is performed without chamfering the end portion of the raw tube made of the high Cr-high Ni alloy. In this case, the edge that is the boundary between the outer surface and the end surface of the tube end is strongly pressed by the perforated roll when cold rolling is performed, and it is assumed that stress concentration occurs and tube end cracking occurs. Is done.

  On the other hand, the seamless pipe cold rolling method of the present invention uses an element pipe that has been chamfered as Lt ≦ 0.75 t and −4 Lt + 4.5 t ≦ Lb. Here, the bevel length in the thickness direction indicated by (t−Lt) in FIG. 1 becomes 0.25 t or more from 0.25 t ≦ t−Lt derived by deforming Lt ≦ 0.75 t. .

  As described above, the seamless pipe cold rolling method according to the present invention has a bevel length (t−Lt) of 0.25 t or more in the thickness direction and a bevel length Lb of the longitudinal direction of −4 Lt + 4.5 t or more. Chamfer the end. As a result, as shown in FIG. 1, the edge of the outer surface 1a and the end surface 1b is removed to form the bevel surface 1c, so that the end of the blank tube is pressed by the perforated roll during cold rolling. It can be received on the surface. As a result, it is estimated that the seamless pipe cold rolling method of the present invention can disperse the stress generated at the end and suppress the pipe end cracking.

  When continuously cold rolling a plurality of blanks, if the end wall thickness Lt after the chamfering process is too thin, the cold rolling of the blanks by the end surface on the rolling start side of the subsequent blanks There is a possibility that rolling may be stopped due to a problem caused by extrusion of the end face on the end side. For this reason, in the seamless pipe cold rolling method of the present invention, the end wall thickness Lt after chamfering is specified to be 0.5 t or more by the above-described equation (1).

  Thus, in the seamless pipe cold rolling method of the present invention, chamfering is performed on the outer surface side of the end portion which is the cold rolling start side and the end side so as to satisfy the above formulas (1) and (2). Use the bare tube. Thereby, when performing cold rolling on the raw tube made of the high Cr-high Ni alloy, it is possible to suppress the occurrence of cracks at the ends on the cold rolling start side and the end side. As a result, rolling stop due to pipe end cracking can be prevented, and a cold working by a cold rolling method having a high workability can be stably applied to a raw pipe made of a high Cr-high Ni alloy.

  If the amount of chamfering applied to the outer surface of the end portion of the raw tube is large, the processing cost increases and the product yield decreases. Therefore, in the seamless pipe cold rolling method of the present invention, the bevel length Lb in the longitudinal direction is preferably set to 3.0 t or less.

  In order to verify the effect of the cold rolling method of the seamless pipe according to the present invention of the present invention, a test was performed in which cold rolling was performed on a blank pipe whose end was chamfered.

[Test method]
In this test, cold rolling by pilger rolling was performed using a seamless pipe obtained by the following procedure as a base pipe.
(1) A hollow billet made of a high Cr-high Ni alloy was hot-made by the Eugene Sejurune method to obtain a seamless pipe.
(2) The seamless tube was heated to 1185 ° C. and held for 2 minutes or more, and then subjected to a heat treatment for water cooling.
(3) Chamfering was performed on the outer surface side of both ends of the seamless pipe subjected to heat treatment.

The material of the base tube used in this test was the above-described steel type B high Cr-high Ni alloy. The processing schedule for cold rolling in this test is as follows.
Seamless pipe as a raw pipe: outer diameter 77.00 mm, wall thickness 13.50 mm,
Raw tube after cold rolling: outer diameter 38.00 mm, wall thickness 8.80 mm,
Cross-sectional reduction rate: 70.0%

Moreover, the dimension of the both ends of the raw pipe in which the chamfering process was given in this test is as follows.
End thickness after chamfering: Lt = 6.8 mm (Lt / t = 0.5),
Bevel length in the longitudinal direction: Lb = 40.0 mm (Lb / t = 3.0)

[Test results]
The cold-rolled blank tube was visually observed at the cold rolling start side and end side to confirm the occurrence of pipe end cracking. As a result, no occurrence of tube end cracking was observed at any end of the cold-rolled tube. From this, the cold-rolling method of the seamless pipe of the present invention uses an element pipe that has been chamfered so as to satisfy the expressions (1) and (2) on the outer surface side of the end portion. It has been confirmed that pipe end cracks generated in the raw pipe when cold rolling is performed on the raw pipe made of a high Cr-high Ni alloy can be suppressed.

The seamless pipe cold rolling method of the present invention has the following remarkable effects.
(1) The cold rolling method for seamless pipes according to the present invention is such that when cold rolling is performed on a raw pipe made of a high Cr-high Ni austenitic alloy, the end portions on the cold rolling start side and end side are arranged. An element pipe that is chamfered so as to satisfy the expressions (1) and (2) on the outer surface side is used.
(2) By said (1), it can suppress that a crack generate | occur | produces in the edge part made into the cold rolling start side and an end side.
(3) Due to the above (2), rolling stop due to pipe end cracking can be prevented, and cold rolling can be performed stably.

  Therefore, if the cold rolling method for seamless pipes of the present invention is applied to the production of seamless steel pipes made of high Cr-high Ni austenitic alloys, it will greatly contribute to the improvement of product yield and production efficiency of seamless steel pipes. can do.

1: Elementary tube, 1a: Outer surface, 1b: End surface, 1c: Bevel surface,
t: blank thickness, Lt: end wall thickness after chamfering, Lb: longitudinal bevel length

Claims (1)

When cold rolling is performed using a seamless tube made of an austenitic alloy containing Cr: 21 to 31% and Ni: 43 to 60% in mass% as the raw tube, the ends on the cold rolling start side and the end side The chamfering process is performed on the outer surface side so that the end wall thickness Lt (mm) and the bevel length Lb (mm) in the longitudinal direction after the chamfering process satisfy the following expressions (1) and (2): A method of cold rolling a seamless tube, characterized by using a tube.
0.5t ≦ Lt ≦ 0.75t (1)
−4Lt + 4.5t ≦ Lb (2)
Here, t is the tube thickness (mm).

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