JP4453082B2 - Piercing method - Google Patents

Description

  The present invention relates to a piercing method in a piercer (piercing and rolling mill) which is a seamless pipe-making facility, and more particularly to a piercer drilling method capable of suppressing inner surface flaws generated on an inner surface of a shell after piercing.

  In the manufacture of seamless pipes by the Mannesmann-Mandrel Mill method, first, a round billet of a material is heated to 1200 to 1260 ° C. in a rotary hearth type heating furnace, and then pierced and rolled by a piercer with a plug and a rolling roll to form a shell (hollow Manufacturing). Next, a mandrel bar is inserted into the inner surface of the shell in a skewer shape, and the outer surface is constrained by a perforated rolling roll with a mandrel mill usually consisting of 5 to 8 stands to reduce the thickness to a predetermined thickness. To do. Then, after extracting the mandrel bar, the thinned tube is shaped and rolled to a predetermined outer diameter with a drawing mill to obtain a product.

  1A and 1B are diagrams showing a schematic configuration of a piercer, in which FIG. 1A shows a side view and FIG. 1B shows a plan view. In addition, illustration of a plug is abbreviate | omitted in FIG.1 (b). As shown in FIG. 1, the piercer 10 includes a pair of rolling rolls 1 a and 1 b inclined with respect to each other, and a bullet-shaped plug 3 whose rear end is supported by a mandrel 2. The pair of rolling rolls 1a and 1b are set such that their axial directions are parallel to each other in a side view or cross at a predetermined crossing angle, and are inclined at an inclination angle FA in opposite directions in a plan view. And are configured to rotate in the same direction. The plug 3 is disposed between the pair of rolling rolls 1a and 1b.

  In order to punch a solid round billet B using the piercer 10 having such a configuration, first, the round billet B is fed between a pair of rolling rolls 1a and 1b. After the round billet B bites into the pair of rolling rolls 1a and 1b, the round billet B is rotated by the frictional force of the rolling rolls 1a and 1b, and at the same time, a force that advances in the axial direction acts. And before reaching the tip of the plug 3, the compressive stress and the tensile stress act alternately and continuously at the center of the round billet B by the rolling rolls 1a and 1b (rotary forging effect), and are easily perforated. It becomes a state. After the round billet B collides with the plug 3, a hole is formed in the center of the billet B, and thereafter, the shell S is obtained by being subjected to wall thickness processing every half rotation between the rolling rolls 1 a and 1 b and the plug 3. It will be.

  Here, if the rotary forging effect acts excessively, as shown in FIG. 2, cracks (Mannesmann failure) are likely to occur at the center of the round billet B, and may remain as inner surface flaws in the shell after drilling. It is known.

  Therefore, conventionally, a piercer drilling method as disclosed in Patent Document 1 has been proposed in order to suppress the occurrence of internal flaws due to the Mannesmann destruction in the piercer. More specifically, in Patent Document 1, the inclination angle of the rolling roll is set to 12 ° to 14 °, and after the tip of the plug is engaged with the round billet, the round billet is swollen by the rolling roll. There has been proposed a piercer drilling method in which the ratio between the length of the billet and the outer diameter of the billet is set to be 0.4 to 0.6.

However, the method described in Patent Document 1 does not consider the material of the round billet to be drilled at all, and only discloses a general method for easily suppressing the occurrence of inner surface flaws. More specifically, conditions for generating internal flaws (conditions leading to Mannesmann fracture) are, for example, a material that can be easily processed such as carbon steel, and a difficult-to-process material such as high Cr-containing steel. Therefore, in the method described in Patent Document 1 in which no consideration is given to the material of the round billet, when it is necessary to drill a round billet of various materials, it is possible to suppress the occurrence of internal flaws in all of them. There is a problem that it is not possible and practical.
JP 2000-140911 A

  The present invention has been made to solve such problems of the prior art, and even when a wide variety of round billets are drilled, a piercer capable of suppressing inner surface flaws generated on the shell inner surface after drilling. It is an object to provide a drilling method.

  In order to solve the above-mentioned problems, the inventors of the present invention have conducted intensive investigations. As a result, a round bar test piece was collected from a round billet actually drilled by a piercer, and the round bar test piece was measured with the temperature of the round billet at the time of drilling. It was found that there is a correlation between the results of conducting a twist test (high temperature twist test) in a state of being heated to a temperature in substantially the same manner and the Mannesmann fracture that occurs in an actual piercer. Therefore, if the piercer conditions (number of rotational forgings) are set based on the results of the high-temperature twist test, it is possible to suppress internal flaws that occur on the inner surface of the shell after drilling, even when drilling a wide variety of round billets. I came up with something. And as a result of earnestly examining various parameters based on this knowledge, the present invention has been completed.

That is, in the present invention, a round billet is fed between a pair of inclined rolling rolls constituting a piercer, and the round billet is perforated using a bullet-shaped plug disposed between the rolling rolls. The piercer drilling method for obtaining the shell of the piercer is characterized in that the piercer conditions are set so that the rotational forging frequency N defined by the following equation (1) satisfies the following equation (2): A drilling method is provided.
N = 2 × L / (Vs / EL) × Brps (1)
N <Nt × (Sr / 2R) / (Brps / Srps) (2)
Here, in the above formula (1) or (2), L is the distance (mm) between the position where the round billet bites into the rolling roll and the tip of the plug (see FIG. 1), and Vs is the shell speed (mm / s). EL is used to calculate the perforation ratio (dimensionless unit), Brps is the rotational speed (rps) of the round billet, Nt is the high temperature twist value (times), and Sr is the high temperature twist value Nt. The diameter (mm) of the round bar test piece, R is the radius of curvature (mm) of the plug tip, and Srps is the rotational speed (rps) of the round bar test piece used to calculate the high temperature twist value Nt. .

  The high-temperature twist value Nt is obtained by heating a round bar specimen taken from the center of a round billet to be drilled (the same kind of round billet to be drilled) to a temperature substantially equal to the temperature of the round billet at the time of drilling. This means the number of rotations until the round bar specimen breaks when a constant twist rotation is given in the state. The shell speed Vs is defined as Vs = V × sin (FA) × η where V is the peripheral speed of the rolling roll, FA is the tilt angle of the rolling roll (see FIG. 1), and η is the piercing efficiency. Value. The perforation ratio EL is a value defined by EL = shell length / billet length. Further, the rotational speed Brps of the round billet is a value defined by Brps = circular length of the round billet / peripheral speed of the rolling roll.

  The high temperature twist value Nt is a value depending on the diameter Sr and the rotational speed Srps of the round bar test piece as well as the material and temperature of the round bar test piece. That is, if the diameter Sr of the round bar test piece is reduced or the rotational speed Srps is reduced, the high-temperature twist value Nt is increased, while the diameter Sr of the round bar test piece is increased or the rotational speed Srps is increased. The high temperature twist value Nt decreases. Therefore, on the right side of the above formula (2) that defines the upper limit of the number N of rotation forgings, the obtained high-temperature twist value Nt is multiplied by the diameter Sr (however, the diameter Sr is plugged in order to make the diameter Sr dimensionless). Normalization by twice the curvature radius R of the tip) and further multiplication by the rotational speed Srps (however, in order to make the rotational speed Srps dimensionless, the rotational speed Srps is normalized by the rotational speed Brps of the round billet) Thus, the influence of the diameter Sr and the rotational speed Srps of the round bar test piece is eliminated.

  According to the present invention, a round bar test piece is collected from a round billet that is actually perforated by a piercer (a round bar test piece corresponding to the material of various round billets is collected), and a high temperature torsion test is performed on the round bar test piece. In order to set the piercer conditions (rotational forging times) based on the results, the material of the round billet to be drilled is considered in the set piercer conditions, and even when drilling a wide variety of round billets. It is possible to suppress inner surface flaws generated on the inner surface of the shell after drilling.

  Hereinafter, an embodiment of a piercer drilling method according to the present invention will be described with reference to the accompanying drawings as appropriate.

  FIG. 3 is a diagram showing a schematic configuration of a round bar test piece for performing a high temperature twist test for calculating a high temperature twist value Nt in the piercer drilling method according to the present embodiment. As shown in FIG. 3, the round bar test piece TP according to the present embodiment has a diameter (a diameter of the central portion) of a material collected from the central portion of a round billet to be drilled (a round billet of the same kind as that to be drilled). It is processed so as to be 10 mm, and screw processing is applied to both ends. When performing a high-temperature torsion test on such a round bar test piece TP, the round bar test piece TP is inserted into a heating furnace with the couplings C screwed to both ends thereof, and the rounds at the time of drilling are inserted. Heat until the temperature is approximately equal to the billet temperature. One of the couplings C is fixedly supported, and the other coupling C is rotated by a rotation mechanism (rotation speed: 5 rps) to give torsional rotation to the round bar test piece TP, and the round bar test piece TP breaks. The number of rotations until the rotation is counted, and the counted number of rotations is set as a high temperature twist value Nt. The rotational speed of the round bar test piece TP in this embodiment (the rotational speed of the other coupling C) is preferably closer to the rotational speed of the round billet when the round billet is actually perforated by the piercer. From the viewpoint, it is set to 5 rps as described above.

After calculating the high temperature twist value Nt as described above, in the piercer drilling method according to the present embodiment, the rotational forging frequency N defined by the following formula (1) satisfies the following formula (2). Set the conditions of piercer.
N = 2 × L / (Vs / EL) × Brps (1)
N <Nt × (Sr / 2R) / (Brps / Srps) (2)
Here, in the above formula (1) or (2), L is the distance (mm) between the position where the round billet bites into the rolling roll and the tip of the plug (see FIG. 1), and Vs is the shell speed (mm / s). EL is used to calculate the perforation ratio (dimensionless unit), Brps is the rotational speed (rps) of the round billet, Nt is the high temperature twist value (times), and Sr is the high temperature twist value Nt. The diameter (mm) of the round bar test piece TP, R means the radius of curvature (mm) of the plug tip, and Srps means the rotational speed (rps) of the round bar test piece used to calculate the high temperature twist value Nt. To do. In the present embodiment, as described above, Sr = 10 (mm) and Srps = 5 (rps).

Table 1 applies the piercer drilling method according to the present embodiment to round billets made of martensitic stainless steel, martensitic stainless steel (6Ni-3.0Mo series), and γ stainless steel (SUS304). (If the piercer conditions are set so as to satisfy the above equation (2)), and if the piercer drilling method of the comparative example is applied (the piercer conditions are set so as not to satisfy the above equation (2)). The results of evaluating the set piercer conditions and the rate of occurrence of shell inner surface defects after drilling (= number of inner surface defects / number of holes × 100 (%)) are shown. FIG. 4 shows a graph in which the data shown in Table 1 is organized with the horizontal axis as the left side to the right side of the above formula (2) and the vertical axis as the internal flaw occurrence rate.

  As shown in Table 1 or FIG. 4, according to the piercer drilling method according to the present embodiment, it was possible to suppress the occurrence rate of inner surface flaws to 5% or less for the round billet made of any material. . In addition, in this embodiment, only the result at the time of perforating the above-mentioned three steel types of round billets was illustrated, but the present invention is not limited to this, and is similarly applied to round billets made of other materials such as carbon steel. It is possible and has the same effect.

FIG. 1 is a diagram showing a schematic configuration of a piercer. FIG. 2 is an explanatory diagram for explaining Mannesmann destruction. FIG. 3 is a diagram showing a schematic configuration of a round bar test piece for carrying out a high-temperature torsion test in the piercer drilling method according to the present invention. FIG. 4 is a graph showing the results of evaluating the occurrence rate of inner surface flaws in the piercer drilling method according to the present invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1a, 1b ... Rolling roll 3 ... Plug 10 ... Piercer P ... Elementary pipe

Claims (1)

Piercer drilling to obtain a hollow shell by feeding a round billet between a pair of inclined rolling rolls constituting a piercer and punching the round billet using a bullet-shaped plug disposed between the rolling rolls A method,
A piercer drilling method, wherein the piercer conditions are set so that the number N of rotational forging defined by the following formula (1) satisfies the following formula (2):
N = 2 × L / (Vs / EL) × Brps (1)
N <Nt × (Sr / 2R) / (Brps / Srps) (2)
Here, in the above formula (1) or (2), L is the distance (mm) between the position where the round billet bites into the rolling roll and the tip of the plug, Vs is the shell speed (mm / s), and EL is the hole. The ratio (dimensionless unit), Brps is the rotation speed (rps) of the round billet, Nt is the high temperature twist value (times), and Sr is the round bar specimen used to calculate the high temperature twist value Nt. The diameter (mm), R represents the radius of curvature (mm) of the plug tip, and Srps represents the rotational speed (rps) of the round bar test piece used to calculate the high temperature twist value Nt.

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