JP2010125492A - Piercing rolling method for billet - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a piercing rolling method for a billet by which the occurrence of scratches on the inner surface at a distal end part of a seamless steel tube is prevented advantageously. <P>SOLUTION: In the piercing rolling method for a billet performing piercing rolling on the billet 10 after heating it, tapering work 20 is applied to the outer surface of the distal end part of the billet before performing piercing rolling and the tapering work is performed so as to satisfy expression (1): G0. 5Db>D100>D0>Gt, wherein Db denotes the diameter of the billet 10 before the tapering work, D0 and D100 denote the diameters in the distal end position and a position of 100 mm inside from the distal end position after tapering work, respectively, and Gt and G0. 5Db denote gaps between rolls in the distal end position of a plug of a piercer used for the piercing rolling and in a position 0.5×Db upstream from the distal end of the plug, respectively. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a method for piercing and rolling a billet (material for seamless steel pipe), and in particular, when a billet is pierced and rolled by a piercing and rolling machine such as a Mannesmann Piercer, it is difficult to generate internal flaws at the tip of a material to be rolled. The present invention relates to a billet piercing and rolling method.

  As shown in FIG. 1, a piercing and rolling mill, for example, Mannesmann Piercer, has rolls 1 and 2 having a shape in which the roll diameter decreases (drum shape) as it goes from the center to the both ends in the roll axis direction. The plug 3 is arranged such that the plug tip is located near the center of the roll bite and the plug rear end is located downstream in the rolling direction. The material 10 is pierced and rolled inside.

  The principle of piercing and rolling is that the center of the cross section of the billet 10 is embrittled by rotary forging (empty stagnation) in the rolling direction range (empty stagnation part 4) from the roll bite start end position to the plug tip position, and then the plug tip In the rolling direction range from the position to the end position of the roll bite (piercing part 5), the embrittled cross-sectional center part is pressed against the plug 3 and drilled to determine the tube thickness.

  In such piercing and rolling, wrinkles tend to occur intensively at the tip of the inner surface of the pipe (the leading end during rolling). The generation mechanism of this wrinkle is considered as follows. That is, as shown in FIG. 2, billet 10 (square slab → billet rolled material or straight cast billet) is segregated at the center of the cross section 15 (linear segregation in square slab → billet rolled material, and spot shaped in direct cast billet. Segregation). In the segregation 15, the portion located at the tip is oxidized by heating before piercing and rolling. At the time of emptying, this oxidized segregation part is the starting point, and cracks are generated and propagated at the tip part. The cracks on the inner surface of the crack are squeezed down by the plug and fall down, resulting in a flange 50 on the inner surface of the rolling tip of the tube 11.

As a countermeasure for reducing the occurrence of wrinkles as described above, as shown in FIG. 3, a method of shifting the segregation 15 to a position deviated from the central portion of the cross-section of the billet 10 that becomes a drilling site (method of offsetting segregation; patent) Reference 1) and as shown in Fig. 4, using cone type piercers (a piercing and rolling machine with cone-shaped rolls 21 and 22 arranged on the top and bottom) to prevent uneven collapse by suppressing shear deformation during rolling. A method (see Patent Document 2) is known.
JP-A-10-128510 Japanese Patent No. 3021664

  However, in the method of offsetting the above-mentioned segregation, segregation remains inside the tube thickness, resulting in non-uniform quality. Moreover, in the method using the cone type piercer described above, a change in roll shape and a facility for tilting the roll axis with respect to the rolling direction are required, and the investment is enormous. That is, the problem that it is difficult to advantageously prevent the generation of wrinkles on the inner surface of the distal end portion of the seamless steel pipe remains with the conventional technology.

The inventors diligently studied the means for solving the above-mentioned problems and made the present invention described below.
(Claim 1)
In the method of heating and piercing and rolling a billet, the billet piercing and rolling method is characterized in that a tip end portion of the billet is tapered to the outer surface before the piercing and rolling.
(Claim 2)
The diameter of the billet before taper machining is D _b , and the diameter at the tip position after taper machining and the diameter at a position 100 mm inside from the tip position is D ₀ and D ₁₀₀ , respectively. tip position and the plug tip 0.5 × from the position D _b only roll gap at the upstream position respectively G _t, a G _{0.5 dB,} so the following equation (1) is satisfied, the tapered The billet piercing and rolling method according to claim 1, wherein the billet is pierced and rolled.

G _{0.5 Db} > D ₁₀₀ > D ₀ > G _t (1)
(Claim 3)
2. The taper processing is performed by pressing the tip of the billet using a die having a hole shape of a billet tip processing target shape after the heating and before the piercing and rolling. Or the piercing-rolling method of billets as described in 2.
(Claim 4)
A seamless steel pipe produced by using the billet piercing and rolling method according to any one of claims 1 to 3.

  ADVANTAGE OF THE INVENTION According to this invention, the generation | occurrence | production of the flaw of a pipe inner surface front-end | tip part can be prevented, without causing the nonuniformity in quality of a seamless steel pipe, and without requiring huge equipment investment.

In the present invention, the billet is heated and pierced and rolled. The heating temperature of the billet may be the heating temperature in normal operation. The piercing and rolling machine used for piercing and rolling may be either the Mannesmann piercer of FIG. 1 or the cone type piercer of FIG. 4, but the Mannesmann piercer is preferable from the viewpoint of equipment cost saving.
In the present invention, before piercing-rolling (before heating or before piercing-rolling after heating), as shown in FIG. 5, the tip of billet 10 (portion leading to piercing-rolling) is tapered to the outer surface. Processing 20 is performed. Needless to say, the processing shape by the taper processing 20 is a tapered shape. As a result, air stagnation with respect to the billet tip is reduced, and segregation at the center of the cross section (segregation at the tip is oxidized during heating) when the tip passes through the air stagnation part 4 (see FIG. 1). The occurrence and progress of cracks starting from the starting point are effectively prevented. Therefore, the generation and progress of cracks having inner surface irregularities that should fall down under the pressure of the plug 3 at the perforated part 5 (see FIG. 1) is suppressed at the center of the cross section of the tip part. The occurrence of soot is effectively prevented.

In the present invention, no segregation offset is required in the steel making process, and this does not lead to uneven quality of the seamless steel pipe. Also, the capital investment for tapering the billet tip is minimal.
In the present invention, as shown in FIG. 6, the processing shape by the taper processing 20 is that the diameter of the billet 10 before taper processing is D _b , the tip position after taper processing, and the diameter at a position 100 mm inside from the tip position. D ₀ and D ₁₀₀ respectively, and the gap between the rolls 1 and 2 at the position upstream of the plug tip position and 0.5 × D _b from the plug tip position of the piercing mill used for the piercing rolling is G _t , As G _0.5Db ,
G _0.5Db > D ₁₀₀ > D ₀ > G _t (1)
It is preferable to have a tapered shape that satisfies the above. In the formula (1), D ₁₀₀ > D ₀ is natural because the tapered shape is a tapered shape. Also, D ₀ > G _t is set so that when D ₀ ≦ G _t , the billet tip collides with the tip of the plug 3 while the billet tip is insufficiently hollowed (that is, the embrittlement at the center of the cross section is insufficient). This is because the plug 3 may be damaged. Further, G _0.5Db > D ₁₀₀ is set such that G _0.5Db ≦ D ₁₀₀ is an unsteady rolling state in which the billet tip has not yet passed through the roll bite and the rolls 1 and 2 are empty. This is because the stagnation-related region extends to the end of the biting side roll, and the load applied to the piercing and rolling mill may become too large.

  The taper processing 20 is a method of pressing the tip portion of the billet 10 at a stage after heating and before piercing and rolling, for example, using a die 30 provided with a hole die 31 having a billet tip portion processing target shape as shown in FIG. Is preferable because it is superior in terms of yield compared to the method by cutting before heating (or after piercing and rolling after heating) and is advantageous in terms of equipment cost compared to the method by cold pressing before heating. . Moreover, since the seamless steel pipe of the present invention is manufactured using the billet piercing and rolling method of the present invention, the inner surface of the pipe tip portion does not have wrinkles.

  A billet (circular cross section, with segregation in the center of the cross section) shown in Table 1 is heated to piercing and rolling (rolling with Mannesmann Piercer) under the conditions shown in Table 1, or heated to tapering the tip (gold). Pressing with a mold) → Punch rolling (rolling with Mannesmann Piercer) to obtain a pipe (seamless steel pipe). Table 1 shows the results obtained by observing the inner surface of the obtained tube from the tip to a position 300 mm in the longitudinal direction of the tube with a fiber scope and examining the presence or absence of wrinkles.

  From Table 1, it can be seen that in the example of the present invention, generation of wrinkles on the inner surface of the tube tip portion was suppressed.

It is explanatory drawing which shows the principle of piercing-rolling. It is explanatory drawing which shows the generation | occurrence | production mechanism of a pipe tip part inner surface flaw. It is explanatory drawing which shows an example of a prior art. It is explanatory drawing which shows an example of a prior art. It is the schematic which shows the front-end | tip part process shape of the billet used for this invention. It is explanatory drawing which shows the suitable front-end | tip part process shape of the billet used for this invention. It is explanatory drawing which shows an example of the suitable tip part processing method of the billet used for this invention.

Explanation of symbols

1 roll (a drum-shaped roll placed on top)
2 rolls (a drum-shaped roll placed below)
3 Plug 4 Empty part 5 Perforated part
10 Billet (material for seamless steel pipe, rolled material)
11 pipe (seamless steel pipe)
15 Segregation
20 Taper processing
21 roll (cone-shaped roll placed on top)
22 roll (cone-shaped roll placed below)
30 mold
31 Hole shape with billet tip machining target shape
50 疵

Claims (4)

  In the method of heating and piercing and rolling a billet, the billet piercing and rolling method is characterized in that, before the piercing and rolling, the tip end portion of the billet is tapered to the outer surface. The diameter of the billet before taper machining is D _b , and the diameter at the tip position after taper machining and the diameter at a position 100 mm inside from the tip position is D ₀ and D ₁₀₀ , respectively. tip position and the plug tip 0.5 × from the position D _b only roll gap at the upstream position respectively G _t, a G _{0.5 dB,} so the following equation (1) is satisfied, the tapered The billet piercing and rolling method according to claim 1, wherein the billet is pierced and rolled.
G _{0.5 Db} > D ₁₀₀ > D ₀ > G _t (1)   The taper processing is performed by pressing the tip portion of the billet using a die having a hole shape of a billet tip portion processing target shape after the heating and before the piercing and rolling. Or the piercing-rolling method of billets as described in 2.   A seamless steel pipe produced by using the billet piercing and rolling method according to any one of claims 1 to 3.

Images