JPH0718407Y2 - Mandrel bar for mandrel mill - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial application] The present invention relates to a mandrel bar used in a mandrel mill for producing a seamless steel pipe.

[Prior Art] A mandrel bar used when manufacturing a seamless steel pipe with a mandrel mill has a small diameter portion 23 at a rear end portion of a bar body 22 having a conical tip end 21 as shown in FIG. Formed,
An engaging portion 24 for restraining the mandrel bar with a restraining bar is formed at the final end portion following the small diameter portion 23. And when manufacturing a seamless steel pipe using this mandrel, after being heated in a heating furnace, the mandrel is inserted in a skewer into the raw pipe perforated by a perforation machine,
Roll with a mandrel mill consisting of 9 stands to a predetermined size in one pass. Then, after pulling out the mandrel bar from the pipe, cut off the defective shape of the pipe end with a hot saw and reheat it in a reheating furnace, then reduce the outer diameter and slightly reduce the wall thickness with a stretch reducer consisting of multiple stands. To obtain the required product.

The mandrel mill will be described in detail below.

(1) Full-float mandrel mill When manufacturing seamless steel pipes with this mill, insert the mandrel bar 31 into the raw pipe 32 as shown in Fig. 4, and simultaneously hold both mandrel bars 31 without restraining them. Roll on mill 33. And the next process strip line 34
Then, the mandrel bar 31 is pulled out from the tube 37 at the intermediate stage by using the burst ripper 35 and the shell stopper 36.

(2) Restrained mandrel mill In the case of this mill, as shown in FIG. 5, while holding the mandrel bar 31 inserted in the raw pipe 32 with the mandrel bar restraint device 38, the raw pipe 32 is mandrel milled. Roll at 33,
The tube 37 in the intermediate stage is manufactured, and then the tube 40 in the final stage is manufactured by the sizing machine 39.

(3) Retaining mandrel mill As in the case of the restraining mandrel mill, the mandrel bar 31 is constrained by the mandrel bar restraint device 38 while the raw pipe 32 is rolled by the mandrel mill 33, and the intermediate stage pipe is rolled. 37 is manufactured, but the intermediate stage pipe 37 is pulled by the extractor 41. (Fig. 6) (4) Semi-float mandrel mill As shown in Fig. 7, the mandrel bar 31 is restrained by the mandrel bar restraint device 38 while the raw pipe 32 is restrained.
Rolling at 33 is similar to restrained mandrel mills and retained mandrel mills, but instead of being fully restrained, mandrel mill 33 still leaves mandrel bar 31 in tube 37 at the intermediate stage. It is restrained to some extent. Therefore, as in the case of the full float mandrel mill, the mandrel bar 31 is configured so that the strip line 34 in the next process is used to pull out the mandrel bar 31 from the tube 37 in the intermediate stage using the burst ripper 35 and the shell stopper 36. There is.

[Problems to be Solved by the Invention] However, when manufacturing a seamless steel pipe using the above-described conventional mandrel bar, there are the following problems. That is, as is common to the various rolling methods described above, when the mandrel bar is pulled out of the pipe or when the mandrel bar is constrained, the small diameter portion of the mandrel bar and the engaging portion for pulling out or restraining the mandrel bar. Stress concentrates on the boundary part of. The force (P _B ) applied to the boundary between the small-diameter portion of the mandrel bar and the engaging portion for pulling out or restraining is expressed by the first equation, and it can be predicted that it will be a large force.

P _B = μ ₁ P ₁ ＋ μ ₂ P ₂ ＋ μ ₃ P ₃ ＋ ………… ＋ μ _n-1 μ _n-1 ＋ μ _n μ _n …… (1) However, μ _i : Tube and mandrel in the i-th stand Coefficient of friction with bar P _i : Roll reaction force at i-th stand When the specific value of P _B was measured under the conditions of Table 1, it was about 60 tons.

In this case, the stress concentration factor at the boundary between the small diameter part of the mandrel bar and the engaging part for pulling out or restraining was confirmed to be about 2.0 by experiment, so the stress applied to this part can be estimated to be 48 kg / mm ^2. . This value is far above the repeated fatigue limit of 30 kg / mm ² , which was also obtained by experiments. Therefore, an accident that this part is broken is apt to occur. For this reason, there are problems that the yield of seamless pipes decreases, the manufacturability of the factory decreases, and the basic unit of the mandrel bar deteriorates.

As a countermeasure to the above-mentioned problem, there is a method of increasing the diameter of the small diameter portion or smoothly connecting the boundary portion between the small diameter portion and the engaging portion with an arc. Moreover, if the diameter of the small diameter part is increased, the pressure receiving area of the engagement part becomes smaller and the engagement part is damaged, and if the boundary part is smoothly connected by an arc, There was a problem that the mandrel bar would come out of the restraint device due to the pulling force,
The same problem as breakage of the small diameter part will occur.

This invention solves the above-mentioned problems of the prior art,
An object of the present invention is to provide a mandrel bar in which stress is not concentrated at the boundary between the small diameter part of the mandrel bar used in the mandrel mill and the engaging part for pulling out or restraining.

[Means for Solving the Problems] A mandrel bar for a mandrel mill according to the present invention has a rear end portion of a bar main body having a small diameter, and a sleeve is fixed to the small diameter portion by shrink fitting, and the bar of the sleeve is fixed. It is a mandrel bar for a mandrel mill in which an engaging step portion with a restraining bar receiving portion is formed by the end on the tip side of the main body.

[Operation] The mandrel bar for a mandrel mill and the rear end portion of the bar body according to the plan are configured to have a small diameter, and the sleeve is fixed to the small diameter portion by shrink fitting, and is restrained by the end of the sleeve on the tip side of the bar body. An engaging step portion is formed with the bar receiving portion. Therefore, when pulling out the mandrel bar from the pipe or restraining the mandrel bar, the tensile stress applied to the mandrel bar is the friction generated on the contact surface between the surface of the small diameter part of the mandrel bar and the inner surface of the sleeve shrink-fitted to the small diameter part. Since it is converted into force, if this contact surface is made large enough, it is possible to reduce the stress concentration that occurs at the boundary of the engagement step between the small diameter part of the mandrel bar and the restraining bar receiving part,
It is possible to reduce breakage accidents of the mandrel bar.

[Embodiment] A mandrel bar for a mandrel mill according to an embodiment of the present invention will be described with reference to FIGS. 1 and 2. 1 and 2 show a mandrel bar (A) of the present invention and a restraining bar (B) for restraining the mandrel bar (A). In this mandrel bar (A), a small diameter portion 3 is formed at a rear end portion of a mandrel bar main body 2 whose tip portion 1 is conically pointed. Then, a sleeve 4 shorter than the length of the small diameter portion 3 is fitted onto the small diameter portion 3 by shrink fitting. The end of the sleeve 4 on the tip side of the mandrel bar body 2 forms an engagement step 6 that engages with the U-shaped receiving portion 5 of the restraining bar (B). The outer diameter of the sleeve 4 is the same as the outer diameter of the mandrel bar body 2. The shrink fit rate of the sleeve 4 is preferably about 1.3 × 10 ^{−3 to} 1.5 × 10 ⁻³ .

Describing the restraint bar (B), the U-shaped receiving portion 5 of the restraint bar opens at the end surface and the upper surface of the restraint bar (B), and the inside thereof engages with the engagement step portion 6. The joining portion 7 is formed. The rear end portion of the mandrel bar including the sleeve 4 is inserted into the U-shaped receiving portion 5 through the opening on the upper surface thereof, and the engaging portions 6 and 7 are engaged with each other, whereby the mandrel bar main body is restrained by the restraining bar. To be done.

Since the mandrel bar for a mandrel mill according to one embodiment of the present invention is configured as described above, the pulling force applied to the boundary between the small diameter portion of the mandrel bar and the engagement step portion and the outer surface of the small diameter portion of the mandrel bar and the sleeve. Since it can be converted into the frictional force of the contact surface with the inner surface, it is possible to avoid stress concentration occurring at the boundary between the small diameter portion of the mandrel bar and the engagement step portion,
The breakage accident of the mandrel bar can be eliminated.

[Effect of the device] With this device, the stress concentration near the root of the engaging step of the mandrel bar can be significantly reduced compared to the conventional structure, and the occurrence of cracks and breakage accidents due to the stress concentration can be eliminated. It is possible to obtain an excellent effect of being able to

[Brief description of drawings]

FIG. 1 is a plan view showing the structure of a mandrel bar for a mandrel mill of the present invention and its holding mechanism, and FIG. 2 is a partially cutaway side view showing the structure of a mandrel bar for a mandrel mill of the present invention and its holding mechanism. The figure is a plan view showing the structure of a conventional mandrel bar for a mandrel mill, FIG. 4 is an explanatory view showing a manufacturing process of a full float mandrel mill, and FIG. 5 is an explanatory view showing a manufacturing process of a wristless mandrel mill. FIG. 6 is an explanatory view showing the manufacturing process of the retained mandrel mill, and FIG. 7 is an explanatory view showing the manufacturing process of the semi-float mandrel mill. 1 ... Mandrel bar tip, 2 ... Mandrel bar body, 3 ... Mandrel bar small diameter part, 4 ... Sleeve.

 ─────────────────────────────────────────────────── --Continued from the front page (56) References JP-A-60-206507 (JP, A) JP-A-61-158302 (JP, A) JP-A-2-197305 (JP, A)

Claims (1)

[Scope of utility model registration request] 1. A rear end portion of a bar body having a small diameter, a sleeve is fixed to the small diameter portion by shrink fitting, and an end of the sleeve on the tip end side of the bar body engages with a restraining bar receiving portion. A mandrel bar for a mandrel mill, which is characterized by forming a part.