JPH03133508A - Drawing and rolling method for thick seamless pipe - Google Patents

Abstract

PURPOSE:To obtain a seamless steel pipe with a specified thickness at its pipe end part and stationary part, by making the interstand tension in zero over the whole length of a pipe and performing the reducing with no tension. CONSTITUTION:A seamless pipe whose thickness is uniform from the pipe end part to stationary part can be manufactured because of the pipe end part thickness of after reducing at each stand being made in a target thickness, in the case of no tension rolling and the yield is improved because of a cropless rolling becoming possible. Also, no offgage is generated by a thinning phenomenon in no tension rolling because of the blank pipe thickness being made larger than the target thickness by the part equivalent to the thinning amount in no tension rolling. Moreover, the elongation length like the thickness becoming constant at the pipe end thickness is found over the whole length of the pipe and the reducing is performed so as to obtain the elongation length, so the reducing at no tension can be facilitated.

Description

[Detailed description of the invention]

(Industrial Field of Application 1) The present invention relates to a method for reducing and rolling thick-walled seamless pipes, and in particular, the present invention relates to a method for reducing and rolling thick-walled seamless pipes. This invention relates to a method for reducing and rolling thick-walled seamless pipes that can be reduced so that After drilling and elongation rolling, the hollow tube is reduced and rolled to control the elongation length in order to obtain a finished tube of a predetermined length.When controlling the elongation length, a stretch reducer is usually used to control the elongation length. The rolls of each stand are given different rotational speeds to apply tension to the hollow tube so that it stretches while being pulled, and the wall thickness of the steel tube is controlled by the tension between the stands. In the case of a seamless pipe, the wall thickness distribution in the pipe axial direction after reduction rolling is as shown by the solid line A in Figure 6. In other words, since sufficient tension is applied to the central part, it is sufficiently elongated to reach the target. While the wall thickness (-dotted chain line B) is obtained,
By adding tension between the stands, at the tip and rear end (tube end),
Due to lack of tension, the wall thickness increases significantly compared to the center part, and this part deviates significantly from the target wall thickness.
This was discarded as crop loss, contributing to a decrease in yield. In FIG. 6, a broken line C indicates the wall thickness of the hollow tube.

[Problem to be achieved by the invention]

On the other hand, in the case of reducing rolling of thick-walled seamless tubes, the situation is different; even if no tension is applied between the stands, the thickness decreases just by squeezing the raw tube, and contrary to the case of thin-walled tubes, the thickness decreases at the tube ends. A flesh phenomenon occurs. This is because, in the case of a thin-walled tube, the stress in the wall thickness direction is almost uniform during reduction rolling, whereas in the case of a thick-walled tube, as shown in FIG. This is considered to be because the stress σt on the inner surface side (12 side) is much larger than that on the inner surface side, and the material on the outer surface side is preferentially processed by the rolls 12, and the processing does not reach the inner surface side. In other words, this thinning phenomenon is more pronounced at the ends of the tube, where there is less material restraint, than at the center, where there is restraint from the front and rear materials. A thinning phenomenon occurs. For such thick-walled pipes where thinning phenomenon occurs at the end of the pipe even when there is no tension, the rolls 12 are used to control the elongation length by applying tension in the same way as for thin-walled pipes. If the tube 10 is rotated quickly, this thinning phenomenon will be accelerated, as shown in FIG. 8, for example. Therefore, in order to ensure the target wall thickness even at the tube ends, a very thick raw tube is required, and the wall thickness at the center becomes excessive, resulting in a decrease in yield. Furthermore, a slip phenomenon occurs at the contact surface between the roll and the material, and the elongated length of each tube varies greatly, so the variation in wall thickness of each tube is greater than that of thin-walled tubes. In addition, when controlling the elongation length assuming that the end of the pipe has increased wall thickness, such as a thin-walled pipe, in order to compensate for the excessive elongation length due to thinning at the end of the pipe, a roll is used as material in the center. However, as shown in Fig. 8, the wall thickness at the center (solid line A) changes from the original pipe wall thickness (dashed line C).
'), which significantly reduces the dimensional accuracy in the longitudinal direction of the tube. The present invention has been made to eliminate the above-mentioned conventional problems, and it is possible to manufacture a seamless pipe with excellent dimensional accuracy even if it is a thick-walled seamless pipe where thinning phenomenon occurs at the end of the pipe. The purpose of the present invention is to provide a method for reducing and rolling thick-walled seamless pipes. [Means for Achieving the Object] The present invention is applicable to reducing the thickness of a thick-walled seamless pipe in which a thinning phenomenon occurs at the end of the pipe, as shown in FIG. The wall thickness to of the pipe end after rolling is the target wall thickness.

[i], the raw tube wall thickness ts is made larger than the target wall thickness ti by an amount Δt equivalent to the amount of thinning in tensionless rolling, and the elongation length is determined so that the wall thickness becomes the tube end wall thickness. In order to obtain the good elongation, the tension between the stands is set to zero over the entire length of the tube, and the reduction rolling is performed without tension, thereby achieving the above object. [Operations and Effects 1] In the present invention, when reducing and rolling a thick-walled seamless pipe in which a thinning phenomenon occurs at the end of the pipe, the tension between the stands is set to zero over the entire length of the pipe, and the reducing rolling is performed without tension. I try to do it. Therefore, slip phenomena do not occur between the rolls and the material gate, and variations in stretchability from tube to tube can be made very small, making it possible to reduce variation in wall thickness from tube to tube. In addition, since there is no compression as in the conventional method, rolling can be performed at low load by lowering the rolling torque, and the thickness limit can be increased without increasing the output of the roll drive motor. Furthermore, during the tensionless rolling, the target wall thickness is the tube end wall thickness after reduction rolling in each stand, so it is possible to manufacture a seamless tube with a uniform wall thickness from the tube end to the stationary portion. This makes it possible to perform cropless rolling, which improves yield. Furthermore, since the wall thickness of the raw tube is made larger than the target wall thickness by an amount equivalent to the amount of thickness reduction during tensionless rolling, off-gauge does not occur due to the thinning phenomenon during tensionless rolling. Furthermore, over the entire length of the tube, the elongation length is determined so that the wall thickness is constant at the wall thickness at the end of the tube, and the reduction rolling is performed so that the elongation length is reduced by 1q. By using the elongation length control of the present invention, tensionless reduction rolling can be easily realized. Embodiments Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. As shown in Fig. 2, the inventor investigated a seamless pipe with a wall thickness of 25 mm during reduction rolling using a 30-L stretch reducer 20, and found that the ratio of the wall thickness t to the outer diameter t It was found that there is a relationship as shown in Fig. 3 between /D (%) and the tube end wall thinning rate (r (%)), which is expressed by the following formula: tr= (tc - ts )/ ts...(1)
Here, ts is the wall thickness of the raw pipe, and to is the wall thickness of the pipe end after reduction rolling. Therefore, in such a seamless pipe, the ratio t/D
In a region where the pressure is approximately 20% or more, a thinning phenomenon occurs at the end of the tension-free tube, and the thinning increases as the amount of reduction in the outer diameter increases. Therefore, when reducing and rolling a thick-walled seamless pipe with a ratio t/D of 20% or more, from the relationship shown in FIG. Find 8t. Δt −f(t/D) ・・・・・・・・・(2
) Even if there is no tension, the wall thickness will be reduced by the amount Δ℃ shown in equation (2), so as shown in the following equation, the element t added to the target wall thickness ti is calculated as shown in the following equation. The pipe wall thickness [S] is made to be obtained by the piercing and elongation rolling in the previous process. ts - tH + Δ℃ (3) In this way, as shown in FIG. By increasing Δt by the amount equivalent to the amount of thickness reduction in tensionless rolling,
Target wall thickness ji (=
jc) can be obtained. In Figure 4, solid line A
is the wall thickness distribution when conventional elongation length control with added tension is performed. Next, the wall thickness over the entire length of the pipe is equal to the target wall thickness ti (=j
c) Find the stretched length and set the roll rotation speed for stretching length control so that the stretched length is obtained.
By setting the inter-stand tension to zero over the entire length of the pipe, reduction rolling can be performed without tension. Therefore, over the entire length of the tube, the deformation pattern at the tube end is applied when the tension between the stands is zero, and stable rolling can be performed at a low load. When conventional elongation length control was performed on a thick-walled seamless steel pipe with outer diameter D = 89.1u and wall thickness T = 25.0um, as shown by the broken line in Figure 5, the end of the pipe When the thinning phenomenon was performed using the control according to the present invention,
Similarly, as shown by the solid line E in FIG. 5, a seamless steel pipe with a uniform wall thickness at both the end portion and the stationary portion could be obtained.

[Brief explanation of the drawing]

FIG. 1 is a flow chart showing the gist of the present invention, FIG. 2 is a perspective view showing an example of a stretch reducer to which the present invention is applied, and FIG. A diagram showing an example of the relationship between the thickness t/outer diameter D) and the tube end thinning rate tr, and FIG. FIG. 5 is a diagram showing an example of the distribution of wall thickness in the tube axis direction after elongation length control in the embodiment and the conventional example; Figure 6 is a diagram showing an example of the original pipe wall thickness, target wall thickness, and wall thickness distribution in the tube axis direction after elongation length control in a conventional thin-walled seamless steel pipe. A cross-sectional view showing an example of the state of material deformation during reduction rolling.
FIG. 3 is a diagram showing an example of the distribution of wall thickness in the tube axis direction after target wall thickness and elongation length control. 10...Steel pipe, 12...0-ru, 20...Stretch reducer, tc...Tube end wall thickness after reduction rolling, [1...Target wall thickness, △[...None] Thickness reduction due to tension rolling, ts...Main pipe wall thickness.

Claims (1)

[Claims] (1) When reducing and rolling a thick-walled seamless pipe where wall thinning occurs at the pipe end, the wall thickness of the pipe end after reducing rolling in each stand is set as the target wall thickness, and the thinning phenomenon occurs during tensionless rolling. Make the raw pipe wall thickness larger than the target wall thickness by an amount equivalent to the amount, find the elongated length so that the wall thickness becomes the pipe end wall thickness, and adjust the length of the pipe so that the elongated length is obtained. A method for reducing and rolling a thick-walled seamless pipe, characterized in that the tension between the stands is zero over the entire length, and the reduction is rolled without tension.