JPS60124405A - Piercing method of hollow blank pipe - Google Patents

Abstract

PURPOSE:To prevent the generation of inner-surface flaws at the front end of a hollow blank piep and to improve the yield of a product by regulating the temperature of a plug arranged at the front side of the billet transferring direction to a prescribed temperature or more at least at the beginning of rolling. CONSTITUTION:A plug 30 arranged at the front of a pair of rolls 28 having each a convex, double circular truncated cone shape, is heated to a prescribed temperature, 200 deg.C, etc., or more at least at the beginning or rolling. When the piercing of a billet 10 by the rolls 28 and the plug 30 is started; a crack 29 generating in the inner surface of billet 10 is perfectly press-welded by the plug 30 and the rolls 28, because the inner surface of billet 10 is not cooled by the plug 30 at the time of beginning the rolling. In this way, a pipe stock 16 is prevented from causing flaws in its inner surface from the time of beginning the rolling, and the quality and yield of a product are improved.

Description

[Detailed description of the invention] The present invention relates to a method for piercing and rolling a hollow shell.

As shown in FIG. 1, one method for manufacturing seamless steel pipes is to first heat a round billet 10 as a raw material in a rotary hearth type heating furnace 12, and then utilize the Mannesmann effect. After drilling with a piercer 14 and inserting a mandrel bar 18 coated with a lubricant into the hollow wooden tube 16 after the drilling, the hollow tube 16 is sent together with the mandrel bar 18 to a mandrel mill 20 to be stretched and rolled. After rolling,
After pulling out the mandrel bar 18 from the tube 22 using a burst stripper or the like and reheating the tube 22 in the reheating furnace 24,
There is a so-called Mannesmann-mandrel mill rolling method in which a stretch reducer 26 applies a reduction in outer diameter and a slight reduction in wall thickness to produce a finished rolled tube having the required product dimensions. According to this Mannesmann-mandrel mill rolling, productivity and rolling efficiency are high, and long length rolling is possible.

Further, as a method for manufacturing seamless steel pipes, there is also the Mannesmann plug mill rolling method which is suitable for manufacturing larger diameter steel pipes, but the piercing rolling using the piercer is the same as the Mannesmann mandrel mill rolling.

By the way, piercing rolling using the piercer 14 takes advantage of the property that when a rod-shaped object is rotationally compressed from both sides, a void is likely to be formed in the center, and is known as the so-called Mannesmann pipe manufacturing method. That is, as shown in FIG. 2, when the billet 1o is interposed between a pair of rolls 28 having a medium and high double truncated cone shape so as to be transportable in the axial direction X, and a rotational compression force is continuously applied to the billet 10. , a tensile force acts on the center of the billet, causing cracks 29 and opening of holes. Since this hole is irregular, a hollow portion 32 is continuously formed in the billet 10 by enlarging the eye hole with a plug 30 placed on the front side of the billet 10 in the transport direction.

Here, the cracks 29 inevitably occur in this type of piercing rolling, but when piercing rolling is in a stable and steady state, they rarely remain as internal flaws. There is no particular problem.

However, when the piercing rolling has not yet become stable and steady, such as immediately after the start of rolling, there is a problem in that the cracks 29 remain as so-called sludge-like flaws. In particular, these internal flaws frequently occur when alloy steel with poor hot workability or billets produced by continuous casting are used, leading to poor quality and decreased yield.

The present invention has been made in view of these conventional problems, and provides a method for piercing-rolling a hollow mother tube, which eliminates the above-mentioned internal defects immediately after the start of piercing-rolling and can perform piercing-rolling with a high yield. Its purpose is to provide a method.

In the present invention, between a pair of rolls having a medium and high double truncated cone shape,
In a method for piercing and rolling a hollow shell, in which a billet is interposed so as to be transportable in the axial direction, and a hollow portion is continuously formed in the billet by a plug placed on the front side in the transport direction of the billet, at least at the start of rolling. The above object is achieved by increasing the temperature of the plug to a predetermined temperature or higher.

The present invention takes into consideration the fact that the above-mentioned internal flaws occur particularly often at the tip of the hollow tube before the piercing and rolling reaches a stable and steady state, and the present invention has been developed by This is because the inner surface of the billet is cooled by the plug, which is in a low temperature state at the start of rolling, and the cracks are not sufficiently compressed by the plug and rollers.On the other hand, internal defects disappear when piercing rolling reaches a stable and steady state. discovered that this is due to the fact that the plug is gradually heated during rolling, and the cracks that occur are completely crimped onto the inner surface of the hollow shell.Based on this finding, they took measures such as preheating the plug to a predetermined temperature during rolling. It was decided to heat and raise the temperature at .

In developing the present invention, a method of heating the tip end of the billet was also considered, but good results were not obtained because the billet was rapidly cooled by the plug.

The present invention will be described in detail below based on the drawings.

In the present invention, for example, as shown in FIG. Billet 1 is removed by the plug 30 placed on the front side.
In the method for piercing and rolling a hollow shell in which a hollow portion 32 is continuously formed in a hollow shell, the temperature of the plug 30 at least at the start of rolling is set to a predetermined temperature or higher, for example, 200'C or higher.

Methods for heating the plug 30 include a method of preheating the plug itself from the outside using gas vanina, steam, waste gas, etc., or a method of making the plug itself a hollow structure and filling the hollow part with steam, waste gas, etc. Conceivable. When the latter method is adopted, there is an advantage that the plug temperature can be controlled during rolling.

In addition to the method of preheating the plug immediately after replacing the plug, plug heating can also be performed by abolishing the cooling conventionally performed during continuous rolling or by stopping the cooling midway through. The effect of reducing internal defects can be obtained.

Further, the preheating temperature of the plug 30 is determined as follows.

Figure 3 shows C=0.25%, cr-1, Q%, Mo=
This figure shows the relationship between plug preheating temperature and internal flaw occurrence rate near the tip of the billet when drilling a seamless steel pipe containing 0.2% of the component, having a diameter of 7 Bφ, and a thickness of 11 days.

According to the relationship shown in the figure, the preheating temperature of the plug 30 is 200°C.
The optimum temperature is between 400'C and 400'C. In other words, when the preheating temperature of the plug is 200°C or lower, the rate of occurrence of internal flaws increases rapidly, whereas even if the preheating temperature of the plug is 400°C or higher, the rate of occurrence of internal flaws does not decrease much, which reduces energy consumption. Alternatively, from the viewpoint of the life of the plug, there is no need to preheat the plug to 400° C. or higher.

Note that this preheating temperature should be changed depending on the material of the billet or the rolling conditions such as the gap between the rollers 28 and the rotational speed. For example, for alloy steels with poor hot workability, it is desirable to preheat to a higher temperature.Also, when using billets produced by continuous casting, which generally have a high incidence of flaking defects, it may be necessary to preheat to a higher temperature. It is desirable to preheat up to the eyes.

FIG. 4 shows the simulation calculation results of the distribution state of the inner wall temperature of the hollow shell during piercing and rolling using the finite difference method. From the same figure, it can be confirmed that the temperature drop on the inner wall of the hollow shell is reduced due to the plug preheating, and it can be inferred that the crimping is performed that much better.

- Note that the present invention does not preclude preheating the vicinity of the tip of the fillet in addition to preheating the plug.

As explained above, according to the present invention, the phenomenon in which the inner surface of the tip of the hollow shell tube is cooled by the plug and prevents crimping during rolling is eliminated, and the occurrence of inner surface flaws at the tip is prevented. The expected effect is that the quality of the product and the yield can be improved.

[Brief explanation of drawings]

Fig. 1 is a schematic process diagram of Mannesmann-mandrel mill rolling, Fig. 2 is an enlarged partial cross-sectional view of the piercer shown in Fig. 1, and Fig. 3 is the principle of the method of piercing and rolling a hollow shell according to the present invention. FIG. 4 is a diagram showing the relationship between plug preheating temperature and internal flaw occurrence rate to explain the above. FIG. 4 is a miniature diagram showing the distribution state of the inner wall temperature of the hollow shell during piercing and rolling using a piercer. 10... Billet, 14... Piasa, 16...
Hollow tube, 28...0-ru, 29...crack, 3
o...Plug, 32...Hollow part, X...Axis direction. Agent Takaya Ron (and 1 other person) Figure 1 Z? Figure 2 Figure 3 Reward i 'J 7u7"fiSffi return (0c)

Claims (1)

[Claims] (1) A billet is interposed between a pair of rolls in the shape of a medium-high double truncated cone so that it can be transferred in the axial direction, and a hollow part is continuously formed in the billet by a plug placed on the front side in the transfer direction of the billet. A method for piercing and rolling a hollow shell, characterized in that the temperature of the plug at least at the start of rolling is set to a predetermined temperature or higher.