COMPARISON OF DOUBLE AND SINGLE PIERCING PROCESS IN ...

Materials Science Research International, Vol.3, No.3 pp. 143-150 (1997)

General paper

COMPARISON OF DOUBLE AND SINGLE PIERCING PROCESS

IN SEAMLESS STEEL TUBE MANUFACTURE

Chihiro HAYASHI and Tomb YAMAKAWA

Corporate Research and Development Laboratories, Sumitomo Metal Industries, Ltd.,

Fusocho, Amagasaki, Hyogo-ken, 660 Japan

Abstract: An investigation work was made on the influence of the piercing process on the rotary forging effects, redundant shear deformations and power consumption. In this work, a study was made to compare the double and single piercing process with reference to the Mannesmann piercing mill and the cone-type piercing mill developed by the authors. If the conventional double piercing process employing the Mannesmann piercing mill is replaced by the single piercing process employing the cone-type piercing mill, the rotary forging effects and redundant shear deformations can be inhibited, and then, the materials with poor workability, such as stainless and high alloy steel can be pierced without inside bore defects. Furthermore, from the viewpoints of the piercing power and power consumption, the single piercing process employing the cone-type piercing mill is very economical. As a result, the single piercing process employing the cone-type piercing mill provides the best economical advantage with respect to both capital and running costs.

Key Words: Seamless tube, Piercing process, Operating conditions, Rotary forging, Shear strain, Power consumption.

1. INTRODUCTION

The so-called Mannesmann piercing mill is based on

the concept that the rotary forging effects are utilized to

make the billet material more brittle than its mother

material in front of the plug as illustrated in Fig. 1. In

this piercing process, the redundant shear deformations,

such as the circumferential shear strain ƒÁrƒÆ, shear strain

due to surface twist ƒÁƒÆl and longitudinal shear strain ƒÁlr

develop noticeably. As already reported, the rotary forging

effects is the cause of the initiation of inside bore defects,

and the redundant shear deformations are the cause of the

propagation of inside bore defects.

In the Mannesmann piercing mill, the piercing ratio

(piercing elongation) is about 3.5 at most. When a larger

piercing ratio is required, piercing operation is subject to a very harsh condition that the rotary forging effects

develop more and more remarkably. Also, the shear stress

fi eld in redundant shear deformations becomes more and

more harsh until the development of further inside bore

defects is no longer avoidable. In such a case, the so-

called double piercing process is employed with two

stands of the Mannesmann piercers arranged in series. For

example, the Mannesmann plug mill process may be the

most popular process for manufacturing medium-diameter

seamless steel tubes. Generally, this process comprises

the steps of piercing a billet with a first piercing mill,

expanding the pierced shell with a second piercing mill

(which is called a rotary elongator), further reducing of

the wall thickness with a plug mill, reeling of the inner

surface of the shell with a reeling mill and sizing of the

shell with a sizing mill to a predetermined outer diameter.

In the present study, a study was made to compare the

double and single piercing process with reference to the Mannesmann piercing mill and the cone-type piercing mill developed by the authors.

The cone-type piercing mill developed by the authors shown in Fig. 2, has a novel concept which contradicts the piercing principles of the Mannesmann piercing mill. The piercing mill is designed: (1) to inhibit as much as possible the development of rotary forging effects, and to make the billet material more ductile than its mother material in front of the plug; (2) to inhibit as much as possible the development of redundant shear deformations in the piercing process. This rotary piercing mill is arranged so that the cone-type rolls supported at both ends may be used as the main rolls whose axes are inclined and crossed so as to enable piercing at a high feed and cross angle, with the disc rolls adopted instead of the plate guide shoes[1-7].

In this report, the influences of the piercing process on the rotary forging effects, redundant shear deformations and power consumption have been examined by using a model piercing mill for experimental use. General views of the model piercing mill are shown in Fig. 3.

2. EXPERIMENTAL PROCEDURE

2.1. Influence of the Piercing Process on the Rotary Forging Effects

The inlet velocity of a billet was measured with a video camera placed at a side of the main roll inlet. At the same time, the rotational velocity of the billet was measured with another video camera facing the main roll inlet. Determined was the number of rotary forging times after the billet material was threaded between the main

Received December 20, 1996

143

Chihiro HAYASHI and Tomio YAMAKAWA

Fig. 1. Principles and features of the Mannesmann

piercing mill.

Fig. 2. Principles and features of the cone-type piercing mill (the super piercing mill).

(a) Entry side view. (b) Exit side view.

Fig. 3. General views of the model piercing mill for experimental use.

rolls until the material reaches the tip of the plug. In this way, the influence of the piercing process on the number of rotary forging times was studied both in the double and in the single piercing process.

The motors of the main rolls and disc rolls were stopped during piercing process, to provide a partially

pierced billet. A micro specimen for a tensile test was cut

out from this billet in front of the tip of the plug in a

diametrical direction of the billet. The tensile test was carried out with this micro specimen at room temperature to determine the elongation value for the specimen, and the influence of the piercing process on the rotary forging effects was examined. In this manner, a study was made to compare the double and single piercing process.

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2.2. Influence of the Piercing Process on the

Redundant Shear Deformations

Billets with plural pins buried radially and in parallel

to their axes were pierced, and the circumferential shear

strain ƒÁrƒÆ was measured. On the basis of the

measurements, the influence of the piercing process on

the circumferential shear strain ƒÁrƒÆ was examined. A

comparison was made between the double and single

piercing process.

Billets with shallow grooves machined on their outer

surface in parallel to their axes were pierced, and the shear

strain due to surface twist ƒÁƒÆl was measured. On the

basis of the measurements, the influence of the piercing

process on the shear strain due to surface twist ƒÁƒÆl was

examined. A comparison was made between the double

and single piercing process.


Piercing Power Consumption

Load cells were mounted on the bearings at the entry

and exit sides of the main rolls and on the thrust block

that is movable while carrying the plug and mandrel bar.

The measurements were made regarding the main roll and

plug loads. The influences of the piercing process on the

main roll and plug load were studied. The double and

single piercing process were compared in this conjunc-

tion.

The rolling torque and rolling power were calculated

fr om the voltages and currents in the main roll motors.

Also, the piercing time was measured to calculate power

consumption. The influences of the piercing process on

the rolling torque, rolling power and power consumption

were studied. The double and single piercing process were

compared in this conjunction.

3. EXPERIMENTAL RESULTS AND

DISCUSSION


Rotary Forging Effects

The influences of the piercing process, feed and cross

angle on the number of rotary forging times are shown in

Fig. 4, and those on the rotary forging effects in Fig. 5.

The results are summarized as follows.

The larger the feed angle is, the more decreased is the

number of rotary forging times. This is because the

amount of advance of the billet at each turn increases as

the feed angle becomes larger The influence of the cross

angle is also apparent. The larger the cross angle is, the

smaller is the number of rotary forging times.

The comparison between the Mannesmann piercing

(cross angle ƒÁ=0•‹) and the cone-type piercing (cross

angle ƒÁ=25•‹) shows a significant fact that the high cross

an gle setting reduces the number of rotary forging times

by about 20%. This decrease of only 20 % brings about

an amazing difference in rotary forging effects. It may

not be appropriate to compare the first piercing operation

at ƒÁ=0•‹ in the double piercing process employing the

Mannesmann piercing mill with the single piercing

operation at ƒÁ=25•‹ employing the cone-type piercing

mill, because there is a noticeable difference in the

piercing ratio between the former and the latter process.

Nevertheless, even when the piercing ratio in the latter

process is twice as large as that in the former process, the

difference between the two processes in the number of

rotary forging times involved up to the tip of the plug is

only one and half times.

The rotary forging effects are in good corelation with

the number of rotary forging times at the plug tip. The

(a) Cross angle ƒÁ=0•‹. (b) Cross angle ƒÁ=25•‹.

Fig. 4. Influences of the piercing process, feed and cross angle on the number of rotary forging times.

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(a) Cross angle ƒÁ=0•‹. (b) Cross angle r=25•‹.

Fig. 5. Influences of the piercing process, feed and cross angle on the rotary forging effects.

larger the feed angle is, the more remarkable is the

increase in the elongation value in the micro tensile test.

The influence of the cross angle is also apparent. The

larger the cross angle is, the more remarkable is the

increase in the elongation value. As the cross angle

becomes larger, the material becomes more ductile in

front the plug than its mother material at a relatively

larger feed angle site. The larger the cross angle is, the

larger is the feed angle range which increases ductility.

The comparison between the Mannesmann piercing

process at ƒÁ=0•‹ and the cone-type piercing process at cross angle ƒÁ=25•‹ shows that in the Mannesmann

piercing mill, or even at the first piercing stage of the

double piercing operation, the billet material tends to

become more brittle in front of the plug than its mother

material, although the feed angle may be large. In

contrast, in the cone-type piercing mill, even in the

single piercing operation, the billet material is rendered

more ductile in front of the plug than its mother material,

only if the larger feed angle is set. Thus, it is possible to

inhibit the inside bore defects.

When a billet material with poor hot workability is

served for the rotary piercing process, voids may occur

fr om any inclusions and/or segregations in front of the

plug. The occurrence of voids can be observed by an

optical microscope as well as an electron microscope. It

seems that the larger the number of rotary forging times

is, the more remarkable is the increase in the number of

voids in front of the plug. The above mentioned

phenomena can be understood by the corelation between

the number of rotary forging times and the number of

voids in front of the plug[5]• The rotary forging effects

may be a cause of the initiation of inside bore defects.


Redundant Shear Deformations

The influences of the piercing process, feed and cross

angle on the circumferential shear strain ƒÁrƒÆ are shown in Fi

g. 6, and those on the shear strain due to surface twist

ƒÁƒÆl in Fig. 7. The circumferential shear strain ƒÁrƒÆ and the

shear strain due to surface twist ƒÁƒÆl were calculated by the

relations ƒÁrƒÆ=rƒÆ/t and ƒÁƒÆl= rƒ³/l. The results are sum-

marized as rfollows.The larger the feed angle is, the more decreased is the

magnitude of the circumferential shear strain ƒÁrƒÆ. The i

nfluence of the piercing process is also apparent. If the

double piercing process is replaced by the single piercing

process, the circumferential shear strain ƒÁrƒÆ decreases

remarkably. The influence of the cross angle is also clear.

The comparison between the Mannesmann piercing at ƒÁ

=0•‹ and the cone-type piercing at ƒÁ=25•‹ shows that the

latter piercing results in a sharp decrease in the

circumferential shear strain ƒÁrƒÆ, to one third or less. In

particular, if the double piercing process employing the

Mannesmann piercing mill (cross angle ƒÁ=0•‹) can be

replaced by the single piercing process employing the

cone-type piercing mill (cross angle ƒÁ=25•‹), the

circumferential shear strain ƒÁrƒÆ, decreases to about one f

ourth or less. Thus, it is possible to reduce the cause of

the propagation of inside bore defects, even when a heavy

reduction piercing is demanded.

The larger the feed angle is, the more decreased is the

absolute value of the shear strain due to surface twist

ƒÁƒÆl.The comparison between the Mannesmann piercing

process at ƒÁ=0•‹ and the cone-type piercing process at ƒÁ=

25•‹ shows that the latter piercing reverses the direction of

surface twist and increases the absolute value of the shear

strain due to surface twist ƒÁƒÆl. However, any shear strain

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Fig. 6. Influences of the piercing process, feed and cross angle on the circumferential shear strain ƒÁrƒÆ.


Fig. 7. Influences of the piercing process, feed and cross angle on the shear strain due to surface twist ƒÁƒÆl

due to surface twist ƒÁƒÆl observed in the single piercing

operation is not so significant. If the double piercing

process employing the Mannesmann piercing mill (cross

angle ƒÁ=0•‹) is replaced by the single piercing process

employing the cone-type piercing mill (cross angle ƒÁ=

25•‹), the direction of the shear strain due to surface twist

ƒÁƒÆ1 will change. But there is no significant change in the absolute value. From the viewpoint of the shear strain

due to surface twist ƒÁƒÆl, the adoption of double piercing

process employing the cone-type piercing mill is not

re commended, because it merely increases the shear strain.

As for the longitudinal shear strain ƒÁlr, the influences

of the feed and cross angle are found to be little by

another experiment using a plasticine billet. Therefore,

the measurement of the longitudinal shear strain ƒÁlr was

neglected in the above experiments.

It is assumed that the magnitude of the circumferen-

tial shear strain ƒÁrƒÆ may be also related to the number of

rotary forging times. Namely, it seems that the smaller

the number of rotary forging times is, the smaller is the

magnitude of the circumferential shear strain ƒÁrƒÆ. Such

redundant shear deformations may be a cause of the

propagation of inside bore defects.

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3.3. Influence of the Piercing Process on the Piercing Power Consumption

The influences of the piercing process, feed and cross angle on the main roll and plug loads are shown in Figs. 8 and 9. Those on the rolling torque, rolling power and

power consumption are in Figs. 10, 11 and 12 respective-ly. The results are summarized as follows.

The larger the feed angle is, the larger are the increases in the main roll and plug loads. The comparison between

the single piercing and the first piercing in the double

piercing process shows that even when the piercing ratio becomes twice as large, there are increases of only about 15% in the main roll load, and about 25% in the plug

load. Both main roll and plug loads in the single piercing

process are smaller than those in the second piercing in the double piercing process. This is because that the

contact length and contact width of the main roll against

the pierced shell, and those of the pierced shell against the

plug in the second piercing of the double piercing process

are larger than those in the single piercing process. In the

case of single piercing, the heat generation due to heavy

working is also related with such difference. The

comparison between the Mannesman piercing at ƒÁ=0•‹

and the cone-type piercing at ƒÁ=25•‹ indicates that the

adoption of the latter process results in the main rolls

being 1.5 times larger in the maximum diameter, but the


Fig. 8. Influences of the piercing process, feed and cross angle on the main roll load.


Fig. 9. Influences of the piercing process, feed and cross angle on the plug load.

148



Fig. 10. Influences of the piercing process, feed and cross angle on the rolling torque.


Fig. 11. Influences of the piercing process, feed and cross angle on the rolling power

increases in the main roll and plug loads are slight.Rolling torque and rolling power are proportional to

the main roll load. The larger the feed angle is, the larger are the increases in rolling torque and rolling power. The comparison between the single piercing process and the first piercing in the double piercing process shows that even when the piercing ratio becomes twice as large, there are about 15% increases at most in the rolling torque and rolling power. The comparison between the single piercing process and the second piercing of the double piercing process indicates that the rolling torque

and rolling power in the single piercing process are

smaller than those in the double piercing process.

Furthermore, the comparison between the Mannesmann

piercing at ƒÁ=0•‹ and the cone-type piercing at ƒÁ=25•‹

shows that the larger cross angle results in a certain

amount of increase in the rolling torque and of decrease in

the rolling powex The rolling power was compared under

the condition of the same delivery speed.

The value of the rolling power in the single piercing

operation is about one half of the algebraic sum of the

rolling power values in the first and second piercing in

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Fig. 12. Influences of the piercing process, feed and cross angle on the power consumption.

the double piercing operation.

The larger the feed angle is, the larger is the decrease

in the power consumption. As already stated, the rolling

power increases as the feed angle becomes larger, whereas the piercing time significantly decreases as the feed angle

becomes larger. As a result, the power consumption

decreases as the feed angle increases. The comparison

between the Mannesmann piercing at ƒÁ=0•‹ and the cone-

type piercing at ƒÁ=25•‹ shows that a larger cross angle

results in a slight decrease in the power consumption.

This can be explained by the formerly mentioned

effects of the feed and cross angle on the decrease in the

redundant shear deformations. The power consumption

was also compared under the condition of the same

delivery speed.

The value of the power consumption in the single

piercing operation is about two thirds of the algebraic

sum of the power consumptions in the first and the

second piercing in the double piercing operation.

Therefore, from the viewpoint of energy consumption, the

double piercing process is not power-saving.

4. CONCLUSIONS

The principal conclusions can be summarized as

follows.

(1) In the Mannesmann piercing process, when the

double piercing process is replaced by the single piercing

process, the rotary forging effects become more

pronounced and the redundant shear deformations also

develop more definitely. Then, the inside bore defects

become a serious problem. If the double piercing process

employing the Mannesmann piercing mill is replaced by

the single piercing process employing the cone-type

piercing mill, the rotary forging effects and the redundant shear deformations are inhibited, even though a large

piercing ratio is required. The causes of the initiation and propagation of the inside bore defects can be simultaneously eliminated. As a result, a heavy piercing ratio can be achieved even for materials with poor workability, such as stainless and high alloy steel.(2) If the double piercing process employing the Mannesmann piercing mill is replaced by the single piercing process employing the cone-type piercing mill, the piercing power can be reduced by 50%, and the power consumption by about 40%.

Therefore, from the viewpoints of rolling power and power consumption, the double piercing process

employing the Mannesmann piercing mill is not power-saving. On the other hand, the single piercing process employing the cone-type piercing mill requires the least capital and running costs.

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K. Nishikawa, The 31st Okochi Memorial Prize (1985) 34.

2. C. Hayashi, M. Utakoji, and K. Yoshioka, Proc. of the 3rd International Conference on Steel Rolling,

Tokyo, (1985) 174.3. C. Hayashi, Proc. of International Conference on

Modernization of Steel Rolling, Beijing, (1989) 57.4. C. Hayashi and T. Yamakawa, The Iron and Steel

Institute of Japan Int., 37, (1997) 146.5. C. Hayashi and T. Yamakawa, The Iron and Steel

Institute of Japan Int., 37, (1997) 153.6. T. Okamoto, C. Hayashi, and M. Nishiguchi, JPN

Patent 808493, US Patent 3719066, UK Patent 1320035.

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