NIPPON STEEL TECHNICAL REPORT No. 90 July 2004
UDC 621 . 774 . 21

Outline of New Forming Equipment for Hikari 24” ERW MillMichitoshi TANIMOTO*1
Osamu SOTOKAWA*2
Tetsuya MAGATANI*1
Hiroyuki MIMURA*3

Isamu KAWATA*2
Tomohiro NAKAJI*1
Eiji TSURU*3
Masanori NAKAMURA*1

Abstract
Nippon Steel Hikari 24” ERW Mill had employed the cage roll forming mill for
long time to get the advantage for producing thin-walled pipes and saving the timefor roll-exchange. However, a new forming mill is desired for meeting the recentmarket needs, such as pipes with ultra-thin wall-thickness and/or high formability.
We remodeled the forming process in 24” ERW Mill and got the planned results:
stable manufacturing of ultra-thin-walled pipes, high dimensional accuracy andhigh forma bility of pipes, and saving the time for roll-exchange.

1. Introduction

strength steel pipes
High-pressure operation of a large-diameter pipeline is effectivein enhancing the transportation efficiency of natural gas and the like.
Increase in the strength of line-pipe products has been pursued forthis reason1). On the other hand, there has been a trend towards reducing material and welding costs in pipeline construction by decreasing the wall thickness of high-strength line-pipes.
(2) High strength and good deformability
Pipelines are often laid underground or under the ocean, and therefore, if the pipes are destructed locally by deformation due to causessuch as seismic force, it will be very costly to repair the damaged
portions. For this reason, line-pipes are required to have good resistance to local buckling, or high deformability2). Yield ratio (yieldstrength (YS) / tensile strength (TS), hereinafter referred to as YR) is
used as an indicator for simple evaluation of deformability, and aline-pipe product having a low YR is preferred. Generally speaking,
the YR of a steel material tends to be higher as its strength increases.
For this reason, it is not easy to cope with the increasing needs forhigh-strength line-pipes and, at the same time, satisfy the market
requirement for a high-grade line-pipe having a low YR. The YR ofa material steel sheet affects not only the properties of a product pipe
but also the work hardening during the pipe forming process, and forthis reason, a technology of pipe forming at a low strain is importantfor producing low-YR line-pipes.

Nippon Steel Corporation produces electric resistance welded(ERW) steel pipes 10.5 to 609.6 mm in outer diameter on 7 pipe
production lines in total. The 24” mill (for an outer diameter up to609.6 mm) of Medium Diameter ERW Pipe Plant, Hikari Pipe &Tube Division (hereinafter referred to as the Hikari 24” Mill), has acapacity for the largest outer-diameter among the lines, and produces
mainly line-pipes. The mill was constructed in 1958 as a 14” mill(for an outer diameter up to 355.6 mm), and various revamps andnew technologies have since been introduced to the mill to respondto changing market needs.
Requirements in the line-pipe market have tended to be evermore sophisticated over the last years as seen typically with increasing demands for ultra-thin-wall pipes, high-deformability pipes andthe like. In order to cope with such market demands, Nippon Steeldetermined that it was necessary to modify the forming equipmentof the Hikari 24” Mill. Therefore, in the autumn of 2003, they installed new forming equipment as a major revamp of the mill.
This paper outlines the new forming equipment and its effects
verified through actual production operation.

2. Market Requirements about Line-pipes
The principal requirements in the line-pipe market related to pipe
forming methods are as follows:(1) Increasing demands for large-diameter, thin-wall and high*1*2*3

Pipe & Tube Division
Environment & Process Technology Center- 122 -Steel Research Laboratories

NIPPON STEEL TECHNICAL REPORT No. 90 July 2004

3. History of Remodeling of Hikari 24” Mill and New
Forming Method

as a long time required for changing the rolls and adjusting theirpositions and a large space for storing a great number of these forming rolls. There was another problem in the forming of thin-wallsteel pipes: spring back of the material steel strip often occurred, alarge forming strain was imposed on it, the strip suffered work hardening, and all these resulted in an increase in the YR of a productpipe. In consideration of these problems, the forming method of themill was changed in 1984 into the cage roll forming method, which
is described in the following sub-section.
3.2 Cage roll forming method
The cage roll forming method, which was developed in the late1960s for medium diameter steel pipes, is characterized by continuously forming a steel strip into a tubular shape using many smallrolls, called cage rolls, arranged along the outer surface of the steel
strip to be formed. The cage forming method introduced to the Hikari
24” Mill was the one where the intermediate forming section of thepreviously used step roll forming method was replaced with a cageroll section. Fig. 3 shows the configuration of the forming equipment. The same 52 small rolls that constituted the cage forming
section could be used for forming product pipes of any size, and as aresult, roll changing time was significantly reduced4). On the occasion of the revamp of the forming equipment, the forming method in

Fig. 1 shows the manufacturing processes of the Hikari 24” Mill.
A flat steel strip is continuously formed into a tubular shape, the butt
seam is welded, and after hydrostatic test, non-destructive inspection and so forth. Then, the ERW steel pipes are shipped as finishedproducts. The material quality including chemical composition of
the steel strip, the technologies of forming and welding and nondestructive inspection constitute the key technologies that determine
the quality of an ERW steel pipe.
The equipment revamps in and introduction of new technologies
to the Hikari 24” Mill3,4) were focused on these key technologies in
addition to the expansion of product size range, as seen in Table 1.
The forming method of the mill, especially, was changed from thestep roll forming method using contoured rolls to the cage roll forming method in 1984, and then to the flexible forming method (hereinafter referred to as the FF method) in 2003.
The characteristics of the old forming methods and the history
leading to the introduction of the FF method are explained below.
3.1 Forming method with contoured rolls
The forming method with contoured rolls, which was the initial
forming method of the Hikari 24” Mill, is a method of bending asteel strip stepwise into a tubular shape using contoured rolls. Fig. 2shows the configuration of the forming equipment of the mill after
the expansion of maximum outer diameter to 24” in 1976. Since
there were many forming stands and the contoured rolls mounted onthese stands had to be changed every time the outer diameter or wall
thickness of the product pipe was changed, there were problems such

Fig. 3 The former forming mill (cage roll forming mill) of NSC Hikari
24” ERW millFig. 2 Step roll forming mill of NSC Hikari 24” ERW mill in 19763)

Fig. 1 Manufacturing process of NSC Hikari 24” electric-resistance-welded (ERW) pipe millTable 1 History of manufacturing facilities of NSC Hikari 24” ERW mill3,4)
YearForming

1958
1960s
1970s

Step roll forming mill

1980s

Cage roll forming mill

1990s

Size range
Welding
Outside diameter
Thickness14”(355.6mm)max. 0.5”(12.7mm)max. Low frequency welder16”(406.4mm)max.
High frequency welder24”(609.6mm)max. 0.63”(16.0mm)max.

Non-destructiveinspection

Seam annealer
Seam UST, rotary UST
Computer-aided quality
control system
Seam QT equipment0.75”(19.1mm)max.
0.87”(22.0mm)max. Replacement of welder

2000s Flexible forming millNote: UST = Ultrasonic Tester, QT = Quenching and Tempering- 123 -Others

Replacement ofseam UST, rotary USTNIPPON STEEL TECHNICAL REPORT No. 90 July 2004
the initial forming stands was changed into the W bending method,
and this, in combination with smooth forming by the cage rolls, reduced the strain at edge forming in the production of thin-wall pipes4).
After the revamp of the forming equipment in 1984, while theabove good effects were fully enjoyed, there still remained problems
such as the roll change of the breakdown (BD) stands and the work
hardening of the material due to the forming by rolling at the BD
stands, which resulted in an increase in YR. Therefore, Nippon Steeljudged that a further change of the method of forming was necessary
in order to cope with the requirements of the line-pipe market flexibly, and began studying introduction of new forming equipment
based on the FF method, which is described in the following subsection.
3.3 Flexible forming method and study of new forming equipment by the method
The FF method is the latest pipe forming method developed inJapan, and its characteristics are as follows5,6):(1) Roll profile in involute curves that comprise all curvatures of
product outer diameters
With this method, it is possible to form a steel strip to the curvature of any outer diameter within the product range by clamping thestrip at the position of a roll, as shown in Fig. 4, the curvature atwhich is best suited for the outer diameter and wall thickness of the
product pipe. As a result, no excessive deformation is imposed, and
it is possible to form thin-wall pipes at a low strain.
(2) Mechanism for freely changing roll position setting
The roll support mechanism of forming stands is so designed as
to allow a very wide freedom of roll positioning such as verticalshifting and axial tilting. Because of this mechanism, as seen in Fig.
5, pipes of different diameters can be formed by simply changing thepositioning of the same set of rolls. Therefore, roll change is notnecessary. The roll stands are designed to have enough strength towithstand the forming force.
(3) Numerical control system
Since the change of the positioning of the rolls is done automatically, the time for roll adjustment is short. In addition, the roll positioning of all the stands for forming a certain product size can be
stored in the mill control computer as a complete package. Therefore, any past forming condition can be replicated instantaneously.
However, at the time of the study for the revamp, the FF method
had been commercially applied to the production of pipes up to 16”
in outer diameter. Thus, in applying the method to forming pipes aslarge as 24” in outer diameter, there were various technical difficulties to overcome. One of the principal difficulties was how to securemill rigidity while enjoying the advantages of forming a wide rangeof product sizes by the flexible positioning of the variable rolls. Theauthors carried out the following studies to solve the problems.
Initially, stress at various portions of roll stands was analyzed
employing the finite element method to identify the weakest portionin each of the stands, and a stand structure having sufficient rigidity

Fig. 5 A typical roll set of flexible forming mill5) (CL: cluster roll)

as a whole was worked out for each forming stand by dispersing thereaction force. Then, the influence of low-strain forming over YR
was also analyzed. A finite element analysis model was constructed
for each of the rolls used in both the cage roll forming method according to Fig. 3 and the FF method according to Fig. 6, and theincremental strain at each of the forming rolls was calculated for awide range of product sizes. Fig. 7 shows an example of the analysis
results with respect to the change of forming strain in the circumferential direction at a point of a pipe opposite (at 180˚ from) the welded
seam. It is understood from the graph that, in the case of the FF
method, the equivalent plastic strain is lower than that by the cageroll forming method by approximately 40%, and that the YS after
pipe forming is lower. Since TS is the same in either of the forming
methods, the YR by the new method is lower than that by the oldmethod.
The introduction of new forming equipment by the FF method
was decided on the basis of these analysis results.

Fig. 4 Forming by rolls with involute profiles (PP: pinching point)

Fig. 6 The new forming mill (FF mill) used for the study- 124 -NIPPON STEEL TECHNICAL REPORT No. 90 July 2004
4.1 Stable production of thin-wall steel pipes
Photo 2 shows examples of steel pipes produced using the newforming equipment. The new equipment enables optimum formingof all the product sizes, and as a result, it has become possible toproduce ultra-thin-wall steel pipes, such as the one in the left-handside of Photo 2, having a t/D ratio (wall thickness divided by diameter) as low as 1.0%, which was difficult by conventional methods.
4.2 Improvement of dimensional accuracy
The new forming method enables to form steel pipes having a(a) Forming strain

Photo 1 Appearance of the new forming mill(b) Yield strength
Fig. 7 Comparison of calculated forming strain and yield strength ofpipes between the cage roll forming mill and the new formingmill

4. Effects of New Forming Equipment
Photo 1 shows the appearance of the new forming equipment
installed in the Hikari 24” Mill.
Some examples of the advantages of the new forming equipmentthat have been verified through actual operation are explained below.

Left: 609.6mmφ × 6.0mmt (24”φ × 0.24”t), t/D = 1.0%,
Right: 609.6mmφ × 22.0mmt (24”φ × 0.87”t), t/D = 3.6%
Photo 2 Examples of pipes manufactured by using the new forming mill(a) The former mill (cage roll forming mill)(b) New forming mill (flexible forming mill)Fig. 8 Examples of improvement in the distribution of outside diameter (406.4mmφ × 16.0mmt (16”φ × 0.63”t))- 125 -

NIPPON STEEL TECHNICAL REPORT No. 90 July 2004
wide range of wall thickness stably, and as a consequence, the dimensional accuracy of products has improved. As exemplified inFig. 8, the dispersion of outer diameter has decreased. The newforming method is effective also in enhancing roundness.
4.3 High forming capacity
Fig. 9 shows the difference of YR before and after the introduction of the new forming equipment. It is clear from the graph thatYR has lowered as a general trend.
4.4 Improvement in productivity
Fig. 10 shows the difference of size changing methods between
the old and the new forming equipment. Whereas, at a product sizechange, all the rolls of BD0 to BD5 of the old equipment shown inFig. 3 had to be changed, the rolls of BD1 to BD5 of the new equipment are variable rolls and thus none of the rolls of the BD stands
have to be changed. As a result, the time it took for product sizechange has been markedly shortened and in addition, the number ofrolls required for forming different product sizes has been drastically decreased.

Fig. 9 Change in yield ratio of pipes by introducing the new formingmill

5. Closing
In order to respond to the increasing demands in the line-pipe
market for high-grand products, typically such as ultra-thin-wall pipes
and high-deformability pipes, new forming equipment based on theflexible forming method has been introduced to the Hikari 24” Mill.
The authors confirmed through commercial operation that the introduction of the new forming equipment brought about advantages
such as stable production of thin-wall steel pipes, enhancement of
the dimensional accuracy and deformability of product pipes andreduction of roll changing time as initially envisaged.

(a) Cage roll forming: fixed rolls

References
1) Kimura, F.: Piping Engineering. Extra Number, 19(2002)
2) Suzuki, N.: Piping Engineering. Extra Number, 43(2002)3) Muramatsu, S. et al.: Seitetsu Kenkyu. (297), 94(1979)
4) Shibano, H. et al.: Proc. 3rd Int. Conf. on Steel Rolling. Tokyo, ISIJ,
1985, p.280
5) Nakata, T.: Journal of the Japan Society for Technology of Plasticity.
38(443), 1061(1997)
6) Kiuchi, M.: Journal of the Japan Society for Technology of Plasticity.
38(443), 1051(1997)(b) Flexible forming: flexible rolls
Fig. 10 Comparison of the way of changing of breakdown roll size
between the former mill and the new forming mill

- 126 -