Nisshin Steel Co. placed the world’s first Sendzimir tandem
mill into operation at its Shunan works in 1969. Located in Nanyo, Japan, the
plant occupies a site of approximately one-third square mile. Occupying about 18
percent of the 315-ft total installation length are four Sendzimir stands: one
ZR 22N-50 and three ZR 21B-50. Once up and running, the mill’s maximum
finished monthly production capacity totaled 18,300 net tons -- 35 percent 400
series; 65 percent 300 series stainless steel.
Nisshin’s tandem installation is the first one designed to
continuously cold roll stainless coils up to 50 inches wide and weighing a
maximum of 24 net tons. Most of the hot band (either 300 or 400 series stainless
ranging in thickness from 0.255 to 0.063 in) comes from the reversing hot mill
at Nisshin’s Shunan works. Some hot band is supplied by the Kure works. Strip
from the tandem ranges from 0.158 in to a minimum of 0.012 in. The maximum
rolling speed is 1969 fpm.
The Nisshin installation is a fully continuous rolling
facility. Incoming coils are fed from payoff reels through a welder that joins
the head and tail of consecutive coils, thereby eliminating tail-out and the
necessity to rethread.
Eleven years before installing the tandem mills, Nisshin
commissioned Japan’s first wide (50-in) high-production Sendzimir mill. A ZR
22-50 (at that time the largest and most powerful Sendzimir reversing mill ever
built) was installed in the Hanshin works near Osaka. This mill was equipped
with twin drives -- the first time such a drive arrangement had been
incorporated in a mill of this type and size -- with a 5000-hp main motor taking
the mill up to a 2800-fpm top speed. The Hanshin plant was dedicated to the
production of low-carbon steel.
In 1964, a second ZR 22-50 mill with solid winders was put into
operation, thereby increasing Nisshin’s wide stainless strip capacity to
56,000 net tons per year.
In 1969, the BOF process successfully produced stainless; in
1970 Nisshin installed two 40-ton converters. In September 1969, the Sendzimir
cold tandem installation, built exclusively for production of stainless strip,
began operation. This installation made it possible to more than triple
production.
Installation of the tandem mill started in the middle of March
1969 and was completed by the middle of August.
A no-load test of each piece of equipment was made between the
middle and end of August 1969.
Adjustments of the control systems for the winding reel
control, accumulator control, speed matching control, strip guiding control, and
so on were made by rolling on both 2-high mills while letting the ZR mills run
idle. This cold-run period lasted from September 1 to 7 and was followed by live
rolling of low-carbon strip on September 8. The trial rolling produced strip
with good shape and no breakage. The first strip rolled was of 43-in-wide,
0.126-in hot band reduced to 0.040 in. The test run with low-carbon steel strip
was continued until September 20, 1969, in order to adjust tension and speed
control.
On September 21, rolling of AISI 430 stainless steel strip
commenced with 0.140-in strip reduced to 0.071 inch in one pass. A total of 550
tons of low-carbon steel and 300 tons of stainless were produced during the 15
days following mill startup on September 8.
Chatter marks were observed following the no. 1 stand. This was
eliminated by changing the amount of tension on both sides of the mill.
Difficulties were experienced with centering the strip at the
entry of the 2-high mill. This problem was solved by adding another EPC device
to the deflector roll at the exit of the accumulator.
Before Nisshin Steel’s tandem mill was even considered, only
a few Sendzimir mills in the world were being used to reduce or skinpass strip
on a once-through basis. Most Sendzimir cluster mills were being used in a
series of reversing operations.
Sendzimir has developed several mill arrangements, but the
dominant layout is the 1-2-3-4 configuration. This design permits the smallest
possible roll diameter for any given backing bearing diameter.
In the classical 1-2-3-4 arrangement, there are eight backing
shafts, numbered A to H in the clockwise direction. Shafts B and C are the main
screwdown shafts and are equipped with large hydraulic cylinders on the top of
the mill. These shafts have roller bearings in the saddle rings and can be
easily rotated under heavy screwdown pressure. All the other shafts have plain
bearings in the saddle rings and can be rotated only under no-load conditions.
These shafts are also self-locking, i.e., in order to open or close the mill,
the shafts have to positively moved.
Shafts A and H are moved by an electric motor located in the
back of the mill. Shafts D and E are also moved by an electric motor. These
shafts are brought closer together or further apart, depending on the size of
the rolls in the mill.
Shafts F and G, the two bottom shafts, are moved by hydraulic
cylinders located in the front of the mill. These shafts are opened or closed in
order to change the work rolls in the mill. The movement of these shafts in the
conventional Sendzimir installation serves two purposes: First, it brings the
bottom work roll to the passline of the mill, therefore providing the even
bearing of the work roll end surfaces against the thrust bearings located in the
front and back of the mill. Second, the closing of the bottom rolls removes the
slack between the rolls and enables the full travel of the top screwdown of the
mill. This permits the operator to reduce heavy hot-rolled gauge down to the
thinnest gauges without changing the work rolls.
In Nisshin’s tandem installation, the design of the lower F
and G shaft assembly differed from the classical Sendzimir reversing mill in
that these shafts were equipped with roller bearings in the saddle rings as in
the upper B and C screwdown shafts. This special feature was incorporated to
provide as much flexibility as possible into the stands to reduce the occurrence
of strip breakage at the welds. The lower screwdown, depending on the weld
condition, could be operated to provide either constant roll pressure or
constant roll gap in addition to the conventional fixed-position locked system.
To accomplish the first two operational modes, two large hydraulic cylinders
substantially the same as the upper screwdown system were utilized on each
stand, replacing the single small cylinder used in the conventional arrangement.
Most modern Sendzimir mills utilize "As-U-Roll"
adjustment, activated through small hydraulic motors that can be controlled from
the operating pulpit during rolling. This adjustment is provided on shafts B and
C acting simultaneously through a very small secondary eccentric gear train. The
adjustment can be made under load so that the operator can change the shape of
the strip while the mill is rolling.
The first intermediate rolls can be axially adjusted from the
front of the mill, and on bigger mills this adjustment is motorized,
necessitating only pushbutton control. This mechanism shifts the rolls to the
front or the back of the mill, which is very important for the operation of the
mill since these rolls are ground with a taper. The top rolls have a taper from
the front side, and the bottom rolls from the rear side. In this way, with an
independent movement of the top and the bottom rolls, it is possible to control
the shape of the edges of the strip to an extremely fine degree. The combination
of crown control and axial shift of the intermediate rolls gives an operator a
means of controlling shape and producing extremely flat strip.
The roll separating force on the 1-2-3-4 mill is distributed
from the work rolls to the intermediate rolls and then to the backup shafts in
such a fashion that the outer shaft takes a heavier force component, i.e.,
absorbs more load than the center shafts. The driven rolls in the mill are the
outer second intermediate rolls. The choice of driving these rolls enables the
designers to incorporate larger pinions. Therefore, the mill can transmit more
torque.
The Sendzimir mills in Nisshin’s tandem installation have all
the modern features listed above.
The single 20-in-diameter collapsible-block winder has an
outboard bearing for additional support during operation. The mandrel is driven
through a gear unit by two 1350-hp armatures providing a maximum tension of 22
tons up to the maximum installation exit speed of 1969 fpm.
To overcome the pressure on the winder block due to the buildup
of large coils of thin strip, Nisshin selected the collapsible mandrel design.
This mandrel differs from conventional designs in that the drum in its expanded
position is filled with solid heat-treated alloy steel as the strip is being
coiled.
Because the mill was designed for continuous operation, a
hydraulically operated horizontal-type two-strand belt wrapper initiates
coiling.
A single coil transfer car similar in design to those at the
entry side payoffs moves the completed coils from the mandrel to the coil
conveyor.
The chain-type coil conveyor is equipped with coil supporting
saddles and has an overall length of approximately 52 feet.
Two soluble-oil roll coolant and backing-bearing spray systems
were installed: one services the entry 2-high mill along with the first and
second Sendzimir stands; the other services the third and fourth Sendzimir
stands. The no. 1 system has a 50,000-gal receiving tank and 41,000-gal clean
tank; system no. 2 has capacities of 40,000 and 33,000 gal, respectively.
Filtration of the oil is by flat bed filters with system capacities of 23,000
and 29,000 gpm for systems no. 1 and 2, respectively. In total, the entire
system consists of two receiving tanks, two clean tanks, six filtering pumps,
six spray pumps, six bearing pumps and two flat-bed filters.
Lubrication of the backup bearings in the Sendzimir stands is
supplied by three oil mist lubrication units servicing each stand.
When the first wide Sendzimir mill went into operation in 1955,
the idea of putting ZR stands in tandem became apparent. Two accomplishments
made it possible to realize the idea: suitable welding of the ends of each strip
and the adoption of accumulators, and improvement of the electrical controls.
In order to use small work rolls most effectively, especially
on materials that are difficult to roll, great tension is required. Under normal
circumstances, the tension provided by a tension reel is limited by the strength
of the reel. Greater tension can be obtained by using entry and exit 2-high
mills. Moreover, by installing a 2-high mill at the entry side, it is not
necessary to use pressure pads for back tension. This not only improves the
surface condition of the strip but eliminates time lost during the exchange of
pressure boards linings.
The second advantage is the improvement of production capacity,
which can be shown by comparing the installation cost of a reversing mill with
that of a 4-stand tandem. If one assumes that the installation cost of a
reversing mill is 1, then for a 4-stand tandem it is 2.4. However, the
production capacity of a 4-stand tandem mill is four times greater, i.e., the
production ratio is 4:1. Combining these ratios provides an increase in
production capacity, as compared with the initial cost of the equipment, of
1.67:1. The tandem mill needs only one winder for several mill stands, while a
reversing mill needs two sets of winders and motors for each stand.
Labor saving costs are also important. Instead of using three
full-time workers, which is required for a single reversing mill, the 4-stand
tandem requires seven full-time workers per shift. Considering the auxiliary
equipment, such as roll grinding, etc., the resulting actual labor productivity
will be 2:1.
The most remarkable merit of the continuous mill is its
production capacity. In order to take advantage of this high capacity, it is
very important that as many as possible mill stops be eliminated. In this way,
the change in gauge produced by the mill will be reduced. Second, single coils
of 50 tons or more, without welds, would be desirable.