Production Technology of Cored Wire Used for Liquid Metal Treatment in Steel Plants: Part Four

Mill Configuration

A single cored wire production line comprises of six integrated operating units and one auxiliary shop. These include; a strip de-coiler, external strip guide rolls, forming/filling unit, coiling unit, packing unit, central monitoring and controlling unit, and powder shop. (additive comminuting and storage system are built in close vicinity of the mill). The movable supporting units like welding set, hoist, coil lifting car etc are kept in readiness. In large mills, the mill lines are arranged according to the convenience of the powder shop operation and distribution, like Ca & Ca- alloy, Ferro-alloy, RE etc and often as per CW diameters.

The different operating units of a mill line are as follows;

1) Twin de-coiler Unit - This is a mechanical device that supports the strip coil while allowing the strip to be fed into the CW forming machine smoothly, the strip coil with its internal diameter is loaded on the expandable drum of the idle de-coiler. When the coil in service is exhausted the idle coil is placed into service & in the meantime the blank spindle is loaded for next change over. The limit switches with upper & lower coil proximities are meant for start & stop coil feeding, avoiding formation of excessive loop.

2) External strip guide rollers are placed after de-coiler & prior to roll forming unit. It functions as a guide for the strip entering into roll forming machine. The rollers ensure that the strip enters the mill at the correct point and angle and prevents the transverse movement.

3) CW Forming & filling Unit - This unit is a conjoined set of two pieces of equipment, i.e. roll forming & powder filling devices. The forming unit is made up of a sturdy steel plate mounted on a rigid structure. This plate supports the stands, shafts, rolls, drive train, gear boxes and other components according to the design. In this unit the tools are chronologically assembled in the following order i.e., strip entry gate, internal guide rolls, sets of pre-forming rolls, powder filling port, forming, finishing stations and coil exit gate. The performance of this set depends on the components chosen for making the set as stated below;
a) Roll stations - tandem sets of rolls used in roll forming to shape the steel strip into a steel hollow tube of designed cross section & lock-seam. The stand holds one complete top and bottom roll set and is used to shape the strip in a series of progressive stages; the total number of stands may be twelve to fifteen or even more.
b) Roll shaft or spindle acting on a revolving axis which mounts roll tooling onto it & provides the mechanical force required for forming. The shaft diameter reflects the capacity of machine.
c) Roll space is the linear space of the shafts where the roll tooling and alignment spacers are mounted.
d) Horizontal center is the distance between the centerline of a shaft on one station to the center line of the shaft on the next station.
e) Vertical center is the distance between the centerline of the bottom shaft and the centerline of the top shaft.
f) Drive (chain, gear, mixed drive etc) in forming machines either the motor drives the reducers directly or it drives via a transmission system that drives the reducers. Chain & sprocket drive is inexpensive, but depends on the type of mill. The main motor horsepower requirement depends on the number of roll stands and the overall machine design used to drive the machine.
g) Gearing - It is necessary to choose a gearing system that can withstand shock loads and heavy loading even at low speeds.
h) Filling station - Mechanical or belt feed is used for powder filling, depending on the nature and feeding rate of additive the filling machine is chosen. The filling station is installed in between performing & forming dies. It may be single point or two point installations; in conventional filling systems the additive is drawn from storage into a middle bucket then to a feeding hopper from there it is delivered to the adjustable spout for pouring into the horizontal U channel continuously.
i) Forming stations - With the help of forming dies the two extending peripheral edges of the U channel are wrapped into a seam lock horizontally throughout the coil length.
j) Finishing station - The role of this unit is to provide appropriate diameter to the CW after seam lock formation.

4) Coiler Machine - the coiler is an assembly of a motor driven rotating drum mounted on steel plate movable on rails. The coiler machine consists of a main shaft, a fixed steel plate with an expandable winding reel, a hexagon-head nut for expanding the mandrel, a removable gap adjusting steel plate, a driving system with bearing, a base which can move on the rail. The coiler is designed to wind the cored wire using a level winding mechanism (easier to wind onto reel and easier to dispense from it).The rail is used to support the coiler and allow the coiler move to and fro upon it.

Back tensioning is very important when installing a CW onto a winch where a layer of coiling exists. If insufficient back tensioning is produced, the CW on the top layers of the drum will be prone to damaging or rarely cutting into the under layers upon load application. The driving system consists of a 3-phase motor, gear-box, sprockets and a chain or alternative arrangements, it is used to support and drive the Main shaft.

5) Packing Unit - the CW Coil is tightened by radial bounding in four directions at 90 degrees, with anti-corrosive steel straps. The bare coil is susceptible to corrosion and handling injuries and such risk is increased during transit. The surface damage may lead to spillage and deterioration in CW quality. To restrict damage each coil is wrapped with corrosion protective plastic shrink materials. Depending on the requirements of smooth & safe handling CW may be packed horizontally / vertically caged or in palletized forms and fastened on it by means of anti-corrosive steel straps. The shelf life and relevant packing details are provided on each & every coil.

6) Monitoring & Control Panel - is the unit from where the working of whole production line is monitored and controlled with the help of electronic gadgets. It consists of the following major components; LCD monitor to watch the working of the forming machine and additive feeding with the help of a digital camera, speed adjusting rheostats for coiler & forming machine motors and their synchronization, converter for setting/altering the working parameters of the powder feeding system and simultaneous monitoring. Besides above several control facilities are made available on the control panel.

7) Powder Shop - This is an auxiliary facility provided to the CW mill. The elements like Ca, C, Al, Bi, Si, Te, Pb, Fe, S, Se, Mo, W, Cr, Mn, B, V, Ti, Nb, Yt, La, misch-metal, etc as such or their alloys or a combination of alloys are used as CW additives. The additives usually arrive in the mill as lump/ briquettes which are added within the specified limits. Classifier blades of a pulverizing machine are set to positions & additive fineness testing is conducted at full load to determine degree of fineness. The classifier is adjusted to achieve >95% passing 8 mesh and < 5% passing through 150 mesh Tyler screen. More precisely Ca & Ca alloy particles between (-) 3mm and (+) 2.5mm, 3% max, & (-) 2.5mm and (+) 0.1mm 92% min, & below 0.1mm 5% max is controlled. However there may be some exceptions depending upon the application & type of the additive used.

Mill Performance - After putting the above machinery components in particular positions, with all consumables and product handling systems, the performance trial of CW production is executed to examine the fulfillment of the project objective and prepare proposals for improvements. The success of the CW forming machine depends mainly, on conformity of steel strip mechanical properties with the properties which designer used for designing the tooling.

Using steel strips of lighter or heavier gauge than specified for roll forming, may lead to failure of finished products. Application of lighter gauge strip needs over tightening of rolls which reduces the service life of the rolls and heavier gauges reduce center contacts with tooling and result in gap differences besides greater finished radii. Importantly, the precise alignment of the roll forming dies which should exactly match with the alignment chart provided by the designer, both horizontally and vertically.

By Dr Pradeep K. Maitra
dipalyconsultants@hotmail.com