CONCEPT OF DIRECT ROLLING IN STEEL RE-ROLLING
The Induction Furnaces melt Scrap and Sponge Iron and produce Ingot through Ingot Molding and some have the continuous Billet Casters also. The cast Ingots or Billets are cooled and sent for rolling in the Rolling Mill. The cooled ingots or Billets are then reheated by placing them into Billet Reheating Furnace which operates on fossil fuel like: “furnace oil or coal”; after the Billet or Ingot is heated up to 1200 degree Celsius, then these are discharged and taken for rolling. This process consumes a lot of fuel in reheating furnace and causes burning loss of around 1.5 -3.5% depending on the type of fuel used. The heating of billet/ingot is not very uniform. The production in Rolling Mill also gets affected many times due to limitation of the manpower handling and Billet Reheating system because in top fired pusher furnace, heating is only from the top surface of the material. However, the re-rolling sector has been operating quite well under the above limitations. It is also important to note that burning of fossil fuel causes generation of CO2 and a large quantity of the same is exhausted through Chimney in addition to other pollutants causing several environmental problems and Global Warming. It is well known that Technology Development is a dynamic phenomena and in this case the constant search by Indian Entrepreneur has given birth to a Revolutionary Technology of Direct Rolling in a SRRM sector. Although, the Technology is in practice in a few steel mills abroad but they are the most modern mega steel plants. In a SME sector, this is indeed a great achievement in the world.
ADVANTAGES OF DIRECT ROLLING PRACTICES
Depending upon the facilities such as capacity, number of induction furnace (IF), mill equipment etc., it would be possible to achieve 75-80% direct rolling. Thus, huge saving of furnace oil and coal required for heating of the billet/ingot which will result into enormous saving in Fuel energy and thereby reduce the Green House Gas (GHG) emission.
Reduction in the scale loss to the extent of 1.5 to 3.5%, which would have been burned in the billet reheating furnace.
Savings due to avoidance of runners risers etc., improving the yield about 4-5%.
Savings in the cost of Ingot moulds, refractory, etc.
It will improve the productivity of the mill and also that of the manpower engaged in the Plant.
It would also result in some saving in power consumption because of the indirect savings of Burning loss and improved liquid metal to finished steel etc.
It will also help in reducing the manpower required in any unit and also reduce the risks of lower production caused due to the manpower engaged in vital function of mould settings; ingots finishing and loading in the billet reheating furnace etc.
It will reduce miss roll, cobbles, etc and improve the product quality, increase % yield etc.
In order to achieve smooth operation, the following facilities are required to be installed in the CCM section of any Induction Furnace and rolling mill complex:
A suitable radius of Concast machine (CCM) with preferably double strand facility. However the single strand can also be set up or three or more strands can also be set up. But the present sizes available in SME units, it is proposed to use single strand with a backup availability of second strand for being put into use as per the requirement.
A suitable radius mould tube having indirect cooling through DM water followed with a secondary cooling facility having direct spray cooling through controlled spray of water, controlled by programmable logic controller. By varying the flow rate of spray water during the secondary cooling. It is also desired to have a control on the flow of cooling water for the Mould Tube Cooling too; so that the controlled cooling can be achieved.
A suitable radius at secondary portion of the CCM is provided up to the withdrawal and withdrawal is provided also with variable speed control through VFD (AFC).
Subsequent to that in each strand one hot billet shearing machine should preferably be installed to ensure that billet being cast are cut to the desire length by consuming minimum time and in line with the casting speed without causing any loss of metal and also without creating any distortion in the end of the billet so that, the ends of the billet do not cause any adverse impact at the entry of first pass. Although at many locations people prefer to install only the manual gas torch cutting facility by using acetylene and oxygen or the LPG with Oxygen. But this has several disadvantages over the on line Hot Shearing Machine.
Once the Hot cast Billets are cut to size by the Billet Shearing Machine then the conveying speed of the cut billet is enhanced to ensure fastest travel of Hot cut piece of the billet to the first pass. The layout may requires that the cut billets to be rolled are shifted at 90 degree angle or may also be possible to convey it straight to reach the rolling mill conveyor. This will depend on the Mill Layout with respect to the Induction Furnace layout.
Rolling Mill conveyors may be required to be designed to provide a linear speed of 1.5 mt/sec or even little more to convey the hot cut billets as fast as possible. The conveyors will be driven by VFD. In case the distance of conveying is more then it may require that the conveying mechanism is so designed that the heat loss during travel is minimum. This may require to even provide certain enclosures canopy linked with refractory material and covers over the conveyors.
The temperature profile from the induction furnace up to the first pass of rolling mill stand should be maintained in the following levels:
Tapping Temperature at Induction furnace should be maintained between 1660 to 1670 Degree C depending on the number of factors.
The ladle designed should be so provided to minimize the loss of heat during transporting the liquid metal up to CONCAST stand. It is expected that not more than around 3 to 4 minutes should take to transport the ladle. Temperature drop during pouring will be 40 Deg. C and during transport shifting of ladle to Concast stand will be 10 to 12 Deg. C. Thus enough heat will be available even with the liquid metal during the casting from ladle.
Subsequently during nitrogen purging another 5 deg. C drop will take place.
Accordingly the opening temperature of liquid metal while start of pouring in the mould will be about 1590 Deg C. and is likely to remain above 1550 Deg by the time the pouring/casting is completed.
The solidification of the liquid metal in the mould will take place due to the indirect primary cooling and thereafter controlled spray of direct cooling water during the secondary cooling. The PLC controlled water spray system will help to ensure that the billet is completely solidified at the withdrawal point and also be having a skin temperature of above 1050 to 1100 Deg. C.
The billet shearing machine should be placed at the closest possible point to so as to avoid any more heat loss during further conveying.
All this protection and minimum time to convey the billet from shearing machine to the first pass should ensure that the maximum temperature drop is not likely to be more than 5 to 10 Degree C.
The insulated cover will also help in protecting the scale losses.
The CCM is likely to operate 24 hours whereas rolling mill will requires at-least 2 hours of maintenance time. Therefore for accommodating the material casted during this rolling mill shutdown period; the cast billet should be conveyed to the cooling bed for which a sufficient length cooling bed should be provided.
It is important to take into consideration the present melting facilities and re rolling facilities capacity.
Once the rolling process is integrated on line with that of Hot cast billet rolling then the entire process must be very well synchronized and it will require very care full and perfect planning in each every aspects of operation from beginning till the end such as:
Proper Selection of raw material for Induction Furnace (IF).
Complete analysis of the charge and proper material balancing before melting itself to target the proper levels of carbon, phosphorus and sulfur to be achieved.
Scheduled predictive/preventive maintenance in time for all the facilities
Proper selection of the Acidic/Neutral Ramming mass and proper application of the same. Timely relining of the crucible well in advance and after every defined number of heats.
Proper and adequate arrangements for proper ramming mass
Controlling the melting temperature and time to achieve the scheduled sequence.
Maintenance of the desired temperature profile as planned
Maintenance of the passes and the mill area equipments in perfect operating conditions.
Adequate stocks of the spares and emergency backup facility along with the emergency backup power. Proper training to the manpower from testing, Operation, maintenance to quality and inventory control.
Reduce the dependence on any manual handling even in the re rolling mill. Hence the Re Rolling mill feeding and discharge of the rolling objects must be automated.
Cooling bed must also be made automatic.
It is to be realized that after the Induction furnaces are synchronized with the re rolling mill through the caster then the success of the plant depends on the failure free and maintenance free operations. Thus all the equipments and machines and spares must be carefully procured from very good and reputed makes and manufacturers.
It is also very important that the design and operations of the plant is given in hand of experienced and expert persons only.
The success of the Plant brings profit and happiness to Industry at the same time protects the environment and makes the steel industry as more sustainable. The life of earth is increased due to reduced GHG emission. Thus a sincere effort would bring.