T R A I N I N G C E N T E R REHEATING FURNACE PRINCIPLES 1) Furnace Descriptions 2) Decarburisation 3) Scale Formation 4) Rolling Temperature 5) Delay Strategies IISCO Steel Plant (ISP) DP03E801/DP03E802 Oct .2012
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T h i s d o c u m e n t c o n t a i n s p r o p r i e t a r
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REHEATING FURNACE PRINCIPLES
DP03E801/DP03E802 / IISCO Oct 2012
Two primary fuels are available to heat the billets up to rolling temperature (1000°C -
1220°C). Although other hydrocarbons could be used , natural gas (methane) andvarious grades of fuel oil are the norm’ for re-heating billets.
Whichever fuel is used, it must be mixed with the oxygen in the air to createcombustion.
For efficient combustion and heat transfer, the correct mixture of fuel and air mustoccur. If too much air is mixed with the fuel, a “lean” mixture results, and the resultingflame will be lower in temperature.
If too little air is mixed with the fuel, a “rich” mixture results. In this case combustion is
not complete resulting in lower temperature steel billets and leaving a dangerous
situation from the explosive gases in the furnace off-take.
The best mixture of fuel and air differs with each fuel, (this because of the calorific valueof each fuel). In the case of natural gas, one cubic foot of gas combined with ten cubic
T h i s d o c u m e n t c o n t a i n s p r o p r i e t a r y i n f o r m a t i o n o f D a n i e l i & C
S p A .
A l l r i g h t r e s e r v e d .
REHEATING FURNACE PRINCIPLES
DP03E801/DP03E802 / IISCO Oct 2012
FURNACE REHEATING PRINCIPLES
Furnace operations
Heat Recovery
Several things can be done to reduce the cost of heating billets, the main one is heat
recovery. The most common way of recovering heat is by recuperation and bylengthening the furnace.
Basically Recuperator is the heat exchanger that pre-heats the furnace combustion air.Pre-heating the combustion air can give enormous savings in fuel. Air temperatures as
high as 500°C. are possible
Another common way to efficiently use waste gas is to lower the waste gas temperatureby lengthening the re-heat furnace. This enables the waste gas to pre-heat newlycharged billets as the gas flows over the top surface on its way to recuperator andstack.
As we saw earlier when discussing the principles of rolling, as steel is heated itbecomes plastic and its resistance to deformation decreases. The higher thetemperature of the steel, the less is its resistance to deformation
T h i s d o c u m e n t c o n t a i n s p r o p r i e t a r y i n f o r m a t i o n o f D a n i e l i & C
S p A .
A l l r i g h t r e s e r v e d .
REHEATING FURNACE PRINCIPLES
DP03E801/DP03E802 / IISCO Oct 2012
Radiation
Heat transfers from the hot flame to the relatively colder refractories and billets, radiationheat transfer also occurs between the hot refractories and the cold billets. The amount
of heat absorbed depends upon a property called emissivity.
The emissivity of an object or material can vary on a scale from 0 to 1. The higher the
emissivity the more energy it can transmit or receive and accept by radiation heattransfer.
Heat Transfer :- 3 types Radiation. Conduction. Convection.
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A l l r i g h t r e s e r v e d .
REHEATING FURNACE PRINCIPLES
DP03E801/DP03E802 / IISCO Oct 2012
The second type of heat transfer is called Conduction heat transfer. Conduction heattransfer is generally heat transfer through contact of materials having a temperaturedifference.
The ability of a material to conduct heat is called the thermal conductivity of a material.A material that conducts heat well is called a good conductor, a material which is poor
at conducting heat is called an insulator.
Steel is a good conductor of heat. Scale has a low conductivity, when it forms it acts asan insulator, it resists conduction of heat, by acting as a layer of insulation. Scaleconducts heat only 1/8th as well as steel.
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A l l r i g h t r e s e r v e d .
REHEATING FURNACE PRINCIPLES
DP03E801/DP03E802 / IISCO Oct 2012
The third type of heat transfer is Convection. Convection heat transfer generally occurswhen you have a fluid or gas at one temperature moving past a surface, which is atanother temperature.
Forced convection occurs between the products of combustion and the billets in thefurnace and between the combustion air and the recuperator metallics.
In the re-heat furnace the chief mode of heat transfer to the steel in the furnace is byradiation.
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REHEATING FURNACE PRINCIPLES
DP03E801/DP03E802 / IISCO Oct 2012
The radiation from the refractory is the major contributor of the heat to the steel.When the steel surface absorbs this radiation, it becomes heated. Once the surface
of the billet begins to heat, it starts transferring the heat to the center of the steel byconduction. As the steel gets hotter, it receives less radiation heat from the furnace.
The furnace operator should understand the term “Heating Quality”. The term in thecase of steel involves:
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REHEATING FURNACE PRINCIPLES
DP03E801/DP03E802 / IISCO Oct 2012
Nominal Temperature Level
This is the temperature that the steel needs to be heated to give the required rolling
performance. This temperature will depend to a great extent on the billet quality and millhorsepower available. It is essential that the furnace operator knows this temperature
and the correct heating curve for the quality of steel being heated. There will be severalgrades of steel rolled in the mill and each of these grades will have a different heating
profile. With the LTR (low temperature finishing rolling process), control of billet
temperature from reheat furnace will be important.
FURNACE REHEATING PRINCIPLES
Furnace operations
It is most important that the billets are heated uniformly from end to end and rightthrough the thickness.
The furnace operator should be able to recognize temperature differentials, based on
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REHEATING FURNACE PRINCIPLES
DP03E801/DP03E802 / IISCO Oct 2012
Minimizing Scale Formation
Scale formation has several disadvantages in a mill:
Increased maintenance of roller tables, pinch roll and roll grooves.Excessive oxidizing scale causing rolled in defects.Scale acts as an insulator, inhibiting transfer of heat into the billets.
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S p A .
A l l r i g h t r e s e r v e d .
REHEATING FURNACE PRINCIPLES
DP03E801/DP03E802 / IISCO Oct 2012
In practice, the amount of time that the billets are in the furnace is determined by theproduction rate in the mill.
Under ideal conditions, the billet should just reach rolling temperature and then bedischarged from the furnace.
When furnaces are fired with a deficiency of air (excess fuel), this is a reducingatmosphere. Scale formed in a reducing atmosphere is tight and adheres firmly to thesteel.
In the event of mill delays the temperatures in the furnace should be reduced byoperating to a furnace delay strategy
5. Close doors: keep furnace doors closed, to reduce O2.
6. Delays: respond to delays by cutting back the furnace temperatures.
7. Communication: the mill rolling personnel must keep furnace operator informed onestimated length of delays, expected stoppage time, and start-up times.