02 reheating furnace basics

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T R A I N I N G

C E N T E RREHEATING

FURNACEPRINCIPLES

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

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

Duty and target of the re heating furnace

The reheating furnace is used to heat the

Billets to the rolling temperature beforeBeing rolled in the rolling mill.

Why do we have to heat the steel?

When steel is cold, its resistance todeformation by rolling is very high.

When steel is heated, it becomes plastic,as the temperature increases so theresistance to deformation decreases.

“A re-heat furnace is a large refractory linedoven, in which billets are brought to rollingtemperature”



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The walking hearth furnace.

+ Good reheating performances ( respect to furnaces of same dimensions but

different design)

+ The refractory in contact with the billets is less stressed due to the design of themoveable beams

+ Higher temperatures of the discharged billets

- Total investment Costs highers than pusher type and walking beams solutions- Needs a dedicated Water Treatment Plant ( Maintenance and cost)

- Does not achieve the performances of the walking beam furnace



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A l l r i g h t r e s e r v e d .



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

feet of air result in 1,000 BTU’s of heat.


Furnace operations



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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 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 .




Maximum temperature is developed only when correct proportions of gas and air isused.

A “perfect” burn is referred to as steichiometric, the subject is steichiometry.


Furnace operations

Best Burn

LEAN FLAME -Gas energy beingused to heat the excess air

RICH FLAME -

Insufficient air, unburnt gasrisk of explosions

Gas content

Air content Steichiometric Burn



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The mechanism of the steel heating inside the furnace

Heat Transfer by Radiation


Furnace operations

BOTTOM FURNACE

BILLETS

FURNACE ROOF



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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



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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.


Furnace operations



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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.

Heat Transfer


Furnace operations



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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 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 .




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.

Heat Transfer


Furnace operations



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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:

Nominal temperature level

Temperature uniformity

Surface quality


Furnace operations



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T [°C]

Position

Passive Pre-heating Heating Soaking

Targ.1

Targ.2

Targ.2

Targ.4 Billet

Average

Target Temperatures


Furnace operations



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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 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

appearance of the heated billets.



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Surface Quality

The surface of the heated steel should have minimum scale build up. Operating

temperatures that are too high cause excessive scale and firing with too much excessair increases scale formation.


Furnace operations



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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.

Scale causes reduced yield (metal loss).

The factors affecting scale formation are:

Scale Growth

Time Temperature Air (O2)


Furnace operations



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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


Furnace operations

Minimizing scale formation



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7 ways to minimize scale formation

1. Heating practice: heat as late as possible, minimize the soaking zone time.

2. Fire on ratio: operate to low levels of free O2 for minimum scale formation.

3. Furnace pressure: maintain a positive furnace pressure to reduce O2.

4. Balance burners: avoid localized over/under heating.

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.


Furnace operations



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Decarburization

Decarburization, a process by which carbon atoms are taken out of the outside surfaces

of the steel. A process occurring in carbon steels, when heated to sufficient temperaturein an appropriate atmosphere.

1. Temperature: as temperature increases, the rate of decarburization increases

decarburization does not take place under 800°C.

2. Time: the amount of decarburization increases with time. The duration that the

steel surface is exposed to temperature increases the amount of decarburization.

3. Atmosphere: a major influence on decarburization is the atmosphere to which the

steel is exposed.


Furnace operations



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Decarburization of the product occurs on all carbon steel. The depth of decarburization is

important, we must reduce the amount such that it is not a problem. Remember that a

large diameter bar will have a greater decarburization depth than a small diameter rod.

Different steel grades will give allowances on the depth.

Decarburization

So what can we do to minimize decarburization? The Furnace operating conditions

given for reducing scale formation are exactly the same for decarburization, thankfully

then if we operate to a low scale growth we can also have low or minimum

decarburization!


Furnace operations



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Furnace delays strategies

Temp Temp

Time Time

Heating Curve

Cooling Curve

Delay


Furnace operations



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ROLLING TEMPERATURE

Here we see the bar enteringthe Stand at temperature T1,and then as the bar rollsthrough the pass thetemperature can be seen to

rise due to the frictionalcontact in the rolling passto TC

The temperature of the bar atthe next stand is T2

02 reheating furnace basics

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