02 Reheating Furnace Basics

IISCO Steel Plant (ISP) DP03E801/DP03E802 Oct .2012

1)

Furnace Descriptions

2)

Decarburisation

3)

Scale Formation

4)

Rolling Temperature

5)

Delay Strategies

TRAINING CENTER

REHEATING FURNACE PRINCIPLES

TRAINING CENTER

FURNACE REHEATING PRINCIPLES This document contains proprietary information of Danieli & C SpA. All right reserved.

Duty and target of the re heating furnace The reheating furnace is used to heat the Billets to the rolling temperature before Being rolled in the rolling mill. Why do we have to heat the steel? When steel is cold, its resistance to deformation by rolling is very high. When steel is heated, it becomes plastic, as the temperature increases so the resistance to deformation decreases. “A re-heat furnace is a large refractory lined oven, in which billets are brought to rolling temperature”

REHEATING FURNACE PRINCIPLES DP03E801/DP03E802 / IISCO Oct 2012

Slide 2

TRAINING CENTER

FURNACE REHEATING PRINCIPLES

+

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 the moveable 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


This document contains proprietary information of Danieli & C SpA. All right reserved.

The walking hearth furnace.

Slide 3

TRAINING CENTER


Furnace operations

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) and various 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 create combustion. For efficient combustion and heat transfer, the correct mixture of fuel and air must occur. If too much air is mixed with the fuel, a “lean” mixture results, and the resulting flame 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 value of 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. REHEATING FURNACE PRINCIPLES DP03E801/DP03E802 / IISCO Oct 2012

Slide 4

TRAINING CENTER

FURNACE REHEATING PRINCIPLES Furnace operations This document contains proprietary information of Danieli & C SpA. All right reserved.

Maximum temperature is developed only when correct proportions of gas and air is used. A “perfect” burn is referred to as steichiometric, the subject is steichiometry. Air content

Steichiometric Burn

LEAN FLAME Gas energy being used to heat the excess air REHEATING FURNACE PRINCIPLES DP03E801/DP03E802 / IISCO Oct 2012

Best Burn

Gas content RICH FLAME Insufficient air, unburnt gas risk of explosions Slide 5

TRAINING CENTER


The mechanism of the steel heating inside the furnace

FURNACE ROOF

BILLETS

BOTTOM FURNACE

Heat Transfer by Radiation


Slide 6

TRAINING CENTER


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 by lengthening 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 temperature by lengthening the re-heat furnace. This enables the waste gas to pre-heat newly charged billets as the gas flows over the top surface on its way to recuperator and stack. As we saw earlier when discussing the principles of rolling, as steel is heated it becomes plastic and its resistance to deformation decreases. The higher the temperature of the steel, the less is its resistance to deformation


Slide 7

TRAINING CENTER

FURNACE REHEATING PRINCIPLES Heat Transfer :- 3 types

Radiation.

Conduction.


Furnace operations Convection.

Radiation Heat transfers from the hot flame to the relatively colder refractories and billets, radiation heat 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 heat transfer.


Slide 8

TRAINING CENTER


Furnace operations Heat Transfer The second type of heat transfer is called Conduction heat transfer. Conduction heat transfer is generally heat transfer through contact of materials having a temperature difference. 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 as an insulator, it resists conduction of heat, by acting as a layer of insulation. Scale conducts heat only 1/8th as well as steel.


Slide 9

TRAINING CENTER


Furnace operations Heat Transfer The third type of heat transfer is Convection. Convection heat transfer generally occurs when you have a fluid or gas at one temperature moving past a surface, which is at another temperature. Forced convection occurs between the products of combustion and the billets in the furnace 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 by radiation.


Slide 10

TRAINING CENTER


Furnace operations 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 by conduction. 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 the case of steel involves: Nominal temperature level Temperature uniformity Surface quality


Slide 11

TRAINING CENTER

FURNACE REHEATING PRINCIPLES Target Temperatures

T [°C]

Passive

Pre-heating

Heating Targ.2

Soaking Targ.4

Billet Average

Targ.2

Targ.1

Position REHEATING FURNACE PRINCIPLES DP03E801/DP03E802 / IISCO Oct 2012

Slide 12


Furnace operations

TRAINING CENTER


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 mill horsepower 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 several grades 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. It is most important that the billets are heated uniformly from end to end and right through the thickness. The furnace operator should be able to recognize temperature differentials, based on appearance of the heated billets.


Slide 13

TRAINING CENTER


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 excess air increases scale formation.


Slide 14

TRAINING CENTER


Scale Growth 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:

Time

Temperature


Air (O2)

Slide 15

TRAINING CENTER


Minimizing scale formation In practice, the amount of time that the billets are in the furnace is determined by the production rate in the mill. Under ideal conditions, the billet should just reach rolling temperature and then be discharged from the furnace. When furnaces are fired with a deficiency of air (excess fuel), this is a reducing atmosphere. Scale formed in a reducing atmosphere is tight and adheres firmly to the steel. In the event of mill delays the temperatures in the furnace should be reduced by operating to a furnace delay strategy


Slide 16

TRAINING CENTER


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 on estimated length of delays, expected stoppage time, and start-up times. REHEATING FURNACE PRINCIPLES DP03E801/DP03E802 / IISCO Oct 2012

Slide 17

TRAINING CENTER


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 temperature in 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.


Slide 18

TRAINING CENTER


Decarburization 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. 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!


Slide 19

TRAINING CENTER

FURNACE REHEATING PRINCIPLES Furnace operations

Temp

Temp


Furnace delays strategies

Delay

Heating Curve

Cooling Curve

Time

Time


Slide 20

ROLLING TEMPERATURE Here we see the bar entering the Stand at temperature T1, and then as the bar rolls through the pass the temperature can be seen to rise due to the frictional contact in the rolling pass to TC The temperature of the bar at the next stand is T2


Slide 21


TRAINING CENTER

02 Reheating Furnace Basics

Recommend Documents