DIFFERENT PROCESSES FOR ANNEALING OF COLD ROLLED STEEL

By | February 20, 2016

                BATCH ANNEALING PROCESS                                     

It is the process in which batches of cold strips which are placed on the base and enclosed  by inner cover and then covered by the furnace.This is different processes for annealing of cold rolled steel.

For this process overhead crane is used for  loading of base and moving of equipment.

Basic equipment required for the annealing of strips are as follows:

  • base unit provide with re circulation fan
  • protective ,gas tight cylindrical cover
  • heating furnace or heating bell.
  • cooling bell or hood.
convector plates

Fig 1. Convector plates

 

Stage 1

BELL

      Fig 2. Bell

At this stage number of cold rolled steel coils are placed one above the other over the work base.

These coils are separated  by the convector plates. After loading of coils in the base unit the interior cover is placed over the base and protective gas is circulated in the enclosure.

After this heating bell or heating furnace is placed over the cover. during this process the burners of heating furnace start to provide heat to the interior cover and it heats the  circulating protective gas. heat from the interior cover is passed to the coils inner and upper surface through convector plates .And the coils get heated by convector plates from circulating protective gas and radiation between coil and interior cover. The furnace brings the charge to the desired temperature.

Cylindrical coil over the work base  called charge.

The temperature to which the cold rolled coils or strips are heated dependent on the chemical and  Mechanical properties of coils or strips.

 

Stage 2

After this steel coils  reached at particular temperature, where soaking process takes place.

During the batch annealing process there is formation of new strain free crystals  (grains) which replaces all deformed crystals of  the cold worked material. Takes place at a temperature known as  re crystallization  temperature. it takes place temperature at  550 deg C.

 

Inner cover

Fig 3. Inner cover

Stage 3

After this heat treatment process , heating furnace is removed that means end of heating.

cooling  is performed by removing of heating furnace or heating bell  Over the base leaving  the interior  cover   to maintain the protective atmosphere.

And then cooled furnace is placed over the base, blower will started  eject the heat to the air and left to desired temperature.

After reached  to desired temperature it will undergoes water cooling process.

In the end coils ejected from the base with new strain free crystals structure on the surface.It is the different processes for annealing of cold rolled steel.

CONTINUOUS ANNEALING PROCESS

Continuous annealing line is defined as process which contains all the steps of cleaning ,cooling,heating, refining in single line.  the over all time required for the process is shortened.It is more demanding requirements in steel mechanical properties.It is also the different processes for annealing of cold rolled steel.

The continous annealing line has many advantages:

  • more productivity in shortened  time
  • corrosion resistance , better shape with  exceptional smoothness
  • can operate easily
  • suitability for coating material.
  • increased in ductility and removal of strains that lead to failure.

 PROCESS

During this process at first stage the coil is placed in the un coiler. due to continuous line process there is two un coiler. after this  the head end of new coil is joined with the tail end of previous coil with the help of welding. due to this joining the movement of continuous line remain maintained.

For this continuous process  accumulators or  loopers  are provided.

  • entry accumulators
  • exit accumulators

Accumulators are used to provide the storage areas between static steel coils and continuous strip running through the furnace or heat treatment  section.

As the un coiler is stopped ,replaced with the new coil  and both the coils welded together during

This time the entry accumulators unwinds to provide the continuous coil or we can say entry accumulator consists of two parallel set of rolls, as the starting stage entry accumulator move apart at some particular distance to hold more coil for the continuous process and the accumulators supplies the coil to the furnace or heat treatment section.

The coil goes through multi stages of annealing process consists of  pre heating, heating ,holding ,rapid and Slow cooling, over ageing and finally cooling.

Then coil go through skin pass process  and set required material properties, Pass through oiling machine for surface protection.

Finally  at the end  during  shearing operation  ,  the exit accumulator winds up while the  full coil is unloaded at the exit.

During  the whole process the temperature is depend upon the chemical and mechanical properties of coils.

so above two are different processes for annealing of cold rolled steel.

                          EFFECT OF HEAT TREATMENT  ON METAL

 Quenching

Varying in the rate of cooling (quenching) of the metal controls grain size and grain patterns.

Grain characteristics are controlled to produce different levels of hardness and tensile strength.

Hardness and tensile strength increase in heat-treated steel toughness and ductility decrease.

Welding

It is used to produces compressive and tensile stresses.

Stresses are minimized by using heat sink welding and annealing.

Annealing

Softens steel and improves ductility

Relieves internal stresses caused by previous processes

Refines grain structure

Advantages of batch annealing process

  • operating cost is low
  • product quality is good
  • good productions rates
  • less capital investment