Proper Cooling of Rolls

Methodology of Wear-Out

1. During hot rolling, high temperature of the stock and the heat of deformation raises the temperature of the rolls in the contact zone (Fig. 3.6) adversely affects the tribological conditions of the working surface.

Variation of roll temperature during the rolling of first slab through the stand

Fig. 3.6 Variation of roll temperature during the rolling of first slab through the stand.

Roll surface layer is subjected to cyclic temperature variation, which leads to development of tensile and compressive stresses on the surface.

2. When die stock leaves the contact zone and air/water cooling of roll surfaces commences, temperature of the siuface gets lowered. Hie cyclic variation of stresses, due to die thermal fatigue, a network (Fig. 3.7) leads to the formation of a network of micro cracks, which is known as ‘Fire cracks’ and flakes of metal get tom-off under high pressure and temperature variations.

Roll surface temperature cycle and comparison of characteristics- Efficient vis-a-vis Inefficient Roll Cooling

Fig. 3.7 Roll surface temperature cycle and comparison of characteristics- Efficient vis-a-vis Inefficient Roll Cooling.

  • 3. Even if roll temperature is insufficient to cause thermal fatigue, higher roll temperature decreases the hardness of the rolls, resulting in excessive abrasive wear. In view of this it is imperative to retard the flow of heat in the rolls by effective and efficient roll cooling techniques.
  • 4. Oxidation of the surface layer also takes place due to high temperature.

Factors Affecting Wear-Out of Rolls

  • (a) Surface Condition of Roll.
  • (.b) Hardness and Toughness of Roll.
  • (c) Chemical Composition and Structure of Rolls and metal to be rolled.

What is Proper Roll Cooling

  • 1. The roll cooling should be such that the surface temperature of rolls should be around 80°C. It means that, at any time during rolling one can touch rolls with bare hands.
  • 2. As the bar touches the rolls, at that contact surface of rolls, the temperature get immediately rises to that of bar temperature, but it stands only for the split of second. Therefore, water, which comes in contact with surface of the roll, is not able to do the cooling, as a layer of steam is fonned on the roll surface. Problem of water-cooling has become more acute in case of slow rolling, since bar remains in contacts with the rolls for a longer period of time, compared to the high speed rolling.
  • 3. Hence, to start a proper cooling, a high pressure jet of water should be impinged on the rolls immediately at the point where bar leaves the rolls i.e., at the exit side of rolls. This will take away the steam fonned on the surface of rolls and by that surface of roll get exposed to the cooling.
  • 4. The cooling of the bottom roll is very important .Water jet has to work against the gravity in case of bottom roll, and because to this, water is not able to reach to the full peripheral surface of the roll, contrary to top roll. Proper care should be taken during the rolling process for checking regularly for water jets installed for bottom rolls, which generally gets blocked by scales generated during rolling process.
  • 5. Cooling water connection to the lower roll should be so set, so that water will not come into the entry side of the pass, as slightly cold front will then lead to the more spread and bar will not able to enter in to the next stand, causing cobbles.
  • 6. Whatever effective is the cooling process, the surface temperature of rolls gets fluctuates from 1000°C to 80°C during the every rotation of roll and by this phenomenon, fire cracks get develops on the roll surface. If these fire cracks will be allowed to go deep inside the rolls, then it will become the cause of roll breakage. The Fig. 3.8, shows that how an effective rolls cooling arrangement is required for a hot rolling mill.
Effective Roll Cooling System for Hot Rolling Mill

Fig. 3.8 Effective Roll Cooling System for Hot Rolling Mill.

7. Roll should not be suddenly get heated or cooled. It is a general practice to run the roll idle for sometimes before the start of rolling to avoid the sudden heating or cooling of the rolls and vice versa, not to stop just after the rolling.

Conventional Roll Cooling

The conventional cooling of work rolls reduces its life and causes production delays due to the frequent roll changes. It also results in production of the poor quality finished products, due to uneven wear out of rolls due to poor roll cooling. It is seen that generally in long products rolling, roll passes are invariably cooled with the help of water hoses, may be placed at some convenient place towards the exit side of the pass, whereas in the flat product mills, roll cooling is carried out by perforated pipes/nozzles on the entry as well as exit side without much of consideration for location and water distribution.

 
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