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31-12-2010, 04:55 PM
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WHAT EXACTLY IS ROLLING?
The majority of steel products start their life by being converted from the ingot or continuous cast form by a process of rolling. To the steel supplied in the ingot form, the preliminary treatment, given is reduced in its section by rolling. Rolling consists of passing a metal between two rolls which grip the sheet and so reduced it in size, and after each pass the rolls are brought closer together and the metal passed through until required reduction in its section has been attained.
FACTS TO REMEMBER :
1. Volume entering ==== Volume leaving
A1×F1 ==== A2×F2
2. Speed increased develops between ‘A’ and ‘C’ on roll surfa- ce. Therefore, since roll surface travels at constant speed, there must be slippage between roll and steel. Theoretically, this slippage of the roll on the steel is forward between ‘A’ and ‘B’, and backward between ‘B’ and ‘C’. ‘B’ is the neutral point.
3. Increase in the speed can be explained by starting that the longitudinal pull in the direction of the ‘BP’ is the result of many tangential pulls at the roll surface introduced immedia- ly at the point of contact with the steel at ‘A’ until the bar leaves at ‘C’. These tangential forces are equal to the force of friction.
4. Therefore, as the pressure of the rolls increases, friction incre- ses. Tangential pulls and subsequent longitudinal pull along ‘CP’ increases, and the bar leaves faster. Thus the increased speed compensates for the decreases area of the bar.
H.R. SLITTER SECTION
PICKLING LINE SECTION
SALENT FEATURES OF COLD ROLLING
• Metal is fed to the rolls when it is below its recrystallisation
• The metal shows the work hardening effect after being cold
• Coefficient or friction between the rolls and the stock is com-
• Theoretical analysis can be easily carried out extensively de-
veloped theory is available.
• Experimental measurements can be easily carried out in cold rolling.
• Heavy reduction is not possible.
• Hardness increases. Excessive cold working generates cracks. Ductility of the metal reduces. Cold rolling increases the tensile strength and yield strength of steel.
• Roll radius is smaller.
• The cold rolled surface is smooth and oxide-free.