In the continuous rolling process, there is a differential metal flow rate between two adjacent rolling mills. When the metal flow rate of the rear mill exceeds that of the front mill, the workpiece between the two mills will be subjected to a pulling force from the front mill. This force is commonly referred to as tension.
Tension acts on the workpiece and reduces its deformation resistance along the rolling direction. As a result, the metal flow in the rolling direction increases, the width of the workpiece decreases, and its length increases. Ultimately, this may result in defective products with incorrect width or length. In severe cases, it can even cause pull-cut accidents.
How can the tension between stands be accurately determined? Common methods include:
- Observing the workpiece condition: When the tail end of the workpiece expands as it exits the previous stand, it indicates the presence of tension between the two stands. If the tension disappears, the width of the workpiece increases. The greater the change in width, the higher the tension.
- Knocking method: Tap the workpiece with an iron rod and observe or feel its vibration. For workpieces with small cross-sections, plucking the center of the workpiece can also help determine tension. If the workpiece easily responds to the movement of the rod, it indicates little or no tension.
- Observing motor load current: Tension between stands can also be judged by monitoring changes in motor load current. When the leading edge of the steel is bitten into the first stand, the current is recorded as reference “A”. If the current remains unchanged when the workpiece enters the second stand, it suggests there is no tension. If the current changes — either below or above “A” — it indicates the presence of tension, either pulling or pushing the steel.
- Loop condition observation: If the top of the loop is directly above the roller but the roller has not reached its set height, it indicates a high-tension state, meaning pull-rolling is occurring. The downward component of force keeps the roller compressed. If the tension increases further, the roller may drop due to the increased force.
Tension Adjustment Principles
The primary method to adjust tension is by regulating the rolling mill speed. However, this must be done while ensuring the height of the workpieces complies with process requirements. Speed and roll gap should not be adjusted simultaneously, as doing so can lead to confusion and instability during tuning.
Once the height of the workpieces at each stand has been confirmed, tension can be eliminated gradually by adjusting the mill speeds stand by stand. The adjustment should start from the first stand and proceed toward the rear. If adjustments are made starting from the last stand, the established tension balance may be disrupted, increasing the risk of accidents.
