Cold rolling is a very important bulk metal working processes which is used in industry. It is a simple process that uses two or more rollers to flatten a metal slab of a certain initial thickness to a certain final thickness by passing the metal slab through the rolls. The rolls are driven by a motor which applies a specified torque to the rolls and the resulting force between the two rolls flattens the metal slab. The amount of force applied controls the material properties and percent reduction of the slab.
The objective of this report is to determine how the roll separating force and roll torque affect the percent reduction of A336 steel strips in the cold rolling processes. This report will also examine the predictive ability of Schey’s method to calculate the roll separating force and roll torque by comparing the calculated results to the actual results obtained during the experiment. The report will discuss why a difference between the actual and theoretical results occur with respect to experimental errors, surface condition of the work piece, speed of rolls, roll deformation, roll bending, roll eccentricity, mill dynamics and temperature rise. A detailed analysis of the experimental results will be included.
To obtain the data this experiment will cold roll four A336 steel specimen through a 2 high rolling mill and using a computer collect roll force and torque data that will be analysed.
· Four A366 steel strips, approximately 25mm Wide x 2mm Thick, of sufficient length to be pulled through the rolling mill
1. The A366 strip samples were deburred using the burr-remover.
2. The surfaces of the A366 strip samples were cleaned with acetone.
3. The width and thickness of each strip was recorded several times over the length of each strip using the vernier callipers. Average initial strip thicknesses and widths were computed and are presented in Table 1.
4. Average initial values were input into the computer.
5. The rolling mill was adjusted to a roll reduction height recommended by the teaching assistant (25 number on the dial on the rolling mill).
6. An A366 strip was passed through the rolling mill using a wooden block. A pair of force transducers and torque transducers recorded the forces and torques exerted upon the rolling wheels during the process.
7. The vernier callipers were used to measure the post-rolling widths and thicknesses across the length of the strip. These values were averaged and are shown in Table 1.
8. Average final values were input into the computer.
9. Steps 5 through 8 were repeated for roll reduction heights of 35, 45, and 55 increments on the rolling mill dial.
Table 1: Average initial and final strip widths and strip thicknesses
1. Thickness and Width (both initial and final): the vernier callipers were used to measure the given dimension at four locations across the length of the strip. The four values were then averaged and recorded (see Table 1).
2. Roll Force and Roll Torque: the force transducers and torque transducers recorded these values and transmitted and saved them to “.txt” files on the connected computer for later acquisition.
During the experiment it was noticed that as the strips exited the rolling mill slightly curved in two planes. It was also noticed that after passing through the mill, the lustre of the strips was increased. After passing through the mill, the strips were warm to the touch, indicating a considerable temperature increase.
The results from the experiment were analysed according to the data obtained from the computer output. A graph of roll force versus percent reduction of both the actual and theoretical results is presented in Figure 1. A graph of roll torque versus percent reduction of both the actual and theoretical results is presented in Figure 2. See Appendix A for data in tabular form and Appendix B for hand calculations.
From Figure 1, it is shown that there is a strong correlation between the percent reduction and the applied roll force. The roll force increases by approximately 3.2 kN for every percent increase in reduction. Similarly from Figure 2, there is a strong correlation between the percent reduction and the applied roll torque. Roll torque increases by approximately 36 Nm for every percent increase in reduction.
From Figure 1, it is shown that the theoretical values predicted from Schey’s method are very close to the actual values for reductions below 25%; however, as the reductions increase beyond this value, the error associated with the predicted values also increases. Similarly from Figure 2, the predicted values for the roll torque are very close for reductions below 25%; however the error associated with the predicted values increases as the percent reduction increases.
The proportionality between the roll force/torque and the percent reduction is to be expected. Increased deformation requires increased energy input to restructure the metal. To increase the energy input, more force and driving torque are required.
The predicted results diverge from the actual results as the percent reduction is increased. The reason for this is that many of the assumptions made in predicting the roll-force/torque become less valid as the reduction is increased. Examples of such assumptions are deformation of the rolling-mill and temperature increase of the strip. It is certain that these factors increased with increased reduction of the strip, though they were assumed to be zero for all calculation purposes.
Figure 1 Roll force versus percent reduction
Figure 2 Roll torque versus percent reduction