Effect of the Number of Passes on the Spread and Elongation of Billets in Hot Rolling

Abstract

This investigation was carried out to study the effect of the number of passes on the spread and elongation of bars. Tests were made by the two-high pinion-drive mill having plain rolls, with a 250mm diameter.
The samples used in this test were square bars of low-carbon steel and the initial sections of them were respectively 25, 35 and 45mm squares. These bars were rolled at temperatures of 800°C and 1200°C down to total reduction of 40 per cent. in height.
The reduction was achieved in one, two, three or four passes. In several passes the draught of each pass was equal. The results obtained were as follows.
(1) As the number of passes increased, the total spread of the rolled bar decreased and the total elongation increased. This tendency was more severe at 800°C than at 1200°C.
(2) In several passes, the spread of each pass was not equal.
It increased with the number of pass.
(3) The side-shapes of the bars after rolling became either concave or barrel shaped and the former shape was found on the bars of 35 and 45mm square, when rolled in three or four passes.
The degree of barrelling increased as the bar size decreased.
(4) The rolling load of each pass was measured by the load cells inserted between the mill screws and the roll chocks.
The maximum load acted at the last pass when the reduction was achieved in several pass.