Siberian Federal University, Krasnoyarsk, Russia:

I. N. Dovzhenko, Assistant Professor, Chair of Metal Forming
N. N. Dovzhenko, Professor, Chair of Metal Forming
S. B. Sidelnikov, Professor, Head of the Chair of Metal Forming, e-mail: sbs270359@yandex.ru
I. L. Konstantinov, Assistant Professor, Chair of Metal Forming

Implemented has been analysis of strain-stress and temperature states during hot rolling process for ingots of an Al - Mg system aluminium alloy doped with scandium. It has been shown that in case of hot rolling of the large-capacity ingots without using vertical (edger) stands under nonuniform deformation conditions, changes of breakdown bar geometry take place, especially on the first rolling passes. This nonuniformity of a metal flow is caused by intensive deformation of the ingot’s outer layer and insignificant metal deformation in the central area, which result in formation of concave edges with a convex near-edge region at front end of the ingot. Metal failure in different zones of the billet may be also caused by casting defects, since essentially nonuniform deformation and stress distribution over thickness as well as that of temperature is typical for hot rolling process, especially during first passes. The metal temperature calculation has shown that sequential deformation is favourable to gradual temperature increase about 5–10 ^oС by pass, but heat abstraction from the central part of an ingot rises as the billet thickness is decreasing and time of pauses between passes is increasing. At the same time, the edges of a strained half-finished product are the very cold areas. A breakdown bar temperature essentially differs over both surface and volume (up to 10–12 ^oС), especially at first passes, and on further rolling, their difference is increasing when temperature on the surface and in the center rise and amount to 25–30 ^oС in comparison with the edge temperature. Based on the Cockroft-Latham criterion, ascertained are the breakdown bar areas with the greatest probability of crack propagation, which is confirmed by results of experimental investigation in an industrial environment. Analysis of changes of the Cockroft-Latham criterion values through passes has demonstrated that values of this criterion at the breakdown bar edge exceed critical value of 1 in pass No. 15 at total deformation of 68.3%.

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