Direct current electrical heat treatment of continuous metal band - in protective atmosphere contg. small amounts of reacting gases

摘要

Electrically heat-treating a continuously travelling band of sheet metal comprises conducting the band in zig-zag fashion over spaced guide rollers with d.c. potentials applied to some of the rollers to generate resistive heating. The length of successive passes from the inlet to the outlet end of the travelling band is reduced to compensate for the increase in the electrical resistivity of the band with the increase in temp. in the course of its travel. This equalises Joule effect in the successive passes. The walls of the housings along successive passes are disposed in close proximity to the faces and edges of the band to maximise the temp. and atmosphere within the housings. Used for the heat treatment of continuous lengths of ferrous metal prior to coating with Al, Zn, Sn. No inductive electric currents are generated in the housing walls as in the case when a.c. potential is applied to the rollers. The lack of any other form of heating other than the d.c. energy results in a low thermal inertia system. This results in fine and rapid control of heating to produce uniform physical and metallurgical properties across the entire width of the band. The reducing gas contains 10% H2 and eliminates the danger of explosions.