Atmosphere control during continuous heat treatment of metal strips

Method for the heat treatment of metallic strips comprising, inside a heat treatment chamber having a pressure greater than atmospheric pressure,

• passing the strip through at least one heating zone of the chamber,
• moving the strip through at least one cooling zone of the chamber, and
• establishing a first protective gas atmosphere containing nitrogen and a first hydrogen and/or helium content in the chamber, with the exception of at least one cooling zone, in which there is adjusted a second protective gas atmosphere containing nitrogen and a second hydrogen and/or helium content greater than the said first content,
• characterised in that it comprises
• at least one introduction of nitrogen into the chamber,
• at least one injection of protective gas containing a third hydrogen and/or helium content greater than the said second content into the said at least one cooling zone having the said second atmosphere
• a gaseous exchange between at least one zone of the chamber having the said first atmosphere of protective gas and the said at least one cooling zone having the said second atmosphere, and
• control of the flow rate of the said at least one introduction and the said at least one injection according to the gaseous exchange between zones, the pressure of the chamber and the hydrogen and/or helium contents to be obtained in the said first and second protective gas atmospheres.