Continuous annealing process for obtaining an improved surface appearance

摘要

Continuous heat treatment of steel sheet involves a divided two stage cooling operation using water vapor and water to furnish two specific and controlled iron oxide layers. The continuous heat treatment of a steel sheet includes subjecting it to a divided cooling operation with the following successive stages: (a) at the start of cooling, whilst the sheet is at high temperatures of between 400 and 900 degrees C, direct contact of the sheet is provoked with water vapor to create a first layer of iron oxides (FexOy) of a specific and controlled composition such that the ratio of the iron (Fe) and oxygen (O) contents of the medium composition of the iron oxide is between 3 and 3.5, the thickness of the oxide layer being between 10 and 100 nm, and preferably between 20 and 50 nm; (b) the sheet is then exposed directly to water in the liquid state, leading to the formation of a second layer of iron oxides, which may be of the type Fe3O4 and Fe2O3. An Independent claim is also included for a device for putting this method of heat treatment into service.