Innovative Annealing Techniques for the Production of Advanced High Strength Steels

Nowadays, it is not evident to produce in a robust way cold rolled and annealed/galvanised high strength steels based on lean chemistries and with consistent mechanical properties over the coil length. The reasons behind this are first the low cooling rates available on the lines which require the use of a large amount of alloying elements for avoiding the unwanted phases such as the pearlite and secondly the difficult control of soaking temperature and time in a narrow range, resulting in a variable austenite content at the end of the soaking and then in a dispersion of the obtained mechanical properties. By considering high speed cooling technologies on a compact annealing/galvanizing line, this production becomes possible. After rapid heating by means of induction heating and short holding at a high soaking temperature, the strip is cooled down to an intermediate temperature, where it is held for a short period for obtaining a stable ferrite-austenite structure. Then it is rapidly cooled by means of cold water (Twice) in the case of a continuous annealing line or by the zinc quench process in the case of the galvanizing line. These rapid cooling equipments allow reaching high tensile strength levels with a significant reduction of the addition of expensive elements (Mn, Cr and Mo).