Simulation of hot-dip galvanizing for automotive applications: Requirements for modern steel product design

With the trend toward increased continuous processing, batch annealing has been partially replaced by continuous annealing during the last years. The prime advantages of continuous annealing are the considerable increase in surface cleanliness and product uniformity along the length of the coil, Also, the versatility of continuous annealing processing lines (CAPL) and continuous galvanizing lines (CGL) has made possible the development of sophisticated processing routes leading to the introduction of a wide variety of new automotive cold rolled steel grades, As an example, the continuous production of hot-dip galvanized steels for automotive applications comprises both thermal (annealing and cooling stages), mechanical (final temper rolling) and chemical treatments (alloying of the Zn coating with the steel substrate). These stages take place continuously within a few seconds, thus demanding excellent on-line control. This situation has led to an increasing need for simulation techniques to reproduce accurately the CA processing conditions, The hot-dip process simulator (HDPS) available at ITMA is being used in conjunction with ACERALIA for the design of new steel products for automotive applications. This equipment has been designed to meet strict control requirements concerning the abrupt changes of cooling conditions within a non oxidizing atmosphere, temperature gradients and thickness of the Zn coating, In this testing machine a cold rolled specimen (120x200 mm(2)) is annealed in a radiation furnace under a H-2-N-2 atmosphere, then controlled cooled by He-blowing and conducted pneumatically into the Zn pot. Additional wiping for thickness control of the deposited Zn layer together with alloying treatment of the coating (galvannealing) by means of infrared or induction furnaces complete the simulation. This article is focused on the description of the processing conditions achievable with the HDPS compared to those of a modem CGL, together with a presentation of the results obtained to date in the field of microalloyed ULC bake-hardening steels.