Surface Quality and Other Quality Features of Zinc-Coated Strips Used in Automobiles

Jean-Claude Herman
CRM, Liège

Steel is the most widely utilised material in the automotive industry, because it exhibits a body of properties which carmakers require to ensure both economy of production and user performances. However, the recent emergence of alternative competitor materials has made its position in this very attractive market somewhat less secure than in the past. The constraints have only been made more severe by the implementation of environmental laws on fuel consumption and exhaust emission control, which have driven a world-wide trend to lighter car bodies, notwithstanding the introduction of additional safety and comfort heavy pieces of onboard equipment.

The response steelmakers could offer in this new situation has been to propose new coated higher-strength grades making it possible to reduce the gauge without sacrificing indispensable easy fabrication and assembling features or user properties. These new grades however achieve their outstanding properties by making use of highly oxidisable alloying elements or by being given out-of-equilibrium microstructures. The presence of the former and the imperfect stability of the latter could well impair the steel’s good response to hotdip galvanising, which long-term corrosion resistance and general durability requirements make absolutely mandatory.

Metallurgists around the world have long identified improper surface preparation and poorly controlled associated selective oxidation during pre-dip annealing as the main culprit for this situation. A global research effort has then been initiated to solve those problems, and the CRM has taken a most significant part in it. Its activity has covered the domains of process metallurgy and new technology, more thorough product characterisation, new alloy development and original laboratory techniques for process simulation and coating evaluation.

The presentation will be organised in distinct sections, dealing with those topics separately. It will first show how metallurgical mechanisms can be taken advantage of to substantially strengthen deep drawing steels without losing anything on their formability and coating capability. Similarly, several metallurgical processes will be described, the aim of which is to prevent the zinc wetting-harmful external selective oxidation of alloying elements added to high Si and Mn grades (DP, TRIP). In this context, special attention will be paid to the metallurgical mechanisms put at work by processes such as preoxidation, oxide maturation and other types of reactive annealing sequences (nitriding, carburising, etc.) developed at the CRM. Similarly, the benefits and limits of Cr or Ni pre-deposits will be commented on. The hot-dip galvanisation of HR steels, often considered a good technical and economical substitute for CR grades used in structural components, will also be analysed. As far as new processes are concerned, a special emphasis will be put on the so-called "Zinc Cushion" technology, whereby hot-dip galvanising can be performed without either a sink roll or touch rolls, thus ensuring better coating surface quality and opening the way to a wide range of new short contact time processes.

Developing processes without having adequate control on their progress is obviously pointless. The CRM has then been particularly active in working out such control techniques, one of the most significant examples of which is given by the on-line microscope for GA transformation monitoring and final property control.

Finally, under the joint sponsorship of Arcelor and Corus, the CRM general approach to new processes and techniques is always based on sequential stages going from the laboratory simulators, to pilot lines, characterisation and final industrial implementation. It goes without saying that laboratory techniques must be made perfectly reliable for this approach to be a success. In this context, details will be provided on original simulation devices and evaluation methods.

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