Effects of Niobium in Galvanized Advanced High Strength Steels for Automotive Applications

摘要

Advanced high strength steel grades are being significantly used in recent automotive bodies and their share is continuously increasing. Typically these steel grades are applied with a hot dip galvanized or galvannealed surface treatment. Therefore the metallurgy of such steels has been designed to achieve the desired properties under the typical thermal cycle of a CCL line. Having gained experience with these steel grades during recent vehicle generations,the improvement of specific properties particularly concerning formability and weldability is being targeted. In this respect,microalloying by niobium has drawn considerable attention. The typical,well-known effect of niobium is that of grain refinement occurring during hot rolling under thermomechanical processing conditions leading to increased strength as well as better toughness. Grain refinement in the hot band is carried over after cold rolling. A resulting secondary effect especially in multiphase steels is a more homogeneous microstructure. This improves the forming behavior under specific conditions such as bending and flanging. Furthermore,the strength gain by grain refinement and/or precipitation hardening principally allows reducing the alloying content improving weldability. Another interesting effect of niobium is its influence on the transformation behavior during the thermal cycle in the CCL line. This influence can be utilized to enhance the processing window and further optimize the steel's application properties.