Metallurgy and continuous galvanizing line processing of high-strength dual-phase steels microalloyed with Niobium and Vanadium

It is well-known that the automobile industry continues to search for stronger, more cost-effective steels to lower the mass of the vehicle for better fuel consumption and to provide better crash worthiness for safely. This movement to higher UTS strength requirements, from the 590-780 range to over 980 MPa, has led to more complex alloy design. In the processing of these steels on continuous, hot-dipped, galvanizing lines (CGL), two major changes in composition have been the addition of hardenability elements and microalloying. For example, very-high strength DP steels, containing high Mn, Cr and Mo along with Nb and V have shown UTS levels in excess of 1100MPa. This paper will present recent research conducted on four experimental steels containing these additions. It will be shown that the choice of intercritical annealing temperature is important when processing microalloyed DP steels, as are the rates of cooling throughout CGL processing. The physical metallurgy of producing ultra-high strength DP steels on CG lines will be presented and discussed.