Development of 590MPa Grade Hot-Dip Galvannealed DP Steels

Recently, the weight reduction of automotive body and crash safety become much more important factors. In addition, the corrosion resistance must be ensured for any material used in a structural part of automotive components. In an effort to satisfy these requirements, high strength galvannealed DP steels with 590 MPa in tensile strength have been developed in Shougang. Steel chemistry with low Si was designed to reduce the iron oxidation with chemical composition (Si, Mn etc.) and to improve the wettability by liquid zinc. And the alloying elements (Cr, Mo etc.) were added to improve hardenability of sheet to obtain DP microstructure. Newly developed 590MPa grade hot dip galvannealed DP steels have good mechanical properties and hole expansibility. The results revealed the Cr addition effectively suppresses the formation of ferrite during the continuous annealing to improve the hole expansibility of steels. The galvannealing temperatures are increased to improve the hole expansibility of steels by generating the appropriate amount pearlite.