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.

The microstructure, formability and corrosion resistance characters of industrially produced galvannealed coatings on high strength interstitial free (IF) steel with different galvannealing temperature have been investigated. With the galvannealing temperature increases, the Fe content of coating is increase, and the powdering resistance gets weaker while the corrosion resistance gets better. The coating microstructure consisting of a δ phase as the main constituent with a ζ layer on the top surface provides a good powdering resistance. The compact δ and Γ phases are good for the corrosion resistance of coating

Two kinds of TRIP steel with different Al and Si content were researched in this article. Different annealing atmosphere in the trial was achieved by the Hot Dip Galvanized Simulator by Iwatani. The segregation of alloying elements after annealing under different atmosphere were investigated and the compounds on the surface were shown. Furhermore, the distribution of oxides was abserved on the surface and the reactive wetting was assessed and the results after galvanizing was provided. We found out the increasing wettability was abserved under more reducing atmosphere with less Si because of a decrease oxide amount and an increase internal oxidation.