Formation oF outburst struCturE in hot dip galvannEalEd Coatings on iF stEEls

Rapid development of automotive industry brings also increased requirements for sheets properties, involving relatively high strength and good formability together with excellent corrosion resistance. Increasing requests for corrosion resistance lead to the production of coated car parts, protected mainly by zinc coatings. Whereas, majority of European producers of passenger cars use preferentially pure zinc for coatings [1], Japan and American automobile factories are oriented in the first place on Zn-Ni, Zn-Fe or Zn-Al alloyed coatings. Fe-Zn coatings obtained by heat treatment of galvanized steel sheets, containing 8 12 % Fe found practical application in production of steel sheets for autobodies. By means of diffusion between zinc and steel substrate initiated by annealing, nearly pure zinc η phase in the coating is transformed into Fe-Zn alloyed coating, containing various Outburst structure in two industrially produced hot dip galvanized interstitial free steel sheets for automotive industry after additional annealing has been examined. Ti IF steel was found to form weak outburst structure in the early stage of annealing, followed by frontal growth of Fe-Zn phases during further heating. The high reactivity of this steel was confirmed by rapid Γ-phase formation. Under the same conditions, Ti-Nb-P IF steel exhibited frontal growth of Fe-Zn compounds without Γ-phase formation due to relatively high phosphorous content, which is known as inhibitor of Fe-Zn reaction, but simultaneously significant occurrence of undesired outburst structures was recorded. It was assumed that the phosphorous content was insufficient and/or ferrite grain was very fine.