PURPOSE: To stably and surely alloy in a short time and efficiently produce a galvannealed steel sheet by forming an FeS film on the surface of a high strength steel sheet, subjecting it to heat treatment, thereafter galvanizing the same and executing alloying treatment under heating.
CONSTITUTION: On the surface of a high strength steel sheet mixed with P and/or Si, a film essentially consisting of FeS is formed, and heat treatment is executed. Preferably, the formation of this FeS film is executed in a heating atmosphere of about 200 to 600°C obtd. by adding about 20 to 200ppm gaseous H2S to about 3 to 25% gaseous H2 and the balance N2 before or in the initial stage of the heat treatment for the steel. The thickness of the FeS film formed by the same is suitably regulated to about 20 to 250. The heat treatment for the steel sheet in which the FeS film is formed is executed at about 600 to 800°C in an atmosphere of gaseous N2 contg. gaseous H2. Successively, this steel sheet is immersed in a plating bath, is galvanized and is furthermore subjected to alloying treatment under heating.
MORI MASAAKI
OGAWA YUTAKA
ITOMI MAKOTO
