PURPOSE: To produce a galvannealed steel sheet excellent in formability and spot weldability by forming a low carbon steel slab into a steel strip by means of hot rolling and cold rolling, subjecting this steel strip to nondecarburizing annealing and further to heat treatment under specific conditions, exerting hot-dip galvanizing, and successively alloying the resulting hot-dip galvanized layer.

CONSTITUTION: A slab of a low carbon steel having a composition containing, by weight, 0.02-0.15% C, <0.20% Si, 0.20-0.50% Mn, <0.15% P, and 0.01-0.10% sol.Al is hot-rolled at 700-950°C finishing temp. and at 200-620°C coiling temp. and successively cold-rolled at <40% reduction of area, by which a steel strip is produced. This steel strip is subjected to nondecarburizing annealing in an open coil annealing furnace or in a gaseous atmosphere annealing furnace of H2 concentration as high as >75%, to soaking treatment, and then to slow cooling through the temp. region between 300 and 500°C in ≥6hr. This steel strip is heated up to 500-650°C, immersed, in the course of cooling, into molten Zn of 420-520°C to undergo hot-dip galvanizing, and then reheated to 520-640°C, by which the hot-dip galvanized layer is formed into a galvannealed layer of Fe-Zn alloy.