To provide a low-yield-ratio high-strength galvannealed steel with the use of a steel containing comparatively few alloying elements, which does not include a precipitate-forming element.
A steel slab has a composition including 0.03-0.18 mass% C, 0.2-2.0 mass% Si, 0.5-3.0 mass% Mn, 0.10 mass% or less P and 0.03% mass% or less S; and has a segregation degree of Mn of 1.05 to 1.10, which is defined by the following expression (1): segregation degree of Mn = (Mn concentration in slab center-average Mn concentration)/average Mn concentration --- (1). The method for manufacturing the galvannealed steel comprises hot-rolling the steel slab, further cold-rolling it, forming a Fe-based plated film with a coating weight of 3 to 15 g/m2, annealing it in a reductive gas atmosphere in a continuous plating and annealing line, immersing it in a hot-dip galvanizing bath to form a galvanized layer, and then skipping a heating step or heating it to a temperature lower than 530°C, to form a galvannealed layer on the surface of a steel sheet.
YAMAMOTO TOMOO
MATSUMOTO TAKASHI
Okada Manri
Next Patent: FILM DEPOSITION SYSTEM, FILM DEPOSITION METHOD, OPTICAL ELEMENT, AND OPTICAL SYSTEM