Patent ID: 9580785
Date: 2017-02-28
CPC Classifications: C21D,C22C,C23C

Claim:
1. A method for manufacturing a 700 MPa or more high-strength galvannealed steel sheet having excellent formability and fatigue resistance, the method comprising: hot-rolling a slab to produce a hot-rolled sheet having a microstructure in which a total area ratio of bainite and martensite is 80% or more, the slab having, by % by mass, C: 0.05% to 0.3%, Si: 0.5% to 2.5%, Mn: 1.0% to 3.5%, P: 0.003% to 0.100%, S: 0.02% or less, Al: 0.010% to 0.1%, and the balance including iron and unavoidable impurities; cold-rolling the hot-rolled sheet to produce a cold-rolled steel sheet; continuously annealing the cold-rolled steel sheet by heating to 750° C. to 900° C. at an average heating rate of 8° C./s or more from 500° C. to an A galvanizing the steel sheet; and further coating-alloying the steel sheet in a temperature region of 540° C. to 600° C. for 5 to 60 seconds.