| 摘要 |
PROBLEM TO BE SOLVED: To provide a hot-dip galvannealed steel sheet which stably shows a high strength of 540 MPa or higher, high ductility and excellent hole expandability even when being mass-produced. SOLUTION: A steel material includes 0.03 to 0.10% C, 0.005 to 0.2% Si, 2.0 to 4.0% Mn, 0.1% or less P, 0.01% or less S, 0.01 to 0.1% sol. Al, 0.01% or less N, and one or two elements of 0.50% or less Ti and 0.50% or less Nb. The manufacturing method comprises the steps of: hot-rolling the steel material at a starting temperature of 1,050 to 1,300°C, at a finishing temperature of 800 to 950°C and at a winding temperature of 450 to 750°C; cold-rolling the plate; annealing the sheet by holding it in a temperature range between the Ac<SB>3</SB>transformation temperature and 950°C for 5 to 200 seconds; cooling the sheet to a temperature range between (galvanizing bath temperature-20°C) and (galvanizing bath temperature+100°C) at an average cooling rate of 1 to 50°C/second in between 750°C and 600°C; holding the sheet for 30 to 1,000 seconds, which include a period during dipping the sheet in the plating bath in the same temperature range; and annealing the sheet at a temperature of 430 to 600°C. The steel sheet contains ferrite of 60% or more by an area rate, and retained austenite of 3.0% or less by an area rate. The ferrite has an average grain size of 1.0 to 6.0μm, and contains precipitates with the particle diameters of 1 to 10 nm in an amount of 100 pieces/μm<SP>2</SP>or more. COPYRIGHT: (C)2009,JPO&INPIT
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