| 摘要 |
The high strength steel sheet consists essentially of 0.05 to 0.15% C, 0.5% or less Si, 1.00 to 2.00% Mn, 0.09% or less P, 0.01% or less S, 0.005% or less N, 0.01 to 0.1% Sol.Al, and balance of Fe and inevitable impurities; and contains 60% or more polygonal ferrite by volume, and 5 to 30% martensite by volume. The steel sheet is manufactured by the steps of: casting a slab having the specified composition; hot-rolling the slab at Ar3 point or more temperature; beginning cooling the hot-rolled steel sheet within 2 seconds after completing the hot-rolling to a temperature of from 750.degree.C to 600.degree.C at a cooling rate of 150.degree.C/s or mor e; holding the cooled steel sheet at a temperature between 750.degree.C and 600.degree. C for 2 to 15 seconds; cooling the steel sheet at a cooling rate of 20.degree.C/s or more; and coiling the cooled steel sheet at a temperature of 400.degree.C or less.
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