| 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 DEG C to 600 DEG C at a cooling rate of 150 DEG C/s or more; holding the cooled steel sheet at a temperature between 750 DEG C and 600 DEG C for 2 to 15 seconds; cooling the steel sheet at a cooling rate of 20 DEG C/s or more; and coiling the cooled steel sheet at a temperature of 400 DEG C or less. <IMAGE> |
