| 摘要 |
A method for manufacturing a cold-rolled and galvanized steel sheet having high yield ratio and high strength is provided to reduce a manufacturing cost by lowering Nb content without adding Ti to the steel sheet. A method for manufacturing a cold-rolled and galvanized steel sheet having high yield ratio and high strength includes the steps of: manufacturing steel slab consisting of C: 0.07~0.09%, Mn: 0.85~0.95%, Si: 0.02~0.3%, P: less than 0.03%, S: less than 0.01%, N: less than 0.006%, acid soluble Al: 0.02~0.05%, Nb: 0.02~0.05%, Fe, and necessary impurities; heating the steel slab up to 1150~1220 deg.c; performing a finish hot-rolling for the heated steel slab at 870~910 deg.c; winding the steel slab in a temperature range of 540~620 deg.c; manufacturing the cold-rolled steel sheet by cold-rolling the steel slab with more than 45% of a reduction rate; recrystallizing-annealing the cold-rolled steel sheet in the temperature range of 760~810 deg.c; skin-passing the cold rolled steel sheet with 0.4~1.6% of the reduction rate. If a thickness of the cold-rolled steel sheet is 0.6~1.2mm, a hot-winding temperature, a recrystallizing-annealing temperature, and a skin-passing reduction rate are to be 580~620 deg.c, 770~810 deg.c, and 1.2~1.6% respectively. If the thickness of the cold rolled steel sheet is 1.2~1.8mm, the hot-winding temperature, the recrystallizing-annealing temperature, and the skin-passing reduction rate are to be 560~600 deg.c, 770~810 deg.c, and 0.6~1.2% respectively. If the thickness of the cold-rolled steel sheet is 1.8~2.5mm, the hot-winding temperature, the recrystallizing-annealing temperature, and the skin-passing reduction rate are to be 540~580 deg.c, 760~800 deg.c, and 0.4~1.0% respectively. |
