Method of manufacturing high-strength steel sheet having excellent processability
Provided is a method of manufacturing a high strength steel sheet having excellent workability, including: reheating a steel slab; hot-rolling the reheated steel slab at an Ar3 transformation point or higher; coiling in a temperature range of 400 to 700° C.; primary cooling at a cooling rate of 0.1° C./s or less to room temperature; producing a cold rolled steel sheet at a cold reduction ratio of 40 to 70%; continuously annealing the cold rolled steel sheet in a temperature range of Ac1+30° C. to Ac3−20° C.; secondary cooling at a cooling rate of 10° C./s or less to 630 to 670° C.; third cooling to 400 to 500° C. at a cooling rate of 5° C./s or more in a hydrogen; maintaining for 70 seconds or more; hot-dip galvanizing; and performing a final cooling to Ms or less at a cooling rate of 1° C./s or more.
1. A method of manufacturing a steel sheet, the method comprising:
reheating, at a temperature in a range of 1050 to 1300° C., a steel slab including, in weight %, 0.06 to 0.18% of carbon (C), 1.5% or less (excluding 0%) of silicon (Si), 1.7 to 2.5% of manganese (Mn), 0.15% or less (excluding 0%) of molybdenum (Mo), 1.0% or less (excluding 0%) of chromium (Cr), 0.1% or less of phosphorus (P), 0.01% or less of sulfur (S), 1.0% or less (excluding 0%) of aluminum (Al), 0.001 to 0.04% of titanium (Ti), 0.001 to 0.04% of niobium (Nb), 0.01% or less of nitrogen (N), 0.01% or less (excluding 0%) of boron (B), 0.05% or less (excluding 0%) of antimony (Sb), a remainder of Fe and other inevitable impurities;
preparing a hot-rolled steel sheet by finishing hot-rolling the reheated steel slab at an Ar3 transformation point or higher;
coiling the hot rolled steel sheet in a temperature range of 400 to 700° C.;
after the coiling, primary cooling at a cooling rate of 0.1° C./s or less to room temperature;
after the cooling, producing a cold rolled steel sheet by cold rolling at a cold reduction ratio of 40 to 70%;
continuously annealing the cold rolled steel sheet in a temperature range of Ac1+30° C. to Ac3-20° C.;
after the continuously annealing, performing a secondary cooling at a cooling rate of 10° C./s or less (excluding 0° C./s) to 630 to 670° C.;
after the secondary cooling, performing a third cooling to 400 to 500° C. at a cooling rate of 5° C./s or more in a hydrogen cooling facility;
maintaining at 400 to 500° C. for 70 seconds or more after the third cooling;
hot-dip galvanizing after the maintaining; and
after the hot-dip galvanizing, performing a final cooling to Ms or less at a cooling rate of 1° C./s or more.
2. The method of claim 1 , wherein in the steel slab, a relationship of C, Si, Al, Mn, Mo and Cr satisfies the following relation 1,
(Si+Al+C)/(Mn+Mo+Cr)≥0.25 [Relationship 1]
where respective elements indicate a weight content.
3. The method of claim 1 , wherein a temperature at an outlet side during the finishing hot-rolling satisfies Ar3 to Ar3+50° C.
4. The method of claim 1 , wherein a bainite phase is formed upon the third cooling.
5. The method of claim 1 , wherein a fresh martensite phase is formed upon the final cooling after the hot-dip galvanizing.
6. The method of claim 1 , wherein the hot-dip galvanizing is performed in a zinc plating bath at 430 to 490° C.
7. The method of claim 1 , further comprising temper rolling at a reduction ratio of less than 1.0% after the final cooling.