Steel for pipes having high fatigue resistance, method of manufacturing the same, and welded steel pipe using the same

1. A steel for pipes, the steel consisting of, by wt %, carbon (C): 0.10% to 0.15%, silicon (Si): 0.30% to 0.50%, manganese (Mn): 0.8% to 1.2%, phosphorus (P): 0.025% or less, sulfur (S): 0.005% or less, niobium (Nb): 0.01% to 0.03%, chromium (Cr): 0.5% to 0.7%, titanium (Ti): 0.01% to 0.03%, copper (Cu): 0.1% to 0.4%, nickel (Ni): 0.1% to 0.3%, nitrogen (N): 0.008% or less, and a balance of iron (Fe) and inevitable impurities, wherein chromium (Cr), copper (Cu), and nickel (Ni) satisfy the following formula, wherein

the steel has a microstructure comprising ferrite having a grain size of 10 pm or less, in an area fraction of 50% to 80%, and pearlite in an area fraction of 20% to 50%,

80<100(Cu+Ni+Cr)+(610−CT)<120   [Formula]

where Cu, Ni, and Cr respectively refer to Cu, Ni, and Cr contents by weight, and CT refers to a coiling temperature ° C. and the steel has a fatigue life of 1000 (Nf or greater).

2. A method of manufacturing a steel for pipes, the method comprising:

preparing a steel slab consisting of, by wt %, carbon (C): 0 .10% to 0.15%, silicon (Si): 0.30% to 0.50%, manganese (Mn): 0.8% to 1.2%, phosphorus (P); 0.025% or less, sulfur (S): 0.005% or less, niobium (Nb): 0.01% to 0.03%, chromium (Cr): 0.5% to 0.7%, titanium (Ti): 0.01% to 0.03%, copper (Cu): 0.1% to 0.4%, nickel (Ni): 0.1% to 0.3%, nitrogen (N): 0.008% or less, a balance of iron (Fe) and inevitable impurities;

reheating the steel slab to a temperature within a range of 1100° C. to 1300° C.: rough rolling the reheated steel slab at a temperature within a range of 900° C. to 1100° C.; after the rough rolling, finish hot rolling the steel slab at a temperature within a range of 800° C. to 900° C. to produce a hot-roiled steel sheet; and

after cooling the hot-rolled steel sheet at a cooling rate of 45° C./s, or less, coiling the steel sheet at a coiling temperature (CT) satisfying the following formula,

80<100(Cu+Ni+Cr)+(610−CT)<120   [Formula]

where Cu, Ni, and Cr respectively refer to Cu, Ni, and Cr contents by weight, and CT refers to the coiling temperature ° C., thereby producing the steel according to claim 1 .

3. The method of claim 2 , wherein the coiling of the steel sheet is performed at a temperature within a range of 590° C. to 630° C.

4. The method of claim 2 , wherein the cooling rate is 43 ° C/s or less.

5. The method of claim 2 , wherein the temperature of the finish hot rolling the steel slab is at a temperature within a range of 800° C. to 851° C. to produce a hot-rolled steel sheet.

6. The method of claim 2 , wherein the temperature of the finish hot rolling the steel slab is at a temperature within a range of 800° C. to 842° C. to produce a hot-rolled steel sheet.

7. The method of claim 2 , wherein the coiling temperature is within a range of 600° C. to 630° C.