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

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).