| 摘要 |
PURPOSE: To reduce the increase of yield strength after pipe forming process by adding boron in place of sol.Al in order to reduce the amount of dissolved nitrogen that causes hardening defects during pipe forming process. CONSTITUTION: The method includes the steps of casting a molten steel comprising 0.05 wt.% or less of C, Mn 0.35 to 0.45 wt.%, 0.05 wt.% or less of Si, 0.03 wt.% or less of P, 0.015 wt.% or less of S, sol.Al 0.001 to 0.10 wt.%, N 0.008 to 0.015 wt.%, B 0.0015 to 0.0035 wt.%, 0.25 wt.% or less of total sum of tramp elements, a balance of Fe into thin slab, wherein the tramp elements are as follows: 0.20 wt.% or less of Cu, 0.05 wt.% or less of Sn, 0.05 wt.% or less of Sb; hot rolling the thin slab, wherein the finish hot rolling temperature is higher than 860°C; and coiling the hot rolled steel sheet at 550 to 700°C.
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