| 摘要 |
PURPOSE: A steel product for API-X65 grade line pipes is provided a strength of which is not deteriorated even after roller bending and stress relief processes, and a method for manufacturing the steel product for API-X65 grade line pipes by applying TMCP(thermo-mechanical controlled process) is provided. CONSTITUTION: The steel product for stress relief heat treatment guaranteed API-X65 grade line pipes comprises 0.07 to 0.09 wt.%, 0.20 to 0.50 wt.% of Si, 1.40 to 1.60 wt.% of Mn, 0.025 wt.% or less of P, 0.005 wt.% or less of S, 0.015 to 0.050 wt.% of soluble Al, 0.05 to 0.15 wt.% of Cr, 0.03 to 0.07 wt.% of Mo, 0.035 to 0.045 wt.% of Nb, 0.040 to 0.050 wt.% of V, 0.005 to 0.015 wt.% of Ti, 20 to 70 parts per million of N and a balance of Fe and other inevitable impurities. The method for manufacturing the steel product for stress relief heat treatment guaranteed API-X65 grade line pipes comprises the processes of heating a steel slab comprising 0.07 to 0.09 wt.%, 0.20 to 0.50 wt.% of Si, 1.40 to 1.60 wt.% of Mn, 0.025 wt.% or less of P, 0.005 wt.% or less of S, 0.015 to 0.050 wt.% of soluble Al, 0.05 to 0.15 wt.% of Cr, 0.03 to 0.07 wt.% of Mo, 0.035 to 0.045 wt.% of Nb, 0.040 to 0.050 wt.% of V, 0.005 to 0.015 wt.% of Ti, 20 to 70 parts per million of N and a balance of Fe and other inevitable impurities in the temperature range of 1100 to 1200 deg.C; hot rolling the heated steel slab in the conditions that a finishing rolling initiation temperature is 850±20 deg.C, a reduction ratio is 62±2 %, and a finishing rolling finishing temperature is 700±20 deg.C; and cooling the hot rolled steel slab to the temperature interval of 540±20 deg.C in a cooling rate of 7±3 deg.C/sec.
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