| 摘要 |
<p>A steel pipe which comprises ultrafine ferrite grains and possesses excellent toughness, ductility, ductility-strength balance, and impact resistance; and a process for manufacturing the same, comprising heating an untreated steel pipe, with the average grain diameter of ferrite being di(νm), comprising C, Si, Mn, and Al each limited in optimal content range and optionally added Cu, Ni, Cr and/or Mo, or Nb, Ti, V and/or B, to a temperature of Ac3 transformation point or below and then subjecting the heated pipe to stretch reduction with a total reduction of Tred (%) at an average rolling temperature of υm (°C) in the transformation temperature range of from 400 to Ac3 in such a manner that di, υm, and Tred satisfy a predetermined relational expression.</p> |
