| 摘要 |
<p>A process for the production of a steel material comprising rolling a steel material having a structure mainly comprising ferrite or ferrite plus pearlite or ferrite plus cementite at a percentage reduction of area of at least 20 % in a ferrite recrystallization temperature region to achieve such characteristics as a crystal particle diameter of not greater than 3 νm, preferably not greater than 1 νm, an elongation of at least 20 %, a value of tensile strength (TS: MPa) x elongation (El: %) of at least 10,000 or a percent ductile fracture of at least 95 %, preferably 100 %, in an actual pipe Charpy impact test at -100 °C. Particularly, this process yields a steel material containing 0.05 to 0.30 wt.% of C, 0.01 to 3.0 wt.% of Si, 0.01 to 2.0 wt.% of Mn and 0.001 to 0.10 wt.% of Al and having a structure comprising ferrite alone or ferrite and a second phase, wherein the ferrite particle diameter is not greater than 3 νm and the areal ratio of the second phase is not greater than 30 %. An untreated steel pipe having the composition described above is heated to (Acl + 50 °C) to 400 °C and subjected to stretch reduction at a cumulative diameter reduction ratio of at least 20 % in a rolling temperature range of (Acl + 50 °C) to 400 °C. In this case, the rolling process preferably contains at least one rolling pass having a diameter reduction ratio of at least 6 % in the stretch reduction. When the contents of C, Si, Mn and other alloy elements are kept at low levels and stretch reduction is carried out in the temperature range described above, a steel pipe having high ductility and strength and improved toughness and stress corrosion crack resistance can be manufactured and the resulting pipe can be used as a line pipe. The fatigue resistance can be improved, too.</p> |
