| 摘要 |
<p><P>PROBLEM TO BE SOLVED: To provide a hot rolled steel sheet and a cold rolled steel sheet which have satisfactory surface properties and excellent formability. <P>SOLUTION: The hot rolled steel sheet has a composition containing 0.0002 to 0.2% C, 0.003 to 3.0% Si, 0.003 to 3.0% Mn and 0.002 to 2.0% Al. The mean crystal grain size of a ferrite phase is≤20μm, and the area ratio thereof is≥70%. The aspect ratio of the ferrite phase is≤3, and≥70% of the ferrite grain boundaries consist of large-angled grain boundaries. Further, the maximum size of the ferrite phase formed of large-angled grain boundaries is≤30μm, and also, the area ratio of precipitates with a minimum size of≥5 nm is≤2% of the metallic structure. The mean crystal grain size of the second phase in which the area ratio is the maximum in the balance phase other than the ferrite phase and the precipitates is≤20μm, and the large-angled grain boundaries of the ferrite phase are present between the nearest second phases. The production method uses the hot rolled steel sheet. The cold rolled steel sheet is produced therefrom. <P>COPYRIGHT: (C)2004,JPO</p> |
