| 摘要 |
PROBLEM TO BE SOLVED: To provide a cold-rolled steel sheet which has excellent slow-aging properties and excellent curability in baking. SOLUTION: A steel stock having a composition comprising, by mass, 0.020 to 0.070% C,≤0.05% Si, 0.1 to 0.5% Mn,≤0.05% P,≤0.02% S, 0.02 to 0.08% Al and 0.005 to 0.02% N is heated at≥1,150°C, is subjected to finish rolling in which finish rolling end temperature is≥850°C so as to be a hot rolled sheet. The hot rolled sheet is coiled at a coiling temperature satisfying the specified relation between the Al and N contents, is then, in two stages of cold rolling at a draft of 60 to 90% and a heating rate, subjected to heating to an annealing temperature satisfying specified relational formula with the Al content, the N content and the coiling temperature, is soaked and is cooled to≤500°C at a cooling rate of≥5°C/s. Further, a galvanizing layer such as a hot dip galvanizing layer may be formed on the surface of the steel sheet. In this way, a cold-rolled steel sheet comprising≥0.0010% solid solution N, comprising a ferritic phase with the average crystal grain size of≤7μm at an area ratio of≥80%, and having a structure where precipitates with a size of 0.05 to 5μm in the average circle-equivalent diameter are precipitated into the crystal grains of the ferritic phase is formed. Thus, occurrence of wrinkles caused by aging can be prevented, and also, after the application of relatively low strain of <2.0%-heat treatment equivalent to coating/baking, a baking/curing amount of≥50 MPa can be secured. COPYRIGHT: (C)2011,JPO&INPIT |
