| 摘要 |
PURPOSE:To improve the phosphate treatability of a steel sheet contg. extremely low carbon in low cost by performing the formation of an oxide layer under specified condition, the reduction thereof and the temper rolling for the cold-rolled steel sheet contg. carbon, nitrogen and carbonitride forming elements in specified proportion. CONSTITUTION:When the cold-rolled steel sheet contg. <=0.01wt% C, <=0.01wt% N and >=0.01wt% total amount of one or more kinds selected among carbonitride forming element groups consisting of Ti, Nb, Zr and Al is continuously annealed, the heat-treatment till 700C is performed under the oxidizing atmosphere to form an oxide layer having 0.1-5mu thickness on the surface of the steel sheet. Then, after the oxide layer is reduced at the temp. between >=700 deg.C and Ac3 point or below under the reducing atmosphere, it is cooled and also the temper rolling of >=0.5% is performed therefor. Thereby, the cold-rolled steel sheet excellent in phosphate treatability is obtained. |
