| 摘要 |
PURPOSE:To prepare a soft non-ageing cold rolled steel plate having quality same to that of one by case hardening without adding an expensive alloying element, in the continuous annealing of the cold rolled steel plate, by limiting a cold and over-ageing condition after continuous annealing so as to obtain an appropriate number of cementite grains. CONSTITUTION:A steel ingot containing 0.01-0.05% C, 0.05-0.25% Mn, 0.003- 0.015% S, 0.005-0.10% Al, N<0.0050% and, if necessary, 0.0005-0.0040% B is hot rolled to form a plate material which is, in turn, wound up at 65 deg.C or more in a coil form. This plate material is cold rolled to adjust the thickness thereof to an objective one while the cold rolled plate material is annealed in a continuous annealing apparatus. In this case, re-crystallization annealing is performed at 700-850 deg.C and, thereafter, quenching is performed from a temp. of 650 deg.C or more at a cooling speed V of 1,000 deg.C/sec or less. Succeedingly, the annealed plate material is held at a temp. ranging from 150 deg.C-[-70X(logV/ 1,000)<2>] to 180-350 deg.C for 10-60sec to form 4X10<4>-2X10<6>/mm.<2> of cementite nuclei before precipitation treatment is applied thereto at 250-400 deg.C for 2- 10min. |
