| 摘要 |
<p>A galvannealed sheet steel suitably used as a steel plate for automobiles, and a process for producing the same. This steel plate has a high powdering resistance during press working, and a high chipping resistance in a cold district. This method is also suitable for the production of a steel plate of a high tensile strength. The galvannealed sheet steel has a chemical composition containing not more than 0.01 wt.% of C, 0.03-0.3 wt.% of Si, 0.05-2.0 wt.% of Mn, 0.017-0.15 wt.% of P, 0.005-0.1 wt.% of Al, 0.005-0.1 wt.% of Ti, not more than 0.1 wt.% of Nb, not more than 0.005 wt.% of B, and the balance comprising Fe and unavoidable impurities. The galvannealed sheet steel has an average crystal grain size at a base metal surface where the plating layer is in contact is not more than 12 νm. The galvannealed steel sheet can be produced easily under the following conditions. A base metal the surface of which has been grind-removed by 1-8 g/m2 is reduced at a high temperature. During this time, the base metal is subjected to recrystallization annealing as necessary. During the cooling process from the high temperature for the reduction, the base metal is held for 10-120 seconds at a temperature of between 600 °C and 500 °C, then cooled to a plating temperature, and plated. The rate of increase of temperature between 420 °C - 480 °C for an alloying treatment of the steel sheet after the plating is not lower than 20 °C/sec.</p> |
