| 摘要 |
<p>The invention relates to a galvannealed steel sheet suited for the use in automobiles, and the method of manufacturing same. Said steel sheet has excellent powdering resistance during press working and also excellent chipping resistance in cold regions. Said method is also applicable to the manufacturing of a steel sheet of high tensile strength. The galvannealed steel sheet has the following chemical composition, on the basis of percent by weight, of C : up to 0.01 %, Si : 0.03 to 0.3 %, Mn : 0.05 to 2 %, P : 0.017 to 0.15 %, Al : 0.005 to 0.1 %, Ti : 0.005 to 0.1 %, Nb : up to 0.1 %, B : up to 0.005 %, balance : Fe and incidental impurities. Further the average grain size of the surface of the base metal of the galvannealed steel sheet is 12 mu m or less. The galvannealed steel sheet is manufactured under processing conditions, wherein after removing 1 to 8 g/m <2> of the surface of the base metal by grinding, the base metal is heated in a hydrogen containing atmosphere at a high temperature, thereby reducing the surface of the base metal. Recrystallization annealing is applied to the base metal as necessary. Then, the base metal is retained in a range of 600 to 500 DEG C for 10 to 120 sec in the cooling stage following said heating, and galvanized. After that, the galvanized steel sheet is heated to a Fe - Zn alloying temperature with the velocity of 20 DEG C /sec or more in the temperature range of 420 to 480 DEG C.</p> |
