METHOD FOR MANUFACTURING HIGH-STRENGTH HOT-DIP GALVANIZED STEEL SHEET

摘要

The invention provides a method for producing high-strength galvannealed steel sheets having excellent workability and fatigue resistance. A steel sheet containing 0.1 to 2.5% Si is subjected to an oxidation treatment, then to reducing annealing, thereafter to a hot-dip galvanizing treatment and to an alloying treatment. In the oxidation treatment, the steel sheet is heated in an upstream stage at a temperature of 400 to 750°C in an atmosphere having an O 2 concentration of not less than 1000 ppm by volume and a H 2 O concentration of not less than 1000 ppm by volume, and is heated in a downstream stage at a temperature of 600 to 850°C in an atmosphere having an O 2 concentration of less than 1000 ppm by volume and a H 2 O concentration of not less than 1000 ppm by volume. In the reducing annealing, the steel sheet is heated at a temperature of 650 to 900°C in an atmosphere containing 5 to 30 vol% of H 2 and 500 to 5000 ppm by volume of H 2 O the balance being N 2 and inevitable impurities. In the alloying treatment, the steel sheet is treated at a temperature T (°C) satisfying the following relation for 10 to 60 seconds: ˆ’ 50 log H 2 O + 650 ‰¤ T ‰¤ ˆ’ 40 log H 2 O + 680 ( H 2 O represents the H 2 O concentration (ppm by volume) during the reducing annealing).