Process for producing galvanized, non-aging cold rolled steel sheets having good formability in a continuous galvanizing line

摘要

A process for producing a non-aging galvanized steel sheet having good formability in a continuous galvanizing production line, which comprises heating a low carbon, Al-killed cold rolled steel sheet at a temperature not lower than a recrystallizing temperature, reducing the surface of the steel sheet thus heated in a reducing atmosphere, cooling the steel sheet to a temperature (TE) ranging from 200 to 350 °C from a temperature not lower than 600 °C at a cooling rate not less than 30 °C/s, holding the steel sheet at the temperature (TE) for 0 to less than 10 seconds, reheating the steel sheet to a temperature ranging from 430 to 500 °C at a heating rate not less than 10 °C/s, immersing the steel sheet into a molten zinc bath, cooling the steel sheet thus galvanized to a temperature not higher than 370 °C, and subjecting the steel sheet to an overaging treatment to a temperature range from 250 to 320 °C for not shorter than 40 seconds. A modified process according to the present invention further comprises reheating the galvanized steel sheet to a temperature ranging from 480 to 600 °C at a heating rate not lower than 10 °C/s, and holding the sheet in this temperature range to perform alloying of the zinc coating layer with the steel substrate.