| 摘要 |
PROBLEM TO BE SOLVED: To provide a high tensile strength hot dip galvanized steel sheet having excellent ductility and slidability and to provide a method for producing the same. SOLUTION: A steel sheet containing 0.05 to 0.20% C, 0.3 to 1.8% Si and 1.0 to 3.0% Mn is subjected to a first stage in which the same is held to (the Ac3 point -50 deg.C) or more for 5s or more and is thereafter cooled to the Ms point or less at a cooling rate of 10 deg.C/s or more, a second stage in which the same is held in the temperature range of (the Ac1 point to the Ac3 point) for 5 to 120s and is thereafter cooled to 500 deg.C or less at a cooling rate of 5 deg.C/s or more, a third stage in which the same is subjected to hot dip galvanizing treatment and is thereafter cooled to 300 deg.C at a cooling rate of 5 deg.C/s or more and a skinpass rolling stage in which the same is subjected to skinpass rolling to control its surface roughness Ra to 0.3 to 1.0μm in succession to obtain the high tensile strength hot dip galvanized steel plate having a composite structure composed of tempered martensite of 20% or more in a volume ratio, retainedγof 2% or more, ferrite of 30% or more and a low temperature transformed phase.
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