Method for manufacturing a hot dip galvanized and galvannealed steel sheet having excellent elongation properties

1. A method for manufacturing a hot-dip galvanized steel sheet having excellent elongation properties in which a hot-dip galvanized layer is formed on a surface of a base steel sheet, the method comprising:

preparing a steel slab and thereafter reheating the steel slab, the steel slab comprising, 0.02-0.08% of carbon (C), 1.3-2.1% of manganese (Mn), 0.3% or less of silicon (Si) (excluding 0%), 1.0% or less of chromium (Cr) (excluding 0%), 0.1% or less of phosphorous (P) (excluding 0%), 0.01% or less of sulfur (S) (excluding 0%), 0.01% or less of nitrogen (N) (excluding 0%), 0.02-0.06% of aluminum (sol, Al), 0.2% or less of molybdenum (Mo) (excluding 0%), 0.003% or less of boron (B) (excluding 0%), and a remainder of Fe and other inevitable impurities, in wt %;

performing finish hot-rolling on the reheated steel slab in a temperature range of Ar3+50° C.-950° C., and thereafter coiling the finish hot-rolled steel sheet at 450-700° C.;

performing cold-rolling on the coiled hot-rolled steel sheet with a reduction ratio of 40-80%, and thereafter performing continuous annealing on the cold-rolled steel sheet in a temperature range of 760-850° C.;

performing a first cooling on the continuous annealed steel sheet to a temperature range of 630-670° C. at an average cooling rate of 2-8° C./s, and thereafter performing a second cooling on the first cooled steel sheet to a temperature range of Ms+20° C. to Ms+50° C. at an average cooling rate of 3-10° C./s, wherein Ms is martensite start temperature; and

performing hot-dip galvanizing on the second cooled steel sheet in a temperature range of 440-480° C., and thereafter cooling the hot-dip galvanized steel sheet to a temperature of Ms−100° C. or lower at an average cooling rate of 4° C./s or higher.

2. The method of claim 1 , wherein the base steel sheet of the hot-dip galvanized steel sheet,

has a microstructure including 90% or more of ferrite, and a remainder of martensite and bainite, wherein B (%), as defined by Equation 1, is 3% or less;

contains a percentage of martensite (M %) having an average particle diameter of 5 μm or less occupying space in ferrite grain boundaries (including grain boundary triple points) of 90% or higher, as defined by Equation 2; and

at ¼t based on the thickness (t) of the sheet, the ratio of a C concentration (a) in the ferrite phase to a C concentration (b) in the martensite phase, i.e., (a)/(b), is 0.7 or less, and the ratio of an Mn concentration (c) in the ferrite phase to an Mn concentration (d) in the martensite phase, i.e., (c)/(d), is 0.8 or less,

B (%)={ BA /( MA+BA )}×  100 [Equation 1]

where BA is the area occupied by bainite, and MA is the area occupied by martensite,

M (%)={ M gb /( M gb +M in )}×100  [Equation 2]

where M gb is the amount of martensite in ferrite grain boundaries, and M in is the amount of martensite within ferrite grains, the martensite having an average particle diameter of 5 μm or less.

3. The method of claim 1 , wherein, in the base steel sheet of the hot-dip galvanized steel sheet, the ferrite phase has an average grain size of 4 μm or greater, and the area occupied by the ferrite phase having an average grain size of 7 μm or greater in the entire ferrite phase is 10% or higher.