The improvement of hole-expansion properties for medium carbon steels by ultra fast cooling (UFC) after hot strip rolling was investigated.It was found that finely dispersed spherical cementite could be formed after ultra fast cooling , coiling and annealing treatment.Tensile strength of the steel after annealing was measured to be about 440MPa.During hole-expansion test , cracks were observed in the edge region around the punched hole because necking or cracking took place when tangential elongation exceeded the forming limit.Cracks were mainly formed by the coalescence of micro-voids.Fine and homogeneous microstructure comprised of ferrite and spheroidized cementite could increase elongation values of the tested sheets by suppressing the combination of the adjacent micro-voids , resulting in the improved hole-expansion property.
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