Dual Phase (DP) steel sheets, mainly 590 MPa TS grade steel, have been applied to
structural parts of automobile because of their good formability, large bake hardenability and high
crash worthiness. Although the concept of DP steel was established as early as the mid-1970th, the
literature contains little discussion of Zn-coated DP steel, which has been the main application in
recent years. In manufacturing Zn-coated DP steel, chemical composition which secures adequate
Zn coatability and appropriate heat cycles, including galvannealing, must be considered.
In this paper, the effect of Nb on the tensile properties, stretch flange formability and bake
hardenability of 590 MPa TS grade DP steel is discussed. The base chemical composition was
0.05%C-2%Mn-0.5%Cr steel. The effect of Nb on the above-mentioned properties was studied
using 0.05%Nb added steel and found that the addition of 0.05%Nb results in improved elongation,
stretch flangeability and bake hardenability with higher tensile strength under both simulated GA
and GI heat cycles. These improved properties by the addition of Nb are brought about by the grain
refinement of ferrite matrix and finer dispersion of martensite.
Effect of Chemical Composition of Nut Material on the Fracture Behavior in Nut Projection Welding of Hot-Stamped Steel Sheet