Effect of Niobium on Zn-Coated Dual Phase Steel Sheet

Abstract

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.