Effect of Chromium on Microstructure and Mechanical Properties of Cold Rolled Hot-dip Galvanizing DP450 Steel

Two cold rolled hot-dip galvanizing dual phase(DP) 450 steels with different amounts of chromium were designed and the effects of the chromium concentration and galvanizing processes on the microstructure and mechanical properties were also investigated. The results show that the experimental steels exhibit typical dual phase microstructure character. However, the ferrite phase of steel with higher chromium is more regular and its boundaries are clearer. Meanwhile, martensite austenite(MA) island in steel No. 2 is diffused and no longer distributes along the grain boundary as net or chain shape. More MA islands enriched with Cr element can be found in the ferrite grains, and the increment of Cr element improves the stablity of the austenite so that the austenite has been reserved in MA islands. In addition, the experimental steel with higher chromium exhibits better elongation, lower yield ratio and better formability. The mean hole expanding ratio of steels No. 1 and No. 2 is 161.70% and 192.70%, respectively.