Effect of Precipitates on Ductility of High Tensile Strength IF Sheet Steel

High strength steel sheet(IF Hi-Ten), in which P,Mn,Si,etc. are added to Ti containing IF steel(Interstitial Free Steel) as substitutional solid-solution strengthening elements, has been developed in response to weight reduction of automobiles. In Ti added interstitial free steel, added P precipitated as (Fe,Ti)P as well as solutes in solid solution. Under microscope, (Fe,Ti)P precipitates were observed in voids of ductile fracture surface. In the developed steel, where Zr is substituted for Ti in order to avoid precipitation of detrimental (Fe,Ti), ZrN or ZrS were observed as large precipitates, although amount was small. The developed steel showed 39% elongation in comparison with 33% of conventional steel at same 440MPa strength level.