Effect of Carbide Forming Elements on the Mechanical Properties of Continuously Annealed Extra-low-carbon Steel Sheets

Abstract

For the purpose of producing a cold-rolled steel sheet with non-aging property and deep drawability by continuous annealing, the effect of strong carbide forming elements such as Ti and Nb on the mechanical properties of extra-low-carbon steel sheets was investigated. It has been clarified that the effect of carbide forming elements is classified into three groups with respect to content of carbon and carbide forming elements (X).(1) The range of high content of carbon and/or carbide forming elements (C>0.005 wt%, X(at%)/C(at%)>1): Conventional interstitial free steels (IF-steels) are included in this range. Precipitation of carbides occurs at the high temperature range in γ phase and the influence of hot-working conditions on the mechanical properties is small.(2) The range of medium content of carbon and carbide forming elements (C=0.003-0.005wt%), X(at%)/C(at%)≅1: The mechanical properties strongly depend on hot-working conditions because carbides such as NbC and TiC start precipitation during hot-rolling. Acceleration of carbide precipitation through the high reduction and high speed hot-rolling decreases the number of fine carbides depositing after hot-rolling, with the result that the ductility and the Lankford-values of the cold-rolled and continuous-annealed steel sheets are improved.(3) The range of low content of carbon and carbide forming elements (C<0.003wt%): Precipitation of carbides occurs very little during hot-rolling in γ phase. In the steel sheets with no carbide forming element, the improvement of mechanical properties by lowering carbon content is relatively small because the planar anisotropy is extremely large even in lower carbon steels. Niobium is the most effective element among Nb, Ti, Cr, V and W for decreasing the planar anisotropy. This mainly results from the fact that the grain size of hot bands effectively decreases by the effect of solute niobium. The effect of carbide forming elements in solution becomes important in this range.