Effect of Second Phase on the r-value of Dual Phase Sheet Steel

Abstract

Study has been made with regard to second phase parameters: hardness ratio (c) of second phase martensite to ferrite phase, second phase volume fraction (f), and second phase spacing (d). The parameters were varied by means of combination of heat treatments; d was controlled by box-annealing, f, by quenching, and c, by tempering.
The r-value was affected strongly by the character of the second phase; it decreased with increasing c and/or f. The effect of c on the r-value was relatively small for small f and marked for large f.
The decrease in d enhanced the effect of c and f on the r-value.
The above effect of the second phase can be interpreted in terms of the internal stress caused by the difference in the deformability of both constituent phases.
Finally, condition is presented for the production of dual phase steel with high r-value; furthermore, this steel has high bake hardenability and shows no yield point elongation after room temperature aging.