The heavy-gauge high strength steel plates with low yield ratio (YR) have been applied to the steel-frames of high-rise buildings to utilize the benefits of their high strength and uniformability. The yielding behavior of steels is influenced by the microstructural factors such as microstructure, grain size, volume fraction and shape of each phase, dislocation density and so on. In the present paper, the microstructural morphologies to control the yielding behavior of the steel were investigated by FEM analysis. The influences of shape, volume fraction and mechanical properties of each phase on yielding behavior were analized by using dual-phase and tri-phase models. From a series of FEM analysis, the appropriate microstructural morphologies to decrease the YR were clarified. Results obtained are summarized as follows; (1) Uniform distribution of circle high-hardness phase in low-hardness phase, (2) Around 50% volume fraction of low-hardness phase, (3) An increase in the ratio of yield strength of high-hardness phase to that of low-hardness phase.
