Achieving Excellent Strength‐Ductility Balance and Lower Yield Ratio in a 690 MPa‐Grade Multiphase Construction Steel

Abstract

In this article, thermomechanical controlled processing (TMCP) plus tempering (T) is adopted to acquire a novel 690 MPa‐grade high‐strength low‐carbon bainitic construction structural steel, which has an optimum balance of strength‐ductility and lower yield ratio (YR). The results show that the as‐rolled steel consists of bainitic ferrite, lath‐like bainite, and granular bainite. With an increase in the tempering temperature, a great deal of carbonitrides precipitates and maintains a small nano‐scaled size without coarsening. Their interaction with dislocations induces an increase in strength. When the tempering temperature further increases, some grain boundaries between adjacent bainitic laths gradually blur or even disappear, and the average width of the bainitic laths increases to ≈860 nm. The recovery of bainitic laths is the main reason for the decreased strength. Due to the synergistic effect of the small dot‐shaped martensite/austenite (M/A) constituents and grain boundaries misorientation distribution, maximum impact absorbed energy is achieved after tempered at 500 °C. Furthermore, the lower YR is achieved by appropriate control of strength difference between the soft bainitic ferrite and hard M/A constituents via tempering.