A phenomenological understanding of the novel design of hierarchical structure for 1 GPa ultrahigh strength and high toughness combination low alloy steel

In the present study we propose a novel design of hierarchical structure with the aim to obtain high strength of 1 GPa and low temperature toughness in a heavy low alloy steel plate by intercritical quenching and tempering (IQ & T). The hierarchical microstructure consisted of bimodal prior austenite grains with complex microstructure obtained via intercritical quenching from 810 degrees C and tempering at 500 degrees C. The large size grains with average diameter of -22 & mu;m consisted of tempered martensite and fine intercritical ferrite. While the surrounding fine grains with average diameter of -12 & mu;m consisted of tempered lower bainite and tempered lath martensite. Ultra-high yield strength of 998 MPa and tensile strength of 1059 MPa were obtained for the novel hierarchical microstructure steel, which was similar to the conventional quenched and tempered (Q & T) steel. Whereas, the low temperature impact energies at both -40 and -80 degrees C were -100% higher for the hierarchical microstructure steel than the conventional Q & T steel. The significant improvement of low temperature toughness is attributed to the synergistical effect of hierarchical microstructure. The fine intercritical ferrite in large grains increased the plastic deformation ability during impact and was beneficial for formation of dimples, and the surrounding fine grains offered high density of large mis-orientation grain boundaries to deflect crack propagation, leading to high impact energy at low temperature.