2017 年 103 巻 10 号 p. 579-588
Better balance of strength and toughness is a strong demand for the ASTM A707 L5 grade steel. In the present study, therefore, the combination of hardening by Cu precipitates and toughening by quenching from a dual-phase (α+γ) region, so-called intercritical quenching or lamellarizing, has been investigated for a better balance of strength and toughness. The combination procedure resulted in a drastic increase in toughness at low temperatures with a slight decrease in yield strength.
The lamellarizing brought about a complicated microstructure with meandered high angle grain boundaries (HAGB) and fine grains bordered by the HAGB. The final microstructure was composed of granular bainitic ferrites without retained γ and basically dual of a softer phase and a harder phase. The softer phase inherited the not-transformed α phase region in lamellarizing and contained coarse Cu precipitates. The harder phase inherited the transformed γ phase region in lamellarizing and contained no Cu precipitates. Hence, over-aging of Cu precipitation in the softer phase might result in the slight decrease in yield strength.
In the present steel, the retained γ has nothing to do with the improved toughness. Hence, the effective grain size (dEFF) approach was verified to account for the microstructural effect on toughness. The unit microstructure to determine the dEFF was identified to be the bainitic ferrite grain bordered by the HAGB. The refinement of the dEFF through lamellarizing can be attributed to the improved toughness.
