Crystallography of Transformation Products with Different Cooling Rates in Low-Carbon Alloy Steel and Its Effect on Low-Temperature Toughness Uniformity of Heavy Plates

The crystallography of transformation products with different cooling rates has been investigated in a typical low-carbon alloy steel, and its effect on low-temperature impact toughness uniformity in the thickness direction of heavy steel plates has been elucidated. The results show that both the full lath martenite (LM) microstructure transformed at the cooling rate of 45 K s−1 and granular bainite (GB) + lath bainite (LB) + LM microstructure transformed at the cooling rates of 3∼6 K s−1 have higher density of high-angle grain boundaries (HAGBs). The density of HAGBs in the full GB microstructure transformed at the cooling rate of 0.5 K s−1 is the lowest, and that in the LB + LM microstructure transformed at the cooling rates of 15∼22 K s−1 is the middle. In the GB + LB + LM microstructure, the division of austenite by multi-stage transformation reduces the LB and LM transformation temperatures, contributing to a weakening of variant selection and a tendency to close-packed plane grouping. Therefore, the GB + LB + LM microstructure achieves the refinement degree similar to the full LM microstructure, but slightly higher than that of the LB + LM microstructure. For the 60 mm thick heavy steel plate with ultra-fast cooling, the cooling rate decreases from 41 K s−1 at the subsurface to 14 K s−1 at the 1/4-thickness and then to 4 K s−1 at the 1/2-thickness. The full LM microstructure, LB + LM microstructure, and GB + LB + LM microstructure are obtained in sequence, resulting in a more uniform density of HAGBs. Consequently, the relatively uniform impact toughness at 143 K in the thickness direction is achieved with a small difference of ∼30 J.