We investigate here the thickness effect on microstructures and mechanical properties of a quenched and tempered 178 mm thickness ASTM A517 GrQ steel. The microstructures at sub-surface, 1/4 thickness (t/4), and 1/2 thickness (t/2) were characterized. A comparison of hardness, strength, and impact toughness of the different positions shows that the lowest strength and toughness occurred at t/2, where a mixture of coarse, tempered martensite and bainite were found, and their inter-lath boundaries were occupied with highly dense, film-like or coarse, spheroidized carbides. The cooling rate for transformation was measured to be 0.6 °C/s at t/2 from the industrial processing data. In addition, the alloy elements at t/2 were heavily segregated, as revealed by electron probe microanalysis (EPMA) and a microhardness test. The resulted coarse microstructures thus lowered both the yield strength and the impact energies significantly, e.g., the crack propagation energy was completely lost at −60 °C. This study correlates the variation of mechanical properties to varied transformed microstructures based on the industrial quenching condition, which shows promise for improving the designing of the hardenability and controlling the carbides for ultra-thick quenched and tempered steel.

The mechanism for refining grains in industrial EH460 heavy‐gauge steel with a thickness of 80 mm and above is investigated using electron backscattered diffraction. Charpy impact and drop‐weight tests are conducted to quantify the effect of microstructures and texture on low‐temperature toughness. Abundant ferrite grains that form and impinge from different prior‐austenite grains increase the high‐angle grain boundary (HAGB) fraction. The result is a refined effective grain size of 3.7 ± 4.2 μm, a high HAGB fraction of 62.1%, and an average Charpy impact absorbed energy of ≈209 J at −80 °C. The excellent impact toughness is due to the high intensity of texture with types of {113}−{112}<110> and {332}<113>, as well as the uniformly distributed {110} slip planes and {001} cleavage planes. The high intensities of {110}<111> and {110}<112> also improve the drop‐weight toughness. However, due to plastic constraint in the larger drop‐weight specimens, increased plane strain in the inner regions is the root cause of the brittle fracture of these specimens at −70 °C. Nevertheless, the improved drop‐weight performance is attributed to the plane stress deformation at the specimen edges and also the occurrence of delamination in the central section.

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In order to compare the hot deformation behavior and microstructure evolution of the rack steels with and without segregation, hot compression tests are carried out in the temperature range of 1173–1423 K and the strain rate range of 0.01–10 s−1 on a DIL805A/D quenching and deformation dilatometer. The flow stress, constitutive relation, processing map, and microstructure characterization are investigated. The results show that the flow stress increases with the increase of strain rate and the decrease of deformation temperature. The calculated activation energy (Q) and stress exponent (n) for the segregated samples are 395.245 kJ mol−1 and 6.046, which are higher than segregated samples, showing 376.815 kJ mol−1 and 5.930. The processing maps show that the workability of the nonsegregated steel is significantly more excellent than segregated steel. The segregated steel shows a higher kernel average misorientation angle and local strain. Complete dynamic recrystallization occurs at a high strain rate, and high deformation temperature can refine the grains.

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