Study on Diversified Carbide Precipitation in High-Strength Low-Alloy Steel during Tempering

Herein, the precipitation and mechanical properties of high-strength low-alloy steel (Q960E) under different tempering temperatures from 560 to 640 degrees C are investigated. Diversified precipitations (including MC, M3C, M23C6, and M7C3) are formed during tempering. The size of the M3C, M23C6, and M7C3 precipitates increases, as the temperature increases, and the precipitates are gradually spheroidized with an average size of less than 100 nm. MC-type precipitates (where M = Nb and Ti) are produced at different tempering temperatures. When tempered at 600 degrees C, Q960E steel exhibits excellent mechanical properties, including a high strength and elongation with a good impact performance. At 600 degrees C, a large number of fine MC carbides with an average size of less than 25 nm are formed. MC and matrix exhibit the Baker-Nutting orientation relationship. Edge dislocations exist in the transition region between the MC nanophase and the matrix. These dislocations reduce the mismatch degree between the MC nanophase and the matrix and promote MC nucleation. Meanwhile, the steel retains many low-angle grain boundaries when tempered at 600 degrees C, which is beneficial to its high strength.