Development of Low-Alloy Steels with High Strength and Good Ductility with the Aid of Nanoscale Troostite
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JMEPEG https://doi.org/10.1007/s11665-019-3868-2
Development of Low-Alloy Steels with High Strength and Good Ductility with the Aid of Nanoscale Troostite J.W. Liang, X. Wang, X.L. Zhang, and Y.F. Shen (Submitted January 31, 2018; in revised form December 13, 2018) We aimed to design nanoscale troositic microstructures in medium-carbon low-alloyed steel by using annealing plus quenching–partitioning–tempering processes to simultaneously obtain high strength and good ductility. The troositic microstructure and the related mechanical properties of the studied steel were studied systematically. After quenching to 200 °C in 5 s and then holding at various partitioning temperatures for 60 s and subsequently tempering at 400 °C for 1800 s, the steel held at the partitioning temperature of 200 °C for 60 s exhibited a high tensile strength (rTS) of 1620 ± 10 MPa and a yield strength (ry) of 1220 ± 10 MPa, together with a good ef of 25% at a strain rate of 1 3 1023 s21. Both rTS and ry values were two times higher than the values for the coarse-grained counterpart with identical chemical composition. The key strengthening factor is associated with the nanoscale troostite phase. The mechanical properties of the steel with an average thickness of troositic lamellae at 200 ± 15 nm are consistent with the line predicted by the H–P relationship. Keywords
low-alloyed steel, nanoscale troostite, quenching– partitioning–tempering (Q–P–T), ultrahigh strength
1. Introduction D6AC steel is a medium-carbon steel with low alloy content; it has a carbon content of 0.42-0.48 wt.% and also contains several other elements within the ranges expected for standard AMS 6431 (i.e., 0.9-1.2 Cr, 0.9-1.1 Mo, 0.4-0.7 Ni, 0.08-0.15 V, 0.6-0.9 Mn, and 0.15-0.3 Si, in wt.%). After heat treatment by high-temperature tempering, D6AC steel shows many advantageous characteristics such as high strength, good corrosion resistance, low cost and reliability in applications; therefore, it is widely used for pressure vessels, aerospace and defense components (Ref 1-3). The chemical composition of D6AC steel is similar to that of AISI4340 and AISI 4140, but D6AC steel also contains V and Cr, which are a strong carbide forming element and a solid solution strengthening element, respectively. Phase formation in Fe-Mn alloys depends on alloy composition and temperature and significantly affects mechanical properties of these alloys. Additions of alloying elements to the Fe-Mn alloys affect their phase stability. For example, small additions of Ni, V, Mo, C and Cu to the Fe-17Mn alloy significantly decrease the temperature of martensitic transformation initiation (Ref 4). Mn is included to retard ferrite, pearlite and bainite formation and to decrease the bainite formation and the martensite-start temperature (Ref 5). However, presently, the application of these steels is limited by their poor ductility at the highest strength levels (Ref 6, 7).
J.W. Liang, X. Wang, X.L. Zhang, and Y.F. Shen, Key Laboratory for Anisotropy and Texture of Materials (MOE), School of Ma
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