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Zhang, Y.*; 丸澤 賢人*; 工藤 航平*; 諸岡 聡; Harjo, S.; 宮本 吾郎*; 古原 忠*
ISIJ International, 64(2), p.245 - 256, 2024/01
被引用回数：0As-quenched martensite in carbon steels needs to be tempered to restore its ductility and toughness for practical applications. During tempering of martensite, microstructural evolutions induced by a series of reactions relevant to carbon diffusion is known to occur. In this study, multi-aspect characterization using advanced techniques such as in-situ neutron diffraction, transmission electron microscopy and three-dimensional atom probe tomography, was performed to investigate the changes in tetragonality, physical properties, microstructure and solute carbon content in high-carbon martensite, with an aim to clarify its low-temperature tempering behaviors. A binary alloy with a chemical composition of Fe-0.78 mass%C was austenitized and quenched to prepare the as-quenched martensite, followed by tempering in continuous heating at different heating rates. It was found that various reactions occurred sequentially during tempering, starting from the structure modulation generated by carbon clustering in the 0th stage, then followed by the precipitation of metastable 
-carbide particles on linear features in the 1st stage, towards the later decomposition of retained austenite and precipitation of cementite in the 2nd and 3rd stages, respectively. After analyzing the experimental results, the solute carbon content in martensite tempered under various conditions was found to be in good agreement with that estimated from the lattice volume expansion, whereas the evaluation based on the tetragonality might lead to some underestimation of the solute carbon content in martensite tempered at high temperatures.
Zhang, Y.*; 宮本 吾郎*; 古原 忠*; 丸澤 賢人*; 工藤 航平*; 諸岡 聡
no journal, ,
In this study, following our previous work on low-temperature tempering kinetics, multi-aspect characterization (in-situ neutron diffraction, TEM and 3DAP) via combining various traditional and advanced experimental approaches was performed, with an aim to thoroughly elucidate the low-temperature tempering behaviors and microstructural evolutions in martensite. As the result, the lattice parameters of a- and c-axes of martensite became increased and decreased, respectively, during continuous heating, resulting in continuous weakening of its tetragonality especially in the temperature range of the 1st stage of 
/-carbide precipitation (340 K 
500 K). On the other hand, the lattice parameter of austenite stopped increasing at 450 K much earlier before the onset of the 2nd stage, indicated the occurrence of carbon depletion.
諸岡 聡; 井川 直樹; 佐々木 未来; 生田目 望; 樹神 克明
no journal, ,
Medium Mn steels have been actively investigated due to their excellent balance between material cost and mechanical properties. In particular, medium Mn steel with a nominal chemical composition of Fe-5.0Mn-0.1C (mass%) fabricated by intercritical annealing 923 K for 1.8 ks after cold-rolling, was the high-strength mechanical properties at low temperature. This strengthening mechanism evaluated by means of in-situ neutron diffraction under low temperature (High Resolution Powder Diffractometer (HRPD) at Japan Research Reactor-3(JRR-3)), electron back scatter diffraction (EBSD), low temperature differential scanning calorimetry (DSC) and low temperature magnetic susceptibility measurement. We found that as the sample temperature decreases, face- centered cubic (FCC) structure transferred face-centered tetragonal (FCT) structure. Namely, it suggests that austenite transformed martensite like Fe-Pd or Fe-Pt alloy. Therefore, the origin of the high-strength mechanical properties at low temperature was in the presence of FCT martensite.
