Austenite Grain Growth Behaviors of La-Microalloyed H13 Steel and Its Effect on Mechanical Properties
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THE actual austenite grain size in steels has a great influence on the final microstructure and properties. Coarse austenite grains often produce a coarse final microstructure. The increase of austenite grain size will change the substructure of the martensite structure, decrease the dislocation density and change the ratio of grain boundaries at different angles. This will lead to the decrease of yield strength, plastic toughness and thermal fatigue properties of the steels.[1–4] Therefore, to improve comprehensive mechanical properties, it is necessary to control the austenite grain size. At present, the austenite grain size is mainly controlled by heat treatment and microalloying. The austenite grain size of 300 M low-alloy ultra-high-strength steel was controlled by isothermal heat treatment and reasonable change of quenching temperature and holding time.[5] Ultra-fine grains were obtained in the 65 Mn steel by the WENJIAN ZHOU and JIAN ZHU are with the Institute for Advanced Materials and Technology, University of Science and Technology Beijing, Beijing 100083, China. ZHIHAO ZHANG is with the Institute for Advanced Materials and Technology, University of Science and Technology Beijing and also with the Key Laboratory for Advanced Materials Processing (MOE), University of Science and Technology Beijing, Beijing 100083, China. Contact e-mail: [email protected] Manuscript submitted January 2, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS A
temperature rise and reheating (TRF) technology, thus obtaining ultra-high strength.[6] There are many studies on controlling austenitic grain growth by adding V,[7] Nb,[8] Ti,[9] Al[10] and Mg.[11] These elements form second-phase particles with carbon or nitrogen, which adhere to grain boundaries and exert a pinning effect on grain boundary migration, thus restraining austenitic grain growth. Another kind of microalloying element is rare earths. Yang et al.[12] reported that the grain size of primary austenite was reduced by adding different contents of rare earth oxide in flux-cored welding wire. Zhou et al.[13] reported that a certain amount of rare earth in 9Cr2Mo steel was beneficial to restrain austenite growth and refine austenite grains. Yan et al.[14] reported that rare earth elements retarded austenitic transformation of 25 Mn steel and refined austenitic grains. H13 steel is widely used in hot forging, hot extrusion, die casting and other hot-working die materials.[15] Because of the impact load in the service environment and the complex stress of cyclic alternating stress, H13 steel is prone to thermal fatigue crack and early failure, which greatly shortens its service life.[16] Therefore, it is important to control the size of austenite grains in H13 steel. Zhang et al.[17] reported that the refinement of multistage deformed austenite grains was optimized by the uniform experimental design. Elias and Viana[18] reported that austenite grains and carbides were refined by substituting some amount of V with Nb in H13 steel, and the strength and toughness were improved. Li
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