The Effects of Grain Size and Precipitate on Strength of Nb-Treated Steels

概要

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Relationship between the strength and the change in grain size and precipitation behavior of NbC due to heat treatments was examined with 0.2% carbon steels containing 0.02 to 0.09% niobium. The results are summarized as follows. (1) Though NbC particles precipitated in niobium-treated steels tend to coalesce with the rise of austenitizing temperature, they remain in steel and prevent the austenite grain from coarsening because of its small solubility and low dissolving rate in austenite. (2) The highest strength is obtained when the cooling rate from austenitizing temperature to room temperature is 25℃/min. When niobium-treated steel is continuously cooled after a full solution treatment for a long time at sufficiently high temperatures such as 1250℃, NbC seems to precipitate not only at temperatures lower than A_3 point but also at temperatures of austenite range, and this precipitation from austenite will cause less strengthening than that from ferrite. (3) Strengthening effect by niobium-treatment may be explained on the assumption that both effects of the grain refinement and the precipitation of fine particles of NbC are additive to each other. An increase of strength in niobium-treated steels austenitized at temperatures lower than about 1050℃ is due mainly to refining of ferrite grain and that at higher temperatures is due mainly to fine precipitates of NbC.
東北大学の論文