Abstract
Nb addition is widely used for strengthening of low-alloy steel. In case of large ingots, however, where the cooling rate during solidification is low, eutectic Nb-carbide can easily coarsen. Nb addition can substantially reduce the ductility and toughness of the material by the influence of such large eutectic carbide. As the effect of Nb on the strengthening is advantageous in high Cr steel, 12% Cr rotor forgings generally contain about 0.05wt% Nb in order to improve the high temperature strength. In case of 12% Cr steel, Nb forms Nb carbo-nitride (Nb(C, N)), because 12% Cr steel usually contains N of several hundred ppm. This research shows a guideline of chemical composition to avoid the formation of Nb(C, N) for a large 12% Cr steel ingot manufactured by conventional method. Formation condition of eutectic Nb(C, N) in 12% Cr steel and low-alloy steel were compared. It is confirmed that Nb(C, N) is formed less easily in 12% Cr steel than in low-alloy steel. The experimental results were arranged such that the amount of eutectic Nb(C, N) was expressed in an experimental formula as a function of contents of Nb, C and N, and the cooling rate. The experimental formula was verified the investigation of a large 12% Cr steel ingot weighing 68 t, and its validity was confirmed.
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