SOME OBSERVATIONS ON GRAIN GROWTH CHARACTERISTICS OF AUSTENITE

Abstract

As the first stage of a study on grain size controll in steel-making plant, the authors observed experimentally some problems as stated below, mainly from the view point of grain-growth characteristics of austenite:
(i) Change of grain size and grain-growth characteristics during melting and teeming practice,
(ii) Relation of grain size and grain-growth characteristics between a ladle sample and a finished product of the same heat,
(iii) Dependence of grain-growth characteristics upon the degree of duplication of grains.
Results obtained were as follows:
(1) In the case of melting of medium-carbon Cr-Mo steel in a basic electric arc furnace, as the reducing proceeds, austenite grains became finer and more uniform, and the coarsening temperature rose. And by the addition of Al, anstenite grains became finer and the coarsening temperature rose higher, but as the time passed on, the coarsening temperature fell.
(2) In the case of 0•3% C (Al-killed) steel and Ni-Cr-Mo case-hardening steel, a finished product had nearly the same grain-growth characteristics as a ladle sample, and at 925°C both had the equal grain-size. But in the duplex-grained steel, a finished product had coarser grain and lower coarsening temperature than a ladle sample.
(3) In the case of Ni-Cr-Mo case-hardening steel, it was quantitatively derived by means of Dr. T. Akutagawa's method that the degree of duplication becomes maximum in the course of abrupt coarsening of austenite.
(4) It was recognized that austenite grain size had a close connection with the acidsoluble Al in steel and that when the percentage of the acid-soluble Al rose beyond 0•020, the grain size in every kinds of steel stated above became finer (No.6-No.7 of McQuaid-Ehn Grain Size Number).
(5) Some considerations on the relation of the cause of duplex-grain formation to the primary segregation, and the grain-coarsening curve were given.