The isothermal decomposition of austenite in hot-rolled microalloyed steels
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I.
INTRODUCTION
O N E of the best studied solid state transformations is the decomposition of austenite to ferrite. Understanding and controlling this reaction are important because the ferrite grain size and other aspects of the product morphology and thus the mechanical properties are determined by its kinetics--the density of ferrite nucleation sites, the rate at which they are activated, and the growth rate of ferrite grains. Two recent reviews ~'2 and much recent work 3-~~have been concerned with the rates of the transformation and the microstructure of the products in various alloys. Most of these studies, however, have been concerned with ideal, high purity alloys and very simple heat treatments; very little work has been done with hot-rolled, low-alloy, ferritepearlite steels. The austenite to proeutectoid ferrite reaction is a diffusional nucleation and growth process. Nucleation occurs mainly at austenite grain boundaries and the kinetics are well described by classical nucleation theory if it is assumed that the ferrite nucleus is semi-coherent with the austenite and that only a small fraction of the total number of sites in the grain boundaries can serve as nucleation sites. 9'~~ Growth of ferrite grains continues until they impinge or the carbon diffusing from the ferrite builds up enough to stabilize the austenite. To refine the ferrite grain size and thus obtain improved strength and toughness in the product, a high density of nucleation sites is desirable. Commercial processes for obtaining grain refinement such as controlled rolling and normalizing increase the density of nucleation sites by reducing the austenite grain size (increasing the grain boundary area per unit volume). Alloying, too, can effectively increase the density of nucleation sites by reducing the growth rate so that further transformation of austenite occurs at additional, less potent sites. In low-alloy, hot-rolled steels, the parent austenite grain size is not as fine as that in normalized or controlled-rolled steels. Therefore, in those steels, it is necessary to explore alternative approaches such as inducing
M. J. CROOKS, Engineer, and J. M. CHILTON, Manager, are with the Materials Science Division, Research Department, Bethlehem Steel Corporation, Bethlehem, PA 18016. Manuscript submitted August 16, 1983.
METALLURGICALTRANSACTIONS A
intragranular nucleation of ferrite or otherwise increasing the density of nucleation sites in order to obtain ferrite grain refinement. Since previous work has shown that microalloying additions can affect the kinetics of austenite decomposition and the product microstructure, 4'"-t3 this work was undertaken to examine the effects of small amounts of vanadium, titanium, and niobium on the austenite-to-ferrite transformation in hot-rolled steels.
II.
EXPERIMENTAL PROCEDURE
The steels used in the work were air induction melted, 227 kg laboratory heats. The compositions, determined after hot rolling the ingots to 12.7 mm plate, are given in Table I. The 12.7 mm plate was cut into 38 x 64 mm
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