The isothermal austenite-ferrite transformation in some deformed vanadium steels
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I.
INTRODUCTION
IT is well recognized that in controlled-rolled microalloyed steels the condition of the deformed austenite has an important influence on the size and shape of the polygonal ferrite which separates on cooling. ~ It is surprising, therefore, that there have been few studies of the isothermal austeniteferrite transformation in these steels. It is also well known that precipitation of carbonitrides accompanying the formation of ferrite affects the kinetics of the transformation. Honeycombe2'3 has studied the interactions of precipitates and growing ferrite grains in detail in a variety of iron alloys containing various combinations of Nb, V, Mo, W, C, and N and has, thereby, greatly enhanced our understanding of the basic phenomena involved. However, studies of precipitation during and after the separation of ferrite from microalloyed austenite deformed similarly to controlled rolled steels have usually been limited to the observation of precipitates in continuously cooled specimens. 4-7 The present study was undertaken with the objective of determining the effects of vanadium on the isothermal separation of ferrite from hot worked austenite in vanadium-nitrogen steels. The importance of studying effects of vanadium in this context can be appreciated when it is recognized that one major reason for adding vanadium to microalloyed steels is to induce precipitation during the transformation and in the resulting ferrite.4
II.
EXPERIMENTAL PROCEDURES
The compositions of the steels used in the experimental work are given in Table I. Steel 1 is a plain carbon steel containing 0.11C and 1.35Mn and Steels 2 and 6 are vanadium-nitrogen steels with the same base composition as Steel 1 and the same nitrogen content as each other. The concentrations of nitrogen reported are those measured in specimens after testing. M. J. CROOKS is with Homer Research Laboratories, Bethlehem Steel Corporation, Bethlehem, PA 18016. A.J. GARRATT-REED, Research Associate, J.B. VANDER SANDE, Professor of Materials Science and Engineering, and W. S. OWEN, Department Head and Professor of Materials Science and Engineering, are all with Massachusetts Institute of Technology, Cambridge, MA 02139. Manuscript submitted September 21, 1981. METALLURGICAL TRANSACTIONS A
Cylindrical specimens 0.9 cm diameter and 1.2 cm long were used for all metallographic studies. They were austenitized at 1250' ~ for one hour to ensure complete solution of carbonitrides and to produce the same austenite grain size (300/zm) in all specimens. Immediately after cooling to the isothermal test temperature, each specimen was compressed 30 pct at a constant strain rate of 0.096 s -t and then unloaded and held for a predetermined time before quenching into water. The progress of the transformation was followed by measuring the volume fraction of ferrite in each specimen on a Magiscan Image Analyser. Many of the measurements were checked by extensive point counting. The hot compression was carried out under "full-stick" conditions; that is, there was no lateral m
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