The kinetics of carbide precipitation in silicon-aluminum steels
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I.
INTRODUCTION
THE effects of second-phase particles on the magnetic properties of nonoriented silicon electrical steels is of major concern in the processing of these materials. Inclusions and precipitates can degrade the AC magnetic properties of these steels by directly inhibiting the motion of domain walls during magnetization. 1-6 Because iron carbides contribute significantly to these effects, the carbon contents of these steels are minimized through special melting practices and by decarburization of the strip during annealing. Rapid cooling after annealing can trap much of the remaining carbon supersaturated in its ferrite matrix. Subsequent precipitation of carbide at room temperature over long periods of time, or at elevated temperatures for shorter periods of time, can reduce magnetic quality. This phenomenon is known as magnetic aging. 7-1~ The major effect of magnetic aging in nonoriented silicon electrical steels is the increase of core loss. Lt Numerous investigations relating to the precipitation of carbon from pure F e , 12-24 F e - M n , 24'2~ Fe-P, 26'27 Fe-A1, 21'2s,29 and Fe-Si 3~ alloys are documented. For a general review of these topics, see References 10 and 29. Much of the work used change in coercive force to measure kinetics of carbide precipitation. The present study determined the effects of carbide precipitation on core loss. Inspection of much of the literature reveals that few data are available for nonoriented silicon electrical steels with commercially relevant carbon supersaturations, which can be about 100 ppm by weight. Recent work H has shown that magnetic aging, as manifested in core loss changes, is a very sensitive function of carbon supersaturation near that concentration in nonoriented silicon electrical steels. The study had two aims: the first was to determine changes in core loss of a typical nonoriented silicon steel as GARY M. MICHAL is Assistant Professor, Department of Metallurgy and Materials Science, Case Western Reserve University, Cleveland, OH 44106. JOHN A. SLANE is Supervisor of Advanced Microstructural Analysis, LTV Steel Corporation Research Center, Independence, OH 44131. This paper is based on a presentation made at the symposium "Physical Metallurgy of Electrical Steels" held at the 1985 annual AIME meeting in New York on February 24-28, 1985, under the auspices of the TMS Ferrous Metallurgy Committee.
METALLURGICAL TRANSACTIONS A
a function of carbon supersaturation and aging temperature and time. The second was to define the kinetics, morphology, and distribution of carbide precipitates responsible for the core-loss changes. This was accomplished by using the change in core loss in conjunction with microstructural examinations by optical and electron microscopy.
II.
EXPERIMENTAL PROCEDURES
The steel used was commercially produced to a coldrolled gauge of 0.635 mm. The composition of the as-received material is listed in Table I. Samples approximately 160 mm by 30 mm were sheared from the cold-rolled sheet. The samples were annealed for various
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