Morphology of nonmetallic inclusions using silicon, aluminum, and calcium-silicon alloy in steel melt
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I.
INTRODUCTION
THE deleterious, influence of inclusions
on most of the important properties of steel, such as toughness, ductility, fatigue resistance, hot and cold workability, corrosion resistahce, etc., is now fairly well understood. It is not only the quantity of inclusions, but their size, shape, morphology, and distribution are equally important in determining the quality of steel. These are generally determined by the type of refining and deoxidation practice used. In the present work, the effect of deoxidation of a stirred steel bath with ferrosilicon, aluminum, and calcium-silicon on the formation, growth modification, and separation of inclusions has been investigated.
II.
EXPERIMENTAL WORK
Mild steel scrap was meltedin a 15 kg induction furnace to obtain a low-carbon and low-silicon bath. Any slag present on the surface of the melt was removed, and the temperature and oxygen activity of the bath were measured, using a Pt-Pt 10 pct Rh thermocouple protected by a silica sheath and an oxygen sensor, respectively. The oxygen sensor used consisted of a ZrO2CaO tablet as the solid electrolyte and Cr-Cr20 3 as the reference electrode, t1'2'3] On obtaining a steady value of the sensor emf, the necessary deoxidizers were added to the bath while the sensor was still operating. Deoxidizers were added either onto the surface of the melt or plunged into it using a mild steel capsule. Samples were drawn from the bath at intervals, using 8 nun diameter pyrex glass/fused silica suction tube. These samples were subjected to chemical analyses as well as inclusion studies. The polished unetched samples were observed under a Carl-Zeiss Neophot-2 microscope at 500 x and the following parameters were determined: (1) Inclusion index (vol pct) by the Jernkontoret point count method, t4] ThirtY fields were examined and the result A.L. KUNDU is Principal Research Engineer, Research and Development Centre for Iron and Steel, SAIL, Sub-Centre, Rourkela Steel Plant, Rourkela-769 011, India. K. M. GUPT and P. KRISHNA RAO are Professors, Department of Metallurgical Engineering, Indian Institute of Technology, Bombay-400 076, India. Manuscript submitted March 31, 1986. METALLURGICAL TRANSACTIONS B
expressed as d30• (2) Inclusion size distribution. (3) Identification of inclusions using plane reflected and polarized light.is-8] For SEM studies 3 mm thick unmounted polished specimens were etched with 5 pct bromine in methanol for 8 to 10 minutes. Secondary electron image, backscattered image, and X-ray images were obtained for typical inclusions at different magnifications, using a Cambridge Stereoscan $4-10 scanning electron microscope.
III.
RESULTS AND DISCUSSION
Typical change in oxygen activity in tile melt on plunging and surface addition of ferrosilicon and aluminum, as measured by the oxygen sensor, are shown in Figures 1 through 3. On plunging the deoxidizer, there was a sharp drop in the oxygen activity. After reaching a minimum within 5 to 10 seconds of the addition, the oxygen activity increased slightly but sharply
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