Effect of rare earth additions on the inclusions and properties of a Ca-AI deoxidized steel
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I.
INTRODUCTION
THEdemand for High Strength Low Alloy (HSLA) steels with higher levels of cleanliness, toughness, and formability has resulted in nonconventional deoxidation practices. Addition of rare earth lanthanides during steelmaking has already proved its effectiveness. ~However, a recent review: has shown that the constitutional and floatational aspects of rare earth (RE) inclusions and their effects on mechanical properties remain to be investigated in detail. The present work is part of a study of the genesis of inclusions and their effects on the mechanical behavior of a case-carburizing steel (20MnCr5).
II.
EXPERIMENTAL PROCEDURE
Misch metal (of Indian origin, average composition Ce-50.0 pet, La-22.5 pet, Pr-5.0 pct, Nd-12.5 pct, and Fe-10 pet) was added at the rates of 0.35, 0.85, and 1.2 kg per tonne of steel to three 2.3 tonne ingots (Numbers 4, 5, and 6) of a 50-tonne heat of commercial grade 20MnCr5. The procedure for REM addition followed was that recommended by Luyckx.~ When the ingot was teemed to the hot-top junction, teeming was halted for 20 to 30 seconds without removing the ladle. At the end of the holding period the whole quantity of misch metal nodules in cloth bags was added and teeming at full throttle was resumed immediately to fill the hot-top volume and to drive the misch metal to the bottom. The steelmaking involved complex deoxidation procedures) At the end of the oxidizing period, the slag was removed, then aluminum and calcium silicide were added simultaneously to the bath. Subsequently, a reducing slag was prepared from lime, ferrosilicon, coke fines, spar, and so forth. Final deoxidation was carried out by adding aluminum and calcium silicide to the ladle during tapping. The teeming stream was not shrouded. S. K. PAUL, formerly Research Scholar, Metallurgical Engineering Department, Indian Institute of Technology, Kharagpur, is now Research Engineer, R & D Centre, Steel Authority of India Ltd., Ranchi-2, India; A.K. CHAKRABARTY, formerly Senior Metallurgist, R & C Laboratory, Alloy Steel Plant, Durgapur, is now SUperintendent (Metallurgy), Mahindra Ugine Steel Company Ltd., Khopoli, Maharastra, India; and S. BASU, formerly Assistant Professor, Materials Science Centre, Indian Institute of Technology, Kharagpur, is now Manager, R & D, Sandvik Asia Ltd., Bombay - Poona Road, Poona - 411012, India. Manuscript submitted June 6, 1979. METALLURGICALTRANSACTIONS B
Samples from the top and bottom of the ingots were collected as billet croppings at an intermediate stage of hot rolling and were subsequently hot forged to 30 mm diameter rods. Nominal composition of the heat was C-0.22 pct, M n - l . 2 9 pct, Cr-1.24 pct, Si-0.27 pct, S-0.009 pct, P-0.029 pct, and A1-0.019 pet. A lineal traverse counting method was employed for stereological analysis 4 of the inclusion size distribution. The inclusions were analyzed by electron microprobe and the results were corrected by Colby's MAGIC-IV computer program. 5 Notched bar tensile tests were carded out at 900 ~ at a cross head speed of 5
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