| The present work is concerned with spheroidization of sulphidic inclusions in the heavy forged carbon steels by Ca treatment. Three kinds of, reduction processes(ⓐ conventional Al-deoxidation ; ⓑ Si-deoxidation with subsequent Ca-and Zr-addition;ⓒ : Al-deoxidation with subsequent Ca-addition) were conducted to correlate the measured oxygen contents during the reduction with the morphology of non-metallic inclusions. Elongated sulphidic inclusions were observed in the forgings made from the process ⓐ. Coarse or spheroidized non-metallic inclusions were observed in the forgings from the process ⓑ, depending upon the amount of dissolved oxygen. However, finely spheroidized inclusions were uniformly distributed in the forgings from the process ⓒ, whose dissolved oxygen was controlled to below 5-l0ppm. The optimum plant conditions for the formation of the finely spheroidized inclusions were obtained as follows: 1) Dissolved oxygen and total oxygen contents in the killed steel should be below 5-l0ppm and 60-70ppm, respectively, before Ca is treated. 2) Al content in steels should be above 0.007-0.019% prior to Ca adding. 3) The effective ratio of % Ca_(eff) to % S is 0.53-0.80, which is provided by addition of 0.020-0.025% Ca. |
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