Abstract: Inclusions in the heavy rail smelting process were analyzed statistically by inclusion analyzer. Methods by a ingredient contrast between typical inclusions and large inclusions were adopted to define the source of the large inclusions and take targeted technical measures. Combination with sample of the inclusion overproof and nonconformity in ultrasonic flaw detection analysis, the common type of large inclusions were summed up and the source was discussed. The results showed that greater than 10 μm inclusions in refining process can be removed and typical inclusions contain 5 %-10 % MgO. After refining process, greater than 10 μm inclusions were the secondary oxidation products or mould powder which is characterized by very low content of MgO. The large inclusions which cause the inclusion overproof and nonconformity in ultrasonic flaw detection were the main source of furnace slag, mould powder and nozzle blocking products, the inclusions generally contain the bulk of spinel. To control of large inclusions in heavy rail steel, in addition to the conventional control methods such as to prevent secondary oxidation, prevent slag entrapment and improve nozzle materials, the spinel should be controlled by using of low Al content of alloy and accessories as far as possible, improving refractory quality of tundish zone and suitable for continuous casting furnace number to prevent spinel product gathered in the nozzle.

Key words: heavy rail steel, large inclusion, flaw detection, MgO, spinel