The effect of some technological factors on the quality of structural steel

Abstract

At the Kulebaki Metallurgical Works, 30KhN3A, 35KhGSA structural steels, and other steels are used for the manufacture of railroad tires and rings for gears. The most common defects of these steels are longitudinal cracks on the surface and gas blowholes under the ingot skin and deep inside the ingot, fractures on the finished rolled rings and large nonmetallic inclusions which axe exposed during the machining of the tires or cutting of the gear teeth. The steel is made in basic open-hearth furnaces fired with oil fuel. Compressed air is used for atomizing the oil Freshly roasted lime, dry iron ore and chamotte chips are used instead of bauxite, with the object of preventing the steel from absorbing hydrogen. The steel is mn from the furnace by a double runner into two ladles. It is bottom poured into big-end-up, twelve-sided molds with hot tops; an ingot weighs 2.5 tons. At the time the tests were carried out, the fountains and the bottom runners were made of chamotte brick manufactured at the Works. A special investigation was carried out at the Works with the object of determining and eliminating the causes of hot cracks, gas blowholes, and large nonmetallic inclusions in the ingots for tires and structural steels. G a s b 1 o w h o I e s. Several causes of the formation of gas blowholes in ingots are known. If the steel is not properly deoxidized, blowholes appear in the ingot because during the solidification of steel, reactions between the dissotved oxygen and carbon take place and gaseous carbon monoxide is formed. To prevent these reactions, the steel should be deoxidized as thoroughly as possible. However, it was shown by the investigation (Fig 1) that it is not possible to eliminate the gas blowholes in the ingot merely by increasing the amount of aluminum added during the ddoxidation. If the deoxidized period of the heat in the furnace is extended, the loss in burning of the deoxidizers in particular aluminum, becomes greater and, hence, the hydrogen content in the steel is higher and this promotes the formation of gas blowholes in ingots. It follows that the longer the deoxidation period, the larger is the number of gas blowholes in the ingots (Fig. 2) With the object of reducing the time of deoxidation, tests were carried out on the complete deoxidation of steel in the ladle. Owing to the replaceanent of the preliminary deoxidation of steel with 12%-ferrosilicon by deoxidation in the ladle, the amount of the ingots affected with gas blowholes fell from 4.66% to 3.55qo (results from 23 heats). During the investigation of the specimens from rejected steel ingots, signs of skin folds in the region of gas blowholes were detected. This fact suggests that the formation of gas blowholes in ingots depends also on the skin folds. With the object of explaining this assumption, some pieces of the skin, which were picked out by means of a steel hook from the mold when it was being filled, were subjected to metallographic analysis. The skin consisted of solid steel full of gas blowholes and large particles of nonmetallic inclusions. As a result of the large number of gas blowholes and large nonmetallic inclusions, the density of the skin (4.5-6.3 g/cm t) is less than that of steel and, therefore, the skin floats on the surface of liquid steel. When it is folded, the skin gets inside the ingot and, thus, introduces the gas blowholes and nonmetallic inclusion: which it contains. Therefore, for the eliminatlon of t h e formation of gas blowholes in ingots, skin folds must be prevented. This can be achieved by increasing the rate of filling the mold. The data given in Table I indicate that an increase in the rate of pouring contributes greatly to a reduction in the percentage of ingots affected by gas blowholes. Since no skin forms on the surface of steel