EFFECT OF CALCIUM ON THE QUALITY OF TUBE STEEL

Abstract

The company Tagmet makes steel in 290-ton open-hearth furnaces by the pig-and-scrap process, tapping the metal into two ladles. The steel is then treated on a two-position “ladle-furnace” unit equipped with a pinch-roller system to introduce cored wire and aluminum wire rod. After the steel has been brought to the desired temperature and chemical composition on the unit and treated with calcium-bearing wire in the final stage, it is bottom-poured into circular tube ingots weighing 1.2‐2.0 tons. In connection with the recent exploitation of gas fields that contain hydrogen sulfide, the pipes used in the equipment for the wells and the gas lines are having to meet higher standards on their content of harmful impurities and nonmetallic inclusions ‐ which affect the physicochemical properties and corrosion resistance of the metal. It is known that micro-additions of different materials to steel can decrease its sulfur content by raising the level of deoxidation of the steel and the refining slag and providing for fuller use of the slag’s sulfide capacity [1]. It has been es tablished that the addition of calcium makes it possible to obtain steel with sulfur content of 0.005% or less [2, 3], and it helps more favorably distribute phosphide eutectics in the steel [4]. The addition of calcium to steel in the needed amount ‐ the amount of calcium being determined by the steel’s content of oxygen, sulfur, and aluminum [3] ‐ helps transform nonmetallic inclusions and convert hard aluminates into globular low-melting calcium aluminates. Proceeding on the basis of the above, Tagmet has developed and introduced a technology for micro-alloying corrosion-resistant tube steels 35G2, 20SA, and other steels used for well casing and drill pipe. The steels are micro-alloyed with a specified amount of calcium-bearing cored wire introduced by a pinch-roller system at a rate of 3‐3.5 m/sec. The wire and the steel are then mixed for 3‐5 min to average out the chemical composition of the latter and ensure that the nonmetallic inclusions rise to the surface of the melt. To study the effect of calcium on the transformation of nonmetallic inclusions in molten steel, we took samples of three heats of steel 20SA having nearly the same chemical composition in terms of their contents of the main elements, aluminum, and harmful impurities (sulfur and phosphorus). Standard procedures were employed on a ladle-furnace unit to obtain the samples with immersion-type samples before and after treatment of the steel (before it was cast). The normal practice was followed in finishing the steel with respect to temperature and chemical composition. Only the steel of two of the heats was treated with the calcium-bearing wire (Table 1). We used the samples to prepare metallographic sections for studying the composition of the nonmetallic inclusions. Thirty fields in all were examined. It was determined that, before the treatment, the nonmetallic inclusions in all three of the heats consisted of accumulations of aluminum spinel and sulfide eutectics present in the form of films along the grain boundaries. After its treatment, the steel of heat No. 1 contained almost no coarse accumulations of oxides (the oxides were probably assimilated by the slag), and the aluminate-type oxides that were precipitated were present in smaller clusters. The sulfides were precipitated in the form of eutectic colonies.