State of the art in the control of inclusions during steel ingot casting
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ALTHOUGH the percentage of steel produced in the world via ingot casting has decreased to 11.2 pct (Figure 1[1]) in 2003, some low-alloy steel grades and steel for special applications can only be produced by this process. These include high carbon chromium bearing steel,[2] thick plate, seamless tube, forgings, bars, and wire rods.[3] Thus, the production of crude steel ingots in 2003 was about 2.5 million metric tonnes in United States, 17.8 million metric tonnes in China, and 108.7 million metric tonnes in the world, which is still important. Top pouring is easy to use, but generates many defects both on the surface and internally, which is not suitable for high-quality steels. Bottom pouring is better, especially for intensive deoxidation, high superheat, low-speed casting, and casting in a nonoxidizing atmosphere. The typical process from steelmaking to steel refining to bottom-poured ingot casting is given in Figure 2.[4,5] During teeming, molten steel flows through the well and controlling side gate at the bottom of the ladle, enters the trumpet (also called the central runner) and passes through the spider, into the runners. The system is often flooded with inert gas (such as argon) to lessen oxidation. Molten steel then enters the ingot mold through an upward-facing ingate near the end of the runner. The rising steel level burns through suspended bags to release mold powder. Then, the powder spreads and melts to form a slag layer, floating on top of the molten steel. The slag layer protects the molten steel from atmospheric oxidation and absorbs inclusions from the moltent steel. After teeming, the ingot stands to solidify for the optimal time for easy removal (stripping) from the mold. The ever-increasing demands for high quality have made the steelmaker increasingly aware of the necessity for products to meet stringent ‘‘cleanliness’’ requirements. Nonmetallic inclusions are a significant problem in cast steels because they may lead to problems in castings that require expensive casting repairs or rejection. The mechanical properties of steel are controlled to a large degree by the LIFENG ZHANG, Professor, is with the Department of Materials Science and Engineering Norwegian University of Science and Technology (NTNU), 7491 Trondheim, Norway. Contact e-mail: lifeng.zhang@material. ntnu.no BRIAN G. THOMAS, Professor, is with the Department of Mechanical and Industrial Engineering, University of Illinois at Urbana– Champaign, Urbana, IL 61801. Manuscript submitted August 22, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS B
volume fraction, size, distribution, composition, and morphology of inclusions and precipitates, which act as stress raisers. For example, ductility is appreciably decreased with increasing amounts of either oxides or sulfides.[6] Fracture toughness decreases when inclusions are present, especially in higher-strength lower-ductility alloys. Similar pronounced property degradation caused by inclusions is observed in tests that reflect slow, rapid, or cyclic strain rates, such as creep,
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