A Novel Technology to Develop a Nickel-Enriched Layer on Slab Surface by Utilizing NiO-Containing Synthetic Powder
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IN these days, most of steelmakers in the world are trying to reduce CO2 emission during ironmaking and steelmaking processes. As one of the activities to reduce the CO2 emission, they have been trying to use ferrous scrap as much as possible during steelmaking process in converter. The use of hot metal containing high content of C is reduced significantly by the so-called ‘‘low hot metal process’’. This process also has an additional merit in such a way that, depending on the market price of the scrap and the hot metal, it can flexibly adjust input amounts of the scrap and the hot metal in order to optimize production cost. However, using the scrap inevitably introduces tramp elements such as Cu and Sn into steel products. As Cu has less affinity with O than Fe has, it is very difficult to remove the Cu by O blowing in the basic oxygen-blowing process.[1] Furthermore, the remained Cu is not oxidized during reheating process, due to a selective oxidation of Fe on slab surface by the same reason. This results in Cu to be enriched at the TAE-IN CHUNG, Senior Researcher, is with the Technical Research Laboratories, POSCO Ltd., Republic of Korea. JUNGWOOK CHO, Research Associate Professor, and YOUN-BAE KANG, Associate Professor, are with the Graduate Institute of Ferrous Technology, Pohang University of Science and Technology, Republic of Korea. Contact e-mail: [email protected] Manuscript submitted May 4, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS B
interface between oxidized scale and steel matrix. When the slab is reheated at temperature over 1373 K (1100 C), the enriched Cu forms at the steel/scale interface, and easily infiltrates into the austenite grain boundary, and induces the inter-granular cracking during hot rolling. This is known as the surface hot shortness by Cu enrichment.[2,3] In order to minimize such undesired aspects of scrap recycling, there have been two major research directions. One is a direct removal of Cu from scrap or liquid steel. This includes low melting point bath,[4] chlorination,[5–8] sulfide flux refining,[9,10] and vacuum distillation.[11–15] However, these have not been yet adapted in industries due to their insufficient Cu removal efficiency, high operation cost, and longer process time. The other direction is to suppress the Cu enrichment during the reheating process. While making Cu to be left in the steel, it aims to suppress harmfulness of the Cu by treating the steel in various ways: Ni/Si alloying,[16–19] reheating Cu-containing slab at higher temperature than fayalite melting point,[3,16] and mechanical shot peening.[20] Among these ideas, the Ni alloying has been widely used in several industries. Alloying Ni increases the solubility of Cu in c matrix. This yields less precipitation of Cu-enriched phase during the reheating. Although not actively used in the industries, increasing the reheating temperature over 1473 K (1200 C) results in the formation of molten fayalite phase which absorbs the Cu-enriched phase at this temperature. This reduces
the harmfulness of the Cu
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