New Slag-less Steelmaking Process Introduced at Fukuyama Works

NKK has succeeded in developing a new "slag-less" steelmaking process in which the volume of slag formed during steelmaking is greatly reduced. This achievement has been made at the company's Fukuyama Works with the combined application of two advanced technologies. One involves low-silicon ironmaking technology to minimize silicon content in hot metal. The other is a new hot metal dephosphorization process carried out in a basic oxygen furnace (BOF) enabling the stable mass-reduction of the phosphorus content in hot metal.

Although slag, an inevitable by-product of steelmaking, has been effectively reused for road bedding materials and other civil engineering applications, decreasing the volume of slag formed is an ever important issue for steelmakers. Less slag also leads to such advantages as improved steel yields, decreased consumption of ferro-alloys in BOFs and the production of ultra-clean steel. NKK has long worked to realize slag-less steelmaking in an effort to enhance the quality and cost competitiveness of its steel products.

NKK's new slag-less steelmaking process starts with low-silicon blast furnace (BF) operations, which have been realized mainly by improving the quality of sintered ores and optimizing the temperature control of hot metal. After tapping, hot metal is dephosphorized by injecting flux into the BF ladle, a process used since 1985. In addition, in 1995 NKK introduced a new BOF-based hot metal dephosphorization process at Fukuyama's No. 3 steelmaking shop. By employing this new refining process, larger-volume, higher-speed hot metal dephosphorization than the BF ladle method has been realized, thereby achieving almost slag-less steelmaking.

The major factors contributing to the achievement of slag-less blowing at the No. 3 steelmaking shop are:

• Optimizing BF operations by use of a newly developed optical fiber thermometer called FIMPIT (fiber in metallic tube for pig iron temperature) and by enhancing the desiliconization process in the runner.
• Reducing the phosphorus content in the hot metal to a level below specification through the optimized addition of lime and technologies to enhance slag melting.

The Fukuyama Works is currently adding a new hot metal desiliconization station, which is scheduled to go into operation in April 1998. The new slag-less steelmaking process will be used at the Keihin Works as well.