Electric Arc Furnace
The centerpiece of the steel plant of Breitenfeld Edelstahl AG is a modern 65t electric arc furnace with an exchangeable container. It is operated as a UHP (Ultra High Power) furnace with help of additional burners and foamed slag technology in order to melt scrap as quickly as possible. Only the essential metallurgy such as reduction of the carbon content or setting of the lowest possible phosphorous concentration is carried out in this furnace. Other types of work, such as the removal of dissolved sulfur, are carried out in the downstream secondary metallurgy section (ladle furnace, vacuum system). Tapping of the steel into the ladle and the complete removal of the oxygen-rich furnace slag occurs after the desired analysis and temperature have been achieved. After de-oxidation with aluminum and silicon, the preliminary alloy is produced and the molten steel is topped with a special protective slag necessary for the further process steps.
The two main functions of the ladle furnace are the heating of the steel melt before the vacuum treatment and the homogenization of the melt with regard to chemical composition after the alloying step. Desulfurization as well as improvement in the oxide and sulfide purity levels is achieved through the purging of the melt with an inert gas (argon) throughout the process. Customer specific analyses are then carried out and final temperature adjustment is made to that required for optimum casting.
Many effects can be achieved through the vacuum treatment of steel. Unwanted gases like hydrogen and nitrogen that are dissolved in the bath can be removed and steel purity can be improved. Furthermore, a deep desulfurization due to the steel / slag reaction is achievable through an intensive purging with argon during the VD treatment (Vacuum Degassing). The treatment takes place at a negative pressure of < 1 mbar.
The VOD (Vacuum Oxygen Decarburisation) method is used for the production of high-chromium containing steels with the lowest carbon content. After placing the vessel into the vacuum facility, a low pressure of < 250 mbar is applied while oxygen is blown over the bath through a water-cooled lance. Special vents installed in the bottom of the vessel allow for argon purging, which leads to good bath agitation. During the purge phase, the pressure is lowered to under 30 mbar, which causes a further reduction of the partial pressure of CO. The oxygen treatment is followed by the VCD (Vacuum Carbon De-oxidation) treatment, which is carried out at a deep vacuum (< 2 mbar) to minimize the concentration of oxygen and carbon.
The oxidation phase is followed by the reduction of slag, or de-oxidation, of the steel bath with aluminum and silicon. The last steps after the application of a reducing slag are the desulphurization and the elimination of non-metallic inclusions under vacuum.
Ingot Casting Process
Ingot Casting refers to the molding of steel, following treatment in the secondary metallurgy, into permanent ingot moulds with a square, rectangular, round or polygonal cross section. This can be a top-cast method or, as practiced at Breitenfeld, a bottom-pour casting method. In this case, 1 to 8 ingot moulds are arranged into a set. After opening the sliding gate closure, the steel flows from the casting ladle into the central funnel, which is connected via fireproof channels with the ingot moulds. The ingot moulds are then simultaneously filled from the bottom to the top with liquid steel according to the principle of “communicating vessels”. This steel, protected from re-oxidation (that is oxidation through the atmospheric air) by casting powder, moves up slowly and starts to solidify. When it is set, the moulds are removed from the ingots and the ingots proceed on for further treatment.