Highly Productive, Reliable and Unique Casting at Badische Stahlwerke

摘要

The two 100% scrap based electric arc furnaces at Badische Stahlwerke currently have a furnace output of 8,559 tgb/d with an average of 116.7 tons tap weight per furnace and an operating time of 305.4 days. The tap-to-tap time of 39.28 min at a power-off time of 9.8 min and an average delay time of 1.91 min/heat, along with the 11 strands of two continuous casting machines present a challenge. An annual output of 2,614,095 tons means 231,500 tons of casted steel per strand. In recent years, attempts have been made with section size extensions, with experiments of up to 8(1/4) inch being successfully carried out on a casting radius of 20.3 feet. The steel plant offers two production lines, consisting of a furnace, a ladle furnace and a continuous casting machine. Continuous improvements have also resulted in an asymmetric casting performance. CCM 2, offers 6 strands with a maximum casting capacity of 231.5 t/h. The CCM 1 on line 1 has 5 strands and reaches a max. casting speed of 170 t/h. For organizational reasons, the billets 6(1/4) inch are produced exclusively on CCM 1. To compensate this casting asymmetry, some heats of line 1 are casted on the CCM 2. This results in a forward planning of the respective heats, especially with different steel grades on both furnaces. Up to two heats are in a buffer position on CCM 2 therefore a precise control of the tap temperature and the cooling rate of the heats is crucial. The delay time on the furnaces waiting for the CCMs was reduced from 134.1 h in 2015 to 80.2 h in 2016. In order to minimize delays on all units, a preventive weekly maintenance takes place every Thursday. Several re-stranding simulations for the function check of all components end the preventive maintenance for the casters. Typical wear components were significantly optimized with regard to installation, removal and work force. The cooling chambers of cooling zone 2 are physically separated from one another to enable a breakthrough cleaning while other strands are in production. Up to 90 heats can be operated on a 5-strand tundish for CCM 1. This equates to 2,100 tons on the tundish nozzle, on the 6-strand tundish for CCM 2 this value is reached at 110 heats but also values of 115 heats are possible. Both continuous casting machines have to change the tundish twice until the next scheduled maintenance day. Tundish changes are made via the casting crane and require between 30 and 45 min. Both CCMs also allow the tundish to tilt in the system and a tundish is deslagged every 8 hours. The ladles are not brought into the casting position with a ladle turret, but this is done via a ladle car. This is a very quick change of a ladle, in less than 90 seconds from the closing gates of the ladles until opening. Furthermore this principle offers more safety to the ladle change. No ladle tundish shroud is used and the flying nozzle changer can be used up to 6 sliding blocks per strand. Due to the open stream for mold lubrication rape seed oil is used. The oil consumption is 0.15 qt/tgb. A specificity is that there are no guides, rolls or plates, below the mold. Despite the lack of foot rolls the breakthrough rate has been reduced from 2.3% in 2015 to 0.1% in 2016. Regarding rhomboidity, diagonal differences of an average of 0.15" (4 mm) are currently achieved with minimal bulging. Malfunctions in the rolling mill caused by billet defects are at 249 minutes for the year 2016 within total of 4 events. For primary cooling, BSW relies on spray mold technology. With regard to the life span of the mold tube, 800 - 1,000 heats per mold tube are possible, which corresponds to up to 18,740 tons of casted steel. Mold tubes close to the end of life time are used exclusively on the CCM 2. This caster has retractable oscillation tables and the molds can be exchanged during casting. In principle, mold tubes are not repaired and returned to the manufacturer after use. A team of one-first and three-second casters per shift are needed for a CC