Improving the Sink Roll life in Galvalume

Abstract

Galvalume is a Continuous Galvanizing Line. JSW Ispat Steel Ltd. Kalmeshwar has to frequently replace the sink roll assembly used in Zn-Al tank of the Galvalume. The mean time between replacements is very less as compared to expected mean time between failures (expected MTBF). This is due to deposition of zinc dross on the surface of roller. This result in uneven or improper Zn-Al coating on sheet surface. To avoid this sink roll has to replaced. The frequent replacement of roller assembly results in Stoppage of production, material loss, start-up loss and increased cost of production. This paper proposes an alternative material to extend the life of sink roll. Galvalume is a continuous hot dipping process in which steel sheet is dipped in Zn-Al pot. Zinc pot contains Aluminum (55%), Zinc (43.5%) and Silicon (1.5%). The hot dipping is carried out at 6050 c. The chemical reaction between molten ZnAl alloy with iron at 6050c results in the formation of complex compound “dross”. This dross deposits on sink roll surface. It causes: 1. Uneven deposition of dross on sink roll. 2. Impressions on sheet and variations in coating on sheet. 3. Sheet showing dross Build-up. Due to this quality of sheet is lowered and sheet is downgraded. This downgrading brings financial loss to the company. To avoid this, frequency of roller replacement is increased. However this has following disadvantages: 1. Increased loss of production due to frequent stoppages (During removal and refitting of assembly). 2. Zinc loss due to frequent change of roller assembly. 3. Additional cost of re-machining of sink roll. 4. Increase idle time in the process. The project is aimed at increasing the life of sink roll or extending the time period between the sink roll replacements. The increased time between replacements will help in overcoming the above disadvantages. Sink Roll TheSink roll, is a roll used in molten zinc tank in continuous galvanizing line. It is generally made of SS316L Ferrite free, stainless steel that can withstand extreme temperatures & high corrosion for extended periods of time. It made by centrifugal casting.[11] There body uses different shapes like straight body, crowned and tapered depending upon applications. A variety of groove patterns and pitches are made on the surface as per as customers specifications. Special grooves are provided for carbide coatings. [8] Sink roll or Pot roll immersed in the molten zinc bath is affected by the dross is also attacked by the molten zinc leading to pitting and build ups. All these factors affect the coil surface. Such rolls have very short life. Sink rolls are works in a very corrosive environment, i.e. molten zinc. They typically see temperatures in excess of 6000C for weeks at a time. Materials used are made to withstand this criteria, so they are also difficult to machine.[16] The roll is made of SS316L material. It is a ferrite free stainless steel grade having surface hardness RC 10. The body is a centrifugal cast stainless steel and the end ball/journal is from a static casting. Most of these rolls have no coatings on them. An uncoated roll may last up to 2 weeks, but with carbide coatings and special bushings they laste up to 5 weeks. However for Galvalume the life is reduced drastically due to dross formation (pick-up) and high tempurature ( i.e above 6000 c ). An apparatus called ‘Scrapper’ is installed in a molten metal plating line includes a blade pressed against a roll dipped in a