Enhanced forming properties of galvannealed steel sheet by polymanganese phosphate coating T.K. Rout a, ⁎ , H.K. Pradhan b , T. Venugopalan b a Research and Development Division, Tata Steel Ltd, Jamshedpur, India b Product Application Group-Flat Product Marketing Division, Tata Steel Ltd, Jamshedpur, 831007 India Received 6 July 2006; accepted in revised form 31 July 2006 Available online 14 September 2006 Abstract Polymanganese phosphate coating is obtained on galvannealed (GA) steel surface by dipping process. Coating weight of 1.1 g/m 2 is found to give dense with uniform crystallites of size 1–2 μm. Energy Dispersive X-ray Spectroscopy and X-ray Diffraction results show that the coating composes of manganese phosphate hydrate and rockbridgeite. The coefficient of the friction of polymanganese coated galvannealed steel sheet is reduced significantly from 0.22 to 0.11 on the die. Manganese phosphate in the coating behaves as an anti-sticking base, thereby the contact zone is found to be smooth. Further, the treated sheet removes only 5.9 mg of Fe powder in the form of flakes as compared to the untreated sheet producing 12 mg Fe powder from its surface. © 2006 Published by Elsevier B.V. Keywords: Galvannealed; Polymanganese phosphate; Rockbridgeite; Lubricant; Powdering 1. Introduction Galvannealed (GA) steel sheets are used for auto components because of its excellent corrosion resistance, good paintability and weldability [1–3]. These steel sheets are produced by hot dip galvanizing (HDG), where molten zinc bath is maintained at 460±5 °C and these sheet are then annealed at about 550–580 °C to form Zn–Fe surface alloy coating. Hence, it is called galvannealed steel sheet. During annealing, 100% zinc on the substrate gets alloyed by diffusion of iron to zinc and vice versa, resulting in different intermetallic Fe–Zn phases in the coating such as ζ—eta (2– 4% Fe), δ—delta(9–11% Fe), Γ—gamma(16–18% Fe) and Γ 1 —gamma 1(19–22% Fe)[4]. A galvannealed (GA) coating weight of 50 g/m 2 /side (coating thickness: 7 μm) is con- sidered to be necessary for providing 10 years perforation corrosion resistance and 5 years cosmetic corrosion resis- tance [5]. However, increase in coating weight of GA causes press formability problem because of flaking of coating (powdering). The flakes thus formed stick to the die face and hence frequent cleaning of die affects its precision. The enrichment of gamma phase in the coating enhances flaking because of wide hardness difference between the substrate and gamma phase. The presence of eta phase (ζ) also in- creases coefficient of friction and hence powdering increases [6]. This problem can be solved by two ways either by controlling coating thickness of GA by optimizing delta phase or by providing an intermediate layer between the die and the sheet. Since, GA coating is brittle by nature and it is difficult to avoid powdering significantly by controlling pro- cess parameters. Therefore, the option for post-treatment is widely tried by many steel industries by using lubricating oils of low/high molecular weight oil chains, inorganic lubricants like borax (Na 2 B 2 O 4 ) [7,8], nickel [9], iron flashing and Fe–P electroplating etc [5]. Sometimes polyethylene sheets are used for reducing coefficient of friction of GA during forming operation to reduce powdering [10]. Electroplated ceramic coating or polymeric particle coating is also being used as post treatment layers on the metal for improving wear and corrosion resistant. This post treatment layer avoids direct contact Surface & Coatings Technology 201 (2006) 3496 – 3501 www.elsevier.com/locate/surfcoat ⁎ Corresponding author. Tel.: +91 0657 2147445; fax: +91 0657 2271510. E-mail address: tapankumarrout@tatasteel.com (T.K. Rout). 0257-8972/$ - see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.surfcoat.2006.07.260