Investigation of a surface defect and its elimination in automotive grade galvannealed steels Anindita Chakraborty ⁎, Avik Mondal, Soham Agnihotri, Rajesh Pais, Monojit Dutta Tata Steel Limited, Jamshedpur, India article info abstract Article history: Received 21 November 2015 Accepted 5 May 2016 Available online 6 May 2016 The galvannealed coating have gained importance as a potential solution for auto body panels owing to their superior performance like weldability, paintability and after painting corrosion resis- tance. Efficient production of galvannealed strip through hot dip galvanising and subsequent an- nealing root is a challenge as occurrence of surface defects results in material downgrading and rejection. The present work deals with one type of surface defect present at the bottom surface of the galvannealed strip with high defect severity. The defect appearance was like a shining spot by naked eye. It was characterised using Optical Microscope, Scanning Electron Microscope coupled with Energy Dispersive Spectroscopy to analyse shape, size, morphology and composition. The de- fect was further analysed by Confocal Laser Scanning Microscope (CLSM). The features of CLSM like high resolution, high depth of focus, 3D surface topography were useful in determining the relative depth of the defect at different regions. The results indicated that the defect occurrence was due to mechanical abrasion of coating with deposited dust of Fe–Zn intermetallic phases at the galvan- nealing tower top roll. The production line parameters were modified to maintain the temperature and cleanliness of the top roll and a reduction in defect severity was achieved. © 2016 Elsevier Ltd. All rights reserved. Keywords: Surface coatings Defects Abrasive wear Confocal Laser Scanning Microscopy Fe–Zn intermetallic 1. Introduction Galvannealed steels have emerged as an important material for auto body outer and inner panels as well as for fuel tanks. The galvannealed coating which is an alloy coating of iron and zinc provides excellent weldability, formability and corrosion resistance after painting. The automotive outer or skin panels are critical grades to produce industrially. Small compositional variation in coating affects the coating formability adversely and might cause powdering, flaking etc. during panel forming. Furthermore the surface defects hamper the aesthetics, average surface roughness and thereby deteriorate the paintability. Therefore accurate process control, galvanising bath management etc. are essential to produce high quality galvannealed coatings. However producing defect-free galvannealed (GA) materials from continuous processing line is difficult. Comprehensive liter- atures are available from researchers and manufacturers on different types of galvanised and galvannealed coating defects [1]. The defects can originate from two sources, firstly defects like sliver, rolled in scale, etc. from the cold rolled strip that further may aggravate post galvanising and galvannealing treatment. It was reported that the presence of defects prior to galvanising bath may result in uncoated spots and improper alloying after galvannealing [2–3]. Secondly the defects originating from the contin- uous galvanising line. The defects generated during galvanising and galvannealing were reported to originate from different sources such as improper air-knife arrangement [4–6], poor bath management [7–11] and improper alloying of GA [12–13]. Surface appearance was also affected due to defects originating from post galvanising treatment [14]. Engineering Failure Analysis 66 (2016) 455–467 ⁎ Corresponding author at: Research and Development, Tata Steel Ltd., Jamshedpur, India. E-mail addresses: anindita.c@tatasteel.com, nindita.ju2008@gmail.com (A. Chakraborty). http://dx.doi.org/10.1016/j.engfailanal.2016.05.010 1350-6307/© 2016 Elsevier Ltd. All rights reserved. Contents lists available at ScienceDirect Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal