Achievement of ultrafast cooling rate in a hot steel plate by air-atomized spray with different surfactant additives Satya V. Ravikumar a , Jay M. Jha a , Ishita Sarkar a , Soumya S. Mohapatra a , Surjya K. Pal b , Sudipto Chakraborty a,⇑ a Department of Chemical Engineering, IIT, Kharagpur 721 302, India b Department of Mechanical Engineering, IIT, Kharagpur 721 302, India article info Article history: Received 27 December 2012 Received in revised form 9 May 2013 Accepted 11 May 2013 Available online 23 May 2013 Keywords: Air-atomized spray Cooling rate Spredability Surfactant Ultrafast cooling Wettability abstract Ultrafast cooling of a 6 mm thick hot stainless steel plate (AISI 304) has been achieved by high flow con- ditions of air and water, which is needed at the runout table of a hot strip mill to develop advanced high strength steels. The present study primarily focuses on the effect of adding different types of surfactants at various concentration levels to air atomized water spray for enhancement of ultrafast cooling rate. The anionic, cationic and non-ionic surfactants used have been characterized by measuring their surface ten- sion, contact angle and viscosity. The surfactant enhanced heat transfer experiments have been con- ducted with a full cone atomizer at a fixed nozzle to surface distance using constant air and water flow rates optimized earlier. The initial surface temperature of the test plate has been maintained at 900 °C in each case. The experimental measurements have been applied to a commercial inverse heat conduction solver (INTEMP) to estimate local surface heat flux as well as surface temperature histories. The concentration of all the surfactants has been optimized based on the highest cooling rate. Depending on their spreadability, wetting characteristics and ability to foam formation, the cationic and non-ionic surfactants are found to produce better cooling effects than the anionic surfactant. These enhanced ultra- fast cooling rates can be useful to produce advanced high strength steels in the steel industries. Ó 2013 Elsevier Inc. All rights reserved. 1. Introduction Steel sheets in today’s steel fabrications have to fulfil increasing requirements. The technical requirements of modern steel con- structions as well as the need for good handling during transporta- tion and fabrication demand for higher strength steel. Regarding the overall efficiency in steel constructions the choice of steel with a more moderate strength and better hardenability can be an advantage. The high tensile strength and toughness combined with good weldability are directly related to the microstructure of the steel. The mechanical properties of hot rolled steel depend on the finished roll temperature (FRT), runout table (ROT) cooling (i.e. cooling rate) and coiling temperature. In a typical production line of ROT of a steel industry, the strips are reheated to a hot rolling temperature close to 900 °C, then rolled through roughing and fin- ishing mills, and finally cooled down to a coiling temperature of 600 °C [1]. Therefore, by cooling in this temperature range, a com- bination of phase microstructures (pearlite–martensite, ferrite– martensite or martensite alone) should be developed to produce advanced high strength steels. But, within a short cooling time, it is very difficult to produce multiphase microstructures with con- ventional laminar cooling technology on ROT because these micro- structures are determined by very high cooling rates. Hence, there is a need to develop a cooling technology, which can give faster cooling rate (140 °C/s for 6 mm thick strip) on ROT and such type of cooling is called Ultrafast cooling (UFC) technology [2–4]. In gen- eral, UFC is said to be achieved when the multiplication product of plate thickness (mm) and cooling rate (°C/s) is greater than 800. There are different types of cooling systems like laminar jet cool- ing, spray cooling and forced jet cooling etc. used for the cooling of hot steel plate. The major problem associated with these cooling technologies is stable film boiling on the test surface which hinders the heat extraction [5–7]. Therefore, one of the most efficient cool- ing systems which can eliminate the stable film formation over the surface is called air atomized spray cooling [8,9]. In air atomized spray method water is atomized into fine drop- lets using compressed air and sprayed on the surface to be cooled. This atomization, in evaporative quenching, should be in such a way that the individual drops on the heated surface completely evaporate, and do not merge in a water film as is the case in water spray cooling [3]. If the time for evaporation is very high then part of the droplet will evaporate and remaining part will be swept away by the superposed flow of air. The droplet evaporation time 0894-1777/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.expthermflusci.2013.05.007 ⇑ Corresponding author. Tel.: +91 3222 283942. E-mail address: sc@che.iitkgp.ernet.in (S. Chakraborty). Experimental Thermal and Fluid Science 50 (2013) 79–89 Contents lists available at SciVerse ScienceDirect Experimental Thermal and Fluid Science journal homepage: www.elsevier.com/locate/etfs