Interface topologies and boiling phenomena are observed within the water jet impingement zone during quenching of a high temperature (300 °C–900 °C) steel plate by direct optical observations. Stable film boiling may occur, but surface asperities may easily penetrate the vapor film and interact with the flowing liquid. By cooling down more rapidly than the remaining solid surface, such asperities act as a kind of micro-fin. Rather surprisingly, non-coalescing bubbles on top of the thin vapor film have been observed, probably formed on top of penetrating surface asperities. After establishing contact between water and solid, so after rewetting, an intense bubble activity is normally seen of bubbles that are footed on a dry plate area with a contact line reflecting the light-emitting diode lights and condensing at later times. These are vapor bubbles. A high surface roughness promotes rewetting that may be initiated at several places simultaneously. For an initial plate temperature of 300 °C and a total height roughness of 5 μm, rewetting took place without the occurrence of a vapor film; surfaces at or above 450 °C exhibited vapor film formation, even for a high jet subcooling of 80 °C. Temperature and time of rewetting are strongly affected by the initial surface temperature and jet subcooling and less by the jet velocity. New correlations are provided to predict these dependencies.

Impinging water jets promote high heat flux extraction rate. Steel industry widely employs the process for accurate temperature control to improve the microstructure and to ensure adequate mechanical properties. The range of surface temperatures, heat fluxes and cooling rates are very large, which makes it important to obtain an accurate value of the heat transfer coefficient. This paper presents an experimental and numerical study of the heat transfer behavior of a high temperature (450°C - 900°C) steel plate cooled by a water jet at 20°C to 70°C. High-speed imaging (up to 20,000 fps) within water jet impingement zone allowed the characterization of the boiling regimes in the early stages of cooling. The effects of initial temperature, water jet temperature and velocity and on the heat transfer coefficient were analyzed by inverse heat conduction method that predicts the heat flux and temperature on the top surface from temperatures measured with thermocouples inserted in test plate. Heat transfer is strongly affected by the initial temperature of the hot steel, water jet temperature and, less intensely, by jet velocity. High cooling rates start when liquid water is in direct contact with surface temperatures above 700°C.The results will contribute to the enhancement of the temperature cooling control on the runout table and cooling model employed at Usiminas Hot Strip Mill.

The microstructure and mechanical properties of the hot rolled steel depend on the laminar jet cooling control on the runout table. An experimental and numerical study of the heat transfer during water jet impingement cooling of a hot steel plate (600 to 900°C) is presented. The effects of plate temperature upon the heat transfer were analyzed by the inverse heat conduction method that provided the heat fluxes on top surface from measured temperatures into the plate. Rewetting (liquid-solid contact) occurs on surface above 880°C. Higher initial temperatures enlarge the heat flux, but delay the onset of the rewetting and the advance of rewetting front radius (R wet ). Heat transfer coefficient (HTC) increases as surface temperature decreases. The size of the non-symmetric effective jet cooling zone depends on strip speed. Correlations to predict HTC and Rwet have been proposed. Results will contribute for enhancement of fast cooling system in hot strip mill.

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For the first time, high-speed imaging (20 kfps) from the boiling phenomenon within water jet impingement zone during quenching of a high temperature (300°C-900°C) steel plate was observed. The inverse heat conduction method gave the heat flux and the temperature at the plate surface from temperatures measured with thermocouples inserted into the plate. Surprisingly, gas bubbles, from a degassing process, on top of a thin vapor film have been observed. Surface roughness induces the occurrence of rewetting on surface temperatures above the critical point of water. For an initial surface temperature of 300°C, rewetting phenomenon took place without occurrence of vapor film, but for surfaces at or above 450°C, even in very high jet subcooling of 80K, vapor film formation was observed. In spite of high subcooling and velocity of jet, an intense vapor bubble activity was observed within wet region and ceased at surface temperature of 246°C for subcooling of 80K. Temperature and time of rewetting are strongly affected by the initial surface temperature and jet subcooling and, less intensely, by jet velocity.