Overcooling in overlap areas during hydraulic descaling

Abstract

The production and processing of high-quality grades of steel are connected with the oxidation at high temperatures. Unwanted scales are formed on the steel surface, which is usually heated to over 900°C. These scales are often removed by hydraulic descaling during the production. In most cases where long, flat products are produced, one row of descaling nozzles is used. As these flat jet nozzles are arranged in a row, the water spray from one nozzle interferes with the spray from the neighboring nozzles. This zone is called an overlap area and often even more scales remain here after the descaling process. An increased amount of the scales left behind results in a lower quality of a final product. A typical configuration with an inclination and twist angle of 15°was studied. Heat-transfer coefficients (HTC) and surface temperatures were measured in the overlap area and compared with the values obtained from undisturbed areas. It was found that the overlap area is grossly overcooled. The results were compared with a new configuration, where the twist angle was changed to 0°, and it was found that the overcooling was significantly reduced. The temperature measurement showed that an increased thickness of the scales in the overlap area can also be caused by surface overcooling because the scales change the material properties with the temperature, and they are therefore more difficult to remove. The new configuration with the twist angle of 0°seems promising for improving the quality of hydraulic descaling. Keywords: scales, steel, water, hydraulic descaling, overlapping, temperature, heat-transfer coefficient, surface Proizvodnja in predelava visoko kvalitetnih jekel je povezana z oksidacijo pri visokih temperaturah. Neza`eljene {kaje nastajajo na povr{ini jekla, ki se ga obi~ajno segreva nad 900°C. Te {kaje se med proizvodnjo pogosto odstranjujejo s hidravli~nim raz{kajanjem. V ve~ini primerov, ko se proizvaja dolge, plo{~ate proizvode, se uporablja ena vrsta raz{kajevalnih {ob. Ker so {obe s plo{~atim curkom razporejene v vrsti, vodni curek iz ene {obe vpliva na vodni curek sosednjih {ob. To podro~je se imenuje podro~je prekrivanja in pogosto na tem podro~ju ostane ve~{kaje po od{kajanju. Pove~an dele`preostale {kaje pa povzro~a slab{o kvaliteto kon~nega proizvoda. Analizirana je bila zna~ilna postavitev z naklonom in kotom zasuka 15°. Na podro~ju prekrivanja je bil izmerjen koeficient prenosa toplote (HTC) in temperatura povr{ine ter primerjava s podatki iz neprizadetih podro~ij. Ugotovljeno je, da so podro~ja prekrivanja mo~no podhlajena. Rezultati so bili primerjani z novo konfiguracijo, kjer je bil kot zasuka 0°in ugotovljeno je, da se je podhladitev mo~no zmanj{ala. Meritve temperature so pokazale, da je pove~ana debelina {kaje v podro~ju prekrivanja lahko tudi posledica podhladitve povr{ine, ker {kaja s temperaturo spreminja lastnosti materiala in se jo zato tudi te`je odstrani. Zdi se, da bo nova postavitev, s kotom zasuka 0°, omogo~ila izbolj{anje kvalitete hidravli~nega raz{kajanja.

optimal hydraulic descaling

“…The right part of Figure 4 shows the measured variations of surface temperature across the width of the surface using an infrared line-scanner placed 0.5 m beyond descaling. To study the heat loss during hydraulic descaling, the hot test plate is equipped with thermocouples (see left diagram in Figure 5), which are located 0.6 mm below the descaled surface [4]. Distributions of heat transfer coefficients are computed for each position of the thermocouple (see right graph in Figure 5) from the recorded temperature history during the descaling experiment using inverse computation [35].…”

“…As a multifactor process, descaling conditions can vary significantly. The most crucial factors that affect the process can be divided into mechanical effects and thermal effects [3,4]. The dominant parameter is impact pressure, influencing both mechanical and thermal action [5,6].…”

“…In other words, designing the descaling unit with a proper spray overlap is difficult [25]. An experimental study confirmed that having neighboring jets which collide with each other can significantly reduce descaling performance [10] and significantly increase cooling [4]. Industrial descaling units usually have nozzles rotated by offset angle β set to 15° (see Figure 1 right) to avoid the water jets colliding before the water hits the descaled surface [5,7,[25][26][27][28][29][30][31][32].…”

The Effect of Water Jet Overlaps in a Descaler on the Quality of Surface of the Hot Rolled Steel

“…Since one nozzle cannot effectively cover the entire width of the steel strip, multiple nozzles with overlapping sprays must be used. The overlap area is often overcooled [14]. This setting can result in uneven descaling of the surface and residual scales.…”

“…The work focuses on the overlap area and the washout area of the sprays. This study draws on research carried out in previous years in our department and presented in papers [14,27]. At the end of the article, procedures for preventing residual scales are proposed.…”