Abstract
The tasks of increasing strength of heat-strengthened rolled products require thorough search for technical solutions, determined by level of understanding of processes occurring in thermal strengthening devices, of which the main is the process of water interaction with hot rolled product. This complex set of phenomena includes movement of water flows relative to moving rolled metal, emergence of vapor gap between water and rolled metal, generation of nanosized droplets and their movement through the layer of vapor, the droplets impact on surface, excitation of elastic waves in rolled material. Analysis of previously derived dispersion equation for Kelvin-Helmholtz instability of vapor-water interface was carried out. It is shown that for 30–60 m/s difference in velocities of liquid and vapor layers, the maximum increment in nanoscale range of wavelengths is observed. Average size of generated drops is determined by wavelength at which the maximum increment dependence is reached. Thus, mechanism of accelerated cooling of rolled steel proposed earlier is confirmed by quantitative calculations. Drops, reaching rolled metal, excite thermoelastic wave which being distributed along its section promotes increase in impact strength. To reveal regularities of propagation of elastic waves created by drops in rolled metal, problem of theory of thermoelasticity in cooling was solved by method of Fourier and Laplace integral transformations. It turned out that the problem posed is analogous to the problem of heating the surface with triangular temperature profile. Evolution of stress waves was investigated. At initial moments, the front of wave appeared to be a stretching wave. When reflected from the free end, the wave becomes a compression wave with stresses leading to cracks closure, and thus it leads to increase in toughness. Obtained results can be used in search for optimal modes of heat treatment of rolled products, providing high mechanical properties.
Highlights
Average size of generated drops is determined by wavelength at which the maximum increment dependence is reached
Mechanism of accelerated cooling of rolled steel proposed earlier is confirmed by quantitative calculations
Drops, reaching rolled metal, excite thermoelastic wave which being distributed along its section promotes increase in impact strength
Summary
Задачи повышения прочности термоупрочненного проката требуют использования целенаправленного поиска технических решений, что определяется уровнем изученности процессов, протекающих в технологических устройствах термоупрочнения, главным из которых является процесс взаимодействия воды с горячим прокатом. Что при разности скоростей движения слоев жидкости и пара 30 – 60 м/с наблюдается максимум инкремента в наноразмерном диапазоне длин волн. Ключевые слова: ускоренное охлаждение проката, коэффициент теплопередачи, неустойчивость Кельвина–Гельмгольца, нанокапли, тепловой удар. Задачи повышения прочности термоупрочненного проката требуют использования целенаправленного поиска технических решений. А это, в свою очередь, определяется уровнем изученности процессов, протекающих в технологических устройствах термоупрочнения, главным из которых является процесс взаимодействия воды. Близкие значения указывались в работе [7], где отмечается, что коэффициент теплопередачи имеет порядок от 1 до 100 кВт/(м2·К). В работе [8] предложен механизм, объясняющий высокое значение коэффициента теплопередачи при ускоренном охлаждении проката.
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