Effect of Rolling Practices on Annihilation Process of Micro-Porosity in Big Ingot

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Abstract
The effect of rolling conditions on the process of the annihilation of the micro-porosities in the big ingot was investigated to clarify the possibility to get the sound ultra-heavy plate only by rolling, eliminating pre-forging process.It was deduced from the results of the experiments that the most important factor for the annihilation of the micro-porosities at the center of the thickness was the compressive stress perpendicular to the rolling plane at the center of the thickness. It was clearly demonstrated that the micro-porosities were easily annihilated in the early stage of rolling under such rolling conditions that enabled the compressive stress to exceed the resistance to deformation significantly. These conditions were realized by the rolling process with the large amount of reduction per rolling pass (“heavy reduction-rolling”) utilizing the work roll with large diameter, while in the case of light reduction-rolling the micro-porosities retained as theelongated imperfections up to the late stage of rolling. It was also confirmed that both slow rolling speed and high rolling temperature assisted the annihilation of thin elongated micro-imperfections. This phenomenon can be interpreted in terms of the effect of time and temperature on the annihilation of interfaces by means of diffusion and viscous flow processes. It was concluded that it should be possible to manufacture the sound ultra-heavy plates without pre-forging process, provided that the combination of aforementioned considerations were taken into account for the rolling process.

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