Application of water jet description on the de-scaling process

Abstract

The paper is dealing with application of theoretical equations derived for rock disintegration onto materials with similar behavior—thick scales on hot metal rolling slabs. Penetration of water jet through a hot, rock-like material (e.g. scale) and work of the steam bubbles emerging from the water jet on the boundary between the scale layer and the hot metal material are described by a set of appropriate equations. The model is applied on the fan jets used for de-scaling process, and it provides both the qualitative and the quantitative results. These results make possible to determine the depth of penetration of water jet into the material of the scales and calculate the sizes of pieces of the disintegrated scales. Both mechanisms of water jet acting on scales, mechanical penetration to a certain depth in the material and the formation of steam bubbles inside the material, create mechanical stresses in the material of scales, especially the tensile and the shear ones. Pieces of scales are separated due to exceeding the limits of the stress and strain in the material of scales. The presented analytical equations describing the process in a simple way yield the quick and apprehensible calculation of applicable results. It is an alternative to solution of a rather complicated set of differential equations describing the mass and heat flow. The proposed theoretical base runs with technical factors and properties that can be obtained from tables or analogies with other materials or processes. The typical water pressure range of rolling mills is 16–24 MPa, the equivalent diameter of the applied water nozzle is 2 mm, the average traverse speed of the rolling slab is set to 1 m s⁻¹, and the mean stand-off distance of the nozzle from the steel slab surface is 150 mm. Calculated depth of penetration into scales is ranging from 5 to 18 mm for these parameters, while the real thickness of scales lies between 1 and 7 mm. Simultaneously, the calculated length of the peeled layer in the direction of the jet movement ranges from 30 to 70 mm and the cutting width determined from the jet shape and the stand-off distance is 80–120 mm. Therefore, the calculated size of the scale debris is 30 × 80 mm for layers thicker than 5 mm and 70 × 120 mm for the ones thinner than 2 mm. These theoretical values correspond with sizes of real scale debris picked at the rolling mill.