Behavior of Structural Defects of Already-Deformed Continuous-Cast Bar on Rolling

Abstract

In the continuous casting of steel ingots, the most significant trend over the last few decades has been the partial transfer of deformation processes from the region of complete solidification (the rolling mill) to the region characterized by a two-phase solid–liquid state (the continuous-casting line). However, such two-stage deformation of the continuous-cast ingot entails changes in the physical modeling of the behavior of surface and interior defects in subsequent rolling. In particular, modifications are required in selecting the geometry and spatial orientation of the defects. In the present work, the influence of the surface-defect orientation and central macrostructure (pore content) of bar billet deformed in the continuous-casting line is investigated by means of laminar physical models. The deformation conditions of reduced-scale (1: 5) continuous- cast billet are studied experimentally in two rolling configurations: (1) the use of smooth rollers to simulate groove-free rolling in the first two stands of the cogging group in the 350 continuous medium-bar mill; (2) rolling in the first and second pairs of straight grooves in the cogging stand of the 500/370 mill at PAO Donetskii Metallurgicheskii Zavod. Since this is a multivariant problem, a universal design has been developed for the physical model, so as to simulate the spatial configuration of both surface and internal defects. The research shows that, in rolling the physical models with an extension coefficient greater than 2.0 and practically 60° inclination of the simulated defects to the rolling axis, their complete elimination is possible. In turn, decreasing the inclination to 30° facilitates greater extension of the simulated defects and only slightly reduces their width. When the inclination of the simulated defects to the rolling axis is 90° (complete lack of coaxiality), broadening of the defects and their compression to the initial length is only observed after 90° rotation. The experimental data provide insights regarding the elimination of internal defects (pores) in the metal as a function of the total extension, the inclination of the defects’ longitudinal axis to the rolling direction, and their distance from the longitudinal–transverse symmetry plane.