Anisotropic configurations to improve bar-metal performance

Abstract

Rolled metal for the manufacture of machine parts, buildings, and other structures must have mechanical properties guaranteeing reliable operation over a long period. These properties are usually ensured by a range of measures, including the creation of new high-performance steels and alloys. A primary consideration here is increasing the strength of the metal, with corresponding increase in its resistance to failure under the action of cyclic and dynamic loads, which is the basic characteristic of a structural material. The entire metallurgical production process serves this goal. Steel smelting usually makes the main contribution to the required metal properties. In rolling, different forms of thermomechanical treatment are employed, without considering deformational effects that may significantly affect the mechanical properties of the metal. The plastic deformation of the metal on rolling is accompanied by phenomena that affect its performance and may even change its physicochemical properties. By controlling these phenomena, we may ensure deformation conditions in which the metal will have the best performance. This may be accomplished by creating an ordered band (fibrous) macrostructure of the bar metal on rolling, on account of shear processes. The appearance of an oriented fibrous macrostructure is associated with both quantitative and qualitative anisotropy of the metal’s mechanical properties. The anisotropy of the properties must be used to best advantage in the final part. The rolling conditions must be such that the orientation of the mechanical texture (band or fibrous structure) formed on deformation in the bar metal is aligned with the action of the maximum normal stress when the final part is in operation. The mechanical texture of the final rolled product is an important characteristic of the metal, which will determine the effectiveness of the corresponding part. The deliberate use of inhomogeneity and anisotropy of the metal properties to increase the structural strength of the parts produced is hindered by inadequate theoretical understanding of the influence of the rolling method on the predominant orientation of the mechanical texture and its relation to the technological, strength, and plastic characteristics of the product. The role of the mechanical texture in ensuring the strength of the final part may be clearly seen for the example of a wire. We know that, with decrease in cross section, its strength increases, on account of the decrease in grain size and increased order of the macrostructure with increase in the extension [1]. In drawing, all the grains in the metal are oriented along the direction of drawing, parallel to the rolling plane, which intensifies the axial orientation of the initial mechanical texture of the wire rod. In general, different deformation methods have different influence on the metal properties. For example, to achieve the standard mechanical properties of the rolled metal, continuous-cast billet must be deformed by no less than tenfold extension ( e = 69%) in longitudinal rolling, with sixfold extension in forging and eightfold extension in pressing [2]. Screw rolling ensures the required properties with deformation at a supply angle β ≥ 15 ° when the extension λ ≥ 4.5 and e = 53% [3]. This significant difference in the deformational effects may be attributed to the different processes by which macrovolumes are shaped and displaced in these treatment methods. They differ in the application of the force, the stress‐strain state of the metal, the boundary conditions, and other factors. The load and kinematic conditions in screw and longitudinal rolling are different. In deformation, the boundary surfaces of the billet (round and square or rectangular, respectively) will be different, which is associated with different trajectories of the macrofluxes and different billet geometry as a whole. By this means, technological inheritance of the metal properties is established for each deformation method, as seen experimentally. To discover the influence of the mechanical texture on the strength and plastic characteristics of the metal, the properties of the profiles (round, hexahedral, square) made on small-bar mills 1 and 2 are analyzed. Longitudinal rolling is used for all the metal in mill 1, whereas screw rolling is also used in mill 2. In the roughing group of mill 2, the screw-rolling cells reduce the billet by 1/3 of the amount required to produce the final form.