Is S275JR angle bar produced by hot-rolling or cold-rolling? Detail the hot-rolling process and its effect on material properties.

S275JR angle bar is exclusively produced by the hot-rolling process. Cold-rolling is not a standard industrial process for manufacturing structural steel angles of this grade and size. The hot-rolling process is fundamental to achieving the required structural properties and dimensions. The process begins with a reheated steel billet (typically at temperatures between 1100°C and 1300°C), where the steel is in a fully austenitic, highly plastic state. This hot billet is then progressively passed through a series of rolling stands containing shaped grooves (passes) in the rolls. Each pass gradually reduces the cross-section and forms the characteristic "L" shape of the angle. The final passes ensure precise control over leg dimensions, thickness, and the radius of the fillet (the inner corner). Effects of Hot-Rolling on Properties: 1) Grain Refinement: The combination of high-temperature deformation and subsequent air cooling refines the coarse as-cast grain structure, resulting in a finer ferrite-pearlite microstructure. This significantly improves toughness and strength compared to the original cast structure. 2) Directional Properties: Mechanical properties are generally isotropic (similar in all directions) for hot-rolled sections, unlike cold-formed products which can exhibit anisotropy. 3) Scale Formation: A layer of iron oxide (mill scale) forms on the surface during rolling. This must be removed by pickling or abrasive blasting before painting for corrosion protection. 4) Residual Stresses: The non-uniform cooling after rolling can introduce minor residual stresses, but these are generally low and redistributed upon cutting or welding. 5) Dimensional Tolerances: Hot-rolled products have larger dimensional tolerances (per EN 10056-1) compared to cold-finished products, but these are perfectly acceptable for structural fabrication. The hot-rolled "as-rolled" state delivers the specified mechanical properties of S275JR directly, with no further thermal treatment required, making it a highly efficient production route for structural sections.