How does the manufacturing process (hot-rolling vs. cold-forming) affect the properties of a Q355B i beam steel section?

Q355B I-beams are exclusively produced by the hot-rolling process. It is important to distinguish this from cold-forming (or cold-rolling), which is used for thinner gauge sheet products that are then formed into light gauge C-sections or Z-purlins, not heavy structural I-beams. The processes have fundamentally different effects on material properties. Hot-Rolling for Q355B I-beams: Steel blooms (rectangular cross-sections) are reheated to a high temperature (typically above 1100°C, in the austenitic region). They are then passed through a series of rolling stands to progressively form the I-shape. The high temperature allows for significant plastic deformation with low forces and results in recrystallization, which refines the austenite grain size. Subsequent controlled cooling (often in air) transforms the austenite into a fine ferrite-pearlite microstructure, which provides the balanced strength and toughness of Q355B. The process relieves internal stresses, improves toughness, and produces a scale (oxide layer) on the surface. The final mechanical properties are isotropic (similar longitudinally and transversely, though some anisotropy exists), and the yield strength is as defined in GB/T 1591. Cold-Forming (for comparison): This process starts with already rolled steel sheet or strip (which could be of grade Q355B, but more commonly lower grades like S350GD). It is formed at room temperature through a series of dies or rolls. The cold working drastically increases yield strength through strain hardening (work hardening) but significantly reduces ductility and toughness. The material becomes anisotropic, with properties varying with direction. Cold-formed sections have sharper corners and tighter tolerances but are generally thinner and used for lighter, secondary structures. They cannot achieve the heavy cross-sections or the combination of high strength and high toughness characteristic of hot-rolled Q355B sections.

*Table: Comparison of Hot-Rolled Q355B I-beam vs. Typical Cold-Formed Steel Section*