What does the B in Q355B i beam specifically require regarding impact testing?

The letter "B" in the Q355B grade designation is a quality grade suffix that primarily dictates the temperature at which Charpy V-notch impact testing must be performed and the minimum energy absorption required. According to GB/T 1591-2018, for grade Q355B, the mandatory impact test is conducted at +20°C (room temperature). The requirement is that the average impact energy (KV2) from three standard specimens must be ≥ 34 Joules (J). Additionally, standard rules usually apply where only one of the three individual values may be below 34 J, but not less than 70% of the average (i.e., not less than about 24 J). This is in contrast to other grades: Q355A has no mandatory impact requirement, Q355C requires testing at 0°C, Q355D at -20°C, and Q355E at -40°C, all with the same 34J minimum. The "B" grade signifies a basic but important level of guaranteed toughness for general structural use in temperate climates where service temperatures are not expected to fall below freezing. It ensures the material has sufficient resistance to brittle fracture under dynamic or shock loading conditions at ambient temperatures. This requirement drives tighter control over chemical composition (particularly harmful elements like P and S) and the steelmaking/rolling process to achieve a fine, uniform microstructure that can absorb energy by plastic deformation rather than cleaving.