The impact toughness of Q235B is closely related to temperature, with its toughness decreasing as the temperature decreases. The specific characteristics and features are as follows:
1. The Standard's Specified Relationship between Temperature and Impact Toughness
According to GB/T 700-2006, the impact test for Q235B is only required to be conducted at room temperature (20°C), with an impact energy (KV2) of ≥27J. This requirement reflects its design intent of ensuring basic toughness at room temperature, not at low temperatures.
2. The Effect of Actual Temperature Changes on Impact Toughness
At room temperature and above (≥20°C):
In this temperature range, Q235B's impact toughness is relatively stable, generally easily meeting the ≥27J requirement. The material exhibits a plastic state, undergoing significant plastic deformation before fracture, and exhibiting a high impact energy. Below 20°C (below room temperature):
As the temperature decreases, the impact toughness of Q235B gradually decreases:
When the temperature is slightly below 20°C (e.g., 0-10°C), the impact energy may still be maintained at around 27J, but the fluctuation increases, and some batches of steel may approach the lower limit.
When the temperature drops further (e.g., below -10°C), the impact energy decreases significantly, possibly below 27J. The material becomes more brittle, and the fracture mode gradually shifts from ductile to brittle. At this point, the risk of brittle fracture increases when subjected to impact loads.
III. Comparison with Other Quality Grades (Reflecting Differences in Temperature Adaptability)
In the Q235 series, grades C and D have lower impact test temperatures (0°C for Grade C, -20°C for Grade D), and also require an impact energy ≥ 27J, indicating superior low-temperature toughness. However, since Q235B does not have mandatory requirements for low-temperature impact resistance, its low-temperature toughness cannot be guaranteed. This is the core difference between it and grades C and D in terms of application scenarios (Q235B is more suitable for room temperature environments, while grades C, D, or other low-temperature steel grades should be preferred for low-temperature environments).
In summary, the impact toughness of Q235B is sensitive to temperature. While toughness is guaranteed at room temperature, it decreases significantly at low temperatures, increasing the risk of brittleness. Therefore, in practical applications, it is important to avoid subjecting it to impact loads at extremely low temperatures (especially below -10°C).