The tensile strength range accommodates normal variations inherent in industrial steel production and allows for the influence of product dimensions.
Inherent Process Variation: Slight fluctuations in chemical composition (especially carbon and manganese content) within the specified limits, rolling temperatures, and cooling rates can affect the final microstructure, leading to variations in strength.
Product Thickness/Size: A key factor is the thickness of the rolled section. Thinner sections cool faster after hot rolling, which can result in a finer grain structure and higher strength. Conversely, thicker sections (like the web or flange of a large H-beam) cool more slowly, leading to a coarser grain structure and potentially lower strength toward the bottom of the specified range. The standard accounts for this by sometimes applying different property requirements for material over a certain thickness.
Deformation History: The amount of reduction during the final rolling passes also influences the final strength.
The range provides manufacturers with practical flexibility while ensuring the material meets a consistent minimum performance threshold (the 400 MPa lower bound). For designers, the conservative approach is to use the minimum expected value (400 MPa) in calculations where tensile strength is a limiting factor.