Comparison of Differences Between Q345B and Q355B

As the foundation of modern construction and mechanical manufacturing, the performance of steel directly impacts engineering quality and safety. With advances in technology and changing industrial demands, steel standards are constantly being updated.

The introduction of the GB/T 1591-2018 standard, particularly the upgrade of Q345B steel to Q355B, marks the alignment of Chinese steel standards with international standards.

Q355B: A Product of International Alignment

The upgrade of Q345B steel is a key component of the GB/T 1591-2018 standard update. The introduction of the new grade Q355B implies, through its naming, its similarity to S355JR in the European standard EN 10025.

Optimization of Chemical Composition

This alignment is not limited to the grade designation but extends to chemical composition and mechanical properties. For example, the upper limit for carbon content has been raised from the original ≤0.20% to 0.24%, matching the European standard’s JR grade, while levels of harmful elements such as Cu have also been aligned as closely as possible with European standards.

Minor Adjustments to Mechanical Properties

In terms of mechanical properties, the yield strength of Q355B steel has been increased from ≥345 MPa in Q345B to ≥355 MPa. However, the original lower yield strength (ReL) criterion has been changed to the upper yield strength (ReH) criterion, consistent with the European standard. This change has a relatively minor impact on production and usage and can even be considered to have little effect on material selection for load-bearing design.

In fact, the primary purpose of changing the yield strength value is to align with European standards, thereby increasing the material’s marketability and reducing the costs associated with design conversion.

Conclusion:

The upgrade from Q345B to Q355B represents a significant step toward aligning Chinese steel standards with international standards. This change is reflected not only in the optimization of chemical composition and mechanical properties but also in the correspondence of steel grades with international standards.

The benefits are primarily twofold:

1. Application: In recent years, cooperation between China and Europe has become increasingly frequent across multiple sectors, with many engineering projects opting to source steel from China. However, differences in standard systems often lead to complications. For example, when procuring S355JR steel, it was typically necessary to re-order from the steel mill. With the implementation of the new national standard, S355JR can now be directly replaced by Q355B in terms of performance, with virtually no difference in compatibility. This means that as long as the design team makes minor adjustments to the drawing specifications, a smooth transition can be achieved.

2. Production: Although demand for European standard products has been rising year by year, differences in standard systems, chemical composition, and performance have made production organization very cumbersome, leading to increased production costs. However, the implementation of the new national standard allows S355JR and Q355B to be almost interchangeable; for steel mills, this essentially boils down to simply modifying the certificate of conformity. Previously, these two grades were not interchangeable. If S355 was in short supply, it could only enter the market as a lower-priced alternative. Additionally, the Q345 series differed from European standards in other aspects, meaning that when European standard orders arrived, production had to be restarted from scratch. Under the new national standard, however, not only is Grade B interchangeable, but other grades—such as S355J0 and J2—are also virtually identical.