Steel, as the foundation of modern construction and machinery manufacturing industries, directly impacts project quality and safety. With technological advancements and changing industrial demands, steel standards are constantly being updated.

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

Q355B: A Product of International Alignment

The upgrade of Q345B steel is a crucial component of the GB/T 1591-2018 standard update. The new grade Q355B, even in its name, hints at its similarity to the European standard EN 10025's S355JR.

Optimized Chemical Composition

This alignment extends beyond the grade designation, delving into chemical composition and mechanical properties. For example, the upper limit for carbon content has been increased from ≤0.20% to 0.24%, aligning with the European standard's JR grade, and harmful elements such as Cu are also kept as consistent as possible with European standards.

Minor Adjustments to Mechanical Properties

Regarding mechanical properties, the yield strength of Q355B steel has been increased from ≥345 MPa for Q345B to ≥355 MPa. However, the original lower yield strength (ReL) criterion has been changed to the upper yield strength (ReH) criterion, consistent with European standards. This change has a relatively small impact on production and use, and can even be considered as having little impact on load design material selection.

In fact, the main purpose of changing the yield strength value is to maintain consistency with European standards, increase material availability, and reduce design conversion costs.

Conclusion:

The upgrade from Q345B to Q355B is an important step in 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 between the grade and international standards.

The benefits are mainly twofold:

1. Usage: In recent years, cooperation between China and Europe in many fields has become increasingly frequent, and many engineering projects choose to purchase steel from China. However, due to different standard systems, this often leads to some problems. For example, when purchasing S355JR steel, it is usually necessary to reorder from the steel mill. However, with the implementation of the new national standard, S355JR can be directly replaced by Q355B in terms of performance, and the two are almost identical. This means that as long as the designer slightly modifies the drawing annotations, they can be successfully interchanged.

2. Production: Although the demand for European standard products has been increasing year by year, the differences in standard systems, composition, and performance have made production organization very inconvenient, leading to increased production costs. However, the implementation of the new national standard makes S355JR and Q355B almost interchangeable. For steel mills, this is basically just a matter of modifying the quality certificate. Previously, these two grades were not interchangeable. If an S355 order failed, it could only be used as a usable product in the market at a lower price. In addition, the Q345 series also differs from the European standard in other aspects, resulting in the need to re-produce European standard orders. But in the new national standard, not only is grade B interchangeable, but several other grades are also almost identical, such as S355J0 and J2.