Comparison of the differences between Q345B and Q355B Steel.

As the foundation of steel structure buildings and machinery manufacturing industries, steel’s performance directly affects project quality and safety. With the advancement of science and technology and changes in industrial needs, steel standards are constantly being updated. The introduction of the GB/T 1591-2018 standard, primarily the upgrade of Q345B steel to Q355B steel, also marks the integration of Chinese steel standards with international standards.

Q355B steel: a product of international integration

The upgrade of Q345B steel is essential to update the GB/T 1591-2018 standard. Introducing the new grade Q355B steel implies its similarity to S355JR of the European standard EN 10025 in its name.

Optimization of chemical composition

This integration is reflected not only in the grade but also in the chemical composition and mechanical properties. For example, the upper limit of some carbon content has been raised from ≤0.20 to 0.24, which is the same as the JR grade of the European standard, and harmful elements such as Cu are also kept consistent with the European standard as much as possible.

Subtle adjustment of mechanical properties

Regarding mechanical properties, the yield strength of Q355B steel has been raised from ≥345 MPa of Q345B to ≥355 MPa, but the original lower yield strength judgment has been changed to the upper yield strength consistent with the European standard. This change has a relatively small impact on production and use and can even be regarded as not a significant change in the selection of load design materials.

The primary purpose of the change in yield value is to remain consistent with European standards, increase material circulation, and reduce the cost of design conversion.

Q345B and Q355B are two different steels, and their main differences are as follows:

1. Different properties: Q345 is a low-alloy steel, while Q355 is a low-alloy, high-strength structural steel widely used in bridges, vehicles, ships, buildings, pressure vessels, special equipment, and other fields.
2. Different yield strength of steel: “Q” stands for yield strength. The yield strength of Q345 is 345MPa, while the yield strength of Q355 is 355MPa. As the thickness of the material increases, the yield value of Q355 decreases.

The main difference between Q345 and Q355 is their properties and yield strength. Q355 has higher strength and a more comprehensive range of applications, while Q345 is a more common low-alloy steel. When choosing which steel to use, it is necessary to consider specific needs and the use environment.

The upgrade of Q345B to Q355B steel is essential to aligning China’s steel standards with international standards. This change is reflected in optimizing chemical composition and mechanical properties and the correspondence between grades and international standards.

The benefits brought are mainly in two aspects:

Use: In recent years, China and Europe have cooperated more frequently in many fields, and many engineering projects have chosen to purchase steel in China. However, due to different standard systems, this often leads to problems.

For example, when purchasing S355JR steel, it is usually necessary to re-order with the steel mill. However, with the implementation of the new national standard, S355JR can be directly replaced with Q355B steel in terms of performance, and the docking of the two can achieve almost zero difference.

This means that as long as the design party slightly modifies the drawing markings, it can be docked smoothly.

Production: Although the demand for European standard products has increased yearly, due to the differences in standard systems and component performance, production organization has become very inconvenient, resulting in rising production costs.

However, implementing the new national standard makes S355JR and Q355B almost interchangeable. For steel mills, this is just a matter of modifying the warranty.

In the past, these two grades were not interchangeable. If S355 is single, it can almost only enter the market as an available product 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 when orders for the European standard come in. However, Class B is interoperable in the new national standard, and several other grades are almost the same, such as S355J0, J2, etc.