Why change the Q345 to Q355 in GB/T1591
The core reason for the modification of Q345 grade to Q355 in GB/T 1591 standard is to achieve alignment with international standards (such as European EN standards), while promoting technological upgrading and global competitiveness in China's steel industry. The following is a detailed analysis of the background, technical logic, and actual impact of this adjustment:
A. Background of Change: Internationalization Integration
1.The impact of European standards
The S355 series steel (such as S355JR, S355J2) in the European EN 10025 standard is a widely used universal structural steel grade worldwide.
The performance of the original Chinese Q345 (yield strength 345MPa) is similar to that of the European S355 (nominal yield strength 355MPa), but the naming difference has led to frequent technical barriers and misunderstandings in international trade.
2.Global market compatibility requirements
As a major exporter of steel, China's unified brand naming can reduce the material selection cost for international buyers. For example, "Q355" can be directly compared to "S355" without the need for additional performance conversion. According to statistics, after the standardization of grades, the certification cycle for Chinese steel exports to the European Union has been shortened by an average of 30%.
B. Technical Logic: Performance Optimization and Standard Upgrade
1. Chemical composition adjustment
| Element | Q345 (旧) | Q355 (新) | adjust purpose |
| 碳(C) | ≤0.20% | ≤0.22% | 放宽限制以适应厚板焊接性能 Relax restrictions to accommodate thick plate weldability |
| 磷(P) | ≤0.035% | ≤0.025% | 降低冷脆性，提升低温韧性 Reduce cold brittleness, improve low temperature toughness |
| 硫(S) | ≤0.035% | ≤0.015% | 减少硫化物夹杂，改善冲击性能 Reduce sulfide inclusion and improve impact performance |
2. Improvement of mechanical properties
Yield strength: The nominal yield strength of Q355 is still 355MPa (with no substantial change from the 345MPa of Q345), but through grain refinement and controlled rolling and cooling (TMCP) process, the actual yield strength fluctuation range is reduced (± 30MPa → ± 25MPa).
Impact toughness: The impact toughness requirement for Q355D at -20 ℃ has been increased from ≥ 34J to * * ≥ 47J * *, significantly enhancing its resistance to brittle fracture in low-temperature environments.
C. Application scenario changes
| Grade | 典型应用场景 Typical application scenario | Standard Comparation |
| Q345 | 普通建筑钢结构 桥梁、船舶 ordinary building steel structure Bridge, ship | 旧标准，逐步淘汰 Old standards, phased out |
| Q355 | 大跨度场馆（如冬奥会速滑馆） 风电塔筒 高寒地区输变电铁塔 Long-span venues (e.g., Olympic speed skating venues) Wind power tower Transmission and transformation tower in cold area | EN S355、ASTM A572 |
D. Industry impact
design optimization
Designers can directly reference the S355 parameters in international standards (such as Eurocode 3) to reduce safety factor redundancy and save 5% to 10% of steel usage.
Production upgrade
Steel companies need to upgrade their smelting processes (such as using LF refining furnaces) to meet stricter requirements for phosphorus and sulfur control, and promote industry technological iteration.
Trade facilitation
According to customs data, the export volume of Q355 steel in 2022 increased by 18% year-on-year, with the EU market share increasing by 7 percentage points.
E. Disputes and Responses
Doubt about 'changing names without changing performance'
Through process improvements such as microalloying with Nb and V, the Z-directional performance (resistance to delamination) of thick plates (>40mm) in Q355 is significantly better than that of Q345.
Transition period issues
After the implementation of the standards in 2018, Q345/Q355 are allowed to be used in parallel until 2020. Currently, new projects are required to adopt Q355.
F. Summary
The change from Q345 to Q355 is not just a simple "numbers game", but a crucial step for China's steel industry to move from "following" to "running parallel" to international standards. Through chemical composition optimization, production process upgrading, and global market integration, Q355 not only solves the problem of "identity recognition" in international trade, but also promotes the comprehensive upgrading of China's steel structure design, manufacturing, and export.
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