The European grade S355K2 (EN 10025-2) has a clear and important equivalent in the Chinese national standard (GB) system.
The direct equivalent is: Q355D according to the primary standard GB/T 1591-2018 (High strength low alloy structural steels).
However, a complete and technically accurate substitution requires careful comparison, as the exact definitions have nuanced differences.
1. Primary Equivalent: Q355D (GB/T 1591-2018)
This is the standard and accepted domestic substitute.
Similarities:
Minimum Yield Strength: Both have a yield strength (ReH) of ≥ 355 MPa for thickness ≤ 16mm.
Toughness Requirement: This is the key matching parameter.
S355K2: Requires impact toughness of 27 J at -20°C.
Q355D: Requires impact toughness of ≥ 34 J at -20°C.
Conclusion: Q355D not only matches but exceeds the toughness requirement of S355K2 at the same temperature (-20°C). This makes it a fully suitable and often superior replacement.
2. Important Distinction: The Meaning of "K2" vs. "D"
S355K2: The "K2" suffix in the old EN 10025-2 standard specifically denoted a steel with guaranteed increased resistance to brittle fracture and was primarily intended for hollow sections (tubes). The "2" indicated testing at -20°C.
Q355D: The "D" suffix in GB/T 1591 is a general toughness grade indicating testing at -20°C, without a specific link to hollow sections. For hollow sections, China has separate product standards (like GB/T 6728 for cold-formed sections) that will specify the use of Q355D as the base material.
3. Comparison with Other S355 Grades
To avoid confusion, here is the mapping:
| European Grade (EN 10025-2) | Key Toughness Requirement | Direct Chinese Equivalent (GB/T 1591) | Key Toughness Requirement |
|---|---|---|---|
| S355J2 | 27J @ -20°C | Q355D | ≥34J @ -20°C |
| S355K2 | 27J @ -20°C (for hollow sections) | Q355D | ≥34J @ -20°C |
| S355J0 | 27J @ 0°C | Q355C | ≥34J @ 0°C |
| S355JR | 27J @ +20°C | Q355B | ≥34J @ +20°C |
Key Insight: In the current GB system, both S355J2 and S355K2 map to Q355D, as the distinction between "J2" and "K2" was not carried over. The Chinese system uses the unified toughness grade "D" for -20°C application.
4. Application-Specific Considerations
For Structural Hollow Sections (Tubes): When procuring square, rectangular, or circular tubes equivalent to S355K2 hollows, you would specify the hollow section product according to standards like:
GB/T 6728 (Cold-formed hollow sections) - Material: Q355D
GB/T 3094 (Cold-drawn seamless hollow sections) - Material: Q355D
The product certificate should confirm the impact toughness at -20°C.
For Plates and Sections: Simply specify Q355D per GB/T 1591-2018.
Procurement Specification
For a direct replacement of S355K2, your material order should state clearly:
"Material: Q355D according to GB/T 1591-2018, with Charpy V-notch impact energy ≥ 34J at -20°C."
For hollow sections:
"Cold-formed hollow section to GB/T 6728, made from base material Q355D per GB/T 1591-2018, with impact toughness at -20°C."
Summary
The Chinese national standard equivalent of S355K2 is Q355D (GB/T 1591-2018).
For all practical purposes of strength and toughness, Q355D is a direct and equivalent substitute.
The historical European distinction between "J2" (general use) and "K2" (for hollow sections) is not mirrored in the GB system; both are covered by Q355D.
When the application involves structural hollow sections, ensure the hollow section product standard (e.g., GB/T 6728) is specified alongside the material grade Q355D.