In the old versions of EN 10025-2 (prior to the 2004 edition), the distinction between the suffixes "K2" and "J2" was significant and application-specific, not merely a difference in toughness at the same temperature.
Here is the essential difference:
Core Distinction: Intended Application & Implicit Quality Requirements
S355J2: The "J" stood for "Impact Tested" (J for the French "de Choc"). This was the general-purpose grade for plates, sections (beams, angles), and bars intended for standard welded, bolted, or riveted structures. Meeting the 27J at -20°C requirement was the primary condition.
S355K2: The "K" stood for "Hollow Sections" (from the German "Kreis" for circle, implying tubular forms). This grade was specifically intended for the manufacture of structural hollow sections (SHS, RHS, CHS).
The specification implied additional, implicit quality requirements that were critical for the cold forming process used to make tubes:
Enhanced Resistance to Ageing: Hollow sections are often cold-formed (bent, shaped) which can cause strain ageing, leading to embrittlement over time. The K2 grade was required to have better resistance to this phenomenon.
Guaranteed Through-Thickness (Ductility) Properties: While not explicitly a Z-quality grade, the steel needed to have sufficient ductility to withstand the severe deformation of cold forming without cracking or laminating.
Consistency for Welding: The tubes made from K2 steel would often be used in highly welded structures (like space frames), so the material's composition and properties were tailored to ensure good weldability in tubular connections.
Summary in a Table (Old EN 10025-2 Context)
| Feature | S355J2 | S355K2 |
|---|---|---|
| Suffix Meaning | J = General Impact Tested Grade. | K = Specifically for K reisformige (circular) hollow sections. |
| Primary Application | General structural plates, beams, bars. | Raw material for manufacturing structural hollow sections (tubes). |
| Key Implicit Requirement | Meet impact toughness at -20°C. | Meet impact toughness AND have superior suitability for cold forming (ageing resistance, ductility). |
| Purchaser's Perspective | You bought "J2" steel to fabricate a structure. | You bought "K2" steel (typically from the mill) to send to a tube mill, which would then cold-form it into hollow sections. The tube would then be sold as a product meeting another standard (like EN 10219). |
Why This Distinction Was Merged and Became Obsolete
The distinction, while technically meaningful, proved cumbersome and was eliminated in the 2004 revision of EN 10025-2 and subsequent editions for several reasons:
Manufacturing Process Evolution: Modern steelmaking (fine-grain practice, tighter composition control) meant that all quality structural steels (J2, J0, K2) inherently possessed good cold-forming properties. The separate "K2" designation became redundant.
Simplification: Maintaining a separate grade for a single product form (hollow section feedstock) was seen as unnecessary complexity.
Current Status: In EN 10025-2:2004 and all current versions, the "K2" grade designation no longer exists. The requirement for hollow section parent material is now covered by specifying "steel grade S355J2" along with the additional requirements of the hollow section product standard itself (EN 10219-1 for cold-formed, EN 10210-1 for hot-finished). These product standards add the necessary clauses for forming, ageing, and testing.
Legacy and Modern Procurement Implications
On Old Drawings: If you see S355K2 on a legacy European drawing, the designer's intent was to use material suitable for hollow sections. The correct modern equivalent is S355J2 (to EN 10025-2), which will then be used to produce hollow sections to EN 10219.
Chinese Equivalent (GB): As discussed, the Chinese system GB/T 1591 never adopted this "J" vs. "K" distinction. Both are mapped to Q355D. However, for hollow sections, one must additionally specify the hollow section product standard (e.g., GB/T 6728) and ensure the base material Q355D meets any supplementary requirements for cold forming.
Conclusion:
The essential historical difference was that S355K2 was a "hollow-section-specific" variant of S355J2, with implied guarantees of better cold-forming behaviour. Modern steelmaking made this separate category unnecessary, and it was officially retired nearly two decades ago. Today, S355J2 (or Q355D) is the universal grade for this strength and toughness level, with any special requirements for hollow sections dictated by the tube manufacturing standard, not the base material standard.