DIN 2391 is the older German seamless precision cold-drawn steel tube standard, withdrawn in 2006 and superseded by EN 10305-1. The two cover the same product with direct grade mappings (St37.4 → E235, St44 → E275, St52 → E355) and renamed tolerance classes (NBK → +N, BKW → +C, BKS → +LC). New designs should reference EN 10305-1.
Grade and tolerance-class mapping
| Attribute | DIN 2391 (withdrawn) | EN 10305-1 (current) |
|---|---|---|
| Standard status | Withdrawn 2006; still referenced on legacy drawings | In force across EU since 2006 |
| Scope | Seamless precision cold-drawn steel tube | Seamless precision cold-drawn steel tube |
| Grade — low strength | St37.4 (340 MPa tensile) | E235 (340 MPa tensile) |
| Grade — medium strength | St44.4 (440 MPa tensile) | E275 (430 MPa tensile) |
| Grade — high strength | St52.4 (490 MPa tensile, 355 MPa yield) | E355 (490 MPa tensile, 355 MPa yield) |
| Normalised condition | NBK (normalised) | +N (normalised) |
| Cold-drawn condition | BKW (bright drawn, no heat treatment) | +C (cold-drawn / hard) |
| Stress-relieved | BKS (stress-relieved cold-drawn) | +LC (cold-drawn light) |
| Typical applications | Hydraulic cylinders, automotive, instrument piping | Hydraulic cylinders, automotive, instrument piping |
| Inspector acceptance | Accepted on legacy drawings; new specs prefer EN | Required for CE marking and current ISO 9001 audits |
The renaming chronology
DIN 2391-1 (dimensions) and DIN 2391-2 (technical delivery conditions) were the German national standards for seamless precision steel tube from the 1960s onwards. As the European Committee for Standardization (CEN) consolidated national specs into EN standards through the 1990s and 2000s, DIN 2391 was superseded by EN 10305-1 “Steel tubes for precision applications — Part 1: Seamless cold drawn tubes” published in 2002 and made fully binding by 2006. The withdrawal did not change the product; it changed the document reference, the grade-naming scheme and the tolerance-class labels.
Tolerance classes — NBK / BKS / BKW vs +N / +LC / +C
The DIN 2391 letter-codes describe the supply condition. NBK (Normalisiert / normalised) is heat-treated to give a uniform soft microstructure with slightly looser OD tolerance — picked when the buyer will form, bend or weld the tube. BKW (Bright cold-drawn, no heat-treatment) is the tightest tolerance condition with the highest hardness, used for hydraulic-cylinder tube where ID and OD are critical. BKS (Bright stress-relieved) sits between — most of the residual stress is removed but dimensional tolerance is preserved. EN 10305-1 renames these to +N, +C and +LC but the underlying mill processing routes are identical.
When DIN 2391 callouts still appear
Legacy German engineering drawings from before 2006, aftermarket parts catalogues for older equipment, replacement-component specifications for hydraulic cylinders supplied 10+ years ago, and Indian OEM drawings that still inherit DIN nomenclature all routinely call DIN 2391 St52 or DIN 2391 St37.4. Mills supply tube that simultaneously meets DIN 2391 and EN 10305-1 — the MTC will typically reference both standards so the buyer can use either reference downstream. See our DIN 2391 St52 precision tube page for stock details.
When to specify EN 10305-1 instead
For any new design, new project specification or RFQ that will be reviewed against current European procurement standards, specify EN 10305-1 with the appropriate grade (E235, E275 or E355) and condition (+N, +C or +LC). CE-marking documentation, ISO 9001 quality-management audits and current OEM purchasing systems expect the in-force standard reference. The mill ships the same product either way; specifying EN keeps the paperwork current.
FAQ — DIN 2391 vs EN 10305-1
What is the difference between DIN 2391 and EN 10305?
Is DIN 2391 still a valid specification?
What does the tolerance class NBK / BKS / BKW mean on DIN 2391 tube?
Which European grade matches DIN 2391 St52?
When should I specify EN 10305-1 instead of DIN 2391?
Related pages
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