DIN 2391 Seamless Precision Steel Tube

Keywords: DIN 2391 Cold-rolled or Cold-drawn Seamless Precision Steel Tube, DIN 2391-C ST35 ST45 ST52 Pipe
DIN 2391 is a German technical specification for cold-rolled or cold-drawn precision seamless steel tubes, mainly used in hydraulic systems, precision machinery, automotive industry and other fields. This standard has strict requirements on the dimensional accuracy, surface quality, mechanical properties and chemical composition of steel tubes, and is suitable for applications requiring high precision, high finish and high strength.

DIN 2391-C (Grade C) is the quality grade of high-precision seamless precision steel tubes specified in the German industrial standard DIN 2391, indicating that the steel tube must meet additional technical requirements (such as higher dimensional accuracy, mechanical properties, etc.) and provide an inspection certificate (such as DIN 50049 3.1.B type).



DIN 2391-C requirements:
1. Dimensional accuracy: Strict outer diameter/wall thickness tolerances, suitable for high-precision hydraulic systems.
2. Surface quality: No oxide layer on the inner and outer walls, high finish, can be directly used in hydraulic pipelines.
3. Mechanical properties: Need to pass tensile, flattening, flaring and other tests (such as St 35 steel tensile strength ≥ 480MPa).
4. Inspection requirements: Chemical composition analysis, non-destructive testing (UT/RT) and other certificates must be provided.

Difference from DIN 2391-A: DIN 2391-A only has basic size and surface requirements, suitable for general use, and no inspection certificate requirements.

Material grade:
DIN 2391 covers a variety of carbon steel and low alloy steel materials, common grades include: DIN 2391 (ST35, ST45, ST52).

Size range:
Outer diameter: 4 mm ~ 260 mm
Wall thickness: 0.5 mm ~ 25 mm

Standard length: 5 m ~ 8 m (usually 6 m fixed length)


DIN 2391 Seamless Precision Steel Tube


Main features of DIN 2391:

1. High-precision dimensional control
Outer diameter tolerance: ±0.08 mm; Inner diameter tolerance: ±0.10~0.15 mm; Wall thickness tolerance: ±0.10~0.15 mm

2. Excellent surface quality
There is no oxide layer on the inner and outer walls, and the finish is high, which can be directly used in hydraulic systems.
Surface treatments such as phosphating and galvanizing can be performed to improve rust resistance.

3. Excellent mechanical properties
No deformation during cold bending, no cracks during expansion and flattening, suitable for high-pressure hydraulic systems.

4. Various delivery states
NBK (normalizing), BKS (stress relief annealing), BK (cold drawn hard state), etc.

Chemical composition of DIN 2391:

Steel grade
Chemical composition (%)
Main characteristics
Approximate material standard (GB)
St 35
C≤0.17, Si≤0.35, Mn≥0.40
Low carbon steel, good weldability, suitable for low pressure hydraulic system
20# 
St 45
C≤0.21, Si≤0.35, Mn≥0.40
Medium carbon steel, high strength, suitable for mechanical structural parts
45#
St 52
C≤0.22, Si≤0.55, Mn≤1.60
High strength low alloy steel, good wear resistance
16Mn
St 52.4
C≤0.20, Si≤0.55, Mn≤1.60
Higher strength, suitable for engineering machinery
Q355

Note: Some special grades such as St 30Si and St 30Al also belong to the DIN 2391 standard and are suitable for specific precision mechanical needs.

Mechanical property requirements of DIN 2391:

The mechanical property requirements for different material grades and delivery states are as follows (taking St 35, St 45, and St 52 as examples):

Steel grade
Delivery state 
Tensile strength (MPa)
Yield strength (MPa)
Elongation (%)
St 35
NBK
340-470
≥235
≥25
St 35
BK ≥480
- ≥6
St 45
NBK
440-570
≥255
≥21
St 45
BK ≥580
- ≥5
St 52
NBK
490-630
≥355
≥22
St 52
BK ≥640
- ≥4

Note:
BK (cold drawn hard state) is usually used in scenarios with high precision and high hardness requirements, but has low plasticity.
NBK (normalizing) is suitable for applications that require good comprehensive mechanical properties.

Similar standards:
European EN 10305-1: a modernized version of DIN 2391, some steel grades can be directly matched.

China GB/T 3639 (the early version is similar to DIN 2391, and the 2021 version has turned to EN 10305-1).


EN 10305-1: E235(St 35)、E355(St 52)

ASTM A519: 1020(St 35)、1045(St 45)

GB/T 3639: 20#(St 35)、45#(St 45)



Application areas of DIN 2391:
Hydraulic system: high-pressure oil pipes, hydraulic cylinders, ship steering gears, engineering machinery hydraulic pipelines.
Automotive industry: fuel injection pipes, brake oil pipes.
Precision machinery: high-precision transmission shafts, pneumatic components.
Engineering machinery: hydraulic pipelines for excavators and cranes.
Shipbuilding: marine hydraulic system pipelines.

Conclusion:

DIN 2391 is an important standard for high-precision seamless steel pipes, suitable for hydraulic, machinery, automotive and other industries. Its material grade (such as St 35, St 45, St 52) and mechanical properties are strictly required, and the appropriate delivery state (NBK, BK, etc.) can be selected according to different working conditions. For more detailed technical parameters, please refer to the DIN 2391-1994 standard document.


Read more: ASTM A519 Seamless Mechanical Tube or Cold Drawn Seamless Tube Size Chart for Mechanical & Hydraulic

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