What Is P355NB Steel?
P355NB is a hot-rolled, normalized non-alloy structural steel grade specified under the EN 10120 standard, designed specifically for the manufacture of welded gas cylinders. It carries the material number 1.0557 in the European steel numbering system (Werkstoffnummer).
Grade Designation Breakdown
The name "P355NB" follows a systematic European steel naming convention, where each character conveys specific information about the material:
P
Pressure application
355
Min. yield strength 355 MPa
N
Normalized delivery
B
Gas cylinder quality
Standard: EN 10120:2017
EN 10120 is the European Standard specifying requirements for hot-rolled steel sheet and strip (up to 5 mm thickness) intended for the manufacture of welded gas cylinders. The standard defines four main grades in the PNB series, each with increasing strength levels:
| Grade | Material No. | Min. Yield Strength (MPa) | Tensile Strength (MPa) |
|---|---|---|---|
| P245NB | 1.0111 | 245 | 360 – 480 |
| P265NB | 1.0423 | 265 | 410 – 500 |
| P310NB | 1.0488 | 310 | 460 – 550 |
| P355NB | 1.0557 | 355 | 510 – 620 |
💡 Key Takeaway
P355NB is the highest-strength grade in the EN 10120 PNB series, offering the best strength-to-weight ratio for gas cylinder manufacturing. It allows designers to reduce wall thickness while maintaining pressure resistance, resulting in lighter and more cost-effective cylinders.
Chemical Composition of P355NB Gas Cylinder Steel
The chemical composition of P355NB steel is carefully controlled to balance strength, ductility, weldability, and formability - all essential properties for gas cylinder production.
Chemical Composition Requirements (EN 10120)
| Element | Symbol |
Maximum / Range (%) |
|---|---|---|
| Carbon | C | ≤ 0.20 |
| Manganese | Mn | ≥ 0.70 (≤ 1.60) |
| Silicon | Si | ≤ 0.50 |
| Phosphorus | P | ≤ 0.025 |
| Sulfur | S | ≤ 0.015 |
| Aluminum | Al | ≥ 0.020 |
Mechanical Properties of P355NB steel
The mechanical properties of P355NB steel are guaranteed under EN 10120 for material in the normalized delivery condition, with thickness ranging from 1.7 mm to 5 mm.
Mechanical Properties at Room Temperature
| Property | Value (t = 1.7 – 3 mm) | Value (t = 3 – 5 mm) | Direction |
|---|---|---|---|
| Yield Strength (min.) | ≥ 355 MPa | ≥ 355 MPa | Transverse |
| Tensile Strength | 510 – 620 MPa | 510 – 620 MPa | Transverse |
| Elongation (min.) | ≥ 19% | ≥ 24% | Transverse |
Note: t = thickness. Properties tested on transverse specimens (perpendicular to rolling direction), which represents the more critical direction for forming operations.
Property Interpretation
- Yield strength ≥ 355 MPa - P355NB is classified as a high-strength steel for gas cylinder applications. This high yield strength allows for thinner wall designs, reducing material weight and cost.
- Tensile strength 510–620 MPa - The controlled tensile range ensures consistent quality and prevents excessive hardness that could compromise weldability.
- Elongation ≥ 19–24% - The high ductility enables deep drawing and cold forming operations required for cylinder manufacturing, with lower risk of cracking or necking.
⚙️ Formability Indicator
The combination of high strength (355 MPa yield) and good ductility (up to 24% elongation) gives P355NB excellent formability - a crucial property for deep-drawing LPG cylinder bodies without defects.
Key Features and Advantages of P355NB Steel for LPG Cylinder
P355NB steel offers a unique combination of properties that make it ideal for gas cylinder manufacturing. Here are its most important features
Excellent Deep Drawability
P355NB is classified as a deep-drawable steel grade. Its fine-grained microstructure and controlled chemical composition allow it to undergo significant plastic deformation during deep drawing operations without cracking or necking. This is essential for forming the domed ends and cylindrical bodies of gas cylinders.
Non-Ageing Properties
One of the distinguishing features of gas cylinder steels is their non-ageing characteristic. P355NB maintains its ductility and mechanical properties over time, even after forming and welding operations. This ensures that the cylinder's structural integrity remains stable throughout its service life, typically 15–30 years.
Outstanding Weldability
With a low carbon equivalent (CEV), P355NB exhibits excellent weldability using all common industrial welding processes, including:
- Gas Metal Arc Welding (GMAW / MIG)
- Submerged Arc Welding (SAW)
- Resistance Welding
- Laser Welding
The weld seam retains sufficient strength and toughness, and post-weld heat treatment (normalization or stress-relief annealing) can be applied without compromising the base material properties.
Applications of P355NB Steel Plate
P355NB steel is specifically engineered for welded gas cylinder manufacturing. Its primary applications include:
LPG Gas Cylinders
The most common application of P355NB steel is in the production of liquefied petroleum gas (LPG) cylinders for both domestic and commercial use. These cylinders store propane, butane, or LPG mixtures under pressure, requiring a material that can withstand cyclic loading and maintain structural integrity over decades of service.
Industrial Gas Cylinders
P355NB is used for manufacturing welded cylinders for industrial gases such as compressed natural gas (CNG), oxygen, nitrogen, and other industrial gases where high strength and reliable performance are essential.
Fire Extinguisher Cylinders
The deep drawability and consistent quality of P355NB make it suitable for fire extinguisher pressure vessels, where safety and reliability are paramount.
Small Pressure Vessels
Beyond gas cylinders, P355NB can be used for various small pressure vessel applications that require good formability and weldability, such as air receivers, buffer tanks, and pressurized storage containers.
Propane & Butane Storage Tanks
For smaller propane and butane storage tanks that are formed from sheet or coil, P355NB provides the necessary strength and corrosion resistance for long-term outdoor service.
International Equivalent Grades of P355NB steel
| Standard / Country | Equivalent Grade | Notes |
|---|---|---|
| Japanese (JIS G 3116) | SG365 | Closest equivalent for gas cylinder steel |
| Chinese (GB) | HP345 | Chinese standard for welded gas cylinders |
| German (DIN) | 19Mn6 (old) | Historical equivalent; P355NB is the modern grade |
⚠️ Important: Don't Confuse These Grades!
P355NB is NOT the same as P355N, P355GH, P355NH, or P355NL1. While these grades share the "P355" designation (indicating 355 MPa yield strength), they belong to different standards and serve different purposes:
- P355NB - EN 10120, gas cylinder steel (deep drawing, non-ageing)
- P355N / P355NH / P355NL1 - EN 10028-3, fine-grain pressure vessel steels (for thicker plates, elevated/low temperature service)
- P355GH - EN 10028-2, high-temperature boiler and pressure vessel steel
Always verify the full grade designation and standard before specifying or ordering.
Summary
P355NB (material number 1.0557) is the highest-strength grade in the EN 10120 series of gas cylinder steels. With a minimum yield strength of 355 MPa, excellent deep drawability, outstanding weldability, and non-ageing properties, it has established itself as the material of choice for high-performance LPG and industrial gas cylinder manufacturing.
Its unique balance of properties enables cylinder manufacturers to produce lighter, stronger, and more cost-effective pressure vessels while maintaining the highest safety standards. When sourcing P355NB steel, always ensure it complies with EN 10120:2017 and verify the full grade designation to avoid confusion with other P355-series steels.
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