What is S355G10+N Offshore Steel Plate?
S355G10+N is a premium normalized offshore structural steel plate manufactured to EN 10225 standard, specifically engineered for heavy-load critical nodes in harsh cold offshore environments such as the North Sea and Arctic regions.
As the highest integrity grade in the S355G offshore steel family, it delivers guaranteed fracture toughness after post-weld heat treatment (PWHT), ultra-low sulfur and phosphorus content, and uniform through-thickness mechanical properties up to 150mm thickness. It directly solves common industry failures - toughness degradation after PWHT, lamellar tearing in thick plates, and low-temperature brittle fracture - that frequently occur when using standard S355G9 or general structural steel in demanding offshore projects.
It is the specified grade for major offshore oil & gas jacket platforms, thick-section offshore wind foundations, and Arctic LNG structures where weld stress relief and extreme cold resistance are mandatory.
Full Breakdown of S355G10+N Grade Code
Each character in the grade name carries clear definition per EN 10225 specification:
S - Structural steel, for load-bearing welded construction
355 - Minimum yield strength of 355MPa for thickness ≤ 16mm; strength values decrease moderately with increased thickness per standard tables
G - Offshore platform dedicated grade, compliant with EN 10225 additional toughness and purity requirements for offshore service
G10 - Top-tier performance grade in the S355G series, with three unique mandatory requirements:
- Mandatory CTOD (Crack Tip Opening Displacement) test after simulated PWHT, ensuring crack resistance remains stable after post-weld stress relief
- Ultra-strict impurity limits: S ≤ 0.005%, P ≤ 0.015%, far tighter than G8 and G9 grades
- Minimum 50J transverse Charpy V-notch impact energy at -40℃
- +N - Normalized delivery condition. Plates undergo full normalizing heat treatment after rolling, producing a uniform fine-grain microstructure across the full plate thickness. This ensures consistent mechanical properties even in heavy-gauge sections, and maintains performance stability after on-site welding and PWHT.
What standard applies to CTOD testing of S355G10+N?
CTOD testing for S355G10+N is generally performed in accordance with BS 7448 or ISO 12135, on specimens that have undergone simulated PWHT thermal cycles to replicate actual post-welding conditions.
Now, you might ask: "Isn't S355G9 or TMCP good enough for most jobs?" The answer is a firm "No" – if your project involves thick nodes, Arctic temperatures, or mandatory PWHT. Below are five exclusive advantages that elevate S355G10+N far beyond a simple specification upgrade – they are the very reasons why engineers choose it when failure is simply not an option.
Exclusive Unique Advantages of S355G10+N structural steel
S355G10+N is not simply an upgraded version of common offshore steel - it addresses specific failure risks that lower grades cannot resolve.
1. Stable toughness retention after full PWHT
This is the most irreplaceable advantage of S355G10+N. Large offshore tubular joints and thick-node structures almost always require overall post-weld stress relief heat treatment. Standard S355G9 and TMCP-grade steels suffer significant toughness drop after such thermal cycles, raising brittle fracture risk. S355G10+N is qualified with PWHT-simulated CTOD testing, and its impact and fracture toughness remain within specification after heat treatment - making it the mandatory grade specified by North Sea design institutes for critical welded nodes.
2. Uniform through-thickness properties up to 150mm maximum thickness
Thanks to the normalizing process, S355G10+N maintains consistent microstructure and mechanical performance from surface to core, even at 150mm thickness. By comparison, S355G10+M (TMCP delivery) is only certified up to 100mm thickness, and core properties degrade noticeably in heavier gauges. For thick-plate foundations and load-bearing nodes above 100mm, S355G10+N is the only qualified S355-grade option.
3. Ultra-clean composition for superior anti-lamellar tearing performance
With sulfur controlled below 0.005% and phosphorus below 0.015%, combined with calcium treatment for sulfide shape control, S355G10+N has inherently excellent through-thickness ductility. It can be supplied with Z15, Z25 or Z35 Z-direction property certification, effectively eliminating lamellar tearing risk during multi-pass thick-plate welding - a common and costly failure mode in offshore jacket fabrication.
4. Reliable low-temperature toughness for Arctic and North Sea service
S355G10+N guarantees minimum 50J transverse impact energy at -40℃, and delivers stable fracture toughness under prolonged dynamic fatigue and cyclic loading in marine environments. This makes it suitable for year-round operation in sub-zero offshore climates, avoiding catastrophic brittle fracture accidents caused by low-temperature material embrittlement.
5. Low carbon equivalent for high welding safety
With strictly controlled carbon and alloy content and low CEV value, S355G10+N has low cold cracking sensitivity during welding. It reduces preheating requirements for on-site thick-plate welding, simplifies construction procedures, and lowers overall fabrication cost for offshore EPC contractors and steel fabricators.
Chemical Composition of S355G10+N offshore steel
| Element | Value | Element | Value |
|---|---|---|---|
| C | ≤0.12 | Si | 0.15–0.55 |
| Mn | ≤1.65 | P | ≤0.015 |
| S | ≤0.005 | Ni | ≤0.7 |
| Cr | ≤0.2 | Mo | ≤0.08* |
| Cu | ≤0.3 | Nb | ≤0.03 |
| V | ≤0.06 | Ti | ≤0.025 |
| Alt | 0.015–0.055 | N | ≤0.010 |
| B | ≤0.0005 | Ca | ≤0.005 |
| Pb | ≤0.010 | Ceq max | ≤0.43 |
| Pcm max | ≤0.22 |
Mechanical Properties of S355G10+N steel
| Thickness (mm) | Delivery condition | ReH (MPa) min | ReH/Rm ratio max | Rm (MPa) | A (%) min | Impact direction | Impact temp. (°C) | Akv (J) min |
|---|---|---|---|---|---|---|---|---|
| ≤ 16 | Normalized or normalized rolling | 355 | 0.87 | 470–630 | 22 | Transverse | –40 | 50 |
| > 16 – 25 | Normalized or normalized rolling | 355 | 0.85 | 470–630 | 22 | Transverse | –40 | 50 |
| > 25 – 40 | Normalized or normalized rolling | 345 | 0.85 | 470–630 | 22 | Transverse | –40 | 50 |
| > 40 – 63 | Normalized or normalized rolling | 335 | 0.85 | 470–630 | 22 | Transverse | –40 | 50 |
| > 63 – 100 | Normalized or normalized rolling | 325 | 0.85 | 470–630 | 22 | Transverse | –40 | 50 |
| > 100 – 150 | Normalized or normalized rolling | 320 | 0.85 | 460–620 | 22 | Transverse | –40 | 50 |
Typical Application Scenarios of S355G10+N Offshore Plate
S355G10+N is specified for high-stakes, thick-section offshore structures where failure is not acceptable.
Fixed offshore oil & gas platform heavy tubular joints
Primary jacket nodes, pile sleeves and riser supports for North Sea and Chile Arctic platforms, where full post-weld stress relief is mandatory per project specification.
Thick-section offshore wind jacket foundations
Main load-bearing nodes and transition piece components with thickness over 100mm, especially for wind farms in cold northern waters with high fatigue load requirements.
Arctic LNG and heavy offshore structures
Low-temperature storage supports, FPSO heavy load brackets, and other thick-plate structures serving in permanently cold marine environments.
When not to choose S355G10+N?
For plates thinner than 60mm, components that do not require overall PWHT, or general secondary structural members, S355G10+M or S355G9 will offer better cost efficiency. We can help you select the most economical grade matching your project requirements.
S355G10+N vs S355G10+M vs S355G9
| Parameter | S355G10+N | S355G10+M | S355G9+N / +M |
|---|---|---|---|
| Delivery process | Normalized heat treatment | TMCP (thermo-mechanical controlled rolling) | Normalized or TMCP |
| Max certified thickness | 150mm | 100mm | 100mm (M) / 150mm (N) |
| PWHT CTOD requirement | Mandatory, qualified after PWHT | Not required after PWHT | No PWHT CTOD requirement |
| S & P limits | S≤0.005%, P≤0.015% | S≤0.005%, P≤0.015% | S≤0.010%, P≤0.020% |
| Typical application | Thick critical nodes requiring overall PWHT; Arctic & North Sea heavy structures | Medium-thickness secondary structures; no overall PWHT | General offshore decks, braces and non-critical components |
| Relative price & lead time | Higher price, longer lead time | Moderate price, shorter lead time | Lower price, shortest lead time |
Can S355G10+M replace S355G10+N for thick plates over 100mm?
No. S355G10+M is produced via TMCP process with a maximum certified thickness of 100mm, and its toughness degrades noticeably after full PWHT. For components thicker than 100mm or structures requiring overall post-weld stress relief, S355G10+N is the only qualified option in the S355G series.
What is the price gap between S355G10+N and S355G9 steel plate?
S355G10+N is typically 25%–40% more expensive per ton than S355G9, due to stricter purity control, normalizing heat treatment, and additional CTOD testing. The exact gap depends on thickness, quantity and additional inspection requirements.
GNEE Steel Supply & Export Support for S355G10+N
As a professional supplier of EN 10225 offshore steel plates with direct access to major certified Chinese mills, we provide full-range supporting services to streamline your procurement:
Full certification package
EN 10204 3.2 MTC issued by third-party inspection bodies, plus DNV GL / LR mill approval documentation, fully compliant with offshore project tender requirements.
Customized inspection options
100% UT ultrasonic inspection per EN 10160, Z-direction tensile test, -40℃ Charpy impact test, and CTOD fracture toughness test with full original reports.
Value-added processing
Precision plasma / flame / laser cutting, V/X/K bevel machining, shot blasting and shop primer pretreatment, ready for direct fabrication upon delivery.
Precision cutting: Flame, plasma and laser cutting according to CAD drawings, realizing near-net-shape blanking and reducing on-site material loss by over 30%.
Export logistics & packaging
Standard seaworthy export packaging, fumigation-free wooden supports, and full sea freight / FOB / CIF solution for offshore project cargo.
Your project's success hangs on the steel you choose – and when PWHT, Arctic cold, and 150mm thickness are on the table, there is literally no substitute. Don't gamble on performance; secure the grade that the world's most demanding offshore operators trust. With our mill‑direct supply, full traceability, and on‑time delivery, we make it easy to specify S355G10+N for your next critical node.
Full EN 10225 Offshore Steel Grades Available at GNEE Steel
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Standard |
Steel Grade |
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EN10225 |
S355G2+N |
S355G3+N |
S355G7+N |
S355G8+N |
|
S355G9+N |
S355G5+M |
S355G6+M |
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|
S355G7+M |
S355G8+M |
S355G9+M |
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|
S420G1+M |
S420G2+M |
S420G1+QT |
S420G2+QT |
|
|
S460G1+M |
S460G2+M |
S460G1+QT |
S460G2+QT |
|
