DNV AH550 Steel Plate | NV A550 Marine Steel Plate

Technical Reviewer: Jack | SHH STEEL Material Solutions Department · Updated: 2026-10-27

DNV AH550, officially stamped NV A550, is an extra-high-strength hull structural steel under DNV rules for classification – Ships. IACS Unified Requirement W16 covers this strength band (yield 550 N/mm²); DNV implements corresponding requirements in its metallic-materials rules for NV grades. Minimum yield is 550 N/mm², tensile 670–835 N/mm², elongation 16 % min., with Charpy at 0 °C. It sits above the AH–FH 40 higher-strength family (max yield 390 N/mm² under UR W11). Commercial enquiries use DNV AH550 or NV A550 marine steel plate.

Equivalent grades: DNV NV A550 ≈ ASTM A131 (no direct grade at this strength) ≈ ABS AH55 / AQ56 ≈ LR AH55 ≈ BV A550 ≈ CCS AH550 ≈ NK KA550 ≈ KR RA56 ≈ RINA A550 ≈ GB 712 Grade AH550 (all class-approved, stamped by the issuing society).

Naming note: commercial and yard enquiries use AH550 / NV A550. ABS often titles the same yield band as AH55 (official ABS AQ56). ASTM A131 has no grade at this strength. Write NV A550 on the DNV purchase order.

NV A550 is almost always produced TM or QT by a DNV-approved mill to order rather than taken from open stock. SHH STEEL sources extra-high-strength plate from mills with DNV approval for the grade, under Surveyor attendance, with optional multi-class and EN 10204 3.2.

Maximum plate thickness depends on delivery condition and the mill’s DNV approval sheet — typically TM up to about 100–150 mm and QT higher, but treat the approval limit as governing (UNCERTAIN without local DNV PDF). Primary commercial scope we quote is within ~100 mm unless the mill approval and DNV survey confirm otherwise.

Product (check) analysis column: DNV rules specify heat (ladle) limits; no numeric product-analysis tolerance is tabulated in the rule text reviewed, so product (check) limits are agreed on the order or taken from the approved mill specification and certificate.

Source: IACS UR W16 chemistry envelope as implemented by DNV for NV A550 — exact ladle limits depend on delivery condition (TM vs QT) and toughness letter. Values above are the rule envelope; the purchase order and mill approval sheet govern. Product analysis: as per standard and mill certificate. All intentionally added elements and Ceq (or CET/Pcm) are to be reported. Flagged UNCERTAIN where a local DNV Pt.2 PDF was not available to quote paragraph numbers.

Final acceptance values are as per standard and mill certificate under DNV survey. For thick plate, additional mid-thickness tensile/impact positions may apply — confirm at enquiry. Test frequency is typically each heat-treatment batch of same cast/condition/thickness.

Charpy temperature is fixed by the grade letter: AH → 0 °C. Indicative average energy 55 / 37 J (L/T) from IACS UR W16 aligned tables — confirm exact figures on the DNV-endorsed certificate. Unlike UR W11 higher-strength tables, EHS energies are usually stated as a single pair rather than rising through many thickness bands; thick-plate mid-thickness rules may still apply.

Allowed delivery conditions are primarily TM and QT within the mill’s approved thickness. Vacuum degassing is commonly required for higher categories and for thicker plate — follow the mill approval. State TM or QT on the PO; do not leave delivery condition blank for AH550.

DNV applies IACS UR W13 unless the PO states otherwise (confirm minus class on PO; see UNCERTAIN). Surface repair limits follow the applicable DNV / UR rules; state primer/surface class on receipt if needed. For QT plate, avoid notching cut edges; follow mill cutting recommendations.

Extra-high grades such as NV A550 are rarely held as open stock; the table shows the size bands we quote against scheduled mill rolls. Quantities are indicative. All plates sold as DNV class carry NV A550 marking and a DNV-endorsed mill certificate. Typical mill lead time including DNV survey is about 6–10 weeks (commercial estimate — confirm per quote). Send your size list and we will reply with the next available rolling slot.

Packing. Plates are bundled by size with steel strapping, marked with heat number, grade and DNV stamp, and shipped in containers (up to about 11.8 m) or break bulk for longer plates.

These are comparisons, not substitutions. Always write the DNV stamp NV A550 on the PO, plus any additional society stamps you need.

Use AH550 when scantlings need yield 550 N/mm². Do not substitute AH40/EH40 (390 N/mm² max under UR W11) without redesign. Within the 550 band, letter choice is toughness: AH 0 °C → DH −20 °C → EH −40 °C → FH −60 °C. Neighbouring strengths are 510 / 590 — swapping strength needs class approval of the drawing change.

When DNV NV A550 is the right choice: typical uses are higher-strength hull girder plating (deck, bottom, side shell) of bulk carriers, tankers and container ships in normal service where 0 °C toughness suffices at extra-high yield — weight-critical hull girder strakes, polar and ice-class primary structure, and offshore units where 550 N/mm² is specified. See offshore platform steel plates for related offshore grades.

Typical uses: higher-strength hull girder plating (deck, bottom, side shell) of bulk carriers, tankers and container ships in normal service where 0 °C toughness suffices at ReH 550 N/mm²; strength deck and shell of large container ships and gas carriers where EHS is specified; ice-belt and polar primary structure; selected offshore members. See offshore platform steel plates.

Welding: NV A550 is an extra-high-strength steel supplied TM or QT. Welding must follow a procedure qualified for this grade and approved by DNV. Ceq/CET/Pcm from the ladle analysis govern hydrogen control, preheat, interpass and heat-input windows. Too much heat input can lower HAZ strength and toughness in QT plate. We send the certificate values before shipment so your welding engineer can confirm the procedure fits. We do not give generic preheat temperatures.

Processing notes: NV A550 owes its strength to TM or QT processing. Hot forming or stress relieving above the tempering temperature can lower the properties, so any heat treatment after delivery must be agreed with the mill and DNV. Cold forming and bending radii should follow the mill’s data sheet. Thermal cutting parameters follow mill recommendations so the cut edge is not damaged.

DNV survey at the mill; certificate includes ladle chemistry, Ceq/CET/Pcm as applicable, tensile (ReH ≥ 550, Rm 670–835, A ≥ 16 %), Charpy at 0 °C, delivery condition, UT if ordered, and plate identity. Optional EN 10204 3.2 and multi-class.

On receipt: verify NV A550 stamp, 0 °C impact data, energies ≥ ordered/certified minima, thickness within tolerance, Surveyor endorsement, UT report if required.

Charpy acceptance for NV A550 (IACS UR W16 aligned; actual results as per standard and mill certificate):

For thickness above ~100 mm, additional mid-thickness rules may apply — confirm at enquiry. If the PO asked for 3.2, the inspector’s report must name the witnessed tests and plate numbers.

• Ordering “DNV 550” without the letter (AH) — toughness missing
• Writing ABS AH55 on a DNV-class PO without arranging ABS survey (wrong stamp)
• Leaving delivery condition blank (TM vs QT changes price, mill and properties)
• Expecting ASTM A131 dual-cert at this strength (A131 has no EHS grade)
• Skipping UT on thick EHS plate when the yard specification requires it
• Missing DNV endorsement / assuming GB 712 Grade AH550 alone is class approval

Write grade, TM/QT, impact temperature, UT level and certificate type into the PO.

Specify DNV AH550 (NV A550), delivery condition TM or QT, thickness × width × length × qty, Z/U/UT level, multi-class, certificate type, port. Send to SHH STEEL.

Sample PO line: “Steel plate to DNV rules for classification – Ships, Grade AH550 (NV A550), delivery condition [TM/QT], 30 × 2500 × 12000 mm, 10 pcs, marked NV A550, certificate endorsed by DNV Surveyor, EN 10204 3.2 [if required], UT [EN 10160 / ASTM A578 level], CIF [port].”

DNV AH550 / NV A550 is extra-high-strength hull steel (IACS UR W16 band): yield 550 N/mm², tensile 670–835 N/mm², Charpy at 0 °C, marking NV A550. Lock on the order: TM or QT, UT level, Ceq/CET/Pcm reporting, multi-class and 3.2.

SHH STEEL supplies NV A550 plate rolled by Chinese mills that hold DNV works approval for the grade, so every plate carries the NV A550 stamp and a Surveyor-endorsed certificate traceable to its heat. Sizes are scheduled into the mill rolling cycle with DNV survey arranged at the mill. On request we add EN 10204 3.2 certificates with third-party witness (TÜV, SGS, BV or another agency) and joint certification with ABS, LR, BV, CCS, NK, KR or RINA.

We accept T/T or L/C payment, with inspection before the balance is paid. Small quantities in several sizes, or NV A550 with neighbouring EHS sizes or higher-strength AH–FH 40 where the drawing allows, can be combined in one container. A dedicated coordinator sends production and shipping updates with photos, and our own workshop cuts to size, bevels, drills and machines plates before shipment so they arrive ready for fabrication.

Request a quotation: send SHH STEEL the grade (NV A550 / DNV AH550), thickness, width, length, quantity per size and destination port, plus any multi-class, Z/U or 3.2 certificate requirement. We reply with price, available stock and lead time.