KR RF56 Shipbuilding Steel Plate

Technical Reviewer: Jack | SHH STEEL Material Solutions Department · Updated: 2026-10-28

KR RF56 (RF56) is the KR −60 °C extra-high-strength grade at 550 N/mm² yield, with F-route chemistry control for Arctic and polar primary structure. The strength band follows IACS Unified Requirement W16; KR implements it in the Rules for the Classification of Steel Ships, Pt 2 (Materials). Minimum yield is 550 N/mm², tensile 670–835 N/mm², elongation 16 % min., Charpy at −60 °C. It sits above the RA–RF 40 higher-strength family (max. 390 N/mm² under UR W11). Commercial enquiries use KR RF56 or KR RF56 shipbuilding steel plate.

Equivalent grades: KR RF56 ≈ ASTM A131 (no direct grade at this strength) ≈ ABS FH55 / FQ56 ≈ DNV NV F550 ≈ LR FH55 ≈ BV F550 ≈ CCS FH550 ≈ NK KF550 ≈ RINA F550 ≈ GB 712 Grade FH550 (all class-approved, stamped by the issuing society).

Naming note: KR’s EHS numbers 43 / 47 / 51 / 56 / 63 / 70 follow the ABS-style official yield-class numbering (420 → 43, 460 → 47, 500 → 51, 550 → 56, 620 → 63, 690 → 70), so 550 N/mm² becomes RF56. ABS titles the same band FH55 commercially (official FQ56); DNV and CCS use FH550; BV uses F550 (no H); LR uses FH55. ASTM A131 has no grade at this strength. The 43…70 mapping is consistent with ABS practice but is UNCERTAIN against a local KR Rules text — confirm the exact designation on the mill’s KR approval certificate before ordering.

KR RF56 is almost always produced TM or QT by a KR-approved mill to order rather than taken from open stock. SHH STEEL sources extra-high-strength plate from mills with Korean Register approval for the grade, under Surveyor attendance, with optional multi-class (ABS/DNV/LR/BV) and EN 10204 3.2.

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

Product (check) analysis column: KR Rules (Part 2) 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 (ABS 2-1-8 Table 4A aligned — the same yield-class numbering KR follows) as implemented by Korean Register for KR RF56 — exact ladle limits depend on delivery condition (TM vs QT) and toughness letter. Class work follows the KR / UR W16 envelope on the mill approval. Values above are the rule envelope with every alloy / microalloy row listed; 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 KR Rules text was not available to quote paragraph numbers.

Final acceptance values are as per standard and mill certificate under KR 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: FH → −60 °C. Indicative average energy 55 / 37 J (L/T) from IACS UR W16 aligned tables — confirm exact figures on the KR-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 RF56.

Korean Register applies IACS UR W13 unless the PO states otherwise (confirm minus class on PO; see UNCERTAIN). Surface repair limits follow the applicable KR / 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 KR RF56 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 KR class carry KR RF56 marking and a Korean Register–endorsed mill certificate. Typical mill lead time including KR 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 KR 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 KR stamp KR RF56 on the PO, plus any additional society stamps you need. Other KR EHS siblings are linked via KR until each product page is live.

Use RF56 when scantlings need yield 550 N/mm². Do not substitute RA40/RE40 (390 N/mm² under UR W11) without redesign. Within the 56 class, letter choice is toughness: RA 0 °C → RD −20 °C → RE −40 °C → RF −60 °C. Neighbouring KR strengths are RF51 / RF63 — swapping strength needs class approval of the drawing change.

When KR RF56 is the right choice: typical uses are extra-high-strength polar and Arctic primary structure, ice belts and cold-region offshore members needing −60 °C toughness 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: extra-high-strength polar and Arctic primary structure, ice belts and cold-region offshore members needing −60 °C toughness 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: KR RF56 is an extra-high-strength steel supplied TM or QT. Welding must follow a procedure qualified for this grade and approved by KR. 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: KR RF56 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 KR. 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.

KR survey at the mill; certificate includes ladle chemistry (full alloy set + Ceq/CET/Pcm as applicable), tensile (ReH ≥ 550, Rm 670–835, A ≥ 16 %), Charpy at −60 °C, delivery condition, UT if ordered, and plate identity. Optional EN 10204 3.2 and multi-class with ABS/DNV/LR/BV.

On receipt: verify KR RF56 stamp, −60 °C impact data, energies ≥ ordered/certified minima, thickness within tolerance, KR Surveyor endorsement, UT report if required.

Charpy acceptance for KR RF56 (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 “KR 56” or “KR 550” without the letter — toughness class missing
• Writing FH550 or F550 on a KR PO — the KR designation is RF56; the mill must stamp KR RF56
• Writing ABS FH55 or DNV FH550 or LR FH55 or BV F550 on a KR-class PO without arranging that society’s survey (wrong stamp)
• Leaving delivery condition blank (TM vs QT changes price, mill and properties)
• Expecting ASTM A131 dual certification at this strength (A131 has no EHS grade)
• Skipping UT on thick EHS plate when the yard specification requires it
• Missing KR Surveyor endorsement on the mill certificate (mill-only or product-standard papers are not KR class approval)
• Omitting alloy / microalloy rows (Al, Nb, V, Ti, Cu, Cr, Ni, Mo, B, N) from the certificate check

Write the KR grade (RF56), TM/QT, impact temperature, UT level and certificate type into the PO.

Specify KR RF56 (KR RF56), 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 KR Rules for the Classification of Steel Ships, Pt 2, Grade RF56 (KR RF56), delivery condition [TM/QT], 30 × 2500 × 12000 mm, 10 pcs, marked KR RF56 / KR RF56, certificate endorsed by KR Surveyor, EN 10204 3.2 [if required], UT [EN 10160 / ASTM A578 level], CIF [port].”

KR RF56 is extra-high-strength hull steel (IACS UR W16 band; KR yield-class number 56): yield 550 N/mm², tensile 670–835 N/mm², Charpy at −60 °C, marking KR RF56. Whatever the IACS family name, only a KR Surveyor-endorsed certificate makes it KR class steel. Lock on the order: TM or QT, UT level, Ceq/CET/Pcm reporting, multi-class and 3.2.

SHH STEEL supplies KR RF56 plate rolled by Chinese mills that hold Korean Register works approval for the grade, so every plate carries the KR RF56 stamp and a Surveyor-endorsed certificate traceable to its heat. Sizes are scheduled into the mill rolling cycle with KR survey arranged at the mill. On request we add EN 10204 3.2 certificates with third-party witness (TÜV, SGS, LR or another agency) and joint certification with ABS, DNV, LR, BV, 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 KR RF56 with neighbouring KR EHS sizes or RA–RF 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 (KR RF56 / KR RF56), 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.