NK KF620 Shipbuilding Steel Plate

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

NK KF620 is the −60 °C grade of the 620 level, for Arctic and polar service. ClassNK names its extra-high-strength steels with the K prefix, the toughness letter and the yield in N/mm²: KA, KD, KE or KF followed by 620. The 620 level is a QT level for offshore and special structures, rarely seen on conventional hulls. It follows IACS UR W16: ReH ≥ 620 N/mm², Rm 720–890 N/mm², A ≥ 15 %.

Equivalent grades: ASTM A131 (no direct grade at this strength) · ABS FH62 / FQ63 · DNV NV F620 · LR FH62 · BV F620 · CCS FH620 · KR RF63 · RINA F620 · GB 712 Grade FH620.

KF620 is made TMCP or quenched and tempered at a mill whose NK approval covers this exact grade and thickness range. It is rolled to order rather than held in stock. SHH STEEL places the order at an NK-approved Chinese mill and arranges the NK survey, with dual class and 3.2 inspection on request.

The maximum thickness depends on the mill’s approval. TM KF620 is often approved to about 50–80 mm and QT thicker, but treat the approval sheet as the limit (UNCERTAIN).

Product (check) analysis column: ClassNK Part K 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.

These rows give the UR W16 envelope. ClassNK fixes the exact limits for each mill at approval, so the approval sheet and the order govern (UNCERTAIN vs Part K text). At 620, Ni–Cr–Mo–B QT chemistry is typical, and Pcm is watched closely for welding. Every intentionally added element must appear on the certificate, along with Ceq, CET or Pcm.

Tests are taken per heat-treatment batch or rolled length, as the approval requires. Thick plate may need extra mid-thickness tests. Results are as per standard and mill certificate.

Charpy specimens are usually taken at quarter thickness and tested at −60 °C. Delivery is TM or QT. The choice changes the mill, price and welding behaviour, so state it on the order. At this toughness level, many owners also ask for CTOD or brittle-crack-arrest data. Those are separate requirements (UNCERTAIN), so specify them.

Minus tolerance follows UR W13. Plus tolerance and flatness follow the agreed standard. QT plate is flatter than heavy TM plate but harder to straighten, so agree the flatness class at enquiry.

KF620 is not held as open stock. These are the sizes we normally quote against mill rolling slots. Lead time is about 6–10 weeks including NK survey (commercial estimate).

Packing: bundled by size with steel strapping, painted with grade and heat number, shipped in containers (≤11.8 m) or break bulk.

All sit in the same UR W16 band. The names differ, and only the NK stamp counts for ClassNK.

The neighbours are KF550 below and KF690 above. Within level 620, the letter sets the test temperature: KA 0 °C, KD −20 °C, KE −40 °C, KF −60 °C. Moving to a different level needs ClassNK plan approval. Typical uses: ice-class and polar hulls, Arctic LNG carriers and cold-region offshore units needing −60 °C, at extra-high strength. See NK Steel Plates and offshore platform steel plates.

Welding needs a procedure qualified on KF620 and approved by ClassNK, with low-hydrogen consumables of matching strength and −60 °C toughness. Heat input must be limited to protect HAZ toughness in QT plate. Preheat is usually needed on thick sections. Do not hot form or stress relieve above the tempering temperature without agreement from the mill and ClassNK.

The certificate gives the full alloy analysis, Ceq/CET/Pcm, tensile results, −60 °C Charpy results, delivery condition, UT result and plate numbers, with the NK endorsement. On receipt, check the KF620 stamp, impact temperature and NK endorsement. Dual class with ABS, DNV, LR or BV and 3.2 witness are available.

• Writing AH620 or A620 without ClassNK, so the plate gets the wrong stamp
• Leaving TM vs QT open
• No UT requirement
• Ceq/Pcm not requested
• Ordering from a mill whose NK approval does not cover the thickness
• Expecting an ASTM A131 dual certificate

State NK KF620, TM or QT, sizes and quantities, UT level, dual class, certificate type and port. Send to SHH STEEL.

Sample line: “Plate to ClassNK Rules Part K, grade KF620, QT, 40 × 2500 × 10000 mm, 8 pcs, UT EN 10160 S1E1, NK-endorsed certificate with Pcm, EN 10204 3.2.”

NK KF620 (KF620) is ClassNK extra-high-strength hull steel under Part K of the Rules for the Survey and Construction of Steel Ships: yield 620 N/mm², tensile 720–890 N/mm², Charpy −60 °C. An NK-classed hull accepts it only with the NK stamp and an endorsed certificate.

SHH STEEL supplies KF620 from NK-approved Chinese mills, with cutting, dual class, UT and EN 10204 3.2 on request. Send grade, sizes, quantities and port for a quotation.