NK KD420 Shipbuilding Steel Plate

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

NK KD420 is the −20 °C grade of ClassNK’s 420 extra-high-strength level. 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 420. The 420 level is the entry level of extra-high strength, used on large container ships to keep hatch-coaming and sheer-strake thickness down. It follows IACS UR W16: ReH ≥ 420 N/mm², Rm 530–680 N/mm², A ≥ 18 %.

Equivalent grades: ASTM A131 (no direct grade at this strength) · ABS DH42 / DQ43 · DNV NV D420 · LR DH42 · BV D420 · CCS DH420 · KR RD43 · RINA D420 · GB 712 Grade DH420.

KD420 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 KD420 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 420 most mills use a lean C–Mn–Nb–Ti TMCP design with little Ni or Mo. 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 −20 °C. Delivery is TM or QT. The choice changes the mill, price and welding behaviour, so state it on the order. Ask for strain-ageing or CTOD data only if your yard specification requires it.

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.

KD420 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 KD40 (390) below and KD460 above. Within level 420, 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: thicker deck, bottom and sheer strakes and hatch side coamings needing −20 °C, at extra-high strength. See NK Steel Plates and offshore platform steel plates.

Welding needs a procedure qualified on KD420 and approved by ClassNK, with low-hydrogen consumables of matching strength and −20 °C toughness. TMCP at this level is generally weldable with modest preheat, but the approved procedure governs. 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, −20 °C Charpy results, delivery condition, UT result and plate numbers, with the NK endorsement. On receipt, check the KD420 stamp, impact temperature and NK endorsement. Dual class with ABS, DNV, LR or BV and 3.2 witness are available.

• Writing AH420 or A420 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 KD420, 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 KD420, QT, 40 × 2500 × 10000 mm, 8 pcs, UT EN 10160 S1E1, NK-endorsed certificate with Pcm, EN 10204 3.2.”

NK KD420 (KD420) is ClassNK extra-high-strength hull steel under Part K of the Rules for the Survey and Construction of Steel Ships: yield 420 N/mm², tensile 530–680 N/mm², Charpy −20 °C. An NK-classed hull accepts it only with the NK stamp and an endorsed certificate.

SHH STEEL supplies KD420 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.