NK Grade B Shipbuilding Steel Plate

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

KB, sold as NK Grade B, is the second step of ClassNK’s mild-steel series. Its strength is the same as KA: 235 N/mm² yield and 400–520 N/mm² tensile. The difference is a 0 °C Charpy test in the thicker bands and a fixed manganese minimum. Japanese drawings usually change from KA to KB where a mild-steel strake gets too thick for the untested grade.

Equivalent grades: ASTM A131 Grade B · ABS B · DNV NV B · LR Grade B · BV Grade B · CCS Grade B · KR RB · RINA Grade B · GB 712 Grade B.

If you have KB on order, the mill may ship KD or KE instead when the chemistry and tensile results also meet KB. You cannot go the other way. Where no Charpy is needed, KA is still cheaper.

Plates are made at a steelworks that holds ClassNK manufacturing approval for KB. They are tested in front of, or under the scheme of, an NK surveyor and stamped with the NK mark and the grade. SHH STEEL buys KB from Chinese mills with that approval. We hold common sizes in stock and roll the rest to order, with the NK survey booked at the mill.

Available sizes. Chinese plate mills roll KB from about 5 mm up to 100 mm or more, in widths from 1500 to about 3500 mm and lengths up to about 18 m. Most trade is 6–30 mm thick and 2000–2500 mm wide. Thin gauges cut from coil need the coil line to be NK-approved as well.

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.

The ladle limits are the IACS UR W11 values, which ClassNK applies in Part K. We have not checked them against a current Part K edition (UNCERTAIN), and we do not quote Part K table numbers. The residual limits for Cu, Cr, Ni and Mo come from the UR W11 notes. The mill reports them as measured. KB needs at least 0.80 % Mn (0.60 % if fully killed or cold-flanging quality) and has a tighter 0.35 % Si cap than KA.

Product (check) analysis has no numeric tolerance in UR W11. If your yard or owner wants check analysis, agree the tolerance at enquiry and write it on the order.

On the flat 200 mm gauge-length test piece, minimum elongation depends on thickness:

KA, KB, KD and KE all share this tensile band. The mill takes one tensile test per batch, with extra tests for large batches or wide thickness spreads, following the UR W11 batch rules.

KB is tested at 0 °C in the thickness and delivery-condition bands where the UR W11 table asks for it. Thin plate in the as-rolled condition is often exempt.

These are average energies for three full-size specimens. One single value may fall below the average if it is at least 70 % of it. Longitudinal specimens are normal unless transverse are ordered. Over 100 mm, agree the requirements with ClassNK.

ClassNK uses the IACS UR W13 thickness tolerances for hull plate, which limit the minus side (commonly 0.3 mm — confirm on the order). The plus side, flatness, width and length follow the agreed standard, for example JIS G3193, EN 10029 or GB/T 709. For Japanese-designed ships ordered in Asia, mills often quote the JIS dimension tables, so state which one applies.

Surface defects may be removed by grinding, provided the remaining thickness stays inside the minus tolerance. If you shot-blast and prime at the yard, say so on the order so the plates are not oiled.

Stock quantities change every week, so confirm before you book. Plates are sold as KB with the NK stamp and endorsed certificate. Sizes not in stock are rolled in about 3–5 weeks including survey.

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

All of these follow the same IACS chemistry and tensile limits, so one heat can be certified to several societies if their surveyors attend together. On paper, though, only the NK stamp satisfies ClassNK, so a GB 712 or A131 mill sheet on its own is not acceptable.

KB sits between untested KA and −20 °C KD. It is the cheapest grade that carries a Charpy test. ClassNK’s material-grade table picks the letter from the member’s category (secondary, primary, special) and its thickness. That table, not the buyer, decides whether a strake is KB. Mild steel stops at 235 N/mm². If the design needs more, look at NK A32/AH32 to F40/FH40 under NK Steel Plates.

Compared with JIS G3106 SM400 or ASTM A36, KB has similar strength. It differs in the C + Mn/6 cap, the surveyed tests and the NK stamp, which a classed hull requires. Typical NK use: shell and deck strakes that move into the 0 °C requirement once the plate gets thicker, plus repair inserts on older Japanese-built bulk carriers.

KB cuts cleanly by gas, plasma or laser without preheat. It welds with consumables approved by ClassNK for mild steel at the right toughness grade (for example grade 1, 2 or 3 for A, D and E respectively, per the shipyard’s qualified procedure). Japanese and Chinese yards weld it with FCAW and one-side SAW on panel lines, under procedures the NK surveyor has approved. Thick or restrained joints may need preheat, which the procedure sets.

Cold bending and line heating are routine. Line heating is used a lot for hull curvature in Japanese practice. For plate ordered for cold flanging, state that quality on the order. For offshore primary steel, see offshore platform steel plates.

The NK surveyor witnesses or audits the tests at the mill and checks the identification. The inspection certificate lists heat number, ladle analysis, tensile results, Charpy results where required, delivery condition and plate numbers, and carries the NK endorsement. On request we add EN 10204 3.2 witness by a third party, ultrasonic testing, or dual certification with ABS, DNV, LR or BV.

At your warehouse: match the stamped KB grade, heat and plate number to the certificate. Check the NK endorsement is present and that Charpy results are at 0 °C where required. Measure thickness at several points against the UR W13 / order tolerance.

• Accepting a KA certificate against a KB order (no 0 °C result)
• Missing the 0.80 % Mn floor
• Leaving the delivery condition off the order
• Coil-cut plate without approval
• No NK endorsement on the certificate

All of these can be avoided at enquiry stage. Put the full NK grade, delivery condition and certificate type on the order.

Send SHH STEEL: grade (NK Grade B / KB), sizes and quantity per size, delivery condition, any dual-class or 3.2 requirement, UT or Z needs, surface finish, and destination.

Sample line: “Hull plate to ClassNK Rules Part K, grade KB (NK Grade B), 16 × 2400 × 12000 mm, 40 pcs, NK-endorsed certificate, EN 10204 3.1, FOB Shanghai.”

NK Grade B (KB) is ClassNK mild hull steel under Part K of the Rules for the Survey and Construction of Steel Ships: yield 235 N/mm², tensile 400–520 N/mm², Charpy 0 °C. An NK-classed hull accepts it only with the NK stamp and an endorsed certificate.

SHH STEEL supplies KB 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.