RINA Grade B Shipbuilding Steel Plate

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

RINA Grade B is the second step of RINA’s normal-strength series in Part D (Materials and Welding) of the Rules for the Classification of Ships. Strength is the same as Grade A — 235 N/mm² yield, 400–520 N/mm² tensile — but B adds a manganese minimum and a 0 °C Charpy test once the plate is thick enough for the RINA material class table to ask for it. Designers use it when a mild-steel shell or deck strake grows past the thickness where Grade A is allowed.

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

Grade B costs very little more than A and gives a documented 0 °C result in the thicker bands. A mill may deliver D or E against a B order when those plates also meet B, but never the other way round.

RINA is the Genoa-based society that classes much of the Italian fleet and a large share of the world’s cruise ships and ferries, many of them built at Fincantieri and other Italian and Mediterranean yards. That background shows in the steel: passenger ships use large areas of thin plate in decks and superstructure, so flatness, weld distortion and consistent supply count as much as the grade letter.

Plates come from a steelworks that holds RINA approval of the manufacturer for this grade, delivery condition and thickness range. Tests are witnessed by, or carried out under the approved scheme of, a RINA surveyor, and each plate is stamped with the RINA mark and grade. SHH STEEL sources RINA Grade B from RINA-approved Chinese plate mills, keeps the common sizes in stock and rolls the rest to order with the RINA survey booked at the mill. The full RINA range is under RINA Grade B.

How RINA names it. For normal-strength steel the RINA designation is just the letter — A, B, D or E — so the plate reads “Grade B” next to the RINA mark. There is no prefix or number to decode, which is why mistakes usually come from leaving the society off the order rather than from the grade name.

Size window. RINA Grade B is rolled from about 5 mm to 100 mm, 1500–3500 mm wide and up to roughly 18 m long. Passenger-ship work uses a lot of thin plate (5–12 mm) for decks and bulkheads, so flatness matters as much as strength — see the tolerance section. Plate cut from coil is acceptable only when the coil line itself is covered by the RINA approval.

Reading the table. The deoxidation and grain-refinement lines matter more for Grade B than the strength lines, which are the same for all four mild letters. They decide whether a plate from a given heat can be certified as Grade B at the ordered thickness, so check them on the certificate before the strength figures.

Product (check) analysis column: RINA Rules Part D 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 that RINA applies in Part D. We have not checked them against a current RINA edition, so the RINA text governs (UNCERTAIN). Grade B needs at least 0.80 % Mn (0.60 % if fully killed or for cold-flanging quality) and tightens silicon to 0.35 %.

Cu, Cr, Ni and Mo are residuals from scrap and should be reported as measured. If the mill adds Al, Nb, V or Ti as grain refiners, the certificate should say so; a blank there on a D or E certificate above 25 mm is a reason to ask questions.

UR W11 gives no numeric check-analysis tolerance. If the yard or owner wants product analysis, fix the tolerance at enquiry and write it on the order.

Minimum elongation on a flat 200 mm gauge-length specimen depends on thickness:

A, B, D and E share this tensile band; only toughness and chemistry change. One tensile test is taken per batch, with more for large batches or wide thickness spreads, under the UR W11 batch rules. Thin cruise-ship deck plate (5–8 mm) falls in the lowest elongation bands. That is how the table is built, not a sign of poor ductility.

Tensile specimens are taken transverse to the rolling direction for plate wider than about 600 mm, as UR W11/W16 practice requires; the certificate should show the specimen orientation. For mild steel, yield is reported as ReH; where no clear yield point appears, Rp0.2 is used.

Grade B is tested at 0 °C in the thickness and supply-condition bands where the UR W11 table asks for it; thin as-rolled plate is often exempt.

Values are averages of three full-size specimens; one result may fall below the average provided it is at least 70 % of it. Longitudinal specimens are standard unless transverse are ordered. Above 100 mm, agree the requirements with RINA.

Condition of supply matters commercially. AR plate is cheapest; N and TM plate cost more but widen the thickness range in which a lighter test regime applies. State the condition on the order so the mill and the surveyor test to the same plan.

Where the yard specification adds requirements beyond the rules — for example Charpy on Grade A below 50 mm — that is an order extra and should be written on the purchase order.

RINA applies the IACS UR W13 thickness tolerances to hull plate. UR W13 limits the minus side (commonly quoted as 0.3 mm — confirm on the order) and leaves plus tolerance, flatness, width and length to the agreed dimensional standard, such as EN 10029 or GB/T 709.

For passenger-ship decks in 5–8 mm plate, flatness is often the real issue: wavy thin plate costs the yard hours of straightening and line heating. If your yard works to a tighter flatness class, put it on the order. Surface defects may be ground out as long as the remaining thickness stays inside the minus tolerance; deeper repairs need the surveyor’s agreement.

UR W13 also allows local grinding of surface defects and sets how the average thickness of a plate is checked; the measurement points and acceptance method should be agreed at enquiry if the yard has its own procedure. For RINA passenger-ship work we suggest stating both the minus tolerance class and the flatness class on the order for any plate under 10 mm.

Quantities change weekly — confirm before booking. Plates are sold as RINA Grade B with the RINA mark and endorsed certificate. Anything outside the stock list goes into the next mill campaign: about 3–5 weeks to certified plate.

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

Processing before shipping. We cut Grade B to drawing by plasma or oxy-fuel, bevel edges for welding, shot-blast and prime with shop primer, and mark each piece with grade, heat number and part number so that RINA traceability survives cutting. Cut parts ship with a cutting list that cross-references every piece to its original plate and certificate.

All of these share the IACS chemistry and tensile limits, so one heat can carry several society marks if every surveyor attends the same tests. On paper, though, only the RINA mark satisfies a RINA-classed hull; a GB 712 or A131 mill sheet alone is not acceptable.

Grade B sits between untested A and −20 °C D, and is the cheapest letter that carries a Charpy test. The RINA material class table picks the letter from the member’s structural category (secondary, primary, special) and its thickness, so the drawing — not the buyer — decides whether a strake is Grade B.

Normal strength stops at 235 N/mm². Passenger ships use a lot of AH32/AH36 in the hull girder and keep mild steel for internal structure; if the design needs more strength, see the AH32–FH40 range under RINA Steel Plates. Typical RINA use for Grade B: shell and deck strakes in the middle thickness bands, car-deck plating of ro-ro ferries, and repair inserts on older Mediterranean tonnage.

Using a higher mild letter than needed adds cost without improving the design; using a lower one is a non-conformity the surveyor will raise. Follow the material class table exactly.

Grade B cuts by oxy-fuel, plasma or laser without preheat. It is welded with RINA-approved consumables for normal-strength steel at the matching toughness grade (typically grade 1, 2 or 3 for A, D and E — confirm in the yard WPS), under procedures the RINA surveyor has approved.

Thin passenger-ship panels are welded on automated panel lines with low heat input to limit distortion; the low C + Mn/6 of mild steel makes this straightforward. Thick or highly restrained joints may still need preheat as set by the procedure.

Cold bending, flanging and line heating are routine. Order cold-flanging quality where needed. For offshore primary steel, see offshore platform steel plates.

Where Grade B fits. Grade B is useful where a mild-steel strake moves just past the thickness limit for Grade A in the class table. Grade B is less common in stock than A or D in some markets; ask early if you need many sizes. Keep mild-steel and high-tensile plates of similar thickness clearly separated in the yard, because they look identical and differ only in the stamp.

At the mill the RINA surveyor witnesses or audits tensile and impact tests, checks identification and endorses the inspection certificate. The certificate lists heat number, ladle analysis, tensile results, Charpy results where required, condition of supply and plate numbers.

On receipt: match the stamped grade, heat and plate number to the certificate; check the RINA endorsement; confirm Charpy results are at the right temperature for Grade B; measure thickness at several points. On request we add EN 10204 3.2 third-party witness, ultrasonic testing or dual certification with another IACS society, provided it is booked before testing.

RINA approval of the manufacturer. A mill is approved by RINA for named grades, steelmaking route, delivery condition and maximum thickness, after approval tests and periodic renewal. Before placing an order for Grade B, ask to see that the approval scope actually lists the grade letter up to the thickness you need — this is the single most useful check a buyer can make (approval document format UNCERTAIN).

Mild-steel certificates are often batch certificates covering many plates; check that every plate number you received is listed and that the heat numbers on the plates match the certificate lines.

• Accepting a Grade A certificate against a B order (no 0 °C result)
• Missing the 0.80 % Mn minimum
• Leaving the condition of supply off the order
• No RINA endorsement on the certificate
• Assuming B is fine-grain — it is not required to be

Every one of these is avoidable at enquiry: write the full RINA grade, the condition of supply, the certificate type and any dual-class need on the purchase order.

With normal-strength plate the costly mistakes are rarely about strength; they are about the toughness letter, the class mark and the condition of supply. A rejected certificate at the yard can stop a block for weeks.

If your yard has never bought from a Chinese mill under RINA survey, we can arrange a trial lot, share sample certificates in advance, and let your inspector or the RINA surveyor visit the mill during rolling and testing.

Send SHH STEEL: grade (RINA Grade B), sizes and quantity per size, condition of supply, flatness class if tighter than standard, any UT, dual-class or 3.2 requirement, surface finish (shot-blasted and primed or mill finish) and destination port.

Sample line: “Hull plate to RINA Rules for the Classification of Ships, Part D, Grade B, 10 × 2500 × 12000 mm, 60 pcs, RINA-endorsed certificate, EN 10204 3.1, CFR Genoa.”

Order checklist:

• Society and rule book: RINA Rules for the Classification of Ships, Part D
• Grade: Grade B
• Condition of supply (AR / N / TM as allowed)
• Sizes and quantity per size; flatness class if tighter than standard
• Certificate: RINA-endorsed, EN 10204 3.1 or 3.2
• Extras: UT, Z-direction, dual class, shot-blast/prime, cutting
• Delivery terms and destination port

RINA Grade B is normal-strength hull steel under RINA’s Rules for the Classification of Ships, Part D (Materials and Welding): yield 235 N/mm², tensile 400–520 N/mm², Charpy at 0 °C. A RINA-classed ship accepts it only with the RINA mark and an endorsed certificate.

SHH STEEL supplies RINA Grade B from RINA-approved Chinese mills, with cutting, UT and EN 10204 3.2 on request. Send grade, sizes, quantities and port for a quotation.