KR Grade D Shipbuilding Steel Plate

Technical Reviewer: Jack | SHH STEEL Material Solutions Department · Updated: 2026-10-27

Korean Register designates the −20 °C ordinary-strength hull steel KR RD; the trade calls it KR Grade D. Under KR Rules Pt 2 it has the same 235 N/mm² yield, 400–520 N/mm² tensile and 22 % elongation as RA and RB, but it must be killed, and killed fine-grain above 25 mm. On KR-classed tankers, bulkers and LNG carriers from Korean yards, RD typically appears where the material class table assigns a −20 °C level to an ordinary-strength strake. Commercial names: KR Grade D / KR RD shipbuilding steel plate.

Equivalent grades: KR RD ≈ ASTM A131 Grade D ≈ ABS D ≈ DNV NV D ≈ LR Grade D ≈ BV Grade D ≈ CCS Grade D ≈ NK KD ≈ RINA Grade D ≈ GB 712 Grade D (all class-approved, stamped by the issuing society).

KR RD is the middle step of the KR ordinary-strength ladder — RA/RB (no test / 0 °C) → RD (−20 °C) → RE (−40 °C). Do not substitute RA or RB where the approved drawing requires RD; RE for RD is normally accepted when properties cover RD.

Plates that pass mill tests under KR survey are stamped with the KR mark and grade KR RD — stencils often add “KR Grade D” for the yard — (plus N/TM marking where the supply condition requires it) (plus Z or U where through-thickness or ultrasonic requirements apply) and are supplied with a mill certificate the KR Surveyor endorses. SHH STEEL supplies KR RD plate from Chinese mills that hold KR works approval for the grade, from stock in common sizes or from a mill rolling cycle, with KR survey arranged at the mill.

Size range. KR RD is rolled on wide plate mills in China from about 5 mm to 100 mm and more, widths 1500–3500 mm and lengths up to about 18 m depending on mill; 6–25 mm plate in 1800–2500 mm width is the bulk of the market. Plates under about 6 mm are often cut from KR-approved hot-rolled coil, which needs separate approval of the coil process. Ask for confirmation for plates over 100 mm or wider than 3200 mm.

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 W11 Table 1 (ladle samples) for normal-strength Grade D, which Korean Register adopts for KR RD in Rules for the Classification of Steel Ships Pt 2 (KR paragraph numbers UNCERTAIN — local KR text not on disk). Because every IACS member uses the same Table 1, one heat can be surveyed for KR together with ABS, DNV, LR, BV or CCS; the KR Surveyor endorsement is what makes it KR RD. Heat (ladle) analysis is determined by the mill on each heat and must meet Table 1. Product (check) analysis on the finished plate is not given numeric tolerances in UR W11 itself; when a purchaser or Surveyor requires product analysis on the certificate, ask at enquiry and record the agreed tolerances — do not invent numbers.

Grade D uses a 0.60 % Mn minimum and a Si range 0.10–0.35 % (minimum waived when acid-soluble Al ≥ 0.015 %). Above 25 mm it must contain a grain-refining element per UR W11. In practice Chinese mills supply aluminium-killed KR RD; treat typical ladle figures as indicative only and check the actual mill certificate.

On the 200 mm gauge full-thickness flat specimen (25 mm width), minimum elongation depends on thickness:

All four normal-strength grades share these tensile values. For each batch, one tensile test is taken from one piece (extra tests when finished weight exceeds 50 t, and for every 10 mm variation in thickness from the same cast), unless otherwise agreed by KR.

CVN at −20 °C; fine grain above 25 mm. Deoxidation: killed ≤25 mm; killed and fine grain >25 mm. Average energies at −20 °C (full-size specimens, long / trans) per IACS UR W11 Table 6:

One individual value may fall below the average if it is at least 70 % of it. Generally only longitudinal specimens are prepared unless the purchaser or KR asks for transverse. The test frequency and permitted delivery condition (AR, CR, N or TM) follow the UR W11 / KR Rules Pt 2 supply-condition table for Grade D, so write the condition into the PO. Beyond 100 mm thickness is special consideration with KR (see UNCERTAIN).

Korean Register applies the thickness tolerances of IACS UR W13 (Thickness tolerances of steel plates and wide flats) unless the purchase order states otherwise. In practice class-approved hull plate is ordered to the class minus-tolerance band commonly associated with UR W13 (often 0.3 mm minus — confirm the exact class and plus tolerance on the PO; see UNCERTAIN). Plus tolerance, flatness, width and length follow the agreed national or international standard (for example ASTM A6, EN 10029 or GB/T 709) unless specified otherwise.

Surface quality follows a recognised standard such as EN 10163 parts 1–2 or the equivalent GB surface standard accepted by KR. Surface defects may be ground within UR W11 limits; grinding areas are checked so remaining thickness stays within the allowed minus tolerance. If you plan to shot-blast and prime at the yard, state any surface class on receipt in the order.

Quantities are approximate and change daily (indicative — confirm before booking); all stock plates carry KR RD marking and KR-endorsed mill certificates when sold as KR class plate. Sizes outside the table (for example 3000 mm width, 60–100 mm thickness, or fixed lengths for a cutting plan) are produced in the next mill rolling cycle, typically 3–6 weeks including KR survey. Send your size list and we will reply with what is in stock and what must be rolled.

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. A KR-classed hull needs plate made by a mill approved by Korean Register for this grade and certified under KR survey. Dual or multi-class certification (for example KR + ABS, KR + DNV, KR + LR or KR + BV) can be arranged at the mill when the heat is surveyed by both societies. Do not treat a GB 712 certificate alone as a substitute for the KR RD stamp.

All four normal-strength grades share the same 235 N/mm² yield and 400–520 N/mm² tensile range; they differ in deoxidation and toughness. Compared with KR Grade A and Grade B, Grade D lowers the impact temperature to −20 °C and requires fine grain above 25 mm. Grade E goes to −40 °C; higher-strength DH grades add yield above 235 N/mm². The hull steel grade for each strake is set by the classification rules according to structural member category and thickness, so check the approved drawing first. Sheer strakes, bilge strakes, deck stringers and ice-belt plating usually call for B, D, E or higher-strength plates under KR.

Compared with land structural steels, KR RD is close in strength to ASTM A36, ASTM A283 Grade C and S275JR class steels but differs in class thickness tolerance practice, the C + Mn/6 ≤ 0.40 cap, and above all the KR survey and KR RD stamp. For a KR-classed vessel only KR-certified plate is accepted.

When KR RD is the right choice: typical uses are strakes and members that the class drawing assigns a −20 °C toughness level, e.g. thicker plating in sheer strake, bilge and deck stringer areas of ordinary-strength designs, and ice-exposed plating without needing −40 °C of Grade E. The Charpy temperature (−20 °C) is fixed by the letter D, and the strength by the normal-strength level (ReH 235 N/mm²). Pick the strength from the scantling calculation and the letter from the material class and design temperature on the approved drawing. Swapping to a different letter or strength needs class approval of the drawing change, not just a new purchase order.

Typical uses: sheer strake, bilge strake, deck stringer and ice-belt plating on ordinary-strength designs, cold-route coastal vessels, and repair steel where the drawing calls for −20 °C. Offshore and port structures also use ordinary-strength grades for secondary steel; see offshore platform steel plates for primary structure grades.

Cutting: oxy-fuel, plasma and laser cutting all work without preheat in normal thicknesses. Welding: with C + Mn/6 at or below 0.40 %, KR RD is welded with ordinary-strength consumables approved for the grade and −20 °C toughness level required by the shipyard’s KR-approved WPS; preheat is generally unnecessary below about 25–30 mm except in cold or humid conditions — follow your approved welding procedure, not a generic number from a catalogue. Forming: plates intended for cold flanging should be ordered with the flanging quality agreed on the PO.

Processing notes for KR RD: cold forming and flanging of normal-strength plate are routine. Keep the cut edges of plate thicker than 50 mm free of notches, because thick plate of this grade is controlled more tightly for deoxidation (killed ≤25 mm; killed and fine grain >25 mm).

Every plate is tested and marked at the mill under Korean Register survey; the mill certificate lists heat number, ladle chemistry (elements required by UR W11 Table 1, plus intentionally added elements), tensile results, impact results at −20 °C where required, condition of supply and plate numbers, and is endorsed by the attending Surveyor. On request we add EN 10204 3.2 certificates with third-party witness by TÜV, SGS, LR or another agency, ultrasonic testing (U mark) or through-thickness testing (Z mark), and multi-class certification with other major class societies (ABS, DNV, LR, BV, NK, KR, RINA).

On receipt, check: KR Grade D / KR RD stamp and heat number on each plate match the certificate; the certificate shows the KR Surveyor endorsement, not just a mill stamp; thickness at several points meets the ordered UR W13 / PO tolerance; impact results agree with the table below for the delivered thickness.

Charpy acceptance for KR RD by thickness (from IACS UR W11 Table 6; actual results as per standard and mill certificate):

On receipt, check that the test temperature is −20 °C and that the energy matches the band for the delivered thickness. Also check that the plate number on the stamp matches the certificate line, and that the KR Surveyor endorsement is present. If UT was ordered (e.g. EN 10160 or ASTM A578 level), check the UT report as well. If the PO asked for 3.2, the inspector’s report must name the witnessed tests and plate numbers.

• Ordering “KR D” without Korean Register certification
• Missing fine-grain practice above 25 mm
• Accepting 0 °C (Grade B) certificates for a −20 °C PO
• Substituting KR Grade D where EH grades are required for yield
• Coil-cut without coil approval
• Ignoring supply-condition / impact lot size on the PO

Each of these is avoided by writing the requirement into the purchase order and checking it on the certificate before the plates are cut.

State the grade and rule (KR Grade D / KR RD, KR Rules for the Classification of Steel Ships, Pt 2 – rolled steels for hull), whether multi-class certification is needed, thickness, width, length and quantity per size, condition of supply, any Z or U requirement, surface or primer requirements, certificate type (mill certificate with KR endorsement, EN 10204 3.2 with third-party witness), packing and destination port. Send the list to SHH STEEL for price, available stock and lead time.

Sample PO line: “Steel plate to KR Rules for the Classification of Steel Ships, Pt 2, Grade D (KR RD), delivery condition [AR/CR/N/TM], 20 × 2500 × 12000 mm, 30 pcs, marked KR Grade D / KR RD, certificate endorsed by KR Surveyor, EN 10204 3.2 [if required], UT [if required], CIF [port].” Each bracketed item changes price, lead time and which mills can supply, so fix them at enquiry stage.

KR Grade D / KR RD is ordinary-strength hull steel under KR Rules for the Classification of Steel Ships, Pt 2 and IACS UR W11: 235 N/mm² minimum yield, 400–520 N/mm² tensile, Charpy at −20 °C where required, and class survey with the KR RD stamp. KR prints it as KR RD; “Grade D” is the commercial name. ASTM A131 or GB 712 Grade D certificates are product-standard papers, not KR class approval. What the rules leave open has to be fixed in the order: KR (or multi-class) certification, the supply condition and impact testing, whether product analysis is required on the certificate, the thickness-tolerance class, and the certificate type (3.1 vs 3.2).

SHH STEEL supplies KR RD plate rolled by Chinese mills that hold Korean Register works approval for the grade, so every plate carries the KR RD stamp and a Surveyor-endorsed certificate traceable to its heat. Common thicknesses are available from stock; other 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 RD mixed with other KR RA/RB/RD/RE or higher-strength grades such as KR RA36, 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 RD / KR Grade D), 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.