GB 712 Grade AH550 shipbuilding steel plate

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

GB 712 Grade AH550 is the 0 °C grade at the 550 N/mm² extra-high-strength level of GB 712. GB 712 keeps the H in its extra-high-strength names and writes the yield in full — AH550 — in line with IACS UR W16 and with Chinese domestic usage. These grades let Chinese mills supply offshore and large-ship projects under the national standard (edition and table numbers not quoted — UNCERTAIN). The 550 level is normally quenched and tempered, used for jack-up legs, crane booms and heavily loaded offshore members.

Equivalent grades: ASTM A131 (no direct grade at this strength) · ABS AH55 / AQ56 · DNV NV A550 · LR AH55 · BV A550 · CCS AH550 · NK KA550 · KR RA56 · RINA A550.

Current edition: GB/T 712-2022 (Structural steel for ships and ocean engineering) (replaced GB 712-2011; implementation date 2023-05-01). English page titles keep the form “GB 712 Grade …”. Chemistry and tensile figures on this page are verified against the GB 712-2011 OCR tables and Caishuku consensus; full 2022 tables were not obtained free — do not assume 2022-only UHSS grades beyond this pack.

Requirements on the UR W16 basis: ReH ≥ 550 N/mm², Rm 670–835 N/mm², A ≥ 16 %, Charpy at 0 °C. The A letter is the least demanding on toughness: sheltered members, lifting structure, internal girders.

AH550 is produced TMCP or QT. In China it comes from the larger plate mills with heavy-plate lines, accelerated cooling and, at the upper levels, roller-quench and tempering furnaces (general context, not a claim about any single mill).

Unlike GB/T 1591 high-strength grades such as Q420 or Q460, GB 712 Grade AH550 carries hull-steel toughness grading, fine-grain practice and full alloy reporting, and it is the grade offshore and ship designers in China actually call for.

AH550 is rolled to order. SHH STEEL places it with a suitable Chinese mill, books any class survey or third-party witness, and adds UT and Z-direction testing on request. Typical uses: jack-up legs, crane booms and highly loaded offshore members, mostly quenched and tempered plate.

How GB 712 names it. AH550 = toughness letter A, H for higher strength, and the full minimum yield 550 N/mm². At 550 QT is the norm and lead times lengthen.

Maximum thickness depends on the mill and route. As a guide, TMCP AH550 is often offered to about 50–80 mm and QT thicker (UNCERTAIN). At this level most enquiries are 20–100 mm QT plate for offshore and lifting structure.

Product (check) analysis column = heat (ladle) limit adjusted by GB/T 222-2006 Table 1 (non-alloy and low-alloy steel), as invoked by GB 712 clause 6.1.3 (upper limit + upper deviation, lower limit − lower deviation; row chosen by the specified upper limit; values for cross-sections up to 6 500 mm², larger sections by agreement); arithmetic shown in brackets.

The table gives the UR W16 envelope that GB 712 follows for extra-high-strength steel; individual mills hold tighter internal limits, and the GB 712 text governs (not checked locally — UNCERTAIN). At 550, a quenched and tempered Ni–Cr–Mo steel, sometimes boron-treated, is usual.

The A letter shares the A/D/E envelope; toughness comes mainly from process route and cleanliness. Every intentionally added element must appear on the certificate together with Ceq, CET or Pcm.

Boron, when added, works in parts per million and only when nitrogen is tied up, usually by titanium; read B, Ti and N together on the certificate.

For EHS plate the certificate should also state the formula used for Ceq, CET and Pcm, since different mills and owners use different ones.

Tests are taken per heat-treatment batch or rolled length; thick plate may need mid-thickness tests. Against AH40 the yield rises by 160 N/mm². The gain only counts if the welded joint keeps up, so qualify the procedure for matching strength and toughness in weld metal and HAZ.

Tensile specimens are taken transverse to the rolling direction for plate, following UR W11/W16 practice; the certificate should show the specimen orientation and type. For QT plate Rp0.2 is often reported, because the yield point is not pronounced.

Charpy specimens are usually taken at quarter thickness and tested at 0 °C. Delivery is TMCP or QT, and the route changes mill choice, price, flatness and welding behaviour, so state it on the order. QT is the usual route at this level, especially in thick plate.

Strain-ageing or CTOD data are not part of the basic grade; ask only if your specification requires them.

TMCP plate gets its strength from controlled rolling and accelerated cooling, so it starts with low carbon and good weldability but can lose strength if overheated. QT plate is austenitised, quenched and tempered, giving uniform properties through thick sections; the tempering temperature then caps any later heat treatment.

Plates are rolled to GB/T 709 or an agreed export standard with a hull-steel minus tolerance (UR W13 practice — confirm). QT plate is generally flatter but harder to straighten later, so agree the flatness class at enquiry. Do not flame-straighten EHS plate above the mill’s stated temperature; it can undo the TMCP or tempering treatment.

If the plate is ordered with a classification certificate, the class minus tolerance applies whatever dimension standard is named, so state both on the order. For EHS plate, ask for ultrasonic thickness readings to be recorded if the design relies on the minimum thickness.

GB 712 Grade AH550 is not held as open stock. The table shows the sizes most often booked into a mill campaign; expect roughly 6–10 weeks from order to certified plate (commercial estimate).

Packing and shipping: bundled by size with steel strapping, stencilled with standard, grade and heat number, and shipped from Shanghai, Tianjin, Qingdao or other Chinese ports in containers (≤ 11.8 m) or break bulk.

Processing before shipping. We cut AH550 to drawing by plasma or oxy-fuel, bevel edges, shot-blast and prime, and transfer grade, heat and plate number to every piece so that GB 712 traceability survives cutting. Cut parts ship with a cutting list cross-referencing each piece to its mother plate and certificate.

GB 712 and the CCS Rules. GB 712 was drafted to mirror the IACS unified requirements, which the China Classification Society (CCS) also applies in its Rules for Materials and Welding. In practice a Chinese mill rolls one heat to the shared chemistry and test plan; the plate becomes GB 712 Grade AH550 through the national standard and CCS AH550 through CCS survey. The two are parallel documents, not one replacing the other (CCS chapter numbering not quoted — UNCERTAIN).

Class certification on a GB 712 base. Export buyers often order “GB 712 AH550 with CCS, ABS or DNV certificate” (or another IACS society). The class surveyor must witness or audit the tests at the mill before shipment; a plain GB 712 certificate cannot be converted into a class certificate afterwards. Several marks on one plate are possible when every society is booked in advance and the mill holds the relevant approvals.

Neighbours are AH500 below and AH620 above. Within level 550 the letter sets the test temperature: A 0 °C, D −20 °C, E −40 °C, F −60 °C.

For a yield-governed member, 30 mm of AH40 becomes roughly 21.3 mm of AH550; buckling, fatigue and corrosion margins usually make the real saving smaller (estimate, not a rule value).

Other GB 712 grades are under GB 712 shipbuilding steel plates; offshore-specific grades under offshore platform steel plates.

EHS grades are usually concentrated in a few highly stressed areas rather than used everywhere. That keeps the number of welding procedures down: the yard only needs AH550 procedures where it pays. An AH plate that happens to test well at −20 °C is still only AH unless it was ordered and tested as DH.

Typical uses: jack-up legs, crane booms and highly loaded offshore members, mostly quenched and tempered plate.

Weld AH550 only under a qualified WPS for this grade, with low-hydrogen consumables reaching matching strength and 0 °C toughness in the weld metal. Limit heat input to protect HAZ toughness in QT plate; thick sections usually need preheat. Do not hot-form or stress-relieve above the tempering temperature without the mill’s agreement.

HAZ softening can occur in TMCP plate under very high heat input and HAZ hardening in QT plate under very low heat input, so respect both ends of the qualified range. Hydrogen control — dry consumables, clean joints, controlled preheat — matters more as strength rises.

The certificate gives full alloy analysis, Ceq/CET/Pcm, tensile results, 0 °C Charpy results, condition of supply, UT result and plate numbers, in Chinese and English.

On receipt, check the AH550 stencil, impact temperature, UT report and heat numbers. Third-party witness and class survey must be booked before rolling.

Mill quality system. Chinese plate mills supplying GB 712 normally run ISO 9001-certified quality systems and hold their own national product certification; when a class certificate is wanted, the mill must also hold that society’s works approval for the grade and thickness (details per mill — UNCERTAIN). Ask for that approval scope before ordering; it is the single most useful check a buyer can make.

• Ordering GB/T 1591 Q550 instead of AH550
• Leaving TMCP vs QT open
• No UT requirement
• Ceq/Pcm not requested
• Choosing a mill without proven production of this grade and thickness
• Expecting an ASTM A131 equivalent (none exists at this level)

EHS plate is rolled for you, so a wrong order line costs a full rolling cycle. Check it twice.

With EHS plate the costly errors cannot be fixed after rolling: wrong route, a thickness outside what the mill has produced, or missing UT.

If you have not bought GB 712 plate from China before, we can send sample certificates in advance, arrange a trial lot, and let your inspector visit the mill during rolling and testing. For buyers outside China we also check that the bilingual certificate wording matches the purchase order line by line before shipment.

State GB 712 AH550, TMCP or QT, sizes and quantities, UT level, any class certificate, certificate type and port, and send to SHH STEEL.

Sample line: “Plate to GB 712, grade AH550, QT, 40 × 2500 × 10000 mm, 8 pcs, UT to GB/T 2970, bilingual certificate with Pcm, EN 10204 3.2.”

Order checklist:

• Standard: GB 712 (state edition if your contract requires one)
• Grade: AH550
• Condition of supply (TMCP or QT)
• Sizes and quantity per size; tolerance standard and flatness class
• Certificate: bilingual GB 712 mill certificate, EN 10204 3.1 or 3.2; class certificate if required
• Extras: UT, Z-direction, shot-blast/prime, cutting
• Delivery terms, destination port and documents (CO, Form E)

GB 712 Grade AH550 is the extra-high-strength hull structural steel of the Chinese national standard GB 712, on the IACS UR W16 basis: yield 550 N/mm², tensile 670–835 N/mm², Charpy 0 °C. Order it by standard and grade, and add a classification certificate on the same heat when the hull is classed.

SHH STEEL supplies GB 712 Grade AH550 from Chinese mills with bilingual certificates, cutting, UT and EN 10204 3.2 on request. Send grade, sizes, quantities and port for a quotation.