Q355D

High-strength low-alloy structural steel · GB/T 1591-2018 — High strength low alloy structural steels

Q355D is the as-rolled, 355 MPa-class quality-D HSLA structural steel defined by GB/T 1591-2018. Its strength, elongation and carbon-equivalent limits depend on product thickness, and it must not be conflated with the normalized Q355ND or TMCP Q355MD grades. Particular care is required when specifying impact toughness because the standard's Table 11 footnote restricts the stated Q355D −20 °C energy values to plate over 250 mm thick; a purchase order should therefore state the required impact temperature, orientation, specimen size and acceptance energy explicitly whenever toughness is design-critical.

As-rolled/hot-rolledDelivery condition · AR or WAR may be omitted, so Q355D denotes the as-rolled variant.
355 MPa at nominal thickness or diameter ≤16 mmMinimum upper yield strength · The specified minimum decreases with increasing section thickness, reaching 265 MPa for >250–400 mm D-grade plate.
470–630 MPa at ≤100 mmTensile strength · The specified range is 450–600 MPa above 100 mm.
≤0.20% at ≤40 mm; ≤0.22% above 40 mmHeat-analysis carbon · Other chemistry limits and carbon equivalent also apply.
0.45–0.49% CEVMaximum carbon equivalent · Limit depends on nominal thickness or diameter.
Do not rely on the letter D alone for routine-thickness plateImpact specification warning · Table 11's Q355D values of 34 J longitudinal and 27 J transverse at −20 °C carry a footnote limiting them to Q355D plate over 250 mm.

Overview

Designation system: Chinese GB/T yield-strength designation
Product forms: Hot-rolled plate and sheet, Hot-rolled strip, Structural sections, Hot-rolled bars, Steel billets (chemical composition requirements only)
Condition: As-rolled/hot-rolled condition; AR or WAR condition symbol is omitted from the designation

What the designation means

Designation breakdown

Element	Meaning
Q	Initial of the Chinese term for yield strength.
355	Specified minimum upper yield strength of 355 MPa in the lowest thickness range.
AR or WAR, omitted	As-rolled/hot-rolled delivery condition.
D	Quality grade D; impact requirements must be read together with Table 11 and its footnotes.

GB/T 1591-2018 covers HSLA steel for general structural and engineering purposes. It addresses plates, strip, sections and bars; its chemical-composition provisions also apply to billets. Q355D is specifically the as-rolled member of the Q355 family. Q355ND and Q355MD are separate designations with normalized/normalizing-rolled and thermomechanically processed delivery conditions, respectively, and have different chemistry, carbon-equivalent and toughness provisions.

The product standard defines grade requirements, but dimensional tolerances come from the applicable form standard—for example, GB/T 709 for hot-rolled plate and strip, GB/T 706 or GB/T 11263 for sections, and GB/T 702 for bars.

Chemical composition and weldability controls

Q355D as-rolled heat analysis

Element	Requirement, mass %	Applicability
C	≤0.20	Nominal thickness or diameter ≤40 mm
C	≤0.22	Nominal thickness or diameter >40 mm
Si	≤0.55	All covered dimensions
Mn	≤1.60	All covered dimensions
P	≤0.025	Quality D
S	≤0.025	Quality D
Cr	≤0.30	All covered dimensions
Ni	≤0.30	All covered dimensions
Cu	≤0.40	All covered dimensions

These are melt/heat-analysis requirements. Finished-product analysis is an optional order item and, when requested, is assessed using the permissible deviations of GB/T 222.

Maximum carbon equivalent CEV by nominal thickness or diameter

Nominal thickness or diameter (mass %)	≤30 mm	>30–63 mm	>63–150 mm	>150–250 mm	>250–400 mm
Maximum carbon equivalent CEV by nominal thickness or diameter	0.45%	0.47%	0.47%	0.49%	0.49%

The >250–400 mm range applies to quality-D plate.

Carbon equivalent used by GB/T 1591

CEV (%) = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15

Calculated from heat-analysis values.

Q355D is normally weldable using established structural-steel practice, but the designation does not define a universal preheat temperature or welding procedure. Procedure qualification should consider the actual certificate chemistry and CEV, combined thickness, restraint, hydrogen level, heat input, consumable strength and required toughness. Increasing thickness permits higher CEV and also lowers the specified yield strength, so a procedure developed for thin Q355D should not automatically be transferred to heavy plate.

GB/T 1591 permits additional alloying elements by agreement to improve performance, with the elements and contents identified on the quality certificate. Q355D should therefore be purchased and qualified by the complete specification and inspection certificate, not from the grade name alone.

Thickness-dependent tensile requirements

Q355D as-rolled mechanical properties

Nominal thickness or diameter	Minimum ReH, MPa	Rm, MPa	Minimum A longitudinal, %	Minimum A transverse, %
≤16 mm	355	470–630	22	20
>16–40 mm	345	470–630	22	20
>40–63 mm	335	470–630	21	19
>63–80 mm	325	470–630	20	18
>80–100 mm	315	470–630	20	18
>100–150 mm	295	450–600	18	18
>150–200 mm	285	450–600	17	17
>200–250 mm	275	450–600	17	17
>250–400 mm	265	450–600	17	17

The >250–400 mm row applies only to quality-D steel plate. Where yielding is not distinct, Rp0.2 may replace upper yield strength ReH.

For plate and strip at least 600 mm wide, the tensile specimen is taken transverse to the rolling direction; other products use a longitudinal specimen. The elongation table nevertheless provides separate longitudinal and transverse minima. Test location and specimen preparation are governed by GB/T 2975-2018, so certificate values should not be compared without considering product form, orientation and sampling position.

Impact toughness: the critical specification issue

Values printed for as-rolled Q355D in GB/T 1591-2018 Table 11

Test temperature	Specimen orientation	Minimum KV2	Table footnote
−20 °C	Longitudinal	34 J	Applicable only to Q355D plate over 250 mm thick
−20 °C	Transverse	27 J	Applicable only to Q355D plate over 250 mm thick

Longitudinal specimens are the standard orientation; transverse testing may be agreed between purchaser and supplier.

The standard generally calls for impact testing when nominal thickness is at least 6 mm or nominal diameter is at least 12 mm. Standard specimens are 10 × 10 × 55 mm. If full-size specimens cannot be prepared, 10 × 7.5 × 55 mm or 10 × 5 × 55 mm specimens are used, with acceptance energies of 75% or 50% of the full-size value, respectively.

Table 11 does not provide a clean, unconditional −20 °C energy guarantee for ordinary Q355D dimensions because its numerical Q355D values are footnoted as applying only above 250 mm. Where brittle-fracture resistance matters, the order should expressly state test temperature, minimum energy, longitudinal or transverse orientation, specimen size, sampling position and testing frequency. Alternatively, assess whether Q355ND or Q355MD is the more appropriate standardized delivery condition.

If specified by the purchaser, grade-D steel may instead be tested at −30 °C with a minimum longitudinal absorbed energy of 27 J. This is an optional requirement and should appear explicitly in the order and inspection documentation.

Forming, surface quality and internal quality

Optional 180° bend test

Nominal thickness or diameter	Mandrel diameter D
≤16 mm	D = 2a
>16–100 mm	D = 3a

a is specimen thickness or diameter. The bend test is performed when requested by the purchaser; inspection may be omitted if the supplier guarantees compliance.

Plate surfaces must be free from harmful defects such as bubbles, scabs, cracks, folds, inclusions and pressed-in scale, and plate must not show visible delamination. Limited local imperfections may be removed by grinding provided the remaining thickness satisfies the permitted minimum. Repair welding of plate defects is subject to agreement and specific restrictions. Alternative plate or strip surface-quality classes may be ordered to GB/T 14977.

Ultrasonic or other nondestructive examination is not an automatic consequence of specifying Q355D. Internal-quality testing, acceptance class and method must be agreed and written into the contract. Similarly, ordinary Q355D does not guarantee through-thickness ductility. If lamellar-tearing resistance is needed, specify a Z-quality suffix such as Q355DZ25 together with the current requirements of GB/T 5313.

Ordering and inspection points

Items that should be fixed in the purchase specification

Item	Why it matters
Standard and grade	State GB/T 1591-2018 and Q355D; do not use only “Q355” or the obsolete Q345D designation.
Product form and dimensions	Mechanical requirements, tolerances and sampling depend on form and thickness.
Delivery condition	Confirm as-rolled Q355D is intended rather than Q355ND or Q355MD.
Dimensional tolerance class	Select the applicable class under the relevant plate, strip, section or bar dimensional standard.
Impact requirements	State temperature, minimum energy, orientation, specimen size and sampling position when toughness is required.
CEV and chemistry reporting	Require heat analysis and CEV on the certificate; request finished-product analysis if needed.
Z-direction properties	Specify Z15, Z25 or Z35 under GB/T 5313 when required for thick welded joints.
Surface and internal quality	State any GB/T 14977 surface class and ultrasonic or other NDT acceptance criteria.
Bend testing	Make the optional bend test contractual if fabrication qualification requires it.
Certification and traceability	Define certificate type, heat identification, batch traceability and any project-specific witness requirements.

Standard acceptance batches normally consist of one designation, heat number, specification and delivery condition, with a maximum batch mass of 60 t; specified exceptions apply to coils and certain large-converter section-steel heats. Unlike Q355B, Q355D is not covered by the standard's mixed-heat batching concession. This distinction is useful when reviewing traceability and test coverage on certificates.

Selection and substitution

Q355D provides a useful strength level for general welded structures, but suitability depends on thickness, structural design temperature, fracture consequence, welding procedure and inspection requirements. It is not a corrosion-resistant or weathering grade, and no general elevated-temperature or pressure-equipment capability follows from the designation.

GB/T 1591-2018 informatively associates Q355D(AR) with the former hot-rolled Q345D and lists S355J2 as a European counterpart. These comparisons are not blanket equivalence statements. Q355D and S355J2 differ in standard structure, yield definition, chemistry limits, thickness ranges, impact wording, sampling and certification. Substitution requires a clause-by-clause comparison for the actual product form and thickness, with particular attention to the Q355D impact footnote.

Sources

GB/T 1591-2018 — High strength low alloy structural steels: official standard recordState Administration for Market Regulation / Standardization Administration of China
GB/T 1591-2018 — Low alloy high strength structural steel: standard textChina Standard Press text hosted by Baosteel
GB/T 5313-2023 — Steel plate with through-thickness propertiesState Administration for Market Regulation / Standardization Administration of China
GB/T 709-2019 — Dimensions, shape, weight and tolerances for hot-rolled steel strip, plate and sheetState Administration for Market Regulation / Standardization Administration of China

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Q345D under GB/T 1591-2008	Predecessor designation	GB/T 1591-2018 replaced the Q345 strength class with Q355 and lists hot-rolled Q345D against Q355D(AR). Existing drawings should be checked for the cited standard edition rather than renaming the material automatically.
Q355ND	Related normalized or normalizing-rolled grade	Not the same delivery condition as Q355D; it has separate chemistry, CEV, tensile and impact provisions.
Q355MD	Related TMCP grade	Not interchangeable solely on strength class; its processing condition, chemistry and toughness requirements differ.
S355J2 under EN 10025-2	Informative cross-standard counterpart	Listed in the informative comparison annex of GB/T 1591-2018, but not automatically equivalent or interchangeable.
Q355DZ15 / Q355DZ25 / Q355DZ35	Q355D plate with specified through-thickness properties	Z-quality must be ordered explicitly under the applicable edition of GB/T 5313.