Q690D
690 MPa-class high-strength low-alloy structural steel; designation may refer historically to either TMCP structural-ste · GB/T 1591-2008 (superseded; TMCP route, now Q690MD in GB/T 1591-2018) · GB/T 16270-1996 (superseded; quenched-and-tempered plate route, now Q690QD in GB/T 16270-2009) · GB/T 1591-2018 — High strength low alloy structural steels · GB/T 16270-2009 — High strength structural steel plates in the quenched and tempered condition · GB/T 34560.4-2017 — Structural steels, Part 4: quenched and tempered high-yield-strength steel plates
Q690D is not a unique current GB/T grade unless the governing standard and edition are stated. Historically it identified both a thermomechanically processed 690 MPa-class structural steel under GB/T 1591 and a quenched-and-tempered high-strength plate under GB/T 16270. Their current designations are respectively Q690MD and Q690QD, with materially different chemistry, product scope, heat-treatment condition and thickness-dependent properties. Procurement or substitution based only on “Q690D” is therefore unsafe.
- Legacy and ambiguousDesignation status · The standard number, edition, product form and delivery condition are essential.
- 690 MPa minimum yield strengthNominal strength class · Applies only to the lower thickness range; specified yield strength decreases with increasing thickness.
- Charpy impact class associated with −20 °CD quality level · Required energy, specimen orientation and sub-size treatment depend on the governing standard.
- Q690MD to GB/T 1591-2018Current TMCP designation · Successor to Q690D supplied by TMCP under GB/T 1591-2008.
- Q690QDCurrent Q&T designation · Successor to legacy Q690D quenched-and-tempered plate; covered by current Q&T plate standards.
- Never order simply as Q690DPrimary procurement rule · State the complete standard, edition, designation, thickness and delivery condition.
Overview
- Designation system
- Chinese GB/T yield-strength designation system
- Product forms
- Plate, Strip, sections and bars under the historical GB/T 1591 route, Plate up to 150 mm under GB/T 16270
- Condition
- Legacy designation requiring standard and edition identification, Thermomechanically processed (historical Q690D; current Q690MD), Quenched and tempered (historical Q690D; current Q690QD)
- Density
- 7850 kg/m³ (Representative density used for mass estimation; not a grade-specific acceptance requirement.)
Why the designation is ambiguous
The letters and number in Q690D identify a yield-strength class and quality level, but the historical designation does not uniquely identify the production route. “Q” is derived from the Chinese term for yield strength, “690” denotes the specified minimum yield strength for the lowest applicable thickness range, and “D” identifies the impact-toughness quality level.
| Historical specification | Material route | Relevant products | Current designation |
|---|---|---|---|
| GB/T 1591-2008 | Thermomechanical processing (TMCP) | Plate, strip, sections and bars within the standard's dimensional scope | Q690MD under GB/T 1591-2018 |
| GB/T 16270-1996 | Quenching and tempering | High-strength structural plate | Q690QD under GB/T 16270-2009; also addressed by GB/T 34560.4-2017 |
TMCP and quenched-and-tempered products are not interchangeable merely because both carry a 690 MPa strength designation. Their alloy design, heat response, weld heat-affected-zone behavior, dimensional scope and certification requirements may differ.
Current TMCP successor: Q690MD to GB/T 1591-2018
Q690MD is the current designation corresponding to Q690D supplied by TMCP under GB/T 1591-2008. The inserted “M” explicitly identifies thermomechanical processing. GB/T 1591-2018 covers general structural and engineering products including plates, strips, sections and bars, although the available dimensions and applicable mechanical-property ranges must be confirmed for the particular product.
| Element or index | Requirement, mass % |
|---|---|
| C | ≤0.18 |
| Si | ≤0.60 |
| Mn | ≤2.00 |
| P | ≤0.030 for D quality |
| S | ≤0.025 for D quality |
| Nb | 0.01–0.11 |
| V | 0.01–0.12 |
| Ti | 0.006–0.050 |
| Cr | ≤1.00 |
| Ni | ≤0.80 |
| Cu | ≤0.80 |
| Mo | ≤0.30 |
| B | ≤0.004 |
| CEV | ≤0.49 through the specified Q690M dimensional range |
| Pcm | ≤0.25 |
The standard permits controlled use of grain-refining elements and contains qualifications concerning nitrogen and aluminium. Product analysis is subject to the applicable permissible deviations.
| Nominal thickness or diameter | Minimum upper yield strength | Tensile strength | Minimum elongation |
|---|---|---|---|
| ≤16 mm | 690 MPa | 770–940 MPa | 14% |
| >16–40 mm | 680 MPa | 770–940 MPa | 14% |
| >40–63 mm | 670 MPa | 750–920 MPa | 14% |
| >63–80 mm | 650 MPa | 730–900 MPa | 14% |
For sections and bars, the relevant nominal dimension is used as defined by the standard. Properties must not be extrapolated beyond the listed dimensional range.
| Quality level | Test temperature | Longitudinal minimum | Transverse minimum |
|---|---|---|---|
| D | −20 °C | 47 J | 27 J |
Impact specimens are normally longitudinal; transverse testing may be agreed. Sub-size specimens and alternative agreed temperatures are treated according to the standard.
Current quenched-and-tempered successor: Q690QD
For the quenched-and-tempered plate lineage, the explicit current designation is Q690QD: the second “Q” identifies quenching and tempering, while “D” remains the toughness quality level. GB/T 16270-2009 applies to quenched-and-tempered high-strength structural plate up to 150 mm. GB/T 34560.4-2017 provides a separate current Q&T structural-steel framework and is based on ISO 630-4. The applicable standard must be chosen contractually; requirements from the two standards should not be mixed.
| Element or index | Requirement, mass % |
|---|---|
| C | ≤0.20 |
| Si | ≤0.80 |
| Mn | ≤1.80 |
| P | ≤0.025 |
| S | ≤0.015 |
| Cu | ≤0.50 |
| Cr | ≤1.50 |
| Ni | ≤2.00 |
| Mo | ≤0.70 |
| B | ≤0.005 |
| V | ≤0.12 |
| Nb | ≤0.06 |
| Ti | ≤0.05 |
| CEV, t≤50 mm | ≤0.65 |
| CEV, >50–100 mm | ≤0.77 |
| CEV, >100–150 mm | ≤0.83 |
These are maximum specification limits, not a typical production analysis. At least one specified grain-refining element is required as described by the standard.
| Plate thickness | Minimum yield strength | Tensile strength | Minimum elongation |
|---|---|---|---|
| ≤50 mm | 690 MPa | 770–940 MPa | 14% |
| >50–100 mm | 650 MPa | 760–930 MPa | 14% |
| >100–150 mm | 630 MPa | 710–900 MPa | 14% |
Tensile testing is based on transverse specimens. The plate is required to be delivered quenched and tempered.
| Quality level | Test temperature | Minimum average energy | Orientation |
|---|---|---|---|
| D | −20 °C | 47 J | Longitudinal |
The result is the arithmetic mean of three specimens. One individual value may fall below the stated average requirement but not below 70% of it. Reduced-size specimens are used where full-size specimens cannot be prepared.
What controls performance in practice
The 690 MPa classification does not mean that every thickness has a 690 MPa guaranteed yield strength. Both principal routes apply thickness reductions, and their thickness bands differ. Design calculations and incoming inspection must use the requirement for the actual thickness and governing specification.
TMCP obtains its strength and toughness through controlled deformation and cooling. Q&T plate relies on a through-thickness heat-treatment cycle. Subsequent heating during welding, hot forming, flame straightening or post-weld heat treatment can alter the microstructure that produced the specified properties. A thermal operation acceptable for one route is not automatically acceptable for the other.
The specification chemistry limits allow substantial producer discretion. Actual carbon equivalent, plate thickness, hydrogen level, restraint and heat input—not the grade name alone—govern cold-cracking risk and preheat requirements. Mill-certificate chemistry should therefore be used for welding assessment rather than calculating from specification maxima, which produces an unrealistically conservative result.
Fabrication and welding implications
| Operation | Technical implication |
|---|---|
| Welding | Use a qualified welding procedure developed for the identified delivery condition. Control hydrogen, joint restraint, preheat/interpass temperature and heat input using actual chemistry, thickness and consumable strength/toughness. |
| Consumable selection | Match the required joint strength and toughness rather than selecting solely by nominal base-metal strength. Overmatching may increase residual stress and cracking sensitivity; undermatching requires design approval. |
| Thermal cutting | Establish preheat and cut-edge inspection requirements from plate thickness, hardness risk and producer guidance. Remove hardened or damaged edge material where required by the fabrication specification. |
| Cold forming | Expect high forming force and springback. Minimum bend radius and allowable forming direction should come from the governing fabrication specification or qualified producer data. |
| Hot forming or flame straightening | Temperature limits must be controlled. Unqualified heating can temper, soften or otherwise alter TMCP and Q&T microstructures. |
| Post-weld heat treatment | Do not apply by analogy with lower-strength structural steel. PWHT can reduce strength or toughness and requires confirmation from the plate producer and a qualified procedure. |
| Machining | Machinable with suitable rigid tooling, but cutting forces and tool wear are greater than for conventional mild structural steel. |
| Inspection | High-restraint welded details may justify enhanced surface or volumetric examination, but acceptance criteria must be specified by the applicable fabrication or project standard. |
GB/T grade compliance alone does not constitute an approved welding or forming procedure.
Ordering and certification
| Item | What to state or verify |
|---|---|
| Governing document | Complete standard number and edition; do not state only “GB/T”. |
| Designation | Use the current complete designation, normally Q690MD or Q690QD, rather than ambiguous Q690D. |
| Product form and dimensions | Plate, strip, section or bar; nominal thickness or diameter; width and length; dimensional and flatness tolerance class. |
| Delivery condition | TMCP, TMCP plus tempering where permitted, or quenched and tempered, exactly as allowed by the selected standard. |
| Mechanical properties | Applicable thickness band, specimen orientation and whether additional testing is required. |
| Impact testing | Test temperature, minimum energy, orientation, specimen size and any project-specific frequency. |
| Chemical controls | Required ladle and product analysis, CEV or Pcm reporting, and any project-specific maximums. |
| Internal soundness | Ultrasonic test standard, examination extent and acceptance class if required. |
| Through-thickness properties | Specify the required Z-quality designation and test standard where lamellar-tearing resistance is necessary. |
| Surface and edge condition | Surface class, shot-blasting or primer requirements, edge condition and limits on repair welding. |
| Documentation | Heat traceability, heat-treatment or rolling condition, test results and the required inspection certificate format. |
| Fabrication compatibility | Confirm that the selected standard and mill product are approved by the design, welding and construction specifications. |
If an existing drawing or bill of materials specifies only Q690D, clarification should be obtained before purchase. The original standard edition, intended product form and delivery condition should be recovered from the design basis or legacy material documentation.
Substitution and cross-standard comparisons
A strength-class match is not sufficient for substitution. Q690MD and Q690QD may satisfy similar nominal design strengths, but substitution can change thickness limits, toughness values, test orientation, chemical limits, weldability indices, heat-treatment response and product certification. Engineering approval should compare the complete requirements for the actual thickness and application.
S690Q to EN 10025-6 is commonly associated with the quenched-and-tempered Q690 strength class, but it is only an approximate counterpart. The EN designation, impact rules, chemistry, options, testing and dimensional standards remain controlling. The bridge-steel designation Q690qD is also a different material governed by GB/T 714 and must not be confused with general structural Q690D.
Standard status
GB/T 1591-2018, GB/T 16270-2009 and GB/T 34560.4-2017 are current standards as of October 1, 2026. A national revision project, 20251175-T-605, is in the approval stage and is intended to replace several current quenched-and-tempered plate standards, including GB/T 16270-2009 and GB/T 34560.4-2017. Until the replacement standard is formally published and implemented, contracts should continue to identify the applicable current edition explicitly.
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 — High strength low alloy structural steelsBaosteel
- GB/T 16270-2009 — High strength structural steel plates in the quenched and tempered condition: official standard recordState Administration for Market Regulation / Standardization Administration of China
- GB/T 16270-2009 — English standard previewChineseStandard.net
- GB/T 34560.4-2017 — Structural steels, Part 4: official standard recordState Administration for Market Regulation / Standardization Administration of China
- 20251175-T-605 — Revision project for quenched-and-tempered structural steel platesNational Public Service Platform for Standards Information
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