Quick Answer
Quick Answer
GB/T 1591 is China's national standard for high-strength low-alloy (HSLA) structural steel, covering grades Q345 through Q690. The 2018 revision redesignated the flagship grade from Q345 to Q355 to align yield strength designations with actual minimum values by thickness. Q345B remains widely cited in legacy projects and is broadly equivalent to ASTM A572 Gr 50 and EN S355JR.
GB/T 1591 (current edition: 2018, which replaced the 2008 edition) governs hot-rolled high-strength low-alloy structural steel plate, strip, sections, and bars. The standard covers grades from Q355 (formerly Q345) up to Q690, with microalloying additions of Nb, V, Ti, Cr, Ni, Cu, and Mo used to achieve yield strengths well above those obtainable with plain carbon steel.
Important transition note: The 2018 revision renamed Q345 → Q355, Q390 → Q390 (unchanged), Q420 → Q420 (unchanged), and added Q500, Q550, Q620, and Q690 as new high-strength grades. The legacy designation Q345 is still encountered in drawings, purchase orders, and MTCs issued before or referencing the 2008 edition. When verifying MTCs, confirm which edition applies. Q345 (2008 edition) and Q355 (2018 edition) have slightly different compositional limits; they are not identical substitutes without verification.
Scope and Applicability
GB/T 1591 applies to rolled HSLA steel used in:
- Heavy structural fabrication: bridges, port cranes, offshore platforms
- Building structures, particularly high-rise and long-span frames
- Construction machinery, pressure equipment (non-vessel), and vehicles
- Wind energy towers and transmission towers
- Shipbuilding structures (in conjunction with classification society rules)
The standard does not cover:
- Pressure vessel plate (use GB 713 Q345R/Q370R)
- Reinforcing bars (use GB 1499.2)
- Seamless structural tubes (use GB/T 8162)
- Fine-grain normalised pressure vessel plate (use GB/T 16270 for Q&T grades)
Available delivery conditions by grade: AR (as-rolled), N (normalised), TMCP (thermo-mechanically controlled process), Q+T (quenched and tempered). Higher grades (Q500 and above) are predominantly supplied in TMCP or Q+T condition.
Grade Coverage
| Grade (2018) | Legacy (2008) | Quality Grades | Yield Strength (≤16 mm), MPa |
|---|---|---|---|
| Q355 | Q345 | A, B, C, D, E | 355 |
| Q390 | Q390 | A, B, C, D, E | 390 |
| Q420 | Q420 | A, B, C, D, E | 420 |
| Q460 | Q460 | C, D, E | 460 |
| Q500 | — (new) | C, D, E | 500 |
| Q550 | — (new) | C, D, E | 550 |
| Q620 | — (new) | C, D, E | 620 |
| Q690 | — (new) | C, D, E | 690 |
Quality grade impact temperatures: A = +20 °C, B = +20 °C, C = 0 °C, D = −20 °C, E = −40 °C. Grades Q460 and above start at quality C (no A or B quality).
Chemical Composition Requirements
Ladle (heat) analysis per GB/T 700 Annex A procedures. All values wt% maximum unless a range is shown.
Q355 / Q390 / Q420 (formerly Q345/Q390/Q420)
| Grade | C max | Mn max | Si max | P max | S max | Nb max | V max | Ti max | Cr max | Ni max | Cu max | N max | CEV max |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Q355 A/B | 0.20 | 1.70 | 0.50 | 0.035 | 0.035 | 0.07 | 0.15 | 0.20 | 0.30 | 0.50 | 0.30 | 0.012 | 0.45 |
| Q355 C | 0.20 | 1.70 | 0.50 | 0.030 | 0.025 | 0.07 | 0.15 | 0.20 | 0.30 | 0.50 | 0.30 | 0.012 | 0.45 |
| Q355 D | 0.18 | 1.70 | 0.50 | 0.025 | 0.020 | 0.07 | 0.15 | 0.20 | 0.30 | 0.50 | 0.30 | 0.012 | 0.43 |
| Q355 E | 0.18 | 1.70 | 0.50 | 0.025 | 0.020 | 0.07 | 0.15 | 0.20 | 0.30 | 0.50 | 0.30 | 0.010 | 0.43 |
| Q390 A/B | 0.20 | 1.70 | 0.50 | 0.035 | 0.035 | 0.07 | 0.20 | 0.20 | 0.30 | 0.50 | 0.30 | 0.012 | 0.45 |
| Q390 C/D | 0.20 | 1.70 | 0.50 | 0.030 | 0.025 | 0.07 | 0.20 | 0.20 | 0.30 | 0.50 | 0.30 | 0.012 | 0.45 |
| Q390 E | 0.18 | 1.70 | 0.50 | 0.025 | 0.020 | 0.07 | 0.20 | 0.20 | 0.30 | 0.50 | 0.30 | 0.010 | 0.43 |
| Q420 A/B | 0.20 | 1.70 | 0.50 | 0.035 | 0.035 | 0.07 | 0.20 | 0.20 | 0.30 | 0.80 | 0.30 | 0.012 | 0.45 |
| Q420 C/D | 0.20 | 1.70 | 0.50 | 0.030 | 0.025 | 0.07 | 0.20 | 0.20 | 0.30 | 0.80 | 0.30 | 0.012 | 0.45 |
| Q420 E | 0.18 | 1.70 | 0.50 | 0.025 | 0.020 | 0.07 | 0.20 | 0.20 | 0.30 | 0.80 | 0.30 | 0.010 | 0.43 |
Q460 / Q500 / Q550 / Q620 / Q690
| Grade | C max | Mn max | Si max | P max | S max | Nb max | V max | Ti max | Cr max | Ni max | Cu max | N max | CEV max |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Q460 C/D | 0.20 | 1.80 | 0.60 | 0.030 | 0.025 | 0.11 | 0.20 | 0.20 | 0.70 | 0.80 | 0.55 | 0.015 | 0.47 |
| Q460 E | 0.18 | 1.80 | 0.60 | 0.025 | 0.020 | 0.11 | 0.20 | 0.20 | 0.70 | 0.80 | 0.55 | 0.010 | 0.46 |
| Q500 C/D | 0.18 | 1.80 | 0.60 | 0.025 | 0.020 | 0.11 | 0.12 | 0.20 | 0.80 | 0.80 | 0.55 | 0.015 | 0.47 |
| Q500 E | 0.16 | 1.80 | 0.60 | 0.020 | 0.010 | 0.11 | 0.12 | 0.20 | 0.80 | 0.80 | 0.55 | 0.010 | 0.47 |
| Q550 C/D | 0.18 | 2.00 | 0.60 | 0.025 | 0.020 | 0.11 | 0.12 | 0.20 | 1.50 | 0.80 | 0.55 | 0.015 | 0.55 |
| Q550 E | 0.16 | 2.00 | 0.60 | 0.020 | 0.010 | 0.11 | 0.12 | 0.20 | 1.50 | 0.80 | 0.55 | 0.010 | 0.55 |
| Q620 C/D | 0.18 | 2.00 | 0.60 | 0.025 | 0.020 | 0.11 | 0.12 | 0.20 | 1.50 | 0.80 | 0.55 | 0.015 | 0.55 |
| Q620 E | 0.16 | 2.00 | 0.60 | 0.020 | 0.010 | 0.11 | 0.12 | 0.20 | 1.50 | 0.80 | 0.55 | 0.010 | 0.55 |
| Q690 C/D | 0.18 | 2.00 | 0.60 | 0.025 | 0.020 | 0.11 | 0.12 | 0.20 | 1.50 | 0.80 | 0.55 | 0.015 | 0.55 |
| Q690 E | 0.16 | 2.00 | 0.60 | 0.020 | 0.010 | 0.11 | 0.12 | 0.20 | 1.50 | 0.80 | 0.55 | 0.010 | 0.55 |
CEV formula: CEV = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15
Microalloying elements (Nb, V, Ti) shall not be added simultaneously in amounts exceeding their individual maximums; total Nb + V + Ti ≤ 0.22% for Q355–Q460, ≤ 0.23% for Q500 and above.
Mechanical Properties
Q355 (formerly Q345) — Minimum Yield Strength (ReH, MPa)
| Thickness (mm) | ≤16 | >16–35 | >35–50 | >50–100 | >100–150 | >150–200 |
|---|---|---|---|---|---|---|
| ReH min (MPa) | 355 | 345 | 335 | 325 | 315 | 295 |
| Rm (MPa) | 470–630 | 470–630 | 470–630 | 470–630 | 450–600 | 450–600 |
| A min (%) | 22 | 22 | 22 | 21 | 20 | 20 |
Q390 — Minimum Yield Strength (ReH, MPa)
| Thickness (mm) | ≤16 | >16–35 | >35–50 | >50–100 | >100–150 | >150–200 |
|---|---|---|---|---|---|---|
| ReH min (MPa) | 390 | 380 | 360 | 340 | 330 | 310 |
| Rm (MPa) | 490–650 | 490–650 | 490–650 | 490–650 | 470–620 | 470–620 |
| A min (%) | 20 | 20 | 20 | 19 | 19 | 18 |
Q420 — Minimum Yield Strength (ReH, MPa)
| Thickness (mm) | ≤16 | >16–35 | >35–50 | >50–100 | >100–150 | >150–200 |
|---|---|---|---|---|---|---|
| ReH min (MPa) | 420 | 410 | 390 | 370 | 360 | 340 |
| Rm (MPa) | 520–680 | 520–680 | 520–680 | 520–680 | 500–650 | 500–650 |
| A min (%) | 19 | 19 | 19 | 18 | 18 | 17 |
Q460 — Minimum Yield Strength (ReH, MPa)
| Thickness (mm) | ≤16 | >16–35 | >35–50 | >50–100 | >100–150 |
|---|---|---|---|---|---|
| ReH min (MPa) | 460 | 440 | 420 | 400 | 385 |
| Rm (MPa) | 550–720 | 550–720 | 550–720 | 550–720 | 530–700 |
| A min (%) | 17 | 17 | 17 | 17 | 16 |
Q500 / Q550 / Q620 / Q690 (TMCP or Q+T only)
| Grade | ReH min ≤50 mm (MPa) | ReH min >50–100 mm (MPa) | Rm (MPa) | A min (%) |
|---|---|---|---|---|
| Q500 | 500 | 480 | 610–770 | 17 |
| Q550 | 550 | 530 | 670–830 | 16 |
| Q620 | 620 | 600 | 710–880 | 15 |
| Q690 | 690 | 670 | 770–940 | 14 |
Impact Test Requirements
Charpy V-notch impact test per GB/T 229, longitudinal specimens (10 × 10 × 55 mm). Minimum average energy: 34 J for all grades and quality levels.
| Quality Grade | Test Temperature | Min Average KV₂ |
|---|---|---|
| A | +20 °C | 34 J |
| B | +20 °C | 34 J |
| C | 0 °C | 34 J |
| D | −20 °C | 34 J |
| E | −40 °C | 34 J |
- Set of 3 specimens per test; individual minimum = 70% of required average (≥ 24 J).
- For Q460 and above, only quality grades C, D, E are available.
- For TMCP and Q+T product, impact energy requirements may be specified at higher values on the purchase order.
Quality Grades / Delivery Condition
| Condition | Code | Applicable Grades | Notes |
|---|---|---|---|
| As-rolled | AR | Q355–Q420 (A/B quality) | Carbon and microalloy strengthening only |
| Normalised | N | Q355–Q420 | Grain refinement; improved toughness |
| TMCP | TM | Q355–Q690 | Controlled rolling + accelerated cooling; lower CEV; better weldability |
| Quenched + Tempered | QT | Q460–Q690 | Required for highest strength grades |
TMCP product typically has lower CEV than equivalent N or AR product, enabling higher-heat-input welding without preheat in some applications.
Cross-Standard Equivalents
| GB/T 1591 Grade | ASTM Equivalent | EN 10025 Equivalent | IS Equivalent | JIS Equivalent |
|---|---|---|---|---|
| Q355B | A572 Gr 50 | S355JR (EN 10025-2) | E350A (IS 2062) | SM490A (JIS G3106) |
| Q355C | A572 Gr 50 | S355J0 | E350B | SM490B |
| Q355D | A572 Gr 50 | S355J2 | E350C | SM490C |
| Q355E | A572 Gr 50 | S355K2 | — | — |
| Q390B | A572 Gr 55 | S390 (EN 10025-3) | — | — |
| Q420B | A572 Gr 60 | S420 (EN 10025-3/4) | — | SM570 (JIS G3106) |
| Q420C | A572 Gr 60 | S420M (EN 10025-4) | — | — |
| Q460C | A572 Gr 65 | S460M (EN 10025-4) | — | — |
| Q460D | A572 Gr 65 | S460ML (EN 10025-4) | — | — |
| Q500C | A514 Gr B (partial) | S500M (EN 10025-4) | — | — |
| Q550C | A514 Gr B | S550M (EN 10025-4) | — | — |
| Q690C | A514 Gr B/F/Q | S690QL (EN 10025-6) | — | — |
Equivalencies are approximate. The EN S355 series under EN 10025-2 (non-alloy) does not mandate microalloying; Q355 may contain Nb/V/Ti additions not present in all S355 products.
MTC Verification Checklist
A 质量证明书 for GB/T 1591 material must include:
- Standard number: GB/T 1591 with edition year (2018 or 2008)
- Grade designation (e.g., Q355B or legacy Q345B) — confirm which edition applies
- Heat number (炉号) and product batch number (批号)
- Product form, dimensions, and heat/delivery condition (AR/N/TMCP/QT)
- Ladle chemical analysis: C, Mn, Si, P, S, Nb, V, Ti, Cr, Ni, Cu, N, CEV — all within limits above
- Mechanical test results: ReH, Rm, A — with thickness band noted
- Charpy impact test: temperature, three individual values, and average (≥ 34 J)
- TMCP or Q+T heat treatment records (if applicable)
- Mill certification authority signature and stamp
Flags for MTC reviewers:
- CEV higher than permitted limit for the quality grade (indicates poor weldability)
- Yield strength test thickness not matching the ordered thickness range
- Q345 designation on a certificate claiming 2018-edition compliance (should be Q355)
- Impact test temperature warmer than required for the specified quality grade
Frequently Asked Questions
What is the difference between Q345 and Q355?
Q345 is the grade designation used in the 2008 edition of GB/T 1591. The 2018 revision renamed it Q355 to better reflect the actual minimum yield strength, which — accounting for thickness — is 355 MPa for material ≤ 16 mm. The chemistry limits also changed slightly: Q355 (2018) has tighter P and S limits for D and E quality grades and a revised CEV formula compared to Q345 (2008). If you are ordering new material, specify Q355 per GB/T 1591-2018. Legacy drawings may still say Q345.
Is Q345B equivalent to ASTM A572 Grade 50?
They are broadly equivalent in yield strength (355 vs. 345 MPa) and tensile strength (470–630 vs. 448–621 MPa). Q345B requires a Charpy impact test at +20 °C (34 J minimum), while A572 Gr 50 does not mandate impact testing unless supplementary requirement S5 is invoked. Q345B also has explicit limits on Nb, V, Ti, and Cu microalloying elements. For most structural applications the substitution is acceptable, but confirm with the design engineer.
What does TMCP mean on a GB/T 1591 MTC?
TMCP stands for Thermo-Mechanically Controlled Process (热机械控制工艺). It involves controlled rolling at specific temperatures followed by accelerated water cooling, producing a fine-grained microstructure without requiring normalising or quenching. TMCP product achieves the same or higher strength as normalised material with lower carbon equivalent (CEV), which improves weldability — particularly relevant for offshore, bridge, and high-heat-input submerged-arc welding applications.
How do I verify the carbon equivalent (CEV) on a GB/T 1591 MTC?
Calculate CEV using: CEV = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15. Insert the ladle chemistry values from the MTC and compare the result against the maximum permitted value for the grade and quality. For Q355D, CEV max is 0.43. If Mo is not listed on the MTC (as is common for microalloyed grades without intentional Mo), treat it as 0. TestCert performs this calculation automatically from the chemistry values on the MTC.
Are Q500, Q550, Q620, and Q690 new grades?
Yes. These four grades were introduced in the 2018 revision of GB/T 1591. They require TMCP or Q+T delivery and are only available in quality grades C, D, and E. These grades compete with European S500–S690 grades and ASTM A514 for high-strength structural applications such as heavy lift cranes, mining equipment, and offshore topside modules.
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