Q355E
Low-alloy high-strength structural steel with specified low-temperature impact toughness · GB/T 1591-2018, High strength low alloy structural steels
Under the current GB/T 1591-2018 designation system, “Q355E” does not fully identify a standard grade: quality class E is available as Q355NE in the normalized/normalizing-rolled condition or Q355ME in the TMCP condition. Both are nominal 355 MPa-class structural steels with Charpy V-notch requirements at −40 °C, but their chemistry, weldability limits, thickness ranges and response to heating differ. Procurement documents should therefore replace bare “Q355E” with the intended complete designation.
- “Q355E” is incomplete under GB/T 1591-2018Designation status · Specify Q355NE or Q355ME; the delivery-condition letter is technically significant.
- GB/T 1591-2018Current specification · Published May 14, 2018, implemented February 1, 2019, and listed as current by China’s national standards system.
- 355 MPa minimum upper yield strength at t ≤ 16 mmStrength class · The required yield strength decreases in steps as nominal thickness or diameter increases.
- KV2 ≥ 31 J longitudinal or ≥ 20 J transverse at −40 °CE-quality toughness · Longitudinal testing is the default; transverse testing is subject to agreement.
- Q355NE: normalized/normalizing rolled; Q355ME: TMCPTwo valid material routes · They must not be treated as interchangeable delivery conditions without engineering review.
- Q345E under GB/T 1591-2008Historical designation · The 2018 edition replaced the Q345 strength series with Q355 and made delivery condition part of the designation.
Overview
- Designation system
- Chinese GB/T structural-steel designation system
- Product forms
- Hot-rolled plate, Hot-rolled strip and cut-to-length plate, Hot-rolled sections, Hot-rolled bars, Steel billets, for chemical composition requirements
- Condition
- Normalized or normalizing rolled: Q355NE, Thermomechanically processed, including permitted TMCP accelerated-cooling variants: Q355ME
The designation must be clarified first
GB/T 1591-2018 constructs a designation from Q, the specified minimum upper yield strength, the delivery-condition symbol and the quality class. The standard gives examples such as Q355ND. For E-quality Q355 steel, the valid choices are Q355NE and Q355ME; there is no plain as-rolled Q355E grade in the 2018 tables.
| Designation | Delivery condition | Quality-class significance | Principal consequence |
|---|---|---|---|
| Q355NE | Normalized or normalizing rolled | E-quality Charpy requirements, principally at −40 °C | Properties are based on a normalized-type metallurgical condition. |
| Q355ME | Thermomechanically processed (TMCP) | E-quality Charpy requirements, principally at −40 °C | Lower-carbon, process-controlled steel with distinct composition and weldability limits. |
| Q355E | Not a complete GB/T 1591-2018 designation | Often used informally to mean a Q355 steel with E-level toughness | Must be clarified before ordering, certification or substitution. |
The “E” suffix defines an impact-test quality class; it does not by itself establish a permissible design temperature or guarantee fracture resistance for a completed structure.
Chemical composition and weldability controls
| Grade | C | Si | Mn | P | S |
|---|---|---|---|---|---|
| Q355NE | ≤ 0.16% | ≤ 0.50% | 0.90–1.65% | ≤ 0.025% | ≤ 0.020% |
| Q355ME | ≤ 0.14% | ≤ 0.50% | ≤ 1.60% | ≤ 0.025% | ≤ 0.020% |
Ladle analysis. GB/T 1591 also controls microalloying and residual elements. The complete chemistry table, rather than only these principal elements, must be used for conformity assessment.
Both routes may use niobium, vanadium and titanium for grain refinement and precipitation strengthening. The standard controls these additions and requires appropriate fine-grain practice. Finished-product analysis is performed when required by the purchaser, with permissible deviations governed by GB/T 222.
| Grade | Nominal thickness or diameter | Maximum |
|---|---|---|
| Q355NE | ≤ 63 mm | CEV 0.43% |
| Q355NE | > 63–250 mm | CEV 0.45% |
| Q355NE | > 250–400 mm | By agreement |
| Q355ME | ≤ 40 mm | CEV 0.39% |
| Q355ME | > 40–63 mm | CEV 0.40% |
| Q355ME | > 63–150 mm | CEV 0.45% |
| Q355ME | All tabulated thicknesses | Pcm 0.20% maximum |
For Q355ME, the standard permits Pcm to be used in place of CEV when carbon is not greater than 0.12%; the applicable metric should be stated in the order when it matters.
Thickness-dependent tensile requirements
| Nominal thickness or diameter | Minimum upper yield strength | Tensile strength |
|---|---|---|
| ≤ 16 mm | 355 MPa | 470–630 MPa |
| > 16–40 mm | 345 MPa | 470–630 MPa |
| > 40–63 mm | 335 MPa | 470–630 MPa |
| > 63–80 mm | 325 MPa | 470–630 MPa |
| > 80–100 mm | 315 MPa | 470–630 MPa |
| > 100–150 mm | 295 MPa | 450–600 MPa |
| > 150–200 mm | 285 MPa | 450–600 MPa |
| > 200–250 mm | 275 MPa | 450–600 MPa |
Where a distinct yield phenomenon is not present, proof strength Rp0.2 may be used as specified by the standard.
| Nominal thickness or diameter | Minimum upper yield strength | Tensile strength |
|---|---|---|
| ≤ 16 mm | 355 MPa | 470–630 MPa |
| > 16–40 mm | 345 MPa | 470–630 MPa |
| > 40–63 mm | 335 MPa | 450–610 MPa |
| > 63–80 mm | 325 MPa | 440–600 MPa |
| > 80–100 mm | 325 MPa | 440–600 MPa |
| > 100–120 mm | 320 MPa | 430–590 MPa |
The general tabulated range for Q355M extends to 120 mm; special product-form provisions and agreed requirements must be checked before applying values beyond the relevant table range.
The designation number 355 is therefore not a universal yield value for every thickness. Structural calculations, purchase specifications and incoming inspection must use the requirement for the actual product thickness or diameter and delivery condition.
Low-temperature impact toughness
| Test temperature | Longitudinal minimum KV2 | Transverse minimum KV2 | Status |
|---|---|---|---|
| 20 °C | 63 J | 40 J | Associated E-quality requirement |
| 0 °C | 55 J | 34 J | Associated E-quality requirement |
| −20 °C | 47 J | 27 J | Associated E-quality requirement |
| −40 °C | 31 J | 20 J | Defining E-quality test temperature |
| −50 °C | 27 J | 16 J | Optional when specifically required and agreed |
The standard normally uses longitudinal specimens. Transverse specimens may be agreed. Standard 10 mm × 10 mm specimens are used when product dimensions permit; prescribed reductions apply to permitted sub-size specimens.
Impact qualification applies to specimens sampled and tested according to the standard. It does not directly establish the lowest operating temperature of a welded component. Service-temperature selection also depends on section thickness, stress state, weld details, heat-affected-zone toughness, strain rate, flaw tolerance and the applicable design code.
Processing and fabrication implications
Both Q355NE and Q355ME are weldable structural steels, but welding controls should be based on the certified ladle chemistry, CEV or Pcm, thickness, restraint, hydrogen level, heat input and ambient temperature. The grade designation alone is insufficient to select preheat, interpass temperature or consumables.
Q355ME obtains part of its property balance from controlled rolling and cooling. GB/T 1591 cautions that hot forming or post-weld heat treatment above 580 °C may reduce the strength of TMCP steel. Any high-temperature forming, stress relief or extended thermal cycle should therefore be reviewed with the mill and the governing fabrication code before Q355ME is selected.
Cold forming limits, minimum bend radii, flame straightening, welding-procedure qualification and heat-treatment rules are fabrication-specific matters. The standard offers an optional bend test but does not replace the requirements of the applicable structural or fabrication code.
Product form, inspection and ordering
GB/T 1591 covers plate, strip, sections, bars and related structural products, while dimensions, shape, mass and tolerances are referred to the applicable product-form standards. A purchase description must therefore identify both GB/T 1591 and the dimensional or product standard appropriate to the supplied form.
| Item | Recommended requirement |
|---|---|
| Complete grade | State Q355NE or Q355ME; do not leave the designation as Q355E. |
| Standard edition | GB/T 1591-2018 unless another edition is contractually required. |
| Product definition | Plate, strip, section or bar; dimensions, tolerances and applicable dimensional standard. |
| Impact testing | E quality, test temperature, longitudinal or transverse orientation and any −50 °C supplementary requirement. |
| Weldability data | Require reported ladle chemistry and applicable CEV or Pcm when needed for welding engineering. |
| Certification | Specify the required inspection document and traceability to heat and test batch. |
| Optional tests | State any agreed bend test, finished-product analysis, ultrasonic examination or other NDT. |
| Through-thickness properties | For plate requiring Z-direction ductility, add the applicable GB/T 5313 class, such as Z25, to the designation and order. |
| Surface quality | State any supplementary surface-quality standard or acceptance class needed by the project. |
A certificate marked only “Q355E” should be queried. Confirm the actual heat-treatment or rolling condition and verify that the chemistry, tensile and impact results correspond to either Q355NE or Q355ME.
Substitution and cross-standard comparison
GB/T 1591-2018 provides an informative comparison table linking Q355NE with the normalized 355 MPa class and Q355ME with the TMCP 355 MPa class in ISO and European standards. These associations are useful for orientation but do not establish automatic interchangeability. Product form, thickness range, composition, tensile properties, impact energy and orientation, dimensional standards, inspection and certification must be compared before substitution.
Sources
- GB/T 1591-2018 — National standard information pageState Administration for Market Regulation / Standardization Administration of China
- GB/T 1591-2018 — High strength low alloy structural steelsNational Standard of the People's Republic of China; hosted by Baosteel
- National Standard GB/T 1591-2018 — detailed recordNational Public Service Platform for Standards Information
- Professional Guide on Selection of Equivalent Steel Materials to European Steel Materials SpecificationsChinese National Engineering Research Centre for Steel Construction, The Hong Kong Polytechnic University
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