Q355C
Hot-rolled high-strength low-alloy structural steel · GB/T 1591-2018 — High strength low alloy structural steels
Q355C is the quality-grade C, as-rolled Q355 structural steel defined by GB/T 1591-2018. It combines a minimum upper yield strength of 355 MPa at nominal thickness or diameter up to 16 mm with a specified Charpy V-notch toughness at 0 °C. Strength, elongation and carbon-equivalent limits depend on product thickness, while dimensions, tolerances and surface requirements also depend on the ordered product form. Q355C must not be confused with the normalized Q355NC or TMCP Q355MC grades.
- 355 MPa minimum at t or d ≤16 mmSpecified yield strength · The minimum decreases progressively with thickness, reaching 275 MPa for >200–250 mm.
- Charpy KV2 at 0 °CQuality class C · Minimum table values are 34 J longitudinal or 27 J transverse; longitudinal testing is the default unless transverse testing is agreed.
- As-rolledDelivery condition · AR or WAR may be omitted from the designation; normalized and TMCP products use Q355NC and Q355MC respectively.
- 470–630 MPa at t or d ≤100 mmTensile strength · The specified range is 450–600 MPa above 100 mm through 250 mm.
- CEV ≤0.45% at ≤30 mmCarbon equivalent · For Q355C, the limit is 0.47% for >30–150 mm; no standard table value is provided for Q355C above 150 mm.
- Q345C under GB/T 1591-2008Predecessor designation · Q355C replaced Q345C in the 2018 revision; legacy designation does not by itself establish compliance with the current edition.
Overview
- Designation system
- Chinese GB/T yield-strength and quality-grade designation
- Product forms
- Hot-rolled plate and sheet, Hot-rolled strip, Hot-rolled sections, including H and cut T sections, Hot-rolled bars
- Condition
- As-rolled (AR or WAR); the delivery-condition code is omitted in the designation Q355C
- Density
- 7.85 g/cm³ (Representative engineering density for structural steel; not a grade-specific requirement of GB/T 1591.)
Designation, scope and delivery condition
The designation separates strength, delivery condition and toughness class. “Q” denotes yield strength, “355” is the specified minimum upper yield strength in MPa for the initial thickness range, and “C” is the quality grade associated with impact testing at 0 °C. Because no N or M appears between 355 and C, Q355C is the as-rolled grade. Normalized or normalizing-rolled material is Q355NC, while thermomechanically processed material is Q355MC; their chemistry and mechanical-property tables are different and must not be merged with Q355C data.
GB/T 1591 covers structural steel plate, strip, sections and bars for general structural and engineering purposes. Product dimensions and tolerances are controlled through the applicable dimensional standard: GB/T 709 for plate and strip, GB/T 706 for general hot-rolled sections, GB/T 11263 for hot-rolled H and cut T sections, and GB/T 702 for round and square bars. Ordering only “Q355C” therefore does not completely define the product.
Chemical composition and weldability control
| Element | Requirement, mass % | Important qualification |
|---|---|---|
| C | ≤0.20 at nominal thickness or diameter ≤40 mm; ≤0.22 above 40 mm | For sections above 30 mm, C may be up to 0.22%; for sections above 100 mm it may be agreed between purchaser and supplier. |
| Si | ≤0.55 | Low-silicon variants may be ordered for processes such as hot-dip galvanizing, subject to the standard's adjusted CEV provisions. |
| Mn | ≤1.60 | Maximum, not a target composition. |
| P | ≤0.030 | For sections and bars, the maximum may be increased by 0.005 percentage point. |
| S | ≤0.030 | For sections and bars, the maximum may be increased by 0.005 percentage point; plates ordered with through-thickness properties must meet GB/T 5313. |
| Cr | ≤0.30 | Included in the CEV calculation. |
| Ni | ≤0.30 | Included in the CEV calculation. |
| Cu | ≤0.40 | Included in the CEV calculation. |
| N | ≤0.012 | The nitrogen limit need not apply when sufficient aluminum or other nitrogen-fixing elements are present; these additions are to be identified on the certificate. |
| Nb, V, Ti, Mo, B | No specific Q355C value stated in the basic hot-rolled composition row | Additional alloying may be agreed to improve properties and must be reported in the quality certificate. |
These are melting-analysis requirements. Finished-product analysis is an optional order requirement, with permissible deviations assessed under GB/T 222.
| Nominal thickness or diameter | CEV maximum |
|---|---|
| ≤30 mm | 0.45% |
| >30–63 mm | 0.47% |
| >63–150 mm | 0.47% |
| >150–250 mm | No Q355C value specified in the standard table |
Where silicon is specially restricted, the standard permits limited increases in maximum CEV: +0.02 percentage point for Si ≤0.030% or +0.01 percentage point for Si ≤0.25%. These exceptions should be visible in the purchase specification and certificate.
Q355C is commonly welded, but the grade name alone is not a welding procedure. Preheat, heat input, consumable strength and hydrogen control should be established from the actual certified CEV and composition, thickness, joint restraint, process and applicable fabrication code. The C quality letter defines impact-test temperature; it does not guarantee a lower CEV than Q355B of the same thickness.
Thickness-dependent tensile properties
| Nominal thickness or diameter | ReH minimum |
|---|---|
| ≤16 mm | 355 MPa |
| >16–40 mm | 345 MPa |
| >40–63 mm | 335 MPa |
| >63–80 mm | 325 MPa |
| >80–100 mm | 315 MPa |
| >100–150 mm | 295 MPa |
| >150–200 mm | 285 MPa |
| >200–250 mm | 275 MPa |
Where yielding is not distinct, Rp0.2 may replace upper yield strength. The value 355 MPa must not be applied indiscriminately to all thicknesses.
| Nominal thickness or diameter | Rm | A longitudinal, min. | A transverse, min. |
|---|---|---|---|
| ≤40 mm | 470–630 MPa | 22% | 20% |
| >40–63 mm | 470–630 MPa | 21% | 19% |
| >63–100 mm | 470–630 MPa | 20% | 18% |
| >100–150 mm | 450–600 MPa | 18% | 18% |
| >150–250 mm | 450–600 MPa | 17% | 17% |
For plate and strip wider than 600 mm, tensile specimens are transverse; other products use longitudinal specimens. The elongation table reports both orientations because orientation materially affects the requirement.
The standard's Q355C tensile-property tables extend through 250 mm. Availability in the upper thickness ranges should not be assumed; confirm product form, mill capability, CEV treatment and certification before purchase.
Impact toughness and test applicability
| Test temperature | Specimen orientation | Minimum KV2 |
|---|---|---|
| 0 °C | Longitudinal | 34 J |
| 0 °C | Transverse | 27 J |
Longitudinal impact specimens are the default. Transverse specimens may be used by agreement between supplier and purchaser.
Impact testing applies to products with nominal thickness above 6 mm or nominal diameter above 12 mm. A full-size specimen is 10 mm × 10 mm × 55 mm. If the product cannot provide a full-size specimen, 7.5 mm or 5 mm sub-size specimens may be used, with required absorbed energies reduced to 75% or 50% of the full-size value respectively. The inspection plan calls for three impact specimens per lot.
Q355C should not be selected merely because the anticipated ambient temperature is near 0 °C. Structural design rules, thickness, stress state, strain rate, weld details and consequence of brittle fracture may require a lower-temperature quality grade or additional fracture-toughness assessment.
Processing and optional requirements
The as-rolled condition gives mills flexibility to achieve the required properties without a specified normalization or TMCP route. If subsequent fabrication includes severe hot forming, unusual heat treatment or demanding welded details, the suitability of the delivered condition should be reviewed rather than assuming that Q355C, Q355NC and Q355MC respond identically.
| Item | Standard treatment | Practical ordering consequence |
|---|---|---|
| Bend test | At purchaser request; supplier may guarantee compliance without testing | For nominal thickness or diameter ≤16 mm, the 180° bend diameter is D=2a; for >16–100 mm, D=3a. |
| Through-thickness properties | Optional under GB/T 5313 | Specify the required Z-quality level when lamellar-tearing resistance is needed. |
| Non-destructive testing | By agreement | State method, acceptance class, coverage and applicable standard in the contract. |
| Enhanced plate surface class | May be agreed under GB/T 14977 | Specify the required surface class rather than relying on the basic visual requirements. |
| Product analysis | Optional | Request when verification of the finished product, rather than only melting analysis, is required. |
| Impact orientation or special sampling location | May be agreed | State transverse testing or project-specific sampling explicitly. |
| Added alloying elements | Permitted by agreement | Require all additions and contents to be reported on the quality certificate. |
Ordering, certification and inspection
A technically complete order should identify GB/T 1591-2018, Q355C, product form, nominal dimensions, dimensional and shape tolerance class, quantity or mass, and the required certificate. It should also address impact orientation where non-default, surface class, ultrasonic or other NDT, through-thickness properties, product analysis and any project-specific chemistry or CEV restrictions.
| Inspection item | Standard sampling frequency |
|---|---|
| Melting analysis | One sample per heat |
| Tensile test | One specimen per lot |
| Impact test | Three specimens per lot |
| Bend test, when applicable | One specimen per lot |
| Through-thickness test, when ordered | Three specimens per lot under GB/T 5313 |
| Dimensions and surface quality | Piece-by-piece or coil-by-coil, as applicable |
A normal lot consists of the same designation, heat, specification and delivery condition and is generally limited to 60 t, subject to the standard's stated exceptions.
The certificate should be checked against the actual product marking, heat number, dimensions and delivery condition. Verify thickness-dependent yield and CEV requirements rather than checking only the headline “355 MPa” designation.
Related grades and substitution limits
GB/T 1591-2018 informatively associates Q355C in the as-rolled condition with ISO S355C and EN 10025-2 S355J0, and identifies Q345C under GB/T 1591-2008 as its predecessor. These comparisons are useful for orientation but do not create automatic interchangeability. Substitution requires comparison of product standard, delivery condition, thickness range, chemistry and CEV, tensile and impact requirements, sampling orientation, dimensional tolerances, inspection, certification and project approval.
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 steelsStandards Press of China reproduction hosted by Baosteel
- Professional Guide on Selection of Equivalent Steel Materials to European Steel Materials SpecificationsChinese National Engineering Research Centre for Steel Construction (Hong Kong Branch), The Hong Kong Polytechnic University
- GB/T 1591-2018 standard status and bibliographic informationNational Public Service Platform for Standards Information
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