Q355B
High-strength low-alloy structural steel · GB/T 1591-2018 — High strength low alloy structural steels · GB/T 2975-2018 — Sampling locations and preparation of mechanical test pieces · GB/T 5313-2023 — Steel plate with through-thickness properties, when Z-quality is ordered
Q355B is the as-rolled, quality-grade B member of the Q355 high-strength structural-steel family in GB/T 1591-2018. It combines a minimum specified upper yield strength of 355 MPa at thicknesses up to 16 mm with a mandatory room-temperature Charpy requirement. Strength, elongation, tensile range and carbon-equivalent limits change with product thickness, so the designation alone is insufficient for procurement or substitution without product form, dimensions, delivery condition and test orientation.
- Q + 355 + BDesignation meaning · Q denotes yield strength; 355 is the minimum specified upper yield strength in MPa for the smallest thickness range; B is the quality grade.
- As-rolledDelivery condition · AR or WAR may be omitted, making Q355B and Q355B(AR) alternative expressions for the same designation.
- 355 MPa minimum at t or d ≤16 mmYield strength · The specified minimum falls progressively to 275 MPa at greater than 200 mm up to 250 mm.
- 470–630 MPa at t or d ≤100 mmTensile strength · The specified range is 450–600 MPa above 100 mm through 250 mm.
- KV2 ≥34 J longitudinal or ≥27 J transverse at 20 °CImpact requirement · Applies to full-size specimens; impact testing is required from specified minimum product dimensions.
- Q345B under GB/T 1591-2008Predecessor designation · Q355B replaced Q345B in the 2018 revision; it should not be treated as an unchanged relabeling.
Overview
- Designation system
- Chinese GB/T strength-based structural-steel designation
- Product forms
- Hot-rolled plate and sheet, Hot-rolled strip, Structural sections, Bars
- Condition
- As-rolled (AR or WAR); the delivery-condition code is omitted from the Q355B designation
- Density
- 7850 kg/m³ (Representative density used for mass calculations; not a grade-specific acceptance requirement in GB/T 1591.)
What the designation controls
GB/T 1591-2018 covers low-alloy high-strength steel plate, strip, sections and bars for general structural and engineering use; its chemical-composition provisions also apply to billets. Q355B specifically identifies the as-rolled Q355 grade with quality level B. Normalized or normalizing-rolled material is designated Q355NB, while thermomechanically processed material is Q355MB. These are separate standardized designations with different chemistry and mechanical-property tables, not merely optional processing descriptions for Q355B.
Quality grade B means the standard Charpy requirement is specified at 20 °C. It does not establish suitability for sub-zero service; Q355C, Q355D or another explicitly qualified grade may be necessary where lower-temperature toughness is required.
Chemical composition and weldability controls
| Element or parameter | Requirement, mass % | Qualification |
|---|---|---|
| C | ≤0.24 | For nominal thickness above 30 mm: ≤0.22 |
| Si | ≤0.55 | Maximum |
| Mn | ≤1.60 | Maximum |
| P | ≤0.035 | Maximum |
| S | ≤0.035 | Maximum |
| Cr | ≤0.30 | Maximum |
| Ni | ≤0.30 | Maximum |
| Cu | ≤0.40 | Maximum |
| N | ≤0.012 | Maximum; may be exceeded when sufficient nitrogen-fixing elements are present |
| B | ≤0.004 | Maximum |
| Al total | ≥0.015 | Applicable to killed steel; an acid-soluble aluminium criterion may be used as provided by the standard |
These are grade requirements for heat analysis, not typical mill values. Product analysis, when ordered, is subject to the permissible deviations of GB/T 222.
Q355B is not defined by mandatory niobium, vanadium or titanium ranges. Additional alloying or microalloying elements may be used by agreement to obtain the required properties, and their contents are to be stated on the quality certificate. A supplier's Nb-V-Ti practice must therefore not be presented as a universal Q355B composition requirement.
| Nominal thickness or diameter | Maximum CEV |
|---|---|
| ≤30 mm | 0.45% |
| >30–63 mm | 0.47% |
| >63–150 mm | 0.47% |
| >150–250 mm | 0.49% |
These limits apply to as-rolled Q355B. The grade-B tables do not provide requirements above 250 mm.
Thickness-dependent tensile requirements
| Nominal thickness or diameter | Minimum upper yield strength ReH | Tensile strength Rm |
|---|---|---|
| ≤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 yielding is not distinct, Rp0.2 may replace upper yield strength ReH. Requirements above 250 mm are not specified for Q355B.
| Nominal thickness or diameter | Longitudinal A | Transverse A |
|---|---|---|
| ≤40 mm | 22% | 20% |
| >40–63 mm | 21% | 19% |
| >63–100 mm | 20% | 18% |
| >100–150 mm | 18% | 18% |
| >150–250 mm | 17% | 17% |
For plate and strip at least 600 mm wide, tensile specimens are normally transverse. Other products are normally tested longitudinally. Orientation must therefore be retained when comparing certificates or standards.
The number 355 is not a thickness-independent design value. For procurement, design verification and substitution, use the requirement corresponding to the actual nominal product thickness or diameter.
Impact toughness, bending and through-thickness properties
| Test temperature | Specimen orientation | Minimum absorbed energy KV2 |
|---|---|---|
| 20 °C | Longitudinal | 34 J |
| 20 °C | Transverse | 27 J |
Impact tests normally use longitudinal specimens unless transverse testing is agreed. Full-size specimens are 10 × 10 × 55 mm.
Impact testing is required for products with nominal thickness of at least 6 mm or nominal diameter of at least 12 mm. Where the product cannot accommodate a full-size specimen, 7.5 mm and 5 mm subsize specimens may be used, with required absorbed energies of 75% and 50% respectively of the full-size value. Larger available specimens take precedence.
| Nominal thickness or diameter a | Mandrel diameter D |
|---|---|
| ≤16 mm | D = 2a |
| >16–100 mm | D = 3a |
The bend test is performed when requested. It may be omitted where the supplier guarantees compliance.
Where resistance to lamellar tearing or verified through-thickness ductility is required, the plate must be ordered with a Z-quality suffix, for example Q355BZ25, and with requirements to the applicable edition of GB/T 5313. Ordinary Q355B does not include a through-thickness reduction-of-area guarantee.
Ordering, inspection and certification
A technically complete order should identify GB/T 1591-2018, the product type, Q355B designation, dimensions, shape and dimensional-tolerance standard, quantity or mass, and all required options. Options that should be considered explicitly include product analysis, impact-test orientation, bending, through-thickness properties, surface-quality class, nondestructive testing and any special smelting or certification requirements. Dimensional requirements are taken from the applicable product standard rather than from the grade designation alone.
| Product form | Referenced standard |
|---|---|
| Hot-rolled bars | GB/T 702 |
| Hot-rolled sections | GB/T 706 |
| Hot-rolled plate and strip | GB/T 709 |
| Hot-rolled H-sections and split T-sections | GB/T 11263 |
The purchase specification should identify the applicable dimensional standard and required tolerance class.
Normal acceptance batches comprise one designation, heat, specification and delivery condition, generally limited to 60 t. Q355B has a special mixed-heat concession: up to six heats may be combined where designation, melting and casting method, dimensions, production route and delivery or heat-treatment system are the same, provided the heat-to-heat carbon difference does not exceed 0.02 percentage points and the manganese difference does not exceed 0.15 percentage points. Purchasers requiring strict single-heat traceability should prohibit mixed-heat batching in the order.
Nondestructive examination is not inherent in the Q355B designation. The method, acceptance level, coverage and governing NDT standard must be agreed in the contract.
Fabrication and substitution considerations
Q355B is intended for welded and fabricated structural work, but compliance with the composition and CEV limits does not by itself define a welding procedure. The fabricator should base consumable selection, preheat, interpass temperature and heat input on the actual certificate chemistry, thickness, restraint and governing fabrication code. Procedures should preserve the specified heat-affected-zone toughness where impact performance is important.
Cold forming must account for thickness, rolling direction, edge condition and local strain. The optional standard bend requirements are acceptance criteria for test pieces, not universal minimum production bend radii for every component geometry. Thermal cutting, flame straightening and post-weld heat treatment should likewise be controlled by qualified fabrication procedures rather than inferred solely from the grade name.
GB/T 1591 lists Q355B(AR), ISO 630-2 S355B and EN 10025-2 S355JR in its informative comparison table. This is useful for market comparison but does not establish unconditional interchangeability. A substitution review must compare product standard, delivery condition, thickness-dependent yield and tensile properties, chemistry and CEV, Charpy temperature and orientation, dimensional tolerances, testing, batch rules and certification.
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 steelChina Standard Press reproduction hosted by Baosteel
- GB/T 2975-2018 — Steel and steel products: location and preparation of samples and test pieces for mechanical testingState Administration for Market Regulation / Standardization Administration of China
- GB/T 5313-2023 — Steel plate with through-thickness propertiesState Administration for Market Regulation / Standardization Administration of China
Ask MetalMate about Q355B
Discussing: Q355B
AI assistant · Responses are generated and can contain errors; verify specifications before acting.