What Q355B Steel Plate Is
Q355B is a low-alloy high-strength structural steel defined by GB/T 1591. In the designation, Q stands for yield, 355 is the minimum specified yield strength in MPa for the common thickness range, and B is the impact quality class, tested at +20 °C with a minimum absorbed energy of 27 J. Q355B is supplied as plate, strip and sections for welded and bolted construction. It is not a seamless pipe grade: describing it as an alloy steel pipe confuses two separate product families, because pipe and tube are ordered to their own standards with their own chemistry and testing rules.
Chemistry and Mechanical Properties
| Element | Typical requirement, GB/T 1591 | Function |
|---|---|---|
| C | 0.20 % maximum | Strength, controlled for weldability |
| Si | 0.55 % maximum | Deoxidation and solid solution strengthening |
| Mn | 1.60 % maximum | Strength and toughness balance |
| P / S | 0.035 % maximum each | Impurity control for toughness and weld quality |
| Nb, V, Ti | Micro additions, within standard limits | Grain refinement and precipitation strengthening |
Mechanical properties fall as thickness rises, which is why the design value must be taken at the actual plate thickness rather than at the headline figure.
| Thickness range | Minimum yield strength | Tensile strength | Elongation |
|---|---|---|---|
| Up to 16 mm | 355 MPa | 470 to 630 MPa | 22 % minimum |
| Over 16 to 40 mm | 345 MPa | 470 to 630 MPa | 22 % minimum |
| Over 40 to 63 mm | 335 MPa | 470 to 630 MPa | 21 % minimum |
| Over 63 to 100 mm | 325 MPa to 315 MPa | 470 to 630 MPa | 20 % minimum |
How Q355B Compares with Other Standards
The closest European counterpart is S355JR of EN 10025-2, which shares a 355 MPa minimum yield in thin sections and an impact test at +20 °C. ASTM A572 Gr.50 is similar in strength level but is not normally ordered with a specified impact energy, and JIS SM490A sits slightly lower at 325 MPa minimum yield. Equivalence is therefore never automatic: the impact class, thickness range, delivery condition and permitted stress values all have to be checked against the governing design code. Where service temperature drops below room temperature, Q355C, Q355D or Q355E should be specified instead of grade B, because they are impact tested at 0 °C, -20 °C and -40 °C respectively.
Wide Application Fields
Building frames: multi-storey steel structures, columns, beams, trusses and prefabricated frames.
Bridges and highway structures: girder webs and flanges, temporary works and support structures.
Industrial plants: equipment frames, platforms, walkways, racks and general fabrication.
Transmission and communication towers, lighting columns and structural masts.
Machinery: excavator and crane structures, mining equipment, formwork and heavy plant supports.
Non-pressure tanks, silos and hoppers, where strength rather than pressure containment governs the design.
Fabrication Notes: Welding, Cutting and Forming
Carbon equivalent and Pcm values should be checked before welding thick sections; preheat is generally required above moderate thickness and at low shop temperature.
Use low-hydrogen consumables matched to the base plate strength, and keep heat input within the qualified range to protect the heat affected zone toughness.
For thick restrained joints carrying through-thickness stress, specify a Z-quality plate such as Q355B-Z15, Z25 or Z35 to reduce the risk of lamellar tearing.
Hot forming is normally followed by normalising to restore the specified properties, while cold forming needs a check on bend radius and edge condition.
Post-weld heat treatment is not routine for Q355B, but it may be specified for thick, highly restrained or sour service assemblies.
How to Specify Q355B Plate Correctly
State the grade, thickness and delivery condition: as-rolled, normalised or thermo-mechanically rolled.
Confirm the impact class against the minimum design metal temperature.
Add dimensional and tolerance requirements, edge condition, and ultrasonic testing for critical joints.
Ask for a mill certificate with heat number, chemistry, mechanical test results and heat treatment record.
Frequently Asked Questions
Q: Is Q355B the same as S355JR?
They are close in strength and impact class, but they are defined by different standards with different chemistry limits and tolerances, so equivalence must be agreed for each project.
Q: Can Q355B be used at low temperature?
Grade B is impact tested at +20 °C only. For cold climates or refrigerated service, specify Q355C, Q355D or Q355E according to the design temperature.
Q: Is Q355B an alloy steel pipe?
No. It is a low-alloy structural steel plate and section grade. Pipes and tubes follow separate product standards and cannot be ordered simply as Q355B plate.
Q: Does Q355B require post-weld heat treatment?
Usually not for ordinary welded structures. It may be required for thick sections, highly restrained joints or specific service environments stated in the specification.
Q: Which thickness should be used for the design stress?
The actual ordered thickness. Yield strength falls in steps as plate thickness increases, so a design based on the thin-section value can be unsafe for heavy plate.
Q: What does the B in Q355B mean?
It is the impact quality class, giving an impact test at +20 °C with a minimum absorbed energy of 27 J.
