Steel Q355B
GB Q355B
Q355B is the Chinese GB/T 1591 low-alloy high-strength structural steel (355 MPa minimum yield for thickness ≤16 mm) that replaced the old Q345B designation; it is essentially the domestic equivalent of S355JR/A572 Gr.50.
Values for the HR condition
Q355B tempers and conditions
Q355B is supplied in 3 conditions, soft to hard. HR is the default FabDigit quotes; the values on this page are for it.
Compare all 3 conditionsHide
| Condition · open for datasheet | Best for | Yield MPa | Tensile MPa | Elong. % | Hardness | Conductivity |
|---|---|---|---|---|---|---|
Normalized | Choose for thick plate or when finer grain, uniform properties through thickness and better low-temperature toughness are needed. | 375 | 510 | 27 | 148 HB | 8.6 % IACS |
HRDefault Hot rolled | Default stock form: hot-rolled plate, sections, coil and bar for general structural fabrication. | 380 | 520 | 25 | 155 HB | — |
Cold rolled | Pick for thin gauge (<3 mm) parts needing tight thickness tolerance, clean scale-free surface and direct painting. | 420 | 550 | 20 | 172 HB | — |
Supply forms, finishes and certifications
Ways Q355B is supplied — form, process route, surface or paperwork — without a datasheet of their own. The property values above apply.
| Option | What it is | Type |
|---|---|---|
Galvanized | Hot-dip zinc coated | Coating |
What is Q355B?
Q355B is the Chinese GB/T 1591 low-alloy high-strength structural steel (355 MPa minimum yield for thickness ≤16 mm) that replaced the old Q345B designation; it is essentially the domestic equivalent of S355JR/A572 Gr.50. It is chosen for bridges, plant structures, pressure-bearing frames, cranes and vehicle chassis because it gives roughly 40 % more yield strength than Q235 at almost the same weldability, cost and fabrication behaviour. Limits are ordinary atmospheric corrosion resistance (needs paint or galvanizing), a guaranteed impact grade only down to +20 °C (34 J), and yield strength that steps down with increasing plate thickness. Q355B has a density of 7.85 g/cm³ (0.284 lb/in³), heavier than 56% of steel grades. It melts at 1,430°C (2,606 °F). Q355B has a yield strength of 380 MPa (55.1 ksi) and a tensile strength of 520 MPa (75.4 ksi) — weaker than 68% of steel grades. Elongation at break is 25% and the elastic modulus is 206 GPa (29.9 Msi).
What is Q355B used for?
- Bridges
- Building frames
- Crane jibs
Advantages
- Readily welded — 85/100, better than 83% of steel grades
- Forms and bends easily — 65/100, better than 81% of steel grades
- Conducts heat well — 48 W/m·K (27.7 BTU/hr·ft·°F), better than 79% of steel grades
- Ductile — tolerates forming and impact — 25%, better than 77% of steel grades
Limitations
- Hard to polish — 45/100, worse than 84% of steel grades
Q355B properties
Typical room-temperature values for Q355B-HR; bars are the position among steel grades.
Physical3
Q355B has a density of 7.85 g/cm³ (0.284 lb/in³), heavier than 56% of steel grades. It melts at 1,430°C (2,606 °F).
Mechanical16
Q355B has a yield strength of 380 MPa (55.1 ksi) and a tensile strength of 520 MPa (75.4 ksi) — weaker than 68% of steel grades. Elongation at break is 25% and the elastic modulus is 206 GPa (29.9 Msi).
Thermal6
Q355B is rated for continuous service to 400°C (752 °F). It conducts heat at 48 W/m·K (27.7 BTU/hr·ft·°F), better than 79% of steel grades. Thermal expansion is 12 µm/m·K (6.67 µin/in·°F).
Electrical3
Q355B conducts electricity at 8.6 % IACS, better than 66% of steel grades.
Chemical & Environmental3
Corrosion resistance is not recommended (14/100), better than 70% of steel grades.
Sustainability4
Producing a kilogram of Q355B takes about 25 MJ of energy and emits 2.1 kg of CO₂ — less than 82% of steel grades. Typical recycled content is 22%.
Manufacturability6
Q355B's machinability is good (55/100, better than 62% of steel grades); weldability excellent (85/100); formability good (65/100).
Common Calculations5
Values are nominal handbook figures for design screening. Certified mill or lot data ships with every FabDigit order on request.
Strength versus weight in the steel family
Yield strength against density for every steel grade FabDigit runs. Q355B is highlighted; hover a dot for its name, or open the interactive family chart.
Working with Q355B
Is Q355B easy to machine?
Machines like a 0.2 %C low-alloy steel — use carbide at ~120–180 m/min, expect stringy chips and moderate tool life; ~60 % of AISI 1212 rating.
Can Q355B be welded?
Readily weldable by SMAW/GMAW/SAW with E50xx (ER50-6) consumables; preheat 75–150 °C for plate above ~30 mm or under restraint, and avoid welding through zinc on galvanized stock.
Can Q355B be formed, bent or molded?
Cold bends to 2× thickness inner radius per GB/T 1591; hot forming 900–1150 °C, and re-normalize heavy sections after severe hot work.
Manufacturability
The process limits FabDigit's quote engine applies to Q355B. Design inside these and the part quotes instantly; outside them it routes to an engineer.
CNC machining
| Machinability | Better55 / 100 |
|---|---|
| ISO material group | P.2 |
| Cutting speed (rough / finish) | 180 / 210 m/min |
| Minimum wall | 0.8 mm |
| Minimum internal radius | 1 mm |
| Drill depth limit | 5× diameter |
| Best surface (machined / polished) | Ra 1.6 / — |
| Hardenable after machining | No |
Sheet metal
| Formability | Better65 / 100 |
|---|---|
| Weldability | Excellent85 / 100 |
| Bendable | Yes |
Q355B chemical composition (wt %)
Limits by weight percent from the governing specification, written the way the spec states them — a single maximum for impurities, a range for alloying elements, and the base element as balance. Nominal is the typical mid-range value.
| Element | Spec limit (wt %) | Nominal |
|---|---|---|
| CGB/T 1591-2018, AR delivery | ≤ 0.24 | 0.18 |
| Si | ≤ 0.55 | 0.3 |
| Mn | ≤ 1.6 | 1.35 |
| P | ≤ 0.04 | 0.02 |
| S | ≤ 0.04 | 0.02 |
| Nbmicroalloying, optional | ≤ 0.05 | 0.02 |
| Vmicroalloying, optional | ≤ 0.12 | |
| Timicroalloying, optional | ≤ 0.2 | |
| Crresidual | ≤ 0.3 | |
| Niresidual | ≤ 0.3 | |
| Curesidual | ≤ 0.4 | |
| Moresidual | ≤ 0.1 | |
| N | ≤ 0.01 | |
| Altotal Al, when Nb/V/Ti not added | ≥ 0.02 | 0.03 |
| Fe | balance |
What surface finishes work on Q355B?
No inherent corrosion protection — shot blast to Sa2.5 then epoxy/zinc-rich primer, or hot-dip galvanize; not suitable for decorative polished exposure.
Surface treatments FabDigit runs on steel parts — pick them in the quote configurator or call them out on the drawing. Each links to its datasheet in the surface-finish library (thickness, specs, call-out, material verdicts).
Plating
CVD Coating
Black / Phosphate
Thermal Spray
How much does Q355B cost?
Q355B is a budget-cost steel ($), cheaper than about 99% of the materials FabDigit quotes. Part price depends far more on geometry, tolerance and quantity than on the raw stock — upload a CAD file for a live quote.
Q355B — frequently asked
What is Q355B used for?
Q355B is typically used for bridges, building frames and crane jibs. In short: high-strength structural plate.
What is the yield strength of Q355B?
Q355B has a typical yield strength of 380 MPa (55.1 ksi) and a tensile strength of 520 MPa (75.4 ksi) — weaker than 68% of steel grades. Strength varies by condition: see the 3 listed tempers.
Is Q355B easy to machine?
Reasonably — machinability is rated good (55/100, better than 62% of steel grades). Machines like a 0.2 %C low-alloy steel — use carbide at ~120–180 m/min, expect stringy chips and moderate tool life; ~60 % of AISI 1212 rating.
Can Q355B be welded?
Yes — weldability is rated excellent (85/100). Readily weldable by SMAW/GMAW/SAW with E50xx (ER50-6) consumables; preheat 75–150 °C for plate above ~30 mm or under restraint, and avoid welding through zinc on galvanized stock.
What surface finishes work on Q355B?
No inherent corrosion protection — shot blast to Sa2.5 then epoxy/zinc-rich primer, or hot-dip galvanize; not suitable for decorative polished exposure.
Can Q355B be formed, bent or molded?
Cold bends to 2× thickness inner radius per GB/T 1591; hot forming 900–1150 °C, and re-normalize heavy sections after severe hot work.
What is the maximum service temperature of Q355B?
Q355B is rated for continuous use to about 400°C (752 °F). Strength falls off well before that limit — check the elevated-temperature data for load-bearing parts.
What is the difference between Q355B-HR and Q355B-CR?
HR is the default condition — default stock form: hot-rolled plate, sections, coil and bar for general structural fabrication. CR: pick for thin gauge (<3 mm) parts needing tight thickness tolerance, clean scale-free surface and direct painting. Yield strength is 380 MPa in HR versus 420 MPa in CR.
How much does Q355B cost?
Q355B is a budget-cost steel ($), cheaper than about 99% of the materials FabDigit quotes. Part price depends far more on geometry, tolerance and quantity than on the raw stock — upload a CAD file for a live quote.
Can FabDigit make parts in Q355B?
Yes — Q355B is available for CNC machining and sheet metal with instant online pricing. Upload a STEP file to get a price and a DFM check.
Sources
- GB/T 700 / GB 50017 (elastic constants, CTE) — MOHURD (2017)
- ASM Handbook Vol.1: Properties and Selection — Irons, Steels and High-Performance Alloys — ASM International (1990)
- ASM Handbook Vol.19: Fatigue and Fracture — ASM International (1996)
- EN 10025-2 S355JR (comparison grade) — CEN (2019)
Property data is compiled from published supplier and standards handbooks and normalised for comparison; hardness values on non-Brinell scales are converted approximately for charting only. Manufacturability limits are FabDigit quote-engine defaults and may be relaxed by engineering review. Nothing on this page is a certification — request mill certs, CoC or material test reports with your order.