Steel Q345B
GB Q345B
Q345B is the classic Chinese low-alloy high-strength structural steel of GB/T 1591 (renamed Q355B in the 2018 edition), with a nominal 345 MPa minimum yield and a +20 °C Charpy V requirement of 34 J.
Values for the HR condition
Q345B tempers and conditions
Q345B is supplied in 4 conditions, soft to hard. HR is the default FabDigit quotes; the values on this page are for it.
Compare all 4 conditionsHide
| Condition · open for datasheet | Best for | Yield MPa | Tensile MPa | Elong. % | Hardness | Conductivity |
|---|---|---|---|---|---|---|
Normalized | Pick for thick plate, pressure parts and anywhere uniform fine grain plus higher, more reliable impact energy is specified. | 360 | 505 | 26 | 155 HB | 8.6 % IACS |
HRDefault Hot rolled | Default stock condition for plate, H-beam, angle and hot-rolled coil; best price and full weldability. | 365 | 510 | 24 | 160 HB | — |
Thermomechanically controlled processed | Choose for heavy welded structures needing low carbon equivalent, high toughness and reduced preheat. | 390 | 520 | 25 | 165 HB | 8.5 % IACS |
Cold rolled | Use for thin sheet needing tight thickness tolerance and a clean bright surface; strength is up, ductility and bend radius suffer. | 450 | 560 | 16 | 180 HB | — |
Supply forms, finishes and certifications
Ways Q345B 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 Q345B?
Q345B is the classic Chinese low-alloy high-strength structural steel of GB/T 1591 (renamed Q355B in the 2018 edition), with a nominal 345 MPa minimum yield and a +20 °C Charpy V requirement of 34 J. It is chosen for bridges, plant structures, pressure-bearing frames, cranes and heavy fabrications where a 30–40 % strength gain over Q235 is wanted at nearly the same weldability and price. Limits are ordinary atmospheric corrosion resistance (needs paint or zinc), yield that de-rates with plate thickness, and only modest low-temperature toughness (grade B is not for service well below about −20 °C). Q345B has a density of 7.85 g/cm³ (0.284 lb/in³), heavier than 56% of steel grades. It melts at 1,460°C (2,660 °F). Q345B has a yield strength of 365 MPa (52.9 ksi) and a tensile strength of 510 MPa (74 ksi) — weaker than 71% of steel grades. Elongation at break is 24% and the elastic modulus is 206 GPa (29.9 Msi).
What is Q345B used for?
- Bridge girders and gussets
- Building and plant structural frames
- Crane booms and gantry structures
- Pressure-bearing frames and skids
- Heavy machinery bases and weldments
- Galvanized structural sections
- Transmission and support towers
Advantages
- Readily welded — 85/100, better than 83% of steel grades
- Tough — resists crack growth — 120 MPa·√m (109.2 ksi·√in), better than 81% of steel grades
- Forms and bends easily — 65/100, better than 81% of steel grades
Limitations
- Hard to polish — 45/100, worse than 84% of steel grades
Q345B properties
Typical room-temperature values for Q345B-HR; bars are the position among steel grades.
Physical3
Q345B has a density of 7.85 g/cm³ (0.284 lb/in³), heavier than 56% of steel grades. It melts at 1,460°C (2,660 °F).
Mechanical16
Q345B has a yield strength of 365 MPa (52.9 ksi) and a tensile strength of 510 MPa (74 ksi) — weaker than 71% of steel grades. Elongation at break is 24% and the elastic modulus is 206 GPa (29.9 Msi).
Thermal6
Q345B is rated for continuous service to 400°C (752 °F). It conducts heat at 47 W/m·K (27.2 BTU/hr·ft·°F), better than 75% of steel grades. Thermal expansion is 12 µm/m·K (6.67 µin/in·°F).
Electrical3
Q345B conducts electricity at 8.6 % IACS, better than 66% of steel grades.
Chemical & Environmental3
Corrosion resistance is not recommended (12/100), worse than 53% of steel grades.
Sustainability4
Producing a kilogram of Q345B takes about 25 MJ of energy and emits 2.1 kg of CO₂ — less than 82% of steel grades. Typical recycled content is 25%.
Manufacturability6
Q345B's machinability is good (60/100, better than 83% 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. Q345B is highlighted; hover a dot for its name, or open the interactive family chart.
Working with Q345B
Is Q345B easy to machine?
Machines like a tough C-Mn steel at roughly 55–65 % of AISI 1212 rates; use rigid setups, coated carbide and generous feed to avoid the gummy chip of the as-rolled ferrite-pearlite structure.
Can Q345B be welded?
Readily welded by SMAW/GMAW/SAW with E50xx-class consumables (ER50-6, J507); CEV ≈ 0.40 so preheat 80–120 °C only for thick section, high restraint or damp conditions.
Can Q345B be formed, bent or molded?
Cold-bends to 2t–3t inner radius per GB/T 1591 in the rolling direction; hot forming 900–1050 °C followed by normalizing restores toughness after heavy strain.
Manufacturability
The process limits FabDigit's quote engine applies to Q345B. Design inside these and the part quotes instantly; outside them it routes to an engineer.
CNC machining
| Machinability | Better60 / 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 |
Q345B 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 1591-2008; ≤0.18 for some thin products | ≤ 0.2 | 0.17 |
| Si | ≤ 0.5 | 0.3 |
| Mn | ≤ 1.7 | 1.45 |
| P | ≤ 0.04 | 0.02 |
| S | ≤ 0.04 | 0.02 |
| Vmicroalloying, optional single or combined with Nb/Ti | 0.02 – 0.15 | |
| Nbmicroalloying | 0.02 – 0.06 | |
| Timicroalloying | 0.02 – 0.2 | |
| Alacid-soluble Al, grain refinement | ≥ 0.02 | 0.03 |
| Crresidual | ≤ 0.3 | |
| Niresidual | ≤ 0.5 | |
| Curesidual | ≤ 0.3 | |
| Moresidual | ≤ 0.1 | |
| N | ≤ 0.01 | |
| Fe | balance |
What surface finishes work on Q345B?
Must be protected — shot-blast Sa2.5 then epoxy/PU paint, or hot-dip galvanize (silicon content controls coating thickness); phosphating and powder coating also standard.
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 Q345B cost?
Q345B 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.
Q345B — frequently asked
What is Q345B used for?
Q345B is typically used for bridge girders and gussets, building and plant structural frames, crane booms and gantry structures, pressure-bearing frames and skids, heavy machinery bases and weldments and galvanized structural sections. In short: workhorse Chinese HSLA plate.
What is the yield strength of Q345B?
Q345B has a typical yield strength of 365 MPa (52.9 ksi) and a tensile strength of 510 MPa (74 ksi) — weaker than 71% of steel grades. Strength varies by condition: see the 4 listed tempers.
Is Q345B easy to machine?
Reasonably — machinability is rated good (60/100, better than 83% of steel grades). Machines like a tough C-Mn steel at roughly 55–65 % of AISI 1212 rates; use rigid setups, coated carbide and generous feed to avoid the gummy chip of the as-rolled ferrite-pearlite structure.
Can Q345B be welded?
Yes — weldability is rated excellent (85/100). Readily welded by SMAW/GMAW/SAW with E50xx-class consumables (ER50-6, J507); CEV ≈ 0.40 so preheat 80–120 °C only for thick section, high restraint or damp conditions.
What surface finishes work on Q345B?
Must be protected — shot-blast Sa2.5 then epoxy/PU paint, or hot-dip galvanize (silicon content controls coating thickness); phosphating and powder coating also standard.
Can Q345B be formed, bent or molded?
Cold-bends to 2t–3t inner radius per GB/T 1591 in the rolling direction; hot forming 900–1050 °C followed by normalizing restores toughness after heavy strain.
What is the maximum service temperature of Q345B?
Q345B 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 Q345B-HR and Q345B-CR?
HR is the default condition — default stock condition for plate, H-beam, angle and hot-rolled coil; best price and full weldability. CR: use for thin sheet needing tight thickness tolerance and a clean bright surface; strength is up, ductility and bend radius suffer. Yield strength is 365 MPa in HR versus 450 MPa in CR.
How much does Q345B cost?
Q345B 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 Q345B?
Yes — Q345B 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 1591-2008 (Q345B edition) — SAC (China) (2008)
- ASM Handbook Vol.1: Properties and Selection: Irons, Steels and High-Performance Alloys — ASM International (1990)
- ASM Handbook Vol.13: Corrosion — ASM International (2005)
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.