Q345B Carbon Steel Plate – GB/T 1591 Factory Direct Supplier | Huaxia-Steel
Q345B is the most widely used Chinese high-strength low-alloy (HSLA) structural plate grade, defined by GB/T 1591 “High Strength Low Alloy Structural Steels”. With minimum yield strength of 345 MPa (50 percent higher than Q235B) and 20 °C impact testing, Q345B has become the standard grade for bridges, high-rise buildings, heavy machinery, mining equipment, vehicle frames, and offshore modules across China and global export markets.
Huaxia-Steel supplies factory-direct Q345B carbon steel plate in thickness 3 mm to 200 mm, width up to 3,500 mm, and length up to 14,000 mm. Hot-rolled, controlled-rolled, normalized, and TMCP deliveries are available. Plates are supplied per GB/T 1591 with EN 10204 3.1 mill test certification and full traceability.
1. Q345B Standard and Naming
Q345B grade is defined by GB/T 1591-2018 “High Strength Low Alloy Structural Steels”. The naming convention breaks down as follows:
- Q – first letter of “Qu” (yield) in Chinese pinyin.
- 345 – minimum yield strength in MPa at thickness up to 16 mm.
- B – quality grade by impact test temperature: A = no impact, B = 20 °C, C = 0 °C, D = -20 °C, E = -40 °C.
Compared to the older GB/T 1591-1994, the 2018 revision unified the grade to all-around “Q345” with sub-grades by impact toughness. Q345B is the most common in service, equivalent to ASTM A572 Gr.50 and EN S355JR in modern cross-reference.
2. Chemical Composition
| Element | Range / Max (percent, plate up to 16 mm) |
|---|---|
| Carbon (C) | 0.20 max (typically 0.16-0.18) |
| Silicon (Si) | 0.50 max |
| Manganese (Mn) | 1.20-1.60 |
| Phosphorus (P) | 0.035 max |
| Sulfur (S) | 0.030 max |
| Nb + V + Ti + Mo | 0.02-0.15 micro-alloy |
| Carbon Equivalent (CEV) | 0.43-0.50 typical |
Q345B uses micro-alloying (Nb, V, Ti, or combinations) to achieve strength through grain refinement and precipitation hardening rather than higher carbon. This gives Q345B good weldability for its strength class – better than A572 Gr.50 with comparable strength.
3. Mechanical Properties
| Property | Thickness 3-16 mm | Thickness 16-40 mm | Thickness 40-63 mm | Thickness 63-80 mm | Thickness 80-200 mm |
|---|---|---|---|---|---|
| Yield Strength (MPa, min) | 345 | 335 | 325 | 315 | 305 |
| Tensile Strength (MPa) | 470-630 | 470-630 | 470-630 | 470-630 | 470-630 |
| Elongation (percent, min) | 21 | 21 | 21 | 20 | 20 |
| Impact @ 20 °C (J, min) | 34 | 34 | 34 | 34 | 34 |
| Bending 180 deg | d = 2a | d = 2a | d = 2a | d = 2a | d = 2a |
The minimum yield drops 10 MPa for each thickness band above 16 mm. The 34 J impact at 20 °C is substantially higher than Q235B’s 27 J, reflecting Q345B’s finer grain and tougher metallurgy.
4. Size Range and Tolerance
| Dimension | Range | Tolerance (per GB/T 709) |
|---|---|---|
| Thickness | 3 – 200 mm | +/- 0.30 to +/- 1.5 mm depending on thickness |
| Width | 1,500 – 3,500 mm | +/- 5 to +/- 30 mm |
| Length | 3,000 – 14,000 mm | +/- 10 to +/- 60 mm |
| Flatness | per GB/T 709 | 3-8 mm per meter |
| Edge | mill or cut | cut +/- 5 mm |
For thicknesses above 100 mm, plate is typically produced as heavy-gauge and the length is limited by rolling mill capacity.
5. Delivery Conditions
Q345B plate is supplied in the following conditions:
- Hot-rolled (HR): Default condition for thick plate. Rolled at 900-1,050 °C, cooled in air. Sufficient mechanical properties for general use.
- Controlled rolling (CR): Final passes at 800-880 °C with controlled reduction. Improved toughness and through-thickness uniformity.
- Normalized (N): Heated to 890-920 °C, held, air-cooled. Refines grain, restores properties after hot rolling, especially for heavy gauges.
- TMCP (Thermo-Mechanical Controlled Processing): Controlled rolling with accelerated cooling. Best combination of strength, toughness, and weldability. Available for thicknesses up to 80 mm.
- Quenched and tempered (Q+T): Available on special request for the highest strength and impact. May be referenced as “Q345D-QT” or “Q345E-QT”.
For most structural applications, normalized Q345B (mill designation “Q345B-N”) meets all requirements. For welded structures that need high through-thickness toughness (e.g., offshore), TMCP is recommended.
6. Cross-Reference to Other Standards
| Standard | Closest Grade | Notes |
|---|---|---|
| ASTM (US) | A572 Gr.50 / A709 Gr.50 | Direct equivalent; A572 Gr.50 has 345 MPa YS |
| EN (Europe) | S355JR / S355J0 | Direct equivalent in chemistry and strength |
| JIS (Japan) | SM490YA / SM490B | Yield 355 MPa; chemistry slightly different |
| ISO | Fe 510 B | Withdrawn but still used in cross-references |
| GOST (Russia) | St5sp / 09G2S | St5sp is older; 09G2S is the modern equivalent |
| DIN (Germany) | St52-3 | Withdrawn, replaced by S355J2 |
Three cross-reference pitfalls: (1) going from Q345B to A572 Gr.50, ensure the carbon equivalent and weldability match – A572 Gr.50 typically has slightly higher CEV; (2) going from S355JR to Q345B, confirm the impact test orientation (subsize Charpy differs); (3) for European structural design, request Q345B with EN 10025-2 S355JR mill certificate as an accepted alternate.
7. Welding and Fabrication
Q345B plate has good weldability for its 345 MPa strength class, but is more sensitive to heat input than Q235B. Recommended consumables:
- SMAW (stick): E7018 (E7018-1 for low hydrogen).
- GMAW (MIG/MAG): ER70S-6 or ER80S-G for matching strength.
- FCAW (flux-cored): E71T-1 or E81T1-GS for higher strength matching.
- SAW (submerged arc): H08MnA / H10Mn2 wire with SJ101 or HJ431 flux.
For plate thicknesses below 25 mm, no preheat is required when ambient temperature is above 5 °C and humidity is moderate. For thicknesses above 25 mm, 75-150 °C preheat is recommended to slow the HAZ cooling rate and prevent HAZ hardening or cracking. For heavy restraint welds in thick sections, hydrogen-controlled consumables (low-hydrogen E7018, basic flux) and a 100-150 °C interpass temperature should be maintained.
Q345B is readily formed, bent, blanked, and machined. Cold-forming reduces yield and tensile locally; for cyclic-loaded parts, normalize after forming.
8. Applications
Q345B plate is the workhorse of structural engineering in China and major export projects. Common applications:
- Bridges: Main girders, deck plates, cross girders, walkways, brackets.
- High-rise buildings: Columns, beams, transfer trusses, outrigger systems.
- Heavy machinery: Mining dump truck frames, excavator booms, shovel handles, crane booms.
- Vehicles: Trailer frames, container chassis, dump truck bodies, railway wagon sides.
- Power and substations: Turbine building columns, boiler steel structures, transformer supports.
- Offshore modules: Deck plates, topside modules (with TMCP), jacket nodes.
- Storage tanks and silos: Non-pressure shells up to 20 m diameter; hopper sections.
- Wind turbine towers: Plate rolled and welded into 3-6 m diameter sections.
For pressure vessels, ASME equivalent grades like Q345R (GB 713) must be used instead. For sour service, Q345B does not qualify unless normalized to NACE hardness limit.
9. Quality Assurance and Certification
Huaxia-Steel Q345B plate is supplied with full QA documentation:
- GB/T 1591 compliance: Chemistry, mechanical, impact per latest revision.
- EN 10204 3.1 mill test certificate with heat traceability and full chemistry table.
- Ultrasonic testing per GB/T 2970 (Class A, B, or C as specified).
- Through-thickness testing (Z-direction test) per GB/T 5313 for plates above 40 mm (Class Z15, Z25, or Z35).
- Dimensional report: Thickness, width, length, flatness, edge.
- Visual examination: No surface defects.
- Third-party inspection by SGS, BV, TUV, or buyer-nominated inspector.
10. Packaging and Shipping
- Standard export packing: Plates bundled with steel straps, edge-protected. Mill scale intact for HR plate. Anti-rust oil applied.
- Shot blasting and priming: Available for shop-ready delivery.
- Marking: Heat number, grade, size, weight on each plate.
- Container loading: 3-30 mm plates in 20-ft or 40-ft containers. Thicker plates on flat-rack or in bulk carriers.
Lead time: 10-20 days for stocked sizes, 25-40 days for custom dimensions.
11. Ordering Information
To order Q345B plate, specify:
- Grade: Q345B (or Q345C / D / E for higher impact temperature)
- Standard: GB/T 1591
- Thickness, width, length (mm)
- Delivery condition: HR / CR / N / TMCP / Q+T
- Surface: mill / shot-blasted + primed / galvanized
- NDT: UT Class A / B / C (per GB/T 2970)
- Through-thickness: Z15 / Z25 / Z35 (per GB/T 5313)
- Quantity: in tonnes or pieces
Huaxia-Steel supplies Q345B carbon steel plate direct from the mill at factory prices. Thickness 3-200 mm, width up to 3,500 mm, length up to 14,000 mm, multiple delivery conditions including TMCP. GB/T 1591, EN 10204 3.1 MTC, container or bulk worldwide shipment. Send your inquiry with size, delivery condition, NDT requirements, and destination port for a same-day quotation.
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