Q345B carbon steel round bar is the high-strength low-alloy (HSLA) structural round bar produced under GB/T 1591 — the Chinese counterpart to EN S355JR-class strength — in diameters from 12 mm to 400 mm hot-rolled and forged. With a 345 MPa minimum yield, 470–630 MPa tensile range, mandatory 20 °C impact energy and microalloyed chemistry, Q345B round bar carries roughly 45 percent more yield than Q235-class bar at a modest price premium, which is why equipment frames, machinery shafts, bridge components, tower sections and heavy fabrications across Chinese and export projects default to it. Huaxia-Steel supplies Q345B round bar factory direct: hot-rolled black bar, peeled and bright options, full EN 10204 3.1 MTCs, dual certification to S355JR on request, and cutting, forging and heat treatment services in-house.
Product Overview
Q345B belongs to the GB/T 1591 family (the modern Q355 series replaced Q345 in the 2018 revision, with Q355B the direct successor — both are produced and certified today, and Q345B remains the dominant trade designation in export markets). The chemistry is a low-carbon manganese steel with microalloy additions: carbon ≤ 0.20, manganese 1.00–1.60, plus controlled silicon, vanadium/niobium/titanium microalloying (one or more, typically 0.02–0.15 total) that delivers the higher yield through grain refinement and precipitation strengthening rather than through carbon — which is precisely why the grade keeps good weldability and toughness. Impact energy of ≥ 34 J at 20 °C is mandatory for the B quality level, with C (0 °C) and D (−20 °C) quality letters available for colder service.
In bar form the grade is used wherever a shaft, pin, tie rod, bolt stock, hinge pin, structural pin or machined component must carry structural loads rather than just fill space. It machines adequately (better than Q235 thanks to consistent microstructure, harder than A36 by about 20 HB as rolled), welds with standard low-hydrogen practice without preheat up to moderate thickness, and responds to normalizing for property homogenization across sections.
Specification Table
| Parameter | Range | Notes |
|---|---|---|
| Diameter (hot-rolled) | 12 – 250 mm | 6–12 mm on request; common: 16, 20, 25, 32, 40, 50, 60, 75, 90, 100, 120, 150, 200 mm |
| Diameter (forged / peeled) | 120 – 400 mm | forged to order with UT |
| Length | 3 – 12 m random; 6 m / 9 m / 12 m fixed | cut-to-length ±2 mm available |
| Chemistry | C ≤ 0.20, Mn 1.00–1.60, Si ≤ 0.55, P ≤ 0.035, S ≤ 0.035, V/Nb/Ti microalloys | per GB/T 1591 (Q355B identical class) |
| Yield strength | ≥ 345 MPa (≤ 16 mm); 335 (16–40); 325 (40–63); 315 (63–80) | decreasing bands with thickness |
| Tensile strength | 470 – 630 MPa | full section |
| Elongation | ≥ 20% (A5.65) / ≥ 22% (A) | proportional gauge conventions differ |
| Impact (B quality) | ≥ 34 J at 20 °C | C: 0 °C; D: −20 °C |
| Hardness (as rolled) | ≈ 150–190 HB typical | not a specified value; for machining planning |
| Tolerance (diameter) | h13–h15 class by size, GB/T 702 | peeled h10 / bright h9 available |
| Certificates | EN 10204 3.1, dual S355JR certification on request | UT per GB/T 6402 or SEP 1921 on forged bar |
Available Grades and Quality Levels
- Q345B / Q355B: the standard export quality, 20 °C impact, the price-performance default.
- Q345C / Q355C: 0 °C impact for colder climates and outdoor equipment.
- Q345D / Q355D: −20 °C impact with tighter sulfur (≤ 0.030); the choice for S355J2-class requirements.
- Q345E / Q355E: −40 °C impact for arctic and offshore-adjacent duty.
- Cross-references: S355JR/J0/J2 (EN 10025-2), ASTM A572 Gr 50 territory (strength class; chemistry and test conventions differ), JIS SM490A territory. Dual-certified MTCs name both standards where chemistry and tests satisfy both scopes.
The most common dual-certification request is Q345B/Q355B to S355JR — the chemistries overlap comfortably, impact temperatures match (both room-temperature B/JR), and mills with tight sulfur control certify both without premium in most size ranges.
Manufacturing Standards
- GB/T 1591 — the grade standard: chemistry, mechanical property bands by thickness, quality letters by impact temperature.
- GB/T 702 — hot-rolled round bar dimensions and tolerances.
- GB/T 699 adjacent — note that Q345B is a structural grade, not a machine-structural grade; for gears and QT shafts buyers move to 40Cr/42CrMo class steels under GB/T 3077.
- EN 10025-2 — target for dual certification to S355JR/J0/J2.
- Delivery conditions: as-rolled standard; normalized on request for heavy sections; controlled rolling available on production heats.
Applications
- Machinery and equipment frames: pins, shafts, tie rods, columns and cross members in construction and agricultural equipment.
- Structural connections: high-strength bolts stock, anchor rods, bearing pins and linkages in bridges and buildings.
- Steel towers and masts: transmission tower legs and guyed mast components where yield margin matters.
- Fabricated pressure-adjacent equipment: support skirts, ladders, clips and external fittings on tanks and vessels.
- General engineering: rollers, gears blanks for low-duty service, dies holders and jigs where S355-class bar is drawn.
Quality Assurance
Each heat is certified with ladle chemistry, tensile results on the size-appropriate band, Charpy V-notch at the quality-letter temperature, and bend performance where the product standard requires it. Forged diameters carry UT examination to GB/T 6402 or SEP 1921 with reportable defect classes. Bar ends are cut clean without shear distortion, and surface defects beyond the standard’s grinding limits are removed before shipment. Heat numbers are paint-stamped or banded along each bar for traceability to the MTC, and third-party inspection (SGS/BV/TUV or your agency) is welcomed at our works. For projects certifying to EN, we issue S355JR dual certificates with EN chemistry limits applied — sample MTCs are provided before booking on request.
Packaging and Shipping
- Standard: straight bars in export bundles, steel-strapped with two-tone bundle tags (grade, heat, size), each heat separated for certificate clarity.
- Container loading: 6 m bars in 20 ft/40 ft with timber dunnage; 12 m bars ship breakbulk or in 40 ft flat-rack systems.
- Protection options: bare, oiled, or VCI-wrapped; peeled/bright bar individually sleeved.
- Processing options: cut-to-length, chamfering, forging to stepped shafts, normalizing and rough turning available before shipment.
Lead time runs 7–15 days for stock diameters and 20–45 days for production heats or forged sections, with FOB, CFR and CIF terms to your port.
Cutting, Forging and Downstream Processing
Diameter choice is the main cost lever on Q345B bar: stock diameters in the 16–120 mm band price at commodity levels with immediate availability, while diameters above 200 mm are forged to order and carry both a longer lead time and a minimum-tonnage logic per heat. When a bill of material mixes many diameters, consolidating to the nearest stock size on non-critical parts can cut the landed cost meaningfully — send us the drawing list and we will mark which diameters can merge without design impact.
- Cut-to-length: saw cutting to blank lengths ±2 mm standard, ±0.5 mm precision available; bundle tagging preserves heat identity on cut blanks.
- Forging: open-die forging from 120 mm blooms up to 400 mm stepped shafts and blocks, with normalizing and rough turning per drawing.
- Peeled / bright finish: peeled h10 or drawn h9 surfaces for machinists who skip the scale-removal pass — 15–30% price premium, immediately recovered in machining time on volume parts.
- UT examination: SEP 1921 or GB/T 6402 classes on forged bar, reportable on the MTC, with edge-zone discrimination on request.
- Sampling coupons: tensile and impact test coupons from the same heat ship with the order for your own lab verification.
For distributors, mixed-diameter bundles per heat can be consolidated in one container while preserving heat-to-certificate mapping — the structure most resellers use to serve fabricators buying small lots from one import shipment.
Bundle tagging on every shipment lists grade, quality letter, heat number and bundle weight — the structure inspectors need to verify a container against its MTC set in minutes rather than hours.
FAQ
Is Q345B the same as S355JR?
Essentially yes at the strength class: both are 355-class HSLA steels with room-temperature impact requirements and similar chemistry. Dual certification is routine. The fine differences sit in the carbon-manganese balance details, residual limits and test frequency conventions, which a dual-standard MTC resolves by satisfying both scopes explicitly.
What replaced Q345B in the new GB standard?
The 2018 revision of GB/T 1591 introduced Q355 as the successor designation; Q355B carries the same strength class and nearly identical requirements. Both designations appear on current certificates, and export buyers should treat Q345B and Q355B as the same material class unless their specification says otherwise.
Can Q345B round bar be used for shafts and machined parts?
Yes for structural-duty shafts, pins and links where 345 MPa yield and good toughness are the design basis. For quenched-and-tempered shaft duty (higher hardness, jominy-controlled) specify 40Cr, 42CrMo or 45# machine-structural steels instead — Q345B is an as-rolled structural grade and is not heat-treated to QT property tables.
How does Q345B compare to ASTM A572 Grade 50?
The strength classes match (345 vs 345 MPa yield), and both are microalloyed HSLA. A572 Gr 50 has a tighter chemistry formula with columbium/vanadium rules and different sampling conventions, while Q345B quotes a wider manganese band and JIS-style proportional elongation. Substitution works with a dual-certified MTC or an engineer’s approval, especially where impact testing is mandatory in the project.
Need Q345B / Q355B / S355JR round bar in stock or forged diameters with dual-standard 3.1 MTCs, UT on forged sections and cut-to-length service? Send diameter, length, quality letter and quantity — Huaxia-Steel replies with stock, pricing and lead time within 24 hours.
