Grade Clarification: Q235B, Q345B and Their International Equivalents
Chinese angle bar designations are frequently mislabelled in catalogue copy. Q235 and Q345 are national grades, not ASTM grades: there is no ASTM Q235 or ASTM Q345 material. Q235B is a carbon structural steel covered by GB/T 700, and Q345B is a high-strength low-alloy structural steel covered by GB/T 1591. Buyers who need a US or European equivalent should compare mechanical property levels rather than designations.
| Chinese grade | Origin standard | Close international equivalent |
|---|---|---|
| Q235B | GB/T 700 | ASTM A36, EN 10025-2 S235JR, JIS G3101 SS400 |
| Q345B | GB/T 1591 | ASTM A572 Grade 50, EN 10025-2 S355JR |
One further point matters for project specifications: GB/T 1591 was revised in 2018 and the Q345 series was renamed Q355. Angle bars produced to the current edition are therefore marked Q355B, while Q345B continues to appear on older drawings. The two designations describe the same class of 355 MPa low-alloy steel, and a substitution note should be agreed before rolling begins.
Chemical Composition and Mechanical Properties
Both grades are produced as hot-rolled equal angle on modern reversing mills, where the L-shaped section is rolled with equal leg length and equal leg thickness. The equal-leg geometry gives a symmetric section with a predictable centroid, which simplifies the design of frames, brackets, supports and truss nodes.
| Property | Q235B | Q345B |
|---|---|---|
| Minimum yield strength, thickness up to 16 mm | 235 MPa | 345 MPa |
| Tensile strength range | 370-500 MPa | 470-630 MPa |
| Minimum elongation | 26 % | 21 % |
| Minimum impact energy at +20 °C | 27 J | 34 J |
| Carbon, maximum | 0.20 % | 0.20 % |
| Manganese, maximum | 1.40 % | 1.70 % |
| Phosphorus and sulphur, maximum each | 0.045 % | 0.035 % |
The higher alloy and microalloying content of Q345B raises tensile strength, stiffness demand tolerance and low-temperature impact resistance, which is why the grade is preferred for heavy machinery frames, long-span bridge components, offshore structures and load-bearing members of high-rise buildings. Q235B remains the economical choice for general structural engineering, where its weldability, formability and ductility support easy cutting, bending, welding and punching.
Size Range and Theoretical Mass
Equal angle bar is commonly supplied with leg lengths from 20 mm to 200 mm and leg thicknesses from 3 mm to 24 mm. Theoretical mass per metre, calculated from nominal dimensions at a steel density of 7.85 g/cm³, is the usual basis for ordering and freight calculation.
| Size, mm | Theoretical mass, kg/m | Size, mm | Theoretical mass, kg/m |
|---|---|---|---|
| 25 x 25 x 3 | 1.12 | 63 x 63 x 6 | 5.72 |
| 30 x 30 x 4 | 1.78 | 75 x 75 x 8 | 9.03 |
| 40 x 40 x 4 | 2.42 | 90 x 90 x 8 | 10.90 |
| 50 x 50 x 5 | 3.77 | 90 x 90 x 9 | 12.20 |
| 50 x 50 x 6 | 4.43 | 125 x 125 x 12 | 22.70 |
Published tables occasionally misprint values for neighbouring sizes: a 90 x 90 x 9 mm angle is listed as 12.20 kg/m, not the 9.59 kg/m that often appears next to the 90 x 90 x 7 mm size. Mass tolerance, leg straightness and out-of-squareness follow the dimensional annex of the applicable product standard, and the actual mass is verified by weighing each bundle against the theoretical figure.
Surface Treatment and Corrosion Protection
Angle bars are supplied black, painted or hot-dip galvanized, and the finish is chosen from the service environment rather than from appearance alone:
Hot-dip galvanizing to ISO 1461 or ASTM A123/A123M, giving a minimum local zinc coating of 70 µm and an average of 85 µm on steel 6 mm and thicker.
Anti-rust primer or epoxy coating for indoor and sheltered structural work.
Black mill finish for members that will be blasted and coated after fabrication.
Galvanized bar should be stored clear of standing water, and any weld made after galvanizing has to be repaired with a zinc-rich coating to restore the barrier. Coastal, humid and industrial atmospheres justify galvanizing even where the structure is not permanently wet.
Fabrication, Inspection and Applications
Equal angle bar is easily drilled, punched, notched and welded, so it suits both shop fabrication and site adjustment. Standard practice includes dimensional verification of leg length, thickness and length; visual inspection of rolled edges for laps or seams; tensile and impact testing of the cast; and a material test certificate traceable to the heat number on every bundle. Where the section will be used in a welded truss, the fabricator checks the weld procedure against the carbon equivalent of the actual heat rather than assuming the grade maximum.
Typical end uses include building steel frames, bridge auxiliary members, mechanical equipment bases, storage racking, transmission towers, agricultural machinery and general fabrication. Because the need is often for mixed sizes on a single project, orders are frequently placed as a size schedule with cut lengths rather than as full mill bundles.
Frequently Asked Questions
Q: Does ASTM define Q235 or Q345 angle bar?
No. Q235B comes from GB/T 700 and Q345B from GB/T 1591. ASTM A36 and ASTM A572 Grade 50 are the closest property equivalents and are the correct references on US-facing specifications.
Q: What is the difference between Q345B and Q355B?
They are the same class of steel. The 2018 edition of GB/T 1591 renamed the Q345 series to Q355, so current mill marking reads Q355B while older drawings still show Q345B.
Q: What sizes and lengths are available?
Equal leg sizes normally run from 20 x 20 mm to 200 x 200 mm with thicknesses of 3 mm to 24 mm, supplied in random lengths or cut to project length on request.
Q: How is the theoretical mass of an angle bar calculated?
From nominal leg length, leg thickness and a steel density of 7.85 g/cm³, expressed as kilograms per metre and used as the basis for order weight and freight.
Q: Which surface finishes are offered?
Black mill finish, anti-rust primer or epoxy paint, and hot-dip galvanizing to ISO 1461 or ASTM A123/A123M with 70 µm minimum local coating and 85 µm average on steel 6 mm and thicker.
Q: Are material test certificates supplied?
Yes. Each shipment carries a certificate giving chemical composition, yield strength, tensile strength, elongation and impact results, traceable to the heat number marked on the bundle.