1. Two Equivalent Construction Grades
A36 and Q235B are the two most widely used general purpose carbon steel grades for construction angles, one defined by ASTM and the other by the Chinese GB/T 700 standard. Both are low carbon steels with a minimum yield strength of 250 and 235 MPa respectively, and both are easy to weld, cut and form. In practice the two grades are frequently accepted as equivalents for structural design, with the governing standard of the project deciding which one is specified.
2. Grade Comparison
| Property | ASTM A36 | Q235B |
| Standard | ASTM A36 / A6 | GB/T 700 |
| Min yield strength | 250 MPa | 235 MPa |
| Tensile strength | 400 - 550 MPa | 370 - 500 MPa |
| Carbon limit | 0.26 percent max | 0.20 percent max |
The B suffix in Q235B refers to a quality class with a guaranteed Charpy impact value of 27 J at 20°C, which makes the grade suitable for welded structures at normal temperatures. A36 has no fixed impact requirement, so the two should be compared against the project specification where toughness is a design condition.
3. Section Range and Availability
Construction angles are available as equal angles from 20x20x3 up to 200x200x24 and as unequal angles up to 200x125, in standard lengths of 6 m and 12 m. The most heavily stocked sizes for building work are 40x40x4, 50x50x5, 63x63x5, 75x75x6 and 100x100x8. Thickness, leg size and the corner radius follow the section tables of GB/T 706 or the equivalent ASTM A6 shape tables.
4. Fabrication and Protection
Both grades are cut, drilled, punched and welded with ordinary carbon steel procedures, which keeps fabrication cost low. For outdoor construction, the angles are hot-dip galvanized or painted; galvanizing to ASTM A123 or GB/T 13912 provides long life protection, while painting systems are specified by the coating designer. Cut edges and drilled holes on galvanized sections are repaired with zinc rich paint.
5. Typical Construction Applications
Bracing and wind struts in portal frame buildings.
Roof purlins and wall girts on light steel buildings.
Truss members and connection gussets.
Racks, platforms and equipment supports.
Frames for gates, canopies and shelters.
Frequently Asked Questions
Q: Can A36 and Q235B be treated as the same steel? They have similar strength and weldability and are often accepted as equivalents, but the project engineer should approve the substitution because the toughness requirements differ.
Q: What does the B mean in Q235B? The B is a quality class in GB/T 700 that guarantees a Charpy impact energy of 27 J at 20°C, which is important for welded structures at normal temperatures.
Q: Which angle size is most common for bracing? For light and medium buildings, 50x50x5 and 63x63x5 are the most common bracing sizes, while heavier trusses use 75x75x6 and above.
Q: Is Q235B suitable for outdoor galvanized use? Yes, Q235B galvanizes well and the combination of the zinc coating and the ductile base steel is widely used for outdoor frames and fences.
Q: What certificate should accompany the angle delivery? The mill certificate should state the grade, heat number, chemical analysis and tensile test results, and for galvanized material the coating thickness report.