Grade Overview
ASTM A36 and GB/T 700 Q235B are the reference carbon steel grades for hot rolled angle bars in general construction. A36 is defined in ASTM A36/A36M with a minimum yield strength of 250 MPa and a tensile strength range of 400 to 550 MPa. Q235B is defined in GB/T 700 with a minimum yield strength of 235 MPa and a tensile strength range of 370 to 500 MPa. The two grades are often treated as interchangeable for statically loaded frames because their strength levels are close and both are fully weldable. Project specifications should state which standard governs, because the chemical limits and the impact requirements are not identical.
Chemical Requirements
A36 limits carbon to 0.26 percent maximum, manganese to 0.80 to 1.20 percent, phosphorus to 0.04 percent and sulphur to 0.05 percent. Q235B limits carbon to 0.20 percent maximum, manganese to 1.40 percent maximum, phosphorus to 0.045 percent and sulphur to 0.045 percent. The lower carbon and tighter sulphur of Q235B give slightly better toughness and weldability. Both grades are killed or semi killed as required by the purchaser. For cold region projects, impact testing at the specified design temperature should be requested, because neither grade guarantees Charpy values unless it is ordered as such.
Dimensions and Section Properties
Angle bars are produced as equal leg and unequal leg sections. Equal angles in common use run from 20 by 20 mm up to 200 by 200 mm, with leg thickness from 3 mm to 20 mm. Unequal angles such as 75 by 50 mm and 100 by 75 mm are used where one leg carries a heavier connection.
| Section | Mass per Metre | Typical Use |
| 50 x 50 x 5 mm | 3.77 kg | Light bracing and frames |
| 63 x 63 x 6 mm | 5.72 kg | Purlin and truss members |
| 75 x 75 x 8 mm | 9.03 kg | Column bracing and girts |
| 100 x 100 x 10 mm | 15.1 kg | Heavy bracing and supports |
Dimensions and tolerances follow GB/T 706 for the metric range and ASTM A6 for inch sizes. Lengths are normally supplied in 6 m, 9 m or 12 m random and fixed lengths.
Fabrication and Connections
Carbon steel angles are easy to shear, punch, drill and weld. Punching holes for bolted connections is common in workshop production, but the hole edge distance must respect the minimum values of the design code so that the net section is not over stressed. Welding uses ordinary carbon steel electrodes or solid wire with no preheat for the thicknesses normally employed. Galvanizing and painting follow standard surface preparation, and the coating thickness should be specified for the corrosion class of the site, particularly for outdoor or coastal construction.
Typical Applications
Roof purlins, wall girts and eave struts.
Diagonal bracing in braced frames.
Support brackets, base plates and equipment frames.
Transmission tower legs and cross arms.
Fencing, stair stringers and handrail supports.
Frequently Asked Questions
Q: Can A36 and Q235B angles be used together?
Yes. Both are weldable carbon steels with similar yield strength, and mixed connections are common in export projects where one grade substitutes for the other.
Q: What is the main difference between equal and unequal angles?
Equal angles have two identical legs and are used for general bracing, while unequal angles have different leg lengths and suit connections that need a wider or deeper bolt group on one side.
Q: Are the dimensions of A36 and Q235B angles the same?
The metric production follows GB/T 706 and inch sizes follow ASTM A6, so the designer should confirm the exact leg dimensions and thickness before ordering.
Q: Do these grades need impact testing?
Impact testing is not mandatory for normal temperature service, but it should be specified for cold climate projects where brittle fracture is a design concern.
Q: How are angles protected against corrosion?
Hot dip galvanizing, epoxy or polyurethane painting and powder coating are the usual options, selected according to the corrosivity category of the site.