Two Decisions: Grade and Dimension Standard
Angle steel is ordered against two independent documents. The first is the material specification, which fixes chemistry and mechanical properties. The second is the dimensional standard, which fixes leg lengths, leg thickness, root radius and permitted tolerances. A quotation that names only the grade is incomplete, because the same grade can be rolled to several dimension systems with different corner radii and mass per metre.
Equal angles have two legs of the same length, while unequal angles have different legs; both are rolled as L sections with the legs meeting at 90 degrees.
Grade Comparison in Numbers
| Grade | Yield strength min | Tensile strength | Elongation min |
|---|---|---|---|
| ASTM A36 | 250 MPa | 400 - 550 MPa | 20% (200 mm gauge) |
| ASTM A53 Grade B (seamless) | 240 MPa | 415 MPa min | Reported per specification |
| Q235B (GB/T 700-2006) | 235 MPa (t up to 16 mm) | 370 - 500 MPa | 26% |
| Q345B (GB/T 1591-2008) | 345 MPa (t up to 16 mm) | 470 - 630 MPa | 21% |
Typical chemistry limits follow the same pattern: A36 permits carbon up to 0.25%, manganese 0.80-1.20%, phosphorus 0.04% max and sulfur 0.05% max; Q235B restricts carbon to 0.20% max, manganese to 1.40% max and both phosphorus and sulfur to 0.045% max; Q345B is a micro-alloyed grade with a lower carbon ceiling and small additions of niobium or vanadium to reach the higher yield strength.
One point of confusion deserves attention: GB/T 1591 was revised in 2018, which replaced the Q345 designation with Q355 at the same minimum yield level for normal thickness. Drawings that still call Q345B can normally be satisfied by either an existing Q345B stock or a Q355B substitute, but the substitution should be confirmed in writing because the impact toughness requirements differ between temperature classes.
Dimension Standards for Angles
Common dimension references are GB/T 706-2016 for Chinese hot-rolled sections, EN 10056-1 for dimensions and EN 10056-2 for tolerances of European angles, and ASTM A6/A6M for the general requirements of rolled structural products in the ASTM system. A typical export range covers equal angles from 20 x 20 x 3 mm up to 200 x 200 x 24 mm and unequal angles from 25 x 16 mm up to 200 x 125 mm, in lengths of 6 m, 9 m or 12 m, subject to mill rolling limits.
Mass per metre matters commercially because it is the basis of most quotations. For equal angles the nominal mass follows directly from the leg lengths and thickness less the corner radii, so a change of dimension standard can move the theoretical weight by a few percent even at identical leg sizes.
Applications and Fabrication
Angle sections are used as bracing, purlins, transmission-tower legs, support brackets, machine frames and base plates. A36 and Q235B angles suit general structural and fabrication work, Q345B angles suit higher-loaded lattice structures and long-span bracing, and A53 Grade B is specified where the angle forms part of a pipe support or a pressure-related assembly. Cutting, punching and drilling are normally performed cold; where the steel will be galvanized, the silicon and phosphorus content should be controlled to avoid a thick, poorly adherent coating.
Frequently Asked Questions
Q: Is Q235B interchangeable with ASTM A36?
A: They are close but not identical. Q235B has a slightly lower minimum yield strength and tighter phosphorus and sulfur limits, so substitution is usually acceptable for static structures but should be confirmed against the project specification.
Q: Why was Q345 replaced by Q355?
A: The 2018 revision of GB/T 1591 realigned the Chinese grade designations with European practice, so Q355 replaced the former Q345 designation at the same minimum yield strength for normal thickness ranges.
Q: What is the difference between equal and unequal angles?
A: Equal angles have two legs of the same length, unequal angles have different leg lengths. Unequal angles are chosen where the two connected planes carry different loads or where the geometry of the connection demands it.
Q: How is the weight of an angle calculated?
A: From the nominal leg lengths, thickness and corner radii defined in the dimension standard, which yield a theoretical mass per metre used as the commercial basis.
Q: Which grade should be used for a galvanized tower?
A: A grade with controlled silicon and phosphorus content, galvanized to ASTM A123/A123M or ISO 1461, because excessive silicon produces an over-thick and brittle zinc layer.
Q: Are angle tolerances different from flat bar tolerances?
A: Yes. Leg length, leg thickness, squareness and length tolerances are set by the section standard, and angles have their own permitted limits for leg squareness and twist.