Why the Steel Grade Matters for Angle Sections
Angle sections are supplied as hot-rolled profiles in carbon and low-alloy steel grades defined by national standards. The grade fixes the minimum yield strength, the tensile range, and for impact-tested classes the guaranteed Charpy energy. Because the same geometric section can be rolled in different grades, specifying the grade is as important as specifying the leg dimensions and thickness.
GB Grades: GB/T 700 and GB/T 1591
Under the GB system, ordinary carbon structural steel is covered by GB/T 700-2006 and low-alloy high-strength steel by GB/T 1591-2018.
Q235: minimum yield strength 235 MPa for thickness up to 16 mm and tensile range 370-500 MPa; sub-grades B, C and D guarantee 27 J Charpy impact at 20, 0 and -20 degrees Celsius respectively
Q355: minimum yield 355 MPa and tensile 470-630 MPa, available as B, C, D and E classes with Charpy requirements down to -40 degrees Celsius; Q355 replaced Q345 in GB/T 1591-2018
Q195 and Q215: lower-strength grades with minimum yields of 195 MPa and 215 MPa, used mainly for light framing and secondary members
ASTM Grades: A36 and A572
ASTM A36/A36M is the most widely specified carbon structural steel in North American projects. It requires a minimum yield strength of 250 MPa (36 ksi) and a tensile range of 400-550 MPa. ASTM A572/A572M Grade 50 is a high-strength low-alloy grade with a minimum yield of 345 MPa (50 ksi), preferred when lighter sections are required for the same load.
JIS Grades: SS400, SS540 and SM490
Japanese structural grades are defined in JIS G3101 and JIS G3106. SS400, the most common general structural grade, guarantees a minimum yield of 245 MPa for material up to 16 mm and a tensile range of 400-510 MPa. SS540 raises the minimum tensile strength to 540 MPa for higher-strength duties. SM490, defined in JIS G3106 for welded structures, provides a minimum yield of 325 MPa up to 16 mm with improved weldability and optional impact classes.
Choosing a Grade for Your Project
Grade selection starts from the design stress and the fabrication route. General building frames, platforms and handrails are usually served economically by Q235, A36 or SS400. Long-span trusses, crane girders and heavy brackets justify Q355, A572 Grade 50 or S355 equivalents, where the higher yield directly reduces section weight and material cost. Where cold-weather toughness is required, specify the impact-tested class of the chosen standard rather than a general-purpose grade.
Frequently Asked Questions
Q: Is Q235 the same material as ASTM A36?
Both are carbon structural steels with similar strength levels and they are frequently treated as equivalent for secondary structures. A36 guarantees a minimum yield of 250 MPa against 235 MPa for Q235, so verify the design stress before direct substitution.
Q: What does the B in Q235B mean?
The suffix letter defines the impact testing class. Q235B requires a minimum Charpy V-notch energy of 27 J at 20 degrees Celsius, while C and D classes are tested at 0 and -20 degrees Celsius respectively.
Q: Is Q345 still specified on new projects?
Q345 was superseded by Q355 in GB/T 1591-2018. New projects should specify Q355, which is the direct successor grade with a minimum yield of 355 MPa.
Q: Which grade should I choose for outdoor angle frames?
For ordinary atmospheric exposure, Q235, A36 or SS400 combined with hot-dip galvanizing to BS EN ISO 1461 is a common and economical solution. For higher loads, move to Q355 or A572 Grade 50.
Q: Can angle sections be supplied in S355 under EN 10025?
Yes. EN 10025-2 defines S235JR, S275JR and S355JR among others, and angle sections rolled to these grades are common for European projects.
Q: How are angle steel dimensions described?
Equal angles are described by leg width and thickness, for example L50x50x5, and unequal angles by both legs, for example L75x50x6, followed by the standard and the steel grade.