Grade Basis: Q235, Q355, SS400 and SM490
Hot rolled angle steel is ordered either to the Chinese system (Q235 and Q355) or to the Japanese system (SS400 and SM490). The difference is not only the number: the impact test requirement, the phosphorus and sulphur limits and the delivery tolerances all differ between the standards. Q235 is covered by GB/T 700-2006, Q355 by GB/T 1591-2018, SS400 by JIS G 3101 and SM490 by JIS G 3136. The section dimensions themselves are covered by GB/T 706-2016 for domestic mill practice and by JIS G 3192 for Japanese dimensions and tolerances.
| Grade | Standard | Min yield (MPa) | Tensile (MPa) | Impact requirement |
|---|---|---|---|---|
| Q235B | GB/T 700-2006 | 235 at t ≤ 16 mm | 370-500 | +20 °C, 27 J |
| Q355B | GB/T 1591-2018 | 355 at t ≤ 16 mm | 470-630 | +20 °C, 34 J |
| SS400 | JIS G 3101 | 245 | 400-510 | not specified |
| SM490A | JIS G 3136 | 325 at t ≤ 16 mm | 490-610 | not specified |
The impact classes within a grade change with the suffix letter. In GB/T 1591-2018 the Q355 quality classes are B at +20 °C, C at 0 °C, D at -20 °C and E at -40 °C, all with 34 J minimum on longitudinal Charpy specimens. Q235 delivered as Q235B carries a 27 J minimum at +20 °C under GB/T 700-2006.
Chemical Composition Control
Q355 gains its extra 120 MPa of yield strength mainly through manganese and micro-alloying rather than through carbon, which keeps weldability realistic at the same section thickness. Typical maximum levels are shown below.
| Element (%) | Q235B | Q355B |
|---|---|---|
| C max | 0.20 | 0.20 |
| Si max | 0.35 | 0.55 |
| Mn max | 1.40 | 1.60 |
| P max | 0.045 | 0.035 |
| S max | 0.045 | 0.035 |
Where the design code applies a carbon equivalent limit, the mill reports CEV with the mechanical results. Fabricators who intend to weld thick legs or use high heat input should check CEV against the welding procedure before the order is placed, because it determines whether preheat is needed.
Section Sizes, Tolerances and Lengths
Angles are supplied as equal leg or unequal leg profiles, commonly from 20 x 20 x 3 mm up to 200 x 200 x 24 mm in the equal leg range and in leg combinations such as 125 x 75 mm in the unequal range. Standard delivery lengths are 6 m and 12 m, with cut-to-length options. Leg length and leg thickness tolerances follow GB/T 706-2016 or JIS G 3192 according to the ordered system, and the root radius is a fixed part of the rolled profile rather than a customer choice.
Fabrication and Field Use
Hot rolled angles are drilled, punched, sheared, sawn and welded with standard low carbon procedures. Because the process leaves a rolled surface scale, painted systems normally require blast cleaning to the preparation grade stated in the coating specification. Angles are used for lattice towers, bracing, roof purlins, cable trays, machine frames, stair stringers and general connection brackets. For structures exposed to sustained vibration or cyclic loading, specify a grade with a tested impact class rather than relying on the base SS400 or SM490 requirement, which does not include Charpy testing.
Frequently Asked Questions
Q: Can SS400 be replaced by Q235 directly?
A: In practice they are often treated as interchangeable for lightly loaded frames, but the numbers differ: Q235B specifies 235 MPa minimum yield and a 27 J impact test at +20 °C, while SS400 specifies 245 MPa minimum yield and no Charpy requirement. Any substitution should be confirmed against the governing drawing.
Q: Which grade should be used for a heavy-duty structure?
A: Q355 or SM490. Q355B gives 355 MPa minimum yield with 470-630 MPa tensile strength and a 34 J impact energy at +20 °C, which is a substantial increase over Q235 and SS400 at almost the same carbon level.
Q: Are the same tolerances applied to GB and JIS angles?
A: No. GB/T 706-2016 and JIS G 3192 set different limits for leg length, leg thickness and out-of-square. State the standard that governs the project so that the mill can certify against the right tolerance table.
Q: What is the standard delivery length?
A: Angles are normally supplied in 6 m and 12 m random or fixed lengths. Custom lengths can be cut at the mill, and cut ends are normally deburred before packing.
Q: Is preheat required when welding Q355 angle steel?
A: It depends on thickness, carbon equivalent and heat input. Thin sections welded with a qualified procedure in normal workshop conditions generally need no preheat, while thick legs or low ambient temperature usually call for preheat. The welding procedure specification governs.