Q355 vs S355: The Chinese and European 355 MPa Structural Steels

Q355 and S355 are near-twins on yield strength and are substituted daily. This guide compares their thickness tables, impact grade ladders and chemistry ceilings, and ends with a substitution checklist.

Headline: near-twins, different paperwork

Q355 (GB/T 1591) and S355 (EN 10025-2) are the two default 355 MPa structural steels of world trade. Both guarantee 355 MPa yield at ≤ 16 mm and both de-rate yield with thickness in almost the same steps — which is why substitution is routine, and why the residual differences deserve a careful look.

Yield by thickness

Minimum yield strength, MPa:

Thickness	Q355	S355
≤ 16 mm	355	355
> 16–40 mm	345	345
> 40–63 mm	335	335
> 63–80 mm	325	325
> 80–100 mm	315	315
> 100–150 mm	305	295

The families track each other almost exactly through the common range; the one visible divergence appears above 100 mm, where the GB table holds 10 MPa more.

Impact grades do not map letter to letter

Test temperature	Q355 quality grade	S355 impact grade
+20 °C	B	JR
0 °C	C	J0
−20 °C	D	J2 (K2 = 40 J)
−40 °C	E	— (use NL/ML families instead)

Two subtleties hide in this ladder. First, GB/T 1591 specifies a minimum absorbed energy of 34 J for these grades, against 27 J in EN 10025-2 — at the same temperature, the Chinese requirement is nominally the tougher one. Second, Q355E (−40 °C) has no EN 10025-2 counterpart: the European route at −40 °C runs through S355NL or S355ML (EN 10025 Parts 3 and 4), not through a J-grade.

Chemistry and weldability

Both standards cap the carbon equivalent at around 0.45 for common grades and thicknesses (each has a thickness-dependent table — check it rather than assuming). Silicon and manganese windows differ in detail, and delivery condition needs explicit attention: Q355 is very commonly supplied thermomechanically rolled, while an S355J2 order may be filled as-rolled or normalized unless the condition is stated. Through-thickness Z-quality (Z15/Z25/Z35) exists in both systems for lamellar-tearing-sensitive joints.

Substitution checklist

Match the impact temperature first: D ↔ J2, C ↔ J0, B ↔ JR. For −40 °C service, change standard family rather than just the letter.
Check yield at your actual thickness, especially above 100 mm.
Confirm delivery condition (as-rolled, normalized, TMCP) and the CEV ceiling your welding procedure was qualified against.
Order certification to the target standard. A Q355D certificate does not by itself prove S355J2 compliance — even when every reported value happens to fall inside the EN window.