TECHNICAL 15 · MATERIAL

Q235 or Q355: what the grade changes, and what it does not.

Grade is yield strength. A higher grade lets a lighter section carry the same load — if the design uses it. On a greenhouse frame the practical effect is often smaller than the price difference suggests.

Round tube bundles, stacked in the mill — XinYonSen

The two grades normally quoted
Property	Q235B	Q355B
Yield strength	≈ 235 MPa	≈ 355 MPa
Tensile strength	375–500 MPa	470–630 MPa
Weldability	Easy	Easy, with matching consumables
Availability in tube	Universal	Common in larger sizes
Cost	Baseline	Higher per tonne
Typical use	Arches, purlins, posts	Heavy spans, trusses, columns

When a higher grade pays

Where bending governs and deflection is not the constraint, a higher grade allows a thinner wall. That saves weight, which saves freight, which matters on export. Where deflection governs — long purlins, wide gutters — extra strength does not help, because stiffness is a function of geometry, not yield.

A common confusion

Buyers ask for Q355 to “make it stronger” and keep the same section. That does raise the load at which the member yields, but it does not reduce deflection. If the complaint is a sagging rail, the answer is a deeper section or closer supports.

What the mill certificate shows

Chemical composition and mechanical properties for the heat number. If a quotation does not state a grade, the certificate will not match the drawing, and an inspector will ask. State the grade on the PO and ask for the certificate to reference the same grade.

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