QIDAMT
Q235B vs Q345B Steel: Strength, Weldability and How to Choose
Buyers comparing structural steel grades often ask: Q235B or Q345B — which is better? The answer depends on strength, weldability, low-temperature service and price. This article compares the two families and explains how the modern Q355B low alloy high strength steel plate relates to the older Q345B designation.
Q235B: The General-Purpose Grade
Q235B is a plain carbon structural steel to GB/T 700 with a minimum yield of 235 MPa and a guaranteed impact energy of 27 J at +20 °C. It is the default material for light and medium structures, fences, brackets, machine frames and general fabrication. Because it contains no micro-alloys, it is the most economical structural grade on the market and is available in every product form — Q235B hot rolled steel plate, channel, angle, square tube and flat bar.
Q345B / Q355B: The High-Strength Family
Q345B is the older low-alloy designation that appeared in the earlier edition of GB/T 1591, with a minimum yield of 345 MPa. In the current GB/T 1591-2018 edition, the grade was renumbered Q355B, with yield ≥ 355 MPa. The chemistry is essentially the same low-alloy system — carbon plus manganese, strengthened by micro-alloy additions of niobium, vanadium and titanium. So when a project specifies Q345B, Q355B is the modern equivalent supplied today.
Chemical Composition Comparison
| Element (%) | Q235B | Q355B (ex-Q345B) |
|---|---|---|
| C | ≤ 0.20 | ≤ 0.24 |
| Si | ≤ 0.35 | ≤ 0.55 |
| Mn | ≤ 1.40 | ≤ 1.60 |
| Nb / V / Ti | — | Added |
| P / S | ≤ 0.045 / 0.045 | ≤ 0.035 / 0.035 |
Mechanical Properties Comparison
| Property | Q235B | Q355B |
|---|---|---|
| Yield Strength | ≥ 235 MPa | ≥ 355 MPa |
| Tensile Strength | 370 – 500 MPa | 470 – 630 MPa |
| Elongation | ≥ 26 % | ≥ 22 % |
| Impact Energy | ≥ 27 J @ +20 °C | ≥ 34 J @ −20 °C |
How to Choose
- Light or non-critical structures: Q235B is the economical default — save cost where strength is not design-limiting.
- Heavy loads or long spans: Q355B reduces section size and weight, saving steel tonnage and freight.
- Cold climates: Q355B’s guaranteed −20 °C impact energy suits outdoor and dynamic structures.
- Weldability: both grades weld readily; Q355B needs slightly more care with preheat in thick sections due to higher carbon equivalent.
For complementary products in the same high-strength family, QIDAMT also supplies Q235B square tube and Q235B angle steel.
Send thickness, width, length and the end use — our engineers will confirm the right grade, price and delivery within 24 hours.
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