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, colchetes, 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

Mechanical Properties Comparison

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 e Q235B angle steel.