Q235B is one of China's most important carbon structural steel grades, defined in GB/T 700. Known for its balanced strength, low cost, and excellent processing behavior, Q235B H-beams are a fundamental material for civil engineering and industrial construction. The grade designation Q235 indicates a minimum yield strength of 235 MPa, and the letter B denotes a guaranteed Charpy impact value at room temperature.
What Is Q235B Steel?
Q235 belongs to the Chinese carbon structural steel family per GB/T 700, available in grades A, B, C, and D that differ mainly in impact toughness requirements. Q235B requires Charpy V-notch impact testing at +20 deg C with a minimum absorbed energy of 27 J, while Q235A carries no impact requirement, Q235C is tested at 0 deg C, and Q235D at -20 deg C. The steel is supplied in hot-rolled form as plates, strips, sections, and bars, and is the default choice for lightly and moderately loaded structures.
Chemical Composition
Q235B is a plain carbon steel with tightly controlled impurity limits. Typical maximum values for the grade are:
| Element | Maximum, % |
|---|---|
| Carbon (C) | 0.20 |
| Manganese (Mn) | 0.30-0.70 |
| Silicon (Si) | 0.35 |
| Phosphorus (P) | 0.045 |
| Sulfur (S) | 0.045 |
The low carbon content ensures good weldability and a low risk of cracking in welded fabrication, and the carbon equivalent of Q235B is low, making it one of the easiest structural steels to weld.
Mechanical Properties
Yield strength (ReL): minimum 235 MPa for thicknesses up to 16 mm, decreasing for thicker sections as specified in GB/T 700.
Tensile strength (Rm): 370-500 MPa.
Elongation: minimum 26% for thin plates, with values by thickness range per the standard.
Impact energy: at least 27 J at +20 deg C for grade B.
These values give engineering designers a reliable basis for structural analysis and safety calculations in light to medium structural applications.
H-Beam Dimensional Range
Hot-rolled H-beams in Q235B are commonly supplied in the following dimensional ranges:
| Parameter | Typical range |
|---|---|
| Web height (H) | 100-900 mm |
| Flange width (B) | 100-300 mm |
| Web thickness (t1) | 5-30 mm |
| Flange thickness (t2) | 5-30 mm |
| Length | 9 m or 12 m standard, other lengths by agreement |
Actual section dimensions and tolerances follow the applicable product standard for hot-rolled H-beams, and the section properties for design should be taken from the current standard tables.
The H profile, with wide flanges and a strong web, distributes load efficiently and offers several advantages over traditional I-sections of similar weight: improved bending resistance, better stability under compression thanks to parallel flanges, and a favorable strength-to-weight ratio. These characteristics make Q235B H-beams suitable for a wide range of buildings and frames while keeping material cost low.
Typical Applications
Industrial workshops and factory buildings.
Warehouse frames and logistics centers.
Bridge auxiliary structures and walkways.
Agricultural steel buildings.
Machinery frames and equipment supports.
Steel formwork supports and residential reinforcement.
Frequently Asked Questions
What does the 235 in Q235 mean? It is the minimum yield strength in megapascals: the steel begins to deform permanently when the stress exceeds 235 MPa. The tensile strength range is 370-500 MPa.
What is the difference between Q235A and Q235B? The grades share the same strength level but differ in toughness: Q235A has no mandatory impact requirement, while Q235B requires at least 27 J Charpy V-notch energy at +20 deg C.
How does Q235 compare with Q355? Q355 (GB/T 1591) has a higher minimum yield strength of 355 MPa and is used for heavy-load or long-span structures. Q235 is more economical and easier to weld, and it covers a large share of standard building structures.
Is Q235 the same as A36 or S235JR? The three grades belong to the same family of mild carbon structural steel and are broadly comparable, but they are not identical. A36 guarantees a minimum yield of 250 MPa for thin sections, while Q235 and S235JR guarantee 235 MPa, and impact requirements differ by grade suffix, so substitution should be confirmed against the design specification.
Does Q235 steel rust easily? Yes. Q235 has no alloying additions for corrosion resistance, so unprotected outdoor use leads to rusting. Painting, hot-dip galvanizing, or other protective systems are required for exterior service.
What are Q235B H-beams mainly used for? They are used for columns and beams in industrial buildings, warehouses, agricultural structures, machinery frames, and light to medium structural frames where the 235 MPa yield level is sufficient.