q235-vs-q355-steel
Q235 vs Q355 Steel: Complete Guide for Steel Structure Buildings
When you request a quote from a Chinese steel structure factory, you'll almost always see Q235B and Q355B on the drawing. These are the two workhorse structural steel grades in China—and choosing between them changes your member sizes, your building weight, and your delivered price.
Q235 vs Q355 steel is not an academic debate. Q235 is a carbon structural steel; Q355 is a low-alloy, higher-strength version. The number after the letter tells you the minimum yield strength in megapascals (MPa). Get the grade wrong and you either overpay for unnecessary strength or under-design your frame.
This guide compares the two grades on chemical composition, mechanical properties, weight and cost impact, international equivalents (A36, A572 Gr.50, S355JR), and gives you a decision rule you can apply on the next quote.
Quick Answer: What's the Difference?
The short version:
- Q235B is a carbon structural steel with minimum yield strength ≥235 MPa for thicknesses up to 16 mm (5/8 in). It is cheap, easy to weld, and ideal for light-to-moderate loads.
- Q355B is a low-alloy high-strength structural steel with minimum yield strength ≥355 MPa. It replaced the old Q345 designation in 2019. It is built for heavy loads, long spans, and crane girders.
That is roughly a 50% strength jump from 235 to 355 MPa. At equal load, a Q355B member can carry the same force on a smaller section, typically shedding 15–30% of the steel weight compared with a Q235B design.
How to think about it at a glance:
- Small warehouses, farm sheds, light cladding systems → Q235B is enough.
- Long-span factory buildings, crane girders, tall frames → Q355B earns its keep.
- The most common and most economical real-world choice is a hybrid: Q355B for the main portal frame columns and rafters, Q235B for secondary members (purlins, girts, bracing).
Why does this matter for an international buyer? A quote that just says "steel structure" without naming a grade is not a serious quote. Insist that both the drawings and the Material Test Certificate (MTC) name the grade explicitly. Otherwise you have no way to enforce what actually ships.
For the broader context of how a prefabricated steel building is put together, our companion guide walks through the fabrication process.
Q235B Profile
Q235B is the default carbon structural steel in China, governed by GB/T 700 (Carbon Structural Steels).
Chemistry and standard
- Carbon content: typically ≤0.20%.
- Standard additions: manganese (Mn), silicon (Si), with residual sulfur and phosphorus kept within standard limits.
- The trailing letter "B" indicates the Charpy V-notch impact test is performed at +20°C (room temperature). Higher letters (C/D/E) mean lower test temperatures.
Mechanical properties (for thickness ≤16 mm)
- Yield strength: ≥235 MPa (≈34 ksi).
- Tensile strength: 370–500 MPa (≈54–72 ksi).
- Elongation after fracture: ≥26%.
- Density: 7.85 g/cm³ (490 lb/ft³).
Typical uses
Q235B is the workhorse for secondary and light primary members:
- C/Z cold-formed purlins and wall girts.
- Roof bracing, tie rods, gusset plates.
- Small single-bay warehouses, agricultural sheds, simple enclosures.
- Light frames where span and height are modest.
Its advantages are straightforward: low price, excellent weldability, and easy sourcing across every major steel mill in China.
Limitations
When span, height, or crane load grows, Q235B forces you into deeper and heavier sections to keep stresses within code. The material cost per ton stays low, but the tons per square meter climbs. On a long-span job, Q235B stops being the cheaper option because you are buying so many extra tons.
If you are pricing a small single-bay building, our steel warehouse cost guide shows how grade choice moves the total.
Q355B Profile
Q355B is China's low-alloy high-strength structural steel, governed by GB/T 1591.
Chemistry and standard
- Small additions of niobium (Nb), vanadium (V), titanium (Ti), and higher manganese content create micro-alloy grain refinement.
- The old Q345 designation was upgraded to Q355 in GB/T 1591-2018, effective 2019. The yield floor was raised slightly and composition tightened. If an old drawing says Q345, treat it as Q355 for ordering purposes.
Mechanical properties (for thickness ≤16 mm)
- Yield strength: ≥355 MPa (≈52 ksi).
- Tensile strength: 470–630 MPa (≈68–91 ksi).
- Elongation: ≥22%.
- Impact grades: B = +20°C, C = 0°C, D = −20°C, E = −40°C. For northern or cold-climate projects, specify C, D, or E.
Typical uses
Q355B is the grade of choice when loads are real:
- Main portal frame columns and tapered rafters.
- Overhead crane girders (bridge crane support beams).
- Long-span trusses and roof rafters.
- Heavy industrial steel factory buildings, aircraft hangars, and multi-story frames.
- High wind, high snow, or high seismic zones.
- Structural members for steel bridge fabrication, where fatigue resistance and weld toughness under live pedestrian loads demand the higher-yield grade.
Cost trade-off
Q355B costs roughly 5–10% more per ton than Q235B. That sounds like a penalty, but the smaller sections usually win back the difference. Because Q355B carries more stress per square millimeter, you use fewer total tons. On long spans and crane-equipped buildings, the total steel package lands at the same price or 5–15% lower than a Q235B design. On small, lightly loaded sheds, Q235B remains cheaper.
Head-to-Head Comparison
Now the two grades side by side.
Table 1 — Q235B vs Q355B: Mechanical Properties
| Property | Q235B | Q355B | Standard |
|---|---|---|---|
| Minimum yield strength (t ≤16 mm) | 235 MPa (34 ksi) | 355 MPa (52 ksi) | GB/T 700 / GB/T 1591 |
| Tensile strength | 370–500 MPa | 470–630 MPa | GB/T 700 / GB/T 1591 |
| Minimum elongation | 26% | 22% | GB/T 700 / GB/T 1591 |
| Charpy V-notch test temp | +20°C (Grade B) | +20°C (B), 0°C (C), −20°C (D), −40°C (E) | GB/T 229 |
| Carbon content | ≤0.20% | ≤0.20% (micro-alloyed) | GB/T 700 / GB/T 1591 |
| Density | 7.85 g/cm³ | 7.85 g/cm³ | Both |
Weight and cost impact
For the same design loads, Q355B sections run 15–30% smaller than Q235B sections. The per-ton premium (5–10%) is usually more than offset by the tonnage saving.
Table 2 — Weight & Cost Impact by Application
| Application | Recommended Grade | Steel Weight Saving vs All-Q235B | Relative Total Cost |
|---|---|---|---|
| Small shed, span ≤18 m (60 ft), no crane | Q235B | Baseline | Cheapest |
| Standard warehouse, span 18–24 m (60–80 ft) | Q355B frame + Q235B purlins | ~10–15% | Roughly equal |
| Crane-equipped workshop, 5–10 ton crane | Q355B | ~15–25% | ~5–10% lower |
| Long-span hall, span ≥30 m (100 ft) | Q355B | ~20–30% | ~10–15% lower |
| Cold climate (below −10°C) | Q355C/D/E | ~15–25% | ~5–10% higher material, but required |
Weldability and construction
Q235B has a low carbon equivalent. It welds well with standard electrodes and no preheat, even in the field.
Q355B has a slightly higher carbon equivalent. For plate thicknesses above 36 mm (1.4 in), most welding procedures require a preheat of 100–150°C (212–302°F) before welding. On-site welding should follow a written Welding Procedure Specification (WPS). Shop welding in the factory is fully controlled; field welds need inspection. Our steel welding process guide details the shop-to-field quality handoff, from submerged-arc welding to UT verification. For a broader look at how connection design interacts with steel grade, our bolted vs welded steel connection comparison walks through when high-strength bolted joints beat welded ones—and vice versa—across both Q235B and Q355B sections.
Low temperature and corrosion
In northern climates or unheated buildings in cold regions, specify Q355C, Q355D, or Q355E so the steel stays ductile at service temperature. In coastal or corrosive atmospheres, both grades rely on the same anti-corrosion coating system (blast to Sa 2.5, prime coat, finish coat). Hot-dip galvanizing is an upgrade option. For aggressive marine exposure, ask our engineers about Q355NH weathering steel—a different family not covered here.
Not Sure Which Grade Your Project Needs?
Send us your span, eaves height, crane capacity (if any), and local wind/snow/seismic loads. Our engineers will recommend Q235B, Q355B, or a hybrid mix—and show you the weight and price difference in writing.
International Equivalents
If your local engineer uses ASTM, EN, JIS, or AS standards, this table is the most useful one in the article.
Table 3 — Steel Grade International Equivalents
| Chinese Grade | ASTM (U.S.) | EN (Europe) | JIS (Japan) | AS (Australia) |
|---|---|---|---|---|
| Q235B | A36 | S235JR | SS400 | AS3679.1-250 |
| Q355B | A572 Gr.50 | S355JR | SM490A | AS3679.1-350 |
| Q355C | A572 Gr.50 (Charpy variant) | S355J0 | SM490B (impact) | AS3679.1-350 (impact variant) |
| Q355D | A572 Gr.50 low-temp | S355J2 | SM490B (low-temp) | AS3679.1-350 (low-temp) |
Important caveat: these are design equivalents, not identical chemical specifications. Yield floors line up closely, but exact CEV limits and Charpy energy requirements differ between standards. Use the table for ordering conversations and customs coding. When you hand drawings to a local approval authority, design to GB 50017, AISC 360, Eurocode 3 (EN 1993), or AS 4100—and state which one in the contract.
Why this table matters to you
Three practical reasons:
- Talk to your local engineer in his own units. Saying "Q355B" means nothing to a Texan inspector; saying "A572 Gr.50" does.
- Import and customs. HS codes and inspection certificates often expect a named national standard.
- Receiving inspection. When the steel arrives, your MTC will quote Chinese grades; the equivalence table lets your local team cross-check.
For U.S. references, the AISC Steel Construction Manual is the governing document. For European equivalents, see Eurocode 3 (EN 1993).
How to Choose: Decision Guide
Use these rules on the next quote.
Choose Q235B when:
- Span is ≤18 m (60 ft) and eave height ≤6 m (20 ft).
- No bridge crane, or only a tiny under-slung hoist.
- Budget is the deciding factor.
- The members are secondary: purlins, girts, bracing, base plates.
Choose Q355B when:
- Span is ≥24 m (80 ft) or eave height ≥8 m (26 ft).
- You plan a bridge or gantry crane.
- Wind, snow, or seismic loads are high.
- The building is a heavy industrial plant, hangar, or multi-story frame.
- Service temperature drops below −10°C (14°F): pick Q355C/D/E.
Choose the hybrid (recommended):
Main frame columns and rafters in Q355B, purlins and girts in Q235B. This is the single most common steel package in export industrial buildings. Ask the supplier to label the grade on every line of the Bill of Materials.
Verify before you pay
Require a Mill Test Certificate (MTC) to EN 10204 3.1 (or 3.2 if your inspector witnesses the test). It must show: heat number, chemical composition, yield and tensile strength, elongation, and Charpy results. Every major member should carry a stenciled heat number matching the MTC. "Certificates will follow" is not acceptable—put the MTC requirement in the purchase order. A full shipment-side acceptance checklist—blast profile, weld NDT, coating thickness, and member marking—is in our steel structure quality inspection guide. If a shop ever proposes swapping grades mid-production—for example, substituting Q355B with Q235B or a domestic equivalent—never accept a verbal swap; follow our steel material substitution rules: equal-strength vs equal-stiffness rechecks, updated MTCs, and engineer re-approval.
Table 4 — Steel Grade Selection Decision Table
| Project Feature | Recommended Grade | Reason |
|---|---|---|
| Span ≤18 m, eave ≤6 m, no crane | Q235B | Lowest first cost; loads are modest |
| Span 18–24 m, small crane (≤5 t) | Q355B frame + Q235B purlins | Hybrid balances weight and cost |
| Span ≥24 m, heavy crane (≥10 t) | Q355B (or Q355C/D/E) | Higher strength needed for girders and columns |
| Cold climate (<−10°C service) | Q355C/D/E | Ductile at low Charpy test temperature |
| Coastal / corrosive atmosphere | Either grade + heavy coating (galvanize if possible) | Grade is not the corrosion answer; coating is |
| Secondary members (purlins, bracing) | Q235B | Secondary loads; weight saving matters less |
For the system-level view of how grade interacts with frame type, read our follow-up on light vs heavy steel structure.
Conclusion
The Q235 vs Q355 steel question is a cost-and-weight optimization, not a competition. Q235B is economical, weldable, and perfectly adequate for small, lightly loaded buildings. Q355B is stronger, slightly more expensive per ton, and usually lighter on large spans, heavy cranes, or harsh climates. The most efficient real-world package is the hybrid: Q355B main frame, Q235B secondary steel.
Whatever you specify, write the grade on the drawings, on the BOM, and in the purchase order. Reject any quote that leaves the grade vague. When in doubt, ask an engineer to compare two designs side by side before you sign.
Specify the Right Steel Grade With Confidence.
Every structure we export ships with full Mill Test Certificates (EN 10204 3.1), shop drawings stamped by a licensed structural engineer, and material declarations you can hand directly to your local approval authority.
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Reference Links
- AISC 360 Specification for Structural Steel Buildings
- Eurocode 3 Design of steel structures
- EN 10204 Metallic products — Types of inspection documents
- GB 50017 Standard for design of steel structures
- ASTM A572/A572M Standard Specification for High-Strength Low-Alloy Columbium-Vanadium Structural Steel
About the Author
Senior Structural Engineer
With over 20 years of hands-on experience in steel structure design and prefabricated building engineering, our in-house senior structural engineer has personally contributed to more than 500 steel building projects—including warehouses, industrial factories, aircraft hangars, agricultural buildings, and commercial structures. The focus is on translating design codes such as AISC 360, ASCE 7, and Eurocode 3 into buildable, cost-effective steel solutions that balance structural performance, fabrication efficiency, and total project cost.
Learn more about our engineering team
Frequently Asked Questions
Beyond grade selection, many buyers also ask about delivery, certification, and cost across our broader steel building FAQ collection.
What does "Q235" and "Q355" actually mean?
The letter "Q" stands for the Chinese word for yield point. The number that follows is the minimum yield strength in megapascals (MPa) for steel thickness ≤16 mm. So Q235 means yield strength ≥235 MPa, and Q355 means yield strength ≥355 MPa. The trailing letter (B/C/D/E) indicates the Charpy V-notch impact test temperature: B = +20°C, C = 0°C, D = −20°C, E = −40°C.
Is Q355 the same as Q345?
Yes, essentially. Q345 was renamed Q355 in the Chinese national standard GB/T 1591-2018 (effective 2019). The chemical composition and yield-strength floor were tightened slightly. Old drawings may still say Q345, but new orders should specify Q355. There is no practical difference for most building applications.
What is Q355 equivalent to in ASTM/A36 terms?
Q355B is the closest equivalent to ASTM A572 Grade 50 in the U.S. (minimum yield strength 345 MPa). Q235B is the rough equivalent of ASTM A36 (minimum yield strength 250 MPa). In European standards, Q355B ≈ S355JR, and Q235B ≈ S235JR. Treat these as design equivalents—not identical chemical specifications.
Is Q355 always better than Q235?
Stronger does not always mean better. Q355B costs roughly 5–10% more per ton, so for small, lightly loaded buildings (≤18 m span, no crane), Q235B is more economical. Q355B pays for itself on large spans, heavy crane loads, or high wind/snow/seismic zones, where its higher strength lets you use smaller sections and less total steel.
How do I verify the steel grade I receive?
Require a Mill Test Certificate (MTC) in the EN 10204 3.1 or 3.2 format, listing the heat number, chemical composition, and mechanical test results. Every major member should have a matching heat number stamped or labeled. Without an MTC, you have no proof that the steel you received is what you paid for.
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