Steel Grades Explained: Q235 vs Q345 Structural Steel Selection Guide
A practical guide to understanding Chinese structural steel grades Q235 and Q345, including yield strength, chemical composition, weldability, and project selection criteria.
Direct Answer: What Are Q235 and Q345 Steel Grades?
Q235 and Q345 are the two most common Chinese structural steel grades used in prefabricated steel buildings. Q235 has a minimum yield strength of 235 MPa and is widely used for secondary members, purlins, and low-load applications. Q345 has a minimum yield strength of 345 MPa and is typically specified for primary load-bearing members such as columns, main beams, and trusses in industrial buildings, warehouses, and large-span structures.
The “Q” stands for Yield Point (Qu Fu in Chinese pinyin), and the number indicates the minimum yield strength in megapascals (MPa). The choice between Q235 and Q345 affects material cost, member sizing, weldability, and overall structural efficiency.
Why Steel Grade Selection Matters
Choosing the correct steel grade is one of the most fundamental decisions in a steel structure project. It directly impacts:
- Structural safety: The grade determines the load-carrying capacity of each member
- Material cost: Higher strength grades generally cost more per tonne but can reduce total steel quantity
- Member dimensions: Q345 allows smaller cross-sections for the same load, saving space and weight
- Weldability: Different grades have different carbon equivalents and welding requirements
- Availability: Q235 and Q345 are widely available in China; other grades may have longer lead times
For most standard industrial buildings and warehouses, a combination of Q345 for primary members and Q235 for secondary members is the most common and cost-effective approach.
Understanding Q235 Steel
Key Properties
Q235 is a carbon structural steel defined by the Chinese standard GB/T 700. It is the most widely used general-purpose structural steel in China.
| Property | Q235 Value |
|---|---|
| Minimum Yield Strength | 235 MPa (for thickness ≤ 16mm) |
| Tensile Strength | 370–500 MPa |
| Elongation | ≥ 26% |
| Carbon Content (C) | ≤ 0.22% |
| Manganese (Mn) | ≤ 1.4% |
| Silicon (Si) | ≤ 0.35% |
| Sulfur (S) | ≤ 0.050% |
| Phosphorus (P) | ≤ 0.045% |
Common Applications
Q235 is typically used for:
- Roof and wall purlins (C/Z sections)
- Secondary beams and joists
- Bracing members
- Platforms and walkways
- Ladders and handrails
- Light-duty support structures
- Cladding accessories and flashings
Advantages
- Excellent weldability due to low carbon content
- Good formability and ductility
- Wide availability and competitive pricing
- Suitable for cold-formed sections
- Easy to source in various profiles and thicknesses
Limitations
- Lower strength means larger member sizes for heavy loads
- Not ideal for long-span primary structures
- May require more material for the same load compared to Q345
- Not recommended for highly loaded columns in multi-story buildings
Understanding Q345 Steel
Key Properties
Q345 is a low-alloy high-strength structural steel defined by GB/T 1591. It is the standard grade for primary load-bearing members in modern steel construction.
| Property | Q345 Value |
|---|---|
| Minimum Yield Strength | 345 MPa (for thickness ≤ 16mm) |
| Tensile Strength | 470–630 MPa |
| Elongation | ≥ 21% |
| Carbon Content (C) | ≤ 0.20% |
| Manganese (Mn) | 1.00–1.60% |
| Silicon (Si) | ≤ 0.55% |
| Niobium (Nb) | ≤ 0.07% |
| Vanadium (V) | ≤ 0.15% |
| Titanium (Ti) | ≤ 0.20% |
| Sulfur (S) | ≤ 0.035% |
| Phosphorus (P) | ≤ 0.035% |
Common Applications
Q345 is typically used for:
- Main building columns (H-section, box-section)
- Primary roof beams and girders
- Crane beams for overhead cranes
- Long-span trusses and arches
- Multi-story steel building frames
- Heavy-load platform structures
- Bridge girders and heavy support structures
- Seismic-zone primary frames
Advantages
- Higher strength-to-weight ratio
- Allows smaller cross-sections, reducing total steel weight
- Suitable for long-span and heavy-load applications
- Good weldability with proper procedures
- Excellent for seismic design due to good ductility
- Can reduce foundation loads due to lighter superstructure
Limitations
- Higher material cost per tonne compared to Q235
- Requires more careful welding procedures due to alloy content
- May require preheating for thick sections or low-temperature welding
- Slightly lower ductility than Q235
- Carbon equivalent must be checked for weldability
Q235 vs Q345: Detailed Comparison
Strength and Weight
The most significant difference is yield strength: Q345 is approximately 47% stronger than Q235. This means:
- A Q345 member can carry more load with the same cross-section
- Or, for the same load, a Q345 member can use a smaller cross-section
- For typical industrial buildings, using Q345 for primary members can reduce total steel weight by 15–25% compared to using Q235 for all members
However, the actual savings depend on the specific project, loading conditions, and member configuration. In some cases, minimum section requirements or deflection limits may prevent full utilization of the higher strength.
Cost Considerations
| Factor | Q235 | Q345 |
|---|---|---|
| Material cost per tonne | Lower | Higher (typically 5–15% more) |
| Total steel quantity | Higher | Lower (for same load) |
| Overall project cost | May be higher for heavy structures | Often lower for large-span/heavy-load buildings |
| Fabrication cost | Similar | Similar (may require more welding control) |
| Transportation cost | Higher (more weight) | Lower (less weight) |
For small, light structures, Q235 may be more cost-effective. For large industrial buildings, warehouses with cranes, and long-span structures, Q345 usually provides better overall value despite the higher per-tonne cost.
Weldability
Both grades are considered weldable, but there are important differences:
-
Q235: Excellent weldability. Low carbon content means low carbon equivalent. Generally no preheating required for normal thicknesses. Can be welded with common electrodes (E43 series) and welding wire (ER49-1).
-
Q345: Good weldability, but requires more attention. Higher alloy content increases carbon equivalent. For thick sections (typically > 25mm) or low-temperature environments, preheating may be required. Use E50 series electrodes or ER50-6 welding wire.
Carbon Equivalent (CEV) is a key indicator:
- Q235: CEV typically ≤ 0.40%
- Q345: CEV typically ≤ 0.45–0.50%
When CEV exceeds 0.45%, preheating and controlled welding procedures are generally recommended.
Ductility and Toughness
- Q235: Higher elongation (≥ 26%), excellent ductility. Good for structures requiring high deformation capacity.
- Q345: Lower elongation (≥ 21%), but still adequate for most structural applications. Good toughness, especially when specified with notch toughness requirements (Q345B, Q345C, Q345D, Q345E grades).
For seismic design, both grades can be used, but Q345 with appropriate toughness grade is often preferred for primary members in seismic zones.
Grade Subgrades and Quality Levels
Both Q235 and Q345 have subgrades based on quality and impact toughness requirements:
Q235 Subgrades
| Grade | Impact Test Temperature | Typical Application |
|---|---|---|
| Q235A | No impact requirement | General secondary structures |
| Q235B | +20°C | Common building structures |
| Q235C | 0°C | Cold-region structures |
| Q235D | -20°C | Severe cold environments |
Q345 Subgrades
| Grade | Impact Test Temperature | Typical Application |
|---|---|---|
| Q345A | No impact requirement | General primary structures |
| Q345B | +20°C | Most common building grade |
| Q345C | 0°C | Cold-region primary structures |
| Q345D | -20°C | Seismic and cold environments |
| Q345E | -40°C | Extreme cold environments |
For most projects in temperate climates, Q235B and Q345B are the standard specifications. For cold regions or seismic design, higher grades (C, D, or E) may be specified.
How to Choose the Right Grade for Your Project
Decision Framework
- Identify member function: Primary load-bearing or secondary?
- Determine loading conditions: Light, medium, or heavy? Crane loads? Seismic zone?
- Check span requirements: Short span (< 15m) or long span (> 20m)?
- Evaluate cost trade-offs: Material cost vs. total steel weight vs. foundation savings
- Consider welding requirements: Field welding vs. shop welding, thickness, temperature
- Review local code requirements: Seismic design codes, building regulations
Typical Project Recommendations
| Project Type | Primary Members | Secondary Members |
|---|---|---|
| Light warehouse (no crane) | Q345B or Q235B | Q235B |
| Standard industrial workshop | Q345B | Q235B |
| Workshop with overhead crane | Q345B (columns and crane beams) | Q235B |
| Long-span warehouse (> 20m) | Q345B | Q235B |
| Multi-story steel building | Q345B or Q345C | Q235B |
| Cold-region structure | Q345C or Q345D | Q235C |
| Seismic-zone primary frame | Q345B or Q345C | Q235B |
| Light canopy / small structure | Q235B | Q235B |
Common Mistakes to Avoid
- Using Q235 for all members in heavy structures: This leads to oversized members, increased weight, and higher foundation costs.
- Specifying Q345 for light secondary members: This is unnecessary and increases cost without benefit.
- Ignoring weldability requirements: For thick Q345 sections, proper welding procedures and preheating are essential.
- Not considering grade availability: While Q235B and Q345B are widely available, higher grades (D, E) may have longer lead times.
- Confusing yield strength with tensile strength: The grade number refers to yield strength, not tensile strength.
- Not checking thickness effects: Yield strength decreases with increasing thickness. For sections over 16mm, the guaranteed yield strength may be lower (e.g., Q345 for 16–35mm is 325 MPa, for 35–50mm is 295 MPa).
International Grade Equivalents
For reference, here are approximate equivalents to common international standards:
| Chinese Grade | ASTM A36 / A572 | EN 10025 | JIS |
|---|---|---|---|
| Q235B | A36 (similar) | S235JR (similar) | SS400 (similar) |
| Q345B | A572 Grade 50 (similar) | S355JR (similar) | SM490 (similar) |
Note: These are approximate equivalents, not exact substitutions. Chemical composition and mechanical properties differ slightly between standards. For international projects, always verify with the applicable code and engineer of record.
Buyer Checklist for Steel Grade Specification
When preparing project specifications or reviewing quotations, confirm:
- Grade specified for each member type: Primary vs. secondary
- Quality subgrade: A, B, C, D, or E based on climate and seismic requirements
- Thickness range: Note that yield strength varies with thickness
- Welding procedures: Especially for Q345 thick sections
- Material certificates: Mill test certificates (EN 10204 3.1) should be provided
- Inspection requirements: Third-party inspection if required
- Surface treatment: Primer, galvanizing, or coating specifications
- Tolerance standards: GB 50205 or applicable fabrication tolerance standard
- Alternative grades: Are equivalent international grades acceptable?
- Lead time: Confirm availability of specified grades and subgrades
Related Capabilities
ZhongSai supports steel structure project supply with:
- Fabricated Steel Beams & Columns: H-section, box-section, and custom-built-up members in Q235 and Q345
- Steel Trusses: Long-span roof trusses and spatial structures
- Steel Purlins: Cold-formed C and Z sections in Q235
- Roof & Wall Cladding: Profiled panels and sandwich panels
- Manufacturing Quality: Cutting, welding, fabrication, and quality control processes
- Export Delivery: Packing, container loading, and export documentation coordination
Project Inquiry
Have structural drawings or a project specification that includes steel grade requirements? Send your available project information for review, including:
- Building dimensions and layout
- Load requirements (crane, snow, wind, seismic)
- Specified steel grades
- Member schedules if available
- Project location and applicable codes
ZhongSai will review the requirements and provide feedback on grade selection, member sizing, and fabrication considerations.
Note: This article provides general guidance on structural steel grades. Specific project design should be based on applicable codes, project location, loads, and project-specific engineering requirements. Always consult a qualified structural engineer for project-specific grade selection.