Q235 vs Q345 – Composition, Heat Treatment, Properties, and Applications

Introduction

Q235 and Q345 are two of the most widely used structural steels in China and in international supply chains where Chinese mill products are specified. Engineers, procurement managers, and manufacturing planners frequently face the choice between them when balancing cost, weldability, formability, and in-service strength requirements. Typical decision contexts include selecting material for building columns and beams, pressure frames, lifting devices, and welded structures where yield strength, toughness at low temperature, and fabrication ease must be weighed.

The essential technical distinction is that Q345 is a higher-strength structural steel than Q235 owing to its alloying and microalloying strategy; this delivers a higher design yield level while retaining acceptable toughness and weldability in most product forms. Because both grades are used for similar structural applications, direct comparisons of composition, processing response, mechanical properties, and fabrication behavior determine which grade is optimal for a given application.

1. Standards and Designations

• GB/T (China): Q235 and Q345 are defined in GB/T 700 (general structural steels) and GB/T 1591 (low-alloy high-strength structural steels for Q345 variants).
• ASTM/ASME: No direct equivalent grade names; comparable families include ASTM A36 (similar to Q235 in many properties) and various ASTM low-alloy grades (for Q345-like strength levels).
• EN (Europe): Structural steels such as S235JR (roughly comparable to Q235) and S355 (roughly comparable to Q345) serve similar strength categories.
• JIS (Japan): JIS structural steel grades are not one-to-one equivalent; comparisons should be made by mechanical and chemical requirements rather than grade name.

Classification: - Q235: Carbon structural steel (low-carbon plain carbon steel). - Q345: Low-alloy structural steel / HSLA (high-strength low-alloy) with controlled microalloying and tighter impurity limits.

2. Chemical Composition and Alloying Strategy

Table below shows typical composition ranges and notable alloying features. Exact limits vary by standard subgrade (e.g., Q235A/B/C/D/E and Q345A/B/C/D/E) and product thickness.

Element	Q235 (typical wt%)	Q345 (typical wt%)	Notes
C	0.14–0.22	0.10–0.20	Q345 often has slightly lower nominal C to improve weldability; strength is gained via Mn and microalloying.
Mn	0.30–1.40	1.00–1.60	Higher Mn in Q345 raises tensile and yield strength and contributes to hardenability.
Si	0.10–0.35	0.10–0.50	Si is deoxidizer; small effect on strength.
P	≤0.045 (typ.)	≤0.035–0.045	Lower P in Q345 variants improves toughness.
S	≤0.045 (typ.)	≤0.045	Kept low in both to avoid embrittlement.
Cr	trace	trace–small (≤0.30)	May be present in trace amounts in Q345 batches.
Ni	trace	trace	Not a deliberate alloying element in standard Q235/Q345.
Mo	trace	trace	Mo may be present in microalloyed variants or special lots.
V, Nb, Ti	trace / microalloying not typical in Q235	≤0.05 (microalloying)	Q345 commonly uses microalloying (Nb, V, Ti) in some subgrades to raise yield via grain refinement and precipitation hardening.
B	trace	trace	Very low if present; can affect hardenability at ppm levels.
N	trace	trace	Controlled to influence weldability and strength in microalloyed steels.

How alloying affects properties: - Increased Mn and controlled microalloying (Nb, V, Ti) in Q345 raise yield strength, refine grain size, and improve toughness without a proportional increase in carbon. - Low carbon in both grades supports weldability, while microalloying and slightly higher Mn in Q345 increase hardenability and yield strength. - Sulfur and phosphorus are limited in both to minimize brittleness and improve toughness.

3. Microstructure and Heat Treatment Response

Typical as-rolled microstructures: - Q235: Predominantly ferrite–pearlite microstructure after hot rolling. Low carbon content produces a relatively soft, ductile matrix with limited strengthening from pearlite. - Q345: Ferrite–pearlite with finer grain size and sometimes dispersed microalloy precipitates (NbC, V(C,N), TiN) that strengthen by precipitation and grain refinement.

Response to thermal processing: - Annealing/normalizing: Both grades respond to normalizing with grain refinement and modest strength adjustment; Q345 benefits more because microalloy precipitates control grain growth. - Quenching and tempering: Both can be quenched and tempered, but Q345 is more amenable to higher strength levels with less carbon because microalloying and Mn increase hardenability. Post-quench tempering gives a balance of toughness and strength. - Thermo-mechanical control processing (TMCP): Widely used for Q345 to achieve fine-grained microstructure and improved toughness at lower alloying levels. Q235 typically uses conventional hot rolling with less benefit from TMCP for strength.

Implication: Q345’s alloying and modern processing routes enable higher yield at comparable toughness, with better control of properties across thickness than plain Q235.

4. Mechanical Properties

Table below shows typical minimum values or ranges; exact values depend on subgrade, thickness, and heat treatment.

Property	Q235 (typical)	Q345 (typical)
Yield Strength (MPa)	~235 (minimum specified)	~345 (minimum specified)
Tensile Strength (MPa)	~375–500	~470–630
Elongation (% in 50 mm)	≥20–26	≥18–22
Impact Toughness (Charpy V-notch)	Varies; adequate at ambient; limited low-temperature toughness unless specified	Generally better low-temperature toughness in TMCP/Q345 variants; often specified at −20 °C
Hardness (HB)	Lower range, depends on product	Higher range but still moderate (designed to be weldable)

Interpretation: - Q345 is the stronger grade in terms of minimum yield and tensile strength, due to alloying and microalloying strategy. - Q235 is generally more ductile in the as-rolled condition; Q345 offers a favorable strength-to-weight ratio with competitive ductility and improved toughness when properly processed. - Impact toughness must be specified for low-temperature service; Q345 variants produced by TMCP often exhibit superior toughness at subambient temperatures.

5. Weldability

Weldability depends on carbon content, carbon equivalent, and presence of microalloying elements. Useful indices:

• International Institute of Welding carbon equivalent: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$

• International predictive index: $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$

Qualitative interpretation: - Q235 typically has a lower $CE_{IIW}$ because of its modest Mn and low C; this yields excellent general weldability with low preheat requirements for thin sections. - Q345 has higher Mn and contains microalloying elements that increase $CE_{IIW}$ and $P_{cm}$ slightly, so preheat and interpass temperature control may be more important for thicker sections to avoid cold cracking and hydrogen-assisted cracking. - Microalloying elements in Q345 raise hardenability locally; weld procedure specifications (WPS) should account for thickness, heat input, and hydrogen control. - Use of low-hydrogen electrodes, controlled heat input, and appropriate preheat/post-weld heat treatment (PWHT) where required will maintain weld integrity for both grades.

6. Corrosion and Surface Protection

• Neither Q235 nor Q345 are stainless steels; corrosion resistance is that of low-alloy plain carbon steel.
• Common protection methods: hot-dip galvanizing, zinc electroplating, organic coatings (paints, powder coatings), and corrosion allowances in design.
• For atmospheric or mild chemical exposures, galvanizing plus painting is the usual practice.

PREN (for stainless alloys) is not applicable to these non-stainless structural steels, but for completeness: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$ This index is not meaningful for Q235/Q345 because Cr, Mo, and N levels are too low to impart passive corrosion resistance.

7. Fabrication, Machinability, and Formability

• Formability: Q235 is more formable for cold bending, roll forming, and deep drawing because of its lower yield and higher ductility. For tight-radius bends or extensive forming, Q235 requires less force and shows fewer spring-back issues.
• Machinability: Both are reasonably machinable; higher strength Q345 may reduce tool life and require slightly higher cutting forces. Machinability also depends on sulfur content (free-cutting grades differ).
• Cutting and punching: Q235 tends to be easier to shear and punch. Q345 may require more robust tooling and higher forces but can still be processed using standard fabrication equipment.
• Surface finishing: Both accept conventional finishing; pre-treatment for galvanizing and coating is the same.

8. Typical Applications

Q235 Applications	Q345 Applications
General structural sections (angles, channels, I-beams) in buildings where ductility and low cost dominate	Structural components requiring higher yield or reduced section thickness for the same load (bridges, cranes, heavy equipment)
Light fabrication, frames, brackets, general platework	Pressure frames, heavy welded structures, offshore platforms (where higher strength and toughness are needed)
Pipes for low-pressure water systems, guardrails, fencing	Welded structures subject to higher static or dynamic loads; machine bases and heavy lifting attachments
Components where extensive forming or fabrication is required and cost sensitivity is high	Applications optimizing weight-to-strength, or where thicker sections must still meet toughness criteria

Selection rationale: - Choose Q235 for uncomplicated structural members and fabricated parts where cost and formability are primary drivers. - Choose Q345 when higher allowable stresses, reduced section thickness, or improved toughness at modest additional cost are required.

9. Cost and Availability

• Q235 is generally less expensive per tonne than Q345 due to simpler chemistry and lower processing control requirements.
• Q345 costs more because of tighter chemical control, microalloy additions, and more advanced processing (TMCP) for consistent properties.
• Availability: Both grades are widely available in plate, coil, bar, and section forms; Q235 is ubiquitous for commodity structural stock, whereas Q345 may need specification for certain subgrades or thicknesses in some markets.
• Procurement tip: Specify exact subgrade, thickness range, and required mechanical/impact properties to avoid confusion and ensure predictable pricing and lead times.

10. Summary and Recommendation

Summary table (qualitative):

Attribute	Q235	Q345
Weldability	Excellent; generally forgiving	Very good but may require additional preheat control on thick sections
Strength–Toughness	Moderate strength, high ductility	Higher yield and tensile strength with good toughness (especially TMCP)
Cost	Lower	Higher
Formability	Better for severe forming	Adequate but requires more force/tooling

Concluding recommendations: - Choose Q235 if: your design prioritizes cost-effectiveness and extensive cold forming or simple welded fabrication for ambient-temperature structural applications where standard yield levels (~235 MPa) are adequate. - Choose Q345 if: you need higher design yield and tensile strength to reduce section thickness or weight, require improved toughness (especially at subambient temperatures), or are specifying welded structures that benefit from HSLA microalloying and TMCP-processed plate.

Final note: Always reference the specific standard subgrade (e.g., Q235B vs Q345C) and required mechanical/property certificates when writing procurement specifications. For welded, thick, or low-temperature applications, include explicit requirements for Charpy impact energy, heat treatment, and weld procedure qualifications to ensure the chosen grade meets in-service demands.