Q355 vs Q390 – Composition, Heat Treatment, Properties, and Applications

Introduction

Q355 and Q390 are high-strength structural steels widely used in construction, heavy equipment, and general fabrication. Engineers, procurement managers, and manufacturing planners frequently weigh trade-offs between material cost, weldability, toughness, and minimum design yield when selecting between them. Typical decision contexts include choosing a lower-cost grade for welded structural frames versus selecting a higher-strength grade to reduce section size or weight under the same loading.

The principal distinction between the two is their strength designation: Q390 is specified to a higher minimum yield than Q355, which drives differences in processing, microalloying strategy, toughness control, and fabrication practice. These steels are commonly compared because they occupy adjacent strength tiers in structural-steel product lines and often compete on performance-per-cost for welded and formed structures.

1. Standards and Designations

• Major standards where these grades appear or have close equivalents:
• GB/T (China): Q355, Q390 (structural steels).
• EN (Europe): comparable strengths are found in S355 and S420 series, but chemical and mechanical requirements differ.
• ASTM/ASME (USA): no exact direct equivalents; nearest common grades include ASTM A572 (Grades 50/60) and A656 (normalized), but equivalence must be verified by chemistry and mechanical data.

• JIS (Japan): similar high-strength structural steels exist, but again direct cross-reference requires verification.

• Classification:

• Both Q355 and Q390 are high-strength, low-alloy structural carbon steels (HSLA family). They are not stainless or tool steels. They commonly rely on controlled carbon content plus microalloying (Nb, V, Ti) and thermo-mechanical processing to obtain a balance of strength and toughness.

2. Chemical Composition and Alloying Strategy

Note: Chemistry varies by supplier, product form, and specific subgrade (e.g., Q355A/B/C/D/E). The table below gives indicative presence and typical ranges commonly encountered in commercial Q355 and Q390 steels; verify actual values from the mill test certificate (MTC) and applicable standard before design or welding.

Alloying strategy summary: - Q355 is typically produced with modest carbon and Mn and relies on controlled rolling and/or normalization to achieve a balance of yield, ductility, and toughness. - Q390 often uses similar base chemistry but may incorporate higher microalloying or slightly adjusted Mn/hardenability to reach the higher minimum yield while maintaining toughness—commonly achieved by thermo-mechanical controlled processing (TMCP) and microalloy precipitation rather than large increases in carbon.

3. Microstructure and Heat Treatment Response

• Typical microstructures:
• Q355: ferrite–pearlite matrix for conventional hot-rolled or normalized products; finer ferrite and dispersion of microalloy precipitates when TMCP is used.

• Q390: finer-grained ferrite with a controlled proportion of bainite in some TMCP products; more controlled precipitation strengthening from Nb/V/Ti is common to raise yield without sacrificing toughness.

• Heat treatment and processing effects:

• Normalizing refines grain size, improves toughness, and homogenizes structure—used when improved impact properties are required.
• Quenching & tempering is not typical for standard Q355/Q390 structural product forms but may be used for plates or bespoke components to achieve higher strength and tailored toughness.

• TMCP (thermo-mechanical controlled processing) is common to both grades to achieve high strength and good low-temperature toughness through controlled rolling, accelerated cooling, and microalloy precipitation—particularly effective for Q390 to meet higher yield targets without significantly increasing carbon.

• Practical note: Both grades are designed for as-rolled or normalized conditions; full hardening heat treatments are usually unnecessary and could compromise weldability and ductility without precise process control.

4. Mechanical Properties

The following table presents representative mechanical property ranges commonly encountered in commercial plate and coil products. Values depend on thickness, subgrade (A/B/C/D/E), and processing—always verify with the supplier's test certificates.

Interpretation: - Q390 is the stronger grade (higher yield and often higher tensile) by specification. Achieving this requires microstructural control that can slightly reduce ductility and, unless controlled, can affect impact toughness—especially for thicker sections or colder service temperatures. - Q355 is generally more ductile and forgiving in fabrication, with easier attainment of specified toughness for thicker sections.

5. Weldability

Key factors: carbon content, carbon equivalent, thickness, and presence of microalloying elements.

Useful empirical indices (interpret and use qualitatively; calculate with actual chemistry for welding procedure qualification): - International Institute of Welding carbon equivalent: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ - Pcm for weldability assessment: $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$

Qualitative interpretation: - Both Q355 and Q390 are designed for good weldability compared with higher-carbon tool steels; however, Q390’s higher strength target often means slightly higher hardenability (from Mn or microalloying), which can raise $CE_{IIW}$ and $P_{cm}$ compared with Q355. - Practical implications: - Preheat and interpass temperatures may need to be higher for thick Q390 sections to avoid cold cracking. - Welding consumable and procedure selection (matching strength, appropriate filler metal toughness) should be determined by procedure qualification using actual MTC values. - Post-weld heat treatment is rarely required for thin sections but may be necessary for thick weldments or where critical service conditions exist.

6. Corrosion and Surface Protection

• Neither Q355 nor Q390 are stainless steels; corrosion resistance is typical of carbon steels.
• Common protection methods:
• Hot-dip galvanizing for long-term atmospheric protection.
• Paint systems, primers and coatings tailored to environment (marine, industrial, rural).
• Cladding or metallizing for severe environments.
• If stainless or corrosion-resistant properties are required, select an appropriate stainless alloy; PREN is not applicable to Q355/Q390. For reference, PREN for stainless alloys is: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
• Clarification: PREN is irrelevant for plain carbon HSLA steels—use coating strategies or corrosion-resistant alloys instead.

7. Fabrication, Machinability, and Formability

• Cutting: Both grades are readily oxy-fuel-cut, plasma, and laser cut; edge quality and scaling depend on thickness and cutting method.
• Forming/bending: Q355 typically has better formability (larger minimum bend radius) due to its lower nominal yield. Q390 can be formed but requires larger bend radii or incremental forming practices to avoid cracking; springback increases with strength.
• Machinability: Higher strength (and associated microalloying) in Q390 can slightly reduce machinability compared to Q355; select tools and speeds accordingly.
• Surface finishing: Both take common surface treatments (shot-blast, priming, painting); welding and fabrication can locally alter properties and require attention to distortion control.

8. Typical Applications

Selection rationale: - Use Q355 when ductility, weldability, and cost are dominant concerns and the design loads are met by 355 MPa yield. - Use Q390 when a higher minimum yield allows reduced section size or weight and when the fabrication process and welding procedures are controlled to ensure toughness and avoid cracking.

9. Cost and Availability

• Cost: Q390 is typically somewhat more expensive per tonne than Q355 due to tighter processing control and potential additional microalloying. Price premium varies by market, supplier, and product form.
• Availability: Q355 is more commonly stocked and available in a broader range of product forms and thicknesses. Q390 availability depends on regional mill production and customer demand; long lead times can occur for uncommon thicknesses or tight-tolerance plate grades.

10. Summary and Recommendation

Recommendation: - Choose Q355 if you need a cost-effective structural steel with reliable weldability, higher ductility, and broad availability—ideal when design yields allow a 355 MPa minimum and ease of fabrication is important. - Choose Q390 if you need a higher minimum yield to reduce section size or weight and are prepared to control processing, welding procedures, and possibly preheat/interpass conditions to maintain required toughness—ideal for high-load structures where material strength delivers clear design or weight advantages.

Final practical note: Always confirm the exact chemical composition and mechanical properties from the mill test certificate and ensure welding procedures are qualified for the actual material lot, thickness, and intended service conditions.