Q345 vs Q390 – Composition, Heat Treatment, Properties, and Applications

Introduction

Engineers, procurement managers, and manufacturing planners commonly face the choice between Q345 and Q390 when specifying structural steels for bridges, cranes, heavy equipment, and pressure-bearing fabrications. The decision typically balances higher guaranteed yield strength and section thickness allowances against factors such as weldability, toughness at low temperature, fabrication cost, and availability.

At a high level, the principal difference between Q345 and Q390 is the guaranteed minimum yield strength: Q345 is specified at 345 MPa and Q390 at 390 MPa. That change in guaranteed strength is achieved by modest adjustments in chemical composition and by metallurgical processing (microalloying, controlled rolling, and heat treatments), which in turn affect hardenability, toughness, and fabrication behavior. These grades are often compared because they occupy adjacent positions in the low-alloy/high-strength structural steel family and are frequently interchangeable in designs where safety factors, weight, or plate thickness make a small change in yield attractive.

1. Standards and Designations

• Common standards and designations where these grades appear:
• GB/T (China): Q345 and Q390 are widely referenced in Chinese standards (e.g., GB/T 1591 and related product specifications for high-strength low-alloy structural steels).
• EN (Europe): rough cross-references include steels in the S355 to S420 range (but direct equivalence is not exact; always check mill certificates).
• ASTM/ASME (USA): similar role is played by ASTM A572/A709 grades (e.g., Grade 50) but direct chemical and mechanical matching must be validated.
• JIS (Japan) and other national standards: local equivalents exist but nomenclature differs.
• Classification: Both Q345 and Q390 are high-strength, low-alloy (HSLA) structural carbon steels. They are not stainless or tool steels; they rely on controlled chemistry and thermo-mechanical processing rather than high alloy levels for their performance.

2. Chemical Composition and Alloying Strategy

Table: Representative (typical) composition ranges by weight percent. These are indicative commercial analyses to illustrate differences; consult the applicable standard and mill certificate for exact limits and subgrade-specific values.

Element	Q345 (typical range, wt%)	Q390 (typical range, wt%)
C	~0.10–0.20	~0.10–0.22
Mn	~0.8–1.6	~0.9–1.8
Si	~0.20–0.50	~0.20–0.50
P	≤ 0.035 (max)	≤ 0.035 (max)
S	≤ 0.035 (max)	≤ 0.035 (max)
Cr	trace–~0.30	trace–~0.30
Ni	trace–~0.30	trace–~0.30
Mo	trace–~0.08	trace–~0.10 (occasionally higher)
V	trace–small (microalloying)	trace–small (microalloying)
Nb (Cb)	trace–small (microalloying)	trace–small (microalloying)
Ti	trace–small (stabilizer)	trace–small
B	trace (rare)	trace (rare)
N (if reported)	typically low, controlled	typically low, controlled

Notes: - Q345 and Q390 are primarily carbon-manganese steels with microalloying additions (Nb, V, Ti) used in some production routes to raise strength without excessive carbon. - Q390 formulations may allow slight increases in carbon, manganese, or controlled microalloy additions and heat-treatment processing to reach the higher yield requirement. - Exact additions (e.g., Mo, Cr) can appear in some product variants to enhance hardenability or elevated-temperature performance, but both grades remain low-alloy steels in general.

How alloying affects performance: - Carbon and manganese are the principal strength formers by solid-solution strengthening and by enabling transformation strengthening. Higher carbon increases strength and hardenability but reduces weldability and toughness if not compensated. - Microalloying elements (Nb, V, Ti) form fine carbides/nitrides that refine grain size and increase yield strength via precipitation and grain refinement without significantly increasing carbon equivalent. - Small additions of Cr, Mo, or Ni (if present) increase hardenability and can help achieve higher strength in thicker sections but may increase the carbon-equivalent and affect weldability.

3. Microstructure and Heat Treatment Response

• Typical microstructures:
• Q345: produced by controlled rolling and normalizing or thermo-mechanical controlled processing (TMCP) to yield a fine-grained ferrite–pearlite or ferrite with dispersed microalloy precipitates. The microstructure emphasizes toughness and ductility at moderate strength levels.
• Q390: similar base microstructure but engineered to provide higher yield via slightly higher dislocation density, more precipitation strengthening, or marginally higher retained pearlite/tempered bainite depending on process. In thicker sections, increased hardenability promotes higher-strength microstructures after controlled cooling.
• Heat-treatment and processing effects:
• Normalizing/refining: both grades benefit from normalizing to refine grain size and homogenize microstructure; Q390 sometimes receives more aggressive TMCP schedules to ensure uniform high strength.
• Quenching & tempering: not typical for standard Q345/Q390 mill products (these are generally delivered as TMCP steels), but quench-and-temper can be applied for higher-strength variants with deliberate transformation to martensite and subsequent tempering—this changes toughness and machinability substantially.
• Thermo-mechanical processing: TMCP (controlled rolling/cooling) is commonly used to achieve high strength with good toughness and weldability, especially for Q390, where processing compensates for modestly higher strength targets without excessive carbon.

4. Mechanical Properties

Table: Representative mechanical properties. Values are indicative and depend on plate thickness, testing standard, and subgrade—use mill certificates for procurement.

Property	Q345 (typical)	Q390 (typical)
Minimum Yield Strength (Rp0.2)	345 MPa	390 MPa
Tensile Strength (Rm)	~470–630 MPa	~520–690 MPa
Elongation (A50mm)	≥ 20% (varies by thickness)	≥ 18% (varies by thickness)
Impact Toughness (Charpy V-notch)	Good at common service temps; subgrade-dependent	Comparable but may require stricter subgrade controls for low-temperature service
Hardness (HBW)	Moderate	Slightly higher (depends on processing)

Interpretation: - Strength: Q390 is the stronger of the two by specification (higher minimum yield), enabling reduced section thickness or higher load capacity for the same geometry. - Toughness and ductility: Q345 typically shows marginally better elongation and sometimes better low-temperature toughness for a given chemistry because of slightly lower strength targets and often lower hardenability. However, modern TMCP processes allow Q390 to achieve good toughness at required testing temperatures—subgrade and plate thickness are critical. - Trade-off: Increased guaranteed yield strength in Q390 typically comes with a modest sacrifice in ductility and may increase sensitivity to welding conditions, unless microalloying and process control compensate.

5. Weldability

Weldability is governed by carbon content, carbon equivalent (hardenability), thickness, and restraint. Useful predictive formulas include:

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

• International Pcm formula (qualitative indicator): $$ P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000} $$

Qualitative interpretation: - Lower $CE_{IIW}$ and $P_{cm}$ values indicate easier weldability and lower tendency to form hard, brittle HAZ microstructures. Both Q345 and Q390 are produced with materials and microalloying optimized for weldability; however: - Q345 tends to have slightly lower carbon-equivalent values on average, making it marginally easier to weld in thicker sections without preheat. - Q390, due to higher strength targets, may have higher manganese or microalloying and thus a higher carbon-equivalent in practice, increasing the need for preheating, controlled heat input, or post-weld heat treatment in thicker sections or restrained joints. - Mitigation: Use of filler metals matched for toughness and strength, controlled interpass temperatures, preheat, and selection of appropriate welding consumables and procedures typically ensures weldable assemblies for both grades. Always qualify welding procedures on representative thicknesses and for the lowest design temperature.

6. Corrosion and Surface Protection

• Both Q345 and Q390 are non-stainless carbon-manganese steels and do not provide intrinsic corrosion resistance beyond bare steel. Standard protection strategies include:
• Hot-dip galvanizing for atmospheric exposure.
• Paint systems (shop primer + finish coats) with surface preparation (e.g., abrasive blasting).
• Thermal spray or polymer coatings for aggressive environments.
• Stainless-specific indices are not applicable to these grades. The PREN formula, used for stainless corrosion resistance, is therefore not relevant here: $$ \text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N} $$
• Selection guidance: Where corrosion performance is required (marine, chemical exposures), choose corrosion-resistant alloys or specify protective systems. The choice between Q345 and Q390 does not materially affect corrosion resistance unless minute alloying differences include corrosion-influencing elements (rare).

7. Fabrication, Machinability, and Formability

• Cutting: Plasma, oxy-fuel, and laser cutting behave similarly for both grades; Q390 may require marginally different thermal inputs for gouging or gouge-free cutting due to slightly higher strength.
• Machinability: Both are moderate in machinability; higher-strength Q390 may exhibit somewhat higher tool wear in heavy machining because of increased strength and possibly harder micro-precipitates.
• Forming/bending: Q345, with slightly greater ductility, is generally more forgiving for cold forming and bending. Q390 can be formed but may require larger bend radii or controlled forming sequences to avoid cracking, especially in thicker plates or in sections with high pre-existing strain.
• Surface finishing: Both accept standard finishing methods; pay attention to stress-relief and distortion control during fabrication when working with Q390 because higher residual stresses can develop at higher strength levels.

8. Typical Applications

Table: Typical uses for each grade with rationale.

Q345 — Typical Applications	Q390 — Typical Applications
General structural components (beams, columns) in buildings and bridges where good toughness and weldability are required	Heavier structural members where reduced section thickness or increased load capacity is required (crane rails, heavy machinery frames)
Welded steel plates for tanks, trailers, general fabrication	Structural sections in transportation and heavy equipment where higher yield allows lighter designs
Cold-formed sections and fabricated frames	Components subject to higher static loads or where margin for fatigue design is limited
Agricultural and general engineering steelwork (cost-sensitive)	Applications where stiffness-to-weight or strength-to-weight benefits justify higher material cost

Selection rationale: - Choose Q345 where ease of fabrication, higher ductility, and cost are prioritized and where the lower yield is sufficient for the design. - Choose Q390 where the design benefits from a higher guaranteed yield (smaller cross-section, weight savings), provided welding procedures, fabrication controls, and toughness requirements are satisfied.

9. Cost and Availability

• Cost: Q390 is typically more expensive per tonne than Q345 due to tighter process controls and potentially higher alloy content or more exacting TMCP schedules. The price premium varies by market, thickness, and geographic region.
• Availability: Both are standard product lines in major steel-producing regions; Q345 is generally more widely available because it is a common structural grade. Q390 is commonly stocked in many markets but availability of certain thicknesses, plate sizes, and subgrades may be more limited—lead times should be checked.
• Product forms: Both are supplied as hot-rolled plates, coils, and sometimes as normalized or thermomechanically rolled plate. For specialized plates (ultra-thick sections or specific impact-tested subgrades), lead times increase.

10. Summary and Recommendation

Table: Quick comparison (qualitative).

Metric	Q345	Q390
Weldability	Good (easier, lower CE)	Good to Fair (may need preheat on thick sections)
Strength–Toughness balance	Balanced toward toughness and ductility	Higher strength; toughness achievable with proper process control
Cost	Lower (generally)	Higher (premium for higher strength)

Recommendations: - Choose Q345 if: - The design can accept 345 MPa yield and the priorities are higher ductility, easier welding, and lower material cost. - Fabrication will involve significant forming or cold-working, or where routine welding without extensive preheat is required. - Stock availability and economy are important.

• Choose Q390 if:
• You need higher guaranteed yield strength (390 MPa) to reduce section thickness or weight, or to meet specific load capacity requirements.
• Fabrication procedures can accommodate slightly higher welding controls (preheat, qualified WPS), and toughness requirements can be met by selecting the proper subgrade and process.
• The project justifies the higher material cost by savings in downstream fabrication, transportation, or weight-sensitive design.

Final note: Q345 and Q390 are adjacent choices in the HSLA structural steel family. The optimal selection is driven by component-level requirements (yield, toughness at lowest service temperature), fabrication constraints (welding and forming), life-cycle cost (coating and maintenance), and availability. Always verify chemical and mechanical certificates from the steel supplier, qualify welding procedures on representative material and thickness, and specify the required impact energy and test temperature for applications exposed to low-temperature service or dynamic loads.