Q390 vs Q420 – Composition, Heat Treatment, Properties, and Applications

Introduction

Q390 and Q420 are high-strength structural steels commonly specified in Chinese and related markets for load-bearing applications where higher yield strength than conventional mild steels is required. Engineers, procurement managers, and manufacturing planners often need to decide between these two grades when balancing strength, toughness, weldability, formability, and cost. Typical selection contexts include heavy fabrication (bridges, cranes, and off‑shore jacket structures), pressure-bearing frames, and structural components that must meet stricter yield or weight-reduction targets.

The core practical difference is that Q420 is specified to provide a higher minimum yield strength than Q390, which affects how the steel is produced (alloying, thermomechanical processing, or heat treatment) and therefore influences toughness, weldability, and forming behavior. Because both are classified as high‑strength low‑alloy (HSLA) steels rather than stainless or tool steels, comparisons focus on strength versus toughness trade-offs, fabrication constraints, and protective measures for corrosion.

1. Standards and Designations

• Common national and international standards where comparable grades are referenced:
• GB/T (China): Q-series steels (e.g., Q390, Q420) are defined in Chinese specifications for structural steels.
• EN (Europe): nearest equivalents are structural S‑grades such as S355 or S420 depending on properties.
• JIS (Japan): no direct one‑to‑one match; JIS grades focus on other designations.
• ASTM/ASME (US): no direct single standard mapping — use mechanical/property matching (e.g., ASTM A572 for high‑strength structural steels).
• Classification: Both Q390 and Q420 are HSLA (high‑strength low‑alloy) carbon steels optimized for structural applications. They are not stainless, tool, or specialty alloy steels.

2. Chemical Composition and Alloying Strategy

Typical composition trends for HSLA Q‑series steels emphasize low carbon plus small controlled additions of manganese, silicon, and microalloying elements to achieve strength while preserving toughness and weldability. The table below gives indicative typical ranges (wt%). Always verify exact composition on mill certificates or the applicable standard for the purchased material.

Table: Typical composition ranges (wt%) — indicative only; consult spec or mill certificate for exact numbers

Element	Q390 (typical range, wt%)	Q420 (typical range, wt%)
C	0.06 – 0.18	0.06 – 0.16
Mn	0.40 – 1.60	0.50 – 1.60
Si	0.10 – 0.50	0.10 – 0.50
P	≤ 0.035	≤ 0.035
S	≤ 0.035	≤ 0.035
Cr	0 – 0.50	0 – 0.50
Ni	0 – 0.30	0 – 0.30
Mo	0 – 0.10	0 – 0.10
V (microalloy)	0 – 0.12	0 – 0.12
Nb (microalloy)	0 – 0.08	0 – 0.08
Ti	0 – 0.02	0 – 0.02
B	0 – 0.002	0 – 0.002
N	0.005 – 0.020	0.005 – 0.020

Explanation - Carbon: Kept low to moderate to limit hardenability and preserve weldability; slightly lower carbon is sometimes used in higher‑strength grades combined with microalloying or thermomechanical rolling. - Manganese and silicon: Strengtheners and deoxidizers; Mn contributes to hardenability and tensile strength. - Microalloying (V, Nb, Ti, B): Small additions enable precipitation strengthening and grain refinement, allowing higher yield strength without large increases in carbon or other alloying elements that would harm weldability. - Minor alloying (Cr, Ni, Mo): Used only when specific hardenability or environmental resistance is required.

3. Microstructure and Heat Treatment Response

Microstructure for both Q390 and Q420 is typically a fine ferrite–pearlite or ferrite–bainite mix, depending on processing route: - As‑rolled/normalized: A fine ferritic matrix with dispersed pearlite is common; normalizing refines grain size and improves toughness. - Thermo‑mechanical processing (TMCP): Controlled rolling and accelerated cooling promote fine polygonal ferrite and bainite, enabling higher yield at lower alloy content — a common route for Q420. - Quenching & tempering: Not typical for routine structural Q‑grades, but used when a tailored combination of high strength and toughness is required; produces tempered martensite or tempered bainite and higher hardness. - Microalloy precipitation: Nb, V, and Ti precipitates pin grain boundaries and inhibit recrystallization, delivering strength through grain refinement and precipitation strengthening with minimal carbon increase.

Effect of processing - Q420 often relies more on TMCP and microalloying to reach the higher guaranteed yield while preserving toughness; this can produce a slightly higher proportion of bainitic microstructure compared with Q390 under equivalent processing. - Heat treatment (normalizing vs quench & temper) can significantly change toughness and hardness; thicker sections have slower cooling and therefore coarser microstructure and lower toughness.

4. Mechanical Properties

Table: Typical mechanical property ranges (indicative; dependent on thickness, processing, and standard)

Property	Q390 (typical)	Q420 (typical)
Minimum yield strength (Rp0.2)	≈ 390 MPa (specified)	≈ 420 MPa (specified)
Tensile strength (Rm)	~ 470 – 630 MPa	~ 520 – 680 MPa
Elongation (A, % on 50 mm)	~ 20 – 26%	~ 17 – 22%
Impact toughness (Charpy V‑notch, J)	Application/grade dependent; often specified at 0°C to −20°C; typical 27 – 47 J	Similar range but tends to be lower on average at the same thickness/processing
Hardness (HB)	Typically lower than Q420 for same processing	Slightly higher, reflective of higher strength

Interpretation - Strength: Q420 provides higher minimum yield strength by specification. Achieving that strength without sacrificing toughness tends to rely on TMCP and microalloying rather than raising carbon content significantly. - Toughness vs ductility: For the same processing, Q420 can be modestly less ductile and may show lower impact energy than Q390; careful control of rolling and cooling is needed to maintain acceptable toughness in thicker sections. - Design implication: Use Q420 where margin against yielding is important or where weight reduction is sought. Use Q390 when slightly better ductility or impact toughness is needed with lower cost.

5. Weldability

Weldability is primarily governed by carbon content, combined alloying (hardenability), and microalloying. Two commonly used empirical indices are the IIW carbon equivalent and the Pcm formula:

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

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

Qualitative interpretation - Both Q390 and Q420 are designed with relatively low carbon to support good weldability. However, Q420 may have slightly higher effective hardenability due to microalloying and TMCP-induced microstructures — increasing the risk of martensite formation in heat‑affected zones (HAZ) for high heat‑input welding or thicker sections. - Use preheat, controlled interpass temperatures, and low hydrogen consumable electrodes for thicker plates or joints with higher Pcm/CE values. Post‑weld heat treatment (PWHT) may be required for critical applications. - For routine shop welding of thin to moderate thicknesses, both grades are readily weldable with appropriate procedures; Q420 often requires closer attention to joint design and heat control.

6. Corrosion and Surface Protection

• Neither Q390 nor Q420 are stainless steels; both are subject to atmospheric and chemical corrosion like most carbon steels.
• Standard protection strategies: hot‑dip galvanizing, zinc lamella coatings, epoxy/urethane painting systems, cathodic protection for marine/off‑shore environments, and corrosion‑resistant design details (drainage, segregation of dissimilar metals).
• PREN is not applicable: For stainless steels the Pitting Resistance Equivalent Number is relevant: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$ This index is not used for carbon/HSLA steels; therefore PREN should not be applied to Q390/Q420.

7. Fabrication, Machinability, and Formability

• Formability: Lower‑strength Q390 generally has better cold formability (bendability, press‑forming) than Q420 at equivalent thickness because of higher ductility. Q420 may require larger bend radii or higher forming forces.
• Machinability: Both grades are similar; machinability is acceptable but decreases with strength. Higher strength (Q420) typically increases tool wear and demands more robust tooling and cutting parameters.
• Cutting/welding tolerance: Q420 may be more sensitive to out‑of‑flatness and springback due to higher yield strength; fabrication tolerances should account for that.
• Surface finishing: Both take paints and galvanizing well; pre‑treatment and surface preparation are standard.

8. Typical Applications

Table: Typical uses by grade

Q390 (typical uses)	Q420 (typical uses)
General structural beams, columns, and frames where cost-efficiency and good ductility are prioritized	Crane components, heavy girders, and bridge members where higher yield reduces section size and weight
Medium‑duty chassis and vehicle frames	Offshore jacket members and platform components where higher strength-to-weight is critical (with corrosion protection)
Agricultural and construction equipment	High‑load structural components (hoists, winches, heavy machinery) where margins against plastic yield are tighter
Storage tanks and non-pressurised structural shells (with appropriate protection)	Fabricated members designed for weight reduction in transport structures

Selection rationale - Choose Q420 when design requires higher yield strength for weight reduction, smaller sections, or to meet higher structural load demands. Q420 is preferred when the fabrication shop can control weld procedures and formwork for the higher‑strength material. - Choose Q390 when slightly better ductility, easier forming, and lower cost/supply risk are priorities.

9. Cost and Availability

• Cost: Q420 is typically more expensive than Q390 on a per‑ton basis because of higher processing demands (TMCP, microalloying control) and tighter property guarantees. Price difference varies by mill, region, and product form (plate, coil, section).
• Availability: Q390 tends to be more commonly stocked in general structural mills and distributors. Q420 may be more available in regions with high demand for HSLA steels; however, specialized product forms or thicknesses may require lead time for both grades.

10. Summary and Recommendation

Table: Quick comparison

Criterion	Q390	Q420
Weldability	Good; slightly better margin in thick sections	Good, but requires stricter heat control for thicker sections
Strength–Toughness balance	Balanced toward ductility/toughness	Higher yield strength; may trade some ductility/toughness
Cost (relative)	Lower	Higher

Recommendations - Choose Q390 if: you need a cost‑effective HSLA steel with good ductility and easier forming/welding for moderate structural loads; when fabrication speed and lower tool wear are priorities; or when stock availability and lower price are decisive. - Choose Q420 if: your design requires a higher guaranteed yield strength to reduce section size or weight, or when structural margins against yield must be increased; provided your fabrication and welding procedures can control heat input and you accept a modest increase in material cost.

Final note Always specify the exact standard, thickness limits, required impact energy (temperature), and weld procedure qualifications in procurement documents. Mill certificates and batch test reports should be reviewed to ensure the delivered chemistry and mechanical results meet the project’s performance and fabrication constraints.