Q355NH vs B450NQR – Composition, Heat Treatment, Properties, and Applications

Introduction

Engineers, procurement managers, and manufacturing planners commonly face the trade-off between toughness, weldability, cost, and strength when selecting structural steels. Q355NH and B450NQR address different points on that trade-off: one is a pressure‑vessel/structural grade optimized for notch toughness and reliable behavior after normalization; the other is a higher-yield structural/microalloyed grade designed to deliver elevated yield strength with reduced cross-section or weight.

The principal practical distinction between the two is their design target for yield strength: Q355NH is a ~355 MPa nominal yield steel produced for good toughness and weldability, while B450NQR targets roughly 450 MPa yield with microalloying and thermomechanical processing used to reach that class. This difference drives choices in fabrication, welding procedure development, part sizing, and cost.

1. Standards and Designations

• Q355NH
• Origin: Chinese GB specification family (Q series). Commonly encountered in pressure-vessel and structural applications in China and in international trade.
• Classification: Carbon-manganese, non-stainless structural/pressure-vessel steel with normalized/normalized-and-tempered designations; falls into the high-toughness structural steels (HSLA-like behavior when normalized).
• B450NQR
• Origin: European-style designation conventions (grade name indicates 450 MPa class). The suffixes NQR indicate normalized/quality/rolled or microalloyed heat treatment variants depending on supplier/standard.
• Classification: High-strength structural steel (high-yield, microalloyed/thermomechanically processed steel).

Both are non-stainless carbon/alloy steels intended for welded structural applications; neither is a tool steel or stainless alloy.

2. Chemical Composition and Alloying Strategy

The two grades use different alloying strategies: Q355NH relies on controlled carbon and moderate Mn with strict limits on P/S and sometimes small additions of Nb/Ti/V for clean steel and toughness. B450NQR reaches higher strength primarily by microalloying (Nb, V, Ti), controlled rolling (TMCP), and precise heat treatments rather than by large additions of costly alloying elements.

Table: Typical alloying focus (qualitative — check the exact purchaser or standard certificate for contract material)

Element	Q355NH (typical control)	B450NQR (typical control)	Role / Comment
C	Low–moderate; controlled for toughness and weldability	Low–moderate; controlled to limit hardness while achieving strength via TMCP	Carbon drives strength/hardenability; both grades keep C relatively low to retain weldability.
Mn	Moderate (for strength and deoxidation)	Moderate to somewhat higher; used with C for base strength	Mn increases hardenability and strength.
Si	Low; deoxidation	Low; deoxidation	Silicon affects deoxidation and slightly increases strength.
P, S	Strictly limited (low) for toughness	Strictly limited (low) for toughness and weldability	Impurities reduce toughness and weldability; both are low-sulfur, low-phosphorus.
Cr, Ni, Mo	Generally minimal in Q355NH	Minimal to low; some variations may include small amounts for hardenability	Large alloying additions are not typical; strength is from processing.
V, Nb, Ti	Possible microalloying at ppm–low weight % to refine grain	Frequently used microalloying (Nb, V, Ti) to raise yield via precipitation strengthening	Microalloying elements strongly raise yield strength without large carbon increases.
B, N	Nitrogen controlled; boron may be present in trace amounts in some steels	Nitrogen controlled; boron rarely used in structural grades	Nitrogen and boron affect hardenability and properties at ppm levels.

Note: The exact chemical limits are given in the controlling standard or mill certificate for each heat. For procurement, always require the mill analysis and material test report.

How alloying affects performance - Carbon and manganese increase strength and hardenability but reduce weldability and toughness if excessive. - Microalloying (Nb, V, Ti) allows higher yield strength via fine precipitates and grain refinement rather than by raising carbon content — thus preserving weldability and ductility to a degree. - Strict control of phosphorus and sulfur is necessary for low-temperature toughness and to avoid weld cracking.

3. Microstructure and Heat Treatment Response

Typical microstructures: - Q355NH - Produced to deliver a normalized microstructure: predominantly fine polygonal ferrite and pearlite with a relatively uniform grain size. Normalizing reduces residual stresses and produces good notch toughness. - If normalized + tempering or other thermal cycles are applied, the microstructure can be adjusted for additional toughness or slightly higher strength. - B450NQR - Typically processed via thermomechanical controlled processing (TMCP) or accelerated cooling to produce a fine-grained bainitic/ferrite-pearlite/microalloyed ferrite matrix with precipitation strengthening (niobium‑ and vanadium‑carbonitrides). - Microalloy precipitates and refined grain structure raise yield strength without large carbide networks.

Effect of standard processing routes - Normalizing (Q355NH): yields balanced strength and toughness, refines grains, and lowers residual stresses — beneficial for pressure vessels and welded fabrications. - Quenching & tempering: not typical for these grades but can be applied to microalloyed steels to increase strength at the expense of cost and distortion; Q355NH is usually not Q&T-treated. - TMCP and controlled rolling (B450NQR): produce high yield at lower carbon content; careful control of finish rolling temperature and cooling rate is essential to achieve the grade.

4. Mechanical Properties

Table: Comparative mechanical characteristics (nominal/qualitative; verify against mill test report and standard)

Property	Q355NH	B450NQR	Comment
Yield Strength (nominal)	~355 MPa	~450 MPa	The grade designation indicates target yield class.
Tensile Strength	Moderate; typically above yield but depends on form/heat	Moderate–high; depends on processing, often higher than Q355NH	Actual tensile values vary with product form and supplier.
Elongation (ductility)	Good (suitable for forming and welding)	Lower than Q355NH for same thickness, but acceptable if designed properly	Higher-strength steels often trade ductility for yield.
Impact Toughness	High notch toughness by design (good CVN at specified temperatures)	Good to variable; depends on TMCP and thickness — may need verification for low-temperature service	Q355NH is often specified for pressure-vessel notch-toughness levels.
Hardness	Moderate	Higher	Hardness rises with strength and microalloy precipitation.

Which is stronger, tougher, or more ductile - Strength: B450NQR typically provides significantly higher yield strength. - Toughness: Q355NH is designed for reliable notch toughness (especially in normalized plate and in the specified temperature range). - Ductility: Q355NH usually offers greater elongation and formability; B450NQR requires design considerations for forming and joining.

5. Weldability

Key factors: carbon equivalent and microalloying impact on hardenability and HAZ properties. Use established formulas to assess weldability qualitatively.

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

More comprehensive parameter for welding: $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$

Interpretation (qualitative) - Q355NH: Lower carbon and controlled alloying usually yield a lower carbon equivalent than higher-strength microalloyed steels, giving easier welding with standard procedures and lower preheat requirements for many thicknesses. Normalizing reduces HAZ hardening susceptibility. - B450NQR: Although carbon is kept low, microalloying and increased hardenability can raise susceptibility to HAZ hardening and cold cracking risk on thick sections or with high heat input. Weld procedure qualification (WPS) with appropriate preheat and interpass temperatures, hydrogen control, and post-weld heat treatment considerations may be necessary for thick plates.

Always perform weld procedure qualification and consult the mill test report to calculate $CE_{IIW}$ or $P_{cm}$ for a particular heat and thickness.

6. Corrosion and Surface Protection

• Both grades are non-stainless carbon/alloy steels; inherent corrosion resistance is limited.
• Standard protection methods:
• Hot-dip galvanizing for general atmospheric protection.
• Paint systems, powder coatings, or specialized linings for aggressive environments.
• Surface treatments (phosphating, passivation where applicable) for pre-painting adhesion.
• PREN (pitting resistance equivalent number) is not applicable to these non‑stainless steels: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$ Use stainless grades if corrosion resistance is required; otherwise protect Q355NH and B450NQR with coatings or cathodic protection as part of design.

7. Fabrication, Machinability, and Formability

• Cutting: Both cut with oxy-fuel, plasma, or laser; higher-strength B450NQR may require slower cutting speeds to avoid localized hardening.
• Forming and bending: Q355NH offers better bendability and springback characteristics at typical plate thicknesses; B450NQR requires larger bend radii and consideration of strain limits due to higher yield and lower elongation.
• Machinability: Slightly better for lower-strength Q355NH; B450NQR can be more abrasive to tooling due to microalloy precipitates; machinability varies with hardness and heat treatment.
• Finishing: Grinding and shot-blasting similar; heat-affected zones during welding require attention on B450NQR for potential hardness peaks.

8. Typical Applications

Q355NH	B450NQR
Pressure vessels, boilers, and tanks where specified notch toughness and normalized plates are required	Structural components requiring high yield strength: cranes, heavy machinery frames, supports where cross‑section reduction is desired
Welded ship deck structures, bridges and general structures where toughness at given service temperature is critical	Wear‑resistant frameworks, heavy equipment, and where designers want to minimize plate thickness for weight savings
General structural plates for civil engineering with low-temperature service needs	High-strength plates for rolling stock, mining equipment frames, and other high-stress structural members

Selection rationale - Choose Q355NH where low-temperature notch toughness, predictable HAZ behavior, and easier weldability are priorities. - Choose B450NQR where higher yield allows reduced section thickness, lower structural weight, or where strength-to-weight economy outweighs additional fabrication controls.

9. Cost and Availability

• Cost: Q355NH is generally lower cost per tonne than higher-grade microalloyed steels because of simpler processing and wider production volumes. B450NQR commands a premium due to TMCP, tighter processing control, and microalloy additions.
• Availability: Q355NH is widely produced and commonly stocked in regions where Chinese plate producers supply the market. B450NQR availability depends on regional producers and market demand for high-yield structural steels; lead times can be longer for larger or certified plates.
• Product forms: Both are available as plates, sections, and sometimes pipes, but specific product mixes depend on mill capability and certification.

10. Summary and Recommendation

Table: Quick comparison

Attribute	Q355NH	B450NQR
Weldability	High (easier, lower preheat in many cases)	Good but requires qualified WPS and attention to HAZ hardening
Strength–Toughness balance	Designed for toughness with moderate strength	Higher yield strength; toughness depends on processing and thickness
Cost	Generally lower	Higher (premium for strength and processing)

Concluding recommendations - Choose Q355NH if: - The design requires verified notch toughness and reliable behavior in welded pressure-vessel or low-temperature structural service. - Ease of welding, forming, and lower procurement cost are important. - You prefer a normalized plate with predictable HAZ properties.

• Choose B450NQR if:
• You need to reduce cross-section or weight and a higher nominal yield (~450 MPa) is essential to achieve the design.
• The project budget allows for higher-cost material and more stringent welding/fabrication controls.
• You accept tighter process control, potential preheat requirements, and the need to verify toughness for thick sections.

Final note: Material selection must be made using the actual mill certificates, thickness- and temperature-specific toughness data, and a formal welding procedure qualification that uses the specific steel heat chemistry and product form. Where in doubt, ask the mill for the heat-specific $CE_{IIW}$ and $P_{cm}$ calculations and require impact test certificates at the service temperature to validate the choice.