S350GD+Z vs S350GD+AZ – Composition, Heat Treatment, Properties, and Applications
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Table Of Content
Table Of Content
Introduction
S350GD+Z and S350GD+AZ are two common surface-finished variants of the EN 10346 family of high-strength structural steels. Both are based on the S350GD substrate — a cold-rolled, high-strength low-alloy (HSLA) steel with a guaranteed minimum yield of 350 MPa — but they differ in surface protection and behaviour in service. Engineers, procurement managers, and manufacturing planners often face a selection dilemma: prioritize lower cost and broad corrosion protection, or prioritize enhanced high-temperature corrosion resistance and barrier performance. Choices revolve around corrosion environment, welding and fabrication methods, coating compatibility with paints, and life-cycle cost.
The principal technical distinction between the two lies in the coating system: one is hot-dip zinc coated (sacrificial galvanizing) and the other uses an aluminium-based alloy coating (typically Al–Si). That coating difference drives differences in corrosion mechanism, high-temperature stability, forming behaviour, and sometimes availability and price — hence the frequent direct comparison in design and procurement.
1. Standards and Designations
- Relevant European standard: EN 10346 — continuously hot-dip coated steel flat products for cold forming.
- International and regional references that may be used alongside: ASTM/ASME (for corrosion and coating practice), JIS (for comparable coated steels), and various national procurement specifications.
- Material class: HSLA (high strength low alloy) structural carbon steel substrate, with metallic surface coatings (zinc or aluminium-silicon).
- Designations:
- S350GD+Z: S350GD substrate with hot-dip zinc coating (galvanized).
- S350GD+AZ: S350GD substrate with an aluminium-based coating (commonly Al–Si alloy, referred to as aluminized or Al–Si coated).
2. Chemical Composition and Alloying Strategy
Below is a qualitative composition table for the substrate alloying and typical trace microalloying elements used in S350GD. Do note the coating elements (Zn or Al-Si) are not part of the substrate chemistry but are applied as metallic layers.
| Element | Typical role in S350GD substrate |
|---|---|
| C (Carbon) | Low carbon level to balance strength and weldability; controlled to limit hardenability. |
| Mn (Manganese) | Principal strengthening element for yield and tensile strength; present at moderate levels. |
| Si (Silicon) | Residual and deoxidation element; limited to avoid reduced toughness if excessive. |
| P (Phosphorus) | Treated as impurity; kept low for toughness. |
| S (Sulfur) | Controlled impurity; low levels to improve formability and weld quality. |
| Cr, Ni, Mo | Not typical major alloying additions in S350GD; may be absent or present only as tramp/trace. |
| V, Nb, Ti | Microalloying elements sometimes used to achieve fine-grain strengthening via precipitation and grain control. |
| B | Rare for this grade; not a defining element. |
| N (Nitrogen) | Controlled during processing; can affect precipitation and strength. |
How alloying affects properties: - Carbon and manganese provide baseline strength. Keeping carbon low improves weldability. - Microalloying elements (Nb, V, Ti), when present, provide strengthening by grain refinement and precipitation, improving yield strength without large carbon increases. - The coating compositions (zinc or aluminium-silicon) are separate metallic layers providing corrosion protection and do not appreciably change substrate bulk mechanical properties, although they influence surface behaviour during forming, welding, and painting.
3. Microstructure and Heat Treatment Response
S350GD is produced by controlled rolling and annealing processes to yield a fine-grained ferritic–pearlitic or ferrite with bainitic islands microstructure depending on processing. Typical processing routes are continuous annealing and thermal-mechanical control processing to achieve the target yield and toughness.
- S350GD+Z and S350GD+AZ substrates share the same bulk microstructure because the coating is applied after cold rolling/annealing and before or after temper rolling depending on mill practice.
- Normalizing: will refine grain size and can raise yield/tensile depending on cooling rate; normally not applied to coated cold-rolled sheet.
- Quenching & tempering: not applicable to commercially supplied S350GD sheet — the grade is provided in a thermomechanically processed/annealed condition rather than hardened and tempered.
- Thermo-mechanical rolling: used by mills to control strength and toughness in the substrate without heavy reliance on carbon. This gives a good combination of strength and ductility.
Coating application effect: - Hot-dip coating baths (zinc or aluminium-silicon) introduce thermal exposure; the substrate microstructure is stable for S350GD, but the coating/substrate intermetallic layer can form differently for Zn and Al–Si systems and influence surface hardness and ductility locally.
4. Mechanical Properties
The table below summarizes typical mechanical characteristics. Numerical ranges for tensile and elongation are indicative; final values depend on supplier, thickness, and temper.
| Property | S350GD+Z | S350GD+AZ |
|---|---|---|
| Yield Strength (min) | 350 MPa (grade designation) | 350 MPa (grade designation) |
| Tensile Strength (typical) | Commonly in a moderate range above yield; supplier-specific (see mill data sheet) | Similar to +Z; substrate determines bulk tensile strength |
| Elongation (A%) | Adequate ductility for cold forming; depends on thickness and roll-anneal practice | Comparable to +Z for the substrate; coating can influence surface crack initiation |
| Impact Toughness | Good at room temperature; low-temperature toughness per mill certification | Similar bulk toughness; surface effects may slightly alter notch behaviour |
| Hardness | Substrate hardness governed by processing; coating slightly alters surface hardness (Zn softer, Al–Si often harder) | See column at left — Al–Si coatings typically result in a harder surface film than Zn |
Which is stronger/tougher/more ductile and why: - Strength and toughness are mainly set by the substrate (S350GD): both coatings do not significantly change bulk mechanical properties. - Surface coatings can influence apparent toughness in thin cross-sections or at the immediate surface due to brittle intermetallic layers (more of a concern with some aluminized coatings). - Ductility for forming is effectively the same in the substrate, but practical formability of coated sheet will depend on coating ductility and adhesion.
5. Weldability
Weldability of S350GD substrate is generally good due to low carbon and controlled alloying, making it suitable for common joining processes (GMAW/MIG, SMAW, laser welding, resistance welding) when best practices are followed.
Useful carbon equivalent and cracking propensity formulas (interpret qualitatively): - International Institute of Welding carbon equivalent: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ - International European Pcm formula: $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
Interpretation: - Low $CE_{IIW}$ and $P_{cm}$ values indicate lower cold-cracking susceptibility and easier weldability. S350GD is engineered to keep carbon and aggressive alloying low, yielding favorable indices. - Coating considerations: - S350GD+Z (Zn): Zinc produces zinc vapor and fume during arc welding; welds must be prepared by removing coating from the joint area to avoid porosity, fume hazards, and embrittlement of the weld metal. - S350GD+AZ (Al–Si): Aluminium-rich coatings can form refractory oxides and higher-melting intermetallics at the weld zone; removal of coating before welding is recommended, and welding parameters may require adjustment to avoid weld defects. - Preheating/post-weld treatment: usually not required for thin S350GD substrates, but follow supplier guidance for thicker sections and for coated surfaces to manage thermal cycles and hydrogen risks.
6. Corrosion and Surface Protection
- S350GD+Z (hot-dip zinc): Provides sacrificial cathodic protection. Zinc corrodes preferentially, protecting steel even where the coating is nicked. Good general-purpose atmospheric corrosion resistance and excellent galvanic protection when steel is in contact with other metals.
- S350GD+AZ (aluminium-silicon): Al–Si coatings act more as a barrier and form a stable aluminium oxide that resists high-temperature oxidation and offers superior performance in some high-temperature and cyclic-oxidation environments. The Al-rich coating is less sacrificial and more barrier-oriented.
When stainless-type indices apply: - PREN (Pitting Resistance Equivalent Number) is not applicable to these non-stainless substrates, but for reference: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$ - Use PREN only for stainless alloys; for coated carbon steels, evaluate the coating’s corrosion mechanism (sacrificial vs barrier), coating thickness, and environmental exposure.
Painting and finishing: - Both coatings accept paints, but surface pre-treatment may differ. Galvanized surfaces require chromate or phosphate conversion layers for optimal adhesion; aluminized surfaces may require different primers for compatibility. Consult coating and paint vendors for system approvals.
7. Fabrication, Machinability, and Formability
- Cutting: Laser, plasma, and shear cutting are commonly used for both coatings. Cutting parameters and dross quality vary by coating type; Al–Si coatings may generate more refractory dross.
- Bending/forming: Substrate formability is similar, but coating behavior differs:
- Zn coatings are relatively ductile and can tolerate tighter bend radii; however, the zinc layer can crack or flake if not roll-annealed properly.
- Al–Si coatings are harder and more brittle — may crack on tight bends or severe stamping operations and can show white oxidation on cracked areas.
- Machinability: Drilling and tapping produce different swarf and tool wear characteristics depending on whether Zn or Al–Si is present; Al–Si may be more abrasive to tooling.
- Surface finishes and edge condition: Edges trimmed after coating may show exposed steel; post-process protection and touch-up painting are common.
8. Typical Applications
| Application area | S350GD+Z (galvanized) | S350GD+AZ (aluminized / Al–Si) |
|---|---|---|
| Building envelope (facade, cladding) | Widely used for general corrosion resistance and cost-effective protection | Used where higher temperature or long-term barrier performance is needed |
| Roofing and rainwater goods | Common choice for atmospheric exposure | Selected for environments with higher thermal cycling or specific aesthetic needs |
| Automotive structural panels | Used for corrosion protection on body-in-white where painting follows | Selected for heat-exposed components or where galvanic compatibility with other metals is a concern |
| HVAC, ductwork | Commonly specified | Used where higher-temperature aluminized resistance is beneficial |
| Industrial equipment (low-medium temp) | Standard choice | Chosen when oxidation resistance at elevated temperatures is required |
| Architectural exposed elements | Economical option with sacrificial protection | Used for longer-life, higher-cost architectural applications where Al-surface finish is desired |
Selection rationale: - Choose S350GD+Z for broad atmospheric corrosion protection at lower cost and where sacrificial protection is beneficial. - Choose S350GD+AZ where exposure to elevated temperatures, oxidation resistance, or specific barrier behaviour is needed, and where the slightly higher cost is justified.
9. Cost and Availability
- S350GD+Z (zinc): Generally more widely available and cost-competitive due to mature galvanizing infrastructure and high demand. Offers a range of coating thicknesses to match service life requirements.
- S350GD+AZ (aluminium-silicon): Less ubiquitous; availability may be more limited and cost somewhat higher due to specialized coating baths and lower production volumes. Lead times can be longer depending on market and mill capability.
- Product forms: Both grades are supplied in coils and sheets. Custom coating thicknesses, temper (surface finish and paintability), and post-coating treatments may affect lead time and cost.
10. Summary and Recommendation
| Parameter | S350GD+Z | S350GD+AZ |
|---|---|---|
| Weldability (practical) | Good substrate weldability; must remove Zn at welds to control fumes/porosity | Good substrate weldability; Al–Si removal recommended and welding parameters adjusted |
| Strength–Toughness | Substrate-determined; similar for both | Substrate-determined; similar for both |
| Cost | Lower / widely available | Higher / more specialized |
Conclusions: - Choose S350GD+Z if you need cost-effective, general-purpose atmospheric corrosion protection with sacrificial behaviour, easy paint over-coating, and broad availability. It is typically the default for building, roofing, and many general industrial uses. - Choose S350GD+AZ if the application involves elevated temperatures, oxidative environments, or where a barrier-type coating with better high-temperature stability and distinct appearance is required; expect higher cost and consider forming/welding implications in procurement and fabrication planning.
Final recommendation: base the decision primarily on service environment and fabrication constraints. For standard structural and exterior applications, S350GD+Z will usually provide the best balance of cost, protection, and ease of fabrication. For specialized thermal or chemical environments where the aluminium-silicon coating’s barrier and high-temperature resilience add measurable life-cycle value, S350GD+AZ is the better technical choice.