Z100 vs Z180 – Composition, Heat Treatment, Properties, and Applications

Introduction

Z100 and Z180 are commonly encountered designations in the cold‑rolled and hot‑rolled steel supply chain when materials are delivered with a hot‑dip zinc coating. The selection dilemma between them is a familiar one for engineers, procurement managers and manufacturing planners: do you prioritize lower material cost and easier forming, or higher sacrificial corrosion protection and longer service life? Decisions typically balance corrosion exposure, forming/welding requirements, finish and coating longevity against unit cost and availability.

At their core the two designations differ in the amount of zinc applied to the steel surface: one has a lighter zinc coating while the other carries a heavier zinc mass per unit area. Because these labels describe surface coating mass rather than different base alloys, they are often compared when the same base steel chemistry and mechanical grade are offered with different zinc coating weights.

1. Standards and Designations

Common international standards and specifications that cover hot‑dip zinc coated steel and the way coatings are designated include: - ASTM A653 / A653M and ASTM A792 (zinc and zinc‑alloy coatings on steel sheet, typically using G‑designations). - EN 10346 (continuous hot‑dip zinc coated flat products) and related EN/ISO documents (some use Z‑prefixes to indicate coating mass). - JIS standards for zinc coatings (commonly use Z‑style nomenclature in Japanese practice). - National standards such as Chinese GB/T specifications for hot‑dip galvanizing and galvanized steel.

Important note: Z100 and Z180 are not steel alloy grades (carbon, HSLA, tool, or stainless); they refer to zinc coating mass on a substrate. The substrate itself is normally carbon or low‑alloy cold‑ or hot‑rolled steel (not stainless). Specification of the underlying steel (for example, cold‑rolled commercial quality, drawing grades, or structural grades) must be read together with the coating designation.

2. Chemical Composition and Alloying Strategy

Comments: - The table intentionally lists "Not applicable to coating" because Z100 and Z180 denote zinc coating mass (mass of zinc per unit area) rather than base‑metal chemistry. The chemical composition of the substrate is determined by the steel grade specified (for example SPCC, DC01, S235, etc.). Typical base steels used for hot‑dip galvanizing are carbon or low‑alloy steels with carbon and alloying controlled to meet the mechanical and formability requirements. - Alloying strategy for the substrate (C, Mn, Si, microalloying elements) primarily targets strength, hardenability, formability and temper response; the zinc coating itself is essentially metallic Zn with occasional small alloy additions (e.g., aluminium in Galfan or zinc–aluminium coatings) introduced to control coating morphology and adherence.

How alloying affects performance (substrate perspective): - Increasing carbon and microalloying (V, Nb, Ti) raises yield and tensile strengths but reduces weldability and may increase susceptibility to hydrogen cracking if not controlled. - Silicon and phosphorus levels influence the reactivity of the steel surface in the galvanizing bath and therefore the resulting coating morphology and thickness uniformity. - Zinc coating mass (lighter vs heavier) affects corrosion lifetime and edge protection but does not change intrinsic bulk substrate strength.

3. Microstructure and Heat Treatment Response

• Microstructure of the substrate: Z100 and Z180 coatings are applied to steels whose microstructures depend on the steel specification and processing route (ferrite–pearlite, ferrite–bainite, martensite in quenched and tempered grades, or dual‑phase in some advanced steels). The zinc layer forms intermetallics at the coating/substrate interface during hot‑dip galvanizing; the amount and thickness of these intermetallic layers depend on steel surface chemistry and bath conditions.
• Coating microstructure: A typical hot‑dip zinc layer has a multilayer structure with an outer pure zinc layer and one or more intermetallic layers (zinc‑iron phases). Heavier coatings (e.g., Z180 relative to Z100) increase the thickness of the outer zinc layer and may increase total intermetallic material depending on the steel surface reactivity and immersion time.
• Heat treatment response: Post‑galvanizing heat treatments are rarely applied to modify coatings directly because heat can change coating morphology. The substrate’s response to normalizing, quenching & tempering, or thermo‑mechanical processing is independent of the zinc mass, but galvanizing is typically applied after forming or at controlled stages to avoid damaging the coating. For pre‑galvanized material (coil‑coated), heat treatments should be validated because high temperatures can cause coating diffusion and changes.

4. Mechanical Properties

Interpretation: - The mechanical strength and ductility values of steel with Z100 vs Z180 coatings are fundamentally controlled by the substrate grade and heat treatment. The zinc coating mass influences surface behavior during forming, local crack initiation at edges or highly strained features, and wear behavior, but it does not materially change bulk tensile or yield strengths. - Thicker coatings are more likely to develop longitudinal cracks or powdering in aggressive bending operations; process validation is required when moving from a lighter to a heavier coating.

5. Weldability

Weldability considerations include the substrate carbon equivalent and the presence of a zinc layer at the weld zone.

Useful carbon‑equivalent and weldability indices: - IIW carbon equivalent: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ - International Institute of Welding Pcm: $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$

Interpretation and practical guidance: - The numerical CE or Pcm should be calculated from the substrate composition to assess preheat requirements and susceptibility to cold cracking. These formulas do not include zinc coating mass. - Welding over a zinc layer introduces additional issues: zinc vaporizes and produces fumes, can cause porosity in fusion welds, and may lead to embrittlement of the weld bead if not removed or driven off. Thicker zinc (Z180) generally increases the volume of zinc vapor and the risk of porosity and fume generation compared with Z100. - Recommended practices: remove zinc from the immediate weld zone by grinding or chemical means; use appropriate ventilation and fume extraction; apply lower heat input welding parameters or backing bars and use consumables suitable for the substrate; consider plug‑welding or mechanical fastening for very heavily coated parts. - For resistance welding and spot welding, thicker coatings change electrical contact resistance and can require adjustments in electrode force, current and time. Heavier coatings often increase the number of welds rejected if process parameters are not optimized.

6. Corrosion and Surface Protection

• For non‑stainless steels with zinc coatings, corrosion protection is sacrificial (cathodic protection) plus barrier protection from the zinc layer. Heavier zinc masses provide longer service life and better protection at cut edges, scratches and crevices because there is more sacrificial metal to consume before exposing the substrate.
• PREN is not applicable to zinc coatings; PREN is relevant to stainless steels and is calculated as: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
• Practical considerations:
• Z100 is typically adequate in mild environments and where painted finishes are used to provide an additional barrier.
• Z180 is chosen for more aggressive outdoor atmospheres, coastal zones, or applications where edge protection and longer maintenance intervals are required.
• Additional coatings (paints, passivation, conversion coatings) can be applied over zinc to enhance aesthetics and corrosion resistance; the combination must be validated for adhesion and compatibility.

7. Fabrication, Machinability, and Formability

• Cutting: Shearing and stamping operations are generally unaffected in the bulk by coating mass, but thicker coatings can produce more scale, flakes or burrs that require post‑processing and can increase tool wear.
• Forming and bending: Thicker zinc coatings are more prone to cracking, powdering and transfer during tight bending or deep drawing. For severe forming operations a lighter coating or post‑forming galvanizing is often preferred, or process parameters (die radii, lubrication) must be adjusted.
• Machinability: Zinc coatings do not substantially change machinability of the steel core for through‑cutting operations, but surface finish and tool life can be affected by transferred zinc smearing. Use of appropriate tooling coatings and coolant is recommended.
• Finishing: Heavier coatings may require more aggressive surface preparation for painting or bonding; however, a thicker zinc layer can also provide a more uniform substrate for topcoats in many processes.

8. Typical Applications

Selection rationale: - Choose the lighter coating when forming requirements are stringent, when the environment is mild, or when the final part will be coated with paint that provides the primary barrier. - Choose the heavier coating for extended outdoor exposure, locations where edge protection is critical, or where maintenance cycles must be extended.

9. Cost and Availability

• Cost: heavier zinc coatings require more zinc metal and longer immersion times or different bath chemistry, so Z180 will typically be more expensive per unit area than Z100. The cost differential should be weighed against lifecycle savings from reduced maintenance and longer corrosion life.
• Availability: both coating masses are commonly available in coil, sheet and pre‑painted products from major mills, but availability by substrate grade and thickness can vary by region and mill. Specialty substrate grades with high formability or high strength may be more limited in heavily coated versions, so early engagement with mills or suppliers is advised.

10. Summary and Recommendation

Recommendations: - Choose Z100 if your product will be formed with tight radii or deep drawing, is installed in a relatively benign environment, or will receive a high‑quality paint or powder coat that serves as the primary corrosion barrier. Z100 minimizes coating flaking and reduces welding fume/porosity issues. - Choose Z180 if the part will be exposed to outdoor or moderately aggressive environments, if extended service life and improved edge protection are required, or if maintenance intervals must be minimized. Z180 provides greater sacrificial zinc reservoir and better long‑term protection at scratches and cut edges.

Final note: Z100 and Z180 are coating mass designations tied to galvanizing practice rather than distinct metallurgical substrate grades. Always specify both the underlying steel grade (chemical and mechanical requirements) and the required coating mass in procurement documents, and validate forming, welding and finishing processes for the chosen combination with supplier data and process trials.