ZF70 vs ZF140 – Composition, Heat Treatment, Properties, and Applications

Introduction

ZF70 and ZF140 are surface‑treated carbon/low‑alloy steel designations commonly used in structural, automotive, and general fabrication applications where corrosion resistance from a metallic coating is required. Engineers, procurement managers, and manufacturing planners frequently weigh trade‑offs between substrate mechanical properties, surface durability, manufacturability, and life‑cycle cost when selecting between these two options. Typical decision contexts include balancing resistance to atmospheric or handling corrosion (and therefore coating durability) against unit cost, formability, and welding process constraints.

The principal operational difference between the two lies in the amount of metallic coating applied to the base steel: one variant carries substantially more protective coating than the other, affecting surface longevity, abrasion resistance, and some fabrication considerations. Because the substrate metallurgy can be very similar for both product labels, they are often compared primarily on surface protection, service life, and cost-per‑unit-area rather than on fundamental bulk alloy chemistry.

1. Standards and Designations

• Major standards that may govern substrate steels and coatings:
• EN (European Norms) — e.g., EN 10147 (galvanized steels), EN 10346 (continuous coated steels)
• ASTM/ASME — various ASTM specifications for galvanized coatings and carbon steels
• JIS — Japanese Industrial Standards for coated steels
• GB — Chinese National Standards for coated steels
• Classification:
• ZF70 and ZF140 are best characterized as carbon or low‑alloy steels with metallic protective coatings (i.e., coated steels). They are not stainless grades; nor are they tool steels or high‑strength alloy tool categories by definition. The underlying substrate may be plain carbon steel, interstitial‑free (IF), or controlled‑yield low‑alloy steel depending on the supplier and intended application.

2. Chemical Composition and Alloying Strategy

Table: typical role of elements for coated carbon/low‑alloy substrates (qualitative)

Notes: - Many coated steel product lines are defined by coating mass and surface finish while the substrate is supplied to an agreed chemical/microstructural grade. Exact compositions vary by mill and product family. - Alloying strategy for these substrates typically favors low carbon and controlled microalloying to preserve formability, weldability, and consistent mechanical properties after coating.

How alloying affects performance: - Carbon and manganese primarily set strength and ductility; increasing C and Mn raises strength and hardenability but reduces weldability and formability. - Microalloying elements (V, Nb, Ti) offer yield‑strength improvements through precipitation and grain refinement without large increases in carbon. - Coating chemistry and adhesion depend on small additions (e.g., Si content) and on the coating process (hot‑dip galvanizing, continuous electrogalvanizing, or Zn‑Al processes).

3. Microstructure and Heat Treatment Response

• Typical microstructure: For these coated steels the substrate is commonly a ferrite–pearlite or ferrite with fine precipitates (in HSLA variants). Microstructure is chosen to balance formability, strength, and toughness.
• Normalizing: Produces a finer, more uniform ferrite–pearlite structure and may be used for higher strength applications. Normalizing can improve toughness and dimensional stability.
• Quenching & tempering: Rare for general coated construction steels because coating operations and formability requirements favor more ductile substrate conditions. Q&T is used when higher strength is required but is typically associated with uncoated or specially treated products.
• Thermo‑mechanical processing: For high‑strength low‑alloy substrates, controlled rolling and accelerated cooling produce fine ferrite–bainite microstructures that raise strength while maintaining ductility. These substrates can be subsequently coated, but process sequencing and thermal exposure are critical to maintain coating integrity.
• Effect of coating: Coating processes (hot‑dip, continuous galvanizing) impose thermal cycles that can slightly temper or alter the near‑surface microstructure. Mills control annealing and cooling to preserve substrate mechanical targets.

4. Mechanical Properties

Table: comparative mechanical behavior (qualitative, substrate dependent)

Interpretation: - The different coating masses do not substantially change the intrinsic mechanical properties of the steel substrate. Selection for strength or toughness should therefore be based on the substrate specification rather than the ZF70/ZF140 label alone. Any small differences in forming behavior or surface fracture initiation at edges can arise from coating thickness and adhesion, not from substrate strength.

5. Weldability

Weldability considerations hinge on substrate carbon equivalent and on how the coating affects arc stability and fume. Useful indices:

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

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

Qualitative interpretation: - Lower carbon and controlled alloy content reduce susceptibility to cold cracking and hydrogen embrittlement; keep $CE_{IIW}$ and $P_{cm}$ as low as possible for easy welding. - Metallic coatings create localized zinc vaporization during welding, which can cause porosity and zinc‑induced fumes; removal of coating at the weld area or use of proper welding parameters is common practice. - Heavier coatings (greater coating mass) require additional edge cleaning or adjustments to welding technique because more coating must be removed or displaced for a sound weld. - Preheating, controlled interpass temperatures, and appropriate filler selection mitigate risks in higher CE substrates or when coating contamination cannot be fully removed.

6. Corrosion and Surface Protection

• For non‑stainless coated steels like ZF70 and ZF140, metallic coatings (typically zinc or zinc‑alloys) provide sacrificial protection and barrier protection. A heavier coating mass prolongs the onset of substrate corrosion and improves resistance to mechanical abrasion and handling damage.
• When assessing corrosion performance in aggressive environments, consider localized breakdown mechanisms, edge protection, and the need for post‑coating passivation or painting.
• For stainless steels only, PREN is relevant: $$ \text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N} $$ This index is not applicable to zinc‑coated carbon steels; use it only when evaluating true stainless alloys.
• Coating systems can be augmented with organic topcoats, inhibitors, or conversion coatings to extend service life, especially in coastal or industrial atmospheres.

7. Fabrication, Machinability, and Formability

• Cutting and machining: Substrate machinability follows standard carbon/low‑alloy behavior. The metallic coating can clog tools or affect surface finish; tooling and coolant strategies should account for this.
• Forming and bending: Heavier coatings change the friction and may crack the coating at sharp bends; process windows (minimum bend radius, punch/ die geometry) must consider coating ductility and adhesion. ZF140‑type heavier coatings will more readily show coating discontinuities on severe forming unless designed for.
• Surface finishing: Heavier coatings offer better post‑processing resistance but may require more careful edge trimming and trimming knife maintenance. Paint adhesion and electrocoating can depend on coating chemistry and surface preparation.

8. Typical Applications

Selection rationale: - Choose a lighter coating where forming complexity, weldability, or lowest initial cost dominate and environmental exposure is limited. - Choose a heavier coating where extended lifetime against corrosion, improved sacrificial protection, or better abrasion resistance during service and handling are required.

9. Cost and Availability

• Relative cost: Heavier coated products carry higher unit cost per area due to additional coating material and processing time. Economies of scale and supplier inventories influence final pricing.
• Availability by product form: Both coatings are commonly offered as coils, sheets, and pre‑painted options. Lead times may vary by regional demand and supplier capability; heavier coated variants can have slightly longer lead times if they are less commonly stocked.
• Procurement tip: Specify required coating mass, adhesion class, and substrate mechanical properties explicitly to avoid ambiguity in supplier offers.

10. Summary and Recommendation

Summary table (qualitative)

Conclusions and guidance: - Choose ZF70 if: the application requires maximum formability and ease of welding, environmental exposure is moderate, and minimizing initial material cost is a priority. ZF70 is often preferred for complex stamping, inner automotive panels, and indoor structural components. - Choose ZF140 if: the application demands extended corrosion protection, improved abrasion/handling resistance, or reduced maintenance frequency in outdoor or aggressive environments. ZF140 makes sense for exposed structural components, external automotive underbody parts, and parts expected to experience frequent mechanical wear or prolonged exposure to corrosive atmospheres.

Final note: Because the substrate chemistry and mechanical requirements can be specified independently from coating mass, always define both the substrate steel grade (mechanical and chemical requirements) and the required coating mass/chemistry in procurement and design documents. This ensures that the selected ZF variant meets both structural and durability expectations without relying on nomenclature alone.