ZA150対ZA200 – 成分、熱処理、特性、および用途

Introduction

ZA150 and ZA200 are designations commonly encountered when selecting coated steel products for structural, architectural, and industrial applications. Engineers and procurement teams frequently face a trade-off between corrosion resistance, formability, weldability, and cost when choosing between these two options. Typical decision contexts include exterior cladding vs. indoor furniture, deep drawing laminates vs. structural members, and welded assemblies vs. bolted or mechanically fastened parts.

Unlike standard steel-grade identifiers (e.g., S355 or A36), ZA150 and ZA200 most often describe zinc–aluminum alloy coated steel products distinguished primarily by the coating mass and the alloy content of the coating. The main technical difference therefore lies in the coating system — its alloying proportion and mass per unit area — which directly influences corrosion protection, adhesion, and some fabrication behaviors.

1. Standards and Designations

• ZA150 / ZA200: Typically used as commercial coated-steel designations where "ZA" indicates a zinc–aluminum type alloy coating and the numeric suffix indicates nominal coating mass (commonly expressed as g/m² total both sides or per side depending on supplier). These are not universal metallurgical steel grade names in ASTM/EN/JIS nomenclature but are commonly used by coil coaters and steel service centers.
• Relevant international standards and specifications that govern coated steels (substrate and coatings) include:
• ASTM A653 / A792 family (hot-dip galvanized and Zn–Al–Mg/Al–Zn coatings) — addresses coated carbon steels.
• EN 10346 (continuously hot-dip metallic coated steel) — covers product definitions and marking for coated steels.
• JIS and GB standards (national specifications) that govern cold-rolled substrates and coating processes.
• Identification by material class:
• The substrate material under ZA coatings is typically carbon (mild) steel (low-carbon or interstitial-free cold-rolled steel) — not stainless, tool, or HSLA in most typical ZA150/ZA200 product lines.
• The coating is an alloy (Zn–Al based), so the finished product is a coated carbon-steel product rather than a different steel class.

2. Chemical Composition and Alloying Strategy

The functional performance difference between ZA150 and ZA200 is controlled predominantly by the coating chemistry (relative proportions of Zn, Al, Mg and minor additions) and the coating mass. The steel substrate chemistry is usually a conventional low-carbon steel optimized for forming or painting.

Table: Typical composition focus areas (substrate vs. coating). Note: supplier-specific compositions vary; the table gives typical element relevance rather than prescriptive values.

Element	Typical low‑carbon steel substrate (relevance)	ZA coating (Zn–Al type) — relevance
C	Low (controls strength and weldability)	Not applicable
Mn	Present (strength/deformation control)	Not applicable
Si	Small amounts (deoxidation; affects coating wetting)	Trace possible
P	Low (embrittlement risk if high)	Trace possible
S	Low (inclusions affect formability)	Trace possible
Cr	Usually absent (unless special substrate)	Rare in ZA coatings (trace, if any)
Ni	Not relevant for substrate unless alloyed	Rare in coating
Mo	Not relevant	Rare
V, Nb, Ti	Possible microalloying in substrate for strength	Not relevant
B	Trace in substrate sometimes	Not relevant
N	Controlled in substrate for some steels	Not relevant
Zn	Not present in substrate	Principal element of the coating (majority)
Al	Not in substrate	Alloying addition to Zn — improves barrier and sacrificial performance
Mg	Not in substrate	Often added in small amounts to enhance corrosion resistance and adhesion
Others (Si, Sn, Bi)	Typically trace in substrate	Minor additions used by some coaters to tailor properties

How alloying affects performance: - Increasing Al content in a Zn–Al coating generally improves barrier protection and paintability, reduces white rust, and can modify sacrificial behavior. - Small Mg additions often improve atmospheric corrosion resistance and reduce undercutting in scratches. - Higher coating mass (e.g., ZA200 vs ZA150) increases the available sacrificial zinc and extends protection lifetime, all else being equal.

3. Microstructure and Heat Treatment Response

ZA150 and ZA200 differ primarily in coating microstructure, not in the bulk steel microstructure (except when special substrates are specified).

• Substrate microstructure:
• The base steel for these coated products is commonly cold-rolled low‑carbon steel with a ferrite–pearlite microstructure (or fully ferritic for interstitial‑free steels). Heat treatment of the substrate is typically limited to annealing cycles used during cold‑rolling processing; the substrate is not intended for additional quench/tempering after coating.
• Coating microstructure:
• Zn–Al coatings solidify to form a duplex structure: an outer Zn–Al alloy layer and an intermetallic reaction layer at the steel–coating interface. Al reduces the growth rate and modifies phase formation compared with pure Zn, resulting in a finer intermetallic layer and different adhesion behavior.
• ZA200 (higher coating mass) will have a thicker alloy layer and a thicker intermetallic zone compared with ZA150 when processed identically.
• Heat treatment/processing response:
• Normalizing or quench & temper processes are not generally applied to coated finished parts because they would damage the coating; thermal exposure during processing (e.g., in forming or welding) can lead to local intermetallic growth or spangle changes.
• Thermal cycles during continuous galvanizing/coating lines (short anneals) control coating wetting, intermetallic formation, and final alloy proportions.

4. Mechanical Properties

Mechanical properties of the finished product are dominated by the substrate steel; the coating contributes marginally to bulk mechanical strength but is critical for surface protection.

Table: Comparative mechanical property expectations (typical ranges for coated low‑carbon steel products).

Property	ZA150 (typical coated low‑carbon steel)	ZA200 (typical coated low‑carbon steel)
Tensile strength (MPa)	Typical substrate range (e.g., 220–420) — coating negligible	Same substrate-controlled range
Yield strength (MPa)	Substrate-controlled (e.g., 140–350)	Same
Elongation (%)	Substrate-controlled (e.g., 15–40%)	Same
Impact toughness	Substrate-dependent; coating has little effect at ambient temps	Same
Surface hardness	Coating slightly harder than pure Zn depending on Al content	Slightly higher surface hardness with thicker/more alloyed coating

Which is stronger, tougher, more ductile, and why: - Strength/toughness/ductility derive from the substrate metallurgy. When ZA150 and ZA200 use identical substrates, mechanical property differences are negligible. - The coating influences surface hardness and local wear/cut edge behavior; a thicker ZA200 coating may show marginally higher resistance to abrasion of the surface layer. - Ductility/formability can be affected by the coating mass and alloying: heavier coatings and more brittle intermetallic layers can reduce stretch‑forming performance and promote coating cracking during severe deformation.

5. Weldability

Weldability is governed largely by the substrate chemistry (carbon and other hardenability elements) and by the presence and composition of the coating.

• Carbon and hardenability:
• Higher carbon and alloying elements increase risk of weld‑zone cracking. Many ZA‑coated products use low‑carbon, low‑alloy substrates to maintain good weldability.
• Coating effects:
• Coatings introduce zinc into the weld pool during arc welding, which can cause porosity, increased spatter, and vaporization-related defects. Thicker coatings (ZA200) yield more zinc in the joint than thinner coatings (ZA150), increasing the potential for these issues.
• Assessment formulas (qualitative use):
• Carbon equivalent index (IIW): $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ Use this to evaluate substrate weldability: higher $CE_{IIW}$ → greater preheat and post‑weld treatment requirements.
• Pcm for cold cracking susceptibility: $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$ Higher $P_{cm}$ increases hydrogen-assisted cold cracking risk.
• Practical implications:
• For manual and robotic arc welding, clean removal of coating at weld area or appropriate welding processes (e.g., MIG with higher travel speed, laser welding, or use of fluxes/fillers designed for galvanized steel) mitigates problems.
• Pre‑heating, controlled heat input, and post‑weld cleaning are common mitigations. ZA200 may require more aggressive measures than ZA150 for the same joint design because of greater zinc content at the weld.

6. Corrosion and Surface Protection

ZA coatings are intended to improve atmospheric corrosion protection compared with uncoated steel. They operate by a combination of barrier and galvanic (sacrificial) protection.

• For non‑stainless substrates:
• Protection strategies include the choice of coating alloy (Zn–Al–Mg formulations), coating mass (ZA150 vs ZA200), painting or conversion coatings over the ZA layer, and edge treatment.
• Thicker coating mass (ZA200) increases service life under the same environment because more sacrificial material is available and the barrier layer is more robust.
• Stainless index (PREN) not applicable:
• For stainless alloys, PREN evaluates pitting resistance: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
• ZA-coated products are not stainless steels; therefore PREN is not relevant to ZA150/ZA200.
• Practical guidance:
• Select ZA200 for harsher outdoor or coastal environments if increased coating mass and possibly modified alloying (e.g., Mg additions) are available.
• For painted systems, a higher-quality ZA coating gives better long‑term adhesion and barrier behavior.

7. Fabrication, Machinability, and Formability

• Cutting (shearing, laser, plasma):
• Thicker coatings (ZA200) can produce more dross and affect edge quality. Laser cutting parameters need adjustment to account for coating composition and thickness.
• Forming and bending:
• ZA150 generally offers better formability in highly worked parts because of lower coating thickness and thinner intermetallic layers; ZA200 can be more prone to coating cracking and powdering at tight bends.
• Use test coupons for critical forming operations to confirm coating behavior under intended strain.
• Machinability:
• Coated products are typically sheared or punched before heavy machining; machining through the coating is possible but produces additional tool wear and requires chip control for zinc alloys.
• Finishing:
• Paint systems bond effectively to Zn–Al coatings, but surface preparation and passivation treatments are important for consistent finish quality.

8. Typical Applications

ZA150 (lighter coating)	ZA200 (heavier coating)
Interior appliance chassis, indoor HVAC ducts, painted panels with controlled environments	Exterior roofing and facades in moderately aggressive atmospheres
Automotive inner panels where formability and paintability are critical	Building trim, gutters, and external flashings requiring extended corrosion life
Light‑duty stamped parts, where cost is prioritized over maximum life	Outdoor signage, coastal or industrial installations needing extra protection

Selection rationale: - Choose ZA150 when high formability, lower cost, and adequate protection for mild environments are primary requirements. - Choose ZA200 when extended corrosion life, thicker sacrificial reserve, and more robust barrier action are priorities despite slightly higher cost and potential reductions in severe forming performance.

9. Cost and Availability

• Cost drivers:
• Coating mass (ZA200 > ZA150) — more alloy consumed, higher material cost.
• Alloy complexity (additional Al/Mg additions increase cost).
• Supplier processing and certification.
• Availability:
• Both ZA150 and ZA200 are commonly available from major coil coaters and service centers in standard sheet/coil forms. Custom alloy formulations or very thick coatings may be lead‑time drivers.
• Product forms:
• Commonly offered as coil, sheet, and prepainted coil (PPCP). Thicker coatings may be less common in thinner gauges or in specialty alloy variants.

10. Summary and Recommendation

Table: Quick comparison

Characteristic	ZA150	ZA200
Weldability (practical)	Easier (less zinc at weld)	More challenging (more zinc vaporization)
Strength–Toughness (substrate)	Substrate-controlled; similar	Substrate-controlled; similar
Corrosion protection	Lower service life than ZA200	Higher sacrificial reserve and longer life
Formability	Better for severe forming	Reduced for severe forming (greater risk of coating cracking)
Cost	Lower	Higher

Recommendations: - Choose ZA150 if: - You need superior formability and tight bending radii in panel or stamped parts. - The service environment is mild to moderate and cost optimization is important. - Welding frequency and weld quality management favor lower coating mass. - Choose ZA200 if: - Longer atmospheric corrosion life is required (outdoor, coastal, or industrial environments). - The design prioritizes coating longevity and reduced maintenance over the marginal increase in cost. - Welded joints are minimized or welding procedures are optimized to handle higher zinc content.

Final note: ZA150 and ZA200 are best evaluated as coated-system choices rather than distinct base-steel grades. For critical projects, request supplier datasheets showing nominal coating mass (g/m²), detailed alloy composition of the coating, adhesion and corrosion test data, and perform forming/welding trials with production-intent materials before final selection.