G90対G60 - 組成、熱処理、特性、および応用

Introduction

G90 and G60 are widely referenced in construction, appliances, and automotive supply chains — but they are not different metallurgical steel grades in the way that A36, S275, or 1020 are. Instead, G90 and G60 are galvanizing/coating designations that communicate minimum zinc coating mass applied to sheet and strip steel under common specifications (for example ASTM A653/A924 family). Engineers, procurement managers, and manufacturing planners commonly face the selection dilemma of trading off corrosion protection and lifecycle performance against incremental material cost and downstream processing needs. Typical decision contexts include exterior exposed structures where long-term corrosion resistance matters versus interior or painted parts where cost and formability may predominate; another common trade-off is whether heavier coatings complicate forming, welding, or painting operations.

The key technical difference between G90 and G60 is the mass of zinc applied to the steel surface: G90 carries a substantially thicker zinc layer than G60. Because the zinc coating — not the bulk substrate chemistry — primarily differentiates these designations, the two coatings are commonly compared when choosing corrosion protection levels while the underlying substrate steel chemistry, strength, and heat-treatment response remain selectable per the purchaser’s required base grade.

1. Standards and Designations

• Common standards that incorporate G-designations and equivalent metric labels:
• ASTM/ASME: ASTM A653 / A924 (hot-dip galvanized and galvalume-coated sheets)
• EN: EN 10346 (continuous hot-dip coated steels); metric coating mass indicated as Z (e.g., Z275)
• JIS and other regional standards use similar coating-mass notations
• GB (China): GB/T standards for galvanized steel
• Classification:
• G90/G60 are coating classes applied to carbon and low-alloy steels (not separate alloy families).
• The base steels to which G90/G60 are applied can be plain carbon, cold-rolled commercial quality, structural steels, or high-strength low-alloy (HSLA) substrates, depending on the purchaser’s specification (e.g., commercial-quality cold-rolled, structural, or structural high-strength grades).

2. Chemical Composition and Alloying Strategy

Table: chemical elements and how they relate to G90/G60 products

Element	Typical relevance for galvanized sheet	Notes for G90 vs G60
C (Carbon)	Controls strength, formability, weldability of the substrate	Substrate C is specified per steel grade; coating class does not prescribe C. Lower C is common for formability.
Mn (Manganese)	Strength and hardenability contributor in substrate	Mn level chosen by substrate grade; galvanizing class independent.
Si (Silicon)	Influences galvanizing zinc-iron reaction and coating appearance	Small Si additions or residual Si can alter coating adherence and morphology; specification may restrict certain Si ranges to control coating quality.
P (Phosphorus)	Controls strength and cold brittleness	Typically limited in cold-formed substrates; coating class does not change P limits.
S (Sulfur)	Affects machinability and defect formation	Kept low; affects coating cleanliness.
Cr, Ni, Mo, V, Nb, Ti, B	Microalloying elements in HSLA or advanced grades	These affect substrate strength and microstructure; selected independently of coating class. Some alloying elements can influence weldability and the galvanizing reaction locally.
N (Nitrogen)	Controlled in some substrate specifications	Nitrogen management is a substrate control, not a coating parameter.

Because G90 and G60 designate zinc mass, alloying strategy for the underlying steel is determined by the structural or formability requirements. Where heavy coatings are required (G90), substrate chemistry controls may also be tightened to ensure post-coating performance (e.g., avoid coating defects due to Si content).

3. Microstructure and Heat Treatment Response

• Microstructure: The intrinsic microstructure (ferrite-pearlite, tempered martensite, bainite, or refined ferrite in HSLA) is set by the steel chemistry and the thermal-mechanical processing of the substrate. The hot-dip galvanizing operation itself produces a metallurgical intermetallic layer (Fe–Zn phases, often zeta, delta, gamma, and eta layers) between the zinc outer layer and the steel substrate.
• Standard processing routes:
• Cold-rolled, annealed, and pickled substrate steels typically develop a ferrite–pearlite microstructure or fully re-crystallized ferrite in commercial grades.
• HSLA or higher-strength substrates may be thermo-mechanically processed to refine grain size and produce stronger ferrite–pearlite matrices or microalloy precipitate strengthening.
• Effect of galvanizing on heat treatment:
• Hot-dip galvanizing is a separate thermal exposure (typically short immersion and air-cooling) and does not substantially change bulk mechanical properties for commonly used low‑ and mid‑strength substrates. However, for quenched and tempered or very high-strength grades, heat exposure can slightly alter temper or residual stress—process parameters and substrate selection should be verified for post-coat performance.
• Post-coating operations (e.g., bending) can crack the zinc layer if the coating is brittle in certain intermetallic condition; choice between G90 and G60 can influence forming recommendations and post-coating annealing decisions.

4. Mechanical Properties

Table: typical comparative presentation (note: mechanical properties determined by substrate steel, not the coating)

Property	G90	G60	Comment
Tensile strength	Same as substrate	Same as substrate	Zinc coating thickness has negligible effect on bulk tensile strength.
Yield strength	Same as substrate	Same as substrate	Controlled by substrate heat treatment and chemistry.
Elongation	Same as substrate	Same as substrate	Coating may slightly affect surface ductility during forming.
Impact toughness	Same as substrate	Same as substrate	Substrate toughness governs; coating layer does not meaningfully change intrinsic toughness.
Hardness (surface)	Zinc layer harder/thicker surface	Thinner zinc surface	Bulk hardness unaffected; thicker coatings may affect surface hardness measurement and wear behavior.

Explanation: Because G90 and G60 refer to zinc mass, neither inherently increases the steel’s yield or tensile strength. The thicker zinc of G90 can marginally affect forming-induced surface cracking and abrasion resistance, but core mechanical properties reflect the underlying steel grade and any heat treatment applied to that substrate.

5. Weldability

Weldability is primarily governed by substrate composition, carbon equivalent, and the presence of surface coatings. Two commonly used empirical indices for weldability are presented for interpretative use:

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

and

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

• Interpretation:
• The zinc coating (G60 or G90) introduces an additional consideration: galvanizing must be removed from joint areas or adjusted for welding technique because zinc vaporizes during fusion welding, generating toxic fumes, promoting porosity, or causing weld defects if not properly managed.
• Substrate carbon and alloying control hardenability and hydrogen cracking susceptibility; the CE and Pcm formulas indicate that higher C, Mn, Cr, Mo, V, and other alloying raise hardenability and risk of cold cracking.
• Practically: weldability of G90 and G60 parts is similar if the substrate chemistry is identical, but heavier coatings (G90) require more careful local coating removal, gas extraction, and fume control to assure weld quality and safety.

6. Corrosion and Surface Protection

• Primary protection method: both G60 and G90 are hot-dip galvanized zinc coatings; zinc provides sacrificial protection (cathodic protection) to exposed steel edges and scratches.
• Comparative corrosion resistance:
• G90, with the higher nominal zinc mass, provides longer time-to-perforation in atmospheric exposure than G60 under the same environmental conditions.
• For painted systems, heavier zinc coatings (G90) can extend primer/paint life or be used where barrier and sacrificial protection are both required.
• Metric/other indices:
• For stainless steels, corrosion resistance indices like PREN would be relevant, for example: $$ \text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N} $$ This is not applicable to G60/G90 because they are zinc-coated carbon/low-alloy steels rather than stainless compositions.
• Note on service environment: selection between G60 and G90 should be based on expected environment (rural, urban, industrial, marine). Heavier coatings improve service life in aggressive environments but do not replace other corrosion control measures (design for drainage, paints, cathodic systems).

7. Fabrication, Machinability, and Formability

• Forming and bending:
• Thicker zinc coatings (G90) can be more prone to cracking or flaking during severe forming operations, potentially exposing bare steel; proper tooling, bend radii, and lubrication help mitigate this.
• G60 offers easier post-forming finish consistency for tight bends; G90 may require post-treatment or touch-up painting on bend lines.
• Cutting and punching:
• Thicker zinc can accumulate on cutting tools more rapidly and may require more frequent tool maintenance. Sheared edges with heavy zinc may show more zinc buildup.
• Machinability:
• The zinc coating itself usually reduces galling but will evaporate during welding. Base metal machinability depends on substrate alloying.
• Finishing:
• Painting over galvanized surfaces typically requires surface preparation (passivation/primer) to ensure adhesion; thicker zinc layers can affect paint sheen and initial adhesion characteristics.

8. Typical Applications

Table: typical uses by coating class

G90 – Typical Uses	G60 – Typical Uses
Exterior building facades, exposed roofing components, guardrails, street furniture, long-life HVAC housings where extended corrosion resistance is required	Interior ductwork, indoor structural members, automotive inner panels, appliance inner components where moderate corrosion resistance suffices
Marine-adjacent structures with additional painting	Cost-sensitive mass-produced panels with lightweight corrosion needs
Parts requiring longer maintenance intervals (bridges, signposts)	Parts likely to be painted or used in sheltered locations

Selection rationale: choose the coating mass based on anticipated lifetime, maintenance intervals, and exposure class. For heavy exposure, G90 reduces corrosion maintenance; for painted or sheltered service, G60 often provides acceptable protection at lower cost.

9. Cost and Availability

• Relative cost: G90 products are typically priced higher than G60 because of the greater zinc consumption and processing time. The premium varies with zinc market price and coating process efficiencies.
• Availability:
• Both G60 and G90 are common commercial designations and widely available in coils, sheets, and formed products from major steel suppliers.
• Availability by substrate strength and thickness may vary; specifying both substrate grade and coating class in purchasing documents helps ensure suppliers can quote accurately (e.g., “Cold-rolled, grade X, G90 hot-dip galvanized per ASTM A653”).
• Logistics: lead times can be influenced by supply of galvanized coil capacity and market demand; heavier coatings like G90 may have slightly longer lead times in constrained facilities.

10. Summary and Recommendation

Table summarizing key trade-offs

Attribute	G90	G60
Weldability (practical)	High (but requires more pre-weld stripping/fume control)	High (less coating removal required)
Strength–Toughness (substrate controlled)	Same as substrate	Same as substrate
Cost	Higher (more zinc)	Lower (less zinc)

Conclusion and recommendations: - Choose G90 if: - The part will be exposed to aggressive environments (coastal, industrial, chloride exposure) or when a longer maintenance-free life is required. - The application prioritizes sacrificial corrosion protection and extended service intervals over incremental material cost. - Choose G60 if: - The component will be used in sheltered, indoor, or painted applications where moderate corrosion protection suffices and cost or formability is a priority. - Downstream forming, tight bending, or welding without extensive coating removal is a critical manufacturing constraint.

Final note: Because G90 and G60 are coating classes, always specify both the required substrate steel grade (chemistry, strength, heat treatment) and the coating class in procurement documents. Confirm whether coating mass is interpreted per side or total surface area for the referenced standard (ASTM vs. EN metric notation) to avoid ordering mismatches and to align expected corrosion performance with the purchaser’s design life.