S220GD vs S250GD – Composition, Heat Treatment, Properties, and Applications

Introduction

S220GD and S250GD are commercially common hot-dip galvanized structural steel grades used for cold-formed sections, building envelopes, and general structural components. Engineers, procurement managers, and manufacturing planners routinely choose between them when balancing cost, formability, weldability, and the required minimum load-bearing capacity.

The most important practical distinction between the two is their guaranteed minimum yield strength: one grade guarantees a lower yield level and the other a higher one. Because both are produced for continuous galvanizing lines and share similar chemistry and processing routes, the selection typically comes down to whether the extra strength of the higher-grade material justifies any trade-offs in formability, weldability, or cost for a given application.

1. Standards and Designations

• EN / European: S220GD, S250GD — common product designations for hot-dip galvanized steels in EN 10346 (continuously hot-dip coated steel).
• ISO: Often referenced via EN / ISO harmonized standards for coated steels.
• Other regional standards: Equivalent structural cold-formed steels exist in JIS, GB, and ASTM product families, but the "SxxxGD" notation is European in origin and widely used by global steel producers supplying galvanized coil and sheet.
• Material family: Both S220GD and S250GD are low-carbon, microalloyed/high-strength low-alloy (HSLA) steels designed for formability and coating; they are not stainless, tool, or high-alloy steels.

2. Chemical Composition and Alloying Strategy

The S220GD and S250GD grades are formulated as low-carbon steels with controlled amounts of manganese, silicon, and small additions of microalloying elements (Nb, Ti, V) when required to achieve higher strength via thermo-mechanical processing. Exact compositions are supplier-specific and governed by product standards and manufacturing route.

Table: Typical composition ranges (wt %). These are indicative ranges used in practice; always consult supplier mill certificates for procurement and welding procedures.

Element	S220GD (typical ranges, wt %)	S250GD (typical ranges, wt %)
C	≤ 0.12 (often ≤ 0.10)	≤ 0.12 (often ≤ 0.10)
Mn	0.3 – 1.0	0.3 – 1.2
Si	≤ 0.50 (often 0.02 – 0.15)	≤ 0.50 (often 0.02 – 0.15)
P	≤ 0.025	≤ 0.025
S	≤ 0.010	≤ 0.010
Cr	≤ 0.30 (trace)	≤ 0.30 (trace)
Ni	≤ 0.30 (trace)	≤ 0.30 (trace)
Mo	≤ 0.10 (if used)	≤ 0.10 (if used)
V	≤ 0.05 (microalloyed variants)	≤ 0.05 (microalloyed variants)
Nb	≤ 0.05 (if microalloyed)	≤ 0.05 (if microalloyed)
Ti	≤ 0.05 (if used)	≤ 0.05 (if used)
B	trace	trace
N	≤ 0.012	≤ 0.012

How alloying affects properties: - Carbon and manganese are the primary strength contributors. Carbon increases tensile/yield strength but reduces weldability and formability if raised. - Silicon and manganese also affect deoxidation and strengthening through solid solution. - Microalloying with Nb, Ti, or V enables higher yield strength through precipitation hardening and refined grain size using thermo-mechanical control, allowing higher strength without excessive carbon. - Low phosphorus/sulfur contents improve toughness and formability.

3. Microstructure and Heat Treatment Response

Microstructure under standard production: - Both grades are typically produced by controlled hot rolling followed by either cooling profiles (TMCP — thermo-mechanical controlled processing) or conventional cold rolling and annealing prior to galvanizing. Typical microstructures are ferrite with controlled amounts of bainite or fine pearlite depending on processing. - S220GD, being lower in guaranteed strength, is often produced with a more ferritic microstructure and fewer microalloy precipitates, favoring ductility and formability. - S250GD generally contains either a slightly higher dislocation density or controlled microalloy precipitates and finer grain size engineered during TMCP to raise the yield strength without large increases in carbon.

Response to heat treatment: - Normalizing and quenching & tempering are not usual production steps for these coated steels; they are produced to meet strength via rolling and TMCP rather than through bulk heat treatment. - If reheated locally (e.g., welding) the microstructure in the heat-affected zone will depend on peak temperature and cooling rate. Low-carbon design limits hardenability and reduces the risk of brittle martensite formation compared with higher-carbon structural steels, but microalloying elements can slightly increase hardenability.

4. Mechanical Properties

Table: Typical mechanical property comparisons. Values are representative; actual supplied values must be verified on mill certificates and depend on thickness and processing.

Property	S220GD	S250GD
Minimum yield strength (Rp0.2)	220 MPa (guaranteed)	250 MPa (guaranteed)
Typical tensile strength (Rm)	Moderate; depends on thickness/process; commonly in low–mid range for structural sheet	Slightly higher than S220GD; varies with processing
Elongation (A%)	Generally higher ductility than S250GD for same thickness	Slightly reduced elongation versus S220GD at equivalent thicknesses
Impact toughness	Generally adequate for cold-formed structural parts; not universally specified	Comparable, but specific toughness depends on TMCP and chemistry
Hardness	Low to moderate, suitable for forming	Slightly higher on average due to increased strength

Interpretation: - S250GD is stronger in terms of minimum yield strength; this is deliberate to enable thinner gauge design or higher load capacity. - S220GD will typically be easier to form and may provide slightly better stretchability and bend recovery. - Toughness differences are subtle and process-dependent — neither grade is inherently brittle; impact performance must be confirmed when low-temperature performance is required.

5. Weldability

Weldability considerations hinge on carbon equivalent and microalloy content. For qualitative assessment of weldability, two commonly used indices are the IIW carbon equivalent and the International Institute of Welding Pcm.

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

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

Qualitative interpretation: - Both S220GD and S250GD are low-carbon steels with relatively low CE$_{IIW}$ and Pcm compared to higher-strength quenched-and-tempered steels. That makes them generally suitable for common arc welding processes (MMA, MIG/MAG, TIG) with standard preheat practices. - S250GD, if strengthened via microalloying or TMCP rather than increased carbon, will typically retain good weldability; however, increased Mn or microalloy content can raise CE and locally increase hardenability, which may require controlled preheat or post-weld heat treatment in thicker sections or cold conditions. - For critical welded structures consult supplier CE/Pcm values and follow recommended welding consumables and preheat/post-heat procedures. Use welded joint design and low hydrogen consumables to minimize risk.

6. Corrosion and Surface Protection

• Both S220GD and S250GD are coated grades: the “GD” suffix denotes hot-dip galvanized coating (zinc) supplied as a continuous coated product. The galvanized layer provides cathodic protection to mild steel.
• Standard corrosion protection strategies: choose appropriate coating mass (g/m²), consider pre-treatment and paint systems for atmospherically exposed or aggressive environments, and specify edge protection or seam sealing where necessary.
• PREN (Pitting Resistance Equivalent Number) applies to stainless alloys and not to zinc-coated low-carbon steels; for reference:

$$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$

This index is not applicable to S220GD/S250GD as they are not stainless steels.

7. Fabrication, Machinability, and Formability

• Cold forming and bending: S220GD, with its lower guaranteed yield, typically offers slightly better formability and greater allowable bend radii for a given thickness. Forming limits should be determined by full-scale trials or supplier datasheets.
• Punching and presswork: both grades are designed for forming operations common in construction and roofing. Tooling life is similar; however, S250GD’s higher strength increases forming loads and may accelerate tool wear.
• Machinability: neither grade is optimized for high-speed machining; both machine similarly to mild structural steels. Machinability can be improved with appropriate tooling and cutting parameters.
• Finishing: the galvanized coating affects painting and adhesive bonding. Surface preparation (e.g., chromate conversion or appropriate primers) is standard for paint systems.

8. Typical Applications

S220GD (typical uses)	S250GD (typical uses)
Roofing and cladding where high formability is required	Structural profiles and sections where higher yield enables reduced gauge
Internal linings and ducting with extensive forming	Cold-formed load-bearing purlins, light structural framing
Non-critical stamped/punched parts	Applications requiring additional margin of safety for load capacity
Economical applications prioritizing cost and ease of fabrication	Situations where weight reduction via thinner gauges is desirable

Selection rationale: - Use S220GD where forming efficiency, bendability, and lower cost are priorities and the required design loads are within the lower yield class. - Use S250GD when a higher minimum yield allows thinner material or when structural requirements demand a higher guaranteed yield.

9. Cost and Availability

• Cost: S250GD is typically marginally more expensive than S220GD because of its higher guaranteed mechanical properties and potential microalloying or TMCP processing required to achieve them. The price premium varies with market conditions.
• Availability: Both grades are commonly produced by major coil suppliers and are widely available in common coil, sheet, and slit formats. Lead times are typically short for standard coating masses and widths but longer for specialty coatings or very tight mechanical tolerances.

10. Summary and Recommendation

Table: Quick comparison (qualitative)

Attribute	S220GD	S250GD
Weldability	Good — easier due to lower strength requirements	Good — slightly higher CE risk if microalloyed
Strength–Toughness balance	Good ductility and toughness for forming	Higher yield strength; toughness similar if processed properly
Cost	Lower	Higher (moderate premium)

Recommendation: - Choose S220GD if: your application emphasizes forming, bending and punching with a need for the most economical galvanized sheet that meets moderate structural requirements; when maximum ductility for complex profiles is essential. - Choose S250GD if: you need a higher guaranteed yield strength to reduce section thickness, achieve a higher factor of safety, or meet specific structural load requirements while retaining the advantages of a galvanized surface.

Final note: For procurement and fabrication, always verify mill certificates for chemical composition, mechanical properties, coating mass, and the supplier’s recommended forming and welding procedures. Where structural safety or low-temperature toughness is critical, specify and test the required properties rather than relying solely on grade name.