Understanding Steel Grade (G250, G350, G550 Explained)

Understanding Steel Grade (G250, G350, G550 Explained)

Complete Technical Guide for Roll Forming & Roofing Steel

Introduction

Steel grade is one of the most important specifications in roll forming.

It determines:

• Yield strength

• Structural capacity

• Springback behavior

• Minimum bend radius

• Forming pressure

• Machine sizing

• Tool wear

• Crack risk

When you see designations like:

• G250

• G300

• G350

• G450

• G550

The number represents the minimum yield strength in megapascals (MPa).

Understanding what these grades mean is essential for:

• Roofing manufacturers

• Structural profile designers

• Roll forming machine buyers

• Coil purchasers

This guide explains steel grades clearly and how they affect roll forming production.

1️⃣ What Does G250, G350, G550 Mean?

The “G” stands for:

Grade

The number represents:

Minimum yield strength in MPa.

Examples:

• G250 → Minimum 250 MPa yield strength

• G350 → Minimum 350 MPa yield strength

• G550 → Minimum 550 MPa yield strength

Higher number = stronger steel.

2️⃣ Steel Grade vs Yield Strength

Steel grade is directly tied to yield strength.

Steel Grade	Minimum Yield Strength
G250	250 MPa
G300	300 MPa
G350	350 MPa
G450	450 MPa
G550	550 MPa

Higher grade steel:

• Requires more forming force

• Produces more springback

• Has greater structural capacity

3️⃣ Steel Grades Used in Roofing Panels

Modern roofing commonly uses:

• G300

• G350

• G550

Why?

Because higher grade allows:

• Thinner material

• Higher stiffness

• Better wind performance

• Reduced weight

Example:

0.42mm G550 steel can match or exceed strength of thicker mild steel.

4️⃣ Steel Grades Used in Structural Profiles

Structural purlins and framing often use:

• G350

• G450

• G550

Higher grade improves:

• Span capability

• Load-bearing performance

• Structural efficiency

But increases forming difficulty.

5️⃣ Steel Grade & Roll Forming Force

Higher grade steel:

• Has higher yield strength

• Requires higher forming pressure

• Increases motor load

• Increases shaft stress

Machine sizing must account for maximum grade expected.

Example:

Machine designed for G250 may struggle with G550.

6️⃣ Steel Grade & Springback

Higher grade = more springback.

Why?

Higher yield strength means:

• Greater elastic recovery

• Greater bend angle opening

Tooling must compensate more for G550 than G250.

7️⃣ Steel Grade & Bend Radius

Higher grade requires:

• Larger minimum bend radius

• More gradual forming progression

Attempting tight bends on G550 can cause:

• Edge cracking

• Coating splitting

• Micro-fractures

Design must respect material limits.

8️⃣ Steel Grade & Tensile Strength

Yield strength and tensile strength are related but different.

Example:

G550 steel may have:

• Yield: 550 MPa

• Tensile: 600–700 MPa

Higher tensile increases fracture resistance but reduces ductility.

9️⃣ Steel Grade & Ductility

As strength increases:

Ductility often decreases.

High-strength steels:

• Are stronger

• But less forgiving during forming

This is why tooling precision becomes more critical at higher grades.

🔟 Steel Grade & Coated Roofing Steel

Roofing steel often includes:

• Galvanized coating

• Galvalume coating

• Painted coating

Steel grade refers to the base steel beneath coating.

Coating does not change grade designation.

1️⃣1️⃣ Why Two Coils of Same Thickness Behave Differently

0.5mm G250
0.5mm G550

Same thickness.
Completely different forming behavior.

G550:

• More springback

• Higher forming force

• Greater tooling stress

Never assume grade based on thickness alone.

1️⃣2️⃣ Steel Grade & Machine Specification

When quoting a roll forming machine, you must specify:

• Thickness range

• Maximum steel grade

Motor sizing, shaft diameter, frame rigidity all depend on maximum grade.

Undersized machine leads to:

• Bearing failure

• Shaft deflection

• Roll wear

• Profile distortion

1️⃣3️⃣ Steel Grade & Oil Canning

Higher grade steel:

• Resists deflection

• Can reduce visible oil canning

But improper forming can still introduce stress imbalance.

Material strength does not eliminate forming defects.

1️⃣4️⃣ Steel Grade Standards Around the World

Australia / Asia:
G250, G350, G550

USA:
Often expressed in ksi (e.g., 50 ksi)

Europe:
S250, S350 (structural designation)

Different naming systems — same principle.

Always confirm MPa value.

1️⃣5️⃣ Common Steel Grade Mistakes

• ❌ Assuming all roofing steel is G550
• ❌ Not confirming grade before tooling design
• ❌ Using mild steel tooling design for high-grade steel
• ❌ Ignoring grade variation between suppliers

Material grade must be confirmed from mill certificate.

1️⃣6️⃣ Real Example Comparison

Profile A:
0.5mm G250

Profile B:
0.5mm G550

Profile B:

• Higher structural capacity

• Greater springback

• Higher forming load

• Greater tooling wear

Machine and tooling must be designed accordingly.

1️⃣7️⃣ Engineering Summary

Steel grade determines:

• Yield strength

• Forming force

• Springback magnitude

• Bend radius requirement

• Machine load

• Structural performance

You cannot design a roll forming line properly without knowing steel grade.

FAQ Section

What does G350 mean?

Minimum 350 MPa yield strength.

Is G550 stronger than G350?

Yes, significantly.

Does higher grade mean better roofing?

Stronger structurally, but harder to form.

Does steel grade affect coil width?

Indirectly, through springback and bend allowance.

Can one machine run multiple grades?

Yes, if designed for highest expected grade.

Where do I find steel grade?

On the mill test certificate.