Steel Grades Explained: Yield Strength, Tensile Strength & MPa Values Compared

When an engineer specifies Grade 350 for a structural beam, they are not picking a number at random. They are specifying an exact yield strength, the stress at which that beam will permanently deform and every downstream decision, from section size to connection design, follows from that number.

For builders, engineers, architects and project managers in Melbourne and Victoria, these numbers appear on structural drawings, mill certificates and fabrication specs. This page is a reference tool: yield strength, tensile strength and MPa values for every Australian structural steel grade, with the comparison tables and AS/NZS standards context needed to read them correctly.

What Is Yield Strength of Steel? 

Yield strength is the stress at which steel permanently deforms rather than returning to its original shape. Below this threshold, steel is elastic, removes the load and the member springs back. Above it, deformation becomes permanent. That threshold is the yield strength (also called yield stress or yield point).

In structural engineering, the yield strength of steel is the critical design threshold. Structures are designed so that stress in steel members stays well below this value under normal loading. When engineers calculate beam sizes, column heights and connection capacities under AS 4100 (Steel Structures), the yield strength of the specified grade is the foundation of every calculation.

Yield strength is expressed in megapascals (MPa) one megapascal equals one newton of force per square millimetre of cross-section. A yield strength of 250 MPa means the steel can withstand 250 N/mm² of stress before permanent deformation begins.

What Is Tensile Strength of Steel?

Tensile strength (also called ultimate tensile strength or UTS) is the maximum stress steel can withstand before it fractures. It is always higher than yield strength. The ratio between the two reveals how much safety margin and ductility the material has before failure.

Where yield strength marks the start of permanent deformation, tensile strength marks the end point of the stress at which the steel actually breaks. For Grade 300 structural steel plate per AS/NZS 3678, yield strength is specified as 300 MPa for thicknesses up to 17mm and 280 MPa for thicknesses greater than 17mm (>17mm), with tensile strength of 430–550 MPa, which provides the safety buffer that allows structures to show visible distress before catastrophic failure.

Both values appear on a mill certificate, the document provided by the steel manufacturer confirming the mechanical properties of each batch (heat) of steel. Any professional fabricator should be able to provide mill certificates for the structural steel in your project. If they cannot, you have no proof that the material meets the specified grade.

For more on what mill certificates contain and why they matter, see the Kelly Steel guide to structural steel fabrication.

Yield Strength vs Tensile Strength What Is the Difference?

These two values are often confused. Here is the precise distinction:

Property	Yield Strength	Tensile Strength (UTS)
What it measures	Stress at permanent deformation	Stress at fracture
Design use	Primary criterion in AS 4100 structural design	Safety margin, ductility assessment
Also called	Yield stress, yield point, proof stress	Ultimate tensile strength, UTS, breaking strength
Mild steel (Grade 250)	250 MPa	410 MPa minimum
Structural Grade 350	360 MPa (≤11mm thick)	480 MPa minimum
Relationship	Always lower	Always higher typically 60–80% above yield

What Is the Compressive Strength of Steel?

For standard structural steel grades, compressive yield strength is equal to tensile yield strength. A Grade 350 beam has the same resistance to being crushed as it does to being pulled apart 360 MPa in both directions. This is what engineers mean when they say steel behaves isotropically under stress.

This equal behaviour in tension and compression is one of the reasons structural steel is so efficient as a building material. A concrete column, by contrast, is strong in compression but weak in tension which is why it needs steel reinforcing bars.

However, compressive capacity for a steel column is not just about compressive yield strength. Slender columns fail by buckling at stresses far below the yield point the column bends sideways rather than crushing. This is why column design under AS 4100 accounts for slenderness ratio, not just material strength. The compressive yield strength of the steel sets the upper limit; buckling calculations determine the actual limit for a given column geometry.

For context on how much load structural steel can carry, including the factors that govern real-world load capacity, see the Kelly Steel guide.

Steel Grades & MPa Values Australian Reference Table

The following values are drawn from AS/NZS 3678 (flat products and plates) and AS/NZS 3679 (hot-rolled bars and sections) the Australian material standards for structural steel. Yield strength decreases with increasing plate thickness; the values below are for sections up to the standard threshold unless noted.

Important note on thickness Yield strength in structural steel decreases as plate or section thickness increases. Grade 350 steel has a yield strength of 360 MPa for sections up to 11mm thick, 340 MPa for 12–17mm, and 330 MPa for 18–11mm (flange thickness). Always check the mill certificate for the actual yield strength of the material in your project especially for heavy sections.

Steel Grade	Standard	Yield Strength (MPa)	Tensile Strength (MPa)	Elongation Min %	Common Use
Grade 250	AS/NZS 3678	250	410 min	22%	Secondary members, platforms, handrails, general fabrication
Grade 300	AS/NZS 3678	300	440 min	22%	General structural where higher strength than 250 is needed
Grade 350	AS/NZS 3678	360 (≤11mm)	480 min	21%	Primary structural steel: beams, columns, trusses the Australian standard structural grade
Grade 350L0	AS/NZS 3678	360	480 min	21%	Same as 350 with improved low-temperature toughness; exposed/dynamic-load applications
Grade 400	AS/NZS 3678	400	480 min	19%	Higher-strength structural applications, heavy plate
Grade 450	AS/NZS 3678	450	520 min	17%	Heavy structural, mining equipment, crane rails, high-load applications
Mild steel (AS 3678-250)	AS/NZS 3678	250	410 min	22%	General fabrication, low-load structural members the everyday grade
Cold-formed LGS (G550)	AS/NZS 4600	550	550 min	2% (limited)	Light gauge steel framing for residential and commercial wall/floor frames

Mild Steel Yield Strength Explained 

Mild steel formally Grade 250 to AS/NZS 3678 is the default material for a huge range of fabricated steel items: general structural members in residential buildings, platforms, walkways, handrails, support brackets, and anything where high yield strength is not the primary driver. Its appeal is practical: it is readily available from Australian steel distributors, straightforward to weld without pre-heating (due to low carbon content), easy to cut and drill, and significantly cheaper than higher-strength grades.

The trade-off is load capacity. For a primary structural beam carrying heavy floor loads in a commercial building, Grade 250 will often require a substantially larger section than Grade 350 to achieve the same structural performance which can eliminate any cost saving from the lower-grade material. This is why engineers specify Grade 350 for primary members and reserve Grade 250 for secondary and lightly loaded elements. 

Mild steel vs structural steel what is the difference?

In Australian construction, the term “structural steel” typically refers to Grade 350 (and occasionally Grade 300 or 400) sections used for primary load-bearing members. “Mild steel” usually refers to Grade 250 material used for general fabrication and secondary elements. Both are hot-rolled carbon steels from the same AS/NZS 3678 family; the difference is grade (and therefore yield strength), not a fundamentally different material.

Structural Steel Yield Strength Grade 350 in Practice 

Grade 350 is the specification that appears on the engineer’s drawings for nearly every primary structural steel member in Australia: beams, columns, portals, trusses. The higher yield strength compared with Grade 250 allows engineers to use smaller, lighter sections to carry the same loads, which typically reduces both material cost and structural weight.

The thickness-dependent yield strength is important and is frequently misunderstood. A 10mm plate cut from Grade 350 coil has a yield strength of 360 MPa. The same grade in a 20mm flange has a yield strength of 330 MPa. This reduction happens because of metallurgical changes during the rolling process; thicker sections cool more slowly and have a slightly different grain structure. The standard accounts for this, and engineers designing to AS 4100 use the appropriate value for the section being specified.

What happens if the wrong grade is used?

Using Grade 250 where Grade 350 was specified means the steel yields at a 28% lower stress than the engineer calculated. Depending on the margin in the design, this may be immediately unsafe or may only become an issue under extreme loading. Either way, it is a non-compliance that a building certifier or inspector will identify when they check the mill certificates against the structural drawings and rectifying it once steel is in place is expensive. This is why mill certificate verification is a mandatory step in Kelly Steel’s quality management system.

What Does MPa Mean for Steel?

MPa stands for megapascal, a unit of pressure and stress in the International System of Units (SI). One megapascal equals one newton of force per square millimetre of area (1 MPa = 1 N/mm²).

In steel specifications, MPa is used to express how much stress force divided by cross-sectional area a material can withstand before yielding (yield strength) or fracturing (tensile strength). A yield strength of 350 MPa means the steel resists 350 newtons of force for every square millimetre of cross-section before permanent deformation begins.

To put that in context: a 250 × 250mm universal column section (UC) of Grade 350 steel, with a cross-sectional area of roughly 11,000 mm², has an axial yield capacity of approximately 3,960 kN, nearly 400 tonnes before it begins to yield. That is the principle that allows relatively slender steel columns to carry enormous building loads.

How Is Steel Strength Tested?

Steel strength is verified through a tensile test (also called a coupon test or mechanical test). A standardised sample coupon is cut from the steel, machined to precise dimensions, and pulled apart in a testing machine at a controlled rate. The machine records the load and elongation continuously, producing a stress-strain curve.

From this curve, the laboratory reads off:

• Yield strength the stress at the yield point, or the 0.2% proof stress for steels without a clear yield plateau
• Ultimate tensile strength the peak stress before fracture
• Elongation the percentage stretch at fracture, which indicates ductility

These results are recorded on the mill certificate (also called a test certificate or heat certificate) and accompany each batch of steel from the mill. The mill certificate is the primary evidence that the steel meets the specified grade. A professional fabricator retains these certificates and includes them in the QA documentation package delivered at project completion.

Choosing the Right Steel Grade A Practical Guide

Steel grade selection is the structural engineer’s decision not the fabricator’s and not the builder’s. However, understanding the logic helps everyone involved ask better questions and avoid specification errors.

Application	Typical Grade	Why
Primary beams and columns in commercial or industrial structures	Grade 350	Higher yield strength allows smaller, lighter sections; standard for AS 4100 design
Secondary members, purlins, girts, bracing	Grade 250 or 350	Lower loads mean Grade 250 is often adequate; engineer confirms
Platforms, walkways, mezzanine floors	Grade 250	Light loads; Grade 250 provides adequate strength at lower cost
Handrails, balustrades, ladders	Grade 250 or RHS/SHS 350	Depends on loading and design; often Grade 250 mild steel
Exposed structural steel in dynamic or cold environments	Grade 350L0	L0 suffix indicates improved Charpy impact toughness at 0°C
Light gauge steel framing (residential/commercial)	G550 (cold-formed)	High yield strength allows thin sections; designed to AS/NZS 4600
Heavy plate, mining, crane structures	Grade 400 or 450	High loads require higher yield strength to keep section sizes manageable
Marine or coastal structural steel	Grade 350 + appropriate coating	Grade selection governs structural capacity; coating governs corrosion protection

For residential projects including steel-framed homes, townhouses and extensions, see Kelly Steel’s light gauge steel framing page for how cold-formed G550 sections are used in wall and floor framing, and the structural steel fabrication page for hot-rolled structural work.

Frequently Asked Questions