Australian Structural Steel Properties — AS/NZS 3679.1 Grades and Mechanical Properties

Complete reference for the mechanical properties of hot-rolled structural steel sections and plates per AS/NZS 3679.1:2016 (Hot-rolled structural steel bars and sections). Covers Grade 250, Grade 300, Grade 300PLUS, Grade 350, Grade 400: yield strength (f_y), tensile strength (f_u), minimum elongation, Charpy V-notch impact requirements, chemical composition limits, and the thickness-dependent yield strength variation.

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Australian Standard Steel Grades for Structural Use

AS/NZS 3679.1 defines the following structural steel grades for hot-rolled sections (UB, UC, PFC, EA, UA) and plates:

Grade Equivalents to International Standards

Note: international equivalents are approximate. Australian grades have specific chemical composition and Charpy requirements that differ from the equivalent international grades. Substitution is not permissible without a formal compliance assessment.

Mechanical Properties — AS/NZS 3679.1 Tables 3.1 and 3.2

Grade 250

Grade 250 is the most basic structural steel grade. It has been largely superseded by Grade 300 in Australian construction because the incremental cost of Grade 300 over Grade 250 is negligible (less than 2% of the finished steel price) while providing 20% greater yield strength. Grade 250 is still specified for non-structural and secondary applications where minimum cost is the primary driver.

Grade 300

Grade 300 is the workhorse grade of Australian structural steel. The majority of UB, UC, PFC, EA, and UA sections supplied in Australia are Grade 300. The yield strength decreases with increasing thickness because the cooling rate during hot rolling is slower in thicker sections, resulting in a coarser grain structure and lower strength.

Key design values for Grade 300:

• f_y_flange: 300 MPa (t_f <= 17 mm typical for UB/UC sections) or 280 MPa (t_f > 17 mm)
• f_y_web: 320 MPa (t_w <= 11 mm typical) or 300 MPa (t_w <= 17 mm)
• f_u: 440 MPa (all thicknesses <= 40 mm)
• E (elastic modulus): 200,000 MPa (all structural steels)
• G (shear modulus): 80,000 MPa
• nu (Poisson's ratio): 0.30

Grade 300PLUS

Grade 300PLUS is a proprietary grade (originally developed by BHP/OneSteel, now InfraBuild) with the same yield and tensile strength as Grade 300, but with a lower carbon equivalent value (CEV) for improved weldability:

The lower CEV of Grade 300PLUS eliminates preheat requirements for thicknesses up to 20 mm (vs 12 mm for standard Grade 300), which significantly reduces fabrication costs for welded built-up sections and connections. Grade 300PLUS is the preferred grade for welded fabrication in Australian practice.

Grade 350

Grade 350 provides approximately 15-20% higher yield strength than Grade 300 at a cost premium of approximately 5-10%. It is used where weight reduction is beneficial: long-span roof beams, transfer structures, and heavy columns in high-rise buildings. Grade 350 plates are commonly used for end plates in moment-resisting connections and for gusset plates in heavy bracing connections.

Grade 400

Grade 400 is a quenched and tempered (Q&T) steel rather than hot-rolled and normalised. It is produced by reheating and rapidly cooling the rolled section, producing a fine-grained tempered martensite/bainite microstructure. The Q&T process gives higher strength but lower ductility (18-20% elongation vs 21-23% for Grade 350) and requires careful control of welding to avoid softening the heat-affected zone.

Charpy V-Notch Impact Requirements

Charpy testing measures the energy absorbed in fracturing a notched specimen, indicating the steel's resistance to brittle fracture at low temperatures. All structural grades in AS/NZS 3679.1 require minimum Charpy values:

For applications in cyclonic regions (Northern Australia, Wind Region C and D per AS 1170.2) or for primary tension members in bridges, Charpy testing at -15 C or -20 C with higher minimum energy values (40 J) may be specified.

Chemical Composition Limits — AS/NZS 3679.1 Table 2.1

Microalloying elements (niobium, vanadium, titanium, aluminium) are used in controlled amounts to achieve grain refinement and precipitation strengthening. The total microalloy content (Nb + V + Ti) is typically 0.02-0.10% and contributes to the higher strength of Grade 350 and 400 without the excessive carbon content that would impair weldability.

Design Yield Stress in AS 4100 — Clause 2.1

Per AS 4100 Clause 2.1, the design yield stress f_y used in capacity calculations is the minimum yield strength from AS/NZS 3679.1 for the relevant thickness, not the nominal grade number. The designer must determine f_y based on the actual plate or section thickness:

• A 310UB40.4 Grade 300 with flange thickness t_f = 10.2 mm: f_yf = 320 MPa (t_f <= 11 mm)
• A 310UB46.2 Grade 300 with flange thickness t_f = 12.5 mm: f_yf = 300 MPa (11 < t_f <= 17 mm)
• A 530UB92.4 Grade 300 with flange thickness t_f = 15.6 mm: f_yf = 300 MPa
• A 700WB173 Grade 350 with flange thickness t_f = 20.0 mm: f_yf = 350 MPa

This thickness-dependent yield strength must be correctly applied for each plate element (flange and web separately) and is critically important for accurate capacity calculations.

Worked Example: Yield Strength Determination for Design

Problem: A welded plate girder uses Grade 300 plates. The flange plate is 25 mm thick and the web plate is 10 mm thick. Determine the design yield strength f_y for the flange and web per AS 4100.

Given:

• Flange: Grade 300 plate, t_f = 25 mm
• Web: Grade 300 plate, t_w = 10 mm (t_w <= 11 mm)

Solution:

Step 1: Flange f_yf (t_f = 25 mm, 17 < t_f <= 40)

From the Grade 300 table: f_yf = 280 MPa

Step 2: Web f_yw (t_w = 10 mm, t_w <= 11)

From the Grade 300 table: f_yw = 320 MPa

Step 3: Ultimate tensile strength f_u (both elements)

f_u = 440 MPa (all thicknesses <= 40 mm for Grade 300)

Step 4: Section moment capacity

The section moment capacity M_s depends on both flange and web yield strengths. For a compact section, M_s = f_yf x S where S is the plastic section modulus. But the web is a different grade...

For a plate girder with a hybrid section (flange f_yf = 280 MPa, web f_yw = 320 MPa), the moment capacity is governed by the weaker element. The flange yield strength (280 MPa) controls the moment capacity because the flange is at the extreme fibre where yielding begins. The web has a higher yield strength (320 MPa) but this does not increase the moment capacity because the flange yields first.

phi M_s = 0.90 x 280 x S x 10^(-6) kNm

Step 5: Shear capacity

V_w = 0.6 x f_yw x A_w (web area) = 0.6 x 320 x d_1 x t_w x 10^(-3) kN

The web shear capacity uses the web yield strength (320 MPa) because shear is resisted by the web. The higher web yield strength is beneficial here.

Result: Flange f_yf governs the moment capacity at 280 MPa. Web f_yw governs the shear capacity at 320 MPa. The hybrid girder has 6.7% less moment capacity than a homogeneous Grade 300 section with t_f <= 11 mm (f_yf = 320 MPa), but the shear capacity is fully developed at the web yield strength.

Frequently Asked Questions

What is the yield strength of Grade 300 steel per AS/NZS 3679.1?

The yield strength of Grade 300 steel depends on the plate/section thickness. For thickness t <= 11 mm: f_y = 320 MPa. For 11 < t <= 17 mm: f_y = 300 MPa. For 17 < t <= 40 mm: f_y = 280 MPa. The grade name "300" refers to the nominal yield strength at the 11-17 mm thickness range. The actual design yield strength must be determined from AS/NZS 3679.1 Table 3.1 based on the specific plate or section element thickness.

What is Grade 300PLUS and why is it used?

Grade 300PLUS is a proprietary brand of Grade 300 steel (originally developed by BHP, now supplied by InfraBuild) with the same yield and tensile strength as standard Grade 300 but with a lower carbon equivalent value (CEV <= 0.42 vs <= 0.44 for standard Grade 300). The lower CEV improves weldability by reducing or eliminating the need for preheat: Grade 300PLUS can be welded without preheat up to 20 mm thickness, compared to 12 mm for standard Grade 300. This reduces fabrication costs for welded connections and built-up sections.

How does the thickness of a steel section affect its yield strength?

Thicker sections have lower yield strength because the cooling rate after hot rolling is slower for thicker sections. Slow cooling produces a coarser grain structure (larger ferrite and pearlite grains), which reduces the yield strength through the Hall-Petch relationship. The effect is quantised in AS/NZS 3679.1 as step reductions at 11 mm, 17 mm, and 40 mm thickness thresholds. A 310UB40.4 (t_f = 10.2 mm) has f_yf = 320 MPa, while a 310UB46.2 (t_f = 12.5 mm) has f_yf = 300 MPa -- a 6.3% reduction from the same nominal grade.

What is the difference between Grade 300 and Grade 350 steel in Australian practice?

Grade 350 has approximately 15-17% higher yield strength than Grade 300 (360 MPa vs 320 MPa at t <= 12 mm) and 9% higher tensile strength (480 MPa vs 440 MPa). The cost premium for Grade 350 is approximately 5-10%. Grade 350 is selected when the higher strength allows a lighter section to be used, offsetting the higher unit cost through reduced steel tonnage. For compression members where buckling governs (capacity proportional to EI, independent of f_y for slender members), the benefit of Grade 350 is less than for tension or bending members where capacity is directly proportional to f_y.

Do AS/NZS 3679.1 properties apply to hollow sections (RHS/SHS/CHS)?

No. Hollow structural sections (RHS, SHS, CHS) are manufactured to a different standard: AS/NZS 1163 (Cold-formed structural steel hollow sections). AS/NZS 1163 specifies different grades (C350, C450) with different chemical composition and mechanical properties. Cold-formed hollow sections typically have higher yield strengths than hot-rolled open sections because the cold-forming process introduces strain hardening. However, the cold-worked yield strength may be reduced by the Bauschinger effect if the section is subsequently straightened or if the corner regions are welded. AS 4100 Table 5.2 provides separate slenderness limits for hollow sections.

Educational reference only. All material properties must be verified against the current edition of AS/NZS 3679.1 and the mill test certificate. This information does not constitute professional engineering advice.