How Steel Gets Its Strength (and Other Mechanical Properties)

Steel is known for its strength, but what exactly makes it strong? To understand how steel achieves its strength, we need to explore the science behind its composition, treatment, and processing. This article will focus primarily on coil/sheet steels (<3mm) that are produced at scale, excluding stainless steel grades for simplicity. While stainless steels share many of the same strengthening mechanisms, such as grain refinement and alloying, they aren’t the focus here. Additionally, ASTM or EN standards will be referenced, particularly as some steel grades aren’t manufactured in Australia or New Zealand. We will also exclude topics such as thickness/size effects and Young's modulus (stiffness), which will be explored in future articles. 

Trade-offs in Steel Strength

Enhancing steel's strength often involves trade-offs. As strength increases, elongation typically decreases, which can reduce formability. Introducing alloying elements to boost strength and maintain ductility may adversely affect weldability and increase hardness. Additionally, the yield ratio - the ratio of yield strength to tensile strength - is a critical factor; a higher yield ratio indicates less capacity for plastic deformation before failure, making the steel more brittle.

Why Stronger Isn’t Always Better

Higher strength doesn’t necessarily mean higher quality. Steel grades are engineered with specific applications in mind, balancing both performance and cost. For example, a Hollow Structural Section (HSS) grade like C450 is relatively basic and economical in both composition and processing - despite its high yield strength. In contrast, an ultra-low carbon Interstitial-Free (IF) or Extra Deep Drawing (EDD) steel may have a much lower yield strength (e.g. ~150 MPa) but require tighter chemistry control and advanced processing to enable very high formability, increasing production costs. Although there are thousands of steel grades and overlapping standards, the ways steel achieves its strength and mechanical properties typically fall into three key categories, discussed below.

1. Work Hardening (Strain Hardening)

When steel is deformed through processes like rolling or drawing, its internal structure becomes more resistant to further deformation. This happens because the dislocation density increases - dislocations are tiny defects or “slips” in the crystal structure. The more dislocations there are, the harder it is for the atoms to move, which increases the steel's strength.

2. Alloying

Adding elements such as carbon, manganese, chromium, or nickel improves the strength and performance of steel by changing its internal structure. One important effect is solid solution strengthening, where these added elements mix into the steel and slightly distort its crystal pattern. This distortion makes it harder for the layers of metal to move past each other, which increases strength and hardness. Alloying can also help form useful microstructures and improve properties like toughness, corrosion resistance, and the steel’s ability to be heat treated.

3. Heat Treatment

Processes like annealing (continuous and batch), quenching and tempering and thermomechanical processing (TMCP) change the steel’s microstructure. These changes adjust properties like strength, hardness, and ductility by altering the arrangement of phases (e.g., ferrite, martensite, bainite) within the steel. Some treatments also refine the grain size, which increases strength by creating more grain boundaries that resist deformation. 

While these three strengthening methods are often discussed separately, they frequently overlap in real-world steel production. As such, the remainder of this article won’t be strictly divided by these categories but will instead follow a progression based on increasing strength.

To start, we’ll focus on low-carbon mild steel (typically containing less than 0.25% carbon by weight), where work hardening plays a central role in increasing strength. In the absence of alloying elements that enable heat treatment and/or contribute to solid solution strengthening, mild steel generally has a yield strength ceiling of ~500 MPa. Because of its simple composition and limited response to heat treatment, work hardening is often the primary method used to enhance its strength, particularly in cold-rolled processing.

Mild Steel

Work hardening, also known as strain hardening, is often considered the simplest of the three main methods for strengthening steel. Unlike alloying or heat treatment, which require changes in chemical composition or precise temperature control, work hardening increases the dislocation density within the steel through plastic deformation. This makes the material harder and stronger but also less ductile. While the process itself is relatively straightforward, its effects on the material’s mechanical behaviour - particularly the balance between strength and formability - can be complex. 

At Industrial Tube Manufacturing, we worked with a customer on a medical durable product that required a tube with higher yield strength to withstand cyclic loading and improve fatigue resistance. The product featured a telescoping insert, so a Precision Tube with tight dimensional tolerances - particularly low thickness variability in the base strip - was essential.

(Howard Wright M10 Ward Bed)

Since no further forming was required after tube fabrication, we selected NZCC-1P - a single-pass cold-rolled product from New Zealand Steel - for its higher yield strength and stable dimensional properties. This grade offers the required mechanical properties, without the brittleness of full-hard steel or the lower strength of annealed grades.

To validate performance, we conducted tensile tests in accordance with AS 1391 on three tube samples, each formed from the below coil stock to dimensions of 1.6x50.8x25.4 mm.

To give context on how work hardening fits into the broader steel processing sequence:

• Hot rolled, pickled strip is further processed by cold rolling to achieve the desired thickness and ends the process in a full-hard condition. At this stage, the material is highly strain-hardened, with elevated yield and tensile strength, but very limited ductility.

• Full-hard (FH) as-rolled steel is typically not suitable for direct forming and is usually sent to: 

 - A metal coating line (e.g. GALVABOND®/GALVSTEEL®). The various grades such as G450, G350 and G250 are in-line annealed to the specified strength. 

 - A batch annealing furnace (close annealed) to restore ductility.

• Temper rolling, in this context, refers to a light cold reduction of the annealed strip to refine mechanical properties and improve shape control. This process slightly increases strength through controlled deformation and stabilises the steel, allowing mechanical properties to be held within narrower tolerances.

• It’s worth noting that annealing or normalising reduces strength, rather than increasing it. These processes are primarily used to soften the steel and restore formability. This applies in both continuous (CGL) and batch annealing processes.

Basic low-carbon steel from a hot strip mill (HSM) - without thermomechanical processing, cold working, or pickling - is an economical option. It offers moderate strength and good ductility, with a typical yield strength of ~200–250 MPa, depending on the steel’s composition and the processing parameters, especially finishing and coiling temperatures.

For higher structural grades such as HA300 and HA350 (AS 1594), the increase in strength is primarily achieved through minimal alloying combined with controlled rolling and coiling practices at the hot strip mill (HSM), a process often referred to as thermomechanical processing. One key factor is coiling temperature, which influences the final microstructure:

• Higher coiling temperatures promote a softer, coarser ferritic structure with lower strength and higher ductility.

• Lower coiling temperatures can encourage finer grains or partial bainitic transformation, increasing strength but reducing ductility.

The elevated manganese content typical of HA350 provides modest solid solution strengthening, while remaining within levels that maintain good weldability and formability.

A practical example of strength enhancement through alloying in hot-rolled steel can be seen in low-cost hollow structural sections (HSS) that are certified as ‘Dual Grade’, such as AS/NZS 1163 C350/C450. In this case, the levels of alloying elements - particularly Manganese and Carbon - are increased within the allowable chemistry limits of the standard. This change in chemistry helps to raise the yield strength to meet the higher strength grade (C450), while still satisfying the minimum elongation and other mechanical requirements of the lower strength grade (C350). Inventory efficiency for the mill is gained by holding less variations of the base material (coil). However, these adjustments can lead to practical trade-offs: higher Carbon Equivalent (CE) values may impact weldability, and increased strength can reduce formability compared to the standalone C350 product. While the product remains compliant with the standards, these real-world implications should be considered during design and fabrication.

High‑Strength Low‑Alloy (HSLA)

As we move beyond 500 MPa yield strength, High‑Strength Low‑Alloy (HSLA) steels become the workhorses of structural, automotive, and engineering applications, where an optimized strength‑to‑weight ratio is critical. These steels combine a total alloy content most commonly in the 2–4 wt % range with microalloy additions and thermomechanical processing at the hot‑strip mill (HSM) to deliver high strength without sacrificing toughness or weldability.

Standards & Products 

HSLA grades are specified in EN 10149‑2 (e.g. S700MC) with some mention in ASTM A1008, A1011 and in automotive contexts, via VDA standards. Commercial products include Strenx® 700MC and Docol® S700MC. Cold‑rolled coil variants are under EN 10268: 2006. Plate grades use different rolling and cooling methods to achieve similar properties and are covered by separate standards.

Key Strengthening Mechanisms

• Grain Refinement (Hall–Petch): Microalloying elements such as Nb, Ti, and V form ultra‑fine precipitates that pin austenite grain boundaries during controlled rolling and cooling, yielding finer ferrite grains that block dislocations.

• Solid‑Solution Strengthening: Mn, Si, and traces of P dissolve in the iron matrix to distort the lattice and impede dislocation motion, boosting yield strength.

Together, these mechanisms deliver steels that are ~500–700 MPa while retaining excellent ductility, weldability, and toughness - ideal for demanding applications.

A Note on Scope 

“HSLA” could be considered a loosely defined category. High-strength carbon–manganese (CMn) steels such as BS4 T45 or grades like AISI 4130 'Chromoly' may offer similar mechanical performance but belong to older material classes that predate the development of more modern steels. Additionally, quenched‑and‑tempered plate products (e.g. Bisalloy®, Hardox®) contain 4–6 wt % alloying to produce martensitic microstructures and are not classified as HSLA steels.

Moving Beyond HSLA: AHSS and UHSS Steels

While HSLA steels typically cover yield strengths up to around 700 MPa, Advanced High-Strength Steels (AHSS) and Ultra High-Strength Steels (UHSS) extend into higher strength ranges, with some overlap. These grades are defined not just by increased tensile strength - often exceeding 1,000 MPa - but by the use of complex, multiphase microstructures engineered to deliver specific combinations of formability, crash energy absorption, and weldability.

Driven by the need to reduce weight without compromising performance, AHSS and UHSS have become central to modern automotive engineering, among other sectors. Their development reflects the shift toward materials that meet demanding structural and safety requirements while enabling greater efficiency and sustainability. Up to 70% of the steel now used in vehicle structures today is AHSS. Manufacturing of these steels typically involves Continuous Annealing Lines (CAL), where cold-rolled coil is rapidly heated into the intercritical temperature range (~750–900 °C), where both ferrite and austenite coexist. The coil is then quenched using gas jet cooling to transform austenite into martensite while retaining ferrite. Precise control of temperature, soak time, and cooling rate is critical to achieving the desired phase balance and mechanical properties. While some variants may contain small amounts of bainite or retained austenite, the primary microstructure consists of ferrite and martensite. Due to the complexity and cost of CAL infrastructure, these steels are typically produced by mills supplying high volumes to the automotive industry. Some hot-rolled grades are produced using advanced hot-strip mills with accelerated cooling and precise control over temperatures during processing.

(Continuous Annealing Line used to process AHSS)

To better understand the balance between strength and formability across these steel families, the Global Formability Diagram provides a useful comparative framework. It positions common steel types according to their tensile strength and formability, helping to visualise the trade-offs that guide material selection.

(Global Formability Diagram)

Dual Phase (DP) steels are the most widely used category of AHSS, particularly in automotive applications, due to their excellent balance of strength, ductility, energy absorption, and cost-effectiveness. As the name suggests, DP steels consist primarily of two phases: a soft, continuous ferrite matrix and hard martensite islands dispersed throughout. This microstructural combination provides a low initial yield strength for good formability, followed by a high work hardening rate and ultimate tensile strength as the martensite phase progressively resists deformation during forming or crash events.

The mechanical properties of DP steels are largely governed by the volume fraction and distribution of martensite. Lower strength DP grades (e.g. 600–800 MPa UTS) may contain as little as 10–20% martensite by volume, while higher strength grades (e.g. DP1000, DP1200) can contain up to 40% or more. The ferrite provides ductility and toughness, while the martensite ensures high strength and strain hardening capacity. The absence of a distinct yield point and the high strain rate sensitivity of DP steels make them particularly well-suited for components requiring controlled deformation under crash loads, such as automotive reinforcements, B-pillars, and crumple zones. 

An interesting side note on chemistry and weldability is that while DP steels have a relatively high CE value (~4.3), largely due to elevated manganese levels (~1.8%), the phase distribution or partitioning of Mn within the microstructure means the material absorbs thermal effects differently, with carbon and manganese less localised in the heat-affected zone (HAZ) compared to fully ferritic steels, improving weldability.

DP steels are commonly designated based on their tensile strength - for example, DP600 or DP980 - though some standards (such as VDA or OEM-specific specs) may include additional suffixes to denote properties like bake hardenability or coating type. Overall, their versatility, consistent performance, and compatibility with existing stamping and joining techniques make Dual Phase steels the backbone of modern AHSS usage.

Other AHSS grades use multiphase microstructures to target specific performance needs. TRIP steels enhance ductility through the transformation of retained austenite during deformation, while CP steels rely on a fine mix of bainite, martensite, and ferrite for strength. MS (martensitic) steels exceed 1,500 MPa tensile strength but have limited ductility. TWIP steels, though not widely adopted, offer exceptional elongation via mechanical twinning, but their high manganese content (15–25 wt %) creates fabrication challenges - particularly in welding, coating, and segregation control. Newer types like DP-HF and DH aim to improve formability at higher strengths but remain less common than standard DP grades. 

Other High-Strength Grades 

High strength steels beyond the scope of this article include maraging steels, which can reach tensile strengths of up to 2500 MPa and are primarily used in aerospace, aircraft, and defence applications. Also excluded are highly alloyed materials and superalloys, some of which have limited commercial availability. Certain superalloys, particularly those with iron (Fe) content below 50% (e.g Inconel 718), fall outside the definition of steel.

In Summary

Throughout this article, we’ve explored the various ways steel’s strength and performance are achieved through composition, processing, and alloying. At Industrial Tube Manufacturing, we utilise a diverse range of steel grades, including advanced materials like Dual Phase (DP) steel, as well as conventional cold-rolled, hot-rolled mild steels, coated high-strength grades and stainless steels to produce high-performance Precision Tube that meets the exacting needs of industries such as automotive, f&b processing, horticulture and manufacturing. As a non-integrated tube mill, we offer the flexibility to source a wide range of steel grades and maintain precise control over tube forming and processing. This allows us to respond quickly to market needs and deliver custom Precision Tube solutions with excellent formability, strength, and durability - ensuring optimal performance for each application. Get in touch to discuss how our manufacturing capabilities can support the specific requirements of your next project.