What is Low-Carbon (Mild) Steel

Steel is comprised mostly of carbon and iron, with a maximum carbon content of 2.1% of the material’s weight. At this carbon level, this type of steel is referred to as high-carbon steel (or carbon tool steel). The material is incredibly hard, but also very brittle, making it very difficult to machine, cut or bend. In other words, high-carbon steel has low ductility.

Related: How Structural Steel is Produced

To increase ductility, high carbon steel can be processed to further reduce carbon. Material that roughly contains between 0.3% and 0.6% carbon is considered medium carbon steel. While there are some applications for medium carbon steel, it still lacks ductility.

Adding up to 0.4% manganese to increase hardness while reducing the carbon content to 0.05%-0.25% (making the material more ductile) produces low-carbon (or mild) steel. Low-carbon steel still has modest ductility and is sensitive to atmospheric elements but can be used in a number of applications.

ASTM A36: The Core Mild Steel Grade

Widely used across a variety of applications, ASTM A36 is easily the most common grade of structural steel used in North America.

A36 is achieved with the addition of manganese, phosphorus, sulfur, silicon and copper. These combined elements increase machinability, hardness and corrosion resistance.

A36 Steel Composition
Element Composition
Carbon 0.25-0.29%
Manganese 0.80-1.2%
Phosphorus 0.03%
Sulfur 0.03%
Silicon 0.4%
Copper 0.2%

A36 features respectable strength, represented below by yield, tensile and elongation.

ASTM A36 Strength
Strength Point Min. Value
Yield Point (psi) 36,000
Tensile Strength (psi) 58,000-80,000
Elongation % (200 mm) 20%

A36 is the sweet spot between hardness, machinability, durability and price. It is one of the most economical steel grades available, and is easily accessible from mills, steel suppliers and steel retailers.

What is HSLA Steel

By adding certain alloys – such as phosphorus, sulfur, silicone and molybdenum – mild steel can be transformed into high-strength low-alloy (HSLA) steel, which boasts high strength, lower weight relative to other materials with similar strength and good machinability and ductility.

HSLA steel encompasses a large range of grades, but three grades comprise a large percentage of HSLA steel demand in North America.


Like A36, ASTM A572 is a widely used, economically priced structural steel material. Used in bridges, buildings, construction equipment, transmission towers and more, A572 has a higher yield point and tensile strength than A36. In other words, in can bear more weight.

ASTM A572 Strength
Strength Point A572-50 A36
Yield Point (psi) 50,000 36,000
Tensile Strength (psi) 65,000 58,000-80,000
Elongation % (200 mm) 18% 20%

Also like A36, the same elements are added to achieve this enhanced strength and hardness:

A572 Steel Composition
Element Grade 42 Grade 50 Grade 60 Grade 65
Carbon 0.21% 0.23% 0.26% 0.23-0.26%
Manganese 1.35% 1.35% 1.35% 1.35-1.65%
Phosphorus 0.03% 0.03% 0.03% 0.03%
Sulfur 0.03% 0.03% 0.03% 0.03%
Silicon 0.15-0.40% 0.15-0.40% 0.4% 0.4%

A572 is available in multiple grades (we look at four grades here) that offer different strength and hardness benefits. The most common of these is ASTM A572-50.


ASTM A656 is an ideal HSLA steel choice for construction equipment, crane booms, truck frames and general fabrication, among other applications. It includes the alloys added to A572 grades, as well as vanadium, columbium and titanium for added strength, and nitrogen for added corrosion resistance.

A656 Steel Composition
Element Composition
Carbon 0.18%
Manganese 1.65%
Phosphorus 0.025%
Sulfur 0.03%
Silicon 0.6%
Vanadium 0.08-0.15%
Nitrogen 0.03%
Columbium 0.008-0.10%
Titanium 0-0.15%

A656 has a similar yield point and tensile strength compared to A572, making it a good option for load-bearing projects, but it can cost more.

ASTM A656 Strength
Strength Point A656-50 A572-50
Yield Point (psi) 50,000 50,000
Tensile Strength (psi) 60,000 65,000-80,000
Elongation % (200 mm) 20% 18%


CSA G40.21

CSA G40.21 steel is a widely used HSLA steel made to Canadian standards. While similar to ASTM A572 in terms of yield point, tensile strength and elongation, CSA G40.21 grades 44W, 50W and 50A have a chemical makeup that is different than its American cousin.

CSA G40.21 Steel Composition
Element Grade 44W Grade 50W Grade 50A
Carbon 0.22% 0.23% 0.02%
Manganese 0.5%-1.5% 0.5%-1.5% 0.75%-1.35%
Phosphorus 0.04% 0.04% 0.03%
Sulfur 0.05% 0.05% 0.04%
Silicon 0.04% 0.04% 0.15%-0.5%
Chromium 0.07%    
Nickel 0.09%    
Copper 0.2%-0.6%    


Other Less Common HSLA Steel Grades

HSLA steel is a large branch of carbon alloy steel. We will look quickly at five other HSLA grades that are less common than the ones already discussed, but widely used across a variety of applications.

To learn more about each specific grade, or to request a quote for that grade, click on the grade below.

Less Common HSLA Grades
Grade Common Uses
A709 Bridges. Available in standard grades as well as higher performance grades for extreme climates. Commonly purchased to also meet AASHTO M270 standards.
AASHTO M270 Bridges. M270 is very similar to A709, but meets AASHTO’s specific standards.
A588 & A606 Welded bridges and buildings, as well as general construction applications. These grades are very corrosion-resistant.
A871 Tubular structures and poles.
A573 Storage tanks. This grade is fully killed and is ideal when atmospheric conditions require improved notch toughness.

Performance Over Composition

While composition guidelines are important for HSLA steel grades, the performance of the material helps engineers determine which HSLA steel is required for their specific project.

Charpy impact tests determine the amount of energy a material can absorb at varying temperatures. This is important in the construction of bridges, where the bridge may need to endure cold atmospheric conditions.

Yield point, tensile strength and elongation percent are other measurements that help engineers determine material need. These measurements identify the point at which material will incur irreparable damage, the point the material will fail and the percentage difference between those two points.

Once engineers determine the material needs for a specific project, partnering with an experienced steel supplier, like Leeco® Steel, will ensure the timely delivery of the desired material, along with documentation showing composition and performance.