What Is Carbon Steel?

Iron and carbon are the most abundant materials present in steel. Pure iron is not particularly strong or hard on its own, so it is the addition of carbon that helps give steel its great strength.

The crude iron used to produce steel has a relatively high amount of carbon. Its carbon composition can be as high as 2.1%, which is the greatest amount of carbon a material can contain and still be considered steel.

However, iron can be processed further to reduce carbon. This manipulation of carbon alters several material properties, including:

Strength: The load a material can bear, measured by yield point and tensile strength. Yield point is the point at which a material deforms, but does not break, and tensile strength is the amount of stress needed to actually break a material.

Ductility: The amount a material can be stretched without becoming brittle. Ductility is measured by elongation, which is the percent the length of a material increases before it breaks.

Hardness: The wear resistance of material and machinability of material.  This is usually measured on the Rockwell hardness scale or Brinell Hardness scale.

The carbon present in steel is typically reduced so that it fits into three main categories of carbon steel: low (or mild), medium and high carbon steel. Each of these categories contain different levels of carbon, show in the chart below.

Type of Carbon Steel

Carbon Composition

Low Carbon/Mild

0.05-0.25%

Medium Carbon

0.26-0.60%

High Carbon

0.61-1.50%

This article will examine the properties, applications and common steel plate grades stocked by suppliers like Leeco® Steel in the different carbon steel categories.

What Is Low Carbon Steel?

Also known as mild steel, low carbon steel has low strength relative to steel with higher carbon levels. Low carbon steel is the most ductile – or machinable – type of carbon steel as well.

Chemical alloys can also be added to low carbon steel to enhance desired properties without increasing the material’s weight. For example, if low carbon steel requires greater hardness for its desired application, manganese can be added to increase hardness without adding weight. Low carbon steel that contains additional alloys is typically referred to as high strength, low alloy (HSLA) steel.

Some of the most common low carbon steel plate grades, all stocked by Leeco, include ASTM A36, A572 Grades 42 & 50 and A830-1020. Each of these grades have moderate strength, high ductility and lighter weight due to the low carbon content and addition of other alloys. These properties make low carbon steel ideal for use in structural applications like building construction, bridges and transmission towers, where materials must be able to withstand high stress while also being easy to form into structural shapes.

Chemical Composition of Common Low Carbon Plate Grades

Grade

Carbon

Manganese

Phosphorus

Sulfur

Silicon

A36 up to 3/4” thick

0.25%

N/A

0.030%

0.030%

0.40%

A36 >3/4 – 1 ½” thick

0.25%

0.80-1.20%

0.030%

0.030%

0.15-0.40%

A36 > 1 ½ - 2 ½” thick

0.26%

0.80-1.20%

0.030%

0.030%

0.15-0.40%

A36 >2 ½ - 4” thick

0.27%

0.85-1.20%

0.030%

0.030%

0.15-0.40%

A36 >4” thick

0.29%

0.85-1.20%

0.030%

0.030%

0.15-0.40%

A572 Grade 42

0.21%

1.35%

0.030%

0.030%

0.15-0.40%

A572 Grade 50

0.23%

1.35%

0.030%

0.030%

0.15-0.40%

A830 Grade 1020

0.18-0.23%

0.30-0.60%

0.030%

0.030%

N/A

Mechanical Properties of Common Low Carbon Plate Grades

Grade

Yield Point

Tensile

A36

36 ksi

58-80 ksi

A572 Grade 42

42 ksi

60 ksi

A572 Grade 50

50 ksi

65 ksi

 

What Is Medium Carbon Steel?

Medium carbon steel provides a balance between low and high carbon steel, offering greater strength and hardness than low carbon steel while still remaining more ductile than high carbon steel. Medium carbon steel also typically contains other alloys, such as manganese, that also contribute to its properties.

In applications where greater toughness and hardness are required, medium carbon steel plate can receive heat treatments – such as quenching and tempering – that enhance these properties without compromising its machinability.

Quenching and tempering is a two-step heat treatment process. In the quenching step of this process, steel is heated to a temperature between 1,500-and 1,650-degrees Fahrenheit, then rapidly cooled with water. In the tempering step, the steel is then re-heated to a below-critical temperature – between 300 and 700 degrees – and air-cooled. This process alters the crystal grain structure of steel to enhance hardness and other mechanical properties.

Two common medium carbon steel plate grades that Leeco stocks are ASTM A516 Grade 70 and A830-1045. The moderate carbon composition and additional alloys give these grades – and other medium carbon grades – a balance of strength, hardness, ductility and wear resistance. These properties make medium carbon steel ideal for use in applications where materials must withstand strong forces without breaking or wearing out, such as machine parts – including gears, axles and bolts – pressure vessel tanks and automotive parts and components.

Chemical Composition of Common Medium Carbon Plate Grades

Grades

Carbon

Manganese

Phosphorus

Sulfur

Silicon

A516 Grade 70 >2” & ≤4” thick

0.30%

0.85-1.20%

0.025%

0.025%

0.15-0.40%

A516 Grade 70 >4” thick

0.31%

0.85-1.20%

0.025%

0.025%

0.15-0.40%

A830-1045

0.43-0.50%

0.60-0.90%

0.030%

0.030%

N/A

Mechanical Properties of Common Medium Carbon Plate Grades

 Grades

Yield Point

Tensile

A516

36 ksi

58-80 ksi

 

What is High Carbon Steel?

High carbon steel offers the greatest strength and hardness compared to mild and medium carbon steel plate. However, high carbon steel is less ductile than lower carbon steels, meaning it is much harder to machine or form.

Like medium carbon steel, high carbon steel can also be heat treated to further enhance hardness and wear resistance for use in applications where steel faces particularly high levels of stress.

The high carbon composition of high carbon plate grades gives them great strength, hardness and wear resistance, which are properties ideal in applications where steel must regularly endure extreme wear without breaking, such as cutting and chiseling tools.

Very High Carbon Steel

Some applications require steel material that can endure forces even greater than that of high carbon steel. In those applications, very high carbon steel, the strongest type of carbon steel, is used. Very high carbon steel is nearly impossible to weld, machine or shape due to its incredible strength and is therefore far less common than the other types of carbon steel.

Sourcing Carbon Steel Plate

When considering a source for your carbon steel plate needs, it is important to consider a supplier who has the following to ensure you receive the best plate product and customer service:

  • A large inventory of mild, medium and high carbon steel plate grades from both domestic and international mills.
  • Strong quality management system, such as one that is ISO-certified, to ensure plates are high quality and meet your required specifications.
  • Wide geographic footprint near your delivery locations to help control your freight costs.

Leeco Steel specializes in steel plate and keeps an ample inventory of carbon steel stocked throughout its 11 strategically-located distribution centers. Leeco is also ISO 9001-certfied in quality management and verifies accuracy throughout all steps of the order fulfillment process to ensure customers receive high-quality plate products.

Contact Leeco or request a quote to discuss your carbon steel plate needs with a knowledgeable sales representative today.