Types of Steel & Steel Grades
February 11, 2021 | Categorised in: Steel 101
Steel is one of the most useful materials in the world. In the simplest context, steel is composed of iron and carbon, but in actuality, steel isn’t so simple. The concentration of carbon and iron or the addition of other elements affect steels’ properties and strength, which aids in steel’s usefulness for an endless variety of end uses.
The Four Types of Steel
Steel is grouped into four categories:
- Carbon steel – Carbon steel has a higher concentration of carbon than other types of steel (up to 2.5% carbon), which adds to carbon steel’s strength. Carbon steel is commonly used to make construction materials, tools, automotive components, and more.
- Alloy steel – Alloy steel is made by combining carbon steel with one or several alloying elements, such as titanium, copper, chromium, aluminum, manganese, silicon, and nickel. This produces specific properties that are not found in standard carbon steel.
- Stainless steel – Stainless steel is a type of steel alloy that contains a minimum of 11% chromium, which helps the material resist corrosion.
- Tool steel – Tool steel is a type of steel alloy that’s hardness, resistance to abrasion, and ability to retain shape at increased temperatures make it a popular choice for hand tools and machine dies.
Steel can also be classified by several different factors, including:
- Composition (carbon range, alloy, stainless, etc.)
- Finishing method (hot rolled, cold rolled, cold finished, etc.)
- Production method (electric furnace, continuous cast, etc.)
- Microstructure (ferritic, pearlitic, martensitic, etc.)
- Physical strength (per ASTM standards)
- De-oxidation process (killed or semi-killed)
- Heat treatment (annealed, tempered, etc.)
- Quality nomenclature (commercial quality, pressure vessel quality, drawing quality, etc.)
Steel Numbering Systems
The steel industry uses two major numbering systems to classify steel grades, the American Iron & Steel Institute (AISI) and Society of Automotive Engineers (SAE) systems. Both the AISI and SAE systems primarily use four-digit numeric codes to identify a material’s base carbon or alloy steel; however, some select alloy steels have five-digit codes.
All carbon steels start with a one (1) in both the SAE & AISI systems. Carbon steels are then further subdivided into four categories by their underlying properties. The four categories are:
- Plain carbon steel – 10xx series (containing 1.00% Mn maximum)
- Resulfurized carbon steel- 11xx series
- Resulfurized and rephosphorized carbon steel – 12xx series
- Non-resulfurized high-manganese – 15xx series (containing up to 1.65% Mn)
The first digit on all other alloy steels (under the SAE-AISI system), are classified as follows:
2 = Nickel steels.
3 = Nickel-chromium steels.
4 = Molybdenum steels.
5 = Chromium steels.
6 = Chromium-vanadium steels.
7 = Tungsten-chromium steels.
8 = Nickel-chromium-molybdenum steels
9 = Silicon-manganese steels and various other SAE grades
The second digit of a series typically indicates the concentration of the major element in percentiles. The last two digits of a series indicate the carbon concentration.
Types of Carbon Steel
Carbon steel is organized into three main categories:
- Low carbon steel (or mild steel) – This is one of the most common types of carbon steel. Mild steel has a carbon content of between 0.04% to 0.30%. Depending on the desired properties needed, other elements can be added or increased.
- Medium carbon steel – Medium carbon steel has a carbon range of 0.31% to 0.60%, and a manganese content ranging from .060% to 1.65%. Medium carbon steel is stronger than low carbon steel but is more difficult to form, weld and cut..
- High carbon steel – High carbon steel, also known as tool steel, has a carbon range between 0.61% and 1.50%. High carbon steel is difficult to cut and bend which makes it ideal for tools.
Steel Grades & Standards
According to the World Steel Association, there are over 3,500 different grades of steel. At its core, steel is composed of iron and carbon, but the amount of carbon, level of impurities and additional alloying elements all contribute to what grade steel is classified. Each steel grade has its unique properties and typical end-use. Below are the common grades of steel and their uses as determined by the American Society for Testing and Materials (ASTM).