Steel Classification: Definition and Categories

Steel is classified based on the elements used in its production, which helps in understanding the specific qualities of each type and its suitability for various applications. One of the most common classification methods is the one proposed by EURONORM, which categorizes steel based on alloying elements. According to this system, steel is grouped into three main categories:

• Non-Alloy Steels
• Low-Alloy Steels
• Alloy Steels

1. Classification Based on Alloying Elements

The table below summarizes the classification of steel based on its alloying elements:

As seen in the table, for steel to be considered low-alloy or alloy steel, it is sufficient for one of the elements to exceed its specified lower limit. However, automobile steels with high S, P, and Pb ratios may not be classified as alloy steels. Some special types of alloy steels for automated machines may have higher S and P values than indicated due to specific manufacturing requirements.

2. Mass-Produced Steels and Noble Steels

Steel can also be classified into two broad categories based on its production method and the presence of impurities such as S, P, and gas content:

• Mass-Produced Steels: This includes non-alloy and low-alloy steels, which are typically used in large-scale production due to their relatively low cost and availability.
• Noble Steels: This category includes non-alloy, low-alloy, and alloy steels with strict impurity limits, typically less than 0.035% sulfur and phosphorus. These steels are used for more demanding applications where high performance is required, such as in aerospace or high-end automotive industries.

3. SAE Number Classification

The SAE (Society of Automotive Engineers) classification system is widely used, especially in the United States, to identify and categorize steel grades based on their chemical composition and intended application. The SAE system is particularly popular in automotive and manufacturing industries, where steel is categorized by its alloy content.

SAE Number Structure:
The SAE designation for steel grades consists of a 4-digit number. Here's the breakdown of this classification system:

• First Two Digits: Represent the type of material (e.g., carbon steel, alloy steel).
  • For example, 1xxx refers to carbon steels, and 2xxx refers to nickel steels.
• Last Two Digits: Indicate the amount of carbon in the steel, typically in hundredths of a percent.
  • For example, in the grade 10xx, the "10" indicates carbon steel, and the "xx" indicates the specific carbon content (the last two digits). For 10xx, this would be a plain carbon steel, with a maximum of 1.00% manganese.

Here’s an overview of the SAE designations and their corresponding types:

The following is a list of common SAE steel grades and their key elements:

• Carbon Steels:

  • 10xx: Plain carbon (Mn 1.00% max.)
  • 11xx: Resulfurized
  • 12xx: Resulfurized and rephosphorized
  • 15xx: Plain Carbon (Mn 1.00–1.65% max.)
  • 15Bxx: Plain Carbon with Boron (Mn 1.00–1.65% max., B 0.0005-0.003% max.)

• Nickel Steels:

  • 23xx: Ni 3.50%
  • 25xx: Ni 5.00%

• Nickel-Chromium Steels:

  • 31xx: Ni 1.25%; Cr 0.65%, or 0.80%
  • 32xx: Ni 1.75%; Cr 1.07%
  • 33xx: Ni 3.50%; Cr 1.50%, or 1.57%

• Chromium-Molybdenum Steels:

  • 41xx: Cr 0.50%, 0.80%, or 0.95%; Mo 0.12%, 0.20%, 0.25%, or 0.30%

• Nickel-Chromium-Molybdenum Steels:

  • 43xx: Ni 1.82%; Cr 0.50–0.80%; Mo 0.25%
  • 43BVxx: Ni 1.82%; Cr 0.50%; Mo 0.12%, or 0.35%; V 0.03% min

• Chromium Steels:

  • 50xx: Cr 0.27%, 0.40%, 0.50%, or 0.65%
  • 51xx: Cr 0.80%, 0.87%, 0.92%, 1.00%, or 1.05%

• Tungsten-Chromium Steels:

  • 72xx: W 1.75%; Cr 0.75%

• Silicon-Manganese Steels:

  • 92xx: Si 1.40%, or 2.00%; Mn 0.65%, 0.82%, or 0.85%

Additionally, there are high-strength low-alloy steels and boron steels (xxBxx) that are used for specific applications requiring improved strength and hardenability.

4. Non-Systematic Classifications

In addition to the standardized classifications, there are non-systematic classifications that focus on the characteristic properties or application areas of the steel. These are typically used in industrial settings where steel types are selected based on performance requirements, such as:

• Tool Steels: These are high-carbon steels designed for making tools. They have excellent hardness and wear resistance.
• Spring Steels: Special steels used for manufacturing springs that need to return to their original shape after deformation.
• Stainless Steels: Steel containing a minimum of 10.5% chromium, offering resistance to corrosion.

Conclusion

Steel classification is essential for understanding the material’s properties and ensuring it is used in the correct application. By categorizing steel based on its alloying elements, carbon content, and production methods, engineers and manufacturers can choose the ideal material for their projects, whether it’s for automotive parts, construction, or specialized machinery. Understanding the SAE system adds another layer of precision to the classification, making it easier to identify the right steel grade for specific needs.