Carbon Steel Classifications

Overview

Engineering reference data for Carbon Steel Classifications in material science and properties.

Key Formulas

Stress

$\sigma = \frac{F}{A}$

Force per unit area.

Strain

$\varepsilon = \frac{\Delta L}{L_0}$

Change in length per original length.

Hooke's Law

$\sigma = E \varepsilon$

Stress proportional to strain in elastic region.

Thermal Expansion

$\Delta L = \alpha L_0 \Delta T$

Length change due to temperature.

Variables

Classification Methods

Carbon steels are classified using several criteria to define their properties, processing, and suitability for applications. The primary methods are:

By Composition

The carbon content is the most fundamental determinant of properties.

• Low-Carbon Steel (Mild Steel): Typically contains 0.04% to 0.30% carbon. It is ductile, tough, and easily welded and formed, making it the most common grade.
• Medium-Carbon Steel: Contains approximately 0.31% to 0.60% carbon. It offers a balance of strength, ductility, and wear resistance, often used in axles, gears, and rails.
• High-Carbon Steel: Contains roughly 0.61% to 1.50% carbon. It is very hard and strong but less ductile, used for cutting tools, springs, and high-strength wires.

By Manufacturing Process

The method of steelmaking influences inclusion content and cleanliness.

• Open Hearth (OH): A batch process allowing precise chemical control, largely superseded by more efficient methods.
• Basic Oxygen Process (BOP): A primary steelmaking process where oxygen is blown through molten iron to reduce carbon content. It is the dominant method for producing carbon steels.
• Electric Arc Furnace (EAF): Melts steel scrap using electric arcs. It is flexible for producing various grades and is increasingly used for recycling.

By Deoxidation Practice

This practice during casting determines the type of ingot structure and soundness.

• Killed Steel: Fully deoxidized with agents like silicon or aluminum, resulting in uniform composition and minimal gas evolution. Used for critical applications.
• Semi-Killed Steel: Partially deoxidized. It is more economical than killed steel and suitable for many structural applications.
• Rimmed Steel: Contains a significant amount of dissolved oxygen, which reacts with carbon during solidification to create a pure iron rim. It has excellent surface quality but variable internal composition.
• Capped Steel: Similar to rimmed steel but capped early in solidification to control the rimming action, offering a compromise between rimmed and killed steel properties.

By Finishing Method

The final rolling process defines the product's surface and mechanical properties.

• Hot Rolling: Performed above the steel's recrystallization temperature. It produces a scaled surface and is used for structural shapes, plates, and bars.
• Cold Rolling: Performed at room temperature. It improves surface finish, dimensional accuracy, and strength through strain hardening, used for sheets, strips, and bars.

By Microstructure

The phases present in the steel determine its properties.

• Ferritic: Body-Centered Cubic (BCC) iron structure, soft, ductile, and magnetic. Typical of low-carbon annealed steels.
• Pearlitic: A lamellar mixture of ferrite and cementite (Fe₃C), providing a good combination of strength and ductility. Common in medium-carbon steels.
• Martensitic: A hard, supersaturated solid solution of carbon in iron formed by rapid quenching. It has very high strength and hardness but low toughness unless tempered.

Classification Criteria Summary

The following table consolidates the primary classification criteria for carbon steels, based on the extracted data.

References

• Original Source