What Is Carbon Steel? Types, Grades, and Key Characteristics Explained
2025/10/7
Carbon steel is an alloy of iron and carbon, with a carbon content ranging from 0.02% to 2.14%. In addition to iron and carbon, it typically contains silicon, manganese, phosphorus, sulfur, and copper. Generally, higher carbon content increases hardness but can also make the steel more brittle.
Under JIS (Japanese Industrial Standards), carbon steel is classified based on carbon content and other properties. Each type has its own advantages and trade-offs, so it’s important to select the right steel according to its intended application.
Carbon Steel Overview
Carbon steel is an alloy based on iron and is commonly referred to simply as steel. According to JIS (Japanese Industrial Standards), iron is classified into pure iron, steel, and cast iron based on carbon content. While this classification differs slightly from common terminology, it highlights the role of carbon in determining steel properties.
As carbon content increases, iron becomes harder and more resistant to deformation, but also more brittle and prone to shattering under impact. Steel balances hardness and strength by adjusting carbon content to achieve optimal performance.
Common Name | Iron | Steel | Cast Iron |
|---|---|---|---|
Carbon Content | Less than 0.02% | 0.02% to 2.14% | 2.14% to 6.7% |
JIS Term | Pure Iron | Steel | Iron |
Carbon steel refers to steel that does not fall under alloy or special steel categories, and it is classified based on carbon content:
Low-Carbon Steel: 0.02% to 0.25% carbon
Medium-Carbon Steel: 0.25% to 0.6% carbon
High-Carbon steel: 0.6% to 2.14% carbon
SS Material (Steel Structure)
SS stands for Steel Structure, a general structural rolled steel with 0.1% to 0.3% carbon (low to medium carbon steel). SS400 is the most common grade, valued for its low cost, good machinability, and weldability. Compared to S-C material, it has lower strength and hardness. The number following “SS” indicates the minimum guaranteed tensile strength (MPa).
S-C Material (Machine Structural Carbon Steel)
S-C material contains 0.1% to 0.6% carbon, making it low to medium carbon steel. Representative grades include S45C and S50C, where the carbon content is indicated in the designation. S-C material has higher mechanical strength than SS material and responds well to heat treatment. However, it can crack during cooling and has lower weldability. S-C is commonly used for precision machinery parts and high-strength components.
SPC Material (Cold-Rolled Steel Sheet)
SPC stands for Steel Plate Cold and refers to low-carbon cold-rolled steel sheets with less than 0.1% carbon. A representative grade is SPCC. SPC is soft, ductile, and highly machinable, making it ideal for bending and forming applications. It is weaker than other carbon steels and prone to oxidation, so coating or plating is recommended after processing.
SK Material
SK, or carbon tool steel, is high-carbon steel containing 0.6% to 2.14% carbon. The abbreviation comes from S (Steel) and K (Kougu, meaning tool in Japanese), followed by two digits indicating carbon content (e.g., SK95 contains 0.95% carbon). SK material offers excellent hardness and wear resistance, making it ideal for tools such as blades. It is typically heat-treated, but hardness decreases at very high temperatures.
Summary
Carbon steel is an iron-based alloy that is neither alloy steel nor special steel.
Classified by carbon content into low, medium, and high-carbon steel.
SPC = low-carbon steel
SS & S-C = low to medium-carbon steel
SK = high-carbon steel
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