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　　In the international steel trade, navigating between different standards (such as DIN, ASTM, GB, and JIS) can be a headache for procurement officers and engineers.

　　However, the **ISO (International Organization for Standardization)** system acts as a universal language. Based largely on the European (EN) and German (DIN) systems since the late 1980s, the ISO designation allows global buyers to identify the exact properties of the metal they are purchasing.

　　Whether you are sourcing structural steel or precision machinery parts, understanding how to read these codes is essential to avoid costly ordering errors. Here is a breakdown of the ISO metal grade system.

　　1. Designations Based on Mechanical Properties

　　For structural and engineering projects, the primary concern is load-bearing capacity. Therefore, the ISO naming convention for these steels focuses on **Yield Strength** (the stress at which a material begins to deform plastically).

　　The "S" Series: Structural Steel

　　This is the most common grade found in construction and general infrastructure.

　　The Format:** It starts with the letter **S** (Structural), followed by a number.

　　The Number:** This number represents the **Minimum Yield Strength** in megapascals (MPa).

　　Example:** **S235** indicates a structural steel with a guaranteed minimum yield strength of 235 MPa.

　　The "E" Series: Engineering Steel

　　Similar to structural steel but typically intended for machine parts and engineering applications.

　　The Format:** It starts with the letter **E** (Engineering), followed by the yield strength value.

　　Example:** **E235** indicates engineering steel with a yield strength of 235 MPa.

　　Important Note: The "Old" Fe Grades vs. New ISO Grades

　　Experienced buyers might remember seeing grades like **Fe360** on older blueprints. This often causes confusion.

　　In the past, grades were designated by their *Tensile Strength* (the maximum stress before breaking), not Yield Strength.

　　Old Designation:** **Fe360** (360 refers to minimum *Tensile* Strength).

　　Current ISO Designation:** This is essentially equivalent to **E235** (235 refers to *Yield* Strength).

　　Buyer Beware:** Although the material is largely the same, the number has changed because the testing criteria changed. Always confirm if your drawing specifies Tensile or Yield strength to ensure you order the correct equivalent.

　　Additional Markings (Impact and Weathering)

　　Quality/Impact Energy:** You will often see letters at the end of the grade, such as **A, B, C, D, or E**. These indicate the temperature at which the steel’s Impact Energy (AKv) is guaranteed. This is critical if your project is located in extremely cold environments.

　　Weathering Steel:** If you see a **W** at the end of the grade, it stands for Weather Resistant. This is commonly known as "Corten" type steel, used for outdoor facades and bridges.

　　High Strength Low Alloy (HSLA) Steels

　　For heavy-duty applications, ISO uses the same "E" format + Yield Strength. This covers grades from **E355** up to **E690**.

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　　2. Designations Based on Chemical Composition

　　For steels intended for **heat treatment** (like quenching and tempering), mechanical properties change during processing. Therefore, the ISO system designates these materials based on their **Chemical Composition**, specifically Carbon content.

　　Heat-Treatable Non-Alloy Steel (Quality Carbon Steel)

　　These are widely used for manufacturing gears, shafts, and axles where hardness is required.

　　The Format:** It starts with the letter **C**, followed by a number.

　　The Number:** This number represents the **Average Carbon Content** multiplied by 100.

　　Example:** **C45**

　　C** stands for Carbon.

　　45** means the steel has an average carbon content of **0.45%**.

　　Quality Indicators

　　Just like structural steel, carbon steels have quality indicators added to the end of the grade:

　　EX:** Indicates "Quality" steel.

　　MX:** Indicates "High Quality" steel.