1.Introduction

Carbon structural steel, commonly referred to as mild steel or plain carbon steel, forms the backbone of global infrastructure and manufacturing. Among the many grades available, Q195, Q235, Q235B, and Q345 are the most widely used in China and are frequently specified in international projects. Understanding their chemical composition, mechanical properties, and global equivalents is essential for sourcing, substitution, and export compliance. This article provides a comprehensive overview of these grades and their applications in steel pipes, coils, and structural sections.

2.Classification of Carbon Structural Steel Grades

In the Chinese standard GB/T 700, carbon structural steel is classified by minimum yield strength. The letter "Q" stands for "yield" in Chinese pinyin, and the number indicates the minimum yield strength in MPa.

2.1 Q195 | Q195  
Q195 is the lowest-strength grade in the GB/T 700 series. Its low carbon content (≤0.06%) gives it excellent plasticity, toughness, and welding performance. It is typically used for cold-rolled and hot-rolled thin sheets, wire rods, welded pipes, nails, and wire products.

2.2 Q235 / Q235B | Q235 / Q235B  
Q235 is the most widely used carbon structural steel in China. It offers a good balance of strength, plasticity, toughness, and weldability. Q235 is divided into quality grades A, B, C, and D, with increasing requirements for impact toughness at lower temperatures. Q235B is the most common grade for structural applications, requiring a minimum impact energy of 27J at 20°C.

2.3 Q345 / Q345B | Q345 / Q345B  
Q345 is a low-alloy high-strength structural steel (GB/T 1591). It contains small amounts of alloying elements such as Mn, Si, V, Nb, and Ti, which enhance its strength and toughness. Q345B is the most commonly used grade, with a minimum yield strength of 345 MPa.

3. Global Equivalents of Carbon Structural Steel Grades

The following table provides approximate equivalents across major international standards. These are not exact equivalents; differences in chemical composition, mechanical properties, and tolerances should be verified for specific applications.

Abbreviations:

• ISO 630: International Organization for Standardization structural steel standard
• ASTM: American Society for Testing and Materials
• JIS: Japanese Industrial Standards
• DIN/EN: German Institute for Standardization / European Norm
• BS/EN: British Standard / European Norm
• NF/EN: French Standard / European Norm
• GOST: Russian/Soviet standard

3.1 Key Equivalence Notes:  
Q195 is a low-strength grade with no exact single equivalent in ASTM. It is often compared to ASTM A285 Gr.B or Gr.B with σs185. In JIS, SS330 is the closest match.

Q235B is most commonly equated with ASTM A36, JIS SS400, EN S235JR, and DIN St37-2. These four grades are frequently substituted for each other in structural applications.

Q345B corresponds to ASTM A572 Gr.50, JIS SM490A, and EN S355JR. The S355 series is often considered the European equivalent of Q345, with S355JR matching Q345B, S355J0 matching Q345C, and S355J2 matching Q345D.

4. Applications in Steel Pipes, Coils, and Sections

4.1 Steel Pipes

4.2 Steel Coils & Sheets

4.3 Structural Sections

5. Selection Guidelines

6. Conclusion

Q195, Q235B, and Q345B represent the three tiers of carbon structural steel strength—low, medium, and high—each with distinct chemical compositions, mechanical properties, and global equivalents. For exporters and traders, understanding these equivalences is critical for meeting international specifications, sourcing the right material, and providing accurate technical documentation. When substituting grades, always verify the specific requirements of the project standard, as “equivalent” grades may differ in impact toughness, tolerance, and traceability requirements.