In steel procurement, the most reliable way to verify the material you are buying is by analyzing the Mill Test Certificate (MTC). While both S355JR and S355MC share a minimum yield strength of 355 MPa, their manufacturing standards and chemical compositions are vastly different.
Here is a breakdown of the critical differences to look for on an MTC.
1. Executive Standard (The First Indicator)
This is the most obvious difference and should be the first thing you check.
- S355JR: Must list EN 10025-2.(Technical delivery conditions for non-alloy structural steels).
- S355MC: Must list EN 10149-2.(Technical delivery conditions for thermomechanically rolled steels for cold forming).
2. Delivery Condition
This indicates the processing method used, which directly affects the steel's ability to be bent.
- S355JR: Usually marked as +AR (As Rolled) or +N (Normalized).
- S355MC: Must be marked as +M (Thermomechanically Rolled).
Note: If you see "+N" (Normalized) on an MTC claiming to be S355MC, it is likely incorrect. MC grades rely on Thermomechanical Control Processing (TMCP) to achieve fine grain structure, not normalizing.
3. Chemical Composition (The "DNA" of the Steel)
S355MC is chemically purer to ensure it does not crack during bending.
| Element | S355JR (General Structural) | S355MC (High Yield / Cold Forming) | What it means |
| Carbon (C) | Higher (≤ 0.24%) | Very Low (Often ≤ 0.12%) | Lower carbon ensures the superior weldability and toughness of MC grades. |
| Sulfur (S) | ≤ 0.035% | Extremely Low (≤ 0.010%) | Low sulfur reduces inclusions, preventing cracks during cold forming. |
| Silicon (Si) | ≤ 0.55% | Usually Low (≤ 0.03%) | MC is typically Aluminium-killed with low silicon for better surface quality. |
| Micro-alloys | Not mandatory | Contains Nb, Ti, V | Look for Niobium (Nb) or Titanium (Ti). These are essential for grain refinement in MC grades. |
4. Mechanical Properties
While both are "355" grade, the testing focus differs.
Impact Test:
- S355JR: Focuses on Impact Energy at 20°C (Room Temp, min 27J).
- S355MC: While it has excellent toughness (often tested at -20°C), the primary focus of the MTC is on ductility.
Elongation:
S355MC: Typically shows significantly higher elongation values (>23%) compared to S355JR, indicating superior ductility for stretching without necking.
5. Technological Properties (Bend Test)
This is the "Deal Breaker" section for cold forming applications.
S355JR: This section is often empty or simply marked "Satisfactory." Strict bend testing is not standard.
S355MC: Must contain Bend Test data.
You will see a Mandrel Diameter (e.g.,0.5t or 1t). This confirms the steel can be bent to a tight radius without cracking.
Summary: How to Identify in 10 Seconds?
"Check the Standard No. and the Sulfur Content."
- If the MTC says EN 10025-2 and Sulfur is around 0.025%→It is S355JR (Use for beams, columns, and general construction).
- If the MTC says EN 10149-2, Sulfur is <0.010%, and it contains Nb/Ti→It is S355MC (Use for cold bent profiles, automotive chassis, and complex shapes).