This is where many buyers get tripped up. The suffix letters indicate the deoxidation method —which directly affects steel quality and suitability for cold heading. Deoxidation is the process of removing that oxygen. Different methods produce different steel qualities—and they’re identified by suffix letters in the grade name.
The Suffix Code: What Each Letter Means
| Suffix | Chinese Term | English Term | What It Means | OK for Fasteners? |
|---|---|---|---|---|
| F | 沸腾钢 | Rimmed steel | Incompletely deoxidized; gas bubbles form during solidification, causing compositional segregation | ❌ Avoid for critical applications |
| b | 半镇静钢 | Semi-killed | Partially deoxidized; intermediate quality | ⚠️ Acceptable for non-critical only |
| Z (or no suffix) | 镇静钢 | Killed steel | Fully deoxidized; uniform composition, no gas evolution | ✅ Minimum standard for quality fasteners |
| TZ | 特殊镇静钢 | Special killed | Extra thorough deoxidation; finest grain size | ✅ Premium quality |
| Al (in grade) | 铝镇静 | Aluminum-killed | Killed with aluminum; fine grain structure | ✅ Best for cold heading |
Here’s how these suffixes appear in real-world specifications:
| Designation | What It Means | Should You Accept It? |
|---|---|---|
| Q235F | Rimmed steel Q235 | ❌ No — for fasteners, avoid |
| Q235b | Semi-killed Q235 | ⚠️ Only for non-critical applications (washers, low-stress parts) |
| Q235 | No suffix = killed steel (Z is implied) | ✅ Acceptable for Class 4.8 fasteners |
| 45 | Killed steel (Z) | ✅ Acceptable for 8.8-grade heat-treated fasteners |
| 45F | Rimmed steel | ❌ Not suitable for heat-treated fasteners |
| ML08Al | Aluminum-killed cold heading steel | ✅ Preferred for all cold-formed fasteners |
| ML35 | Killed steel for cold heading | ✅ Excellent for 8.8-grade cold-headed bolts |
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