I know the sinking feeling of receiving a shipment and wondering if the paperwork actually matches the steel sitting on your dock. As a manufacturer in China with decades of experience, I understand that trusting a piece of paper is hard when your reputation and food safety are on the line.
To verify a Mill Test Certificate (MTC), you must cross-reference the heat and coil numbers on the document with the physical tags on the material. Additionally, validate the chemical composition and mechanical properties against industry standards like ASTM A623 or EN 10202, and contact the issuing mill directly to confirm the certificate’s origin.
This process might sound tedious, but it is the only wall between you and a production disaster. Let me walk you through the specific steps I take to ensure every sheet of metal is exactly what it claims to be.
Can I cross-check the heat number with the coil tag?
I always tell my warehouse team that the label on the coil is just as important as the metal itself. If the numbers on the sticker do not match the numbers on the paper, we stop everything immediately.
The heat number is the primary link between the physical coil and the Mill Test Certificate. You can cross-check this unique identifier found on the coil’s packaging tag against the "Heat No." or "Cast No." column on the MTC to ensure the data belongs to that specific batch of steel.
When we talk about traceability in the steel industry, the "Heat Number" is king. This number represents the specific batch of molten steel from the ladle where your material was born. However, simply seeing a number on a paper is not enough. You have to play detective in your own warehouse to ensure the physical chain of custody has not been broken.
The Anatomy of a Mill Tag
The first thing I do is look at the physical tag attached to the coil or the bundle of sheets. This tag is like the passport for the material. It usually comes from the original steel mill (like Baosteel, Shougang, or my own Huajiang lines). You need to look for two specific numbers: the Heat Number (sometimes called Cast Number) and the Coil Number.
The Heat Number covers a large batch of metal, potentially hundreds of tons. The Coil Number is unique to that specific roll. I often see fake certificates where the Heat Number is correct, but the Coil Numbers listed do not exist in that batch.
Common Tag Discrepancies
I have listed a checklist below that I use when a new shipment arrives. It helps my team spot issues before we even open the package.
| Check Point | What to Look For | What It Means |
|---|---|---|
| Tag Style | Is the tag a generic white label or an official mill tag with a logo? | Generic tags are a red flag. Original mill tags usually have specific branding, barcodes, and sometimes holograms. |
| Font Consistency | Do the numbers look like they were stamped or printed at the same time? | If the Heat Number looks like it was written with a different printer or font than the rest of the text, be careful. |
| Package Markings | Does the stencil on the metal sheet (if visible) match the tag? | The ink stencil on the inner diameter of the coil must match the paper tag perfectly. |
| Barcodes | Can you scan the barcode? | A real mill tag often has a QR or Barcode that scans to a text string containing the coil number. Fake tags often have "dummy" barcodes that do not scan. |
The "Ghost Coil" Phenomenon
One specific trick I have seen traders use is the "Ghost Coil." They buy one legitimate coil of high-quality tinplate and get one real certificate. Then, they buy ten coils of cheaper, lower-quality material. They photocopy the legitimate tag and stick it on the cheap coils.
To beat this, check the Net Weight. The certificate will list the exact weight of the coil, for example, "5,432 kg". If the tag on your coil says "5,432 kg" but the scale in your factory reads "4,900 kg", the tag is likely a fake copy. Real coils rarely deviate from their tagged weight by more than a few kilograms.
Why This Matters
If these numbers do not match, you have broken the Chain of Custody 1. This means you cannot prove that the mechanical properties listed on the paper actually apply to the steel you are about to put into your press. For food safety, this is critical. If a problem happens later, like a can leaking, you need to be able to trace it back to the exact day and furnace where the steel was made. Without a matching heat number, you are flying blind.
Does the certificate show the chemical composition and mechanical properties?
I get frustrated when I see certificates that look like they were copied and pasted because they lack the specific details that make tinplate usable. A real certificate tells a detailed chemical and physical story that must align with your specific order.
A valid MTC must explicitly list the chemical elements (Carbon, Manganese, Phosphorus, Sulfur) and mechanical properties like Rockwell Hardness (HR30T) and Yield Strength. For Electrolytic Tin Plate, it must also detail the tin coating weight for both surfaces to confirm it meets the ordered specifications.
The numbers on an MTC are not just random data; they are the recipe for your steel. When I review a certificate, I am not just checking if the fields are filled in. I am checking if the recipe makes sense for the product I ordered. You need to use critical thinking to interpret these chemical and physical values.
Analyzing the Chemical Makeup
For Electrolytic Tin Plate 2 (ETP), the chemical composition is strictly controlled. The most important element I look at is Carbon.
- Low Carbon (<0.06%): If you ordered soft material for deep drawing (like a 2-piece can or a drawn-and-ironed can), the carbon should be very low.
- High Carbon (0.10%-0.12%): If you ordered stiff material for the body of a 3-piece can or an easy-open end (EOE), the carbon will be higher to provide strength.
If I see a certificate for "Double Reduced" (DR) material but the chemistry looks like soft "Single Reduced" (SR) steel, I know something is wrong. The certificate must match the physical reality of the steel grades. Also, pay attention to Sulfur. High sulfur content can lead to sulfide staining 3 (black spots) when the can is filled with protein-rich foods like fish or meat. A good certificate will show sulfur levels well controlled below 0.020%.
Tin Coating Weight Verification
This is the area where I see the most cheating. ETP is defined by its tin coating. The certificate must show the test results for both sides of the sheet.
- Equal Coating: Example: 2.8/2.8 g/m² (or EIK).
- Differential Coating: Example: 2.8/5.6 g/m² (or D 2.8/5.6).
If you ordered differential coating, the certificate must clearly show two different numbers. Furthermore, legitimate mills will often list the "Average" value and the "Minimum" value. A fake cert often just lists the target value (like exactly 2.80) for every single coil. In reality, production varies slightly. Seeing "2.81, 2.79, 2.82" looks much more real than "2.80, 2.80, 2.80".
Mechanical Properties Checklist
Here is a table of what I verify regarding the physical strength of the material.
| Property | Symbol | Why it Matters | Typical Value Example |
|---|---|---|---|
| Hardness | HR30T | Determines if the can will collapse or crack during forming. | 57 +/- 3 (for T-3 temper) |
| Yield Strength | MPa | Vital for Double Reduced (DR) materials to ensure stiffness and pressure resistance. | 550 MPa (for DR-8) |
| Tin Coating | g/m² | Prevents rust and protects food integrity. | 2.8 / 5.6 |
| Tempering | Code | Indicates the heat treatment method. | CA (Continuous Annealing) or BA (Batch Annealing) |
Regulatory Compliance and Standards
Finally, because we deal with food packaging, the certificate should mention compliance standards. I always look for references to ASTM A623 4, EN 10202 5, or GB/T 2520.
More importantly, for food safety, look for a note about FDA 21 CFR 6 or EU Regulation 1935/2004 7. If a certificate is for "Prime Food Grade" tinplate but has no mention of food safety standards, I view it with suspicion. The chemical limits for things like lead and cadmium are strict in food-grade material, and the certificate is your proof that the mill checked for them. If these references are missing, you might have bought industrial-grade steel that is not safe for food contact.
Can you send the MTC digitally before the vessel arrives?
I hate waiting until the container doors open to find out if the paperwork is right. I always insist on getting the documents while the goods are still on the water to ensure a smooth customs clearance and production start.
You should request a digital copy of the Mill Test Certificate as soon as the cargo is loaded. This allows you to verify the document’s authenticity using QR codes, digital signatures, or by contacting the mill directly before the shipment arrives, saving you from demurrage costs.
In today’s world, there is no excuse for slow paperwork. I find that the time between the ship leaving the loading port and arriving at your destination is the "Golden Window" for verification. Using this time wisely saves me money and headaches.
Digital Forensics on PDF Documents
Most certificates today arrive as PDF files. Sadly, this makes them easy to forge using software like Photoshop or PDF editors. I have learned a few tricks to spot a fake digital file that any buyer can use.
- Zoom In Analysis: I zoom in to 400% on the PDF. I look closely at the area around the Heat Numbers and the Tin Coating weights. If the background looks "pixelated," "noisy," or "dirty" while the numbers look perfectly clean and sharp, it usually means someone erased the old numbers and typed in new ones.
- Font Consistency: I compare the font of the static text (template text like "Description of Goods") with the dynamic text (variable text like "Thickness: 0.19mm"). In an original system-generated PDF, the fonts should be identical or complementary. In a fake, the specific numbers often use a slightly different font (like Arial vs. Helvetica) or a different size.
- Digital Signatures: Modern mills like Baosteel or my factory use encrypted digital signatures. If you click on the signature field in Adobe Reader, it should say "Signature is Valid" and show the signer’s identity. If it is just a flat, scanned image of a stamp, it is harder to verify, but not necessarily fake.
The Power of QR Codes
Top-tier Chinese mills are now placing QR codes on their MTCs to combat fraud. This is a game-changer for verification.
- How it works: You verify the document by scanning the QR code with your phone camera or a dedicated app.
- The Result: It takes you to a hosted page on the steel mill’s official website (e.g., baosteel.com).
- The Check: You compare the data on your phone screen with the PDF in your email. If the PDF says "2.8g coating" but the website says "1.1g coating," you know the seller changed the PDF to make the cheap material look expensive.
Verification Timeline
Here is how I structure my verification process to avoid port delays and demurrage costs 8.
| Timeframe | Action Step | Goal |
|---|---|---|
| 2 Days After Loading | Request MTC PDF from supplier. | Get the data early to review against the contract. |
| While at Sea | Check QR codes and email the Mill’s Quality Dept if suspicious. | Confirm validity before arrival. |
| Arrival Day | Compare PDF data with physical coil tags. | Final physical confirmation. |
By doing this "pre-arrival" check, I avoid the nightmare of having containers stuck at the port while I argue about paperwork. If the MTC is fake, I can stop the payment or alert the bank before the funds are released, preventing a total loss.
What should I do if the tested hardness doesn’t match the certificate?
I feel a knot in my stomach when a client tells me their production line jammed because the tinplate was too hard or too soft. When the metal acts differently than the paper says, you have to act fast to identify the root cause.
If your independent testing shows hardness values that differ from the MTC, you must immediately retain samples, check your testing calibration, and notify the supplier. You should then arrange for a third-party laboratory inspection to generate an official report for a potential claim.
Disagreements between the "Paper Value" and the "Actual Value" are common. Steel is not perfect, and neither are testing machines. However, you need to know how to handle this professionally to protect your business and maintain a good relationship with your supplier.
Step 1: Internal Verification
Before I accuse the supplier, I check my own house. Errors often happen in the testing method itself.
- Machine Calibration: Is my Rockwell hardness tester calibrated? I use a standard test block to verify the machine is reading correctly.
- Sampling Method: Did I test the right spot? The head and tail of a coil often have slightly different properties than the middle due to the speed of the line during production. I usually cut a sample at least 5 meters into the coil to get a representative reading.
- The "Anvil Effect": For very thin tinplate (like 0.15mm), if you use the wrong anvil or load, you might be measuring the hardness of the machine’s support table, not the sheet. This gives a falsely high reading.
- The "Aging" Effect: Tinplate can undergo strain aging 9. This means it gets slightly harder over time, especially if it sits in a hot warehouse (like in Thailand or Mexico). A difference of 1-2 points in hardness might be natural aging, not fraud.
Step 2: Third-Party Arbitration
If my machine is right and the steel is wrong, I call in the experts. I cannot just tell the mill "It feels wrong." I need proof.
- SGS 10 / Intertek: These are global inspection agencies. I hire them to come to the warehouse, take a sample, and test it in their certified labs.
- The Report: Their report is the "Judge." If SGS says the hardness is HR30T 65, but the MTC says 57, the mill cannot argue. This report is your golden ticket for a refund or replacement.
Step 3: Understanding Tolerances
It is important to understand that steel standards allow for a range. A "Mismatch" is only a claim if it exceeds the standard tolerance.
- Aim Value: The target might be 57.
- Tolerance: The standard usually allows +/- 3 or 4 points. So, anything from 53 to 61 is technically "good."
- The Dispute: I only file a claim if the result is outside this tolerance range. If the certificate says 57 and I test 59, that is acceptable. If I test 64, that is a rejection.
Communicating with the Supplier
When I send the complaint, I include three things:
- Photos of the coil tag (to prove identity).
- My internal test data.
- The Third-Party Report (if available).
A good supplier will look at this data and say, "Okay, let’s solve this." They might offer a discount, or they might send a technician to check. A bad supplier will stop answering emails. This moment is the ultimate test of who you are doing business with.
Conclusion
Verifying a Mill Test Certificate is about protecting your business from risk. By checking heat numbers, validating chemical specs, using digital tools, and knowing how to handle disputes, you ensure that you get the quality you paid for. It is not just about paperwork; it is about ensuring the safety of the food inside the can and the efficiency of your production line.
Footnotes
1. Explanation of materials tracking through the supply chain. ↩︎
2. Overview of Electrolytic Tin Plate manufacturing and properties. ↩︎
3. Definition of sulfide staining corrosion in food cans. ↩︎
4. Standard specification for tin mill products and weights. ↩︎
5. European standard specifications for cold reduced tinmill products. ↩︎
6. US regulations governing substances in contact with food. ↩︎
7. European framework for materials contacting food products. ↩︎
8. Fees incurred for delayed cargo container returns. ↩︎
9. Phenomenon causing increased hardness in steel over time. ↩︎
10. Global inspection, verification, testing, and certification company. ↩︎
