Tinplate, also known as electrolytic tinplate, refers to the common name for tin-plated thin steel sheets.
Definition
It is a cold-rolled low-carbon thin steel sheet or strip with commercial pure tin plated on both sides, abbreviated as SPTE in English.
Characteristics
It has properties such as corrosion resistance, non-toxicity, high strength, and good ductility. It combines the strength and formability of steel with the corrosion resistance, solderability, and attractive appearance of tin in one material.
Classification
- By tin-plating process: Divided into hot-dip tinplate and electroplated tinplate. Hot-dip tinplate has a thicker coating and higher tin consumption; electroplated tinplate uses an electroplating process to uniformly deposit a tin film on the steel substrate.
- By tin coating thickness: Divided into equal-thickness tinplate and differential-thickness tinplate. The former has the same tin coating weight on both sides, while the latter has different tin coating weights on the front and back.
- By application: Divided into tinplate for food packaging, beverage packaging, spray and cosmetic packaging, chemical packaging, etc.
Production Process
The process flow is: steelmaking → hot rolling → pickling → cold rolling → continuous annealing (or electrolytic degreasing → batch annealing) → temper rolling (or secondary cold rolling) → steel coil preparation line → electroplating → oiling.
Development History
Tinplate was first produced in Bohemia, Europe, starting from the 14th century, mainly used to make tableware and drinking utensils. Britain first realized the industrial production of hot-dip tinplate in 1730, and the British obtained the invention patent for food cans in 1810. The United States built the first continuous electroplating tin production line for steel strips in 1937, promoting the large-scale application of tinplate.
Application Fields
It is widely used in can making, various containers for packaging food, stamping containers and other industries. It can also be used to make lids or bottoms for containers made of other materials.
Chemical Composition of Tinplate
Tinplate's composition is divided into two parts: the steel substrate (the main structural component) and the tin coating (the protective/functional layer). The substrate type varies by application (e.g., food packaging vs. industrial use), while the tin coating follows strict purity standards.
1.1 Chemical Composition of the Steel Substrate
The steel substrate is typically low-carbon steel (or ultra-low-carbon steel) to ensure good formability (e.g., for shaping into cans) and weldability. The most common substrate grades are MR (Mild Steel for Drawing), L (Low Carbon), and D (Drawing Quality), with compositions compliant with international standards (e.g., ASTM A623M, GB/T 2520).
| Component (Element) |
Content Range (Mass Fraction) |
Role & Function |
| Carbon (C) |
≤ 0.12% (MR grade: ~0.05-0.10%) |
Improves strength; too high reduces formability. Low carbon ensures the substrate can be deep-drawn (e.g., for can bodies). |
| Manganese (Mn) |
≤ 0.60% |
Enhances hardenability and tensile strength without significantly harming formability. |
| Silicon (Si) |
≤ 0.03% |
Minimizes surface defects (e.g., oxide scale) and ensures good tin coating adhesion. High Si can cause coating peeling. |
| Phosphorus (P) |
≤ 0.025% |
Lowers ductility if excessive; controlled to balance strength and formability. |
| Sulfur (S) |
≤ 0.025% |
Reduces hot workability (causes "hot shortness") if too high; strict limits prevent cracking during processing. |
| Iron (Fe) |
Balanced (≥ 99.2%) |
The base element of the steel substrate, providing structural rigidity. |
| Trace Elements |
(e.g., Al: ≤ 0.05%, N: ≤ 0.008%) |
Aluminum stabilizes nitrogen (prevents "aging" of steel), maintaining consistent mechanical properties over time. |
1.2 Chemical Composition of the Tin Coating
The tin coating must be high-purity to ensure corrosion resistance (critical for food/chemical packaging) and compatibility with coatings (e.g., lacquers). International standards (e.g., ISO 11950) mandate:
Tin (Sn) Purity: ≥ 99.85% (high-purity tin avoids impurities like lead, which are toxic for food contact).
Allowed Impurities:
Lead (Pb): ≤ 0.02% (strictly limited for food-grade tinplate to meet FDA/EFSA safety requirements).
Antimony (Sb): ≤ 0.10% (prevents brittleness in the coating).
Copper (Cu): ≤ 0.08% (avoids discoloration and coating defects).
Post-Coating Treatments (Optional):
Chromate Passivation: A thin chromium-rich layer (Cr content: ~2-10 mg/m²) is applied to enhance corrosion resistance and lacquer adhesion.
Oil Coating: A food-grade mineral oil (e.g., dioctyl sebacate) is sprayed on the surface to prevent scratches during processing.
2. Mechanical Properties of Tinplate
Mechanical properties determine tinplate's ability to withstand processing (e.g., bending, deep drawing, stamping) and end-use stress (e.g., pressure in cans, impact in transportation). They are mainly determined by the steel substrate, while the tin coating (thin, ~2-15 g/m² per side) has minimal impact on overall mechanical performance.
Key mechanical indicators and their typical ranges (compliant with ASTM A623M, GB/T 2520) are listed below, categorized by substrate grade (since grades are designed for different processing needs):
| Mechanical Property |
Testing Standard |
MR Grade (General-Purpose) |
L Grade (Low-Strength, High Ductility) |
D Grade (Deep-Drawing Quality) |
Core Function |
| Tensile Strength (σb) |
GB/T 228.1 |
320-420 MPa |
280-360 MPa |
300-400 MPa |
Resists breaking under tension (e.g., can sealing). |
| Yield Strength (σs) |
GB/T 228.1 |
180-280 MPa |
150-250 MPa |
160-260 MPa |
Prevents permanent deformation (e.g., during stacking of cans). |
| Elongation (A) |
GB/T 228.1 |
≥ 28% (A80mm) |
≥ 32% (A80mm) |
≥ 30% (A80mm) |
Measures ductility (critical for deep drawing into can bodies; higher = easier to shape). |
| Vickers Hardness (HV) |
GB/T 4340.1 |
60-80 HV |
50-70 HV |
55-75 HV |
Indicates surface hardness (affects scratch resistance and stamping precision). |
| Bending Property |
GB/T 232 |
180° cold bending (d=0, no cracks) |
180° cold bending (d=0, no cracks) |
180° cold bending (d=0, no cracks) |
Ensures no cracking during folding (e.g., can lids, decorative tin edges). d=0 means bending over the same thickness. |
| ** Erichsen Index (IE)** |
GB/T 4156 |
≥ 7.0 mm |
≥ 8.0 mm |
≥ 7.5 mm |
Evaluates deep-drawing ability (higher = more suitable for complex shapes like seamless cans). |
2.1 Key Notes on Mechanical Properties
Directionality: Tinplate is a rolled product, so mechanical properties (e.g., elongation) differ slightly between the rolling direction (RD) and transverse direction (TD). Standards typically specify values for the direction with lower performance (e.g., TD for elongation) to ensure consistency.
Temper Designation: Tinplate is often labeled by "temper" (hardness/ductility level), such as T1 (soft, high ductility) to T5 (hard, low ductility). For example:
T2 temper (MR grade): Used for can bodies (requires deep drawing).
T4 temper (L grade): Used for can lids (requires rigidity for sealing).
3. Industry-Specific Requirements
Mechanical and chemical properties are tailored to end-use scenarios:
Food Packaging: Low carbon (≤0.08%), lead-free tin coating (≤0.005%), high elongation (≥30%) for shaping, and chromate passivation for corrosion resistance.
Electronics (Shielding): Higher tensile strength (≥380 MPa) for structural stability, and thin tin coating (2-5 g/m²) for conductivity.
Construction (Roofing): Thick substrate (0.3-0.5mm), high hardness (HV ≥70) for impact resistance, and heavy tin coating (10-15 g/m²) for weather resistance.
By adhering to these composition and property standards, tinplate balances functionality, safety, and processability across diverse industries.
| Mechanical Properties |
| Temper Grade |
Hardness(HR30Tm) |
Yield Strength(MPa) |
| T-1 |
49±3 |
330 |
| T-2 |
53±3 |
350 |
| T-3 |
57±3 |
370 |
| T-4 |
61±3 |
415 |
| T-5 |
65±3 |
450 |
| T-6 |
70±3 |
530 |
| DR-7M |
71±5 |
520 |
| DR-8 |
73±5 |
550 |
| DR-8M |
73±5 |
580 |
| DR-9 |
76±5 |
620 |
| DR-9M |
77±5 |
660 |
| DR-10 |
80±5 |
690 |
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