In this article, I will introduce what tinplate is, how temper and tin coating are specified, where can makers use it, and how it differs from tin-free steel and from galvanized sheet.
What is tinplate?
Tinplate is thin low-carbon steel sheet coated with tin. The steel provides the strength and the stiffness. The tin is the surface that meets food, a lid, a crown or a printed can. Almost all commercial tinplate is electrolytic tinplate: the tin is deposited from a solution and then melted to a bright or a matte finish. It is not stainless steel, and it is not galvanized steel. Zinc and tin are different coatings for different duties.
A related product is tin-free steel, also called electrolytic chromium coated steel. It has a very thin chromium and chromium-oxide film instead of tin. It is used for some ends, crowns and drawn cans where the specification calls for it. It is not a light grade of tinplate. If the drawing says tinplate, do not offer chromium-coated plate, and the reverse is also true.
How temper and coating are named
The base steel is blackplate, cold rolled to tin-mill thickness, then annealed and temper rolled. Single-reduced plate is reduced once before annealing. Double-reduced plate is reduced again after annealing, which makes it thinner and stronger and is used where the can maker wants to save metal. The two routes are not substitutes.
Temper is the hardness of that base steel. Under ASTM A623, single-reduced tempers run from T-1, the softest, toward T-5, the hardest, based on the Rockwell 30T test. EN 10202 names a similar idea by proof strength, with TS grades for the softer plate and TH grades for the harder plate. The equivalents are approximate because the standards do not measure the same property. Copy the temper system the can specification uses. Do not translate T-3 into a TH number unless the specification already gives that equivalent.
The tin coating is written as a mass on each side, in grams per square metre, such as an equal coating 2.8/2.8 or a differential coating with a heavier side and a lighter side. The heavier side is chosen for the face that needs it, often the outside of a printed can or the inside against a particular product. ASTM A624 is the electrolytic tinplate specification often named with A623. The exact code, equal or differential, comes from the can drawing. It is not chosen from a catalog photo.
What a complete tinplate line protects
Can lines are set up for one temper, one thickness and one coating. A near miss does not run.
A temper the die can form
T-1 and the softer TS grades are the usual conversation for deep drawing. Harder tempers, toward T-4 and T-5 or the TH grades, are the usual conversation for ends, crowns and bodies that have to hold a pressure. A hard temper in a deep-draw die cracks. A soft temper in an end may not hold the rigidity the line was set for.
A coating the product can live with
The tin mass and which side is heavier are part of corrosion protection and of how the lacquer will take. A code copied from the specification keeps the inside and the outside on the faces the can maker planned.
A width the line already has tools for
Coil width and sheet size follow the scroll or the cut the line uses. A coil with the right coating and the wrong width is still scrap to that line.
When tinplate is the right material
Tinplate is specified for food cans, beverage ends, aerosol components, crown corks, closures and some dry or industrial containers, when the specification is a tin-mill product. Sheet is the delivery when the plant cuts from flat packs. Coil is the delivery when the line unwinds. Double-reduced plate is specified when the can is designed around that higher strength and lower thickness.
It is the wrong order for outdoor structure, ducting or a part that asks for zinc. That is galvanized steel. It is the wrong order for a part that asks for 304 or 316. That is stainless. It is the wrong order when the drawing says tin-free or ECCS and the tin coating is not what the lacquer system expects.
Tinplate versus tin-free steel
Tinplate has a tin layer that can be melted to a bright finish and that solders in the ways can lines have used for decades. Tin-free steel has a chromium and oxide film, no tin, and is chosen where the specification and the lacquer are built for it. Price is not the way to choose. The can specification is.
Surface finish is a third line, independent of that choice. Bright, stone, fine stone, silver and matte are ordered as surface names. They change the look under lacquer and print. Write the finish the artwork was approved on.
FAQ
These are the lines a tin-mill enquiry is incomplete without.
Is tinplate a kind of galvanized sheet?
No. Galvanized sheet is zinc on carbon steel, usually much thicker, for building and general fabrication. Tinplate is a tin-mill product for cans and closures.
What does T-3 mean?
Under ASTM A623 it is a mid-range single-reduced temper, harder than T-1 and softer than T-5. EN 10202 uses TS and TH names instead. Use the system written on the specification.
What does 2.8/2.8 mean?
It is an equal tin coating written as grams per square metre on each side. A pair of different numbers is a differential coating. The specification, not a default, picks the pair.
Sheet or coil?
Sheet when the plant cuts from packs. Coil when the line unwinds. The tinplate series is these two forms only.
Can the temper be swapped to use a thinner gauge?
Only if the can specification allows that double-reduced or harder grade. Thickness and temper are designed together. They are not a free substitution.
What to send with the enquiry
State sheet or coil, single reduced or double reduced, the temper in the standard the specification uses, the thickness, the width or the sheet size, the coating code, the surface finish and the quantity. Name ASTM, EN or JIS if the drawing names it. The coating mass and the temper are confirmed from that specification on the enquiry, not assumed from the word tinplate.