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Tin Mill Products: Tinplate, Black Plate, TFS (Tin Free Steel) and Tin Coated Sheet

HISTORY / ELECTROLYTIC COATING / PRODUCT CHARACTERISTICS / CLASSIFICATION OF TIN MILL PRODUCTS / TEMPERS / FINISHES / BASE WEIGHTS / TIN COATINGS - STANDARD COATING WEIGHTS / TIN PLATE TYPICAL MECHANICAL PROPERTIES (Chart) / GLOSSARY

HISTORY

Tin mill products include electrolytic tinplate, electrolytic chromium coated steel (also referred to as tin free steel or TFS), and black plate, the uncoated steel. About 90% of tin mill products made in the world are used by the container industry in the manufacture of cans, ends, and closures for the food and beverage industry. The balance of production is used for automotive components, shelving, computer chassis, telecommunications cable, cookware, etc.

Tin-coated vessels are known to have existed as early as 23 A.D., but the tin was apparently only used for decoration. The first tinplate appears to have been made in Bavaria in the fourteenth century, and by the sixteenth century, a thriving tinplate industry existed in Saxony and Bohemia. Tinplate manufacture spread to England in the late seventeenth and early eighteenth centuries.

The tinplate industry as we know it today is founded on the invention of the process of preserving sterilized food for long periods of time by Nicolas Appert in 1810. By 1812, Appert was successfully packaging a variety of products in glass containers for a growing market.

The Appert process was adapted to the preservation of food in tinplate containers by John Hall in 1812 in London. By the 1820's, canned foods were widely sold in England and France, and by 1839, foods were being canned in the United States. The American can industry grew explosively as a result of the Civil War, the subsequent settling of the west, and the growth of the oil industry.

Starting in the 1880's, a series of technical innovations transformed the tinplate industry. These included the replacement of wrought iron with steel for the black plate in about 1880; the development of continuous cold reduction in 1927, that eliminated hot-pack rolling; the introduction of continuous electro-tinning on a small scale in Germany in 1934 and on a commercial scale in the United States in 1937, which replaced the hot dip process; the invention of double cold reduction in 1960; and the invention of TFS in the early 1960's in Japan and the United States.

Concurrently, many advances occurred in canmaking technology that exploited the improvements in the quality of tin mill products. Tin mill products are also used by the automotive, building, appliance, and furniture industries, all of which take advantage of the unique properties of these light-gauge steel products.

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ELECTROLYTIC COATING

Electrolytic Tinplating
Black plate (basically a clean, light gauge cold rolled steel, see Steelmaking) destined for coating goes directly from either the temper mill or double cold reduction mill to the coating line. The sequence of operations that occurs in a tinplating line, at speeds up to 2000 feet per minute, are the following:

  1. Charging the Coil - The black plate coil is placed on an uncoiling mandrel and fed into the unit. The charged coil is then welded to the preceding coil in order to run the facility continuously. Looping towers accumulate a length of material that is varied to enable the welding of two coils while not stopping the operation.

  2. Side Trimming - The material is side trimmed to the final width. The scrap is automatically baled, removed, and recycled to the steelmaking furnaces.

  3. Clean & Pickle - The strip is next electrolytically cleaned, pickled, and rinsed by running through a series of tanks to ensure the surface is clean and suitable to accept the electro-deposition of tin.

  4. Tin_plating_lineElectroplating - The strip passes through tanks containing tin anodes on either side of the strip and in electrolyte. Tin dissolves from the anode and is transported through the electrolyte to deposit on the strip. The quantity of tin that deposits on the strip is determined by the quantity of electric current flowing between the anode and the strip. To produce differential coatings, different current settings are used for each of the anode banks. The dull as-plated tinplate is sometimes referred to as having a matte finish.

  5. Melting Tower (Tin Reflow) - The electrolytic tinplate next passes through a melting tower. In this section, the temperature of the strip is raised by resistance or induction heating to just above the melting point of tin, then is immediately quenched in cold water. The tin begins to melt and reflows uniformly across the strip. The product now takes on the more typical bright or shiny surface appearance. Should a customer require a matte or unmelted tin finish, the melting tower can be turned off. When differentially coated tinplate is being produced, an identifying mark may be placed on either side just prior to melting.

  6. Chemical Treatment - A sodium dichromate solution is next applied to the electrolytic tinplate to create a light protective chromium oxide film. This passivation process protects the surface from the formation of tin oxides, which will decrease lacquerability. Two commonly used chemical treatments for electrolytic tinplate that have been adopted by the industry are:

    • Sodium Dichromate Dip (Abbr: SDCD)
      Has a moderate resistance to tin oxide formation with limited storage stability and is used where a highly passivated tin surface is not required or is detrimental to the end use. Sodium Dichromate Dip has an aim not to exceed 0.15 milligrams (mg) of chromium/sq. ft. of surface.

    • Cathodic Sodium Dichromate (Abbr:CDC)
      With the addition of a cathodic electric current, a highly passivated surface against the formation of tin oxide is provided. Cathodic Sodium Dichromate treatment normally has an aim of 0.5 milligrams (mg) of chromium/sq. ft. of surface.

  7. Oiling - A very thin film of oil is then electrostatically applied to the finished tinplate. The oil is applied to minimize scratching the tin surface in transit and handling. The primary function of oiling is not prohibiting rust formation as it is with uncoated products. The oil applied to electrolytic tinplate, almost exclusively, is acetyl tributyl citrate or ATBC. The oil is applied uniformly to both sides.

  8. tin-plate-coilRecoiling - The finished tinplate product is finally recoiled at the exit end of the electrolytic tin line on a take up reel that most commonly has a 16-1/2 inch inside diameter (ID).

  9. Quality Inspection - The entire process is monitored automatically and manually to ensure that the material conforms to specification and meets customer expectations.

Electrolytic Tin Coated Sheet
Sheet and tin mill products are differentiated by gauge. Electrolytic tinplate has a maximum thickness of 135 lb. base weight or 0.0149 inch nominal thickness. Tin coated sheet starts at 0.0150 inch nominal thickness and is produced up to 0.0359 inch nominal thickness.

Use of Chromium Coating
Chromium and chromium oxide coatings, developed for food packaging in the 1960's, offer superior lacquer adhesion and good storage properties. These coatings are mixtures of chromium metal and chromium oxides. Unlike tinplate with its multiplicity of coating weights, only a single standardized chromium-coated product is manufactured.

Electrolytic Chromium Coated Black Plate
This product, commonly referred to as tin-free steel or TFS, follows the same processing sequence as electrolytic tinplate. The coating lines are mechanically similar. During the electrolytic deposition process, chromium and chromium oxide are deposited. The metallic chromium coating on each surface is applied 5.0 milligrams (mg)/sq. ft. of area. The oxide film ranges from 0.7 to 2.0 milligrams (mg)/sq. ft., but is generally on the lower side of this range.

Unlike tin, the chromium layers cannot be reflowed, therefore a coating line dedicated to chromium coating will not have melting towers as used on the tin line to reflow the tin into a bright state.

The product is also electrostatically oiled before it exits the coating line. Historically, the industry used butyl stearate oil (BSO), which was developed for its lubricity to prevent scratching. In some instances, it has been determined that acetyl tributyl citrate (ATBC) oil used on electrolytic tinplate is more compatible with some specific lacquering and paint systems. In this case, ATBC has been preferred at the expense of the greater lubricity of the BSO.

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PRODUCT CHARACTERISTICS

Tin mill products are produced with certain standardized product characteristics, including terminology, composition, mechanical properties, surface finish, coating weights, and the like. These characteristics are covered in detail in the ASTM Standard Specifications.

CLASSIFICATION OF TIN MILL PRODUCTS

The steel industry has adopted standard product names, nomenclature, and order sequencing as published by ASTM.

Black Plate
Single Reduced Black Plate (1 CR BP) - through 135 lb. base weight
Double Reduced Black Plate (2 CR BP) - through 100 lb. base weight

Electrolytic Tin Coated
Single Reduced Electrolytic Tinplate (1 CR ETP) - through 135 lb. base weight
Double Reduced Electrolytic Tinplate (2 CR ETP) - through 100 lb. base weight
Single Reduced Electrolytic Tin Coated Sheet (1 CR TCS) - starting at 0.0150 inch nominal

Electrolytic Chromium Coated
Single Reduced Electrolytic Chromium Coated Black Plate (1 CR ECCS) - through .020 inch nominal
Double Reduced Electrolytic Chromium Coated Black Plate (2 CR ECCS) - through 100 lb. base weight

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TEMPERS

Designation

Rockwell Hardness
Aim 30-T*

Approximate Longitudinal
Yield Strength, ksi*

T-1 BA
T-2 BA
T-3 BA
T-4 CA
T-5 CA
DR-8 BA
DR-9 CA

49 +/- 4
53 +/- 4
57 +/- 4
61 +/- 4
65 +/- 4
73 Mean
76 Mean

25 - 42
34 - 46
40 - 52
48 - 60
57 - 58
70 - 85
85 - 100

*The values shown are the approximate range only and are not specification requirements.

 

FINISHES

Designation

Approximate A-A*
Microinch Range

Characteristic

5C

30 - 60

Standard blasted surface for black plate and some tinplate and TFS requirements.  For tinplate, when the tin is unmelted, and when the 5C finish is used for black plate and TFS, the finish is often referred to as “matte” finish.  On tinplate only, when the tin is melted, the finish is sometimes referred to as “silver” or “SBF”, or “brite grit”.

7A

5 or less

A lustrous, smooth surface typically found only on black plate intended for electroplating. It is currently not available in the U.S.

7B

7 - 15

A smooth finish that may contain fine grit lines.  Usually used only for tinplate that is melted after plating and then it’s often referred to as “bright” finish because the surface is very smooth and reflective.  Limited availability, only made by a few mills.

7C

12 - 22

A smooth finish with grit lines. This is the most common finish for tin mill products, especially tinplate and TFS though black plate can be ordered with this finish.  When used for tinplate that is melted after plating, or for black plate or TFS, the finish is commonly referred to as "stone" finish

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BASE WEIGHTS

Base Weight

Thickness
in/mm

Base Weight

Thickness
in/mm

53
55
57
60
63
65
68
70
73
75
78
80
83
85

0.0058 / 0.15
0.0061 / 0.155
0.0063 / 0.16
0.0066 / 0.17
0.0069 / 0.175
0.0072 / 0.18
0.0075 / 0.19
0.0077 / 0.195
0.0080 / 0.20
0.0083 / 0.21
0.0086 / 0.22
0.0088 / 0.22
0.0091 / 0.23
0.0094 / 0.24

90
93
95
98
100
103
105
107
112
115
118
123
128
135

0.0099 / 0.25
0.0102 / 0.26
0.0105 / 0.27
0.0108 / 0.275
0.0110 / 0.28
0.0113 / 0.29
0.0116 / 0.295
0.0118 / 0.30
0.0123 / 0.31
0.0127 / 0.32
0.0130 / 0.33
0.0135 / 0.34
0.0141 / 0.36
0.0149 / 0.38

For base weights not included in the table, the decimal thickness can be calculated by multiplying the base weight by 0.00011.


TIN COATINGS - STANDARD COATING WEIGHTS

Coating Weight
Designation (US) [EURO]

Nominal Tin Coating Weight
Each Surface, Lb./Base Box (US) [EURO]

(05) [0.55/0.55]
(10) [1.1/1.1]
(20/05) [2.2/0.55]
(15) [1.7/1.7]
(20/10) [2.2/1.1]
(20) [2.2/2.2]
(25/20) [2.8/2.2]
(25) [2.8/2.8]
(50/20) [5.6/2.2]
(50/25) [5.6/2.8]
(75/20) [8.4/2.2]
(50) [5.6/5.6]
(75) [8.4/8.4]
(100) [11.2/11.2]
(75/25) [8.4/2.8]
(100/20) [11.2/2.2]
(100/25) [11.2/2.8]

(0.025/0.025) [0.55/0.55]
(0.05/0.05) [1.1/1.1]
(0.10/0.025) [2.2/0.55]
(0.075/0.075) [1.7/1.7]
(0.10/0.05) [2.2/1.1]
(0.10/0.10) [2.2/2.2]
(0.125/0.10) [2.8/2.2]
(0.125/0.125) [2.8/2.8]
(0.25/0.10) [5.6/2.2]
(0.25/0.125) [5.6/2.8]
(0.375/0.10) [8.4/2.2]
(0.25/0.25) [5.6/5.6]
(0.375/0.375) [8.4/8.4]
(0.50/0.50) [11.2/11.2]
(0.375/0.125) [8.4/2.8]
(0.50/0.10) [11.2/2.2]
(0.50/0.125) [11.2/2.8]

One pound of tin per base box (shared over both surfaces) is equivalent to approximately 0.000060 inch thickness on each surface.
This list represents standard coating weights. Others available upon inquiry.

 

TIN PLATE TYPICAL MECHANICAL PROPERTIES

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GLOSSARY

Acetyl Tributyl Citrate (ATBC) Oil - A lubricating film applied to both surfaces of tin mill products to prevent abrasion during transit or handling.

Anode - The electrode, in an electrolytic process, that has the positive charge - in tin plating the metallic tin is the anode, and the steel strip the cathode, or negative electrode.

Base Weight - The term used to describe the thickness of tin mill products is base weight. The designated base weight multiplied by .00011 (see table on page 10) is the exact decimal thickness in inches of the material. Although it is still customary to use the word "pound" following the base weight designation, base weight is used only to describe material thickness and is not a measure of the weight of a base box.

Black Plate - A low-carbon cold-reduced steel intended for use in the uncoated state or for coating with tin or chromium.

Box Annealing - A process to soften steel and relieve stresses produced during cold rolling, which involves slow heating to, holding at, and cooling of coils from a high temperature. It is accomplished in a closed box and a relatively bright surface is maintained by introducing a slightly reducing gas during the annealing process.

Butyl Stearate (BSO) Oil - A lubricating film applied to both surfaces of chromium coated black plate.

Cathode - See Anode.

Chemical Treatment - A chemical or electrochemical treatment that stabilizes the tinplate surface, minimizing the oxidation of tin during storage or lacquer baking, and improves lacquer performance.

Chromium Coated Steel - Single or double reduced black plate having a coating of chromium and chromium oxide applied electrolytically. Also referred to as TFS, tin-free steel, electrolytic chromium-coated steel or ECCS.

Coating Weight - A term used in reference to the specified amount of tin per base box of plate.

Cold Reduction - The process of reducing the thickness of the strip at ambient temperature, generally accomplished by rolling through a series of four-high or six-high rolling mills arranged in tandem.

Continuous Annealing - This process consists of passing the cold reduced strip continuously and in a single thickness through a series of vertical passes within a furnace consisting of heating, soaking, and cooling zones. A slightly reducing gas is maintained in the furnace to obtain a relatively bright strip.

Differentially Coated Tinplate - Electrolytic tinplate with a different weight of tin coating on each surface.

Double Cold Reduced - A product that is given a cold reduction to an intermediate gauge then annealed and given another cold reduction to the final gauge. The resulting product is stiffer, harder, and stronger than single reduced product and offers economies, in many instances, to the consuming industry by enabling the use of lighter base weight plate.

Drawing - Forming a cup from a flat circular blank so that the cup bottom and sidewalls possess essentially the same thickness as the original blank.

Earing - The formation of scallops (ears) around the top edge of a drawn part caused by directional differences in the strength properties of the sheet metal used.

Electrolyte - The conductive medium employed in an electrolytic process.

Electrolytic Tinplate - Black plate on which tin has been electrolytically deposited.

Hot Rolling - Method of reducing thickness by hot rolling the steel (usually above 1400°F).

Ironing - Reducing can sidewall thickness by advancing a punch-supported cup through progressively smaller diameter dies or "ironing" rings.

Melted Finish - A bright or fused finish, generally on a smooth finish (#7C) base steel, though it can be on a rough finish (#5).

Pickling - A process of removing scale (iron oxide) from sheets or coils by immersion in an acid solution. The process may be accelerated by introduction of an electric current.

Rockwell Hardness Test - A test for determining temper. For most tin mill products, the hardness is measured on the 30-T scale.

Single Reduced - Steel sheet that is rolled in multiple-stand reduction mills while cold, then annealed and temper rolled to produce thin gauges for canmaking. Besides reducing gauge and permitting fabrication of lighter weight cans, cold rolling also improves the steel's surface and metallurgical properties.

Temper - A number designation to indicate the mechanical properties of tinplate.

Temper Mill - A four-high mill for rolling strip after annealing to obtain proper temper, flatness, and surface qualities. It usually consists of two stands arranged in tandem.

Tin Coating Weight - The amount of tin per base box of plate, e.g., no. 25 designates 0.25 lb. per base box (.125 lb. is distributed on each side of the strip).

Tin-free Steel - Electrolytic chromium coated black plate (TFS).

Unmelted Finish - An as-deposited tin coating that has a dull matte appearance, generally on a roughened base steel. The tin is not reflowed.

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