US3174917A - Method of making tin plate - Google Patents

Description

March 23, 1965 Filed July 10. 1961 A. LESNEY ETAL 3,174,917

METHOD OF MAKING TIN PLATE 2 Sheets-Sheet 1 ANDREW LES/V5) 0/70 R/CHARD A NE/SH March 23, 19 65 A, LESNEY ETAL 3,174,917

METHOD OF MAKING TIN PLATE Filed July 10. 1961 2 Sheets-Sheet 2 any NEAL ELECTROFLATE .05 .l5/b Tin /bb TTi-E m E F0802 0.01 0.08 10/1 Sn 0J0 fa 0,90 [0/ F050 0.05 to 0./5/b/bb INVENTORS ANDREW LES/V5) and RICHARD A. IVE/SH Afforney I 3,3 74,01? Patentd hliar. 23, 1965 has 3,174,917 METHOD F MAKING TIN PLATE Andrew Lesney, Frazer Township, Allegheny County,

and Richard A. Neish, Snowden Township, Allegheny County, Pa., assignors to United States Steel Corporation, a corporation of New Jersey Filed July 10, 1961, Ser. No. 122,957 1 Claim. (Cl. 204-37) This invention relates to a method for producing sheet steel having exceptional resistance to corrosion, either in the as-produced condition or after the application of a coating of tin of normal thickness, e.g., 0.50 pound per base box.

This is a continuation-in-part of our application Serial No. 49,371, filed August 12, 1960.

Tin-plate users are constantly seeking a higher-quality product, particularly in respect to corrosion resistance. There is, furthermore, a wide range of potential uses for sheet steel without a tin coating of normal thickness thereon, but for the corrodibility of the product. Accordingly, it is an object of our invention to provide a novel sheet steel having exceptional rust resistance comprising a base of sheet steel covered with a thin, adherent coating of FeSn alloy. We have discovered a method for making sheet steel characterized by high resistance to rusting during prolonged storage under humid conditions, which may be used as such for many purposes without further protection or may be further coated with tin by methods conventional in making tin plate.

Our sheet steel may be easily produced by depositing an electrolytic tin coating of 0.05 to 0.15 pound per base box (hereinafter abbreviated as lb./bb.) on a base of sheet steel and then heating it to 1000 F. or more and preferably 1150 F. or more, in a nonoxidizing atmosphere. The heating may conveniently be effected in a continuous, tower-type annealing furnace. The tin coating may be applied to steel strip that has been cold-reduced to final gage before it is annealed. The heating of the lightly tin-plated steel strip to 1000 F. or more efiects annealing of the strip and simultaneously the conversion of the deposited tin to the alloy, FeSn.

It has long been known that a tin coating of less than about 0.25 lb./bb. (i.e., 0.000015 inch thick) was insufficient to give sheet steel satisfactory corrosion resistance. Commercial tin plate usually carries either a 0.50-lb./bb. coating on both sides or 1.00-lb./bb. coating on one side and a 0.25-lb./bb. coating on the other. Application of coatings of tin thinner than 0.25 lb./bb. for special purposes, however, has also been suggested.

In the production of electrolytic tin plate according to methods now in commercial use, a tin coating of 0.25 to 1.00 lb./bb. is deposited on the steel base, which has previously been annealed. The strip is then heated, usually by electrical resistance or induction, to a temperature slightly above the melting point of tin but in no event greater than about 500 F. This causes the tin to melt and gives the product a bright, attractive appearance. In this conventional practice, a thin layer of the melted tin adjacent to the basis steel reacts or combines with the steel to form a tin-iron alloy, FeSn In the conventional practice, the flow-brightened tin plate is then rapidly cooled or quenched to a temperature at least below the melting point of tin. Since the reaction between iron and tin to form FeSn proceeds rapidly at temperatures above the melting point of tin, there is danger that much of or all the electro-deposited tin will alloy with the iron and thus leave on the surface of the product an amount of unalloyed tin less than that required to insure satisfactory performance during the subsequent soldering of the tin plate by the user, e.g., to form cans. Hence the use of higher temperatures; such as 500 F. or more, is avoided as well as prolonged holding times at temperatures above the melting point of tin.

Thus conventional electrolytic tin plate exhibits a threelayer structure: (1) the steel base; (2) a thin (0.02 to 0.10 lb./bb.) layer of FeSn and (3) an overlying layer of unalloyed tin (0.15 to 0.98 lb./bb.). Cans made of conventional tin plate having about 0.98 lb./bb. of free tin exhibit an average pack life (the time at which 50% of the cans in a pack will have failed) of about weeks at 100 F. when filled with grapefruit juice. To obtain an indication of pack life without conducting a time-consuming pack test, it is customary to determine the alloytin couple (ATC) current. The ATC test is performed by detinning a known area of tin-plate sample down to the FeSn alloy layer, immersing the sample in deaerated grapefruit juice at about F. for several hours, and then determining the current flow between the sample and a tin electrode also immersed in the juice. Low ATC currents are indicative of long pack life. Conventional tin plate usually gives an ATC current of from 0.15 to 0.25 microampere per square centimeter. Differences of about 0.05 microampere per square centimeter are considered significant. Accordingly, it is a further object of our invention to produce a novel kind of tin plate having superior corrosion resistance, as indicated by an ATC current of about 0.09 microampere or less per square centimeter.

Our novel tin plate is unique in that it has a four-layer structure: (1) the steel base; (2) a thin (0.05 to 0.15 lb./bb.) layer of FeSn alloy; (3) a thin layer of FeSn alloy; and (4) an overlying layer of unalloyed tin. This product may be made by coating our steel sheet (steel base bearing a thin coating of FeSn alloy) with electrodeposited tin, then melting to tiow-brighten the electrodeposited tin.

A complete understanding of our invention may be obtained from the following detailed description and explanation, which refer to the accompanying drawings. In the drawings:

FIGURE 1 is an electron photomicrograph of a replica of the FeSn layer present on tin plate produced by con ventional methods, i.e., electrolytic tin coating and heating to just above the melting point of tin;

FIGURE 2 is an electron photomicrograph of a replica of the FeSn layer present on tin-coated steel prepared in accordance with our invention;

FIGURE 3 is a diagram representing the steps of our method and illustrating the present preferred practice in producing a corrosion-resistant steel sheet;

FIGURE 4 is a photograpth showing the result of a comparative rust test on conventional sheet steel and sheet steel made in accordance with our invention;

FIGURE 5 is a diagrammatic showing of a cross section through the novel sheet steel of our invention; and

FIGURE 6 is a similar representation of a cross section through the novel tin plate of our invention.

When iron and tin are heated together to a temperature above the melting point of tin but below 925 F., the alloy formed is FeSn At temperatures above 925 F., the alloy FeSn is formed. The two kinds of tin-iron alloy differ remarkably in microstructure, as will appear from a comparison of FIGURES 1 and 2. Each photomicrograph is at a magnification of 14,000 diameters, and each received the same primary-chromium, secondarycarbon shadowing. FIGURE 1 shows the typical individual-platelet microstructure of the FcSn alloy produced according to conventional methods, whereas FIG- URE 2 shows the FeSn layer produced in accordance with our invention.

In a. present preferred practice of our method, we take low-carbon steel strip which has been cold-reduced to tinplate gage, i.e., the material ordinarily designated black plate. After cleaning and rinsing it in the conventional manner, we pass it through a known electroplating line 10 of any convenient or desired type, as shown in FIGURE 3, to effect the electrodeposition of a very light coating of Effects of annealing atmosphere and temperature on various qualities of fin plate Amount of Sn Solder Tin in Preplated, Annealing Temp, ATC, ISV 1 Bond, Alloy as lb./bb

None. 5% Hz in HNX"... 1,200 0.15 48 47 0. 06 0.1 N2 1,200 0.04 32 19 0.03 0.1 2% H2111 HNX 1,200 0. 08 21 8 0. 01 0.1 5% H2 in HNX 1,200 0.05 13 21 0.03 0.1- 8% H in HNX 1,200 0.05 12 22 0.03 0.l 5% H2 in HNX 1,000 0.07 39 0. 0.1 5% H2 in I-INX 1, 050 0.08 35 0 0. 06 0.1 5% H2111 HNX 1, 150 0. 05 13 0. 05 0.1. 5% H in HNX 1,200 0. 04 20 0. 01 0.1 5% Hz in HNX 1, 200 0. 06 18 28 0. 02

1 Mierograms of iron dissolved when a 4-square-ineh-area of tin is exposed to approximately 1 N HzSOi for 2 hours at 80 F.

tin thereon, from 0.05 to 0.15 1b./bb., for example 0.1 lb./bb. The extreme thinness and light weight of such coatings are emphasized by comparison with the coating of about 0.5 lb./bb. or more on conventional tin plate.

On leaving the plating line 10, the strip passes successively through a rinse tank 11 and a drier 12 of a known type. The strip next enters a continuous annealing tower 13, also of known construction, where it is heated in a nonoxidizing atmosphere to a temperature of from 1000 to 1300 F., preferably 1150 to 1300 F. In passing through a tower-type annealer at a speed of about 1000 f.p.m., the strip is brought up to temperature in about 20 seconds, is held at that temperature for about 10 seconds, and is then cooled in about 60 seconds to a temperature of about 300 F. or below.

The product of the method described above may be used as such. Without further tinning, the product shows no rust after four weeks exposure to an atmosphere having a relative humidity of 95% and a temperature of 100 F. We have, furthermore, conducted an experiment in which two samples, 4 by 6 inches, were suspended in a humid atmosphere (100 F., 95% relative humidity). The first sample was conventional steel (black plate). The second sample was coated with 0.15 lb./bb. of tin and then heated, in accordance with our invention, toa temperature of 1200 F. After one week, light over-all rusting of the first sample was'apparent, but the second sample was virtually unchanged. After two weeks, the first sample was heavily coated with rust, but the second sample was almost totally bright and unrusted. FIGURE 4 shows the two samples after 10 weeks.

The product of the method described above may also be passed through a second conventional tinning line (not shown) to apply additional tin to bring the total up to 0.25, 0.50, 0.75, or 1.0 lb./bb. For this purpose the FeSn-coated sheet steel described above is further processed by temper rolling, cleaning, pickling and electrolytic tin plating to the desired total weight of tin coating, e.g., 0.50 lb./bb., followed by melting, quenching and other conventional steps in tin-plate manufacture. The finished product exhibits a greatly improved alloy-tincouple current and iron solution value. In one instance, :an ATC current of 0.02 .a./ :m. and an ironsolution value of 13 were obtained, compared to 0.27 a/cm. and 179 for the control samples of conventional electrolytic tin plate. In the case of both factors, corrosion is proportional to the magnitude thereof.

Additional data obtained when the effects of the annealing atmosphere and holding temperature were studied are shown in the following table. The first sample treated was a control, i.e., conventional electrolytic tin plate hav- In the production of our novel sheet steel coated with FeSn alloy, the use of tin coatings lighter than 0.05 lb./bb. is to be avoided because such lighter coatings are not consistently effective. Although coatings heavier than 0.15 lb./bb. could be used, they have no economic advantage.

The atmosphere in which the lightly tin-coated steel is heated must be nonoxidizing or the product will have an undesirable blue or black oxidized appearance. Suitable atmospheres include HNX gas (consisting of 0.1% maximum water vapor, 2 to 16% hydrogen, remainder essentially nitrogen with usual commercial impurities such as up to 1.0% carbon monoxide and up to 0.1% of carbon dioxide), high-purity nitrogen, argon, ammonia, etc. Since HNX gas containing about 5% or more of hydrogen is commonly used for continuous annealing of black plate and is readily available, we prefer to use it in our process.

The temperature to which the lightly tin-coated steel is heated must be at least 1000 F. or more, to ensure conversion of the tin to FeSn alloy. The FeSn alloy characterizing our novel sheet steel and tin plate is not formed at temperatures below about 925 F., and for best results, the temperature to which the lightly tinned steel is raised should be in the range of about 1150 to about 1300" F. If FeSn-coated steel is to be further processed to fully protected tin plate, the results of the experiments reported in the table above indicate the desirability of using temperatures of 1150 F. or more, to obtain desirably low ATC current values indicative of improved corrosion performance.

The product of our method, of course, is sheet steel covered with the iron-tin alloy, FeSn, as shown in FIGURE 5 but the thickness of the alloy coat is limited by the extremely thin coating of tin which has been deposited on the sheet steel. It has a resistance to corrosion much better than that of black plate and, if subsequently coated with tin, as shown in FIGURE 6, superior to that of ordinary electrolytic tin plate as now produced.

Although We have disclosed herein the preferred embodiment of our invention, we intend to cover as well any change or modification therein which may be made without departing from the spirit and scope of the invention.

We claim:

A method of making tin plate which comprises (a) electrodepositing on cold reduced low-carbon steel strip prior to any heat treatment after cold-reduction thereof, a tin coating of from 0.05 to 0.15 pound per base box,

(b) then heating the coated strip to a temperature of 1000 to 1300" E, and then cooling it to a temper- 5 ature of about 300 F. in a non-oxidizing atmosphere, thereby forming on the strip a layer of the alloy FeSn, (c) then electrodepositing additional tin on the strip to bring the total tin deposited up to from 0.25 to 1.0 5

pound per base box,

(d) and again heating the strip to a temperature between the melting point of tin and 500 F., thereby forming over said FeSn layer a layer of the alloy Fesn 5 References fited in the file of this patent UNITED STATES PATENTS 2,078,868 Oplinger Apr. 27, 1937 2,215,278 SWartz et al Sept. 17, 1940 2,643,975 Neish June 30, 1953 OTHER REFERENCES Metal Progress, Continuous Furnace for Fast Annealing of Tin-Plate, February 1952, pp. 62-65.

Metals Handbook, 8th edition, 1961, p. 1134.

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