BACKGROUND OF THE INVENTION
Tinplate and other coated sheet steels used in the fabrication of food containers or cans are usually manufactured by hot rolling a conventional low-carbon steel to about 0.08 inch. This product, called "hot band", is subsequently cleaned of all surface scale and cold rolled to less than about 0.020 inch, more particularly, to about 0.017 inch for beverage can end stock and to about 0.010 inch for beverage can body stock. After the initial cold rolling the cold rolled strip is annealed in a non-oxidizing atmosphere at about 1,200° F. or more to relieve the cold rolling stresses. For the respective gages mentioned a second cold reduction to about 0.012 inch and 0.004 inch is employed after annealing. The strip is then coated with a corrosion resistant material such as tin, chromium or organic coating.
While these are the steps presently employed the "hot band" may be reduced to the finish gage (i.e. about 0.012 inch or about 0.004 inch), annealed as above, and then temper rolled to achieve shape and flatness. The strip is then coated as above.
Even though the steel strip is tinned or chromium plated, and the interior surfaces of the manufactured containers are usually further coated with a protective resin, the container's life may be severely limited in the presence of some corrosive food products. For example, many carbonated beverages packed in tinplate or tin-free steel cans are known to quickly attack the container material exposed through flaws in the resin coating. Therefore, the resistance of the base steel to direct attack by the carbonated beverage, is considered to be the main factor controlling the rate of perforation at such areas of exposed metal throughout the major part of the can life.
It is generally well known in the industry that steels containing a small amount of sulfur, approximately 0.03 percent, have a substantially improved resistance to the corrosive effects of carbonated beverages. Therefore, in the manufacture of steel container products for use by the carbonated beverage industry, it has been the practice to use a resulfurized steel for the base metal, that is, a steel to which sulfur has been added to achieve a concentration of about 0.03 percent. It follows therefore that even though a carbonated beverage container may have flaws in the resin coating, thereby permitting the carbonated beverage to directly contact the base metal, the base metal being a resulfurized steel, will resist the corrosive action thereby greatly enhancing the life of the container.
Resulfurization of the steel is usually accomplished by adding elemental sulfur to the molten metal in the ladle prior to casting and rolling, with the usual aim being about 0.03 percent sulfur. This method of resulfurizing steel, i.e. direct sulfur additions, does however present numerous problems to the mill operator. For example, the desired sulfur concentration in hot rolled steel may be sufficient to physically segregate, resulting in non-uniform corrosion resistance in the final product. In the newer, commercially, continuous cast steels, sulfur levels of about 0.025 percent or higher result in substantial surface defects in the cast slabs which require excessive scarfing prior to hot rolling.
SUMMARY OF THE INVENTION
This invention is predicated upon our conception and development of a new and improved method for manufacturing a sulfur-containing sheet steel product for use in making carbonated beverage containers which overcomes the above noted disadvantages associated with the use of conventional resulfurized steel. By this process a low-sulfur steel, i.e. nonresulfurized, is hot and cold rolled to the required gage, followed by a unique surface sulfurizing anneal to produce a sheet having excellent resistance to the corrosive effects of carbonated beverages. Because the steel does not contain an appreciable amount of sulfur during the hot and primary cold rolling operations, the usual problems as noted above are overcome to virtually eliminate rejected strip. The resulting cost efficiencies are obvious.
It is therefore, an object of this invention to provide an improved process for producing sulfur containing sheet steels for use in the fabrication of carbonated beverage containers.
It is another object of this invention to provide a method for annealing cold rolled sheet products that will impart a sufficient surface sulfur concentration to the steel to render the steel more resistant to the corrosive effects of carbonated beverages.
It is a further object of this invention to improve the conventional methods of manufacturing sheet steel for carbonated beverage containers by providing a surface sulfurizing anneal thereby permitting the utilization of more conventional nonresulfurized steels through the rolling processes.
It is yet a further object of this invention to provide a new and improved sheet steel for use in the fabrication of carbonated beverage containers having a low core sulfur concentration, i.e., less than 0.025 percent and a surface sulfur enrichment corresponding to a sulfur weight gain of about 0.05 (mg/cm.2) or more.
BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENT
Since the crux of this invention resides in the unique annealing procedure following primary cold reduction of the hot rolled steel strip, the other steps of the inventive process may be substantially the same as those practiced in the prior art, and may be varied in accordance with prior art techniques. However, the primary object of the invention is to avoid the use of resulfurized steel during the hot rolling and primary reduction operations and, therefore, conventional nonresulfurized steel is the required starting material if the advantages of this invention are to be fully realized. Accordingly, the preferred embodiment of this invention requires the casting and hot rolling of a conventional low-sulfur steel. The typical composition for such a steel would be carbon 0.05 to 0.12 percent, manganese 0.30 to 0.60 percent, phosphorus 0.015 percent (max.), sulfur 0.014 to 0.025 percent, the balance being iron, with other minor impurities in the usual residual amounts.
As noted above, the process of this invention is commenced in accordance with prior art practices. Hence, the nonresulfurized steel, as described above, is cast into ingot form and hot rolled in accordance with prior art practice to conventional hot band gage, i.e. about 0.08 inch. The extent of hot reduction is not critical. Thereafter, the hot rolled steel is cleaned to remove all surface scale and then cold rolled by prior art techniques, as previously described, to about 0.017 in. or 0.010 in. Although other gages may be produced by our process, these gages mentioned are the most common for container applications.
The crux of this invention resides in the annealing atmosphere which, in addition to being nonoxidizing, is further provided with a small amount of hydrogen sulfide sufficient to cause sulfur to diffuse into the subsurface portion of the steel strip to render a surface sulfur enrichment corresponding to a sulfur weight gain between about 0.05 and 5 mg./cm.2. Hence the steel strip, having its surface impregnated with sulfur, will be resistant to the corrosive effects of carbonated beverages as would be a conventional resulfurized steel. Yet, since high levels of sulfur are not present during the preceding rolling operations, the strip can be readily rolled without the attendant problems associated with rolling a resulfurized steel.
As in prior art practices, the annealing step in this inventive process may be performed in a continuous strip annealing furnace, or by open-coil annealing. In either event, the strip is heated, annealed and cooled in a non-oxidizing atmosphere to prevent surface oxidation of the steel. Although any non-oxidizing atmosphere will suffice, a moderate reducing atmosphere, such as HNX, is preferred. Commercial HNX atmospheres typically contain from 2 to 10 percent hydrogen by volume with the remainder being substantially nitrogen. Minor amounts of impurity gases may be present without harmful effect. In accordance with this invention, we utilize such prior art annealing gases, but further add hydrogen sulfide thereto in amounts of at least about 0.10 percent by volume. Exposing the steel surfaces to such amounts of hydrogen sulfide at such elevated temperatures will cause sulfur to diffuse into the steel in sufficient quantities to render the steel more resistant to the corrosive effects of carbonated beverages. For optimum results, the sulfur should diffuse into the steel at a rate sufficient to yield a weight gain as sulfur between about 0.05 and 5 mg./cm.2.
Although the amount of hydrogen sulfide in the annealing atmosphere may be varied substantially, the stated minimum of 0.10 percent by volume is preferred for the purpose of maintaining other annealing parameters consistent with prior art practices. For example, in commercial continuous annealing processes, the strip is rather quickly heated to an annealing temperature, i.e. 1,200° to 1,350° F., and there maintained from about 15 to 30 seconds. In adjoining chambers, the strip is cooled to about 240° F. over a period of about 30 to 60 seconds before the annealed strip is exposed to ambient atmosphere. For such annealing durations, hydrogen sulfide concentrations as low as 0.10 percent by volume are more than sufficient to render the desired sulfur content in the strip. It follows, however, that concentrations even below 0.10 percent hydrogen sulfide could be used to advantage, especially if annealing durations and/or annealing temperatures were increased.
Although there does not appear to be a critical maximum limit for the amount of hydrogen sulfide that may be present in the annealing atmosphere, we have learned that concentrations substantially exceeding about 1.0 percent by volume do not provide any appreciable improvement. Therefore, to assure optimum results at a minimum cost, we have maintained hydrogen sulfide concentrations within the range 0.10 to 1.5 percent by volume, with a preference for range 0.3 to 1.0 percent by volume.
It is well accepted in the prior art that temperatures exceeding the recrystallization temperature (i.e. about 1,050° F.) are essential to sufficiently anneal and recrystallize the cold rolled steel strip. Therefore, the usual annealing temperature is within the range 1,200° to 1,350° F. For the purposes of the sulfurizing treatment of this invention, however, such high temperatures are not absolutely essential. That is to say, temperatures of 1,200° to 1,350° F. are suitable to effect sulfurization as taught herein, and in fact some improvement in the product may be noted by sulfurizing towards the upper end of the temperature range. Nevertheless, temperatures as low as 1,050° F. or even lower may be used to sulfurize the steel, if a recrystallizing anneal is of no concern or if a recrystallizing anneal took place in a non-oxidizing atmosphere without H2 S.
As noted above, the steel strip can be suitably sulfurized in a hydrogen sulfide containing atmosphere at exposure times as brief as about 15 seconds. Depending upon temperature and hydrogen sulfide concentration, short durations of about 15 seconds will provide a sulfur weight gain between about 0.05 and 1 mg./cm.2. Increasing the exposure time will of course increase the ultimate sulfur content of the steel surfaces, but without any significant improvement in the corrosion resistance characteristics of the final product. Exposure durations of as much as 20 minutes may provide sulfur weight gains in the steel as high as 10 mg./cm.2 or more. Although this is substantially higher than the sulfur contents in conventional resulfurized steels, the excessive amount of sulfur does not appreciably affect the corrosion characteristics of the steel in carbonated beverages. Therefore, increasing the exposure time substantially beyond 15 seconds, as is necessary for sulfurizing during conventional open-coil annealing, will neither improve nor diminish the desired corrosion resistance characteristics of the final steel product.
It is apparent that since the sulfurizing anneal of this invention need not be performed at temperatures exceeding the recrystallization temperature, and need not exceed exposure durations of about 15 seconds, the total process could be readily modified without appreciably affecting the final product. For example, the steel strip can be given a separate sulfurizing anneal below the recrystallization temperature either before or after the usual anneal. In conventional continuous annealing apparatus, therefore, it is possible to limit the sulfurizing treatment to the preheat, holding, or cooling zones thereof.
Since the sulfurization of this process is more than a superficial surface treatment, it is possible to further cold reduce the steel strip after sulfurizing. We have found that cold reductions after annealing of about 50 percent usually decrease the corrosion resistance of the final product if our sulfurization treatment is not used. Using our sulfurization treatment, second cold reductions of up to about 50 percent to 70 percent may be used without serious impairment of corrosion resistance.
The following examples are presented to aid in a more complete understanding of this invention.
Examples 1-6 simulated annealing for 1.5 minutes in the heating zone of a continuous annealing furnace. The experimental furnace used was a hinged-top Hevi-Duty electric furnace provided with a Vycor-glass tube, wherein a cold zone and a 1,200° F. zone were provided. Examples 1-3 were conducted on three different commercial, continuously cast low carbon steels having a low sulfur content of about 0.014 percent. Examples 4-6 were conducted on three different commercial, open-hearth low-carbon steels, resulfurized for tin-plate service. The carbon, manganese, phosphorus and sulfur contents are set forth in Table 1. The remainder of each steel was iron with the usual percentages or other constituents. Specimens measuring 3 × 0.75 inches were cut from about 0.009 to about 0.010 inch thick sheets received in the "full hard" condition (after primary cold reduction). The specimens were attached to nichrome wires for pulling them into and out of the hot and cold zones of the furnace. In these examples, specimens were held in the hot zone for 11/2 minutes, pulled to the unheated end of the Vycor tube and the test atmosphere purged from the system with a stream of dried nitrogen gas. After the specimens had cooled they were removed for testing. It was estimated the specimens took about 55 seconds to reach the 1,200° F. annealing temperature. The test atmosphere of HNX gas contained 7.95 volume per cent hydrogen with the remainder nitrogen. The other atmosphere, by volume per cent, contained 7.55 percent hydrogen, 91.41 percent nitrogen and 1.04 percent hydrogen sulfide. When the furnace was at temperature the specific gas mixture was passed through the Vycor tube at about two volume changes per minute.
In order to compare results, the annealed test specimens were weighed and the average weight gain in per cent was determined. Assuming this weight gain to be sulfur it was expressed in (mg./cm.2). Blue dye tests (BDT) were also conducted on all specimens. The blue dye test was developed by the American Can Company for predicting base metal performance in carbonated beverages. The test is described in "Corrosion Resistant Tin Plate for Carbonated Beverage Containers", by M. D. Mittelman, J. F. Collins and J. A. Lawson, Reg. Techn. Meetings Am. Iron Steel Inst., 1965, pp. 279-97. The test is based on a weight loss, after 96 hours immersion of specimens of the metal (surface area, two sides, of 8.0 cm.2) in a test medium made up of phosphoric acid, deionized water, and FD & C (Federal Drug and Cosmetics approved) blue dye No. 1. Weight loss in milligrams is multiplied by 1.25 for conversion to microamperes per square centimeter (μa./cm.2). The lower the BDT value, the better the product. At present, BDT values below 60 μa./cm.2 are considered acceptable by this company.
The test results are set forth in Table 1. It can be seen therefrom that for Example 1, annealing with HNX plus H2 S resulted in a much lower BDT value than with HNX. The results with Examples 2 and 3 were substantially the same. For Examples 4- 6, the BDT values showed hardly any improvement in the resulfurized steels, when annealing with or without H2 S addition to the HNX gas. This was so despite a weight gain when annealing in HNX plus H2 S. It is surprising to find that under similar test conditions, when annealing with HNX plus H2 S, the average weight gain for the Example 1-3 specimens was about twice as great as for the Example 4-6 specimens. This would indicate that diffusion of sulfur into the steel surface is an important factor in the surprising improvement in corrosion resistance to carbonated beverages for low-sulfur steel as opposed to resulfurized steel containing at least twice as much core sulfur. In other words, by following our invention teachings, the continuously cast steels need not be subjected to costly resulfurization treatments to make them available as container stock for carbonated beverages.
Example 1 was repeated as Examples 7-12 except that all tests were made with an Example 1 steel and at temperatures of 1,100°, 1,200° and 1,300° F. The atmospheres were HNX, HNX plus 0.10 percent H2 S, HNX plus 0.29 percent H2 S and HNX plus 1.04 percent H2 S. After removal from the furnace, some specimens were blue-dye tested in the as-annealed condition. Other annealed specimens were given a second cold reduction from an 0.009 inch to a 0.005 inch thickness (44 percent cold reduction) and then BDT values were determined. These tests differed also from the Example 1 tests in that they simulated the practice of our invention in the holding zone of a continuous annealing furnace rather than in the heating zone as for Example 1. To this end the specimens were pulled into the hot zone, maintained at the indicated temperature and kept there for 1.5 minutes. During this time span a specimen was heated to and maintained at temperature for 1.0 minute in HNX gas, then the hold zone simulation took place with annealing at temperature for 0.25 minute in HNX plus H2 S. Then the specimen was annealed at temperature for 0.25 minute in HNX gas. Comparison tests were run using only HNX gas. The BDT values are set forth in Table 2 for as-annealed specimens and for as-annealed, then second cold-reduced specimens. A review of Table 2 permits a number of generalizations. When tests were conducted on as-annealed specimens, the BDT values of specimens exposed to HNX plus H2 S were lower than the BDT values of specimens annealed only in HNX gas. Higher annealing temperatures tended to decrease BDT values, especially where no H2 S was used. When tested after the second cold reduction, the specimens annealed in HNX plus H2 S exhibited much smaller increases in BDT values than for specimens annealed only in HNX. For as-annealed as well as for as-annealed, then second cold-reduced specimens, the same lowest BDT value of 18 was obtained at a 1,300° F. anneal in HNX plus 1.04% H2 S. It is evident therefrom that in the preferred embodiment of our invention more than about a 50 percent cold reduction should be avoided for maximum corrosion resistance with the most economical processing. It is also evident, however, that where cold reductions in excess of 50 percent are required, our process can be readily modified to provide a good product. The most important finding is that annealing at temperature for about 15 seconds in an atmosphere of HNX plus only 0.10% H2 S yields a product having a BDT value considerably lower than when annealing only in HNX. The examples demonstrate the continuously cast steels need not be subjected to costly resulfurization treatments to yield annealed and double cold-reduced material which can be used to make carbonated beverage containers.
Example 1 was repeated as Examples 13-20, except that all tests were made at 1,200° F. with an Example 1 steel. The atmospheres were HNX and HNX plus H2 S in volume per cent of 0.10, 0.29 and 1.04 respectively. After removal from the furnace the average weight gain of the specimens was determined and the weight gain as sulfur in mg./cm.2 determined. Some annealed specimens were given a second cold reduction of about 44 percent. BDT values were determined for specimens, as-annealed and as-annealed, then second cold-reduced. These tests were conducted to determine the effect of increasing the annealing time from 11/2 minutes to 5 minutes. The test results are set forth in Table 3. It was found with HNX plus 1.04% H2 S the weight gain as sulfur increased from 1.27 mg./cm.2 to 4.25 mg./cm.2 when the annealing time was increased from 11/2 to 5 minutes. The corresponding weight increases when annealing in HNX plus 0.29% H2 S and HNX plus 0.10% H2 S were relatively small. Surprisingly, the BDT values did not differ very much from each other for the as-annealed and as-annealed, then second cold-reduced specimens when annealed for 5 minutes in HNX plus H2 S. The specimens annealed only in HNX gas showed BDT values between 43 and 48 as annealed and between 142 and 150 after second cold reduction for the annealing period from 11/2 to 5 minutes. It is evident from these examples that if for some reason any of the low-sulfur steels are annealed with HNX plus H2 S for longer periods than 11/2 minutes, there will be no impairment in BDT values. In some instances the BDT values will be somewhat lower. For economical commercial operations, our invention may be practiced to advantage with HNX plus 0.10% H2 S for about 15 seconds.
Although we have disclosed herein the preferred practice 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. ##SPC1## ##SPC2## ##SPC3##