US2814578A - Process of fabricating coated steel products - Google Patents

Description

P. B. WHITE Nov. 26, 1957 PROCESS OF FABRICATING COATED STEEL PRODUCTS whats-sheen Filed Dec. 7, 195;

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Nov. 26, 1957 P. B. WHITE 2,814,578

PROCESS OF FABRICATING COATED STEEL PRODUCTS Filed Dec. '7, 1953 2 Sheets-Sheet 2 FIG. 21. FIG. 4--

FIG. 5. FIG. 5.

INVENTOR. PAUL B. WH/TE',

his Attorney.

Patented Nov. 26, 1957 ice PROCESS OF FABRICATING COATED STEEL PRODUCTS Paul B. White, Gary, Ind, assignor to United States Steel Corporation, a corporation of New Jersey Application December 7, 1953, Serial N 0. 396,715 2 Claims. (Cl. 148-12) This invention relates to the production of thin gauge, low-carbon sheet steel suitable for products requiring severe bending or seaming operations and particularly material having a protective coating of tin, lead, zinc or lacquer, the application of which involves heat. Examples of such products are metal containers, roofing sheets or the like.

The ordinary method of making protectively coated sheet steel such as tin-plate includes hot-rolling steel strip to intermediate gauge, pickling, cold-rolling to final gauge, cleaning, annealing, temper-rolling, cleaning, pickling and coating. The tin-plate made by this method performs satisfactorily under most normal fabricating operations. A certain number of failures by fracture occur, however, because of so-called brittle plate, in making the side-seam of ordinary containers or in the use of keyopening containers having hinge covers wherein the hinge portion is subjected to repeated bending back and forth. The side-seams of containers, for example, are subject to failure when slightly crushed or expanded in testing. The acid-cleaning to which the material must be subjected causes hydrogen entrapment and the steel used, furthermore, is of an aging character. Both these factors contribute to the brittleness defect, the aging being accelerated by the heating incident to coating. There is almost no observable difference, however, in analysis, ferrite grain size, gauge thickness, tensile strength or Rockwell hardness, between material which successfully withstands severe forming operations and that which fails either during fabrication or subsequent use. Such failures entail a substantial loss but no explanation or remedy therefor has resulted from previous laboratory investigation.

By prolonged and extensive experiment and research, I have discovered that a series of properties of the steel base have a hitherto unsuspected effect on the ability of tin-plate to stand up under severe deforming operations. I designate these properties collectively as the carbide characteristic since they all relate to the iron carbides which precipitate at the grain boundaries in steel, to a greater or less extent and in a variable pattern, depending on the manner in which the steel is cooled through the transformation range and on the treatment subsequent thereto. The carbide characteristic, as I employ the term herein, includes the number or frequency of agglomerations of carbide observable in a photomicrograph of a specimen of sheet steel, the shape and size of such agglomerations and the pattern in which they are distributed. I have found that tin-plate, in order to successfully withstand flatbending and other severe deformations involved in the fabrication and use of containers such as those mentioned above, should have a carbide characteristic marked by a relatively low frequency or a small number of carbide agglomerations of a small size and generally spheroidal shape rather than angular, distributed substantially uniformly, and not noticeably collected in groups.

- I have invented a method of making sheet steel whereby the various factors or steps affecting the carbide characten'stic of the finished product are so controlled that such characteristic will meet the requirements stated above for tin plate capable of withstanding fiat bending and the other deformations incident to container fabrication and use. Among such factors are the temperature range in which the final hot-rolling of the strip is performed, the amount of the final draft on the strip in the hot mill and, most important of all, the temperature range in which the hot-rolled strip is coiled before final slow cooling to atmospheric temperature. I have further discovered that if such factors are properly controlled, subsequent processing (pickling, cold-rolling, cleaning, annealing, temperrolling, cleaning, pickling and coating), will not affect the carbide characteristic thus imparted, so long as the material is not again heated above its lower critical temperature. My invention may therefore be considered as a method of making strip suited for difficult fabrication, whereby the carbide formation is controlled so that the final product develops and retains the desirable carbide characteristic as explained above.

More particularly, I have discovered that low-carbon steel strip given a suitable reduction in the final stand of a continuous hot mill, at the proper temperature, if quenched Ito a temperature substantially below the final rolling temperature and coiled, will exhibit the desired carbide characteristic after further processing into tinplate by pickling, cold-rolling, cleaning, annealing, temperrolling, cleaning and coating. A complete understanding of the invention may be gained from the following detailed description and explanation of the preferred practice and by reference to the accompanying drawings and photomicrographs. In the drawings,

Figure l is a schematic diagram illustrating certain of the steps in making sheet steel according to my invention;

Figure 2 is a set of curves showing the variation of strip temperature between hot rolling and coiling;

Figures 3, 4 and 5 are photomicrographs of diiferent examples of the product of my invention; and

Figure 6 is a photomicrograph of a product which does not possess the desired carbide characteristic, for the purpose of contrast.

Referring now in detail to the drawings and, for the moment, to Figure 1, I reduce a steel slab 10 of suitable composition to strip 11 by passing it through a continuous mill 12 composed of a series of roughing stands 13 and a series of finishing stands 14. The steel should be of lowcarbon, rimmed or mechanically capped steel (i. e., aging steel) containing less than .12% carbon and preferably from .05 to .10% carbon with from .25-.50% manganese, less than .15% phosphorus, less than 050% sulphur and the balance substantially iron except for the usual undesir able elements in the maximum amounts as follows: .0l% silicon, .2% copper, .l% nickel and .10% chromium. The last finishing stand of mill 12 should be adjusted K0 take a draft of from 10 to 15%, reducing the strip to an intermediate gauge of from .065" to .100". The temperature of the slab at the start of rolling and its progress through the mill should be so correlated that the strip is at a temperature between 1500 and 1650 F., preferably about 1560 F., when it leaves the last stand 14.

The strip 11 is delivered by the hot mill to a run-out table 15 at the far end of which is a coiler 16. Before reaching the coiler, however, the strip is quenched by high-pressure water sprays 17 to a temperature substantially below that at which it leaves the mill. This quenching has the effect of preventing the material from subsequently acquiring an undesirable carbide characteristic if not reheated above the lower critical temperature. To this end, the strip should be coiled only when it has been rapidly quenched to a point between 1000 and 1125 F. and preferably between 1025 and 1075 F. or about 1050 F. The sprays 17 should be sufiicient in number and the pressure of the water adequate to reduce the tem perature of the strip rapidly and progressively throughout its length, in a cooling curve lying between those shown in Figure 2, the rate of cooling preferably being of the order of 100 F. per second. When the strip is quenched to within the temperature range stated, the desired carbide characteristic is fixed and the strip may be further cooled to atmospheric temperature in any convenient manner without affecting the carbide characteristic of the final product.

For producing sprays 17, I employe nozzles having an arcuate discharge mouth in the form of a narrow slot creating a fan-shaped sheet of water transverse to the length of the strip and extending substantially the full width thereof. In a specific example, I employed a series of thirty nozzles above the strip and a similar number below, spaced along about 150' of the length of the runout table. Each nozzle 'had an outlet about 6" x A" and was supplied with water at about 100 p. s. i. -It is important that the sprays have suificient impact -on the strip to penetrate the film of stream forming immediately adjacent the surfaces thereof, in order to insure effective quenching progressively or on the fly as the strip passes from the mill to the coiler.

The hot-rolled strip made and coiled as indicated above, is then processed into tin-plate in the known manner by uncoiling, pickling, cold-rolling to final gauge, cleaning, annealing, temper-rolling, cleaning and coating. Each of these steps may be performed in any of the ways currently practiced, so long as one precaution is observed, viz., the material must not be heated above its lower critical temperature, i. e., 1300-1350 F. The preferred carbide characteristic, i. e., small agglomerations, few in number, spheroidal in character and dispersed in distribution, will be obtained from the practice explained and maintained through further processing. Departure from the practice outlined or reheating to a temperature above the lower critical, will cause the carbides to be too numerous, too large, angular in shape or arranged in groups, any one of which will Prevent the material from satisfactorily withstanding severe deformation.

Figures 3, 4 and are photomicrographs at a magnification of 500, of specimens of material made in accordance with the invention. Figure 3 shows a good carbide characteristic since the carbide iagglomerations are relatively few, small in size, spheroidal in character and rather uniformly dispersed. Figure 4 shows a somewhat less desinable characteristic, since the carbides are more numerous, but it is still acceptable since they are mostly small and spheroidal and are well distributed. Figure 5 shows a satisfactory carbide characteristic marked by agglomerations more numerous than those of Figure 3 yet quite small, spheroidal and almost uniformly distributed.

Figure 6 shows, by way of contrast, a photomicrograph at the same magnification, of a specimen having an undesirable carbide characteristic. While the agglomerations are not too numerous, they are large, angular and collected in groups instead of being well distributed. Material exhibiting such a characteristic cannot be depended upon to withstand the severe deformations necessary in can fabrication without a substantial percentage of failure. This is true if any one of the specified properties is absent, i. e., low to moderate frequency or number, spheroidal shape, small size and wide distribution.

The shape of the carbides, i. e., spheroidal or angular, is a function of the coiling temperature. High coiling temperatures and the resulting long cooling cycles produce angularly shaped carbides. Conversely, the quenching to a low temperature develops spheroidal shaped carbides which remain unaffected by subsequent cooling. Thus, by controlling the temperature at which hot-rolled strip is coiled, the shape of carbide may be brought to the desirable form, providing that the steel is not subjected 4 to a subsequent heat treatment at a temperature above its lower critical temperature.

The distribution of the carbides throughout the finished hot-rolled steel appears to be a function of the temperature of the steel at the finishing pass of the hot mill, the draft taken in this pass and the temperature at which the strip is coiled. High finishing temperatures tend to produce coarse ferrite grains and it appears that, when the carbides are precipitated to the grain boundaries, the inherent quantity of carbide for the given composition of the steel is distributed to a smaller number of nuclei than would be the case with a finer grain, so the distribution tends to be grouped rather than scattered. Lower finishing temperatures, so long as they are not low enough to develop a cold-worked surface structure (large, distorted, ferrite grain), tend to produce finer ferrite grain and are believed to provide more nuclei for carbide precipitation. Thus the inherent quantity of carbide for the given composition of the steel is distributed more widely and tends toward dispersed distribution.

The draft in the finishing pass of the hot mill also has an influence on the carbide distribution in the finished steel. It is believed that the draft at the finishing pass creates strain areas in the strip and that the areas of greatest strain become nuclei for carbide precipitation. Further, it is my opinion that, as the draft at the finishing pass is increased within reasonable limits, more highstrain areas are created and these provide more nuclei for the precipitation of the carbides, tending to a dispersed carbide distribution. The temperature at which the hotrolled strip is coiled also has an influence on carbide distribution, in a manner similar to that discussed above in connection with finishing temperatures, the distribution tending to be grouped with high temperatures and dispersed with lower temperatures. is apparent that the carbide distribution may be controlled to predetermined patterns by varying the finishing temperature, draft and temperature of coiling.

The size of the carbides in the finished steel is a function of the cooling range and the rate of cooling through this range. The initial temperature at which hot-rolled material is coiled establishes the top of the range. Conventional practice has been to permit the hot-rolled material, coiled immediately after rolling, to cool unaided in the atmosphere; thus the bottom of the range is established as approximately room temperature. Therefore the carbide size is controlled by varying the temperature at which the hot-rolled steel is coiled. Coiling at high temperature makes the cooling period longer which provides a greater opportunity for the carbides to migrate and they therefore tend to agglomerate into larger formations as shown in Figure 6. Coiling at a lower temperature reduces the cooling time and the resulting carbide formation is in the direction of small size (Figure 5). Thus, by controlling the temperature at which hot-rolled steel is coiled, a predetermined carbide size in the finished material may be produced.

In the foregoing, I have broadly disclosed methods which may be applied to the production of finished steel having a predetermined carbide characteristic. It Will be noted that, in the foregoing discussion of carbide characteristic as to frequency, shape, distribution and size, the control of the temperature at which the hot-rolled steel strip is coiled is common to the control of shape, distribution and size of the carbide agglomerations and that the frequency thereof is governed by the steel analysis. The control of the coiling temperature is the most critical of all the factors involved and the additional controls mentioned serve as further refinements to improve the carbide characteristic in the finished steel and give better fabricating experience. I have found, however, that the control of the carbides afiorded by coiling at the proper temperature produces a product of improved fabricating qualities.

While I have explained the invention in detail by specific reference to the manufacture of tin plate, it should be From the foregoing it i understood that the principle thereof is equally applicable to the production of galvanized sheets, terne plate and the like. It may also be advantageous in making sheets for use as deep-drawing stock, which do not have a protective coating but are ordinarily made of steel which ages.

I claim:

1. In a process of manufacturing articles having flat bends from a protectively coated steel product, which process includes hot rolling a slab of low-carbon aging steel to an intermediate strip gauge and at a finishing temperature between 1500 and 1650 F., coiling and cooling the strip, cold-rolling the strip substantially to final gauge, annealing the cold-rolled strip, applying a protective coating to the strip accompanied by application of heat thereto, and severely fiat-bending the resulting coated product while fabricating articles therefrom, a method of improving the ability of the coated product to withstand such bending comprising dispersing carbides therein into small agglomerations spheroidal in character by rapidly quenching the hot-rolled strip from its finishing temperature to about 1050 F., and retaining this carbide characteristic in the coated product by maintaining the strip below its lower critical temperature as it is heated during the coating operation.

2. In a process of manufacturing tinplate articles having flat bends, which process includes hot rolling a slab of low-carbon rimmed steel to an intermediate strip gauge and at a finishing temperature between 1500 and 1650 F., coiling and cooling the strip, cold-rolling the strip substantially to final gauge, annealing the cold-rolled strip, tinplating the strip accompanied by application of heat thereto, and severely flat-bending the resulting tinplate while fabricating articles therefrom, a method of improving the ability of the tinplate to withstand such bending comprising dispersing carbides therein into small agglomerations spheroidal in character by rapidly quenching the hot-rolled strip from its finishing temperature to about 1050 F., and retaining this carbide characteristic in the tinplate by maintaining the strip below its lower critical temperature as it is heated during the plating.

References Cited in the file of this patent UNITED STATES PATENTS 2,309,801 Veeder Feb. 2, 1943 2,381,435 Burns et a1 Aug. 7, 1945 2,666,722 Epstein Jan. 19, 1954 OTHER REFERENCES Steel Processing, November 1948, pages 605-607. The Making, Shaping and Treating of Steel, by Camp and Francis, 6th edition, pages 873, 874, 969.

Claims (1)

1. 2. IN A PROCESS OF MANUFACTURING ARTICLES HAVING FLAT BENDS FROM A PROTECTIVELY COATED STELL PRODUCT, WHICH PROCESS INCLUDES HOT ROLLING A SLAB OF LOW-CARBON AGING STEEL TO AN INTERMEDIATE STRIP GAUGE AND AT A FINISHING TEMPERATURE BETWEEN 1500 AND 1650*F., COILING AND COOLING THE STRIP, COLD-ROLLING THE STRIP SUBSTANTIALLY TO FINAL GAUGE, ANNEALING THE COLD-ROLLED STRIP, APPLYING A PROTECTIVE COATING TO THE STRIP ACCOMPANIED BY APPLICATION OF HEAT THERETO, AND SEVERELY FLAT-BENDING THE RESULTING COATED PRODUCT WHILE FABRICATING ARTICULES THEREFROM, A METHOD OF IMPROVING THE ABILITY OF THE COATED PRODUCT TO WITHSTAND SUCH BENDING COMPRISING DISPERSING CARBIDES THEREIN INTO SMALL AGGLOMERATIONS SPHEROIDAL IN CHARACTER BY RAPIDLY QUENCHING THE HOT-ROOLED STRIP FROM ITS FINISHING TEMPERATURE TO ABOUT 1050*F., AND RETAINING THE CARBIDE CHARACTERISTIC IN THE COATED PRODUCT BY MAINTAINING THE STRIP BELOW ITS LOWER CRISTICAL TEMPERATURE AS IT IS HEATED DURING THE COATING OPERATION.

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