US2956906A - Enameled sheet steel - Google Patents

Description

Oct. 18, 1960 D. J. BLlcKwEDE ETAL 2,956,906

ENAMELED SHEET STEEL Filed June 22. 1959 2 sheets-sheet 2 \C`mo/v ./Z MAX. "\-57E5L 5,455

MANGANESE .20% ra 50% INVENTORS Dona/J J /ic/fwea/e John n( Frame war Mayer Unite 2,956,906 ENAMELED SHEET STEEL Donald I. Blickwede, John W. Frame, and Edward H. Mayer, Bethlehem, Pa., assignors to Bethlehem Steel Company, a corporation of Pennsylvania Filed June 22, 1959, Ser. No. 821,931

13 Claims. (Cl. 117129) This invention relates to enameled sheet steel, and more particularly to an extremely low carbon sheet steel which is to be subsequently coated with vitreous enamel.

Among the principal diliiculties encountered in enameling sheet steel is the formation of fish scale, boiling and blisters Fish scale, which is the chipping oi of small pieces of enamel coating after the firing A causes small craters or black specks to appear on the surface of the enamel, which are especially noticeable on direct-on White or color coatings.

Blisters, on the other hand, are manifest as pimples n the surface of the enamel. They are also caused by the evolution of gaseous products of the reaction between the steel and the frit. Since sheets or particles which are vitreous enameled must generally possess a smooth and unbroken surface if they are to be found acceptable, it is a prerequisite that enameled articles be substantially free from fish scales and blisters.

A further requisite for sheet steel which is to be subsequently enameled is that said sheets must exhibit satisfactory sag resistance, i.e. resistance to warping or sagging during the tiring of the enamel.

Conventional enameling iron, i.e. steel having a carbon content of :O2-.03% and a manganese content of .G7-.08%, has substantial freedom from ish scale and blisters and also exhibits fairly good sag resistance.

States Patent However, there are certain processing disadvantages due l to the composition of conventional enameling iron, these disadvantages being substantially reduced or completely w eliminated by reason of the composition of the superior enameling steel which is the subject of the present invention.

One of the ditliculties in processing conventional enameling iron occurs during the rolling of the metal into sheets and strip. Steel of compositions in which the total carbon and manganese content is less than .10%, such as conventional enameling iron, often exhibit a hot short temperature range between l650 and l900 F., since there is insufficient manganese to effectively combine with the sulfur in the metal. As hot rolling in this range may produce deep cracks on the edge of the prodl uct, it is the usual practice to complete the roughing l operations above this range, allow the steel to cool through this range, and then to resume rolling below the hotshort range. It is obvious that this cessation during hot rolling is undesirable, as close temperature control and '2,956,906 Patented Oct. 18,- 19,60

2 tinuity in the entire operation thus reduces the tonnage output of the mill.

Steel that has been subjected to cold reduction, e.g. sheet or strip, is inherently hard and fairly brittle because of internal strains produced by this cold reduction. It is therefore necessary to heat treat the steel in order to impart the desired mechanical properties thereto prior t0 subsequent forming into finished products. The two general methods of heat treatment are annealing and normalizing, both of which satisfactorily relieve the strains from the metal. Conventional enameling iron, because of a low manganese content of .D7-.08%, does not respond to ordinary annealing temperatures, e.g. 1200- l300 F., but rather must be normalized at temperatures approximating l700 F. This low manganese content is thought to be necessary for good sag resistance, and it indeed appears to be necessary for steels of the general composition of conventional enameling iron containing .O2-.03% carbon. Our` steel, on the other hand, by Virtue of Iits higher Mn content, can be annealed at l200-1300 F. with consequent advantages in the simplicity of the softening heat treatment. At the same time it has good sag resistance in spite of its high Mn content because its carbon content is so low.

Enameling iron is widely used in applications Wherein deep drawing precedes the enameling operation. AIt is therefore desirable to produce an enameling iron which is both soft and ductile. Although conventional enameling iron possesses these qualities, we believe our new steel to be generally superior for deep drawing applications.

lt is a principal object of our invention to provide a new product which not only satisfies the aforementioned requisites for steel to be enemaled, but is actually superior to conventional enameling iron in satisfying these conditions.

It is a further object of our invention to provide ja new method of making a vitreous enameled sheet steel.

Other objects will become apparent from the following description taken in connection with the drawings, in which f Fig. l is a graph showing sag versus temperature;1

Fig. 2 is a plan view of a vitreous enameled sheet embodying our invention, and

Fig. 3 is a view in transverse section of the sheet shown in Fig. 2. t f 4 Compositions which have been found to have exceptionally good qualities in achieving the objects of the present invention as above set forth come within the following approximate ranges: Y

Percent YManganese .2O-F50.

-' additional equipment are necessary, and this lack of con-V Sulfur .030 max. H Phosphorus .01 max. Remainder Essentially iron.

Although the sulfur and phosphorus contents mentioned are excellent for steel sheet which is to be deep drawn prior to thev enameling operation, other ranges areof course permissible, the ranges mentioned being for the purposes of example only, and in no way intended to limit the scope of the invention. Y

It will become apparent later in the specification that although the carbon range is given as .010% max., a lower carbon content results in an even betterproduct,

Y and the preferred embodiment of the invention is aY steel having a maximum carbon content of approximately .O03%.

One method of attaining these low carbon'levels comprises decarburizing sheet steel of e.g. .0o-.08% carbon by heating to a temperature of 1200 to,1300 F. for an vitreous enameled in the usual manner.

Steels of these compositions are free from fish scale, boiling defects, and blisters upon enameling. Furthermore, as will later be pointed out `in greater detail in connection with the drawing, these steels have at least as good sag resistance as conventional enameling iron up to l550 F., and considerably better sag resistance between 1550 and 1600 F. Because of the composition there is no hot-shortness during the rolling operation, and our steels can also be annealed at ordinary annealing temperatures.

It is well known that primary boiling defects in enameled articles are caused by the evolution of carbonaceous gases that are the products of the reaction between the iron oxide scale on the steel surface and the carbon in the steel. During the enameling operation these gases bubble through the molten frit and either carry with them bits of scale or leave a crater on the surface. Both result in poor appearance, especially in direct-on white or color coats. By maintaining a maxi` mum carbon content of .003% in the preferred embodiment of the invention, blisters are greatly minimized, and excellent results have been obtained in single coat enameling, as well as in applications when more than one coating is desired. It is also well known that fish scaling and reboiling are caused by the evolution of hydrogen from the steel. The hydrogen is formed at the enameling temperature during the tiring, and is dissolved in the steel at that temperature. At room temperature it is less soluble in the steel and hence evolves, causing fish scales or blisters. Or it may be evolved on reheating for the second coat firing in which case it is called reboiling. For some reason, perhaps the lower solubility of hydrogen in our very low carbon steel, the hydrogen dissolved in the material as a result of the ring is not as great as in ordinary enameling irons, and hence fish scaling does not occur even with direct-on white coating on both sides of the steel.

The enamel coating applied to the steel may be fired at high temperatures, e.g. between 1550 and 1600 F. In ring at such temperatures, we have found, for example, that 20 gauge steel embodying our invention (specifically having a carbon content of 003%, manganese .20% to .50%, and the remainder essentially iron) has excellent sag resistance.

Fig. 1 shows a comparison of the sag resistance of ordinary sheet steel, therein designated by solid line C, conventional enameling iron, designated by broken line A, and compositions comprising the preferred embodiment of the present invention, designated by solid line B. All these sheets were of the same gauge, in this case 20, although other gauges will, of course, have similar characteristics. The ordinary sheet steel is of the same composition as steels of the present invention except for carbon, i.e. having a manganese content of .20 to .50% and a carbon content of .06 to .08%. The conventional enameling iron has a manganese content of .07 to .08% and .02 to .03% carbon, while the preferred steel of this invention has .20 to .50% manganese and carbon of the order of .003% max. While it is obvious that both enameling iron and steel of this invention are far superior to ordinary sheet steel, it is readily apparent that our new compositions have better sag resistance than enameling iron in the temperature range 1550 .to

,4 1600 F., our steel showing an increase in sag of less than .10 in. in a 10 inch span, as measured in the center of said span between these temperature limits, while conventional enameling iron shows a sag increase of more than .60 in. in a 10 inch span in this same temperature range. The total sag of the enameling iron at 1600" F. can be seen to be more than twice as great as that of steels comprising the preferred embodiment of the present invention, while the sag of both these products is approximately the same between 1400 and 1550 F.

rPbis sag apparently occurs principally at temperatures where the steel consists essentially of ferrite and austenite, there being very little tendency to sag at lower temperatures where the steel consists essentially of ferrite. Since, as has been heretofore mentioned but will be discussed in greater detail later in the specication, it is highly desirable for the steel to have a manganese content of .20 to .50%, the carbon content of our steel is of prime importance. Due to the much lower carbon content of compositions of the present invention, a considerably higher manganese content causes no decrease in sag resistance, since the higher transformation temperature associated with a lower carbon content offsets the lowering of the transformation temperature caused by increasing the manganese, and our steel thus has better sag resistance than conventional enameling iron between 1550 and1600 F.

The high manganese content of our new steel is one of the salient features of the invention, as the aforementioned diiculties in the processing of conventional enameling iron are substantially reduced or completely eliminated by the presence of .20 to .50% manganese in the steel. Enameling iron, with a carbon content of .02 to .03%, is necessarily limited to a manganese content of .G7-.08% to insure good sag resistance, whereas the effects of higher manganese are offset by the much lower carbon in our steel. A carbon content of :O2-.03% therein is not low enough to allow a manganese content substantially greater than .G7-.08% and still impart good sag resistance.

There is no trouble with hot-shortness during the hot rolling operations, as there is suicient manganese present in the steel to efficiently combine with the sulfur to form manganese sulde, a uon-embrittling compound, thus preventing the formation of ferrous sulfide, an embrittling compound. It is therefore unnecessary to inter rupt the hot rolling of the steel to allow the metal t0 cool through the temperature range in which hot shortness occurred in prior enameling irons. Furthermore, our steel can be annealed at ordinary annealing temperatures of 1200-1300 F., thus eliminating the additional cost incurred in the normalizing necessary for conventional enameling iron, such normalizing being done at about 1700 F.

It has been found that after box annealing at ordinary temperatures of 1200-1300 F. our steel 'has slightly coarser grains, the sheet is softer, more ductile, and generally superior to enameling iron for deep drawing applications.

Although we have described our operation in considerable detail, we do not wish to be limited to the exact compositions shown and described but may use such substitutions, modiiications or equivalents thereof as are embraced within the scope of our invention or as pointed out in the claims.

We claim:

1. A vitreous enameled sheet formed from steel containing carbon .010% max., manganese .20 to .50%, sulfur .030% max., phosphorus .01% max., and the remainder being iessentially iron.

2. A vitreous enameled article formed of steel containing carbon .010% max., manganese .20 to .50%, and the remainder being essentially iron.

y3. A vitreous enameled steel sheet containing carbon .010% max., manganese .20 to .50%, and the remainder being essentially iron.

4. A vitreous enameled steel sheet containing carbon .010% max., manganese .20 to .50%, sulfur .030% max., phosphorus .010% max., and the remainder being essentially iron.

5. A vitreous enameled `article formed of steel containing carbon .003% max., manganese .20 to .50%, and the remainder being essentially iron.

6. A vitreous enameled steel sheet containing carbon .003% max., manganese .20 to .50%, and the remainder being essentially iron.

7. A vitreous enameled steel sheet containing carbon .003% max., manganese .20 to .50%, sulfur .030% max., phosphorus .01% max., remainder essentially iron.

8. An enameled sheet formed from steel containing carbon .003% max., manganese .20 to .50%, the remainder essentially iron, said sheet during the enameling operation having a maximum difference of sag at 20 gauge thickness of .10 inch in a 10 inch span between 1550 and 1600 F.

9. An enameled sheet formed from steel containing carbon .003% max., manganese .20 to .50%, the remainder essentially iron, said sheet having the property at 20 gauge thickness of sagging less than .40 in. in a 10 inch span at a temperature of 1600 F.

10. A method of making a vitreous enameled article comprising applying an enameling material to an article formed of steel containing carbon .01% max., manganese 6 .20% to .50%, and the remainder being essentially iron, and firing the coated article.

11. A method of making a vitreous enameled article comprising `applying an enameling material to an article formed of steel containing carbon .003% max., manganese .20% to .50%, the remainder being essentially iron, and firing the coated article.

12. A method of making a vitreous enameled steel sheet comprising applying an enameling material to a steel sheet containing carbon .003% max., manganese .20% to .50%, sulfur .030% max., phosphorus .01% max., remainder essentially iron, and firing the coated sheet.

13. A method of making a vitreous enameled steel sheet comprising applying an enameling material to a steel sheet containing carbon .003% max., manganese .20% to .50%, the remainder essentially iron, and tiring the coated sheet, said sheet having the property at 20 gauge thickness of sagging less than .40 inch in a 10 inch span at a temperature of 1600 F.

References Cited in the le of this patent UNITED STATES PATENTS 1,496,505 Whittaker et al. June 3, 1924 1,996,568 Butts Apr. 2, 1935 2,109,271 Krause IFeb. 22, 1938 2,455,331 Eckel et al. Nov. 30, 1948 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 2,956,906 october le, 1

Donald J Blickwede et al.,

It is hereby certified that error appears in the printed specificati of the above numbered patent requiring correction and that the said Lette Patent should read as corrected below.

Column l, line 38, for "particles" read articles column 2, line 34, for "enemaled" read enameled Signed and sealed thi 25th day of April l9l (SEAL) Attest:

ERNEST W, SWIDER DAVID L LADD Attesting Ofcer i Commissioner of Pate]

Claims (1)

1. 2. A VITREOUS ENAMELED ARTICLE FORMED OF STEEL CONTAINING CARBON .010% MAX., MANGANESE .20 TO .50%, AND THE REMAINDER BEING ESSENTIALLY IRON.

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