US3127289A

US3127289A - hoursx

Info

Publication number: US3127289A
Authority: US
Publication date: 1964-03-31
Anticipated expiration: 1981-03-31

Description

March 31, 1964 F. W. BEALL METHOD OF DECARBURIZING STEEL COIL Filed Dec. 4, 1961 Emperafure f) 2 Sheets-Sheet 2 7400 Reduc'me To Iron I300 R oxldizine +0 Iron r200 Fig. 4

INVENTOR.

FRANCIS W. BEALL,

Awausvs United States Patent 3,127,289 METHOD OF DECARBURIZING STEEL CGIL francis W. Beall, deceased, late of Middletown, Olno, by

Edythe E. Beall, exeeutrix, Middletown, Ohio, assignor to Armco Steel Corporation, Middletown, OlllO, a corporation of Ohio Filed Dec. 4, 1961, Ser. No. 157,322 4 Claims. (Cl. 148-46) The invention relates to the decarburization of steel coil and more particularly to decarburization effected in an opened coil annealing operation.

The decarburizing anneal of steel to remove the strains of cold rolling as well as to remove the carbon from the strip is a known expedient, and hitherto various procedures have been proposed for enhancing the effectiveness of the decarburizing operation. For example, it has been recognized for some time that water vapor is a particularly elfective decarburizing substance, and this is particularly true if water vapor is present in controlled amounts in gases containing hydrogen. The atmospheres usually employed are the commercially available and relatively inexpensive gases such as HNX i -8% H balance N or DX (11.5% H CO 10% CO, and the balance N Dissociated ammonia also may be employed. If HNX or DX gas are employed they are normally admixed with sulficient water vapor and hydrogen or dissociated ammonia to establish a dew point in the range of +90 to +130 F. Ideally, the most efiective gas is straight hydrogen made as wet as possible while maintaining a non-oxidizing potential; however gas mixtures of various hydrogen contents may be used with suitable dew points to attain comparable results. In any event, in all cases the major decarburization reaction is: C+H O==CO+H and ideally, the H /H O ratio should be maintained in equilibrium so that the atmosphere in the annealing furnace will be decarburizing and yet nonoxidizing at all times.

Considerable difficulty has heretofore been encountered in endeavoring to maintain the desired equilibrium. As decarburization is effected, some of the water vapor in the incoming gas will be consumed, the consumption varying with the rate of the reaction. If insufficient water vapor is present to off-set the quantity consumed, the decarburization process is slowed for want of the decarburizing reactant. On the other hand, if an excessive amount of water vapor is present, the excess moisture will cause scaling which again slows or stops decarburization until the scale is reduced. In addition, such reduced scale is an undesirable contaminant for future processing, such as enameling.

Prior to the instant invention, the customary practice for establishing the desired dew point in the furnace was to maintain the input dew point at a constant level. That is, during the decarburizing portion of the anneal the dew point of the incoming gas was adjusted to the equilibrium level either by admixing water vapor with the gas prior to its introduction into the furnace or by admitting steam along with the gas. However, the difficulty with such prior practice lies in the many variable factors which affect the rate of decarburization and hence the consumption of the water vapor. For example, such factors as the rate at which the carbon in the steel diffuses to the surface where it can react with the water vapor, the rapidity at which the reaction between the carbon and the water vapor takes place, together with the rate at which the moisture laden gas is brought into contact with the surface of the steel, all introduce variables which affect the rate of decarburization. Since ideally, the maximum rate of decarburization will occur under conditions wherein the optimum amount of water vapor is available for reaction no matter how fast or slow the decarburization may be atmosphere introduced in the furnace during 3,127,289 Patented Mar. 31, 1964 ice 2 taking place and yet insufficient water vapor is present to cause scaling, the prior art practice of maintaining the input dew point at the equilibrium level does not assure the maintenance of such conditions in the annealing furnace due to the numerous variables which are encountered and which cannot be accurately predicted.

Accordingly, a principal object of the instant invention is the provision of an improved procedure by means of which the H /H O equilibrium of the atmosphere in the furnace is automatically maintained, thereby permitting decarburization to take place at the maximum rate no matter how fast or slow the reaction may be proceeding at any given time during the decarburizing cycle.

A further object of the instant invention contemplates the control of the atmosphere in the annealing furnace during decarburization by maintaining the desired H /H O equilibrium in the exit gas, the hydrogen content and/or the dew point of the incoming gas being varied so as to maintain the desired H /H O equilibrium in the exit gas. In other words, the exit H /H O ratio is maintained constant rather than the input fi /H O ratio, as practiced by the prior art.

Still a further object of the instant invention is the provision of a process for decarburizing steel in an annealing furnace in a materially shortened decarburizing cycle utilizing a single control, namely, dew point control of the effluent gas, to maintain essentially optimum decarburizing conditions without scaling. This control is based upon the discovery that, if the exit gas is decarburizing and non-oxidizing, the steel will react accordingly irrespective of inlet gas composition due to the high recirculation rate of the atmosphere and the high rate of the water gas reaction which take place within the furnace.

Still a further object of the instant invention is the provision of decarburization-annealing procedures which may be used in conjunction with any annealing atmosphere gases which are reducing in nature, inclusive of DX gas, HNX gas, hydrogen, or dissociated ammonia, all of which are readily available and inexpensive.

The foregoing, together with other objects of the instant invention which will appear hereinafter or which will be apparent to the skilled worker in the art upon reading this specification, are accomplished by those procedures of which exemplary embodiments shall now be described.

Reference is now made to the accompanying drawings wherein:

FIGURE a side elevational view with parts broken away and in section of a coil heating furnace.

FIGURE 2 is a graph illustrating a conventional decarburization operation.

FIGURE 3 is a graph similar to FIGURE 2 but illustrating the decarburizing operation of the instant invention.

FIGURE 4 is a graph illustrating the dependence of the H2/H2O ratio upon furnace temperature.

Referring first to FIGURE 4, the graph plots H /H O equilibriums for various annealing temperatures. In order to prevent the steel from oxidizing, the atmosphere within the furnace must lie to the right of the curve, with optimum results being obtained when the Bi /H O ratio lies on the curve. Thus, Where the decarburization portion of the anneal is conducted at a temperature of 1250 F., optimum results will be achieved when the ratio of hydrogen to water vapor is 2.5:1. For a given atmosphere having a known hydrogen content, such equilibrium ratio may be initially established by elevating the dew point of the atmosphere to a predetermined value, which may be on the order of +l30 F.

In accordance with prior art practices, and as illustrated the graph shown in FIGURE 2, the moisture laden decarburiza- 2% tion would be maintained at a constant dew point !+130 F. in the example given) and it will be noted that throughout the decarburizing period the dew point of the efiluent gas is substantially below that of tron process will be slowed for reactant.

In accordance with the instant invention, it has been discovered that the H /H O ratio of the exit gas acconstant reducing potential at substantially equilibrium conditions. It is believed that the input gas, so as to thereby maintain equilibrium conditions in the efiluent gas. However, such controls are relatively complex due to the number of variables which must be calibrated. Consequently, while it is possible to control the H /H O ratio of the eflluent gases, it has also been discovered that the desired the amount of carbon being removed. However, a typical peak value for input dew point would be 1+160 F. when controlling the exit dew point at +130 F.

Essentially, the instant invention may be practiced in conjunction with known decarburizing and annealing techniques. Thus, the operation begins by the open winding of the steel strip into coils. Such open or loose winding contemplates the insertion of a spacer, which may comprise string, a metal tape, or a twisted Wire, between each convolution of the coil. This produces a uniform space between each convolution, thereby permitting free passage of the gas between the all over contact of the gas with the entire surface areas of the loose coiled strip. Commercial coiling units are avai able for this purpose.

Subsequent to the formation of the opened coil, it i transferred to the annealing base of an annealing furnace cover or furnace is vided with suitable being pro- 5. A gas inlet is prosteam will be injected into the incoming gas as the moisture content of the efiiuent gas drops below the predetermined level; and similarly, should the dew point of the efiluent gas rise above the desired level, the dew meter will act to reduce or shut off the steam flow until the desired etfiuent dew point has been reestablished.

constantly controlled annealing at- In other words, one of the reactants, namely, water vapor, is supplied at a rate equal to its consumption and demand in the decarburizing re- In addition to increasing productivity, the constant control of the exit dew point acts to prevent oxidation of the metal. Yet it has been found that exceedingly high initial relatively dry gas already present in the furnace.

The instant procedure contemplates the use of any annealing atmosphere gases which are reducing in nature, such as the aforementioned DX, HNX, dissociated ammonia or hydrogen gases and mixtures of the same. maximum efiluent gas dew point is limited only by the H /H O equilibrium at the desired annealing temperature in order to prevent scaling. As a practical matter, the maximum useful dew point will be about +l60 F. and the minimum useful dew point will be about +70 F. The desired annealing temperature will be determined by the type of steel being treated; but in general the temperature range will be from about 1100 to 1500 F. By and large, it is preferred to decarburize at a temperature of about 1250 to 1350 F. Hydrogen may be added to the common bright annealing gases during the soak period of the annealing cycle in order to increase the allowable dew point, thereby further increasing the decarburizing efiiciency.

Having thus described the invention in certain eXemplary embodiments, and with the understanding that modifications may be made therein without departing from its spirit and purpose, What is desired to secure and protect by Letters Patent is:

1. The method of controlling the hydrogen/Water vapor volume ratio of a hydrogen containing reducing atmosphere in an annealing furnace during the decarburization of steel coil, which methodcomprises maintaining the atmosphere in the furnace at a temperature of from about 1100 F. to 1500 F. during decarburization, and varying the dew point of the atmosphere as it is introduced into the furnace to the extent necessary to maintain the dew point of the effluent atmosphere at an essentially constant level, at which level the hydrogen/ water vapor volume ratio of the efiiuent atmosphere is within the range of 2.021 to 3.5 :1, the hydrogen/water vapor volume ratio at any given temperature being so chosen that the effluent atmosphere is at all times decarburizing and non-oxidizing to the steel coil being treated.

2. The method claimed in claim 1 wherein the dew point of the effluent atmosphere is maintained at the said essentially constant level until the carbon monoxide content of the effluent atmosphere due to the major decarburization reaction C+H O=CO+H has fallen below a value on the order of about one percent.

3. A method of decarburizating steel coil in a con tinuously circulating hydrogen containing reducing atmosphere having a controlled hydrogen/water vapor volume ratio, which method comprises maintaining the circulating atmosphere in the furnace at a temperature of from about 1100 F. to 1500 F. during decarburization, and maintaining the hydrogen/water vapor volume ratio of the efiiuent atmosphere at an essentially constant level by varying the quantity of moisture introduced into the entering atmosphere, the quantity of moisture introduced into the entering atmosphere being the amount necessary to maintain the dew point of the efiiuent atmosphere at from about +70 F. to about +l F., the dew point at any given temperature being so chosen that the efiluent atmosphere is at all times decarburizing and non-oxidizing to the steel coil being treated.

4. The method claimed in claim 3 wherein the dew point of the efliuent atmosphere is maintained at the chosen essentially constant level until the carbon monoxide content of the effluent atmosphere due to the major decarburization reaction C+H O=CO+H has fallen below a value on the order of about one percent.

References Cited in the file of this patent UNITED STATES PATENTS 2,287,467 Carpenter et al. June 23, 1942 2,557,379 Hancock et al June 19, 1951 2,753,170 Ness et al. July 3, 1956 3,021,237 Henke Feb. 13, 1962 3,030,240 Crombie et al. Apr. 17, 1962

Claims

1. THE METHOD OF CONTROLLING THE HYDROGEN/WATER VAPOR VOLUME RATIO OF A HYDROGEN CONTAINING REDUCING ATMOSPHERE IN AN ANNEALING FURNACE DURING THE DECARBURIZATION OF STEEL COIL, WHICH METHOD COMPRISES MAINTAINING THE ATMOSPHERE IN THE FURNACE OF A TEMPERATURE OF FROM ABOUT 1100*F. TO 1500*F. DURING DECARBURIZATION, AND VARYING THE DEW POINT OF THE ATMOSPHERE AS IT IS INTRODUCED INTO THE FURNACE TO THE EXTENT NECESSARY TO MAINTAIN THE DEW POINT OF THE EFFLUENT ATMOSPHERE AT AN ESSENTIALLY CONSTANT LEVEL, AT WHICH LEVEL THE HYDROGEN/ WATER VAPOR VOLUME RATIO OF THE EFFLUENT ATMOSPHERE IS WITHIN THE RANGE OF 2.0:1 TO 3.5:1, THE HYDROGEN/WATER VAPOR VOLUME RATIO AT ANY GIVEN TEMPERATURE BEING SO CHOSEN THAT THE EFFLUENT ATMOSPHERE IS AT ALL TIMES DECARBURIZING AND NON-OXIDIZING TO THE STEEL COIL BEING TREATED.