US3531333A - Method of heat treating steel strip or the like - Google Patents

Description

United States Patent O "ice 3,531,333 METHOD OF HEAT TREATING STEEL STRIP OR THE LIKE Robert R. Hill, Westlake, Ohio, assignor to Wilson, Lee

Engineering Company, Inc., Cleveland, Ohio, a corporation of Ohio No Drawing. Filed June 24, 1968, Ser. No. 739,180 Int. Cl. C21d 1/26 US. Cl. 148--16.7 4 Claims ABSTRACT OF THE DISCLOSURE Coils of strip steel are subjected to heat treatment in a protective atmosphere of hydrogen and nitrogen. During the heat treatment, the percentage of hydrocarbon gas, produced from oil on the surface of the strip, in the protective atmosphere is determined. The composition of the protective atmosphere is controlled during treatment so that the percentage of the hydrocarbon gas relative to the percentage of hydrogen in the protective atmosphere is maintained below that at which carbon deposition will occur.

BACKGROUND OF THE INVENTION This invention relates to a method of heat treating metallic articles and more particularly to a method of annealing steel strip in coils.

The production of steel strip commonly includes final cold rolling to desired gauge and finish. The strip as it leaves the last pass of the cold rolling mill is coiled on a mandrel of substantial diameter to form a tightly wound coil. During the cold rolling operation, the steel loses ductility and in order for the desired ductility to be regained, the coils of steel are annealed.

During annealing the coils of steel may be left in the tight coils into which the strip is wound as it leaves the mill and wherein each successive lap of the steel strip contacts the adjacent lap, or the tight coil may be opened into an open coil wherein a space is provided between successive laps. Both tight coils and open coils are annealed in bell-type furnaces in which a coil, or a stack of coils, is supported with its axis vertical and an inner cover placed thereover. A furnace bell, carrying heating means such as combustion tubes, is placed over the inner cover and, in well-known manner, provides the heating means for raising the temperature of the strip steel to annealing value and holding it there for the desired time.

To prevent oxidation of the steel during annealing, a protective atmosphere, consisting primarily of hydrogen and nitrogen, is circulated through the inner cover in, around and, in the case of open coils, through the coils of strip steel.

In the prior art utilization of bell-type furnaces for annealing coils of strip steel, either in open or tight coil form, difficulties have been encountered in the formation of black spots which may occur anywhere on the surface of the strip in open coil annealing or adjacent the edges of the strip in tight coil annealing. These black spots are undesirable and lower the quality of the product.

In the cold rolling of steel to form coils of strip, a light oil is applied to the surface of the strip. Although as much of this oil as possible is removed prior to annealing, a residue frequently remains on the surfaces and edges of the strip and black spots may occur when a coil of such oil-contaminated strip is heated to annealing temperature.

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a method of annealing steel strip that does not Patented Sept. 29, 1970 have the disadvantages of the prior art methods. Another object is the provision of a method for annealing open or tight coils of strip steel in a furnace wherein the possibility of deposition of carbon on the surface or edges of the steel with resulting undesirable black spots is subtantially eliminated.

This present procedure proposes introducing into the furnace containing the coil-s of strip steel an inert protective gas comprising a mixture of hydrogen and nitrogen and raising the temperature of the steel to about 1150 F. (the cracking temperature of the oil but below the desired annealing temperature for the steel). The temperature is held at this point until the oil on the strip has broken down or cracked forming hydrocarbon gases, largely methane (CH At this point the percentage of methane in the atmosphere within the furnace is determined by suitable and well known procedures. Next the temperature of the coils is raised to the annealing temperature and held there for a desired soaking period while the composition of the protective atmosphere is controlled so that the percentage of methane or other hydrocarbon gas relative to the percentage of hydrogen in the enclosure will always be below that at which carbon deposition would occur on the steel strip.

DESCRIPTION OF PREFERRED EMBODIMENTS A description of the preferred embodiment of this invention will now be presented. A coil or stack of coils of strip steel is placed on the base of a conventional furnace with the coil axes vertical. An inner cover is placed over the stack of coils and sealed to the base by suitable means such as a solid or liquid seal. The furnace bell is then placed over the inner cover which is purged of air by the introduction of a protective atmosphere, for example a mixture of hydrogen and nitrogen containing from about 4% to about 10% hydrogen and the remainder nitrogen. Heat is generated by radiant heating tubes or other conventional means in the furnace bell to raise the temperature of the coils to between about ll2S F. and about 1200 F., preferably to about 1150 F. During this heating, oil that has remained on the surface of the coils after rolling will be cracked forming gaseous hydrocarbons, largely methane (CH which gas will become mixed with the protective atmosphere of hydrogen and nitrogen.

By referring to the gas equilibrium curve-s it will be seen that the amount of methane (CH.,) that will remain as a gas in the presence of an atmosphere gas containing, for example, 10% hydrogen (H diminishes considerably as the temperature of the gases rise. Thus, in an atmosphere containing a total of 10% CH +H at 1200 F., 0.23% CH can be maintained as a gas. Any additional amount will break down to carbon whereas, if the temperature of the gas is increased to 1400" F., only 0.069% CH will remain as a gas. With these figures in mind, it has been determined that by measuring the quantity of CH, in the atmosphere gas and properly controlling the composition of the furnace atmosphere before heating it to the elevated annealing temperature, the possibility of carbon drop-out from the gases may be completely eliminated Several procedures can be utilized to effect such control of the furnace atmosphere after determining the percentage of CH, in the atmosphere and before heating to the elevated annealing temperature. In one method the hydrogen content of the gas is increased before heating above 1200 F. so as to maintain a quantity of hydrogen great enough to permit the quantity of CH; present to be maintained as a gas at the elevated annealing temperature which will be used. This may be accomplished by changing the protective atmosphere from the usual relatively inexpensive gas with 4% to hydrogen and the remainder nitrogen, to a high hydrogen content gas such as dissociated ammonia (75% H 25% N However, the cost of using such a gas would probably be prohibitive for ordinary strip annealing applications.

A second method is to determine the quantity of methane (CH in the furnace atmosphere at the time a temperature of about 1150 F. is attained, then to increase the total fiow volume of standard 4% to 10% hydrogen-nitrogen gas through the furnace, and monitor the percentage of methane in the gas while maintaining the temperature of the steel at about 1150 F. until the percentage of methane is low enough so that final heating to annealing temperature, for example 1400 F., can be brought about without carbon drop-out at that temperature.

A third control method is to increase the total atmosphere fiow through the furnace while holding the temperacarbon gas in said enclosure when said article reaches said temperature, and increasing the temperature of said article to the desired maximum while controlling the composition of said protective atmosphere in said enclosure during said temperature increase whereby the percentage of said hydrocarbon gas relative to the percentage of hydrogen in said enclosure is maintained below that at which carbon drop-out 'will occur on said article.

2. A method of heat treating a steel article as defined in claim 1 in which said steel article is a coil of strip steel and said hydrocarbon gas is methane.

3. A method of heat treating a steel article as defined in claim 1 in which the control of the composition of ture of the coil at about 1150 F. and then, using a higher percentage of methane as an end point than in the second method described above, increasing the hydrogen content in the gas to 15% to 18% hydrogen during the final heating to temperatures between 1150 F. and 1375 F. After reaching the final annealing temperature (for example 1400 F.) and allowing time for the methane content to drop to a level where there will be no carbon drop-out danger, use of the standard H N atmosphere gas with a hydrogen (H content of between 4% and 10% could be resumed.

It can be seen from the foregoing that this invention makes it possible to anneal in a protective atmosphere coils of strip steel having an oil residue on the strip without the formation of the black spots frequently found following prior art annealing procedures. It will also be understood that various modifications in the specific procedures followed may be made within the scope of this invention as set forth in the appended claims.

What is claimed is:

1. A method of heat treating a steel article having a non-gaseous hydrocarbon on its surface which includes the steps of placing said article in an enclosure, passing a protective atmosphere comprising from about 4% to about 10% hydrogen and the remainder nitrogen through said enclosure around said article, heating said article to a predetermined temperature high enough to create a hydrocarbon gas from said non-gaseous hydrocarbon on said article, but not high enough to cause said hydrocarbon gas to break down into carbon and hydrogen, determining the percentage of said hydrosaid protective atmosphere is elfected by determining the percentage of methane in the atmosphere when the article attains a temperature of about 1150 F., monitoring the percentage of methane in the atmosphere while increasing the fioW volume of said protective atmosphere and maintaining the temperatureof the steel article at about '1150 F. until the percentage of methane in said atmosphere is below the level at which methane will break down into carbon and hydrogen upon reaching a predetermined temperature, and then increasing the temperature of the article to said predetermined temperature.

4. A method of heat treating a steel article as defined in claim 1 in which said control of the composition of said protective atmosphere is effected by increasing the hydrogen content of said atmosphere to between about 15% and 18% hydrogen while raising the temperature of the article from about 1150 F. to about 1400 F., and, after the methane content of the atmosphere has dropped to a level at which carbon will not drop out at the final heat treating temperature with a hydrogen content of from 4% to 10%, reducing the hydrogen content of said atmosphere to between about 4% to about 10%.

References Cited UNITED STATES PATENTS 2,152,154 3/1939 Robiette 148-203 2,177,031 10/1939 Tonner 148-16.7 2,504,808 4/1950 Dailey 14816.7 3,167,459 1/l965 Daguier 148--13.1

CHARLES N. LOVELL, Primary Examiner US. Cl. X.R.