US2255016A - Process for treating iron and steel - Google Patents

Description

Patented Sept. 2,1941

PROCESS FOR. TREATING IRON AND STEEL Earle C. Smith, Cleveland Heights, and George T.

' phorus,

Motok; Massillon, Ohio, assignors to Republic Steel Corporation, Cleveland, Ohio, a corporatlon= of New Jersey No Drawing. Application May 13, 1940,

Serial :No. 334,891

. -6 Claims.

This invention relates to the art of makin -irons and steels,'and is particularly concerned with a new and improved process for making irons and steels having new and unexpected properties and characteristics, including improved resistance to corrosion.

.The present invention is based on the discovery that nitrogen improves th properties of irons and steels, including corrosion resisting irons and steels. We have found that, contrary to generally accepted belief, nitrogen in the proper form and amount is not an element to be avoided, but in fact can be used to improve the properties and characteristics of irons and steels of a wide variety ofrcompositions, including corrosion resisting irons having the fundamental composition of between about .01% and about .10% of carbon and between about .15% and about 1.5%. of copper, with or without small quantities of other elements, such as molybdenum and mangane'se. Other elements whichmay be present in such compositions include-sulphur, phosphorus and silicon in the usual small amounts. We have found that when an iron or this fundamental composition contains between about .005% and about .2% of nitrogen the resistance of the iron to corrosion is considerably-increased.

we have also discovered "that, contrary to previously generally accepted belief, ,'phos'phorus in proper amount is not an'element to be avoided in corrosion resisting irons of the foregoing fundamental composition but, in fact, can be used to advantage to confer desirable characteristics thereon if and when present in about .04% and about 50% In practicing the present invention on irons and steels of the foregoing fundamental composition to improve the corrosion resisting properties thereof, the process may be carried out conveniently as follows: An iron or steel is made in the ordinary manner, as in an electric furnace, or in an acid or basic open hearth furnace, or in a Bessemer vessel 'and/or-converter and may contain between about .15% and about 1.5%

amounts between of copper and between about 0l% and about .10% of carbon, with or without between about 95% and about 55% of molybdenum and with or without between about .01% and about 50% of manganese. Such an iron or steel would orcontaining material is added to bring the total phosphorus content of the melt to between about .04% and about 50%. When the molten metal is poured from the ladle into the ingot molds, nitrogen bearing material may be thrown into the molds in an amount sufllcient to insure the presence of between about .005% and about 20%- of nitrogen in cold ingots. Alternatively the nitrogen containing material may be added when the phosphorous is added, and both, or either, may be added to the molten metal in. the furnace, in the ladle or in the molds.

vCalcium cyanamid has been found to be a most suitable material for the foregoing purposes. The

addition of from about two to about five pounds.

of calcium cyanamid per ton of molten metal will carry a sufllclent amount of nltrogen'into the iron for the desired purposes.

In practicing the present process on compositions of the foregoing general analysis for improved-resistance to corrosion, it is preferable to have ingredients present in the molten metal which will act as nitrogen fixers, i. e., elements which will combine readily with nitrogen liberated in the metal to form stable nitrides. Several metals have this property including titanium,

vanadium, zirconium, columbium,, tantalum, silicon, aluminum, boron and beryllium. In the absence of nitrogen flxers the amount of nitrogenretained in the. metal may vary with variations inthe temperature of themetal, the pouringrate of the metal, the oxygen content-{oi the.

metal, the time-the metal is held before pouring, and the amount of mechanical movement of the steel. When nitrogen fixers are present these variables do not materially afl'ect the amount r nitrogen retained by the metal, for the nitrogen forms stable nitrides with the flxers and hence makes it pomible to control the amount oi nitrogen in the metal without the necessity of making any attempts to control those several variables.

.The process described in our former applica-- tion-Ber. No. 119,264 inherently possesses many dinarily contain up to about .065% of micsv when the melt is tappedlnto a ladle a sumcient amount of phosphorus or phosphorus advantages. For example, when calcium cyanamid is added -to molten metal it decomposes, liberating nitrogen in a nascent state and also liberating th element or elements which were combined with the nitrogen in that compound. The nascent nitrogen so liberated is quite eilec-' tive in removing oxygen dissolved in the molten metal, probably by displacement of the oxygen by the nitrogen, and also seemsfto have some eilect on combined oxygen. Such displaced oxygen and also free and combined oxygen apparently comblneswith the element or elements released when the nitrogen containing compound decomposed, and the resulting oxide is removed from the metal, partly by the agitation or ebullition taking place in the molten metal when the nitrogen is thus liberated. The elements liberated when the nitrogen containing compound decomposes, also apparently combine with sulphur in the metal and the resulting sulphur compounds are carried out of the metal by the agitation thereof, thus reducing the sulphur conent of the metal,

We have discovered, as a result of the appreciation of the foregoing inherent results of the process as disclosed in our application Ser. No. 119,264, that our process can be advantageously practiced, not only on corrosion resisting irons and steels, but also on irons and steels of a wide range of compositions, and with the result that the properties and characteristics of irons and steels so treated may be surprisingly improved. For example, by applying our process to molten irons-and steels, we have been able to increase notch sensitivity and permeability; to reduce their tendency toward caustic embrittlement,

characteristics obtained by our invention in large 1 part to the marked reduction in oxygen content and marked increase in nitrogen content as compared with the amounts of those substances normally present in the untreated metal, and in part to. the reduction of sulphur content of the metal.

A plain steel containing about .05% of carbon, and made by the most advanced practice with which we are familiar, normally contains between about .06% and about .10% of oxygen, much of which is present in a dissolved state. By means of our process we have been able to reduce such an oxygen content to less than about .02% and in some instances to as low as about .005%. High carbon steels, for example those containing about .50% of carbon, and alloy steels containing various amounts of carbon, chromium, nickel, and other alloying elements, when made according to the most highly advanced practice with which we are familiar. normally contain from about .005% to about .08% of oxygen. By our process we are able to eifect a marked decrease in the oxygen content of these steels and to attain substantial improvements in the properties and characteristics thereof, particularly in the respect above mentioned.

Irons and steels, made according to the best present day practice with which we are familiar,

. normally contain oxygen contents, such as have been described above, and nitrogen contents which vary depending on the type of furnace or process used in making such irons or steels. For -example, in basic open hearth steel the nitrogen content is about 004%, in acid open hearth steel it is about .008%, in basic electric furnace steel it is between about .005% and .01%, in acid electric furnace steel it is about .0l% and in Bessemer steel it ranges from about .01% to about .03%. When'such. steels are treated by our process, the oxygen content is decreased from a small amount where the oxygen content was normally very low to as much as or more, in a plain low carbon steel where the oxygen content was as high as .10% or higher.

At the same time the amount of nitrogen normally present in the steel is increased by our process to the extent of from 40% or 50% to more than several hundred per cent and simultaneously the marked improvement in physical properties and characteristics of the metal so treated is obtained.

Additional improvements treaceable to our process include an increase in hardness of ferrite grains, particularly in the presence of aluminum and vanadium; an increase in toughness and particularly in low carbon steels; an increase in their strength, hardness, ductility, elastic limit,

the plasticity of thevmetal at rolling temperatures and a considerable extension of the hot working temperature ranges; and the development of austenitic and martensitic structures in low carbon compositions which are analogous to structures present in compositions containing much higher amounts of carbon.

The present invention may be practiced by bringing into nitrogen-liberating contact with the molten iron or steel to be treated when it is in a furnace, Bessemer vessel, or ladle, or while it is being poured into a mold, a quantity of a calcium cyanamid which is capable of decomposing in the molten metal and. liberating nascent or active nitrogen in -the metal in amounts sufflcient for the purpose desired. Cal-: cium cyanamid has-been found to be quite satisfactory. In contact with the molten metal it decomposes into its constituent elements, libera'ting nascent nitrogen and calcium and canbon. The nitrogen displaces oxygen dissolved in the metal and the calcium and carbon combine with free, combined, and displaced oxygen to form compounds which will be carried out of the metal partly by the ebullition or agitation of'the metal.

Furthermore, calcium cyanamid apparently aids in the desulphurization of the metal and 1 tends to break down non-metallic inclusions in the metal.

The amounts of calcium cyanamid which may be used advantageously in carrying out the present invention are rather critical and, accordingly, care should be taken to use such amounts as will not exert any harmful efiect on the metal.

Generally speaking, the amount of calcium cyanamid used should range between about pound and about 8 pounds per ton of metal. The amount of such material which may be safely used depends partly upon the content of carbon or carbide forming alloying elements present in the metal and partly on the amounts of nitrogen flxers which are in the metal. When an iron or steel is to be treated by this invention and contains either a low carbon content, such as .10%, or a small amount of carbide forming alloying elements, and also contains small amounts of added nitrogen flxers, as much as 8 pounds of calcium cyanamid per ton of metal can be safely brought into the molten metal. As illustrative of this condition, reference is made to the two first above described plain irons or steels which are suitable for use for corrosion resisting purposes. I These compositions contain up to about .10% or .15% of carbon, up to about 1.5% of copper, up to about 25% of molybdenum and up to about .50% of manganese. In these compositions the carbon and carbide forming alloying elements, and also the nitrogen ilxers, are all sufliciently low so that when thecomposition is treated according to this process and with up toas much as 8 pounds of calcium cyanamid, the oxygen content will be reduced,

the nitrogen content will be increased and the physical properties will be improved, as has been described herein.

when the metal to be treated contains higher amountsof carbon, such as 50%, or a somewhat lower carbon content coupled with carbide forming alloying elements, such as manganese, chromium, tungsten and molybdenum, the amount of calcium cyanamid should be decreased to avoid increase in brittleness of the treated metal. For example, a steel containing about 50% of carbon and about 1.50% of carbide forming alloying elements such as manganese, chromium, molyb denum and tungsten may be improved asde-.

scribed herein provided the amount of calcium cyanamid brought into the metal does not exceed about 1 pound per ton of metal. When the content of carbon or carbide forming alloying elements is-fairly low and the content of nitrogen fixers is fairly high,-the amount of calcium cyanamid which may be used may be somewhat higher than when the carbon or carbide forming alloying elements are high. For examplea steel containing about 20% of carbon, about .8% of carbide forming alloying elements and about .25% of nitrogen fixers, including titanium, zirconium, vanadium, columbium, boron, beryllium, tantalum and aluminum, as much as about 6 pounds of calcium cyanamid per ton of metal may .be. safely employed and will produce the improvements-herein described.

In general, it may be said that with a plain low carbon steel containing substantially no carbide forming alloying elements and substantially no added nitrogen fixers, up to about 6 pounds of calcium cyanamid may be safely brought into the metal and, when substantial amounts of fixers are present, up to about 8 pounds of calcium cyanamid may be similarly used; with a plain low carbon steel containing substantial amounts of carbide forming alloying elements or a high carbon steelwith small amounts of carbide forming alloying elements, up to about 2 pounds of calcium cyanamid may be similarly used, this amount being lowered as the amounts of carbon and carbide formers are increased, and, when example. -A fewp'oundsof one or more of such nitrogen flxers per ton'of molten-metal has been found to be suflicient to retain in the latter several hundred percent more nitrogen than would be retained by our process if such fixers were not present.

We have discovered that our .invention is not limited to the introduction of calcium cyanamid directly" into the molten metal, but that the in- .vention may be advantageously carried out by the calcium cyanamid is added to or incorporated in such second slag. It is added to or incorporated' in the slag of the Bessemer or' open hearthfurnace preferably as soon as the steel has begun to melt.;' In all such instances the calcium cyanamid may constitute as much as 25% by volume of the slag, this amount'havlng been found to contain suflicient nitrogen to impart .the desired properties to the steel.

We believe that calcium cyanamid in the slag deoxidizes. theslag, which, in turn, deoxidizes the metal and that the nitrogen exerts its beneficial influence on the molten metal in much the same wayas itt-does when it is added directly to the, molten metal; We believe that these actions occur in much the same manner regardless of the typev of furnace used.

The manner of practicing this form of our invention in'the various types of non-electric fur naces will be understood from the following de- 5 oxygen the metal itself contains a high amount of oxygen'and when the slag is low in oxygen the metal is likewise low in oxygen. Sometimes this ratio is as high as 100 to 1.

vIn'the manufacture of irons and steels by the electric furnace'method, the second'slag which such a steel also contains substantial amounts-of nitrogen fixers,- up to about 6 pounds of calcium cyanamid may be similarly used.

We believe that the nitrogen liberated in,

These fixers may be added to the moltenmetal' before, or together with, the addition of the calcium cyanamid. Suitable nitrogen'fixers include metals'such as copper, aluminum, manganese, molybdenum, 'arsenic, titanium, tantalum, vanadium, columbium, silicon, zirconium, boron and beryllium, and any one or more-of such metals may be used together. Such nitrogen fixers may.

be introduced into the molten metal in metallic form ,as in the case of copper and aluminum for example, or in the form of a ferro-alloy as in the case of aluminum, silicon and manganese for contains less oxygen than the slag it replaced changes the previously existing physico-chemical equilibrium and also lowers the ratio of oxygen for the higher oxygen containing slag which it replaces exerts a metal. I when the oxy en content in electric furnace irons or steels is to be reduced to small amounts and this is to be accomplished by use of a second slag, the ingredients of the second slag are charged into the furnace and are there converted deoxidizing effect on the molten into molten slag by the electric arc. Often about.

three hours time is required for such melting. and before the slag becomes effective. The heating is continued thereafter for about two hours or until the metal has been deoxidized.

We have discovered that if calcium cyanamid, as such, is added to the substances which are to 7 form the slag, much time can be saved; and the 7 resulting metal will possess'improved properties and characteristics, both as compared with the prior practice. By adding such calcium cyanamid gen content and to bring about; the deoxidation of the metal thereby. The amount; of calcium cyanamid which may be so used may amount to as much as half of the total slag" rials. g Furthermore, we believe the; he calcium cyanamid, added as a part of .th second slag in an electric furnace process, works into. the metal and decomposes therein, and its components act as has been described above, that is, the nascent nitrogen liberated by such decomposition displaces oxygen dissolved in the metal and exposes such oxygen to the liberated calcium and carbon, and such oxygen, as well as some free and some combined oxygen in the metal, combines with the calcium and carbon and is taken into the slag. We also believe that the liberated calcium and carbon attack oxygen in the slag and eliminate part of it as oxides of carbon, thereby reducingthe amountofoxygen in the slag and enabling the slag to reduce further the amount of oxygen in the metal.

Certain of the above described features of our invention were disclosed in our application Ser. No. 119,264 flied January 6, 1937, .now Patent No. 2,121,055 issued June 21, 1938, and also in our rining mate- 'application Ser. No. 213,420 which was filed on June 13, 1938, and which was copending with application Ser. No. 119,264. The present application is a continuation in part of our applications Ser. Nos. 119,264 and 213,420, with the latter of which this application is copending.

Having thus described our invention so that those skilled in the art may be able to understand and practice thesame, we state that what we desire-to secure by Letters Patent is defined in what is claimed.

What is claimed is:

1. In the manufacture of iron or steel, the method of decreasing the oxygen content, increasing the nitrogen content, and improving the physical properties of the metal which includes the step of bringing into nitrogen-liberating contact with the molten metal between about onehalf pound and eight pounds per ton of metal, of calcium cyanamid.

2. In the manufacture of iroh or steel, the method of decreasing the oxygen content, increasing the nitrogen content and improving the physical properties of the metal which includes the step of incorporating in the slag forming materials in the furnace a quantity, approximating 25% of the slag, of calcium cyanamid.

3. In the manufacture of iron or steel, the method of decreasing the oxygen content, increasing the nitrogen content and improving the 'physical properties of the metal which includes the step of bringing into the metal containing substantial amounts of carbon and carbide forming alloying elements and substantial amounts of nitrogen fixers between about one-half pound and about six pounds per ton of the metal, of calcium cyanamid.

4.- In the manufacture of iron or steel, the method of decreasing the oxygen content, increasing the nitrogen content and improving the physical properties of the-metal which includes the step of bringing into the metal containing a small amount of carbon and substantial amounts -of added nitrogen fixers about six pounds of calcium cyanamid per ton of the metal.

5. The method of making iron articles to be' exposed to corroding conditions during use which includes the steps of making molten iron containing between about .01% and about .10% of carbon and ordinary amounts of impurities such as sulphur and manganese, adding thereto between about .15% and about 1.5% copper and between about .05% and about 55% of molybdenum, and

bringing into the molten metal between about I pound and about 8 pounds of calcium cyanamid per ton of metal.

6. In the manufacture of iron or steel, the method of decreasing the oxygen content, increasing the nitrogen content, and improving the physical properties of the metal which includes the step of bringing into nitrogen-liberating contact with the molten metal a quantity of commercial calcium cyanamid containing between about .105 pound and about 1.68 pounds oi nitrogen per ton of metal.

EARLE C. SMITH. GEORGE T. MOTOK.