US2370364A - Alloy steel process - Google Patents

Description

Patented Feb. 27, 1945 ALLOY STEEL PROCESS Donald L. Loveless, Baltimore, Md., assignor to Rustless Iron and Steel Corporation, a corporation of Delaware No Drawing. Application February 28,- 1941. Serial No. 381,137

1 Claim.

This invention relates to stainless steels and to an art of producing the same.

One object of my invention is the provision of a process for producing clean, sound nitrogen-containing stainless steel, which in practice requires inexpensive and readily available materials, which is easy to perform, requiring little time and labor and which in practice lends improvement to certain qualities of various types of stainless steel.

Another object of my invention is to provide a process for introducing nitrogen into stainless steel which is economically practiced. which is highly efiective in character, which can be employed using an inexpensive nitrogen-containing material which keeps well and is easily and conveniently stored and which is easy to use in conjunction with operations now employed in producing stainless steel.

A further object of my invention is that of providing a process for introducing nitrogen into stainless steel; the employment of which enables the production, in a simple, direct manner, of stainless steels which are of great strength and durability, of fine even grain, which are heatresistant and corrosion resistant, that lend themselves to hot working, cold working and polishing, which possess a high yield point and, in the case of the austenitic steels, a higher rate of vwork hardening, which have good ductility in the workhardened state, that have high impact resistance and which are very resistant to decarburization and grain growth under the many conditions of fabrication and use.

Other objects in part will be obvious and in part pointed out hereinafter.

The invention accordingly consists in the combination of elements, composition of ingredients. and in the several steps and the relation of each of the same to one or more of the others as described herein and the scope of the application of which is indicated in the following claim.

As conducive to a clearer understanding of my invention, it may be noted at this point that stain less steel is a, low carbon, iron-chromium alloy which usually contains some to 35% chromium, with or without nickel, and supplemental additions of manganese, silicon. copper, tungsten. vanadium, molybdenum, titanium, columbium. sulphur, phosphorus, selenium, tellurium and the like, for special purposes. Through the use of heretofore known process for introducin nitrogen into stainless steel many valuable results have been achieved. For example, Patent No. 2,118,- 693 .to Arness, issued May 24, 1938, discloses nitrogen-treated stainless steel of the ferritic iron-chromium grade comprising 10% to 30% chromium, 0.06% to 0.30% carbon and 0.06% to 0.20% nitrogen. The presence of nitrogen in the metal causes a decrease in size of grain structure of the metal and gives the metal greater resistance to decarburization and grain growth in working. Such alloy products are more ductile, more workable, and lend themselves quite readily to hardening by heat treatment over a wider range of chromium content. Cold forming operations may be carried out more successfully over a wider range of temperatures. Further, the tensile strength of such products is much improved.

In Patent No. 2,121,391 to Arness, issued June 21, 1938, there is disclosed a nitrogen-treated austenitic iron chromium nickel alloy. Nitrogen, in a stainless alloy of the austenitic grade, improves the yield strength of the metal. The cold working properties of such non-corrosive ferrous alloys are, accordingly, much improved.

Prior processes for introducing nitrogen into non-corrosive ferrous alloys have proven to be very expensive in practice and otherwise not completely satisfactory. The nitrogen containing treating materials used in these processes are, for example, expensive or difficult to procure, or costly to prepare. Moreover, such materials frequently are difficult to handle without great waste, while others contain certain impurities which affect the treated metal detrimentally. Still other nitrogen-containing materials are objectionable for, through their use, nitrogen cannot be uniformly distributed throughout the metal or because nitrogen is introduced into the metal in amounts either too low or too high.

The properties conveyed by the nitrogen con-' tent of non-corrosive ferrous alloys are in some respects similar to the well known properties resulting from the use of a small increased amount of carbon without, however, the accompanying undesirable effects of carbon on corrosion resistance and without decarburization and surface grain growth. A certain nitrogen content is necessary to impart desirable characteristics to non-corrosive ferrous alloy products,-

while a nitrogen content which is too high causes the metal to be unsound and to possess other gas defects. In the process of the Arness patent, high-nitrogen ferrochrome is used to introduce nitrogen into a melt of stainless steel. High nitrogen ferrochrome lends the right amount of nitrogen to stainless alloy metal baths when the treating agent is introduced in proper quantity. This treating agent, however, is expensive to prepare or to procure. The present cost of introducing some 0.03% of nitrogen by way of high nitrogen ferrochrome in the preparation of a commercial 13-ton heat of metal is about one hundred dollars.

Accordingly, an object of my invention is the provision of a new process and material for introducing nitrogen into non-corrosive ferrous alloys, which enables nitrogen-containing stainless steel to be produced more cheaply than heretofore possible, which gives a metal product uniformly containing the proper amount of nitrogen, which enables nitrogen to be introduced into a non-corrosive alloy without adding an objectionable amount of carbon to the metal, or without adding other injurious ingredients to the metal, and which in use causes a high percentage of available nitrogen to be released into the alloy metal with highly eflfective and beneficial results.

Referring now more particularly to the practice of my invention; I add nitrogen to anon-corrosive ferrous alloy during its production by way of calcium cyanamide or nitrolim (CaCm). Calcium cyanamide comprises a plentiful amount of nitro- Ben and is commonly used as a fertilizer. The nitrogen treating agent is of a heavy nature and therefore does not fiy to waste when handled during my metal treating process. Calcium cyanamide is a readily available, inexpensive material which is free from objectionable impurities. In use, my nitrogen-containing agent, whether used in lump or powdered form, yields a high percentage of its nitrogen content to the metal. Therefore, since calcium cyanamide so effectively lends nitrogen to a metal bath, only small amounts of the agent need be used. Further, once the amount of nitrogen which is to be introduced into a metal is known, it is but a simple matter to place sufllcient quantities of calcium cyanamide in the metal to release the determined amount of nitrogen. The quantity of nitrogen released by a given amount of calcium cyanamide in a particular non-corrosive ferrous alloy can .be ascertained by experiment. Thereafter, the necessary amount of calcium cyanamide needed to impart a certain amount of nitrogen to that partlcular metal alloy can be based upon experimental data.

As illustrative of the practice of my invention, a heat of stainless steel is produced in any well known manner, for example, by using stainless steel scrap and high-carbon ferrochrome as sources of chromium, as disclosed in Patent No. 1,925,182 to Feild, issued September 5, 1933; or in accordance with the method described and claimed in the Arness Patent No. 1,954,400, issued April 10, 1934, wherein rustless iron scrap and chromium ore are used as the principal sources of chromium; or in accordance with the method described and claimed in the Amess Patent No. 2,056,162, issued October 6, 1936, wherein rustless iron scrap, high-carbon-ferrochrome and chrome ore are used.

I make up a finishing slag on the bath of metal, using ferro-silicon and ferro-manganese. As desired, along with the ferro-silicon and ferro-manganese," there is added sufiicient ferrochrome to adjust th analysis of the bath of the instant specification. Thereafter, I tap the metal into a ladle for teeming.

In accordance with my invention, the finishing slag is held back during the initial stages of the tapping operation. The metal is run into the ladle until it is about one foot deep. At this point, a certain amount of finely divided calcium cyanamide agent is shoveled into the ladle. The turbulent action of the metal as it is poured into the crucible, eflects a stirring action which is highly eflective in rapidly distributing the calcium cyanamide throughout the metal. Nitrogen is released as the calcium cyanamide decomposes under the heat of the molten steel. The released nitrogen is quickly disseminated throughout the molten metal and is absorbed by the metal. The finishing slag previously held back i then poured into the ladle along with the remainder of the metal. The metal then is teemed into suitable ingot molds where it is permitted to solidify and cool. The molds are then stripped from the innote and the ingots stored and converted into various semi-finished products as desired.

To illustrate a specific result of the practice of my invention, I produce a iii-ton heat of stainless steel of the 17% chromium grade in any well known manner. An analysis in the furnace just prior to tapping shows that the stainless steel contains 0.025% of nitrogen. The metal is tapped into a ladle and while the metal flows into the ladle, I add pounds of calcium cyanamide, which by analysis, comprises 21% of nitrogen. The finished steel analyzes 0.06% nitrogen, which indicate that there is an increase of 0.035% of nitrogen through th calcium cyanamide addition.

Accordingly, it is to be observed that approxb mately 44% of the nitrogen available in the 100 pounds of calcium cyanamide is picked up by the 13-ton heat of metal. Thus, my nitrogen-containing agent is highly effective in use. It is economical for it can be used in small quantities, and yet the proper amount of nitrogen necessary to be supplied to stainless steels is present. To'introduce nitrogen into the above 13-ton heat in accordance with my invention so as to obtain a nitrogen pick-up of 0.035%, the present cost involved is about $3.00 as compared to approximately $100 if high nitrogen ferrochrome had been used.

My process and nitrogen agent can be employed to good advantage in introducing nitrogen into any type of iron or steel alloy. I find that the qualities of non-corrosive ferrous alloys of various types and grades are materially improved in the practice of my invention. I prefer to apply my invention to low carbon, stainless steel alloys in which there'is present an alloying element or alloying elements such as chromium (or chromium and nickel) with or without supplementary amounts of other alloying elements.

My invention, in practice, readily lends itself to metallurgical operations now in use, without requiring the purchase of new equipment or the shifting or re-designing of old equipment. In carrying out my process, the cost of either labor or materials is unusually small and the time expended is practically negligible.

Further, the non-corrosive ferrous alloys upon which my invention is practiced, in consequence, are greatly improved in quality. Nitrogen is uniformly distributed throughout the metal, and thereby the nitrogen imparts a more uniform and even grain structure and a consistently finer grain structure to the metal. This is true because my invention enables nitrogen to mix and, in turn, to combine moreuniformly with the metal. Moreover, the metal is un sually clean, this being attributed to the deoxidizing effects of the calcium cyanamide particularly through the liberated calcium acting on the chromic oxides distributed in the metal. Such metals are strong and durable throughout and are free from gas flaws and impurities.

Thus it will be seen that in the present invention, a new process for introducing nitrogen into stainless irons and steels has been provided, by the use of which the various objects hereinbefore noted, along with many thoroughly practical advantages. are successfully achieved. It will be seen that the physical properties of rustless ferrous alloys are greatly improved by the practice of my invention and that these improved characteristics are achieved at great savings in manufacture heretofore unrealized; and that the useful fields of application of these products have been broadened both from the standpoints of economy and improved quality.

As many possible embodiments may be made of my invention and as many changes may be made in the embodiment hereinbefore set forth, it is to be understood that all matter described herein is to be interpreted as illustrative and not in a limiting sense.

I claim:

In the production of stainless steel of appreciable nitrogen contents, the art which includes preparing a bath of such steel, forming a finishing slag comprising ierro-manganese and ferrosilicon on the surface of said bath, tapping said metal into a crucible while withholding the finishing slag, adding a quantity of finely divided calcium cyanamideto the first of the tapped metal, and thereafter permitting said finishing slag together with the remainder of said tapped metal to mix turbulently with the calcium cyanamide and first tapped metal.

DONALD L. LOVELESS.