US2069203A - Production of rustless iron - Google Patents

Description

Patented. Feb. 2, 1937 ,EUNITED STATES PRODUCTION OF RUSTLESS IRON William B. Arness, Baltimore, Md., assignor, by mesne assignments, to Rustless Iron and Steel Corporation, Baltimore, Md., a corporation of Delaware No Drawing. Application July 20,1933,

Serial No. 681,373

9 Claim.

This invention relates to corrosion resistant irons and steels and more particularly to an art of producing the same.

Among the objects of my invention is the simple, eflicient and economical production of rustless irons and steels of a desirable fine grain size and of improved physical characteristics, made to a desired specification of iron, carbon, chromium and nitrogen, either with or without supplementary elements such as nickel, copper, cobalt, titanium, molybdenum, tungsten, vanadium and the like.

The invention accordingly consists in the combination of elements, composition of ingredients and mixture of materials and in the several steps and the relation of each of the same to one or 1932, many highly beneficial characteristics are given rustless iron and steel by adding to the metal a small percentage of nitrogen. The irons and steels containing nitrogen are of -an inherently fine, even-grain structure; they are of improved workability over a wider range of working temperatures; they are less subject to grain growth, decarburization, brittleness and fatigue: and they are more durable and of higher impact values than heretofore known alloys of the class indicated.

One of the outstanding objects of my inven-. tion is the eflicient, economical and thoroughly practical production of rustless irons and steels to a required analysis of carbon and chromium, either with or without supplementary additions indicated above, wherein a desired nitrogen content is reliably and economically achieved.

In the practice of my invention a bath of ferrous metal containing chromium is prepared as by melting down ordinary low carbon steel scrap and/or rustless iron scrap together with suitable chromium containing ingredients such as high carbon ferrochrome or chrome ore, or by adding low carbon ferrochrome to a bath of molten iron prepared as indicated above.

To the bath containing a substantially desired final percentage of chromium there is added an alloy of appreciable nitrogen content, such as a, ferro alloy of high nitrogen content (preferably a nitrogen containing ferrochrome), in such quantity as to give a desired percentage of nitrogen in the final product. The nitrogen containing ferro alloy is preferably added shortly prior to tapping the heat of metal in order to permit a free dissemination of nitrogen throughout the well as the loss of chromium (where a nitrogen containing ferrochrome is employed). Certain advantages are achieved, however, where the nitrogen containing ferro alloy is added along with the initial charge of ingredients or at a subsequent period in the operation of the particular process utilized, all as more particularly described and claimed in my copending application entitled "Production of rustless iron, Serial No. 681,374 filed July 20, 1933.

' The alloy content of the metal, is adjusted and the metal finished in accordance with standard practice after which the heat of metal is poured into suitable molds and permitted to cool. The tapped metal containing a desired percentage of nitrogen is of an inherently fine grain structure which permits an improved workability over a wider range of temperatures, in addition, is less subject to grain growth, decarburization, brittleness and fatigue, and which is more durable and of higher impact values than heretofore known alloy irons and steels of the class indicated, all

as more particularly pointed out in my copending application, Serial No. 645,637 referred to above. 1

As illustrative of the practice of my invention a 6-ton Heroult electric arc furnace, having graphite electrodes and rated three-phase, cycle, 1500 kva. at 110 to 180 volts, is first prepared for the reception of a charge by arcing on electrodebutts to heat up the furnace. The furnace is preferably provided with a chromite brick bottom which is carried up to a height somewhat above the siag line of the furnace. For reasons of economy, a lining containing chrome ore is preferably rammed in over the chromite brick hearth lining, sodium silicate being used as a convenient binder. The furnace side walls and roof are conveniently lined with silica brick.

After the preheating, as indicated above, the furnace is, illustratively, charged with 12,600 pounds of rustless iron scrap, analyzing about 17% chromium and about .10% carbon; 3250 pounds of ordinary low-carbon steel scrap; 2850 pounds of chrome ore, analyzing about 48% chromium oxide (CrzOa) and about 19% iron oxide (FeO); and 1,000 pounds of roll scale which is substantially 100% magnetic iron oxide. I

Electric power is applied to the furnace and the charge of ingredients is rapidly melted down to form a bath of molten iron containing chromium with a small percentage of carbon and'an overlying slag containing the oxides of iron and chromium. With the continued application of power, the temperatureof the bath of molten metal and the supernatant slag is brought up to a point considerably higher than that ordinarily employed in usual steel melting practice in an electric furnace.

While no reliable method is known for precisely determining the temperature of the metal bath beneath the slag blanket, it is estimated that this temperature, designated as a temperature of superheat, is approximately 3000" F. to 3200 E, which is some 100 F. to 300 F. higher than the temperature ordinarily employed in electric steel melting practice. At this temperature of superheat the oxidizing slag is more active in combining with carbon contained in the metal bath and carbon coming from the furnace atmosphere toeil'ectively remove and/or exclude carbon from the metal bath.

Incidental to the oxidation of carbon, there is an oxidation of chromium. from the bath,- the chromium oxide entering the slag. The extent of the loss of chromiuminto the slag is minimized by the rapidity of the initial melt-down and bringing the bath and slag to the elevated temperature of superheat indicated above.

Under the strongly oxidizing action of the slag overlying the metal bath the carbon content is continuously lowered. When tests on samples taken from the bath indicate a desired low-carbon content is reached (about 05%) the melt-down and oxidizing stage is at an end.

In order to effect a recovery of the metals contained in the slag as oxides of iron and chromium, there is added, illustratively, 1400 pounds of crushed 75% ferrosilicon and 4500 pounds of hot dry burnt lime as rapidly as furnace conditions permit. The additions of ferrosilicon and lime fuse and become incorporated in the slag and efiect a reduction of the oxides of iron and chromium contained therein, producing metal which goes into the underlying metal bath, and various silicates which remain in the slag and tend to render it acid in character. A. neutral orbasic slag is obtained as desired by the introduction of large quantities of lime as more particularly indicated above.

As the lime and ferrosilicon are added to the slag the oxides contained therein are progressively reduced and the character of the slag changes from a black to a light green color. The reducing action is accompanied by a transfer of the metal from the slag to the underlying bath of molten iron and chromium. When the color of the slag indicates that substantially all of the oxide content has been reduced this slag is preferably completely removed from the surface of the metal bath and a basic finishing slag of lime, ferrosilicon, and fiuorspar, or like flux, is formed in accordance with standard practice.

In order to achieve tapped metal of a desired nitrogen content there is preferably added to the bath low-carbon ferrochrome of high nitrogen content in an amount sufficient to give the desired percentage. For the proportions of ingredients illustratively set forth above about 700 pounds of nitrogen containing low-carbon ferrochrome, analyzing 63% chromium, 2% nitrogen, 20% carbon and the balance substantially iron and produced for example as described in the co-pending application of James N. Ostrofsky entitled Ferro alloy and art of producing the same, Serial No. 681,371, filed July 20, 1933. The lump low-carbon ferrochrome nitride is quickly dissolved by the bath of molten metal thus directly introducing a desired nitrogen content,

the amount of which is precisely controlled, to-

gether with a supplementary amount of chro mium which adjusts the chromium content of the heat.

Final additions of lump low-carbon ferrosilicon and low-carbon ferromanganese are added to adjust the analysis of the bath to the desired specifications of silicon and manganese. The addition of supplementary alloying elements, nickel, copper, cobalt, titanium, tungsten, vanadium, and the like are made as desired.

The heat of metal is then tapped into suitable molds and allowed to cool. The tapped metal,

, for the embodiment illustratlvely set forth above,

weighs 17,500 pounds and analyzes about .08% carbon, 18.0% chromium, .08% nitrogen, 40% manganese, .35% silicon with the desired supplementary alloy additions indicated above, with the usual low percentages 01' sulphur and phosphorous, and the balance principally iron.

Thus it will be seen that there has been provided in this invention an art in which the various objects hereinbefore noted together with many thoroughly practical advantages are successfully achieved. It will be seen that the process of producing rustless irons and steels of a desired nitrogen content is simple, practical, thoroughly reliable, and that the percentage of nitrogen added lends itself to a direct and precise control, all at minimum cost and maximum eillclency. r.

While in the above illustrative embodiment of my invention a bath of metal of substantially a desired analysis is first prepared by melting down a charge of rustless iron scrap, ordinary low-carbon steel scrap, chrome ore and iron oxide to form a bath of metal covered by a slag containing the oxides of carbon and chromium which are subsequently reduced, thus permitting a recovery by the bath of iron and chromium, and wherein lowcarbon ferrochrome of high nitrogen content is added to this bath just prior to tapping the heat of metal, it will be understood that good results are achieved where the metal of desired analysis is prepared in any desired manner from satisfactory raw materials, such as by melting down lowcarbon steel scrap and/or rustless iron scrap, high-carbon ferrochrome and iron oxide to form a ferrous metal bath containing chromium covered by a slag containing the oxides of iron and chromium (which are subsequently reduced to exclude a recovery of the metal oxides in the slag) and high nitrogen containing ferrochrome is directly added thereto, all as more particularly set forth above. Or, for example, a ferrous metal bath of substantially a. desired final analysis of carbon and chromium may be prepared from ordinary low-carbon scrap and low-carbon ferrochrome to which is added ferrochrome of high nitrogen content, as indicated above, to give tapped metal of a desired percentage of nitrogen.

While in the practice of my invention, as i1lustratively set forth above, low-carbon ferrochrome of high nitrogen content is preferably employed to introduce a desired percentage of nitrogen in the finished metal it will be understood that lowcarbon ferrochrome of either lower or higher nitrogen content may be employed where desired, although where a ferrochrome of lower nitrogen content is used, a greater quantity of this relatively expensive material is necessary in order to achieve a desired percentage of nitrogen: lowcarbon ferrochrome of maximum nitrogen content is preferred since a mimimum of this material introduces a desired quantity of nitrogen, this is specially true where a relatively high nitroen content is desired in the tapped alloy.

Where the introduction of a small amount of carbon into the final metal is permissible, as for example, where the carbon content at the finishing stage is below the permissible maximum limit, good results are achieved by adding the considerably cheaper high-carbon ferrochrome of high nitrogen content, produced, for example, as described in the co-pending application of James N. Ostrofsky entitled Ferro-alloy and art of producing the same, Serial No. 681,371, filed July 20, 1933, and containing about 50% to 72% chromium, .5% to 15% nitrogen, 1% to 8% carbon and the balance substantially iron, and preferably comprising about 59% to 70% chromium, .5% to 7% nitrogen, 1% to 7% carbon and the balance 1 substantially iron, as a complete or partial substitute for the low-carbon ferrochrome of high nitrogen content.

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

I claim:

1. In the production of nitrogen-containing rustless iron and steel of fine grain structure, the art which includes, preparing a bath of molten iron covered by an overlying slag containing oxides of chromium, reducing theoxides of chromium contained in the slag thereby enriching said bath, and after withdrawing the slag from which the oxides have been reduced and forming on said bath a basic finishing slag, adding to said bath a ferrochrome' of high nitrogen content whereby metal of a desired nitrogen content is achieved.

2. In the production of nitrogen-containing rustless iron and steel of fine grain structure, the art which includes, melting down a charge of rustless iron. scrap, iron oxides and chrome ore to form a bath covered by an oxidizing slag, maintaining said bath and slag at high temperature thereby oxidizing carbon and chromium from the bath, the chromium oxides entering the slag, reducing the chromium oxides contained in the slag thereby enriching the bath of metal and achieving a heat of metal of approximately the desired chromium content, and after withdrawing the slag from which the oxides have been reduced and forming on said bath a basic finishing slag, adding to said metal a ferrochrome of high nitrogen content whereby a desired amount of nitrogen is added to the metal and the final chromium content is adjusted.

3. In the production of nitrogen-containing rustless iron and steel of fine grain structure, the art which includes, preparing a ferrous metal bath of appreciable chromium content, and then adding to said bath an iron-chromium alloy analyzing approximately, 59 per cent to 70 per cent chromium, .5 per cent to '7 per cent nitrogen and the balance substantially iron, the chromium of said bath being available to take up nitrogen coming from said nitrogen-containing iron-chromium alloy whereby loss of nitrogen is minimized and. sound metal of desired nitrogen content is achieved. 1

4. In the production of nitrogen-containing rustless iron and steel of fine grain structure, the art which includes, preparing a ferrous metal bath of appreciable chromium content, and then adding to said bath an iron-chromium alloy compresence of a basic slag reducing in character, the chromium of said bath being available to take up nitrogen coming from said iron-chromium alloy of high nitrogen content and the basic slag aiding in the absorption and retention of nitrogen by the bath whereby loss of nitrogen is minimized and sound metal of desired nitrogen content is achieved.

5. In the production of nitrogen-containing rustless iron and steel of fine grain structure, the art which includes, preparing a bath of molten iron covered by an overlying slag containing oxides of chromium, reducing the oxides of chromium contained in the slag thereby enriching said bath in chromium, and adding to said bath a nitrogen-containing iron-chromium alloy comprising about 50 per cent to 72 per cent chromium, .5 per cent to 15 per cent nitrogen and the balance substantially iron, whereby metal of a desired nitrogen content is achieved.

6. In the production of nitrogen-containing rustless iron and steel of fine grain structure, the art which includes, preparing a bath of molten iron containing chromium in substantial amounts, and then adding to said bath an iron-chromium alloy of low-carbon content and containing 59 per cent to 70 per cent chromium, .5 per cent to 7 per cent nitrogen and the balance substantially iron, the chromium of said bath-being available to take up nitrogen coming from the iron-chromium alloy of high nitrogen content whereby loss of nitrogen is minimized and sound metal of desired nitrogen content is achieved.

7. In the production of nitrogen-containing rustless iron and steel of fine grain structure, the art which includes, preparing a ferrous metal bath of appreciable chromium content, and then adding to said bath an iron-chromium alloy of high nitrogen content in the presence of abasic slag mildlyreducing in character, the chromium of said bath' being available to take up nitrogen coming from said iron-chromium alloy of high nitrogen content and the basic slag aiding in the absorption and retention of nitrogen by the bath whereby loss of nitrogen is minimized and sound metal of desired nitrogen content is achieved.

8. In the production of nitrogen-containing rustless iron and steel of fine grain structure, the art which includes, preparing a bath of ferrous metal containing substantial amounts of chromium, and then adding togaid bath a metallic ingredient containingfi'siibstantial quantities of nitrogen, the chromiurn'of said bath being available to take up nitrogen coming from said nitrogen-containing ingredient whereby loss of nitroand slag a metallic ingredient containing sub stantial quantities of nitrogen, the chromium of said bath being available to take up the nitrogen coming from said ingredient and the basic slag aiding in the absorption and retention of nitrogen by the bath whereby loss of nitrogen is minimized and sound. metal of desired nitrogen content is achieved.

WILLIAM B. ARNESS.