US1715979A - Low-carbon chromium steel - Google Patents

Description

Patented June 4, 1929.

UNITED STATES PATENT OFFICE.

HENRY C. BIGGE AN!) CHARLES R. ELLICOTT, 0F BETHLEHEM, PETINSYLVANIA, ASSIGNOR-S TO BETHLEHEM STEEL COMPANY.

LOW-CARBON CHROMIUM STEEL.

No Drawing.

treme wildness when poured into the ingot molds, the consequent lack of homogeneity in the metal and the abundance of blow holes in ingots resulting therefrom. The precautions, well known in the steel making art for securing sound ingots, have appeared hitherto to be quite ineffective in overcoming the aforementioned difficulties. These difficulties chrome steel is the high content of oxidized.

chromium in the final slag. It has even been suggested that such high content of chromium 1n the slag is a necessary attribute of low carbon chromium steels, and that by maintaining this loss in the slag the difficulties in casting may be'to some extent overcome.

To the best of our knowledge, this has hitherto been the accepted view and no success-' ful process of eliminating the loss of chromium to the slag and withal producing sound ingots has hitherto been developed. The object of our invention is, therefore, to insure the production of sound ingots having a low carbon content'and a substantial content of chromium, without incurring any great losses of chromiun r other metallic values in the Application filed November 15, 1924. Serial No. 750,194.

furnace slag. A further object of our invention is to improve the control of the furnace reactions and to appreciably reduce the duration of the heat A further object of our invention is to develop a furnace process for the production of sound metal that does not depend on corrective measures of'scavenging and degasification with possible inclusion in the metal of such additions.

Because of its well known advantages, we prefer to use an electric furnace for the op eration of our process. The preliminary steps of charging into the furnace the desired amount of commercial steel scrap of low carbon content and iron ore, and eflecting a fusion of the charge. are carried out in the customary manner. The slag is run off and a sufficient amount of burnt lime is added to the bath so as to form an adequate slag blanket for the bath. For example, if We charge 5200 lbs. of low carbon scrap and 300 lbs. of iron ore, we make an addition, after slagging off, of lbs. of lime, to form a slag blanket. Before making any additions of chromium in the form of ferrochrome, We add to the bath approximately 75 lbs. of ferrosilicon containing 50% of silicon, said addition being preferably made in two portions of 50 lbs. and 25 lbs. respectively. The first portion is preferably added immediately after the addition of lime, and the second portion added about 10 minutes later. The function of the ferrosilicon additions is to effect the deoxidation of the bath with increasing formation of SiO in the slag. At the same time the ferrous oxide content of the slag is rapidly reduced and the metallic iron passed into the bath. \Ve have found that an interval of from 10 to 15 minutes is sufficient to completely effect this reaction. The bath being now in a substantially deoxidized condition we proceed to make the necessary additions of heated ferrochromium to ob tain the final chromium content desired. To

avoid excessive chilling of the bath and an undesirable loss of fluidity in the slag, we

case we have mentioned for the purpose of describing our process, we make a first addition of approximately 550 lbs. of ferrochrome followed by 20 lbs. of lime and 20 lbs. of fluorspar. We prefer to make the two subsequent and equal additions. of ferrochrome at 20 minute intervals without addition of lime or of fluorspar between the second and third additions of ferrochrome. Immediately after the last addition of ferrochrome, we add 10 lbs. of fiuorspar and 20 lbs; of lime and maintain the bath tempera ture for a further period of 20 minutes. A slag test made at this point exhibits a characteristic disintegrating lime slag. Hitherto it has been considered that the chemical reactions of a basic steel electric process have been completed as soon as this disintegrating slag has been obtained. Upon pouring the bath at this stage the metal shows considerable wildness, which has generally been attributed to the presence in the metal of oceluded gases. homogeneity due partly to these gases and partly to inclusion in the metal of suspended oxides and silicates in a state of line subdivison. To overcome these defects, various corrective measures have been proposed.- For example, it has been suggested that a quiescent period should be allowed to the metal in the furnace before tapping, and the temperature allowed to decrease by. switching off the current applied to the electrodes, or by raising the electrodes clear of the molten charge; It has also been proposed to add to the metal bath, either in the furnace before tapping or in the ladle before pouring, degasifying agents or scavengers for the purpose of combining with the oxygen occluded in the metal, the oxides soformed passing to the slag. To the best of our knowledge, none of these measures have been effective in improving the quality of ingots made of low carbon chromium steel.

In the case of the quiescent period proposed, apart from the loss of time and pouring temperature, we have found that further reactions may take place tending to dis: turb the equilibrium of the bath as is evidenced by the lowering ofthe silica content of the slag. Moreover, the advantages claimed for said quiescent period are not borne outby practice. Non-metallic inclusions in the bath which it is expected will, under quiescent conditions, have the opportunity of rising to the slag do so under a slag, the fluidity of which has been impaired by the loss of temperature during this peri-.

od. This accumulation at the interface of the slag and the bath does not materially prevent their being washed back" into the metal when the furnace is tapped, though possibly they may, as a result of the quiescent period, he in a state of greater coalescence and aggregation. 7 Similarly we have not The ingots in cooling lack found that any specific advantage is derived therefrom for the removal of occluded gases; on the contrary, we consider this purpose is best attained by keeping the bath in a condition of circulation or agitation.

The addition of degasifying agents to the bath is objectionable for several reasons. In the first place, it is difficult to insure the entry of the degasifier into the bath, and various methods of achieving this have been devised, for example, by introducing the degasifying agent in the form of a briquette or where its specific gravity is low, relying upon the addition of a heavier metal compounded with the degasifying agent to insure its passage to the lower part of the bath. In any case, an even distribution of the degasifier in the bath is impossible to attain and a certain period of time must elapse before its action can be effected to the desired degree. We have experimented with degasifying additions to the bath both in the furnace and in the ladle without obtaining'any material advantage therefrom. As the extent of occlusion in any bath is an unknown quantity it is evident that a purely arbitrary addition must be made, mpre than suflicient to combine with the occluded gases or the suspended oxides. The result of such addition-will be an inclusion of the excess of uncombined reagent in the metal. For example, we have made additions to the bath of silicon. the scavenging properties of which are well known, in the form of ferrosilicon, but beyond a tendency to increase the silicon content of the metal, which we particularly desired to avoid, we failed to effect any substantial improvement in the pouring conditions of the metal.

As we have already stated, after the charactristic disintegrating slag has been formed, there is a tendency towards loss of silica if the temperature of the bath is maintained and a quiescent period allowed. .Analysis of the slag atjthis point shows an appreciable content of chromium in the slag, also some iron, possibly existing as oxidesand silicates. We also consider itprobable that the early additions offerrosili'con to the bath have contributed to the formation of suspended metallic silicates in the bath. The presence of said suspended silicates in the finalbath un-' doubtedly tends to produce faulty ingots, and the addition of scavengers thereto does not materially assist their elimination.

It is well known that the presence of carbon in a metal bath favors the reduction of oxides contained therein. Hence in the case of a bath that has been almost completely decarburized, there is a danger of over-oxidation and the consequent inclusion in the metal of'suspended metallic oxides and silicates and of uncombined oxygen, all of which tend to cause bad ingots. Silicon which is capable of removing these inclusions at appropriate temperatures cannot be added to the bath at this stage, because of the danger of increasing the final silicon content in the bath, it being impossible to determine in the course of a furnace heat the precise content of oxides in the decarburized bath and, therefore, difficult to avoid adding an excess of silicon thereto. As we have already stated, it is our particular object to produce a low carbon chromium steel or a chrome iron that shall be low in silicon. Residual aluminum, titanium, manganese, etc., are all likewise undesirable and it is therefore evident t-hat'no addition to the bath itself for the purpose of removing occluded oxygen or oxide inclusions can be contemplated.

As one of the objects of our invention is to reduce the metallic loss in the slag, it is clear that any treatment contemplated must be confined to the slag. We have found that by reconstituting a strongly reducing slag and maintaining the circulation of the bath, both the occluded gases and the solid metallic and non-metallic inclusions may be removed. Increase in the temperature of the slag and of the bath flavors these reactions, hence any addition to the slag should be such as to produce a strongly exothermic reaction. A further advantage of this development of heat during these reactions is that the circulation of the (path beneath the slag is thereby facilitate We have found that our objects are completely attained by a final addition to the slag of a silicon alloy in finely pulverized condition. Such addition should be made quickly n idevenly to the surface of the slag without allowing its introduction to the bath.

Careful analyses before and after the addition of the pulverized silicon to the slag indicate that the mechanism of our process is probably as follows: The slag before such addition contains from 6 to 8% iron, mainly as an oxide and probably also as silicate and from 2.00 to 5.00% of chromic oxide, (Cr O On the addition of pulverized silicon, part of the iron oxide is first reduced, the temperature resulting therefrom in excess of the silica formed tending to also promote decomposition of any silicates present. The iron oxides having been substantially reduced and the temperature of the slag increased thereby, the chromium oxide (cr O is then attacked. Although the slag is still basic the free silica now present, because of its acid reaction, no doubt favors the reduction of the chromium, which duly takes place with further developmentof heat. The reduced metals, iron and chromium, because of their high specific gravity descend to the bath and probably efiect some metallic displacement of the suspended oxides or silicates of lighter specific gravity. Analyses which we have made of ingots exhibiting the wildness characteristic of low carbon chrome steels, and of ingots produced by our process,

has a slight desiliconizing effect on the metal,

which we attribute to the removal from the latter of suspended silicates. A further phenomenon of our process is the fact that loss of chromium by oxidation, during transfer of the metal to the ingot mold is very considerably diminished, owing to the high temperature at which our metal is poured, the oxidation of chromium on contact with the air taking place less readily at this high temperature. v

The essential feature of our process may therefore be described as a final reduction of the slag and the elimination from the metal of inclusions, said elimination resulting indirectly from the aforesaid slag reduction. The reactions of our process take place rapidly and at high temperature and the metal should be poured as expeditiously as possible as soon as the point of equilibrium between the bath and the slag has been reached. This point is very readily determined by the fact that the appearance of our slag is changed from that of a typical disintegrating slag to that of a dark consistent slag. We consider the greater cohesiveness of this final slag also possesses the physical advantageofpreventing the washing down of slag particles into the metal when the furnace is being tapped.

It is evident that our processhas nothing in common with the addition of silicon to the bath itself. The deoxidizing effect of ferrosilicon, for example, is well known, but it has also been recognized that such addition has certain objections and should be sparingly used in the final bath, because of the tendency of silica to unite with unreduced metallic oxides and form silicates therewith. I

WVhile a portion of said silicates will pass to the slag, appreciable quantities remain in the bath in a condition of fine suspension as we have determined by .analysis. Besides increasing the silicon content of.thefinal metal, which we desire to avoid, these said silicates are frequently the cause of undesirable inclusions in the metal which segregate during the cooling of the ingot.

In'our process we prefer to use finely pul verized ferrosilicon for ourfinal slag addition, but any alloy, of silicon is equally applicable, such as alloys of silicon with calcium or with manganese. Silicomanganese would present certain advantages if an increase in the manganese content of the steel were not objectionable.

Referring to the specific furnace charge we have described,we add 45 lbs. of pulverized 50% ferrosili'con to the surface-of the slag, and tap the heat about 10 minutes after such. addition. The comminution of the ferrosilicon and the greater reaction surface exposed thereby effects a very rapid reduc tion of the metallic oxides in the slag.

While we have shown and described our invention as applied to the production of chrome iron in an electric furnace, it will .be obvious to those skilled in the art that it is not so limited, but is equally applicable to other types of furnace and to other alloys having similar characteristics. It is also susceptibleof various changes and modifications without departing from the spirit thereof, and we desire, therefore, that only such limitations shall be placed thereupon as are imposed by the prior art or as are specifically set forth in the appended claims. Having thus described the invention What we claim as new and desire to secure by Letters Patent is:

1. In a process of preparing a low carbon, ferrous alloy having a metal component of the chromium group, normally tending to be Wild, the steps of adding a reducing agent to the slag to produce a strongly exothermic reaction, and pouring the alloy while the thermal effects of said reaction are still present in the slag.

2. In a process of preparing a low carbon, ferrous alloy containing chromium in substantial amounts, the steps of adding a reduc- 30 ing agent to the slag to produce a strongly exothermic reaction, and casting While the thermal effects of said reaction are still present in the slag.

3. In the process of manufacturing a low carbon steel containing chromium in substantial amounts, the steps of preparing a fused bath of the desired composition and having a basic slag, next adding a strongly exothermic reducing agent to the slag, and casting the metal While the slag still retains thermal effects of said reaction.

4. In the process of manufacturing a low carbon steel containing chromium in substantial amounts, the steps of preparing a fused bath of substantially the desired composition and having a basic slag, next adding an agent containing silicon as a reducing agent to produce a strongly exothermic reaction in the slag, and then casting the metal before the effects of said reaction have been lost.

In testimony whereof I hereunto aflix my signature this 7th day of November 1924.

HENRY C. BIGGE.

In testimony whereof I hereunto affix my signature this 7 th day of November 1924.

CHARLES R. ELLICOTT.