US2232403A - Process for obtaining steels having high micrographic purity - Google Patents

Description

- tion to non-metallic inclusions.

Patented F eb. 18, 1941 UNITED STATES PROCESS FOR OBTAINING STEELS HAYING HIGH MICROGRAPHIC RURITY Ren Perrin, Paris, France, assignor to Socit dElectrochimie, dElectromtallnrgie et des Aciries Electriques dUgine, Paris, France, a

corporation of France No Drawing. Application February 4, 1939, Se-

rial No. 254;728. In France February 10, 1938 '12 Claims.

Recent metallurgical studies have brought to light the considerable influence exerted on the properties of steels in general by the presence therein of relatively minute quantities of aluminum, and they have made it evident that slight variations of this content are sufficient to deeply modify a number of the properties of steel, such as McQuaid-Ehn grain size, normality, depth of hardening, resistance to superheating and mechanical properties.

It has been shown in particular that substantial additions of aluminum to the steel, without having recourse to costly additions like vanadium, permit the production of fine grain steels which are advantageous for numerous applications. It the aluminum is added in the ladle or in the ingot mould it is then necessary to use," according to the authors, additions of aluniinum ranging from 0.025 to 0.050% and even more. But it is acknowledged in general by the authors that such additions have unfavourable efiects on the micrographic purity of the steel and that in any casein order to obviate such effects it is necessary to bring the deoxidisatlon to a very high degree, although no indication is given as to how this result can be obtained.

The additions of aluminum also tend to increase the number and size of the internal defects in the steel, which appear when the steel is submitted to 1:001 working (step down test) in the form of capillary lines called hair lines. The operation defined as tool working (step down test) refers to a test which is made for investigating the purity 01' the metal in rela- This test is made by taking-oil metal from the steel ingot by means of a cutting tool. This is commonly done on a lathe by turning a rod drawn out from an ingot of the steel to be tested. The cutting tool should have a very sharp edge and the depth ofthe cut should be very small. When operating under such conditions, the surface of the metal after the cutting operation shows surface defects which correspond to the non-metallic inclusions. Some of these defects appear as little spots of limited area. Some others appear as lines because the non-metallic inclusion or (a part of it has been taken away by the tool and the tool has dug a small line in the surface of the metal. This test is called a stepdown test because several successive cutting operations are generally made 'on the same piece of steel by successive steps which .diminish the diameter of the rod. The aluminum additions likewise tend to give rise to the formation of groups of inclusions called clusters, alumina clusters or "alumina stringers which are very unfavourable as-concerns the micrographic appearance of the steel.

0n the other hand slags having alumina as a basis and comprising besides alumina, lime, silica and coke, have been used with a view to desulfurizingand deoxidising steel; The results of the use of such slags have shown:

(1) That it is possible, when such aluminouslime-carbonaceous slags are used, to obtain a desulfurization and even a deoxidization, by operating as is usual with purely calcareous slag in an electric furnace, namely by charging onto the steel bath a slag composed of alumina, lime, silica and coke and thus forming a carbide slag which is caused to act upon the bath during a considerable period of time. Such a process is a mere application of the use of an aluminouscarbonaceous-slag, to the well known method of deoxidization and desulfurization by means of a carbide slag as commonly used in the electric fur- ,nace. Such a process does not lead to any particular advantage in comparison with such method. It has the same drawbacks: long duration, progressive carburation of the metal rendering it more difllcult to obtain a desired content of carbon, very great difficulty of applying the process to extra-mild steels. On the other hand it has with regard to the process using carbide slags very serious inconveniences: greater injuring of the lining of the furnace by-a slag which is corrosive in relation with the said lining and besides this an important increase of cost of the raw materials used and accordingly, of the operation, this increase not being compensated either by a technical advantage or by a saving due to a shortening of the operation. (2) Such a process as above defined depends upon the presence of a reducing atmosphere and a very high temperature of the operation which when considering the manufacture of steel, can only be reached with electric furnaces. The said process is thus practically limited to the use of an electric furnace, or the common processes in which white or carbide" slags are used.

(3) The aluminous-carbonaceous slags used in the previous process hereabov'e referred to get thicker as carbides are formed. Accordingly their fluidity decreases as their efilciency increases. Now-the applicant has discovered a process according to which it is possible to regularly and very simply obtain steels showing a very thorough micrographic cleanliness, whatever may be the fineness of their McQuaid-Ehn grain size and even when they are treated in the ladle or in the ingot moulds, with relatively important proportions of aluminum (up to 0.0200.030% or even more). Such a result is obtained as well with extra-mild steels as with mild or hard steels, although it is well known that it is much more diflicult to obtain clean extra-mild steels especially when they show a fine grain size.

Applicants invention consists in causing a bath of steel containing a strong reducing agent the oxide of which has an acid character, to act upon a slag comprising principally alumina and alkali or earth-alkaline bases, the action of said slag on the added metal being continued until the slag contains but a very small proportion of iron oxide and manganese oxide.

The reducing agent which can more particularly be silicon or even titanium or zirconium is added to the steel in suitable proportion sufficient to reduce down to a very small proportion the iron'oxide in the slag and as the case may be the manganese oxide and to ensure the deoxidising, and desulfurising of the bath of steel by combined action with the slag.

The process according to the invention does not necessitate an abnormally high temperature and can be applied as well to an operation in an open hearth furnace as to an electric furnace or to any other apparatus for making steel. There is no carburation of the slag by means of coke in the process for this is absolutely useless, the success of the operation being totally independent of the formation of carbides. It is sufficient as hereabove stated that the added metal acts upon the slag for a period of time suificient that the content of the slag in metallic oxides such as FeO or/and MnO is extremely low. This takes place in a relatively short time or in an extremely short time when the operation is performed by a violent intermixing as will be described hereafter. Although the slag becomes poorer and poorer in metallic oxides reducible by the strong reducing agent, it remains fluid during the whole operation and is for this reason especially adapted to rapidly react with the added metal. This strong reducing agent, namely silicon, is added to the bath before or during the action of the slag. The addition of the reducing agent may be made in a highly oxidised steel containing 0.080% of oxygen and a very remarkable fact is that even in such a case, a very high final micrographic cleanliness is obtained.

The action of the strong reducing agent has essentially as its object when combined with the action of the aluminous slag according to the invention, to reduce alumina and to simultaneouslyintroduce aluminum in the metal. This is practically obtained only when the action of the reducing agent has been conducted in such a manner as to lower the content in iron oxide to extremely low figures as is shown in the examples given below, viz. that a close contact has been realised for a sufficient time between the slag and the added metal until the saidQlow figures of the iron oxide in the slag have been obtained. This said action of the reducing agent is proved'by the fact that if for instance a steel which does not contain silicon or another equivalent reducing agent is violently intermixed with a slag which possesses a composition according to the invention and which initially has a very low content in iron oxidee. g. 0.04%there is no decrease of the McQuaid-Ehn grain size which would result from an introduction of aluminum as stated hereabove, and this even with steel having a high content of carbon.

It can be acknowledged that the few very small oxidic inclusions existing after solidification in the steel treated according to the invention are of a crystalline character, that they are as a rule spaced apart and that they do not elongate on rolling and forging. The steel exhibits no or only extremely few capillary flaws or hair lines and these when they exist are of very small dimensions. Moreover and in contradistinction with the conclusion which can be drawn from the old process above referred to,

the process according to the invention ensures a desulphurization of the steel, even when starting from sulphur contents which are already low, for example of the order of 0.020%, so that steels are obtained likewise which are very clean from the viewpoint of inclusions of sulphides.

Experience has shown that steel treated according to the invention contains a certain quantity of aluminum. It is observed in fact that even if aluminum has not been added to the metal during the course of the operation or subsequently, and if no deoxidising agents containing aluminum have been introduced into the slag, the steel obtained, which pipes deeply in the ingot mould with dead heads (sinkheads or sullage pieces) and charcoal on the top of the ingots, exhibits a few rare inclusions containing alumina, proving an introduction of aluminum into the metal. On the other hand, the introduction of aluminum and the presence of aluminum dissolved in the metal likewise is proved by the fact that extremely slight additions of aluminum in the ladle or in the ingot moulds, for example of the order of 0.005% are sufficient to obtain steels showing a fine Mc- Quaid-Ehn grain size, which in the normal course can only be obtained, even with steels well previously deoxidised, by additions of aluminum from 3 to 10 times as large. This fact constitutes moreover one of the advantages of the process according to the invention.

Another advantage of this process is that it is thereby possible to obtain very clean surfaces of the ingots, which reduces the cost of chipping and grinding.

The following are a few examples of remarkable results obtained by the application of the invention.

Example 1.A cast of fifteen tons of steel has been melted in an electric furnace in order to make steel specially prepared for cementation treatment. After this fusion, the slag has been removed, additions of chromium, manganese and silicon have been made to the overoxidized bath, the silicon having been added in the proportion of 0.400% by weight, and the so added bath has been violently intermixed with 4% by weight of an aluminous earth-alkaline slag previously molten and which was placed in the bottom of the casting ladle. The said aluminous slag had the following composition:

Per cent Per cent A1203 43 FeO 1.5 CaO 47 TiOz 1.4 SiO2 7 After the violent intermixing of the slag and metal had taken place, the slag which floated over the treated metal had the following com- The remainder being substantially iron. The content of oxygen of the metal was 0.072

immediately after -slagging off of the initial slag;

0.021% after the silicon, the manganese and the chromium had been added, and it was 0.0025% after the violent intermixing with the slag. The content of sulphur of the metal has been lowered to 0.006% which is extremely low for a mild steel.

An addition of 0.005%of aluminum when casting in ingot moulds, has been suiiicient for obtaining with this metal a uniform McQuaid-Ehn grain size of 67. A test made by turning on a lathe at successive diameters of 75, 65 and 55 mm. a cylindrical bar of 85 mm. diameter and 1 m. long; which test consisted in scrutinizing the surface of the bar for all the visible capillary defects, has not rendered it possible to altogether detect for the three cuts more than one defect of 1.5 mm. and nine small defects having a length smaller than 1 mm.

A test has been made which consisted in taking from the middle partof an ingot cast from a steel bath which had received no addition of aluminum, a square billet having a length of 80 mm. This billet was longitudinally cut in two pieces parallel to one face of the billet and the oxidic inclusions which appeared on a surface of this section, having a width of 1.76 mm. and the length of the billet, were counted. Measurements were made to determine the length of the inclusions which were apparent on said surface. When there were separate inclusions, the

length of each inclusion was measured. When,

on the contrary, the inclusions appeared as forming groups, the total length of the group was measured as being the length of the so grouped inclusions. The examination showed only four inclusions having lengths between 0.07

mm. and 0.17 mm. and one inclusion of 0.20 mm. The same scrutinizing made on an ingot of the same cast as above but having had added 0.020% of aluminum in the ingot moulds has shown five inclusions comprised between 0.07 mm. and 0.17

added to the steel. After the process according to the inventiton had been performed the slag had the following composition:

Per cent Per cent A1203 39 FeO 0.6 Ca() 47 MnO 0.4 $10: 1.0.5 r102 1.1 MgO 1.5 v

7 After having been treated by the slag the metal showed the following composition:

Per cent I Per cent C 1.030 C1 1.37 Si 0.265 3--."; 0.005 Mn 0.300 P 0.008

The remainder being substantially iron. As a result of the treatment the content of total oxygen in the metal which after the" addition of silicon, manganese and chromium was 0.0075% had been lowered down to 0.00l5% and the content of sulphur down to 0.005%.

The scrutinizing test by turning on a lathe as above described has permitted of detecting under the same conditions as previously recited, four small defects having each a length which was smaller than 1 mm., the ingot showing a fine McQuaid-Ehn grain size.

The counting of the oxidic inclusions, all of which consisted of alumina, was done under the same conditions and showed only four inclusions having a length comprised between 0.07 mm. and

0.17 mm. but none having a larger dimension. An ingot containing an addition of 0.020% of alumimum made in the ingot mould after slag treatment according to the present process showed three inclusions the length of which was comprised between 0.07 mm. and 0.17 mm., and one inclusion of 0.22 mm. in length; the austenitic grain obtained by a.McQuaid-Ehn test has been found to be uniformly fine as soon as the aluminum added reached 0.007% by weight.

Clearly it would be possible without departing from the scope of the invention to add aluminum to the steel besides silicon (or if desired titanium or zirconium) v before or during the reaction with the aluminous slag containing alkaline or earth-alkaline bases. tion nevertheless does not appear to present any specialadvantage, the essential point being the action of the silicon. (or if desired of the titanium or zirconium) upon such a slag, the said action being continued until the slag contains very little iron oxide and relatively little manganese oxide.

On the other hand the slag may initially contain a certain proportion of oxidesof iron and manganese, provided that the final slag is finally very poor in these compounds; but the inconvenience of such a method of operating would be to give rise to a useless consumption of the reducing agent the oxide of-which is acid, such as silicon, incorporated in the steel, since a part of the reducing agent would then have to act for reducing the oxides of iron and manganese, and this would require the addition of a larger quantity thereof. It is true that the reduction of these oxides could be eiiected by aluminum, but this would likewise lead to additional expense. It is therefore advantageous to start with slags containing no excessive proportion of the abovementloned oxides.

However, the essential consideration which must enter into account is, as already emphasized above, the final composition of the slag after reaction of the silicon at the time the metal containing silicon is ready to be cast in the ingot mould. Besides the slight content in oxides of iron and manganese this composition must correspond to a. slag containing essentially alumina and alkali and/or earth-alkaline The final slag may nevertheless contain bases. a certain proportion of silica, and if this amount isincreased, no abrupt change in the results ob- ,tained is observed, but these results become progressively less good. An example will now be given which shows by comparison with theresuits indicated above, the unfavorable influence which may be exerted by a slag rich in alumina and in earth-alkaline bases but containing in addition to these substances, a relatively large pro-,

Such an addithe treatment the slag had the following composition:

Per cent Per cent Si0 40. 8 MnO 3. 3 A1 0 21 MgO 3. 5

a0 27. 5 TiO l. 2 FeO 0. 7

After treatment by the aluminous slag the composition of the metal was:

Per cent Per cent C 0.090 -Cr 0.74 Si 0.415 S 0.015

Mn 0.375 P 0.012 N1 2. 68 Fe- Substantially the remainder.

With an addition of 0.012% of aluminum in the ingot mould the steel still showed a mixed Mc- Quaid-Ehn grain of non-uniform fineness. "A count of the inclusions carried out on an ingot which has received such an addition of aluminum showed twenty-five inclusions from 0.07 mm. to 0.17 mm. in length, five inclusions from 0.17 mm. to 0.35 mm. in length, three inclusions from 0.35 mm. to 0.70 mm. in length and two inclusions of a length greater than 0.70 mm. The cleanliness of this steel was thus distinctly inferior to that of the steels obtained in the two examples previously described.

When the slags used as starting material are made up for instance solely or principally of alumina and lime with an addition of silica it is preferable that the proportions of these constituents are such that the final content of the slag in silica after the operation according to the invention has been performed, does not exceed 30% while the alumina content reaches at least 20%. By way of examples but with no limiting character, excellent results can be obtained with slags finally having the following compositions:

Per cent Per cent A1203 8 CaO 40 FeO 0 8 SiO .20 MnO 1 2 a0 44 MnO 0 5 SiO; 15

or again A1203 35 MgO 4 CaO 47 FeO 0 5 SiO; 10 -TiO 2 Of course the addition of fluxes such as fluorspar, titanic acid or zirconia even in proportion larger than in the previous examples, would not involve departure from the scope of the invention.

The process according to the invention. may be carried out by any desired method of operation, provided that a slag having the starting characteristics previously defined is caused to act upon a metal containing a strong reducing agent the oxide of which is acid, such as silicon, provided also that the slag has finally the hereabove general or particular composition hereabove described. Two methods of procedure will now be described by way of example.

Dealing with a steel prepared in an electric furnace or in an open hearth furnace, the slag existing above the metal in the furnace, for instance after melting the charge and bringi it to the required temperature is eliminated, then the constituents of the aluminous slag are charged onto the metal. Melting takes place more rapidly if a synthetic slag of the desired composition has previously been formed by fusion and if this slag is charged in the solid state in small pieces. The slag can also be charged in liquid form, which gives still more rapid results. Additions comprising silicon are fed to the metal at the same time as the slag is charged onto the metal or before or after charging of the latter, and the metal containing the silicon is allowed to act until the slag has the desired composition and is in particular practically free of iron oxide. The addition of manganese and the additions of alloying constituents can be made at any time during the operation.

It sometimes takes a relatively long period of time for obtaining in a furnace, without any other precautions, a very far reaching reducing action of the silicon contained in the metal on the aluminous slag. Therefore, in contradistinction with a practice which is widespread and recommended by experts, which consists in keeping back the slag as much as possible when the metal is cast, in order to avoid a mixture of the steel and slag, an extremely favorable means which shortens in any case the duration of the operation, consists in casting the slag in the ladle before or at the same time. as the metal in such a manner as to violently and thoroughly intermix the metal and the slag. This has a very great advantage besides saving time of minimising the attack on the lining'of the furnace or of the ladle by slags which corrosively act upon it, and also to diminish the modification of the slag and especially an enrichment of the same in silica and metallic oxides coming either from the lining or from the bricks of the roof or from any other source. The examples given hereafter illustrate the great advantage of this way of operating.

Example IV.A cast of fifteen tons of case hardening nickel-chromium steel and having a composition which is close to the one described in Example I has been treated, after slagging off the fusion slag and after additions of manganese. chromium. and 0.350% silicon, with 4% by Weight of an aluminous earth-alkaline synthetic slag previously formed which has been charged in solid state in the electric furnace-and has been melted over the liquid steel bath. The melting of the slag lasted 10 minutes. The slag and metal has then been cast in a thick jet from a height into a ladle in order to perform an intermixing of the slag and metal as violently as possible. The slag before tapping'from the furnace had the following composition:

Per cent Per cent A1 0 32 FeO 1. 6 02.0 43 MnO 0. 4 SiOg 14. 3 TiO 2. 2 Mg O 5. 4

After the operation in the ladle the slag floating over the metal had the following composition:

Per cent Per cent A1 0 30. 5 FeO 0. 7 CaO 42 MnO 0. 7 sio2 17. 5 T102 2 MgO 5. 5

The composition of the steel after it had been treated by the slag was the following:

Per cent 1 Per cent C 0.130 Cr 0.7 S1 0. S 0.008 Mn 0.445 P 0.010 Ni 2. 8

The metal showed the following contents of sulfur and of oxygen:

After the silicon was added and before the slag was fused s 0.020% 0.015% After the slag was fuseds 0.015% o -f0.007% After violent intermixing- These figures illustrate the very great advantage of the final violent intermixing.

A scrutinizing of the metal by turning on a lathe and by numbering the inclusions under the same conditions as above described have respectively shown: two defects of 1 mm. and six small defects less than 1 mm. in length, and five inclusions of 0.07 mm. to 0.17 mm. in length, and two inclusions having 0.25 mm. in length The Mc- Quaid-Ehn grain size was a fine one and was obtained with an addition of 0.005% of aluminum.

Example V.A melt of fifteen tons of chromium hard steel to which 0.450% of silicon has been added has been treated under the same conditions as the heat described in the preceding example.

The composition of the fused slag floating over the bath in the furnace just before tapping was the following:

' Per cent Per cent FeO 0. 9 MnQ 0. 3 TiO 1. 2

In the ladle, after the operation had been added, the composition of the slag had become the following:

Per cent Per cent A1203 F60 O. 4 C110 46 MnO 0. 7 SiO, 18 TiO 1. 2 MgO 4. whilst the composition of the metal was:

Per cent Per cent C 1. 020 Or 1. 42 Si 0. 245 S 0. 006 Mn 0.320 P 0.010

The remainder being substantially iron.

The content of sulfur had dropped from 0.019% just before tapping to 0.006% after violent intermixing. The content of oxygen determined under the same conditions had dropped from 0.009%

A scrutinizing by turning on a lathe has shown a single defect longer than 1 mm. and six small defects. The numbering of the oxidic inclusions has only shown five inclusions which were shorter than 0.17 mm. and one comprised between 0.1! mm. and 0.34 mm.

The process according to the present invention can be carried out as well in a high frequency furnace, or neutral or basic hearth, or in an open-hearth furnace or in any suitable receptacle. But a particularly advantageous procedure which is applicable whatever may be the way in which the steel'is made and especially to the well known classicalv processes with an open-hearth furnace or with a converter, consists in using for the intermiidng of the aluminous slag and the metal containing the strong reducing a ent, a process such as the one descrlbed'in the U. S. A. Patent No. 2,015,691 and in the French Patents No. 724,869 of September 18, 1931, and No. 747,074

steel whatever may be the viewpoint from which the metal is considered: micrographic or macrographic cleanliness, chemical composition, longitudinal and transverse mechanical properties etc. Owing to the small proportion of slag which is necessary for carrying-out the process it is possible by using a pretty hot metal to add the slag in the form of solid elements, preferably in the form of a previously molten synthetic slag which has been comminuted after cooling and which is used either cold or hot. In these cases the slag fuses as soon as the pouring commences and the intermixing with a molten slag is thus performed. In a very short. period of time the reaction of the silicon in the metal is obtained upon a'slag which when the operation is ended is very poor in iron oxide. When operating as above described the slag may initially contain a certain proportion of iron oxide and manganese oxide, which only necessitates to add a greater excess of silicon in relation to the content which is to be kept in the metal, the said oxides being widely reduced by the silicon during the violent intermixing, so that the final slag is poor in FeO and mo.

The reduction of oxidic compounds contained in the slag besides the alumina may receive an interesting practical application when it is desired to obtain steels containing alloying constitwith violent intermixing when working with anopen-hearth furnace of 40 tons is given below.

Example VI.--A bath of steel of 40 tons was made in an open hearth furnace. The composition of the said steel was, just before adding silicon:

Per cent Per cent C 0. 60 P 0. 022 Mn 0. 45 S 0. 024

the remainder being substantially iron.

A proportion of 0.400% by weight of silicon was added to the metal i. e. 160 kg. of silicon. The so-added steel has then been immediately tapped by keeping back in the furnace the slag which floated over the metallic bath, the tapping operation being carried out in a thick jet from a considerable height into a ladle containing 1400 kg. of a very fluid previously molten slag of the following composition: Y

Per cent Per cent A1 0; 43. 2 TiO 2. 1 CaO 47. 5 F--- 0. 8" SiO, 5. 3

After the violent pouring of the steel onto the aluminous slag which lasted about one minute and a half, and after natural separation of theslag the compositions of the metal and of the slag were respectively the following:

Steel:

Per cent Per cent C 0. 60 P 0. 021 Mn 0. 45 S 0. 007 Si 0. 27

Slag:

A1 40. 2 TiO 1. 9 CaO 47. 2 FeO 0. 5 SiO 8. 9 MnO 0. 5

When cast in ingot moulds without any addition of aluminum the steel was perfectly quiet and its McQuaid-Ehn grain size was 7 which proved an important introduction of aluminum into the steel.

The numbering of the inclusions according to the same method as that described in the previ ous examples showed only 3 inclusions of 0.07 mm. to 0.17 mm. in length.

The steel has been subjected to transverse Mesnager impact value tests after the steel had been hardened and then tempered at 650 C. A transverse impact value of 5.7 has been acknowledged, which is extremely remarkable for such a hard steel type.

The cast when transformed into very thin wires has shown a very high transformation suitability and the yield from the billet to the wire has been increased by 30% compared to a cast of steel of the same composition obtained in openhearth furnaces but not treated according to the present process.

No absolute figures have been given hereabove asconcerns the proportion of reducing agent to be added to the steel in order to cooperate with the aluminous slag according to the present invention because the said proportion varies according to the circumstances. Propontions of silicon have been given in the examples; when the steel to be treated does not initially contain silicon a proportion of about 0.150% to 0.750% is in most cases suitable for carrying out the process.

The proportion of silicon to be added to the steel will obviously depend upon the final content of silicon which is desired in the steel and upon theproportion of FeO, MnO or other oxidic compounds to be reduced.

The aluminous slag after it has been used for cleansing steel according to the present invention canbe utilised in the manufacture of cement, which constitutes a supplementary advantage of the method.

What I claim is:

1. The process for obtaining steel having a high degree of micrographic cleanliness, which comprises adding to a. bath of steel a strong reducing agent of the group consisting of silicon, titanium and zirconium, causing to act upon the so treated steel a basic slag consisting principally of alumina and oxide of at least one member of the group consisting of alkali-metals and alkaline earth metals, the proportion of alumina being higher than the proportion of the total of all the acid constituents of the slag, and carrying out the action of the slag upon the steel until the slag contains only a very small quantity of iron oxide.

2. The process for obtaining steel having a high degree of micrographic cleanliness, which comprises adding to a bath of steel 9. strong reducing agent of the group consisting of silicon,

titanium and zirconium, causing to act upon the so treated steel a basic slag consisting principally of alumina and oxide of at least one member of the group consisting of alkali-metals and alkaline earth metals, theproportion of alumina being higher than the proportion of the total of all the acid constituents of the slag, and carrying out the action of the slag upon the steel until the slag contains not more than about 1% of iron oxide.

3. The process for obtaining steel having a high degree of micrographic cleanliness, which comprises adding silicon to a bath of steel, causing to act upon the so treated steel a basic slag consisting principally of alumina and oxide ofat least one member of the group consisting of alkali-metals and alkaline earth metals, the proportion of alumina being higher than the proportion of the total of all the acid constituents of the slag, and carrying out the action of the slag upon the steel until the slag contains only a very small quantity of iron oxide.

4. The process for obtaining steel having a high degree of micrographic cleanliness, which comprises adding to a bath of steel 2. strong reducing agent of the group consisting of silicon, titanium and zirconium, causing to act upon the so treated steel a basic slag initially containing silica, a proportion of alumina larger than the silica, and oxide of at least one member of the group consisting of the alkali-metals'and alkaline earth metals, and carrying out the said action until the slag contains only a very small amount of iron oxide, the proportions of the constituents used for preparing the slag and the proportion of slag being such that at the end of the action the slag contains not more than 30% silica, not less than 20% alumina and is poor in iron oxide.

5. The process for obtaining steel having a high degree of micrographic cleanliness, which comprises adding to a. bath of steel at least about 0.15% but not more than about 0.75% of a strong reducing agent of the group consisting of silicon, titanium and zirconium, and treating the so added steel with a basic slag containing a higher proportion of alumina than silica and also containing oxide of at least one member of the group consisting of alkali-metals and alkaline earth metals until the said slag contains only a very small quantity of iron oxide.

6. The process for obtaining steel having a high degree of micrographic cleanliness, which comprises adding to a bath of steel a proportion of silicon between about 0.15% and 0.75% and treating the so added steel with a basic slag containing a higher proportion of alumina than silica, and also containing lime, the remainder of the slag being substantially substances which act as fluidifying agents for the alumina and lime, until the slag contains only a very small quantity of iron oxide, the proportions of alumina and silica in the slag being such and the proportion of slag used for the treatment being such that the slag after the treatment has been performed contains more than 20% of alumina and less than 30% of silica.

7. The process for obtaining steel having a high degree of micrographic cleanliness, which silica and also containing oxide of at least one member of the group consisting of alkali-metals and alkaline earth metals, until the said slag contains only a very small proportion of iron oxide.

8. The process for obtaining steel having a high degree of micrographic cleanliness, which comprises producing a vigorous intermixing between steel containing a strong reducing agent of the group consisting of silicon, titanium and zirconium, and a fused synthetic basic slag containing more alumina than silica and also containing oxide of at least one member of the group consisting of alkali-metals and alkaline earth metals, until the slag contains only a very small quantity of iron oxide. 1

9. The process for obtaining steelhaving a high degree of micrographic cleanliness, which comprises adding to a bath of steel a strong reducing agent of the group consisting of silicon, titanium and zirconium, causing to act' upon the so-treated steel a basic slag consisting principally of alumina and oxide of at least one member of the group consisting of alkaliunetals and alkaline earth metals. said slag containing about '20 to 60% of alumina, and carrying out the action of the slag upon the steel until the slag contains only a very small quantity of iron oxide. 10. The process for obtaining steel having a high degree of micrographic cleanliness, which comprises adding to a bath of steel such an amount of a strong reducing agent of the group consisting of silicon, titanium and zirconium that the oxides reducible by said agent which are present in the bath are transformed into oxides of I said agent by a part of the said strong reducing agent and that a part of thesaid agent remains untransformed, causing to act upon the so treated steel a basic slag consisting principally of alumina and oxide of at least one mem-- ber of the group consisting of alkali-metals and alkaline earth metals, the proportion of alumina) being higher than the proportion of the total of all the acid constituents of the slag, and carrying out the action of the slag upon the steel until the slag contains only a very small proportion of iron oxide.

11. The process for obtaining steel having a high degree of micrographic cleanliness, which 7 comprises adding to a bath of steel such an amount of a reducing agent stronger than aluminum that when causing the so added steel to react upon a basic slag consisting principally of alumina and oxide of at least one member of the group consisting of alkali-metals and alkaline earth metals and in which the proportion of alumina is higher than the proportion of the total of all the acid, constituents of the slag, the action of the said added metal on the said slag being carried out until the slag contains only a very small proportion of iron oxide, the stronger reducing agent reduces alumina of the slag.

12. The process for obtaining steel having a high degree of micrographic cleanliness which comprises introducing aluminum in statu nascendi into a bath of steel containing a tree reducing agent stronger than aluminum, by the action of a basic slag consisting principally oi alumina and oxide of at least one member 01 the group consisting of alkali-metals and alkaline earth metals and in which the proportion of alumina is higher than'the proportion of the total of all the acid constituents of the slag, the action of said slag being carried out upon the steel until the slag contains only a very small proportion of iron oxide. I

RENE PIERRIN.