US2288836A - Process for economically and rapidly obtaining high quality steels - Google Patents

Description

July 7, 1942-. R. PERRIN 7 2,288,836

rnoclsss FOR acououxcnu AND RAPIDLY mmmme urea QUALITY swans Filed Oct. 17, 1939 v INVENTOR J? 617.6 Perrz'n Patented July 7, 1942 OFFICE LY OBTAINING HIGH QUALITY STEELS Ren Pen-in, Paris, France, assignmto- Societe D'Electrochimie, DElectrometailnrgie et des Acleries Electrlques D'Ugine, Paris, France, a corporation of France Application October 17, 14239, Serial No. 299,907

In France November 17, 1938 6 Claims. (01. 15-55) The recent metallurgical studies tend to infer that all sorts of properties of steels: micro- Ki'aphic cleanliness, Mac-Quaid-Ehn grain size,

standardization, mechanical characteristics, brittleness, etc. are-essentially determined by thev composition of the steel when in liquid state having regard to all the substances contained therein, some elements having a capital influence upon certain properties, even whenin very small proportions, whilst higher contents of other elements have much less influence.

n the other hand a long metallurgical practice has shown that the steels obtained in a basic electric furnace, when they are especially manufactured by a melting process without oxidization, owing, to the neutral atmosphere of the electric furnace, then deoxidized and desulphurrized under carburized slag for a long time which may often reach three hours and more, show properties that are distinctly higher than those of the steels obtained in a basic Martin furnace and of course so much more than those obtained in an acid or a basic converter.

ium or like contents, according as the sulphur I content is 0.020 per cent or 0.012 per cent.

Such a fact which is of the highest importance, makes it obvious that it is a considerable technical improvement for the world's metallurgy to be able to regularly manufacture, otherwise than with the expensive electric furnace practice under white or carburized slag, low sulphur content steels and particularly to reduce to the above mentioned amounts, in a regular way, the sulphur contents of steel baths obtained in a converter or in a basic Martin furnace from any sort'of charge, the said improvement being effective only if really low contents are attained.

By way of an example of the said influence of sulphur, nickel-chromium cementation steel bodies have been manufactured under absolutely identical conditions and with substantially the same amounts of oxygen, nitrogen, hydrogen, Si,

The said superiority is shown-by a number of properties, particularly: micrographic cleanliness, dendritic homogeneity, absence ofinternal defects, longitudinal and still more transverse mechanical properties, specially as to resilience and the reduction ,of tempering brittleness.

But if in a general way the metallurgists endeavoured to obtain low sulphur content steels,

the importance of actually low sulphur contents did not clearly appear until now and the capital endeavours of the metallurgists were principally directed towards the reduction of oxygen content,

a the difference in quality between Martin steel and electric'steel being first of all attributed to the more complete deoxidization obtained in an electric furnace. In this way and more generally the specification of customers only state sulphur contents below or equal to 0.025'per cent or 0.030 per cent-sometimes even more.

The applicant has discovered that all other conditions being equal, 1. e. particularly the oxygen, nitrogen, Si, Mn, Al contents of steel, very important improvements in the qualities of steel are obtained and most specially of longitudinal and still more of transverse mechanical characteristics, particularly of the impact characteristics as well as ductility of cold .steels, when the sulphur content is reduced to about 0.015 per cent and even less.

For instance, a difi'erencelwhich is extremely substantial and important in practice is noticed between steels having the same oxygen, alumin- Mn, Al but differing only in the sulphur-contents.

The characteristics obtained with the said baths were as follows:

Example 1 Longitu- Trans- Tensile a dinal reverse remmgth slliency silienc-y I K gs. Oil hardened at 850 C..... 5 136 7. l 2 S=0.027% Oil hardened at 850 0.,

tempered at 500 C 94 12.5 2.9 Oil hardened at 850 C. 135 10.4 b S=0.013% Oil hardened at 850 0.,

tempered at 5(XJ C 96 17. 4 8 5 Example 2 Longitu- Trans- .taelt e a siliency sihency Kg. Oil hardened at 850 (1.... 99 0. 3 3. 5 S==0.020% Oil hardened at 850- C.,

tempered at 500 C 87 i3, 5 4. 7 Oil hardened at 850 C. 96 17.4 9. 5 S=0.0l4% Oil hardened at 850 0.,

tempered at 500 0 23. 4 11.8

There is a. considerable difference in the values of the longitudinal resiliency and still more in the transverse resilience, the ratio transverse resiliency longitudinal resiliency being considerably increased by reducing sulphur to contents slightly below 0.015 per cent.

The applicant discovered that in applying procphurizing power.

esses in which the slag is distributed in fine drops into the steel 'it is possible to obtain in a very short time, even from highly oxidized steels (whatever may be the melting process, be it electric, Martin, converter or like process) steels reproducing all the characteristics of electric steels which have been manufactured without having been highly oxidized and which have been treated by a carburlzed slag, the following conditions being fulfilled, i. e. that these steels not only have a low oxygen content but also show very low content of sulphur, this content being of the order stated hereabove.

The advantages set forth in the basic electric furnace practice naturally induced to use in a method applying previously molten slags, the well-known high limestone contents basic slags of the electric furnace and attempts have been made in that direction. But the results heretofore obtained gave insufficient and irregular desulphurizatons, consequently without any industrialinterest and did not in. any case correspond to the results that the above mentioned studies of the applicant show as being of capital impor-- tance, i. e. securing sulphur contents lower or at the most equal to 0.015 per cent. Indeed it is not suflicient for obtaining an effective, rapid and regular desulphurization to pour steel in a slag, even in a very high limestone and very low iron oxide content slag, even in a slag whitened by gether high lime contents and very low iron oxide contents, had by themselves a very high desul- In the said treatments mild steel with about 0.10 per cent carbon content was intimately intermixed with highly basic and very low iron oxide slags, possessing theoretically a high desulphurizing power, the said slags being rendered by addition of fluor spar.- The analysis of the slag before intermixing is given hereafter as a mere indication:

Per cent CaO 70.3 $102 14.6 A1203 6.4 MgO 3.1 MnO 0.2 FeO 0.8

In the above test, the sulphur content of the metal before inter-mixing was 0.052 per cent; after intermixing 0.048 per cent.

Other tests made with similar slags gave the following results:

Per cent Sulphur 0.058 per cent before intermixing;

after inter iximr Sulphur 0.071 per cent before intermixing;

after intermixing 0.064

the said content is not already very low sulphur contents quite equivalent to those obtained in an electric furnace by using carburized slags--the desulphurizing power of such slags being however well known as being much higher-and this under industrial conditions, i. e.. quite regularly and whatever may be the carbon content of the steel, be it hard or halfmild steel, mild or very mild steel (although it is well known that it is extremely diflicult to reach very low sulphur contents in mild steels even in an electric furnace).

This process appears consequently as having a more general scope for desulphurization than the classic process of the electric furnace using white or carburized slags.

This result is obtained according to the invention, under the essential condition of applying simultaneously a number of means disclosed hereafter:

(a) First of all the slag should essentially comprise lime, alumina and silica with a more or less important addition of fluxes such as fluor spar; it may further contain small proportions of iron oxide and manganese oxide.

Without departing from the scope of the invention, the lime may be partially or completely replaced by other compounds such as: magnesia, baryta, strontia or soda. Silica may be for instance replaced by alumina, titanium oxide or zirconium oxide; but the said substitutions, ex-

theoretical desulphurizing powerit is possible to obtain in a a very short tme together with a very substantial reduction of oxygen content (if cept that of TiO: which increases the fluidity of slag, increase its cost without any great practical interest.

(b) Furthermore the steel to be treated by the slag should contain a suflicient amount of silicon to insure the reduction of the sulphur content to the very low values stated above (for instance of the order of 0.015% and less).

(0) Finally, the initial composition of the slag and its proportion in relation to the steel, account being taken of the chemical analysis of the steel, should be such that after intimate mixing of the steel with the slag:

(1.) the iron oxide content of the slag will be very low (for instance a maximum of 1 per cent), the manganese oxide content having a corresponding value;

(2.) the final silicon content of the steel will be over 0.050 per cent;

(3.) the final slag will be distinctly basic.

It is advantageous to maintain the composition of the slag as regards lime, alumina and silica in a zone of the triangular diagram silica, alumina, lime deflnedas follows:

0n the onehand a straight line is traced on the said diagram joining the points Per cent Ca0=60 S102=40 A1203: 0

and

Ca0=41 Si0z=22 A12Oa=37 on the other hand a straight line latter point to the point:

Per cent Ca0=45 S102: 0

The zone in which the compositions of the slags should be chosen is defined by the two above joining this low) very v assaase i straight lines and by the sides of the triangle (on the side CaO=100) Of course only compositions corresponding to fusible slags or to slags which may be rendered fusible by proper additions of usual fluxes such as for instancefiuor spar, will be used in the said zone.-

Concerning themost'favorable points of the above mentioned zone, it will be observed that desulphurization will be so much the better-all other conditions being equal--as the lower are ;the silica and alumina contents.

It will be advantageous when'using relatively high silica content slags, to use low sulph r conascertained during the said treatments that the sulphur content maybe reduced 0.040 per cent which corresponds to a calcium reduction from lime of 0.050per cent. It is absolutely remarkable that with very reduced silicon concentrations in'the metal, for example of 0.300 per cent, it ispossible to very regularly reduce in a very short time under ordinary steelworks temperatures by a simple intimate intermixing, amount of calcium corresponding to the sixth of the weight of the silicon notwithstanding the considerable difference of the formation heats of the oxides of the said elements. 7

In order to obtain the above mentioned results in accordance with the theoretical data it is necessary to eliminate or compensate as far as possteel a final silicon content which is higher when 'The examples cited above correspond intentionally to slags that cannot cause such drawbacks. V

It will be noticed that the range of the synthetic molten slags to be used in carrying out this invention comprises the aluminous slags described in the United States patent application Ser. No. 254,728 for Process for obtaining steels having high micrographic purity which matured into Patent No. 2,232,403, the saidslags maintaining the advantage with respect to the other slags of the present invention of giving steels comprising the properties described in the said patent application and having still higher mechanical propertiesfor equal sulphur contents.

The applicant discovered that the silicon present in the metal, notwithstanding its very low concentration, of the order of some thousandths, and notwithstanding the very considerable difwith the synthetic slag whatever may be the preference in the formation heats, referred to the oxygen molecule of lime on the one hand and of silica on another hand, partially reduces the lime of the slag according to the reaction:

This is confirmed by the fact that m the absence of silicon in the metal theresult is not attained. It is particularly interesting to point out in this connection that although the addi- Per cent S102 CaOa 43.2 A1203 V 7 M20 9.3 MnO 4.6 FeO 2.2

However, the said reduction of lime by silicon and also the desulphurization will be important only if the treatment is carried out in such a way that the slag after intermixing, only contains reduced amounts of FeO as well as of MnO, since practice has shown that the said elements are preferentially reduced by silicon. Even ifthe initial slag contains only a very reduced amount.

of FeO or MnO, it tends to get richer in the said substances on account of practically unavoidable circumstances. 7 a

It will be noticed that the proportion of lime reduced by silicon may be relatively high with respect to the weight .of the latter; it is indeed sible the noxious accessory causes which occur during the treatment. The said causes are as follows:

(1.) The existence of residual slag which'mixes cautions taken.

If for instance in an electric furnace slagging of! of the oxidizing slag is performed as perfect- 1y as possible in the first part of the treatment, the total amount of the said slag cannot however be eliminated. Furthermore the walls of the furnace remain soaked with such oxidized slag and it is ascertained that if additions are made to the steel in a barelaath, the metal will be progressively covered with 'a thin layer of slag.

By way of investigation the said phenomenon has been studied in the operation of a 15 tons electric ,furnace. After a thorough slagging off, 0.500 per cent ferro-manganese and 0.100 per cent ferro-silicon were added on the bare bath and a sample of the new slag which formed itself over the metal was taken with great care; this slag had the following analysis:

I I Per cent SiOa 24 'CaO 38.8

A1203 10 MgO 9.6 MnO 11.2 FeO 5.2

Then 0.400% silicon was added and a sample of the slag was again taken and showed the following analysis:

It will be easily understood how much such slags, which may constitute 1 per cent and even more of the weight of the metal, will trouble the intermi'xing treatments with molten synthetic slags.

If the slag is retained in the furnace by any proper means, as in Martin furnaces, it is practically unavoidable that a given amount of the slag be entrained together with the metal thus causing an important trouble .in the treatment. In any case it is advantageous to slag off the metal as far as possible, in order to reduce at theleast the said trouble, since the correction .ap- V plied willthen lead to so much more nearly accurate results from one operation to another.

(2) Furthermore intermixing being performed in free air, 1. e. in an oxidizing atmosphere, there is some oxidization of the metaiby the air and a ducing agent of whic said oxidization is increased owing to the fact that during the violent pouring in the ladle from a considerable height, the metal entrains air by a suction action and-that the air bubbles thus entrained into the metal oxidize it. 'Such oxidizations also form parasitic slags.

The importance of parasitic causes is often so considerable that the excess of silicon to be added in industrial manufacturing for correcting such parasitic causes may reach 0.250 per cent and even more of the weight of the metal, this being Y very large; and when desulphuri'zation is to be performed it cannot be secured in an important and regular way, i. e. industrially, if the said correction is not made. The value of such correcting element, i. e. the value of the excess oi silicon to be added for taking into account the parasitic causes, depends of course on the operating conditions: atmosphere oi the furnace, i'urnace walls being more or less soaked with metal oxides, more on less perfect stoppage of the slag during pouring.

An important feature consists in that the applicant discoveredthat by intermixing with the slag, contaminated by the action of parasitic causes, the metalcontaining a suflicient amount of silicon, a reduction of iron and manganese oxides by silicon down to extremely low. proportions of such elements is obtained in a very short time, and, what is still more surprising the silicon simultaneously reduces the lime of the slag up to an amount sumcient for securing a strong desulphurization. I

The composition and amount oi the synthetic slag should be sufficient for the said slag to remain basic notwithstanding the increase of silica content due to an oxidization oi the silicon.

The amount of silicon to be added in excess and the corresponding increase of silica content of the slag, may be easily determined by some previously made tests. v

Without departing from the scope of the in-/ vention, a portion of the silicon may be replaced by aluminum or an ther equivalent strong'rethe formation heat of the oxide is higher than that of silica. The saidreduction agent will partially or completely protect silicon from the parasitic causes mentioned above since it will be oxidized before silicon. The said substitution however increases the cost of the treatment oi. the steel without any practical interest since the result may be obtained with silicon alone.

Finally it is also-possible, but this is not advantageous, to introduce into the ladle a synthetic previously formed slag, in solid state, advantageously strongly preheated, since if the slag has been previously formed it melts very rapidly under the influence of the heat of the metal.

Some treatments are described hereunder by way of non-limitative examples:

(1) To several baths of steel made in a Martin furnace, after having slagged oil the furnace as far as possible, silicon as well as manganese were added to the metal baths which at the time contained 0.025 to 0.035 per cent of sulphur, in'order to obtain a. content of silicon 0.15 per cent higher than the minimum final content wanted in the steels; the residual over-floating slags had been retained so far as possible'in the furnace and the steels have been violently poured from a considerable height (five meters) into a ladle containing about 5 per cent slag showing the follow-' ing composition: CaO, 50 per cent; $102, 6 per 2,288,888 combustion of silicon, manganese and iron. The v cent; A120:, 42 per cent; the remainder being mainly iron oxide and manganese oxide.

The sulphur contents, after intimate intermixing resulting from the violent pouring of the metal onto the slag, were all below 0.015 per cent and of the order of 0.010 per cent-the total oxygen contents being of the order of 0.002 per cent, the final slags containing an average amount of 10 per cent of silica.

(2) In electric furnaces, after deoxidizatlon and dephosphorization, the oxidized slags were very carefully slagged off 'and on thebare baths manganese was then added to the metal which contained 0.020 to 0.025 sulphur and for some of the treatments, also nickel chromium alloys, then silicon was added to the metal in order to have a final content 0.300 per cent higher than the minimum-final desired content in the steels, thus leading in most cases to a silicon addition twice hi her than the amount theoretically necessary for the steel alone.

Two minutes after the addition of silicon the metal was violently poured in a ladle containing a slag having approximately theiollowing com-* position: (380, 58 to 62 percent; SiOz, to 17 per cent; A1203, 18 to 20 per cent; the said slag After intermixing the sulphur contents of the metal were regularly of the order of 0.011 per cent and the oxygen contents of the orderof 0.005 per cent. Such very low sulphur contents were obtained even with steels containing less than 0.010 per cent of carbon.

The silica content of the slags increased to about 26 per cent. I y

' (3) Identical treatments have been performed with steels produced in an electric furnace, with slags containing initially of about: 63 to 66 per cent of lime, 24 to 26 per cent of silica, 9 to 11 per cent of alumina, fluxed by means of fiuor spar.

The, final sulphur contents were of the order of 0.013 per cent, the oxygen contents of the order of 0.005 per cent and the final silica contents of the slags increased to about 30 per cent.

What I claim is:

l. A process for economically and rapidly obtaining high quality steel having a sulphur content not 'over about 015%, which comprises violently intermixing a bath of steel and a-basic slag containing lime, alumina and silica, the steel bath containing such a high proportion of silicon and the amount of slag used being so large that after said violent intermixing the content of irorioxide in the slag is not over about 1%, the slag is distinctly basic, and the final content of silicon in the steel is greater thanabout .05%.

2. A process for economically and rapidly obtaining high quality steel having a sulphurcon- .tent not over about .015%, which comprises viotent not over about 015%, which comprises violently intermixing a bath of steel and a basic slag containing a strong base of the group consisting of calcium oxide, magnesium oxide, barium oxide, strontium oxide and sodium oxide and also containing a substantial proportion of alumina and silica, the steel bath containing such a high proportion of silicon and the amount of slag used being so large that after said violent intermixing the content of iron oxide in the slag is not over about 1%, the slag is distinctly basic, and the iinal content of silicon in the steel is greater than about .05%.

4. A process for economically and rapidly obtaining high quality steel having a sulphur con-, tent not over about .0-15%, which comprises violently intermixing a bath of mild steel (carlently intermixing a bath of steel and a basic slag containing lime, alumina and silica, the steel bath containing such a high proportion of silicon and a reducing metal the oxide 0! which has a formation heat greater than that of silica, and the amount of slag used being so large that after said violent intermixing the content of iron oxide in'the slag is not over about 1%, the slag is distinctly basic, and the final content of silicon in the steel is greater than about .05%.

6. A process for economically and rapidly obtaining high quality steel having a sulphur content not over about .015%, which comprises violently intermixing a bath of steel and a basic slag containing lime, alumina and silica, the steel bath containing such a high proportion of a reducing metal the oxide of which has a formation heat greater than that of silica, and the amount oi! slag usedbeing so large that after said violent intermixing the content 01 iron oxide in the slag is not over about 1%, the slag is distinctly basic, and the final content of said reducing metal in the steel is greater than about .05%.

' RENE PERRIN.

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