US2440564A - Process for the manufacture of basic steel of low phosphorus content - Google Patents

Description

Patented Apr. 27, 1948 rnocnss FOR THE MANUFACTURE or BASIC s'rmzr. F LOW rnosrnonus CONTENT Marc Louis Allard, Valenciennes, France, assignor to Socit Anonyme dos-Forges et. Aciries du Nord & de lEst, Paris, France 1 l No Drawing. Application May 6, 1939, Serial No, I

272,267, In France May 10,1938

Section 3, Public Law 690, August 8, 19 46 Patent expires May 10, 1958 r 7 Claims.

This invention relates to manufacture of basic steelfof low phosphorus content.

It is known that in the process of manufacturing steel by the Thomas converter, thejelimination of the phosphorus results from the formation, on thesurface of the bath, of a slag constituted by the lime added to the bath and by the phosphorus pentoxide and other oxides, particularly oxides of iron, resulting from the blow.

For obtaining basic steel of low phosphorus content (about 0.025%), there will now be described aparticular process of dephosphorisation in two stages, in each of which certain detersuccessively the combustion of the silicon, then the carbon, the phosphorus and a partial oxidation of the iron, and the temperature of the bath rises. The slag-formed on contact with the lime is removed. In the second stage, a fresh quantity of lime is added in order to eliminate a fresh quantity of phosphorus and a fresh blow is carried out. The fresh addition of lime and ofblast necessary for the formation of the second slag nevertheless involve a certain cooling; but particularly when it is desired to leave the metal to stand before tapping in order to decant the impurities, or when it is desired to tap slowly, it a may be desirable to obtain a refined steel at a high temperature. 1

It is furthermore known to effect the dephosphorisation in the second stage with the aid of an oxidising fused basic slag, known as second slag, and, in the presence of this slag, to proceed with a blow-intended to effect intense mixing of the slag and the metal to be treated, leading practically to complete chemical equilibrium between the slag and the'metal.

When, in one of these two-stage processes, it is desired to obtain hard steels of low phosphorus content quality, the use of ordinary recarburising cast irons must be abandoned, because they are always too high in phosphorus and it would not T a process for the '7 .air introduced in order to efiect the oxidations:

be possible to eliminate the phosphorus, whilst.

introducing them at the end of the second phase. If, besides, and in order-to thus eliminatethe phosphorus, said addition would be made at the end of the first stage. the carbon would be burnt mined quantities of lime are added anda blow is carried out. In the first stage, the blow efiects 2 during the blow in the second stage and the recarburising addition would consequently be ineffective, l

The, present invention aims at providing an economic dephosphorising processv enabling, a high temperature to be attained at the end of the operation,,if required, while obtaining a phosphorus content of at most 0.025% whether the steels concerned are softer carbonisedsteels, particularly irail steels, the final steel exhibiting an extremely low degree of oxidation and possessing excellent qualities. This process embodies the following steps. A first dephosphorisation is ,efiected in accordance with the, customary ,Thomas process (thephosphorus content of the steel may onlyjbeneduced to between 0.2 and 0.06%, for example); the slag resulting from this first. dephosphorisation is eliminated as com:-

pletely as possible, andthereupon a convenient proportion--of very hot and liquid oxidised basic slag is poured on to the surface of the bath. Good superficial contact between said slag and the surface of the bath is ensured by light rocking of the converter, ,allowing just sufiicient blast to prevent the .stopping jup of the holes, and taking precautions to ensure that no appreciable stirring up of the bath takes place. The slag is .dried by a streanrof lime, after having removed immediately a fraction" thereof it there, is too largea quantityof it, and finally the tapping is carried out with dams, making the usual additions. i l The process of the invention differs from processes already known in that not only is no-complete equilibrium attained between metal and slag, but, on the contrary, every endeavour is -made to avoid intense stirring up which would be liable to lead to said equilibrium,

Experience has shown in factthat, by having recourse simply to very short contact as described above, providing that asuflicient quantity of a slag of suitable composition be employed, extensive dephosphorisation is obtained simultaneously with practical freedom from oxidation of the metal in the caseof the dephosphorisation of soft steels, aswell as the possibility of recarburising the steel in the converter and then of effecting th final dephosphorisation of this reeerburlsedsteel.

all the inconveniences involved thereby, particua larly extensive combustion of the carbonin the...

- bejas ,shortlaspossible. When operating in a case of hard steels.

The chemical equilibrium between the slag the metal is not obtained. However, one observes smaller extent.

The amounts of slag to be employedjalliotherfthings being equal, must naturally be-largenth'arr for operations in which equilibrium is desired:

By virtue of the process of the present invention, it is possible, by selecting abasicvcastiron sufficiently rich in. thermogenous element's, to

obtain in the first stage of the treatmentla bath the temperature of whichis relatively high. and to effect, after. removalof the first slag;.a fresh dephosphorisationr bythe; addition .of .the'iliquid slag without, lowering .the temperature. of the bath. Itisalso possible to addtothe bath, before adding. the liquid slag, alrecarburising, castiron, even one high in .phosphoruShpr-ovided" that the quantity of. slag utilisedfi's. inlproportionto the average. phosphorus content of themixture, and to effect the, second dephospho'risation. almost instantaneously without'needing a vigorous stirring up--and hence without combustion of. the recarburisingcarbon. of; the bath. By virtue of the process of, the present, invention,. it is. thus possible to obtaingin the converter hardidephosphorised steels, the recarburisationbeing effected by means. ofa cast iron. eyen. of a phosphorise'd, .c

'dephosphorised, theother conditions remaining nature. v 1 I h In order to carryoutthe processofithe invention, theprocedure. may be. as. follows:

The molten ,cast iron being .in. the converter, the procedure-'atlfirst followsthe usualilines, :but at a temperaturewhichimay be higher than normal, and the ,cperaticnlis' stopped (which is. easy for a trained.operator) at aphosphorus. content sufficiently hiehtqiensure that the; bath is not highly. oxidised ,This. content. dependingon the tapping temperature, generally: varies between 0.060% and 0.200 and the slag formedisquite is made of a neighbouring. converter, whichmay beone of the service converters :or: arconverter of very. small dimensions-reserved:- for thisapurpose, in which the: oxidised: basic slagzis formed from a certainq ntity of-unrefineds'steel taken .from

the preceding tapping. In-order' toprepare this slag, when utilising a service converter, 2,. quantity of steel will be charged .into the apparatusithe latter beingv well cleaned. from .its phosphatised slag) and will be after-blown, burning, ironand proceeding progressively to the addition O 50 quality granulated lime necessary for obtaining finally a slag containing 40 to 45% of lime. In addition, this slag must normally have an iron content of 30 to 35%, in the form of oxides, the 5 whole of the steel not being incorporated in the slag. These contents are not imperative, the essential condition being to-have a quite fluid and verylhotbasic oxidised slag. The oxidising blow to which reference has been made above will be ,efiected very rapidly, so that the durationof the contact oi the oxidised iron and the lining will converter of very small dimensions, the quantity of steelutilisedmay be much more restricted and mayiniore closely approximate to the quantity 'str-ictlwnecessary for obtaining the above-specifiedcorripositionzr' Accordingto asecond method of preparation, -th-.-slag,is+taken during an operation in a steel refining furnace, for example a Siemens-Martin for ielectricifurnace. In this case, this slag could,

" ii -necessary, be corrected by additions of oxides ,ofJiron of fluxes and/or of lime or like substances.

be utilisedfthisvlatter method ofpreparation permitting" greater adjustability 'in the composition of the slag to be prepared. 3 i Q;

The'qua-ntity of-slag tobe utilised depends at once on the phosphorus content ofthe 'metal at the end'of the first'stage, bn-the'greateror lesser perfection of the cleaning, on the initial content of P205 in the slag itself, onflthe temperature and also 'onthe"final'phosphorusloontent to be attllitainedi By way pf indication, if thesl aghas been prepared in Y aconverter and contains, for example,;2% 10f P205; and if themetal contains 0.06% of'phosphorusQthe quantity of slag to be "introducedj'into the operation is of the order of '40 2.5 aftera cleaning carriedoutnormally, without particularprecautiona-and at a temperature slightly-above the usual temperature' of basic steel at the end of I the" operation. Fora phosphoruscoritent-of-*0.200% in the metal'to be the same, the quantity of jslagto be utilised is of the'order'of 8%i 1'nthese"twocases, the final phosphorus content-fobtained is lower than 0.025 5; 7 V

The percentages of lslag can be reduced considerablyif particularly carefulicleaning be carried out, if the-temperatureislower, and ifthe initial v contentof P205 is very loww hich may beachieved particularly when preparing the slag ma special furnace: j

It should, moreover, be' noted that, conversely,

if it is desired to effect appreciable desu1phurisa-' tion'offthebath at thesame' time 'a s'dephos phorisation', the'pr'oportionof slag to be added must belarger, another-things being equal. 3

Similarly, the addition of dephos'phorising slag,

and'al'so'its composition, can'bevaried in such a manner asto compensate-forthe-retarding ac,- tion ofthe raising of thetemperature by-means of an increase in thequantity of slag or by 'means of an increase in its basicity or'its content of oxidisediron; It shouldbe observed that in accordance with the foregoingprin'ciples; theuse of .aiurnace for thepreparation' of the secondslag enables the compositionjd'esirabl'e] for each particular case tobe obtained at. will," and; in consequence, vfurnishes additional flexibility} in the application of the process." In" f a'ct, the'preparation of the slag in a Thomas converter leads to' compositions Finally, a special slag melting furnacemay also the foregoin'gmethods of. preparation is removed,

while carefully. avoiding the removal of steel at the' same time, in a ladle of dimensions as suitable as possible and lined with a refricatory linin'g; for example having a high alumina content, and it is poured into the converter on to the bath of cleaned steel to be dephosphorised.

In order that good superficial contact may be established betwen metal and slag, the converter is gently rocked, providing just sufiicient blast to prevent the stopping up of the base holes. On the other hand, it is important that no vigorous and prolonged stirring is effected, which, by emulsifying the slag into the steel, might leave large quantities of iron oxides in the latter. The activity of the slag and the contact over a large surface are sufficient to ensure that the dephosphorisation is effected-very rapidly during the very short time necessary and suflicient to effect a single gentle rocking under the above-specified conditions. The decantation of the slag and the drying thereof by superficial projection of granulated lime may then be carried out immediately.

The great rapidity of the operation and the very early decline in activity of the slag due to its cooling on contact with a steel at a lower temperature, and above all to its almost immediate drying, have the result that, contrary to current opinion, the metal is only extremely slightly oxidised in practice through its contact with the added slag.

That is moreover the reason that recarburization is possible in the case of hard steel, by adding to the bath of soft skimmed steel, before the slag treatment, recarburising cast iron. The latter may be high in phosphorus, provided that the quantity of slag utilised is then in proper proportion to the mean phosphorus content of the mixture. The recarburising cast iron must nevertheless be almost completely free from silicon, in order to avoid any difficulty; that is why, when recarbonising with basic cast iron, it is preferable to take from a, neighbouring converter cast iron in which the combustion of the silicon is completed and the decarburisation of which has not commenced.

Once the soft or carburised steel has been dephosphorised in the aforedescribed manner, it may be brought to the desired manganese content by the usual additions, either in the converter or in the ladle. An excellent yield, and hence a low consumption of added ferro-manganese, is observed.

The slightly phosphatised slag, containing an appreciable quantity of iron which it is desirable to recover, may, as will be seen later on, be incorporated in the succeeding charge. To this end, if there is a large amount thereof, the bulk of it may be removed very rapidly before drying, this removed fraction being introduced into the next tap, while the fraction remaining may be retained in the converter by careful damming. On the other hand, this skimming is useless in the majority of cases, the amount of slag being too small to be conveniently retained by the dam. During the next tapping, carried out with the slag remaining or re-introduced into the converter, the iron in said slag is regenerated by the cast-iron and thus recovered without any appreciablecooling resulting therefrom, although such cooling could easily be compensated by suitable dosing of the scrap iron.

If the slag isprepared in a normal service converter, the steel, and possibly the excess slag,

, may be left in the apparatus and are recovered in the next tap carried out in that apparatus. The process is particularly economical in the case of the preparation of slag by blowing, be-

cause it does not require any supply of heat from external sources and, despite special refining, needs no heating up of the steel. The recovery, on the one hand, of the slag remaining in the converter and, on the other hand, of the residue from the preparation of said slag, obviates any supplementary loss of iron and thus the process not only does not increase the loss, but cannot fail to reduce it.

In a general way, moreover, the final slag, if it is not re-incorporated in another Thomas operation, can be re-utilised (when skimmed and placed aside) in any metallurgical operation requiring originating materials of this composition.

In addition, with a dephosphorised steel, it is possible to obtain a higher bath temperature than normal and a steel which may be either soft or hard, owing to the possibiltiy of adding into the converter recarburising material not necessarily constituted by cast iron of a very low phosphorus content, since this phorphorus can be eliminated by the action of the liquid slag added, without any appreciable fresh blowing. Finally, the bath being safeguarded against any appreciable oxidation both during the first dephosphorisation (because the latter is incomplete) and during the second, as indicated in the foregoing, the total.

consumption of manganese can be considerably reduced. In fact, not only is the yield of the final addition of manganese improved but, all other things being equal, it is possible to treat by this process, without inconvenience, cast irons having lower manganese contents than normal, the importance of the protective role of this element being reduced by reason of the smaller final oxidation of the bath.

I claim:

1. A process for obtaining in the Thomas converter steel of low phosphorus and oxygen contents and possessing a high temperature, comprising blowing molten cast iron in the converter to reduce the phosphorus content, skimming off the slag formed, adding to the skimmed metal a fused basic oxidizing slag, slightly raising the converter and strictly maintaining the blast necessary for preventing the stopping up of the blow holes of the converter whereby superficial contact of short duration is insured between the second slag and the molten iron, and avoiding any appreciable stirring in order to prevent the attainment of equilibrium between the molten iron and slag.

2. A process according to claim 1, in which, for the manufacture of hard steel, recarburizing material is added before the addition of the second slag,

3. A process according to claim 1, in which the first blow is limited so that the metal has; a phosphorus content comprised between 0.06 and 0.2%.

4. A process according to claim 1, in which the second slag is a slag prepared in a converter by quickly blowing a steel and adding lime.

5. A process according to claim 1, in which the second slag is a synthetic slag prepared in a spe-- cial slag melting furnace.

imam

6. Aprocess according to claim 1, in which the second slag aflslag of the group cons-istingpf Martin and electric steel refining sslajg, if necessary corrected by additions of cxides.

. Z'. Aprocess. according to-claim 1, in which :the w final oxidized and- -slightly phospha-tized .slag is Number reincorporated, after use as an element of the 21,40,167 change, in a similar process in another Thomas Ya-(50303 .ccnverter. 2,109,264

MARC LOUIS. 2,111,89

mmaumidns 01mm The following reerences' are =0! record iii the me of this pqitcntz UNI'I'ED "STATES PATENTS Name Date Crosby "May 12, 1:936

Pemn.. Aug. 1131936