US3575695A - Deoxidation method of molten steel - Google Patents

Abstract

METHOD OF DEOXIDIZING MOLTEN STEEL BY ADDING CALCIUM OR CALCIUM ALLOY TO THE LOWER PART OF THE MOLTEN STEEL IN A VESSEL AFTER OR DURING DEOXIDATION. BY INCREASING THE YIELD OF CALCIUM IN THE MOLTEN STEEL, FLOATATION OF SOME INCLUSIONS ARE RAPIDLY ACCELERATED AND THEN THOSE ARE EASILY SEPARATED, AND THE PRESENCE OF HARMFUL MACRO INCLUSIONS MAY BE DECREASED IN A GREAT QUANTITY.

Description

April 20, 1971 YQSHIQ MlYAsHlTA ETAL 3 ,71,595

DEOXIDATION METHOD OF MOLTEN STEEL 4 Sheets-Sheet 1 Filed Oct. 10, 1968 I 1/ 1 COO I r I April 20, 1971 Filed Oct. 10, 1968 FIG.2.

YOSHIO MlYASHlTA ETAL DEOXIDATION METHOD OF MOLTEN STEEL Oxygen concentration in the molten steel de oxidation 4 Sheets-Sheet 2 0; Oxygen in the molte steel before I Behaviour of total oxygen deoxided in a normal method at t,

Calcium added at t Calcium added at t 2 Behaviour of free oxygen deoxided in a normal method at t 3 Behaviour. of total oxygen with 4 Behaviour of free oxygen with (Difference between i and 2, and 3 and 4 are the amount of oxygen in the molten steel) Total oxygen (7.) O

\ Calcium added Metallic Si l l l I l 4 6 Metallic Si 2 3 4 5 6 Time after deoxidation with Si (minutes) April 20, 11971 YOSHIO MIYASHITA H mzoxxmvrxou mmnon 0F, MQLTEN STEEL Filed 001;. 10, 1968 4 Sheets-Sheet 3 Metallic Si 0 Toml oxygen Im 2:8: n w w m m m mw mu O .0 0 0 h d M d O O O m o m m W F Time after deoxidation with Si (minutes) @L E 222 2 w 0 Metallic Al April 1971 YOSHIO MIYASHITA A 3,575,695

DEOXIDA'I'ION METHOD OF MOLTEN STEEL Filed Oct. 10 1968 v 4 Sheets-Sheet 4 F G'U g g Metallic Al 3 2 M-\; 0.10 2 3 0.04 E I Cl d s ac' a d d E a 0.02 e

OCT-0N" oxygen o 1 l l l I O Time after deoxidcnion with Al (minutes) United States Patent O U.S. Cl. 75-51 6 Claims ABSTRACT OF THE DISCLOSURE Method of deoxidizing molten steel by adding calcium or calcium alloy to the lower part of the molten steel in a vessel after or during deoxidation. By increasing the yield of calcium in the molten steel, floatation of some inclusions are rapidly accelerated and then those are easily separated, and the presence of harmful macro inclusions may be decreased in a great quantity.

to conduct experiments on the laboratory or industrial scale and to publish similar conclusions. The phenomenal effect of calcium on the form and quantitative variation of inclusions in steel is also recognized in a number of papers. However, Sims et al. do not admit the effect of calcium by comparing a deoxidation with Ca-Si with one using Si. A number of workers hold that calcium is not effective.

One of the reasons for such contrasting results is that calcium is considered unstable and its effective yield in the steel low because its melting point is 850 C. and its boiling point is 1440 C. Also it is simply vaporized when it is added to the surface of steel bath. Accordingly, various researches have been conducted to obtain improved results. For example, the use of Fe-Ca base alloy will decrease oxide inclusions, but its effectiveness is small considering the large amount of calcium addition (more than 0.2% as (a)), as is shown in Table 1 and the final amount of inclusions after addition is large. Moreover, analyzing for inclusions as well as the reliability of the results are not without problems.

TABLE 1.ANALYSIS OF OXIDE INCLUSIONS Fe-Ca Oxide inclusions, weight/percent Heat addition alloy FeO MnO SiOz A1203 2 oxide Number:

II None 0. 0045 0. 0013 0. 0120 0. 0074 0. 0252 III None 0. 0055 0. 0023 0. 0097 0. 0075 O, 0250 l. 5 0. 051 0. 0009 0. 0055 O. 0068 O. 0183 IV None 0. 004 0. 003 0. 001 0. 017 0. 025

V None 0. 004 0. 0042 0. 0002 0, 018 0, 0262 VI None 0. 004 0. 001 0. 0015 0, 014 0. 0205 VII None 0. 003 131' O. 0014 0. 016 0. 0204 l. 0 0. 005 tr 0. 0001 0. 006 0. 0111 SUMMARY OF INVENTION This invention relates to a method of manufacturing steel having higher cleanliness by adding calcium or calcium allov to the molten steel and effectively improving yield of calcium. This eliminates some non-metallic inclusions from the molten steel and further effectively desulphurizes and degasses the steel.

BACKGROUND OF INVENTION Previously Mn, Si, Al, Ti, and Cr have been widely used for the deoxidation of steel. Ordinarily, deoxidation is conducted in the ladle at the period of tapping. The amount of oxygen in a molten steel is generally higher than that being in equilibrium with metallic Mn,

Si or A1 of said molten steel. That is, it has been difficult to obtain steel with little inclusions and to keep the amount of inclusions to lower'value, no matter how much deoxidizer is added to keep the amount of oxygen low, because the deoxidation products formed in the molten steel and the oxide inclusions formed by erosion of refractory materials and slag dragged into said molten steel amountto a considerable percentage. Moreover, deoxidized steel with Si, Al, Ti, and the like tends to contain harmful macro inclusions for the steel, so that this often results in defects even when the amount of inclusions in the steel are negligible.

Many researchers relating to deoxidation with calcium have been conducted in order to solve such problems. It was known as early as 50 years ago that calcium alloy is a powerful deoxidizing and desulphurizing agent and that it can be used in refining steel. Goldschmidt published work which showed that the use of Ca-Si for deoxidation eliminates oxygen and sulphur from molten steel and form melting .slags which can easily be separated from the molten steel. This fundamental research led many others In work, on a large scale, to confirm the effect of calcium alloy, the effect of calcium alloy is generally recognized with as much as 5.2 kg./ton addition. This is an undesirable amount both because of production cost and because of the aimed composition of the steel.

Finally, research on the deoxidation with calcium have been conducted by adding calcium or calcium alloy on the top of the molten steel. Because of the inferior yields of calcium resulting from its vaporisation, it becomes effective only after a large amount has been added.

Vacuum melting methods, vacuum degassing methods and electro slag methods are used in order to decrease inclusions in the steel. These all require complicated devices and are skilled work, which are therefore unsuitable for mass-production.

The drawbacks to the use of calcium and some conclusions on its use may be summarized as follows:

(1) The yield of calcium in the molten steel is quite low when calcium or calcium alloy is added for deoxidation purposes. This inferior yield tends to deteriorate as the amount of the molten metal increases.

(2) Accordingly, small amounts of calcium addition are not effective.

(3) However, the effect after a great amount of calcium addition is not noteworthy. (The decrease of oxide inclusion is small and its final amount high.)

(4) Calcium is generally added to the steel in alloy form and its quantity is naturally restricted in accordance with the aimed steel compositions. Therefore, the addition of calcium in a large quantity will not result in a significant effect.

(5) As the reaction of the calcium at the time of addition is not known, there is no certain method established for the preliminary deoxidation or the timing of the calcium addition.

(6) Because of the vaporation of the calcium on the bath surface and its violent reaction, with its low melting and boiling points, the amount of vaporization or reaction cannot be controlled at the time of addition.

(7) Accordingly, there is no known deoxidizing method effective in decreasing inclusions or oxygen with a small amount of calcium.

(8) Improvement of the steel properties with calcium is not to be expected because of the low yield of calcium in the molten steel.

Mn, Si, or Al may be used in deoxidation without calcium. When the value of equilibrating oxygen with metallic Mn, Si or Al in the molten bath is quite low, the concentration of total oxygen in the steel (the sum of free oxygen and oxide inclusions, and the oxygen found by oxygen analysis) is kept at a considerably high level. Because of inclusions not removable from the molten bath, it is difficult to keep the total oxygen stable below 0.004% no matter how much deoxidizer is added. Owing to various conditions at the time of deoxidation, inclusions that remain in steel tend to be unstable and create macro inclusions which are harmful to properties of steel.

According to the present invention there is provided a method of deoxidising molten steel, wherein calcium or calcium-containing material is added to the lower part of molten bath which has been subjected to a preliminary deoxidation step.

For a better understanding of the invention, reference will now be made to the accompanying drawings, in which FIG. 1 illustrates an embodiment of the invention;

FIG. 2 shows graphs illustrating the decrease of oxygen in steel by using the present invention;

FIGS. 3 and 4 are graphs illustrating the decrease in total oxygen with the addition of calcium at the time of Si-deoxidation;

FIGS. 5 and 6 are graphs, similar to FIGS. 3 and 4, obtained from another series of tests (Al-deoxidation).

Referring now to FIG. 1, calcium alloy is added through hollow cylinder or tubing 3 to the lower part of the molten bath 5. The calcium alloy is stored in tank 1 and its flow is controlled by adjusting valve 2 as gas is blown into said cylinder. Refractory lining 4 surrounds cylinder or tubing 3, where it enters into the said bath 5. The calcium rises to the surface rapidly and eliminates inclusions in said bath and prevents or decreases the occurences of macro-inclusions. The requirement for preliminary deoxidation is to have the value of dissolving oxygen below the predetermined value of oxygen in steel. This matter must be conducted prior to calcium addition or during addition of calcium.

The calcium, melting point 1440 C. and boiling point 1440 0., when added to the lower part of the molten bath immediately liquifies or vaporises itself and as it rises to the surface, it melts into the bath, .thereby reacting with free oxygen and oxide inclusions. Inclusions reacting with added calcium rapidly rise to the surface as changes of its components are brought about and are eliminated so that the value of the total oxygen decreases surely.

By adjusting the valve 2 at the bottom of the tank 1 as shown in FIG. 1, and regulating the amount of the calcium addition, a degree of reaction can be regulated at the time of addition to the molten bath.

Several refractory hollow cylinders can be used simul taneously instead of the single cylinder shown in FIG. 1.

Thus, the yield of calcium or calcium alloy added to said bath is excellent. (According to our experiments it is more than 70% in some cases.)

An analysis of the remaining calcium in steel is shown in Table 2 which is a comparison between a prior art addition of 0.1% calcium to the surface of 50 kg. molten bath, and an addition of the same amount made by the method of the present invention.

4 TABLE 2.CALCIUM ANALYSIS IS STEEL PERCENT Percent The prior art 0.0015 The present invention 0.013

This shows that the amount of calcium to be added is small as compared with the prior art method. (In the prior art, more than 0.2% calcium was used. According to this invention, 0.05% addition is quite sufficient.) The more the amount of the molten steel increases, the longer becomes the contact period of calcium either in liquid or vapour form. In some cases the static pressure of the molten bath is higher than the vapour pressure of the calcium, thereby making the present method more effective than ever.

The calcium or calcium alloy required for the present invention may be a powder, granule or in small lump form. Besides the calcium or calcium alloy or calcium compound, magnesium, sodium or potassium or their alloys and compounds may be added.

The calcium or calcium alloy is usually added to ladle containing the molten steel but it can also be added to open hearth, converted, electric furnace or mould.

Inert gases like argon or nitrogen gas may be used when the calcium or calcium alloy is added. In the case of nitrogen addition, formation of nitrides can be effectively achieved. Other additives aimed at the formation of nitrides can also be added along 'with the calcium or calcium alloy. In this case, the partial pressure of oxygen in the molten bath contacting nitrogen gas is kept at a very low level by calcium and nitrogen absorption in said bath is very effective.

Because of an exothermic reaction by addition of the calcium or calcium alloy, the deoxidizing agent can be added without lowering the temperature.

The molten steel is stirred well as the calcium vaporizes and rises to the surface of said bath, thereby making composition of said bath uniform.

As is indicated in Table 2, the contents of calcium remaining in steel is smaller, so that calcium has the possibility of improving properties of steel greatly.

Much of inclusions in the molten bath that have hitherto been unable to rise to the surface of said bath are made to rise rapidly and in stable condition by the addition of the calcium. The decrease in percentage of inclusions is large, the final level of contents reached is small and stable.

By adding 0.05% calcium to 50 kg. molten steel which has been preliminarily deoxidized with Mn, Si or Al, it is possible to keep the concentration of oxygen below 0.002%. By the prior art, it is diflicult to keep it below 0.005%. The composition of inclusions remaining in steel is made to change to one which prevents harmful macro inclusions from being formed. Accordingly, the size of inclusions in a steel ingot is extremely small.

Any aimed composition of steel is possible to obtain easily as the amount of added calcium is small.

When this method is compared with the vacuum melting process, the vacuum degassing process or the electro slag process, the apparatus is extremely simple requiring no skilled work and is therefore suited for mass production.

The nature and effectiveness of the present invention will now be more fully discussed with reference to FIGS. 2 to 6.

FIG. 2 contains curves 1, 2, 3 and 4. O is the level of oxygen in the molten steel before deoxidation. Curve 1 shows the behaviour of total oxygen, when deoxidation is effected in a normal manner at time T while curve 2 shows the behaviour of free oxygen under the same conditions. In curve 3 calcium has been added at time T and the total oxygen is shown on the curve 3, while the behaviour of the free oxygen under the same conditions is shown in curve 4. The difference between curves 1 and 2 respectively and curves 3 and 4 respectively gives 5 contents of oxygen present as inclusions in the molten steel.

FIGS. 3 and 4 show the behaviour of total oxygen in electrolytic iron, 1 kg., which has been melted in a high frequency furnace, with the temperature kept at 1600 C. and to which is added 0.05 calcium after it has been deoxidized with 0.3% silicon. In FIG. 3, the total oxygen reaches a constant value of 0.015% approximately in one minute after calcium has been added. Under the same conditions but without calcium addition, it will also reach the set value of 0.015% in six to eight minutes after Si addition (see FIG. 4). In these experiments, the metallic Si present was 0.10% and the equilibrating oxygen was 0.015%. Free oxygen at the period of Si deoxidation is shown to decrease in approximate equilibrium with metallic Si. That is to say, of the oxygen value of 0.065% in one minute after Si addition, most of it is SiO the primary deoxidation product. This is found to rise rapidly to the surface of molten bath and is eliminated by the calcium addition. The size of inclusions in steel after the calcium addition are extremely small, and its composition has been shown to be different from the composition prior to addition.

FIG. 4 shows an embodiment where calcium has been added to the bath after Si constituting the primary deoxidation product had almost risen to the surface of said bath and eliminated. In this example, it is found that a constant value of total oxygen, that is, about 0.015 is obtained in about six minutes. Accordingly, in the process of Si-deoxidation, an addition of calcium during Sideoxidation is desirable.

FIG. 5 shows that the total oxygen reaches 0.0016% and less if calcium addition has been made. Under the same conditions, but without calcium, it reaches about 0.004% but not less in 6 to 8 minutes after the aluminum addition (see FIG. 6). In these experiments, the metallic Al was 0.14% and the equilibrating oxygen with Al is about 0.0001%. In the case of Al deoxidation, fine particles formed in the primary deoxidation cannot rise to the surface of said bath even when the free oxygen decreases. The value of total oxygen is thus kept at a considerably higher level than that of the free oxygen and does not decrease. Of the oxygen value of 0.046% in one minute after Al addition, most of it is A1 0 the primary deoxidation product. It rapidly rises to the surface of said bath and is eliminated, when calcium is added. The value of the total oxygen tends to approach the value of free oxygen.

FIG. 6 shows an example where calcium addition is made to the molten steel where considerable A1 0 remains as the primary product. This figure indicates that oxygen rapidly decreases and the value of the total oxygen approaches the value of free oxygen.

In FIGS. 5 and 6, the size of inclusions in steel after calcium addition becomes extremely small and its composition is difterent from that before the addition made. A1 0 generally tends to cluster, but such tendency is not to be found in inclusions to which calcium addition has been made. This shows that the efiective addition of calcium prevents the macro inclusions peculiar to A1 0 inclusions.

The use of calcium alloy, when calcium content of said alloy is equal, gave similar results.

The foregoing results have hitherto been unknown. These facts reveal that calcium addition to the molten bath after or during deoxidation so as to keep a level of free oxygen contents at a point below the finally desired level of oxygen will decrease, rapidly and in considerable quantity, the primary deoxidation products.

FIGS. 3 to 6 show the results of small scale experiments with the molten steel of 1 kg., but the same or a similar effect is found when the amount of molten metal is increased to that of the scale of industrial production. The method is eflective because the manner of addition keeps the amount of calcium escaping into the atmosphere as small as possible by keeping the contact period of calcium in liquid or vapor form longer after it has been added to the lower part of the molten bath. Accordingly, the more the amount of the molten steel, the longer the contact period. In some cases the static pressure of molten bath becomes higher than the vapor pressure of the calcium, thereby making the method more elfective. This has been confirmed in large scale tests.

The amount of deoxidizing element needed to keep free oxygen below 0.004% is, in the case of metallic Al about 0.0004%, and in the case of metallic Ti according to the work of Chipman et al. about 0.02%. That is to say, by adding calcium or calcium alloy to molten steel deoxidized preliminarily to make metallic Al 0.0004% and metallic Ti 0.02% the total oxygen can be decreased to below 0.004%. In this case, the composition of the inclusions change so that it prevents harmful macro in clusions in steel.

In a process of adding calcium or calcium-base alloy not only the devices mentioned above are used, but also blowing and adding through a penetrating hole on the bottom of a vessel may be employed.

What we claim is:

1. In the steel deoxidation process wherein molten steel containing free oxygen is deoxidized in a vessel by the addition of a deoxidizer to said molten steel to substantially eliminate said free oxygen and to form non-metallic inclusions in said steel resultant from the deoxidation, the improvement comprising blowing into said molten steel after the addition of said deoxidizer thereto about 0.05% by weight of said molten steel of at least one calcium additive selected from the group consisting of calcium and calcium alloys, said calcium additive being blown into the lower part of said vessel containing said molten steel by a blowing gas, whereby the amount and size of said nonmetallic inclusions in the steel are substantially reduced.

2. A method as defined in claim 1, wherein said molten steel is deoxidized to substantially eliminate said free oxygen by addition of at least one deoxidizer selected from the group consisting of manganese, silicon and aluminum.

3. A method as defined in any one of claims 1 and 2, wherein said blowing gas is nitrogen or argon.

4. A method as defined in claim 3, wherein said calcium additive is blown through a refractory hollow cylinder.

5. A method as defined in any of claims 3, 4, 1 and 2, wherein said vessel is a ladle into which the molten steel has been poured.

6. A method as defined in any one of claims 3, 4, 5, 1 and 2, wherein said calcium additive is calcium.

References Cited UNITED STATES PATENTS 2,049,004 7/1936 Flannery 7557X 2,819,956 1/1958 Strauss 7557 3,157,492 11/ 1964 Matuschkovitz 7551 3,215,525 11/ 1965 Sprankle 7558X 3,269,828 8/1966 Hale 7558 3,467,167 9/1969 Mahin 75-57X FOREIGN PATENTS 776,315 6/ 1957 Great Britain 755 1 OTHER REFERENCES The Making, Shaping and Treating of Steel, US. Steel, 7th edition, 1957, pages 396 and 397.

L. DEWAYNE RUTLEDGE, Primary Examiner G. K. WHITE, Assistant Examiner US. Cl. X.R.

* g;;g UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent NO, 1 D t d 20,

Inventor) YOSHIO MIYASHITA et al It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 64 "researchers" should be deleted and replaced with "researches", as indicated in specification at page 2, line 20.

Column 3,- line 52 l440C. should be deleted and replaced with "850C. as indicated in specification at page 6, line 9.

Column 5, line 56 after "addition" insert "is" as indicated in specification at page 10, line 12.

Signed and sealed this fi th day of September 1971 (SEAL) Attest:

ilgfifi lgmg sggmmm. Refit-Eu GOTTSCHALK g 1061' Acting Commissioner of Patents

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