IL26401A - Method for continuous casting of rimming steel - Google Patents

Description

7Ι1ΠΠ |HD fJinj"T 'Π PATENT ATTORNEYS · □ 1 D ] D 9 ' D 111J PATENTS AND DESIGNS ORDINANCE SPECIFICATION Method for Continuous Casting of Rimming Steel X (we) GOHOASP IHC TlPOaATSB, a tJS body corporate, of 83 Maiden lane, Hew York N.Y. 10005, U.S.A. do hereby declare the nature of this invention and in what manner the same is to be performed, to be particularly described and ascertained in and by the following statement:- fejTto<¾--i ^-G Qfria¾Q ie--Goet-irftg—ef—Riam-FBg-StreQl This invention relates to a method for continuous casting of rimming steel, wherein deoxidants are added and molten steel is poured into a cooled mould, from which the solidifying strand is withdrawn by withdrawing means, guided and further cooled in a strand guiding means.

In continuous casting plants already known, molten steel is poured into a water-cooled open-ended mould. The bottom of this mould is closed by a dummy bar which is inserted before the pouring begins. The mould cools the metal which solidifies and forms a skin around the molten steel. To enable the cast strand in the mould to be withdrawn by the dummy bar, the head of the dummy bar is formed to make a connection between the solidified strand and the dummy bar. The dummy bar is withdrawn by 24.8.66 - 1 - Fi/Ze/st withdrawing means and disconnected from the strand as soon as the strand is engaged by the withdrawing means. Thus, a continuous strand is formed.

The skin of the strand formed in the mould is relatively thin when the cast strand emerges from the bottom of the mould. The molten metal in the center of the strand is contained by the skin. Guiding and cooling means are situated beneath the mould for guiding and further cooling the strand.

It has been known to add aluminium to rimming steel during casting, to deoxidize the steel and to influence the rimming action and thickness of the rim, i.e. the zone free from blowholes. Thereby, non-metallic inclusions are formed which have ill-effects on the quality of the cast and the rolled product. This addition of aluminium is also unsatisfactory for controlling the rimming action. An intensive cooling and a deep liquid crater favour an irregular and erratic rimming action, whereby the steel boils over the mould. To decrease this erratic boiling and frothing action, additional aluminium was added to the steel in the mould with the effect that the above-mentioned disadvantages and irregularities of the cast product were further increased.

In continuous casting a substantial portion of the molten metal is cooled and solidifies after the strand has left the mould. Where the collecting gas can escape upward through the molten metal, it vigorously agitates, boils and froths the molten metal. Such vigorous agitation and boiling interferes with the desired cooling of the molten metal by the cooled mould wall and hinders a proper formation of the skin of solid metal around the slab, vital to continuous casting.

Excess oxygen can be removed from the metal before it is continuously cast. This can be accomplished by the addition of material which will react with the oxygen and absorb all or the greater part of the excess oxygen.

However, when rimming steel is produced a controlled amount of uncombined oxygen is required to obtain the necessary rimming action and to receive sound material. Rimming action is caused by gas, formed by reaction of carbon and oxygen which agitates and moves the molten metal. By regulating the uncombined oxygen, this agitation of the molten metal can be controlled so that the metal is agitated in a desired pattern in the mould. Thus, a relatively dense, pure rim solidifies from the surface of the cast inwardly and impurities are moved away from the surface of the cast.

Attempts to adapt ingot casting deoxidation techniques for rimming steel to continuous casting have met with limited success and have resulted in the formation of relatively thin rimmed areas, high proportions of oxide impurities and casts which, when rolled, produced rolled sheets with streaks and flaws on or close to the surfaces of the sheets.

The object of the present invention is to regulate the intensity of the rimming action in the mould during casting, to avoid frothing over and to obtain a satisfactory thick and sound rim of the cast which can be rolled into relatively thin sheets without impurities or flaws on the surfaces of the sheets.

This can be achieved by adding as deoxidants into a pouring vessel vanadium in an amount of approximately up to 0.09% and aluminium in an amount of approximately up to 0.05% of molten steel.

Further features of this invention are apparent from the following description which is based on an average charge of 43 t of molten metal. Any substantial change in weight would, of course, lead to corresponding changes in the additions to be described.

In the present method the steel for casting is prepared in the customary manner. It is teemed into a pouring vessel which is preferably a ladle. Therefrom, it is poured into a tundish and hereafter into a mould of a continuous casting plant. When the steel is ready for teeming into the ladle, the iron oxide content of the slag and the residual elements of the steel can be measured in accordance with customary practices. The steel is teemed into the ladle at a temperature of about 1620°C to 1680°C. Difficulties of sampling and the available time do not allow a precise measurement of the iron oxide content of the slag. However, and as will be more apparent from the following description, such a measurement has been found sufficient for the purpose of the present invention. In addition to the measuring of the iron oxide content of the slag, the carbon, manganese, phosphorus and sulfur content of the steel can also be measured. These latter measurements are primarily made for the purpose of determining the additions required to produce the desired steel quality but they are only indirectly related to the oxygen content.

For rimming purposes it is desirable to control the carbon content of the molten metal to be cast.

Tne initial carbon content of the molten metal may be regulated by the oxygen blowing of the metal. This blowing is adjusted such that the carbon content of the molten metal, when it is ready for producing low carbon rimming steel, does not substantially exceed 0.07 %. Where higher carbon rimming steel is to be continuously cast, the blowing is adjusted for a higher carbon content.

Where the carbon content of the metal is 0.07% or less, approximately 4.5 kg of carbon is added in form of breeze to the ladle during teeming of the metal. If the metal has been over-oxidized and the carbon content is substantially less than 0.05%, the addition of carbon may be increased to 9.0 kg.

The addition of carbon to the molten metal, whether as breeze or in the form of alloy, does to a certain extent react with and remove some of the uncombined oxygen in the molten metal in a gaseous state. The amount of carbon that can be added, however, is limited.

According to the present invention, the balance of the uncombined oxygen is removed to attain a substantially uniform pattern of agitation in the molten metal during continuous casting and, at the same time, avoids excess agitation, boiling and froth in the mould by reacting with vanadium and aluminium additions to the molten metal.

Aluminium is well-known as a deoxidant in steelmaking. The disadvantages, arising from an excess addition of aluminium, i.e. relatively thin rims, agglomerates of aluminium oxides at or near the surface of the cast, have already been pointed out.

In the present invention it has been found that the formation of these agglomerates can be avoided by combining an addition of vanadium and aluminium as deoxidants. It has been further discovered that the amount of each material which is added is critical for the production of a well-rimmed strand, free of oxide flaws at or near the surface.

The amount of deoxidant addition depends, of course, on the oxygen content of the metal for which the iron oxide content of the slag in the steelmaking furnace may be a satisfactory estimate for present purposes.

Vanadium, in the form of ferro-vanadium, has been found to be acceptable. In the following description and examples ferro-vanadium having an average analysis of 55.0% vanadium, 4.00% silicon, 2.50% carbon, and the balance iron is employed.

In carrying out the present invention, it has been discovered that there is an upper limit of the amount of each deoxidant that can be added to obtain the required results. For example, it has been found that the addition of substantially more than 0.09% vanadium or 0.16% ferro-vanadium does not produce the required results. Likewise, it has been found that the addition of substantially more than 0.05% of aluminium does not produce the required results. Within these limits, however, the amounts can be varied according to the circumstances, but adding an amount of vanadium less than 0.03% or 0.05% ferro-vanadium and adding an amount of aluminium less than 0.01% gives no suitable answer to the case in question.

When applying the method of the invention, ferro-vanadium is added to stabilize the melt, and aluminium is added to control the rimming action. As mentioned above, the amount of ferro-vanadium added to a particular melt can be determined from the iron oxide content of the slag and the chemistry of the metal.

When for instance the measured iron oxide content of the slag is 16% or less, 0.075% of ferro-vanadium is sufficient to maintain the required addition of aluminium well under 0.05%. As soon as the iron oxide content exceeds 16%, the ferro-vanadium may be increased to 0.11% to maintain the addition of aluminium at a low level.

In the method of the present invention, the vanadium apparently acts as a buffer and lowers the un-combined oxygen content of the molten metal to a point where it can be effectively controlled by the aluminium addition which does not diminish the quality of the cast. It is preferred to add the ferro-vanadium in the first place, because the measurements of the iron oxide content of the slag and of the residual elements in the metal are not precise ones. Also, the amount of uncombined oxygen in the metal may not have been precisely determined.

Aluminium is then added and the amount of aluminium is determined to some extent by the activity of the tapping stream and the metal in the ladle. Therefore, the activity of the gas is lowered to a point where suitable pouring of the required steel is possible.

An effective method for regulating the addition of aluminium and at the same time attaining the required results is to estimate the amount of aluminium required from the iron oxide measurement and the ferro vanadium addition. A portion of the estimated aluminium is then added. If the metal in the ladle is relatively calm and is not excessively boiling and frothing, the remaining portion is not added. If, on the other hand, the metal in the ladle remains wild, boils excessively and froths, the additional aluminium is then added.

In some instances, even after the addition of the remaining portion further addition of aluminium may be required to reduce the activity of the metal in the ladle.

As mentioned above, the melt for continuous casting is prepared at a temperature of about 1620°C to 1650°C and the temperature in the tundlsh, as the molten metal is being delivered to the continuous casting mould, is about 1535°C to 1570°C. As the temperature drops, the molten metal may become more active and the activity may exceed the one necessary for good and effective rimming. When such activity occurs, additional aluminium can be added into the tundish. Usually, however, any addition into the tundish is unnecessary, and it is preferred to add the whole amount of the required aluminium to the ladle.

The following table lists examples of casts which were cast according to the method of the present invention. In each example the average charge of molten metal was 43 t.

Preliminary analysis Additions , kg Cast C n FeO FeMn FeV C Al No. 1 0.060 0.200 12.5 136. 1 22.7 22.7 6.8 2 0.045 0. 120 18.2 192.8 45.4 4.5 15.8 3 0.050 0.200 14.5 79.3 34.0 4.5 6.8 4 0.050 0.250 17.7 136. 1 34.0 4.5 11.3 0.055 0.180 22.3 158.7 45.4 4,5 11.3 6 0.040 0.120 20.9 158.7 45.4 6.8 13.6 7 0.050 0.300 16.0 68.0 45.4 4.5 6.8 8 0.045 0.230 16.8 272.2 34.0 4.5 9.1 9 0.035 0.170 21.0 204. 1 45.4 4.5 18. 1 0.060 0.330 13.1 113.3 34.0 4.5 13.6 11 0.045 0.210 13.3 136.1 34.0 4.5 9.1 12 0.045 0.200 13.5 136. 1 34.0 4.5 13.6 13 0.045 0.210 17.2 136. 1 34.0 4.5 11.3 14 0.055 0.280 12.5 90.7 34.0 4.5 11.3 0.045 0.120 18.7 204. 1 34.0 , 5 15.8 16 0.045 0.240 16.5 113.3 34.0 4.5 6.8 17 0.050 0.320 13.3 90.7 34.0 4.5 4.5 18 0.050 0. 180 13.2 113.3 34.0 4.5 9. 1 19 0.055 0.320 16.2 68.0 34.0 4.5 9.1 0.060 0.220 19.4 181.4 34.0 4.5 11.3 21 0.070 0.200 22.7 90.7 34.0 0 13.6 22 0.065 0.260 19.2 113.3 34.0 4.5 9. 1 23 0.070 0.400 14.0 68.0 34.0 0 11.3 24 0.070 0.390 14.0 68.0 34.0 0 9. 1 0.045 0. 140 15.5 181.4 34.0 4.5 9.1 26 0.050 0.240 20.9 113 , 3 34.0 4.5 11.3 27 0.050 0. 160 22.5 181.4 34.0 4.5 11, 3 28 0.045 0,310 16.5 90.7 34.0 4.5 6.8 29 0.060 0.210 15.7 90.7 34.0 0 6.8 0.050 0.270 17.3 68.0 34.0 4.5 9.1 Carbon Manganese ferrous oxide Ferro manganese Ferro-vanadium Aluminium It must be pointed out that the present invention is not restricted to these examples and can be varied within the indicated limits.

Claims (8)

HAVING NOW particularly described and ascertained the nature our said invention and in what manner the same is to be performed, declare that what we claim is

1. Λ method for continuous casting of rimming steel, wherein deoxidants are added and molten steel is poured into a cooled mould, from which the solidifying strand is withdrawn by withdrawing means, guided and further cooled in a strand guiding means, characterized in that the deoxidants added into a pouring vessel are vanadium in an amount of approximately up to 0,09% and aluminium in an amount of approximately up to 0.05% of the molten steel.

2. A method according to claim 1, characterized in that the amount of deoxidants added is a function of the previously determined iron oxide content of the slag the f and/residual element content of the molten metal.

3. A method according to claim 1, characterized in that the vanadium is added as ferro-vanadium in an amount of approximately up to 0.16%.

4. A method according to claim 2, characterized in that the amount of vanadium added is between approximately 0.01% and approximately 0.09% and the amount of aluminium added is between approximately 0.01% and approximately 0.05%.

5. A method according to claim 4, characterized in that the amount of vanadium added as ferro-vanadium is between approximately 0.05% and approximately 0.16%.

6. A method according to claim 1 or 2, characterized in that the aluminium is added according to the intensity of the rimming action of the molten steel in an amount sufficient to reduce the gas activity to a point where suitable pouring of the desired steel is possible.

7. A method according to claim 1 or 6, characterized in that the amount of vanadium added is between approximately 0.03% and approximately 0.09% of the molten steel.

8. A method according to claim 7, characterized iinn tthhaatt tthhee aammoouunntt ooff vvaannaaddiiuumm aaddddeedd aass ffeerrrroo--vvaannaaddiiuumm iiss bbeettwweeeenn aapppprrooxxiimmaatteellyy 00..0055%% aanndd aapppprrooxxiimmaatteellyy 00..1166%%.. 9 9.. ΔA mmeetthhoodd aaccccoorrddiinngg ttoo ccllaaiimm 88,, cchhaarraacctteerriizzeedd iinn tthhaatt tthhee aalluummiinniiuumm iiss aaddddeedd iinn aann aammoouunntt ooff uupp ttoo 00..0055%%.. 1 100.. AA mmeetthhoodd aaccccoorrddiinngg ttoo ccllaaiimm 88,, cchhaarraacctteerriizzeedd iinn tthhaatt tthhee aalluummiinniiuumm iiss aaddddeedd iinnttoo tthhee ttuunnddiisshh.. 1 111.. AA mmeetthhoodd ffoorr ccoonnttiinnuuoouuss ccaassttiinngg ooff rriimmmmiinngg sstteeeell ssuubbssttaannttiiaallllyy aass ddeessccrriibbeedd hheerreeiinnbbeeffoorree.. D Daatteedd tthhiiss 2288tthh ddaayy ooff AAuugguusstt,, For the Applicants DR. REINHOLD COHN & CO.