GB1569158A - Methods of and apparatus for vacuum refining molten steel - Google Patents

Description

(54) METHODS OF AND APPARATUS FOR VACUUM REFINING MOLTEN STEEL (71) We, NIPPON STEEL CORPORATION, a Japanese Company, of No. 6-3, 2chome, Ote-machi, Chiyoda-ku, Tokyo, Japan, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to a method of and an apparatus for the vacuum treatments of various molten steels, such as decarburization, degassing, the removal of inclusions and bath temperature adjustment.

With the increased demands for higher grades of speciality steels, more efficient and economical production processes are required. To meet with these requirements the refining of molten steels under vacuum has proved indispensible. However, the conventional vacuum degassing processes, such as the Ruhrstahl-Heraeus (R-H) process and the D-H (Dortmund-Hoerder) process, display the following problems which have hindered their commerical operation.

i) The conventional processes aim only to degas molten steels and are not able to decarburize molten steels; since they cannot prevent an increase in the carbon content in the molten steel, they require the use of expensive low-carbon or super-low carbon ferro-alloys.

ii) During treatment, the bath temperature of a conventional process cannot be increased, so that any lowering of the bath temperature during the treatment must be compensated by increasing the blow-off temperature of a convertor. This unavoidably causes wear of the convertor refractory lining.

In order to solve the above difficulties, it has been proposed to oxidize the molten steel bath under a reduced pressure and it is known to blow pure oxygen on to the molten steel flowing in a vacuum refining vessel of a R-H degassing apparatus from a tuyere provided above the molten steel bath. This method has solved only problem i) above and leads to the following further problems.

1) As the gas is blown to the molten steel bath from above the bath surface, the shock of the blown gas causes much splashing of the molten steel.

2) Insufficient contact between the bath and the gas is obtained so that the efficiency of useage of the 2 (for example, the oxygen effeciency in respect to decarburisation) is low.

3) As the gas is blown from above the bath, any slag or the like floating on the bath surface will hinder the contact between the gas and the molten steel, so that the efficiency of the 2 iS lowered.

4) The oxidation loss of alloying elements cannot completely be eliminated. For example, when vacuum degassing of stainless steels, 0.2 to 0.6% of the chromium content is lost in any of the known processes, thus causing an increase in the production cost.

5) The finishing temperature cannot be controlled precisely due to the oxidation loss of alloying elements.

6) As the oxygen effeciency for decarburization varies widely over a relatively low range of from 40 to 70%, the final carbon content cannot be controlled satisfactorily.

The present invention seeks to provide a method of and an apparatus for vacuum refining of molten steel which at least reduces or solves the problems discussed above with known bath oxidation processes.

According to one aspect of this invention, there is provided a method for vacuumrefining a molten steel, in which a non-oxidizing gas or a non-oxidizing gas together with an oxidizing gas is blown into a bath of molten steel in a substantially horizontal direction in a vacuum refining vessel of a Ruhrstahl-Heraeus vacuum degassing apparatus at a position between the bath surface and a bath depth of not more than 50 cm from the bath surface.It is preferred to perform the method of this invention by maintaining the pressure of the atmosphere contacting the molten steel at a higher level at an initial stage of the refining than at a subsequent stage of the refining to allow initiation of the refining with a relatively large gas blow, during the progress of the refining at least one of the amount of the gas blown, and the ratio of the amounts of the non-oxidizing gas to oxidizing gas is lowered either step-wise or continuously.

According to another aspect of this invention, there is provided a modified R-H type of degassing apparatus which comprises at least one gas blowing tuyere provided through a side wall of the vacuum chamber at a position between the level of the surface of molten steel bath (when in use) and a bath depth of not greater than 50 cm from the molten steel bath surface.

In short, the present invention is based on the R-H vacuum refining process and relates to improvements in the R-H process and apparatus.

Hereinafter, unless otherwise qualified, the blowing of a non-oxidixing gas either together or without an oxidizing gas is referred to simply as "gas blowing". For illustration, the oxidizing gas can include pure oxygen, CO2 and water vapour, whereas the non-oxidizing gas can include argon and nitrogen.

In order that the invention may better be understood, it will now be described in greater detail and certain examples thereof given, reference being made to the accompanying drawings, in which: Figure 1 is a graph showing the relation between the decarburization oxygen efficiency and the position of a tuyere used for the gas blowing; Figure 2 is a graph showing the relation between the chromium loss and the position of a tuyere used for the gas blowing; Figure 3 shows schematically a degassing apparatus of the present invention; Figure 4 is a graph showing the decarburization zone where there is no oxidation loss of chromium; Figure 5 is a graph showing the relation between the metal deposition on the uppermost portion of a vacuum vessel and the maximum exhaust gas flow rate generated from the steel bath surface;; Figures 6a and 6b show schematically the gas blowing directions, respectively for the cases of a single tuyere and two tuyeres; and Figure 7 shows schematically the tuyere incorporated in a vacuum refining vessel.

The present invention stems from considerations of the causes of the problems occurring in the prior art methods, especially from the view-point that the gas-liquid reaction interface to which an oxidizing gas such as 2 blown from above the bath surface is limited.

For the purpose of increasing the gas-liquid reaction interface, decarburization experiments were conducted by lowering the gas blowing position. Thus, in a refining vessel of the VAC type vacuum refining apparatus, a common form of tuyere of double-pipe structure was used for blowing 02, the position of the tuyere being lowered to a position in which it was immersed under the circulating flow of the molten steel.

Using a conventional VAC apparatus (comprising a ladle having at its bottom a gas stirring device, and a vacuum tank in which the ladle is contained, decarburization being performed under vacuum using oxygen), the gas blowing position was varied within the range from 1.0 m above the bath surface to 1.5 m below the bath surface and the oxygen efficiency for decarburization and the lowering of the chromium content in the case of 18% Cr stainless steel were compared. Oxygen was blown through the inner pipe and argon was blown through the outer pipe. The results are shown in Figure 1 and Figure 2, from which it is clear that as the double-pipe tuyere position approaches the bath surface from above, the oxygen efficiency for decarburization increases, whilst the oxidation loss of the chromium content is reduced.

In the graphs shown in Figure 1 and Figure 2: The decarburization oxygen efficiency =Consumption of 2 by CO generation x 100 Amount of blown 02 The pressure inside the vessel was 40 - 60 Torr. The tuyere was positioned below the bath surface and directed horizontally and in the same direction as the circulating flow of the molten steel. The gas blowing rate was 1000 Nm3/h.

The above results are remarkable particularly when the gas is blown from below the bath surface. The average decarburization oxygen efficiency then increases to 85% as compared with 40% obtainable in the conventional VAC operation, and the lowering of the chromium content becomes zero as compared with the average lowering of 0.4% in the conventional VAC operation. With regard to the position of the tuyere, the above results are obtained by providing no gas blowing after degassing when the tuyere is positioned below the bath surface level.

One known treatment method of molten steel under a reduced pressure uses a top-blowing immersion lance. In this method, the operation is very unstable because of peeling-off and erosion of the lance refractory material and because of metal deposition on the lance which is caused by mechanical vibration due to foaming of the gas blow under the high temperature conditions of above 1600"C. Also, it is very difficult to attain the required decarburization rate using a high gas blowing rate.

In accordance with a preferred aspect of the present invention, a tuyere for example of a double-pipe structure is provided in the side wall of a vacuum refining vessel in order to assure the security of the refractories surrounding the tuyere and the defects of the conventional immersion lance are eliminated.

The advantageous results of the present invention are attained by blowing gas into the molten steel from a relatively shallow position below the bath surface which contacts the evacuated region of the refining vessel. This feature is advantageous when the apparatus used is a modified R-H vacuum degassing equipment.

In a conventional R-H vacuum degassing apparatus, a part of the molten steel is introduced into the vacuum vessel and circulated for degassing, and the bath depth of the molten steel is limited. However, the method of this invention can be used to perform both the gas blowing and the decarburization in an R-H type apparatus.

It is most preferred to use an R-H type of vacuum refining apparatus in performing the present invention for the following reasons.

(1) In this type of apparatus, circulation of the bath occurs automatically by the vacuum effect (pressure difference) and a constantly changing bath surface contacts with evacuated atmosphere. Thus this type is ideal for refining.

(2) It is not convenient to move the refining vessel because a gas supply pipe is provided for the gas blowing.

In addition the further advantageous results set out below can be obtained by the method and apparatus of the present invention.

When a non-oxidizing gas is blown in the same direction as the bath circulation in the vacuum refining vessel, the amount of molten steel circulation (T/min) employed normally in a R-H process is increased by the pumping action of the blown gas, so that the total amount of molten steel which is exposed to the evacuated atmosphere within a given time is increased. In this way, the gas blowing rate (Nm3/h) can be increased so that the treatment time is significantly shortened.

From the results shown in Figures 1 and 2, it will be understood that advantageous results can be obtained when the tuyere is positioned below the bath surface. However, in order to avoid the danger that the tuyere is exposed above the bath surface if there is a change in the bath surface level, caused for instance by changes in the degree of vacuum or by wear of the refractories, it is desirable to position the tuyere about 20 cm or more below the molten steel bath surface. If the tuyere did become exposed, it could cause rapid wear of the refractories lining the opposed wall, and also could cause the temperatures of the refractories to be increased by oxidation of any metal deposited on the vacuum vessel walls.

In a R-H degassing apparatus, it would theoretically be possible to maintain the depth of the circulating molten steel flow at 50 cm or more, but in order to avoid the blown gas attacking the refractories lining the vessel bottom, the position of the tuyere is limited to be not further than 50 cm below the bath surface.

According to the results of investigations into the tuyere position, it has been found the most preferable range is from 20 to cm below the bath surface.

As mentioned above, by blowing the gas in a substantially horizontal direction below the bath surface, a large gas-liquid interface is maintained and the gas is retained in the bath for a longer time so that the reaction efficiency is considerably improved. Thus the ratio of oxygen (02) used for the decarburization - i.e. the decarburization oxygen efficiency - is both stable and high.

When the gas is blown obliquely upwards at an excessive angle from the horizontal direction, the reaction time within the bath is shorter giving insufficient time for a complete reaction. This lowers the effeciency of the gas and also causes a loss of the molten steel. On the other hand, when the gas is blown obliquely downwards at an excessive angle from the horizontal direction, the gas is blown against the bottom of the vacuum vessel, causing damage to the refractory lining.

For performing the present invention, the following technical considerations as set out below are required.

If the operation is done under the same degree of vacuum (usually 80 Torr or less) as a conventional top-blown process, more sever spitting is caused and in an extreme case the opening through which alloying elements are added to the melt becomes clogged. Also the spitting can deposit metal on the vacuum generating system to lower the gas exhausting ability, thus severely hindering its operation. However, it is possible to prevent substantially completely deposition of metal on the vessel in the method of this invention while maintaining the advantages of the reactions in the vessel obtained in the conventional way whilst significantly shortening the decarburization time by increasing the gas blowing rate.

Decarburization tests were made on 18% Cr molten stainless steel with different O2/Ar ratios and vacuum degrees using the apparatus as shown in Figure 3. There shown is a double-pipe tuyere 2 provided through the side wall of a vacuum vessel 1 at a depth of 30 cm below the bath surface. A ladle 3, an upward flow pipe 4, and a downward flow pipe 5 are also shown, together with the molten steel 6. In the tests, argon was blown through the outer pipe of the tuyere and oxygen or an oxygen-argon mixture was blown through the inner pipe in a horizontal direction and in the same direction as the circulating flow of the molten steel.

The ratio of O2/Ar (by volume) was maintained within the range of from 30/1 to 1/1, and the vacuum degree in the vessel was maintained in the range from 200 to 20 Torr during the decarburization by the ejector operation. The results of the tests revealed that as the O2/Ar ratio decreased, and as the vacuum degree increased, the decarburization can be completed to a greater extent giving a lower carbon content without lowering the chromium content.

Based on the results of the tests, the conditions for decarburization without loss of the chromium content could be found with respect to the carbon content, the O2/Ar ratio and the vacuum degrees being shown by the hatched-lined zone shown in Figure 4 (16 - 17% Cr stainless steels). At the same time the thickness of metal deposited on the metal fittings attached to the uppermost portion (5 m above the bath surface) of the vacuum vessel was measured, and it was revealed from the measurements that the thickness increased in proportion to an increase in the maximum flow rate of the exhaust gas generating from the bath surface, as shown in Figure 5.

The exhaust gas flow is determined by the gas blowing rate, the vacuum degree, the decarburization oxygen efficiency, the bath temperature, and so on. As for the conditions for preventing metal deposition, the following formula was obtained.

(2llVo2+vAr)e ' To273 . 760 1 6100(f/min/cm2) .... (1) 273 P S in which: TI : decarburization oxygen efficiency V02 : O2 gas blowing rate (/min) VAr : Ar gas blowing rate (fimin) S : Surfacial area of the bath in the vacuum vessel (cm2) T : molten steel temperature ("C) P : pressure in the vessel (Torr) Preferably, the formula sets the flow rate to not more than 80 (e/min/cm2).

In order to prevent metal deposition under various O2/Ar ratios, it is found necessary to consider the oxygen blowing rate as well as the pressure in the vessel. Therefore, on the basis of the above relation, metal deposition can be prevented by increasing the pressure in the vessel even when the oxygen blowing rate is increased considerably.

In the case of the decarburization of ordinary steels other than high alloy steels such as stainless steels, for example low-alloy steels and silicon steels, it is found not to be necessary to change the ratio between the non-oxidizing and the oxidizing gas, and it is sufficient only to control the gas blowing rate.

However, for controlling and especially for adjusting the ratio between the non-oxidizing gas the the oxidizing gas, it is advantageous to use a tuyere having a double-pipe structure, such as is used in the AOD process.

The maximum gas blowing rate for each tuyere is about 1500 Nm3/h, and a preferable range is from 1200 to 500 Nm3/h for the following reasons.

(1) To ensure the gas blown does not contact with the opposed side wall of the vacuum refining vessel.

(2) If a tuyere of large diameter is used for blowing a large amount of gas, the refractories surrounding the tuyere can be damaged if the operation is done with a low gas blowing rate.

On On the other hand, the minimum gas flow rate is determined from the fact that when the amount of gas blown from the tuyere is small, it is pushed back by the static pressure of the molten steel and imparts shock waves and damage to the refractories surrounding the tuyere. Therefore, at least 200 Nm3/h for the gas blowing rate should be used.

When there are present in the molten steel bath elements such as Al and Si, which generate heat by oxidation, it is possible to increase the bath temperature by causing such elements to react with the oxidizing gas, giving an average heat generation efficiency of about 73%.

The apparatus of the present invention will now be described in greater detail referring to Figure 3.

In order to allow consistent blowing of the gas, the tuyere should be protected by cooling and for this purpose, a tuyere of a double-pipe structure is preferably used, since this can be cooled by the non-oxidizing gas blown through the outer pipe.

The position of the tuyere is determined from the following considerations.

(1) The bath depth: The end of the tuyere opening into the inside wall of the vacuum vessel should be not greater than 50 cm from the bath surface for the reasons set forth hereinbefore. This position should be maintained irrespective to the number of the tuyere.

(2) The molten steel flow in the vacuum refining vessel.

Regarding the relation between the gas blowing direction and the flow direction of the molten steein the vacuum tank, it is desirable that the blown gas and the molten steel flow are maintained in essentially the same direction so as to increase the circulation of the molten steel and thus to improve the reaction efficiency. When more than one tuyere is provided to increase the gas flow, it is desirable that the tuyeres are provided in such a manner that the intersection of their projected axes lies over the downward flow pipe, and that they are disposed at an angle 0 of between 5 and 15 to each other, measured about the said intersection, or that they are spaced from each other by 20 to 70 cm, measured between their inner opening ends.The above numerical limitations are determined from the following considerations.

The gas blown into the molten steel bath spreads at an angle of about 20 . Therefore, in order to avoid interference between the foams immediately after the blowing and to attain desirable foam distribution, the above angular range for the tuyere should be observed.

The above defined distance between the opening ends of the tuyere is calculated from the centre angle, taking into account a typical diameter of a vacuum refining vessel, and the minimum distance is determined from the physical space required from the tuyere arrangement.

The arrangement either of one or of two tuyeres are shown in Figures 6a and 6b respectively. However, it will be understood that these are examples of possible arrangements and other arrangements of tuyeres could be employed in performing the invention.

(3) The direction of the gas blowing.

It is desirable that the gas blowing direction is substantially horizontal for the reasons set forth below, although there is no practical hindrance if the blowing direction does not lie outside +5 from the horizontal.

(i) When the gas blowing is directed downwardly from the horizontal, there is some possibility that the gas will contact the bottom of the vacuum chamber. If this occurs the refractory lining can be damaged.

(ii) When the gas blowing is directed upwardly above the horizontal, there is the possibility that the oxidizing gas is blown and leaves the molten steel surface without a sufficient reaction effect, so that the efficiency of the 2 is lowered.

The present invention is not limited to any specific steel and is useful for the decarburization and degassing of ordinary steels, silicon steels and special steels. The invention is however most useful for the decarburization of chromium-containing steels, and the following description of specific Examples will be made in connection with such steels. The Examples all use the same equipment, defined below.

Equipment Vacuum refining vessel inside diameter: 1.6 m (able to treat 60 ton of molten steel in one charge) Arragement of tuyere: the tuyere is arranged so that the gas is blown in a horizontal direction and in the same direction as the circulating flow of the molten steel in the vacuum vessel, namely at a position through the side wall of the vessel 20 cm below the static bath surface.

Structure of tuyere : Double-pipe structure; the inner pipe is used for blowing either oxidizing gas only or a mixture of oxidizing and non-oxidizing gas, and the outer pipe is used for blowing non-oxidizing gas only through the space between the inner pipe and the outer pipe.

The equipment as above specified is shown in Figure 7.

The results of decarburization using the above apparatus are shown in Table 2 in comparison with those obtained by a conventional top-blown process.

The operation conditions were changed step-wise in correspondence to the carbon content in the bath as shown in Table 1, from the initial conditions set forth above.

The results of the conventional top-blown operation were obtained under ordinary conditions; the gas blowing rate was 600 - 800Nm /h and the degree of vacuum was 60 to 30 Torr.

TABLE 1 [% Cj Gas blowing O2/Ar Vacuum degree rate (Nm3/h) (by volume) in vacuum vessel Initial C% -0.15 800 - 1200 10 - 30 100 - 150 Torr 0.15 - 0.05 700 - 1100 - 5 100 - 150 0.05 - 0.01 200 - 400 - 1 50 - 80 TABLE 2 Decar- Yield Maximum buriza- of #[%Cr] Exhaust Composition before Composition after tion Molten Gas Decarburization (%) Decarburization (%) Time Steel Volume C Si Mn Cr C Si Mn Cr min % % l/min/cm2 Method 0.50 0.05 0.20 16.30 0.041 0.05 0.15 16.35 25 99.2 0.05 75 of this 0.45 0.03 0.22 16.81 0.045 0.04 0.20 16.80 21 99.4 -0.01 70 invention 0.59 0.04 0.18 16.55 0.045 0.05 0.14 16.65 26 99.3 0.10 80 *Conven- 0.40 0.02 0.15 16.10 0.030 0.02 0.08 15.70 40 85.2 -0.30 90 tional to to to to to to to to to to to to Method 0.60 0.09 0.25 17.30 0.090 0.07 0.17 16.90 55 85.9 -0.75 120 (Top-blown) " Range of variation over about 100 typical heats.

As will be clearly be understood from the results shown in Table 2, the present invention has the following technical advantages over the conventional top-blown decarburization process.

1) The loss of alloying elements by oxidation is significantly lowered. Particularly for the case of the decarburization of stainless steels, the oxidation of chromium is completely prevented.

2) It is possible significantly to increase the gas blowing rate, thus shortening the decarburization time.

3) It is possible to prevent almost completely metal deposition on the inside of the vacuum refining vessel, caused in the known process by spitting This assures great advantages in respect of the equipment reliability, its operation and the production yield.

4) Control of the final carbon content and the final bath temperature is greatly improved - these can vary widely in the known process.

Although not shown in Table 2, the following further advantages are obtained by the present invention.

5) It is possible to treat steel comps containing 100 ppm or less of carbon.

6) Contents of H, N and O in the molten steel after refining are similar to or lower than those obtained by the conventional top-blown process.

7) It is possible to utilize about 73% of the oxidation heat of elements such as C, Al, Si contained in the bath for increasing the bath temperature.

WHAT WE CLAIM IS: 1. A method for vacuum-refining a molten steel, in which a non-oxidizing gas or a non-oxidizing gas together with an oxidizing gas is blown into a bath of molten steel in a substantially horizontal direction in a vacuum refining vessel of Ruhrstahl-Heraeus (R-H) vacuum degassing apparatus at a position between the bath surface and a bath depth of not more than 50 cm from the bath surface.

2. A method according to claim 1, in which at least one of the pressure of the atmosphere contacting the molten steel bath, the amount of the non-oxidizing gas blown and the amount of the oxidizing gas blown is adjusted so as to maintain a volume of exhaust gas generated from the surface of molten steel not higher than 100 t/minlcm2 of the cross section of the bath.

3. A method according to claim 2, in which the value of the exhaust gas generation is maintained not higher than 80 e/min/cm2.

4. A method according to any of claims 1 to 3, in which the pressure of the atmosphere contacting the molten steel is maintained at a higher level at an initial stage of the refining than at a subsequent stage of the refining to allow initiation of the refining with a relatively large gas blow, and during the progress of the refining at least one of the amount of the gas blown, and the ratio of the amounts of non-oxidizing gas to oxidizing gas is lowered either step-wise or continuously.

5. A method according to any of claims 1 to 4, in which the non-oxidizing gas or the non-oxidizing gas together with the oxidizing gas is blown into the molten steel bath from a single blowing point at a blowing rate not lower than 200 Nm3/h.

6. A method according to claims 1 to 5, in which the non-oxidizing gas or the non-oxidizing gas together with the oxidizing gas is blown into the molten steel bath from a single blowing point at a blowing rate not higher than 1500 Nm3/h.

7. A method according to any of claims 1 to 4, in which the non-oxidizing gas or the non-oxidizing gas together with the oxidizing gas is blown into the molten steel bath from a single blowing point at a blowing rate not lower than 500 Nm3/h and not nigher than 1200 Nm3/h.

8. A method according to any of claims 1 to 7, in which the gas blowing takes place from a position lying in the range of from 20 cm to 40 cm below the bath surface.

9. A method for vacuum refining a molten steel according to claim 1 and substantially as hereinbefore described with reference to the accompanying drawings.

10. A modified R-H (Ruhrstahl-Heraeus) type of degassing apparatus which comprises at least one gas blowing tuyere provided through a side wall of the vacuum chamber at a position between the level of the surface of molten steel bath (when in use) and a bath depth of not greater than 50 cm from the molten steel bath surface.

11. A modified R-H type of degassing apparatus according to claim 10, wherein the tuyere is directed to blow the gas at substantially the same direction as the flow direction of molten steel circulating in the apparatus when in use.

12. A modified R-H type of degassing apparatus according to either of claims 10 and 11, wherein the tuyere is directed to blow the gas in a direction substantially parallel to the flow of molten steel circulating in the apparatus when in use.

13. A modified R-H type of degassing apparatus according to any of claims 10 to 12, wherein more than one tuyere is provided, their axes being angularly spaced by from 5 to

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (19)

**WARNING** start of CLMS field may overlap end of DESC **. As will be clearly be understood from the results shown in Table 2, the present invention has the following technical advantages over the conventional top-blown decarburization process.

1) The loss of alloying elements by oxidation is significantly lowered. Particularly for the case of the decarburization of stainless steels, the oxidation of chromium is completely prevented.

2) It is possible significantly to increase the gas blowing rate, thus shortening the decarburization time.

3) It is possible to prevent almost completely metal deposition on the inside of the vacuum refining vessel, caused in the known process by spitting This assures great advantages in respect of the equipment reliability, its operation and the production yield.

4) Control of the final carbon content and the final bath temperature is greatly improved - these can vary widely in the known process.

Although not shown in Table 2, the following further advantages are obtained by the present invention.

5) It is possible to treat steel comps containing 100 ppm or less of carbon.

6) Contents of H, N and O in the molten steel after refining are similar to or lower than those obtained by the conventional top-blown process.

7) It is possible to utilize about 73% of the oxidation heat of elements such as C, Al, Si contained in the bath for increasing the bath temperature.

WHAT WE CLAIM IS: 1. A method for vacuum-refining a molten steel, in which a non-oxidizing gas or a non-oxidizing gas together with an oxidizing gas is blown into a bath of molten steel in a substantially horizontal direction in a vacuum refining vessel of Ruhrstahl-Heraeus (R-H) vacuum degassing apparatus at a position between the bath surface and a bath depth of not more than 50 cm from the bath surface.

2. A method according to claim 1, in which at least one of the pressure of the atmosphere contacting the molten steel bath, the amount of the non-oxidizing gas blown and the amount of the oxidizing gas blown is adjusted so as to maintain a volume of exhaust gas generated from the surface of molten steel not higher than 100 t/minlcm2 of the cross section of the bath.

3. A method according to claim 2, in which the value of the exhaust gas generation is maintained not higher than 80 e/min/cm2.

4. A method according to any of claims 1 to 3, in which the pressure of the atmosphere contacting the molten steel is maintained at a higher level at an initial stage of the refining than at a subsequent stage of the refining to allow initiation of the refining with a relatively large gas blow, and during the progress of the refining at least one of the amount of the gas blown, and the ratio of the amounts of non-oxidizing gas to oxidizing gas is lowered either step-wise or continuously.

5. A method according to any of claims 1 to 4, in which the non-oxidizing gas or the non-oxidizing gas together with the oxidizing gas is blown into the molten steel bath from a single blowing point at a blowing rate not lower than 200 Nm3/h.

6. A method according to claims 1 to 5, in which the non-oxidizing gas or the non-oxidizing gas together with the oxidizing gas is blown into the molten steel bath from a single blowing point at a blowing rate not higher than 1500 Nm3/h.

7. A method according to any of claims 1 to 4, in which the non-oxidizing gas or the non-oxidizing gas together with the oxidizing gas is blown into the molten steel bath from a single blowing point at a blowing rate not lower than 500 Nm3/h and not nigher than 1200 Nm3/h.

8. A method according to any of claims 1 to 7, in which the gas blowing takes place from a position lying in the range of from 20 cm to 40 cm below the bath surface.

9. A method for vacuum refining a molten steel according to claim 1 and substantially as hereinbefore described with reference to the accompanying drawings.

10. A modified R-H (Ruhrstahl-Heraeus) type of degassing apparatus which comprises at least one gas blowing tuyere provided through a side wall of the vacuum chamber at a position between the level of the surface of molten steel bath (when in use) and a bath depth of not greater than 50 cm from the molten steel bath surface.

11. A modified R-H type of degassing apparatus according to claim 10, wherein the tuyere is directed to blow the gas at substantially the same direction as the flow direction of molten steel circulating in the apparatus when in use.

12. A modified R-H type of degassing apparatus according to either of claims 10 and 11, wherein the tuyere is directed to blow the gas in a direction substantially parallel to the flow of molten steel circulating in the apparatus when in use.

13. A modified R-H type of degassing apparatus according to any of claims 10 to 12, wherein more than one tuyere is provided, their axes being angularly spaced by from 5 to

15 .

14. A modified R-H type of degassing apparatus according to any of claims 10 to 12, wherein more than one tuyere is provided, the tuyeres being spaced from each other around the chamber wall by from 20 to 70 cm.

15. A modified R-H type of degassing apparatus according to any of claims 10 to 14, wherein more than one tuyere is provided, the axis of each tuyere intersecting at a common point disposed over the downward flow of the R-H type of apparatus.

16. A modified R-H type of degassing apparatus according to any of claims 10 to 15, wherein the gas is blown into the molten steel at not more than +5" from the horizontal.

17. A modified R-H type of degassing apparatus according to any of claims 10 to 16, wherein the or each tuyere is from 20 to 40 cm below the surface of the molten steel bath when in use.

18. A modified R-H type of degassing apparatus according to claim 10 and substantially as hereinbefore described, with reference to and as illustrated in Figure 3 or in Figures 6(a) or 6(b) or in Figure 7 of the accompanying drawings.

19. Refined steel whenever produced according to a method of any one of claims 1 to 9.