US3879192A

US3879192A - Electroslag-remelting method

Info

Publication number: US3879192A
Application number: US414842A
Authority: US
Inventors: Kiyoshi Segawa; Yasushi Nakamura; Naoki Tokumitsu; Kazuumi Harashima
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp; Warner Lambert Technologies Inc
Priority date: 1970-12-31
Filing date: 1973-11-12
Publication date: 1975-04-22
Anticipated expiration: 1992-04-22

Abstract

An electroslag-remelting method for refining metals by using a slag composed of at least one of metallic calcium, strontium, magnesium and barium in an amount of 0.5 to 6% by weight, the remainder of the slag being at least one of fluorides of calcium, strontium, magnesium and barium, and optionally, the oxides of these metals, the impurities in the slag being restricted to less than 0.5% by weight.

Description

United States Patent n91 Segawa et al.

[ Apr. 22, 1975 ELECTROSLAG-REMELTING METHOD [75] Inventors: Kiyoshi Segawa; Yasushi Nakamura.

both of Tokyo; Naoki Tokumitsu, Yokohama; Kazuumi Harashima. Kawasaki. all of Japan [73] Assignee: Nippon Steel Corporation, Tokyo.

Japan [22] Filed: Nov. 12, 1973 [21] Appl. No.: 414,842

Related US. Application Data [63] Continuation-impart of Ser. No. 212.724. Dec. 27.

1971, abandoned.

[30] Foreign Application Priority Data Dec. 31. 1970 Japan 45-129162 [52] US. Cl. 75/10 C; 75/94 [51] Int. Cl C22d 7/00 [58] Field of Search 75/10 R, 10 C. 94

[56] References Cited FORElGN PATENTS OR APPLICATIONS l l75.453 l2/l969 United Kingdom 75/l0 C 560.260 7/1958 Canada 75/10 C Primary E.\'uminerL. Dewayne Rutledge Assistant E.\'aminerM. J. Andrews Alrurney. Agent. or FirmWenderoth. Lind & Ponack [57] ABSTRACT 8 Claims, S'Drawing Figures ELECTROSLAG-REMELTING METHOD This application is a continuation-in-part of application Ser. No. 212,724, filed Dec. 27, 1971, now abandoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a method for producing mainly high-quality steels such as superhigh tensile strength steels, heat-resistant steels, superheat-resistant alloys, industrial chemical stainless steels and welding materials or nickel alloys, chromium alloys, cobalt alloys and the like.

Such alloy steels or alloys as above mentioned (referred to as metals" hereinafter) are required to contain a very slight amount of all such impurities as phosphorus, sulfur, nitrogen, oxygen, arsenic and antimony, or at least some of them, depending upon the kinds of metals. There is also the case in which the nitrogen content is kept at a certain level, whereas other impurities are required to be of a very slight content, as in the case of some kinds of industrial chemical stainless steels.

2. Description of the Prior Art Heretofore, in order to meet such a severe requirement as above mentioned, said metals have often been manufactured through so-called multiple steps, comprising a careful selection of materials, melting in an electric furnace and subsequent vacuum-treatment, which is followed by vacuum-arc-remelting or an electroslag-remelting, or further an electroslag reremelting, as occasion demands. However, the multiple steps are accompanied by the disadvantages that much labor and power are required, and the yield is lowered, so that the production cost becomes high. Further, even adopting the multiple steps as above mentioned, it is sometimes difficult in some kinds of steels to reduce the content of the above-mentioned impurities to such a degree as may satisfy the necessary requirements.

On the other hand, in US. Pat. No. 3,507,968 there are disclosed a special electroslag melting apparatus and method, in which dephosphorization and desulphurization are accomplished even in the case of a high chromium steel, without causing a loss of chromium. The special electroslag melting method of this patent is characterized in that a mixture of powdered metal and powdered non-metal flux ingredients is fed into a pool of molten slag, while causing the mixture to magnetically adhere to a strip electrode, and the strip electrode is traversed in a crystallizer, in which the electroslag melting is accomplished, so as to maintain a heat pattern of large section, notwithstanding the fact that the electrode itself is of small section, in contrast with the known normal electroslag melting, in which a consumable electrode of a large section containing a great part of alloy ingredients in advance is employed. Accordingly, the electroslag melting of this reference shows aspects considerably different from the conventional one in regard to the slag-metal contact, movement of slag pool and electrode current density, and may be said to be epoch-making in the effect that the dephosphorization of a high chromium steel, which has been regarded as very difficult by the steel-making processes of the present time, including the known normal electroslag melting method, can be accomplished.

However, the electroslag melting method of this patent is accompanied by the disadvantages that l) the apparatus is complicated in structure and is expensive, because it must be provided with a plurality of bins for storingpowdered metal and powdered flux ingredients. respective metering feeders of powdered metals and fluxing ingredients and a powder mixing mechanism, and the electric connecting parts are of a friction contact type and (2) alloying powders that are expensive and are difficult to treat, are used as materials. Moreover, the electroslag melting of this patent is insufficient in its effect, because the desulphurization is incomplete, and a loss of silicon is unavoidable. Particularly, in view of the fact that silicon is the most oxidizable among the components shown in the Exampleof this patent, it must be allowed to be very difficult to produce a metal, which contains a large amount of titanium and aluminium, much easier to oxidize than silicon, such as for instance, a heat-resistant steel.

Further, it was heretofore also known to include in the electroslag remelting slag metallic calcium and magnesium in an amount of less than 10%, as disclosed in Electroslag Remelting, Duckworth et al., Chapman & Hall Ltd., London, 1969, pages 47 and 48, but with the understanding that the addition of minor components serves mainly to modify the chemical rather than the physical properties of the slag. However, there is no concrete explanation in this publication as to how the chemical properties of the slag are modified. Calcium and magnesium are used only in the capacity of a metallic deoxidizer, just as aluminum, silicon, titanium, vanadium and the like.

SUMMARY OF THE INVENTION In order to solve the above mentioned problems this invention has for its primary object the provision of an electroslag remelting method for obtaining a metal, in which the contents of metal impurities such as phosphorus, sulfur, nitrogen, oxygen, arsenic, antimony and the like are reduced to their utmost, without causing a loss of alloy components due to the oxidation thereof in an ordinary type electroslag remelting method using a consumable electrode of a large cross section.

Another object of the present invention is the provision of an electroslag remelting method for obtaining a metal in which the contents of nitrogen, arsenic and antimony contained in a steel ingot are regulated to an arbitrary amount, and further, are uniformly distributed therein.

These objects can be achieved by refining a metal in an atmosphere of at least one gas selected from the group consisting of argon, helium and nitrogen, by employing a slag composed of 0.5 to 6% by weight, based on the weight of the slag, of at least one metallic element selected from the group consisting of metallic calcium, strontium, magnesium and barium, the remainder 0f the slag being essentially composed of at least one of the fluorides selected from the group consisting of calcium, strontium, magnesium and barium fluorides, impurities such as alumina, silica, titania and manganese oxide being restricted to less than 0.5% by weight, each.

Further, the objects of this invention can be achieved by refining a metal in an atmosphere of at least one of the gases selected from the group consisting of argon, helium and nitrogen, employing a slag composed of 0.5 to 6% by weight of at least one metallic element selected from the group consisting of metallic calcium, strontium, magnesium and barium, the remainder of the slag being essentially composed of at least one of the fluorides selected from the group consisting of calcium. strontium, magnesium and barium fluorides and at least one of the oxides selected from the group consisting of calcium, strontium, magnesium and barium oxides in an amount of not more than 30%. impurities such as alumina, silica, titania and manganese oxide being restricted to less than 0.5% by weight, each.

Further, the objects of this invention can be achieved by increasing the concentration of at least one of the metallic components selected from the group consisting of calcium, strontium, magnesium and barium contained in the slag having the compositions as above mentioned.

Still further. the objects of this invention can be achieved by regulating the amounts of nitrogen, phosphorus, arsenic and antimony by the addition thereof in the form of the elements themselves or their compounds.

In the following the essential features of this invention shall be explained.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing the distribution of the nitrogen content in the lengthwise direction of the metal ingot prepared in Example 6.

FIG. 2 is a diagram showing the distribution of the phosphorus content in the lenghwise direction of the metal ingot prepared in Example 6.

FIG. 3 is a diagram showing the distribution of the arsenic content in the lenghwise direction of the metal ingot prepared in Example 6.

FIG. 4 is a diagram showing the distribution of the antimony content in the lengthwise direction of the metal ingot prepared in Example 6.

FIG. 5 is a diagram showing the distribution of the phosphorus and nitrogen contents in the lengthwise direction of the metal ingot prepared in Example 7.

DESCRIPTION OF THE PREPARED EMBODIMENTS This invention is based on the technical concept of carrying out an electroslag remelting method in a nonoxidative atmosphere by using a slag containing as a solute at least one metal which is soluble in the slag phase but hardly soluble in the metal phase to be remelted, and further is chemically very reactive with phosphorus, sulfur, nitrogen, oxygen, arsenic and antimony, which method is therefore quite different from the conventional methods.

According to such concept, as above mentioned, such metals as aluminum, titanium, silicon and vandium, which have a strong affinity for elements constituting the main components of a metal to be remelted, such as iron, chromium, nickel and the like, and are highly soluble therein, can not act as a solute of a slag, though they are highly chemically reactive.

However, alkaline earth metals such as Ca, Sr, Mg and Ba, can meet the conditions required of the technical concept of this invention, because they are highly chemically reactive with phosphorus, sulfur, nitrogen, oxygen, arsenic and antimony, and further are hardly soluble in the metal to be remelted but soluble in a slag containing as a solvent the fluorides of Ca, Sr, Mg and Ba.

Each of the metallic components of the slag, that is, Ca, Sr, Mg and Ba, has a function of removing phosphorus, sulfur, nitrogen, oxygen, arsenic and antimony from the metal to be remelted, in coexistence with the above mentioned fluorides. The higher the concentration of these metallic components, the greater the refining effect. The total content of these metallic components is required to be 0.5% by weight, based on the weight of the slag. However, when strong deoxidizing and desulphurizing effects are particularly desired, their total content is preferably more than 1.0% by weight, and when strong dephosphorizing, denitrating, arsenic-removing and antimony-removing effects are desired, their total content is preferably more than 3.0% by weight.

Among these alkaline earth metals, Ca, Sr and Ba are the most preferable from the point of stability in the slag, and Mg follows. But, from the point of a noxious character to the human body, Ca and Mg are preferable because of their least noxious character, whereas Sr and Ba are not desirable, because they are noxious if they are inhaled in a large amount. Therefore, special precautions against toxic effects are necessary in the case of using Sr and Ba, not only in the metallic state but also in the form of their fluorides and oxides. From the point of material price Ca and Mg are advantageous. Considering all of these factors, Ca is the most preferable.

On the other hand, with the increase in the concentration of these metals in the slag, the electric resistance of the slag is decreased, since the metal has a better electric conductivity than the fluorides. Then, the role of the slag as a resistance heat generator will be lost. Therefore, the total concentration of the metals should not be greater than 6% by weight.

The respective fluorides of calcium, strontium, magnesium and barium are important components of the slag to be used in this invention, since they enable the slag to fulfill such roles in the electroslag-remelting method as (l) generating Joule heat in the slag, (2) exerting a favorable influence on the removal of nonmetallic inclusions by a slag-metal reaction, (3) obtaining a steel ingot having a favorable skin and (4) protecting the casting mold from high temperature due to its presence between the ingot and the casting mold walls. Further, these fluorides act as a solvent for the metallic components of the slag, i.e. calcium, strontium, magnesium and barium. Therefore, it is advantageous that the total concentration of the fluorides in the slag is preferably at least by weight of the slag. From various viewpoints, such as purity, steam pressure, noxiousness and price, calcium fluoride can be said to be the most advantageous.

Into the slag used in this invention one or more of'the oxides of calcium, strontium, magnesium and barium may be added, as required. By the addition of these oxides, there can be obtained various advantages, such as an improvement of surface properties of the ingot, an acceleration of desulphurization and an increase in the electric resistance of the slag and the like. In addition thereto, as these oxides are lower in price than the fluorides, they can be advantageously used to secure the absolute amount of the slag. However, if the content of the oxides is too high, the solubility limit of the metallic components of the slag, calcium, strontium, magnesium and barium, is lowered, so that the desired concentration of the metallic components can no longer be secured, and moreover, the melting point of the slag rises, with the result that the cast skin of the produced steel ingot is deteriorated. Therefore, the content of these oxides is not more than 30%, preferably by weight, based on the weight of the slag.

Such oxides as alumina, silica. titania, ferrous oxide and manganese oxide should not be added to the slag. In case these oxides are included as impurities in the slag, the amount of these oxides should be limited to below 0.5% for each oxide.

Among the above mentioned oxides, alumina and silica are used as the slag component according to conventional electroslag remelting methods. But, in the method of this invention the existence of alumina and The method of this invention is carried out. while using a water-cooled metal mold as in an ordinary electroslag remelting method, but in a nonoxidative atmosphere of an inert gas such as argon, helium and the like, or further with nitrogen gas added thereto, instead of open air. particularly, when it is necessary to regulate the nitrogen content of the metal to be remelted, the method of this invention is carried out in an atmosphere having a nitrogen partial pressure regulated to a specified value.

The slag is used in a previously melted state or is melted by electric resistance or an arc after being supplied in the form of powder.

The invention shall be explained with reference to silica is particularly restricted for reasons explained in 15 the following examples. the following.

Firstly, if more than 1% of the respective oxides are EXAMPLE 1 contained in the slag, the metallic components of the A chromium steel (SUS 51) of the undermentioned s ag. a iu tium. magn sium and barium. composition as a mother material was remelted under react preferentially with the above mentioned oxides, the undermentioned conditions in a water-cooled castso that their effect of removing phosphorus, sulfur, niing mold of a diameter of 69 mm. made of copper to trogen, arsenic and antimony can not sufficiently be reobtain an ingot of a weight of 5 kg: alized. Secondly, the reduction products of the above mentioned oxides are being admixed into the metal to Electric cufrrpm:l ac of 1000 A. 12v.

Amount 0 t c s ag: 00 g. be rernelted, thereby causing a change m the compost Mcmng time: 58 minutes tron DI the metal. Slag temperature: about 1800C.

In this disclosure the metallic components of the slag used in this invention are expressed as those existing in The results were as follows:

Table 1 Chemical Degree composition C Si Mn Cr P 5 As Sb 0 N of puri- (wt.%) fication Mother 0.13 0.43 0.35 12.34 0.019 0.009 0.009 0.0017 29""'" 300" 0.138 material E.S.R.

material of the present 0.13 0.42 0.36 12.60 0.003 0.005 0.00l 0.0002 10 68 0.016 invention E.S.R. material of Reference 0.13 0.40 0.31 12.29 0.019 0.005 0.009 0.0016 25 312 0.038 Example (1) E.S.R. material of Reference 0.13 0.39 0.31 12.31 0.019 0.007 0.008 0.0016 20 305 0038 Example (2) Remarks:

1. The E.S.R. material means a steel material obtained by the "electroslag remelting method". 2. The slag used in (conventional) Reference Example 1 consisted of 70% CaF. and 30% A1 0 3. The slag used in (conventional) Reference Example 2 consisted of 100% CaF 4. The slag used in the present invention consisted of 94% CaF and 6% Ca. 5. The analysis of S was made according to the method of 11S 6-1215 (Japanese Industrial Standard) and that of 0 according to an argon-conveying method. The degree of purification was measured by the method of 11S 6-0555.

the metallic state in the slag. However, in connection therewith, it is to be noted that the expression metallic component as used herein does not always denote the metallic state in the strict scientific meaning, but that, if taking, for instance, the case of Ca, the balance obtained by subtracting Ca as CaF and CaO from the total Ca is regarded as the metallic component. This is also the same for the cases of Mg, Sr and Ba.

In preparing the slag used in this invention metallic calcium, strontium, magnesium and barium may be added to the fluorides, thereof, or the slag may be made to contain an increased amount of the metallic components by removing fluorine from the fluorides by means of electrolysis and the like.

As is apparent from the above-mentioned results, by the method of the present invention a steel having the contents of P, S and 0 being remarkably reduced and an excellent degree of purification can be obtained.

EXAMPLE 2 Table 2 Chemical composition C Si Mn P S O Mother material 0.4) 0.30 0.73 0.030 0.012 l6""" E.S.R. material of the present 0.47 0.29 0.74 0.007 0.005 10 invention E.S.R. material of Reference 0.48 0.28 0.71 0.030 0.010 30 Example (3) E.S.R. material of Reference 0.48 0.31 0.70 0.028 0.012 19 Example (4) Remarks:

1. The slag used in the present invention consisted of 98% CaF and 2% Ca. 2. The slag used in Reference Example 3 was the same as in Reference Example 1. 3. The slag used in Reference Example 4 was the same as in Reference Example 2.

EXAMPLE 3 The results were as follows:

A mother material in the undermentioned table was 20 EXAMPLE 5 melted under the same conditions as in Example 1. The results are shown in the follgwing; The undermentioned alloy Stl (A181 661) used 38 a mother material was remelted under the undermen- EXAMPLE 4 tioned conditions to obtain an ingot of a weight of 5 A low chromium-content steel (SUJ 2,) of the undermentioned composition as a mother material was remelted under the undermentioned conditions in a water-cooled casting mold of a diameter of 69 mm. made Elccmc fi g gg 'fv gyp Slag) of copper to obtain an ingot of a weight of kg.: 30 c 0f 1000 25 v,

(in the case of other slags) Electric current: A.C. of 1250 A. 15V. Melting time: 23 minutes (in the case of Melting time: 36 minutes g z-Mg g) Table 3 Chemical I composition C Si Mn Ni Cr P S O (WLY!) Mother material (SUS 27) 0.067 0.51 1.85 8.54 18.60 0.026 0.010 79"""' E.S.R. material of the present 0.071 0.47 1.84 8.59 18.57 0.006 0.005 10 invention E.S.R. material of Reference 0071 0.46 1.84 8.60 18.57 0.026 0.010 46 Example (5) E.S.R. material of Reference 0.070 0.46 1.85 8.61 18.52 0.024 0.008 46 Example (6) Remarks:

1. The slag used in the present invention consisted of CaF 10% C210 and 5% Ca. 2. The slag used in Reference Example 5 was the same as in Reference Example 1.

Table 4 Chemical composition C Si Mn Cr P S O Mother material 0.93 0.34 0.39 1.57 0.012 0.035 1 1 1 E.S.R. material of the present 0.93 0.28 0.40 1.61 0.007 0.005 l0 invention v E.S.R. material of Reference 0.95 0.21 0.39 1.61 0.011 0.012 83 Example (7) E.S.R. material of Reference 0.95 0.26 0.39 1.60 0.012 0.018 53 Example (8) Remarks:

1. The slag used in the present invention consists of 82% CaF 157: BaO and 37: Ba. 2. The slag used in Reference Example 7 was the same as in Reference Example 1.

3. The slag used in Reference 8 consisted of 85% CaF, and 15% BaO.

Table Chemical 1 composition C Si Mn Ni Cr Mo W Co Nh Ta N P S O (wtf/r Mothermaterial 0.12 0.66 1.74 20.3 22.34 2.85 2.35 19.47 0.38 0.52 012 0.025 0.013 6 E.S.R. material oftheprcsent 0.13 0.56 1.72 20.3 22.29 2.85 2.35 19.37 0.36 0.50 0.10 0.020 0.005 invention E.S.R. material ofRcferencc 0.13 0.42 1.70 20.4 22.21 2.86 2.34 19.39 0.35 0.52 0.10 0.026 0.010 58 Examplc(9 E.S.R. material ofReferencc 0.13 0.51 1.69 20.4 22.35 2.86 2.35 19.44 0.36 0.50 0.11 0.027 0.010 42 Example 1 l0) Remarks:

1. The slag used in the present invention consisted of 9671 MgF: and 4% Mg. 2. The slag used in Reference Example 9 was the same as in Reference Example 1. 3. The slag used in Reference Example 10 consisted of 100% MgF The present invention makes possible the attainment nent). In determining the amount of metallic compoof its primary object of providing an electroslag remeltnents to be added, their loss caused by evaporation, oxing method for producing a metal, in which such impu- 2O idation and the like must be taken into consideration. rities as phosphorus. sulfur, nitrogen, oxygen, arsenic As for the concentration up to which the metallic com and antimony have been extremely reduced, without ponents is to be increased, it varies according to the opcausing a loss of alloy components due to the oxidation erating conditions. Therefore, the optimum concentrathereof as occurs in an ordinary type electroslag retion and the amount ofmetallic component to be added melting, in which a consumable electrode having a must be determined in each instance. However, the fealarge cross-section is used. sible upper limit of concentration is 6% by weight.

However, since the method of this invention is a re- In the following, the above mentioned concepts shall melting method, it is conditioned by a phenomenon be explained concretely with reference to examples. which occurs in the remelting method in general, that with the progress of the refining of an ingot from the EXAMPLE 6 bottom part thereof in the initial period of refining to A stainless steel (SUS 304) of a diameter of mm, the head part thereofin the final period of refining, the containing 0.046% N, 0.028% P, 0.006% As and slag in the final period is correspondingly more CO 0.0022% Sb as a mother material, was remelted under taminated than in the initial period, by the conce the undermentioned conditions as shown in Table 6 in tion of impurities absorbed and removed from the 35 a water-cooled ti ld h i a di f 70 metal. Therefore, in case the refining operation is not mm d d f copper, i an argon atmosphere t bsufficiently carefully conducted, there will be a differrain an ingot f a i ht f b t 5 k Table 6 Charge Composition of Weight Amount of C a Concentration No. slag charged of supplcmentally of Ca in slag [weight 7: slag added (x) after remelting 1 CaF- 90%;Ca0 47!,CZ1 6% 500g none 3.8% 2 Same as No. 1 500g 0.3g/min. 5.8% 3 Cal- 90%;Ca0 fi /ucu 4'7( 500g 0.7g/min. 6.0%

(x) The addition was carried out through the remelting period.

ence in the ratio of removing the V A group elements In FIGS. 1,2,3 and 4 there are shown the distribution (such as phosphorus, nitrogen, arsenic and antimony) 50 of the content of each of the elements, nitrogen, phosbetween the head part and the bottom part of the ingot. phorus, arsenic and antimony, in the lengthwise direc- Further, it is to be noted that in the remelting method tion of the ingot, respectively. As is seen from these figthese V A group elements are removed with the same ures, there is substantial uniformity in the ratio of retendency. moving the respective elements as above mentioned,

The second object of this invention, i.e. regulating only when the concentration of the metalic compothe contents and effecting uniform distribution of phosnents, calcium, strontium, magnesium and barium, in phorus, nitrogen, arsenic and antimony in the ingot, the slag is increased, as is in the case of Charge No. 3. can be achieved by increasing the concentration of me- [It is to be noted that in the figures, Nos. 1, 2 and 3 reptallic components in the slag, such as calcium, stronresent Charge Nos. 1, 2 and 3 in Table 6, respectively] tium, magnesium or barium, with the lapse of time, as Heretofore, it was very difficult to cause a slag used above mentioned. The details of this shall be explained in an ordinary electroslag remelting to stably contain in the following. nitrogen, phosphorus, arsenic and antimony, if a Thus, as regards the method of increasing the conmelted metal containing more than a fixed amount of centration of the metallic components in the-slag with components such as chromium, silicon, titanium, aluthe lapse of time, each of the metallic components may minium and the like, which are easily oxidized, coexists be added to the slag in the form of an element or its th the g- On he ontrary, in the case Of the Slag alloy or a mixture of the fluorides and metallic compo- Used n t s invention, it is easy to permit the Slag to nents (but high in the concentration of metallic compocontain a large amount of the elemental impurities, as

above mentioned, even under the co-existence of the slag with the melted metal containing components which are easily oxidized. This is apparent from the fact that the slag used in this invention has a great ability for removing nitrogen, phosphorus, sulfur, arsenic, antimony and oxygen from a refined metal. This is regarded as an important feature of this invention.

Therefore, under the utilization of the phenomenon above mentioned, this invention has succeeded in establishing a method in which the slag is caused to contain at least one element selected from the group consisting of nitrogen, phosphorus, sulfur, arsenic and antimony in advance, to determine the distribution faculties of the respective elements between the slag and the metal, whereby the transfer of these elements from the melted metal to the slag may be restricted, and the content of these elements in the ingot may be freely regulated. This is also one of the important features of this invention.

As for the method of causing the slag to contain elements belonging to the V A group of the periodic system, each of them may be added in elemental form or its compound. In the case of using the compound form, it is necessary that the temperature is high and the vapour pressure is low, and that the compound is decomposed in the slag. Further, in the case of nitrogen, a gaseous nitrogen may be used in such a manner that the slag is caused to absorb the same. It is also permissible to use a nitrogen compound, such as Cr N, Fe,N and the like, under the fulfillment of the above mentioned conditions.

However, in the case of using gaseous nitrogen, when causing the slag to absorb the gaseous nitrogen it is necessary to regulate the partial pressure of the nitrogen gas, because the absorbing velocity depends upon the partial pressure of nitrogen in the atmosphere.

Further, in the case of adding phosphorus, arsenic and antimony, they are added in the form of their respective compounds, such as, for instance, P Ca P0,) and the like.

An explanation shall be given with reference to an example.

EXAMPLE 7 In the known electroslag remelting method there has of course usually been carried out a technique of adding alloying components through a slag. However, for the purpose of regulating the objective components to a fixed amount in this known electroslag remelting method, the amount of the. alloying components to be added is determined while considering loss due to the oxidation thereof during the adding operation carried out through the remelting period. The method of this invention differs from this known electroslag remelting method in this point. That is, according to the method of this invention the amount of the alloying components to be added is determined in consideration of the distribution ratio of the components between the slag and the metal. Consequently, according to the method of this invention certain advantages can be realized. Thus, the content of the added elements belonging to the V A group of the periodic system in the slag may reach the concentration of several times or even a hundred times as much as that in the metal. Moreover, as the distribution ratio betweeen the slag and the metal changes very gradually, fluctuation in the concentration of the objective components in the metal can be made very slight, because of a buffer effect caused by the slag phase, even if the addition is not carried out in an even amount with the lapse of time.

In short, the method of this invention is based on the findings'that the elements such as phosphorus, nitrogen, arsenic and antimony are distributed between the slag and the metal in a fixed ratio, and that this distribu tion ratio depends upon the composition and temperature of the steel and the composition of the slag, and particularly upon the concentration of the metallic components such as calcium, strontium, magnesium and barium contained in the slag. The prominent effects of removing phosphorus, nitrogen, arsenic and antimony, and freely regulating the content of these elements in the metal, as achieved by the method of this invention, are easily understood as a logical consequence of these findings.

In summary, such prominent effects as will be explained in the following can be achieved by the method of this invention.

1. An electroslag-remelting ingot, in which elemental impurities such as phosphorus, sulfur, nitrogen, oxygen, arsenic and antimony have been reduced to an extremely small amount, can be obtained. Particularly, it is easy to produce a metal containing less than 0.005% sulfur and less than 0.001% oxygen.

2. An ingot of a uniform structure, in which the content of phosphorus, nitrogen, arsenic and antimony can Table 7 C harge No. Composition of slag charged Atmosphere l CaF %;Ca0 6%;Ca 4% Ar 2 CaF 90%;Ca0 6%;Ca 4%.Cr N 0.2% Ar 98%;N 271 3 CaF 90%;Ca0 5%;Ca 4'/(;Ca,,(PO 0.8% Ar FIG. 5 shows the distribution of phosphorus and nitrogen in the ingot, respectively. As is seen from this figure, it is possible to regulate the content of nitrogen by adding nitride to the slag or regulating the atmosphere. g

In the same manner it is also possible to regulate the content of phosphorus by adding a phosphorus compound to the slag.

be freely regulated, can be obtained.

3. In addition to the above mentioned advantages, the following is to be mentioned as a further advantage, when compared with the conventional electroslag remelting method.

In contrast with the conventional method, in which the deoxidation is usually conducted by using calcium, strontium, magnesium of barium in the form of metal or its compound, the method of this invention does not directly incorporate these metals into a metal bath to be refined, so that the reaction proceeds calmly and in safety, and moreover there is no produced inclusion remaining in the metal.

4. Still further, as compared with the electroslag melting method of a special type, as proposed in US. Pat. No. 3,507,968, the method of this invention has the following advantages:

i. As a consumable electrode of a usual shape and size is employed, the device is so simple that its construction, maintenance and management are easy and further there occurs less chance of mishap to the device. Also the life of the electric contact portion of the device is long.

ii. As the operation is not required to function as a magnetic body, there is no limitation to the rise in temperature thereof.

iii. As the operation is carried out in a non-oxidative atmosphere, the electrode is less oxidized.

iv. It is so easy to produce an ingot of a large crosssection.

v. As there is no need of using an expensive alloy powder, the material cost is low.

What we claim is:

1. An electroslag-remelting method for refining a metal which comprises immersing the unrefined metal in a slag contained in a water-cooled casting mold, the slag having an electric current applied thereto and consisting essentially of l) 0.5 to 6% by weight of at least one metallic element selected from the group consisting of calcium, strontium and barium and (2) at least one fluoride selected from the group consisting of calcium, strontium, magnesium and barium fluorides, any impurities in the form of alumina, silica, titania and manganese oxide being present in an amount of less than 0.5% each, by weight, and carrying out remelting and refining of the metal in an atmosphere of at least one gas selected from the group consisting of argon, helium and nitrogen.

2. The method according to claim 1, wherein the concentration of at least one of the metallic elements contained in the slag is increased with the lapse of time commencing from the remelting of the metal.

3. The method according to claim 1, further comprising adding at least one of nitrogen, phosphorus, arsenic and antimony to the slag in the form of their elements or compounds.

4. The method according to claim 3, wherein nitrogen is added to the slag in gaseous elemental form and the partial pressure of the gaseous nitrogen is regulated to permit the slag to absorb the gaseous nitrogen.

5. An electroslag-remelting method for refining a metal which comprises immersing the unrefined metal in a slag contained in a water-cooled casting mold, the slag having an electric current applied thereto and consisting essentially of l 0.5 to 6% by weight of at least one metallic element selected from the group consisting of calcium, strontium and barium, (2) at least one oxide selected from the group consisting of calcium, strontium, magnesium and barium oxides in an amount of less than 30% by weight, based on the total weight of the slag and (3) at least one fluoride selected from the group consisting of calcium, strontium, magnesium and barium fluorides, any impurities in the form of alumina, silica, titania and manganese oxide being present in an amount of less than 0.5% each, by weight, and carrying out remelting and refining of the metal in an atmosphere of at least one gas selected from the group consisting of argon, helium and nitrogen.

6. The method according to claim 5, wherein the concentration of at least one of the metallic elements contained in the slag is increased with the lapse of time commencing from the remelting of the metal.

7. The method according to claim 5, further comprising adding at least one of nitrogen, phosphorus, arsenic and antimony to the slag in the form of their elements or compounds.

8. The method according to claim 7, wherein nitrogen is added to the slag in gaseous elemental form and the partial pressure of the gaseous nitrogen is regulated to permit the slag to absorb the gaseous nitrogen.

Claims

1. AN ELECTROSLAG-REMELTING METHOD FOR REFINING A METAL WHICH COMPRISES IMMERSING HTE UNREFINED METAL IN A SLAG CONTAINED IN A WATER-COOLED CASTING MOLD, THE SLAG HAVING AN ELECTRIC CURRENT APPLIED THERETO AND CONSISTING ESSENTIALLY OF (1) 0.5 TO 6% BY WEIGHT OF AT LEAST ONE METALLIC ELEMENT SELEXTED FROM THE GROUP CONSISTING OF CALCIUM, STRONTIUM AND BARIUM AND (2) AT LEAST ONE FLOURIDE SELECTED FROM THE GROUP CONSISTING OF CALCIUM , STRONTIUM, MAGNESIUM AND BARIUM FLOURIDES, ANY IMPURITIES IN THE FORM OF ALUMINA, SILICA, TITANIA AND MANGANESE OXIDE BEING PRESENT IN AN AMOUNT OF LESS THAN 0.5% EACH, BY WEIGHT, AND CARRYING OUT REMELTING AND REFINING OF THE METAL IN AN ATMOSPHERE OF AT LEAST ONE GAS SELECTED FROM THE GROUP CONSISTING OF ARGON, HELIUM AND NITROGEN.

1. An electroslag-remelting method for refining a metal which comprises immersing the unrefined metal in a slag contained in a water-cooled casting mold, the slag having an electric current applied thereto and consisting essentially of (1) 0.5 to 6% by weight of at least one metallic element selected from the group consisting of calcium, strontium and barium and (2) at least one fluoride selected from the group consisting of calcium, strontium, magnesium and barium fluorides, any impurities in the form of alumina, silica, titania and manganese oxide being present in an amount of less than 0.5% each, by weight, and carrying out remelting and refining of the metal in an atmosphere of at least one gas selected from the group consisting of argon, helium and nitrogen.

5. An electroslag-remelting method for refining a metal which comprises immersing the unrefined metal in a slag contained in a water-cooled casting mold, the slag having an electric current applied thereto and consisting essentially of (1) 0.5 to 6% by weight of at least one metallic element selected from the group consisting of calcium, strontium and barium, (2) at least one oxide selected from the group consisting of calcium, strontium, magnesium and barium oxides in an amount of less than 30% by weight, based on the total weight of the slag and (3) at least one fluoride selected from the group consisting of calcium, strontium, magnesium and barium fluorides, any impurities in the form of alumina, silica, titania and manganese oxide being present in an amount of less than 0.5% each, by weight, and carrying out remelting and refining of the metal in an atmosphere of at least one gas selected from the group consisting of argon, helium and nitrogen.