More particularly, the present invention relates to an additive composition which shows excellent activity in deoxidation, desulphurization and dephosphorization and makes slag to be of high fluidity. Also, the present invention is concerned with a method for making special steel superior in mechanical properties, by use of the additive composition.

BACKGROUND ART When making steel, lime (CaO) or fluorite (CaF2), amounting up to 10% by weight of the amount of the ore to be fed, is conventionally added in metal blast furnaces to remove impurities such as phosphorous (P) and sulfur (S) and to make the fluidity of slag better.

Also, in order to give the mechanical properties required for the use of special steel, metal additives are frequently used. Such additives, however, play an incomplete role in removing the impurities from iron melts.

In the case of using the metal additives for alloy, their specific gravities are different from that of iron, the base material, so that an imbalance occurs upon formulation of metals. In addition, the metal components of the additives are often oxidized, which makes it more difficult to obtain desired steel products. After all, expensive, high purity metal additives are

rising as an alternative, but give rise to an increase in the production cost.

The problems ascribed to the difference in specific gravity may be overcome by stirring the iron melts of metal blast furnaces at constant speeds, but this is extremely difficult. With the aim of avoiding the difference of specific gravity between iron components and alloy additive components in an iron melt, attempts have been made to make alloys in space, which is in a gravity-free state (e. g., M42 steel according to ASTM rule).

The resulting alloys, however, are extremely expensive.

Catalytic agents formulated with rare metals were developed to improve such situations. For instance, 0.003% (30 ppm) of bromine element was added to a molten metal to remove P and S therefrom and ultimately to make CBM steel of high hardenability.

Recent research for rare earth element combinations of lanthanum (La) and yttrium (Y) has allowed the making of special steel superior in wear resistance, impact resistance and toughness as well as reduced the amounts of conventional metal additives. This method, however, has such a disadvantage that the rare earth elements require the processes necessary for dressing and smelting.

DISCLOSURE OF THE INVENTION Therefore, it is an object of the present invention to overcome the above problems encountered in prior arts and to provide an additive composition for use in steel making, which is superior in deoxidation, desulphurization and dephosphorization and improves the fluidity of slag.

It is another object of the present invention to provide a method for making special

steel superior in mechanical properties, including impact resistance, wear resistance and thermal resistance.

In accordance with an aspect of the present invention, there is provided an additive an additive composition for use in steel making, comprising: 84.8-99.3% by weight of an oxide component; 0.5-1.6% by weight of a metal component; and 0.04-0.07% by weight of a rare-earth element component.

In accordance with another aspect of the present invention, there is provided a method for making special steel, comprising the steps of: adding over four times, four aliquots of the additive composition comprising 84.8-99.3% by weight of an oxide component; 0.5-1.6% by weight of a metal component; and 0.04-0.07% by weight of a rare-earth element component at an amount of 15-25% by weight of a scrap iron base to be molten, in a blast furnace while the temperature is maintained at removing slag from an iron melt in the furnace; and carburizing if the iron melt is short of carbon content.

BRIEF DESCRIPTION OF THE DRAWINGS The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: Figs. 1 a and I b are microphotographs showing the structures of a conventional steel product and a special high-speed tool steel of the present invention, respectively.

BEST MODES FOR CARRYING OUT THE INVENTION The additive composition useful in making special steel, according to the present invention comprises rare-earth elements (RE), metal elements and metal oxides.

Belonging to the group IIIa in the Periodic Table, the rare-earth elements are composed of 17 elements, that is, scandium (Sc) with an atomic number of 21, yttrium (Y) with an atomic number of 39, and the rare-earth metals, which are divided into the light rare- earth metals ranging, in atomic number, from 57 to 64 and the heavy rare-earth metals ranging, in atomic number, from 65 to 71. In the present invention, all or combinations of the rare-earth elements inclusive essentially of Ce, Nd, Sm, Pr, Eu, Gd and Dy, may be used.

La with atomic number 57, which is believed to give thermal resistance to the steel of the present invention, is preferably added at an amount of 0.009-0.01% by weight (90-100 ppm) based on the total weight of the scrap iron to be fed. Regarding the amounts of the other rare earth elements, by weight (120-140 ppm) are preferable for Ce with atomic number 58,0.0004-0.0006% by weight (4-6 ppm) for Pr with atomic number 59, 0.0009-0.0011 % by weight (9-11 ppm) for Nd with atomic number 60,0.0009-0.0011 % by weight (9-11 ppm) for Sm with atomic number 62,0.0001-0.0003% by weight (1-3 ppm) for Eu with atomic number 63,0.0009-0.0011% by weight (9-11 ppm) for Gd with atomic number 64,0.0005-0.0007% by weight (5-7 ppm) for Dy with atomic number 66,0.0001- 0.0002% by weight (1-2 ppm) for each of Ho, Er and Tu with atomic numbers 67,68 and 69, respectively, 0.0002-0.0004% by weight (2-4 ppm) for Yb with atomic number 70,0.002- 0.004% by weight (20-40 ppm) for Sc with atomic number 21, and 0.004-0.005% by weight (40-50 ppm) for Y with atomic number 39. The total amount of all of the rare earth elements

is preferably on the order of 0.03-0.05% by weight (300-500 ppm) based on the weight of the scrap iron to be fed.

Examples of the metals useful for the present invention include titanium (Ti) with atomic number 22, manganese (Mn) with atomic number 25, chromium (Cr) with atomic number 24, nickel (Ni) with atomic number 28, strontium (Sr) with atomic number 38, barium (Ba) with atomic number 56, and germanium (Ge) with atomic number 32. In accordance with the present invention, all or combinations of the metals, inclusive essentially of Ti, Mn and Cr, are used. In a preferable formulation of the composition, Ti is present at an amount of 1.1-1.5% by weight, Mn at 0.10-0.15% by weight, Cr at 0.01-0.05% by weight, Ni at by weight, Sr at 0.05-0.07% by weight and Ba at 0.04-0.06% by weight.

As for the metal oxides, they comprise SiO2, Al203, CaO, MgO, Na2O, K20, CoO3 and Fe203. When being molten along with an iron base material in a blast furnace, CaO improves the fluidity of slag in cooperation with the rare earth elements. SiO2 contributes to the absorption of impurities while MgO is preventive of mesotherm. Deoxidation of the iron melt is effected by Au203.

In accordance with the present invention, the metal oxides are contained at an amount of 84.8-99.3% by weight, based on the total weight of the composition, in order to make use of the functions of the oxides.

In accordance with the present invention, the composition comprises 8-10.4% by weight of CaO, 43-45.5% by weight of Six2,8-10.4% by weight of MgO, 13-15.5% by weight of Al203,3-4.8% by weight of NaO, 1.5-1.9% by weight of K20,0.3-0.5% by weight of CoO3, and 8-10.3% by weight of FeO3, based on the total weight of the composition.

Preferably, the additive composition has a particle size of 10 mesh or less. In order

not to lose calories while melting iron materials in a blast furnace, the temperature mist be maintained at at least 1,600°C upon feeding the additive composition.

For making general steel, the additive composition is used at an amount of 0.01- 0.10% by weight of the weight of the iron material to be fed. However, the amount of the additive composition depends on the kinds of the steel to be produced. The additive composition is added at an amount of 15-17% by weight of the iron material to be fed, for special carbonic tool steel, at 18-20% by weight for special tool and die steel, and at 21-25% by weight for special high-speed tool steel.

When scrap iron is melted in a blast furnace, metal carbides (MXCy) and iron carbide (Fe3C) are formed by the actions of the metal components while the rare-earth elements exert potent deoxidation, desulphurization and dephosphorization in the iron melt. In addition, the oxides play a role in improving the fluidity of the iron melt. As a result, the metal carbides and iron carbides together form tight cementite structures in which hexagonal systems are introduced, shortening intercarbonic distances. The carbon steel having such a hexagonal spheroidite is highly resistant to impact, wear and heat. Also, it is greatly improved in hardness and tension and easy to heat-treat without deformation. Consequently, the additive composition of the present invention penetrates into steel by virtue of the affinity and catalytic activity of itself upon melting in a furnace, leading to a great improvement in quality of steel.

A better understanding of the present invention may be obtained in light of the following examples which are set forth to illustrate, but are not to be construed to limit the present invention.

EXAMPLE I Additive Composition for Use in Steel-Making An additive composition was prepared as indicated in Table 1, below.

TABLE 1 Oxides Metals Rare-Earth Elements Component wt% Component wt% Component wt% Y 0.0043 La 0.0095 Ce 0.0131 SiO2 45.45 Ti 1.13 Nd 0.00715 Al203 15.33 Mn 0.13 Sm 0.0010 CaO 10.19 Cr 0.04 Pr 0.0005 MgO 10.07 Ni 0.02 Eu 0.0002 Na2O 4.55 Sr 0.06 Gd 0.00107 KO 1.79 Ba 0.05 Dy 0.00062 Co03 0.48 Ge Ho 0.00010 Fe 10.18 Er 0.00011 Tm 0.00013 Yb 0.0005 Sc 0.0035 U 0.00007 Sum 0.04185 The total weight was 99.51185% while ignition loss was 0.48815%.

EXAMPLE II In making special steel, the additive composition of Example I was used at an amount Df 0.08% by weight on the total weight of raw scrap iron (with a content of ca. 0.3% of C, : a. 0.15% of Si and ca. 0.1 % of Mn). While four aliquots of the additive composition were

each added to the blast furnace, the temperature was always maintained at 1,650°C to completely dissolve the composition. At this time, a large ignition loss was highly apt to be produced. Thus, slag must be removed with caution against the ignition loss. Thereafter, carburization was executed if the carbon content was measured to be short.

Increasing the fluidity of the slag, the rare-earth elements served to remove phosphorous and sulfur from the molten metal. They were associated with sulphur to give particles such as RE20,-S and RE, S3 or sulfides such as RES which were, then, removed as slag. Some rare-earth elements were associated with Al203 to form REAI, Og particles which were also removed as slag. That is, rare-earth elements also served as a potent deoxidizer of depriving dissolved oxygens of the metal molts. Therefore, this deoxidation effect could considerably reduce the amount of the deoxidizer added, such as ferro-silicon (Fe-Si) and ferro-manganese (Fe-Mn).

Of the rare-earth elements, La, Y and Ce showed potent catalytic actions. They each combine with carbon at a ratio of one or two molecules per one carbon to give metal carbides which can make the structure of steel better than can iron-carbide (Fe3C) called cementite.

Taken together, the functions of the rare-earth elements make it possible to make high quality special steel which is remarkably free of phosphorous and sulfur, both causing brittleness, and is greatly improved in mechanical properties. Consequently, the additive composition according to the present invention allows the making of special steel superior in mechanical properties, at lower production costs than do conventional additive compositions.

The rare-earth elements added in an electric furnace fiercely react with the iron melts boiling therein, so they are spontaneously mixed together. The iron melts are greatly

improved in fluidity by virtue of the oxides of the additive composition, such as CaO. Meanwhile, the impurities resulting from the dephosphorization, desulphurization and deoxidation, such as RE202S, REAI"O, 8, RES and RE2S3, are absorbed in the slag formed by SiO2, CaO, MgO. The slag rises to t he surface of the iron melts to shield the iron melts from being in contact with the sir, thereby preventing the alloy elements from being oxidized.

The various metals of ores are associated with carbon under the potent catalytic action of La, Y and Ce to form metal carbides (MXCy) in the presence of which the resulting alloy can have superior alloy structures. Cooperating with one another, the additives of the present invention make desired alloy. After taking off the slag and executing a final deoxidizing process, the iron melts are introduced into ingot cases which have a size suitable for rolling and forging. The ingots thus obtained are immediately subjected to annealing before quenching, to rolling or forging at 1,200°C, and to heat treatments depending on their uses.

EXAMPLE III Special Carbonic Tool Steel Using the additive composition as indicated in Table 1, special carbon tool steel was made in a similar manner as that of Example II.

That is, an additive composition comprising 0.04% by weight or more of the rare- earth elements, 98% by weight or more of the oxides and 1.4% by weight or more of the metal elements, was added at an amount of 15-17% by weight per ton of scrap iron, followed by addition of 1.5 kg of ferro-titan (Fe-Ti) with a grade of 40%. Impurities were removed from iron melt by the cooperation of the deoxidation due to the catalytic action of the rare-

earth elements with the deoxidation, desulphurization and dephosphorization due to the reactions of the oxides, and 40% by weight of the ferro-titan (Fe-Ti) added was made to incorporate in the iron melt, to give five types of novel rare-earth special carbonic tool steel called KRS-2300 series, as shown in Table 2, below.

TABLE 2 Rare-Earth Special Carbonic Tool Steel in KRS-2300 Series KRS-2300 series JIS AST C Si Mn P S M KRS-2301 SK-1 W-13 1.30-1.50 0.35 0.50 <0.020 <0.020 KRS-2302 SK-2 W-11 1.10-1.30 0.35 0.50 <0.020 <0.020 KRS-2303 SK-3 W1-9 0.90-1.10 0.35 0.50 <0.020 <0.020 KRS-2304 SK-4 W1-8 0.70-0.90 0.35 0.50 <0.020 <0.020 KRS-2305 SK-5 WI-7 0.60-0.70 0.35 0.50 <0.020 <0.020 EXAMPLE IV Special Tool and Die Steel Using the additive composition as indicated in Table 1, special tool and die steel was made in a similar manner as that of Example II.

An additive composition comprising 0.04% by weight or more of the rare-earth elements, 98% by weight or more of the oxides and 1.4% by weight or more of the metal elements, was added at an amount of 18-20% by weight per ton of scrap iron, followed by addition of 2 kg of ferro-titan (Fe-Ti) with a grade of 40%. Impurities were removed from

the iron melt by the cooperative action of the rare-earth elements, the oxides and the ferro- titan. That is, the rare-earth elements catalyzed deoxidation while the oxides showed deoxidation, desulphurization and dephosphorization. In addition, when 40% of the added ferro-titan was incorporated in the iron melt, its potent deoxidation and catalytic action promoted to form iron carbide structures and titanium carbide structures, which led the alloy to spherodized structures, together. Ten types of novel rare-earth special tool and die steel called KRS-2200 series as shown in Table 3, below, were made.

TABLE 3 Rare-earth Special Tool and Die Steel in KRS-2200 SERIES KRS JIS ASTM C Si Mn P S Ni Cr Mo W V Series KRS-SKS51 L6 <0.02 1. 3-0. 20- 2201 0. 85 2.0 0.50 KRS-SKS2i 1. 00-0. 35 0. 50 <0.02 <0.02-0. 20--0. 50- 0. 10- 2202 1. 10 0.50 1.00 0. 25 KRS-SKSII F2 1. 20-0.35 0.50 <0.02 <0.02 - 0.20- - 3.00- 0.10- 2203 1 30. 1.00 4.00 0. 30 KRS-SKS4 <0.02 - 0.20- 0.50- - 2204 0. 55 0.50 1.00 KRS-SKS44 W2,0. 80-0.35 <0.02--0. 10- 2205 0. 90 0.25 KRS-SKS95 W5 <0.02 0. 20-- 2206 0. 90 0.60 KRS-SKS94 0.8- <0.02 <0.02- 0.20- - - - 2207 1. 00 1. 1 0.60 KRS-SKS3 OI <0.02 0. 20-- 2208 1.00 1.2 0.60 KRS- SKD11 D2 1.40- 0.40 0.60 <0.02 <0.02 11. 0- 0. 80- 0.20- 2209 1. 60 1 13. 0 1. KRS-SKD12 A2 0.6-<0.02 <0.02 4 50-0. 20-0.20- 2210 1. 05 0. 9 _ 5. 50 1.20 0.50 The qualities of the steel obtained were examined and the results are given as shown in Table 4, below.

TABLE 4 Mechanical Properties of KRS-2200 Series Special Tool and Die Steel* Samples Tensile Yield Elongation Reduction Impact Quenching Hardness Strength Point (%) Area (%) (kgf/nun=) (kgf/mm2) (kg/mm) Avg. High Mid. Low KRS-90.5 45 15.5 28 12 63.5 64 63.5 63 2205 KRS-92.5 41 15 8.5 9 64.5 65 64.5 64 2206 * tested in Korea Advanced Institute of Science and Technology (KAIST), Korea Hardness was measured to be homogeneous over many parts of the samples. They were higher in tensile strength and elongation than corresponding JIS'. In addition, they were also measured to be strong and very resistant to wear and impact.

Heat treatments were made on the special tool and die steel in KRS-2200 series, according to the present invention, and corresponding mechanical properties were analyzed and their results are given as shown in Table 5, below.

TABLE 5 Heat Treatments and Mechanical Properties of KRS-2200 Series KRS series JIS ASTM Annealing Quenching Tempering KRS-2201 SKS51 L6 900-925°C HB207 80-1, 025°C 205-540°C HRC>62 SlowCooling Oil. Water Air KRS-2202 SKS21 900-925°C HB201 80-1, 025°C 205-540°C HRC>63 Slow Cooling Oil. Water Air KRS-2203 SKS 11 F2 900-925°C HB217 80-1, 025°C 205-540°C HRC>65 Slow Cooling Oil. Water Air KRS-2204 SKS4 900-925°C HB207 80-1, 025°C 205-540°C HRC>63 Slow Cooling Oil. Water Air KRS-2205 SKS4 W2_R" 900-925°C HB201 80-1, 025°C 205-540°C HRC>66 Slow Cooling Oil. Water Air KRS-2206 SKS95 W5 900-925°C HB212 80-1, 025°C 205-540°C HRC>63 Slow Cooling Oil. Water Air KRS-2207 SKS94 900-925°CHB2) 7 80-), 025°C205-540°C HRC>64 Slow Cooling Oil. Water Air KRS-2208 SKS3 O1 900-925°C HB217 80-1. 025°C 205-540°C HRC>64 Slow Cooling Oil. Water Air KRS-2209 SKDI I D2 900-925°C HB223 80-1, 025°C 205-540°C HRC>65 SlowCooling Oil. Water Air KRS-2210 SKD12 A2 900-925°C HB217 80-1, 025°C 205-540°C HRC>64 Slow Cooling Oil. Water Air Comparison between the KRS-2200 series special tool and die steel and the corresponding JIS'was made with regard to mechanical properties and the results are given as shown in Table 6, below.

TABLE 6 Comparison between KRS-2200 Series and JIS Heat Treatment/KRS-2205 SKS44 (JIS) KRS-2208 SKS-3 (JIS) Properties Heat Treatment Quenching 980-1,025°C 760-820°C 980-1, 025°C 830-880°C Water Oil Oil. Water Oil Tempering 205-540°C 150-200°C 205-540°C 150-200°C Air Air Air Air HRC >66 >62 >66 >62 Annealing 925-980°C 730-760°C 920-980°C 750-200°C Slow Slow Slow Slow HB <203 <213 <207 <217 Properties Impact <10 >6.5 >8.3 >5.5 Tensile Strength (kg/m) >90 >50 >90 >55 Elongation (%) >18 >8.5 >16 >7.5 As apparent from Table 6, the KRS-2200 series of the present invention are superior to corresponding JISs in various mechanical properties, including hardenability, hardness, tensile strength, impact resistance, wear resistance and processability. Upon heat treatment, the special tool and die steel of the present invention showed almost no decarburization or deformation.

EXAMPLE V High-Speed Tool Steel Using the additive composition as indicated in Table 1, high-speed tool steel was made in a similar manner as that of Example II.

An additive composition comprising 0.04% by weight or more of the rare-earth elements, 98% by weight or more of the oxides and 1.4% by weight or more of the metal elements, was added at an amount of 21-25% by weight per ton of scrap iron, followed by

addition of 3 kg of ferro-titan (Fe-Ti) with a grade of 40%. Impurities were removed from the iron melt by the cooperative action of the rare-earth elements, the oxides and the ferro- titan. That is, the rare-earth elements catalyzed deoxidation while the oxides showed deoxidation, desulphurization and dephosphorization. In addition, when 40% of the added ferro-titan was incorporated in the iron melt, its potent deoxidation and catalytic action promoted to form iron carbide structures and titanium carbide structures, which together led the alloy to spherodized structures. Ten types of novel rare-earth high-speed tool steel called KRS-2100 series as shown in Table 7, below, were made.

TABLE 7 Rare-Earth High-Speed Tool Steel in KRS-2100 Series KRS JIS ASTM C Si Mn P S Cr Mo W V Co series KRS-SKH2 T 1 <0.40 <0.02 <0.02 3.80--17. 0- 0. 80-- 2101 0.85 4. 50 19. 0 1.20 KRS-SKH3 T4 <0.40 <0.02 <0.02 3.80--17. 0- 0. 80- 4.50- 2102 0.80 _ 4. 50 19. 0 1.20 5.50 KRS-SKH4 T5 <0.40 <0.02 <0.02 3.80--17. 0- 1. 00- 10.0- 2103 0.80 4. 50 19. 0 1. 50 11.0 KRS-SKHI0 T10 1. 45-<0.40 <0.40 <0.02 <0.02 3.80--11. 5- 4. 20- 4.20- 2104 1.60 4.50 13.5 5.20 5.20 KRS-SKH51 M 2 <0.40 <0.02 <0.02 3.80- 4. 50- 5. 50- 1. 60-- 2105 0.90 4. 50 5. 50 6. 70 2.20 _ KRS-SKH52 M 3 1. 00-<0.40 <0.40 <0.02 <0.02 3.80-4. 50-5. 50-2. 30- 2106 1. 10 4. 50 5. 50 6. 70 2.80 KRS-SKH54 M 4 1.25- <0.40 <0.40 <0.02 <0.02 3.80-4. 50-5. 50-3. 90- 2107 1. 40 4. 50 5. 50 6. 50 4.50 KRS-SKH56 M36 <0.40 <0.02 <0.02 3.80- 4. 60- 5. 70- 1. 70- 5.50- 2108 0.95 4. 50 5. 30 6. 70 2.20 7.00 KRS-SKH58 M 7 <0.50 <0.02 <0.02 3.80- 8. 20- 1. 50- 1. 70-- 2109 1 05 4 50 9. 20 2. 10 2.20 KRS-SKH59 M 42 1. 00-<0.40 <0.50 <0.02 <0.02 3.80- 9. 00- 1. 20- 0. 90- 7.50- 2110 1. 15 4. 50 10. 0 1. 50 1.40 8.50

The qualities of the steel obtained were examined and the results are given as shown in Table 8, below.

TABLE 8 Mechanical Properties of KRS-2100 Series Special High-Speed Tool* Wear Hot Quenching Hardness (HRC) Sample Resistance Harness Toughness Grindability Avg. High Mid. Low KRS-2110 5.3.8.5.67.5 67.67.5 68.

* tested in KAIST, Korea Hardness was measured to be high and homogeneous over many parts of the sample.

It was superior to corresponding JIS'in grindability and toughness. It was also measured to be high in wear resistance and impact resistance, low in impurity content and homogeneous in quality.

With reference to Figs. la and I b, there are microphotographs showing the structure of a conventional product and that of the special high-speed tool steel of the present invention. As shown, the structure of the tool steel according to the present invention is tighter by 200% or more than that of the conventional product.

INDUSTRIAL APPLICABILITY As described hereinbefore, the additive composition in accordance with the present invention makes steel tight in structure and provides it with a great improvement in the resistance to impact, wear and heat. So, the present invention is very useful to regenerate

scrap iron.

The present invention has been described in an illustrative manner, and it is to be understood the terminology used is intended to be in the nature of description rather than of limitation. Many modification and variations of the present invention are possible in light of the above teachings. Therefore, it is to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.